Olfactory discrimination largely persists in mice with defects in odorant receptor expression and axon guidance

Thomas K Knott,Pasil A Madany,Timothy R Henion,Mei Xu,Jörg Strotmann,Gerald A Schwarting,Ashley A Faden

doi:10.1186/1749-8104-7-17

Thomas K Knott, Pasil A Madany + Show 5 more

Open Access

https://doi.org/10.1186/1749-8104-7-17

Copy DOI

Abstract

BackgroundThe defining feature of the main olfactory system in mice is that each olfactory sensory neuron expresses only one of more than a thousand different odorant receptor genes. Axons expressing the same odorant receptor converge onto a small number of targets in the olfactory bulb such that each glomerulus is made up of axon terminals expressing just one odorant receptor. It is thought that this precision in axon targeting is required to maintain highly refined odor discrimination. We previously showed that β3GnT2−/− mice have severe developmental and axon guidance defects. The phenotype of these mice is similar to adenylyl cyclase 3 (AC3) knockout mice largely due to the significant down-regulation of AC3 activity in β3GnT2−/− neurons.ResultsMicroarray analysis reveals that nearly one quarter of all odorant receptor genes are down regulated in β3GnT2−/− mice compared to controls. Analysis of OR expression by quantitative PCR and in situ hybridization demonstrates that the number of neurons expressing some odorant receptors, such as mOR256-17, is increased by nearly 60% whereas for others such as mOR28 the number of neurons is decreased by more than 75% in β3GnT2−/− olfactory epithelia. Analysis of axon trajectories confirms that many axons track to inappropriate targets in β3GnT2−/− mice, and some glomeruli are populated by axons expressing more than one odorant receptor. Results show that mutant mice perform nearly as well as control mice in an odor discrimination task. In addition, in situ hybridization studies indicate that the expression of several activity dependent genes is unaffected in β3GnT2−/− olfactory neurons.ConclusionsResults presented here show that many odorant receptors are under-expressed in β3GnT2−/− mice and further demonstrate that additional axon subsets grow into inappropriate targets or minimally innervate glomeruli in the olfactory bulb. Odor evoked gene expression is unchanged and β3GnT2−/− mice exhibit a relatively small deficit in their ability to discriminate divergent odors. Results suggest that despite the fact that β3GnT2−/− mice have decreased AC3 activity, decreased expression of many ORs, and display many axon growth and guidance errors, odor-evoked activity in cilia of mutant olfactory neurons remains largely intact.

Highlights

The defining feature of the main olfactory system in mice is that each olfactory sensory neuron expresses only one of more than a thousand different odorant receptor genes
Odorant receptor alterations in β3GnT2−/− mice Five separate samples of Olfactory epithelium (OE) were dissected from adult β3GnT2−/− mice and littermate controls and mRNA was isolated from each sample
A null/Wild type (WT) ratio was generated that represents the mean of the five pairwise comparisons, Of 1,144 probe sets for Odorant receptor olfactory sensory neurons (OSNs) (OR) transcripts, results indicated that mRNAs for 867 of the ORs were expressed at similar levels in WT and β3GnT2−/− OEs, including OR37 and M72

Summary

Introduction

The defining feature of the main olfactory system in mice is that each olfactory sensory neuron expresses only one of more than a thousand different odorant receptor genes. Much of the influence that was initially attributed to odorant receptors (ORs) themselves has been shown to result from ORdependent cAMP signaling [7] In this regard, an important question is how olfactory sensory neurons (OSNs) regulate adenylyl cyclase 3 (AC3) activity, since cAMP is responsible for activation of the cyclic nucleotide-gated channel (CNG) in cilia and for downstream signaling in axons via pathways that may regulate the activities of protein kinase A and the transcription factor, CREB [4]. The severe olfactory developmental and axon guidance abnormalities in β3GnT2−/− mice appear in many respects to phenocopy wiring defects described for AC3 knockout mice where olfactory cAMP signaling is reduced [10,11,12] In both models, M72 axons are misguided to multiple heterotypic glomeruli primarily in the ventromedial OB, and P2 axons are mostly lost from the postnatal OB. These changes in axon growth and guidance are due in part to the decrease in cAMP-dependent signaling in both mouse models resulting in altered expression of important axon guidance cues, such as neuropilin-1 and semaphorin-3A [6,13]

Methods

Results

Discussion

Conclusion