Orthodenticle Is Required for the Expression of Principal Recognition Molecules That Control Axon Targeting in the Drosophila Retina

A TRiP RNAi screen to identify molecules necessary for Drosophila photoreceptor differentiation

Drosophila rhabdomeric terminal photoreceptor differentiation is an extended process taking several days to complete. Following ommatidial patterning by the morphogenetic furrow, photoreceptors are sequentially recruited and specified, and terminal differentiation begins. Key events of terminal differentiation include the establishment of apical and basolateral domains, rhabdomere and stalk formation, inter-rhabdomeral space formation, and expression of phototransduction machinery. While many key regulators of these processes have been identified, the complete network of transcription factors to downstream effector molecules necessary for regulating each of these major events remains incomplete. Here, we report an RNAi screen to identify additional molecules and cellular pathways required for photoreceptor terminal differentiation. First, we tested several eye-specific GAL4 drivers for correct spatial and temporal specificity and identified Pph13-GAL4 as the most appropriate GAL4 line for our screen. We screened lines available through the Transgenic RNAi Project and isolated lines that when combined with Pph13-GAL4 resulted in the loss of the deep pseudopupil, as a readout for abnormal differentiation. In the end, we screened 6,189 lines, representing 3,971 genes, and have identified 64 genes, illuminating potential new regulatory molecules and cellular pathways for the differentiation and organization of Drosophila rhabdomeric photoreceptors.

Glial insulin regulates cooperative or antagonistic Golden goal/Flamingo interactions during photoreceptor axon guidance

Transmembrane protein Golden goal (Gogo) interacts with atypical cadherin Flamingo (Fmi) to direct R8 photoreceptor axons in the Drosophila visual system. However, the precise mechanisms underlying Gogo regulation during columnar- and layer-specific R8 axon targeting are unknown. Our studies demonstrated that the insulin secreted from surface and cortex glia switches the phosphorylation status of Gogo, thereby regulating its two distinct functions. Non-phosphorylated Gogo mediates the initial recognition of the glial protrusion in the center of the medulla column, whereas phosphorylated Gogo suppresses radial filopodia extension by counteracting Flamingo to maintain a one axon-to-one column ratio. Later, Gogo expression ceases during the midpupal stage, thus allowing R8 filopodia to extend vertically into the M3 layer. These results demonstrate that the long- and short-range signaling between the glia and R8 axon growth cones regulates growth cone dynamics in a stepwise manner, and thus shapes the entire organization of the visual system.

The Cytoplasmic LIM Domain Protein Espinas Contributes to Photoreceptor Layer Selection in the Visual System

During circuit assembly it is essential that neurons connect with their specific synaptic partners. To facilitate this process, a common strategy in many organisms is the organization of brain regions, including the fly visual system, in layers and columns. The atypical-cadherin Flamingo (Fmi) and the receptor Golden Goal (Gogo) were proposed to regulate both the temporary and final layer selection of the R8 photoreceptor, through the cytoplasmic domain of Gogo. Our data suggests that Fmi intracellular signaling is also relevant for R8 final layer selection. The LIM-domain cytoplasmic molecule Espinas (Esn) binds Fmi, and they cooperatively control dendritic self-avoidance in sensory neurons. We observed defects in R8 layer selection in esn mutants with axons overshooting the final target layer, and we demonstrated that the LIM domain is necessary for layer selection. fmi knockdown in photoreceptors results in most R8 axons stalling at the temporary layer, however, we also detected R8 axons projecting past the final-target layer, and showed that fmi and esn genetically interact. Based on the previously described physical and genetic interactions between Fmi/Esn and the findings presented here, we propose that Esn signals downstream of Fmi to stabilize R8 axons in their final target layer.

Age dependent trans-cellular propagation of human tau aggregates in Drosophila disease models

Tauopathies is a class of neurodegenerative disorders which involves the transformation of physiological tau into pathogenic tau. One of the prime causes reported to drive this conversion is tau hyperphosphorylation and the subsequent propagation of pathogenic protein aggregates across the nervous system. Although past attempts have been made to deduce the details of tau propagation, yet not much is known about its mechanism. A better understanding of this aspect of disease pathology can prove to be beneficial for the development of diagnostic and therapeutic approaches. For the first time, we demonstrate that the human tau possesses an intrinsic property to spread trans-cellularly in the fly nervous system irrespective of the tau allele or the neuronal tissue type. Aggregate migration restricted by targeted down-regulation of a specific kinase, elucidates the role of hyper-phosphorylation in its movement. On the contrary to the previous models, our study delivers an easy and rapid in-vivo model for comprehensive examination of tau migration pathology. Henceforth, the developed model would not only be immensely helpful in uncovering the mechanistic in-depths of tau propagation pathology but also aid in modifier and/or drug screening for amelioration of tauopathies.

Gene regulatory networks during the development of the Drosophila visual system

The Drosophila visual system integrates input from 800 ommatidia and extracts different features in stereotypically connected optic ganglia. The development of the Drosophila visual system is controlled by gene regulatory networks that control the number of precursor cells, generate neuronal diversity by integrating spatial and temporal information, coordinate the timing of retinal and optic lobe cell differentiation, and determine distinct synaptic targets of each cell type. In this chapter, we describe the known gene regulatory networks involved in the development of the different parts of the visual system and explore general components in these gene networks. Finally, we discuss the advantages of the fly visual system as a model for gene regulatory network discovery in the era of single-cell transcriptomics.

Visualisation of ribosomes in Drosophila axons using Ribo-BiFC

The distribution of assembled, and potentially translating, ribosomes within cells can be visualised in Drosophila by using Bimolecular Fluorescence Complementation (BiFC) to monitor the interaction between tagged pairs of 40S and 60S ribosomal proteins (RPs) that are close neighbours across inter-subunit junctions in the assembled 80S ribosome. Here we describe transgenes expressing two novel RP pairs tagged with Venus-based BiFC fragments that considerably increase the sensitivity of this technique we termed Ribo-BiFC. This improved method should provide a convenient way of monitoring the local distribution of ribosomes in most Drosophila cells and we suggest that it could be implemented in other organisms. We visualised 80S ribosomes in different neurons, particularly photoreceptors in the larva, pupa and adult brain. Assembled ribosomes are most abundant in the various neuronal cell bodies, but they are also present along the full length of axons. They are concentrated in growth cones of developing photoreceptors and are apparent at the terminals of mature larval photoreceptors targeting the larval optical neuropil. Surprisingly, there is relatively less puromycin incorporation in the distal portion of axons in the larval optic stalk, suggesting that some of the ribosomes that have initiated translation may not be engaged in elongation in growing axons.This article has an associated First Person interview with the first author of the paper.

Two temporal functions of Glass: Ommatidium patterning and photoreceptor differentiation

Much progress has been made in elucidating the molecular networks required for specifying retinal cells, including photoreceptors, but the downstream mechanisms that maintain identity and regulate differentiation remain poorly understood. Here, we report that the transcription factor Glass has a dual role in establishing a functional Drosophila eye. Utilizing conditional rescue approaches, we confirm that persistent defects in ommatidium patterning combined with cell death correlate with the overall disruption of eye morphology in glass mutants. In addition, we reveal that Glass exhibits a separable role in regulating photoreceptor differentiation. In particular, we demonstrate the apparent loss of glass mutant photoreceptors is not only due to cell death but also a failure of the surviving photoreceptors to complete differentiation. Moreover, the late reintroduction of Glass in these developmentally stalled photoreceptors is capable of restoring differentiation in the absence of correct ommatidium patterning. Mechanistically, transcription profiling at the time of differentiation reveals that Glass is necessary for the expression of many genes implicated in differentiation, i.e. rhabdomere morphogenesis, phototransduction, and synaptogenesis. Specifically, we show Glass directly regulates the expression of Pph13, which encodes a transcription factor necessary for opsin expression and rhabdomere morphogenesis. Finally, we demonstrate the ability of Glass to choreograph photoreceptor differentiation is conserved between Drosophila and Tribolium, two holometabolous insects. Altogether, our work identifies a fundamental regulatory mechanism to generate the full complement of cells required for a functional rhabdomeric visual system and provides a critical framework to investigate the basis of differentiation and maintenance of photoreceptor identity.

Ancient default activators of terminal photoreceptor differentiation in the pancrustacean compound eye: the homeodomain transcription factors Otd and Pph13

The origin of the Drosophila compound eye predates the ancestor of Pancrustacea, the arthropod clade that includes insects and Crustaceans. Recent studies in emerging model systems for pancrustacean development-the red flour beetle Tribolium castaneum and water flea Daphnia pulex-have begun to shed light on the evolutionary conservation of transcriptional mechanisms found for the Drosophila compound eye. Here, we discuss the conserved roles of the transcription factors Otd and Pph13, which complement each other in two terminal events of photoreceptor differentiation: rhabdomere morphogenesis and transcriptional default activation of opsin gene expression. The synthesis of these data allows us to frame an evolutionary developmental model of the earliest events that generated the wavelength-specific photoreceptor subtypes of pancrustacean compound eyes.