Troponin T Nuclear Localization and its Role in Aging Skeletal Muscle

Abstract

Troponin T (TnT) is known to mediate the interaction between Tn complex and tropomyosin (Tm), which is essential for calcium-activated striated muscle contraction. This regulatory function takes place in the myoplasm, where TnT binds Tm. However, recent findings of Troponin I and Tm nuclear translocation in Drosophila and mammalian cells imply other roles for the Tn-Tm complex. We hypothesized that TnT plays a nonclassical role through nuclear translocation. By immunoblotting with two different antibodies that target the NH2- or COOH-terminal region, respectively, we show that fast skeletal muscle TnT3 is localized in the muscle nucleus in addition to the cytoplasm as either an intact or fragmented protein. By transient overexpressing DsRed fusion proteins of different TnT3 fragments in C2C12 skeletal muscle cells, full-length or the COOH-terminal fragment are predominantly localized in the nucleus, closely related to nucleolus. In contrast, TnT3 middle and NH2-terminal fragments are predominantly localized in the cytoplasm, suggesting divergent functions. RNA polymerase inhibition redistributed TnT3 and its COOH-terminus from the nucleolus, implying a role for TnT3 in rRNA processing. More importantly, the full-length TnT3 protein is less abundant in myonuclei from old (24 months) vs young (4 months) mice while the COOH-terminal fragment is more abundant in older mice. We also found that apoptosis markers increased in cells expressing the fluorescence-tagged full length or COOH-terminal (CT) TnT3 proteins, in contrast to those expressing the middle and NH2-terminal fragments. We propose that TnT3 plays an important role in aging muscle, by contributing to decreased myonuclear numbers, thus mediating age-related muscle damage and disease.