Gamete formation resets the aging clock in yeast

Spatiotemporal Dynamic Regulation of Organelles During Meiotic Development, Insights From Fungi

Eukaryotic cell development involves precise regulation of organelle activity and dynamics, which adapt the cell architecture and metabolism to the changing developmental requirements. Research in various fungal model organisms has disclosed that meiotic development involves precise spatiotemporal regulation of the formation and dynamics of distinct intracellular membrane compartments, including peroxisomes, mitochondria and distinct domains of the endoplasmic reticulum, comprising its peripheral domains and the nuclear envelope. This developmental regulation implicates changes in the constitution and dynamics of these organelles, which modulate their structure, abundance and distribution. Furthermore, selective degradation systems allow timely organelle removal at defined meiotic stages, and regulated interactions between membrane compartments support meiotic-regulated organelle dynamics. This dynamic organelle remodeling is implicated in conducting organelle segregation during meiotic differentiation, and defines quality control regulatory systems safeguarding the inheritance of functional membrane compartments, promoting meiotic cell rejuvenation. Moreover, organelle remodeling is important for proper activity of the cytoskeletal system conducting meiotic nucleus segregation, as well as for meiotic differentiation. The orchestrated regulation of organelle dynamics has a determinant contribution in the formation of the renewed genetically-diverse offspring of meiosis.

Ready, SET, Go: Post-translational regulation of the histone lysine methylation network in budding yeast

Histone lysine methylation is a key epigenetic modification that regulates eukaryotic transcription. Here, we comprehensively review the function and regulation of the histone methylation network in the budding yeast and model eukaryote, Saccharomyces cerevisiae. First, we outline the lysine methylation sites that are found on histone proteins in yeast (H3K4me1/2/3, H3K36me1/2/3, H3K79me1/2/3, and H4K5/8/12me1) and discuss their biological and cellular roles. Next, we detail the reduced but evolutionarily conserved suite of methyltransferase (Set1p, Set2p, Dot1p, and Set5p) and demethylase (Jhd1p, Jhd2p, Rph1p, and Gis1p) enzymes that are known to control histone lysine methylation in budding yeast cells. Specifically, we illustrate the domain architecture of the methylation enzymes and highlight the structural features that are required for their respective functions and molecular interactions. Finally, we discuss the prevalence of post-translational modifications on yeast histone methylation enzymes and how phosphorylation, acetylation, and ubiquitination in particular are emerging as key regulators of enzyme function. We note that it will be possible to completely connect the histone methylation network to the cell's signaling system, given that all methylation sites and cognate enzymes are known, most phosphosites on the enzymes are known, and the mapping of kinases to phosphosites is tractable owing to the modest set of protein kinases in yeast. Moving forward, we expect that the rich variety of post-translational modifications that decorates the histone methylation machinery will explain many of the unresolved questions surrounding the function and dynamics of this intricate epigenetic network.

The ESCRT-III complex is required for nuclear pore complex sequestration and regulates gamete replicative lifespan in budding yeast meiosis

Cellular aging occurs as a cell loses its ability to maintain homeostasis. Aging cells eliminate damaged cellular compartments and other senescence factors via self-renewal. The mechanism that regulates cellular rejuvenation remains to be further elucidated. Using budding yeast gametogenesis as a model, we show here that the endosomal sorting complex required for transport (ESCRT) III regulates nuclear envelope organization. During gametogenesis, the nuclear pore complex (NPC) and other senescence factors are sequestered away from the prospore nuclei. We show that the LEM-domain protein Heh1 (Src1) facilitates the nuclear recruitment of ESCRT-III, which is required for meiotic NPC sequestration and nuclear envelope remodeling. Furthermore, ESCRT-III-mediated nuclear reorganization appears to be critical for gamete rejuvenation, as hindering this process curtails either directly or indirectly the replicative lifespan in gametes. Our findings demonstrate the importance of ESCRT-III in nuclear envelope remodeling and its potential role in eliminating senescence factors during gametogenesis.

Forever young: the key to rejuvenation during gametogenesis

Cell aging is the result of deteriorating competence in maintaining cellular homeostasis and quality control. Certain cell types are able to rejuvenate through asymmetric cell division by excluding aging factors, including damaged cellular compartments and extrachromosomal rDNA circles, from entering the daughter cell. Recent findings from the budding yeast S. cerevisiae have shown that gametogenesis represents another type of cellular rejuvenation. Gametes, whether produced by an old or a young mother cell, are granted a renewed replicative lifespan through the formation of a fifth nuclear compartment that sequesters the harmful senescence factors accumulated by the mother. Here, we describe the importance and mechanism of cellular remodeling at the nuclear envelope mediated by ESCRT-III and the LEM-domain proteins, with a focus on nuclear pore biogenesis and chromatin interaction during gamete rejuvenation.

The dynamic nuclear periphery as a facilitator of gamete health and rejuvenation

The nuclear periphery is a hotspot for the accumulation of age-induced damage in eukaryotic cells. The types of damage that occur at the periphery and their phenotypic consequences have begun to be characterized; however, the mechanisms by which cells repair or eliminate nuclear damage remain poorly understood. Using budding yeast meiosis as a natural system to study cellular rejuvenation, we recently discovered a novel nuclear quality control event, in which age-induced damage is sequestered away from dividing chromosomes to a discarded nuclear compartment that we term the GUNC (for "Gametogenesis Uninherited Nuclear Compartment"). Interestingly, extensive nuclear remodeling occurs even in young cells, including a surprising modularity of the nuclear pore complex, suggesting a general contribution to gamete fitness. In this review, we discuss these findings in the context of recent evidence that the nuclear periphery is a highly dynamic region critical for cellular health.

Meiotic cellular rejuvenation is coupled to nuclear remodeling in budding yeast

Production of healthy gametes in meiosis relies on the quality control and proper distribution of both nuclear and cytoplasmic contents. Meiotic differentiation naturally eliminates age-induced cellular damage by an unknown mechanism. Using time-lapse fluorescence microscopy in budding yeast, we found that nuclear senescence factors - including protein aggregates, extrachromosomal ribosomal DNA circles, and abnormal nucleolar material - are sequestered away from chromosomes during meiosis II and subsequently eliminated. A similar sequestration and elimination process occurs for the core subunits of the nuclear pore complex in both young and aged cells. Nuclear envelope remodeling drives the formation of a membranous compartment containing the sequestered material. Importantly, de novo generation of plasma membrane is required for the sequestration event, preventing the inheritance of long-lived nucleoporins and senescence factors into the newly formed gametes. Our study uncovers a new mechanism of nuclear quality control and provides insight into its function in meiotic cellular rejuvenation.

The life of [PSI]

The AAA+ disaggregase Hsp104 is essential for the maintenance and inheritance of nearly all known prions of the yeast Saccharomyces cerevisiae. Uniquely for [PSI⁺], the prion form of the Sup35 protein, there seem to be two activities, involving differing co-chaperones, by which Hsp104 affects the inheritance of [PSI⁺], the prion form of the Sup35 protein. Each pathway is also involved in protection against ageing, one through disaggregation of damaged proteins and the other through their retention in the mother cell during budding. Mutations in both Hsp104 and Sup35 affect prion inheritance by one or other of these pathways, as does manipulation of either Hsp104 enzyme activity or expression, in both vegetative (budding) divisions and in sporulation. Based on our recent finding (Ness et al. in Molec Microbiol 104:125–143, 2017) we suggest that the management of the heritable prion forms of Sup35 in [PSI⁺] cells in sporulation may be a marker for a role for Hsp104 in rejuvenation during sporulation.

p53, p63 and p73 in the wonderland of S. cerevisiae

Since its discovery in 1979, p53 has been on the forefront of cancer research. It is considered a master gene of cancer suppression and is found mutated in around 50% of all human tumors. In addition, the progressive identification of p53-related transcription factors p63 and p73 as well as their multiple isoforms have added further layers of complexity to an already dense network. Among the numerous models used to unravel the p53 family mysteries, S. cerevisiae has been particularly useful. This seemingly naive model allows the expression of a functional human p53 and thus the assessment of p53 intrinsic transcriptional activity. The aim of this article is to review the various contributions that the budding yeast has made to the understanding of p53, p63 and p73 biology and to envision new possible directions for yeast-based assays in the field of cancer as well as other p53-family-related diseases.

Remodelling germ cells by intercellular cannibalism

Work from the early 1980s reported strange lobes protruding from Caenorhabditis elegans germ cell precursors. However, the fate and potential significance of these lobes remained unexplored for decades. Now, neighbouring endodermal cells are shown to sever and digest these lobes, in an unexpected process of 'intercellular cannibalism', which could play an important part in regulating primordial germ cells.

Effect of temperature on replicative aging of the budding yeast Saccharomyces cerevisiae

The use of the budding yeast Saccharomyces cerevisiae in gerontological studies was based on the assumption that the reproduction limit of a single cell (replicative aging) is a consequence of accumulation of a hypothetical universal "senescence factor" within the mother cell. However, some evidence suggests that molecules or structures proposed as the "aging factor", such as rDNA circles, oxidatively damaged proteins (with carbonyl groups) or mitochondria, have little effect on replicative lifespan of yeast cells. Our results also suggest that protein aggregates associated with Hsp104, treated as a marker of yeast aging, do not seem to affect the numeric value of replicative lifespan of yeast. What these results indicate, however, is the need for finding a different way of expressing age and longevity of yeast cells instead of the commonly used number of daughters produced over units of time, as in the case of other organisms. In this paper, we show that the temperature has a stronger influence on the time of life (the total lifespan) than on the reproductive potential of yeast cells.

A budding yeast's perspective on aging: the shape I'm in

Aging is exemplified by progressive, deleterious changes that increase the probability of death. However, while the effects of age are easy to recognize, identification of the processes involved has proved to be much more difficult. Somewhat surprisingly, research using the budding yeast has had a profound impact on our current understanding of the mechanisms involved in aging. Herein, we examine the biological significance and implications surrounding the observation that genetic pathways involved in the modulation of aging and the determination of lifespan in yeast are highly complicated and conserved.

Maintaining integrity of germline DNA: individuals age, species do not

All life forms are under constant assault, resulting in an accumulation of damage within each individual, in both somatic and germline cells. The obvious causes are: (1) mutations from radiation, chemical reactions like peroxidation and errors in replicating genetic material; (2) injury due to environmental insults, such as chemical alteration of proteins by reactive oxygen species; (3) epigenetic errors, such as failure of appropriate maintenance methylation of cytosines of DNA; and (4) numerous other problems, including retroviral invasions, inflammation and unhealthy microbiomes. Collectively, these phenomena constitute aging and/or certain disease states. Nature has developed numerous mechanisms to counteract these problems, such as proofreading enzymes, ubiquitous antioxidants and apoptotic death of unfit cells. However, none of these is completely effective. Although individuals accumulate damage, species usually do not become increasingly damaged; however, this could be one of the mechanisms for eventual extinction or evolution to a different species, the apparent fate of essentially all species. Nevertheless, germline DNA appears to remain sufficiently pristine to maintain fairly stable phenotypes over many generations. How do species avoid accumulating damage when composed of individuals that do? One broad answer seems to be reproductive redundancy followed by elimination of defects through the death of gametes, embryos, fetuses, neonates and postpubertal individuals, with the culling pressure increasing as potential parents age. Another major force appears to be evolutionary pressure; individuals that best fit the environment out-reproduce those that fit less well. What is impressive is that older and older parents continue to have offspring that are nearly as pristine as those of younger parents, even though their germline cells have continued to age. Although the offspring of old parents are not as fit, on average, as those of young parents, differences are small and, in some species, compensated for by superior parenting with accumulated experience. To conclude, it appears that species do not age, even though they are composed of individuals whose somatic and germline cells have aged.

Mouse primordial germ cells produce cysts that partially fragment prior to meiosis

Mammalian germ cells divide mitotically and form nests of associated cells just prior to entering meiosis. At least some nests contain germline cysts that arise by synchronous, incomplete mitotic divisions, but others may form by aggregation. To systematically investigate early murine germ cell development, we lineage marked the progeny of individual, newly arrived primordial germ cells in the E10.5 gonad. All the marked germ cells initially develop into clones containing two, four or eight cells, indicating cyst formation. Surprisingly, growing cysts in both sexes partially fragment into smaller cysts prior to completion and associate with cysts from unrelated progenitors. At the time divisions cease, female clones comprise five cysts on average that eventually give rise to about six primordial follicles. Male cyst cells break apart and probably become spermatogonial stem cells. Thus, cysts are invariant units of mouse germ cell development and cyst fragmentation provides insight into the amplification of spermatogonial stem cells and the origin of primordial follicles.

The Sum1/Ndt80 transcriptional switch and commitment to meiosis in Saccharomyces cerevisiae

Cells encounter numerous signals during the development of an organism that induce division, differentiation, and apoptosis. These signals need to be present for defined intervals in order to induce stable changes in the cellular phenotype. The point after which an inducing signal is no longer needed for completion of a differentiation program can be termed the "commitment point." Meiotic development in the yeast Saccharomyces cerevisiae (sporulation) provides a model system to study commitment. Similar to differentiation programs in multicellular organisms, the sporulation program in yeast is regulated by a transcriptional cascade that produces early, middle, and late sets of sporulation-specific transcripts. Although critical meiosis-specific events occur as early genes are expressed, commitment does not take place until middle genes are induced. Middle promoters are activated by the Ndt80 transcription factor, which is produced and activated shortly before most middle genes are expressed. In this article, I discuss the connection between Ndt80 and meiotic commitment. A transcriptional regulatory pathway makes NDT80 transcription contingent on the prior expression of early genes. Once Ndt80 is produced, the recombination (pachytene) checkpoint prevents activation of the Ndt80 protein. Upon activation, Ndt80 triggers a positive autoregulatory loop that leads to the induction of genes that promote exit from prophase, the meiotic divisions, and spore formation. The pathway is controlled by multiple feed-forward loops that give switch-like properties to the commitment transition. The conservation of regulatory components of the meiotic commitment pathway and the recently reported ability of Ndt80 to increase replicative life span are discussed.