Molecular tuning of odorant receptors and its implication for odor signal processing

Abstract

The discovery of the odorant receptor (OR) family by Buck and Axel in 1991 provided a quantum jump in our understanding of olfactory function. However, the study of the responsiveness of ORs to odor ligands was challenging due to the difficulties in deorphanizing the receptors. In this manuscript, we review recent findings of OR responsiveness that have come about through improved OR deorphanization methods, site-directed mutagenesis, structural modeling studies, and studies of OR responses in situ in olfactory sensory neurons. Although there has been a major leap in our understanding of receptor-ligand interactions and how these contribute to the input to the olfactory system, an improvement of our understanding of receptor structure and dynamics and interactions with intracellular and extracellular proteins is necessary.

Figure 1

Odorant responses of rat olfactory sensory neurons in situ. Two electrodes were inserted though an opening in the dorsal surface of a rat head to reach the olfactory epithelium. A large diameter tip electrode recorded the electroolfactogram (A), whereas a metal-filled sharp electrode recorded single olfactory sensory neuron action potential activity (B). The odorant ethyl butyrate was delivered in the air phase for the duration of the bar at the top of each recording. Reprinted from Gesteland and Sigwart (1977) with permission from Elsevier.

Figure 2

Responsiveness of OR I7 to C8 aldehydes. This figure is taken with permission from the study of Araneda et al. (2000) where they recorded odor responses for I7 through EOG recordings in rats whose olfactory epithelium had been transduced with an adenovirus that expressed rat I7 (Ad-I7). (A) Comparison of the EOG responses to analogs of octanal. All compounds are shown at 10⁻³ M except for citral (10⁻² M). Citral, only at this high concentration, produced a small but significantly increased response (P < 0.001). At all concentrations tested, 2,5,7-trimethyl-2-octenal did not produce a significant increase in responses in infected animals. All other C8 analogs had increased responses (P < 0.001) at 10⁻³ M. Control responses were EOG responses from epithelium transduced with adenovirus that did not carry the coding region for I7. (B) Citral reduced octanal responses, as shown in Ca²⁺ imaging, in isolated olfactory neurons expressing I7 receptors. I7-expressing cells were recognized by the presence of GFP. Octanal (10 and 30 μM, top and bottom panels, respectively) produced an increase in Ca²⁺ as shown by the change in the emitted light. In the presence of citral (100 μM), top, but not in the presence of 2,5,5-trimethyl-2-octenal (100 μM), bottom, the response to octanal is reduced.

Figure 3

Recording odorant-induced responses from olfactory sensory neurons expressing the M71 OR. (A) The introduction of IRES-tauGFP into the mouse genome following the coding sequence for the M71 OR allowed the identification and visualization of all M71-expressing olfactory sensory neurons, which are found in the most dorsal zone on the olfactory turbinates. Scalebar, 250 μm. (B) Screening of GFP-positive olfactory sensory neurons using Ca²⁺ imaging and a variety of odorants revealed the M71 agonists acetophenone (Acp) and benzaldehyde (Bnz). Note the ∼10-fold shift difference in sensitivity. Modified from Bozza et al. (2002) with permission from the Society of Neuroscience.

Figure 4

Structure of LUSH^D118A Mutant Protein Mimics cVA-Bound LUSH Protein. (A) Ribbon diagram comparing one monomer from the asymmetric unit of the LUSH–cVA complex (blue) with the LUSH^D118A (cyan), apo-LUSH (green), and LUSH–butanol complex (yellow). The cVA is shown as a stick model in magenta. Arrow indicates position of structural changes in the C-terminal loop shared by LUSH^D118A (cyan) and LUSH–cVA (blue) but distinct from the LUSH–butanol complex (yellow) and apo-LUSH (data not shown). (B) Comparison of the structures of the 117–121 loop in the LUSH–butanol (yellow) and LUSH–cVA (blue) complexes. (C) Comparison of the structures of the same regions between LUSH–cVA (blue) and apo-LUSH^D118A (cyan). LUSH–cVA and LUSH^D118A adopt similar conformations. (D) Stereo representation of the electron density defining the loop between residues E115–M122 in the LUSH^D118A structure solved without cVA. The position of A118 is indicated in red. The electron density is from a 2F_o−F_c map contoured at 1σ. Reproduced with permission from Laughlin et al. (2008).