UC San Diego

Skeletal muscles generate force through crossbridge interactions between actin thin filaments and myosin thick filaments within sarcomeres, which are, in turn, organized into a myofibrillar lattice in muscle fibers. Sarcomere assembly involves a complex and poorly understood process wherein protein constituents are synthesized and precisely localized to their target sites. One protein, nebulin, is believed to be a template or "ruler" that regulates thin filament length during actin polymerization. This dissertation seeks to understand the role of nebulin in vivo through a series of physiological experiments using a neonatal-lethal nebulin-knockout mouse model. Because functional reference data from neonatal mouse skeletal muscle were not available in the scientific literature, a detailed analysis of the morphological, biochemical, and contractile properties of muscle was first performed in wild-type mice from postnatal days 1 to 28. Measurements showed that the mouse tibialis anterior muscle exhibits intrinsic enhancement of functional quality during postnatal growth independently of absolute size. Possible explanations for this phenomenon include a developmental transition to mature myosin heavy chain isoform expression and increased myofibrillar packing within fibers. The functional role of nebulin was evaluated by comparing wild -type and nebulin-knockout mice at postnatal days 1 and 7. Nebulin had a dramatic impact on the active mechanical properties of skeletal muscle, with nebulin-deficient muscles exhibiting progressively inferior isometric stress generation and reduced functional integrity during cyclic contractions. The length-tension curve of nebulin- deficient muscle was also shifted in a manner consistent with reduced thin filament length. Short thin filaments alone could not explain the functional deficit of nebulin- deficient muscle, suggesting an additional role for nebulin in lateral force transmission. Finally, to further probe nebulin-mediated force transmission in muscle, the phenotype of I6611X nebulin-mutant mice was examined. This mutant exhibits a truncation of the nebulin protein at the extreme C-terminus, thereby eliminating the Src homology 3 (SH3) domain that anchors nebulin in the Z-disk. Skeletal muscle from the I6611X nebulin-mutant mice exhibited normal isometric stress production but was more susceptible to eccentric contraction-induced injury. It is conceivable that the nebulin SH3 domain acts as a Z-disk stabilizer during muscle injury.