Role of components of the phagocytic NADPH oxidase in oxygen sensing

Carotid body type I cells engage flavoprotein and Pin1 for oxygen sensing

Carotid body (CB) type I cells sense the blood Po₂ and generate a nervous signal for stimulating ventilation and circulation when blood oxygen levels decline. Three oxygen-sensing enzyme complexes may be used for this purpose: 1) mitochondrial electron transport chain metabolism, 2) heme oxygenase 2 (HO-2)-generating CO, and/or 3) an NAD(P)H oxidase (NOX). We hypothesize that intracellular redox changes are the link between the sensor and nervous signals. To test this hypothesis type I cell autofluorescence of flavoproteins (Fp) and NAD(P)H within the mouse CB ex vivo was recorded as Fp/(Fp+NAD(P)H) redox ratio. CB type I cell redox ratio transiently declined with the onset of hypoxia. Upon reoxygenation, CB type I cells showed a significantly increased redox ratio. As a control organ, the non-oxygen-sensing sympathetic superior cervical ganglion (SCG) showed a continuously reduced redox ratio upon hypoxia. CN^-, diphenyleneiodonium, or reactive oxygen species influenced chemoreceptor discharge (CND) with subsequent loss of O₂ sensitivity and inhibited hypoxic Fp reduction only in the CB but not in SCG Fp, indicating a specific role of Fp in the oxygen-sensing process. Hypoxia-induced changes in CB type I cell redox ratio affected peptidyl prolyl isomerase Pin1, which is believed to colocalize with the NADPH oxidase subunit p47_phox in the cell membrane to trigger the opening of potassium channels. We postulate that hypoxia-induced changes in the Fp-mediated redox ratio of the CB regulate the Pin1/p47_phox tandem to alter type I cell potassium channels and therewith CND.

NADPH oxidase is the major source of placental superoxide in early pregnancy: association with MAPK pathway activation

First-trimester placenta (<10 gestational weeks (GW)) develops in a low oxygen environment (≈2%). Early oxygen exposure can cause oxidative damage leading to pregnancy disorders. The aim of this work was to determine the major sources of placental superoxide during early pregnancy - more specifically before 10 GW - and to study redox adaptation to increased oxygen pressure after 12 GW. Our results show that NADPH oxidase (Nox) is the main source of superoxide in first-trimester chorionic villi. Its activity is higher before 10 GW and concomitant with the location on the syncytiotrophoblast apical pole of p47phox, the Nox organizer subunit. After the increase in pO₂ pressure (12–14 GW), the activities of the antioxidant enzymes SOD1, catalase and GPX1 are increased. The redox-sensitive MAPK pathways show increased phosphorylated-p38 expression, but no variation in the phosphorylation of stress-activated protein kinase/c-Jun NH2-terminal kinase (SAPK/JNK) during first trimester, suggesting a physiological redox adaptation, whilst ERK1/2 phosphorylation is higher after 12 GW. Nox is the major superoxide source in early pregnancy (<10 GW). Increased superoxide production at 7–9 GW is associated with p38 MAPK pathway activation, suggesting that it is involved in physiological placental function and healthy early development of the placenta, through MAPK pathways.

Oxygen sensing by the carotid body: mechanisms and role in adaptation to hypoxia

Oxygen (O2) is fundamental for cell and whole-body homeostasis. Our understanding of the adaptive processes that take place in response to a lack of O2(hypoxia) has progressed significantly in recent years. The carotid body (CB) is the main arterial chemoreceptor that mediates the acute cardiorespiratory reflexes (hyperventilation and sympathetic activation) triggered by hypoxia. The CB is composed of clusters of cells (glomeruli) in close contact with blood vessels and nerve fibers. Glomus cells, the O2-sensitive elements in the CB, are neuron-like cells that contain O2-sensitive K(+)channels, which are inhibited by hypoxia. This leads to cell depolarization, Ca(2+)entry, and the release of transmitters to activate sensory fibers terminating at the respiratory center. The mechanism whereby O2modulates K(+)channels has remained elusive, although several appealing hypotheses have been postulated. Recent data suggest that mitochondria complex I signaling to membrane K(+)channels plays a fundamental role in acute O2sensing. CB activation during exposure to low Po2is also necessary for acclimatization to chronic hypoxia. CB growth during sustained hypoxia depends on the activation of a resident population of stem cells, which are also activated by transmitters released from the O2-sensitive glomus cells. These advances should foster further studies on the role of CB dysfunction in the pathogenesis of highly prevalent human diseases.

Type I cell ROS kinetics under hypoxia in the intact mouse carotid body ex vivo: a FRET-based study

Reactive oxygen species (ROS) mainly originating from NADPH oxidases have been shown to be involved in the carotid body (CB) oxygen-sensing cascade. For measuring ROS kinetics, type I cells of the mouse CB in an ex vivo preparation were transfected with the ROS sensor construct FRET-HSP33. After 2 days of tissue culture, type I cells expressed FRET-HSP33 as shown by immunohistochemistry. In one population of CBs, 5 min of hypoxia induced a significant and reversible decrease of type I cell ROS levels (n = 9 CBs; P < 0.015), which could be inhibited by 4-(2-aminoethyl)benzensulfonylfluorid (AEBSF), a highly specific inhibitor of the NADPH oxidase subunits p47(phox) and p67(phox). In another population of CBs, however, 5 min of hypoxia induced a significant and reversible increase of ROS levels in type I cells (n = 8 CBs; P < 0.05), which was slightly enhanced by administration of 3 mM AEBSF. These different ROS kinetics seemed to coincide with different mice breeding conditions. Type I cells of both populations showed a typical hypoxia-induced membrane potential (MP) depolarization, which could be inhibited by 3 mM AEBSF. ROS and MP closely followed the hypoxic decrease in CB tissue oxygen as measured with an O2-sensitive dye. We conclude that attenuated p47(phox) subunit activity of the NADPH oxidase under hypoxia is the physiological trigger for type I cell MP depolarization probably due to ROS decrease, whereas the observed ROS increase has no influence on type I cell MP kinetics under hypoxia.

Immunohistochemical localization of Nox1, Nox4 and Mn-SOD in mouse femur during endochondral ossification

Enzymes synthesizing reactive oxygen (Nox family) have recently been identified. Elucidation of the production mechanism has been initiated, and the involvement of reactive oxygen in metabolism, intracellular transport, signal transmission and apoptosis has been reported. We immunohistochemically investigated expression and localization of the Nox family in endochondral ossification using a normal mouse femur. Weakly positive reactions with Nox1, Noxa1, and Noxo1 were observed in the zones of proliferative and prehypertrophic chondrocytes at 3 weeks of age. Nox4 was widely positive from the resting over the hypertrophic cell zone. At 18 weeks of age, none of the Nox types was expressed in chondrocytes as the zones disappeared. On the other hand, positive reactions with Nox1, Noxa1, Noxo1, and Nox4 were observed in osteoblasts in the zone of ossification at 3 weeks of age, and each Nox was also positive in osteoblasts arranged on the bone marrow side in the epiphyseal cartilage at 18 weeks of age. In addition, a reactive oxygen-eliminating enzyme, Mn-SOD, was observed only in prehypertrophic chondrocytes at 3 weeks of age, and not detected in osteoblasts. It was suggested that the Nox family is closely associated with endochondral ossification of the mouse femur, and Nox1 and Nox4 are closely involved in the chondrocyte maturation process and bone matrix formation.

Carotid chemoreceptor development in mice

Mice are the most suitable species for understanding genetic aspects of postnatal developments of the carotid body due to the availability of many inbred strains and knockout mice. Our study has shown that the carotid body grows differentially in different mouse strains, indicating the involvement of genes. However, the small size hampers investigating functional development of the carotid body. Hypoxic and/or hyperoxic ventilatory responses have been investigated in newborn mice, but these responses are indirect assessment of the carotid body function. Therefore, we need to develop techniques of measuring carotid chemoreceptor neural activity from young mice. Many studies have taken advantage of the knockout mice to understand chemoreceptor function of the carotid body, but they are not always suitable for addressing postnatal development of the carotid body due to lethality during perinatal periods. Various inbred strains with well-designed experiments will provide useful information regarding genetic mechanisms of the postnatal carotid chemoreceptor development. Also, targeted gene deletion is a critical approach.

Differential roles of NADPH oxidases in vascular physiology and pathophysiology

Reactive oxygen species (ROS) are produced by all vascular cells and regulate the major physiological functions of the vasculature. Production and removal of ROS are tightly controlled and occur in discrete subcellular locations, allowing for specific, compartmentalized signaling. Among the many sources of ROS in the vessel wall, NADPH oxidases are implicated in physiological functions such as control of vasomotor tone, regulation of extracellular matrix and phenotypic modulation of vascular smooth muscle cells. They are involved in the response to injury, whether as an oxygen sensor during hypoxia, as a regulator of protein processing, as an angiogenic stimulus, or as a mechanism of wound healing. These enzymes have also been linked to processes leading to disease development, including migration, proliferation, hypertrophy, apoptosis and autophagy. As a result, NADPH oxidases participate in atherogenesis, systemic and pulmonary hypertension and diabetic vascular disease. The role of ROS in each of these processes and diseases is complex, and a more full understanding of the sources, targets, cell-specific responses and counterbalancing mechanisms is critical for the rational development of future therapeutics.

A revisit to O2 sensing and transduction in the carotid body chemoreceptors in the context of reactive oxygen species biology

Oxygen-sensing and transduction in purposeful responses in cells and organisms is of great physiological and medical interest. All animals, including humans, encounter in their lifespan many situations in which oxygen availability might be insufficient, whether acutely or chronically, physiologically or pathologically. Therefore to trace at the molecular level the sequence of events or steps connecting the oxygen deficit with the cell responses is of interest in itself as an achievement of science. In addition, it is also of great medical interest as such knowledge might facilitate the therapeutical approach to patients and to design strategies to minimize hypoxic damage. In our article we define the concepts of sensors and transducers, the steps of the hypoxic transduction cascade in the carotid body chemoreceptor cells and also discuss current models of oxygen- sensing (bioenergetic, biosynthetic and conformational) with their supportive and unsupportive data from updated literature. We envision oxygen-sensing in carotid body chemoreceptor cells as a process initiated at the level of plasma membrane and performed by a hemoprotein, which might be NOX4 or a hemoprotein not yet chemically identified. Upon oxygen-desaturation, the sensor would experience conformational changes allosterically transmitted to oxygen regulated K+ channels, the initial effectors in the transduction cascade. A decrease in their opening probability would produce cell depolarization, activation of voltage dependent calcium channels and release of neurotransmitters. Neurotransmitters would activate the nerve endings of the carotid body sensory nerve to convey the information of the hypoxic situation to the central nervous system that would command ventilation to fight hypoxia.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Modulation of chronic hypoxia-induced chemoreceptor hypersensitivity by NADPH oxidase subunits in rat carotid body

Previous studies in our laboratory established that reactive oxygen species (ROS) generated by NADPH oxidase (NOX) facilitate the open state of a subset of K+ channels in oxygen-sensitive type I cells of the carotid body. Thus pharmacological inhibition of NOX or deletion of a NOX gene resulted in enhanced chemoreceptor sensitivity to hypoxia. The present study tests the hypothesis that chronic hypoxia (CH)-induced hypersensitivity of chemoreceptors is modulated by increased NOX activity and elevated levels of ROS. Measurements of dihydroethidium fluorescence in carotid body tissue slices showed that increased ROS production following CH (14 days, 380 Torr) was blocked by the specific NOX inhibitor 4-(2-amino-ethyl)benzenesulfonyl fluoride (AEBSF, 3 microM). Consistent with these findings, in normal carotid body AEBSF elicited a small increase in the chemoreceptor nerve discharge evoked by an acute hypoxic challenge, whereas after 9 days of CH the effect of the NOX inhibitor was some threefold larger (P<0.001). Evaluation of gene expression after 7 days of CH showed increases in the isoforms NOX2 (approximately 1.5-fold) and NOX4 (approximately 3.8-fold) and also increased presence of the regulatory subunit p47phox (approximately 4.2-fold). Involvement of p47phox was further implicated in studies of isolated type I cells that demonstrated an approximately 8-fold and an approximately 11-fold increase in mRNA after 1 and 3 days, respectively, of hypoxia in vivo. These findings were confirmed in immunocytochemical studies of carotid body tissue that showed a robust increase of p47phox in type I cells after 14 days of CH. Our findings suggest that increased ROS production by NOX enzymes in type I cells dampens CH-induced hypersensitivity in carotid body chemoreceptors.

Mitochondrial reactive oxygen species are required for hypoxia-induced degradation of keratin intermediate filaments

Hypoxia can cause stress and structural changes to the epithelial cytoskeleton. The intermediate filament (IF) network is known to reorganize in response to stress. We examined whether rats exposed to hypoxia had altered keratin IF expression in their alveolar epithelial type II (ATII) cells. There was a significant decrease in keratin protein levels in hypoxic ATII cells compared with those in ATII cells isolated from normoxic rats. To define the mechanisms regulating this process we studied changes to the keratin IF network in A549 cells (an alveolar epithelial cell line) exposed to 1.5% oxygen. We observed a time-dependent disassembly-degradation of keratin 8 and 18 proteins, which was associated with an increase in reactive oxygen species (ROS). Hypoxia-treated A549 cells deficient in mitochondrial DNA or A549 cells treated with a small interfering RNA against the Rieske iron-sulfur protein of mitochondrial complex III did not have increased levels of ROS nor was the keratin IF network disassembled and degraded. The superoxide dismutase (SOD)/catalase mimetic (EUK-134) prevented the hypoxia-mediated keratin IF degradation as did the overexpression of SOD1 but not of SOD2. Accordingly, we provide evidence that hypoxia promotes the disassembly and degradation of the keratin IF network via mitochondrial complex III-generated reactive oxygen species.-Na, N., Chandel, N. S., Litvan, J., Ridge, K. M. Mitochondrial reactive oxygen species are required for hypoxia-induced degradation of keratin intermediate filaments.

NADPH oxidase activity is necessary for acute intermittent hypoxia-induced phrenic long-term facilitation

Phrenic long-term facilitation (pLTF) following acute intermittent hypoxia (AIH) is a form of spinal, serotonin-dependent synaptic plasticity that requires reactive oxygen species (ROS) formation. We tested the hypothesis that spinal NADPH oxidase activity is a necessary source of ROS for pLTF. Sixty minutes post-AIH (three 5-min episodes of 11% O(2), 5 min intervals), integrated phrenic and hypoglossal (XII) nerve burst amplitudes were increased from baseline, indicative of phrenic and XII LTF. Intrathecal injections (approximately C(4)) of apocynin or diphenyleneiodonium chloride (DPI), two structurally and functionally distinct inhibitors of the NADPH oxidase complex, attenuated phrenic, but not XII, LTF. Immunoblots from soluble (cytosolic) and particulate (membrane) fractions of ventral C(4) spinal segments revealed predominantly membrane localization of the NADPH oxidase catalytic subunit, gp91(phox), whereas membrane and cytosolic expression were both observed for the regulatory subunits, p47(phox) and RAC1. Immunohistochemical analysis of fixed tissues revealed these same subunits in presumptive phrenic motoneurons of the C(4) ventral horn, but not in neighbouring astrocytes or microglia. Collectively, these data demonstrate that NADPH oxidase subunits localized within presumptive phrenic motoneurons are a major source of ROS necessary for AIH-induced pLTF. Thus, NADPH oxidase activity is a key regulator of spinal synaptic plasticity, and may be a useful pharmaceutical target in developing therapeutic strategies for respiratory insufficiency in patients with, for example, cervical spinal injury.

The NOX family of ROS-generating NADPH oxidases: physiology and pathophysiology

For a long time, superoxide generation by an NADPH oxidase was considered as an oddity only found in professional phagocytes. Over the last years, six homologs of the cytochrome subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the phagocyte NADPH oxidase itself (NOX2/gp91(phox)), the homologs are now referred to as the NOX family of NADPH oxidases. These enzymes share the capacity to transport electrons across the plasma membrane and to generate superoxide and other downstream reactive oxygen species (ROS). Activation mechanisms and tissue distribution of the different members of the family are markedly different. The physiological functions of NOX family enzymes include host defense, posttranlational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. NOX enzymes also contribute to a wide range of pathological processes. NOX deficiency may lead to immunosuppresion, lack of otoconogenesis, or hypothyroidism. Increased NOX activity also contributes to a large number or pathologies, in particular cardiovascular diseases and neurodegeneration. This review summarizes the current state of knowledge of the functions of NOX enzymes in physiology and pathology.

The cellular basis for diverse responses to oxygen

Mammalian cells have divergent responses to varying oxygen levels. Cells exposed to low oxygen levels (hypoxia) activate the transcription factor hypoxia-inducible factor-1 (HIF-1) as an adaptive response. Cells exposed to hypoxia do not undergo senescence or cell death and do not diminish ATP levels. By contrast, cells exposed to high oxygen levels (hyperoxia) undergo senescence and cell death and decrease their ATP levels, yet do not activate HIF-1. Despite these divergent responses with respect to senescence, cell death, metabolism, and gene expression, the signaling events in both systems are mediated by the generation of mitochondrial-derived reactive oxygen species (ROS). This perspective reviews the role of signaling through mitochondrial ROS in hypoxic and hyperoxic environments.

The role of NADPH oxidase in carotid body arterial chemoreceptors

O(2)-sensing in the carotid body occurs in neuroectoderm-derived type I glomus cells where hypoxia elicits a complex chemotransduction cascade involving membrane depolarization, Ca(2+) entry and the release of excitatory neurotransmitters. Efforts to understand the exquisite O(2)-sensitivity of these cells currently focus on the coupling between local P(O2) and the open-closed state of K(+)-channels. Amongst multiple competing hypotheses is the notion that K(+)-channel activity is mediated by a phagocytic-like multisubunit enzyme, NADPH oxidase, which produces reactive oxygen species (ROS) in proportion to the prevailing P(O2). In O(2)-sensitive cells of lung neuroepithelial bodies (NEB), multiple studies confirm that ROS levels decrease in hypoxia, and that E(M) and K(+)-channel activity are indeed controlled by ROS produced by NADPH oxidase. However, recent studies in our laboratories suggest that ROS generated by a non-phagocyte isoform of the oxidase are important contributors to chemotransduction, but that their role in type I cells differs fundamentally from the mechanism utilized by NEB chemoreceptors. Data indicate that in response to hypoxia, NADPH oxidase activity is increased in type I cells, and further, that increased ROS levels generated in response to low-O(2) facilitate cell repolarization via specific subsets of K(+)-channels.

A Phenotypic Perspective on Mammalian Oxygen Sensor Candidates

Chronic hypoxic stimulation in mammals can induce several phenotypic changes, such as polycythemia, pulmonary vascular changes, pulmonary hypertension, and carotid body (CB) enlargement. These phenotypic alterations provide a tool to test whether an oxygen sensor candidate is involved in an organism's response to environmental hypoxia. Here I evaluate the phenotypic evidence for several commonly considered oxygen sensor candidates. Germline mutations in NADPH oxidase, mitochondrial complexes I, III, IV, and heme oxygenase 2 genes cause different phenotypic consequences, suggesting distinct physiological roles rather than oxygen sensing. Germline mutations in VHL and HIF1 prolyl hydroxylase 2 genes cause polycythemia consistent with their role in oxygen homeostasis. However, it is unclear whether environmental variations affecting oxygen availability modify their phenotype, as would be expected from a defect in an oxygen sensor. Succinate dehydrogenase (SDH); mitochondrial complex II) germline mutations cause CB paragangliomas and there is evidence that the severity and the population genetics of paragangliomas may be influenced by altitude. Thus, from a phenotypic perspective, succinate dehydrogenase (SDH) appears to be a well-supported oxygen sensor candidate. It is suggested that a universal oxygen sensor candidate must be supported by evidence from multiple layers of biological complexity.

Reactive oxygen species in the control of hypoxia-inducible factor-mediated gene expression

Reactive oxygen species (ROS) have long been considered as cytotoxic. However, recent evidence indicates a prominent role of ROS as signaling molecules in the response to hormones, growth and coagulation factors, cytokines and other factors as well as to changes in oxygen tension. The hypoxia-inducible transcription factors (HIFs) are key players in the cellular response to changes in oxygen tension. Recently, HIFs have also been shown to respond to the above-mentioned non-hypoxic stimuli. In this article, the role of ROS in the regulation of HIF-1 under hypoxic and non-hypoxic conditions is summarized.

The oxygen sensing signal cascade under the influence of reactive oxygen species

Structural and functional integrity of organ function profoundly depends on a regular oxygen and glucose supply. Any disturbance of this supply becomes life threatening and may result in severe loss of organ function. Particular reductions in oxygen availability (hypoxia) caused by respiratory or blood circulation irregularities cannot be tolerated for longer periods due to an insufficient energy supply by anaerobic glycolysis. Complex cellular oxygen sensing systems have evolved to tightly regulate oxygen homeostasis. In response to variations in oxygen partial pressure (PO2), these systems induce adaptive and protective mechanisms to avoid or at least minimize tissue damage. These various responses might be based on a range of oxygen sensing signal cascades including an isoform of the neutrophil NADPH oxidase, different electron carrier units of the mitochondrial chain such as a specialized mitochondrial, low PO2 affinity cytochrome c oxidase (aa3) and a subfamily of 2-oxoglutarate dependent dioxygenases termed HIF (hypoxia inducible factor) prolyl-hydroxylase and HIF asparaginyl hydroxylase called factor-inhibiting HIF (FIH-1). Thus, specific oxygen sensing cascades involving reactive oxygen species as second messengers may by means of their different oxygen sensitivities, cell-specific and subcellular localization help to tailor various adaptive responses according to differences in tissue oxygen availability.

NADPH oxidases in cardiovascular health and disease

Increased oxidative stress plays an important role in the pathophysiology of cardiovascular diseases such as hypertension, atherosclerosis, diabetes, cardiac hypertrophy, heart failure, and ischemia-reperfusion. Although several sources of reactive oxygen species (ROS) may be involved, a family of NADPH oxidases appears to be especially important for redox signaling and may be amenable to specific therapeutic targeting. These include the prototypic Nox2 isoform-based NADPH oxidase, which was first characterized in neutrophils, as well as other NADPH oxidases such as Nox1 and Nox4. These Nox isoforms are expressed in a cell- and tissue-specific fashion, are subject to independent activation and regulation, and may subserve distinct functions. This article reviews the potential roles of NADPH oxidases in both cardiovascular physiological processes (such as the regulation of vascular tone and oxygen sensing) and pathophysiological processes such as endothelial dysfunction, inflammation, hypertrophy, apoptosis, migration, angiogenesis, and vascular and cardiac remodeling. The complexity of regulation of NADPH oxidases in these conditions may provide the possibility of targeted therapeutic manipulation in a cell-, tissue- and/or pathway-specific manner at appropriate points in the disease process.

Effect of p47phoxgene deletion on ROS production and oxygen sensing in mouse carotid body chemoreceptor cells

Membrane potential in oxygen-sensitive type I cells in carotid body is controlled by diverse sets of voltage-dependent and -independent K+channels. Coupling of Po2to the open-closed state of channels may involve production of reactive oxygen species (ROS) by NADPH oxidase. One hypothesis suggests that ROS are produced in proportion to the prevailing Po2and a subset of K+channels closes as ROS levels decrease. We evaluated ROS levels in normal and p47phoxgene-deleted [NADPH oxidase knockout (KO)] type I cells using the ROS-sensitive dye dihydroethidium (DHE). In normal cells, hypoxia elicited an increase in ROS, which was blocked by the specific NADPH oxidase inhibitor 4-(2-aminoethyl)-benzenesulfonyl fluoride (AEBSF, 3 mM). KO type I cells did not respond to hypoxia, but the mitochondrial uncoupler azide (5 μM) elicited increased fluorescence in both normal and KO cells. Hypoxia had no effect on ROS production in sensory and sympathetic neurons. Methodological control experiments showed that stimulation of neutrophils with a cocktail containing the chemotactic peptide N-formyl-Met-Leu-Phe (1 μM), arachidonic acid (10 μM), and cytochalasin B (5 μg/ml) elicited a rapid increase in DHE fluorescence. This response was blocked by the NADPH oxidase inhibitor diphenyleneiodonium (10 μM). KO neutrophils did not respond; however, azide (5 μM) elicited a rapid increase in fluorescence. Physiological studies in type I cells demonstrated that hypoxia evoked an enhanced depression of K+current and increased intracellular Ca2+levels in KO vs. normal cells. Moreover, AEBSF potentiated hypoxia-induced increases in intracellular Ca2+and enhanced the depression of K+current in low O2. Our findings suggest that local compartmental increases in oxidase activity and ROS production inhibit the activity of type I cells by facilitating K+channel activity in hypoxia.

Regulation of oxygen sensing by ion channels

O(2) sensing is of critical importance for cell survival and adaptation of living organisms to changing environments or physiological conditions. O(2)-sensitive ion channels are major effectors of the cellular responses to hypoxia. These channels are preferentially found in excitable neurosecretory cells (glomus cells of the carotid body, cells in the neuroepithelial bodies of the lung, and neonatal adrenal chromaffin cells), which mediate fast cardiorespiratory adjustments to hypoxia. O(2)-sensitive channels are also expressed in the pulmonary and systemic arterial smooth muscle cells where they participate in the vasomotor responses to low O(2) tension (particularly in hypoxic pulmonary vasoconstriction). The mechanisms underlying O(2) sensing and how the O(2) sensors interact with the ion channels remain unknown. Recent advances in the field give different support to the various current hypotheses. Besides the participation of ion channels in acute O(2) sensing, they also contribute to the gene program developed under chronic hypoxia. Gene expression of T-type calcium channels is upregulated by hypoxia through the same hypoxia-inducible factor-dependent signaling pathway utilized by the classical O(2)-regulated genes. Alteration of acute or chronic O(2) sensing by ion channels could participate in the pathophysiology of human diseases, such as sudden infant death syndrome or primary pulmonary hypertension.

Cellular oxygen sensing need in CNS function: physiological and pathological implications

Structural and functional integrity of brain function profoundly depends on a regular oxygen and glucose supply. Any disturbance of this supply becomes life threatening and may result in severe loss of brain function. In particular, reductions in oxygen availability (hypoxia) caused by systemic or local blood circulation irregularities cannot be tolerated for longer periods due to an insufficient energy supply to the brain by anaerobic glycolysis. Hypoxia has been implicated in central nervous system pathology in a number of disorders including stroke, head trauma, neoplasia and neurodegenerative disease. Complex cellular oxygen sensing systems have evolved for tight regulation of oxygen homeostasis in the brain. In response to variations in oxygen partial pressure (PO2) these induce adaptive mechanisms to avoid or at least minimize brain damage.

A significant advance in our understanding of the hypoxia response stems from the discovery of the hypoxia inducible factors (HIF), which act as key regulators of hypoxia-induced gene expression. Depending on the duration and severity of the oxygen deprivation, cellular oxygen-sensor responses activate a variety of short- and long-term energy saving and cellular protection mechanisms. Hypoxic adaptation encompasses an immediate depolarization block by changing potassium, sodium and chloride ion fluxes across the cellular membrane, a general inhibition of protein synthesis, and HIF-mediated upregulation of gene expression of enzymes or growth factors inducing angiogenesis, anaerobic glycolysis, cell survival or neural stem cell growth. However, sustained and prolonged activation of the HIF pathway may lead to a transition from neuroprotective to cell death responses. This is reflected by the dual features of the HIF system that include both anti- and proapoptotic components.

These various responses might be based on a range of oxygen-sensing signal cascades, including an isoform of the neutrophil NADPH oxidase, different electron carrier units of the mitochondrial chain such as a specialized mitochondrial, low PO2 affinity cytochrome c oxidase (aa3) and a subfamily of 2-oxoglutarate dependent dioxygenases termed HIF prolyl-hydroxylase (PHD) and HIF asparaginyl hydroxylase, known as factor-inhibiting HIF (FIH-1). Thus specific oxygen-sensing cascades, by means of their different oxygen sensitivities,cell-specific and subcellular localization, may help to tailor various adaptive responses according to differences in tissue oxygen availability.

Structure and regulation of the neutrophil respiratory burst oxidase: comparison with nonphagocyte oxidases

Neutrophils play an essential role in the body's innate defense against pathogens and are one of the primary mediators of the inflammatory response. To defend the host, neutrophils use a wide range of microbicidal products, such as oxidants, microbicidal peptides, and lytic enzymes. The generation of microbicidal oxidants by neutrophils results from the activation of a multiprotein enzyme complex known as the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which is responsible for transferring electrons from NADPH to O2, resulting in the formation of superoxide anion. During oxidase activation, cytosolic oxidase proteins translocate to the phagosome or plasma membrane, where they assemble around a central membrane-bound component known as flavocytochrome b. This process is highly regulated, involving phosphorylation, translocation, and multiple conformational changes. Originally, it was thought that the NADPH oxidase was restricted to phagocytes and used solely in host defense. However, recent studies indicate that similar NADPH oxidase systems are present in a wide variety of nonphagocytic cells. Although the nature of these nonphagocyte NADPH oxidases is still being defined, it is clear that they are functionally distinct from the phagocyte oxidases. It should be noted, however, that structural features of many nonphagocyte oxidase proteins do seem to be similar to those of their phagocyte counterparts. In this review, key structural and functional features of the neutrophil NADPH oxidase and its protein components are described, including a consideration of transcriptional and post-translational regulatory features. Furthermore, relevant details about structural and functional features of various nonphagocyte oxidase proteins will be included for comparison.

Metabolic regulation of potassium channels

Potassium (K+) channels exist in all three domains of organisms: eubacteria, archaebacteria, and eukaryotes. In higher animals, these membrane proteins participate in a multitude of critical physiological processes, including food and fluid intake, locomotion, stress response, and cognitive functions. Metabolic regulatory factors such as O2, CO2/pH, redox equivalents, glucose/ATP/ADP, hormones, eicosanoids, cell volume, and electrolytes regulate a diverse group of K+ channels to maintain homeostasis.

Effects of reducing agents on glutathione metabolism and the function of carotid body chemoreceptor cells

Two current hypotheses of O2 sensing in the carotid body (CB) chemoreceptors suggest participation of oxygen reactive (ROS) species, but they are mechanistically opposed. One postulates that hypoxia decreases ROS levels; the other that hypoxia increases them. Yet, both propose that the ensuing alteration in the cellular redox environment is the key signal triggering hypoxic chemoreception. Since the glutathione redox pair is the main cellular buffer for ROS and the main determinant of the general redox environment of the cells, a way to test whether ROS participate in chemoreception is to determine glutathione levels and to correlate them with the activity of CB chemoreceptor cells. We found that hypoxia does not alter the glutathione reduction potential but that it activates chemoreceptor cell neurosecretion. Incubation of tissues with reduced glutathione increases the glutathione-reducing potential but does not activate chemoreceptor cells in normoxia nor does it modify hypoxic activation. Like reduced glutathione, N-acetylcysteine promoted a general reducing environment in the cells without alteration of chemoreceptor cell activity. N-(mercaptopropionyl)-glycine, like the two previous agents, increases the reduction potential of glutathione. In contrast, the compound activated chemoreceptor cells in normoxia, promoting a dose- and Ca(2+)-dependent neurosecretion and a potentiation of the hypoxic responses. The existence of multiple relationships between glutathione reduction potential in the cells and their activity indicates that the general cellular redox environment is not a factor determining chemoreceptor cell activation. It cannot be excluded that the local redox environments of restricted microdomain(s) in the cells with specific regulating mechanisms are important signals for chemoreceptor cell activity.

The role of endogenous NADPH oxidases in airway and pulmonary vascular smooth muscle function

Reactive oxygen species generated from NADPH oxidase(s) in airway smooth muscle cells and pulmonary artery smooth muscle cells are important signaling intermediates. Nox4 appears to be the predominant gp91 homologue in these cells. However, expression of NADPH oxidase components is dependent on phenotype, and different homologues may be expressed during different functional states of the cell. NADPH oxidase(s) appear to be important not only for mitogenesis by these cells, but also for O(2) sensing. The regulation of NADPH oxidase(s) in airway and pulmonary artery smooth muscle cells has important implications for the pathobiochemistry of asthma and pulmonary vascular diseases.

Oxygen and glucose sensing by carotid body glomus cells

Carotid body glomus cells sense hypoxia through the inhibition of plasmalemmal K(+) channels, which leads to Ca(2+) influx and transmitter release. Although the mechanism of O(2) sensing remains enigmatic, it does not seem to depend on cellular redox status or inhibition of mitochondrial electron transport. Hypoxia inducible factors appear to be necessary for the expression of the O(2) sensor and carotid body remodeling in chronic hypoxia, but are not directly involved in acute O(2) sensing. Glomus cells are also rapidly activated by reductions of glucose concentration due to inhibition of K(+) channels. These cells function as combined O(2) and glucose sensors that help to prevent neuronal damage by acute hypoxia and/or hypoglycemia.