The mechanisms of HIF-2α action in the carotid body

The carotid body (CB) is an arterial chemoreceptor central to ventilatory adaptations to reduced oxygen arterial pressure (hypoxia) and responds both rapidly, by increasing ventilation within seconds of the drop in O2 saturation, and chronically, by potentiating the acute hypoxic ventilatory response (HVR), which is associated with CB hyperplasia. Mechanisms behind these adaptations are not fully understood, but the Hypoxia Inducible Factor (HIF) signalling pathway has been implicated in mediating the CB chronic response to hypoxia. Indeed, HIF-2α isoform has been found to be very highly expressed in the CB. Using a recently developed pharmacological HIF-2α inhibitor (PT2385), we demonstrate that HIF-2α is necessary for the CB-mediated acclimatisation to chronic hypoxia, and plays a major role in acute HVR (Chapter 2). The response to PT2385 was blocked in mice with HIF-2α S305M mutation, obstructing binding of allosteric inhibitors without altering the HIF transcriptional function, demonstrating that the PT2385 effects are on-target to HIF-2α (Chapter 3). Further, a hypomorphic ventilatory phenotype was found in S305M mutants (Chapter 3), suggesting a potential HIF-β-independent role of HIF-2α in CB function. This was investigated in Arnt1/2dKO mice, which showed incomplete loss of CB responses to chronic hypoxia with Arnt1/2 loss in adulthood (Chapter 4). The role of G-protein signalling regulators: RGS4 and RGS5, which are highly CB-enriched and reportedly HIF-target genes, was further investigated using genetic models, and demonstrated a function of RGS5 in ventilatory acclimatisation to hypoxia, uncovering a potential HIF-2α downstream mediator (Chapter 5). Finally, the role of HIF-2α in sympathoadrenal tumours was investigated by developmental, tyrosine-hydroxylase-expressing cell-restricted Phd2 inactivation (Chapter 6). This study showed that HIF-2α stabilisation (due to Phd2 inactivation) preserves the chromaffin cells in an ‘immature’, noradrenergic state, with induction of CB-specific genes, and is sufficient to preserve acute oxygen sensitivity of chromaffin cells, typically only found in neonates. Together, presented results demonstrate that HIF-2α is both necessary and sufficient for oxygen sensitivity in the carotid body and related tissues, and provides insight into the possible transcriptional and HIF-β-independent mechanisms of HIF-2α action in hypoxia.