Unravelling the complexity of the mitochondrial Ca2+ uniporter: regulation, tissue specificity, and physiological implications

Calcium (Ca2+) signalling acts a pleiotropic message within the cell that is decoded by the mitochondria through a sophisticated ion channel known as the Mitochondrial Ca2+ Uniporter (MCU) complex. Under physiological conditions, mitochondrial Ca2+ signalling is crucial for coordinating cell activation with energy production. Conversely, in pathological scenarios, it can determine the fine balance between cell survival and death. Over the last decade, significant progress has been made in understanding the molecular bases of mitochondrial Ca2+ signalling. This began with the elucidation of the MCU channel components and extended to the elucidation of the mechanisms that regulate its activity. Additionally, increasing evidence suggests molecular mechanisms allowing tissue-specific modulation of the MCU complex, tailoring channel activity to the specific needs of different tissues or cell types. This review aims to explore the latest evidence elucidating the regulation of the MCU complex, the molecular factors controlling the tissue-specific properties of the channel, and the physiological and pathological implications of mitochondrial Ca2+ signalling in different tissues.

Metabolic dysfunction induced by HFD + L-NAME preferentially affects hippocampal mitochondria, impacting spatial memory in rats

High-fat diet-induced metabolic changes are not restricted to the onset of cardiovascular diseases, but also include effects on brain functions related to learning and memory. This study aimed to evaluate mitochondrial markers and function, as well as cognitive function, in a rat model of metabolic dysfunction. Eight-week-old male Wistar rats were subjected to either a control diet or a two-hit protocol combining a high fat diet (HFD) with the nitric oxide synthase inhibitor L-NAME in the drinking water. HFD plus L-NAME induced obesity, hypertension, and increased serum cholesterol. These rats exhibited bioenergetic dysfunction in the hippocampus, characterized by decreased oxygen (O2) consumption related to ATP production, with no changes in H2O2 production. Furthermore, OPA1 protein expression was upregulated in the hippocampus of HFD + L-NAME rats, with no alterations in other morphology-related proteins. Consistently, HFD + L-NAME rats showed disruption of performance in the Morris Water Maze Reference Memory test. The neocortex did not exhibit either bioenergetic changes or alterations in H2O2 production. Calcium uptake rate and retention capacity in the neocortex of HFD + L-NAME rats were not altered. Our results indicate that hippocampal mitochondrial bioenergetic function is disturbed in rats exposed to a HFD plus L-NAME, thus disrupting spatial learning, whereas neocortical function remains unaffected.

Mitochondrial sodium/calcium exchanger (NCLX) regulates basal and starvation-induced autophagy through calcium signaling

Mitochondria shape intracellular Ca2+ signaling through the concerted activity of Ca2+ uptake via mitochondrial calcium uniporters and efflux by Na+ /Ca2+ exchangers (NCLX). Here, we describe a novel relationship among NCLX, intracellular Ca2+ , and autophagic activity. Conditions that stimulate autophagy in vivo and in vitro, such as caloric restriction and nutrient deprivation, upregulate NCLX expression in hepatic tissue and cells. Conversely, knockdown of NCLX impairs basal and starvation-induced autophagy. Similarly, acute inhibition of NCLX activity by CGP 37157 affects bulk and endoplasmic reticulum autophagy (ER-phagy) without significant impacts on mitophagy. Mechanistically, CGP 37157 inhibited the formation of FIP200 puncta and downstream autophagosome biogenesis. Inhibition of NCLX caused decreased cytosolic Ca2+ levels, and intracellular Ca2+ chelation similarly suppressed autophagy. Furthermore, chelation did not exhibit an additive effect on NCLX inhibition of autophagy, demonstrating that mitochondrial Ca2+ efflux regulates autophagy through the modulation of Ca2+ signaling. Collectively, our results show that the mitochondrial Ca2+ extrusion pathway through NCLX is an important regulatory node linking nutrient restriction and autophagy regulation.

Adiponectin reverses β-Cell damage and impaired insulin secretion induced by obesity

Obesity significantly decreases life expectancy and increases the incidence of age-related dysfunctions, including β-cell dysregulation leading to inadequate insulin secretion. Here, we show that diluted plasma from obese human donors acutely impairs β-cell integrity and insulin secretion relative to plasma from lean subjects. Similar results were observed with diluted sera from obese rats fed ad libitum, when compared to sera from lean, calorically restricted, animals. The damaging effects of obese circulating factors on β-cells occurs in the absence of nutrient overload, and mechanistically involves mitochondrial dysfunction, limiting glucose-supported oxidative phosphorylation and ATP production. We demonstrate that increased levels of adiponectin, as found in lean plasma, are the protective characteristic preserving β-cell function; indeed, sera from adiponectin knockout mice limits β-cell metabolic fluxes relative to controls. Furthermore, oxidative phosphorylation and glucose-sensitive insulin secretion, which are completely abrogated in the absence of this hormone, are restored by the presence of adiponectin alone, surprisingly even in the absence of other serological components, for both the insulin-secreting INS1 cell line and primary islets. The addition of adiponectin to cells treated with plasma from obese donors completely restored β-cell functional integrity, indicating the lack of this hormone was causative of the dysfunction. Overall, our results demonstrate that low circulating adiponectin is a key damaging element for β-cells, and suggest strong therapeutic potential for the modulation of the adiponectin signaling pathway in the prevention of age-related β-cell dysfunction.

Regulation of Kidney Mitochondrial Function by Caloric Restriction

Caloric restriction (CR) prevents obesity and increases resilience against pathological stimuli in laboratory rodents. At the mitochondrial level, protection promoted by CR in the brain and liver is related to higher calcium uptake rates and capacities, avoiding Ca²⁺-induced mitochondrial permeability transition. Dietary restriction has also been shown to increase kidney resistance against damaging stimuli, but if these effects are related to similar mitochondrial adaptations has not been uncovered. Here, we characterized changes in mitochondrial function in response to six months CR in rats, measuring bioenergetic parameters, redox balance and calcium homeostasis. CR promoted an increase in succinate-supported mitochondrial oxygen consumption rates. While CR prevents mitochondrial reactive oxygen species production in many tissues, in kidney we found that mitochondrial H₂O₂ release was enhanced in a succinate-dependent manner. Surprisingly, and opposite to the effects observed in brain and liver, mitochondria from CR animals are more prone to Ca²⁺-induced mitochondrial permeability transition, in a manner reversed by antioxidant dithiothreitol. CR mitochondria also displayed higher calcium uptake rates, which were not accompanied by changes in calcium efflux rates, nor related to altered inner mitochondrial membrane potentials or amounts of the mitochondrial calcium uniporter (MCU). Instead, increased mitochondrial calcium uptake rates in CR kidneys correlate with a loss of MICU2, an MCU modulator. Interestingly, MICU2 is also modulated by CR in liver, suggesting it has a broader diet-sensitive regulatory role controlling mitochondrial calcium homeostasis. Together, our results highlight the organ-specific bioenergetic, redox, and ionic transport effects of CR, with some unexpected deleterious effects in kidney.

Disruption of polycystin-1 cleavage leads to cardiac metabolic rewiring in mice

Cardiovascular manifestations account for marked morbi-mortality in autosomal dominant polycystic kidney disease (ADPKD). Pkd1- and Pkd2-deficient mice develop cardiac dysfunction, however the underlying mechanisms remain largely unclear. It is unknown whether impairment of polycystin-1 cleavage at the G-protein-coupled receptor proteolysis site, a significant ADPKD mutational mechanism, is involved in this process. We analyzed the impact of polycystin-1 cleavage on heart metabolism using Pkd1^V/V mice, a model unable to cleave this protein and with early cardiac dysfunction. Pkd1^V/V hearts showed lower levels of glucose and amino acids and higher lipid levels than wild-types, as well as downregulation of p-AMPK, p-ACCβ, CPT1B-Cpt1b, Ppara, Nppa and Acta1. These findings suggested decreased fatty acid β-oxidation, which was confirmed by lower oxygen consumption by Pkd1^V/V isolated mitochondria using palmitoyl-CoA. Pkd1^V/V hearts also presented increased oxygen consumption in response to glucose, suggesting that alternative substrates may be used to generate energy. Pkd1^V/V hearts displayed a higher density of decreased-size mitochondria, a finding associated with lower MFN1, Parkin and BNIP3 expression. These derangements were correlated with increased apoptosis and inflammation but not hypertrophy. Notably, Pkd1^V/V neonate cardiomyocytes also displayed shifts in oxygen consumption and p-AMPK downregulation, suggesting that, at least partially, the metabolic alterations are not induced by kidney dysfunction. Our findings reveal that disruption of polycystin-1 cleavage leads to cardiac metabolic rewiring in mice, expanding the understanding of heart dysfunction associated with Pkd1 deficiency and likely with human ADPKD.

Calorie restriction changes muscle satellite cell proliferation in a manner independent of metabolic modulation

Calorie restriction is known to promote healthy aging, which includes prevention of muscle loss. We investigated the effect of rodent calorie restriction on mitochondrial respiration and clonogenic capacity of muscle satellite stem cells, since metabolic alterations are known to regulate stem cell activity. Surprisingly, short or long-term calorie restriction do not change mitochondrial or glycolytic function. Nevertheless, both short- and long-term calorie restriction enhance myogenic colony formation. Overall, our results show that not all changes in satellite stem cell function are accompanied by metabolic remodeling.