Dendritic Eph organizes dendrodendritic segregation in discrete olfactory map formation in Drosophila

The role of glial and neuronal Eph/ephrin signaling in Drosophila mushroom body development and sleep and circadian behavior

The Eph receptor, a prototypically large receptor protein tyrosine kinase, interacts with ephrin ligands, forming a bidirectional signaling system that impacts diverse brain functions. Eph receptors and ephrins mediate forward and reverse signaling, affecting neurogenesis, axon guidance, and synaptic signaling. While mammalian studies have emphasized their roles in neurogenesis and synaptic plasticity, the Drosophila counterparts are less studied, especially in glial cells, despite structural similarities. Using RNAi to modulate Eph/ephrin expression in Drosophila neurons and glia, we studied their roles in brain development and sleep and circadian behavior. Knockdown of neuronal ephrin disrupted mushroom body development, while glial knockdown had minimal impact. Surprisingly, disrupting ephrin in neurons or glial cells altered sleep and circadian rhythms, indicating a direct involvement in these behaviors independent from developmental effects. Further analysis revealed distinct sleep phenotypes between neuronal and glial knockdowns, underscoring the intricate interplay within the neural circuits that govern behavior. Glia-specific knockdowns showed altered sleep patterns and reduced circadian rhythmicity, suggesting an intricate role of glia in sleep regulation. Our findings challenge simplistic models of Eph/ephrin signaling limited to neuron-glia communication and emphasize the complexity of the regulatory networks modulating behavior. Future investigations targeting specific glial subtypes will enhance our understanding of Eph/ephrin signaling's role in sleep regulation across species.

Modular and Distinct Plexin-A4/FARP2/Rac1 Signaling Controls Dendrite Morphogenesis

Diverse neuronal populations with distinct cellular morphologies coordinate the complex function of the nervous system. Establishment of distinct neuronal morphologies critically depends on signaling pathways that control axonal and dendritic development. The Sema3A-Nrp1/PlxnA4 signaling pathway promotes cortical neuron basal dendrite arborization but also repels axons. However, the downstream signaling components underlying these disparate functions of Sema3A signaling are unclear. Using the novel PlxnA4^KRK-AAA knock-in male and female mice, generated by CRISPR/cas9, we show here that the KRK motif in the PlxnA4 cytoplasmic domain is required for Sema3A-mediated cortical neuron dendritic elaboration but is dispensable for inhibitory axon guidance. The RhoGEF FARP2, which binds to the KRK motif, shows identical functional specificity as the KRK motif in the PlxnA4 receptor. We find that Sema3A activates the small GTPase Rac1, and that Rac1 activity is required for dendrite elaboration but not axon growth cone collapse. This work identifies a novel Sema3A-Nrp1/PlxnA4/FARP2/Rac1 signaling pathway that specifically controls dendritic morphogenesis but is dispensable for repulsive guidance events. Overall, our results demonstrate that the divergent signaling output from multifunctional receptor complexes critically depends on distinct signaling motifs, highlighting the modular nature of guidance cue receptors and its potential to regulate diverse cellular responses.SIGNIFICANCE STATEMENT The proper formation of axonal and dendritic morphologies is crucial for the precise wiring of the nervous system that ultimately leads to the generation of complex functions in an organism. The Semaphorin3A-Neuropilin1/Plexin-A4 signaling pathway has been shown to have multiple key roles in neurodevelopment, from axon repulsion to dendrite elaboration. This study demonstrates that three specific amino acids, the KRK motif within the Plexin-A4 receptor cytoplasmic domain, are required to coordinate the downstream signaling molecules to promote Sema3A-mediated cortical neuron dendritic elaboration, but not inhibitory axon guidance. Our results unravel a novel Semaphorin3A-Plexin-A4 downstream signaling pathway and shed light on how the disparate functions of axon guidance and dendritic morphogenesis are accomplished by the same extracellular ligand in vivo.

Novel Strategies for the Generation of Neuronal Diversity: Lessons From the Fly Visual System

Among all organs of an adult animal, the central nervous system stands out because of its vast complexity and morphological diversity. During early development, the entire central nervous system develops from an apparently homogenous group of progenitors that differentiate into all neural cell types. Therefore, understanding the molecular and genetic mechanisms that give rise to the cellular and anatomical diversity of the brain is a key goal of the developmental neurobiology field. With this aim in mind, the development of the central nervous system of model organisms has been extensively studied. From more than a century, the mechanisms of neurogenesis have been studied in the fruit fly Drosophila melanogaster. The visual system comprises one of the major structures of the Drosophila brain. The visual information is collected by the eye-retina photoreceptors and then processed by the four optic lobe ganglia: the lamina, medulla, lobula and lobula plate. The molecular mechanisms that originate neuronal diversity in the optic lobe have been unveiled in the past decade. In this article, we describe the early development and differentiation of the lobula plate ganglion, from the formation of the optic placode and the inner proliferation center to the specification of motion detection neurons. We focused specifically on how the precise combination of signaling pathways and cell-specific transcription factors patterns the pool of neural stem cells that generates the different neurons of the motion detection system.

Multiple functions of the ER-resident VAP and its extracellular role in neural development and disease

VAP (VAMP-associated protein) is a type II integral membrane protein of the endoplasmic reticulum (ER), and its N-terminal major sperm protein (MSP) domain faces the cytoplasmic side. VAP functions as a tethering molecule at the membrane contact sites between the ER and intracellular organelles and regulates a wide variety of cellular functions, including lipid transport, membrane trafficking, microtubule reorganization and unfolded protein response. VAP-point mutations in human vapb are strongly associated with amyotrophic lateral sclerosis. Importantly, the MSP domain of VAP is cleaved, secreted and interacts with the axon growth cone guidance receptors (Eph, Robo, Lar), suggesting that VAP could function as a circulating hormone similar to the Caenorhabditis elegans MSP protein. In this review, we discuss not only the intracellular functions of VAP but also the recently discovered extracellular functions and their implications for neurodegenerative disease.

Eph Receptor Effector Ephexin Mediates Olfactory Dendrite Targeting in Drosophila

Deciphering the mechanisms of sensory neural map formation is a central aim in neurosciences. Failure to form a correct map frequently leads to defects in sensory processing and perception. The olfactory map develops in subsequent steps initially forming a rough and later a precise map of glomeruli in the antennal lobe (AL), mainly consisting of olfactory receptor neuron (ORN) axons and projection neuron (PN) dendrites. The mechanisms underpinning the later stage of class-specific glomerulus formation are not understood. Recent studies have shown that the important guidance molecule Eph and its ligand ephrin play a role in class-specific PN targeting. Here, we reveal aspects of the mechanism downstream of Eph signaling during olfactory map formation. We show that the Eph-specific RhoGEF Ephexin (Exn) is required to fine tune PN dendrite patterning within specific glomeruli. We provide the first report showing an in vivo neurite guidance defect in an exn mutant. Interestingly, the quality of the phenotypes is different between eph and exn mutants; while loss of Eph leads to strong misprojections of DM3/Or47a neurons along the medial-lateral axis of the antennal lobe (AL), loss of Exn induces ventral ectopic innervation of a neighboring glomerulus. Genetic interaction experiments suggest that differential signaling of the small GTPases Rac1 and Cdc42 mediated by Exn-dependent and -independent Eph signaling fine tunes spatial targeting of PN dendrites within the olfactory map. We propose that their distinct activities on the actin cytoskeleton are required for precise navigation of PN dendrites within the olfactory map. Taken together, our results suggest that the precise connectivity of an individual neuron can depend on different modes of signaling downstream of a single guidance receptor. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.