Mitochondrial Perturbations Couple mTORC2 to Autophagy in C. elegans

A fluorescent reporter for rapid assessment of autophagic flux reveals unique autophagy signatures during C. elegans post-embryonic development and identifies compounds that modulate autophagy

Autophagy is important for many physiological processes; and disordered autophagy can contribute to the pathogenesis of a broad range of systemic disorders. C. elegans is a useful model organism for studying the genetics of autophagy, however, current methods for studying autophagy are labor-intensive and not readily amenable to high-throughput procedures. Here we describe a fluorescent reporter, GFP::LGG-1::mKate2, which is useful for monitoring autophagic flux in live animals. In the intestine, the fusion protein is processed by endogenous ATG-4 to generate GFP::LGG-1 and mKate2 proteins. We provide data indicating that the GFP:mKate ratio is a suitable readout for measuring cellular autophagic flux. Using this reporter, we measured autophagic flux in L1 larvae to day 7 adult animals. We show that basal autophagic flux is relatively low during larval development but increases markedly in reproductive adults before decreasing with age. Furthermore, we show that wild-type, eat-2, and daf-2 mutant animals have distinct autophagic flux profiles through post-embryonic development. Finally, we demonstrate the utility of this reporter by performing a high-content small molecule screen to identify compounds that alter autophagic flux in C. elegans.

mTORC2: A neglected player in aging regulation

Mammalian target of rapamycin (mTOR) is a serine/threonine kinase that plays a pivotal role in various biological processes, through integrating external and internal signals, facilitating gene transcription and protein translation, as well as by regulating mitochondria and autophagy functions. mTOR kinase operates within two distinct protein complexes known as mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), which engage separate downstream signaling pathways impacting diverse cellular processes. Although mTORC1 has been extensively studied as a pro-proliferative factor and a pro-aging hub if activated aberrantly, mTORC2 received less attention, particularly regarding its implication in aging regulation. However, recent studies brought increasing evidence or clues for us, which implies the associations of mTORC2 with aging, as the genetic elimination of unique subunits of mTORC2, such as RICTOR, has been shown to alleviate aging progression in comparison to mTORC1 inhibition. In this review, we first summarized the basic characteristics of mTORC2, including its protein architecture and signaling network. We then focused on reviewing the molecular signaling regulation of mTORC2 in cellular senescence and organismal aging, and proposed the multifaceted regulatory characteristics under senescent and nonsenescent contexts. Next, we outlined the research progress of mTOR inhibitors in the field of antiaging and discussed future prospects and challenges. It is our pleasure if this review article could provide meaningful information for our readers and call forth more investigations working on this topic.

Autophagy alterations in obesity, type 2 diabetes, and metabolic dysfunction-associated steatotic liver disease: the evidence from human studies

Autophagy is an evolutionarily conserved process that plays a pivotal role in the maintenance of cellular homeostasis and its impairment has been implicated in the pathogenesis of various metabolic diseases including obesity, type 2 diabetes (T2D), and metabolic dysfunction-associated steatotic liver disease (MASLD). This review synthesizes the current evidence from human studies on autophagy alterations under these metabolic conditions. In obesity, most data point to autophagy upregulation during the initiation phase of autophagosome formation, potentially in response to proinflammatory conditions in the adipose tissue. Autophagosome formation appears to be enhanced under hyperglycemic or insulin-resistant conditions in patients with T2D, possibly acting as a compensatory mechanism to eliminate damaged organelles and proteins. Other studies have proposed that prolonged hyperglycemia and disrupted insulin signaling hinder autophagic flux, resulting in the accumulation of dysfunctional cellular components that can contribute to β-cell dysfunction. Evidence from patients with MASLD supports autophagy inhibition in disease progression. Nevertheless, given the available data, it is difficult to ascertain whether autophagy is enhanced or suppressed in these conditions because the levels of autophagy markers depend on the overall metabolism of specific organs, tissues, experimental conditions, or disease duration. Owing to these constraints, determining whether the observed shifts in autophagic activity precede or result from metabolic diseases remains challenging. Additionally, autophagy-modulating strategies are shortly discussed. To conclude, more studies investigating autophagy impairment are required to gain a more comprehensive understanding of its role in the pathogenesis of obesity, T2D, and MASLD and to unveil novel therapeutic strategies for these conditions.

2,4,6-trinitrotoluene causes mitochondrial toxicity in Caenorhabditis elegans by affecting electron transport

As a typical energetic compound widely used in military activities, 2,4,6-trinitrotoluene (TNT) has attracted great attention in recent years due to its heavy pollution and wide distribution in and around the training facilities, firing ranges, and demolition sites. However, the subcellular targets and the underlying toxic mechanism of TNT remain largely unknown. In this study, we explored the toxic effects of TNT biological reduction on the mitochondrial function and homeostasis in Caenorhabditis elegans (C. elegans). With short-term exposure of L4 larvae, 10-1000 ng/mL TNT reduced mitochondrial membrane potential and adenosine triphosphate (ATP) content, which was associated with decreased expression of specific mitochondrial complex involving gas-1 and mev-1 genes. Using fluorescence-labeled transgenic nematodes, we found that fluorescence expression of sod-3 (muls84) and gst-4 (dvls19) was increased, suggesting that TNT disrupted the mitochondrial antioxidant defense system. Furthermore, 10 ng/mL TNT exposure increased the expression of the autophagy-related gene pink-1 and activated mitochondrial unfolded protein response (mt UPR), which was indicated by the increased expression of mitochondrial stress activated transcription factor atfs-1, ubiquitin-like protein ubl-5, and homeobox protein dve-1. Our findings demonstrated that TNT biological reduction caused mitochondrial dysfunction and the development of mt UPR protective stress responses, and provided a basis for determining the potential risks of energetic compounds to living organisms.

Icariin Ameliorates D-galactose-induced Cell Injury in Neuron-like PC12 Cells by Inhibiting MPTP Opening

OBJECTIVE

Icariin (ICA) has a good neuroprotective effect and can upregulate neuronal basal autophagy in naturally aging rats. Mitochondrial dysfunction is associated with brain aging-related neurodegenerative diseases. Abnormal opening of the mitochondrial permeability transition pore (mPTP) is a crucial factor in mitochondrial dysfunction and is associated with excessive autophagy. This study aimed to explore that ICA protects against neuronal injury by blocking the mPTP opening and down-regulating autophagy levels in a D-galactose (D-gal)-induced cell injury model.

METHODS

A cell model of neuronal injury was established in rat pheochromocytoma cells (PC12 cells) treated with 200 mmol/L D-gal for 48 h. In this cell model, PC12 cells were pre-treated with different concentrations of ICA for 24 h. MTT was used to detect cell viability. Senescence associated β-galactosidase (SA-β-Gal) staining was used to observe cell senescence. Western blot analysis was performed to detect the expression levels of a senescence-related protein (p21), autophagy markers (LC3B, p62, Atg7, Atg5 and Beclin 1), mitochondrial fission and fusion-related proteins (Drp1, Mfn2 and Opa1), and mitophagy markers (Pink1 and Parkin). The changes of autophagic flow were detected by using mRFP-GFP-LC3 adenovirus. The intracellular ultrastructure was observed by transmission electron microscopy. Immunofluorescence was used to detect mPTP, mitochondrial membrane potential (MMP), mitochondrial reactive oxygen species (mtROS) and ROS levels. ROS and apoptosis levels were detected by flow cytometry.

RESULTS

D-gal treatment significantly decreased the viability of PC12 cells, and markedly increased the SA-β-Gal positive cells as compared to the control group. With the D-gal stimulation, the expression of p21 was significantly up-regulated. Furthermore, D-gal stimulation resulted in an elevated LC3B II/I ratio and decreased p62 expression. Meanwhile, autophagosomes and autolysosomes were significantly increased, indicating abnormal activation of autophagy levels. In addition, in this D-gal-induced model of cell injury, the mPTP was abnormally open, the ROS generation was continuously increased, the MMP was gradually decreased, and the apoptosis was increased. ICA effectively improved mitochondrial dysfunction to protect against D-gal-induced cell injury and apoptosis. It strongly inhibited excessive autophagy by blocking the opening of the mPTP. Cotreatment with ICA and an mPTP inhibitor (cyclosporin A) did not ameliorate mitochondrial dysfunction. However, the protective effects were attenuated by cotreatment with ICA and an mPTP activator (lonidamine).

CONCLUSION

ICA inhibits the activation of excessive autophagy and thus improves mitochondrial dysfunction by blocking the mPTP opening.

The interplay between mitophagy and mitochondrial ROS in acute lung injury

Mitochondria orchestrate the production of new mitochondria and the removal of damaged ones to dynamically maintain mitochondrial homeostasis through constant biogenesis and clearance mechanisms. Mitochondrial quality control particularly relies on mitophagy, defined as selective autophagy with mitochondria-targeting specificity. Most ROS are derived from mitochondria, and the physiological concentration of mitochondrial ROS (mtROS) is no longer considered a useless by-product, as it has been proven to participate in immune and autophagy pathway regulation. However, excessive mtROS appears to be a pathogenic factor in several diseases, including acute lung injury (ALI). The interplay between mitophagy and mtROS is complex and closely related to ALI. Here, we review the pathways of mitophagy, the intricate relationship between mitophagy and mtROS, the role of mtROS in the pathogenesis of ALI, and their effects and related progression in ALI induced by different conditions.

A novel MTORC2-AKT-ROS axis triggers mitofission and mitophagy-associated execution of colorectal cancer cells upon drug-induced activation of mutant KRAS

RAS is one of the most commonly mutated oncogenes associated with multiple cancer hallmarks. Notably, RAS activation induces intracellular reactive oxygen species (ROS) generation, which we previously demonstrated as a trigger for autophagy-associated execution of mutant KRAS-expressing cancer cells. Here we report that drug (merodantoin; C1)-induced activation of mutant KRAS promotes phospho-AKT S473-dependent ROS-mediated S616 phosphorylation and mitochondrial localization of DNM1L/DRP1 (dynamin 1 like) and cleavage of the fusion-associated protein OPA1 (OPA1 mitochondrial dynamin like GTPase). Interestingly, accumulation of the outer mitochondrial membrane protein VDAC1 (voltage dependent anion channel 1) is observed in mutant KRAS-expressing cells upon exposure to C1. Conversely, silencing VDAC1 abolishes C1-induced mitophagy, and gene knockdown of either KRAS, AKT or DNM1L rescues ROS-dependent VDAC1 accumulation and stability, thus suggesting an axis of mutant active KRAS-phospho-AKT S473-ROS-DNM1L-VDAC1 in mitochondrial morphology change and cancer cell execution. Importantly, we identified MTOR (mechanistic target of rapamycin kinsase) complex 2 (MTORC2) as the upstream mediator of AKT phosphorylation at S473 in our model. Pharmacological or genetic inhibition of MTORC2 abrogated C1-induced phosphorylation of AKT S473, ROS generation and mitophagy induction, as well as rescued tumor colony forming ability and migratory capacity. Finally, increase in thermal stability of KRAS, AKT and DNM1L were observed upon exposure to C1 only in mutant KRAS-expressing cells. Taken together, our work has unraveled a novel mechanism of selective targeting of mutant KRAS-expressing cancers via MTORC2-mediated AKT activation and ROS-dependent mitofission, which could have potential therapeutic implications given the relative lack of direct RAS-targeting strategies in cancer.Abbreviations: ACTB/ß-actin: actin beta; AKT: AKT serine/threonine kinase; C1/merodantoin: 1,3-dibutyl-2-thiooxo-imidazoldine-4,5-dione; CAT: catalase; CETSA: cellular thermal shift assay; CHX: cycloheximide; DKO: double knockout; DNM1L/DRP1: dynamin 1 like; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; H2O2: hydrogen peroxide; HSPA1A/HSP70-1: heat shock protein family A (Hsp70) member 1A; HSP90AA1/HSP90: heat shock protein 90 alpha family class A member 1; KRAS: KRAS proto-oncogene, GTPase; MAP1LC3B/LC3B, microtubule associated protein 1 light chain 3 beta; LC3B-I: unlipidated form of LC3B; LC3B-II: phosphatidylethanolamine-conjugated form of LC3B; MAPKAP1/SIN1: MAPK associated protein 1; MAPK1/ERK2: mitogen-activated protein kinase 1; MAPK3/ERK1: mitogen-activated protein kinase 3; MFI: mean fluorescence intensity; MiNA: Mitochondrial Network Analysis; MTOR: mechanistic target of rapamycin kinase; MTORC1: mechanistic target of rapamycin kinase complex 1; MTORC2: mechanistic target of rapamycin kinase complex 2; O2.-: superoxide; OMA1: OMA1 zinc metallopeptidase; OPA1: OPA1 mitochondrial dynamin like GTPase; RICTOR: RPTOR independent companion of MTOR complex 2; ROS: reactive oxygen species; RPTOR/raptor: regulatory associated protein of MTOR complex 1; SOD1: superoxide dismutase 1; SOD2: superoxide dismutase 2; SQSTM1/p62: sequestosome 1; VDAC1: voltage dependent anion channel 1; VDAC2: voltage dependent anion channel 2.

Non-cell-autonomous regulation of mTORC2 by Hedgehog signaling maintains lipid homeostasis

Organisms must appropriately allocate energetic resources between essential cellular processes to maintain homeostasis and in turn, maximize fitness. The nutritional and homeostatic regulators of energy homeostasis have been studied in detail; however, how developmental signals might impinge on these pathways to govern cellular metabolism is poorly understood. Here, we identify a non-canonical role for Hedgehog (Hh), a classic regulator of development, in maintaining intestinal lipid homeostasis in C. elegans . We find that expression of two Hh ligands, GRD-3 and GRD-4, is controlled by the LIN-29/EGR transcription factor in the hypodermis, where the Hh secretion factor CHE-14/Dispatched also facilitates non-cell autonomous Hh signaling. We demonstrate, using C. elegans and mouse hepatocytes, that Hh metabolic regulation does not occur through the canonical Hh transcription factor, TRA-1/GLI, but rather through non-canonical signaling that engages mTOR Complex 2 (mTORC2) in the intestine. Hh mutants display impaired lipid homeostasis, including reduced lipoprotein synthesis and fat accumulation, decreased growth, and upregulation of autophagy factors, mimicking loss of mTORC2. Additionally, we found that Hh inhibits p38 MAPK signaling in parallel to mTORC2 activation and that both pathways act together to modulate of lipid homeostasis. Our findings show a non-canonical role for Hedgehog signaling in lipid metabolism via regulation of core homeostatic pathways and reveal a new mechanism by which developmental timing events govern metabolic decisions.

Rictor/mTORC2 signalling contributes to renal vascular endothelial-to-mesenchymal transition and renal allograft interstitial fibrosis by regulating BNIP3-mediated mitophagy

BACKGROUND

Renal allograft interstitial fibrosis/tubular atrophy (IF/TA) constitutes the principal histopathological characteristic of chronic allograft dysfunction (CAD) in kidney-transplanted patients. While renal vascular endothelial-mesenchymal transition (EndMT) has been verified as an important contributing factor to IF/TA in CAD patients, its underlying mechanisms remain obscure. Through single-cell transcriptomic analysis, we identified Rictor as a potential pivotal mediator for EndMT. This investigation sought to elucidate the role of Rictor/mTORC2 signalling in the pathogenesis of renal allograft interstitial fibrosis and the associated mechanisms.

METHODS

The influence of the Rictor/mTOR2 pathway on renal vascular EndMT and renal allograft fibrosis was investigated by cell experiments and Rictor depletion in renal allogeneic transplantation mice models. Subsequently, a series of assays were conducted to explore the underlying mechanisms of the enhanced mitophagy and the ameliorated EndMT resulting from Rictor knockout.

RESULTS

Our findings revealed a significant activation of the Rictor/mTORC2 signalling in CAD patients and allogeneic kidney transplanted mice. The suppression of Rictor/mTORC2 signalling alleviated TNFα-induced EndMT in HUVECs. Moreover, Rictor knockout in endothelial cells remarkably ameliorated renal vascular EndMT and allograft interstitial fibrosis in allogeneic kidney transplanted mice. Mechanistically, Rictor knockout resulted in an augmented BNIP3-mediated mitophagy in endothelial cells. Furthermore, Rictor/mTORC2 facilitated the MARCH5-mediated degradation of BNIP3 at the K130 site through K48-linked ubiquitination, thereby regulating mitophagy activity. Subsequent experiments also demonstrated that BNIP3 knockdown nearly reversed the enhanced mitophagy and mitigated EndMT and allograft interstitial fibrosis induced by Rictor knockout.

CONCLUSIONS

Consequently, our study underscores Rictor/mTORC2 signalling as a critical mediator of renal vascular EndMT and allograft interstitial fibrosis progression, exerting its impact through regulating BNIP3-mediated mitophagy. This insight unveils a potential therapeutic target for mitigating renal allograft interstitial fibrosis.

LPD-3 as a megaprotein brake for aging and insulin-mTOR signaling in C. elegans

Insulin-mechanistic target of rapamycin (mTOR) signaling drives anabolic growth during organismal development; its late-life dysregulation contributes to aging and limits lifespans. Age-related regulatory mechanisms and functional consequences of insulin-mTOR remain incompletely understood. Here, we identify LPD-3 as a megaprotein that orchestrates the tempo of insulin-mTOR signaling during C. elegans aging. We find that an agonist insulin, INS-7, is drastically overproduced from early life and shortens lifespan in lpd-3 mutants. LPD-3 forms a bridge-like tunnel megaprotein to facilitate non-vesicular cellular lipid trafficking. Lipidomic profiling reveals increased hexaceramide species in lpd-3 mutants, accompanied by up-regulation of hexaceramide biosynthetic enzymes, including HYL-1. Reducing the abundance of HYL-1, insulin receptor/DAF-2 or mTOR/LET-363, normalizes INS-7 levels and rescues the lifespan of lpd-3 mutants. LPD-3 antagonizes SINH-1, a key mTORC2 component, and decreases expression with age. We propose that LPD-3 acts as a megaprotein brake for organismal aging and that its age-dependent decline restricts lifespan through the sphingolipid-hexaceramide and insulin-mTOR pathways.

The role of serum-glucocorticoid regulated kinase 1 in reproductive viability: implications from prenatal programming and senescence

OBJECTIVE

Organisms and cellular viability are of paramount importance to living creatures. Disruption of the balance between cell survival and apoptosis results in compromised viability and even carcinogenesis. One molecule involved in keeping this homeostasis is serum-glucocorticoid regulated kinase (SGK) 1. Emerging evidence points to a significant role of SGK1 in cell growth and survival, cell metabolism, reproduction, and life span, particularly in prenatal programming and reproductive senescence by the same token. Whether the hormone inducible SGK1 kinase is a major driver in the pathophysiological processes of prenatal programming and reproductive senescence?

METHOD

The PubMed/Medline, Web of Science, Embase/Ovid, and Elsevier Science Direct literature databases were searched for articles in English focusing on SGK1 published up to July 2023 RESULT: Emerging evidence is accumulating pointing to a pathophysiological role of the ubiquitously expressed SGK1 in the cellular and organismal viability. Under the regulation of specific hormones, extracellular stimuli, and various signals, SGK1 is involved in several biological processes relevant to viability, including cell proliferation and survival, cell migration and differentiation. In line, SGK1 contributes to the development of germ cells, embryos, and fetuses, whereas SGK1 inhibition leads to abnormal gametogenesis, embryo loss, and truncated reproductive lifespan.

CONCLUTION

SGK1 integrates a broad spectrum of effects to maintain the homeostasis of cell survival and apoptosis, conferring viability to multiple cell types as well as both simple and complex organisms, and thus ensuring appropriate prenatal development and reproductive lifespan.

Rab11 regulates autophagy at dendritic spines in an mTOR- and NMDA-dependent manner

Synaptic plasticity is a process that shapes neuronal connections during neurodevelopment and learning and memory. Autophagy is a mechanism that allows the cell to degrade its unnecessary or dysfunctional components. Autophagosomes appear at dendritic spines in response to plasticity-inducing stimuli. Autophagy defects contribute to altered dendritic spine development, autistic-like behavior in mice, and neurological disease. While several studies have explored the involvement of autophagy in synaptic plasticity, the initial steps of the emergence of autophagosomes at the postsynapse remain unknown. Here, we demonstrate a postsynaptic association of autophagy-related protein 9A (Atg9A), known to be involved in the early stages of autophagosome formation, with Rab11, a small GTPase that regulates endosomal trafficking. Rab11 activity was necessary to maintain Atg9A-positive structures at dendritic spines. Inhibition of mTOR increased Rab11 and Atg9A interaction and increased the emergence of LC3 positive vesicles, an autophagosome membrane-associated protein marker, in dendritic spines when coupled to NMDA receptor stimulation. Dendritic spines with newly formed LC3+ vesicles were more resistant to NMDA-induced morphologic change. Rab11 DN overexpression suppressed appearance of LC3+ vesicles. Collectively, these results suggest that initiation of autophagy in dendritic spines depends on neuronal activity and Rab11a-dependent Atg9A interaction that is regulated by mTOR activity.

The role of serum and glucocorticoid-regulated kinase 1 in cellular signaling: Implications for drug development

Serum and glucocorticoid-regulated kinase 1 (SGK1) is a ubiquitously expressed protein belonging to the Ser/Thr kinase family. It regulates diverse physiological processes, including epithelial sodium channel activity, hypertension, cell proliferation, and insulin sensitivity. Due to its significant role in the pathogenesis of numerous diseases, SGK1 can be exploited as a potential therapeutic target to address challenging health problems. SGK1 is associated with the development of obesity, and its overexpression enhances the sodium-glucose co-transporter 1 activity, which absorbs intestinal glucose. This review highlighted the detailed functional significance of SGK1 signaling and role in different diseases and subsequent therapeutic targeting. We aim to provide deeper mechanistic insights into understanding the pathogenesis and recent advancements in the SGK1 targeted drug development process. Small-molecule inhibitors are being developed with excellent binding affinity and improved SGK1 inhibition with desired selectivity. We have discussed small molecule inhibitors designed explicitly as potent SGK1 inhibitors and their therapeutic implications in various diseases. We further addressed the therapeutic potential and mechanism of action of these SGK1 inhibitors and provided a strong scientific foundation for developing effective therapeutics.

SGK-1 mediated inhibition of iron import is a determinant of lifespan in C. elegans

Maintaining iron levels is crucial for health, but iron overload has been associated with tumorigenesis. Therefore, critical enzymes involved in iron homeostasis are under tight, typically posttranslational control. In C. elegans , the mTORC2 and insulin/IGF-1 activated kinase SGK-1 is induced upon exogenous iron overload to couple iron storage and fat accumulation. Here we show that, already at physiological iron conditions, sgk-1 loss-of-function increases intracellular iron levels that may impair lifespan. Reducing iron levels by diminishing cellular or mitochondrial iron import is sufficient to extend the short lifespan of sgk-1 loss-of-function animals. Our results indicate another regulatory level of sgk-1 in iron homeostasis via negative feedback regulation on iron transporters.

SKN-1/Nrf2-dependent regulation of mitochondrial homeostasis modulates transgenerational toxicity induced by nanoplastics with different surface charges in Caenorhabditis elegans

The toxic effects of nanoplastics on transgenerational toxicity in environmental organisms and the involved mechanisms remain poorly comprehended. This study aimed to identify the role of SKN-1/Nrf2-dependent regulation of mitochondrial homeostasis in response to transgenerational toxicity caused by changes in nanoplastic surface charges in Caenorhabditis elegans (C. elegans). Our results revealed that compared with the wild-type control and PS exposed groups, exposure to PS-NH₂ or PS-SOOOH at environmentally relevant concentrations (ERC) of ≥ 1 μg/L caused transgenerational reproductive toxicity, inhibited mitochondrial unfolded protein responses (UPR) by downregulating the transcription levels of hsp-6, ubl-5, dve-1, atfs-1, haf-1, and clpp-1, membrane potential by downregulating phb-1 and phb-2, and promoted mitochondrial apoptosis by downregulating ced-4 and ced-3 and upregulating ced-9, DNA damage by upregulating hus-1, cep-1, egl-1, reactive oxygen species (ROS) by upregulating nduf-7 and nuo-6, ultimately resulting in mitochondrial homeostasis. Additionally, further study indicated that SKN-1/Nrf2 mediated antioxidant response to alleviate PS-induced toxicity in the P0 generation and dysregulated mitochondrial homeostasis to enhance PS-NH₂ or PS-SOOOH-induced transgenerational toxicity. Our study highlights the momentous role of SKN-1/Nrf2 mediated mitochondrial homeostasis in the response to nanoplastics caused transgenerational toxicity in environmental organisms.

Overexpressed SIRT6 ameliorates doxorubicin-induced cardiotoxicity and potentiates the therapeutic efficacy through metabolic remodeling

Since the utilization of anthracyclines in cancer therapy, severe cardiotoxicity has become a major obstacle. The major challenge in treating cancer patients with anthracyclines is minimizing cardiotoxicity without compromising antitumor efficacy. Herein, histone deacetylase SIRT6 expression was reduced in plasma of patients treated with anthracyclines-based chemotherapy regimens. Furthermore, overexpression of SIRT6 alleviated doxorubicin-induced cytotoxicity in cardiomyocytes, and potentiated cytotoxicity of doxorubicin in multiple cancer cell lines. Moreover, SIRT6 overexpression ameliorated doxorubicin-induced cardiotoxicity and potentiated antitumor efficacy of doxorubicin in mice, suggesting that SIRT6 overexpression could be an adjunctive therapeutic strategy during doxorubicin treatment. Mechanistically, doxorubicin-impaired mitochondria led to decreased mitochondrial respiration and ATP production. And SIRT6 enhanced mitochondrial biogenesis and mitophagy by deacetylating and inhibiting Sgk1. Thus, SIRT6 overexpression coordinated metabolic remodeling from glycolysis to mitochondrial respiration during doxorubicin treatment, which was more conducive to cardiomyocyte metabolism, thus protecting cardiomyocytes but not cancer cells against doxorubicin-induced energy deficiency. In addition, ellagic acid, a natural compound that activates SIRT6, alleviated doxorubicin-induced cardiotoxicity and enhanced doxorubicin-mediated tumor regression in tumor-bearing mice. These findings provide a preclinical rationale for preventing cardiotoxicity by activating SIRT6 in cancer patients undergoing chemotherapy, but also advancing the understanding of the crucial role of SIRT6 in mitochondrial homeostasis.

Preservation of mitochondrial membrane potential is necessary for lifespan extension from dietary restriction

Dietary restriction (DR) increases lifespan in many organisms, but its underlying mechanisms are not fully understood. Mitochondria play a central role in metabolic regulation and are known to undergo changes in structure and function in response to DR. Mitochondrial membrane potential (Δψ_m) is the driving force for ATP production and mitochondrial outputs that integrate many cellular signals. One such signal regulated by Δψ_m is nutrient-status sensing. Here, we tested the hypothesis that DR promotes longevity through preserved Δψ_m during adulthood. Using the nematode Caenorhabditis elegans, we find that Δψ_m declines with age relatively early in the lifespan, and this decline is attenuated by DR. Pharmacologic depletion of Δψ_m blocked the longevity and health benefits of DR. Genetic perturbation of Δψ_m and mitochondrial ATP availability similarly prevented lifespan extension from DR. Taken together, this study provides further evidence that appropriate regulation of Δψ_m is a critical factor for health and longevity in response to DR.

Natural compounds from botanical drugs targeting mTOR signaling pathway as promising therapeutics for atherosclerosis: A review

Atherosclerosis (AS) is a chronic inflammatory disease that is a major cause of cardiovascular diseases (CVDs), including coronary artery disease, hypertension, myocardial infarction, and heart failure. Hence, the mechanisms of AS are still being explored. A growing compendium of evidence supports that the activity of the mechanistic/mammalian target of rapamycin (mTOR) is highly correlated with the risk of AS. The mTOR signaling pathway contributes to AS progression by regulating autophagy, cell senescence, immune response, and lipid metabolism. Various botanical drugs and their functional compounds have been found to exert anti- AS effects by modulating the activity of the mTOR signaling pathway. In this review, we summarize the pathogenesis of AS based on the mTOR signaling pathway from the aspects of immune response, autophagy, cell senescence, and lipid metabolism, and comb the recent advances in natural compounds from botanical drugs to inhibit the mTOR signaling pathway and delay AS development. This review will provide a new perspective on the mechanisms and precision treatments of AS.

mTORC2: a multifaceted regulator of autophagy

Autophagy is a multi-step catabolic process that delivers cellular components to lysosomes for degradation and recycling. The dysregulation of this precisely controlled process disrupts cellular homeostasis and leads to many pathophysiological conditions. The mechanistic target of rapamycin (mTOR) is a central nutrient sensor that integrates growth signals with anabolism to fulfil biosynthetic and bioenergetic requirements. mTOR nucleates two distinct evolutionarily conserved complexes (mTORC1 and mTORC2). However, only mTORC1 is acutely inhibited by rapamycin. Consequently, mTORC1 is a well characterized regulator of autophagy. While less is known about mTORC2, the availability of acute small molecule inhibitors and multiple genetic models has led to increased understanding about the role of mTORC2 in autophagy. Emerging evidence suggests that the regulation of mTORC2 in autophagy is mainly through its downstream effector proteins, and is variable under different conditions and cellular contexts. Here, we review recent advances that describe a role for mTORC2 in this catabolic process, and propose that mTORC2 could be a potential clinical target for the treatment of autophagy-related diseases. Video abstract.

Toxic Effects Induced by Diuron and Its Metabolites in Caenorhabditis elegans

The toxicity of diuron herbicide and its metabolites has been extensively investigated; however, their precise toxic mechanisms have yet to be fully appreciated. In this context, we evaluated the toxic mechanism of diuron, 3,4-dichloroaniline (DCA) and 3-(3,4-dichlorophenyl)-1-methylurea (DCPMU), using Caenorhabditis elegans (C. elegans) in the L1 larval stage. For this purpose, worms were acutely exposed to the test chemicals with a preliminary concentration range of 0.5 to 500 μM and first analyzed for lethality (%). Next, the highest concentration (500 μM) was considered for survival (%), reactive oxygen and nitrogen species (RONS), glutathione (GSH) and ATP levels, autophagy index, behavior, and dopaminergic neurodegeneration parameters. Interestingly, increased lethality (%) was found for all chemicals at the higher concentrations tested (100 and 500 μM), with significant differences at 500 μM DCA (p < 0.05). A decrease in the median survival was observed mainly for DCA. Although no changes were observed in RONS production, GSH levels were significantly increased upon diuron and DCA treatment, likely reflecting an attempt to restore the redox status. Moreover, diuron and its metabolites impaired ATP levels, suggesting an alteration in mitochondrial function. The latter may trigger autophagy as an adaptive survival mechanism, but this was not observed in C. elegans. Dopaminergic neurotoxicity was observed upon treatment with all the tested chemicals, but only diuron induced alterations in the worms' locomotor behavior. Combined, these results indicate that exposure to high concentrations of diuron and its metabolites elicit distinct adverse outcomes in C. elegans, and DCA in particular, plays an important role in the overall toxicity observed in this experimental model.

PCB153 suppressed autophagy via PI3K/Akt/mTOR and RICTOR/Akt/mTOR signaling by the upregulation of microRNA-155 in rat primary chondrocytes

Polychlorinated biphenyls (PCBs) are a typical type of persistent organic pollutant. PCB exposure is associated to the occurrence and development of osteoarthritis (OA); however, the involved mechanisms have yet to be elucidated. Here, we investigated the pro-osteoarthritic effect of 2, 2', 4, 4', 5, 5'-hexachlorobiphenyl (PCB153), and the involvement of the phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt)/ mammalian target of rapamycin (mTOR) and the RICTOR/Akt/mTOR signaling pathways. PCB153 of 20 and 30 μM increased the expression of MMP13 and decreased the expression of type II collagen, in a concentration-dependent manner. PCB153 treatment reduced the expression of Beclin 1 and LC3B, but increased the expression of p62 by upregulating miR-155 levels. PCB153 treatment activated the PI3K/Akt/mTOR signaling pathway by upregulating miR-155 levels. RICTOR was involved in activating the Akt/mTOR signaling pathway, and was also regulated by miR-155. In conclusion, PCB153 could promote the degradation of the extracellular matrix of chondrocytes by upregulating miR-155 via a mechanism related to the activation of the PI3K/Akt/mTOR and RICTOR/Akt/mTOR signaling pathway, which suppressed autophagy and facilitated the development of OA. MiR-155 may represent potential therapeutic targets to alleviate the development of OA.

mTORC2 protects the heart from high-fat diet-induced cardiomyopathy through mitochondrial fission in Drosophila

High-fat diet (HFD)-induced obesity has become the major risk factor for the development of cardiovascular diseases, but the underlying mechanisms remain poorly understood. Here, we use Drosophila as a model to study the role of mTORC2 in HFD-induced mitochondrial fission and cardiac dysfunction. We find that knockdown of mTORC2 subunit rictor blocks HFD-induced mitochondrial fragmentation and Drp1 recruitment. Knockdown of rictor further impairs cardiac contractile function under HFD treatment. Surprisingly, knockdown of Akt, the major effector of mTORC2, did not affect HFD-induced mitochondrial fission. Similar to mTORC2 inhibition, knockdown of Drp1 blocks HFD-induced mitochondrial fragmentation and induces contractile defects. Furthermore, overexpression of Drp1 restored HFD-induced mitochondrial fragmentation in rictor knockdown flies. Thus, we uncover a novel function of mTORC2 in protecting the heart from HFD treatment through Drp1-dependent mitochondrial fission.

SIN-3 functions through multi-protein interaction to regulate apoptosis, autophagy, and longevity in Caenorhabditis elegans

SIN3/HDAC is a multi-protein complex that acts as a regulatory unit and functions as a co-repressor/co-activator and a general transcription factor. SIN3 acts as a scaffold in the complex, binding directly to HDAC1/2 and other proteins and plays crucial roles in regulating apoptosis, differentiation, cell proliferation, development, and cell cycle. However, its exact mechanism of action remains elusive. Using the Caenorhabditis elegans (C. elegans) model, we can surpass the challenges posed by the functional redundancy of SIN3 isoforms. In this regard, we have previously demonstrated the role of SIN-3 in uncoupling autophagy and longevity in C. elegans. In order to understand the mechanism of action of SIN3 in these processes, we carried out a comparative analysis of the SIN3 protein interactome from model organisms of different phyla. We identified conserved, expanded, and contracted gene classes. The C. elegans SIN-3 interactome -revealed the presence of  well-known proteins, such as DAF-16, SIR-2.1, SGK-1, and AKT-1/2, involved in autophagy, apoptosis, and longevity. Overall, our analyses propose  potential mechanisms by which SIN3 participates in multiple biological processes and their conservation across species and identifies candidate genes for further experimental analysis.

Organismal and Cellular Stress Responses upon Disruption of Mitochondrial Lonp1 Protease

Cells engage complex surveillance mechanisms to maintain mitochondrial function and protein homeostasis. LonP1 protease is a key component of mitochondrial quality control and has been implicated in human malignancies and other pathological disorders. Here, we employed two experimental systems, the worm Caenorhabditis elegans and human cancer cells, to investigate and compare the effects of LONP-1/LonP1 deficiency at the molecular, cellular, and organismal levels. Deletion of the lonp-1 gene in worms disturbed mitochondrial function, provoked reactive oxygen species accumulation, and impaired normal processes, such as growth, behavior, and lifespan. The viability of lonp-1 mutants was dependent on the activity of the ATFS-1 transcription factor, and loss of LONP-1 evoked retrograde signaling that involved both the mitochondrial and cytoplasmic unfolded protein response (UPR^mt and UPR^cyt) pathways and ensuing diverse organismal stress responses. Exposure of worms to triterpenoid CDDO-Me, an inhibitor of human LonP1, stimulated only UPR^cyt responses. In cancer cells, CDDO-Me induced key components of the integrated stress response (ISR), the UPR^mt and UPR^cyt pathways, and the redox machinery. However, genetic knockdown of LonP1 revealed a genotype-specific cellular response and induced apoptosis similar to CDDO-Me treatment. Overall, the mitochondrial dysfunction ensued by disruption of LonP1 elicits adaptive cytoprotective mechanisms that can inhibit cancer cell survival but diversely modulate organismal stress response and aging.

Molecular mechanisms and consequences of mitochondrial permeability transition

Mitochondrial permeability transition (mPT) is a phenomenon that abruptly causes the flux of low molecular weight solutes (molecular weight up to 1,500) across the generally impermeable inner mitochondrial membrane. The mPT is mediated by the so-called mitochondrial permeability transition pore (mPTP), a supramolecular entity assembled at the interface of the inner and outer mitochondrial membranes. In contrast to mitochondrial outer membrane permeabilization, which mostly activates apoptosis, mPT can trigger different cellular responses, from the physiological regulation of mitophagy to the activation of apoptosis or necrosis. Although there are several molecular candidates for the mPTP, its molecular nature remains contentious. This lack of molecular data was a significant setback that prevented mechanistic insight into the mPTP, pharmacological targeting and the generation of informative animal models. In recent years, experimental evidence has highlighted mitochondrial F₁F_o ATP synthase as a participant in mPTP formation, although a molecular model for its transition to the mPTP is still lacking. Recently, the resolution of the F₁F_o ATP synthase structure by cryogenic electron microscopy led to a model for mPTP gating. The elusive molecular nature of the mPTP is now being clarified, marking a turning point for understanding mitochondrial biology and its pathophysiological ramifications. This Review provides an up-to-date reference for the understanding of the mammalian mPTP and its cellular functions. We review current insights into the molecular mechanisms of mPT and validated observations - from studies in vivo or in artificial membranes - on mPTP activity and functions. We end with a discussion of the contribution of the mPTP to human disease. Throughout the Review, we highlight the multiple unanswered questions and, when applicable, we also provide alternative interpretations of the recent discoveries.

Tourniquet-induced lower limb ischemia/reperfusion reduces mitochondrial function by decreasing mitochondrial biogenesis in acute kidney injury in mice

The mechanisms by which lower limb ischemia/reperfusion induces acute kidney injury (AKI) remain largely uncharacterized. We hypothesized that tourniquet-induced lower limb ischemia/reperfusion (TILLIR) would inhibit mitochondrial function in the renal cortex. We used a murine model to show that TILLIR of the high thigh regions inflicted time-dependent AKI as determined by renal function and histology. This effect was associated with decreased activities of mitochondrial complexes I, II, V and citrate synthase in the kidney cortex. Moreover, TILLIR reduced mRNA levels of a master regulator of mitochondrial biogenesis PGC-1α, and its downstream genes NDUFS1 and ATP5o in the renal cortex. TILLIR also increased serum corticosterone concentrations. TILLIR did not significantly affect protein levels of the critical regulators of mitophagy PINK1 and PARK2, mitochondrial transport proteins Tom20 and Tom70, or heat-shock protein 27. TILLIR had no significant effect on mitochondrial oxidative stress as determined by mitochondrial ability to generate reactive oxygen species, protein carbonylation, or protein levels of MnSOD and peroxiredoxin1. However, TILLIR inhibited classic autophagic flux by increasing p62 protein abundance and preventing the conversion of LC3-I to LC3-II. TILLIR increased phosphorylation of cytosolic and mitochondrial ERK1/2 and mitochondrial AKT1, as well as mitochondrial SGK1 activity. In conclusion, lower limb ischemia/reperfusion induces distal AKI by inhibiting mitochondrial function through reducing mitochondrial biogenesis. This AKI occurs without significantly affecting PINK1-PARK2-mediated mitophagy or mitochondrial oxidative stress in the kidney cortex.

The role of metabolic ecosystem in cancer progression — metabolic plasticity and mTOR hyperactivity in tumor tissues

Despite advancements in cancer management, tumor relapse and metastasis are associated with poor outcomes in many cancers. Over the past decade, oncogene-driven carcinogenesis, dysregulated cellular signaling networks, dynamic changes in the tissue microenvironment, epithelial-mesenchymal transitions, protein expression within regulatory pathways, and their part in tumor progression are described in several studies. However, the complexity of metabolic enzyme expression is considerably under evaluated. Alterations in cellular metabolism determine the individual phenotype and behavior of cells, which is a well-recognized hallmark of cancer progression, especially in the adaptation mechanisms underlying therapy resistance. In metabolic symbiosis, cells compete, communicate, and even feed each other, supervised by tumor cells. Metabolic reprogramming forms a unique fingerprint for each tumor tissue, depending on the cellular content and genetic, epigenetic, and microenvironmental alterations of the developing cancer. Based on its sensing and effector functions, the mechanistic target of rapamycin (mTOR) kinase is considered the master regulator of metabolic adaptation. Moreover, mTOR kinase hyperactivity is associated with poor prognosis in various tumor types. In situ metabolic phenotyping in recent studies highlights the importance of metabolic plasticity, mTOR hyperactivity, and their role in tumor progression. In this review, we update recent developments in metabolic phenotyping of the cancer ecosystem, metabolic symbiosis, and plasticity which could provide new research directions in tumor biology. In addition, we suggest pathomorphological and analytical studies relating to metabolic alterations, mTOR activity, and their associations which are necessary to improve understanding of tumor heterogeneity and expand the therapeutic management of cancer.

How the Innate Immune DNA Sensing cGAS-STING Pathway Is Involved in Autophagy

The cGAS–STING pathway is a key component of the innate immune system and exerts crucial roles in the detection of cytosolic DNA and invading pathogens. Accumulating evidence suggests that the intrinsic cGAS–STING pathway not only facilitates the production of type I interferons (IFN-I) and inflammatory responses but also triggers autophagy. Autophagy is a homeostatic process that exerts multiple effects on innate immunity. However, systematic evidence linking the cGAS–STING pathway and autophagy is still lacking. Therefore, one goal of this review is to summarize the known mechanisms of autophagy induced by the cGAS–STING pathway and their consequences. The cGAS–STING pathway can trigger canonical autophagy through liquid-phase separation of the cGAS–DNA complex, interaction of cGAS and Beclin-1, and STING-triggered ER stress–mTOR signaling. Furthermore, both cGAS and STING can induce non-canonical autophagy via LC3-interacting regions and binding with LC3. Subsequently, autophagy induced by the cGAS–STING pathway plays crucial roles in balancing innate immune responses, maintaining intracellular environmental homeostasis, alleviating liver injury, and limiting tumor growth and transformation.

Identification of enzymes that have helminth-specific active sites and are required for Rhodoquinone-dependent metabolism as targets for new anthelmintics

Soil transmitted helminths (STHs) are major human pathogens that infect over a billion people. Resistance to current anthelmintics is rising and new drugs are needed. Here we combine multiple approaches to find druggable targets in the anaerobic metabolic pathways STHs need to survive in their mammalian host. These require rhodoquinone (RQ), an electron carrier used by STHs and not their hosts. We identified 25 genes predicted to act in RQ-dependent metabolism including sensing hypoxia and RQ synthesis and found 9 are required. Since all 9 have mammalian orthologues, we used comparative genomics and structural modeling to identify those with active sites that differ between host and parasite. Together, we found 4 genes that are required for RQ-dependent metabolism and have different active sites. Finding these high confidence targets can open up in silico screens to identify species selective inhibitors of these enzymes as new anthelmintics.

Sirtuins and Autophagy in Age-Associated Neurodegenerative Diseases: Lessons from the C. elegans Model

Age-associated neurodegenerative diseases are known to have "impaired protein clearance" as one of the key features causing their onset and progression. Hence, homeostasis is the key to maintaining balance throughout the cellular system as an organism ages. Any imbalance in the protein clearance machinery is responsible for accumulation of unwanted proteins, leading to pathological consequences-manifesting in neurodegeneration and associated debilitating outcomes. Multiple processes are involved in regulating this phenomenon; however, failure to regulate the autophagic machinery is a critical process that hampers the protein clearing pathway, leading to neurodegeneration. Another important and widely known component that plays a role in modulating neurodegeneration is a class of proteins called sirtuins. These are class III histone deacetylases (HDACs) that are known to regulate various vital processes such as longevity, genomic stability, transcription and DNA repair. These enzymes are also known to modulate neurodegeneration in an autophagy-dependent manner. Considering its genetic relevance and ease of studying disease-related endpoints in neurodegeneration, the model system Caenorhabditis elegans has been successfully employed in deciphering various functional outcomes related to critical protein molecules, cell death pathways and their association with ageing. This review summarizes the vital role of sirtuins and autophagy in ageing and neurodegeneration, in particular highlighting the knowledge obtained using the C. elegans model system.

SGK1, autophagy and cancer: an overview

BACKGROUND

The serum and glucocorticoid-induced kinase-1 (SGK1) belonging to the AGC protein kinase family phosphorylates serine and threonine residues of target proteins. It regulates numerous ion channels and transporters and promotes survival under cellular stress. Unique to SGK1 is a tight control at transcriptional and post-transcriptional levels. SGK1 regulates multiple signal transduction pathways related to tumor development. Several studies have reported that SGK1 is upregulated in different types of human malignancies and induces resistance against inhibitors, drugs, and targeted therapies.

RESULTS AND CONCLUSION

This review highlights the cellular functions of SGK1, its crucial role in cancer development, and clinical insights for SGK1 targeted therapies. Furthermore, the role of SGK1-mediated autophagy as a potential therapeutic target for cancer has been discussed.

A genetic titration of membrane composition in Caenorhabditis elegans reveals its importance for multiple cellular and physiological traits

Communicating editor: B. Grant The composition and biophysical properties of cellular membranes must be tightly regulated to maintain the proper functions of myriad processes within cells. To better understand the importance of membrane homeostasis, we assembled a panel of five Caenorhabditis elegans strains that show a wide span of membrane composition and properties, ranging from excessively rich in saturated fatty acids (SFAs) and rigid to excessively rich in polyunsaturated fatty acids (PUFAs) and fluid. The genotypes of the five strain are, from most rigid to most fluid: paqr-1(tm3262); paqr-2(tm3410), paqr-2(tm3410), N2 (wild-type), mdt-15(et14); nhr-49(et8), and mdt-15(et14); nhr-49(et8); acs-13(et54). We confirmed the excess SFA/rigidity-to-excess PUFA/fluidity gradient using the methods of fluorescence recovery after photobleaching (FRAP) and lipidomics analysis. The five strains were then studied for a variety of cellular and physiological traits and found to exhibit defects in: permeability, lipid peroxidation, growth at different temperatures, tolerance to SFA-rich diets, lifespan, brood size, vitellogenin trafficking, oogenesis, and autophagy during starvation. The excessively rigid strains often exhibited defects in opposite directions compared to the excessively fluid strains. We conclude that deviation from wild-type membrane homeostasis is pleiotropically deleterious for numerous cellular/physiological traits. The strains introduced here should prove useful to further study the cellular and physiological consequences of impaired membrane homeostasis.

Thioredoxin interacting protein drives astrocytic glucose hypometabolism in corticosterone-induced depressive state

Brain energetics disturbance is a hypothesized cause of depression. Glucose is the predominant fuel of brain energy metabolism, however, the cell-specific change of glucose metabolism and underlying molecular mechanism in depression remain unclear. In this study, we firstly applied ¹⁸ F-FDG PET and observed brain glucose hypometabolism in prefrontal cortex (PFC) of corticosterone-induced depression of rats. Next, astrocytic glucose hypometabolism was identified in PFC slices in in both corticosterone-induced depression of rats and cultured primary astrocytes from newborn rat PFC after stress-level corticosterone (100 nM) stimulation. Furthermore, we found the blockage of glucose uptake and the decrease of plasma membrane (PM) translocation of glucose transporter 1 (GLUT1) in astrocytic glucose hypometabolism under depressive condition. Interestingly, thioredoxin interacting protein (TXNIP), a glucose metabolism sensor and controller, was found to be overexpressed in corticosterone-stimulated astrocytes in vivo and in vitro. High TXNIP level could restrict GLUT1-mediated glucose uptake in primary astrocytes in vitro. Adeno-associated virus vector-mediated astrocytic TXNIP overexpression in rat medial PFC suppressed GLUT1 PM translocation, consequently developed depressive-like behavior. Conversely, TXNIP siRNA facilitated GLUT1 PM translocation to recover glucose hypometabolism in corticosterone-exposed cultured astrocytes. Notably, astrocyte-specific knockdown of TXNIP in medial PFC of rats facilitated astrocytic GLUT1 PM translocation, showing obvious antidepressant activity. These findings provide a new astrocytic energetic perspective in the pathogenesis of depression, more importantly, provide TXNIP as a promising molecular target for novel depression therapy.

Alteration in the mRNA expression profile of the autophagy-related mTOR pathway in schizophrenia patients treated with olanzapine

BACKGROUND

The mammalian target of rapamycin protein (mTOR) signaling pathway is involved in the pathogenesis of schizophrenia and the mechanism of extrapyramidal adverse reactions to antipsychotic drugs, which might be mediated by an mTOR-dependent autophagy impairment. This study aimed to examine the expression of mTOR pathway genes in patients with schizophrenia treated with olanzapine, which is considered an mTOR inhibitor and autophagy inducer.

METHODS

Thirty-two patients with acute schizophrenia who had been treated with olanzapine for four weeks (average dose 14.24 ± 4.35 mg/d) and 32 healthy volunteers were recruited. Before and after olanzapine treatment, the Positive and Negative Syndrome Scale (PANSS) was used to evaluate the symptoms of patients with schizophrenia, and the mRNA expression levels of mTOR pathway-related genes, including MTOR, RICTOR, RAPTOR, and DEPTOR, were detected in fasting venous blood samples from all subjects using real-time quantitative PCR.

RESULTS

The MTOR and RICTOR mRNA expression levels in patients with acute schizophrenia were significantly decreased compared with those of healthy controls and further significantly decreased after four weeks of olanzapine treatment. The DEPTOR mRNA expression levels in patients with acute schizophrenia were not significantly different from those of healthy controls but were significantly increased after treatment. The expression levels of the RAPTOR mRNA were not significantly different among the three groups. The pairwise correlations of MTOR, DEPTOR, RAPTOR, and RICTOR mRNA expression levels in patients with acute schizophrenia and healthy controls were significant. After olanzapine treatment, the correlations between the expression levels of the DEPTOR and MTOR mRNAs and between the DEPTOR and RICTOR mRNAs disappeared.

CONCLUSIONS

Abnormalities in the mTOR pathway, especially DEPTOR and mTORC2, might play important roles in the autophagy mechanism underlying the pathophysiology of schizophrenia and effects of olanzapine treatment.

mTORC2: The other mTOR in autophagy regulation

The mechanistic target of rapamycin (mTOR) has gathered significant attention as a ubiquitously expressed multimeric kinase with key implications for cell growth, proliferation, and survival. This kinase forms the central core of two distinct complexes, mTORC1 and mTORC2, which share the ability of integrating environmental, nutritional, and hormonal cues but which regulate separate molecular pathways that result in different cellular responses. Particularly, mTORC1 has been described as a major negative regulator of endosomal biogenesis and autophagy, a catabolic process that degrades intracellular components and organelles within the lysosomes and is thought to play a key role in human health and disease. In contrast, the role of mTORC2 in the regulation of autophagy has been considerably less studied despite mounting evidence this complex may regulate autophagy in a different and perhaps complementary manner to that of mTORC1. Genetic ablation of unique subunits is currently being utilized to study the differential effects of the two mTOR complexes. RICTOR is the best‐described subunit specific to mTORC2 and as such has become a useful tool for investigating the specific actions of this complex. The development of complex‐specific inhibitors for mTORC2 is also an area of intense interest. Studies to date have demonstrated that mTORC1/2 complexes each signal to a variety of exclusive downstream molecules with distinct biological roles. Pinpointing the particular effects of these downstream effectors is crucial toward the development of novel therapies aimed at accurately modulating autophagy in the context of human aging and disease.

Adding Some "Splice" to Stress Eating: Autophagy, ESCRT and Alternative Splicing Orchestrate the Cellular Stress Response

Autophagy is a widely studied self-renewal pathway that is essential for degrading damaged cellular organelles or recycling biomolecules to maintain cellular homeostasis, particularly under cellular stress. This pathway initiates with formation of an autophagosome, which is a double-membrane structure that envelopes cytosolic components and fuses with a lysosome to facilitate degradation of the contents. The endosomal sorting complexes required for transport (ESCRT) proteins play an integral role in controlling autophagosome fusion events and disruption to this machinery leads to autophagosome accumulation. Given the central role of autophagy in maintaining cellular health, it is unsurprising that dysfunction of this process is associated with many human maladies including cancer and neurodegenerative diseases. The cell can also rapidly respond to cellular stress through alternative pre-mRNA splicing that enables adaptive changes to the cell's proteome in response to stress. Thus, alternative pre-mRNA splicing of genes that are involved in autophagy adds another layer of complexity to the cell's stress response. Consequently, the dysregulation of alternative splicing of genes associated with autophagy and ESCRT may also precipitate disease states by either reducing the ability of the cell to respond to stress or triggering a maladaptive response that is pathogenic. In this review, we summarize the diverse roles of the ESCRT machinery and alternative splicing in regulating autophagy and how their dysfunction can have implications for human disease.

VHL suppresses RAPTOR and inhibits mTORC1 signaling in clear cell renal cell carcinoma

Inactivation of the tumor suppressor von Hippel-Lindau (VHL) gene is a key event in hereditary and sporadic clear cell renal cell carcinomas (ccRCC). The mechanistic target of rapamycin (mTOR) signaling pathway is a fundamental regulator of cell growth and proliferation, and hyperactivation of mTOR signaling is a common finding in VHL-dependent ccRCC. Deregulation of mTOR signaling correlates with tumor progression and poor outcome in patients with ccRCC. Here, we report that the regulatory-associated protein of mTOR (RAPTOR) is strikingly repressed by VHL. VHL interacts with RAPTOR and increases RAPTOR degradation by ubiquitination, thereby inhibiting mTORC1 signaling. Consistent with hyperactivation of mTORC1 signaling in VHL-deficient ccRCC, we observed that loss of vhl-1 function in C. elegans increased mTORC1 activity, supporting an evolutionary conserved mechanism. Our work reveals important new mechanistic insight into deregulation of mTORC1 signaling in ccRCC and links VHL directly to the control of RAPTOR/mTORC1. This may represent a novel mechanism whereby loss of VHL affects organ integrity and tumor behavior.

The mTOR-Autophagy Axis and the Control of Metabolism

The mechanistic target of rapamycin (mTOR), master regulator of cellular metabolism, exists in two distinct complexes: mTOR complex 1 and mTOR complex 2 (mTORC1 and 2). MTORC1 is a master switch for most energetically onerous processes in the cell, driving cell growth and building cellular biomass in instances of nutrient sufficiency, and conversely, allowing autophagic recycling of cellular components upon nutrient limitation. The means by which the mTOR kinase blocks autophagy include direct inhibition of the early steps of the process, and the control of the lysosomal degradative capacity of the cell by inhibiting the transactivation of genes encoding structural, regulatory, and catalytic factors. Upon inhibition of mTOR, autophagic recycling of cellular components results in the reactivation of mTORC1; thus, autophagy lies both downstream and upstream of mTOR. The functional relationship between the mTOR pathway and autophagy involves complex regulatory loops that are significantly deciphered at the cellular level, but incompletely understood at the physiological level. Nevertheless, genetic evidence stemming from the use of engineered strains of mice has provided significant insight into the overlapping and complementary metabolic effects that physiological autophagy and the control of mTOR activity exert during fasting and nutrient overload.

Prohibitin depletion extends lifespan of a TORC2/SGK-1 mutant through autophagy and the mitochondrial UPR

Mitochondrial prohibitins (PHB) are highly conserved proteins with a peculiar effect on lifespan. While PHB depletion shortens lifespan of wild‐type animals, it enhances longevity of a plethora of metabolically compromised mutants, including target of rapamycin complex 2 (TORC2) mutants sgk‐1 and rict‐1. Here, we show that sgk‐1 mutants have impaired mitochondrial homeostasis, lipogenesis and yolk formation, plausibly due to alterations in membrane lipid and sterol homeostasis. Remarkably, all these features are suppressed by PHB depletion. Our analysis shows the requirement of SRBP1/SBP‐1 for the lifespan extension of sgk‐1 mutants and the further extension conferred by PHB depletion. Moreover, although the mitochondrial unfolded protein response (UPR^mt) and autophagy are induced in sgk‐1 mutants and upon PHB depletion, they are dispensable for lifespan. However, the enhanced longevity caused by PHB depletion in sgk‐1 mutants requires both, the UPR^mt and autophagy, but not mitophagy. We hypothesize that UPR^mt induction upon PHB depletion extends lifespan of sgk‐1 mutants through autophagy and probably modulation of lipid metabolism.

Perspectives on ROCK2 as a Therapeutic Target for Alzheimer's Disease

Rho-associated coiled-coil containing kinase isoform 2 (ROCK2) is a member of the AGC family of serine/threonine kinases and an extensively studied regulator of actin-mediated cytoskeleton contractility. Over the past decade, new evidence has emerged that suggests ROCK2 regulates autophagy. Recent studies indicate that dysregulation of autophagy contributes to the development of misfolded tau aggregates among entorhinal cortex (EC) excitatory neurons in early Alzheimer's disease (AD). While the accumulation of tau oligomers and fibrils is toxic to neurons, autophagy facilitates the degradation of these pathologic species and represents a major cellular pathway for tau disposal in neurons. ROCK2 is expressed in excitatory neurons and pharmacologic inhibition of ROCK2 can induce autophagy pathways. In this mini-review, we explore potential mechanisms by which ROCK2 mediates autophagy and actin dynamics and discuss how these pathways represent therapeutic avenues for Alzheimer's disease.

The endolysosomal adaptor PLEKHM1 is a direct target for both mTOR and MAPK pathways

The lysosome is a cellular signalling hub at the point of convergence of endocytic and autophagic pathways, where the contents are degraded and recycled. Pleckstrin homology domain-containing family member 1 (PLEKHM1) acts as an adaptor to facilitate the fusion of endocytic and autophagic vesicles with the lysosome. However, it is unclear how PLEKHM1 function at the lysosome is controlled. Herein, we show that PLEKHM1 coprecipitates with, and is directly phosphorylated by, mTOR. Using a phosphospecific antibody against Ser432/S435 of PLEKHM1, we show that the same motif is a direct target for ERK2-mediated phosphorylation in a growth factor-dependent manner. This dual regulation of PLEKHM1 at a highly conserved region points to a convergence of both growth factor- and amino acid-sensing pathways, placing PLEKHM1 at a critical juncture of cellular metabolism.

Relevance of Autophagy and Mitophagy Dynamics and Markers in Neurodegenerative Diseases

During the past few decades, considerable efforts have been made to discover and validate new molecular mechanisms and biomarkers of neurodegenerative diseases. Recent discoveries have demonstrated how autophagy and its specialized form mitophagy are extensively associated with the development, maintenance, and progression of several neurodegenerative diseases. These mechanisms play a pivotal role in the homeostasis of neural cells and are responsible for the clearance of intracellular aggregates and misfolded proteins and the turnover of organelles, in particular, mitochondria. In this review, we summarize recent advances describing the importance of autophagy and mitophagy in neurodegenerative diseases, with particular attention given to multiple sclerosis, Parkinson’s disease, and Alzheimer’s disease. We also review how elements involved in autophagy and mitophagy may represent potential biomarkers for these common neurodegenerative diseases. Finally, we examine the possibility that the modulation of autophagic and mitophagic mechanisms may be an innovative strategy for overcoming neurodegenerative conditions. A deeper knowledge of autophagic and mitophagic mechanisms could facilitate diagnosis and prognostication as well as accelerate the development of therapeutic strategies for neurodegenerative diseases.

The interplay of ROS and the PI3K/Akt pathway in autophagy regulation

Autophagy causes the breakdown of damaged proteins and organelles to their constituent components. The phosphatidylinositol 3-kinase (PI3K) pathway played an important role in regulating the autophagic response of cells in response to changing reactive oxygen species (ROS) levels. The PI3K α catalytic subunit inhibits autophagy, while its β catalytic subunit promotes autophagy in response to changes in ROS levels. The downstream Akt protein acts against autophagy initiation in response to increases in ROS levels under nutrient-rich conditions. Akt acts by activating a mechanistic target of the rapamycin complex 1 (mTORC1) and by arresting autophagic gene expression. The AMP-activated protein kinase (AMPK) protein counteracts the Akt actions. mTORC1 and mTORC2 inhibit autophagy under moderate ROS levels, but under high ROS levels, mTORC2 can promote cellular senescence via autophagy. Phosphatase and tensin homolog (PTEN) protein are the negative regulators of the PI3K pathway, and it has proautophagic activities. Studies conducted on cells treated with flavonoids and ionizing radiation showed that the moderate increase in ROS levels in the flavonoid-treated groups corresponded with higher PTEN levels and lowered Akt levels leading to a higher occurrence of autophagy. In contrast, higher ROS levels evoked by ionizing radiation caused a lowering of the incidence of autophagy.

The Mitochondrial Permeability Transition: Nexus of Aging, Disease and Longevity

The activity of the mitochondrial permeability transition pore, mPTP, a highly regulated multi-component mega-channel, is enhanced in aging and in aging-driven degenerative diseases. mPTP activity accelerates aging by releasing large amounts of cell-damaging reactive oxygen species, Ca²⁺ and NAD⁺. The various pathways that control the channel activity, directly or indirectly, can therefore either inhibit or accelerate aging or retard or enhance the progression of aging-driven degenerative diseases and determine lifespan and healthspan. Autophagy, a catabolic process that removes and digests damaged proteins and organelles, protects the cell against aging and disease. However, the protective effect of autophagy depends on mTORC2/SKG1 inhibition of mPTP. Autophagy is inhibited in aging cells. Mitophagy, a specialized form of autophagy, which retards aging by removing mitochondrial fragments with activated mPTP, is also inhibited in aging cells, and this inhibition leads to increased mPTP activation, which is a major contributor to neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases. The increased activity of mPTP in aging turns autophagy/mitophagy into a destructive process leading to cell aging and death. Several drugs and lifestyle modifications that enhance healthspan and lifespan enhance autophagy and inhibit the activation of mPTP. Therefore, elucidating the intricate connections between pathways that activate and inhibit mPTP, in the context of aging and degenerative diseases, could enhance the discovery of new drugs and lifestyle modifications that slow aging and degenerative disease.

mTORC2 Assembly Is Regulated by USP9X-Mediated Deubiquitination of RICTOR

The mechanistic target of rapamycin complex 2 (mTORC2) controls cell metabolism and survival in response to environmental inputs. Dysregulation of mTORC2 signaling has been linked to diverse human diseases, including cancer and metabolic disorders, highlighting the importance of a tightly controlled mTORC2. While mTORC2 assembly is a critical determinant of its activity, the factors regulating this event are not well understood, and it is unclear whether this process is regulated by growth factors. Here, we present data, from human cell lines and mice, describing a mechanism by which growth factors regulate ubiquitin-specific protease 9X (USP9X) deubiquitinase to stimulate mTORC2 assembly and activity. USP9X removes Lys63-linked ubiquitin from RICTOR to promote its interaction with mTOR, thereby facilitating mTORC2 signaling. As mTORC2 is central for cellular homeostasis, understanding the mechanisms regulating mTORC2 activation toward its downstream targets is vital for our understanding of physiological processes and for developing new therapeutic strategies in pathology.

Monitoring autophagic flux in Caenorhabditis elegans using a p62/SQST-1 reporter

Autophagy is a well-conserved self-degrading mechanism, which involves the elimination of unnecessary or damaged cellular constituents. Although extensively studied, many aspects regarding its tight regulation and its implication in health and disease remain elusive. The nematode Caenorhabditis elegans has been widely used as a simple multicellular model organism for studying the autophagic machinery per se, and uncover its multidimensional roles in the maintenance of cellular and organismal homeostasis. The current protocol describes the in vivo detection and biochemical analysis of the autophagic substrate SQST-1, as an indicator of autophagic flux in C. elegans.

Autophagy in kidney homeostasis and disease

Autophagy is a conserved lysosomal pathway for the degradation of cytoplasmic components. Basal autophagy in kidney cells is essential for the maintenance of kidney homeostasis, structure and function. Under stress conditions, autophagy is altered as part of the adaptive response of kidney cells, in a process that is tightly regulated by signalling pathways that can modulate the cellular autophagic flux - mammalian target of rapamycin, AMP-activated protein kinase and sirtuins are key regulators of autophagy. Dysregulated autophagy contributes to the pathogenesis of acute kidney injury, to incomplete kidney repair after acute kidney injury and to chronic kidney disease of varied aetiologies, including diabetic kidney disease, focal segmental glomerulosclerosis and polycystic kidney disease. Autophagy also has a role in kidney ageing. However, questions remain about whether autophagy has a protective or a pathological role in kidney fibrosis, and about the precise mechanisms and signalling pathways underlying the autophagy response in different types of kidney cells and across the spectrum of kidney diseases. Further research is needed to gain insights into the regulation of autophagy in the kidneys and to enable the discovery of pathway-specific and kidney-selective therapies for kidney diseases and anti-ageing strategies.

ULK1-ATG13 and their mitotic phospho-regulation by CDK1 connect autophagy to cell cycle

Unc-51-like autophagy activating kinase 1 (ULK1)–autophagy-related 13 (ATG13) is the most upstream autophagy initiation complex that is phosphorylated by mammalian target-of-rapamycin complex 1 (mTORC1) and AMP-activated protein kinase (AMPK) to induce autophagy in asynchronous conditions. However, their phospho-regulation and functions in mitosis and cell cycle remain unknown. Here we show that ULK1-ATG13 complex is differentially regulated throughout the cell cycle, especially in mitosis, in which both ULK1 and ATG13 are highly phosphorylated by the key cell cycle machinery cyclin-dependent kinase 1 (CDK1)/cyclin B. Combining mass spectrometry and site-directed mutagenesis, we found that CDK1-induced ULK1-ATG13 phosphorylation promotes mitotic autophagy and cell cycle progression. Moreover, double knockout (DKO) of ULK1 and ATG13 could block cell cycle progression and significantly decrease cancer cell proliferation in cell line and mouse models. Our results not only bridge the mutual regulation between the core machinery of autophagy and mitosis but also illustrate the positive function of ULK1-ATG13 and their phosphorylation by CDK1 in mitotic autophagy regulation.

This study shows that the ULK1-ATG13 autophagy initiation complex is differentially regulated throughout the cell cycle, especially in mitosis, in which both ULK1 and ATG13 are highly phosphorylated by CDK1/cyclin B, promoting mitotic autophagy and cell cycle progression.

Pro-Senescence and Anti-Senescence Mechanisms of Cardiovascular Aging: Cardiac MicroRNA Regulation of Longevity Drug-Induced Autophagy

Chronological aging as well as biological aging accelerated by various pathologies such as diabetes and obesity contribute to cardiovascular aging, and structural and functional tissue damage of the heart and vasculature. Cardiovascular aging in humans is characterized by structural pathologic remodeling including cardiac and vascular fibrosis, hypertrophy, stiffness, micro- and macro-circulatory impairment, left ventricular diastolic dysfunction precipitating heart failure with either reduced or preserved ejection fraction, and cardiovascular cell death. Cellular senescence, an important hallmark of aging, is a critical factor that impairs repair and regeneration of damaged cells in cardiovascular tissues whereas autophagy, an intracellular catabolic process is an essential inherent mechanism that removes senescent cells throughout life time in all tissues. Several recent reviews have highlighted the fact that all longevity treatment paradigms to mitigate progression of aging-related pathologies converge in induction of autophagy, activation of AMP kinase (AMPK) and Sirtuin pathway, and inhibition of mechanistic target of rapamycin (mTOR). These longevity treatments include health style changes such as caloric restriction, and drug treatments using rapamycin, the first FDA-approved longevity drug, as well as other experimental longevity drugs such as metformin, rapamycin, aspirin, and resveratrol. However, in the heart tissue, autophagy induction has to be tightly regulated since evidence show excessive autophagy results in cardiomyopathy and heart failure. Here we discuss emerging evidence for microRNA-mediated tight regulation of autophagy in the heart in response to treatment with rapamycin, and novel approaches to monitor autophagy progression in a temporal manner to diagnose and regulate autophagy induction by longevity treatments.

TORC2-SGK-1 signaling integrates external signals to regulate autophagic turnover of mitochondria via mtROS

Macroautophagy/autophagy is an evolutionarily conserved cellular degradation and recycling process that is tightly regulated by external stimuli, diet, and stress. Our recent findings suggest that in C. elegans, a nutrient sensing pathway mediated by MTORC2 (mechanistic target of rapamycin kinase complex 2) and its downstream effector kinase SGK-1 (serum- and glucocorticoid-inducible kinase homolog 1) suppresses autophagy, involving mitophagy. Induced autophagy/mitophagy in MTORC2-deficient animals slows down development and impairs reproduction independently of the SGK-1 effectors DAF-16/FOXO and SKN-1/NFE2L2/NRF2. In this punctum, we discuss how TORC2-SGK-1 signaling might regulate autophagic turnover and its impact on mitochondrial homeostasis via linking mitochondria-derived reactive oxygen species (mtROS) production to mitophagic turnover.