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Key points: The carotid body is a peripheral arterial chemoreceptor that regulates ventilation in response to both acute and sustained hypoxia. Type I cells in this organ respond to low oxygen both acutely by depolarization and dense core vesicle secretion and, over the longer term, via cellular proliferation and enhanced ventilatory responses. Using lineage analysis, the present study shows that the Type I cell lineage itself proliferates and expands in response to sustained hypoxia. Inactivation of HIF‐2α in Type I cells impairs the ventilatory, proliferative and cell intrinsic (dense core vesicle) responses to hypoxia. Inactivation of PHD2 in Type I cells induces multilineage hyperplasia and ultrastructural changes in dense core vesicles to form paraganglioma‐like carotid bodies. These changes, similar to those observed in hypoxia, are dependent on HIF‐2α. Taken together, these findings demonstrate a key role for the PHD2–HIF‐2α couple in Type I cells with respect to the oxygen sensing functions of the carotid body. Abstract: The carotid body is a peripheral chemoreceptor that plays a central role in mammalian oxygen homeostasis. In response to sustained hypoxia, it manifests a rapid cellular proliferation and an associated increase in responsiveness to hypoxia. Understanding the cellular and molecular mechanisms underlying these processes is of interest both to specialized chemoreceptive functions of that organ and, potentially, to the general physiology and pathophysiology of cellular hypoxia. We have combined cell lineage tracing technology and conditionally inactivated alleles in recombinant mice to examine the role of components of the HIF hydroxylase pathway in specific cell types within the carotid body. We show that exposure to sustained hypoxia (10% oxygen) drives rapid expansion of the Type I, tyrosine hydroxylase expressing cell lineage, with little transdifferentiation to (or from) that lineage. Inactivation of a specific HIF isoform, HIF‐2α, in the Type I cells was associated with a greatly reduced proliferation of Type I cells and hypoxic ventilatory responses, with ultrastructural evidence of an abnormality in the action of hypoxia on dense core secretory vesicles. We also show that inactivation of the principal HIF prolyl hydroxylase PHD2 within the Type I cell lineage is sufficient to cause multilineage expansion of the carotid body, with characteristics resembling paragangliomas. These morphological changes were dependent on the integrity of HIF‐2α. These findings implicate specific components of the HIF hydroxylase pathway (PHD2 and HIF‐2α) within Type I cells of the carotid body with respect to the oxygen sensing and adaptive functions of that organ. [ABSTRACT FROM AUTHOR]

Key points Sustained hypoxic exposure increases ventilatory sensitivity to hypoxia as part of physiological acclimatisation., Oxygen-sensitive signals are transduced in animal cells by post-translational hydroxylation of transcription factors termed hypoxia-inducible factors (HIFs)., Mice heterozygous for the principal 'oxygen-sensing' HIF hydroxylase PHD2 (prolyl hydroxylase domain 2) show enhanced ventilatory sensitivity to hypoxia., To analyse the underlying mechanisms, functional (hypoxic ventilatory responses, HVRs) and anatomical (cellular proliferation within carotid bodies) responses were studied in genetic models of inducible and constitutive inactivation of PHD2 and its principal hydroxylation substrates, HIF-1α and HIF-2α., Inducible PHD2 inactivation enhanced HVR, similar to constitutive inactivation; both responses were almost entirely compensated for by specific inactivation of HIF-2α., Inducible inactivation of HIF-2α, but not HIF-1α, strikingly reduced ventilatory acclimatisation to hypoxia and associated carotid body cell proliferation., These findings demonstrate a key role for PHD2 and HIF-2α in ventilatory control and carotid body biology., Abstract Ventilatory sensitivity to hypoxia increases in response to continued hypoxic exposure as part of acute acclimatisation. Although this process is incompletely understood, insights have been gained through studies of the hypoxia-inducible factor (HIF) hydroxylase system. Genetic studies implicate these pathways widely in the integrated physiology of hypoxia, through effects on developmental or adaptive processes. In keeping with this, mice that are heterozygous for the principal HIF prolyl hydroxylase, PHD2, show enhanced ventilatory sensitivity to hypoxia and carotid body hyperplasia. Here we have sought to understand this process better through comparative analysis of inducible and constitutive inactivation of PHD2 and its principal targets HIF-1α and HIF-2α. We demonstrate that general inducible inactivation of PHD2 in tamoxifen-treated Phd2f/f;Rosa26+/CreERT2 mice, like constitutive, heterozygous PHD2 deficiency, enhances hypoxic ventilatory responses (HVRs: 7.2 ± 0.6 vs. 4.4 ± 0.4 ml min−1 g−1 in controls, P < 0.01). The ventilatory phenotypes associated with both inducible and constitutive inactivation of PHD2 were strongly compensated for by concomitant inactivation of HIF-2α, but not HIF-1α. Furthermore, inducible inactivation of HIF-2α strikingly impaired ventilatory acclimatisation to chronic hypoxia (HVRs: 4.1 ± 0.5 vs. 8.6 ± 0.5 ml min−1 g−1 in controls, P < 0.0001), as well as carotid body cell proliferation (400 ± 81 vs. 2630 ± 390 bromodeoxyuridine-positive cells mm−2 in controls, P < 0.0001). The findings demonstrate the importance of the PHD2/HIF-2α enzyme-substrate couple in modulating ventilatory sensitivity to hypoxia. [ABSTRACT FROM AUTHOR]