BL-1249 [(5,6,7,8-tetrahydro-naphthalen-1-yl)-[2-(1H-tetrazol-5-yl)-phenyl]-amine]: a putative potassium channel opener with bladder-relaxant properties

Long-term follow-up of TREK-1 KO mice reveals the development of bladder hypertrophy and impaired bladder smooth muscle contractility with age

Stretch-activated two-pore domain K+ (K2P) channels play important roles in many visceral organs, including the urinary bladder. The TWIK-related K+ channel TREK-1 is the predominantly expressed K2P channel in the urinary bladder of humans and rodents. Downregulation of TREK-1 channels was observed in the urinary bladder of patients with detrusor overactivity, suggesting their involvement in the pathogenesis of voiding dysfunction. This study aimed to characterize the long-term effects of TREK-1 on bladder function with global and smooth muscle-specific TREK-1 knockout (KO) mice. Bladder morphology, bladder smooth muscle (BSM) contractility, and voiding patterns were evaluated up to 12 mo of age. Both sexes were included in this study to probe the potential sex differences. Smooth muscle-specific TREK-1 KO mice were used to distinguish the effects of TREK-1 downregulation in BSM from the neural pathways involved in the control of bladder contraction and relaxation. TREK-1 KO mice developed enlarged urinary bladders (by 60.0% for males and by 45.1% for females at 6 mo; P < 0.001 compared with the age-matched control group) and had a significantly increased bladder capacity (by 137.7% at 12 mo; P < 0.0001) and compliance (by 73.4% at 12 mo; P < 0.0001). Bladder strips isolated from TREK-1 KO mice exhibited decreased contractility (peak force after KCl at 6 mo was 1.6 ± 0.7 N/g compared with 3.4 ± 2.0 N/g in the control group; P = 0.0005). The lack of TREK-1 channels exclusively in BSM did not replicate the bladder phenotype observed in TREK-1 KO mice, suggesting a strong neurogenic origin of TREK-1-related bladder dysfunction.NEW & NOTEWORTHY This study compared voiding function and bladder phenotypes in global and smooth muscle-specific TREK-1 KO mice. We found significant age-related changes in bladder contractility, suggesting that the lack of TREK-1 channel activity might contribute to age-related changes in bladder smooth muscle physiology.

Validation of TREK1 ion channel activators as an immunomodulatory and neuroprotective strategy in neuroinflammation

Modulation of two-pore domain potassium (K_2P) channels has emerged as a novel field of therapeutic strategies as they may regulate immune cell activation and metabolism, inflammatory signals, or barrier integrity. One of these ion channels is the TWIK-related potassium channel 1 (TREK1). In the current study, we report the identification and validation of new TREK1 activators. Firstly, we used a modified potassium ion channel assay to perform high-throughput-screening of new TREK1 activators. Dose-response studies helped to identify compounds with a high separation between effectiveness and toxicity. Inside-out patch-clamp measurements of Xenopus laevis oocytes expressing TREK1 were used for further validation of these activators regarding specificity and activity. These approaches yielded three substances, E1, B3 and A2 that robustly activate TREK1. Functionally, we demonstrated that these compounds reduce levels of adhesion molecules on primary human brain and muscle endothelial cells without affecting cell viability. Finally, we studied compound A2 via voltage-clamp recordings as this activator displayed the strongest effect on adhesion molecules. Interestingly, A2 lacked TREK1 activation in the tested neuronal cell type. Taken together, this study provides data on novel TREK1 activators that might be employed to pharmacologically modulate TREK1 activity.

Activation of TREK-1 (K2P2.1) potassium channels protects against influenza A-induced lung injury

Influenza-A virus (IAV) infects yearly an estimated one billion people worldwide, resulting in 300,000-650,000 deaths. Preventive vaccination programs and antiviral medications represent the mainstay of therapy, but with unacceptably high morbidity and mortality rates, new targeted therapeutic approaches are urgently needed. Since inflammatory processes are commonly associated with measurable changes in the cell membrane potential (Em), we investigated whether Em hyperpolarization via TREK-1 (K2P2.1) K+ channel activation can protect against influenza-A virus (IAV)-induced pneumonia. We infected mice with IAV, which after 5 days caused 10-15% weight loss and a decrease in spontaneous activity, representing a clinically relevant infection. We then started a 3-day intratracheal treatment course with the novel TREK-1 activating compounds BL1249 or ML335. We confirmed TREK-1 activation with both compounds in untreated and IAV-infected primary human alveolar epithelial cells (HAECs) using high-throughput fluorescent imaging plate reader (FLIPR) assays. In mice, TREK-1 activation with BL1249 and ML335 counteracted IAV-induced histological lung injury and decrease in lung compliance and improved BAL fluid total protein levels, cell counts, and inflammatory IL-6, IP-10/CXCL-10, MIP-1α, and TNF-α levels. To determine whether these anti-inflammatory effects were mediated by activation of alveolar epithelial TREK-1 channels, we studied the effects of BL1249 and ML335 in IAV-infected HAEC, and found that TREK-1 activation decreased IAV-induced inflammatory IL-6, IP-10/CXCL10, and CCL-2 secretion. Dissection of TREK-1 downstream signaling pathways and construction of protein-protein interaction (PPI) networks revealed NF-κB1 and retinoic acid-inducible gene-1 (RIG-1) cascades as the most likely targets for TREK-1 protection. Therefore, TREK-1 activation may represent a novel therapeutic approach against IAV-induced lung injury.

Convergence of Multiple Stimuli to a Single Gate in TREK1 and TRAAK Potassium Channels

Inhibitory potassium channels of the TREK1/TRAAK family are integrators of multiple stimuli, including temperature, membrane stretch, polyunsaturated fatty acids and pH. How these signals affect the gating of these channels is the subject of intense research. We have previously identified a cytoplasmic domain, pCt, which plays a major role in controlling channel activity. Here, we use pharmacology to show that the effects of pCt, arachidonic acid, and extracellular pH converge to the same gate within the channel. Using a state-dependent inhibitor, fluoxetine, as well as natural and synthetic openers, we provide further evidence that the “up” and “down” conformations identified by crystallography do not correspond to open and closed states of these channels.

Bepridil, a class IV antiarrhythmic agent, can block the TREK-1 potassium channel

Background

The TWIK-related potassium channel (TREK-1) can be regulated by different stimuli. However, it is not clear whether some antiarrhythmics affect the activity of TREK-1. In the present study, the effect of bepridil on the TREK-1 currents is investigated.

Methods

In a TREK-1 stably-expressed HEK-293 cell line (HEK-TREK-1), U251MG cells, and isolated mouse ventricular myocytes, the TREK-1 current and action potentials were recorded by the patch-clamp technique. The standard voltage protocol was a 200 ms constant potential at 20 mV, followed bya 500 ms ramp from –90 to +20 mV (HEK-TREK-1) or +80 mV (U251MG cells and myocytes) every 10 s. The currents at +20 mV or +80 mV were used for analysis. The docking study of bepridil’s binding model in the TREK-1 channel was performed using the Swissdock web service.

Results

In HEK-TREK-1 cells, BL1249 induced a significantly large outwardly rectifying current with similar baseline TREK-1 current characteristic, with a reversal potential (−70 mV). The concentration of half-maximal activation (EC₅₀) of BL1249 was 3.45 µM. However, bepridil decreased the baseline TREK-1 currents, with a concentration of half-maximal inhibition (IC₅₀) 0.59 µM and a Hill coefficient of 1.1. Also, bepridil inhibited BL1249-activated TREK-1 currents, with an IC₅₀ 4.08 µM and a Hill coefficient of 3.22. The outside-out patch-clamp confirmed bepridil inhibited BL1249-activated TREK-1 currents. In U251MG cells and myocytes, BL1249 activated outwardly rectifying endogenous TREK-1 currents, which could be inhibited by bepridil. BL1249 (10 µM) could decrease the peak value and reduce the duration of the action potential. Bepridil (10 µM) prolonged the duration of action potential of myocytes. The docking study revealed that bepridil might affect the K⁺ pore domain and the M4 modulator pocket.

Conclusions

Bepridil may be a blocker for the TREK-1K⁺channel at a clinically therapeutic concentration, providing a new mechanism of TREK-1 regulation and bepridil's antiarrhythmic effect.

Negative Influence by the Force: Mechanically Induced Hyperpolarization via K2P Background Potassium Channels

The two-pore domain K_2P subunits form background (leak) potassium channels, which are characterized by constitutive, although not necessarily constant activity, at all membrane potential values. Among the fifteen pore-forming K_2P subunits encoded by the KCNK genes, the three members of the TREK subfamily, TREK-1, TREK-2, and TRAAK are mechanosensitive ion channels. Mechanically induced opening of these channels generally results in outward K⁺ current under physiological conditions, with consequent hyperpolarization and inhibition of membrane potential-dependent cellular functions. In the past decade, great advances have been made in the investigation of the molecular determinants of mechanosensation, and members of the TREK subfamily have emerged among the best-understood examples of mammalian ion channels directly influenced by the tension of the phospholipid bilayer. In parallel, the crucial contribution of mechano-gated TREK channels to the regulation of membrane potential in several cell types has been reported. In this review, we summarize the general principles underlying the mechanical activation of K_2P channels, and focus on the physiological roles of mechanically induced hyperpolarization.

TREK-1 potassium channels participate in acute and long-lasting nociceptive hypersensitivity induced by formalin in rats

TREK-1 channels are expressed in small nociceptive dorsal root ganglion (DRG) neurons where they participate in acute inflammatory and neuropathic pain. However, the role of TREK-1 in persistent pain is not well understood. The aim of this study was to investigate the local peripheral and spinal participation of TREK-1 in formalin-induced acute and long-lasting nociceptive hypersensitivity. Local peripheral or intrathecal pre-treatment with spadin, selective blocker of TREK-1, increased acute flinching behavior and secondary mechanical allodynia and hyperalgesia behavior observed 6 days after formalin injection. Local peripheral or intrathecal pre-treatment with BL-1249, selective opener of TREK-1, decreased long-lasting secondary mechanical allodynia and hyperalgesia induced by formalin. Pre-treatment with BL-1249 prevented the pro-nociceptive effect of spadin on acute nociception and long-lasting mechanical allodynia and hyperalgesia in rats. Pre-treatment with two recombinant channels that produce a high TREK-1 current, S300A and S333A (non-phosphorylated state of TREK-1), reduced formalin-induced acute pain and long-lasting mechanical allodynia and hyperalgesia. Besides, post-treatment with S300A, S333A or BL-1249 reversed long-lasting mechanical allodynia and hyperalgesia induced by formalin. Formalin increased TREK-1 expression at 1 and 6 days in DRG and dorsal spinal cord in rats, whereas that it increased c-fos expression at the DRG. Intrathecal repeated transfection of rats with S300A and S333A or injection with BL-1249 reduced formalin-induced enhanced c-fos expression. Data suggest that TREK-1 activity at peripheral and spinal sites reduces neuronal excitability in the process of acute and long-lasting nociception induced by formalin in rats.

The Prostacyclin Analogue, Treprostinil, Used in the Treatment of Pulmonary Arterial Hypertension, is a Potent Antagonist of TREK-1 and TREK-2 Potassium Channels

Pulmonary arterial hypertension (PAH) is an aggressive vascular remodeling disease that carries a high morbidity and mortality rate. Treprostinil (Remodulin) is a stable prostacyclin analogue with potent vasodilatory and anti-proliferative activity, approved by the FDA and WHO as a treatment for PAH. A limitation of this therapy is the severe subcutaneous site pain and other forms of pain experienced by some patients, which can lead to significant non-compliance. TWIK-related potassium channels (TREK-1 and TREK-2) are highly expressed in sensory neurons, where they play a role in regulating sensory neuron excitability. Downregulation, inhibition or mutation of these channels leads to enhanced pain sensitivity. Using whole-cell patch-clamp electrophysiological recordings, we show, for the first time, that treprostinil is a potent antagonist of human TREK-1 and TREK-2 channels but not of TASK-1 channels. An increase in TASK-1 channel current was observed with prolonged incubation, consistent with its therapeutic role in PAH. To investigate treprostinil-induced inhibition of TREK, site-directed mutagenesis of a number of amino acids, identified as important for the action of other regulatory compounds, was carried out. We found that a gain of function mutation of TREK-1 (Y284A) attenuated treprostinil inhibition, while a selective activator of TREK channels, BL-1249, overcame the inhibitory effect of treprostinil. Our data suggests that subcutaneous site pain experienced during treprostinil therapy may result from inhibition of TREK channels near the injection site and that pre-activation of these channels prior to treatment has the potential to alleviate this nociceptive activity.

Structural Insights into the Mechanisms and Pharmacology of K2P Potassium Channels

Leak currents, defined as voltage and time independent flows of ions across cell membranes, are central to cellular electrical excitability control. The K_2P (KCNK) potassium channel class comprises an ion channel family that produces potassium leak currents that oppose excitation and stabilize the resting membrane potential in cells in the brain, cardiovascular system, immune system, and sensory organs. Due to their widespread tissue distribution, K_2Ps contribute to many physiological and pathophysiological processes including anesthesia, pain, arrythmias, ischemia, hypertension, migraine, intraocular pressure regulation, and lung injury responses. Structural studies of six homomeric K_2Ps have established the basic architecture of this channel family, revealed key moving parts involved in K_2P function, uncovered the importance of asymmetric pinching and dilation motions in the K_2P selectivity filter (SF) C-type gate, and defined two K_2P structural classes based on the absence or presence of an intracellular gate. Further, a series of structures characterizing K_2P:modulator interactions have revealed a striking polysite pharmacology housed within a relatively modestly sized (~70 kDa) channel. Binding sites for small molecules or lipids that control channel function are found at every layer of the channel structure, starting from its extracellular side through the portion that interacts with the membrane bilayer inner leaflet. This framework provides the basis for understanding how gating cues sensed by different channel parts control function and how small molecules and lipids modulate K_2P activity. Such knowledge should catalyze development of new K_2P modulators to probe function and treat a wide range of disorders.

Development of Non-opioid Analgesics Targeting Two-pore Domain Potassium Channels

Two-pore domain potassium (K2P) channels are a diverse family of potassium channels. K2P channels generate background leak potassium currents to regulate cellular excitability and are thereby involved in a wide range of neurological disorders. K2P channels are modulated by a variety of physicochemical factors such as mechanical stretch, temperature, and pH. In the the peripheral nervous system (PNS), K2P channels are widely expressed in nociceptive neurons and play a critical roles in pain perception. In this review, we summarize the recent advances in the pharmacological properties of K2P channels, with a focus on the exogenous small-molecule activators targeting K2P channels. We emphasize the subtype-selectivity, cellular and in vivo pharmacological properties of all the reported small-molecule activators. The key underlying analgesic mechanisms mediated by K2P are also summarized based on the data in the literature from studies using small-molecule activators and genetic knock-out animals. We discuss advantages and limitations of the translational perspectives of K2P in pain medicine and provide outstanding questions for future studies in the end.

A "Target Class" Screen to Identify Activators of Two-Pore Domain Potassium (K2P) Channels

Two-pore domain potassium (K2P) channels carry background (or leak) potassium current and play a key role in regulating resting membrane potential and cellular excitability. Accumulating evidence points to a role for K2Ps in human pathophysiologies, most notably in pain and migraine, making them attractive targets for therapeutic intervention. However, there remains a lack of selective pharmacological tools. The aim of this work was to apply a “target class” approach to investigate the K2P superfamily and identify novel activators across all the described subclasses of K2P channels. Target class drug discovery allows for the leveraging of accumulated knowledge and maximizing synergies across a family of targets and serves as an additional approach to standard target-based screening. A common assay platform using baculovirus (BacMam) to transiently express K2P channels in mammalian cells and a thallium flux assay to determine channel activity was developed, allowing the simultaneous screening of multiple targets. Importantly, this system, by allowing precise titration of channel function, allows optimization to facilitate the identification of activators. A representative set of channels (THIK-1, TWIK-1, TREK-2, TASK-3, and TASK-2) were screened against a library of Food and Drug Administration (FDA)-approved compounds and the LifeArc Index Set. Activators were then analyzed in concentration–response format across all channels to assess selectivity. Using the target class approach to investigate the K2P channels has enabled us to determine which of the K2Ps are amenable to small-molecule activation, de-risk multiple channels from a technical point of view, and identify a diverse range of previously undescribed pharmacology.

K2P2.1 (TREK-1) potassium channel activation protects against hyperoxia-induced lung injury

No targeted therapies exist to counteract Hyperoxia (HO)-induced Acute Lung Injury (HALI). We previously found that HO downregulates alveolar K_2P2.1 (TREK-1) K⁺ channels, which results in worsening lung injury. This decrease in TREK-1 levels leaves a subset of channels amendable to pharmacological intervention. Therefore, we hypothesized that TREK-1 activation protects against HALI. We treated HO-exposed mice and primary alveolar epithelial cells (AECs) with the novel TREK-1 activators ML335 and BL1249, and quantified physiological, histological, and biochemical lung injury markers. We determined the effects of these drugs on epithelial TREK-1 currents, plasma membrane potential (Em), and intracellular Ca²⁺ (iCa) concentrations using fluorometric assays, and blocked voltage-gated Ca²⁺ channels (Ca_V) as a downstream mechanism of cytokine secretion. Once-daily, intra-tracheal injections of HO-exposed mice with ML335 or BL1249 improved lung compliance, histological lung injury scores, broncho-alveolar lavage protein levels and cell counts, and IL-6 and IP-10 concentrations. TREK-1 activation also decreased IL-6, IP-10, and CCL-2 secretion from primary AECs. Mechanistically, ML335 and BL1249 induced TREK-1 currents in AECs, counteracted HO-induced cell depolarization, and lowered iCa²⁺ concentrations. In addition, CCL-2 secretion was decreased after L-type Ca_V inhibition. Therefore, Em stabilization with TREK-1 activators may represent a novel approach to counteract HALI.

PKC- and PKA-dependent phosphorylation modulates TREK-1 function in naïve and neuropathic rats

PKC and PKA phosphorylation inhibit TREK-1 channels downstream of G_s protein-coupled receptor activation in vitro. However, the role of phosphorylation of TREK-1 in neuropathic pain is unknown. The purpose of this study was to investigate whether altered TREK-1 channel function by PKA and PKC modulators contributes to antiallodynia in neuropathic rats. Furthermore, we investigated if the in vitro described sites for PKC and PKA phosphorylation (S300 and S333, respectively) participate in the modulation of TREK-1 in naïve and neuropathic rats. L5/L6 spinal nerve ligation (SNL) induced tactile allodynia. Intrathecal injection of BL-1249 (TREK-1 activator) reversed nerve injury-induced tactile allodynia, whereas spadin (TREK-1 blocker) produced tactile allodynia in naïve rats and reversed the antiallodynic effect induced by BL-1249 in neuropathic rats. Intrathecal administration of rottlerin or Rp-cAMPs (PKC and PKA inhibitors, respectively) enhanced the antiallodynia observed with BL-1249 in neuropathic rats. In contrast, pretreatment with PdBu or forskolin (PKC and PKA activators, respectively) reduced the BL-1249-induced antiallodynia. Intrathecal injection of two high-activity TREK-1 recombinant channels, using a in vivo transfection method with lipofectamine, with mutations at PKC/PKA phosphosites (S300A and S333A) reversed tactile allodynia in neuropathic rats, with no effect in naïve rats. In contrast, transfection of two low-activity TREK-1 recombinant channels with phosphomimetic mutations at those sites (S300D and S333D) produced tactile allodynia in naïve rats and interfered with antiallodynic effects of rottlerin/BL-1249 or Rp-cAMPs/BL-1249. Data suggest that TREK-1 channel activity can be dynamically tuned in vivo by PKC/PKA to provoke allodynia and modulate its antiallodynic role in neuropathic pain.

Urinary bladder smooth muscle ion channels: expression, function, and regulation in health and disease

Urinary bladder smooth muscle (UBSM), also known as detrusor smooth muscle, forms the bladder wall and ultimately determines the two main attributes of the organ: urine storage and voiding. The two functions are facilitated by UBSM relaxation and contraction, respectively, which depend on UBSM excitability shaped by multiple ion channels. In this review, we summarize the current understanding of key ion channels establishing and regulating UBSM excitability and contractility. They include excitation-enhancing voltage-gated Ca2+ (Cav) and transient receptor potential channels, excitation-reducing K+ channels, and still poorly understood Cl- channels. Dynamic interplay among UBSM ion channels determines the overall level of Cav channel activity. The net Ca2+ influx via Cav channels increases global intracellular Ca2+ concentration, which subsequently triggers UBSM contractility. Here, for each ion channel type, we describe UBSM tissue/cell expression (mRNA and protein) profiles and their role in regulating excitability and contractility of UBSM in various animal species, including the mouse, rat, and guinea pig, and, most importantly, humans. The currently available data reveal certain interspecies differences, which complicate the translational value of published animal research results to humans. This review highlights recent developments, findings on genetic knockout models, pharmacological data, reports on UBSM ion channel dysfunction in animal bladder disease models, and the very limited human studies currently available. Among all gaps in present-day knowledge, the unknowns on expression and functional roles for ion channels determined directly in human UBSM tissues and cells under both normal and disease conditions remain key hurdles in the field.

Two-Pore Domain Potassium Channels as Drug Targets: Anesthesia and Beyond

Two-pore domain potassium (K2P) channels stabilize the resting membrane potential of both excitable and nonexcitable cells and, as such, are important regulators of cell activity. There are many conditions where pharmacological regulation of K2P channel activity would be of therapeutic benefit, including, but not limited to, atrial fibrillation, respiratory depression, pulmonary hypertension, neuropathic pain, migraine, depression, and some forms of cancer. Up until now, few if any selective pharmacological regulators of K2P channels have been available. However, recent publications of solved structures with small-molecule activators and inhibitors bound to TREK-1, TREK-2, and TASK-1 K2P channels have given insight into the pharmacophore requirements for compound binding to these sites. Together with the increasing availability of a number of novel, active, small-molecule compounds from K2P channel screening programs, these advances have opened up the possibility of rational activator and inhibitor design to selectively target K2P channels.

PIP2 Mediated Inhibition of TREK Potassium Currents by Bradykinin in Mouse Sympathetic Neurons

Bradykinin (BK), a hormone inducing pain and inflammation, is known to inhibit potassium M-currents (I_M) and to increase the excitability of the superior cervical ganglion (SCG) neurons by activating the Ca²⁺-calmodulin pathway. M-current is also reduced by muscarinic agonists through the depletion of membrane phosphatidylinositol 4,5-biphosphate (PIP₂). Similarly, the activation of muscarinic receptors inhibits the current through two-pore domain potassium channels (K2P) of the “Tandem of pore-domains in a Weakly Inward rectifying K⁺ channel (TWIK)-related channels” (TREK) subfamily by reducing PIP₂ in mouse SCG neurons (mSCG). The aim of this work was to test and characterize the modulation of TREK channels by bradykinin. We used the perforated-patch technique to investigate riluzole (RIL) activated currents in voltage- and current-clamp experiments. RIL is a drug used in the palliative treatment of amyotrophic lateral sclerosis and, in addition to blocking voltage-dependent sodium channels, it also selectively activates the K2P channels of the TREK subfamily. A cell-attached patch-clamp was also used to investigate TREK-2 single channel currents. We report here that BK reduces spike frequency adaptation (SFA), inhibits the riluzole-activated current (I_RIL), which flows mainly through TREK-2 channels, by about 45%, and reduces the open probability of identified single TREK-2 channels in cultured mSCG cells. The effect of BK on I_RIL was precluded by the bradykinin receptor (B₂R) antagonist HOE-140 (d-Arg-[Hyp³, Thi⁵, d-Tic⁷, Oic⁸]BK) but also by diC₈PIP₂ which prevents PIP₂ depletion when phospholipase C (PLC) is activated. On the contrary, antagonizing inositol triphosphate receptors (IP₃R) using 2-aminoethoxydiphenylborane (2-APB) or inhibiting protein kinase C (PKC) with bisindolylmaleimide did not affect the inhibition of I_RIL by BK. In conclusion, bradykinin inhibits TREK-2 channels through the activation of B₂Rs resulting in PIP₂ depletion, much like we have demonstrated for muscarinic agonists. This mechanism implies that TREK channels must be relevant for the capture of information about pain and visceral inflammation.

Pharmacologically reversible, loss of function mutations in the TM2 and TM4 inner pore helices of TREK-1 K2P channels

A better understanding of the gating of TREK two pore domain potassium (K2P) channels and their activation by compounds such as the negatively charged activator, flufenamic acid (FFA) is critical in the search for more potent and selective activators of these channels. Currents through wild-type and mutated human K2P channels expressed in tsA201 cells were measured using whole-cell patch-clamp recordings in the presence and absence of FFA. Mutation of the TM2.6 residue of TREK-1 to a phenylalanine (G171F) and a similar mutation of TM4.6 (A286F) substantially reduced current through TREK-1 channels. In complementary experiments, replacing the natural F residues at the equivalent position in TRESK channels, significantly enhanced current. Known, gain of function mutations of TREK-1 (G137I, Y284A) recovered current through these mutated channels. This reduction in current could be also be reversed pharmacologically, by FFA. However, an appropriate length MTS (MethaneThioSulfonate) cross-linking reagent (MTS14) restricted the activation of TREK-1_A286C channels by repeated application of FFA. This suggests that the cross-linker stabilises the channel in a conformation which blunts FFA activation. Pharmacologically reversible mutations of TREK channels will help to clarify the importance of these channels in pathophysiological conditions such as pain and depression.

Role of TREK-1 in Health and Disease, Focus on the Central Nervous System

TREK-1 is the most studied background K_2P channel. Its main role is to control cell excitability and maintain the membrane potential below the threshold of depolarization. TREK-1 is multi-regulated by a variety of physical and chemical stimuli which makes it a very promising and challenging target in the treatment of several pathologies. It is mainly expressed in the brain but also in heart, smooth muscle cells, endocrine pancreas, and prostate. In the nervous system, TREK-1 is involved in many physiological and pathological processes such as depression, neuroprotection, pain, and anesthesia. These properties explain why many laboratories and pharmaceutical companies have been focusing their research on screening and developing highly efficient modulators of TREK-1 channels. In this review, we summarize the different roles of TREK-1 that have been investigated so far in attempt to characterize pharmacological tools and new molecules to modulate cellular functions controlled by TREK-1.

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.

Protein and Chemical Determinants of BL-1249 Action and Selectivity for K2P Channels

K_2P potassium channels generate leak currents that stabilize the resting membrane potential of excitable cells. Various K_2P channels are implicated in pain, ischemia, depression, migraine, and anesthetic responses, making this family an attractive target for small molecule modulator development efforts. BL-1249, a compound from the fenamate class of nonsteroidal anti-inflammatory drugs is known to activate K_2P2.1(TREK-1), the founding member of the thermo- and mechanosensitive TREK subfamily; however, its mechanism of action and effects on other K_2P channels are not well-defined. Here, we demonstrate that BL-1249 extracellular application activates all TREK subfamily members but has no effect on other K_2P subfamilies. Patch clamp experiments demonstrate that, similar to the diverse range of other chemical and physical TREK subfamily gating cues, BL-1249 stimulates the selectivity filter "C-type" gate that controls K_2P function. BL-1249 displays selectivity among the TREK subfamily, activating K_2P2.1(TREK-1) and K_2P10.1(TREK-2) ∼10-fold more potently than K_2P4.1(TRAAK). Investigation of mutants and K_2P2.1(TREK-1)/K_2P4.1(TRAAK) chimeras highlight the key roles of the C-terminal tail in BL-1249 action and identify the M2/M3 transmembrane helix interface as a key site of BL-1249 selectivity. Synthesis and characterization of a set of BL-1249 analogs demonstrates that both the tetrazole and opposing tetralin moieties are critical for function, whereas the conformational mobility between the two ring systems impacts selectivity. Together, our findings underscore the landscape of modes by which small molecules can affect K_2P channels and provide crucial information for the development of better and more selective K_2P modulators of the TREK subfamily.

Activation of TREK currents by riluzole in three subgroups of cultured mouse nodose ganglion neurons

Two-pore domain potassium channels (K2P) constitute major candidates for the regulation of background potassium currents in mammalian cells. Channels of the TREK subfamily are also well positioned to play an important role in sensory transduction due to their sensitivity to a large number of physiological and physical stimuli (pH, mechanical, temperature). Following our previous report describing the molecular expression of different K2P channels in the vagal sensory system, here we confirm that TREK channels are functionally expressed in neurons from the mouse nodose ganglion (mNG). Neurons were subdivided into three groups (A, Ah and C) based on their response to tetrodotoxin and capsaicin. Application of the TREK subfamily activator riluzole to isolated mNG neurons evoked a concentration-dependent outward current in the majority of cells from all the three subtypes studied. Riluzole increased membrane conductance and hyperpolarized the membrane potential by approximately 10 mV when applied to resting neurons. The resting potential was similar in all three groups, but C cells were clearly less excitable and showed smaller hyperpolarization-activated currents at -100 mV and smaller sustained currents at -30 mV. Our results indicate that the TREK subfamily of K2P channels might play an important role in the maintenance of the resting membrane potential in sensory neurons of the autonomic nervous system, suggesting its participation in the modulation of vagal reflexes.

GI-530159, a novel, selective, mechanosensitive two-pore-domain potassium (K2P ) channel opener, reduces rat dorsal root ganglion neuron excitability

BACKGROUND AND PURPOSE

TREK two-pore-domain potassium (K2P ) channels play a critical role in regulating the excitability of somatosensory nociceptive neurons and are important mediators of pain perception. An understanding of the roles of TREK channels in pain perception and, indeed, in other pathophysiological conditions, has been severely hampered by the lack of potent and/or selective activators and inhibitors. In this study, we describe a new, selective opener of TREK channels, GI-530159.

EXPERIMENTAL APPROACH

The effect of GI-530159 on TREK channels was demonstrated using 86 Rb efflux assays, whole-cell and single-channel patch-clamp recordings from recombinant TREK channels. The expression of K2P 2.1 (TREK1), K2P 10.1 (TREK2) and K2P 4.1 (TRAAK) channels was determined using transcriptome analysis from single dorsal root ganglion (DRG) cells. Current-clamp recordings from cultured rat DRG neurons were used to measure the effect of GI-530159 on neuronal excitability.

KEY RESULTS

For recombinant human TREK1 channels, GI-530159 had similar low EC50 values in Rb efflux experiments and electrophysiological recordings. It activated TREK2 channels, but it had no detectable action on TRAAK channels nor any significant effect on other K channels tested. Current-clamp recordings from cultured rat DRG neurones showed that application of GI-530159 at 1 μM resulted in a significant reduction in firing frequency and a small hyperpolarization of resting membrane potential.

CONCLUSIONS AND IMPLICATIONS

This study provides pharmacological evidence for the presence of mechanosensitive TREK K2P channels in sensory neurones and suggests that development of selective K2P channel openers like GI-530159 could aid in the development of novel analgesic agents.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

TREK-1 Channel Expression in Smooth Muscle as a Target for Regulating Murine Intestinal Contractility: Therapeutic Implications for Motility Disorders

Gastrointestinal (GI) motility disorders such as irritable bowel syndrome (IBS) can occur when coordinated smooth muscle contractility is disrupted. Potassium (K⁺) channels regulate GI smooth muscle tone and are key to GI tract relaxation, but their molecular and functional phenotypes are poorly described. Here we define the expression and functional roles of mechano-gated K_2P channels in mouse ileum and colon. Expression and distribution of the K_2P channel family were investigated using quantitative RT-PCR (qPCR), immunohistochemistry and confocal microscopy. The contribution of mechano-gated K_2P channels to mouse intestinal muscle tension was studied pharmacologically using organ bath. Multiple K_2P gene transcripts were detected in mouse ileum and colon whole tissue preparations. Immunohistochemistry confirmed TREK-1 expression was smooth muscle specific in both ileum and colon, whereas TREK-2 and TRAAK channels were detected in enteric neurons but not smooth muscle. In organ bath, mechano-gated K_2P channel activators (Riluzole, BL-1249, flufenamic acid, and cinnamyl 1-3,4-dihydroxy-alpha-cyanocinnamate) induced relaxation of KCl and CCh pre-contracted ileum and colon tissues and reduced the amplitude of spontaneous contractions. These data reveal the specific expression of mechano-gated K_2P channels in mouse ileum and colon tissues and highlight TREK-1, a smooth muscle specific K_2P channel in GI tract, as a potential therapeutic target for combating motility pathologies arising from hyper-contractility.

K2P2.1 (TREK-1)-activator complexes reveal a cryptic selectivity filter binding site

Polymodal K_2P (KCNK) thermo- and mechanosensitive TREK¹ potassium channels, generate ‘leak’ currents that regulate neuronal excitability, respond to lipids, temperature, and mechanical stretch, and influence pain, temperature perception, and anesthetic responses^1–3. These dimeric voltage-gated ion channel (VGIC) superfamily members have a unique topology comprising two pore forming regions per subunit^4–6. Contrasting other potassium channels, K_2Ps use a selectivity filter ‘C-type’ gate^7–10 as the principal gating site. Despite recent advances^3,11,12, K_2Ps suffer from a poor pharmacologic profile limiting mechanistic and biological studies. Here, we describe a new small molecule TREK activator class that directly stimulates the C-type gate by acting as molecular wedges that restrict interdomain interface movement behind the selectivity filter. Structures of K_2P2.1(TREK-1) alone with two selective K_2P2.1(TREK-1) and K_2P10.1(TREK-2) activators, an N-aryl-sulfonamide, ML335, and a thiophene-carboxamide, ML402, define a cryptic binding pocket unlike other ion channel small molecule binding sites and, together with functional studies, identify a cation-π interaction that controls selectivity. Together, our data unveil a previously unknown, druggable K_2P site that stabilizes the C-type gate ‘leak mode’ and provide direct evidence for K_2P selectivity filter gating.

Swelling-activated and arachidonic acid-induced currents are TREK-1 in rat bladder smooth muscle cells

Using the perforated patch voltage clamp, we investigated swelling-activated ionic channels (SACs) in rat urinary bladder smooth muscle cells.　Hypo-osmotic (60%) bath solution increased a membrane current which was inhibited by the SAC inhibitor, gadolinium.　The reversal potential of the hypotonicity-induced current shifted in the positive direction by increasing external K(+) concentration.　The hypotonicity-induced current was inhibited by extracellular acidic pH, phorbol ester and forskolin.　These pharmacological properties are identical to those of arachidonic acid-induced current present in these cells, suggesting the presence of TREK-1, a four-transmembrane two pore domain K(+) channel.　Using RT-PCR we screened rat bladder smooth muscles and cerebellum for expression of TREK-1, TREK-2 and TRAAK mRNAs.　Only TREK-1 mRNA was expressed in the bladder, while all three were expressed in the cerebellum.　We conclude that a mechanosensitive K(+) channel is present in rat bladder myocytes, which is activated by arachidonic acid and most likely is TREK-1.　This K(+) channel may have an important role in the regulation of bladder smooth muscle tone during urine storage.

Pioneering drugs for overactive bladder and detrusor overactivity: Ongoing research and future directions

The ongoing research on pioneering drug candidates for the overactive bladder (OAB) aimed to overcome the limitations of currently licensed pharmacotherapies, such as antimuscarinics, β3-adrenergic agents, and botulinum neurotoxin, has been reviewed performing a systematic literature review and web search. The review covers the exploratory agents alternative to available medications for OAB and that may ultimately prove to be therapeutically useful in the future management of OAB patients based on preclinical and early clinical data. It emerges that many alternative pharmacological strategies have been discovered or are under investigation in disease-oriented studies. Several potential therapeutics are known for years but still find obstacles to pass the clinical stages of development, while other completely novel compounds, targeting new pharmacological targets, have been recently discovered and show potential to translate into clinical therapeutic agents for idiopathic and neurogenic OAB syndrome. The global scenario of investigational drugs for OAB gives promise for the development of innovative therapeutics that may ultimately prove effective as first, combined or second-line treatments within a realistic timescale of ten years.

Innovative pharmacotherapies for women with overactive bladder: where are we now and what is in the pipeline?

INTRODUCTION AND HYPOTHESIS

The impressive prevalence of overactive bladder (OAB) and the relevant limitations of current treatments urge the need for novel therapeutic approaches.

METHODS

A systematic literature and web search was performed to identify investigational drugs that entered the early and late phases of clinical development for women with OAB symptoms.

RESULTS

Approved pharmacological therapies for OAB (antimuscarinics, beta-3 agonists, and botulinum toxin) are evolving with the development of alternative administration methods, combination strategies, and novel compounds, expected to improve effectiveness, bladder selectivity, and dose flexibility. A wealth of investigational compounds, developed with both public and companies' indoor nonclinical disease-oriented studies, entered the early and late stages of clinical development in the last decade. Most non-anticholinergic compounds in ongoing clinical trials target central and peripheral neurotransmitter receptors involved in neurological modulation of micturition, nonadrenergic-noncholinergic mechanisms, cyclic nucleotide metabolism, different subtypes of ion channels or peripheral receptors of prostaglandins, vanilloids, vitamin D3, and opioids. Fascinating advances are ongoing also in the field of genetic therapy.

CONCLUSIONS

New pharmaceutical formulations and drug combinations are expected to be available in the next decade in order to overcome the limitations of current drugs for OAB. Although proof-of-concept, patient-oriented studies yielded disappointing results for several tentative drugs, a lot of clinical research is ongoing that is expected to provide clinicians with novel therapeutic agents in the near future.

Response of the human detrusor to stretch is regulated by TREK-1, a two-pore-domain (K2P) mechano-gated potassium channel

The mechanisms of mechanosensitivity underlying the response of the human bladder to stretch are poorly understood. Animal data suggest that stretch-activated two-pore-domain (K2P) K(+) channels play a critical role in bladder relaxation during the filling phase. The objective of this study was to characterize the expression and function of stretch-activated K2P channels in the human bladder and to clarify their physiological role in bladder mechanosensitivity. Gene and protein analysis of the K2P channels TREK-1, TREK-2 and TRAAK in the human bladder revealed that TREK-1 is the predominantly expressed member of the mechano-gated subfamily of K2P channels. Immunohistochemical labelling of bladder wall identified higher levels of expression of TREK-1 in detrusor smooth muscle cells in comparison to bladder mucosa. Functional characterization and biophysical properties of the predominantly expressed member of the K2P family, the TREK-1 channel, were evaluated by in vitro organ bath studies and the patch-clamp technique. Electrophysiological recordings from single smooth muscle cells confirmed direct activation of TREK-1 channels by mechanical stretch and negative pressure applied to the cell membrane. Inhibition of TREK-1 channels in the human detrusor significantly delayed relaxation of the stretched bladder smooth muscle strips and triggered small-amplitude spontaneous contractions. Application of negative pressure to cell-attached patches (-20 mmHg) caused a 19-fold increase in the open probability (NPo) of human TREK-1 channels. l-Methionine (1 mm), a specific TREK-1 inhibitor, dramatically decreased the NPo of TREK-1 channels from 0.045 ± 0.003 to 0.008 ± 0.001 (n = 8, P ≤ 0.01). Subsequent addition of arachidonic acid (10 μm), a channel opener, increased the open probability of methionine-inhibited unitary currents up to 0.43 ± 0.05 at 0 mV (n = 9, P ≤ 0.05). The results of our study provide direct evidence that the response of the human detrusor to mechanical stretch is regulated by activation of mechano-gated TREK-1 channels. Impaired mechanosensation and mechanotransduction associated with the changes in stretch-activated K2P channels may underlie myogenic bladder dysfunction in humans.

A high-throughput functional screen identifies small molecule regulators of temperature- and mechano-sensitive K2P channels

K_2P (KCNK) potassium channels generate “leak” potassium currents that strongly influence cellular excitability and contribute to pain, somatosensation, anesthesia, and mood. Despite their physiological importance, K_2Ps lack specific pharmacology. Addressing this issue has been complicated by the challenges that the leak nature of K_2P currents poses for electrophysiology-based high-throughput screening strategies. Here, we present a yeast-based high-throughput screening assay that avoids this problem. Using a simple growth-based functional readout, we screened a library of 106,281 small molecules and identified two new inhibitors and three new activators of the mammalian K_2P channel K_2P2.1 (KCNK2, TREK-1). By combining biophysical, structure–activity, and mechanistic analysis, we developed a dihydroacridine analogue, ML67-33, that acts as a low micromolar, selective activator of temperature- and mechano-sensitive K_2P channels. Biophysical studies show that ML67-33 reversibly increases channel currents by activating the extracellular selectivity filter-based C-type gate that forms the core gating apparatus on which a variety of diverse modulatory inputs converge. The new K_2P modulators presented here, together with the yeast-based assay, should enable both mechanistic and physiological studies of K_2P activity and facilitate the discovery and development of other K_2P small molecule modulators.

Role of potassium ion channels in detrusor smooth muscle function and dysfunction

Contraction and relaxation of the detrusor smooth muscle (DSM), which makes up the wall of the urinary bladder, facilitates the storage and voiding of urine. Several families of K(+) channels, including voltage-gated K(+) (K(V)) channels, Ca(2+)-activated K(+) (K(Ca)) channels, inward-rectifying ATP-sensitive K(+) (K(ir), K(ATP)) channels, and two-pore-domain K(+) (K(2P)) channels, are expressed and functional in DSM. They control DSM excitability and contractility by maintaining the resting membrane potential and shaping the action potentials that determine the phasic nature of contractility in this tissue. Defects in DSM K(+) channel proteins or in the molecules involved in their regulatory pathways may underlie certain forms of bladder dysfunction, such as overactive bladder. K(+) channels represent an opportunity for novel pharmacological manipulation and therapeutic intervention in human DSM. Modulation of DSM K(+) channels directly or indirectly by targeting their regulatory mechanisms has the potential to control urinary bladder function. This Review summarizes our current state of knowledge of the functional role of K(+) channels in DSM in health and disease, with special emphasis on current advancements in the field.

Emerging Families of Ion Channels Involved in Urinary Bladder Nociception

The expression of multiple ion channels and receptors is essential for nociceptors to detect noxious stimuli of a thermal, mechanical or chemical nature. The peripheral sensory transduction systems of the urinary bladder include sensory nerve endings, urothelial cells and others whose location is suitable for transducing mechanical and chemical stimuli. There is an increasing body of evidence implicating the Deg/ENaC and TRP channel families in the control of bladder afferent excitability under physiological and pathological conditions. Pharmacological interventions targeting these ion channels may provide a new strategy for the treatment of pathological bladder sensation and pain.

Molecular background of leak K+ currents: two-pore domain potassium channels

Two-pore domain K(+) (K(2P)) channels give rise to leak (also called background) K(+) currents. The well-known role of background K(+) currents is to stabilize the negative resting membrane potential and counterbalance depolarization. However, it has become apparent in the past decade (during the detailed examination of the cloned and corresponding native K(2P) channel types) that this primary hyperpolarizing action is not performed passively. The K(2P) channels are regulated by a wide variety of voltage-independent factors. Basic physicochemical parameters (e.g., pH, temperature, membrane stretch) and also several intracellular signaling pathways substantially and specifically modulate the different members of the six K(2P) channel subfamilies (TWIK, TREK, TASK, TALK, THIK, and TRESK). The deep implication in diverse physiological processes, the circumscribed expression pattern of the different channels, and the interesting pharmacological profile brought the K(2P) channel family into the spotlight. In this review, we focus on the physiological roles of K(2P) channels in the most extensively investigated cell types, with special emphasis on the molecular mechanisms of channel regulation.

Inhibition of intestinal motility by the putative BK(Ca) channel opener LDD175

LDD175 (4-chloro-7-trifluoromethyl-10H-benzo[4,5]furo[3,2-b]indole-1-carboxylic acid) is a benzofuroindole compound characterized previously as a potent opener of the large conductance calcium activated (BK(Ca)) channels. Activators of the BK(Ca) channels are potential therapies for smooth muscle hyperactivity disorders. The present study investigates the influence of LDD175 on the mechanical activity of the ileum smooth muscle. LDD175 inhibited spontaneous contractions of the ileum in a concentration-dependent manner (pEC(50)=5.9 +/- 0.1) (E (max)=96 +/- 1.0% at 100 muM, n=3). It also remarkably inhibited contractions due to acetylcholine (ACh) (pEC(50)=5.3 +/- 0.1)(E (max)=97.7 +/- 2.3%, n=6) and electrical field stimulation (EFS) (pEC(50)=5.5 +/- 0.1) (E (max)=83.3 +/- 6.0%, n=6). In strips precontracted by 20 mM KCl, LDD175 significantly reduced the contractions yielding a pEC(50) of 6.1 +/- 0.1 and E (max) of 96.6 +/- 0.9%, (n=6). In 60 mM KCl, a concentration-dependent inhibition was observed with respective pEC(50) and E (max) values of 4.1 +/- 0.1 and 50.8 +/- 5.0% (n=3). BK(Ca) channel blockers iberiotoxin (IbTX) and tetraethylammonium chloride (TEA, 1 mM) attenuated the relaxative effect of LDD175 but not barium chloride (BaCl(2)), and glibenclamide (K(IR) and K(ATP) channel blockers, respectively). These data demonstrate the antispasmodic activity of LDD175 attributable to the potentiation of the BK(Ca) channels.

Roles of mechanosensitive ion channels in bladder sensory transduction and overactive bladder

In the storage phase, mechanical stretch stimulates bladder afferents. These signals generate sensations and trigger voiding responses, however the precise mechanisms by which mechanical stimuli excite bladder afferents are yet to be explored. For mechanosensory transduction, the presence of mechanosensors is essential in the peripheral sensory systems including sensory nerve endings, urothelium and others. There is increasing evidence that mechanosensitive ion channels, such as degenerin/epithelial Na(+) channel (ENaC) and transient receptor potential (TRP) channel families, play key roles in the mechanosensory transduction of the urinary bladder. Pharmacological interventions targeting mechanosensitive ion channels may provide a new strategy for the treatment of bladder dysfunction.

Potentiation of potassium currents by beta-adrenoceptor agonists in human urinary bladder smooth muscle cells: a possible electrical mechanism of relaxation

We examined the effects of beta-adrenoceptor agonists on the membrane currents of smooth muscle cells from the human urinary bladder using a whole-cell patch clamp to investigate the involvement of Ca(2+)-activated K(+) (K(Ca)) channels in relaxation by beta-adrenergic agonists. With 0.05 mmol/l EGTA in the patch pipette, depolarizing pulses evoked outward rectifying currents. Isoproterenol (1 micromol/l) significantly increased the membrane currents by 75% at +80 mV with 0.05 mmol/l EGTA pipette solution. BRL 37344 (1 micromol/l) significantly increased the membrane currents by 44% at +80 mV. Iberiotoxin (100 nmol/l) significantly decreased the membrane currents by 60% at +80 mV. In the presence of iberiotoxin, the potentiation of the outward currents by isoproterenol was greatly suppressed and, in the presence of iberiotoxin and apamin (1 micromol/l), the potentiation by isoproterenol was totally abolished. On the other hand, with 5 mmol/l EGTA pipette solution, depolarizing pulses evoked smaller outward currents. Isoproterenol (1 micromol/l) did not change the membrane currents with 5 mmol/l EGTA pipette solution. The real-time PCR analysis revealed the expression of beta(2)-adrenoceptors in the cells. These results suggest that Ca(2+)-activated and iberiotoxin- and apamin-sensitive currents via both large-conductance and small-conductance K(Ca) channels could be increased by stimulation of beta(2)-adrenoceptors.

Developments in the pharmacotherapy of the overactive bladder

PURPOSE OF REVIEW

The overactive bladder is a common and distressing condition that has a significant impact on the quality of life of many people worldwide. Anticholinergics remain the first line in pharmacotherapy, however the use of these agents is hindered by adverse effects and limited efficacy. Thus there is a need for more effective treatments. Recently, there has been a move towards targeting novel pathways thought to play a role in overactivity. This review aims to provide an insight into the recent developments in pharmacotherapy of the overactive bladder.

RECENT FINDINGS

With recent advances in our understanding of the basic science of the overactive bladder it is becoming clear that the control of bladder functioning is far more complex than previously believed. Peripherally, a prominent role has emerged for the urothelium and the underlying suburothelium in mechanosensory control, and the role of afferent pathways in pathophysiology is increasingly recognized.

SUMMARY

Recent research has highlighted several potential targets for treatment of the overactive bladder, particularly within the mechanosensory pathways. With the exception of botulinum toxin, however, few new therapies have emerged showing clinical benefits. A clearer understanding of the pathophysiology of the bladder will hopefully lead to more effective and tolerated treatments.

Investigation of the role of TASK-2 channels in rat pulmonary arteries; pharmacological and functional studies following RNA interference procedures

In the present study, we investigated the ability of RNA interference technology to suppress TASK-2 potassium channel expression in human embryonic kidney (HEK293) cells stably transfected with TASK-2 cDNA and in rat isolated intact pulmonary arteries. Lipofectamine-induced transfection of a specific siRNA sequence targeted against TASK-2 resulted in a dose- and time-dependent decrease in TASK-2 channel protein expression. In siRNA-transfected cells the TASK-2 peak currents were significantly smaller than in control cells at every investigated pH, while the pH sensitivity was not altered. Using scrambled siRNA as a negative control, there were no significant changes in TASK-2 protein expression or current compared to mock-transfected cells. In TASK-2 siRNA-transfected small pulmonary arteries, but not in scrambled siRNA-treated vessels, myocyte resting membrane potential at pH 7.4 was significantly less negative and the hyperpolarisations in response to increasing pH from 6.4 to 8.4 were significantly smaller compared with control. The application of levcromakalim (10 microM), NS1619 (33 microM) and a potassium channel inhibitor cocktail (5 mM 4-aminopyridine, 10 mM tetraethylammonium chloride, 30 microM Ba2+ and 10 microM glibenclamide) had similar effects in control and in siRNA-transfected vessels. The TASK-1 (anandamide-sensitive) contribution to resting membrane potential was comparable in each group. Clofilium (100 microM) generated significantly smaller responses in transfected artery segments. These results suggest that RNA interference techniques are effective at inhibiting TASK-2 channel expression in cultured cells and in intact vessels and that TASK-2 channels have a functional role in setting the membrane potential of pulmonary artery myocytes.