The role of macroautophagy in the ageing process, anti-ageing intervention and age-associated diseases

5'-Adenosine monophosphate-activated protein kinase--mammalian target of rapamycin axis as therapeutic target for age-related macular degeneration

Age-related macular degeneration (AMD) is the most common reason for blindness in developed countries. AMD essentially involves chronic oxidative stress, increased accumulation of lipofuscin in retinal pigment epithelial (RPE) cells, and extracellular drusen formation, as well as presence of chronic inflammation in the retina. The capacity to prevent the accumulation of cellular cytotoxic protein aggregates is decreased in senescent cells, which may evoke lipofuscin accumulation into lysosomes in postmitotic RPE cells. The formation of lipofuscin, in turn, decreases the lysosomal enzyme activity and impairs the autophagic clearance of damaged proteins destined for cellular removal. 5'-Adenosine monophosphate-activated protein kinase (AMPK) is a well-known inhibitor of mammalian target of rapamycin (mTOR) that subsequently evokes induction of autophagy. This review examines the novel potential therapeutic targets on the AMPK-mTOR axis and the ways in which autophagy clearance can suppress or prevent RPE degeneration and development of AMD.

Role of insulin and growth hormone/insulin-like growth factor-I signaling in lifespan extension: rodent longevity models for studying aging and calorie restriction

Insulin/insulin-like growth factor-I (IGF-I) pathways are recognized as critical signaling pathways involved in the control of lifespans in lower organisms to mammals. Caloric restriction (CR) reduces plasma concentration of insulin, growth hormone (GH), and IGF-I. CR retards various age-dependent disorders such as nuerodegenerative diseases and extends lifespan in laboratory rodents. These beneficial effects of CR are partly mimicked in spontaneous or genetically engineered rodent models of reduced insulin and GH/IGF-I axis. Most of these long-living rodents show increased insulin sensitivity; however, recent study has revealed that some other rodents show normal or reduced insulin sensitivity. Thus, increased insulin sensitivity might be not prerequisite for lifespan extension in insulin/GH/IGF-I altered longevity rodent models. These results highlighted that, for lifespan extension, the intracellular signaling molecules of insulin/GH/IGF-I pathways might be more important than actual peripheral or systemic insulin action.

The emerging role of autophagy in the pathophysiology of diabetes mellitus

An emerging body of evidence supports a role for autophagy in the pathophysiology of type 1 and type 2 diabetes mellitus. Persistent high concentrations of glucose lead to imbalances in the antioxidant capacity within the cell resulting in oxidative stress-mediated injury in both disorders. An anticipated consequence of impaired autophagy is the accumulation of dysfunctional organelles such as mitochondria within the cell. Mitochondria are the primary site of the production of reactive oxygen species (ROS), and an imbalance in ROS production relative to the cytoprotective action of autophagy may lead to the accumulation of ROS. Impaired mitochondrial function associated with increased ROS levels have been proposed as mechanisms contributing to insulin resistance. In this article we review and interpret the literature that implicates a role for autophagy in the pathophysiology of type 1 and type 2 diabetes mellitus as it applies to β-cell dysfunction, and more broadly to organ systems involved in complications of diabetes including the cardiovascular, renal and nervous systems.

Role of autophagy in β-cell function and mass

Type 2 diabetes (T2D) is characterized by decreased insulin secretion and action. Decreased insulin secretion results from a reduction in pancreatic β-cell mass and/or function. Apoptosis, oxidative stress, mitochondrial dysfunction and endoplasmic reticulum (ER) stress responses including JNK activation have been suggested as mechanisms for the changes of pancreatic β-cells in T2D; however, the underlying causes were not clearly elucidated. Autophagy is an intracellular process that plays crucial roles in cellular homeostasis through degradation and recycling of organelles. We have reported increased apoptosis and decreased proliferation of β-cells with resultant reduction in the β-cell mass in β-cell-specific autophagy-deficient mice. Morphological analysis of β-cells revealed accumulation of ubiquitinated proteins, swollen mitochondria and distended ER. Insulin secretory function ex vivo was also impaired. As a result, β-cell-specific autophagy-deficient mice showed hypoinsulinaemia and hyperglycaemia. These results suggested that autophagy is necessary to maintain the structure, mass and function of pancreatic β-cells. In addition, as autophagy may play a protective role against ER stress and rejuvenates organelle function, impaired autophagy may lead to mitochondrial dysfunction and ER stress, which have been implicated as potential causes of insulin resistance. Therefore, in addition to β-cell homeostasis, dysregulated autophagy may possibly be involved in diverse aspects of the pathogenesis of diabetes.

Autophagy induced by ischemic preconditioning is essential for cardioprotection

Based on growing evidence linking autophagy to preconditioning, we tested the hypothesis that autophagy is necessary for cardioprotection conferred by ischemic preconditioning (IPC). We induced IPC with three cycles of 5 min regional ischemia alternating with 5 min reperfusion and assessed the induction of autophagy in mCherry-LC3 transgenic mice by imaging of fluorescent autophagosomes in cryosections. We found a rapid and significant increase in the number of autophagosomes in the risk zone of the preconditioned hearts. In Langendorff-perfused hearts subjected to an IPC protocol of 3 x 5 min ischemia, we also observed an increase in autophagy within 10 min, as assessed by Western blotting for p62 and cadaverine dye binding. To establish the role of autophagy in IPC cardioprotection, we inhibited autophagy with Tat-ATG5(K130R), a dominant negative mutation of the autophagy protein Atg5. Cardioprotection by IPC was reduced in rat hearts perfused with recombinant Tat-ATG5(K130R). To extend the potential significance of autophagy in cardioprotection, we also assessed three structurally unrelated cardioprotective agents--UTP, diazoxide, and ranolazine--for their ability to induce autophagy in HL-1 cells. We found that all three agents induced autophagy; inhibition of autophagy abolished their protective effect. Taken together, these findings establish autophagy as an end-effector in ischemic and pharmacologic preconditioning.

Clearing the brain's cobwebs: the role of autophagy in neuroprotection

Protein aggregates or inclusion bodies are common hallmarks of age-related neurodegenerative disorders. Why these aggregates form remains unclear. Equally debated is whether they are toxic, protective, or simple by-products. Increasing evidence, however, supports the notion that in general aggregates confer toxicity and disturb neuronal function by hampering axonal transport, synaptic integrity, transcriptional regulation, and mitochondrial function. Thus, neuroscientists in search of effective treatments to slow neural loss during neurodegeneration have long been interested in finding new ways to clear inclusion bodies. Intriguingly, two studies using conditional neuron-specific gene ablations of autophagy regulators in mice revealed that autophagy loss elicits inclusion body formation and a neurodegenerative cascade.Such studies indicate autophagy may be a built-in defense mechanism to clear the nervous system of inclusion bodies.This new finding has implications for our understanding of aging and neurodegeneration and the development of new therapies. First, we discuss the pathways underlying autophagy and its controversial role in cell death and survival regulation.We then discuss the physiological role of autophagy in the aging process of the nervous system. In the final portion of this review, we discuss the therapeutic promise of inducing autophagy and the potential side effects of such treatments.

Vitamin E as a novel enhancer of macroautophagy in rat hepatocytes and H4-II-E cells

Autophagy is an intracellular bulk degradation process induced by nutrient starvation, and contributes to macromolecular turnover and rejuvenation of cellular organelles. We demonstrated that vitamin E was a novel nutritional enhancer of autophagy in freshly isolated rat hepatocytes and rat hepatoma H4-II-E cells. Supplementation of fresh hepatocytes with vitamin E (up to 100 microM) increased proteolysis significantly in the presence or absence of amino acids in a dose-dependent manner. The cytosolic LC3 ratio, a newly established index of autophagic flux, was significantly increased by vitamin E, strongly suggesting that the possible site of action is the LC3 conversion step, an early step in autophagosome formation. A typical antioxidant, alpha-lipoic acid, exerted autophagy suppression, while H(2)O(2) stimulated autophagy. It is conceivable that autophagy was stimulated by oxidative stress and this stimulation was cancelled by cellular antioxidative effects. However, in our studies, vitamin E could have enhanced autophagy over-stimulation by H(2)O(2), rather than suppress it. From these results, using a new cytosolic LC3 ratio, vitamin E increases autophagy by accelerating LC3 conversion through a new signaling pathway, emerging as a novel enhancer of autophagy.

Molecular medicine in ophthalmic care

BACKGROUND

Lipofuscin is the most consistent and phylogenically constant morphologic marker of cellular aging. Autofluorescence of the A2E fluorophore within retinal pigment epithelial (RPE) lipofuscin affords the opportunity for noninvasive evaluation of age- and disease-related pathophysiological changes in the human retina. It is being used in National Eye Institute/Age-Related Eye Disease Study II to evaluate age-related macular degeneration (AMD) geographic atrophy expansion. Experiments show lipofuscin can be reversed in cell culture and animal models in heart, brain, spinal cord, and retinal tissues, using an array of antioxidants and iron chelators.

METHODS

An 80-year-old man with a gastric resection presented with complaints of unremitting night driving difficulty despite treatment with lutein and omega III fatty acids. Notable parafoveal deposition of retinal lipofuscin by 50 degrees fundus auto-fluorescence (580 nm excitation/660 barrier filters) and concurrent abnormalities in non-Snellen measures of visual function-Contrast Sensitivity Function, 6.5 degrees large field tritan threshold, 10 degrees threshold visual fields, and deficits in the National Institutes of Health/National Eye Institute Visual Function Questionnaire (VFQ) 25 subjective night driving/mental health subscale questionnaire were obtained. The patient was placed on an over-the-counter daily oral polyphenolic mixture containing resveratrol and re-evaluated 5 months later.

RESULTS

The data reveal improvements in all measures of visual function, subjective improvement in vision and mental functioning on the VFQ 25, and visible clearing of RPE lipofuscin.

CONCLUSION

To our knowledge, we believe this to be the first reported human clinical case of lipofuscin reversal in the human eye correlated with measured clinical and subjective improvement in visual and mental function after nutraceutical intervention.

Autophagy in health and disease. 1. Regulation and significance of autophagy: an overview

Macroautophagy is a vacuolar degradation pathway that terminates in the lysosomal compartment after formation of a cytoplasmic vacuole or autophagosome that engulfs macromolecules and organelles. The identification of ATG (autophagy-related) genes that are involved in the formation of autophagosomes has greatly increased our knowledge of the molecular basis of macroautophagy, and its roles in cell function, which extend far beyond degradation and quality control of the cytoplasm. Macroautophagy, which plays a major role in tissue homeostasis, is now recognized as contributing to innate and adaptive immune responses. Recently, several mediators of apoptosis have been shown to control macroautophagy. Deciphering the cross talk between macroautophagy and apoptosis probably should help increase understanding of the role of macroautophagy in human disease and is likely to be of therapeutic importance.

Roles of mitophagy and the mitochondrial permeability transition in remodeling of cultured rat hepatocytes

In primary culture, hepatocytes dedifferentiate, and their cytoplasm undergoes remodeling. Here, our aim was to characterize changes of mitochondria during remodeling. Hepatocytes were cultured one to five days in complete serumcontaining Waymouth's medium. In rat hepatocytes loaded with MitoTracker Green (MTG), tetramethylrhodamine methylester (TMRM), and/or LysoTracker Red (LTR), confocal microscopy revealed that mitochondria number and mass decreased by approximately 50% between Day 1 and Day 3 of culture. As mitochondria disappeared, lysosomes/autophagosomes proliferated five-fold. Decreased mitochondrial content correlated with (a) decreased cytochrome c oxidase activity and mitochondrial number observed by electron microscopy and (b) a profound decrease of PGC-1alpha mRNA expression. By contrast, mtDNA content per cell remained constant from the first to the third day of culture, although ethidium bromide (de novo mtDNA synthesis inhibitor) caused mtDNA to decrease by half from the first to the third culture day. As mitochondria disappeared, their MTG label moved into LTR-labeled lysosomes, which was indicative of autophagic degradation. A multiwell fluorescence assay revealed a 2.5-fold increase of autophagy on Day 3 of culture, which was decreased by 3-methyladenine, an inhibitor of autophagy, and also by cyclosporin A and NIM811, both selective inhibitors of the mitochondrial permeability transition (MPT). These findings indicate that mitochondrial autophagy (mitophagy) and the MPT underlie mitochondrial remodeling in cultured hepatocytes.

Skeletal muscle autophagy and apoptosis during aging: effects of calorie restriction and life-long exercise

Sarcopenia, loss of muscle mass and function, is a common feature of aging. Oxidative damage and apoptosis are likely underlying factors. Autophagy, a process for the degradation of cellular constituents, may be a mechanism to combat cell damage and death. We investigated the effect of age on autophagy and apoptosis in plantaris muscle of male Fischer 344 rats that were either fed ad libitum, or mild, life-long calorie restricted (CR) alone or combined with life-long voluntary exercise. Upstream autophagy-regulatory proteins were either upregulated with age (Beclin-1) or unchanged (Atg7 and 9). LC3 gene and protein expression pattern as well as LAMP-2 gene expression, both downstream regulators of autophagy, however, suggested an age-related decline in autophagic degradation. Atg protein expression and LC3 and LAMP-2 gene expression were improved in CR rats with or without exercise. The age-related increase in oxidative damage and apoptosis were attenuated by the treatments. Both, oxidative damage and apoptosis correlated negatively with autophagy. We conclude that mild CR attenuates the age-related impairment of autophagy in rodent skeletal muscle, which might be one of the mechanisms by which CR attenuates age-related cellular damage and cell death in skeletal muscle in vivo.

Cellular mechanisms of cardioprotection by calorie restriction: state of the science and future perspectives

Evidence from animal models and preliminary studies in humans indicates that calorie restriction (CR) delays cardiac aging and can prevent cardiovascular disease. These effects are mediated by a wide spectrum of biochemical and cellular adaptations, including redox homeostasis, mitochondrial function, inflammation, apoptosis, and autophagy. Despite the beneficial effects of CR, its large-scale implementation is challenged by applicability issues as well as health concerns. However, preclinical studies indicate that specific compounds, such as resveratrol, may mimic many of the effects of CR, thus potentially obviating the need for drastic food intake reductions. Results from ongoing clinical trials will reveal whether the intriguing alternative of CR mimetics represents a safe and effective strategy to promote cardiovascular health and delay cardiac aging in humans.

BACE and gamma-secretase characterization and their sorting as therapeutic targets to reduce amyloidogenesis

Secretases are named for enzymes processing amyloid precursor protein (APP), a prototypic type-1 membrane protein. This led directly to discovery of novel Aspartyl proteases (beta-secretases or BACE), a tetramer complex gamma-secretase (gamma-SC) containing presenilins, nicastrin, aph-1 and pen-2, and a new role for metalloprotease(s) of the ADAM family as a alpha-secretases. Recent advances in defining pathways that mediate endosomal-lysosomal-autophagic-exosomal trafficking now provide targets for new drugs to attenuate abnormal production of fibril forming products characteristic of AD. A key to success includes not only characterization of relevant secretases but mechanisms for sorting and transport of key metabolites to abnormal vesicles or sites for assembly of fibrils. New developments we highlight include an important role for an 'early recycling endosome' coated in retromer complex containing lipoprotein receptor LRP-II (SorLA) for switching APP to a non-amyloidogenic pathway for alpha-secretases processing, or to shuttle APP to a 'late endosome compartment' to form Abeta or AICD. LRP11 (SorLA) is of particular importance since it decreases in sporadic AD whose etiology otherwise is unknown.

Preserving organelle vitality: peroxisomal quality control mechanisms in yeast

Cellular proteins and organelles such as peroxisomes are under continuous quality control. Upon synthesis in the cytosol, peroxisomal proteins are kept in an import-competent state by chaperones or specific proteins with an analogous function to prevent degradation by the ubiquitin-proteasome system. During protein translocation into the organelle, the peroxisomal targeting signal receptors (Pex5, Pex20) are also continuously undergoing quality control to enable efficient functioning of the translocon (RADAR pathway). Even upon maturation of peroxisomes, matrix enzymes and peroxisomal membranes remain subjected to quality control. As a result of their oxidative metabolism, peroxisomes are producers of reactive oxygen species (ROS), which may damage proteins and lipids. To counteract ROS-induced damage, yeast peroxisomes contain two important antioxidant enzymes: catalase and an organelle-specific peroxiredoxin. Additionally, a Lon-type protease has recently been identified in the peroxisomal matrix, which is capable of degrading nonfunctional proteins. Finally, cellular housekeeping processes keep track of the functioning of peroxisomes so that dysfunctional organelles can be quickly removed via selective autophagy (pexophagy). This review provides an overview of the major processes involved in quality control of yeast peroxisomes.

Molecular architecture of myelinated peripheral nerves is supported by calorie restriction with aging

Peripheral nerves from aged animals exhibit features of degeneration, including marked fiber loss, morphological irregularities in myelinated axons and notable reduction in the expression of myelin proteins. To investigate how protein homeostatic mechanisms change with age within the peripheral nervous system, we isolated Schwann cells from the sciatic nerves of young and old rats. The responsiveness of cells from aged nerves to stress stimuli is weakened, which in part may account for the observed age-associated alterations in glial and axonal proteins in vivo. Although calorie restriction is known to slow the aging process in the central nervous system, its influence on peripheral nerves has not been investigated in detail. To determine if dietary restriction is beneficial for peripheral nerve health and glial function, we studied sciatic nerves from rats of four distinct ages (8, 18, 29 and 38 months) kept on an ad libitum (AL) or a 40% calorie restricted diet. Age-associated reduction in the expression of the major myelin proteins and widening of the nodes of Ranvier are attenuated by the dietary intervention, which is paralleled with the maintenance of a differentiated Schwann cell phenotype. The improvements in nerve architecture with diet restriction, in part, are underlined by sustained expression of protein chaperones and markers of the autophagy-lysosomal pathway. Together, the in vitro and in vivo results suggest that there might be an age-limit by which dietary intervention needs to be initiated to elicit a beneficial response on peripheral nerve health.

Melatonin alters cell death processes in response to age-related oxidative stress in the brain of senescence-accelerated mice

We studied the effect of age and melatonin on cell death processes in brain aging. Senescence-accelerated prone mice 8 (SAMP8) and senescence-accelerated resistant mice (SAMR1) at 5 and 10 months of age were used as models of the study. Melatonin (10 mg/kg) or its vehicle (ethanol at 0.066%) was administered in the drinking water from 1 to 9 months of age. Neurodegeneration, previously shown in the aged brain of SAMP8 and SAMR1 at 10 months of age, may be due to a drop in age-related proteolytic activities (cathepsin D, calpains, and caspase-3). Likewise, lack of apoptotic and macroautophagic processes were found, without apparent modification by melatonin. However, the caspase-independent cell death, owing to high p53 and apoptosis-inducing factor (AIF) levels, might be an alternative pathway of cell death in the aged brain. The main effects of melatonin treatment were observed in the aged SAMR1 mice; in this strain we observed a marked increase in antioxidant activity (catalase and superoxide dismutase). Likewise, a key antioxidant role of apoptosis-related proteins, Bcl-2 and AIF, was suggested in the aged brain of SAM mice, which was clearly influenced by melatonin. Moreover, the age-related increase of lysosomal activity of cathepsin B and a lysosomal membrane-associated protein 2 supports the possibility of the maintenance of lysosomal viability in addition to age-related impairments of the proteolytic or macroautophagic activities. The effectiveness of melatonin against the oxidative stress-related impairments and apoptosis during the aging process is, once more, corroborated in this article.

Physiological functions of autophagy

The field of autophagy research has advanced rapidly in recent years, with important discoveries made in relation to both molecular mechanisms and physiological functions. Initially, autophagy was thought to be primarily a response to starvation. Although this might be true in lower eukaryotes, this catabolic process exerts various physiological functions in higher eukaryotes. This review summarizes the physiological roles of autophagy in amino acid pool maintenance, intracellular quality control, development, cell death, tumor suppression and anti-aging.

Fish as model systems for the study of vertebrate apoptosis

Apoptosis is a process of pivotal importance for multi-cellular organisms and due to its implication in the development of cancer and degenerative disease it is intensively studied in humans and mammalian model systems. Invertebrate models of apoptosis have been well-studied, especially in C. elegans and D. melanogaster, but as these are evolutionarily distant from mammals the relevance of findings for human research is sometimes limited. Presently, a non-mammalian vertebrate model for studying apoptosis is missing. However, in the past few years an increasing number of studies on cell death in fish have been published and thus new model systems may emerge. This review aims at highlighting the most important of these findings, showing similarities and dissimilarities between fish and mammals, and will suggest topics for future research. In addition, the outstanding usefulness of fish as research models will be pointed out, hoping to spark future research on this exciting, often underrated group of vertebrates.

The cell biology of autophagy in metazoans: a developing story

The cell biological phenomenon of autophagy (or ;self-eating') has attracted increasing attention in recent years. In this review, we first address the cell biological functions of autophagy, and then discuss recent insights into the role of autophagy in animal development, particularly in C. elegans, Drosophila and mouse. Work in these and other model systems has also provided evidence for the involvement of autophagy in disease processes, such as neurodegeneration, tumorigenesis, pathogenic infection and aging. Insights gained from investigating the functions of autophagy in normal development should increase our understanding of its roles in human disease and its potential as a target for therapeutic intervention.

How autophagy is related to programmed cell death during the development of the nervous system

Programmed cell death, together with proliferation and differentiation, is an essential process during the development of the nervous system. During neurogenesis, neurons and glia are generated in large numbers and, subsequently, they die in a process that depends on trophic signalling that refines the cytoarchitecture and connectivity of the nervous system. In addition, programmed cell death also affects proliferating neuroepithelial cells and recently differentiated neuroblasts. Autophagy is a lysosomal degradative pathway that allows the recycling of cell constituents, and seems to be able to play a dual role. It may serve to protect the cell by preventing the accumulation of deleterious products and organelles and supplying energy and amino acids. On the other hand, it has been considered a type of cell death. The role of autophagy during development is little characterized. The retina provides an excellent model system to study autophagy in the context of neural development, and to establish its relationship with proliferation, differentiation and cell death. In the present review, we summarize recent findings showing that autophagy contributes to the development of the nervous system by providing energy for cell corpse removal after physiological cell death, a process associated with retinal neurogenesis.

Sex-specific hippocampus-dependent cognitive deficits and increased neuronal autophagy in DEspR haploinsufficiency in mice

Aside from abnormal angiogenesis, dual endothelin-1/VEGF signal peptide-activated receptor deficiency (DEspR(-/-)) results in aberrant neuroepithelium and neural tube differentiation, thus elucidating DEspR's role in neurogenesis. With the emerging importance of neurogenesis in adulthood, we tested the hypothesis that nonembryonic-lethal DEspR haploinsufficiency (DEspR(+/-)) perturbs neuronal homeostasis, thereby facilitating aging-associated neurodegeneration. Here we show that, in male mice only, DEspR-haploinsufficiency impaired hippocampus-dependent visuospatial and associative learning and induced noninflammatory spongiform changes, neuronal vacuolation, and loss in the hippocampus, cerebral cortex, and subcortical regions, consistent with autophagic cell death. In contrast, DEspR(+/-) females exhibited better cognitive performance than wild-type females and showed absence of neuropathological changes. Signaling pathway analysis revealed DEspR-mediated phosphorylation of activators of autophagy inhibitor mammalian target of rapamycin (mTOR) and dephosphorylation of known autophagy inducers. Altogether, the data demonstrate DEspR-mediated diametrical, sex-specific modulation of cognitive performance and autophagy, highlight cerebral neuronal vulnerability to autophagic dysregulation, and causally link DEspR haploinsufficiency with increased neuronal autophagy, spongiosis, and cognitive decline in mice.

Autophagic dysfunction in mucolipidosis type IV patients

Mutations in Mucolipin 1 (MCOLN1) have been linked to mucolipidosis type IV (MLIV), a lysosomal storage disease characterized by several neurological and ophthalmological abnormalities. It has been proposed that MCOLN1 might regulate transport of membrane components in the late endosomal-lysosomal pathway; however, the mechanisms by which defects of MCOLN1 function result in mental and psychomotor retardation remain largely unknown. In this study, we show constitutive activation of autophagy in fibroblasts obtained from MLIV patients. Accumulation of autophagosomes in MLIV cells was due to the increased de novo autophagosome formation and to delayed fusion of autophagosomes with late endosomes/lysosomes. Impairment of the autophagic pathway led to increased levels and aggregation of p62, suggesting that abnormal accumulation of ubiquitin proteins may contribute to the neurodegeneration observed in MLIV patients. In addition, we found that delivery of platelet-derived growth factor receptor to lysosomes is delayed in MCOLN1-deficient cells, suggesting that MCOLN1 is necessary for efficient fusion of both autophagosomes and late endosomes with lysosomes. Our data are in agreement with recent evidence showing that autophagic defects may be a common characteristic of many neurodegenerative disorders.

Mucolipidosis type IV: the importance of functional lysosomes for efficient autophagy

Mucolipidosis IV (MLIV) is a lysosomal storage disorder characterized by severe neurological and ophthalmologic abnormalities. In contrast with most lysosomal storage disorders, which are attributed to the absence of specific lysosomal hydrolases, accumulation of material in MLIV results from defects in membrane transport along the late endocytic pathway. Mutations in MCOLN1 are the cause of MLIV; however, how the lack of MCOLN1 function ultimately leads to neurodegeneration remains largely unknown. We found that MCOLN1 is required for efficient fusion of both late endosomes and autophagosomes with lysosomes. Impaired autophagosome degradation results in accumulation of autophagosomes in MLIV fibroblasts. In addition, we found increased levels and aggregation of p62, suggesting that abnormal accumulation of ubiquitinated protein inclusions may contribute to the neurodegenerative phenotype observed in MLIV patients. These findings corroborate recent evidence indicating that defects in autophagy may be a common feature of many neurodegenerative disorders.

Nutrition and cancer: essential elements for a roadmap

Personalizing nutrition for cancer prevention and therapy will require a comprehensive understanding of "genotypes/phenotypes" in order to identify, evaluate, and prioritize appropriate points for dietary intervention. This nutritional preemption roadmap must begin with accurately assessing intakes/exposures of which bioactive food component(s) is needed to bring about a desired response in critical cellular processes (carcinogen metabolism, DNA repair, cell proliferation, apoptosis, inflammation, immunity, differentiation, angiogenesis, hormonal regulation and cellular energetic) within an individual. Understanding this "individuality" through a better understanding of the "omics" is fundamental to arriving at the correct destination and thus interpreting biological variables which establish the magnitude or direction of a response to bioactive food components.

The involvement of lysosomes in myocardial aging and disease

The myocardium is mainly composed of long-lived postmitotic cells with, if there is any at all, a very low rate of replacement through the division and differentiation of stem cells. As a consequence, cardiac myocytes gradually undergo pronounced age-related alterations which, furthermore, occur at a rate that inversely correlates with the longevity of species. Basically, these alterations represent the accumulation of structures that have been damaged by oxidation and that are useless and often harmful. These structures (so-called 'waste' materials), include defective mitochondria, aberrant cytosolic proteins, often in aggregated form, and lipofuscin, which is an intralysosomal undegradable polymeric substance. The accumulation of 'waste' reflects the insufficient capacity for autophagy of the lysosomal compartment, as well as the less than perfect functioning of proteasomes, calpains and other cellular digestive systems. Senescent mitochondria are usually enlarged, show reduced potential over their inner membrane, are deficient in ATP production, and often produce increased amounts of reactive oxygen species. The turnover of damaged cellular structures is hindered by an increased lipofuscin loading of the lysosomal compartment. This particularly restricts the autophagic turnover of enlarged, defective mitochondria, by diverting the flow of lysosomal hydrolases from autophagic vacuoles to lipofuscin-loaded lysosomes where the enzymes are lost, since lipofuscin is not degradable by lysosomal hydrolases. As a consequence, aged lipofuscin-rich cardiac myocytes become overloaded with damaged mitochondria, leading to increased oxidative stress, apoptotic cell death, and the gradual development of heart failure. Defective lysosomal function also underlies myocardial degeneration in various lysosomal storage diseases, while other forms of cardiomyopathies develop due to mitochondrial DNA mutations, resulting in an accumulation of abnormal mitochondria that are not properly eliminated by autophagy. The degradation of iron-saturated ferritin in lysosomes mediates myocardial injury in hemochromatosis, an acquired or hereditary disease associated with iron overload. Lysosomes then become sensitized to oxidative stress by the overload of low mass, redox-active iron that accumulates when iron-saturated ferritin is degraded following autophagy. Lysosomal destabilization is of importance in the induction and/or execution of programmed cell death (either classical apoptotic or autophagic), which is a common manifestation of myocardial aging and a variety of cardiac pathologies.

Autophagy in neuroprotection and neurodegeneration: A question of balance

A central issue in developing therapies for neurodegenerative diseases involves understanding why adaptive responses to stress or injury fail to prevent synaptic dysfunction and neuronal cell death. Macroautophagy is a major, evolutionarily conserved response to nutrient and bioenergetic stresses, which has the capacity to remove aggregated proteins and damaged organelles such as mitochondria. This has prompted intense interest in autophagy-related therapies for Huntington's, Alzheimer's, Parkinson's, stroke and other neurological diseases. However, excessive or imbalanced induction of autophagic recycling can actively contribute to neuronal atrophy, neurite degeneration and cell death. Oxidative-, aging- and disease-related increase in demand for autophagy, coupled with declining axonal trafficking, lysosomal degradation or biosynthetic efficiencies promote increased susceptibility to a harmful state of autophagic stress. A more complete understanding of dysfunction along the entire spectrum of autophagic recycling, from autophagosome formation through clearance and regeneration of new cellular components is necessary to restore balance to the system, promote neuronal health and maximize therapeutic potentials.

Activation of innate immunity system during aging: NF-kB signaling is the molecular culprit of inflamm-aging

Innate and adaptive immunity are the major defence mechanisms of higher organisms against inherent and environmental threats. Innate immunity is present already in unicellular organisms but evolution has added novel adaptive immune mechanisms to the defence armament. Interestingly, during aging, adaptive immunity significantly declines, a phenomenon called immunosenescence, whereas innate immunity seems to be activated which induces a characteristic pro-inflammatory profile. This process is called inflamm-aging. The recognition and signaling mechanisms involved in innate immunity have been conserved during evolution. The master regulator of the innate immunity is the NF-kB system, an ancient signaling pathway found in both insects and vertebrates. The NF-kB system is in the nodal point linking together the pathogenic assault signals and cellular danger signals and then organizing the cellular resistance. Recent studies have revealed that SIRT1 (Sir2 homolog) and FoxO (DAF-16), the key regulators of aging in budding yeast and Caenorhabditis elegans models, regulate the efficiency of NF-kB signaling and the level of inflammatory responses. We will review the role of innate immunity signaling in the aging process and examine the function of NF-kB system in the organization of defence mechanisms and in addition, its interactions with the protein products of several gerontogenes. Our conclusion is that NF-kB signaling seems to be the culprit of inflamm-aging, since this signaling system integrates the intracellular regulation of immune responses in both aging and age-related diseases.

Manipulation of caloric content but not diet composition, attenuates the deficit in learning and memory of senescence-accelerated mouse strain P8

Calorie restriction (CR) is an experimental intervention in laboratory animals that attenuates age-associated increases in morbidity, mortality, and functional impairment. It is characterized by mild ketosis, hypoinsulinemia and hypoglycemia. In this study, we examined whether metabolic simulation of CR by a diet of isocaloric ketogenic or hypoinsulinemic diets ameliorated the learning and memory deficit in a strain of senescence-accelerated prone mice (SAMP8), a mouse model of age-dependent impairments in learning and memory. Male SAMP8 mice were fed high carbohydrate (CHO), high fat (FAT), or high protein (PRO) diets after weaning, and calorie intake was adjusted to 95% (sub ad libitum, sAL) or 70% (CR) of the mean calorie intake of control mice. At 28 weeks of age, we found CR ameliorated the performance defects of SAMP8 mice in a passive avoidance task. Neither FAT nor PRO diets affected performance of the task when fed sAL level, although a diet of these compositions partially mimicked the serum parameters of CR mice. These results suggest restriction of calorie intake is important for the prevention of learning and memory deficits, and that the simulation of serum changes induced by CR is not sufficient to prevent the cognitive defects of SAMP8 mice.

Neurosecretases provide strategies to treat sporadic and familial Alzheimer disorders

Recent discoveries on neurosecretases and their trafficking to release fibril-forming neuropeptides or other products, are of interest to pathology, cell signaling and drug discovery. Nomenclature arose from the use of amyloid precursor protein (APP) as a prototypic type-1 substrate leading to the isolation of beta-secretase (BACE), multimeric complexes (gamma-secretase, gamma-SC) for intramembranal cleavage, and attributing a new function to well-characterized metalloproteases of the ADAM family (alpha-secretase) for normal APP turnover. While purified alpha/beta-secretases facilitate drug discovery, gamma-SC presents greater challenges for characterization and mechanisms of catalysis. The review comments on links between mutation or polymorphisms in relation to enzyme mechanisms and disease. The association between lipoprotein receptor LRP11 variants and sporadic Alzheimer's disease (SAD) offers scope to integrate components of pre- and post-Golgi membranes, or brain clathrin-coated vesicles within pathways for trafficking as targets for intervention. The presence of APP and metabolites in brain clathrin-coated vesicles as significant cargo with lipoproteins and adaptors focuses attention as targets for therapeutic intervention. This overview emphasizes the importance to develop new therapies targeting neurosecretases to treat a major neurological disorder that has vast economic and social implications.

Autophagy in the heart and liver during normal aging and calorie restriction

Autophagy is a highly regulated intracellular process for the degradation of cellular constituents and essential for the maintenance of a healthy cell. We evaluated the effects of age and life-long calorie restriction on autophagy in heart and liver of young (6 months) and old (26 months) Fisher 344 rats. We observed that the occurrence of autophagic vacuoles was higher in heart than liver. The occurrence of autophagic vacuoles was not affected by age in either tissue, but was increased with calorie restriction in heart but not in liver. Next, we examined the expression of proteins involved in the formation and maturation of autophagosomes (beclin-1, LC3, Atg7, Atg9) or associated with autolysosomes and lysosomes (LAMP-1; cathepsin D). In hearts of both ad libitum-fed and calorie-restricted rats, we observed an increase in expression of beclin-1 and procathepsin D, but not mature cathepsin D, and a decrease in expression of LAMP-1 because of aging. In hearts, calorie restriction stimulated the expression of Atg7 and Atg9 and the lipidation of Atg8 (elevated LC3-II/I ratios) in aged rats. In hearts of ad libitum-fed rats, expression of Atg7 and lipidation of Atg8 were unaffected by age, while the cellular levels of Atg9 were lower in aged animals. Furthermore, we observed that the age- and diet-dependent expression levels of those proteins differed between heart and liver. In conclusion, autophagy in heart and liver did not decrease with age in ad libitum-fed rats, but was enhanced by calorie restriction in the heart. Thus, calorie restriction may mediate some of its beneficial effects by stimulating autophagy in the heart, indicating the potential for cardioprotective therapies.

Low-level caloric restriction rescues proteasome activity and Hsc70 level in liver of aged rats

Proteasome activity is known to decrease with aging in ad libitum (AL) fed rats. Severe caloric restriction (CR) significantly extends the maximum life-span of rats, and counteracts the age-associated decrease in liver proteasome activities. Since few investigations have explored whether lower CR diets might positively counteract the age associated decrease in proteasome activity, we then investigated the effects of a mild CR regimen on animal life-span, proteasome content and function. In addition, we addressed the question whether both CR regimens might also affect the expression of Hsc70 protein, a constitutive chaperone reported to share a role in the function of proteasome complex and in the repair of proteotoxic damage, and whose level decreased during aging. In contrast to severe CR, mild CR had a poor effect on life-span; however, it better counteracted the decrease of proteasome activities. Both regimens, however, maintain Hsc70 in liver of old rats at level comparable to that of young rats. Interestingly, the effects of aging and CRs on liver proteasome enzyme activities did not appear to be associated with parallel changes in the amount of proteasome proteins suggesting that the quality (molecular activity of the enzymes) rather than the quantity are likely to be modified with age. In conclusion, the results presented in this work show that a mild CR can have beneficial effects on liver function of aging rats because is adequate to counteract the decrease of proteasome function and Hsc70 chaperone level.

Cloning and characterization of an autophagy-related gene, ATG12, from the three-host tick Haemaphysalis longicornis

Ticks are obligate hematophagous ectoparasites with a life cycle characterized by a period of starvation; many ticks spend more than 95% of their life off the host. Autophagy, which is the process of bulk cytoplasmic degradation in eukaryotic cells, is induced by starvation and is essential for extension of the lifespan. Therefore, we hypothesized that autophagy also occurs in ticks; however, there has been no report on autophagy-related (ATG) genes in ticks. Here, we show the homologue of an ATG gene, ATG12, and its expression pattern from the nymphal to adult stages in the three-host tick Haemaphysalis longicornis. The sequence analysis showed that H. longicornis ATG12 (HlATG12) cDNA is 649bp, has a 411bp ORF coding for a 136-amino acid polypeptide with the carboxy-terminal glycine residue, and has a predicted molecular mass of 15.2kDa. Moreover, RT-PCR revealed that HlATG12 was downregulated at the beginning of feeding, upregulated after engorgement, and downregulated again after molting. The expression level of HlATG12 was highest at 3 months after engorgement. By immuno-electron microscopy, it was demonstrated that HlAtg12 was localized to the region around granule-like structures within midgut cells of unfed adults. In conclusion, HlATG12 might function during unfed and molting stages.

The role of autophagy in mediating cell survival and death during ischemia and reperfusion in the heart

Autophagy is a major mechanism for degrading long-lived cytosolic proteins and the only known pathway for degrading organelles. Autophagy is activated by many forms of stress, including nutrient and energy starvation, oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress, and infections. Although autophagy recycles amino acids and fatty acids to produce energy and removes damaged organelles, thereby playing an essential role in cell survival, inappropriate activation of autophagy leads to cell death. In the heart, activation of autophagy can be observed in response to nutrient starvation, ischemia/reperfusion, and heart failure. In this review, the signaling mechanism and the functional significance of autophagy during myocardial ischemia and reperfusion are discussed.

Autophagy mitigates metabolic stress and genome damage in mammary tumorigenesis

Autophagy is a catabolic process involving self-digestion of cellular organelles during starvation as a means of cell survival; however, if it proceeds to completion, autophagy can lead to cell death. Autophagy is also a haploinsufficient tumor suppressor mechanism for mammary tumorigenesis, as the essential autophagy regulator beclin1 is monoallelically deleted in breast carcinomas. However, the mechanism by which autophagy suppresses breast cancer remains elusive. Here we show that allelic loss of beclin1 and defective autophagy sensitized mammary epithelial cells to metabolic stress and accelerated lumen formation in mammary acini. Autophagy defects also activated the DNA damage response in vitro and in mammary tumors in vivo, promoted gene amplification, and synergized with defective apoptosis to promote mammary tumorigenesis. Therefore, we propose that autophagy limits metabolic stress to protect the genome, and that defective autophagy increases DNA damage and genomic instability that ultimately facilitate breast cancer progression.

Autophagy and neurodegeneration: when the cleaning crew goes on strike

Intracellular accumulation of altered and misfolded proteins is the basis of most neurodegenerative disorders. Altered proteins are usually organised in the form of toxic multimeric complexes that eventually promote neuronal death. Cells rely on surveillance mechanisms that take care of the removal of these toxic products. What then goes wrong in these pathologies? Recent studies have shown that a primary failure in autophagy, a mechanism for clearance of intracellular components in lysosomes, could be responsible for the accumulation of these altered proteins inside the affected neurons. In this Review we summarise our current knowledge on the contribution of autophagy to the maintenance of normal cellular homoeostasis, its changes in neurodegenerative disorders, and the role of aggravating factors such as oxidative stress and ageing on autophagic failure in these pathologies.

Regulation of autophagy by extracellular signal-regulated protein kinases during 1-methyl-4-phenylpyridinium-induced cell death

Increased autophagic vacuoles (AVs) occur in injured or degenerating neurons, under both developmental and pathological situations. Although regulation of starvation-induced autophagy has been extensively studied, less is known about autophagic responses to pathological damage. The neurotoxin 1-methyl-4-phenylpyridinium (MPP(+)) produces mitochondria-targeted injury, which contributes to parkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydro-pyridine in mammals. Here, we demonstrate that MPP(+) elicited increased autophagy in SH-SY5Y cells, as assessed by electron microscopy, immunofluorescence for the autophagy protein LC3/Atg8, LC3 electrophoretic mobility shift, mitochondrial degradation, and monodansylcadaverine staining for late AVs/autolysosomes. During nutrient deprivation, class III phosphatidylinositol-3 kinase (PI3K) stimulates autophagy in concert with the autophagy-regulatory protein beclin 1/Atg6. Although PI3K inhibitors and RNA interference knockdown of beclin 1 effectively inhibited autophagy elicited by amino acid deprivation, neither reduced MPP+-induced autophagic stress. In contrast, inhibition of mitogen-activated protein kinase/extracellular signal-regulated protein kinase kinase reduced AV content, mitochondrial degradation, and cell death in MPP+-treated cells. RNA interference studies targeting core Atg proteins also reduced AV content and cell death. Likewise, in primary midbrain dopaminergic neurons, MPP+ elicited increased AV content, which was reversed by inhibition of mitogen-activated protein kinase/extracellular signal-regulated protein kinase kinase but not PI3K. These results implicate a role for extracellular signal-regulated protein kinase (ERK) signaling upstream of MPP+-elicited autophagic stress. Moreover, pathological stimulation of beclin 1-independent autophagy is associated with neuronal cell death.

Autophagy: A cell repair mechanism that retards ageing and age-associated diseases and can be intensified pharmacologically

The process of ageing denotes a post-maturational deterioration of cells and organisms with the passage of time, an increased vulnerability to challenges and prevalence of age-associated diseases, and a decreased ability to survive. Causes may be found in an enhanced production of reactive oxygen species (ROS) and oxidative damage and not completed housekeeping, with an accumulation of altered ROS-hypergenerating organelles in older cells. It has been shown that autophagy is the only tier of defence against the accumulation of effete mitochondria and peroxisomes; that functioning of autophagy declines with increasing age and determinates cell and individual lifespan; that autophagy can be intensified by drugs; and that the pharmacological intensification of autophagy may be a big step towards retardation of ageing and prevention and therapy of age-associated diseases including neurodegeneration.

Nutrient control of macroautophagy in mammalian cells

A growing number of evidences indicate a strict causality between the reduction of autophagic functionality and aging. In this context the preservation of a proper autophagic response is of paramount importance to preserve the cellular processes in aging cell. Nutrients availability, especially for amino acids, is the most physiological key regulator of macroautophagy. In mammalian cells the knowledge of the mechanism and the underlying regulation of macroautophagy has been greatly improved in recent years and we focus on the role of nutrients, in particular on their involvement in preventing cellular aging through the modulation of autophagy. This review covers the main features of macroautophagy regulation by nutrients, in particular amino acids as well as glucose and vitamins, and its mechanisms, focusing primarily on the mammalian hepatocyte, which has been extensively utilized to dissect signaling pathways underlying the regulation of macroautophagy.

The involvement of macroautophagy in aging and anti-aging interventions

Macroautophagy is a process that sequesters and degrades organelles and macromolecular constituents of cytoplasm for cellular restructuring and repair, and as a source of nutrients for metabolic use in early starvation. Extensive evidence has been reported that macroautophagy process declines with increasing age. This impairment, probably due to ad libitum feeding, may cause accumulation of altered structures leading to the age-related decline in cell functions. It has been suggested that caloric restriction (CR) and disruption of insulin-like signals contrast the process of aging by prolonged stimulation of macroautophagy. According to this hypothesis, it is shown that life-long weekly administration of an anti-lipolytic drug decreases glucose and insulin levels, stimulates autophagy and intensifies anti-aging effects of submaximal CR.

Chaperone-mediated autophagy in aging and neurodegeneration: lessons from alpha-synuclein

Different conditions, ranging from genetic mutation to post-translational modification, result in the intracellular presence of misfolded or conformationally altered proteins. These abnormal proteins tend to organize in toxic oligomeric structures often resulting in cellular death. Alterations in the function of the surveillance systems that normally repair or remove abnormal proteins are the basis of many neurodegenerative disorders. In this review, we focus on such protein conformational disorders and on the role that altered function of intracellular proteolytic systems, in particular autophagy, plays in the evolution of these diseases. Using Parkinson disease as a main example, we recapitulate the different stages of this protein conformational disorder at the cellular level and relate them with changes in the different types of autophagy. Finally, we also comment on the effect that aggravating conditions, such as oxidative stress and aging, have on the functioning of the autophagic system and its ability to cope with altered proteins.

Targets for antiageing drugs

The process of ageing denotes a postmaturational deterioration of cells and organisms with the passage of time, an increased vulnerability to challenges and a decreased ability to survive. Causes may be found in oxidative damage and incomplete cell maintenance, and both of these factors may be considered the primary targets of any antiageing therapy. Levels of lipophylic anti-oxidant defences may be the limiting factor of longevity in ad-libitum fed animals. An appropriate administration of lipophylic antioxidants (e.g., polyunsaturated fatty acids, vitamin E) and the stimulation of cytomembrane and organelle renewal, by calorie restriction and pharmacological intensification of autophagy, are currently available interventions that may extend longevity and retard the onset, as well as aid the treatment, of most age-associated diseases.

Autophagic stress in neuronal injury and disease

Autophagy is the regulated process by which cytoplasmic organelles and long-lived proteins are delivered for lysosomal degradation. Increased numbers of autophagosomes and autolysosomes often represent prominent ultrastructural features of degenerating or dying neurons. This morphology is characteristic not only of neurons undergoing pathologic degeneration, but also during developmental programmed cell death of some neuronal populations. In recent years, a growing number of reports highlight potentially important roles for autophagy-related processes in relation to protein aggregation, regulated cell death pathways, and neurodegeneration. While starvation-induced autophagy involves nonselective bulk degradation of cytoplasm, mechanisms that regulate selective targeting of damaged organelles form an emerging area. As the study of autophagy evolves from physiologic homeostasis to pathologic situations, consideration of terminology and definitions becomes important. Increased autophagic vacuoles do not necessarily correlate with increased autophagic activity or flux. Instead, the striking accumulation of autophagic vacuoles in dying or degenerating neurons likely reflects an imbalance between the rates of autophagic sequestration and completion of the degradative process. In other words, these cells can be thought of as undergoing "autophagic stress." The concept of autophagic stress may reconcile apparently conflicting roles of autophagy-related processes in adaptive, homeostatic responses and in pathways of neurodegeneration and cell death.

Protein degradation and aging

Continuous turnover of intracellular proteins is essential for the maintenance of cellular homeostasis and for the regulation of multiple cellular functions. The first reports showing a decrease in total rates of protein degradation with age are dated more than 50 years ago, when the major players in protein degradation where still to be discovered. The current advances in the molecular characterization of the two main intracellular proteolytic systems, the lysosomal and the ubiquitin proteasome system, offer now the possibility of a systematic search for the defect(s) that lead to the declined activity of these systems in old organisms. We discuss here, in light of the current findings, how malfunctioning of these two proteolytic systems can contribute to different aspects of the phenotype of aging and to the pathogenesis of some age-related diseases.

Calorie restriction mimetics: an emerging research field

When considering all possible aging interventions evaluated to date, it is clear that calorie restriction (CR) remains the most robust. Studies in numerous species have demonstrated that reduction of calories 30-50% below ad libitum levels of a nutritious diet can increase lifespan, reduce the incidence and delay the onset of age-related diseases, improve stress resistance, and decelerate functional decline. A current major focus of this research area is whether this nutritional intervention is relevant to human aging. Evidence emerging from studies in rhesus monkeys suggests that their response to CR parallels that observed in rodents. To assess CR effects in humans, clinical trials have been initiated. However, even if results from these studies could eventually substantiate CR as an effective pro-longevity strategy for humans, the utility of this intervention would be hampered because of the degree and length of restriction required. As an alternative strategy, new research has focused on the development of 'CR mimetics'. The objective of this strategy is to identify compounds that mimic CR effects by targeting metabolic and stress response pathways affected by CR, but without actually restricting caloric intake. For example, drugs that inhibit glycolysis (2-deoxyglucose), enhance insulin action (metformin), or affect stress signaling pathways (resveratrol), are being assessed as CR mimetics (CRM). Promising results have emerged from initial studies regarding physiological responses which resemble those observed in CR (e.g. reduced body temperature and plasma insulin) as well as protection against neurotoxicity (e.g. enhanced dopamine action and up-regulated neurotrophic factors). Ultimately, lifespan analyses in addition to expanded toxicity studies must be accomplished to fully assess the potential of any CRM. Nonetheless, this strategy clearly offers a very promising and expanding research endeavor.

The pleiotropic role of autophagy: from protein metabolism to bactericide

Autophagy is in principle a nonselective, bulk degradation system within cells, with a contribution to intracellular protein degradation estimated to be as large as that of the ubiquitin--proteasome system. The primary roles of autophagy are baseline turnover of intracellular proteins and organelles, production of amino acids in nutrient emergency, and regression of retired tissues. These functions guarantee rejuvenation and adaptation to adverse conditions, and even underlie dynamic processes such as development/metamorphosis. In addition, several other roles for autophagy have recently been discovered, such as presentation of endogenous antigens and degradation of invasive bacteria. This review will discuss the biological significance of autophagy from yeast to higher eukaryotes.

Autophagy and signaling: their role in cell survival and cell death

Macroautophagy is a vacuolar, self-digesting mechanism responsible for the removal of long-lived proteins and damaged organelles by the lysosome. The discovery of the ATG genes has provided key information about the formation of the autophagosome, and about the role of macroautophagy in allowing cells to survive during nutrient depletion and/or in the absence of growth factors. Two connected signaling pathways encompassing class-I phosphatidylinositol 3-kinase and (mammalian) target of rapamycin play a central role in controlling macroautophagy in response to starvation. However, a considerable body of literature reports that macroautophagy is also a cell death mechanism that can occur either in the absence of detectable signs of apoptosis (via autophagic cell death) or concomitantly with apoptosis. Macroautophagy is activated by signaling pathways that also control apoptosis. The aim of this review is to discuss the signaling pathways that control macroautophagy during cell survival and cell death.

Molecular mechanism and regulation of autophagy

Autophagy is a major cellular pathway for the degradation of long-lived proteins and cytoplasmic organelles in eukaryotic cells. A large number of intracellular/extracellular stimuli, including amino acid starvation and invasion of microorganisms, are able to induce the autophagic response in cells. The discovery of the ATG genes in yeast has greatly advanced our understanding of the molecular mechanisms participating in autophagy and the genes involved in regulating the autophagic pathway. Many yeast genes have mammalian homologs, suggesting that the basic machinery for autophagy has been evolutionarily conserved along the eukaryotic phylum. The regulation of autophagy is a very complex process. Many signaling pathways, including target of rapamycin (TOR) or mammalian target of rapamycin (mTOR), phosphatidylinositol 3-kinase-I (PI3K-I)/PKB, GTPases, calcium and protein synthesis all play important roles in regulating autophagy. The molecular mechanisms and regulation of autophagy are discussed in this review.

Autophagy

Autophagy is a major intracellular pathway for the degradation and recycling of long-lived proteins and cytoplasmic organelles. Like apoptotic programmed cell death, autophagy is an essential part of growth regulation and maintenance of homeostasis in multicellular organisms. Autophagic vacuole formation is also activated as an adaptive response to a variety of extracellular and intracellular stimuli, including nutrient deprivation, hormonal or therapeutic treatment, bacterial infection, aggregated and misfolded proteins and damaged organelles. Mediators of class I and class III PI3 kinase signaling pathways and trimeric G proteins play major roles in regulating autophagosome formation during the stress response. Defective autophagy is the underlying cause of a number of pathological conditions, including vacuolar myopathies, neurodegenerative diseases, liver disease, and some forms of cancer. This chapter provides an overview of the morphology and molecular basis of autophagosome formation and offers a glimpse into the role of autophagy in normal growth and development, while discussing the pathological implications of its deregulation.