Deficiency of beta 1 integrins in teratoma interferes with basement membrane assembly and laminin-1 expression

Defective perlecan-associated basement membrane regeneration and altered modulation of transforming growth factor beta in corneal fibrosis

In the cornea, the epithelial basement membrane (EBM) and corneal endothelial Descemet's basement membrane (DBM) critically regulate the localization, availability and, therefore, the functions of transforming growth factor (TGF)β1, TGFβ2, and platelet-derived growth factors (PDGF) that modulate myofibroblast development. Defective regeneration of the EBM, and notably diminished perlecan incorporation, occurs via several mechanisms and results in excessive and prolonged penetration of pro-fibrotic growth factors into the stroma. These growth factors drive mature myofibroblast development from both corneal fibroblasts and bone marrow-derived fibrocytes, and then the persistence of these myofibroblasts and the disordered collagens and other matrix materials they produce to generate stromal scarring fibrosis. Corneal stromal fibrosis often resolves completely if the inciting factor is removed and the BM regenerates. Similar defects in BM regeneration are likely associated with the development of fibrosis in other organs where perlecan has a critical role in the modulation of signaling by TGFβ1 and TGFβ2. Other BM components, such as collagen type IV and collagen type XIII, are also critical regulators of TGF beta (and other growth factors) in the cornea and other organs. After injury, BM components are dynamically secreted and assembled through the cooperation of neighboring cells-for example, the epithelial cells and keratocytes for the corneal EBM and corneal endothelial cells and keratocytes for the corneal DBM. One of the most critical functions of these reassembled BMs in all organs is to modulate the pro-fibrotic effects of TGFβs, PDGFs and other growth factors between tissues that comprise the organ.

Basement membrane-like structures containing NTH α1(IV) are formed around the endothelial cell network in a novel in vitro angiogenesis model

Angiogenesis is a process through which new blood vessels are formed by sprouting and elongating from existing blood vessels. Several methods have been used to replicate angiogenesis in vitro, including culturing vascular endothelial cells on Matrigel and coculturing with endothelial cells and fibroblasts. However, the angiogenesis elongation process has not been completely clarified in these models. We therefore propose a new in vitro model of angiogenesis, suitable for observing vascular elongation, by seeding a spheroid cocultured from endothelial cells and fibroblasts into a culture dish. In this model, endothelial cells formed tubular networks elongated from the spheroid with a lumen structure and were connected with tight junctions. A basement membrane (BM)-like structure was observed around the tubular network, similarly to blood vessels in vivo. These results suggested that blood vessel-like structure could be reconstituted in our model. Laminin and type IV collagen, main BM components, were highly localized around the network, along with nontriple helical form of type IV collagen α1-chain [NTH α1(IV)]. In an ascorbic acid-depleted condition, laminin and NTH α1(IV) were observed around the network but not the triple-helical form of type IV collagen and the network was unstable. These results suggest that laminin and NTH α1(IV) are involved in the formation of tubular network and type IV collagen is necessary to stabilize the network.

The Ultrastructural Localization of Type II, IV, and VI Collagens at the Vitreoretinal Interface

Background

The vitreoretinal interface is the border of the cortical vitreous and the inner surface of the retina. The adhesion of the cortical vitreous to the ILM, namely vitreoretinal adhesion, involves a series of complex molecular adhesion mechanisms and has been considered as an important pathogenic factor in many vitreoretinal diseases. The presence of type VI collagen at the vitreoretinal interface and its possible interaction with collagens and glycoproteins indicates that type VI collagen may contribute to the vitreoretinal adhesion.

Purpose

To clarify the ultrastructural location of type VI collagen and its relationship to type II and IV collagens at the vitreoretinal interface.

Methods

The ultrastructural localization of type II, IV and VI collagens in the adult human vitreoretinal interface of five donor eyes was evaluated by transmission electron microscopy using immunogold labeling.

Results

In the pre-equatorial region, we observed densely packed vitreous lamellae with a partly intraretinal course containing type II and VI collagens, reticular structures containing type IV and VI collagens and a thin inner limiting membrane (ILM) containing type IV and VI collagens in a linear distribution pattern. From the anterior to the posterior retina, the linear pattern of type IV and VI collagen labeling gradually became more diffusely present throughout the entire thickness of the ILM.

Conclusions

The presence of type VI collagen in vitreous lamellae penetrating the ILM into the superficial retina suggests that type VI collagen may be involved in the organization of vitreous fibers into lamellae and in the adhesion of the vitreous fibers to the retina. The close relation of type VI to type IV collagen in the ILM suggests that type VI collagen is an important collagen type in the ILM. The topographic variations of type IV and VI collagens in the different regions of the ILM suggest a regional heterogeneity of the ILM. The reticular labeling pattern of type IV and VI collagens observed in the anterior vitreous are highly similar to labeling patterns of blood vessel walls. In the anterior vitreous, they may represent remnants of the regressed embryonic hyaloid blood vessel system. Their presence is in support of the theory on interactive remodeling of the developing vitreous as opposed to the main stream theory of displacement and compression of the primary by the secondary vitreous.

β1 integrin is a crucial regulator of pancreatic β-cell expansion

Development of the endocrine compartment of the pancreas, as represented by the islets of Langerhans, occurs through a series of highly regulated events encompassing branching of the pancreatic epithelium, delamination and differentiation of islet progenitors from ductal domains, followed by expansion and three-dimensional organization into islet clusters. Cellular interactions with the extracellular matrix (ECM) mediated by receptors of the integrin family are postulated to regulate key functions in these processes. Yet, specific events regulated by these receptors in the developing pancreas remain unknown. Here, we show that ablation of the β1 integrin gene in developing pancreatic β-cells reduces their ability to expand during embryonic life, during the first week of postnatal life, and thereafter. Mice lacking β1 integrin in insulin-producing cells exhibit a dramatic reduction of the number of β-cells to only ∼18% of wild-type levels. Despite the significant reduction in β-cell mass, these mutant mice are not diabetic. A thorough phenotypic analysis of β-cells lacking β1 integrin revealed a normal expression repertoire of β-cell markers, normal architectural organization within islet clusters, and a normal ultrastructure. Global gene expression analysis revealed that ablation of this ECM receptor in β-cells inhibits the expression of genes regulating cell cycle progression. Collectively, our results demonstrate that β1 integrin receptors function as crucial positive regulators of β-cell expansion.

β1 integrin-extracellular matrix interactions are essential for maintaining exocrine pancreas architecture and function

Integrin receptors are responsible for integrating extracellular matrix signals inside the cell. The most prominent integrin receptor, β1 integrin, has a role in cell function, survival and differentiation. Recently, we demonstrated a profound in vivo role of β1 integrin expression in the pancreas on glucose homeostasis and islet function. Here, we extend these results by examining the role of β1 integrin in exocrine pancreatic structure and function. Adult C57Bl/6 mice hemizygous for a collagen type Iα2 (Col1a2) promoter-controlled tamoxifen-inducible Cre recombinase gene and homozygous for loxP-β1 integrin were injected with tamoxifen or corn oil to generate mice deleted or not for β1 integrin. Pancreata derived from these male mice were analyzed by quantitative reverse transcriptase-polymerase chain reaction, western blot and immunofluorescence. Our results showed that β1 integrin-deficient mice displayed a significant decrease in pancreas weight with a significant reduction of amylase, regenerating islet-derived protein II and carboxypeptidase-A expression (P<0.05-0.01). Compared with control pancreata, β1 integrin-deficient pancreata showed reduced mRNA expression of extracellular matrix (collagen type Iα2, fibronectin and laminin) genes (P<0.05), detached acini clusters and lost focal adhesion structure. Moreover, β1 integrin-deficient pancreatic acinar cells displayed decreased proliferation (P<0.05) and increased apoptosis (P<0.001). Apoptosis was reduced to that of controls when isolated exocrine clusters were cultured in media supplemented with extracellular matrix proteins. Taken together, these results implicate β1 integrin as an essential component for maintaining exocrine pancreatic structure and function.

Rotational motion during three-dimensional morphogenesis of mammary epithelial acini relates to laminin matrix assembly

Our understanding of the mechanisms by which ducts and lobules develop is derived from model organisms and three-dimensional (3D) cell culture models wherein mammalian epithelial cells undergo morphogenesis to form multicellular spheres with a hollow central lumen. However, the mechanophysical properties associated with epithelial morphogenesis are poorly understood. We performed multidimensional live-cell imaging analysis to track the morphogenetic process starting from a single cell to the development of a multicellular, spherical structure composed of polarized epithelial cells surrounding a hollow lumen. We report that in addition to actively maintaining apicobasal polarity, the structures underwent rotational motions at rates of 15-20 μm/h and the structures rotated 360° every 4 h during the early phase of morphogenesis. Rotational motion was independent of the cell cycle, but was blocked by loss of the epithelial polarity proteins Scribble or Pard3, or by inhibition of dynein-based microtubule motors. Interestingly, none of the structures derived from human cancer underwent rotational motion. We found a direct relationship between rotational motion and assembly of endogenous basement membrane matrix around the 3D structures, and that structures that failed to rotate were defective in weaving exogenous laminin matrix. Dissolution of basement membrane around mature, nonrotating acini restored rotational movement and the ability to assemble exogenous laminin. Thus, coordinated rotational movement is a unique mechanophysical process observed during normal 3D morphogenesis that regulates laminin matrix assembly and is lost in cancer-derived epithelial cells.

Beta1 integrin cytoplasmic variants differentially regulate expression of the antiangiogenic extracellular matrix protein thrombospondin 1

Beta(1) integrins play an important role in regulating cell proliferation and survival. Using small interfering RNA or an inhibitory antibody to beta(1), we show here that, in vivo, beta(1) integrins are essential for prostate cancer growth. Among the five known beta(1) integrin cytoplasmic variants, two have been shown to differentially affect prostate cell functions. The beta(1A) variant promotes normal and cancer cell proliferation, whereas the beta(1C) variant, which is down-regulated in prostate cancer, inhibits tumor growth and appears to have a dominant effect on beta(1A). To investigate the mechanism by which beta(1C) inhibits the tumorigenic potential of beta(1A), we analyzed changes in gene expression in cells transfected with either beta(1C) or beta(1A). The results show that beta(1C) expression increases the levels of an extracellular matrix protein, thrombospondin 1 (TSP1), an angiogenesis inhibitor. TSP1 protein levels are increased upon beta(1C) expression in prostate cancer cells as well as in beta(1)-null GD25 cells. We show that TSP1 does not affect proliferation, apoptosis, or anchorage-independent growth of prostate cancer cells. In contrast, the newly synthesized TSP1, secreted by prostate cancer cells expressing beta(1C), prevents proliferation of endothelial cells. In conclusion, our novel findings indicate that expression of the beta(1C) integrin variant in prostate glands prevents cancer progression by up-regulation of TSP1 levels and inhibition of angiogenesis.

Tissue-specific KO of ECM proteins

Nearly 20 years after its first description, gene targeting and generation of transgenic mice by homologous recombination in embryonic stem cells still are cutting edge tools for the postgenomic era. Understanding the function of the large number of genes encoding extracellular matrix proteins and their cellular receptors appears a daunting task that can very much profit from a genetic approach. The generation of new mutant alleles remains essential to define the different biochemical properties of such proteins. While in the past, gene targeting represented a complex procedure, restricted to few laboratories, recent breakthroughs, such as the publication of the mouse genome sequence and the perfection of recombineering techniques in bacteria, made generation of transgenic mice faster and easier. This chapter will thus focus on the recent advances in gene-targeting technology with a special eye on the study of genes involved in cell adhesion and migration.

Integrins are required for the differentiation of visceral endoderm

Integrins of the beta1 subfamily are highly expressed in the early mouse embryo and are essential for the formation of primitive germ layers from the inner cell mass (ICM). We investigated the mechanisms by which alphabeta1 integrins regulate ICM morphogenesis by using the embryonic-stem-cell-derived embryoid body (EB), a model for peri-implantation development. Ablation of integrin beta1 in EBs resulted in endoderm detachment and in maturation defects, which were manifested by the mislocalization of GATA4 in the cytoplasm and the markedly reduced synthesis of basement membrane (BM) components and the lineage marker disabled homolog 2. The mutant endoderm cells failed to spread on BM substrates, but could spread on vitronectin, which induced upregulation of alphavbeta3 integrin and integrin-dependent GATA4 nuclear translocation. Forced expression of integrin beta3 in the mutant EBs completely rescued endoderm morphogenesis, suggesting that integrin beta3 can substitute for integrin beta1 in the endoderm. Furthermore, the mitogen-activated protein kinases (MAPKs) ERK1 and ERK2 (ERK1/2) and p38 were activated in endoderm in an integrin-dependent fashion. Pharmacological inhibition of ERK1/2 or p38 MAPK blocked vitronectin-induced GATA4 nuclear translocation and endoderm maturation, whereas expression of a constitutively active ERK kinase (MEK1) or p38 MAPK in the mutant cells rescued endoderm maturation in integrin-beta1-null endoderm cells. Collectively, these results suggest that integrins are required for both the stable adhesion and maturation of visceral endoderm, the latter being mediated through the activation of ERK1/2 and p38 MAPK.

Integrin α6β1-laminin interactions regulate early myotome formation in the mouse embryo

We addressed the potential role of cell-laminin interactions during epaxial myotome formation in the mouse embryo. Assembly of the myotomal laminin matrix occurs as epaxial myogenic precursor cells enter the myotome. Most Myf5-positive and myogenin-negative myogenic precursor cells localise near assembled laminin, while myogenin-expressing cells are located either away from this matrix or in areas where it is being assembled. In Myf5nlacZ/nlacZ (Myf5-null) embryos, laminin,collagen type IV and perlecan are present extracellularly near myogenic precursor cells, but do not form a basement membrane and cells are not contained in the myotomal compartment. Unlike wild-type myogenic precursor cells, Myf5-null cells do not express the α6β1 integrin, a laminin receptor, suggesting that integrin α6β1-laminin interactions are required for myotomal laminin matrix assembly. Blockingα6β1-laminin binding in cultured wild-type mouse embryo explants resulted in dispersion of Myf5-positive cells, a phenotype also seen in Myf5nlacZ/nlacZ embryos. Furthermore, inhibition ofα6β1 resulted in an increase in Myf5 protein and ectopic myogenin expression in dermomyotomal cells, suggesting that α6β1-laminin interactions normally repress myogenesis in the dermomyotome. We conclude that Myf5 is required for maintaining α6β1 expression on myogenic precursor cells, and that α6β1 is necessary for myotomal laminin matrix assembly and cell guidance into the myotome. Engagement of laminin byα6β1 also plays a role in maintaining the undifferentiated state of cells in the dermomyotome prior to their entry into the myotome.

Beta1-integrin orients epithelial polarity via Rac1 and laminin

Epithelial cells polarize and orient polarity in response to cell-cell and cell-matrix adhesion. Although there has been much recent progress in understanding the general polarizing machinery of epithelia, it is largely unclear how this machinery is controlled by the extracellular environment. To explore the signals from cell-matrix interactions that control orientation of cell polarity, we have used three-dimensional culture systems in which Madin-Darby canine kidney (MDCK) cells form polarized, lumen-containing structures. We show that interaction of collagen I with apical beta1-integrins after collagen overlay of a polarized MDCK monolayer induces activation of Rac1, which is required for collagen overlay-induced tubulocyst formation. Cysts, comprised of a monolayer enclosing a central lumen, form after embedding single cells in collagen. In those cultures, addition of a beta1-integrin function-blocking antibody to the collagen matrix gives rise to cysts that have defects in the organization of laminin into the basement membrane and have inverted polarity. Normal polarity is restored by either expression of activated Rac1, or the inclusion of excess laminin-1 (LN-1). Together, our results suggest a signaling pathway in which the activation of beta1-integrins orients the apical pole of polarized cysts via a mechanism that requires Rac1 activation and laminin organization into the basement membrane.

Myosin-mediated cytoskeleton contraction and Rho GTPases regulate laminin-5 matrix assembly

Laminin-5 is a major structural element of epithelial tissue basement membranes. In the matrix of cultured epithelial cells, laminin-5 is arranged into intricate patterns. Here we tested a hypothesis that myosin II-mediated actin contraction is necessary for the proper assembly of a laminin-5 matrix by cultured SCC12 epithelial cells. To do so, the cells were treated with ML-7, a myosin II light chain kinase inhibitor, or Y-27632, an inhibitor of Rho-kinase (ROCK), both of which block actomyosin contraction. Under these conditions, laminin-5 shows an aberrant localization in dense patches at the cell periphery. Since ROCK activity is regulated by the small GTPase Rho, this suggests that members of the Rho family of GTPases may also be important for laminin-5 matrix assembly by SCC12 cells. We confirmed this hypothesis since SCC12 cells expressing mutant proteins that inhibit RhoA, Rac, and Cdc42 assemble the same aberrant laminin-5 protein arrays as drug-treated cells. We have also evaluated the organization of the laminin-5 receptors alpha3beta1 and alpha6beta4 integrin and hemidesmosome proteins in ML-7- and Y-27632-treated cells or in cells in which RhoA, Rac, and Cdc42 activity were inhibited. In all instances, alpha3beta1 and alpha6beta4 integrin heterodimers, as well as hemidesmosome proteins, localize precisely with laminin-5 in the matrix of the cells. In summary, our results provide evidence that myosin II-mediated actin contraction and the activity of Rho GTPases are necessary for the proper organization of a laminin-5 matrix and localization of hemidesmosome protein arrays in epithelial cells.

The muscle integrin binding protein (MIBP) interacts with alpha7beta1 integrin and regulates cell adhesion and laminin matrix deposition

Integrins are alphabeta transmembrane receptors that function in key cellular processes, including cell adhesion, differentiation, and extracellular matrix deposition through interactions with extracellular, membrane, and cytoplasmic proteins. We previously identified and cloned a muscle beta1 integrin cytoplasmic binding protein termed MIBP and found that the expression level of MIBP is critical in the decision-making process of terminal myogenic differentiation. We report here that MIBP interacts with the alpha7beta1 integrin but not the alpha5beta1 integrin in C2C12 myoblasts, suggesting an important role of integrin alpha chains in the regulation of the beta1-MIBP interaction. Furthermore, consistent with its selective binding activity toward the alpha7beta1 laminin receptor, we have found that overexpression of MIBP in C2C12 myoblasts resulted in a significant reduction of cell adhesion to laminin and inhibition of laminin matrix deposition. By contrast, neither cell adhesion to fibronectin nor fibronectin matrix deposition was significantly altered in cells overexpressing MIBP. Finally, we show that both the protein level and tyrosine phosphorylation of paxillin, a key signaling molecule involved in the cellular control of myogenic differentiation, are increased by MIBP. These results suggest that MIBP functions in the control of myogenic differentiation by regulating alpha7beta1 integrin-mediated cell interactions with laminin matrix and intracellular signaling through paxillin.

Expression of laminin receptors in schwann cell differentiation: evidence for distinct roles

Schwann cells require laminin-2 throughout nerve development, because mutations in the alpha2 chain in dystrophic mice interfere with sorting of axons before birth and formation of myelin internodes after birth. Mature Schwann cells express several laminin receptors, but their expression and roles in development are poorly understood. Therefore, we correlated the onset of myelination in nerve and synchronized myelinating cultures to the appearance of integrins and dystroglycan in Schwann cells. Only alpha6beta1 integrin is expressed before birth, whereas dystroglycan and alpha6beta4 integrin appear perinatally, just before myelination. Although dystroglycan is immediately polarized to the outer surface of Schwann cells, alpha6beta4 appears polarized only after myelination. We showed previously that Schwann cells lacking beta1 integrin do not relate properly to axons before birth. Here we show that the absence of beta1 before birth is not compensated by other laminin receptors, whereas coexpression of both dystroglycan and beta4 integrin is likely required for beta1-null Schwann cells to myelinate after birth. Finally, both beta1-null and dystrophic nerves contain bundles of unsorted axons, but they are predominant in different regions: in spinal roots in dystrophic mice and in nerves in beta1-null mice. We show that differential compensation by laminin-1, but not laminin receptors may partially explain this. These data suggest that the action of laminin is mediated by beta1 integrins during axonal sorting and by dystroglycan, alpha6beta1, and alpha6beta4 integrins during myelination.

Basement membranes: structure, assembly and role in tumour angiogenesis

In recent years, the basement membrane (BM)--a specialized form of extracellular matrix (ECM)--has been recognized as an important regulator of cell behaviour, rather than just a structural feature of tissues. The BM mediates tissue compartmentalization and sends signals to epithelial cells about the external microenvironment. The BM is also an important structural and functional component of blood vessels, constituting an extracellular microenvironment sensor for endothelial cells and pericytes. Vascular BM components have recently been found to be involved in the regulation of tumour angiogenesis, making them attractive candidate targets for potential cancer therapies.

Dystrophin isoform Dp7l is present in lamellipodia and focal complexes in human astrocytoma cells U-373 MG

Dp71 is the most abundant product of the dmd gene in the brain. There are at least 2 isoforms derived from alternative splicing of exon 78 (Dp71d, which contains exon 78 and Dp71f, the spliced isoform) but the precise localization and function of each isoform is still unknown. In the present study, we demonstrate by RT-PCR that the Dp71f isoform is present in an astrocytoma cell line U-373 MG, and its subcellular localization was determined in the cytoplasm, particularly in perinuclear areas, with lower amounts towards the periphery but increasing in the leader borders of lamellipodia and focal complexes. Double labeling indirect immunofluorescence showed that Dp71f colocalized with actin-like beta-dystroglycan and beta-1 integrin. We also demonstrated by triple labeling that Dp71f was colocalized with actin and two members of integrin complexes, alpha-actinin and vinculin, in focal complexes. Ventral plasma membranes were enriched and in those containing focal complex proteins, we found colocalization of Dp71f, actin and vinculin. It is concluded that U-373 MG cells express Dp71f as part of lamellipodia and focal complex proteins, and possibly connected via distroglycan complexes to integrin complexes.

Expression and biological role of laminin-1

Of the approximately 15 laminin trimers described in mammals, laminin-1 expression seems to be largely limited to epithelial basement membranes. It appears early during epithelial morphogenesis in most tissues of the embryo, and remains present as a major epithelial laminin in some adult tissues. Previous organ culture studies with embryonic tissues have suggested that laminin-1 is important for epithelial development. Recent data using genetically manipulated embryonic stem (ES) cells grown as embryoid bodies provide strong support for the view of a specific role of laminin-1 in epithelial morphogenesis. One common consequence of genetic ablation of FGF signaling, beta1-integrin or laminin gamma1 chain expression in ES cells is the absence of laminin-1, which correlates with failure of BM assembly and epiblast differentiation. Partial but distinct rescue of epiblast differentiation has been achieved in all three mutants by exogenously added laminin-1. Laminin-1 contains several biologically active modules, but several are found in beta1 or gamma1 chains shared by at least 11 laminins. However, the carboxytermini of the alpha chains contain five laminin globular (LG) modules, distinct for each alpha chain. There is increasing evidence for a particular role of alpha1LG4 binding to its receptors for epithelial tubulogenesis. The biological roles of this and other domains of laminin-1 are currently being explored by genetic means. The pathways controlling laminin-1 synthesis have remained largely unknown, but recent advances raise the possibility that laminin-1 and collagen IV synthesis can be regulated by pro-survival kinases of the protein kinase B/Akt family.

Applications of adhesion molecule gene knockout cell lines

Cell lines derived from mice carrying targeted mutations in adhesion or ECM genes, and ES cell lines homozygous for mutations in these genes, have proved to be valuable tools to examine the functions of these molecules in development, and at a molecular level by in vitro experimentation. In many cases, the development of cell lines has exposed novel phenotypes not apparent from in vivo observations, possibly because the in vitro system under observation is more dependent on a specific molecule than is an entire organ or organism. These cell lines should continue to provide interesting model systems to study the molecular function of adhesion receptors.

Evidence of nidogen-2 compensation for nidogen-1 deficiency in transgenic mice

Previous studies have shown that inhibition of nidogen-laminin binding interferes with basement membrane stabilization in various mouse organ cultures while no overt phenotype has been observed following inactivation of the nidogen-1 gene in mice. We have now used recombinant mouse nidogen-1 and nidogen-2 in order to evaluate a possible compensation between the two isoforms in the knock-out mice. Essentially, a comparable in vitro binding of nidogens-1 and -2 to the same laminin gamma1 chain structure and to several other basement membrane proteins has been revealed. Quantitative radioimmuno-assays have demonstrated high concentrations of nidogen-1 exceeding those of laminin gamma1 and nidogen-2 by factors of 5 and 20-50, respectively, in tissue extracts of wild-type mice. A three- to sevenfold increase in nidogen-2 was observed in heart and muscle of mice with nidogen-1 deficiency and confirmed by a similar increase in the intensity of immunogold staining of these tissues. However, a few of the tissues from mice with the gene knock-out still contained some nidogen-1-like immunoreactivity (1% of wild-type). Furthermore, both nidogen isoforms showed a similar distribution in various organs during embryonic development which, however, as shown previously, changed in some adult tissues. The data support the nidogen-2 compensation hypothesis to explain the limited phenotype observed following elimination of the nidogen-1 gene.

Assembly, stability and integrity of basement membranes in vivo

Basement membranes are layered structures of the extracellular matrix which separate cells of various kinds from the surrounding stroma. One of the frequently recurring questions about basement membranes is how these structures are formed in vivo. Up to a few years ago, it was thought that basement membranes were formed spontaneously by a process of self-assembly of their components. However, it has now become clear that cell membrane receptors for basement membrane components are essential factors for the formation and stability of basement membranes in vivo. The present review highlights the modern concepts of basement membrane formation.

Expression of basal lamina protein mRNAs in the early embryonic chick eye

Laminin, collagen IV, collagen XVIII, agrin, and nidogen are major protein constituents of the chick retinal basal lamina. To determine their sites of synthesis during de novo basal lamina assembly in vivo, we localized their mRNA expression in the eye during maximum expansion of the retina between embryonic day (E) 2.5 and E6. Our in situ hybridization studies showed that the expression pattern of every basal lamina protein mRNA in the developing eye is unique. Collagen IV and perlecan originate predominantly from the lens epithelium, whereas collagen XVIII, nidogen, and the laminin gamma 1 and beta1 chains are synthesized mainly by the ciliary body. Agrin, collagen XVIII, collagen IV, and laminin gamma 1 also originate from cells of the optic disc. The only basal lamina protein that is synthesized by the neural retina throughout development is agrin with ganglion cells as its main source. Some of the mRNAs have short, transient expressions in the retina, most notably that of collagen IV and laminin gamma 1, both of which appear in the ventral retina between E4 and E5. That most retinal basal lamina proteins originate from extraretinal tissues infers that the basal lamina proteins have to be shed from the lens, optic disc, and ciliary body into the vitreous body. The assembly of the retinal basal lamina then occurs by the binding of these proteins by cellular receptor proteins on the vitreal endfeet of the retinal neuroepithelial cells.

Specific ablation of the nidogen-binding site in the laminin γ1 chain interferes with kidney and lung development

Basement membrane assembly is of crucial importance in the development and function of tissues and during embryogenesis. Nidogen 1 was thought to be central in the assembly processes, connecting the networks formed by collagen type IV and laminins, however, targeted inactivation of nidogen 1 resulted in no obvious phenotype. We have now selectively deleted the sequence coding for the 56 amino acid nidogen-binding site, γ1III4, within the Lamc1 gene by gene targeting. Here, we show that mice homozygous for the deletion die immediately after birth, showing renal agenesis and impaired lung development. These developmental defects were attributed to locally restricted ruptures in the basement membrane of the elongating Wolffian duct and of alveolar sacculi. These data demonstrate that an interaction between two basement membrane proteins is required for early kidney morphogenesis in vivo.

Laminin assembles into separate basement membrane and fibrillar matrices in Schwann cells

Laminins are important for Schwann cell basement membrane assembly and axonal function. In this study, we found that exogenous laminin-1, like neuromuscular laminins-2/4, formed two distinct extracellular matrices on Schwann cell surfaces, each facilitated by laminin polymerization. Assembly of one, a densely-distributed reticular matrix, was accompanied by a redistribution of cell-surface dystroglycan and cytoskeletal utrophin into matrix-receptor-cytoskeletal complexes. The other, a fibrillar matrix,accumulated in separate zones associated with pre-existing β1-integrin arrays. The laminin-1 fragment E3 (LG-modules 4-5), which binds dystroglycan and heparin, inhibited reticular-matrix formation. By contrast,β1-integrin blocking antibody (Ha2/5) prevented fibrillar assembly. Ultrastructural analysis revealed that laminin treatment induced the formation of a linear electron-dense extracellular matrix (lamina densa)separated from plasma membrane by a narrow lucent zone (lamina lucida). This structure was considerably reduced with non-polymerizing laminin, fully blocked by E3, and unaffected by Ha2/5. Although it formed in the absence of type IV collagen, it was nonetheless able to incorporate this collagen. Finally, cell competency to bind laminin and form a basement membrane was passage-dependent. We postulate that laminin induces the assembly of a basement membrane on competent cell surfaces probably mediated by anchorage through LG 4-5. Upon binding, laminin interacts with dystroglycan,mobilizes utrophin, and assembles a `nascent' basement membrane, independent of integrin, that is completed by incorporation of type IV collagen. However,the fibrillar β1-integrin dependent matrix is unlikely to be precursor to basement membrane.

Dystroglycan binding to laminin alpha1LG4 module influences epithelial morphogenesis of salivary gland and lung in vitro

Dystroglycan is a receptor for the basement membrane components laminin-1, -2, perlecan, and agrin. Genetic studies have revealed a role for dystroglycan in basement membrane formation of the early embryo. Dystroglycan binding to the E3 fragment of laminin-1 is involved in kidney epithelial cell development, as revealed by antibody perturbation experiments. E3 is the most distal part of the carboxyterminus of laminin alpha1 chain, and is composed of two laminin globular (LG) domains (LG4 and LG5). Dystroglycan-E3 interactions are mediated solely by discrete domains within LG4. Here we examined the role of this interaction for the development of mouse embryonic salivary gland and lung. Dystroglycan mRNA was expressed in epithelium of developing salivary gland and lung. Immunofluorescence demonstrated dystroglycan on the basal side of epithelial cells in these tissues. Antibodies against dystroglycan that block binding of alpha-dystroglycan to laminin-1 perturbed epithelial branching morphogenesis in salivary gland and lung organ cultures. Inhibition of branching morphogenesis was also seen in cultures treated with polyclonal anti-E3 antibodies. One monoclonal antibody (mAb 200) against LG4 blocked interactions between a-dystroglycan and recombinant laminin alpha1LG4-5, and also inhibited salivary gland and lung branching morphogenesis. Three other mAbs, also specific for the alpha1 carboxyterminus and known not to block branching morphogenesis, failed to block binding of alpha-dystroglycan to recombinant laminin alpha1LG4-5. These findings clarify why mAbs against the carboxyterminus of laminin alpha1 differ in their capacity to block epithelial morphogenesis and suggest that dystroglycan binding to alpha1LG4 is important for epithelial morphogenesis of several organs.

The distribution and regulation of integrin-linked kinase in normal and diabetic kidneys

Alteration in cell adhesion and extracellular matrix deposition is a hallmark of diabetic glomerulosclerosis. Integrin-linked kinase (ILK) is a recently identified integrin cytoplasmic-binding protein that has been implicated in the regulation of cell adhesion and extracellular matrix deposition. To begin to investigate whether ILK is involved in the pathogenesis of diabetic glomerulosclerosis, we have analyzed the distribution and regulation of ILK in normal and diabetic kidneys as well as in isolated mesangial cells. We have found that ILK is normally expressed at high concentration in visceral epithelial cells. In diabetic glomeruli, ILK expression in the mesangium is dramatically increased. The increase in ILK level is associated with diffuse mesangial expansion. In glomeruli where advanced nodular sclerosis and global sclerosis were dominant, ILK level was reduced, suggesting that the increase in ILK expression likely associates with relatively early glomerulosclerosis. Additionally, we have found that exposure of mesangial cells to high concentrations of glucose significantly increased the ILK level. Finally, we show that ILK localizes to regions of cell membranes that are in close contact with mesangial fibronectin matrix. These results suggest that ILK is likely involved in mesangial matrix expansion in response to hyperglycemia in the pathogenesis of diabetic glomerulosclerosis.

Localization of extracellular matrix receptors in ICGN mice, a strain of mice with hereditary nephrotic syndrome

Fibrotic degeneration was examined in the kidneys of ICR-derived glomerulonephritis (ICGN) mice, a novel inbred mouse line with a hereditary nephrotic syndrome of unknown etiology considered to be a good model of human idiopathic nephrotic syndrome. In the present study, we histochemically revealed changes in accumulation of extracellular matrix (ECM) components and in localization of integrins, cellular receptors for ECM, in the kidneys of ICGN mice with the progression of renal failure. Excessive accumulation of basement membrane (laminin and collagen IV) and interstitial (type III collagen) ECM components were demonstrated in the glomeruli and tubulointerstitum of ICGN mice. Marked deposition of type I collagen and tenascin was seen only in the glomeruli of ICGN mice but not in those of ICR mice as normal controls. Increased expression of integrin alpha1-, alpha2-, alpha5- and beta1-subunits in glomeruli with fibrotic degeneration and abnormal distribution of alpha6-subunit were noted in the kidneys of ICGN mice. Excessive laminin, a ligand of alpha6beta1-integrin, was demonstrated on the tubular basement membrane, but alpha6-subunit diffusely disappeared on the basal side of the tubular epithelial cells. We presumed that abnormal integrin expression in renal tubules causes epithelial cell detachment, and consequently tubular nephropathy, and results in disorder of ECM metabolism causing excessive accumulation of ECM components in the kidneys of ICGN mice.

Temporary disruption of the retinal basal lamina and its effect on retinal histogenesis

An experimental paradigm was devised to remove the retinal basal lamina for defined periods of development: the basal lamina was dissolved by injecting collagenase into the vitreous of embryonic chick eyes, and its regeneration was induced by a chase with mouse laminin-1 and alpha2-macroglobulin. The laminin-1 was essential in reconstituting a new basal lamina and could not be replaced by laminin-2 or collagen IV, whereas the macroglobulin served as a collagenase inhibitor that did not directly contribute to basal lamina regeneration. The regeneration occurred within 6 h after the laminin-1 chase by forming a morphologically complete basal lamina that included all known basal lamina proteins from chick embryos, such as laminin-1, nidogen-1, collagens IV and XVIII, perlecan, and agrin. The temporary absence of the basal lamina had dramatic effects on retinal histogenesis, such as an irreversible retraction of the endfeet of the neuroepithelial cells from the vitreal surface of the retina, the formation of a disorganized ganglion cell layer with an increase in ganglion cells by 30%, and the appearance of multiple retinal ectopias. Finally, basal lamina regeneration was associated with aberrant axons failing to correctly enter the optic nerve. The present data demonstrate that a transient disruption of the basal lamina leads to dramatic and probably irreversible aberrations in the histogenesis in the developing central nervous system.

Functional consequences of integrin gene mutations in mice

Integrins are cell-surface receptors responsible for cell attachment to extracellular matrices and to other cells. The application of mouse genetics has significantly increased our understanding of integrin function in vivo. In this review, we summarize the phenotypes of mice carrying mutant integrin genes and compare them with phenotypes of mice lacking the integrin ligands.

The laminins in the murine inner ear: developmental transitions and expression in cochlear basement membranes

The laminins are a family of heterotrimeric extracellular matrix molecules that form suprastructural networks in basement membranes and elsewhere. They interact with integrin receptors, playing key roles in modulating programs of cytodifferentiation and maintaining tissue homeostasis in animals. Earlier studies have demonstrated an extensive laminin network in both the developing and adult cochlea, primarily associated with the basement membranes. These studies, however, did not address the laminin chain composition of these networks. In this study, we used antibodies specific for the known laminin chains to examine the composition of laminins in both the developing and adult murine cochlea. The results illustrate a complex and dynamic postnatal developmental regulation pattern for most of these chains, and suggest that an unusually large number of laminin heterotrimers are present in both the developing and adult cochlea. The laminin composition at postnatal day 2 is relatively simple. By postnatal day 7, however, activation of several laminin chains results in a very complex laminin composition. In the basement membrane underlying the region of the basilar membrane under the developing organ of Corti, eight of the 11 known basement membrane laminins are possible by co-localization inference. Dynamic changes in expression continue through day 14, but simplify by adulthood. Thus, the most dynamic period for laminin expression in the mouse cochlea coincides with terminal cytodifferentiation of the cochlear epithelial structures. Considering the well established role of laminins in regulating both embryonic and organ development in other systems, these data suggest a closer look at the role of the laminins in cochlear development and function may be warranted.

Assembly of laminin polymers is dependent on beta1-integrins

Recent reports suggest that laminin deposition is controlled by the cell via specific receptors, one of which is dystroglycan. In this study, the involvement of beta1-integrins in this process was investigated by comparing beta1-integrin-deficient cells of different phenotypes with their normal counterparts. Normal embryonic stem (ES) cells and embryoid bodies (EBs) derived from them were found to deposit cell-associated laminin into fibrillar networks, and in the EBs a basement membrane was assembled under the primitive endoderm. beta1-deficient ES cells and their EBs formed only small amounts of dot-like laminin deposits. Skeletal myotubes formed after prolonged differentiation in EBs were found to be surrounded by laminin, nidogen, and perlecan by immunofluorescent staining irrespective of the presence of beta1-integrins on the myotubes. However, at the electron microscope level only very thin sheet-like structures were detected close to the beta1-deficient myotubes, while the wt myotubes formed thick basement membranes. An epithelial cell line, GE11, derived from the beta1-integrin-deficient ES cells was also unable to assemble laminin on the cell surface, while transfection of the cells with the integrin beta1 subunit resulted in formation of a dense laminin network. Taken together, these results suggest that dystroglycan and beta1-integrins can both contribute to the recruitment of laminin to cell surfaces and that integrins are required at a subsequent step in the formation of basement membranes.

Distinct roles for dystroglycan, (β)1 integrin and perlecan in cell surface laminin organization

Dystroglycan (DG) is a cell surface receptor for several extracellular matrix (ECM) molecules including laminins, agrin and perlecan. Recent data indicate that DG function is required for the formation of basement membranes in early development and the organization of laminin on the cell surface. Here we show that DG-mediated laminin clustering on mouse embryonic stem (ES) cells is a dynamic process in which clusters are consolidated over time into increasingly more complex structures. Utilizing various null-mutant ES cell lines, we define roles for other molecules in this process. In (β)1 integrin-deficient ES cells, laminin-1 binds to the cell surface, but fails to organize into more morphologically complex structures. This result indicates that (β)1 integrin function is required after DG function in the cell surface-mediated laminin assembly process. In perlecan-deficient ES cells, the formation of complex laminin-1 structures is defective, implicating perlecan in the laminin matrix assembly process. Moreover, laminin and perlecan reciprocally modulate the organization of the other on the cell surface. Taken together, the data support a model whereby DG serves as a receptor essential for the initial binding of laminin on the cell surface, whereas (β)1 integrins and perlecan are required for laminin matrix assembly processes after it binds to the cell.

The localized assembly of extracellular matrix integrin ligands requires cell-cell contact

The assembly of an organism requires the interaction between different layers of cells, in many cases via an extracellular matrix. In the developing Drosophila larva, muscles attach in an integrin-dependent manner to the epidermis, via a specialized extracellular matrix called tendon matrix. Tiggrin, a tendon matrix integrin ligand, is primarily synthesized by cells distant to the muscle attachment sites, yet it accumulates specifically at these sites. Previous work has shown that the PS integrins are not required for tiggrin localization, suggesting that there is redundancy among tiggrin receptors. We have examined this by testing whether the PS2 integrin can recruit tiggrin to ectopic locations within the Drosophila embryo. We found that neither the wild type nor modified forms of the PS2 integrin, which have higher affinity for tiggrin, can recruit tiggrin to new cellular contexts. Next, we genetically manipulated the fate of the muscles and the epidermal muscle attachment cells, which demonstrated that muscles have the primary role in recruiting tiggrin to the tendon matrix and that cell-cell contact is necessary for this recruitment. Thus we propose that the inherent polarity of the muscle cells leads to a molecular specialization of their ends, and interactions between the ends produces an integrin-independent tiggrin receptor. Thus, interaction between cells generates an extracellular environment capable of nucleating extracellular matrix assembly.

Integrins as receptors for laminins

Laminins are a family of trimeric glycoproteins present in the extracellular matrix and the major constituents of basement membranes. Integrins are alpha beta transmembrane receptors that play critical roles in both cell-matrix and cell-cell adhesion. Several members of the integrin family, including alpha 1 beta 1, alpha 2 beta 1, alpha 3 beta 1, alpha 6 beta 1, alpha 7 beta 1 and alpha 6 beta 4 heterodimers serve as laminin receptors on a variety of cell types. This review summarizes recent advances in understanding the involvement of individual integrins in cell interactions with laminins and the roles of laminin-binding integrins in adhesion-mediated events in vertebrates, including embryonic development, cell migration and tumor cell invasiveness, cell proliferation and differentiation, as well as basement membrane assembly. We discuss the regulation of integrin function via alternative splicing of cytoplasmic domains of alpha and beta subunits of the integrin receptors for laminins and present examples of functional collaboration between laminin-binding integrins and non-integrin laminin receptors. Advances in our understanding of the laminin-binding integrins continue to demonstrate the essential roles these receptors play in maintaining cell polarity and tissue architecture.

Defects in adhesion and migration, but not in proliferation and differentiation, of embryonic stem cells upon replacement of integrin subunit beta1A by beta1D

Beta1D is a skeletal muscle-specific splice variant of the beta1 integrin subunit, while beta1A integrin subunit has a wide tissue distribution. We have previously shown that replacement of beta1A by beta1D by homologous recombination (knockin) in all mouse tissues was embryonic lethal. Through two successive rounds of homologous recombination, we have now produced embryonic stem (ES) cells expressing beta1D instead of beta1A, and analyzed the ability of beta1D to support ES cell differentiation in vitro and in teratomas in vivo. Beta1D knockin (KI) ES cells grew at a similar rate but as more compact colonies than the beta1A-expressing cells. Increased cell cohesiveness, however, did not appear to involve changes in cadherin activity. Although in both beta1A and beta1D-KI ES cells only one beta1 allele is active; the expression of beta1 integrins in the beta1D-KI ES cells was reduced by 50%, compared with that in the beta1A-expressing cells; this correlated with impaired adhesive and migratory capacities. It appeared that during in vitro cardiac differentiation, in spite of a slight delay in the induction of two cardiac-specific transcripts, the alpha- and beta-myosin heavy chains, contracting cardiomyocytes were detected in similar numbers and at the same time in embryoid bodies (EB) derived from beta1D-KI and from beta1A cells. Furthermore, replacement of beta1A by beta1D in ES cells did not affect neurite differentiation in embryoid bodies in the presence of retinoic acid suggesting that beta1D supports neurogenesis. However, the impaired migration of other cells from the EB, including endodermal cells, prevented the normal outgrowth of neurites in beta1D-KI EB. Finally, injection of beta1D-KI ES cells in the flank of syngeneic mice gave rise to fully developed teratomas containing simple and pluristratified epithelia, muscle, cartilage, blood vessels, and tissues from the neural lineage. These results show that the muscle-specific splice variant beta1D, in spite of its specific cytoplasmic domain, supports the differentiation of many cell types. This further suggests that the embryonic lethality in the beta1D-KI embryos was mainly due to the different ability of beta1 A and beta1D to mediate cell adhesion and migration.

Conditional ablation of beta1 integrin in skin. Severe defects in epidermal proliferation, basement membrane formation, and hair follicle invagination

The major epidermal integrins are alpha3beta1 and hemidesmosome-specific alpha6beta4; both share laminin 5 as ligand. Keratinocyte culture studies implicate both integrins in adhesion, proliferation, and stem cell maintenance and suggest unique roles for alphabeta1 integrins in migration and terminal differentiation. In mice, however, whereas ablation of alpha6 or beta4 results in loss of hemidesmosomes, epidermal polarity, and basement membrane (BM) attachment, ablation of alpha3 only generates microblistering due to localized internal shearing of BM. Using conditional knockout technology to ablate beta1 in skin epithelium, we have uncovered biological roles for alphabeta1 integrins not predicted from either the alpha3 knockout or from in vitro studies. In contrast to alpha3 null mice, beta1 mutant mice exhibit severe skin blistering and hair defects, accompanied by massive failure of BM assembly/organization, hemidesmosome instability, and a failure of hair follicle keratinocytes to remodel BM and invaginate into the dermis. Although epidermal proliferation is impaired, a spatial and temporal program of terminal differentiation is executed. These results indicate that beta1's minor partners in skin are important, and together, alphabeta1 integrins are required not only for extracellular matrix assembly but also for BM formation. This, in turn, is required for hemidesmosome stability, epidermal proliferation, and hair follicle morphogenesis. However, beta1 downregulation does not provide the trigger to terminally differentiate.

Skin and hair follicle integrity is crucially dependent on beta 1 integrin expression on keratinocytes

beta 1 integrins are ubiquitously expressed receptors that mediate cell-cell and cell-extracellular matrix interactions. To analyze the function of beta1 integrin in skin we generated mice with a keratinocyte-restricted deletion of the beta 1 integrin gene using the cre-loxP system. Mutant mice developed severe hair loss due to a reduced proliferation of hair matrix cells and severe hair follicle abnormalities. Eventually, the malformed hair follicles were removed by infiltrating macrophages. The epidermis of the back skin became hyperthickened, the basal keratinocytes showed reduced expression of alpha 6 beta 4 integrin, and the number of hemidesmosomes decreased. Basement membrane components were atypically deposited and, at least in the case of laminin-5, improperly processed, leading to disruption of the basement membrane and blister formation at the dermal-epidermal junction. In contrast, the integrity of the basement membrane surrounding the beta 1-deficient hair follicle was not affected. Finally, the dermis became fibrotic. These results demonstrate an important role of beta 1 integrins in hair follicle morphogenesis, in the processing of basement membrane components, in the maintenance of some, but not all basement membranes, in keratinocyte differentiation and proliferation, and in the formation and/or maintenance of hemidesmosomes.

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.

Composition, synthesis, and assembly of the embryonic chick retinal basal lamina

To study the biology of basal laminae in the developing nervous system the protein composition of the embryonic retinal basal lamina was investigated, the site of synthesis of its proteins in the eye was determined, and basal lamina assembly was studied in vivo in two assay systems. Laminin, nidogen, agrin, collagen IV, and XVIII are major constituents of the retinal basal lamina. However, only agrin is synthesized by the retina, whereas the other matrix constituents originate from cells of the ciliary body, the lens, or the optic disc. The synthesis from extraretinal tissues infers that the retinal basal lamina proteins must be shed from their tissues of origin into the vitreous body and from there bind to receptor proteins provided by the retinal neuroepithelium. The fact that all proteins typical for the retinal basal lamina are abundant in the vitreous body and a new basal lamina is only formed when the vitreous body was directly adjacent to the retina is consistent with the contention of the vitreous body having a function in retinal basal lamina formation. Basal lamina assembly was also studied after disrupting the retinal basal lamina by intraocular injection of collagenase. The basal lamina regenerated after chasing the collagenase with Matrigel, which served as a collagenase inhibitor. The basal lamina was reconstituted within 6 h. However, the regenerated basal lamina was located deeper in the retina than normal by reconstituting along the retracted neuroepithelial endfeet demonstrating that these endfeet are the preferred site of basal lamina assembly.

Impaired bone formation in transgenic mice resulting from altered integrin function in osteoblasts

To determine the role of integrins in mature osteoblasts in vivo, we expressed in transgenic mice a dominant-negative integrin subunit (beta1-DN) consisting of the beta1 subunit cytoplasmic and transmembrane domains, driven by the osteoblast-specific osteocalcin promoter. Immature osteoblasts isolated from transgenic animals differentiated normally in vitro until the osteocalcin promoter became active; thereafter they detached from the substratum, suggesting that beta1-DN was impairing adhesion in mature osteoblasts. Transgenic animals had reduced bone mass, with increased cortical porosity in long bones and thinner flat bones in the skull. At 35 days, the rate of bone formation was reduced in cortical bone, and the parietal bones were 45% thinner than in wild-type animals. Active osteoblasts were less polar and had larger areas of cytoplasm with intracellular stores of matrix molecules. Osteocyte lacunae appeared normal around the cell body but did not have normal canilicular structures. At 90 days, the parietal bone of transgenic males was of normal width, suggesting that the original defect in matrix deposition had been repaired or compensated for. In contrast, transgenic females still had decreased bone mass in the parietal bone at 90 days. The decreased bone mass in TG females was accompanied by increased staining for osteoclast activity, suggesting that there was a sex-specific defect in mature animals.

Altered synthesis of laminin 1 and absence of basement membrane component deposition in (beta)1 integrin-deficient embryoid bodies

Basement membranes are the earliest extracellular matrices produced during embryogenesis. They result from synthesis and assembly into a defined supramolecular architecture of several components, including laminins, collagen IV, nidogen, and proteoglycans. In vitro studies have allowed us to propose an assembly model based on the polymerisation of laminin and collagen IV in two separate networks associated together by nidogen. How nucleation of polymers and insolubilisation of the different components into a basement membrane proceed in vivo is, however, unknown. A most important property of several basement membrane components is to provide signals controling the activity of adjacent cells. The transfer of information is mediated by interactions with cell surface receptors, among them integrins. Mouse genetics has demonstrated that the absence of these interactions is not compatible with development as deletion of either laminin (gamma)1 chain or integrin (beta)1 chain lead to lethality of mouse embryos at the peri-implantation stage. We have used embyoid bodies as a model system recapitulating the early steps of embryogenesis to unravel the respective roles of laminin and (beta)1 integrins in basement membrane formation. Our data show that there is formation of a basal lamina in wild-type, but not in (beta)1-integrin deficient, embryoid bodies. Surprisingly, in the absence of (beta)1 integrins, laminin 1 was not secreted in the extracellular space due to a rapid switch off of laminin (alpha)1 chain synthesis which normally drives the secretion of laminin heterotrimers. These results indicate that (beta)1 integrins are required for the initiation of basement membrane formation, presumably by applying a feed-back regulation on the expression of laminin (alpha)1 chain and other components of basement membranes.

Perlecan maintains the integrity of cartilage and some basement membranes

Perlecan is a heparan sulfate proteoglycan that is expressed in all basement membranes (BMs), in cartilage, and several other mesenchymal tissues during development. Perlecan binds growth factors and interacts with various extracellular matrix proteins and cell adhesion molecules. Homozygous mice with a null mutation in the perlecan gene exhibit normal formation of BMs. However, BMs deteriorate in regions with increased mechanical stress such as the contracting myocardium and the expanding brain vesicles showing that perlecan is crucial for maintaining BM integrity. As a consequence, small clefts are formed in the cardiac muscle leading to blood leakage into the pericardial cavity and an arrest of heart function. The defects in the BM separating the brain from the adjacent mesenchyme caused invasion of brain tissue into the overlaying ectoderm leading to abnormal expansion of neuroepithelium, neuronal ectopias, and exencephaly. Finally, homozygotes developed a severe defect in cartilage, a tissue that lacks BMs. The chondrodysplasia is characterized by a reduction of the fibrillar collagen network, shortened collagen fibers, and elevated expression of cartilage extracellular matrix genes, suggesting that perlecan protects cartilage extracellular matrix from degradation.

Cell adhesion molecules and extracellular-matrix constituents in kidney development and disease

Functional analyses of cell-matrix interactions during kidney organogenesis have provided compelling evidence that extracellular-matrix glycoproteins and their receptors play instructive roles during kidney development. Two concepts are worthy of emphasis. First, matrix molecules appear to regulate signal transduction pathways, either by activating cell-surface receptors such as integrins directly or by modulating the activity of signaling molecules such as WNTs. Second, basement membranes are highly organized structures and have distinct molecular compositions, which are optimized for their diverse functions. The importance of these findings is highlighted by the fact that mutations affecting basement-membrane components lead to inherited forms of kidney disease.

Fibronectin fibrillogenesis: a paradigm for extracellular matrix assembly

Fibronectin matrix assembly is a regulated stepwise process. In the past year, analyses of fibronectin domains, integrin and cytoskeletal contributions, and fibril architecture have provided new insights into assembly mechanisms and matrix control of cell functions. Like fibronectin, laminin polymerization is cell-mediated. Thus a common pathway for extracellular matrix assembly is emerging.

Dystroglycan inside and out

Dystroglycan connects the extracellular matrix and cytoskeleton. Key findings in the past year indicate that dystroglycan interacts with a wider repertoire of extracellular ligands than originally appreciated, that dystroglycan plays a critical role in organizing extracellular matrix molecules on the cell surface and in basement membranes, and that at least two human pathogens utilize dystroglycan to gain access to host cells. Together, these advances begin to help elucidate important biological roles for dystroglycan in development and disease.

Mutation of a basic sequence in the laminin alpha2LG3 module leads to a lack of proteolytic processing and has different effects on beta1 integrin-mediated cell adhesion and alpha-dystroglycan binding

A RRKRRQ sequence unique to the LG3 module of the laminin alpha2 chain was previously shown to be sensitive to endogenous proteolysis during the recombinant production of the tandem array alpha2LG1-3. Mutation of RQ surrounding the cleaved peptide bond did not prevent this processing and intracellular degradation. Alanine mutagenesis of three alternate basic residues, however, was shown to prevent the cleavage in alpha2LG1-3, allowing for the alpha2LG3 module to be obtained as a folded, globular fragment. The mutation did not change heparin and sulfatide binding or cell adhesion of alpha2LG1-3 which can be mediated by alpha3beta1 and alpha6beta1 integrins. It did, however, cause a 10-fold reduction in alpha-dystroglycan binding. The data favor the interpretation that binding epitopes for heparin/sulfatides, beta1 integrins and alpha-dystroglycan occupy different parts of the alpha2LG1-3 structure.

Laminin polymerization induces a receptor-cytoskeleton network

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