Cardiac Troponin I Directly Binds Mitochondrial Proteins And Inhibits Mitochondrial Functions

The heart relies on mitochondria for its extensive energetic demands, and some cytoskeletal elements can regulate mitochondrial function. Cardiac troponin I (cTnI) is a structural protein involved in sarcomeric contraction. Here we demonstrate that some cTnI localizes to mitochondria, inhibits mitochondrial functions, and increases opening of the mitochondrial permeability transition pore under oxidative stress. Purified cTnI directly binds immune-captured ATP synthase in saturable fashion and doubles "reverse" F 1 F 0 -ATPase activity in vitro , whereas skeletal troponin I (ssTnI) does not. Relevant to human disease, several pathogenic variants of cTnI associated with hypertrophic cardiomyopathy do not increase F 1 F 0 -ATPase activity. Using a rational search, we identified the d subunit of ATP synthase as a potential cTnI-binding partner, and a ten-amino acid peptide inhibitor of cTnI’s interaction with ATP synthase, peptide P888, inhibited cTnI binding to ATP synthase and abolished the cTnI-induced increase in ATPase activity. P888 treatment greatly reduced cardiac LDH release and improved fractional shortening in vivo after transient coronary artery occlusion in rats, indicating that mitochondrial cTnI exacerbates cardiac ischemia-reperfusion injury. We explore the mechanisms governing cTnI’s mitochondrial localization and additional mitochondrial binding partners for cTnI. Together, our studies suggest that mitochondria-associated cTnI inhibits ATP synthase under basal conditions. Preventing cTnI from binding to ATP synthase during cardiac ischemia may increase the reservoir of functional mitochondria to mitigate post-ischemic cardiac injury. We also suggest that cTnI has additional roles in regulating mitochondrial functions. Finally, pathogenic cTnI mutations associated with hypertrophic cardiomyopathy do not affect ATP synthase activity, suggesting a potential mechanism that contributes to the diverse pathologies associated with these mutations.