There is a Limit to the Changes in Myofilament Ca2+-Sensitivity due to Myopathies

Abstract

We have investigated Ca2+-sensitivity changes in cardiac and skeletal muscle due to mutations or phosphorylation of troponin I (TnI) using the in vitro motility assay.In normal heart, TnI phosphorylation alters myofibrillar Ca2+-sensitivity and increases the speed of Ca2+-dissociation (lusitropic effect). When end-stage heart failure samples with a reduced level of phosphorylation were looked at, the average Ca2+-sensitivity change was x2.6±0.24.Eight dilated cardiomyopathy (DCM) mutations have been looked at in all components of the thin filament (Actin E361G, TnT R141W and ΔK210, TnI K36Q, TnC G159D and Tm E40K, E54K and D230N). Although they all have differing effects on Ca2+-sensitivity (some increased it and some decreased it) the average Ca2+-sensitivity change was x2.0±0.16.The actin E99K hypertrophic cardiomyopathy (HCM) mutation causes an increase in Ca2+-sensitivity of x2.5 but a literature search looking at 21 other HCM mutations in every sarcomeric protein has found an average Ca2+-sensitivity change of x1.8±0.13.A wide range of skeletal myopathies were looked at which either caused a gain of function (TPM2 ΔK49, ΔE139, ACTA1 K326N and TPM3 ΔK7) or loss of function (TPM2 E117K, R91P and TPM3 A4V, L100M and R167C). The mean changes of Ca2+-sensitivity were x1.8±0.3 and x2.7±0.4 respectively.It is remarkable that mutations causing a range of different striated muscle myopathies have such a similar and limited effect on myofilament Ca2+-sensitivity and that phosphorylation of troponin I regulates Ca2+-sensitivity over the same range. We have found in DCM and HCM that Ca2+-sensitivity is uncoupled from the level of TnI phosphorylation and tends to the Ca2+-sensitivity characteristic of phosphorylated wild-type muscle at all phosphorylation levels. It is tempting to speculate that the 2-fold change represents a transition between two activation states of the thin filament.