The alpha7beta1 integrin is a laminin receptor on the surface of skeletal myoblasts and myofibers. Alternative forms of both the alpha7 and beta1 chains are expressed in a developmentally regulated fashion during myogenesis. These different alpha7beta1 isoforms localize at specific sites on myofibers and appear to have distinct functions in skeletal muscle. These functions include the migration and proliferation of developing myoblasts, the formation and integrity of neuromuscular and myotendinous junctions, and the "gluing" together of muscle fibers that is essential to the generation of contractile force. The alpha7beta1 integrin appears to be both directly and indirectly causally related to several muscle diseases. Enhanced expression of alpha7beta1-mediated linkage of the extracellular matrix is seen in Duchenne muscular dystrophy and may compensate for the absence of the dystrophin-mediated linkage. Downregulation of expression of the integrin may contribute to the development of pathology in congenital laminin deficiencies. Mutations in the alpha7 integrin gene underlie additional congenital muscle diseases. The functional roles of this integrin in the formation and stability of the neuromuscular and myotendinous junctions and its localization between fibers suggest that altered expression or function of this integrin may have widespread involvement in other myopathies. The localization of the alpha7 gene at human chromosome 12q13 is a useful clue for focusing such studies.

Muscle fibers attach to laminin in the basal lamina using two distinct mechanisms: the dystrophin glycoprotein complex and the α7β1 integrin. Defects in these linkage systems result in Duchenne muscular dystrophy (DMD), α2 laminin congenital muscular dystrophy, sarcoglycan-related muscular dystrophy, and α7 integrin congenital muscular dystrophy. Therefore, the molecular continuity between the extracellular matrix and cell cytoskeleton is essential for the structural and functional integrity of skeletal muscle. To test whether the α7β1 integrin can compensate for the absence of dystrophin, we expressed the rat α7 chain in mdx/utr−/− mice that lack both dystrophin and utrophin. These mice develop a severe muscular dystrophy highly akin to that in DMD, and they also die prematurely. Using the muscle creatine kinase promoter, expression of the α7BX2 integrin chain was increased 2.0–2.3-fold in mdx/utr−/− mice. Concomitant with the increase in the α7 chain, its heterodimeric partner, β1D, was also increased in the transgenic animals. Transgenic expression of the α7BX2 chain in the mdx/utr−/− mice extended their longevity by threefold, reduced kyphosis and the development of muscle disease, and maintained mobility and the structure of the neuromuscular junction. Thus, bolstering α7β1 integrin–mediated association of muscle cells with the extracellular matrix alleviates many of the symptoms of disease observed in mdx/utr−/− mice and compensates for the absence of the dystrophin- and utrophin-mediated linkage systems. This suggests that enhanced expression of the α7β1 integrin may provide a novel approach to treat DMD and other muscle diseases that arise due to defects in the dystrophin glycoprotein complex. A video that contrasts kyphosis, gait, joint contractures, and mobility in mdx/utr−/− and α7BX2-mdx/utr−/−mice can be accessed at http://www.jcb.org/cgi/content/full/152/6/1207.

The dystrophin glycoprotein complex links laminin in the extracellular matrix to the cell cytoskeleton. Loss of dystrophin causes Duchenne muscular dystrophy, the most common human X-chromosome-linked genetic disease. The α7β1 integrin is a second transmembrane laminin receptor expressed in skeletal muscle. Mutations in the α7 integrin gene cause congenital myopathy in humans and mice. The α7β1 integrin is increased in the skeletal muscle of Duchenne muscular dystrophy patients and mdx mice. This observation has led to the suggestion that dystrophin and α7β1 integrin have complementary functional and structural roles. To test this hypothesis, we generated mice lacking both dystrophin and α7 integrin (mdx/α7-/-). The mdx/α7-/- mice developed early-onset muscular dystrophy and died at 2-4 weeks of age. Muscle fibers from mdx/α7-/- mice exhibited extensive loss of membrane integrity, increased centrally located nuclei and inflammatory cell infiltrate, greater necrosis and increased muscle degeneration compared to mdx or α7-integrin null animals. In addition, loss of dystrophin and/or α7 integrin resulted in altered expression of laminin-α2 chain. These results point to complementary roles for dystrophin and α7β1 integrin in maintaining the functional integrity of skeletal muscle.

Alpha7beta1-integrin links laminin in the extracellular matrix with the cell cytoskeleton and therein mediates transduction of mechanical forces into chemical signals. Muscle contraction and stretching ex vivo result in activation of intracellular signaling molecules that are integral to postexercise injury responses. Because alpha7beta1-integrin stabilizes muscle and provides communication between the matrix and cytoskeleton, the role of this integrin in exercise-induced cell signaling and skeletal muscle damage was assessed in wild-type and transgenic mice overexpressing the alpha7BX2 chain. We report here that increasing alpha7beta1-integrin inhibits phosphorylation of molecules associated with muscle damage, including the mitogen-activated protein kinases (JNK, p38, and ERK), following downhill running. Likewise, activation of molecules associated with hypertrophy (AKT, mTOR, and p70(S6k)) was diminished in mice overexpressing integrin. While exercise resulted in Evans blue dye-positive fibers, an index of muscle damage, increased integrin protected mice from injury. Moreover, exercise leads to an increase in alpha7beta1 protein. These experiments provide the first evidence that alpha7beta1-integrin is a negative regulator of mechanotransduction in vivo and provides resistance to exercise-induced muscle damage.

Duchenne muscular dystrophy (DMD) is a devastating neuromuscular disease caused by mutations in the gene encoding dystrophin. Loss of dystrophin results in reduced sarcolemmal integrity and increased susceptibility to muscle damage. The α 7 β 1 -integrin is a laminin-binding protein up-regulated in the skeletal muscle of DMD patients and in the mdx mouse model. Transgenic overexpression of the α 7 -integrin alleviates muscle disease in dystrophic mice, making this gene a target for pharmacological intervention. Studies suggest laminin may regulate α 7 -integrin expression. To test this hypothesis, mouse and human myoblasts were treated with laminin and assayed for α 7 -integrin expression. We show that laminin-111 (α 1 , β 1 , γ 1 ), which is expressed during embryonic development but absent in normal or dystrophic skeletal muscle, increased α 7 -integrin expression in mouse and DMD patient myoblasts. Injection of laminin-111 protein into the mdx mouse model of DMD increased expression of α 7 -integrin, stabilized the sarcolemma, restored serum creatine kinase to wild-type levels, and protected muscle from exercised-induced damage. These findings demonstrate that laminin-111 is a highly potent therapeutic agent for the mdx mouse model of DMD and represents a paradigm for the systemic delivery of extracellular matrix proteins as therapies for genetic diseases.

The ␣7␤1 integrin is a laminin receptor that has been implicated in muscle disease and the development of neuromuscular and myotendinous junctions. Studies have shown the ␣7␤1 integrin is also expressed in nonskeletal muscle tissues. To identify the expression pattern of the ␣7 integrin in these tissues during embryonic development, ␣7 integrin chain knockout mice were generated by a LacZ knockin strategy. In these mice, expression from the ␣7 promoter is reported by ␤-galactosidase. From embryonic day (ED) 11.5 to ED14.5, ␤-galactosidase was detected in the developing central and peripheral nervous systems and vasculature. The loss of the ␣7 integrin gene resulted in partial embryonic lethality. Several ␣7 null embryos were identified with cerebrovascular hemorrhages and showed reduced vascular smooth muscle cells and cerebral vascularization. The ␣7 null mice that survived to birth exhibited vascular smooth muscle defects, including hyperplasia and hypertrophy. In addition, altered expression of ␣5 and ␣6B integrin chains was detected in the cerebral arteries of ␣7 null mice, which may contribute to the vascular phenotype. Our results demonstrate for the first time that the ␣7␤1 integrin is important for the recruitment or survival of cerebral vascular smooth muscle cells and that this integrin plays an important role in vascular development and integrity. Developmental Dynamics 234:11-21, 2005.

Sarcospan (SSPN) is a core component of the major adhesion complexes in skeletal muscle, the dystrophin- and utrophin (Utr)-glycoprotein complexes (DGC and UGC). We performed a rigorous analysis of SSPN-null mice and discovered that loss of SSPN decreased DGC and UGC abundance, leading to impaired laminin-binding activity and susceptibility to eccentric contraction-induced injury in skeletal muscle. We show that loss of SSPN increased levels of α7β1 integrin. To genetically test whether integrin compensates for the loss of DGC and UGC function in SSPN-nulls, we generated mice lacking both SSPN and α7 integrin (DKO, double knockout). Muscle regeneration, sarcolemma integrity and fibrosis were exacerbated in DKO mice and were remarkably similar to muscle from Duchenne muscular dystrophy (DMD) patients, suggesting that secondary loss of integrin contributes significantly to pathogenesis. Expression of the DGC and UGC, laminin binding and Akt signaling were negatively impacted in DKO muscle, resulting in severely diminished specific force properties. We demonstrate that SSPN is a necessary component of dystrophin and Utr function and that SSPN modulation of integrin signaling is required for extracellular matrix attachment and muscle force development.

Objective: We aimed to determine the frequency of all known forms of congenital muscular dystrophy (CMD) in a large Australasian cohort. Methods:We screened 101 patients with CMD with a combination of immunofluorescence, Western blotting, and DNA sequencing to identify disease-associated abnormalities in glycosylated ␣-dystroglycan, collagen VI, laminin ␣2, ␣7-integrin, and selenoprotein.Results: A total of 45% of the CMD cohort were assigned to an immunofluorescent subgroup based on their abnormal staining pattern. Abnormal staining for glycosylated ␣-dystroglycan was present in 25% of patients, and approximately half of these had reduced glycosylated ␣-dystroglycan by Western blot. Sequencing of the FKRP, fukutin, POMGnT1, and POMT1 genes in all patients with abnormal ␣-dystroglycan immunofluorescence identified mutations in one patient for each of these genes and two patients had mutations in POMT2. Twelve percent of patients had abnormalities in collagen VI immunofluorescence, and we identified disease-causing COL6 mutations in eight of nine patients in whom the genes were sequenced. Laminin ␣2 deficiency accounted for only 8% of CMD. ␣7-Integrin staining was absent in 12 of 45 patients studied, and ITGA7 gene mutations were excluded in all of these patients. LGMD2I ϭ limb-girdle muscular dystrophy type 2I; MDC1C ϭ congenital muscular dystrophy type 1C; MEB ϭ muscle-eyebrain disease; PVDF ϭ polyvinylidene fluoride; SSCP ϭ single strand conformational polymorphism; UCMD ϭ Ullrich congenital muscular dystrophy; WWS ϭ Walker-Warburg syndrome. Conclusions: