*, **, and *** denote 0.05, 0.01, and 0.001, respectively. To examine the impact of MCU knockdown on mitochondrial N6,N6-Dimethyladenosine Ca2+ uptake, we determined the effect of extramitochondrial Ca2+ ([Ca2+]in -toxin-permeabilized INS-1E cells expressing mitochondria-targeted ratiopericam (RPmit). and metabolism-secretion coupling in insulin-releasing cells. activates several matrix enzymes including -ketoglutarate dehydrogenase in the TCA cycle (8). The ATP synthase is also directly activated by a rise in [Ca2+](9). In pancreatic -cells [Ca2+]is strictly required for ATP synthase-dependent respiration stimulated by glucose (10). Given its importance, mitochondrial Ca2+ uptake has been a research focus for decades, starting with the functional characterization in isolated mitochondria. Nevertheless, it took 50 years to elucidate the molecular identity of the mitochondrial Ca2+ uniporter (MCU) (11, 12). Mitochondrial Ca2+ uptake through MCU is regulated by a number of recently discovered proteins, including mitochondrial Ca2+ uptake 1 and 2 (MICU1/2) (13,C15), mitochondrial Ca2+ uniporter regulator 1 (MCUR1) (16), and essential MCU regulator (EMRE) (17). Especially MICU1/2 negatively regulate MCU activity under resting cytosolic Ca2+ ([Ca2+]( 10 m), however, MICU1 activates MCU activity, implying that the regulatory subunits of the MCU complex modulate mitochondrial Ca2+ loads of under resting conditions. Nutrient stimulation causes matrix alkalinization without any marked cytosolic pH change (29). Preventing the resulting nutrient-induced increase of the pH(29,C31). Therefore, pathogenic conditions causing a reduction of pHmay seriously deteriorate ATP generation and insulin secretion in pancreatic -cells. Several recent reports demonstrate the functional role of MCU in pancreatic -cells (26, 32). MCU mediates glucose-stimulated [Ca2+]rise and second phase ATP/ADP increase (26). Knockdown of either MCU or MICU1 diminishes insulin secretion associated with defects in mitochondrial Ca2+ uptake (32). Mice lacking MCU show a significant reduction of [Ca2+]and Ca2+-stimulated oxygen consumption in muscle mitochondria, without changes in the basal respiration in embryonic fibroblasts (33). It remains unclear, however, how reduced MCU activity attenuates mitochondrial signal generation in pancreatic -cell metabolism-secretion coupling. N6,N6-Dimethyladenosine In this study, we observed that reduced mitochondrial Ca2+ uptake following silencing of MCU significantly attenuated respiratory chain N6,N6-Dimethyladenosine activity and pHincrease in permeabilized as well as in intact insulin-secreting cells. These defects lead to impaired ATP synthesis and insulin secretion, demonstrating the crucial role of mitochondrial Ca2+ uptake for the establishment of the pHin metabolism-secretion coupling. We also provide evidence for a novel role of the putative Ca2+/H+ antiporter leucine zipper-EF hand-containing transmembrane protein 1 (LETM1) as a Ca2+ efflux route in insulin secreting cells, the role of which is altered in the absence of MCU. EXPERIMENTAL PROCEDURES Cell Culture and Drugs Rat insulinoma INS-1E cells were cultured in a humidified atmosphere (37 C) containing 5% CO2 in a complete medium composed N6,N6-Dimethyladenosine of RPMI 1640 (Invitrogen, Carlsbad, CA) supplemented with 10% FBS (Invitrogen), 1 mm sodium pyruvate, 50 m 2-mercaptoethanol, 2 mm glutamine, 10 mm HEPES, 100 units/ml penicillin, and 100 g/ml streptomycin (HyClone, Thermo Fisher Scientific Inc., Lafayette, CO). Experiments were performed with cells of passage number 80C120. Most chemicals were purchased from Sigma except JC-1 from Molecular Probes (Eugene, OR). Pancreatic islets were isolated from 200C300-g male Sprague-Dawley rats (Orient Bio, Seongnam, Korea) by collagenase (Sigma) digestion (29) and dispersed by a brief incubation with trypsin (Invitrogen). Islet cells N6,N6-Dimethyladenosine were seeded on multi-well-plates coated with 804G matrix and cultured in RPMI 1640 medium supplemented with 10% FBS, 10 mm HEPES, 100 units/ml penicillin, and 100 g/ml streptomycin (7). Permeabilization with -Hemolysin Toxin INS-1E cells were seeded and cultured onto well-plates or coverslips coated with 804G matrix. Cells were washed with Ca2+-free Krebs-Ringer bicarbonate (KRB) solution (mm; 140 NaCl, 3.6 KCl, 0.5 NaH2PO4, 0.5 MgSO4, 1.5 CaCl2, 10 HEPES, 2 NaHCO3, 5.5 glucose, pH 7.4 titrated with NaOH) and then incubated for 10 min at 37 C with 1 g/ml of Rabbit Polyclonal to MT-ND5 -hemolysin toxin (Sigma) in an intracellular buffer (mm; 140 KCl, 5 NaCl, 7 MgSO4, 1 KH2PO4, 20 HEPES, 10.2 EGTA, 1.65 CaCl2, 0.1 ATP, pH 7.0 with KOH), which has about 120 nm of free Ca2+ concentration. After -toxin permeabilization, cells were washed once with 0.5% bovine serum albumin (BSA) containing intracellular buffer and used for experiments (29). siRNA Transfection Cells were transfected with non-targeting or target-specific small interfering RNA (siRNA).