Arachidonic acid and ion channels: an update

CYP2J2-mediated metabolism of arachidonic acid in heart: A review of its kinetics, inhibition and role in heart rhythm control

Cytochrome P450 2 J2 (CYP2J2) is primarily expressed extrahepatically and is the predominant epoxygenase in human cardiac tissues. This highlights its key role in the metabolism of endogenous substrates. Significant scientific interest lies in cardiac CYP2J2 metabolism of arachidonic acid (AA), an omega-6 polyunsaturated fatty acid, to regioisomeric bioactive epoxyeicosatrienoic acid (EET) metabolites that show cardioprotective effects including regulation of cardiac electrophysiology. From an in vitro perspective, the accurate characterization of the kinetics of CYP2J2 metabolism of AA including its inhibition and inactivation by drugs could be useful in facilitating in vitro-in vivo extrapolations to predict drug-AA interactions in drug discovery and development. In this review, background information on the structure, regulation and expression of CYP2J2 in human heart is presented alongside AA and EETs as its endogenous substrate and metabolites. The in vitro and in vivo implications of the kinetics of this endogenous metabolic pathway as well as its perturbation via inhibition and inactivation by drugs are elaborated. Additionally, the role of CYP2J2-mediated metabolism of AA to EETs in cardiac electrophysiology will be expounded.

Selenomethionine in gelatin methacryloyl hydrogels: Modulating ferroptosis to attenuate skin aging

During skin aging, the degeneration of epidermal stem cells (EpiSCs) leads to diminished wound healing capabilities and epidermal disintegration. This study tackles this issue through a comprehensive analysis combining transcriptomics and untargeted metabolomics, revealing age-dependent alterations in the Gpx gene family and arachidonic acid (AA) metabolic networks, resulting in enhanced ferroptosis. Selenomethionine (Se-Met) could enhance GPX4 expression, thereby assisting EpiSCs in countering AA-induced mitochondrial damage and ferroptosis. Additionally, Se-Met demonstrates antioxidative characteristics and extensive ultraviolet absorption. For the sustained and controllable release of Se-Met, it was covalently grafted to UV-responsive GelMA hydrogels via AC-PEG-NHS tethers. The Se-Met@GelMA hydrogel effectively accelerated wound healing in a chronological aging mice model, by inhibiting lipid peroxidation and ferroptosis with augmented GPX4 expression. Moreover, in a photoaging model, this hydrogel significantly mitigated inflammatory responses, extracellular matrix remodeling, and ferroptosis in UV-exposed mice. These characteristics render Se-Met@GelMA hydrogel valuable in practical clinical applications.

TMEM175: A lysosomal ion channel associated with neurological diseases

Lysosomes are acidic intracellular organelles with autophagic functions that are critical for protein degradation and mitochondrial homeostasis, while abnormalities in lysosomal physiological functions are closely associated with neurological disorders. Transmembrane protein 175 (TMEM175), an ion channel in the lysosomal membrane that is essential for maintaining lysosomal acidity, has been proven to coordinate with V-ATPase to modulate the luminal pH of the lysosome to assist the digestion of abnormal proteins and organelles. However, there is considerable controversy about the characteristics of TMEM175. In this review, we introduce the research progress on the structural, modulatory, and functional properties of TMEM175, followed by evidence of its relevance for neurological disorders. Finally, we discuss the potential value of TMEM175 as a therapeutic target in the hope of providing new directions for the treatment of neurodegenerative diseases.

Evaluation of Cx43 Gap Junction Inhibitors Using a Quantitative Structure-Activity Relationship Model

Gap junctions (GJs) made of connexin-43 (Cx43) are necessary for the conduction of electrical impulses in the heart. Modulation of Cx43 GJ activity may be beneficial in the treatment of cardiac arrhythmias and other dysfunctions. The search for novel GJ-modulating agents using molecular docking allows for the accurate prediction of binding affinities of ligands, which, unfortunately, often poorly correlate with their potencies. The objective of this study was to demonstrate that a Quantitative Structure-Activity Relationship (QSAR) model could be used for more precise identification of potent Cx43 GJ inhibitors. Using molecular docking, QSAR, and 3D-QSAR, we evaluated 16 known Cx43 GJ inhibitors, suggested the monocyclic monoterpene d-limonene as a putative Cx43 inhibitor, and tested it experimentally in HeLa cells expressing exogenous Cx43. The predicted concentrations required to produce 50% of the maximal effect (IC50) for each of these compounds were compared with those determined experimentally (pIC50 and eIC50, respectively). The pIC50ies of d-limonene and other Cx43 GJ inhibitors examined by our QSAR and 3D-QSAR models showed a good correlation with their eIC50ies (R = 0.88 and 0.90, respectively) in contrast to pIC50ies obtained from molecular docking (R = 0.78). However, molecular docking suggests that inhibitor potency may depend on their docking conformation on Cx43. Searching for new potent, selective, and specific inhibitors of GJ channels, we propose to perform the primary screening of new putative compounds using the QSAR model, followed by the validation of the most suitable candidates by patch-clamp techniques.

Investigating the relevance of CYP2J2 inhibition for drugs known to cause intermediate to high risk torsades de pointes

Cardiac cytochrome P450 2J2 (CYP2J2) metabolizes endogenous polyunsaturated fatty acid, arachidonic acid (AA), to bioactive regioisomeric epoxyeicosatrienoic acid (EET) metabolites. This endogenous metabolic pathway has been postulated to play a homeostatic role in cardiac electrophysiology. However, it is unknown if drugs that cause intermediate to high risk torsades de pointes (TdP) exhibit inhibitory effects against CYP2J2 metabolism of AA to EETs. In this study, we demonstrated that 11 out of 16 drugs screened with intermediate to high risk of TdP as defined by the Comprehensive in vitro Proarrhythmia Assay (CiPA) initiative are concurrently reversible inhibitors of CYP2J2 metabolism of AA, with unbound inhibitory constant (K_i,AA,u) values ranging widely from 0.132 to 19.9 µM. To understand the physiological relevancy of K_i,AA,u, the in vivo unbound drug concentration within human heart tissue (C_u,heart) was calculated via experimental determination of in vitro unbound partition coefficient (K_puu) for 10 CYP2J2 inhibitors using AC16 human ventricular cardiomyocytes as well as literature-derived values of fraction unbound in plasma (f_u,p) and plasma drug concentrations in clinical scenarios leading to TdP. Notably, all CYP2J2 inhibitors screened belonging to the high TdP risk category, namely vandetanib and bepridil, exhibited highest K_puu values of 18.2 ± 1.39 and 7.48 ± 1.16 respectively although no clear relationship between C_u,heart and risk of TdP could eventually be determined. R values based on basic models of reversible inhibition as per FDA guidelines were calculated using unbound plasma drug concentrations (C_u,plasma) and adapted using C_u,heart which suggested that 4 out of 10 CYP2J2 inhibitors with intermediate to high risk of TdP demonstrate greatest potential for clinically relevant in vivo cardiac drug-AA interactions. Our results shed novel insights on the relevance of CYP2J2 inhibition in drugs with risk of TdP. Further studies ascertaining the role of CYP2J2 metabolism of AA in cardiac electrophysiology, characterizing inherent cardiac ion channel activities of drugs with risk of TdP as well as in vivo evidence of drug-AA interactions will be required prior to determining if CYP2J2 inhibition could be an alternative mechanism contributing to drug-induced TdP.

Phenotypic and Transcriptomics Analyses Reveal Underlying Mechanisms in a Mouse Model of Corneal Bee Sting

Corneal bee sting (CBS) is one of the most common ocular traumas and can lead to blindness. The ophthalmic manifestations are caused by direct mechanical effects of bee stings, toxic effects, and host immune responses to bee venom (BV); however, the underlying pathogenesis remains unclear. Clinically, topical steroids and antibiotics are routinely used to treat CBS patients but the specific drug targets are unknown; therefore, it is imperative to study the pathological characteristics, injury mechanisms, and therapeutic targets involved in CBS. In the present study, a CBS injury model was successfully established by injecting BV into the corneal stroma of healthy C57BL/6 mice. F-actin staining revealed corneal endothelial cell damage, decreased density, skeletal disorder, and thickened corneal stromal. The terminal-deoxynucleotidyl transferase mediated nick end labeling (TUNEL) assay showed apoptosis of both epithelial and endothelial cells. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that cytokine–cytokine interactions were the most relevant pathway for pathogenesis. Protein–protein interaction (PPI) network analysis showed that IL-1, TNF, and IL-6 were the most relevant nodes. RNA-seq after the application of Tobradex^® (0.3% tobramycin and 0.1% dexamethasone) eye ointment showed that Tobradex^® not only downregulated relevant inflammatory factors but also reduced corneal pain as well as promoted nerve regeneration by repairing axons. Here, a stable and reliable model of CBS injury was successfully established for the first time, and the pathogenesis of CBS and the therapeutic targets of Tobradex^® are discussed. These hub genes are expected to be biomarkers and therapeutic targets for the diagnosis and treatment of CBS.

Role of Arachidonic Acid and Its Metabolites in the Biological and Clinical Manifestations of Idiopathic Nephrotic Syndrome

Studies concerning the role of arachidonic acid (AA) and its metabolites in kidney disease are scarce, and this applies in particular to idiopathic nephrotic syndrome (INS). INS is one of the most frequent glomerular diseases in childhood; it is characterized by T-lymphocyte dysfunction, alterations of pro- and anti-coagulant factor levels, and increased platelet count and aggregation, leading to thrombophilia. AA and its metabolites are involved in several biological processes. Herein, we describe the main fields where they may play a significant role, particularly as it pertains to their effects on the kidney and the mechanisms underlying INS. AA and its metabolites influence cell membrane fluidity and permeability, modulate platelet activity and coagulation, regulate lymphocyte activity and inflammation, preserve the permeability of the glomerular barrier, influence podocyte physiology, and play a role in renal fibrosis. We also provide suggestions regarding dietary measures that are able to prevent an imbalance between arachidonic acid and its parental compound linoleic acid, in order to counteract the inflammatory state which characterizes numerous kidney diseases. On this basis, studies of AA in kidney disease appear as an important field to explore, with possible relevant results at the biological, dietary, and pharmacological level, in the final perspective for AA to modulate INS clinical manifestations.

A Brief Review of Brain's Blood Flow-Metabolism Coupling and Pressure Autoregulation

BACKGROUND

 The human brain, depending on aerobic glycolysis to cover its metabolic needs and having no energy reserves whatsoever, relies on a constant and closely regulated blood supply to maintain its structural and functional integrity. Cerebral autoregulation, that is, the brain's intrinsic ability to regulate its own blood flow independently from the systemic blood pressure and cardiac output, is an important physiological mechanism that offers protection from hypoperfusion injury.

DISCUSSION

 Two major independent mechanisms are known to be involved in cerebral autoregulation: (1) flow-metabolism coupling and (2) myogenic responses of cerebral blood vessels to changes in transmural/arterial pressure. A third, less prominent component of cerebral autoregulation comes in the form of neurogenic influences on cerebral vasculature.

CONCLUSION

 Although fragmentation of cerebral autoregulation in separate and distinct from each other mechanisms is somewhat arbitrary, such a scheme is useful for reasons of simplification and to better understand their overall effect. Comprehension of cerebral autoregulation is imperative for clinicians in order for them to mitigate consequences of its impairment in the context of traumatic brain injury, subarachnoid hemorrhage, stroke, or other pathological conditions.

Arachidonic acid effect on the allosteric gating mechanism of BK (Slo1) channels associated with the β1 subunit

Arachidonic acid (AA) is a fatty acid involved in the modulation of several ion channels. Previously, we reported that AA activates the high conductance Ca²⁺- and voltage-dependent K⁺ channel (BK) in vascular smooth muscle depending on the expression of the auxiliary β1 subunit. Here, using the patch-clamp technique on BK channel co-expressed with β1 subunit in a heterologous cell expression system, we analyzed whether AA modifies the three functional modules involved in the channel gating: the voltage sensor domain (VSD), the pore domain (PD), and the intracellular calcium sensor domain (CSD). We present evidence that AA activates BK channel in a direct way, inducing VSD stabilization on its active configuration observed as a significant left shift in the Q-V curve obtained from gating currents recordings. Moreover, AA facilitates the channel opening transitions when VSD are at rest, and the CSD are unoccupied. Furthermore, the activation was independent of the intracellular Ca²⁺ concentration and reduced when the BK channel was co-expressed with the Y74A mutant of the β1 subunit. These results allow us to present new insigths in the mechanism by which AA modulates BK channels co-expressed with its auxiliary β1 subunit.

Phosphoproteomics identify arachidonic-acid-regulated signal transduction pathways modulating macrophage functions with implications for ovarian cancer

Arachidonic acid (AA) is a polyunsaturated fatty acid present at high concentrations in the ovarian cancer (OC) microenvironment and associated with a poor clinical outcome. In the present study, we have unraveled a potential link between AA and macrophage functions.

Methods: AA-triggered signal transduction was studied in primary monocyte-derived macrophages (MDMs) by phosphoproteomics, transcriptional profiling, measurement of intracellular Ca²⁺ accumulation and reactive oxygen species production in conjunction with bioinformatic analyses. Functional effects were investigated by actin filament staining, quantification of macropinocytosis and analysis of extracellular vesicle release.

Results: We identified the ASK1 - p38δ/α (MAPK13/14) axis as a central constituent of signal transduction pathways triggered by non-metabolized AA. This pathway was induced by the Ca²⁺-triggered activation of calmodulin kinase II, and to a minor extent by ROS generation in a subset of donors. Activated p38 in turn was linked to a transcriptional stress response associated with a poor relapse-free survival. Consistent with the phosphorylation of the p38 substrate HSP27 and the (de)phosphorylation of multiple regulators of Rho family GTPases, AA impaired actin filament organization and inhibited actin-driven macropinocytosis. AA also affected the phosphorylation of proteins regulating vesicle biogenesis, and consistently, AA enhanced the release of tetraspanin-containing exosome-like vesicles. Finally, we identified phospholipase A₂ group 2A (PLA2G2A) as the clinically most relevant enzyme producing extracellular AA, providing further potentially theranostic options.

Conclusion: Our results suggest that AA contributes to an unfavorable clinical outcome of OC by impacting the phenotype of tumor-associated macrophages. Besides critical AA-regulated signal transduction proteins identified in the present study, PLA2G2A might represent a potential prognostic tool and therapeutic target to interfere with OC progression.

Dietary ARA Improves COX Activity in Broodstock and Offspring Survival Fitness of a Model Organism (Medaka Oryzias latipes)

A 3-week feeding trial was conducted in medaka broodstock (age five months) to examine the effect of dietary arachidonic acid (ARA) level (range: 4-23 mg g^-1 of total fatty acids (TFAs)) on fertility, cyclooxygenase (COX) activity, egg size, sperm motility parameters, hatching rate and weight of hatch, survival and growth of larvae. After spawn induction and dietary exposure to 4 mg ARA g^-1 TFA, broodstock were fed a diet containing ARA in the amounts: 4 (continued, as control), 5, 14 and 23 mg g^-1 TFA. COX1 activity in the liver and the number of COX2-positive cells in the ovaries was increased in females fed the diets containing the two highest amounts of ARA. The highest sperm motility parameters were observed in males fed a diet containing 23 mg ARA g^-1 TFA. The hatchability rate and bodyweight of hatchlings were higher in the group obtained from broodstock fed a diet containing 23 mg ARA g^-1 TFA (79% and 0.66 mg fish^-1, respectively) compared with 4 mg ARA g^-1 TFA (50% and 0.40 mg fish^-1). The average mortality of offspring obtained from this group at 7 days post hatching was significantly higher than that of all other groups.

In vitro exposure to Hymenoptera venom and constituents activates discrete ionotropic pathways in mast cells

Calcium entry is central to the functional processes in mast cells and basophils that contribute to the induction and maintenance of inflammatory responses. Mast cells and basophils express an array of calcium channels, which mediate responses to diverse stimuli triggered by small bioactive molecules, physicochemical stimuli and immunological inputs including antigens and direct immune cell interactions. These cells are also highly responsive to certain venoms (such as Hymenoptera envenomations), which cause histamine secretion, cytokine release and an array of pro-inflammatory functional responses. There are gaps in our understanding of the coupling of venom exposure to specific signaling pathways such as activation of calcium channels. In the present study, we performed a current survey of a model mast cell line selected for its pleiotropic responsiveness to multiple pro-inflammatory inputs. As a heterogenous stimulus, Hymenoptera venom activates multiple classes of conductance at the population level but tend to lead to the measurement of only one type of conductance per cell, despite the cell co-expressing multiple channel types. The data show that I_CRAC, I_ARC, and TRPV-like currents are present in the model mast cell populations and respond to venom exposure. We further assessed individual venom components, specifically secretagogues and arachidonic acid, and identified the conductances associated with these stimuli in mast cells. Single-cell calcium assays and immunofluorescence analysis show that there is heterogeneity of channel expression across the cell population, but this heterogeneity does not explain the apparent selectivity for specific channels in response to exposure to venom as a composite stimulus.

The Lectin LecA Sensitizes the Human Stretch-Activated Channel TREK-1 but Not Piezo1 and Binds Selectively to Cardiac Non-myocytes

The healthy heart adapts continuously to a complex set of dynamically changing mechanical conditions. The mechanical environment is altered by, and contributes to, multiple cardiac diseases. Mechanical stimuli are detected and transduced by cellular mechano-sensors, including stretch-activated ion channels (SAC). The precise role of SAC in the heart is unclear, in part because there are few SAC-specific pharmacological modulators. That said, most SAC can be activated by inducers of membrane curvature. The lectin LecA is a virulence factor of Pseudomonas aeruginosa and essential for P. aeruginosa-induced membrane curvature, resulting in formation of endocytic structures and bacterial cell invasion. We investigate whether LecA modulates SAC activity. TREK-1 and Piezo1 have been selected, as they are widely expressed in the body, including cardiac tissue, and they are “canonical representatives” for the potassium selective and the cation non-selective SAC families, respectively. Live cell confocal microscopy and electron tomographic imaging were used to follow binding dynamics of LecA, and to track changes in cell morphology and membrane topology in human embryonic kidney (HEK) cells and in giant unilamellar vesicles (GUV). HEK cells were further transfected with human TREK-1 or Piezo1 constructs, and ion channel activity was recorded using the patch-clamp technique. Finally, freshly isolated cardiac cells were used for studies into cell type dependency of LecA binding. LecA (500 nM) binds within seconds to the surface of HEK cells, with highest concentration at cell-cell contact sites. Local membrane invaginations are detected in the presence of LecA, both in the plasma membrane of cells (by 17 min of LecA exposure) as well as in GUV. In HEK cells, LecA sensitizes TREK-1, but not Piezo1, to voltage and mechanical stimulation. In freshly isolated cardiac cells, LecA binds to non-myocytes, but not to ventricular or atrial cardiomyocytes. This cell type specific lack of binding is observed across cardiomyocytes from mouse, rabbit, pig, and human. Our results suggest that LecA may serve as a pharmacological tool to study SAC in a cell type-preferential manner. This could aid tissue-based research into the roles of SAC in cardiac non-myocytes.

PUFA and their derivatives in neurotransmission and synapses: a new hallmark of synaptopathies

PUFA of the n-3 and n-6 families are present in high concentration in the brain where they are major components of cell membranes. The main forms found in the brain are DHA (22 :6, n-3) and arachidonic acid (20:4, n-6). In the past century, several studies pinpointed that modifications of n-3 and n-6 PUFA levels in the brain through dietary supply or genetic means are linked to the alterations of synaptic function. Yet, synaptopathies emerge as a common characteristic of neurodevelopmental disorders, neuropsychiatric diseases and some neurodegenerative diseases. Understanding the mechanisms of action underlying the activity of PUFA at the level of synapses is thus of high interest. In this frame, dietary supplementation in PUFA aiming at restoring or promoting the optimal function of synapses appears as a promising strategy to treat synaptopathies. This paper reviews the link between dietary PUFA, synapse formation and the role of PUFA and their metabolites in synaptic functions.

Spadin Selectively Antagonizes Arachidonic Acid Activation of TREK-1 Channels

TREK-1 channel activity is a critical regulator of neuronal, cardiac, and smooth muscle physiology and pathology. The antidepressant peptide, spadin, has been proposed to be a TREK-1-specific blocker. Here we sought to examine the mechanism of action underlying spadin inhibition of TREK-1 channels. Heterologous expression in Xenopus laevis oocytes and electrophysiological analysis using two-electrode voltage clamp in standard bath solutions was used to characterize the pharmacological profile of wild-type and mutant murine TREK-1 and TREK-2 channels using previously established human K_2P activators; arachidonic acid (AA), cis-4,7,10,13,16,19-docosahexaenoic acid (DHA), BL-1249, and cinnamyl-3,4-dihydroxy-α-cyanocinnamate (CDC) and inhibitors; spadin and barium (Ba²⁺). Mouse TREK-1 and TREK-2 channel currents were both significantly increased by AA, BL-1249, and CDC, similar to their human homologs. Under basal conditions, both TREK-1 and TREK-2 currents were insensitive to application of spadin, but could be blocked by Ba²⁺. Spadin did not significantly inhibit either TREK-1 or TREK-2 currents either chemically activated by AA, BL-1249, or CDC, or structurally activated via a gating mutation. However, pre-exposure to spadin significantly perturbed the subsequent activation of TREK-1 currents by AA, but not TREK-2. Furthermore, spadin was unable to prevent activation of TREK-1 by BL-1249, CDC, or the related bioactive lipid, DHA. Spadin specifically antagonizes the activation of TREK-1 channels by AA, likely via an allosteric mechanism. Lack of intrinsic activity may explain the absence of clinical side effects during antidepressant therapy.

MS-based targeted metabolomics of eicosanoids and other oxylipins: Analytical and inter-individual variabilities

Oxylipins, including the well-known eicosanoids, are potent lipid mediators involved in numerous physiological and pathological processes. Therefore, their quantitative profiling has gained a lot of attention during the last years notably in the active field of health biomarker discovery. Oxylipins include hundreds of structurally and stereochemically distinct lipid species which today are most commonly analyzed by (ultra) high performance liquid chromatography-mass spectrometry based ((U)HPLC-MS) methods. To maximize the utility of oxylipin profiling in clinical research, it is crucial to understand and assess the factors contributing to the analytical and biological variability of oxylipin profiles in humans. In this review, these factors and their impacts are summarized and discussed, providing a framework for recommendations expected to enhance the interlaboratory comparability and biological interpretation of oxylipin profiling in clinical research.

Cytokine and inflammatory mediator effects on TRPV4 function in choroid plexus epithelial cells

The choroid plexus (CP), composed of capillaries surrounded by a barrier epithelium, is the main producer of cerebrospinal fluid (CSF). The CP epithelium regulates the transport of ions and water between the blood and the ventricles, contributing to CSF production and composition. Several studies suggest a connection between the cation channel transient receptor potential vanilloid-4 (TRPV4) and transepithelial ion movement. TRPV4 is a nonselective, calcium-permeable cation channel present in CP epithelia reported to be activated by cytokines and inflammatory mediators. Utilizing the PCP-R (porcine choroid plexus-Riems) cell line, we investigated the effects of various cytokines and inflammatory mediators on TRPV4-mediated activity. Select proinflammatory cytokines (TNF-α, IL-1β, TGF-β1) had inhibitory effects on TRPV4-stimulated transepithelial ion flux and permeability changes, whereas anti-inflammatory cytokines (IL-10, IL-4, and IL-6) had none. Quantitative mRNA analysis showed that these cytokines had no effect on TRPV4 transcription levels. Inhibition of the transcription factor NF-κB, involved in the production and regulation of several inflammatory cytokines, inhibited TRPV4-mediated activity, suggesting a link between TRPV4 and cytokine production. Contrary to published studies, the proinflammatory mediator arachidonic acid (AA) had inhibitory rather than stimulatory effects on TRPV4-mediated responses. However, inhibition of AA metabolism also caused inhibitory effects on TRPV4, suggesting a complex interaction of AA and its metabolites in the regulation of TRPV4 activity. Together these data imply that TRPV4 activity is involved in the inflammatory response; it is negatively affected by proinflammatory mediators. Furthermore, arachidonic acid metabolites, but not arachidonic acid itself, are positive regulators of TRPV4.

Reactions at Biomembrane Interfaces

Membranes surrounding the biological cell and its internal compartments host proteins that catalyze chemical reactions essential for the functioning of the cell. Rather than being a passive structural matrix that holds membrane-embedded proteins in place, the membrane can largely shape the conformational energy landscape of membrane proteins and impact the energetics of their chemical reaction. Here, we highlight the challenges in understanding how lipids impact the conformational energy landscape of macromolecular membrane complexes whose functioning involves chemical reactions including proton transfer. We review here advances in our understanding of how chemical reactions occur at membrane interfaces gleaned with both theoretical and experimental advances using simple protein systems as guides. Our perspective is that of bridging experiments with theory to understand general physicochemical principles of membrane reactions, with a long term goal of furthering our understanding of the role of the lipids on the functioning of complex macromolecular assemblies at the membrane interface.

Arachidonic acid hyperpolarizes mesenchymal stromal cells from the human adipose tissue by stimulating TREK1 K+ channels

The current knowledge of electrogenesis in mesenchymal stromal cells (MSCs) remains scarce. Earlier, we demonstrated that in MSCs from the human adipose tissue, transduction of certain agonists involved the phosphoinositide cascade. Its pivotal effector PLC generates DAG that can regulate ion channels directly or via its derivatives, including arachidonic acid (AA). Here we showed that AA strongly hyperpolarized MSCs by stimulating instantly activating, outwardly rectifying TEA-insensitive K⁺ channels. Among AA-regulated K⁺ channels, K2P channels from the TREK subfamily appeared to be an appropriate target. The expression of K2P channels in MSCs was verified by RT-PCR, which revealed TWIK-1, TREK-1, and TASK-5 transcripts. The TREK-1 inhibitor spadin antagonized the electrogenic action of AA, which was simulated by the channel activator BL 1249. This functional evidence suggested that TREK-1 channels mediated AA-dependent hyperpolarization of MSCs. Being mostly silent at rest, TREK-1 negligibly contributed to the “background” K⁺ current. The dramatic stimulation of TREK-1 channels by AA indicates their involvement in AA-dependent signaling in MSCs.

TREK-1 channels regulate pressure sensitivity and calcium signaling in trabecular meshwork cells

The trabecular meshwork (TM) plays a fundamental role in intraocular pressure regulation, but its mechanotransduction pathway is poorly understood. Yarishkin et al. show that the mechanosensing channel TREK-1 regulates TM membrane potential, pressure sensitivity, calcium homeostasis, and impedance.

Mechanotransduction by the trabecular meshwork (TM) is an essential component of intraocular pressure regulation in the vertebrate eye. This process is compromised in glaucoma but is poorly understood. In this study, we identify transient receptor potential vanilloid isoform 4 (TRPV4) and TWIK-related potassium channel-1 (TREK-1) as key molecular determinants of TM membrane potential, pressure sensitivity, calcium homeostasis, and transcellular permeability. We show that resting membrane potential in human TM cells is unaffected by “classical” inhibitors of voltage-activated, calcium-activated, and inwardly rectifying potassium channels but is depolarized by blockers of tandem-pore K⁺ channels. Using gene profiling, we reveal the presence of TREK-1, TASK-1, TWIK-2, and THIK transcripts in TM cells. Pressure stimuli, arachidonic acid, and TREK-1 activators hyperpolarize these cells, effects that are antagonized by quinine, amlodipine, spadin, and short-hairpin RNA–mediated knockdown of TREK-1 but not TASK-1. Activation and inhibition of TREK-1 modulates [Ca²⁺]_TM and lowers the impedance of cell monolayers. Together, these results suggest that tensile homeostasis in the TM may be regulated by balanced, pressure-dependent activation of TRPV4 and TREK-1 mechanotransducers.

A Click Cage: Organelle-Specific Uncaging of Lipid Messengers

Lipid messengers exert their function on short time scales at distinct subcellular locations, yet most experimental approaches for perturbing their levels trigger cell-wide concentration changes. Herein, we report on a coumarin-based photocaging group that can be modified with organelle-targeting moieties by click chemistry and thus enables photorelease of lipid messengers in distinct organelles. We show that caged arachidonic acid and sphingosine derivatives can be selectively delivered to mitochondria, the ER, lysosomes, and the plasma membrane. By comparing the cellular calcium transients induced by localized uncaging of arachidonic acid and sphingosine, we show that the precise intracellular localization of the released second messenger is crucial for the signaling outcome. Ultimately, we anticipate that this new class of caged compounds will greatly facilitate the study of cellular processes on the organelle level.

Optical control of GPR40 signalling in pancreatic β-cells

Fatty acids activate GPR40 and K+ channels to modulate β-cell function. Herein, we describe the design and synthesis of FAAzo-10, a light-controllable GPR40 agonist based on Gw-9508. FAAzo-10 is a potent GPR40 agonist in the trans-configuration and can be inactivated on isomerization to cis with UV-A light. Irradiation with blue light reverses this effect, allowing FAAzo-10 activity to be cycled ON and OFF with a high degree of spatiotemporal precision. In dissociated primary mouse β-cells, FAAzo-10 also inactivates voltage-activated and ATP-sensitive K+ channels, and allows us to control glucose-stimulated Ca2+ oscillations in whole islets with light. As such, FAAzo-10 is a useful tool to study the complex effects, with high specificity, which FA-derivatives such as Gw-9508 exert at multiple targets in mouse β-cells.

Undercover Power of Endocannabinoids: Postsynaptic Ion-Channel Modulator

In this issue of Neuron, Gantz and Bean (2017) show that the endocannabinoid 2-arachidonoyl glycerol (2-AG) can directly alter the properties of native ion-channel Kv_4.3 and accelerate the pacemaker activity of rodent dopamine neurons. These findings are one of the first demonstrations of postsynaptic, cell-autonomous actions of endocannabinoids in the mammalian brain.

Investigation of Diacylglycerol Lipase Alpha Inhibition in the Mouse Lipopolysaccharide Inflammatory Pain Model

Diacylglycerol lipase (DAGL) α and β, the major biosynthetic enzymes of the endogenous cannabinoid (endocannabinoid) 2-arachidonylglycerol (2-AG), are highly expressed in the nervous system and immune system, respectively. Genetic deletion or pharmacological inhibition of DAGL-β protects against lipopolysaccharide (LPS)-induced inflammatory responses in mouse peritoneal macrophages and reverses LPS-induced allodynia in mice. To gain insight into the contribution of DAGL-α in LPS-induced allodynia, we tested global knockout mice as well as DO34, a dual DAGL-α/β inhibitor. Intraperitoneal administration of DO34 (30 mg/kg) significantly decreased whole-brain levels of 2-AG (∼83%), anandamide (∼42%), and arachidonic acid (∼58%). DO34 dose-dependently reversed mechanical and cold allodynia, and these antinociceptive effects did not undergo tolerance after 6 days of repeated administration. In contrast, DO34 lacked acute thermal antinociceptive, motor, and hypothermal pharmacological effects in naive mice. As previously reported, DAGL-β (-/-) mice displayed a protective phenotype from LPS-induced allodynia. However, DAGL-α (-/-) mice showed full allodynic responses, similar to their wild-type littermates. Interestingly, DO34 (30 mg/kg) fully reversed LPS-induced allodynia in DAGL-α (+/+) and (-/-) mice, but did not affect the antinociceptive phenotype of DAGL-β (-/-) mice in this model, indicating a DAGL-α-independent site of action. These findings suggest that DAGL-α and DAGL-β play distinct roles in LPS-induced nociception. Whereas DAGL-α appears to be dispensable for the development and expression of LPS-induced nociception, DAGL-β inhibition represents a promising strategy to treat inflammatory pain.

Role of TRP Channels in Dinoflagellate Mechanotransduction

Transient receptor potential (TRP) ion channels are common components of mechanosensing pathways, mainly described in mammals and other multicellular organisms. To gain insight into the evolutionary origins of eukaryotic mechanosensory proteins, we investigated the involvement of TRP channels in mechanosensing in a unicellular eukaryotic protist, the dinoflagellate Lingulodinium polyedra. BLASTP analysis of the protein sequences predicted from the L. polyedra transcriptome revealed six sequences with high similarity to human TRPM2, TRPM8, TRPML2, TRPP1, and TRPP2; and characteristic TRP domains were identified in all sequences. In a phylogenetic tree including all mammalian TRP subfamilies and TRP channel sequences from unicellular and multicellular organisms, the L. polyedra sequences grouped with the TRPM, TPPML, and TRPP clades. In pharmacological experiments, we used the intrinsic bioluminescence of L. polyedra as a reporter of mechanoresponsivity. Capsaicin and RN1734, agonists of mammalian TRPV, and arachidonic acid, an agonist of mammalian TRPV, TRPA, TRPM, and Drosophila TRP, all stimulated bioluminescence in L. polyedra. Mechanical stimulation of bioluminescence, but not capsaicin-stimulated bioluminescence, was inhibited by gadolinium (Gd³⁺), a general inhibitor of mechanosensitive ion channels, and the phospholipase C (PLC) inhibitor U73122. These pharmacological results are consistent with the involvement of TRP-like channels in mechanosensing by L. polyedra. The TRP channels do not appear to be mechanoreceptors but rather are components of the mechanotransduction signaling pathway and may be activated via a PLC-dependent mechanism. The presence and function of TRP channels in a dinoflagellate emphasize the evolutionary conservation of both the channel structures and their functions.

Valnoctamide, which reduces rat brain arachidonic acid turnover, is a potential non-teratogenic valproate substitute to treat bipolar disorder

BACKGROUND

Valproic acid (VPA), used for treating bipolar disorder (BD), is teratogenic by inhibiting histone deacetylase. In unanaesthetized rats, chronic VPA, like other mood stabilizers, reduces arachidonic acid (AA) turnover in brain phospholipids, and inhibits AA activation to AA-CoA by recombinant acyl-CoA synthetase-4 (Acsl-4) in vitro. Valnoctamide (VCD), a non-teratogenic constitutional isomer of VPA amide, reported effective in BD, also inhibits recombinant Acsl-4 in vitro.

HYPOTHESIS

VCD like VPA will reduce brain AA turnover in unanaesthetized rats.

METHODS

A therapeutically relevant (50mg/kg i.p.) dose of VCD or vehicle was administered daily for 30 days to male rats. AA turnover and related parameters were determined using our kinetic model, following intravenous [1-¹⁴C]AA in unanaesthetized rats for 10min, and measuring labeled and unlabeled lipids in plasma and high-energy microwaved brain.

RESULTS

VCD, compared with vehicle, increased λ, the ratio of brain AA-CoA to unesterified plasma AA specific activities; and decreased turnover of AA in individual and total brain phospholipids.

CONCLUSIONS

VCD's ability like VPA to reduce rat brain AA turnover and inhibit recombinant Acsl-4, and its efficacy in BD, suggest that VCD be further considered as a non-teratogenic VPA substitute for treating BD.

Lipids as central modulators of sensory TRP channels

The transient receptor potential (TRP) ion channel family is involved in a diversity of physiological processes including sensory and homeostatic functions, as well as muscle contraction and vasomotor control. Their dysfunction contributes to the etiology of several diseases, being validated as therapeutic targets. These ion channels may be activated by physical or chemical stimuli and their function is highly influenced by signaling molecules activated by extracellular signals. Notably, as integral membrane proteins, lipid molecules also modulate their membrane location and function either by direct interaction with the channel structure or by modulating the physico-chemical properties of the cellular membrane. This lipid-based modulatory effect is being considered an alternative and promising approach to regulate TRP channel dysfunction in diseases. Here, we review the current progress in this exciting field highlighting a complex channel regulation by a large diversity of lipid molecules and suggesting some diseases that may benefit from a membrane lipid therapy. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Cell-Autonomous Excitation of Midbrain Dopamine Neurons by Endocannabinoid-Dependent Lipid Signaling

The major endocannabinoid in the mammalian brain is the bioactive lipid 2-arachidonoylglycerol (2-AG). The best-known effects of 2-AG are mediated by G-protein-coupled cannabinoid receptors. In principle, 2-AG could modify neuronal excitability by acting directly on ion channels, but such mechanisms are poorly understood. Using a preparation of dissociated mouse midbrain dopamine neurons to isolate effects on intrinsic excitability, we found that 100 nM 2-AG accelerated pacemaking and steepened the frequency-current relationship for burst-like firing. In voltage-clamp experiments, 2-AG reduced A-type potassium current (I_A) through a cannabinoid receptor-independent mechanism mimicked by arachidonic acid, which has no activity on cannabinoid receptors. Activation of orexin, neurotensin, and metabotropic glutamate G_q/11-linked receptors mimicked the effects of exogenous 2-AG and their actions were prevented by inhibiting the 2-AG-synthesizing enzyme diacylglycerol lipase α. The results show that 2-AG and related lipid signaling molecules can directly tune neuronal excitability in a cell-autonomous manner by modulating I_A.

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Animal toxins and renal ion transport: Another dimension in tropical nephrology

Renal vascular and tubular ion channels and transporters involved in toxin injury are reviewed. Vascular ion channels modulated by animal toxins, which result in haemodynamic alterations and changes in blood pressure, include ENaC/Degenerin/ASIC, ATP sensitive K channels (KATP ), Ca activated K channels (Kca) and voltage gated Ca channels, mostly L-type. Renal tubular Na channels and K channels are also targeted by animal toxins. NHE3 and ENaC are two important targets. NCC and NKCC may be involved indirectly by vasoactive mediators induced by inflammation. Most renal tubular K channels including voltage gated K channels (Kv1), KATP , ROMK1, BK and SK are blocked by scorpion toxins. Few are inhibited by bee, wasp and spider venoms. Due to small envenoming, incomplete block and several compensatory mechanisms in renal tubules, serum electrolyte charges are not apparent. Changes in serum electrolytes are observed in injury by large amount of venom when several channels or transporters are targeted. Envenomings by scorpions and bees are examples of toxins targeting multiple ion channels and transporters.

Arachidonic acid mediates the formation of abundant alpha-helical multimers of alpha-synuclein

The protein alpha-synuclein (αS) self-assembles into toxic beta-sheet aggregates in Parkinson’s disease, while it is proposed that αS forms soluble alpha-helical multimers in healthy neurons. Here, we have made αS multimers in vitro using arachidonic acid (ARA), one of the most abundant fatty acids in the brain, and characterized them by a combination of bulk experiments and single-molecule Fӧrster resonance energy transfer (sm-FRET) measurements. The data suggest that ARA-induced oligomers are alpha-helical, resistant to fibril formation, more prone to disaggregation, enzymatic digestion and degradation by the 26S proteasome, and lead to lower neuronal damage and reduced activation of microglia compared to the oligomers formed in the absence of ARA. These multimers can be formed at physiologically-relevant concentrations, and pathological mutants of αS form less multimers than wild-type αS. Our work provides strong biophysical evidence for the formation of alpha-helical multimers of αS in the presence of a biologically relevant fatty acid, which may have a protective role with respect to the generation of beta-sheet toxic structures during αS fibrillation.

Arachidonic acid-evoked Ca2+ signals promote nitric oxide release and proliferation in human endothelial colony forming cells

Arachidonic acid (AA) stimulates endothelial cell (EC) proliferation through an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i), that, in turn, promotes nitric oxide (NO) release. AA-evoked Ca²⁺ signals are mainly mediated by Transient Receptor Potential Vanilloid 4 (TRPV4) channels. Circulating endothelial colony forming cells (ECFCs) represent the only established precursors of ECs. In the present study, we, therefore, sought to elucidate whether AA promotes human ECFC (hECFC) proliferation through an increase in [Ca²⁺]_i and the following activation of the endothelial NO synthase (eNOS). AA induced a dose-dependent [Ca²⁺]_i raise that was mimicked by its non-metabolizable analogue eicosatetraynoic acid. AA-evoked Ca²⁺ signals required both intracellular Ca²⁺ release and external Ca²⁺ inflow. AA-induced Ca²⁺ release was mediated by inositol-1,4,5-trisphosphate receptors from the endoplasmic reticulum and by two pore channel 1 from the acidic stores of the endolysosomal system. AA-evoked Ca²⁺ entry was, in turn, mediated by TRPV4, while it did not involve store-operated Ca²⁺ entry. Moreover, AA caused an increase in NO levels which was blocked by preventing the concomitant increase in [Ca²⁺]_i and by inhibiting eNOS activity with NG-nitro-l-arginine methyl ester (l-NAME). Finally, AA per se did not stimulate hECFC growth, but potentiated growth factors-induced hECFC proliferation in a Ca²⁺- and NO-dependent manner. Therefore, AA-evoked Ca²⁺ signals emerge as an additional target to prevent cancer vascularisation, which may be sustained by ECFC recruitment.

Chronic Omega-3 Polyunsaturated Fatty Acid Treatment Variably Affects Cellular Repolarization in a Healed Post-MI Arrhythmia Model

Introduction: Over the last 40 years omega-3 polyunsaturated fatty acids (PUFAs) have been shown to be anti-arrhythmic or pro-arrhythmic depending on the method and duration of administration and model studied. We previously reported that omega-3 PUFAs do not confer anti-arrhythmic properties and are pro-arrhythmic in canine model of sudden cardiac death (SCD). Here, we evaluated the effects of chronic omega-3 PUFA treatment in post-MI animals susceptible (VF+) or resistant (VF−) to ventricular tachyarrhythmias.

Methods: Perforated patch clamp techniques were used to measure cardiomyocyte action potential durations (APD) at 50 and 90% repolarization and short term variability of repolarization. The early repolarizing transient outward potassium current I_to was also studied.

Results: Omega-3 PUFAs prolonged the action potential in VF− myocytes at both 50 and 90% repolarization. Short term variability of repolarization was increased in both untreated and treated VF− myocytes vs. controls. I_to was unaffected by omega-3 PUFA treatment. Omega-3 PUFA treatment attenuated the action potential prolongation in VF+ myocytes, but did not return repolarization to control values.

Conclusions: Omega-3 PUFAs do not confer anti-arrhythmic properties in the setting of healed myocardial infarction in a canine model of SCD. In canines previously resistant to ventricular fibrillation (VF−), omega-3 PUFA treatment prolonged the action potential in VF− myocytes, and may contribute to pro-arrhythmic responses.

Differential volume regulation and calcium signaling in two ciliary body cell types is subserved by TRPV4 channels

Fluid secretion by the ciliary body plays a critical and irreplaceable function in vertebrate vision by providing nutritive support to the cornea and lens, and by maintaining intraocular pressure. Here, we identify TRPV4 (transient receptor potential vanilloid isoform 4) channels as key osmosensors in nonpigmented epithelial (NPE) cells of the mouse ciliary body. Hypotonic swelling and the selective agonist GSK1016790A (EC50 ∼33 nM) induced sustained transmembrane cation currents and cytosolic [Formula: see text] elevations in dissociated and intact NPE cells. Swelling had no effect on [Formula: see text] levels in pigment epithelial (PE) cells, whereas depolarization evoked [Formula: see text] elevations in both NPE and PE cells. Swelling-evoked [Formula: see text] signals were inhibited by the TRPV4 antagonist HC067047 (IC50 ∼0.9 μM) and were absent in Trpv4(-/-) NPE. In NPE, but not PE, swelling-induced [Formula: see text] signals required phospholipase A2 activation. TRPV4 localization to NPE was confirmed with immunolocalization and excitation mapping approaches, whereas in vivo MRI analysis confirmed TRPV4-mediated signals in the intact mouse ciliary body. Trpv2 and Trpv4 were the most abundant vanilloid transcripts in CB. Overall, our results support a model whereby TRPV4 differentially regulates cell volume, lipid, and calcium signals in NPE and PE cell types and therefore represents a potential target for antiglaucoma medications.

The endocannabinoid system in guarding against fear, anxiety and stress

The endocannabinoid (eCB) system has emerged as a central integrator linking the perception of external and internal stimuli to distinct neurophysiological and behavioural outcomes (such as fear reaction, anxiety and stress-coping), thus allowing an organism to adapt to its changing environment. eCB signalling seems to determine the value of fear-evoking stimuli and to tune appropriate behavioural responses, which are essential for the organism's long-term viability, homeostasis and stress resilience; and dysregulation of eCB signalling can lead to psychiatric disorders. An understanding of the underlying neural cell populations and cellular processes enables the development of therapeutic strategies to mitigate behavioural maladaptation.

Effect of oleic acid on store-operated calcium entry in immune-competent cells

PURPOSE

To study the mechanism by which oleic acid (OA) (C18:1) exerts its beneficial effects on immune-competent cells. Since store-operated Ca²⁺ entry (SOCE) is a Ca²⁺ influx pathway involved in the control of multiple physiological processes including cell proliferation, we studied the effect of OA in Ca²⁺ signals of Jurkat T cells and THP-1 monocytes, paying particular attention to SOCE.

METHODS

Changes in [Ca²⁺]_i were measured using the Fura-2 fluorescence dye. Mn²⁺ uptake was monitored as a rate of quenching of Fura-2 fluorescence measured at the Ca²⁺-insensitive wavelengths. Thapsigargin was used to induce SOCE in Fura-2-loaded cells.

RESULTS

We showed a clear dose-dependent SOCE-inhibitory effect of OA in both cell lines. Such an inhibitory effect was PKC independent and totally restored by albumin, suggesting that OA exerts its effect somewhere in the membrane. We also demonstrated that OA induces increases in [Ca²⁺]_i partly mediated by an extracellular Ca²⁺ influx through econazole-insensitive channels. Finally, we compared the effect of OA with stearic acid (C18:0), assuming the emerged evidence concerning the link between saturated fats and inflammation disorders. Stearic acid failed to inhibit SOCE, independently on the concentration tested, thus intensifying the physiological relevance of our findings.

CONCLUSION

We suggest a physiological pathway for the beneficial effects of OA in inflammation.

The Effects of Yin, Yang and Qi in the Skin on Pain

The most effective and safe treatment site for pain is in the skin. This chapter discusses the reasons to treat pain in the skin. Pain is sensed in the skin through transient receptor potential cation channels and other receptors. These receptors have endogenous agonists (yang) and antagonists (yin) that help the body control pain. Acupuncture works through modulation of these receptor activities (qi) in the skin; as do moxibustion and liniments. The treatment of pain in the skin has the potential to save many lives and improve pain therapy in most patients.

Effects of unsaturated fatty acids on the kinetics of voltage-gated proton channels heterologously expressed in cultured cells

KEY POINTS

Arachidonic acid (AA) greatly enhances the activity of the voltage-gated proton (Hv) channel, although its mechanism of action and physiological function remain unclear. In the present study, we analysed the effects of AA on proton currents through Hv channels heterologously expressed in HEK293T cells. The dramatic increase in proton current amplitude elicited by AA was accompanied by accelerated activation kinetics and a leftward shift in the voltage-dependence of activation. Mutagenesis studies suggest the two aforementioned effects of AA reflect two distinct structural mechanisms. Application of phospholipase A2 , which liberates AA from phospholipids in the membrane, also enhances Hv channel activity, supporting the idea that AA modulates Hv channel activity within physiological contexts. Unsaturated fatty acids are key components of the biological membranes of all cells, and precursors of mediators for cell signalling. Arachidonic acid (AA) is an unsaturated fatty acid known to modulate the activities of various ion channels, including the voltage-gated proton (Hv) channel, which supports the rapid production of reactive oxygen species (ROS) in phagocytes through regulation of pH and membrane potential. However, the molecular mechanisms and physiological functions of the effects of AA on Hv channels remain unclear. In the present study, we report an electrophysiological analysis of the effects of AA on the mouse Hv channel (mHv1) heterologously expressed in HEK293T cells. Application of AA to excised inside-out patch membranes rapidly induced a robust increase in the amplitude of the proton current through mHv1. The current increase was accompanied by accelerated activation kinetics and a small leftward shift of the current-voltage relationship. In monomeric channels lacking the coiled-coil region of the channel protein, the shift in the current-voltage relationship was diminished but activation and deactivation remained accelerated. Studies with several AA derivatives showed that double bonds and hydrophilic head groups are essential for the effect of AA, although charge was not important. The application of phospholipase A2 (PLA2), which generates AA from cell membrane phospholipids, stimulated mHv1 activity to a similar extent as direct application of ∼ 20 μM AA, suggesting that endogenous AA may regulate Hv channel activity.

Profiling the role of mammalian target of rapamycin in the vascular smooth muscle metabolome in pulmonary arterial hypertension

Increased proliferation and resistance to apoptosis of pulmonary arterial vascular smooth muscle cells (PAVSMCs), coupled with metabolic reprogramming, are key components of pulmonary vascular remodeling, a major and currently irreversible pathophysiological feature of pulmonary arterial hypertension (PAH). We recently reported that activation of mammalian target of rapamycin (mTOR) plays a key role in increased energy generation and maintenance of the proliferative, apoptosis-resistant PAVSMC phenotype in human PAH, but the downstream effects of mTOR activation on PAH PAVSMC metabolism are not clear. Using liquid and gas chromatography-based mass spectrometry, we performed pilot metabolomic profiling of human microvascular PAVSMCs from idiopathic-PAH subjects before and after treatment with the selective adenosine triphosphate-competitive mTOR inhibitor PP242 and from nondiseased lungs. We have shown that PAH PAVSMCs have a distinct metabolomic signature of altered metabolites-components of fatty acid synthesis, deficiency of sugars, amino sugars, and nucleotide sugars-intermediates of protein and lipid glycosylation, and downregulation of key biochemicals involved in glutathione and nicotinamide adenine dinucleotide (NAD) metabolism. We also report that mTOR inhibition attenuated or reversed the majority of the PAH-specific abnormalities in lipogenesis, glycosylation, glutathione, and NAD metabolism without affecting altered polyunsaturated fatty acid metabolism. Collectively, our data demonstrate a critical role of mTOR in major PAH PAVSMC metabolic abnormalities and suggest the existence of de novo lipid synthesis in PAVSMCs in human PAH that may represent a new, important component of disease pathogenesis worthy of future investigation.

Exclusive photorelease of signalling lipids at the plasma membrane

Photoactivation of caged biomolecules has become a powerful approach to study cellular signalling events. Here we report a method for anchoring and uncaging biomolecules exclusively at the outer leaflet of the plasma membrane by employing a photocleavable, sulfonated coumarin derivative. The novel caging group allows quantifying the reaction progress and efficiency of uncaging reactions in a live-cell microscopy setup, thereby greatly improving the control of uncaging experiments. We synthesized arachidonic acid derivatives bearing the new negatively charged or a neutral, membrane-permeant coumarin caging group to locally induce signalling either at the plasma membrane or on internal membranes in β-cells and brain slices derived from C57B1/6 mice. Uncaging at the plasma membrane triggers a strong enhancement of calcium oscillations in β-cells and a pronounced potentiation of synaptic transmission while uncaging inside cells blocks calcium oscillations in β-cells and causes a more transient effect on neuronal transmission, respectively. The precise subcellular site of arachidonic acid release is therefore crucial for signalling outcome in two independent systems.

Caged signalling intermediates are powerful cell biological tools, however it can be challenging to precisely control where activation occurs. Nadler et al. develop a caging group that specifically targets the plasma membrane, and demonstrate spatially controlled activation of arachidonic acid signalling.

Putative binding sites for arachidonic acid on the human cardiac Kv 1.5 channel

BACKGROUND AND PURPOSE

In human heart, the Kv 1.5 channel contributes to repolarization of atrial action potentials. This study examined the electrophysiological and molecular mechanisms underlying arachidonic acid (AA)-induced inhibition of the human Kv 1.5 (hKv 1.5) channel.

EXPERIMENTAL APPROACH

Site-directed mutagenesis was conducted to mutate amino acids that reside within the pore domain of the hKv 1.5 channel. Whole-cell patch-clamp method was used to record membrane currents through wild type and mutant hKv 1.5 channels heterologously expressed in CHO cells. Computer docking simulation was conducted to predict the putative binding site(s) of AA in an open-state model of the Kv 1.5 channel.

KEY RESULTS

The hKv 1.5 current was minimally affected at the onset of depolarization but was progressively reduced during depolarization by the presence of AA, suggesting that AA acts as an open-channel blocker. AA itself affected the channel at extracellular sites independently of its metabolites and signalling pathways. The blocking effect of AA was attenuated at pH 8.0 but not at pH 6.4. The blocking action of AA developed rather rapidly by co-expression of Kv β1.3. The AA-induced block was significantly attenuated in H463C, T480A, R487V, I502A, I508A, V512A and V516A, but not in T462C, A501V and L510A mutants of the hKv 1.5 channel. Docking simulation predicted that H463, T480, R487, I508, V512 and V516 are potentially accessible for interaction with AA.

CONCLUSIONS AND IMPLICATIONS

AA itself interacts with multiple amino acids located in the pore domain of the hKv 1.5 channel. These findings may provide useful information for future development of selective blockers of hKv 1.5 channels.

From Heart Failure to Highly Unsaturated Fatty Acid Deficiency and Vice Versa: Bidirectional Heart and Liver Interactions

BACKGROUND

In several trials, beneficial prognostic effects of highly unsaturated fatty acids (HUFAs) in heart failure were shown. Because other studies showed no incremental benefit in nearly preserved cardiac function, the question arises, whether the degree of cardiac dysfunction is involved. It is hypothesized that increased left ventricular (LV) wall stress affects the endogenous hepatic HUFA metabolism, which in turn exhibits adverse cardiac consequences.

METHODS

Cardiac magnetic resonance imaging was performed in 30 patients with suspected cardiomyopathy. The serum fatty acid profile was assessed using gas chromatography/mass spectrometry.

RESULTS

Docosahexaenoic acid (DHA; P = 0.002) and eicosapentaenoic acid (EPA; by trend) levels were decreased in patients with reduced LV ejection fraction (≤ 50%) or LV dilatation (≥ 90 mL/m(2)). Decreased DHA (P = 0.003) and EPA (P = 0.022) levels were associated with a reduced LV ejection fraction. Decreased DHA level was correlated with increased end-diastolic (P = 0.047) and end-systolic LV wall stress (P = 0.001). Pseudocholinesterase activity was inversely correlated with end-diastolic (P = 0.020) and end-systolic LV wall stress (P = 0.025).

CONCLUSIONS

DHA level was significantly reduced in heart failure. Similar, but less pronounced effects were found for EPA and arachidonic acid by trend. Increased LV wall stress was correlated with a reduced DHA level. Increased LV wall stress exhibits various adverse consequences (eg, increased oxygen consumption, favouring of arrhythmias, and an unfavourable remodelling). The increase of wall stress was paralleled by reduced HUFA level. Increased LV wall stress was correlated with reduced pseudocholinesterase, which is suggestive of hepatic congestion (ie, a cardiohepatic syndrome, involved in the altered fatty acid profile in heart failure) and has major consequences regarding the dose-efficacy of HUFA treatment.

Differential regulation of proton-sensitive ion channels by phospholipids: a comparative study between ASICs and TRPV1

Protons are released in pain-generating pathological conditions such as inflammation, ischemic stroke, infection, and cancer. During normal synaptic activities, protons are thought to play a role in neurotransmission processes. Acid-sensing ion channels (ASICs) are typical proton sensors in the central nervous system (CNS) and the peripheral nervous system (PNS). In addition to ASICs, capsaicin- and heat-activated transient receptor potential vanilloid 1 (TRPV1) channels can also mediate proton-mediated pain signaling. In spite of their importance in perception of pH fluctuations, the regulatory mechanisms of these proton-sensitive ion channels still need to be further investigated. Here, we compared regulation of ASICs and TRPV1 by membrane phosphoinositides, which are general cofactors of many receptors and ion channels. We observed that ASICs do not require membrane phosphatidylinositol 4-phosphate (PI(4)P) or phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) for their function. However, TRPV1 currents were inhibited by simultaneous breakdown of PI(4)P and PI(4,5)P₂. By using a novel chimeric protein, CF-PTEN, that can specifically dephosphorylate at the D3 position of phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P₃), we also observed that neither ASICs nor TRPV1 activities were altered by depletion of PI(3,4,5)P₃ in intact cells. Finally, we compared the effects of arachidonic acid (AA) on two proton-sensitive ion channels. We observed that AA potentiates the currents of both ASICs and TRPV1, but that they have different recovery aspects. In conclusion, ASICs and TRPV1 have different sensitivities toward membrane phospholipids, such as PI(4)P, PI(4,5)P₂, and AA, although they have common roles as proton sensors. Further investigation about the complementary roles and respective contributions of ASICs and TRPV1 in proton-mediated signaling is necessary.