Mutation in the silencing gene SIR4 can delay aging in S. cerevisiae

Chromatin and epigenetics in aging biology

This book chapter will focus on modifications to chromatin itself, how chromatin modifications are regulated, and how these modifications are deciphered by the cell to impact aging. In this chapter, we will review how chromatin modifications change with age, examine how chromatin-modifying enzymes have been shown to regulate aging and healthspan, discuss how some of these epigenetic changes are triggered and how they can regulate the lifespan of the individual and its naïve descendants, and speculate on future directions for the field.

Antiageing strategy for neurodegenerative diseases: from mechanisms to clinical advances

In the context of global ageing, the prevalence of neurodegenerative diseases and dementia, such as Alzheimer's disease (AD), is increasing. However, the current symptomatic and disease-modifying therapies have achieved limited benefits for neurodegenerative diseases in clinical settings. Halting the progress of neurodegeneration and cognitive decline or even improving impaired cognition and function are the clinically meaningful goals of treatments for neurodegenerative diseases. Ageing is the primary risk factor for neurodegenerative diseases and their associated comorbidities, such as vascular pathologies, in elderly individuals. Thus, we aim to elucidate the role of ageing in neurodegenerative diseases from the perspective of a complex system, in which the brain is the core and peripheral organs and tissues form a holistic network to support brain functions. During ageing, the progressive deterioration of the structure and function of the entire body hampers its active and adaptive responses to various stimuli, thereby rendering individuals more vulnerable to neurodegenerative diseases. Consequently, we propose that the prevention and treatment of neurodegenerative diseases should be grounded in holistic antiageing and rejuvenation means complemented by interventions targeting disease-specific pathogenic events. This integrated approach is a promising strategy to effectively prevent, pause or slow down the progression of neurodegenerative diseases.

The enigmatic role of SIRT2 in the cardiovascular system: Deciphering its protective and detrimental actions to unlock new avenues for therapeutic intervention

Cardiovascular diseases (CVDs) are leading causes of mortality throughout the world, and hence, there is a critical need to elucidate their molecular mechanisms. The Sirtuin (SIRT) family of NAD+-dependent enzymes has recently been shown to play a critical role in cardiovascular health and disease, and several SIRT isoforms, especially SIRT1 and SIRT3, have been amply investigated. However, the precise function of SIRT2 is only partially explored. Here, we review the current understanding of the involvement of SIRT2 in various cardiovascular pathologies, such as cardiac hypertrophy, ischemia-reperfusion injury, diabetic cardiomyopathy, and vascular dysfunction, with emphasis placed on the context-dependent protective or deleterious actions of SIRT2, including its wide array of catalytic activities which span beyond deacetylation. Furthermore, the review uncovers several unresolved research gaps for SIRT2 mechanisms by which SIRT2 modulates cardiac and vascular function during development and aging, thereby paving the way for the discovery of novel therapeutic targets as well as SIRT2-targeted interventions in the prevention and treatment of various cardiovascular diseases.

Exploring the anti-aging potential of natural products and plant extracts in budding yeast Saccharomyces cerevisiae: A review

Historically, plant derived natural products and their crude extracts have been used to treat a wide range of ailments across the world. Biogerontology research aims to explore the molecular basis of aging and discover new anti-aging therapeutic compounds or formulations to combat the detrimental effects of aging and promote a healthy life span. The budding yeast Saccharomyces cerevisiae has been, and continues to be, an indispensable model organism in the field of biomedical research for discovering the molecular basis of aging S. cerevisiae has preserved nutritional signaling pathways (such as the target of rapamycin (TOR)-Sch9 and the Ras-AC-PKA (cAMP-dependent protein kinase) pathways, and shows two distinct aging paradigms chronological life span (CLS) and replicative life span (RLS). This review explores the anti-aging properties of natural products, predominantly derived from plants, and phytoextracts using S. cerevisiae as a model organism.

Exploring the anti-aging potential of natural products and plant extracts in budding yeast Saccharomyces cerevisiae: A review

Historically, plant derived natural products and their crude extracts have been used to treat a wide range of ailments across the world. Biogerontology research aims to explore the molecular basis of aging and discover new anti-aging therapeutic compounds or formulations to combat the detrimental effects of aging and promote a healthy life span. The budding yeast Saccharomyces cerevisiae has been, and continues to be, an indispensable model organism in the field of biomedical research for discovering the molecular basis of aging S. cerevisiae has preserved nutritional signaling pathways (such as the target of rapamycin (TOR)-Sch9 and the Ras-AC-PKA (cAMP-dependent protein kinase) pathways, and shows two distinct aging paradigms chronological life span (CLS) and replicative life span (RLS). This review explores the anti-aging properties of natural products, predominantly derived from plants, and phytoextracts using S. cerevisiae as a model organism.

Roles of chromatin and genome instability in cellular senescence and their relevance to ageing and related diseases

Ageing is a complex biological process in which a gradual decline in physiological fitness increases susceptibility to diseases such as neurodegenerative disorders and cancer. Cellular senescence, a state of irreversible cell-growth arrest accompanied by functional deterioration, has emerged as a pivotal driver of ageing. In this Review, we discuss how heterochromatin loss, telomere attrition and DNA damage contribute to cellular senescence, ageing and age-related diseases by eliciting genome instability, innate immunity and inflammation. We also discuss how emerging therapeutic strategies could restore heterochromatin stability, maintain telomere integrity and boost the DNA repair capacity, and thus counteract cellular senescence and ageing-associated pathologies. Finally, we outline current research challenges and future directions aimed at better comprehending and delaying ageing.

The Role of Molecular and Cellular Aging Pathways on Age-Related Hearing Loss

Aging, a complex process marked by molecular and cellular changes, inevitably influences tissue and organ homeostasis and leads to an increased onset or progression of many chronic diseases and conditions, one of which is age-related hearing loss (ARHL). ARHL, known as presbycusis, is characterized by the gradual and irreversible decline in auditory sensitivity, accompanied by the loss of auditory sensory cells and neurons, and the decline in auditory processing abilities associated with aging. The extended human lifespan achieved by modern medicine simultaneously exposes a rising prevalence of age-related conditions, with ARHL being one of the most significant. While our understanding of the molecular basis for aging has increased over the past three decades, a further understanding of the interrelationship between the key pathways controlling the aging process and the development of ARHL is needed to identify novel targets for the treatment of AHRL. The dysregulation of molecular pathways (AMPK, mTOR, insulin/IGF-1, and sirtuins) and cellular pathways (senescence, autophagy, and oxidative stress) have been shown to contribute to ARHL. However, the mechanistic basis for these pathways in the initiation and progression of ARHL needs to be clarified. Therefore, understanding how longevity pathways are associated with ARHL will directly influence the development of therapeutic strategies to treat or prevent ARHL. This review explores our current understanding of the molecular and cellular mechanisms of aging and hearing loss and their potential to provide new approaches for early diagnosis, prevention, and treatment of ARHL.

Sirt1: An Increasingly Interesting Molecule with a Potential Role in Bone Metabolism and Osteoporosis

Osteoporosis (OP) is a common metabolic bone disease characterized by low bone mass, decreased bone mineral density, and degradation of bone tissue microarchitecture. However, our understanding of the mechanisms of bone remodeling and factors affecting bone mass remains incomplete. Sirtuin1 (SIRT1) is a nicotinamide adenine dinucleotide-dependent deacetylase that regulates a variety of cellular metabolisms, including inflammation, tumorigenesis, and bone metabolism. Recent studies have emphasized the important role of SIRT1 in bone homeostasis. This article reviews the role of SIRT1 in bone metabolism and OP and also discusses therapeutic strategies and future research directions for targeting SIRT1.

Search for protein kinase(s) related to cell growth or viability maintenance in the presence of ethanol in budding and fission yeasts

Alcohol fermentation comprises two phases: phase 1, alcohol fermentation occurs while yeast cells proliferate; phase 2, growth stops and alcohol fermentation continues. We categorized genes related to proliferation in low ethanol (phase 1) and viability in high ethanol (phase 2) as Alcohol Growth Ability (AGA) and Alcohol Viability (ALV), respectively. Although genes required for phase 1 are examined in budding yeast, those for phase 2 are unknown. We set conditions for ALV screening, searched for protein kinases (PKs) related to ALV in budding yeast, and expanded two screenings to fission yeast. Bub1 kinase was important for proliferation in low ethanol but not for viability in high ethanol, suggesting that the important PKs differ between the two phases. It was indeed the case. Further, 3 common PKs were identified as AGA in both yeasts, suggesting that the important cellular mechanism in phase 1 is conserved in both yeasts, at least partially.

Dual activities of a silencing information regulator complex in yeast transcriptional regulation and DNA-damage response

The Saccharomyces cerevisiae silencing information regulator (SIR) complex contains up to four proteins, namely Sir1, Sir2, Sir3, and Sir4. While Sir2 encodes a NAD-dependent histone deacetylase, other SIR proteins mainly function as structural and scaffold components through physical interaction with various proteins. The SIR complex displays different conformation and composition, including Sir2 homotrimer, Sir1-4 heterotetramer, Sir2-4 heterotrimer, and their derivatives, which recycle and relocate to different chromosomal regions. Major activities of the SIR complex are transcriptional silencing through chromosomal remodeling and modulation of DNA double-strand-break repair pathways. These activities allow the SIR complex to be involved in mating-type maintenance and switching, telomere and subtelomere gene silencing, promotion of nonhomologous end joining, and inhibition of homologous recombination, as well as control of cell aging. This review explores the potential link between epigenetic regulation and DNA damage response conferred by the SIR complex under various conditions aiming at understanding its roles in balancing cell survival and genomic stability in response to internal and environmental stresses. As core activities of the SIR complex are highly conserved in eukaryotes from yeast to humans, knowledge obtained in the yeast may apply to mammalian Sirtuin homologs and related diseases.

Sirtuins, a key regulator of ageing and age-related neurodegenerative diseases

Sirtuins are Nicotinamide Adenine Dinucleotide (NAD+) dependent class ІΙΙ histone deacetylases enzymes (HDACs) present from lower to higher organisms such as bacteria (Sulfolobus solfataricus L. major), yeasts (Saccharomyces cerevisiae), nematodes (Caenorhabditis elegans), fruit flies (Drosophila melanogaster), humans (Homo sapiens sapiens), even in plants such as rice (Oryza sativa), thale cress (Arabidopsis thaliana), vine (Vitis vinifera L.) tomato (Solanum lycopersicum). Sirtuins play an important role in the regulation of various vital cellular functions during metabolism and ageing. It also plays a neuroprotective role by modulating several biological pathways such as apoptosis, DNA repair, protein aggregation, and inflammatory processes associated with ageing and neurodegenerative diseases. In this review, we have presented an updated Sirtuins and its role in ageing and age-related neurodegenerative diseases (NDDs). Further, this review also describes the therapeutic potential of Sirtuins and the use of Sirtuins inhibitor/activator for altering the NDDs disease pathology.

The Information Theory of Aging

Information storage and retrieval is essential for all life. In biology, information is primarily stored in two distinct ways: the genome, comprising nucleic acids, acts as a foundational blueprint and the epigenome, consisting of chemical modifications to DNA and histone proteins, regulates gene expression patterns and endows cells with specific identities and functions. Unlike the stable, digital nature of genetic information, epigenetic information is stored in a digital-analog format, susceptible to alterations induced by diverse environmental signals and cellular damage. The Information Theory of Aging (ITOA) states that the aging process is driven by the progressive loss of youthful epigenetic information, the retrieval of which via epigenetic reprogramming can improve the function of damaged and aged tissues by catalyzing age reversal.

Roles of Stress Response in Autophagy Processes and Aging-Related Diseases

The heat shock factor 1 (HSF1)-mediated stress response pathway and autophagy processes play important roles in the maintenance of proteostasis. Autophagy processes are subdivided into three subtypes: macroautophagy, chaperone-mediated autophagy (CMA), and microautophagy. Recently, molecular chaperones and co-factors were shown to be involved in the selective degradation of substrates by these three autophagy processes. This evidence suggests that autophagy processes are regulated in a coordinated manner by the HSF1-mediated stress response pathway. Recently, various studies have demonstrated that proteostasis pathways including HSF1 and autophagy are implicated in longevity. Furthermore, they serve as therapeutic targets for aging-related diseases such as cancer and neurodegenerative diseases. In the future, these studies will underpin the development of therapies against various diseases.

Saccharomyces cerevisiae as a research tool for RNA-mediated human disease

The budding yeast, Saccharomyces cerevisiae, has been used for decades as a powerful genetic tool to study a broad spectrum of biological topics. With its ease of use, economic utility, well-studied genome, and a highly conserved proteome across eukaryotes, it has become one of the most used model organisms. Due to these advantages, it has been used to study an array of complex human diseases. From broad, complex pathological conditions such as aging and neurodegenerative disease to newer uses such as SARS-CoV-2, yeast continues to offer new insights into how cellular processes are affected by disease and how affected pathways might be targeted in therapeutic settings. At the same time, the roles of RNA and RNA-based processes have become increasingly prominent in the pathology of many of these same human diseases, and yeast has been utilized to investigate these mechanisms, from aberrant RNA-binding proteins in amyotrophic lateral sclerosis to translation regulation in cancer. Here we review some of the important insights that yeast models have yielded into the molecular pathology of complex, RNA-based human diseases. This article is categorized under: RNA in Disease and Development > RNA in Disease.

Assessing chronological aging in Saccharomyces cerevisiae

Chronological age represents the time that passes between birth and a given date. To understand the complex network of factors contributing to chronological lifespan, a variety of model organisms have been implemented. One of the best studied organisms is the yeast Saccharomyces cerevisiae, which has greatly contributed toward identifying conserved biological mechanisms that act on longevity. Here, we discuss high- und low-throughput protocols to monitor and characterize chronological lifespan and chronological aging-associated cell death in S. cerevisiae. Included are propidium iodide staining with the possibility to quantitatively assess aging-associated cell death via flow cytometry or qualitative assessments via microscopy, cell viability assessment through plating and cell counting and cell death characterization via propidium iodide/AnnexinV staining and subsequent flow cytometric analysis or microscopy. Importantly, all of these methods combined give a clear picture of the chronological lifespan under different conditions or genetic backgrounds and represent a starting point for pharmacological or genetic interventions.

Neural cell state shifts and fate loss in ageing and age-related diseases

Most age-related neurodegenerative diseases remain incurable owing to an incomplete understanding of the disease mechanisms. Several environmental and genetic factors contribute to disease onset, with human biological ageing being the primary risk factor. In response to acute cellular damage and external stimuli, somatic cells undergo state shifts characterized by temporal changes in their structure and function that increase their resilience, repair cellular damage, and lead to their mobilization to counteract the pathology. This basic cell biological principle also applies to human brain cells, including mature neurons that upregulate developmental features such as cell cycle markers or glycolytic reprogramming in response to stress. Although such temporary state shifts are required to sustain the function and resilience of the young human brain, excessive state shifts in the aged brain might result in terminal fate loss of neurons and glia, characterized by a permanent change in cell identity. Here, we offer a new perspective on the roles of cell states in sustaining health and counteracting disease, and we examine how cellular ageing might set the stage for pathological fate loss and neurodegeneration. A better understanding of neuronal state and fate shifts might provide the means for a controlled manipulation of cell fate to promote brain resilience and repair.

Research Progress on the Role of Sirtuin 1 in Cerebral Ischemia

A significant amount of evidence from the past few years has shown that Sirtuin 1 (SIRT1), a histone deacetylase dinucleotide of nicotinamide adenine dinucleotide (NAD+) is closely related to the cerebral ischemia. Several potential neuroprotective strategies like resveratrol, ischemia preconditioning, and caloric restriction exert their neuroprotection effects through SIRT1-related signaling pathway. However, the potential mechanisms and neuroprotection of SIRT1 in the process of cerebral ischemia injury development and recovery have not been systematically elaborated. This review summarized the the deacetylase activity and distribution of SIRT1 as well as analyzed the roles of SIRT1 in astrocytes, microglia, neurons, and brain microvascular endothelial cells (BMECs), and the molecular mechanisms of SIRT1 in cerebral ischemia, providing a theoretical basis for exploration of new therapeutic target in future.

Cellular and molecular biology of sirtuins in cardiovascular disease

Sirtuins (SIRTs) are a nicotinic adenine dinucleotide (+) -dependent histone deacetylase that regulates critical signaling pathways in prokaryotes and eukaryotes. Studies have identified seven mammalian homologs of the yeast SIRT silencing message regulator 2, namely, SIRT1-SIRT7. Recent in vivo and in vitro studies have successfully demonstrated the involvement of SIRTs in key pathways for cell biological function in physiological and pathological processes of the cardiovascular system, including processes including cellular senescence, oxidative stress, apoptosis, DNA damage, and cellular metabolism. Emerging evidence has stimulated a significant evolution in preventing and treating cardiovascular disease (CVD). Here, we review the important roles of SIRTs for the regulatory pathways involved in the pathogenesis of cardiovascular diseases and their molecular targets, including novel protein post-translational modifications of succinylation. In addition, we summarize the agonists and inhibitors currently identified to target novel specific small molecules of SIRTs. A better understanding of the role of SIRTs in the biology of CVD opens new avenues for therapeutic intervention with great potential for preventing and treating CVD.

New Insights into the Genetics and Epigenetics of Aging Plasticity

Biological aging is characterized by irreversible cell cycle blockade, a decreased capacity for tissue regeneration, and an increased risk of age-related diseases and mortality. A variety of genetic and epigenetic factors regulate aging, including the abnormal expression of aging-related genes, increased DNA methylation levels, altered histone modifications, and unbalanced protein translation homeostasis. The epitranscriptome is also closely associated with aging. Aging is regulated by both genetic and epigenetic factors, with significant variability, heterogeneity, and plasticity. Understanding the complex genetic and epigenetic mechanisms of aging will aid the identification of aging-related markers, which may in turn aid the development of effective interventions against this process. This review summarizes the latest research in the field of aging from a genetic and epigenetic perspective. We analyze the relationships between aging-related genes, examine the possibility of reversing the aging process by altering epigenetic age.

Cell-cell metabolite exchange creates a pro-survival metabolic environment that extends lifespan

Metabolism is deeply intertwined with aging. Effects of metabolic interventions on aging have been explained with intracellular metabolism, growth control, and signaling. Studying chronological aging in yeast, we reveal a so far overlooked metabolic property that influences aging via the exchange of metabolites. We observed that metabolites exported by young cells are re-imported by chronologically aging cells, resulting in cross-generational metabolic interactions. Then, we used self-establishing metabolically cooperating communities (SeMeCo) as a tool to increase metabolite exchange and observed significant lifespan extensions. The longevity of the SeMeCo was attributable to metabolic reconfigurations in methionine consumer cells. These obtained a more glycolytic metabolism and increased the export of protective metabolites that in turn extended the lifespan of cells that supplied them with methionine. Our results establish metabolite exchange interactions as a determinant of cellular aging and show that metabolically cooperating cells can shape the metabolic environment to extend their lifespan.

Introduction: Progression of the Science of Ageing

We outline the progression of ageing research from ancient history to present day geroscience. Calorie restriction, genetic mutations, and the involvement of the sirtuins are highlighted, along with pharmaceutical interventions, in particular rapamycin. At the cellular level, replicative senescence and telomere shortening are presented in the history of ageing studies. We discuss the roles of macromolecular damage in ageing including damage to nuclear, and mitochondrial DNA, epigenetic and protein damage. The importance inflammation during ageing "inflammageing" is becoming increasingly recognized. Omics-based biomarkers are now proving to be a promising approach, along with comparative studies on long-lived animals. The science is getting closer to understanding the mechanisms of ageing and developing reliable interventions to improve human health.

Epigenetic regulation of aging: implications for interventions of aging and diseases

Aging is accompanied by the decline of organismal functions and a series of prominent hallmarks, including genetic and epigenetic alterations. These aging-associated epigenetic changes include DNA methylation, histone modification, chromatin remodeling, non-coding RNA (ncRNA) regulation, and RNA modification, all of which participate in the regulation of the aging process, and hence contribute to aging-related diseases. Therefore, understanding the epigenetic mechanisms in aging will provide new avenues to develop strategies to delay aging. Indeed, aging interventions based on manipulating epigenetic mechanisms have led to the alleviation of aging or the extension of the lifespan in animal models. Small molecule-based therapies and reprogramming strategies that enable epigenetic rejuvenation have been developed for ameliorating or reversing aging-related conditions. In addition, adopting health-promoting activities, such as caloric restriction, exercise, and calibrating circadian rhythm, has been demonstrated to delay aging. Furthermore, various clinical trials for aging intervention are ongoing, providing more evidence of the safety and efficacy of these therapies. Here, we review recent work on the epigenetic regulation of aging and outline the advances in intervention strategies for aging and age-associated diseases. A better understanding of the critical roles of epigenetics in the aging process will lead to more clinical advances in the prevention of human aging and therapy of aging-related diseases.

H3K9me1/2 methylation limits the lifespan of daf-2 mutants in C. elegans

Histone methylation plays crucial roles in the development, gene regulation, and maintenance of stem cell pluripotency in mammals. Recent work shows that histone methylation is associated with aging, yet the underlying mechanism remains unclear. In this work, we identified a class of putative histone 3 lysine 9 mono/dimethyltransferase genes (met-2, set-6, set-19, set-20, set-21, set-32, and set-33), mutations in which induce synergistic lifespan extension in the long-lived DAF-2 (insulin growth factor 1 [IGF-1] receptor) mutant in Caenorhabditis elegans. These putative histone methyltransferase plus daf-2 double mutants not only exhibited an average lifespan nearly three times that of wild-type animals and a maximal lifespan of approximately 100 days, but also significantly increased resistance to oxidative and heat stress. Synergistic lifespan extension depends on the transcription factor DAF-16 (FOXO). mRNA-seq experiments revealed that the mRNA levels of DAF-16 Class I genes, which are activated by DAF-16, were further elevated in the daf-2;set double mutants. Among these genes, tts-1, F35E8.7, ins-35, nhr-62, sod-3, asm-2, and Y39G8B.7 are required for the lifespan extension of the daf-2;set-21 double mutant. In addition, treating daf-2 animals with the H3K9me1/2 methyltransferase G9a inhibitor also extends lifespan and increases stress resistance. Therefore, investigation of DAF-2 and H3K9me1/2 deficiency-mediated synergistic longevity will contribute to a better understanding of the molecular mechanisms of aging and therapeutic applications.

Breaking the aging epigenetic barrier

Aging is an inexorable event occurring universally for all organisms characterized by the progressive loss of cell function. However, less is known about the key events occurring inside the nucleus in the process of aging. The advent of chromosome capture techniques and extensive modern sequencing technologies have illuminated a rather dynamic structure of chromatin inside the nucleus. As cells advance along their life cycle, chromatin condensation states alter which leads to a different epigenetic landscape, correlated with modified gene expression. The exact factors mediating these changes in the chromatin structure and function remain elusive in the context of aging cells. The accumulation of DNA damage, reactive oxygen species and loss of genomic integrity as cells cease to divide can contribute to a tumor stimulating environment. In this review, we focus on genomic and epigenomic changes occurring in an aged cell which can contribute to age-related tumor formation.

DNA repair as a shared hallmark in cancer and ageing

Increasing evidence demonstrates that DNA damage and genome instability play a crucial role in ageing. Mammalian cells have developed a wide range of complex and well‐orchestrated DNA repair pathways to respond to and resolve many different types of DNA lesions that occur from exogenous and endogenous sources. Defects in these repair pathways lead to accelerated or premature ageing syndromes and increase the likelihood of cancer development. Understanding the fundamental mechanisms of DNA repair will help develop novel strategies to treat ageing‐related diseases. Here, we revisit the processes involved in DNA damage repair and how these can contribute to diseases, including ageing and cancer. We also review recent mechanistic insights into DNA repair and discuss how these insights are being used to develop novel therapeutic strategies for treating human disease. We discuss the use of PARP inhibitors in the clinic for the treatment of breast and ovarian cancer and the challenges associated with acquired drug resistance. Finally, we discuss how DNA repair pathway‐targeted therapeutics are moving beyond PARP inhibition in the search for ever more innovative and efficacious cancer therapies.

Regulation of Gene Expression through Food—Curcumin as a Sirtuin Activity Modulator

The sirtuin family comprises NAD⁺-dependent protein lysine deacylases, mammalian sirtuins being either nuclear (SIRT1, SIRT2, SIRT6, and SIRT7), mitochondrial (SIRT3, SIRT4, and SIRT5) or cytosolic enzymes (SIRT2 and SIRT5). They are able to catalyze direct metabolic reactions, thus regulating several physiological functions, such as energy metabolism, stress response, inflammation, cell survival, DNA repair, tissue regeneration, neuronal signaling, and even circadian rhythms. Based on these data, recent research was focused on finding molecules that could regulate sirtuins’ expression and/or activity, natural compounds being among the most promising in the field. Curcumin (1,7-bis-(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) can induce, through SIRT, modulation of cancer cell senescence, improve endothelial cells protection against atherosclerotic factors, enhance muscle regeneration in atrophy models, and act as a pro-longevity factor counteracting the neurotoxicity of amyloid-beta. Although a plethora of protective effects was reported (antioxidant, anti-inflammatory, anticancer, etc.), its therapeutical use is limited due to its bioavailability issues. However, all the reported effects may be explained via the bioactivation theory, which postulates that curcumin’s observed actions are modulated via its metabolites and/or degradation products. The present article is focused on bringing together the literature data correlating the ability of curcumin and its metabolites to modulate SIRT activity and its consequent beneficial effects.

NKT-like (CD3 + CD56+) cells differ from T cells in expression level of cellular protective proteins and sensitivity to stimulation in the process of ageing

BACKGROUND

NKT-like cells are T lymphocytes coexpressing several NK cell-associated receptors. They are effector lymphocytes of innate and adaptive immunity, and their number increases with age. The study aimed to analyze the expression of cellular protective proteins, i.e. sirtuin 1 (SIRT1), heat shock protein 70 (HSP70) and manganese superoxide dismutase (SOD2) in NKT-like and T cells of the young ('young', 31 subjects, age range 19-24 years), seniors aged under 85 ('old'; 30 subjects, age range 65-84 years) and seniors aged over 85 ('oldest', 24 subjects, age range 85-94 years). Both NKT-like and T cells were cultured for 48 h and stimulated with IL-2, LPS and PMA with ionomycin and compared with unstimulated control cells.

RESULTS

The oldest seniors varied from the other age groups by significantly increased expression of SIRT1 and HSP70 in both NKT-like and T cells observed in both stimulated and nonstimulated cells. The analyzed lymphocyte populations of the oldest revealed not only the highest expression of these proteins but also insensitivity to all types of applied stimulation. When NKT-like cells were compared to T cells, higher expression of the studied protective proteins was observed in both stimulated and unstimulated NKT-like cells. Neither CD3 + CD56+ nor CD3+ cells revealed elevated expression of SOD2, and these cells responded to stimulation until very advanced age. T cells revealed higher sensitivity to stimulation with IL-2 regarding SIRT1 and HSP70 expression. NKT-like cells were more sensitive to stimulation with PMA and ionomycin concerning the expression of these proteins. IL-2 did not induce a significant increase in SOD2 expression in the studied age groups.

CONCLUSIONS

The oldest seniors developed an adaptive stress response in both T and NKT-like cells regarding the expression of SIRT1 and HSP70, which was increased and insensitive to further stimulation in contrast to SOD2, which showed a more inducible pattern of expression. CD3 + CD56+ cells exhibited higher expression of cellular protective proteins than CD3+ cells in both stimulated and control, nonstimulated cells. NKT-like and T cells showed a distinct sensitivity to the applied stimulatory factors in the respective age groups.

Adaptive Response of Saccharomyces Hosts to Totiviridae L-A dsRNA Viruses Is Achieved through Intrinsically Balanced Action of Targeted Transcription Factors

Totiviridae L-A virus is a widespread yeast dsRNA virus. The persistence of the L-A virus alone appears to be symptomless, but the concomitant presence of a satellite M virus provides a killer trait for the host cell. The presence of L-A dsRNA is common in laboratory, industrial, and wild yeasts, but little is known about the impact of the L-A virus on the host’s gene expression. In this work, based on high-throughput RNA sequencing data analysis, the impact of the L-A virus on whole-genome expression in three different Saccharomyces paradoxus and S. cerevisiae host strains was analyzed. In the presence of the L-A virus, moderate alterations in gene expression were detected, with the least impact on respiration-deficient cells. Remarkably, the transcriptional adaptation of essential genes was limited to genes involved in ribosome biogenesis. Transcriptional responses to L-A maintenance were, nevertheless, similar to those induced upon stress or nutrient availability. Based on these data, we further dissected yeast transcriptional regulators that, in turn, modulate the cellular L-A dsRNA levels. Our findings point to totivirus-driven fine-tuning of the transcriptional landscape in yeasts and uncover signaling pathways employed by dsRNA viruses to establish the stable, yet allegedly profitless, viral infection of fungi.

Genetic and Epigenetic Inheritance at Telomeres

Transgenerational inheritance can occur at telomeres in distinct contexts. Deficiency for telomerase or telomere-binding proteins in germ cells can result in shortened or lengthened chromosome termini that are transmitted to progeny. In human families, altered telomere lengths can result in stem cell dysfunction or tumor development. Genetic inheritance of altered telomeres as well as mutations that alter telomeres can result in progressive telomere length changes over multiple generations. Telomeres of yeast can modulate the epigenetic state of subtelomeric genes in a manner that is mitotically heritable, and the effects of telomeres on subtelomeric gene expression may be relevant to senescence or other human adult-onset disorders. Recently, two novel epigenetic states were shown to occur at C. elegans telomeres, where very low or high levels of telomeric protein foci can be inherited for multiple generations through a process that is regulated by histone methylation.Together, these observations illustrate that information relevant to telomere biology can be inherited via genetic and epigenetic mechanisms, although the broad impact of epigenetic inheritance to human biology remains unclear.

Bacterial Sirtuins Overview: An Open Niche to Explore

Sirtuins are deacetylase enzymes widely distributed in all domains of life. Although for decades they have been related only to histones deacetylation in eukaryotic organisms, today they are considered global regulators in both prokaryotes and eukaryotes. Despite the important role of sirtuins in humans, the knowledge about bacterial sirtuins is still limited. Several proteomics studies have shown that bacterial sirtuins deacetylate a large number of lysines in vivo, although the effect that this deacetylation causes in most of them remains unknown. To date, only the regulation of a few bacterial sirtuin substrates has been characterized, being their metabolic roles widely distributed: carbon and nitrogen metabolism, DNA transcription, protein translation, or virulence. One of the most current topics on acetylation and deacetylation focuses on studying stoichiometry using quantitative LC-MS/MS. The results suggest that prokaryotic sirtuins deacetylate at low stoichiometry sites, although more studies are needed to know if it is a common characteristic of bacterial sirtuins and its biological significance. Unlike eukaryotic organisms, bacteria usually have one or few sirtuins, which have been reported to have closer phylogenetic similarity with the human Sirt5 than with any other human sirtuin. In this work, in addition to carrying out an in-depth review of the role of bacterial sirtuins in their physiology, a phylogenetic study has been performed that reveals the evolutionary differences between sirtuins of different bacterial species and even between homologous sirtuins.

Evaluating the beneficial effects of dietary restrictions: A framework for precision nutrigeroscience

Dietary restriction (DR) has long been viewed as the most robust nongenetic means to extend lifespan and healthspan. Many aging-associated mechanisms are nutrient responsive, but despite the ubiquitous functions of these pathways, the benefits of DR often vary among individuals and even among tissues within an individual, challenging the aging research field. Furthermore, it is often assumed that lifespan interventions like DR will also extend healthspan, which is thus often ignored in aging studies. In this review, we provide an overview of DR as an intervention and discuss the mechanisms by which it affects lifespan and various healthspan measures. We also review studies that demonstrate exceptions to the standing paradigm of DR being beneficial, thus raising new questions that future studies must address. We detail critical factors for the proposed field of precision nutrigeroscience, which would utilize individualized treatments and predict outcomes using biomarkers based on genotype, sex, tissue, and age.

Sirtuin1 in vascular endothelial function, an overview

Sirtuin1 is a nutrient-sensitive class III histone deacetylase which is a well-known regulator of organismal lifespan. It has been extensively studied for its role in metabolic regulation as well. Along with its involvement in ageing and metabolism, Sirtuin1 directly deacetylates many critical proteins controlling cardiovascular pathophysiology. Studies using conditional expression and deletion of Sirtuin1 have revealed that it functions in a highly tissue/organ-specific manner. In the vasculature, Sirtuin1 controls endothelial homoeostasis by governing the expression of inflammatory mediators, oxidants and essential transcription factors. Adding to this complexity, Sirtuin1 expression and/or function is also governed by some of these target proteins. Therefore, the importance of better understanding the organ and tissue specificity of Sirtuin1 is highly desirable. Considering the huge volume of research done in this field, this review focuses on Sirtuin1 targets regulating vascular endothelial function. Here, we summarize the discovery of Sirtuin1 as a transcription controller and the further identification of direct target proteins involved in the vascular physiology. Overall, this review presents a holistic picture of the complex cross-talk involved in the molecular regulation of vascular physiology by Sirtuin1.

Hairless regulates heterochromatin maintenance and muscle stem cell function as a histone demethylase antagonist

Skeletal muscle possesses remarkable regenerative ability because of the resident muscle stem cells (MuSCs). A prominent feature of quiescent MuSCs is a high content of heterochromatin. However, little is known about the mechanisms by which heterochromatin is maintained in MuSCs. By comparing gene-expression profiles from quiescent and activated MuSCs, we found that the mammalian Hairless (Hr) gene is expressed in quiescent MuSCs and rapidly down-regulated upon MuSC activation. Using a mouse model in which Hr can be specifically ablated in MuSCs, we demonstrate that Hr expression is critical for MuSC function and muscle regeneration. In MuSCs, loss of Hr results in reduced trimethylated Histone 3 Lysine 9 (H3K9me3) levels, reduced heterochromatin, increased susceptibility to genotoxic stress, and the accumulation of DNA damage. Deletion of Hr leads to an acceleration of the age-related decline in MuSC numbers. We have also demonstrated that despite the fact that Hr is homologous to a family of histone demethylases and binds to di- and trimethylated H3K9, the expression of Hr does not lead to H3K9 demethylation. In contrast, we show that the expression of Hr leads to the inhibition of the H3K9 demethylase Jmjd1a and an increase in H3K9 methylation. Taking these data together, our study has established that Hr is a H3K9 demethylase antagonist specifically expressed in quiescent MuSCs.

Epigenetic age prediction

Advanced age is the main common risk factor for cancer, cardiovascular disease and neurodegeneration. Yet, more is known about the molecular basis of any of these groups of diseases than the changes that accompany ageing itself. Progress in molecular ageing research was slow because the tools predicting whether someone aged slowly or fast (biological age) were unreliable. To understand ageing as a risk factor for disease and to develop interventions, the molecular ageing field needed a quantitative measure; a clock for biological age. Over the past decade, a number of age predictors utilising DNA methylation have been developed, referred to as epigenetic clocks. While they appear to estimate biological age, it remains unclear whether the methylation changes used to train the clocks are a reflection of other underlying cellular or molecular processes, or whether methylation itself is involved in the ageing process. The precise aspects of ageing that the epigenetic clocks capture remain hidden and seem to vary between predictors. Nonetheless, the use of epigenetic clocks has opened the door towards studying biological ageing quantitatively, and new clocks and applications, such as forensics, appear frequently. In this review, we will discuss the range of epigenetic clocks available, their strengths and weaknesses, and their applicability to various scientific queries.

The RNA-binding protein Puf5 contributes to buffering of mRNA upon chromatin-mediated changes in nascent transcription

Gene expression involves regulation of chromatin structure and transcription, as well as processing of the transcribed mRNA. While there are feedback mechanisms, it is not clear whether these include crosstalk between chromatin architecture and mRNA decay. To address this, we performed a genome-wide genetic screen using a Saccharomyces cerevisiae strain harbouring the H3K56A mutation, which is known to perturb chromatin structure and nascent transcription. We identified Puf5 (also known as Mpt5) as essential in an H3K56A background. Depletion of Puf5 in this background leads to downregulation of Puf5 targets. We suggest that Puf5 plays a role in post-transcriptional buffering of mRNAs, and support this by transcriptional shutoff experiments in which Puf5 mRNA targets are degraded slower in H3K56A cells compared to wild-type cells. Finally, we show that post-transcriptional buffering of Puf5 targets is widespread and does not occur only in an H3K56A mutant, but also in an H3K4R background, which leads to a global increase in nascent transcription. Our data suggest that Puf5 determines the fate of its mRNA targets in a context-dependent manner acting as an mRNA surveillance hub balancing deregulated nascent transcription to maintain physiological mRNA levels.

Molecular and cellular pathways contributing to brain aging

Aging is the leading risk factor for several age-associated diseases such as neurodegenerative diseases. Understanding the biology of aging mechanisms is essential to the pursuit of brain health. In this regard, brain aging is defined by a gradual decrease in neurophysiological functions, impaired adaptive neuroplasticity, dysregulation of neuronal Ca2+ homeostasis, neuroinflammation, and oxidatively modified molecules and organelles. Numerous pathways lead to brain aging, including increased oxidative stress, inflammation, disturbances in energy metabolism such as deregulated autophagy, mitochondrial dysfunction, and IGF-1, mTOR, ROS, AMPK, SIRTs, and p53 as central modulators of the metabolic control, connecting aging to the pathways, which lead to neurodegenerative disorders. Also, calorie restriction (CR), physical exercise, and mental activities can extend lifespan and increase nervous system resistance to age-associated neurodegenerative diseases. The neuroprotective effect of CR involves increased protection against ROS generation, maintenance of cellular Ca2+ homeostasis, and inhibition of apoptosis. The recent evidence about the modem molecular and cellular methods in neurobiology to brain aging is exhibiting a significant potential in brain cells for adaptation to aging and resistance to neurodegenerative disorders.

Smc5/6 in the rDNA modulates lifespan independently of Fob1​

The ribosomal DNA (rDNA) in Saccharomycescerevisiae is in one tandem repeat array on Chromosome XII. Two regions within each repetitive element, called intergenic spacer 1 (IGS1) and IGS2, are important for organizing the rDNA within the nucleolus. The Smc5/6 complex localizes to IGS1 and IGS2. We show that Smc5/6 has a function in the rDNA beyond its role in homologous recombination (HR) at the replication fork barrier (RFB) located in IGS1. Fob1 is required for optimal binding of Smc5/6 at IGS1 whereas the canonical silencing factor Sir2 is required for its optimal binding at IGS2, independently of Fob1. Through interdependent interactions, Smc5/6 stabilizes Sir2 and Cohibin at both IGS and its recovery at IGS2 is important for nucleolar compaction and transcriptional silencing, which in turn supports rDNA stability and lifespan.

Sirtuins as pharmacological targets in neurodegenerative and neuropsychiatric disorders

Histone deacetylases (HDACs) are enzymes that regulate several processes, such as transcription, cell proliferation, differentiation and development. HDACs are classified as either Zn²⁺ -dependent or NAD⁺ -dependent enzymes. Over the years, experimental and clinical evidence has demonstrated that HDAC modulation is a critical process in neurodegenerative and psychiatric disorders. Nevertheless, most of the studies have focused on the role of Zn²⁺ -dependent HDACs in the development of these diseases, although there is growing evidence showing that the NAD⁺ -dependent HDACs, known as sirtuins, are also very promising targets. This possibility has been strengthened by reports of decreased levels of NAD⁺ in CNS disorders, which can lead to alterations in sirtuin activation and therefore result in increased pathology. In this review, we discuss the role of sirtuins in neurodegenerative and neuropsychiatric disorders as well the possible rationale for them to be considered as pharmacological targets in future therapeutic interventions.

Progresses in both basic research and clinical trials of NAD+ in Parkinson's disease

The decline of nicotinamide adenine dinucleotide (NAD+) levels is a hallmark of aging in multiple organisms and tissues, including the human brain. Hence, agents that increase intracellular NAD+ could have beneficial effects in aging and age-related neurodegenerative diseases. Disturbances in NAD+ metabolism have also been observed in Parkinson's disease (PD), supporting a link between neuronal bioenergetics failure and disease pathogenesis. Here, we review emerging findings revealing key roles for NAD+ and related metabolites in experimental models of dopaminergic neurodegeneration and in PD patients. We discuss how increased NAD+ levels might ameliorate disease phenotypes by restoring neuronal mitochondrial energy metabolism, promoting cellular proteostasis, and modulating the immune system. Finally, we describe ongoing clinical trials targeting NAD+ in PD and highlight the need for further investigations to better delineate the association between NAD+, brain aging and disease, and optimal strategies for efficiently and safely raising NAD+ levels. A more comprehensive understanding of the basic mechanisms linking NAD+, energy metabolism, and PD, and of the impact of life-long NAD+ targeting strategies, are critical to inform future clinical applications.

Skeletal Muscle Gene Expression Profile in Response to Caloric Restriction and Aging: A Role for SirT1

SirT1 plays a crucial role in the regulation of some of the caloric restriction (CR) responsive biological pathways. Aging suppresses SirT1 gene expression in skeletal muscle, suggesting that aging may affect the role of CR in muscle. To determine the role of SirT1 in the regulation of CR regulated pathways in skeletal muscle, we performed high-throughput RNA sequencing using total RNA isolated from the skeletal muscles of young and aged wild-type (WT), SirT1 knockout (SirT1-KO), and SirT1 overexpression (SirT1-OE) mice fed to 20 wk ad libitum (AL) or 40% CR diet. Our data show that aging repressed the global gene expression profile, which was restored by CR via upregulating transcriptional and translational process-related pathways. CR inhibits pathways linked to the extracellular matrix and cytoskeletal proteins regardless of aging. Mitochondrial function and muscle contraction-related pathways are upregulated in aged SirT1 KO mice following CR. SirT1 OE did not affect whole-body energy expenditure or augment skeletal muscle insulin sensitivity associated pathways, regardless of aging or diet. Overall, our RNA-seq data showed that SirT1 and CR have different functions and activation of SirT1 by its activator or exercise may enhance SirT1 activity that, along with CR, likely have a better functional role in aging muscle.

Protection of nuclear DNA by lifespan-extending compounds in the yeast Saccharomyces cerevisiae

DNA damage has been hypothesized to be a driving force of the aging process. At the same time, there exists multiple compounds that can extend lifespan in model organisms, such as yeast, worms, flies, and mice. One possible mechanism of action for these compounds is a protective effect against DNA damage. We investigated whether five of these lifespan-extending compounds, dinitrophenol, metformin, rapamycin, resveratrol, and spermidine, could protect nuclear DNA in the yeast Saccharomyces cerevisiae at the same doses under which they confer lifespan extension. We found that rapamycin and spermidine were able to decrease the spontaneous mutation rate at the CAN1 locus, whereas dinitrophenol, metformin, and resveratrol were able to protect yeast against CAN1 mutations induced by ethyl methanesulfonate (EMS). We also tested whether these compounds could enhance survival against EMS, ultraviolet (UV) light, or hydrogen peroxide (H₂O₂) insult. All five compounds conferred a protective effect against EMS, while metformin and spermidine protected yeast against UV light. Somewhat surprisingly, none of the compounds were able to afford a significant protection against H₂O₂, with spermidine dramatically sensitizing cells. We also examined the ability of these compounds to increase lifespan when growth-arrested by hydroxyurea; only spermidine was found to have a positive effect. Overall, our results suggest that lifespan-extending compounds may act in part by protecting nuclear DNA.

Role of sirtuins in bone biology: Potential implications for novel therapeutic strategies for osteoporosis

The decline in bone mass and bone strength and musculoskeletal problems associated with aging constitute a major challenge for affected individuals and the healthcare system globally. Sirtuins 1-7 (SIRT1-SIRT7) are a family of nicotinamide adenine dinucleotide-dependent deacetylases with remarkable abilities to promote longevity and counteract age-related diseases. Sirtuin knockout and transgenic models have provided novel insights into the function and signaling of these proteins in bone homeostasis. Studies have revealed that sirtuins play a critical role in normal skeletal development and homeostasis through their direct action on bone cells and that their dysregulation might contribute to different bone diseases. Preclinical studies have demonstrated that mice treated with sirtuin agonists show protection against age-related, postmenopausal, and immobilization-induced osteoporosis. These findings suggest that sirtuins could be potential targets for the modulation of the imbalance in bone remodeling and treatment of osteoporosis and other bone disorders. The aim of this review was to provide a comprehensive updated review of the current knowledge on sirtuin biology, focusing specifically on their roles in bone homeostasis and osteoporosis, and potential pharmacological interventions targeting sirtuins for the treatment of osteoporosis.

Longevity pathways in stress resistance: targeting NAD and sirtuins to treat the pathophysiology of hemorrhagic shock

Stress resistance correlates with longevity and this pattern has been exploited to help identify genes that can influence lifespan. Reciprocally, genes and pharmacological agents that have been studied primarily in the context of longevity may be an untapped resource for treating acute stresses. Here we summarize the evidence that targeting SIRT1, studied primarily in the context of longevity, can improve outcomes in hemorrhagic shock and resuscitation. Hemorrhagic shock is a potentially fatal condition that occurs when blood loss is so severe that tissues no longer receive adequate oxygen. While stabilizing the blood pressure and reperfusing tissues are necessary, re-introducing oxygen to ischemic tissues generates a burst of reactive oxygen species that can cause secondary tissue damage. Reactive oxygen species not only exacerbate the inflammatory cascade but also can directly damage mitochondria, leading to bioenergetic failure in the affected tissues. Treatments with polyphenol resveratrol and with nicotinamide adenine dinucleotide (NAD) precursors have both shown promising results in rodent models of hemorrhagic shock and resuscitation. Although a number of different mechanisms may be at play in each case, a common theme is that resveratrol and NAD both enhance the activity of SIRT1. Moreover, many of the physiologic improvements observed with resveratrol and NAD precursors are consistent with modulation of known SIRT1 targets. Because small blood vessels and limited blood volume make mice very challenging for the development of hemorrhagic shock models, there is a paucity of direct genetic evidence testing the role of SIRT1. However, the development of more robust methods in mice as well as genetic modifications in rats should allow the study of SIRT1 transgenic and KO rodents in the near future. The potential therapeutic effect of SIRT1 in hemorrhagic shock may serve as an important example supporting the value of considering "longevity" pathways in the mitigation of acute stresses.

High-throughput small molecule screening reveals Nrf2-dependent and -independent pathways of cellular stress resistance

Aging is the dominant risk factor for most chronic diseases. Development of antiaging interventions offers the promise of preventing many such illnesses simultaneously. Cellular stress resistance is an evolutionarily conserved feature of longevity. Here, we identify compounds that induced resistance to the superoxide generator paraquat (PQ), the heavy metal cadmium (Cd), and the DNA alkylator methyl methanesulfonate (MMS). Some rescue compounds conferred resistance to a single stressor, while others provoked multiplex resistance. Induction of stress resistance in fibroblasts was predictive of longevity extension in a published large-scale longevity screen in Caenorhabditis elegans, although not in testing performed in worms and flies with a more restricted set of compounds. Transcriptomic analysis and genetic studies implicated Nrf2/SKN-1 signaling in stress resistance provided by two protective compounds, cardamonin and AEG 3482. Small molecules identified in this work may represent attractive tools to elucidate mechanisms of stress resistance in mammalian cells.

The role of the Acanthamoeba castellanii Sir2-like protein in the growth and encystation of Acanthamoeba

BACKGROUND

The encystation of Acanthamoeba leads to the development of resilient cysts from vegetative trophozoites. This process is essential for the survival of parasites under unfavorable conditions. Previous studies have reported that, during the encystation of A. castellanii, the expression levels of encystation-related factors are upregulated. However, the regulatory mechanisms for their expression during the encystation process remains unknown. Proteins in the sirtuin family, which consists of nicotinamide adenine dinucleotide-dependent deacetylases, are known to play an important role in various cellular functions. In the present study, we identified the Acanthamoeba silent-information regulator 2-like protein (AcSir2) and examined its role in the growth and encystation of Acanthamoeba.

METHODS

We obtained the full-length sequence for AcSir2 using reverse-transcription polymerase chain reaction. In Acanthamoeba transfectants that constitutively overexpress AcSir2 protein, SIRT deacetylase activity was measured, and the intracellular localization of AcSir2 and the effects on the growth and encystation of trophozoites were examined. In addition, the sirtuin inhibitor salermide was used to determine whether these effects were caused by AcSir2 overexpression RESULTS: AcSir2 was classified as a class-IV sirtuin. AcSir2 exhibited functional SIRT deacetylase activity, localized mainly in the nucleus, and its transcription was upregulated during encystation. In trophozoites, AcSir2 overexpression led to greater cell growth, and this growth was inhibited by treatment with salermide, a sirtuin inhibitor. When AcSir2 was overexpressed in the cysts, the encystation rate was significantly higher; this was also reversed with salermide treatment. In AcSir2-overexpressing encysting cells, the transcription of cellulose synthase was highly upregulated compared with that of control cells, and this upregulation was abolished with salermide treatment. Transmission electron microscope-based ultrastructural analysis of salermide-treated encysting cells showed that the structure of the exocyst wall and intercyst space was impaired and that the endocyst wall had not formed.

CONCLUSIONS

These results indicate that AcSir2 is a SIRT deacetylase that plays an essential role as a regulator of a variety of cellular processes and that the regulation of AcSir2 expression is important for the growth and encystation of A. castellanii.