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Biological Sciences Department Faculty
Postdoctoral Research Fellow, Samuel Lunenfeld Research Institute, 2006
Ph.D. University of Toronto, 1998
B.Sc. University of Toronto, 1988
My research interests are focused on the development and function of the nervous system. Specifically, my lab is working to better define the molecular mechanisms that underlie neuronal growth cone migrations during nervous system development (axon guidance) and the subsequent communication that occurs at synapses between neurons after development is completed (neurotransmission). A fundamental comprehension of neurotransmission will help us understand information storage and processing in the brain and combined with a through knowledge of axon guidance we may eventually be capable of using drugs to recreate functional neural circuits in those who have suffered damage to the central nervous system.
In my lab we are taking a genetic approach to understanding axon guidance and neurotransmission using the soil nematode Caenorhabditis elegans as a model organism. The C. elegans nervous system is less complex than that of vertebrates, with only 302 neurons, but there is significant conservation at the molecular level between the two systems. For example, C. elegans shares essentially all of the same neurotransmitters found in vertebrates and at a mechanistic level the same receptors and extracellular guidance cues are used to guide commissural axons in the two systems.
Our focus in the lab is the study of the molecular pathways that function downstream of the guidance cue receptors in the process of axon guidance. How is the information provided by the extracellular guidance cues transduced to the internal structural components of the neuron, which are ultimately responsible for changes in the extent and direction of growth cone migration? A component of this pathway is the gene unc-73 (uncoordinated movement) which encodes an activator of Rho-family GTPases and is one of the first proteins proposed to act downstream of guidance cue receptors to regulate the organization of the actin cytoskeleton (Steven et al., 1998). More recent work indicates that the unc-73 locus is complex and encodes several different protein isoforms. Analysis of unc-73 behavioral mutants obtained by reverse genetics and the use of these mutants in isoform specific transgenic rescue experiments revealed that specific UNC-73 protein isoforms are required for normal neurotransmission (Steven et al., 2005). Our current hypothesis is that the UNC-73B isoform regulates the actin cytoskeleton through the activation of Rac-like GTPases at the leading edge of migrating growth cones to influence the direction of migration. The UNC-73 C1, C2 and E isoforms act through the GTPase Rho and the Gαs heterotrimeric G protein signaling pathway to modulate neurotransmission through a large dense core vesicle mediated pathway (Hu et al., 2011).
Our focus for the future will be to use the advantages of the C. elegans model system to further our understanding of nervous system development and function. UNC-73 and other regulatory proteins are being used as entrance points into the important Rho GTPase pathways that are required for neuronal development and also function to modulate neural activity. Biochemical and genetic methods are being used to identify additional proteins in the Rho GTPase pathways and these molecules will be characterized to determine how they function within the nervous system whether at the synapse or in extending axons.
Lin, L., Tran, T., Hu, S., Cramer, T., Komuniecki, R. and Steven, R. (2012). RHGF-2 is an essential Rho-1 specific RhoGEF that binds to the multi-PDZ domain scaffold protein MPZ-1 in Caenorhabditis elegans. PLoS ONE (In press).
Hu, S., Pawson, T. and Steven, R. (2011). UNC-73/Trio RhoGEF-2 activity modulates C. elegans motility through changes in neurotransmitter signaling upstream of the GSA-1/Gαs pathway. Genetics 189:137-151.
Harris, P., Hapiak, V., Wragg, R., Miller, S., Hughes, L., Hobson, R., Steven, R., Bamber, B. and Komuniecki, R. (2009). Three distinct amine receptors operating at different levels within the locomotory circuit are each essential for the serotonergic modulation of chemosensation in Caenorhabditis elegans. J. Neurosci. 29:1446-1456.
Steven, R., Zhang, L., Culotti, J. and Pawson, T. (2005). The UNC-73/Trio RhoGEF-2 domain is required in separate isoforms for the regulation of pharynx pumping and normal neurotransmission in C. elegans. Genes Dev. 19:2016-2029.
Killeen, M., Tong, J., Krizus, A., Steven, R., Scott, I., Pawson, T. and Culotti, J. (2002). UNC-5 function requires phosphorylation of cytoplasmic tyrosine 482, but its UNC-40-independent functions also require a region between the ZU-5 and death domains. Dev. Biol. 251:348-366.
Tong, J., Killeen, M., Steven, R., Binns, K., Culotti, J. and Pawson, T. (2001). Netrin stimulates tyrosine phosphorylation of the UNC-5 family of netrin receptors and induces shp2 binding to the RCM cytodomain. J. Biol. Chem. 276:40917-40925.
Steven, R., Kubiseski, T., Zheng, H., Kulkarni, S., Mancillas, J., Ruiz Morales, A., Hogue, C., Pawson, T. and Culotti, J. (1998). UNC-73 activates the Rac GTPase and is required for cell and growth cone migrations in C. elegans. Cell 92:785-795.
Run, J.-Q., Steven, R., Hung, M., van Weeghel, R., Culotti, J.G. and Way, J.C. (1996). Suppressors of the unc-73 gene of Caenorhabditis elegans. Genetics 143:225-236.