The Super-Relaxed State of Myosin is Altered by Estradiol in Skeletal Muscle of Aged Female Mice

Abstract

We have used quantitative epifluorescence microscopy of fluorescent ATP to measure single-nucleotide turnover in skinned skeletal muscle fibers from old male and female mice. Aging causes declines in muscle strength, often leading to weakness-related health problems for the elderly. Female muscle has additional functional decrements with age due to reduced ovarian hormone production. Estradiol (E2) is the major sex hormone signal to skeletal muscle in females, and strength loss is rescued by E2 treatment in ovariectomized (OVX) mice. We previously showed that E2-mediated signaling reversibly regulates slow ATP turnover by myosin in single fibers isolated from OVX mice. To investigate E2 mechanisms on aged skeletal muscle, single fibers were isolated from young (2 months) or old (28 months) male and female mice, and were incubated with mantATP. Old female mice receiving 60-day E2 treatment were also studied. We measured the decay of mantATP fluorescence in an ATP-chase experiment to characterize the slow nucleotide turnover, called the super-relaxed state (SRX), detected in approximately one-third of the myosin heads. The SRX turnover was faster in aged female fibers compared to young female fibers. In contrast, ATP turnover in old male fibers was not significantly different than in young males, and even trended toward slower rather than faster turnover with aging. We conclude that E2-mediated signaling reversibly regulates slow ATP turnover by myosin in aging female mice. Age- and hormone-related functional deficits may be targetable at the level of myosin and other contractile protein structure/function for strategies to offset muscle weakness and metabolic changes that occur with age. This work was supported by NIH grants to DDT (R37 AG026160 and R01 AR032961), to DL (R01 AG31743), to BAC (R00 HL122397), and to KJP and BCC (T32 AR7612).