M.P.4.06 Effects of ageing and gender on the spatial organization of nuclei in single human skeletal muscle cells

Abstract

Skeletal muscle fibres have hundreds of nuclei sharing a common cytoplasm,and each myonucleus regulates the gene products in a specific region of cytoplasm known as myonuclear domain (MND). We analyzed ageing and gender-related effects on myonuclei organization and the MND size, at single fiber level, in a total of 208 single chemically skinned muscle fibers from the right vastus lateralis of young and old men and women. The 3D spatial organization of myonuclei and the individual MND size were analyzed by applying a novel image analysis algorithm to confocal image of single muscle fiber segments mounted at a fixed sarcomere length, corresponding to optimal filament overlap for force generation. Actin and myonuclei were stained with Rhodamine phalloidin and DAPI, respectively. The muscle fibers were classified according to Myosin Heavy Chain isoform expression. Significant ageing- and gender-related differences were observed in myonuclei organization and MND size in type IIa fibers, while the type I remained unaffected. The MND size is in part related to muscle fiber size and significant linear relation was observed between MND size and fiber CSA in the men irrespective age and MyHC isoform expression. However fiber size was a poor predictor of the MND size in women and a significant relationship was only observed in muscle finers expressing type I MyHC isoform among the old women. MND size variability increased in old age irrespective gender and MyHC isoform expression. The spatial organization of individual myonuclei, calculated as the distribution of nearest neighbour distance in 3D, was affected in old age. Muscle fibers expressing the type II MyHC isoform were preferentially affected irrespective gender, although changes were more pronounced among women. It is suggested that these changes have a significant impact on the protein synthesis and degradation during the ageing process and that some of the changes are dependent on gender and MyHC isoform expression.