The role of nervous system extracellular matrix remodeling in chronic pain

1. Introduction

Chronic pain remains a major clinical challenge, and although the role of maladaptive neuronal and glial activity has been well documented, emerging research suggests that remodeling of the nervous system extracellular matrix (ECM) also regulates chronic pain.^{51,68,70,71,76} Extracellular matrix remodeling in non-nervous system tissues contributes to pain disorders such as osteoarthritis and intervertebral disc degeneration.^42,53,58 In addition, specific fragments of ECM components can be pronociceptive,^38,47 whereas other matrix components like high molecular weight hyaluronic acid can attenuate pain.^9,10,23 It is becoming increasingly clear that ECM remodeling in nervous system compartments also contributes to pain. Here, we will discuss the ECM composition of the brain, spinal cord (SC), and dorsal root ganglia (DRG) and how its remodeling contributes to pain.

2. Extracellular matrix biology

The ECM is a meshwork of macromolecules and ECM composition is tissue specific, conferring distinct structural, biomechanical, and biochemical properties.^11,22,59 In general, ECM is a mix of proteoglycans, glycosaminoglycans (GAGs), glycoproteins, and fibrous proteins like fibrillar collagens.^30,59 Extracellular matrix typically consists of a hyaluronic acid (HA) backbone that forms a scaffold^8,40 to which proteoglycans (PGs) are connected.⁸ Proteoglycans are classified by their attached GAGs, most notably chondroitin sulfate (CS) and heparan sulfate (HS) in the nervous system.^22,81 Glycoproteins, such as fibronectin, laminin, and thrombospondins, are involved in cell adhesion, matrix scaffolding, and basement membrane structure.^27,40,79,85 Fibrous collagens, such as type I and II, provide tensile strength and structural support to tissues,^59,61 and nonfibrillar collagens contribute to basement membranes and ECM–cell interactions.⁵⁹ In addition to providing structural support, the ECM functions as a signaling hub that mediates intercellular communication and regulates diffusion of soluble factors such as cytokines.^11,59

Extracellular matrix remodeling occurs through changes in the balance between ECM synthesis and ECM catabolism.^11,78 Extracellular matrix catabolism is mediated by proteases, including matrix metalloproteinases (MMPs) and additional protease families, which cleave matrix components in the nervous system.¹¹

3. Nervous system extracellular matrix composition and organization

3.1. Brain

The ECM makes up ∼40% of the volume of the developing brain and 20% of the adult brain.^49,54,64,67 Brain ECM is organized into distinct structures, including perineuronal nets (PNNs), basement membranes, and the interstitial matrix (Table 1, Fig. 1). Perineuronal nets are dense ECM structures that typically surround the neuronal somas and proximal dendrites of parvalbumin-positive (PV⁺) inhibitory interneurons and play an important role in regulating synaptic plasticity.^{12,13,22,46,64} Perineuronal nets are rich in the CSPG aggrecan and contain HA, tenascin-R, and other CSPGs. The interstitial matrix is distributed throughout the brain parenchyma and consists of proteoglycans, HA, glycoproteins, and nonfibrillar collagens that provide structural support and facilitate diffusion of molecules and nutrients.^40,48 Basement membranes separate endothelial cells from the CNS parenchyma and form a key component of the blood–brain barrier (BBB).^12,64 They contain collagen type IV, glycoproteins, and the HSPG perlecan.^22,40 Basement membranes regulate nutrient and waste exchange and restrict cellular and macromolecule entry into the CNS.⁴⁰

Table 1.

Extracellular matrix composition across the brain, spinal cord, and dorsal root ganglia.

	Brain	Spinal cord	DRG
ECM volume/abundance	20% of adult brain volume 40% of developing brain49,54,64,67	Similar to brain	Not quantified/less is known
Major proteoglycans	High: CSPGs (aggrecan, brevican, neurocan)12,15,81 Low: HSPGs16,17	High: CSPGs (aggrecan, neurocan, brevican)24,75 Low: HSPGs17	High: HSPGs (perlecan) Small leucine-rich PGs (lumican, decorin, brevican)21,76 Low/absent: CSPGs (aggrecan, neurocan)1,57,76
Glycosaminoglycans	Predominantly chondroitin sulfate (CS)17,49 Low heparan sulfate (HS)17,49 - CS:HS = 9:1 overall in the CNS17 - CS:HS = 7:3 (in PNNs)16	Predominantly CS17,49 Low HS17,49 - CS:HS = 9:1 overall in the CNS17 - CS:HS = 7:3 (in PNNs)16	Higher proportion of HS than CS1,21,57,76 Distinct GAG composition from CNS
Fibrillar collagens	Very low/absent type I and II in parenchyma Restricted to meninges4,29	Very low type I and II in parenchyma4,28,80 Present in meninges and around vessels	High: type I collagen (Col1a1, Col1a2)76
Nonfibrillar collagens	Type IV (basement membranes)25,29 Type XV (small amounts in the interstitial matrix)32	Type IV (basement membranes)25,29	Type IV (basement membrane-like structures around neuron-SGC units)19,21,84
Glycoproteins	TSPs Tenascins (C, R) Laminin (basement membranes) Fibronectin (basement membranes)2,40,55,56	TSPs Tenascins (C, R) Laminin (basement membranes) Fibronectin (basement membranes)40,55,56	SPARC, SPARCL1 Fibronectin76 Laminin-1 (basement membrane-like structure)19,21,84 Nidogen-1 (basement membrane-like structure)19,21,84
Hyaluronic acid (HA)	High abundance, forms ECM scaffold8,22,40 and a component of PNNs12,15,22	High abundance,31 similar to brain	Present, but quantity not well characterized
Specialized ECM structures	PNNs: - Around PV+ interneurons12,15,22,64 - Rich in aggrecan, HA, tenascin-R, link proteins12,15,22 Basement membranes: - Collagen IV, XVIII, laminins, fibronectin, nidogen, perlecan15,22,40 Interstitial matrix: - Loosely dispersed HA, CSPGs, tenascins, link proteins, collagen XV, laminins, fibronectin22,40,48	PNNs: - In dorsal horn lamina I around spinoparabrachial projection neurons39,71 - Similar composition to brain Basement membranes: - Collagen IV, laminins, nidogens, HSPGs (perlecan, agrin)6,39 - Blood-spinal cord barrier20 Interstitial matrix: - Similar to brain39	Basement membrane–like structures: - Surround neuron-SGC units - Collagen IV, nidogen-1, laminin-1, HSPGs19,21,84 - Separate individual neuron-SGC pairs84
Primary ECM-producing cells	Astrocytes (predominant): HA, tenascins, CSPGs (brevican, versican, aggrecan), TSPs3,65 Neurons: tenascin-C, tenascin-R, reelin, CSPGs (brevican, neurocan)65 Oligodendrocytes: CSPGs, tenascin-R65 No fibroblasts within BBB18,60,74	Astrocytes: CSPGs (brevican, aggrecan), HA, tenascin-C, TSPs3,79 Neurons: similar to brain65 Oligodendrocytes: tenascin-C, tenascin-R, brevican5 No fibroblasts within blood–SC barrier18,60	Fibroblasts (predominant): Col1a1, Lum, Bgn, proteoglycans, glycoproteins, collagens1,57,76,84 Satellite glia cells (minor): Bcan, collagen genes43

ACC, anterior cingulate cortex; BBB, blood–brain barrier; CAG, glycosaminoglycan; CCI, chronic constriction injury; CFA, complete Freund adjuvant; CNS, central nervous system; CSPG, chondroitin sulfate proteoglycan; DRG, dorsal root ganglia; HA, hyaluronic acid; HSPG, heparan sulfate proteoglycan; LTP, long-term potentiation; mPFC, medial prefrontal cortex; PG, proteoglycan; PNN, perineuronal net; PV, parvalbumin; SC, spinal cord; SGC, satellite glia cell; SNI, spared nerve injury; SNL, spinal nerve ligation; SSC, somatosensory cortex; TSP, thrombospondin.

Figure 1.

Overview of extracellular matrix (ECM) composition in the brain, spinal cord, and dorsal root ganglia (DRG). ECM in the brain, spinal cord, and DRG consists of a diffuse interstitial matrix composed of hyaluronic acid (HA), proteoglycans, glycoproteins (GPs), nonfibrillar collagens, and fibrillar collagens in the DRG. There are also distinct ECM structures including perineuronal nets (PNN), basement membranes that separate blood vessels from nervous tissue, and a specialized neuron-satellite glia cell (SGC) basement membrane in the DRG. Brain and spinal cord ECM are synthesized by neurons, astrocytes, and oligodendrocytes, whereas DRG ECM is mainly produced by fibroblasts as well as an SGC subpopulation.

Within the BBB, ECM components are synthesized by astrocytes, neurons, and oligodendrocytes,^3,65 rather than by a specialized, matrix synthesizing cell population like fibroblasts. Fibroblasts are largely excluded from the CNS parenchyma but contribute to the ECM in the meninges and perivascular spaces outside the BBB.^18,60,74

3.2. Spinal cord

The SC ECM is similar in composition and structure to the brain, with high levels of HA, CSPGs such as aggrecan, and various glycoproteins.^24,75 Perineuronal nets enwrap spinoparabrachial projection neurons, mainly in the dorsal horn lamina I, and have a similar composition to brain PNNs.^39,71 The SC interstitial ECM also mirrors the composition of the brain interstitial matrix.³⁹ Basement membranes in the SC are critical to the blood–SC barrier and consist of collagen IV, laminin, nidogen, and HSPGs.^6,31,39 Neurons, astrocytes, and oligodendrocytes produce most ECM components in the SC.^3,79,80

3.3. Dorsal root ganglia

The DRG ECM has been less studied than the CNS but has a distinct composition.^1,43,57,76 Both mouse and human DRG have high expression of collagen type I,⁷⁶ which is largely absent from the CNS parenchyma.^45,82 Aggrecan, a key CSPG in the CNS, is absent or expressed at very low levels in the DRG,^1,57,76 whereas the DRG ECM contains HSPGs (eg, perlecan), small leucine-rich proteoglycans, and several glycoproteins.^21,76 The different ECM composition likely impacts biochemical and biomechanical properties, which could have a downstream effect on pain.

Extracellular matrix composition differs across the DRG meninges, parenchyma, and basement membranes. Neuron-satellite glia cell (SGC) units are ensheathed by a basement membrane–like structure containing collagen IV, nidogen, laminin, and HSPGs,^19,21,84 which separate each neuron-SGC unit and likely has functional consequences.⁸⁴ There is also evidence of a PNN-like structure that surrounds DRG neuronal cell bodies, but it remains poorly characterized.⁶⁶

In contrast to the CNS, fibroblasts are the main ECM-producing cells in the DRG.^{1,57,63,76,84} Fibroblasts are located adjacent to neurons within the blood barrier and fibroblast-synthesized ECM can directly interact with neurons. In addition, a subpopulation of satellite glia cells express ECM genes.⁴³

4. Nervous system extracellular matrix remodeling in pain

Extracellular matrix remodeling in the nervous system has been identified to have a role in the development and maintenance of pain. Bulk RNA sequencing of the brain, SC, and DRG from mouse models of neuropathic pain (spared nerve injury, SNI) and inflammatory pain (complete Freund adjuvant, CFA) identified ECM organization as a significantly enriched pathway in all 3 tissues, suggesting that ECM remodeling is an underlying pain mechanism across tissues and types of pain.⁵¹

4.1. Brain

Evidence from neuropathic and inflammatory pain models demonstrates that brain ECM remodeling is associated with pain and pain-related cognitive deficits. In the anterior cingulate cortex (ACC), SNI reduces laminin after 28 and 56 days, and this reduction targets CaMKII-expressing excitatory pyramidal neurons (Table 2, Fig. 2).⁴¹ Reduced laminin increases presynaptic neurotransmitter release and destabilizes dendritic spines, thereby enhancing cortical excitability and pain sensitivity.⁴¹ Similarly, in a model of complex regional pain syndrome (CRPS), loss of hippocampal aggrecan and PNNs around PV⁺ interneurons disrupts long-term potentiation and promotes mechanical hypersensitivity and memory deficits.⁷⁰ Downregulation of MMP8 3 weeks after injury normalized hippocampal ECM and alleviated mechanical hypersensitivity and cognitive deficits when evaluated 7 to 9 weeks after injury.⁷⁰ These findings suggest that protecting ECM integrity in the ACC or hippocampus is important for maintaining normal neuronal function and preventing maladaptive plasticity that underlies chronic pain.

Table 2.

Pain associated extracellular matrix remodeling across the brain, spinal cord, and dorsal root ganglia.

	Brain	Spinal cord	DRG
Pain models	Neuropathic: - SNI41,51 Traumatic: - Tibia fracture and cast immobilization70 Inflammatory: - CFA44,51	Neuropathic: - SNI51,71 - SNL35,36,52,83 Traumatic: - Tibia fracture69 - Spinal cord injury (SCI)36,52,83 Inflammatory: - CFA51	Neuropathic: - SNI51 - SNL35 - CCI19 Inflammatory: - Intraplantar CFA50,51 Chemotherapy: - Paclitaxel72,73 Postoperative: - Paw incision26,33 Human: - Patients with neuropathic pain76
Areas studied	• ACC41 • Hippocampus70 • SCC44 • mPFC44	• Dorsal horn (lamina I predominantly)71 • Throughout spinal cord20,36,52,68,83	• Neuron-satellite glia cell units19,84 • Throughout entire DRG26,33,35,72,73
ECM structural changes	ACC: ↓ Disrupted basement membrane41 Hippocampus: ↓ Disrupted PNNs70 SSC/mPFC: ↑ PNN density except for aggrecan gene expression44 ↑ Gene expression of tenascin-R44	↓ PNN integrity (microglia-mediated degradation)71 ↓ Blood–spinal cord barrier integrity20,68	↓ Disrupted basement membrane around neuron-SGC units19 Human DRG: Differential expression of ECM genes in patients with neuropathic pain76
ECM modification mechanisms	ACC: - Increased presynaptic neurotransmitter release41 - Destabilized dendritic spines41 Hippocampus: - Decreased PNNs around PV+ interneurons70 - Disrupted LTP70 SSC/mPFC: - Increased PNN density around PV+ interneurons44 - Enhanced inhibitory synaptic transmission onto layer V pyramidal neurons44 - Circuit hyperstability44	Microglia: - Active ECM degradation (phagocytosis)71 - PNN dismantling around inhibitory interneurons71 - Begins 3 days postinjury71 MMPs: - Proteolytic degradation of the ECM, such as cleaving aggrecan, collagens, GAGs35,37 - Disrupt blood–spinal cord barrier68 - Plasma extravasation68 - Immune cell infiltration68 TSP4: - Promotes maladaptive synaptogenesis36,52,83 - Increases dorsal horn excitability36,52,83 SPARCL1/Hevin: - Elevated in inflammatory, neuropathic, and postsurgical pain models14,34,62,77 - Modulates synaptic trafficking of AMPA receptors and NMDA receptor currents14,34 - Promotes maladaptive synaptogenesis in dorsal horn14,34	Basement membrane disruption: - Loss of neuron-SGC unit isolation19,84 - Increased coupled activation of adjacent neurons84 Fibroblasts: - Altered ECM synthesis84 - Reduced SMOC2 production84 MMPs: - Proteolytic degradation of the ECM26,33,35,50,72,73
Downstream effects of structural ECM changes	ACC: - Enhanced cortical excitability41 - Impaired affective pain processing41 Hippocampus: - Impaired working memory70 - Cognitive deficits70 SSC: - Maladaptive circuit rigidity44 - Locked hyperstable state44 - Enhanced nociceptive sensitization44	Dorsal horn: - Disinhibition of lamina I spinoparabrachial projection neurons71 - Reduced inhibitory tone71 - Increased neuronal excitability71 - Enhanced pain signal transmission to brain71	Primary afferents: - Direct regulation of nociceptor activity84 - Increased neuronal coupling84 - Enhanced cross-excitation84
Pain phenotypes	• Mechanical hypersensitivity41 • Pain-related anxiety and depression41 • Cognitive impairments41	• Mechanical hypersensitivity/allodynia71 • Thermal hypersensitivity71 • Enhanced pain transmission71	• Mechanical hypersensitivity84 • Primary afferent hyperexcitability84
Reversal and protection intervention studies	• MMP8 downregulation in hippocampus: - Restored mechanical sensitivity70 - Improved memory70 - Normalized LTP70 - Increased PV+ neurons with PNNs70	Protective: • Microglia depletion: prevented PNN loss and mechanical hypersensitivity71 • Disruption of microglia phagocytosis: protected from PNN degradation71 Reversal: • MMP8 inhibition (genetic/pharmacological): alleviated mechanical allodynia69 • MMP9 inhibition: reduced early-phase neuropathic pain35 • MMP2 inhibition: reduced late-phase neuropathic pain35 • TSP4 blocking/downregulation/knockout: reversed thermal and mechanical hypersensitivity36,52,83 • SPARCL1 knockout/knockdown: faster resolution of pain behaviour14	Protective: • SMOC2 injection into DRG: prevented increased neuronal coupling84 Reversal: • MMP inhibition (broad or specific): - Reduced pain onset and maintenance35,72,73
Active ECM degradation studies	SSC: • Chondroitinase ABC (enzymatic PNN degradation): - Reduced mechanical hypersensitivity44 - Reduced thermal hypersensitivity44 - Normalized synaptic transmission44	Dorsal horn lamina I: • Chondroitinase ABC expression in projection neurons: - Reduced inhibitory tone71 - Disinhibition of lamina I projection neurons71 - Increased excitability of projection neurons underlying mechanical and thermal hypersensitivity71	Knockout: • Smoc2 knockout: increased mechanical hypersensitivity84
Key insights	• Region-specific effects integrate to sustain chronic pain across sensory, affective, and cognitive dimensions	• ECM remodeling linked to pain onset and maintenance • Loss of inhibitory control, heightened responsiveness, spinal cord as sensitized relay center71 • Elevated astrocyte secretion of glycoproteins TSP4 and SPARCL1/Hevin promotes maladaptive synaptogenesis and dorsal horn hyperexcitability14,34,62,77	• ECM remodeling in DRG directly regulates nociceptor activity84 • Basement membrane integrity prevents pronociceptive coupling84 • ECM changes also occur in human DRG from patients with neuropathic pain76

ACC, anterior cingulate cortex; BBB, blood–brain barrier; CAG, glycosaminoglycan; CCI, chronic constriction injury; CFA, complete Freund adjuvant; CNS, central nervous system; CSPG, chondroitin sulfate proteoglycan; DRG, dorsal root ganglia; HA, hyaluronic acid; HSPG, heparan sulfate proteoglycan; LTP, long-term potentiation; mPFC, medial prefrontal cortex; PG, proteoglycan; PNN, perineuronal net; PV, parvalbumin; SC, spinal cord; SGC, satellite glia cell; SNI, spared nerve injury; SNL, spinal nerve ligation; SSC, somatosensory cortex; TSP, thrombospondin.

Figure 2.

Overview of pain-associated extracellular matrix (ECM) remodeling. ECM remodeling in the brain, spinal cord, and dorsal root ganglia (DRG) promotes pain behaviour, nociceptor hyperexcitability, and synaptogenesis. In the somatosensory cortex (SSC) and medial prefrontal cortex (mPFC), increased PNN density around PV⁺ interneurons promotes pain behaviour, whereas loss of PNNs and proteoglycan degradation in the anterior cingulate cortex (ACC) and hippocampus results in pain behaviour and enhanced cortical excitability. In the spinal cord, proteoglycan degradation, PNN loss, and increased expression of the proteoglycans (PGs) thrombospondin-4 (TSP4) and SPARCL1 regulate pain and promote nociceptor excitability. In the DRG, disruption of the neuron-satellite glia cell (SGC) basement membrane results in elevated nociceptor activity and pain behaviour. In all 3 structures, ECM degradation is mediated by proteases, with most research having focused on MMP2, 8, and 9. In the spinal cord, microglia phagocytose ECM during pain-associated remodeling while the precise cell types responsible for pain-associated remodeling in the brain and DRG are still being determined.

Adverse ECM remodeling also includes excessive ECM accumulation. Perineuronal net density around PV⁺ interneurons and gene expression of tenascin-R, a key PNN component, increases in the somatosensory cortex 3 and 7 days after inflammatory pain is induced with intraplantar CFA injection. Perineuronal net density also increases in the medial prefrontal cortex 7 days after CFA injection.⁴⁴ These brain regions process tactile information including pain signals. Increased PNN density enhances inhibitory synaptic transmission onto layer V pyramidal neurons, likely stabilizing maladaptive circuits that maintain nociceptive sensitization.⁴⁴ Enzymatic digestion of PNNs in the SCC with chondroitinase ABC (a GAG-cleaving enzyme) reverses CFA-induced pain behaviour, suggesting that excessive ECM deposition also contributes to chronic pain.⁴⁴

These findings illustrate that brain ECM remodeling exerts region-specific effects on pain-associated neuronal plasticity. Extracellular matrix loss in the ACC and hippocampus destabilizes excitatory synapses, whereas ECM accumulation in cortical sensory regions stabilizes maladaptive circuits. Thus, region-specific ECM remodeling contributes to the sensory and cognitive components of pain.

4.2. Spinal cord

In the SC, pain-associated ECM remodeling affects how nociceptive information is processed and transmitted to the brain. After SNI, microglia degrade PNNs surrounding spinoparabrachial projection neurons in lamina I of the dorsal horn. Perineuronal net loss occurs as early as 3 days post-SNI and persists for at least 14 days.⁷¹ Perineuronal net loss after SNI, or PNN degradation via chondroitinase ABC, reduces inhibitory tone, disinhibits lamina I projection neurons, and results in heightened excitability and pain behaviour. Protection of PNNs by microglia depletion or disruption of microglia phagocytosis reduces pain behaviour.⁷¹ Together, these findings show that microglia-mediated disruption of spinal ECM has a role initiating and maintaining neuropathic pain.⁷¹

Spinal MMP activity is linked to pain-associated inflammation and neuroplasticity. In the SC, MMPs are produced by microglia, astrocytes, and neurons.^35,80 In a CRPS model, spinal MMP8 is elevated and inhibition or downregulation of MMP8 reduces mechanical hypersensitivity.⁶⁹ After spinal nerve ligation (SNL), elevated microglial MMP9 contributes to pain behaviour onset, whereas MMP2 expression in astrocytes is linked to the maintenance of pain behaviour from day 7 through 21 postinjury.³⁵ Matrix metalloproteinases also disrupt the blood–SC barrier after SC injury (SCI), causing elevated permeability and immune cell infiltration for at least 28 days.^20,68 Although these studies did not directly investigate the ECM, elevated MMP activity will increase ECM degradation, suggesting that MMPs have multiple roles in spinal ECM remodeling that are linked to pain.

Increased spinal astrocyte secretion of the glycoproteins thrombospondin-4 (TSP4) and SPARCL1/HEVIN contributes to pain mechanisms. Thrombospondin-4 is elevated after SNL and SCI for at least 14 to 28 days, binds and activates Ca_vα2δ1, and promotes maladaptive synaptogenesis and hyperexcitability within dorsal horn circuits.^36,52,83 Blocking, downregulating, or genetic inactivation of TSP4 after SNL or SCI reversed pain behaviour.^36,52,83 SPARCL1 increases in models of inflammatory, neuropathic, and postsurgical pain over the duration of pain behaviour.^14,34,62,77 Knockout or knockdown of SPARCL1 results in faster resolution of pain behaviour.¹⁴ SPARCL1 likely modulates pain by regulating synapse formation and increasing synaptic trafficking of AMPA receptors and by potentiating NMDA receptor currents.^14,34

Extracellular matrix remodeling in the SC, including loss of PNNs, elevated protease activity and glycoprotein regulation of synapses converge on a common outcome: a progressive loss of inhibitory control and heightened excitability. In the dorsal horn, ECM remodeling is an underlying mechanism that contributes to sensitization and the development of pain.

4.3. Dorsal root ganglia

Emerging evidence indicates that the ECM of the DRG undergoes remodeling in rodent models of pain and in humans with chronic pain. Bulk RNA-sequencing identified ECM organization as a top differentially regulated pathway in DRG across neuropathic and inflammatory pain models.⁵¹ Similarly, ECM genes are differentially expressed in DRG from humans with neuropathic pain.⁷⁶

Some of the most compelling evidence for a mechanistic role of ECM remodeling in the DRG comes from studies examining the basement membrane surrounding neuronal cell body-SGC units. Chronic constriction injury and SNL in rats disrupts the basement membrane by reducing laminin-1, nidogen-1, and collagen IV for at least 14 days after injury.¹⁹ In mice, genetic deletion or knockdown of Smoc2, which is expressed by DRG fibroblasts, also disrupts the neuron-SGC basement membrane and causes mechanical hypersensitivity.⁸⁴ Peripheral inflammation induced with intraplantar CFA reduces SMOC2 levels 3 days later and disrupts the neuron-SGC basement membrane, resulting in increased coupled activation of adjacent neurons and mechanical hypersensitivity. Increased neuronal firing and mechanical hypersensitivity can be reversed by SMOC2 injection.⁸⁴ These findings suggest that an intact ECM prevents activation of adjacent DRG neurons and indicate that DRG ECM directly regulates nociceptor activity.

Proteolytic ECM remodeling also occurs in the DRG. MMP2 and MMP9 activity, protein levels, and gene expression are elevated in the L4-L6 DRG in models of neuropathic,^35,72,73 postoperative,^26,33 and inflammatory pain.⁵⁰ MMP2 and MMP9 are reportedly expressed by DRG neurons,^26,35 and MMP2 is also expressed by SGCs.^35,73 In addition, single cell RNA sequencing indicates that fibroblasts express Mmp2 and immune cells express Mmp9.⁷ After SNL and the paw incision model, elevated MMP9 is detected for 6 to 24 hours, and elevated MMP2 is detected after 1 day and for up to 21 days.^26,35 Similar to the SC, MMP9 activity was linked to pain behaviour onset, and MMP2 was linked to maintenance of pain behaviour after SNL.³⁵ However, intraplantar CFA and paclitaxel treatment both cause sustained MMP9 elevation, suggesting that the timing of matrix protease activity is context dependent.^50,72

Pharmacological inhibition of MMPs reduces pain behaviour in these models, linking MMP activity to the onset and maintenance of pain behaviour.^35,72,73 Of note, protease inhibition studies were performed using spinal delivery of drugs, which could affect proteases in the DRG as well as the SC. Although these studies have linked elevated MMP expression and activity to pain behaviour, none directly investigated how MMP-mediated degradation of ECM in the DRG regulates pain. Therefore, elevated MMP activity may underlie pain-associated ECM remodeling the DRG, but further studies are needed to directly link DRG protease activity, ECM degradation, and pain mechanisms.

5. Conclusion and future directions

Extracellular matrix remodeling in the brain, SC, and DRG occurs in inflammatory and neuropathic pain models and regulates pain mechanisms. Extracellular matrix regulation of pain is location and context specific. For example, pain is linked to increased PNN density in the SCC and mPFC and decreased PNN density in other brain regions. In the SC and DRG, research has mainly focused on how loss of ECM structures like PNNs and increased spinal levels of TSP4/SPARCL1 potentiate pain. It remains unclear if increased deposition of matrix components like PNNs can be pronociceptive in the DRG and SC. Pain-associated ECM remodeling mechanisms have similarities and distinct differences between each compartment that stems from different ECM composition and the cellular mediators of ECM remodeling.

Changes in the abundance of ECM components and proteases are linked to pain, but how changes in ECM architecture and biomechanical properties modulate pain remains poorly understood. Scanning electron microscopy can be used to analyze ECM microarchitecture, such as fiber diameter. Similarly, atomic force microscopy can assess tissue stiffness and elasticity. These techniques have been used in a CRPS mouse model and linked decreased hippocampal ECM fiber diameter and tissue stiffness to pain.⁷⁰ Expanding the use of these approaches, as well as techniques like second harmonic generation imaging to examine collagen fibrils, will increase our understanding of how ECM biomechanics regulate pain.

Several key questions remain and are exciting areas for future research: how does ECM remodeling separately influence pain onset, maintenance, and resolution? Can nervous system ECM self-repair, or do these changes persist to maintain pain and sensitize the nervous system to future, potentially painful insults? As new protease inhibitors are being developed, such as monoclonal antibodies against MMPs, it will be important to determine whether blocking protease-mediated ECM remodeling is sufficient to reduce pain clinically. Addressing these gaps will determine whether targeting ECM remodeling or stimulating ECM repair are potential therapeutic strategies for pain management.

Conflict of interest statement

The authors have no conflicts of interest to declare.