Visualization of the large heparan sulfate proteoglycan from basement membrane

Vascular basement membrane thickening in diabetic retinopathy

Vascular basement membrane (BM) thickening is a fundamental structural alteration of small blood vessels in diabetes. Over two decades of research has established hyperglycemia as the primary causal factor mediating this alteration. Various high glucose-induced mechanisms have been investigated and excess synthesis of BM components has been identified as a major contributing factor to BM thickening. Although BM thickening has been long hailed as the histological hallmark of diabetic microangiopathy, the consequences of BM thickening on the functionality of target organs of diabetes remain elusive even today. This review presents an overview of our current understanding of the BM structure and function, and focuses on how capillary BM thickening develops, its effect on retinal vascular function, and potential strategies for preventing the development of BM thickening in diabetic retinopathy.

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.

Heparan sulfate proteoglycans in experimental models of diabetes: a role for perlecan in diabetes complications

Proteoglycans are ubiquitous extracellular proteins that serve a variety of functions throughout the organism. Unlike other glycoproteins, proteoglycans are classified based on the structure of the glycosaminoglycan carbohydrate chains, not the core proteins. Perlecan, a member of the heparan sulfate proteoglycan (HSPG) family, has been implicated in many complications of diabetes. Decreased levels of perlecan have been observed in the kidney and in other organs, both in patients with diabetes and in animal models. Perlecan has an important role in the maintenance of the glomerular filtration barrier. Decreased perlecan in the glomerular basement membrane has a central role in the development of diabetic albuminuria. The involvement of this proteoglycan in diabetic complications and the possible mechanisms underlying such a role have been addressed using a variety of models. Due to the importance of nephropathy among diabetic patients most of the studies conducted so far relate to diabetes effects on perlecan in different types of kidney cells. The various diabetic models used have provided information on some of the mechanisms underlying perlecan's role in diabetes as well as on possible factors affecting its regulation. However, many other aspects of perlecan metabolism still await full elucidation. The present review provides a description of the models that have been used to study HSPG and in particular perlecan metabolism in diabetes and some of the factors that have been found to be important in the regulation of perlecan.

A beta amyloidogenesis: unique, or variation on a systemic theme?

For more than a century amyloid was considered to be an interesting, unique, but inconsequential pathologic entity that rarely caused significant clinical problems. We now recognize that amyloid is not one entity. In vivo it is a uniform organization of a disease, or process, specific protein co-deposited with a set of common structural components. Amyloid has been implicated in the pathogenesis of diseases affecting millions of patients. These range from Alzheimer's disease, adult-onset diabetes, consequences of prolonged renal dialysis, to the historically recognized systemic forms associated with inflammation and plasma cell disturbances. Strong evidence is emerging that even when deposited in local organ sites significant physiologic effects may ensue. With emphasis on A beta amyloid, we review the present definition, classification, and general in vivo pathogenetic events believed to be involved in the deposition of amyloids. This encompasses the need for an adequate amyloid precursor protein pool, whether precursor proteolysis is required prior to deposition, amyloidogenic amino acid sequences, fibrillogenic nucleating particles, and an in vivo microenvironment conducive to fibrillogenesis. The latter includes several components that seem to be part of all amyloids. The role these common components may play in amyloid accumulation, why amyloids tend to be associated with basement membranes, and how one may use these findings for anti-amyloid therapeutic strategies is also examined.

Identification of sites in domain I of perlecan that regulate heparan sulfate synthesis

Perlecan is primarily a heparan sulfate containing proteoglycan found in all basement membranes. Rotary shadowed images of perlecan show it to contain three glycosaminoglycan (GAG) side chains extending from one end of its core protein. Domain I is at the N terminus of perlecan and contains three closely spaced Ser-Gly-Asp sequences that may serve in GAG attachment. We evaluated the serines in these three sequences for GAG attachment by preparing a cDNA construct encoding for the N-terminal half (domains I, II, and III) of perlecan and then a series of constructs containing deletions and mutations within domain I of the domain I/II/III construct, expressing these constructs in COS-7 cells, and then analyzing the recombinant product for GAG side chains and GAG type. The results showed that all three serine residues in the Ser-Gly-Asp sequences in domain I can accept both chondroitin and heparan sulfate side chains but that a cluster of acidic residues N-terminal to these sequences is the primary determinant responsible for targeting these sites for heparan sulfate. Furthermore, there are two elements that can enhance heparan sulfate synthesis at a targeted site: 1) the presence of a the SEA module in the C-terminal region of domain I and 2) the presence of multiple acceptors in close proximity. These results indicate that the proportion of heparan and chondroitin sulfate at any one site in domain I of perlecan is regulated by multiple factors.

The biochemistry of cancer dissemination

The progression of a tumor cell from one of benign delimited proliferation to invasive and metastatic growth is the major cause of poor clinical outcome of cancer patients. Recent research has revealed that this complex process requires many components for successful dissemination and growth of the tumor cell at secondary sites. These include angiogenesis, enhanced extracellular matrix degradation via tumor and host-secreted proteases, tumor cell migration, and modulation of tumor cell adhesion. Each individual component is multifaceted and is discussed within this review with respect to historical and recent findings. The identification of components and their interrelationship have yielded new therapeutic targets leading to the development of agents that may prove effective in the treatment of cancer and its metastatic progression.

Structural and cell-adhesive properties of three recombinant fragments derived from perlecan domain III

Domain III of the basement membrane proteoglycan perlecan was produced as three overlapping fragments in stably transfected mammalian cell clones. These recombinant fragments (43-48 kDa) were obtained in purified form and showed complete immunological cross-reactivity with perlecan, indicating their native structure. Rotary shadowing electron microscopy of each fragment demonstrated a small globular structure connected to a short rod. These data were interpreted to indicate that domain III has an elongated shape of 30 nm in length and consists of alternating globular domains (L4 modules) and short connecting segments attributed to tandem arrays of LE (laminin-type of EGF-like) modules which form rod-like segments in laminins. Sequence analyses of pepsin fragments were consistent with the disulfide-bonding patterns known for these modules from studies with laminin fragments, but two additional disulfide loops were also identified. Several cell lines which attached to mouse perlecan and/or human fibronectin failed to adhere to the domain III fragments, despite the fact that one of them contained an RGD (Arg-Gly-Asp) site in the L4 module. Furthermore, no significant binding was observed in solid phase binding assays with alpha 5 beta 1 and alpha v beta 3 integrins underscoring the low activity or accessibility of the RGD site.

Recombinant domain III of perlecan promotes cell attachment through its RGDS sequence

Perlecan has been previously been shown to support attachment of a wide variety of cells through interactions of its core protein with the cell surface. The core protein domains involved in cell adhesion are, however, unknown. The laminin-like domain III of murine perlecan contains an RGDS sequence and is a likely candidate for supporting integrin-mediated cell attachment. We made a cDNA construct corresponding to domain III and containing an in frame signal peptide at the 5' end as well as in frame a stop codon at the 3' end by using cDNA clones to perlecan. The construct was inserted into the pRC/CMV vector and transfected into HT1080 cells, and the secreted recombinant domain III, a 130-kDa protein, was purified from the medium. The size of proteolytic fragments produced by digestion with V8 protease as well as analysis of the rotary shadowed image of the recombinant protein indicated it was produced in a native conformation. Recombinant domain III coated on tissue culture dishes, supports adhesion of an epithelial-like mouse mammary tumor cell line MMT 060562 in a dose-dependent manner. This interaction was inhibited specifically by the RGDS synthetic peptide and intact perlecan, but not laminin. This domain III RGD-dependent cell attachment activity indicates a role for perlecan in integrin-mediated signaling.

Distribution of heparan sulfate proteoglycans in embryonic chicken neural retina and isolated inner limiting membrane

Quantitative distribution of proteoglycans was studied in retinal neural epithelium and its basement membrane (inner limiting membrane). Heparan sulfate proteoglycans (HSPGs) were primarily associated with both inner and outer plexiform (synaptic) layers, and inner limiting membrane (ILM), as determined by autoradiographs of lyase-digested cryosections. Based on distribution of 35S-sulfate-labeled proteoglycans, the isolated ILM contained on average approximately three fourths of its proteoglycans as HSPGs and one fourth as chondroitin sulfate/dermatan sulfate proteoglycans (CS/DSPGs), whereas the remaining retina contained approximately equal amounts of the two proteoglycans (PGs). Immunohistochemical staining indicates that the core proteins of the HSPGs in the ILM are distinct from those of the plexiform layers. The photoreceptor layer, which other studies have shown to contain much of the extracellular CS/DSPGs, was not examined. Enrichment of distinct HSPGs in the ILM and plexiform layers support the conclusion that the HSPGs may be intimately involved in the different developmental events characterizing the two regions: development and extension of ganglion cell axons in the former, synaptogenesis and neuronal function in the latter.

Perlecan: a gem of a proteoglycan

Perlecan, the main proteoglycan of basement membranes and pericellular spaces, is one of the largest single-chain polypeptides of vertebrate animals. The five modules of perlecan are collated from protein building blocks evolutionarily related to molecules involved in nutrient metabolism, mitogenesis and adhesion. These structural motifs, when translated into multimeric functional units, could be effectively utilized by diverse tissues during development, remodelling or neoplastic growth. The protein is highly conserved across species and the available data indicate that this modular proteoglycan has evolved from ancient ancestors by gene duplication and exon shuffling. The discovery of a related molecule in the nematode C. elegans, and the development of skeletal muscle abnormalities in this animal when the perlecan-like molecule is truncated, opens new avenues of research.

Proteoglycans of basement membranes

Proteoglycans carrying either heparan sulfate and/or chondroitin sulfate side chains are typical constituents of basement membranes. The most prominent proteoglycan (perlecan) consists of a 400-500 kDa core protein and three heparan sulfate chains. Electron microscopy and cDNA sequencing show a complex and elongated domain structure for the core protein which in part is homologous to that of the laminin A chain. This structure may be varied by alternative splicing and proteolysis. Integration into basement membranes probably occurs by heparan sulfate binding to laminin and collagen IV, core protein binding to nidogen and by limited self assembly. The proteoglycan is in addition a cell-adhesive protein which is recognized by beta 1 integrins. Several more proteoglycans with smaller core proteins (10-160 kDa) apparently exist in basement membranes but are less well characterized. Biological functions include control of filtration through basement membranes and binding of growth factors and protease inhibitors.

Proteoglycans of basement membranes

Proteoglycans carrying either heparan sulfate and/or chondroitin sulfate side chains are typical constituents of basement membranes. The most prominent proteoglycan (perlecan) consists of a 400-500 kDa core protein and three heparan sulfate chains. Electron microscopy and cDNA sequencing show a complex and elongated domain structure for the core protein which in part is homologous to that of the laminin A chain. This structure may be varied by alternative splicing and proteolysis. Integration into basement membranes probably occurs by heparan sulfate binding to laminin and collagen IV, core protein binding to nidogen and by limited self assembly. The proteoglycan is in addition a cell-adhesive protein which is recognized by beta 1 integrins. Several more proteoglycans with smaller core proteins (10-160 kDa) apparently exist in basement membranes but are less well characterized. Biological functions include control of filtration through basement membranes and binding of growth factors and protease inhibitors.

Purification of a basic fibroblast growth factor-binding proteoglycan from bovine cardiac plasma membrane

A heparan sulfate proteoglycan (HSPG) from bovine cardiac plasma membrane was purified to homogeneity using either isoelectric focusing or anion-exchange chromatography, followed by affinity chromatography on immobilized basic fibroblast growth factor (bFGF). Fractions were assayed for bFGF-binding activity using 125I-bFGF as a probe. Purified proteoglycan ran as a broad band on SDS-PAGE, spanning an apparent molecular mass range of 100-200 kDa, and could be incorporated into liposomes. Digestion of radioiodinated proteoglycan with heparitinase yielded a product of 73 kDa, while digestion with chondroitinase ABC did not change the apparent molecular mass. Monoclonal antibody directed against the ectodomain of another plasma membrane HSPG, syndecan, failed to recognize the purified cardiac proteoglycan on immunoblots. We conclude that adult bovine myocardium contains a membrane-associated bFGF-binding heparan sulfate proteoglycan containing little or no chondroitin sulfate and that this HSPG may be distinct from those of the syndecan family of heparan sulfate proteoglycans.

Perlecan, the large low-density proteoglycan of basement membranes: structure and variant forms

The complete primary structure of perlecan, the large low-density proteoglycan of basement membranes, has been deduced by cDNA cloning for the mouse and more recently the human gene products. Mouse perlecan contains a 396 kDa core protein with five distinct domains: a heparan sulfate attachment domain, a LDL receptor-like domain, two different laminin-like domains and an N-CAM-like domain. These domains are conserved to a striking degree between mouse and human, including alternate splicing of the N-CAM domain to generate variations of perlecan. These variant sequences also appear to be highly conserved between mouse and human. The strong conservation of these domains, including highly repetitive elements and potential alternative splices, suggest they have vital functions.

Ultracytochemical localization of basal lamina anionic sites in the rat epithelial attachment apparatus

The basal lamina anionic sites of the epithelial attachment apparatus (EAA) were investigated at the electron microscopic level in adult rat periodontium. After 1M NaCl junctional epithelium detachment, an irregular and fluffy basal lamina-like structure appeared to cover the cementum surface. This structure reacted positively with polyethyleneimine (PEI), a strongly cationized ultrastructural tracer, appearing to be composed of highly electron-dense microaggregates. Depending on section plane, double-tracked structures of undefined length were found within PEI precipitates and closely related to cementum collagen fibrils. After nitrous acid de-N-sulphation, 8 nm wide sets of two parallel lines were clearly identified. "Double tracks", i.e., sets of paired lines with peripherical PEI electron-dense material, were found to self-assemble to form dimers, clusters or more complex organizational patterns. From sensitivity towards nitrous acid oxidation and positive control observations, it was concluded that basal lamina anionic sites in the EAA, represented by PEI microaggregates, contain heparan sulfate proteoglycans (HSPGs). Furthermore, high resolution ultrastructural images demonstrated that HSPGs adopt a morphological appearance of "double tracks" in the tissue. On the other hand, the present findings suggest that HSPGs clusters, never found in the mucosal basement membrane used as positive control, may be related to a functional specificity of the tissue at the dento-gingival junction.

Immunolocalization of laminin, heparan-sulfate proteoglycan, entactin, and type IV collagen in the rat anterior pituitary. I. An in vivo study

The distribution of three components of basement membranes (BM): heparan-sulfate proteoglycan (HSPG), entactin (ENT), and type IV collagen (Coll. IV), was studied in the adult rat anterior pituitary and compared to the distribution of laminin (LAM) described in an earlier report (Vila-Porcile et al., 1987, J. Histochem. Cytochem., 35, 287). Several immunocytochemical methods were applied at both light and electron microscope levels. The three components were detected in all the pituitary BM and in endothelial and perivascular connective cells, as previously observed for LAM. In contrast to the prominent labeling previously observed for LAM within epithelial cells, however, only a faint immunoreaction could be detected for the other components, with nonetheless a discrete signal for Coll. IV. These findings indicate that 1) the four studied components are present in all the BM of the rat anterior pituitary; 2) pituitary endocrine cells contain LAM, and to a lesser extent Coll. IV, and thus could participate to the elaboration of the BM; and 3) the presence of the four components in non-epithelial cells also suggests their cooperation in BM building. The questions of the turnover and intracellular pathways of each of these components were addressed but remain unanswered.

Basement membrane proteins: structure, assembly, and cellular interactions

Basement membranes are thin layers of a specialized extracellular matrix that form the supporting structure on which epithelial and endothelial cells grow, and that surround muscle and fat cells and the Schwann cells of peripheral nerves. One common denominator is that they are always in close apposition to cells, and it has been well demonstrated that basement membranes do not only provide a mechanical support and divide tissues into compartments, but also influence cellular behavior. The major molecular constituents of basement membranes are collagen IV, laminin-entactin/nidogen complexes, and proteoglycans. Collagen IV provides a scaffold for the other structural macromolecules by forming a network via interactions between specialized N- and C-terminal domains. Laminin-entactin/nidogen complexes self-associate into less-ordered aggregates. These two molecular assemblies appear to be interconnected, presumably via binding sites on the entactin/nidogen molecule. In addition, proteoglycans are anchored into the membrane by an unknown mechanism, providing clusters of negatively charged groups. Specialization of different basement membranes is achieved through the presence of tissue-specific isoforms of laminin and collagen IV and of particular proteoglycan populations, by differences in assembly between different membranes, and by the presence of accessory proteins in some specialized basement membranes. Many cellular responses to basement membrane proteins are mediated by members of the integrin class of transmembrane receptors. On the intracellular side some of these signals are transmitted to the cytoskeleton, and result in an influence on cellular behavior with respect to adhesion, shape, migration, proliferation, and differentiation. Phosphorylation of integrins plays a role in modulating their activity, and they may therefore be a part of a more complex signaling system.

Effects of methylprednisolone on glomerular and medullary mRNA levels for extracellular matrices in puromycin aminonucleoside nephrosis

We examined the effects of methylprednisolone (MPSL) on type IV collagen, laminin and heparan sulfate proteoglycan (HSPG) mRNA levels in the renal glomeruli and medulla of puromycin aminonucleoside (PAN) nephrosis. mRNA levels encoding for type IV collagen and laminin increased markedly, whereas those for HSPG decreased significantly in glomeruli of PAN nephrosis. Administration of MPSL partially ameliorated the abnormal gene expression for basement membrane components. Furthermore, we showed that medullary mRNA levels for all these basement membrane components decreased with age in PAN nephrosis with or without MPSL treatment, suggesting that neither PAN nor MPSL has any effect on basement membrane component mRNA levels in the renal medulla. In contrast, mRNA levels for the interstitial collagens including alpha 1 (I) and alpha 1 (III) chains in glomeruli showed little change with or without MPSL treatment, whereas those in medulla increased significantly in PAN nephrosis when compared with the control. MPSL ameliorated the abnormal gene expression of alpha 1 (I) and alpha 1 (III) collagen in renal medulla. These results indicate that PAN affects both glomerular mRNA encoding for basement membrane components and medullary mRNA encoding for interstitial collagens, and that MPSL has marked effects on the amelioration of abnormal gene expression in both glomeruli and medulla of PAN nephrosis.

Immunogold localization of basal laminar heparan sulfate proteoglycan in rat brain and retinal capillaries

Heparan sulfate proteoglycan was localized in the basal lamina of the brain and retinal capillaries, using an antibody to the core protein of the proteoglycan and a postembedding immunogold labeling method at the ultrastructural level. Gold particles appeared to be randomly distributed in the basal lamina of both types of capillaries, in contrast to discrete ruthenium red-staining sites near the endothelial plasma membrane, as shown in previous studies of the retinal vessel. The present study not only agrees with previous biochemical and immunofluorescence studies, but also provides a more precise localization of the antigen. Furthermore, the method can be useful for quantitative studies.

Matrix-associated heparan sulfate proteoglycan: core protein-specific monoclonal antibodies decorate the pericellular matrix of connective tissue cells and the stromal side of basement membranes

Cultured human lung fibroblasts produce a large, nonhydrophobic heparan sulfate proteoglycan that accumulates in the extracellular matrix of the monolayer (Heremans, A., J. J. Cassiman, H. Van den Berghe, and G. David. 1988. J. Biol. Chem. 263: 4731-4739). A panel of four monoclonal antibodies, specific for four distinct epitopes on the 400-kD core protein of this extracellular matrix heparan sulfate proteoglycan, detects similar proteoglycans in human epithelial cell cultures. Immunohistochemistry of human tissues with the monoclonal antibodies reveals that these proteoglycans are concentrated at cell-matrix interfaces. Immunogold labeling of ultracryosections of human skin indicates that the proteoglycan epitopes are nonhomogeneously distributed over the width of the basement membrane. Immunochemical investigations and amino acid sequence analysis indicate that the proteoglycan from the fibroblast matrix shares several structural features with the large, low density heparan sulfate proteoglycan isolated from the Engelbreth-Holm-Swarm sarcoma. Thus, both epithelial cell sheets and individual mesenchymal cells accumulate a large heparan sulfate proteoglycan(s) at the interface with the interstitial matrix, where the proteoglycan may adopt a specific topological orientation with respect to this matrix.

In situ hybridization reveals temporal and spatial changes in cellular expression of mRNA for a laminin receptor, laminin, and basement membrane (type IV) collagen in the developing kidney

The appearance of extracellular matrix molecules and their receptors represent key events in the differentiation of cells of the kidney. Steady-state mRNA levels for a laminin receptor, the laminin B1, B2, and A chains, and the alpha 1-chain of collagen IV (alpha 1[IV]), were examined in mouse kidneys at 16 d gestation and birth, when cell differentiation is active, and 1-3 wk after birth when this activity has subsided. Northern analysis revealed that mRNA expression of laminin receptor precedes the alpha 1(IV) and laminin B chains whereas laminin A chain mRNA expression was very low. In situ hybridization reflected this pattern and revealed the cells responsible for expression. At 16 d gestation, laminin receptor mRNA was elevated in cells of newly forming glomeruli and proximal and distal tubules of the nephrogenic zone located in the kidney cortex. These cells also expressed mRNA for alpha 1(IV) and laminin chains. At birth, mRNA expression of receptor and all chains remained high in glomeruli but was reduced in proximal and distal tubules. At 1 wk after birth, expression was located in the medulla over collecting ducts and loops of Henle. Little expression was detectable by 3 wk. These results suggest that cellular expression of steady-state mRNA for laminin receptor, laminin, and collagen IV is temporally linked, with laminin receptor expression proceeding first and thereafter subsiding.

Structure, composition, and assembly of basement membrane

Basement membranes are thin layers of matrix separating parenchymal cells from connective tissue. Their ultrastructure consists of a three-dimensional network of irregular, fuzzy strands referred to as "cords"; the cord thickness averages 3-4 nm. Immunostaining reveals that the cords are composed of at least five substances: collagen IV, laminin, heparan sulfate proteoglycan, entactin, and fibronectin. Collagen IV has been identified as a filament of variable thickness persisting after the other components have been removed by plasmin digestion or salt extraction. Heparan sulfate proteoglycan appears as sets of two parallel lines, referred to as "double tracks," which run at the surface of the cords. Laminin is detected in the cords as diffuse material within which thin wavy lines may be distinguished. The entactin and fibronectin present within the cords have not been identified as visible structures. The ability of laminin, heparan sulfate proteoglycan, fibronectin, and entactin to bind to collagen IV has been demonstrated by visualization with rotary shadowing and/or biochemical studies. Incubation of three of these substances-collagen IV, laminin (with small entactin contamination), and proteoglycan-at 35 degrees C for 1 hr resulted in a precipitate that was sectioned for electron microscopic examination and processed for gold immunolabeling for each of the three incubated substances. Three structures are present in the precipitate: 1) a lacework, exclusively composed of heparan sulfate proteoglycan in the form of two parallel lines, similar to double tracks; 2) semi-solid, irregular accumulations, composed of the three initial substances distributed on a cord network; and 3) convoluted sheets, which are also composed of the three initial substances distributed on a cord network but which, in addition, have the uniform appearance and thickness of the lamina densa of basement membrane. Hence these sheets are closely similar to the main component of authentic basement membranes.