Primary structure of the human laminin A chain. Limited expression in human tissues

Laminin N-terminus (LaNt) proteins, laminins and basement membrane regulation

Basement membranes (BMs) are structured regions of the extracellular matrix that provide multiple functions including physical support and acting as a barrier, as a repository for nutrients and growth factors, and as biophysical signalling hubs. At the core of all BMs is the laminin (LM) family of proteins. These large heterotrimeric glycoproteins are essential for tissue integrity, and differences between LM family members represent a key nexus in dictating context and tissue-specific functions. These variations reflect genetic diversity within the family, which allows for multiple structurally and functionally distinct heterotrimers to be produced, each with different architectures and affinities for other matrix proteins and cell surface receptors. The ratios of these LM isoforms also influence the biophysical properties of a BM owing to differences in their relative ability to form polymers or networks. Intriguingly, the LM superfamily is further diversified through the related netrin family of proteins and through alternative splicing leading to the generation of non-LM short proteins known as the laminin N-terminus (LaNt) domain proteins. Both the netrins and LaNt proteins contain structural domains involved in LM-to-LM interaction and network assembly. Emerging findings indicate that one netrin and at least one LaNt protein can potently influence the structure and function of BMs, disrupting the networks, changing physical properties, and thereby influencing tissue function. These findings are altering the way that we think about LM polymerisation and, in the case of the LaNt proteins, suggest a hitherto unappreciated form of LM self-regulation.

Laminin Polymerization and Inherited Disease: Lessons From Genetics

The laminins (LM) are a family of basement membranes glycoproteins with essential structural roles in supporting epithelia, endothelia, nerves and muscle adhesion, and signaling roles in regulating cell migration, proliferation, stem cell maintenance and differentiation. Laminins are obligate heterotrimers comprised of α, β and γ chains that assemble intracellularly. However, extracellularly these heterotrimers then assemble into higher-order networks via interaction between their laminin N-terminal (LN) domains. In vitro protein studies have identified assembly kinetics and the structural motifs involved in binding of adjacent LN domains. The physiological importance of these interactions has been identified through the study of pathogenic point mutations in LN domains that lead to syndromic disorders presenting with phenotypes dependent on which laminin gene is mutated. Genotype-phenotype comparison between knockout and LN domain missense mutations of the same laminin allows inferences to be drawn about the roles of laminin network assembly in terms of tissue function. In this review, we will discuss these comparisons in terms of laminin disorders, and the therapeutic options that understanding these processes have allowed. We will also discuss recent findings of non-laminin mediators of laminin network assembly and their implications in terms of basement membrane structure and function.

Laminin and Integrin in LAMA2-Related Congenital Muscular Dystrophy: From Disease to Therapeutics

Laminin-α2-related congenital muscular dystrophy (LAMA2-CMD) is a devastating neuromuscular disease caused by mutations in the LAMA2 gene. These mutations result in the complete absence or truncated expression of the laminin-α2 chain. The α2-chain is a major component of the laminin-211 and laminin-221 isoforms, the predominant laminin isoforms in healthy adult skeletal muscle. Mutations in this chain result in progressive skeletal muscle degeneration as early as neonatally. Laminin-211/221 is a ligand for muscle cell receptors integrin-α7β1 and α-dystroglycan. LAMA2 mutations are correlated with integrin-α7β1 disruption in skeletal muscle. In this review, we will summarize laminin-211/221 interactions with integrin-α7β1 in LAMA2-CMD muscle. Additionally, we will summarize recent developments using upregulation of laminin-111 in the sarcolemma of laminin-α2-deficient muscle. We will discuss potential mechanisms of action by which laminin-111 is able to prevent myopathy. These published studies demonstrate that laminin-111 is a disease modifier of LAMA2-CMD through different methods of delivery. Together, these studies show the potential for laminin-111 therapy as a novel paradigm for the treatment of LAMA2-CMD.

Comparative Analysis of the Distribution of Laminin Chains in the Basement Membranes in Some Malignant Epithelial Tumors: The α1 Chain of Laminin Shows a Selected Expression Pattern in Human Carcinomas

Laminins (Ln), together with Type IV collagen and nidogen-1, form the structural integrity of the basement membranes (BM). In this study we used immunohistochemistry to show the distribution of laminin chains α1, α3, α5, β1, β2, β3, γ1, γ2, as well as Type IV collagen, in various types of carcinomas and in normal tissues. Except for diffuse gastric carcinomas and infiltrative breast carcinomas, the malignant epithelial tumor clusters were surrounded by quite a continuous BM in most tumors. These BMs comprised most abundantly Ln α5, β1, and γ1 chains. Conversely, the Ln α1 chain, a component of laminins-1 and -3, showed the most restricted distribution in BMs of both normal tissues and malignancies, being moderately present in carcinomas of thyroid gland and ovary and in intraductal carcinomas of breast. In other types of carcinomas, immunoreactivity for Ln α1 chain was found more randomly and was practically negative in carcinomas of tongue, stomach, and colon. These findings were comparable to those observed by in situ hybridization, which showed that carcinomas of thyroid gland and intraductal carcinomas of breast constitutively expressed Ln α1 mRNA and that the epithelial tumor cells were the main producers of it. The results suggest that epithelial malignancies, except for infiltrative breast and diffuse gastric carcinomas, produce more notable amounts of BM macromolecules in their growth substratum than has previously been anticipated. Corroborating their widespread distribution in normal epithelial tissues, the chains of Lns-5 and -10 are the most abundant Ln molecules in the corresponding carcinomas.

Potential molecular chaperones involved in laminin chain assembly

To explore potential molecular chaperones involved in the intracellular assembly of laminin chains, bovine aortic endothelial cells were treated with a thiol cleavable divalent cross-linking reagent, dithio-bis-(succinimidylpropionate), and cellular proteins cross-linked to laminin chains were co-immunoprecipitated with anti-laminin antiserum. Sodium dodecylsulfate (SDS) gel electrophoresis of the precipitate under reducing condition showed polypeptides with estimated sizes of 80, 60 and 50 kDa together with laminin chains. Two dimensional electrophoresis, in which non-reducing and reducing SDS electrophoresis were combined, suggested that many molecules of these polypeptides were cross-linked to each laminin chain. Sepharose CL-4B beads conjugated with E8 fragment of mouse laminin-1 was prepared. Affinity chromatography with the beads of microsomal proteins from rat liver showed that Bip and HSP70 associated to laminin chains and dissociated upon ATP hydrolysis. Protein-disulfide isomerase also showed affinity to the column. GRP94 and calnexin showed strong affinity and were washed out only with a detergent solution. Thus, many molecular chaperones are suggested to be involved in the intracellular assembly of laminin chains.

Three heterotrimeric laminins produced by human keratinocytes

Laminins are a family of glycoproteins composed of alpha,beta and gamma chains. Five alpha(alpha1-alpha5), three beta (beta1-beta3) and twogamma (gamma1 and gamma2) chains have been cloned fromhuman and their replaceable assembly into heterotrimers producesthe variety of laminins. Reverse transcription-polymerase chainreaction of mRNAs showed that human keratinocytes express thealpha3, alpha5, beta1, beta3, gamma1 andgamma2 genes at high level among the ten cloned lamininchains. Western blot and immunoprecipitation of the cell lysatewith antiserum directed against mouse laminin-1(alpha1beta1gamma1) detected two trimers with thecomposition of alphaxbeta1gamma1 (probablylaminin-10 with the composition of alpha5beta1gamma1and alphaybeta1gamma1. Meanwhile, antiserum directedagainst a synthetic peptide of human alpha3 detected onlyalpha3beta3gamma2 trimer (laminin-5). We thus show thatkeratinocytes produce three heterotrimeric laminins. We couldnot detect the assembly of alpha3 with beta1 and gamma1chains to form alpha3beta1gamma1 (laminin-6) in keratinocytes.

The association of a single nucleotide polymorphism in the promoter region of the LAMA1 gene with susceptibility to Chinese high myopia

PURPOSE

High myopia is a severe hereditary ocular disease leading to blindness. LAMA1 (alpha subunit of laminin) is a promising candidate gene for high myopia present in the MYP2 (myopia 2) region. The purpose of this study was to determine if high myopia is associated with single nucleotide polymorphism (SNP) variants in LAMA1 in Chinese subjects.

METHODS

Ninety-seven Chinese subjects with high myopia and ethnically and sexually matched 103 normal controls were enrolled. Genomic DNA was prepared from peripheral blood. The 5 SNPs of LAMA1 were analyzed using PCR and SNaPshot. Allele frequencies were tested for Hardy-Weinberg disequilibrium. The genotype and allele frequencies were evaluated using the χ(2) tests or the Fisher exact tests.

RESULTS

One of the 5 SNPs showed a significant difference between patients and control subjects (rs2089760: p(genotype)=0.005, p(allel)=0.003). There were no statistically significant differences between patients and control subjects for the other four SNPs: rs566655, rs11664063, rs607230, and rs3810046.

CONCLUSIONS

Our results indicate that the polymorphism of rs2089760, located in the promoter region of LAMA1, may be associated with high myopia in the Chinese population and should be investigated further.

Self-assembly of peptides into spherical nanoparticles for delivery of hydrophilic moieties to the cytosol

We report a novel class of self-assembling peptide nanoparticles formed by mixing aqueous solutions of K(16) peptide and a 20 amino acid peptide of net charge -5 (GLFEALLELLESLWELLLEA). Particle formation is salt-dependent and yields perfectly spherical nanoparticles of approximately 120 to approximately 800 nm diameter, depending on buffer composition and temperature, with a stoichiometry of approximately 1:2.5 for the cationic and anionic peptides. The anionic peptide forms an alpha-helix in aqueous solution, has all five glutamates on one side of the helix, and exists entirely as a discrete oligomer of 9-10 peptides. A rigid oligomer with 45-50 negative charges almost certainly represents the core component of these nanoparticles, held together by electrostatic interactions with the unstructured K(16) peptide. Cells internalize these particles by an endocytic process, and free particles are frequently seen in the cytosol, presumably because of the acid-dependent fusogenic properties of the anionic peptide. Among other applications, these particles have potential for the targeted delivery of single or multiple therapeutic moieties directly to the cytosol, and we report the successful delivery of a K(16)-linked pro-apoptosis peptide.

A single nucleotide polymorphism analysis of the LAMA1 gene in Japanese patients with high myopia

Although a myopia susceptibility gene has not yet been elucidated, ten candidate regions (MYP1-MYP10) have been associated with myopia by linkage analysis employing large pedigrees. We report herein on the results of our analysis pertaining to polymorphisms of LAMA1 (alpha subunit of laminin), a promising candidate gene for high myopia present in the MYP2 region of Japanese subjects with high myopia. Three hundred and thirty Japanese subjects with high myopia at a level of greater than -9.25 D and ethnically and sex matched 330 normal controls without high myopia was enrolled in this study. The thirteen SNPs located on the LAMA1 gene were analyzed using PCR and SNP-specific fluorogenic probes. Two of the SNPs were monomorphic and none of the 11 SNPs showed statistically significant association with high myopia in the Japanese population. There is no convincing evidence to prove a connection between nucleotide sequence variations in LAMA1 and high myopia. The pairwise linkage disequilibrium (LD) mapping disclosed a strong value (D' > 0.8) and narrow ranged block within these SNPs.

Endothelial extracellular matrix: biosynthesis, remodeling, and functions during vascular morphogenesis and neovessel stabilization

The extracellular matrix (ECM) is critical for all aspects of vascular biology. In concert with supporting cells, endothelial cells (ECs) assemble a laminin-rich basement membrane matrix that provides structural and organizational stability. During the onset of angiogenesis, this basement membrane matrix is degraded by proteinases, among which membrane-type matrix metalloproteinases (MT-MMPs) are particularly significant. As angiogenesis proceeds, ECM serves essential functions in supporting key signaling events involved in regulating EC migration, invasion, proliferation, and survival. Moreover, the provisional ECM serves as a pliable scaffold wherein mechanical guidance forces are established among distal ECs, thereby providing organizational cues in the absence of cell-cell contact. Finally, through specific integrin-dependent signal transduction pathways, ECM controls the EC cytoskeleton to orchestrate the complex process of vascular morphogenesis by which proliferating ECs organize into multicellular tubes with functional lumens. Thus, the composition of ECM and therefore the regulation of ECM degradation and remodeling serves pivotally in the control of lumen and tube formation and, finally, neovessel stability and maturation.

Extracellular matrices of the avian ovarian follicle. Molecular characterization of chicken perlecan

In egg-laying species, such as the chicken, the mode of transport of lipoprotein particles from the capillary plasma to endocytic receptors on the oocyte surface is largely unknown. Here we show by molecular characterization that the large prominent heparan sulfate proteoglycan of extracellular matrices, termed perlecan or HSPG2 (the product of the hspg2 gene), is a component of ovarian follicles that may participate in this process. However, although normally a major HSPG of basement membranes or basal laminae, in chicken follicles, perlecan is absent from the membranous structure between the theca interna and granulosa cell layers, which to date has been considered a bona fide basement membrane. Rather, the protein is localized in the extracellular matrix of theca externa cells, which produce this HSPG. Furthermore, in chicken testes, perlecan is localized in the peritubular spaces but in less organized fashion than the classical basement membrane components, agrin and laminin. All five domains and structural hallmarks of chicken perlecan (4071 residues) have been conserved in its mammalian counterparts. We have produced the recombinant domain II (containing low density lipoprotein (LDL) receptor-like binding repeats) of chicken perlecan and demonstrate its capacity to bind LDL and very low density lipoprotein (VLDL), apolipoprotein B-containing lipoproteins ultimately destined for uptake into oocytes via members of the low density lipoprotein receptor family. Binding to perlecan heparan sulfate side chains may facilitate the interaction of lipoproteins with domain II. Based on the current results and on domain-domain interactions revealed by recent ultrastructural investigations of the LDL receptor, nidogen, and laminin (Rudenko, G., Henry, L., Henderson, K., Ichtchenko, K., Brown, M. S., Goldstein, J. L., and Deisenhofer, J. (2002) Science 298, 2353-2358 and Takagi, J., Yang, Y., Liu, J. H., Wang, J. H., and Springer, T. A. (2003) Nature 424, 969-974), we propose a novel role of perlecan in mediating plasma-to-oocyte surface transport of VLDL particles.

High affinity binding between laminin and laminin binding protein of Leishmania is stimulated by zinc and may involve laminin zinc-finger like sequences

In the course of trying to understand the pathogenesis of leishmaniasis in relation to extracellular matrix (ECM) elements, laminin, a major ECM protein, has been found to bind saturably and with high affinity to a 67-kDa cell surface protein of Leishmania donovani. This interaction involves a single class of binding sites, which are ionic in nature, conformation-dependent and possibly involves sulfhydryls. Binding activity was significantly enhanced by Zn2+, an effect possibly mediated through Cys-rich zinc finger-like sequences on laminin. Inhibition studies with monoclonals against polypeptide chains and specific peptides with adhesive properties revealed that the binding site was localized in one of the nested zinc finger consensus sequences of B1 chain containing the specific pentapeptide sequence, YIGSR. Furthermore, incubation of L. donovani promastigotes with C(YIGSR)3-NH2 peptide amide or antibody directed against the 67-kDa laminin-binding protein (LBP) induced tyrosine phosphorylation of proteins with a molecular mass ranging from 115 to 130 kDa. These studies suggest a role for LBP in the interaction of parasites with ECM elements, which may mediate one or more downstream signalling events necessary for establishment of infection.

Purification and characterization of human laminin-8. Laminin-8 stimulates cell adhesion and migration through alpha3beta1 and alpha6beta1 integrins

Recently identified laminin isoforms containing the alpha4 chain have been shown to be expressed in the basement membrane of restricted organs such as heart, skeletal muscle, and blood vessels, especially those in embryos. We screened 38 human cell lines for the expression of the laminin alpha4 chain by reverse transcriptase-polymerase chain reaction and found that T98G glioblastoma cells express only alpha4, but not other alpha chains. Laminin-8, an isoform containing the alpha4 and beta1 chains, was purified from conditioned medium of T98G cells by gel filtration and immunoaffinity chromatography using a monoclonal antibody against laminin beta1 chain. The purified laminin isoform was composed of disulfide-linked 230-, 220-, and 200-kDa subunits, which immunoblot analysis identified as the beta1, gamma1, and alpha4 chains. Purified laminin-8 had cell adhesive activity comparable to laminin-1 but significantly weaker than laminin-5 and laminin-10/11. T98G cells adhering to laminin-8 became more elongated than those adhering to other laminin isoforms and extended multiple pseudopods. Cell adhesion to laminin-8 was abolished by an antibody against the integrin beta1 subunit or a combination of antibodies against the integrin alpha3 and alpha6 subunits, but not by either anti-alpha3 or anti-alpha6 antibody alone, suggesting that both alpha3beta1 and alpha6beta1 integrins serve as adhesion receptors for laminin-8. Consistent with these observations, K562 erythroleukemic cells transfected with either integrin alpha3 or alpha6 cDNA were capable of adhering to laminin-8 when beta1 integrins were stimulated by the beta1-activating antibody 8A2. Despite its moderate cell adhesive activity, laminin-8 was significantly potent in promoting cell migration when compared with other laminin isoforms and fibronectin. Cell migration on laminin-8 was completely inhibited by a combination of antibodies against alpha3 and alpha6 integrins, and substantially inhibited by anti-alpha3 antibody alone, suggesting that laminin-8-mediated cell migration is predominantly mediated by alpha3beta1 integrin. Given its potency to stimulate cell migration and preferential localization to the basement membrane of capillaries and embryonic tissues, laminin-8 may play a role in processes requiring enhanced cell migration during development, wound healing, and angiogenesis.

Characterization and functional analysis of laminin isoforms in human bone marrow

Laminins are a family of disulfide-linked heterotrimeric proteins consisting of 3 different subunits termed α, β, and γ chains. Combinations of 11 characterized laminin subunits (α1-α5, β1-β3, and γ1-γ3) generate at least 12 laminin isoforms, which can serve different functions. Although expression of laminin in the hematopoietic microenvironment has been known for many years, the nature of the laminin isoforms present in the human bone marrow is poorly characterized. The present study attempts to clarify this issue. Reverse transcriptase–polymerase chain reaction analysis of human bone marrow stromal cells suggested the expression of many laminin isoforms in the marrow. Northern blot and immunoblot analysis, however, showed that laminin-8/9 and laminin-10/11 are the most abundant laminin isoforms synthesized by human bone marrow stromal cells. Other isoforms, if present, certainly play a minor role in the hematopoietic microenvironment. Functionally, laminin-10/11 preparations showed strong adhesive interactions with human CD34+ cell lines. Antibodies against the β1 integrin subunit inhibited these interactions. Other laminin isoforms, especially laminin-1 and laminin-2/4, showed only weak or no adhesive interactions with the hematopoietic cell lines tested, explaining former negative results. In addition to its adhesion-mediating properties, laminin-10/11 preparations also showed a mitogenic activity for human hematopoietic progenitor cells. Taken together, these data suggest that laminin in the bone marrow plays a hitherto unexpected important function in the development of hematopoietic progenitor cells.

Characterization and functional analysis of laminin isoforms in human bone marrow

Laminins are a family of disulfide-linked heterotrimeric proteins consisting of 3 different subunits termed α, β, and γ chains. Combinations of 11 characterized laminin subunits (α1-α5, β1-β3, and γ1-γ3) generate at least 12 laminin isoforms, which can serve different functions. Although expression of laminin in the hematopoietic microenvironment has been known for many years, the nature of the laminin isoforms present in the human bone marrow is poorly characterized. The present study attempts to clarify this issue. Reverse transcriptase–polymerase chain reaction analysis of human bone marrow stromal cells suggested the expression of many laminin isoforms in the marrow. Northern blot and immunoblot analysis, however, showed that laminin-8/9 and laminin-10/11 are the most abundant laminin isoforms synthesized by human bone marrow stromal cells. Other isoforms, if present, certainly play a minor role in the hematopoietic microenvironment. Functionally, laminin-10/11 preparations showed strong adhesive interactions with human CD34+ cell lines. Antibodies against the β1 integrin subunit inhibited these interactions. Other laminin isoforms, especially laminin-1 and laminin-2/4, showed only weak or no adhesive interactions with the hematopoietic cell lines tested, explaining former negative results. In addition to its adhesion-mediating properties, laminin-10/11 preparations also showed a mitogenic activity for human hematopoietic progenitor cells. Taken together, these data suggest that laminin in the bone marrow plays a hitherto unexpected important function in the development of hematopoietic progenitor cells.

Still more complexity in mammalian basement membranes

At the epithelial/mesenchymal interface of most tissues lies the basement membrane (BM). These thin sheets of highly specialized extracellular matrix vary in composition in a tissue-specific manner, and during development and repair. For about two decades it has been apparent that all BMs contain laminins, entactin-1/nidogen-1, Type IV collagen, and proteoglycans. However, within the past few years this complexity has increased as new components are described. The entactin/nidogen (E/N) family has expanded with the recent description of a new isoform, E/N-2/osteonidogen. Agrin and Type XVIII collagen have been reclassified as heparan sulfate proteoglycans (HSPGs), expanding the repertoire of HSPGs in the BM. The laminin family has become more diverse as new alpha-chains have been characterized, increasing the number of laminin isoforms. Interactions between BM components are now appreciated to be regulated through multiple, mostly domain-specific mechanisms. Understanding the functions of individual BM components and their assembly into macromolecular complexes is a considerable challenge that may increase as further BM and cell surface ligands are discovered for these proteins.

Cytotoxic mAb from rheumatic carditis recognizes heart valves and laminin

Anti-streptococcal antibodies cross-reactive with N-acetyl-betaD-glucosamine (GlcNAc) and myosin are present in the sera of patients with rheumatic fever (RF). However, their role in tissue injury is not clear. In this study, we show that anti-GlcNAc/anti-myosin mAb 3.B6 from a rheumatic carditis patient was cytotoxic for human endothelial cell lines and reacted with human valvular endothelium and underlying basement membrane. Reactivity of mAb 3.B6 with the valve was inhibited by human cardiac myosin > laminin > GlcNAc. The mAb 3.B6 epitopes were localized in fragments of human cardiac myosin, including heavy meromyosin (HMM), the S1 subfragment, and two light meromyosin (LMM) peptides containing amino acid sequences KEALISSLTRGKLTYTQQ (LMM 1) and SERVQLLHSQNTSLINQK (LMM 33). A novel feature of mAb 3.B6 was its reactivity with the extracellular matrix protein laminin, which may explain its reactivity with the valve surface. A laminin A-chain peptide (HTQNT) that includes homology to LMM33 inhibited the reactivity of mAb 3.B6 with human valve. These data support the hypothesis that cross-reactive antibodies in rheumatic carditis cause injury at the endothelium and underlying matrix of the valve.

Laminin alpha1-chain shows a restricted distribution in epithelial basement membranes of fetal and adult human tissues

Two novel monoclonal antibodies were raised and used to study the expression of laminin (Ln) alpha1-chain in developing and adult human tissues. In both fetal and adult kidney, a distinct immunoreactivity was seen in basement membranes (BM) of most proximal tubules but not in the distal tubular or glomerular BM or in the basal laminae of blood vessels. Immunoprecipitation of metabolically labeled cultured human renal proximal tubular cells showed an abundant production and deposition of Ln alpha1-chain to the extracellular matrix, suggestive of an epithelial origin of kidney Ln-1. Quantitative cell adhesion experiments with JAR choriocarcinoma cells showed that purified human Ln-1 is a good substrate for cell adhesion that it is differently recognized by integrin receptors when compared to mouse Ln-1. In fetal and adult testes immunoreactivity was solely confined to BM of the seminiferous epithelium. In the airways BM-confined reaction was only seen in fetal budding bronchial tubules (16-19 weeks) at the pseudoglandular stage of development. In the skin a distinct immunoreactivity was confined to BM of developing hair buds but not in epithelial BMs of adult epidermis or of epidermal appendages. In other adult tissues, immunoreactivity was found in BMs of thyroid, salivary, and mammary glands as well as in BMs of endometrium and endocervix, but not of ectocervix or vagina. No immunoreactivity was found in BMs of most of the digestive tract, including the liver and pancreas, except for BMs of esophageal submucosal glands and duodenal Brunner's glands. In fetal specimens, BMs of the bottoms of the intestinal and gastric glands were positive. Basal laminae of blood vessels were generally negative for Ln alpha1 chain with the exception of specimens of both fetal and adult central nervous system in which immunoreactivity for Ln alpha1 chain was prominently confined to capillary walls. The results suggest that outside the central nervous system, Ln alpha1 chain shows a restricted and developmentally regulated expression in BMs of distinct epithelial tissues.

Form and function: the laminin family of heterotrimers

The laminins are a family of glycoproteins that provide an integral part of the structural scaffolding of basement membranes in almost every animal tissue. Each laminin is a heterotrimer assembled from alpha, beta, and gamma chain subunits, secreted and incorporated into cell-associated extracellular matrices. The laminins can self-assemble, bind to other matrix macromolecules, and have unique and shared cell interactions mediated by integrins, dystroglycan, and other receptors. Through these interactions, laminins critically contribute to cell differentiation, cell shape and movement, maintenance of tissue phenotypes, and promotion of tissue survival. Recent advances in the characterization of genetic disruptions in humans, mice, nematodes and flies have revealed developmental roles for the different laminin subunits in diverse cell types, affecting differentiation from blastocyst formation to the post-natal period. These genetic defects have challenged some of the previous concepts about basement membranes and have shed new light on the diversity and complexity of laminin functions as well as established the molecular basis of several human diseases.

Identification of epi-1 locus as a laminin alpha chain gene in the nematode Caenorhabditis elegans and characterization of epi-1 mutant alleles

A new genetic locus, epi-1, has been identified and mapped, which affects epithelialization of various tissues in the nematode, Caenorhabditis elegans. Seven independent epi-1 mutant alleles have been obtained. These mutants have a wide spectrum of abnormalities, all seem to be caused by a primary defect of basement membrane. We have identified the epi-1 gene as a structural gene of laminin alpha chain. The sequence analyses of the gene and cDNAs revealed that the gene consists of 15 exons and encodes a protein of 3704 amino acids in an open reading frame of 11115 base pairs. The nematode alpha chain is similar to its vertebrate and fly orthologs in the domain structure. The mRNA is trans-spliced to SL1 leader RNA as many of the nematode mRNAs. Mutation sites have been identified in four alleles. Two alleles have nonsense mutations and produce truncated proteins lacking the domain necessary for the formation of a heterotrimeric laminin molecule. The other two alleles have missense mutations in domains VI and IIIb, respectively.

Prenatal diagnosis of a fetus with a cryptic translocation 4p;18p and Wolf-Hirschhorn syndrome (WHS)

Wolf-Hirschhorn Syndrome (WHS) is caused by distal deletion of the short arm of chromosome 4 and is characterized by growth deficiency, mental retardation, a distinctive, 'greek-helmet' facial appearance, microcephaly, ear lobe anomalies, and sacral dimples. We report a family with a balanced chromosomal translocation 4;18(p15.32;p11.21) in the father and an unbalanced translocation resulting in partial monosomy 4 and partial trisomy 18 in one living boy and a prenatally diagnosed male fetus. Both showed abnormalities consistent with WHS and had in addition aplasia of one umbilical artery. Karyotyping of another stillborn fetus revealed a supernumerary derivative chromosome der(18)t(4;18)(p15.32;p11.21) of paternal origin and two normal chromosomes 4. The umbilical cord had three normal vessels. A third stillborn fetus with the same balanced translocation as the father had a single umbilical artery and hygroma colli.

Restricted distribution of laminin alpha1 chain in normal adult mouse tissues

The distribution of laminin alpha1 chain in adult mouse tissue was determined by immunofluorescence using monoclonal antibody 200, reacting with the globular carboxyterminus E3 fragment of alpha1 chain. Strong reactivity was noted only in a few tissues. Reactivity was restricted to epithelial basement membranes. Expression was noted in several epithelial basement membranes of the urinary tract, and male and female reproductive organs. In addition, expression was seen in some parts of the nervous system. Expression was seen in pia mater which surrounds the brain, and in the extracellular matrices covering the vitreous chamber and the lens of the eye. Staining was seen in the adrenal gland cortex, with strongest staining in the zona glomerulosa. Staining was negative in all other studied epithelial basement membranes, such as the lung (trachea or lung epithelium), epidermis, and all parts of the gastrointestinal tract (liver, gut) except for weak staining in the ventricle and Brunner's glands. No expression was seen in basement membranes of fat, Schwann, or endothelial cells in any studied parts of the body. Both small- and large-size vessel walls were negative both in endothelial basement membranes and blood vessel walls, with the exception of some larger brain blood vessels in locations where epithelial cells have invaginated. Neither smooth muscle, myocardium or striated muscle expressed alpha1 chain. We conclude that alpha1-containing heterotrimers including laminin-1 (alpha1beta1gamma1) have a very restricted tissue distribution.

Distribution of ten laminin chains in dystrophic and regenerating muscles

Using immunohistochemical methods, we assessed the distribution of all 10 known laminin chains (alpha1-5, beta1-3, gamma1 and gamma2) in skeletal muscles from patients with Duchenne, congenital, limb girdle, or Emery-Dreifuss muscular dystrophies. The alpha2, beta1 and gamma1 chains were abundant in the basal lamina surrounding muscle fibers in normal controls; alpha1, alpha3-alpha5, beta3, and gamma2 were undetectable; and beta2 was present at a low level. Compared to controls, levels of the alpha5 chain were increased in muscles from many dystrophic patients; levels of beta1 were reduced and/or levels of beta2 were increased in a minority. However, these changes were neither specific for, nor consistent within, diagnostic categories. In contrast, levels of alpha4 were increased in muscles from all patients with alpha2 laminin (merosin)-deficient congenital muscular dystrophy. Loss of alpha2 laminin in congenital dystrophy is disease-specific but some other changes in laminin isoform expression in dystrophic muscles could be secondary consequences of myopathy, denervation, regeneration or immaturity. To distinguish among these possibilities, we compared the laminins of embryonic, denervated, regenerating, and mutant mouse muscles with those in normal adult muscle. Embryonic muscle basal lamina contained alpha4 and alpha5 along with alpha2, and regenerating muscle re-expressed alpha5 but not alpha4. Levels of alpha5 but not alpha4 were increased in dystrophin (mdx) mutants and in dystrophin/utrophin double mutants (mdx:utrn -/-), models for Duchenne dystrophy. In contrast, laminin alpha4 was upregulated more than alpha5 in muscles of laminin alpha2 mutant mice (dy/dy; a model for alpha2-deficient congenital dystrophy). Based on these results, we hypothesize that the expression of alpha5 in many dystrophies reflects the regenerative process, whereas the selective expression of alpha4 in alpha2-deficient muscle is a specific compensatory response to loss of alpha2.

Expression of laminins 1 and 10 in carcinoma cells and comparison of their roles in cell adhesion

The expression pattern of laminin (Ln) alpha1 chain has been a controversial topic due to discrepancies between mRNA and protein studies. Recently it was reported that the monoclonal antibody 4C7, previously thought to recognize Ln alpha1 chain, actually detects Ln alpha5 chain. This finding makes it necessary to reestimate the role of Ln alpha1 chain and to compare the expression and functions of Ln alpha1 and alpha5 chains. We studied the expression of Ln alpha1 and alpha5 chains and production of Ln-1 and Ln-10 in cultured human carcinoma cells. Ln alpha1 chain mRNA was detected in JAR choriocarcinoma cells and in all four renal cell carcinoma cell lines studied. In contrast, pancreatic, colon, and lung alveolar carcinoma cell lines did not express or produce Ln alpha1 chain, suggesting that Ln-1 (alpha1 beta1 gamma1) is produced only by certain carcinoma cells. Ln alpha5 chain mRNA was expressed in all carcinoma cells, but was not incorporated into extracellular matrix in vitro, as shown with JAR cells. Immunoprecipitation of metabolically labeled cells showed that cells expressing Ln alpha1 mRNA also produced 400-kDa Ln alpha1 chain, whereas all cells produced 380-kDa Ln alpha5 chain. Adhesion to Ln-1 was inhibited by a functionally blocking antibody against alpha6-integrin subunit, whereas adhesion to Ln-10 was inhibited by an antibody against alpha6-integrin in JAR cells and by an antibody against alpha3-integrin in PANC-1 cells. The results suggest that Ln-10 is a ubiquitously expressed Ln isoform in carcinoma cells, and the mechanism of adhesion to Ln-10 is cell-type specific.

Efficacy and specificity of antisense laminin chain-specific expression vectors in blocking laminin induction by TGFbeta1: effect of laminin blockade on TGFbeta1-mediated cellular responses

Transforming growth factorbeta1 (TGFbeta1) elicits a multitude of cellular responses from the epithelial-derived human colon cancer Moser cells. TGFbeta1 induces the expression of laminin and fibronectin, and previous studies show that the induction of fibronectin is functionally associated with the regulation of carcinoembryonic antigen (CEA) expression by TGFbeta1 (Huang and Chakrabarty, 1994, J Biol Chem 269:28764-28768). In this study we constructed antisense laminin chain-specific expression vectors and determined their efficacy in blocking the expression and the induction of the large multichain laminin molecule by TGFbeta1. We also determined the functional role of laminin in several TGFbeta1-mediated responses: growth inhibition, downmodulation of anchorage-independent growth, and cellular invasion. Expression of either antisense laminin chain A, B1, or B2 RNA resulted in a downmodulation of endogenous laminin mRNA expression and blocked the induction of laminin protein by TGFbeta1 without affecting the induction of other adhesion molecules such as fibronectin or CEA. It is concluded that antisense RNA directed to only one of the laminin chains was sufficient to disrupt the induction of the complex laminin molecule in quite a specific manner. Expression of antisense laminin RNA downregulated cellular adhesion to extracellular matrix (ECM) laminin and blocked the ability of TGFbeta1 to upmodulate adhesion to ECM laminin. Expression of antisense laminin RNA, however, did not alter the downregulating effect of TGFbeta1 on cellular proliferation, anchorage-independent growth, or cellular invasion, suggesting that the induction of laminin did not play a significant functional role in these TGFbeta1-mediated cellular responses. It is likely that other adhesion pathways may be involved in mediating the action of TGFbeta1 in this cell line.

Isolation and characterization of laminin-10/11 secreted by human lung carcinoma cells. laminin-10/11 mediates cell adhesion through integrin alpha3 beta1

A panel of human tumor cell lines was screened for selective expression of laminin alpha5 chain, a newly identified laminin subunit comprising laminin-10 (alpha5 beta1 gamma1) and -11 (alpha5 beta2 gamma1). The lung adenocarcinoma cell line A549 was found to express the alpha5 chain at relatively high levels but no detectable amounts of other alpha chains. The laminin variants containing alpha5 chain were purified from the conditioned medium of A549 cells by immunoaffinity chromatography using the anti-laminin monoclonal antibody 4C7 which was shown recently to recognize the laminin alpha5 chain (Tiger, C.-F., Champliaud, M.-F., Pedrosa-Domellof, F., Thornell, L.-E., Ekblom, P., and Gullberg, D. (1997) J. Biol. Chem. 272, 28590-28595). The purified laminin variants consisted of three chains with molecular masses of 350, 220, and 210 kDa. The 350-kDa chain was specifically recognized by another anti-alpha5 chain monoclonal antibody capable of recognizing denatured alpha5 chain on immunoblots, whereas the 210-kDa chain was recognized by an anti-gamma1 chain antibody. The purified alpha5 chain-containing laminin variants (hereafter referred to as laminin-10/11) were highly active in mediating adhesion of A549 cells to the substratum with potency as high as that of laminin-5 and significantly higher than those of laminin-1, laminin-2/4, or fibronectin. Adhesion to substrata coated with laminin-10/11 was specifically inhibited by anti-integrin antibodies directed against the integrin alpha3 or beta1 subunit but not by those against alpha2 or alpha6 subunit, indicating that laminin-10/11 is specifically recognized by integrin alpha3 beta1. Given the wide distribution of laminin-10/11 in the basement membrane of various tissue types and dominant expression of integrin alpha3 beta1 in most epithelial cells, specific interaction of laminin-10/11 with integrin alpha3 beta1 may play an important role in in vivo regulation of proliferation and differentiation of epithelial cells through the basement membrane.

BeWo choriocarcinoma cells produce laminin 10

BeWo is a choriocarcinoma cell line that generates an extracellular matrix (ECM) rich in laminin and is a useful model for human trophoblast. Immunofluorescence with monoclonal antibodies demonstrates that BeWo ECM contains laminin subunits beta1 and gamma1. Immunoprecipitation from conditioned medium shows that the cells secrete two distinct laminin trimers both containing beta1 and gamma1 but with alpha subunits of approx. 400 and 450 kDa. The culture medium also contains a species thought to be beta1 gamma1 dimer. Immunoprecipitation with monoclonal antibody 4C7, previously thought to recognize the alpha1 subunit, isolates complexes containing only the smaller alpha subunit. A second complex containing the larger alpha subunit along with beta1, gamma1 and a 150 kDa polypeptide is precipitated from 4C7-depleted medium with an anti-(laminin 1) polyclonal antibody. Peptide sequencing demonstrates that the 4C7-reactive species is alpha5, which is present as two similarly sized polypeptides. mRNA species encoding laminin subunits alpha1, alpha5, beta1, beta2 and gamma1 are all present in the cells. These results demonstrate the secretion of a novel laminin isoform, laminin 10, the subunit composition of which is alpha5 beta1 gamma1. Laminin 1 is also produced. No evidence for the secretion of beta2-containing laminin isoforms could be derived despite the presence of beta2 mRNA. Analysis with reverse transcriptase-mediated PCR also showed the presence of laminin alpha5 in first-trimester placenta and decidua.

Presence of laminin alpha5 chain and lack of laminin alpha1 chain during human muscle development and in muscular dystrophies

There is currently a great interest in identifying laminin isoforms expressed in developing and regenerating skeletal muscle. Laminin alpha1 has been reported to localize to human fetal muscle and to be induced in muscular dystrophies based on immunohistochemistry with the monoclonal antibody 4C7, suggested to recognize the human laminin alpha1 chain. Nevertheless, there seems to be no expression of laminin alpha1 protein or mRNA in developing or dystrophic mouse skeletal muscle fibers. To address the discrepancy between the results obtained in developing and dystrophic human and mouse muscle we expressed the E3 domain of human laminin alpha1 chain as a recombinant protein and made antibodies specific for human laminin alpha1 chain (anti-hLN-alpha1G4/G5). We also made antibodies to the human laminin alpha5 chain purified from placenta. In the present report we show that hLN-alpha1G4/G5 antibodies react with a 400-kDa laminin alpha1 chain and that 4C7 reacts with a 380-kDa laminin alpha5 chain. Immunohistochemistry with the hLN-alpha1G4/G5 antibody and 4C7 revealed that the two antibodies stained human kidney, developing and dystrophic muscle in distinct patterns. Our data indicate that the previously reported expression patterns in developing, adult, and dystrophic human muscle tissues with 4C7 should be re-interpreted as an expression of laminin alpha5 chain. Our data are also consistent with earlier work in mouse, indicating that laminin alpha1 is largely an epithelial laminin chain not present in developing or dystrophic muscle fibers.

The human alpha3b is a 'full-sized' laminin chain variant with a more widespread tissue expression than the truncated alpha3a

We report the molecular cloning of the human laminin alpha3b chain variant and its mRNA expression pattern in adult human tissues when compared to the alpha3a variant. The mRNA encoding for the alpha3b variant is about 11 kb and the predicted translation product carries the complete set of domains typical for a 'full-sized' laminin alpha chain. Apart from the similar domain structure of alpha3b also the sequence of alpha3 resulted more closely related to the alpha5 than to the alpha4 chain. Quantitative analysis of the RNA expression in a broad panel of adult human tissues indicated that the alpha3b variant is more widely distributed than the alpha3a shorter variant.

Primary structure, developmental expression, and immunolocalization of the murine laminin alpha4 chain

The complete primary structure of the mouse laminin alpha4 chain was derived from cDNA clones. The translation product contains a 24-residue signal peptide preceding the mature alpha4 chain of 1,792 residues. Northern analysis on whole mouse embryos revealed that the expression was weak at day 7, but it later increased and peaked at day 15. In adult tissues the strongest expression was observed in lung and cardiac and skeletal muscles. Weak expression was also seen in other adult tissues such as brain, spleen, liver, kidney, and testis. By in situ hybridization of fetal and newborn tissues, expression of the laminin alpha4 chain was mainly localized to mesenchymal cells. Strong expression was seen in the villi and submucosa of the developing intestine, the mesenchymal stroma surrounding the branching lung epithelia, and the external root sheath of vibrissae follicles, as well as in cardiac and skeletal muscle fibers. In the developing kidney, intense but transient expression was associated with the differentiation of epithelial kidney tubules from the nephrogenic mesenchyme. Immunohistologic staining with affinity-purified IgG localized the laminin alpha4 chain primarily to lung septa, heart, and skeletal muscle, capillaries, and perineurium.

Differentiation-dependent expression of laminin-8 (alpha 4 beta 1 gamma 1) mRNAs in mouse 3T3-L1 adipocytes

We report that laminin-8 (alpha 4 beta 1 gamma 1) is the specific isoform of laminin synthesized in adipocytes. Reverse transcription-polymerase chain reaction (RT-PCR) of mRNA from mouse 3T3-L1 cells with paired primers for alpha 1, alpha 2, alpha 3, alpha 4, alpha 5, beta 1, beta 2, beta 3, gamma 1 and gamma 2 laminins yielded amplified fragments only for alpha 4, beta 1 and gamma 1. A polyclonal antibody against mouse laminin-1 (alpha 1 beta 1 gamma 1) precipitated alpha 4 in addition to beta 1 and gamma 1, while the antibody against a deduced peptide sequence of mouse alpha 4 in addition to beta 1 and gamma 1 in addition to alpha 4. Thus, laminin-8 (alpha 4 beta 1 gamma 1) is the only isoform expressed in 3T3-L1 cells. Northern blots showed that the levels of alpha 4, beta 1 and gamma 1 mRNAs increased 2.5-fold during adipose conversion of 3T3-L1 cells. A 1062 bp cDNA fragment cloned by RT-PCR demonstrated a polymorphism in the mouse alpha 4 gene which would lead to five amino acid changes in the domain G.

The alpha7beta1 integrin mediates adhesion and migration of skeletal myoblasts on laminin

Many aspects of myogenesis are believed to be regulated by myoblast interactions with specific components of the extracellular matrix. For example, laminin has been found to promote adhesion, migration, and proliferation of mammalian myoblasts. Based on affinity chromatography, the alpha7beta1 integrin has been presumed to be the major receptor mediating myoblast interactions with laminin. We have prepared a monoclonal antibody, O26, that specifically reacts with both the X1 and the X2 extracellular splice variants of the alpha7 integrin chain. This antibody completely and selectively blocks adhesion and migration of rat L8E63 myoblasts on laminin-1, but not on fibronectin. In contrast, a polyclonal antibody to the fibronectin receptor, alpha5beta1 integrin, blocks myoblast adhesion on fibronectin, but not on laminin-1. The alpha7beta1 integrin also binds to a mixture of laminin-2 and laminin-4, the major laminin isoforms in developing and adult skeletal muscle, but O26 is a much less potent inhibitor of myoblast adhesion on the laminin-2/4 mixture than on laminin-1. Based on affinity chromatography, we suggest that this may be due to higher affinity binding of alpha7X1 to laminin-2/4 than to laminin-1.

Extrasynaptic location of laminin beta 2 chain in developing and adult human skeletal muscle

We have investigated the distribution of the laminin beta 2 chain (previously s-laminin) in human fetal and adult skeletal muscle and compared it to the distribution of laminin beta 1. Immunoblotting and transfection assays were used to characterize a panel of monoclonal and polyclonal antibodies to the laminin beta 2 chain. We found that laminin beta 1 chain was detected at all times during development from 10 weeks of gestation. Laminin beta 2 chain was first detected in 15 to 22-week-old fetal skeletal muscle as distinct focal immunoreactivity in the sarcolemmal basement membrane area of some myofibers. In the adult skeletal muscle, laminin beta 2 chain immunoreactivity was found along the entire perimeter of each of the individual myofibers in a large series of different muscles studied. Laminin beta 2 chain was similarly found in the skeletal muscle basement membranes in patients with Duchenne and Becker muscular dystrophy. Immunoaffinity chromatography of muscle extracts with a monoclonal antibody to the laminin alpha 2 chain followed by immunoblotting with various antibodies to the beta 2 chain demonstrated the presence of the laminin-4 (alpha 2-beta 2-gamma 1) isoform. Together the present results demonstrate a prominent extrasynaptic localization of laminin beta 2 in the human muscle, suggesting that it may have an important function in the sarcolemmal basement membrane.

Binding site for C4b-binding protein in vitamin K-dependent protein S fully contained in carboxy-terminal laminin-G-type repeats. A study using recombinant factor IX-protein S chimeras and surface plasmon resonance

The interaction between vitamin K-dependent protein S and the C4b-binding protein (C4BP) was studied using surface plasmon resonance and genetic engineering. The affinity, as well as association and dissociation rates of the complex, was measured for human and bovine protein S at five different calcium concentrations. The binding to C4BP of six protein hybrids containing different parts of coagulation factor IX and protein S was studied in the absence and presence of calcium. The results show that dissociation of the human protein S-C4BP complex is extremely slow in the presence of > or = 10 microM calcium (k(off) = 7 x 10(-6) s(-1)) and the association rate constant is k(on) = 7 x 10(4) M(-1) s(-1). Human and bovine protein S were found to bind to human C4BP with the same affinity, K(D) = 0.1 nM, but the rates of association and dissociation were higher for the bovine protein S (k(on) = 2 x 10(5) M(-1) s(-1), k(off) = 2 x 10(-5) s(-1)). In the absence of calcium, the affinity for C4BP was reduced by a factor of 65 for human protein S and by a factor of 40 for bovine protein S. The decreased affinity could be mainly attributed to an increased off-rate (12-17-fold), while the on-rate decreased 3-4-fold. The studies using chimeric proteins show that the portion of protein S that is responsible for binding to C4BP is fully contained in the two laminin-G-type repeats, which are homologous to the sex hormone binding globulin (SHBG). All hybrids that contain the laminin-G-type repeats bind to C4BP with the same affinity as recombinant protein S, whereas hybrids lacking these repeats show no detectable binding to C4BP. The present data also suggest that the effect of calcium on the C4BP-binding properties is mediated by calcium binding site(s) in the laminin-G-type repeats.

The functions of laminins: lessons from in vivo studies

This series of three short reviews is an attempt to summarize our current knowledge of the in vivo tests of hypotheses of laminin functions. The structures of the laminins have been thoroughly reviewed recently (P. Ekblom and R. Timpl, in press), and I will not attempt to repeat this information here. Instead, I will focus on the recent evidence gathered from gene knock out experiments in mice and from naturally occurring human and mouse gene mutations. The most obvious lesson from the above studies--other than demonstrating the importance of laminins in general--is that the structural diversity of the laminin family members makes highly specialized functions possible. While all laminins may share many functional properties, the individual chains are involved in interactions which cannot be substituted for by other laminins or by other basement membrane components. While this concept is not new, it is very satisfying to see its validity so dramatically confirmed. It is therefore predictable that additional gene ablation experiments using other known and yet undescribed laminin genes will be equally interesting and informative. To me, one of the most striking lessons from these studies is how strongly the induced mouse mutations mimic human disease. With all the concerns with genetic background differences and species specific effects, manipulation of the laminin genes appears to be a particularly good first approach to identifying the causes of human disease. There is an abundant literature accumulated from biochemical and, more recently, molecular structural analyses, and from in vitro systems, suggesting a role of laminins contributing directly to the stability of the basement membrane. There is an equally vast literature supporting an indirect role in mediating cellular behavior, through interactions with various receptors. It is interesting that the in vivo studies summarized above support both activities. In the case of laminin 5 mutations, the phenotypic consequence appears to be due primarily to the loss of an important structural link between the epithelial cytokeratins and the dermal anchoring fibrils. The ultrastructure of the epithelium appears normal, as does the architecture of the papillary dermis. Only the anchoring complex itself is aberrant. The absence of laminin 5 appears not to compromise the development or viability of the epidermis. The basement membrane appears normal-other than the anchoring complex itself. The pathology observed in the newborn is believed to be due to the frictional trauma of birth, with the expectation that the function of the fetal skin is normal in utero. The Herlitz epidermolysis bullosa phenotype is obvious immediately at birth, and it does not progress postnatally beyond the extent to which the affected individual experiences additional frictional trauma or secondary consequences such as infection or fluid loss. Since laminin 5 is only one of a series of structural links within the anchoring complex, one would predict that a loss of any of these links would result in the same phenotype. Current evidence supports this view, as the absence of integrin alpha 6 beta 4 (Vidal et al., 1995; Dowling et al., 1996; Georges-Labouesse et al., 1996; van der Neut et al., 1996) or of collagen VII (A. M. Christiano and J. Uitto, in press) also results in dramatic neonatal dermal-epidermal fragility. The differences in phenotype, such as the pyloric atresia in the case of loss of integrin alpha 6 beta 4, are presumably due to additional functions of the integrin in other tissues or in other developmental processes. Therefore, the laminin 5 mutations may be unique, in that the in vivo studies suggest that the primary role of the molecule is in the elaboration and stability of the anchoring complex, but not in the basement membrane itself. Of course, since the in vivo phenotype reflects only losses that cannot be compensated, this interpretation may be much too narrow. (ABSTRACT TRUNCATED)

Structure of the human laminin alpha2-chain gene (LAMA2), which is affected in congenital muscular dystrophy

We have determined the structure and complete exon size pattern of the human laminin alpha2-chain gene (LAMA2), which has been shown to be affected in congenital muscular dystrophy (Helbling-Leclerc, A., Zhang, X., Topaloglu, H., Cruaud, C., Tesson, F., Weissenbach, J., Tomé, F. M. S., Schwartz, K., Fardeau, M., Tryggvason, K., and Guicheney, P. (1995) Nat. Genet. 11, 216-218). The gene is over 260, 000 base pairs and contains 64 exons. The sequence of all exon-intron borders was determined. Two of the exons, i.e. exons 43 and 52, are extremely small in size, 6 and 12 base pairs, respectively. Comparison of the exon pattern of the human LAMA2 gene with that of the Drosophila LAMA gene revealed that only 2 of 63 intron locations in the 5'-end of the human gene match the intron locations in the Drosophila gene, which contains 14 introns.

Characterization of monoclonal antibodies recognizing human merosin and their use in affinity purification of native merosin

Two monoclonal antibodies (MAbs) to human placenta laminin (pl-LAM), 1D8 (IgG1) and 6G5 (IgG2b) were generated and shown by ELISA and immunoblot analysis to recognize only native pl-LAM, but not denatured, reduced pl-LAM or mouse EHS laminin. Intact pl-LAM was easily isolated and purified in large scale from human placenta by 1D8-conjugated affinity chromatography. Electrophoretic analysis of the purified pl-LAM revealed the presence of a major 750-kDa component composed of 320-, 220-, and 200-kDa polypeptides and a minor 800-kDa component composed of 320-, 240-, and 220-kDa polypeptides. Neither molecule had a 400-kDa component corresponding to the A chain. It has already been shown that the 320-kDa polypeptide is identical to the M chain of human merosin (Hori et al. J. Biochem. 1994;116:1212-1219). Electron microscopy revealed that isolated merosin was composed of three short arms and one long arm. By immunohistochemistry, MAbs showed positive staining in human adult kidney and liver. These results indicate that these MAbs recognize only native merosin and can be used to study merosin structure and function by rapid purification of native merosin and by immunohistochemical analysis.

Conserved neuron promoting activity in Drosophila and vertebrate laminin alpha1

Drosophila S2 cells were transfected with constructs that code for two portions of the Drosophila laminin alpha chain. Construct recalphaL coded for domains III, I/II, and G of laminin alpha. Construct recalphaS coded for only the COOH-most 12% of the I/II domain and the G domain. The corresponding polypeptides were isolated and characterized from the culture media. The recalphaL chain partly formed disulfide-linked heterotrimers with the endogenously produced beta and gamma laminin chains. Like normal Drosophila laminin, a substrate coating of either recalphaL or recalphaS supported neuron differentiation and neurite extension of primary Drosophila embryo cell cultures. However, at the same low concentrations, only Drosophila laminin-1, but neither recalphaL nor recalphaS supported myogenesis in these cultures. Previously, an overlapping set of dodecapeptides that covered a region of the murine laminin alpha1 chain similar to recalphaS had been synthesized and tested for cell culture support properties (Nomizu, M., Kim, W. H., Yamamura, K., Utani, A., Otaka, A., Roller, P. P., Kleinman, H. K., and Yamada, Y. (1995) J. Biol. Chem. 270, 20583-20590). The Drosophila laminin alpha homologues of the six most active vertebrate dodecapeptides were now synthesized and tested as substrates for differentiation of primary Drosophila embryo cells. Peptides that contained either the Drosophila sequence SIKVGV or the murine homologue, SIKVAV, provided support for neurite extension.

Substitution of a conserved cysteine-996 in a cysteine-rich motif of the laminin alpha2-chain in congenital muscular dystrophy with partial deficiency of the protein

Congenital muscular dystrophies (CMDs) are autosomal recessive muscle disorders of early onset. Approximately half of CMD patients present laminin alpha2-chain (merosin) deficiency in muscle biopsies, and the disease locus has been mapped to the region of the LAMA2 gene (6q22-23) in several families. Recently, two nonsense mutations in the laminin alpha2-chain gene were identified in CMD patients exhibiting complete deficiency of the laminin alpha2-chain in muscle biopsies. However, a subset of CMD patients with linkage to LAMA2 show only partial absence of the laminin alpha2-chain around muscle fibers, by immunocytochemical analysis. In the present study we have identified a homozygous missense mutation in the alpha2-chain gene of a consanguineous Turkish family with partial laminin alpha2-chain deficiency. The T-->C transition at position 3035 in the cDNA sequence results in a Cys996-->Arg substitution. The mutation that affects one of the conserved cysteine-rich repeats in the short arm of the laminin alpha2-chain should result in normal synthesis of the chain and in formation and secretion of a heterotrimeric laminin molecule. Muscular dysfunction is possibly caused either by abnormal disulfide cross-links and folding of the laminin repeat, leading to the disturbance of an as yet unknown binding function of the laminin alpha2-chain and to shorter half-life of the muscle-specific laminin-2 and laminin-4 isoforms, or by increased proteolytic sensitivity, leading to truncation of the short arm.

Differential expression of laminin chain-specific mRNA transcripts during mouse preimplantation embryo development

Laminin is the first extracellular matrix protein that has been shown to be synthesized in preimplantation mouse embryos. In the present study, chain-specific expression patterns of laminin mRNAs were examined by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). During preimplantation mouse embryo development, temporal expression patterns of laminin chain mRNAs were somewhat differential: B1 chain mRNA was first detectable at the late two-cell stage and its level was gradually increased by the blastocyst stage. In contrast, B2 and A chain mRNAs first appeared at the morula and blastocyst stages, respectively. At the blastocyst stage, all of the laminin chain mRNAs were highly detected compared to the earlier stages. When embryos were flushed at the morula stage and cultured in vitro, all laminin chain mRNA levels were decreased or not changed in the process of blastocoele expansion. In contrast, in the in vivo condition where embryos at different stages of blastocyst were flushed at different time points, laminin chain mRNA levels were increased as a function of blastocoele expansion. These changes in laminin mRNAs were parallel with its receptors such as integrin alpha 3 and alpha 6. 3-Isobutyl-1-methylxanthine (IBMX), which is known to be a potent activator of blastocoele expansion and regulates cAMP metabolism, upregulated laminin expression (except B1 chain) in blastocysts cultured in vitro. In vitro cultured embryos normally developed up to the late blastocyst, although their development was delayed compared with the in vivo condition where laminin gene expression was gradually increased as the blastocoele expanded. These results indicate that laminin expression may not be involved directly in the regulation of blastocoele expansion. The uterine environment enclosing the preimplantation embryos appears, therefore, to play an important role in the regulation of laminin gene expression during blastocyst development.

Murine laminin B1 gene regulation during the retinoic acid- and dibutyryl cyclic AMP-induced differentiation of embryonic F9 teratocarcinoma stem cells

Retinoic acid (RA) and cyclic AMP analogs cause the differentiation of F9 embryonic teratocarcinoma stem cells into parietal endoderm, an epithelial cell of the early mouse embryo. Laminin B1 is induced in this differentiation process, but is not transcriptionally activated until 24-48 h after RA addition and is not maximally induced until approximately 72 h. Cyclic AMP analogs enhance this transcriptional activation. Although several DNase I hypersensitive sites (DHSS) were observed in the LAMB1 5 -flanking DNA, one of the sites, DHSS2, was detected only after 72 h of RA treatment. Transient transfections have demonstrated that the DHSS2 region functions as a "late-acting RA-inducible enhancer," and motifs in this enhancer contain the homeobox protein-binding site TTATTAACA. Greater binding is observed at these sites by electrophoretic mobility shift assay when cells are cultured with RA and cyclic AMP analogs versus RA alone, and no binding is seen in extracts from RA-treated F9 RAR gamma-/- cells which lack RAR gamma mRNA and protein. Laminin B1 mRNA is not induced by RA in the RAR gamma-/- cells (Boylan, J. F., Lohnes, D., Taneja, R., Chambon, P., and Gudas, L. J. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 9601-9605). Our data show that these DNA regulatory elements contribute to the transcriptional activation of the LAMB1 gene during the later stages of the differentiation process.

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

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

Structural and functional analysis of the globular domain IVa of the laminin alpha 1 chain and its impact on an adjacent RGD site

The globular domain IVa (about 250 residues) of the laminin alpha1 chain was obtained in recombinant form from mammalian cell clones. It was prepared either with (alpha1IVa-R) or without (alpha1IVa) an adjacent cell-adhesive RGD site which seems to be masked in laminin-1. The recombinant products could be visualized as globular structures by rotary shadowing, were resistant to trypsin and shared immunological epitopes with laminin-1, indicating folding into a native structure. Sequence analysis of pepsin fragments demonstrated the insertion of the globular domain into an epidermal growth factor-like scaffold which is characteristic of the extracellular laminin domain IV (L4) module. Only little immunological cross-reaction was found, however, with other L4 modules from perlecan and different laminin isoforms. Fragment alpha1IVa-R, but not fragment alpha1IVa, bound to alphaVbeta3 integrin, although to a distinctly lower level than a laminin fragment where the RGD site is fully exposed. The fragments also had no or only little cell attachment activity. This confirmed previous predictions that the globular domain alpha 1IVa masks the RDG site in laminin-1. Domain alpha 1IVa showed, in addition, a weak binding activity for the basement-membrane protein fibulin-1.

Laminins: a family of diverse multifunctional molecules of basement membranes

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

Isolation of pepsin-resistant laminin fragments from human placenta: effect on epithelial cells cultured from the kidneys of patients with autosomal dominant polycystic kidney disease (ADPKD)

Laminin isolated from human placenta was subjected to prolonged pepsin digestion. Seven peptide fragments (designated N1 to N7) were separated by ion-exchange chromatography and gel filtration and characterised by SDS-polyacrylamide gel electrophoresis and immunoblotting. The molecular size of the laminin fragments varied from approx. 900,000 (N1) to 28,000 (N7). Epithelial cells obtained from normal kidneys and patients with autosomal dominant polycystic kidney disease (ADPKD) were cultured. The incorporation of [3H]thymidine was measured over 96 h to determine the effect of the addition of the different fragments and whole laminin from EHS tumour to the cells. The rate of growth of both normal and polycystic cells was increased in the presence of the laminin fragments but this effect was more pronounced in the ADPKD cells.

Abnormal expression and distribution of nidogen in Hirschsprung's disease

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

Molecular cloning of a novel laminin chain, alpha 5, and widespread expression in adult mouse tissues

We have identified a fifth member of the alpha subfamily of vertebrate laminin chains. Sequence analysis revealed a close relationship of alpha 5 to the only known Drosophila alpha chain, suggesting that the ancestral alpha gene was more similar to alpha 5 than to alpha 1-4. Analysis of RNA expression showed that alpha 5 is widely expressed in adult tissues, with highest levels in lung, heart, and kidney. Our results suggest that alpha 5 may be a major laminin chain of adult basal laminae.

Identification of cell binding sites in the laminin alpha 1 chain carboxyl-terminal globular domain by systematic screening of synthetic peptides

The laminin alpha 1 chain carboxyl-terminal globular domain has been identified as a site of multiple biological activities. Using a systematic screening for cell binding sites with 113 overlapping synthetic peptide beads that covered this domain, we found 19 potential active sequences. Corresponding synthetic peptides were evaluated for direct cell attachment, spreading, and inhibition of cell spreading to a laminin-1 substrate using several cell lines. Five peptides (AG-10, AG-22, AG-32, AG-56, and AG-73) showed cell attachment activities with cell-type specificities. Cell spreading on AG-10 was inhibited by beta 1 and alpha 6 integrin antibodies and on AG-32 was inhibited by beta 1, alpha 2, and alpha 6 integrin antibodies. In contrast, cell adhesion and spreading on peptide AG-73 were not inhibited by these antibodies. The minimum active sequences of AG-10, AG-32, and AG-73 were determined to be SIYITRF, IAFQRN, and LQVQLSIR, respectively. These sequences are highly conserved among the different species and different laminin alpha chains, suggesting that they play a critical role for biological function and for interaction with cell surface receptors.