A Sensor for Low Environmental Oxygen in the Mouse Main Olfactory Epithelium

Abstract

Sensing the level of oxygen in the external and internal environments is essential for survival. Organisms have evolved multiple mechanisms to sense oxygen. No function in oxygen sensing has been attributed to any mammalian olfactory system. Here, we demonstrate that low environmental oxygen directly activates a subpopulation of sensory neurons in the mouse main olfactory epithelium. These neurons express the soluble guanylate cyclase Gucy1b2 and the cation channel Trpc2. Low oxygen induces calcium influx in these neurons, and Gucy1b2 and Trpc2 are required for these responses. In vivo exposure of a mouse to low environmental oxygen causes Gucy1b2-dependent activation of olfactory bulb neurons in the vicinity of the glomeruli formed by axons of Gucy1b2+ sensory neurons. Low environmental oxygen also induces conditioned place aversion, for which Gucy1b2 and Trpc2 are required. We propose that this chemosensory function enables a mouse to rapidly assess the oxygen level in the external environment.

Figure 1

Type B Cells Respond to Low Oxygen (A) Confocal image of intrinsic fluorescence of type B cells from a Gucy1b2-GFP −/− mouse (8 weeks) in an ex vivo en face whole-mount preparation. Scale bar, 5 μm. (B) Decreasing the PO₂ from 161 to 138 mmHg in the bath solution evoked a transient rise in intracellular Ca²⁺ in type B cells within seconds. No responses occurred in non-type B cells. A Ca²⁺ response is defined as a stimulus-dependent deviation in fluorescence ratio exceeding twice the SD of the mean baseline noise. (C) Repeatability of type B cell responses to low oxygen (138 mmHg). KCl, 30 mM. (D) Examples of Ca²⁺ responses to various levels of low oxygen (in mmHg) in a type B cell. (E) Stimulus-response curves obtained from single type B cells (n = 18; dashed gray lines). Each cell was stimulated with 20 s pulses of solution containing distinct oxygen levels (mmHg). Relative changes in peak fluorescence ratio induced by a given stimulus were normalized to the maximum peak fluorescence ratio of a given cell. Curves are fit using the Hill equation combined with an iterative Levenberg-Marquardt nonlinear, least-squares fitting routine, defining EC₅₀, slope (−22.9 ± 3.0), and saturation (1.0 ± 0.05). Solid black curve represents a fit of the mean stimulus-response curve using all measured data points (chi-square 0.201823; p = 0.99). Responses were recorded during a 4 min trial period with a 1 min recovery period between trials (illumination off). Each stimulus was tested one to three times per cell. Numbers are presented as mean ± SD. (F) Responses of a type B cell to various chemostimuli. Low oxygen (122 mmHg), Henkel 100 (1:100), diluted urine (1:100), predator odor mix (TMT, [2,5-dihydro-2,4,5-trimethylthiazole], PEA [β-phenylethylamine], 2-PT [2-propylthietane], and 2,6-DMP [2,6-dimethylpyrazine], each at 100 μM), and KCl (30 mM).

Figure 2

Low-Oxygen-Evoked Ca²⁺ Responses of Type B Cells Require Ca²⁺ Entry, Gucy1b2, and Trpc2 (A) Recording example indicating that Ca²⁺ responses to low oxygen (122 mmHg) depend on Ca²⁺ entry into type B cells. Removal of external Ca²⁺ eliminated the response in a reversible manner. (B) Recording examples of Ca²⁺ responses in type B cells from Gucy1b2-GFP −/− mice, Gucy1b2-KO^∗ −/− mice, and Trpc2-KO^∗ −/− mice. Cells were stimulated with low oxygen (122 mmHg) or KCl (30 mM). (C) Group data indicating that the Ca²⁺ responses to low oxygen require Gucy1b2 (Gucy1b2-KO^∗ −/−, n = 4 mice) and Trpc2 (Trpc2-KO^∗ −/−, n = 5 mice) (PO₂: 138, 129, or 122 mmHg) (ANOVA: F(14,398) = 12.791, p < 0.0001; LSD: ^∗p < 0.05, ^∗∗p < 0.001, ^∗∗∗p < 0.0001). Responses to 8-bromo-cGMP (8-br-cG; 500 μM) or KCl (30 mM) do not differ between the three genotypes (LSD: p = 0.66–0.71). Maximum fluorescence ratios are plotted; number of cells for each genotype and stimulus condition is indicated above each bar. No significant difference was observed between Gucy1b2-GFP −/− mice and Gucy1b2-KO^∗ +/− mice in the responsiveness of their type B cells (n = 6; LSD: p = 0.55). (D and E) Recording example (D) and group data (E) (n = 3 mice) indicating that type B cells in Cnga2-KO^∗ MUT mice have normal Ca²⁺ responses to low oxygen but lack responsiveness to 8-br-cGMP (500 μM). Number of cells is indicated above each bar. (F) Recording example indicating that Ca²⁺ responses to low oxygen in a type B cell (Gucy1b2-GFP −/− mouse) are eliminated after a 10 min treatment with ODQ (10 μM) (paired t test: t = 3.835, ^∗p < 0.05), whereas the response to 8-br-cGMP (500 μM) is not. (G–I) Recording examples (G and I) and group data (H) showing the effects of the PKG inhibitor KT5823 (10 μM) on type B cell Ca²⁺ responses to low oxygen (138 mmHg) or 8-br-cGMP (50 μM). Responses to low oxygen are blocked by KT5823 in a reversible manner (ANOVA: F(2,20) = 26.856, p < 0.0001; LSD: ^∗∗∗p < 0.0001), whereas responses to 8-br-cGMP remain unaffected (p = 0.17). Number of independent recordings is indicated above each bar. (J and K) Lack of effect of the NOS inhibitor L-NAME (100 μM) on type B cell Ca²⁺ responses to low oxygen (138 mmHg) (paired t test: t = 2.359, p = 0.078). Results in histograms are presented as mean ± SEM.

Figure 3

Low Environmental Oxygen Causes Gucy1b2-Dependent Activation of Olfactory Bulb Neurons in the Vicinity of Type B Cell Glomeruli in Freely Breathing Mice (A–F) c-Fos immunoreactivity in the olfactory bulbs of Gucy1b2-GFP −/− mice (A and B), of Gucy1b2-KO^∗ +/− mice (C and D), and of Gucy1b2-KO^∗ −/− mice (E and F). Mice were exposed to normal (147 mmHg, 20%) (A, C, and E) or low (120 mmHg, 16%) (B, D, and F) oxygen. Sagittal sections were derived from olfactory bulbs. The intrinsic fluorescence of GFP or the immunoreactivity for mCherry identifies presynaptic axons and outlines the dense glomerular neuropil. The c-Fos immunoreactivity denotes the activity of postsynaptic olfactory bulb neurons. Scale bar, 50 μm. (G) Quantification of c-Fos+ nuclei reveals that exposure to low environmental oxygen caused a significant increase in neuronal activity in the vicinity of type B cell glomeruli of Gucy1b2-GFP −/− mice (used as reference mice) and Gucy1b2-KO^∗ +/− mice. Responses in these two genotypes were indistinguishable from each other (LSD: p = 0.95). By contrast, this response was absent in Gucy1b2-KO^∗ −/− mice (LSD: p = 0.14). ANOVA genotype: F(2,229) = 28.767, p < 0.0001; ANOVA treatment: F(1,229) = 105.600, p < 0.0001; LSD: ^∗∗∗p < 0.0001. The number of sections analyzed ranged from 23 to 57 per genotype and treatment. Number of mice is indicated above each bar. (H) c-Fos immunoreactivity in the vicinity of type A cell glomeruli in the olfactory bulbs of Gucy1b2-KO^∗ +/− mice (left panels) and Gucy1b2-KO^∗ −/− mice (right panels). Mice were exposed to normal (147 mmHg, 20%) or low (120 mmHg, 16%) oxygen. Scale bar, 50 μm. No c-Fos immunoreactivity was detetectable under these conditions. (I) Quantification of c-Fos+ nuclei reveals that exposure to low oxygen caused no significant increase in neuronal activity in the vicinity of type A cell glomeruli of Gucy1b2-KO^∗ +/− or Gucy1b2-KO^∗ −/− mice (ANOVA: F(1,251) = 0.134, p = 0.72). Basal c-Fos immunoreactivity was very low, and the two genotypes were indistinguishable from each other (ANOVA: F(1,251) = 2.712, p = 0.10). The number of sections analyzed ranged from 18 to 124 per genotype and treatment. Number of mice is indicated above each bar. (J) Plot of the oxygen levels measured in the ambient air of the behavioral chambers as used during in vivo experiments of activation of olfactory bulb neurons and conditioned place aversion. Concentrations were measured in percentage and converted into PO₂ (mmHg). Number of independent measurements is indicated. PCO₂ was <0.1% (0.76 mmHg). Results in histograms are presented as mean ± SEM.

Figure 4

Gucy1b2 and Trpc2 Are Required for Conditioned Place Aversion of a Low-Oxygen Environment (A) Behavioral arena. The opening between the two chambers is indicated with a stipple line. Experiments were performed under infrared illumination. (B) Heatmap graphs obtained from trajectory plots of all tested mice in the conditioned place aversion assay demonstrating their preference of the 20%-paired chamber versus the 16%-paired chamber. (C and D) Average dwell times and preference index of Gucy1b2-GFP −/− mice (used as reference mice), Gucy1b2-KO +/− mice and Gucy1b2-KO −/− mice for the 20%- versus 16%-paired chamber. ANOVA: F(5,22) = 3.728, p < 0.042; LSD: ^∗∗p < 0.001; n.s., p = 0.79 (C); ANOVA: F(5,45) = 10.256, p < 0,002; LSD: ^∗∗p < 0.001; n.s., p = 0.28) (D). (E and F) Average dwell times and preference index of Trpc2-KO^∗ +/− mice and Trpc2-KO^∗ −/− mice for the 20%- versus the 16%-paired chamber. ANOVA: F(3,40) = 6.7629, p < 0.001; LSD: ^∗∗∗p < 0.001; n.s., p = 0.81 (E); paired t test: t(20) = 2.2353, ^∗p < 0.05 (F). The number of mice is indicated at each bar. (G) Plot of latency-to-find platform versus trial number in a Morris water maze assay for Gucy1b2-GFP −/− mice (n = 6) and Gucy1b2-KO −/− mice (n = 7). ANOVA genotype: F(1,103) = 1.22, p = 0.27. (H and I) Analysis of reference memory in a Morris water maze assay using single probe trials in the absence of the platform (P) for Gucy1b2-GFP −/− mice (n = 6) and Gucy1b2-KO −/− mice (n = 7). Percentage of time spent in each quadrant (H; ANOVA genotype: F(1,51) = 0.043, p = 0.84) and number of platform zone crossings in quadrant 1 (I; t test: t(11) = 0.364, p = 0.72) were analyzed. Results are presented as mean ± SEM.