Type 3 inositol 1,4,5-trisphosphate receptor is dispensable for sensory activation of the mammalian vomeronasal organ

Pablo Chamero,Jan Weiss,María Teresa Alonso,Frank Zufall,Chihiro Hisatsune,Trese Leinders-Zufall,Katsuhiko Mikoshiba,Macarena Rodríguez-Prados

doi:10.1038/s41598-017-09638-8

Abstract

Signal transduction in sensory neurons of the mammalian vomeronasal organ (VNO) involves the opening of the canonical transient receptor potential channel Trpc2, a Ca2+-permeable cation channel that is activated by diacylglycerol and inhibited by Ca2+-calmodulin. There has been a long-standing debate about the extent to which the second messenger inositol 1,4,5-trisphosphate (InsP3) and type 3 InsP3 receptor (InsP3R3) are involved in the opening of Trpc2 channels and in sensory activation of the VNO. To address this question, we investigated VNO function of mice carrying a knockout mutation in the Itpr3 locus causing a loss of InsP3R3. We established a new method to monitor Ca2+ in the endoplasmic reticulum of vomeronasal sensory neurons (VSNs) by employing the GFP-aequorin protein sensor erGAP2. We also performed simultaneous InsP3 photorelease and Ca2+ monitoring experiments, and analysed Ca2+ dynamics, sensory currents, and action potential or field potential responses in InsP3R3-deficient VSNs. Disruption of Itpr3 abolished or minimized the Ca2+ transients evoked by photoactivated InsP3, but there was virtually no effect on sensory activation of VSNs. Therefore, InsP3R3 is dispensable for primary chemoelectrical transduction in mouse VNO. We conclude that InsP3R3 is not required for gating of Trpc2 in VSNs.

Highlights

The mammalian olfactory system has evolved two major signaling systems for chemoelectrical transduction: one that depends on cyclic nucleotide-gated (CNG) channel activation and involving cAMP or cGMP signaling, respectively, and another that depends on transient receptor potential (TRP) channel activation, mainly involving the Trpc[2] cation channel[1, 2]
By applying a combination of state-of-the-art Ca2+ imaging and electrophysiological methods using wild-type and InsP3R3-deficient vomeronasal sensory neurons (VSNs), we asked the following: (1) Are intracellular Ca2+ stores significantly mobilized during VSN sensory activation? (2) Does InsP3 induce Ca2+ signals in VSNs? (3) Is InsP3R3 necessary for stimulus-evoked Ca2+ signaling, the generation of sensory currents, and action potential or field potential responses in VSNs? With this approach, we provide compelling evidence that sensory activation of the mouse vomeronasal organ (VNO) is largely independent of InsP3R3, ruling out a crucial role for InsP3 signaling in the primary chemotransduction process of mouse VSNs
We focused on sensory neurons of the VNO because these have been shown previously to express InsP3R3 but not InsP3R1 or InsP3R228

Summary

Introduction

The mammalian olfactory system has evolved two major signaling systems for chemoelectrical transduction: one that depends on cyclic nucleotide-gated (CNG) channel activation and involving cAMP or cGMP signaling, respectively, and another that depends on transient receptor potential (TRP) channel activation, mainly involving the Trpc[2] cation channel[1, 2]. There has been considerable evidence for the existence of additional, sequential or parallel VSN signaling mechanisms some of which require elevated intracellular Ca2+ levels and could depend on InsP3R3 activation and store-dependent Ca2+ mobilization[16]. These mechanisms include the activity of Ca2+-activated chloride channels[33,34,35,36,37], several types of Ca2+-activated potassium channels[38], as well as Ca2+-activated cation channels[32, 39]. By applying a combination of state-of-the-art Ca2+ imaging and electrophysiological methods using wild-type and InsP3R3-deficient VSNs, we asked the following: (1) Are intracellular Ca2+ stores significantly mobilized during VSN sensory activation? (2) Does InsP3 induce Ca2+ signals in VSNs? (3) Is InsP3R3 necessary for stimulus-evoked Ca2+ signaling, the generation of sensory currents, and action potential or field potential responses in VSNs? With this approach, we provide compelling evidence that sensory activation of the mouse VNO is largely independent of InsP3R3, ruling out a crucial role for InsP3 signaling in the primary chemotransduction process of mouse VSNs

Methods

Results

Conclusion