Keep in touch: a perspective on the mitochondrial social network and its implication in health and disease

Mitochondria have been the focus of extensive research for decades since their dysfunction is linked to more than 150 distinct human disorders. Despite considerable efforts, researchers have only been able to skim the surface of the mitochondrial social complexity and the impact of inter-organelle and inter-organ communication alterations on human health. While some progress has been made in deciphering connections among mitochondria and other cytoplasmic organelles through direct (i.e., contact sites) or indirect (i.e., inter-organelle trafficking) crosstalk, most of these efforts have been restricted to a limited number of proteins involved in specific physiological pathways or disease states. This research bottleneck is further narrowed by our incomplete understanding of the cellular alteration timeline in a specific pathology, which prevents the distinction between a primary organelle dysfunction and the defects occurring due to the disruption of the organelle's interconnectivity. In this perspective, we will (i) summarize the current knowledge on the mitochondrial crosstalk within cell(s) or tissue(s) in health and disease, with a particular focus on neurodegenerative disorders, (ii) discuss how different large-scale and targeted approaches could be used to characterize the different levels of mitochondrial social complexity, and (iii) consider how investigating the different expression patterns of mitochondrial proteins in different cell types/tissues could represent an important step forward in depicting the distinctive architecture of inter-organelle communication.

Metformin Disrupts Signaling and Metabolism in Fetal Hepatocytes

Metformin is used by women during pregnancy to manage diabetes and crosses the placenta, yet its effects on the fetus are unclear. We show that the liver is a site of metformin action in fetal sheep and macaques, given relatively abundant OCT1 transporter expression and hepatic uptake following metformin infusion into fetal sheep. To determine the effects of metformin action, we performed studies in primary hepatocytes from fetal sheep, fetal macaques, and juvenile macaques. Metformin increases AMP-activated protein kinase (AMPK) signaling, decreases mammalian target of rapamycin (mTOR) signaling, and decreases glucose production in fetal and juvenile hepatocytes. Metformin also decreases oxygen consumption in fetal hepatocytes. Unique to fetal hepatocytes, metformin activates stress pathways (e.g., increased PGC1A gene expression, NRF-2 protein abundance, and phosphorylation of eIF2α and CREB proteins) alongside perturbations in hepatokine expression (e.g., increased growth/differentiation factor 15 [GDF15] and fibroblast growth factor 21 [FGF21] expression and decreased insulin-like growth factor 2 [IGF2] expression). Similarly, in liver tissue from sheep fetuses infused with metformin in vivo, AMPK phosphorylation, NRF-2 protein, and PGC1A expression are increased. These results demonstrate disruption of signaling and metabolism, induction of stress, and alterations in hepatokine expression in association with metformin exposure in fetal hepatocytes.

ARTICLE HIGHLIGHTS

The major metformin uptake transporter OCT1 is expressed in the fetal liver, and fetal hepatic uptake of metformin is observed in vivo. Metformin activates AMPK, reduces glucose production, and decreases oxygen consumption in fetal hepatocytes, demonstrating similar effects as in juvenile hepatocytes. Unique to fetal hepatocytes, metformin activates metabolic stress pathways and alters the expression of secreted growth factors and hepatokines. Disruption of signaling and metabolism with increased stress pathways and reduced anabolic pathways by metformin in the fetal liver may underlie reduced growth in fetuses exposed to metformin.

LonP1 Links Mitochondria-ER Interaction to Regulate Heart Function

Interorganelle contacts and communications are increasingly recognized to play a vital role in cellular function and homeostasis. In particular, the mitochondria-endoplasmic reticulum (ER) membrane contact site (MAM) is known to regulate ion and lipid transfer, as well as signaling and organelle dynamics. However, the regulatory mechanisms of MAM formation and their function are still elusive. Here, we identify mitochondrial Lon protease (LonP1), a highly conserved mitochondrial matrix protease, as a new MAM tethering protein. The removal of LonP1 substantially reduces MAM formation and causes mitochondrial fragmentation. Furthermore, deletion of LonP1 in the cardiomyocytes of mouse heart impairs MAM integrity and mitochondrial fusion and activates the unfolded protein response within the ER (UPR^ER). Consequently, cardiac-specific LonP1 deficiency causes aberrant metabolic reprogramming and pathological heart remodeling. These findings demonstrate that LonP1 is a novel MAM-localized protein orchestrating MAM integrity, mitochondrial dynamics, and UPR^ER, offering exciting new insights into the potential therapeutic strategy for heart failure.

Histone deacetylase 6 inhibition restores leptin sensitivity and reduces obesity

The adipose tissue-derived hormone leptin can drive decreases in food intake while increasing energy expenditure. In diet-induced obesity, circulating leptin levels rise proportionally to adiposity. Despite this hyperleptinemia, rodents and humans with obesity maintain increased adiposity and are resistant to leptin's actions. Here we show that inhibitors of the cytosolic enzyme histone deacetylase 6 (HDAC6) act as potent leptin sensitizers and anti-obesity agents in diet-induced obese mice. Specifically, HDAC6 inhibitors, such as tubastatin A, reduce food intake, fat mass, hepatic steatosis and improve systemic glucose homeostasis in an HDAC6-dependent manner. Mechanistically, peripheral, but not central, inhibition of HDAC6 confers central leptin sensitivity. Additionally, the anti-obesity effect of tubastatin A is attenuated in animals with a defective central leptin-melanocortin circuitry, including db/db and MC4R knockout mice. Our results suggest the existence of an HDAC6-regulated adipokine that serves as a leptin-sensitizing agent and reveals HDAC6 as a potential target for the treatment of obesity.

Anterograde regulation of mitochondrial genes and FGF21 signaling by hepatic LSD1

Mitochondrial biogenesis and function are controlled by anterograde regulatory pathways involving more than 1000 nuclear-encoded proteins. Transcriptional networks controlling the nuclear-encoded mitochondrial genes remain to be fully elucidated. Here, we show that histone demethylase LSD1 KO from adult mouse liver (LSD1-LKO) reduces the expression of one-third of all nuclear-encoded mitochondrial genes and decreases mitochondrial biogenesis and function. LSD1-modulated histone methylation epigenetically regulates nuclear-encoded mitochondrial genes. Furthermore, LSD1 regulates gene expression and protein methylation of nicotinamide mononucleotide adenylyltransferase 1 (NMNAT1), which controls the final step of NAD+ synthesis and limits NAD+ availability in the nucleus. Lsd1 KO reduces NAD+-dependent SIRT1 and SIRT7 deacetylase activity, leading to hyperacetylation and hypofunctioning of GABPβ and PGC-1α, the major transcriptional factor/cofactor for nuclear-encoded mitochondrial genes. Despite the reduced mitochondrial function in the liver, LSD1-LKO mice are protected from diet-induced hepatic steatosis and glucose intolerance, partially due to induction of hepatokine FGF21. Thus, LSD1 orchestrates a core regulatory network involving epigenetic modifications and NAD+ synthesis to control mitochondrial function and hepatokine production.

OPA1 deletion in brown adipose tissue improves thermoregulation and systemic metabolism via FGF21

Adrenergic stimulation of brown adipocytes alters mitochondrial dynamics, including the mitochondrial fusion protein optic atrophy 1 (OPA1). However, direct mechanisms linking OPA1 to brown adipose tissue (BAT) physiology are incompletely understood. We utilized a mouse model of selective OPA1 deletion in BAT (OPA1 BAT KO) to investigate the role of OPA1 in thermogenesis. OPA1 is required for cold-induced activation of thermogenic genes in BAT. Unexpectedly, OPA1 deficiency induced fibroblast growth factor 21 (FGF21) as a BATokine in an activating transcription factor 4 (ATF4)-dependent manner. BAT-derived FGF21 mediates an adaptive response by inducing browning of white adipose tissue, increasing resting metabolic rates, and improving thermoregulation. However, mechanisms independent of FGF21, but dependent on ATF4 induction, promote resistance to diet-induced obesity in OPA1 BAT KO mice. These findings uncover a homeostatic mechanism of BAT-mediated metabolic protection governed in part by an ATF4-FGF21 axis, which is activated independently of BAT thermogenic function.

Autophagy and cancer cell metabolism

Autophagy is an ancient catabolic process used by cells to clear excess or dysfunctional organelles and large subcellular structures and thus performs an important housekeeping role for the cell. Autophagy is acutely sensitive to nutrient availability and is upregulated at a transcriptional and posttranslational level in response to nutrient deprivation. This serves to promote turnover of cellular content and recycling of nutrients for continued growth and survival. While important for most normal tissues, tumor cells appear to be particularly dependent on autophagy for survival under ischemic or therapeutic stress, and in response to loss of matrix attachment; autophagy is upregulated markedly in cancers as they progress to malignancy. Ras-driven tumors appear to be particularly dependent on autophagy and thus inhibition of autophagy is being pursued as a productive clinical approach for such cancers. However, this enthusiasm needs to be offset against possible negative effects of autophagy inhibition on normal tissue function and on limiting antitumor immune responses. In addressing all of these topics, we focus in on understanding how autophagy is induced by nutrient stress, its role in recycling metabolites for growing tumors, how selective forms of autophagy, such as mitophagy and ribophagy contribute specifically to tumorigenesis, how autophagy in the tumor microenvironment and throughout the animal affects access of the tumor to nutrients, and finally how different oncogenic pathways may determine which tumors respond to autophagy inhibition and which ones will not.

Intestinal serine protease inhibition increases FGF21 and improves metabolism in obese mice

Trypsin is the major serine protease responsible for intestinal protein digestion. An inhibitor, camostat (CS), reduced weight gain, hyperglycemia, and dyslipidemia in obese rats; however, the mechanisms for these are largely unknown. We reasoned that CS creates an apparent dietary protein restriction, which is known to increase hepatic fibroblast growth factor 21 (FGF21). Therefore, metabolic responses to CS and a gut-restricted CS metabolite, FOY-251, were measured in mice. Food intake, body weight, blood glucose, branched-chain amino acids (LC/MS), hormone levels (ELISA), liver pathology (histology), and transcriptional changes (qRT-PCR) were measured in ob/ob, lean and diet-induced obese (DIO) C57BL/6 mice. In ob/ob mice, CS in chow (9-69 mg/kg) or FOY-251 (46 mg/kg) reduced food intake and body weight gain to a similar extent as pair-fed mice. CS decreased blood glucose, liver weight, and lipidosis and increased FGF21 gene transcription and plasma levels. In lean mice, CS increased liver FGF21 mRNA and plasma levels. Relative to pair feeding, FOY-251 also increased plasma FGF21 and induced liver FGF21 and integrated stress response (ISR) transcription. In DIO mice, FOY-251 (100 mg/kg po) did not alter peak glucose levels but reduced the AUC of the glucose excursion in response to an oral glucose challenge. FOY-251 increased plasma FGF21 levels. In addition to previously reported satiety-dependent (cholecystokinin-mediated) actions, intestinal trypsin inhibition engages non-satiety-related pathways in both leptin-deficient and DIO mice. This novel mechanism improves metabolism by a liver-integrated stress response and increased FGF21 expression levels in mice. NEW & NOTEWORTHY Trypsin inhibitors, including plant-based consumer products, have long been associated with metabolic improvements. Studies in the 1980s and 1990s suggested this was due to satiety hormones and caloric wasting by loss of protein and fatty acids in feces. This work suggests an entirely new mechanism based on the lower amounts of digested protein available in the gut. This apparent protein reduction may cause beneficial metabolic adaptation by the intestinal-liver axis to perceived nutrient stress.

Fibroblast growth factor 21 as a biomarker for long-term complications in organic acidemias

BACKGROUND

There is increasing evidence that long-term complications in organic acidemias are caused by impaired mitochondrial metabolism. Currently, there is no specific biomarker to monitor mitochondrial dysfunction in organic acidemias. Serum fibroblast growth factor 21 (FGF-21) is a biomarker for mitochondrial disease and could be a candidate to monitor mitochondrial function in the deleterious course of disease.

METHODS

Data of 17 patients with classical organic acidemias (11 propionic acidemia (PA), four methylmalonic acidemia (MMA) and two isovaleric acidemia (IVA) patients) were included. The clinical course was evaluated; metabolic decompensations and long-term complications were correlated with plasma FGF-21 levels. Cardiomyopathy, prolonged QT interval, renal failure, and optic neuropathy were defined as long-term complications.

RESULTS

Patients ages ranged from 16 months up to 32 years. Serious long-term complications occurred in eight patients (five PA and three MMA patients). In MMA and PA patients plasma FGF-21 levels during stable metabolic periods were significantly higher in patients with long-term complications (Mdn = 2556.0 pg/ml) compared to patients without (Mdn = 287.0 pg/ml). A median plasma FGF-21 level above 1500 pg/ml during a stable metabolic period, measured before the occurrence of long-term complications, had a positive predictive value of 0.83 and a negative predictive value of 1.00 on long-term complications in MMA and PA patients.

CONCLUSION

This study demonstrates the potential role of FGF-21 as a biomarker for long-term complications in classical organic acidemias, attributed to mitochondrial dysfunction.

Deciphering the metabolic secret of longevity through the analysis of metabolic response to stress on long-lived species

Despite intensive research, no satisfactory therapeutic options have been found for aging and age-related diseases. The British scientist Leslie Orgel stated that evolution is cleverer than we are. This assumption seems correct considering that some species are naturally able to resist the age-related diseases that remain unsolved by our modern medicine. Indeed, bowhead whales can live for more than two hundred years and are suspected to possess efficient antitumor mechanisms. Naked mole-rats are exceptionally long-lived compared to similar-sized mammals and are protected from senescence and age-related diseases. Consequently, the characterization of protective molecular mechanisms in long-lived species (i.e. bowhead whale, naked mole-rat, microbat) could be of great interest for therapeutic applications in human. Cellular stress response is considered to be an anti-aging process dedicated to the prevention of damage accumulation and the maintenance of homeostasis. Interestingly, cellular stress response in plants and animals involves the production of health-promoting metabolites such as resveratrol, nicotinamide adenine dinucleotide and spermidine. Do anti-aging metabolites formed during stress exposure differ between human and extreme longevity species in terms of their nature, their quantity or their production? These questions remain unsolved and deserve to be considered. Indeed, the mimicking of anti-aging strategies selected throughout evolution in long-lived species could be of high therapeutic value for humans. This paper suggests that metabolomic studies on extreme longevity species cells exposed to mild stressors may lead to the characterization of health-promoting metabolites. If confirmed, this would provide new avenues of research for the development of innovative anti-aging strategies for humans.

The effect of Tmem135 overexpression on the mouse heart

Tissues with high-energy demand including the heart are rich in the energy-producing organelles, mitochondria, and sensitive to mitochondrial dysfunction. While alterations in mitochondrial function are increasingly recognized in cardiovascular diseases, the molecular mechanisms through which changes in mitochondria lead to heart abnormalities have not been fully elucidated. Here, we report that transgenic mice overexpressing a novel regulator of mitochondrial dynamics, transmembrane protein 135 (Tmem135), exhibit increased fragmentation of mitochondria and disease phenotypes in the heart including collagen accumulation and hypertrophy. The gene expression analysis showed that genes associated with ER stress and unfolded protein response, and especially the pathway involving activating transcription factor 4, are upregulated in the heart of Tmem135 transgenic mice. It also showed that gene expression changes in the heart of Tmem135 transgenic mice significantly overlap with those of aged mice in addition to the similarity in cardiac phenotypes, suggesting that changes in mitochondrial dynamics may be involved in the development of heart abnormalities associated with aging. Our study revealed the pathological consequence of overexpression of Tmem135, and suggested downstream molecular changes that may underlie those disease pathologies.

The Mitochondrial Basis of Aging and Age-Related Disorders

Aging is a natural phenomenon characterized by progressive decline in tissue and organ function leading to increased risk of disease and mortality. Among diverse factors that contribute to human aging, the mitochondrial dysfunction has emerged as one of the key hallmarks of aging process and is linked to the development of numerous age-related pathologies including metabolic syndrome, neurodegenerative disorders, cardiovascular diseases and cancer. Mitochondria are central in the regulation of energy and metabolic homeostasis, and harbor a complex quality control system that limits mitochondrial damage to ensure mitochondrial integrity and function. The intricate regulatory network that balances the generation of new and removal of damaged mitochondria forms the basis of aging and longevity. Here, I will review our current understanding on how mitochondrial functional decline contributes to aging, including the role of somatic mitochondrial DNA (mtDNA) mutations, reactive oxygen species (ROS), mitochondrial dynamics and quality control pathways. I will further discuss the emerging evidence on how dysregulated mitochondrial dynamics, mitochondrial biogenesis and turnover mechanisms contribute to the pathogenesis of age-related disorders. Strategies aimed to enhance mitochondrial function by targeting mitochondrial dynamics, quality control, and mitohormesis pathways might promote healthy aging, protect against age-related diseases, and mediate longevity.

Mitotherapy for Fatty Liver by Intravenous Administration of Exogenous Mitochondria in Male Mice

Mitochondrial dysfunction is a major and common mechanism in developing non-alcoholic fatty liver disease (NAFLD). Replacement of dysfunctional mitochondria by functional exogenous mitochondria may attenuate intrahepatic excessive lipid and recover hepatocyte function. However, no data shows that mitochondria can be systemically administrated to animals to date. Here we suggest that mitochondria isolated from hepatoma cells are used as a mitotherapy agent to treat mouse fatty liver induced by high-fat diets. When the mitochondria were intravenously injected into the mice, serum aminotransferase activity and cholesterol level decreased in a dose-dependent manner. Also, the mitotherapy reduced lipid accumulation and oxidation injury of the fatty liver mice, improved energy production, and consequently restored hepatocyte function. The mitotherapy strategy offers a new potential therapeutic approach for treating NAFLD.

A Role for Photobiomodulation in the Prevention of Myocardial Ischemic Reperfusion Injury: A Systematic Review and Potential Molecular Mechanisms

Myocardial ischemia reperfusion injury is a negative pathophysiological event that may result in cardiac cell apoptosis and is a result of coronary revascularization and cardiac intervention procedures. The resulting loss of cardiomyocyte cells and the formation of scar tissue, leads to impaired heart function, a major prognostic determinant of long-term cardiac outcomes. Photobiomodulation is a novel cardiac intervention that has displayed therapeutic effects in reducing myocardial ischemia reperfusion related myocardial injury in animal models. A growing body of evidence supporting the use of photobiomodulation in myocardial infarct models has implicated multiple molecular interactions. A systematic review was conducted to identify the strength of the evidence for the therapeutic effect of photobiomodulation and to summarise the current evidence as to its mechanisms. Photobiomodulation in animal models showed consistently positive effects over a range of wavelengths and application parameters, with reductions in total infarct size (up to 76%), decreases in inflammation and scarring, and increases in tissue repair. Multiple molecular pathways were identified, including modulation of inflammatory cytokines, signalling molecules, transcription factors, enzymes and antioxidants. Current evidence regarding the use of photobiomodulation in acute and planned cardiac intervention is at an early stage but is sufficient to inform on clinical trials.

MOTS-c: A novel mitochondrial-derived peptide regulating muscle and fat metabolism

Mitochondria are ancient organelles that are thought to have emerged from once free-living α-proto-bacteria. As such, they still possess several bacterial-like qualities, including a semi-autonomous genetic system, complete with an independent genome and a unique genetic code. The bacterial-like circular mitochondrial DNA (mtDNA) has been described to encode 37 genes, including 22 tRNAs, 2 rRNAs, and 13 mRNAs. Two additional peptides reported to originate from the mtDNA, namely humanin (Hashimoto et al., 2001; Ikone et al., 2003; Guo et al., 2003) [1-3] and MOTS-c (mitochondrial ORF of the twelve S c) (Lee et al., 2015) [4], indicate a larger mitochondrial genetic repertoire (Shokolenko and Alexeyev, 2015) [5]. These mitochondrial-derived peptides (MDPs) have profound and distinct biological activities and provide a paradigm-shifting concept of active mitochondrial-encoded signals that act at the cellular and organismal level (i.e. mitochondrial hormone) (da Cunha et al., 2015; Quiros et al., 2016) [6,7]. Considering that mitochondria are the single most important metabolic organelle, it is not surprising that these MDPs have metabolic actions. MOTS-c has been shown to target the skeletal muscle and enhance glucose metabolism. As such, MOTS-c has implications in the regulation of obesity, diabetes, exercise, and longevity, representing an entirely novel mitochondrial signaling mechanism to regulate metabolism within and between cells.

Hepatic Fgf21 Expression Is Repressed after Simvastatin Treatment in Mice

Fibroblast growth factor 21 (Fgf21) is a hormone with emerging beneficial roles in glucose and lipid homeostasis. The interest in Fgf21 as a potential antidiabetic drug and the factors that regulate its production and secretion is growing. Statins are the most widely prescribed drug for the treatment of dyslipidemia. However, the function of statins is not limited to the lowering of cholesterol as they are associated with pleiotropic actions such as antioxidant, anti-inflammatory and cytoprotective effects. The recently described effect of statins on mitochondrial function and the induction of Fgf21 by mitochondrial stress prompted us to investigate the effect of statin treatment on Fgf21 expression in the liver. To this end, C57BL6J male mice and primary mouse hepatocytes were treated with simvastatin, and Fgf21 expression was subsequently assessed by immunoblotting and quantitative real-time PCR. Hepatic Fgf21 protein and mRNA and circulating levels of FGF21significantly decreased in mice that had received simvastatin in their food (0.1% w/w) for 1 week. This effect was also observed with simvastatin doses as low as 0.01% w/w for 1 week or following 2 intraperitoneal injections within a single day. The reduction in Fgf21 mRNA levels was further verified in primary mouse hepatocytes, indicating that the effect of simvastatin is cell autonomous. In conclusion, simvastatin treatment reduced the circulating and hepatic Fgf21 levels and this effect warrants further investigation with reference to its role in metabolism.

X-linked intellectual disability gene CASK regulates postnatal brain growth in a non-cell autonomous manner

The phenotypic spectrum among girls with heterozygous mutations in the X-linked intellectual disability (XLID) gene CASK (calcium/calmodulin-dependent serine protein kinase) includes postnatal microcephaly, ponto-cerebellar hypoplasia, seizures, optic nerve hypoplasia, growth retardation and hypotonia. Although CASK knockout mice were previously reported to exhibit perinatal lethality and a 3-fold increased apoptotic rate in the brain, CASK deletion was not found to affect neuronal physiology and their electrical properties. The pathogenesis of CASK associated disorders and the potential function of CASK therefore remains unknown. Here, using Cre-LoxP mediated gene excision experiments; we demonstrate that deleting CASK specifically from mouse cerebellar neurons does not alter the cerebellar architecture or function. We demonstrate that the neuron-specific deletion of CASK in mice does not cause perinatal lethality but induces severe recurrent epileptic seizures and growth retardation before the onset of adulthood. Furthermore, we demonstrate that although neuron-specific haploinsufficiency of CASK is inconsequential, the CASK mutation associated human phenotypes are replicated with high fidelity in CASK heterozygous knockout female mice (CASK^(+/-)). These data suggest that CASK-related phenotypes are not purely neuronal in origin. Surprisingly, the observed microcephaly in CASK^(+/-) animals is not associated with a specific loss of CASK null brain cells indicating that CASK regulates postnatal brain growth in a non-cell autonomous manner. Using biochemical assay, we also demonstrate that CASK can interact with metabolic proteins. CASK knockdown in human cell lines cause reduced cellular respiration and CASK^(+/-) mice display abnormalities in muscle and brain oxidative metabolism, suggesting a novel function of CASK in metabolism. Our data implies that some phenotypic components of CASK heterozygous deletion mutation associated disorders represent systemic manifestation of metabolic stress and therefore amenable to therapeutic intervention.

Transcriptional activator TAp63 is upregulated in muscular atrophy during ALS and induces the pro-atrophic ubiquitin ligase Trim63

Mechanisms of muscle atrophy are complex and their understanding might help finding therapeutic solutions for pathologies such as amyotrophic lateral sclerosis (ALS). We meta-analyzed transcriptomic experiments of muscles of ALS patients and mouse models, uncovering a p53 deregulation as common denominator. We then characterized the induction of several p53 family members (p53, p63, p73) and a correlation between the levels of p53 family target genes and the severity of muscle atrophy in ALS patients and mice. In particular, we observed increased p63 protein levels in the fibers of atrophic muscles via denervation-dependent and -independent mechanisms. At a functional level, we demonstrated that TAp63 and p53 transactivate the promoter and increased the expression of Trim63 (MuRF1), an effector of muscle atrophy. Altogether, these results suggest a novel function for p63 as a contributor to muscular atrophic processes via the regulation of multiple genes, including the muscle atrophy gene Trim63.