Intracellular Ca2+ signals govern numerous physiological phenomena. Often these signals are generated by opening of ryanodine receptor (RyR) or inositol trisphosphate receptor (IP3R) Ca2+ release channels on the endoplasmic reticulum (ER) membrane. In tissues where one of these channels dominates, functional cross talk with the other is often present. Such cross talk between Ca2+ release channels has been documented in smooth, cardiac and recently skeletal muscle. In other words, function of each has ramifications on both. Here, our focus is the local Ca2+ control of the type-1 IP3R. IP3R-mediated Ca2+ release is controlled by the concerted actions of local IP3 and Ca2+. The IP3-control signal (source, size, kinetics) is fairly well defined. In contrast, the Ca2+-control signa is poorly understood because of its multi-modal nature. The Ca2+ control signal modes include cytosolic Ca2+ activation, cytosolic Ca2+ inactivation, intra-ER Ca2+ regulation (luminal regulation), auto-FT (Feed Through) and inter-FT Ca2+ regulation. Each has been explored separately but never have all been defined as a whole. The ill-defined local multi-modal Ca2+ control of the IP3R is a significant obstacle that has limited our understanding of both IP3R hierarchical Ca2+ signaling (channel to puff to wave) and quantal Ca2+ release. The least understood Ca2+ control modes are auto-FT, inter-FT and intra-ER Ca2+ regulation. Often, their existence (let alone significance) has been questioned due to the lack of definitive information (as the overlapping signals and feedback-loops have precluded separate testing of variables). This obstacle is overcome here and multi- modal IP3R local Ca2+ control defined as a whole for the first time. This is significant because IP3R governs Ca2+ signaling in nearly all cells and its abnormal function can have lethal ramifications. This means IP3R local control mechanisms are both potential points of pathological failure and logical targets for therapeutic intervention, making knowledge of their molecular basis highly significant to human health. Here, 3 laboratories with unique complementary resources and expertise combine to address the hypothesis: Multi-modal IP3R local Ca2+ control (activation, inactivation & feed through) includes a luminal intra-pore Ca2+-Mg2+ sensing site that governs the channel's IP3 sensitivity, the latter newly discovered mechanism explains IP3R-mediated quantal Ca2+ release in cells. This proposal applies a multidisciplinary (molecular to cellular) carefully crafted approach to tes this hypothesis.
The specific aims are to 1) define multi-modal local Ca2+ control of type-1 IP3R function and 2) identify the molecular basis of quantal type-1 IP3R Ca2+ release. This work will establish the biophysical basis of multi-modal local Ca2+ control of single IP3R1 function and likely will identify a novel (and long sought) single IP3R molecular mechanism that explains quantal release. This will represent a significant advancement in our understanding of IP3R1 local control.
Intracellular calcium signals drive a myriad of cellular phenomena. The signals are made by calcium release from intracellular deposits (stores). For the signals to be effective, calcium release must start, terminate, stop as sharply as it starts. Failure or malfunction of this process has severe ramifications in nearly all types of cells. For example, exaggerated calcium release from IP3R-gated stores has been a consistent observation in familial Alzheimer's disease. Here, we define local control mechanisms that govern intracellular calcium release, as these are potential sites of pathological failure and/or therapeutic intervention.
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