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bioRxiv · 10.1101/2021.05.12.443789

The effect of magnesium on calcium binding to cardiac troponin C related hypertrophic cardiomyopathy mutants

Rayani, K.·Hantz, E.·Haji-Ghassemi, O.·Li, A. Y.·Spuches, A. M.·Van Petegem, F.·Solaro, R. J.·Lindert, S.·Tibbits, G. F.
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Abstract

Cardiac troponin C (cTnC) is the calcium (Ca2+) sensing component of the troponin complex. Binding of Ca2+ to cTnC triggers a cascade of myofilament conformational changes that culminate in force production. Mutations in cTnC linked to hypertrophic myocardial myopathy (HCM) induce a a greater degree and duration of Ca2+ binding, which may underly the hypertrophic phenotype. Recent evidence from our laboratories demonstrated novel modifications of cTnC Ca2+ binding by cellular magnesium (Mg2+) that we hypothesize may be of significance in promoting HCM. Regulation of contraction has long been thought to occur exclusively through Ca2+ binding to site II of cTnC. However, abundant cellular Mg2+ is a potential competitor for binding to the same sites; work by several groups also suggests this is possible. We have used isothermal titration calorimetry (ITC) to explore the thermodynamic properties associated with the interaction between Ca2+/Mg2+ and site II of cTnC; these experiments demonstrated that physiological concentrations of Mg2+ may compete with Ca2+ to bind site II of cTnC. In experiments reported here, we studied a series of mutations in cTnC thought to be causal in HCM. Three mutants (A8V, L29Q, and A31S) slightly elevated the affinity for both Ca2+ and Mg2+, whereas other mutants (L48Q, Q50R, and C84Y), that are closer to the C-terminal domain and surrounding the EF hand binding motif of site II had a more significant effect on affinity and the thermodynamics of the binding interaction. To the best of our knowledge, this work is the first to explore the role of Mg2+ in modifying the Ca2+ affinity ofcTnC mutations linked to HCM. Our results indicate a physiologically significant role for cellular Mg2+ at baseline conditions and when elevated on the control of the dynamics of contraction by modifications in the Ca2+ binding properties of cTnC.

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Rayani, K., Hantz, E., Haji-Ghassemi, O., Li, A. Y., Spuches, A. M., Van Petegem, F., Solaro, R. J., Lindert, S., Tibbits, G. F.. 2021-05-13. The effect of magnesium on calcium binding to cardiac troponin C related hypertrophic cardiomyopathy mutants. https://doi.org/10.1101/2021.05.12.443789

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