Ceroid/lipofuscin-loaded human fibroblasts show decreased survival time and diminished autophagocytosis during amino acid starvation

Toward Systemic Lipofuscin Removal

Lipofuscin is indigestible garbage that accumulates in the autophagic vesicles and cytosol of postmitotic cells with age. Drs. Brunk and Terman postulated that lipofuscin accumulation is the main or at least a major driving factor in aging. They even posited that the evolution of memory is the reason why we get lipofuscin at all, as stable synaptic connections must be maintained over time, meaning that the somas of neurons must also remain in the same locale. In other words, they cannot dilute out their garbage over time through cell division. Mechanistically, their position certainly makes sense given that rendering a large percentage of a postmitotic cell's lysosomes useless must almost certainly negatively affect that cell and the surrounding microenvironment. It may be the case that lipofuscin accumulation is the main issue with regard to current age-related disease. Degradation in situ may be an insurmountable task currently. However, a method of systemic lipofuscin removal is discussed herein.

Promoting TFEB nuclear localization with curcumin analog C1 attenuates sensory hair cell injury and delays age-related hearing loss in C57BL/6 mice

Sensory hair cell (HC) injuries, especially outer hair cell (OHC) loss, are well-documented to be the primary pathology of age-related hearing loss (AHL). Recent studies have demonstrated that autophagy plays an important role in HC injury in the inner ear. In our previous works, a decline in autophagy levels and HC loss were found to occur simultaneously in the inner ears of aged C57BL/6 mice, and the administration of rapamycin promoted autophagy levels, which reduced OHC loss and delayed AHL, but the underlying mechanism of autophagy in AHL has not been well elucidated. Transcription factor EB (TFEB), an autophagy regulator and the downstream target of mammalian target of rapamycin (mTOR), is involved in the pathological development of neurodegenerative disease. This study would address the link between autophagy and TFEB in aged C57BL/6 mouse cochleae and clarify the effect of the TFEB activator curcumin analog C1 (C1) in aged cochleae. Decreased TFEB nuclear localization (p = 0.0371) and autophagy dysfunction (p = 0.0273) were observed in the cochleae of aged C57BL/6 mice that exhibited AHL, HCs loss and HCs senescence. Treatment with C1 promoted TFEB nuclear localization and restored autophagy, subsequently alleviating HC injury and delaying AHL. The protective effect of C1 on HEI-OC1 cells against autophagy disorder and aging induced by D-galactose was abolished by chloroquine, which is one of the commonly used autophagy inhibitors. Overall, our results demonstrated that the capacity to perform autophagy is mediated by the nuclear localization of TFEB in aged C57BL/6 mouse cochleae. C1 promotes the nuclear localization of TFEB, subsequently alleviating HC injury and delaying AHL by restoring the impaired autophagy function. TFEB may serve as a new therapeutic target for AHL treatment.

The role of lysosomal cathepsins in neurodegeneration: Mechanistic insights, diagnostic potential and therapeutic approaches

Lysosomes are ubiquitous organelles with a fundamental role in maintaining cellular homeostasis by mediating degradation and recycling processes. Cathepsins are the most abundant lysosomal hydrolyses and are responsible for the bulk degradation of various substrates. A correct autophagic function is essential for neuronal survival, as most neurons are post-mitotic and thus susceptible to accumulate cellular components. Increasing evidence suggests a crucial role of the lysosome in neurodegeneration as a key regulator of aggregation-prone and disease-associated proteins, such as α-synuclein, β-amyloid and huntingtin. Particularly, alterations in lysosomal cathepsins CTSD, CTSB and CTSL can contribute to the pathogenesis of neurodegenerative diseases as seen for neuronal ceroid lipofuscinosis, synucleinopathies (Parkinson's disease, Dementia with Lewy Body and Multiple System Atrophy) as well as Alzheimer's and Huntington's disease. In this review, we provide an overview of recent evidence implicating CTSD, CTSB and CTSL in neurodegeneration, with a special focus on the role of these enzymes in α-synuclein metabolism. In addition, we summarize the potential role of lysosomal cathepsins as clinical biomarkers in neurodegenerative diseases and discuss potential therapeutic approaches by targeting lysosomal function.

Redox changes in obesity, metabolic syndrome, and diabetes

"Life is an instantaneous encounter of circulating matter and flowing energy" (Jean Giaja, Serbian physiologist), is one of the most elegant definitions not only of life but the relationship of redox biology and metabolism. Their evolutionary liaison has created inseparable yet dynamic homeostasis in health, which, when disrupted, leads to disease. This interconnection is even more pertinent today, in an era of increasing metabolic diseases of epidemic proportions such as obesity, metabolic syndrome, and diabetes. Despite great advances in understanding the molecular mechanisms of redox and metabolic regulation, we face significant challenges in preventing, diagnosing, and treating metabolic diseases. The etiological association and temporal overlap of these syndromes present significant challenges for the discrimination of appropriate clinical biomarkers for diagnosis, treatment, and outcome prediction. These multifactorial, multiorgan metabolic syndromes with complex etiopathogenic mechanisms are accompanied by disturbed redox equilibrium in target tissues and circulation. Free radicals and reactive species are considered both a causal factor and a consequence of disease status. Thus, determining the subtypes and levels of free radicals and reactive species, oxidatively damaged biomolecules (lipids, proteins, and nucleic acids) and antioxidant defense components as well as redox-sensitive transcription factors and fluxes of redox-dependent metabolic pathways will help define existing and establish novel redox biomarkers for stratifying metabolic diseases. This review aims to discuss diverse redox/metabolic aspects in obesity, metabolic syndrome, and diabetes, with the imperative to help establish a platform for emerging and future redox-metabolic biomarkers research in precision medicine. Future research warrants detailed investigations into the status of redox biomarkers in healthy subjects and patients, including the use of emerging 'omic' profiling technologies (e.g., redox proteomes, lipidomes, metabolomes, and transcriptomes), taking into account the influence of lifestyle (diet, physical activity, sleep, work patterns) as well as circadian ~24h fluctuations in circulatory factors and metabolites.

An update in toxicology of ageing

The field of ageing research has been rapidly advancing in recent decades and it had provided insight into the complexity of ageing phenomenon. However, as the organism-environment interaction appears to significantly affect the organismal pace of ageing, the systematic approach for gerontogenic risk assessment of environmental factors has yet to be established. This puts demand on development of effective biomarker of ageing, as a relevant tool to quantify effects of gerontogenic exposures, contingent on multidisciplinary research approach. Here we review the current knowledge regarding the main endogenous gerontogenic pathways involved in acceleration of ageing through environmental exposures. These include inflammatory and oxidative stress-triggered processes, dysregulation of maintenance of cellular anabolism and catabolism and loss of protein homeostasis. The most effective biomarkers showing specificity and relevancy to ageing phenotypes are summarized, as well. The crucial part of this review was dedicated to the comprehensive overview of environmental gerontogens including various types of radiation, certain types of pesticides, heavy metals, drugs and addictive substances, unhealthy dietary patterns, and sedentary life as well as psychosocial stress. The reported effects in vitro and in vivo of both recognized and potential gerontogens are described with respect to the up-to-date knowledge in geroscience. Finally, hormetic and ageing decelerating effects of environmental factors are briefly discussed, as well.

Implication of Dietary Iron-Chelating Bioactive Compounds in Molecular Mechanisms of Oxidative Stress-Induced Cell Ageing

One of the prevailing perceptions regarding the ageing of cells and organisms is the intracellular gradual accumulation of oxidatively damaged macromolecules, leading to the decline of cell and organ function (free radical theory of ageing). This chemically undefined material known as "lipofuscin," "ceroid," or "age pigment" is mainly formed through unregulated and nonspecific oxidative modifications of cellular macromolecules that are induced by highly reactive free radicals. A necessary precondition for reactive free radical generation and lipofuscin formation is the intracellular availability of ferrous iron (Fe2+) ("labile iron"), catalyzing the conversion of weak oxidants such as peroxides, to extremely reactive ones like hydroxyl (HO•) or alcoxyl (RO•) radicals. If the oxidized materials remain unrepaired for extended periods of time, they can be further oxidized to generate ultimate over-oxidized products that are unable to be repaired, degraded, or exocytosed by the relevant cellular systems. Additionally, over-oxidized materials might inactivate cellular protection and repair mechanisms, thus allowing for futile cycles of increasingly rapid lipofuscin accumulation. In this review paper, we present evidence that the modulation of the labile iron pool distribution by nutritional or pharmacological means represents a hitherto unappreciated target for hampering lipofuscin accumulation and cellular ageing.

Hallmarks and detection techniques of cellular senescence and cellular ageing in immune cells

The ageing of the global population brings about unprecedented challenges. Chronic age-related diseases in an increasing number of people represent an enormous burden for health and social care. The immune system deteriorates during ageing and contributes to many of these age-associated diseases due to its pivotal role in pathogen clearance, tissue homeostasis and maintenance. Moreover, in order to develop treatments for COVID-19, we urgently need to acquire more knowledge about the aged immune system, as older adults are disproportionally and more severely affected. Changes with age lead to impaired responses to infections, malignancies and vaccination, and are accompanied by chronic, low-degree inflammation, which together is termed immunosenescence. However, the molecular and cellular mechanisms that underlie immunosenescence, termed immune cell senescence, are mostly unknown. Cellular senescence, characterised by an irreversible cell cycle arrest, is thought to be the cause of tissue and organismal ageing. Thus, better understanding of cellular senescence in immune populations at single-cell level may provide us with insight into how immune cell senescence develops over the life time of an individual. In this review, we will briefly introduce the phenotypic characterisation of aged innate and adaptive immune cells, which also contributes to overall immunosenescence, including subsets and function. Next, we will focus on the different hallmarks of cellular senescence and cellular ageing, and the detection techniques most suitable for immune cells. Applying these techniques will deepen our understanding of immune cell senescence and to discover potential druggable pathways, which can be modulated to reverse immune ageing.

The dialogue between the ubiquitin-proteasome system and autophagy: Implications in ageing

Dysregulated proteostasis is one of the hallmarks of ageing. Damaged proteins may impair cellular function and their accumulation may lead to tissue dysfunction and disease. This is why protective mechanisms to safeguard the cell proteome have evolved. These mechanisms consist of cellular machineries involved in protein quality control, including regulators of protein translation, folding, trafficking and degradation. In eukaryotic cells, protein degradation occurs via two main pathways: the ubiquitin-proteasome system (UPS) and the autophagy-lysosome pathway. Although distinct pathways, they are not isolated systems and have a complementary nature, as evidenced by recent studies. These findings raise the question of how autophagy and the proteasome crosstalk. In this review we address how the two degradation pathways impact each other, thereby adding a new layer of regulation to protein degradation. We also analyze the implications of the UPS and autophagy in ageing.

Oxidized protein aggregates: Formation and biological effects

The study of protein aggregates has a long history. While in the first decades until the 80ies of the 20th century only the observation of the presence of such aggregates was reported, later the biochemistry of the formation and the biological effects of theses aggregates were described. This review focusses on the complexity of the biological effects of protein aggregates and its potential role in the aging process.

Autophagy in Female Fertility: A Role in Oxidative Stress and Aging

Significance: The precipitous age-related decline in female fertility is intimately associated with a reduction in both the quantity and quality of the germline (oocytes). Although complex etiologies undoubtedly contribute to the deterioration of oocyte quality, increasing attention has focused on the pervasive impact of oxidative stress. Indeed, the prolonged lifespan of the meiotically arrested oocyte places this cell at heightened risk of oxidative lesions, which commonly manifest in dysregulation of protein homeostasis (proteostasis). Although oocytes are able to mitigate this threat via the mobilization of a sophisticated network of surveillance, repair, and proteolytic pathways, these defenses are themselves prone to age-related defects, reducing their capacity to eliminate oxidatively damaged proteins. Recent Advances: Here, we give consideration to the quality control mechanisms identified within the ovary that afford protection to the female germline. Our primary focus is to review recent advances in our understanding of the autophagy pathway and its contribution to promoting oocyte longevity and modulating pathophysiological responses to oxidative stress. In addition, we explore the therapeutic potential of emerging strategies to fortify autophagic activity. Critical Issues: The complex interplay of oxidative stress and autophagy has yet to be fully elucidated within the context of the aging oocyte and surrounding ovarian environment. Future Directions: Emerging evidence provides a strong impetus to resolve the causal link between autophagy and oxidative stress-driven pathologies in the aging oocyte. Such research may ultimately inform novel therapeutic strategies to combat the age-related loss of female fertility via fortification of intrinsic autophagic activity.

Myocardial lipofuscin accumulation in ageing and sudden cardiac death

Lipofuscin is an intracellular aggregate of highly oxidized proteins that cannot be digested in the ubiquitin-proteasome system and accumulate mainly in lysosomes, especially in aged cells and pathological conditions. However, no systematic study has evaluated the cardiac accumulation of lipofuscin during human ageing and sudden cardiac death (SCD). Age estimation in unidentified bodies and postmortem SCD diagnosis are important themes in forensics. Thus, we aimed to elucidate their correlations with myocardial lipofuscin accumulation. We collected 76 cardiac samples from autopsy patients aged 20–97 years. After histopathological examination, myocardial lipofuscin was measured using its autofluorescence. Lipofuscin accumulated mainly in the perinuclear zone, and its accumulation rate positively correlated with chronological ageing (r = 0.82). Meanwhile, no significant change in lipofuscin level was observed with different causes of death, including SCD. There was also no significant change in lipofuscin level in relation to body mass index, serum brain natriuretic peptide level, or heart weight. Moreover, we performed LC3 and p62 immunoblotting to evaluate autophagic activity, and no change was observed in ageing. Therefore, lipofuscin accumulation more directly reflects chronological ageing rather than human cardiac pathology. Our study reveals the stability and utility of cardiac lipofuscin measurement for age estimation during autopsy.

Restoring autophagic flux attenuates cochlear spiral ganglion neuron degeneration by promoting TFEB nuclear translocation via inhibiting MTOR

Macroautophagy/autophagy dysfunction is associated with many neurodegenerative diseases. TFEB (transcription factor EB), an important molecule that regulates lysosomal and autophagy function, is regarded as a potential target for treating some neurodegenerative diseases. However, the relationship between autophagy dysfunction and spiral ganglion neuron (SGN) degeneration and the role of TFEB in SGN degeneration has not yet been established. Here, we showed that in degenerated SGNs, induced by sensory epithelial cell loss in the cochlea of mice following kanamycin and furosemide administration, the lipofuscin area and oxidative stress level were increased, the nuclear-to-cytoplasmic TFEB ratio was decreased, and the late stage of autophagic flux was impaired. After autophagy dysfunction was partially ameliorated with an MTOR inhibitor, which promoted TFEB translocation into the nucleus from the cytoplasm, we found that the lysosomal deficits were significantly relieved, the oxidative stress level was reduced, and the density of surviving SGNs and auditory nerve fibers was increased. The results in the present study reveal that autophagy dysfunction is an important component of SGN degeneration, and TFEB may be a potential target for attenuating SGN degeneration following sensory epithelial cell loss in the cochlea of mice.

Abbreviations: 3-NT: 3-nitrotyrosine; 4-HNE: 4-hydroxynonenal; 8-OHdG: 8-hydroxy-2ʹ-deoxyguanosine; ABR: auditory brainstem response; APP: amyloid beta (A4) precursor protein; CLEAR: coordinated lysosomal expression and regulation; CTSB: cathespin B; CTSD: cathespin D; SAMR1: senescence-accelerated mouse/resistance 1; SAMP8: senescence-accelerated mouse/prone 8; MAPK1/ERK2: mitogen-activated protein kinase 1; MTOR: mechanistic target of rapamycin kinase; SGN: spiral ganglion neuron; SQSTM1/p62: sequestosome 1; TEM: transmission electron microscope; TFEB: transcription factor EB

A Sequential Study of Age-Related Lipofuscin Accumulation in Hippocampus and Striate Cortex of Rats

Background

The age-pigment, lipofuscin that accumulates in cells intrinsically and progressively with age is considered as the hallmark of aging. This accumulation is more prominent in post-mitotic cells like neurons and also appears in glia.

Purpose

The aim of the present study was to assess the age-associated occurrence and distribution pattern of lipofuscin both in neurons and microglia in various regions of hippocampus and striate cortex.

Methods

The accumulation pattern of lipofuscin in hippocampus and striate cortex was observed in the female Wistar rats of 6 age groups, that is, 3, 6, 12, 18, 24, and 30 months using the autofluorescent property of lipofuscin and its specific localization in neurons and microglia by dual immunolabeling with NeuN and Iba-1 antibodies respectively. Cytoarchitectural and the morphological age-related changes were observed with cresyl violet staining.

Results

Lipofuscin pigments accumulate progressively through the normal aging process in hippocampus and striate cortex. However, in hippocampus, lipofuscin accumulates in a region-specific manner with the highest accumulation observed in cornu ammonis (CA) 1 and 3 subregions. Furthermore, the lipofuscin accumulation was also observed in microglia in the senile brains both in the hippocampus and striate cortex.

Conclusion

The progressive deposition of lipofuscin could result in cellular dysfunction. This encouraged us to forward the idea that microglia not only participate in the removal of the pigment from the neurons but also accumulate these waste products in itself. Such senescent microglia may contribute to age- related neurodegeneration as they lose the neuroprotective potential due to oxidative stress and thus may be unable to effectively phagocytose the age pigment.

Autophagy-A key pathway for cardiac health and longevity

As average life expectancy continues to rise in the developed world, age-associated pathologies are increasing in prevalence. The hallmarks of cardiac ageing include cardiomyocyte loss, fibrosis and hypertrophy, all of which contribute to an increased incidence of cardiac disease. At the molecular level, cellular ageing is characterized by increased ROS production, mitochondrial dysfunction and the accumulation of damaged proteins and organelles. Cardiomyocytes and other senescent cell types rely upon autophagy, a lysosome-mediated degradation pathway, to remove potentially toxic protein aggregates and damaged organelles from the cellular milieu. However, increasing lines of evidence point to an age-associated decrease in cardiomyocyte autophagy, with predictably negative consequences for cardiac function and health. Conversely, stimulation of autophagy has been shown to improve cellular health and cardiac function and to increase lifespan in numerous model organisms. Clearly, autophagy represents a critical pathway for cellular vitality, as well as a promising therapeutic target for the treatment of age-related cardiac pathologies. In this review, we will discuss the mechanism of autophagy and its regulation in the cell, the role of autophagy in the ageing heart, and how the autophagy pathway might be targeted to improve cardiac health.

The Role of Free Radicals in Autophagy Regulation: Implications for Ageing

Reactive oxygen and nitrogen species (ROS and RNS, resp.) have been traditionally perceived solely as detrimental, leading to oxidative damage of biological macromolecules and organelles, cellular demise, and ageing. However, recent data suggest that ROS/RNS also plays an integral role in intracellular signalling and redox homeostasis (redoxtasis), which are necessary for the maintenance of cellular functions. There is a complex relationship between cellular ROS/RNS content and autophagy, which represents one of the major quality control systems in the cell. In this review, we focus on redox signalling and autophagy regulation with a special interest on ageing-associated changes. In the last section, we describe the role of autophagy and redox signalling in the context of Alzheimer's disease as an example of a prevalent age-related disorder.

Young plasma reverses age-dependent alterations in hepatic function through the restoration of autophagy

Recent studies showing the therapeutic effect of young blood on aging‐associated deterioration of organs point to young blood as the solution for clinical problems related to old age. Given that defective autophagy has been implicated in aging and aging‐associated organ injuries, this study was designed to determine the effect of young blood on aging‐induced alterations in hepatic function and underlying mechanisms, with a focus on autophagy. Aged rats (22 months) were treated with pooled plasma (1 ml, intravenously) collected from young (3 months) or aged rats three times per week for 4 weeks, and 3‐methyladenine or wortmannin was used to inhibit young blood‐induced autophagy. Aging was associated with elevated levels of alanine transaminase and aspartate aminotransferase, lipofuscin accumulation, steatosis, fibrosis, and defective liver regeneration after partial hepatectomy, which were significantly attenuated by young plasma injections. Young plasma could also restore aging‐impaired autophagy activity. Inhibition of the young plasma‐restored autophagic activity abrogated the beneficial effect of young plasma against hepatic injury with aging. In vitro, young serum could protect old hepatocytes from senescence, and the antisenescence effect of young serum was abrogated by 3‐methyladenine, wortmannin, or small interfering RNA to autophagy‐related protein 7. Collectively, our data indicate that young plasma could ameliorate age‐dependent alterations in hepatic function partially via the restoration of autophagy.

Implications of aging and the endoplasmic reticulum unfolded protein response on the molecular modality of breast cancer

The endoplasmic reticulum (ER) is an important subcellular organelle that is involved in numerous activities required to achieve and maintain functional proteins in addition to its role in the biosynthesis of lipids and as a repository of intracellular Ca²⁺. The inability of the ER to cope with protein folding beyond its capacity causes disturbances that evoke ER stress. Cells possess molecular mechanisms aimed at clearing unwanted cargo from the ER lumen as an adaptive response, but failing to do so navigates the system towards cell death. This systemic approach is called the unfolded protein response. Aging insults cells through various perturbations in homeostasis that involve curtailing ER function by mitigating the expression of its resident chaperones and enzymes. Here the unfolded protein response (UPR) cannot protect the cell due to the weakening of its protective arm, which exacerbates imbalanced homeostasis. Aging predisposed breast malignancy activates the UPR, but tumor cells maneuver the mechanistic details of the UPR, favoring tumorigenesis and thereby eliciting a treacherous condition. Tumor cells exploit UPR pathways via crosstalk involving various signaling cascades that usher tumor cells to immortality. This review aims to present a collection of data that can delineate the missing links of molecular signatures between aging and breast cancer.

The ER stress regulator Bip mediates cadmium-induced autophagy and neuronal senescence

Autophagy is protective in cadmium (Cd)-induced oxidative damage. Endoplasmic reticulum (ER) stress has been shown to induce autophagy in a process requiring the unfolded protein response signalling pathways. Cd treatment significantly increased senescence in neuronal cells, which was aggravated by 3-MA or silencing of Atg5 and abolished by rapamycin. Cd increased expression of ER stress regulators Bip, chop, eIf2α, and ATF4, and activated autophagy as evidenced by upregulated LC3. Moreover, the ER stress inhibitor mithramycin inhibited the expression of ER stress protein chaperone Bip and blocked autophagic flux. Downregulating Bip significantly blocked the conversion of LC3-I to LC3-II, decreased LC3 puncta formation, and prevented the increase of senescence in PC12 cells. Interestingly, knocking down Bip regulated the expression of p-AMPK, p-AKT and p-s6k induced by Cd. BAPTA, a Bip inhibitor, decreased the expression of p-AMPK and LC3-II, but enhanced neuronal senescence. In addition, we found that siRNA for Bip enhanced GATA4 expression after 6 h Cd exposure in PC12 cells, while rapamycin treatment decreased GATA4 levels induced by 24 h Cd exposure. These results indicate that autophagy degraded GATA4 in a Bip-dependent way. Our findings suggest that autophagy regulated by Bip expression after ER stress suppressed Cd-induced neuronal senescence.

Autophagy and Immune Senescence

With extension of the average lifespan, aging has become a heavy burden in society. Immune senescence is a key risk factor for many age-related diseases such as cancer and increased infections in the elderly, and hence has elicited much attention in recent years. As our body's guardian, the immune system maintains systemic health through removal of pathogens and damage. Autophagy is an important cellular 'clearance' process by which a cell internally delivers damaged organelles and macromolecules to lysosomes for degradation. Here, we discuss the most current knowledge of how impaired autophagy can lead to cellular and immune senescence. We also provide an overview, with examples, of the clinical potential of exploiting autophagy to delay immune senescence and/or rejuvenate immunity to treat various age-related diseases.

Change in the antioxidative capacity of extraocular muscles in patients with exotropia

BACKGROUND

We compared the oxidative stress and antioxidant capacities of the medial rectus muscles (MRMs) between the patients with constant exotropia and control subjects.

METHODS

A total of 40 MRMs from patients with constant exotropia and 40 MRMs from normal donor eyes (controls) were assessed. Neuronal nitric oxide synthase (nNOS), superoxide dismutase (SOD), and catalase levels were compared between the two groups using western blot analysis. In addition, the lipofuscin accumulation level was compared between the exotropic and control groups.

RESULTS

According to western blot analysis, the nNOS level was significantly higher in the exotropic group than in the control group (P < 0.05). On the other hand, a catalase expression level was higher in the control group than in the exotropic group (P < 0.05). The SOD1 level did not show a significant difference between the two groups. The relative lipofuscin fluorescence intensity was higher in the exotropic group than in the control group (P < 0.001).

CONCLUSIONS

Based on these findings, the MRMs of patients with exotropia had a redox imbalance status. Further study is needed to investigate whether this imbalance in antioxidant capacity is present in the extraocular muscles of patients with other strabismus.

Pathophysiological importance of aggregated damaged proteins

Reactive oxygen species (ROS) are formed continuously in the organism even under physiological conditions. If the level of ROS in cells exceeds the cellular defense capacity, components such as RNA/DNA, lipids, and proteins are damaged and modified, thus affecting the functionality of organelles as well. Proteins are especially prominent targets of various modifications such as oxidation, glycation, or conjugation with products of lipid peroxidation, leading to the alteration of their biological function, nonspecific interactions, and the production of high-molecular-weight protein aggregates. To ensure the maintenance of cellular functions, two proteolytic systems are responsible for the removal of oxidized and modified proteins, especially the proteasome and organelles, mainly the autophagy-lysosomal systems. Furthermore, increased protein oxidation and oxidation-dependent impairment of proteolytic systems lead to an accumulation of oxidized proteins and finally to the formation of nondegradable protein aggregates. Accordingly, the cellular homeostasis cannot be maintained and the cellular metabolism is negatively affected. Here we address the current knowledge of protein aggregation during oxidative stress, aging, and disease.

Deficiency of insulin-like growth factor 1 attenuates aging-induced changes in hepatic function: role of autophagy

BACKGROUND & AIMS

Circulating insulin-like growth factor-1 (IGF-1) plays a pivotal role in mediating the aging process. This study was designed to evaluate the effect of liver IGF-1 deficiency (LID) on aging-induced changes in hepatic function and underlying mechanisms, with a focus on autophagy.

METHODS

Plasma and liver samples were obtained from young (3-mo) and aged (24-mo) wild type (WT) and LID mice. Levels of AST, ALT, triglyceride, hepatic lipofuscin, steatosis, fibrosis, and nuclear morphology were analyzed. Western blot was employed to evaluate autophagy. Human HepG2 cells were treated with free fatty acid (FFA) to mimic hepatic aging in the absence or presence of IGF-1 siRNA. SA-β-gal activity was detected using flow cytometry and a fluorescence microplate reader. GFP-LC3 was used to assess autophagy activity in HepG2 cells.

RESULTS

Median survival was longer in LID mice compared with WT mice. Aging was associated with elevated levels of triglyceride, AST and ALT, lipofuscin accumulation, steatosis, fibrosis and nuclear injury, which were significantly attenuated by liver IGF-1 deficiency. Levels of autophagy were suppressed in senescent livers, the effect was reversed in the liver of IGF-1 deficient mice. In HepG2 cells, FFA induced the accumulation of β-gal, which was dramatically suppressed by IGF-1 knockdown. Importantly, inhibiting autophagy using 3-methyladenine mitigated IGF-1 knockdown-induced preservation of autophagic vacuole formation and inhibition of β-gal accumulation in the presence of FFA in HepG2 cells.

CONCLUSIONS

Our data revealed that IGF-1 deficiency ameliorated aging-induced hepatic injury, possibly through preventing a concomitant diminution in autophagy. These data provide new insight into the role of IGF-1 and autophagy in the management of aging-induced hepatic injury.

Lysosomal iron, iron chelation, and cell death

SIGNIFICANCE

Lysosomes are acidic organelles containing more than fifty hydrolases that provide for the degradation of intracellular and endocytosed materials by autophagy and heterophagy, respectively. They digest a variety of macromolecules, as well as all organelles, and their integrity is crucial. As a result of the degradation of iron-containing macromolecules (e.g., ferritin and mitochondrial components) or endocytosed erythrocytes (by macrophages), lysosomes can accumulate large amounts of iron. This iron occurs often as Fe(II) due to the acidic and reducing lysosomal environment. Fe(II) is known to catalyze Fenton reactions, yielding extremely reactive hydroxyl radicals that may jeopardize lysosomal membrane integrity during oxidative stress. This results in the release of hydrolases and redox-active iron into the cytosol with ensuing damage or cell death. Lysosomes play key roles not only in apoptosis and necrosis but also in neurodegeneration, aging, and atherosclerosis.

RECENT ADVANCES

The damaging effect of intralysosomal iron can be hampered by endogenous or exogenous iron chelators that enter the lysosomal compartment by membrane permeation, endocytosis, or autophagy.

CRITICAL ISSUES

Cellular sensitivity to oxidative stress is enhanced by lysosomal redox-active iron or by lysosomal-targeted copper chelators binding copper (from degradation of copper-containing macromolecules) in redox-active complexes. Probably due to higher copper levels, lysosomes of malignant cells may be specifically sensitized by such chelators.

FUTURE DIRECTIONS

By increasing lysosomal redox-active iron or exposing cells to lysosomal-targeted copper chelators, it should be possible to enhance the sensitivity of cancer cells to radiation-induced oxidative stress or treatment with cytostatics that induce such stress.

HEART AND AGE (PART I): AGEING THEORIES AND MORPHOLOGICAL CHANGES

Heart ageing is a complex process including multiple cellular and molecular-level changes and resulting in different ageing phenotypes within the same biological species. According to the existing ageing theories, the multiple interacting mechanisms of ageing include somatic mutations, telomere shortening, oxidative stress, and mitochondrial defects. The review presents the most accepted ageing theories and discusses morphological characteristics of heart ageing on subcellular, cellular, and organ levels.

Protein oxidation in aging and the removal of oxidized proteins

Reactive oxygen species (ROS) are generated constantly within cells at low concentrations even under physiological conditions. During aging the levels of ROS can increase due to a limited capacity of antioxidant systems and repair mechanisms. Proteins are among the main targets for oxidants due to their high rate constants for several reactions with ROS and their abundance in biological systems. Protein damage has an important influence on cellular viability since most protein damage is non-repairable, and has deleterious consequences on protein structure and function. In addition, damaged and modified proteins can form cross-links and provide a basis for many senescence-associated alterations and may contribute to a range of human pathologies. Two proteolytic systems are responsible to ensure the maintenance of cellular functions: the proteasomal (UPS) and the lysosomal system. Those degrading systems provide a last line of antioxidative protection, removing irreversible damaged proteins and recycling amino acids for the continuous protein synthesis. But during aging, both systems are affected and their proteolytic activity declines significantly. Here we highlight the recent advantages in the understanding of protein oxidation and the fate of these damaged proteins during aging. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Oxidative stress and myocardial remodeling in chronic mitral regurgitation

Mechanisms of left ventricular (LV) dysfunction in isolated mitral regurgitation (MR) are not well understood. Vasodilator therapy in other forms of LV dysfunction reduces LV wall stress and improves LV function; however, studies in isolated MR show no beneficial effect on LV remodeling using vasodilator drugs or renin-angiotensin system blockade. Therefore, the search for new therapies that improve LV remodeling and function in isolated MR is clinically significant. Recent work in the authors' laboratory has demonstrated increased oxidants from a number of sources including the enzyme xanthine oxidase (XO) in the LV of patients with isolated MR. In addition to being a major source of reactive oxygen species, XO is linked to bioenergetic dysfunction because its substrates derive from adenosine triphosphate catabolism. Correspondingly, there was also evidence of aggregates of small mitochondria in cardiomyocytes, which is generally considered a response to bioenergetic deficit in cells. Future studies are required to determine whether XO and persistent oxidative stress are causative in maladaptive LV remodeling and offer potential therapeutic targets in ameliorating LV damage in patients with isolated MR.

The role of lysosomes in iron metabolism and recycling

Iron is the most abundant transition metal in the earth's crust. It cycles easily between ferric (oxidized; Fe(III)) and ferrous (reduced; Fe(II)) and readily forms complexes with oxygen, making this metal a central player in respiration and related redox processes. However, 'loose' iron, not within heme or iron-sulfur cluster proteins, can be destructively redox-active, causing damage to almost all cellular components, killing both cells and organisms. This may explain why iron is so carefully handled by aerobic organisms. Iron uptake from the environment is carefully limited and carried out by specialized iron transport mechanisms. One reason that iron uptake is tightly controlled is that most organisms and cells cannot efficiently excrete excess iron. When even small amounts of intracellular free iron occur, most of it is safely stored in a non-redox-active form in ferritins. Within nucleated cells, iron is constantly being recycled from aged iron-rich organelles such as mitochondria and used for construction of new organelles. Much of this recycling occurs within the lysosome, an acidic digestive organelle. Because of this, most lysosomes contain relatively large amounts of redox-active iron and are therefore unusually susceptible to oxidant-mediated destabilization or rupture. In many cell types, iron transit through the lysosomal compartment can be remarkably brisk. However, conditions adversely affecting lysosomal iron handling (or oxidant stress) can contribute to a variety of acute and chronic diseases. These considerations make normal and abnormal lysosomal handling of iron central to the understanding and, perhaps, therapy of a wide range of diseases.

Autophagy impairment induces premature senescence in primary human fibroblasts

BACKGROUND

Recent studies have demonstrated that activation of autophagy increases the lifespan of organisms from yeast to flies. In contrast to the lifespan extension effect in lower organisms, it has been reported that overexpression of unc-51-like kinase 3 (ULK3), the mammalian homolog of autophagy-specific gene 1 (ATG1), induces premature senescence in human fibroblasts. Therefore, we assessed whether the activation of autophagy would genuinely induce premature senescence in human cells.

METHODOLOGY/PRINCIPAL FINDINGS

Depletion of ATG7, ATG12, or lysosomal-associated membrane protein 2 (Lamp2) by transfecting siRNA or infecting cells with a virus containing gene-specific shRNA resulted in a senescence-like state in two strains of primary human fibroblasts. Prematurely senescent cells induced by autophagy impairment exhibited the senescent phenotypes, similar to the replicatively senescent cells, such as increased senescence associated β-galactosidase (SA-β-gal) activity, reactive oxygen species (ROS) generation, and accumulation of lipofuscin. In addition, expression levels of ribosomal protein S6 kinase1 (S6K1), p-S6K1, p-S6, and eukaryotic translation initiation factor 4E (eIF4E) binding protein 1 (4E-BP1) in the mammalian target of rapamycin (mTOR) pathway and beclin-1, ATG7, ATG12-ATG5 conjugate, and the sequestosome 1 (SQSTM1/p62) monomer in the autophagy pathway were decreased in both the replicatively and the autophagy impairment-induced prematurely senescent cells. Furthermore, it was found that ROS scavenging by N-acetylcysteine (NAC) and inhibition of p53 activation by pifithrin-α or knockdown of p53 using siRNA, respectively, delayed autophagy impairment-induced premature senescence and restored the expression levels of components in the mTOR and autophagy pathways.

CONCLUSION

Taken together, we concluded that autophagy impairment induces premature senescence through a ROS- and p53-dependent manner in primary human fibroblasts.

Redox activity within the lysosomal compartment: implications for aging and apoptosis

The lysosome is a redox-active compartment containing low-mass iron and copper liberated by autophagic degradation of metalloproteins. The acidic milieu and high concentration of thiols within lysosomes will keep iron in a reduced (ferrous) state, which can react with endogenous or exogenous hydrogen peroxide. Consequent intralysosomal Fenton reactions may give rise to the formation of lipofuscin or "age pigment" that accumulates in long-lived postmitotic cells that cannot dilute it by division. Extensive accumulation of lipofuscin seems to hinder normal autophagy and may be an important factor behind aging and age-related pathologies. Enhanced oxidative stress causes lysosomal membrane permeabilization, with ensuing relocation to the cytosol of iron and lysosomal hydrolytic enzymes, with resulting apoptosis or necrosis. Lysosomal copper is normally not redox active because it will form non-redox-active complexes with various thiols. However, if cells are exposed to lysosomotropic chelators that do not bind all the copper coordinates, highly redox-active complexes may form, with ensuing extensive lysosomal Fenton-type reactions and loss of lysosomal stability. Because many malignancies seem to have increased amounts of copper-containing macromolecules that are turned over by autophagy, it is conceivable that lysosomotropic copper chelators may be used in the future in ROS-based anticancer therapies.

Mitochondrial turnover and aging of long-lived postmitotic cells: the mitochondrial-lysosomal axis theory of aging

It is now generally accepted that aging and eventual death of multicellular organisms is to a large extent related to macromolecular damage by mitochondrially produced reactive oxygen species, mostly affecting long-lived postmitotic cells, such as neurons and cardiac myocytes. These cells are rarely or not at all replaced during life and can be as old as the whole organism. The inherent inability of autophagy and other cellular-degradation mechanisms to remove damaged structures completely results in the progressive accumulation of garbage, including cytosolic protein aggregates, defective mitochondria, and lipofuscin, an intralysosomal indigestible material. In this review, we stress the importance of crosstalk between mitochondria and lysosomes in aging. The slow accumulation of lipofuscin within lysosomes seems to depress autophagy, resulting in reduced turnover of effective mitochondria. The latter not only are functionally deficient but also produce increased amounts of reactive oxygen species, prompting lipofuscinogenesis. Moreover, defective and enlarged mitochondria are poorly autophagocytosed and constitute a growing population of badly functioning organelles that do not fuse and exchange their contents with normal mitochondria. The progress of these changes seems to result in enhanced oxidative stress, decreased ATP production, and collapse of the cellular catabolic machinery, which eventually is incompatible with survival.

Increased oxidative stress and cardiomyocyte myofibrillar degeneration in patients with chronic isolated mitral regurgitation and ejection fraction >60%

OBJECTIVES

This study assessed myocardial damage in patients with chronic isolated mitral regurgitation (MR) and left ventricular ejection fraction (LVEF) >60%.

BACKGROUND

Typically, MR patients have decreased LVEF after mitral valve (MV) repair despite normal pre-operative LVEF.

METHODS

Twenty-seven patients with isolated MR had left ventricular (LV) biopsies taken at time of MV repair. Magnetic resonance imaging with tissue tagging was performed in 40 normal subjects and in MR patients before and 6 months after MV repair.

RESULTS

LVEF (66 +/- 5% to 54 +/- 9%, p < 0.0001) and LV end-diastolic volume index (108 +/- 28 ml/m(2) to 78 +/- 24 ml/m(2), p < 0.0001) decreased, whereas left ventricular end-systolic (LVES) volume index was 60% above normal pre- and post-MV repair (p < 0.05). The LV circumferential and longitudinal strain rates decreased below normal following MV repair (6.38 +/- 1.38 vs. 5.11 +/- 1.28, p = 0.0009, and 7.51 +/- 2.58 vs. 5.31 +/- 1.61, percentage of R to R interval, p < 0.0001), as LVES stress/LVES volume index ratio was depressed at baseline and following MV repair versus normal subjects (0.25 +/- 0.10 and 0.28 +/- 0.05 vs. 0.33 +/- 0.12, p < 0.01). LV biopsies demonstrated cardiomyocyte myofibrillar degeneration versus normal subjects (p = 0.035). Immunostaining and immunoblotting demonstrated increased xanthine oxidase in MR versus normal subjects (p < 0.05). Lipofuscin deposition was increased in cardiomyocytes of MR versus normal subjects (0.62 +/- 0.20 vs. 0.33 +/- 0.11, percentage of area: p < 0.01).

CONCLUSIONS

Decreased LV strain rates and LVES wall stress/LVES volume index following MV repair indicate contractile dysfunction, despite pre-surgical LVEF >60%. Increased oxidative stress could cause myofibrillar degeneration and lipofuscin accumulation resulting in LV contractile dysfunction in MR.

Lipofuscin: detection and quantification by microscopic techniques

Since lipofuscin, the so-called "aging pigment", turned out to play a fundamental role in the aging process, particularly in the postmitotic senescence of muscle or neuronal cells, it became a focus of aging and stress research. During normal aging, lipofuscin accumulates in a nearly linear way, whereas its rate of formation can increase in the final stages of senescence or in the progress of several pathologic processes. Thus, both in senescence and pathologic processes, lipofuscin can be used as a detectable "marker" to estimate the remaining lifetime of single cells, the amount of long-term oxidative stress cells were subjected to or to quantify and qualify a pathologic progress in vivo or in vitro. To enable this, a quick and easy applicable method of detection and quantification of lipofuscin has to be used, as is provided by fluorescence microscopy determining the autofluorescence via of the "aging pigment". In this review, we take a look at different methods of detection and quantification of lipofuscin in single cells by using its physical or chemical features.

Aging: central role for autophagy and the lysosomal degradative system

The lysosomal network is the major intracellular proteolytic system accounting for more than 98% of long-lived bulk protein degradation and recycling particularly in tissues such as liver and muscles. Lysosomes are the final destination of intracellular damaged structures, identified and sequestered by the processes of macroautophagy and chaperone-mediated autophagy (CMA). In the process of macroautophagy, long-lived proteins and other macromolecular aggregates and damaged intracellular organelles are first engulfed by autophagosomes. Autophagosomes themselves have limited degrading capacity and rely on fusion with lysosomes. Unlike macroautophagy, CMA does not require intermediate vesicle formation and the cytosolic proteins recognized by this pathway are directly translocated to the lysosomal membrane. Aging is a universal phenomenon characterized by progressive deterioration of cells and organs due to accumulation of macromolecular and organelle damage. The continuous removal of worn-out components and replacement with newly synthesized ones ensures cellular homeostasis and delays the aging process. Growing evidence indicate that the rate of autophagosome formation and maturation and the efficiency of autophagosome/lysosome fusion decline with age. In addition, a progressive increase in intralysosomal concentration of free radicals and the age pigment lipofuscin further diminish the efficiency of lysosomal protein degradation. Therefore, integrity of the autophagosomal-lysosomal network appears to be critical in the progression of aging. Discovery of the genes involved in the process of autophagy has provided insight into the various molecular pathways that may be involved in aging and senescence. In this review, we discuss the cellular and molecular mechanisms involved in autophagy and the role of autophagosome/lysosome network in the aging process.

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.

Loss of macroautophagy promotes or prevents fibroblast apoptosis depending on the death stimulus

Macroautophagy has been implicated as a mechanism of cell death. However, the relationship between this degradative pathway and cell death is unclear as macroautophagy has been shown recently to protect against apoptosis. To better define the interplay between these two critical cellular processes, we determined whether inhibition of macroautophagy could have both pro-apoptotic and anti-apoptotic effects in the same cell. Embryonic fibroblasts from mice with a knock-out of the essential macroautophagy gene atg5 were treated with activators of the extrinsic and intrinsic death pathways. Loss of macroautophagy sensitized these cells to caspase-dependent apoptosis from the death receptor ligands Fas and tumor necrosis factor-alpha (TNF-alpha). Atg5-/- mouse embryonic fibroblasts had increased activation of the mitochondrial death pathway in response to Fas/TNF-alpha in concert with decreased ATP levels. Fas/TNF-alpha treatment failed to up-regulate macroautophagy, and in fact, decreased activity at late time points. In contrast to their sensitization to Fas/TNF-alpha, Atg5-/- cells were resistant to death from menadione and UV light. In the absence of macroautophagy, an up-regulation of chaperone-mediated autophagy induced resistance to these stressors. These results demonstrate that inhibition of macroautophagy can promote or prevent apoptosis in the same cell and that the response is governed by the nature of the death stimulus and compensatory changes in other forms of autophagy. Experimental findings that an inhibition of macroautophagy blocks apoptosis do not prove that autophagy mediates cell death as this effect may result from the protective up-regulation of other autophagic pathways such as chaperone-mediated autophagy.

Lysosomes in iron metabolism, ageing and apoptosis

The lysosomal compartment is essential for a variety of cellular functions, including the normal turnover of most long-lived proteins and all organelles. The compartment consists of numerous acidic vesicles (pH approximately 4 to 5) that constantly fuse and divide. It receives a large number of hydrolases ( approximately 50) from the trans-Golgi network, and substrates from both the cells' outside (heterophagy) and inside (autophagy). Many macromolecules contain iron that gives rise to an iron-rich environment in lysosomes that recently have degraded such macromolecules. Iron-rich lysosomes are sensitive to oxidative stress, while 'resting' lysosomes, which have not recently participated in autophagic events, are not. The magnitude of oxidative stress determines the degree of lysosomal destabilization and, consequently, whether arrested growth, reparative autophagy, apoptosis, or necrosis will follow. Heterophagy is the first step in the process by which immunocompetent cells modify antigens and produce antibodies, while exocytosis of lysosomal enzymes may promote tumor invasion, angiogenesis, and metastasis. Apart from being an essential turnover process, autophagy is also a mechanism by which cells will be able to sustain temporary starvation and rid themselves of intracellular organisms that have invaded, although some pathogens have evolved mechanisms to prevent their destruction. Mutated lysosomal enzymes are the underlying cause of a number of lysosomal storage diseases involving the accumulation of materials that would be the substrate for the corresponding hydrolases, were they not defective. The normal, low-level diffusion of hydrogen peroxide into iron-rich lysosomes causes the slow formation of lipofuscin in long-lived postmitotic cells, where it occupies a substantial part of the lysosomal compartment at the end of the life span. This seems to result in the diversion of newly produced lysosomal enzymes away from autophagosomes, leading to the accumulation of malfunctioning mitochondria and proteins with consequent cellular dysfunction. If autophagy were a perfect turnover process, postmitotic ageing and several age-related neurodegenerative diseases would, perhaps, not take place.

Lysosomes and oxidative stress in aging and apoptosis

The lysosomal compartment consists of numerous acidic vesicles (pH approximately 4-5) that constantly fuse and divide. It receives a large number of hydrolases from the trans-Golgi network, while their substrates arrive from both the cell's outside (heterophagy) and inside (autophagy). Many macromolecules under degradation inside lysosomes contain iron that, when released in labile form, makes lysosomes sensitive to oxidative stress. The magnitude of generated lysosomal destabilization determines if reparative autophagy, apoptosis, or necrosis will follow. Apart from being an essential turnover process, autophagy is also a mechanism for cells to repair inflicted damage, and to survive temporary starvation. The inevitable diffusion of hydrogen peroxide into iron-rich lysosomes causes the slow oxidative formation of lipofuscin in long-lived postmitotic cells, where it finally occupies a substantial part of the volume of the lysosomal compartment. This seems to result in a misdirection of lysosomal enzymes away from autophagosomes, resulting in depressed autophagy and the accumulation of malfunctioning mitochondria and proteins with consequent cellular dysfunction. This scenario might put aging into the category of autophagy disorders.

The role of autophagy in age-related neurodegeneration

Most age-related neurodegenerative diseases are characterized by accumulation of aberrant protein aggregates in affected brain regions. In many cases, these proteinaceous deposits are composed of ubiquitin conjugates, suggesting a failure in the clearance of proteins targeted for degradation. The 2 principal routes of intracellular protein catabolism are the ubiquitin proteasome system and the autophagy-lysosome system (autophagy). Both of these degradation pathways have been implicated as playing important roles in the pathogenesis of neurodegenerative disease. Here we describe autophagy and review the evidence suggesting that impairment of autophagy contributes to the initiation or progression of age-related neurodegeneration. We also review recent evidence indicating that autophagy may be exploited to remove toxic protein species, suggesting novel strategies for therapeutic intervention for a class of diseases for which no effective treatments presently exist.

Autophagy, organelles and ageing

As a result of insufficient digestion of oxidatively damaged macromolecules and organelles by autophagy and other degradative systems, long-lived postmitotic cells, such as cardiac myocytes, neurons and retinal pigment epithelial cells, progressively accumulate biological 'garbage' ('waste' materials). The latter include lipofuscin (a non-degradable intralysosomal polymeric substance), defective mitochondria and other organelles, and aberrant proteins, often forming aggregates (aggresomes). An interaction between senescent lipofuscin-loaded lysosomes and mitochondria seems to play a pivotal role in the progress of cellular ageing. Lipofuscin deposition hampers autophagic mitochondrial turnover, promoting the accumulation of senescent mitochondria, which are deficient in ATP production but produce increased amounts of reactive oxygen species. Increased oxidative stress, in turn, further enhances damage to both mitochondria and lysosomes, thus diminishing adaptability, triggering mitochondrial and lysosomal pro-apoptotic pathways, and culminating in cell death.

Autophagy, ageing and apoptosis: the role of oxidative stress and lysosomal iron

As an outcome of normal autophagic degradation of ferruginous materials, such as ferritin and mitochondrial metalloproteins, the lysosomal compartment is rich in labile iron and, therefore, sensitive to the mild oxidative stress that cells naturally experience because of their constant production of hydrogen peroxide. Diffusion of hydrogen peroxide into the lysosomes results in Fenton-type reactions with the formation of hydroxyl radicals and ensuing peroxidation of lysosomal contents with formation of lipofuscin that amasses in long-lived postmitotic cells. Lipofuscin is a non-degradable polymeric substance that forms at a rate that is inversely related to the average lifespan across species and is built up of aldehyde-linked protein residues. The normal accumulation of lipofuscin in lysosomes seems to reduce autophagic capacity of senescent postmitotic cells--probably because lipofuscin-loaded lysosomes continue to receive newly formed lysosomal enzymes, which results in lack of such enzymes for autophagy. The result is an insufficient and declining rate of autophagic turnover of worn-out and damaged cellular components that consequently accumulate in a way that upsets normal metabolism. In the event of a more substantial oxidative stress, enhanced formation of hydroxyl radicals within lysosomes jeopardizes the membrane stability of particularly iron-rich lysosomes, specifically of autophagolysosomes that have recently participated in the degradation of iron-rich materials. For some time, the rupture of a limited number of lysosomes has been recognized as an early upstream event in many cases of apoptosis, particularly oxidative stress-induced apoptosis, while necrosis results from a major lysosomal break. Consequently, the regulation of the lysosomal content of redox-active iron seems to be essential for the survival of cells both in the short- and the long-term.

Mitochondrial damage and intralysosomal degradation in cellular aging

Normal mitochondrial respiration is associated with a continuous production of superoxide and hydrogen peroxide, inevitably resulting in minor macromolecular damage. Damaged cellular components are not completely turned over by autophagy and other cellular repair systems, leading to a progressive age-related accumulation of biological "garbage" material, such as defective mitochondria, cytoplasmic protein aggregates and an intralysosomal undegradable material, lipofuscin. These changes primarily affect neurons, cardiac myocytes and other long-lived postmitotic cells that neither dilute this "garbage" by mitotic activity, nor are replaced by newly differentiated cells. Defective mitochondria are insufficient in ATP production and often generate increased amounts of reactive oxygen species, further enhancing oxidative stress. Lipofuscin-loaded lysosomes, in turn, poorly turn over mitochondria that gradually leads to the overload of long-lived postmitotic cells with "garbage" material, decreased adaptability and eventual cell death.

Increased resistance of lipofuscin-loaded prematurely senescent fibroblasts to starvation-induced programmed cell death

Alterations of cellular structures often found in ageing cells is mainly the result of production of reactive oxygen species and a consequence of aerobic life. Both oxidative stress and decreased degradative capacity of lysosomal system cause accumulation of intralysosomal age-related pigment called lipofuscin. To investigate the influence of lipofuscin on cell function, we compared survival of lipofuscin-loaded and control human fibroblasts following complete starvation induced by exposure to phosphate-buffered saline (PBS). Starving of control fibroblasts resulted in lysosomal alkalinisation, relocation of cathepsin D to the cytosol, caspase-3 activation and, finally, cell death, which became evident 72 h after the start of exposure to PBS. Increase of lysosomal pH was significantly less prominent in lipofuscin-loaded cells than in controls and was accompanied neither by leakage of cathepsin D nor by caspase-3 activation even 96 h after the initiation of starvation. Suppression of autophagy by 3-methyladenine (3-MA) accelerated cell death, while inhibition of cathepsin D delayed it, implying an important role of autophagy in cell survival during starvation and showing the involvement of lysosomes in starvation-induced cell death. Disturbed apoptotic response found in lipofuscin-loaded cells can be interpreted as an example of hormesis--an adaptation to low doses of otherwise harmful agents, in this case of lipofuscin, which has a protective effect at moderate amounts but becomes toxic at large quantities.

Catabolic Insufficiency and Aging

Cellular degradative processes, which include lysosomal (autophagic) and proteasomal degradation, as well as the activity of cytosolic and mitochondrial proteases, provide for a continuous turnover of damaged and obsolete biomolecules and organelles. Inherent insufficiency of these degradative processes results in progressive accumulation within long-lived postmitotic cells of biological "garbage" ("waste" material), such as indigestible protein aggregates, defective mitochondria, and lipofuscin (age pigment), an intralysosomal, polymeric, undegradable material. Intracellular "garbage" is neither completely catabolized, nor exocytosed to any considerable extent. Heavy lipofuscin loading of lysosomes, typical of old age, seems to pronouncedly decrease autophagic potential. As postulated in the mitochondrial-lysosomal axis theory of aging, this occurs on account of the transport of newly synthesized lysosomal enzymes to lipofuscin-loaded lysosomes rather than to active lysosomes/late endosomes, making the enzyme content of autophagolysosomes insufficient for proper degradation. Consequently, the turnover of mitochondria progressively declines, resulting in decreased ATP synthesis and enhanced formation of reactive oxygen species, inducing further mitochondrial damage and additional lipofuscin formation. With advancing age, lipofuscin-loaded lysosomes and defective mitochondria occupy increasingly larger parts of long-lived postmitotic cells, leaving less and less capability for normal turnover and ATP production, finally resulting in cell death.