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Olfactory Receptor Proteins in Axonal Processes of Chemosensory Neurons

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Olfactory Receptor Proteins in Axonal Processes of Chemosensory Neurons 7754 • The Journal of Neuroscience, September 1, 2004 • 24(35):7754–7761 Cellular/Molecular Olfactory Receptor Proteins in Axonal Processes of Chemosensory Neurons Joerg Strotmann, Olga Levai, Joerg Fleischer, Karin Schwarzenbacher, and Heinz Breer Institute of Physiology, University of Hohenheim, 70593 Stuttgart, Germany Olfactoryreceptorsaresupposedtoactnotonlyasmolecularsensorsforodorantsbutalsoascellrecognitionmoleculesguidingtheaxons of olfactory neurons to their appropriate glomerulus in the olfactory bulb. This concept implies that olfactory receptor proteins are located in sensory cilia and in the axons. To approach this critical issue, antibodies were generated against two peptides, one derived from olfactory receptor mOR256–17, one derived from the “mOR37” subfamily. By means of immunohistochemistry and double-labeling studies using transgenic mouse lines as well as Western blot analyses, it was demonstrated that the newly generated antibodies specifi- cally recognized the receptor proteins. To scrutinize the hypothesis that olfactory receptor proteins may also be present in the axonal processes and the nerve terminals, serial sections through the olfactory bulb were probed with the antibodies. Two glomeruli in each bulb were stained by anti-mOR256–17, one positioned in the medial, one in the lateral hemisphere. Fiber bundles approaching the glomeruli through the outer nerve layer also displayed intense immunofluorescence. A similar picture emerged for the antibody anti-mOR37, a small number of glomeruli in the ventral domain of the bulb was stained. On serial sections through the olfactory bulb of mOR37- transgenic mouse lines, double-labeling experiments demonstrated that distinct immunoreactive glomeruli corresponded to glomeruli that were targeted by neurons expressing a particular member of the mOR37 receptor subfamily. These data indicate that olfactory receptor (OR) proteins are indeed present in the axonal processes and nerve terminals of olfactory sensory neurons, thus supporting the notion that ORs may participate in the molecular processes underlying the fasciculation and targeting of olfactory axons. Key words: olfactory receptor; axon guidance; antibody; olfactory bulb; projection; transgenic mouse Introduction cells with the coding region for one OR gene replaced by that of The projection patterns of olfactory sensory neurons (OSNs) are another redirected their axons to a location expected for the in- supposed to provide the basis for encoding the quality of a che- troduced receptor (Mombaerts et al., 1996; Wang et al., 1998; mosensory stimulus in a topographic map of activity in the olfac- Bozza et al., 2002). Moreover, deletion of one particular OR gene tory bulb (for review, see Buck, 1996, 2000; Mori et al., 1999; coding region destroyed glomerular convergence (Wang et al., Bozza and Mombaerts, 2001; Mombaerts, 2004). Although neu- 1998); in more recent studies it was found that genetic disruption rons expressing a given olfactory receptor (OR) gene appear to be of one OR gene permitted the expression of other ORs in that randomly dispersed within one of the four epithelial expression particular neuron population and these cells did not converge zones (Ressler et al., 1993; Vassar et al., 1993; Strotmann et al., any longer, but targeted multiple glomeruli, apparently directed 1994b), their axonal projections converge onto one medial and by the novel OR they expressed (Serizawa et al., 2003; Lewcock one lateral glomerulus in the olfactory bulb. This convergence, and Reed, 2004). Thus, the OR protein seems to be critical for initially observed by in situ hybridization that detected receptor axon sorting, converging, and targeting. The tight linkage be- mRNA in the axon terminals (Ressler et al., 1994; Vassar et al., tween the choice of a receptor type and the site of axonal conver- 1994), was subsequently clearly documented by means of genet- gence in the bulb raised the possibility that in OSNs, receptor ically manipulated mice that generated a bicistronic mRNA en- proteins may fulfill two distinct roles; in the cilia recognizing coding the OR as well as a marker protein. This approach made it odorous molecules from the environment and in the axons rec- possible to selectively visualize the axons and nerve terminals of ognizing molecular cues in the olfactory bulb. all cells expressing the same OR (Mombaerts et al., 1996; Wang et The question whether OR proteins are indeed present in the al., 1998; Strotmann et al., 2000; Zheng et al., 2000; Potter et al., axons and nerve terminals of OSNs is thus of fundamental im- 2001). Several lines of evidence suggest that the OR proteins may portance toward an understanding of the functional wiring in the be directly or indirectly involved in convergent axon projection olfactory system. In this study, antibodies were generated against and glomerulus formation. It was found in transgenic mice that unique epitopes of distinct OR types and used in immunohisto- chemical experiments to visualize the receptor proteins in whole- mount preparations and tissue sections of the olfactory system. Received June 30, 2004; accepted July 21, 2004. This work was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 495, and the Leibniz Program. We thank Kerstin Bach and Olaf Selchow for excellent technical assistance. Materials and Methods CorrespondenceshouldbeaddressedtoDr.HeinzBreerattheaboveaddress.E-mail:[email protected]. Animals and tissue preparation. Wild-type C57/BL6 mice were obtained DOI:10.1523/JNEUROSCI.2588-04.2004 from Charles River (Sulzfeld, Germany). Transgenic mice from the lines Copyright © 2004 Society for Neuroscience 0270-6474/04/247754-08$15.00/0 mOR37A-green fluorescent protein (GFP), mOR37-B-lacZ and Strotmann et al. • Olfactory Receptor Proteins in Chemosensory Axons J. Neurosci., September 1, 2004 • 24(35):7754–7761 • 7755 tion at 4000 ϫ g and 4°C for 5 min. The supernatant was collected, and the pellet was resuspended in Ringer’s solution containing 10 mM CaCl2 and centrifuged as described above. The supernatants were combined and centrifuged at 19,000 ϫ g and 4°C for 15 min. The resulting pellet containing the cilia was resuspended in a small volume of MES buffer (in mM: 10 Tris, 3 MgCl2, and 2 EGTA, pH 7.4), frozen in liquid nitrogen and stored at –70°C. Preparation of a ciliary membrane fraction. Olfactory cilia resuspended in MES buffer were centrifuged at 15,000 ϫ g and 4°C for 30 min. The pellet was resuspended in 5 mM Tris, pH 7.5. After 30 min on ice, followed by centrifugation at 15,000 ϫ g and 4°C for 30 min, the pellet was resus- pended in 5 mM Tris, pH 7.5. Protein concentrations were determined by standard procedures (Bradford, 1976). SDS-PAGE and Western blot analysis (immunoblotting). Ten micro- grams of the ciliary membrane fraction were supplemented with electro- phoresis buffer (7.5 mM DTT, 2.5% SDS, 45% glycerol, and 0.3 M Tris, pH 6.8, 0.045% bromphenol blue), boiled for 3 min, separated by SDS- PAGE, and transferred to nitrocellulose membranes using standard pro- tocols. For Western blot analysis, nonspecific binding sites were blocked with 5% milk powder in TBST (10 mM Tris, pH 7.4, 150 mM NaCl, and 0.05% Tween 20) and probed with the primary antibody (4.76 ϫ 10 Ϫ4 ␮ ␮ ␣ g/ l) in TBST with 2.5% milk powder [the rabbit anti-G s/olf antibody used for the loading control was purchased from Santa Cruz Biotechnol- ogy (Santa Cruz, CA) and diluted 1:1500]. To test the specificity of anti- mOR37, the antibody was preincubated with a fivefold excess of the corresponding blocking peptide. The secondary antibody (anti-rabbit- peroxidase; Bio-Rad, Hercules, CA) was diluted in 2.5% milk powder in TBST and detected using the ECL system (Amersham Biosciences, Ar- lington Heights, IL). Immunohistochemistry on tissue sections. For immunohistochemistry on nasal epithelium, the bones covering the lateral nasal walls of adult animals were carefully removed using a bone cutter. Heads from postna- tal day 6 and embryonic animals were entirely processed after removal of the lower jaw. Olfactory bulbs from adult animals were carefully re- moved from the cranium. The specimens were then either fixed for 50 min (adult animals) or 4 hr (embryos) on ice in 4% paraformaldehyde for anti-mOR256–17 or 30 min in 4% paraformaldehyde on ice for anti-mOR37 (postnatal day 6 animals). Before sectioning, the specimen Figure 1. A, B, View onto the whole-mount nasal turbinates probed with anti-mOR256–17 were incubated in 25% sucrose overnight. Subsequently, 16–20 ␮m sec- antibody (A) or anti-mOR37 antibody (B), respectively. II—IV, Endoturbinates. Scale bars: (in tions were generated using a Leica (Nussloch, Germany) CM3500 cryo- ␮ ␮ A) 500 m; (in B) 200 m. C, D, View onto the medial surface of endoturbinate II after incuba- stat at –20°C and transferred to Superfrost slides (Menzel). Immunohis- tion of anti-mOR256–17 antibody (C) or anti-mOR37 antibody (D). The knobs and cilia of tochemical stainings were performed with anti-mOR256–17 (1.5 ϫ ␮ Ϫ distinct OSNs are strongly immunoreactive. Scale bars, 40 m. E, F, High magnification of 10 3 ␮g/␮l) in PBS containing 10% normal goat serum (NGS), 0.3% anti-mOR256–17-immunoreactive (E) or anti-mOR37-immunoreactive (F) OSN; the olfactory Triton X-100 overnight at 4°C, or with anti-mOR37 (1.48 ϫ 10 Ϫ3 ␮g/␮l) ␮ knob and cilia radiating in all directions are fluorescent. Scale bars, 10 m. in PBS containing 10% NGS and 0.1% Triton X-100 at 4°C overnight. After washing in PBS, the sections were incubated with anti-rabbit Al- mOR37-C-lacZ (Strotmann et al., 2000) were kept at the University of exa488 (Molecular Probes Europe BV, Leiden, Netherlands) for 2 hr at Hohenheim transgenic core facility; housing conditions fulfilled the an- room temperature. imal welfare guidelines. Timed pregnant mice were obtained by 2 hr For double-labeling experiments with anti-mOR37 in the mOR37- mating period and subsequent vaginal plug inspection.
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