Molecular Mechanisms of Acute Oxygen Sensing by Arterial Chemoreceptor Cells. Role of Hif2α

Hyperbaric oxygen treatment reveals spatiotemporal OXPHOS plasticity in the porcine heart

Cardiomyocytes meet their high ATP demand almost exclusively by oxidative phosphorylation (OXPHOS). Adequate oxygen supply is an essential prerequisite to keep OXPHOS operational. At least two spatially distinct mitochondrial subpopulations facilitate OXPHOS in cardiomyocytes, i.e. subsarcolemmal (SSM) and interfibrillar mitochondria (IFM). Their intracellular localization below the sarcolemma or buried deep between the sarcomeres suggests different oxygen availability. Here, we studied SSM and IFM isolated from piglet hearts and found significantly lower activities of electron transport chain enzymes and F1FO-ATP synthase in IFM, indicative for compromised energy metabolism. To test the contribution of oxygen availability to this outcome, we ventilated piglets under hyperbaric hyperoxic (HBO) conditions for 240 min. HBO treatment raised OXPHOS enzyme activities in IFM to the level of SSM. Complexome profiling analysis revealed that a high proportion of the F1FO-ATP synthase in the IFM was in a disassembled state prior to the HBO treatment. Upon increased oxygen availability, the enzyme was found to be largely assembled, which may account for the observed increase in OXPHOS complex activities. Although HBO also induced transcription of genes involved in mitochondrial biogenesis, a full proteome analysis revealed only minimal alterations, meaning that HBO-mediated tissue remodeling is an unlikely cause for the observed differences in OXPHOS. We conclude that a previously unrecognized oxygen-regulated mechanism endows cardiac OXPHOS with spatiotemporal plasticity that may underlie the enormous metabolic and contractile adaptability of the heart.

Fish gill chemosensing: knowledge gaps and inconsistencies

In this review, we explore the inconsistencies in the data and gaps in our knowledge that exist in what is currently known regarding gill chemosensors which drive the cardiorespiratory reflexes in fish. Although putative serotonergic neuroepithelial cells (NEC) dominate the literature, it is clear that other neurotransmitters are involved (adrenaline, noradrenaline, acetylcholine, purines, and dopamine). And although we assume that these agents act on neurons synapsing with the NECs or in the afferent or efferent limbs of the paths between chemosensors and central integration sites, this process remains elusive and may explain current discrepancies or species differences in the literature. To date it has been impossible to link the distribution of NECs to species sensitivity to different stimuli or fish lifestyles and while the gills have been shown to be the primary sensing site for respiratory gases, the location (gills, oro-branchial cavity or elsewhere) and orientation (external/water or internal/blood sensing) of the NECs are highly variable between species of water and air breathing fish. Much of what has been described so far comes from studies of hypoxic responses in fish, however, changes in CO2, ammonia and lactate have all been shown to elicit cardio-respiratory responses and all have been suggested to arise from stimulation of gill NECs. Our view of the role of NECs is broadening as we begin to understand the polymodal nature of these cells. We begin by presenting the fundamental picture of gill chemosensing that has developed, followed by some key unanswered questions about gill chemosensing in general.

Low oxygen in inspired air causes severe cerebrocortical hypoxia and cell death in the cerebral cortex of awake rats

Type 1 respiratory failure (T1RF) is associated with secondary acute brain injury (sABI). The underlying mechanisms of sABI could include injury to brain cells mediated either by hypoxia or by lung injury-triggered inflammation. To elucidate to what extent T1RF causes hypoxia and a consequent hypoxic injury in the brain in the absence of lung injury, we exposed healthy, conscious Sprague-Dawley rats to 48 h long low partial pressure of O2 in inspired air (PiO2) (7.5-8 % O2 in N2, CO2 < 0.5 %, normal barometric pressure) and measured the partial pressure of oxygen in the premotor cortex (PtO2), cerebral blood flow (CBF), lactate concentrations, and cell death. Low PiO2 significantly affected PtO2, which was 52.3 (SD 2.1) mmHg when PiO2 was normal but declined to 6.4 (SD 3.8) mmHg when PiO2 was low for 1 h. This was accompanied by increased lactate concentrations in plasma, CSF, and premotor cortex. Low PiO2 elevated the number of dead cells in the cerebral cortex from 5.6 (SD 4.8) % (when PiO2 was normal) to 20.5 (SD 4.1) % and 32.37 (SD 6.5) % after 24 h and 48 h exposure to low PiO2, respectively. The Mann-Kendall test could not detect any monotonic increase or decrease in pial blood flow during the 48 h exposure to low PiO2. In summary, our findings suggest that exposure to low PiO2 caused a severe hypoxia in the cerebral cortex, which triggers a massive cell death. Since these conditions mimic T1RF, hypoxic injury could be an important underlying cause of T1RF-induced sABI.

Mitochondrial complex I ROS production and redox signaling in hypoxia

Mitochondria are a main source of cellular energy. Oxidative phosphorylation (OXPHOS) is the major process of aerobic respiration. Enzyme complexes of the electron transport chain (ETC) pump protons to generate a protonmotive force (Δp) that drives OXPHOS. Complex I is an electron entry point into the ETC. Complex I oxidizes nicotinamide adenine dinucleotide (NADH) and transfers electrons to ubiquinone in a reaction coupled with proton pumping. Complex I also produces reactive oxygen species (ROS) under various conditions. The enzymatic activities of complex I can be regulated by metabolic conditions and serves as a regulatory node of the ETC. Complex I ROS plays diverse roles in cell metabolism ranging from physiologic to pathologic conditions. Progress in our understanding indicates that ROS release from complex I serves important signaling functions. Increasing evidence suggests that complex I ROS is important in signaling a mismatch in energy production and demand. In this article, we review the role of ROS from complex I in sensing acute hypoxia.

Insights into the control and consequences of breathing adjustments in fishes-from larvae to adults

Adjustments of ventilation in fishes to regulate the volume of water flowing over the gills are critically important responses to match branchial gas transfer with metabolic needs and to defend homeostasis during environmental fluctuations in O₂ and/or CO₂ levels. In this focused review, we discuss the control and consequences of ventilatory adjustments in fish, briefly summarizing ventilatory responses to hypoxia and hypercapnia before describing the current state of knowledge of the chemoreceptor cells and molecular mechanisms involved in sensing O₂ and CO₂. We emphasize, where possible, insights gained from studies on early developmental stages. In particular, zebrafish (Danio rerio) larvae have emerged as an important model for investigating the molecular mechanisms of O₂ and CO₂ chemosensing as well as the central integration of chemosensory information. Their value stems, in part, from their amenability to genetic manipulation, which enables the creation of loss-of-function mutants, optogenetic manipulation, and the production of transgenic fish with specific genes linked to fluorescent reporters or biosensors.

Neurobiology of the carotid body

The carotid body (CB) is a bilateral arterial chemoreceptor located in the carotid artery bifurcation with an essential role in cardiorespiratory homeostasis. It is composed of highly perfused cell clusters, or glomeruli, innervated by sensory fibers. Glomus cells, the most abundant in each glomerulus, are neuron-like multimodal sensory elements able to detect and integrate changes in several physical and chemical parameters of the blood, in particular O₂ tension, CO₂ and pH, as well as glucose, lactate, or blood flow. Activation of glomus cells (e.g., during hypoxia or hypercapnia) stimulates the afferent fibers which impinge on brainstem neurons to elicit rapid compensatory responses (hyperventilation and sympathetic activation). This chapter presents an updated view of the structural organization of the CB and the mechanisms underlying the chemosensory responses of glomus cells, with special emphasis on the molecular processes responsible for acute O₂ sensing. The properties of the glomus cell-sensory fiber synapse as well as the organization of CB output are discussed. The chapter includes the description of recently discovered CB stem cells and progenitor cells, and their role in CB growth during acclimatization to hypoxemia. Finally, the participation of the CB in the mechanisms of disease is briefly discussed.

Motor patterning, ion regulation and spreading depolarization during CNS shutdown induced by experimental anoxia in Locusta migratoria

Anoxia induces a reversible coma in insects. Coma onset is triggered by the arrest of mechanisms responsible for maintaining membrane ion homeostasis in the CNS, resulting in a wave of neuronal and glial depolarization known as spreading depolarization (SD). Different methods of anoxia influence the behavioural response but their effects on SD are unknown. We investigated the effects of CO₂, N₂, and H₂O on the characteristics of coma induction and recovery in Locusta migratoria. Water immersion delayed coma onset and recovery, likely due to involvement of the tracheal system and the nature of asphyxiation but otherwise resembled N₂ delivery. The main difference between N₂ and CO₂ was that CO₂ hastened onset of neural failure and SD and delayed recovery. In the CNS, this was associated with CO₂ inducing an abrupt and immediate decrease of interstitial pH and increase of extracellular [K⁺]. Recording of the transperineurial potential showed that SD propagation and a postanoxic negativity (PAN) were similar with both gases. The PAN increased with ouabain treatment, likely due to removal of the counteracting electrogenic effect of Na⁺/K⁺-ATPase, and was inhibited by bafilomycin, a proton pump inhibitor, suggesting that it was generated by the electrogenic effect of a Vacuolar-type ATPase (VA). Muscle fibres depolarized by ~20 mV, which happened more rapidly with CO₂ compared with N₂. Wing muscle motoneurons depolarized nearly completely in two stages, with CO₂ causing more rapid onset and slower recovery than N₂. Other parameters of SD onset and recovery were similar with the two gases. Electrical resistance across the ganglion sheath increased during anoxia and at SD onset. We provisionally attribute this to cell swelling reducing the dimensions of the interstitial pathway from neuropil to the bathing saline. Neuronal membrane resistance decreased abruptly at SD onset indicating opening of an unidentified membrane conductance. Consideration of the intracellular recording relative to the saline suggests that the apical membrane of perineurial glia depolarizes prior to neuron depolarization. We propose that SD is triggered by events at the perineurial sheath and then propagates laterally and more deeply into the neuropil. We conclude that the fundamental nature of SD is not dependent on the method of anoxia however the timing of onset and recovery are influenced; water immersion is complicated by the tracheal system and CO₂ delivery has more rapid and longer lasting effects, associated with severe interstitial acidosis.