Structural and functional roles of nebulin in skeletal muscle
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.