Cellular quality control during gametogenesis

Mitochondrial fission surveillance is coupled to Caenorhabditis elegans DNA and chromosome segregation integrity

Mitochondrial fission and fusion are tightly regulated to specify mitochondrial abundance, localization, and arrangement during cell division as well as in the diverse differentiated cell types and physiological states. However, the regulatory pathways for such mitochondrial dynamics are less explored than the mitochondrial fission and fusion components. Here we report a large-scale screen for genes that regulate mitochondrial fission. Mitochondrial fission defects cause a characteristic uneven fluorescent pattern in embryos carrying mitochondrial stress reporter genes. Using this uneven activation, we performed RNAi screens that identified 3 kinase genes from a ~ 500-kinase library and another 11 genes from 3,300 random genes that function in mitochondrial fission. Many of these identified genes play roles in chromosome segregation. We found that chromosome missegregation and genome instability lead to dysregulation of mitochondrial fission, possibly independent of DRP-1. ATL-1, the C. elegans ATR orthologue, plays a potentially protective role in alleviating the mitochondrial fission defect caused by chromosome missegregation. This establishes a screening paradigm for identifying mitochondrial fission regulators, which reveals the potential role of ATR in surveilling mitochondrial fission to mitigate dysregulation caused by improper chromosome segregation.

The localization and function of the moonlighting protein Clathrin during oocyte maturation

Clathrin is one of the leading players in the endocytic process during oocyte maturation. Immunofluorescence and transmission electron analysis on fully-grown germinal vesicle (GV) mouse oocytes shows Clathrin localization on the cortical region with three peculiar patterns: complete, incomplete, and half-moon. The first configuration is characterized by Clathrin lattices along the cortex; the second is represented by Clathrin lattices interrupted by invaginations forming coated vesicles as an indication of active endocytosis. The half-moon profile, the less frequent but the most interesting one, refers to Clathrin lattices distributed to one-half of the cell. The in vivo analysis of organelles' positioning and cytoplasmic rearrangements, performed to understand the possible relation between endocytosis and oocyte maturation, suggests that the half-moon pattern indicates those fully-grown oocytes that may have likely undergone Germinal Vesicle Breakdown, MI, and MII. Our results show that, before oocytes undergo maturation, Clathrin localizes on the side of the cell, opposite to future spindle migration, thus marking spindle orientation in mouse oocytes.

Remodeling of the secretory pathway is coordinated with de novo membrane formation in budding yeast gametogenesis

Gametogenesis in budding yeast involves a large-scale rearrangement of membrane traffic to allow the de novo formation of a membrane, called the prospore membrane (PSM). However, the mechanism underlying this event is not fully elucidated. Here, we show that the number of endoplasmic reticulum exit sites (ERES) per cell fluctuates and switches from decreasing to increasing upon the onset of PSM formation. Reduction in ERES number, presumably accompanying a transient stall in membrane traffic, resulting in the loss of preexisting Golgi apparatus from the cell, was followed by local ERES regeneration, leading to Golgi reassembly in nascent spores. We have revealed that protein phosphatase-1 (PP-1) and its development-specific subunit, Gip1, promote ERES regeneration through Sec16 foci formation. Furthermore, sed4Δ, a mutant with impaired ERES formation, showed defects in PSM growth and spore formation. Thus, ERES regeneration in nascent spores facilitates the segregation of membrane traffic organelles, leading to PSM growth.

Waves of regulated protein expression and phosphorylation rewire the proteome to drive gametogenesis in budding yeast

Sexually reproducing eukaryotes employ a developmentally regulated cell division program-meiosis-to generate haploid gametes from diploid germ cells. To understand how gametes arise, we generated a proteomic census encompassing the entire meiotic program of budding yeast. We found that concerted waves of protein expression and phosphorylation modify nearly all cellular pathways to support meiotic entry, meiotic progression, and gamete morphogenesis. Leveraging this comprehensive resource, we pinpointed dynamic changes in mitochondrial components and showed that phosphorylation of the FoF1-ATP synthase complex is required for efficient gametogenesis. Furthermore, using cryoET as an orthogonal approach to visualize mitochondria, we uncovered highly ordered filament arrays of Ald4ALDH2, a conserved aldehyde dehydrogenase that is highly expressed and phosphorylated during meiosis. Notably, phosphorylation-resistant mutants failed to accumulate filaments, suggesting that phosphorylation regulates context-specific Ald4ALDH2 polymerization. Overall, this proteomic census constitutes a broad resource to guide the exploration of the unique sequence of events underpinning gametogenesis.

Gametogenesis: Exploring an Endogenous Rejuvenation Program to Understand Cellular Aging and Quality Control

Gametogenesis is a conserved developmental program whereby a diploid progenitor cell differentiates into haploid gametes, the precursors for sexually reproducing organisms. In addition to ploidy reduction and extensive organelle remodeling, gametogenesis naturally rejuvenates the ensuing gametes, leading to resetting of life span. Excitingly, ectopic expression of the gametogenesis-specific transcription factor Ndt80 is sufficient to extend life span in mitotically dividing budding yeast, suggesting that meiotic rejuvenation pathways can be repurposed outside of their natural context. In this review, we highlight recent studies of gametogenesis that provide emerging insight into natural quality control, organelle remodeling, and rejuvenation strategies that exist within a cell. These include selective inheritance, programmed degradation, and de novo synthesis, all of which are governed by the meiotic gene expression program entailing many forms of noncanonical gene regulation. Finally, we highlight critical questions that remain in the field and provide perspective on the implications of gametogenesis research on human health span.

Conservation of oocyte development in germline cysts from Drosophila to mouse

Recent studies show that pre-follicular mouse oogenesis takes place in germline cysts, highly conserved groups of oogonial cells connected by intercellular bridges that develop as nurse cells as well as an oocyte. Long studied in Drosophila and insect gametogenesis, female germline cysts acquire cytoskeletal polarity and traffic centrosomes and organelles between nurse cells and the oocyte to form the Balbiani body, a conserved marker of polarity. Mouse oocyte development and nurse cell dumping are supported by dynamic, cell-specific programs of germline gene expression. High levels of perinatal germ cell death in this species primarily result from programmed nurse cell turnover after transfer rather than defective oocyte production. The striking evolutionary conservation of early oogenesis mechanisms between distant animal groups strongly suggests that gametogenesis and early embryonic development in vertebrates and invertebrates share even more in common than currently believed.

Meiotic cDNA libraries reveal gene truncations and mitochondrial proteins important for competitive fitness in Saccharomyces cerevisiae

Gametogenesis is an evolutionarily conserved developmental program whereby a diploid progenitor cell undergoes meiosis and cellular remodeling to differentiate into haploid gametes, the precursors for sexual reproduction. Even in the simple eukaryotic organism Saccharomyces cerevisiae, the meiotic transcriptome is very rich and complex, thereby necessitating new tools for functional studies. Here, we report the construction of 5 stage-specific, inducible complementary DNA libraries from meiotic cells that represent over 84% of the genes found in the budding yeast genome. We employed computational strategies to detect endogenous meiotic transcript isoforms as well as library-specific gene truncations. Furthermore, we developed a robust screening pipeline to test the effect of each complementary DNA on competitive fitness. Our multiday proof-of-principle time course revealed 877 complementary DNAs that were detrimental for competitive fitness when overexpressed. The list included mitochondrial proteins that cause dose-dependent disruption of cellular respiration as well as library-specific gene truncations that expose a dominant negative effect on competitive growth. Together, these high-quality complementary DNA libraries provide an important tool for systematically identifying meiotic genes, transcript isoforms, and protein domains that are important for a specific biological function.

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.

Comparative analysis of vertebrates reveals that mouse primordial oocytes do not contain a Balbiani body

Oocytes spend the majority of their lifetime in a primordial state. The cellular and molecular biology of primordial oocytes is largely unexplored; yet, studying these is necessary to understand the mechanisms through which oocytes maintain cellular fitness for decades, and why they eventually fail with age. Here, we develop enabling methods for live-imaging based comparative characterization of Xenopus, mouse and human primordial oocytes. We show that primordial oocytes in all three vertebrate species contain active mitochondria, Golgi apparatus and lysosomes. We further demonstrate that human and Xenopus oocytes have a Balbiani body characterized by a dense accumulation of mitochondria in their cytoplasm. However, despite previous reports, we did not find a Balbiani body in mouse oocytes. Instead, we demonstrate what was previously used as a marker for the Balbiani body in mouse primordial oocytes is in fact a ring-shaped Golgi apparatus that is not functionally associated with oocyte dormancy. Our work provides the first insights into the organisation of the cytoplasm in mammalian primordial oocytes, and clarifies relative advantages and limitations of choosing different model organisms for studying oocyte dormancy.

Programmed cortical ER collapse drives selective ER degradation and inheritance in yeast meiosis

The endoplasmic reticulum (ER) carries out essential and conserved cellular functions, which depend on the maintenance of its structure and subcellular distribution. Here, we report developmentally regulated changes in ER morphology and composition during budding yeast meiosis, a conserved differentiation program that gives rise to gametes. A subset of the cortical ER collapses away from the plasma membrane at anaphase II, thus separating into a spatially distinct compartment. This programmed collapse depends on the transcription factor Ndt80, conserved ER membrane structuring proteins Lnp1 and reticulons, and the actin cytoskeleton. A subset of ER is retained at the mother cell plasma membrane and excluded from gamete cells via the action of ER-plasma membrane tethering proteins. ER remodeling is coupled to ER degradation by selective autophagy, which relies on ER collapse and is regulated by timed expression of the autophagy receptor Atg40. Thus, developmentally programmed changes in ER morphology determine the selective degradation or inheritance of ER subdomains by gametes.