TRP channels as sensors of oxygen availability

Metal profiling in coronary ischemia-reperfusion injury: Implications for KEAP1/NRF2 regulated redox signaling

Coronary ischemia-reperfusion (IR) injury results from a blockage of blood supply to the heart followed by restoration of perfusion, leading to oxidative stress induced pathological processes. Nuclear factor erythroid 2-related factor 2 (NRF2), a master antioxidant transcription factor, plays a key role in regulating redox signaling. Over the past decades, the field of metallomics has provided novel insights into the mechanism of pro-oxidant and antioxidant pathological processes. Both redox-active (e.g. Fe and Cu) and redox-inert (e.g. Zn and Mg) metals play unique roles in establishing redox balance under IR injury. Notably, Zn protects against oxidative stress in coronary IR injury by serving as a cofactor of antioxidant enzymes such as superoxide dismutase [Cu-Zn] (SOD1) and proteins such as metallothionein (MT) and KEAP1/NRF2 mediated antioxidant defenses. An increase in labile Zn2+ inhibits proteasomal degradation and ubiquitination of NRF2 by modifying KEAP1 and glycogen synthase kinase 3β (GSK3β) conformations. Fe and Cu catalyse the formation of reactive oxygen species via the Fenton reaction and also serve as cofactors of antioxidant enzymes and can activate NRF2 antioxidant signaling. We review the evidence that Zn and redox-active metals Fe and Cu affect redox signaling in coronary cells during IR and the mechanisms by which oxidative stress influences cellular metal content. In view of the unique double-edged characteristics of metals, we aim to bridge the role of metals and NRF2 regulated redox signaling to antioxidant defenses in IR injury, with a long-term aim of informing the design and application of novel therapeutics.

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.

Intracellular activation of full-length human TREK-1 channel by hypoxia, high lactate, and low pH denotes polymodal integration by ischemic factors

TREK-1, a two-pore domain potassium channel, responds to ischemic levels of intracellular lactate and acidic pH to provide neuroprotection. There are two splice variants of hTREK1: the shorter splice variant having a shorter N-terminus compared with the full-length hTREK1 with similar C-terminus sequence that is widely expressed in the brain. The shorter variant was reported to be irresponsive to hypoxia-a condition attributed to ischemia, which has put the neuroprotective role of hTREK-1 channel into question. Since interaction between N- and C-terminus of different ion channels shapes their gating, we re-examined the sensitivity of the full-length as well as the shorter hTREK-1 channel to intracellular hypoxia along with lactate. Single-channel data obtained from the excised inside-out patches of the full-length channel expressed in HEK293 cells indicated an increase in activity as opposed to a decrease in activity in the shorter isoform. However, both the isoforms showed an increase in activity under combined hypoxia, 20mM lactate, and low pH 6 condition, albeit with subtle differences in their individual actions, confirming the neuroprotective role played by hTREK-1 irrespective of the differences in the N-terminus among the splice variants. Furthermore, E321A mutant that disrupts the interaction of the C-terminus with the membrane showed a decrease in activity with hypoxia indicating the importance of the C-terminus in the hypoxic response of the full-length hTREK-1. We propose an increase in activity of both the splice variants of hTREK-1 in combined hypoxia, high lactate, and low pH conditions typically associated with ischemia provides neuroprotection.

Transient receptor potential (TRP) channels: a metabolic TR(i)P to obesity prevention and therapy

Cellular transport of ions, especially by ion channels, regulates physiological function. The transient receptor potential (TRP) channels, with 30 identified so far, are cation channels with high calcium permeability. These ion channels are present in metabolically active tissues including adipose tissue, liver, gastrointestinal tract, brain (hypothalamus), pancreas and skeletal muscle, which suggests a potential role in metabolic disorders including obesity. TRP channels have potentially important roles in adipogenesis, obesity development and its prevention and therapy because of their physiological properties including calcium permeability, thermosensation and taste perception, involvement in cell metabolic signalling and hormone release. This wide range of actions means that organ-specific actions are unlikely, thus increasing the possibility of adverse effects. Delineation of responses to TRP channels has been limited by the poor selectivity of available agonists and antagonists. Food constituents that can modulate TRP channels are of interest in controlling metabolic status. TRP vanilloid 1 channels modulated by capsaicin have been the most studied, suggesting that this may be the first target for effective pharmacological modulation in obesity. This review shows that most of the TRP channels are potential targets to reduce metabolic disorders through a range of mechanisms.

TRPA1 channels: expression in non-neuronal murine lung tissues and dispensability for hyperoxia-induced alveolar epithelial hyperplasia

Transient receptor potential A1 (TRPA1) channels were originally characterized in neuronal tissues but also identified in lung epithelium by staining with fluorescently coupled TRPA1 antibodies. Its exact function in non-neuronal tissues, however, is elusive. TRPA1 is activated in vitro by hypoxia and hyperoxia and is therefore a promising TRP candidate for sensing hyperoxia in pulmonary epithelial cells and for inducing alveolar epithelial hyperplasia. Here, we isolated tracheal, bronchial, and alveolar epithelial cells and show low but detectable TRPA1 mRNA levels in all these cells as well as TRPA1 protein by Western blotting in alveolar type II (AT II) cells. We quantified changes in intracellular Ca²⁺ ([Ca²⁺]_i) levels induced by application of hyperoxic solutions in primary tracheal epithelial, bronchial epithelial, and AT II cells isolated from wild-type (WT) and TRPA1-deficient (TRPA1-/-) mouse lungs. In all cell types, we detected hyperoxia-induced rises in [Ca²⁺]_i levels, which were not significantly different in TRPA1-deficient cells compared to WT cells. We also tested TRPA1 function in a mouse model for hyperoxia-induced alveolar epithelial hyperplasia. A characteristic significant increase in thickening of alveolar tissues was detected in mouse lungs after exposure to hyperoxia, but not in normoxic WT and TRPA1-/- controls. Quantification of changes in lung morphology in hyperoxic WT and TRPA1-/- mice, however, again revealed no significant changes. Therefore, TRPA1 expression does neither appear to be a key player for hyperoxia-induced changes in [Ca²⁺]_i levels in primary lung epithelial cells, nor being essential for the development of hyperoxia-induced alveolar epithelial hyperplasia.

Hypoxia Sensing in Plants: On a Quest for Ion Channels as Putative Oxygen Sensors

Over 17 million km2 of land is affected by soil flooding every year, resulting in substantial yield losses and jeopardizing food security across the globe. A key step in resolving this problem and creating stress-tolerant cultivars is an understanding of the mechanisms by which plants sense low-oxygen stress. In this work, we review the current knowledge about the oxygen-sensing and signaling pathway in mammalian and plant systems and postulate the potential role of ion channels as putative oxygen sensors in plant roots. We first discuss the definition and requirements for the oxygen sensor and the difference between sensing and signaling. We then summarize the literature and identify several known candidates for oxygen sensing in the mammalian literature. This includes transient receptor potential (TRP) channels; K+-permeable channels (Kv, BK and TASK); Ca2+ channels (RyR and TPC); and various chemo- and reactive oxygen species (ROS)-dependent oxygen sensors. Identified key oxygen-sensing domains (PAS, GCS, GAF and PHD) in mammalian systems are used to predict the potential plant counterparts in Arabidopsis. Finally, the sequences of known mammalian ion channels with reported roles in oxygen sensing were employed to BLAST the Arabidopsis genome for the candidate genes. Several plasma membrane and tonoplast ion channels (such as TPC, AKT and KCO) and oxygen domain-containing proteins with predicted oxygen-sensing ability were identified and discussed. We propose a testable model for potential roles of ion channels in plant hypoxia sensing.

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

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

Gene expressions of TRP channels in glioblastoma multiforme and relation with survival

Glioblastoma multiforme (GBM) is one of the most lethal forms of cancer in humans, with a median survival of 10 to 12 months. Glioblastoma is highly malignant since the cells are supported by a great number of blood vessels. Although new treatments have been developed by increasing knowledge of molecular nature of the disease, surgical operation remains the standard of care. The TRP (transient receptor potential) superfamily consists of cation-selective channels that have roles in sensory physiology such as thermo- and osmosensation and in several complex diseases such as cancer, cardiovascular, and neuronal diseases. The aim of this study was to investigate the expression levels of TRP channel genes in patients with glioblastoma multiforme and to evaluate the relationship between TRP gene expressions and survival of the patients. Thirty-three patients diagnosed with glioblastoma were enrolled to the study. The expression levels of 21 TRP genes were quantified by using qRT-PCR with dynamic array 48 × 48 chip (BioMark HD System, Fluidigm, South San Francisco, CA, USA). TRPC1, TRPC6, TRPM2, TRPM3, TRPM7, TRPM8, TRPV1, and TRPV2 were found significantly higher in glioblastoma patients. Moreover, there was a significant relationship between the overexpression of TRP genes and the survival of the patients. These results demonstrate for the first time that TRP channels contribute to the progression and survival of the glioblastoma patients.

TRP channels and STIM/ORAI proteins: sensors and effectors of cancer and stroma cell migration

Cancer cells are strongly influenced by host cells within the tumour stroma and vice versa. This leads to the development of a tumour microenvironment with distinct physical and chemical properties that are permissive for tumour progression. The ability to migrate plays a central role in this mutual interaction. Migration of cancer cells is considered as a prerequisite for tumour metastasis and the migration of host stromal cells is required for reaching the tumour site. Increasing evidence suggests that transient receptor potential (TRP) channels and STIM/ORAI proteins affect key calcium-dependent mechanisms implicated in both cancer and stroma cell migration. These include, among others, cytoskeletal remodelling, growth factor/cytokine signalling and production, and adaptation to tumour microenvironmental properties such as hypoxia and oxidative stress. In this review, we will summarize the current knowledge regarding TRP channels and STIM/ORAI proteins in cancer and stroma cell migration. We focus on how TRP channel or STIM/ORAI-mediated Ca(2+) signalling directly or indirectly influences cancer and stroma cell migration by affecting the above listed mechanisms.

LINKED ARTICLES

This article is part of a themed section on Cytoskeleton, Extracellular Matrix, Cell Migration, Wound Healing and Related Topics. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-24.

Prolonged (9 h) poikilocapnic hypoxia (12% O2) augments cutaneous thermal hyperaemia in healthy humans

The primary aim of this study was to investigate the effect of systemic poikilocapnic hypoxia on forearm cutaneous thermal hyperaemia. A secondary aim was to examine the relationship between the individual susceptibility to oxygen desaturation and cutaneous vasodilator capacity. Twelve healthy participants (seven male) were exposed to 9 h of normoxia and 12% poikilocapnic hypoxia in a temperature- and humidity-controlled environmental chamber. Skin blood flow was assessed at the ventral forearm using laser Doppler flowmetry combined with rapid local heating. After 6 min at baseline (skin temperature clamped at 33°C), local skin temperature was elevated at a rate of 0.5°C every 5 s up to 42°C to elicit a sensory axon response and then held constant for 30 min to cause a plateau. Skin blood flow was calculated as cutaneous vascular conductance [CVC; in perfusion units/mean arterial blood pressure (APU mmHg(-1))] and expressed in raw format and relative to heating at 44°C in normoxia (%CVC44). During hypoxaemia, vasodilatation was greater during the initial peak (raw, Δ0.35 APU mmHg(-1), P = 0.09; %CVC44, Δ18%, P = 0.05) and the plateau phase (raw, Δ0.55 APU mmHg(-1), P = 0.03; %CVC44, Δ26%, P = 0.02). The rate of rise in cutaneous blood flow during the initial peak was significantly greater during poikilocapnic hypoxia (P < 0.01). We observed a negative relationship between oxygen saturation in poikilocapnic hypoxia and the change in baseline (P = 0.06), initial peak (P = 0.01) and plateau phase of thermal hyperaemia (P = 0.01). Prolonged poikilocapnic hypoxia causes robust increases in CVC during both phases of thermal hyperaemia that are dependent on the oxygen saturation of the individual.

TRPs as chemosensors (ROS, RNS, RCS, gasotransmitters)

The transient receptor potential (trp) gene superfamily encodes TRP proteins that act as multimodal sensor cation channels for a wide variety of stimuli from outside and inside the cell. Upon chemical or physical stimulation of cells, TRP channels transduce electrical and/or Ca(2+) signals via their cation channel activities. These functional features of TRP channels allow the body to react and adapt to different forms of environmental changes. Indeed, members of one class of TRP channels have emerged as sensors of reactive oxygen species (ROS), reactive nitrogen species (RNS), reactive carbonyl species (RCS), and gaseous messenger molecules including molecular oxygen (O2), hydrogen sulfide (H2S), and carbon dioxide (CO2). Hydrogen peroxide (H2O2), an ROS, triggers the production of ADP-ribose, which binds and activates TRPM2. In addition to TRPM2, TRPC5, TRPV1, and TRPA1 are also activated by H2O2 via modification of cysteine (Cys) free sulfhydryl groups. Nitric oxide (NO), a vasoactive gaseous molecule, regulates TRP channels directly via Cys S-nitrosylation or indirectly via cyclic GMP (cGMP)/protein kinase G (PKG)-dependent phosphorylation. Anoxia induced by O2-glucose deprivation and severe hypoxia activates TRPM7 and TRPC6, respectively, whereas TRPA1 serves as a sensor of mild hypoxia and hyperoxia in vagal and sensory neurons. TRPA1 also detects other gaseous molecules, such as hydrogen sulfide (H2S) and carbon dioxide (CO2). In this review, we highlight our current knowledge of TRP channels as chemosensors for ROS, RNS, RCS, and gaseous molecules and discuss their functional impacts on physiological and pathological events.