Discoidin domain receptors are a family of receptor tyrosine kinases activated by collagens. Here we characterize the role of the two discoidin domain receptors, ddr-1 and ddr-2, of the nematode C. elegans during nervous system development. ddr-2 mutant animals exhibit axon guidance defects in major longitudinal tracts most prominently in the ventral nerve cord. ddr-1 mutants show no significant phenotype on their own but significantly enhance guidance defects of ddr-2 in double mutants. ddr-1 and ddr-2 GFP-reporter constructs are expressed in neurons with axons in all affected nerve tracts. DDR-1 and DDR-2 GFP fusion proteins localize to axons. DDR-2 is required cell-autonomously in the PVPR neuron for the guidance of the PVPR pioneer axon, which establishes the left ventral nerve cord tract and serves as substrate for later outgrowing follower axons. Our results provide the first insight on discoidin domain receptor function in invertebrates and establish a novel role for discoidin domain receptors in axon navigation and axon tract formation.

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This study was aimed to characterize the earliest phases of synapse development in mouse retinal ganglion cells (RGCs) by recording spontaneous postsynaptic currents (PSCs). First PSCs were detected at embryonic day 17 and completely suppressed by bicuculline, demonstrating their GABAergic nature. Starting from postnatal day 3 a small fraction of RGCs had rapidly decaying, most likely glutamatergic currents. The present results suggest that functional GABAergic synapses with RGCs appear before birth and that GABAergic synaptic transmission precedes that of glutamate in the retina. In this early period GABA acts in a depolarizing manner and takes over an excitatory function.

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