Phospholipase C and Diacylglycerol Mediate Olfactory Responses to Amino Acids in the Main Olfactory Epithelium of an Amphibian

Abstract

The semi-aquatic lifestyle of amphibians represents a unique opportunity to study the molecular driving forces involved in the transition of aquatic to terrestrial olfaction in vertebrates. Most amphibians have anatomically segregated main and vomeronasal olfactory systems, but at the cellular and molecular level the segregation differs from that found in mammals. We have recently shown that amino acid responses in the main olfactory epithelium (MOE) of larval Xenopus laevis segregate into a lateral and a medial processing stream, and that the former is part of a vomeronasal type 2 receptor expression zone in the MOE. We hypothesized that the lateral amino acid responses might be mediated via a vomeronasal-like transduction machinery. Here we report that amino acid-responsive receptor neurons in the lateral MOE employ a phospholipase C (PLC) and diacylglycerol-mediated transduction cascade that is independent of Ca2+ store depletion. Furthermore, we found that putative transient receptor potential (TRP) channel blockers inhibit most amino acid-evoked responses in the lateral MOE, suggesting that ion channels belonging to the TRP family may be involved in the signaling pathway. Our data show, for the first time, a widespread PLC- and diacylglycerol-dependent transduction cascade in the MOE of a vertebrate already possessing a vomeronasal organ.

Coordinated shift of olfactory amino acid responses and V2R expression to an amphibian water nose during metamorphosis

“…In larval MOE, amino acid responses have been shown to be mediated primarily by microvillous neurons, but to some extent also by forskolin-responsive (and forskolin-insensitive) ciliated neurons [7,10,16]. A small minority of both forskolin-and amino acid-responsive neurons is also observed in the middle cavity (this study), but not in the postmetamorphotic PC.…”

Sensitivity of Olfactory Sensory Neurons to food cues is tuned to nutritional states by Neuropeptide Y signalling

“…The microvillar subtype of OSNs employ vomeronasal type 2 receptors (V2Rs) to mediate amino acid sensing in teleost fish and amphibia, like Xenopus (Koide et al , 2009; Sansone et al , 2014). Unlike classical OSN signal transduction, V2R signalling involves phospholipase C activity and opening of the diacylglycerol (DAG) -gated TrpC2 channels resulting in OSN activation (Lucas et al , 2003; Sato et al , 2005; Koide et al , 2009; Sansone et al , 2014). To test if amino acid mediated activation of OSNs in zebrafish was limited to the microvillar OSNs, we directly evaluated OSN activation by monitoring ERK1/2 phosphorylation (pERK).…”

“…Attenuation of OSN activation by blocking TrpC2 activity would be confirmatory but there are no specific TrpC2 inhibitors currently available. We therefore used 2-APB, a general Trp channel blocker (not specific for TrpC2), that has been shown to block OSN responses in the vomeronasal organ in rodents (Lucas et al , 2003; Zhang et al , 2010) and amino acid responses in the OE of Xenopus (Sansone et al , 2014). Pretreatment of zebrafish with 2-APB administered via icv route significantly attenuated OSN activation by amino acids in starved zebrafish (Figure 6A, p<0.0001).…”

“…Amino acids are typically perceived by microvillar OSNs expressing the vomeronasal class II receptors (V2Rs) that are coupled to PLC activity. PLC activation, in turn, leads to OSN depolarization via the opening of DAG-gated TrpC2 channels (Lucas et al , 2003; Sansone et al , 2014). Zebrafish and Xenopus , lack a distinct vomeronasal organ, instead, vomeronasal receptor (VR) expressing OSNs are found in the OE (Sato et al , 2005; Koide et al , 2009; Sansone et al , 2014).…”

Metamorphic remodeling of the olfactory organ of the African clawed frog, Xenopus laevis

“…V2Rs have been shown to be expressed in the adult VNO and MC (Hagino‐Yamagishi et al, ). Distinct classes of sensory neurons can be characterized by olfactory receptor gene family expression, transduction machinery, and morphology, and at least two different neuronal populations coexist in the larval PC (Gliem et al, ; Syed et al, ; Sansone et al, ). It is still not clear how the reorganization in the PC and the formation of the MC impact the distribution of these neuronal subtypes with regard to the connected transduction pathways coupled either to cAMP or phospholipase C/transient receptor potential cation channel 2 (Gliem et al, ; Syed et al, ; Sansone et al, ).…”