Time-restricted feeding improves aortic endothelial relaxation by enhancing mitochondrial function and attenuating oxidative stress in aged mice

Age-related endothelial dysfunction is a pivotal factor in the development of cardiovascular diseases, stemming, at least in part, from mitochondrial dysfunction and a consequential increase in oxidative stress. These alterations are central to the decline in vascular health seen with aging, underscoring the urgent need for interventions capable of restoring endothelial function for preventing cardiovascular diseases. Dietary interventions, notably time-restricted feeding (TRF), have been identified for their anti-aging effects on mitochondria, offering protection against age-associated declines in skeletal muscle and other organs. Motivated by these findings, our study aimed to investigate whether TRF could similarly exert protective effects on endothelial health in the vasculature, enhancing mitochondrial function and reducing oxidative stress. To explore this, 12-month-old C57BL/6 mice were placed on a TRF diet, with food access limited to a 6-h window daily for 12 months. For comparison, we included groups of young mice and age-matched controls with unrestricted feeding. We evaluated the impact of TRF on endothelial function by measuring acetylcholine-induced vasorelaxation of the aorta. Mitochondrial health was assessed using fluororespirometry, and vascular reactive oxygen species (ROS) production was quantified with the redox-sensitive dye dihydroethidium. We also quantified 4-hydroxynonenal (4-HNE) levels, a stable marker of lipid peroxidation, in the aorta using ELISA. Our findings demonstrated that aged mice on a standard diet exhibited significant impairments in aortic endothelial relaxation and mitochondrial function, associated with elevated vascular oxidative stress. Remarkably, the TRF regimen led to substantial improvements in these parameters, indicating enhanced endothelial vasorelaxation, better mitochondrial function, and reduced oxidative stress in the aortas of aged mice. This investigation establishes a vital foundation, paving the way for subsequent clinical research aimed at exploring the cardiovascular protective benefits of intermittent fasting.

The mitochondrially targeted peptide elamipretide (SS-31) improves ADP sensitivity in aged mitochondria by increasing uptake through the adenine nucleotide translocator (ANT)

Aging muscle experiences functional decline in part mediated by impaired mitochondrial ADP sensitivity. Elamipretide (ELAM) rapidly improves physiological and mitochondrial function in aging and binds directly to the mitochondrial ADP transporter ANT. We hypothesized that ELAM improves ADP sensitivity in aging leading to rescued physiological function. We measured the response to ADP stimulation in young and old muscle mitochondria with ELAM treatment, in vivo heart and muscle function, and compared protein abundance, phosphorylation, and S-glutathionylation of ADP/ATP pathway proteins. ELAM treatment increased ADP sensitivity in old muscle mitochondria by increasing uptake of ADP through the ANT and rescued muscle force and heart systolic function. Protein abundance in the ADP/ATP transport and synthesis pathway was unchanged, but ELAM treatment decreased protein s-glutathionylation incuding of ANT. Mitochondrial ADP sensitivity is rapidly modifiable. This research supports the hypothesis that ELAM improves ANT function in aging and links mitochondrial ADP sensitivity to physiological function. ELAM binds directly to ANT and ATP synthase and ELAM treatment improves ADP sensitivity, increases ATP production, and improves physiological function in old muscles. ADP (adenosine diphosphate), ATP (adenosine triphosphate), VDAC (voltage-dependent anion channel), ANT (adenine nucleotide translocator), H+ (proton), ROS (reactive oxygen species), NADH (nicotinamide adenine dinucleotide), FADH2 (flavin adenine dinucleotide), O2 (oxygen), ELAM (elamipretide), -SH (free thiol), -SSG (glutathionylated protein).

A novel mitochondrial complex I ROS inhibitor partially improves muscle regeneration in adult but not old mice

It is unclear whether mitochondrial dysfunction and redox stress contribute to impaired age-related muscle regenerative capacity. Here we characterized a novel compound, BI4500, that inhibits the release of reactive oxygen species (ROS) from the quinone site in mitochondrial complex I (site IQ). We tested the hypothesis that ROS release from site IQ contributes to impaired regenerative capacity in aging muscle. Electron transfer system site-specific ROS production was measured in adult and aged mouse isolated muscle mitochondria and permeabilized gastrocnemius fibers. BI4500 inhibited ROS production from site IQ in a concentration-dependent manner (IC50 = ∼985 nM) by inhibiting ROS release without impairing complex I-linked respiration. In vivo BI4500 treatment decreased ROS production from site IQ. Muscle injury and sham injury were induced using barium chloride or vehicle injection to the tibialis anterior (TA) muscle in adult and aged male mice. On the same day as injury, mice began a daily gavage of 30 mg/kg BI4500 (BI) or placebo (PLA). Muscle regeneration (H&E, Sirius Red, Pax7) was measured at 5 and 35 days after injury. Muscle injury increased centrally nucleated fibers (CNFs) and fibrosis with no treatment or age effect. There was a significant age by treatment interaction for CNFs at 5- and 35-days post injury with significantly more CNFs in BI adults compared to PLA adults. Muscle fiber cross-sectional area (CSA) recovered significantly more in adult BI mice (-89 ± 365 μm2) compared to old PLA (-599 ± 153 μm2) and old BI (-535 ± 222 μm2, mean ± SD). In situ TA force recovery was measured 35 days after injury and was not significantly different by age or treatment. Inhibition of site IQ ROS partially improves muscle regeneration in adult but not old muscle demonstrating a role for CI ROS in the response to muscle injury. Site IQ ROS does not contribute to impaired regenerative capacity in aging.

Elamipretide Improves ADP Sensitivity in Aged Mitochondria by Increasing Uptake through the Adenine Nucleotide Translocator (ANT)

Aging muscle experiences functional decline in part mediated by impaired mitochondrial ADP sensitivity. Elamipretide (ELAM) rapidly improves physiological and mitochondrial function in aging and binds directly to the mitochondrial ADP transporter ANT. We hypothesized that ELAM improves ADP sensitivity in aging leading to rescued physiological function. We measured the response to ADP stimulation in young and old muscle mitochondria with ELAM treatment, in vivo heart and muscle function, and compared protein abundance, phosphorylation, and S-glutathionylation of ADP/ATP pathway proteins. ELAM treatment increased ADP sensitivity in old muscle mitochondria by increasing uptake of ADP through the ANT and rescued muscle force and heart systolic function. Protein abundance in the ADP/ATP transport and synthesis pathway was unchanged, but ELAM treatment decreased protein s-glutathionylation incuding of ANT. Mitochondrial ADP sensitivity is rapidly modifiable. This research supports the hypothesis that ELAM improves ANT function in aging and links mitochondrial ADP sensitivity to physiological function.

Abstract Figure

Determining the contributions of protein synthesis and breakdown to muscle atrophy requires non-steady-state equations

BACKGROUND

Ageing and cachexia cause a loss of muscle mass over time, indicating that protein breakdown exceeds protein synthesis. Deuterium oxide (D₂ O) is used for studies of protein turnover because of the advantages of long-term labelling, but these methods introduce considerations that have been largely overlooked when studying conditions of protein gain or loss. The purpose of this study was to demonstrate the importance of accounting for a change in protein mass, a non-steady state, during D₂ O labelling studies while also exploring the contribution of protein synthesis and breakdown to denervation-induced muscle atrophy.

METHODS

Adult (6 months) male C57BL/6 mice (n = 14) were labelled with D₂ O for a total of 7 days following unilateral sciatic nerve transection to induce denervation of hindlimb muscles. The contralateral sham limb and nonsurgical mice (n = 5) were used as two different controls to account for potential crossover effects of denervation. We calculated gastrocnemius myofibrillar and collagen protein synthesis and breakdown assuming steady-state or using non-steady-state modelling. We measured RNA synthesis rates to further understand ribosomal turnover during atrophy.

RESULTS

Gastrocnemius mass was less in denervated muscle (137 ± 9 mg) compared with sham (174 ± 15 mg; P < 0.0001) or nonsurgical control (162 ± 5 mg; P < 0.0001). With steady-state calculations, fractional synthesis and breakdown rates (FSR and FBR) were lower in the denervated muscle (1.49 ± 0.06%/day) compared with sham (1.81 ± 0.09%/day; P < 0.0001) or nonsurgical control (2.27 ± 0.04%/day; P < 0.0001). When adjusting for change in protein mass, FSR was 4.21 ± 0.19%/day in denervated limb, whereas FBR was 4.09 ± 0.22%/day. When considering change in protein mass (k_syn ), myofibrillar synthesis was lower in denervated limb (2.44 ± 0.14 mg/day) compared with sham (3.43 ± 0.22 mg/day; P < 0.0001) and non-surgical control (3.74 ± 0.12 mg/day; P < 0.0001), whereas rate of protein breakdown (k_deg, 1/t) was greater in denervated limb (0.050 ± 0.003) compared with sham (0.019 ± 0.001; P < 0.0001) and nonsurgical control (0.023 ± 0.000; P < 0.0001). Muscle collagen breakdown was completely inhibited during denervation. There was a strong correlation (r = 0.83, P < 0.001) between RNA and myofibrillar protein synthesis in sham but not denervated muscle.

CONCLUSIONS

We show conflicting results between steady- and non-steady-state calculations on myofibrillar protein synthesis and breakdown during periods of muscle loss. We also found that collagen accumulation was largely from a decrease in collagen breakdown. Comparison between sham and non-surgical control demonstrated a crossover effect of denervation on myofibrillar protein synthesis and ribosomal biogenesis, which impacts study design for unilateral atrophy studies. These considerations are important because not accounting for them can mislead therapeutic attempts to maintain muscle mass.

Reduced adenosine diphosphate sensitivity in skeletal muscle mitochondria increases reactive oxygen species production in mouse models of aging and oxidative stress but not denervation

Background

Mitochondrial bioenergetics are sensitive to adenosine diphosphate (ADP) concentration. Reactive oxygen species (ROS) production and respiration [oxygen consumption rate (OCR)] are altered at physiological ADP concentrations (i.e. ADP insensitivity) in aged human muscle. Here, we investigate ADP sensitivity in mouse muscle mitochondria.

Methods

We measured OCR and ROS production in permeabilized gastrocnemius fibres using an ADP titration protocol and the Oroboros O2k respirometer and fluorometer. We measured changes in ADP sensitivity in muscle from mice at different ages, after sciatic nerve transection (denervation), and in response to increased oxidative stress (Sod1 -/- mice). Further, we asked whether the mitochondrial-targeted peptide SS-31 can modulate ADP insensitivity and contractile function in the Sod1 -/- mouse model.

Results

Reduced ADP sensitivity is associated with increases in mitochondrial ROS production in aged (62%) and Sod1 -/- (33%) mice. The maximal capacity to produce ROS does not increase with age, and there is no effect of age on ADP sensitivity for OCR in mouse gastrocnemii. Denervation does not induce ADP insensitivity for either ROS generation or OCR. Treatment of Sod1 -/- mice with SS-31 increases ADP sensitivity for both OCR and ROS, decreases maximal ROS production (~40%), and improves resistance to muscle fatigue.

Conclusions

Adenosine diphosphate sensitivity for ROS production decreases in aged mouse gastrocnemius muscle fibres, although aged mice do not exhibit a difference in OCR. Denervation does not induce ADP insensitivity; however, insensitivity to ADP is induced in a model of oxidative stress. ADP insensitivity could contribute to muscle fatigue, and SS-31 may be the first drug capable of targeting this aging phenotype.

Cancer cachexia in a mouse model of oxidative stress

BACKGROUND

Cancer is associated with muscle atrophy (cancer cachexia) that is linked to up to 40% of cancer-related deaths. Oxidative stress is a critical player in the induction and progression of age-related loss of muscle mass and weakness (sarcopenia); however, the role of oxidative stress in cancer cachexia has not been defined. The purpose of this study was to examine if elevated oxidative stress exacerbates cancer cachexia.

METHODS

Cu/Zn superoxide dismutase knockout (Sod1KO) mice were used as an established mouse model of elevated oxidative stress. Cancer cachexia was induced by injection of one million Lewis lung carcinoma (LLC) cells or phosphate-buffered saline (saline) into the hind flank of female wild-type mice or Sod1KO mice at approximately 4 months of age. The tumour developed for 3 weeks. Muscle mass, contractile function, neuromuscular junction (NMJ) fragmentation, metabolic proteins, mitochondrial function, and motor neuron function were measured in wild-type and Sod1KO saline and tumour-bearing mice. Data were analysed by two-way ANOVA with Tukey-Kramer post hoc test when significant F ratios were determined and α was set at 0.05. Unless otherwise noted, results in abstract are mean ±SEM.

RESULTS

Muscle mass and cross-sectional area were significantly reduced, in tumour-bearing mice. Metabolic enzymes were dysregulated in Sod1KO mice and cancer exacerbated this phenotype. NMJ fragmentation was exacerbated in tumour-bearing Sod1KO mice. Myofibrillar protein degradation increased in tumour-bearing wild-type mice (wild-type saline, 0.00847 ± 0.00205; wildtype LLC, 0.0211 ± 0.00184) and tumour-bearing Sod1KO mice (Sod1KO saline, 0.0180 ± 0.00118; Sod1KO LLC, 0.0490 ± 0.00132). Muscle mitochondrial oxygen consumption was reduced in tumour-bearing mice compared with saline-injected wild-type mice. Mitochondrial protein degradation increased in tumour-bearing wild-type mice (wild-type saline, 0.0204 ± 0.00159; wild-type LLC, 0.167 ± 0.00157) and tumour-bearing Sod1KO mice (Sod1KO saline, 0.0231 ± 0.00108; Sod1 KO LLC, 0.0645 ± 0.000631). Sciatic nerve conduction velocity was decreased in tumour-bearing wild-type mice (wild-type saline, 38.2 ± 0.861; wild-type LLC, 28.8 ± 0.772). Three out of eleven of the tumour-bearing Sod1KO mice did not survive the 3-week period following tumour implantation.

CONCLUSIONS

Oxidative stress does not exacerbate cancer-induced muscle loss; however, cancer cachexia may accelerate NMJ disruption.

Targeting cPLA2 derived lipid hydroperoxides as a potential intervention for sarcopenia

Defects in neuromuscular innervation contribute significantly to the age-related decline in muscle mass and function (sarcopenia). Our previous studies demonstrated that denervation induces muscle mitochondrial hydroperoxide production (H2O2 and lipid hydroperoxides (LOOHs)). Here we define the relative contribution of mitochondrial electron transport chain (ETC) derived H2O2 versus cytosolic phospholipase A2 (cPLA2) derived LOOHs in neurogenic muscle atrophy. We show that denervation increases muscle cPLA2 protein content, activity, and metabolites downstream of cPLA2 including LOOHs. Increased scavenging of mitochondrial H2O2 does not protect against denervation atrophy, suggesting ETC generated H2O2 is not a critical player. In contrast, inhibition of cPLA2 in vivo mitigates LOOH production and muscle atrophy and maintains individual muscle fiber size while decreasing oxidative damage. Overall, we show that loss of innervation in several muscle atrophy models including aging induces generation of LOOHs produced by arachidonic acid metabolism in the cPLA2 pathway contributing to loss of muscle mass.

Disparate Central and Peripheral Effects of Circulating IGF-1 Deficiency on Tissue Mitochondrial Function

Age-related decline in circulating levels of insulin-like growth factor (IGF)-1 is associated with reduced cognitive function, neuronal aging, and neurodegeneration. Decreased mitochondrial function along with increased reactive oxygen species (ROS) and accumulation of damaged macromolecules are hallmarks of cellular aging. Based on numerous studies indicating pleiotropic effects of IGF-1 during aging, we compared the central and peripheral effects of circulating IGF-1 deficiency on tissue mitochondrial function using an inducible liver IGF-1 knockout (LID). Circulating levels of IGF-1 (~ 75%) were depleted in adult male Igf1^f/f mice via AAV-mediated knockdown of hepatic IGF-1 at 5 months of age. Cognitive function was evaluated at 18 months using the radial arm water maze and glucose and insulin tolerance assessed. Mitochondrial function was analyzed in hippocampus, muscle, and visceral fat tissues using high-resolution respirometry O2K as well as redox status and oxidative stress in the cortex. Peripherally, IGF-1 deficiency did not significantly impact muscle mass or mitochondrial function. Aged LID mice were insulin resistant and exhibited ~ 60% less adipose tissue but increased fat mitochondrial respiration (20%). The effects on fat metabolism were attributed to increases in growth hormone. Centrally, IGF-1 deficiency impaired hippocampal-dependent spatial acquisition as well as reversal learning in male mice. Hippocampal mitochondrial OXPHOS coupling efficiency and cortex ATP levels (~ 50%) were decreased and hippocampal oxidative stress (protein carbonylation and F₂-isoprostanes) was increased. These data suggest that IGF-1 is critical for regulating mitochondrial function, redox status, and spatial learning in the central nervous system but has limited impact on peripheral (liver and muscle) metabolism with age. Therefore, IGF-1 deficiency with age may increase sensitivity to damage in the brain and propensity for cognitive deficits. Targeting mitochondrial function in the brain may be an avenue for therapy of age-related impairment of cognitive function. Regulation of mitochondrial function and redox status by IGF-1 is essential to maintain brain function and coordinate hippocampal-dependent spatial learning. While a decline in IGF-1 in the periphery may be beneficial to avert cancer progression, diminished central IGF-1 signaling may mediate, in part, age-related cognitive dysfunction and cognitive pathologies potentially by decreasing mitochondrial function.

Metabolic and Stress Response Changes Precede Disease Onset in the Spinal Cord of Mutant SOD1 ALS Mice

Many Amyotrophic Lateral Sclerosis (ALS) patients experience hypermetabolism, or an increase in measured vs. calculated metabolic rate. The cause of hypermetabolism and the effects on neuronal metabolism in ALS are currently unknown, but the efficacy of dietary interventions shows promise for metabolism as an ALS therapeutic target. The goal of this study is to measure changes in metabolic pathways as a function of disease progression in spinal cords of the SOD1^G93A mouse model of ALS. We conducted a comprehensive assessment of protein expression for metabolic pathways, antioxidants, chaperones, and proteases in lumbar spinal cord from male SOD1^G93A mice at pre-onset, onset, and end-stages of the disease using targeted proteomic analysis. These results reveal that protein content of metabolic proteins including proteins involved in glycolysis, β-oxidation, and mitochondrial metabolism is altered in SOD1^G93A mouse spinal cord well before disease onset. The changes in mitochondrial metabolism proteins are associated with decreased maximal respiration and glycolytic flux in SOD1^G93A dermal fibroblasts and increased hydrogen peroxide and lipid hydroperoxide production in mitochondria from sciatic nerve and gastrocnemius muscle fibers at end stage of disease. Consistent with redox dysregulation, expression of the glutathione antioxidant system is decreased, and peroxiredoxins and catalase expression are increased. In addition, stress response proteases and chaperones, including those involved in the mitochondrial unfolded protein response (UPR^mt), are induced before disease onset. In summary, we report that metabolic and stress response changes occur in SOD1^G93A lumbar spinal cord before motor symptom onset, and are primarily caused by SOD1^G93A expression and do not vary greatly as a function of disease course.

Mitochondrial oxidative stress impairs contractile function but paradoxically increases muscle mass via fibre branching

BACKGROUND

Excess reactive oxygen species (ROS) and muscle weakness occur in parallel in multiple pathological conditions. However, the causative role of skeletal muscle mitochondrial ROS (mtROS) on neuromuscular junction (NMJ) morphology and function and muscle weakness has not been directly investigated.

METHODS

We generated mice lacking skeletal muscle-specific manganese-superoxide dismutase (mSod2KO) to increase mtROS using a cre-Lox approach driven by human skeletal actin. We determined primary functional parameters of skeletal muscle mitochondrial function (respiration, ROS, and calcium retention capacity) using permeabilized muscle fibres and isolated muscle mitochondria. We assessed contractile properties of isolated skeletal muscle using in situ and in vitro preparations and whole lumbrical muscles to elucidate the mechanisms of contractile dysfunction.

RESULTS

The mSod2KO mice, contrary to our prediction, exhibit a 10-15% increase in muscle mass associated with an ~50% increase in central nuclei and ~35% increase in branched fibres (P < 0.05). Despite the increase in muscle mass of gastrocnemius and quadriceps, in situ sciatic nerve-stimulated isometric maximum-specific force (N/cm² ), force per cross-sectional area, is impaired by ~60% and associated with increased NMJ fragmentation and size by ~40% (P < 0.05). Intrinsic alterations of components of the contractile machinery show elevated markers of oxidative stress, for example, lipid peroxidation is increased by ~100%, oxidized glutathione is elevated by ~50%, and oxidative modifications of myofibrillar proteins are increased by ~30% (P < 0.05). We also find an approximate 20% decrease in the intracellular calcium transient that is associated with specific force deficit. Excess superoxide generation from the mitochondrial complexes causes a deficiency of succinate dehydrogenase and reduced complex-II-mediated respiration and adenosine triphosphate generation rates leading to severe exercise intolerance (~10 min vs. ~2 h in wild type, P < 0.05).

CONCLUSIONS

Increased skeletal muscle mtROS is sufficient to elicit NMJ disruption and contractile abnormalities, but not muscle atrophy, suggesting new roles for mitochondrial oxidative stress in maintenance of muscle mass through increased fibre branching.

Lifelong reduction in complex IV induces tissue-specific metabolic effects but does not reduce lifespan or healthspan in mice

Loss of SURF1, a Complex IV assembly protein, was reported to increase lifespan in mice despite dramatically lower cytochrome oxidase (COX) activity. Consistent with this, our previous studies found advantageous changes in metabolism (reduced adiposity, increased insulin sensitivity, and mitochondrial biogenesis) in Surf1^−/− mice. The lack of deleterious phenotypes in Surf1^−/− mice is contrary to the hypothesis that mitochondrial dysfunction contributes to aging. We found only a modest (nonsignificant) extension of lifespan (7% median, 16% maximum) and no change in healthspan indices in Surf1^−/− vs. Surf1^+/+ mice despite substantial decreases in COX activity (22%–87% across tissues). Dietary restriction (DR) increased median lifespan in both Surf1^+/+ and Surf1^−/− mice (36% and 19%, respectively). We measured gene expression, metabolites, and targeted expression of key metabolic proteins in adipose tissue, liver, and brain in Surf1^+/+ and Surf1^−/− mice. Gene expression was differentially regulated in a tissue‐specific manner. Many proteins and metabolites are downregulated in Surf1^−/− adipose tissue and reversed by DR, while in brain, most metabolites that changed were elevated in Surf1^−/− mice. Finally, mitochondrial unfolded protein response (UPR^mt)‐associated proteins were not uniformly altered by age or genotype, suggesting the UPR^mt is not a key player in aging or in response to reduced COX activity. While the changes in gene expression and metabolism may represent compensatory responses to mitochondrial stress, the important outcome of this study is that lifespan and healthspan are not compromised in Surf1^−/− mice, suggesting that not all mitochondrial deficiencies are a critical determinant of lifespan.

Loss of mitochondrial protease ClpP protects mice from diet-induced obesity and insulin resistance

Caseinolytic peptidase P (ClpP) is a mammalian quality control protease that is proposed to play an important role in the initiation of the mitochondrial unfolded protein response (UPR^mt), a retrograde signaling response that helps to maintain mitochondrial protein homeostasis. Mitochondrial dysfunction is associated with the development of metabolic disorders, and to understand the effect of a defective UPR^mt on metabolism, ClpP knockout (ClpP^-/-) mice were analyzed. ClpP^-/- mice fed ad libitum have reduced adiposity and paradoxically improved insulin sensitivity. Absence of ClpP increased whole-body energy expenditure and markers of mitochondrial biogenesis are selectively up-regulated in the white adipose tissue (WAT) of ClpP^-/- mice. When challenged with a metabolic stress such as high-fat diet, despite similar caloric intake, ClpP^-/- mice are protected from diet-induced obesity, glucose intolerance, insulin resistance, and hepatic steatosis. Our results show that absence of ClpP triggers compensatory responses in mice and suggest that ClpP might be dispensable for mammalian UPR^mt initiation. Thus, we made an unexpected finding that deficiency of ClpP in mice is metabolically beneficial.

A fish oil diet induces mitochondrial uncoupling and mitochondrial unfolded protein response in epididymal white adipose tissue of mice

White adipose tissue (WAT) mitochondrial dysfunction is linked to the pathogenesis of obesity driven insulin resistance. Dietary conditions that alter fat mass are known to affect white adipocyte mitochondrial function, however, the impact of high calorie diets on white adipocyte mitochondria is not fully understood. The aim of this study is to assess the effect of a diet rich in saturated or polyunsaturated fat on mitochondrial unfolded protein response (UPR^mt), a retrograde signaling response that maintains mitochondrial homeostasis, in epididymal WAT (eWAT). Mice were fed a low fat diet (LFD), saturated fat diet (SFD) or fish oil (unsaturated fat diet, UFD) and assessed changes in eWAT mitochondria. Compared to mice fed a LFD, SFD-fed mice have reduced mitochondrial biogenesis markers, mitochondrial fatty acid oxidation enzymes and TCA cycle enzymes, suggesting an impaired mitochondrial function that could contribute to increased fat mass. In contrast, isocaloric UFD-fed mice have increased expression of mitochondrial uncoupling protein 1 (UCP1) and peroxisomal fatty acid oxidation enzymes suggesting that elevated mitochondrial uncoupling and peroxisomal fatty acid oxidation could contribute to the reduction in fat mass. Interestingly, expression of UPR^mt-associated proteins caseinolytic peptidase (ClpP) and heat shock protein 60 (Hsp60) are induced by UFD, whereas SFD reduced the expression of ClpP. Based on our data, we propose that induction of UPR^mt helps to preserve a functional mitochondria and efficient utilization of fat by UFD whereas a dampened UPR^mt response might impair mitochondrial function and promote fat accumulation by SFD. Thus, our findings suggest a potential role of UPR^mt in mediating the beneficial effects of fish oil.

Ablation of the mitochondrial complex IV assembly protein Surf1 leads to increased expression of the UPR(MT) and increased resistance to oxidative stress in primary cultures of fibroblasts

Mice deficient in the electron transport chain (ETC) complex IV assembly protein SURF1 have reduced assembly and activity of cytochrome c oxidase that is associated with an upregulation of components of the mitochondrial unfolded protein response (UPR^MT) and increased mitochondrial number. We hypothesized that the upregulation of proteins associated with the UPR^MT in response to reduced cytochrome c oxidase activity in Surf1^−/− mice might contribute to increased stress resistance. To test this hypothesis we asked whether primary cultures of fibroblasts from Surf1^−/− mice exhibit enhanced resistance to stressors compared to wild-type fibroblasts. Here we show that primary dermal fibroblasts isolated from Surf1^−/− mice have increased expression of UPR^MT components ClpP and Hsp60, and increased expression of Lon protease. Fibroblasts from Surf1^−/− mice are significantly more resistant to cell death caused by oxidative stress induced by paraquat or tert-Butyl hydroperoxide compared to cells from wild-type mice. In contrast, Surf1^−/− fibroblasts show no difference in sensitivity to hydrogen peroxide stress. The enhanced cell survival in response to paraquat or tert-Butyl hydroperoxide in Surf1^−/− fibroblasts compared to wild-type fibroblasts is associated with induced expression of Lon, ClpP, and Hsp60, increased maximal respiration, and increased reserve capacity as measured using the Seahorse Extracellular Flux Analyzer. Overall these data support a protective role for the activation of the UPR^MT in cell survival.

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Surf1^−/− mice fibroblasts exhibit upregulation of proteins involved in the UPR^MT.

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Mitochondrial specific oxidative stressors induce UPR^MT in mammalian fibroblasts.

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Surf1^−/− fibroblasts exhibit enhanced mitochondrial specific stress resistance.

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Surf1^−/− fibroblasts have increased maximal respiration and respiratory reserve.