Jean-René Huynh

Transposable elements (TEs) pose a major threat to genome integrity in the germline, where the piRNA pathway ensures heritable TEs silencing. How the chromatin environment that enables piRNA biogenesis is first established during oogenesis remains unclear. Here, we identify the histone variant His2Av, the single H2A variant in Drosophila combining H2A.Z (transcriptional) and H2A.X (DNA repair) features, as a critical regulator of the earliest steps of piRNA pathway activation. Germline-specific depletion of His2Av disrupts transcription of piRNA pathway genes, abolishes dual-strand piRNA cluster transcription, and triggers strong TEs derepression. These defects are associated with loss of Rhino recruitment despite intact H3K9me3, suggesting that His2Av contributes to the establishment of a specialized heterochromatin permissive for noncanonical piRNA cluster transcription. His2Av depletion also causes DNA damage, replication stress, and activation of Chk2- and Claspin-dependent checkpoints, leading to oogenesis arrest. Remarkably, overexpression of RNase H, but not a catalytic-dead variant, robustly rescues oocyte development, suggesting that replication stress is a major source of DNA damage in His2Av mutants. Finally, using a separation-of-function approach with a C-terminal truncation inhibiting H2A.X-like activity, we show that the essential germline role of His2Av is transcriptional (H2A.Z-like). Together, our findings reveal that His2Av primes germline chromatin for piRNA pathway initiation while limiting transcription-replication conflicts during early oogenesis.

Recherche Pages web : https://germcells.fr ; https://www.college-de-france.fr/evolution-and-development-of-germ-cells. Germ cells are the only cells, which are transmitted from one generation to the next and can be considered immortal. Germ cells produce highly specialized cells, called gametes, which carry the genetic and cytoplasmic information defining a given species and which can initiate the formation of an entire organism. Understanding how germ cells develop is not only of paramount m...

Cytokinetic abscission is the final step of cell division, resulting in two separate daughter cells. While abscission is typically complete across most cell types, germline cells, which produce sexual gametes, do not finish cytokinesis, maintaining connections between sister cells. These connections are essential for sharing cytoplasm as they differentiate into oocyte and sperm. First, we outline the molecular events of cytokinesis during both complete and delayed abscission, highlighting the role of the ESCRT-III proteins. We then focus on recent discoveries that reveal the molecular mechanisms blocking abscission in Drosophila germline cells. The enzyme Usp8 was identified as vital for ensuring incomplete cytokinesis through the regulation of ESCRT-III ubiquitination and localization. Finally, we explore how the processes of incomplete cytokinesis could hold evolutionary importance, suggesting additional studies into choanoflagellates to comprehend the origins of multicellularity.

Germline cells produce gametes, which are specialized cells essential for sexual reproduction. Germline cells first amplify through several rounds of mitosis before switching to the meiotic program, which requires specific sets of proteins for DNA recombination, chromosome pairing, and segregation. Surprisingly, we previously found that some proteins of the synaptonemal complex, a prophase I meiotic structure, are already expressed and required in the mitotic region of Drosophila females. Here, to assess if additional meiotic genes were expressed earlier than expected, we isolated mitotic and meiotic cell populations to compare their RNA content. Our transcriptomic analysis reveals that all known meiosis I genes are already expressed in the mitotic region; however, only some of them are translated. As a case study, we focused on mei-W68, the Drosophila homolog of Spo11, to assess its expression at both the mRNA and protein levels and used different mutant alleles to assay for a premeiotic function. We could not detect any functional role for Mei-W68 during homologous chromosome pairing in dividing germ cells. Our study paves the way for further functional analysis of meiotic genes expressed in the mitotic region.

Germline cells produce gametes, which are specialized cells essential for sexual reproduction. Germline cells first amplify through several rounds of mitosis before switching to the meiotic program, which requires specific sets of proteins for DNA recombination, chromosome pairing and segregation. Surprisingly, we previously found that some proteins of the synaptonemal complex, a prophase I meiotic structure, are already expressed and required in the mitotic region of Drosophila females. Here, to assess if additional meiotic genes were expressed earlier than expected, we isolated mitotic and meiotic cell populations to compare their RNA content. Our transcriptomic analysis reveals that all known meiosis I genes are already expressed in the mitotic region, however, only some of them are translated. As a case study, we focused on mei-W68 , the Drosophila homologue of Spo11 , to assess its expression at both the mRNA and protein levels, and used different mutant alleles to assay for a pre-meiotic function. We could not detect any functional role for Mei-W68 during homologous chromosome pairing in dividing germ cells. Our study paves the way for further functional analysis of meiotic genes expressed in the mitotic region. Article Summary: Germline cells, crucial for sexual reproduction, were thought to switch to meiosis only after several rounds of mitosis. Surprisingly, a few meiotic proteins were found active in the mitotic phase of female flies. Here, we discovered that all known meiosis genes were expressed during mitosis, but only some produced proteins. This study suggests that genes related to reproduction are active earlier than expected, prompting further exploration into their functions during early cell division.

Abscission is the final step of cytokinesis that allows the physical separation of sister cells through the scission of the cellular membrane. This deformation is driven by ESCRT-III proteins, which can bind membranes and form dynamic helices. A crucial step in abscission is the recruitment of ESCRT-III proteins at the right time and place. Alix is one of the best characterized proteins that recruits ESCRT-III proteins from yeast to mammals. However, recent studies in vivo have revealed that pathways acting independently or redundantly with Alix are also required at abscission sites in different cellular contexts. Here, we show that Lgd acts redundantly with Alix to properly localize ESCRT-III to the abscission site in germline stem cells (GSCs) during Drosophila oogenesis. We further demonstrate that Lgd is phosphorylated at multiple sites by the CycB/Cdk1 kinase. We found that these phosphorylation events potentiate the activity of Shrub, a Drosophila ESCRT-III, during abscission of GSCs. Our study reveals that redundancy between Lgd and Alix, and coordination with the cell cycle kinase Cdk1, confers robust and timely abscission of Drosophila germline stem cells.

Recherche Responsable : Jean-René Huynh Pages web : https://www.college-de-france.fr/evolution-and-development-of-germ-cells ; www.germcells.fr. Germ cells are the only cells, which are transmitted from one generation to the next and can be considered immortal. Germ cells produce highly specialized cells, called gametes, which carry the genetic and cytoplasmic information defining a given species and which can initiate the formation of an entire organism. Understanding how germ cells develop ...

Médecine/sciences (M/S), revue internationale dans le domaine de la recherche biologique, médicale et en santé