The effect of mitochondrial inhibitors on membrane currents in isolated neonatal rat carotid body type I cells

Blood oxygen regulation via P2Y12R expressed in the carotid body

BACKGROUND

Peripheral blood oxygen monitoring via chemoreceptors in the carotid body (CB) is an integral function of the autonomic cardiorespiratory regulation. The presence of the purinergic P2Y12 receptor (P2Y12R) has been implicated in CB; however, the exact role of the receptor in O2 sensing and signal transduction is unknown.

METHODS

The presence of P2Y12R was established by immunoblotting, RT qPCR and immunohistochemistry. Primary glomus cells were used to assess P2Y12R function during hypoxia and hypercapnia, where monoamines were measured by HPLC; calcium signal was recorded utilizing OGB-1 and N-STORM Super-Resolution System. Ingravescent hypoxia model was tested in anaesthetized mice of mixed gender and cardiorespiratory parameters were recorded in control and receptor-deficient or drug-treated experimental animals.

RESULTS

Initially, the expression of P2Y12R in adult murine CB was confirmed. Hypoxia induced a P2Y12R-dependent release of monoamine transmitters from isolated CB cells. Receptor activation with the endogenous ligand ADP promoted release of neurotransmitters under normoxic conditions, while blockade disrupted the amplitude and duration of the intracellular calcium concentration. In anaesthetised mice, blockade of P2Y12R expressed in the CB abrogated the initiation of compensatory cardiorespiratory changes in hypoxic environment, while centrally inhibited receptors (i.e. microglial receptors) or receptor-deficiency induced by platelet depletion had limited influence on the physiological adjustment to hypoxia.

CONCLUSIONS

Peripheral P2Y12R inhibition interfere with the complex mechanisms of acute oxygen sensing by influencing the calcium signalling and the release of neurotransmitter molecules to evoke compensatory response to hypoxia. Prospectively, the irreversible blockade of glomic receptors by anti-platelet drugs targeting P2Y12Rs, propose a potential, formerly unrecognized side-effect to anti-platelet medications in patients with pulmonary morbidities.

The Emerging Role of the Mitochondrial Respiratory Chain in Skeletal Aging

Maintenance of mitochondrial homeostasis is crucial for ensuring healthy mitochondria and normal cellular function. This process is primarily responsible for regulating processes that include mitochondrial OXPHOS, which generates ATP, as well as mitochondrial oxidative stress, apoptosis, calcium homeostasis, and mitophagy. Bone mesenchymal stem cells express factors that aid in bone formation and vascular growth. Positive regulation of hematopoietic stem cells in the bone marrow affects the differentiation of osteoclasts. Furthermore, the metabolic regulation of cells that play fundamental roles in various regions of the bone, as well as interactions within the bone microenvironment, actively participates in regulating bone integrity and aging. The maintenance of cellular homeostasis is dependent on the regulation of intracellular organelles, thus understanding the impact of mitochondrial functional changes on overall bone metabolism is crucially important. Recent studies have revealed that mitochondrial homeostasis can lead to morphological and functional abnormalities in senescent cells, particularly in the context of bone diseases. Mitochondrial dysfunction in skeletal diseases results in abnormal metabolism of bone-associated cells and a secondary dysregulated microenvironment within bone tissue. This imbalance in the oxidative system and immune disruption in the bone microenvironment ultimately leads to bone dysplasia. In this review, we examine the latest developments in mitochondrial respiratory chain regulation and its impacts on maintenance of bone health. Specifically, we explored whether enhancing mitochondrial function can reduce the occurrence of bone cell deterioration and improve bone metabolism. These findings offer prospects for developing bone remodeling biology strategies to treat age-related degenerative diseases.

Mitochondria in hypoxic pulmonary hypertension, roles and the potential targets

Mitochondria are the centrol hub for cellular energy metabolisms. They regulate fuel metabolism by oxygen levels, participate in physiological signaling pathways, and act as oxygen sensors. Once oxygen deprived, the fuel utilizations can be switched from mitochondrial oxidative phosphorylation to glycolysis for ATP production. Notably, mitochondria can also adapt to hypoxia by making various functional and phenotypes changes to meet the demanding of oxygen levels. Hypoxic pulmonary hypertension is a life-threatening disease, but its exact pathgenesis mechanism is still unclear and there is no effective treatment available until now. Ample of evidence indicated that mitochondria play key factor in the development of hypoxic pulmonary hypertension. By hypoxia-inducible factors, multiple cells sense and transmit hypoxia signals, which then control the expression of various metabolic genes. This activation of hypoxia-inducible factors considered associations with crosstalk between hypoxia and altered mitochondrial metabolism, which plays an important role in the development of hypoxic pulmonary hypertension. Here, we review the molecular mechanisms of how hypoxia affects mitochondrial function, including mitochondrial biosynthesis, reactive oxygen homeostasis, and mitochondrial dynamics, to explore the potential of improving mitochondrial function as a strategy for treating hypoxic pulmonary hypertension.

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.

Mitochondrial oxygen sensing of acute hypoxia in specialized cells - Is there a unifying mechanism?

Acclimation to acute hypoxia through cardiorespiratory responses is mediated by specialized cells in the carotid body and pulmonary vasculature to optimize systemic arterial oxygenation and thus oxygen supply to the tissues. Acute oxygen sensing by these cells triggers hyperventilation and hypoxic pulmonary vasoconstriction which limits pulmonary blood flow through areas of low alveolar oxygen content. Oxygen sensing of acute hypoxia by specialized cells thus is a fundamental pre-requisite for aerobic life and maintains systemic oxygen supply. However, the primary oxygen sensing mechanism and the question of a common mechanism in different specialized oxygen sensing cells remains unresolved. Recent studies unraveled basic oxygen sensing mechanisms involving the mitochondrial cytochrome c oxidase subunit 4 isoform 2 that is essential for the hypoxia-induced release of mitochondrial reactive oxygen species and subsequent acute hypoxic responses in both, the carotid body and pulmonary vasculature. This review compares basic mitochondrial oxygen sensing mechanisms in the pulmonary vasculature and the carotid body.

Mitochondrial Redox Signaling in O2-Sensing Chemoreceptor Cells

Significance: Acute responses to hypoxia are essential for the survival of mammals. The carotid body (CB), the main arterial chemoreceptor, contains glomus cells with oxygen (O₂)-sensitive K⁺ channels, which are inhibited during hypoxia to trigger adaptive cardiorespiratory reflexes. Recent Advances: In this review, recent advances in molecular mechanisms of acute O₂ sensing in CB glomus cells are discussed, with a special focus on the signaling role of mitochondria through regulating cellular redox status. These advances have been achieved thanks to the use of genetically engineered redox-sensitive green fluorescent protein (roGFP) probes, which allowed us to monitor rapid changes in ROS production in real time in different subcellular compartments during hypoxia. This methodology was used in combination with conditional knockout mice models, pharmacological approaches, and transcriptomic studies. We have proposed a mitochondria-to-membrane signaling model of acute O₂ sensing in which H₂O₂ released in the mitochondrial intermembrane space serves as a signaling molecule to inhibit K⁺ channels on the plasma membrane. Critical Issues: Changes in mitochondrial reactive oxygen species (ROS) production during acute hypoxia are highly compartmentalized in the submitochondrial regions. The use of redox-sensitive probes targeted to specific compartments is essential to fully understand the role of mitochondrial ROS in acute O₂ sensing. Future Directions: Further studies are needed to specify the ROS and to characterize the target(s) of ROS in chemoreceptor cells during acute hypoxia. These data may also contribute to a more complete understanding of the implication of ROS in acute responses to hypoxia in O₂-sensing cells in other organs. Antioxid. Redox Signal. 37, 274-289.

Chemoreflex Control as the Cornerstone in Immersion Water Sports: Possible Role on Breath-Hold

Immersion water sports involve long-term apneas; therefore, athletes must physiologically adapt to maintain muscle oxygenation, despite not performing pulmonary ventilation. Breath-holding (i.e., apnea) is common in water sports, and it involves a decrease and increases PaO₂ and PaCO₂, respectively, as the primary signals that trigger the end of apnea. The principal physiological O₂ sensors are the carotid bodies, which are able to detect arterial gases and metabolic alterations before reaching the brain, which aids in adjusting the cardiorespiratory system. Moreover, the principal H⁺/CO₂ sensor is the retrotrapezoid nucleus, which is located at the brainstem level; this mechanism contributes to detecting respiratory and metabolic acidosis. Although these sensors have been characterized in pathophysiological states, current evidence shows a possible role for these mechanisms as physiological sensors during voluntary apnea. Divers and swimmer athletes have been found to displayed longer apnea times than land sports athletes, as well as decreased peripheral O₂ and central CO₂ chemoreflex control. However, although chemosensitivity at rest could be decreased, we recently found marked sympathoexcitation during maximum voluntary apnea in young swimmers, which could activate the spleen (which is a reservoir organ for oxygenated blood). Therefore, it is possible that the chemoreflex, autonomic function, and storage/delivery oxygen organ(s) are linked to apnea in immersion water sports. In this review, we summarized the available evidence related to chemoreflex control in immersion water sports. Subsequently, we propose a possible physiological mechanistic model that could contribute to providing new avenues for understanding the respiratory physiology of water sports.

Review: A history and perspective of mitochondria in the context of anoxia tolerance

Symbiosis is found throughout nature, but perhaps nowhere is it more fundamental than mitochondria in all eukaryotes. Since mitochondria were discovered and mechanisms of oxygen reduction characterized, an understanding gradually emerged that these organelles were involved not just in the combustion of oxygen, but also in the sensing of oxygen. While multiple hypotheses exist to explain the mitochondrial involvement in oxygen sensing, key elements are developing that include potassium channels and reactive oxygen species. To understand how mitochondria contribute to oxygen sensing, it is informative to study a model system which is naturally adapted to survive extended periods without oxygen. Amongst air-breathing vertebrates, the most highly adapted are western painted turtles (Chrysemys picta bellii), which overwinter in ice-covered and anoxic water bodies. Through research of this animal, it was postulated that metabolic rate depression is key to anoxic survival and that mitochondrial regulation is a key aspect. When faced with anoxia, excitatory neurotransmitter receptors in turtle brain are inhibited through mitochondrial calcium release, termed "channel arrest". Simultaneously, inhibitory GABAergic signalling contributes to the "synaptic arrest" of excitatory action potential firing through a pathway dependent on mitochondrial depression of ROS generation. While many pathways are implicated in mitochondrial oxygen sensing in turtles, such as those of adenosine, ATP turnover, and gaseous transmitters, an apparent point of intersection is the mitochondria. In this review we will explore how an organelle that was critical for organismal complexity in an oxygenated world has also become a potentially important oxygen sensor.

Are Multiple Mitochondrial Related Signalling Pathways Involved in Carotid Body Oxygen Sensing?

It is generally acknowledged that the carotid body (CB) type I cell mitochondria are unique, being inhibited by relatively small falls in PaO2 well above those known to inhibit electron transport in other cell types. This feature is suggested to allow for the CB to function as an acute O2 sensor, being stimulated and activating systemic protective reflexes before the metabolism of other cells becomes compromised. What is less clear is precisely how a fall in mitochondrial activity links to type I cell depolarisation, a process that is required for initiation of the chemotransduction cascade and post-synaptic action potential generation. Multiple mitochondrial/metabolic signalling mechanisms have been proposed including local generation of mitochondrial reactive oxygen species (mitoROS), a change in mitochondrial/cellular redox status, a fall in MgATP and an increase in lactate. Although each mechanism is based on compelling experimental evidence, they are all not without question. The current review aims to explore the importance of each of these signalling pathways in mediating the overall CB response to hypoxia. We suggest that there is unlikely to be a single mechanism, but instead multiple mitochondrial related signalling pathways are recruited at different PaO2s during hypoxia. Furthermore, it still remains to be determined if mitochondrial signalling acts independently or in partnership with extra-mitochondrial O2-sensors.

Oxygen Sensing: Physiology and Pathophysiology

Oxygen is such an essential element for life that multiple mechanisms have evolved to maintain oxygen homeostasis, including those which detect decreases in arterial O2 and generate adaptive responses to hypoxia [...].

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.

Carotid Body Inflammation: Role in Hypoxia and in the Anti-inflammatory Reflex

Emergent evidence indicates that the carotid body (CB) chemoreceptors may sense systemic inflammatory molecules, and is an afferent-arm of the anti-inflammatory reflex. Moreover, a pro-inflammatory milieu within the CB is involved in the enhanced CB chemosensory responsiveness to oxygen following sustained and intermittent hypoxia. In this review, we focus on the physio-pathological participation of CBs in inflammatory diseases, such as sepsis and intermittent hypoxia.

Mitochondrial acute oxygen sensing and signaling

Oxygen (O₂) is essential for life and therefore the supply of sufficient O₂ to the tissues is a major physiological challenge. In mammals, a deficit of O₂ (hypoxia) triggers rapid cardiorespiratory reflexes (e.g. hyperventilation and increased heart output) that within a few seconds increase the uptake of O₂ by the lungs and its distribution throughout the body. The prototypical acute O₂-sensing organ is the carotid body (CB), which contains sensory glomus cells expressing O₂-regulated ion channels. In response to hypoxia, glomus cells depolarize and release transmitters which activate afferent fibers terminating at the brainstem respiratory and autonomic centers. In this review, we summarize the basic properties of CB chemoreceptor cells and the essential role played by their specialized mitochondria in acute O₂ sensing and signaling. We focus on recent data supporting a "mitochondria-to-membrane signaling" model of CB chemosensory transduction. The possibility that the differential expression of specific subunit isoforms and enzymes could allow mitochondria to play a generalized adaptive O₂-sensing and signaling role in a wide variety of cells is also discussed.

Production of IL-6 and Phagocytosis Are the Most Resilient Immune Functions in Metabolically Compromised Human Monocytes

At sites of inflammation, monocytes carry out specific immune functions while facing challenging metabolic restrictions. Here, we investigated the potential of human monocytes to adapt to conditions of gradually inhibited oxidative phosphorylation (OXPHOS) under glucose free conditions. We used myxothiazol, an inhibitor of mitochondrial respiration, to adjust two different levels of decreased mitochondrial ATP production. At these levels, and compared to uninhibited OXPHOS, we assessed phagocytosis, production of reactive oxygen species (ROS) through NADPH oxidase (NOX), expression of surface activation markers CD16, CD80, CD11b, HLA-DR, and production of the inflammatory cytokines IL-1β, IL-6 and TNF-α in human monocytes. We found phagocytosis and the production of IL-6 to be least sensitive to metabolic restrictions while surface expression of CD11b, HLA-DR, production of TNF-α, IL-1β and production of ROS through NOX were most compromised by inhibition of OXPHOS in the absence of glucose. Our data demonstrate a short-term hierarchy of immune functions in human monocytes, which represents novel knowledge potentially leading to the development of new therapeutics in monocyte-mediated inflammatory diseases.

Plate-Based Respirometry to Assess Thermal Sensitivity of Zebrafish Embryo Bioenergetics in situ

Oxygen consumption allows measuring the metabolic activity of organisms. Here, we adopted the multi-well plate-based respirometry of the extracellular flux analyzer (Seahorse XF96) to investigate the effect of temperature on the bioenergetics of zebrafish embryos (Danio rerio) in situ. We show that the removal of the embryonic chorion is beneficial for oxygen consumption rates (OCR) and penetration of various mitochondrial inhibitors, and confirm that sedation reduces the variability of OCR. At 48h post-fertilization, embryos (maintained at a routine temperature of 28°C) were exposed to different medium temperatures ranging from 18°C to 37°C for 20h prior OCR measurement. Measurement temperatures from 18°C to 45°C in the XF96 were achieved by lowering the room temperature and active in-built heating. At 18°C assay temperature, basal OCR was low due to decreased ATP-linked respiration, which was not limited by mitochondrial power, as seen in substantial spare respiratory capacity. Basal OCR of the embryos increased with assay temperature and were stable up to 37°C assay temperature, with pre-exposure of 37°C resulting in more thermo-resistant basal OCR measured at 41°C. Adverse effects of the mitochondrial inhibitor oligomycin were seen at 37°C and chemical uncouplers disrupted substrate oxidation gradually with increasing assay temperature. Proton leak respiration increased at assay temperatures above 28°C and compromised the efficiency of ATP production, calculated as coupling efficiency. Thus, temperature impacts mitochondrial respiration by reduced cellular ATP turnover at lower temperatures and by increased proton leak at higher temperatures. This conclusion is coherent with the assessment of heart rate, an independent indicator of systemic metabolic rate, which increased with exposure temperature, peaking at 28°C, and decreased at higher temperatures. Collectively, plate-based respirometry allows assessing distinct parts of mitochondrial energy transduction in zebrafish embryos and investigating the effect of temperature and temperature acclimation on mitochondrial bioenergetics in situ.

Carotid body chemoreceptors: physiology, pathology, and implications for health and disease

The carotid body (CB) is the main peripheral chemoreceptor for arterial respiratory gases O2 and CO2 and pH, eliciting reflex ventilatory, cardiovascular, and humoral responses to maintain homeostasis. This review examines the fundamental biology underlying CB chemoreceptor function, its contribution to integrated physiological responses, and its role in maintaining health and potentiating disease. Emphasis is placed on 1) transduction mechanisms in chemoreceptor (type I) cells, highlighting the role played by the hypoxic inhibition of O2-dependent K+ channels and mitochondrial oxidative metabolism, and their modification by intracellular molecules and other ion channels; 2) synaptic mechanisms linking type I cells and petrosal nerve terminals, focusing on the role played by the main proposed transmitters and modulatory gases, and the participation of glial cells in regulation of the chemosensory process; 3) integrated reflex responses to CB activation, emphasizing that the responses differ dramatically depending on the nature of the physiological, pathological, or environmental challenges, and the interactions of the chemoreceptor reflex with other reflexes in optimizing oxygen delivery to the tissues; and 4) the contribution of enhanced CB chemosensory discharge to autonomic and cardiorespiratory pathophysiology in obstructive sleep apnea, congestive heart failure, resistant hypertension, and metabolic diseases and how modulation of enhanced CB reactivity in disease conditions may attenuate pathophysiology.

Scavenging of reactive oxygen species mimics the anoxic response in goldfish pyramidal neurons

Goldfish are one of a few species able to avoid cellular damage during month-long periods in severely hypoxic environments. By suppressing action potentials in excitatory glutamatergic neurons, the goldfish brain decreases its overall energy expenditure. Coincident with reductions in O2 availability is a natural decrease in cellular reactive oxygen species (ROS) generation, which has been proposed to function as part of a low-oxygen signal transduction pathway. Using live-tissue fluorescence microscopy, we found that ROS production decreased by 10% with the onset of anoxia in goldfish telencephalic brain slices. Employing whole-cell patch-clamp recording, we found that, similar to severe hypoxia, the ROS scavengers N-acetyl cysteine (NAC) and MitoTEMPO, added during normoxic periods, depolarized membrane potential (severe hypoxia -73.6 to -61.4 mV, NAC -76.6 to -66.2 mV and MitoTEMPO -71.5 mV to -62.5 mV) and increased whole-cell conductance (severe hypoxia 5.7 nS to 8.0 nS, NAC 6.0 nS to 7.5 nS and MitoTEMPO 6.0 nS to 7.6 nS). Also, in a subset of active pyramidal neurons, these treatments reduced action potential firing frequency (severe hypoxia 0.18 Hz to 0.03 Hz, NAC 0.27 Hz to 0.06 Hz and MitoTEMPO 0.35 Hz to 0.08 Hz). Neither severe hypoxia nor ROS scavenging impacted action potential threshold. The addition of exogenous hydrogen peroxide could reverse the effects of the antioxidants. Taken together, this supports a role for a reduction in [ROS] as a low-oxygen signal in goldfish brain.

Mitochondrial Succinate Metabolism and Reactive Oxygen Species Are Important but Not Essential for Eliciting Carotid Body and Ventilatory Responses to Hypoxia in the Rat

Reflex increases in breathing in response to acute hypoxia are dependent on activation of the carotid body (CB)—A specialised peripheral chemoreceptor. Central to CB O₂-sensing is their unique mitochondria but the link between mitochondrial inhibition and cellular stimulation is unresolved. The objective of this study was to evaluate if ex vivo intact CB nerve activity and in vivo whole body ventilatory responses to hypoxia were modified by alterations in succinate metabolism and mitochondrial ROS (mitoROS) generation in the rat. Application of diethyl succinate (DESucc) caused concentration-dependent increases in chemoafferent frequency measuring approximately 10–30% of that induced by severe hypoxia. Inhibition of mitochondrial succinate metabolism by dimethyl malonate (DMM) evoked basal excitation and attenuated the rise in chemoafferent activity in hypoxia. However, approximately 50% of the response to hypoxia was preserved. MitoTEMPO (MitoT) and 10-(6′-plastoquinonyl) decyltriphenylphosphonium (SKQ1) (mitochondrial antioxidants) decreased chemoafferent activity in hypoxia by approximately 20–50%. In awake animals, MitoT and SKQ1 attenuated the rise in respiratory frequency during hypoxia, and SKQ1 also significantly blunted the overall hypoxic ventilatory response (HVR) by approximately 20%. Thus, whilst the data support a role for succinate and mitoROS in CB and whole body O₂-sensing in the rat, they are not the sole mediators. Treatment of the CB with mitochondrial selective antioxidants may offer a new approach for treating CB-related cardiovascular–respiratory disorders.

Role of IP3 Receptors in Shaping the Carotid Chemoreceptor Response to Hypoxia But Not to Hypercapnia in the Rat Carotid Body: An Evidence Review

This article addresses the disparity in the transduction pathways for hypoxic and hypercapnic stimuli in carotid body glomus cells. We investigated and reviewed the experimental evidence showing that the response to hypoxia, but not to hypercapnia, is mediated by 1,4,5-inositol triphosphate receptors (IP₃R/s) regulating the intracellular calcium content [Ca²⁺]_c in glomus cells. The rationale was based on the past observations that inhibition of oxidative phosphorylation leads to the explicit inhibition of the hypoxic chemoreflex. [Ca²⁺]_c changes were measured using cellular Ca²⁺-sensitive fluorescent probes, and carotid sinus nerve (CSN) sensory discharge was recorded with bipolar electrodes in in vitro perfused-superfused rat carotid body preparations. The cell-permeant, 2-amino-ethoxy-diphenyl-borate (2-APB; 100 μM) and curcumin (50 μM) were used as the inhibitors of IP₃R/s. These agents suppressed the [Ca²⁺]_c, and CSN discharge increases in hypoxia but not in hypercapnia, leading to the conclusion that only the hypoxic effects were mediated via modulation of IP₃R/s. The ATP-induced Ca²⁺ release from intracellular stores in a Ca²⁺-free medium was blocked with 2-APB, supporting this conclusion.

A bioluminescent probe for longitudinal monitoring of mitochondrial membrane potential

Mitochondrial membrane potential (ΔΨm) is a universal selective indicator of mitochondrial function and is known to play a central role in many human pathologies, such as diabetes mellitus, cancer and Alzheimer's and Parkinson's diseases. Here, we report the design, synthesis and several applications of mitochondria-activatable luciferin (MAL), a bioluminescent probe sensitive to ΔΨm, and partially to plasma membrane potential (ΔΨp), for non-invasive, longitudinal monitoring of ΔΨm in vitro and in vivo. We applied this new technology to evaluate the aging-related change of ΔΨm in mice and showed that nicotinamide riboside (NR) reverts aging-related mitochondrial depolarization, revealing another important aspect of the mechanism of action of this potent biomolecule. In addition, we demonstrated application of the MAL probe for studies of brown adipose tissue (BAT) activation and non-invasive in vivo assessment of ΔΨm in animal cancer models, opening exciting opportunities for understanding the underlying mechanisms and for discovery of effective treatments for many human pathologies.

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

Carotid body glomus cells are multimodal arterial chemoreceptors able to sense and integrate changes in several physical and chemical parameters in the blood. These cells are also essential for O₂ homeostasis. Glomus cells are prototypical peripheral O₂ sensors necessary to detect hypoxemia and to elicit rapid compensatory responses (hyperventilation and sympathetic activation). The mechanisms underlying acute O₂ sensing by glomus cells have been elusive. Using a combination of mouse genetics and single-cell optical and electrophysiological techniques, it has recently been shown that activation of glomus cells by hypoxia relies on the generation of mitochondrial signals (NADH and reactive oxygen species), which modulate membrane ion channels to induce depolarization, Ca²⁺ influx, and transmitter release. The special sensitivity of glomus cell mitochondria to changes in O₂ tension is due to Hif2α-dependent expression of several atypical mitochondrial subunits, which are responsible for an accelerated oxidative metabolism and the strict dependence of mitochondrial complex IV activity on O₂ availability. A mitochondrial-to-membrane signaling model of acute O₂ sensing has been proposed, which explains existing data and provides a solid foundation for future experimental tests. This model has also unraveled new molecular targets for pharmacological modulation of carotid body activity potentially relevant in the treatment of highly prevalent medical conditions.

Green fluorescent protein-based lactate and pyruvate indicators suitable for biochemical assays and live cell imaging

Glycolysis is the metabolic pathway that converts glucose into pyruvate, whereas fermentation can then produce lactate from pyruvate. Here, we developed single fluorescent protein (FP)-based lactate and pyruvate indicators with low EC₅₀ for trace detection of metabolic molecules and live cell imaging and named them “Green Lindoblum” and “Green Pegassos,” respectively. Green Lindoblum (EC₅₀ of 30 µM for lactate) and Green Pegassos (EC₅₀ of 70 µM for pyruvate) produced a 5.2- and 3.3-fold change in fluorescence intensity in response to lactate and pyruvate, respectively. Green Lindoblum measured lactate levels in mouse plasma, and Green Pegassos in combination with D-serine dehydratase successfully estimated D-serine levels released from mouse primary cultured neurons and astrocytes by measuring pyruvate level. Furthermore, live cell imaging analysis revealed their utility for dual-colour imaging, and the interplay between lactate, pyruvate, and Ca²⁺ in human induced pluripotent stem cell-derived cardiomyocytes. Therefore, Green Lindoblum and Green Pegassos will be useful tools that detect specific molecules in clinical use and monitor the interplay of metabolites and other related molecules in diverse cell types.

Role of Plasma Gelsolin Protein in the Final Stage of Erythropoiesis and in Correction of Erythroid Dysplasia In Vitro

Gelsolin, an actin-remodeling protein, is involved in cell motility, cytoskeletal remodeling, and cytokinesis and is abnormally expressed in many cancers. Recently, human recombinant plasma gelsolin protein (pGSN) was reported to have important roles in cell cycle and maturation of primary erythroblasts. However, the role of human plasma gelsolin in late stage erythroblasts prior to enucleation and putative clinical relevance in patients with myelodysplastic syndrome (MDS) and hemato-oncologic diseases have not been reported. Polychromatic and orthochromatic erythroblasts differentiated from human cord blood CD34+ cells, and human bone marrow (BM) cells derived from patients with MDS, were cultured in serum-free medium containing pGSN. Effects of pGSN on mitochondria, erythroid dysplasia, and enucleation were assessed in cellular and transcriptional levels. With pGSN treatment, terminal maturation at the stage of poly- and ortho-chromatic erythroblasts was enhanced, with higher numbers of orthochromatic erythroblasts and enucleated red blood cells (RBCs). pGSN also significantly decreased dysplastic features of cell morphology. Moreover, we found that patients with MDS with multi-lineage dysplasia or with excess blasts-1 showed significantly decreased expression of gelsolin mRNA (GSN) in their peripheral blood. When BM erythroblasts of MDS patients were cultured with pGSN, levels of mRNA transcripts related to terminal erythropoiesis and enucleation were markedly increased, with significantly decreased erythroid dysplasia. Moreover, pGSN treatment enhanced mitochondrial transmembrane potential that is unregulated in MDS and cultured cells. Our findings demonstrate a key role for plasma gelsolin in erythropoiesis and in gelsolin-depleted MDS patients, and raises the possibility that pGSN administration may promote erythropoiesis in erythroid dysplasia.

The Synthetic Cannabinoids THJ-2201 and 5F-PB22 Enhance In Vitro CB1 Receptor-Mediated Neuronal Differentiation at Biologically Relevant Concentrations

Recreational use of synthetic cannabinoids (SCs) before and during pregnancy poses a major public health risk, due to the potential onset of neurodevelopmental disorders in the offspring. Herein, we report the assessment of the neurotoxic potential of two commonly abused SCs, THJ-2201 and 5F-PB22, particularly focusing on how they affect neuronal differentiation in vitro. Differentiation ratios, total neurite length, and neuronal marker expression were assessed in NG108-15 neuroblastoma x glioma cells exposed to the SCs at non-toxic, biologically relevant concentrations (≤1 μM), either in acute or repeated exposure settings. Both SCs enhanced differentiation ratios and total neurite length of NG108-15 cells near two-fold compared to vehicle-treated cells, in a CB₁R activation-dependent way, as the CB₁R blockade with a specific antagonist (SR141718) abrogated SC-induced effects. Interestingly, repeated 5F-PB22 exposure was required to reach effects similar to a single THJ-2201 dose. Cell viability and proliferation, mitochondrial membrane potential, and intracellular ATP levels were also determined. The tested SCs increased mitochondrial tetramethyl rhodamine ethyl ester (TMRE) accumulation after 24 h at biologically relevant concentrations but did not affect any of the other toxicological parameters. Overall, we report firsthand the CB₁R-mediated enhancement of neurodifferentiation by 5F-PB22 and THJ-2201 at biologically relevant concentrations.

AMPK and the Need to Breathe and Feed: What's the Matter with Oxygen?

We live and to do so we must breathe and eat, so are we a combination of what we eat and breathe? Here, we will consider this question, and the role in this respect of the AMP-activated protein kinase (AMPK). Emerging evidence suggests that AMPK facilitates central and peripheral reflexes that coordinate breathing and oxygen supply, and contributes to the central regulation of feeding and food choice. We propose, therefore, that oxygen supply to the body is aligned with not only the quantity we eat, but also nutrient-based diet selection, and that the cell-specific expression pattern of AMPK subunit isoforms is critical to appropriate system alignment in this respect. Currently available information on how oxygen supply may be aligned with feeding and food choice, or vice versa, through our motivation to breathe and select particular nutrients is sparse, fragmented and lacks any integrated understanding. By addressing this, we aim to provide the foundations for a clinical perspective that reveals untapped potential, by highlighting how aberrant cell-specific changes in the expression of AMPK subunit isoforms could give rise, in part, to known associations between metabolic disease, such as obesity and type 2 diabetes, sleep-disordered breathing, pulmonary hypertension and acute respiratory distress syndrome.

Synthetic Cannabinoids JWH-122 and THJ-2201 Disrupt Endocannabinoid-Regulated Mitochondrial Function and Activate Apoptotic Pathways as a Primary Mechanism of In Vitro Nephrotoxicity at In Vivo Relevant Concentrations

The widespread recreational use of synthetic cannabinoids (SCBs) represents a major public health issue, as reports of intoxications and deaths following SCB use rapidly mount up. Specifically, a direct link between SCB use and acute kidney injury (AKI) has been established, although the pathophysiologic mechanisms remain undefined. Here we assessed the in vitro nephrotoxicity of 3 commonly detected and structurally distinct SCBs-AB-FUBINACA, JWH-122, and THJ-2201-in human proximal tubule cells (HK-2), to ascertain potential similarities and/or differences regarding their nephrotoxicity signatures. We showed that 2 of the 3 SCBs tested, namely JWH-122 and THJ-2201, at in vivo relevant concentrations (1 nM-1 μM), triggered apoptotic cell death pathways, mainly through a shared mechanism involving the deregulation of mitochondrial function (ie, with mitochondrial membrane hyperpolarization and increased intracellular ATP levels), as the primary molecular signature of nephrotoxicity mechanism. Noteworthy, no SCB affected cell viability (MTT reduction, lactate dehydrogenase release, Neutral Red inclusion). Use of the cannabinoid receptor (CBR) antagonists SR141716A and SR144528, as well as HEK293T cells, which do not express CBRs, confirmed the involvement of these receptors in SCB-mediated mitochondrial membrane hyperpolarization but not on other events, suggesting an off-target action regulating SCB-induced kidney cell death. Our results further strengthen the relevance of the endocannabinoid system in maintaining mitochondrial function in kidney cells, as we demonstrate that HK-2 incubation with CBR antagonists or inhibitors of endocannabinoid biosynthesis (ie, methyl arachydonyl fluorophosphonate, tetrahydrolipstatin) alone produced deleterious effects similar to those now reported for SCBs. Overall, SCB-induced nephrotoxicity seems to be mainly regulated at the mitochondrial level, but the specific mechanisms involved require further clarification.

Physiology of the Carotid Body: From Molecules to Disease

The carotid body (CB) is an arterial chemoreceptor organ located in the carotid bifurcation and has a well-recognized role in cardiorespiratory regulation. The CB contains neurosecretory sensory cells (glomus cells), which release transmitters in response to hypoxia, hypercapnia, and acidemia to activate afferent sensory fibers terminating in the respiratory and autonomic brainstem centers. Knowledge of the physiology of the CB has progressed enormously in recent years. Herein we review advances concerning the organization and function of the cellular elements of the CB, with emphasis on the molecular mechanisms of acute oxygen sensing by glomus cells. We introduce the modern view of the CB as a multimodal integrated metabolic sensor and describe the properties of the CB stem cell niche, which support CB growth during acclimatization to chronic hypoxia. Finally, we discuss the increasing medical relevance of CB dysfunction and its potential impact on the mechanisms of disease.

Acute O2 sensing through HIF2α-dependent expression of atypical cytochrome oxidase subunits in arterial chemoreceptors

Acute cardiorespiratory responses to O₂ deficiency are essential for physiological homeostasis. The prototypical acute O₂-sensing organ is the carotid body, which contains glomus cells expressing K⁺ channels whose inhibition by hypoxia leads to transmitter release and activation of nerve fibers terminating in the brainstem respiratory center. The mechanism by which changes in O₂ tension modulate ion channels has remained elusive. Glomus cells express genes encoding HIF2α (Epas1) and atypical mitochondrial subunits at high levels, and mitochondrial NADH and reactive oxygen species (ROS) accumulation during hypoxia provides the signal that regulates ion channels. We report that inactivation of Epas1 in adult mice resulted in selective abolition of glomus cell responsiveness to acute hypoxia and the hypoxic ventilatory response. Epas1 deficiency led to the decreased expression of atypical mitochondrial subunits in the carotid body, and genetic deletion of Cox4i2 mimicked the defective hypoxic responses of Epas1-null mice. These findings provide a mechanistic explanation for the acute O₂ regulation of breathing, reveal an unanticipated role of HIF2α, and link acute and chronic adaptive responses to hypoxia.

Differential Mechanism of ATP Production Occurs in Response to Succinylacetone in Colon Cancer Cells

Our aim was to verify the potential ability of succinylacetone (SA) to inhibit mitochondrial function, thereby suppressing cancer cell proliferation. SA treatment caused apoptosis in HCT116 and HT29 cells, but not in SW480 cells, with mitochondria playing a key role. We checked for dysfunctional mitochondria after SA treatment. Mitochondria of HT29 cells were swollen, indicating damage, whereas in HCT116 cells, several mitochondria had a diminished size. Damaged mitochondria decreased ATP production and induced reactive oxygen species (ROS) in the cells. To understand SA-induced reduction in ATP production, we investigated the electron transfer chains (ETC) and pyruvate dehydrogenase kinase (PDK) activity, which prevents the transfer of acetyl-CoA to the TCA (tricarboxylic acid) cycle by inhibiting PDH (pyruvate dehydrogenase) activity. In each cell line, the inhibitory mechanism of ATP by SA was different. The activity of complex III consisting of the mitochondrial ETCs in HT29 cells was decreased. In contrast, PDH activity in HCT116 cells was reduced. Nicotinamide nucleotide transhydrogenase (NNT)-removing reactive oxygen species (ROS) was upregulated in HT29 cells, but not in HCT116 cells, indicating that in HT29 cells, a defense mechanism was activated against ROS. Collectively, our study showed a differential mechanism occurs in response to SA in colon cancer cells.

Physiologic Implications of Reactive Oxygen Species Production by Mitochondrial Complex I Reverse Electron Transport

Mitochondrial reactive oxygen species (ROS) can be either detrimental or beneficial depending on the amount, duration, and location of their production. Mitochondrial complex I is a component of the electron transport chain and transfers electrons from NADH to ubiquinone. Complex I is also a source of ROS production. Under certain thermodynamic conditions, electron transfer can reverse direction and reduce oxygen at complex I to generate ROS. Conditions that favor this reverse electron transport (RET) include highly reduced ubiquinone pools, high mitochondrial membrane potential, and accumulated metabolic substrates. Historically, complex I RET was associated with pathological conditions, causing oxidative stress. However, recent evidence suggests that ROS generation by complex I RET contributes to signaling events in cells and organisms. Collectively, these studies demonstrate that the impact of complex I RET, either beneficial or detrimental, can be determined by the timing and quantity of ROS production. In this article we review the role of site-specific ROS production at complex I in the contexts of pathology and physiologic signaling.

A readily usable two-photon fluorescence lifetime microendoscope

A two-photon fluorescence lifetime (2P-FLIM) microendoscope, capable of energetic metabolism imaging through the intracellular nicotinamide adenine dinucleotide (NADH) autofluorescence, at sub-cellular resolution, is demonstrated. It exhibits readily usable characteristics such as convenient endoscope probe diameter (≈2 mm), fiber length (>5 m) and data accumulation rate (16 frames per second (fps)), leading to a FLIM refreshing rate of ≈0.1 to 1 fps depending on the sample. The spiral scanning image formation does not influence the instrument response function (IRF) characteristics of the system. Near table-top microscope performances are achieved through a comprehensive system including a home-designed spectro-temporal pulse shaper and a custom air-silica double-clad photonic crystal fiber, which enables to reach up to 40 mW of ≈100 fs pulses @ 760 nm with a 80 MHz repetition rate. A GRadient INdex (GRIN) lens provides a lateral resolution of 0.67 μm at the focus of the fiber probe. Intracellular NADH fluorescence lifetime data are finally acquired on cultured cells at 16 fps.

Effects of lactate ions on the cardiorespiratory system in rainbow trout (Oncorhynchus mykiss)

Lactate ions are involved in several physiological processes, including a direct stimulation of the carotid body, causing increased ventilation in mammals. A similar mechanism eliciting ventilatory stimulation in other vertebrate classes has been demonstrated, but it remains to be thoroughly investigated. Here, we investigated the effects of lactate ions on the cardiorespiratory system in swimming rainbow trout by manipulating the blood lactate concentration. Lactate elicited a vigorous, dose-dependent elevation of ventilation and bradycardia at physiologically relevant concentrations at constant pH. After this initial confirmation, we examined the chiral specificity of the response and found that only l-lactate induced these effects. By removal of the afferent inputs from the first gill arch, the response was greatly attenuated, and a comparison of the responses to injections up- and downstream of the gills collectively demonstrated that the lactate response was initiated by branchial cells. Injection of specific receptor antagonists revealed that a blockade of serotonergic receptors, which are involved in the hypoxic ventilatory response, significantly reduced the lactate response. Finally, we identified two putative lactate receptors based on sequence homology and found that both were expressed at substantially higher levels in the gills. We propose that lactate ions modulate ventilation by stimulating branchial oxygen-sensing cells, thus eliciting a cardiorespiratory response through receptors likely to have originated early in vertebrate evolution.

Working with Hypoxia

A hypoxic environment can be defined as a region of the body or the whole body that is deprived of oxygen. Hypoxia is a feature of many diseases, such as cardiovascular disease, tissue trauma, stroke, and solid cancers. A loss of oxygen supply usually results in cell death; however, when cells gradually become hypoxic, they may survive and continue to thrive as described for conditions that promote metastatic growth. The role of hypoxia in these pathogenic pathways is therefore of great interest, and understanding the effect of hypoxia in regulating these mechanisms is fundamentally important. This chapter gives an extensive overview of these mechanisms. Moreover, given the challenges posed by tumor hypoxia we describe the current methods to simulate and detect hypoxic conditions followed by a discussion on current and experimental therapies that target hypoxic cells.

Recent advances in understanding the physiology of hypoxic sensing by the carotid body

Hypoxia resulting from reduced oxygen (O ₂) levels in the arterial blood is sensed by the carotid body (CB) and triggers reflex stimulation of breathing and blood pressure to maintain homeostasis. Studies in the past five years provided novel insights into the roles of heme oxygenase-2 (HO-2), a carbon monoxide (CO)-producing enzyme, and NADH dehydrogenase Fe-S protein 2, a subunit of the mitochondrial complex I, in hypoxic sensing by the CB. HO-2 is expressed in type I cells, the primary O2-sensing cells of the CB, and binds to O ₂ with low affinity. O ₂-dependent CO production from HO-2 mediates hypoxic response of the CB by regulating H ₂S generation. Mice lacking NDUFS2 show that complex I-generated reactive oxygen species acting on K ⁺ channels confer type I cell response to hypoxia. Whether these signaling pathways operate synergistically or independently remains to be studied.

Investigation of mitochondrial calcium uniporter role in embryonic and adult motor neurons from G93AhSOD1 mice

Amyotrophic lateral sclerosis is characterized by progressive death of motor neurons (MNs) with glutamate excitotoxicity and mitochondrial Ca²⁺ overload as critical mechanisms in disease pathophysiology. We used MNs from G93A^hSOD1 and nontransgenic embryonic cultures and adult mice to analyze the expression of the main mitochondrial calcium uniporter (MCU). MCU was overexpressed in cultured embryonic G93A^hSOD1 MNs compared to nontransgenic MNs but downregulated in MNs from adult G93A^hSOD1 mice. Furthermore, cultured embryonic G93A^hSOD1 were rescued from kainate-induced excitotoxicity by the Ca²⁺/calmodulin-dependent protein kinase type II inhibitor; KN-62, which reduced MCU expression in G93A^hSOD1 MNs. MCU activation via kaempferol neither altered MCU expression nor influenced MN survival. However, its acute application served as a fine tool to study spontaneous Ca²⁺ activity in cultured neurons which was significantly altered by the mutated hSOD1. Pharmacological manipulation of MCU expression might open new possibilities to fight excitotoxic damage in amyotrophic lateral sclerosis.

Pleiotropic Effects of GLP-1 and Analogs on Cell Signaling, Metabolism, and Function

The incretin hormone Glucagon-Like Peptide-1 (GLP-1) is best known for its "incretin effect" in restoring glucose homeostasis in diabetics, however, it is now apparent that it has a broader range of physiological effects in the body. Both in vitro and in vivo studies have demonstrated that GLP-1 mimetics alleviate endoplasmic reticulum stress, regulate autophagy, promote metabolic reprogramming, stimulate anti-inflammatory signaling, alter gene expression, and influence neuroprotective pathways. A substantial body of evidence has accumulated with respect to how GLP-1 and its analogs act to restore and maintain normal cellular functions. These findings have prompted several clinical trials which have reported GLP-1 analogs improve cardiac function, restore lung function and reduce mortality in patients with obstructive lung disease, influence blood pressure and lipid storage, and even prevent synaptic loss and neurodegeneration. Mechanistically, GLP-1 elicits its effects via acute elevation in cAMP levels, and subsequent protein kinase(s) activation, pathways well-defined in pancreatic β-cells which stimulate insulin secretion in conjunction with elevated Ca2+ and ATP. More recently, new studies have shed light on additional downstream pathways stimulated by chronic GLP-1 exposure, findings which have direct relevance to our understanding of the potential therapeutic effects of longer lasting analogs recently developed for clinical use. In this review, we provide a comprehensive description of the diverse roles for GLP-1 across multiple tissues, describe downstream pathways stimulated by acute and chronic exposure, and discuss novel pleiotropic applications of GLP-1 mimetics in the treatment of human disease.

A1899, PK-THPP, ML365, and Doxapram inhibit endogenous TASK channels and excite calcium signaling in carotid body type-1 cells

Sensing of hypoxia and acidosis in arterial chemoreceptors is thought to be mediated through the inhibition of TASK and possibly other (e.g., BKCa ) potassium channels which leads to membrane depolarization, voltage-gated Ca-entry, and neurosecretion. Here, we investigate the effects of pharmacological inhibitors on TASK channel activity and [Ca2+ ]i -signaling in isolated neonatal rat type-1 cells. PK-THPP inhibited TASK channel activity in cell attached patches by up to 90% (at 400 nmol/L). A1899 inhibited TASK channel activity by 35% at 400 nmol/L. PK-THPP, A1899 and Ml 365 all evoked a rapid increase in type-1 cell [Ca2+ ]i . These [Ca2+ ]i responses were abolished in Ca2+ -free solution and greatly attenuated by Ni2+ (2 mM) suggesting that depolarization and voltage-gated Ca2+ -entry mediated the rise in [Ca2+ ]i. Doxapram (50 μmol/L), a respiratory stimulant, also inhibited type-1 cell TASK channel activity and increased [Ca2+ ]i. . We also tested the effects of combined inhibition of BKCa and TASK channels. TEA (5 mmol/L) slightly increased [Ca2+ ]i in the presence of PK-THPP and A1899. Paxilline (300 nM) and iberiotoxin (50 nmol/L) also slightly increased [Ca2+ ]i in the presence of A1899 but not in the presence of PK-THPP. In general [Ca2+ ]i responses to TASK inhibitors, alone or in combination with BKCa inhibitors, were smaller than the [Ca2+ ]i responses evoked by hypoxia. These data confirm that TASK channel inhibition is capable of evoking membrane depolarization and robust voltage-gated Ca2+ -entry but suggest that this, even with concomitant inhibition of BKCa channels, may be insufficient to account fully for the [Ca2+ ]i -response to hypoxia.

Acute oxygen sensing-Role of metabolic specifications in peripheral chemoreceptor cells

Acute oxygen sensing is essential for humans under hypoxic environments or pathologic conditions. This is achieved by the carotid body (CB), the key arterial chemoreceptor, along with other peripheral chemoreceptor organs, such as the adrenal medulla (AM). Although it is widely accepted that inhibition of K⁺ channels in the plasma membrane of CB cells during acute hypoxia results in the activation of cardiorespiratory reflexes, the molecular mechanisms by which the hypoxic signal is detected to modulate ion channel activity are not fully understood. Using conditional knockout mice lacking mitochondrial complex I (MCI) subunit NDUFS2, we have found that MCI generates reactive oxygen species and pyridine nucleotides, which signal K⁺ channels during acute hypoxia. Comparing the transcriptomes from CB and AM, which are O₂-sensitive, with superior cervical ganglion, which is practically O₂-insensitive, we have found that CB and AM contain unique metabolic gene expression profiles. The "signature metabolic profile" and their biophysical characteristics could be essential for acute O₂ sensing by chemoreceptor cells.

Acute oxygen sensing by the carotid body: a rattlebag of molecular mechanisms

The molecular underpinnings of the oxygen sensitivity of the carotid body Type I cells are becoming better defined as research begins to identify potential interactions between previously separate theories. Nevertheless, the field of oxygen chemoreception still presents the general observer with a bewildering array of potential signalling pathways by which a fall in oxygen levels might initiate Type I cell activation. The purpose of this brief review is to address five of the current oxygen sensing hypotheses: the lactate-Olfr 78 hypothesis of oxygen chemotransduction; the role mitochondrial ATP and metabolism may have in chemotransduction; the AMP-activated protein kinase hypothesis and its current role in oxygen sensing by the carotid body; reactive oxygen species as key transducers in the oxygen sensing cascade; and the mechanisms by which H2 S, reactive oxygen species and haem oxygenase may integrate to provide a rapid oxygen sensing transduction system. Over the previous 15 years several lines of research into acute hypoxic chemotransduction mechanisms have focused on the integration of mitochondrial and membrane signalling. This review places an emphasis on the subplasmalemmal-mitochondrial microenvironment in Type I cells and how theories of acute oxygen sensing are increasingly dependent on functional interaction within this microenvironment.

High-performance reoxygenation from PLGA-PEG/PFOB emulsions: a feedback relationship between ROS and HIF-1α

Background

Hypoxemia is one of the most common pathological processes in various clinical diseases.

Methods

A novel emulsion of poly(lactide-co-glycolide)-poly(ethylene glycol)/perfluorooctyl bromide has been developed to improve arterial hypoxemia through pulmonary drug delivery. Hypoxia-reoxygenation experiment was used to investigate the ability of the emulsion to supply oxygen and the saline lavage acute lung injury model was established to evaluate oxygen supply of the emulsion.

Results

It has been demonstrated that an apparent increase has been detected in the cytotoxicity test of the emulsion, indicating its lower cell toxicity. A hypoxia-reoxygenation experiment uncovered the fact that notable cell growth was observed after reoxygenation with poly(lactide-co-glycolide)-poly(ethylene glycol)/perfluorooctyl bromide emulsion because of the ability of the emulsion to supply oxygen adequately and reasonably. Moreover, the level of intracellular reactive oxygen species was significantly enhanced during hypoxia, which further influenced the concentration and activity of hypoxia-inducible factor-1α (HIF-1α). Furthermore, the upregulated expression of HIF-1α during hypoxia has verified that certain emulsions can increase HIF-1α content and relieve hypoxia, which further indicates HIF-1α plays an essential role in improving cell viability. Afterwards, the saline lavage acute lung injury model was established to evaluate oxygen supply of the emulsion and the result shows considerable improvement of lung ventilation of rabbits.

Conclusion

We recommend that the feedback relationship between reactive oxygen species and HIF-1 plays an essential role in improving cell viability. It is anticipated that the emulsion will be applied in the field of alleviating hypoxemia.

Is Carotid Body Physiological O2 Sensitivity Determined by a Unique Mitochondrial Phenotype?

The mammalian carotid body (CB) is the primary arterial chemoreceptor that responds to acute hypoxia, initiating systemic protective reflex responses that act to maintain O2 delivery to the brain and vital organs. The CB is unique in that it is stimulated at O2 levels above those that begin to impact on the metabolism of most other cell types. Whilst a large proportion of the CB chemotransduction cascade is well defined, the identity of the O2 sensor remains highly controversial. This short review evaluates whether the mitochondria can adequately function as acute O2 sensors in the CB. We consider the similarities between mitochondrial poisons and hypoxic stimuli in their ability to activate the CB chemotransduction cascade and initiate rapid cardiorespiratory reflexes. We evaluate whether the mitochondria are required for the CB to respond to hypoxia. We also discuss if the CB mitochondria are different to those located in other non-O2 sensitive cells, and what might cause them to have an unusually low O2 binding affinity. In particular we look at the potential roles of competitive inhibitors of mitochondrial complex IV such as nitric oxide in establishing mitochondrial and CB O2-sensitivity. Finally, we discuss novel signaling mechanisms proposed to take place within and downstream of mitochondria that link mitochondrial metabolism with cellular depolarization.

The synthetic cannabinoid XLR-11 induces in vitro nephrotoxicity by impairment of endocannabinoid-mediated regulation of mitochondrial function homeostasis and triggering of apoptosis

Synthetic cannabinoids (SCBs)-related intoxications and deaths have been increasingly reported, turning its widespread recreational use into a major public health concern. Specifically, a direct link between SCBs and acute kidney injury (AKI) has been established. XLR-11 is an SCB commonly found in the toxicological analysis of patients with SCB-associated AKI. However, the pathophysiology of AKI among SCB consumers remains unknown. This work thus represents the first in vitro assessment of SCB nephrotoxicity, as a first approach to identify its cellular targets. We demonstrate that XLR-11, at biologically relevant concentrations (in the nanomolar range), primarily targets mitochondrial function in human proximal tubule (HK-2) cells, inducing a transient hyperpolarization of the mitochondrial membrane and increasing ATP production, accompanied by Bax translocation from cytosol into mitochondria. These phenomena further triggered energy-dependent apoptotic cell death pathways, indicated by increased caspase-3 activity and chromatin condensation. Experiments using SR141716A and SR144258, specific antagonists for CB1 and CB2 receptors, respectively, as well as HEK293T cells (which do not express CBRs) highlighted these processes' dependence on CBR activation. Nevertheless, ATP formation seemed to follow a CBR-independent pathway. Our findings using specific inhibitors of endogenous cannabinoids biosynthesis (i.e. MAFP and THL) further evidenced the involvement of the endocannabinoid system in the regulation of these processes, as XLR-11 binding to CBRs seemed to compromise endocannabinoid-mediated preservation of mitochondrial function. Nevertheless, the exact mechanisms involved require further clarification.

Acute oxygen sensing by the carotid body: from mitochondria to plasma membrane

Maintaining oxygen homeostasis is crucial to the survival of animals. Mammals respond acutely to changes in blood oxygen levels by modulating cardiopulmonary function. The major sensor of blood oxygen that regulates breathing is the carotid body (CB), a small chemosensory organ located at the carotid bifurcation. When arterial blood oxygen levels drop in hypoxia, neuroendocrine cells in the CB called glomus cells are activated to signal to afferent nerves that project to the brain stem. The mechanism by which hypoxia stimulates CB sensory activity has been the subject of many studies over the past 90 years. Two discrete models emerged that argue for the seat of oxygen sensing to lie either in the plasma membrane or mitochondria of CB cells. Recent studies are bridging the gap between these models by identifying hypoxic signals generated by changes in mitochondrial function in the CB that can be sensed by plasma membrane proteins on glomus cells. The CB is important for physiological adaptation to hypoxia, and its dysfunction contributes to sympathetic hyperactivity in common conditions such as sleep-disordered breathing, chronic heart failure, and insulin resistance. Understanding the basic mechanism of oxygen sensing in the CB could allow us to develop strategies to target this organ for therapy. In this short review, I will describe two historical models of CB oxygen sensing and new findings that are integrating these models.

Gene expression analyses reveal metabolic specifications in acute O2 -sensing chemoreceptor cells

KEY POINTS

Glomus cells in the carotid body (CB) and chromaffin cells in the adrenal medulla (AM) are essential for reflex cardiorespiratory adaptation to hypoxia. However, the mechanisms whereby these cells detect changes in O₂ tension are poorly understood. The metabolic properties of acute O₂ -sensing cells have been investigated by comparing the transcriptomes of CB and AM cells, which are O₂ -sensitive, with superior cervical ganglion neurons, which are practically O₂ -insensitive. In O₂ -sensitive cells, we found a characteristic prolyl hydroxylase 3 down-regulation and hypoxia inducible factor 2α up-regulation, as well as overexpression of genes coding for three atypical mitochondrial electron transport subunits and pyruvate carboxylase, an enzyme that replenishes tricarboxylic acid cycle intermediates. In agreement with this observation, the inhibition of succinate dehydrogenase impairs CB acute O₂ sensing. The responsiveness of peripheral chemoreceptor cells to acute hypoxia depends on a 'signature metabolic profile'.

ABSTRACT

Acute O₂ sensing is a fundamental property of cells in the peripheral chemoreceptors, e.g. glomus cells in the carotid body (CB) and chromaffin cells in the adrenal medulla (AM), and is necessary for adaptation to hypoxia. These cells contain O₂ -sensitive ion channels, which mediate membrane depolarization and transmitter release upon exposure to hypoxia. However, the mechanisms underlying the detection of changes in O₂ tension by cells are still poorly understood. Recently, we suggested that CB glomus cells have specific metabolic features that favour the accumulation of reduced quinone and the production of mitochondrial NADH and reactive oxygen species during hypoxia. These signals alter membrane ion channel activity. To investigate the metabolic profile characteristic of acute O₂ -sensing cells, we used adult mice to compare the transcriptomes of three cell types derived from common sympathoadrenal progenitors, but exhibiting variable responsiveness to acute hypoxia: CB and AM cells, which are O₂ -sensitive (glomus cells > chromaffin cells), and superior cervical ganglion neurons, which are practically O₂ -insensitive. In the O₂ -sensitive cells, we found a characteristic mRNA expression pattern of prolyl hydroxylase 3/hypoxia inducible factor 2α and up-regulation of several genes, in particular three atypical mitochondrial electron transport subunits and some ion channels. In addition, we found that pyruvate carboxylase, an enzyme fundamental to tricarboxylic acid cycle anaplerosis, is overexpressed in CB glomus cells. We also observed that the inhibition of succinate dehydrogenase impairs CB acute O₂ sensing. Our data suggest that responsiveness to acute hypoxia depends on a 'signature metabolic profile' in chemoreceptor cells.

Redox signaling in acute oxygen sensing

Acute oxygen (O₂) sensing is essential for individuals to survive under hypoxic conditions. The carotid body (CB) is the main peripheral chemoreceptor, which contains excitable and O₂-sensitive glomus cells with O₂-regulated ion channels. Upon exposure to acute hypoxia, inhibition of K⁺ channels is the signal that triggers cell depolarization, transmitter release and activation of sensory fibers that stimulate the brainstem respiratory center to produce hyperventilation. The molecular mechanisms underlying O₂ sensing by glomus cells have, however, remained elusive. Here we discuss recent data demonstrating that ablation of mitochondrial Ndufs2 gene selectively abolishes sensitivity of glomus cells to hypoxia, maintaining responsiveness to hypercapnia or hypoglycemia. These data suggest that reactive oxygen species and NADH generated in mitochondrial complex I during hypoxia are signaling molecules that modulate membrane K⁺ channels. We propose that the structural substrates for acute O₂ sensing in CB glomus cells are “O₂-sensing microdomains” formed by mitochondria and neighboring K⁺ channels in the plasma membrane.

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Acute O₂ sensing by peripheral chemoreceptors depends on K⁺ channels.

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Mitochondrial complex I function is required for acute O₂ sensing.

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Reactive oxygen species inhibits background K⁺ channels during acute hypoxia.

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Pyridine nucleotides may signal voltage-gated K⁺ channels during acute hypoxia.

Voltage- and receptor-mediated activation of a non-selective cation channel in rat carotid body glomus cells

A recent study showed that hypoxia activates a Ca²⁺-sensitive, Na⁺-permeable non-selective cation channel (NSC) in carotid body glomus cells. We studied the effects of mitochondrial inhibitors that increase Ca²⁺ influx via Ca²⁺ channel (Ca_v), and receptor agonists that release Ca²⁺ from endoplasmic reticulum (ER) on NSC. Mitochondrial inhibitors (NaCN, FCCP, H₂S, NO) elevated [Ca²⁺]_i and activated NSC. Angiotensin II and acetylcholine that elevate [Ca²⁺]_i via the Gq-IP₃ pathway activated NSC. However, endothelin-1 (Gq) and 5-HT (Gq) showed little or no effect on [Ca²⁺]_i and did not activate NSC. Adenosine (Gs) caused a weak rise in [Ca²⁺]_i but did not activate NSC. Dopamine (Gs) and γ-aminobytyric acid (Gi) were ineffective in raising [Ca²⁺]_i and failed to activate NSC. Store-operated Ca²⁺ entry (SOCE) produced by depletion of Ca²⁺ stores with cyclopiazonic acid activated NSC. Our results show that Ca²⁺ entry via Ca_v, ER Ca²⁺ release and SOCE can activate NSC. Thus, NSC contributes to both voltage- and receptor-mediated excitation of glomus cells.

Moderate inhibition of mitochondrial function augments carotid body hypoxic sensitivity

A functional role for the mitochondria in acute O2 sensing in the carotid body (CB) remains undetermined. Whilst total inhibition of mitochondrial activity causes intense CB stimulation, it is unclear whether this response can be moderated such that graded impairment of oxidative phosphorylation might be a mechanism that sets and modifies the O2 sensitivity of the whole organ. We assessed NADH autofluorescence and [Ca2+]i in freshly dissociated CB type I cells and sensory chemoafferent discharge frequency in an intact CB preparation, in the presence of varying concentrations of nitrite (NO2 −), a mitochondrial nitric oxide (NO) donor and a competitive inhibitor of mitochondrial complex IV. NO2 − increased CB type I cell NADH in a manner that was dose-dependent and rapidly reversible. Similar concentrations of NO2 − raised type I cell [Ca2+]i via L-type channels in a PO2-dependent manner and increased chemoafferent discharge frequency. Moderate inhibition of the CB mitochondria by NO2 − augmented chemoafferent discharge frequency during graded hypoxia, consistent with a heightened CB O2 sensitivity. Furthermore, NO2 − also exaggerated chemoafferent excitation during hypercapnia signifying an increase in CB CO2 sensitivity. These data show that NO2 − can moderate the hypoxia sensitivity of the CB and thus suggest that O2 sensitivity could be set and modified in this organ by interactions between NO and mitochondrial complex IV.

The emerging role of AMPK in the regulation of breathing and oxygen supply

Regulation of breathing is critical to our capacity to accommodate deficits in oxygen availability and demand during, for example, sleep and ascent to altitude. It is generally accepted that a fall in arterial oxygen increases afferent discharge from the carotid bodies to the brainstem and thus delivers increased ventilatory drive, which restores oxygen supply and protects against hypoventilation and apnoea. However, the precise molecular mechanisms involved remain unclear. We recently identified as critical to this process the AMP-activated protein kinase (AMPK), which is key to the cell-autonomous regulation of metabolic homoeostasis. This observation is significant for many reasons, not least because recent studies suggest that the gene for the AMPK-α1 catalytic subunit has been subjected to natural selection in high-altitude populations. It would appear, therefore, that evolutionary pressures have led to AMPK being utilized to regulate oxygen delivery and thus energy supply to the body in the short, medium and longer term. Contrary to current consensus, however, our findings suggest that AMPK regulates ventilation at the level of the caudal brainstem, even when afferent input responses from the carotid body are normal. We therefore hypothesize that AMPK integrates local hypoxic stress at defined loci within the brainstem respiratory network with an index of peripheral hypoxic status, namely afferent chemosensory inputs. Allied to this, AMPK is critical to the control of hypoxic pulmonary vasoconstriction and thus ventilation-perfusion matching at the lungs and may also determine oxygen supply to the foetus by, for example, modulating utero-placental blood flow.