Chondroitin Sulfate Proteoglycans in CNS Development and Pathophysiology

ABSTRACT

Chondroitin sulfate proteoglycans (CSPGs) are major components of the matrix in many tissues including the central nervous system (CNS). Interactions between extracellular CSPGs and different cell types are crucial for the development of the CNS as CSPGs are heavily involved in maintaining the pool of progenitors, neurogenesis, neuronal migration and maturation, cortical lamination, synapse formation and stabilization, neuronal plasticity, and memory formation. CSPGs play distinct roles in CNS development and pathology. While physiologic levels of CSPGs have key roles in CNS development, CNS pathologies result in upregulation of CSPGs that pose a barrier to neuroregeneration. Extensive evidence shows that pathologic CSPGs interfere with various regenerative mechanisms including axonal elongation, immunomodulation, synaptogenesis, cellular replacement, and remyelination. At the cellular level, CSPGs' effects are mainly mediated through activation of leukocyte common antigen‐related receptor (LAR) and protein tyrosine phosphatase sigma (PTP‐σ) receptors. Various approaches have been developed to overcome the inhibitory effects of pathologic CSPGs including enzymatic degradation of CSPGs, blocking CSPG/LAR/PTP‐σ axis, and inhibition of CSPGs synthesis. Here, we will discuss the current understanding on the role and mechanisms of CSPGs in CNS development and pathologies and signaling pathways that mediate CSPGs' effects in the CNS. We will also review how CSPGs have been modulated in neurological disorders.

Summary

Chondroitin sulfate proteoglycans (CSPGs) are key extracellular matrix components that regulate central nervous system (CNS) development, plasticity, and disease. During development, CSPGs control neural precursor cell maintenance, neuronal migration, axonal guidance, synapse formation, and perineuronal net assembly. In contrast, pathological CSPG upregulation after injury or in neurodegenerative conditions restricts axonal regeneration, remyelination, and synaptic remodeling. These effects are mediated primarily through protein tyrosine phosphatase receptors PTP‐σ and LAR, with additional context‐dependent roles for integrins, EGFR, and Nogo receptors. This review synthesizes evidence from developmental and disease models, including stroke, spinal cord injury, multiple sclerosis, and Alzheimer's disease, and discusses therapeutic strategies targeting CSPG degradation, synthesis, and signaling to promote CNS repair.

Chondroitin sulfate proteoglycans (CSPGs) are critical components of the extracellular matrix in the central nervous system (CNS). In the developing CNS, CSPGs regulate neural precursor cells, neuronal migration, axon guidance, synapse organization, and peri‐neuronal net assembly. In neuropathological conditions, CSPG upregulation contributes to impaired axonal regeneration, remyelination, synaptic remodeling, and neuroinflammation. Therapeutic strategies that target CSPGs and their signalling receptors have shown promise for CNS repair in preclinical models.

1. Overview

Chondroitin sulfate proteoglycans (CSPGs) are a main component of the extracellular matrix (ECM) in the central nervous system (CNS) during development and adulthood (Dyck and Karimi‐Abdolrezaee ²⁰¹⁸). CSPGs are produced by different cell types in the CNS, including astrocytes, microglia, NG2 expressing cells, and pericytes (Mencio et al. 2021). Physiologic levels of CSPGs play important roles in the development of the CNS, including regulating proliferation of neural precursor cells (NPCs), neurogenesis, and guidance of growth cones (Mencio et al. 2021). Moreover, CSPGs are main components of the perineuronal net (PNN), which is crucial in modulation of synaptic plasticity, synaptic maturation, and stabilization (Gáti et al. 2010; Nakamura et al. ²⁰⁰⁹). Although physiological levels of CSPGs support neurodevelopmental and homeostasis processes, upregulation of CSPGs following CNS pathologies is predominantly inhibitory for the neuroregeneration process (Barritt et al. 2006; Bradbury et al. ²⁰⁰²; Hosseini et al. ²⁰²²; Karimi‐Abdolrezaee et al. ²⁰¹⁰; Lang et al. ²⁰¹⁵).

In response to CNS injuries and neurodegenerative conditions, the ECM undergoes significant pathological remodeling, which includes upregulation of CSPGs alongside changes in other matrix components such as tenascins, hyaluronan, and laminin (Cregg et al. 2014; Wareham and Calkins ²⁰²⁵). Upregulation of CSPGs aids in limiting the expansion of tissue injury in neurotraumatic pathologies (Anderson et al. 2016; Bradbury and Burnside ²⁰¹⁹); however, the excess deposits of CSPGs pose a chemical and physical barrier to neuroregeneration (Siebert et al. 2014; Mukherjee et al. ²⁰²⁰; Lin et al. ²⁰²¹; Yang et al. ²⁰²⁴). To target CSPGs' inhibitory effects on regeneration, different approaches such as degradation of CSPGs, suppressing CSPGs signaling, and inhibiting CSPGs synthesis have been developed (Siebert et al. 2014; Yang et al. ²⁰²⁴). Early studies showed that enzymatic degradation of CSPGs with chondroitinase ABC (ChABC) can improve axon regeneration and functional recovery in rodents with SCI (Bradbury et al. 2002). At the cellular level, extensive evidence shows that genetic deletion or pharmacological inhibition of CSPG receptors, the leukocyte common antigen‐related receptor (LAR) and protein tyrosine phosphatase sigma (PTP‐σ), reduces CSPG‐induced growth cone collapse and improves regenerative responses in vivo (Hosseini and Karimi‐Abdolrezaee ²⁰²⁵). Moreover, in vitro mechanistic studies demonstrate involvement of Nogo Receptor (NgR) and Epidermal Growth Factor Receptor (EGFR) in CSPG‐induced inhibition of CNS regeneration (Koprivica et al. 2005; Sami et al. ²⁰²⁰).

In this review, we provide a timely update on the recent knowledge related to CSPGs mechanisms of actions in the CNS including various signaling receptors and intracellular pathways that mediate CSPGs effects. We will critically discuss converging and diverging findings across multiple model systems and compare complementary therapeutic strategies aimed at modulating CSPGs signaling in the pathologic CNS with the aim to provide an updated and interdisciplinary perspective on how CSPGs regulate neural plasticity and pathology.

2. Structure of CSPGs

CSPGs are composed of a core protein and sulfated glycosaminoglycan (GAG) side chains that are formed by covalent binding (Lin et al. 2021). To synthesize chondroitin sulfate (CS)‐GAG chains, xylose links to the core protein followed by conjugation of two galactose residues to xylose (Lin et al. 2021). Then, chondroitin polymerization occurs by sequential addition of a repeating disaccharide unit containing D‐glucuronic acid (GlcUA) and N‐acetyl‐D‐galactosamine (GalNAc) (Avram et al. 2014). Modifications of the GAG side chains continue through sulfation at defined positions. This process is carried out by a diverse family of chondroitin sulfotransferases rather than a single enzyme (Kitagawa et al. 2003, 2001). In humans and mice, at least seven sulfotransferases, including CHST3, CHST7, CHST11, CHST12, CHST13, CHST14, and CHST15, catalyze the transfer of sulfate to specific hydroxyl groups on GalNAc or glucuronic acid residues that generate distinct motifs such as C4‐sulfated, C6‐sulfated, and over‐sulfated CS structures. Through this process, each enzyme exhibits strict substrate and positional specificity that results in the structural heterogeneity and functional diversity of CSPGs (Izumikawa et al. 2008; Kitagawa et al. ^{2003, 2001}; Mikami and Kitagawa ²⁰¹³). The specific sulfation pattern of the carbohydrate part defines the protein binding characteristics of these polysaccharides and the overall functions of CSPGs in the CNS (Gama et al. 2006). Lecticans such as aggrecan, brevican, neurocan, and versican are the main sub‐forms of CSPGs in the CNS which have a similar N‐terminal hyaluronan binding domain, central domain, and C‐terminal globular domain (Yamaguchi 2000). We will discuss the main members of the CSPGs family in the following sections.

3. Members of CSPGs Family

3.1. Aggrecan

Aggrecan is encoded by the Acan gene, and it consists of three globular domains (G1, G2, and G3) and three main extended domains including the inter‐globular domain (IGD), keratan sulfate (KS), and the chondroitin sulfate (CS) domain (Kiani et al. 2002; Matthews et al. ²⁰⁰²). Among these domains, globular domains are highly conserved, while extended domains vary among different species (Kiani et al. 2002). Functionally, the G1 domain is mainly involved in cell interaction, cell recognition, and immune response (Aspberg 2012; Grover and Roughley ¹⁹⁹⁴). G2 interacts with hyaluronic acid (HA) (Kiani et al. 2002), and G3 binds to fucose, galactose, heparin, and heparan sulfate (Aspberg et al. 1995; Julkanen et al. ¹⁹⁸⁸). IGD extended domain is situated between G1 and G2 domains and it is the action site of proteolytic enzyme and its degradation results in rapid loss of GAG chain (Kiani et al. 2002). KS domain of aggrecan has different structures depending on the tissue of origin. For example, this domain is implicated in the migration of neural crest cells during development (Hayes and Melrose 2020; Kiani et al. ²⁰⁰¹). The CS domain is the largest domain in aggrecan, which contains approximately 100 chains of chondroitin sulfates (Kiani et al. 2002). The CS domain is the main functional element in aggrecan's structure (Haynesworth et al. 1991; Kiani et al. ²⁰⁰¹).

In the CNS, aggrecan contributes to the formation of ECM and is a major component of PNN, a specialized matrix surrounding neurons with a crucial role in synaptic stability (Brückner et al. 2006; Morawski et al. ²⁰¹²; Rowlands et al. ²⁰¹⁸). Aggrecan has a high capacity to hold water and balance osmotic pressure due to its negatively charged CS domain (Kiani et al. 2001). Removal of aggrecan disrupts PNN structure and leads to a re‐emergence of juvenile‐like plasticity, which in turn enhances object recognition memory in adult mice (Rowlands et al. 2018). Taken together, aggrecan plays key roles in the integration of CNS matrix and PNN which modulate neuronal plasticity in the brain.

3.2. Versican

Versican (CSPG2) is encoded by VCAN gene which has multiple functions in the ECM of cardiac, cartilage, skin and CNS tissues (Lin et al. 2021). Versican has five different isotypes (V0, V1, V2, V3, and V4) because of alternative splicing patterns of exons which encode the attachment region for CS‐GAG to the core protein (Ito et al. 1995; Kischel et al. ²⁰¹⁰; Lemire et al. ¹⁹⁹⁹; Wight et al. ²⁰²³; Zako et al. ¹⁹⁹⁵; Zimmermann and Ruoslahti ¹⁹⁸⁹). In four of the versican isotypes (V0, V1, V2, and V4), CS‐GAG chains bind to the core protein by covalent linkage; however, V3 isotype does not have any GAG domain due to splicing of both C‐terminal and N‐terminals (Lemire et al. 1999). In the CNS, Versican is expressed and released by both neural and non‐neural cells including epithelial cells, leukocytes, astrocytes, neurons, and immature oligodendrocytes (Dyck and Karimi‐Abdolrezaee ²⁰¹⁵; Popp et al. ²⁰⁰⁴; Wight et al. ²⁰²⁰; Yamagata and Sanes ²⁰⁰⁵). In the CNS, V1 isotype is predominantly expressed in late neurodevelopmental stage by NPCs, astrocytes, neurons, and oligodendrocytes and it regulates proliferation and neurogenesis (Gu et al. 2007; Zhang et al. ²⁰¹¹). V2 isotype is mainly found in the adult CNS, and it negatively regulates neurite outgrowth and cell proliferation, and it induces apoptosis (Schmalfeldt et al. 2000). Following CNS injury, inflammatory cytokines such as tumor necrosis factor‐α (TNF‐α) and interferon‐γ (IFN‐γ) trigger upregulation of V2 isoform in astrocytes, NPCs, and oligodendrocytes which hinders the neuroregeneration process (Gu et al. 2007; Schmalfeldt et al. ²⁰⁰⁰). In conclusion, versican is a versatile member of CSPGs family, it is widely distributed in different tissue, and it plays differential roles in CNS development and injury.

3.3. Brevican

The core protein of Brevican (CSPG7), another member of the CSPG family, is encoded by the BCAN gene (Lin et al. 2021). Brevican is constituted of an N‐terminal globular domain (G1), central CS, and a C‐terminal globular domain (G3) (Yamada et al. 1994). G1 contains an immunoglobulin‐like loop which binds to HA (Frischknecht and Seidenbecher 2012; Yamada et al. ¹⁹⁹⁴), while G3 consists of three different components including an epidermal growth factor (EGF)‐like module, a C‐type lectin‐like domain, and a complement regulatory (CRP)‐like domain (Yamada et al. 1994). Brevican has an alternatively spliced Glycosylphosphatidylinositol (GPI)‐anchored form (Seidenbecher et al. 1995) which does not have a G3 domain, and the GPI anchor binds to G1 and NH domains (Seidenbecher et al. 1995).

Brevican is only found in the CNS with gradual increase in its expression during development that reaches the steady level in adulthood (Seidenbecher et al. 1998; Yamada et al. ¹⁹⁹⁴). It is mostly synthesized by astrocytes, immature oligodendrocytes, and neurons (John et al. 2006; Ogawa et al. ²⁰⁰¹). Brevican is a major component of PNN. Under physiological conditions in mice, brevican is involved in maintaining long‐term potentiation (LTP) (Brakebusch et al. 2002) and connecting ECM to the axon initial segment and node of Ranvier through neurofascin‐186 (Bekku et al. 2009). Altogether, brevican is a CNS‐specific CSPG with a key role in the structure and function of neurons, facilitating the interaction of neurons with their surrounding PNN.

3.4. Neurocan

Neurocan is another CNS specific CSPG that is mainly distributed in the adult cortex, cerebellum, striatum and hippocampus (Deepa et al. 2006; Kurazono et al. ²⁰⁰¹; Yamaguchi ²⁰⁰⁰). Neurocan (CSPG3) core protein is encoded by the NCAN gene (Lin et al. 2021) and comprises a CS chain and O‐linked oligosaccharide chains which make it similar to aggrecan (Rauch et al. 2001; Yamaguchi ²⁰⁰⁰). N‐terminal of neurocan consists of one immunoglobulin module and two link modules (Rauch et al. 2001). The central region of neurocan is formed by nearly 600 amino acids which are mostly serine, threonine, and proline (Rauch et al. 2001). C‐terminal of neurocan is composed of two EGF‐like modules, one C‐type lectin module, and a 45‐amino acid C‐terminal extension module (Rauch et al. 2001; Yamaguchi ²⁰⁰⁰).

Neurocan is synthesized and released by neurons, and it is involved in PNN formation, axonal development, and morphological maturation of cortical neurons (Fontanil et al. 2019; Mohan et al. ²⁰¹⁸). During cortical development, neurocan is produced by cortical neurons in the subplate and intermediate zones, and it binds to HA and tenascin‐C (TNC) (Mubuchi et al. 2024). This ternary complex of neurocan, TNC, and HA plays a major role in the migration of radial glial cells and lamination of the cortical plate (Mubuchi et al. 2024). In addition to neurons, astrocytes also synthesize neurocan during development and in the early postnatal stage in mice (Irala et al. 2024). Astrocyte‐secreted neurocan has a central role in the formation and function of inhibitory synapses in the mouse brain (Irala et al. 2024). Taken together, neurocan is specific to the CNS, and it is associated with cortical development, neuronal morphogenesis, and circuit formation in the brain.

3.5. Phosphacan

Phosphacan is also specifically found in the CNS. It consists of the N‐terminal amino acid of core glycoprotein, internal CNBr domain, tryptic and endoproteinase Lys‐C peptide (Maurel et al. 1994). Phosphacan is the splice alternative of receptor‐type protein tyrosine phosphatase (RPTP) without the two intracellular phosphatase domains that are present in RPTP (Galtrey and Fawcett 2007; Siebert et al. ²⁰¹⁴). Developmentally, phosphacan is mainly found in proliferative zones such as the ventricular zone suggesting its potential role in regulating cell division (Busch and Silver 2007; Viapiano and Matthews ²⁰⁰⁶). Phosphacan stabilizes the PNN structure to the surface of neurons and modulates the spatial distribution of the PNN (Eill et al. 2020).

Phosphacan can both inhibit and promote neurite outgrowth (Faissner et al. 2006; Hayashi et al. ²⁰⁰⁵; Hayes and Melrose ²⁰²¹). Phosphacan is shown to inhibit neurite outgrowth of dorsal root ganglia (DRG) neurons (Sango et al. 2003), while it enhances neurite outgrowth and dendritic morphogenesis of mouse cortical neurons (Maeda and Noda 1996). In addition to neuronal morphology, phosphacan is involved in neuron–glia cross talk, myelination, neuronal differentiation, and axonal regeneration mostly because of extracellular carbonic anhydrase (CAH) and fibronectin‐ΙΙΙ repeat domains (Adamsky et al. 2001; Garwood et al. ²⁰⁰³). In conclusion, phosphacan is one of the CNS specific members of the CSPG family with various roles during development and adulthood.

3.6. NG2

NG2 is a membrane‐spanning proteoglycan, which is encoded by the CSPG4 gene. It is composed of three main subdomains including the N‐terminal D1 subdomain, D2 central nonglobular subdomain, and D3 globular juxtamembrane subdomain (Chelyshev et al. 2022). The N‐terminal D1 subdomain consists of laminin/neurexin/sex‐hormone (LNS) domains which integrate with other binding sites to facilitate cell–cell and matrix‐cell interactions (Rudenko et al. 1999). The D2 central nonglobular subdomain binds to GAGs and collagens, in particular collagen V and VΙ (Tillet et al. 1997). The D3 globular juxtamembrane subdomain contains β1 integrin and N‐linked oligosaccharide binding sites as well as proteolytic cleavage (Asher et al. 2005; Nishiyama et al. ¹⁹⁹⁵; Stallcup and Huang ²⁰⁰⁸; Wen et al. ²⁰⁰⁶).

NG2 is mainly secreted from oligodendrocyte precursor cells (OPCs) and modulates N‐methyl‐D‐aspartate (NMDA) receptor‐mediated LTP of pyramidal neurons of the somatosensory cortex in mice (Sakry et al. 2014). NG2 can be inhibitory or permissive for neurite outgrowth (Chelyshev et al. 2022). In mice with spinal cord transection, NG2 facilitates regeneration of serotonergic and sensory axons after injury (De Castro et al. 2005). In contrast, NG2 is reported to inhibit neurite outgrowth of DRG neurons and cerebellar granular neurons (CGNs) in vitro (Dou and Levine 1994; Fidler et al. ¹⁹⁹⁹). Altogether, NG2/CSPG4 is a distinct membrane‐associated form of CSPGs with versatile roles in synaptic plasticity and neurite outgrowth.

4. Developmental Roles of CSPGs

CSPGs family is heavily involved in the development of CNS, in particular brain development (Dyck and Karimi‐Abdolrezaee ²⁰¹⁵; Mencio et al. ²⁰²¹). Extensive studies have shown that CSPGs play critical roles in maintaining the NPCs population, regulating functions of NPCs, neurogenesis, axonal guidance, synapse formation, and myelination (Letourneau et al. 1994; Snow et al. ^{2003, 1991}). We will discuss different roles of CSPGs during CNS development in the following sub‐sections (Figure 1).

FIGURE 1.

Developmental roles of CSPGs. Physiological level of CSPGs is involved in various aspects of CNS development. CSPGs play major roles in regulating NPCs and neurogenesis, neuronal migration, and axonal elongation. CSPGs are also critical for the formation of perineuronal net (PNN), perinodal matrix, synapse formation, maturation, and stabilization.

4.1. Regulation of Neural Precursor Cells (NPCs) and Neurogenesis

Extensive evidence demonstrates that the composition of matrix in the sub‐ventricular zone (SVZ) and sub‐granular zone (SGZ) regulates the proliferation of NPCs and maintains the progenitor pool in the brain (Bandtlow and Zimmermann 2000; Ida et al. ²⁰⁰⁶; Yamada et al. ²⁰¹⁸). CSPGs are found in the ECM of the progenitor niches in the developing brain and are mainly secreted by NPCs (Bandtlow and Zimmermann 2000; Ida et al. ²⁰⁰⁶). Degradation of CSPGs in mice on embryonic day 13 (E13) reduces the ability of NPCs to form neurosphere in vitro, and it decreases their proliferation and neurogenic capacity (Ida et al. 2006). Physiological levels of CSPGs are essential for proliferation and neuronal differentiation of NPCs. In vitro digestion of CSPGs, using ChABC treatment, enhances astrocytes differentiation of E13 mouse cortical NPCs at the expense of neurogenesis (Sirko et al. 2010). In the hippocampus of adult mice, removal of CSPGs using ChABC enzyme significantly decreases proliferation of NPCs in the SGZ and consequently hippocampal neurogenesis, in particular generation of parvalbumin neurons (Yamada et al. 2018). In support of the positive role of CSPGs in neurogenesis, other studies have also shown that upregulation of genes related to biosynthesis of CSPGs such as CSGalNAcT1 and C6ST is associated with neurogenesis and neuronal distribution in mouse hippocampus (Maeda et al. 2022).

Mechanistically, CSPGs are essential for fibroblast growth factor‐2 (FGF‐2) signaling in NPCs as digestion of CSPGs impairs self‐renewal capacity of NPCs in response to FGF‐2 (Sirko et al. 2010). CSPGs activate FGF‐2 signaling in NPCs through phosphorylation of mitogen‐activated protein (MAP) kinase (Sirko et al. 2010). CSPGs also bind to other neurotrophic factors such as brain‐derived neurotrophic factor (BDNF) and pleiotrophin (PTN) and facilitate their signaling in NPCs, which further regulates proliferation and neurogenesis of NPCs (Karumbaiah et al. 2015; Miller and Hsieh‐Wilson ²⁰¹⁵; Yamada et al. ²⁰¹⁸). Additionally, CSPGs enhance proliferation and sphere formation of NPCs by activating EGFR signaling, which results in activation of the PI3K/JAK/STAT signaling pathway (Tham et al. 2010). In vitro studies show that CSPGs can also regulate growth and proliferation of NPCs by activating integrin signaling as a major surface receptor involved in interactions between NPCs and their surrounding matrix (Frost et al. 1999; Gu et al. ²⁰⁰⁹). In conclusion, CSPGs play important roles in regulating proliferation, growth, and neuronal differentiation of developing NPCs through various signaling pathways.

4.2. Neuronal Migration, Axonal Elongation, and Guidance

In CNS development, newly generated neurons in the ventricular zone are multipolar (Barry et al. 2014). Excitatory neurons migrate to the intermediate zone and change their morphology from multipolar to bipolar shape in the subplate which allows them to adhere to radial glial cell processes and migrate radially to the cortical plate and marginal zone (Hatanaka and Yamauchi 2013). In the cortical plate, neurons detach from radial glial cell processes and form cortical layers from deeper to superficial layers (Dehay and Huttner 2024). Phosphacan, aggrecan, and versican are upregulated in the subplate and marginal zone suggesting a possible role for CSPGs in cortical lamination (Meyer‐Puttlitz et al. ¹⁹⁹⁵; Oohira et al. ¹⁹⁹⁴; Popp et al. ²⁰⁰⁴). Various functions of CSPGs are mediated through binding of over‐sulfated structures of CSPGs to different proteins such as neural cell adhesion molecule (NCAM), laminin, tenascin‐C, and PTN (Long and Huttner 2021; Maeda et al. ²⁰¹⁰; Mencio et al. ²⁰²¹). The generation of over‐sulfated structures in CSPGs is mainly conducted by two CS sulfotransferase enzymes, uronyl 2‐O‐sulfotransferase (UST) and N‐acetyl‐galactosamine 4‐sulfate 6‐O‐sulfotransferase (4,6‐ST) (Maeda et al. 2010). Expression of UST and 4,6‐ST is increased in the ventricular zone and SVZ during cortical development in the late embryonic stage in mice (Ishii and Maeda 2008). Interestingly, downregulation of UST and 4,6‐ST in the mouse embryo resulted in impaired migration of newly born neurons and accumulation of these neurons in the intermediate zone and SVZ (Ishii and Maeda 2008). These neurons, which lack UST and 4,6‐ST, were unable to transit from multipolar to bipolar shape for radial migration supporting the critical roles that CSPGs play in neuronal migration and cortical lamination in neurodevelopment (Ishii and Maeda 2008). Moreover, reducing over‐sulfated structures in CSPGs, by downregulation of uronyl 2‐O‐sulfotransferase and N‐acetyl‐galactosamine 4‐sulfate 6‐O‐sulfotransferase enzymes, interrupts the roles of integrin and PTN in neuronal radial migration in the developing cortex (Ishii and Maeda 2008).

The matrix of marginal zone, subplate, and developing cortex is enriched by the presence of CSPGs family, in particular neurocan and phosphacan (Meyer‐Puttlitz et al. ¹⁹⁹⁵; Sheppard et al. ¹⁹⁹¹). These CSPG‐enriched regions prevent aberrant neuronal migration and neuronal morphogenesis by providing a non‐permissive environment for neuronal arborization (Zluhan et al. 2020). Interestingly, Cajal–Retzius cells in marginal zone express Reelin which counteracts CSPGs and facilitates neuronal migration (Noctor et al. 2020; Zluhan et al. ²⁰²⁰). Furthermore, gradual decrease in the production of CSPGs plays an important role in axonal elongation in the developing retina of mammals (Brittis et al. 1992). Removal of CSPGs in retinal cells of rats leads to disrupted localization of retinal ganglionic cells (RGCs) with random neurite outgrowth rather than guided axonal elongation (Brittis et al. 1992; Snow et al. ¹⁹⁹¹). Similarly, the mixture of CSPGs and laminin guides the axonal elongation of chicken DRG neurons in culture which is characterized by axonal fasciculation (Snow et al. 2003). Altogether, CSPGs exhibit important roles in neurite outgrowth and axonal guidance during the development of different regions of the CNS including the cortex, DRGs, and RGCs.

4.3. Perineuronal Net and Perinodal Matrix

PNN is a CSPG‐enriched and condensed matrix that surrounds neurons, in particular soma, proximal and middle parts of dendrites, and proximal axonal segments (Fawcett et al. 2019). In cortical area, mostly inhibitory parvalbumin‐expressing (PV+) fast‐firing GABAergic interneurons and some pyramidal excitatory neurons of deep cortical layers are embedded in PNN (Dauth et al. 2016; Matthews et al. ²⁰⁰²). PNN also enwraps excitatory neurons in other parts of the brain such as the amygdala, hippocampal CA2 region, and ventromedial hypothalamus (Morikawa et al. 2017). In the spinal cord, the majority of neurons in the ventral horns, intermediate horns, and the ventral part of the dorsal horns are embedded in PNN (Galtrey et al. 2008; Jäger et al. ²⁰¹³).

The structure of PNN is mainly composed of hyaluronan, CSPGs, link proteins, and tenascin glycoproteins (Tn‐C and Tn‐R) (Tewari et al. 2022). Among CSPGs family, aggrecan is the predominant component of PNN while the proportion of brevican, neurocan, and phosphacan varies depending on the CNS region (Fawcett et al. 2019). Ablation of aggrecan substantially disrupts the PNN structure in the mouse cortex, which results in a high plasticity state of inhibitory interneurons and visual cortex and improvement of object recognition memory (Rowlands et al. 2018). The brevican component of PNN is the main modulator of PV+ interneurons' synaptic plasticity through regulating the localization of potassium channels and AMPA receptors (Favuzzi et al. 2017). In the mouse cortex, neurocan is shown to stabilize peri‐somatic synapses in the PNN structure by regulating postnatal remodeling of interneuron axons through inhibition of NCAM/EphA3 signaling (Sullivan et al. 2018). Recent evidence shows that PNN phosphacan in association with tenascin‐R acts as an anchor to stabilize the components of PNN to the neuronal surface (Eill et al. 2020; Sinha et al. ²⁰²³). In the visual cortex, reducing chondroitin sulfate by deleting the CS‐synthesizing enzyme CSGalNAcT1 delays the onset of ocular dominance plasticity and prolongs its duration, showing that CS accumulation is important for both the opening and closing of the critical period (Hou et al. 2017). Additionally, mice lacking the PNN link protein Crtl1 (Hapln1) display attenuated PNNs that are associated with persistent plasticity in the visual cortex into adulthood, suggesting an important role for intact CSPG‐based nets in closing developmental plasticity (Carulli et al. 2010).

Otx2, a homeoprotein that binds to aggrecan‐containing PNNs, has also been implicated in the closing of the critical period, as disruption of Otx2‐PNN interactions in the adult visual cortex reopens plasticity and enables recovery from amblyopia in mice (Beurdeley et al. 2012; Spatazza et al. ²⁰¹³). Furthermore, conditional loss of aggrecan has shown that this CSPG is a key structural organizer of PNNs, and its deletion causes a long‐lasting juvenile‐like state of plasticity and enhances object recognition memory in adult mice (Rowlands et al. 2018). These findings collectively support an active role for CSPG‐rich PNNs in regulating the onset and closure of critical periods rather than serving as passive structural elements.

CSPGs also contribute to the matrix surrounding the nodes of Ranvier that is called perinodal matrix (Fawcett et al. 2019). Perinodal matrix is formed after myelination, and it is a barrier to ion‐diffusion which facilitates fast axonal conduction (Oohashi et al. 2002). The structure of perinodal net consists of hyaluronan, proteoglycan link protein 2 (HAPLN2), and CSPGs family, mainly brevican, V2 isoform of versican, and neurocan (Fawcett et al. 2019; Kwon and Koh ²⁰²⁰). Astrocytes secrete brevican and V2 versican where they contact axons at the nodes of Ranvier (Susuki et al. 2013). Astrocytes‐secreted brevican, neurocan, and V2 versican, in conjunction with neurofascin and HAPLN2 shape the perinodal matrix (Bekku and Oohashi 2010; Susuki et al. ²⁰¹³). Oligodendrocytes are also a major contributor to the formation of PNN and perinodal matrix by production of CSPGs, integrins, and laminin (Elbaz et al. 2024). The role of oligodendrocytes in this process is regulated by transcription factor Osterix (also known as Sp7) (Elbaz et al. 2024). Genetic ablation of Sp7 results in significant disruption of PNN and perinodal matrix and aberrant formation of node of Ranvier (Elbaz et al. 2024). Taken together, CSPGs critically contribute to the structure and function of PNN and perinodal matrix, with various roles such as synapse stabilization, cell‐matrix interaction, and axonal conductance.

4.4. Synapse Formation, Maturation, and Stabilization

All the synapses in the developing CNS are immersed in a complex structure of matrix such as HSPGs, CSPGs, Tenascin‐R, Tenascin‐C, and laminin (Fawcett et al. 2022; Hockfield et al. ¹⁹⁹⁰; Pintér et al. ²⁰²⁰). Genetic and direct in vitro studies have unraveled the specific roles of CSPGs in regulating synaptogenesis. For instance, in cultures of mouse primary cortical neurons, CSPGs decrease dendritic spine formation through acting on the PTP‐σ receptor and dephosphorylation of tropomyosin‐related kinase B (TrkB) as the main receptor for brain‐derived neurotrophic factor (BDNF) (Kurihara and Yamashita 2012). Brevican plays an important role in the coupling of pre‐ and post‐synaptic structures in PNN, and transgenic ablation of brevican results in substantial impairment of spatial coupling of pre and post‐synaptic elements in the inner ear in mice (Sonntag et al. 2018).

Developmentally, termination of the critical period of plasticity is temporally and spatially correlated with the expression of CSPGs in the CNS (Orlando et al. 2012). In mice, CSPGs that surround the developing synapses aid in stabilizing dendritic spines as their enzymatic digestion, using ChABC, increases the dynamicity of dendritic spines and spine head protrusion in CA1 pyramidal neurons of the hippocampus (Orlando et al. 2012). Similarly, digestion of CSPGs enhances the motility of dendritic spines of pyramidal neurons in the mouse visual cortex, which further enhances plasticity and visual evoked potential in mice (De Vivo et al. 2013). CSPGs are also involved in the formation and function of inhibitory synapses (Irala et al. 2024). For example, in postnatal development, neurocan is produced by astrocytes, and it is further cleaved in the C‐terminal and N‐terminal (Irala et al. 2024). The C‐terminal of neurocan is localized to somatostatin inhibitory neurons and is shown to regulate the formation and function of inhibitory synapses in the anterior cingulate cortex of mice (Irala et al. 2024). Collectively, in neurodevelopment, CSPGs are implicated in the stabilization of dendritic spines, coupling of pre‐ and post‐synaptic elements, and functional regulation of excitatory and inhibitory synapses.

CSPGs also influence synaptic pruning and stabilization of mature circuits. Studies in the spinal cord and cortex suggest that PNNs assembly around active neurons stabilizes newly formed synapses, whereas synapses on non‐PNN‐ensheathed neurons are more likely to be pruned during the critical period (Sánchez‐Ventura et al. ²⁰²²). Electrophysiological studies on mouse cortical neurons in culture show that enzymatic digestion of PNNs with ChABC alters synaptic plasticity and spontaneous synaptic activity onto cortical neurons and increases structural flexibility of inhibitory circuits (Paylor et al. 2018; Willis et al. ²⁰²²). In mice, microglia‐mediated disassembly of PNNs, following repeated ketamine anesthesia or 60‐Hz light stimulation, reopens ocular dominance plasticity in the adult visual cortex (Venturino et al. 2021). Thus, current evidence signifies CSPG‐PNN remodeling as a key step in integrating synaptic pruning, circuit reconfiguration, and restored juvenile‐like plasticity. Overall, these findings support a model in which CSPG‐containing PNNs both protect and stabilize selected synapses while permitting the pruning of unprotected connections during and after the closure of the critical period.

The developmental roles of CSPGs in regulation of axon guidance, synaptic refinement and critical period closure have important implications for adult CNS function. The maturation of PNNs and other CSPG‐rich matrices establishes long‐lasting constraints on plasticity that shape circuit stability throughout life (Reichelt et al. 2019). When these developmental programs are disrupted, adult neurons remain overly plastic and form incorrect connections, affecting sensory processing, learning, and susceptibility to neurological disorders. Changes in the timing or structure of CSPG‐based ECM remodeling can also influence how the brain reacts to injury, including the way glial scars form, how well synapses recover, and how much circuits can reorganize during disease (Reichelt et al. 2019). Thus, understanding CSPG functions during early development provides critical context for understanding their roles in adult pathologies. Although our understanding of the roles of CSPGs in CNS development has advanced, more studies are needed to dissect the underlying extracellular and intracellular mechanisms of CSPGs in human neurodevelopmental processes. Recent development of human cerebral and spinal organoids provides the opportunity to fill these gaps.

5. CSPGs Signaling and Mechanisms

Various receptors have been identified to mediate CSPGs function in the CNS. The main receptors that can bind to CSPGs include: (1) PTP‐σ receptor (Lang et al. 2015; Ohtake et al. ²⁰¹⁶; Shen et al. ²⁰⁰⁹), (2) LAR receptor (Fisher et al. 2011), and (3) Nogo receptor (NgR) family members NgR1 and NgR3 (Dickendesher et al. 2012). In addition to these main pathways, CSPGs act through interaction with the laminin/integrin pathway (Gu et al. 2009; Tan et al. ²⁰¹¹) and binding to EGFR (Koprivica et al. 2005; Tham et al. ²⁰¹⁰). Among these receptors, LAR and PTP‐σ represent the most characterized receptors that mediate CSPG‐induced inhibition of regeneration, particularly in vivo models. However, CSPGs also interact with NgR family members (NgR1 and NgR3), suggesting potential crosstalk between CSPG and myelin‐associated inhibitory pathways. Moreover, CSPGs can modulate integrin‐mediated adhesion and cytoskeletal dynamics, and signal on EGFR to enhance RhoA/ROCK signaling (Tan et al. 2011; Dickendesher et al. ²⁰¹²; Cheah et al. ²⁰¹⁶). Here, we will discuss the diverse signaling receptors and pathways that are associated with the CSPGs network, signifying their importance in neural injury and repair.

5.1. Protein Tyrosine Phosphatase Receptors: PTP‐σ and LAR

LAR and PTP‐σ are the most specific and studied signaling receptors of CSPGs. These receptors are members of the transmembrane protein tyrosine phosphatase (PTPs) family. PTPs are capable of transducing extracellular responses intracellularly due to their transmembrane‐cytosolic structure (Chagnon et al. 2004). In 2009, Shen and colleagues identified that the PTP‐σ receptor can bind to CSPGs. In a cell‐free system, they showed that the CS part of CSPGs binds to the ectodomain of the PTP‐σ receptor, which is a conserved positively charged surface immunoglobulin‐like domain (Shen et al. 2009). This binding was abolished by degradation of CS‐GAG chains of CSPGs with ChABC treatment, confirming the involvement of CS chains of CSPGs in the interactions with the PTP‐σ receptor (Shen et al. 2009). Additional in vitro and in vivo functional studies verified that downregulation of PTP‐σ in neurons can perturb CSPGs inhibitory effects on axonal growth in cultured DRG sensory axons in a mouse model of spinal cord dorsal column crush injury (Shen et al. 2009). Genetic downregulation of PTP‐σ also significantly enhances neurite outgrowth of CGNs on CSPGs substrate in vitro and promotes long‐distance regeneration of corticospinal tract axons in a contusion model of SCI in mice and an optic nerve micro‐crush injury in rats (Fry et al. 2010; Sapieha et al. ²⁰⁰⁵). Intracellularly, the PTP‐σ receptor mediates the inhibitory effects of CSPGs on neurite outgrowth by inhibiting Erk1/2 phosphorylation and activity of mitogen‐activated protein kinase (MAPK)/Akt (Lang et al. 2015; Sapieha et al. ²⁰⁰⁵).

LAR receptor is another member of PTPs family that is widely expressed in the brain and spinal cord and is physiologically involved in neuronal morphogenesis, maturity and synapse formation (Dunah et al. 2005; Hoogenraad et al. ²⁰⁰⁷). LAR activity plays a major role in NMDA receptor‐mediated synaptic transmission (Sclip and Südhof 2020). LAR also mediates the inhibitory effects of CSPGs on axonal growth, and its genetic deletion or pharmacological blockade reverses CSPGs effects on neurite outgrowth of rodent‐derived DRG neurons and CGNs (Fisher et al. 2011; Ohtake et al. ²⁰¹⁶). Intracellularly, CSPG/LAR axis partially mediates the inhibitory effects of CSPGs on neurite outgrowth through activation of Rho/ROCK and suppression of Akt/GSK‐3β and Erk1/2 pathways (Fisher et al. 2011). These intracellular pathways are downstream to both LAR and PTP‐σ receptors in response to CSPGs (Ohtake et al. 2016). LAR can also specifically and independently mediate CSPGs inhibitory effects on neurite outgrowth by inhibiting liver kinase B1 (LKB1), protein kinase C‐zeta (PKC ζ) and cofilin (Ohtake et al. 2016). LKB1, or the serine/threonine kinase 11, regulates neuronal polarization and neurite outgrowth by targeting SAD/MAPK signaling (Barnes et al. 2007). Solitary activation of LAR (but not PTP‐σ) receptor significantly decreases phosphorylation of LKB1, and deletion of LAR results in restoration of LKB1 activity in mouse CGNs (Ohtake et al. 2016). PKC‐ζ is shown to phosphorylate LKB1 in endothelial cells and modulate the activity of Notch signaling by regulating intracellular localization of Notch1 in the CNS of chicken embryo and human embryonic kidney 293 (HEK293) cells (Dunah et al. 2005). Activation of CSPG/LAR signaling significantly decreases activity and phosphorylation of PKC‐ζ (Ohtake et al. 2016). Cofilin is a critical regulator of actin dynamics, and its activation facilitates actin reassembly (Hsieh et al. 2006; Ichetovkin et al. ²⁰⁰²). CSPGs signaling through LAR receptor promotes cofilin phosphorylation that inhibits actin polymerization and neurite outgrowth in cultured CGNs (Ohtake et al. 2016). Previous studies also identified that CSPGs suppress several properties of both mouse and human NPCs including cell growth, survival, attachment, proliferation, and fate specification by signaling on both PTP‐σ and LAR receptors (Dyck et al. 2015, 2019; Hosseini et al. ²⁰²²). Genetic and pharmacological co‐inhibition of both receptors was sufficient to reverse the inhibitory effects of CSPGs on regenerative properties of NPCs in culture. Role of CSPGs/LAR/PTP‐σ axis in NPC regulation will be specifically discussed in a following section.

Altogether, among the known receptors for CSPGs, PTP‐σ and LAR have emerged as the most potent and consistently validated mediators of CSPG‐induced inhibition of regeneration across neuronal populations (Shen et al. 2009; Fry et al. ²⁰¹⁰). These receptors individually or synergistically mediate CSPG‐mediated effects on target cells (Figure 2). PTP‐σ and LAR receptors converge on conserved intracellular pathways, including RhoA/ROCK, Akt/GSK‐3β, and Erk1/2 signaling, with LAR additionally regulating LKB1 and PKC‐ζ. Overall, these downstream interactions highlight that PTP‐σ and LAR act as the core signaling hubs for CSPGs functions in the CNS (Fisher et al. 2011; Ohtake et al. ²⁰¹⁶; Sami et al. ²⁰²⁰; Shen et al. ²⁰⁰⁹).

FIGURE 2.

Intracellular signaling pathways downstream of CSPGs. CSPGs mainly act through two transmembrane receptors, LAR and PTP‐σ. Intracellularly, activation of CSPG/LAR/PTP‐σ axis modulates activity of Rho/ROCK, Akt and Erk1/2 pathways. Activation of LAR receptor also inhibits PKC ζ signaling pathway. PKC ζ: Protein Kinase C‐zeta, LKB1: Liver Kinase B1, CRMP2: Collapsin Response Mediator Protein‐2, Akt: Activated protein kinase, GSK‐3β: Glycogen synthase kinase, Erk1/2: Extracellular signal‐regulated kinase 1/2, CREB: CAMP‐response element binding protein, mTOR: Mammalian target of rapamycin, APC: Adenomatous Polyposis Coli.

5.2. Nogo Receptors

Nogo receptors, NgR1 and NgR3, are also shown to partly participate in CSPGs signaling (Sami et al. 2020). Nogo membrane proteins are involved in diverse physiological and developmental events in the CNS such as migration of precursor cells, neurite outgrowth, and morphogenesis, regulation of secretases, composition of endoplasmic reticulum, and cell survival (Schwab 2010). Nogo66‐receptor 1 (NgR1) is a main receptor for myelin‐associated inhibitors such as myelin‐associated glycoprotein (MAG) and oligodendrocyte myelin glycoprotein (OMgp) (Fournier et al. 2001; Schwab ²⁰¹⁰). Interestingly, NgR1 and NgR3 contain two evolutionarily conserved sequence motifs which facilitate their binding to CS‐GAGs chains of CSPGs (Dickendesher et al. 2012). One study has shown that CSPGs partially mediate their inhibitory effects on neurite outgrowth of CGNs through NgRs (Dickendesher et al. 2012). This study showed that genetic downregulation of NgR1, NgR2, and NgR3 significantly increases axon regeneration following optic nerve crush injury in mice (Dickendesher et al. 2012). Nogo receptors, NgR1 and NgR3, can also bind to CS‐GAG chains and contribute to CSPG signaling, although their relative importance remains context‐dependent and controversial (Dickendesher et al. 2012). Overall, current findings suggest CSPG–NgR interactions display lower specificity and weaker affinity compared with PTP‐σ and LAR (Dickendesher et al. 2012).

5.3. Epidermal Growth Factor Receptor

CSPGs also mediate their effects through the kinase function of EGFR signaling (Koprivica et al. 2005; Tham et al. ²⁰¹⁰). Early studies identified that inhibition of EGFR signaling significantly attenuates the inhibitory effects of CSPGs on neurite outgrowth of mouse CGNs in vitro and enhances axon regeneration in mice with optic nerve crush injury (Koprivica et al. 2005). Introduction of EGFR kinase inhibitors (AG1478 or PD168393) or a dominant‐negative EGFR construct restored CSPG‐mediated inhibition of neurite outgrowth in cultured CGN. Interestingly, this study further showed that the inhibitory effects of CSPGs on neurite outgrowth are partially mediated through phosphorylation of EGFR in a calcium‐dependent manner as calcium chelation reduced CSPG‐induced EGFR activation (Koprivica et al. 2005). Further in vivo evidence also identified that inhibition of EGFR by intrathecal administration of PD168393 increases regeneration of serotonergic axons and remyelination that was associated with improved functional recovery of rats with contusion SCI (Erschbamer et al. 2007). It is also reported that physiological levels of CSPGs are critical for NPCs to form neurospheres in culture that are mediated by EGFR through activation of JAK/STAT and PI3K/Akt pathways (Tham et al. 2010). Overall, existing evidence supports the involvement of EGFR in mediating CSPG‐induced inhibition of axon regeneration and the potential of blocking this pathway in targeting CSPGs (Koprivica et al. 2005; Sami et al. ²⁰²⁰).

5.4. Integrin Signaling

Integrin signaling has been also implicated in CSPGs effects on neurons (Fawcett 2020; Sami et al. ²⁰²⁰). CSPGs inhibit axonal growth of rat DRGs by inactivating integrin signaling through FAK dephosphorylation (Tan et al. 2011). Integrin receptor and signaling play a major role in axonal elongation by facilitating adhesion of growth cone to the substrate and regulating cytoskeleton dynamics and activating focal adhesion kinase (FAK) (Nieuwenhuis et al. 2018). Under CSPGs exposure, forced activation of integrin pathway by manganese and TS2/16 effectively overcomes CSPGs effects on axons of rat DRG neurons and human embryonic stem‐cell derived motoneurons, respectively (Tan et al. 2011). Similarly, activation of integrin signaling by kindlin‐1 in combination with administration of tenascin‐binding integrin, α9β1, significantly enhances regeneration of sensory axons and sensory circuit re‐establishment in a rat model of dorsal root crush injury (Cheah et al. 2016). Moreover, physiological level of CSPGs secreted by NPCs partially acts through inhibiting integrin signaling pathway (Gu et al. 2009). Inhibition of integrin signaling in NPCs by Echistatin abolishes the regulatory effects of CSPGs on neurogenesis, and morphogenesis of neurons and oligodendrocytes differentiated from NPCs (Gu et al. 2009). Altogether, CSPGs regulate their biological functions in the CNS through an intricate network which recruits various types of receptors and intracellular pathways to convey its effects on different cell types. Current evidence indicates that LAR and PTP‐σ receptors play the main role in mediating CSPGs effects.

Taken together, current findings show that CSPG‐mediated inhibition of CNS repair arises from a complex interplay of various receptor signaling, intracellular cytoskeletal dynamics, and interactions with other ECM constituents. Although LAR and PTP‐σ have been the most extensively characterized receptors of CSPGs, additional receptors and pathways have been implicated in modulating or amplifying CSPGs function including integrins, NgR, EGFR, and interactions with hyaluronan and tenascins. Existing evidence suggests that PTP‐σ and LAR function as the primary inhibitory receptors for CSPGs, whereas NgRs and EGFR act in a context‐dependent manner and their role may vary by cell type and mode of injury (Dickendesher et al. 2012; Koprivica et al. ²⁰⁰⁵). It is also noteworthy to mention that the mechanistic knowledge of CSPG signaling heavily relies on in vitro studies, which may not fully capture the diversity of ECM organization across CNS regions and the potential differences in species, injury models, and cell‐specific receptor expression profiles. Hence, a more integrated understanding of how various components of CSPG signaling converge in vivo remains an essential direction for future research.

6. Roles of CSPGs in CNS Pathologies

CSPGs have also emerged as a critical regulator of CNS pathologies with distinct roles from neurodevelopment. Pathologic upregulation of CSPGs is a shared hallmark of many CNS disorders that include traumatic, ischemic, and neurodegenerative conditions (Dyck and Karimi‐Abdolrezaee ²⁰¹⁵; Alizadeh et al. ²⁰¹⁹; Yang et al. ²⁰²⁴). In response to CNS pathology, various cell types including pericytes, astrocytes, NG2+ cells, and microglia/macrophages over‐express CSPGs at the site of injury. Although upregulation of CSPGs in the glial and fibrotic scar aids in limiting the expansion of neurodegeneration, the longstanding abundance of CSPGs becomes inhibitory for the repair and regeneration processes (Dyck and Karimi‐Abdolrezaee ²⁰¹⁵; Lin et al. ²⁰²¹). The same CSPG‐mediated mechanisms that stabilize maturing circuits during development can become maladaptive in the adult CNS, where elevated levels of CSPGs restrict plasticity, neural repair, and synaptic remodeling after injury or degeneration. The persistence of PNNs and CSPG‐rich extracellular matrices into adulthood may also create a structural framework that could influence how the CNS responds to inflammatory signals, demyelination, or neurodegeneration. Hence, insights from developmental functions of CSPGs may inform how CSPGs influence disease mechanisms in adult pathological states. In this section, we will discuss different roles of CSPGs in various types of CNS pathologies including ischemic stroke, traumatic SCI, multiple sclerosis (MS), and Alzheimer disease (AD) (Figure 3).

FIGURE 3.

Pathologic CSPG is implemented in CNS diseases. Stroke, spinal cord injury, multiple sclerosis, and Alzheimer's disease result in upregulation of CSPGs. Pathologic long‐lasting CSPGs deposits impede neuroregeneration by promoting pro‐inflammatory response, suppressing synaptic plasticity, axonal regeneration, and response of NPCs.

6.1. Stroke

Stroke is caused by disruption of brain blood supply resulting in brain damage and neurological deficits (Qin et al. 2022; Unnithan et al. ²⁰²³). Stroke is categorized into ischemic and hemorrhagic stroke (Qin et al. 2022; Unnithan et al. ²⁰²³). In ischemic stroke, brain blood flow is abrupted by thrombosis, atherosclerosis, or embolism (Qin et al. 2022), while hemorrhagic stroke is caused by disruption of brain blood supply due to a ruptured blood vessel (Unnithan et al. 2023). Studies in the middle cerebral artery occlusion (MCAO) model of ischemic stroke in rats have shown that ischemia drives activation of astrocytes surrounding the lesion where they form CSPG‐enriched glial scar, in particular neurocan and phosphacan, through activation of the TGF‐β1/SMAD2/3 signaling pathway (Zhang et al. 2018). A recent study in endothelin1‐induced ischemic stroke in rats shows that degradation of CSPGs, using ChABC 37‐point mutations (ChASE37), significantly increases axonal regeneration and penetration into the brain stroke lesion (Letko Khait et al. 2025). Mechanistically, Sox9 transcription factor is shown to regulate CSPGs production following MCAO ischemic stroke in mice, and downregulation of Sox9 can significantly decline CSPGs deposition and glial scar formation (Xu et al. 2018). Conditional knockout of Sox9 also increases tissue sparing, axonal sprouting of corticorubral and corticospinal tracts, and improves neurological recovery of mice with MCAO‐induced ischemic stroke (Xu et al. 2018). Blockade of the PTP‐σ receptor by ISP significantly enhances migration of brain NPCs and neuroblasts into the lesion epicenter and promotes axonal sprouting after MCAO‐induced ischemic stroke in mice (Luo et al. 2022). Notably, ISP treatment significantly improves general and fine motor functions and cognition of mice with MCAO ischemic stroke (Luo et al. 2022).

In a mouse model of hemorrhagic stroke induced by collagenase, brain microglia and resident astrocytes overproduce CSPGs, particularly neurocan and V1/V2 versican (Li et al. 2023). Importantly, inhibition of the PTP‐σ receptor, using ISP peptide, modulates the inflammatory response, increases remyelination and motor axonal sprouting, and enhances nerve conductance in collagenase‐induced hemorrhagic stroke in mice (Yao et al. 2022). Importantly, upregulation of neurocan by astrocytes and microglia has been detected in brain lesions of individuals with hemorrhagic stroke (Li et al. 2023). Recent evidence suggests that CSPGs contribute to stroke recovery. In a mouse model of MCAO, ischemic injury results in a transient degradation of CSPG‐enriched PNN around parvalbumin interneurons (Dzyubenko et al. 2023). Importantly, this study showed that the restoration of CSPGs and re‐compaction of PNNs are important for remodeling of GABAergic synapses and precede improvements in motor coordination, indicating that adaptive PNN plasticity supports post‐stroke recovery. Altogether, upregulation of CSPGs is shown to generally impede neural repair and axonal regeneration in stroke, and strategies that target CSPG signaling or digest excess CSPGs hold promise to facilitate neuroregeneration. However, emerging evidence suggests that CSPG‐rich matrices may also support the recovery process in certain contexts that need to be taken into consideration when CSPGs are therapeutically targeted.

6.2. Traumatic Spinal Cord Injury (SCI)

Upregulation of CSPGs in the glial scar is a hallmark of SCI that is extensively studied in the past two decades. In SCI lesion, CSPGs are produced by astrocytes, microglia/macrophages, fibroblasts, pericytes, and NG2+ glia, and are shown to hinder the regeneration process following injury (Bradbury and Burnside 2019; Dyck and Karimi‐Abdolrezaee ²⁰¹⁸; Hussein et al. ²⁰²⁰). Time course analysis in a rat model of dorsal column SCI has shown that deposition of neurocan, brevican, and versican starts acutely 1 day after SCI around the lesion (Jones et al. 2003). Production of neurocan and versican consistently continues up to 4 weeks, and brevican is over‐expressed for up to 2 months in rats with dorsal column SCI (Jones et al. 2003), indicating the long‐lasting presence and effects of pathologic CSPGs in the lesion.

Inhibition of axonal growth is one of the main pathological effects of CSPGs after SCI, which is shown to be mediated by LAR and PTP‐σ receptors (Busch and Silver 2007; Lang et al. ²⁰¹⁵; Tran et al. ²⁰¹⁸). Intracellularly, the inhibitory effects of CSPGs on axonal regeneration after SCI are through activation of protein kinase C (PKC) and the Rho/ROCK pathway (Monnier et al. 2003; Sivasankaran et al. ²⁰⁰⁴). CSPGs also inhibit neurite outgrowth and synapse formation by suppression of activated autophagy flux through the PTP‐σ receptor in the transected spinal cord of mice (Wang et al. 2025). Degradation of CSPGs by the ChABC enzyme significantly increases expression of growth associated protein‐43 (GAP43) and enhances regeneration of ascending sensory as well as descending corticospinal tract pathways in rats with dorsal column lesion and contusion SCI (Bradbury et al. 2002; Burnside et al. ²⁰¹⁸). In contrast, in a mouse model of crush SCI, astrocyte‐derived CSPG4 and CSPG5 are shown to support axonal sprouting and early tissue organization (Anderson et al. 2016). Interestingly, these CSPG isoforms help guide regenerating axons toward the lesion border and contribute to the structural stabilization of regenerating axons during the repair process. This finding emphasizes that certain CSPGs may support the regeneration process in SCI (Anderson et al. 2016). Moreover, in optic nerve crush injury in adult mice, CSPG4 expression by NG2 glia also creates a permissive ECM which facilitates the alignment and extension of regenerating retinal ganglion cell axons (Vilallongue et al. 2022). These contrasting effects might reflect differential expression of CSPG isoforms in various CNS injuries and/or the differential effects of CSPG subtypes on axons. Altogether, while CSPG isoforms might play various roles in SCI repair, extensive findings from SCI studies have confirmed an inhibitory role for the pathologic upregulation of CSPGs on axon regeneration.

CSPG inhibition of axon regeneration has been primarily attributed to activation of PTP‐σ receptor by inducing dystrophic growth cones and immobilizing them within CSPG‐enriched matrix (Lang et al. 2015). Of note, blockade of PTP‐σ intracellular wedge domain using ISP overcomes CSPGs inhibitory effects on axonal regeneration and enhances serotonergic innervation to the caudal side of the injury that leads to improved functional recovery in a rat model of contusion SCI (Lang et al. 2015). Interestingly, this study showed that inhibition of LAR intracellular wedge domain using ILP does not improve functional recovery of SCI rates, which highlights the differential roles that PTP‐σ and LAR receptors play in mediating inhibitory effects of CSPGs on axonal regeneration following SCI (Lang et al. 2015). Suppression of CSPG/PTP‐σ axis also promotes axonal regeneration of contralateral medullary rostral ventral respiratory group into the caudal side of the lesion and sprouting of bulbospinal respiratory axons in a rat model of cervical SCI (Urban et al. 2020). While PTP‐σ receptor has been extensively implicated in mediating CSPG‐induced inhibition of axon regeneration, there are some reports that also indicate the involvement of LAR in these CSPG inhibitory effects. Pharmacological inhibition of LAR receptor, using LAR inhibitory peptide, significantly increases regeneration of ipsilateral medullary rostral ventral respiratory axons into the caudal side of the spinal cord lesion and enhances their synapse formation with phrenic motor neurons in cervical hemi‐section SCI in rats (Cheng et al. 2021). Similarly, pharmacological inhibition of LAR receptor, using ILP, significantly restores inhibitory effects of CSPGs on axonal growth and regeneration of serotonergic fibers in mice with thoracic transection SCI (Fisher et al. 2011). Taken together, more evidence suggests a pronounced role for CSPG/PTP‐σ signaling in mediating the inhibitory effects of CSPGs on neurite outgrowth and axonal regeneration, although LAR has also been implicated in inhibition of axon regeneration.

Pathological upregulation of CSPGs also induces cell death in a variety of cells after SCI (Dyck and Karimi‐Abdolrezaee ²⁰¹⁵; Yang et al. ²⁰²⁴). Exposure to CSPGs induces caspase 3‐mediated cell death in OPCs and NPCs in culture through activation of LAR and PTP‐σ, and it increases oligodendrocyte apoptosis in rats with contusive/compression SCI (Dyck et al. 2015, 2019). Likewise, upregulated CSPGs have been associated with caspase 3‐mediated cell death in neurons after SCI in Lamprey, which was reversed by digestion of CSPGs using ChABC (Hu et al. 2021). Notably, transplantation studies in SCI have shown that the pathologic deposits of CSPGs inhibit survival of engrafted NPCs in the injured spinal cord through co‐activation of LAR and PTP‐σ receptors (Hosseini et al. 2022; Karimi‐Abdolrezaee et al. ²⁰¹⁰; Nori et al. ²⁰¹⁸). While the underlying mechanisms of cell death by pathologic CSPGs require further elucidation, this might be linked to the effects of CSPGs in promoting neuroinflammation in the injured spinal cord (Dyck and Karimi‐Abdolrezaee ²⁰¹⁸). We have shown that CSPGs through LAR and PTP‐σ receptors exacerbate neutrophil infiltration and promote the population of pro‐inflammatory microglia/macrophages and effector T‐cells that augment the production of pro‐inflammatory cytokines such as IL‐1β and TNF‐α (Didangelos et al. 2014; Dyck et al. ²⁰¹⁸; Francos‐Quijorna et al. ²⁰²²). Interestingly, CSPGs also prevent clearance of immune cells and their conversion to a homeostatic state through activation of toll‐like receptor 4 (TLR4) signaling (Francos‐Quijorna et al. ²⁰²²). In conclusion, extensive evidence in the past two decades has established that upregulation of CSPGs negatively regulates SCI repair through multiple mechanisms. Emerging work has identified LAR and PTP‐σ as the key signaling receptors for CSPG function in SCI. However, further research is warranted to fully understand the contributions of each receptor to various aspects of CSPG‐mediated effects in SCI pathophysiology.

6.3. Multiple Sclerosis (MS)

MS is the leading immune‐mediated disease of the CNS, which is characterized by demyelination, inflammation, gliosis, axonal injury and neuronal loss leading to progressive neurodegeneration (Tafti et al. 2024). Accumulation of CSPGs has been reported in MS both in animal and human studies. Upregulated levels of neurocan, versican, and aggrecan have been detected in active MS plaques, which is associated with astrogliosis (Sobel and Ahmed 2001). In mice with experimental autoimmune encephalomyelitis (EAE) and lysolecithin (LPC)‐induced demyelination, accumulated CSPGs in demyelinated lesions of spinal cord suppress proliferation of OPCs, decrease the number of oligodendrocytes, and promote a pro‐inflammatory response (Kataria et al. 2018; Lau et al. ²⁰¹²; Luo et al. ²⁰¹⁸). CSPGs secreted by astrocytes significantly suppress differentiation of oligodendrocytes from OPCs and decline their morphological maturation and myelination ability in vitro (Feliu et al. 2020). Inhibition of CSPGs synthesis, particularly brevican and neurocan, using 2‐aracgidonoglycerol (2‐AG), restores the suppressing effects of reactive astrocytes on differentiation and maturation of oligodendrocytes (Feliu et al. 2020). Mechanistically, CSPGs inhibit OPCs and oligodendrocytes mainly through PTP‐σ and LAR receptors and activation of downstream Rho/Rock pathway (Dyck et al. 2019; Pendleton et al. ²⁰¹³). In LPC‐induced demyelination of optic chiasm in mice, CSPGs/PTP‐σ activation hinders remyelination, aggravates immune response, and hinders functional recovery of optic pathway that can be reversed by ISP treatment (Niknam et al. 2019). Systemic blockade of PTP‐σ receptor, using ISP peptide, at the beginning of EAE induction significantly increases remyelination and myelin thickness, and attenuates disease severity (Luo et al. 2018). In EAE mouse, upregulation of CSPGs also drives neuroinflammation and prevents polarization of microglia/macrophages toward a pro‐regenerative phenotype which is reversed by blocking CSPG/PTP‐σ axis (Luo et al. 2018). Of note, upregulation of versican‐1 is associated with low number of immature BCAS1+ oligodendrocytes in human MS lesions and EAE mice which significantly inhibits maturation of oligodendrocytes and remyelination (Ghorbani et al. 2022). Versican‐1 polarizes naïve T cells toward pro‐inflammatory T helper‐17 (Th17) which are inhibitory to OPCs by decreasing their survival and differentiation into oligodendrocytes (Ghorbani et al. 2022). Interestingly, this effect of versican‐1 on Th17 is mediated thorough integrin β‐3 surface protein and not via LAR and PTP‐σ receptors (Ghorbani et al. 2022). Inhibition of CSPGs production by difluoramine also decreases Th17 polarization and enhances remyelination in the spinal cord of EAE mice (Ghorbani et al. 2022). Conversely, in LPC‐induced demyelination model, conditional deletion of NG2/CSPG4 impairs remyelination while the initial demyelination remains unaffected (Kucharova and Stallcup 2015). Mice expressing NG2 exhibit increased OPC recruitment, enhanced myelin debris clearance, and a more efficient remyelination. These findings show that NG2/CSPG4 supports effective remyelination and plays a pro‐repair role in demyelinating pathology (Kucharova and Stallcup 2015).

Upregulation of CSPGs by astrocytes and macrophages/microglia in the peri‐vascular cuff of human MS brains and EAE mice enhances infiltration of peripheral leukocytes into the CNS (Kataria et al. 2021; Stephenson et al. ²⁰¹⁸). Pathologic CSPGs around the vasculature induce the expression levels of several members of matrix metalloproteinase (MMPs), including MMP‐2, MMP‐3, MMP‐8, MMP‐9, and MMP‐12, that facilitate the infiltration of CD45+ leukocytes to EAE lesions (Stephenson et al. 2018). This was confirmed by suppression of CSPGs synthesis by fluorosamine treatment, in which significantly reduced infiltration of CD45+ cells in the spinal cord of EAE mice (Stephenson et al. 2018).

Collectively, recent findings have also implicated CSPGs in the pathogenesis and progression of MS through their roles in facilitating the infiltration of peripheral leukocytes into the CNS and inhibiting oligodendrocyte maturation and remyelination.

6.4. Alzheimer Disease (AD)

AD is a progressive neurodegenerative disease with various underlying mechanisms such as neuroinflammation, changes in brain vasculature, aging, pathologic matrix remodeling, accumulation of amyloid β1 (Aβ1), and tau pathology (Ma et al. 2020; Scheltens et al. ²⁰²¹). One of the major alterations in the brain matrix of individuals diagnosed with AD is the deposition of CSPGs, in particular brevican and neurocan, in the superior frontal gyrus and hippocampal regions (Howell et al. 2015; Lendvai et al. ²⁰¹³; Végh et al. ²⁰¹⁴). The extent of CSPGs accumulation is associated with the severity of AD since CSPGs bind to oligomeric and fibrillar Aβ1 and reduce synaptic density around Aβ1 plaques (Howell et al. 2015; Lendvai et al. ²⁰¹³). Studies on the APPswe/PS1dE9 mouse model of AD show that enzymatic degradation of hippocampal CSPGs significantly restores LTP and memory in the pre‐pathological state of the disease, indicating a possible link between CSPGs accumulation and synaptic loss (Végh et al. 2014). Similarly, ChABC treatment significantly decreases Aβ burden and restores synaptic loss in Aβ plaques in the stratum lacunosum moleculare brain region of APPswe/PS1dE9 mice (Howell et al. 2015). A recent study in the 5xFAD mouse model of AD also shows that CSPGs degradation in the medial prefrontal cortex restores short‐term memory loss and enhances clearance of Aβ plaques by reactive astrocytes through activation of the autophagy lysosome pathway and MERTK‐mediated phagocytosis (Yang et al. 2024). Among CSPGs family, aggrecan is shown to inhibit brain plasticity and recognition memory in adult mice (Romberg et al. 2013; Yang et al. ²⁰¹⁷). Neutralizing aggrecan using the Cat316 antibody can therapeutically restore object recognition memory in the P301S tau mouse model of AD (Yang et al. 2017).

Although alterations in CSPG accumulation and PNN composition are often associated with impaired plasticity and cognitive decline in AD, some studies suggest that CSPG‐rich PNN can exert protective effects by providing structural stability and support to vulnerable neurons against pathological insults. In vitro studies demonstrated that cortical neurons ensheathed in CSPG‐rich PNNs show strong resistance to Aβ‐induced toxicity (Miyata et al. 2007). Removal of CS‐GAG chains with ChABC eliminated this protection, which suggests neuroprotective effects of PNN‐associated CSPGs in modulating Aβ toxicity and preserving neuronal viability (Miyata et al. 2007). In human AD tissue, neurons surrounded by aggrecan‐rich PNNs display markedly reduced tau pathology compared with neurons with less developed PNNs (Morawski et al. 2010). Regions with sparse PNNs show heavy accumulation of neurofibrillary tangles, whereas PNN‐rich regions exhibit higher neuronal preservation (Morawski et al. 2010). These findings suggest that CSPG‐containing PNNs provide biochemical and structural protection against tau aggregation (Morawski et al. 2010; Miyata et al. ²⁰⁰⁷). Collectively, current evidence suggests that upregulation of CSPGs contributes to cognitive impairment in AD, largely through restricting plasticity and disrupting neuronal function. However, some studies indicate that CSPG‐rich PNN may also exert context‐dependent protective effects, such as buffering neurons against Aβ toxicity or tau pathology. Hence, future studies are needed to unravel the underlying mechanisms of CSPG actions and their signaling pathways in AD pathology.

In conclusion, across CNS pathologies, CSPG accumulation has been associated with neuroinflammation, synaptic alterations, impaired plasticity, and impeded repair (Cregg et al. 2014). However, the strength and specificity of these effects vary by disease models. For instance, findings from SCI and demyelination models frequently rely on acute injury paradigms (Hosseini and Karimi‐Abdolrezaee ²⁰²⁵), whereas studies in neurodegenerative diseases such as AD employ chronic transgenic systems with distinct ECM remodeling patterns (Bonneh‐Barkay and Wiley ²⁰⁰⁹). These methodological differences complicate direct comparison and may explain why CSPG‐driven mechanisms appear more prominent in some conditions than others. Furthermore, species‐specific differences in CSPG core proteins and sulfation patterns introduce additional layers of complexity when extrapolating rodent findings to human pathology. Integrating these model‐dependent insights suggests that CSPGs act through both common and disease‐specific pathways, underscoring the need for comparative studies that examine how CSPG biology diverges across injury types and CNS regions.

7. Therapeutic Approaches to Overcome Pathologic CSPGs in CNS

CSPGs have become an attractive therapeutic target for CNS injury and disease given their significant implications in various pathophysiological processes. As we discussed in earlier sections, different approaches have been developed to overcome the pathologic effects of CSPGs. These strategies include enzymatic degradation of CSPGs, suppressing CSPGs synthesis, and perturbing CSPGs signaling by blocking CSPG receptors that will be discussed in this section.

7.1. Enzymatic Degradation of CSPGs

Degrading CSPGs by ChABC has been extensively studied to overcome the inhibitory effects of lesional CSPGs in different CNS pathologies such as SCI, MS, AD and stroke (Muir et al. 2019). ChABC is a bacterial enzyme derived from bacterium Proteus vulgaris that degrades CSPGs by cleaving the glycoside bond between GalNAc and glucuronic acid (GlcA) or iduronic acid (IdoA) residues within the GAG chains of CSPGs (Dalal et al. 2025). Extensive data from animal studies, in particular SCI models, show the potential of ChABC treatment in facilitating neuroregeneration by enhancing remyelination, synapse formation, cell survival, immunomodulation, and axonal regeneration and sprouting (Barritt et al. 2006; Bartus et al. ²⁰¹²; Bradbury et al. ²⁰⁰²; Carter et al. ²⁰¹¹; Didangelos et al. ²⁰¹⁴; Führmann et al. ²⁰¹⁸; Gherardini et al. ²⁰¹⁵; Hu et al. ²⁰²¹; Jevans et al. ²⁰²¹; Karimi‐Abdolrezaee et al. ²⁰¹⁰; Muir et al. ²⁰¹⁹; Rosenzweig et al. ²⁰¹⁹). However, controlling the release of ChABC enzyme and the drug stability have been major limitations for translating ChABC therapy to clinic (Burnside et al. 2018). Extensive efforts have been made to overcome these limitations. Burnside and colleagues established an immune‐evasive and controlled gene delivery system by fusing the reverse tetracycline‐controlled trans‐activator with a glycine alanine repeat (GARrtTA) in which the delivery of ChABC can be controlled by doxycycline administration (Burnside et al. 2018). These studies showed that short‐term delivery (2.5‐week) of ChABC, using doxycycline‐inducible system increases conduction of sensory axons, and its long‐term delivery (8‐week) significantly improves skilled hand functions in rats with lower cervical contusion SCI (Burnside et al. 2018). Thermal instability of ChABC enzyme is another drawback of in vivo administration of this enzyme (Khalil et al. 2022). In addition, expression of bacterial ChABC in eukaryote cells results in over‐glycosylation of ChABC which interferes with extracellular secretion of ChABC (Muir et al. 2010). This has been mitigated by guided mutation of N‐glycosylation sites of ChABC (mtChABC) that can facilitate secretion of ChABC in eukaryotic cells (Muir et al. 2010). Intraspinal delivery of mtChABC‐encoding micro‐RNA (mRNA) using mineral‐coated microparticle induces local expression of ChABC in the damaged spinal cord of rats. This approach significantly decreases CSPGs deposition and enhances sprouting of serotonergic axons which results in hind limb functional improvement (Khalil et al. 2022). To overcome thermal instability of ChABC, approaches for local and sustained expression of ChABC have been developed (Wei and Andrews 2022). A recent study employed a highly mutated ChABC with 37‐point mutation (ChASE37) that is thermally stable and resistant to proteolytic activity, and is shown to be more effective compared with ChABC (Letko Khait et al. 2025). Local delivery of ChASE37 embedded in carboxymethylcellulose (CMC) with SH3‐binidng affinity effectively digest CSPGs in the brain of rats with Endothelin‐1 induced stroke (Letko Khait et al. 2025).

Beyond the issues of thermal instability and short enzymatic half‐life, several additional factors have limited the translational potential of ChABC. As a bacterial enzyme, ChABC may elicit immune responses that can reduce its activity and generate complications with repeated delivery (Cunha et al. 2025). Moreover, there are concerns about off‐target effects of ChABC, because the enzyme broadly digests glycosaminoglycan chains and can degrade physiological CSPGs that are essential for PNNs, synaptic stability, and circuit homeostasis in intact neural tissue as well as CSPGs in the ECM of non‐neural tissues if administered systemically (Galtrey and Fawcett 2007). Moreover, differences between rodent and human ECM composition, particularly in CSPG core proteins and sulfation patterns, may alter substrate interactions and limit the predictability of ChABC effects across species (Fawcett et al. 2019; Sorg et al. ²⁰¹⁶). For human applications, regulatory translation requires an enzyme with predictable pharmacokinetics, controllable dosing, and long‐term stability, criteria that existing bacterial ChABC preparations or engineered variants have not yet fulfilled. Hence, while ChABC remains a viable approach for targeting the inhibitory effects of CSPGs, its clinical translation will require strategies that retain specificity while minimizing immunogenicity and off‐target disruption of ECM (Cunha et al. 2025). To this end, several mitigation strategies have emerged in the preclinical pipeline, including stabilized or human‐compatible ChABC constructs (Cunha et al. 2025). Moreover, newer strategies have been developed to circumvent enzymatic degradation of CSPGs including receptor‐inhibiting peptides and approaches that modulate CSPG synthesis or downstream signaling (Ohtake et al. 2016). These multidimensional strategies highlight active efforts to address the translational barriers in the clinical use of ChABC for targeting CSPGs.

ADAMTS (A Disintegrin and Metalloproteinases with Thrombospondin motifs) enzymes can also digest CSPGs. Several members of the ADAMTS family, ADAMTS1, ADAMTS4, ADAMTS5, ADAMTS8, ADAMTS9, ADAMTS15, and ADAMTS20, have been identified as endogenous proteinases with the ability to digest CSPGs (Kelwick et al. 2015; Lemarchant et al. ²⁰¹³). Intraspinal administration of ADAMTS4 markedly increases sprouting and regeneration of serotonergic axons and improves functional recovery in a rat model of compressive SCI (Lemarchant et al. 2014). Overexpression of ADAMTS4 in astrocytes using AAV5‐mediated gene therapy also significantly reduces the lesion size, increases sprouting of corticospinal and serotonergic fibers, and improves functional recovery of rats with contusion SCI (Griffin et al. 2020). It is recently discovered that astrocytes derived from NPCs are a source of ADAMTS1 and ADAMTS9 enzymes and can digest inhibitory CSPGs in the damaged spinal cord of rats with contusive/compressive SCI (Hosseini et al. 2024). This shows one of the beneficial effects of NPC‐based therapies in regenerative remodeling of CSPGs in SCI lesions, which can be applicable for other CNS pathologies. In conclusion, clearing CSPGs from CNS lesions by CSPG‐degrading enzymes appears to be an effective approach for enhancing regeneration; however, its translation to clinical testing needs further investigations and optimizations.

7.2. Suppressing CSPG Synthesis

Blocking CSPG production by interfering with the CSPG synthesis pathways is another strategy that has been pursued to overcome the inhibitory effects of CSPGs (Ghorbani and Yong 2021; Hosseini et al. ²⁰²⁴). Fluorosamine is a fluorinated analogue of GlcNAc, which effectively reduces CSPGs synthesis by blocking conversion of UDP‐GlcNAc to UDP‐N‐acetyl‐galactosamine (Nigro et al. 2009; Stephenson et al. ²⁰¹⁹). Fluorosamine treatment is shown to reduce CSPG synthesis by astrocytes that promote remyelination and modulate immune response in mice with LPC‐induced demyelination (Keough et al. 2016). Suppressing CSPG synthesis in peri‐vascular cuffs also reduces infiltration of CD45‐expressing leukocytes in the spinal cord of EAE mice (Stephenson et al. 2018). β‐D‐xyloside is another treatment that suppresses proteoglycan synthesis by competing with the xylosylated peptide core (Kolset et al. 1990). Xyloside treatment has shown efficacy in increasing the number of OPCs and oligodendrocytes and enhancing remyelination in LPC‐induced demyelinated lesions of spinal cord in mice (Lau et al. 2012). In vitro co‐culture studies also show that xyloside can reverse the CSPG‐mediated inhibitory effects of activated astrocytes on NPC properties and neurogenesis (Hosseini et al. 2024). 2‐AG, an endocannabinoid in the CNS, also suppresses CSPG production in Theriler's murine encephalomyelitis virus‐induced progressive demyelination that modulates neuroinflammation and significantly increases oligodendrocyte number, maturation and remyelination (Feliú et al. 2017). Mechanistically, 2‐AG decreases production of CSPGs in astrocytes by modulating the TGF‐β1/SMAD pathway that supports differentiation of OPCs to oligodendrocytes on astrocytes matrix (Feliu et al. 2020). Under physiological conditions, CSPGs interact with different types of ECM molecules, in particular HA (Stepánková et al. 2023). In uninjured rats, 4‐methylumbelliferone (4‐MU) substantially inhibits synthesis of HA and CSPGs by decreasing synthesis of CS through suppressing production of uridine diphosphate GlcA enzyme (Stepánková et al. 2023). Although administration of 4‐MU at a dose of 1.2/kg/day reduces overexpression of HA, this dose was not effective in hindering overexpression of CSPGs contusion SCI in rats (Stepánková et al. 2023). Overall, these findings indicate the efficacy of treatment strategies aimed at blocking CSPGs synthesis pathway for targeting their inhibitory effects. However, delivery route, off‐target effects, and disruption in the production of physiological CSPGs are among the considerations that need to be addressed in this strategy.

7.3. Perturbing CSPG Signaling

Disruption of CSPGs receptors has been employed as a targeted approach to overcome CSPGs inhibitory effects without removal of CSPGs from the ECM. As described earlier, among identified receptors for CSPGs in the CNS, LAR and PTP‐σ are the most specific receptors for CSPGs signaling (Dyck and Karimi‐Abdolrezaee ²⁰¹⁵). Development of inhibitory peptides to functionally block LAR and PTP‐σ receptors has been successful in perturbing CSPGs signaling in SCI and other neurological disorders (Dyck et al. 2018, 2019; Hosseini et al. ²⁰²²; Lang et al. ²⁰¹⁵; Luo et al. ²⁰²²; Niknam et al. ²⁰¹⁹; Yao et al. ²⁰²²). Notably, these peptides are conjugated to a TAT domain (transcription of human immunodeficiency) to facilitate their intracellular delivery (Xie et al. 2006; Fisher et al. ²⁰¹¹; Lang et al. ²⁰¹⁵). These original studies showed that ISP treatment relieves tight stabilization of growth cone on CSPGs substrate, which results in extensive regeneration of serotonergic fibers and improvement of functional recovery in rats with contusion SCI (Lang et al. 2015). Likewise, ILP treatment enhances neurite outgrowth in mouse DRG neurons, increases regeneration of serotonergic fibers, and improves functional recovery of mice with transection SCI (Fisher et al. 2011). ILP and ISP co‐treatment reverses the inhibitory effects of CSPGs on growth, survival, and proliferation of mouse NPCs and OPCs (Dyck et al. 2015). Moreover, we demonstrated that blockade of CSPG/LAR/PTP‐σ axis, using ILP/ISP co‐treatment, restores inhibitory effects of CSPGs on neurogenesis and synaptogenesis of human NPC‐derived neurons (Hosseini et al. 2022). In conclusion, targeting LAR and PTP‐σ receptors is shown to be an effective approach to overcome CSPG inhibitory effects also in a more translationally relevant approach.

8. Conclusions and Future Perspectives

Accumulating evidence has unveiled a key role for CSPGs in neurodevelopment, homeostasis and pathophysiology of the CNS. Developmentally, CSPGs are a major component of the ECM in the niche of neural stem cells. Interactions of CSPGs with these stem cells are a determining factor in their proliferation and fate specification in the developing CNS. CSPGs also play an important role in guiding migratory neurons and cortical expansion and lamination. In addition, CSPGs are the main compounds of PNN, which has a significant role in synapse stabilization and circuit establishment in the postnatal CNS. Although the neurodevelopmental role of CSPGs is prominent, the level of CSPGs in the adult CNS remains minimal under physiological conditions. However, in neuropathology, CNS matrix undergoes significant changes which can be associated with the disease phase, mechanisms, and progression. The change in the production level of CSPGs is a hallmark characteristic of a variety of neurodegenerative, ischemic and traumatic diseases such as SCI, stroke, MS, and AD. Extensive evidence highlights the important roles that the pathologic levels of CSPGs play in disease processes such as axonal degeneration, demyelination, neuroinflammation, as well as suppression of neuroregeneration, neuroplasticity and circuit reassembly. These studies have established that modulation of CSPGs can improve the outcome of these CNS pathologies. CSPGs have been targeted through various approaches including CSPG degradation, inhibition of CSPGs synthesis, and manipulation of CSPG signaling mostly through suppression of LAR and PTP‐σ receptors. A wealth of evidence from preclinical studies shows that targeting CSPGs is an effective approach to enhance regeneration processes in the injured/diseased CNS. Among various therapeutic strategies that have been developed for inhibiting CSPGs, modulation of PTP‐σ receptor using small molecules has moved to clinical trials (Clinicaltrials.gov ID: NCT05965700). These clinical studies have confirmed the safety of this approach for humans, and its efficacy is currently being evaluated in individuals with SCI. Overall, great strides have been made in studying CSPGs and future studies are needed to translate the beneficial effects of targeting CSPGs in treating neurological disorders.

Author Contributions

S.M.H. and S.K.‐A. have contributed to the design and writing of this manuscript and approved the final version of the manuscript.

Funding

This work was supported by the Canadian Institutes of Health Research, PJT‐191850, PJT‐186168 and Wings for Life, WFL‐CA‐07/22.

Disclosure

Declaration of Transparency: The authors, reviewers and editors affirm that in accordance to the policies set by the Journal of Neuroscience Research, this manuscript presents an accurate and transparent account of the study being reported and that all critical details describing the methods and results are present.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Data S1: Transparent Science Questionnaire for Authors.

Data Availability Statement

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.