The broad goal of our proposed studies is to extend our novel insights into biochemical and functional interactions of the inositol trisphosphate receptor (InsP3R) Ca2+ release ion channel and anti-apoptotic Bcl-2 family proteins, and into the discovery of an essential requirement of InsP3R activity to suppress macroautophagy and maintain efficient mitochondrial respiration and normal cell bioenergetics. The InsP3R participates in generation of complex Ca2+ signals that regulate many physiological processes in cells, including, as we have discovered, autophagy and basal metabolism, as well as cell survival and death decisions. We discovered that all three mammalian InsP3R isoforms interact with anti-apoptotic proteins Bcl-xL, Mcl-1 and Bcl-2. Using nuclear patch clamp electrophysiology, we found that these interactions profoundly alter gating of the channels, sensitizing them to low [InsP3] that exist in unstimulated cells and causing low- level Ca2+ signaling. Importantly, these interactions provide apoptosis resistance, although the mechanisms by which the Bcl-2-InsP3R interactions impinge on apoptosis protection remain to be defined. However, a possible insight has emerged in our recent discovery that constitutive low-level Ca2+ release by the InsP3R is required for efficient mitochondrial respiration and maintenance of normal cell bioenergetics. In its absence, cells become metabolically compromised due to diminished mitochondrial Ca2+ uptake. Mitochondrial uptake of InsP3R released Ca2+ is fundamentally required to provide optimal basal bioenergetics by providing sufficient reducing equivalents to support oxidative phosphorylation. Absence of this Ca2+ transfer results in enhanced phosphorylation of pyruvate dehydrogenase and activation of AMP Kinase, which activates pro-survival mTOR-independent macroautophagy. The identification of the InsP3R at the nexus of bioenergetics, apoptosis resistance and macroautophagy is of fundamental importance because of the roles of these processes in normal and path-physiology, including cancer, diabetes and cardiovascular disease, as well as in ischemia/reperfusion, aging and neurodegeneration. We will employ biophysical (electrophysiology, single live cell optical imaging), biochemical, genetic and cell biological approaches to define the mechanistic and structural bases for anti-apoptotic Bcl-2 family protein regulation of the InsP3R channel and the role that these interactions play in apoptosis. These studies will provide a basis to understand novel regulation of apoptosis by other signaling pathways that impinge on these interactions, including oncogenic Ras. Furthermore, they will provide insights to guide the development of novel molecules designed to modify apoptosis sensitivity, particularly in cancer cells. We will define the mechanisms of bioenergetics regulation by InsP3R signaling and determine the roles that these mechanisms play in the Bcl-2 protein-mediated ER gateway to programmed cell death, autophagy, and in diseases associated with altered cell metabolism.
The InsP3R participates in generation of complex Ca2+ signals that regulate many physiological processes in cells. Our discovery that it interacts with Bcl-2 proteins to regulate programmed cell death, and that its activity is critical for regulating cellular basal bioenergetics places InsP3R-mediated Ca2+ signaling at the nexus of cell survival/death signaling, autophagy and metabolism, processes involved in normal cell function and in many pathological conditions in humans.
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