Myofibrillar Ca2+ sensitivity is uncoupled from troponin I phosphorylation in hypertrophic obstructive cardiomyopathy due to abnormal troponin T

We studied the relationship between myofilament Ca-2 sensitivity and troponin I (TnI) phosphorylation by protein kinase A at serines 22/23 in human heart troponin isolated from donor hearts and from myectomy samples from patients with hypertrophic obstructive cardiomyopathy (HOCM). We used a quantitative in vitro motility assay. With donor heart troponin, Ca-2 sensitivity is two- to three-fold higher when TnI is unphosphorylated. In the myectomy samples from patients with HOCM, the mean level of TnI phosphorylation was low: 0.38 0.19 mol Pi/mol TnI compared with 1.60 0.19 mol Pi/mol TnI in donor hearts, but no difference in myofilament Ca-2 sensitivity was observed. Thus, troponin regulation of thin filament Ca-2 sensitivity is abnormal in HOCM hearts. HOCM troponin (0.29 mol Pi/mol TnI) was treated with protein kinase A to increase the level of phosphorylation to 1.56 mol Pi/mol TnI. No difference in EC50 was found in thin filaments containing high and low TnI phosphorylation levels. This indicates that Ca-2 sensitivity is uncoupled from TnI phosphorylation in HOCM heart troponin. Coupling could be restored by replacing endogenous troponin T (TnT) with the recombinant TnT T3 isoform. No difference in Ca-2 sensitivity was observed if TnI was exchanged into HOCM heart troponin or if TnT was exchanged into the highly phosphorylated donor heart troponin. Comparison of donor and HOCM heart troponin by mass spectrometry and with adduct-specific antibodies did not show any differences in TnT isoform expression, phosphorylation or any post-translational modifications. An abnormality in TnT is responsible for uncoupling myofibrillar Ca-2 sensitivity from TnI phosphorylation in the septum of HOCM patients.