replying to H. Torres-Torrelo et al. Nature 561, https://doi.org/10.1038/s41586-018-0545-9 (2018)

In the accompanying Comment, Torres-Torrelo et al.1, and others previously2, confirmed the following key findings of our paper3: Olfr78 is highly and selectively expressed in carotid body glomus cells; Olfr78 can be activated by lactate, as measured in Olfr78-transfected cultured cells; and lactate activates glomus cells at low-millimolar concentrations, within the physiological concentration range of lactate and the range that we measured for lactate activation (half-maximum effective concentration (EC50) of around 4 mM) of Olfr78-transfected cultured cells.

The major difference that can be compared is in the observed behavioural response of Olfr78−/− mutant mice to acute hypoxia (hypoxic ventilatory response). The Olfr78tm1Mom mutants used in our original study were cryorecovered by JAX in 2010 from sperm in a mixed 129P2/OlaHsd × C57BL/6J background3,4, but a marked decline in breeding in heterozygous intercrossed mice within three years of obtaining the animals prevented us from retesting the same cohort now. To address differences between the results presented by Torres-Torrelo et al.1 and our previous study3, we instead tested female progeny of newly cryorecovered Olfr78+/− mutant mice from JAX as well as Olfr78+/− mutant mice from the Frankfurt colony using behavioural protocols similar to our original study and the protocol used by Torres-Torrelo et al.1. Surprisingly, we did not observe a consistent defect in the hypoxic ventilatory response of either current strain under these conditions. Although we cannot exclude an effect due to differences in our current versus previous test conditions (the animal facility and equipment used in our original experiments in 2011 are no longer available), it seems more likely that the original cohort had an unusual genetic background that rendered a defect in this response more apparent in Olfr78−/− mutants. Because the carotid body response to hypoxia is still partially intact in Olfr78−/− mutants, including a fully functional response to acid3 that contributes to carotid body oxygen sensing5, the behavioural response to hypoxia is likely to be highly sensitive to differences in assay conditions and genetic background6, as observed for mutants in the acid-sensing pathway5,7. We proposed that glomus cells separately sense both the extracellular lactate and hydrogen ions from lactic acid produced during acute hypoxia3; it may be necessary to disrupt both pathways simultaneously and acutely to observe a robust effect in behavioural assays.

Although we did not find a consistent defect in the hypoxic ventilatory response in the current Olfr78 mutant strains, curiously we did observe a greater variance in their ventilatory frequency in hypoxia compared to wild-type littermates. Therefore, in addition to potential functional redundancy between Olfr78 and acid-sensing pathways, there may be a variable compensatory response induced by Olfr78 loss. This could involve either of these carotid body pathways, or the peripheral and central adaptation mechanisms that restore a hypoxic ventilatory response following bilateral carotid body denervation8,9.

The results of Torres-Torrelo et al.1 from carotid body slices and isolated cells cannot be directly compared to our results derived from monitoring the integrated sensory output of the intact carotid body by recordings of the carotid sinus nerve. There is growing evidence that sensory signalling involves communication between glomus cells and other carotid body cells10, so the effect of Olfr78 on signalling may be apparent only in the intact organ. Furthermore, Torres-Torrelo et al.1 measured release of dopamine, which is not an excitatory transmitter in the rodent carotid body11, and calcium responses of the glomus cells. Olfr78 may regulate a signalling step in glomus cells that is downstream or parallel to these responses, so the observations of Torres-Torrelo et al.1 are not incompatible with our nerve recordings. A similar pair of seemingly contradictory observations was previously described for acid-sensing mutants, in which the carotid sinus nerve response to hypoxia was partially defective in mutants but the dopamine response was intact5,12. It will be important to determine not only how Olfr78 activity in glomus cells is modulated by lactate produced under hypoxia, but also which signalling step(s) the activated receptor regulates.

We thank Torres-Torrelo et al.1 for their experiments showing that the Olfr78 mutant phenotype is more complex than our original experiments revealed, and for motivating us and others to carefully define the relationship of Olfr78 to other pathways implicated in oxygen sensing in the carotid body and the physiological and genetic compensatory mechanisms that can influence them and other aspects of the hypoxic ventilatory response.

Three of the listed authors (N.S.K., H.H. and A.D.) contributed only to the work contained in this Reply.