Mouse heart laminin

Laminin α5 in the keratinocyte basement membrane is required for epidermal-dermal intercommunication

Laminin α5 is broadly expressed in the epidermal basement membrane (BM) of mature mice and its elimination at this site (Lama5^Ker5 mouse) results in hyperproliferation of basal keratinocytes and a delay in hair follicle development, which correlated with upregulation of the dermally-derived laminin α2 and laminin α4 chains in the epidermal BM and of tenascin-C subjacent to the BM. In vitro studies revealed laminin 511 to be strongly adhesive for primary keratinocytes and that loss of laminin α5 does not result in cell autonomous defects in proliferation. Flow cytometry reveals that the loss of laminin α5 resulted in increased numbers of CD45⁺, CD4⁺ and CD11b⁺ immune cells in the skin, which temporo-spatial analyses revealed were detectable only subsequent to the loss of laminin α5 and the appearance of the hyperproliferative keratinocyte phenotype. These findings indicate that immune cell changes are the consequence and not the cause of keratinocyte hyperproliferation. Loss of laminin α5 in the epidermal BM was also associated with changes in the expression of several dermally-derived growth factors involved in keratinocyte proliferation and hair follicle development in adult but not new born Lama5^Ker5 skin, including KGF, EGF and KGF-2. In situ binding of FGF-receptor-2α (IIIb)-Fc chimera (FGFR2IIIb) to mouse skin sections revealed decoration of several BMs, including the epidermal BM, which was absent in Lama5^Ker5 skin. This indicates reduced levels of FGFR2IIIb ligands, which include KGF and KGF-2, in the epidermal BM of adult Lama5^Ker5 skin. Our data suggest an initial inhibitory effect of laminin α5 on basal keratinocyte proliferation and migration, which is exacerbated by subsequent changes in growth factor expression by epidermal and dermal cells, implicating laminin α5 in epidermal-dermal intercommunication.

Structural characterization of N-linked oligosaccharides of laminin from rat kidney: changes during diabetes and modulation by dietary fiber and butyric acid

Carbohydrates of laminin, a family of large multidomain glycoproteins, have been implicated in various cellular activities including maintaining the protein structure, its function and also basement membrane integrity. During the course of our investigation, we observed that purified laminin from kidneys of control, diabetic, and dietary fiber- and butyric acid-treated diabetic rats showed differences in binding to extracellular matrix components. This prompted us to determine whether there are structural changes in laminin oligosaccharides. In this study, we have characterized a few major N-linked oligosaccharides isolated from purified laminin in various experimental groups, viz. normal, diabetic and diabetic rats fed with dietary fiber and butyric acid. Sugar composition, as identified by GLC, revealed the presence of mannose, galactose and N-acetylglucosamine. In order to study fine structures of the oligosaccharides, N-linked oligosaccharides of laminin were released by Peptide-N-glycosidase F digestion, end-labeled with 2-anthranilic acid and fractionated by lectin affinity chromatography. Furthermore, structural elucidation carried out by MALDI-TOF MS/MS analysis showed variations in the oligosaccharide sequence of laminin during diabetes which were altered by the feeding of dietary fiber and butyric acid.

Laminin alpha4-null mutant mice develop chronic kidney disease with persistent overexpression of platelet-derived growth factor

Each extracellular matrix compartment in the kidney has a unique composition, with regional specificity in the expression of various laminin isoforms. Although null mutations in the majority of laminin chains lead to specific developmental abnormalities in the kidney, Lama4-/- mice have progressive glomerular and tubulointerstitial fibrosis. These mice have a significant increase in expression of platelet-derived growth factor (PDGF)-BB, PDGF-DD, and PDGF receptor beta in association with immature glomerular and peritubular capillaries. In addition, mesangial cell exposure to alpha4-containing laminins, but not other isoforms, results in down-regulation of PDGF receptor mRNA and protein, suggesting a direct effect of LN411/LN421 on vessel maturation. Given the known role of overexpression of PDGF-BB and PDGF-DD on glomerular and tubulointerstitial fibrosis, these data suggest that failure of laminin alpha4-mediated down-regulation of PDGF activity contributes to the progressive renal lesions in this animal model. Given the recent demonstration that individuals with laminin alpha4 mutations develop cardiomyopathy, these findings may be relevant to kidney disease in humans.

α7β1 integrin is a receptor for laminin-2 on Schwann cells

The Schwann cell basal lamina acts as an organizer of peripheral nerve tissue and influences many aspects of cell behavior during development and regeneration. A principal component of the Schwann cell basal lamina is laminin-2. This study was undertaken to identify Schwann cell receptors for laminin-2. We found that among several Schwann cell integrins that can potentially interact with laminin-2, only alpha7beta1 bound to laminin-2-Sepharose. Dystroglycan, a non-integrin Schwann cell receptor for laminin-2 identified previously, was also found to bind to laminin-2-Sepharose. Antibody to the alpha7 integrin subunit partially inhibited Schwann cell adhesion to laminin-2. Small interfering RNA-mediated suppression of either alpha7 integrin or dystroglycan expression decreased adhesion and spreading of Schwann cells on laminin-2, whereas knocking down both proteins together inhibited adhesion and spreading on laminin-2 almost completely. alpha7 integrin and dystroglycan both colocalized with laminin-2 containing basal lamina tubes in differentiating neuron-Schwann cell cocultures. The alpha7beta1 integrin also coprecipitates with focal adhesion kinase in differentiating cocultures. These findings strongly suggest that alpha7beta1 integrin is a Schwann cell receptor for laminin-2 that provides transmembrane linkage between the Schwann cell basal lamina and cytoskeleton.

The Conduit System Transports Soluble Antigens from the Afferent Lymph to Resident Dendritic Cells in the T Cell Area of the Lymph Node

Resident dendritic cells (DC) within the T cell area of the lymph node take up soluble antigens that enter via the afferent lymphatics before antigen carrying DC arrive from the periphery. The reticular network within the lymph node is a conduit system forming the infrastructure for the fast delivery of soluble substances from the afferent lymph to the lumen of high endothelial venules (HEVs). Using high-resolution light microscopy and 3D reconstruction, we show here that these conduits are unique basement membrane-like structures ensheathed by fibroblastic reticular cells with occasional resident DC embedded within this cell layer. Conduit-associated DC are capable of taking up and processing soluble antigens transported within the conduits, whereas immigrated mature DC occur remote from the reticular fibers. The conduit system is, therefore, not a closed compartment that shuttles substances through the lymph node but represents the morphological equivalent to the filtering function of the lymph node.

Molecular Analysis of Laminin N-terminal Domains Mediating Self-interactions

The ability of laminins to self-polymerize is crucial for the formation of basement membranes. Development of this polymerized network has profound effects upon tissue architecture as well as on the intracellular organization and differentiation of neighboring cells. The laminin N-terminal (LN) domains have been shown to mediate this interaction and studies using proteolytic fragments derived from laminin-1 led to the theory that network assembly depends on the formation of a heterotrimeric complex between LN domains derived from alpha, beta, and gamma chains in different laminin molecules with homologous interactions being insignificant. The laminin family consists of 15 known isoforms formed from five alpha, three beta, and three gamma chains, of which some are truncated and lack the N-terminal LN domain. To address whether the model of heterotrimeric complex formation is applicable to laminin isoforms other than laminin-1, eight LN domains found in the laminin protein family were recombinantly expressed and tested in three different assays for homologous and heterologous interactions. The results showed that the lack of homologous interactions is an exception, with such interactions being seen for LN domains derived from all alpha chains and from the beta2 and beta3 subunits. The gamma chain-derived LN domains showed a far more limited binding repertoire, particularly in the case of the gamma3 chain, which is found present in a range of non-basement membrane locations. Further, whereas the interactions depended upon Ca2+ ions, with EDTA reversibly abrogating binding, EDTA-induced conformational changes were not reversible. Together these results demonstrate that the assembly model proposed on the basis of laminin-1 may be a simplification, with the assembly of naturally occurring laminin networks being far more complex and highly dependent upon which laminin isoforms are present.

Preferential locomotion of leukemic cells towards laminin isoforms 8 and 10

To identify the laminin isoforms of the basement membranes that could be implicated in the extravasation process of neoplastic lymphocytes, a number of purified laminins and one native renal laminin complex were comparatively investigated for their ability to promote migration of neoplastic lymphocytes in vitro. The identity/composition of a human placental laminin complex was asserted by combining immunochemical assays, sequence determination of tryptic peptides, and ultrastructural analysis to be composed predominantly of laminin-10 in which the coiled-coil C-terminal regions and the G globular domain of the alpha5 chain were preserved intact despite the enzymatic treatment used for its isolation. Lymphoma and leukemic cell lines failed to migrate towards laminin-4, -9, -11, moved poorly in response to laminin-1, -2/4, -5 and the renal laminin complex, but markedly locomoted towards the subendothelial laminin-8 and -10. The motility-promoting interaction with these latter laminins was interchangeably mediated by the alpha3beta1 and alpha6beta1 integrins. Lymphocyte locomotion on laminins assayed in the presence of cytokines was either reduced or enhanced suggesting that local cytokine milieu could further influence motility response.

Alternative splice variants of alpha 7 beta 1 integrin selectively recognize different laminin isoforms

The integrin alpha(7)beta(1) occurs in several cytoplasmic (alpha(7A), alpha(7B)) and extracellular splice variants (alpha(7X1), alpha(7X2)), which are differentially expressed during development of skeletal and heart muscle. The extracellular variants result from the alternative splicing of exons X1 and X2, corresponding to a segment within the putative ligand binding domain. To study the specificity and affinity of the X1/X2 variants to different laminin isoforms, soluble alpha(7)beta(1) complexes were prepared by recombinant coexpression of the extracellular domains of the alpha- and beta-subunits. The binding of these complexes to purified ligands was measured by solid phase binding assays. Surprisingly, the alternative splice variants revealed different and specific affinities to different laminin isoforms. While the alpha(7X2) variant bound much more strongly to laminin-1 than the alpha(7X1) variant, the latter showed a high affinity binding to laminins-8 and -10/11. Laminin-2, the major laminin isoform in skeletal muscle, was recognized by both variants, whereas none of the two variants were able to interact with laminin-5. A specific blocking antibody inhibited the binding of both variants to all laminins tested, indicating the involvement of common epitopes in alpha(7X1)beta(1) and alpha(7X2)beta(1). Because laminin-8 and -10/11 as well as alpha(7X1) are expressed in developing skeletal and cardiac muscle, these findings suggest that alpha(7X1)beta(1) may represent a physiological receptor with novel specificities for laminin-8 and -10.

Cell adhesion and migration properties of beta 2-integrin negative polymorphonuclear granulocytes on defined extracellular matrix molecules. Relevance for leukocyte extravasation

Regulated adhesion of leukocytes to the extracellular matrix is essential for transmigration of blood vessels and subsequent migration into the stroma of inflamed tissues. Although beta(2)-integrins play an indisputable role in adhesion of polymorphonuclear granulocytes (PMN) to endothelium, we show here that beta(1)- and beta(3)-integrins but not beta(2)-integrin are essential for the adhesion to and migration on extracellular matrix molecules of the endothelial cell basement membrane and subjacent interstitial matrix. Mouse wild type and beta(2)-integrin null PMN and the progranulocytic cell line 32DC13 were employed in in vitro adhesion and migration assays using extracellular matrix molecules expressed at sites of extravasation in vivo, in particular the endothelial cell laminins 8 and 10. Wild type and beta(2)-integrin null PMN showed the same pattern of ECM binding, indicating that beta(2)-integrins do not mediate specific adhesion of PMN to the extracellular matrix molecules tested; binding was observed to the interstitial matrix molecules, fibronectin and vitronectin, via integrins alpha(5)beta(1) and alpha(v)beta(3), respectively; to laminin 10 via alpha(6)beta(1); but not to laminins 1, 2, and 8, collagen type I and IV, perlecan, or tenascin-C. PMN binding to laminins 1, 2, and 8 could not be induced despite surface expression of functionally active integrin alpha(6)beta(1), a major laminin receptor, demonstrating that expression of alpha(6)beta(1) alone is insufficient for ligand binding and suggesting the involvement of accessory factors. Nevertheless, laminins 1, 8, and 10 supported PMN migration, indicating that differential cellular signaling via laminins is independent of the extent of adhesion. The data demonstrate that adhesive and nonadhesive interactions with components of the endothelial cell basement membrane and subjacent interstitium play decisive roles in controlling PMN movement into sites of inflammation and illustrate that beta(2)-integrins are not essential for such interactions.

Endothelial cell laminin isoforms, laminins 8 and 10, play decisive roles in T cell recruitment across the blood-brain barrier in experimental autoimmune encephalomyelitis

An active involvement of blood-brain barrier endothelial cell basement membranes in development of inflammatory lesions in the central nervous system (CNS) has not been considered to date. Here we investigated the molecular composition and possible function of the extracellular matrix encountered by extravasating T lymphocytes during experimental autoimmune encephalomyelitis (EAE). Endothelial basement membranes contained laminin 8 (alpha4beta1gamma1) and/or 10 (alpha5beta1gamma1) and their expression was influenced by proinflammatory cytokines or angiostatic agents. T cells emigrating into the CNS during EAE encountered two biochemically distinct basement membranes, the endothelial (containing laminins 8 and 10) and the parenchymal (containing laminins 1 and 2) basement membranes. However, inflammatory cuffs occurred exclusively around endothelial basement membranes containing laminin 8, whereas in the presence of laminin 10 no infiltration was detectable. In vitro assays using encephalitogenic T cell lines revealed adhesion to laminins 8 and 10, whereas binding to laminins 1 and 2 could not be induced. Downregulation of integrin alpha6 on cerebral endothelium at sites of T cell infiltration, plus a high turnover of laminin 8 at these sites, suggested two possible roles for laminin 8 in the endothelial basement membrane: one at the level of the endothelial cells resulting in reduced adhesion and, thereby, increased penetrability of the monolayer; and secondly at the level of the T cells providing direct signals to the transmigrating cells.

Altered expression of cell cycle proteins and prolonged duration of cardiac myocyte hyperplasia in p27KIP1 knockout mice

-The precise role of cell cycle-dependent molecules in controlling the switch from cardiac myocyte hyperplasia to hypertrophy remains to be determined. We report that loss of p27(KIP1) in the mouse results in a significant increase in heart size and in the total number of cardiac myocytes. In comparison to p27(KIP1)+/+ myocytes, the percentage of neonatal p27(KIP1)-/- myocytes in S phase was increased significantly, concomitant with a significant decrease in the percentage of G(0)/G(1) cells. The expressions of proliferating cell nuclear antigen, G(1)/S and G(2)/M phase-acting cyclins, and cyclin-dependent kinases (CDKs) were upregulated significantly in ventricular tissue obtained from early neonatal p27(KIP1)-/- mice, concomitant with a substantial decrease in the expressions of G(1) phase-acting cyclins and CDKs. Furthermore, mRNA expressions of the embryonic genes atrial natriuretic factor and alpha-skeletal actin were detectable at significant levels in neonatal and adult p27(KIP1)-/- mouse hearts but were undetectable in p27(KIP1)+/+ hearts. In addition, loss of p27(KIP1) was not compensated for by the upregulation of other CDK inhibitors. Thus, the loss of p27(KIP1) results in prolonged proliferation of the mouse cardiac myocyte and perturbation of myocyte hypertrophy.

Immunochemical analysis of laminin during postnatal development of the rat submandibular gland

Anti-laminin serum was used to investigate distribution patterns and chain composition of laminin during postnatal development of rat submandibular gland. Immunohistochemical analysis showed that in glands of newborn rats laminin was not uniformly present around growing acini. Staining was frequently weak or absent in clefts formed between adjacent cells. This irregular staining pattern decreased progressively over the subsequent periods, and in 30-day-old animals immunoreactivity was observed only at the periphery of glands. Immunoblot analysis showed that laminin was composed of bands corresponding to the alpha 1, beta 1 and gamma 1 polypeptides. The correlation between the pattern of laminin expression and gland maturation suggests a role of laminin in the functional maturation of acinar cells.

The intrinsic factor-vitamin B12 receptor, cubilin, is assembled into trimers via a coiled-coil alpha-helix

A large protein was purified from bovine kidney, using selective extraction with EDTA to solubilize proteins anchored by divalent cation-dependent interactions. An antiserum raised against the purified protein labeled the apical cell surface of the epithelial cells in proximal tubules and the luminal surface of small intestine. Ten peptide sequences, derived from the protein, all matched the recently published sequences for rat (Moestrup, S. K., Kozyraki, R., Kristiansen, M., Kaysen, J. H., Holm Rasmussen, H., Brault, D., Pontillon, F., Goda, F. O., Christensen, E. I., Hammond, T. G., and Verroust, P. J. (1998) J. Biol. Chem. 273, 5235-5242) and human cubilin, a receptor for intrinsic factor-vitamin B12 complexes, identifying the protein as bovine cubilin. In electron microscopy, a three-armed structure was seen, indicating an oligomerization of three identical subunits. This model was supported by the Mr values of about 1,500,000 for the intact protein and 440,000 for its subunits obtained by analytical ultracentrifugation. In a search for a potential assembly domain, we identified a region of heptad repeats in the N-terminal part of the cubilin sequence. Computer-assisted analysis supported the presence of a coiled-coil alpha-helix between amino acids 103 and 132 of the human cubilin sequence and predicted the formation of a triple coiled-coil. We therefore conclude that cubilin forms a noncovalent trimer of identical subunits connected by an N-terminal coiled-coil alpha-helix.

Laminins of the dermo-epidermal junction

Laminins are the most abundant structural non-collagenous glycoproteins ubiquitously present in basement membranes. They are multidomain molecules constituting a family of possibly more than 50 members. Some members such as laminins 5, 6 and 10 are specific of the basal lamina present under stratified epithelia. Although only few intact laminin isoforms have been purified from cultivated cells or tissues, genetic engineering has opened the way for a rapid development of laminin structural biology. Moreover, the phenotypes resulting from gene targeting in mouse or from laminin defects in acquired or inherited human diseases highlight the pivotal role of laminins in morphogenesis, development, and physiology. Indeed, the laminins display a remarkable repertoire of functions, most importantly as structural elements forming a network throughout the basement membrane to which other collagenous or non-collagenous glycoproteins and proteoglycans attach. Furthermore, they are signaling molecules providing adjacent cells with diverse information by interacting with cell surface components.

Expression of laminin alpha1, alpha2, alpha4, and alpha5 chains, fibronectin, and tenascin-C in skeletal muscle of dystrophic 129ReJ dy/dy mice

The dy/dy mouse is an animal model for human merosin-negative congenital muscular dystrophy (CMD), which has been reported to have reduced or no expression of the basement membrane protein laminin alpha2. We here investigate various myogenic and nonmyogenic tissues of mature dy/dy and control 129ReJ mice histologically and for laminin alpha2 expression. In addition, expression patterns of laminin alpha1, alpha2, alpha4, and alpha5 chains, the interstitial proteins fibronectin and tenascin-C, and the adhesion molecules VCAM-1, ICAM-1, and alpha4 integrin were characterized in skeletal muscle of 1- and 7-day and mature (>6 weeks old) dy/dy and control 129ReJ mice. The laminin alpha2 chain remained detectable in myogenic tissues of dy/dy mice by immunofluorescence using two different monoclonal antibodies and by Northern blot analysis. However, laminin alpha2 expression was significantly reduced or not detectable in nonmyogenic tissues of dy/dy mice, including skin, lung, kidney, brain, thymus, and eye. Focal lesions were observed in mature skeletal muscle only, characterized by necrotic tissue, isolated VCAM-1- and ICAM-1-positive cells indicative of inflammatory processes, and regenerating muscle fibers surrounded by intense tenascin-C and fibronectin expression. In contrast to studies on human CMD muscle, laminin alpha1 was not detectable in either dy/dy or control skeletal muscle using immunofluorescence or Northern blot analysis. Immunofluorescence localized laminin alpha4 to basement membranes of blood vessels, the endoneurium of the intramuscular nerves, and the neuromuscular junction in skeletal muscle of 1- and 7-day-old dy/dy and control mice. In mature muscle, laminin alpha4 expression shifted to the perineurium of intramuscular nerves in both dy/dy and control mice. Furthermore, strong upregulation of laminin alpha4 in the basement membranes of blood vessels, the perineurium of intramuscular nerves, and of isolated regenerating muscle fibers in the dy/dy mice was apparent. Investigation of 1-day-old animals revealed expression of laminin alpha5 in skeletal muscle fiber basement membranes of dy/dy but not control animals. This difference between dy/dy and control animals was no longer apparent at 7 days after birth, indicating a temporary shift in expression pattern of laminin alpha5 in dy/dy animals. Analysis of the extracellular matrix components of 1- and 7-day-old dy/dy and control skeletal muscle revealed an early onset of the dystrophy, even before histopathological features of the disease were evident. Our data confirm the absence of laminin alpha1 chain in myogenic tissues of both dy/dy and control mice and suggest compensation for reduced laminin alpha2 in dy/dy skeletal muscle by laminin alpha4 and, in early development, also laminin alpha5. These results have significant ramifications in the diagnosis of human merosin-negative CMD.

Differential heparin sensitivity of alpha-dystroglycan binding to laminins expressed in normal and dy/dy mouse skeletal muscle

The alpha-dystroglycan binding properties of laminins extracted from fully differentiated skeletal muscle were characterized. We observed that the laminins expressed predominantly in normal adult rat or mouse skeletal muscle bound alpha-dystroglycan in a Ca2+-dependent, ionic strength-sensitive, but heparin-insensitive manner as we had observed previously with purified placental merosin (Pall, E. A., Bolton, K. M., and Ervasti, J. M. 1996 J. Biol. Chem. 271, 3817-3821). Rat skeletal muscle laminins partially purified by heparin-agarose affinity chromatography also bound alpha-dystroglycan without sensitivity to heparin. We also confirm previous studies of dystrophic dy/dy mouse skeletal muscle showing that the alpha2 chain of merosin is reduced markedly and that the laminin alpha1 chain is not up-regulated detectably. However, we further observed a quantitative decrease in the expression of laminin beta/gamma chain immunoreactivity in alpha2 chain-deficient dy/dy skeletal muscle and reduced alpha-dystroglycan binding activity in laminin extracts from dy/dy muscle. Most interestingly, the alpha-dystroglycan binding activity of residual laminins expressed in merosin-deficient dy/dy skeletal muscle was inhibited dramatically (69 +/- 19%) by heparin. These results identify a potentially important biochemical difference between the laminins expressed in normal and dy/dy skeletal muscle which may provide a molecular basis for the inability of other laminin variants to compensate fully for the deficiency of merosin in some forms of muscular dystrophy.

Immunochemical analysis of laminin in duct-ligated submandibular glands of rats

Immunochemical methods have been used to study the expression of laminin during experimental atrophy of the submandibular gland of rats caused by ductal ligation. In normal submandibular glands, laminin immunoreactivity appeared as continuous linear staining around acini and ducts. In the ligated glands, it exhibited an irregular pattern and intensity. Staining was usually stronger around small ducts and acini, which were most prominent in glands ligated for 30 days. Immunoblot analysis showed that the laminin of the rat submandibular gland contains bands that correspond to the EHS alpha 1, beta 1 and gamma 1 chains, and that the composition of the laminin chains does not change during the atrophy.

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.

Differential neural crest cell attachment and migration on avian laminin isoforms

A number of laminin isoforms have recently been identified and proposed to exert different functions during embryonic development. In the present study, we describe the purification and partial characterization of several isoforms isolated from chick heart and gizzard, and provide data on the molecular mechanisms underlying the interaction of avian neural crest cells with these molecules in vitro. Laminins extracted from heart and gizzard tissues were separated by gel filtration and purified to homogeneity by sequential lectin and immunoaffinity chromatography by utilizing monoclonal antibodies directed against the avian alpha 2, beta 2 and gamma 1 laminin chains. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) banding pattern of the polypeptide complexes obtained and immunoblotting with polyclonal antisera allowed the identification of Laminin-2 (alpha 2 beta 1 gamma 1), Laminin-4 (alpha 2 beta 2 gamma 1), and laminins comprising the beta 1, beta 2 and gamma 1 chains associated with a shorter alpha chain which, in SDS-PAGE, co-migrate with the beta/gamma complex in the 200 kDa region. These latter laminins, which are here arbitrarily denoted Laminin-alpha x (heart tissue) and Laminin-G (gizzard tissue), are somewhat distinct in their apparent molecular weight, are differentially associated with nidogen, and appear as "T"-shaped particles similar to Laminin-6 and Laminin-7 when analyzed by transmission electron microscopy following rotary shadowing. In contrast, the avian Laminin-2 and Laminin-4 isoforms exhibit the characteristic cruciform shape described previously for their mammalian counterparts. Isolated neural crest cells differentially attached and migrated on these laminin isoforms, showing a clear preference for Laminin-G. Similarly to the EHS Laminin-1, neural crest cells recognized all avian isoforms through their alpha 1 beta 1 integrin, shown previously to be the primary laminin-binding receptor on these cells. Neural crest cell interaction with the avian laminins was dependent upon maintenance of the secondary and tertiary structure of the molecules, as shown by the marked reduction in cell attachment and migration upon disruption of the alpha-helical coiled-coil structure of their constituent chains. The results demonstrate that different laminin isoforms may be differentially involved in the regulation of neural crest cell migration and suggest that this regulation operates through interaction of the cells with a structurally conserved cell binding site recognized by the alpha 1 beta 1 integrin.

Differential expression of alpha-6 and other subunits of laminin binding integrins during development of the murine heart

The development of the heart from a single heart tube to a four chambered organ with two separated unidirectional flows is a highly complex process. Events like looping, septation, tissue remodelling, and development of valves take place in a time period in which the heart already exerts its pump function. Adhesion of cells to each other and to their extracellular matrix as well as the capability to migrate in such a dynamic environment are extremely important. Integrins and extracellular matrix components have already been implicated in this process. In this report, we describe in detail the differential expression of the alpha-6 integrin subunit during late murine heart development, e.g., in the process from looping to the end of septation. We compare mRNA and protein expression patterns with those of beta-1 and other subunits of laminin-binding integrins, alpha-3 and alpha-7. We show a constant and high expression of alpha-6 in the atrial myocardium and a decrease in expression in the ventricular trabecular myocardium. The compact myocardial wall and the ventricular septum do not express alpha-6, except for the myocardium of the distal outflow tract at early stages. Moreover, we describe expression of this integrin subunit in the endocardial cushions that contribute to the development of the atrioventricular and semilunar valves. We propose a role for the alpha-6-beta-1 laminin receptor in the adhesion of cells to their extracellular matrix at sites of high stress due to cardiac contraction or blood flow induced shear stress. Moreover, site specific endothelial expression within the heart and surrounding extracardiac tissue is discussed. This study suggests a distinct role for alpha-6-beta-1 in the heart and provides insight concerning probably important roles of integrins and their extracellular matrix ligands during embryonic 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.

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.

Regulated expression of a novel laminin beta subunit during the development of the chick embryo

In order to define specific laminin variants implicated in organogenesis, we have undertaken a systematic search to detect and characterize novel laminin subunits, the expression of which is both developmentally regulated and tissue-specific. cDNA prepared from embryonic chick tissues was amplified by the polymerase chain reaction (PCR) with degenerate primers based on conserved sequences in domains V and VI of members of the laminin beta subunit family. Restriction mapping, cloning and sequencing of PCR products demonstrated a novel cDNA, the derived protein sequence of which displayed greatest homology with laminin beta subunits. However, the degree of amino acid divergence, comparison of sequence motifs and pattern of expression indicates that the cloned cDNA does not code for the avian orthologue of a previously characterized laminin beta subunit. Northern blot analysis showed that the expression of the 6-kb mRNA coding for the novel subunit was restricted to the skin. The mRNA was not detectable before day 5 of chick embryonic development, after which in situ hybridization showed expression only in surface ectoderm cells and subsequently in the epidermis. The developmentally regulated ectodermal expression of the novel beta subunit, prior to condensation of mesenchymal cells to form the dermis, is consistent with a specific role for this laminin isoform in the development and maintenance of the skin.

Distinct heparin-binding and neurite-promoting properties of laminin isoforms isolated from chick heart

Laminin isolated from chick heart is composed of several heterotrimeric variants of 800 and 700 kDa. Here, we used monoclonal antibodies against chick laminin to purify different laminin isoforms from this mixture. Antibody 8D3 specifically removed laminin containing alpha 2 chain from chick heart laminin preparations, leaving behind 700 kDa variants. Using antibody C4 against the laminin beta 2 chain, alpha 2 chain containing variants were further separated into alpha 2 beta 1 gamma 1 and alpha 2 beta 2 gamma 1 laminin, respectively. Laminins containing alpha 2 chain and recognized by antibody 8D3 are cross-shaped molecules. Their expression during embryogenesis is tightly regulated. In 5-day embryos staining with monoclonal antibody 8D3 is restricted to the dermamyotome. Older embryos (8 days) express alpha 2 chain containing variants at myotendinous junction primordia of skeletal muscle, and only late in development these variants are generally expressed in skeletal and heart muscle basement membranes. The 700 kDa laminin variants contain beta 1, beta 2, and gamma 1 subunits affiliated with an immunologically distinct, shorter alpha × chain and appear to be T-shaped in the electron microscope. Whereas laminins with an alpha 2 subunit bind to heparin, variants with the novel alpha × chain do not. Experiments using cultured sympathetic neurons showed that laminins with alpha × chain are less potent than alpha 2 chain containing variants in promoting neurite outgrowth. In contrast, sympathetic neurons cannot discriminate between alpha 2 beta 1 gamma 1 and alpha 2 beta 2 gamma 1 laminin substrates, respectively, and show identical high rates of neurite formation.

Immunochemical characterization and distribution of laminin in the rat tongue

Anti-laminin serum was used to investigate the distribution and composition of laminin in the rat tongue. Immunohistochemical analysis showed that laminin is a useful marker to study the microscopic anatomy of the tongue. Laminin is expressed in most tissues that form this organ, such as smooth and striated muscle, epithelium, nerves, and endothelium, allowing the observation of fine details of neuromuscular junctions, taste papillae and microvasculature. Western blot analysis revealed that laminin extracted from the tongue differs from EHS tumor laminin. Tongue laminin contains B chains, while A chain seems to be present only in small amounts.

Anti-La (SS-B) but not anti-Ro52 (SS-A) antibodies cross-react with laminin--a role in the pathogenesis of congenital heart block?

Cross-reactions between maternally derived autoantibodies and fetal cardiac antigens have been postulated to play a role in the pathogenesis of congenital heart block (CHB). We have explored the cross-reactivity of autoantibodies to the small ribonuclear autoantigens, La/SS-B and Ro/SS-A, with laminin, the major component of cardiac sarcolemmal membrane using affinity-purified antibodies from patients with Sjögren's syndrome (SS). Anti-La antibodies purified from eight of 10 patients cross-reacted significantly with mouse laminin by ELISA. In contrast, purified antibodies to Ro52 from the same 10 patients showed little or no binding to laminin. Laminin inhibited up to 70% binding of anti-La antibodies to La antigen, and La inhibited up to 65% binding of anti-La antibodies to laminin. The cross-reaction was further examined on cryosections of 10 human fetal hearts aged from 8.7 to 14.9 weeks of gestation, two normal adult hearts, and one pathological adult heart with a diagnosis of dilated cardiomyopathy. Anti-Ro52 antibodies did not bind to the surface of cardiac cells. However, anti-La antibodies from seven of 10 patients tested bound to the surface of fetal myocytes from hearts aged 9.4 to 14.9 weeks of gestation, and also to the myocytes from the pathological adult heart but not to normal adult hearts. Preincubation with La antigen abolished the binding of anti-La antibodies to the surface of adult heart myocytes with dilated cardiomyopathy, and pre-incubation with mouse laminin could partially block this binding. These results suggest that molecular mimicry between laminin and La, but not Ro52, may act as a target for specific maternal autoantibodies, and contribute to the pathogenesis of CHB at a critical stage during fetal cardiac development.

Specific attachment and migration of human astrocytoma cells on human but not murine laminin

Attachment sites and biological functions of laminin isolated from murine EHS sarcoma have been well studied. Recently several variants of laminin including human placental laminin have been shown to be distinct from EHS-laminin. This study was undertaken to determine attachment, proliferation, and migration phenomena of human astrocytoma cell lines to human and murine sarcoma EHS-laminin. Using short-term attachment assays human placental laminin was shown to be the better substrate for cell adhesion. EHS-laminin mediated approximately 30-50% of the effect observed on human laminin. The astrocytoma cells expressed beta 1, beta 3, and beta 4 subunit mRNA as determined by RT-PCR. Anti-beta 1 antibodies blocked adhesion to EHS-laminin, but antibodies against beta 1, beta 4, and alpha v subunits were all ineffective in blocking adhesion to human laminin. A migration assay showed that astrocytoma cells on human laminin dispersed from a central seeding area, while cells on EHS-laminin remained where they were seeded. The pattern of dispersion could not be accounted for by changes in growth rates of astrocytoma cells on the different proteins, since both cell lines grew equally well on the two laminins. We conclude that unique epitopes on human laminin are recognized by novel receptors on human astrocytoma cells which confer a migratory phenotype to the cells.

Expression of the alpha 2-subunit of laminin correlates with increased cell adhesion and metastatic propensity

Previous studies have indicated that laminin from neoplastic cells of high tumorigenicity is less active in promoting cell adhesion than aminin from normal cells or tissues. In the present study, we tested the hypothesis that laminin of metastatic tumor cells differs from that of nonmetastatic cells. Accordingly, we determined the subunit composition of laminin in highly metastatic, ras-transformed cells (4R) and compared it with laminin produced by nonmetastatic cells transformed with ras plus E1a (RE4). Metastatic 4R cells produced three to four times more of the alpha 2-subunit of laminin than RE4 cells did. Furthermore, the highly metastatic human melanoma cells (1205 and A2058) made and secreted into the medium, laminin containing significantly more of the alpha 2-subunit than laminin from the highly tumorigenic but nonmetastatic melanoma WM793 or HT1080 fibrosarcoma cells. Using HT1080 cells, laminin (250 ng/well) from 4R cells showed more adhesion promoting activity (68%) than laminin from RE4 cells (39%). Similarly, laminin isolated from human placenta, which expresses both the alpha 1 beta 1 gamma 1 and alpha 2 beta 1 gamma 1 isoforms, promoted cell adhesion better (63%) than EHS laminin (26%), which contains only the former isoform, at 250 ng/well. In addition, both 4R and RE4 cells attached more efficiently to 4R laminin-coated substratum than to RE4 laminin at 0.3 and 0.6 microgram/well.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Partial cloning of the M subunit of laminin from adult rat lipocytes: expression of the M subunit by cells isolated from normal and injured liver

Laminin is a heterotrimeric glycoprotein found in the perisinusoidal space of adult rat liver. The principal cellular source of laminin in liver is the lipocyte, with its three subunits measuring 324, 200 and 200 kD. The large subunit of lipocyte-derived laminin is distinct from the A subunit of murine laminin (440 kD); its size suggests that it represents a peptide, called M, recently cloned from human placenta. Using oligonucleotide primers derived from the human M-subunit cDNA, we amplified a 445-bp sequence encoding a fragment of M-laminin from adult rat lipocytes. The rat cDNA is 90% homologous to the human M-subunit cDNA and recognizes an mRNA in lipocytes measuring about 10 kb. M-subunit transcripts were identified only in lipocytes from normal adult liver; they could not be identified in hepatocytes, endothelial cells or Kupffer cells. Lipocytes were screened for M-subunit protein with a polyclonal M antiserum. Cells stained specifically for the M-subunit after 36 hr in primary culture; the protein was also identified in freshly isolated cells by means of immunoblotting. To determine whether lipocytes alter their expression of the laminin M subunit during liver injury, we monitored M-subunit mRNA in these cells at various intervals after carbon tetrachloride administration. M-subunit transcripts increased twofold within 12 hr of toxin exposure, returning to below baseline by 48 hr. The results indicate that lipocytes produce the M subunit of laminin in place of A. Production of this subunit by lipocytes may facilitate cell growth and reorganization during liver regeneration.

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

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

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

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

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

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

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.

Isolation of sympathonectin; a substrate-bound protein which induces preferential growth of sympathetic fibers in vitro

A protein named 'sympathonectin', was purified from chick heart cell-conditioned medium (HCM), on the basis of its ability to direct the neurite outgrowth of cultured sympathetic neurons. The most purified fraction showed a doublet band on SDS-PAGE with an apparent molecular weight of 370 kDa. The biological activity of sympathonectin was over 100 times higher than that of laminin. Immunoblotting with anti-sympathonectin of the 100,000 x g pellet (particulate) fraction of HCM identified a distinct 370 kDa band; this molecule did not react to the anti-laminin serum. The immunohistochemical analysis showed that the antibody against sympathonectin stained heart tissue but not skeletal muscle tissue, whereas anti-laminin serum stained both tissues. These results suggest that sympathonectin may play a role during sympathetic innervation of target organs.

The role of laminin in attachment, growth, and differentiation of cultured cells: a brief review

Laminin, a major structural multidomain protein of basement membranes, has been shown to exert a variety of biological activities. Prominent among those are mediation of cell attachment as well as influences on cellular proliferation, differentiation and motility. Distinct domains of laminin have been identified which carry these activities. The active sites on laminin are recognized by cellular receptors, several of which belong to the integrin class of heterodimeric transmembrane proteins. These are in direct contact with intracellular proteins and mediate signals from the extracellular matrix to the cytoskeleton and possibly to other intracellular regulatory systems. The biological activities of laminin may be used to design optimal conditions for the expression of a differentiated phenotype of cells in culture.

The extracellular matrix glycoproteins BM-90 and tenascin are expressed in the mesenchyme at sites of endothelial-mesenchymal conversion in the embryonic mouse heart

BM-90 is a novel glycoprotein initially isolated from the extracellular matrix of a mouse tumor. We here studied the expression of BM-90 during embryonic development of the mouse heart and compared its expression pattern with that of tenascin and laminin. Distribution was studied by immunofluorescence using antibodies specifically raised against mouse BM-90, laminin and tenascin. Some expression of BM-90 was seen in myocardial basement membranes at early developmental stages, but expression abruptly decreased from these sites at day 12 of embryogenesis. Laminin B chains were also found in the muscle basement membranes early but did not decrease with advancing development. The most striking observation was the markedly enriched expression of BM-90 in the endocardial cushion tissue (ECT). The ECT is derived from mesenchymal cells converted from endothelium and they will form the cardiac valves and septa. In the ECT, BM-90 showed considerable co-distribution with tenascin, but tenascin expression was more focal and did not mark all areas of the ECT. Northern blot data show that BM-90 and tenascin were produced by the developing heart. With antibodies detecting A, B1 and B2 chains of mouse laminin, no immunoreactivity was seen in the ECT. Our data thus show clear-cut differences in the molecular composition of the ECT and muscle basement membranes in the developing heart. The focal expression of BM-90 in the ECT suggests that BM-90 could be involved in epithelial-mesenchymal transitions.

Dystrophin-glycoprotein complex and laminin colocalize to the sarcolemma and transverse tubules of cardiac muscle

The expression and subcellular distribution of the dystrophin-glycoprotein complex and laminin were examined in cardiac muscle by immunoblot and immunofluorescence analysis of rabbit and sheep papillary muscle. The five dystrophin-associated proteins (DAPs), 156-DAG, 59-DAP, 50-DAG, 43-DAG, and 35-DAG, were identified in rabbit ventricular muscle and found to codistribute with dystrophin in both papillary myofibers and Purkinje fibers. The DAPs and dystrophin codistributed not only in the free surface sarcolemma but also in interior regions of the myofibers where T tubules are present. Neither the DAPs nor dystrophin were detected in intercalated discs, a specialized region of cardiac sarcolemma where neighboring myocardial cells are physically joined by cell-cell junctions. Similarly, in bundles of Purkinje fibers, which lack T tubules, DAPs and dystrophin were also found to codistribute at the free surface sarcolemma but were not detected either in the region of surface sarcolemma closely apposed to a neighboring Purkinje fiber or in interior regions of these myofibers. Comparison between the distribution of the dystrophin-glycoprotein complex and laminin showed that laminin codistributes with the components of this complex in both papillary myofibers and Purkinje fibers. These results are consistent with previous findings demonstrating that the extracellularly exposed 156-DAG (dystroglycan) of the skeletal muscle dystrophin-glycoprotein complex binds laminin, a component of the basement membrane. Although we demonstrate that DAPs, dystrophin, and laminin colocalize to the sarcolemma in rabbit and sheep papillary myofibers as they do in skeletal myofibers, the most striking difference between the subcellular distribution of these proteins in cardiac and skeletal muscle is that the dystrophin-glycoprotein complex and laminin also localize to regions of the fibers where T tubules are distributed in cardiac but not in skeletal muscle. These results imply that the protein composition and thus possibly some functions of T tubules in cardiac muscle are distinct from those of skeletal muscle.

A variant form of laminin is responsible for the neurite outgrowth-promoting activity in conditioned medium from a squamous carcinoma cell line

We examined the effects of conditioned medium (CM) obtained from a squamous carcinoma cell line, termed SAS, on chick sympathetic neuritic outgrowth. Neurons grown on a substratum coated with CM extended their neurites. Antisera raised against human laminin and mouse EHS laminin immunoprecipitated the neurite outgrowth-promoting factor in CM. The most purified fraction of CM contained a 740-kDa protein which reacted with the anti-laminin sera and was composed of three polypeptides of 330 kDa, 215 kDa and 195 kDa. The 215-kDa and 195-kDa polypeptides, but not the 330-kDa polypeptide were shown to be antigenically related to mouse laminin and human laminin by immunoblotting. The presence of merosin M chain in SAS cells was ruled out by using the method of reverse transcription polymerase chain reaction (RT-PCR). These results suggest that the 740-kDa protein is a laminin variant having a novel heavy chain of 330 kDa and is responsible for neurite outgrowth-promoting activity in CM.

Adhesion promoting property of laminin from normal tissue and from a tumorigenic cell line

The cell adhesion promoting activity of laminin isolated from normal human placenta was compared with that isolated from mouse EHS tumor and from the cultures of a mouse epithelial cell line B82 and its tumorigenic derivative, B82HT. The adhesion promoting properties of commercial merosin isolated from placenta was also compared with the above preparations using the human fibrosarcoma HT1080 cells. Percent attachment was defined as (radioactivity extracted from attached cells)/(radioactivity in cells added to assay) x 100. HT1080 cells adhered more efficiently on laminin (0.5 micrograms/well), isolated from the B82 rather than B82HT cell conditioned medium, (82% vs 64%). Percent attachment of HT1080 cells on isolated native placental laminin or commercial merosin was significantly higher compared to laminin from the EHS tumor (at 0.75 micrograms/well, 69%, 73% and 20% respectively). In parallel experiments the steady-state levels of mRNAs for subunits A, M, B1 and B2 in cultures of B82 and B82HT cells were determined. The ratio of mRNA for the laminin subunits in B82 and B82HT cells was 1:0.9 for the A chain, 1:0.6 for the M chain, 1:0.4 for the B1 chain, and 1:0.3 for the B2 chain. Protein studies indicated that the M subunit is absent in laminin preparations from the EHS tumor whereas it is abundant in the laminin from placenta and in commercial merosin. Laminin isolated from B82 cells contains a higher proportion of the M subunit compared to that from B82HT cells. The data suggest that there are functional differences between the laminin found in normal tissue and that present in a solid tumor. Functional differences were noted between the laminins synthesized by the B82 cell line and its tumorigenic counterpart, B82HT. These differences may result from the lack of gene expression for the laminin subunit M by the EHS tumor and by the lower degree of gene expression for this subunit by B82HT cells. The possibility that the laminin synthesized by the tumorigenic cell line may be structurally different from that synthesized by the B82 cells should also be considered.

Co-localization and molecular association of dystrophin with laminin at the surface of mouse and human myotubes

In Duchenne muscular dystrophy (DMD), deficiency of the protein dystrophin results in necrosis of muscle myofibres, associated with lesions in the sarcolemma and surrounding basal lamina. Dystrophin has been proposed to be a major component of the sub-sarcolemmal cytoskeleton involved in maintaining the integrity of the myofibre plasma membrane, and is known to associate with a group of sarcolemmal glycoproteins, one of which exhibits high affinity binding to the basal lamina component laminin. However, a direct or indirect transmembrane association of dystrophin in muscle cells with the myofibre basal lamina has not been demonstrated. To address this question we have examined dystrophin immunostaining and immunoprecipitation patterns in cultured mouse and human myotubes in comparison with that of the basal lamina component, laminin. Dual-immunolabelling revealed virtually complete co-localization of dystrophin on the inside surface of the muscle cell sarcolemma with plaques and veined arrays of laminin accumulating on the extracellular face. This pattern of laminin and dystrophin distribution was distinct from that of other cell surface molecules expressed in myotubes such as the neural cell adhesion molecule, NCAM, and the beta 1 integrin receptor, and immunoprecipitation of dystrophin from solubilized myotube extracts resulted in co-purification of laminin B1 chain confirming an association between these two components. The results thus provide the first direct cellular evidence of a transmembrane linkage between dystrophin in the sarcolemmal cytoskeleton with laminin in the overlying basal lamina. While the immunocytochemical distribution of laminin was apparently normal in dystrophin-deficient muscle cells, elevated levels of soluble laminin were present in extracts of mdx compared with normal mouse skeletal muscle.(ABSTRACT TRUNCATED AT 250 WORDS)