Structural study of long arm fragments of laminin. Evidence for repetitive C-terminal sequences in the A-chain, not present in the B-chains

Laminin 511-E8 Fragment Offers Superior Adhesion Properties for Gastric Cancer Cells Compared with Full-Length Laminin 511

Simple Summary

Numerous studies over the past few decades have revealed that the interactions of gastric cancer cells with laminins through integrins play important roles in tumor cell proliferation, infiltration, and metastasis. However, the association between gastric cancer cells and the laminin E8 fragment, which is the smallest integrin-binding component, has not been investigated. In this study, we revealed that the laminin 511-E8 fragment had a greater impact on the adhesion, morphology, and proliferation of gastric cancer cells than full-length laminin 511. Thus, the laminin 511-E8 fragment is considered to be suitable for investigating the interaction between gastric cancer cells and extracellular matrices in tumor invasion and metastasis. Further, the involvement of Cdc42 in the laminin 511-E8 fragment-induced enhanced adhesion of gastric cancer cells was suggested.

Abstract

Background: The interaction between cancer cells and laminin (Ln) is a key event in tumor invasion and metastasis. Previously, we determined the effect of full-length Ln511 on gastric cancer cells. However, the interactions between the Ln511-E8 fragment, a truncated protein of Ln511, and gastric cancer cells have not been investigated. Methods: We investigated the adhesion properties of gastric cancer cells to full-length Ln511 and Ln511-E8 fragments. Results: The proliferation of four gastric cancer cell lines (SH-10-TC, MKN74, SC-6-JCK, and MKN45) was highest on the Ln511-E8 fragment. Further, a larger cytoplasm was observed in SH-10-TC and MKN74 cells cultured on full-length Ln511 or Ln511-E8 fragments. The percentage of adhesive cells was highest on the Ln511-E8 fragment in all four cell lines. Moreover, adhesion of the gastric cancer cells to Ln511-E8 fragment-coated plates was reduced by the Cdc42 GTPase inhibitor in a dose-dependent manner, suggesting the involvement of Cdc42 in the Ln511-E8 fragment-induced enhanced adhesion of gastric cancer cells. Conclusions: The Ln511-E8 fragment had a greater impact on the adhesion, morphology, and proliferation of gastric cancer cells than full-length laminin. Thus, the Ln511-E8 fragment is suitable for investigating the interaction between gastric cancer cells and extracellular matrices in tumor invasion and metastasis.

Structural biology of laminins

Laminins are large cell-adhesive glycoproteins that are required for the formation and function of basement membranes in all animals. Structural studies by electron microscopy in the early 1980s revealed a cross-shaped molecule, which subsequently was shown to consist of three distinct polypeptide chains. Crystallographic studies since the mid-1990s have added atomic detail to all parts of the laminin heterotrimer. The three short arms of the cross are made up of continuous arrays of disulphide-rich domains. The globular domains at the tips of the short arms mediate laminin polymerization; the surface regions involved in this process have been identified by structure-based mutagenesis. The long arm of the cross is an α-helical coiled coil of all three chains, terminating in a cell-adhesive globular region. The molecular basis of cell adhesion to laminins has been revealed by recent structures of heterotrimeric integrin-binding fragments and of a laminin fragment bound to the carbohydrate modification of dystroglycan. The structural characterization of the laminin molecule is essentially complete, but we still have to find ways of imaging native laminin polymers at molecular resolution.

Phosphorylation mapping of laminin α1-chain: Kinases in association with active sites

Laminin-111 is a trimeric glycoprotein of the extracellular matrix (ECM) that holds a significant role in cell adhesion, migration and differentiation. Laminin-111 is the most studied laminin isoform, composed of three chains; α1, β1 and γ1. Phosphorylation is the most common eukaryotic post - translational modification and has regulatory effect on protein function. Using bioinformatic tools we computationally predicted all the possible phosphorylation sites on human laminin α1-chain sequence (LAMA1) according to kinases binding motifs. Thus, we predicted, for the first time, the possibly responsible kinases for fifteen of the nineteen already published experimentally observed phosphorylated residues in LAMA1. Searching the literature extensively, we recorded all the known functional sites (active sites) in LAMA1. We combined the experimentally observed and predicted phosphorylated residues as well as the active sites in LAMA1, generating an analytic phosphorylation map of human laminin α1-chain, which is useful for further analysis. Our results indicated fourteen kinases that might be important for the phosphorylation of human laminin α1-chain, out of which three kinases with reported ecto-phosphorylation activity (PKA, PKC and CKII) were suggested to have a more significant role. Six cancer associated-active sites were correlated with kinases, three out which were correlated with only the above ecto - kinases.

Extracellularly Extruded Syntaxin-4 Binds to Laminin and Syndecan-1 to Regulate Mammary Epithelial Morphogenesis

Epithelial morphogenesis in the mammary gland proceeds as a consequence of complex cell behaviors including apoptotic cell death and epithelial-mesenchymal transition (EMT); the extracellular matrix (ECM) protein laminin is crucially involved. Syntaxins mediate intracellular vesicular fusion, yet certain plasmalemmal members have been shown to possess latent extracellular functions. In this study, the extracellular subpopulation of syntaxin-4, extruded in response to the induction of differentiation or apoptosis in mammary epithelial cells, was detected. Using a tetracycline-repressive transcriptional system and clonal mammary epithelial cells, SCp2, we found that the expression of cell surface syntaxin-4 elicits EMT-like cell behaviors. Intriguingly, these cells did not up-regulate key transcription factors associated with the canonical EMT such as snail, slug, or twist, and repressed translation of E-cadherin. Concurrently, the cells completely evaded the cellular aggregation/rounding triggered by a potent EMT blocker laminin-111. We found that the recombinant form of syntaxin-4 not only bound to laminin but also latched onto the glycosaminoglycan (GAG) side chains of syndecan-1, a laminin receptor that mediates epithelial morphogenesis. Thus, temporal extracellular extrusion of syntaxin-4 emerged as a novel regulatory element for laminin-induced mammary epithelial cell behaviors. J. Cell. Biochem. 118: 686-698, 2017. © 2016 Wiley Periodicals, Inc.

TFPI cofactor function of protein S: essential role of the protein S SHBG-like domain

Protein S is a cofactor for tissue factor pathway inhibitor (TFPI), accelerating the inhibition of activated factor X (FXa). TFPI Kunitz domain 3 residue Glu226 is essential for enhancement of TFPI by protein S. To investigate the complementary functional interaction site on protein S, we screened 44 protein S point, composite or domain swap variants spanning the whole protein S molecule for their TFPI cofactor function using a thrombin generation assay. Of these variants, two protein S/growth arrest-specific 6 chimeras, with either the whole sex hormone-binding globulin (SHBG)-like domain (Val243-Ser635; chimera III) or the SHBG laminin G-type 1 subunit (Ser283-Val459; chimera I), respectively, substituted by the corresponding domain in growth arrest-specific 6, were unable to enhance TFPI. The importance of the protein S SHBG-like domain (and its laminin G-type 1 subunit) for binding and enhancement of TFPI was confirmed in FXa inhibition assays and using surface plasmon resonance. In addition, protein S bound to C4b binding protein showed greatly reduced enhancement of TFPI-mediated inhibition of FXa compared with free protein S. We show that binding of TFPI to the protein S SHBG-like domain enables TFPI to interact optimally with FXa on a phospholipid membrane.

A Laminin G-EGF-Laminin G module in Neurexin IV is essential for the apico-lateral localization of Contactin and organization of septate junctions

Septate junctions (SJs) display a unique ultrastructural morphology with ladder-like electron densities that are conserved through evolution. Genetic and molecular analyses have identified a highly conserved core complex of SJ proteins consisting of three cell adhesion molecules Neurexin IV, Contactin, and Neuroglian, which interact with the cytoskeletal FERM domain protein Coracle. How these individual proteins interact to form the septal arrays that create the paracellular barrier is poorly understood. Here, we show that point mutations that map to specific domains of neurexin IV lead to formation of fewer septae and disorganization of SJs. Consistent with these observations, our in vivo domain deletion analyses identified the first Laminin G-EGF-Laminin G module in the extracellular region of Neurexin IV as necessary for the localization of and association with Contactin. Neurexin IV protein that is devoid of its cytoplasmic region is able to create septae, but fails to form a full complement of SJs. These data provide the first in vivo evidence that specific domains in Neurexin IV are required for protein-protein interactions and organization of SJs. Given the molecular conservation of SJ proteins across species, our studies may provide insights into how vertebrate axo-glial SJs are organized in myelinated axons.

Laminin chain assembly is regulated by specific coiled-coil interactions

Laminins are large heterotrimeric, multidomain proteins that play a central role in organising and establishing all basement membranes. Despite a total of 45 potential heterotrimeric chain combinations formed through the coiled-coil domain of the 11 identified laminin chains (α1–5, β1–3, γ1–3), to date only 15 different laminin isoforms have been reported. This observation raises the question whether laminin assembly is regulated by differential gene expression or specific chain recognition. To address this issue, we here perform a complete analysis of laminin chain assembly and specificity. Using biochemical and biophysical techniques, all possible heterotrimeric combinations from recombinant C-terminal coiled-coil fragments of all chains were analysed. Apart from laminin 323 (α3, β2, γ3), for which no biochemical evidence of its existence in vivo is available, these experiments confirmed all other known laminin isoforms and identified two novel potential chain combinations, laminins 312 (α3, β1, γ2) and 422 (α4, β2, γ4). Our findings contribute to the understanding of basement membrane structure, function and diversity.

Distinct Acidic Clusters and Hydrophobic Residues in the Alternative Splice Domains X1 and X2 of α7 Integrins Define Specificity for Laminin Isoforms

The binding specificity of alpha7beta1 integrins for different laminin isoforms is defined by the X1 and X2 splice domains located in the beta-propeller domain of the alpha7 subunit. In order to gain insight into the mechanism of specific laminin-integrin interactions, we defined laminin-binding epitopes of the alpha7X1 and -X2 domains by single amino acid substitutions and domain swapping between X1 and X2. The interaction of mutated, recombinantly prepared alpha7X1beta1 and alpha7X2beta1 heterodimers with various laminin isoforms was studied by surface plasmon resonance and solid phase binding assays. The data show that distinct clusters of surface-exposed acidic residues located in different positions of the X1 and the X2 loops are responsible for the specific recognition of laminins. These residues are conserved between the respective X1 or X2 splice domains of the alpha7 chains of different species, some also in the corresponding X1/X2 splice domains of alpha6 integrin. Interestingly, ligand binding was also modulated by mutating surface-exposed hydrophobic residues (alpha7X1L205, alpha7X2Y208) at positions corresponding to the fibronectin binding synergy site in alpha5beta1 integrin. Mutations in X1 that affected binding to laminin-1 also affected binding to laminin-8 and -10, but not to the same extent, thus allowing conclusions on the specific role of individual surface epitopes in the selective recognition of laminin-1 versus laminins -8 and -10. The role of the identified epitopes was confirmed by molecular dynamics simulations of wild-type integrins and several inactivating mutations. The analysis of laminin isoform interactions with various X1/X2 chimaera lend further support to the key role of negative surface charges and pointed to an essential contribution of the N-terminal TARVEL sequence of the X1 domain for recognition of laminin-8 and -10. In conclusion, specific surface epitopes containing charged and hydrophobic residues are essential for ligand binding and define specific interactions with laminin isoforms.

Crystal structure and cell surface anchorage sites of laminin alpha1LG4-5

The laminin G-like (LG) domains of laminin-111, a glycoprotein widely expressed during embryogenesis, provide cell anchoring and receptor binding sites that are involved in basement membrane assembly and cell signaling. We now report the crystal structure of the laminin alpha1LG4-5 domains and provide a mutational analysis of heparin, alpha-dystroglycan, and galactosylsulfatide binding. The two domains of alpha1LG4-5 are arranged in a V-shaped fashion similar to that observed with laminin alpha2 LG4-5 but with a substantially different interdomain angle. Recombinant alpha1LG4-5 binding to heparin, alpha-dystroglycan, and sulfatides was dependent upon both shared and unique contributions from basic residues distributed in several clusters on the surface of LG4. For heparin, the greatest contribution was detected from two clusters, 2719RKR and 2791KRK. Binding to alpha-dystroglycan was particularly dependent on basic residues within 2719RKR, 2831RAR, and 2858KDR. Binding to galactosylsulfatide was most affected by mutations in 2831RAR and 2766KGRTK but not in 2719RKR. The combined analysis of structure and activities reveal differences in LG domain interactions that should enable dissection of biological roles of different laminin ligands.

Pregnancy-associated plasma protein-A (PAPP-A): a local regulator of IGF bioavailability through cleavage of IGFBPs

Pregnancy-associated plasma protein-A (PAPP-A) was originally isolated in 1974, as one of four proteins of placental origin found in high concentrations in the blood of pregnant women. In the early 1990s several laboratories reported novel protease activity against insulin-like growth factor binding protein-4 (IGFBP-4) in media conditioned by several cell types. This activity was unique, as it appeared to require the presence of IGF to cleave IGFBP-4. In 1999, this IGF-dependent IGFBP-4 protease activity was isolated from human fibroblast conditioned media and identified as PAPP-A. Subsequently, PAPP-A was shown to be expressed by a variety of cell types, and thus no longer could be considered to be just "pregnancy-associated". This review will describe what is currently known about the structure of PAPP-A and about its function as an IGFBP protease, with a focus on new insights obtained through study of a PAPP-A knock-out mouse model and on potential clinical applications.

Definition, expression, and characterization of a protein domain in the N-terminus of pregnancy-associated plasma protein-A distantly related to the family of laminin G-like modules

Although pregnancy-associated plasma protein-A (PAPP-A), a modulator of insulin-like growth factor (IGF) activity through its cleavage of IGF-binding protein (IGFBP)-4 and -5, has been known for more than two decades, knowledge about its domain architecture is still incomplete. Using position-specific iterative BLAST, we have identified distant relatives of the PAPP-A N-terminal sequence stretch of 250 residues. We present evidence that a protein domain with weak similarity to known laminin G-like (LG) modules is contained within this region, and we propose that PAPP-A and PAPP-A2 are new and unique members in the group of LG proteins as the pappalysins represent the first examples where LG modules are associated with proteinases. Fourteen beta-strands characteristic for the LG structure were tentatively located within the PAPP-A LG (PA-LG) module using secondary structure prediction and sequence alignment. Upon mammalian expression of PAPP-A truncation mutants, we defined domain boundaries showing that PA-LG is an autonomously folding unit, which spans the first 243 residues. We were unable to express PAPP-A variants which lack the PA-LG module, suggesting a possible role in stabilization of the proteolytic domain. To obtain larger amounts of protein for functional and structural analysis, the defined PA-LG domain was expressed in bacteria and folded in vitro. In addition, the availability of recombinant PA-LG module may potentially improve diagnostic assays based on the measurement of PAPP-A antigen, and also facilitate the study of PAPP-A in animal model systems.

Functional properties of the sex-hormone-binding globulin (SHBG)-like domain of the anticoagulant protein S

Protein S (PS) possesses a sex-hormone-binding globulin (SHBG)-like domain in place of the serine-protease domain found in other vitamin K-dependent plasma proteins. This SHBG-like domain is able to bind a complement fraction, C4b-binding protein (C4b-BP). To establish whether the PS SHBG-like domain can fold normally in the absence of other domains, and to obtain information on the specific functions of this region, we expressed the PS SHBG-like domain alone or together with its adjacent domain EGF4. The folding of the two recombinant modules was studied by analyzing their binding to C4b-BP. The apparent dissociation constants of this interaction indicated that both recombinant modules adopted the conformation of native PS, indicating that the PS SHBG-like region is an independent folding unit. We also obtained the first direct evidence that the SHBG-like domain alone is sufficient to support the interaction with C4b-BP. In addition, both recombinant modules were able to bind Ca2+ directly, as shown by the migration shift in agarose gel electrophoresis in the presence of Ca2+, together with the results of equilibrium dialysis and the functional effect of Ca2+ on the C4b-BP/PS interaction, confirming the presence of one Ca2+ binding site within the SHBG-like domain. Neither recombinant module exhibited activated protein C (aPC) cofactor activity in a clotting assay, suggesting that the PS SHBG-like region must be part of the intact molecule for it to contribute to aPC cofactor activity, possibly by constraining the different domains in a conformation that permits optimal interaction with aPC.

LG/LNS domains: multiple functions -- one business end?

The three-dimensional structures of LG/LNS domains from neurexin, the laminin alpha 2 chain and sex hormone-binding globulin reveal a close structural relationship to the carbohydrate-binding pentraxins and other lectins. However, these LG/LNS domains appear to have a preferential ligand-interaction site distinct from the carbohydrate-binding sites found in lectins, and this interaction site accommodates not only sugars but also steroids and proteins. In fact, the LG/LNS domain interaction site has features reminiscent of the antigen-combining sites in immunoglobulins. The LG/LNS domain presents an interesting case in which the fold has remained conserved but the functional sites have evolved; consequently, making predictions of structure-function relationships on the basis of the lectin fold alone is difficult.

Structure of the C-terminal laminin G-like domain pair of the laminin alpha2 chain harbouring binding sites for alpha-dystroglycan and heparin

The laminins are large heterotrimeric glycoproteins with fundamental roles in basement membrane architecture and function. The C-terminus of the laminin alpha chain contains a tandem of five laminin G-like (LG) domains. We report the 2.0 A crystal structure of the laminin alpha2 LG4-LG5 domain pair, which harbours binding sites for heparin and the cell surface receptor alpha-dystroglycan, and is 41% identical to the laminin alpha1 E3 fragment. LG4 and LG5 are arranged in a V-shaped fashion related by a 110 degrees rotation about an axis passing near the domain termini. An extended N-terminal segment is disulfide bonded to LG5 and stabilizes the domain pair. Two calcium ions, one each in LG4 and LG5, are located 65 A apart at the tips of the domains opposite the polypeptide termini. An extensive basic surface region between the calcium sites is proposed to bind alpha-dystroglycan and heparin. The LG4-LG5 structure was used to construct a model of the laminin LG1-LG5 tandem and interpret missense mutations underlying protein S deficiency.

The structure of the ligand-binding domain of neurexin Ibeta: regulation of LNS domain function by alternative splicing

Neurexins are expressed in hundreds of isoforms on the neuronal cell surface, where they may function as cell recognition molecules. Neurexins contain LNS domains, folding units found in many proteins like the G domain of laminin A, agrin, and slit. The crystal structure of neurexin Ibeta, a single LNS domain, reveals two seven-stranded beta sheets forming a jelly roll fold with unexpected structural similarity to lectins. The LNS domains of neurexin and agrin undergo alternative splicing that modulates their affinity for protein ligands in a neuron-specific manner. These splice sites are localized within loops at one edge of the jelly roll, suggesting a distinct protein interaction surface in LNS domains that is regulated by alternative splicing.

Characterization of Bone Marrow Laminins and Identification of 5-Containing Laminins as Adhesive Proteins for Multipotent Hematopoietic FDCP-Mix Cells

Laminins are extracellular matrix glycoproteins that influence the phenotype and functions of many types of cells. Laminins are heterotrimers composed of , β, and γ polypeptides. So far five , three β, and two γ polypeptide chains, and 11 variants of laminins have been proposed. Laminins interact in vitro with mature blood cells and malignant hematopoietic cells. Most studies have been performed with laminin-1 (1β1γ1), and its expression in bone marrow is unclear. Employing an antiserum reacting with most laminin isoforms, we found laminins widely expressed in mouse bone marrow. However, no laminin 1 chain but rather laminin 2, 4, and 5 polypeptides were found in bone marrow. Our data suggest presence of laminin-2 (2β1γ1), laminin-8 (4β1γ1), and laminin-10 (5β1γ1) in bone marrow. Northern blot analysis showed expression of laminin 1, 2, 4, and 5 chains in long-term bone marrow cultures, indicating upregulation of laminin 1 chain expression in vitro. Laminins containing 5 chain, in contrast to laminin-1, were strongly adhesive for multipotent hematopoietic FDCP-mix cells. Integrin 6 and β1 chains mediated this adhesion, as shown by antibody perturbation experiments. Our findings indicate that laminins other than laminin-1 are functional in adhesive interactions in bone marrow.

Characterization of Bone Marrow Laminins and Identification of 5-Containing Laminins as Adhesive Proteins for Multipotent Hematopoietic FDCP-Mix Cells

Laminins are extracellular matrix glycoproteins that influence the phenotype and functions of many types of cells. Laminins are heterotrimers composed of , β, and γ polypeptides. So far five , three β, and two γ polypeptide chains, and 11 variants of laminins have been proposed. Laminins interact in vitro with mature blood cells and malignant hematopoietic cells. Most studies have been performed with laminin-1 (1β1γ1), and its expression in bone marrow is unclear. Employing an antiserum reacting with most laminin isoforms, we found laminins widely expressed in mouse bone marrow. However, no laminin 1 chain but rather laminin 2, 4, and 5 polypeptides were found in bone marrow. Our data suggest presence of laminin-2 (2β1γ1), laminin-8 (4β1γ1), and laminin-10 (5β1γ1) in bone marrow. Northern blot analysis showed expression of laminin 1, 2, 4, and 5 chains in long-term bone marrow cultures, indicating upregulation of laminin 1 chain expression in vitro. Laminins containing 5 chain, in contrast to laminin-1, were strongly adhesive for multipotent hematopoietic FDCP-mix cells. Integrin 6 and β1 chains mediated this adhesion, as shown by antibody perturbation experiments. Our findings indicate that laminins other than laminin-1 are functional in adhesive interactions in bone marrow.

Analysis of heparin, alpha-dystroglycan and sulfatide binding to the G domain of the laminin alpha1 chain by site-directed mutagenesis

The 395-residue proteolytic fragment E3, which comprises the two most C-terminal LG modules of the mouse laminin alpha1 chain, was previously shown to contain major binding sites for heparin, alpha-dystroglycan and sulfatides. The same fragment (alpha1LG4-5) and its individual alpha1LG4 and alpha1LG5 modules have now been obtained by recombinant production in mammalian cells. These fragments were apparently folded into a native form, as shown by circular dichroism, electron microscopy and immunological assays. Fragment alpha1LG4-5 bound about five- to tenfold better to heparin, alpha-dystroglycan and sulfatides than E3. These binding activities could be exclusively localized to the alpha1LG4 module. Side-chain modifications and proteolysis demonstrated that Lys and Arg residues in the C-terminal region of alpha1LG4 are essential for heparin binding. This was confirmed by 14 single to triple point mutations, which identified three non-contiguous basic regions (positions 2766-2770, 2791-2793, 2819-2820) as contributing to both heparin and sulfatide binding. Two of these regions were also recognized by monoclonal antibodies which have previously been shown to inhibit heparin binding. The same three regions and a few additional basic residues also make major contributions to the binding of the cellular receptor alpha-dystroglycan, indicating a larger binding epitope. The data are also consistent with previous findings that heparin competes for alpha-dystroglycan binding.

Binding of the G domains of laminin alpha1 and alpha2 chains and perlecan to heparin, sulfatides, alpha-dystroglycan and several extracellular matrix proteins

The C-terminal G domain of the mouse laminin alpha2 chain consists of five lamin-type G domain (LG) modules (alpha2LG1 to alpha2LG5) and was obtained as several recombinant fragments, corresponding to either individual modules or the tandem arrays alpha2LG1-3 and alpha2LG4-5. These fragments were compared with similar modules from the laminin alpha1 chain and from the C-terminal region of perlecan (PGV) in several binding studies. Major heparin-binding sites were located on the two tandem fragments and the individual alpha2LG1, alpha2LG3 and alpha2LG5 modules. The binding epitope on alpha2LG5 could be localized to a cluster of lysines by site-directed mutagenesis. In the alpha1 chain, however, strong heparin binding was found on alpha1LG4 and not on alpha1LG5. Binding to sulfatides correlated to heparin binding in most but not all cases. Fragments alpha2LG1-3 and alpha2LG4-5 also bound to fibulin-1, fibulin-2 and nidogen-2 with Kd = 13-150 nM. Both tandem fragments, but not the individual modules, bound strongly to alpha-dystroglycan and this interaction was abolished by EDTA but not by high concentrations of heparin and NaCl. The binding of perlecan fragment PGV to alpha-dystroglycan was even stronger and was also not sensitive to heparin. This demonstrated similar binding repertoires for the LG modules of three basement membrane proteins involved in cell-matrix interactions and supramolecular assembly.

Glomerular extracellular matrix components and integrins

It has become apparent that extracellular matrix components and their cellular receptors, the integrins, are important regulators of glomerular development and function. In this rapidly evolving field we studied the production of extracellular matrix components and integrins by rat glomerular visceral epithelial and mesangial cells, using molecular probes and antibodies that have recently become available. Special attention was paid to laminin isoforms and to splice variants of the integrin subunits alpha 3 and alpha 6. Results were compared to the in vivo expression in human fetal, newborn and adult kidneys. The mesangial cells were found to produce laminin-1, nidogen and two as yet unidentified laminin isoforms with putative alpha chains of about 395 (alpha x) and of 375 kDa (alpha y), tentatively described before as bovine kidney laminin. Furthermore, they expressed the integrins alpha 1 beta 1, alpha 2 beta 1, alpha 3A beta 1, alpha 5 beta 1, alpha v beta 3, alpha v beta 5, and small amounts of alpha 6A beta 1 and alpha 6B beta 1. The glomerular visceral epithelial cells produced the two new laminin isoforms mentioned above, laminin-5, but no laminin-1 or nidogen. The integrins alpha 2 beta 1, alpha 3A beta 1, alpha 6A beta 4, alpha 6B beta 4 and the integrin subunit alpha v were found to be expressed. We show that during nephrogenesis, the laminin alpha 1 chain disappears and is replaced by another alpha chain, possibly one of the two as yet unidentified alpha chains mentioned above. The laminin beta 1 chain is replaced by the beta 2 chain somewhat later in glomerular development. In general, the integrins found to be expressed in glomeruli of adult kidney were consistent with those found in cultured glomerular visceral epithelial and mesangial cells. No splice variant switch of the integrin alpha 3 or alpha 6 subunits could be demonstrated during nephrogenesis. Our results suggest an important role for the mesangial cell in providing nidogen as a crucial component of the supramolecular structure of the glomerular basement membrane. Furthermore our results indicate that laminin alpha x beta 2 gamma 1 and alpha y beta 2 gamma 1 isoforms are important in the glomerulus of adult kidney and that the integrin alpha 3A beta 1 is the main integrin receptor for laminin isoforms on glomerular visceral epithelial and mesangial cells, both in vitro and in vivo.

Merging extracellular domains: fold prediction for laminin G-like and amino-terminal thrombospondin-like modules based on homology to pentraxins

Using a new method for construction and database searches of sequence consensus strings, we have identified a new superfamily of protein modules comprising laminin G, thrombospondin N and the pentraxin families. The conserved patterns correspond mainly to hydrophobic core residues located in central beta strands of the known three-dimensional structures of two pentraxins, the human C-reactive protein and the serum amyloid P-component. Thus, we predict a similar jellyroll fold for all members of this superfamily. In addition, the conservation of two exposed aspartate residues in the majority of superfamily members suggests hitherto unrecognised functional sites.

Neurexins: three genes and 1001 products

The human brain has approximately 10(12) neurons, three orders of magnitude more than there are basepairs in the human genome. Each neuron is connected to other neurons by thousands of synapses, creating a dense network of communicating neurons. Cell-recognition events between neurons at, and outside of synapses, are likely to guide the development and maintenance of the complex network formed by neurons. However, little is known about which proteins are important for neuronal cell recognition. Neurexins, a family of polymorphic cell-surface proteins, might mediate some of these cell recognition events. Thousands of neurexin isoforms are generated from three genes by usage of alternative promoters and alternative splicing. These isoforms are displayed on the neuronal cell surface, with different classes of neurons expressing distinct combinations of isoforms. Neurexins probably have a multitude of ligands, some of which interact only with subsets of neurexin isoforms. This review describes the properties of the neurexin protein family and their potential roles in neuronal cell adhesion and intercellular signaling.

Sequence and functional relationships between androgen-binding protein/sex hormone-binding globulin and its homologs protein S, Gas6, laminin, and agrin

Androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) is an extracellular binding protein that regulates the bioavailability of sex steroids. ABP/SHBG is closely related to the globular (G) domain of vitamin K-dependent protein S family of proteins and more distantly related to the G domains of several extracellular matrix proteins. ABP/SHBG appears to have evolved from the fusion of two ancestral G domains. Expanding evidence suggests that ABP/SHBG has other functions that are mediated through membrane binding, including signal transduction; however, the types of binding proteins (receptors) have not been identified. Sequence comparisons of ABP/SHBG with G domains of its homologs protein S, Gas6, laminin, and agrin have identified regions of ABP/SHBG that may bind receptors related to homolog receptors. These membrane receptors include beta-integrins, alpha-dystroglycan, and receptor tyrosine kinases. The G domains of laminin and related proteins have clearly evolved from a common ancestor to interact with specific receptors and binding proteins. It remains to be determined if ABP/SHBG followed this evolutionary pathway.

Cloning of the mouse laminin alpha 4 cDNA. Expression in a subset of endothelium

Endothelial cells express a 400-kDa or 240-kDa laminin alpha chain, depending on their tissue of origin or physiological state [1, 2]. Using differential display and subsequent screening of a mouse endothelial cell cDNA library we here identify the gene coding for the 240-kDa laminin chain as the laminin alpha 4 gene. The complete mouse laminin alpha 4 cDNA sequence is reported and compared with other laminin alpha chains. In situ hybridization of embryonic and new born mouse tissues revealed expression of laminin alpha 4 mRNA in a subset of endothelium, in particular aortic endothelium, endocardium and endothelium of blood vessels in the skin and in the brain. Strong laminin alpha 4 expression by aortic endothelia was confirmed by data obtained from cultured bovine aortic endothelial cells (BAEC). Isolation of laminin from BAEC conditioned medium revealed a Y-shaped molecule in rotary shadowing. Subsequent sequencing of BAEC laminin resulted in laminin alpha 4, beta 1 and gamma 1 amino acid sequences, confirming that laminin alpha 4 is one of the major laminin alpha chains expressed by aortic endothelium not only in the mouse. In addition, strong laminin alpha 4 mRNA expression occurred in peripheral nerves, cardiac muscle, fat, the dermis of the skin and lung stroma of mouse tissues. The data demonstrate a cytokine and progesterone-regulated differential expression of laminin alpha 4 mRNA in mouse endothelium, suggesting a distinct functional role for this laminin chain in endothelium.

HNK-1 carbohydrate-mediated cell adhesion to laminin-1 is different from heparin-mediated and sulfatide-mediated cell adhesion

The sulfated HNK-1 carbohydrate present on glycolipids and on several neural recognition molecules has been shown to mediate the adhesion of murine small cerebellar neurons and astrocytes to the extracellular matrix molecule laminin-1. In this study, we characterized the binding of the HNK-1 carbohydrate to laminin-1 extracted from the Engelbreth-Holm-Swarm (EHS) sarcoma and distinguished it unequivocally from binding sites for other sulfated carbohydrates. Electron microscopic analysis of rotary shadowed complexes of laminin-1 and a HNK-1 neoglycoprotein revealed a major binding site on the G domain that comprises the C-terminal globule of the laminin alpha1 chain. The HNK-1 carbohydrate also interacted with placental laminin isoforms containing an alpha chain variant. It bound to the proteolytic laminin-1 fragment E8 comprising the domains G1-G3, but not to fragment E3 that carries the major heparin-binding site on domains G4-G5. No binding was observed to the short arm containing fragments E1XNd or P1. Binding studies with native or denatured laminin E8 fragments and proteolytic or recombinant fragments of the G domain localized the HNK-1 carbohydrate binding site to domain G2. The binding could be clearly distinguished from binding sites for other sulfated carbohydrates such as heparin and sulfatides. Further, the binding could not be abolished by reduction and alkylation or by urea treatment of laminin-1 and was independent of the native conformation of laminin-1 and of Ca2+. The G2 domain is also involved in the adhesion of HNK-1 carbohydrate expressing early postnatal cerebellar neurons and is different from heparin- and sulfatide-mediated cell adhesion to laminin-1. HNK-1 carbohydrate-mediated cell adhesion appears, however, to be dependent on the native conformation of laminin-1 indicating a more complex cellular recognition process.

The biochemistry of cancer dissemination

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

Collagen COL4A3 knockout: a mouse model for autosomal Alport syndrome

A mouse model for the autosomal form of Alport syndrome was produced. These mice develop a progressive glomerulonephritis with microhematuria and proteinuria, consistent with the human disease. End-stage renal disease develops at approximately 14 weeks of age. TEM analysis of the glomerular basement membranes (GBM) during development of renal pathology revealed focal multilaminated thickening and thinning beginning in the external capillary loops at 4 weeks and spreading throughout the GBM by 8 weeks. By 14 weeks, half of the glomeruli were fibrotic with collapsed capillaries. Immunofluorescence analysis of the GBM showed the absence of type IV collagen alpha-3, alpha-4, and alpha-5 chains and a persistence of alpha-1 and alpha-2 chains (these chains normally localize to the mesangial matrix). Northern blot analysis using probes specific for the collagen chains illustrate the absence of COL4A3 in the knockout, whereas mRNAs for the remaining chains are unchanged. An accumulation of fibronectin, heparan sulfate proteoglycan, laminin-1, and entactin was observed in the GBM of the affected animals. The temporal and spatial pattern of accumulation was consistent with that for thickening of the GBM as observed by TEM. Thus, expression of these basement membrane-associated proteins may be involved in the progression of Alport renal disease pathogenesis. The levels of mRNAs encoding the basement membrane-associated proteins at 7 weeks were unchanged.

Inhibition of laminin alpha 1-chain expression leads to alteration of basement membrane assembly and cell differentiation

The expression of the constituent alpha 1 chain of laminin-1, a major component of basement membranes, is markedly regulated during development and differentiation. We have designed an antisense RNA strategy to analyze the direct involvement of the alpha 1 chain in laminin assembly, basement membrane formation, and cell differentiation. We report that the absence of alpha 1-chain expression, resulting from the stable transfection of the human colonic cancer Caco2 cells with an eukaryotic expression vector comprising a cDNA fragment of the alpha 1 chain inserted in an antisense orientation, led to (a) an incorrect secretion of the two other constituent chains of laminin-1, the beta 1/gamma 1 chains, (b) the lack of basement membrane assembly when Caco2-deficient cells were cultured on top of fibroblasts, assessed by the absence of collagen IV and nidogen deposition, and (c) changes in the structural polarity of cells accompanied by the inhibition of an apical digestive enzyme, sucrase-isomaltase. The results demonstrate that the alpha 1 chain is required for secretion of laminin-1 and for the assembly of basement membrane network. Furthermore, expression of the laminin alpha 1-chain gene may be a regulatory element in determining cell differentiation.

Expression of laminin isoforms in mouse myogenic cells in vitro and in vivo

The expression of laminin-1 (previously EHS laminin) and laminin-2 (previously merosin) isoforms by myogenic cells was examined in vitro and in vivo. No laminin alpha 2 chainspecific antibodies react with mouse tissues, 50 rat monoclonal antibodies were raised against the mouse laminin alpha 2 chain: their characterization is described here. Myoblasts and myotubes from myogenic cell lines and primary myogenic cultures express laminin beta 1 and gamma 1 chains and form a complex with a 380 kDa alpha chain identified as laminin alpha 2 by immunofluorescence, immunoprecipitation and PCR. PCR from C2C12 myoblasts and myotubes for the laminin alpha 2 chain gene (LamA2) provided cDNA sequences which were used to investigate the in vivo expression of mouse LamA2 mRNA in embryonic tissues by in situ hybridization. Comparisons were made with specific probes for the laminin alpha 1 chain gene (LamA1). LamA2 but not LamA1 mRNA was expressed in myogenic tissues of 14- and 17-day-old mouse embryos, while the laminin alpha 2 polypeptide was localized in adjacent basement membranes in the muscle fibres. In situ hybridization also revealed strong expression of the LamA2 mRNA in the dermis, indicating that laminin alpha 2 is expressed other than by myogenic cells in vivo. Immunofluorescence studies localized laminin alpha 2 in basement membranes of basal keratinocytes and the epithelial cells of hair follicles, providing new insight into basement membrane assembly during embryogenesis. In vitro cell attachment assays revealed that C2C12 and primary myoblasts adhere to laminin-1 and -2 isoforms in a similar manner except that myoblast spreading was significantly faster on laminin-2. Taken together, the data suggest that laminins 1 and 2 play distinct roles in myogenesis.

Extracellular matrix components in intestinal development

Intestinal morphogenesis and differentiation are dependent on heterotypic cell interactions between embryonic epithelial cells (endoderm) and stromal cells (mesenchyme). Extracellular matrix molecules represent attractive candidates for regulators of these interactions. The structural and functional diversity of the extracellular matrix as intestinal development proceeds is demonstrated by 1) spatio-temporal specific expression of the classically described constituents, 2) the finding of laminin and collagen IV variants, 3) changes in the ratio of individual constituent chains, and 4) a stage-specific regulation of basement membrane molecule production, in particular by glucocorticoids. The orientation/assembly of these extracellular matrix molecules could direct precise cellular functions through interactions via integrin molecules. The involvement of extracellular matrix, and in particular basement membrane molecules in heterotypic cell interactions leading to epithelial cell differentiation, has been highlighted by the use of experimental models such as cocultures, hybrid intestines and antisense approaches. These models allowed us to conclude that a correct elaboration and assembly of the basement membrane, following close contacts between epithelial and fibroblastic cells, is necessary for the expression of differentiation markers such as digestive enzymes.

Collagen binding activity of recombinant and N-terminally modified annexin V (anchorin CII)

We have cloned the full coding cDNA sequence of chicken annexin V and of a mutant lacking 8 amino acid residues of the N-terminal tail for prokaryotic expression. Both proteins were synthesized in Escherichia coli upon induction with isopropyl thio-beta-D-galactoside, and were purified following two different protocols: one based on the ability of these proteins to interact reversibly with liposomes in the presence of calcium, and the other based on two sequential ion-exchange chromatographic steps. Spectroscopical analysis of recombinant annexin V revealed that binding of calcium did not change the circular dichroism spectra indicating no significant changes on the secondary structure; however, a conformational change affecting the exposition to the solvent of the tryptophan residue 187 was detected by analysis of fluorescence emission spectra. Recombinant annexin V binds with high affinity to collagen types II and X, and with lower affinity to collagen type I in a calcium-independent manner. Heat denaturing of collagen decreases this interaction while pepsin-treatment of collagen almost completely abolishes annexin V binding. Mutated annexin V interacts with collagen in a similar way as the nonmutated recombinant protein, indicating that the N-terminal tail of annexin V is not essential for collagen binding.

Expression of novel 400-kDa laminin chains by mouse and bovine endothelial cells

Laminin expression was studied in endothelial cells derived from different mouse tissues and primary cultures of bovine aortic endothelium (BAEC). Immunoprecipitation with polyclonal anti-laminin-1 revealed two 200-kDa chains, a novel 400-kDa chain and nidogen in all cells studied. Two-dimensional electrophoresis of laminin complexes immunoprecipitated from endothelial-cell-conditioned medium demonstrated that the 400-kDa chain was disulphide-linked to the laminin beta 1 and gamma 1 chains. In addition, rotary shadowing illustrated the secretion of molecules from the endothelial cells which were morphologically similar to laminin-1. Immunoblotting and Northern blot analysis confirmed the presence of beta 1 and gamma 1 polypeptides and the corresponding LamB1 and LamC1 mRNAs in all cells investigated. However, both polyclonal anti-laminin-1 and alpha 1-specific monoclonal antibodies failed to react with the 400-kDa polypeptide in immunoblots and immunoprecipitations. Similarly, the expression of the gene coding for alpha 1, LamA1, was not detected in the majority of cells by Northern blot analysis using three different cDNAs. Only in two cases, BAEC and a Simian-virus-40-transformed mouse endothelial cell line (SVEC), a 10-kb mRNA was detected by Northern analysis using a cDNA specific for the 3' coding region (LAC) of LamA1 mRNA. However, cDNAs specific for the central and 5' coding region of LamA1 mRNA did not show any reaction. Hybridization of LAC to BAEC and SVEC mRNA is consistent with the laminin-like nature of the 400-kDa chain expressed by these cells. The results demonstrate the existence of at least one and possibly two novel 400-kDa laminin chains which complex with beta 1 and gamma 1 and occur in mouse and bovine endothelial cell basement membranes.

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.

Cell and heparin binding in the distal long arm of laminin: identification of active and cryptic sites with recombinant and hybrid glycoprotein

The long arm of laminin, which binds heparin and cells, consists of three polypeptides (A, B1, and B2) joined in a coiled-coil rod attached to a terminal A chain globule (G). Previously, we found that recombinant globular domain (rG) supported heparin and myoblast binding (Yurchenco, P. D., U. Sung, M. D. Ward, Y. Yamada, and J. J. O'Rear. 1993. J. Biol. Chem. 268:8356-8365). To further analyze long arm functions, we expressed the distal moiety of the mouse laminin A chain extending from the middle of the rod to the carboxyl terminus (rAiG). This larger glycoprotein, secreted by Sf9 insect cells infected with recombinant baculovirus, was intercalated in vitro into the corresponding disulfide-linked B chain segments of laminin fragment E8 (distal long arm rod and proximal globule). The hybrid molecule (B-rAiG) possessed a structure similar to laminin long arm as judged by electron microscopy and limited proteolysis. By joining rAiG with E8-B chains, the affinity of G domain for heparin decreased from that observed with rAiG and rG to one similar to native protein. HT1080 cells adhered to E8, rAiG, and B-rAiG, less well to rG, and not to denatured E8/B-rAiG, the A and B chain moieties of E8, or to a mixture of rG and E8-B chains. Cell adhesion to E8 and B-rAiG, in contrast to rAiG, was inhibited with antibodies specific for alpha 6 and beta 1 integrin chains. Since intercalation (a) restored a conformationally dependent alpha 6 beta 1 integrin recognition site present in native protein, (b) inactivated a cryptic cell binding activity in the A chain, and (c) inhibited a heparin binding site present in proximal G domain, we conclude that biological activities of laminin are different from that of its isolated subunits.

Differential expression of laminin, nidogen and collagen IV genes in the midgestation mouse placenta

The distribution of laminin A, B1, B2, nidogen and collagen alpha 1(IV) mRNA was studied in the 12.5-day mouse placenta and uterus. This was compared to the pattern of laminin, nidogen and collagen IV immunoreactivity in the placenta at this time. High levels of B2 mRNA were distributed throughout the decidual layer, compared to lower levels of laminin B1, nidogen and collagen IV. In contrast, laminin B1 and nidogen mRNA were found at very high levels in the trophoblast giant cells and cytotrophoblast. Laminin B2 was much lower in trophoblast cells than in the decidua. Nidogen mRNA levels were low in the decidual cells, but high in endothelial cells lining the placental blood spaces in the decidual layer. Immunofluorescence staining of the placenta showed colocalization of laminin, nidogen and collagen IV in fetal and maternal layers of the placenta. In the antimesometrial area where the uterine epithelium was reforming laminin B1 and B2 mRNA were uniformly distributed between the epithelium and stroma, whereas nidogen and collagen alpha 1(IV) mRNA were only produced by the stromal cells. In all cell types, apart from parietal endoderm cells forming Reichert's membrane, laminin A chain mRNA was very low or absent. These results demonstrate that laminin and nidogen genes are not coordinately expressed in the midgestation mouse placenta. Comparison of the distribution of these mRNAs with collagen alpha 1(IV) mRNA suggests that different regions of the placenta produce specialized extracellular matrices which may contain different ratios of these polypeptides.

Expression of the laminin-A chain is down-regulated by a non-canonical polyadenylation signal

It is well accepted that 3' untranslated regions (UTR) are an essential part of mRNA. However, little is known in detail about the contribution of different regions of 3' UTR on synthesis, stability and translatability of their mRNA. In addition to the highly conserved hexanucleotide AAUAAA, some consensus sequences for 3'-end processing and polyadenylation have been characterized, but most of this work has been done with viral mRNA or beta-globin mRNA. We have studied the influence of the 3' UTR of the mRNA for the three chains A, B1, B2 of laminin on the expression of a reporter gene (galK). Laminin is a large glycoprotein of basement membranes and all three polypeptide chains are needed in equal amounts for a functional molecule. The three 3' UTR of the laminin mRNA differ widely with respect to length, number of polyadenylation signals and other consensus sequences. Nevertheless, all three 3' UTR reduce the expression of the reporter gene at least three-fold, when the corresponding cDNA sequences are inserted downstream of the reporter gene instead of the 3' UTR of simian virus 40 early genes. The 3' UTR of laminin-A mRNA contains the non-canonical polyadenylation signal AUUAAA which seems to be responsible for the limiting amounts of laminin-A mRNA and protein compared to those for laminin B1 and B2 [Speth and Oberbäumer (1993) Exp. Cell Res. 204, 302-310]. Mutation of the laminin-A polyadenylation signal to the canonical form AAUAAA increases expression by a factor of 2.5.

Variants of the alpha 6 beta 1 laminin receptor in early murine development: distribution, molecular cloning and chromosomal localization of the mouse integrin alpha 6 subunit

Laminin (A:B1:B2) is a major component of the first basement membrane to appear in the developing mouse embryo. Its effects on morphogenesis and differentiation are mediated by interaction with cell surface receptors that are members of the integrin family. We have studied the expression of the alpha 6 subunit of murine alpha 6 beta 1 and its ligand, laminin, in preimplantation mouse embryos, embryo outgrowths and in embryonic stem (ES) cells and embryonal carcinoma (EC) cells. The alpha 6 subunit is present in the oocyte and throughout preimplantation development. Laminin A chain appears later than alpha 6 and has a more restricted distribution until the late blastocyst stage. alpha 6 beta 1 is strongly expressed in ES and EC cells; the levels of mRNA expression are not altered by differentiation. Molecular cloning of cDNA for the murine integrin alpha 6 subunit from a mammary gland lambda gt11 library showed, as in man, an open reading frame encoding two variants of alpha 6, alpha 6A and alpha 6B. The identity of the alpha 6 amino acid sequence to that in man and chicken is 93% and 73%, respectively. The gene for murine alpha 6 was mapped to chromosome 2. While undifferentiated ES and EC cells express only alpha 6B, alpha 6A is co-expressed in ES cells after differentiation is induced by retinoic acid. alpha 6B is also the only variant expressed in blastocyst stage embryos, but when blastocysts have grown out in culture both alpha 6A and alpha 6B are expressed reflecting the results in the cell lines. We suggest that the deposition of laminin in the embryo is a receptor-mediated process and that the shift in the expression of the variants, as the inner cell mass forms its first differentiated progeny, reflects a change in functional properties.

Role of laminin carbohydrates on cellular interactions

Laminins, a family of large multidomain glycoproteins of the basal lamina, have been implicated in the development and maintenance of cellular and tissue organization. Considerable interest has arisen concerning the ways in which laminin carries out its biological functions. Previously these biologic responses have been primarily attributed to the peptide sequences of laminin, however, newer studies suggest that laminin carbohydrates may also participate in such cellular activities. Recently, a subpopulation of laminin molecules purified from EHS sarcoma by lectin affinity chromatography has been shown to contain about 25 to 30% carbohydrate. Most of the carbohydrates present are complex-type asparagine-linked oligosaccharides encompassing many different structures, some of which are unique to laminin. To date, the biological function of the carbohydrates of laminin remains somewhat unclear. They do not appear to be needed for heparin binding or to enhance proteinase stability, however, current evidence suggests they are important in cellular spreading and neurite outgrowth. It is our hypothesis that in the covalently-linked carbohydrate moieties of laminin will ultimately prove to be involved in information transfer to responsive cells. It is the purpose of this review to delineate current concepts of the structure and function of this unique glycoprotein's sugar chains.

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

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

Monoclonal antibodies against laminin A chain fragment E3 and their effects on binding to cells and proteoglycan and on kidney development

Rat monoclonal antibodies were raised against fragment E3 of the mouse Engelbreth-Holm-Swarm (EHS) tumor laminin and selected according to their exclusive reaction with laminin A chain by immunoblotting and staining pattern in embryonic kidneys by immunofluorescence. Immunochemical studies of nine purified antibodies showed a comparable reaction with unfragmented laminin and fragment E3 but no cross-reaction with several other, unrelated laminin fragments including the major cell-binding fragment E8. Reduction or pepsin digestion of fragment E3 reduced or abolished antibody binding indicating that most of the epitopes involved are conformation dependent and do not include carbohydrates. They are, however, not identical as shown by different reactivities after proteolytic or chemical cleavage of E3. Four of the antibodies were highly active in inhibiting cell adhesion of the teratocarcinoma cell line F9 and the Schwannoma cell line RN22 on fragment E3 (IC50 approximately 1 microgram/ml), while the others were distinctly less active. No inhibition was observed for cell adhesion on unfragmented laminin, consistent with previous findings that this is largely mediated by binding of fragment E8 to alpha 6 beta 1 integrin. A distinct correlation was observed between cell adhesion inhibition and the inhibition of heparansulfate proteoglycan and heparin binding to fragment E3. Since heparin is not very efficient in inhibiting cell adhesion, it indicates that heparin- and cell-binding sites on fragment E3 are in close proximity but not identical. Two of the antibodies also showed partial inhibition of kidney tubule formation in organ culture of embryonic kidney mesenchyme while the other antibodies were inactive. It suggests some but probably minor involvement of the fragment E3 structure of laminin in this developmental process.

Evaluation of mRNA steady-state and protein levels for basement membrane proteins in cultured murine cells

We have determined the mRNA steady-state levels for the six constituent polypeptide chains of the basement membrane proteins collagen IV, laminin and nidogen in murine cell lines derived from a teratocarcinoma, and in some other cell lines of different origin in stationary cultures and during different growth phases. The mRNA and protein levels change in response to growth phase. The amounts of the mRNAs for the single chains do not agree with the ratios needed for the different peptide chains of collagen IV and laminin. While the mRNA and protein levels for laminin are in a similar range for the teratocarcinoma-derived cell lines, the mRNA and protein levels vary by at least a factor of 10 for collagen IV. These results point to complex posttranscriptional regulatory mechanisms for the biosynthesis of basement membrane proteins.

A synthetic peptide derived from the carboxy terminus of the laminin A chain represents a binding site for the alpha 3 beta 1 integrin

The purpose of this study was to identify the binding site(s) within laminin for the alpha 3 beta 1 integrin receptor. It has been previously shown, using proteolytic fragments and anti-laminin antibodies, that the region in laminin for alpha 3 beta 1 integrin binding is localized to the carboxy-terminal region at the end of the long arm (Gehlsen, K. R., E. Engvall, K. Dickerson, W. S. Argraves, and E. Ruoslahti. 1989. J. Biol. Chem. 264:19034-19038; Tomaselli, K. J., D. E. Hall, L. T. Reichardt, L. A. Flier, K. R. Gehlsen, D. C. Turner, and S. Carbonetto. 1990. Neuron. 5:651-662). Using synthetic peptides, we have identified an amino acid sequence within the carboxy-terminal region of the laminin A chain that is recognized by the alpha 3 beta 1 integrin. The amino acid sequence represented by the synthetic peptide GD-6 (KQNCLSSRASFRGCVRNLRLSR residues numbered 3011 to 3032) of the globular domain of the murine A chain supports cell attachment and inhibits cell adhesion to laminin-coated surfaces. By affinity chromatography, peptide GD-6-Sepharose specifically bound solubilized alpha 3 beta 1 from extracts of surface-iodinated cells in a cation-dependent manner, while it did not bind other integrins. In addition, exogenous peptide GD-6 specifically eluted bound alpha 3 beta 1 from laminin-Sepharose columns but did not elute the alpha 3 beta 1 integrin from a fibronectin-Sepharose column. Using integrin subunit-specific monoclonal antibodies, only those antibodies against the alpha 3 and beta 1 subunits inhibited cell adhesion to peptide GD-6-coated surfaces. Finally, a polyclonal antibody made against peptide GD-6 reacted specifically with both murine and human laminin and significantly inhibited cell adhesion to laminin-coated surfaces but not those coated with other matrix proteins. These results identify the laminin A chain amino acid sequence of peptide GD-6 as representing a binding site in laminin for the alpha 3 beta 1 integrin.

Basement membrane proteins: structure, assembly, and cellular interactions

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

Laminin A-related domains in crb protein of Drosophila and their possible role in epithelial polarization

It is shown that four domains of the integral membrane protein crb of Drosophila are homologous with five C-terminal domains of the laminin A chain and merosin. Since the latter domains of laminin A have been implicated in determining epithelial polarity, the homology may suggest a similar function for these domains of crb. Such a function would be consistent with the known importance of crb for organization of epithelia in Drosophila.