Aberrant splicing of an alternative exon in the Drosophila troponin-T gene affects flight muscle development

During myofibrillogenesis, many muscle structural proteins assemble to form the highly ordered contractile sarcomere. Mutations in these proteins can lead to dysfunctional muscle and various myopathies. We have analyzed the Drosophila melanogaster troponin T (TnT) up' mutant that specifically affects the indirect flight muscles (IFM) to explore troponin function during myofibrillogenesis. The up' muscles lack normal sarcomeres and contain zebra bodies, a phenotypic feature of human nemaline myopathies. We show that the up' mutation causes defective splicing of a newly identified alternative TnTexon (10a) that encodes part of the TnT C terminus. This exon is used to generate a TnT isoform specific to the IFM and jump muscles, which during IFM development replaces the exon I Ob isoform. Functional differences between the 10a and 10b TnT isoforms may be due to different potential phosphorylation sites, none of which correspond to known phosphorylation sites in human cardiac TnT. The absence of TnT mRNA in up' IFM reduces mRNA levels of an IFM-specific troponin I (TnI) isoform, but not actin, tropomyosin, or troponin C, suggesting a mechanism controlling expression of TnTand TnI genes may exist that must be examined in the context of human myopathies caused by mutations of these thin filament proteins.