New Findings: Hindlimb Unloading Causes Nucleocytoplasmic Ca 2+ Overload and DNA Damage in Skeletal Muscle

Disuse atrophy of skeletal muscle is associated with a severe imbalance in cellular Ca homeostasis and marked increase in nuclear apoptosis. Nuclear Ca is involved in the regulation of cellular Ca homeostasis. However, it remains unclear whether nuclear Ca levels change under skeletal muscle disuse conditions, and whether changes in nuclear Ca levels are associated with nuclear apoptosis. In this study, changes in Ca levels, Ca transporters, and regulatory factors in the nucleus of hindlimb unloaded rat soleus muscle were examined to investigate the effects of disuse on nuclear Ca homeostasis and apoptosis. Results showed that, after hindlimb unloading, the nuclear envelope Ca levels ([Ca ] ) and nucleocytoplasmic Ca levels ([Ca ] ) increased by 78% ( < 0.01) and 106% ( < 0.01), respectively. The levels of Ca -ATPase type 2 (Ca -ATPase2), Ryanodine receptor 1 (RyR1), Inositol 1,4,5-tetrakisphosphate receptor 1 (IP R1), Cyclic ADP ribose hydrolase (CD38) and Inositol 1,4,5-tetrakisphosphate (IP ) increased by 470% ( < 0.001), 94% ( < 0.05), 170% ( < 0.001), 640% ( < 0.001) and 12% ( < 0.05), respectively, and the levels of Na /Ca exchanger 3 (NCX3), Ca /calmodulin dependent protein kinase II (CaMK II) and Protein kinase A (PKA) decreased by 54% ( < 0.001), 33% ( < 0.05) and 5% ( > 0.05), respectively. In addition, DNase X is mainly localized in the myonucleus and its activity is elevated after hindlimb unloading. Overall, our results suggest that enhanced Ca uptake from cytoplasm is involved in the increase in [Ca ] after hindlimb unloading. Moreover, the increase in [Ca ] is attributed to increased Ca release into nucleocytoplasm and weakened Ca uptake from nucleocytoplasm. DNase X is activated due to elevated [Ca ] , leading to DNA fragmentation in myonucleus, ultimately initiating myonuclear apoptosis. Nucleocytoplasmic Ca overload may contribute to the increased incidence of myonuclear apoptosis in disused skeletal muscle.