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.

Mutations in the ␣7 integrin gene cause congenital myopathy characterized by delayed developmental milestones and impaired mobility. Previous studies in dystrophic mice suggest the ␣7␤1 integrin may be critical for muscle repair. To investigate the role that ␣7␤1 integrin plays in muscle regeneration, cardiotoxin was used to induce damage in the tibialis anterior muscle of ␣7 integrin-null mice. Unlike wild-type muscle, which responded rapidly to repair damaged myofibers , ␣7 integrin-deficient muscle exhibited defective regeneration. Analysis of Pax7 and MyoD expression revealed a profound delay in satellite cell activation after cardiotoxin treatment in ␣7 integrin-null animals when compared with wild type. We have recently demonstrated that the muscle of ␣7 integrin-null mice exhibits reduced laminin-␣2 expression. To test the hypothesis that loss of laminin contributes to the defective muscle regeneration phenotype observed in ␣7 integrin-null mice , mouse laminin-111 (␣1 , ␤1 , ␥1) protein was injected into the tibialis anterior muscle 3 days before cardiotoxin-induced injury. The injected laminin-111 protein infiltrated the entire muscle and restored myogenic repair and muscle regeneration in ␣7 integrin-null muscle to wild-type levels. Our data demonstrate a critical role for a laminin-rich microenvironment in muscle repair and suggest laminin-111 protein may serve as an unexpected and novel therapeutic agent for patients with congenital myopathies.

Duchenne muscular dystrophy is a lethal neuromuscular disease that currently has no effective therapy. Transgenic overexpression of the ␣7 integrin in mdx/ utrn ؊/؊ mice, a model of Duchenne muscular dystrophy ameliorates the disease. We have isolated and used ␣7 ؉/؊ muscle cells expressing ␤-galactosidase, driven by the endogenous ␣7 promoter, to identify compounds that increase ␣7 integrin levels. Valproic acid (VPA) was found to enhance ␣7 integrin levels, induce muscle hypertrophy, and inhibit apoptosis in myotubes by activating the Akt/mTOR/p70S6K pathway. This activation of the Akt pathway occurs within 1 hour of treatment and is mediated by phosphatidylinositol 3-OH kinase. To evaluate the potential use of VPA to treat muscular dystrophy, mdx/utrn ؊/؊ mice were injected with the drug. Treatment with VPA lowered collagen content and fibrosis, and decreased hind limb contractures. VPA-treated mice also had increased sarcolemmal integrity and decreased damage , decreased CD8-positive inflammatory cells , and higher levels of activated Akt in their muscles. Thus , VPA has important biological effects that may be applicable for the treatment of muscular dystrophy.

The ␣7␤1 integrin, dystrophin, and utrophin glycoprotein complexes are the major laminin receptors in skeletal muscle. Loss of dystrophin causes Duchenne muscular dystrophy, a lethal muscle wasting disease. Duchenne muscular dystrophy-affected muscle exhibits increased expression of ␣7␤1 integrin and utrophin, which suggests that these laminin binding complexes may act as surrogates in the absence of dystrophin. Indeed, mice that lack dystrophin and ␣7 integrin (mdx/␣7 ؊/؊ ), or dystrophin and utrophin (mdx/utr ؊/؊ ), exhibit severe muscle pathology and die prematurely. To explore the contribution of the ␣7␤1 integrin and utrophin to muscle integrity and function, we generated mice lacking both ␣7 integrin and utrophin. Surprisingly, mice that lack both ␣7 integrin and utrophin (␣7/utr ؊/؊ ) were viable and fertile. However, these mice had partial embryonic lethality and mild muscle pathology, similar to ␣7 integrin-deficient mice. Dystrophin levels were increased 1.4-fold in ␣7/utr ؊/؊ skeletal muscle and were enriched at neuromuscular junctions. Ultrastructural analysis revealed abnormal myotendinous junctions, and functional tests showed a ninefold reduction in endurance and 1.6-fold decrease in muscle strength in these mice. The ␣7/utr ؊/؊ mouse, therefore, demonstrates the critical roles of ␣7 integrin and utrophin in maintaining myotendinous junction structure and enabling force transmission during muscle contraction. Together, these results indicate that the ␣7␤1 integrin, dystrophin, and utrophin complexes act in a concerted manner to maintain the structural and functional integrity of skeletal muscle. (Am J Pathol