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

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