The function of the IRM protein Kirre in sensory pathway formation

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This grant application aims at the elucidation of IRM protein function in synaptogenesis, in both the visual and olfactory system of Drosophila. IRM proteins are a small conserved family of cell adhesion proteins of the immunoglobulin superfamily. In Drosophila this protein family comprises Irregular Chiasm C/Roughest (IrreC/Rst), Kin-of-irre/Dumbfounded (Kirre/Duf), and their interacting protein partners Sticks-and-Stones (SNS) and Hibris (Hbs). They are related to Syg-1 and Syg-2 in C. elegans and to Neph1, Neph2, Neph3 and Nephrin in mammals. We will analyze the role of these proteins from Drosophila and mouse in the establishment of neuronal connectivity by focusing on single neuronal cell types in the fly. L4 neurons are the only monopolar cells in the fly lamina that invade neighbouring cartridges. In the proximal lamina layer they form synapses with three L2 neurons and with six other L4 neurons. Kirre is specifically expressed at sites of mutual contact between these collaterals and its absence leads to reduced synapse numbers. In the olfactory system, Kirre and Rst in Drosophila and their orthologs in mouse are also expressed on axon terminals of olfactory receptor neurons. Using mutant analysis and various methods based on binary gene expression and flipase systems we will manipulate the genetic constitution of the Kirre expressing neurons and their interacting cellular partners, e. g. by overexpressing IRM transgenes or by knocking down genes expressed using UAS-RNAi. Cell autonomous and non-autonomous effects will be distinguished by the use of genetic mosaic analysis and the induced loss of Gal4 inhibitor Gal80 by mitotic recombination. Effects on synaptogenesis will be tested by use of synaptic markers and by direct inspection in the electron microscope. Behavioural consequences of altered connectivity in mutants and transformants will be used as sensitive sensors, e.g. rescue of sensitivity to specific odorants will be done by reintroducing cell type specifically kirre+ on kirre- genetic background.