Mfn2 deficiency links age-related sarcopenia and impaired autophagy to activation of an adaptive mitophagy pathway

Hallmarks of Aging: An Autophagic Perspective

Autophagy is a major protein turnover pathway by which cellular components are delivered into the lysosomes for degradation and recycling. This intracellular process is able to maintain cellular homeostasis under stress conditions, and its dysregulation could lead to the development of physiological alterations. The autophagic activity has been found to decrease with age, likely contributing to the accumulation of damaged macromolecules and organelles during aging. Interestingly, failure of the autophagic process has been reported to worsen aging-associated diseases, such as neurodegeneration or cancer, among others. Likewise, it has been proposed in different organisms that maintenance of a proper autophagic activity contributes to extending longevity. In this review, we discuss recent papers showing the impact of autophagy on cell activity and age-associated diseases, highlighting the relevance of this process to the hallmarks of aging. Thus, understanding how autophagy plays an important role in aging opens new avenues for the discovery of biochemical and pharmacological targets and the development of novel anti-aging therapeutic approaches.

Intensified mitophagy in skeletal muscle with aging is downregulated by PGC-1alpha overexpression in vivo

Mitochondrial dysfunction plays an important role in the etiology of age-related muscle atrophy known as sarcopenia. PGC-1α is positioned at the center of crosstalk in regulating mitochondrial quality control, but its role in mitophagy in aged skeletal muscle is currently unclear. The present study investigated the effects of aging and PGC-1α overexpression via in vivo DNA transfection on key mitophagy protein markers, as well as mitochondrial dynamics related proteins, metabolic function and antioxidant capacity in mouse muscle. C57BL/6J mice at the age of 2 mo (young, Y; N = 14) and 24 mo (old, O; N = 14) were transfected in vivo with either PGC-1α DNA (OE, N = 7) or GFP (N = 7) into the tibialis anterior (TA) muscle followed by electroporation. PINK1 and Parkin protein contents were 3.6 and 1.4-fold higher (P < 0.01), whereas mitochondrial ubiquitination (Ub) increased 1.5-fold (P < 0.05), in O vs. Y mice. PGC-1 OE suppressed PINK and Parkin protein levels by 50-60% (P < 0.01), and decreased Ub by 20% (P < 0.05) in old mice. Aging significantly increased the protein content of LC3II (30%, P < 0.05), p62 (42%, P < 0.05), RheB (5.5-fold, P < 0.01), Beclin-1 (3-fold, P < 0.01) and Mfn2 (~4-fold, P < 0.01) in the TA muscle. However, these age-related increases in mitophagy markers were attenuated by PGC-1α OE. Furthermore, aging dramatically increased Fis-1 protein content by 14-fold (P < 0.01), along with a severe reduction of citrate synthase activity (64%, P < 0.01) and cytochrome c oxidase subunit IV (COXIV) protein content (85%, P < 0.01). PGC-1α OE mitigated the age effects on Fis-1 and Drp-1 (P < 0.05). Moreover, PGC-1α OE enhanced mitochondrial oxidative function and antioxidant enzyme activities, and decreased lipid peroxidation and inner membrane damage found in old mice (P < 0.01). In summary, our data demonstrate that mitophagy protein expression in skeletal muscle was enhanced at old age driven possibly by increased mitochondrial dysfunction, damage, and fission. PGC-1α OE was effective in ameliorating mitochondrial deficits but did not restore muscle fiber atrophy.

Targeting mitochondrial quality control for treating sarcopenia: lessons from physical exercise

INTRODUCTION

Mitochondrial dysfunction is a hallmark of aging and hence is a candidate target for intervention. Sarcopenia of aging is a prevalent condition and is associated with numerous negative health outcomes. Alterations in mitochondrial homeostasis have been reported in sarcopenic muscle. Area covered: We discuss the evidence that points to mitochondrial dysfunction having a causative role in sarcopenia and the mechanisms involved in the accumulation of damaged mitochondria in the aged muscle. We also discuss the effects of physical exercise on mitochondrial quality control and muscle health in advanced age. Expert opinion: In the aged muscle, the mitochondrial quality control axis is altered at several levels, including proteostasis, biogenesis, dynamics, and autophagy. Mitochondrial dysfunction arising from impaired quality control is thought to play a major role in the pathogenesis of sarcopenia. Physical exercise is the most effective strategy for the management of sarcopenia. Improvements in mitochondrial health and plasticity may mediate several beneficial effects of exercise in muscle. A greater understanding of the molecular changes that occur in the aged muscle following exercise and how they impact mitochondrial homeostasis is necessary for the exploration of potential targets that are amenable for interventions.

Mitochondrial quality orchestrates muscle-adipose dialog to alleviate dietary obesity

Skeletal muscle fitness is vital for human health and disease and is determined by the capacity for burning fuel, mitochondrial ATP production, and contraction. High quality mitochondria in skeletal muscle are essential for maintaining energy homeostasis in response to a myriad of physiologic or pathophysiological stresses. A sophisticated mitochondrial quality control system including mitochondrial autophagy, dynamics, and proteolysis has been identified, which maintains their functional integrity. In this review, we discuss recent studies highlighting mitochondrial quality control mechanisms that govern systemic metabolism by skeletal muscles. Increasing evidence suggests that mitochondria can "communicate" with the nucleus and triggers adaptive genomic re-programming during stress response. We focus on participation of the mitochondrial quality control system in the regulation of mitochondrial communications that drive the muscle to adipose dialog and suggest that muscle-specific regulation of mitochondrial quality impacts systemic homeostasis.

Mitochondrial Dynamics in Stem Cells and Differentiation

Mitochondria are highly dynamic organelles that continuously change their shape. Their main function is adenosine triphosphate (ATP) production; however, they are additionally involved in a variety of cellular phenomena, such as apoptosis, cell cycle, proliferation, differentiation, reprogramming, and aging. The change in mitochondrial morphology is closely related to the functionality of mitochondria. Normal mitochondrial dynamics are critical for cellular function, embryonic development, and tissue formation. Thus, defects in proteins involved in mitochondrial dynamics that control mitochondrial fusion and fission can affect cellular differentiation, proliferation, cellular reprogramming, and aging. Here, we review the processes and proteins involved in mitochondrial dynamics and their various associated cellular phenomena.

Oxidative muscles have better mitochondrial homeostasis than glycolytic muscles throughout life and maintain mitochondrial function during aging

Preservation of mitochondrial function, which is dependent on mitochondrial homeostasis (biogenesis, dynamics, disposal/recycling), is critical for maintenance of skeletal muscle function. Skeletal muscle performance declines upon aging (sarcopenia) and is accompanied by decreased mitochondrial function in fast-glycolytic muscles. Oxidative metabolism promotes mitochondrial homeostasis, so we investigated whether mitochondrial function is preserved in oxidative muscles. We compared tibialis anterior (predominantly glycolytic) and soleus (oxidative) muscles from young (3 mo) and old (28-29 mo) C57BL/6J mice. Throughout life, the soleus remained more oxidative than the tibialis anterior and expressed higher levels of markers of mitochondrial biogenesis, fission/fusion and autophagy. The respiratory capacity of mitochondria isolated from the tibialis anterior, but not the soleus, declined upon aging. The soleus and tibialis anterior exhibited similar aging-associated changes in mitochondrial biogenesis, fission/fusion, disposal and autophagy marker expression, but opposite changes in fiber composition: the most oxidative fibers declined in the tibialis anterior, while the more glycolytic fibers declined in the soleus. In conclusion, oxidative muscles are protected from mitochondrial aging, probably due to better mitochondrial homeostasis ab initio and aging-associated changes in fiber composition. Exercise training aimed at enriching oxidative fibers may be valuable in preventing mitochondria-related aging and its contribution to sarcopenia.

The Causal Role of Mitochondrial Dynamics in Regulating Insulin Resistance in Diabetes: Link through Mitochondrial Reactive Oxygen Species

Background

Mitochondrial dynamics (mtDYN) has been proposed as a bridge between mitochondrial dysfunction and insulin resistance (IR), which is involved in the pathogenesis of type 2 diabetes (T2D). Our previous study has identified that mitochondrial DNA (mtDNA) haplogroup B4 is a T2D-susceptible genotype. Using transmitochondrial cybrid model, we have confirmed that haplogroup B4 contributes to cellular IR as well as a profission mtDYN, which can be reversed by antioxidant treatment. However, the causal relationship between mtDYN and cellular IR pertaining to T2D-susceptible haplogroup B4 remains unanswered.

Methods

To dissect the mechanisms between mtDYN and IR, knockdown or overexpression of MFN1, MFN2, DRP1, and FIS1 was performed using cybrid B4. We then examined the mitochondrial network and mitochondrial oxidative stress (mtROS) as well as insulin signaling IRS-AKT pathway and glucose transporters (GLUT) translocation to plasma membrane stimulated by insulin. We employed Drp1 inhibitor, mdivi-1, to interfere with endogenous expression of fission to validate the pharmacological effects on IR.

Results

Overexpression of MFN1 or MFN2 increased mitochondrial network and reduced mtROS, while knockdown had an opposing effect. In contrast, overexpression of DRP1 or FIS1 decreased mitochondrial network and increased mtROS, while knockdown had an opposing effect. Concomitant with the enhanced mitochondrial network, activation of the IRS1-AKT pathway and GLUT translocation stimulated by insulin were improved. On the contrary, suppression of mitochondrial network caused a reduction of the IRS1-AKT pathway and GLUT translocation stimulated by insulin. Pharmacologically inhibiting mitochondrial fission by the Drp1 inhibitor, mdivi-1, also rescued mitochondrial network, reduced mtROS, and improved insulin signaling of diabetes-susceptible cybrid cells.

Conclusion

Our results discovered the causal role of mtDYN proteins in regulating IR resulted from diabetes-susceptible mitochondrial haplogroup. The existence of a bidirectional interaction between mtDYN and mtROS plays an important role. Direct intervention to reverse profission in mtDYN provides a novel therapeutic strategy for IR and T2D.

Restoration of SERCA ATPase prevents oxidative stress-related muscle atrophy and weakness

Molecular targets to reduce muscle weakness and atrophy due to oxidative stress have been elusive. Here we show that activation of Sarcoplasmic Reticulum (SR) Ca²⁺ ATPase (SERCA) with CDN1163, a novel small molecule allosteric SERCA activator, ameliorates the muscle impairment in the CuZnSOD deficient (Sod1^-/-) mouse model of oxidative stress. Sod1^-/- mice are characterized by reduced SERCA activity, muscle weakness and atrophy, increased oxidative stress and mitochondrial dysfunction. Seven weeks of CDN1163 treatment completely restored SERCA activity and reversed the 23% reduction in gastrocnemius mass and 22% reduction in specific force in untreated Sod1^-/- versus wild type mice. These changes were accompanied by restoration of autophagy protein markers to the levels found in wild-type mice. CDN1163 also reversed the increase in mitochondrial ROS generation and oxidative damage in muscle tissue from Sod1^-/- mice. Taken together our findings suggest that the pharmacological restoration of SERCA is a promising therapeutic approach to counter oxidative stress-associated muscle impairment.

Adult skeletal muscle deletion of Mitofusin 1 and 2 impedes exercise performance and training capacity

Endurance exercise has been shown to be a positive regulator of skeletal muscle metabolic function. Changes in mitochondrial dynamics (fusion and fission) have been shown to influence mitochondrial oxidative capacity. We therefore tested whether genetic disruption of mitofusins (Mfns) affected exercise performance in adult skeletal muscle. We generated adult-inducible skeletal muscle-specific Mfn1 (iMS-Mfn1KO), Mfn2 (iMS-Mfn2KO), and Mfn1/2 (iMS-MfnDKO) knockout mice. We assessed exercise capacity by performing a treadmill time to exhaustion stress test before deletion and up to 8 wk after deletion. Analysis of either the iMS-Mfn1KO or the iMS-Mfn2KO did not reveal an effect on exercise capacity. However, analysis of iMS-MfnDKO animals revealed a progressive reduction in exercise performance. We measured individual electron transport chain (ETC) complex activity and observed a reduction in ETC activity in both the subsarcolemmal and intermyofibrillar mitochondrial fractions specifically for NADH dehydrogenase (complex I) and cytochrome- c oxidase (complex IV), which was associated with a decrease in ETC subunit expression for these complexes. We also tested whether voluntary exercise training would prevent the decrease in exercise capacity observed in iMS-MfnDKO animals ( n = 10/group). However, after 8 wk of training we did not observe any improvement in exercise capacity or ETC subunit parameters in iMS-MfnDKO animals. These data suggest that the decrease in exercise capacity observed in the iMS-MfnDKO animals is in part the result of impaired ETC subunit expression and ETC complex activity. Taken together, these results provide strong evidence that mitochondrial fusion in adult skeletal muscle is important for exercise performance. NEW & NOTEWORTHY This study is the first to utilize an adult-inducible skeletal muscle-specific knockout model for Mitofusin (Mfn)1 and Mfn2 to assess exercise capacity. Our findings reveal a progressive decrease in exercise performance with Mfn1 and Mfn2 deletion. The decrease in exercise capacity was accompanied by impaired oxidative phosphorylation specifically for complex I and complex IV. Furthermore, voluntary exercise training was unable to rescue the impairment, suggesting that normal fusion is essential for exercise-induced mitochondrial adaptations.

Parkin is required for exercise-induced mitophagy in muscle: impact of aging

The maintenance of muscle health with advancing age is dependent on mitochondrial homeostasis. While reductions in mitochondrial biogenesis have been observed with age, less is known regarding organelle degradation. Parkin is an E3 ubiquitin ligase implicated in mitophagy, but few studies have examined Parkin's contribution to mitochondrial turnover in muscle. Wild-type (WT) and Parkin knockout (KO) mice were used to delineate a role for Parkin-mediated mitochondrial degradation in aged muscle, in concurrence with exercise. Aged animals exhibited declines in muscle mass and mitochondrial content, paralleled by a nuclear environment endorsing the transcriptional repression of mitochondrial biogenesis. Mitophagic signaling was enhanced following acute endurance exercise in young WT mice but was abolished in the absence of Parkin. Basal mitophagy flux of the autophagosomal protein lipidated microtubule-associated protein 1A/1B-light chain 3 was augmented in aged animals but did not increase additionally with exercise when compared with young animals. In the absence of Parkin, exercise increased the nuclear localization of Parkin-interacting substrate, corresponding to a decrease in nuclear peroxisome proliferator gamma coactivator-1α. Remarkably, exercise enhanced mitochondrial ubiquitination in both young WT and KO animals. This suggested compensation of alternative ubiquitin ligases that were, however, unable to restore the diminished exercise-induced mitophagy in KO mice. Under basal conditions, we demonstrated that Parkin was required for mitochondrial mitofusin-2 ubiquitination. We also observed an abrogation of exercise-induced mitophagy in aged muscle. Our results demonstrate that acute exercise-induced mitophagy is dependent on Parkin and attenuated with age, which likely contributes to changes in mitochondrial content and quality in aging muscle.

Skeletal muscle mitochondrial remodeling in exercise and diseases

Skeletal muscle fitness and plasticity is an important determinant of human health and disease. Mitochondria are essential for maintaining skeletal muscle energy homeostasis by adaptive re-programming to meet the demands imposed by a myriad of physiologic or pathophysiological stresses. Skeletal muscle mitochondrial dysfunction has been implicated in the pathogenesis of many diseases, including muscular dystrophy, atrophy, type 2 diabetes, and aging-related sarcopenia. Notably, exercise counteracts the effects of many chronic diseases on skeletal muscle mitochondrial function. Recent studies have revealed a finely tuned regulatory network that orchestrates skeletal muscle mitochondrial biogenesis and function in response to exercise and in disease states. In addition, increasing evidence suggests that mitochondria also serve to "communicate" with the nucleus and mediate adaptive genomic re-programming. Here we review the current state of knowledge relevant to the dynamic remodeling of skeletal muscle mitochondria in response to exercise and in disease states.

Autophagy and mitophagy flux in young and aged skeletal muscle following chronic contractile activity

KEY POINTS

A healthy mitochondrial pool is dependent on the removal of dysfunctional organelles via mitophagy, but little is known about how mitophagy is altered with ageing and chronic exercise. Chronic contractile activity (CCA) is a standardized exercise model that can elicit mitochondrial adaptations in both young and aged muscle, albeit to a lesser degree in the aged group. Assessment of mitophagy flux revealed enhanced targeting of mitochondria for degradation in aged muscle, in contrast to previous theories. Mitophagy flux was significantly reduced as an adaptation to CCA suggesting that an improvement in organelle quality reduces the need for mitochondrial turnover. CCA enhances lysosomal capacity and may ameliorate lysosomal dysfunction in aged muscle.

ABSTRACT

Skeletal muscle exhibits deficits in mitochondrial quality with age. Central to the maintenance of a healthy mitochondrial pool is the removal of dysfunctional organelles via mitophagy. Little is known on how mitophagy is altered with ageing and chronic exercise. We assessed mitophagy flux using colchicine treatment in vivo following chronic contractile activity (CCA) of muscle in young and aged rats. CCA evoked mitochondrial biogenesis in young muscle, with an attenuated response in aged muscle. Mitophagy flux was higher in aged muscle and was correlated with the enhanced expression of mitophagy receptors and upstream transcriptional regulators. CCA decreased mitophagy flux in both age groups, suggesting an improvement in organelle quality. CCA also reduced the exaggerated expression of TFEB evident in aged muscle, which may be promoting the age-induced increase in lysosomal markers. Thus, aged muscle possesses an elevated drive for autophagy and mitophagy which may contribute to the decline in organelle content observed with age, but which may serve to maintain mitochondrial quality. CCA improves organelle integrity and reduces mitophagy, illustrating that chronic exercise is a modality to improve muscle quality in aged populations.

Loss of sirtuin 1 and mitofusin 2 contributes to enhanced ischemia/reperfusion injury in aged livers

Ischemia/reperfusion (I/R) injury is a causative factor contributing to morbidity and mortality during liver resection and transplantation. Livers from elderly patients have a poorer recovery from these surgeries, indicating reduced reparative capacity with aging. Mechanisms underlying this age‐mediated hypersensitivity to I/R injury remain poorly understood. Here, we investigated how sirtuin 1 (SIRT1) and mitofusin 2 (MFN2) are affected by I/R in aged livers. Young (3 months) and old (23–26 months) male C57/BL6 mice were subjected to hepatic I/R in vivo. Primary hepatocytes isolated from each age group were also exposed to simulated in vitro I/R. Biochemical, genetic, and imaging analyses were performed to assess cell death, autophagy flux, mitophagy, and mitochondrial function. Compared to young mice, old livers showed accelerated liver injury following mild I/R. Reperfusion of old hepatocytes also showed necrosis, accompanied with defective autophagy, onset of the mitochondrial permeability transition, and mitochondrial dysfunction. Biochemical analysis indicated a near‐complete loss of both SIRT1 and MFN2 after I/R in old hepatocytes, which did not occur in young cells. Overexpression of either SIRT1 or MFN2 alone in old hepatocytes failed to mitigate I/R injury, while co‐overexpression of both proteins promoted autophagy and prevented mitochondrial dysfunction and cell death after reperfusion. Genetic approaches with deletion and point mutants revealed that SIRT1 deacetylated K655 and K662 residues in the C‐terminus of MFN2, leading to autophagy activation. The SIRT1‐MFN2 axis is pivotal during I/R recovery and may be a novel therapeutic target to reduce I/R injury in aged livers.

SILAC-based quantitative proteomic analysis of the livers of spontaneous obese and diabetic rhesus monkeys

Type 2 diabetes mellitus (T2DM) is a severe metabolic disorder that affects more than 10% of the population worldwide. Obesity is a major cause of insulin resistance and contributes to the development of T2DM. Liver is an essential metabolic organ that plays crucial roles in the pathogenesis of obesity and diabetes. However, the underlying mechanisms of liver in the transition of obesity to diabetes are not fully understood. The nonhuman primate rhesus monkey is an appropriate animal for research of human diseases. Here, we first screened and selected three individuals of spontaneously diabetic rhesus monkeys. Interestingly, the diabetic monkeys were obese with a high body mass index at the beginning, but gradually lost their body weight during one year of observation. Furthermore, we performed stable isotope labeling with amino acids in cell culture-based quantitative proteomics to identify proteins and signaling pathways with altered expression in the liver of obese and diabetic monkeys. In total, 3,509 proteins were identified and quantified, of which 185 proteins displayed an altered expression level. Gene ontology analysis revealed that the expression of proteins involved in fatty acids β-oxidation and galactose metabolism was increased in obese monkeys; while proteins involved in oxidative phosphorylation and branched chain amino acid (BCAA) degradation were upregulated in diabetic monkeys. In addition, we observed mild apoptosis in the liver of diabetic monkeys, suggesting liver injury at the late onset of diabetes. Taken together, our liver proteomics may reveal a distinct metabolic transition from fatty acids β-oxidation in obese monkey to BCAA degradation in diabetic monkeys.

H2O2 Signaling-Triggered PI3K Mediates Mitochondrial Protection to Participate in Early Cardioprotection by Exercise Preconditioning

Previous studies have shown that early exercise preconditioning (EEP) imparts a protective effect on acute cardiovascular stress. However, how mitophagy participates in exercise preconditioning- (EP-) induced cardioprotection remains unclear. EEP may involve mitochondrial protection, which presumably crosstalks with predominant H₂O₂ oxidative stress. Our EEP protocol involves four periods of 10 min running with 10 min recovery intervals. We added a period of exhaustive running and a pretreatment using phosphoinositide 3-kinase (PI3K)/autophagy inhibitor wortmannin to test this protective effect. By using transmission electron microscopy (TEM), laser scanning confocal microscopy, and other molecular biotechnology methods, we detected related markers and specifically analyzed the relationship between mitophagic proteins and mitochondrial translocation. We determined that exhaustive exercise associated with various elevated injuries targeted the myocardium, oxidative stress, hypoxia-ischemia, and mitochondrial ultrastructure. However, exhaustion induced limited mitochondrial protection through a H₂O₂-independent manner to inhibit voltage-dependent anion channel isoform 1 (VDAC1) instead of mitophagy. EEP was apparently safe to the heart. In EEP-induced cardioprotection, EEP provided suppression to exhaustive exercise (EE) injuries by translocating Bnip3 to the mitochondria by recruiting the autophagosome protein LC3 to induce mitophagy, which is potentially triggered by H₂O₂ and influenced by Beclin1-dependent autophagy. Pretreatment with the wortmannin further attenuated these effects induced by EEP and resulted in the expression of proapoptotic phenotypes such as oxidative injury, elevated Beclin1/Bcl-2 ratio, cytochrome c leakage, mitochondrial dynamin-1-like protein (Drp-1) expression, and VDAC1 dephosphorylation. These observations suggest that H₂O₂ generation regulates mitochondrial protection in EEP-induced cardioprotection.

Regulation of apoptosis and autophagy in mouse and human skeletal muscle with aging and lifelong exercise training

Exercise training has been reported to prevent the age-induced decline in muscle mass and fragmentation of mitochondria, as well as to affect autophagy and mitophagy. The interaction between these pathways during aging as well as the similarity between such changes in human and mouse skeletal muscle is however not fully understood. Therefore the aim of the present study was to test the hypothesis that cellular degradation pathways, including apoptosis, autophagy and mitophagy are coordinately regulated in mouse and human skeletal muscle during aging and lifelong exercise training through a PGC-1α-p53 axis. Muscle samples were obtained from young untrained, aged untrained and aged lifelong exercise trained men, and from whole-body PGC-1α knockout mice and their littermate controls that were either lifelong exercise trained or sedentary young and aged. Lifelong exercise training prevented the aging-induced reduction in PGC-1α, p53 and p21 mRNA as well as the increase in LC3II and BNIP3 protein in mouse skeletal muscle, while aging decreased the BAX/Bcl-2 ratio, LC3I and BAX protein in mouse skeletal muscle without effects of lifelong exercise training. In humans, aging was associated with reduced PGC-1α mRNA as well as decreased p62 and p21 protein in skeletal muscle, while lifelong exercise training increased BNIP3 protein and decreased p53 mRNA. In conclusion, there was a divergent regulation of autophagy and apoptosis in mouse muscle with aging and lifelong exercise training, whereas healthy aged human skeletal muscle seemed rather robust to changes in apoptosis, autophagy and mitophagy markers compared with mouse muscle at the investigated age.

Mitochondrial Dynamics: Shaping Metabolic Adaptation

Despite their classic bean-shaped depiction, mitochondria have very different aspects in each cell type. From long filamentous structures to punctuated small round organelles. These shapes can dynamically change in response to nutrients and in situations of metabolic disease. However, why do mitochondria adapt different shapes and how is this determined? In this review, we will aim to understand different visions on how metabolic cues influence mitochondrial shape and vice-versa. This response can be dramatically different between tissues and cells, as illustrated by a large array of genetically engineered mouse models reported to date. We will use these models to understand the role of different mitochondrial dynamics-related proteins and processes.

Autophagy and lysosomal pathways in nervous system disorders

Autophagy is an evolutionarily conserved pathway for delivering cytoplasmic cargo to lysosomes for degradation. In its classically studied form, autophagy is a stress response induced by starvation to recycle building blocks for essential cellular processes. In addition, autophagy maintains basal cellular homeostasis by degrading endogenous substrates such as cytoplasmic proteins, protein aggregates, damaged organelles, as well as exogenous substrates such as bacteria and viruses. Given their important role in homeostasis, autophagy and lysosomal machinery are genetically linked to multiple human disorders such as chronic inflammatory diseases, cardiomyopathies, cancer, and neurodegenerative diseases. Multiple targets within the autophagy and lysosomal pathways offer therapeutic opportunities to benefit patients with these disorders. Here, I will summarize the mechanisms of autophagy pathways, the evidence supporting a pathogenic role for disturbed autophagy and lysosomal degradation in nervous system disorders, and the therapeutic potential of autophagy modulators in the clinic.

TDP-43 interacts with mitochondrial proteins critical for mitophagy and mitochondrial dynamics

Transactive response DNA-binding protein of 43 kDa (TDP-43) functions as a heterogeneous nuclear ribonucleoprotein and is the major pathological protein in frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis/motor neuron disease (ALS/MND). TDP-43 pathology may also be present as a comorbidity in approximately 20-50% of sporadic Alzheimer's disease cases. In a mouse model of MND, full-length TDP-43 increases association with the mitochondria and blocking the TDP-43/mitochondria interaction ameliorates motor dysfunction. Utilizing a proteomics screen, several mitochondrial TDP-43-interacting partners were identified, including voltage-gated anion channel 1 (VDAC1) and prohibitin 2 (PHB2), a crucial mitophagy receptor. Overexpression of TDP-43 led to an increase in PHB2 whereas TDP-43 knockdown reduced PHB2 expression in cells treated with carbonyl cyanide m-chlorophenylhydrazone (CCCP), an inducer of mitophagy. These results suggest that TDP-43 expression contributes to metabolism and mitochondrial function however we show no change in bioenergetics when TDP-43 is overexpressed and knocked down in HEK293T cells. Furthermore, the fusion protein mitofusin 2 (MFN2) interacts in complex with TDP-43 and selective expression of human TDP-43 in the hippocampus and cortex induced an age-dependent change in Mfn2 expression. Mitochondria morphology is altered in 9-month-old mice selectively expressing TDP-43 in an APP/PS1 background compared with APP/PS1 littermates. We further confirmed TDP-43 localization to the mitochondria using immunogold labeled TDP-43 transmission electron microscopy (TEM) and mitochondrial isolation methods There was no increase in full-length TDP-43 localized to the mitochondria in APP/PS1 mice compared to wild-type (littermates); however, using C- and N-terminal-specific TDP-43 antibodies, the N-terminal (27 kDa, N27) and C-terminal (30 kDa, C30) fragments of TDP-43 are greatly enriched in mitochondrial fractions. In addition, when the mitochondrial peptidase (PMPCA) is overexpressed there is an increase in the N-terminal fragment (N27). These results suggest that TDP-43 processing may contribute to metabolism and mitochondrial function.

Mitochondrial DNA and TLR9 drive muscle inflammation upon Opa1 deficiency

Opa1 participates in inner mitochondrial membrane fusion and cristae morphogenesis. Here, we show that muscle-specific Opa1 ablation causes reduced muscle fiber size, dysfunctional mitochondria, enhanced Fgf21, and muscle inflammation characterized by NF-κB activation, and enhanced expression of pro-inflammatory genes. Chronic sodium salicylate treatment ameliorated muscle alterations and reduced the muscle expression of Fgf21. Muscle inflammation was an early event during the progression of the disease and occurred before macrophage infiltration, indicating that it is a primary response to Opa1 deficiency. Moreover, Opa1 repression in muscle cells also resulted in NF-κB activation and inflammation in the absence of necrosis and/or apoptosis, thereby revealing that the activation is a cell-autonomous process and independent of cell death. The effects of Opa1 deficiency on the expression NF-κB target genes and inflammation were absent upon mitochondrial DNA depletion. Under Opa1 deficiency, blockage or repression of TLR9 prevented NF-κB activation and inflammation. Taken together, our results reveal that Opa1 deficiency in muscle causes initial mitochondrial alterations that lead to TLR9 activation, and inflammation, which contributes to enhanced Fgf21 expression and to growth impairment.

Mitochondrial content is preserved throughout disease progression in the mdx mouse model of Duchenne muscular dystrophy, regardless of taurine supplementation

Mitochondrial dysfunction is a pathological feature of Duchenne muscular dystrophy (DMD), a debilitating and fatal neuromuscular disorder characterized by progressive muscle wasting and weakness. Mitochondria are a source of cellular ATP involved in Ca2+ regulation and apoptotic signaling. Ameliorating aberrant mitochondrial function has therapeutic potential for reducing DMD disease severity. The dystrophic mdx mouse exhibits peak muscle damage at 21–28 days, which stabilizes after 8 wk. The amino acid taurine is implicated in mitochondrial health and function, with endogenous concentrations low when measured during the cycle of peak muscle damage in mdx mice. Using whole soleus and extensor digitorum longus (EDL) muscle homogenates from 28- and 70-day mdx mice, we found that there was no change in native state mitochondrial complexes using blue native-PAGE. NADH:ubiquinone oxidotreductase subunit-A9 (NDUFA9) protein abundance was lower in soleus muscle of 28- and 70-day mdx mice and EDL muscle of 70-day mdx mice compared with same muscles in WT (C57/BL10ScSn) animals. There were age-dependent increases in both NDUFA9 protein abundance and citrate synthase activity in soleus muscles of mdx and wild-type mice. There was no change in abundances of mitochondrial dynamics proteins mitofusin 2 (Mfn2) and mitochondrial dynamics protein 49 (MiD49). Taurine administration essentially did not affect any measurements of mitochondria. Collectively, these findings suggest mitochondrial content and dynamics are not reduced in the mdx mouse regardless of disease severity. We also elucidate that taurine affords no significant benefit to mitochondrial content or dynamics in the mdx mouse at either 28 or 70 days.

Yap regulates gastric cancer survival and migration via SIRT1/Mfn2/mitophagy

Gastric cancer is the fifth most common cancer worldwide and Hippo-Yap is the novel signaling pathway which plays an important role in gastric cancer tumor development and progression. However, little insight is available to date regarding the specific role of Yes-associated protein (Yap) in gastric cancer. In the present study, we identified the mechanism through which Yap sustains gastric cancer viability and migration. Yap was greatly upregulated in gastric cancer cells and its expression promoted cellular migration and survival. Functional studies found that knockdown of Yap reduced the mitophagy activity, which subsequently caused mitochondrial apoptosis and cellular oxidative stress. The latter impaired adhesive protein expression, alleviated F-actin expression, blunted lamellipodium formation, leading to inhibition of cancer cell motility. Mechanistically, Yap preserved Sirtuin 1 (SIRT1) activity which manipulated mitofusin 2 (Mfn2) expression and subsequent mitophagy. Loss of Yap reduced SIRT1 expression and inhibited Mfn2-mediated mitophagy. Collectively, our results identified Hippo-Yap as a tumor promoter in gastric cancer that was mediated via activation of the SIRT1/Mfn2/mitophagy axis, with potential applications to gastric cancer therapy involving cancer survival and migration.

Antagonistic effects of selenium on lead-induced autophagy by influencing mitochondrial dynamics in the spleen of chickens

Lead (Pb) may damage the immune function in human and animal. Selenium (Se) has antagonistic effects on Pb. In our study, brown layer chickens were randomly allocated to control group, Se group (1 mg/kg Se), Se+Pb group (1 mg/kg Se and 350 mg/kg Pb), and Pb group (350 mg/kg Pb). The chickens were sacrificed on the 90^th day; spleen tissues were subjected to observation of ultrastructure and detection of spleen-related indexes. The results revealed that in the Pb group, expression levels of the cytokines IL-1 and TNF-α significantly increased, and expression levels of IL-2 and INF-γ significantly decreased; activities of antioxidant enzyme GPX, SOD and CAT significantly decreased, and expression level of malondialdehyde (MDA) significantly increased; expression levels of mitochondrial fission-related genes (Mff and Drp1) significantly increased, and expression levels of mitochondrial fusion-related genes (Opa1, Mfn1 and Mfn2) significantly decreased; expression of autophagy-related genes (Beclin 1, Dynein, Atg 5, LC3-I and LC-II) was upregulated, while expression of mammalian target of rapamycin (mTOR) was downregulated. The results of transmission electron microscopy indicated that Pb induced mitochondrial fragmentation, and triggered autophagy in the spleen of chickens. The Se and Pb co-treatment remarkably alleviated these injuries induced by Pb in the spleen of chickens. In conclusion, Pb can induce the oxidative stress to influence the mitochondrial dynamics balance and lead to autophagy, which triggers the immune dysfunction in the spleen of chickens; the Se exhibits the antagonistic effects on lead-induced autophagy by influencing mitochondrial dynamics in the spleen of chickens.

Mitofusin 2: from functions to disease

Mitochondria are highly dynamic organelles whose functions are essential for cell viability. Within the cell, the mitochondrial network is continuously remodeled through the balance between fusion and fission events. Moreover, it dynamically contacts other organelles, particularly the endoplasmic reticulum, with which it enterprises an important functional relationship able to modulate several cellular pathways. Being mitochondria key bioenergetics organelles, they have to be transported to all the specific high-energy demanding sites within the cell and, when damaged, they have to be efficiently removed. Among other proteins, Mitofusin 2 represents a key player in all these mitochondrial activities (fusion, trafficking, turnover, contacts with other organelles), the balance of which results in the appropriate mitochondrial shape, function, and distribution within the cell. Here we review the structural and functional properties of Mitofusin 2, highlighting its crucial role in several cell pathways, as well as in the pathogenesis of neurodegenerative diseases, metabolic disorders, cardiomyopathies, and cancer.

Update on mitochondria and muscle aging: all wrong roads lead to sarcopenia

Sarcopenia is a well-known geriatric syndrome that has been endorsed over the years as a biomarker allowing for the discrimination, at a clinical level, of biological from chronological age. Multiple candidate mechanisms have been linked to muscle degeneration during sarcopenia. Among them, there is wide consensus on the central role played by the loss of mitochondrial integrity in myocytes, secondary to dysfunctional quality control mechanisms. Indeed, mitochondria establish direct or indirect contacts with other cellular components (e.g. endoplasmic reticulum, peroxisomes, lysosomes/vacuoles) as well as the extracellular environment through the release of several biomolecules. The functional implications of these interactions in the context of muscle physiology and sarcopenia are not yet fully appreciated and represent a promising area of investigation. Here, we present an overview of recent findings concerning the interrelation between mitochondrial quality control processes, inflammation and the metabolic regulation of muscle mass in the pathogenesis of sarcopenia highlighting those pathways that may be exploited for developing preventive and therapeutic interventions against muscle aging.

Highlighting the endoplasmic reticulum-mitochondria connection: Focus on Mitofusin 2

The endoplasmic reticulum (ER) and the mitochondrial network are two highly interconnected cellular structures. By proteinaceous tethers, specialized membrane domains of the ER are tightly associated with the outer membrane of mitochondria, allowing the assembly of signaling platforms where different cell functions take place or are modulated, such as lipid biosynthesis, Ca²⁺ homeostasis, inflammation, autophagy and apoptosis. The ER-mitochondria coupling is highly dynamic and contacts between the two organelles can be modified in their number, extension and thickness by different stimuli. Importantly, several pathological conditions, such as cancer, neurodegenerative diseases and metabolic syndromes show alterations in this feature, underlining the key role of ER-mitochondria crosstalk in cell physiology. In this contribution, we will focus on one of the major modulator of ER-mitochondria apposition, Mitofusin 2, discussing the structure of the protein and its debated role on organelles tethering. Moreover, we will critically describe different techniques commonly used to investigate this crucial issue, highlighting their advantages, drawbacks and limits.

Hypothalamic Mitochondrial Dysfunction as a Target in Obesity and Metabolic Disease

Mitochondria are important organelles for the adaptation to energy demand that play a central role in bioenergetics metabolism. The mitochondrial architecture and mitochondrial machinery exhibits a high degree of adaptation in relation to nutrient availability. On the other hand, its disruption markedly affects energy homeostasis. The brain, more specifically the hypothalamus, is the main hub that controls energy homeostasis. Nevertheless, until now, almost all studies in relation to mitochondrial dysfunction and energy metabolism have focused in peripheral tissues like brown adipose tissue, muscle, and pancreas. In this review, we highlight the relevance of the hypothalamus and the influence on mitochondrial machinery in its function as well as its consequences in terms of alterations in both energy and metabolic homeostasis.

Identification of New Activators of Mitochondrial Fusion Reveals a Link between Mitochondrial Morphology and Pyrimidine Metabolism

Mitochondria are dynamic organelles that produce most of the cellular ATP, and are involved in many other cellular functions such as Ca²⁺ signaling, differentiation, apoptosis, cell cycle, and cell growth. One key process of mitochondrial dynamics is mitochondrial fusion, which is catalyzed by mitofusins (MFN1 and MFN2) and OPA1. The outer mitochondrial membrane protein MFN2 plays a relevant role in the maintenance of mitochondrial metabolism, insulin signaling, and mutations that cause neurodegenerative disorders. Therefore, modulation of proteins involved in mitochondrial dynamics has emerged as a potential pharmacological strategy. Here, we report the identification of small molecules by high-throughput screen that promote mitochondrial elongation in an MFN1/MFN2-dependent manner. Detailed analysis of their mode of action reveals a previously unknown connection between pyrimidine metabolism and mitochondrial dynamics. Our data indicate a link between pyrimidine biosynthesis and mitochondrial dynamics, which maintains cell survival under stress conditions characterized by loss of pyrimidine synthesis.

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.

Dietary Restriction and AMPK Increase Lifespan via Mitochondrial Network and Peroxisome Remodeling

Mitochondrial network remodeling between fused and fragmented states facilitates mitophagy, interaction with other organelles, and metabolic flexibility. Aging is associated with a loss of mitochondrial network homeostasis, but cellular processes causally linking these changes to organismal senescence remain unclear. Here, we show that AMP-activated protein kinase (AMPK) and dietary restriction (DR) promote longevity in C. elegans via maintaining mitochondrial network homeostasis and functional coordination with peroxisomes to increase fatty acid oxidation (FAO). Inhibiting fusion or fission specifically blocks AMPK- and DR-mediated longevity. Strikingly, however, preserving mitochondrial network homeostasis during aging by co-inhibition of fusion and fission is sufficient itself to increase lifespan, while dynamic network remodeling is required for intermittent fasting-mediated longevity. Finally, we show that increasing lifespan via maintaining mitochondrial network homeostasis requires FAO and peroxisomal function. Together, these data demonstrate that mechanisms that promote mitochondrial homeostasis and plasticity can be targeted to promote healthy aging.

Mitochondrial Function and Mitophagy in the Elderly: Effects of Exercise

Aging is a natural, multifactorial and multiorganic phenomenon wherein there are gradual physiological and pathological changes over time. Aging has been associated with a decrease of autophagy capacity and mitochondrial functions, such as biogenesis, dynamics, and mitophagy. These processes are essential for the maintenance of mitochondrial structural integrity and, therefore, for cell life, since mitochondrial dysfunction leads to an impairment of energy metabolism and increased production of reactive oxygen species, which consequently trigger mechanisms of cellular senescence and apoptotic cell death. Moreover, reduced mitochondrial function can contribute to age-associated disease phenotypes in model organisms and humans. Literature data show beneficial effects of exercise on the impairment of mitochondrial biogenesis and dynamics and on the decrease in the mitophagic capacity associated to aging. Thus, exercise could have effects on the major cell signaling pathways that are involved in the mitochondria quality and quantity control in the elderly. Although it is known that several exercise protocols are able to modify the activity and turnover of mitochondria, further studies are necessary in order to better identify the mechanisms of interaction between mitochondrial functions, aging, and physical activity, as well as to analyze possible factors influencing these processes.

Mitochondria Initiate and Regulate Sarcopenia

We present the hypothesis that an accumulation of dysfunctional mitochondria initiates a signaling cascade leading to motor neuron and muscle fiber death and culminating in sarcopenia. Interactions between neural and muscle cells that contain dysfunctional mitochondria exacerbate sarcopenia. Preventing sarcopenia will require identifying mitochondrial sources of dysfunction that are reversible.

Exercise-mediated modulation of autophagy in skeletal muscle

Although exercise exerts multiple beneficial health effects, it may also damage cellular structures. Damaged elements are continuously degraded and its constituents recycled to produce renovated structures through a process called autophagy, which is essential for the adaptation to training. Autophagy is particularly active in skeletal muscle, where it can be evaluated using specific molecular markers of activation (unc-51-like kinase 1 [ULK1] phosphorylation) and specific proteins indicating increased autophagosome content (increased total LC3, LC3-II, LC3-II/LC3-I ratio). Studies in humans are technically limited but have provided evidence suggesting the activation of autophagy in skeletal muscle through AMP-activated protein kinase (AMPK) and its downstream target ULK1. Autophagy activation is more likely when the intensity is elevated and the exercise performed in the fasted state. The autophagy-gene program and autophagosome content are upregulated after ultraendurance running competitions. However, autophagosome content is reduced after endurance exercise at moderate intensities (50% and 70% of VO₂ max) for 60-120 minutes. Autophagosome content is decreased within the first few hours after resistance training. The effects of regular endurance and strength training on basal autophagy remain to be established in humans. One study has reported that acute severe hypoxia increases autophagosome content in human skeletal muscle, which is reverted by 20 minutes of low-intensity exercise. Experiments with transgenic mice have shown that autophagy is necessary for skeletal muscle adaptation to training. Little is known on how genetic factors, environment, nutrition, drugs and diseases may interact with exercise to modulate autophagy at rest and during exercise in humans.

Activation of mitophagy leads to decline in Mfn2 and loss of mitochondrial mass in Fuchs endothelial corneal dystrophy

Human corneal endothelial cells (HCEnCs) are terminally differentiated cells that have limited regenerative potential. The large numbers of mitochondria in HCEnCs are critical for pump and barrier function required for corneal hydration and transparency. Fuchs Endothelial Corneal Dystrophy (FECD) is a highly prevalent late-onset oxidative stress disorder characterized by progressive loss of HCEnCs. We previously reported increased mitochondrial fragmentation and reduced ATP and mtDNA copy number in FECD. Herein, carbonyl cyanide m-chlorophenyl hydrazone (CCCP)-induced mitochondrial depolarization decreased mitochondrial mass and Mfn2 levels, which were rescued with mitophagy blocker, bafilomycin, in FECD. Moreover, electron transport chain complex (I, V) decrease in FECD indicated deficient mitochondrial bioenergetics. Transmission electron microscopy of FECD tissues displayed an increased number of autophagic vacuoles containing degenerated and swollen mitochondria with cristolysis. An elevation of LC3-II and LAMP1 and downregulation of Mfn2 in mitochondrial fractions suggested that loss of fusion capacity targets fragmented mitochondria to the pre-autophagic pool and upregulates mitophagy. CCCP-induced mitochondrial fragmentation leads to Mfn2 and LC3 co-localization without activation of proteosome, suggesting a novel Mfn2 degradation pathway via mitophagy. These data indicate constitutive activation of mitophagy results in reduction of mitochondrial mass and abrogates cellular bioenergetics during degeneration of post-mitotic cells of ocular tissue.

Lack of Glycogenin Causes Glycogen Accumulation and Muscle Function Impairment

Glycogenin is considered essential for glycogen synthesis, as it acts as a primer for the initiation of the polysaccharide chain. Against expectations, glycogenin-deficient mice (Gyg KO) accumulate high amounts of glycogen in striated muscle. Furthermore, this glycogen contains no covalently bound protein, thereby demonstrating that a protein primer is not strictly necessary for the synthesis of the polysaccharide in vivo. Strikingly, in spite of the higher glycogen content, Gyg KO mice showed lower resting energy expenditure and less resistance than control animals when subjected to endurance exercise. These observations can be attributed to a switch of oxidative myofibers toward glycolytic metabolism. Mice overexpressing glycogen synthase in the muscle showed similar alterations, thus indicating that this switch is caused by the excess of glycogen. These results may explain the muscular defects of GSD XV patients, who lack glycogenin-1 and show high glycogen accumulation in muscle.

Epigallocatechin-3-gallate increases autophagy signaling in resting and unloaded plantaris muscles but selectively suppresses autophagy protein abundance in reloaded muscles of aged rats

We have previously found that Epigallocatechin-3-gallate (EGCg), an abundant catechin in green tea, reduced apoptotic signaling and improved muscle recovery in response to reloading after hindlimb suspension (HS). In this study, we investigated if EGCg altered autophagy signaling in skeletal muscle of old rats in response to HS or reloading after HS. Fischer 344×Brown Norway inbred rats (age 34months) were given 1ml/day of purified EGCg (50mg/kg body weight), or the same sample volume of the vehicle by gavage. One group of animals received HS for 14days and the second group of rats received 14days of HS, then the HS was removed and they were allowed to recover by ambulating normally around the cage for two weeks. EGCg decreased a small number of autophagy genes in control muscles, but it increased the expression of other autophagy genes (e.g., ATG16L2, SNCA, TM9SF1, Pink1, PIM-2) and HS did not attenuate these increases. HS increased Beclin1, ATG7 and LC3-II/I protein abundance in hindlimb muscles. Relative to vehicle treatment, EGCg treatment had greater ATG12 protein abundance (35.8%, P<0.05), but decreased Beclin1 protein levels (-101.1%, P<0.05) after HS. However, in reloaded muscles, EGCg suppressed Beclin1 and LC3-II/I protein abundance as compared to vehicle treated muscles. EGCg appeared to "prime" autophagy signaling before and enhance autophagy gene expression and protein levels during unloading in muscles of aged rats, perhaps to improve the clearance of damaged organelles. However, EGCg suppressed autophagy signaling after reloading, potentially to increase the recovery of hindlimb muscles mass and function after loading is restored.

Impact of Aging and Exercise on Mitochondrial Quality Control in Skeletal Muscle

Mitochondria are characterized by its pivotal roles in managing energy production, reactive oxygen species, and calcium, whose aging-related structural and functional deteriorations are observed in aging muscle. Although it is still unclear how aging alters mitochondrial quality and quantity in skeletal muscle, dysregulation of mitochondrial biogenesis and dynamic controls has been suggested as key players for that. In this paper, we summarize current understandings on how aging regulates muscle mitochondrial biogenesis, while focusing on transcriptional regulations including PGC-1α, AMPK, p53, mtDNA, and Tfam. Further, we review current findings on the muscle mitochondrial dynamic systems in aging muscle: fusion/fission, autophagy/mitophagy, and protein import. Next, we also discuss how endurance and resistance exercises impact on the mitochondrial quality controls in aging muscle, suggesting possible effective exercise strategies to improve/maintain mitochondrial health.

Mitochondria and endoplasmic reticulum: Targets for a better insulin sensitivity in skeletal muscle?

Obesity and its associated metabolic disorders represent a major health burden, with economic and social consequences. Although adapted lifestyle and bariatric surgery are effective in reducing body weight, obesity prevalence is still rising. Obese individuals often become insulin-resistant. Obesity impacts on insulin responsive organs, such as the liver, adipose tissue and skeletal muscle, and increases the risk of cardiovascular diseases, type 2 diabetes and cancer. In this review, we discuss the effects of obesity and insulin resistance on skeletal muscle, an important organ for the control of postprandial glucose. The roles of mitochondria and the endoplasmic reticulum in insulin signaling are highlighted and potential innovative research and treatment perspectives are proposed.

Autophagy as an Emerging Common Pathomechanism in Inherited Peripheral Neuropathies

The inherited peripheral neuropathies (IPNs) comprise a growing list of genetically heterogeneous diseases. With mutations in more than 80 genes being reported to cause IPNs, a wide spectrum of functional consequences is expected to follow this genotypic diversity. Hence, the search for a common pathomechanism among the different phenotypes has become the holy grail of functional research into IPNs. During the last decade, studies on several affected genes have shown a direct and/or indirect correlation with autophagy. Autophagy, a cellular homeostatic process, is required for the removal of cell aggregates, long-lived proteins and dead organelles from the cell in double-membraned vesicles destined for the lysosomes. As an evolutionarily highly conserved process, autophagy is essential for the survival and proper functioning of the cell. Recently, neuronal cells have been shown to be particularly vulnerable to disruption of the autophagic pathway. Furthermore, autophagy has been shown to be affected in various common neurodegenerative diseases of both the central and the peripheral nervous system including Alzheimer's, Parkinson's, and Huntington's diseases. In this review we provide an overview of the genes involved in hereditary neuropathies which are linked to autophagy and we propose the disruption of the autophagic flux as an emerging common pathomechanism. We also shed light on the different steps of the autophagy pathway linked to these genes. Finally, we review the concept of autophagy being a therapeutic target in IPNs, and the possibilities and challenges of this pathway-specific targeting.

Preservation of skeletal muscle mitochondrial content in older adults: relationship between mitochondria, fibre type and high-intensity exercise training

KEY POINTS

Ageing is associated with an upregulation of mitochondrial dynamics proteins mitofusin 2 (Mfn2) and mitochondrial dynamics protein 49 (MiD49) in human skeletal muscle with the increased abundance of Mfn2 being exclusive to type II muscle fibres. These changes occur despite a similar content of mitochondria, as measured by COXIV, NDUFA9 and complexes in their native states (Blue Native PAGE). Following 12 weeks of high-intensity training (HIT), older adults exhibit a robust increase in mitochondria content, while there is a decline in Mfn2 in type II fibres. We propose that the upregulation of Mfn2 and MiD49 with age may be a protective mechanism to protect against mitochondrial dysfunction, in particularly in type II skeletal muscle fibres, and that exercise may have a unique protective effect negating the need for an increased turnover of mitochondria.

ABSTRACT

Mitochondrial dynamics proteins are critical for mitochondrial turnover and maintenance of mitochondrial health. High-intensity interval training (HIT) is a potent training modality shown to upregulate mitochondrial content in young adults but little is known about the effects of HIT on mitochondrial dynamics proteins in older adults. This study investigated the abundance of protein markers for mitochondrial dynamics and mitochondrial content in older adults compared to young adults. It also investigated the adaptability of mitochondria to 12 weeks of HIT in older adults. Both older and younger adults showed a higher abundance of mitochondrial respiratory chain subunits COXIV and NDUFA9 in type I compared with type II fibres, with no difference between the older adults and young groups. In whole muscle homogenates, older adults had higher mitofusin-2 (Mfn2) and mitochondrial dynamics protein 49 (MiD49) contents compared to the young group. Also, older adults had higher levels of Mfn2 in type II fibres compared with young adults. Following HIT in older adults, MiD49 and Mfn2 levels were not different in whole muscle and Mfn2 content decreased in type II fibres. Increases in citrate synthase activity (55%) and mitochondrial respiratory chain subunits COXIV (37%) and NDUFA9 (48%) and mitochondrial respiratory chain complexes (∼70-100%) were observed in homogenates and/or single fibres. These findings reveal (i) a similar amount of mitochondria in muscle from young and healthy older adults and (ii) a robust increase of mitochondrial content following 12 weeks of HIT exercise in older adults.

Human biallelic MFN2 mutations induce mitochondrial dysfunction, upper body adipose hyperplasia, and suppression of leptin expression

MFN2 encodes mitofusin 2, a membrane-bound mediator of mitochondrial membrane fusion and inter-organelle communication. MFN2 mutations cause axonal neuropathy, with associated lipodystrophy only occasionally noted, however homozygosity for the p.Arg707Trp mutation was recently associated with upper body adipose overgrowth. We describe similar massive adipose overgrowth with suppressed leptin expression in four further patients with biallelic MFN2 mutations and at least one p.Arg707Trp allele. Overgrown tissue was composed of normal-sized, UCP1-negative unilocular adipocytes, with mitochondrial network fragmentation, disorganised cristae, and increased autophagosomes. There was strong transcriptional evidence of mitochondrial stress signalling, increased protein synthesis, and suppression of signatures of cell death in affected tissue, whereas mitochondrial morphology and gene expression were normal in skin fibroblasts. These findings suggest that specific MFN2 mutations cause tissue-selective mitochondrial dysfunction with increased adipocyte proliferation and survival, confirm a novel form of excess adiposity with paradoxical suppression of leptin expression, and suggest potential targeted therapies.

Age-dependent obesity and mitochondrial dysfunction

Aging is associated with progressive visceral white adipose tissue (WAT) expansion both in human and mouse. Importantly, WAT enlargement is initiated early in life, suggesting that molecular mechanisms underlying age-dependent obesity are activated at early stages of lifetime. Our recent study found that age-dependent obesity was associated with a specific decline in mitochondrial complex IV activity, which leads to reduced fatty acid oxidation and subsequent adipocyte hypertrophy. At the molecular level, global mitochondrial complex IV inhibition was driven by hypoxia-inducible factor-1α (HIF1α)-mediated repression of some of its key subunits, including cytochrome c oxidase 5b (Cox5b). In this commentary, we compare age-dependent WAT responses with those observed in the high fat diet model of extreme obesity. Furthermore, we discuss the potential scenarios that could initiate age-dependent WAT expansion as well as the mechanisms by which HIF1α could be activated in WAT.