Drosophila hedgehog signaling and engrailed-runt mutual repression direct midline glia to alternative ensheathing and non-ensheathing fates

Engrailed transcription factors direct excitatory cerebellar neuron diversity and survival

The neurons of the three cerebellar nuclei (CN) are the primary output neurons of the cerebellum. The excitatory neurons (e) of the medial (m) CN (eCNm) were recently divided into molecularly defined subdomains in the adult; however, how they are established during development is not known. We define molecular subdomains of the mouse embryonic eCNm using single-cell RNA-sequencing and spatial expression analysis, showing that they evolve during embryogenesis to prefigure the adult. Furthermore, eCNm are transcriptionally divergent from cells in the other nuclei by embryonic day 14.5. We previously showed that loss of the homeobox genes En1 and En2 leads to loss of approximately half of the embryonic eCNm. We demonstrate that mutation of En1/2 in the embryonic eCNm results in death of specific posterior eCNm molecular subdomains and downregulation of TBR2 (EOMES) in an anterior embryonic subdomain, as well as reduced synaptic gene expression. We further reveal a similar function for EN1/2 in mediating TBR2 expression, neuron differentiation and survival in the other excitatory neurons (granule and unipolar brush cells). Thus, our work defines embryonic eCNm molecular diversity and reveals conserved roles for EN1/2 in the cerebellar excitatory neuron lineage.

Engrailed transcription factors direct excitatory cerebellar neuron diversity and survival

The excitatory neurons of the three cerebellar nuclei (eCN) form the primary output for the cerebellar circuit. The medial eCN (eCNm) were recently divided into molecularly defined subdomains in the adult, however how they are established during development is not known. We define molecular subdomains of the eCNm using scRNA-seq and spatial expression analysis and show they evolve during embryogenesis to resemble the adult. Furthermore, the eCNm is transcriptionally divergent from the rest of the eCN by E14.5. We previously showed that loss of the homeobox genes En1 and En2 leads to death of a subset of embryonic eCNm. We demonstrate that mutation of En1/2 in embryonic eCNm results in cell death of specific posterior eCNm molecular subdomains and loss of TBR2 (EOMES) expression in an anterior subdomain, as well as reduced synaptic gene expression. We further reveal a similar function for EN1/2 in mediating TBR2 expression, neuron differentiation and survival in the two other cerebellar excitatory neuron types. Thus, our work defines embryonic eCNm molecular diversity and reveals conserved roles for EN1/2 in the cerebellar excitatory neuron lineage.

Patched-Related Is Required for Proper Development of Embryonic Drosophila Nervous System

Patched-related (Ptr), classified primarily as a neuroectodermal gene, encodes a protein with predicted topology and domain organization closely related to those of Patched (Ptc), the canonical receptor of the Hedgehog (Hh) pathway. To investigate the physiological function of Ptr in the developing nervous system, Ptr null mutant embryos were immunolabeled and imaged under confocal microscopy. These embryos displayed severe alterations in the morphology of the primary axonal tracts, reduced number, and altered distribution of the Repo-positive glia as well as peripheral nervous system defects. Most of these alterations were recapitulated by downregulating Ptr expression, specifically in embryonic nerve cells. Because similar nervous system phenotypes have been observed in hh and ptc mutant embryos, we evaluated the Ptr participation in the Hh pathway by performing cell-based reporter assays. Clone-8 cells were transfected with Ptr-specific dsRNA or a Ptr DNA construct and assayed for changes in Hh-mediated induction of a luciferase reporter. The results obtained suggest that Ptr could act as a negative regulator of Hh signaling. Furthermore, co-immunoprecipitation assays from cell culture extracts premixed with a conditioned medium revealed a direct interaction between Ptr and Hh. Moreover, in vivo Ptr overexpression in the domain of the imaginal wing disc where Engrailed and Ptc coexist produced wing phenotypes at the A/P border. Thus, these results strongly suggest that Ptr plays a crucial role in nervous system development and appears to be a negative regulator of the Hh pathway.

Early lineage segregation of the retinal basal glia in the Drosophila eye disc

The retinal basal glia (RBG) is a group of glia that migrates from the optic stalk into the third instar larval eye disc while the photoreceptor cells (PR) are differentiating. The RBGs are grouped into three major classes based on molecular and morphological characteristics: surface glia (SG), wrapping glia (WG) and carpet glia (CG). The SGs migrate and divide. The WGs are postmitotic and wraps PR axons. The CGs have giant nucleus and extensive membrane extension that each covers half of the eye disc. In this study, we used lineage tracing methods to determine the lineage relationships among these glia subtypes and the temporal profile of the lineage decisions for RBG development. We found that the CG lineage segregated from the other RBG very early in the embryonic stage. It has been proposed that the SGs migrate under the CG membrane, which prevented SGs from contacting with the PR axons lying above the CG membrane. Upon passing the front of the CG membrane, which is slightly behind the morphogenetic furrow that marks the front of PR differentiation, the migrating SG contact the nascent PR axon, which in turn release FGF to induce SGs' differentiation into WG. Interestingly, we found that SGs are equally distributed apical and basal to the CG membrane, so that the apical SGs are not prevented from contacting PR axons by CG membrane. Clonal analysis reveals that the apical and basal RBG are derived from distinct lineages determined before they enter the eye disc. Moreover, the basal SG lack the competence to respond to FGFR signaling, preventing its differentiation into WG. Our findings suggest that this novel glia-to-glia differentiation is both dependent on early lineage decision and on a yet unidentified regulatory mechanism, which can provide spatiotemporal coordination of WG differentiation with the progressive differentiation of photoreceptor neurons.

Drosophila Embryonic CNS Development: Neurogenesis, Gliogenesis, Cell Fate, and Differentiation

The Drosophila embryonic central nervous system (CNS) is a complex organ consisting of ∼15,000 neurons and glia that is generated in ∼1 day of development. For the past 40 years, Drosophila developmental neuroscientists have described each step of CNS development in precise molecular genetic detail. This has led to an understanding of how an intricate nervous system emerges from a single cell. These studies have also provided important, new concepts in developmental biology, and provided an essential model for understanding similar processes in other organisms. In this article, the key genes that guide Drosophila CNS development and how they function is reviewed. Features of CNS development covered in this review are neurogenesis, gliogenesis, cell fate specification, and differentiation.

Chromatin profiling of Drosophila CNS subpopulations identifies active transcriptional enhancers

One of the key issues in studying transcriptional regulation during development is how to employ genome-wide assays that reveals sites of open chromatin and transcription factor binding to efficiently identify biologically relevant genes and enhancers. Analysis of Drosophila CNS midline cell development provides a useful system for studying transcriptional regulation at the genomic level due to a large, well-characterized set of midline-expressed genes and in vivo validated enhancers. In this study, FAIRE-seq on FACS-purified midline cells was performed and the midline FAIRE data were compared with whole-embryo FAIRE data. We find that regions of the genome with a strong midline FAIRE peak and weak whole-embryo FAIRE peak overlap with known midline enhancers and provide a useful predictive tool for enhancer identification. In a complementary analysis, we compared a large dataset of fragments that drive midline expression in vivo with the FAIRE data. Midline enhancer fragments with a midline FAIRE peak tend to be near midline-expressed genes, whereas midline enhancers without a midline FAIRE peak were often distant from midline-expressed genes and unlikely to drive midline transcription in vivo.

Origins of glial cell populations in the insect nervous system

Glia of vertebrates and invertebrates alike represents a diverse population of cells in the nervous system, divided into numerous classes with different structural and functional characteristics. In insects, glia fall within three basic classes: surface, cell body, and neuropil glia. Due to the glial subclass-specific markers and genetic tools available in Drosophila, it is possible to establish the progenitor origin of these different populations and reconstruct their migration and differentiation during development. We review, and posit when appropriate, recently elucidated aspects of glial developmental dynamics. In particular, we focus on the relationships between mature glial subclasses of the larval nervous system (primary glia), born in the embryo, and glia of the adult (secondary glia), generated in the larva.

Hedgehog signaling regulates gene expression in planarian glia

Hedgehog signaling is critical for vertebrate central nervous system (CNS) development, but its role in CNS biology in other organisms is poorly characterized. In the planarian Schmidtea mediterranea, hedgehog (hh) is expressed in medial cephalic ganglia neurons, suggesting a possible role in CNS maintenance or regeneration. We performed RNA sequencing of planarian brain tissue following RNAi of hh and patched (ptc), which encodes the Hh receptor. Two misregulated genes, intermediate filament-1 (if-1) and calamari (cali), were expressed in a previously unidentified non-neural CNS cell type. These cells expressed orthologs of astrocyte-associated genes involved in neurotransmitter uptake and metabolism, and extended processes enveloping regions of high synapse concentration. We propose that these cells are planarian glia. Planarian glia were distributed broadly, but only expressed if-1 and cali in the neuropil near hh+ neurons. Planarian glia and their regulation by Hedgehog signaling present a novel tractable system for dissection of glia biology.

The Hedgehog Signalling Pathway in Cell Migration and Guidance: What We Have Learned from Drosophila melanogaster

Cell migration and guidance are complex processes required for morphogenesis, the formation of tumor metastases, and the progression of human cancer. During migration, guidance molecules induce cell directionality and movement through complex intracellular mechanisms. Expression of these molecules has to be tightly regulated and their signals properly interpreted by the receiving cells so as to ensure correct navigation. This molecular control is fundamental for both normal morphogenesis and human disease. The Hedgehog (Hh) signaling pathway is evolutionarily conserved and known to be crucial for normal cellular growth and differentiation throughout the animal kingdom. The relevance of Hh signaling for human disease is emphasized by its activation in many cancers. Here, I review the current knowledge regarding the involvement of the Hh pathway in cell migration and guidance during Drosophila development and discuss its implications for human cancer origin and progression.

Coordinated repression and activation of two transcriptional programs stabilizes cell fate during myogenesis

Molecular models of cell fate specification typically focus on the activation of specific lineage programs. However, the concurrent repression of unwanted transcriptional networks is also essential to stabilize certain cellular identities, as shown in a number of diverse systems and phyla. Here, we demonstrate that this dual requirement also holds true in the context of Drosophila myogenesis. By integrating genetics and genomics, we identified a new role for the pleiotropic transcriptional repressor Tramtrack69 in myoblast specification. Drosophila muscles are formed through the fusion of two discrete cell types: founder cells (FCs) and fusion-competent myoblasts (FCMs). When tramtrack69 is removed, FCMs appear to adopt an alternative muscle FC-like fate. Conversely, ectopic expression of this repressor phenocopies muscle defects seen in loss-of-function lame duck mutants, a transcription factor specific to FCMs. This occurs through Tramtrack69-mediated repression in FCMs, whereas Lame duck activates a largely distinct transcriptional program in the same cells. Lineage-specific factors are therefore not sufficient to maintain FCM identity. Instead, their identity appears more plastic, requiring the combination of instructive repressive and activating programs to stabilize cell fate.

Enhancer diversity and the control of a simple pattern of Drosophila CNS midline cell expression

Transcriptional enhancers integrate information derived from transcription factor binding to control gene expression. One key question concerns the extent of trans- and cis-regulatory variation in how co-expressed genes are controlled. The Drosophila CNS midline cells constitute a group of neurons and glia in which expression changes can be readily characterized during specification and differentiation. Using a transgenic approach, we compare the cis-regulation of multiple genes expressed in the Drosophila CNS midline primordium cells, and show that while the expression patterns may appear alike, the target genes are not equivalent in how these common expression patterns are achieved. Some genes utilize a single enhancer that promotes expression in all midline cells, while others utilize multiple enhancers with distinct spatial, temporal, and quantitative contributions. Two regulators, Single-minded and Notch, play key roles in controlling early midline gene expression. While Single-minded is expected to control expression of most, if not all, midline primordium-expressed genes, the role of Notch in directly controlling midline transcription is unknown. Midline primordium expression of the rhomboid gene is dependent on cell signaling by the Notch signaling pathway. Mutational analysis of a rhomboid enhancer reveals at least 5 distinct types of functional cis-control elements, including a binding site for the Notch effector, Suppressor of Hairless. The results suggest a model in which Notch/Suppressor of Hairless levels are insufficient to activate rhomboid expression by itself, but does so in conjunction with additional factors, some of which, including Single-minded, provide midline specificity to Notch activation. Similarly, a midline glial enhancer from the argos gene, which is dependent on EGF/Spitz signaling, is directly regulated by contributions from both Pointed, the EGF transcriptional effector, and Single-minded. In contrast, midline primordium expression of other genes shows a strong dependence on Single-minded and varying combinations of additional transcription factors. Thus, Single-minded directly regulates midline primordium-expressed genes, but in some cases plays a primary role in directing target gene midline expression, and in others provides midline specificity to cell signaling inputs.

A comparison of midline and tracheal gene regulation during Drosophila development

Within the Drosophila embryo, two related bHLH-PAS proteins, Single-minded and Trachealess, control development of the central nervous system midline and the trachea, respectively. These two proteins are bHLH-PAS transcription factors and independently form heterodimers with another bHLH-PAS protein, Tango. During early embryogenesis, expression of Single-minded is restricted to the midline and Trachealess to the trachea and salivary glands, whereas Tango is ubiquitously expressed. Both Single-minded/Tango and Trachealess/Tango heterodimers bind to the same DNA sequence, called the CNS midline element (CME) within cis-regulatory sequences of downstream target genes. While Single-minded/Tango and Trachealess/Tango activate some of the same genes in their respective tissues during embryogenesis, they also activate a number of different genes restricted to only certain tissues. The goal of this research is to understand how these two related heterodimers bind different enhancers to activate different genes, thereby regulating the development of functionally diverse tissues. Existing data indicates that Single-minded and Trachealess may bind to different co-factors restricted to various tissues, causing them to interact with the CME only within certain sequence contexts. This would lead to the activation of different target genes in different cell types. To understand how the context surrounding the CME is recognized by different bHLH-PAS heterodimers and their co-factors, we identified and analyzed novel enhancers that drive midline and/or tracheal expression and compared them to previously characterized enhancers. In addition, we tested expression of synthetic reporter genes containing the CME flanked by different sequences. Taken together, these experiments identify elements overrepresented within midline and tracheal enhancers and suggest that sequences immediately surrounding a CME help dictate whether a gene is expressed in the midline or trachea.

Probing the enigma: unraveling glial cell biology in invertebrates

Despite their predominance in the nervous system, the precise ways in which glial cells develop and contribute to overall neural function remain poorly defined in any organism. Investigations in simple model organisms have identified remarkable morphological, molecular, and functional similarities between invertebrate and vertebrate glial subtypes. Invertebrates like Drosophila and Caenorhabditis elegans offer an abundance of tools for in vivo genetic manipulation of single cells or whole populations of glia, ease of access to neural tissues throughout development, and the opportunity for forward genetic analysis of fundamental aspects of glial cell biology. These features suggest that invertebrate model systems have high potential for vastly improving the understanding of glial biology. This review highlights recent work in Drosophila and other invertebrates that reveal new insights into basic mechanisms involved in glial development.

Formation and specification of a Drosophila dopaminergic precursor cell

Dopaminergic neurons play important roles in animal behavior, including motivation, reward and locomotion. The Drosophila dopaminergic H-cell interneuron is an attractive system for studying the genetics of neural development because analysis is focused on a single neuronal cell type. Here we provide a mechanistic understanding of how MP3, the precursor to the H-cell, forms and acquires its identity. We show that the gooseberry/gooseberry-neuro (gsb/gsb-n) transcription factor genes act to specify MP3 cell fate. It is proposed that single-minded commits neuroectodermal cells to a midline fate, followed by a series of signaling events that result in the formation of a single gsb(+)/gsb-n(+) MP3 cell per segment. The wingless signaling pathway establishes a midline anterior domain by activating expression of the forkhead transcription factors sloppy paired 1 and sloppy paired 2. This is followed by hedgehog signaling that activates gsb/gsb-n expression in a subgroup of anterior cells. Finally, Notch signaling results in the selection of a single MP3, with the remaining cells becoming midline glia. In MP3, gsb/gsb-n direct H-cell development, in large part by activating expression of the lethal of scute and tailup H-cell regulatory genes. Thus, a series of signaling and transcriptional events result in the specification of a unique dopaminergic precursor cell. Additional genetic experiments indicate that the molecular mechanisms that govern MP3/H-cell development might also direct the development of non-midline dopaminergic neurons.

Time-lapse imaging reveals stereotypical patterns of Drosophila midline glial migration

The Drosophila CNS midline glia (MG) are multifunctional cells that ensheath and provide trophic support to commissural axons, and direct embryonic development by employing a variety of signaling molecules. These glia consist of two functionally distinct populations: the anterior MG (AMG) and posterior MG (PMG). Only the AMG ensheath axon commissures, whereas the function of the non-ensheathing PMG is unknown. The Drosophila MG have proven to be an excellent system for studying glial proliferation, cell fate, apoptosis, and axon-glial interactions. However, insight into how AMG migrate and acquire their specific positions within the axon-glial scaffold has been lacking. In this paper, we use time-lapse imaging, single-cell analysis, and embryo staining to comprehensively describe the proliferation, migration, and apoptosis of the Drosophila MG. We identified 3 groups of MG that differed in the trajectories of their initial inward migration: AMG that migrate inward and to the anterior before undergoing apoptosis, AMG that migrate inward and to the posterior to ensheath commissural axons, and PMG that migrate inward and to the anterior to contact the commissural axons before undergoing apoptosis. In a second phase of their migration, the surviving AMG stereotypically migrated posteriorly to specific positions surrounding the commissures, and their final position was correlated with their location prior to migration. Most noteworthy are AMG that migrated between the commissures from a ventral to a dorsal position. Single-cell analysis indicated that individual AMG possessed wide-ranging and elaborate membrane extensions that partially ensheathed both commissures. These results provide a strong foundation for future genetic experiments to identify mutants affecting MG development, particularly in guidance cues that may direct migration. Drosophila MG are homologous in structure and function to the glial-like cells that populate the vertebrate CNS floorplate, and study of Drosophila MG will provide useful insights into floorplate development and function.

Morphological characterization of the entire interneuron population reveals principles of neuromere organization in the ventral nerve cord of Drosophila

Decisive contributions to our understanding of the mechanisms underlying the development of the nervous system have been made by studies performed at the level of single, identified cells in the fruit fly Drosophila. While all the motor neurons and glial cells in thoracic and abdominal segments of the Drosophila embryo have been individually identified, few of the interneurons, which comprise the vast majority of cells in the CNS, have been characterized at this level. We have applied a single cell labeling technique to carry out a detailed morphological characterization of the entire population of interneurons in abdominal segments A1-A7. Based on the definition of a set of spatial parameters specifying axonal projection patterns and cell body positions, we have identified 270 individual cell types as the complete hemisegmental set of interneurons and placed these in an interactive database. As well as facilitating analyses of developmental processes, this comprehensive set of data sheds light on the principles underlying the formation and organization of an entire segmental unit of the CNS.

Mastermind mutations generate a unique constellation of midline cells within the Drosophila CNS

Background

The Notch pathway functions repeatedly during the development of the central nervous system in metazoan organisms to control cell fate and regulate cell proliferation and asymmetric cell divisions. Within the Drosophila midline cell lineage, which bisects the two symmetrical halves of the central nervous system, Notch is required for initial cell specification and subsequent differentiation of many midline lineages.

Methodology/Principal Findings

Here, we provide the first description of the role of the Notch co-factor, mastermind, in the central nervous system midline of Drosophila. Overall, zygotic mastermind mutations cause an increase in midline cell number and decrease in midline cell diversity. Compared to mutations in other components of the Notch signaling pathway, such as Notch itself and Delta, zygotic mutations in mastermind cause the production of a unique constellation of midline cell types. The major difference is that midline glia form normally in zygotic mastermind mutants, but not in Notch and Delta mutants. Moreover, during late embryogenesis, extra anterior midline glia survive in zygotic mastermind mutants compared to wild type embryos.

Conclusions/Significance

This is an example of a mutation in a signaling pathway cofactor producing a distinct central nervous system phenotype compared to mutations in major components of the pathway.

Embryonic expression of Drosophila ceramide synthase schlank in developing gut, CNS and PNS

Schlank is a member of the highly conserved ceramide synthase family and controls growth and body fat in Drosophila. Ceramide synthases are key enzymes in the sphingolipid de novo synthesis pathway. Ceramide synthase proteins and the (dihydro)ceramide produced are involved in a variety of biological processes among them apoptosis and neurodegeneration. The full extent of their involvement in these processes will require a precise analysis of the distribution and expression pattern of ceramide synthases. Paralogs of the ceramide synthase family have been found in all eukaryotes studied, however the mRNA and protein expression patterns have not yet been analysed systematically. In this study, we use antibodies that specifically recognize Schlank, a schlank mRNA probe and an endogenous schlank promoter driven LacZ reporter line to reveal the expression pattern of Schlank throughout embryogenesis. We found that Schlank is expressed in all embryonic epithelia during embryogenesis including the developing epidermis and the gastrointestinal tract. In addition, Schlank is upregulated in the developing central (CNS) and peripheral nervous system (PNS). Co-staining experiments with neuronal and glial markers revealed specific expression of Schlank in glial and neuronal cells of the CNS and PNS.