Exon 12 of slow skeletal troponin t affects calcium sensitivity of force development and interactions with other thin filament components (1102.30)

Abstract

Little information exists concerning the functional roles of the human slow skeletal troponin T isoforms (HSSTnT isoforms) in slow skeletal muscle. Three HSSTnT isoforms have been found in human slow skeletal muscle: HSSTnT1 (+ exons 5 and 12), HSSTnT2 (+5, ‐12), and HSSTnT3 (‐5, ‐12). Another potential isoform HSSTnT‐Hyp (‐5, +12) was recently found at the mRNA level. The objective of this study was to determine the physiological role of these SSTnT isoforms in slow skeletal muscle. To investigate these SSTnT isoforms several methods including skinned fiber mechanics, peptide spot blot, and mammalian two‐hybrid assays were utilized. Skinned rabbit slow soleus muscle fibers were displaced with HSSTnT1, 2, 3 or Hyp and reconstituted with the human slow skeletal troponin I (HSSTnI)/human cardiac troponin C (HCTnC) complex. The calcium sensitivity increased between SSTnT isoforms: isoform 1 (pCa50 = 5.74) < Hyp isoform (pCa50 = 5.80) < isoform 2 (pCa50 = 5.81) < isoform 3 (pCa50 = 5.84). In a reconstituted skeletal muscle system, the HSSTnT 1‐3 isoforms had similar actomyosin ATPase activation or inhibition in the presence or absence of calcium. Peptide spot blot analysis revealed novel interactions between HSSTnT peptides and HCTnC, RSTm and HCTnI. Mammalian two‐hybrid studies showed that the alternatively spliced regions affect the interaction of HSSTnT with other thin filament components. These results suggest that the functional differences that occur between HSSTnT isoforms may be partially due to alternative splicing of exon 12. Overall, HSSTnT isoforms were found to have distinct functional properties in slow skeletal muscle regulation and are likely to be relevant for the understanding of skeletal muscle diseases.Grant Funding Source: UC Davis research funds and NIH