Drosophila ovarian stem cells communicate with somatic cells to define the size of the stem cell niche.

The stem cell niche, formed by surrounding stromal cells, provides extrinsic signals that maintain stem cell self-renewal. However, it remains unclear how these extrinsic signals are regulated. In the Drosophila female germline stem cell (GSC) niche, Decapentaplegic (DPP) is an important niche factor for GSC self-renewal. The exact source of the DPP and how its transcription is regulated in this niche remain unclear. We show that dpp is expressed in somatic cells of the niche including the cap cells, a subtype of niche cells. Furthermore, our data show that the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway positively regulates dpp expression in the cap cells, suggesting that JAK/STAT activity is required in somatic niche cells to prevent precocious GSC differentiation. Our data suggest that the JAK/STAT pathway functions downstream/independently of cap cell formation induced by Notch signaling. JAK/STAT signaling may also regulate dpp expression in the male GSC niche, suggesting a common origin of female and male GSC niches.

The evolutionarily conserved Par3/Par6/aPKC complex regulates the polarity establishment of diverse cell types and distinct polarity-driven functions. However, how the Par complex is concentrated beneath the membrane to initiate cell polarization remains unclear. Here we show that the Par complex exhibits cell cycle-dependent condensation in Drosophila neuroblasts, driven by liquid–liquid phase separation. The open conformation of Par3 undergoes autonomous phase separation likely due to its NTD-mediated oligomerization. Par6, via C-terminal tail binding to Par3 PDZ3, can be enriched to Par3 condensates and in return dramatically promote Par3 phase separation. aPKC can also be concentrated to the Par3N/Par6 condensates as a client. Interestingly, activated aPKC can disperse the Par3/Par6 condensates via phosphorylation of Par3. Perturbations of Par3/Par6 phase separation impair the establishment of apical–basal polarity during neuroblast asymmetric divisions and lead to defective lineage development. We propose that phase separation may be a common mechanism for localized cortical condensation of cell polarity complexes.

Stem cell self-renewal versus differentiation is regulated by the niche, which provides localized molecules that favor self-renewal. In the Drosophila melanogaster female germline stem cell (GSC) niche, Decapentaplegic (Dpp), a fly transforming growth factor β molecule and well-established long-range morphogen, acts over one cell diameter to maintain the GSCs. Here, we show that Thickveins (Tkv; a type I receptor of Dpp) is highly expressed in stromal cells next to Dpp-producing cells and functions to remove excess Dpp outside the niche, thereby spatially restricting its activity. Interestingly, Tkv expression in these stromal cells is regulated by multiple Wnt ligands that are produced by the niche. Our data demonstrate a self-restraining mechanism by which the Drosophila ovarian GSC niche acts to define its own boundary.

In Drosophila postembryonic neuroblasts, transition in gene expression programs of a cascade of transcription factors (also known as the temporal series) acts together with the asymmetric division machinery to generate diverse neurons with distinct identities and regulate the end of neuroblast proliferation. However, the underlying mechanism of how this “temporal series” acts during development remains unclear. Here, we show that Hh signaling in the postembryonic brain is temporally regulated; excess (earlier onset of) Hh signaling causes premature neuroblast cell cycle exit and under-proliferation, whereas loss of Hh signaling causes delayed cell cycle exit and excess proliferation. Moreover, the Hh pathway functions downstream of Castor but upstream of Grainyhead, two components of the temporal series, to schedule neuroblast cell cycle exit. Interestingly, hh is likely a target of Castor. Hence, Hh signaling provides a link between the temporal series and the asymmetric division machinery in scheduling the end of neurogenesis. In almost all metazoans, neurons are produced by a group of neural stem cells/progenitors in a precise temporal manner, which is important for generating a functional nervous system. In Drosophila, this “timing” mechanism is mainly governed by the sequential switching of transcription factors in neural stem cells called neuroblasts, such that neuronal fate is associated with its birth order. These temporal factors also coordinate the termination of neuroblast division towards the end of neurogenesis. In this study, we show that Hedgehog (Hh) signaling also regulates the division rate of neuroblasts during their proliferative phase at larval stage, as well as the cessation of proliferation at early pupal stage. Excessive Hh signaling causes premature neuroblast cell cycle exit and early termination of neurogenesis, while loss of Hh signaling results in prolonged proliferation of neuroblasts beyond its physiological window. We also find that Hh signaling acts in concert with the temporal transcription factors, and is itself regulated by these factors. We hypothesize that this mode of interaction (temporal transcription factors with developmentally regulated signals like Hh) during neurogenesis could be widely conserved in other organisms.