Analyzing Specificty of Gustatory Neuron Innervation to Taste Receptor Cells in the Taste Bud

In mammals, the taste receptor cells (TRCs) in taste buds have a short life span of 5-20 days and they are constantly replenished. These TRCs constitute 5 basic taste qualities- bitter, sweet, sour, salty and umami [1,2]. It is important to maintain the connections between the new TRCs formed and the respective partner ganglion neurons which transmit the taste signals from tongue to the brain. A study by Lee et al. (2017) shows that Semaphorin 3A (Sema3A), an axon guidance molecule may have a role in bitter taste wiring. Studies in other systems have concluded that Neuropilin-1 (Nrp1) is an essential co-receptor for Sema3A signaling [3–6]. We hypothesize that Nrp1 receptors are present on the partner bitter ganglion neurons to aid in the wiring process. To establish the role of Nrp1 in gustation, we performed experiments to label the Nrp1 population in the ganglion neurons using known gustatory markers. Using Nrp1-Cre; Ai9 mice, we examined the innervation of Nrp1 fibers to the taste buds within the fungiform and circumvallate papillae. We injected AAV virus carrying AAV-Flex-tdTomato in Nrp1-Cre mice to analyze the association of Nrp1 fibers with two categories of taste cells. One category consisted of sour cells (anti-CAR4 labelled) and other category consisted of bitter, sweet and umami (anti-TRPM5 labelled) populations of taste receptor cells. The current study was aimed to investigate the role of Nrp1, by assessing the innervation of Nrp1 fibers in the taste buds and whether they preferentially associate with bitter TRCs. Our experiments conclude that Nrp1 fibers associate with both categories of labelled taste cells. More experiments need to be performed and quantitative analysis of Nrp1 fiber association may help to ascertain a role for Nrp1-expressing gustatory neurons innervating specific taste receptor cell populations.