Olfactory signal transduction
Our lab investigates how olfactory receptor neurons, the principle neurons which detect odorants and convert their presence into a nervous signal, function. In vertebrates olfactory receptor neurons are located in the olfactory epithelium in the nasal cavity. Odor molecules, carried by the inspired air, dissolve into the mucus which lines the nasal cavity to contact and activate olfactory receptor proteins located in the membrane of cilia which extend from olfactory receptor neurons into the mucus (Figure 1).
Figure 1: A schematic of the olfactory epithelium with olfactory receptor neurons, supporting and basal cells. B. Cross-section of a tip of a cilium with transduction components. Arrows in green and red indicate events involved in response activation and termination respectively.
The cilia are the site of olfactory transduction and they contain the signaling molecules to generate the odorant-induced depolarization. In short, odorant receptor activation leads via a G protein-coupled cascade to an increase in intracellular cAMP and opening of the olfactory cyclic nucleotide-gated (CNG) ion channel. The ensuing influx of Na+ and Ca2+ begins to depolarize the neurons and this depolarization is amplified by the opening of an excitatory Ca2+-activated Cl- conductance, which can carry up to 80 % of the odorant-induced receptor current. The high intracellular Cl- concentration to support excitatory Cl- efflux is maintained by a Na+/K+/Cl- cotransporter. The odor response is terminated by several negative feedback mechanisms which often involve Ca2+, degradation of cAMP by phosphodiesterases and removal of Ca2+ by Na+/Ca2+ exchange.
Figure 2: Suction pipette recordings from a frog olfactory receptor neuron to demonstrate the presence and role of Na+/Ca2+ exchange. Upon odorant stimulation and Ca2+ influx removal of external Na+ incapacitates Na+/Ca2+ exchange, thus preventing the fall for intracilary Ca2+ and closure of the Ca2+ avtivated Cl- channel. Modified from Reisert & Matthews, 1998.
Figure 3: Antibody staining of an olfactory epithelium to localize the Na+/K+/Cl- cotransporter. The ciliary layer shows strong staining for the A2 subunit of the CNG channel (red), while dendrites and cell bodies stained for the Na+/K+/Cl- cotransporter (green). Blue: nuclear stain. Modified from Reisert et al. 2005.
Our research has contributed to understanding of:
- Action potential coding of odorant stimulation and the effect of adaptation on the odor response.
- The role of Na+/Ca2+ exchange in response termination and recovery from adaptation in frog and mouse olfactory receptor neurons.
- Odor-evoked ciliary Ca2+ dynamics.
- The interactions of CNG channel, Cl- channel and the Na+/Ca2+ exchanger.
- The role of CNG channel subunits and their calmodulin-binding sites in cellular targeting and response termination.
- The function of Na+/K+/Cl- contransport in olfaction and its cellular localization.
- Binding kinetics of odorants to the odorant receptor.
- The contribution of olfactory marker protein (OMP) as a central player in the regulation of response kinetics.
Current collaborations:
Signal transduction components and their role in olfaction. This project utilizes electrophysiological methods and genetically modified mice to investigate the contribution of individual players in olfactory transduction in shaping the odorant response. Collaboration with Dr. H. Zhao, Department of Biology, Johns Hopkins University, Baltimore, USA.
Towards a mathematical understanding of olfactory transduction. Combining genetics, physiology and mathematics we are developing a mathematical model of olfactory transduction. Collaboration with Dr. D. Dougherty, Dept. of Statistics and Probability, Michigan State University, East Lansing, USA and Dr. H. Zhao, Department of Biology, Johns Hopkins University, Baltimore, USA.
The role of membrane structure on olfactory transduction. We are studying the relation between membrane structure and the proteins embedded therein using olfactory receptor neurons as a model system. Collaboration with Dr. K Gaus, Centre for Vascular Research, University of New South Wales, Sydney, Australia.
Ca2+ dynamics and homeostatic mechanisms in salamander olfactory cilia. Simultaneous measurement of odorant-induced ciliary Ca2+ signals and receptor currents are performed to investigate homeostatic mechanisms of Ca2+ regulations. Collaboration with Dr. H.R. Matthews and S. Antolin, Department of Physiology, Development and Neuroscience, Cambridge University, Cambridge, United Kingdom.
References
1. Reisert, J. & Matthews, H. R. (1998). Na+-dependent Ca2+ extrusion governs response recovery in frog olfactory receptor cells. Journal of General Physiology 112, 529-535.
2. Reisert, J. & Matthews, H. R. (1999). Adaptation of the odour-induced response in frog olfactory receptor cells. Journal of Physiology 519, 801-813.
3. Reisert, J. & Matthews, H. R. (2000). Adaptation-induced changes in sensitivity in frog olfactory receptor cells. Chemical Senses 25, 483-486.
4. Reisert, J. & Matthews, H. R. (2001). Response properties of isolated mouse olfactory receptor cells. Journal of Physiology 530, 113-122.
5. Reisert, J. & Matthews, H. R. (2001). Responses to prolonged odour stimulation in frog olfactory receptor cells. Journal of Physiology 534, 179-191.
6. Reisert, J. & Matthews, H. R. (2001). Simultaneous recording of receptor current and intraciliary Ca2+ concentration in salamander olfactory receptor cells. Journal of Physiology 535, 637-645.
7. Reisert, J., Bauer, P. J., Yau, K. W. & Frings, S. (2003). The Ca-activated Cl channel and its control in rat olfactory receptor neurons. Journal of General Physiology 122, 349-364.
8. Reisert, J., Lai, J., Yau, K. W. & Bradley, J. (2005). Mechanism of the excitatory Cl- response in mouse olfactory receptor neurons. Neuron 45, 553-561.
9. Bhandawat, V., Reisert, J. & Yau, K. W. (2005). Elementary response of olfactory receptor neurons to odorants. Science 308, 1931-1934.
10. Michalakis, S., Reisert, J., Geiger, H., Wetzel, C., Zong, X., Bradley, J., Spehr, M., Hüttl, S., Gerstner, A., Pfeifer, A., Hatt, H., Yau, K. W. & Biel, M. (2006). Loss of CNGB1 protein leads to olfactory dysfunction and subciliary cyclic nucleotide-gated channel trapping. Journal of Biological Chemistry 281, 35156-35166
11. Reisert, J., Yau, K. W. & Margolis, F. L. (2007). Olfactory marker protein modulates the cAMP kinetics of the odour-induced response in cilia of mouse olfactory receptor neurons, Journal of Physiology, 585, 731-740.
12. Song, Y., Cygnar, K. D., Sagdullaev, B., Valley, M., Hirsh, S., Stephen, A., Reisert, J, Zhao, H. (2008). Ca2+/calmodulin-mediated fast desensitization by the B1b subunit of the CNG channel affects response termination but not sensitivity to recurring stimulation in olfactory sensory neurons. Neuron, 58, 374-386.
Reviews, Book Chapters and Perspectives
1. Matthews, H. R. & Reisert, J. (2003). Calcium, the two-faced messenger of olfactory transduction and adaptation. Current Opinion in Neurobiology 13, 469-475.
2. Reisert, J. & Bradley, J. (2004). Vertebrate olfactory signal transduction and the interplay of excitatory anionic and cationic currents. In Transduction channels in sensory cells. ed Frings, S. & Bradley, J. Wiley-VCH Verlag, Weinheim.
3. Reisert, J (2005). Signal transduction in vertebrate olfactory receptor cells. In "Lectures in Mathematical Biosciences, Calcium in Signal Transduction". ed Sneyd, J. Springer Verlag.
4. Bradley, J., Reisert, J. & Frings, S. (2005). Regulation of cyclic nucleotide-gated channels. Current Opinion in Neurobiology 15, 343-349.
5. Reisert, J. & Bradley, J. (2005). Activation of olfactory cyclic-nucleotide gated channels revisited. Journal of Physiology, 569, 4-5.
6. Reisert, & Margolis, F. L. (2008). Olfactory marker protein: a gift to molecular biologists, an enigma to physiologists, Physiology News, 73, 27-29.