Mitochondrial Complex IV Subunit 4 Isoform 2 Is Essential for Acute Pulmonary Oxygen Sensing

Piezo1 in PASMCs: Critical for Hypoxia-Induced Pulmonary Hypertension Development

BACKGROUND

Pulmonary hypertension (PH) is a life-threatening and progressive yet incurable disease. The hallmarks of PH comprise (1) sustained contraction and (2) excessive proliferation of pulmonary arterial smooth muscle cells (PASMCs). A major stimulus to which PASMCs are exposed during PH development is altered mechanical stress, originating from increased blood pressure, changes in blood flow velocity, and a progressive stiffening of pulmonary arteries. Mechanosensitive ion channels, including Piezo1 (Piezo-type mechanosensitive ion channel component-1), perceive such mechanical stimuli and translate them into a variety of cellular responses, including contractility or proliferation. Thus, the objective of the present study was to elucidate the specific role of Piezo1 in PASMCs for PH development and progression.

METHODS

The cell-type specific function of Piezo1 in PH was assessed in (1) PASMCs and lung tissues from patients with PH and (2) 2 mouse strains characterized by smooth muscle cell-specific, conditional Piezo1 knockout. Taking advantage of these strains, the smooth muscle cell-specific role of Piezo1 in PH development and progression was assessed in isolated, perfused, and ventilated mouse lungs, wire myography, and proliferation assays. Finally, in vivo function of smooth muscle cell-specific Piezo1 knockout was evaluated upon induction of chronic hypoxia-induced PH in these mice with insights into pulmonary vascular cell senescence.

RESULTS

Compared with healthy controls, PASMCs from patients with PH featured an elevated Piezo1 expression and increased proliferative phenotype. Smooth muscle cell-specific Piezo1 deletion, as confirmed via quantitative real-time polymerase chain reaction and patch clamp recordings, prevented the hypoxia-induced increase in PASMC proliferation in mice. Moreover, Piezo1 knockout reduced hypoxic pulmonary vasoconstriction in isolated, perfused, and ventilated mouse lungs, endothelial-denuded pulmonary arteries, and hemodynamic measurements in vivo. Consequently, Piezo1-deficient mice were considerably protected against chronic hypoxia-induced PH development with ameliorated right heart hypertrophy and improved hemodynamic function. In addition, distal pulmonary capillaries were preserved in the Piezo1-knockout mice, associated with a lower number of senescent endothelial cells.

CONCLUSIONS

This study provides evidence that Piezo1 expressed in PASMCs is critically involved in the pathogenesis of PH by controlling pulmonary vascular tone, arterial remodeling, and associated lung capillary rarefaction due to endothelial cell senescence.

The carotid body oxygen sensor

Carotid body (CB) chemoreceptor glomus cells sense hypoxia through the inhibition of plasmalemmal K+ channels, which leads to the opening of Ca2+ channels, Ca2+ influx, and neurotransmitter release. The mechanism of O2 sensing and the regulation of membrane ion channels by O2 have remained undefined and a subject of debate. Here, we summarize the molecular pathway that underlies acute O2 sensing in the CB. This process does not rely on a single-molecule O2 sensor expressed in glomus cells but rather on HIF2α-dependent genetically specialized mitochondria that can detect changes in O2 tension, within physiological ranges, and generate biochemical signals that regulate membrane ion channels. The acute O2-sensing pathway in glomus cells could provide new targets for respiratory and cardiovascular pharmacology.

Short-term sustained hypoxia distinctly affects subpopulations of carotid body glomus cells from rats

The main O2 arterial chemoreceptors are the carotid bodies (CBs), which mediate hyperventilation in response to short-term sustained hypoxia (SH). CBs contain glomus cells expressing K+ channels, which are inhibited by hypoxia, leading to neurotransmitter release. ATP released by CBs and type II cells has been considered essential for chemosensory processing under physiological and pathophysiological conditions. Although the systemic effects of chronic activation of CBs by SH are well known, the early (first 24 h) cellular and molecular mechanisms in CBs as well as the effects of short-term SH on populations of glomus cells are still poorly understood. Here, we show that SH (10% O2 for 24 h) depolarizes the membrane potential of one population of glomus cells, mediated by increases in inward current, but does not affect the ATP release by CBs. In addition, SH promotes a reduction in their maximum outward current, mediated by voltage-gated K+ channels. SH also affected sensitivity to acute hypoxia in one glomus cell subpopulation. As for the content of mitochondrial proteins, we observed increases in the citrate synthase, Tom-20, and succinate dehydrogenase (mitochondrial complex II) per cell of CBs after SH. Our results demonstrate important cellular and molecular mechanisms of plasticity in CBs from rats after only 24 h of SH, which may contribute to the generation of cardiovascular and ventilatory adjustments observed in this experimental model.NEW & NOTEWORTHY Our study revealed two subpopulations of glomus cells of carotid bodies (CBs) with specific electrophysiological properties, which were differentially affected by short-term sustained hypoxia (SH; 10% O2 for 24 h). Our experiments showed that SH also affected the sensitivity to acute hypoxia of these glomus cell subpopulations differently. Our molecular analyses allowed us to identify important adaptations in the content of CB mitochondrial proteins that participate in the Krebs cycle and form the electron transport chain.

Emerging insights into pulmonary hypertension: the potential role of mitochondrial dysfunction and redox homeostasis

Pulmonary hypertension (PH) is heterogeneous diseases that can lead to death due to progressive right heart failure. Emerging evidence suggests that, in addition to its role in ATP production, changes in mitochondrial play a central role in their pathogenesis, regulating integrated metabolic and signal transduction pathways. This review focuses on the basic principles of mitochondrial redox status in pulmonary vascular and right ventricular disorders, a series of dysfunctional processes including mitochondrial quality control (mitochondrial biogenesis, mitophagy, mitochondrial dynamics, mitochondrial unfolded protein response) and mitochondrial redox homeostasis. In addition, we will summarize how mitochondrial renewal and dynamic changes provide innovative insights for studying and evaluating PH. This will provide us with a clearer understanding of the initial signal transmission of mitochondria in PH, which would further improve our understanding of the pathogenesis of PH.

Dysfunction in mitochondrial electron transport chain drives the pathogenesis of pulmonary arterial hypertension: insights from a multi-omics investigation

BACKGROUND

Pulmonary arterial hypertension (PAH) is a progressive disorder that can lead to right ventricular failure and severe consequences. Despite extensive efforts, limited progress has been made in preventing the progression of PAH. Mitochondrial dysfunction is implicated in the development of PAH, but the key mitochondrial functional alterations in the pathogenesis have yet to be elucidated.

METHODS

We integrated three microarray datasets from the Gene Expression Omnibus (GEO), including 222 lung samples (164 PAH, 58 controls), for differential expression and functional enrichment analyses. Machine learning identified key mitochondria-related signaling pathways. PAH and control lung tissue samples were collected, and transcriptomic and metabolomic profiling were performed. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis investigated shared pathways, and canonical correlation analysis assessed gene-metabolite relationships.

RESULTS

In the GEO datasets, mitochondria-related signaling pathways were significantly enriched in PAH samples, in particular the electron transport chain (ETC) in mitochondrial oxidative phosphorylation system. Notably, the electron transport from cytochrome c to oxygen in ETC was identified as the most crucial mitochondria-related pathway, which was down-regulated in PAH samples. Transcriptomic profiling of the clinical lung tissue analysis identified 14 differentially expressed genes (DEGs) related to mitochondrial function. Metabolomic analysis revealed three differential metabolites in PAH samples: increased 3-phenyllactic acid and ADP, and decreased citric acid. Mitochondria-related genes highly correlated with these metabolites included KIT, OTC, CAMK2A, and CHRNA1.

CONCLUSIONS

Down-regulation of electron transport from cytochrome c to oxygen in mitochondrial ETC and disruption of the citric acid cycle homeostasis may contribute to PAH pathogenesis. 3-phenyllactic acid emerges as a potential novel diagnostic biomarker for PAH. These findings offer insights for developing novel PAH therapies and diagnostics.

Regulation of mitochondrial oxidative phosphorylation through tight control of cytochrome c oxidase in health and disease - Implications for ischemia/reperfusion injury, inflammatory diseases, diabetes, and cancer

Mitochondria are essential to cellular function as they generate the majority of cellular ATP, mediated through oxidative phosphorylation, which couples proton pumping of the electron transport chain (ETC) to ATP production. The ETC generates an electrochemical gradient, known as the proton motive force, consisting of the mitochondrial membrane potential (ΔΨm, the major component in mammals) and ΔpH across the inner mitochondrial membrane. Both ATP production and reactive oxygen species (ROS) are linked to ΔΨm, and it has been shown that an imbalance in ΔΨm beyond the physiological optimal intermediate range results in excessive ROS production. The reaction of cytochrome c oxidase (COX) of the ETC with its small electron donor cytochrome c (Cytc) is the proposed rate-limiting step in mammals under physiological conditions. The rate at which this redox reaction occurs controls ΔΨm and thus ATP and ROS production. Multiple mechanisms are in place that regulate this reaction to meet the cell's energy demand and respond to acute stress. COX and Cytc have been shown to be regulated by all three main mechanisms, which we discuss in detail: allosteric regulation, tissue-specific isoforms, and post-translational modifications for which we provide a comprehensive catalog and discussion of their functional role with 55 and 50 identified phosphorylation and acetylation sites on COX, respectively. Disruption of these regulatory mechanisms has been found in several common human diseases, including stroke and myocardial infarction, inflammation including sepsis, and diabetes, where changes in COX or Cytc phosphorylation lead to mitochondrial dysfunction contributing to disease pathophysiology. Identification and subsequent targeting of the underlying signaling pathways holds clear promise for future interventions to improve human health. An example intervention is the recently discovered noninvasive COX-inhibitory infrared light therapy that holds promise to transform the current standard of clinical care in disease conditions where COX regulation has gone awry.

Mitochondrial proteins as therapeutic targets in diabetic ketoacidosis: evidence from Mendelian randomization analysis

Introduction

Diabetic ketoacidosis (DKA) is a severe and potentially fatal acute complication in diabetic patients, commonly occurring in type 1 diabetes (T1D) but also seen in type 2 diabetes (T2D). The pathogenesis of DKA involves complex physiological processes that are not fully understood, especially the role of mitochondria. Mitochondria, known as the powerhouse of cells, plays a crucial role in oxidative phosphorylation and ATP production, which is vital in various metabolic diseases, including diabetes. However, the exact causal relationship between mitochondrial dysfunction and DKA remains unclear.

Methods

This study employed Mendelian randomization (MR) analysis and protein-protein interaction (PPI) networks to systematically explore the causal relationships between mitochondrial DNA copy number (mtDNA-CN) and specific mitochondrial proteins with DKA. We used bidirectional MR analysis and genome-wide association study (GWAS) data from openGWAS database to investigate the causal effects of mtDNA-CN and 64 mitochondrial-related proteins on DKA and its subtypes (T1DKA, T2DKA, unspecified-DKA).

Results

The study revealed that increased mtDNA-CN significantly reduces the risk of DKA, whereas the effect of DKA on mtDNA-CN was not significant. Mitochondrial-related proteins such as MRPL32, MRPL33, COX5B, DNAJC19, and NDUFB8 showed a negative causal relationship with DKA, indicating their potential protective roles. Conversely, ATP5F1B and COX4I2 have a positive causal relationship with DKA, indicating that excessive ATP production in diabetic patients may be detrimental to health and increase the risk of severe complications such as DKA.

Discussion

The results emphasize the necessity of protecting mitochondrial function in order to reduce the risk of DKA. The study offers novel perspectives on the molecular pathways involved in DKA, emphasizing the critical functions of mt-DNA and distinct proteins. These evidences not only enhance our comprehension of the implications of mitochondrial dysfunction in diabetes-related complications but also identify potential therapeutic targets for individualized treatment approaches, thereby making a substantial contribution to clinical care and public health initiatives.

Revisiting reactive oxygen species production in hypoxia

Cellular responses to hypoxia are crucial in various physiological and pathophysiological contexts and have thus been extensively studied. This has led to a comprehensive understanding of the transcriptional response to hypoxia, which is regulated by hypoxia-inducible factors (HIFs). However, the detailed molecular mechanisms of HIF regulation in hypoxia remain incompletely understood. In particular, there is controversy surrounding the production of mitochondrial reactive oxygen species (ROS) in hypoxia and how this affects the stabilization and activity of HIFs. This review examines this controversy and attempts to shed light on its origin. We discuss the role of physioxia versus normoxia as baseline conditions that can affect the subsequent cellular response to hypoxia and highlight the paucity of data on pericellular oxygen levels in most experiments, leading to variable levels of hypoxia that might progress to anoxia over time. We analyze the different outcomes reported in isolated mitochondria, versus intact cells or whole organisms, and evaluate the reliability of various ROS-detecting tools. Finally, we examine the cell-type and context specificity of oxygen's various effects. We conclude that while recent evidence suggests that the effect of hypoxia on ROS production is highly dependent on the cell type and the duration of exposure, efforts should be made to conduct experiments under carefully controlled, physiological microenvironmental conditions in order to rule out potential artifacts and improve reproducibility in research.

Hif1α-dependent mitochondrial acute O2 sensing and signaling to myocyte Ca2+ channels mediate arterial hypoxic vasodilation

Vasodilation in response to low oxygen (O2) tension (hypoxic vasodilation) is an essential homeostatic response of systemic arteries that facilitates O2 supply to tissues according to demand. However, how blood vessels react to O2 deficiency is not well understood. A common belief is that arterial myocytes are O2-sensitive. Supporting this concept, it has been shown that the activity of myocyte L-type Ca2+channels, the main ion channels responsible for vascular contractility, is reversibly inhibited by hypoxia, although the underlying molecular mechanisms have remained elusive. Here, we show that genetic or pharmacological disruption of mitochondrial electron transport selectively abolishes O2 modulation of Ca2+ channels and hypoxic vasodilation. Mitochondria function as O2 sensors and effectors that signal myocyte Ca2+ channels due to constitutive Hif1α-mediated expression of specific electron transport subunit isoforms. These findings reveal the acute O2-sensing mechanisms of vascular cells and may guide new developments in vascular pharmacology.

Use of FRET-Sensor 'Mermaid' to Detect Subtle Changes in Membrane Potential of Primary Mouse PASMCs

Subtle changes in the membrane potential of pulmonary arterial smooth muscle cells (PASMCs) are pivotal for controlling pulmonary vascular tone, e.g., for initiating Hypoxic Pulmonary Vasoconstriction, a vital mechanism of the pulmonary circulation. In our study, we evaluated the ability of the fluorescence resonance energy transfer (FRET)-based voltage-sensor Mermaid to detect such subtle changes in membrane potential. Mouse PASMCs were isolated and transduced with Mermaid-encoding lentiviral vectors before the acceptor/donor emission ratio was assessed via live cell FRET-imaging. Mermaid's sensitivity was tested by applying specific potassium chloride (KCl) concentrations. These KCl concentrations were previously validated by patch clamp recordings to induce depolarization with predefined amplitudes that physiologically occur in PASMCs. Mermaid's emission ratio dose-dependently increased upon depolarization with KCl. However, Mermaid formed unspecific intracellular aggregates, which limited the usefulness of this voltage sensor. When analyzing the membrane rim only to circumvent these unspecific signals, Mermaid was not suitable to resolve subtle changes in the membrane potential of ≤10 mV. In summary, we found Mermaid to be a suitable alternative for reliably detecting qualitative membrane voltage changes of more than 10 mV in primary mouse PASMCs. However, one should be aware of the limitations associated with this voltage sensor.

Hyperbaric oxygen treatment reveals spatiotemporal OXPHOS plasticity in the porcine heart

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

The ratio of cytochrome c oxidase subunit 4 isoform 4I1 and 4I2 mRNA is changed in permanent atrial fibrillation

AIMS

The conditions of hypoxia are suggested to induce permanent atrial fibrillation (AF). The regulation of COX4I2 and COX4I1 depends on oxygen availability in tissues. A role of COX4I2 in the myocardium of AF patients is supposed for pathogenesis of AF and subsequent alterations in the electron transfer chain (ETC) under hypoxia.

METHODS AND RESULTS

In vitro, influence of hypoxia on HeLa 53 cells was studied and elevated parts of COX 4I2 were confirmed. Myocardial biopsies were taken ex vivo from the patients' Right Atria with SR (n = 31) and AF (n = 11), respectively. RT- PCR for mRNA expresson, mitochondrial respiration by polarography and the protein content of cytochrome c oxidase (CytOx) subunit 4I1 and CytOx subunit 4I2 by ELISA were studied. Clinical data were correlated to the findings of gene expressions in parallel. Patients with permanent AF had a change in isoform 4I2/4I1 expression along with a decrease of isoform COX 4I1 expression. The 4I2/4I1 ratio of mRNA expression was increased from 0.630 to 1.058 in comparison. However, the protein content of CytOx subunit 4 was much lower in the AF group, whereas the respiration/units enzyme activity in both groups remained the same.

CONCLUSIONS

This study describes a possible molecular correlate for the development of AF. Due to the known functional significance of COX 4I2, mitochondrial dysfunction can be assumed as a part of the pathogenesis of AF.

Low-Salt Diet Regulates the Metabolic and Signal Transduction Genomic Fabrics, and Remodels the Cardiac Normal and Chronic Pathological Pathways

Low-salt diet (LSD) is a constant recommendation to hypertensive patients, but the genomic mechanisms through which it improves cardiac pathophysiology are still not fully understood. Our publicly accessible transcriptomic dataset of the left ventricle myocardium of adult male mice subjected to prolonged LSD or normal diet was analyzed from the perspective of the Genomic Fabric Paradigm. We found that LSD shifted the metabolic priorities by increasing the transcription control for fatty acids biosynthesis while decreasing it for steroid hormone biosynthesis. Moreover, LSD remodeled pathways responsible for cardiac muscle contraction (CMC), chronic Chagas (CHA), diabetic (DIA), dilated (DIL), and hypertrophic (HCM) cardiomyopathies, and their interplays with the glycolysis/glucogenesis (GLY), oxidative phosphorylation (OXP), and adrenergic signaling in cardiomyocytes (ASC). For instance, the statistically (p < 0.05) significant coupling between GLY and ASC was reduced by LSD from 13.82% to 2.91% (i.e., -4.75×), and that of ASC with HCM from 10.50% to 2.83% (-3.71×). The substantial up-regulation of the CMC, ASC, and OXP genes, and the significant weakening of the synchronization of the expression of the HCM, CHA, DIA, and DIL genes within their respective fabrics justify the benefits of the LSD recommendation.

Transcriptomic Analysis of Postnatal Rat Carotid Body Development

The carotid body (CB), located bilaterally at the carotid artery bifurcations, is the primary sensory organ for monitoring arterial blood O2 levels. Carotid bodies are immature at birth, exhibiting low sensitivity to hypoxia, and become more sensitive with maturation during the first few weeks of neonatal life. To understand the molecular basis for the postnatal developmental hypoxic responses of CB, we isolated CBs from 5-day and 21-day-old Sprague-Dawley rats and performed RNA sequencing, which allows comprehensive analysis of gene expression. Differentially expressed genes (DEGs) were generated using Edge R, while functional enrichment analysis was performed using gene-set enrichment analysis (GSEA). Analysis of RNA-Seq data showed 2604 DEGs of the total 12,696 genes shared between neonates and adults. Of the 2604 DEGs, 924 genes were upregulated, and 1680 genes were downregulated. Further analysis showed that genes related to oxidative phosphorylation (Ox/phos) and hypoxia-signaling pathways were significantly upregulated in neonatal CBs compared to adult CBs, suggesting a possible link to differential developmental hypoxic responses seen in CB. Genes related to cytokine signaling (INFγ and TNFα) and transcription factors (CREB and NFΚB) mediated pathways were enriched in adult CBs, suggesting that expression of these pathways may be linked to developmental regulation. The RNA-Seq results were verified by analyzing mRNA changes in selected genes by qRT-PCR. Our results of enrichment analysis of biological pathways offer valuable insight into CB hypoxic sensing responses related to the development process.

Spatial adjustment of bioenergetics, a possible determinant of contractile adaptation and development of contractile failure

Cardiomyocytes depend on mitochondrial oxidative phosphorylation (OXPHOS) for energy metabolism, which is facilitated by the mitochondrial electron transfer system (ETS). In a series of thermogenic redox reactions, electrons are shuttled through the ETS to oxygen as the final electron acceptor. This electron transfer is coupled to proton translocation across the inner mitochondrial membrane, which itself is the main driving force for ATP production. Oxygen availability is thus a prerequisite for ATP production and consequently contractility. Notably, cardiomyocytes are exceptionally large cells and densely packed with contractile structures, which constrains intracellular oxygen distribution. Moreover, oxygen must pass through layers of actively respiring mitochondria to reach the ones located in the innermost contractile compartment. Indeed, uneven oxygen distribution was observed in cardiomyocytes, suggesting that local ATP supply may also vary according to oxygen availability. Here, we discuss how spatial adjustment of bioenergetics to intracellular oxygen fluctuations may underlie cardiac contractile adaptation and how this adaptation may pose a risk for the development of contractile failure.

Endothelial FIS1 DeSUMOylation Protects Against Hypoxic Pulmonary Hypertension

BACKGROUND

Hypoxia is a major cause and promoter of pulmonary hypertension (PH), a representative vascular remodeling disease with poor prognosis and high mortality. However, the mechanism underlying how pulmonary arterial system responds to hypoxic stress during PH remains unclear. Endothelial mitochondria are considered signaling organelles on oxygen tension. Results from previous clinical research and our studies suggested a potential role of posttranslational SUMOylation (small ubiquitin-like modifier modification) in endothelial mitochondria in hypoxia-related vasculopathy.

METHODS

Chronic hypoxia mouse model and Sugen/hypoxia rat model were employed as PH animal models. Mitochondrial morphology and subcellular structure were determined by transmission electron and immunofluorescent microscopies. Mitochondrial metabolism was determined by mitochondrial oxygen consumption rate and extracellular acidification rate. SUMOylation and protein interaction were determined by immunoprecipitation.

RESULTS

The involvement of SENP1 (sentrin-specific protease 1)-mediated SUMOylation in mitochondrial remodeling in the pulmonary endothelium was identified in clinical specimens of hypoxia-related PH and was verified in human pulmonary artery endothelial cells under hypoxia. Further analyses in clinical specimens, hypoxic rat and mouse PH models, and human pulmonary artery endothelial cells and human embryonic stem cell-derived endothelial cells revealed that short-term hypoxia-induced SENP1 translocation to endothelial mitochondria to regulate deSUMOylation (the reversible process of SUMOylation) of mitochondrial fission protein FIS1 (mitochondrial fission 1), which facilitated FIS1 assembling with fusion protein MFN2 (mitofusin 2) and mitochondrial gatekeeper VDAC1 (voltage-dependent anion channel 1), and the membrane tethering activity of MFN2 by enhancing its oligomerization. Consequently, FIS1 deSUMOylation maintained the mitochondrial integrity and endoplasmic reticulum-mitochondria calcium communication across mitochondrial-associated membranes, subsequently preserving pulmonary endothelial function and vascular homeostasis. In contrast, prolonged hypoxia disabled the FIS1 deSUMOylation by diminishing the availability of SENP1 in mitochondria via inducing miR (micro RNA)-138 and consequently resulted in mitochondrial dysfunction and metabolic reprogramming in pulmonary endothelium. Functionally, introduction of viral-packaged deSUMOylated FIS1 within pulmonary endothelium in mice improved pulmonary endothelial dysfunction and hypoxic PH development, while knock-in of SUMO (small ubiquitin-like modifier)-conjugated FIS1 in mice exaggerated the diseased cellular and tissue phenotypes.

CONCLUSIONS

By maintaining endothelial mitochondrial homeostasis, deSUMOylation of FIS1 adaptively preserves pulmonary endothelial function against hypoxic stress and consequently protects against PH. The FIS1 deSUMOylation-SUMOylation transition in pulmonary endothelium is an intrinsic pathogenesis of hypoxic PH.

Nicotine promotes e-cigarette vapour-induced lung inflammation and structural alterations

BACKGROUND

Electronic cigarette (e-cigarette) vapour is gaining popularity as an alternative to tobacco smoking and can induce acute lung injury. However, the specific role of nicotine in e-cigarette vapour and its long-term effects on the airways, lung parenchyma and vasculature remain unclear.

RESULTS

In vitro exposure to nicotine-containing e-cigarette vapour extract (ECVE) or to nicotine-free e-cigarette vapour extract (NF ECVE) induced changes in gene expression of epithelial cells and pulmonary arterial smooth muscle cells (PASMCs), but ECVE in particular caused functional alterations (e.g. a decrease in human and mouse PASMC proliferation by 29.3±5.3% and 44.3±8.4%, respectively). Additionally, acute inhalation of nicotine-containing e-cigarette vapour (ECV) but not nicotine-free e-cigarette vapour (NF ECV) increased pulmonary endothelial permeability in isolated lungs. Long-term in vivo exposure of mice to ECV for 8 months significantly increased the number of inflammatory cells, in particular lymphocytes, compared to control and NF ECV in the bronchoalveolar fluid (BALF) (ECV: 853.4±150.8 cells·mL-1; control: 37.0±21.1 cells·mL-1; NF ECV: 198.6±94.9 cells·mL-1) and in lung tissue (ECV: 25.7±3.3 cells·mm-3; control: 4.8±1.1 cells·mm-3; NF ECV: 14.1±2.2 cells·mm-3). BALF cytokines were predominantly increased by ECV. Moreover, ECV caused significant changes in lung structure and function (e.g. increase in airspace by 17.5±1.4% compared to control), similar to mild tobacco smoke-induced alterations, which also could be detected in the NF ECV group, albeit to a lesser degree. In contrast, the pulmonary vasculature was not significantly affected by ECV or NF ECV.

CONCLUSIONS

NF ECV components induce cell type-specific effects and mild pulmonary alterations, while inclusion of nicotine induces significant endothelial damage, inflammation and parenchymal alterations.

Reprogramming of Mitochondrial Respiratory Chain Complex by Targeting SIRT3-COX4I2 Axis Attenuates Osteoarthritis Progression

Mitochondrial homeostasis is of great importance for cartilage integrity and associated with the progression of osteoarthritis (OA); however, the underlying mechanisms are unknown. This study aims to investigate the role of mitochondrial deacetylation reaction and investigate the mechanistic relationship OA development. Silent mating type information regulation 2 homolog 3 (SIRT3) expression has a negative correlation with the severity of OA in both human arthritic cartilage and mice inflammatory chondrocytes. Global SIRT3 deletion accelerates pathological phenotype in post-traumatic OA mice, as evidenced by cartilage extracellular matrix collapse, osteophyte formation, and synovial macrophage M1 polarization. Mechanistically, SIRT3 prevents OA progression by targeting and deacetylating cytochrome c oxidase subunit 4 isoform 2 (COX4I2) to maintain mitochondrial homeostasis at the post-translational level. The activation of SIRT3 by honokiol restores cartilage metabolic equilibrium and protects mice from the development of post-traumatic OA. Collectively, the loss of mitochondrial SIRT3 is essential for the development of OA, whereas SIRT3-mediated proteins deacetylation of COX4I2 rescues OA-impaired mitochondrial respiratory chain functions to improve the OA phenotype. Herein, the induction of SIRT3 provides a novel therapeutic candidate for OA treatment.

Xenotopic expression of alternative oxidase (AOX) to study mechanisms of mitochondrial disease

The mitochondrial respiratory chain or electron transport chain (ETC) facilitates redox reactions which ultimately lead to the reduction of oxygen to water (respiration). Energy released by this process is used to establish a proton electrochemical gradient which drives ATP formation (oxidative phosphorylation, OXPHOS). It also plays an important role in vital processes beyond ATP formation and cellular metabolism, such as heat production, redox and ion homeostasis. Dysfunction of the ETC can thus impair cellular and organismal viability and is thought to be the underlying cause of a heterogeneous group of so-called mitochondrial diseases. Plants, yeasts, and many lower organisms, but not insects and vertebrates, possess an enzymatic mechanism that confers resistance to respiratory stress conditions, i.e., the alternative oxidase (AOX). Even in cells that naturally lack AOX, it is autonomously imported into the mitochondrial compartment upon xenotopic expression, where it refolds and becomes catalytically engaged when the cytochrome segment of the ETC is blocked. AOX was therefore proposed as a tool to study disease etiologies. To this end, AOX has been xenotopically expressed in mammalian cells and disease models of the fruit fly and mouse. Surprisingly, AOX showed remarkable rescue effects in some cases, whilst in others it had no effect or even exacerbated a condition. Here we summarize what has been learnt from the use of AOX in various disease models and discuss issues which still need to be addressed in order to understand the role of the ETC in health and disease.

Alternative respiratory oxidases to study the animal electron transport chain

Oxidative phosphorylation is a common process to most organisms in which the main function is to generate an electrochemical gradient across the inner mitochondrial membrane (IMM) and to make energy available to the cell. However, plants, many fungi and some animals maintain non-energy conserving oxidases which serve as a bypass to coupled respiration. Namely, the alternative NADH:ubiquinone oxidoreductase NDI1, present in the complex I (CI)-lacking Saccharomyces cerevisiae, and the alternative oxidase, ubiquinol:oxygen oxidoreductase AOX, present in many organisms across different kingdoms. In the last few years, these alternative oxidases have been used to dissect previously indivisible processes in bioenergetics and have helped to discover, understand, and corroborate important processes in mitochondria. Here, we review how the use of alternative oxidases have contributed to the knowledge in CI stability, bioenergetics, redox biology, and the implications of their use in current and future research.

Acute oxygen sensing by vascular smooth muscle cells

An adequate supply of oxygen (O₂) is essential for most life forms on earth, making the delivery of appropriate levels of O₂ to tissues a fundamental physiological challenge. When O₂ levels in the alveoli and/or blood are low, compensatory adaptive reflexes are produced that increase the uptake of O₂ and its distribution to tissues within a few seconds. This paper analyzes the most important acute vasomotor responses to lack of O₂ (hypoxia): hypoxic pulmonary vasoconstriction (HPV) and hypoxic vasodilation (HVD). HPV affects distal pulmonary (resistance) arteries, with its homeostatic role being to divert blood to well ventilated alveoli to thereby optimize the ventilation/perfusion ratio. HVD is produced in most systemic arteries, in particular in the skeletal muscle, coronary, and cerebral circulations, to increase blood supply to poorly oxygenated tissues. Although vasomotor responses to hypoxia are modulated by endothelial factors and autonomic innervation, it is well established that arterial smooth muscle cells contain an acute O₂ sensing system capable of detecting changes in O₂ tension and to signal membrane ion channels, which in turn regulate cytosolic Ca²⁺ levels and myocyte contraction. Here, we summarize current knowledge on the nature of O₂ sensing and signaling systems underlying acute vasomotor responses to hypoxia. We also discuss similarities and differences existing in O₂ sensors and effectors in the various arterial territories.

Deletion of classical transient receptor potential 1, 3 and 6 alters pulmonary vasoconstriction in chronic hypoxia-induced pulmonary hypertension in mice

Chronic hypoxia-induced pulmonary hypertension (CHPH) is a severe disease that is characterized by increased proliferation and migration of pulmonary arterial smooth muscle cells (PASMCs) leading to pulmonary vascular remodeling. The resulting increase in pulmonary vascular resistance (PVR) causes right ventricular hypertrophy and ultimately right heart failure. In addition, increased PVR can also be a consequence of hypoxic pulmonary vasoconstriction (HPV) under generalized hypoxia. Increased proliferation and migration of PASMCs are often associated with high intracellular Ca²⁺ concentration. Recent publications suggest that Ca²⁺-permeable nonselective classical transient receptor potential (TRPC) proteins-especially TRPC1 and 6-are crucially involved in acute and sustained hypoxic responses and the pathogenesis of CHPH. The aim of our study was to investigate whether the simultaneous deletion of TRPC proteins 1, 3 and 6 protects against CHPH-development and affects HPV in mice. We used a mouse model of chronic hypoxia as well as isolated, ventilated and perfused mouse lungs and PASMC cell cultures. Although right ventricular systolic pressure as well as echocardiographically assessed PVR and right ventricular wall thickness (RVWT) were lower in TRPC1, 3, 6-deficient mice, these changes were not related to a decreased degree of pulmonary vascular muscularization and a reduced proliferation of PASMCs. However, both acute and sustained HPV were almost absent in the TRPC1, 3, 6-deficient mice and their vasoconstrictor response upon KCl application was reduced. This was further validated by myographical experiments. Our data revealed that 1) TRPC1, 3, 6-deficient mice are partially protected against development of CHPH, 2) these changes may be caused by diminished HPV and not an altered pulmonary vascular remodeling.

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.

Single-cell transcriptomics reveals skewed cellular communication and phenotypic shift in pulmonary artery remodeling

A central feature of progressive vascular remodeling is altered smooth muscle cell (SMC) homeostasis; however, the understanding of how different cell populations contribute to this process is limited. Here, we utilized single-cell RNA sequencing to provide insight into cellular composition changes within isolated pulmonary arteries (PAs) from pulmonary arterial hypertension and donor lungs. Our results revealed that remodeling skewed the balanced communication network between immune and structural cells, in particular SMCs. Comparative analysis with murine PAs showed that human PAs harbored heterogeneous SMC populations with an abundant intermediary cluster displaying a gradient transition between SMCs and adventitial fibroblasts. Transcriptionally distinct SMC populations were enriched in specific biological processes and could be differentiated into 4 major clusters: oxygen sensing (enriched in pericytes), contractile, synthetic, and fibroblast-like. End-stage remodeling was associated with phenotypic shift of preexisting SMC populations and accumulation of synthetic SMCs in neointima. Distinctly regulated genes in clusters built nonredundant regulatory hubs encompassing stress response and differentiation regulators. The current study provides a blueprint of cellular and molecular changes on a single-cell level that are defining the pathological vascular remodeling process.

Tissue-specific mitochondrial HIGD1C promotes oxygen sensitivity in carotid body chemoreceptors

Mammalian carotid body arterial chemoreceptors function as an early warning system for hypoxia, triggering acute life-saving arousal and cardiorespiratory reflexes. To serve this role, carotid body glomus cells are highly sensitive to decreases in oxygen availability. While the mitochondria and plasma membrane signaling proteins have been implicated in oxygen sensing by glomus cells, the mechanism underlying their mitochondrial sensitivity to hypoxia compared to other cells is unknown. Here, we identify HIGD1C, a novel hypoxia-inducible gene domain factor isoform, as an electron transport chain complex IV-interacting protein that is almost exclusively expressed in the carotid body and is therefore not generally necessary for mitochondrial function. Importantly, HIGD1C is required for carotid body oxygen sensing and enhances complex IV sensitivity to hypoxia. Thus, we propose that HIGD1C promotes exquisite oxygen sensing by the carotid body, illustrating how specialized mitochondria can be used as sentinels of metabolic stress to elicit essential adaptive behaviors.

Oxygen regulation of breathing is abolished in mitochondrial complex III-deficient arterial chemoreceptors

Oxygen sensing by chemoreceptor glomus cells in the carotid body plays an essential adaptive function in health and disease; however, the underlying mechanisms are not fully understood. Glomus cells survive genetic disruption of mitochondrial complex III, although this results in a functional disconnection between the distal and proximal components of the mitochondrial electron transport chain (ETC). These cells exhibit selective abolition of mitochondrial and cellular responsiveness to hypoxia, as well as altered systemic hyperventilation and acclimatization to hypoxia, indicating that acute oxygen-sensing and -signaling during hypoxia result from the integrated action of mitochondrial ETC components. The mitochondrial ETC emerges as a complex oxygen-sensing and -signaling system of potential pathophysiological relevance in maladaptive responses to hypoxia.

Acute oxygen (O₂) sensing is essential for adaptation of organisms to hypoxic environments or medical conditions with restricted exchange of gases in the lung. The main acute O₂-sensing organ is the carotid body (CB), which contains neurosecretory chemoreceptor (glomus) cells innervated by sensory fibers whose activation by hypoxia elicits hyperventilation and increased cardiac output. Glomus cells have mitochondria with specialized metabolic and electron transport chain (ETC) properties. Reduced mitochondrial complex (MC) IV activity by hypoxia leads to production of signaling molecules (NADH and reactive O₂ species) in MCI and MCIII that modulate membrane ion channel activity. We studied mice with conditional genetic ablation of MCIII that disrupts the ETC in the CB and other catecholaminergic tissues. Glomus cells survived MCIII dysfunction but showed selective abolition of responsiveness to hypoxia (increased [Ca²⁺] and transmitter release) with normal responses to other stimuli. Mitochondrial hypoxic NADH and reactive O₂ species signals were also suppressed. MCIII-deficient mice exhibited strong inhibition of the hypoxic ventilatory response and altered acclimatization to sustained hypoxia. These data indicate that a functional ETC, with coupling between MCI and MCIV, is required for acute O₂ sensing. O₂ regulation of breathing results from the integrated action of mitochondrial ETC complexes in arterial chemoreceptors.

Circulating Microparticles Are Differentially Increased in Lowlanders and Highlanders with High Altitude Induced Pulmonary Hypertension during the Cold Season

The role of microparticles (MPs) and cold in high altitude pulmonary hypertension (HAPH) remains unexplored. We investigated the impact of long-term cold exposure on the pulmonary circulation in lowlanders and high-altitude natives and the role of MPs. Pulmonary hemodynamics were evaluated using Doppler echocardiography at the end of the colder and warmer seasons. We further examined the miRNA content of MPs isolated from the study participants and studied their effects on human pulmonary artery smooth muscle (hPASMCs) and endothelial cells (hPAECs). Long-term exposure to cold environment was associated with an enhanced pulmonary artery pressure in highlanders. Plasma levels of CD62E-positive and CD68-positive MPs increased in response to cold in lowlanders and HAPH highlanders. The miRNA-210 expression contained in MPs differentially changed in response to cold in lowlanders and highlanders. MPs isolated from lowlanders and highlanders increased proliferation and reduced apoptosis of hPASMCs. Further, MPs isolated from warm-exposed HAPH highlanders and cold-exposed highlanders exerted the most pronounced effects on VEGF expression in hPAECs. We demonstrated that prolonged exposure to cold is associated with elevated pulmonary artery pressures, which are most pronounced in high-altitude residents. Further, the numbers of circulating MPs are differentially increased in lowlanders and HAPH highlanders during the colder season.

Fibroblasts mediate the angiogenesis of pheochromocytoma by increasing COX4I2 expression

Objective

Our previous work found COX4I2 was associated with angiogenesis in pheochromocytoma. The purpose of this study was to explore the role of COX4I2 in regulating angiogenesis in pheochromocytoma.

Methods

Distribution of COX4I2 was evaluated by scRNA-seq in one case of pheochromocytoma and the findings were verified by immunostaining. COX4I2 was further knocked down in target cells. Changes of angiogenesis-related genes were evaluated by qPCR in target cells.

Results

The scRNA-seq revealed high mRNA expression of COX4I2 in fibroblasts rather than tumor cells. Immunostaining of COX4I2 confirmed its distribution in fibroblasts. Knocking down COX4I2 in NIH3T3 cell line led to significant reduction of angiogenesis-related genes, especially ANG1 and HGF.

Conclusions

Fibroblasts mediate the angiogenesis of pheochromocytoma by increasing COX4I2 expression, possibly by affecting ANG1 and HGF.

Mitochondrial Regulation of the Hypoxia-Inducible Factor in the Development of Pulmonary Hypertension

Pulmonary hypertension (PH) is a severe progressive lung disorder characterized by pulmonary vasoconstriction and vascular remodeling, culminating in right-sided heart failure and increased mortality. Data from animal models and human subjects demonstrated that hypoxia-inducible factor (HIF)-related signaling is essential in the progression of PH. This review summarizes the regulatory pathways and mechanisms of HIF-mediated signaling, emphasizing the role of mitochondria in HIF regulation and PH pathogenesis. We also try to determine the potential to therapeutically target the components of the HIF system for the management of PH.

AMPK deficiency in smooth muscles causes persistent pulmonary hypertension of the new-born and premature death

AMPK has been reported to facilitate hypoxic pulmonary vasoconstriction but, paradoxically, its deficiency precipitates pulmonary hypertension. Here we show that AMPK-α1/α2 deficiency in smooth muscles promotes persistent pulmonary hypertension of the new-born. Accordingly, dual AMPK-α1/α2 deletion in smooth muscles causes premature death of mice after birth, associated with increased muscularisation and remodeling throughout the pulmonary arterial tree, reduced alveolar numbers and alveolar membrane thickening, but with no oedema. Spectral Doppler ultrasound indicates pulmonary hypertension and attenuated hypoxic pulmonary vasoconstriction. Age-dependent right ventricular pressure elevation, dilation and reduced cardiac output was also evident. KV1.5 potassium currents of pulmonary arterial myocytes were markedly smaller under normoxia, which is known to facilitate pulmonary hypertension. Mitochondrial fragmentation and reactive oxygen species accumulation was also evident. Importantly, there was no evidence of systemic vasculopathy or hypertension in these mice. Moreover, hypoxic pulmonary vasoconstriction was attenuated by AMPK-α1 or AMPK-α2 deletion without triggering pulmonary hypertension.

Mitochondria-targeted hydrogen sulfide donors versus acute oxidative gastric mucosal injury

Hydrogen sulfide (H2S) as a gaseous molecule prevents gastrointestinal (GI)-tract against various injuries. This study aimed to evaluate for the first time the detailed molecular mechanism of mitochondria-targeting H2S-prodrugs, AP39 and RT01 in gastroprotection against ischemia/reperfusion (I/R)-induced lesions. Wistar rats exposed to I/R were pretreated i.g. with vehicle, AP39 (0.004-2 mg/kg), RT01 (0.1 mg/kg), or with AP219 (0.1 mg/kg) as structural control without ability to release H2S. AP39 was also administered with mTOR1 inhibitor, rapamycin (1 mg/kg i.g.). Gastric damage area was assessed micro-/macroscopically, gastric blood flow (GBF) by laser flowmetry, mRNA level of HIF-1α, GPx, SOD1, SOD2, annexin-A1, SOCS3, IL-1RA, IL-1β, IL-1R1, IL-1R2, TNFR2, iNOS by real-time PCR. Gastric mucosal and/or serum content of IL-1β, IL-4, IL-5, IL-10, G-CSF, M-CSF, VEGFA, GRO, RANTES, MIP-1α, MCP1, TNF-α, TIMP1, FABP3, GST-α, STAT3/5 and phosphorylation of mTOR, NF-κB, ERK, Akt was evaluated by microbeads-fluorescent assay. Mitochondrial complexes activities were measured biochemically. RNA damage was assessed as 8-OHG by ELISA. AP39 and RT01 reduced micro-/macroscopic gastric I/R-injury increasing GBF. AP39-gastroprotection was accompanied by maintained activity of mitochondrial complexes, prevented RNA oxidation and enhanced mRNA/protein expression of SOCS3, IL-1RA, annexin-A1, GST-α, HIF-1α. Rapamycin reversed AP-39-gastroprotection. AP39-gastroprotection was followed by decreased NF-κB, ERK, IL-1β and enhanced Akt and mTOR proteins phosphorylation. AP39-prevented gastric mucosal damage caused by I/R-injury, partly by mitochondrial complex activity maintenance. AP39-mediated attenuation of gastric mucosal oxidation, hypoxia and inflammation involved mTOR1 and Akt pathways activity and modulation of HIF-1α, GST-α, SOCS3, IL1RA and TIMP1 molecular interplay.

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.

Differential HIF2α Protein Expression in Human Carotid Body and Adrenal Medulla under Physiologic and Tumorigenic Conditions

Hypoxia-inducible factors (HIF) 2α and 1α are the major oxygen-sensing molecules in eukaryotic cells. HIF2α has been pathogenically linked to paraganglioma and pheochromocytoma (PPGL) arising in sympathetic paraganglia or the adrenal medulla (AM), respectively. However, its involvement in the pathogenesis of paraganglioma arising in the carotid body (CB) or other parasympathetic ganglia in the head and neck (HNPGL) remains to be defined. Here, we retrospectively analyzed HIF2α by immunohistochemistry in 62 PPGL/HNPGL and human CB and AM, and comprehensively evaluated the HIF-related transcriptome of 202 published PPGL/HNPGL. We report that HIF2α is barely detected in the AM, but accumulates at high levels in PPGL, mostly (but not exclusively) in those with loss-of-function mutations in VHL and genes encoding components of the succinate dehydrogenase (SDH) complex. This is associated with upregulation of EPAS1 and the HIF2α-regulated genes COX4I2 and ADORA2A. In contrast, HIF2α and HIF2α-regulated genes are highly expressed in CB and HNPGL, irrespective of VHL and SDH dysfunctions. We also found that HIF2α and HIF1α protein expressions are not correlated in PPGL nor HNPGL. In addition, HIF1α-target genes are almost exclusively overexpressed in VHL-mutated HNPGL/PPGL. Collectively, the data suggest that involvement of HIF2α in the physiology and tumor pathology of human paraganglia is organ-of-origin-dependent and HIF1α-independent.

Oxygen Sensor of the Heart

How oxygen is sensed by the heart and what mechanisms mediate its sensing remain poorly understood. Since recent reports show that low PO₂ levels are detected by the cardiomyocytes in a few seconds, the rapid and short applications of low levels of oxygen (acute hypoxia), that avoid multiple effects of chronic hypoxia may be used to probe the oxygen sensing pathway of the heart. Here we explore the oxygen sensing pathway, focusing primarily on cellular surface membrane proteins that are first exposed to low PO₂. Such studies suggest that acute hypoxia primarily targets the cardiac calcium channels, where either the channel itself or moieties closely associated with it, for instance, heme-oxygenase-2 (HO-2) interacting through kinase phosphorylation, signals the α-subunit of the channel as to the altered levels of PO₂. Amino acids 1572-1651, the CaMKII phosphorylation sites (S1487 and S1545), CaM-binding site (I1624, Q1625) and Ser1928 of the carboxyl tail of the α-subunit appear to be critical residues that sense oxygen. Future studies in HO-2 knockout mice or CRISPR/Cas9 gene-edited hiPSC-CMs that reduce CaM-binding affinity are likely to provide deeper insights in the O₂-sensinsing mechanisms.

Mitochondrien als universelle Sensoren der akuten Hypoxie?

Adaptation to acute hypoxia through cardiorespiratory responses is mediated by specialized cells in the carotid body and pulmonary vasculature to optimize systemic arterial oxygenation. Acute oxygen sensing thus is a fundamental pre-requisite for aerobic life. Recent studies unravelled basic oxygen sensing mechanisms involving the mitochondrial cytochrome c oxidase subunit 4 isoform 2 that regulates the release of mitochondrial reactive oxygen species and subsequent acute hypoxic responses.

Systemic Effects of mitoTEMPO upon Lipopolysaccharide Challenge Are Due to Its Antioxidant Part, While Local Effects in the Lung Are Due to Triphenylphosphonium

Mitochondria-targeted antioxidants (mtAOX) are a promising treatment strategy against reactive oxygen species-induced damage. Reports about harmful effects of mtAOX lead to the question of whether these could be caused by the carrier molecule triphenylphosphonium (TPP). The aim of this study was to investigate the biological effects of the mtAOX mitoTEMPO, and TPP in a rat model of systemic inflammatory response. The inflammatory response was induced by lipopolysaccharide (LPS) injection. We show that mitoTEMPO reduced expression of inducible nitric oxide synthase in the liver, lowered blood levels of tissue damage markers such as liver damage markers (aspartate aminotransferase and alanine aminotransferase), kidney damage markers (urea and creatinine), and the general organ damage marker, lactate dehydrogenase. In contrast, TPP slightly, but not significantly, increased the LPS-induced effects. Surprisingly, both mitoTEMPO and TPP reduced the wet/dry ratio in the lung after 24 h. In the isolated lung, both substances enhanced the increase in pulmonary arterial pressure induced by LPS observed within 3 h after LPS treatments but did not affect edema formation at this time. Our data suggest that beneficial effects of mitoTEMPO in organs are due to its antioxidant moiety (TEMPO), except for the lung where its effects are mediated by TPP.

Experimental Setup for Investigation of Acute Mitochondrial Oxygen Sensing in Primary Cells

The ability to sense and respond to acute changes in oxygen is essential for the viability of cells and organisms. To study molecular mechanisms of acute oxygen sensing, we established a setup for the adjustment of acute hypoxic conditions in cultured cells, exemplified here for the use of primary pulmonary arterial smooth muscle cells (PASMCs). The mitochondrial electron transport chain (ETC) is the main consumer of oxygen but recently also emerged as essential oxygen sensor suggesting that the ETC itself adapts its electron flux to oxygen availability. To test this assumption and to experimentally manipulate electron flux through the ETC, we used alternative oxidase (AOX), which bypasses the cytochrome pathway of the ETC when blocked. The described combination of our experimental setup and AOX allowed us in previous publications unprecedented insights into the role of the ETC in cellular oxygen sensing and cellular response mechanisms in living cells. Against this background, we here describe and discuss this method in detail, which will allow transfer to other cell types and research questions.

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.

Mitochondrial iron-sulfur clusters: Structure, function, and an emerging role in vascular biology

Iron-sulfur (Fe-S) clusters are essential cofactors most commonly known for their role mediating electron transfer within the mitochondrial respiratory chain. The Fe-S cluster pathways that function within the respiratory complexes are highly conserved between bacteria and the mitochondria of eukaryotic cells. Within the electron transport chain, Fe-S clusters play a critical role in transporting electrons through Complexes I, II and III to cytochrome c, before subsequent transfer to molecular oxygen. Fe-S clusters are also among the binding sites of classical mitochondrial inhibitors, such as rotenone, and play an important role in the production of mitochondrial reactive oxygen species (ROS). Mitochondrial Fe-S clusters also play a critical role in the pathogenesis of disease. High levels of ROS produced at these sites can cause cell injury or death, however, when produced at low levels can serve as signaling molecules. For example, Ndufs2, a Complex I subunit containing an Fe-S center, N2, has recently been identified as a redox-sensitive oxygen sensor, mediating homeostatic oxygen-sensing in the pulmonary vasculature and carotid body. Fe-S clusters are emerging as transcriptionally-regulated mediators in disease and play a crucial role in normal physiology, offering potential new therapeutic targets for diseases including malaria, diabetes, and cancer.

A Microfluidic System for Simultaneous Raman Spectroscopy, Patch-Clamp Electrophysiology, and Live-Cell Imaging to Study Key Cellular Events of Single Living Cells in Response to Acute Hypoxia

The ability to sense changes in oxygen availability is fundamentally important for the survival of all aerobic organisms. However, cellular oxygen sensing mechanisms and pathologies remain incompletely understood and studies of acute oxygen sensing, in particular, have produced inconsistent results. Current methods cannot simultaneously measure the key cellular events in acute hypoxia (i.e., changes in redox state, electrophysiological properties, and mechanical responses) at controlled partial pressures of oxygen (pO₂ ). The lack of such a comprehensive method essentially contributes to the discrepancies in the field. A sealed microfluidic system that combines i) Raman spectroscopy, ii) patch-clamp electrophysiology, and iii) live-cell imaging under precisely controlled pO₂ have therefore been developed. Merging these modalities allows label-free and simultaneous observation of oxygen-dependent alterations in multiple cellular redox couples, membrane potential, and cellular contraction. This technique is adaptable to any cell type and allows in-depth insight into acute oxygen sensing processes underlying various physiologic and pathologic conditions.

Impairment of hypoxic pulmonary vasoconstriction in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a serious complication of severe systemic or local pulmonary inflammation, such as caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. ARDS is characterised by diffuse alveolar damage that leads to protein-rich pulmonary oedema, local alveolar hypoventilation and atelectasis. Inadequate perfusion of these areas is the main cause of hypoxaemia in ARDS. High perfusion in relation to ventilation (V/Q<1) and shunting (V/Q=0) is not only caused by impaired hypoxic pulmonary vasoconstriction but also redistribution of perfusion from obstructed lung vessels. Rebalancing the pulmonary vascular tone is a therapeutic challenge. Previous clinical trials on inhaled vasodilators (nitric oxide and prostacyclin) to enhance perfusion to high V/Q areas showed beneficial effects on hypoxaemia but not on mortality. However, specific patient populations with pulmonary hypertension may profit from treatment with inhaled vasodilators. Novel treatment targets to decrease perfusion in low V/Q areas include epoxyeicosatrienoic acids and specific leukotriene receptors. Still, lung protective ventilation and prone positioning are the best available standard of care. This review focuses on disturbed perfusion in ARDS and aims to provide basic scientists and clinicians with an overview of the vascular alterations and mechanisms of V/Q mismatch, current therapeutic strategies, and experimental approaches.

Peptide Blocking Self-Polymerization of Extracellular Calcium-Sensing Receptor Attenuates Hypoxia-Induced Pulmonary Hypertension

[Figure: see text].

COX4I2 is a novel biomarker of blood supply in adrenal tumors

Background

Previous study has been reported that COX4I2 expression level demonstrated a positive correlation with microvessel density in pheochromocytomas (PCC) samples, suggesting that the expression of COX4I2 maybe related to blood supply level in other adrenal tumors as well. The aim of this study is to clarify the correlation of COX4I2 expression and blood supply in adrenal tumors.

Methods

A total of 84 patients were recruited, among which 46 was diagnosed as adrenocortical adenoma (ACA) and 38 was diagnosed as PCC. Contrast-enhanced CT values were used to evaluate the blood supply levels in those patients. The expression of mRNA was examined by quantitative real-time polymerase chain reaction (qPCR) and protein was detected by immunohistochemistry (IHC).

Results

The COX4I2 expression level in PCC group is significantly higher than that in ACA group (P<0.01). The expression of angiogenesis-related genes EPAS1, VEGFA and KDR mRNA in PCC group is higher than that of ACA group (P<0.05). Correlation analysis shows COX4I2 expression level is correlated with CT values (P<0.001), intraoperative blood loss (P<0.05) and operation time (P<0.05), and the expression of COX4I2 mRNA is correlated with EPAS1, VEGFA and KDR mRNA (P<0.01).

Conclusions

The results displayed a distinct expression level of COX4I2 between ACA and PCC, suggesting that COX4I2 is a novel biomarker of blood supply in adrenal tumors. This research also opens the possibility for further research on COX4I2 as a novel target for anti-tumor angiogenesis.

Oxygen sensing, mitochondrial biology and experimental therapeutics for pulmonary hypertension and cancer

The homeostatic oxygen sensing system (HOSS) optimizes systemic oxygen delivery. Specialized tissues utilize a conserved mitochondrial sensor, often involving NDUFS2 in complex I of the mitochondrial electron transport chain, as a site of pO₂-responsive production of reactive oxygen species (ROS). These ROS are converted to a diffusible signaling molecule, hydrogen peroxide (H₂O₂), by superoxide dismutase (SOD2). H₂O₂ exits the mitochondria and regulates ion channels and enzymes, altering plasma membrane potential, intracellular Ca²⁺ and Ca²⁺-sensitization and controlling acute, adaptive, responses to hypoxia that involve changes in ventilation, vascular tone and neurotransmitter release. Subversion of this O₂-sensing pathway creates a pseudohypoxic state that promotes disease progression in pulmonary arterial hypertension (PAH) and cancer. Pseudohypoxia is a state in which biochemical changes, normally associated with hypoxia, occur despite normal pO₂. Epigenetic silencing of SOD2 by DNA methylation alters H₂O₂ production, activating hypoxia-inducible factor 1α, thereby disrupting mitochondrial metabolism and dynamics, accelerating cell proliferation and inhibiting apoptosis. Other epigenetic mechanisms, including dysregulation of microRNAs (miR), increase pyruvate dehydrogenase kinase and pyruvate kinase muscle isoform 2 expression in both diseases, favoring uncoupled aerobic glycolysis. This Warburg metabolic shift also accelerates cell proliferation and impairs apoptosis. Disordered mitochondrial dynamics, usually increased mitotic fission and impaired fusion, promotes disease progression in PAH and cancer. Epigenetic upregulation of dynamin-related protein 1 (Drp1) and its binding partners, MiD49 and MiD51, contributes to the pathogenesis of PAH and cancer. Finally, dysregulation of intramitochondrial Ca²⁺, resulting from impaired mitochondrial calcium uniporter complex (MCUC) function, links abnormal mitochondrial metabolism and dynamics. MiR-mediated decreases in MCUC function reduce intramitochondrial Ca²⁺, promoting Warburg metabolism, whilst increasing cytosolic Ca²⁺, promoting fission. Epigenetically disordered mitochondrial O₂-sensing, metabolism, dynamics, and Ca²⁺ homeostasis offer new therapeutic targets for PAH and cancer. Promoting glucose oxidation, restoring the fission/fusion balance, and restoring mitochondrial calcium regulation are promising experimental therapeutic strategies.

Cholate Disrupts Regulatory Functions of Cytochrome c Oxidase

Cytochrome c oxidase (CytOx), the oxygen-accepting and rate-limiting enzyme of mitochondrial respiration, binds with 10 molecules of ADP, 7 of which are exchanged by ATP at high ATP/ADP-ratios. These bound ATP and ADP can be exchanged by cholate, which is generally used for the purification of CytOx. Many crystal structures of isolated CytOx were performed with the enzyme isolated from mitochondria using sodium cholate as a detergent. Cholate, however, dimerizes the enzyme isolated in non-ionic detergents and induces a structural change as evident from a spectral change. Consequently, it turns off the "allosteric ATP-inhibition of CytOx", which is reversibly switched on under relaxed conditions via cAMP-dependent phosphorylation and keeps the membrane potential and ROS formation in mitochondria at low levels. This cholate effect gives an insight into the structural-functional relationship of the enzyme with respect to ATP inhibition and its role in mitochondrial respiration and energy production.

Oxygen-dependent regulation of ion channels: acute responses, post-translational modification, and response to chronic hypoxia

Oxygen is a vital element for the survival of cells in multicellular aerobic organisms such as mammals. Lack of O₂ availability caused by environmental or pathological conditions leads to hypoxia. Active oxygen distribution systems (pulmonary and circulatory) and their neural control mechanisms ensure that cells and tissues remain oxygenated. However, O₂-carrying blood cells as well as immune and various parenchymal cells experience wide variations in partial pressure of oxygen (P_O2) in vivo. Hence, the reactive modulation of the functions of the oxygen distribution systems and their ability to sense P_O2 are critical. Elucidating the physiological responses of cells to variations in P_O2 and determining the P_O2-sensing mechanisms at the biomolecular level have attracted considerable research interest in the field of physiology. Herein, we review the current knowledge regarding ion channel-dependent oxygen sensing and associated signalling pathways in mammals. First, we present the recent findings on O₂-sensing ion channels in representative chemoreceptor cells as well as in other types of cells such as immune cells. Furthermore, we highlight the transcriptional regulation of ion channels under chronic hypoxia and its physiological implications and summarize the findings of studies on the post-translational modification of ion channels under hypoxic or ischemic conditions.

Mitochondrial redox systems as central hubs in plant metabolism and signaling

Plant mitochondria are indispensable for plant metabolism and are tightly integrated into cellular homeostasis. This review provides an update on the latest research concerning the organization and operation of plant mitochondrial redox systems, and how they affect cellular metabolism and signaling, plant development, and stress responses. New insights into the organization and operation of mitochondrial energy systems such as the tricarboxylic acid cycle and mitochondrial electron transport chain (mtETC) are discussed. The mtETC produces reactive oxygen and nitrogen species, which can act as signals or lead to cellular damage, and are thus efficiently removed by mitochondrial antioxidant systems, including Mn-superoxide dismutase, ascorbate-glutathione cycle, and thioredoxin-dependent peroxidases. Plant mitochondria are tightly connected with photosynthesis, photorespiration, and cytosolic metabolism, thereby providing redox-balancing. Mitochondrial proteins are targets of extensive post-translational modifications, but their functional significance and how they are added or removed remains unclear. To operate in sync with the whole cell, mitochondria can communicate their functional status via mitochondrial retrograde signaling to change nuclear gene expression, and several recent breakthroughs here are discussed. At a whole organism level, plant mitochondria thus play crucial roles from the first minutes after seed imbibition, supporting meristem activity, growth, and fertility, until senescence of darkened and aged tissue. Finally, plant mitochondria are tightly integrated with cellular and organismal responses to environmental challenges such as drought, salinity, heat, and submergence, but also threats posed by pathogens. Both the major recent advances and outstanding questions are reviewed, which may help future research efforts on plant mitochondria.

The inevitability of ATP as a transmitter in the carotid body

Atmospheric oxygen concentrations rose markedly at several points in evolutionary history. Each of these increases was followed by an evolutionary leap in organismal complexity, and thus the cellular adaptions we see today have been shaped by the levels of oxygen within our atmosphere. In eukaryotic cells, oxygen is essential for the production of adenosine 5'-triphosphate (ATP) which is the 'Universal Energy Currency' of life. Aerobic organisms survived by evolving precise mechanisms for converting oxygen within the environment into energy. Higher mammals developed specialised organs for detecting and responding to changes in oxygen content to maintain gaseous homeostasis for survival. Hypoxia is sensed by the carotid bodies, the primary chemoreceptor organs which utilise multiple neurotransmitters one of which is ATP to evoke compensatory reflexes. Yet, a paradox is presented in oxygen sensing cells of the carotid body when during periods of low oxygen, ATP is seemingly released in abundance to transmit this signal although the synthesis of ATP is theoretically halted because of its dependence on oxygen. We propose potential mechanisms to maintain ATP production in hypoxia and summarise recent data revealing elevated sensitivity of purinergic signalling within the carotid body during conditions of sympathetic overactivity and hypertension. We propose the carotid body is hypoxic in numerous chronic cardiovascular and respiratory diseases and highlight the therapeutic potential for modulating purinergic transmission.

Chronic Obstructive Pulmonary Disease and the Cardiovascular System: Vascular Repair and Regeneration as a Therapeutic Target

Chronic obstructive pulmonary disease (COPD) is a major cause of morbidity and mortality worldwide and encompasses chronic bronchitis and emphysema. It has been shown that vascular wall remodeling and pulmonary hypertension (PH) can occur not only in patients with COPD but also in smokers with normal lung function, suggesting a causal role for vascular alterations in the development of emphysema. Mechanistically, abnormalities in the vasculature, such as inflammation, endothelial dysfunction, imbalances in cellular apoptosis/proliferation, and increased oxidative/nitrosative stress promote development of PH, cor pulmonale, and most probably pulmonary emphysema. Hypoxemia in the pulmonary chamber modulates the activation of key transcription factors and signaling cascades, which propagates inflammation and infiltration of neutrophils, resulting in vascular remodeling. Endothelial progenitor cells have angiogenesis capabilities, resulting in transdifferentiation of the smooth muscle cells via aberrant activation of several cytokines, growth factors, and chemokines. The vascular endothelium influences the balance between vaso-constriction and -dilation in the heart. Targeting key players affecting the vasculature might help in the development of new treatment strategies for both PH and COPD. The present review aims to summarize current knowledge about vascular alterations and production of reactive oxygen species in COPD. The present review emphasizes on the importance of the vasculature for the usually parenchyma-focused view of the pathobiology of COPD.

Generation of Reactive Oxygen Species by Mitochondria

Reactive oxygen species (ROS) are series of chemical products originated from one or several electron reductions of oxygen. ROS are involved in physiology and disease and can also be both cause and consequence of many biological scenarios. Mitochondria are the main source of ROS in the cell and, particularly, the enzymes in the electron transport chain are the major contributors to this phenomenon. Here, we comprehensively review the modes by which ROS are produced by mitochondria at a molecular level of detail, discuss recent advances in the field involving signalling and disease, and the involvement of supercomplexes in these mechanisms. Given the importance of mitochondrial ROS, we also provide a schematic guide aimed to help in deciphering the mechanisms involved in their production in a variety of physiological and pathological settings.