Essential Regulation of Cell Bioenergetics by Constitutive InsP3 Receptor Ca2+ Transfer to Mitochondria

Abstract

Mechanisms that regulate cellular metabolism are a fundamental requirement of all cells. Most eukaryotic cells rely on aerobic mitochondrial metabolism to generate ATP. Nevertheless, regulation of mitochondrial activity is incompletely understood. Here we identified an unexpected and essential role for constitutive InsP3R-mediated Ca2+ release in maintaining cellular bioenergetics. Macroautophagy provides eukaryotes with an adaptive response to nutrient deprivation that prolongs survival. Constitutive InsP3R Ca2+ signaling is required for macroautophagy suppression in cells in nutrient-replete media. 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 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 AMPK, which activates pro-survival macroautophagy. Thus, constitutive InsP3R Ca2+ release to mitochondria is an essential cellular process that is required for efficient mitochondrial respiration and maintenance of normal cell bioenergetics.

Reduced mitochondrial Ca2+ transients stimulate autophagy in human fibroblasts carrying the 13514A>G mutation of the ND5 subunit of NADH dehydrogenase

“…As previously reported, an impairment of mitochondrial Ca 2+ handling is associated with mitochondrial diseases. 35 Recently, Cardenas et al 6 reported that constitutively Ca 2+ release from the ER controls cellular bioenergetics and autophagy. Based on these observations, we investigated cellular Ca 2+ Figure S3d) and in permeabilized cells, where the contribution of plasma membrane or ER channels to the mitochondrial Ca 2+ response is absent (Supplementary Figure S3c).…”

“…6,39,40 Western blotting analysis of AMPK phosphorylation on Ser172 (p-AMPK) 40 revealed low p-AMPK level in resting conditions in control cells. Conversely, all patient-derived fibroblasts displayed a slight increase in p-AMPK levels, as expected in cells with deficiency of the respiratory chain complexes, but in Pat#1 this increase was dramatic.…”

“…30,31 In addition, mitochondrial Ca 2+ signaling has a role in the regulation of autophagy. 6 This complex relationship could allow, in the context of a mitochondrial disorder, the triggering of a cellular response that compensate, at least in part, the energetic defect, dampening the clinical phenotype. In this work, we investigated autophagy and mitochondrial Ca 2+ signaling in human fibroblasts from control subjects and from a set of patients carrying mutations in the mtDNA-encoded ND5 subunit of the ETC complex I.…”

“…5 In addition, Ca 2+ uptake in this organelle has recently been demonstrated to be a fundamental regulator of autophagy. 6,7 Autophagy is involved in physiological organelle turnover and in the removal of damaged or non-functional …”

“…5 In addition, Ca 2+ uptake in this organelle has recently been demonstrated to be a fundamental regulator of autophagy. 6,7 Autophagy is involved in physiological organelle turnover and in the removal of damaged or non-functional mitochondria by autophagy (called 'mitophagy') [8][9][10][11] and is critical for organelle quality control. Given the pivotal role of mitochondrial Ca 2+ in the adaptation of adenosine triphosphate (ATP) production to cellular energy demand, the recent identification of the channel responsible for Ca 2+ entry into the organelle, the mitochondrial Ca 2+ uniporter (MCU), is instrumental for the understanding of the regulation of mitochondrial Ca 2+ transport in both physiological and pathological conditions.…”