Argos and the Spitz Group/der Pathway Function to Regulate Midline Glial Cell Numbers in Drosophila Embryos

Abstract

The midline glia (MG) perform an active role in the pioneering and morphogenesis ofthe commissural axons of the central nervous system (CNS) during Drosophila embryogenesis. Following the establishment of these commissural axon pathways, a subset of the MG undergoes apoptosis and the surviving MG remain to ensheath the commissures. Previous studies demonstrated that the pattern ofthis developmental apoptosis is stochastic. The surviving MG exhibit intersegment variability in their final number and position relative to the commissures. Interestingly, argos, a gene which encodes a diffusible extracellular factor with an epidermal growth factor (EGF) motif, is only expressed in MG which survive. argos expression in this subset of MG is initiated at the onset of apoptosis, reflecting the temporal pattern of cell death of the other MG. In argos loss-of-function mutants, there are extra surviving MG in each segment while ectopic over-expression of argos results in increased apoptosis among the MG. Therefore, Argos has a negative regulatory effect on MG survival. Its effects are opposite to the spitz group/Drosophila EGF receptor (DER) pathway, a cassette of genes required for MG survival. However, argos is hypostatic to the spitz group/DER pathway function and its expression requires a certain threshold of spitz group and DER pathway activity. Argos is postulated to act as an EGF receptor antagonist. It attenuates signaling of the DER pathway. Therefore, the regulation of MG survival is mediated by a balance of extracellular inductive and inhibitory signals. How can this signaling pathway be reconciled with the stochastic pattern of MG survival and the observation that only argos expressing MG survive? We propose a model in which the adherens junction along with possible accessory proteins like Rhomboid and Star mediate the close apposition of MG and promote intense Spitz mediated DER pathway signaling. The MG with the most concentrated coupling of this adhesion and signaling pathway network achieve sufficient DER signaling to express argos. These MG are in tum fated to assume a final differentiated identity and will remain to ensheathe the commissures. Argos is secreted and it mediates the apoptosis of MG with reduced levels of adhesion contacts and lower DER signaling (MG not expressing argos). The other mesectodermal cell (MEC) lineages appear to develop independently of argos function. The repercussions of HSargos elicited MG loss to the commissural axons and CNS cytoarchitecture was also examined. Removal of the MG through ectopic expression ofargos results in a loss of ensheathment and the commissural axon tracts are exposed to the haemolymph. Furthermore, the commissural axons are wider and they misexpress Fasciclin II, a phenotype reminiscent of mutations in roundabout.