Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCA039

Poster Communications

Oxygen regulation of breathing through an olfactory receptor activated by lactate

A. J. Chang1,4, F. E. Ortega1,4, J. Riegler2, D. V. Madison3, M. A. Krasnow1,4

1. Biochemistry, Stanford University School of Medicine, Stanford, California, United States. 2. Medicine, Stanford University School of Medicine, Stanford, California, United States. 3. Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, California, United States. 4. Howard Hughes Medical Institute, Stanford, California, United States.

Animals have evolved homeostatic responses to changes in oxygen availability that act on different time scales. On the acute time scale, the carotid body (CB) is the major sensor of arterial blood oxygen that stimulates ventilation within seconds of hypoxia. While cellular and neuroanatomical pathways for the hypoxic ventilatory response mediated by the CB have been elucidated, the molecular mechanism of oxygen sensing within the organ is not well understood. We took an unbiased, genomic approach to identify putative oxygen sensors in the CB by comparing gene expression of the CB and adrenal medulla (AM) from wild-type adult C57BL/6 mice using microarrays and RNA sequencing. Looking for genes encoding fast signaling molecules, we found a gene encoding an olfactory receptor (Olfr78) expressed at 92-fold higher level in the CB versus AM. Using mice expressing reporters from the endogenous Olfr78 locus, we showed that Olfr78 is highly and selectively expressed in oxygen-sensitive glomus cells of the CB. Olfr78 null mice failed to increase ventilation in hypoxia (10% O2) but responded normally to hypercapnia (5% CO2) by unrestrained, unanesthesized whole body plethysmography. Body temperature and metabolic responses to hypoxia were not different between wild-type and Olfr78 mutant animals. Visualized by immunohistochemistry and electron microscopy, glomus cells were present in normal numbers and appear structurally intact in Olfr78 mutant carotid bodies. However, carotid body activity in hypoxia was reduced by half in electrophysiological recordings of the carotid sinus nerve in Olfr78 mutant preparations. Lactate, a metabolite that rapidly accumulates in hypoxia and induces hyperventilation (Hardarson et al., 1998; Kirsch and D'Alecy, 1983; Lee et al., 1996; Marina et al., 2015), activated Olfr78 in heterologous expression experiments in a luciferase assay. In the CB, lactate induced calcium transients in glomus cells, as reported by the genetically encoded calcium indicator GCaMP3, and stimulated carotid sinus nerve activity through Olfr78. Based on these results, we propose a model for CB oxygen sensing in which Olfr78 acts as a hypoxia sensor by detecting lactate produced when oxygen levels decline. All experimental details were previously described (Chang et al., 2015).

Where applicable, experiments conform with Society ethical requirements