Spatial regulation of the actin cytoskeleton by HSF-1 during aging

Argonaute CSR-1A promotes H3K9me3 maintenance to protect somatic development in offspring

Parental stress can be encoded into altered epigenetic information to influence their offspring. Concurrently, it is vital for the preservation of a parent's epigenetic information, despite environmental challenges, to ensure accurate inheritance by the next generation. Nevertheless, the complexities of this process and the specific molecular mechanisms involved are not yet fully understood. Here we report that Argonaute CSR-1A potentiates the recovery of histone H3 lysine 9 trimethylation (H3K9me3) in spermatocyte to secure the developmental competence of male offspring. CSR-1A employs its repetitive RG motif to engage with putative histone 3 lysine 9 (H3K9) methyltransferases SET-25 and -32, and helps to restore repressive H3K9me3 chromatin marks following heat-stress, protecting the late development of somatic cells in the progeny. Finally, among the genes regulated by CSR-1A, we identified dim-1, at which decreased H3K9me3 persists in the progeny, and RNAi of dim-1 mitigates the somatic defects associated with csr-1a loss under stress. Thus, CSR-1A coordinates a paternal epigenetic program that shields development from the influences of the paternal environment. We speculate that, driven by both natural environmental stressors and the unique characteristics of spermatogenic chromatin, the emergence of multiple RG motif-featured and spermatogenesis-specific CSR-1A and small RNA serves as a protective strategy to safeguard against variability in the orchestration of inherited developmental programs from the paternal lineage.

Overexpression of Lifeact in the C. elegans body wall muscle causes sarcomere disorganization and embryonic or larval lethality

Lifeact is a short peptide that is widely utilized as a probe for actin filaments in live imaging. However, high concentrations of Lifeact can alter actin filament dynamics and cause artificial modifications to the actin cytoskeleton. Here, I evaluated Caenorhabditis elegans strains expressing Lifeact fused to fluorescent proteins in the body wall muscle. I found that, while low-level expression of Lifeact from a single-copy transgene was appropriate for labeling sarcomeric actin filaments, overexpression of Lifeact from an extrachromosomal array causes severe disorganization of muscle sarcomeres and lethality at an embryonic or larval stage. Therefore, for imaging studies in C. elegans, Lifeact needs to be kept at a low level by proper management of the expression system.

HSF-1 promotes longevity through ubiquilin-1-dependent mitochondrial network remodelling

Increased activity of the heat shock factor, HSF-1, suppresses proteotoxicity and enhances longevity. However, the precise mechanisms by which HSF-1 promotes lifespan are unclear. Using an RNAi screen, we identify ubiquilin-1 (ubql-1) as an essential mediator of lifespan extension in worms overexpressing hsf-1. We find that hsf-1 overexpression leads to transcriptional downregulation of all components of the CDC-48-UFD-1-NPL-4 complex, which is central to both endoplasmic reticulum and mitochondria associated protein degradation, and that this is complemented by UBQL-1-dependent turnover of NPL-4.1. As a consequence, mitochondrial network dynamics are altered, leading to increased lifespan. Together, our data establish that HSF-1 mediates lifespan extension through mitochondrial network adaptations that occur in response to down-tuning of components associated with organellar protein degradation pathways.

Accumulation of F-actin drives brain aging and limits healthspan in Drosophila

The actin cytoskeleton is a key determinant of cell structure and homeostasis. However, possible tissue-specific changes to actin dynamics during aging, notably brain aging, are not understood. Here, we show that there is an age-related increase in filamentous actin (F-actin) in Drosophila brains, which is counteracted by prolongevity interventions. Critically, decreasing F-actin levels in aging neurons prevents age-onset cognitive decline and extends organismal healthspan. Mechanistically, we show that autophagy, a recycling process required for neuronal homeostasis, is disabled upon actin dysregulation in the aged brain. Remarkably, disrupting actin polymerization in aged animals with cytoskeletal drugs restores brain autophagy to youthful levels and reverses cellular hallmarks of brain aging. Finally, reducing F-actin levels in aging neurons slows brain aging and promotes healthspan in an autophagy-dependent manner. Our data identify excess actin polymerization as a hallmark of brain aging, which can be targeted to reverse brain aging phenotypes and prolong healthspan.

Age-dependent heat shock hormesis to HSF-1 deficiency suggests a compensatory mechanism mediated by the unfolded protein response and innate immunity in young Caenorhabditis elegans

The transcription factor HSF-1 (heat shock factor 1) acts as a master regulator of heat shock response in eukaryotic cells to maintain cellular proteostasis. The protein has a protective role in preventing cells from undergoing ageing, and neurodegeneration, and also mediates tumorigenesis. Thus, modulating HSF-1 activity in humans has a promising therapeutic potential for treating these pathologies. Loss of HSF-1 function is usually associated with impaired stress tolerance. Contrary to this conventional knowledge, we show here that inactivation of HSF-1 in the nematode Caenorhabditis elegans results in increased thermotolerance at young adult stages, whereas HSF-1 deficiency in animals passing early adult stages indeed leads to decreased thermotolerance, as compared to wild-type. Furthermore, a gene expression analysis supports that in young adults, distinct cellular stress response and immunity-related signaling pathways become induced upon HSF-1 deficiency. We also demonstrate that increased tolerance to proteotoxic stress in HSF-1-depleted young worms requires the activity of the unfolded protein response of the endoplasmic reticulum and the SKN-1/Nrf2-mediated oxidative stress response pathway, as well as an innate immunity-related pathway, suggesting a mutual compensatory interaction between HSF-1 and these conserved stress response systems. A similar compensatory molecular network is likely to also operate in higher animal taxa, raising the possibility of an unexpected outcome when HSF-1 activity is manipulated in humans.

Growth on stiffer substrates impacts animal health and longevity in C. elegans

Mechanical stress is a measure of internal resistance exhibited by a body or material when external forces, such as compression, tension, bending, etc. are applied. The study of mechanical stress on health and aging is a continuously growing field, as major changes to the extracellular matrix and cell-to-cell adhesions can result in dramatic changes to tissue stiffness during aging and diseased conditions. For example, during normal aging, many tissues including the ovaries, skin, blood vessels, and heart exhibit increased stiffness, which can result in a significant reduction in function of that organ. As such, numerous model systems have recently emerged to study the impact of mechanical and physical stress on cell and tissue health, including cell-culture conditions with matrigels and other surfaces that alter substrate stiffness and ex vivo tissue models that can apply stress directly to organs like muscle or tendons. Here, we sought to develop a novel method in an in vivo model organism setting to study the impact of altering substrate stiffness on aging by changing the stiffness of solid agar medium used for growth of C. elegans. We found that greater substrate stiffness had limited effects on cellular health, gene expression, organismal health, stress resilience, and longevity. Overall, our study reveals that altering substrate stiffness of growth medium for C. elegans has only mild impact on animal health and longevity; however, these impacts were not nominal and open up important considerations for C. elegans biologists in standardizing agar medium choice for experimental assays.

How do neurons age? A focused review on the aging of the microtubular cytoskeleton

Aging is the leading risk factor for Alzheimer's disease and other neurodegenerative diseases. We now understand that a breakdown in the neuronal cytoskeleton, mainly underpinned by protein modifications leading to the destabilization of microtubules, is central to the pathogenesis of Alzheimer's disease. This is accompanied by morphological defects across the somatodendritic compartment, axon, and synapse. However, knowledge of what occurs to the microtubule cytoskeleton and morphology of the neuron during physiological aging is comparatively poor. Several recent studies have suggested that there is an age-related increase in the phosphorylation of the key microtubule stabilizing protein tau, a modification, which is known to destabilize the cytoskeleton in Alzheimer's disease. This indicates that the cytoskeleton and potentially other neuronal structures reliant on the cytoskeleton become functionally compromised during normal physiological aging. The current literature shows age-related reductions in synaptic spine density and shifts in synaptic spine conformation which might explain age-related synaptic functional deficits. However, knowledge of what occurs to the microtubular and actin cytoskeleton, with increasing age is extremely limited. When considering the somatodendritic compartment, a regression in dendrites and loss of dendritic length and volume is reported whilst a reduction in soma volume/size is often seen. However, research into cytoskeletal change is limited to a handful of studies demonstrating reductions in and mislocalizations of microtubule-associated proteins with just one study directly exploring the integrity of the microtubules. In the axon, an increase in axonal diameter and age-related appearance of swellings is reported but like the dendrites, just one study investigates the microtubules directly with others reporting loss or mislocalization of microtubule-associated proteins. Though these are the general trends reported, there are clear disparities between model organisms and brain regions that are worthy of further investigation. Additionally, longitudinal studies of neuronal/cytoskeletal aging should also investigate whether these age-related changes contribute not just to vulnerability to disease but also to the decline in nervous system function and behavioral output that all organisms experience. This will highlight the utility, if any, of cytoskeletal fortification for the promotion of healthy neuronal aging and potential protection against age-related neurodegenerative disease. This review seeks to summarize what is currently known about the physiological aging of the neuron and microtubular cytoskeleton in the hope of uncovering mechanisms underpinning age-related risk to disease.

Mechanical stress through growth on stiffer substrates impacts animal health and longevity in C. elegans

Mechanical stress is a measure of internal resistance exhibited by a body or material when external forces, such as compression, tension, bending, etc. are applied. The study of mechanical stress on health and aging is a continuously growing field, as major changes to the extracellular matrix and cell-to-cell adhesions can result in dramatic changes to tissue stiffness during aging and diseased conditions. For example, during normal aging, many tissues including the ovaries, skin, blood vessels, and heart exhibit increased stiffness, which can result in a significant reduction in function of that organ. As such, numerous model systems have recently emerged to study the impact of mechanical and physical stress on cell and tissue health, including cell-culture conditions with matrigels and other surfaces that alter substrate stiffness and ex vivo tissue models that can apply stress directly to organs like muscle or tendons. Here, we sought to develop a novel method in an in vivo, model organism setting to study the impact of mechanical stress on aging, by increasing substrate stiffness in solid agar medium of C. elegans. To our surprise, we found shockingly limited impact of growth of C. elegans on stiffer substrates, including limited effects on cellular health, gene expression, organismal health, stress resilience, and longevity. Overall, our studies reveal that altering substrate stiffness of growth medium for C. elegans have only mild impact on animal health and longevity; however, these impacts were not nominal and open up important considerations for C. elegans biologists in standardizing agar medium choice for experimental assays.

Anemia and Its Connections to Inflammation in Older Adults: A Review

Anemia is a common hematological disorder that affects 12% of the community-dwelling population, 40% of hospitalized patients, and 47% of nursing home residents. Our understanding of the impact of inflammation on iron metabolism and erythropoiesis is still lacking. In older adults, anemia can be divided into nutritional deficiency anemia, bleeding anemia, and unexplained anemia. The last type of anemia might be caused by reduced erythropoietin (EPO) activity, progressive EPO resistance of bone marrow erythroid progenitors, and the chronic subclinical pro-inflammatory state. Overall, one-third of older patients with anemia demonstrate a nutritional deficiency, one-third have a chronic subclinical pro-inflammatory state and chronic kidney disease, and one-third suffer from anemia of unknown etiology. Understanding anemia's pathophysiology in people aged 65 and over is crucial because it contributes to frailty, falls, cognitive decline, decreased functional ability, and higher mortality risk. Inflammation produces adverse effects on the cells of the hematological system. These effects include iron deficiency (hypoferremia), reduced EPO production, and the elevated phagocytosis of erythrocytes by hepatic and splenic macrophages. Additionally, inflammation causes enhanced eryptosis due to oxidative stress in the circulation. Identifying mechanisms behind age-related inflammation is essential for a better understanding and preventing anemia in older adults.

Mitochondrial protein and organelle quality control-Lessons from budding yeast

Mitochondria are essential for normal cellular function and have emerged as key aging determinants. Indeed, defects in mitochondrial function have been linked to cardiovascular, skeletal muscle and neurodegenerative diseases, premature aging, and age-linked diseases. Here, we describe mechanisms for mitochondrial protein and organelle quality control. These surveillance mechanisms mediate repair or degradation of damaged or mistargeted mitochondrial proteins, segregate mitochondria based on their functional state during asymmetric cell division, and modulate cellular fitness, the response to stress, and lifespan control in yeast and other eukaryotes.

Imaging actin organisation and dynamics in 3D

The actin cytoskeleton plays a critical role in cell architecture and the control of fundamental processes including cell division, migration and survival. The dynamics and organisation of F-actin have been widely studied in a breadth of cell types on classical two-dimensional (2D) surfaces. Recent advances in optical microscopy have enabled interrogation of these cytoskeletal networks in cells within three-dimensional (3D) scaffolds, tissues and in vivo. Emerging studies indicate that the dimensionality experienced by cells has a profound impact on the structure and function of the cytoskeleton, with cells in 3D environments exhibiting cytoskeletal arrangements that differ to cells in 2D environments. However, the addition of a third (and fourth, with time) dimension leads to challenges in sample preparation, imaging and analysis, necessitating additional considerations to achieve the required signal-to-noise ratio and spatial and temporal resolution. Here, we summarise the current tools for imaging actin in a 3D context and highlight examples of the importance of this in understanding cytoskeletal biology and the challenges and opportunities in this domain.

Accumulation of F-actin drives brain aging and limits healthspan in Drosophila

The actin cytoskeleton is a key determinant of cell and tissue homeostasis. However, tissue-specific roles for actin dynamics in aging, notably brain aging, are not understood. Here, we show that there is an age-related increase in filamentous actin (F-actin) in Drosophila brains, which is counteracted by prolongevity interventions. Critically, modulating F-actin levels in aging neurons prevents age-onset cognitive decline and extends organismal healthspan. Mechanistically, we show that autophagy, a recycling process required for neuronal homeostasis, is disabled upon actin dysregulation in the aged brain. Remarkably, disrupting actin polymerization in aged animals with cytoskeletal drugs restores brain autophagy to youthful levels and reverses cellular hallmarks of brain aging. Finally, reducing F-actin levels in aging neurons slows brain aging and promotes healthspan in an autophagy-dependent manner. Our data identify excess actin polymerization as a hallmark of brain aging, which can be targeted to reverse brain aging phenotypes and prolong healthspan.

Lipid homeostasis is essential for a maximal ER stress response

Changes in lipid metabolism are associated with aging and age-related diseases, including proteopathies. The endoplasmic reticulum (ER) is uniquely a major hub for protein and lipid synthesis, making its function essential for both protein and lipid homeostasis. However, it is less clear how lipid metabolism and protein quality may impact each other. Here, we identified let-767, a putative hydroxysteroid dehydrogenase in Caenorhabditis elegans, as an essential gene for both lipid and ER protein homeostasis. Knockdown of let-767 reduces lipid stores, alters ER morphology in a lipid-dependent manner, and blocks induction of the Unfolded Protein Response of the ER (UPRER). Interestingly, a global reduction in lipogenic pathways restores UPRER induction in animals with reduced let-767. Specifically, we find that supplementation of 3-oxoacyl, the predicted metabolite directly upstream of let-767, is sufficient to block induction of the UPRER. This study highlights a novel interaction through which changes in lipid metabolism can alter a cell's response to protein-induced stress.

Age-associated anatomical and physiological alterations in Caenorhabditis elegans

Since its introduction by Sydney Brenner, Caenorhabditis elegans has become a widely studied organism. Given its highly significant properties, including transparency, short lifespan, self-fertilization, high reproductive yield and ease in manipulation and genetic modifications, the nematode has contributed to the elucidation of several fundamental aspects of biology, such as development and ageing. Moreover, it has been extensively used as a platform for the modelling of ageing-associated human disorders, especially those related to neurodegeneration. The use of C. elegans for such purposes requires, and at the same time promotes the investigation of its normal ageing process. In this review we aim to summarize the major organismal alterations during normal worm ageing, in terms of morphology and functionality.

Evolutionary conservation of the fidelity of transcription

Accurate transcription is required for the faithful expression of genetic information. However, relatively little is known about the molecular mechanisms that control the fidelity of transcription, or the conservation of these mechanisms across the tree of life. To address these issues, we measured the error rate of transcription in five organisms of increasing complexity and found that the error rate of RNA polymerase II ranges from 2.9 × 10-6 ± 1.9 × 10-7/bp in yeast to 4.0 × 10-6 ± 5.2 × 10-7/bp in worms, 5.69 × 10-6 ± 8.2 × 10-7/bp in flies, 4.9 × 10-6 ± 3.6 × 10-7/bp in mouse cells and 4.7 × 10-6 ± 9.9 × 10-8/bp in human cells. These error rates were modified by various factors including aging, mutagen treatment and gene modifications. For example, the deletion or modification of several related genes increased the error rate substantially in both yeast and human cells. This research highlights the evolutionary conservation of factors that control the fidelity of transcription. Additionally, these experiments provide a reasonable estimate of the error rate of transcription in human cells and identify disease alleles in a subunit of RNA polymerase II that display error-prone transcription. Finally, we provide evidence suggesting that the error rate and spectrum of transcription co-evolved with our genetic code.

Genomic instability caused by Arp2/3 complex inactivation results in micronucleus biogenesis and cellular senescence

The Arp2/3 complex is an actin nucleator with well-characterized activities in cell morphogenesis and movement, but its roles in nuclear processes are relatively understudied. We investigated how the Arp2/3 complex affects genomic integrity and cell cycle progression using mouse fibroblasts containing an inducible knockout (iKO) of the ArpC2 subunit. We show that permanent Arp2/3 complex ablation results in DNA damage, the formation of cytosolic micronuclei, and cellular senescence. Micronuclei arise in ArpC2 iKO cells due to chromatin segregation defects during mitosis and premature mitotic exits. Such phenotypes are explained by the presence of damaged DNA fragments that fail to attach to the mitotic spindle, abnormalities in actin assembly during metaphase, and asymmetric microtubule architecture during anaphase. In the nuclei of Arp2/3-depleted cells, the tumor suppressor p53 is activated and the cell cycle inhibitor Cdkn1a/p21 mediates a G1 arrest. In the cytosol, micronuclei are recognized by the DNA sensor cGAS, which is important for stimulating a STING- and IRF3-associated interferon response. These studies establish functional requirements for the mammalian Arp2/3 complex in mitotic spindle organization and genome stability. They also expand our understanding of the mechanisms leading to senescence and suggest that cytoskeletal dysfunction is an underlying factor in biological aging.

Large-scale genetic screens identify BET-1 as a cytoskeleton regulator promoting actin function and life span

The actin cytoskeleton is a three-dimensional scaffold of proteins that is a regulatory, energyconsuming network with dynamic properties to shape the structure and function of the cell. Proper actin function is required for many cellular pathways, including cell division, autophagy, chaperone function, endocytosis, and exocytosis. Deterioration of these processes manifests during aging and exposure to stress, which is in part due to the breakdown of the actin cytoskeleton. However, the regulatory mechanisms involved in preservation of cytoskeletal form and function are not well-understood. Here, we performed a multipronged, cross-organismal screen combining a whole-genome CRISPR-Cas9 screen in human fibroblasts with in vivo Caenorhabditis elegans synthetic lethality screening. We identified the bromodomain protein, BET-1, as a key regulator of actin function and longevity. Overexpression of bet-1 preserves actin function at late age and promotes life span and healthspan in C. elegans. These beneficial effects are mediated through actin preservation by the transcriptional regulator function of BET-1. Together, our discovery assigns a key role for BET-1 in cytoskeletal health, highlighting regulatory cellular networks promoting cytoskeletal homeostasis.

The Thermal Stress Coping Network of the Nematode Caenorhabditis elegans

Response to hyperthermia, highly conserved from bacteria to humans, involves transcriptional upregulation of genes involved in battling the cytotoxicity caused by misfolded and denatured proteins, with the aim of proteostasis restoration. C. elegans senses and responds to changes in growth temperature or noxious thermal stress by well-defined signaling pathways. Under adverse conditions, regulation of the heat shock response (HSR) in C. elegans is controlled by a single transcription factor, heat-shock factor 1 (HSF-1). HSR and HSF-1 in particular are proven to be central to survival under proteotoxic stress, with additional roles in normal physiological processes. For years, it was a common belief that upregulation of heat shock proteins (HSPs) by HSF-1 was the main and most important step toward thermotolerance. However, an ever-growing number of studies have shown that targets of HSF-1 involved in cytoskeletal and exoskeletal integrity preservation as well as other HSF-1 dependent and independent pathways are equally important. In this review, we follow the thermal stimulus from reception by the nematode nerve endings till the activation of cellular response programs. We analyze the different HSF-1 functions in HSR as well as all the recently discovered mechanisms that add to the knowledge of the heat stress coping network of C. elegans.

Cell Architecture-Dependent Constraints: Critical Safeguards to Carcinogenesis

Animal cells display great diversity in their shape. These morphological characteristics result from crosstalk between the plasma membrane and the force-generating capacities of the cytoskeleton macromolecules. Changes in cell shape are not merely byproducts of cell fate determinants, they also actively drive cell fate decisions, including proliferation and differentiation. Global and local changes in cell shape alter the transcriptional program by a multitude of mechanisms, including the regulation of physical links between the plasma membrane and the nuclear envelope and the mechanical modulation of cation channels and signalling molecules. It is therefore not surprising that anomalies in cell shape contribute to several diseases, including cancer. In this review, we discuss the possibility that the constraints imposed by cell shape determine the behaviour of normal and pro-tumour cells by organizing the whole interconnected regulatory network. In turn, cell behaviour might stabilize cells into discrete shapes. However, to progress towards a fully transformed phenotype and to acquire plasticity properties, pro-tumour cells might need to escape these cell shape restrictions. Thus, robust controls of the cell shape machinery may represent a critical safeguard against carcinogenesis.

Auxin-Inducible Degron System Reveals Temporal-Spatial Roles of HSF-1 and Its Transcriptional Program in Lifespan Assurance

HSF-1 is a key regulator of cellular proteotoxic stress response and is required for animal lifespan. In C. elegans, HSF-1 mediated heat shock response (HSR) declines sharply on the first day of adulthood, and HSF-1 was proposed to function primarily during larval stages for lifespan assurance based on studies using RNAi. The tissue requirement for HSF-1 in lifespan, however, is not well understood. Using the auxin-inducible degron (AID) system, we manage to uncouple the roles of HSF-1 in development and longevity. In wild-type animals, we find HSF-1 is required during the whole self-reproductive period for lifespan. This period is extended in long-lived animals that have arrested germline stem cells (GSC) or reduced insulin/IGF-1 signaling (IIS). While depletion of HSF-1 from any major somatic tissues during development results in severe defects, HSF-1 primarily functions in the intestine and likely neural system of adults to support lifespan. Finally, by combining AID and genome-wide transcriptional analyses, we find HSF-1 directly activates the transcription of constitutively-expressed chaperone and co-chaperone genes among others in early adulthood, which underlies its roles in longevity assurance.

Surveying Low-Cost Methods to Measure Lifespan and Healthspan in Caenorhabditis elegans

The discovery and development of Caenorhabditis elegans as a model organism was influential in biology, particularly in the field of aging. Many historical and contemporary studies have identified thousands of lifespan-altering paradigms, including genetic mutations, transgenic gene expression, and hormesis, a beneficial, low-grade exposure to stress. With its many advantages, including a short lifespan, easy and low-cost maintenance, and fully sequenced genome with homology to almost two-thirds of all human genes, C. elegans has quickly been adopted as an outstanding model for stress and aging biology. Here, several standardized methods are surveyed for measuring lifespan and healthspan that can be easily adapted into almost any research environment, especially those with limited equipment and funds. The incredible utility of C. elegans is featured, highlighting the capacity to perform powerful genetic analyses in aging biology without the necessity of extensive infrastructure. Finally, the limitations of each analysis and alternative approaches are discussed for consideration.

Identification of a modulator of the actin cytoskeleton, mitochondria, nutrient metabolism and lifespan in yeast

In yeast, actin cables are F-actin bundles that are essential for cell division through their function as tracks for cargo movement from mother to daughter cell. Actin cables also affect yeast lifespan by promoting transport and inheritance of higher-functioning mitochondria to daughter cells. Here, we report that actin cable stability declines with age. Our genome-wide screen for genes that affect actin cable stability identified the open reading frame YKL075C. Deletion of YKL075C results in increases in actin cable stability and abundance, mitochondrial fitness, and replicative lifespan. Transcriptome analysis revealed a role for YKL075C in regulating branched-chain amino acid (BCAA) metabolism. Consistent with this, modulation of BCAA metabolism or decreasing leucine levels promotes actin cable stability and function in mitochondrial quality control. Our studies support a role for actin stability in yeast lifespan, and demonstrate that this process is controlled by BCAA and a previously uncharacterized ORF YKL075C, which we refer to as actin, aging and nutrient modulator protein 1 (AAN1).

Actin cables affect lifespan by supporting movement and inheritance of fitter mitochondria to daughter cells in yeast. Here the authors show that branched-chain amino acid (BCAA) levels affect actin cable stability and a role for YKL075C/AAN1 in control of BCAA metabolism and actin cable stability and function.

Dyskerin Downregulation Can Induce ER Stress and Promote Autophagy via AKT-mTOR Signaling Deregulation

Dyskerin is an evolutionarily conserved nucleolar protein implicated in a wide range of fundamental biological roles, including telomere maintenance and ribosome biogenesis. Germline mutations of DKC1, the human gene encoding dyskerin, cause the hereditary disorders known as X-linked dyskeratosis congenita (X-DC). Moreover, dyskerin is upregulated in several cancers. Due to the pleiotropic functions of dyskerin, the X-DC clinical features overlap with those of both telomeropathies and ribosomopathies. In this paper, we evaluate the telomerase-independent effects of dyskerin depletion on cellular physiology by using inducible DCK1 knockdown. This system allows the downregulation of DKC1 expression within a short timeframe. We report that, in these cellular systems, dyskerin depletion induces the accumulation of unfolded/misfolded proteins in the endoplasmic reticulum, which in turn induces the activation of the PERK branch of the unfolded protein response. We also demonstrate that the PERK-eIF2a-ATF4-CHOP signaling pathway, activated by dyskerin downregulation, triggers a functional autophagic flux through the inhibition of the PI3K/AKT/mTOR pathway. By revealing a novel unpredicted connection between the loss of dyskerin, autophagy and UPR, our results establish a firm link between the lowering of dyskerin levels and the activation of the ER stress response, that plays a key role in the pathogenesis of several diseases.

Hijacking Cellular Stress Responses to Promote Lifespan

Organisms are constantly exposed to stress both from the external environment and internally within the cell. To maintain cellular homeostasis under different environmental and physiological conditions, cell have adapted various stress response signaling pathways, such as the heat shock response (HSR), unfolded protein responses of the mitochondria (UPR^MT), and the unfolded protein response of the endoplasmic reticulum (UPR^ER). As cells grow older, all cellular stress responses have been shown to deteriorate, which is a major cause for the physiological consequences of aging and the development of numerous age-associated diseases. In contrast, elevated stress responses are often associated with lifespan extension and amelioration of degenerative diseases in different model organisms, including C. elegans. Activating cellular stress response pathways could be considered as an effective intervention to alleviate the burden of aging by restoring function of essential damage-clearing machinery, including the ubiquitin-proteosome system, chaperones, and autophagy. Here, we provide an overview of newly emerging concepts of these stress response pathways in healthy aging and longevity with a focus on the model organism, C. elegans.

Imaging of Actin Cytoskeletal Integrity During Aging in C. elegans

The actin cytoskeleton plays a fundamental role in the regulation of multiple cellular pathways, including trafficking and locomotion. The functional integrity of the cytoskeleton is important during aging, as the decline of cytoskeletal integrity contributes to the physiological consequence of aging. Moreover, improving cytoskeletal form and function throughout aging is sufficient to drive life span extension and promote organismal health in multiple model systems. For these reasons, optimized protocols for visualization of the actin cytoskeleton and its downstream consequences on health span and life span are critical for understanding the aging process. In C. elegans, the actin cytoskeleton shows diverse morphologies across tissues, potentially due to the significantly different functions of each cell type. This chapter describes an imaging platform utilizing LifeAct to visualize the actin cytoskeleton in live, whole nematodes throughout the aging process and methods to perform follow-up studies on the life span and health span of these organisms.

Cross-species screening platforms identify EPS-8 as a critical link for mitochondrial stress and actin stabilization

The dysfunction of mitochondria is associated with the physiological consequences of aging and many age-related diseases. Therefore, critical quality control mechanisms exist to protect mitochondrial functions, including the unfolded protein response of the mitochondria (UPRMT). However, it is still unclear how UPRMT is regulated in mammals with mechanistic discrepancies between previous studies. Here, we reasoned that a study of conserved mechanisms could provide a uniquely powerful way to reveal previously uncharacterized components of the mammalian UPRMT. We performed cross-species comparison of genetic requirements for survival under—and in response to—mitochondrial stress between karyotypically normal human stem cells and the nematode Caenorhabditis elegans. We identified a role for EPS-8/EPS8 (epidermal growth factor receptor pathway substrate 8), a signaling protein adaptor, in general mitochondrial homeostasis and UPRMT regulation through integrin-mediated remodeling of the actin cytoskeleton. This study also highlights the use of cross-species comparisons in genetic screens to interrogate cellular pathways.

Overexpression of Lifeact-GFP Disrupts F-Actin Organization in Cardiomyocytes and Impairs Cardiac Function

Lifeact-GFP is a frequently used molecular probe to study F-actin structure and dynamic assembly in living cells. In this study, we generated transgenic zebrafish models expressing Lifeact-GFP specifically in cardiac muscles to investigate the effect of Lifeact-GFP on heart development and its application to study cardiomyopathy. The data showed that transgenic zebrafish with low to moderate levels of Lifeact-GFP expression could be used as a good model to study contractile dynamics of actin filaments in cardiac muscles in vivo. Using this model, we demonstrated that loss of Smyd1b, a lysine methyltransferase, disrupted F-actin filament organization in cardiomyocytes of zebrafish embryos. Our studies, however, also demonstrated that strong Lifeact-GFP expression in cardiomyocytes was detrimental to actin filament organization in cardiomyocytes that led to pericardial edema and early embryonic lethality of zebrafish embryos. Collectively, these data suggest that although Lifeact-GFP is a good probe for visualizing F-actin dynamics, transgenic models need to be carefully evaluated to avoid artifacts induced by Lifeact-GFP overexpression.

Imaging of Actin Cytoskeleton in the Nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans is one of the major model organisms in cell and developmental biology. This organism is easy to culture in laboratories and suitable for microscopic investigation of the cytoskeleton. Because the worms are small and transparent, the actin cytoskeleton in many tissues and cells can be observed with appropriate visualization techniques without sectioning or dissection. This chapter describes the introduction to representative methods for imaging the actin cytoskeleton in C. elegans and a protocol for staining worms with fluorescent phalloidin.

iTRAQ-based proteomics of testicular interstitial fluid during aging in mice

Advancing age is associated with a decline in fertility and testicular function in males. The testicular interstitial fluid (TIF) bathing the seminiferous tubules and testicular interstitium is considered an important part of the testicular microenvironment. However, the TIF proteome in mice and whether it changes with age remain unclear. This study aimed to map the TIF proteome and identify differentially abundant proteins (DEPs) among young, middle-aged, and old mice using isobaric tags for relative and absolute quantification (iTRAQ) coupled with liquid chromatography-tandem mass spectrometry. A total of 1477 proteins were identified in murine TIF. The abundance of 706 proteins showed a linear change trend with age, of which 360 and 346 proteins increased and decreased, respectively. In addition, 45 age-related DEPs were identified (P < 0.05, with at least 1.2-fold up- or downregulation). Bioinformatic analyses revealed that these proteins were involved in "actin cytoskeleton organization," "intrinsic apoptotic signaling pathway," and "regulation of protein transport." Comparative analysis with relevant proteomes previously reported further revealed the characteristics of mouse TIF proteome. Moreover, two of the age-related DEPs (Fga and Qsox1) were also found to be age-related differentially expressed proteins in human blood plasma and senescence-related secretome of human peritubular myoid cells. Taken together, these findings may represent the foundation for a better understanding of the molecular function of TIF and testicular aging.

Measuring expression heterogeneity of single-cell cytoskeletal protein complexes

Multimeric cytoskeletal protein complexes orchestrate normal cellular function. However, protein-complex distributions in stressed, heterogeneous cell populations remain unknown. Cell staining and proximity-based methods have limited selectivity and/or sensitivity for endogenous multimeric protein-complex quantification from single cells. We introduce micro-arrayed, differential detergent fractionation to simultaneously detect protein complexes in hundreds of individual cells. Fractionation occurs by 60 s size-exclusion electrophoresis with protein complex-stabilizing buffer that minimizes depolymerization. Proteins are measured with a ~5-hour immunoassay. Co-detection of cytoskeletal protein complexes in U2OS cells treated with filamentous actin (F-actin) destabilizing Latrunculin A detects a unique subpopulation (~2%) exhibiting downregulated F-actin, but upregulated microtubules. Thus, some cells may upregulate other cytoskeletal complexes to counteract the stress of Latrunculin A treatment. We also sought to understand the effect of non-chemical stress on cellular heterogeneity of F-actin. We find heat shock may dysregulate filamentous and globular actin correlation. In this work, our assay overcomes selectivity limitations to biochemically quantify single-cell protein complexes perturbed with diverse stimuli.

Rewiring of the ubiquitinated proteome determines ageing in C. elegans

Ageing is driven by a loss of cellular integrity1. Given the major role of ubiquitin modifications in cell function2, here we assess the link between ubiquitination and ageing by quantifying whole-proteome ubiquitin signatures in Caenorhabditis elegans. We find a remodelling of the ubiquitinated proteome during ageing, which is ameliorated by longevity paradigms such as dietary restriction and reduced insulin signalling. Notably, ageing causes a global loss of ubiquitination that is triggered by increased deubiquitinase activity. Because ubiquitination can tag proteins for recognition by the proteasome3, a fundamental question is whether deficits in targeted degradation influence longevity. By integrating data from worms with a defective proteasome, we identify proteasomal targets that accumulate with age owing to decreased ubiquitination and subsequent degradation. Lowering the levels of age-dysregulated proteasome targets prolongs longevity, whereas preventing their degradation shortens lifespan. Among the proteasomal targets, we find the IFB-2 intermediate filament4 and the EPS-8 modulator of RAC signalling5. While increased levels of IFB-2 promote the loss of intestinal integrity and bacterial colonization, upregulation of EPS-8 hyperactivates RAC in muscle and neurons, and leads to alterations in the actin cytoskeleton and protein kinase JNK. In summary, age-related changes in targeted degradation of structural and regulatory proteins across tissues determine longevity.

Trauma-induced regulation of VHP-1 modulates the cellular response to mechanical stress

Mechanical stimuli initiate adaptive signal transduction pathways, yet exceeding the cellular capacity to withstand physical stress results in death. The molecular mechanisms underlying trauma-induced degeneration remain unclear. In the nematode C. elegans, we have developed a method to study cellular degeneration in response to mechanical stress caused by blunt force trauma. Herein, we report that physical injury activates the c-Jun kinase, KGB-1, which modulates response elements through the AP-1 transcriptional complex. Among these, we have identified a dual-specificity MAPK phosphatase, VHP-1, as a stress-inducible modulator of neurodegeneration. VHP-1 regulates the transcriptional response to mechanical stress and is itself attenuated by KGB-1-mediated inactivation of a deubiquitinase, MATH-33, and proteasomal degradation. Together, we describe an uncharacterized stress response pathway in C. elegans and identify transcriptional and post-translational components comprising a feedback loop on Jun kinase and phosphatase activity.

Cytoskeletal disorganization underlies PABPN1-mediated myogenic disability

Muscle wasting and atrophy are regulated by multiple molecular processes, including mRNA processing. Reduced levels of the polyadenylation binding protein nucleus 1 (PABPN1), a multifactorial regulator of mRNA processing, cause muscle atrophy. A proteomic study in muscles with reduced PABPN1 levels suggested dysregulation of sarcomeric and cytoskeletal proteins. Here we investigated the hypothesis that reduced PABPN1 levels lead to an aberrant organization of the cytoskeleton. MURC, a plasma membrane-associated protein, was found to be more abundant in muscles with reduced PABPN1 levels, and it was found to be expressed at regions showing regeneration. A polarized cytoskeletal organization is typical for muscle cells, but muscle cells with reduced PABPN1 levels (named as shPAB) were characterized by a disorganized cytoskeleton that lacked polarization. Moreover, cell mechanical features and myogenic differentiation were significantly reduced in shPAB cells. Importantly, restoring cytoskeletal stability, by actin overexpression, was beneficial for myogenesis, expression of sarcomeric proteins and proper localization of MURC in shPAB cell cultures and in shPAB muscle bundle. We suggest that poor cytoskeletal mechanical features are caused by altered expression levels of cytoskeletal proteins and contribute to muscle wasting and atrophy.

Age-Onset Phosphorylation of a Minor Actin Variant Promotes Intestinal Barrier Dysfunction

Age-associated decay of intercellular interactions impairs the cells' capacity to tightly associate within tissues and form a functional barrier. This barrier dysfunction compromises organ physiology and contributes to systemic failure. The actin cytoskeleton represents a key determinant in maintaining tissue architecture. Yet, it is unclear how age disrupts the actin cytoskeleton and how this, in turn, promotes mortality. Here, we show that an uncharacterized phosphorylation of a low-abundant actin variant, ACT-5, compromises integrity of the C. elegans intestinal barrier and accelerates pathogenesis. Age-related loss of the heat-shock transcription factor, HSF-1, disrupts the JUN kinase and protein phosphatase I equilibrium which increases ACT-5 phosphorylation within its troponin binding site. Phosphorylated ACT-5 accelerates decay of the intestinal subapical terminal web and impairs its interactions with cell junctions. This compromises barrier integrity, promotes pathogenesis, and drives mortality. Thus, we provide the molecular mechanism by which age-associated loss of specialized actin networks impacts tissue integrity.

Measurements of Physiological Stress Responses in C. Elegans

Organisms are often exposed to fluctuating environments and changes in intracellular homeostasis, which can have detrimental effects on their proteome and physiology. Thus, organisms have evolved targeted and specific stress responses dedicated to repair damage and maintain homeostasis. These mechanisms include the unfolded protein response of the endoplasmic reticulum (UPR^ER), the unfolded protein response of the mitochondria (UPR^MT), the heat shock response (HSR), and the oxidative stress response (OxSR). The protocols presented here describe methods to detect and characterize the activation of these pathways and their physiological consequences in the nematode, C. elegans. First, the use of pathway-specific fluorescent transcriptional reporters is described for rapid cellular characterization, drug screening, or large-scale genetic screening (e.g., RNAi or mutant libraries). In addition, complementary, robust physiological assays are described, which can be used to directly assess sensitivity of animals to specific stressors, serving as functional validation of the transcriptional reporters. Together, these methods allow for rapid characterization of the cellular and physiological effects of internal and external proteotoxic perturbations.

Roles of the actin cytoskeleton in aging and age-associated diseases

The integrity of the cytoskeleton is essential to diverse cellular processes such as phagocytosis and intracellular trafficking. Disruption of the organization and dynamics of the actin cytoskeleton leads to age-associated symptoms and diseases, ranging from cancer to neurodegeneration. In addition, changes in the integrity of the actin cytoskeleton disrupt the functioning of not only somatic and stem cells but also gametes, resulting in aberrant embryonic development. Strategies to preserve the integrity and dynamics of the cytoskeleton are, therefore, potentially therapeutic to age-related disorders. The objective of this article is to revisit the current understanding of the roles played by the actin cytoskeleton in aging, and to review the opportunities and challenges for the transition of basic research into intervention development. It is hoped that, with the snapshot of evidence regarding changes in actin dynamics with advanced age, insights into future research directions can be attained.

UPRER promotes lipophagy independent of chaperones to extend life span

Longevity is dictated by a combination of environmental and genetic factors. One of the key mechanisms to regulate life-span extension is the induction of protein chaperones for protein homeostasis. Ectopic activation of the unfolded protein response of the endoplasmic reticulum (UPR^ER) specifically in neurons is sufficient to enhance organismal stress resistance and extend life span. Here, we find that this activation not only promotes chaperones but also facilitates ER restructuring and ER function. This restructuring is concomitant with lipid depletion through lipophagy. Activation of lipophagy is distinct from chaperone induction and is required for the life-span extension found in this paradigm. Last, we find that overexpression of the lipophagy component, ehbp-1, is sufficient to deplete lipids, remodel ER, and promote life span. Therefore, UPR induction in neurons triggers two distinct programs in the periphery: the proteostasis arm through protein chaperones and metabolic changes through lipid depletion mediated by EH domain binding protein 1 (EHBP-1).

The Cytoskeleton as a Modulator of Aging and Neurodegeneration

The cytoskeleton consists of filamentous protein polymers that form organized structures, contributing to a multitude of cell life aspects. It includes three types of polymers: the actin microfilaments, the microtubules and the intermediate filaments. Decades of research have implicated the cytoskeleton in processes that regulate cellular and organismal aging, as well as neurodegeneration associated with injury or neurodegenerative disease, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, Amyotrophic Lateral Sclerosis, or Charcot Marie Tooth disease. Here, we provide a brief overview of cytoskeletal structure and function, and discuss experimental evidence linking cytoskeletal function and dynamics with aging and neurodegeneration.