Fig. 8
Model of the OR class specification of OSNs and terrestrial adaptation. a Schematic representation of the class-specific expression of Bcl11b in the OE of frog and mouse and the molecular mechanism of the OR class specification of OSNs. In tadpoles, the OE called “water-nose” expresses class I genes. During metamorphosis, the OE undergoes remodeling to form two distinct OE, called “water nose” and “air nose”, and Bcl11b starts expressing in the future air nose to allow the expression of class II ORs. In mice, class I and class II OSNs are Bcl11b-negative and -positive, respectively, as in adult frog. In mouse OSNs, the class I OR enhancer J-element is active in the absence of Bcl11b, whereas the presence of Bcl11b suppresses the J-element enhancer activity to permit choosing a functional class II enhancer from the class II enhancer repertoire spread through chromosomes to activate transcription of class II OR gene. b Schematic representation of changes in olfactory behavior caused by the biased OR class choice of OSNs. Behavioral outputs against two distinct aversive odorants, 2MBA (spoiled foods odor) and TMT (predators’ odor) depend on the populations of class I and class II OSNs. 2MBA, which is mainly detected by class I OSNs, induces stronger aversive response in Bcl11b cKO mice (class I-dominant) and less aversive in Bcl11b gain-of-function mutant mice (class II dominant). Class II-responsible odorant, TMT induces weaker aversive response when the population of class II OSNs is decreased
