Expression and characterization of human perlecan domains I and II synthesized by baculovirus-infected insect cells

The Role of Heparan Sulfate and Perlecan in Bone-regenerative Procedures

Tissue engineering, grafting procedures, regeneration, and tissue remodeling are developing therapeutic modalities with great potential medical value, but these regenerative modalities are not as effective or predictable as clinicians and patients would like. Greater understanding of growth factors, cytokines, extracellular matrix molecules, and their roles in cell-mediated healing processes have made these regenerative therapies more clinically viable and will continue advancing the fields of tissue engineering and grafting. However, millions of oral and non-oral bone-grafting procedures are performed annually, and only a small percentage yield the most desirable results. Here we review the heparan-sulfate-decorated extracellular biomolecule named perlecan and the research relating to its potential as an adjunct in bone-regenerative procedures. The review includes an overview of bone graft substitutes and biological adjuncts to bone-regenerative procedures in medicine as they apply to periodontal disease, alveolar ridge augmentation, and barrier membrane therapy. Perlecan is discussed as a potential biological adjunct in terms of growth factor sequestration and delivery, and promoting cell adhesion, proliferation, differentiation, and angiogenesis. Further, we propose delivery and application schemes for perlecan and/or its domains in bone-regenerative procedures, with particular emphasis on its heparan-sulfate-decorated domain I. The perlecan molecule, with its heparan sulfate glycosylation, may provide a multi-faceted approach for the delivery of a more comprehensive stimulus than other single potential adjuncts currently available for bone-regenerative procedures.

Heparan sulfate proteoglycans in healthy and diseased systems

Heparin and heparan sulfate (HS) are glycosaminoglycans (GAGs) that are synthesized in the tissues and organs of mammals. They are synthesized and attached to a core protein as proteoglycans through serine-glycine concensus motifs along the core protein. These GAGs are linear polysaccharides composed of repeating disaccharide saccharide units that are variously modified along their length. As a consequence of these modifications naturally occurring heparin and HS are extremely heterogeneous in their structures. A diverse range of proteins bind heparin and HS. The types of proteins that bind are dictated by the structure of the HS or heparin chains with which they are interacting. Heparan sulfates play major roles in tissue development and in maintaining homeostasis within healthy individuals. Recent genetic studies illustrate that alterations in their structural organization can have important consequences often giving rise to, or directly causing, a disease situation. A greater understanding of the repertoire of proteins with which heparin and HS interact and the diseases that can be caused by perturbations in the structures of heparin and HS proteoglycan may provide insights into possible therapeutic interventions. These issues are discussed with a focus on musculoskeletal phenotypes and diseases.

Similarity of recombinant human perlecan domain 1 by alternative expression systems bioactive heterogenous recombinant human perlecan D1

Background

Heparan sulfate glycosaminoglycans are diverse components of certain proteoglycans and are known to interact with growth factors as a co-receptor necessary to induce signalling and growth factor activity. In this report we characterize heterogeneously glycosylated recombinant human perlecan domain 1 (HSPG2 abbreviated as rhPln.D1) synthesized in either HEK 293 cells or HUVECs by transient gene delivery using either adenoviral or expression plasmid technology.

Results

By SDS-PAGE analysis following anion exchange chromatography, the recombinant proteoglycans appeared to possess glycosaminoglycan chains ranging, in total, from 6 kDa to >90 kDa per recombinant. Immunoblot analysis of enzyme-digested high M_r rhPln.D1 demonstrated that the rhPln.D1 was synthesized as either a chondroitin sulfate or heparan sulfate proteoglycan, in an approximately 2:1 ratio, with negligible hybrids. Secondary structure analysis suggested helices and sheets in both recombinant species. rhPln.D1 demonstrated binding to rhFGF-2 with an apparent k_D of 2 ± 0.2 nM with almost complete susceptibility to digestion by heparinase III in ligand blot analysis but not to chondroitinase digestion. Additionally, we demonstrate HS-mediated binding of both rhPln.D1 species to several other GFs. Finally, we corroborate the augmentation of FGF-mediated cell activation by rhPln.D1 and demonstrate mitogenic signalling through the FGFR1c receptor.

Conclusions

With importance especially to the emerging field of DNA-based therapeutics, we have shown here that proteoglycan synthesis, in different cell lines where GAG profiles typically differ, can be directed by recombinant technology to produce populations of bioactive recombinants with highly similar GAG profiles.

Microfibrils at basement membrane zones interact with perlecan via fibrillin-1

Mutational defects in fibrillin-rich microfibrils give rise to a number of heritable connective tissue disorders, generally termed microfibrillopathies. To understand the pathogenesis of these microfibrillopathies, it is important to elucidate the supramolecular composition of microfibrils and their interaction properties with extracellular matrix components. Here we demonstrate that the proteoglycan perlecan is an associated component of microfibrils typically close to basement membrane zones. Double immunofluorescence studies demonstrate colocalization of fibrillin-1, the major backbone component of microfibrils, with perlecan in fibroblast cultures as well as in dermal and ocular tissues. Double immunogold labeling further confirms colocalization of perlecan to microfibrils in various tissues at the ultrastructural level. Extraction studies revealed that perlecan is not covalently associated with microfibrils. High affinity interactions between fibrillin-1 and perlecan were found by kinetic binding studies with dissociation constants in the low nanomolar range. A detailed mapping study of the interaction epitopes by solid phase binding assays primarily revealed interactions of perlecan domains I and II with a central region of fibrillin-1. Analysis of perlecan null embryos showed less microfibrils at the dermal-epidermal junction as compared with wild-type littermates. The data presented indicate a functional significance for perlecan in anchoring microfibrils to basement membranes and in the biogenesis of microfibrils.

Extracellular matrices of the avian ovarian follicle. Molecular characterization of chicken perlecan

In egg-laying species, such as the chicken, the mode of transport of lipoprotein particles from the capillary plasma to endocytic receptors on the oocyte surface is largely unknown. Here we show by molecular characterization that the large prominent heparan sulfate proteoglycan of extracellular matrices, termed perlecan or HSPG2 (the product of the hspg2 gene), is a component of ovarian follicles that may participate in this process. However, although normally a major HSPG of basement membranes or basal laminae, in chicken follicles, perlecan is absent from the membranous structure between the theca interna and granulosa cell layers, which to date has been considered a bona fide basement membrane. Rather, the protein is localized in the extracellular matrix of theca externa cells, which produce this HSPG. Furthermore, in chicken testes, perlecan is localized in the peritubular spaces but in less organized fashion than the classical basement membrane components, agrin and laminin. All five domains and structural hallmarks of chicken perlecan (4071 residues) have been conserved in its mammalian counterparts. We have produced the recombinant domain II (containing low density lipoprotein (LDL) receptor-like binding repeats) of chicken perlecan and demonstrate its capacity to bind LDL and very low density lipoprotein (VLDL), apolipoprotein B-containing lipoproteins ultimately destined for uptake into oocytes via members of the low density lipoprotein receptor family. Binding to perlecan heparan sulfate side chains may facilitate the interaction of lipoproteins with domain II. Based on the current results and on domain-domain interactions revealed by recent ultrastructural investigations of the LDL receptor, nidogen, and laminin (Rudenko, G., Henry, L., Henderson, K., Ichtchenko, K., Brown, M. S., Goldstein, J. L., and Deisenhofer, J. (2002) Science 298, 2353-2358 and Takagi, J., Yang, Y., Liu, J. H., Wang, J. H., and Springer, T. A. (2003) Nature 424, 969-974), we propose a novel role of perlecan in mediating plasma-to-oocyte surface transport of VLDL particles.

Perlecan and tumor angiogenesis

Perlecan is a major heparan sulfate proteoglycan (HSPG) of basement membranes (BMs) and connective tissues. The core protein of perlecan is divided into five domains based on sequence homology to other known proteins. Commonly, the N-terminal domain I of mammalian perlecan is substituted with three HS chains that can bind a number of matrix molecules, cytokines, and growth factors. Perlecan is essential for metazoan life, as shown by genetic manipulations of nematodes, insects, and mice. There are also known human mutations that can be lethal. In vertebrates, new functions of perlecan emerged with the acquisition of a closed vascular system and skeletal connective tissues. Many of perlecan's functions may be related to the binding and presentation of growth factors to high-affinity tyrosine kinase (TK) receptors. Data are accumulating, as discussed here, that similar growth factor-mediated processes may have unwanted promoting effects on tumor cell proliferation and tumor angiogenesis. Understanding of these attributes at the molecular level may offer opportunities for therapeutic intervention.

Accumulation of heparan sulfate proteoglycans in cerebellar senile plaques

Alzheimer's disease (AD) brains are characterized by the presence of senile plaques (SPs), which primarily consist of amyloid beta protein (Abeta). Besides Abeta, several other proteins with the ability to modulate amyloid fibril formation accumulate in SPs, e.g. heparan sulfate proteoglycans (HSPGs). Cerebellar SPs are predominantly of the diffuse type, whereas fibrillar SPs are rarely observed. Furthermore, because of the spatial separation of non-fibrillar and fibrillar SPs in the cerebellum, this brain region provides a model for the study of the association of Abeta-associated factors with various stages of SP formation. In the present study, we performed an immunohistochemical analysis to investigate the expression of the HSPG species agrin, perlecan, glypican-1 and the syndecans 1-3 as well as glycosaminoglycan side-chains in cerebellar SPs. We demonstrated that agrin and glypican-1 were expressed in both non-fibrillar and fibrillar cerebellar SPs, whereas the syndecans were only associated with fibrillar cerebellar SPs. Perlecan expression was absent in all cerebellar SPs. Since fibrillar and non-fibrillar SPs may develop independently in the cerebellum, it is likely that agrin, glypican-1 as well as heparan sulfate glycosaminoglycans may contribute to the formation of both cerebellar plaque types, whereas syndecan only seems to play a role in the generation of cerebellar fibrillar plaques.

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.

Epidermal transformation leads to increased perlecan synthesis with heparin-binding-growth-factor affinity

Perlecan, a proteoglycan of basement membrane and extracellular matrices, has important roles in both normal biological and pathological processes. As a result of its ability to store and protect growth factors, perlecan may have crucial roles in tumour-cell growth and invasion. Since the biological functions of different types of glycosaminoglycan vary with cellular origin and structural modifications, we analysed the expression and biological functions of perlecan produced by a normal epidermal cell line (JB6) and its transformed counterpart (RT101). Expression of perlecan in tumorigenic cells was significantly increased in both mRNA and protein levels. JB6 perlecan was exclusively substituted with heparan sulphate, whereas that of RT101 contained some additional chondroitin sulphate. Detailed structural analysis of the heparan sulphate (HS) chains from perlecan of both cell types revealed that their overall sulphation and chain length were similar (approximately 60 kDa), but the HS chains of tumour-cell-derived perlecan were less sulphated. This resulted from reduced 2-O- and 6-O-sulphation, but not N-sulphation, and an increase in the proportion of unsulphated disaccharides. Despite this, the heparan sulphate of RT101- and JB6-derived perlecan bound fibroblast growth factor-1, -2, -4 and -7 and heparin-binding epidermal growth factor with similar affinity. Therefore abundant tumour-derived perlecan may support the angiogenic responses seen in vivo and be a key player in tumorigenesis.

Perlecan domain V of Drosophila melanogaster. Sequence, recombinant analysis and tissue expression

The C-terminal domain V of the basement membrane proteoglycan perlecan was previously shown to play a major role in extracellular matrix and cell interactions. A homologous sequence of 708 amino-acid residues from Drosophila has now been shown to be 33% identical to mouse perlecan domain V. It consists of three laminin G-type (LG) and epidermal growth factor-like (EG) modules but lacks the EG3 module and a link region found in mammalian perlecans. Recombinant production of Drosophila perlecan domain V in mammalian cells yielded a 100-kDa protein which was folded into a linear array of three globular LG domains. Unlike the mouse counterpart, domain V from Drosophila was not modified by glycosaminoglycans and endogenous proteolysis, due to the absence of the link region. It showed moderate affinities for heparin and sulfatides but did not bind to chick alpha-dystroglycan or to various mammalian basement membrane proteins. A single RGD sequence in LG3 of Drosophila domain V was also incapable of mediating cell adhesion. Production of a proteoglycan form of perlecan (approximately 450 kDa) in one Drosophila cell line could be demonstrated by immunoblotting with antibodies against Drosophila domain V. A strong expression was also found by in situ hybridization and immunohistology at various stages of embryonic development and expression was localized to several basement membrane zones. This indicates, as for mammalian species, a distinct role of perlecan during Drosophila development.

Expression of perlecan proteoglycan in the infarct zone of mouse myocardial infarction

Perlecan, a basal lamina proteoglycan, has been shown to interact with other extracellular matrix (ECM) components, especially type IV collagen, and is thus involved in ECM formation. Perlecan has also been postulated to promote growth factor-receptor interactions, including the binding of basic fibroblast growth factor (bFGF) to its receptor, and to enhance mitogenesis and angiogenesis. To test our hypothesis that perlecan is increased in the myocardial infarct zone, we examined perlecan expression after experimentally induced myocardial infarction in BALb/c mice by the methods of in situ hybridization, Northern blotting, and immunohistochemistry. In situ hybridization revealed mRNA signals for perlecan in the infarct marginal zone on day 2 and in the infarct interior zone around infarct granulation tissue on day 7. On day 14 the signals were observed at the center point of the infarct. The signals were detected in spindle-shaped mesenchymal cells (fibroblasts and myofibroblasts). Some surviving myocytes in the infarct marginal zone also showed positive signals. The sequential changes in the perlecan mRNA signal distribution paralleled those for type IV collagen mRNA. Northern blotting demonstrated increased expression of perlecan consistent with the observations of in situ hybridization. Immunopositive staining for perlecan was observed in the infarct zone around granulation tissue on day 7 and in the entire infarct zone on days 14-28. Immunostaining for bFGF was localized surrounding the infarct granulation tissue on day 7 and overlapped with perlecan immunostaining. The present results demonstrated the expression of perlecan by spindle-shaped mesenchymal cells (fibroblasts and myofibroblasts) and some surviving myocytes in the myocardial infarct, indicating the contribution of perlecan to the pathological course of myocardial infarction.

Agrin is a major heparan sulfate proteoglycan accumulating in Alzheimer's disease brain

Heparan sulfate proteoglycans (HSPGs) have been suggested to play an important role in the formation and persistence of senile plaques and neurofibrillary tangles in dementia of the Alzheimer's type (DAT). We performed a comparative immunohistochemical analysis of the expression of the HSPGs agrin, perlecan, glypican-1, and syndecans 1-3 in the lesions of DAT brain neocortex and hippocampus. Using a panel of specific antibodies directed against the protein backbone of the various HSPG species and against the glycosaminoglycan (GAG) side-chains, we demonstrated the following. The basement membrane-associated HSPG, agrin, is widely expressed in senile plaques, neurofibrillary tangles and cerebral blood vessels, whereas the expression of the other basement membrane-associated HSPG, perlecan, is lacking in senile plaques and neurofibrillary tangles and is restricted to the cerebral vasculature. Glypican and three different syndecans, all cell membrane-associated HSPG species, are also expressed in senile plaques and neurofibrillary tangles, albeit at a lower frequency than agrin. Heparan sulfate GAG side chains are also associated with both senile plaques and neurofibrillary tangles. Our results suggest that glycosaminoglycan side chains of the HSPGs agrin, syndecan, and glypican, but not perlecan, may play an important role in the formation of both senile plaques and neurofibrillary tangles. In addition, we speculate that agrin, because it contains nine protease-inhibiting domains, may protect the protein aggregates in senile plaques and neurofibrillary tangles against extracellular proteolytic degradation, leading to the persistence of these deposits.

Structural basis of glycosaminoglycan modification and of heterotypic interactions of perlecan domain V

The C-terminal perlecan domain V of about 90 kDa consists of laminin-type G domain modules (LG) (25 kDa) and epidermal growth factor-like modules (EG) (4 kDa) in the tandem arrangement LG1-EG1-EG2-LG2-EG3-EG4-LG3. Several shorter fragments have been prepared by recombinant production in mammalian cells and used to map the single glycosaminoglycan (GAG) substitution site and the binding of several carbohydrate and protein ligands. This identified a Ser3511 residue located in a short link region between EG4 and LG3 as being involved in GAG attachment. Electron microscopy provided evidence that the same substitution exists in tissue forms of perlecan. Heparan sulphate attached to this site was shown to bind to the alpha1LG4 module of laminin-1, indicating a role in basement membrane assembly and cell-matrix interactions. This site is also close to an Asn-Asp bond which is readily cleaved by an endogenous protease that depends on the presence of Asp and the LG2 module. A weak heparin binding site was shown to include the EG2 module, which contains five basic residues. Binding to sulphatides and the alpha-dystroglycan receptor was much stronger and required at least two LG modules. However, single LG modules appear to be sufficient for the interaction with the laminin-nidogen complex, while EG3-4 and some flanking regions are apparently involved in fibulin-2 binding. These observations indicate that a complex modular structure is required for domain V in order to provide a rich repertoire of potential biological functions.

Human perlecan immunopurified from different endothelial cell sources has different adhesive properties for vascular cells

Perlecan, a major heparan sulfate proteoglycan of vascularized tissues, was immunopurified from media conditioned by human endothelial cells of both arterial and venous origin. The heparan sulfate moiety of perlecan from cultured arterial cells differed in amount and/or composition from that produced by a transformed cell line of venous origin. Both forms of perlecan bound basic fibroblast growth factor with Kd approximately 70 nM. In ELISA experiments, perlecan and its protein core bound to various extracellular matrix components in a manner that was strongly influenced by the format of the assay. Human vascular smooth muscle cells and human endothelial cells adhered to perlecan-coated surfaces, and both cell types adhered better to the venous cell-derived than to the arterial cell-derived perlecan. Removal of the heparan sulfate chains abolished this difference and increased the ability of both types of perlecan to adhere vascular cells. Denaturation of perlecan and its protein core also rendered each of them more adhesive, indicating the presence of conformation-independent adhesion determinants in the polypeptide sequence. Their location was investigated using recombinant perlecan domains. Overall, our results represent the first demonstration of human perlecan acting as an adhesive molecule for human vascular cells and suggest that it may play a role in vascular wound healing.

Expression of human perlecan domain I as a recombinant heparan sulfate proteoglycan with 20-kDa glycosaminoglycan chains

Recombinant forms of human perlecan domain I were secreted as proteoglycans by stably transfected human 293 cells. A recombinant domain I-only proteoglycan spanned the 95- to 265-kDa region in SDS-PAGE and appeared to be 160 kDa in denaturing gel filtration. Its glycosaminoglycan (GAG) content was approximately 67% heparan sulfate, and its average GAG chain size of 20 kDa suggested that the true molecular mass of the proteoglycan was 90 kDa. Domain I with enhanced green fluorescent protein fused to its C-terminus had an apparent molecular mass of 210-220 kDa and contained approximately 100% heparan sulfate. Its average GAG chain size (also 20 kDa) suggested a true molecular mass of 117 kDa for this proteoglycan. Its sulfate content of 53-77 mol SO2-4 per mole of protein indicated the presence of one sulfate group per 4-7 GAG sugar residues.

Recombinant domain IV of perlecan binds to nidogens, laminin-nidogen complex, fibronectin, fibulin-2 and heparin

Domain IV of mouse perlecan, which consists of 14 immunoglobulin superfamily (IG) modules, was prepared from recombinant human cell culture medium in the form of two fragments, IV-1 (IG2-9, 100 kDa) and IV-2 (IG10-15, 66 kDa). Both fragments bound to a heparin column, being eluted at ionic strengths either below (IV-2) or above (IV-1) physiological level, and could thus be readily purified. Electron microscopy demonstrated an elongated shape (20-25 nm), and folding into a native structure was indicated by immunological assay and CD spectroscopy. Solid-phase and surface plasmon resonance assays demonstrated strong binding of fragment IV-1 to fibronectin, nidogen-1, nidogen-2 and the laminin-1-nidogen-1 complex, with Kd values in the range 4-17 nM. The latter binding apparently occurs through nidogen-1, as shown by the formation of ternary complexes. Only moderate binding was observed for fibulin-2 and collagen IV and none for fibulin-1 and BM-40. Fragment IV-2 showed a more restricted pattern of binding, with only weaker binding to fibronectin and fibulin-2. None of these activities could be demonstrated for recombinant fragments corresponding to the N-terminal perlecan domains I to III. This indicates a special role for domain IV in the integration of perlecan into basement membranes and other extracellular structures via protein-protein interactions.

Matrix proteoglycans: from molecular design to cellular function

The proteoglycan superfamily now contains more than 30 full-time molecules that fulfill a variety of biological functions. Proteoglycans act as tissue organizers, influence cell growth and the maturation of specialized tissues, play a role as biological filters and modulate growth-factor activities, regulate collagen fibrillogenesis and skin tensile strength, affect tumor cell growth and invasion, and influence corneal transparency and neurite outgrowth. Additional roles, derived from studies of mutant animals, indicate that certain proteoglycans are essential to life whereas others might be redundant. The review focuses on the most recent genetic and molecular biological studies of the matrix proteoglycans, broadly defined as proteoglycans secreted into the pericellular matrix. Special emphasis is placed on the molecular organization of the protein core, the utilization of protein modules, the gene structure and transcriptional control, and the functional roles of the various proteoglycans. When possible, proteoglycans have been grouped into distinct gene families and subfamilies offering a simplified nomenclature based on their protein core design. The structure-function relationship of some paradigmatic proteoglycans is discussed in depth and novel aspects of their biology are examined.

Inhibition of glycosaminoglycan modification of perlecan domain I by site-directed mutagenesis changes protease sensitivity and laminin-1 binding activity

Glycosaminoglycan attachment to perlecan domain I (173 residues) was completely prevented by site-directed mutagenesis of Ser-65, Ser-71 and Ser-76 as shown by recombinant production in mammalian cells. This did not interfere with the proper folding of the domain's SEA module but enhanced its sensitivity to neutral proteases. Lack of substitution also abolished binding to the two major heparin binding sites of laminin-1.

Agrin is a major heparan sulfate proteoglycan in the human glomerular basement membrane

Agrin is a heparan sulfate proteoglycan (HSPG) that is highly concentrated in the synaptic basal lamina at the neuromuscular junction (NMJ). Agrin-like immunoreactivity is also detected outside the NMJ. Here we show that agrin is a major HSPG component of the human glomerular basement membrane (GBM). This is in addition to perlecan, a previously characterized HSPG of basement membranes. Antibodies against agrin and against an unidentified GBM HSPG produced a strong staining of the GBM and the NMJ, different from that observed with anti-perlecan antibodies. In addition, anti-agrin antisera recognized purified GBM HSPG and competed with an anti-GBM HSPG monoclonal antibody in ELISA. Furthermore, both antibodies recognized a molecule that migrated in SDS-PAGE as a smear and had a molecular mass of approximately 200-210 kD after deglycosylation. In immunoelectron microscopy, agrin showed a linear distribution along the GBM and was present throughout the width of the GBM. This was again different from perlecan, which was exclusively present on the endothelial side of the GBM and was distributed in a nonlinear manner. Quantitative ELISA showed that, compared with perlecan, the agrin-like GBM HSPG showed a sixfold higher molarity in crude glomerular extract. These results show that agrin is a major component of the GBM, indicating that it may play a role in renal ultrafiltration and cell matrix interaction. (J Histochem Cytochem 46:19-27, 1998)

The C-terminal domain V of perlecan promotes beta1 integrin-mediated cell adhesion, binds heparin, nidogen and fibulin-2 and can be modified by glycosaminoglycans

Domain V of the major basement-membrane proteoglycan perlecan, a domain which consists of three laminin type G (LG) and four epidermal-growth-factor-like (EG) modules, was obtained in recombinant form by transfecting embryonic kidney cells with an episomal expression vector. A major 90-kDa fragment V was obtained together with fragments Va (74 kDa) and Vb (26 kDa) which were generated by endogenous proteolysis in front of the most C-terminal LG module. All three fragments bound to a heparin affinity column and could be displaced at a moderate (0.2 M) NaCl concentration. Rotary-shadowing electron microscopy demonstrated a three-globule structure for fragment V. Fragment V also showed a strong immunological cross-reaction with tissue-derived perlecan, indicating that it was folded into a native structure. A further, larger fragment, Vc, was apparently substituted with heparan sulphate and/or chondroitin sulphate chains and failed to bind to heparin. Fragment V but not fragment Vc promoted a distinct adhesion of several cell lines and this could be blocked by antibodies against the integrin beta1 chain. This domain may, however, represent only one of several cell-adhesive sites of perlecan. The recombinant perlecan fragment V bound in surface plasmon resonance assays to fibulin-2, laminin-nidogen complex, nidogen and two nidogen fragments. This indicated two different nidogen-binding epitopes on perlecan domain V with about a 10-fold difference in their affinities (Kd = 0.05-0.2 microM and about 2 microM). Perlecan domain V therefore seems to participate in the supramolecular assembly and cell connections of basement membranes.

Developmental expression of perlecan during murine embryogenesis

Perlecan is a modular heparan sulfate proteoglycan that is an intrinsic constituent of all basement membranes and extracellular matrices. Because of its strategic position and unique structure, perlecan has been implicated in modulating the activity of various growth factors required for normal development and tissue homeostasis. To gain insights into the potential function of perlecan in vivo, we examined the spatiotemporal distribution of its mRNA and protein core during murine embryogenesis. We utilized a new affinity-purified antibody that recognizes specifically the protein core of perlecan together with an in situ RT-PCR approach to perform a systematic analysis of perlecan expression and deposition during murine ontogeny. Perlecan appeared early (E10.5) in tissues of vasculogenesis including heart, pericardium, and major blood vessels. Its early expression coincided with the development of the cardiovascular system. Subsequently (E11-13), the greatest deposition of perlecan occurred within the developing cartilage, especially the cartilage undergoing endochondral ossification, where it remained elevated throughout all the developmental stages, and up to adulthood. Interestingly, the mRNA levels of perlecan were always higher in all the vascularized tissues, principally within endothelial cells, while chondrocytes displayed relatively low mRNA levels. This suggests a higher biosynthesis and turnover rates in the blood vessels vis-à-vis those of cartilaginous and other mesenchymal tissues. During later stages of development (E13-17.5) perlecan mRNA levels progressively increased and its expression correlated with the onset of tissue differentiation of various parenchymal organs including the developing kidneys, lungs, liver, spleen, and gastrointestinal tract. The central nervous system showed no perlecan expression with the exception of the calvaria and choroid plexus. Collectively, the results indicate that perlecan may play crucial roles not only in vasculogenesis but also in the maturation and maintenance of differentiated tissues, including cartilage.

Evidence for the existence of multiple heparan sulfate proteoglycans in the human glomerular basement membrane and mesangial matrix

Heparan sulfate proteoglycans (HSPGs) are essential components of the glomerular basement membrane (GBM) carrying a strong anionic charge. A well-characterized extracellular HSPG is perlecan, ubiquitously expressed in basement membranes. A cDNA construct encoding domains I and II of human perlecan was expressed as a fusion protein with glutathione S-transferase. This fusion protein was used to generate monoclonal antibody 95J10. We compared the staining pattern of 95J10 with that of M215, a previously prepared mAb that recognizes HSPG isolated from human GBM. In kidney cortex, the anti-perlecan mAb 95J10 showed a strong staining of the mesangium, Bowman's capsule, the tubular basement membrane, and stained the GBM only slightly. In contrast, M215 predominantly stained the GBM in a linear fashion. Immunoelectron microscopy supported these results, showing concentrations of perlecan in some regions of the GBM, whereas the unidentified M215 antigen was homogenously distributed throughout the GBM. In other human tissues, both antibodies also produced a different staining pattern. Furthermore, a polyclonal antiserum recognizing HSPG isolated from the GBM did not recognize perlecan from EHS tumors. These results provide evidence for the presence of another HSPG in the GBM that is immunologically distinct from perlecan. The absence of perlecan splice variants in the kidney suggests that this component is encoded by a different gene than perlecan. Given its marked expression in the GBM, this component could be a determining factor in the maintenance of selective glomerular permeability.