Simultaneous Loss of NCKX4 and CNG Channel Desensitization Impairs Olfactory Sensitivity

In vertebrate olfactory sensory neurons (OSNs), Ca2+ plays key roles in both mediating and regulating the olfactory response. Ca2+ enters OSN cilia during the response through the olfactory cyclic nucleotide-gated (CNG) channel and stimulates a depolarizing chloride current by opening the olfactory Ca2+-activated chloride channel to amplify the response. Ca2+ also exerts negative regulation on the olfactory transduction cascade, through mechanisms that include reducing the CNG current by desensitizing the CNG channel via Ca2+/calmodulin (CaM), to reduce the response. Ca2+ is removed from the cilia primarily by the K+-dependent Na+/Ca2+ exchanger 4 (NCKX4), and the removal of Ca2+ leads to closure of the chloride channel and response termination. In this study, we investigate how two mechanisms conventionally considered negative regulatory mechanisms of olfactory transduction, Ca2+ removal by NCKX4, and desensitization of the CNG channel by Ca2+/CaM, interact to regulate the olfactory response. We performed electro-olfactogram (EOG) recordings on the double-mutant mice, NCKX4(-/-); CNGB1(Delta CaM), which are simultaneously lacking NCKX4 (NCKX4(-/-)) and Ca2+/CaM-mediated CNG channel desensitization (CNGB1(Delta CaM)). Despite exhibiting alterations in various response attributes, including termination kinetics and adaption properties, OSNs in either NCKX4(-/-) mice or CNGB1(Delta CaM) mice show normal resting sensitivity, as determined by their unchanged EOG response amplitude. We found that OSNs in NCKX4(-/-); CNGB1(Delta CaM) mice displayed markedly reduced EOG amplitude accompanied by alterations in other response attributes. This study suggests that what are conventionally considered negative regulatory mechanisms of olfactory transduction also play a role in setting the resting sensitivity in OSNs.