Monoallelic expresion of the odourant receptor gene and axonal projection of olfactory sensory neurones

Background: We have previously generated transgenic mice carrying the murin e odourant receptor gene, MOR28, tagged with lacZ. In this animal, the endo genous MOR28 is differently tagged with GFP. It was found that the transgen ic and endogenous MOR28 genes are expressed in a mutually exclusive manner and that the two sets of olfactory sensory neurones (OSNs), each expressing either the transgenic or the endogenous MOR28, project their axons to sepa rate glomeruli. Results: Our fluorescent in situ hybridization (FISH) revealed that the two endogenous alleles of MOR28 are also mutually excluded for their transcrip tional activation. Therefore, we studied whether there would be any segrega tion in the projection of the two subsets of OSNs: one set expressing the p aternal and the other expressing the maternal allele. It was found that the OSNs for both alleles shared the same glomerulus for their projection, but the projection targets were segregated within the glomerular structure. Conclusion: Two subsets of neurones expressing either the transgenic or the endogenous MOR28 target their axons to two separate glomeruli based on the differences in the genetic backgrounds, nature of tagging, and chromosomal locations. In contrast, neurones expressing a maternal or paternal allele share the same glomeruli, but tend to target to segregated areas within the glomerular structure. The segregation was more prominent with increased di fferences in the genetic background between the two alleles.