Dampened Activity of Single Ryanodine Receptor Channels in Mice Devoid of TRIC-A

Abstract

TRIC-A and TRIC-B are proteins localized to endo/sarcoplasmic reticulum (ER/SR). Their physiological roles are not established although there is evidence that they can function as monovalent cation channels (Yazawa et al., 2007). Mice lacking both TRIC-A and TRIC-B die in embryonic heart failure displaying serious defects in SR Ca2+-release (Yazawa et al., 2007). TRIC-A knockout (KO) mice survive but exhibit hypertension and abnormalities in cardiac and skeletal excitation-contraction coupling. We incorporated SR vesicles derived from the skeletal muscle of wild type (WT) and TRIC-A KO mice into planar lipid bilayers under voltage-clamp conditions to investigate if the single-channel properties of ryanodine receptors (RyR) and SR K+ channels are altered.We found no significant difference in single channel conductance, open probability (Po) and voltage-dependence of SR K+ channels from wild type (WT) or TRIC-A KO mice. [3H]ryanodine binding studies indicate that the Ca2+ sensitivity of RyR derived from TRIC-A KO or WT mice is similar (WT EC50 =4.14 µM; TRIC-A KO EC50 =3.65 µM; n=6). This was confirmed by the similar Po of RyR from WT (0.0023±0.0011, SEM, n=14) and TRIC-A KO (0.0017±0.0006, SEM, n=21) mice with 10 µM cytosolic Ca2+ as the sole activator. However, in the presence of 1 mM free Mg2+, 3 mM ATP was significantly less effective at activating RyR from TRIC-A KO mice (Po=0.0464±0.0133; SEM, n=21) than from WT mice (Po =0.1149±0.0290; n=14, SEM; p<0.05). Our results indicate that TRIC-A indirectly modulates RyR channel function either by reducing the inhibitory effect of Mg2+ or by increasing sensitivity to the effects of ATP and may, at least in part, explain why SR Ca2+-release is impaired in TRIC-A KO mice.Funded by the BHF.