Plasma membrane Ca²⁺‐ATPase in the cilia of olfactory receptor neurons: possible role in Ca²⁺ clearance

Abstract

Olfactory sensory neurons respond to odorants increasing Ca(2+) concentrations in their chemosensory cilia. Calcium enters the cilia through cAMP-gated channels, activating Ca(2+)-dependent chloride or potassium channels. Calcium also has a fundamental role in odour adaptation, regulating cAMP turnover rate and the affinity of the cyclic nucleotide-gated channels for cAMP. It has been shown that a Na(+)/Ca(2+) exchanger (NCX) extrudes Ca(2+) from the cilia. Here we confirm previous evidence that olfactory cilia also express plasma membrane Ca(2+)-ATPase (PMCA), and show the first evidence supporting a role in Ca(2+) removal. Both transporters were detected by immunoblot of purified olfactory cilia membranes. The pump was also revealed by immunocytochemistry and immunohistochemistry. Inside-out cilia membrane vesicles transported Ca(2+) in an ATP-dependent fashion. PMCA activity was potentiated by luminal Ca(2+) (K(0.5) = 670 nm) and enhanced by calmodulin (CaM; K(0.5) = 31 nm). Both carboxyeosin (CE) and calmidazolium reduced Ca(2+) transport, as expected for a CaM-modulated PMCA. The relaxation time constant (tau) of the Ca(2+)-dependent Cl(-) current (272 +/- 78 ms), indicative of luminal Ca(2+) decline, was increased by CE (2181 +/- 437 ms), by omitting ATP (666 +/- 49 ms) and by raising pH (725 +/- 65 ms), suggesting a role of the pump on Ca(2+) clearance. Replacement of external Na(+) by Li(+) had a similar effect (tau = 442 +/- 8 ms), confirming the NCX involvement in Ca(2+) extrusion. The evidence suggests that both Ca(2+) transporters contribute to re-establish resting Ca(2+) levels in the cilia following olfactory responses.

Ca2+ Extrusion by NCX Is Compromised in Olfactory Sensory Neurons of OMP−/− Mice

“…Using electrophysiological criteria it has been reported that NCX is the major means of Ca 2+ extrusion following odor-evoked stimulation of the apical region of amphibian and mouse OSNs [50], [51]. The demonstration of NCX1 in isolated cilia by immunoblotting [52] is consistent with these reports. This has not been previously analyzed using Ca 2+ imaging of mouse OSN knobs.…”

“…This has not been previously analyzed using Ca 2+ imaging of mouse OSN knobs. Recent reports suggests that PMCA may also play a significant role in Ca 2+ removal in the rat [52] and mouse [53] OSNs. Therefore, we evaluated the contribution of each Ca 2+ clearance mechanism to the removal of elevated Ca 2+ i .…”

Scaffolding proteins in highly purified rat olfactory cilia membranes

“…The latter activate Ca 2 + -dependent Cl À channels (Cl Ca ) involved in excitatory odour transduction and/or Ca 2 + -dependent K + channels (K Ca ), which act in an inhibitory manner [1,2]. Ca 2 + levels are restored by a Na + /Ca 2 + exchanger and a plasma membrane Ca 2 + -ATPase [3]. Moreover, Ca 2 + binding to calmodulin mediates adaptation to odour signals by interaction with various targets [4].…”

Mechanisms of Regulation of Olfactory Transduction and Adaptation in the Olfactory Cilium

“…Additionally, Ca 2+ /Calmodulin (Ca 2+ /CaM) can bind to CNG channels, promoting a decrease in its affinity to cAMP [15], a process that seems to be involved in STA. Since cAMP opens CNG channels leading to Ca 2+ influx that induces STA [11], Ca 2+ extrusion mainly through Na + /Ca 2+ and K + exchangers (NCKX) [16]–[18] and, possibly, plasma membrane Ca 2+ -ATPases (PMCA) [19]–[22] leads to the resetting the cellular resting state. There are evidences that K + -independent Na + /Ca 2+ exchangers (NCX) and NCKX are present in mammalian OSNs [23], [24].…”