Characterization of a novel glycoprotein isolated from the basement membrane matrix of the Engelbreth-Holm-Swarm tumor

A previously undescribed protein has been isolated and purified from the extracellular matrix of the Engelbreth-Holm-Swarm (EHS) tumor, a murine tumor that synthesizes an extensive matrix composed of basement membrane molecules. Molecular characterization of the molecule determined that it is a glycoprotein with internal disulfide bonds and an isoelectric point of 6.0. Electrophoretic mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the glycoprotein migrated as a diffuse band with a molecular weight of approximately 72,000-80,000. The amino acid composition was significantly different from known basement membrane components. Polyclonal antibodies that specifically recognize the glycoprotein localized it to the kidney glomerular basement membrane. These antibodies did not cross-react with either known basement membrane components (laminin, type IV collagen, and heparan sulfate proteoglycan), with 70K "culture shock" protein or with components of normal mouse serum (including mouse transferrin, albumin, or alpha-fetoprotein), when analyzed by "Western" immunoblots. Our data indicate that the glycoprotein is synthesized by the EHS tumor cells and is present at relatively high levels in the EHS tumor matrix.     

Laminins in basement membrane assembly

The heterotrimeric laminins are a defining component of all basement membranes and self-assemble into a cell-associated network. The three short arms of the cross-shaped laminin molecule form the network nodes, with a strict requirement for one α, one β and one γ arm. The globular domain at the end of the long arm binds to cellular receptors, including integrins, α-dystroglycan, heparan sulfates and sulfated glycolipids. Collateral anchorage of the laminin network is provided by the proteoglycans perlecan and agrin. A second network is then formed by type IV collagen, which interacts with the laminin network through the heparan sulfate chains of perlecan and agrin and additional linkage by nidogen. This maturation of basement membranes becomes essential at later stages of embryo development.     

Immunogold quantitation of laminin, type IV collagen, and heparan sulfate proteoglycan in a variety of basement membranes

A series of basement membranes was immunolabeled for laminin, type IV collagen, and heparan sulfate proteoglycan in the hope of comparing the content of these substances. The basement membranes, including thin ones (less than 0.3 micron) from kidney, colon, enamel organ, and vas deferens, and thick ones (greater than 2 micron), i.e., Reichert's membrane, Descemet's membrane, and EHS tumor matrix, were fixed in formaldehyde, embedded in Lowicryl, and treated with specific antisera or antibodies followed by anti-rabbit immunoglobulin bound to gold. The density of gold particles, expressed per micron2, was negligible in controls (less than or equal to 1.1), but averaged 307, 146, and 23, respectively, for laminin, collagen IV, and proteoglycan over the thick basement membranes (except for Descemet's membranes, over which the density was 16, 5, and 34, respectively) and 117, 72, and 64, respectively, over the lamina densa of the thin basement membranes. Lower but significant reactions were observed over the lamina lucida. Interpretation of the gold particle densities was based on (a) the similarity between the ultrastructure of most thick basement membranes and of the lamina densa of most thin basement membranes, and (b) the biochemical content of the three substances under study in the EHS tumor matrix (Eur J Biochem 143:145, 1984). It was proposed that thick basement membranes (except Descemet's) contained more laminin and collagen IV but less heparan sulfate proteoglycan than the lamina densa of thin basement membranes. In the latter, there was a fair variation from tissue to tissue, but a tendency towards a similar molar content of the three substances.     

Mapping of three major heparin-binding sites on laminin and identification of a novel heparin-binding site on the B1 chain

Laminin, a major basement membrane glycoprotein, interacts with many basement membrane- and cell surface-associated heparin-like macromolecules. In order to understand these interactions better, we have tried to map heparin-binding sites on laminin precisely. Electron microscopy revealed three major heparin-binding sites: 1) on the globule of the long arm; 2) on the outer globule of the short arms; and 3) on the inner globule of the short arms. Elution of heparin bound to a laminin affinity column with a linear salt gradient produced three peaks at 0.15, 0.17, and 0.20 M NaCl. When the laminin-heparin interaction was examined in the presence of increasing salt concentrations by the technique of rotary shadowing, the weakest binding was assigned to the inner globule of the short arms and the strongest to the globule of the long arm. One peptide termed AC15, with the sequence Arg-Ile-Gln-Asn-Leu-Leu-Lys-Ile-Thr-Asn-Leu-Arg-Ile-Lys-Phe-Val-Lys from the B1 chain, was identified as a heparin-binding sequence localized on the outer globule of the lateral short arm. Because the two stronger heparin-binding sites were mapped in domains participating in laminin self-association, the effect of heparin on this phenomenon was examined using turbidity and electron microscopy. At low heparin concentrations, laminin oligomer and polymer formation was slightly enhanced. At high heparin concentrations, a drastic inhibition of polymerization was observed, and laminin was detected to be mainly monomeric in rotary-shadowed samples. These results suggest that local variation in the concentration of heparin-like macromolecules might be a crucial factor in determining the association of matrix macromolecules and therefore the structure of basement membranes.     

The N terminus of laminin A chain is homologous to the B chains

A major proteolytic fragment (E1/E1-4) of the basement membrane protein laminin, comprising the three short arms with some terminal globules missing, was isolated by elastase digestion, and partial protein sequence data were determined for several tryptic peptides. Sequences which corresponded to A-chain structures were used to synthesize oligonucleotides for the construction and screening of a primer-extended cDNA library from mouse PYS-2 cells. A clone of 1.1 kb was obtained and shown by sequencing to correspond to the 5' end of the 10-kb mRNA of the A chain of laminin. The clone contains 77 nucleotides of 5' untranslated sequence and a region coding for 334 amino acids, including a presumptive signal peptide of 24 amino acids. The sequence is 30% homologous to the corresponding N-terminal part of the B1 chain of laminin, suggesting the same structure for both domains. The data present further evidence for a recent structural model which postulates that each of the three laminin polypeptide chains forms a distinct short arm.     

Inhibition of laminin self-assembly and interaction with type IV collagen by antibodies to the terminal domain of the long arm

Laminin is a major glycoprotein of the basement membrane. Although its precise localization and orientation within this structure is unknown, it is presumably anchored to other macromolecules such as type IV collagen or proteoheparan sulfate. In vitro, laminin has the ability to self-assemble and to bind to type IV collagen molecules at distinct sites. To identify more precisely the domains of the complex, cross-shaped laminin molecule that are involved in these interactions, images of laminin-laminin dimers and laminin-type IV collagen complexes obtained by the rotary shadowing method were analyzed. We observed that the complex domain at the end of the long arm of laminin is predominantly involved in these interactions. By using Fab fragments of antibodies specific for a peptide fragment derived from this complex domain, it is shown that laminin self-assembly is inhibited in their presence, as measured by turbidity and by electron microscopy. In addition, these antibodies inhibit the specific interaction of laminin with type IV collagen. These data suggest that the complex domain at the end of the long arm of laminin contains binding sites of potential importance for the assembly of basement membranes.     

Structure of laminin variants. The 300-kDa chains of murine and bovine heart laminin are related to the human placenta merosin heavy chain and replace the a chain in some laminin variants

A variant of laminin has previously been isolated from murine heart and shown to be distinct from laminin purified from a traditional source, the murine Engelbreth-Holm-Swarm (EHS) tumor (Paulsson, M., and Saladin, K. (1989) J. Biol. Chem. 264, 18726-18732). It contains a novel polypeptide chain designated as 300 kDa, which is not found in laminin from the EHS tumor. In the present study, heart laminin was purified from bovine tissue and shown to be structurally and immunochemically closely related to the murine protein. Further, heart laminins were compared with merosin, a laminin-like protein isolated from human placenta (Ehrig, K., Leivo, I., Argraves, W. S., Ruoslahti, E., and Engvall, E. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 3264-3268). The 300-kDa chain of bovine heart laminin cross-reacted with the heavy chain of merosin, showing that these polypeptides are closely related, albeit from different species. Heart laminin is more resistant to proteolysis than laminin derived from the EHS tumor. A large fragment could be prepared by digestion with thermolysin, which consisted of an almost intact long arm structure and variably long, residual short arm structures. Analysis of its structure shows that the 300-kDa heavy chain is disulfide-bonded to the B1 and B2 chains in the center of the laminin cross and forms the long arm together with these chains. It thereby replaces the A chain, well known from tumor sources, in the laminin structure.     

Localization of binding sites for laminin, heparan sulfate proteoglycan and fibronectin on basement membrane (type IV) collagen

Rotary shadowing electron microscopy was used to examine complexes formed by incubating combinations of the basement membrane components: type IV collagen, laminin, large heparan sulfate proteoglycan and fibronectin. Complexes were analyzed by length measurement from the globular (COOH) domain of type IV collagen, and by examination of the four arms of laminin and the two arms of fibronectin. Type IV collagen was found to contain binding sites for laminin, heparan sulfate proteoglycan and fibronectin. With laminin the most frequent site was centered approximately 81 nm from the carboxy end of type IV collagen. Less frequent sites appeared to be present at approximately 216 nm and approximately 291 nm, although this was not apparent when the sites were expressed as a fraction of the length of type IV collagen to which they were bound. For heparan sulfate proteoglycan the most frequent site occurred at approximately 206 nm with a less frequent site at approximately 82 nm. For fibronectin, a single site was present at approximately 205 nm. Laminin bound to type IV collagen through its short arms, particularly through the end of the lateral short arms and to heparan sulfate proteoglycan mainly through the end of its long arm. Fibronectin bound to type IV collagen through the free end region of its arms. Using a computer graphics program, the primary laminin binding sites of two adjacent type IV collagen molecules were found to align in the "polygonal" model of type IV collagen, whereas with the "open network" model, a wide meshed matrix is predicted. It is proposed that basement membrane may consist of a lattice of type IV collagen coated with laminin, heparan sulfate proteoglycan and fibronectin.     

Selective immunoreactivities of kidney basement membranes to monoclonal antibodies against laminin: localization of the end of the long arm and the short arms to discrete microdomains

To examine the ultrastructural distribution of laminin within kidney basement membranes, we prepared rat anti-mouse laminin mAbs to use in immunolocalization experiments. Epitope domains for these mAbs were established by immunoprecipitation, immunoblotting, affinity chromatography, and rotary shadow EM. One mAb bound to the laminin A and B chains on blots and was located to a site approximately 15 nm from the long arm-terminal globular domain as shown by rotary shadowing. Conjugates of this long arm-specific mAb were coupled to horseradish peroxidase (HRP) and intravenously injected into mice. Kidney cortices were fixed for microscopy 3 h after injection. HRP reaction product was localized irregularly within the renal glomerular basement membrane (GBM) and throughout mesangial matrices. In addition, this mAb bound in linear patterns specifically to the laminae rarae of basement membranes of Bowman's capsule and proximal tubule. This indicates the presence of the long arm immediately beneath epithelial cells in these sites. The laminae densae of these basement membranes were negative by this protocol. In contrast, the lamina rara and densa of distal tubular basement membranes (TBM) were both heavily labeled with this mAb. A different ultrastructural binding pattern was seen with eight other mAbs, including two that mapped to different sites on the short arms by rotary shadowing and five that blotted to a large pepsin-resistant laminin fragment (P1). These latter mAbs bound weakly or not at all to GBM but all bound throughout mesangial matrices. In contrast, discrete spots of HRP reaction product were seen across all layers of Bowman's capsule BM and proximal TBM. These same mAbs, however, bound densely across the full width of distal TBM. Our findings therefore show that separate strata of different basement membranes are variably immunoreactive to these laminin mAbs. The molecular orientation or integration of laminin into the three dimensional BM meshwork therefore varies with location. Alternatively, there may be a family of distinct laminin-like molecules distributed within basement membranes.     

Collagen IV is essential for basement membrane stability but dispensable for initiation of its assembly during early development

Basement membranes are specialized extracellular matrices consisting of tissue-specific organizations of multiple matrix molecules and serve as structural barriers as well as substrates for cellular interactions. The network of collagen IV is thought to define the scaffold integrating other components such as, laminins, nidogens or perlecan, into highly organized supramolecular architectures. To analyze the functional roles of the major collagen IV isoform alpha1(IV)(2)alpha2(IV) for basement membrane assembly and embryonic development, we generated a null allele of the Col4a1/2 locus in mice, thereby ablating both alpha-chains. Unexpectedly, embryos developed up to E9.5 at the expected Mendelian ratio and showed a variable degree of growth retardation. Basement membrane proteins were deposited and assembled at expected sites in mutant embryos, indicating that this isoform is dispensable for matrix deposition and assembly during early development. However, lethality occurred between E10.5-E11.5, because of structural deficiencies in the basement membranes and finally by failure of the integrity of Reichert's membrane. These data demonstrate for the first time that collagen IV is fundamental for the maintenance of integrity and function of basement membranes under conditions of increasing mechanical demands, but dispensable for deposition and initial assembly of components. Taken together with other basement membrane protein knockouts, these data suggest that laminin is sufficient for basement membrane-like matrices during early development, but at later stages the specific composition of components including collagen IV defines integrity, stability and functionality.     

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)     

Artificial laminin polymers assembled in acidic pH mimic basement membrane organization

Natural laminin matrices are formed on cell membranes by a cooperative process involving laminin self-polymerization and binding to cognate cellular receptors. In a cell-free system, laminin can self-polymerize, given that a minimal critical concentration is achieved. We have previously described that pH acidification renders self-polymerization independent of protein concentration. Here we studied the ultrastructure of acid-induced laminin polymers using electron and atomic force microscopies. Polymers presented the overall appearance of natural matrices and could be described as homogeneous polygonal sheets, presenting struts of 21 +/- 5 and 86 +/- 3 nm of height, which approximately correspond to the sizes of the short and the long arms of the molecule, respectively. The addition of fragment E3 (the distal two domains of the long arm) did not affect the polymerization in solution nor the formation of adsorbed matrices. On the other hand, the addition of fragment E1', which contains two intact short arms, completely disrupted polymerization. These results indicate that acid-induced polymers, like natural ones, involve only interactions between the short arms. The electrostatic surface map of laminin alpha1 LG4-5 shows that acidification renders the distal end in the long arms exclusively positive, precluding homophylic interactions between them. Therefore, acidification reproduces in vitro, and at a physiological protein concentration, what receptor interaction does in the cellular context, namely, it prevents the long arm from disturbing formation of the homogeneous matrix involving the short arms only. We propose that acid-induced polymers are the best tool to study cellular response to laminin in the future.     

The incubation of laminin, collagen IV, and heparan sulfate proteoglycan at 35 degrees C yields basement membrane-like structures

Three basement membrane components, laminin, collagen IV, and heparan sulfate proteoglycan, were mixed and incubated at 35 degrees C for 1 h, during which a precipitate formed. Centrifugation yielded a pellet which was fixed in either potassium permanganate for ultrastructural studies, or in formaldehyde for Lowicryl embedding and immunolabeling with protein A-gold or anti-rabbit immunoglobulin-gold. Three types of structures were observed and called types A, B, and C. Type B consisted of 30-50-nm-wide strips that were dispersed or associated into a honeycomb-like pattern, but showed no similarity with basement membranes. Immunolabeling revealed that type B strips only contained heparan sulfate proteoglycan. The structure was attributed to self-assembly of this proteoglycan. Type A consisted of irregular strands of material that usually accumulated into semisolid groups. Like basement membrane, the strands contained laminin, collagen IV, and heparan sulfate proteoglycan, and, at high magnification, they appeared as a three-dimensional network of cord-like elements whose thickness averaged approximately 3 nm. But, unlike the neatly layered basement membranes, the type A strands were arranged in a random, disorderly manner. Type C structures were convoluted sheets composed of a uniform, dense, central layer which exhibited a few extensions on both surfaces and was similar in appearance and thickness to the lamina densa of basement membranes. Immunolabeling showed that laminin, collagen IV, and proteoglycan were colocalized in the type C sheets. At high magnification, the sheets appeared as a three-dimensional network of cords averaging approximately 3 nm. Hence, the organization, composition, and ultrastructure of type C sheets made them similar to the lamina densa of authentic basement membranes.     

Laminin alpha5-chain is expressed early and widely during the development of the anterior segment of rat eye

The distribution of a novel laminin alpha5-chain in the basement membranes of the anterior segment of rat eye was studied. Frozen sections of embryonic day (E)16--17, post-natal day (P)2, 5, 10, 15 and 30 and adult rat eyes were immunostained for laminin chains alpha2, alpha5, beta1, beta2 and gamma1 and for laminin-5, as well as for EHS-laminin, to visualize all basement membranes. Laminin alpha5-, beta1- and gamma1-chain immunoreactivities were found in the basement membranes of the inner and outer layers of optic cup, lens epithelium, further corneal epithelium and skin of the eyelids in E16--17 rat eyes. In P2 and older rat eyes, laminin alpha5-, beta1- and gamma1-chains were all seen in the basement membranes of the corneal and conjunctival epithelium, Descemet's membrane, lens epithelium, ciliary processes, blood vessels and skin of the eyelids. There was a change in the expression pattern of laminin alpha5, beta1- and gamma1-chains in Descemet's membrane from the endothelial side of the membrane (P2--P15 eyes) to both sides of the membrane after P30. Immunoreactivity for laminin-5 was weak in the basement membrane of E16--17 epidermis, but strong in the basement membrane of corneal, conjunctival and eyelid epithelium in P2 and older rat eyes. Laminin alpha2- and beta2-chains were seen in conjunctival and uveal blood vessels in P15 and older rat eyes. The laminin beta2-chain emerged into the basement membrane of conjunctival epithelium in P30 and older rat eyes, suggesting a role for the laminin beta2-chain in the maturation of conjunctiva. The results suggest that laminin alpha5-chain, possibly in laminin-10 (alpha5beta1gamma1), is early and widely expressed in the basement membranes of developing and adult rat eye and, further, that laminin alpha5-chain is a major laminin alpha-chain, partly in coexpression with the alpha3-chain of laminin-5 in the basement membranes of the anterior segment of the eye in developing and adult rats. © 1998 Chapman & Hall     

Compositional and structural requirements for laminin and basement membranes during mouse embryo implantation and gastrulation

Laminins are components of all basement membranes and have well demonstrated roles in diverse developmental processes, from the peri-implantation period onwards. Laminin 1 (alpha1beta1gamma1) is a major laminin found at early stages of embryogenesis in both embryonic and extraembryonic basement membranes. The laminin gamma1 chain has been shown by targeted mutation to be required for endodermal differentiation and formation of basement membranes; Lamc1(-/-) embryos die within a day of implantation. We report the generation of mice lacking laminin alpha1 and laminin beta1, the remaining two laminin 1 chains. Mutagenic insertions in both Lama1 and Lamb1 were obtained in a secretory gene trap screen. Lamb1(-/-) embryos are similar to Lamc1(-/-) embryos in that they lack basement membranes and do not survive beyond embryonic day (E) 5.5. However, in Lama1(-/-) embryos, the embryonic basement membrane forms, the embryonic ectoderm cavitates and the parietal endoderm differentiates, apparently because laminin 10 (alpha5beta1gamma1) partially compensates for the absent laminin 1. However, such compensation did not occur for Reichert's membrane, which was absent, and the embryos died by E7. Overexpression of laminin alpha5 from a transgene improved the phenotype of Lama1(-/-) embryos to the point that they initiated gastrulation, but this overexpression did not rescue Reichert's membrane, and trophoblast cells did not form blood sinuses. These data suggest that both the molecular composition and the integrity of basement membranes are crucial for early developmental events.     

Laminin alpha 5 is required for lobar septation and visceral pleural basement membrane formation in the developing mouse lung

Laminin alpha/beta/gamma heterotrimers are the major noncollagenous components of all basement membranes. To date, five alpha, three beta, and three gamma chains have been identified. Laminin alpha 5 is expressed early in lung development and colocalizes with laminin alpha1. While laminin alpha1 expression in the lung is restricted to the embryonic period, laminin alpha 5 expression persists throughout embryogenesis and adulthood. Targeted mutation of the mouse laminin alpha 5 gene Lama5 causes embryonic lethality at E14-E17 associated with exencephaly, syndactyly, placentopathy, and kidney defects, all attributable to abnormal basement membranes. In this investigation, lung development in Lama5(-/-) mice up to E16.5 was examined. We observed normal lung branching morphogenesis and vasculogenesis, but incomplete lobar septation and absence of the visceral pleura basement membrane. Preservation of branching morphogenesis was associated with ectopic deposition of laminin alpha 4 in the airway basement membrane. Perturbation of pleural basement membrane formation and right lung septation correlated with absence of laminin alpha 5, which was found to be the only laminin alpha chain present in the normal visceral pleura basement membrane. Our finding of normal lung branching morphogenesis with abnormal lobar septation demonstrates that these processes are not obligatorily linked.     

Isolation of a tumor cell laminin receptor

BL6 murine melanoma cells contain approximately 110,000 cell surface binding sites for the basement membrane glycoprotein laminin. Treatment of isolated melanoma cell plasma membranes with detergent yields a single class of laminin receptor. The receptor was purified 900 fold by laminin affinity chromatography. The isolated receptor has a Mr of 67,000 and binds laminin with high affinity: kd = 2 nm. The binding affinity of the isolated receptor was similar to that of the plasma membranes or the whole cells. Such a laminin receptor, isolated here for the first time, could facilitate the interaction of metastasizing tumor cells with the basement membrane.     

Laminin polymerization induces a receptor-cytoskeleton network

The transition of laminin from a monomeric to a polymerized state is thought to be a crucial step in the development of basement membranes and in the case of skeletal muscle, mutations in laminin can result in severe muscular dystrophies with basement membrane defects. We have evaluated laminin polymer and receptor interactions to determine the requirements for laminin assembly on a cell surface and investigated what cellular responses might be mediated by this transition. We found that on muscle cell surfaces, laminins preferentially polymerize while bound to receptors that included dystroglycan and alpha7beta1 integrin. These receptor interactions are mediated through laminin COOH-terminal domains that are spatially and functionally distinct from NH2-terminal polymer binding sites. This receptor-facilitated self-assembly drives rearrangement of laminin into a cell-associated polygonal network, a process that also requires actin reorganization and tyrosine phosphorylation. As a result, dystroglycan and integrin redistribute into a reciprocal network as do cortical cytoskeleton components vinculin and dystrophin. Cytoskeletal and receptor reorganization is dependent on laminin polymerization and fails in response to receptor occupancy alone (nonpolymerizing laminin). Preferential polymerization of laminin on cell surfaces, and the resulting induction of cortical architecture, is a cooperative process requiring laminin- receptor ligation, receptor-facilitated self-assembly, actin reorganization, and signaling events.     

Effect of brown spider venom on basement membrane structures

Loxoscelism or necrotic arachnidism are terms used to describe lesions and reactions induced by bites (envenomation) from spiders of the genus Loxosceles. Envenomation has been reported to provoke dermonecrosis and haemorrhage at the bite site and haemolysis, disseminated intravascular coagulation and renal failure. The purpose of this work was to study the effect of the venom of the brown spider Loxosceles intermedia on basement membrane structures and on its major constituent molecules. Light microscopy observations showed that L. intermedia venom obtained through electric shock, which reproduces two major signals of Loxoscelism in the laboratory, exhibits activity toward basement membrane structures in mouse Engelbreth-Holm-Swarm (EHS) sarcoma. Basement degradation was seen by a reduced periodic acid-Schiff (PAS) and alcian blue staining as well as by a reduced immunostaining for laminin when compared to control experiments. Electron microscopy studies confirmed the above results, showing the action of the venom on EHS-basement membranes and demonstrating that these tissue structures are susceptible to the venom. Using purified components of the basement membrane, we determined through SDS-PAGE and agarose gel that the venom is not active toward laminin or type IV collagen, but is capable of cleaving entactin and endothelial heparan sulphate proteoglycan. In addition, when EHS tissue was incubated with venom we detected a release of laminin into the supernatant, corroborating the occurrence of some basement membrane disruption. The venom-degrading effect on entactin was blocked by 1,10-phenanthroline, but not by other protease inhibitors such as PMSF, NEM or pepstatin-A. By using light microscopy associated with PAS staining we were able to identify that 1,10-phenanthroline also inhibits EHS-basement membrane disruption evoked by venom, corroborating that a metalloprotease of venom is involved in these effects. Degradation of these extracellular matrix molecules and the observed susceptibility of the basement membrane could lead to loss of vessel and glomerular integrity, resulting in haemorrhage and renal problems after envenomation.     

Mapping of domains in human laminin using monoclonal antibodies: localization of the neurite-promoting site

Monoclonal antibodies were made against a truncated form of human laminin isolated from placenta. 12 antibodies were isolated and characterized. All antibodies stained basement membranes in placenta and immunoprecipitated laminin from media of cultured choriocarcinoma cells. Three antibodies, 3E5, 4C7, and 4E10, partially blocked the neurite-promoting activity of laminin. Addition of a second antibody, goat anti-mouse IgG, caused more complete blocking of the activity. Two of the blocking antibodies, 4C7 and 4E10, reacted with epitopes within the globular domain at the end of the long arm of laminin, and the third one, 3E5, reacted at the end of the rod-like portion of the long arm adjacent to the globular domain, as shown by electron microscopy after rotary shadowing. Five nonblocking antibodies used in the same test reacted with epitopes in other domains of the molecule. Blocking antibodies 3E5 and 4E10 could be used in immunoblotting and both antibodies reacted with the same polypeptides in pepsin fragments of human laminin, the predominant polypeptides being approximately 400 kD. When a crude extract of human amnion was used as a source of intact laminin, the 4E10 antibody detected a single polypeptide of approximately 400 kD. A nonblocking antibody, 2E8, which reacted at the center of the laminin cross, reacted predominantly with a 200-kD polypeptide in human laminin fragments and exclusively with a 200-kD polypeptide in amnion extract and in rat laminin. Our results with human laminin match the results by Edgar, D., R. Timpl, and H. Thoenen, 1984, EMBO (Eur. Mol. Biol. Organ.) J., 3:1463-1468, in which the neurite-promoting activity of mouse laminin resides at the end of the long arm, which is also the site for heparin binding. However, since the active fragments of human laminin did not bind to heparin, the neurite-promoting site should be different from the heparin-binding site. Our results further suggest that the neurite-promoting site may be contained in or close to the 400-kD component of laminin.