The evolutionary shift from aqueous to airborne olfaction: intermediate and novel properties in an amphibian, transitional species
Zusammenfassung der Projektergebnisse
The greater goal of our research was to obtain a thorough understanding of the evolution of olfactory coding in vertebrates using the amphibian species Xenopus laevis as a transition point in the conversion from aquatic to terrestrial olfaction during vertebrate evolution. In contrast to the single sensory surface present in teleost fishes, the mammalian olfactory system is defined by spatially segregated subsystems with distinct molecular and functional characteristics, chief among them the main olfactory epithelium and the vomeronasal organ. Most amphibians, including Xenopus laevis, also have anatomically segregated main and vomeronasal olfactory systems, but it was not known how this anatomical separation was reflected at the cellular and molecular level. We analyzed the olfactory system of the secondarily aquatic pipid frog Xenopus laevis, and report the existence of two odor-processing streams in the main olfactory system, sharply segregated in the olfactory bulb and partially segregated in the olfactory epithelium of premetamorphic larvae. The two streams consist of morphologically different receptor neuron types and exhibit diverse transduction pathways and different odorant sensitivity. We have characterized expression patterns of four different olfactory receptor gene families, and concomitantly have analyzed odorevoked neuronal activity in olfactory epithelium and olfactory bulb. The most striking finding was the segregation of Xenopus v2r gene expression. Earlier-diverging (more ancestral) v2r genes are expressed exclusively in the main epithelium and later-diverging (more modern) genes are restricted to the epithelium of the vomeronasal organ. We could show a high degree of correlation between expression of taar genes and amine responses and between expression of main epithelium-specific v2r genes and amino acid responses. We characterized the signal transduction cascade for amino acid responses and could show that it involves phospholipase C, diacylglyerol, and TRPC2. We analyzed the cellular and molecular composition of the vomeronasal organ of larval Xenopus and identified proper natural stimuli for vomeronasal receptor neurons. A group of sulfated steroids, excreted by larval and adult Xenopus, turned out to be strong vomeronasal stimuli, but also activated receptor neurons of the main epithelium. We successfully finished virtually all experiments which we proposed for the first funding period. This ended in 5 publications resulting directly and exclusively from our SPP1392 work. Efforts concerning Experiment B are still ongoing and we expect to finish this project in the next year. We have performed extensive experiments for Experiment C (validation of the polygenic expression of olfactory receptor genes), and obtained some positive results. However, we realized that our approach (double ISH) could not rigorously exclude technical causes resulting in double labeling, and we did not have enough time, money, and manpower to do a single cell transcriptome analysis, which would have been an unambiguous way to answer the question. Experiment D was performed as described and resulted in 2 additional publications. Here we studied the cellular organization of the epithelium of the vomeronasal organ of Xenopus laevis, and found that it is generally structured like the main olfactory epithelium. Furthermore, we identified sulfated steroids as the first known class of vomeronasal stimuli in Xenopus, and obtained good evidence that these stimuli are likely to convey intraspecific information. Together, our results show that the olfactory regionalization in Xenopus is still incomplete compared to mammals. While the anatomical segregation of the main olfactory system and the vomeronasal system is already complete, the functional segregation is only partial, and the main olfactory system is still very similar to that of teleost fishes, including cellular and molecular components that are already confined to the vomeronasal system in fully terrestrial vertebrates. The intermediate segregation of the Xenopus olfactory system results in an excellent model to investigate the molecular driving forces regulating the evolution of segregation in the vertebrate olfactory system.
Projektbezogene Publikationen (Auswahl)
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(2011) The peripheral olfactory system of vertebrates: molecular, structural and functional basics of the sense of smell. e-Neuroforum 2(3), 68-77
Manzini I, Korsching S
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(2012) Amino acid- vs. peptide-odorants: responses of individual olfactory receptor neurons in an aquatic species. PLoS One 7:e53097
Hassenklöver T, Pallesen LP, Schild D, Manzini I
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(2013) Ancestral amphibian v2rs are expressed in the main olfactory epithelium. Proc. Natl. Acad. Sci. USA. 110:7714-7719
Syed AS, Sansone A, Nadler W, Manzini, Korsching SI
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(2013) Bimodal processing of olfactory information in an amphibian nose: odor responses segregate into a medial and a lateral stream. Cell. Mol. Life Sci. 70:1965-1984
Gliem S, Syed AS, Sansone A, Kludt E, Tantalaki E, Hassenklöver T, Korsching SI, Manzini I
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(2013) Olfactory wiring logic in amphibians challenges the basic assumptions of the unbranched axon concept. J. Neurosci. 33:17247-17252
Hassenklöver T, Manzini I
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(2014) Phospholipase C and diacylglycerol mediate olfactory responses to amino acids in the main olfactory epithelium of an amphibian. PLoS One. 9:e87721
Sansone A, Hassenklöver T, Syed AS, Korsching SI, Manzini I
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(2014) Purinergic receptor-induced Ca2+ signaling in the neuroepithelium of the vomeronasal organ of larval Xenopus laevis. Purinergic Signal. 10:327-336
Dittrich K, Sansone A, Hassenklöver T, Manzini I
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(2014) Trpc2 is expressed in two olfactory subsystems, the main and the vomeronasal system of larval Xenopus laevis. J. Exp. Biol. 217:2235-2238
Sansone A, Syed AS, Tantalaki E, Korsching SI, Manzini I
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(2015) Distinct expression domains for two closely related amphibian TAARs generate a bimodal distribution similar to that of neuronal responses to amine odors. Sci. Rep. 5: 13935
Syed AS, Sansone A, Röner S, Nia SB, Manzini I, Korsching SI
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(2015) Dual processing of sulfated steroids in the olfactory system of an anuran amphibian. Front. Cell. Neurosci. 9:373
Sansone AT, Hassenklöver T, Offner XFu, Holy TE, Manzini I
