Abstract 13109: Troponin I Phosphorylation is Uncoupled From Myofibrillar Ca2+-sensitivity in Septal Myectomy Samples From Hocm Patients Due to an Abnormality in Troponin T
Using quantitative in vitro motility assay with human heart troponin we have shown that myofilament Ca2+-sensitivity is dependent upon the level of troponin I (Tn I) phosphorylation by PKA at Serines 22/23. In failing hearts, reduced TnI phosphorylation leads to a 2-3 fold higher Ca2+-sensitivity compared with donor heart. In myectomy samples from patients with hypertrophic obstructive cardiomyopathy (HOCM: 40% of patients have an identified HCM -causing mutation) Tn I phosphorylation was as low as in failing heart: 0.38±0.19 molsPi/mol TnI compared with 1.60±0.19 molsPi/mol TnI in donor heart, but no difference in Ca2+-sensitivity was observed (EC50 HOCM/donor troponin = 0.88±0.22). Thus troponin regulation of thin filament Ca2+-sensitivity is abnormal in HOCM hearts. HOCM troponin (0.29 molsPi/mol TnI) was then treated with PKA to increase the level of phosphorylation to 1.56 molsPi/mol TnI. No difference in EC50 was found in thin filaments containing high and low Tn I phosphorylation levels (EC50 HOCM/PKA-HOCM = 1.08±0.25). This confirms that Ca2+-sensitivity is uncoupled from Tn I phosphorylation in HOCM heart troponin. When thin filaments were reconstituted with HOCM troponin exchanged with recombinant TnT there was a 1.9-fold higher Ca2+-sensitivity as was expected with the low level of phosphorylation. No difference was observed if TnI was exchanged into HOCM heart troponin or if TnT was exchanged into the highly phosphorylated donor heart troponin. Therefore a change in troponin T is responsible for the observed disruption of the Ca2+-sensitivity-phosphorylation relationship. We conclude that in HOCM heart muscle TnI phosphorylation is uncoupled from Ca2+-sensitivity due to an abnormality in troponin T independently of the disease-causing mutation. Consequently the expected high Ca2+-sensitivity due to both the HCM-causing mutation and the low level of Tn I phosphorylation is attenuated. The abnormality of Tn T seems to be due to a post-translational modification or an isoform shift.
- © 2011 by American Heart Association, Inc.