No 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.

SUMMARY Mitochondrial Ca2+ (Ca2+m) uptake is mediated by an inner membrane Ca2+ channel called the uniporter. Ca2+ uptake is driven by the considerable voltage present across the inner membrane (ΔΨm) generated by proton pumping by the respiratory chain. Mitochondrial matrix Ca2+ concentration is maintained 5–6 orders of magnitude lower than its equilibrium level, but the molecular mechanisms for how this is achieved are not clear. Here we demonstrate that the mitochondrial protein MICU1 is required to preserve normal [Ca2+]m under basal conditions. In its absence, mitochondria become constitutively loaded with Ca2+, triggering excessive reactive oxygen species generation and sensitivity to apoptotic stress. MICU1 interacts with the uniporter pore-forming subunit MCU and sets a Ca2+ threshold for Ca2+m uptake without affecting the kinetic properties of MCU-mediated Ca2+ uptake. Thus, MICU1 is a gatekeeper of MCU-mediated Ca2+m uptake that is essential to prevent [Ca2+]m overload and associated stress.

Mutations in presenilins (PS) are the major cause of familial Alzheimer's disease (FAD) and have been associated with calcium (Ca2+) signaling abnormalities. Here, we demonstrate that FAD mutant PS1 (M146L)and PS2 (N141I) interact with the inositol 1,4,5-trisphosphate receptor (InsP3R) Ca2+ release channel and exert profound stimulatory effects on its gating activity in response to saturating and suboptimal levels of InsP3. These interactions result in exaggerated cellular Ca2+ signaling in response to agonist stimulation as well as enhanced low-level Ca2+signaling in unstimulated cells. Parallel studies in InsP3R-expressing and -deficient cells revealed that enhanced Ca2+ release from the endoplasmic reticulum as a result of the specific interaction of PS1-M146L with the InsP3R stimulates amyloid beta processing,an important feature of AD pathology. These observations provide molecular insights into the "Ca2+ dysregulation" hypothesis of AD pathogenesis and suggest novel targets for therapeutic intervention.