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

Transmembrane protein Golden goal (Gogo) interacts with atypical cadherin Flamingo 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 shape the entire organization of the visual system.

Drosophila ClC-a is required in glia of the stem cell niche for proper neurogenic proliferation and wiring of neural circuits

ABSTRACTGlial cells form part of the neural stem cell niche and express a wide variety of ion channels; however, the contribution of these channels to nervous system development is poorly understood. We explored the function of the Drosophila ClC-a chloride channel, since its mammalian ortholog CLCN2 is expressed in glial cells, and defective channel function results in leukodystrophies, which in humans are accompanied by cognitive impairment. We found that ClC-a was expressed in the niche in cortex glia, which are closely associated with neurogenic tissues. Characterization of loss-of-function ClC-a mutants revealed that these animals had smaller brains and widespread wiring defects. We showed that ClC-a is required in cortex glia for neuroepithelia and neuroblast proliferation and identified defects in a neuroblast lineage that generates guidepost glial cells essential for photoreceptor axon guidance. We propose that glia-mediated ionic homeostasis could non-autonomously affect neurogenesis, and consequently, the correct assembly of neural circuits.

Analysis of Drosophila photoreceptor axon guidance in eye-specific mosaics

During development of the adult Drosophila visual system, axons of the eight photoreceptors in each ommatidium fasciculate together and project as a single bundle towards the optic lobes of the brain. Within the brain, individual photoreceptor axons from each bundle then seek specific targets in distinct layers of the optic lobes. The axons of photoreceptors R1-R6 terminate in the lamina, while R7 and R8 axons pass through the lamina to terminate in separate layers of the medulla. To identify genes required for photoreceptor axon guidance, including those with essential functions during early development, we have devised a strategy for the simple and efficient generation of genetic mosaics in which mutant photoreceptor axons innervate a predominantly wild-type brain. In a large-scale saturation mutagenesis performed using this system, we recovered new alleles of the gene encoding the receptor tyrosine phosphatase PTP69D. PTP69D has previously been shown to function in the correct targeting of motor axons in the embryo and R1-R6 axons in the visual system. Here, we show that PTP69D is also required for correct targeting of R7 axons. Whereas mutant R1-R6 axons occasionally extend beyond their normal targets in the lamina, mutant R7 axons often fail to reach their targets in the medulla, stopping instead at the same level as the R8 axon. These targeting errors are difficult to reconcile with models in which PTP69D plays an instructive role in photoreceptor axon targeting, as previously proposed. Rather, we suggest that PTP69D plays a permissive role, perhaps reducing the adhesion of R1-R6 and R7 growth cones to the pioneer R8 axon so that they can respond independently to their specific targeting cues.

Migration and function of glia in the developing Drosophila eye

Although glial cells have been implicated widely in the formation of axon tracts in both insects and vertebrates, their specific function appears to be context-dependent, ranging from providing essential guidance cues to playing a merely facilitory role. Here we examine the role of the retinal basal glia (RBG) in photoreceptor axon guidance in Drosophila. The RBG originate in the optic stalk and have been thought to migrate into the eye disc along photoreceptor axons, thus precluding any role in axon guidance. Here we show the following. (1) The RBG can, in fact, migrate into the eye disc even in the absence of photoreceptor axons in the optic stalk; they also migrate to ectopic patches of differentiating photoreceptors without axons providing a continuous physical substratum. This suggests that glial cells are attracted into the eye disc not through haptotaxis along established axons, but through another mechanism, possibly chemotaxis. (2) If no glial cells are present in the eye disc, photoreceptor axons are able to grow and direct their growth posteriorly as in wild type, but are unable to enter the optic stalk. This indicates that the RBG have a crucial role in axon guidance, but not in axonal outgrowth per se. (3) A few glia close to the entry of the optic stalk suffice to guide the axons into the stalk, suggesting that glia instruct axons by local interaction.