Growth factors in the development and physiology of geniculate taste neurons

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Project Summary/Abstract The sense of taste starts with taste buds, clusters of sensory cells, that communicate chemosensory information to afferent neurons whose cell bodies are located in the geniculate and nodose/petrosal ganglia. Geniculate ganglion (GG) oral sensory neurons project via the chorda tympani (CT) nerve to innervate taste buds located in fungiform papillae that are distributed across the anterior tongue. Importantly, fungiform papillae are multimodal in that CT nerve fibers respond to all five taste qualities, tactile stimulation of the tongue surface, and temperature. Our understanding of the different subtypes of sensory neurons that communicate these varied stimuli to the brain, and their role in feeding and the perception of flavor, is rudimentary. Likewise, the molecular mechanisms responsible for cell fate specification of these GG oral sensory subpopulations, and the maintenance of their functional connections throughout life, are poorly understood. Neurotrophic factor signaling pathways, along with downstream transcription factors, are critical for the emergence of neuronal diversity in other sensory systems. To identify cell fate specification pathways in the taste system, we used ribosomal profiling to identify genes that were enriched specifically in GG oral sensory neurons. From this screen we identified the growth factor receptor anaplastic lymphoma kinase (ALK), which is expressed in other visceral neurons such as sympathetic neurons. We also identified the transcription factor early growth response 4 (EGR4), and both ALK and EGR4 were confirmed to be expressed in GG oral sensory neurons. We will test the hypothesis that the ALK receptor complex is required for oral sensory neuron differentiation, target innervation and maintenance throughout the lifespan by examining Alk-/- mice. We will also determine whether whether Alk-/- mice, as compared to Alk+/+ mice, have impaired responses to chemical and tactile stimulation of the tongue using electrophysiological recordings of the CT. Likewise, the function of EGR4 in the cell fate specification of geniculate oral sensory neurons will be evaluated through the analysis of Egr4-/- mice. Whether Egr4-/- mice have impaired CT responses to taste and tactile stimulation of the tongue will be evaluated, along with whether BDNF, a neurotrophic factor critical for the development of GG neurons, is the signal that initiates EGR4 expression. Lastly, we identified a population of GG oral sensory neurons that are mechanosensory and selectively express the receptor Ret. Using intersectional genetics, we will distinguish Ret+ GG neuron projections in fungiform papillae from intermingled trigeminal projections. We will also determine which Ret ligands support the innervation pattern of these oral mechanosensory neurons. Linking this subpopulation to cell fate pathways, we will determine whether the emergence of Ret+ GG neurons requires EGR4 and/or ALK complexes. These experiments will provide a greater understanding of the organization and maintenance of the peripheral gustatory system, and may reveal therapeutic strategies for taste impairments caused by neurodegerative disorders, cancer treatments, viral infections and aging.