Biophysical characteristics of TRIC-A and TRIC-B channels and their regulation of RYR2

Abstract

Trimeric intracellular cation channels (TRIC-A and TRIC-B), found in the sarco/endoplasmic reticulum (SR/ER) and nuclear membranes, are thought to provide countercurrents to balance Ca2+-movements across the SR, but there is also evidence that they physically interact with ryanodine receptors (RYR). We therefore investigated if TRIC channels could modulate the single-channel function of RYR2 after incorporation of vesicles isolated from HEK293 cells expressing TRIC-A or TRIC-B with RYR2 into artificial membranes under voltage clamp. We also examined the gating and conductance properties of TRIC channels. Co-expression of RYR2 with either TRIC-A or TRIC-B significantly altered the gating behavior of RYR2; however, co-expression with TRIC-A was particularly effective at potentiating the activating effects of cytosolic Ca2+. Fusing membrane vesicles containing TRIC-A or TRIC-B together with RYR2 into bilayers produced large currents of rapidly gating current fluctuations of multiple amplitudes. In 740 cytosolic/210 luminal mM KCl gradient, current-voltage relationships of macroscopic currents revealed average reversal potentials (Erev) of -13.67 ± 9.02 (n = 7), -2.11 ± 3.84 (n = 11), and 13.19 ± 3.23 (n = 13, **, P = 0.0025) from vesicles from RYR2 only, RyR2 + TRIC-A, or RyR2 + TRIC-B cells, respectively. Thus, with the incorporation of TRIC channels, the Erevs depart further from the calculated Erev for ideally selective cation channels than occurs when vesicles from RYR2-only cells are incorporated, suggesting that TRIC channels are permeable to both K+ and Cl-. In conclusion, our results indicate that both TRIC-A and TRIC-B regulate the gating of RYR2, but that TRIC-A has greater capacity to stimulate the RYR2 opening. The results also suggest that TRIC channels may be relatively nonselective ion channels being permeable to both cations and anions. This property would enable TRIC channels to be versatile providers of counter-ion current throughout the SR of many cell types.