The structure, location, and function of perlecan, a prominent pericellular proteoglycan of fetal, postnatal, and mature hyaline cartilages

Short-term response of primary human meniscus cells to simulated microgravity

BACKGROUND

Mechanical unloading of the knee articular cartilage results in cartilage matrix atrophy, signifying the osteoarthritic-inductive potential of mechanical unloading. In contrast, mechanical loading stimulates cartilage matrix production. However, little is known about the response of meniscal fibrocartilage, a major mechanical load-bearing tissue of the knee joint, and its functional matrix-forming fibrochondrocytes to mechanical unloading events.

METHODS

In this study, primary meniscus fibrochondrocytes isolated from the inner avascular region of human menisci from both male and female donors were seeded into porous collagen scaffolds to generate 3D meniscus models. These models were subjected to both normal gravity and mechanical unloading via simulated microgravity (SMG) for 7 days, with samples collected at various time points during the culture.

RESULTS

RNA sequencing unveiled significant transcriptome changes during the 7-day SMG culture, including the notable upregulation of key osteoarthritis markers such as COL10A1, MMP13, and SPP1, along with pathways related to inflammation and calcification. Crucially, sex-specific variations in transcriptional responses were observed. Meniscus models derived from female donors exhibited heightened cell proliferation activities, with the JUN protein involved in several potentially osteoarthritis-related signaling pathways. In contrast, meniscus models from male donors primarily regulated extracellular matrix components and matrix remodeling enzymes.

CONCLUSION

These findings advance our understanding of sex disparities in knee osteoarthritis by developing a novel in vitro model using cell-seeded meniscus constructs and simulated microgravity, revealing significant sex-specific molecular mechanisms and therapeutic targets.

Application of an in silico approach identifies a genetic locus within ITGB2, and its interactions with HSPG2 and FGF9, to be associated with anterior cruciate ligament rupture risk

We developed a Biomedical Knowledge Graph model that is phenotype and biological function-aware through integrating knowledge from multiple domains in a Neo4j, graph database. All known human genes were assessed through the model to identify potential new risk genes for anterior cruciate ligament (ACL) ruptures and Achilles tendinopathy (AT). Genes were prioritised and explored in a case-control study comparing participants with ACL ruptures (ACL-R), including a sub-group with non-contact mechanism injuries (ACL-NON), to uninjured control individuals (CON). After gene filtering, 3376 genes, including 411 genes identified through previous whole exome sequencing, were found to be potentially linked to AT and ACL ruptures. Four variants were prioritised: HSPG2:rs2291826A/G, HSPG2:rs2291827G/A, ITGB2:rs2230528C/T and FGF9:rs2274296C/T. The rs2230528 CC genotype was over-represented in the CON group compared to ACL-R (p < 0.001) and ACL-NON (p < 0.001) and the TT genotype and T allele were over-represented in the ACL-R group and ACL-NON compared to CON (p < 0.001) group. Several significant differences in distributions were noted for the gene-gene interactions: (HSPG2:rs2291826, rs2291827 and ITGB2:rs2230528) and (ITGB2:rs2230528 and FGF9:rs2297429). This study substantiates the efficiency of using a prior knowledge-driven in silico approach to identify candidate genes linked to tendon and ACL injuries. Our biomedical knowledge graph identified and, with further testing, highlighted novel associations of the ITGB2 gene which has not been explored in a genetic case control association study, with ACL rupture risk. We thus recommend a multistep approach including bioinformatics in conjunction with next generation sequencing technology to improve the discovery potential of genomics technologies in musculoskeletal soft tissue injuries.HighlightsA biomedical knowledge graph was modelled for musculoskeletal soft tissue injuries to efficiently identify candidate genes for genetic susceptibility analyses.The biomedical knowledge graph and sequencing data identified potential biologically relevant variants to explore susceptibility to common tendon and ligament injuries. Specifically genetic variants within the ITGB2 and FGF9 genes were associated with ACL risk.Novel allele combinations (HSPG2-ITGB2 and ITGB2-FGF9) showcase the potential effect of ITGB2 in influencing risk of ACL rupture.

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.

Transcriptomic response of bioengineered human cartilage to parabolic flight microgravity is sex-dependent

Spaceflight and simulated spaceflight microgravity induced osteoarthritic-like alterations at the transcriptomic and proteomic levels in the articular and meniscal cartilages of rodents. But little is known about the effect of spaceflight or simulated spaceflight microgravity on the transcriptome of tissue-engineered cartilage developed from human cells. In this study, we investigate the effect of simulated spaceflight microgravity facilitated by parabolic flights on tissue-engineered cartilage developed from in vitro chondrogenesis of human bone marrow mesenchymal stem cells obtained from age-matched female and male donors. The successful induction of cartilage-like tissue was confirmed by the expression of well-demonstrated chondrogenic markers. Our bulk transcriptome data via RNA sequencing demonstrated that parabolic flight altered mostly fundamental biological processes, and the modulation of the transcriptome profile showed sex-dependent differences. The secretome profile analysis revealed that two genes (WNT7B and WNT9A) from the Wnt-signaling pathway, which is implicated in osteoarthritis development, were only up-regulated for female donors. The results of this study showed that the engineered cartilage tissues responded to microgravity in a sex-dependent manner, and the reported data offers a strong foundation to further explore the underlying mechanisms.

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.

The Extracellular Matrix of Articular Cartilage Controls the Bioavailability of Pericellular Matrix-Bound Growth Factors to Drive Tissue Homeostasis and Repair

The extracellular matrix (ECM) has long been regarded as a packing material; supporting cells within the tissue and providing tensile strength and protection from mechanical stress. There is little surprise when one considers the dynamic nature of many of the individual proteins that contribute to the ECM, that we are beginning to appreciate a more nuanced role for the ECM in tissue homeostasis and disease. Articular cartilage is adapted to be able to perceive and respond to mechanical load. Indeed, physiological loads are essential to maintain cartilage thickness in a healthy joint and excessive mechanical stress is associated with the breakdown of the matrix that is seen in osteoarthritis (OA). Although the trigger by which increased mechanical stress drives catabolic pathways remains unknown, one mechanism by which cartilage responds to increased compressive load is by the release of growth factors that are sequestered in the pericellular matrix. These are heparan sulfate-bound growth factors that appear to be largely chondroprotective and displaced by an aggrecan-dependent sodium flux. Emerging evidence suggests that the released growth factors act in a coordinated fashion to drive cartilage repair. Thus, we are beginning to appreciate that the ECM is the key mechano-sensor and mechano-effector in cartilage, responsible for directing subsequent cellular events of relevance to joint health and disease.

The role of the cell surface glycocalyx in drug delivery to and through the endothelium

Cell membranes are key interfaces where materials engineering meets biology. Traditionally regarded as just the location of receptors regulating the uptake of molecules, we now know that all mammalian cell membranes are 'sugar coated'. These sugars, or glycans, form a matrix bound at the cell membrane via proteins and lipids, referred to as the glycocalyx, which modulate access to cell membrane receptors crucial for interactions with drug delivery systems (DDS). Focusing on the key blood-tissue barrier faced by most DDS to enable transport from the place of administration to target sites via the circulation, we critically assess the design of carriers for interactions at the endothelial cell surface. We also discuss the current challenges for this area and provide opportunities for future research efforts to more fully engineer DDS for controlled, efficient, and targeted interactions with the endothelium for therapeutic application.

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.

Engineered Human Meniscus in Modeling Sex Differences of Knee Osteoarthritis in Vitro

Background: Osteoarthritis (OA) primarily affects mechanical load-bearing joints. The knee joint is the most impacted by OA. Knee OA (KOA) occurs in almost all demographic groups, but the prevalence and severity are disproportionately higher in females. The molecular mechanism underlying the pathogenesis and progression of KOA is unknown. The molecular basis of biological sex matters of KOA is not fully understood. Mechanical stimulation plays a vital role in modulating OA-related responses of load-bearing tissues. Mechanical unloading by simulated microgravity (SMG) induced OA-like gene expression in engineered cartilage, while mechanical loading by cyclic hydrostatic pressure (CHP), on the other hand, exerted a pro-chondrogenic effect. This study aimed to evaluate the effects of mechanical loading and unloading via CHP and SMG, respectively, on the OA-related profile changes of engineered meniscus tissues and explore biological sex-related differences.

Methods: Tissue-engineered menisci were made from female and male meniscus fibrochondrocytes (MFCs) under static conditions of normal gravity in chondrogenic media and subjected to SMG and CHP culture. Constructs were assayed via histology, immunofluorescence, GAG/DNA assays, RNA sequencing, and testing of mechanical properties.

Results: The mRNA expression of ACAN and COL2A1, was upregulated by CHP but downregulated by SMG. COL10A1, a marker for chondrocyte hypertrophy, was downregulated by CHP compared to SMG. Furthermore, CHP increased GAG/DNA levels and wet weight in both female and male donors, but only significantly in females. From the transcriptomics, CHP and SMG significantly modulated genes related to the ossification, regulation of ossification, extracellular matrix, and angiogenesis Gene Ontology (GO) terms. A clear difference in fold-change magnitude and direction was seen between the two treatments for many of the genes. Furthermore, differences in fold-change magnitudes were seen between male and female donors within each treatment. SMG and CHP also significantly modulated genes in OA-related KEGG pathways, such as mineral absorption, Wnt signalling pathway, and HIF-1 signalling pathway.

Conclusion: Engineered menisci responded to CHP and SMG in a sex-dependent manner. SMG may induce an OA-like profile, while CHP promotes chondrogenesis. The combination of SMG and CHP could serve as a model to study the early molecular events of KOA and potential drug-targetable pathways.

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.

Molecular Engineering of Pericellular Microniche via Biomimetic Proteoglycans Modulates Cell Mechanobiology

Molecular engineering of biological tissues using synthetic mimics of native matrix molecules can modulate the mechanical properties of the cellular microenvironment through physical interactions with existing matrix molecules, and in turn, mediate the corresponding cell mechanobiology. In articular cartilage, the pericellular matrix (PCM) is the immediate microniche that regulates cell fate, signaling, and metabolism. The negatively charged osmo-environment, as endowed by PCM proteoglycans, is a key biophysical cue for cell mechanosensing. This study demonstrated that biomimetic proteoglycans (BPGs), which mimic the ultrastructure and polyanionic nature of native proteoglycans, can be used to molecularly engineer PCM micromechanics and cell mechanotransduction in cartilage. Upon infiltration into bovine cartilage explant, we showed that localization of BPGs in the PCM leads to increased PCM micromodulus and enhanced chondrocyte intracellular calcium signaling. Applying molecular force spectroscopy, we revealed that BPGs integrate with native PCM through augmenting the molecular adhesion of aggrecan, the major PCM proteoglycan, at the nanoscale. These interactions are enabled by the biomimetic "bottle-brush" ultrastructure of BPGs and facilitate the integration of BPGs within the PCM. Thus, this class of biomimetic molecules can be used for modulating molecular interactions of pericellular proteoglycans and harnessing cell mechanosensing. Because the PCM is a prevalent feature of various cell types, BPGs hold promising potential for improving regeneration and disease modification for not only cartilage-related healthcare but many other tissues and diseases.

Mechanical Cues: Bidirectional Reciprocity in the Extracellular Matrix Drives Mechano-Signalling in Articular Cartilage

The composition and organisation of the extracellular matrix (ECM), particularly the pericellular matrix (PCM), in articular cartilage is critical to its biomechanical functionality; the presence of proteoglycans such as aggrecan, entrapped within a type II collagen fibrillar network, confers mechanical resilience underweight-bearing. Furthermore, components of the PCM including type VI collagen, perlecan, small leucine-rich proteoglycans—decorin and biglycan—and fibronectin facilitate the transduction of both biomechanical and biochemical signals to the residing chondrocytes, thereby regulating the process of mechanotransduction in cartilage. In this review, we summarise the literature reporting on the bidirectional reciprocity of the ECM in chondrocyte mechano-signalling and articular cartilage homeostasis. Specifically, we discuss studies that have characterised the response of articular cartilage to mechanical perturbations in the local tissue environment and how the magnitude or type of loading applied elicits cellular behaviours to effect change. In vivo, including transgenic approaches, and in vitro studies have illustrated how physiological loading maintains a homeostatic balance of anabolic and catabolic activities, involving the direct engagement of many PCM molecules in orchestrating this slow but consistent turnover of the cartilage matrix. Furthermore, we document studies characterising how abnormal, non-physiological loading including excessive loading or joint trauma negatively impacts matrix molecule biosynthesis and/or organisation, affecting PCM mechanical properties and reducing the tissue’s ability to withstand load. We present compelling evidence showing that reciprocal engagement of the cells with this altered ECM environment can thus impact tissue homeostasis and, if sustained, can result in cartilage degradation and onset of osteoarthritis pathology. Enhanced dysregulation of PCM/ECM turnover is partially driven by mechanically mediated proteolytic degradation of cartilage ECM components. This generates bioactive breakdown fragments such as fibronectin, biglycan and lumican fragments, which can subsequently activate or inhibit additional signalling pathways including those involved in inflammation. Finally, we discuss how bidirectionality within the ECM is critically important in enabling the chondrocytes to synthesise and release PCM/ECM molecules, growth factors, pro-inflammatory cytokines and proteolytic enzymes, under a specified load, to influence PCM/ECM composition and mechanical properties in cartilage health and disease.

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.

Enzymatic Degradation of Glycosaminoglycans and Proteoglycan-Mimetic Materials in Solution and on Polyelectrolyte Multilayer Surfaces

Proteoglycans (PGs) play many important roles in biology, contributing to the mechanical properties of tissues, helping to organize extracellular matrix components, and participating in signaling mechanisms related to mechanotransduction, cell differentiation, immune responses, and wound healing. Our lab has designed two different types of PG mimics: polyelectrolyte complex nanoparticles (PCNs) and PG-mimetic graft copolymers (GCs), both of which are prepared using naturally occurring glycosaminoglycans. This work evaluates the enzymatic stability of these PG mimics using hyaluronidases (I-S, IV-S, and II), chondroitinase ABC, and lysozyme, for PG mimics suspended in solution and adsorbed onto surfaces. Hyaluronan (HA)- and chondroitin sulfate (CS)-containing PG mimics are degraded by the hyaluronidases. PCNs prepared with CS and GCs prepared with heparin are the only CS- and HA-containing PG mimics protected from chondroitinase ABC. None of the materials are measurably degraded by lysozyme. Adsorption to polyelectrolyte multilayer surfaces protects PG mimics from degradation, compared to when PG mimics are combined with enzymes in solution; all surfaces are still intact after 21 days of enzyme exposure. This work reveals how the stability of PG mimics is controlled by both the composition and macromolecular assembly of the PG mimic and also by the size and specificity of the enzyme. Understanding and tuning these degradation susceptibilities are essential for advancing their applications in cardiovascular materials, orthopedic materials, and growth factor delivery applications.

Perlecan in Pericellular Mechanosensory Cell-Matrix Communication, Extracellular Matrix Stabilisation and Mechanoregulation of Load-Bearing Connective Tissues

In this study, we review mechanoregulatory roles for perlecan in load-bearing connective tissues. Perlecan facilitates the co-acervation of tropoelastin and assembly of elastic microfibrils in translamellar cross-bridges which, together with fibrillin and elastin stabilise the extracellular matrix of the intervertebral disc annulus fibrosus. Pericellular perlecan interacts with collagen VI and XI to define and stabilize this matrix compartment which has a strategic position facilitating two-way cell-matrix communication between the cell and its wider extracellular matrix. Cues from the extracellular matrix are fed through this pericellular matrix back to the chondrocyte, allowing it to perceive and respond to subtle microenvironmental changes to regulate tissue homeostasis. Thus perlecan plays a key regulatory role in chondrocyte metabolism, and in chondrocyte differentiation. Perlecan acts as a transport proteoglycan carrying poorly soluble, lipid-modified proteins such as the Wnt or Hedgehog families facilitating the establishment of morphogen gradients that drive tissue morphogenesis. Cell surface perlecan on endothelial cells or osteocytes acts as a flow sensor in blood and the lacunar canalicular fluid providing feedback cues to smooth muscle cells regulating vascular tone and blood pressure, and the regulation of bone metabolism by osteocytes highlighting perlecan's multifaceted roles in load-bearing connective tissues.

Perlecan in the Natural and Cell Therapy Repair of Human Adult Articular Cartilage: Can Modifications in This Proteoglycan Be a Novel Therapeutic Approach?

Articular cartilage is considered to have limited regenerative capacity, which has led to the search for therapies to limit or halt the progression of its destruction. Perlecan, a multifunctional heparan sulphate (HS) proteoglycan, promotes embryonic cartilage development and stabilises the mature tissue. We investigated the immunolocalisation of perlecan and collagen between donor-matched biopsies of human articular cartilage defects (n = 10 × 2) that were repaired either naturally or using autologous cell therapy, and with age-matched normal cartilage. We explored how the removal of HS from perlecan affects human chondrocytes in vitro. Immunohistochemistry showed both a pericellular and diffuse matrix staining pattern for perlecan in both natural and cell therapy repaired cartilage, which related to whether the morphology of the newly formed tissue was hyaline cartilage or fibrocartilage. Immunostaining for perlecan was significantly greater in both these repair tissues compared to normal age-matched controls. The immunolocalisation of collagens type III and VI was also dependent on tissue morphology. Heparanase treatment of chondrocytes in vitro resulted in significantly increased proliferation, while the expression of key chondrogenic surface and genetic markers was unaffected. Perlecan was more prominent in chondrocyte clusters than in individual cells after heparanase treatment. Heparanase treatment could be a means of increasing chondrocyte responsiveness to cartilage injury and perhaps to improve repair of defects.

Perlecan, a modular instructive proteoglycan with diverse functional properties

This study reviewed some new aspects of the modular proteoglycan perlecan, a colossal proteoglycan with a 467 kDa core protein and five distinct functional domains. Perlecan is a heparan sulphate proteoglycan that transiently displays native CS sulphation motifs 4-C-3 and 7-D-4 during tissue morphogenesis these are expressed by progenitor cell populations during tissue development. Perlecan is susceptible to fragmentation by proteases during tissue development and in pathological tissues particularly in domains IV and V. The fragmentation pattern of domain IV has been suggested as a means of grading prostate cancer. Domain V of perlecan is of interest due to its interactive properties with integrin α5β1 that promotes pericyte migration enhancing PDGF-BB-induced phosphorylation of PDGFRβ, Src homology region 2 domain-containing phosphatase-2, and focal adhesion kinase supporting the repair of the blood brain barrier following ischaemic stroke. Fragments of domain V can also interact with α2β1 integrin disrupting tube formation by endothelial cells. LG1-LG2, LG3 fragments can antagonise VEGFR2, and α2β1 integrin interactions preventing angiogenesis by endothelial cells. These domain V fragments are of interest as potential anti-tumour agents. Perlecan attached to the luminal surfaces of endothelial cells in blood vessels acts as a flow sensor that signals back to endothelial and smooth muscle cells to regulate vascular tone and blood pressure. Perlecan also acts as a flow sensor in the lacuno-canalicular space regulating osteocytes and bone homeostasis. Along with its biomechanical regulatory properties in cartilaginous tissues this further extends the functional repertoire of this amazingly diverse functional proteoglycan.

Mechanotransduction pathways in the regulation of cartilage chondrocyte homoeostasis

Mechanical stress plays a critical role in cartilage development and homoeostasis. Chondrocytes are surrounded by a narrow pericellular matrix (PCM), which absorbs dynamic and static forces and transmits them to the chondrocyte surface. Recent studies have demonstrated that molecular components, including perlecan, collagen and hyaluronan, provide distinct physical properties for the PCM and maintain the essential microenvironment of chondrocytes. These physical signals are sensed by receptors and molecules located in the cell membrane, such as Ca²⁺ channels, the primary cilium and integrins, and a series of downstream molecular pathways are involved in mechanotransduction in cartilage. All mechanoreceptors convert outside signals into chemical and biological signals, which then regulate transcription in chondrocytes in response to mechanical stresses. This review highlights recent progress and focuses on the function of the PCM and cell surface molecules in chondrocyte mechanotransduction. Emerging understanding of the cellular and molecular mechanisms that regulate mechanotransduction will provide new insights into osteoarthritis pathogenesis and precision strategies that could be used in its treatment.

Discovery of HSPG2 (Perlecan) as a Therapeutic Target in Triple Negative Breast Cancer

In recent years, there have been significant advances in the treatment of breast cancer resulting in remarkably high survival rates. However, treatment options for metastatic triple negative breast cancer (TNBC) are quite limited due to a lack of identifiable, unique markers. Using a phage display-based whole cell biopanning procedure, we developed two human antibodies that bind to tumor cells with a metastatic TNBC phenotype. Our studies further identified domain 1 of HSPG2 (perlecan) protein as the cognate cell surface antigen bound by the antibody. Immunohistochemistry studies utilizing patient tissue samples revealed significant cell surface expression of HSPG2 in both primary tumors and metastatic lesions. Further, higher HSPG2 expression correlated with poor survival in TNBC. The affinity-matured antibody inhibited the growth of triple negative MDA-MB-231 tumors to a greater extent in nude mice than in NSG mice, pointing to the potential role of natural killer cell-mediated antibody-dependent cell cytotoxicity. This mechanism of action was confirmed through in vitro assays using mouse splenocytes and human peripheral blood mononuclear cells (PBMCs). These results suggest that HSPG2 is a promising target in metastatic TNBC and HSPG2-targeted antibodies could represent a potentially novel class of targeted therapeutics for TNBC.

Perlecan-targeted nanoparticles for drug delivery to triple-negative breast cancer

Aim:

We previously developed two antibodies that bind to a cell surface protein, perlecan, overexpressed in triple-negative breast cancer (TNBC). The goal of this study was to investigate these antibodies as targeting ligands for nanoparticle-mediated drug delivery.

Methods:

Paclitaxel-loaded poly(D,L-lactide-co-glycolide) nanoparticles were functionalized with antibodies using thiol–maleimide chemistry. Effect of antibody functionalization on therapeutic efficacy of drug-loaded nanoparticles was investigated using in vitro and in vivo models of TNBC.

Results:

The antibodies were covalently conjugated to nanoparticles without affecting antibody binding affinity or nanoparticle properties. Perlecan-targeted nanoparticles showed improved cell uptake, retention, cytotoxicity in vitro and enhanced tumor growth inhibition in vivo.

Conclusion:

The data presented here indicates that perlecan-targeted nanoparticles can improve tumor drug delivery to TNBC.

IL-1 and TGF-β Modulation of Epithelial Basement Membrane Components Perlecan and Nidogen Production by Corneal Stromal Cells

Purpose

To determine whether (1) the in vitro expression of epithelial basement membrane components nidogen-1, nidogen-2, and perlecan by keratocytes, corneal fibroblasts, and myofibroblasts is modulated by cytokines/growth factors, and (2) perlecan protein is produced by stromal cells after photorefractive keratectomy.

Methods

Marker-verified rabbit keratocytes, corneal fibroblasts, myofibroblasts were stimulated with TGF-β1, IL-1α, IL-1β, TGF-β3, platelet-derived growth factor (PDGF)-AA, or PDGF-AB. Real-time quantitative RT-PCR was used to detect expression of nidogen-1, nidogen-2, and perlecan mRNAs. Western blotting evaluated changes in protein expression. Immunohistochemistry was performed on rabbit corneas for perlecan, alpha-smooth muscle actin, keratocan, vimentin, and CD45 at time points from 1 day to 1 month after photorefractive keratectomy (PRK).

Results

IL-1α or -1β significantly upregulated perlecan mRNA expression in keratocytes. TGF-β1 or -β3 markedly downregulated nidogen-1 or -2 mRNA expression in keratocytes. None of these cytokines had significant effects on nidogen-1, -2, or perlecan mRNA expression in corneal fibroblasts or myofibroblasts. IL-1α significantly upregulated, while TGF-β1 significantly downregulated, perlecan protein expression in keratocytes. Perlecan protein expression was upregulated in anterior stromal cells at 1 and 2 days after −4.5 or −9 diopters (D) PRK, but the subepithelial localization of perlecan became disrupted at 7 days and later time points in −9-D PRK corneas when myofibroblasts populated the anterior stroma.

Conclusions

IL-1 and TGF-β1 have opposing effects on perlecan and nidogen expression by keratocytes in vitro. Proximate participation of keratocytes is likely needed to regenerate normal epithelial basement membrane after corneal injury.

Changes in stiffness and biochemical composition of the pericellular matrix as a function of spatial chondrocyte organisation in osteoarthritic cartilage

OBJECTIVE

During osteoarthritis (OA), chondrocytes seem to change their spatial arrangement from single to double strings, small and big clusters. Since the pericellular matrix (PCM) appears to degrade alongside this reorganisation, it has been suggested that spatial patterns act as an image-based biomarker for OA. The aim of this study was to establish the functional relevance of spatial organisation in articular cartilage.

METHOD

Cartilage samples were selected according to their predominant spatial cellular pattern. Young's modulus of their PCM was measured by atomic force microscopy (AFM) (∼500 measurements/pattern). The distribution of two major PCM components (collagen type VI and perlecan) was analysed by immunohistochemistry (8 patients) and protein content quantified by enzyme-linked immunosorbent assay (ELISA) (58 patients).

RESULTS

PCM stiffness significantly decreased with the development from single to double strings (p = 0.030), from double strings to small clusters (p = 0.015), and from small clusters to big clusters (p < 0.001). At the same time, the initially compact collagen type VI and perlecan staining progressively weakened and was less focalised. The earliest point with a significant reduction in protein content as shown by ELISA was the transition from single strings to small clusters for collagen type VI (p = 0.016) and from double strings to small clusters for perlecan (p = 0.008), with the lowest amounts for both proteins seen in big clusters.

CONCLUSIONS

This study demonstrates the functional relevance of spatial chondrocyte organisation as an image-based biomarker. At the transition from single to double strings PCM stiffness decreases, followed by protein degradation from double strings to small clusters.

Roles of blood-brain barrier integrins and extracellular matrix in stroke

Ischemicstroke is a leading cause of death and disability in the United States, but recent advances in treatments [i.e., endovascular thrombectomy and tissue plasminogen activator (t-PA)] that target the stroke-causing blood clot, while improving overall stroke mortality rates, have had much less of an impact on overall stroke morbidity. This may in part be attributed to the lack of therapeutics targeting reperfusion-induced injury after the blood clot has been removed, which, if left unchecked, can expand injury from its core into the surrounding at risk tissue (penumbra). This occurs in two phases of increased permeability of the blood-brain barrier, a physical barrier that under physiologic conditions regulates brain influx and efflux of substances and consists of tight junction forming endothelial cells (and transporter proteins), astrocytes, pericytes, extracellular matrix, and their integrin cellular receptors. During, embryonic development, maturity, and following stroke reperfusion, cerebral vasculature undergoes significant changes including changes in expression of integrins and degradation of surrounding extracellular matrix. Integrins, heterodimers with α and β subunits, and their extracellular matrix ligands, a collection of proteoglycans, glycoproteins, and collagens, have been modestly studied in the context of stroke compared with other diseases (e.g., cancer). In this review, we describe the effect that various integrins and extracellular matrix components have in embryonic brain development, and how this changes in both maturity and in the poststroke environment. Particular focus will be on how these changes in integrins and the extracellular matrix affect blood-brain barrier components and their potential as diagnostic and therapeutic targets for ischemic stroke.

The Pathobiology of the Meniscus: A Comparison Between the Human and Dog

Serious knee pain and related disability have an annual prevalence of approximately 25% on those over the age of 55 years. As curative treatments for the common knee problems are not available to date, knee pathologies typically progress and often lead to osteoarthritis (OA). While the roles that the meniscus plays in knee biomechanics are well characterized, biological mechanisms underlying meniscus pathophysiology and roles in knee pain and OA progression are not fully clear. Experimental treatments for knee disorders that are successful in animal models often produce unsatisfactory results in humans due to species differences or the inability to fully replicate disease progression in experimental animals. The use of animals with spontaneous knee pathologies, such as dogs, can significantly help addressing this issue. As microscopic and macroscopic anatomy of the canine and human menisci are similar, spontaneous meniscal pathologies in canine patients are thought to be highly relevant for translational medicine. However, it is not clear whether the biomolecular mechanisms of pain, degradation of extracellular matrix, and inflammatory responses are species dependent. The aims of this review are (1) to provide an overview of the anatomy, physiology, and pathology of the human and canine meniscus, (2) to compare the known signaling pathways involved in spontaneous meniscus pathology between both species, and (3) to assess the relevance of dogs with spontaneous meniscal pathology as a translational model. Understanding these mechanisms in human and canine meniscus can help to advance diagnostic and therapeutic strategies for painful knee disorders and improve clinical decision making.

The multifaceted roles of perlecan in fibrosis

Perlecan, or heparan sulfate proteoglycan 2 (HSPG2), is a ubiquitous heparan sulfate proteoglycan that has major roles in tissue and organ development and wound healing by orchestrating the binding and signaling of mitogens and morphogens to cells in a temporal and dynamic fashion. In this review, its roles in fibrosis are reviewed by drawing upon evidence from tissue and organ systems that undergo fibrosis as a result of an uncontrolled response to either inflammation or traumatic cellular injury leading to an over production of a collagen-rich extracellular matrix. This review focuses on examples of fibrosis that occurs in lung, liver, kidney, skin, kidney, neural tissues and blood vessels and its link to the expression of perlecan in that particular organ system.

Biodiversity of CS–proteoglycan sulphation motifs: chemical messenger recognition modules with roles in information transfer, control of cellular behaviour and tissue morphogenesis

Chondroitin sulphate (CS) glycosaminoglycan chains on cell and extracellular matrix proteoglycans (PGs) can no longer be regarded as merely hydrodynamic space fillers. Overwhelming evidence over recent years indicates that sulphation motif sequences within the CS chain structure are a source of significant biological information to cells and their surrounding environment. CS sulphation motifs have been shown to interact with a wide variety of bioactive molecules, e.g. cytokines, growth factors, chemokines, morphogenetic proteins, enzymes and enzyme inhibitors, as well as structural components within the extracellular milieu. They are therefore capable of modulating a panoply of signalling pathways, thus controlling diverse cellular behaviours including proliferation, differentiation, migration and matrix synthesis. Consequently, through these motifs, CS PGs play significant roles in the maintenance of tissue homeostasis, morphogenesis, development, growth and disease. Here, we review (i) the biodiversity of CS PGs and their sulphation motif sequences and (ii) the current understanding of the signalling roles they play in regulating cellular behaviour during tissue development, growth, disease and repair.

Targeted sequencing in FGF/FGFR genes and association analysis of variants for mandibular prognathism

To identify variants of the genes in fibroblast growth factors/fibroblast growth factor receptors (FGF/FGFR) signal pathway that predispose to mandibular prognathism (MP) in the general Chinese population systematically.Targeted sequencing of the FGF/FGFR genes was conducted in 176 MP individuals and 155 class I malocclusion controls. The associations of common and rare variants with MP as a categorical phenotype and also continuous malocclusion phenotypes generated by principal component (PC) analysis were analyzed.One common variant, rs372127537, located in the 3'-untranslated region of FGF7 gene, was significantly related to PC1 (P  =  4.22 × 10), which explained 23.23% of the overall phenotypic variation observed and corresponded to vertical discrepancies ranging from short anterior face height to long anterior face height, after Bonferroni correction. Also, 15 other variants were associated with PC1-4, although not significant after multiple corrections (P < .05). We also identified 3 variants: rs13317 in FGFR1, rs149242678 in FGF20, and rs79176051 FGF12 associated with MP (P < .05). With respect to rare variant analysis, variants within the FGF12 gene showed significant association with MP (P  =  .001).Association between FGF/FGFR signaling pathway and MP has been identified. We found a previously unreported SNP in FGF7 significantly related to increased facial height. Also, rare variants within the FGF12 were associated with MP. Our results provide new clues for genetic mechanisms of MP and shed light on strategies for evaluating rare variants that underlie complex traits. Future studies with larger sample sizes and more comprehensive genome coverage, and also in other population are required to replicate these findings.

Platelet Factor 4 Binds to Vascular Proteoglycans and Controls Both Growth Factor Activities and Platelet Activation

Platelet factor 4 (PF4) is produced by platelets with roles in both inflammation and wound healing. PF4 is stored in platelet α-granules bound to the glycosaminoglycan (GAG) chains of serglycin. This study revealed that platelet serglycin is decorated with chondroitin/dermatan sulfate and that PF4 binds to these GAG chains. Additionally, PF4 had a higher affinity for endothelial-derived perlecan heparan sulfate chains than serglycin GAG chains. The binding of PF4 to perlecan was found to inhibit both FGF2 signaling and platelet activation. This study revealed additional insight into the ways in which PF4 interacts with components of the vasculature to modulate cellular events.

Use of FGF-2 and FGF-18 to direct bone marrow stromal stem cells to chondrogenic and osteogenic lineages

Aim:

Intervertebral disc degeneration/low back pain is the number one global musculoskeletal condition in terms of disability and socioeconomic impact.

Materials & methods

Multipotent mesenchymal stem cells (MSCs) were cultured in micromass pellets ± FGF-2 or -18 up to 41 days, matrix components were immunolocalized and gene expression monitored by quantitative-reverse transcription PCR.

Results:

Chondrogenesis occurred earlier in FGF-18 than FGF-2 cultures. Lower COL2A1, COL10A1 and ACAN expression by day 41 indicated a downregulation in chondrocyte hypertrophy. MEF2c, ALPL, were upregulated; calcium, decorin and biglycan, and 4C3 and 7D4 chondroitin sulphate sulfation motifs were evident in FGF-18 but not FGF-2 pellets.

Conclusion:

FGF-2 and -18 preconditioned MSCs produced cell lineages which promoted chondrogenesis and osteogenesis and may be useful in the production of MSC lineages suitable for repair of cartilaginous tissue defects.

Intervertebral disc degeneration and low back pain is the number one global musculoskeletal disorder effecting 80% of the general population. A remedy for this condition is being eagerly sought as part of a WHO research priority. Stem cells are one potential therapy that shows promise in animal models, laboratory studies, and preclinical and early clinical trials. Conditioning of stem cells in the laboratory before injection may improve their efficacy for the alleviation of low back pain. In the present study we have developed a means of improving how stem cells form cartilage and bone, which should be of application in the repair of spinal defects.

The CS Sulfation Motifs 4C3, 7D4, 3B3[-]; and Perlecan Identify Stem Cell Populations and Their Niches, Activated Progenitor Cells and Transitional Areas of Tissue Development in the Fetal Human Elbow

We compared the immunohistochemical distribution of (1) the novel chondroitin sulfate (CS) sulfation motifs 7D4, 4C3, and 3B3[-], (2) native heparan sulfate (HS) and Δ-HS "stubs" generated by heparitinase III digestion and (3) the HS-proteoglycan (PG), perlecan, in the fetal human elbow joint. Putative stem cell populations associated with hair bulbs, humeral perichondrium, humeral and ulnar rudiment stromal/perivascular tissues expressed the CS motifs 4C3, 7D4, and 3B3[-] along with perlecan in close association but not colocalized. Chondrocytes in the presumptive articular cartilage of the fetal elbow expressed the 4C3 and 7D4 CS sulfation motifs consistent with earlier studies on the expression of these motifs in knee cartilage following joint cavitation. This study also indicated that hair bulbs, skin, perichondrium, and rudiment stroma were all perlecan-rich progenitor cell niches that contributed to the organization and development of the human fetal elbow joint and associated connective tissues. One of the difficulties in determining the precise role of stem cells in tissue development and repair processes is their short engraftment period and the lack of specific markers, which differentiate the activated stem cell lineages from the resident cells. The CS sulfation motifs 7D4, 4C3, and 3B3[-] decorate cell surface PGs on activated stem/progenitor cells and thus can be used to identify these cells in transitional areas of tissue development and in repair tissues and may be applicable to determining a more precise mode of action of stem cells in these processes. Isolation of perlecan from 12 to 14 week gestational age fetal knee rudiments demonstrated that perlecan in these fetal tissues was a HS-CS hybrid PG further supporting roles for CS in tissue development.

The heparan sulphate deficient Hspg2 exon 3 null mouse displays reduced deposition of TGF-β1 in skin compared to C57BL/6 wild type mice

This was an observational study where we examined the role of perlecan HS on the deposition of TGF-β1 in C57BL/6 and Hspg2(∆3-/∆3-) perlecan exon 3 null mouse skin. Despite its obvious importance in skin repair and tissue homeostasis no definitive studies have immunolocalised TGF-β1 in skin in WT or Hspg2(∆3-/∆3-) perlecan exon 3 null mice. Vertical parasagittal murine dorsal skin from 3, 6 and 12 week old C57BL/6 and Hspg2(∆3-/∆3-) mice were fixed in neutral buffered formalin, paraffin embedded and 4 μm sections stained with Mayers haematoxylin and eosin (H & E). TGF-β1 was immunolocalised using a rabbit polyclonal antibody, heat retrieval and the Envision NovaRED detection system. Immunolocalisation of TGF-β1 differed markedly in C57BL/6 and Hspg2(∆3-/∆3-) mouse skin, ablation of exon 3 of Hspg2 resulted in a very severe reduction in the deposition of TGF-β1 in skin 3-12 weeks postnatally. The reduced deposition of TGF-β1 observed in the present study would be expected to impact detrimentally on the remodelling and healing capacity of skin in mutant mice compounding on the poor wound-healing properties already reported for perlecan exon 3 null mice due to an inability to signal with FGF-2 and promote angiogenic repair processes. TGF-β1 also has cell mediated effects in tissue homeostasis and matrix stabilisation a reduction in TGF-β1 deposition would therefore be expected to detrimentally impact on skin homeostasis in the perlecan mutant mice.

The cartilage extracellular matrix as a transient developmental scaffold for growth plate maturation

The cartilage growth plate is a specialized developmental tissue containing characteristic zonal arrangements of chondrocytes. The proliferative and differentiative states of chondrocytes are tightly regulated at all stages including the initial limb bud and rudiment cartilage stages of development, the establishment of the primary and secondary ossification centers, development of the growth plates and laying down of bone. A multitude of spatio-temporal signals, including transcription factors, growth factors, morphogens and hormones, control chondrocyte maturation and terminal chondrocyte differentiation/hypertrophy, cell death/differentiation, calcification and vascular invasion of the growth plate and bone formation during morphogenetic transition of the growth plate. This involves hierarchical, integrated signaling from growth and factors, transcription factors, mechanosensory cues and proteases in the extracellular matrix to regulate these developmental processes to facilitate progressive changes in the growth plate culminating in bone formation and endochondral ossification. This review provides an overview of selected components which have particularly important roles in growth plate biology including collagens, proteoglycans, glycosaminoglycans, growth factors, proteases and enzymes.

Soluble LILRA3 promotes neurite outgrowth and synapses formation through high affinity interaction with Nogo 66

Inhibitory proteins, particularly Nogo 66, a highly conserved 66 amino acid loop of Nogo A, play key roles in limiting the intrinsic capacity of the central nervous system to regenerate after injury. Ligation of surface Nogo receptors (NgRs) and/or leukocyte immunoglobulin like receptor B2 (LILRB2) and its mouse orthologue the paired-immunoglobulin-like receptor B (PIRB) by Nogo 66 transduces inhibitory signals that potently inhibit neurite outgrowth. Here we show that soluble leukocyte immunoglobulin-like receptor A3 (LILRA3) is a high affinity receptor for Nogo 66, suggesting that LILRA3 might be a competitive antagonist to these cell surface inhibitory receptors. Consistent with this, LILRA3 significantly reversed Nogo 66-mediated inhibition of neurite outgrowth and promoted synapse formation in primary cortical neurons via regulation of the MEK/ERK pathway. LILRA3 represents a new antagonist to Nogo 66-mediated inhibition of neurite outgrowth in the CNS, a function distinct from its immune-regulatory role in leukocytes. This report is also the first to demonstrate that a member of LILR family normally not expressed in rodents exerts functions on mouse neurons through the highly homologous Nogo 66 ligand.

Preparation of Proteoglycan Mimetic Graft Copolymers

Proteoglycans are proteins with pendant glycosaminoglycan polysaccharide side chains. The method described here enables the preparation of graft copolymers with glycosaminoglycan side chains, which mimic the structure and composition of proteoglycans. By controlling the stoichiometry, graft copolymers can be obtained with a wide range of glycosaminoglycan side-chain densities. The method presented here uses a three-step reaction mechanism to first functionalize a hyaluronic acid backbone, followed by reductive amination to couple the glycosaminoglycan side chain to the backbone, by the reducing end. Proteoglycan mimics like the ones proposed here could be used to study the structure-property relationships of proteoglycans and to introduce the biochemical and biomechanical properties of proteoglycans into biomaterials and therapeutic formulations.

The Basement Membrane Proteoglycans Perlecan and Agrin: Something Old, Something New

Several members of the proteoglycan family are integral components of basement membranes; other proteoglycan family members interact with or bind to molecular residents of the basement membrane. Proteoglycans are polyfunctional molecules, for they derive their inherent bioactivity from the amino acid motifs embedded in the core protein structure as well as the glycosaminoglycan (GAG) chains that are covalently attached to the core protein. The presence of the covalently attached GAG chains significantly expands the "partnering" potential of proteoglycans, permitting them to interact with a broad spectrum of targets, including growth factors, cytokines, chemokines, and morphogens. Thus proteoglycans in the basement membrane are poised to exert diverse effects on the cells intimately associated with basement membranes.

Angiogenic growth factors interactome and drug discovery: The contribution of surface plasmon resonance

Angiogenesis is implicated in several pathological conditions, including cancer, and in regenerative processes, including the formation of collateral blood vessels after stroke. Physiological angiogenesis is the outcome of a fine balance between the action of angiogenic growth factors (AGFs) and anti-angiogenic molecules, while pathological angiogenesis occurs when this balance is pushed toward AGFs. AGFs interact with multiple endothelial cell (EC) surface receptors inducing cell proliferation, migration and proteases upregulation. On the contrary, free or extracellular matrix-associated molecules inhibit angiogenesis by sequestering AGFs (thus hampering EC stimulation) or by interacting with specific EC receptors inducing apoptosis or decreasing responsiveness to AGFs. Thus, angiogenesis results from an intricate network of interactions among pro- and anti-angiogenic molecules, EC receptors and various modulators. All these interactions represent targets for the development of pro- or anti-angiogenic therapies. These aims call for suitable technologies to study the countless interactions occurring during neovascularization. Surface plasmon resonance (SPR) is a label-free optical technique to study biomolecular interactions in real time. It has become the golden standard technology for interaction analysis in biomedical research, including angiogenesis. From a survey of the literature it emerges that SPR has already contributed substantially to the better understanding of the neovascularization process, laying the basis for the decoding of the angiogenesis "interactome" and the identification of "hub molecules" that may represent preferential targets for an efficacious modulation of angiogenesis. Here, the still unexploited full potential of SPR is enlightened, pointing to improvements in its use for a deeper understanding of the mechanisms of neovascularization and the identification of novel anti-angiogenic drugs.

The structure and function of the pericellular matrix of articular cartilage

Chondrocytes in articular cartilage are surrounded by a narrow pericellular matrix (PCM) that is both biochemically and biomechanically distinct from the extracellular matrix (ECM) of the tissue. While the PCM was first observed nearly a century ago, its role is still under investigation. In support of early hypotheses regarding its function, increasing evidence indicates that the PCM serves as a transducer of biochemical and biomechanical signals to the chondrocyte. Work over the past two decades has established that the PCM in adult tissue is defined biochemically by several molecular components, including type VI collagen and perlecan. On the other hand, the biomechanical properties of this structure have only recently been measured. Techniques such as micropipette aspiration, in situ imaging, computational modeling, and atomic force microscopy have determined that the PCM exhibits distinct mechanical properties as compared to the ECM, and that these properties are influenced by specific PCM components as well as disease state. Importantly, the unique relationships among the mechanical properties of the chondrocyte, PCM, and ECM in different zones of cartilage suggest that this region significantly influences the stress-strain environment of the chondrocyte. In this review, we discuss recent advances in the measurement of PCM mechanical properties and structure that further increase our understanding of PCM function. Taken together, these studies suggest that the PCM plays a critical role in controlling the mechanical environment and mechanobiology of cells in cartilage and other cartilaginous tissues, such as the meniscus or intervertebral disc.

Confocal microscopy demonstrates association of LTBP-2 in fibrillin-1 microfibrils and colocalisation with perlecan in the disc cell pericellular matrix

Comparative immunolocalisations of latent transforming growth factor-beta-1 binding protein (LTBP)-2, fibrillin-1, versican and perlecan were undertaken in foetal human and wild type C57BL/6 mouse and Hspg2 exon 3 null HS deficient mouse intervertebral discs (IVDs). LTBP-2 was a prominent pericellular component of annular fibrochondrocytes in the posterior annulus fibrosus (AF), interstitial matrix adjacent to nucleus pulposus (NP) cells and to fibrillar and cell associated material in the anterior AF of the human foetal IVD and also displayed a pericellular localisation pattern in murine IVDs. Perlecan and LTBP-2 displayed strong pericellular colocalisation patterns in the posterior AF and to fibrillar material in the outer anterior AF in the foetal human IVD. Versican was a prominent fibril-associated component in the posterior and anterior AF, localised in close proximity to fibrillin-1 in fibrillar arrangements in the cartilaginous vertebral rudiments around paraspinal blood vessels, to major collagen fibre bundles in the anterior and posterior AF and shorter fibres in the NP. Fibrillin-1 was prominent in the outer anterior AF of the human foetal IVD and in fibres extending from the AF into the cartilaginous vertebral rudiments. LTBP-2 was prominently associated with annular fibrils containing fibrillin-1, versican was localised in close proximity to these but not specifically with LTBP-2. The similar deposition levels of LTBP-2 observed in the AF of the Hspg2 exon 3 null and wild type murine IVDs indicated that perlecan HS was not essential for LTBP-2 deposition but colocalisation of LTBP-2 with perlecan in the foetal human IVD was consistent with HS mediated interactions which have already been demonstrated in-vitro.

The role of vascular-derived perlecan in modulating cell adhesion, proliferation and growth factor signaling

Smooth muscle cell proliferation can be inhibited by heparan sulfate proteoglycans whereas the removal or digestion of heparan sulfate from perlecan promotes their proliferation. In this study we characterized the glycosaminoglycan side chains of perlecan isolated from either primary human coronary artery smooth muscle or endothelial cells and determined their roles in mediating cell adhesion and proliferation, and in fibroblast growth factor (FGF) binding and signaling. Smooth muscle cell perlecan was decorated with both heparan sulfate and chondroitin sulfate, whereas endothelial perlecan contained exclusively heparan sulfate chains. Smooth muscle cells bound to the protein core of perlecan only when the glycosaminoglycans were removed, and this binding involved a novel site in domain III as well as domain V/endorepellin and the α2β1 integrin. In contrast, endothelial cells adhered to the protein core of perlecan in the presence of glycosaminoglycans. Smooth muscle cell perlecan bound both FGF1 and FGF2 via its heparan sulfate chains and promoted the signaling of FGF2 but not FGF1. Also endothelial cell perlecan bound both FGF1 and FGF2 via its heparan sulfate chains, but in contrast, promoted the signaling of both growth factors. Based on this differential bioactivity, we propose that perlecan synthesized by smooth muscle cells differs from that synthesized by endothelial cells by possessing different signaling capabilities, primarily, but not exclusively, due to a differential glycanation. The end result is a differential modulation of cell adhesion, proliferation and growth factor signaling in these two key cellular constituents of blood vessels.

Transcriptional complexity of the HSPG2 gene in the human mast cell line, HMC-1

The mammalian HSPG2 gene encodes the proteoglycan protein core perlecan, which has important functions in biology including cell adhesion via integrins, binding to the extracellular matrix via various protein-protein interactions and binding of growth factors via the heparan sulfate chains decorating the N-terminal domain I. Here we show that, in the human mast cell line HMC-1, the transcription of this gene results in a population of mRNA that is processed in such a way to provide a relative increase of transcripts corresponding to domain V or the C-terminus compared to transcripts from either domain III or the N-terminal domain I. This paper also presents evidence of splicing of the HSPG2 gene in HMC-1 cells at exons 2/3 and after comparing this sequence with those published in various databases, a model is postulated to explain what might be happening in these cells with regard to the transcription of the HSPG2 gene. As domain V of perlecan contains the α2β1 integrin binding site that modulates angiogenesis, we hypothesize that the transcriptional control of the HSPG2 gene in mast cells to synthesize these transcripts supports their stimulatory and specific role in wound healing and tissue regeneration.

High resistance of the mechanical properties of the chondrocyte pericellular matrix to proteoglycan digestion by chondroitinase, aggrecanase, or hyaluronidase

In articular cartilage, the extracellular matrix (ECM) and chondrocyte-associated pericellular matrix (PCM) are characterized by a high concentration of proteoglycans (PGs) and their associated glycosaminoglycans (GAGs). These molecules serve important biochemical, structural, and biomechanical roles in the tissue and differences in their regional distributions suggest that different GAG/PG species contribute to the specific biomechanical properties of the ECM and PCM. The objective of this study was to investigate region-specific contributions of aggrecan, chondroitin and dermatan sulfate, and hyaluronan to the micromechanical properties of articular cartilage PCM and ECM in situ. Cryosections of porcine cartilage underwent digestion with ADAMTS-4, chondroitinase ABC, bacterial hyaluronidase or human leukocyte elastase. Guided by immunofluorescence for type VI collagen, AFM stiffness mapping was used to evaluate the elastic properties of matched PCM and ECM regions in paired control and digested cartilage sections. These methods were used to test the hypotheses that specific enzymatic digestion of GAGs or PGs would reduce both PCM and ECM elastic moduli. Elastase, which digests a number of PGs, some types of collagen, and non-collagenous proteins, was used as a positive control. ECM elastic moduli were significantly reduced by all enzyme treatments. However, PCM micromechanical properties were unaffected by enzymatic digestion of aggrecan, chondroitin/dermatan sulfate, and hyaluronan but were significantly reduced by 24% following elastase digestion. Our results provide new evidence for high resistance of PCM micromechanical properties to PG digestion and suggest a potential role for elastase in the degradation of the ECM and PCM.

Border patrol: insights into the unique role of perlecan/heparan sulfate proteoglycan 2 at cell and tissue borders

The extracellular matrix proteoglycan (ECM) perlecan, also known as heparan sulfate proteoglycan 2 or HSPG2, is one of the largest (>200 nm) and oldest (>550 M years) extracellular matrix molecules. In vertebrates, perlecan's five-domain structure contains numerous independently folding modules with sequence similarities to other ECM proteins, all connected like cars into one long, diverse complex train following a unique N-terminal domain I decorated with three long glycosaminoglycan chains, and an additional glycosaminoglycan attachment site in the C-terminal domain V. In lower invertebrates, perlecan is not typically a proteoglycan, possessing the majority of the core protein modules, but lacking domain I where the attachment sites for glycosaminoglycan chains are located. This suggests that uniting the heparan sulfate binding growth factor functions of domain I and the core protein functions of the rest of the molecule in domains II-V occurred later in evolution for a new functional purpose. In this review, we surveyed several decades of pertinent literature to ask a fundamental question: Why did nature design this protein uniquely as an extraordinarily long multifunctional proteoglycan with a single promoter regulating expression, rather than separating these functions into individual proteins that could be independently regulated? We arrived at the conclusion that the concentration of perlecan at functional borders separating tissues and tissue layers is an ancient key function of the core protein. The addition of the heparan sulfate chains in domain I likely occurred as an additional means of binding the core protein to other ECM proteins in territorial matrices and basement membranes, and as a means to reserve growth factors in an on-site depot to assist with rapid repair of those borders when compromised, such as would occur during wounding. We propose a function for perlecan that extends its role from that of an extracellular scaffold, as we previously suggested, to that of a critical agent for establishing and patrolling tissue borders in complex tissues in metazoans. We also propose that understanding these unique functions of the individual portions of the perlecan molecule can provide new insights and tools for engineering of complex multi-layered tissues including providing the necessary cues for establishing neotissue borders.

Basement membrane molecule expression attendant to chondrogenesis by nucleus pulposus cells and mesenchymal stem cells

Bone marrow-derived mesenchymal stem cells (MSCs) represent an autologous cell source for nucleus pulposus (NP) tissue engineering and regeneration. Although studies have demonstrated the ability of MSCs to differentiate to NP-like chondrocytic cells, few have comparatively studied the matrix synthesis and composition of the cartilaginous tissue formed in vitro from both cell types, particularly with respect to the expression of basement membrane (BM) molecules. The objective of this study was to evaluate chondrogenesis and expression of BM molecules, laminin and type IV collagen, in monolayer and in pellet cultures of caprine NP cells and MSCs. Both cell types demonstrated comparable levels of chondrogenesis, indicated by the percentage of chondrocytic cells, and the amounts of glycosaminoglycan and type II collagen. Laminin and type IV collagen were expressed intracellularly by NP cells and MSCs cultured in monolayer. During chondrogenesis in pellet cultures, the deposition of BM molecules in NP and MSC pellets followed an orderly spatiotemporal shift in pattern from a diffuse territorial and interterritorial distribution to a defined pericellular localization, as seen in normal adult NP. These results inform the use of MSCs for NP regeneration and suggest the possible involvement of certain BM molecules in chondrogenesis and cartilage regeneration.