Mol. Cells 2018; 41(1): 65-72
Published online January 23, 2018
https://doi.org/10.14348/molcells.2018.2333
© The Korean Society for Molecular and Cellular Biology
Correspondence to : *Correspondence: tamyoshi@fbs.osaka-u.ac.jp
Autophagy is an evolutionally conserved cytoplasmic degradation system in which varieties of materials are sequestered by a double membrane structure, autophagosome, and delivered to the lysosomes for the degradation. Due to the wide varieties of targets, autophagic activity is essential for cellular homeostasis. Recent genetic evidence indicates that autophagy has a crucial role in the regulation of animal lifespan. Basal level of autophagic activity is elevated in many longevity paradigms and the activity is required for lifespan extension. In most cases, genes involved in autophagy and lysosomal function are induced by several transcription factors including HLH-30/TFEB, PHA-4/FOXA and MML-1/Mondo in long-lived animals. Pharmacological treatments have been shown to extend lifespan through activation of autophagy, indicating autophagy could be a potential and promising target to modulate animal lifespan. Here we summarize recent progress regarding the role of autophagy in lifespan regulation.
Keywords aging, autophagy,
Macroautophagy, hereafter referred to as autophagy, is a catabolic process targeting wide varieties of cellular contents. Autophagy occurs at basal level in normal condition, but is accelerated by varieties of stresses such as starvation, accumulation of abnormal proteins, organelle damage and pathogen infection. Autophagy was originally considered to be a bulk and non-selective degradation system. But subsequent studies show autophagy selectively degrades cargos and by doing so contribute to the intracellular homeostasis. During autophagy, a small cisterna, called isolation membrane elongates and surrounds a portion of cytoplasm to form a double-membraned structure, called the autophagosome. Autophagosomes are then transported and fuse with lysosomes to form autolysosomes for the digestion of sequestered contents (Fig. 1). During autophagy, several autophagy-related (ATG) genes are engaged sequentially in a highly regulated manner. Genetic studies in yeast have identified more than 30 ATG genes that are required for autophagy, most of which are conserved from yeast to mammals. Essential ATG genes are organized into at least five functional groups that allow for the initiation, formation, elongation, and fusion of the autophagosome. These functional groups are the Atg1/ULK initiation complex, the class III PI3 Kinase nucleation complex, the phosphatidylinositol 3-phosphate (PI3P)-binding Atg18/Atg2 complex, the Atg5–Atg12 conjugation system, and the Atg8/LC3-PE (Atg8/LC3-phosphatidylethanolamine) conjugation system. First step of autophagy initiates from the activation of Atg1/ULK complex, which lead to the formation of isolation membrane. The next step involves membrane nucleation by the Class III Vps34/PI3-kinase nucleation complex (consisting of Vps34, Atg6/Beclin1, and Vps15/p150) via production of PI3P, to start formation of a double-membrane structure, isolation membrane (or phagophore). In mammals, the isolation membrane originates from the endoplasmic reticulum (ER)-mitochondria contact site and from others including Golgi, endosomes, and plasma membrane (Chan and Tang, 2013; Hamasaki et al., 2013). To start elongation, the isolation membrane recruits the PI3P-binding complex consisting of Atg18/WIPI and Atg2, which regulates the distribution of Atg9, a transmembrane protein that has been proposed to deliver lipids to the isolation membrane and the growing autophagosome. During the next step, the isolation membrane expands into a double-membrane structure called the autophagosome. Autophagosome elongation is dependent on two ubiquitin-like conjugation systems, the Atg5–Atg12 conjugation system and the Atg8/LC3-PE conjugation system. In Atg5–Atg12 conjugation system, Atg7 and Atg10 (E1- and E2-like enzymes, respectively) conjugate Atg12 to Atg5 and this complex associates with Atg16. Then, the Atg12–Atg5 conjugate promotes the conjugation of phosphatidylethanolamine (PE) to cytosolic Atg8/LC3, which is formed by cleavage of the ubiquitin-like protein Atg8/LC3 by the protease Atg4. During this process, PE-conjugated LC3 associates with the autophagosomal membrane and therefore LC3 is most commonly used as an experimental marker of autophagosomes (Fujita et al., 2008; Kabeya et al., 2000; Mizushima and Levine, 2010). The autophagosome eventually matures into a closed cargo-containing vesicle, which then fuses with the lysosome to become the autolysosome, and its contents are finally degraded for recycling. Autophagosome fusion step is mediated by HOPS complex, phosphoinositides, Rab proteins and SNEREs. For the detailed molecular mechanism of autophagosome formation and autophagosome-lysosome fusion, please refer to the recent specific review paper (Nakamura and Yoshimori, 2017).
Aging represents the functional deterioration of an organism. For long time, aging is not considered as a tightly regulated process. During last 20 decades, the evolutionally conserved molecular mechanisms which delay animal aging and extend lifespan have been identified using several model organisms including yeast, worms, fly and mice. These pathways, for instance, include reduced Insulin/IGF-1 signaling, dietary restriction, reduced TOR signaling, germline removal and reduced mitochondrial respiration. Extensive efforts to identify the downstream mechanism in each longevity pathway reveals that numerous but different sets of factors or biological processes mediate in each longevity pathways, although some factors work in common. Notably, recent studies mainly from
Reduced Insulin/IGF-1 signaling has been shown to extend the lifespan in several species (Kenyon, 2010). Initial discoveries were made in
Dietary restriction is one of most prominent way to slow aging and extend lifespan in many species. Dietary restriction was first observed to slow down aging in rat about 100 years ago. Since then the beneficial effects to extend lifespan was confirmed in numerous species including yeast, worms, fly, fish, dogs, mice and apes (Mair and Dillin, 2008). Multiple molecular mechanisms have been proposed to mediate the effect of dietary restriction on longevity, including TOR and Insulin/IGF-1 signaling. The lifespan of the budding yeast
Reproduction is negatively correlated with longevity in many species. Removal of germline stem cells by laser microsurgery or genetic mutation extends lifespan in
The free radical theory proposes that aging is the cumulative result of oxidative damages to cells and tissues over time. These molecular damages are caused by reactive oxygen species (ROS) which is generated primarily from mitochondrial respiration. Although oxidative damages increase with age, it is still unclear if this is indeed causative effect to organism aging. Importantly, reduced mitochondrial respiration is known to extend lifespan of many organism from yeast to mice (Hur et al., 2010; Kirchman et al., 1999). In worms, the reduction of electron transport chain components extends lifespan, when they are inhibited during larval stages. Several mitochondrial mutants including ubiquinone synthetase mutant
Loss of autophagic activity has been shown to cause premature aging phenotypes in many species. An unbiased screening for genes involved in chronological lifespan in yeast, identified several short-lived mutants which have mutation in macroautophagy genes (Matecic et al., 2010). Decreased lifespan is also observed in
In the following section, we summarize several pharmacological treatments which have been shown to extend animal lifespan and healthspan through the activation of autophagy (Table 1).
Administration of a natural polyamine, spermidine provides beneficial for health in a number of species and extends lifespan of yeast, worms, flies and mice (Eisenberg et al., 2009; 2016). Survival of cultured mammalian cells is also promoted by treatment with spermidine, and this is accompanied by epigenetic hypoacetylation of histone H3 via inhibition of histone acetyltransferase activity. This, in turn, correlates with transcriptional upregulation of multiple autophagy-related genes, including
Resveratrol is a naturally occurring polyphenolic compound found in grapes and an activator of the NAD-dependent histone deacetylase sirtuin (SIRT1). Administration of resveratrol is known to extend the lifespan of several model organisms (Park et al., 2013). Especially, the lifespan extension in
Urolithin A as a first-in-class natural compound that induces mitophagy both
Tomatidine, a natural compound abundant in unripe tomatoes, inhibits age-related skeletal muscle atrophy in mice. Recent study shows that tomatidine extends lifespan and healthspan in
In many cases, autophagic activation at the transcript level seems essential for longevity. Several autophagy and lysosomal genes are regulated by different transcription factors, microRNA and chromatin modifying enzymes, which are described below.
TFEB originally identified as a master regulator of lysosomal biogenesis is subsequently is shown to regulate autophagy and fat metabolism (Sardiello et al., 2009; Settembre et al., 2011; 2013a). TFEB is known to be negatively regulated by nutrient sensor TOR. At nutrient rich condition, TFEB is phosphorylated on lysosome. Phosphorylated TFEB is bound to 14-3-3 and is mainly localized on cytosol. Upon starvation, TOR becomes inactivated and TFEB is then dephosphorylated and translocated in the nucleus to initiate the transcription of target genes (Settembre et al., 2013b).
Other bHLH transcription factor complex, MML-1/MXL-2 has been identified as a novel regulator of longevity (Nakamura et al., 2016). MML-1/MXL-2 belongs to Myc and Mondo family member and their homologs, MondoA/MLX or ChREBP/MXL functions as a glucose sensor. MML-1/MXL-2 is required for the longevity conferred by germline removal, reduced Insulin/IGF-1 signaling, reduced mitochondrial respiration, reduced TOR signaling in
In
Many microRNA has been shown to regulate animal lifespan. Among them, miR-34 is related to autophagy and aging in some species. In worms, loss of function of miR-34 extends lifespan and this longevity is abolished by RNAi knockdown of several autophagy regulators,
NAD-dependent deacetylase SIRT1 (sirtuin 1) is a particularly well-known modulator of aging. Specifically, the life spans of yeast, worms, and flies can be extended by overexpression and/or pharmacological activation of SIRT1 and the lifespan of mice is extended by ubiquitous overexpression of SIRT6, or brain-specific overexpression of SIRT1 (Giblin et al., 2014; Park et al., 2013). In
As we described above, accumulating evidences show activation of autophagy seems essential for longevity. However, the several fundamental questions remain elusive. How is autophagy regulated during aging? Cells, tissues and timing specific roles of autophagy also need to be considered. Recently and unexpectedly, neuron specific knockdown of autophagy after reproductive period has been shown to extend lifespan in worms (Wilhelm et al., 2017). Thus, it is crucial to understand spatio- and temporal-regulation of autophagy and their physiological relevance to aging. It is essential to determine how autophagy contribute to lifespan extension and which autophagy cargo are relevant for aging and longevity. Clearance of lipids (lipophagy) and mitochondria (mitophagy) are relevance to
Longevity through activation of autophagy
Genetic or pharmacological manipulations | Animals | Phenotypes | Epistatic analysis by inhibition of autophagy genes | References |
---|---|---|---|---|
Reduced insulin/IGF-1 signaling | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Meléndez et al., 2003 |
Calorie restriction | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Jia et al., 2007 |
Reduced TOR signaling | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Hansen et al., 2008 |
Reduced mitochondrial respiration | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Toth et al., 2008 |
Germline removal | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Lapierre et al., 2011 |
HLH-30 overexpression | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Lapierre et al., 2013 |
Urolithin A | Worm, mouse | Lifespan extension(worms), improved muscle function(mouse), activation of mitophagy | Cancelation of longevity | Ryu et al., 2016 |
Resveratrol | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Morselli et al., 2010 |
Spermidine | Worm, | Lifespan extension, activation of autophagy | Cancelation of longevity | Eisenberg et al., 2009 |
Rapamycin | Lifespan extension, activation of autophagy | Cancelation of longevity | Bjedov et al., 2010 | |
Tomatidine | Worm | Lifespan extension, activation of autophagy | ND | Fang et al., 2017 |
Brain specific Atg8 overexpression | Lifespan extension in female | ND | Simonsen et al., 2008 | |
ATG5 overexpression | Mouse | Lifespan extension, activation of autophagy | ND | Pyo et al., 2013 |
Mol. Cells 2018; 41(1): 65-72
Published online January 31, 2018 https://doi.org/10.14348/molcells.2018.2333
Copyright © The Korean Society for Molecular and Cellular Biology.
Shuhei Nakamura1,2, and Tamotsu Yoshimori1,2,*
1Department of Genetics, Graduate School of Medicine, Osaka University, Osaka, Japan, 2Department of Intracellular Membrane Dynamics, Graduate School of Frontier Biosciences, Osaka University, Osaka, Japan
Correspondence to:*Correspondence: tamyoshi@fbs.osaka-u.ac.jp
Autophagy is an evolutionally conserved cytoplasmic degradation system in which varieties of materials are sequestered by a double membrane structure, autophagosome, and delivered to the lysosomes for the degradation. Due to the wide varieties of targets, autophagic activity is essential for cellular homeostasis. Recent genetic evidence indicates that autophagy has a crucial role in the regulation of animal lifespan. Basal level of autophagic activity is elevated in many longevity paradigms and the activity is required for lifespan extension. In most cases, genes involved in autophagy and lysosomal function are induced by several transcription factors including HLH-30/TFEB, PHA-4/FOXA and MML-1/Mondo in long-lived animals. Pharmacological treatments have been shown to extend lifespan through activation of autophagy, indicating autophagy could be a potential and promising target to modulate animal lifespan. Here we summarize recent progress regarding the role of autophagy in lifespan regulation.
Keywords: aging, autophagy,
Macroautophagy, hereafter referred to as autophagy, is a catabolic process targeting wide varieties of cellular contents. Autophagy occurs at basal level in normal condition, but is accelerated by varieties of stresses such as starvation, accumulation of abnormal proteins, organelle damage and pathogen infection. Autophagy was originally considered to be a bulk and non-selective degradation system. But subsequent studies show autophagy selectively degrades cargos and by doing so contribute to the intracellular homeostasis. During autophagy, a small cisterna, called isolation membrane elongates and surrounds a portion of cytoplasm to form a double-membraned structure, called the autophagosome. Autophagosomes are then transported and fuse with lysosomes to form autolysosomes for the digestion of sequestered contents (Fig. 1). During autophagy, several autophagy-related (ATG) genes are engaged sequentially in a highly regulated manner. Genetic studies in yeast have identified more than 30 ATG genes that are required for autophagy, most of which are conserved from yeast to mammals. Essential ATG genes are organized into at least five functional groups that allow for the initiation, formation, elongation, and fusion of the autophagosome. These functional groups are the Atg1/ULK initiation complex, the class III PI3 Kinase nucleation complex, the phosphatidylinositol 3-phosphate (PI3P)-binding Atg18/Atg2 complex, the Atg5–Atg12 conjugation system, and the Atg8/LC3-PE (Atg8/LC3-phosphatidylethanolamine) conjugation system. First step of autophagy initiates from the activation of Atg1/ULK complex, which lead to the formation of isolation membrane. The next step involves membrane nucleation by the Class III Vps34/PI3-kinase nucleation complex (consisting of Vps34, Atg6/Beclin1, and Vps15/p150) via production of PI3P, to start formation of a double-membrane structure, isolation membrane (or phagophore). In mammals, the isolation membrane originates from the endoplasmic reticulum (ER)-mitochondria contact site and from others including Golgi, endosomes, and plasma membrane (Chan and Tang, 2013; Hamasaki et al., 2013). To start elongation, the isolation membrane recruits the PI3P-binding complex consisting of Atg18/WIPI and Atg2, which regulates the distribution of Atg9, a transmembrane protein that has been proposed to deliver lipids to the isolation membrane and the growing autophagosome. During the next step, the isolation membrane expands into a double-membrane structure called the autophagosome. Autophagosome elongation is dependent on two ubiquitin-like conjugation systems, the Atg5–Atg12 conjugation system and the Atg8/LC3-PE conjugation system. In Atg5–Atg12 conjugation system, Atg7 and Atg10 (E1- and E2-like enzymes, respectively) conjugate Atg12 to Atg5 and this complex associates with Atg16. Then, the Atg12–Atg5 conjugate promotes the conjugation of phosphatidylethanolamine (PE) to cytosolic Atg8/LC3, which is formed by cleavage of the ubiquitin-like protein Atg8/LC3 by the protease Atg4. During this process, PE-conjugated LC3 associates with the autophagosomal membrane and therefore LC3 is most commonly used as an experimental marker of autophagosomes (Fujita et al., 2008; Kabeya et al., 2000; Mizushima and Levine, 2010). The autophagosome eventually matures into a closed cargo-containing vesicle, which then fuses with the lysosome to become the autolysosome, and its contents are finally degraded for recycling. Autophagosome fusion step is mediated by HOPS complex, phosphoinositides, Rab proteins and SNEREs. For the detailed molecular mechanism of autophagosome formation and autophagosome-lysosome fusion, please refer to the recent specific review paper (Nakamura and Yoshimori, 2017).
Aging represents the functional deterioration of an organism. For long time, aging is not considered as a tightly regulated process. During last 20 decades, the evolutionally conserved molecular mechanisms which delay animal aging and extend lifespan have been identified using several model organisms including yeast, worms, fly and mice. These pathways, for instance, include reduced Insulin/IGF-1 signaling, dietary restriction, reduced TOR signaling, germline removal and reduced mitochondrial respiration. Extensive efforts to identify the downstream mechanism in each longevity pathway reveals that numerous but different sets of factors or biological processes mediate in each longevity pathways, although some factors work in common. Notably, recent studies mainly from
Reduced Insulin/IGF-1 signaling has been shown to extend the lifespan in several species (Kenyon, 2010). Initial discoveries were made in
Dietary restriction is one of most prominent way to slow aging and extend lifespan in many species. Dietary restriction was first observed to slow down aging in rat about 100 years ago. Since then the beneficial effects to extend lifespan was confirmed in numerous species including yeast, worms, fly, fish, dogs, mice and apes (Mair and Dillin, 2008). Multiple molecular mechanisms have been proposed to mediate the effect of dietary restriction on longevity, including TOR and Insulin/IGF-1 signaling. The lifespan of the budding yeast
Reproduction is negatively correlated with longevity in many species. Removal of germline stem cells by laser microsurgery or genetic mutation extends lifespan in
The free radical theory proposes that aging is the cumulative result of oxidative damages to cells and tissues over time. These molecular damages are caused by reactive oxygen species (ROS) which is generated primarily from mitochondrial respiration. Although oxidative damages increase with age, it is still unclear if this is indeed causative effect to organism aging. Importantly, reduced mitochondrial respiration is known to extend lifespan of many organism from yeast to mice (Hur et al., 2010; Kirchman et al., 1999). In worms, the reduction of electron transport chain components extends lifespan, when they are inhibited during larval stages. Several mitochondrial mutants including ubiquinone synthetase mutant
Loss of autophagic activity has been shown to cause premature aging phenotypes in many species. An unbiased screening for genes involved in chronological lifespan in yeast, identified several short-lived mutants which have mutation in macroautophagy genes (Matecic et al., 2010). Decreased lifespan is also observed in
In the following section, we summarize several pharmacological treatments which have been shown to extend animal lifespan and healthspan through the activation of autophagy (Table 1).
Administration of a natural polyamine, spermidine provides beneficial for health in a number of species and extends lifespan of yeast, worms, flies and mice (Eisenberg et al., 2009; 2016). Survival of cultured mammalian cells is also promoted by treatment with spermidine, and this is accompanied by epigenetic hypoacetylation of histone H3 via inhibition of histone acetyltransferase activity. This, in turn, correlates with transcriptional upregulation of multiple autophagy-related genes, including
Resveratrol is a naturally occurring polyphenolic compound found in grapes and an activator of the NAD-dependent histone deacetylase sirtuin (SIRT1). Administration of resveratrol is known to extend the lifespan of several model organisms (Park et al., 2013). Especially, the lifespan extension in
Urolithin A as a first-in-class natural compound that induces mitophagy both
Tomatidine, a natural compound abundant in unripe tomatoes, inhibits age-related skeletal muscle atrophy in mice. Recent study shows that tomatidine extends lifespan and healthspan in
In many cases, autophagic activation at the transcript level seems essential for longevity. Several autophagy and lysosomal genes are regulated by different transcription factors, microRNA and chromatin modifying enzymes, which are described below.
TFEB originally identified as a master regulator of lysosomal biogenesis is subsequently is shown to regulate autophagy and fat metabolism (Sardiello et al., 2009; Settembre et al., 2011; 2013a). TFEB is known to be negatively regulated by nutrient sensor TOR. At nutrient rich condition, TFEB is phosphorylated on lysosome. Phosphorylated TFEB is bound to 14-3-3 and is mainly localized on cytosol. Upon starvation, TOR becomes inactivated and TFEB is then dephosphorylated and translocated in the nucleus to initiate the transcription of target genes (Settembre et al., 2013b).
Other bHLH transcription factor complex, MML-1/MXL-2 has been identified as a novel regulator of longevity (Nakamura et al., 2016). MML-1/MXL-2 belongs to Myc and Mondo family member and their homologs, MondoA/MLX or ChREBP/MXL functions as a glucose sensor. MML-1/MXL-2 is required for the longevity conferred by germline removal, reduced Insulin/IGF-1 signaling, reduced mitochondrial respiration, reduced TOR signaling in
In
Many microRNA has been shown to regulate animal lifespan. Among them, miR-34 is related to autophagy and aging in some species. In worms, loss of function of miR-34 extends lifespan and this longevity is abolished by RNAi knockdown of several autophagy regulators,
NAD-dependent deacetylase SIRT1 (sirtuin 1) is a particularly well-known modulator of aging. Specifically, the life spans of yeast, worms, and flies can be extended by overexpression and/or pharmacological activation of SIRT1 and the lifespan of mice is extended by ubiquitous overexpression of SIRT6, or brain-specific overexpression of SIRT1 (Giblin et al., 2014; Park et al., 2013). In
As we described above, accumulating evidences show activation of autophagy seems essential for longevity. However, the several fundamental questions remain elusive. How is autophagy regulated during aging? Cells, tissues and timing specific roles of autophagy also need to be considered. Recently and unexpectedly, neuron specific knockdown of autophagy after reproductive period has been shown to extend lifespan in worms (Wilhelm et al., 2017). Thus, it is crucial to understand spatio- and temporal-regulation of autophagy and their physiological relevance to aging. It is essential to determine how autophagy contribute to lifespan extension and which autophagy cargo are relevant for aging and longevity. Clearance of lipids (lipophagy) and mitochondria (mitophagy) are relevance to
. Longevity through activation of autophagy.
Genetic or pharmacological manipulations | Animals | Phenotypes | Epistatic analysis by inhibition of autophagy genes | References |
---|---|---|---|---|
Reduced insulin/IGF-1 signaling | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Meléndez et al., 2003 |
Calorie restriction | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Jia et al., 2007 |
Reduced TOR signaling | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Hansen et al., 2008 |
Reduced mitochondrial respiration | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Toth et al., 2008 |
Germline removal | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Lapierre et al., 2011 |
HLH-30 overexpression | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Lapierre et al., 2013 |
Urolithin A | Worm, mouse | Lifespan extension(worms), improved muscle function(mouse), activation of mitophagy | Cancelation of longevity | Ryu et al., 2016 |
Resveratrol | Worm | Lifespan extension, activation of autophagy | Cancelation of longevity | Morselli et al., 2010 |
Spermidine | Worm, | Lifespan extension, activation of autophagy | Cancelation of longevity | Eisenberg et al., 2009 |
Rapamycin | Lifespan extension, activation of autophagy | Cancelation of longevity | Bjedov et al., 2010 | |
Tomatidine | Worm | Lifespan extension, activation of autophagy | ND | Fang et al., 2017 |
Brain specific Atg8 overexpression | Lifespan extension in female | ND | Simonsen et al., 2008 | |
ATG5 overexpression | Mouse | Lifespan extension, activation of autophagy | ND | Pyo et al., 2013 |
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