Cellular & molecular Ca²⁺ microdomains in olfactory cilia support low signaling amplification of odor transduction

Signal transduction depends critically on the spatial localization of protein constituents. A key question in odor transduction is whether chemotransduction proteins organize into discrete molecular complexes throughout olfactory cilia or distribute homogeneously along the ciliary membrane. Our recordings of Ca²⁺ changes in individual cilia with unprecedented spatial and temporal resolution, by the use of two-photon microscopy, provide solid evidence for Ca²⁺ microdomains (transducisomes). Dissociated frog olfactory neurons were preloaded with caged-cAMP and fluo-4 acetoxymethyl ester probe Ca²⁺ indicator. Ca²⁺ influx through cyclic nucleotide-gated (CNG) channels was evoked by uniformly photoreleasing cAMP, while ciliary Ca²⁺ was measured. Discrete fluorescence events were clearly resolved. Events were missing in the absence of external Ca ²⁺, consistent with the absence of internal Ca²⁺ sources. Fluorescence events at individual microdomains resembled single-CNG channel fluctuations in shape, mean duration and kinetics, indicating that transducisomes typically contain one to three CNG channels. Inhibiting the Na⁺/Ca²⁺ exchanger or the Ca²⁺-ATPase prolonged the decay of evoked intraciliary Ca²⁺ transients, supporting the participation of both transporters in ciliary Ca²⁺ clearance, and suggesting that both molecules localize close to the CNG channel. Chemosensory transducisomes provide a physical basis for the low amplification and for the linearity of odor responses at low odor concentrations.