A comprehensive temporal patterning gene network in Drosophila medulla neuroblasts revealed by single-cell RNA sequencing

Abstract

During development, neural progenitors are temporally patterned to sequentially generate a variety of neural types. In Drosophila neural progenitors called neuroblasts, temporal patterning is regulated by cascades of Temporal Transcription Factors (TTFs). However, known TTFs were mostly identified through candidate approaches and may not be complete. In addition, many fundamental questions remain concerning the TTF cascade initiation, progression, and termination. In this work, we use single-cell RNA sequencing of Drosophila medulla neuroblasts of all ages to identify a list of previously unknown TTFs, and experimentally characterize their roles in temporal patterning and neuronal specification. Our study reveals a comprehensive temporal gene network that patterns medulla neuroblasts from start to end. Furthermore, the speed of the cascade progression is regulated by Lola transcription factors expressed in all medulla neuroblasts. Our comprehensive study of the medulla neuroblast temporal cascade illustrates mechanisms that may be conserved in the temporal patterning of neural progenitors.

Cutting edge technologies expose the temporal regulation of neurogenesis in the Drosophila nervous system

“…Several scRNA-seq analyses have been performed in the OL, revealing gene expression profiles in a large number of neurons [ 22–26 , 63 ]. In addition, scRNA-seq analysis has been performed focusing on the developing OL, which has led to a better understanding of the temporal patterning of neurogenesis [ 27 , 29 ]. As a result, many TTFs were newly identified in medulla NBs, and their regulatory network was revealed.…”

“…SoxN and Dmrt99B are expressed in NEs and younger NBs overlapping with the Hth domain and are required for the production of Bsh-positive neurons ( Figure 2c ) [ 27 ]. While Hth and SoxN are not required to turn on the expression of later TTFs, Dmrt99B turns on the expression of a subsequent TTF, Odd-paired (Opa), which then represses the previous TTF, Hth.…”

“…Earmuff (Erm; ortholog of mammalian Fezf2) and Opa show striped expression patterns with two expression domains. Erm expression domains in NEs and NBs are separated with a gap found in the youngest NBs, where the first Opa domain is found ( Figure 2c ) [ 27 ]. The second Opa domain is found in NBs subsequent to the second Erm domain.…”

“…However, its loss of function caused overproliferation of NBs, masking its involvement in Run-positive neuron production [ 15 ]. In contrast, Opa was shown to be necessary for the production of Run-positive neurons in a loss-of-function experiment [ 27 ]. It is possible to speculate that Klu and Opa act together to control the production of Run-positive neurons.…”

“…Hbn and Scro were found to be expressed in the regions of Ey- and Slp-positive NBs. They promote the transition from Ey- to Slp-positive NBs [ 27 , 29 ]. These TTFs have been identified as target genes of Ey by TaDa analysis using the Ey-Dam fusion protein [ 50 ].…”