Mitochondria associate with lipid droplets (LDs) in fat-oxidizing tissues, but the functional role of these peridroplet mitochondria (PDM) is unknown. Microscopic observation of interscapular brown adipose tissue reveals that PDM have unique protein composition and cristae structure and remain adherent to the LD in the tissue homogenate. We developed an approach to isolate PDM based on their adherence to LDs. Comparison of purified PDM to cytoplasmic mitochondria reveals that (1) PDM have increased pyruvate oxidation, electron transport, and ATP synthesis capacities; (2) PDM have reduced β-oxidation capacity and depart from LDs upon activation of brown adipose tissue thermogenesis and β-oxidation; (3) PDM support LD expansion as Perilipin5-induced recruitment of mitochondria to LDs increases ATP synthase-dependent triacylglyceride synthesis; and (4) PDM maintain a distinct protein composition due to uniquely low fusion-fission dynamics. We conclude that PDM represent a segregated mitochondrial population with unique structure and function that supports triacylglyceride synthesis.
Changes in mitochondrial size and shape have been implicated in several physiologic processes, but their role in mitochondrial Ca 2+ uptake regulation and overall cellular Ca 2+ homeostasis is largely unknown. Here we show that modulating mitochondrial dynamics toward increased fusion through expression of a dominant negative (DN) form of the fission protein [dynamin-related protein 1 (DRP1)] markedly increased both mitochondrial Ca 2+ retention capacity and Ca 2+ uptake rates in permeabilized C2C12 cells. Similar results were seen using the pharmacological fusion-promoting M1 molecule. Conversely, promoting a fission phenotype through the knockdown of the fusion protein mitofusin (MFN)-2 strongly reduced the mitochondrial Ca 2+ uptake speed and capacity in these cells. These changes were not dependent on modifications in mitochondrial calcium uniporter expression, inner membrane potentials, or the mitochondrial permeability transition. Implications of mitochondrial morphology modulation on cellular calcium homeostasis were measured in intact cells; mitochondrial fission promoted lower basal cellular calcium levels and lower endoplasmic reticulum (ER) calcium stores, as indicated by depletion with thapsigargin. Indeed, mitochondrial fission was associated with ER stress. Additionally, the calcium-replenishing process of store-operated calcium entry was impaired in MFN2 knockdown cells, whereas DRP1-DN-promoted fusion resulted in faster cytosolic Ca 2+ increase rates. Overall, our results show a novel role for mitochondrial morphology in the regulation of mitochondrial Ca 2+ uptake, which impacts cellular Ca 2+ homeostasis.-
Changes in mitochondrial size and shape have been implicated in several physiological processes, but their role in mitochondrial Ca 2+ uptake regulation and overall cellular Ca 2+ homeostasis is largely unknown. Here we show that modulating mitochondrial dynamics towards increased fusion through expression of a dominant negative form of the fission protein DRP1 (DRP1-DN) markedly increased both mitochondrial Ca 2+ retention capacity and Ca 2+ uptake rates in permeabilized C2C12 cells. Similar results were seen using the pharmacological fusion-promoting M1 molecule. Conversely, promoting a fission phenotype through the knockdown of the fusion protein mitofusin 2 (MFN2) strongly reduced mitochondrial Ca 2+ uptake speed and capacity in these cells. These changes were not dependent on modifications in inner membrane potentials or the mitochondrial permeability transition. Implications of mitochondrial morphology modulation on cellular calcium homeostasis were measured in intact cells; mitochondrial fission promoted lower basal cellular calcium levels and lower endoplasmic reticulum (ER) calcium stores, as measured by depletion with thapsigargin. Indeed, mitochondrial fission was associated with ER stress. Additionally, the calcium-replenishing process of store-operated calcium entry (SOCE) was impaired in MFN2 knockdown cells, while DRP1-DN-promoted fusion resulted in faster cytosolic Ca 2+ increase rates. Overall, our results show a novel role for mitochondrial morphology in the regulation of mitochondrial Ca 2+ uptake, which impacts on cellular Ca 2+ homeostasis.
Chronic exposure of pancreatic β cells to high concentrations of free fatty acids leads to lipotoxicity (LT)‐mediated suppression of glucose‐stimulated insulin secretion. This effect is in part caused by a decline in mitochondrial function as well as by a reduction in lysosomal acidification. Because both mitochondria and lysosomes can alter one another's function, it remains unclear which initiating dysfunction sets off the detrimental cascade of LT, ultimately leading to β‐cell failure. Here, we investigated the effects of restoring lysosomal acidity on mitochondrial function under LT. Our results show that LT induces a dose‐dependent lysosomal alkalization accompanied by an increase in mitochondrial mass. This increase is due to a reduction in mitochondrial turnover as analyzed by MitoTimer, a fluorescent protein for which the emission is regulated by mitochondrial clearance rate. Mitochondrial oxygen consumption rate, citrate synthase activity, and ATP content are all reduced by LT. Restoration of lysosomal acidity using lysosome‐targeted nanoparticles is accompanied by stimulation of mitochondrial turnover as revealed by mitophagy measurements and the recovery of mitochondrial mass. Remarkably, re‐acidification restores citrate synthase activity and ATP content in an insulin secreting β‐cell line (INS‐1). Furthermore, nanoparticle‐mediated lysosomal reacidification rescues mitochondrial maximal respiratory capacity in both INS‐1 cells and primary mouse islets. Therefore, our results indicate that mitochondrial dysfunction is downstream of lysosomal alkalization under lipotoxic conditions and that recovery of lysosomal acidity is sufficient to restore the bioenergetic defects.—Assali, E. A., Shlomo, D., Zeng, J., Taddeo, E. P., Trudeau, K. M., Erion, K. A., Colby, A. H., Grinstaff, M. W., Liesa, M., Las, G., Shirihai, O. S. Nanoparticle‐mediated lysosomal reacidification restores mitochondrial turnover and function in β cells under lipotoxicity. FASEB J. 33, 4154–4165 (2019). http://www.fasebj.org
Calcium dynamics control synaptic transmission. Calcium triggers synaptic vesicle fusion, determines release probability, modulates vesicle recycling, participates in long-term plasticity and regulates cellular metabolism. Mitochondria, the main source of cellular energy, serve as calcium signaling hubs. Mitochondrial calcium transients are primarily determined by the balance between calcium influx, mediated by the mitochondrial calcium uniporter (MCU), and calcium efflux through the sodium/lithium/calcium exchanger (NCLX). We identified a human recessive missense SLC8B1 variant that impairs NCLX activity and is associated with severe mental retardation. On this basis, we examined the effect of deleting NCLX in mice on mitochondrial and synaptic calcium homeostasis, synaptic activity, and plasticity. Neuronal mitochondria exhibited basal calcium overload, membrane depolarization, and a reduction in the amplitude and rate of calcium influx and efflux. We observed smaller cytoplasmic calcium transients in the presynaptic terminals of NCLX-KO neurons, leading to a lower probability of release and weaker transmission. In agreement, synaptic facilitation in NCLX-KO hippocampal slices was enhanced. Importantly, deletion of NCLX abolished long term potentiation of Schaffer collateral synapses. Our results show that NCLX controls presynaptic calcium transients that are crucial for defining synaptic strength as well as short- and long-term plasticity, key elements of learning and memory processes.
Combined fatty acid esterification and lipolysis, termed lipid cycling, is an ATP‐consuming process that contributes to energy expenditure. Therefore, interventions that stimulate energy expenditure through lipid cycling are of great interest. Here we find that pharmacological and genetic inhibition of the mitochondrial pyruvate carrier (MPC) in brown adipocytes activates lipid cycling and energy expenditure, even in the absence of adrenergic stimulation. We show that the resulting increase in ATP demand elevates mitochondrial respiration coupled to ATP synthesis and fueled by lipid oxidation. We identify that glutamine consumption and the Malate‐Aspartate Shuttle are required for the increase in Energy Expenditure induced by MPC inhibition in Brown Adipocytes (MAShEEBA). We thus demonstrate that energy expenditure through enhanced lipid cycling can be activated in brown adipocytes by decreasing mitochondrial pyruvate availability. We present a new mechanism to increase energy expenditure and fat oxidation in brown adipocytes, which does not require adrenergic stimulation of mitochondrial uncoupling.
A sharp increase in mitochondrial Ca 2+ marks the activation of brown adipose tissue (BAT) thermogenesis, yet the mechanisms preventing Ca 2+ deleterious effects are poorly understood. Here, we show that adrenergic stimulation of BAT activates a PKA-dependent mitochondrial Ca 2+ extrusion via the mitochondrial Na + /Ca 2+ exchanger, NCLX. Adrenergic stimulation of NCLX-null brown adipocytes (BA) induces a profound mitochondrial Ca 2+ overload and impaired uncoupled respiration. Core body temperature, PET imaging of glucose uptake and VO 2 measurements confirm a thermogenic defect in NCLX-null mice. We show that Ca 2+ overload induced by adrenergic stimulation of NCLX-null BAT, triggers the mitochondrial permeability transition pore (mPTP) opening, leading to a remarkable mitochondrial swelling and cell death. Treatment with mPTP inhibitors rescue mitochondrial function and thermogenesis in NCLX-null BAT, while calcium overload persists. Our findings identify a key pathway through which BA evade apoptosis during adrenergic stimulation of uncoupling. NCLX deletion transforms the adrenergic pathway responsible for thermogenesis activation into a death pathway.
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