Perlecan, a modular instructive proteoglycan with diverse functional properties

Diverse and multifunctional roles for perlecan (HSPG2) in repair of the intervertebral disc

Perlecan is a widely distributed, modular, and multifunctional heparan sulfate proteoglycan, which facilitates cellular communication with the extracellular environment to promote tissue development, tissue homeostasis, and optimization of biomechanical tissue functions. Perlecan-mediated osmotic mechanotransduction serves to regulate the metabolic activity of cells in tissues subjected to tension, compression, or shear. Perlecan interacts with a vast array of extracellular matrix (ECM) proteins through which it stabilizes tissues and regulates the proliferation or differentiation of resident cell populations. Here we examine the roles of the HS-proteoglycan perlecan in the normal and destabilized intervertebral disc. The intervertebral disc cell has evolved to survive in a hostile weight bearing, acidic, low oxygen tension, and low nutrition environment, and perlecan provides cytoprotection, shields disc cells from excessive compressive forces, and sequesters a range of growth factors in the disc cell environment where they aid in cellular survival, proliferation, and differentiation. The cells in mechanically destabilized connective tissues attempt to re-establish optimal tissue composition and tissue functional properties by changing the properties of their ECM, in the process of chondroid metaplasia. We explore the possibility that perlecan assists in these cell-mediated tissue remodeling responses by regulating disc cell anabolism. Perlecan's mechano-osmotic transductive property may be of potential therapeutic application.

Biomimetic strategies for the deputization of proteoglycan functions

Proteoglycans (PGs), which have glycosaminoglycan chains attached to their protein cores, are essential for maintaining the morphology and function of healthy body tissues. Extracellular PGs perform various functions, classified into the following four categories: i) the modulation of tissue mechanical properties; ii) the regulation and protection of the extracellular matrix; iii) protein sequestration; and iv) the regulation of cell signaling. The depletion of PGs may significantly impair tissue function, encompassing compromised mechanical characteristics and unregulated inflammatory responses. Since PGs play critical roles in the function of healthy tissues and their synthesis is complex, the development of PG mimetic molecules that recapitulate PG functions for tissue engineering and therapeutic applications has attracted the interest of researchers for more than 20 years. These approaches have ranged from semisynthetic graft copolymers to recombinant PG domains produced by cells that have undergone genetic modifications. This review discusses some essential extracellular PG functions and approaches to mimicking these functions.

CNS/PNS proteoglycans functionalize neuronal and astrocyte niche microenvironments optimizing cellular activity by preserving membrane polarization dynamics, ionic microenvironments, ion fluxes, neuronal activation, and network neurotransductive capacity

Central and peripheral nervous system (CNS/PNS) proteoglycans (PGs) have diverse functional roles, this study examined how these control cellular behavior and tissue function. The CNS/PNS extracellular matrix (ECM) is a dynamic, responsive, highly interactive, space-filling, cell supportive, stabilizing structure maintaining tissue compartments, ionic microenvironments, and microgradients that regulate neuronal activity and maintain the neuron in an optimal ionic microenvironment. The CNS/PNS contains a high glycosaminoglycan content (60% hyaluronan, HA) and a diverse range of stabilizing PGs. Immobilization of HA in brain tissues by HA interactive hyalectan PGs preserves tissue hydration and neuronal activity, a paucity of HA in brain tissues results in a pro-convulsant epileptic phenotype. Diverse CS, KS, and HSPGs stabilize the blood-brain barrier and neurovascular unit, provide smart gel neurotransmitter neuron vesicle storage and delivery, organize the neuromuscular junction basement membrane, and provide motor neuron synaptic plasticity, and photoreceptor and neuron synaptic functions. PG-HA networks maintain ionic fluxes and microgradients and tissue compartments that contribute to membrane polarization dynamics essential to neuronal activation and neurotransduction. Hyalectans form neuroprotective perineuronal nets contributing to synaptic plasticity, memory, and cognitive learning. Sialoglycoprotein associated with cones and rods (SPACRCAN), an HA binding CSPG, stabilizes the inter-photoreceptor ECM. HSPGs pikachurin and eyes shut stabilize the photoreceptor synapse aiding in phototransduction and neurotransduction with retinal bipolar neurons crucial to visual acuity. This is achieved through Laminin G motifs in pikachurin, eyes shut, and neurexins that interact with the dystroglycan-cytoskeleton-ECM-stabilizing synaptic interconnections, neuronal interactive specificity, and co-ordination of regulatory action potentials in neural networks.

Hippo cell signaling and HS-proteoglycans regulate tissue form and function, age-dependent maturation, extracellular matrix remodeling, and repair

This review examined how Hippo cell signaling and heparan sulfate (HS)-proteoglycans (HSPGs) regulate tissue form and function. Despite being a nonweight-bearing tissue, the brain is regulated by Hippo mechanoresponsive cell signaling pathways during embryonic development. HS-proteoglycans interact with growth factors, morphogens, and extracellular matrix components to regulate development and pathology. Pikachurin and Eyes shut (Eys) interact with dystroglycan to stabilize the photoreceptor axoneme primary cilium and ribbon synapse facilitating phototransduction and neurotransduction with bipolar retinal neuronal networks in ocular vision, the primary human sense. Another HSPG, Neurexin interacts with structural and adaptor proteins to stabilize synapses and ensure specificity of neural interactions, and aids in synaptic potentiation and plasticity in neurotransduction. HSPGs also stabilize the blood-brain barrier and motor neuron basal structures in the neuromuscular junction. Agrin and perlecan localize acetylcholinesterase and its receptors in the neuromuscular junction essential for neuromuscular control. The primary cilium is a mechanosensory hub on neurons, utilized by YES associated protein (YAP)-transcriptional coactivator with PDZ-binding motif (TAZ) Hippo, Hh, Wnt, transforming growth factor (TGF)-β/bone matrix protein (BMP) receptor tyrosine kinase cell signaling. Members of the glypican HSPG proteoglycan family interact with Smoothened and Patched G-protein coupled receptors on the cilium to regulate Hh and Wnt signaling during neuronal development. Control of glycosyl sulfotransferases and endogenous protease expression by Hippo TAZ YAP represents a mechanism whereby the fine structure of HS-proteoglycans can be potentially modulated spatiotemporally to regulate tissue morphogenesis in a similar manner to how Hippo signaling controls sialyltransferase expression and mediation of cell-cell recognition, dysfunctional sialic acid expression is a feature of many tumors.

Molecular interactions between perlecan LG3 and the SARS-CoV-2 spike protein receptor binding domain

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has caused a global health crisis with significant clinical morbidity and mortality. While angiotensin-converting enzyme 2 (ACE2) is the primary receptor for viral entry, other cell surface and extracellular matrix proteins may also bind to the viral receptor binding domain (RBD) within the SARS-CoV-2 spike protein. Recent studies have implicated heparan sulfate proteoglycans, specifically perlecan LG3, in facilitating SARS-CoV-2 binding to ACE2. However, the role of perlecan LG3 in SARS-CoV-2 pathophysiology is not well understood. In this study, we investigated the binding interactions between the SARS-CoV-2 spike protein RBD and perlecan LG3 through molecular modeling simulations and surface plasmon resonance (SPR) experiments. Our results indicate stable binding between LG3 and SARS-CoV-2 spike protein RBD, which may potentially enhance RBD-ACE2 interactions. These findings shed light on the role of perlecan LG3 in SARS-CoV-2 infection and provide insight into SARS-CoV-2 pathophysiology and potential therapeutic strategy for COVID-19.

The Glycosaminoglycan Side Chains and Modular Core Proteins of Heparan Sulphate Proteoglycans and the Varied Ways They Provide Tissue Protection by Regulating Physiological Processes and Cellular Behaviour

This review examines the roles of HS-proteoglycans (HS-PGs) in general, and, in particular, perlecan and syndecan as representative examples and their interactive ligands, which regulate physiological processes and cellular behavior in health and disease. HS-PGs are essential for the functional properties of tissues both in development and in the extracellular matrix (ECM) remodeling that occurs in response to trauma or disease. HS-PGs interact with a biodiverse range of chemokines, chemokine receptors, protease inhibitors, and growth factors in immune regulation, inflammation, ECM stabilization, and tissue protection. Some cell regulatory proteoglycan receptors are dually modified hybrid HS/CS proteoglycans (betaglycan, CD47). Neurexins provide synaptic stabilization, plasticity, and specificity of interaction, promoting neurotransduction, neurogenesis, and differentiation. Ternary complexes of glypican-1 and Robbo-Slit neuroregulatory proteins direct axonogenesis and neural network formation. Specific neurexin-neuroligin complexes stabilize synaptic interactions and neural activity. Disruption in these interactions leads to neurological deficits in disorders of functional cognitive decline. Interactions with HS-PGs also promote or inhibit tumor development. Thus, HS-PGs have complex and diverse regulatory roles in the physiological processes that regulate cellular behavior and the functional properties of normal and pathological tissues. Specialized HS-PGs, such as the neurexins, pikachurin, and Eyes-shut, provide synaptic stabilization and specificity of neural transduction and also stabilize the axenome primary cilium of phototoreceptors and ribbon synapse interactions with bipolar neurons of retinal neural networks, which are essential in ocular vision. Pikachurin and Eyes-Shut interactions with an α-dystroglycan stabilize the photoreceptor synapse. Novel regulatory roles for HS-PGs controlling cell behavior and tissue function are expected to continue to be uncovered in this fascinating class of proteoglycan.

Hyaluronan hydrates and compartmentalises the CNS/PNS extracellular matrix and provides niche environments conducive to the optimisation of neuronal activity

The central nervous system/peripheral nervous system (CNS/PNS) extracellular matrix is a dynamic and highly interactive space-filling, cell-supportive, matrix-stabilising, hydrating entity that creates and maintains tissue compartments to facilitate regional ionic micro-environments and micro-gradients that promote optimal neural cellular activity. The CNS/PNS does not contain large supportive collagenous and elastic fibrillar networks but is dominated by a high glycosaminoglycan content, predominantly hyaluronan (HA) and collagen is restricted to the brain microvasculature, blood-brain barrier, neuromuscular junction and meninges dura, arachnoid and pia mater. Chondroitin sulphate-rich proteoglycans (lecticans) interactive with HA have stabilising roles in perineuronal nets and contribute to neural plasticity, memory and cognitive processes. Hyaluronan also interacts with sialoproteoglycan associated with cones and rods (SPACRCAN) to stabilise the interphotoreceptor matrix and has protective properties that ensure photoreceptor viability and function is maintained. HA also regulates myelination/re-myelination in neural networks. HA fragmentation has been observed in white matter injury, multiple sclerosis, and traumatic brain injury. HA fragments (2 × 105  Da) regulate oligodendrocyte precursor cell maturation, myelination/remyelination, and interact with TLR4 to initiate signalling cascades that mediate myelin basic protein transcription. HA and its fragments have regulatory roles over myelination which ensure high axonal neurotransduction rates are maintained in neural networks. Glioma is a particularly invasive brain tumour with extremely high mortality rates. HA, CD44 and RHAMM (receptor for HA-mediated motility) HA receptors are highly expressed in this tumour. Conventional anti-glioma drug treatments have been largely ineffective and surgical removal is normally not an option. CD44 and RHAMM glioma HA receptors can potentially be used to target gliomas with PEP-1, a cell-penetrating HA-binding peptide. PEP-1 can be conjugated to a therapeutic drug; such drug conjugates have successfully treated dense non-operative tumours in other tissues, therefore similar applications warrant exploration as potential anti-glioma treatments.

Glycocalyx-Sodium Interaction in Vascular Endothelium

The glycocalyx generally covers almost all cellular surfaces, where it participates in mediating cell-surface interactions with the extracellular matrix as well as with intracellular signaling molecules. The endothelial glycocalyx that covers the luminal surface mediates the interactions of endothelial cells with materials flowing in the circulating blood, including blood cells. Cardiovascular diseases (CVD) remain a major cause of morbidity and mortality around the world. The cardiovascular risk factors start by causing endothelial cell dysfunction associated with destruction or irregular maintenance of the glycocalyx, which may culminate into a full-blown cardiovascular disease. The endothelial glycocalyx plays a crucial role in shielding the cell from excessive exposure and absorption of excessive salt, which can potentially cause damage to the endothelial cells and underlying tissues of the blood vessels. So, in this mini review/commentary, we delineate and provide a concise summary of the various components of the glycocalyx, their interaction with salt, and subsequent involvement in the cardiovascular disease process. We also highlight the major components of the glycocalyx that could be used as disease biomarkers or as drug targets in the management of cardiovascular diseases.

Proteoglycans orchestrate remodeling of prostatic cytoarchitecture after androgenic blockade in old gerbils

BACKGROUND

The aim of this study was to evaluate modifications in proteoglycan morphology and composition in the prostatic stroma of 18-month-old gerbils after surgical castration, in association or not with an androgenic blockade.

METHODS

The animals (n = 5) were sorted into groups subjected or not to antiandrogen treatment (flutamide 10 mg/kg/day) administered for the total postsurgery period and euthanized at 7- or 30-day postcastration; the control group consisted of intact animals. Tissue analysis included immunohistochemical assessment (perlecan and chondroitin sulfate) and proteoglycan morphology was analyzed by transmission electron microscopy.

RESULTS

Chondroitin sulfate frequency was increased 7 days postcastration with an androgenic blockade. The presence of these carbohydrates was rare after 30 days of androgenic blockade treatment. There was a significant increase in the amount of perlecan in the prostate stroma from groups subjected to castration plus flutamide for 7 or 30 days. Ultrastructural analysis showed that the incidence of areas occupied by proteoglycans and basement membrane was altered by treatment. In addition, androgenic blockade results in changes in the amount, thickness, and morphology of these structures. At 30 days postcastration, with or without flutamide treatment, larger proteoglycans were common.

CONCLUSIONS

In this study, in particular, the decrease in chondroitin sulfate after the longer period might be understood as a prostatic response to androgenic deprivation, while the high frequency and permanence of perlecan led to the assumption that its modulation could be androgen-independent. Length and form alterations in proteoglycans as well as associations among them and with the basement membrane were dynamic events in the prostate microenvironment.

HS, an Ancient Molecular Recognition and Information Storage Glycosaminoglycan, Equips HS-Proteoglycans with Diverse Matrix and Cell-Interactive Properties Operative in Tissue Development and Tissue Function in Health and Disease

Heparan sulfate is a ubiquitous, variably sulfated interactive glycosaminoglycan that consists of repeating disaccharides of glucuronic acid and glucosamine that are subject to a number of modifications (acetylation, de-acetylation, epimerization, sulfation). Variable heparan sulfate chain lengths and sequences within the heparan sulfate chains provide structural diversity generating interactive oligosaccharide binding motifs with a diverse range of extracellular ligands and cellular receptors providing instructional cues over cellular behaviour and tissue homeostasis through the regulation of essential physiological processes in development, health, and disease. heparan sulfate and heparan sulfate-PGs are integral components of the specialized glycocalyx surrounding cells. Heparan sulfate is the most heterogeneous glycosaminoglycan, in terms of its sequence and biosynthetic modifications making it a difficult molecule to fully characterize, multiple ligands also make an elucidation of heparan sulfate functional properties complicated. Spatio-temporal presentation of heparan sulfate sulfate groups is an important functional determinant in tissue development and in cellular control of wound healing and extracellular remodelling in pathological tissues. The regulatory properties of heparan sulfate are mediated via interactions with chemokines, chemokine receptors, growth factors and morphogens in cell proliferation, differentiation, development, tissue remodelling, wound healing, immune regulation, inflammation, and tumour development. A greater understanding of these HS interactive processes will improve therapeutic procedures and prognoses. Advances in glycosaminoglycan synthesis and sequencing, computational analytical carbohydrate algorithms and advanced software for the evaluation of molecular docking of heparan sulfate with its molecular partners are now available. These advanced analytic techniques and artificial intelligence offer predictive capability in the elucidation of heparan sulfate conformational effects on heparan sulfate-ligand interactions significantly aiding heparan sulfate therapeutics development.

Separation and Identification of Native Proteoglycans by Composite Agarose-Polyacrylamide Gel Electrophoresis and Immunoblotting

Composite agarose-polyacrylamide gel electrophoresis (CAPAGE) in gels of 1.2% w/v polyacrylamide and 0.6% w/v agarose can be used to examine the heterogeneity of full-length native proteoglycan populations and their fragments in crude tissue extracts, and when used in conjunction with immunoblotting and specific antibodies to proteoglycan core protein and glycosaminoglycan, side chain epitopes can provide significant information on the level of proteoglycan polydispersity/heterogeneity and a number of proteoglycan populations present in tissue samples. This can be a technically difficult technique, but it reveals significant information on proteoglycans from small tissue samples not possible by any other separation methodology. Native full-length and proteoglycan fragments are examined in this technique something which cannot be done in the popular SDS-PAGE format unless the glycosaminoglycan side chains are first removed. Furthermore, since proteoglycans do not require renaturation from SDS-protein complexes, the proteoglycan populations separated by native electrophoresis are highly reactive with antibodies in immunoblotting procedures. Despite the massive sizes of proteoglycans, transfer conditions have been determined which provide close to quantitative transfer to nitrocellulose membranes without exceeding the binding capacity of such membranes, avoiding bleed-through of the transferred proteoglycans. Development of biotinylated hyaluronan and its application in an affinity blotting procedure has also yielded significant information on aggregatable proteoglycan populations separated by CAPAGE from a number of cartilages and vascular tissues in health and disease. While the CAPAGE system can be a technically demanding technique to master particularly in gel preparation, all other steps are straightforward, and the method yields invaluable information on proteoglycan populations extracted from connective tissues in health and disease that cannot be ascertained by any other technique. Further improvements in the detection of proteoglycan features with the development of novel bio-affinity probes or new antibody preparations are expected to further improve the utility of CAPAGE separation methodology.

Perlecan: Roles in osteoarthritis and potential treating target

Osteoarthritis (OA) is the most common joint disease, affecting hundreds of millions of people globally, which leads to a high cost of treatment and further medical care and an apparent decrease in patient prognosis. The recent view of OA pathogenesis is that increased vascularity, bone remodeling, and disordered turnover are influenced by multivariate risk factors, such as age, obesity, and overloading. The view also reveals the gap between the development of these processes and early stage risk factors. This review presents the latest research on OA-related signaling pathways and analyzes the potential roles of perlecan, a typical component of the well-known protective structure against osteoarthritic pericellular matrix (PCM). Based on the experimental results observed in end-stage OA models, we summarized and analyzed the role of perlecan in the development of OA. In normal cartilage, it plays a protective role by maintaining the integrin of PCM and sequesters growth factors. Second, perlecan in cartilage is required to not only activate vascular epithelium growth factor receptor (VEGFR) signaling of endothelial cells for vascular invasion and catabolic autophagy, but also for different signaling pathways for the catabolic and anabolic actions of chondrocytes. Finally, perlecan may participate in pain sensitization pathways.

Proteolysis: a key post-translational modification regulating proteoglycans

Proteoglycans are composite molecules comprising a protein backbone, i.e., the core protein, with covalently attached glycosaminoglycan chains of distinct chemical types. Most proteoglycans are secreted or attached to the cell membrane. Their specialized structures, binding properties, and biophysical attributes underlie diverse biological roles, which include modulation of tissue mechanics, cell adhesion, and the sequestration and regulated release of morphogens, growth factors, and cytokines. As an irreversible post-translational modification, proteolysis has a profound impact on proteoglycan function, abundance, and localization. Proteolysis is required for molecular maturation of some proteoglycans, clearance of extracellular matrix proteoglycans during tissue remodeling, generation of bioactive fragments from proteoglycans, and ectodomain shedding of cell-surface proteoglycans. Genetic evidence shows that proteoglycan core protein proteolysis is essential for diverse morphogenetic events during embryonic development. In contrast, dysregulated proteoglycan proteolysis contributes to osteoarthritis, cardiovascular disorders, cancer, and inflammation. Proteolytic fragments of perlecan, versican, aggrecan, brevican, collagen XVIII, and other proteoglycans are associated with independent biological activities as so-called matrikines. Yet, proteoglycan proteolysis has been investigated to only a limited extent to date. Here, we review the actions of proteases on proteoglycans and illustrate their functional impact with several examples. We discuss the applications and limitations of strategies used to define cleavage sites in proteoglycans and explain how proteoglycanome-wide proteolytic mapping, which is desirable to fully understand the impact of proteolysis on proteoglycans, can be facilitated by integrating classical proteoglycan isolation methods with mass spectrometry-based proteomics.

Importance of Matrix Cues on Intervertebral Disc Development, Degeneration, and Regeneration

Back pain is one of the leading causes of disability worldwide and is frequently caused by degeneration of the intervertebral discs. The discs’ development, homeostasis, and degeneration are driven by a complex series of biochemical and physical extracellular matrix cues produced by and transmitted to native cells. Thus, understanding the roles of different cues is essential for designing effective cellular and regenerative therapies. Omics technologies have helped identify many new matrix cues; however, comparatively few matrix molecules have thus far been incorporated into tissue engineered models. These include collagen type I and type II, laminins, glycosaminoglycans, and their biomimetic analogues. Modern biofabrication techniques, such as 3D bioprinting, are also enabling the spatial patterning of matrix molecules and growth factors to direct regional effects. These techniques should now be applied to biochemically, physically, and structurally relevant disc models incorporating disc and stem cells to investigate the drivers of healthy cell phenotype and differentiation. Such research will inform the development of efficacious regenerative therapies and improved clinical outcomes.

Fractone Stem Cell Niche Components Provide Intuitive Clues in the Design of New Therapeutic Procedures/Biomatrices for Neural Repair

The aim of this study was to illustrate recent developments in neural repair utilizing hyaluronan as a carrier of olfactory bulb stem cells and in new bioscaffolds to promote neural repair. Hyaluronan interacts with brain hyalectan proteoglycans in protective structures around neurons in perineuronal nets, which also have roles in the synaptic plasticity and development of neuronal cognitive properties. Specialist stem cell niches termed fractones located in the sub-ventricular and sub-granular regions of the dentate gyrus of the hippocampus migrate to the olfactory bulb, which acts as a reserve of neuroprogenitor cells in the adult brain. The extracellular matrix associated with the fractone stem cell niche contains hyaluronan, perlecan and laminin α5, which regulate the quiescent recycling of stem cells and also provide a means of escaping to undergo the proliferation and differentiation to a pluripotent migratory progenitor cell type that can participate in repair processes in neural tissues. Significant improvement in the repair of spinal cord injury and brain trauma has been reported using this approach. FGF-2 sequestered by perlecan in the neuroprogenitor niche environment aids in these processes. Therapeutic procedures have been developed using olfactory ensheathing stem cells and hyaluronan as a carrier to promote neural repair processes. Now that recombinant perlecan domain I and domain V are available, strategies may also be expected in the near future using these to further promote neural repair strategies.

Impact of the Heparan Sulfate Proteoglycan Perlecan on Human Disease and Health

Perlecan, a basement membrane-type heparan sulfate proteoglycan, is an important molecule in the functional diversity of organisms because of the diversity of its glycan chains and the multifunctionality of its core proteins. Human diseases associated with perlecan have been identified using gene-deficient mice. Two human diseases related to perlecan have been reported. One is Silverman-Handmaker type Dyssegmental Dysplasia, resulting from complete loss of function of the HSPG2 gene which encods perlecan core protein which maps to chromosome 1p36. The other is Schwartz-Jampel syndrome from partial loss of function of the HSPG2 gene. Subsequent in vivo and in vitrostudies have revealed the organ-specific functions of perlecan, suggesting its involvement in the pathogenesis of various human diseases. In this review, we discuss the role of perlecan in human diseases and summarize our knowledge about perlecan as a future therapeutic target to treat the related diseases and for healthy longevity.

Perlecan, A Multi-Functional, Cell-Instructive, Matrix-Stabilizing Proteoglycan With Roles in Tissue Development Has Relevance to Connective Tissue Repair and Regeneration

This review highlights the multifunctional properties of perlecan (HSPG2) and its potential roles in repair biology. Perlecan is ubiquitous, occurring in vascular, cartilaginous, adipose, lymphoreticular, bone and bone marrow stroma and in neural tissues. Perlecan has roles in angiogenesis, tissue development and extracellular matrix stabilization in mature weight bearing and tensional tissues. Perlecan contributes to mechanosensory properties in cartilage through pericellular interactions with fibrillin-1, type IV, V, VI and XI collagen and elastin. Perlecan domain I - FGF, PDGF, VEGF and BMP interactions promote embryonic cellular proliferation, differentiation, and tissue development. Perlecan domain II, an LDLR-like domain interacts with lipids, Wnt and Hedgehog morphogens. Perlecan domain III binds FGF-7 and 18 and has roles in the secretion of perlecan. Perlecan domain IV, an immunoglobulin repeat domain, has cell attachment and matrix stabilizing properties. Perlecan domain V promotes tissue repair through interactions with VEGF, VEGF-R2 and α2β1 integrin. Perlecan domain-V LG1-LG2 and LG3 fragments antagonize these interactions. Perlecan domain V promotes reconstitution of the blood brain barrier damaged by ischemic stroke and is neurogenic and neuroprotective. Perlecan-VEGF-VEGFR2, perlecan-FGF-2 and perlecan-PDGF interactions promote angiogenesis and wound healing. Perlecan domain I, III and V interactions with platelet factor-4 and megakaryocyte and platelet inhibitory receptor promote adhesion of cells to implants and scaffolds in vascular repair. Perlecan localizes acetylcholinesterase in the neuromuscular junction and is of functional significance in neuromuscular control. Perlecan mutation leads to Schwartz-Jampel Syndrome, functional impairment of the biomechanical properties of the intervertebral disc, variable levels of chondroplasia and myotonia. A greater understanding of the functional working of the neuromuscular junction may be insightful in therapeutic approaches in the treatment of neuromuscular disorders. Tissue engineering of salivary glands has been undertaken using bioactive peptides (TWSKV) derived from perlecan domain IV. Perlecan TWSKV peptide induces differentiation of salivary gland cells into self-assembling acini-like structures that express salivary gland biomarkers and secrete α-amylase. Perlecan also promotes chondroprogenitor stem cell maturation and development of pluripotent migratory stem cell lineages, which participate in diarthrodial joint formation, and early cartilage development. Recent studies have also shown that perlecan is prominently expressed during repair of adult human articular cartilage. Perlecan also has roles in endochondral ossification and bone development. Perlecan domain I hydrogels been used in tissue engineering to establish heparin binding growth factor gradients that promote cell migration and cartilage repair. Perlecan domain I collagen I fibril scaffolds have also been used as an FGF-2 delivery system for tissue repair. With the availability of recombinant perlecan domains, the development of other tissue repair strategies should emerge in the near future. Perlecan co-localization with vascular elastin in the intima, acts as a blood shear-flow endothelial sensor that regulates blood volume and pressure and has a similar role to perlecan in canalicular fluid, regulating bone development and remodeling. This complements perlecan's roles in growth plate cartilage and in endochondral ossification to form the appendicular and axial skeleton. Perlecan is thus a ubiquitous, multifunctional, and pleomorphic molecule of considerable biological importance. A greater understanding of its diverse biological roles and functional repertoires during tissue development, growth and disease will yield valuable insights into how this impressive proteoglycan could be utilized successfully in repair biology.

Regulation of FGF-2, FGF-18 and Transcription Factor Activity by Perlecan in the Maturational Development of Transitional Rudiment and Growth Plate Cartilages and in the Maintenance of Permanent Cartilage Homeostasis

The aim of this study was to highlight the roles of perlecan in the regulation of the development of the rudiment developmental cartilages and growth plate cartilages, and also to show how perlecan maintains permanent articular cartilage homeostasis. Cartilage rudiments are transient developmental templates containing chondroprogenitor cells that undergo proliferation, matrix deposition, and hypertrophic differentiation. Growth plate cartilage also undergoes similar changes leading to endochondral bone formation, whereas permanent cartilage is maintained as an articular structure and does not undergo maturational changes. Pericellular and extracellular perlecan-HS chains interact with growth factors, morphogens, structural matrix glycoproteins, proteases, and inhibitors to promote matrix stabilization and cellular proliferation, ECM remodelling, and tissue expansion. Perlecan has mechanotransductive roles in cartilage that modulate chondrocyte responses in weight-bearing environments. Nuclear perlecan may modulate chromatin structure and transcription factor access to DNA and gene regulation. Snail-1, a mesenchymal marker and transcription factor, signals through FGFR-3 to promote chondrogenesis and maintain Acan and type II collagen levels in articular cartilage, but prevents further tissue expansion. Pre-hypertrophic growth plate chondrocytes also express high Snail-1 levels, leading to cessation of Acan and CoI2A1 synthesis and appearance of type X collagen. Perlecan differentially regulates FGF-2 and FGF-18 to maintain articular cartilage homeostasis, rudiment and growth plate cartilage growth, and maturational changes including mineralization, contributing to skeletal growth.

Pentosan Polysulphate (PPS), a Semi-Synthetic Heparinoid DMOAD With Roles in Intervertebral Disc Repair Biology emulating The Stem Cell Instructive and Tissue Reparative Properties of Heparan Sulphate

This review highlights the attributes of pentosan polysulphate (PPS) in the promotion of intervertebral disc (IVD) repair processes. PPS has been classified as a disease modifying osteoarthritic drug (DMOAD) and many studies have demonstrated its positive attributes in the countering of degenerative changes occurring in cartilaginous tissues during the development of osteoarthritis (OA). Degenerative changes in the IVD also involve inflammatory cytokines, degradative proteases and cell signalling pathways similar to those operative in the development of OA in articular cartilage. PPS acts as a heparan sulphate (HS) mimetic to effect its beneficial effects in cartilage. The IVD contains small cell membrane HS-proteoglycans (HSPGs) such as syndecan, and glypican and a large multifunctional HS/chondroitin sulphate (CS) hybrid proteoglycan (HSPG2/perlecan) that have important matrix stabilising properties and sequester, control and present growth factors from the FGF, VEGF, PDGF and BMP families to cellular receptors to promote cell proliferation, differentiation and matrix synthesis. HSPG2 also has chondrogenic properties and stimulates the synthesis of extracellular matrix (ECM) components, expansion of cartilaginous rudiments and has roles in matrix stabilisation and repair. Perlecan is a perinuclear and nuclear proteoglycan in IVD cells with roles in chromatin organisation and control of transcription factor activity, immunolocalises to stem cell niches in cartilage, promotes escape of stem cells from quiescent recycling, differentiation and attainment of pluripotency and migratory properties. These participate in tissue development and morphogenesis, ECM remodelling and repair. PPS also localises in the nucleus of stromal stem cells, promotes development of chondroprogenitor cell lineages, ECM synthesis and repair and discal repair by resident disc cells. The availability of recombinant perlecan and PPS offer new opportunities in repair biology. These multifunctional agents offer welcome new developments in repair strategies for the IVD.

The Glomerular Endothelium Restricts Albumin Filtration

Inflammatory activation and/or dysfunction of the glomerular endothelium triggers proteinuria in many systemic and localized vascular disorders. Among them are the thrombotic microangiopathies, many forms of glomerulonephritis, and acute inflammatory episodes like sepsis and COVID-19 illness. Another example is the chronic endothelial dysfunction that develops in cardiovascular disease and in metabolic disorders like diabetes. While the glomerular endothelium is a porous sieve that filters prodigious amounts of water and small solutes, it also bars the bulk of albumin and large plasma proteins from passing into the glomerular filtrate. This endothelial barrier function is ascribed predominantly to the endothelial glycocalyx with its endothelial surface layer, that together form a relatively thick, mucinous coat composed of glycosaminoglycans, proteoglycans, glycolipids, sialomucins and other glycoproteins, as well as secreted and circulating proteins. The glycocalyx/endothelial surface layer not only covers the glomerular endothelium; it extends into the endothelial fenestrae. Some glycocalyx components span or are attached to the apical endothelial cell plasma membrane and form the formal glycocalyx. Other components, including small proteoglycans and circulating proteins like albumin and orosomucoid, form the endothelial surface layer and are bound to the glycocalyx due to weak intermolecular interactions. Indeed, bound plasma albumin is a major constituent of the endothelial surface layer and contributes to its barrier function. A role for glomerular endothelial cells in the barrier of the glomerular capillary wall to protein filtration has been demonstrated by many elegant studies. However, it can only be fully understood in the context of other components, including the glomerular basement membrane, the podocytes and reabsorption of proteins by tubule epithelial cells. Discovery of the precise mechanisms that lead to glycocalyx/endothelial surface layer disruption within glomerular capillaries will hopefully lead to pharmacological interventions that specifically target this important structure.

Impact of perlecan, a core component of basement membrane, on regeneration of cartilaginous tissues

As an indispensable component of the extracellular matrix, perlecan (Pln) plays an essential role in cartilaginous tissue function. Although there exist studies suggesting that Pln expressed by cartilaginous tissues is critical for chondrogenesis, few papers have discussed the potential impact Pln may have on cartilage regeneration. In this review, we delineate Pln structure, biomechanical properties, and interactive ligands-which together contribute to the effect Pln has on cartilaginous tissue development. We also review how the signaling pathways of Pln affect cartilage development and scrutinize the potential application of Pln to divisions of cartilage regeneration, spanning vascularization, stem cell differentiation, and biomaterial improvement. The aim of this review is to deepen our understanding of the spatial and temporal interactions that occur between Pln and cartilaginous tissue and ultimately apply Pln in scaffold design to improve cell-based cartilage engineering and regeneration. STATEMENT OF SIGNIFICANCE: As a key component of the basement membrane, Pln plays a critical role in tissue development and repair. Recent findings suggest that Pln existing in the pericellular matrix surrounding mature chondrocytes is actively involved in cartilage regeneration and functionality. We propose that Pln is essential to developing an in vitro matrix niche within biological scaffolds for cartilage tissue engineering.

Congenital Disorders of Deficiency in Glycosaminoglycan Biosynthesis

Glycosaminoglycans (GAGs) including chondroitin sulfate, dermatan sulfate, and heparan sulfate are covalently attached to specific core proteins to form proteoglycans, which are distributed at the cell surface as well as in the extracellular matrix. Proteoglycans and GAGs have been demonstrated to exhibit a variety of physiological functions such as construction of the extracellular matrix, tissue development, and cell signaling through interactions with extracellular matrix components, morphogens, cytokines, and growth factors. Not only connective tissue disorders including skeletal dysplasia, chondrodysplasia, multiple exostoses, and Ehlers-Danlos syndrome, but also heart and kidney defects, immune deficiencies, and neurological abnormalities have been shown to be caused by defects in GAGs as well as core proteins of proteoglycans. These findings indicate that GAGs and proteoglycans are essential for human development in major organs. The glycobiological aspects of congenital disorders caused by defects in GAG-biosynthetic enzymes including specific glysocyltransferases, epimerases, and sulfotransferases, in addition to core proteins of proteoglycans will be comprehensively discussed based on the literature to date.

The CNS/PNS Extracellular Matrix Provides Instructive Guidance Cues to Neural Cells and Neuroregulatory Proteins in Neural Development and Repair

BACKGROUND

The extracellular matrix of the PNS/CNS is unusual in that it is dominated by glycosaminoglycans, especially hyaluronan, whose space filling and hydrating properties make essential contributions to the functional properties of this tissue. Hyaluronan has a relatively simple structure but its space-filling properties ensure micro-compartments are maintained in the brain ultrastructure, ensuring ionic niches and gradients are maintained for optimal cellular function. Hyaluronan has cell-instructive, anti-inflammatory properties and forms macro-molecular aggregates with the lectican CS-proteoglycans, forming dense protective perineuronal net structures that provide neural and synaptic plasticity and support cognitive learning.

AIMS

To highlight the central nervous system/peripheral nervous system (CNS/PNS) and its diverse extracellular and cell-associated proteoglycans that have cell-instructive properties regulating neural repair processes and functional recovery through interactions with cell adhesive molecules, receptors and neuroregulatory proteins. Despite a general lack of stabilising fibrillar collagenous and elastic structures in the CNS/PNS, a sophisticated dynamic extracellular matrix is nevertheless important in tissue form and function.

CONCLUSIONS

This review provides examples of the sophistication of the CNS/PNS extracellular matrix, showing how it maintains homeostasis and regulates neural repair and regeneration.

What Are the Potential Roles of Nuclear Perlecan and Other Heparan Sulphate Proteoglycans in the Normal and Malignant Phenotype

The recent discovery of nuclear and perinuclear perlecan in annulus fibrosus and nucleus pulposus cells and its known matrix stabilizing properties in tissues introduces the possibility that perlecan may also have intracellular stabilizing or regulatory roles through interactions with nuclear envelope or cytoskeletal proteins or roles in nucleosomal-chromatin organization that may regulate transcriptional factors and modulate gene expression. The nucleus is a mechano-sensor organelle, and sophisticated dynamic mechanoresponsive cytoskeletal and nuclear envelope components support and protect the nucleus, allowing it to perceive and respond to mechano-stimulation. This review speculates on the potential roles of perlecan in the nucleus based on what is already known about nuclear heparan sulphate proteoglycans. Perlecan is frequently found in the nuclei of tumour cells; however, its specific role in these diseased tissues is largely unknown. The aim of this review is to highlight probable roles for this intriguing interactive regulatory proteoglycan in the nucleus of normal and malignant cell types.

A guide to the composition and functions of the extracellular matrix

Extracellular matrix (ECM) is a dynamic 3-dimensional network of macromolecules that provides structural support for the cells and tissues. Accumulated knowledge clearly demonstrated over the last decade that ECM plays key regulatory roles since it orchestrates cell signaling, functions, properties and morphology. Extracellularly secreted as well as cell-bound factors are among the major members of the ECM family. Proteins/glycoproteins, such as collagens, elastin, laminins and tenascins, proteoglycans and glycosaminoglycans, hyaluronan, and their cell receptors such as CD44 and integrins, responsible for cell adhesion, comprise a well-organized functional network with significant roles in health and disease. On the other hand, enzymes such as matrix metalloproteinases and specific glycosidases including heparanase and hyaluronidases contribute to matrix remodeling and affect human health. Several cell processes and functions, among them cell proliferation and survival, migration, differentiation, autophagy, angiogenesis, and immunity regulation are affected by certain matrix components. Structural alterations have been also well associated with disease progression. This guide on the composition and functions of the ECM gives a broad overview of the matrisome, the major ECM macromolecules, and their interaction networks within the ECM and with the cell surface, summarizes their main structural features and their roles in tissue organization and cell functions, and emphasizes the importance of specific ECM constituents in disease development and progression as well as the advances in molecular targeting of ECM to design new therapeutic strategies.