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

Chrdl1-mediated BMP4 inhibition disrupts the balance between retinal neurons and Müller Glia

Chordin-like 1 (CHRDL1) is a secreted protein that serves as an endogenous antagonist of bone morphogenetic proteins (BMPs). In the developing retina, Bmp4 has been demonstrated to be essential for sustaining the proliferation of progenitor cells and facilitating the differentiation of glial cells. Despite these efforts, the precise effects of Bmp4 inhibition on the developing retina are yet to be fully understood. We sought to address this question by overexpressing Chrdl1 in the developing retina. In this study, we explored the impact of Bmp4 inhibition on the developing mouse retina by conditionally overexpressing the Bmp4 inhibitor Chrdl1. Initially, we characterized the expression patterns of Bmp4 and Chrdl1 in the developing mouse retina from E10.5 to P12.5. Additionally, we utilized various molecular markers to demonstrate that Bmp4 inhibition disrupts both neuronal and Müller glial differentiation in the developing mouse retina. Moreover, through the application of RNA-seq analysis, distinctively expressed retinal genes under the modulation of Bmp4 signaling were discerned, encompassing the upregulation of Id1/2/3/4 and Hes1/5, as well as the downregulation of Neurod1/2/4 and Bhlhe22/23. Lastly, electroretinogram (ERG) and optomotor response (OMR) assays were conducted to illustrate that Bmp4 inhibition impairs the functional connectivity of various cells in the retina and consequently affects visual function. Collectively, this study demonstrates that inhibiting Bmp4 promotes the differentiation of retinal neurons over Müller glia by activating the expression of genes associated with neuron specification. These findings offer molecular insights into the role of Bmp4 signaling in mammalian retinal development.

Evolution of glial cells: a non-bilaterian perspective

Nervous systems of bilaterian animals generally consist of two cell types: neurons and glial cells. Despite accumulating data about the many important functions glial cells serve in bilaterian nervous systems, the evolutionary origin of this abundant cell type remains unclear. Current hypotheses regarding glial evolution are mostly based on data from model bilaterians. Non-bilaterian animals have been largely overlooked in glial studies and have been subjected only to morphological analysis. Here, we provide a comprehensive overview of conservation of the bilateral gliogenic genetic repertoire of non-bilaterian phyla (Cnidaria, Placozoa, Ctenophora, and Porifera). We overview molecular and functional features of bilaterian glial cell types and discuss their possible evolutionary history. We then examine which glial features are present in non-bilaterians. Of these, cnidarians show the highest degree of gliogenic program conservation and may therefore be crucial to answer questions about glial evolution.

Separable Roles for Neur and Ubiquitin in Delta Signalling in the Drosophila CNS Lineages

The execution of a Notch signal at the plasma membrane relies on the mechanical force exerted onto Notch by its ligand. It has been appreciated that the DSL ligands need to collaborate with a ubiquitin (Ub) ligase, either Neuralized or Mindbomb1, in order to exert this pulling force, but the role of ubiquitylation per se is uncertain. Regarding the Delta-Neur pair, it is documented that neither the Neur catalytic domain nor the Delta intracellular lysines (putative Ub acceptors) are needed for activity. Here, we present a dissection of the Delta activity using the Delta-Notch-dependent expression of Hey in newborn Drosophila neurons as a sensitive in vivo assay. We show that the Delta-Neur interaction per se, rather than ubiquitylation, is needed for activity, pointing to the existence of a Delta-Neur signaling complex. The Neur catalytic domain, although not strictly needed, greatly improves Delta-Neur complex functionality when the Delta lysines are mutated, suggesting that the ubiquitylation of some component of the complex, other than Delta, can enhance signaling. Since Hey expression is sensitive to the perturbation of endocytosis, we propose that the Delta-Neur complex triggers a force-generating endocytosis event that activates Notch in the adjacent cell.

A Notch-dependent transcriptional mechanism controls expression of temporal patterning factors in Drosophila medulla

Temporal patterning is an important mechanism for generating a great diversity of neuron subtypes from a seemingly homogenous progenitor pool in both vertebrates and invertebrates. Drosophila neuroblasts are temporally patterned by sequentially expressed Temporal Transcription Factors (TTFs). These TTFs are proposed to form a transcriptional cascade based on mutant phenotypes, although direct transcriptional regulation between TTFs has not been verified in most cases. Furthermore, it is not known how the temporal transitions are coupled with the generation of the appropriate number of neurons at each stage. We use neuroblasts of the Drosophila optic lobe medulla to address these questions and show that the expression of TTFs Sloppy-paired 1/2 (Slp1/2) is directly regulated at the transcriptional level by two other TTFs and the cell-cycle dependent Notch signaling through two cis-regulatory elements. We also show that supplying constitutively active Notch can rescue the delayed transition into the Slp stage in cell cycle arrested neuroblasts. Our findings reveal a novel Notch-pathway dependent mechanism through which the cell cycle progression regulates the timing of a temporal transition within a TTF transcriptional cascade.

Regulation of Neural Stem Cell Competency and Commitment during Indirect Neurogenesis

Indirect neurogenesis, during which neural stem cells generate neurons through intermediate progenitors, drives the evolution of lissencephalic brains to gyrencephalic brains. The mechanisms that specify intermediate progenitor identity and that regulate stem cell competency to generate intermediate progenitors remain poorly understood despite their roles in indirect neurogenesis. Well-characterized lineage hierarchy and available powerful genetic tools for manipulating gene functions make fruit fly neural stem cell (neuroblast) lineages an excellent in vivo paradigm for investigating the mechanisms that regulate neurogenesis. Type II neuroblasts in fly larval brains repeatedly undergo asymmetric divisions to generate intermediate neural progenitors (INPs) that undergo limited proliferation to increase the number of neurons generated per stem cell division. Here, we review key regulatory genes and the mechanisms by which they promote the specification and generation of INPs, safeguarding the indirect generation of neurons during fly larval brain neurogenesis. Homologs of these regulators of INPs have been shown to play important roles in regulating brain development in vertebrates. Insight into the precise regulation of intermediate progenitors will likely improve our understanding of the control of indirect neurogenesis during brain development and brain evolution.

Tumor Allotransplantation in Drosophila melanogaster with a Programmable Auto-Nanoliter Injector

This protocol describes the allotransplantation of tumors in Drosophila melanogaster using an auto-nanoliter injection apparatus. With the use of an autoinjector apparatus, trained operators can achieve more efficient and consistent transplantation results compared to those obtained using a manual injector. Here, we cover topics in a chronological fashion: from the crossing of Drosophila lines, to the induction and dissection of the primary tumor, transplantation of the primary tumor into a new adult host and continued generational transplantation of the tumor for extended studies. As a demonstration, here we use Notch intracellular domain (NICD) overexpression induced salivary gland imaginal ring tumors for generational transplantation. These tumors can first be reliably induced in a transition-zone microenvironment within larval salivary gland imaginal rings, then allografted and cultured in vivo to study continued tumor growth, evolution, and metastasis. This allotransplantation method can be useful in potential drug screening programs, as well as for studying tumor-host interactions.