Research Focus
Research Focus
The function of the nervous system depends on the creation of precisely defined patterns of connectivity between neurons. Despite the importance of this process, the biological rules governing neural specificity are poorly understood. What are the molecular cues that result in the creation of synapses between specific sets of neurons? The complexity of the vertebrate nervous system coupled with the dearth of biochemical information about synaptic choice have hindered efforts to answer this question in mammals. Our strategy to circumvent these problems is, first, to address this question in a simple, well-defined nervous system and, second, to employ a genetic approach which does not require prior assumptions about the molecular mechanism of neural specificity.
In the nematode, Caenorhabditis elegans, the nervous system is composed of exactly 302 neurons. Every contact between these neurons has been catalogued to construct a complete wiring diagram. With this detailed information in hand, it is possible to correlate mutations that produce abnormal or "uncoordinated" movement in C.elegans with specific changes in the structure of the nervous system. A mutation in one of these genes, unc-4, alters the pattern of synaptic input to one class of motor neurons in the ventral nerve cord and results in a strong movement defect (i.e. no backward locomotion). We have cloned the unc-4 gene and established that it encodes a homeodomain transcription factor (Miller et al., 1992). We used a classical genetic strategy (Miller et al., 1993) to show that the unc-37 locus encodes a highly conserved Groucho-like transcriptional cofactor that functions with unc-4 to regulate target genes (Pflugrad et al., 1997).We hypothesize that unc-4 defines a specific motor neuron trait that is recognized by potential presynaptic partners and that these traits are encoded by downstream genes that unc-4 and unc-37 regulate (Winnier et al, 1999). A major goal in the Miller lab is to identify these unc-4 target genes. To achieve this aim, we recently developed a method for generating C. elegans motor neurons in primary cell cultures (Christensen et al., 2002). In these cultures, the motor neurons in which unc-4 functions are marked by expression of an unc-4::GFP transgene and thus can be purified by Fluorescence Activated Cell Sorting (FACS). The Affymetrix DNA microarray will be probed with labeled mRNA isolated from these cells. By comparing gene expression in these motor neurons isolated from wild type versus unc-4 and unc-37 mutants, we expect to identify the small subset of genes that are specifically regulated by unc-4 and unc-37 in these cells. Reverse genetic approaches will be used to test the function of these proteins in the mechanism of synaptic choice.
Our long term aim is to work out the molecular and cellular mechanism of neural specificity in this simple model system and then to extend our findings to complex vertebrate nervous systems which could not otherwise be dissected by this genetic approach.
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last updated November 25, 2002