Determining TrkB intracellular signalling pathways required for specific aspects of gustatory development
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Neurotrophins BDNF and NT4 influence the development of the rodent gustatory system. Despite binding to the same receptor, TrkB, they have different roles. BDNF is chemo-attractive for gustatory neurons and regulates gustatory neuron targeting and number during development. NT4 regulates gustatory neuron number earlier in development than BDNF, but it is not chemo-attractive and does not regulate gustatory neuron targeting. To elucidate the mechanisms that regulate these processes we have examined which TrkB intracellular signalling pathways are required for specific aspects of gustatory development by studying the effect of specific point mutations in TrkB docking sites. We found that the TrkB/Shc docking site is involved in regulating the survival of geniculate ganglion neurons as a point mutation in this adaptor site (TrkbS/S) caused large losses of these neurons as early as E12.5. These losses were exacerbated throughout development until after birth. A point mutation in the TrkB/PLCγ (TrkbP/P) docking site did not cause loss of geniculate ganglion neurons at any point during development. Animals with a point mutation in both docking sites (TrkbD/D) caused a further decrease in neuron numbers compared to animals with a mutation in only one of the docking sites, similarly to what has previously been shown in Trkb null animals. We concluded that the TrkB/Shc docking site is crucial for determining the survival of geniculate ganglion neurons during mouse gustatory development, while the TrkB/PLCγ docking site does not affect the neuronal survival directly and likely plays a role in maintenance of these neurons. Examining the targeting of geniculate ganglion afferents into the tongue revealed large deficits in innervated neural bud and taste bud numbers in TrkbS/S animals both before and after birth. This was concluded to be reflecting the lack of neuronal survival in this ganglion, a result that was mirrored in TrkbD/D animals. TrkbP/P animals, on the other hand, exhibited a developmental delay in innervation. This was indicated by a low amount of innervated neural buds following the initial innervation period, which was compensated for by a large increase in the number of innervated taste buds by birth. By adulthood, the numbers of taste buds present on the tongues of TrkbP/P animals reached normal numbers compared to control animals. This suggested that the TrkB/PLCγ docking site is involved primarily in innervation. Finally, we examined the morphology of taste buds in newly born and adult animals. We found that the low amount of geniculate ganglion afferents innervating the tongue in TrkbS/S and TrkbD/D animals caused a decrease in size of taste buds. This effect was seen to be partially rescued by adulthood in TrkbS/S animals but not in TrkbD/D animals due to lack of viability. The morphology of taste buds was unaffected in TrkbP/P animals until adulthood, at which point the size of the taste buds was increased. These results are in agreement with previous findings showing dependency of taste bud morphology on the amount of innervation. Overall, our findings show a differential role of TrkB adaptor sites in gustatory development. Despite activated by the same ligands, the docking sites on this receptor are able to exert different influence on signalling pathways downstream of TrkB affecting neuronal survival, targeting and morphology of taste buds.