Activin signaling balances proliferation and differentiation of ovarian niche precursors and enables adjustment of niche numbers

Abstract

How the numbers of niches and resident stem cells within a particular organ are determined during development and how they may be modulated or corrected is a question with significant medical implications. In the larval ovary of Drosophila melanogaster, somatic precursors for niches, and germ cells that will become germline stem cells, co-develop. Somatic precursors proliferate during the first 3 days of larval development. By mid-third instar, adult terminal filament (TF) ( part of the germline stem cell niche) cells first appear, and differentiation terminates 24 h later when 16-20 TFs fully form. The developmental sequence responsible for TF cell determination and final TF numbers is only partially understood. We show that TF formation proceeds through several, hitherto uncharacterized stages, which include an early exit from the cell cycle to form TF precursors and two steps of cell shape change to form the mature TF cells. The Activin receptor Baboon (Babo) is required for somatic precursor cell proliferation and therefore determines the pool of TF precursors available for TF differentiation. During the final differentiation stage, Babo facilitates TF and germ cell differentiation, and promotes the accumulation of Broad-Z1, which is also a target of the steroid hormone ecdysone. Epistasis analysis shows that Activin controls cell proliferation in an ecdysone-independent manner and TF differentiation by affecting ecdysone targets. We propose that this mode of function allows Activin to balance proliferation and differentiation, and to equilibrate niche numbers. These results suggest a novel model for how niche numbers are corrected during development.

Combgap Promotes Ovarian Niche Development and Chromatin Association of EcR-Binding Regions in BR-C

“…To follow TF cell accumulation and stack formation we used anti-Engrailed (En), which specifically stains TF cells [27]. At LL3, TF cells in WT or heterozygous ovaries were already stacked to form the adult number of niches (Fig 1H, Table 1) [8,12,28]. By contrast, TF formation was defective in cg mutants.…”

“…Vital dye staining (Propidium Iodide, PI) indicated that cell death did not increase in cg -mutant ovaries (of 18 cg 1 /cg KG00882 ovaries, 16 were not labeled by PI at all, and 2 had 1–3 dead cells, a similar level to WT ovaries)[8]. On the other hand, phospho-Histone H3 (pH3) staining, which marks mitotic cells, was significantly decreased in cg -mutants (0.15±0.03 pH3-positive cells per 1 μm 3 in WT ovarian volume, n = 21, as compared to 0.08±0.02 per 1 μm 3 in cg 1 /cg KG00882 ovaries, n = 14, t-test p-value 1.35e-6).…”

“…Such phenotypes suggest that Cg may somehow affect the ecdysone pathway, which induces molting and pupariation [30]. Since ecdysone signaling is required for ovarian development [8,13,31,32], we investigated whether Cg affected the ovarian ecdysone response, and in particular the Broad-complex (BR-C), its major ovarian target during larval development [8,13]. …”

“…Somatic niche formation initiates when a pool of proliferating somatic cells become non-proliferating Terminal Filament (TF) precursor cells (Fig 1A). TF precursors accumulate throughout third instar, acquire adult TF markers from mid-larval third instar (ML3), and then form filaments by convergent extension [8]. By Late Larval third instar (LL3) all 16–20 TF stacks, which are contained within a single ovary, have formed [9,10,11,12].…”

“…Our previous work underlined the importance of the hormone ecdysone as a switch signal between proliferation and differentiation and as a coordinator of somatic niche formation with GSC establishment [7,8,13]. At early larval stages, the ecdysone receptors EcR and Ultraspiracle (Usp) act as repressors of niche and PGC differentiation.…”