Signal transduction through G protein-coupled signaling pathways must be tightly controlled to allow animals to respond to changes in the environment. RGS/GAP (GTPase activating) proteins are important regulators of G protein signaling. They bind to G?-GTP and stabilize its catalytically active conformation, thus accelerating the rate of GTP hydrolysis and dampening signaling through G?. Downregulation of G protein signaling protects cells from overstimulation and allows cells to integrate information from multiple inputs.
Ce-rgs-3 (C. elegans regulator of G protein signaling-3, C29H12.3) encodes an RGS protein that is most similar to mammalian RGS-8. However, Ce-RGS-3 is unique among RGS proteins as it contains two RGS domains.
Ce-rgs-3 is expressed exclusively in eight pairs of sensory neurons (ASH, ADL, AWB, AWC, ASJ, ASK, PHA and PHB), suggesting that it may act to regulate signal transduction and the behavioral responses mediated by these neurons. We identified a recessive deletion allele of
Ce-rgs-3,
vs19, that removes the second RGS domain. Since RGS proteins generally act to dampen signaling, we expected
Ce-rgs-3 mutant animals to have enhanced signaling in sensory neurons and, therefore, to be hypersensitive to environmental stimuli. Instead, we find that
Ce-rgs-3 animals are defective in their ability to respond to many chemical stimuli. For two odorants, however,
Ce-rgs-3 animals respond as well as N2 animals when the concentration of the odorant is reduced. Surprisingly, we find that feeding status alters the sensitivity of
Ce-rgs-3 mutant animals to strong stimuli. We are working to determine if this effect is due to serotonin or dopamine, two neurotransmitters released upon exposure to food. One possible explanation for the
Ce-rgs-3 sensory defects is that in the absence of normal Ce-RGS-3 function, there is actually too much signaling in response to strong stimuli and animals are unable to respond. Consistent with this hypothesis, we find that mutations that decrease G protein-coupled signal transduction in sensory neurons are sufficient to restore
Ce-rgs-3 response to strong stimuli. Since calcium influx into sensory neurons is a downstream consequence of G protein-coupled signal transduction,
Ce-rgs-3 animals may have altered neuronal calcium signaling. We are using cameleon (a genetically encoded fluorescent calcium sensor) to analyze stimulus-evoked calcium fluxes in the ASH sensory neurons of
Ce-rgs-3 animals to address this hypothesis.