Alternatively spliced troponin T isoforms in myocardial function and disease

Contraction in striated muscle is regulated by Ca+2 through the tropomyosin (Tm)-troponin (Tn) thin filament associated regulatory complex. The Tn complex is composed of three subunits: troponin C (TnC), troponin I (TnI), and troponin T (TnT). Troponin T has been proposed as a molecular modulator of Ca+2 signaling in the Tm-Tn system. Troponin T is encoded by three homologous genes expressed in a fiber specific manner. In skeletal muscle alternative splicing of the NH₂-terminal region of TnT leads to the expression of multiple isoforms. However, in the normal adult heart only one TnT isoform or a second slight variant is expressed. The function of the NH₂-terminus of TnT is debated. The NH ₂-terminus of TnT does not bind other thin filament proteins, but the NH₂-terminus may modulate the overall conformation of TnT resulting in altered binding to Tm, TnC and TnI. Altered TnT function in the thin filament can lead to the development of cardiomyopathy. Mutations within cardiac TnT (cTnT) cause altered contraction and the development of cardiomyopathy while cTnT isoform expression is altered in cardiomyopathy. For the first time we demonstrate a number of species that develop spontaneous dilated cardiomyopathy also express multiple cTnT in the adult heart. Cloning from four of these species demonstrated skipping of the exon 7 segment in turkey and canine cTnT, exclusion of the exon 6-encoded segment from guinea pig cTnT and abnormal retention of the embryonic exon 5 in the adult cat, dog and guinea pig cTnT. The exclusion of exon 7 from turkey cTnT induced an overall conformational change altering the binding to Tm and TnI. ATPase experiments of reconstituted thin filaments demonstrated those containing the exon 7 excluded TnT were more sensitive to Ca+2. Cardiomyocytes from mice transgenic for cTnT either abnormally including exon 5 or excluding exon 7 exhibited altered shortening. Together these data support a role of the TnT NH ₂-terminus as a modulatory region altering the Ca+2 activation of the thin filament. We propose the expression of multiple cTnT isoforms in the heart resulting from aberrant NH₂-terminal splicing decreases the contractile efficiency leading to the development of cardiomyopathy.