Genetic analysis of fibroblast growth factor signaling in the Drosophila eye

Comparative transcriptome analyses of the Drosophila pupal eye

Tissue function is dependent on correct cellular organization and behavior. As a result, the identification and study of genes that contribute to tissue morphogenesis is of paramount importance to the fields of cell and developmental biology. Many of the genes required for tissue patterning and organization are highly conserved between phyla. This has led to the emergence of several model organisms and developmental systems that are used to study tissue morphogenesis. One such model is the Drosophila melanogaster pupal eye that has a highly stereotyped arrangement of cells. In addition, the pupal eye is postmitotic that allows for the study of tissue morphogenesis independent from any effects of proliferation. While the changes in cell morphology and organization that occur throughout pupal eye development are well documented, less is known about the corresponding transcriptional changes that choreograph these processes. To identify these transcriptional changes, we dissected wild-type Canton S pupal eyes and performed RNA-sequencing. Our analyses identified differential expression of many loci that are documented regulators of pupal eye morphogenesis and contribute to multiple biological processes including signaling, axon projection, adhesion, and cell survival. We also identified differential expression of genes not previously implicated in pupal eye morphogenesis such as components of the Toll pathway, several non-classical cadherins, and components of the muscle sarcomere, which could suggest these loci function as novel patterning factors.

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.

Receptor Tyrosine Kinases in Development: Insights from Drosophila

Cell-to-cell communication mediates a plethora of cellular decisions and behaviors that are crucial for the correct and robust development of multicellular organisms. Many of these signals are encoded in secreted hormones or growth factors that bind to and activate cell surface receptors, to transmit the cue intracellularly. One of the major superfamilies of cell surface receptors are the receptor tyrosine kinases (RTKs). For nearly half a century RTKs have been the focus of intensive study due to their ability to alter fundamental aspects of cell biology, such as cell proliferation, growth, and shape, and because of their central importance in diseases such as cancer. Studies in model organisms such a Drosophila melanogaster have proved invaluable for identifying new conserved RTK pathway components, delineating their contributions, and for the discovery of conserved mechanisms that control RTK-signaling events. Here we provide a brief overview of the RTK superfamily and the general mechanisms used in their regulation. We further highlight the functions of several RTKs that govern distinct cell-fate decisions in Drosophila and explore how their activities are developmentally controlled.

Proper splicing contributes to visual function in the aging Drosophila eye

Changes in splicing patterns are a characteristic of the aging transcriptome; however, it is unclear whether these age-related changes in splicing facilitate the progressive functional decline that defines aging. In Drosophila, visual behavior declines with age and correlates with altered gene expression in photoreceptors, including downregulation of genes encoding splicing factors. Here, we characterized the significance of these age-regulated splicing-associated genes in both splicing and visual function. To do this, we identified differential splicing events in either the entire eye or photoreceptors of young and old flies. Intriguingly, aging photoreceptors show differential splicing of a large number of visual function genes. In addition, as shown previously for aging photoreceptors, aging eyes showed increased accumulation of circular RNAs, which result from noncanonical splicing events. To test whether proper splicing was necessary for visual behavior, we knocked down age-regulated splicing factors in photoreceptors in young flies and examined phototaxis. Notably, many of the age-regulated splicing factors tested were necessary for proper visual behavior. In addition, knockdown of individual splicing factors resulted in changes in both alternative splicing at age-spliced genes and increased accumulation of circular RNAs. Together, these data suggest that cumulative decreases in splicing factor expression could contribute to the differential splicing, circular RNA accumulation, and defective visual behavior observed in aging photoreceptors.

Endocrine regulation of airway clearance in Drosophila

Fluid clearance from the respiratory system during developmental transitions is critically important for achieving optimal gas exchange in animals. During insect development from embryo to adult, airway clearance occurs episodically each time the molt is completed by performance of the ecdysis sequence, coordinated by a peptide-signaling cascade initiated by ecdysis-triggering hormone (ETH). We find that the neuropeptide Kinin (also known as Drosokinin or Leukokinin) is required for normal respiratory fluid clearance or "tracheal air-filling" in Drosophila larvae. Disruption of Kinin signaling leads to defective air-filling during all larval stages. Such defects are observed upon ablation or electrical silencing of Kinin neurons, as well as RNA silencing of the Kinin gene or the ETH receptor in Kinin neurons, indicating that ETH targets Kinin neurons to promote tracheal air-filling. A Kinin receptor mutant fly line (Lkr^f02594 ) also exhibits tracheal air-filling defects in all larval stages. Targeted Kinin receptor silencing in tracheal epithelial cells using breathless or pickpocket (ppk) drivers compromises tracheal air-filling. On the other hand, promotion of Kinin signaling in vivo through peptide injection or Kinin neuron activation through Drosophila TrpA1 (dTrpA1) expression induces premature tracheal collapse and air-filling. Moreover, direct exposure of tracheal epithelial cells in vitro to Kinin leads to calcium mobilization in tracheal epithelial cells. Our findings strongly implicate the neuropeptide Kinin as an important regulator of airway clearance via intracellular calcium mobilization in tracheal epithelial cells of Drosophila.

Unique patterns of organization and migration of FGF-expressing cells during Drosophila morphogenesis

Fibroblast growth factors (FGF) are essential signaling proteins that regulate diverse cellular functions in developmental and metabolic processes. In Drosophila, the FGF homolog, branchless (bnl) is expressed in a dynamic and spatiotemporally restricted pattern to induce branching morphogenesis of the trachea, which expresses the Bnl-receptor, breathless (btl). Here we have developed a new strategy to determine bnl- expressing cells and study their interactions with the btl-expressing cells in the range of tissue patterning during Drosophila development. To enable targeted gene expression specifically in the bnl expressing cells, a new LexA based bnl enhancer trap line was generated using CRISPR/Cas9 based genome editing. Analyses of the spatiotemporal expression of the reporter in various embryonic stages, larval or adult tissues and in metabolic hypoxia, confirmed its target specificity and versatility. With this tool, new bnl expressing cells, their unique organization and functional interactions with the btl-expressing cells were uncovered in a larval tracheoblast niche in the leg imaginal discs, in larval photoreceptors of the developing retina, and in the embryonic central nervous system. The targeted expression system also facilitated live imaging of simultaneously labeled Bnl sources and tracheal cells, which revealed a unique morphogenetic movement of the embryonic bnl- source. Migration of bnl- expressing cells may create a dynamic spatiotemporal pattern of the signal source necessary for the directional growth of the tracheal branch. The genetic tool and the comprehensive profile of expression, organization, and activity of various types of bnl-expressing cells described in this study provided us with an important foundation for future research investigating the mechanisms underlying Bnl signaling in tissue morphogenesis.

Mactosylceramide prevents glial cell overgrowth by inhibiting insulin and fibroblast growth factor receptor signaling

Receptor tyrosine kinase (RTK) signaling controls key aspects of cellular differentiation, proliferation, survival, metabolism, and migration. Deregulated RTK signaling also underlies many cancers. Glycosphingolipids (GSL) are essential elements of the plasma membrane. By affecting clustering and activity of membrane receptors, GSL modulate signal transduction, including that mediated by the RTK. GSL are abundant in the nervous system, and glial development in Drosophila is emerging as a useful model for studying how GSL modulate RTK signaling. Drosophila has a simple GSL biosynthetic pathway, in which the mannosyltransferase Egghead controls conversion of glucosylceramide (GlcCer) to mactosylceramide (MacCer). Lack of elongated GSL in egghead (egh) mutants causes overgrowth of subperineurial glia (SPG), largely due to aberrant activation of phosphatidylinositol 3-kinase (PI3K). However, to what extent this effect involves changes in upstream signaling events is unresolved. We show here that glial overgrowth in egh is strongly linked to increased activation of Insulin and fibroblast growth factor receptors (FGFR). Glial hypertrophy is phenocopied when overexpressing gain-of-function mutants of the Drosophila insulin receptor (InR) and the FGFR homolog Heartless (Htl) in wild type SPG, and is suppressed by inhibiting Htl and InR activity in egh. Knockdown of GlcCer synthase in the SPG fails to suppress glial overgrowth in egh nerves, and slightly promotes overgrowth in wild type, suggesting that RTK hyperactivation is caused by absence of MacCer and not by GlcCer accumulation. We conclude that an early product in GSL biosynthesis, MacCer, prevents inappropriate activation of insulin and fibroblast growth factor receptors in Drosophila glia.

dMyc is required in retinal progenitors to prevent JNK-mediated retinal glial activation

In the nervous system, glial cells provide crucial insulation and trophic support to neurons and are important for neuronal survival. In reaction to a wide variety of insults, glial cells respond with changes in cell morphology and metabolism to allow repair. Additionally, these cells can acquire migratory and proliferative potential. In particular, after axonal damage or pruning the clearance of axonal debris by glial cells is key for a healthy nervous system. Thus, bidirectional neuron-glial interactions are crucial in development, but little is known about the cellular sensors and signalling pathways involved. In here, we show that decreased cellular fitness in retinal progenitors caused by reduced Drosophila Myc expression triggers non cell-autonomous activation of retinal glia proliferation and overmigration. Glia migration occurs beyond its normal limit near the boundary between differentiated photoreceptors and precursor cells, extending into the progenitor domain. This overmigration is stimulated by JNK activation (and the function of its target Mmp1), while proliferative responses are mediated by Dpp/TGF-β signalling activation.

For a functional nervous system, neurons transmit information from cell to cell while glial cells provide crucial insulation and trophic support to neurons, which is important for neuronal survival. Glial cells are one of the most plastic cell types being able to adapt and respond to changing environmental stimuli. In this work we inhibit the function of the growth regulator dMyc in Drosophila retinal primordium, the eye imaginal discs. Glial cell numbers and migration pattern to the eye disc are tightly controlled but in dMyc-depleted retinas the glial cells overcome their normal barriers and overmigrate into the eye progenitors domain. We show evidence that this process is mediated by JNK activation in the presence of metalloproteinases. We discuss the biological role of overmigrating glia in tissue regeneration and/or confinement of the damaged area.

Functional Conservation of the Glide/Gcm Regulatory Network Controlling Glia, Hemocyte, and Tendon Cell Differentiation in Drosophila

High-throughput screens allow us to understand how transcription factors trigger developmental processes, including cell specification. A major challenge is identification of their binding sites because feedback loops and homeostatic interactions may mask the direct impact of those factors in transcriptome analyses. Moreover, this approach dissects the downstream signaling cascades and facilitates identification of conserved transcriptional programs. Here we show the results and the validation of a DNA adenine methyltransferase identification (DamID) genome-wide screen that identifies the direct targets of Glide/Gcm, a potent transcription factor that controls glia, hemocyte, and tendon cell differentiation in Drosophila. The screen identifies many genes that had not been previously associated with Glide/Gcm and highlights three major signaling pathways interacting with Glide/Gcm: Notch, Hedgehog, and JAK/STAT, which all involve feedback loops. Furthermore, the screen identifies effector molecules that are necessary for cell-cell interactions during late developmental processes and/or in ontogeny. Typically, immunoglobulin (Ig) domain-containing proteins control cell adhesion and axonal navigation. This shows that early and transiently expressed fate determinants not only control other transcription factors that, in turn, implement a specific developmental program but also directly affect late developmental events and cell function. Finally, while the mammalian genome contains two orthologous Gcm genes, their function has been demonstrated in vertebrate-specific tissues, placenta, and parathyroid glands, begging questions on the evolutionary conservation of the Gcm cascade in higher organisms. Here we provide the first evidence for the conservation of Gcm direct targets in humans. In sum, this work uncovers novel aspects of cell specification and sets the basis for further understanding of the role of conserved Gcm gene regulatory cascades.

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.

Quantitative Genetics of Food Intake in Drosophila melanogaster

Food intake is an essential animal activity, regulated by neural circuits that motivate food localization, evaluate nutritional content and acceptance or rejection responses through the gustatory system, and regulate neuroendocrine feedback loops that maintain energy homeostasis. Excess food consumption in people is associated with obesity and metabolic and cardiovascular disorders. However, little is known about the genetic basis of natural variation in food consumption. To gain insights in evolutionarily conserved genetic principles that regulate food intake, we took advantage of a model system, Drosophila melanogaster, in which food intake, environmental conditions and genetic background can be controlled precisely. We quantified variation in food intake among 182 inbred, sequenced lines of the Drosophila melanogaster Genetic Reference Panel (DGRP). We found significant genetic variation in the mean and within-line environmental variance of food consumption and observed sexual dimorphism and genetic variation in sexual dimorphism for both food intake traits (mean and variance). We performed genome wide association (GWA) analyses for mean food intake and environmental variance of food intake (using the coefficient of environmental variation, CV_E, as the metric for environmental variance) and identified molecular polymorphisms associated with both traits. Validation experiments using RNAi-knockdown confirmed 24 of 31 (77%) candidate genes affecting food intake and/or variance of food intake, and a test cross between selected DGRP lines confirmed a SNP affecting mean food intake identified in the GWA analysis. The majority of the validated candidate genes were novel with respect to feeding behavior, and many had mammalian orthologs implicated in metabolic diseases.

FGF /FGFR signal induces trachea extension in the drosophila visual system

The Drosophila compound eye is a large sensory organ that places a high demand on oxygen supplied by the tracheal system. Although the development and function of the Drosophila visual system has been extensively studied, the development and contribution of its tracheal system has not been systematically examined. To address this issue, we studied the tracheal patterns and developmental process in the Drosophila visual system. We found that the retinal tracheae are derived from air sacs in the head, and the ingrowth of retinal trachea begin at mid-pupal stage. The tracheal development has three stages. First, the air sacs form near the optic lobe in 42-47% of pupal development (pd). Second, in 47-52% pd, air sacs extend branches along the base of the retina following a posterior-to-anterior direction and further form the tracheal network under the fenestrated membrane (TNUFM). Third, the TNUFM extend fine branches into the retina following a proximal-to-distal direction after 60% pd. Furthermore, we found that the trachea extension in both retina and TNUFM are dependent on the FGF(Bnl)/FGFR(Btl) signaling. Our results also provided strong evidence that the photoreceptors are the source of the Bnl ligand to guide the trachea ingrowth. Our work is the first systematic study of the tracheal development in the visual system, and also the first study demonstrating the interactions of two well-studied systems: the eye and trachea.

Receptor tyrosine kinases in Drosophila development

Tyrosine phosphorylation plays a significant role in a wide range of cellular processes. The Drosophila genome encodes more than 20 receptor tyrosine kinases and extensive studies in the past 20 years have illustrated their diverse roles and complex signaling mechanisms. Although some receptor tyrosine kinases have highly specific functions, others strikingly are used in rather ubiquitous manners. Receptor tyrosine kinases regulate a broad expanse of processes, ranging from cell survival and proliferation to differentiation and patterning. Remarkably, different receptor tyrosine kinases share many of the same effectors and their hierarchical organization is retained in disparate biological contexts. In this comprehensive review, we summarize what is known regarding each receptor tyrosine kinase during Drosophila development. Astonishingly, very little is known for approximately half of all Drosophila receptor tyrosine kinases.