TRIC‐A channels rock out alone while TRIC‐B get high with a little help from their friends

The bulk of Ca2+ required for the contraction of striated muscles arises from the sarcoplasmic reticulum (SR) intracellular Ca2+ stores. SR Ca2+ mobilization is mediated by arrays of ryanodine receptor (RyRs) Ca2+ release channels at the terminal cisternae of the SR. We have a good understanding of RyRs-mediated SR Ca2+ release and the role of other molecular transporters and regulators involved with intracellular Ca2+ homeostasis (Bers, 2001). However, technical limitations have prevented a detailed understanding of the dynamic interplay between molecules in t-tubular invaginations of the sarcolemma and at the junctional face membrane of the SR terminal cisterna. We have yet to understand the molecular cross-talk between cytosolic and SR luminal compartments. Patch clamp and microelectrode studies paved the road for understanding how membrane permeability of monovalent ions (i.e. K+, Na+, and Cl−) affect the action potential and the processes of SR Ca2+ release, linking excitation to contraction (Bers, 2001). However, current electrophysiological methods cannot access intracellular compartments, preventing the ability to define the SR membrane electrical properties. Consequently, there is only limited knowledge about SR monovalent ion permeability in cells, as well as how changes in permeability affect SR Ca2+ release and sequestration. Early studies found a high K+ permeability in the SR, which was assumed to provide cationic counter-current flow into the SR, thus preventing SR membrane polarization during RyRs-mediated Ca2+ release (McKinley & Meissner, 1978). Still, the importance of K+ counterfluxes for SR Ca2+ mobilization in cells remained unclear for decades as a result of SR membrane ‘leakage’ of various other ions, including Cl−. This provided various alternative pathways for short-circuiting RyRs-mediated Ca2+ currents. Subsequently, Yazawa et al. (2007) confirmed the molecular identity of the K+ permeable SR counter-ion channels as subtypes of the trimeric intracellular cation (TRIC) channels. Subsequent studies provided support with respect to TRIC channels being crucial for SR Ca2+ release in skeletal muscle and heart (Yazawa et al. 2007; El-Ajouz et al. 2017). TRIC channels also are important for Ca2+ release from the endoplasmic reticulum (ER) in non-muscle systems, including bone, where several genetic mutations within the TRIC-B locus are associated with osteogenesis imperfecta (Ichimura & Takeshima, 2016). Because TRIC channels are the ‘new kids on the block’ in SR/ER mediated Ca2+ signalling, we still need to define their function and regulatory roles in ER/SR homeostasis, including Ca2+ release, in both normal and pathological states. Addressing the unknown functional characteristics of TRIC channels, in this issue of the Journal of Physiology, O’Brien et al. (2019) report an investigation of TRIC-A and TRIC-B channel function after reconstituting channels in planar lipid bilayers. One major difference between the TRIC isoforms was uncovered, through comparative analysis, with respect to the gating characteristics of channels that reconstitute together as groups (multichannels) vs. those channels that are found to gate in isolation (single channels). Although being single or grouped channels does not change TRIC-A behaviour, grouped TRIC-B channels display 5–10 times higher open probability (Po) compared to ‘lonely’ single channels (O’Brien et al. 2019). Thus, TRIC-B channels need to establish mutual relationships to sustain a higher Po. Few studies have described channel–channel interactions, and we know little about the physical nature of this phenomenon. As demonstrated by O’Brien et al. (2019), channel connectivity typically is found in native membranes, suggesting that this results from physiological spatial arrangements for neighbouring interactions and/or ‘bridging’ through ancillary proteins. These interactions would probably be lost if channels are solubilized with non-denaturing detergents, as is the case in isolation and purification studies of intracellular channels. Channel co-operativity is associated with multichannel behaviour where the probability of observing 0 to n channels open deviates from the prediction of a binomial distribution for independently gating, identical channels. However, the results serve as a reminder that co-operativity is not the only category of channel–channel relationships. Whatever ‘glues’ the TRIC-B channels together decidedly helps to stabilize their open-state conformations without apparent interference of channels independency for deciding on when to open or close. O’Brien et al. (2019) also found that both TRIC channels show high K+ conductance. This is quite appropriate for channels serving as ionic counter-currents, with the SR membrane voltage as the primary modulator of both channels types. This is not uncommon for SR channels because voltage-dependence is found in RyRs and some Cl− permeable channels. Considering that the SR membranes are assumed to be essentially clamped at 0 mV, the TRIC-A channel will be ‘constitutively active’, whereas the clustering of TRIC-B in the SR could also play a modulatory role by generating membrane nanodomains with large K+ conductance and a high capacity to counter-current a locally generated ionic flow. In summary, O’Brien et al. (2019) found that TRIC-A channels, comprising the main isoform in heart SR, are ready to serve as a counter-current source. Their contribution in this role appears to be significant because dysfunction of TRIC-A genes significantly affects SR Ca2+ signalling and impedes the normal function of the heart, muscles and other tissues. However, several composite factors (developmental, hormonal, etc.) may be involved in these observed pathologies. We hope that future TRIC channel function studies will identify specific channel blockers, a highly consequential discovery for ‘the million dollar question’ that still remains unanswered: what would happen to SR/ER Ca2+ release following the sudden block of TRIC channels?