Mitochondrial redox state, bioenergetics, and calcium transport in caloric restriction: A metabolic nexus.

Mitochondria congregate central reactions in energy metabolism, many of which involve electron transfer. As such, they are expected to both respond to changes in nutrient supply and demand and also provide signals that integrate energy metabolism intracellularly. In this review, we discuss how mitochondrial bioenergetics and reactive oxygen species production is impacted by dietary interventions that change nutrient availability and impact on aging, such as calorie restriction. We also discuss how dietary interventions alter mitochondrial Ca2+ transport, regulating both mitochondrial and cytosolic processes modulated by this ion. Overall, a plethora of literature data support the idea that mitochondrial oxidants and calcium transport act as integrating signals coordinating the response to changes in nutritional supply and demand in cells, tissues, and animals.

How and when to measure mitochondrial inner membrane potentials.

The scientific literature on mitochondria has increased significantly over the years due to findings that these organelles have widespread roles in the onset and progression of pathological conditions such as metabolic disorders, neurodegenerative and cardiovascular diseases, inflammation, and cancer. Researchers have extensively explored how mitochondrial properties and functions are modified in different models, often using fluorescent inner mitochondrial membrane potential (ΔΨm) probes to assess functional mitochondrial aspects such as protonmotive force and oxidative phosphorylation. This review provides an overview of existing techniques to measure ΔpH and ΔΨm, highlighting their advantages, limitations, and applications. It discusses drawbacks of ΔΨm probes, especially when used without calibration, and conditions where alternative methods should replace ΔΨm measurements for the benefit of the specific scientific objectives entailed. Studies investigating mitochondria and their vast biological roles would be significantly advanced by the understanding of the correct applications as well as limitations of protonmotive force measurements and use of fluorescent ΔΨm probes, adopting more precise, artifact-free, sensitive, and quantitative measurements of mitochondrial functionality.

A practical and robust method to evaluate metabolic fluxes in primary pancreatic islets.

OBJECTIVE: Evaluation of mitochondrial oxygen consumption and ATP production is important to investigate pancreatic islet pathophysiology. Most studies use cell lines due to difficulties in measuring primary islet respiration, which requires specific equipment and consumables, is expensive and poorly reproducible. Our aim was to establish a practical method to assess primary islet metabolic fluxes using standard commercial consumables. METHODS: Pancreatic islets were isolated from mice/rats, dispersed with trypsin, and adhered to pre-coated standard Seahorse or Resipher microplates. Oxygen consumption was evaluated using a Seahorse Extracellular Flux Analyzer or a Resipher Real-time Cell Analyzer. RESULTS: We provide a detailed protocol with all steps to optimize islet isolation with high yield and functionality. Our method requires a few islets per replicate; both rat and mouse islets present robust basal respiration and proper response to mitochondrial modulators and glucose. The technique was validated by other functional assays, which show these cells present conserved calcium influx and insulin secretion in response to glucose. We also show that our dispersed islets maintain robust basal respiration levels, in addition to maintaining up to 89% viability after five days in dispersed cultures. Furthermore, OCRs can be measured in Seahorse analyzers and in other plate respirometry systems, using standard materials. CONCLUSIONS: Overall, we established a practical and robust method to assess islet metabolic fluxes and oxidative phosphorylation, a valuable tool to uncover basic ß-cell metabolic mechanisms as well as for translational investigations, such as pharmacological candidate discovery and islet transplantation protocols.

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.

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.

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

Obesity significantly decreases life expectancy and increases the incidence of age-related dysfunctions, including ß-cell dysregulation leading to inadequate insulin secretion. Here, we show that diluted plasma from obese human donors acutely impairs ß-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 ß-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 ß-cell function; indeed, sera from adiponectin knockout mice limits ß-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 ß-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 ß-cells, and suggest strong therapeutic potential for the modulation of the adiponectin signaling pathway in the prevention of age-related ß-cell dysfunction.

Mitochondrial Ca2+ handling as a cell signaling hub: lessons from astrocyte function.

Astrocytes are a heterogenous population of macroglial cells spread throughout the central nervous system with diverse functions, expression signatures, and intricate morphologies. Their subcellular compartments contain a distinct range of mitochondria, with functional microdomains exhibiting widespread activities, such as controlling local metabolism and Ca2+ signaling. Ca2+ is an ion of utmost importance, both physiologically and pathologically, and participates in critical central nervous system processes, including synaptic plasticity, neuron-astrocyte integration, excitotoxicity, and mitochondrial physiology and metabolism. The mitochondrial Ca2+ handling system is formed by the mitochondrial Ca2+ uniporter complex (MCUc), which mediates Ca2+ influx, and the mitochondrial Na+/Ca2+ exchanger (NCLX), responsible for most mitochondrial Ca2+ efflux, as well as additional components, including the mitochondrial permeability transition pore (mtPTP). Over the last decades, mitochondrial Ca2+ handling has been shown to be key for brain homeostasis, acting centrally in physiopathological processes such as astrogliosis, astrocyte-neuron activity integration, energy metabolism control, and neurodegeneration. In this review, we discuss the current state of knowledge regarding the mitochondrial Ca2+ handling system molecular composition, highlighting its impact on astrocytic homeostasis.

Goldilocks calcium concentrations and the regulation of oxidative phosphorylation: Too much, too little, or just right.

Calcium (Ca2+) is a key regulator in diverse intracellular signaling pathways and has long been implicated in metabolic control and mitochondrial function. Mitochondria can actively take up large amounts of Ca2+, thereby acting as important intracellular Ca2+ buffers and affecting cytosolic Ca2+ transients. Excessive mitochondrial matrix Ca2+ is known to be deleterious due to opening of the mitochondrial permeability transition pore (mPTP) and consequent membrane potential dissipation, leading to mitochondrial swelling, rupture, and cell death. Moderate Ca2+ within the organelle, on the other hand, can directly or indirectly activate mitochondrial matrix enzymes, possibly impacting on ATP production. Here, we aimed to determine in a quantitative manner if extra- or intramitochondrial Ca2+ modulates oxidative phosphorylation in mouse liver mitochondria and intact hepatocyte cell lines. To do so, we monitored the effects of more modest versus supraphysiological increases in cytosolic and mitochondrial Ca2+ on oxygen consumption rates. Isolated mitochondria present increased respiratory control ratios (a measure of oxidative phosphorylation efficiency) when incubated with low (2.4 ± 0.6 µM) and medium (22.0 ± 2.4 µM) Ca2+ concentrations in the presence of complex I-linked substrates pyruvate plus malate and α-ketoglutarate, respectively, but not complex II-linked succinate. In intact cells, both low and high cytosolic Ca2+ led to decreased respiratory rates, while ideal rates were present under physiological conditions. High Ca2+ decreased mitochondrial respiration in a substrate-dependent manner, mediated by mPTP. Overall, our results uncover a Goldilocks effect of Ca2+ on liver mitochondria, with specific "just right" concentrations that activate oxidative phosphorylation.

Mitochondrial sodium/calcium exchanger NCLX regulates glycolysis in astrocytes, impacting on cognitive performance.

Intracellular Ca2+ concentrations are strictly controlled by plasma membrane transporters, the endoplasmic reticulum, and mitochondria, in which Ca2+ uptake is mediated by the mitochondrial calcium uniporter complex (MCUc), while efflux occurs mainly through the mitochondrial Na+ /Ca2+ exchanger (NCLX). RNAseq database repository searches led us to identify the Nclx transcript as highly enriched in astrocytes when compared with neurons. To assess the role of NCLX in mouse primary culture astrocytes, we inhibited its function both pharmacologically or genetically. This resulted in re-shaping of cytosolic Ca2+ signaling and a metabolic shift that increased glycolytic flux and lactate secretion in a Ca2+ -dependent manner. Interestingly, in vivo genetic deletion of NCLX in hippocampal astrocytes improved cognitive performance in behavioral tasks, whereas hippocampal neuron-specific deletion of NCLX impaired cognitive performance. These results unveil a role for NCLX as a novel modulator of astrocytic glucose metabolism, impacting on cognition.

Cold Exposure and the Metabolism of Mice, Men, and Other Wonderful Creatures.

Laboratory rodents and cold-adapted animals in the wild use a significant amount of the energy derived from food intake for heat generation. Thermogenesis involving mitochondrial uncoupling in the brown adipose tissue differs quantitatively in mice, humans, and cold-adapted animals and could be an important ally to combat obesity if humans were prepared to deviate slightly from thermoneutral living conditions to activate this pathway.

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 Ca2+ uptake rates and capacities, avoiding Ca2+-induced mitochondrial permeability transition. Dietary restriction has also been shown to increase kidney resistance against damaging stimuli; if these effects are related to similar mitochondrial adaptations has not been uncovered. Here, we characterized changes in mitochondrial function in response to 6 mo of CR in rats and measured bioenergetic parameters, redox balance, and Ca2+ homeostasis. CR promoted an increase in succinate-supported mitochondrial oxygen consumption rates. Although 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 the brain and liver, mitochondria from CR animals were more prone to Ca2+-induced mitochondrial permeability transition, in a manner reversed by the antioxidant dithiothreitol. CR mitochondria also displayed higher Ca2+ uptake rates, which were not accompanied by changes in Ca2+ efflux rates or related to altered inner mitochondrial membrane potentials or amounts of the mitochondrial Ca2+ uniporter. Instead, increased mitochondrial Ca2+ uptake rates in CR kidneys correlated with loss of mitochondrial Ca2+ uptake protein 2 (MICU2), a mitochondrial Ca2+ uniporter modulator. Interestingly, MICU2 is also modulated by CR in the liver, suggesting that it has a broader diet-sensitive regulatory role controlling mitochondrial Ca2+ homeostasis. Together, our results highlight the organ-specific bioenergetic, redox, and ionic transport results of CR, with some unexpected deleterious effects in the kidney.NEW & NOTEWORTHY Prevention of obesity through caloric restriction (CR) is well known to protect many tissues but has been poorly studied in kidneys. Here, we determined the effects of long-term CR in rat kidney mitochondria, which are central players in energy metabolism and aging. Surprisingly, we found that the diet increased mitochondrial reactive oxygen production and permeability transition. This suggests that the kidneys respond differently to restricted diets and may be more susceptible under CR.

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 Pkd1V/V mice, a model unable to cleave this protein and with early cardiac dysfunction. Pkd1V/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-ACCß, CPT1B-Cpt1b, Ppara, Nppa and Acta1. These findings suggested decreased fatty acid ß-oxidation, which was confirmed by lower oxygen consumption by Pkd1V/V isolated mitochondria using palmitoyl-CoA. Pkd1V/V hearts also presented increased oxygen consumption in response to glucose, suggesting that alternative substrates may be used to generate energy. Pkd1V/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, Pkd1V/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.

MS-Driven Metabolic Alterations Are Recapitulated in iPSC-Derived Astrocytes.

OBJECTIVE: Astrocytes play a significant role in the pathology of multiple sclerosis (MS). Nevertheless, for ethical reasons, most studies in these cells were performed using the Experimental Autoimmune Encephalomyelitis model. As there are significant differences between human and mouse cells, we aimed here to better characterize astrocytes from patients with MS (PwMS), focusing mainly on mitochondrial function and cell metabolism. METHODS: We obtained and characterized induced pluripotent stem cell (iPSC)-derived astrocytes from three PwMS and three unaffected controls, and performed electron microscopy, flow cytometry, cytokine and glutamate measurements, gene expression, in situ respiration, and metabolomics. We validated our findings using a single-nuclei RNA sequencing dataset. RESULTS: We detected several differences in MS astrocytes including: (i) enrichment of genes associated with neurodegeneration, (ii) increased mitochondrial fission, (iii) increased production of superoxide and MS-related proinflammatory chemokines, (iv) impaired uptake and enhanced release of glutamate, (v) increased electron transport capacity and proton leak, in line with the increased oxidative stress, and (vi) a distinct metabolic profile, with a deficiency in amino acid catabolism and increased sphingolipid metabolism, which have already been linked to MS. INTERPRETATION: Here we describe the metabolic profile of iPSC-derived astrocytes from PwMS and validate this model as a very powerful tool to study disease mechanisms and to perform non-invasive drug targeting assays in vitro. Our findings recapitulate several disease features described in patients and provide new mechanistic insights into the metabolic rewiring of astrocytes in MS, which could be targeted in future therapeutic studies. ANN NEUROL 2022;91:652-669.

Mitochondrial K+ Transport: Modulation and Functional Consequences.

The existence of a K+ cycle in mitochondria has been predicted since the development of the chemiosmotic theory and has been shown to be crucial for several cellular phenomena, including regulation of mitochondrial volume and redox state. One of the pathways known to participate in K+ cycling is the ATP-sensitive K+ channel, MitoKATP. This channel was vastly studied for promoting protection against ischemia reperfusion when pharmacologically activated, although its molecular identity remained unknown for decades. The recent molecular characterization of MitoKATP has opened new possibilities for modulation of this channel as a mechanism to control cellular processes. Here, we discuss different strategies to control MitoKATP activity and consider how these could be used as tools to regulate metabolism and cellular events.

Increased glycolysis is an early consequence of palmitate lipotoxicity mediated by redox signaling.

Exposure to toxic levels of fatty acids (lipotoxicity) leads to cell damage and death and is involved in the pathogenesis of the metabolic syndrome. Since the metabolic consequences of lipotoxicity are still poorly understood, we studied the bioenergetic effects of the saturated fatty acid palmitate, quantifying changes in mitochondrial morphology, real-time oxygen consumption, ATP production sources, and extracellular acidification in hepatoma cells. Surprisingly, glycolysis was enhanced by the presence of palmitate as soon as 1 h after stimulus, while oxygen consumption and oxidative phosphorylation were unchanged, despite overt mitochondrial fragmentation. Palmitate only induced mitochondrial fragmentation if glucose and glutamine were available, while glycolytic enhancement did not require glutamine, showing it is independent of mitochondrial morphological changes. Redox state was altered by palmitate, as indicated by NAD(P)H quantification. Furthermore, the mitochondrial antioxidant mitoquinone, or a selective inhibitor of complex I electron leakage (S1QEL) further enhanced palmitate-induced glycolysis. Our results demonstrate that palmitate overload and lipotoxicity involves an unexpected and early increase in glycolytic flux, while, surprisingly, no changes in oxidative phosphorylation are observed. Interestingly, enhanced glycolysis involves signaling by mitochondrially-generated oxidants, uncovering a novel regulatory mechanism for this pathway.

Autophagy in Hepatic Steatosis: A Structured Review.

Steatosis is the accumulation of neutral lipids in the cytoplasm. In the liver, it is associated with overeating and a sedentary lifestyle, but may also be a result of xenobiotic toxicity and genetics. Non-alcoholic fatty liver disease (NAFLD) defines an array of liver conditions varying from simple steatosis to inflammation and fibrosis. Over the last years, autophagic processes have been shown to be directly associated with the development and progression of these conditions. However, the precise role of autophagy in steatosis development is still unclear. Specifically, autophagy is necessary for the regulation of basic metabolism in hepatocytes, such as glycogenolysis and gluconeogenesis, response to insulin and glucagon signaling, and cellular responses to free amino acid contents. Also, genetic knockout models for autophagy-related proteins suggest a critical relationship between autophagy and hepatic lipid metabolism, but some results are still ambiguous. While autophagy may seem necessary to support lipid oxidation in some contexts, other evidence suggests that autophagic activity can lead to lipid accumulation instead. This structured literature review aims to critically discuss, compare, and organize results over the last 10 years regarding rodent steatosis models that measured several autophagy markers, with genetic and pharmacological interventions that may help elucidate the molecular mechanisms involved.

Responsible Science Assessment: downplaying indexes, boosting quality.

Scientists are facing enormous pressures posed by growing scientific communities and stagnant/reduced funding. In this scenario, mechanisms of knowledge achievement and management, as well as how recruitment, progression and evaluation are carried out should be reevaluated. We argue here that knowledge has become a profitable commodity and, as a consequence, excessive academic quantification, individual output assessment problems and abusive editorial market strategies have reached unsustainable levels. We propose to reinforce existing guidelines and to establish new ones to overcome these issues. Our proposal, the Initiative for Responsible Scientific Assessment (IRSA), has the main goal to strengthen and expand previous movements in the scientific community to promote higher quality research assessment, focused on better Science.

Unveiling the contribution of the reproductive system of individual Caenorhabditis elegans on oxygen consumption by single-point scanning electrochemical microscopy measurements.

Metabolic analysis in animals is usually either evaluated as whole-body measurements or in isolated tissue samples. To reveal tissue specificities in vivo, this study uses scanning electrochemical microscopy (SECM) to provide localized oxygen consumption rates (OCRs) in different regions of single adult Caenorhabditis elegans individuals. This is achieved by measuring the oxygen reduction current at the SECM tip electrode and using a finite element method model of the experiment that defines oxygen concentration and flux at the surface of the organism. SECM mapping measurements uncover a marked heterogeneity of OCR along the worm, with high respiration rates at the reproductive system region. To enable sensitive and quantitative measurements, a self-referencing approach is adopted, whereby the oxygen reduction current at the SECM tip is measured at a selected point on the worm and in bulk solution (calibration). Using genetic and pharmacological approaches, our SECM measurements indicate that viable eggs in the reproductive system are the main contributors in the total oxygen consumption of adult Caenorhabditis elegans. The finding that large regional differences in OCR exist within the animal provides a new understanding of oxygen consumption and metabolic measurements, paving the way for tissue-specific metabolic analyses and toxicity evaluation within single organisms.