J Exp Biol
July 1, 2014;
Trpc2 is expressed in two olfactory subsystems, the main and the vomeronasal system of larval Xenopus laevis.
Complete segregation of the main olfactory epithelium
(MOE) and the vomeronasal epithelium
is first observed in amphibians. In contrast, teleost fishes possess a single olfactory surface, in which genetic components of the main and vomeronasal olfactory systems are intermingled. The transient receptor potential channel TRPC2
, a marker of vomeronasal neurons, is present in the single fish sensory surface, but is already restricted to the vomeronasal epithelium
in a terrestrial amphibian, the red-legged salamander (Plethodon shermani). Here we examined the localization of TRPC2
in an aquatic amphibian and cloned the Xenopus laevis trpc2
gene. We show that it is expressed in both the MOE and the vomeronasal epithelium
. This is the first description of a broad trpc2
expression in the MOE of a tetrapod. The expression pattern of trpc2
in the MOE is virtually undistinguishable from that of MOE-specific v2rs, indicating that they are co-expressed in the same neuronal subpopulation.
J Exp Biol
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Fig. 1. Cloning of trpc2 and analysis of tissue-specific expression. (A) Schematic representation of the predicted trpc2 transcript of Xenopus tropicalis and degenerate primers used for the PCR shown in B. Two fragments of the trpc2 multi-species alignment are shown below. The black boxes highlight the conserved regions chosen to design the degenerate primers. (B) Touchdown RT-PCR with degenerate primers (see A). An amplification product of 1402 bp was detected in the olfactory organ (OO) including both the main olfactory epithelium (MOE) and the vomeronasal organ (VNO). The obtained fragment was sequenced, and in BLAST searches (http://blast.ncbi.nlm.nih.gov/) gave the best score with the predicted X. tropicalis trpc2 sequence (90% nucleotide identity). (C) For analysis of tissue specificity, an RT-PCR (35 cycles) for trpc2 was performed with specific primers (see Materials and methods) under stringent conditions. OB, olfactory bulb. An amplification product of the expected size was detected in the VNO and MOE, whereas no signal was detected from other organs.
Fig. 2. Distribution of trpc2-positive cells closely mimics that of v2r-C-expressing cells in the MOE. (A) Cryosections of larval Xenopus laevis were hybridized with antisense probes for the trpc2 gene. The micrograph shown is from a horizontal section of larval head tissue, which contains both the MOE and the VNO. A zone of trpc2-positive cells was detected in the MOE and widespread labeling was visible in the VNO. The arrow is pointing at the region enlarged in the inset. (B) Cryosections of larval head tissue were hybridized with antisense probes for the v2r-C gene. Orientation and region as explained in A. Consistent with previous results (see Syed et al., 2013), v2r-C-positive cells were only found in the MOE, and occupy a discrete zone there. The arrow is pointing at the region enlarged in the inset. (C) Basal-to-apical distribution (0, most basal; 1, most apical position) of trpc2 (314 cells, 5 sections) and v2r-C-expressing cells [data taken from Syed et al. (Syed et al., 2013) and shown here for comparison]. Data are given as mid-bin values (0.1 bin size); y-axis shows total number of cells per bin. (D) Characteristic parameters for the distribution of trpc2-expressing cells; values for v2r-C taken from our earlier work (Syed et al., 2013) are shown for comparison.
Role of a ubiquitously expressed receptor in the vertebrate olfactory system.