Transient transcriptional silencing alters the cell cycle to promote germline stem cell differentiation in Drosophila

Abstract

Transcriptional silencing is a conserved process used by embryonic germ cells to repress somatic fate and maintain totipotency and immortality. In Drosophila, this transcriptional silencing is mediated by polar granule component (pgc). Here, we show that in the adult ovary, pgc is required for timely germline stem cell (GSC) differentiation. Pgc is expressed transiently in the immediate GSC daughter (pre-cystoblast), where it mediates a pulse of transcriptional silencing. This transcriptional silencing mediated by pgc indirectly promotes the accumulation of Cyclin B (CycB) and cell cycle progression into late-G2 phase, when the differentiation factor bag of marbles (bam) is expressed. Pgc mediated accumulation of CycB is also required for heterochromatin deposition, which protects the germ line genome against selfish DNA elements. Our results suggest that transient transcriptional silencing in the pre-cystoblast “re-programs” it away from self-renewal and toward the gamete differentiation program.

Drosophilaovarian germline stem cell cytocensor projections dynamically receive and attenuate BMP signaling

“…Genes up-regulated in GSCs are associated with gonad development, chromatin organisation and transcriptional regulation. This is consistent with germline differentiation being accompanied by a reduction in transcriptional activity and altered chromatin organisation (Flora et al, 2018;Zhang et al, 2014). Genes upregulated in CBs include those encoding factors involved in cellular metabolism, growth and protein production.…”

Coordinating Proliferation, Polarity, and Cell Fate in the Drosophila Female Germline

“…Membrane cleavage at abscission requires the Endosomal Sorting Complex Required for Transport-III (ESCRT-III) complex and the scaffold protein ALIX, and is negatively regulated by Aurora B (Eikenes et al, 2015;Matias et al, 2015). Following abscission, a brief pulse of transcriptional silencing in the presumptive cystoblast prior to Bam expression is induced by the transcriptional repressor Polar granule component (Flora et al, 2018). This allows for the accumulation of CycB, promoting cell cycle resumption and differentiation in the cystoblast.…”

Sequential regulation of maternal mRNAs through a conserved cis-acting element in their 3’UTRs

“…All strains were grown at 25°C, except RNAi crosses, which were grown at 29°C. pgcGFP and pgc Δ used in this study have been previously reported (Martinho et al 2004;Flora et al 2018).…”

“…The P-P-P (pgc promoter-pgc 5'UTR-eGFP-pgc 3'UTR) construct was generated by cloning eGFP coding sequence into a plasmid with the pgc 5'UTR and pgc 3'UTR as previously described (Flora et al 2018). The P-P-T (pgc promoter-pgc 5'UTR-eGFPtubulin 3'UTR) and P-P-K (pgc promoter-pgc 5'UTR-eGFP-K10 3'UTR) constructs were assembled by PCR amplifying a XhoI-KpnI fragment containing the α-tubulin 3'UTR (T) or K10 3'UTR (K) was then cloned into the XhoI-KpnI site of the P-P-P plasmid, respectively.…”

“…Loss of pum and nos results in failure to maintain GSCs, and this defect is thought to be 341 the result of dysregulation of differentiation-promoting mRNAs in the GSCs (Forbes and 342 Lehmann 1998;Wang and Lin 2005;Gilboa and Lehmann 2004b;Joly et al 2013). We 343 have previously shown that pgc promotes timely differentiation in the pre-CBs where it 344 is expressed (Flora et al 2018). Thus, we hypothesized that in nos, pum and twin 345 mutants, Pgc is upregulated in the GSCs, forcing the cells to prematurely differentiate 346 and resulting in a loss of GSCs.…”