**1.1 Adhesion of muscle fibres to the extracellular matrix**

Skeletal muscle is composed of many multinucleated myofibres each of which is surrounded by a connective tissue matrix that is essential for the function and the structural integrity of muscle. Apposed to each myofibre is a basement membrane, composed of a mixture of extracellular matrix (ECM) proteins, including collagen, fibronectin, glycoproteins (laminins, perlecan and nidogen) and proteoglycans. The proteins bind to multiple receptors expressed on the surface of muscle fibres: this is most notable at the level of the Z discs where an assembly of cytoskeletal proteins including dystrophin and integrins maintain continuity between the contractile apparatus, cytoskeleton and the ECM. This association of proteins is commonly referred to as the costamere, which is derived from the Latin word *costa,* meaning rib, because they encircle the whole muscle fibre and are arranged at regular intervals, thus conferring the appearance of a rib-like structure (Ervasti, 2003). Costameres are the means by which mechanical stress generated by contraction is diffused laterally across the myofibre. An additional structure where stress is transmitted to the ECM is the myotendinous junction (MTJ), where a connection to the tendon is made at the termini of muscle fibres (Tidball, 1991). This tight association between the muscle fibre and its surrounding matrix not only confers tensile strength to the entire muscle but also plays an important role in development, regeneration and synaptogenesis (Sanes, 2003). Indeed genetic defects in proteins that localise to the costameres and MTJs are a common cause of muscle disease, underscoring their importance in maintaining normal muscle function (Campbell and Stull, 2003).

The two main adhesion systems recognised in striated muscle are the dystrophin-associated protein complex (DPC) and the integrins. Each system is composed of transmembrane

<sup>\*</sup> Corresponding Author

Integrins in the Development and Pathology of Skeletal Muscle 3

subunits is also regulated: 7a is found at the MTJ, 7b at the sarcolemma, MTJ and neuromuscular junction (NMJ), 3- and v-integrins are localized to the NMJ, and 9 integrin appears to be uniformly distributed along the sarcolemma (Wang et al., 1995; Martin et al., 1996). We will discuss here the functions identified for integrins in muscle, and refer the reader to recent reviews for details on the regulation of somitogenesis and NMJ formation by integrin-ECM interactions (Singhal and Martin, 2011; Thorsteinsdottir et al.,

While the expression pattern of the different subunits is well characterized, the precise function of many remains to be addressed. Genetic ablation of integrins in mice has not always been informative in this regard. For instance, mice with an ablation of 1-, 9- and v-integrins present no defects in skeletal muscle (Gardner et al., 1996; Bader et al., 1998; Huang et al., 2000). Mice with a genetic ablation of 3- and 6-integrins, die too early to study the long-term functions of integrins in skeletal muscle maintenance (Georges-Labouesse et al., 1996; Kreidberg et al., 1996). The distribution of laminin 5, which is the main ligand for 3-integrin, suggests a possible function in maturation of the muscle fibre and of the NMJ, since its initial expression throughout the basal lamina of developing myotubes becomes restricted to the NMJ in the first 3 weeks following birth (Nishimune et al., 2008). This is also consistent with 3-integrin expression being concentrated at the

Muscle defects have also been identified in mice with ablation of 5- and 7-integrins (Taverna et al., 1998; Mayer et al., 1997). 5-integrin is a receptor for fibronectin, and is expressed transiently during myotube differentiation. Ablation of 5-integrin in mice leads to early embryonic lethality with defects in mesoderm, vascular development and neural crest (Yang et al., 1993; Goh et al., 1997), but mice chimeric for this subunit survive postnatally and develop a form of muscular dystrophy (Taverna et al., 1998). No patients have been identified with mutations in 5-integrin possibly because, extrapolating from the data obtained in the mutant mouse models, null mutations are likely to be non viable. 7 integrin has been shown to play important functions in muscle in animal models and human patients, where mutations lead to a form of congenital muscular dystrophy (Mayer et al., 1997; Hayashi et al., 1998). Whilst it is possible that an in-depth analysis of the integrin subunit knockout mice would reveal muscle defects, for example in response to stressors such as exercise or mechanical damage, the apparent absence of a reported phenotype for some of these mice might be explained by redundancy. This possibility is supported by the generation of mice with a muscle specific ablation of the 1-subunit, which leads to the concomitant ablation of all 1-integrins (Schwander et al., 2003). These mice die shortly after birth, probably because of respiratory failure, with severe developmental defects in the muscle caused by impaired myoblast fusion and altered assembly of the

Fusion of myoblasts is essential for the formation of a syncytial myofibre, and it occurs in distinct steps: (i) migration of myoblasts to achieve cell proximity; (ii) contact between

2011).

sarcomere.

presynaptic NMJ (Martin et al., 1996).

**3. Integrins in skeletal muscle development** 

proteins that bind to the ECM, and of cytoplasmic proteins that connect to the cytoskeleton and transmit biochemical signals. The DPC is composed of several proteins, which include - and -dystroglycan that bind to laminin, dystrophin that connects to the cytoskeleton, and associated proteins such as sarcoglycans and neuronal nitric oxide synthase (nNOS). These proteins have the important function to confer mechanical integrity to the plasma membrane, which otherwise would break following muscle contraction. Indeed, this occurs in patients with mutations in DPC components, and present with several types of severe muscle disease, including Duchenne Muscular Dystrophy (DMD) and various forms of Limb Girdle Muscular Dystrophy (Bushby, 1999; Barresi and Campbell, 2006).

While integrins also establish a connection between the ECM and cytoskeletal and signalling proteins, the two complexes are biochemically distinct. As we will see below, integrins appear dispensable for the mechanical integrity of the sarcolemma, but have important functions during all stages of muscle development.
