Steroid hormone induction of temporal gene expression in Drosophila brain neuroblasts generates neuronal and glial diversity

Abstract

An important question in neuroscience is how stem cells generate neuronal diversity. During Drosophila embryonic development, neural stem cells (neuroblasts) sequentially express transcription factors that generate neuronal diversity; regulation of the embryonic temporal transcription factor cascade is lineage-intrinsic. In contrast, larval neuroblasts generate longer ˜50 division lineages, and currently only one mid-larval molecular transition is known: Chinmo/Imp/Lin-28+ neuroblasts transition to Syncrip+ neuroblasts. Here we show that the hormone ecdysone is required to down-regulate Chinmo/Imp and activate Syncrip, plus two late neuroblast factors, Broad and E93. We show that Seven-up triggers Chinmo/Imp to Syncrip/Broad/E93 transition by inducing expression of the Ecdysone receptor in mid-larval neuroblasts, rendering them competent to respond to the systemic hormone ecdysone. Importantly, late temporal gene expression is essential for proper neuronal and glial cell type specification. This is the first example of hormonal regulation of temporal factor expression in Drosophila embryonic or larval neural progenitors.SummaryHormone induction of temporal gene expression in neural progenitors

Playing Well with Others: Extrinsic Cues Regulate Neural Progenitor Temporal Identity to Generate Neuronal Diversity

“…Third, we expressed an EcR dominant negative protein specifically in larval neuroblasts, which delayed the early-late gene expression switch. This experiment also shows that failure to make the neuroblast gene expression switch is not due to an indirect effect on organismal growth or development, but rather a neuroblast-autonomous effect of ecdysone signaling [56]. As expected, loss of ecdysone signaling had no effect on the timing of EcR expression.…”

“…Two temporal windows are not sufficient to generate the known diversity of neurons made by each larval neuroblast [52-54]. To identify additional, novel candidate temporal factors expressed by larval neuroblasts, unbiased transcriptomic screens were performed [55, 56]. Our screen identified the above-mentioned factors, plus additional factors including the ecdysone receptor, raising the possibility that the steroid hormone ecdysone may be used to generate temporal identity within larval neuroblasts (Figure 3).…”

“…Ecdysone is produced by the prothoracic gland (outside the CNS) and acts systemically throughout the animal, via binding cell type specific Ecdysone receptor (EcR) isoforms. We found that the EcR-B1 isoform was temporally expressed in most or all larval central brain neuroblasts from ∼60h ALH onwards [56]. Interestingly, most other temporal factors were expressed fully before or after this mid-larval timepoint: young neuroblasts expressed Castor, Seven-up, Chinmo, Imp, and Lin-28 whereas old neuroblasts expressed EcRB1, Broad, E93 and Syncrip; this raises the hypothesis that ecdysone signaling could induce a major temporal gene expression transition in larval neuroblasts.…”

“…As expected, loss of ecdysone signaling had no effect on the timing of EcR expression. Rather, EcR expression was fully dependent on the prior expression of the early temporal factor, Seven-up, an orphan nuclear hormone receptor [56]. Loss of Seven-up not only completely prevented EcR expression, but all late temporal factor expression, with continued expression of early genes such as Imp, Lin-28, and Chinmo [49, 55, 56].…”

“…Rather, EcR expression was fully dependent on the prior expression of the early temporal factor, Seven-up, an orphan nuclear hormone receptor [56]. Loss of Seven-up not only completely prevented EcR expression, but all late temporal factor expression, with continued expression of early genes such as Imp, Lin-28, and Chinmo [49, 55, 56]. It is likely that the difference in phenotypes between loss of Seven-up and manipulations of ecdysone are due to the fact that we were unable to remove all ecdysone signaling.…”