Myonuclear Domain Size and 3d Myonuclear Organization in Single Muscle Fibers from Myostatin Deficient Or IGF1 Overexpressing Mice

Abstract

Myostatin deprived or IGF-1 over-expressing mice are characterized by a 2-3 fold increase in muscle size compared to controls. Despite the hypertrophy these mice show significant difference in force generating capacity, i.e., maximum force normalized to muscle fiber cross-sectional area or specific force. That is, specific tension in IGF1 overexpressing transgenic mice is similar to controls while significantly lower in the myostatin knock out mice. The mechanism underlying this compromised muscle function is unknown. In an attempt to explore this mechanism we have investigated the size of cytoplasmic volume (myonuclear domain MND) supported by individual myonuclei in single muscle fiber segments from myostatin deficient, IGF1 over-expressing and control mice, using a novel algorithm to measure the MND in 3D. Single skinned muscle fiber segments were mounted at fixed sarcomere length corresponding to optimum filament overlap for force generation and stained with DAPI (myonuclei) and rhodamine (actin). Our image analysis algorithm was highly effective in determining the spatial organization of myonuclei and distribution of MNDs along the length of the fiber. Early results point towards an inverse relationship between MND and specific force. This implies that hypertrophy is primarily due to expansion of existing MNDs in myostatin knock-outs, and addition of more myonuclei in IGF1 over-expressing mice. This is suggested to have significant effects on transcriptional control of protein synthesis/degradation, turnover rates and/or posttranslational modifications of contractile proteins. We conclude that a maintained MND size is a prerequisite for force generation capacity in hypertrophied muscle fibers.