Glial cell development in Drosophila.

In the Drosophila central nervous system (CNS) about 10% of the cells are of glial nature. A set of molecular markers has allowed unraveling a number of genes controlling glial cell fate determination as well as genes required for glial cell differentiation. Here we focus on the embryonic CNS glia and review the recent progress in the field.

gcm and pointed synergistically control glial transcription of the Drosophila gene loco.

In Drosophila lateral glial cell development is initiated by the transcription factor encoded by glial cells missing. glial cells missing activates downstream transcription factors such as repo and pointed which subsequently control terminal glial differentiation. The gene loco has been identified as a potential target gene of pointed and is involved in terminal glial differentiation. It encodes an RGS domain protein expressed specifically by the lateral glial cells in the developing embryonic CNS. Here we analyzed the loco promoter and the control of the glial-specific transcription pattern. Using promoter-reporter gene fusions we identified a 1.9 kb promoter element capable of directing the almost complete loco gene expression pattern. Sequence analysis suggested the presence of gcm and pointed DNA binding sites. Following in vitro mutagenesis of these sites we demonstrated their relevance in vivo. The expression of loco is initially dependent on gcm. During subsequent stages of embryonic development gcm and pointed appear to activate loco transcription synergistically. In addition, at least two other factors appear to repress loco expression in the ectoderm and in the CNS midline cells.

Glia development in the embryonic CNS of Drosophila.

Glial cells are pivotal players during the development and function of complex nervous systems. In Drosophila, recent genetic analyses have revealed several genes that control differentiation and function of CNS glial cells and their interactions with neurons can be studied in detail at the CNS midline, where it is essential for the correct establishment of the commissural axon pattern.

loco encodes an RGS protein required for Drosophila glial differentiation.

In Drosophila, glial cell development depends on the gene glial cells missing (gcm). gcm activates the expression of other transcription factors such as pointed and repo, which control subsequent glial differentiation. In order to better understand glial cell differentiation, we have screened for genes whose expression in glial cells depends on the activity of pointed. Using an enhancer trap approach, we have identified loco as such a gene. loco is expressed in most lateral CNS glial cells throughout development. Embryos lacking loco function have an normal overall morphology, but fail to hatch. Ultrastructural analysis of homozygous mutant loco embryos reveals a severe glial cell differentiation defect. Mutant glial cells fail to properly ensheath longitudinal axon tracts and do not form the normal glial-glial cell contacts, resulting in a disruption of the blood-brain barrier. Hypomorphic loco alleles were isolated following an EMS mutagenesis. Rare escapers eclose which show impaired locomotor capabilities. loco encodes the first two known Drosophila members of the family of Regulators of G-protein signalling (RGS) proteins, known to interact with the alpha subunits of G-proteins. loco specifically interacts with the Drosophila alphai-subunit. Strikingly, the interaction is not confined to the RGS domain. This interaction and the coexpression of LOCO and Galphai suggests a function of G-protein signalling for glial cell development.

CNS midline development in Drosophila.

The first cells specified during CNS development of vertebrates and invertebrates are the cells located at the midline of the neuroepithelium. In Drosophila the development of these cells requires inductive signals from the mesoderm. Later in CNS development, the midline cells are in turn influencing the flanking neuroectoderm, contributing to the establishment of dorsoventral positional information. During axonal pattern formation the midline cells are required in guiding commissural growth cones towards and across the midline. The midline consists of only few, easily identifiable neuronal and glial cells per segment. The development of midline glial cells is relatively well understood. Their differentiation appears to be controlled by the concomitant expression of two different sets of transcription factors. Activation of glial differentiation mediated by the ETS transcription factor encoded by pointed (whose activity depends on EGF-receptor signalling) occurs in concert with repression of neuronal differentiation mediated by the Zn-finger transcription factor encoded by tramtrack.