News and Publications

Molecular Mechanisms of Sensory Neuron Specification and Function

A. Patapoutian

The members of my laboratory are interested in the mechanisms that control the specification and function of sensory neurons in vertebrates. Sensory neurons of the dorsal root ganglia (DRG) have distinct axonal projections and perceive varied sensory stimuli, such as pain, temperature, and touch. Neurotrophins are expressed at the target tissues of DRG axons and promote survival of these distinct neuronal subtypes. Neurotrophins act through trk receptor tyrosine kinases that are expressed on the cell bodies and projections of DRGs, and expression of trk receptors distinguishes subclasses of sensory neurons.

We showed that in mice deficient in neurotrophin-3, cell death occurs in newly differentiated neurons, suggesting a role for neurotrophins before axons reach their target. Consistent with this novel idea, we found that neurotrophin-3 is dynamically expressed at the tips of extending axons from the time of early neuronal differentiation. We showed that this pattern of expression does not require neurons, and in limb buds, neurotrophin-3 can be induced by epithelially derived Wnt factors. Because few Wnt targets in mammalian cells have been identified, the limb organ culture system I developed provides a unique opportunity to investigate genes that act downstream of and together with Wnt factors to activate expression of neurotrophin-3.

The dynamic, neuronally independent expression of neurotrophin-3 raises the possibility that neurotrophins actively guide growing sensory axons. To pursue this possibility, we used gene targeting technology to replace mouse genomic trkA with trkC (the receptor for neurotrophin-3), creating a situation in which presumptive trkA-expressing neurons and their projections express trkC instead of trkA. Our hypothesis is that these ectopic trkC-expressing neurons will now follow the neurotrophin-3 path and switch neuronal fate.

Aside from neurotrophin receptors, early markers of DRG subsets have not been characterized. We are using DRGs from wild-type and subtype-deficient animals to catalog gene expression profiles by means of DNA chip technology. Differences in these expression profiles will help us understand how DRG neurons that have a common origin become functionally distinct. Animals with genes deleted in specific tissues will be used to uncover the roles of the individual genes identified.

Finally, we are involved in a collaborative effort with the Genomic Institute of the Novartis Research Foundation, San Diego, California, to conduct a large-scale screening for behavioral phenotypes in mice with mutations induced by N-ethyl-N-nitrosourea. A screen for altered sensitivity to pain, temperature, and proprioception should reveal mutants with defects in the development or function of sensory neurons.

PUBLICATIONS

Patapoutian, A., Backus, C., Kispert, A., Reichardt, L.F. Regulation of neurotrophin-3 expression by epithelial-mesenchymal interactions: The role of Wnt factors. Science 283:1180, 1999.

Patapoutian, A., Reichardt, L.F. Roles of Wnt proteins in neural development and maintenance. Curr. Opin. Neurobiol. 10:392, 2000.