Role(s) of nucleoli and phosphorylation of ribosomal protein S6 and/or HSP27 in the regulation of muscle mass.
Effects of 14 days of hindlimb unloading or synergist ablation-related overloading with or without deafferentation on the fiber cross-sectional area, myonuclear number, size, and domain, the number of nucleoli in a single myonucleus, and the levels in the phosphorylation of the ribosomal protein S6 (S6) and 27-kDa heat shock protein (HSP27) were studied in rat soleus. Hypertrophy of fibers (+24%), associated with increased nucleolar number (from 1-2 to 3-5) within a myonucleus and myonuclear domain (+27%) compared with the preexperimental level, was induced by synergist ablation. Such phenomena were associated with increased levels of phosphorylated S6 (+84%) and HSP27 (+28%). Fiber atrophy (-52%), associated with decreased number (-31%) and domain size (-28%) of myonuclei and phosphorylation of S6 (-98%) and HSP27 (-63%), and with increased myonuclear size (+19%) and ubiquitination of myosin heavy chain (+33%, P > 0.05), was observed after unloading, which inhibited the mechanical load. Responses to deafferentation, which inhibited electromyogram level (-47%), were basically similar to those caused by hindlimb unloading, although the magnitudes were minor. The deafferentation-related responses were prevented and nucleolar number was even increased (+18%) by addition of synergist ablation, even though the integrated electromyogram level was still 30% less than controls. It is suggested that the load-dependent maintenance or upregulation of the nucleolar number and/or phosphorylation of S6 and HSP27 plays the important role(s) in the regulation of muscle mass. It was also indicated that such regulation was not necessarily associated with the neural activity.
Kawano
F
F
Graduate School of Medicine, Osaka University, Suita City, Osaka, Japan.
Matsuoka
Y
Y
Oke
Y
Y
Higo
Y
Y
Terada
M
M
Wang
X D
XD
Nakai
N
N
Fukuda
H
H
Imajoh-Ohmi
S
S
Ohira
Y
Y
D000818:Animals; D002466:Cell Nucleolus / Q000378:metabolism* / Q000473:pathology; D004195:Disease Models, Animal; D004576:Electromyography; D055551:HSP27 Heat-Shock Proteins; D006360:Heat-Shock Proteins / Q000378:metabolism*; D019417:Hindlimb Suspension; D006984:Hypertrophy; D008297:Male; D009121:Muscle Denervation; D018485:Muscle Fibers, Skeletal / Q000378:metabolism / Q000473:pathology; D053580:Muscle Strength; D018482:Muscle, Skeletal / Q000294:innervation / Q000378:metabolism* / Q000473:pathology / Q000503:physiopathology; D009133:Muscular Atrophy / Q000378:metabolism* / Q000473:pathology / Q000503:physiopathology; D009135:Muscular Diseases / Q000378:metabolism* / Q000473:pathology / Q000503:physiopathology; D018995:Myosin Heavy Chains / Q000378:metabolism; D009363:Neoplasm Proteins / Q000378:metabolism*; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D051381:Rats; D017208:Rats, Wistar; D038601:Ribosomal Protein S6 / Q000378:metabolism*; D025801:Ubiquitin / Q000378:metabolism
D055551:HSP27 Heat-Shock Proteins; D006360:Heat-Shock Proteins; C486114:Hspb1 protein, rat; D009363:Neoplasm Proteins; D038601:Ribosomal Protein S6; D025801:Ubiquitin; D018995:Myosin Heavy Chains