Wrapping Glial Morphogenesis and Signaling Control the Timing and Pattern of Neuronal Differentiation in the Drosophila Lamina

Various regions of the developing brain coordinate their construction so that the correct types and numbers of cells are generated to build a functional network. We previously discovered that wrapping glia in the Drosophila visual system are essential for coordinating retinal and lamina development. We showed that wrapping glia, which ensheath photoreceptor axons, respond to an epidermal growth factor cue from photoreceptors by secreting insulins. Wrapping glial insulins activate the mitogen-activated protein kinase (MAPK) pathway downstream of insulin receptor in lamina precursors to induce neuronal differentiation. The signaling relay via wrapping glia introduces a delay that allows the lamina to assemble the correct stoichiometry and physical alignment of precursors before differentiating and imposes a stereotyped spatiotemporal pattern that is relevant for specifying the individual lamina neuron fates. Here, we further describe how wrapping glia morphogenesis correlates with the timing of lamina neuron differentiation by 2-photon live imaging. We also show that although MAPK activity in lamina precursors drives neuronal differentiation, the upstream receptor driving MAPK activation in lamina precursors and the ligand secreted by wrapping glia to trigger it differentially affect lamina neuron differentiation. These results highlight differences in MAPK signaling properties and confirm that communication between photoreceptors, wrapping glia, and lamina precursors must be precisely controlled to build a complex neural network.