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

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.

Binding of fibulin-1 to nidogen depends on its C-terminal globular domain and a specific array of calcium-binding epidermal growth factor-like (EG) modules

The calcium-binding basement membrane protein fibulin-1C was shown to bind nidogen in a calcium-dependent fashion. Fibulin-1C consists of small N (domain 1) and C-terminal (domain III) globular structures connected by a central rod (domain II) composed of nine epidermal growth factor (EG) modules, eight of which possess a consensus sequence for calcium binding. Several point and deletion mutants and chimeric protein constructs were used to define the nidogen binding epitope of fibulin-1C by surface plasmon resonance and solid phase assays. All recombinant products were obtained from transfected kidney cells in a folded form as shown by CD spectroscopy, electron microscopy and proteolysis. They were used to demonstrate that calcium-binding is essentially due to the EG modules possessing the consensus binding sequence. Deletion of domain III caused a 30-fold reduction in nidogen binding, whereas deletion of domain I had no effect, yet domain III alone was also inactive. Successive deletions of two to seven EG modules of domain II also caused partial of complete inactivation of binding depending on how many were deleted or their position relative to domain III. Site-directed mutagenesis within the calcium binding consensus sequences demonstrated a similar dependence. Replacement of seven of the calcium-binding modules by a similar tandem array from a related protein showed a distinct (fibulin-2) to almost complete loss of binding (fibrillin-1). This indicates a complex epitope structure involving domains II and III, which each may provide binding epitopes or stabilize each other.

Dystroglycan is essential for early embryonic development: disruption of Reichert's membrane in Dag1-null mice

Dystroglycan is a central component of the dystrophin-glycoprotein complex (DGC), a protein assembly that plays a critical role in a variety of muscular dystrophies. In order to better understand the function of dystroglycan in development and disease, we have generated a null allele of dystroglycan (Dag1neo2) in mice. Heterozygous Dag1neo2 mice are viable and fertile. In contrast, homozygous Dag1neo2 embryos exhibit gross developmental abnormalities beginning around 6.5 days of gestation. Analysis of the mutant phenotype indicates that an early defect in the development of homozygous Dag1neo2 embryos is a disruption of Reichert's membrane, an extra-embryonic basement membrane. Consistent with the functional defects observed in Reichert's membrane, dystroglycan protein is localized in apposition to this structure in normal egg cylinder stage embryos. We also show that the localization of two critical structural elements of Reichert's membrane--laminin and collagen IV--are specifically disrupted in the homozygous Dag1neo2 embryos. Taken together, the data indicate that dystroglycan is required for the development of Reichert's membrane. Furthermore, these results suggest that disruption of basement membrane organization might be a common feature of muscular dystrophies linked to the DGC.

Agrin binds to the nerve-muscle basal lamina via laminin

Agrin is a heparan sulfate proteoglycan that is required for the formation and maintenance of neuromuscular junctions. During development, agrin is secreted from motor neurons to trigger the local aggregation of acetylcholine receptors (AChRs) and other proteins in the muscle fiber, which together compose the postsynaptic apparatus. After release from the motor neuron, agrin binds to the developing muscle basal lamina and remains associated with the synaptic portion throughout adulthood. We have recently shown that full-length chick agrin binds to a basement membrane-like preparation called Matrigel. The first 130 amino acids from the NH2 terminus are necessary for the binding, and they are the reason why, on cultured chick myotubes, AChR clusters induced by full-length agrin are small. In the current report we show that an NH2-terminal fragment of agrin containing these 130 amino acids is sufficient to bind to Matrigel and that the binding to this preparation is mediated by laminin-1. The fragment also binds to laminin-2 and -4, the predominant laminin isoforms of the muscle fiber basal lamina. On cultured myotubes, it colocalizes with laminin and is enriched in AChR aggregates. In addition, we show that the effect of full-length agrin on the size of AChR clusters is reversed in the presence of the NH2-terminal agrin fragment. These data strongly suggest that binding of agrin to laminin provides the basis of its localization to synaptic basal lamina and other basement membranes.

Drosophila laminin binds to mammalian nidogen and to heparan sulfate proteoglycan

A Drosophila laminin that has the chain composition alpha5 beta1 gamma1, relative to mammalian laminins, bound human and mouse nidogen almost as strongly as mouse laminin-1 (alpha1 beta1 gamma1) in solid-phase assays, and had only a fourfold lower affinity in a radioligand competition test. This is due to a short, highly conserved sequence that occurs in both laminin gamma1 chains and which binds nidogen. When the single conservative amino acid difference between the two sequences (Tyr-->His) was introduced into the mouse laminin binding module gamma1 III4 it failed to cause any change of binding. A high affinity between Drosophila laminin and mouse nidogen resulted in the formation of a stable complex in solution. Drosophila laminin also bound to the mouse heparan sulfate proteoglycan perlecan and the formation of this complex was inhibited by heparin, but not by chondroitin sulfate. In addition, a weaker connection between the core protein of mouse perlecan and Drosophila laminin can be mediated through nidogen. Elastase and other proteases degraded Drosophila laminin to a restricted number of larger fragments (40-300 kDa), almost all of which were bound to a heparin affinity column. Three fragments could be displaced at low salt concentration and were derived from the short arms of the Drosophila laminin, as shown by sequence analysis. A more strongly bound 50-kDa fragment apparently comprised the globular domains LG2 and LG3 derived from the C-terminal part of its alpha chain. Therefore, Drosophila laminin and mouse laminin-1 differ in certain aspects of protease stability and heparin-binding sites that, in part, can be attributed to their different alpha chains. The data suggest the existence of a nidogen analog and heparan sulfate proteoglycans in Drosophila, which remain to be identified.

Laminins in lung development

Laminins are essential components of basement membranes, playing important roles in cell adhesion, proliferation, and differentiation. These heterotrimeric glycoproteins are composed of an alpha, beta, and gamma chains held together by disulfide bonds. The first laminin identified, from the mouse Engelbreth-Holm-Swarm (EHS) tumor, is now referred to as laminin-1. Laminin-1 is expressed in the mouse developing lung by epithelial and mesenchymal cells and plays a role in branching morphogenesis. Since laminins are multidomain proteins, different laminin sites are engaged in promoting lung organogenesis by serving different functions at different stages of development. This study shows that the cross region of the molecule selectively promotes epithelial cell proliferation. The outer globular region of alpha 1 and beta 1 chains mediates laminin polymerization and thereby basement membrane formation and epithelial cell polarization. The inner globular region of laminin beta 1 chain binds to heparan sulfate proteoglycan and both stimulate lumen formation. While the combined effect of these laminin active sites results in normal lung tissue structure and branching morphogenesis, different developmental abnormalities of the lung may result from alterations in each of them.

Importance of nidogen binding to laminin gamma1 for branching epithelial morphogenesis of the submandibular gland

Epithelial-mesenchymal interactions are major driving forces for the development of most solid organs. The importance of these interactions was first shown for the embryonic submandibular gland more than 40 years ago. We here present evidence that interactions between two basement membrane components, nidogen (entactin) and laminin gamma1 chain, could be important for epithelial-mesenchymal interactions in this gland. Nidogen mRNA was detected by in situ hybridization in the mesenchyme, and yet the protein was detected in epithelial and endothelial basement membranes. The role of nidogen-laminin interactions for epithelial morphogenesis was studied by applying antibodies to submandibular gland organ cultures. Antibodies reacting strongly with the nidogen-binding site of laminin gamma1 chain drastically perturbed branching epithelial morphogenesis. Electron microscopy of the epithelial-mesenchymal interface showed that blocking antibodies disrupted the formation of the basement membrane. Epidermal growth factor was shown to increase the expression of nidogen in mesenchyme, and could counteract the effect of the blocking antibodies. We suggest that nidogen could be an important mesenchymal factor for submandibular gland development.

Integrin and basement membrane normalization in mouse grafts of human keratinocytes--implications for epidermal homeostasis

Integrin patterns and formation of basement membrane (BM) were investigated in correlation to epidermal growth and differentiation during skin regeneration in human keratinocyte transplants on nude mice. Immuno-fluorescence and transmission electron microscopy (TEM) showed that different stages of tissue reconstruction were characterized by a sequence of coordinated events. Features of the initial tissue activation, with rapid keratinocyte proliferation around day 4, including cells in a suprabasal position, were: (1) a marked increase in and extended distribution of the integrin chains alpha 2, alpha 3, beta 1 and alpha 6, while beta 4 already showed a preferential basal location; (2) de novo expression of alpha 5 and alpha v; and (3) marked deposition of laminin-5 and nidogen but low levels of other BM components. Tissue normalization during the 2nd week, initiated by a drastic decrease in the number of proliferating cells after day 4, now strictly in basal position, was signified: by (1) orthotopic staining for basal-type keratins (K5, K14) together with a regular pericellular alpha 2 beta 1 and alpha 3 beta 1 distribution, (2) linear, balanced deposition of BM components (e.g. laminin-1, type IV collagen) and (3) colocalization of integrin alpha 6 beta 4 and bullous pemphigoid antigen. Simultaneously at 7 days hemidesmosomes and a defined BM had developed (TEM), becoming continuous at 14 days. This coincided with the regular distribution of suprabasal keratins (K1, K10) as well as intermediate (involucrin) and late differentiation markers (filaggrin, loricrin). Type-VII collagen deposition, still irregular at 14 days, became continuous at 22 days together with developing BM-anchoring fibrils indicating final tissue consolidation. This model mimics principal stages of epidermal wound healing in human skin and implies a linkage between BM assembly, integrin distribution and the compartment of proliferation competent cells, which in turn determines the onset of differentiation. Thus, apart from the balance of diffusible growth regulators, this positional control of keratinocytes, largely accomplished by integrin-matrix interactions, seems to be prerequisite to establishment and maintenance of tissue homeostasis.

Fibulin-2 binds to the short arms of laminin-5 and laminin-1 via conserved amino acid sequences

Epithelial cell-specific laminin-5, consisting of three chains, alpha3, beta3, and gamma2, is a component of the anchoring filament that traverses the lamina lucida beneath the hemidesmosomes of epidermal cells and functions to link these cells to the basement membrane. We have studied the molecular interaction between laminin-5 and extracellular matrix proteins using recombinant proteins and synthetic peptides. Affinity chromatography assays with recombinant fragments of the laminin gamma2 short arm identified a 195-kDa binding protein in the conditioned media from the mouse epidermal cell line Pam 212 and from primary dermal fibroblasts. This molecule was identified by Western blotting as fibulin-2, a recently identified extracellular matrix protein. Using deletion mutants and various synthetic peptides in competition assays, the 9-amino acid sequence SADFSVHKI (residues 199-207) in domain IV of the gamma2 chain was defined as a critical site for fibulin-2 binding. An anti-gamma2 antibody co-immunoprecipitated fibulin-2 from the conditioned media, further confirming the interaction of fibulin-2 with laminin-5. Fibulin-2 was also found to interact with laminin-1 (alpha1beta1gamma1) through a region (residues 654-665) of the alpha1 chain short arm whose sequence is similar to that of the fibulin-2 binding site of the gamma2 chain. Together these results suggest that fibulin-2 functions to bridge laminin-1 and laminin-5 with other extracellular matrix proteins, providing a linkage between the cell surface and the basement membrane.

Characterization of high affinity binding between laminin and the acute-phase protein, serum amyloid A

Serum amyloid A isoforms, apoSAA1 and apoSAA2, are acute-phase proteins of unknown function and can be precursors of amyloid AA peptides (AA) found in animal and human amyloid deposits. These deposits are often a complication of chronic inflammatory disorders and are associated with a local disturbance in basement membrane (BM). In the course of trying to understand the pathogenesis of this disease laminin, a major BM glycoprotein, has been discovered to bind saturably, and with high affinity to murine acute-phase apoSAA. This interaction involves a single class of binding sites, which are ionic in nature, conformation-dependent, and possibly involve sulfhydryls. Binding activity was significantly enhanced by Zn2+, an effect possibly mediated through Cys-rich zinc finger-like sequences on laminin. Collagen type IV also bound apoSAA but with lower affinity. Unexpectedly, no binding was detected for perlecan, a BM proteoglycan previously implicated in AA fibrillogenesis, although a low affinity interaction cannot be excluded. Entactin, another BM protein that functions to cross-link the BM matrix and is normally complexed with laminin, could inhibit laminin-apoSAA binding suggesting apoSAA does not bind to normal BM. Since laminin binds apoSAA with high affinity and has previously been shown to codeposit with AA amyloid fibrils, we postulate that laminin interacts with apoSAA and facilitates nucleation events leading to fibrillogenesis. This work also provides further support for the hypothesis that a disturbance in BM metabolism contributes to the genesis of amyloid. The specificity and avidity of the laminin-apoSAA interaction also implies that it may be a normal event occurring during the inflammatory process, which mediates one or more of the functions recently proposed for apoSAA.

Basement-membrane stromal relationships: interactions between collagen fibrils and the lamina densa

Collagens, the most abundant molecules in the extracellular space, predominantly form either fibrillar or sheet-like structures-the two major supramolecular conformations that maintain tissue integrity. In connective tissues, other than cartilage, collagen fibrils are mainly composed of collagens I, III, and V at different molecular ratios, exhibiting a D-periodic banding pattern, with diameters ranging from 30 to 150 nm, that can form a coarse network in the extracellular matrix in comparison with a fine meshwork of lamina densa. The lamina densa represents a stable sheet-like meshwork composed of collagen IV, laminin, nidogen, and perlecan compartmentalizing tissue from one another. We hypothesize that the interactions between collagen fibrils and the lamina densa are crucial for maintaining tissue-tissue interactions. A detailed analysis of these interactions forms the basis of this review article. Here, we demonstrate that there is a direct connection between collagen fibrils and the lamina densa and propose that collagen V may play a crucial role in this connection. Collagen V might also be involved in regulation of collagen fibril diameter and anchoring of epithelia to underlying connective tissues.

Biosynthesis and processing of type XVI collagen in human fibroblasts and smooth muscle cells

The alpha 1(XVI) collagen chain, recently identified by cDNA cloning, exhibits structural similarity to a subgroup of collagens that associate with collagen fibrils. Recombinant alpha 1(XVI) collagen chains produced in embryonic kidney cells are able to form stable homotrimers, which are rapidly converted into smaller polypeptides after secretion into the culture medium. In this study, we investigated the biosynthesis of native type XVI collagen by immunoprecipitation of metabolically labeled human cells. Dermal fibroblasts and arterial smooth muscle cells were precipitated with three antibodies raised against distinct regions in the N- and C-terminal part of the human alpha 1(XVI) collagen chain. A disulfide-bonded polypeptide of 220 kDa was obtained from the culture medium, cells and extracellular matrix with all three antibodies. This polypeptide is sensitive to bacterial collagenase digestion and partially resistant to pepsin digestion, suggesting that it is the endogenous alpha 1(XVI) collagen chain. Pulse/chase experiments showed that the newly synthesized alpha 1(XVI) chains are secreted into the medium and deposited in the extracellular matrix in a time-dependent manner. Unlike the recombinant chain, the native type XVI collagen does not undergo extensive proteolytic processing upon secretion. Both cell types deposit a substantial amount of the newly synthesized alpha 1(XVI) chain into the extracellular matrix, in which the 220-kDa polypeptide is the only product immunoprecipitated. There is little evidence for the presence of another constituent chain. The data are consistent with a nomotrimeric chain composition for type XVI collagen. No apparent difference exists in the rate of synthesis and secretion between fibroblasts and smooth muscle cells. Indirect immunofluorescence microscopy showed an extracellular distribution of type XVI collagen, which is located close to cells but not associated with fibrillar structures.

Domain-specific interactions between entactin and neutrophil integrins. G2 domain ligation of integrin alpha3beta1 and E domain ligation of the leukocyte response integrin signal for different responses

Extracellular matrix proteins activate neutrophils to up-regulate many physiologic functions that are necessary at sites of tissue injury. To elucidate the ligand-receptor interactions that mediate these functions, we examined neutrophil activation by the basement membrane protein, entactin. Entactin is structurally and functionally organized into distinct domains; therefore, we utilized glutathione S-transferase -fusion proteins encompassing its four major domains, G1, G2, E, and G3, to assess interactions between entactin and neutrophil integrin receptors. We show that the E domain, which contains the single RGD sequence of entactin, is sufficient for ligation of the beta3-like integrin, leukocyte response integrin, and signaling for chemotaxis. Moreover, the G2 domain signals for stimulation of Fc receptor-mediated phagocytosis via ligation of alpha3beta1. This receptor-ligand interaction was revealed only after stimulation of neutrophil by immune complexes or phorbol esters. Interestingly, the E domain does not enhance phagocytosis, and the G2 domain is not chemotactic. Furthermore, cleavage of entactin with the matrix metalloproteinase, matrilysin, liberates peptides that retain E domain-mediated chemotaxis and G2 domain-mediated enhancement of phagocytosis. These studies indicate that multiple domains of entactin have the ability to ligate individual integrins expressed by neutrophils and to activate distinct functions.

Elements of neural adhesion molecules and a yeast vacuolar protein sorting receptor are present in a novel mammalian low density lipoprotein receptor family member

Normal cell development depends to a large part on multifunctional proteins that have evolved by recombination of proven modular elements. We now have discovered and characterized in rabbit such a multi-domain protein, and classify it as novel member of the low density lipoprotein (LDL) receptor gene family. The extracellular portion of the approximately 250-kDa membrane protein, termed LR11, contains a cluster of 11 LDL receptor ligand binding repeats, a group of 5 LDL receptor "YWTD" repeats, a large hexarepeat domain of structural elements found in neural cell adhesion molecules, and a domain with similarity to a yeast receptor for vacuolar protein sorting, VPS10. The cytoplasmic domain exhibits features typical of endocytosis-competent coated-pit receptors. The mosaic, and presumably multifunctional, receptor is expressed abundantly in brain, in particular the hippocampus, dentate gyrus, and cerebral cortex, and is present at significant levels in liver, adrenal glands, and testis. Western blotting of tissues and ligand blotting of LR11-transfected cells demonstrated that the novel protein binds apolipoprotein E-containing lipoproteins. In contrast to the LDL receptor, hepatic expression of LR11 is unaffected by hyperlipidemia. The identification of this highly conserved and superbly complex protein offers the opportunity to gain new insights into the emergence of multifunctional mosaic proteins akin to the ever expanding LDL receptor gene family.

Site-directed mutagenesis and structural interpretation of the nidogen binding site of the laminin gamma1 chain

A precise molecular map of the nidogen binding site of laminins was obtained by site-directed mutagenesis and structural analysis of the 56 residue LE module gamma1III4 of their gamma1 chain. This demonstrated the crucial importance of the sequence DPNAV (position 800-804) in the disulfide-bonded loop a, with major contributions made by all residues except P801. Different substitutions of these residues emphasized the essential role of the negative charge (D800) and carboxamide group (N802) as well as their spacings and hydrophobic contacts (V804) for interaction, and predict direct contacts of these three residues with a complementary binding region of nidogen. An inactivating A803-V substitution, however, may lead to a distorted loop structure. A lower but still significant contribution originates from the non-contiguous link/loop c sequence LKCIY (positions 815-819) which is spatially close to the loop a sequence. The link residues (L815 and K816) provide main chain hydrogen bonds to N806 and a side chain hydrogen bond to the V804 carbonyl and thus stabilize the conformation of loop a. The side chains of I818 and Y819 together with P842 from loop d form hydrophobic contacts that provide further stability but could possibly also participate in direct ligation. The nidogen binding epitope is therefore localized on a narrow ridge and has a length of approximately 17 angstroms. The data also indicate a strong conservation of the epitope in the laminin gamma1 chains of several invertebrates.

Entactin immunoreactivity in immature and adult rat brain

Entactin (nidogen) is a glycoprotein of 150 kDa mainly found in the basement membranes of peripheral tissues where it is co-localized and forms a very tight complex with the outgrowth-promoting molecule laminin. In the present report we tested by immunoblotting the specificity of polyclonal antibodies to laminin and entactin isolated from Engelbreth-Holm-Swarm (EHS) mouse sarcoma and investigated laminin and entactin immunoreactivities in the hippocampus of newborn, adult control and kainate-injured rats. The three polyclonal antibodies to laminin (two of them commercial) used in the present study stained somas of neurons, blood vessels and reactive glial cells, in agreement with previous reports. Nevertheless, all of them cross-reacted with entactin. The anti-entactin serum, which specifically recognized entactin protein, but not laminin or fibronectin, stained mainly the walls of blood vessels in rat brain slices. We observed a stronger entactin expression in immature than in adult brain, and a dramatic increase of vascular staining in kainate-injured hippocampus, suggesting a contribution of entactin to both development and reactive angiogenesis.

Structural and electron microscopic analysis of neurocan and recombinant neurocan fragments

Neurocan, a nervous tissue-specific chondroitin sulfate proteoglycan of the aggrecan family which has been shown to interact with neural cell adhesion molecules and tenascin, could be visualized by rotary shadowing electron microscopy as two globular domains interconnected by an extended flexible filament of 60-90 nm. Several recombinant neurocan fragments generated in the human embryonic kidney cell line 293 represent as observed by electron microscopy the expected parts of this structure, which indicates a correct folding of these molecules. Biological activity of the recombinant N-terminal globular domain could be demonstrated by its coelution with hyaluronan in gel permeation chromatography. In addition, the modification of the recombinant fragments with certain carbohydrate structures was analyzed. High mannose oligosaccharides could be mapped to the N-terminal globular domain of the brain-derived molecule. Only recombinant fragments containing parts of the central region of the molecule were modified with chondroitin sulfate chains and with the HNK-1 epitope, and could be considerably altered in their migratory behavior on SDS-polyacrylamide gel electrophoresis by neuraminidase treatment. These findings and the electron microscopic shape indicate a mucin-like character for the central neurocan region.

Human trophoblast cell adhesion to extracellular matrix protein, entactin

PROBLEM

Trophoblast interaction with endometrial extracellular matrix (ECM) is crucial during human embryo implantation and placentation. Entactin, a ubiquitous basement membrane glycoprotein, plays a central role in ECM assembly, cell attachment, and chemotaxis. The present study was conducted to examine the possible role of entactin in promoting human trophoblast adhesion.

METHODS

Using an extended life span first trimester trophoblast cell line HTR-8/SVneo (HTR) and a cell adhesion assay, we measured the adherence of human first trimester trophoblasts to recombinant entactin and its domains. Also, we used flow cytometry and indirect immunofluorescence to detect the presence of integrins that may be involved in human trophoblast-entactin interaction; these methods were used to analyze HTR cells, as well as tissue sections and freshly isolated human trophoblasts from first trimester and term placenta.

RESULTS

We found that first trimester trophoblast cells were highly adherent to entactin and its E and G2 domains but not to G1 or G3 domains. Using indirect immunofluorescence and flow cytometry, we found that both beta 1 and beta 3 integrin subunits were expressed on the surface of HTR trophoblast cells adhering to entactin; in contrast, beta 2 and beta 4 integrin subunits were not detected. In addition, we found that alpha v beta 3 was expressed on freshly isolated villous cytotrophoblasts and cytotrophoblast and syncytiotrophoblasts in tissue sections from term placenta. The beta 3 integrin subunit was expressed in cytotrophoblasts and syncytiotrophoblasts in villi of first trimester placental tissue sections.

CONCLUSION

Recombinant entactin promotes human trophoblast cell adhesion through both its E and G2 domains and these specific adhesive interactions may be mediated by beta 1 and/or beta 3 class integrins.

Conversion of the Kunitz-type module of collagen VI into a highly active trypsin inhibitor by site-directed mutagenesis

The recombinant Kunitz protease inhibitor module (domain C5) of human collagen alpha 3(VI) chain was previously shown to lack inhibitory activity for proteases with trypsin-like specificity and some other proteases. We have now prepared mutants in the binding loop region including the P1' site (D2889-->A), the P2' site (F2890-->R) and the P3 site (T2886-->P) and in a more remote region (W2907-->V) either as individual substitutions or combinations of them. These mutants were analyzed for their kinetics of binding to trypsin by surface plasmon resonance and for their capacity to inhibit various proteases. Single substitutions (D-->A, T-->P, W-->V) showed an effect only for D->A which bound to trypsin with Kd = 0.25 microM. A 25-100-fold increase in affinity was observed for the double mutants T-->P/D-->A and F-->R/D-->A and approached the affinity of aprotinin (Kd approximately 0.01 nM) in two different triple mutants. These affinities correlated well with the inhibitory capacities of the mutants for trypsin in the cleavage of a large protein and a small peptide substrate. A similar but not completely identical improvement in inhibitory capacity was also observed for leucocyte elastase but not for thrombin. These data could be interpreted in terms of steric interferences or lack of hydrogen bonding of a few critical residues based on three-dimensional structures available for the C5 domain.

Laminin interactions important for basement membrane assembly are promoted by zinc and implicate laminin zinc finger-like sequences

Laminin is an abundant basement membrane (BM) glycoprotein which regulates specific cellular functions and participates in the assembly and maintenance of the BM superstructure. The assembly of BM is believed to involve the independent polymerization of collagen type IV and laminin, as well as high affinity interactions between laminin, entactin/nidogen, perlecan, and collagen type IV. We report here that Zn2+ can influence laminin binding activity, in vitro. Laminin contains 42 cysteine-rich repeats of which 12 contained nested zinc finger consensus sequences. Recently, the entactin binding site was mapped to one of these zinc finger-containing repeats on the laminin gamma chain (Mayer, U., Nischt, R., Poschl, E., Mann, K., Fukuda, K., Gerl, M., Yamada, Y., and Timpl, R. (1993) EMBO J. 12, 1879-1885). Based on these observations, the effect of a series of essential ions (Ca2+, Cd2+, Cu2+, Mg2+, Mn2+, and Zn2+) on laminin binding activity was evaluated. Zn2+ was found to be the most effective at enhancing laminin-entactin and laminin-collagen type IV binding. Laminin-bound Zn2+ was detected by flame atomic absorption spectroscopy at a maximum of 8 mol/mol of laminin. Furthermore, Ca2+-dependent laminin polymerization was unaffected by Zn2+, an observation consistent with the lack of zinc finger-containing repeats in the terminal globular domains required for polymerization. We conclude that Zn2+-laminin complexes may generate high affinity binding sites which contribute to BM cross-linking important for its assembly and homeostasis. Zinc is likely a cofactor for 2 kinds of cross-linking interactions; one involving direct binding between laminin and collagen type IV and the other a ternary complex of laminin-entactin-collagen type IV.

Supramolecular assembly of basement membranes

Basement membranes are thin sheets of extracellular proteins situated in close contact with cells at various locations in the body. They have a great influence on tissue compartmentalization and cellular phenotypes from early embryonic development onwards. The major constituents of all basement membranes are collagen IV and laminin, which both exist as multiple isoforms and each form a huge irregular network by self assembly. These networks are connected by nidogen, which also binds to several other components (proteoglycans, fibulins). Basement membranes are connected to cells by several receptors of the integrin family, which bind preferentially to laminins and collagen IV, and via some lectin-type interactions. The formation of basement membranes requires cooperation between different cell types since nidogen, for example, is usually synthesized by cells other than those exposed to the basement membranes. Thus many molecular interactions, of variable affinities, determine the final shape of basement membranes and their preferred subanatomical localization.

Laminins: a family of diverse multifunctional molecules of basement membranes

Laminins represent a growing family of disulfide-linked heterotrimers constituted by the association of three genetically different polypeptides, the alpha, beta, and gamma chains. Laminins are endowed with structural and biological functions. They play a direct critical role in the control of cellular behavior by providing cells with specific information through interactions with cell surface receptors. Because of their structural properties, they represent crucial building blocks for tissue assembly, architecture, and stability. The expression of laminin chain variants is spatio-temporally regulated, which suggests that laminin isoforms might have different functions responsible for the biological and morphological polymorphism of basement membranes. The different cells present in the skin express several laminin chains, which lead to the deposition of various laminin isoforms, whose mechanical and biological functions are likely to be adapted to the properties of the dermo-epidermal junction. Recently, defective laminin isoforms have been shown to be associated with several inborn and acquired diseases, illustrating a major structural function for laminins in skin integrity.

Abnormal expression and distribution of nidogen in Hirschsprung's disease

Hirschsprung's disease (congenital colonic aganglionosis) is associated with abnormalities in the distributions and amounts of basement membrane and other extracellular matrix components in the human gut. The authors have investigated the possible significance of nidogen in Hirschsprung's disease, because this glycoprotein is necessary for the formation of ternary complexes with the other basement membrane components, laminin and collagen type IV, and thus may contribute the pathology of the disease. Increased nidogen immunoreactivity in the smooth muscle basement membranes and muscularis mucosae of the distal aganglionic zone in Hirschsprung's bowel was observed, the nidogen immunoreactivity demonstrating that the thickness of the muscularis mucosae was increased in this region. However, steady-state nidogen mRNA levels were found to be significantly lower in both proximal and distal Hirschsprung's bowel (relative to controls). In contrast, no significant differences were observed in the steady-state levels of the mRNAs coding for laminin subunits. These results indicate that although abnormalities in the amount or distribution of nidogen may contribute to the abnormalities seen in the extracellular matrix in Hirschsprung's disease, the levels of expression of the genes coding for either nidogen or laminin are unlikely to be primarily responsible for the lesions.

Skin fibroblasts are the only source of nidogen during early basal lamina formation in vitro

The purpose of this study was to determine whether nidogen, the linkage protein of the basal lamina, is of epidermal or dermal origin. The development of the basal lamina was studied in an in vitro skin model. Preputial fibroblasts seeded onto a nylon mesh attached, proliferated, and developed a rich extracellular matrix (dermal model). Preputial keratinocytes were added to the dermal model to form a keratinocyte dermal model that ultrastructurally resembled in many respects human skin. Ultrastructural analysis revealed early stages of dermal development, including an incomplete basal lamina, aggregates of dermal filamentous material connecting to the lamina densa, bundles of 10-nm microfibrils, formation of premature hemidesmosomes, anchoring filaments, and anchoring fibrils. The cell origin of nidogen was determined in the dermal model and in the epidermal and dermal components of the keratinocyte dermal model. Specific antibodies and a cDNA probe for nidogen were used for immunofluorescence microscopy, Western and Northern blots, and for in situ hybridization studies. Our data show that fibroblasts are the only source of nidogen during early basal lamina formation. Although fibroblasts can synthesize nidogen and deposit it in the dermal matrix, no basal lamina will form unless they are recombined with keratinocytes. This suggests that the epidermis plays a major regulatory role in the production and assembly of nidogen into the basal lamina.

Role of laminin-nidogen complexes in basement membrane formation during embryonic development

Laminin and nidogen (entactin) are major glycoprotein components of basement membranes. At least seven different isoforms of laminin have been identified. Laminin and nidogen form high affinity complexes in basement membranes by specific binding between the laminin gamma 1 chain and the G3 globule of nidogen. Additional interactions between nidogen and collagen IV, perlecan and other basement membrane components result in the formation of ternary complexes between these matrix components. Nidogen is highly susceptible to proteolytic cleavage, and binding to laminin protects nidogen from degradation. Nidogen is considered to have a crucial role as a link protein in the assembly of basement membranes. Basement membrane components are synthesized at high levels during tissue growth and development, and sites of morphogenesis correlate with localized remodelling of basement membranes. The formation of distinct basement membrane matrices in the developing embryo is influenced by the laminin isoforms produced and by whether laminin and nidogen are co-expressed and secreted as a complex or are produced by cooperation between two cell layers. The potential roles of laminin-nidogen complexes, cell-matrix interactions, and other intermolecular interactions within the matrix in basement membrane assembly and stability are discussed in this review.

Two distinct cell attachment sites in entactin are revealed by amino acid substitutions and deletion of the RGD sequence in the cysteine-rich epidermal growth factor repeat 2

The basement membrane glycoprotein, entactin, has previously been shown to promote cell attachment and chemotaxis. We have constructed a panel of glutathione S-transferase fusion proteins that encompasses the four major structural domains of entactin, G1, G2, E, and G3. These proteins have been synthesized in bacteria and purified by affinity chromatography. The connecting stalk of entactin, E, which contains four cysteine-rich EGF homology repeats and the integrin receptor RGD recognition sequence, has been modified by deletion of the RGD sequence and substituting glutamic acid for aspartic acid. Attachment assays reveal that the RGD sequence is one of the major cell attachment sites in entactin and that this sequence is recognized by the alpha v beta 3 integrin receptor. Analysis of cell attachment on mutant forms of full-length entactin expressed in the baculovirus expression system revealed a second attachment site that was independent of the RGD sequence. This second site was localized to a peptide of 39 amino acid residues in the second globular G2 domain of entactin. This peptide represents a cysteine-rich EGF repeat. Inhibition of cell attachment by anti-integrin receptor antibodies indicates that the second attachment site is recognized by a member of the beta 1 family of integrin receptors, possibly alpha 3 beta 1.

Autoantibodies to the laminin P1 fragment in HgCl2-induced membranous glomerulopathy

Exposure to mercuric chloride induces the development of a membranous glomerulopathy with high proteinuria in DZB rats, in which immunoglobulin (Ig)G1 and IgG2a bound in the glomeruli were previously found to react with laminin of the EHS tumor and several unidentified glomerular basement membrane components. Monoclonal antibodies were prepared by fusing cervical and mandibular lymph node cells from a HgCl2-treated DZB rat with a nonsecreting mouse myeloma. Monoclonal antibodies were screened for reactivity with collagenase-digested glomerular basement membrane and kidney sections; upon subcloning, eight stable hybridomas were obtained, named MEC1 to MEC8. MEC2 (IgG1, kappa), MEC3 (IgM, kappa), and MEC5 (IgG1, kappa), as well as the polyclonal glomerular eluate, reacted preferentially with the P1 fragment of the laminin-1 (alpha 1 beta 1 gamma 1) isoform. MEC8 (IgM, kappa) reacted with the P1 and the E4 fragment of laminin. Both MEC6 (IgM, kappa) and MEC8 bound to actin and to various other, unidentified cellular antigens, indicating that MEC6 and MEC8 are polyreactive antibodies. MEC7 (IgM, kappa) bound to a cytoskeleton-linked cell membrane antigen, present on various epithelial cells and between heart muscle fibers and associated with small peripheral, intramuscular nerves. Several of the MEC monoclonal antibodies bound in vivo along the glomerular capillary wall. Although discrete electron-dense subepithelial immune aggregates were not detected and proteinuria was not induced, MEC3 localization changed from a continuous pattern into a fine granular pattern along the glomerular basement membrane, and focally along the TBM, upon passive transfer into naive DZB rats. These findings suggest a pathogenetic role for the P1 fragment of laminin either in the induction phase of HgCl2-induced membranous glomerulopathy as an immunogen or in the effector phase as a target antigen.

A novel role for alpha 3 beta 1 integrins in extracellular matrix assembly

To study the biological role of alpha 3 beta 1 integrins in cell adhesion, migration, and in the deposition of extracellular matrix, we stably expressed the human alpha 3 integrin subunit in the alpha 4, alpha 5 integrin deficient CHO cell line B2. The expression of alpha 3 beta 1 integrins enhanced cell adhesion on entactin (also known as nidogen), but not on fibronectin. Using recombinant GST-fusion proteins that span the entire length of the entactin molecule, we located cell adhesive activity to the G2 domain of entactin. These results suggest that the alpha 3 beta 1 integrin functions as an adhesion receptor interacting with the G2 domain of entactin. On the other hand, the expression of alpha 3 beta 1 integrins did not confer the ability to migrate on entactin. Strikingly, the expression of alpha 3 beta 1 dramatically increased the deposition of entactin and fibronectin into the pericellular matrix. This was accompanied by increased binding activity of the 29 kDa amino-terminal domain of fibronectin. Thus, similar to alpha 5 beta 1 integrins, alpha 3 beta 1 integrins can play an important role in modulating the assembly of pericellular matrices. However, unlike fibronectin deposition supported by alpha 5 beta 1, alpha 3 beta 1 supported fibronectin deposition into pericellular matrix was not inhibited by antibodies binding to the RGD containing cell adhesion domain of fibronectin, demonstrating that the two processes are mechanistically distinct. The role of alpha 3 beta 1 in pericellular matrix assembly potentially implicates this receptor in the assembly and/or recognition of entactin-containing pericellular matrices, an observation consistent with its apparent role in the renal glomerulus of the mammalian kidney.

Evidence for the presence of collagenous domains in Candida albicans cell surface proteins

Rabbit polyclonal antibodies (PAbs) directed towards the amino-terminal cysteine-rich 7S domain (PAb anti-7S), the major internal collagenous domain (PAb anti-type IV), and the C-terminal noncollagenous region (PAb anti-NC1) of the type IV collagen molecule were probed by indirect immunofluorescence against Candida albicans blastoconidia and germinated blastoconidia. Most nongerminating cells and mother blastoconidia from which germ tubes originated showed strong fluorescence when PAb anti-7S was used, whereas with PAb anti-type IV, fluorescence was found almost exclusively on the surface of filamentous forms. A patched fluorescent pattern rather than a homogenous confluent fluorescence was observed in all cases. No fluorescent cells were observed with PAb anti-NC1. By Western immunoblotting, PAb anti-type IV cross-reacted primarily with a polypeptide of 37 kDa present in wall extracts obtained from intact cells of both growth forms by treatment with beta-mercaptoethanol, whereas PAb anti-7S recognized a major 58-kDa antigen also present in both extracts, along with some other high-molecular-mass (> 106-kDa) polydisperse species present only in the material released from blastoconidia with beta-mercaptoethanol. No reactive bands were observed when PAb anti-NC1 was used as a probe in Western immunoblotting experiments. The sensitivities or resistances to collagenase digestion of the different polypeptides that cross-reacted with PAbs anti-type IV and anti-7S suggest the existence of cell wall components in C. albicans that contain epitopes that mimic the collagenous domains of the type IV collagen molecule.

Low nidogen affinity of laminin-5 can be attributed to two serine residues in EGF-like motif gamma 2III4

High affinity nidogen binding of laminin-1 (chain composition alpha 1 beta 1 gamma 1) has been previously mapped to a single EGF-like motif gamma 1III4 of its gamma 1 chain. Two more isoforms, laminin-5 (alpha 3 beta 3 gamma 2) and laminin-7 (alpha 3 beta 2 gamma 1), show low and high binding activity, respectively, indicating that the gamma 2 chain is of low affinity. This was confirmed by recombinant production of the homologous EGF-like motif gamma 2III4 of the gamma 2 chain, which has a 100,000-fold lower binding activity than gamma 1III4. The crucial heptapeptide binding sequence Asn-Ile-Asp-Pro-Asn-Ala-Val of gamma 1III4 is modified in gamma 2III4 by replacing both the central Asn and Val by Ser. Changing these replacements to Asn and Val by site-directed mutagenesis enhanced the activity of gamma 2III4 to a level which was only 5-fold lower than that of gamma 1III4. Despite their high sequence identity (77%) motifs gamma 1III4 and gamma 2III4 were also shown to differ considerably in immunological epitopes. This indicates distinctly different functions for laminins which differ in the gamma chain isoform.

Mapping of network-forming, heparin-binding, and alpha 1 beta 1 integrin-recognition sites within the alpha-chain short arm of laminin-1

Cell-interactive and architecture-forming functions are associated with the short arms of basement membrane laminin-1. To map and characterize these functions, we expressed recombinant mouse laminin-1 alpha-chain extending from the N terminus through one third of domain IIIb. This dumbbell-shaped glycoprotein (r alpha 1(VI-IVb)'), secreted by mammalian cells, was found to possess three activities. 1) Laminin polymerization was quantitatively inhibited by recombinant protein, supporting an alpha-chain role for a three-short arm interaction model of laminin self-assembly. 2) r alpha 1(VI-IVb)' bound to heparin, and the activity was localized to a subfragment corresponding to domain VI by 125I-heparin blotting. 3) PC12 rat pheochromocytoma cells adhered to, and rapidly extended branching neurites on, r alpha 1(VI-IVb)', with adhesion inhibited by alpha 1 and beta 1 integrin chain-specific antibodies. The ability of anti-laminin antibody to block PC12 cell adhesion to laminin was selectively prevented by absorption with r alpha 1(VI-IVb)' or alpha-chain domain VI fragment. This active integrin-recognition site could furthermore be distinguished from a second cryptic alpha 1 beta 1-binding site exposed by heat treatment of fragment P1', a short arm fragment lacking globules. Thus, a polymer-forming, a heparin-binding, and the active alpha 1 beta 1 integrin-recognition site are all clustered at the end of the alpha-chain short arm, the latter two resident solely in domain VI.

Exon organization of the mouse entactin gene corresponds to the structural domains of the polypeptide and has regional homology to the low-density lipoprotein receptor gene

Entactin is a widespread basement membrane protein of 150 kDa that binds to type IV collagen and laminin. The complete exon-intron structure of the mouse entactin gene has been determined from lambda genomic DNA clones. The gene spans at least 65 kb and contains 20 exons. The exon organization of the mouse entactin gene closely corresponds to the organization of the polypeptide into distinct structural and functional domains. The two amino-terminal globular domains are encoded by three exons each. Single exons encode the two protease-sensitive, O-glycosylated linking regions. The six EGF-like repeats and the single thyroglobulin-type repeat are each encoded by separate exons. The carboxyl-terminal half of entactin displays sequence homology to the growth factor-like region of the low-density lipoprotein receptor, and in both genes this region is encoded by eight exons. The positions of four introns are also conserved in the homologous region of the two genes. These observations suggest that the entactin gene has evolved via exon shuffling. Finally, several sequence polymorphisms useful for gene linkage analysis were found in the 3' noncoding region of the last exon.

Binding properties and protease stability of recombinant human nidogen

Recombinant human nidogen was obtained from transfected kidney cell clones as a 150-kDa protein with a three-globule structure. It was modified by sulfation and O-glycosylation and a lower level of N-glycosylation than mouse nidogen. Recombinant nidogens of both species were, however, indistinguishable in their affinities for laminin-1 and a recombinant laminin gamma 1 chain fragment and showed a similar binding to collagen IV and the heparan sulfate proteoglycan perlecan. The two nidogens were also equivalent in the promotion of ternary complex formation between these ligands, indicating that this function has been conserved during mammalian evolution. Fewer zinc-binding sites could be identified in human nidogen and correlated with a lower capacity of zinc to prevent binding to laminin and collagen IV. Most remarkable was the greater sensitivity of human nidogen to endogenous proteolysis in cell culture, yielding fragments of 90-145 kDa. Studies with several exogenous proteases, including thrombin and leucocyte elastase, showed lack of stability of the N-terminal globular domain G1 in contrast to what was found for mouse nidogen. Since such degradation could be important for basement membrane remodelling, this difference between human and mouse may be biologically significant.

Binding of purified collagen receptors (alpha 1 beta 1, alpha 2 beta 1) and RGD-dependent integrins to laminins and laminin fragments

Integrins alpha 1 beta 1 and alpha 2 beta 1 when purified by collagen affinity chromatography, showed distinct binding to mouse tumor laminin-1, which has the chain composition alpha 1 beta 1 gamma 1. The binding was, however, about 10-fold lower than to collagen IV. Only little (alpha 1 beta 1) or no binding (alpha 2 beta 1) was observed to two different laminin isoforms (alpha 2 beta 1 gamma 1, alpha 2 beta 2 gamma 1) from human placenta. Binding to laminin-1 was abolished by EDTA and could be specifically inhibited by antibodies to the respective integrin alpha subunit. These antibodies also inhibited cell adhesion to collagens. The binding of soluble integrins was weaker than that of immobilized integrins but could be enhanced by an activating anti(beta 1 integrin). No enhancement was observed for immobilized integrins. Studies with laminin-1 fragments demonstrated lack of binding to the major cell-adhesive fragment E8 from the long arm, fragments E3 and E4, involved in heparin-binding and self-assembly, respectively, and fragment P1, corresponding to the inner segments of the short arms. A larger short-arm fragment (E1XNd), which lacks the N-terminal beta 1 chain domains V and VI, was as active as laminin. Together, these results, suggested the localization of the binding sites for alpha 1 beta 1 and alpha 2 beta 1 to the N-terminal region of the laminin alpha 1 chain. Fragment P1 but not intact laminin-1 bound to alpha V beta 3 integrin in an EDTA-sensitive and RGD-sensitive manner, underscoring previous data on the cryptic nature of the RGD site in laminin-1. Further analyses by surface plasmon resonance assays demonstrated a KD = 50 nM for alpha 2 beta 1/laminin-1 binding and a KD = 450 nM for alpha V beta 3/fragment P1 binding and confirmed the anti-beta 1-mediated increase in affinity for alpha 2 beta 1.

Influence of nidogen complexed or not with laminin on attachment, spreading, and albumin and laminin B2 mRNA levels of rat hepatocytes

Nidogen/entactin is a Mr = 150,000 glycoprotein which is present within basement membranes in a noncovalent stable complex with laminin. We have studied the effects of nidogen/entactin complexed or not with laminin on attachment, spreading, and functions of adult rat hepatocytes in primary culture. Freshly isolated hepatocytes attached on either recombinant or EHS-derived nidogen, although to a lesser extent than on laminin/nidogen complex, laminin, and E8 and P1 fragments of laminin. Hepatocytes bound on a nidogen fragment bearing the N-terminal and rod-like domains but not on either the N-terminal globules or the rod-like domain which contains a RGD sequence. Attachment of hepatocytes on nidogen and laminin/nidogen complex was inhibited by anti-beta 1 integrin antibodies. Hepatocytes remained rounded on nidogen and laminin, whereas they rapidly spread on laminin/nidogen complex and collagen IV. Nidogen, laminin, and laminin/nidogen complex transiently maintained high steady-state albumin mRNA levels in cultured hepatocytes, but a decrease in albumin mRNA content was observed after 24 h, independently of the substrates. Actinomycin D and cycloheximide treatment indicated that the transient effect of these substrates on albumin expression was related to post-transcriptional mechanisms. Laminin B2 mRNAs were not detectable in freshly isolated hepatocytes but were expressed in 4 h hepatocyte cultures. After 24 h, a dramatic increase in the steady-state level of laminin B2 mRNA was found in hepatocytes cultured on nidogen and laminin/nidogen complex. This effect was slightly prevented in hepatocytes plated on laminin. These results show that interactions of hepatocytes with nidogen/entactin in vitro result only in a transient modulation of hepatocyte functions.

Recombinant expression and properties of the Kunitz-type protease-inhibitor module from human type VI collagen alpha 3(VI) chain

The Kunitz-type inhibitor motif (domain C5) present at the C-terminus of the human collagen alpha 3(VI) chain was prepared in a recombinant form from the culture medium of stably transfected kidney cell clones. The 76-residue protein was disulfide bonded and showed a high stability against protease treatment. The recombinant protein lacked, however, any inhibitory activity for trypsin, thrombin, kallikrein and several other proteases, which could be due to a few unusual substitutions in the region crucial for inhibitor binding. A sensitive radioimmunoassay detected low concentrations of C5 epitopes in normal human serum and fibroblast culture medium and showed a lack of cross-reaction with aprotinin. Antibodies against C5 immunoprecipitated collagen VI obtained from fibroblast medium. The C5 epitopes could not be detected on intact collagen VI purified from guanidine extracts of human placenta. Collagen VI was shown to possess several alpha 3(VI) chain bands (approximately 200 kDa) and reacted strongly with antibodies to an N-terminal recombinant fragment. Immunofluorescence with anti-C5 antibodies failed to stain several human tissues but produced a distinct intracellular staining of cultured fibroblasts. The data indicate the rapid loss of the C5 domain after biosynthesis of collagen VI.

The laminins

Laminins are extracellular matrix proteins which consist of alpha, beta and gamma chains with molecular masses of 140-400 kDa. Chain association occurs through a large triple alpha-helical coiled-coil domain towards the C-terminus of each chain. Eight genetically distinct laminin chains (alpha 1, alpha 2, alpha 3, beta 1, beta 2, beta 3, gamma 1, gamma 2) and seven different assembly forms (laminins-1 to -7) are known so far. The most extensively characterized laminin-1 (alpha 1 beta 1 gamma 1) shows calcium-dependent self assembly and heterotypic binding to perlecan, nidogen, fibulin-1 and other matrix components. This binding indicates a crucial role in the supramolecular organization of basement membranes. Laminins also possess binding sites for at least six different integrin receptors and are thus involved in many cell-matrix interactions. Such interactions have been shown to be important during embryonic development and for tissue homeostasis and remodelling.

Role of mesenchymal nidogen for epithelial morphogenesis in vitro

Recent biochemical studies suggested that the extracellular matrix protein nidogen is a binding molecule linking together basement membrane components. We studied its expression and role during development. By immunofluorescence and northern blotting, nidogen was found early during epithelial cell development of kidney and lung. Yet, in situ hybridization revealed that nidogen was not produced by epithelium but by the adjacent mesenchyme in both organs. Binding of mesenchymal nidogen to epithelial laminin may thus be a key event during epithelial development. This is supported by antibody perturbation experiments. Antibodies against the nidogen binding site on laminin B2 chain perturbed epithelial development in vitro in embryonic kidney and lung. Mesenchymal nidogen could be important for early stages of epithelial morphogenesis.

Recombinant expression and structural and binding properties of alpha 1(VI) and alpha 2(VI) chains of human collagen type VI

Full-length alpha 1(VI) and alpha 2(VI) cDNAs in an eukaryotic expression vector were used to obtain stably transfected human kidney cell clones and to purify these collagen-VI chains in substantial quantities from the culture medium. Both chains appeared mainly as monomers together with some dimers that were disulfide linked through their C-terminal globular domains. Despite sufficient hydroxylation of proline and lysine residues, the chains did not form a triple-helix, as shown by electronmicroscopy, CD spectra and pepsin sensitivity. Digestion of the chains with bacterial collagenase released the N-terminal and C-terminal globular domains, which were identified by their size and partial sequences. They showed a substantial content of alpha-helical conformation and a distinct globular structure after rotary shadowing. Antibodies could be raised that distinguished between the two chains and reacted with the globular domains. The alpha 2(VI) but not the alpha 1(VI) chain showed binding to a heparan sulfate proteoglycan (perlecan), fibronectin and pepsin-solubilized collagen VI. Purified globular domains did not bind these ligands indicating the localization of binding sites within the triple-helical domain. Both chains showed a distinct affinity for heparin but failed to bind to various collagen types.

Expression of collagen alpha 1(IV), laminin and nidogen genes in the embryonic mouse lung: implications for branching morphogenesis

The patterns of laminin A, B1, B2, nidogen and collagen alpha 1(IV) gene expression in the embryonic mouse lung were determined using in situ hybridization histochemistry at a stage when branching morphogenesis is taking place. Collagen alpha 1(IV), laminin B1 and B2 genes were expressed throughout the mesenchyme and epithelium. Nidogen gene expression was uniform throughout the mesenchyme but was not detected in epithelial cells. Laminin A mRNA was localized to cells closely associated with a basement membrane at the epithelial-mesenchymal interface. However, expression of the laminin A gene was limited to the mesenchymal cells in bronchial regions and to epithelial cells in distal terminal lobules. We propose that the pattern of laminin A gene expression in different regions of the developing lung will influence the structure of the basement membrane at the epithelial-mesenchymal interface and thus have a role in branching morphogenesis.

Glomerular matrix: synthesis, turnover and role in mesangial expansion

The extracellular matrix has an integral role in development, homeostasis and pathology of the glomerulus. Three spatially distinct matrices are present in the glomerulus: the mesangium, and basement membranes of the capillary loops and Bowman's capsule. Each is dominated by basement membrane components, but is distinct in organization and composition. Many matrix components influence cell behavior directly, through specific interactions with receptors, or indirectly through growth factor sequestration. Growth factors may be of great importance in development and disease progression in the glomerulus, and may be central to mesangial expansion. In addition, changes in matrix composition accompany and contribute to the pathological condition, such as the accumulation of matrix in diabetes mellitus.

Characterization of native laminin from bovine kidney and comparison with other laminin variants

A comprehensive characterization of laminin isoforms requires access to native preparations of laminins of a defined subunit composition. For this purpose an optimized isolation procedure was developed and shown to be broadly applicable to normal mammalian tissues. The protocol does in addition yield side fractions highly enriched in collagens XII and XIV. The major laminin purified from bovine kidney is indistinguishable from mouse Engelbreth-Holm-Swarm (EHS) tumor laminin in electron microscopy, but contains an A chain that migrates in a position intermediate to the Ae and the Am chains on SDS/PAGE. Antisera raised against mouse EHS-tumor laminin crossreact with B chains, but not with the A chain, of kidney laminin. Further, this A chain is not recognized by antisera raised against the Am chain. Laminins from heart and kidney both contain a significant subpopulation with a 190-kDa polypeptide identified as the B1s chain. The Am-containing laminins from heart and placenta differ morphologically from the Ae-containing EHS laminin in having one short arm that does not have the characteristic globule-rod-globule appearance. Further, the Am-containing laminins show a significantly higher thermal stability of the coiled-coil alpha-helical region in the long arm than does Ae-containing EHS laminin, indicating that certain combinations of laminin chains interact more strongly than others.

Hemorrhagic metalloproteinases from snake venoms

One of the more significant consequences of crotalid envenomation is hemorrhage. Over the past 50 years of investigation, it is clear that the primary factors responsible for hemorrhage are metalloproteinases present in the venom of these snakes. The biochemical basis for their activity is the proteolytic destruction of basement membrane and extracellular matrix surrounding capillaries and small vessels. These proteinase toxins may also interfere with coagulation, thus complementing loss of blood from the vasculature. Structural studies have shown that these proteinases are synthesized as zymogens and are processed at both the amino and carboxy termini to give the mature protein. The variety of hemorrhagic toxins found in snake venoms is due to the presence of structurally related proteins composed of various domains. The type of domains found in each toxin plays an important role in the hemorrhagic potency of the protein. Recently, structural homologs to the venom hemorrhagic metalloproteinases have been identified in several mammalian reproductive systems. The functional significance of the reproductive proteins is not clear, but in light of the presence of similar domains shared with the venom metalloproteinases, their basic biochemical activities may be similar but with very different consequences. This review discusses the history of hemorrhagic toxin research with emphasis on the Crotalus atrox proteinases. The structural similarities observed among the hemorrhagic toxins are outlined, and the structural relationships of the toxins to the mammalian reproductive proteins are described.

Merosin is synthesized by thyroid cells in primary culture irrespective of cellular organization

The in vitro synthesis and deposition of laminin family glycoproteins were studied using primary porcine thyroid cells cultured as monolayers or in follicles. The latter organization mimics the in vivo state of these polarized epithelial cells. In both cell systems a trimeric molecule was immunoprecipitated by using polyclonal antibodies against EHS-laminin. When the cells were fully polarized the protein was found at the basal pole of cells, irrespective of their organization. However, this molecule was different from laminin purified from a traditional source, the murine Engelbreth-Holm-Swarm (EHS) tumor. Thyroid cell laminin was composed of two light chains, analogous to EHS B1 and B2, and a disulfide-bonded heavy chain not found in EHS-laminin. The heavy chain was first synthesized as a 380 kDa polypeptide, then rapidly cleaved to a doublet of 350–380 kDa, which was subsequently found in both cell extracts and conditioned culture media. This thyroid laminin variant was compared with merosin, another variant found in the basement membranes of trophoblast, Schwann cells, striated muscle and liver. The heavy chain (M) of merosin shows homology to EHS-laminin heavy chain at the C-terminal domain, and is usually found as two polypeptides of 80 kDa and 300 kDa (Ehrig K., Leivo I., Argraves W. S., Ruoslahti E. and Engvall E. Proc. Nat. Acad. Sci. 87, 3264–3268, 1990). mRNA of the M chain was identified by RT-PCR in freshly isolated thyrocytes as well as in 6-day-old cultured thyroid cells. Furthermore, both the classical laminin heavy chain and the 350 kDa variant were detected by immunoblotting and immunofluorescence in the thyroid gland in vivo. All these results suggest strongly that merosin is a basement membrane component of thyroid cells in vivo and in vitro.

Protein binding and cell adhesion properties of two laminin isoforms (AmB1eB2e, AmB1sB2e) from human placenta

Two isoforms of laminin were extracted from human placenta by neutral buffer containing EDTA, copurified through several steps and finally separated by Mono Q anion exchange chromatography. One variant consisted of disulphide-linked 340, 230 and 190 kDa subunits, which were identified by immunoblotting as Am, B1e and B2e chains. In the other variant, the B1e chain was replaced by B1s of 180 kDa. After rotary shadowing, both variants showed a similar cross-shaped structure. The nidogen content of these laminins was substoichiometric and variable (3-70%), indicating loss by endogenous proteolysis. Yet both human isoforms were able to bind mouse nidogen with an affinity (Kd approximately 0.5 nM) comparable to that of AeB1eB2e laminin from a mouse tumour. Since the binding site is known to be contributed by a single EGF-like motif of the B2e chain, this demonstrates that activity of this site is independent of chain assembly. Binding activity of both isoforms to collagen IV and the heparan sulphate proteoglycan perlecan was correlated to the nidogen content and could be enhanced by adding nidogen. Binding to heparin was only partial and heparin did not inhibit perlecan binding. This indicated a crucial role for nidogen in mediating the integration of these laminin isoforms into basement membranes. Variant AmB1sB2e showed calcium-dependent binding to fibulin-1, while only a little activity was found for AmB1eB2e. Both isoforms promoted adhesion and spreading of several cell lines. Adhesion could be completely inhibited by antibodies to the integrin beta 1 subunit but not, or only weakly, by antibodies against beta 3, alpha 2, alpha 3, alpha 5 and alpha 6 subunits. No inhibition was observed with an Arg-Gly-Asp-containing peptide.

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.