A challenging problem in the field of neuroscience is to understand how a functional synapse is formed. This complex process involves many steps, including the differentiation of numerous cell types, the establishment of synaptic connections, and the localization of molecules, such as ligand-gated ion channels, to the appropriate synapses. It is critical that these processes be carried out with great precision in order for the nervous system to function properly. We have undertaken a genetic approach to identify molecules that are required to build a glutamatergic synapse. Recently, we described a novel transmembrane protein, SOL-1, that is required for glutamatergic neurotransmission in C. elegans (Zheng, et al., 2004). The
sol-1 gene was isolated in a genetic screen for suppressors of GLR-1(A/T) - a dominantly active variant of the GLR-1 ionotropic glutamate receptor (iGluR) subunit. GLR-1(A/T) carries an alanine to threonine mutation that results in a partially ligand-independent ion channel (Zheng et al., 1999). Transgenic worms that express GLR-1(A/T) have severe locomotion defects characterized by rapid switching between forward and backward movement. A null mutation in
sol-1 suppresses the hyper-reversal behavior such that mutant worms reverse at a similar frequency to wild-type. Electrophysiological analysis revealed that SOL-1 is required for GLR-1 dependent glutamate-gated currents in the AVA interneurons. The AVA neurons are one of 5 pairs of command interneurons that express GLR-1. Interestingly, the
sol-1 mutation does not effect GLR-1 expression, synaptic localization or membrane insertion, and may have a role in receptor gating. By screening for additional suppressors of the GLR-1(A/T) hyper-reversal phenotype, we hope to identify novel genes that are necessary for the function of iGluRs. We expect to find genes required for GLR-1 expression and thus neuronal differentiation. We also anticipate the identification of genes that function to traffic GLR-1 from the cell body to the appropriate post-synaptic sites. C. elegans expresses both non-NMDA and NMDA iGluRs - the two major receptor subtypes. GLR-1 is a member of the non-NMDA class, whereas NMR-1 and NMR-2 belong to the NMDA class. We have undertaken a similar approach to that described above to identify genes required for NMDA receptor function. By introducing various mutations in the NMR-1 and NMR-2 subunits, we hope to generate transgenic worms with locomotion defects. Genes that suppress these defects when mutated are predicted to encode molecules involved in the regulation of this important class of ligand-gated ion channels. Zheng, et al., (1999). Neuron 24(2), 347-361. Zheng, et al., (2004). Nature 427(6973), 451-457.