Regulation of postsynaptic AMPA receptor trafficking by MAPK pathways in Caenorhabditis elegans neurons

AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazolepropionate)-type glutamate receptors mediate the majority of the rapid ongoing excitatory neurotransmission in the brain. The spatio-temporal regulation of AMPA receptor trafficking, which includes endocytosis and recycling, is a crucial process for synaptic plasticity, and is therefore important for learning and memory. Signaling cascades, including the MAPK pathways, have been implicated in the regulation of synaptic plasticity in mammals. However, little is known about the molecular mechanisms by which MAPK pathways regulate AMPA receptor trafficking. In C. elegans, the AMPA receptor subunit GLR-1 is trafficked to synapses within a circuit that regulate locomotion reversal. GLR-1 undergoes clathrin-dependent endocytosis, and its recycling is mediated by the PDZ protein LIN-10; mutations in lin-10 result in the accumulation of GLR-1 in an internal compartment. From the screening of genetic modifiers of lin-10, we identified mutations in rpm-1 as an enhancer of lin-10. We find that RPM-1, an E3 ubiquitin ligase, negatively regulates PMK-3 (p38 MAPK) activity by decreasing upstream DLK-1(MAPKKK) levels, and that PMK-3 is involved in clathrin-dependent/RAB-5-mediated endocytosis of GLR-1. However, intriguingly, the loss of function mutations of two known components of PMK-3 pathways, dlk-1(mapkkk) and mkk-4 (mapkk), do not suppress GLR-1 accumulation, unlike the mutations of pmk-3. By examining other known MAPKKs in C. elegans, we find that mutations in jkk-1 suppress the GLR-1 accumulation in lin-10 mutants, similar to mutations in pmk-3. Moreover, we find that mutations in sek-1, another MAPKK, result in the accumulation of GLR-1. Finally, by examining the orthologs of Ras family protein known to function upstream of MAPK pathways in mammals, we find that RAP-1 regulates GLR-1 trafficking. Taken together, our results suggest that multiple MAPK pathways regulate AMPA receptor trafficking, sometimes with opposite outcomes. Moreover, our results indicate that there is crosstalk between MAPK pathways, and that factors like RPM-1 act to regulate such crosstalk.Ph.D.Includes bibliographical references (p. 148-165)