Projects
1. Establishment of a molecular genetic program for the generation of GABAergic motor neurons (type-D neurons)
The D type neurons control the locomotion pattern of the worm. When they are defective, the animals hypercontract body muscles, resulting in a phenotype nicknamed "shrinker." By characterizing "shrinker" genes, our early work revealed a transcriptional regulation of GABA fate. unc-30 (unc, for uncoordinated) encodes a homeodomain protein and is both necessary and sufficient for specifying aspects of the terminal differentiation of the D neurons. unc-25 encodes the GABA biosynthetic enzyme glutamic acid decarboxylase. unc-47 encodes the GABA vesicular transporter. We have shown that unc-30 controls the GABAergic property of these type D neurons by regulating the expression of both unc-25 and unc-47 at the transcriptional level. Furthermore, by using a variety of molecular genetic and genomic methods, we have identified over 200 genes that are enriched in the these D neurons acquire other properties. Understanding the functional interactions among these genes will reveal how individual type of neuron acquire their specific characteristics.
2. Defining the molecular pathways guiding neurons to their targets
Neurons sense a variety of cues to extend their axons towards their targets. Through forward genetic screens for mutant animals exhibiting altered axon trajectory, we have identified several genes that function in motor neuron axon guidance. MAX-1 defines a family of evolutionarily conserved proteins that function downstream of netrin receptor to mediate motor axon repulsion. UNC-71 ADAM protein and integrins act to regulate specific choice points for axon extension and guidance. We have also performed genome-wide RNAi screen to dissect additional axon guidance pathways.
3. Identification of the molecular components in the presynaptic terminal formation
Synapses are the function units in our brain. All synapses are composed of a presynaptic terminal, specialized synaptic cleft and a postsynaptic site. At the presynaptic terminal, neurons develop elaborate subcellular structures to facilitate the accumulation and release of synaptic vesicles. Although much of the progress has been made in understanding neurotransmitter release, how the cytoarchitecture of a presynaptic terminal is built is poorly understood. Using GFP markers to label the presynaptic regions of the motor neurons, we have performed large scale forward genetic screens for mutants displaying altered morphology of the presynaptic termini. Through molecular and cellular characterizations of genes affected in these mutants, we have uncovered multi-layer regulatory mechanisms involving MAP kinases, ubiquitin-mediated protein degradation and other signaling modules to specify distinct spatial domains at presynaptic terminals. For example, RPM-1 (for Regulator of Presynaptic Morphology), a conserved ubiquitin E3 ligase, controls the synaptic abundance of the Dual-Leucine Zipper MAP kinase to promote proper organization of presynaptic terminals. SYD-2 (for Synapse defective) liprin protein is a key regulator in the assembly of presynaptic active zone.
4. Characterization of the developmental remodeling of DD motorneurons
An intriguing feature of the DD neurons is that they remodel their synaptic connections during development. This remodeling is unusual because it involves a complete reversal of information flow without dramatic changes in neuronal morphology. We are using a variety of markers to visualize this remodeling process in vivo. We have found that the timing of the remodeling is under the control of a nuclear protein, LIN-14. We are particularly interested in exploring this phenomenon in the hope that our analysis will shed light on other types of synaptic plasticity that are related to growth, aging, learning and memory.