Molecular Organization of the Olfactory Septal Organ

Abstract

The septal organ in the mammalian nose is a distinct chemosensory organ sitting in the air path. To gain insights into its organization and function, we analyzed the chemoreceptors expressed in this area. By combining cDNA cloning, Affymetrix (Santa Clara, CA) genechips covering all the mouse olfactory receptor genes, and in situ hybridization, we achieved a relatively complete expression profile of the olfactory receptor genes in the septal organ. The majority of the septal neurons express only a few receptor genes in varying patterns, with the top one in ϳ50% of the cells and the top eight together in ϳ93% of the cells. We demonstrated that a single neuron expresses only one receptor by a thorough combination of all the major septal receptor genes in double-labeling studies. These septal receptor genes do not form a single subfamily. Instead, these genes are distributed on a few major branches of the phylogenetic tree covering all the mouse olfactory receptors. Most of these genes are also concentrated in certain areas within the most ventral zone of the main olfactory epithelium, although their expression patterns do not match those in the septal organ. In contrary to the previous view of random distribution, our results indicate that certain olfactory receptors form "hot spots" in the nose.

Activity plays a role in eliminating olfactory sensory neurons expressing multiple odorant receptors in the mouse septal organ

“…This is distinct from the previous reports in rat (Rawson et al, 2000), zebrafish (Sato et al, 2007), and Drosophila (Goldman et al, 2005), in which certain receptors are coexpressed in a subset of OSNs all the time or with a high tendency. Second, there are additional ORs (identified or unidentified) expressed in the septal organ (Kaluza et al, 2004;Tian and Ma, 2004). Third, detection of coexpression is essentially limited by the sensitivity of in situ hybridization, and individual RNA probes in a mixture may have different labeling efficiencies leading to an underestimation of colocalization.…”

“…Coexpression of multiple ORs in single neurons is likely to occur in the main olfactory epithelium as well since all the predominant septal organ receptors are also expressed in the most ventral zone (Tian and Ma, 2004). However, individual OSNs with multiple receptors are difficult to detect in the main epithelium using the same approach, because it requires many more receptor types to cover a significant portion of the sensory neurons.…”

“…The nine specific probes used here (the sequences are listed in Supplementary sTable 2) presumably only detect their target OR genes, because seven of them share less than 40% of nucleotide homology with any other OR in the genome, and the other two (MOR235-1 and MOR236-1 sharing 75% homology with each other) label different subsets of cells in double in situ hybridization (Tian and Ma, 2004). The tissue sections were hybridized with mixed FLU-and DIG-labeled probes overnight at 65 °C in the hybridization solution (50% deionized formamide, 10 mM Tris-Cl (pH 8.0), 10% dextran sulfate, 1X Denhardt's solution, 200 μg/ml tRNA, 0.6 M NaCl, 0.25% SDS and 1 mM EDTA), followed by high-stringency washing steps sequentially in 2x, 0.2x and 0.1x SSC at 65 °C.…”

Differential development of odorant receptor expression patterns in the olfactory epithelium: A quantitative analysis in the mouse septal organ

“…To minimize variations from the tissue source, adjacent sections from each animal (E16 to P7) were tested for different OR probes and each OR probe was hybridized onto only one section from a single septal organ. The two sides of a single animal were treated as two independent septal organs, due to significant variations between them (Kaluza et al, 2004;Tian and Ma, 2004). NBT/BCIP stained cells appeared as dark brown dots in the tissue sections and ~85-90% of the dots were unequivocally individual OSNs with a characteristic cell contour.…”

“…The presumptive septal organ starts to appear at about E16 in the rat and gradually separates from the MOE (Giannetti et al, 1995;Oikawa et al, 2001). In adult mice, the septal organ expresses nine abundant ORs in greater than 90% of the cells (Kaluza et al, 2004;Tian and Ma, 2004), which makes it practical to perform a detailed analysis on development of the OR expression patterns in this region. In the current study, we quantified the densities of OSNs expressing each of the nine ORs in the mouse septal organ from E16 to 3 months by in situ hybridization.…”