Single-cell mapping of neural and glial gene expression in the developing Drosophila CNS midline cells

Multiple Notch signaling events control Drosophila CNS midline neurogenesis, gliogenesis and neuronal identity

The study of how transcriptional control and cell signaling influence neurons and glia to acquire their differentiated properties is fundamental to understanding CNS development and function. The Drosophila CNS midline cells are an excellent system for studying these issues because they consist of a small population of diverse cells with well-defined gene expression profiles. In this paper, the origins and differentiation of midline neurons and glia were analyzed. Midline precursor (MP) cells each divide once giving rise to two neurons; here, we use a combination of single-cell gene expression mapping and time-lapse imaging to identify individual MPs, their locations, movements and stereotyped patterns of division. The role of Notch signaling was investigated by analyzing 37 midline-expressed genes in Notch pathway mutant and misexpression embryos. Notch signaling had opposing functions: it inhibited neurogenesis in MP1,3,4 and promoted neurogenesis in MP5,6. Notch signaling also promoted midline glial and median neuroblast cell fate. This latter result suggests that the median neuroblast resembles brain neuroblasts that require Notch signaling, rather than nerve cord neuroblasts, the formation of which is inhibited by Notch signaling. Asymmetric MP daughter cell fates also depend on Notch signaling. One member of each pair of MP3-6 daughter cells was responsive to Notch signaling. By contrast, the other daughter cell asymmetrically acquired Numb, which inhibited Notch signaling, leading to a different fate choice. In summary, this paper describes the formation and division of MPs and multiple roles for Notch signaling in midline cell development, providing a foundation for comprehensive molecular analyses.

Postmitotic specification of Drosophila insulinergic neurons from pioneer neurons

Insulin and related peptides play important and conserved functions in growth and metabolism. Although Drosophila has proved useful for the genetic analysis of insulin functions, little is known about the transcription factors and cell lineages involved in insulin production. Within the embryonic central nervous system, the MP2 neuroblast divides once to generate a dMP2 neuron that initially functions as a pioneer, guiding the axons of other later-born embryonic neurons. Later during development, dMP2 neurons in anterior segments undergo apoptosis but their posterior counterparts persist. We show here that surviving posterior dMP2 neurons no longer function in axonal scaffolding but differentiate into neuroendocrine cells that express insulin-like peptide 7 (Ilp7) and innervate the hindgut. We find that the postmitotic transition from pioneer to insulin-producing neuron is a multistep process requiring retrograde bone morphogenetic protein (BMP) signalling and four transcription factors: Abdominal-B, Hb9, Fork Head, and Dimmed. These five inputs contribute in a partially overlapping manner to combinatorial codes for dMP2 apoptosis, survival, and insulinergic differentiation. Ectopic reconstitution of this code is sufficient to activate Ilp7 expression in other postmitotic neurons. These studies reveal striking similarities between the transcription factors regulating insulin expression in insect neurons and mammalian pancreatic β-cells.

Genetic studies using invertebrate model organisms such as Drosophila have provided many new insights into the functions of insulin and related peptides. It has, however, been more difficult to use Drosophila to study the regulation of insulin, at least in part because the relevant insulinergic cell lineages were not well characterised. Here, we have identified a cell lineage that generates a single Drosophila insulin-producing neuron. This neuron first functions as a pioneer, guiding the axons of other neurons within the central nervous system of the embryo. It then develops long axons that exit the central nervous system to innervate the gut and also begins to express an insulin-like peptide. Genetic analysis identifies four transcription factors and one extrinsic signal that instruct the pioneer neuron to become an insulin-producing neuron. The analysis also reveals similarities between the genetic programmes specifying insulin production by Drosophila neurons and mammalian pancreatic ß-cells. This suggests that Drosophila may, in the future, prove a useful model system for identifying new regulators of human insulin production.

A genetic analysis in the fruit fly reveals similarities between the transcriptional programmes regulating insulin production in mammalian pancreatic β-cells and insect neurons.

Identification of motifs that are conserved in 12 Drosophila species and regulate midline glia vs. neuron expression

Functional complexity of the central nervous system (CNS) is reflected by the large number and diversity of genes expressed in its many different cell types. Understanding the control of gene expression within cells of the CNS will help reveal how various neurons and glia develop and function. Midline cells of Drosophila differentiate into glial cells and several types of neurons and also serve as a signaling center for surrounding tissues. Here, we examine regulation of the midline gene, wrapper, required for both neuron-glia interactions and viability of midline glia. We identify a region upstream of wrapper required for midline expression that is highly conserved (87%) between 12 Drosophila species. Site-directed mutagenesis identifies four motifs necessary for midline glial expression: (1) a Single-minded/Tango binding site, (2) a motif resembling a pointed binding site, (3) a motif resembling a Sox binding site, and (4) a novel motif. An additional highly conserved 27 bp are required to restrict expression to midline glia and exclude it from midline neurons. These results suggest short, highly conserved genomic sequences flanking Drosophila midline genes are indicative of functional regulatory regions and that small changes within these sequences can alter the expression pattern of a gene.

An illustrated review of early pancreas development in the mouse

Pancreas morphogenesis and cell differentiation are highly conserved among vertebrates during fetal development. The pancreas develops through simple budlike structures on the primitive gut tube to a highly branched organ containing many specialized cell types. This review presents an overview of key molecular components and important signaling sources illustrated by an extensive three-dimensional (3D) imaging of the developing mouse pancreas at single cell resolution. The 3D documentation covers the time window between embryonic days 8.5 and 14.5 in which all the pancreatic cell types become specified and therefore includes gene expression patterns of pancreatic endocrine hormones, exocrine gene products, and essential transcription factors. The 3D perspective provides valuable insight into how a complex organ like the pancreas is formed and a perception of ventral and dorsal pancreatic growth that is otherwise difficult to uncover. We further discuss how this global analysis of the developing pancreas confirms and extends previous studies, and we envisage that this type of analysis can be instrumental for evaluating mutant phenotypes in the future.