No abstract

Dermatopontin is a widely distributed small molecular weight protein in the extracellular matrix (ECM) and today its homologues are known in five mammals and several invertebrates. The structures of these homologues are relatively well conserved among the species. In the skin, dermatopontin is located mainly on the surface of the collagen fibers. It is found in the conditioned medium and also in the cytoplasm of cultured fibroblasts. Early studies focused on ECM assembly (collagen fibrillogenesis) and interactions (with the proteoglycan decorin). Subsequently, a targeted disruption of dermatopontin resulted in a phenotype similar to Ehlers-Danlos syndrome. In addition, a cell adhesion activity of this protein for dermal fibroblasts and several other cells was found, and this activity might suggest this protein's involvement in wound healing. The expression of dermatopontin around an infarct zone of experimental myocardial infarction may support this possibility. In invertebrates, dermatopontin homologues act mainly as adhesion/agglutination molecules. In addition, we found that transforming growth factor-beta1 interacts with dermatopontin and the function of this cytokine is modified by dermatopontin. Recently, the involvement of this protein in cell proliferation has been indicated. In this review we describe the reported functions of this protein and speculate on the multiple roles of this largely uncharacterized matrix molecule.

Nidogen was purified from a mouse tumor basement membrane where it accounted for 2 -3 of the total proteins. It was isolated as two forms (A and B) of a monomer (Mr = 80000) each consisting of a single polypeptide chain folded into a globular head connected to a small tail. The B form of the monomer was shown to be capable of aggregating into a nest-like structure (Mr > 250000). A smaller form (M, = 45000) was observed in some of the extracts. The amino acid composition of nidogen was different to that of other basement membrane proteins. It contained about 10 % carbohydrate, with N-linked and 0-linked oligosaccharide chains in similar proportions.Isoelectrofocussing demonstrated a limited heterogeneity of nidogen with PI in the range 6.5 -7. Monomeric nidogen failed to interact with other basement membrane components and heparin. Aggregation could be induced by limited proteolysis and was reversed by detergents or high salt concentrations. Together with the observation that most of the nidogen could be solubilized only after destroying the collagenous matrix, the data indicate that aggregation of nidogen reflects an activity involved in matrix assembly. Specific antibodies raised against nidogen did not distinguish between the monomeric and aggregated form of the protein but showed that the fragment was antigenically deficient. These antibodies did not cross-react with collagen type IV, laminin, entactin and heparansulfate proteoglycan. Immunofluorescence staining and absorption studies demonstrated that nidogen is a common component of authentic basement membranes. Larger forms of nidogen (Mr about 100000 and 150000) were found in organ cultures of Reichert's membrane suggesting that it is synthesized in precursor forms.Basement membranes are extracellular sheets of proteins in close vicinity to epithelial, endothelial, muscle and other cells of the body. They separate these cells from underlying connective tissue and serve as major filtration barriers and for maintaining a proper tissue organization [1,2]. Major components of basement membranes are collagen type IV [3], laminin [4] and heparansulfate proteoglycan [5, 61. The collagenous proteins are responsible for the mechanical stability of these matrices and are integrated in a kind of a network using their terminal domains for self-assembly [7]. Laminin and proteoglycan have the ability to interact with each other and presumably mediate binding of cells to the network [2,8,9]. The precise architecture of these extracellular structures has not yet been elucidated. The difficulties of structural studies are due in part to the complexity and extraordinary size (length 100 -400 nm) of the known components, but also to the incomplete knowledge of additional constituents which, although minor by weight, may nevertheless be important for the structural integrity and function of basement membranes.A considerable diversity of basement membrane components was indicated in previous immunological studies [3,10] and in electrophoretic analyses of glomerular and other...

A 450-kDa human epidermal autoantigen was originally identified as a protein that reacted with the serum from an individual with a subepidermal blistering disease. Molecular cloning of this protein has now shown that it contains 5065 amino acids and has a molecular mass of 552 kDa. As reported previously this protein, which we call epiplakin, belongs to the plakin family, but it has some very unusual features. Epiplakin has 13 domains that are homologous to the B domain in the COOH-terminal region of desmoplakin. The last five of these B domains, together with their associated linker regions, are particularly strongly conserved. However, epiplakin lacks a coiled-coil rod domain and an aminoterminal domain, both of which are found in all other known members of the plakin family. Furthermore, no dimerization motif was found in the sequence. Thus, it is likely that epiplakin exists in vivo as a single-chain structure. Epitope mapping experiments showed that the original patient's serum recognized a sequence unique to epiplakin, which was not found in plectin. Immunofluorescence staining revealed the presence of epiplakin in whole sheets of epidermis and esophagus, in glandular cells of eccrine sweat and parotid glands and in mucous epithelial cells in the stomach and colon.Clarification of the basic structure of desmoplakin has been followed by the identification of many related proteins, such as BPAG1, 1 plectin, envoplakin, and periplakin (1-7). These proteins form a family known as the "plakin family" (8). Almost all members of this family have a common structure, with predicted globular amino-terminal and COOH-terminal domains that are separated by a central rod domain. Some homologous domain structures have been identified in both globular domains of many plakins, while the central domain is rich in heptad repeats and is believed to form a parallel ␣-helical coiled-coil structure with a dimerization partner (9). As suggested by this model, it has been demonstrated that desmoplakin I can form homodimers in vitro (10). Early investigations revealed that the COOH-terminal domains of plakins are involved in binding to intermediate filaments (11)(12)(13). The amino-terminal domains of desmoplakin and of BPAG1 are believed to bind to desmosomes or hemidesmosomes. Furthermore, some splicing variants of plectin and BPAG1 have actinor microtubule-binding domains at their amino termini, and it has been proposed that these domains form cross-links between microfilaments and/or microtubules and intermediate filaments (14 -16). Studies of a few inheritable diseases that appear to involve plectin or desmoplakin and of a BPAG1 null mouse have shown that each plakin plays a critical role in the tissue integrity in specific tissues (5,(17)(18)(19). Moreover, it seems likely that, in many autoimmune blistering diseases, plakins, located in the epidermis, might be target antigens, and these plakins are used for markers of specific diseases (2, 20 -22). However, their pathological roles remain to be clarified. Several years ago ...

Various forms of heparan sulfate proteoglycan were solubilized from the mouse Engelbreth-Holm-Swarm (EHS) sarcoma by extraction with 0.5 M NaCl, collagenase digestion and extraction with 4 M guanidine. They could be separated into high (3 1.65 giml) and low (1.38 g/ml) buoyant density variants. The high-density form from the NaCl extract and collagenase digest had M,= 130000 and s&,, =4.5 S and contained 4-10% protein, indicating M , = 5000 -12000 for the protein core. This proteoglycan exhibited polydispersity as shown by rotary shadowing electron microscopy and ultracentrifugation. An average molecule consisted of four heparan sulfate chains (M,=29000) each with a length of 32flOnm. The low-density form ( M , about 400000) could not be completely purified and contained about 50 % protein. As shown by radioimmunoassay, the various proteoglycans shared similar protein cores. Labeling of the tumor in vivo or in vitro demonstrated preferential incorporation of radioactive sulfate in the high-density form.The high-density proteoglycan interacted in affinity chromatography by virtue of its heparan sulfate chains with laminin, fibronectin, the globular domain NCI and the triple helix of collagen IV. These interactions were abolished at moderate concentrations of NaCl (0.1 -0.2 M) and in the presence of heparin, chondroitin sulfate or dextran sulfate. Interactions with the globule NCI could also be demonstrated by velocity band centrifugation in sucrose ~ was derived. gradients and a binding constant of about lo6 M Heparan sulfate proteoglycans are a special class of acidic glycoconjugates occurring predominantly in the extracellular matrix [I, 21. They are prominent components of basement membranes where they show a regular and restricted distribution [3,4]. Proteoglycans in these matrices can serve as selective filtration barriers [5] and have the potential to interact with other matrix components such as laminin [6,7] or procollagen IV [8]. Aggregating forms of heparan sulfate are known [9] and may contribute to interactions between similar proteoglycan components [2]. Some of these molecular interactions may become stabilized by covalent cross-links as demonstrated for heparan sulfate proteoglycan from glomerular basement membrane which is connected to collagenous structures [lo]. This data suggests that proteoglycans have a central role in maintaining the structural integrity and biological activity of basement membranes.Current data indicate a considerable diversity of heparan sulfate proteoglycans in various tissues. The relative molecular mass of these components is often in the range 75000 -350000 as shown for material from hepatocytes [Ill and fibroblasts [12], respectively. Heparan sulfate proteoglycan with M , = 130 000 was detected in glomerular basement membranes [ 131 but smaller and larger variants have been described from the same source or other basement membranes [14,15]. Similar variations are found for proteoglycans synthesized by cultured cell lines [16 -201 including cells which also produc...