A protein in the regulator of G-protein signaling (RGS) family, RGS-3 is a key regulatory checkpoint in the sensory pathway that controls the response of C. elegans to external stimuli. This finding, from the lab of Anne Hart, HMS associate professor of pathology at Massachusetts General Hospital, marks the first in-depth analysis of RGS protein function in the well-characterized C. elegans sensory neurons. Conducted by lead author and postdoctoral fellow Denise Ferkey, BBS graduate student Hana Fukuto, and colleagues, the study appears in the Jan. 4 Neuron.
The researchers used a mutant C. elegans, rgs-3, to assess the role that the RGS-3 protein normally plays in G-protein signaling in response to attractive or aversive stimuli that were either strong or weak. Compared to their wild-type counterparts, mutant animals responded either slowly or poorly to strong stimuli (which included 100 percent octanol and 10 mM of quinine). Surprisingly, when the animals were exposed to diluted stimulants, the mutants responded as well as the wild-type roundworms.
Genetic and molecular analysis demonstrated that loss of RGS-3 does cause increased signaling after exposure to strong stimuli. At the molecular level, the result is an excess influx of calcium ions into sensory neurons. The researchers were able to sequester this extra calcium in the neurons by employing a calcium-binding “sponge” protein, cameleon. By thus decreasing the pool of calcium ions available for signaling, the researchers could restore responses in mutant animals. Alternatively, enhancing the synaptic transmission of glutamate from sensory neurons also rescued the behavior of rgs-3 mutant animals.
Based on these findings, the research team proposes a model whereby the excess calcium influx into the sensory neurons is detected by an unknown feedback mechanism that subsequently decreases synaptic output. The final result is a defective response observed in rgs-3 mutant animals. These observations and previous studies indicate the presence of multiple checkpoints in the sensory signaling pathway that can compensate for loss of function of individual components.
Hart, the principal investigator on the study, notes that the defective response of mutants to strong sensory stimuli was an unexpected outcome of mutating RGS-3. Since the protein is a negative regulator of G-protein signaling, a loss of its function was expected to lead to increased signaling and consequently higher neurotransmitter release, mechanisms that suggest a hyperactive response to sensory stimuli. The counterintuitive observations noted in this study, however, demonstrate that it is difficult to predict the consequences of RGS protein loss.
Further studies are under way to examine the role of additional genes that modulate behavior as a function of changes in the environment.
