Recombinant nidogen consists of three globular domains and mediates binding of laminin to collagen type IV

Laser scanning confocal examination and comparison of nidogen (entactin) with laminin in term human amniochorion

Indirect immunofluorescence confocal microscopy was used to compare the distribution of nidogen (entactin) and laminin in term human amniochorion. Rabbit antiserum raised against recombinant mouse nidogen was immunoreactive with components of the amniotic epithelial basement membrane, chorion laeve basement membrane and maternally derived capillary basement membranes. Overall anti-nidogen immunoreactivity co-distributes with laminin except in the extracellular matrix of the maternally derived decidua, the embryonically derived trophoblast and the apical surfaces of the amniotic epithelia. These differences lead us to suggest that nidogen and laminin may be independently expressed and are not always complexed together. Original observations include dramatic parallel nidogen rich striations in the decidual layer, the fact that the trophoblastic basement membrane appears to extend inwards forming a boundary layer to degenerate chorionic villi and the finding that the recently discovered collagen type IV rich microtrabeculae of the fibroblast layer also contain nidogen and laminin.

Formation of hemidesmosomes in cells of a transformed murine mammary tumor cell line and mechanisms involved in adherence of these cells to laminin and kalinin

Keratinocytes attach to an underlying basement membrane by adhesion junctions called hemidesmosomes. We have characterized a cell line, RAC-11P/SD, established from a murine mammary tumor, which differentiates into squamous epithelium and forms well defined hemidesmosomes. These hemidesmosomes contain the integrin alpha 6 beta 4 as well as the hemidesmosomal plaque proteins BP230 and HD1 and are associated with a matrix containing kalinin and laminin. We examined how these cells adhere to laminin and to kalinin present in matrices as well as immunopurified kalinin. We show that adhesion to laminin is energy dependent but does not require an intact actin-containing cytoskeleton. The affinity for kalinin proved to be greater and binding to kalinin was still observed when cells had been treated with deoxyglucose and azide to inhibit metabolic energy. Binding to laminin (or fragment E8), but not to kalinin was partially blocked by a monoclonal antibody specific for the integrin alpha 6 subunit, and only in the initial phase of adhesion. The antibody efficiently blocked adhesion to laminin of cells treated with the microfilament disrupting drug cytochalasin B, but only partially blocked the adhesion of cytochalasin B-treated cells to kalinin, while adherence of cells treated with deoxyglucose and azide to kalinin was blocked completely. The integrin alpha 6 beta 4 is redistributed to the basal surface during adhesion and then is organized into ring-like structures when cells are bound to laminin and localized into hemidesmosomes in cells adhered to kalinin. We suggest that anti-alpha 6 hinders the binding of the alpha 6 beta 4 integrins to its ligands laminin and kalinin, but cannot prevent adhesion when clustering of the integrin has become complete. In addition, there is evidence that adhesion to kalinin is mediated by a second receptor, which associates with the actin-containing cytoskeleton. The presence of such a second receptor is suggested because the cells can spread on kalinin, but not when they have been treated with cytochalasin B. On laminin spreading does not occur, irrespective of whether cells have been treated with cytochalasin B or not. The integrin alpha 3 beta 1, which has been identified as a receptor for kalinin but not for laminin, is strongly expressed in RAC-11P/SD cells and it seems likely that this integrin is responsible for spreading of cells on kalinin.

Sites of nidogen cleavage by proteases involved in tissue homeostasis and remodelling

The cleavage of recombinant mouse nidogen in its native form was examined with granule-stored proteases (leucocyte elastase, mast-cell chymase), blood proteases (thrombin, plasmin, kallikrein), matrix metalloproteinases (stromelysin, matrilysin, collagenases) and, for comparison, with trypsin and the endoproteinase Glu-C. More than 50 major cleavage sites were identified by Edman degradation of several large fragments and smaller peptides. The data show an almost exclusive localization of protease-sensitive sites to the flexible segment, connecting the N-terminal globular domains G1 and G2, and within the C-terminal, laminin-binding domain G3. Domains G1, G2 and the rod-like segment were much more stable against proteolysis. Kinetic analysis indicated a fast cleavage of several different sites in the link region followed by destruction of G3 but this was to some extent variable depending on the particular protease. Leucocyte elastase was identified as the most active protease in the cleavage of nidogen whilst stromelysin, matrilysin, plasmin and kallikrein were of distinctly lower activity. No cleavage could be detected with interstitial collagenase and gelatinase A. The peptide analyses also allowed the location of two disulfide bridges within the G3 domain. Complex formation between nidogen and laminin fragments caused some protection against cleavage by thrombin, leucocyte elastase and stromelysin particularly in domain G3. The data indicate a relatively uniform cleavage pattern of nidogen which may be relevant in the context of protein/ligand-binding activities associated with domains G2 and G3. The proteolytic processes involved in remodelling and the cellular penetration of basement membranes could therefore be essential for the modulation of the mediator function of nidogen.

Appearance and distribution of entactin in the early chick embryo

Entactin is a sulfated glycoprotein of basement membranes and recent data indicate that it may play a major role in extracellular matrix (ECM) assembly and in modulating the activities of the other molecular components. We investigated the time of appearance and subsequent distribution of entactin during the earliest stages of morphogenesis and its involvement in the first major cellular migrations and interactions in the chick embryo. Entactin is first detected in the epiblast and in the hypoblast at the blastula stage. The accumulating ECM displays intense presence of entactin in the space between the epiblast and the hypoblast at late blastula. Entactin is increasingly abundant in the neural plate and in the ECM and also at least transiently in many mesodermal tissues such as the notochord, the developing heart and somites in the early chick embryo. Immunogold labeling revealed a punctate pattern of entactin distribution in the ECM during the gastrula, neurula and at later stages and at all levels within the embryo. Because of its early appearance in more than one germ layer, entactin may be important in the formation of most embryonic structures. Entactin is detected at the same developmental time and co-localizes with laminin. Antibodies to entactin do not interfere with triggering of the first major cell movements but perturb directional migration of these cells. It would seem that entactin plays a functional role in the directed migration of cells and does not seem to affect cell adhesion during the period of the first morphogenetic events in the early chick embryo.

Sequence of extracellular mouse protein BM-90/fibulin and its calcium-dependent binding to other basement-membrane ligands

Partial sequence comparisons have recently indicated that two extracellular components, fibulin from human placenta and BM-90 from a basement-membrane-producing mouse tumor, are either identical or closely related proteins. In this study, a complete sequence analysis of mouse BM-90 cDNA showed a 539-residue N-terminal core structure (domains I and II), which was 85% identical with the same core structure of human fibulin. A 137-residue C-terminal sequence (domain III) was unique for BM-90 and could also be identified by Edman degradation. This suggested a novel splice product, variant D, which is characteristic for the mouse tumor. A second 117-residue C-terminal sequence (domain III) was identified in additional mouse cDNA clones and showed 91% identity with the region specific for variant C of fibulin. Northern blots using mouse cells demonstrated two mRNA species, 2.7 kb and 2.3 kb, which encoded the variants D and C, respectively. The sequence of BM-90/fibulin indicates the presence of nine epidermal-growth-factor-like repeats in the core domain-II structure, eight of which contain consensus motifs for calcium binding. This binding is apparently important for the interaction of BM-90 with laminin and nidogen and for some weaker interactions with collagen IV. Further binding of BM-90 was demonstrated to fibronectin and BM-90 itself, but did not depend on calcium. Major binding sites for BM-90 were identified at a C-terminal segment of laminin A chain and at the N-terminus of nidogen. The broad interaction repertoire of BM-90 is comparable to that of nidogen and both proteins may have similar roles as connecting elements in the extracellular matrix.

Structure and localization of O- and N-linked oligosaccharide chains on basement membrane protein nidogen

The carbohydrate content of mouse nidogen predicts the occupation of two N- and about seven O-linked acceptor sites. The corresponding oligosaccharides were examined by sequential exoglycosidase digestions. The data indicate N-linked substitutions by several bi-, tri- and tetraantennary complex types of oligosaccharides which are further modified by additional lactosamines and terminal alpha-galactose and/or sialic acid. Mannose-rich oligosaccharides were of low abundance. O-linked structures included a di- and tetrasaccharide core structure that were in addition sialylated and may be similar to structures found in fetuin. Evidence is provided that the two sequence-predicted asparagine acceptors are almost fully substituted. Sequence analysis of tryptic peptides identified Thr-271, Ser-303, Thr-309, Thr-317, Thr-320, Thr-892 and Thr-905 as the most likely sites for galactosamine substitutions. These residues are located in the flexible link connecting the N-terminal globular domains G1 and G2 of nidogen and at the border between the rod and the C-terminal globe G3. Four of them showed Pro in the -1 or +3 position. All these Ser, Thr and Pro residues but not the N-linked attachment sites are identical in human nidogen.

Ascidian entactin/nidogen. Implication of evolution by shuffling two kinds of cysteine-rich motifs

Entactin/nidogen, a major component of the basement membrane, has a domain structure comprising three globular domains, and thread-like and rod-like domains connecting them. It contains six epidermal-growth-factor-(EGF)-like motifs and one thyroglobulin-like motif. In the present study, ascidian entactin/nidogen has been identified by a monoclonal antibody technique. We prepared anti-(ascidian entactin/nidogen)IgG, named anti-AsEnt1, then cloned the cDNA of ascidian entactin/nidogen using anti-AsEnt1 as a probe, and determined its entire sequence. Mainly because the deduced amino acid sequence exhibited high similarity to mouse entactin and human nidogen, and because the antigen localized in basement membrane of ascidian body-wall muscle, we have concluded that the antigen anti-AsEnt1 corresponds to the ascidian entactin/nidogen homologue. The deduced amino acid sequence of ascidian entactin/nidogen clearly showed that the ascidian homologue also has a domain structure. However, the ascidian homologue lacked the thread-like domain, and the rod-like domain differed from that of mouse entactin in composition, consisting of two kinds of cysteine-rich motifs, that is, the EGF-like motif and the thyroglobulin-like motif. These results suggest that entactin/nidogen have evolved by modifying the domains, especially by shuffling the two kinds of cysteine-rich motifs.

The laminins: a family of basement membrane glycoproteins important in cell differentiation and tumor metastases

Laminins are a family of basement membrane-derived glycoproteins that are very biologically active with a number of diverse cell types. The response of the cells is dependent on the cell type and various cell-specific intracellular events are activated. Multiple active sites on laminin and cellular receptors have been described. Both laminin and the synthetic peptides that define the active sites may have important clinical uses. For example, the neurite-promoting peptides may be useful in vivo in regeneration studies because of their potent activity with neural cells and their lack of antigenicity. Also, peptides, such as YIGSR, that inhibit angiogenesis are potentially useful for treating the vascularization of the eye that occurs in conditions such as diabetes mellitus. Likewise, the angiogenic peptide SIKVAV, because of its role in endothelial cell block vessel formation, may be useful for treating ischemia. The recent progress that has been made in characterizing basic mechanisms of action of laminin has laid the groundwork for more direct studies of its clinical relevance.

Nidogen mediates the formation of ternary complexes of basement membrane components

Using a recombinant nidogen we have probed the calcium binding potential of various nidogen domains, examined the binding of nidogen to various basement membrane proteins and assessed the ability of nidogen to mediate the formation of ternary complexes between laminin and heparan sulfate proteoglycan and collagen IV and laminin. The results of these experiments indicate that the Ca+2 binding is on the rod-like domain with additional binding observed on the N-terminal G1 domain. With regard to the role of nidogen in mediating complex formation among basement membrane components it was demonstrated that nidogen effectively promotes the formation of a ternary complex between laminin and collagen IV, with both of these components interacting independently with nidogen. Similarly, nidogen mediates a ternary complex formation between laminin and proteoglycan. Interestingly, the interaction between proteoglycan and nidogen is through the protein core of the proteoglycan. We have localized the major interaction sites on nidogen with the proteoglycan core and collagen IV to a region on the globular G2 domain while the C-terminal globe G3 binds to laminin. Ca+2 binding does not appear to be important in either of the binary or ternary complex formations. The data reported allow us to hypothesize that, via the multiple interactions of nidogen with other basement membrane components, nidogen plays a crucial structural role in basement membrane organization and stabilization.

Laminins and other strange proteins

Laminins are large multidomain proteins of the extracellular matrix (ECM) with important functions in the development and maintenance of cellular organization and supramolecular structure, in particular in basement membranes. Each molecule is composed of three polypeptide chains, A (300-400 kDa) and B1 and B2 (180-200 kDa), which together form the characteristic cross-shaped laminin structure with three short arms and one long arm. Many different domains have been identified in laminin by sequence analysis, structural investigations, and functional studies. Each short arm is formed by homologous N-terminal portions of one of the three chains. Structurally, each short arm contains two or three globular domains which are connected by rows of manyfold-repeated Cys-rich "EGF-like" domains. In all three chains this region is followed by a long heptad repeat region similar to those found in many alpha-helical coiled-coil proteins. These parts of the three laminin chains constitute a triple-stranded coiled-coil domain, which forms the extended rodlike structure of the long arm. This is the only domain in the protein which is made up of more than one chain and consequently serves the function of chain assembly. The two B chains are terminated by the coiled-coil domain, but the A chain contains an additional C-terminal segment which accounts for five globular domains located at the tip of the long arm. Several important functions of laminin have been assigned to individual domains in either the short arms or terminal regions of the long arm.(ABSTRACT TRUNCATED AT 250 WORDS)

Assembly of extracellular matrix

A great challenge in understanding how different extracellular matrices assemble is to sort through the vast number of possible interactions between and among matrix molecules. The most profound insights are likely to come from patients with defined defects of matrix molecules and the use of transgenic mice or other experimental technologies that mimic the complexity of the human system.

The junction between cytokines and cell adhesion

Several aspects of the interactions between growth factors and cell adhesion are described. Recent advances in the field come from the identification of molecules resembling growth factors or growth factor receptors, which bear cell adhesion motifs as well as molecules participating in both cell growth control and adhesion.

Basement-membrane heparan sulfate proteoglycan binds to laminin by its heparan sulfate chains and to nidogen by sites in the protein core

A large, low-density form of heparan sulfate proteoglycan was isolated from the Engelbreth-Holm-Swarm (EHS) tumor and demonstrated to bind in immobilized-ligand assays to laminin fragment E3, collagen type IV, fibronectin and nidogen. The first three ligands mainly recognize the heparan sulfate chains, as shown by inhibition with heparin and heparan sulfate and by the failure to bind to the proteoglycan protein core. Nidogen, obtained from the EHS tumor or in recombinant form, binds exclusively to the protein core in a heparin-insensitive manner. Studies with other laminin fragments indicate that the fragment E3 possesses a unique binding site of laminin for the proteoglycan. A major binding site of nidogen was localized to its central globular domain G2 by using overlapping fragments. This allows for the formation of ternary complexes between laminin, nidogen and proteoglycan, suggesting a key role for nidogen in basement-membrane assembly. Evidence is provided for a second proteoglycan-binding site in the C-terminal globule G3 of nidogen, but this interaction prevents the formation of such ternary complexes. Therefore, the G3-mediated nidogen binding to laminin and proteoglycan are mutually exclusive.

Urokinase binding to laminin-nidogen. Structural requirements and interactions with heparin

Recently we have shown that heparin and related sulfated polyanions are low-affinity ligands of the kringle domain in the amino-terminal region (ATF) of human urokinase (u-PA), and proposed that this may facilitate loading of u-PA onto its receptor at the focal contacts between adherent cells and their matrix. We have now tested other components of the cell matrix (fibronectin, vitronectin, thrombospondin and laminin-nidogen) for u-PA binding, and found that laminin-nidogen is also a ligand of the u-PA ATF. Direct binding assays and competition binding assays with defined fragments of laminin-nidogen showed that there are u-PA binding sites in fragment E4 of laminin as well as in nidogen. The long-arm terminal domain of laminin (fragment E3), which contains a heparin-binding site, competed for binding of u-PA to immobilised heparin. However nidogen, which does not bind to heparin, also inhibited binding of u-PA to heparin, and this effect was also observed with recombinant nidogen and with a fragment of nidogen lacking the carboxy-terminal domain. Direct binding assays confirmed that u-PA binds to nidogen through a site in the u-PA ATF. We conclude that u-PA binds to laminin-nidogen by interactions involving the ATF region of u-PA, the E4 domain of laminin and the rod or amino-terminal regions of nidogen. Since nidogen is suggested to be an important bridging molecule in the maintenance of the supramolecular organization in basement membranes, the presence of a binding site for u-PA in nidogen indicates a role for plasminogen activation in basement membrane remodelling.