Abstract
BackgroundOdor transduction, occurring in the chemosensory cilia of vertebrate olfactory sensory neurons, is triggered by guanosine triphosphate-coupled odor receptors and mediated by a cyclic adenosine monophosphate (cAMP) signaling cascade, where cAMP opens cationic non-selective cyclic nucleotide-gated (CNG) channels. Calcium enters through CNG gates Ca2+-activated Cl− channels, allowing a Cl− inward current that enhances the depolarization initiated by the CNG-dependent inward current. The anoctamin channel 2, ANO2, is considered the main Ca2+-activated Cl− channel of olfactory transduction. Although Ca2+-activated Cl− channel-dependent currents in olfactory sensory neurons were reported to be suppressed in ANO2-knockout mice, field potentials from their olfactory epithelium were only modestly diminished and their smell-dependent behavior was unaffected, suggesting the participation of additional Ca2+-activated Cl− channel types. The Bestrophin channel 2, Best2, was also detected in mouse olfactory cilia and ClCa4l, belonging to the ClCa family of Ca2+-activated Cl− channels, were found in rat cilia. Best2 knock-out mice present no electrophysiological or behavioral impairment, while the ClCa channels have not been functionally studied; therefore, the overall participation of all these channels in olfactory transduction remains unresolved.ResultsWe explored the presence of detectable Ca2+-activated Cl− channels in toad olfactory cilia by recording from inside-out membrane patches excised from individual cilia and detected unitary Cl− current events with a pronounced Ca2+ dependence, corresponding to 12 and 24 pS conductances, over tenfold higher than the aforementioned channels, and a approx. fivefold higher Ca2+ affinity (K0.5 = 0.38 µM). Remarkably, we observed immunoreactivity to anti-ClCa and anti-ANO2 antibodies in the olfactory cilia, suggesting a possible cooperative function of both channel type in chemotransduction.ConclusionsThese results are consistent with a novel olfactory cilia channel, which might play a role in odor transduction.
Highlights
Odor transduction, occurring in the chemosensory cilia of vertebrate olfactory sensory neurons, is triggered by guanosine triphosphate-coupled odor receptors and mediated by a cyclic adenosine monophosphate signaling cascade, where cAMP opens cationic non-selective cyclic nucleotide-gated (CNG) channels
Odor transduction is confined to the chemosensory cilia of olfactory sensory neurons (OSNs), where it gives origin to a depolarizing receptor potential
Ca2+ dependence of a ciliary Cl− channel A typical dissociated toad olfactory neuron is shown in Fig. 1a in which the chemosensory cilia are clearly visible
Summary
Odor transduction, occurring in the chemosensory cilia of vertebrate olfactory sensory neurons, is triggered by guanosine triphosphate-coupled odor receptors and mediated by a cyclic adenosine monophosphate (cAMP) signaling cascade, where cAMP opens cationic non-selective cyclic nucleotide-gated (CNG) channels. Odorants bind to G-protein coupled receptors, which activate adenylyl cyclase type III by coupling to a heterotrimeric G-protein (Golf ), producing cyclic-AMP This nucleotide directly opens non-selective cationic cyclic nucleotide. Delgado et al BMC Neurosci (2016) 17:17 diameter of the olfactory cilia, bordering the limit of resolution of light microscopy [8]. For this reason, only macroscopic current measurements had been performed on OSNs by means of whole cell recording [6], excised entire individual cilium [3] and patch clamping the dendritic knob [9], an apical round-shape structure of the dendrite from where the olfactory cilia emanate. No single Ca2+-activated Cl− currents have been directly recorded from the ciliary membrane, limiting the biophysical characterization of the olfactory Cl− conductance
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