The bHLH factor deadpan is a direct target of Notch signaling and regulates neuroblast self-renewal in Drosophila

Abstract

A defining feature of stem cells is their capacity to renew themselves at each division while producing differentiated progeny. How these cells balance self-renewal versus differentiation is a fundamental issue in developmental and cancer biology. The Notch signaling pathway has long been known to influence cell fate decisions during development. Indeed, there is a great deal of evidence correlating its function with the regulation of neuroblast (NB) self-renewal during larval brain development in Drosophila. However, little is known about the transcription factors regulated by this pathway during this process. Here we show that deadpan (dpn), a gene encoding a bHLH transcription factor, is a direct target of the Notch signaling pathway during type II NB development. Type II NBs undergo repeated asymmetric divisions to self-renew and to produce immature intermediate neural progenitors. These cells mature into intermediate neural progenitors (INPs) that have the capacity to undergo multiple rounds of asymmetric division to self-renew and to generate GMCs and neurons. Our results indicate that the expression of dpn at least in INPs cells depends on Notch signaling. The ectopic expression of dpn in immature INP cells can transform these cells into NBs-like cells that divide uncontrollably causing tumor over-growth. We show that in addition to dpn, Notch signaling must be regulating other genes during this process that act redundantly with dpn.

Rationally designed fluorogenic protease reporter visualizes spatiotemporal dynamics of apoptosis in vivo

“…5 D and E, and Fig. S4 C and D), consistent with previous Dpn overexpression studies (22)(23)(24)27). To examine the spatiotemporal dynamics of apoptosis during tumorigenesis of the larval brain, we imaged dpn OE and wild-type larval brains at different time points throughout larval development.…”

“…5A and Fig. S4) (22)(23)(24). Throughout neural development, the stem cells of Drosophia, known as neuroblasts (NBs), proliferate to generate the nervous system of both larval and adult stages via asymmetric division, whereby NBs self-renew and generate daughter cells (Fig.…”

“…5C) (25). Dpn is normally expressed in the NBs and intermediate neural progenitors (INPs) of the developing Drosophila brain and has roles in maintaining NB selfrenewal and specification of the type II NB identity in larval brains (22)(23)(24). We ectopically overexpressed Dpn (dpn OE ) using the insc-GAL4 (GAL4 1407 inserted in inscuteable promoter) driving expression in NBs, INPs, and GMCs (ganglion mother cells), which has been shown to lead to excessive NB proliferation at the expense of more differentiated cells, representing a tumorlike state (22)(23)(24).…”

Drosophila neuroblasts: a model for stem cell biology

“…It is thought that the reduction of Notch activity induced by Numb in one of the two daughter cells is key for driving this daughter cell to differentiate. In addition, Notch enhances cellular re-growth after division in both type I and type II NBs (San-Juán and Baonza, 2011;Song and Lu, 2011). These Notch functions seem to be more important in type II lineages, as knock down of Notch in the brain reduces overall NB numbers (Wang et al, 2006), but causes a complete loss of all type II lineages (Bowman et al, 2008).…”

Opportunities lost and gained: Changes in progenitor competence during nervous system development

“…An attractive model is that Ey recruits SWI/SNF proteins to prevent activation of Notch target genes in GMCs. 43,44 Consistent with this model, murine Pax6 protein directly binds the SWI/ SNF-related BAF complex to promote neuronal differentiation in murine adult neural progenitors. 45 In addition, a BAF subunit switch triggers the transition from proliferation to differentiation in mammalian neural progenitors, 46 raising the possibility that both Drosophila and mammals use similar pathways to regulate progenitor choice of differentiation or proliferation.…”