Dynamic Regulation of Mitochondrial Import by the Ubiquitin System.

Mitochondria import nearly their entire proteome from the cytoplasm by translocating precursor proteins through the translocase of the outer membrane (TOM) complex. Here, we show dynamic regulation of mitochondrial import by the ubiquitin system. Acute pharmacological inhibition or genetic ablation of the mitochondrial deubiquitinase (DUB) USP30 triggers accumulation of Ub-substrates that are normally localized inside the mitochondria. Mitochondrial import of USP30 substrates is impaired in USP30 knockout (KO) cells, suggesting that deubiquitination promotes efficient import. Upstream of USP30, the E3 ligase March5 ubiquitinates mitochondrial proteins whose eventual import depends on USP30. In USP30 KOs, exogenous March5 expression induces accumulation of unimported translocation intermediates that are degraded by the proteasomes. In USP30 KO mice, TOM subunits have reduced abundance across multiple tissues. Together these data highlight how protein import into a subcellular compartment can be regulated by ubiquitination and deubiquitination by E3 ligase and DUB machinery positioned at the gate.

High-fat diet and FGF21 cooperatively promote aerobic thermogenesis in mtDNA mutator mice.

Mitochondria are highly adaptable organelles that can facilitate communication between tissues to meet the energetic demands of the organism. However, the mechanisms by which mitochondria can nonautonomously relay stress signals remain poorly understood. Here we report that mitochondrial mutations in the young, preprogeroid polymerase gamma mutator (POLG) mouse produce a metabolic state of starvation. As a result, these mice exhibit signs of metabolic imbalance including thermogenic defects in brown adipose tissue (BAT). An unexpected benefit of this adaptive response is the complete resistance to diet-induced obesity when POLG mice are placed on a high-fat diet (HFD). Paradoxically, HFD further increases oxygen consumption in part by inducing thermogenesis and mitochondrial biogenesis in BAT along with enhanced expression of fibroblast growth factor 21 (FGF21). Collectively, these findings identify a mechanistic link between FGF21, a long-known marker of mitochondrial disease, and systemic metabolic adaptation in response to mitochondrial stress.

Burmese pythons exhibit a transient adaptation to nutrient overload that prevents liver damage.

As an opportunistic predator, the Burmese python (Python molurus bivittatus) consumes large and infrequent meals, fasting for up to a year. Upon consuming a large meal, the Burmese python exhibits extreme metabolic responses. To define the pathways that regulate these postprandial metabolic responses, we performed a comprehensive profile of plasma metabolites throughout the digestive process. Following ingestion of a meal equivalent to 25% of its body mass, plasma lipoproteins and metabolites, such as chylomicra and bile acids, reach levels observed only in mammalian models of extreme dyslipidemia. Here, we provide evidence for an adaptive response to postprandial nutrient overload by the python liver, a critical site of metabolic homeostasis. The python liver undergoes a substantial increase in mass through proliferative processes, exhibits hepatic steatosis, hyperlipidemia-induced insulin resistance indicated by PEPCK activation and pAKT deactivation, and de novo fatty acid synthesis via FASN activation. This postprandial state is completely reversible. We posit that Burmese pythons evade the permanent hepatic damage associated with these metabolic states in mammals using evolved protective measures to inactivate these pathways. These include a transient activation of hepatic nuclear receptors induced by fatty acids and bile acids, including PPAR and FXR, respectively. The stress-induced p38 MAPK pathway is also transiently activated during the early stages of digestion. Taken together, these data identify a reversible metabolic response to hyperlipidemia by the python liver, only achieved in mammals by pharmacologic intervention. The factors involved in these processes may be relevant to or leveraged for remediating human hepatic pathology.

Whole transcriptome analysis of the fasting and fed Burmese python heart: insights into extreme physiological cardiac adaptation.

The infrequently feeding Burmese python (Python molurus) experiences significant and rapid postprandial cardiac hypertrophy followed by regression as digestion is completed. To begin to explore the molecular mechanisms of this response, we have sequenced and assembled the fasted and postfed Burmese python heart transcriptomes with Illumina technology using the chicken (Gallus gallus) genome as a reference. In addition, we have used RNA-seq analysis to identify differences in the expression of biological processes and signaling pathways between fasted, 1 day postfed (DPF), and 3 DPF hearts. Out of a combined transcriptome of ∼2,800 mRNAs, 464 genes were differentially expressed. Genes showing differential expression at 1 DPF compared with fasted were enriched for biological processes involved in metabolism and energetics, while genes showing differential expression at 3 DPF compared with fasted were enriched for processes involved in biogenesis, structural remodeling, and organization. Moreover, we present evidence for the activation of physiological and not pathological signaling pathways in this rapid, novel model of cardiac growth in pythons. Together, our data provide the first comprehensive gene expression profile for a reptile heart.

PPEF2 Opposes PINK1-Mediated Mitochondrial Quality Control by Dephosphorylating Ubiquitin.

Dysregulation of mitophagy, whereby damaged mitochondria are labeled for degradation by the mitochondrial kinase PINK1 and E3 ubiquitin ligase Parkin with phosphorylated ubiquitin chains (p-S65 ubiquitin), may contribute to neurodegeneration in Parkinson's disease. Here, we identify a phosphatase antagonistic to PINK1, protein phosphatase with EF-hand domain 2 (PPEF2), that can dephosphorylate ubiquitin and suppress PINK1-dependent mitophagy. Knockdown of PPEF2 amplifies the accumulation of p-S65 ubiquitin in cells and enhances baseline mitophagy in dissociated cortical cultures. Overexpressing enzymatically active PPEF2 reduces the p-S65 ubiquitin signal in cells, and partially purified PPEF2 can dephosphorylate recombinant p-S65 ubiquitin chains in vitro. Using a mass spectrometry approach, we have identified several p-S65-ubiquitinated proteins following mitochondrial damage that are inversely regulated by PPEF2 and PINK1. Interestingly, many of these proteins are involved in nuclear processes such as DNA repair. Collectively, PPEF2 functions to suppress mitochondrial quality control on a cellular level through dephosphorylation of p-S65 ubiquitin.

Metabolic and Organelle Morphology Defects in Mice and Human Patients Define Spinocerebellar Ataxia Type 7 as a Mitochondrial Disease.

Spinocerebellar ataxia type 7 (SCA7) is a retinal-cerebellar degenerative disorder caused by CAG-polyglutamine (polyQ) repeat expansions in the ataxin-7 gene. As many SCA7 clinical phenotypes occur in mitochondrial disorders, and magnetic resonance spectroscopy of patients revealed altered energy metabolism, we considered a role for mitochondrial dysfunction. Studies of SCA7 mice uncovered marked impairments in oxygen consumption and respiratory exchange. When we examined cerebellar Purkinje cells in mice, we observed mitochondrial network abnormalities, with enlarged mitochondria upon ultrastructural analysis. We developed stem cell models from patients and created stem cell knockout rescue systems, documenting mitochondrial morphology defects, impaired oxidative metabolism, and reduced expression of nicotinamide adenine dinucleotide (NAD+) production enzymes in SCA7 models. We observed NAD+ reductions in mitochondria of SCA7 patient NPCs using ratiometric fluorescent sensors and documented alterations in tryptophan-kynurenine metabolism in patients. Our results indicate that mitochondrial dysfunction, stemming from decreased NAD+, is a defining feature of SCA7.

PPARδ Promotes Running Endurance by Preserving Glucose.

Management of energy stores is critical during endurance exercise; a shift in substrate utilization from glucose toward fat is a hallmark of trained muscle. Here we show that this key metabolic adaptation is both dependent on muscle PPARδ and stimulated by PPARδ ligand. Furthermore, we find that muscle PPARδ expression positively correlates with endurance performance in BXD mouse reference populations. In addition to stimulating fatty acid metabolism in sedentary mice, PPARδ activation potently suppresses glucose catabolism and does so without affecting either muscle fiber type or mitochondrial content. By preserving systemic glucose levels, PPARδ acts to delay the onset of hypoglycemia and extends running time by ∼100 min in treated mice. Collectively, these results identify a bifurcated PPARδ program that underlies glucose sparing and highlight the potential of PPARδ-targeted exercise mimetics in the treatment of metabolic disease, dystrophies, and, unavoidably, the enhancement of athletic performance.

Growth differentiation factor 15 is a myomitokine governing systemic energy homeostasis.

Reduced mitochondrial electron transport chain activity promotes longevity and improves energy homeostasis via cell-autonomous and -non-autonomous factors in multiple model systems. This mitohormetic effect is thought to involve the mitochondrial unfolded protein response (UPRmt), an adaptive stress-response pathway activated by mitochondrial proteotoxic stress. Using mice with skeletal muscle-specific deficiency of Crif1 (muscle-specific knockout [MKO]), an integral protein of the large mitoribosomal subunit (39S), we identified growth differentiation factor 15 (GDF15) as a UPRmt-associated cell-non-autonomous myomitokine that regulates systemic energy homeostasis. MKO mice were protected against obesity and sensitized to insulin, an effect associated with elevated GDF15 secretion after UPRmt activation. In ob/ob mice, administration of recombinant GDF15 decreased body weight and improved insulin sensitivity, which was attributed to elevated oxidative metabolism and lipid mobilization in the liver, muscle, and adipose tissue. Thus, GDF15 is a potent mitohormetic signal that safeguards against the onset of obesity and insulin resistance.

Nuclear receptors and AMPK: can exercise mimetics cure diabetes?

Endurance exercise can lead to systemic improvements in insulin sensitivity and metabolic homeostasis, and is an effective approach to combat metabolic diseases. Pharmacological compounds that recapitulate the beneficial effects of exercise, also known as 'exercise mimetics', have the potential to improve disease symptoms of metabolic syndrome. These drugs, which can increase energy expenditure, suppress hepatic gluconeogenesis, and induce insulin sensitization, have accordingly been highly scrutinized for their utility in treating metabolic diseases including diabetes. Nevertheless, the identity of an efficacious exercise mimetic still remains elusive. In this review, we highlight several nuclear receptors and cofactors that are putative molecular targets for exercise mimetics, and review recent studies that provide advancements in our mechanistic understanding of how exercise mimetics exert their beneficial effects. We also discuss evidence from clinical trials using these compounds in human subjects to evaluate their efficacy in treating diabetes.

Fatty acids identified in the Burmese python promote beneficial cardiac growth.

Burmese pythons display a marked increase in heart mass after a large meal. We investigated the molecular mechanisms of this physiological heart growth with the goal of applying this knowledge to the mammalian heart. We found that heart growth in pythons is characterized by myocyte hypertrophy in the absence of cell proliferation and by activation of physiological signal transduction pathways. Despite high levels of circulating lipids, the postprandial python heart does not accumulate triglycerides or fatty acids. Instead, there is robust activation of pathways of fatty acid transport and oxidation combined with increased expression and activity of superoxide dismutase, a cardioprotective enzyme. We also identified a combination of fatty acids in python plasma that promotes physiological heart growth when injected into either pythons or mice.