Genetic dissection of novel myopathy models reveals a role of CapZα and Leiomodin 3 during myofibril elongation.

Myofibrils within skeletal muscle are composed of sarcomeres that generate force by contraction when their myosin-rich thick filaments slide past actin-based thin filaments. Although mutations in components of the sarcomere are a major cause of human disease, the highly complex process of sarcomere assembly is not fully understood. Current models of thin filament assembly highlight a central role for filament capping proteins, which can be divided into three protein families, each ascribed with separate roles in thin filament assembly. CapZ proteins have been shown to bind the Z-disc protein α-actinin to form an anchoring complex for thin filaments and actin polymerisation. Subsequent thin filaments extension dynamics are thought to be facilitated by Leiomodins (Lmods) and thin filament assembly is concluded by Tropomodulins (Tmods) that specifically cap the pointed end of thin filaments. To study thin filament assembly in vivo, single and compound loss-of-function zebrafish mutants within distinct classes of capping proteins were analysed. The generated lmod3- and capza1b-deficient zebrafish exhibited aspects of the pathology caused by variations in their human orthologs. Although loss of the analysed main capping proteins of the skeletal muscle, capza1b, capza1a, lmod3 and tmod4, resulted in sarcomere defects, residual organised sarcomeres were formed within the assessed mutants, indicating that these proteins are not essential for the initial myofibril assembly. Furthermore, detected similarity and location of myofibril defects, apparent at the peripheral ends of myofibres of both Lmod3- and CapZα-deficient mutants, suggest a function in longitudinal myofibril growth for both proteins, which is molecularly distinct to the function of Tmod4.

The sarcoglycan complex in skeletal muscle.

In skeletal muscle, the dystrophin-associated glycoprotein complex forms a link between the actin cytoskeleton and the extracellular matrix that is critical for muscle integrity. Within this complex resides the sarcoglycan subcomplex, which consists of four transmembrane glycoproteins (alpha-, beta-, gamma-, and delta-sarcoglycan). During assembly, beta-sarcoglycan tightly associates with delta-sarcoglycan to form a functional core that then recruits gamma- and alpha-sarcoglycan to form the sarcoglycan complex. Together with sarcospan, the sarcoglycan complex binds other components of the dystrophin-associated glycoprotein complex and integrates into the myofibre's membrane. Once integrated, the sarcoglycan complex plays a pivotal role in mechanically stabilising the sarcolemma as well as the dystrophin-associated glycoprotein complex. Additionally, the sarcoglycan complex undergoes chemical modifications in response to muscle contractions, thereby transducing mechanical information into a cellular signal. Mutations in the sarcoglycans induce limb girdle muscular dystrophy, and several animal models have been established to study the molecular biology and function of the sarcoglycan complex. This review discusses the role of the sarcoglycan complex in skeletal muscle and describes the functional deficiencies that lead to muscular dystrophies.

Loss of Tropomodulin4 in the zebrafish mutant träge causes cytoplasmic rod formation and muscle weakness reminiscent of nemaline myopathy.

Nemaline myopathy is an inherited muscle disease that is mainly diagnosed by the presence of nemaline rods in muscle biopsies. Of the nine genes associated with the disease, five encode components of striated muscle sarcomeres. In a genetic zebrafish screen, the mutant träge (trg) was isolated based on its reduction in muscle birefringence, indicating muscle damage. Myofibres in trg appeared disorganised and showed inhomogeneous cytoplasmic eosin staining alongside malformed nuclei. Linkage analysis of trg combined with sequencing identified a nonsense mutation in tropomodulin4 (tmod4), a regulator of thin filament length and stability. Accordingly, although actin monomers polymerize to form thin filaments in the skeletal muscle of tmod4(trg) mutants, thin filaments often appeared to be dispersed throughout myofibres. Organised myofibrils with the typical striation rarely assemble, leading to severe muscle weakness, impaired locomotion and early death. Myofibrils of tmod4(trg) mutants often featured thin filaments of various lengths, widened Z-disks, undefined H-zones and electron-dense aggregations of various shapes and sizes. Importantly, Gomori trichrome staining and the lattice pattern of the detected cytoplasmic rods, together with the reactivity of rods with phalloidin and an antibody against actinin, is reminiscent of nemaline rods found in nemaline myopathy, suggesting that misregulation of thin filament length causes cytoplasmic rod formation in tmod4(trg) mutants. Although Tropomodulin4 has not been associated with myopathy, the results presented here implicateTMOD4 as a novel candidate for unresolved nemaline myopathies and suggest that the tmod4(trg) mutant will be a valuable tool to study human muscle disorders.