Engineering longevity-design of a synthetic gene oscillator to slow cellular aging.

Synthetic biology enables the design of gene networks to confer specific biological functions, yet it remains a challenge to rationally engineer a biological trait as complex as longevity. A naturally occurring toggle switch underlies fate decisions toward either nucleolar or mitochondrial decline during the aging of yeast cells. We rewired this endogenous toggle to engineer an autonomous genetic clock that generates sustained oscillations between the nucleolar and mitochondrial aging processes in individual cells. These oscillations increased cellular life span through the delay of the commitment to aging that resulted from either the loss of chromatin silencing or the depletion of heme. Our results establish a connection between gene network architecture and cellular longevity that could lead to rationally designed gene circuits that slow aging.

Functional tug of war between kinases, phosphatases, and the Gcn5 acetyltransferase in chromatin and cell cycle checkpoint controls.

Covalent modifications of chromatin regulate genomic structure and accessibility in diverse biological processes such as transcriptional regulation, cell cycle progression, and DNA damage repair. Many histone modifications have been characterized, yet understanding the interactions between these and their combinatorial effects remains an active area of investigation, including dissecting functional interactions between enzymes mediating these modifications. In budding yeast, the histone acetyltransferase Gcn5 interacts with Rts1, a regulatory subunit of protein phosphatase 2A (PP2A). Implicated in the interaction is the potential for the dynamic phosphorylation of conserved residues on histone H2B and the Cse4 centromere-specific histone H3 variant. To probe these dynamics, we sought to identify kinases which contribute to the phosphorylated state. In a directed screen beginning with in silico analysis of the 127 members of yeast kinome, we have now identified 16 kinases with genetic interactions with GCN5 and specifically found distinct roles for the Hog1 stress-activated protein kinase. Deletion of HOG1 (hog1Δ) rescues gcn5Δ sensitivity to the microtubule poison nocodazole and the lethality of the gcn5Δ rts1Δ double mutant. The Hog1-Gcn5 interaction requires the conserved H2B-T91 residue, which is phosphorylated in vertebrate species. Furthermore, deletion of HOG1 decreases aneuploidy and apoptotic populations in gcn5Δ cells. Together, these results introduce Hog1 as a kinase that functionally opposes Gcn5 and Rts1 in the context of the spindle assembly checkpoint and suggest further kinases may also influence GCN5's functions.

Age-dependent aggregation of ribosomal RNA-binding proteins links deterioration in chromatin stability with challenges to proteostasis.

Chromatin instability and protein homeostasis (proteostasis) stress are two well-established hallmarks of aging, which have been considered largely independent of each other. Using microfluidics and single-cell imaging approaches, we observed that, during the replicative aging of Saccharomyces cerevisiae, a challenge to proteostasis occurs specifically in the fraction of cells with decreased stability within the ribosomal DNA (rDNA). A screen of 170 yeast RNA-binding proteins identified ribosomal RNA (rRNA)-binding proteins as the most enriched group that aggregate upon a decrease in rDNA stability induced by inhibition of a conserved lysine deacetylase Sir2. Further, loss of rDNA stability induces age-dependent aggregation of rRNA-binding proteins through aberrant overproduction of rRNAs. These aggregates contribute to age-induced proteostasis decline and limit cellular lifespan. Our findings reveal a mechanism underlying the interconnection between chromatin instability and proteostasis stress and highlight the importance of cell-to-cell variability in aging processes.

Cell cycle roles for GCN5 revealed through genetic suppression.

The conserved acetyltransferase Gcn5 is a member of several complexes in eukaryotic cells, playing roles in regulating chromatin organization, gene expression, metabolism, and cell growth and differentiation via acetylation of both nuclear and cytoplasmic proteins. Distinct functions of Gcn5 have been revealed through a combination of biochemical and genetic approaches in many in vitro studies and model organisms. In this review, we focus on the unique insights that have been gleaned from suppressor studies of gcn5 phenotypes in the budding yeast Saccharomyces cerevisiae. Such studies were fundamental in the early understanding of the balance of counteracting chromatin activities in regulating transcription. Most recently, suppressor screens have revealed roles for Gcn5 in early cell cycle (G1 to S) gene expression and regulation of chromosome segregation during mitosis. Much has been learned, but many questions remain which will be informed by focused analysis of additional genetic and physical interactions.

A programmable fate decision landscape underlies single-cell aging in yeast.

Chromatin instability and mitochondrial decline are conserved processes that contribute to cellular aging. Although both processes have been explored individually in the context of their distinct signaling pathways, the mechanism that determines which process dominates during aging of individual cells is unknown. We show that interactions between the chromatin silencing and mitochondrial pathways lead to an epigenetic landscape of yeast replicative aging with multiple equilibrium states that represent different types of terminal states of aging. The structure of the landscape drives single-cell differentiation toward one of these states during aging, whereby the fate is determined quite early and is insensitive to intracellular noise. Guided by a quantitative model of the aging landscape, we genetically engineered a long-lived equilibrium state characterized by an extended life span.

Advances in quantitative biology methods for studying replicative aging in Saccharomyces cerevisiae.

Aging is a complex, yet pervasive phenomenon in biology. As human cells steadily succumb to the deteriorating effects of aging, so too comes a host of age-related ailments such as neurodegenerative disorders, cardiovascular disease and cancer. Therefore, elucidation of the molecular networks that drive aging is of paramount importance to human health. Progress toward this goal has been aided by studies from simple model organisms such as Saccharomyces cerevisiae. While work in budding yeast has already revealed much about the basic biology of aging as well as a number of evolutionarily conserved pathways involved in this process, recent technological advances are poised to greatly expand our knowledge of aging in this simple eukaryote. Here, we review the latest developments in microfluidics, single-cell analysis and high-throughput technologies for studying single-cell replicative aging in S. cerevisiae. We detail the challenges each of these methods addresses as well as the unique insights into aging that each has provided. We conclude with a discussion of potential future applications of these techniques as well as the importance of single-cell dynamics and quantitative biology approaches for understanding cell aging.

Divergent Aging of Isogenic Yeast Cells Revealed through Single-Cell Phenotypic Dynamics.

Although genetic mutations that alter organisms' average lifespans have been identified in aging research, our understanding of the dynamic changes during aging remains limited. Here, we integrate single-cell imaging, microfluidics, and computational modeling to investigate phenotypic divergence and cellular heterogeneity during replicative aging of single S. cerevisiae cells. Specifically, we find that isogenic cells diverge early in life toward one of two aging paths, which are characterized by distinct age-associated phenotypes. We captured the dynamics of single cells along the paths with a stochastic discrete-state model, which accurately predicts both the measured heterogeneity and the lifespan of cells on each path within a cell population. Our analysis suggests that genetic and environmental factors influence both a cell's choice of paths and the kinetics of paths themselves. Given that these factors are highly conserved throughout eukaryotes, divergent aging might represent a general scheme in cellular aging of other organisms.

Connecting GCN5's centromeric SAGA to the mitotic tension-sensing checkpoint.

Multiple interdependent mechanisms ensure faithful segregation of chromosomes during cell division. Among these, the spindle assembly checkpoint monitors attachment of spindle microtubules to the centromere of each chromosome, whereas the tension-sensing checkpoint monitors the opposing forces between sister chromatid centromeres for proper biorientation. We report here a new function for the deeply conserved Gcn5 acetyltransferase in the centromeric localization of Rts1, a key player in the tension-sensing checkpoint. Rts1 is a regulatory component of protein phopshatase 2A, a near universal phosphatase complex, which is recruited to centromeres by the Shugoshin (Sgo) checkpoint component under low-tension conditions to maintain sister chromatid cohesion. We report that loss of Gcn5 disrupts centromeric localization of Rts1. Increased RTS1 dosage robustly suppresses gcn5∆ cell cycle and chromosome segregation defects, including restoration of Rts1 to centromeres. Sgo1's Rts1-binding function also plays a key role in RTS1 dosage suppression of gcn5∆ phenotypes. Notably, we have identified residues of the centromere histone H3 variant Cse4 that function in these chromosome segregation-related roles of RTS1. Together, these findings expand the understanding of the mechanistic roles of Gcn5 and Cse4 in chromosome segregation.

Critical genomic regulation mediated by Enhancer of Polycomb.

Enhancer of Polycomb (EPC) was first identified for its contributions to development in Drosophila and was soon-thereafter purified as a subunit of the NuA4/TIP60 acetyltransferase complex. Since then, EPC has often been left in the shadows as an essential, yet non-catalytic subunit of NuA4/TIP60; however, its deep conservation and disease association make clear that it warrants additional attention. In fact, recent studies in yeast demonstrated that its Enhancer of Polycomb, Epl1, was just as important for gene expression and acetylation as is the catalytic subunit of NuA4. Despite its conservation, studies of EPC have often remained siloed between organisms. Here, our goal is to provide a cohesive view of the current state of the EPC literature as it stands among the major model organisms in which it has been studied. EPC is involved in multiple processes, beginning with its cardinal role in regulating global and targeted histone acetylation. EPC also frequently serves as an important interaction partner in these basic cellular functions, as well as in multicellular development, such as in hematopoiesis and skeletal muscle differentiation, and in human disease. Taken together, a unifying theme from these studies highlights EPC as a critical genomic regulator.

Multigenerational silencing dynamics control cell aging.

Cellular aging plays an important role in many diseases, such as cancers, metabolic syndromes, and neurodegenerative disorders. There has been steady progress in identifying aging-related factors such as reactive oxygen species and genomic instability, yet an emerging challenge is to reconcile the contributions of these factors with the fact that genetically identical cells can age at significantly different rates. Such complexity requires single-cell analyses designed to unravel the interplay of aging dynamics and cell-to-cell variability. Here we use microfluidic technologies to track the replicative aging of single yeast cells and reveal that the temporal patterns of heterochromatin silencing loss regulate cellular life span. We found that cells show sporadic waves of silencing loss in the heterochromatic ribosomal DNA during the early phases of aging, followed by sustained loss of silencing preceding cell death. Isogenic cells have different lengths of the early intermittent silencing phase that largely determine their final life spans. Combining computational modeling and experimental approaches, we found that the intermittent silencing dynamics is important for longevity and is dependent on the conserved Sir2 deacetylase, whereas either sustained silencing or sustained loss of silencing shortens life span. These findings reveal that the temporal patterns of a key molecular process can directly influence cellular aging, and thus could provide guidance for the design of temporally controlled strategies to extend life span.

The replicative lifespan-extending deletion of SGF73 results in altered ribosomal gene expression in yeast.

Sgf73, a core component of SAGA, is the yeast orthologue of ataxin-7, which undergoes CAG-polyglutamine repeat expansion leading to the human neurodegenerative disease spinocerebellar ataxia type 7 (SCA7). Deletion of SGF73 dramatically extends replicative lifespan (RLS) in yeast. To further define the basis for Sgf73-mediated RLS extension, we performed ChIP-Seq, identified 388 unique genomic regions occupied by Sgf73, and noted enrichment in promoters of ribosomal protein (RP)-encoding genes. Of 388 Sgf73 binding sites, 33 correspond to 5' regions of genes implicated in RLS extension, including 20 genes encoding RPs. Furthermore, half of Sgf73-occupied, RLS-linked RP genes displayed significantly reduced expression in sgf73Δ mutants, and double null strains lacking SGF73 and a Sgf73-regulated, RLS-linked RP gene exhibited no further increase in replicative lifespan. We also found that sgf73Δ mutants display altered acetylation of Ifh1, an important regulator of RP gene transcription. These findings implicate altered ribosomal protein expression in sgf73Δ yeast RLS and highlight altered acetylation as a pathway of relevance for SCA7 neurodegeneration.

Phosphorylation of the 19S regulatory particle ATPase subunit, Rpt6, modifies susceptibility to proteotoxic stress and protein aggregation.

The ubiquitin proteasome system (UPS) is a highly conserved and tightly regulated biochemical pathway that degrades the majority of proteins in eukaryotic cells. Importantly, the UPS is responsible for counteracting altered protein homeostasis induced by a variety of proteotoxic stresses. We previously reported that Rpt6, the ATPase subunit of the 19S regulatory particle (RP) of the 26S proteasome, is phosphorylated in mammalian neurons at serine 120 in response to neuronal activity. Furthermore, we found that Rpt6 S120 phosphorylation, which regulates the activity and distribution of proteasomes in neurons, is relevant for proteasome-dependent synaptic remodeling and function. To better understand the role of proteasome phosphorylation, we have constructed models of altered Rpt6 phosphorylation in S. cerevisiae by introducing chromosomal point mutations that prevent or mimic phosphorylation at the conserved serine (S119). We find that mutants which prevent Rpt6 phosphorylation at this site (rpt6-S119A), had increased susceptibility to proteotoxic stress, displayed abnormal morphology and had reduced proteasome activity. Since impaired proteasome function has been linked to the aggregation of toxic proteins including the Huntington's disease (HD) related huntingtin (Htt) protein with expanded polyglutamine repeats, we evaluated the extent of Htt aggregation in our phospho-dead (rpt6-S119A) and phospho-mimetic (rpt6-S119D) mutants. We showed Htt103Q aggregate size to be significantly larger in rpt6-S119A mutants compared to wild-type or rpt6-S119D strains. Furthermore, we observed that phosphorylation of endogenous Rpt6 at S119 is increased in response to various stress conditions. Together, these data suggest that Rpt6 phosphorylation at S119 may play an important function in proteasome-dependent relief of proteotoxic stress that can be critical in protein aggregation pathologies.

Chromatin Regulation by the NuA4 Acetyltransferase Complex Is Mediated by Essential Interactions Between Enhancer of Polycomb (Epl1) and Esa1.

Enzymes that modify and remodel chromatin act in broadly conserved macromolecular complexes. One key modification is the dynamic acetylation of histones and other chromatin proteins by opposing activities of acetyltransferase and deacetylase complexes. Among acetyltransferases, the NuA4 complex containing Tip60 or its Saccharomyces cerevisiae ortholog Esa1 is of particular significance because of its roles in crucial genomic processes including DNA damage repair and transcription. The catalytic subunit Esa1 is essential, as are five noncatalytic NuA4 subunits. We found that of the noncatalytic subunits, deletion of Enhancer of polycomb (Epl1), but not the others, can be bypassed by loss of a major deacetylase complex, a property shared by Esa1 Noncatalytic complex subunits can be critical for complex assembly, stability, genomic targeting, substrate specificity, and regulation. Understanding the essential role of Epl1 has been previously limited, a limitation now overcome by the discovery of its bypass suppression. Here, we present a comprehensive in vivo study of Epl1 using the powerful tool of suppression combined with transcriptional and mutational analyses. Our results highlight functional parallels between Epl1 and Esa1 and further illustrate that the structural role of Epl1 is important for promotion of Esa1 activity. This conclusion is strengthened by our dissection of Epl1 domains required in vivo for interaction with specific NuA4 subunits, histone acetylation, and chromatin targeting. These results provide new insights for the conserved, essential nature of Epl1 and its homologs, such as EPC1/2 in humans, which is frequently altered in cancers.

Promotion of Cell Viability and Histone Gene Expression by the Acetyltransferase Gcn5 and the Protein Phosphatase PP2A in Saccharomyces cerevisiae.

Histone modifications direct chromatin-templated events in the genome and regulate access to DNA sequence information. There are multiple types of modifications, and a common feature is their dynamic nature. An essential step for understanding their regulation, therefore, lies in characterizing the enzymes responsible for adding and removing histone modifications. Starting with a dosage-suppressor screen in Saccharomyces cerevisiae, we have discovered a functional interaction between the acetyltransferase Gcn5 and the protein phosphatase 2A (PP2A) complex, two factors that regulate post-translational modifications. We find that RTS1, one of two genes encoding PP2A regulatory subunits, is a robust and specific high-copy suppressor of temperature sensitivity of gcn5∆ and a subset of other gcn5∆ phenotypes. Conversely, loss of both PP2A(Rts1) and Gcn5 function in the SAGA and SLIK/SALSA complexes is lethal. RTS1 does not restore global transcriptional defects in gcn5∆; however, histone gene expression is restored, suggesting that the mechanism of RTS1 rescue includes restoration of specific cell cycle transcripts. Pointing to new mechanisms of acetylation-phosphorylation cross-talk, RTS1 high-copy rescue of gcn5∆ growth requires two residues of H2B that are phosphorylated in human cells. These data highlight the potential significance of dynamic phosphorylation and dephosphorylation of these deeply conserved histone residues for cell viability.

Functions for diverse metabolic activities in heterochromatin.

Growing evidence demonstrates that metabolism and chromatin dynamics are not separate processes but that they functionally intersect in many ways. For example, the lysine biosynthetic enzyme homocitrate synthase was recently shown to have unexpected functions in DNA damage repair, raising the question of whether other amino acid metabolic enzymes participate in chromatin regulation. Using an in silico screen combined with reporter assays, we discovered that a diverse range of metabolic enzymes function in heterochromatin regulation. Extended analysis of the glutamate dehydrogenase 1 (Gdh1) revealed that it regulates silent information regulator complex recruitment to telomeres and ribosomal DNA. Enhanced N-terminal histone H3 proteolysis is observed in GDH1 mutants, consistent with telomeric silencing defects. A conserved catalytic Asp residue is required for Gdh1's functions in telomeric silencing and H3 clipping. Genetic modulation of α-ketoglutarate levels demonstrates a key regulatory role for this metabolite in telomeric silencing. The metabolic activity of glutamate dehydrogenase thus has important and previously unsuspected roles in regulating chromatin-related processes.

The Set3 Complex Antagonizes the MYST Acetyltransferase Esa1 in the DNA Damage Response.

Acetylation is a dynamic posttranslational modification that contributes to chromatin-regulated processes, including DNA replication, repair, recombination, and gene expression. Acetylation is controlled by complexes containing opposing lysine and histone acetyltransferase (KAT and HAT) and deacetylase (KDAC and HDAC) activities. The essential MYST family Esa1 KAT acetylates core histones and many nonhistone substrates. Phenotypes of esa1 mutants include transcriptional silencing and activation defects, impaired growth at high temperatures, and sensitivity to DNA damage. The KDAC Rpd3 was previously identified as an activity opposing Esa1, as its deletion suppresses growth and silencing defects of esa1 mutants. However, loss of Rpd3 does not suppress esa1 DNA damage sensitivity. In this work, we identified Hos2 as a KDAC counteracting ESA1 in the damage response. Deletion of HOS2 resulted in changes of esa1's transcriptional response upon damage. Further, loss of HOS2 or components of the Set3 complex (Set3C) in which it acts specifically suppressed damage sensitivity and restored esa1 histone H4 acetylation. This rescue was mediated via loss of either Set3C integrity or of its binding to dimethylated histone H3K4. Our results thus add new insight into the interactions of an essential MYST acetyltransferase with diverse deacetylases to respond specifically to environmental and physiological challenges.

A moonlighting metabolic protein influences repair at DNA double-stranded breaks.

Catalytically active proteins with divergent dual functions are often described as 'moonlighting'. In this work we characterize a new, chromatin-based function of Lys20, a moonlighting protein that is well known for its role in metabolism. Lys20 was initially described as homocitrate synthase (HCS), the first enzyme in the lysine biosynthetic pathway in yeast. Its nuclear localization led to the discovery of a key role for Lys20 in DNA damage repair through its interaction with the MYST family histone acetyltransferase Esa1. Overexpression of Lys20 promotes suppression of DNA damage sensitivity of esa1 mutants. In this work, by taking advantage of LYS20 mutants that are active in repair but not in lysine biosynthesis, the mechanism of suppression of esa1 was characterized. First we analyzed the chromatin landscape of esa1 cells, finding impaired histone acetylation and eviction. Lys20 was recruited to sites of DNA damage, and its overexpression promoted enhanced recruitment of the INO80 remodeling complex to restore normal histone eviction at the damage sites. This study improves understanding of the evolutionary, structural and biological relevance of independent activities in a moonlighting protein and links metabolism to DNA damage repair.

Tyrosine phosphorylation of histone H2A by CK2 regulates transcriptional elongation.

Post-translational histone modifications have a critical role in regulating transcription, the cell cycle, DNA replication and DNA damage repair. The identification of new histone modifications critical for transcriptional regulation at initiation, elongation or termination is of particular interest. Here we report a new layer of regulation in transcriptional elongation that is conserved from yeast to mammals. This regulation is based on the phosphorylation of a highly conserved tyrosine residue, Tyr 57, in histone H2A and is mediated by the unsuspected tyrosine kinase activity of casein kinase 2 (CK2). Mutation of Tyr 57 in H2A in yeast or inhibition of CK2 activity impairs transcriptional elongation in yeast as well as in mammalian cells. Genome-wide binding analysis reveals that CK2α, the catalytic subunit of CK2, binds across RNA-polymerase-II-transcribed coding genes and active enhancers. Mutation of Tyr 57 causes a loss of H2B mono-ubiquitination as well as H3K4me3 and H3K79me3, histone marks associated with active transcription. Mechanistically, both CK2 inhibition and the H2A(Y57F) mutation enhance H2B deubiquitination activity of the Spt-Ada-Gcn5 acetyltransferase (SAGA) complex, suggesting a critical role of this phosphorylation in coordinating the activity of the SAGA complex during transcription. Together, these results identify a new component of regulation in transcriptional elongation based on CK2-dependent tyrosine phosphorylation of the globular domain of H2A.

The SAGA histone deubiquitinase module controls yeast replicative lifespan via Sir2 interaction.

We have analyzed the yeast replicative lifespan of a large number of open reading frame (ORF) deletions. Here, we report that strains lacking genes SGF73, SGF11, and UBP8 encoding SAGA/SLIK complex histone deubiquitinase module (DUBm) components are exceptionally long lived. Strains lacking other SAGA/SALSA components, including the acetyltransferase encoded by GCN5, are not long lived; however, these genes are required for the lifespan extension observed in DUBm deletions. Moreover, the SIR2-encoded histone deacetylase is required, and we document both a genetic and physical interaction between DUBm and Sir2. A series of studies assessing Sir2-dependent functions lead us to propose that DUBm strains are exceptionally long lived because they promote multiple prolongevity events, including reduced rDNA recombination and altered silencing of telomere-proximal genes. Given that ataxin-7, the human Sgf73 ortholog, causes the neurodegenerative disease spinocerebellar ataxia type 7, our findings indicate that the genetic and epigenetic interactions between DUBm and SIR2 will be relevant to neurodegeneration and aging.

Bypassing the requirement for an essential MYST acetyltransferase.

Histone acetylation is a key regulatory feature for chromatin that is established by opposing enzymatic activities of lysine acetyltransferases (KATs/HATs) and deacetylases (KDACs/HDACs). Esa1, like its human homolog Tip60, is an essential MYST family enzyme that acetylates histones H4 and H2A and other nonhistone substrates. Here we report that the essential requirement for ESA1 in Saccharomyces cerevisiae can be bypassed upon loss of Sds3, a noncatalytic subunit of the Rpd3L deacetylase complex. By studying the esa1∆ sds3∆ strain, we conclude that the essential function of Esa1 is in promoting the cellular balance of acetylation. We demonstrate this by fine-tuning acetylation through modulation of HDACs and the histone tails themselves. Functional interactions between Esa1 and HDACs of class I, class II, and the Sirtuin family define specific roles of these opposing activities in cellular viability, fitness, and response to stress. The fact that both increased and decreased expression of the ESA1 homolog TIP60 has cancer associations in humans underscores just how important the balance of its activity is likely to be for human well-being.