Decreased Fatigue Tolerance In Diaphragm Muscle Of Slow Troponin T Knockdown Mice

Abstract

The loss of slow skeletal muscle troponin T (TnT) results in a severe type of nemaline myopathy in the Amish (ANM). The genes encoding TnT and troponin I (TnI) are closely linked in pairs in which the 5′-enhancer region of the slow TnT gene overlaps with the cardiac TnI gene. In a mouse line with the entire cardiac TnI gene deleted, a partial destruction of the slow TnT gene promoter produces a knockdown effect. By crossing with transgenic mouse lines that over-express a core structure of cardiac TnI (cTnI-ND) under the control of cloned alpha-MHC promoter, we rescued the postnatal lethality of the cardiac TnI gene-deleted mice with no detrimental cardiac phenotypes or leaking expression in non-cardiac tissues. The double transgenic mice exhibited decreased expression of slow TnT mRNA and protein in adult diaphragm muscle. Functional analysis of isolated muscle strips showed that the slow TnT deficient (sTnT-KD) diaphragm had significantly decreased fatigue tolerance evident by the faster decrease in force and slower rate of recovery as compared with that in wild type controls. As a consequence of slow TnT deficiency, the sTnT-KD diaphragm muscle contained a higher proportion of fast TnT, decreased slow TnI with increased fast TnI, and decreased type I myosin with increased type II myosin. Consistent with the switch toward fast myofilament contents, the sTnT-KD diaphragm muscle produced higher specific tension in twitch and tetanic contractions as well as shorter time to develop peak tension in twitch contractions. The decreased fatigue tolerance of sTnT-KD diaphragm muscle explains the terminal respiratory failure seen in virtually all ANM patients and this double transgenic mouse model provides a useful experimental system to study the pathogenesis and treatment of ANM.