The formation of a complex nervous system requires the intricate interaction of neurons and glial cells. Glial cells generally migrate over long distances before they initiate their differentiation, which leads to wrapping and insulation of axonal processes. The molecular pathways coordinating the switch from glial migration to glial differentiation are largely unknown. Here we demonstrate that, within the Drosophila eye imaginal disc, fibroblast growth factor (FGF) signalling coordinates glial proliferation, migration and subsequent axonal wrapping. Glial differentiation in the Drosophila eye disc requires a succession from glia-glia interaction to glia-neuron interaction. The neuronal component of the fly eye develops in the peripheral nervous system within the eye-antennal imaginal disc, whereas glial cells originate from a pool of central-nervous-system-derived progenitors and migrate onto the eye imaginal disc. Initially, glial-derived Pyramus, an FGF8-like ligand, modulates glial cell number and motility. A switch to neuronally expressed Thisbe, a second FGF8-like ligand, then induces glial differentiation. This switch is accompanied by an alteration in the intracellular signalling pathway through which the FGF receptor channels information into the cell. Our findings reveal how a switch from glia-glia interactions to glia-neuron interactions can trigger formation of glial membrane around axonal trajectories. These results disclose an evolutionarily conserved control mechanism of axonal wrapping, indicating that Drosophila might serve as a model to understand glial disorders in humans.

Kinesin heavy chain (Khc) is crucially required for axonal transport and khc mutants show axonal swellings and paralysis. Here, we demonstrate that in Drosophila khc is equally important in glial cells. Glial-specific downregulation of khc by RNA interference suppresses neuronal excitability and results in spastic flies. The specificity of the phenotype was verified by interspecies rescue experiments and further mutant analyses. Khc is mostly required in the subperineurial glia forming the blood-brain barrier. Following glial-specific knockdown, peripheral nerves are swollen with maldistributed mitochondria. To better understand khc function, we determined Khcdependent Rab proteins in glia and present evidence that Neurexin IV, a well known blood-brain barrier constituent, is one of the relevant cargo proteins. Our work shows that the role of Khc for neuronal excitability must be considered in the light of its necessity for directed transport in glia.

Organs are characterized by a specific shape that is often remodeled during development. The dynamics of organ shape is in particular evident during the formation of the Drosophila nervous system. During embryonic stages the central nervous system compacts, whereas selective growth occurs during larval stages. The nervous system is covered by a layer of surface glial cells that form the blood brain barrier and a thick extracellular matrix called neural lamella. The size of the neural lamella is dynamically adjusted to the growing nervous system and we show here that perineurial glial cells secrete proteases to remodel this matrix. Moreover, an imbalance in proteolytic activity results in an abnormal shape of the nervous system. To identify further components controlling nervous system shape we performed an RNAi based screen and identified the gene nolo, which encodes an ADAMTS-like protein. We generated loss of function alleles and demonstrate a requirement in glial cells. Mutant nolo larvae, however, do not show an abnormal nervous system shape. The only predicted off-target of the nolo(dsRNA) is Oatp30B, which encodes an organic anion transporting protein characterized by an extracellular protease inhibitor domain. Loss of function mutants were generated and double mutant analyses demonstrate a genetic interaction between nolo and Oatp30B which prevented the generation of maternal zygotic mutant larvae.

In all complex organisms, the peripheral nerves ensure the portage of information from the periphery to central computing and back again. Axons are in part amazingly long and are accompanied by several different glial cell types. These peripheral glial cells ensure electrical conductance, most likely nature the long axon, and establish and maintain a barrier towards extracellular body fluids. Recent work has revealed a surprisingly similar organization of peripheral nerves of vertebrates and Drosophila. Thus, the genetic dissection of glial differentiation in Drosophila may also advance our understanding of basic principles underlying the development of peripheral nerves in vertebrates.

The Drosophila nervous system is ideally suited to study glial cell development and function, because it harbors only relatively few glial cells, and nervous system development is very well conserved during evolution. In the past, enhancer trap studies provided tools allowing to study glial cells with a single-cell resolution and, moreover, disclosed a surprising molecular heterogeneity among the different glial cells. The peripheral nervous system in the embryo comprises only 12 glial cells in one hemisegment and thus offers a unique opportunity to decipher the mechanisms directing glial development. Here, we focus on transcriptional regulators that have been reported to function during gliogenesis. To uncover additional regulators, we have conducted a genetic screen and report the identification of two additional transcriptional regulators involved in the control of peripheral glial migration: nejire and tango.