The broad goal of our proposed studies is to exploit our new insights into the identity of CALHM1 as a founding member of a novel ion channel family. CALHM1, a gene of unknown function, was identified as a susceptibility factor for late-onset Alzheimer's disease that influences the age of onset. CALHM1 encodes a membrane protein expressed throughout the brain and in taste buds that lacks significant homology to other functionally characterized proteins, although five human homologs have been identified, and CALHM1 is conserved across species. We recently identified CALHM1 as the pore-forming subunit of Ca2+ permeable ion channel with unusual permeation properties and gating regulation by both voltage and extracellular Ca2+ concentration (Ca2+o). We have discovered that CALHM1 is essential for the perceptions of sweet, bitter and umami tastes, since CALHM1 knockout mice cannot perceive these tastants. Furthermore, we have identified the molecular mechanism that links CALHM1 expression to taste perception by discovering that CALHM1 is an ATP permeable channel, and that tastants activate ATP release as a neurotransmitter through CALHM1 channels by a voltage dependent mechanism that transduces taste in the periphery to the central nervous system. We will employ a combination of biophysical (electrophysiology, optical imaging), biochemical and cell biological approaches to define the molecular physiology of CALHM1 in taste perception. We will record the electrical properties of taste cells from wild-type mice and mice with CALHM1 genetically deleted and fully characterize the properties of CALHM channels in taste cells. We will define the role of CALHM1 in taste cell signal transduction by single cell imaging of intracellular Ca2+ and whole cell electrophysiology. Because a CALHM1 channel is multimeric, type II taste cells also express CALHM2 and CALHM3, and co-expression of CALHMs 1 and 3 generates a novel ATP-permeable channel, we will determine the biochemical and functional interactions and roles of all three CALHMs to define their channel properties with the goal to reconstitute the ATP release channel currents in taste cells. Finally we will use electrophysiological recordings of heterologous cells and taste cells to understand the mechanisms by which taste cell electrical responses activate CALHM channels and are in turn modified by CALHM channel activation. The results of these studies will provide new insights into the properties and regulation of this unique voltage-gated Ca2+ and ATP permeable ion channel and its essential role in taste sensory perception.
The sense of taste plays an important role in the quality of human life. Identification of a mechanism that is essential for perception of sweet, bitter and fa tastes may provide new targets for pharmacological manipulation of these appetitive and aversive taste sensations, which may be important in obesity, diabetes and formulations of medicines.
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