We are interested in examining the role of Rho GTPase signaling in nervous system function as it affects behavior. Central to our study is the Rho-family GTPase activator UNC-73. The
unc-73 gene encodes multiple isoforms that contain either one or two RhoGEF domains. In the nervous system the UNC-73 RhoGEF-1 domain specifically activates Rac GTPases in the process of axon guidance, while the RhoGEF-2 domain specifically activates Rho to influence the rate of locomotion by affecting neurotransmission (Steven et al, 1998, 2005). By using different cell-specific or inducible promoters to drive the expression of the UNC-73E isoform in transgenic
unc-73 mutant animals we have begun to identify the temporal and spatial requirements of UNC-73 RhoGEF-2 isoform function. We have found that UNC-73E acts within the nervous system, but not the muscles, to physiologically regulate the rate of locomotion. It was recently reported that UNC-73E is an effector of Gαq (Williams et al, 2007), however, it is still not clear how the UNC- 73 isoforms and the Rho signaling pathway function to regulate neurotransmission. The lethargic
unc-73 RhoGEF-2 mutants do not appear to have a general defect in synaptic vesicle function as these mutants are not resistant to the acetylcholinesterase inhibitor aldicarb. Alternatively, these
unc-73 mutants exhibit a decreased level of NLP-21::YFP neuropeptide release from motor neurons as monitored indirectly through coelomocyte uptake. The defect in release is likely due to a decrease in NLP-21::YFP transport in the cell as a decreased level of fluorescence is also observed in the dorsal cord axons of
unc-73 RhoGEF-2 mutants, while wild-type levels of fluorescence are seen in cell somas. These results suggest that the UNC-73 RhoGEF-2 isoforms may play a neuromodulatory role in the regulation of locomotion through neuropeptides. We have also observed that activating mutations in different components of the Gαs pathway rescue the
unc-73 lethargic movement phenotype caused by the loss of UNC-73 RhoGEF-2 isoforms. Expression of a
gsa-1(gf) cDNA in muscles cells partially rescues the
unc-73 movement defects, but neuronal expression of GSA-1 pathway gain of function proteins does not rescue. Together our data suggest UNC-73 RhoGEF-2 isoforms are required for the intracellular transport of a neuropeptide modulator that functions to activate Gαs-coupled receptors on muscle cells upon release.