bHLH-O proteins balance the self-renewal and differentiation of Drosophila neural stem cells by regulating Earmuff expression

Seven-up acts in neuroblasts to specify adult central complex neuron identity and initiate neuroblast decommissioning

An unanswered question in neurobiology is how are diverse neuron cell types generated from a small number of neural stem cells? In the Drosophila larval central brain, there are eight bilateral Type 2 neuroblast (T2NB) lineages that express a suite of early temporal factors followed by a different set of late temporal factors and generate the majority of the central complex (CX) neurons. The early-to-late switch is triggered by the orphan nuclear hormone receptor Seven-up (Svp), yet little is known about how this Svp-dependent switch is involved in specifying CX neuron identities. Here, we: (1) birth date the CX neurons P-EN and P-FN (early and late, respectively); (2) show that Svp is transiently expressed in all early T2NBs; and (3) show that loss of Svp expands the population of early born P-EN neurons at the expense of late born P-FN neurons. Furthermore, in the absence of Svp, T2NBs fail decommissioning and abnormally extend their lineage into week-old adults. We conclude that Svp is required to specify CX neuron identity, as well as to initiate T2NB decommissioning.

Seven-up acts in neuroblasts to specify adult central complex neuron identity and initiate neuroblast decommissioning

An open question in neurobiology is how diverse neuron cell types are generated from a small number of neural stem cells. In the Drosophila larval central brain, there are eight bilateral Type 2 neuroblast (T2NB) lineages that express a suite of early temporal factors followed by a different set of late temporal factors and generate the majority of the central complex (CX) neurons. The early-to-late switch is triggered by the orphan nuclear hormone receptor Seven-up (Svp), yet little is known about this Svp-dependent switch in specifying CX neuron identities. Here, we (i) birthdate the CX neurons P-EN and P-FN (early and late, respectively); (ii) show that Svp is transiently expressed in all early T2NBs; and (iii) show that loss of Svp expands the population of early born P-EN neurons at the expense of late born P-FN neurons. Furthermore, in the absence of Svp, T2NBs fail decommissioning and abnormally extend their lineage into week-old adults. We conclude that Svp is required to specify CX neuron identity, as well as to initiate T2NB decommissioning.

The Ets protein Pointed P1 represses Asense expression in type II neuroblasts by activating Tailless

Intermediate neural progenitors (INPs) boost the number and diversity of neurons generated from neural stem cells (NSCs) by undergoing transient proliferation. In the developing Drosophila brains, INPs are generated from type II neuroblasts (NBs). In order to maintain type II NB identity and their capability to produce INPs, the proneural protein Asense (Ase) needs to be silenced by the Ets transcription factor pointed P1 (PntP1), a master regulator of type II NB development. However, the molecular mechanisms underlying the PntP1-mediated suppression of Ase is still unclear. In this study, we utilized genetic and molecular approaches to determine the transcriptional property of PntP1 and identify the direct downstream effector of PntP1 and the cis-DNA elements that mediate the suppression of ase. Our results demonstrate that PntP1 directly activates the expression of the transcriptional repressor, Tailless (Tll), by binding to seven Ets-binding sites, and Tll in turn suppresses the expression of Ase in type II NBs by binding to two hexameric core half-site motifs. We further show that Tll provides positive feedback to maintain the expression of PntP1 and the identity of type II NBs. Thus, our study identifies a novel direct target of PntP1 and reveals mechanistic details of the specification and maintenance of the type II NB identity by PntP1.

Type II neuroblasts (NBs) are the neural stem cells (NSCs) in Drosophila central brains that produce neurons by generating intermediate neural progenitors (INPs) to boost brain complexity, as mammalian NSCs do during the development of neocortex. The key to the generation of INPs from type II NBs is the suppression of proneural protein Asense (Ase) in type II NBs by the Ets family transcription factor Pointed P1 (PntP1), but how PntP1 suppresses Ase expression remains unclear. In this study, we provided evidence to demonstrate that PntP1 directly activates the orphan nuclear receptor Tailless (Tll), which in turn suppresses Ase expression to maintain the capability of type II NBs to produce INPs. Meanwhile, Tll provides positive feedback to maintain the expression of PntP1 and type II NB identity. We further identified seven PntP1 binding sites in the tll enhancer regions and two Tll binding sites in the ase regulatory regions that mediate the activation of tll and the suppression of ase, respectively. Our work reveals detailed mechanisms of the specification and maintenance of the type II NB identity by PntP1.

Enhancer analysis of the Drosophila zinc finger transcription factor Earmuff by gene targeting

BACKGROUND

Many transcription factors are involved in the formation of the brain during the development of Drosophila melanogaster. The transcription factor Earmuff (Erm), a member of the forebrain embryonic zinc finger family (Fezf), is one of these important factors for brain development. One major function of Earmuff is the regulation of proliferation within type II neuroblast lineages in the brain; here, Earmuff is expressed in intermediate neural progenitor cells (INPs) and balances neuronal differentiation versus stem cell maintenance. Erm expression during development is regulated by several enhancers.

RESULTS

In this work we show a functional analysis of erm and some of its enhancers. We generated a new erm mutant allele by gene targeting and reintegrated Gal4 to make an erm enhancer trap strain that could also be used on an erm mutant background. The deletion of three of the previously analysed enhancers showing the most prominent expression patterns of erm by gene targeting resulted in specific temporal and spatial defects in defined brain structures. These defects were already known but here could be assigned to specific enhancer regions.

CONCLUSION

This analysis is to our knowledge the first systematic analysis of several large enhancer deletions of a Drosophila gene by gene targeting and will enable deeper analysis of erm enhancer functions in the future.

The molecular landscape of neural differentiation in the developing Drosophila brain revealed by targeted scRNA-seq and multi-informatic analysis

The Drosophila type II neuroblast lineages present an attractive model to investigate the neurogenesis and differentiation process as they adapt to a process similar to that in the human outer subventricular zone. We perform targeted single-cell mRNA sequencing in third instar larval brains to study this process of the type II NB lineage. Combining prior knowledge, in silico analyses, and in situ validation, our multi-informatic investigation describes the molecular landscape from a single developmental snapshot. 17 markers are identified to differentiate distinct maturation stages. 30 markers are identified to specify the stem cell origin and/or cell division numbers of INPs, and at least 12 neuronal subtypes are identified. To foster future discoveries, we provide annotated tables of pairwise gene-gene correlation in single cells and MiCV, a web tool for interactively analyzing scRNA-seq datasets. Taken together, these resources advance our understanding of the neural differentiation process at the molecular level.

Homeodomain protein Six4 prevents the generation of supernumerary Drosophila type II neuroblasts and premature differentiation of intermediate neural progenitors

In order to boost the number and diversity of neurons generated from neural stem cells, intermediate neural progenitors (INPs) need to maintain their homeostasis by avoiding both dedifferentiation and premature differentiation. Elucidating how INPs maintain homeostasis is critical for understanding the generation of brain complexity and various neurological diseases resulting from defects in INP development. Here we report that Six4 expressed in Drosophila type II neuroblast (NB) lineages prevents the generation of supernumerary type II NBs and premature differentiation of INPs. We show that loss of Six4 leads to supernumerary type II NBs likely due to dedifferentiation of immature INPs (imINPs). We provide data to further demonstrate that Six4 inhibits the expression and activity of PntP1 in imINPs in part by forming a trimeric complex with Earmuff and PntP1. Furthermore, knockdown of Six4 exacerbates the loss of INPs resulting from the loss of PntP1 by enhancing ectopic Prospero expression in imINPs, suggesting that Six4 is also required for preventing premature differentiation of INPs. Taken together, our work identified a novel transcription factor that likely plays important roles in maintaining INP homeostasis.

Intermediate neural progenitors (INPs) are descendants of neural stem cells that can proliferate for a short term to amplify the number of nerve cells generated in the brain. INPs play critical roles in determining how big and complex a brain can grow. To perform their function, INPs need to maintain their own population and must not adopt the identity of neural stem cells, a process called dedifferentiation, or acquire the fate of their own daughter cells and stop proliferation too soon, a process called premature differentiation. However, how INPs avoid dedifferentiation and premature differentiation is not fully understood. In this study, we identified a protein called Six4 as a novel factor that plays important roles in preventing the generation of extra neural stem cells and premature differentiation of INPs in developing fruit fly brains. We described how Six4 functionally and physically interacts with other factors that are involved in regulating INP cell fate specification. Our work provides novel insights into the mechanisms regulating INP development and could have important implications in understanding how complex brains are generated during normal development and how abnormal brain development or brain tumor can occur when INPs fail to avoid premature differentiation or dedifferentiation.

Dissecting Hes-centred transcriptional networks in neural stem cell maintenance and tumorigenesis in Drosophila

Neural stem cells divide during embryogenesis and juvenile life to generate the entire complement of neurons and glia in the nervous system of vertebrates and invertebrates. Studies of the mechanisms controlling the fine balance between neural stem cells and more differentiated progenitors have shown that, in every asymmetric cell division, progenitors send a Delta-Notch signal to their sibling stem cells. Here, we show that excessive activation of Notch or overexpression of its direct targets of the Hes family causes stem-cell hyperplasias in the Drosophila larval central nervous system, which can progress to malignant tumours after allografting to adult hosts. We combined transcriptomic data from these hyperplasias with chromatin occupancy data for Dpn, a Hes transcription factor, to identify genes regulated by Hes factors in this process. We show that the Notch/Hes axis represses a cohort of transcription factor genes. These are excluded from the stem cells and promote early differentiation steps, most likely by preventing the reversion of immature progenitors to a stem-cell fate. We describe the impact of two of these 'anti-stemness' factors, Zfh1 and Gcm, on Notch/Hes-triggered tumorigenesis.