Purinergic receptor-induced Ca2+ signaling in the neuroepithelium of the vomeronasal organ of larval Xenopus laevis

Nucleotide receptor P2RY4 is required for head formation via induction and maintenance of head organizer in Xenopus laevis

Vertebrates have unique head structures that are mainly composed of the central nervous system, the neural crest, and placode cells. These head structures are brought about initially by the neural induction between the organizer and the prospective neuroectoderm at early gastrula stage. Purinergic receptors are activated by nucleotides released from cells and influence intracellular signaling pathways, such as phospholipase C and adenylate cyclase signaling pathways. As P2Y receptor is vertebrate‐specific and involved in head formation, we expect that its emergence may be related to the acquisition of vertebrate head during evolution. Here, we focused on the role of p2ry4 in early development in Xenopus laevis and found that p2ry4 was required for the establishment of the head organizer during neural induction and contributed to head formation. We showed that p2ry4 was expressed in the head organizer region and the prospective neuroectoderm at early gastrula stage, and was enriched in the head components. Disruption of p2ry4 function resulted in the small head phenotype and the reduced expression of marker genes specific for neuroectoderm and neural border at an early neurula stage. Furthermore, we examined the effect of p2ry4 disruption on the establishment of the head organizer and found that a reduction in the expression of head organizer genes, such as dkk1 and cerberus, and p2ry4 could also induce the ectopic expression of these marker genes. These results suggested that p2ry4 plays a key role in head organizer formation. Our study demonstrated a novel role of p2ry4 in early head development.

Dual processing of sulfated steroids in the olfactory system of an anuran amphibian

Chemical communication is widespread in amphibians, but if compared to later diverging tetrapods the available functional data is limited. The existing information on the vomeronasal system of anurans is particularly sparse. Amphibians represent a transitional stage in the evolution of the olfactory system. Most species have anatomically separated main and vomeronasal systems, but recent studies have shown that in anurans their molecular separation is still underway. Sulfated steroids function as migratory pheromones in lamprey and have recently been identified as natural vomeronasal stimuli in rodents. Here we identified sulfated steroids as the first known class of vomeronasal stimuli in the amphibian Xenopus laevis. We show that sulfated steroids are detected and concurrently processed by the two distinct olfactory subsystems of larval Xenopus laevis, the main olfactory system and the vomeronasal system. Our data revealed a similar but partially different processing of steroid-induced responses in the two systems. Differences of detection thresholds suggest that the two information channels are not just redundant, but rather signal different information. Furthermore, we found that larval and adult animals excrete multiple sulfated compounds with physical properties consistent with sulfated steroids. Breeding tadpole and frog water including these compounds activated a large subset of sensory neurons that also responded to synthetic steroids, showing that sulfated steroids are likely to convey intraspecific information. Our findings indicate that sulfated steroids are conserved vomeronasal stimuli functioning in phylogenetically distant classes of tetrapods living in aquatic and terrestrial habitats.

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

The amphibian olfactory system undergoes massive remodeling during metamorphosis. The transition from aquatic olfaction in larvae to semiaquatic or airborne olfaction in adults requires anatomical, cellular, and molecular modifications. These changes are particularly pronounced in Pipidae, whose adults have secondarily adapted to an aquatic life style. In the fully aquatic larvae of Xenopus laevis, the main olfactory epithelium specialized for sensing water-borne odorous substances lines the principal olfactory cavity (PC), whereas a separate olfactory epithelium lies in the vomeronasal organ (VNO). During metamorphosis, the epithelium of the PC is rearranged into the adult "air nose," whereas a new olfactory epithelium, the adult "water nose," forms in the emerging middle cavity (MC). Here we performed a stage-by-stage investigation of the anatomical changes of the Xenopus olfactory organ during metamorphosis. We quantified cell death in all olfactory epithelia and found massive cell death in the PC and the VNO, suggesting that the majority of larval sensory neurons is replaced during metamorphosis in both sensory epithelia. The moderate cell death in the MC shows that during the formation of this epithelium some cells are sorted out. Our results show that during MC formation some supporting cells, but not sensory neurons, are relocated from the PC to the MC and that they are eventually eliminated during metamorphosis. Together our findings illustrate the structural and cellular changes of the Xenopus olfactory organ during metamorphosis.

Purinergic signaling negatively regulates activity of an olfactory receptor in an odorant-dependent manner

Extracellular purines and pyrimidines are important signaling molecules that mediate diverse biological functions via cell surface purinergic receptors. Although purinergic modulation to olfactory activity has been reported, cell-specific expression and action of purinergic receptors deserve further exploration. We physiologically characterized expression of purinergic receptors in a set of olfactory sensory neurons that are responsive to both acetophenone and benzaldehyde (AB-OSNs). Sparsely distributed in the most ventral olfactory receptor zone, AB-OSNs were activated by P2 purinergic receptor agonists but not by P1 purinergic receptor agonist adenosine. Both P2X-selective agonist α,β-methylene ATP and P2Y-selective agonist uridine 5'-triphosphate (UTP) were stimulatory to AB-OSNs, indicating expression of both P2X and P2Y purinergic receptors in AB-OSNs. Pharmacological characterization of receptor specificity using various P2X and P2Y agonists and antagonists illustrated that P2X1 and P2Y2 receptors played major roles in purinergic signaling in AB-OSNs. Interestingly, the results of purinergic modulation to acetophenone-evoked responses were different from those to benzaldehyde-evoked responses within the same neurons. Activation of P2X1 receptors had more profound inhibitory effects on benzaldehyde-evoked intracellular calcium elevation than on acetophenone-evoked responses within the same neurons, and the reverse was true when P2Y2 receptors were activated. Cross-adaptation data showed that acetophenone and benzaldehyde bound to the same olfactory receptor. Thus, our study has demonstrated that purinergic signaling of P2X and P2Y receptors has different effects on olfactory transduction mediated by a defined olfactory receptor and the consequences of purinergic modulation of olfactory activity might depend on stereotypic structures of the odorant-receptor complex.