This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Sensory input to the nervous system is mediated by specialized neurons that reside in the dorsal root ganglia (DRG) that flank the spinal cord. During embryonic development, these neurons are generated from migrating neural crest cells that ultimately give rise to all of the neurons and glial cells in the DRG. As the neural crest cells reach the DRG and divide, they give rise to progenitor cells that must first decide whether to become neurons or glia (neural fate determination). The progenitors that become committed to a neural fate then must decide to differentiate into one of several subtypes of neurons that sense touch, temperature, pain, limb movements, and limb position (subtype specification). While the analysis of knockout animals has shown that several genes are required for the normal development of the different subtypes of neurons in the DRG, it is not clear if they are involved in the process of neural cell fate determination or neural subtype specification. To learn more about the molecular regulation of neural fate determination and subtype specification in the DRG, we propose to investigate the role of the basic helix-loop-helix transcription factor neurogenin1 (ngn1) which is required for the development of one subtype of DRG neuron;the pain-sensing nociceptive neurons. The lack of nociceptive neurons in ngn1 knockout animals shows that this gene is required for the development of these cells but does not tell us whether ngn1 is involved in neural fate determination or subtype specification. Based on previous work from the lab demonstrating that a related basic helix-loop-helix transcription factor (mash1) is involved in neural fate determination in the olfactory epithelium, we hypothesize that ngn1 is involved in neural fate determination in DRG progenitor cells. To test this hypothesis, the following specific aims will be pursued: 1) the developmental fate of cells that express ngn1 will be determined using a conditional lineage tracing approach to determine if cells other than neurons are derived from these progenitors. If ngn1 is involved in neural fate determination, a low level of ngn1 expression could be found in cells that give rise to other cell types (glia), whereas if ngn1 is involved in neural subtype specification, only neurons should be generated;2) the proportion of different cell types in the DRG of wild-type and ngn1 knockout embryos will be determined using a developmental time course with cell-type specific markers. If ngn1 is involved in neural fate determination then the loss of ngn1 function in the knockout should result in a decrease in the proportion of neurons and an increase in the number of other (glial) cells. On the other hand, if ngn1 is involved in neural subtype specification, the loss of ngn1 function in the knockout should result in a decrease of one subtype of neuron and an increase in other subtypes. 3) A third specific aim will be pursued to identify the DRG cell types that express a myelin proteolipid protein-lacZ (PLP-LacZ) fusion gene product in a transgenic mouse line to determine if this line could be used to follow the development of glial cells in the DRG. Together, these specific aims will contribute to our understanding of cell fate determination in the mammalian nervous system which is a crucial step in generating the correct balance of neurons and glia in the developing embryo.
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