A feedback loop between heterochromatin and the nucleopore complex controls germ-cell to oocyte transition duringDrosophilaoogenesis

Abstract

SummaryGerm cells differentiate into oocytes that become totipotent upon fertilization. How the highly specialized oocyte acquires this distinct cell fate is poorly understood. During Drosophila oogenesis, H3K9me3 histone methyltransferase SETDB1 translocates from the cytoplasm to the nucleus of germ cells concurrent with oocyte specification. Here, we discovered that nuclear SETDB1 is required to silence a cohort of differentiation-promoting genes by mediating their heterochromatinization. Intriguingly, SETDB1 is also required for the upregulation of 18 of the ~30 nucleoporins (Nups) that comprise the nucleopore complex (NPC). NPCs in turn anchor SETDB1-dependent heterochromatin at the nuclear periphery to maintain H3K9me3 and gene silencing in the egg chambers. Aberrant gene expression due to loss of SETDB1 or Nups results in loss of oocyte identity, cell death and sterility. Thus, a feedback loop between heterochromatin and NPCs promotes transcriptional reprogramming at the onset of oocyte specification that is critical to establish oocyte identity.

Oo-site: A dashboard to visualize gene expression during Drosophila oogenesis suggests meiotic entry is regulated post-transcriptionally

“…Our bulk RNA-seq data, as well as the available scRNA-seq data indicated that RpS19b was highly expressed in GSCs, decreased during differentiation in the cyst stages and was greatly decreased in expression in early egg chambers, consistent with previous reports ( Fig. 2 A,B) ( McCarthy et al, 2021 ; Sarkar et al, 2021 preprint). To further validate this expression pattern, we probed the expression of RpS19b in vivo using in situ hybridization as well as an RpS19b::GFP line that is under endogenous control elements ( McCarthy et al, 2021 ).…”

“…A modified in situ hybridization procedure for Drosophila ovaries was followed from Sarkar et al (2021) . Probes were designed and generated by LGC Biosearch Technologies using Stellaris ® RNA FISH Probe Designer, with specificity to target base pairs of target mRNAs.…”

Oo-site: A dashboard to visualize gene expression during Drosophila oogenesis reveals meiotic entry is regulated post-transcriptionally

“…To determine if the bulk RNA-seq data recapitulates genuine changes in gene expression, we compared the expression of ribosomal small subunit protein 19b ( RpS19b ) in bulk RNA-seq to scRNA-seq data. Our bulk RNA-seq data, as well as the available scRNA-seq data indicated that RpS19b was highly expressed in GSCs, decreased during differentiation in the cyst stages and was greatly decreased in expression in early egg chambers, consistent with previous reports ( Fig 2A-B ) (McCarthy et al, 2021; Sarkar et al, 2021). To further validate this expression pattern, we probed the expression of RpS19b in vivo using in situ hybridization as well as an RpS19b::GFP line that is under endogenous control elements (McCarthy et al, 2021).…”

“…A modified in situ hybridization procedure for Drosophila ovaries was followed from Sarkar et al . (2021).…”

SafeguardingDrosophilafemale germ cell identity depends on an H3K9me3 mini domain guided by a ZAD zinc finger protein

“…One type includes genes usually expressed in the testis [9]. A second class includes genes that are typically expressed in undifferentiated female germ cells but are silenced once the oocyte is specified, such as ribosomal protein S19b [70]. These few examples of context dependent H3K9me3 gene silencing suggest a finely tuned guidance mechanism in the female germline.…”