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^ΔCaM, which are simultaneously lacking NCKX4 (NCKX4^-/-) and Ca²⁺/CaM-mediated CNG channel desensitization (CNGB1^ΔCaM). Despite exhibiting alterations in various response attributes, including termination kinetics and adaption properties, OSNs in either NCKX4^-/- mice or CNGB1^ΔCaM mice show normal resting sensitivity, as determined by their unchanged EOG response amplitude. We found that OSNs in NCKX4^-/-;CNGB1^ΔCaM 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.

Sensory receptor cells maintain high sensitivity at rest. Although the mechanisms responsible for setting the resting sensitivity of sensory receptor cells are not well understood, it has generally been assumed that the sensitivity is set primarily by how effectively the components in the activation cascade of sensory transduction can be stimulated. Our findings in mouse olfactory sensory neurons suggest that mechanisms that are primarily responsible for terminating the olfactory response are also critical for proper resting sensitivity.