Troponin T isoform expression in humans. A comparison among normal and failing adult heart, fetal heart, and adult and fetal skeletal muscle.

Abstract

The expression of troponin (Tn) T, a thin-filament regulatory protein, was examined in left ventricular myocardium from normal and from failing adult human hearts. The differences in isoform expression between normal and failing myocardium led us to examine the ontogenic expression of TnT in human striated muscle. Left ventricular samples were obtained from patients with severe heart failure undergoing cardiac transplantation and normal adult organ donors. Fetal muscle was obtained from aborted fetuses after 14-15 weeks of gestation, and adult skeletal muscle was obtained from surgical biopsies. Western blots of normal and failing adult heart proteins demonstrated that two isoforms, TnT1 and TnT2, are expressed in different amounts, with TnT2 being significantly greater in failing hearts (p less than 0.004). Western blots of two-dimensional gels of these proteins resolved two predominant spots of both TnT1 and TnT2 and several minor TnT species. Alkaline phosphatase treatment converted the two major spots of each isoform into the single more basic spots. A comparison of the ATPase activities and the TnT2 percentage of total TnT in individual failing and normal adult hearts demonstrated an inverse and negative relation (r = 0.7, p less than 0.02). In the fetal heart, four TnT isoforms were found, two of which had the same electrophoretic mobilities as the adult cardiac isoforms TnT1 and TnT2. Fetal skeletal muscle expressed two of the four fetal cardiac TnT isoforms, one of which comigrated with adult cardiac TnT1. These cardiac isoforms were expressed in low abundance in fetal skeletal muscle relative to seven fast skeletal muscle TnT isoforms. No cardiac isoforms were present in adult skeletal muscle. Because many etiologies caused heart failure in the transplant patients, we propose that the disease-associated increased expression of the TnT isoform TnT2 is an adaptation to the heart failure state and a partial recapitulation of the fetal expression of cardiac TnT isoforms.