alpha11beta1 integrin constitutes a recent addition to the integrin family. Here, we present the first in vivo analysis of alpha11 protein and mRNA distribution during human embryonic development. alpha11 protein and mRNA were present in various mesenchymal cells around the cartilage anlage in the developing skeleton in a pattern similar to that described for the transcription factor scleraxis. alpha11 was also expressed by mesenchymal cells in intervertebral discs and in keratocytes in cornea, two sites with highly organized collagen networks. Neither alpha11 mRNA nor alpha11 protein could be detected in myogenic cells in human embryos. The described expression pattern is compatible with alpha11beta1 functioning as a receptor for interstitial collagens in vivo. To test this hypothesis in vitro, full-length human alpha11 cDNA was stably transfected into the mouse satellite cell line C2C12, lacking endogenous collagen receptors. alpha11beta1 mediated cell adhesion to collagens I and IV (with a preference for collagen I) and formed focal contacts on collagens. In addition, alpha11beta1 mediated contraction of fibrillar collagen gels in a manner similar to alpha2beta1, and supported migration on collagen I in response to chemotactic stimuli. Our data support a role for alpha11beta1 as a receptor for interstitial collagens on mesenchymally derived cells and suggest a multifunctional role of alpha11beta1 in the recognition and organization of interstitial collagen matrices during development.

There is currently a great interest in identifying laminin isoforms expressed in developing and regenerating skeletal muscle. Laminin ␣1 has been reported to localize to human fetal muscle and to be induced in muscular dystrophies based on immunohistochemistry with the monoclonal antibody 4C7, suggested to recognize the human laminin ␣1 chain. Nevertheless, there seems to be no expression of laminin ␣1 protein or mRNA in developing or dystrophic mouse skeletal muscle fibers. To address the discrepancy between the results obtained in developing and dystrophic human and mouse muscle we expressed the E3 domain of human laminin ␣1 chain as a recombinant protein and made antibodies specific for human laminin ␣1 chain (anti-hLN-␣1G4/G5). We also made antibodies to the human laminin ␣5 chain purified from placenta. In the present report we show that hLN-␣1G4/G5 antibodies react with a 400-kDa laminin ␣1 chain and that 4C7 reacts with a 380-kDa laminin ␣5 chain. Immunohistochemistry with the hLN-␣1G4/G5 antibody and 4C7 revealed that the two antibodies stained human kidney, developing and dystrophic muscle in distinct patterns. Our data indicate that the previously reported expression patterns in developing, adult, and dystrophic human muscle tissues with 4C7 should be re-interpreted as an expression of laminin ␣5 chain. Our data are also consistent with earlier work in mouse, indicating that laminin ␣1 is largely an epithelial laminin chain not present in developing or dystrophic muscle fibers.

The fibroblast integrin ␣11␤1 is a key receptor for fibrillar collagens. To study the potential function of ␣11 in vivo, we generated a null allele of the ␣11 gene. Integrin ␣11 ؊/؊ mice are viable and fertile but display dwarfism with increased mortality, most probably due to severely defective incisors. Mutant incisors are characterized by disorganized periodontal ligaments, whereas molar ligaments appear normal. The primary defect in the incisor ligament leads to halted tooth eruption. ␣11␤1-defective embryonic fibroblasts displayed severe defects in vitro, characterized by (i) greatly reduced cell adhesion and spreading on collagen I, (ii) reduced ability to retract collagen lattices, and (iii) reduced cell proliferation. Analysis of matrix metalloproteinase in vitro and in vivo revealed disturbed MMP13 and MMP14 synthesis in ␣11 ؊/؊ cells. We show that ␣11␤1 is the major receptor for collagen I on mouse embryonic fibroblasts and suggest that ␣11␤1 integrin is specifically required on periodontal ligament fibroblasts for cell migration and collagen reorganization to help generate the forces needed for axial tooth movement. Our data show a unique role for ␣11␤1 integrin during tooth eruption.Integrins constitute a family of multifunctional cell adhesion receptors involved in a variety of biological processes. In higher vertebrates the integrin family is composed of 18 ␣ subunits and 8 ␤ subunits. Out of the 24 integrin heterodimers, ␣1␤1, ␣2␤1, ␣10␤1, and ␣11␤1 act as primary receptors for native collagens. The ␣3␤1 integrin, initially described as a collagen receptor, was later shown to act as a receptor for laminin-5 and does not display any measurable affinity for collagen I (21). The observed role of ␣v␤3 integrin in collagen lattice remodeling in vitro (13, 33) and in vivo (33) may be related to indirect binding to RGD ligands which are locally deposited in the collagen matrix.Collagen-binding integrins bind native collagens via their ␣I domain, recognizing a GFOGER motif (30, 52) or similar sequences with varying specificities and affinities depending on the collagen type and fibrillar status (27). In addition to mediating cell adhesion, cell spreading, and cell migration (22), collagen-binding integrins regulate collagen turnover (32) and take part in assembling (31, 49) and reorganizing three-dimensional collagen matrices (23).Fibroblasts express characteristic collagen receptor repertoires in a tissue-specific manner, partly reflecting their different embryonic origins. We have previously demonstrated that ␣11␤1 is the only detectable collagen-binding integrin in the incisor periodontal ligament (PDL) fibroblasts of mice (38). The PDL in the mouse incisor is composed of a relatively dense connective tissue on the side facing the tooth and a loose connective tissue region rich in blood vessels on the side facing the bone. The odontogenic ectomesenchyme is the source of the cells populating these two domains in the PDL, which are sometimes referred to as tooth-associated fibroblasts and bone-associated...

The integrins ␣ 1 ␤ 1 , ␣ 2 ␤ 1 , ␣ 10 ␤ 1 , and ␣ 11 ␤ 1 are referred to as a collagen receptor subgroup of the integrin family. Recently, both ␣ 1 ␤ 1 and ␣ 2 ␤ 1 integrins have been shown to recognize triple-helical GFOGER (where single letter amino acid nomenclature is used, O ‫؍‬ hydroxyproline) or GFOGER-like motifs found in collagens, despite their distinct binding specificity for various collagen subtypes. In the present study we have investigated the mechanism whereby the latest member in the integrin family, ␣ 11 ␤ 1 , recognizes collagens using C2C12 cells transfected with ␣ 11 cDNA and the bacterially expressed recombinant ␣ 11 I domain. The ligand binding properties of ␣ 11 ␤ 1 were compared with those of ␣ 2 ␤ 1 . Mg 2؉ -dependent ␣ 11 ␤ 1 binding to type I collagen required micromolar Ca 2؉ but was inhibited by 1 mM Ca 2؉ , whereas ␣ 2 ␤ 1 -mediated binding was refractory to millimolar concentrations of Ca 2؉ . The bacterially expressed recombinant ␣ 11 I domain preference for fibrillar collagens over collagens IV and VI was the same as the ␣ 2 I domain. Despite the difference in Ca 2؉ sensitivity, ␣ 11 ␤ 1 -expressing cells and the ␣ 11 I domain bound to helical GFOGER sequences in a manner similar to ␣ 2 ␤ 1 -expressing cells and the ␣ 2 I domain. Modeling of the ␣ I domain-collagen peptide complexes could partially explain the observed preference of different I domains for certain GFOGER sequence variations. In summary, our data indicate that the GFOGER sequence in fibrillar collagens is a common recognition motif used by ␣ 1 ␤ 1 , ␣ 2 ␤ 1 , and also ␣ 11 ␤ 1 integrins. Although ␣ 10 and ␣ 11 chains show the highest sequence identity, ␣ 2 and ␣ 11 are more similar with regard to collagen specificity. Future studies will reveal whether ␣ 2 ␤ 1 and ␣ 11 ␤ 1 integrins also show overlapping biological functions.The collagen family currently includes at least 24 members (1, 2), and four different collagen-binding integrins ␣ 1 ␤ 1 , ␣ 2 ␤ 1 , ␣ 10 ␤ 1 (3) and ␣ 11 ␤ 1 (4) are known. The ␣ 3 ␤ 1 integrin does not interact directly with collagen, but it does act as a laminin receptor (5) that can affect the activity of the collagen receptor ␣ 2 ␤ 1 through receptor cross-talk (6).

Murine retroviruses may cause malignant tumors in mice by insertional mutagenesis of host genes. The use of retroviral tagging as a means of identifying cancer-causing genes has, however, almost entirely been restricted to hematopoietic tumors. The aim of this study was to develop a system allowing for the retroviral tagging of candidate genes in malignant brain tumors. Mouse gliomas were induced by a recombinant Moloney murine leukemia virus encoding platelet-derived growth factor (PDGF) B-chain. The underlying idea was that tumors evolve through a combination of PDGF-mediated autocrine growth stimulation and insertional mutagenesis of genes that cooperate with PDGF in gliomagenesis. Common insertion sites (loci that were tagged in more than one tumor) were identified by cloning and sequencing retroviral flanking segments, followed by BLAST searches of mouse genome databases. A number of candidate brain tumor loci (Btls) were identified. Several of these Btls correspond to known tumor-causing genes; these findings strongly support the underlying idea of our experimental approach. Other Btls harbor genes with a hitherto unproven role in transformation or oncogenesis. Our findings indicate that retroviral tagging with a growth factor-encoding virus may be a powerful means of identifying candidate tumorcausing genes in nonhematopoietic tumors.