Cell-to-Cell Signaling Networks at the Blood-Brain Barrier in Health and Disease

Abstract

The blood-brain barrier (BBB) is a highly specialized interface between the central nervous system (CNS) and the peripheral circulation, crucial for maintaining neuronal homeostasis and protecting the brain parenchyma from potentially harmful blood-borne substances. This review examines the molecular and cellular organization of the BBB and explores how defective cell adhesion and signaling networks lead to BBB pathologies. I discuss the intricate architecture of brain endothelial cells (ECs) within the context of the larger multicellular neurovascular unit (NVU), highlighting the roles of pericytes, astrocytes, as well as extracellular matrix (ECM) proteins and growth factors in vascular basement membranes. Recent advances in understanding EC tight junction dynamics, transport mechanisms, and communication pathways within the NVU are presented, with a particular emphasis on astrocyte-endothelial communication. Furthermore, I detail how abnormal astrocyte-endothelial signaling leads to BBB breakdown and contributes to various neurological disorders. In summary, this review synthesizes current knowledge of BBB biology with particular emphasis on recent discoveries in signaling pathways, intercellular adhesion, and dynamic regulatory mechanisms that govern barrier function. By integrating findings from functional studies across multiple models, this review provides critical insights into both fundamental BBB biology and the potential development of translational approaches for treating human cerebrovascular disorders. Understanding these complex mechanisms not only advances our knowledge of normal brain homeostasis but also illuminates promising therapeutic targets and strategies for addressing conditions ranging from stroke to neurodegenerative diseases, ultimately paving the way for more effective clinical interventions that can preserve or restore BBB integrity.

Graphical Abstract

Introduction

The BBB is a highly selective border that separates the circulating blood from the neural parenchyma in the CNS ¹. First described by Paul Ehrlich in the late 19th century and subsequently characterized by Edwin Goldmann, the BBB plays a fundamental role in maintaining the precise ionic and molecular composition of the brain microenvironment necessary for proper synaptic transmission and neural function ². The concept of the NVU (Fig. 1) has emerged as a more comprehensive framework for understanding vascular homeostasis, recognizing that the BBB is not merely an EC barrier but a dynamic, metabolically active interface comprising multiple cell types that work in concert to regulate blood flow, nutrient delivery, waste removal, and immune surveillance ³.

Figure 1. Neurovascular unit cytoarchitecture and cell-cell signaling networks.

Schematic diagram (left) showing the multicellular composition of a brain capillary neurovascular unit (NVU). Schematic summary (right) showing ECM and soluble factors that mediate communication between astrocytes and brain vascular ECs to promote BBB stability. Created with BioRender.com.

The NVU comprises ECs, pericytes, astrocytic end-feet, basement membranes, neurons, and microglia ⁴. Each component contributes unique structural and functional properties that collectively establish BBB integrity and regulate cerebral perfusion in response to neuronal activity. Dysfunction of the NVU has been increasingly recognized as a central mechanism in numerous pathologies, including ischemic and hemorrhagic stroke, vascular cognitive impairment and dementia, and neurodegenerative diseases. Given the extensive literature covering BBB biology broadly, this review focuses specifically on signaling pathways operating between ECs, astrocytes, pericytes, and vascular basement membranes that regulate BBB maturation, maintenance, and pathophysiological disruption. I highlight recent advances in understanding BBB biology, with particular emphasis on discoveries from the past ten years that have transformed our appreciation of roles for astrocytes and ECM components in NVU maturation and physiology and pathological BBB breakdown mechanisms. Among the many signaling systems that influence BBB biology, this review focuses on Wnt/β-catenin, APOE-LRP1, TGFβ, integrin-mediated ECM signaling, and MMP-dependent matrix remodeling because recent work has established these pathways as central regulators of astrocyte-endothelial communication and BBB integrity across multiple neurological diseases.

Cellular Architecture of the Neurovascular Unit

Endothelial Cells

Brain microvascular ECs form the anatomical basis of the BBB and possess unique characteristics that distinguish them from peripheral endothelium, including low rates of transcytosis, lack of fenestrations, and tight junction proteins that restrict paracellular permeability ⁵. The bulk transcriptional profile of brain ECs is fundamentally different from peripheral ECs, with elevated expression of genes encoding tight junction factors (claudin-5/CLDN5, occludin/OCLN, zonula occluden/TJP1), efflux transporters (P-glycoprotein/ABCB1, breast cancer resistance protein/ABCG2), nutrient transporters (glucose transporter-1/GLUT1, large amino acid transporter-1/LAT1), and receptor-mediated transcytosis systems (transferrin receptor/TFRC, insulin receptor/INSR) ⁶. Age-dependent alterations in expression of BBB transporters and other factors correlates with reduced receptor-mediated transcytosis in ECs ⁷.

Single-cell RNA sequencing studies have revealed remarkable molecular heterogeneity among brain ECs across different vascular zones (capillary, arterial, and veinous) and brain regions ⁸, with capillary ECs exhibiting the highest expression of tight junction proteins, solute transporters, and BBB-associated factors including Claudin-5, Mfsd2a, Slc2a1, and ABC transporters ⁹. In contrast, post-capillary venules display relatively reduced barrier specialization and greater expression of leukocyte adhesion molecules such as ICAM-1, VCAM-1, and selectins, consistent with their role as primary sites of immune cell trafficking into the CNS. Furthermore, regional specialization exists, with ECs in different brain areas exhibiting unique molecular profiles that may reflect local metabolic demands and functional requirements ¹⁰. This diversity has important implications for understanding region-specific vulnerabilities to cerebrovascular disease. Functional imaging studies further demonstrate that BBB disruption is not spatially uniform during neuroinflammatory and neurodegenerative disease, but instead preferentially occurs within venular and capillary segments where endothelial inflammatory activation, pericyte loss, and basement membrane remodeling are most pronounced ¹¹. Leukocyte extravasation similarly occurs predominantly at post-capillary venules, where lower shear stress, specialized endothelial adhesion programs, and distinct ECM composition facilitate immune cell arrest and diapedesis ^12,13. These findings highlight that vascular zonation reflects not only EC transcriptional heterogeneity but also regional specialization in BBB stability, inflammatory responsiveness, and susceptibility to pathological disruption ¹⁴.

Tight junctions between adjacent brain ECs are the primary structural determinants of BBB integrity (Fig. 2) ¹⁵. These multiprotein complexes create a selective paracellular barrier that restricts the passage of hydrophilic molecules while permitting highly regulated transcellular transport. The core transmembrane proteins include claudins, occludin, and junctional adhesion molecules (JAMs). These proteins interact with cytoplasmic scaffolding proteins, including zonula occludens (ZO-1, ZO-2, ZO-3), which anchor the tight junction complex to the actin cytoskeleton and coordinate intracellular signaling ¹⁶. Tight junction proteins undergo dynamic remodeling in response to physiological and pathological stimuli, involving regulated endocytosis, degradation, and membrane reinsertion ^17,18. Claudin-5 is the predominant claudin protein at the BBB and is absolutely required for the barrier's selectivity to small molecules. Genetic deletion of Cldn5 in mice results in size-selective loosening of the BBB, permitting passage of molecules smaller than 800 Da while maintaining restriction of larger molecules ¹⁹.

Figure 2. Junctional proteins at the BBB.

Brain vascular EC junctions (tight junctions and adherens junctions) are interconnected through various multiprotein complexes that mediate BBB formation and physiology. Magnified view of the boxed NVU (left) is shown on the right, highlighting select junctional components. Created with BioRender.com.

A defining property of brain capillary ECs is their exceptionally low rate of vesicular transcytosis compared with peripheral endothelium (Fig. 3). Whereas fenestrated capillaries in peripheral organs depend heavily on caveolae-mediated transcytosis for macromolecule exchange, brain ECs actively suppress this pathway to maintain CNS homeostasis ²⁰. This suppression is not a passive default state but is enforced by a coordinated molecular program involving major facilitator superfamily domain containing 2a (Mfsd2a), which is expressed at high levels in brain ECs and suppresses transcytosis ²¹. Mfsd2a was initially discovered as a transporter for docosahexaenoic acid (DHA), providing the brain with an essential omega-3 fatty acid that cannot be synthesized de novo in sufficient quantities ²². Loss-of-function mutations in MFSD2A cause a severe autosomal recessive neurodevelopmental syndrome characterized by microcephaly, intellectual disability, seizures, and spastic quadriplegia ^23,24. Mfsd2a was also reported to promote BBB integrity, with mice that lack Mfsd2a due to targeted gene deletion developing a leaky BBB characterized by increases in caveolar vesicles without detectable disruption of tight junctions ²⁵. While recent reports have confirmed roles for Mfsd2a in DHA transport, its functions in maintaining BBB integrity have been challenged ^26,27.

Figure 3. BBB transport pathways.

Diagram illustrating different active and passive transport mechanisms that selectively regulate the passage of circulating molecules across the BBB and into the neural parenchyma. Created with BioRender.com.

Plasmalemmal vesicle-associated protein (PLVAP, also known as PV-1) is a type II transmembrane glycoprotein that forms the structural diaphragms bridging the stomatal openings of caveolae and fenestrae in peripheral endothelium ²⁸. Unlike Mfsd2a, PLVAP is virtually absent from normal healthy brain ECs, with its absence serving as a hallmark of the mature BBB phenotype. In brain endothelium, the absence of PLVAP contributes to the marked reduction in functional caveolae and suppression of the transcytotic route. Genetic deletion of PLVAP in mice results in a surprising phenotype: although caveolae lack their diaphragms, overall transcytosis is not dramatically increased, suggesting that PLVAP's role is more nuanced than simply inhibiting all vesicular transport ²⁹. Importantly, upregulation of PLVAP and loss of Mfsd2a appear to be reciprocally linked, suggesting these two molecules operate in an inverse relationship that shifts the endothelial transcytotic setpoint toward either BBB phenotype (PLVAP⁻/MFSD2A⁺) or peripheral endothelial phenotype (PLVAP⁺/MFSD2A⁻) ³⁰.

PLVAP expression is suppressed by CNS-specific inductive signals, including Wnt/β-catenin signaling from glial progenitors during BBB development ^31,32. Indeed, The canonical Wnt pathway is a key regulator of BBB development and CNS angiogenesis ³³. Wnt ligands instruct invading ECs to acquire BBB-specific properties during embryogenesis. Wnt/β-catenin signaling is selectively activated in ECs during vascular ingression into the neural tube, where Wnt7a/Wnt7b and related ligands stabilize β-catenin and drive angiogenesis ³⁴. Loss of endothelial β-catenin signaling results in severe CNS vascular defects, hemorrhage, and impaired expression of BBB-associated genes. Endothelial β-catenin signaling directly promotes maturation of tight junction architecture and BBB identity through induction of claudin genes including CLDN3 and CLDN5 ³⁵. Endothelial-specific activation of β-catenin enhances BBB maturation, whereas β-catenin/CTNNB1 gene loss disrupts junctional organization and increased vascular permeability ³⁶. Various EC co-factors play important roles in regulating β-catenin activation, including Gpr124 and Reck, mainly via control of Fzd receptor affinities for Wnt ligands ^37–39. Together, these studies establish the paradigm that Wnts are key developmental cues that coordinates angiogenesis and acquisition of BBB phenotypes. Interestingly, and as detailed below, EC gene expression profiling efforts reveal that Wnt/β-catenin and transforming growth factor-β (TGFβ)/Smad pathways cooperatively communicate to regulate ECM deposition and BBB maturation ^40,41.

While the BBB restricts paracellular diffusion, it must simultaneously facilitate the exchange of essential nutrients, metabolites, and signaling molecules between blood and brain. This is accomplished through sophisticated transcellular transport systems that maintain brain homeostasis while excluding potentially harmful substances. Brain ECs express an extensive array of transporters that can be broadly categorized into solute carrier (SLC) transporters that mediate facilitated diffusion or secondary active transport, and ATP-binding cassette (ABC) transporters that actively efflux substrates against concentration gradients ⁴². GLUT1 (SLC2A1) is the predominant glucose transporter at the BBB, expressed at high levels to meet the brain's energetic demands. Large neutral amino acids are transported by LAT1 (SLC7A5), while monocarboxylic acids including lactate, pyruvate, and ketone bodies utilize MCT1 (SLC16A1). On the efflux side, P-glycoprotein (ABCB1) and BCRP (ABCG2) restrict brain penetration of numerous lipophilic compounds, including many therapeutic drugs, thereby limiting CNS drug delivery but also protecting the brain from xenobiotics ^43,44. Lastly, receptor-mediated transcytosis pathways enable BBB penetration of larger molecules that cannot utilize carrier-mediated transport. The transferrin receptor mediates brain iron uptake, the insulin receptor facilitates insulin entry, and the low-density lipoprotein receptor-related protein 1 (LRP1) is involved in clearance of amyloid-β (Aβ) peptides and other ligands from brain to blood (see below). Recent studies have exploited RMT pathways for drug delivery, engineering antibodies or fusion proteins that undergo transcytosis via transferrin receptor or other BBB receptors to deliver therapeutics to the CNS ^45,46.

Pericytes

Pericytes are essential regulators of BBB integrity and play a particularly critical role in suppressing endothelial transcytosis ⁴⁷. Endothelial-derived platelet-derived growth factor-B (PDGF-B) signaling through PDGFRβ is essential for pericyte recruitment to developing CNS vessels ⁴⁸. Pericyte deficiency results in profound BBB disruption despite relatively preserved tight junction ultrastructure, establishing that increased endothelial transcytosis represents a major mechanism of barrier failure independent of junctional breakdown. ^49,50. Pericytes suppress caveolae-mediated vesicular trafficking in ECs, in part, via control of MFSD2A expression. Pericyte loss leads to reduced MFSD2A expression, increased caveolin-1-positive vesicles, and enhanced transport of plasma-derived proteins across the BBB ⁵¹. Pericytes additionally regulate endothelial transcytosis through platelet-derived growth factor-B/PDGFRβ signaling, angiopoietin-Tie2 signaling, and ECM interactions that promote endothelial quiescence and barrier specialization ⁴⁷.

Pericyte loss and/or dysfunction is increasingly recognized as a key pathological feature in subsets of patients with Alzheimer's disease. Recent imaging studies in humans have revealed that pericyte injury, measured by elevated levels of soluble PDGFRβ in cerebrospinal fluid, correlates with cognitive decline and predicts conversion to dementia ⁵². In pathological conditions including Alzheimer’s disease, ischemic stroke, and diabetic vasculopathy, pericyte degeneration is associated with increased endothelial vesicular transport, leakage of circulating factors into the brain parenchyma, neuroinflammation, and impaired cerebral blood flow ⁵³. Studies using pericyte-deficient mouse models have demonstrated that lack of pericytes leads to BBB breakdown and accumulation of neurotoxic blood-derived proteins in brain parenchyma ⁵⁴. It is important to note, however, that several studies have reported relatively preserved pericyte coverage in certain brain regions, suggesting that BBB dysfunction can occur in the absence of overt pericyte depletion. These findings highlight regional heterogeneity and indicate that altered pericyte signaling may be as important as pericyte loss itself.

It is important to note that brain pericytes exhibit diverse morphologies and variable blood vessel coverage ⁵⁵. For example, ‘mesh pericytes’ are embedded within the capillary basement membrane and form highly specialized peg-and-socket contacts with ECs that are enriched in adhesion molecules and signaling proteins that facilitate bidirectional communication ⁵⁶. The ‘thin-stranded pericyte’ subtype extend fine processes longitudinally along the abluminal capillary endothelial surface but cover only a portion of the vessel circumference ⁵⁷. Pre-capillary arterioles contain ‘ensheathing pericytes’, that cover most but not all of the abluminal EC surface. Because pericyte coverage is incomplete in many vascular zones, astrocytes frequently interact directly with ECs through the intervening basement membrane, highlighting the importance of astrocyte-endothelial signaling independent of direct pericyte contact.

Astrocytes

Astrocytes contribute to BBB induction, maturation, and long-term maintenance through highly coordinated bidirectional signaling with ECs and pericytes, mediated by both soluble trophic factors and ECM interactions (Fig. 1) ^5,58. During development, astrocyte progenitors secrete soluble factors which collectively promote endothelial tight junction formation, suppress endothelial transcytosis, and stabilize BBB-specific transporter expression ⁵⁹. For example, astrocyte-derived Shh acts through endothelial Smoothened receptors to enhance expression of tight junction proteins including Claudin-5, Occludin, and ZO-1 while suppressing inflammatory EC states ⁶⁰.

Astrocyte endfeet contact exposed pericyte surfaces and contribute to local ECM organization that stabilize both EC and pericyte phenotypes. This mosaic cellular arrangement emphasizes that the NVU is not organized as rigidly stratified cellular layers, but rather as an integrated and spatially heterogeneous signaling interface in which ECs, pericytes, astrocytic endfeet, and the basement membrane forming highly dynamic and regionally specialized interactions. Astrocyte end-feet are highly enriched in aquaporin-4 (AQP4), Kir4.1 potassium channels, connexins, and dystrophin-associated protein complexes that regulate water flux, ion buffering, and neurovascular coupling ^61,62. Proper polarization of AQP4 to astrocytic end-feet is essential for glymphatic fluid transport, potassium homeostasis, and cerebrovascular responsiveness to neuronal activity. Disruption of end-foot polarity occurs in many neurological diseases and is frequently associated with detachment of astrocytes from the laminin-rich vascular basement membrane and degradation of dystroglycan complexes ⁶³. Loss of polarized AQP4 localization impairs clearance of interstitial solutes, disrupts neurovascular coupling, promotes vasogenic edema, and exacerbates inflammatory leukocyte infiltration across the BBB ⁶⁴.

Recent studies have identified astrocyte-derived apolipoprotein E (APOE) as a key regulator of cerebrovascular integrity and pericyte-endothelial homeostasis, linking astrocyte biology directly to BBB dysfunction in neurodegenerative disease ⁶⁵. Astrocytes are the primary source of APOE within the CNS, where APOE-containing lipoprotein particles mediate lipid transport, membrane repair, synaptic remodeling, and NVU signaling through interactions with LDL receptor family members including LRP1. Importantly, APOE isoforms exert distinct effects on BBB stability. APOE3 maintains vascular homeostasis in part by suppressing pro-inflammatory signaling in pericytes through LRP1-dependent inhibition of the cyclophilin A pathway ^66,67. In contrast, the APOE4 isoform-the strongest genetic risk factor for late-onset Alzheimer’s disease-fails to adequately suppress this pathway, leading to chronic pericyte activation, elevated MMP9 expression, degradation of tight junction proteins and basement membrane components, and progressive BBB leakage ⁶⁸. APOE4-associated BBB disruption occurs independently of Aβ deposition in early disease stages and is accompanied by accumulation of blood-derived neurotoxic proteins including fibrinogen, thrombin, and albumin within the brain parenchyma. These vascular changes are associated with reduced cerebral blood flow, impaired clearance of metabolic waste products, and enhanced neuroinflammation. APOE4 alters astrocyte lipid metabolism, inflammatory reactivity, mitochondrial function, and ECM regulation, driving a maladaptive astrocyte phenotype that further destabilizes the NVU ⁶⁹. Together, these findings position astrocyte-derived APOE signaling as a critical molecular interface connecting pericyte degeneration, ECM remodeling, and BBB breakdown in neurodegenerative disease.

TGF-β is another critical factor that mediates bidirectional communication between astrocytes and brain ECs ⁷⁰. TGF-β ligands, particularly TGF-β1, are secreted by multiple NVU cell types and signal through type I and type II serine/threonine kinase receptors to activate canonical SMAD-dependent and non-canonical SMAD-independent pathways ⁷¹. This signaling system plays essential roles in BBB development, maintenance, and pathological remodeling. During development, astrocytes express αvβ8 integrin which mediates activation of latent-TGF-β in the ECM ⁷⁰ (Fig. 1). TGF-β subsequently signals to ECs to promote barrier formation, including upregulation of tight junction proteins and downregulation of transcytotic machinery ^72–74. TGF-β activates the ALK1 and ALK5 receptors on ECs, with ALK5/SMAD2/3 signaling generally promoting barrier maturation and stability, while ALK1/SMAD1/5/8 signaling can influence angiogenic responses ^75,76. The balance between these pathways is crucial for proper BBB formation and maintenance. Leucine-Rich Repeat Containing 33 secreted by perivascular microglia sequesters latent-TGF-β and acts cooperatively with αvβ8 integrin to promote TGF-β activation ⁷⁷. Disruption of TGF-β receptors in ECs, including neuropilin-1 (Nrp1), results in BBB dysfunction, vascular abnormalities, and neurological deficits ^78,79.

TGF-β signaling also modulates basement membrane composition and stability, a critical aspect of NVU integrity. TGF-β stimulates production of ECM components including collagen IV, laminin, and fibronectin. This TGF-β-mediated matrix deposition strengthens the structural composition of basement membranes (see below), thus supporting the BBB and provides biochemical cues that reinforce endothelial barrier properties ⁸⁰. Dysregulated TGF-β signaling can lead to excessive ECM deposition and fibrosis, contributing to vascular stiffening and impaired NVU function in aging and disease ⁸¹. Collectively, these observations illustrate that astrocytes actively regulate BBB integrity not only through secretion of morphogens and ECM proteins, but also through maintenance of polarized end-foot protein complexes that coordinate ion and water homeostasis. Disruption of these astrocytic signaling networks underlies several neurological disorders discussed later in this review. For example, inherited mutations affecting the MLC1/GlialCAM complex disrupt localization and function of Kir4.1 and AQP4 at astrocyte end-feet, whereas age-dependent impairment of APOE-LRP1 signaling contributes to defective Aβ clearance and progressive BBB dysfunction in vascular cognitive impairment and Alzheimer's disease.

Basement Membranes

The cerebral microvasculature is ensheathed by basement membranes, with an endothelial basement membrane closely juxtaposed to capillary ECs and a parenchymal basement membrane adjacent to astrocytic end-feet, separated by pericyte cell bodies ^82–84. These ECM structures are composed primarily of laminin, collagen IV, nidogen, and heparan sulfate proteoglycans (perlecan, agrin). The basement membrane provides structural support, anchors cellular components of the NVU, and serves as a repository for growth factors and signaling molecules including Wnts and TGF-β ⁸¹.

Basement membrane composition and integrity are critical for BBB function. ECs and pericytes express distinct laminin isoforms-laminin-α4 and laminin-α5-that differentially regulate BBB properties ⁸³. Integrins are the primary cell surface receptors for ECM proteins and regulate intracellular signaling pathways and cytoskeletal dynamics ⁸⁵. For example, astrocytes and pericytes utilize α7 integrin to stabilize laminin ligands in vascular basement membranes ⁸⁶. Deficiency in laminin-α2, which is expressed in the parenchymal basement membrane, results in astrocytic end-foot detachment and impaired AQP4 polarization ⁸⁷. The α5β1 integrin in pericytes is also critical for communication with NVU ECs via adhesion to vitronectin in the vascular basement membrane ⁸⁸.

Type IV collagen is the primary structural component of all basement membranes and is present at exceptional density in the cerebral vascular basement membrane, reflecting the specialized mechanical and barrier demands of the NVU. The type IV collagen family consists of six α-chains (α1–α6) that assemble into three distinct heterotrimeric networks: α1-α1-α2(IV), α3-α4-α5(IV), and α5-α5-α6(IV). In the brain vasculature, the endothelial basement membrane is predominantly composed of the α1-α1-α2(IV) network encoded by COL4A1 and COL4A2, genes that co-localize on chromosome 13 and share a bidirectional promoter. The parenchymal basement membrane in contact with astrocyte end-feet also contains α1-α1-α2(IV), though its laminin isoform composition differs from the endothelial layer ^89,90. COL4A1 and COL4A2 mutations have emerged as important genetic causes of cerebrovascular disease, establishing collagen IV as essential for vascular integrity in the human brain. Mutations in COL4A1 cause a pleiotropic syndrome termed COL4A1-related disease or Gould’s disease, encompassing porencephaly, hereditary angiopathy with nephropathy, and aneurysms. Patients also present with retinal arteriopathy, and the features of cerebral small vessel disease including lacunar stroke, white matter lesions, and hemorrhage (see below). These mutations typically affect conserved glycine residues in the triple helix domain, impairing proper collagen IV folding, secretion, and assembly into basement membrane networks. The resulting structurally defective basement membranes are prone to rupture under hemodynamic stress, leading to vessel fragility and hemorrhage ^89,90. Heterozygous COL4A1 mutations in mice faithfully recapitulate the human phenotype, including perinatal intracerebral hemorrhage, progressive vascular basement membrane abnormalities, and pericyte loss. COL4A2 mutations also cause similar cerebrovascular disease, highlighting the non-redundant importance of both chains ^89,90.

Collagen XVIII is an atypical collagen family member that functions as a HSPG and is expressed at the inner surface of vessel walls in the brain, particularly at the luminal aspect of the endothelial basement membrane ⁹¹. Its C-terminal domain, endostatin, is proteolytically released and functions as a potent endogenous inhibitor of angiogenesis by blocking vascular endothelial growth factor-A (VEGF) signaling and EC migration ⁹². In the healthy brain, endostatin derived from collagen XVIII contributes to the suppression of pathological angiogenesis and maintenance of vascular quiescence ⁹². Abnormal endostatin processing and accumulation have been linked to cerebral amyloid angiopathy, where collagen XVIII deposits alongside Aβ in vessel walls, and to impaired vascular responses following stroke ⁹³.

Fibronectin is a multidomain ECM glycoprotein with critical roles in vascular biology ⁹⁴. Although fibronectin is present at low levels in the mature cerebrovascular basement membrane, it is rapidly upregulated and deposited following vascular injury, inflammation, or hypoxia, forming a provisional matrix scaffold that supports EC migration, proliferation, and adhesion. Two major forms exist: plasma fibronectin, secreted by hepatocytes and circulating in blood, and cellular fibronectin, produced locally by ECs, smooth muscle cells, and activated astrocytes. In the healthy NVU, fibronectin acts through integrin receptors (α5β1, αvβ3) to promote endothelial survival and support basement membrane assembly ⁹⁵. In cerebrovascular disease, fibronectin undergoes abnormal accumulation and isoform switching. In chronic hypertension and aging, fibronectin deposition in arteriolar walls increases, contributing to vascular stiffening and impaired autoregulation. Following ischemic stroke, cellular fibronectin is upregulated in peri-infarct vasculature, where it promotes angiogenesis and vascular remodeling during recovery. However, excessive fibronectin deposition can impede axonal regeneration and synaptogenesis by creating a non-permissive ECM environment. The alternatively spliced EDA and EDB domains of fibronectin, which are absent from plasma fibronectin, are selectively expressed in pathological vasculature and have been exploited as targets for tumor and wound vasculature imaging and therapy ⁸¹.

Heparan sulfate proteoglycans (HSPGs) are essential non-collagenous components of the cerebrovascular basement membrane that serve as a reservoir for signaling molecules and regulate their bioavailability at the NVU. Perlecan (HSPG2) and agrin are the two major HSPGs in the brain vascular basement membrane. Perlecan, the largest known proteoglycan, contains five distinct protein domains that interact with laminin, nidogen, collagen IV, and cell-surface receptors, acting as an organizing scaffold for basement membrane assembly. Its heparan sulfate chains bind numerous growth factors and morphogens including fibroblast growth factor (FGF), VEGF, Shh, and Wnt ligands, sequestering them in the matrix and presenting them to cell-surface receptors in a spatially controlled manner ⁸¹. Perlecan has a complex role in cerebrovascular integrity: while it contributes to basement membrane organization and acts as a sink for pro-angiogenic factors, its heparan sulfate chains are substrates for heparanase, an enzyme upregulated during neuroinflammation and ischemia that cleaves HSPGs to release bound growth factors and disrupt matrix integrity. Agrin, the other major HSPG, plays a specific role in organizing the gliovascular interface by anchoring AQP4-containing complexes to astrocyte end-feet through interactions with α-dystroglycan, and its disruption contributes to end-foot depolarization and impaired water homeostasis ⁶⁴.

Nidogens (nidogen-1 and nidogen-2, also called entactins) are glycoproteins that bridge the two major scaffold networks of basement membranes — the laminin polymer and the collagen IV network — through high-affinity binding interactions. Double nidogen-1/nidogen-2 knockout mice develop severe brain vascular defects including hemorrhage, disrupted basement membrane assembly, and BBB breakdown, demonstrating the combined importance of these proteins in vascular integrity ⁹⁶. Nidogen fragments released during basement membrane proteolysis are detectable in CSF and have been investigated as biomarkers of NVU damage in neurological disease.

Molecular Pathways that Drive BBB Breakdown

Recent work has also highlighted the importance of basement membrane mechanics in BBB integrity, with alterations in ECM stiffness affecting EC phenotypes and barrier properties ⁹⁷. Matrix metalloproteinases (MMPs) are zinc-dependent endopeptidases ⁹⁸ that degrade tight junction proteins and basement membrane components, directly increasing BBB permeability. Excessive activation of MMP-2 and MMP-9 is implicated in BBB breakdown during stroke, inflammation, and other pathological conditions ⁹⁹. MMP-9 is rapidly induced following stroke, traumatic brain injury, and during neuroinflammation. Its expression and activity are regulated by inflammatory cytokines, oxidative stress, and growth factors. Genetic deletion or pharmacological inhibition of MMP-9 reduces BBB breakdown and improves outcomes in experimental stroke models, although clinical translation has proven challenging due to the complex biology of MMPs and their roles in beneficial vascular remodeling ¹⁰⁰.

Inflammatory signaling profoundly impacts BBB integrity. Pro-inflammatory cytokines including TNF-α, IL-1β, and IL-6 disrupt tight junctions through multiple mechanisms: inducing MMP expression, promoting internalization and degradation of junction proteins, reorganizing the actin cytoskeleton, and activating downstream signaling cascades including NF-κB and Rho kinase pathways. T cells selectively open the BBB via production of interferon-γ, which facilitates their passage through EC tight junctions ¹⁰¹. Glial-derived VEGF, while promoting angiogenesis, signals through the VEGFR2 receptor tyrosine kinase to increase vascular permeability ¹⁰² via multiple mechanisms including claudin phosphorylation, VE-cadherin internalization, and enhanced transcytosis.

Oxidative stress is both a consequence and driver of BBB dysfunction. Reactive oxygen species generated during ischemia-reperfusion, neuroinflammation, or because of mitochondrial dysfunction directly damage ECs, modify proteins through oxidation, and activate redox-sensitive transcription factors that alter gene expression. NADPH oxidase enzymes, particularly NOX2 and NOX4, are important sources of superoxide in cerebral vessels and contribute to BBB breakdown in hypertension, stroke, and other pathologies. Conversely, antioxidant systems including superoxide dismutase, catalase, and glutathione peroxidase protect BBB integrity, and their dysfunction exacerbates neurovascular injury ¹⁰³.

BBB disruption permits entry of circulating proteins and coagulation factors into the brain parenchyma, where they can exert potent neurotoxic and pro-inflammatory effects. Fibrinogen is among the most extensively studied blood-derived mediators and promotes activation of microglia, resulting in oxidative stress, synaptic dysfunction, and neurodegeneration ¹⁰⁴. Thrombin activates protease-activated receptors on ECs, astrocytes, and neurons, triggering inflammatory responses, vascular instability, and neuronal injury ¹⁰⁵. Albumin, normally excluded from the CNS, accumulates within the brain following BBB disruption and activates TGF-β signaling pathways in astrocytes, promoting reactive gliosis, aberrant neuronal excitability, and cognitive impairment ¹⁰⁶. Following hemorrhage, hemoglobin and iron released from erythrocytes generate reactive oxygen species and contribute to oxidative tissue damage. Collectively, these blood-derived factors amplify neuroinflammation and neuronal dysfunction, establishing a pathogenic feed-forward cycle in which BBB breakdown accelerates disease progression across multiple neurological disorders, including Alzheimer's disease, vascular dementia, and stroke.

Megalencephalic leukoencephalopathy with subcortical cysts (MLC): A leukodystrophy linked to defective glial regulation of BBB integrity

The astrocyte-endothelial signaling pathways described above provide a framework for understanding mechanisms of BBB dysfunction. Among these, MLC represents one of the clearest examples in which disruption of astrocyte end-foot protein complexes impairs ion and water homeostasis at the glial-EC interface (Fig. 4). MLC is characterized by macrocephaly apparent from early infancy, slowly progressive cerebellar ataxia and spasticity, delayed cognitive decline, and a distinctive MRI pattern of diffuse white matter signal abnormality with subcortical cysts predominantly in the anterior temporal and frontoparietal regions ¹⁰⁷. Pathologically, the white matter shows vacuolization within myelin sheaths and intramyelinic edema, without inflammatory infiltrates or primary demyelination. Mutations in one of two genes: MLC1 (the most common form, accounting for approximately 75% of cases) and HEPACAM, encoding the cell-adhesion molecule GlialCAM (accounting for approximately 20% of cases) drive MLC pathogenesis, with mutations in AQP4 or GPRC5B linked to rare cases of MLC ¹⁰⁸.

Figure 4. The MLC1/GlialCAM protein complex in astrocytes promotes normal NVU stability and is disrupted in the neurodevelopmental disorder, MLC.

(A); The Mlc1 and GlialCAM form protein complexes with CLC-2, KiR4.1, and AQP4 in perivascular astrocyte endfeet to promote normal NVU cytoarchitecture and brain physiology. (B); In patients with MLC disease, mutations in MLC1, HEPACAM (GlialCAM), or AQP4 disrupt astrocyte endfoot polarity leading to imbalances in ion and water homeostasis, white matter degeneration, and resulting neurological deficits. Disruption of the BBB likely results from the astrocyte defects in MLC. Created with BioRender.com.

MLC1 is a membrane protein with eight predicted transmembrane domains that belongs to the major facilitator transporter superfamily, although its substrate remains unknown. MLC1 is expressed almost exclusively in astrocytes, with highest levels in end-feet ensheathing blood vessels and in astrocytes lining the pial surface, glia limitans, and ependyma — all locations where astrocytes interface with fluid compartments (blood, CSF, subarachnoid space) ^109,110. This selective gliovascular expression pattern strongly implicates MLC1 in ion and water homeostasis at these interfaces. Immunoprecipitation and co-localization studies demonstrate that MLC1 forms a molecular complex with GlialCAM, Kir4.1, the ClC-2 chloride channel, and AQP4 at astrocyte end-feet, suggesting it is part of a broader ion/water regulatory complex at the glial-EC junction ¹¹¹.

GlialCAM (encoded by HEPACAM) is a member of the immunoglobulin superfamily of cell-adhesion molecules expressed by astrocytes and oligodendrocytes. At astrocyte end-feet, GlialCAM localizes to cell junctions and the gliovascular interface, where it functions as an indispensable chaperone for MLC1, regulating its trafficking to and retention at the plasma membrane. In the absence of GlialCAM, MLC1 is mislocalized to intracellular compartments and fails to reach the plasma membrane in astrocyte end-feet. Beyond its role as an MLC1 chaperone, GlialCAM independently regulates the activity of the CLC-2 chloride channel - a critical mediator of chloride homeostasis and cell volume regulation in astrocytes - by targeting ClC-2 to cell junctions and modifying its gating properties. GlialCAM also engages in homophilic and heterophilic adhesion interactions that stabilize astrocyte-astrocyte and astrocyte-myelin junctions, contributing to structural integrity of the white matter NVU ^112–114.

The central pathophysiological mechanism in MLC disease is a defect in astrocytic ion and water homeostasis at the NVU, leading to impaired regulation of the brain water content and consequent intramyelinic edema. Under normal conditions, astrocyte end-feet actively regulate interstitial fluid volume by taking up excess K⁺ and water through Kir4.1 and AQP4, respectively, and by mediating transcellular water fluxes across the glial-endothelial interface. In MLC1-deficient and GlialCAM-deficient mice, astrocytes exhibit impaired activation of volume-regulated anion channels (VRACs), impaired regulatory volume decrease (RVD) following hypotonic swelling, and disrupted end-foot polarity of Kir4.1 and AQP4. These defects result in astrocyte swelling, particularly at end-feet in contact with blood vessels, and progressive accumulation of fluid within myelin sheaths, generating the intramyelinic vacuoles and subcortical cysts that are pathological hallmarks of MLC ¹¹³. Although overt BBB leakage is modest in most leukodystrophies, these disorders and especially MLC, reveal how defects in astrocyte-endothelial signaling, end-foot polarity, ion transport, and fluid homeostasis can destabilize BBB maturation and NVU function.

MLC is a rare genetic disease; however, it provides a valuable conceptual model for understanding how disruption of astrocyte polarity can compromise NVU function. Unlike inflammatory disorders in which EC tight junction disruption predominates, MLC demonstrates that selective impairment of astrocyte end-foot protein complexes is sufficient to perturb BBB physiology by disrupting coordinated regulation of water, potassium, and chloride transport. These findings emphasize that maintenance of BBB integrity depends on integrated signaling among endothelial cells, astrocytes, pericytes, and basement membrane components rather than on endothelial cells alone.

BBB Dysregulation in Age-Related Dementia

Accumulating evidence indicates that cerebrovascular dysfunction and BBB breakdown contribute to cognitive decline not only in pure vascular dementia but also in Alzheimer's disease and mixed pathology dementia. The working model is that initial vascular injury leads to BBB disruption, hypoperfusion, and accumulation of neurotoxic blood-derived proteins, which then render the brain more vulnerable to neurodegenerative pathology ¹¹⁵. Longitudinal neuroimaging studies have demonstrated that BBB breakdown occurs early in cognitive decline, even before structural brain changes or overt cognitive symptoms. Using dynamic contrast-enhanced MRI, BBB permeability in the hippocampus was shown to increase with age and correlates with mild cognitive impairment, independent of Alzheimer's biomarkers ¹¹⁶. Subsequent studies confirmed that early BBB dysfunction, particularly in medial temporal lobe regions, predicts cognitive decline and conversion to dementia ¹¹⁷. CSF biomarkers of pericyte injury (soluble/shed PDGFRβ) and BBB breakdown (albumin ratio) further support early neurovascular dysfunction in the pathogenesis of cognitive impairment.

Age-related dementia similarly illustrates how dysfunction of multiple NVU cell types converges on BBB failure. Whereas MLC primarily reflects disruption of astrocytic end-foot function, Alzheimer's disease and vascular cognitive impairment involve coordinated abnormalities in endothelial transport, pericyte survival, astrocyte reactivity, and extracellular matrix remodeling. Together, these changes progressively impair BBB homeostasis and reduce clearance of toxic metabolites, including Aβ. Aβ homeostasis is tightly regulated by BBB transport systems that control peptide clearance from the brain and limit influx from the circulation. Under physiological conditions, endothelial LRP1 mediates efflux of Aβ from brain interstitial fluid into the blood, while receptor for advanced glycation end products (RAGE) promotes transport of circulating Aβ into the CNS. Aging and Alzheimer's disease are associated with reduced endothelial LRP1 expression and increased RAGE activity, resulting in impaired Aβ clearance and progressive accumulation of vascular and parenchymal amyloid deposits ¹¹⁸. Aβ itself directly contributes to BBB dysfunction by inducing oxidative stress, inflammatory signaling, and endothelial activation, while promoting degradation of tight junction proteins and basement membrane components ¹¹⁹. Deposition of Aβ within cerebral vessel walls, a condition termed cerebral amyloid angiopathy, further compromises vascular integrity, impairs cerebral blood flow, and increases susceptibility to microhemorrhage ¹²⁰. In cerebral amyloid angiopathy, Aβ deposits in vessel walls cause vascular degeneration, inflammation, and ultimately fragility that predisposes to both hemorrhage and ischemia. Affected vessels exhibit thickened, fragmented basement membranes, smooth muscle cell loss, and BBB dysfunction even in regions without overt hemorrhage ^121,122. Following hemorrhage, blood breakdown products including thrombin and hemoglobin directly injure brain tissue, disrupt the BBB, and trigger robust neuroinflammation that contributes to secondary injury and poor outcomes. These findings highlight a bidirectional relationship in which BBB dysfunction promotes Aβ accumulation, while Aβ accumulation further exacerbates BBB breakdown and NVU dysfunction.

Cerebral small vessel disease (CSVD) encompasses a group of pathologies affecting cerebral arteries, arterioles, capillaries, and venules, resulting in white matter lesions, microbleeds, and enlarged perivascular spaces. CSVD is the leading vascular cause of cognitive impairment and dementia, accounting for approximately 20% of all strokes and contributing to up to 45% of dementia cases. BBB dysfunction plays a central role in CSVD pathogenesis, with chronic low-grade BBB leakage in white matter leading to accumulation of blood-derived proteins, oxidative stress, inflammation, and progressive white matter injury ¹²³.

Advanced MRI techniques including dynamic contrast-enhanced imaging have demonstrated BBB leakage in CSVD patients, with permeability increases correlating with white matter lesion severity and cognitive decline. Pathological studies confirm blood-derived protein deposition (fibrinogen, albumin, immunoglobulins) in white matter of CSVD patients, along with evidence of EC activation, pericyte loss, and basement membrane thickening. The white matter appears particularly vulnerable to BBB dysfunction due to its watershed location, relatively low capillary density, and unique metabolic demands ¹²³. Pericyte degeneration is a prominent early feature in CSVD and may represent both a cause and consequence of BBB breakdown, creating a vicious cycle of neurovascular dysfunction ¹¹⁵.

Therapeutic Strategies Targeting the NVU/BBB

Given the central role of NVU dysfunction in cerebrovascular disease, therapeutic strategies aimed at protecting or restoring BBB integrity and NVU function represent promising approaches for treating stroke, vascular dementia, and related disorders. Multiple complementary strategies are being pursued, ranging from targeting specific molecular pathways to promoting vascular repair and regeneration. For example, stabilizing tight junctions and reducing paracellular permeability is a direct approach to BBB protection. Activated protein C (APC), an endogenous anticoagulant with cytoprotective properties, protects the BBB through PAR1- and PAR3-mediated signaling that stabilizes tight junctions, reduces MMP expression, and exhibits anti-inflammatory effects. 3K3A-APC, an APC variant with reduced anticoagulant activity but preserved cytoprotective function, showed promising results in preclinical stroke models and advanced to clinical trials, though results have been mixed ¹²⁴. Pharmacological activation of the Wnt/β-catenin pathway can enhance tight junction expression and reduce permeability. Similarly, Rho kinase inhibitors prevent cytoskeletal reorganization and tight junction disruption induced by inflammatory stimuli ¹²⁵.

Promoting angiogenesis and vascular repair is an alternative strategy that aims to restore NVU function following injury. VEGF, while increasing acute permeability, drives long-term angiogenesis and vascular remodeling that may aid recovery. However, the timing and duration of VEGF delivery are critical-early VEGF can worsen edema, while delayed or chronic administration promotes beneficial neovascularization ¹²⁶. Careful titration of pro-angiogenic signals, possibly combined with tight junction stabilizers, may optimize vascular repair while minimizing permeability ¹²⁷. Pericyte transplantation or strategies to prevent pericyte loss represent novel approaches based on recent understanding of pericyte importance in BBB maintenance. Preclinical studies have shown that pericyte transplantation or delivery of pericyte-derived factors can reduce BBB breakdown and improve outcomes in stroke and brain injury models ¹²⁸.

Targeting inflammation and oxidative stress provides broader BBB protection through multiple mechanisms. Anti-inflammatory approaches including IL-1 receptor antagonists, TNF-α and Interferon-γ inhibitors, and modulators of microglial activation are being explored ¹²⁹. Antioxidant therapies targeting specific ROS sources (NADPH oxidase inhibitors) or enhancing endogenous antioxidant systems (Nrf2 activators) have shown promise in preclinical studies. For example, edaravone, a free radical scavenger approved for acute ischemic stroke treatment in Japan, exhibits BBB-protective effects, though its clinical benefits remain debated ¹³⁰.

Emerging Technologies and Future Directions

Recent technological advances are enabling unprecedented insights into NVU biology and opening new therapeutic avenues. Single-cell RNA sequencing and spatial transcriptomics have revealed remarkable cellular heterogeneity within the NVU, identifying molecularly distinct EC populations, pericyte subtypes, and region-specific astrocyte phenotypes. These approaches are mapping the transcriptional landscape of the healthy and diseased NVU at single-cell resolution, identifying novel molecular targets and biomarkers ^8,10,131.

Many of these studies demonstrate that astrocytes actively shape EC phenotype through secretion of morphogens, cytokines, lipid mediators, and ECM proteins, while ECs reciprocally influence astrocyte maturation and polarization through angiocrine signaling pathways ¹³². Advanced imaging technologies are also providing new opportunities to study astrocyte-endothelial interactions in vivo. High-resolution two-photon microscopy and dynamic contrast-enhanced MRI permit visualization of BBB permeability changes and astrocyte calcium dynamics in real time, while emerging PET tracers targeting reactive astrocytes, matrix remodeling enzymes, and endothelial transport systems provide molecular insight into NVU dysfunction in living patients. In malignant brain tumors, advanced brain tumor imaging platforms combining quantitative PET and MRI approaches are beginning to reveal spatial heterogeneity in BBB disruption, vascular remodeling, and ECM deposition within the tumor microenvironment. These imaging modalities may ultimately enable patient stratification based on specific neurovascular and matrix-associated signatures.

Human induced pluripotent stem cell (iPSC)-derived BBB models and vascularized organoid systems are similarly advancing the field by allowing mechanistic interrogation of astrocyte-endothelial crosstalk using human cells. Recent organ-on-chip platforms incorporate ECs, astrocytes, pericytes, and engineered ECM scaffolds under physiologic flow conditions, recapitulating key aspects of BBB maturation and dysfunction. These systems have highlighted critical roles for astrocyte-derived laminins, integrin signaling, and basement membrane organization in maintaining endothelial barrier properties. Patient-derived iPSC models carrying disease-associated mutations further provide opportunities to define how altered ECM composition and astrocyte reactivity contribute to BBB instability in neurodegenerative disease and brain cancer.

Therapeutically, targeting astrocyte-EC signaling and ECM remodeling represents an emerging translational frontier. Strategies aimed at stabilizing basement membrane composition, restoring astrocyte endfoot polarization, modulating integrin signaling, or suppressing pathological matrix metalloproteinase activity may help preserve BBB integrity in neurological disease. Conversely, controlled modulation of ECM architecture and endothelial transcytosis pathways may improve CNS drug delivery. The growing recognition that the ECM functions as a dynamic regulator of neurovascular signaling rather than a static structural element is likely to reshape future therapeutic approaches to cerebrovascular and neurodegenerative disorders.

Conclusions

The NVU is a highly integrated multicellular system that maintains CNS homeostasis through coordinated interactions among ECs, astrocytes, pericytes, neurons, and the vascular basement membrane. Increasing evidence indicates that astrocyte-endothelial communication and ECM organization are central determinants of BBB integrity, cerebral perfusion, and neurovascular stability. Astrocyte endfeet not only physically ensheath ~80% of the cerebral vasculature but also actively regulate endothelial barrier phenotype through secretion of trophic factors, lipid mediators, and matrix proteins, while endothelial-derived signals reciprocally shape astrocyte maturation and function. The vascular basement membrane serves as a critical signaling interface that integrates these bidirectional interactions and organizes the molecular architecture of the BBB.

Over the past several years, major advances in transcriptomics, imaging, and human stem cell modeling have revealed previously unrecognized heterogeneity within astrocyte and endothelial populations and identified diverse ECM-associated signaling pathways that regulate NVU function in health and disease. BBB dysfunction is now recognized as an early and potentially causative event in stroke, vascular cognitive impairment, and neurodegeneration. In these conditions, disruption of astrocyte-endothelial signaling, loss of endfoot polarity, and pathological ECM remodeling contribute to tight junction destabilization, neuroinflammation, leukocyte infiltration, and accumulation of neurotoxic blood-derived factors within the CNS parenchyma.

Emerging studies further suggest that reactive astrocytes and altered basement membrane composition can either promote vascular repair or exacerbate BBB dysfunction depending on disease context and regional microenvironment. Understanding how signaling pathways such as TGF-β, Wnt/β-catenin, ECM factors, and extracellular protease networks coordinate astrocyte-endothelial interactions will therefore be essential for developing targeted neurovascular therapies. Future therapeutic strategies aimed at restoring BBB integrity will likely require approaches that stabilize both cellular signaling networks and the surrounding extracellular matrix environment.

Highlights.

• The BBB is maintained by coordinated signaling among ECs, pericytes, astrocytes, and vascular basement membrane components within the NVU.

• Astrocyte-EC communication through Wnt/β-catenin, TGF-β/Smads, APOE-LRP1, and ECM signaling pathways is a central regulator of BBB development, maintenance, and remodeling.

• Vascular basement membrane proteins actively regulate NVU signaling and BBB integrity rather than serving solely as structural scaffolds.

• Disruption of NVU signaling contributes to BBB dysfunction in disorders including MLC, CSVD, and age-related dementias.

• Emerging single-cell, spatial transcriptomic, and human stem cell-based technologies are revealing new therapeutic opportunities to restore NVU function and BBB integrity.

Non-standard Abbreviations and Acronyms

Aβ

amyloid-β

APC

activated protein C

APOE

apolipoprotein E

AQP4

aquaporin-4

BBB

blood-brain barrier

CSVD

cerebral small vessel disease

EC

endothelial cell

ECM

extracellular matrix

GlialCAM

glial cell adhesion molecule

iPSC

induced pluripotent stem cell

LRP1

lipoprotein receptor-related protein 1

Mfsd2a

major facilitator superfamily domain containing 2a

MLC

megalencephalic leukoencephalopathy with subcortical cysts

MMP

matrix metalloproteinase

Nrp1

neuropilin-1

NVU

neurovascular unit

RAGE

receptor for advanced glycation end products

TGFβ

transforming growth factor-β

VEGF

vascular endothelial growth factor-A