Functional characterization of the Ca 2 + - activated Cl – channel Ano 2 in the olfactory system

Functional characterization of the Ca-activated Cl channel Ano2 in the olfactory system. Ca-activated Cl currents have been described in a plethora of physiological processes including sensory transduction. In olfaction, Ca-activated Cl channels (CaCCs) are thought to play a crucial role as amplifiers of the olfactory signal. Binding of odorants to olfactory sensory neurons (OSNs) generates a primary transduction current that is mediated by cyclic nucleotide-gated (CNG) channels. The concomitant Ca influx then activates CaCCs that may account for up to 90% of the total receptor current. However, the unknown molecular identity of the underlying channel has precluded direct functional testing. Recently, Ano1 (Anoctamin-1, Tmem16a) has been cloned as the first bona fide CaCC with physiological functions in fluid secretion and smooth muscle contraction. Ano2 (Anoctamin-2, Tmem16b) is its closest homolog and likewise gives rise to Ca-activated Cl currents. A physiological role of Ano2, though, has not been explored yet. We investigated the function of Ano2 in mice and identified Ano2 as the CaCC of the olfactory transduction cascade. Expression of Ano2 was restricted to neuronal tissues with highest levels in the two sensory systems of smell and vision and low expression in several brain regions. In the olfactory system, the Ano2 channel localized to sensory cilia of olfactory neurons in the main olfactory epithelium (MOE) and to microvilli of sensory neurons in the vomeronasal organ (VNO). In the VNO, as in the retina, Ano2 co-localized with the related CaCC Ano1 which otherwise was mainly detected in apical membranes of glandular cells, consistent with its function in epithelial secretion. Disruption of Ano2 in mice abolished Ca-activated Cl currents in the MOE and the VNO. Surprisingly, the loss of CaCC activity from OSNs only moderately affected the receptor potential in electro-olfactogram (EOG) recordings. Odor responses were reduced by ~40% in the fluid-phase configuration, while in air-phase EOGs we could not detect any changes. Neuronal input activity to the olfactory bulb and convergence of OSN axons to the olfactory glomeruli, where Ano2 is also located, were unchanged. Consistently, Ano2 mice showed normal olfaction-guided behaviors and did perform undistinguishable from their littermates in olfactory behavioral tasks. Neither olfactory discrimination ability nor odor sensitivity was affected. Our results show that CNG channels do not need a boost by CaCC to achieve near-physiological levels of olfaction. We conclude that in contrast to the prevailing view, Ca-activated Cl currents are dispensable for olfactory signaling.