Faculty Opinions recommendation of Fat cells reactivate quiescent neuroblasts via TOR and glial insulin relays in Drosophila.

Many stem, progenitor and cancer cells undergo periods of mitotic quiescence from which they can be reactivated [1][2][3][4][5] .The signals triggering entry into and exit from this reversible dormant state are not well understood.In the developing Drosophila central nervous system (CNS), multipotent self-renewing progenitors called neuroblasts [6][7][8][9] undergo quiescence in a stereotypical spatiotemporal pattern 10 .Entry into quiescence is regulated by Hox proteins and an internal neuroblast timer [11][12][13] .Exit from quiescence (reactivation) is subject to a nutritional checkpoint requiring dietary amino acids 14 .Organ co-cultures also implicate an unidentified signal from an adipose/hepatic-like tissue called fat body 14 .Here, we provide in vivo evidence that Slimfast amino-acid sensing and Target-of-Rapamycin (TOR) signalling 15 activate a fat-body derived signal (FDS) required for neuroblast reactivation.Downstream of the FDS, Insulin-like receptor (InR) signalling and the Phosphatidylinositol 3-Kinase (PI3K)/TOR network are required in neuroblasts for exit from quiescence.We demonstrate that nutritionally regulated glial cells provide the source of Insulin-like Peptides (Ilps) relevant for timely neuroblast reactivation but not for overall larval growth.Conversely, Ilps secreted into the hemolymph by median neurosecretory cells (mNSCs) systemically control organismal size [16][17][18] but do not reactivate neuroblasts.Drosophila thus contains two segregated Ilp pools, one regulating proliferation within the CNS and the other controlling tissue growth systemically.Together, our findings support a model in which amino acids trigger the cell cycle re-entry of neural progenitors via a fat body→glia→neuroblasts relay.This mechanism highlights that dietary nutrients and remote organs, as well as local niches, are key regulators of transitions in stem-cell behaviour.In fed larvae, Drosophila neuroblasts (Fig. 1a) exit quiescence from the late first instar (L1) stage onwards.This reactivation involves cell enlargement and entry into S-phase, monitored in this study using the thymidine analogue, 5-ethynyl-2′-deoxyuridine (EdU).Consistent with a previous study 10 , we observed that reactivated neuroblast lineages (neuroblasts and their progeny, Fig 1b) reproducibly incorporated EdU in a characteristic spatiotemporal sequence: central brain (CB) → thoracic (Th) → abdominal (Ab) neuromeres (Fig. 1c and Supplementary Fig. 1).Mushroom-body neuroblasts (MB NBs) and one ventrolateral neuroblast, however, are known not to undergo quiescence and to continue dividing for several days in the absence of dietary amino acids 14 (Fig. 1a,c,f).This indicates that dietary amino acids are more than mere "fuel", providing a specific signal that reactivates neuroblasts.However, explanted CNSs incubated with amino acids do not * To whom correspondence and requests for materials should be addressed to A.P.

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