Our primary goal is to understand the organization and control of taste signaling i.e. how individual pathways involved in taste transduction function and interact with each other. In the long term, this will involve defining the various components and the organization required for taste responses both peripherally and centrally and will help to elucidate the logic of taste coding. Initially, we would like to know what receptors mediate sweet, sour, salty, umami (savory) and bitter tastes; how tastant specificity and taste discrimination are accomplished; what topographic organization exists in the various taste buds and papillae; and how the information is transmitted and encoded in the afferent nerves. Our focus has been on the isolation and characterization of genes encoding taste receptors and using these to mark the cells, define the corresponding signaling pathways, dissect receptor specificity, generate topographic maps, and trace the respective neuronal connectivity circuits. This work has identified and characterized two families of G-protein coupled receptors, T1Rs and T2Rs, that are expressed in distinct subsets of taste receptor cells and that include functionally validated sweet, amino acid and bitter taste receptors. In addition, we have shown that the TRP-ion channel PKD2L1 is selectively expressed in sour sensing cells. We have also developed a number of genetically engineered mouse lines that have had a major impact in our understanding of how sweet, bitter and umami taste are encoded at the periphery.? ? In this reporting period we (in our continuing collaboration with Charles Zuker's group at UCSD) have focused on sour taste. Using a bioinformatics based approach we identified a TRP-channel PKD2L1 that is highly expressed in a subset of taste receptor cells. Immunohistochemistry revealed that the ion-channel itself is concentrated towards the taste pore implying a role in taste reception. Double labeling studies showed that PKD2L1 is selectively expressed in taste receptor cells that do not contain sweet, bitter or umami receptors. Moreover, mice engineered to express a toxin selectively within the PKD2L1 cells and thus kill these cells showed a remarkable loss of sour taste responses. In contrast, responses to all other taste modalities remained unimpaired. This proves that sour taste is mediated by the PKD2L1-expressing population of taste receptor cells and demonstrates that PKD2L1-expressing cells are not involved in detecting other taste modalities. In combination with our previous demonstration that sweet, bitter and umami are each mediated by specific and largely non-overlapping populations of taste receptor cells, this result strengthens our hypothesis that there specific subsets of taste cell for each taste modality. Is PKD2L1 (part of) the sour taste receptor itself? Immunohistochemistry suggests that it is expressed in the relevant region of the taste receptor cell. Moreover, PKD2L1 is also expressed in a class of neurons that surround the central canal of the spinal cord and brain stem. We have shown that these neurons fire action potentials in response to mild acidification. In contrast, neurons surrounding the spinal cord that do not express PKD2L1 are insensitive to such pH-changes. Thus PKD2L1 is a strong candidate acid sensor and thus sour receptor, but additional work will be needed to elucidate its exact role in mediating taste and pH-responses in the spinal cord neurons.
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