Cellular stress and protein misfolding during aging

Crosstalk between protein misfolding and endoplasmic reticulum stress during ageing and their role in age-related disorders

Maintaining the proteome is crucial to retaining cell functionality and response to multiple intrinsic and extrinsic stressors. Protein misfolding increased the endoplasmic reticulum (ER) stress and activated the adaptive unfolded protein response (UPR) to restore cell homeostasis. Apoptosis occurs when ER stress is prolonged or the adaptive response fails. In healthy young cells, the ratio of protein folding machinery to quantities of misfolded proteins is balanced under normal circumstances. However, the age-related deterioration of the complex systems for handling protein misfolding is accompanied by ageing-related disruption of protein homeostasis, which results in the build-up of misfolded and aggregated proteins. This ultimately results in decreased cell viability and forms the basis of common age-related diseases called protein misfolding diseases. Proteins or protein fragments convert from their ordinarily soluble forms to insoluble fibrils or plaques in many of these disorders, which build up in various organs such as the liver, brain, or spleen. Alzheimer's, Parkinson's, type II diabetes, and cancer are diseases in this group commonly manifest in later life. Thus, protein misfolding and its prevention by chaperones and different degradation paths are becoming understood from molecular perspectives. Proteodynamics information will likely affect future interventional techniques to combat cellular stress and support healthy ageing by avoiding and treating protein conformational disorders. This review provides an overview of the diverse proteostasis machinery, protein misfolding, and ER stress involvement, which activates the UPR sensors. Here, we will discuss the crosstalk between protein misfolding and ER stress and their role in developing age-related diseases.

Impact of Obesity-Related Endoplasmic Reticulum Stress on Cancer and Associated Molecular Targets

Obesity, characterized by excessive body fat, is closely linked to endoplasmic reticulum (ER) stress, leading to insulin resistance and type 2 diabetes. Inflammatory pathways like c-Jun N-terminal kinase (JNK) worsen insulin resistance, impacting insulin signaling. Moreover, ER stress plays a substantial role in cancer, influencing tumor cell survival and growth by releasing factors like vascular endothelial growth factor (VEGF). The unfolded protein response (UPR) is pivotal in this process, offering both pro-survival and apoptotic pathways. This review offers an extensive exploration of the sophisticated connection between ER stress provoked by obesity and its role in both the onset and advancement of cancer. It delves into the intricate interplay between oncogenic signaling and the pathways associated with ER stress in individuals who are obese. Furthermore, this review sheds light on potential therapeutic strategies aimed at managing ER stress induced by obesity, with a focus on addressing cancer initiation and progression. The potential to alleviate ER stress through therapeutic interventions, which may encompass the use of small molecules, FDA-approved medications, and gene therapy, holds great promise. A more in-depth examination of pathways such as UPR, ER-associated protein degradation (ERAD), autophagy, and epigenetic regulation has the potential to uncover innovative therapeutic approaches and the identification of predictive biomarkers.

Effect of In Vitro Solvation Conditions on Inter- and Intramolecular Assembly of Full-Length TDP-43

Cellular stress is a major cause of neurodegenerative diseases. In particular, in amyotrophic lateral sclerosis (ALS), around 90% of the cases are believed to occur due to aggregation and misfolding of TDP-43 protein in neurons due to aging and chronic environmental stress. However, the physicochemical basis of how TDP-43 senses the change in solvation conditions during stress and misfolds remains very poorly understood. We show here that the full-length human TDP-43 can exist in equilibrium with multiple structural states. The equilibrium between these states is highly sensitive to changes in solvation conditions. We show that upon thermal and pH stress, amyloidogenic oligomers can form amyloid-like fibrils. However, the internal structure of the fibril depends upon the physicochemical nature of stress. Our results present a physical basis of the effect of solvation conditions on inter- and intramolecular assembly formation of TDP-43 and reconcile why the nature and the internal structure of the aggregated form have been found to be different when extracted from the brain of different ALS patients.

Heat Shock Proteins: Potential Modulators and Candidate Biomarkers of Peripartum Cardiomyopathy

Peripartum cardiomyopathy (PPCM) is a potentially life-threatening condition in which heart failure and systolic dysfunction occur late in pregnancy or within months following delivery. To date, no reliable biomarkers or therapeutic interventions for the condition exist, thus necessitating an urgent need for identification of novel PPCM drug targets and candidate biomarkers. Leads for novel treatments and biomarkers are therefore being investigated worldwide. Pregnancy is generally accompanied by dramatic hemodynamic changes, including a reduced afterload and a 50% increase in cardiac output. These increased cardiac stresses during pregnancy potentially impair protein folding processes within the cardiac tissue. The accumulation of misfolded proteins results in increased toxicity and cardiac insults that trigger heart failure. Under stress conditions, molecular chaperones such as heat shock proteins (Hsps) play crucial roles in maintaining cellular proteostasis. Here, we critically assess the potential role of Hsps in PPCM. We further predict specific associations between the Hsp types Hsp70, Hsp90 and small Hsps with several proteins implicated in PPCM pathophysiology. Furthermore, we explore the possibility of select Hsps as novel candidate PPCM biomarkers and drug targets. A better understanding of how these Hsps modulate PPCM pathogenesis holds promise in improving treatment, prognosis and management of the condition, and possibly other forms of acute heart failure.

Stem Cells of the Aging Brain

The adult central nervous system (CNS) contains resident stem cells within specific niches that maintain a self-renewal and proliferative capacity to generate new neurons, astrocytes, and oligodendrocytes throughout adulthood. Physiological aging is associated with a progressive loss of function and a decline in the self-renewal and regenerative capacities of CNS stem cells. Also, the biggest risk factor for neurodegenerative diseases is age, and current in vivo and in vitro models of neurodegenerative diseases rarely consider this. Therefore, combining both aging research and appropriate interrogation of animal disease models towards the understanding of the disease and age-related stem cell failure is imperative to the discovery of new therapies. This review article will highlight the main intrinsic and extrinsic regulators of neural stem cell (NSC) aging and discuss how these factors impact normal homeostatic functions within the adult brain. We will consider established in vivo animal and in vitro human disease model systems, and then discuss the current and future trajectories of novel senotherapeutics that target aging NSCs to ameliorate brain disease.

Sugar in mind: Untangling a sweet and sour relationship beyond type 2 diabetes

It is widely recognised that type 2 diabetes (T2D) represents a major disease burden but it is only recently that its role in neurodegeneration has attracted more attention. This research has shown that T2D is associated with impaired cerebral health, cognitive decline and dementia. However, the impact on the brain of progressive metabolic changes associated with the pre-clinical development of the disease is less clear. The aim of this review is to comprehensively summarise how the emergence of risk factors and co-morbid conditions linked to the development of T2D impact cerebral health. Particular attention is directed at characterising how normal but elevated blood glucose levels in individuals without T2D contribute to neurodegenerative processes, and how the main risk factors for T2D including obesity, physical activity and diet modulate these effects. Where available, evidence from the animal and human literature is contrasted, and sex differences in risk and outcomes are highlighted.

Featured Article: Deterioration of visual function mediated by senescence-associated endoplasmic reticulum stress in inflammatory tie2-TNF mice

Stress-associated premature senescence plays a major role in retinal diseases. In this study, we investigated the relationship between endothelial dysfunction, endoplasmic reticulum (ER) stress, and cellular senescence in the development of retinal dysfunction. We tested the hypothesis that constant endothelial activation by transmembrane tumor necrosis factor-α (tmTNF-α) exacerbates age-induced visual deficits via senescence-mediated ER stress in this model. To address this, we employed a mouse model of chronic vascular activation using endothelial-specific TNF-α-expressing (tie2-TNF) mice at 5 and 10 months of age. Visual deficits were exhibited by tie2-TNF mice at both 5 months and 10 months of age, with the older mice showing statistically significant loss of visual acuity compared with tie2-TNF mice at age 5 months. The neural defects, as measured by electroretinogram (ERG), also followed a similar trend in an age-dependent fashion, with 10-month-old tie2-TNF mice showing the greatest decrease in "b" wave amplitude at 25 cd.s.m² compared with age-matched wildtype (WT) mice and five-month-old tie2-TNF mice. While gene and protein expression from the whole retinal extracts demonstrated increased inflammatory (Icam1, Ccl2), stress-associated premature senescence (p16, p21, p53), and ER stress (Grp78, p-Ire1α, Chop) markers in five-month-old tie2-TNF mice compared with five-month-old WT mice, a further increase was seen in 10-month-old tie2-TNF mice. Our data demonstrate that tie2-TNF mice exhibit age-associated increases in visual deficits, and these data suggest that inflammatory endothelial activation is at least partly at play. Given the correlation of increased premature senescence and ER stress in an age-dependent fashion, with the loss of visual functions and increased endothelial activation, our data suggest a possible self-enhanced loop of unfolded protein response pathways and senescence in propagating neurovascular defects in this model. Impact statement Vision loss in most retinal diseases affects the quality of life of working age adults. Using a novel animal model that displays constant endothelial activation by tmTNF-α, our results demonstrate exacerbated age-induced visual deficits via premature senescence-mediated ER stress. We have compared mice of 5 and 10 months of age, with highly relevant human equivalencies of approximately 35- and 50-year-old patients, representing mature adult and middle-aged subjects, respectively. Our studies suggest a possible role for a self-enhanced loop of ER stress pathways and senescence in the propagation of retinal neurovascular defects, under conditions of constant endothelial activation induced by tmTNF-α signaling.

Different Expression Levels of Human Mutant Ubiquitin B+1 (UBB+1) Can Modify Chronological Lifespan or Stress Resistance of Saccharomyces cerevisiae

The ubiquitin-proteasome system (UPS) is the main pathway responsible for the degradation of misfolded proteins, and its dysregulation has been implicated in several neurodegenerative diseases, including Alzheimer's disease (AD). UBB⁺¹, a mutant variant of ubiquitin B, was found to accumulate in neurons of AD patients and it has been linked to UPS dysfunction and neuronal death. Using the yeast Saccharomyces cerevisiae as a model system, we constitutively expressed UBB⁺¹ to evaluate its effects on proteasome function and cell death, particularly under conditions of chronological aging. We showed that the expression of UBB⁺¹ caused inhibition of the three proteasomal proteolytic activities (caspase-like (β1), trypsin-like (β2) and chymotrypsin-like (β5) activities) in yeast. Interestingly, this inhibition did not alter cell viability of growing cells. Moreover, we showed that cells expressing UBB⁺¹ at lower level displayed an increased capacity to degrade induced misfolded proteins. When we evaluated cells during chronological aging, UBB⁺¹ expression at lower level, prevented cells to accumulate reactive oxygen species (ROS) and avert apoptosis, dramatically increasing yeast life span. Since proteasome inhibition by UBB⁺¹ has previously been shown to induce chaperone expression and thus protect against stress, we evaluated our UBB⁺¹ model under heat shock and oxidative stress. Higher expression of UBB⁺¹ caused thermotolerance in yeast due to induction of chaperones, which occurred to a lesser extent at lower expression level of UBB⁺¹ (where we observed the phenotype of extended life span). Altering UPS capacity by differential expression of UBB⁺¹ protects cells against several stresses during chronological aging. This system can be valuable to study the effects of UBB⁺¹ on misfolded proteins involved in neurodegeneration and aging.

Omics Approaches for Identifying Physiological Adaptations to Genome Instability in Aging

DNA damage causally contributes to aging and age-related diseases. The declining functioning of tissues and organs during aging can lead to the increased risk of succumbing to aging-associated diseases. Congenital syndromes that are caused by heritable mutations in DNA repair pathways lead to cancer susceptibility and accelerated aging, thus underlining the importance of genome maintenance for withstanding aging. High-throughput mass-spectrometry-based approaches have recently contributed to identifying signalling response networks and gaining a more comprehensive understanding of the physiological adaptations occurring upon unrepaired DNA damage. The insulin-like signalling pathway has been implicated in a DNA damage response (DDR) network that includes epidermal growth factor (EGF)-, AMP-activated protein kinases (AMPK)- and the target of rapamycin (TOR)-like signalling pathways, which are known regulators of growth, metabolism, and stress responses. The same pathways, together with the autophagy-mediated proteostatic response and the decline in energy metabolism have also been found to be similarly regulated during natural aging, suggesting striking parallels in the physiological adaptation upon persistent DNA damage due to DNA repair defects and long-term low-level DNA damage accumulation occurring during natural aging. These insights will be an important starting point to study the interplay between signalling networks involved in progeroid syndromes that are caused by DNA repair deficiencies and to gain new understanding of the consequences of DNA damage in the aging process.

Mitochondrial proteostasis as a shared characteristic of slowed aging: the importance of considering cell proliferation

Proteostasis is one of the seven "pillars of aging research" identified by the Trans-NIH Geroscience Initiative and loss of proteostasis is associated with aging and age-related chronic disease. Accumulated protein damage and resultant cellular dysfunction are consequences of limited protein repair systems and slowed protein turnover. When relatively high rates of protein turnover are maintained despite advancing age, damaged proteins are more quickly degraded and replaced, maintaining proteome fidelity. Therefore, maintenance of protein turnover represents an important proteostatic mechanism. However, measurement of protein synthesis without consideration for cell proliferation can result in an incomplete picture, devoid of information about how new proteins are being allocated. Simultaneous measurement of protein and DNA synthesis provides necessary mechanistic insight about proteins apportioned for newly proliferating cells versus for somatic maintenance. Using this approach with a number of murine models of slowed aging shows that, compared to controls, energetic resources are directed more toward somatic maintenance and proteostasis, and away from cell growth and proliferation. In particular, slowed aging models are associated with heightened mechanisms of mitochondrial proteostatic maintenance. Taking cell proliferation into account may explain the paradoxical findings that aging itself and slowed aging interventions can both be characterized by slower rates of protein synthesis.

Small Glutamine-Rich Tetratricopeptide Repeat-Containing Protein Alpha (SGTA) Ablation Limits Offspring Viability and Growth in Mice

Small glutamine-rich tetratricopeptide repeat-containing protein α (SGTA) has been implicated as a co-chaperone and regulator of androgen and growth hormone receptor (AR, GHR) signalling. We investigated the functional consequences of partial and full Sgta ablation in vivo using Cre-lox Sgta-null mice. Sgta(+/-) breeders generated viable Sgta(-/-) offspring, but at less than Mendelian expectancy. Sgta(-/-) breeders were subfertile with small litters and higher neonatal death (P < 0.02). Body size was significantly and proportionately smaller in male and female Sgta(-/-) (vs WT, Sgta(+/-) P < 0.001) from d19. Serum IGF-1 levels were genotype- and sex-dependent. Food intake, muscle and bone mass and adiposity were unchanged in Sgta(-/-). Vital and sex organs had normal relative weight, morphology and histology, although certain androgen-sensitive measures such as penis and preputial size, and testis descent, were greater in Sgta(-/-). Expression of AR and its targets remained largely unchanged, although AR localisation was genotype- and tissue-dependent. Generally expression of other TPR-containing proteins was unchanged. In conclusion, this thorough investigation of SGTA-null mutation reports a mild phenotype of reduced body size. The model's full potential likely will be realised by genetic crosses with other models to interrogate the role of SGTA in the many diseases in which it has been implicated.

Gene expression profiling to investigate tyrosol-induced lifespan extension in Caenorhabditis elegans

PURPOSE

We have previously reported that tyrosol (TYR) promotes lifespan extension in the nematode Caenorhabditis elegans, also inducing a stronger resistance to thermal and oxidative stress in vivo. In this study, we performed a whole-genome DNA microarray in order to narrow down the search for candidate genes or signaling pathways potentially involved in TYR effects on C. elegans longevity.

METHODS

Nematodes were treated with 0 or 250 μM TYR, total RNA was isolated at the adult stage, and derived cDNA probes were hybridized to Affymetrix C. elegans expression arrays. Microarray data analysis was performed, and relative mRNA expression of selected genes was validated using qPCR.

RESULTS

Microarray analysis identified 208 differentially expressed genes (206 over-expressed and two under-expressed) when comparing TYR-treated nematodes with vehicle-treated controls. Many of these genes are linked to processes such as regulation of growth, transcription, reproduction, lipid metabolism and body morphogenesis. Moreover, we detected an interesting overlap between the expression pattern elicited by TYR and those induced by other dietary polyphenols known to extend lifespan in C. elegans, such as quercetin and tannic acid.

CONCLUSIONS

Our results suggest that important cellular mechanisms directly related to longevity are influenced by TYR treatment in C. elegans, supporting our previous notion that this phenol might act on conserved genetic pathways to increase lifespan in a whole organism.

ER Stress Response in Human Cellular Models of Senescence

The aging process is characterized by progressive accumulation of damaged biomolecules in the endoplasmic reticulum, as result of increased oxidative stress accompanying cellular senescence. In agreement, we hypothesized that WI-38 human cellular models of replicative senescence and stress-induced premature senescence (SIPS) induced by hydrogen peroxide (H2O2-SIPS) or copper sulfate (CuSO4-SIPS) would present endoplasmic reticulum chaperoning mechanisms impairment and unfolded protein response activation. Results show that in replicative senescence and CuSO4-SIPS, immunoglobulin binding protein, calnexin, protein disulfide isomerase, and ER oxireductin-1 levels adjust to restore proteostasis and inositol-requiring enzyme-1 (IRE1)-, activating transcription factor 6 (ATF6)-, and pancreatic ER kinase (PERK)-mediated unfolded protein response are activated. However, H2O2-SIPS does not exhibit IRE1 and ATF6 pathways activation but a PERK-mediated upregulation of CCAAT/enhancer-binding protein homologous protein, showing that CuSO4-SIPS mimics better the endoplasmic reticulum molecular events of replicative senescence than H2O2-SIPS. Moreover, unfolded protein response activation is required for both SIPS models induction, because PERK and IRE1 inhibitors decreased senescence-associated beta-galactosidase appearance. In CuSO4-SIPS, the decrease in senescence levels is associated with PERK-driven, but IRE1 independent, cell cycle arrest while in H2O2-SIPS cell proliferation is PERK independent. These results add a step further on the molecular mechanisms that regulate senescence induction; moreover, they validate CuSO4-SIPS model as a useful tool to study cellular stress responses during aging, hoping to postpone age-related health decline.

Dystrophin deficiency leads to disturbance of LAMP1-vesicle-associated protein secretion

Duchenne muscular dystrophy results from loss of the protein dystrophin, which links the intracellular cytoskeletal network with the extracellular matrix, but deficiency in this function does not fully explain the onset or progression of the disease. While some intracellular events involved in the degeneration of dystrophin-deficient muscle fibers have been well characterized, changes in their secretory profile are undescribed. To analyze the secretome profile of mdx myotubes independently of myonecrosis, we labeled the proteins of mdx and wild-type myotubes with stable isotope-labeled amino acids (SILAC), finding marked enrichment of vesicular markers in the mdx secretome. These included the lysosomal-associated membrane protein, LAMP1, that co-localized in vesicles with an over-secreted cytoskeletal protein, myosin light chain 1. These LAMP1/MLC1-3-positive vesicles accumulated in the cytosol of mdx myotubes and were secreted into the culture medium in a range of abnormal densities. Restitution of dystrophin expression, by exon skipping, to some 30 % of the control value, partially normalized the secretome profile and the excess LAMP1 accumulation. Together, our results suggest that a lack of dystrophin leads to a general dysregulation of vesicle trafficking. We hypothesize that disturbance of the export of proteins through vesicles occurs before, and then concurrently with, the myonecrotic cascade and contributes chronically to the pathophysiology of DMD, thereby presenting us with a range of new potential therapeutic targets.

Rationale and methods of discovering hormetins as drugs for healthy ageing

INTRODUCTION

Mild stress-induced hormesis is becoming increasingly attractive as an ageing interventional strategy and is leading to the discovery of hormesis-inducing compounds called hormetins. Almost 50 years of modern biogerontolgical research has established a clear framework regarding the biological basis of ageing and longevity, and it is now generally accepted that ageing occurs in spite of the presence of complex pathways of maintenance, repair and defense, and there is no 'enemy within.' This viewpoint makes modulation of ageing different from the treatment of one or more age-related diseases. A promising strategy to slow down ageing and prevent or delay the onset of age-related diseases is that of mild stress-induced hormesis by using hormetins.

AREAS COVERED

The article presents the rationale and a strategy for discovering novel hormetins as potential drugs for ageing intervention by elucidating multiple stress responses of normal human cells. Furthermore, it discusses the first steps in identifying prospective hormetin drugs and provides a recent example of successful product development, based on the ideas of hormesis and by following the strategy described here.

EXPERT OPINION

As a biomedical issue, the biological process of ageing underlies several major diseases, and although the optimal treatment of every disease, irrespective of age, is a social and moral necessity, preventing the onset of age-related diseases by intervening in the basic process of ageing is the best approach for achieving healthy ageing and for extending the healthspan.