Role of intracellular calcium stores in hair-cell ribbon synapse

Ca2+ mobilization from intracellular stores represents an important cell signaling process 1 which is regulated, in mammalian cells, by inositol 1,4,5-trisphosphate (InsP3), cyclic ADP ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). InsP3 and cADPR release Ca2+ from sarco / endoplasmic reticulum (S/ER) stores through activation of InsP3 and ryanodine receptors (InsP3Rs and RyRs). By contrast, the nature of the intracellular stores targeted by NAADP and molecular identity of the NAADP receptors remain controversial 1,2, although evidence indicates that NAADP mobilizes Ca2+ from lysosome-related acidic compartments 3,4. Here we show that two-pore channels (TPCs) comprise a family of NAADP receptors, with TPC1 and TPC3 being expressed on endosomal and TPC2 on lysosomal membranes. Membranes enriched with TPC2 exhibit high affinity NAADP binding and TPC2 underpins NAADP-induced Ca2+ release from lysosome-related stores that is subsequently amplified by Ca2+-induced Ca2+ release via InsP3Rs. Responses to NAADP were abolished by disrupting the lysosomal proton gradient and by ablating TPC2 expression, but only attenuated by depleting ER Ca2+ stores or blocking InsP3Rs. Thus, TPCs form NAADP receptors that release Ca2+ from acidic organelles, which can trigger additional Ca2+ signals via S/ER. TPCs therefore provide new insights into the regulation and organization of Ca2+ signals in animal cells and will advance our understanding of the physiological role of NAADP.

The ryanodine receptors (RyRs) are a family of Ca2+ release channels found on intracellular Ca2+ storage/release organelles. The RyR channels are ubiquitously expressed in many types of cells and participate in a variety of important Ca2+ signaling phenomena (neurotransmission, secretion, etc.). In striated muscle, the RyR channels represent the primary pathway for Ca2+ release during the excitation-contraction coupling process. In general, the signals that activate the RyR channels are known (e.g., sarcolemmal Ca2+ influx or depolarization), but the specific mechanisms involved are still being debated. The signals that modulate and/or turn off the RyR channels remain ambiguous and the mechanisms involved unclear. Over the last decade, studies of RyR-mediated Ca2+ release have taken many forms and have steadily advanced our knowledge. This robust field, however, is not without controversial ideas and contradictory results. Controversies surrounding the complex Ca2+ regulation of single RyR channels receive particular attention here. In addition, a large body of information is synthesized into a focused perspective of single RyR channel function. The present status of the single RyR channel field and its likely future directions are also discussed.