Maintenance of mitochondrial morphology by autophagy and its role in high glucose effects on chronological lifespan of Saccharomyces cerevisiae

The phosphorylation status of Hsp82 regulates mitochondrial homeostasis during glucose sensing in Saccharomyces cerevisiae

Sensing extracellular glucose, budding yeast switches from aerobic glycolysis to oxidative phosphorylation to adapt to environmental changes. During the conversion of metabolic mode, mitochondrial function and morphology change significantly. Mitochondria are the main supply factories of energy for various life activities in cells. However, the research on the signal pathways from glucose sensing to changes in mitochondrial function and morphology is still scarce and worthy of further exploration. In this study, we found that in addition to the known involvement of molecular chaperone Hsp82 in stress response during the conversion of metabolic mode, the phosphorylation status of Hsp82 at S485 residue regulates mitochondrial function and morphology to maintain mitochondrial homeostasis. The Hsp82^S485A mutant that mimics dephosphorylation at S485 residue showed abnormal growth phenotypes related to mitochondrial defects, such as the petite phenotype, slow growth rates, and inability to use non-fermentable carbon sources. Further exploring the causes of growth defects, we found that the Hsp82^S485A mutant caused mitochondrial dysfunction, including a decrease in cellular oxygen consumption rate, defects in mitochondrial electron transport chain, decreased mitochondrial membrane potential and complete loss of mtDNA. Furthermore, the Hsp82^S485A mutant displayed fragmented or globular mitochondria, which may be responsible for its mitochondrial dysfunction. Our findings suggested that the phosphorylation status of Hsp82 at S485 residue might regulate mitochondrial function and morphology by affecting the stability of mitochondrial fission and fusion-related proteins. Thus, Hsp82 might be a key molecule in the signal pathway from glucose sensing to changes in mitochondrial function and morphology.

Yeast Chronological Lifespan: Longevity Regulatory Genes and Mechanisms

S. cerevisiae plays a pivotal role as a model system in understanding the biochemistry and molecular biology of mammals including humans. A considerable portion of our knowledge on the genes and pathways involved in cellular growth, resistance to toxic agents, and death has in fact been generated using this model organism. The yeast chronological lifespan (CLS) is a paradigm to study age-dependent damage and longevity. In combination with powerful genetic screening and high throughput technologies, the CLS has allowed the identification of longevity genes and pathways but has also introduced a unicellular “test tube” model system to identify and study macromolecular and cellular damage leading to diseases. In addition, it has played an important role in studying the nutrients and dietary regimens capable of affecting stress resistance and longevity and allowing the characterization of aging regulatory networks. The parallel description of the pro-aging roles of homologs of RAS, S6 kinase, adenylate cyclase, and Tor in yeast and in higher eukaryotes in S. cerevisiae chronological survival studies is valuable to understand human aging and disease. Here we review work on the S. cerevisiae chronological lifespan with a focus on the genes regulating age-dependent macromolecular damage and longevity extension.

Stress and ageing in yeast

There has long been speculation about the role of various stresses in ageing. Some stresses have beneficial effects on ageing-dependent on duration and severity of the stress, others have negative effects and the question arises whether these negative effects are causative of ageing or the result of the ageing process. Cellular responses to many stresses are highly coordinated in a concerted way and hence there is a great deal of cross-talk between different stresses. Here the relevant aspects of the coordination of stress responses and the roles of different stresses on yeast cell ageing are discussed, together with the various functions that are involved. The cellular processes that are involved in alleviating the effects of stress on ageing are considered, together with the possible role of early stress events on subsequent ageing of cells.

In Vitro Nephrotoxicity Induced by Herb-Herb Interaction between Radix Glycyrrhizae and Radix Euphorbiae Pekinensis

Radix Glycyrrhizae (RG)-Radix Euphorbiae Pekinensis (REP) is a representative incompatible herbal pair of Eighteen Incompatible Medicaments (EIM) and has been disputed in clinical application for a long time. The present study was performed with the Madin-Darby canine kidney (MDCK) cell line using cell cytotoxicity assay, apoptosis detection, cell cycle measurement, reactive oxygen species (ROS) determination, and high content analysis (HCA) in combination with high-performance liquid chromatography (HPLC) fingerprint comparison to clarify whether RG and REP can be concomitantly used from the perspective of cytotoxicity, investigate the major correlated compounds, and elucidate the underlying mechanisms. The results showed that the toxicity of REP could be significantly enhanced through its concomitant use with RG in the ratio of 1 : 1, and this increased toxicity could be weakened with the further increased proportion of RG. 3,3′-di-O-methylellagic acid-4′-O-β-D-xylopyranoside (DEAX) and 3,3′-di-O-methylellagic acid (DEA) were shown to be mainly responsible for the toxicity induced by concomitant use of REP and RG. Both RG-REP decoctions and the above two compounds boosted cell apoptosis, cellular morphological change, ROS accumulation, and mitochondrial membrane potential (MMP) disruption. In conclusion, the incompatible use of RG and REP is conditionally established because of the bidirectional regulatory effect of RG, and the major compounds responsible for RG-REP incompatibility are DEAX and DEA, which result in toxicity through activation of mitochondria-dependent apoptosis induced by increased ROS production. This study provided a basis for understanding the incompatible use of RG and REP and the EIM theory.

Slm35 links mitochondrial stress response and longevity through TOR signaling pathway

In most eukaryotic cells mitochondria are essential organelles involved in a great variety of cellular functions. One of the physiological processes linked to mitochondria is aging, a gradual process of damage accumulation that eventually promotes cell death. Aging depends on a balance between mitochondrial biogenesis, function and degradation. It has been previously shown that Tor1, Sch9 and Ras2 are activated in response to nutrient availability and regulate cell growth and division. A deficiency in any of these genes promotes lifespan extension and cell protection during oxidative and heat shock stress. In this work we report that in Saccharomyces cerevisiae, the uncharacterized mitochondrial protein Slm35 is functionally linked with the TOR signaling pathway. A Δtor1Δslm35 strain shows a severe decrease in lifespan and is unable to contend with oxidative and heat shock stresses. Specifically, this mutant shows decreased catalase activity indicating a misregulation of ROS scavenging mechanisms. In this study we show that Slm35 is also relevant for mitochondrial network dynamics and mitophagy. The results presented here suggest that Slm35 plays an important role connecting mitochondrial function with cytosolic responses and cell adaptation to stress and aging.

The m6A methyltransferase Ime4 and mitochondrial functions in yeast

In eukaryotes, the precise transcriptional and post-transcriptional regulations of gene expression are crucial for the developmental processes. More than 100 types of post-transcriptional RNA modifications have been identified in eukaryotes. The deposition of N⁶-methyladenosine (m⁶A) into mRNA is among the most common post-transcriptional RNA modifications known in eukaryotes. It has been reported that m⁶A RNA modification can regulate gene expression. The role of yeast m⁶A methyltransferase (Ime4) in meiosis and sporulation in diploid cells is very well proven, but its physiological role in haploid cells has remained unknown until recently. Previously, we have shown that Ime4 epitranscriptionally regulates triacylglycerol (TAG) metabolism and vacuolar morphology in haploid cells. Mitochondrial dysfunction leads to TAG accumulation as lipid droplets (LDs) in the cells; besides, LDs are physically connected to the mitochondria. As of now there are no reports on the role of Ime4 in mitochondrial biology. Here we report the important role played by Ime4 in the mitochondrial morphology and functions in Saccharomyces cerevisiae. The confocal microscopic analysis showed that IME4 gene deletion causes mitochondrial fragmentation; besides, the ime4Δ cells showed a significant decrease in cytochrome c oxidase and citrate synthase activities compared to the wild-type cells. IME4 gene deletion causes mitochondrial dysfunction, and it will be interesting to find out the target genes of Ime4 related to the mitochondrial biology. The determination of the role of Ime4 and its targets in mitochondrial biology could probably help in formulating potential cures for the mitochondria-linked rare genetic disorders.

Fructose-Induced Carbonyl/Oxidative Stress in S. cerevisiae: Involvement of TOR

The TOR (target of rapamycin) signaling pathway first described in the budding yeast Saccharomyces cerevisiae is highly conserved in eukaryotes effector of cell growth, longevity, and stress response. TOR activation by nitrogen sources, in particular amino acids, is well studied; however its interplay with carbohydrates and carbonyl stress is poorly investigated. Fructose is a more potent glycoxidation agent capable of producing greater amounts of reactive carbonyl (RCS) and oxygen species (ROS) than glucose. The increased RCS/ROS production, as a result of glycoxidation in vivo, is supposed to be involved in carbonyl/oxidative stress, metabolic disorders, and lifespan shortening of eukaryotes. In this work we aim to expand our understanding of how TOR is involved in carbonyl/oxidative stress caused by reducing monosaccharides. It was found that in fructose-grown compared with glucose-grown cells the level of carbonyl/oxidative stress markers was higher. The defects in the TOR pathway inhibited metabolic rate and suppressed generation of glycoxidation products in fructose-grown yeast.

Oxidative stress in aging human skin

Oxidative stress in skin plays a major role in the aging process. This is true for intrinsic aging and even more for extrinsic aging. Although the results are quite different in dermis and epidermis, extrinsic aging is driven to a large extent by oxidative stress caused by UV irradiation. In this review the overall effects of oxidative stress are discussed as well as the sources of ROS including the mitochondrial ETC, peroxisomal and ER localized proteins, the Fenton reaction, and such enzymes as cyclooxygenases, lipoxygenases, xanthine oxidases, and NADPH oxidases. Furthermore, the defense mechanisms against oxidative stress ranging from enzymes like superoxide dismutases, catalases, peroxiredoxins, and GSH peroxidases to organic compounds such as L-ascorbate, α-tocopherol, beta-carotene, uric acid, CoQ10, and glutathione are described in more detail. In addition the oxidative stress induced modifications caused to proteins, lipids and DNA are discussed. Finally age-related changes of the skin are also a topic of this review. They include a disruption of the epidermal calcium gradient in old skin with an accompanying change in the composition of the cornified envelope. This modified cornified envelope also leads to an altered anti-oxidative capacity and a reduced barrier function of the epidermis.

Tetramethylpyrazine (TMP) protects against sodium arsenite-induced nephrotoxicity by suppressing ROS production, mitochondrial dysfunction, pro-inflammatory signaling pathways and programed cell death

Although kidney is a target organ of arsenic cytotoxicity, the underlying mechanisms of arsenic-induced nephrotoxicity remain poorly understood. As tetramethylpyrazine (TMP) has recently been found to be a renal protectant in multiple kidney injuries, we hypothesize that TMP could suppress arsenic nephrotoxicity. In this study, human renal proximal tubular epithelial cell line HK-2 was used to elucidate the precise mechanisms of arsenic nephrotoxicity as well as the protective mechanism of TMP in these cells. Sodium arsenite exposure dramatically increased cellular reactive oxygen species (ROS) production, decreased levels of cellular glutathione (GSH), decreased cytochrome c oxidase activity and mitochondrial membrane potential, which indicated mitochondrial dysfunction. On the other hand, sodium arsenite activated pro-inflammatory signals, including β-catenin, nuclear factor-κB (NF-κB), p38 mitogen-activated protein kinase (MAPK), tumor necrosis factor alpha and cyclooxygenase-2 (COX-2). Small molecule inhibitors of NF-κB and p38 MAPK blocked arsenic-induced COX-2 expression, suggesting arsenic-induced COX-2 up-regulation was NF-κB- and p38 MAPK-dependent. Finally, sodium arsenite induced autophagy in HK-2 cells at early phase (6 h) and the subsequent apoptosis at 24 h. Treatment by TMP or by the antioxidant N-acetylcysteine decreased arsenic-induced ROS production, enhanced GSH levels, prevented mitochondria dysfunction and suppressed the activation of pro-inflammatory signals and the development of autophagy and apoptosis. Our results suggested that TMP may be used as a new potential therapeutic agent to prevent arsenic-induced nephrotoxicity by suppressing these pathological processes.

Reduced TORC1 signaling abolishes mitochondrial dysfunctions and shortened chronological lifespan of Isc1p-deficient cells

The target of rapamycin (TOR) is an important signaling pathway on a hierarchical network of interacting pathways regulating central biological processes, such as cell growth, stress response and aging. Several lines of evidence suggest a functional link between TOR signaling and sphingolipid metabolism. Here, we report that the TORC1-Sch9p pathway is activated in cells lacking Isc1p, the yeast orthologue of mammalian neutral sphingomyelinase 2. The deletion of TOR1 or SCH9 abolishes the premature aging, oxidative stress sensitivity and mitochondrial dysfunctions displayed by isc1Δ cells and this is correlated with the suppression of the autophagic flux defect exhibited by the mutant strain. The protective effect of TOR1 deletion, as opposed to that of SCH9 deletion, is not associated with the attenuation of Hog1p hyperphosphorylation, which was previously implicated in isc1Δ phenotypes. Our data support a model in which Isc1p regulates mitochondrial function and chronological lifespan in yeast through the TORC1-Sch9p pathway although Isc1p and TORC1 also seem to act through independent pathways, as isc1Δtor1Δ phenotypes are intermediate to those displayed by isc1Δ and tor1Δ cells. We also provide evidence that TORC1 downstream effectors, the type 2A protein phosphatase Sit4p and the AGC protein kinase Sch9p, integrate nutrient and stress signals from TORC1 with ceramide signaling derived from Isc1p to regulate mitochondrial function and lifespan in yeast. Overall, our results show that TORC1-Sch9p axis is deregulated in Isc1p-deficient cells, contributing to mitochondrial dysfunction, enhanced oxidative stress sensitivity and premature aging of isc1Δ cells.