Dedifferentiation is the reversion of mature cells to a stem cell‐like fate, whereby gene expression programs are altered and genes associated with multipotency are (re)expressed. Misexpression of multipotency factors and pathways causes the formation of ectopic neural stem cells (NSCs). Whether dedifferentiated NSCs faithfully produce the correct number and types of progeny, or undergo timely terminal differentiation, has not been assessed. Here, we show that ectopic NSCs induced via bHLH transcription factor Deadpan (Dpn) expression fail to undergo appropriate temporal progression by constantly expressing mid‐temporal transcription factor(tTF), Sloppy‐paired 1/2 (Slp). Consequently, this resulted in impaired terminal differenation and generated an excess of Twin of eyeless (Toy)‐positive neurons at the expense of Reversed polarity (Repo)‐positive glial cells. Preference for a mid‐temporal fate in these ectopic NSCs is concordant with an enriched binding of Dpn at mid‐tTF loci and a depletion of Dpn binding at early‐ and late‐tTF loci. Retriggering the temporal series via manipulation of the temporal series or cell cycle is sufficient to reinstate neuronal diversity and timely termination.

Dedifferentiation is the reversion of differentiated cells to a stem cell like fate, whereby, the gene expression program of mature cells is altered and genes associated with multipotency are expressed. Appropriate terminal differentiation of NSCs is essential for restricting the overall number of neurons produced; in addition, faithful production of neuronal subtypes that populate the brain is important for NSC function. Both characteristics of NSCs are specified through temporal patterning of the NSCs driven by the successive expression of temporal transcription factors (tTFs). In this study, we found that ectopic NSCs induced via bHLH transcription factor Deadpan (Dpn) expression fail to undergo timely expression of temporal transcription factors (tTFs), where they express mid-tTF, Sloppy-paired 1 (Slp-1) and fail to express late-tTF Tailless (Tll); consequently generating an excess of Twin of eyeless (Toy) positive neurons at the expense of Reversed polarity (Repo) positive glial cells. In addition to disrupted production of neuronal/glial progeny, Dpn overexpression also resulted in stalled progression through the cell cycle, and a failure to undergo timely terminal differentiation. Mechanistically, DamID studies demonstrated that Dpn directly binds to both Dichaete (D), a Sox-box transcription factor known to repress Slp-1, as well as a number of cell cycle genes. Promoting cell cycle progression or overexpression of D were able to re-trigger the progression of the temporal series in dedifferentiated NBs, restoring both neuronal diversity and timely NB terminal differentiation.

The formation of a functional circuitry in the central nervous system (CNS) requires the correct number and subtypes of neural cells. In the developing brain, neural stem cells (NSCs) self-renew while giving rise to progenitors that in turn generate differentiated progeny. As such, the size and the diversity of cells that make up the functional CNS depend on the proliferative properties of NSCs. In the fruit fly Drosophila, where the process of neurogenesis has been extensively investigated, extrinsic factors such as the microenvironment of NSCs, nutrients, oxygen levels and systemic signals have been identified as regulators of NSC proliferation. Here, we review decades of work that explores how extrinsic signals non-autonomously regulate key NSC characteristics such as quiescence, proliferation and termination in the fly.