Perlecan maintains the integrity of cartilage and some basement membranes

Beta1 integrins regulate chondrocyte rotation, G1 progression, and cytokinesis

Beta1 integrins are highly expressed on chondrocytes, where they mediate adhesion to cartilage matrix proteins. To assess the functions of beta1 integrin during skeletogenesis, we inactivated the beta1 integrin gene in chondrocytes. We show here that these mutant mice develop a chondrodysplasia of various severity. beta1-deficient chondrocytes had an abnormal shape and failed to arrange into columns in the growth plate. This is caused by a lack of motility, which is in turn caused by a loss of adhesion to collagen type II, reduced binding to and impaired spreading on fibronectin, and an abnormal F-actin organization. In addition, mutant chondrocytes show decreased proliferation caused by a defect in G1/S transition and cytokinesis. The G1/S defect is, at least partially, caused by overexpression of Fgfr3, nuclear translocation of Stat1/Stat5a, and up-regulation of the cell cycle inhibitors p16 and p21. Altogether these findings establish that beta1-integrin-dependent motility and proliferation of chondrocytes are mandatory events for endochondral bone formation to occur.

Perlecan, the multidomain heparan sulfate proteoglycan of basement membranes, is also a prominent component of the cartilaginous primordia in the developing human fetal spine

The aim of this study was to localize perlecan in human fetal spine tissues. Human fetal spines (12-20 weeks; n=6) were fixed in either Histochoice or 10% neutral buffered formalin, routinely processed, paraffin-embedded, and 4-microm sagittal sections were cut and stained with toluidine blue, H&E, and von Kossa. Perlecan, types I, II, IV, and X collagen, CD-31, aggrecan core protein, and native and delta-HS 4, 5 hexuronate stub epitopes were immunolocalized. Toluidine blue staining visualized the cartilaginous vertebral body (VB) rudiments and annular lamellae encompassing the nucleus pulposus (NP). Von Kossa staining identified the VB primary center of ossification. Immunolocalization of type IV collagen, CD-31, and perlecan delineated small blood vessels in the outer annulus fibrosus (AF) and large canals deep within the VBs. Perlecan and type X collagen were also prominently expressed by the hypertrophic vertebral growth plate chondrocytes. Aggrecan was extracellularly distributed in the intervertebral disk (IVD) with intense staining in the posterior AF. Notochordal tissue stained strongly for aggrecan but negatively for perlecan and types I and II collagen. Type I collagen was prominent in the outer AF and less abundant in the NP, while type II collagen was localized throughout the IVD and VB. The immunolocalization patterns observed indicated key roles for perlecan in vasculogenic, chondrogenic, and endochondral ossification processes associated with spinal development.

FAK deficiency in cells contributing to the basal lamina results in cortical abnormalities resembling congenital muscular dystrophies

Targeted deletion of focal adhesion kinase (fak) in the developing dorsal forebrain resulted in local disruptions of the cortical basement membrane located between the neuroepithelium and pia-meninges. At disruption sites, clusters of neurons invaded the marginal zone. Retraction of radial glial endfeet, midline fusion of brain hemispheres, and gliosis also occurred, similar to type II cobblestone lissencephaly as seen in congenital muscular dystrophy. Interestingly, targeted deletion of fak in neurons alone did not result in cortical ectopias, indicating that fak deletion from glia is required for neuronal mislocalization. Unexpectedly, fak deletion specifically from meningeal fibroblasts elicited similar cortical ectopias in vivo and altered laminin organization in vitro. These observations provide compelling evidence that FAK plays a key signaling role in cortical basement membrane assembly and/or remodeling. In addition, FAK is required within neurons during development because neuron-specific fak deletion alters dendritic morphology in the absence of lamination defects.

Antiangiogenic antithrombin down-regulates the expression of the proangiogenic heparan sulfate proteoglycan, perlecan, in endothelial cells

Antithrombin, a key serpin family regulator of blood coagulation proteases, is transformed into a potent antiangiogenic factor by limited proteolysis or mild heating. Here, we show by cDNA microarray, semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR), Northern blotting, and immunoblotting analyses that the expression of the proangiogenic heparan sulfate proteoglycan (HSPG), perlecan, but not other HSPGs, is dramatically down-regulated in human umbilical vein endothelial cells (HUVECs) treated with antiangiogenic cleaved and latent forms of antithrombin but not with the native form. Down-regulation of perlecan expression by cleaved and latent antithrombins was observed in both basic fibroblast growth factor (bFGF)-stimulated and unstimulated cells, whereas the antiangiogenic antithrombins inhibited the proliferation of only bFGF-stimulated HUVECs by arresting cells at the G(1) cell cycle phase. The importance of perlecan expression levels in mediating the antiproliferative effect of the antiangiogenic antithrombins was suggested by the finding that transforming growth factor-beta 1, a potent stimulator of perlecan expression in endothelial cells, blocked the down-regulation of perlecan expression and antiproliferative activity of cleaved antithrombin on endothelial cells. The previously established key role of perlecan in mediating bFGF stimulation of endothelial cell proliferation and angiogenesis suggests that a primary mechanism by which antiangiogenic antithrombins exert their effects is through the down-regulation of perlecan expression.

Genetic rescue of chondrodysplasia and the perinatal lethal effect of cartilage link protein deficiency

The targeted disruption of cartilage link protein gene (Crtl1) in homozygous mice resulted in a severe chondrodysplasia and perinatal lethality. This raised the question of whether the abnormalities seen in Crtl1 null mice are all caused by the absence of link protein in cartilage or whether the deficiency of the protein in other tissues and organs contributed to the phenotype. To address this question we have generated transgenic mice overexpressing cartilage link protein under the control of a cartilage-specific promoter, and then these transgenic mice were used for a genetic rescue of abnormalities in Crtl1 null mice. While the overexpression of cartilage link protein resulted in no abnormal phenotype, the cartilage-specific transgene expression of link protein could completely prevent the perinatal mortality of link protein-deficient mice and, depending on the level of the link protein expression, rescue skeletal abnormalities. Although link protein was originally isolated from cartilage, we found and determined Crtl1 transcripts and corresponding proteins in every organ tested from mouse embryos to aging animals. We also identified three additional members of the link protein family, all co-localized with hyaluronic acid-binding proteoglycans in the mouse genome. The ubiquitous presence of link protein suggests a general and systemic function of link protein in the organization of extracellular matrix in a number of tissues, possibly interacting with other proteoglycans, such as versican, brevican, and neurocan.

A novel interaction between perlecan protein core and progranulin: potential effects on tumor growth

In an in vivo search of novel partners for perlecan, a major heparan sulfate proteoglycan of basement membranes and cell surfaces, we identified progranulin, a secreted growth factor, as a strong interacting protein. Unambiguous interaction, first observed with the yeast two-hybrid system, was corroborated by co-immunoprecipitation studies using cell-free transcription/translation and transient cell transfection assays. The interaction of progranulin with perlecan domain V involved the first two laminin- and epidermal growth factor-like repeats. Within progranulin, the subdomains interacting most with perlecan harbored granulins F and B. Kinetics analysis of the interaction using surface plasmon resonance showed a saturable binding of relative low affinity (KD approximately 1 microM). These results were supported by significant expression overlap of these two proteins in a series of ovarian tumor tissue microarrays. Progranulin was present within proliferating blood vessels of ovarian carcinomas and perivascular matrices, with a distribution similar to perlecan. Notably, both progranulin and domain V stimulated the growth of adrenal carcinoma cells. However, when used together in equimolar amounts, the two proteins counteracted each other's activity. Thus, progranulin/perlecan interaction could contribute to a fine regulation of tumor angiogenesis and could ultimately affect cancer growth.

Atypical development of the tracheal basement membrane zone of infant rhesus monkeys exposed to ozone and allergen

Development of the basement membrane zone (BMZ) occurs postnatally in the rhesus monkey. The purpose of this study was to determine whether house dust mite allergen (HDMA) plus ozone altered this process. Rhesus monkeys were exposed to a regimen of HDMA and/or ozone or filtered air for 6 mo. To detect structural changes in the BMZ, we measured immunoreactivity of collagen I. To detect functional changes in the BMZ, we measured perlecan and fibroblast growth factor-2 (FGF-2). We also measured components of the FGF-2 ternary signaling complex [fibroblast growth factor receptor-1 (FGFR-1) and syndecan-4]. The width of the BMZ was irregular in the ozone groups, suggesting atypical development of the BMZ. Perlecan was also absent from the BMZ. In the absence of perlecan, FGF-2 was not bound to the BMZ. However, FGF-2 immunoreactivity was present in basal cells, the lateral intercellular space (LIS), and attenuated fibroblasts. FGFR-1 immunoreactivity was downregulated, and syndecan-4 immunoreactivity was upregulated in the basal cells. This suggests that FGF-2 in basal cells and LIS may be bound to the syndecan-4. We conclude that ozone and HDMA plus ozone effected incorporation of perlecan into the BMZ, resulting in atypical development of the BMZ. These changes are associated with specific alterations in the regulation of FGF-2, FGFR-1, and syndecan-4 in the airway epithelial-mesenchymal trophic unit, which may be associated with the developmental problems of lungs associated with exposure to ozone.

High binding capacity of cyclophilin B to chondrocyte heparan sulfate proteoglycans and its release from the cell surface by matrix metalloproteinases: possible role as a proinflammatory mediator in arthritis

OBJECTIVE

To study cyclophilin B, a protein newly identified as a secretion product of cultured chondrocytes, in the context of chondrocyte pathobiology.

METHODS

Cyclophilin B was purified by sequential chromatographic processing of the secretion medium of cultured guinea pig chondrocytes. Its presence both at the surface of chondrocyte monolayers and in cartilage was demonstrated by immunohistochemistry. Binding sites at the surface of chondrocytes were characterized by Scatchard plot analysis using (125)I-labeled cyclophilin B, and by glycosidase treatments. The release of cyclophilin B from chondrocytes by activated matrix metalloproteinases (MMPs) was studied by Western blot analysis.

RESULTS

Cyclophilin B was present at the surface of cultured chondrocytes and within cartilage, both in cells and in the extracellular matrix, with a particularly intense staining in the superficial layer. It was secreted constitutively by chondrocytes and cartilage explants. Its secretion was enhanced after treatment with its pharmacologic binding partner, cyclosporin A (CSA). Experiments with (125)I-labeled cyclophilin B demonstrated the presence of high-capacity, low-affinity, NaCl-sensitive binding sites at the surface of chondrocytes. Cell-bound cyclophilin B could be released by heparinase treatment, demonstrating binding to pericellular heparan sulfate proteoglycans (HSPGs). Chondroitinase or keratanase treatments had no effect. MMPs 1, 2, 3, 9, and 13 released intact cyclophilin B from the cell surface, probably by cleavage of HSPGs. This effect was reversed by the broad-spectrum MMP inhibitor, marimastat.

CONCLUSION

Cyclophilin B is a secreted CSA-binding protein involved in inflammatory events. It can induce chemotaxis in human neutrophils and T lymphocytes. The finding that cyclophilin B is an intrinsic component of cartilage and that it can be released by MMPs suggests that it has a role in the pathogenesis of arthritic diseases, even more so since its signaling receptor is present within the inflamed joint both on T cells and in the rheumatoid synovium.

CTGF/Hcs24, hypertrophic chondrocyte-specific gene product, interacts with perlecan in regulating the proliferation and differentiation of chondrocytes

Connective tissue growth factor/hypertrophic chondrocyte-specific gene product 24 (CTGF/Hcs24) plays important roles in the control of the proliferation and differentiation of chondrocytes in vitro. To clarify the mechanisms of regulation by CTGF/Hcs24 with respect to cartilage metabolism, we investigated the interaction between CTGF/Hcs24 and heparan sulfate proteoglycan perlecan. An immunofluorescence study showed that CTGF/Hcs24 was colocalized with heparan sulfate and perlecan in human chondrosarcoma-derived chondrocytic cell line HCS-2/8 in vitro. Northern blot analysis showed that perlecan, syndecan-1, -2, and -4 transcripts were detected in HCS-2/8 cells. Particularly, expression of the perlecan gene increased markedly in HCS-2/8 cells by recombinant CTGF/Hcs24 (rCTGF/Hcs24) treatment. We also found that CTGF/Hcs24 interacted with perlecan from HCS-2/8 cells in vitro. Furthermore, CTGF/Hcs24-stimulated gene expression of the aggrecan gene, as well as DNA/proteoglycan synthesis, was diminished when HCS-2/8 cells were pretreated with heparinase, indicating that the effects of CTGF/Hcs24 on chondrocytes occurred through the interaction between CTGF/Hcs24 and heparan sulfate on the cells. An in vivo study using mouse growth plate revealed that CTGF/Hcs24 produced by hypertrophic chondrocytes was localized from the proliferative to the hypertrophic zone, whereas perlecan was predominantly localized in the prehyphertrophic zone. Consistent with such findings in vivo, the binding of (125)I-rCTGF/Hcs24 to maturing chondrocytes was at higher levels than that to chondrocytes in hypertrophic stages. These findings suggest that CTGF/Hcs24 produced in the hypertrophic region may act on chondrocytes in the proliferative and maturative zone via some heparan sulfate proteoglycan, such as perlecan.

Heparan sulphate proteoglycans in Alzheimer's disease and amyloid-related disorders

Proteoglycans are associated with all kinds of amyloid deposits in the human body. These complex macromolecules, in particular heparan sulphate proteoglycans, have also been implicated in several features of the pathogenesis of Alzheimer's disease (AD), including the genesis of senile plaques, cerebrovascular amyloid, and neurofibrillary tangles. In this review we focus on the role of proteoglycans and glycosaminoglycans in amyloidogenesis in general and in AD in particular. Heparan sulphate proteoglycans may promote amyloid-beta peptide (Abeta) or tau fibrillisation on the one hand, and provide resistance against proteolytic breakdown on the other. Knowledge about the role of proteoglycans in AD pathology may eventually be of therapeutic use, because small polysulphated compounds, which can interfere with the interaction between proteoglycan and Abeta, have been shown to stop or even prevent amyloidogenesis.

Genetically altered mouse models: the good, the bad, and the ugly

Targeted gene disruption in mice is a powerful tool for generating murine models for human development and disease. While the human genome program has helped to generate numerous candidate genes, few genes have been characterized for their precise in vivo functions. Gene targeting has had an enormous impact on our ability to delineate the functional roles of these genes. Many gene knockout mouse models faithfully mimic the phenotypes of the human diseases. Because some models display an unexpected or no phenotype, controversy has arisen about the value of gene-targeting strategies. We argue in favor of gene-targeting strategies, provided they are used with caution, particularly in interpreting phenotypes in craniofacial and oral biology, where many genes have pleiotropic roles. The potential pitfalls are outweighed by the unique opportunities for developing and testing different therapeutic strategies before they are introduced into the clinic. In the future, we believe that genetically engineered animal models will be indispensable for gaining important insights into the molecular mechanisms underlying development, as well as disease pathogenesis, diagnosis, prevention, and treatment.

Basement membranes: structure, assembly and role in tumour angiogenesis

In recent years, the basement membrane (BM)--a specialized form of extracellular matrix (ECM)--has been recognized as an important regulator of cell behaviour, rather than just a structural feature of tissues. The BM mediates tissue compartmentalization and sends signals to epithelial cells about the external microenvironment. The BM is also an important structural and functional component of blood vessels, constituting an extracellular microenvironment sensor for endothelial cells and pericytes. Vascular BM components have recently been found to be involved in the regulation of tumour angiogenesis, making them attractive candidate targets for potential cancer therapies.

Localisation of specific heparan sulfate proteoglycans during the proliferative phase of brain development

Early brain development is characterised by the proliferation of neural precursor cells. Several families of signalling molecules such as the fibroblast growth factors (FGFs) and Wnts are known to play important roles in this early phase of brain development. Accumulating evidence demonstrates that signalling of these molecules requires the presence of heparan sulfate chains attached to a proteoglycan core protein (HSPG). However, the specific identity of the HSPG components in the developing brain is unknown. To determine which HSPGs might be involved at this early phase, we analysed the expression of the major cell surface HSPG families in the developing brain at a time of most active proliferation. Syndecan-1 and glypican-4 were the most highly expressed in the developing brain during the time of peak proliferation and localise to ventricular regions of the brain, where the precursor cells are proliferating. Syndecan-4, although less abundant, also localises to cells in the ventricular zone. We have also examined HSPG involvement in brain development using cultures of embryonic neural precursor cells. We find that FGF2 stimulation of proliferation is inhibited in the presence of sodium chlorate, an inhibitor of heparan sulfate synthesis, and is rescued by addition of exogenous heparan sulfate. These data support a requirement for heparan sulfate in FGF signalling for proliferation of brain precursor cells. The expression of these specific HSPGs within the proliferative zone of the brain suggests that they may be involved in regulation of early brain development, such as FGF-stimulated proliferation.

Perlecan protein core interacts with extracellular matrix protein 1 (ECM1), a glycoprotein involved in bone formation and angiogenesis

The goal of this study was to discover novel partners for perlecan, a major heparan sulfate proteoglycan of basement membranes, and to examine new interactions through which perlecan may influence cell behavior. We employed the yeast two-hybrid system and used perlecan domain V as bait to screen a human keratinocyte cDNA library. Among the strongest interacting clones, we isolated a approximately 1.6-kb cDNA insert that encoded extracellular matrix protein 1 (ECM1), a secreted glycoprotein involved in bone formation and angiogenesis. The sequencing of the clone revealed the existence of a novel splice variant that we name ECM1c. The interaction was validated by co-immunoprecipitation studies, using both cell-free systems and mammalian cells, and the specific binding site within each molecule was identified employing various deletion mutants. The C terminus of ECM1 interacted specifically with the epidermal growth factor-like modules flanking the LG2 subdomain of perlecan domain V. Perlecan and ECM1 were also co-expressed by a variety of normal and transformed cells, and immunohistochemical studies showed a partial expression overlap, particularly around dermal blood vessels and adnexal epithelia. ECM1 has been shown to regulate endochondral bone formation, stimulate the proliferation of endothelial cells, and induce angiogenesis. Similarly, perlecan plays an important role in chondrogenesis and skeletal development, as well as harboring pro- and anti-angiogenic activities. Thus, a physiological interaction could also occur in vivo during development and in pathological events, including tissue remodeling and tumor progression.

Perlecan up-regulation of FRNK suppresses smooth muscle cell proliferation via inhibition of FAK signaling

We previously reported that fully assembled basement membranes are nonpermissive to smooth muscle cell (SMC) replication and that perlecan (PN), a basement membrane heparan sulfate proteoglycan, is a dominant effector of this response. We report here that SMC adhesion to basement membranes, and perlecan in particular, up-regulate the expression of focal adhesion kinase-related nonkinase (FRNK), a SMC-specific endogenous inhibitor of FAK, which subsequently suppresses FAK-mediated, ERK1/2-dependent growth signals. Up-regulation of FRNK by perlecan is actively and continuously regulated. Relative to the matrix proteins studied, the effects are unique to perlecan, because plating of SMCs on several other basement membrane proteins is associated with low levels of FRNK and corresponding high levels of FAK and ERK1/2 phosphorylation and SMC growth. Perlecan supports SMC adhesion, although there is reduced cell spreading compared with fibronectin (FN), laminin (LN), or collagen type IV (IV). Despite the reduction in cell spreading, we report that perlecan-induced up-regulation of FRNK is independent of cell shape changes. Growth inhibition by perlecan was rescued by overexpressing a constitutively active FAK construct, but overexpressing kinase-inactivated mutant FAK or FRNK attenuated fibronectin-stimulated growth. These data indicate that perlecan functions as an endogenously produced inhibitor of SMC growth at least in part through the active regulation of FRNK expression. FRNK, in turn, may control SMC growth by downregulating FAK-dependent signaling events.

Hyaluronan and CD44: strategic players for cell-matrix interactions during chondrogenesis and matrix assembly

Embryonic induction, soluble and insoluble factors, receptors, and signal transduction are orchestrated for the morphogenesis of the cartilage elements. The interaction of cells with the extracellular matrix (ECM) may lead to altered cellular response to morphogens based on the formation of new adhesive contacts, or the uncoupling of cell-matrix interactions. Hyaluronan's influence on cell behavior, and its intimate association with cells are accomplished by a wide variety of specific binding proteins for hyaluronan. The temporal expression of the hyaluronan receptor CD44 (which is expressed as several alternatively spliced variants) may be strategic to many of these cell-matrix interactions during chondrogenesis. CD44 expression is temporally coincident with the reduction of intercellular spaces at the regions of future cartilage deposition. The spatial organization of CD44 at the cell surface may function to establish or regulate the structure of the pericellular matrix dependent on a hyaluronan scaffold. As the ECM is modified during embryogenesis, the cellular response to inductive signals may be altered. An uncoupling of chondrocyte-hyaluronan interaction leads to chondrocytic chondrolysis. Thus, consideration of cell-matrix interactions during chondrogenesis, in the light of our current understanding of the temporal and spatial expression of signaling morphogens, should become a promising focus of future research endeavors.

The role of laminin in embryonic cell polarization and tissue organization

Genetic analyses have revealed that members of the laminin glycoprotein family are required for basement membrane assembly and cell polarization, with subsequent effects on cell survival and tissue organization during metazoan embryogenesis. These functions depend upon the cooperation between laminin polymerization and cell anchorage mediated via interactions with beta1-integrins, dystroglycan, and other cell surface receptors.

Phenotypic knock out of heparan sulfates in myotubes impairs excitation-induced calcium spiking

Little is known about the physiological functions of heparan sulfates (HSs), which are present in the basal lamina surrounding skeletal muscle fibers. Here, we present a new system in which HS is phenotypically knocked out by endogenous expression of epitope-specific anti-HS antibodies. Single-chain antibodies, containing an immunoglobulin leader peptide, were produced by using various expression systems. Antibodies were detected in the Golgi apparatus, the site of HS biosynthesis. Likewise, the HS-degrading enzyme heparanase was expressed. Endogenous expression of antibodies or heparanase in myoblasts resulted in HS-defective myotubes. Excitability and calcium kinetics of HS-defective myotubes were severely compromised, as determined by analysis of electrically induced calcium spikes via video-speed UV confocal laser scanning microscopy. Phenotypically knocking out of individual HS epitopes resulted in specific effects on excitability and calcium kinetics. These data indicate important roles for HSs in skeletal muscle calcium kinetics.

Role of collagen type II and perlecan in skeletal development

The cartilage extracellular matrix is composed of a dense collagen network that entraps a range of other specialized proteins important for the proper formation and function of the tissue. Loss of two abundant cartilage components, type II collagen and perlecan, has drastic effects on skeletal development. Both collagen II and perlecan mutants have severe and lethal chondrodysplasia characterized by disorganized growth plate, lack of collagen network, defective endochondral bone formation, and abnormal intervertebral disk development. To test whether the reduced collagen density in the perlecan-null cartilage is due to enhanced activity of collagen-degrading proteinases, we have analyzed gelatinase expression and activity in the mutant tissue. Immunohistochemical analysis revealed a weak, but clear, expansion of MMP-9 deposition into the hypertrophic zone of the perlecan-null growth plate. However, in situ and SDS-PAGE zymography showed that the activity of gelatinases (MMP-2 and MMP-9) is not altered in perlecan-null cartilage, suggesting that they are not primarily linked to the reduced fibrillar network observed in the mutant. Likewise, intercrossing of perlecan mutants onto an MMP-9-null background could not rescue the ultrastructural abnormalities of the perlecan-deficient cartilage.

Sox5 and Sox6 are required for notochord extracellular matrix sheath formation, notochord cell survival and development of the nucleus pulposus of intervertebral discs

The notochord has major roles in vertebral column formation: indirectly by inducing sclerotome cell differentiation; and directly by forming the nucleus pulposus of intervertebral discs. Sox5 and Sox6 encode Sry-related HMG box transcription factors that act redundantly to promote chondroblast differentiation in all cartilages of the mouse embryo. We show that Sox5 and Sox6 are expressed in the notochord cell lineage and required for notochord late development. In Sox5(-/-)/Sox6(-/-) embryos, the notochord formed a typical rod-like structure. It fulfilled its inductive functions, as indicated by expression of sonic hedgehog and sclerotome specification. However, the notochord failed to become surrounded with an extracellular matrix sheath. This phenotype was associated with a downregulation of extracellular matrix genes, including the genes for collagen 2, aggrecan and perlecan in both notochord cells and surrounding chondrocytic cells of presumptive inner annuli and vertebral bodies. The mutant notochord then underwent an aberrant, fatal dismantling after sclerotome cell migration. Its cells became removed first from intervertebral spaces and then from vertebral bodies, and it progressively underwent apoptosis. Meanwhile, the development of inner annuli and vertebral bodies was dramatically impaired. Consequently, the vertebral column of Sox5(-/-)/Sox6(-/-) fetuses consisted of a very deficient cartilage and was devoid of nuclei pulposi. In Sox5(-/-)/Sox6(+/-) and more severely in Sox5(+/-)/Sox6(-/-) embryos, the notochord sheath was thinner, but cells survived. By birth, nuclei pulposi were rudimentary, and its cells poorly swelled and still expressing sonic hedgehog. Hence, Sox5 and Sox6 are required for notochord extracellular matrix sheath formation, notochord cell survival and formation of nuclei pulposi. Through these roles and essential roles in cartilage formation, they are central transcriptional regulators of vertebral column development.

Endorepellin, a novel inhibitor of angiogenesis derived from the C terminus of perlecan

Perlecan, a ubiquitous basement membrane heparan sulfate proteoglycan, plays key roles in blood vessel growth and structural integrity. We discovered that the C terminus of perlecan potently inhibited four aspects of angiogenesis: endothelial cell migration, collagen-induced endothelial tube morphogenesis, and blood vessel growth in the chorioallantoic membrane and in Matrigel plug assays. The C terminus of perlecan was active at nanomolar concentrations and blocked endothelial cell adhesion to fibronectin and type I collagen, without directly binding to either protein; henceforth we have named it "endorepellin." We also found that endothelial cells possess a significant number of high affinity (K(d) of 11 nm) binding sites for endorepellin and that endorepellin binds endostatin and counteracts its anti-angiogenic effects. Thus, endorepellin represents a novel anti-angiogenic product, which may retard tumor neovascularization and hence tumor growth in vivo.

Drosophila perlecan modulates FGF and hedgehog signals to activate neural stem cell division

Mutations in the Drosophila trol gene cause cell cycle arrest of neuroblasts in the larval brain. Here, we show that trol encodes the Drosophila homolog of Perlecan and regulates neuroblast division by modulating both FGF and Hh signaling. Addition of human FGF-2 to trol mutant brains in culture rescues the trol proliferation phenotype, while addition of a MAPK inhibitor causes cell cycle arrest of the regulated neuroblasts in wildtype brains. Like FGF, Hh activates stem cell division in the larval brain in a Trol-dependent fashion. Coimmunoprecipitation studies are consistent with interactions between Trol and Hh and between mammalian Perlecan and Shh that are not competed with heparin sulfate. Finally, analyses of mutations in trol, hh, and ttv suggest that Trol affects Hh movement. These results indicate that Trol can mediate signaling through both of the FGF and Hedgehog pathways to control the onset of stem cell proliferation in the developing nervous system.

Heparan sulfate chains of perlecan are indispensable in the lens capsule but not in the kidney

Mice lacking exon 3 of perlecan (Hspg2) gene were generated by gene targeting. Exon deletion does not alter the expression or the reading frame but causes loss of attachment sites for three heparan sulfate (HS) side chains. Hspg2(Delta 3 / Delta 3) mice are viable and fertile but have small eyes. Apoptosis and leakage of cellular material through the lens capsule are observed in neonatal lenses, and lenses degenerate within 3 weeks of birth. Electron microscopy revealed altered structure of the lens capsule through which cells had formed extensions. No kidney malfunction, such as protein uria, was detected in Hspg2(Delta 3 / Delta 3) mutant mice, nor were ultrastructural changes observed in the glomerular basement membranes (BMs). To achieve further depletion in the HS content of the BMs, Hspg2(Delta 3 / Delta 3) mice were bred with collagen XVIII null mice. Lens defects were more severe in the newborn Col18a1(-/-) x Hspg2(Delta 3 / Delta 3) mice and degeneration proceeded faster than in Hspg2(Delta 3 / Delta 3) mice. The results suggest that in the lens capsule, HS chains have a structural function and are essential in the insulation of the lens from its environment and in regulation of incoming signals.

Perlecan-stimulated nodules undergo chondrogenic maturation in response to rhBMP-2 treatment in vitro

The heparan sulfate proteoglycan, perlecan, is localized to hypertrophic chondrocytes in the growth plates of long bones. Mice mutants for perlecan display severe cartilage and skeletal defects. Previously, we demonstrated that C3H10T1/2 fibroblasts cultured on perlecan stimulated extensive formation of dense nodules reminiscent of embryonic cartilaginous condensations. These nodules stain intensely with Alcian blue, and antibodies specific for collagen type II and aggrecan; however, nodules do not express collagen type X, a marker of chondrogenic maturation. In this investigation, we tested the hypothesis that addition of rhBMP-2 to perlecan-induced nodules would promote chondrogenic maturation in vitro. C3H10T1/2 fibroblasts were seeded in Lab-Tek chambered "Permanox" slides uncoated or coated with perlecan (B&D, 5 microg/well), at a density of 2 x 10(5) cells/well. The cells were maintained in CMRL-1066 media supplemented with ascorbic acid, citrate, and pyruvate (50 ng/ml). C3H10T1/2 fibroblasts seeded on perlecan-coated wells began to condense and form cell aggregates within 15 min. On the third day postplating, the media was replaced and supplemented with or without rhBMP-2 (50 ng/ml, Genetics Institute). On day 6 of culture, microscopy revealed that rhBMP-2-treated cultures had significantly proliferated; however, untreated cultures had not. By day 12 of culture, confocal microscopy revealed that perlecan-stimulated nodules treated with rhBMP-2 express a late stage marker of chondrogenesis (collagen type X). Morphologically, cells expressing collagen type X in rhBMP-2-treated nodules appear larger in diameter, relative to cells not expressing collagen type X. Cells cultured on plastic and treated with rhBMP-2 did not form nodules, but attached and spread, yielding a high-density monolayer. In response to rhBMP-2 treatment, these cells also express collagen type X. However, the appearance of collagen type X occurs at a later time point relative to the appearance of collagen type X in perlecan-stimulated nodules. Thus, perlecan-stimulated nodules do mature at a faster rate when treated with rhBMP-2 relative to monolayer cells.

Perlecan: an important component of the cartilage pericellular matrix

Perlecan (Pln) is a large proteoglycan that can bear HS (heparan sulfate) and chondroitin sulfate glycosaminoglycans. Previous studies have demonstrated that Pln can interact with growth factors and cell surfaces either via its constituent glycosaminoglycan chains or core protein. Herein, we summarize studies demonstrating spatially and temporally regulated expression of Pln mRNA and protein in developing and mature cartilage. Mutations either in the Pln gene or in genes involved in glycosaminoglycan assembly result in severe cartilage phenotypes seen in both human syndromes and mouse model systems. In vitro studies demonstrate that Pln can trigger chondrogenic differentiation of multipotential mouse CH310T1/2 stem cells as well as maintain the phenotype of adult human chondrocytes. Structural mapping indicates that these activities lie entirely within domain I, a region unique to Pln, and that they require glycosaminoglycans. We also discuss data indicating that Pln cooperates with the key chondrogenic growth factor, BMP-2, to promote expression of hypertrophic chondrocyte markers. Collectively, these studies indicate that Pln is an important component of human cartilage and may have useful applications in tissue engineering and cartilage-directed therapeutics.

Neurologic defects and selective disruption of basement membranes in mice lacking entactin-1/nidogen-1

Entactin-1 (nidogen-1) is an ubiquitous component of basement membranes. From in vitro experiments, entactin-1 was assigned a role in maintaining the structural integrity of the basement membrane because of its binding affinity to other components, such as type IV collagen and laminin. Entactin-1 also interacts with integrin receptors on the cell surface to mediate cell adhesion, spreading, and motility. Targeted disruption of the entactin-1 gene in the mouse presented in this study revealed a duplication of the entacin-1 locus. Homozygous mutants for the functional locus lacked entactin-1 mRNA and protein and often displayed seizure-like symptoms and loss of muscle control in the hind legs. The behavior patterns suggested the presence of neurologic deficits in the central nervous system, thus providing genetic evidence linking entactin-1 to proper functions of the neuromuscular system. In homozygous mutants, structural alterations in the basement membranes were found only in selected locations including brain capillaries and the lens capsule. The morphology of the basement membranes in other tissues examined superficially appeared to be normal. These observations suggest that the lost functions of entactin-1 result in pathologic changes that are highly tissue specific.

MEC-8 regulates alternative splicing ofunc-52transcripts inC. eleganshypodermal cells

Previous work has shown that C. elegans MEC-8 is a putative RNA-binding protein that promotes specific alternative splices ofunc-52 transcripts. unc-52 encodes homologs of mammalian perlecan that are located extracellularly between muscle and hypodermis and are essential for muscle development in both embryos and larvae. We show that MEC-8 is a nuclear protein found in hypodermis at most stages of development and not in most late embryonic or larval body-wall muscle. We have also found that overexpression of MEC-8 in hypodermis but not muscle can suppress certainunc-52 mutant phenotypes. These are unexpected results because it has been proposed that UNC-52 is produced exclusively by muscle. We have constructed various tissue-specific unc-52 minigenes fused to a gene for green fluorescent protein that have allowed us to monitor tissue-specificmec-8-dependent alternative splicing; we show that mec-8must be expressed in the same cell type as the unc-52 minigene in order to regulate its expression, supporting the view that MEC-8 acts directly on unc-52 transcripts and that UNC-52 must be synthesized primarily by the hypodermis. Indeed, our analysis of unc-52 genetic mosaics has shown that the focus of unc-52 action is not in body-wall muscle but most likely is in hypodermis.

Binding of mouse nidogen-2 to basement membrane components and cells and its expression in embryonic and adult tissues suggest complementary functions of the two nidogens

Nidogen-1 binds several basement membrane components by well-defined, domain-specific interactions. Organ culture and gene targeting approaches suggest that a high-affinity nidogen-binding site of the laminin gamma1 chain (gamma1III4) is important for kidney development and for nerve guidance. Other proteins may also bind gamma1III4, although human nidogen-2 binds poorly to the mouse laminin gamma1 chain. We therefore characterized recombinant mouse nidogen-2 and its binding to basement membrane proteins and cells. Mouse nidogen-1 and -2 interacted at comparable levels with collagen IV, perlecan, and fibulin-2 and, most notably, also with laminin-1 fragments P1 and gamma1III3-5, which both contain the gamma1III4 module. In embryos, nidogen-2 mRNA was produced by mesenchyme at sites of epithelial-mesenchymal interactions, but the protein was deposited on epithelial basement membranes, as previously shown for nidogen-1. Hence, binding of both nidogens to the epithelial laminin gamma1 chain is dependent on epithelial-mesenchymal interactions. Epidermal growth factor stimulated expression of both nidogens in embryonic submandibular glands. Both nidogens were found in all studied embryonic and adult basement membranes. Nidogen-2 was more adhesive than nidogen-1 for some cell lines and was mainly mediated by alpha3beta1 and alpha6beta1 integrins as shown by antibody inhibition. These findings revealed extensive coregulation of nidogen-1 and -2 expression and much more complementary functions of the two nidogens than previously recognized.

Gene structure and functional analysis of the mouse nidogen-2 gene: nidogen-2 is not essential for basement membrane formation in mice

Nidogens are highly conserved proteins in vertebrates and invertebrates and are found in almost all basement membranes. According to the classical hypothesis of basement membrane organization, nidogens connect the laminin and collagen IV networks, so stabilizing the basement membrane, and integrate other proteins. In mammals two nidogen proteins, nidogen-1 and nidogen-2, have been discovered. Nidogen-2 is typically enriched in endothelial basement membranes, whereas nidogen-1 shows broader localization in most basement membranes. Surprisingly, analysis of nidogen-1 gene knockout mice presented evidence that nidogen-1 is not essential for basement membrane formation and may be compensated for by nidogen-2. In order to assess the structure and in vivo function of the nidogen-2 gene in mice, we cloned the gene and determined its structure and chromosomal location. Next we analyzed mice carrying an insertional mutation in the nidogen-2 gene that was generated by the secretory gene trap approach. Our molecular and biochemical characterization identified the mutation as a phenotypic null allele. Nidogen-2-deficient mice show no overt abnormalities and are fertile, and basement membranes appear normal by ultrastructural analysis and immunostaining. Nidogen-2 deficiency does not lead to hemorrhages in mice as one may have expected. Our results show that nidogen-2 is not essential for basement membrane formation or maintenance.

A critical function of the pial basement membrane in cortical histogenesis

Mice with a targeted deletion of the nidogen-binding site of laminin gamma1 were used to study the function of the pial basement membrane in cortical histogenesis. The pial basement membrane in the mutant embryos assembled but was unstable and disintegrated at random segments. In segments with a disrupted basement membrane, radial glia cells were retracted from the pial surface, and radially migrating neurons, including Cajal-Retzius cells and cortical plate neurons, passed the meninges or terminated their migration prematurely. By correlating the disruptions in the pial basal lamina with changes in the morphology of radial glia cells, the aberrant migration of Cajal-Retzius cells, and subsequent dysplasia of cortical plate neurons, the present data establish a causal relationship of proper cortical histogenesis with the presence of an intact pial basement membrane.

Assembly, stability and integrity of basement membranes in vivo

Basement membranes are layered structures of the extracellular matrix which separate cells of various kinds from the surrounding stroma. One of the frequently recurring questions about basement membranes is how these structures are formed in vivo. Up to a few years ago, it was thought that basement membranes were formed spontaneously by a process of self-assembly of their components. However, it has now become clear that cell membrane receptors for basement membrane components are essential factors for the formation and stability of basement membranes in vivo. The present review highlights the modern concepts of basement membrane formation.

Hyperplastic conotruncal endocardial cushions and transposition of great arteries in perlecan-null mice

Perlecan is a heparan-sulfate proteoglycan abundantly expressed in pericellular matrices and basement membranes during development. Inactivation of the perlecan gene in mice is lethal at two developmental stages: around E10 and around birth. We report a high incidence of malformations of the cardiac outflow tract in perlecan-deficient embryos. Complete transposition of great arteries was diagnosed in 11 out of 15 late embryos studied (73%). Three of these 11 embryos also showed malformations of semilunar valves. Mesenchymal cells in the outflow tract were abnormally abundant in mutant embryos by E9.5, when the endocardial-mesenchymal transformation starts in wild-type embryos. At E10.5, mutant embryos lacked well-defined spiral endocardial ridges, and the excess of mesenchymal cells obstructed sometimes the outflow tract lumen. Most of this anomalous mesenchyme expressed the smooth muscle cell-specific alpha-actin isoform, a marker of the neural crest in the outflow tract of the mouse. In wild-type embryos, perlecan is present in the basal surface of myocardium and endocardium, as well as surrounding presumptive neural crest cells. We suggest that the excess of mesenchyme at the earlier stages of conotruncal development precludes the formation of the spiral ridges and the rotation of the septation complex in order to achieve a concordant ventriculoarterial connection. The observed mesenchymal overpopulation might be due to an uncontrolled migration of neural crest cells, which would arrive prematurely to the heart. Thus, perlecan is involved in the control of the outflow tract mesenchymal population size, underscoring the importance of the extracellular matrix in cardiac morphogenesis.

Zebrafish mutants identify an essential role for laminins in notochord formation

Basement membranes are thought to be essential for organ formation, providing the scaffold on which individual cells organize to form complex tissues. Laminins are integral components of basement membranes. To understand the development of a simple vertebrate organ, we have used positional cloning to characterize grumpy and sleepy, two zebrafish loci known to control notochord formation, and find that they encode laminin β1 and laminin γ1, respectively. Removal of either chain results in the dramatic loss of laminin 1 staining throughout the embryo and prevents formation of the basement membrane surrounding the notochord. Notochord cells fail to differentiate and many die by apoptosis. By transplantation, we demonstrate that, for both grumpy and sleepy, notochord differentiation can be rescued by exogenous sources of the missing laminin chain, although notochordal sources are also sufficient for rescue. These results demonstrate a clear in vivo requirement for laminin β1 and laminin γ1 in the formation of a specific vertebrate organ and show that laminin or the laminin-dependent basement membrane is essential for the differentiation of chordamesoderm to notochord.

Basic FGF mediates an immediate response of articular cartilage to mechanical injury

The extracellularly regulated kinase (ERK), one of the three types of mitogen-activated kinases, was rapidly activated after cutting porcine articular cartilage either when maintained as explants or in situ. Cutting released a soluble ERK-activating factor from the cartilage, which was purified and identified by MS as basic fibroblast growth factor (bFGF). Experiments with neutralizing Abs to bFGF and an FGFR1 tyrosine kinase inhibitor showed that this growth factor was the major ERK-activating factor released after injury. Treating cartilage with the heparin-degrading enzyme heparitinase also caused release of bFGF, suggesting the presence of an extracellular store that is sequestered in the matrix and released upon damage. Basic FGF induced the synthesis of a number of chondrocyte proteins including matrix metalloproteinases 1 and 3, tissue inhibitor of metalloproteinases-1, and glycoprotein 38, which were identified by MS. The strong induction of matrix metalloproteinases and tissue inhibitor of metalloproteinases-1 suggests that bFGF could have a role in remodeling damaged tissue.

Dyssegmental dysplasia, Silverman-Handmaker type: unexpected role of perlecan in cartilage development

Dyssegmental dysplasia, Silverman-Handmaker type (DDSH), is a lethal autosomal recessive form of dwarfism with characteristic anisospondylic micromelia. The remarkable similarities in the radiographic, clinical, and chondroosseous morphology of DDSH patients to those of perlecan-null mice led to the identification of mutations in the perlecan gene (HSPG2) of DDSH. Perlecan, a large heparan sulfate proteoglycan, is expressed in various tissues and is a component of all basement membrane extracellular matrices. A chondrodysplasia phenotype caused by the loss of perlecan was unexpected, because cartilage does not have basement membranes. Insertion and splicing mutations in HSPG2 of DDSH were found that were predicted to create a premature termination codon. Immunostaining and biochemical analysis revealed that the mutant perlecan molecules were unstable and not secreted into the extracellular matrix. These results indicate that DDSH is caused by functional null mutations of HSPG2 and that perlecan is essential for cartilage development. Published 2002 Wiley-Liss, Inc.

Specific ablation of the nidogen-binding site in the laminin γ1 chain interferes with kidney and lung development

Basement membrane assembly is of crucial importance in the development and function of tissues and during embryogenesis. Nidogen 1 was thought to be central in the assembly processes, connecting the networks formed by collagen type IV and laminins, however, targeted inactivation of nidogen 1 resulted in no obvious phenotype. We have now selectively deleted the sequence coding for the 56 amino acid nidogen-binding site, γ1III4, within the Lamc1 gene by gene targeting. Here, we show that mice homozygous for the deletion die immediately after birth, showing renal agenesis and impaired lung development. These developmental defects were attributed to locally restricted ruptures in the basement membrane of the elongating Wolffian duct and of alveolar sacculi. These data demonstrate that an interaction between two basement membrane proteins is required for early kidney morphogenesis in vivo.

Smooth, rough and upside-down neocortical development

Lissencephaly, which means 'smooth cortex', is caused by defective neuronal migration during development of the cerebral cortex and has devastating clinical consequences. 'Classical' lissencephaly seems to reflect mutations in regulators of the microtubule cytoskeleton, whereas 'cobblestone' lissencephaly is caused by mutations in genes needed for the integrity of the basal lamina of the central nervous system. Reelin, which is mutated in a third type of lissencephaly, may represent a unifying link because it encodes an extracellular protein that regulates neuronal migration and may also regulate the microtubule cytoskeleton.

Differentiation of the RN33B cell line into forebrain projection neurons after transplantation into the neonatal rat brain

The rat neural cell line RN33B has a remarkable ability to undergo region-specific neuronal differentiation after transplantation into the CNS. To further study its neurogenic properties in vivo, we used a recombinant lentiviral vector to genetically label the cells with the Green Fluorescent Protein (GFP) gene before implantation into the striatum/cortex, hippocampus, or mesencephalon of newborn rats. Three weeks after implantation, about 1-2% of the GFP-expressing cells had developed morphologies typical of neurons, astrocytes, or oligodendrocytes, the rest remained as either immature or undifferentiated nestin-positive cells. At 15-17 weeks postgrafting, the immature cells had disappeared in most graft recipients and only cells with neuronal or glial morphologies remained in similar numbers as at 3 weeks. The GFP distributed throughout the expressing cells, revealing fine morphological details, including dendrites with spines and extensive axonal projections. In all forebrain regions, the grafted cells differentiated into neurons with morphologies characteristic for each site, including large numbers of pyramidal-like cells in the cortex and the hippocampus, giving rise to dense projections to normal cortical target regions and to the contralateral hippocampus, respectively. In lower numbers, it was also possible to identify GFP-positive granulelike cells in the hippocampus, as well as densely spiny neurons in the striatum. In the mesencephalon by contrast, cells with astrocytic features predominated. The ability of the grafted RN33B cells to undergo region-specific differentiation into highly specialized types of forebrain projection neurons and establish connections with appropriate targets suggests that cues present in the microenvironment of the neonatal rat brain can effectively guide the development of immature progenitors, also in the absence of ongoing neurogenesis.

Isolation and identification of the major heparan sulfate proteoglycans in the developing bovine rib growth plate

Heparan sulfate proteoglycans are thought to mediate the action of growth factors. The heparan sulfate-containing proteoglycans in extracts of the bovine fetal rib growth plate were detected using the monoclonal antibody 3G10, which recognizes a neoepitope generated by heparitinase digestion (David, G., Bai, X. M., Van der Schueren, B., Cassiman, J. J., and Van den Berghe, H. (1992) J. Cell Biol. 119, 961-975). The heparan sulfate proteoglycans that react with this antibody were identified using antisera to known proteoglycans; purified using CsCl density gradient centrifugation, molecular sieve, and ion exchange chromatography; and then characterized. The major heparan sulfate proteoglycans in the growth plate had core proteins of 200 kDa and larger and were identified as perlecan and aggrecan. These two heparan sulfate proteoglycans could be effectively separated from each other by CsCl density gradient centrifugation alone. Perlecan contained 25% heparan sulfate and 75% chondroitin sulfate. The heparan sulfate chains on growth plate perlecan were considerably smaller than the chondroitin sulfate chains, and the heparan sulfate disaccharide content was different than that found for heparan sulfate from either kidney, tumor tissue, or growth plate aggrecan. Aggrecan contained only 0.1% heparan sulfate, which was localized to the CS-1 domain of aggrecan. These results indicate that perlecan and aggrecan would be the principal candidate proteoglycans involved in the action of heparan sulfate-binding proteins in the developing growth plate.

Role of perlecan in skeletal development and diseases

Perlecan, a large heparan sulfate proteoglycan (HSPG), is present in the basement membrane and other extracellular matrices. Its protein core is 400 kDa in size and consists of five distinct structural domains. A number of in vitro studies suggest multiple functions of perlecan in cell growth and differentiation and tissue organization. Recent studies with gene knockout mice and human diseases revealed critical in vivo roles of perlecan in cartilage development and neuromuscular junction activity.

Structural and functional mutations of the perlecan gene cause Schwartz-Jampel syndrome, with myotonic myopathy and chondrodysplasia

Perlecan, a large heparan sulfate proteoglycan, is a component of the basement membrane and other extracellular matrices and has been implicated in multiple biological functions. Mutations in the perlecan gene (HSPG2) cause two classes of skeletal disorders: the relatively mild Schwartz-Jampel syndrome (SJS) and severe neonatal lethal dyssegmental dysplasia, Silverman-Handmaker type (DDSH). SJS is an autosomal recessive skeletal dysplasia characterized by varying degrees of myotonia and chondrodysplasia, and patients with SJS survive. The molecular mechanism underlying the chondrodystrophic myotonia phenotype of SJS is unknown. In the present report, we identify five different mutations that resulted in various forms of perlecan in three unrelated patients with SJS. Heterozygous mutations in two patients with SJS either produced truncated perlecan that lacked domain V or significantly reduced levels of wild-type perlecan. The third patient had a homozygous 7-kb deletion that resulted in reduced amounts of nearly full-length perlecan. Unlike DDSH, the SJS mutations result in different forms of perlecan in reduced levels that are secreted to the extracellular matrix and are likely partially functional. These findings suggest that perlecan has an important role in neuromuscular function and cartilage formation, and they define the molecular basis involved in the difference in the phenotypic severity between DDSH and SJS.

The leucine-rich repeat protein PRELP binds perlecan and collagens and may function as a basement membrane anchor

PRELP (proline arginine-rich end leucine-rich repeat protein) is a heparin-binding leucine-rich repeat protein in connective tissue extracellular matrix. In search of natural ligands and biological functions of this molecule, we found that PRELP binds the basement membrane heparan sulfate proteoglycan perlecan. Also, recombinant perlecan domains I and V carrying heparan sulfate bound PRELP, whereas other domains without glycosaminoglycan substitution did not. Heparin, but not chondroitin sulfate, inhibited the interactions. Glycosaminoglycan-free recombinant perlecan domain V and mutated domain I did not bind PRELP. The dissociation constants of the PRELP-perlecan interactions were in the range of 3-18 nm as determined by surface plasmon resonance. As expected, truncated PRELP, without the heparin-binding domain, did not bind perlecan. Confocal immunohistochemistry showed that PRELP outlines basement membranes with a location adjacent to perlecan. We also found that PRELP binds collagen type I and type II through its leucine-rich repeat domain. Electron microscopy visualized a complex with PRELP binding simultaneously to the triple helical region of procollagen I and the heparan sulfate chains of perlecan. Based on the location of PRELP and its interaction with perlecan heparan sulfate chains and collagen, we propose a function of PRELP as a molecule anchoring basement membranes to the underlying connective tissue.

Not all perlecans are created equal: interactions with fibroblast growth factor (FGF) 2 and FGF receptors

Human basement membrane heparan sulfate proteoglycan (HSPG) perlecan binds and activates fibroblast growth factor (FGF)-2 through its heparan sulfate (HS) chains. Here we show that perlecans immunopurified from three cellular sources possess different HS structures and subsequently different FGF-2 binding and activating capabilities. Perlecan isolated from human umbilical arterial endothelial cells (HUAEC) and a continuous endothelial cell line (C11 STH) bound similar amounts of FGF-2 either alone or complexed with FGFRalpha1-IIIc or FGFR3alpha-IIIc. Both perlecans stimulated the growth of BaF3 cell lines expressing FGFR1b/c; however, only HUAEC perlecan stimulated those cells expressing FGFR3c, suggesting that the source of perlecan confers FGF and FGFR binding specificity. Despite these differences in FGF-2 activation, the level of 2-O- and 6-O-sulfation was similar for both perlecans. Interestingly, perlecan isolated from a colon carcinoma cell line that was capable of binding FGF-2 was incapable of activating any BaF3 cell line unless the HS was removed from the protein core. The HS chains also exhibited greater bioactivity after digestion with heparinase III. Collectively, these data clearly demonstrate that the bioactivity of HS decorating a single PG is dependent on its cell source and that subtle changes in structure including secondary interactions have a profound effect on biological activity.

Chondrodysplasias due to proteoglycan defects

The proteoglycans, especially the large chondroitin sulfate proteoglycan aggrecan, have long been viewed as important components of the extracellular matrix of cartilage. The drastic change in expression during differentiation from mesenchyme to cartilage, the loss of tissue integrity associated with proteoglycan degradation in several disease processes and, most important, the demonstration of abnormalities in proteoglycan production concomitant with the aberrant growth patterns exhibited by the brachymorphic mouse, the cartilage matrix deficient mouse, and the nanomelic chick provide the strongest evidence that the proteoglycan aggrecan is essential during differentiation and for maintenance of the skeletal elements. More recently, mutations associated with proteoglycans other than aggrecan, especially the heparan sulfate proteoglycans, glypican and perlecan, suggest an important role for these molecules in skeletal development as well. This review focuses on the molecular bases of the hereditary proteoglycan defects in animal models, as well as of some human chondrodysplasias, that collectively are providing a better understanding of the role of proteoglycans in the development and maintenance of the skeletal elements.

Antibody response against perlecan and collagen types IV and VI in chronic renal allograft rejection in the rat

Chronic rejection is the leading cause of late renal transplant failure. Various structural lesions are observed in grafts undergoing chronic rejection including glomerular basement membrane (GBM) duplications. The well-established Fisher (F344) to Lewis (LEW) rat renal transplant model for chronic rejection was used to assess the presence and role of the humoral immune response against graft antigens during chronic rejection. LEW recipients of F344 allografts develop transplant glomerulopathy and produce IgG1 antibodies directed against F344 GBM preparations that are detectable 3 weeks after transplantation. Glomerular IgG1 deposition was observed that in vitro co-localized with a rabbit anti-rat GBM antiserum in rejecting F344 grafts; elution experiments of isolated glomeruli yielded IgG1 antibodies reactive in vitro with F344 GBM, but not LEW GBM. Prevention of acute rejection by transient treatment of the recipients with cyclosporin A completely abrogated the production of anti-GBM antibodies. Using proteomic techniques we identified the antigens recognized by the LEW posttransplant sera as being the heparan sulfate proteoglycan perlecan and the alpha1 chain of collagen type VI in association with the alpha5 chain of collagen type IV. In conclusion, LEW recipients of F344 kidney grafts produce IgG1 antibodies against donor type perlecan and alpha1(VI)/alpha5(IV) collagen and develop transplant glomerulopathy. These data implicate an important role for the humoral immune response in the development of glomerulopathy during chronic rejection.

Absence of acetylcholinesterase at the neuromuscular junctions of perlecan-null mice

The collagen-tailed form of acetylcholinesterase (AChE) is concentrated at the vertebrate neuromuscular junction (NMJ), where it is responsible for rapidly terminating neurotransmission. This unique oligomeric form of AChE, consisting of three tetramers covalently attached to a collagen-like tail, is more highly expressed in innervated regions of skeletal muscle fibers, where it is externalized and attached to the synaptic basal lamina interposed between the nerve terminal and the receptor-rich postsynaptic membrane. Although it is clear that the enzyme is preferentially synthesized in regions of muscle contacted by the motoneuron, the molecular events underlying its localization to the NMJ are not known. Here we show that perlecan, a multifunctional heparan sulfate proteoglycan concentrated at the NMJ, is the unique acceptor molecule for collagen-tailed AChE at sites of nerve-muscle contact and is the principal mechanism for localizing AChE to the synaptic basal lamina.

The permissive cue laminin is essential for growth cone turning in vivo

The proper guidance of migrating growth cones relies on the balance of multiple guidance cues in the embryonic environment. In addition to guidance cues, growth cones are in contact with other substrates that may contribute to the pathfinding of neurons. For example, in the developing insect peripheral nervous system, pioneer neurons migrate on and between layers of the basal lamina. Previous studies have demonstrated that one basal lamina molecule, laminin, promotes outgrowth of many classes of neurons in vitro. In this study, the simple grasshopper nervous system was used to investigate the role of laminin in neuronal pathfinding. Laminin expression precedes axonogenesis of the Tibial (Ti1) pioneer neurons in the developing limb bud, and expression continues during outgrowth and guidance of the pioneer neurons. The role of a nidogen-binding motif on laminin was investigated using subunit-specific antibodies and peptides as blocking reagents in vivo. Antibodies and peptides that block the nidogen-binding site on laminin resulted in stalled Ti1 axon migration, predominantly at the precise location where they normally turn ventrally. After prolonged culturing, Ti1 axons remained stalled at the same location. Therefore, although Ti1 axons were capable of outgrowth in the presence of blocking reagents, they were not able to navigate an essential turn. This study indicates that the interaction of the Ti1 growth cone with the nidogen-binding site on laminin is vital for neuronal pathfinding in vivo and suggests that permissive cues may be essential for growth cone steering.

Chondrogenic activity of the heparan sulfate proteoglycan perlecan maps to the N-terminal domain I

C3H10T1/2 cells differentiate along a chondrogenic pathway when plated onto the extracellular matrix (ECM) protein perlecan (Pln). To identify the region(s) within the large Pln molecule that provides a differentiation signal, recombinant Pln-sequence-based polypeptides representing distinct structural domains were assayed for their ability to promote chondrogenesis in C3H10T1/2 cells. Five distinct domains, along with structural variations, were tested. The N-terminal domain I was tested in two forms (IA and IB) that contain only heparan sulfate (HS) chains or both HS and chondroitin sulfate (CS) chains, respectively. A mutant form of domain I lacking attachment sites for both HS and CS (Pln I(mut)) was tested also. Other constructs consecutively designated Pln domains II, III(A-C), IV(A,B), and V(A,B) were used to complete the structure-function analysis. Cells plated onto Pln IA or Pln IB but no other domain rapidly assembled into cellular aggregates of 40-120 microm on average. Aggregate formation was dependent on the presence of glycosaminoglycan (GAG) chains, because Pln I-based polypeptides lacking GAG chains either by enzymatic removal or mutation of HS/CS attachment sites were inactive. Aggregates formed on GAG-bearing Pln IA stained with Alcian Blue and were recognized by antibodies to collagen type II and aggrecan but were not recognized by an antibody to collagen type X, a marker of chondrocyte hypertrophy. Collectively, these studies indicate that the GAG-bearing domain I of Pln provides a sufficient signal to trigger C3H10T1/2 cells to enter a chondrogenic differentiation pathway. Thus, this matrix proteoglycan (PG) found at sites of cartilage formation in vivo is likely to enhance early stage differentiation induced by soluble chondrogenic factors.