Abstract
Stem cell self-renewal and the daughter cell differentiation are tightly regulated by the respective niches, which produce extrinsic cues to support the proper development. In Drosophila ovary, Dpp is secreted from germline stem cell (GSC) niche and activates the BMP signaling in GSCs for their self-renewal. Escort cells (ECs) in differentiation niche restrict Dpp outside the GSC niche and extend protrusions to help with proper differentiation of the GSC daughter cells. Here we provide evidence that loss of large Maf transcriptional factor Traffic jam (Tj) blocks GSC progeny differentiation. Spatio-temporal specific knockdown experiments indicate that Tj is required in pre-adult EC lineage for germline differentiation control. Further molecular and genetic analyses suggest that the defective germline differentiation caused by tj-depletion is partly attributed to the elevated dpp in the differentiation niche. Moreover, our study reveals that tj-depletion induces ectopic En expression outside the GSC niche, which contributes to the upregulated dpp expression in ECs as well as GSC progeny differentiation defect. Alternatively, loss of EC protrusions and decreased EC number elicited by tj-depletion may also partially contribute to the germline differentiation defect. Collectively, our findings suggest that Tj in ECs regulates germline differentiation by controlling the differentiation niche characteristics.
Highlights
The Drosophila ovaries continually generate mature eggs in adulthood due to a stable population of self-renewable ovarian germline stem cells (GSCs)
Beginning at late third larval instar (LL3), primordial germ cells (PGCs) -derived germ cell differentiation initiates and proceeds in Drosophila female gonads[40,41], producing differentiating germ cells including CBs and actively dividing cysts. Those germ cells at different developmental stages can be morphologically distinguished by the fusome, a germline specific cytoplasmic organelle which appears spherical in PGCs or GSCs and grows new branches after each cyst division[42]
We argue that depletion of tj in pre-adult Escort cells (ECs) may induce an elevation of dpp outside the GSC niche
Summary
The Drosophila ovaries continually generate mature eggs in adulthood due to a stable population of self-renewable ovarian germline stem cells (GSCs). Proper GSC progeny differentiation is controlled extrinsically by a differentiation niche containing somatic ECs, which encapsulate GSC progeny with protrusions[8,9]. Suppression of Dally[16] and expression of Tkv[17] in ECs are necessary to restrict diffusion of GSC niche-derived Dpp within the range of one-cell-diameter. Lsd[120,21], Piwi[22,23], COP9/ Hh24,25 and H126 have been reported to control GSC progeny differentiation through repressing Dpp expression in ECs. Rho[110] and Bre1/Set[127] function in ECs to inhibit both Dpp and Dally production. Tj-deficient pre-adult ECs produce ectopic En, which promotes dpp transcription and subsequently up-regulates BMP signaling activity in CBs, thereby causing differentiation defect. This study suggests new mechanisms by which Tj preserves the properties of differentiation niche to promote proper germline differentiation
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