Mouse primordial germ cells lacking beta1 integrins enter the germline but fail to migrate normally to the gonads

Primordial germ cells are the founder cells of the gametes. They are set aside at the initial stages of gastrulation in mammals, become embedded in the hind-gut endoderm, then actively migrate to the sites of gonad formation. The molecular basis of this migration is poorly understood. Here we sought to determine if members of the integrin family of cell surface receptors are required for primordial germ cell migration, as integrins have been implicated in the migration of several other motile cell types. We have established a line of mice which express green fluorescent protein in germline cells that has enabled us to efficiently purify primordial germ cells at different stages by flow cytometry. We have catalogued the spectrum of integrin subunit expression by primordial germ cells during and after migration, using flow cytometry, immunocytochemistry and RT-PCR. Through analysis of integrin beta1(−/−)-->wild-type chimeras, we show that embryonic cells lacking beta1 integrins can enter the germline. However, integrin beta1(−/−) primordial germ cells do not colonize the gonad efficiently. Embryos with targeted deletion of integrin subunit alpha3, alpha6, or alphaV show no major defects in primordial germ cell migration. These results demonstrate a role for beta1-containing integrins in the development of the germline, although an equivalent role for * integrin subunit(s) has yet to be established.

Fibromodulin-null mice have abnormal collagen fibrils, tissue organization, and altered lumican deposition in tendon

Fibromodulin is a member of a family of connective tissue glycoproteins/proteoglycans containing leucine-rich repeat motifs. Several members of this gene family bind to fibrillar collagens and are believed to function in the assembly of the collagen network in connective tissues. Here we show that mice lacking a functional fibromodulin gene exhibit an altered morphological phenotype in tail tendon with fewer and abnormal collagen fiber bundles. In fibromodulin-null animals virtually all collagen fiber bundles are disorganized and have an abnormal morphology. Also 10-20% of the bundles in heterozygous mice are similar to the abnormal bundles in fibromodulin-null tail tendon. Ultrastructural analysis of Achilles tendon from fibromodulin-null mice show collagen fibrils with irregular and rough outlines in cross-section. Morphometric analysis show that fibromodulin-null mice have on the average thinner fibrils than wild type animals as a result of a larger preponderance of very thin fibrils in an overall similar range of fibril diameters. Protein and RNA analyses show an approximately 4-fold increase in the content of lumican in fibromodulin-null as compared with wild type tail tendon, despite a decrease in lumican mRNA. These results demonstrate a role for fibromodulin in collagen fibrillogenesis and suggest that the orchestrated action of several leucine-rich repeat glycoproteins/proteoglycans influence the architecture of collagen matrices.

The vasodilator-stimulated phosphoprotein (VASP) is involved in cGMP- and cAMP-mediated inhibition of agonist-induced platelet aggregation, but is dispensable for smooth muscle function

The vasodilator-stimulated phosphoprotein (VASP) is associated with actin filaments and focal adhesions, which form the interface between the cytoskeleton and the extracellular matrix. VASP is phosphorylated by both the cAMP- and cGMP-dependent protein kinases in a variety of cells, including platelets and smooth muscle cells. Since both the cAMP and cGMP signalling cascades relax smooth muscle and inhibit platelet activation, it was speculated that VASP mediates these effects by modulating actin filament dynamics and integrin activation. To study the physiological relevance of VASP in these processes, we inactivated the VASP gene in mice. Adult VASP-deficient mice had normal agonist-induced contraction, and normal cAMP- and cGMP-dependent relaxation of intestinal and vascular smooth muscle. In contrast, cAMP- and cGMP-mediated inhibition of platelet aggregation was significantly reduced in the absence of VASP. Other cAMP- and cGMP-dependent effects in platelets, such as inhibition of agonist-induced increases in cytosolic calcium concentrations and granule secretion, were not dependent on the presence of VASP. Our data show that two different cyclic, nucleotide-dependent mechanisms are operating during platelet activation: a VASP-independent mechanism for inhibition of calcium mobilization and granule release and a VASP-dependent mechanism for inhibition of platelet aggregation which may involve regulation of integrin function.

Functional beta1-integrins release the suppression of fibronectin matrix assembly by vitronectin

beta1-null GD25 fibroblasts adherent to vitronectin fail to bind the N-terminal 70-kDa matrix assembly domain of fibronectin or to assemble fibronectin (Sakai, T., Zhang, Q., Fässler, R., and Mosher, D. F. (1998) J. Cell Biol. 141, 527-538). We have made four observations that extend this finding. First, the presence of vitronectin on a substrate that otherwise can support fibronectin assembly has a dominant-negative effect on assembly. Second, the dominant-negative effect is lost when active beta1A is expressed. Third, beta1A containing the extracellular D130A inactivating mutation has a dominant-negative effect on fibronectin assembly. Fourth, beta1-null cells adherent to vitronectin are flat and lack filopodia, whereas beta1-null cells adherent to fibronectin or beta1A-expressing cells adherent to either vitronectin or fibronectin are contracted and exhibit numerous filopodia. These results reveal, therefore, that GD25 cells adherent to vitronectin can only assume a shape suitable for assembly of fibronectin when there is a countervailing signal from functional beta1-integrins.

Long-term potentiation in the hippocampal CA1 region of mice lacking cGMP-dependent kinases is normal and susceptible to inhibition of nitric oxide synthase

Long-term potentiation (LTP) is a potential cellular mechanism for learning and memory. The retrograde messenger nitric oxide (NO) is thought to induce LTP in the CA1 region of the hippocampus via activation of soluble guanylyl cyclase (sGC) and, ultimately, cGMP-dependent protein kinase (cGK). Two genes code for the isozymes cGKI and cGKII in vertebrates. The functional role of cGKs in LTP was analyzed using mice lacking the gene(s) for cGKI, cGKII, or both. LTP was not altered in the mutant mice lineages. However, LTP was reduced by inhibition of NO synthase and NMDA receptor antagonists, respectively. The reduced LTP was not recovered by the cGK-activator 8-(4 chlorophenylthio)-cGMP. Moreover, LTP was not affected by the sGC inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]-quiloxalin-1-one. In contrast, it was effectively suppressed by nicotinamide, a blocker of the ADP-ribosyltransferase. These results show that cGKs are not involved in LTP in mice and that NO induces LTP through an alternative cGMP-independent pathway, possibly ADP-ribosylation.

Collagen II is essential for the removal of the notochord and the formation of intervertebral discs

Collagen II is a fibril-forming collagen that is mainly expressed in cartilage. Collagen II-deficient mice produce structurally abnormal cartilage that lacks growth plates in long bones, and as a result these mice develop a skeleton without endochondral bone formation. Here, we report that Col2a1-null mice are unable to dismantle the notochord. This defect is associated with the inability to develop intervertebral discs (IVDs). During normal embryogenesis, the nucleus pulposus of future IVDs forms from regional expansion of the notochord, which is simultaneously dismantled in the region of the developing vertebral bodies. However, in Col2a1-null mice, the notochord is not removed in the vertebral bodies and persists as a rod-like structure until birth. It has been suggested that this regional notochordal degeneration results from changes in cell death and proliferation. Our experiments with wild-type mice showed that differential proliferation and apoptosis play no role in notochordal reorganization. An alternative hypothesis is that the cartilage matrix exerts mechanical forces that induce notochord removal. Several of our findings support this hypothesis. Immunohistological analyses, in situ hybridization, and biochemical analyses demonstrate that collagens I and III are ectopically expressed in Col2a1-null cartilage. Assembly of the abnormal collagens into a mature insoluble matrix is retarded and collagen fibrils are sparse, disorganized, and irregular. We propose that this disorganized abnormal cartilage collagen matrix is structurally weakened and is unable to constrain proteoglycan-induced osmotic swelling pressure. The accumulation of fluid leads to tissue enlargement and a reduction in the internal swelling pressure. These changes may be responsible for the abnormal notochord removal in Col2a1-null mice. Our studies also show that chondrocytes do not need a collagen II environment to express cartilage-specific matrix components and to hypertrophy. Furthermore, biochemical analysis of collagen XI in mutant cartilage showed that alpha1(XI) and alpha2 (XI) chains form unstable collagen XI molecules, demonstrating that the alpha3(XI) chain, which is an alternative, posttranslationally modified form of the Col2a1 gene, is essential for assembly and stability of triple helical collagen XI.

Roles of integrins and fibronectin in the entry of Streptococcus pyogenes into cells via protein F1

Entry of group A streptococcus (GAS) into cells has been suggested as an important trait in GAS pathogenicity. Protein F1, a fibronectin (Fn) binding protein, mediates GAS adherence to cells and the extracellular matrix, and efficient cell internalization. We demonstrate that the cellular receptors responsible for protein F1-mediated internalization of GAS are integrins capable of Fn binding. In HeLa cells, bacterial entry is blocked by anti-beta1 integrin monoclonal antibody. In the mouse cell line GD25, a beta1 null mutant, the alphavbeta3 integrin promotes GAS entry. Internalization of these cells by GAS is blocked by a peptide that specifically binds to alphavbeta3 integrin. In both cell lines, entry of GAS requires the occupancy of protein F1 by Fn. Neither the 29 kDa nor the 70 kDa N-terminal fragments or the 120 kDa cell-binding fragment of Fn promote bacterial entry. Fn-coated beads are taken up efficiently by HeLa cells. Both the entry of GAS via protein F1 and the uptake of Fn-coated beads are blocked by anti-beta1 antibody but are unaffected by a large excess of soluble Fn. Internalization of HeLa cells by bacteria bearing increasing amounts of prebound Fn to protein F1 reveals a sigmoidal ultrasensitive curve. These suggest that the ability of particles to interact via Fn with multiple integrin sites plays a central role in their ability to enter cells.

Endochondral ossification is dependent on the mechanical properties of cartilage tissue and on intracellular signals in chondrocytes

Skeletal elements are formed either by replacing a performed cartilagenous matrix template in a process called endochondral ossification or directly from mesenchyme by a process known as membranous ossification. Longitudinal growth of bones is achieved by growth plates where calcified cartilage is converted into bone. To investigate the role of extracellular matrix as well as intracellular signaling pathways in the formation and growth of bone, the genes coding for type II collagen and cyclic guanosine 3',5'-monophosphate (cGMP)-dependent protein kinase (cGK) II, were disrupted. It is demonstrated that loss of Col2a1 or cGKII led to abnormal endochondral ossification and skeletal development. In cGKII -/- mice, bones derived by membranous ossification developed normally while bones derived by endochondral ossification were shortened. This growth defect was not associated with a general metabolic disturbance. In Col2a1 knockout mice, endochondral ossification was completely absent, whereas membraneous ossification was not affected. Despite the defects in bone formation, invasion of blood vessels into bone cavities and formation of bone marrow occurred in Col2a1-null mice. Taken together, the phenotypes of these two knockout mice show that chondrocytes need a well-functioning extracellular matrix scaffold and a normal cGMP-signaling system for endochondral ossification to form a normal skeleton.

Loss of beta1 integrin function results in a retardation of myogenic, but an acceleration of neuronal, differentiation of embryonic stem cells in vitro

Integrin cell surface receptors play an important role for cell adhesion, migration, and differentiation during embryonic development by mediating cell-cell and cell-matrix interactions. Less is known about the function of integrins during commitment and lineage determination of early embryogenesis. Homozygous inactivation of the beta1 integrin gene results in embryonal death in mice around the time of implantation. In vitro, differentiation of embryonic stem (ES) cells which lack beta1 integrin (beta1-/-) into the cardiogenic lineage is delayed and results in a disordered cellular specification (Fässler et al., J. Cell Sci. 109, 2989-2999, 1996). To analyze beta1 integrin function during myogenesis and neurogenesis we studied differentiation of beta1-/- ES cells via embryoid bodies into skeletal muscle and neuronal cells in vitro. beta1-/- cells showed delayed and reduced myogenic differentiation compared to wildtype and heterozygous (beta1+/-) ES cells. RT-PCR analysis demonstrated delayed expression of skeletal muscle-specific genes in the absence of beta1 integrin. Immunofluorescence studies with antibodies against the sarcomeric proteins myosin heavy chain, titin, nebulin, and slow C-protein showed that myotubes formed, but their number was reduced and the assembly of sarcomeric structures was retarded. In contrast, neuronal cells differentiating from beta1-/- ES cells appeared earlier than wildtype and heterozygous (beta1+/-) ES cells. This was shown by the accelerated expression of neuron-specific genes and an increased number of neuronal cells in beta1-/- embryoid bodies. However, neuronal outgrowth was retarded in the absence of beta1 integrin. No functional difference between wildtype and beta1-/- cells was found with respect to secretion of gamma-aminobutyric acid, the main neurotransmitter of ES cell-derived neuronal cells. The lineage-specific effects of loss of beta1 integrin function, that is the inhibition of mesodermal and acceleration of neuroectodermal differentiation, were supported by differential expression of genes encoding lineage-specific transcription factors (Brachyury, Pax-6, Mash1) and signaling molecules (BMP-4 and Wnt-1). Because of the reduced and delayed expression of the BMP-4 encoding gene in beta1-/- cells, we analyzed in wildtype and beta1-/- cells the regulatory role of exogenously added BMP-4 on the expression of the mesodermal and neuronal marker genes, Brachyury and wnt-1, respectively. The data suggest that BMP-4 plays a regulatory role during differentiation of wildtype and beta1-/- cells by modifying mesodermal and neuronal pathways. The reduced expression of BMP-4 in beta1-/- cells may account for the accelerated neuronal differentiation in beta1-/- ES cells.

Mouse myoblasts can fuse and form a normal sarcomere in the absence of beta1 integrin expression

Antibody perturbation experiments suggested that migration, terminal differentiation and fusion of myoblasts are dependent on beta1 integrin expression. In addition, several studies have postulated that beta1 integrins have a role in the formation of sarcomeres. In the present report we have analysed skeletal myogenesis in wild-type/beta1-null chimeric mice and beta1-null embryoid bodies. Trunk and limbs of beta1-null chimeric mice contained muscle tissue composed of normal and beta1-null myoblasts indicating that all myotomic sublineages can form, migrate to their peripheral targets and fuse in the absence of beta1 integrin expression. Pure populations of beta1-null myoblasts and satellite cells isolated from beta1-null chimeric embryos and chimeric newborn mice, respectively, were able to differentiate in vitro and to fuse into multinucleated myotubes. Quantitative and qualitative comparisons between normal and beta1-null myoblasts revealed no apparent difference in their capacity to terminally differentiate and fuse. Furthermore, beta1-null myotubes developed sarcomeres which were indistinguishable from wild-type controls. When normal and beta1-null ES cells were differentiated into embryoid bodies, they contained fully differentiated myotubes with normal sarcomeres and normal deposition of costameric components. However, formation of beta1-null myotubes was delayed and was less efficient in beta1-null embryoid bodies than in wild-type controls. High expression of alphav integrin subunit at the tips of normal as well as beta1-null myotubes indicated that the lack of beta1 integrins did not result in a significant redistribution of alphav-containing receptors.

Beta1 integrin deficiency impairs migration and differentiation of mouse embryonic stem cell derived neurons

Cell-matrix interaction plays an important role during neuronal development, which is demonstrated by comparing wild type (D3)- and beta1 integrin-deficient (G201) embryonic stem cell derived neurons. In D3 preparations complex networks of functionally coupled neurons with bi- and multipolar morphologies develop. In contrast, neuronal differentiation is retarded in G201 derived neurons, recognised by limited migration and restricted morphological differentiation. Furthermore, beta1 integrin deficiency causes a delay in expression of major neurotransmitters like GABA and glutamate as well as of synaptophysin. These findings indicate a prominent role of beta1 integrin for both morphological and chemical differentiation.

Restoration of beta1A integrins is required for lysophosphatidic acid-induced migration of beta1-null mouse fibroblastic cells

Cells lacking the beta1 integrin subunit or expressing beta1A with certain cytoplasmic mutations have poor directed cell migration to platelet-derived growth factor or epidermal growth factor, ligands of receptor tyrosine kinases (Sakai, T., Zhang, Q., Fässler, R., and Mosher, D. F. (1998) J. Cell Biol. 141, 527-538). We investigated the effect of expression of beta1A integrins on lysophosphatidic acid (LPA)-induced migration of fibroblastic cells derived from beta1-null mouse embryonic stem cells. These cells expressed edg-2, a G-protein-linked receptor for LPA, as well as the related edg-1 receptor. Cells expressing wild type beta1A demonstrated enhanced cell migration across filters coated with gelatin or adhesive proteins in response to LPA, whereas beta1-deficient cells lacked LPA-induced cell migratory ability. Checkerboard analyses indicated that LPA causes both chemotaxis and chemokinesis of beta1-replete cells. Cells expressing beta1A with mutations of prolines or tyrosines in conserved cytoplasmic NPXY motifs, threonine in the inter-motif sequence, or a critical aspartic acid in the extracellular domain had low migratory responses to LPA. These findings indicate that active beta1A integrin is required for cell migration induced by LPA and that the cytoplasmic domain of ligated beta1A interacts with pathways that are common to both receptor tyrosine kinase and G-protein-linked receptor signaling.

B lymphocytes producing demyelinating autoantibodies: development and function in gene-targeted transgenic mice

We studied the cellular basis of self tolerance of B cells specific for brain autoantigens using transgenic mice engineered to produce high titers of autoantibodies against the myelin oligodendrocyte glycoprotein (MOG), a surface component of central nervous system myelin. We generated "knock-in" mice by replacing the germline JH locus with the rearranged immunoglobulin (Ig) H chain variable (V) gene of a pathogenic MOG-specific monoclonal antibody. In the transgenic mice, conventional B cells reach normal numbers in bone marrow and periphery and express exclusively transgenic H chains, resulting in high titers of MOG-specific serum Igs. Additionally, about one third of transgenic B cells bind MOG, thus demonstrating the absence of active tolerization. Furthermore, peritoneal B-1 lymphocytes are strongly depleted. Upon immunization with MOG, the mature transgenic B cell population undergoes normal differentiation to plasma cells secreting MOG-specific IgG antibodies, during which both Ig isotype switching and somatic mutation occur. In naive transgenic mice, the presence of this substantial autoreactive B cell population is benign, and the mice fail to develop either spontaneous neurological disease or pathological evidence of demyelination. However, the presence of the transgene both accelerates and exacerbates experimental autoimmune encephalitis, irrespective of the identity of the initial autoimmune insult.

Defective smooth muscle regulation in cGMP kinase I-deficient mice

Regulation of smooth muscle contractility is essential for many important biological processes such as tissue perfusion, cardiovascular haemostasis and gastrointestinal motility. While an increase in calcium initiates smooth muscle contraction, relaxation can be induced by cGMP or cAMP. cGMP-dependent protein kinase I (cGKI) has been suggested as a major mediator of the relaxant effects of both nucleotides. To study the biological role of cGKI and its postulated cross-activation by cAMP, we inactivated the gene coding for cGKI in mice. Loss of cGKI abolishes nitric oxide (NO)/cGMP-dependent relaxation of smooth muscle, resulting in severe vascular and intestinal dysfunctions. However, cGKI-deficient smooth muscle responded normally to cAMP, indicating that cAMP and cGMP signal via independent pathways, with cGKI being the specific mediator of the NO/cGMP effects in murine smooth muscle.

Modulation of beta1A integrin functions by tyrosine residues in the beta1 cytoplasmic domain

beta1A integrin subunits with point mutations of the cytoplasmic domain were expressed in fibroblasts derived from beta1-null stem cells. beta1A in which one or both of the tyrosines of the two NPXY motifs (Y783, Y795) were changed to phenylalanines formed active alpha5 beta1 and alpha6 beta1 integrins that mediated cell adhesion and supported assembly of fibronectin. Mutation of the proline in either motif (P781, P793) to an alanine or of a threonine in the inter-motif sequence (T788) to a proline resulted in poorly expressed, inactive beta1A. Y783,795F cells developed numerous fine focal contacts and exhibited motility on a surface. When compared with cells expressing wild-type beta1A or beta1A with the D759A activating mutation of a conserved membrane-proximal aspartate, Y783, 795F cells had impaired ability to transverse filters in chemotaxis assays. Analysis of cells expressing beta1A with single Tyr to Phe substitutions indicated that both Y783 and Y795 are important for directed migration. Actin-containing microfilaments of Y783,795F cells were shorter and more peripheral than microfilaments of cells expressing wild-type beta1A. These results indicate that change of the phenol side chains in the NPXY motifs to phenyl groups (which cannot be phosphorylated) has major effects on the organization of focal contacts and cytoskeleton and on directed cell motility.

Mutational analysis of the potential phosphorylation sites in the cytoplasmic domain of integrin beta1A. Requirement for threonines 788–789 in receptor activation

To investigate the role of the potential phosphorylation sites in the cytoplasmic domain of integrin beta1A, point mutated variants of the protein were stably expressed in the beta1-deficient cell line GD25. Mutants T777A, Y783F, S785A, and Y795F were fully active in promoting cell adhesion, de novo formation of focal contacts, formation of fibronectin fibrils, and activation of focal adhesion kinase. Thus, phosphorylation of these residues is not required for several basic functions of integrin beta1A. On the other hand, the TT788-9AA mutant, was defective in mediating cell attachment and did not contribute to fibronectin fibril formation. The conformation of the extracellular domain was shifted towards an inactive state as measured by binding of the monoclonal antibody 9EG7. Antibody induced clustering of beta1ATT788-9AA demonstrated that the mutant cytoplasmic part was functional in mediating activation of focal adhesion kinase. Therefore, we conclude that threonines 788–789, which are conserved among most integrin beta subunits, are of critical importance for integrin function due to effects on the extracellular conformation of the receptor.

beta1-integrin cytoplasmic subdomains involved in dominant negative function

The beta1-integrin cytoplasmic domain consists of a membrane proximal subdomain common to the four known isoforms ("common" region) and a distal subdomain specific for each isoform ("variable" region). To investigate in detail the role of these subdomains in integrin-dependent cellular functions, we used beta1A and beta1B isoforms as well as four mutants lacking the entire cytoplasmic domain (beta1TR), the variable region (beta1COM), or the common region (beta1 deltaCOM-B and beta1 deltaCOM-A). By expressing these constructs in Chinese hamster ovary and beta1 integrin-deficient GD25 cells (Wennerberg et al., J Cell Biol 132, 227-238, 1996), we show that beta1B, beta1COM, beta1 deltaCOM-B, and beta1 deltaCOM-A molecules are unable to support efficient cell adhesion to matrix proteins. On exposure to Mn++ ions, however, beta1B, but none of the mutants, can mediate cell adhesion, indicating specific functional properties of this isoform. Analysis of adhesive functions of transfected cells shows that beta1B interferes in a dominant negative manner with beta1A and beta3/beta5 integrins in cell spreading, focal adhesion formation, focal adhesion kinase tyrosine phosphorylation, and fibronectin matrix assembly. None of the beta1 mutants tested shows this property, indicating that the dominant negative effect depends on the specific combination of common and B subdomains, rather than from the absence of the A subdomain in the beta1B isoform.

Identification of beta1C-2, a novel variant of the integrin beta1 subunit generated by utilization of an alternative splice acceptor site in exon C

A new splice variant of the human integrin subunit beta1 has been identified and designated beta1C-2. It differs from the previously reported beta1C (in this report designated beta1C-1) by 18 nucleotides, and is generated by splicing from exon 6 to an alternative splice acceptor site within exon C, causing an in-frame deletion of six amino acids of the cytoplasmic region of beta1C-1. The beta1C-2 mRNA is present in several human cell lines and tissues at low levels, similarly to beta1C-1. In peripheral T-lymphocytes, beta1C-2 is the selectively expressed isoform. Neither beta1C-1 nor beta1C-2 mRNA could be detected in mouse tissues, and Southern hybridization of a mouse genomic beta1 clone with a human exon-C-specific probe failed to identify a corresponding mouse exon. The antisense orientation of exon C is highly homologous to an Alu element. Since Alu elements are restricted to primates, the beta1C-1 and beta1C-2 variants of the integrin subunit beta1 are specific for these species. The protein coded for by the beta1C-2 cDNA can be expressed and localized to the surface of beta1 deficient mouse cells. However, while stable transformed clones expressing high levels of the beta1A were commonly found, the beta1C-1 and beta1C-2 expressing clones expressed barely detectable amounts of the beta1 protein. Hence, high levels of beta1C-2 may be incompatible with cell proliferation, as previously suggested for beta1C-1.

Mouse models for extracellular matrix diseases

Mutations of a number of genes encoding for extracellular matrix (ECM) proteins in mice have provided new insights regarding their role during development and disease. Many mouse strains have helped to verify the link between mutation and disease in humans, and others have produced unexpected phenotypes and identified new functions for ECM proteins. Finally, some null mutations in ECM genes provide no phenotypic alterations in mice, confronting the scientific community with a new challenge to search for their functions. This review lists all mouse strains with spontaneous and experimentally induced mutations in ECM genes. The phenotypes of these mice are discussed in comparison with the human diseases.

Sequence, structure and chromosomal localization of Crtm gene encoding mouse cartilage matrix protein and its exclusion as a candidate for murine achondroplasia

The mouse cartilage matrix protein gene (Crtm) was isolated from a cosmid library using a mouse Crtm cDNA fragment as probe. Crtm spans 12.2 kb from the start of translation to the polyadenylation signal sequence and comprises eight exons. Sequencing of the 1.9 kb 5' flanking region revealed a TATA-like box 72 bp upstream from the initiator Met codon as well as several cis-acting motifs known to bind eukaryotic transcription factors. Analysis of the exon-intron junctions demonstrated that the last intron does not follow the gt/ag rule but belongs to the minor class of pre-mRNA introns that contain "at" and "ac" at their 5'and 3' ends, respectively. Single-strand conformation polymorphism analysis was used to map Crtm to the distal part of chromosome 4 between the microsatellite markers D4Mit16 and D4Mit339. Achodroplasia (cn), a recessive skeletal disorder in mice, has already been mapped to this region. Immunostaining for CMP and sequence of Crtm in cn/cn mice failed to reveal any disease-specific mutations, suggesting that mutations in Crtm do not cause achondroplasia.

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

Subcutaneous injection of beta 1 integrin-deficient embryonic stem cells in mice causes the formation of teratomas although they occur with a lower frequency and are smaller than wild-type cells. Immunofluorescence analysis of these deficient tumors indicates a disorganized deposition of several basement membrane proteins. This was confirmed by electron microscopy which demonstrated frequent gaps in cell-associated basement membranes or loss of close contacts to the cells. Further aberrant features were multilaminar structures and amorphous deposits, indicating a strong impairment of correct basement membrane assembly. Quantitative radioimmunoassays were used to determine the levels of specific proteins in successive tissue extracts with neutral buffer in the absence and presence of EDTA and with 6 M guanidine. This demonstrated a more than 90% decrease in the content of laminin-1 (alpha 1 beta 1 gamma 1) and a 70% decrease in nidogen in the beta 1 integrin-deficient teratomas. No significant changes were detected for other matrix proteins (perlecan, fibronectin, fibulins). This selective change impaired the formation of laminin-nidogen complex and enhanced nidogen degradation. Northern blots also demonstrated a distinctly reduced expression of laminin alpha 1, beta 1, and gamma 1 chains. Similar reductions were also observed in cultured embryonic stem cells prior to any differentiation. No or only smaller changes were observed for laminin alpha 2 and beta 2 chain, nidogen, and perlecan mRNA. These data emphasize a distinct role of beta 1 integrins in the correct assembly of basement membranes which may occur through direct ligand binding and/or regulatory events at the transcriptional level.

The mouse chondroadherin gene: characterization and chromosomal localization

The mouse chondroadherin gene was isolated from a cosmid genomic library by the use of a rat chondroadherin cDNA probe. Southern blot analysis of mouse genomic DNA revealed a simple pattern of hybridization indicating a single copy gene for chondroadherin. The mouse chondroadherin gene encompasses 4.1 kb and consists of four exons separated by one large intron of 1929 bp followed by two smaller introns of 247 and 225 bp, respectively. Most of the translated region, including the start codon and the main part of a leucine-rich region, is contained within the first exon. Two small exons of 164 and 146 bp encode the rest of the protein. Interestingly, 4 bases from the stop codon, in the 3'-UTR, a third intron is located. A putative promoter region of 669 bp was sequenced and shown to contain a potential TATAA-box signal 29 bp upstream of the transcription start site and several recognition sites for transcription factors. The exon/intron organization of the chondroadherin gene differs from those of the other known genes of the leucine-rich repeat (LRR) family in the extracellular matrix. Taken together with comparison of protein sequences of other members of the LRR family in the extracellular matrix, the data suggest that chondroadherin has evolved along a different pathway. The chondroadherin gene was mapped to mouse chromosome 11, near D11Mit14, by single-strand conformation polymorphism linkage analysis.

Genetic analysis of beta1 integrin function: confirmed, new and revised roles for a crucial family of cell adhesion molecules

Integrins are heterodimeric cell adhesion proteins connecting the extracellular matrix to the cytoskeleton and transmitting signals in both directions. These integrins are suggested to be involved in many different biological processes such as growth, differentiation, migration, and cell death. Of more than 20 known integrins, 10 contain the nearly ubiquitously expressed beta1 integrin subunit. Disruption of the beta1 integrin gene by homologous recombination allows us to assess the supposed functions of beta1 containing integrins in vivo in a new way. This review will present and discuss recent findings derived from such studies concerning the biological roles of beta1 integrins in early development, differentiation and migration, hematopoiesis, tumorigenesis, and supramolecular assembly of extracellular matrix proteins. While several former results were confirmed, others were contradicted and new functions found, significantly changing the previous view of beta1 integrin function in vivo.

Targeted inactivation of plectin reveals essential function in maintaining the integrity of skin, muscle, and heart cytoarchitecture

Previous studies suggest that plectin, a versatile cytoskeletal linker protein, has an important role in maintaining the structural integrity of diverse cells and tissues. To establish plectin's function in a living organism, we have disrupted its gene in mice. Plectin (-/-) mice died 2-3 days after birth exhibiting skin blistering caused by degeneration of keratinocytes. Ultrastructurally, hemidesmosomes and desmosomes appeared unaffected. In plectin-deficient mice, however, hemidesmosomes were found to be significantly reduced in number and apparently their mechanical stability was altered. The skin phenotype of these mice was similar to that of patients suffering from epidermolysis bullosa simplex (EBS)-MD, a hereditary skin blistering disease with muscular dystrophy, caused by defects in the plectin gene. In addition, plectin (-/-) mice revealed abnormalities reminiscent of minicore myopathies in skeletal muscle and disintegration of intercalated discs in heart. Our results clearly demonstrate a general role of plectin in the reinforcement of mechanically stressed cells. Plectin (-/-) mice will provide a useful tool for the study of EBS-MD, and possibly other types of plectin-related myopathies involving skeletal and cardiac muscle, in an organism amenable to genetic manipulation.

Absence of integrin alpha 7 causes a novel form of muscular dystrophy

Integrin alpha 7 beta 1 is a specific cellular receptor for the basement membrane protein laminin-1 (refs 1,2), as well as for the laminin isoforms -2 and -4 (ref. 3). The alpha 7 subunit is expressed mainly in skeletal and cardiac muscle and has been suggested to be involved in differentiation and migration processes during myogenesis. Three cytoplasmic and two extracellular splice variants that have been described are developmentally regulated and expressed in different sites in the muscle. In adult muscle, the alpha 7A and alpha 7B subunits are concentrated in myotendinous junctions but can also be detected in neuromuscular junctions and along the sarcolemmal membrane. To study the potential involvement of alpha 7 integrin, during myogenesis and its role in muscle integrity and function, we generated a null allele of the alpha 7 gene (Itga7) in the germline of mice by homologous recombination in embryonic stem (ES) cells. Surprisingly, mice homozygous for the mutation are viable and fertile, indicating that the alpha 7 beta 1 integrin is not essential for myogenesis. However, histological analysis of skeletal muscle revealed typical symptoms of a progressive muscular dystrophy starting soon after birth, but with a distinct variability in different muscle types. The observed histopathological changes strongly indicate an impairment of function of the myotendinous junctions. These findings demonstrate that alpha 7 beta 1 integrin represents an indispensable linkage between the muscle fibre and the extracellular matrix that is independent of the dystrophin-dystroglycan complex-mediated interaction of the cytoskeleton with the muscle basement membrane.