α-Lipoic acid promotes α-tubulin hyperacetylation and blocks the turnover of mitochondria through mitophagy

The interplay between mitochondrial dynamics and autophagy: From a key homeostatic mechanism to a driver of pathology

The complex relationship between mitochondrial dynamics and autophagy illustrates how two cellular housekeeping processes are intimately linked, illuminating fundamental principles of cellular homeostasis and shedding light on disparate pathological conditions including several neurodegenerative disorders. Here we review the basic tenets of mitochondrial dynamics i.e., the concerted balance between fusion and fission of the organelle, and its interplay with macroautophagy and selective mitochondrial autophagy, also dubbed mitophagy, in the maintenance of mitochondrial quality control and ultimately in cell viability. We illustrate how conditions of altered mitochondrial dynamics reverberate on autophagy and vice versa. Finally, we illustrate how altered interplay between these two key cellular processes participates in the pathogenesis of human disorders affecting multiple organs and systems.

Metabolic control of mitophagy

Mitochondrial dysfunction is a major hallmark of ageing and related chronic disorders. Controlled removal of damaged mitochondria by the autophagic machinery, a process known as mitophagy, is vital for mitochondrial homeostasis and cell survival. The central role of mitochondria in cellular metabolism places mitochondrial removal at the interface of key metabolic pathways affecting the biosynthesis or catabolism of acetyl-coenzyme A, nicotinamide adenine dinucleotide, polyamines, as well as fatty acids and amino acids. Molecular switches that integrate the metabolic status of the cell, like AMP-dependent protein kinase, protein kinase A, mechanistic target of rapamycin and sirtuins, have also emerged as important regulators of mitophagy. In this review, we discuss how metabolic regulation intersects with mitophagy. We place special emphasis on the metabolic regulatory circuits that may be therapeutically targeted to delay ageing and mitochondria-associated chronic diseases. Moreover, we identify outstanding knowledge gaps, such as the ill-defined distinction between basal and damage-induced mitophagy, which must be resolved to boost progress in this area.

Chemoproteomic target deconvolution reveals Histone Deacetylases as targets of (R)-lipoic acid

Lipoic acid is an essential enzyme cofactor in central metabolic pathways. Due to its claimed antioxidant properties, racemic (R/S)-lipoic acid is used as a food supplement but is also investigated as a pharmaceutical in over 180 clinical trials covering a broad range of diseases. Moreover, (R/S)-lipoic acid is an approved drug for the treatment of diabetic neuropathy. However, its mechanism of action remains elusive. Here, we performed chemoproteomics-aided target deconvolution of lipoic acid and its active close analog lipoamide. We find that histone deacetylases HDAC1, HDAC2, HDAC3, HDAC6, HDAC8, and HDAC10 are molecular targets of the reduced form of lipoic acid and lipoamide. Importantly, only the naturally occurring (R)-enantiomer inhibits HDACs at physiologically relevant concentrations and leads to hyperacetylation of HDAC substrates. The inhibition of HDACs by (R)-lipoic acid and lipoamide explain why both compounds prevent stress granule formation in cells and may also provide a molecular rationale for many other phenotypic effects elicited by lipoic acid.

Saturated fatty acids increase LPI to reduce FUNDC1 dimerization and stability and mitochondrial function

Ectopic lipid deposition and mitochondrial dysfunction are common etiologies of obesity and metabolic disorders. Excessive dietary uptake of saturated fatty acids (SFAs) causes mitochondrial dysfunction and metabolic disorders, while unsaturated fatty acids (UFAs) counterbalance these detrimental effects. It remains elusive how SFAs and UFAs differentially signal toward mitochondria for mitochondrial performance. We report here that saturated dietary fatty acids such as palmitic acid (PA), but not unsaturated oleic acid (OA), increase lysophosphatidylinositol (LPI) production to impact on the stability of the mitophagy receptor FUNDC1 and on mitochondrial quality. Mechanistically, PA shifts FUNDC1 from dimer to monomer via enhanced production of LPI. Monomeric FUNDC1 shows increased acetylation at K104 due to dissociation of HDAC3 and increased interaction with Tip60. Acetylated FUNDC1 can be further ubiquitinated by MARCH5 for proteasomal degradation. Conversely, OA antagonizes PA-induced accumulation of LPI, and FUNDC1 monomerization and degradation. A fructose-, palmitate-, and cholesterol-enriched (FPC) diet also affects FUNDC1 dimerization and promotes its degradation in a non-alcoholic steatohepatitis (NASH) mouse model. We thus uncover a signaling pathway that orchestrates lipid metabolism with mitochondrial quality.

The Role of HDAC6 in Autophagy and NLRP3 Inflammasome

Autophagy fights against harmful stimuli and degrades cytosolic macromolecules, organelles, and intracellular pathogens. Autophagy dysfunction is associated with many diseases, including infectious and inflammatory diseases. Recent studies have identified the critical role of the NACHT, LRR, and PYD domain-containing protein 3 (NLRP3) inflammasomes activation in the innate immune system, which mediates the secretion of proinflammatory cytokines IL-1β/IL-18 and cleaves Gasdermin D to induce pyroptosis in response to pathogenic and sterile stimuli. Accumulating evidence has highlighted the crosstalk between autophagy and NLRP3 inflammasome in multifaceted ways to influence host defense and inflammation. However, the underlying mechanisms require further clarification. Histone deacetylase 6 (HDAC6) is a class IIb deacetylase among the 18 mammalian HDACs, which mainly localizes in the cytoplasm. It is involved in two functional deacetylase domains and a ubiquitin-binding zinc finger domain (ZnF-BUZ). Due to its unique structure, HDAC6 regulates various physiological processes, including autophagy and NLRP3 inflammasome, and may play a role in the crosstalk between them. In this review, we provide insight into the mechanisms by which HDAC6 regulates autophagy and NLRP3 inflammasome and we explored the possibility and challenges of HDAC6 in the crosstalk between autophagy and NLRP3 inflammasome. Finally, we discuss HDAC6 inhibitors as a potential therapeutic approach targeting either autophagy or NLRP3 inflammasome as an anti-inflammatory strategy, although further clarification is required regarding their crosstalk.

Aging-dependent mitochondrial dysfunction mediated by ceramide signaling inhibits antitumor T cell response

We lack a mechanistic understanding of aging-mediated changes in mitochondrial bioenergetics and lipid metabolism that affect T cell function. The bioactive sphingolipid ceramide, induced by aging stress, mediates mitophagy and cell death; however, the aging-related roles of ceramide metabolism in regulating T cell function remain unknown. Here, we show that activated T cells isolated from aging mice have elevated C14/C16 ceramide accumulation in mitochondria, generated by ceramide synthase 6, leading to mitophagy/mitochondrial dysfunction. Mechanistically, aging-dependent mitochondrial ceramide inhibits protein kinase A, leading to mitophagy in activated T cells. This aging/ceramide-dependent mitophagy attenuates the antitumor functions of T cells in vitro and in vivo. Also, inhibition of ceramide metabolism or PKA activation by genetic and pharmacologic means prevents mitophagy and restores the central memory phenotype in aging T cells. Thus, these studies help explain the mechanisms behind aging-related dysregulation of T cells' antitumor activity, which can be restored by inhibiting ceramide-dependent mitophagy.

Nutrition and Genetics in NAFLD: The Perfect Binomium

Nonalcoholic fatty liver disease (NAFLD) represents a global healthcare burden since it is epidemiologically related to obesity, type 2 diabetes (T2D) and Metabolic Syndrome (MetS). It embraces a wide spectrum of hepatic injuries, which include simple steatosis, nonalcoholic steatohepatitis (NASH), fibrosis, cirrhosis and hepatocellular carcinoma (HCC). The susceptibility to develop NAFLD is highly variable and it is influenced by several cues including environmental (i.e., dietary habits and physical activity) and inherited (i.e., genetic/epigenetic) risk factors. Nonetheless, even intestinal microbiota and its by-products play a crucial role in NAFLD pathophysiology. The interaction of dietary exposure with the genome is referred to as 'nutritional genomics,' which encompasses both 'nutrigenetics' and 'nutriepigenomics.' It is focused on revealing the biological mechanisms that entail both the acute and persistent genome-nutrient interactions that influence health and it may represent a promising field of study to improve both clinical and health nutrition practices. Thus, the premise of this review is to discuss the relevance of personalized nutritional advices as a novel therapeutic approach in NAFLD tailored management.

Autophagic degradation of KAT2A/GCN5 promotes directional migration of vascular smooth muscle cells by reducing TUBA/α-tubulin acetylation

Macroautophagy/autophagy, a fundamental process for degradation of macromolecules and organelles, occurs constitutively at a basal level and is upregulated in response to stress. Whether autophagy regulates protein acetylation and microtubule stability in vascular smooth muscle cells (VSMCs) migration, however, remains unknown. Here, we demonstrate that the histone acetyltransferase KAT2A/GCN5 (lysine acetyltransferase 2) binds directly to the autophagosome protein MAP1LC3/LC3 (microtubule associated protein 1 light chain 3) via a conserved LC3-interacting region (LIR) domain. This interaction is required for KAT2A sequestration in autophagosomes and degradation by lysosomal acid hydrolases. Suppression of autophagy results in KAT2A accumulation. KAT2A functions as an acetyltransferase to increase TUBA/α-tubulin acetylation, promote microtubule polymerization and stability, ultimately inhibiting directional cell migration. Our findings indicate that deacetylation of TUBA and perturbation of microtubule stability via selective autophagic degradation of KAT2A are essential for autophagy-promoting VSMC migration. Abbreviations: ACTB: actin beta; ATAT1: alpha tubulin acetyltransferase 1; ATG: autophagy-related; BECN1: beclin 1; CQ: chloroquine; FBS: fetal bovine serum; GST: glutathione S-transferase; H4K16ac: histone H4 lysine 16 acetylation; HASMCs: human aortic smooth muscle cells; HBSS: Hank's buffered salt solution; HDAC6: histone deacetylase 6; hMOF: human males absent on the first; IP: immunoprecipitation; KAT2A/GCN5: lysine acetyltransferase 2A; Lacta: lactacystin; LIR: LC3-interaction region; MAP1LC3: microtubule associated protein 1 light chain 3; MEFs: mouse embryonic fibroblasts; MTOC: microtubule-organizing center; PE: phosphatidylethanolamine; PtdIns3K: class III phosphatidylinositol 3-kinase; RUNX2: runt-related transcription factor 2; SIRT1: sirtuin 1; SIRT2: sirtuin 2; SQSTM1/p62: sequestosome 1; ULK1: unc-51 like autophagy activating kinase 1; VSMCs: vascular smooth muscle cells; WT: wild-type.

Epigenetic Treatment of Neurodegenerative Ophthalmic Disorders: An Eye Toward the Future

Eye disease is one of the primary medical conditions that requires attention and therapeutic intervention in ageing populations worldwide. Further, the global burden of diabetes and obesity, along with heart disease, all lead to secondary manifestations of ophthalmic distress. Therefore, there is increased interest in developing innovative new approaches that target various mechanisms and sequelae driving conditions that result in adverse vision. The research challenge is even greater given that the terrain of eye diseases is difficult to landscape into a single therapeutic theme. This report addresses the burden of eye disease due to mitochondrial dysfunction, including antioxidant, autophagic, epigenetic, mitophagic, and other cellular processes that modulate the biomedical end result. In this light, we single out lipoic acid as a potent known natural activator of these pathways, along with alternative and potentially more effective conjugates, which together harness the necessary potency, specificity, and biodistribution parameters required for improved therapeutic outcomes.

GCN5L1 modulates cross-talk between mitochondria and cell signaling to regulate FoxO1 stability and gluconeogenesis

The mitochondrial enriched GCN5-like 1 (GCN5L1) protein has been shown to modulate mitochondrial protein acetylation, mitochondrial content and mitochondrial retrograde signaling. Here we show that hepatic GCN5L1 ablation reduces fasting glucose levels and blunts hepatic gluconeogenesis without affecting systemic glucose tolerance. PEPCK and G6Pase transcript levels are downregulated in hepatocytes from GCN5L1 liver specific knockout mice and their upstream regulator, FoxO1 protein levels are decreased via proteasome-dependent degradation and via reactive oxygen species mediated ERK-1/2 phosphorylation. ERK inhibition restores FoxO1, gluconeogenic enzyme expression and glucose production. Reconstitution of mitochondrial-targeted GCN5L1 blunts mitochondrial ROS, ERK activation and increases FoxO1, gluconeogenic enzyme expression and hepatocyte glucose production. We suggest that mitochondrial GCN5L1 modulates post-translational control of FoxO1, regulates gluconeogenesis and controls metabolic pathways via mitochondrial ROS mediated ERK activation. Exploring mechanisms underpinning GCN5L1 mediated ROS signaling may expand our understanding of the role of mitochondria in gluconeogenesis control.Hepatic gluconeogenesis is tightly regulated at transcriptional level and is essential for survival during prolonged fasting. Here Wang et al. show that the mitochondrial enriched GCN5-like 1 protein controls hepatic glucose production by regulating FoxO1 protein levels via proteasome-dependent degradation and, in turn, gluconeogenic gene expression.