A mutation in the cardiac KV7.1 channel possibly disrupts interaction with Yotiao protein

Here, we characterized the p.Arg583His (R583H) Kv7.1 mutation, identified in two unrelated families suffered from LQT syndrome. This mutation is located in the HС-HD linker of the cytoplasmic portion of the Kv7.1 channel. This linker, together with HD helix are responsible for binding the A-kinase anchoring protein 9 (AKAP9), Yotiao. We studied the electrophysiological characteristics of the mutated channel expressed in CHO-K1 along with KCNE1 subunit and Yotiao protein, using the whole-cell patch-clamp technique. We found that R583H mutation, even at the heterozygous state, impedes IKs activation. Molecular modeling showed that HС and HD helixes of the C-terminal part of Kv7.1 channel are swapped along the C-terminus length of the channel and that R583 position is exposed to the outer surface of HC-HD tandem coiled-coil. Interestingly, the adenylate cyclase activator, forskolin had a smaller effect on the mutant channel comparing with the WT protein, suggesting that R583H mutation may disrupt the interaction of the channel with the adaptor protein Yotiao and, therefore, may impair phosphorylation of the KCNQ1 channel.

Progression of axonal excitability abnormalities with increasing clinical severity of diabetic peripheral neuropathy

OBJECTIVE

Diabetic peripheral neuropathy (DPN) is a frequent complication for persons with type 2 diabetes. Previous studies have failed to demonstrate any significant impact of treatment for DPN. The present study assessed the role of axonal ion channel dysfunction in DPN and explored the hypothesis that there may be a progressive change in ion channel abnormalities that varied with disease stage.

METHODS

Neurophysiological studies were conducted using axonal excitability techniques, a clinical method of assessing ion channel dysfunction. Studies were conducted in 178 persons with type 2 diabetes, with participants allocated into four groups according to clinical severity of neuropathy, assessed using the Total Neuropathy Grade.

RESULTS

Analysis of excitability data demonstrated a progressive and stepwise reduction in two parameters that are related to the activity of Kv1.1 channels, namely superexcitability and depolarizing threshold electrotonus at 10-20 ms (p < 0.001), and mathematical modelling of axonal excitability findings supported progressive upregulation of Kv1.1 conductances with increasing greater disease severity.

CONCLUSION

The findings are consistent with a progressive upregulation of juxtaparanodal Kv1.1 conductances with increasing clinical severity of diabetic peripheral neuropathy.

SIGNIFICANCE

From a translational perspective, the study suggests that blockade of Kv1.1 channels using 4-aminopyridine derivatives such as fampridine may be a potential treatment for DPN.

Cryo-EM structure of the Slo1 potassium channel with the auxiliary γ1 subunit suggests a mechanism for depolarization-independent activation

Mammalian Ca2+-dependent Slo K+ channels can stably associate with auxiliary γ subunits which fundamentally alter their behavior. By a so far unknown mechanism, the four γ subunits reduce the need for voltage-dependent activation and, thereby, allow Slo to open independently of an action potential. Here, using cryo-EM, we reveal how the transmembrane helix of γ1/LRRC26 binds and presumably stabilizes the activated voltage-sensor domain of Slo1. The activation is further enhanced by an intracellular polybasic stretch which locally changes the charge gradient across the membrane. Our data provide a possible explanation for Slo1 regulation by the four γ subunits and also their different activation efficiencies. This suggests a novel activation mechanism of voltage-gated ion channels by auxiliary subunits.

Potential Benefit of Channel Activators in Loss-of-Function Primary Potassium Channelopathies Causing Heredoataxia

Potassium channels (KCN) are transmembrane complexes that regulate the resting membrane potential and the duration of action potentials in cells. The opening of KCN brings about an efflux of K+ ions that induces cell repolarization after depolarization, returns the transmembrane potential to its resting state, and enables for continuous spiking ability. The aim of this work was to assess the role of KCN dysfunction in the pathogenesis of hereditary ataxias and the mechanisms of action of KCN opening agents (KCO). In consequence, a review of the ad hoc medical literature was performed. Among hereditary KCN diseases causing ataxia, mutated Kv3.3, Kv4.3, and Kv1.1 channels provoke spinocerebellar ataxia (SCA) type 13, SCA19/22, and episodic ataxia type 1 (EA1), respectively. The K+ efflux was found to be reduced in experimental models of these diseases, resulting in abnormally prolonged depolarization and incomplete repolarization, thereby interfering with repetitive discharges in the cells. Hence, substances able to promote normal spiking activity in the cerebellum could provide symptomatic benefit. Although drugs used in clinical practice do not activate Kv3.3 or Kv4.3 directly, available KCO probably could ameliorate ataxic symptoms in SCA13 and SCA19/22, as verified with acetazolamide in EA1, and retigabine in a mouse model of hypokalemic periodic paralysis. To summarize, ataxia could possibly be improved by non-specific KCO in SCA13 and SCA19/22. The identification of new specific KCO agents will undoubtedly constitute a promising therapeutic strategy for these diseases.

Interaction of ROMK2 channel with lipid kinases DGKE and AGK: Potential channel activation by localized anionic lipid synthesis

In this study, we utilized enzyme-catalyzed proximity labeling with the engineered promiscuous biotin ligase Turbo-ID to identify the proxisome of the ROMK2 channel. This channel resides in various cellular membrane compartments of the cell including the plasma membrane, endoplasmic reticulum and mitochondria. Within mitochondria, ROMK2 has been suggested as a pore-forming subunit of mitochondrial ATP-regulated potassium channel (mitoKATP). We found that ROMK2 proxisome in addition to previously known protein partners included two lipid kinases: acylglycerol kinase (AGK) and diacylglycerol kinase ε (DGKE), which are localized in mitochondria and the endoplasmic reticulum, respectively. Through co-immunoprecipitation, we confirmed that these two kinases are present in complexes with ROMK2 channels. Additionally, we found that the products of AGK and DGKE, lysophosphatidic acid (LPA) and phosphatidic acid (PA), stimulated the activity of ROMK2 channels in artificial lipid bilayers. Our molecular docking studies revealed the presence of acidic lipid binding sites in the ROMK2 channel, similar to those previously identified in Kir2 channels. Based on these findings, we propose a model wherein localized lipid synthesis, mediated by channel-bound lipid kinases, contributes to the regulation of ROMK2 activity within distinct intracellular compartments, such as mitochondria and the endoplasmic reticulum.

All-potassium channel CRISPR screening reveals a lysine-specific pathway of insulin secretion

OBJECTIVE

Genome-scale CRISPR-Cas9 knockout coupled with single-cell RNA sequencing (scRNA-seq) has been used to identify function-related genes. However, this method may knock out too many genes, leading to low efficiency in finding genes of interest. Insulin secretion is controlled by several electrophysiological events, including fluxes of KATP depolarization and K+ repolarization. It is well known that glucose stimulates insulin secretion from pancreatic β-cells, mainly via the KATP depolarization channel, but whether other nutrients directly regulate the repolarization K+ channel to promote insulin secretion is unknown.

METHODS

We used a system involving CRISPR-Cas9-mediated knockout of all 83 K+ channels and scRNA-seq in a pancreatic β cell line to identify genes associated with insulin secretion.

RESULTS

The expression levels of insulin genes were significantly increased after all-K+ channel knockout. Furthermore, Kcnb1 and Kcnh6 were the two most important repolarization K+ channels for the increase in high-glucose-dependent insulin secretion that occurred upon application of specific inhibitors of the channels. Kcnh6 currents, but not Kcnb1 currents, were reduced by one of the amino acids, lysine, in both transfected cells, primary cells and mice with β-cell-specific deletion of Kcnh6.

CONCLUSIONS

Our function-related CRISPR screen with scRNA-seq identifies Kcnh6 as a lysine-specific channel.

Maternal Embryo Effect Arrest 31 (MEE31) is a moonlighting protein involved in GDP-D-mannose biosynthesis and KAT1 potassium channel regulation

Due to anthropogenic global warming, droughts are expected to increase and water availability to decrease in the coming decades. For this reason, research is increasingly focused on developing plant varieties and crop cultivars with reduced water consumption. Transpiration occurs through stomatal pores, resulting in water loss. Potassium plays a significant role in stomatal regulation. KAT1 is an inward-rectifying potassium channel that contributes to stomatal opening. Using a yeast high-throughput screening of an Arabidopsis cDNA library, MEE31 was found to physically interact with KAT1. MEE31 was initially identified in a screen for mutants with delayed embryonic development. The gene encodes a conserved phosphomannose isomerase (PMI). We report here that MEE31 interacts with and increases KAT1 activity in yeast and this interaction was also confirmed in plants. In addition, MEE31 complements the function of the yeast homologue, whereas the truncated version recovered in the screening does not, thus uncoupling the enzymatic activity from KAT1 regulation. We show that MEE31 overexpression leads to increased stomatal opening in Arabidopsis transgenic lines. Our data suggest that MEE31 is a moonlighting protein involved in both GDP-D-mannose biosynthesis and KAT1 regulation.

Ca2+-activated K+ channels modulate membrane potential in the human parathyroid cell: Possible role in exocytosis

The relationships between parathyroid hormone (PTH) secretion and parathyroid cell membrane potential, including the identities and roles of K+ channels that regulate and/or modulate membrane potential are not well defined. Here we have used Western blot/immunohistochemistry as well as patch-clamp and perifusion techniques to identify and localize specific K+ channels in parathyroid cells and to investigate their roles in the control of membrane potential and PTH secretion. We also re-investigated the relationship between membrane potential and exocytosis. We showed that in single human parathyroid cells K+ current is dependent on at least two types of Ca2+-activated K+ channels: a small-conductance Ca2+-activated K+ channel (KSK) and a large-conductance voltage and Ca2+-activated K+ channel (KBK). These channels were sensitive to specific peptide blocking toxins including apamin, charybdotoxin, and iberiotoxin. These channels confer sensitivity of the membrane potential in single cells to high extracellular K+, TEA, and peptide toxins. Blocking of KBK potently inhibited K+ channel current, and KBK was shown to be expressed in the plasma membrane of parathyroid cells. In addition, when using the capacitance technique as an indicator of exocytosis, clamping the parathyroid cell at -60 mV prevented exocytosis, whereas holding the membrane potential at 0 mV facilitated it. Taken together, the results show that human parathyroid cells have functional KBK and KSK channels but the data presented herein suggest that KBK/KSK channels likely contribute to the maintenance of the membrane potential, and that membrane potential, per se, modulates exocytosis independently of [Ca2+]i.

Environmental Enrichment Improves the Recognition Memory in Adult Mice Following Social Isolation via Downregulation of Kv4.2 Potassium Channels

Recognition memory is a cognitive process that enables us to distinguish familiar objects and situations from new items, which is essential for mammalian survival and adaptation to a changing environment. Social isolation (SI) has been implicated as a detrimental factor for recognition memory. The medial prefrontal cortex (mPFC) has been shown to carry information concerning the relative familiarity of individual stimuli, and modulating neuronal function in this region may contribute to recognition memory. The present study aimed to investigate the neuronal mechanisms in the mPFC of environmental enrichment (EE) on recognition memory in adult mice following SI. Mice were assigned into three groups: control, SI, and SI + EE groups. Novel location recognition (NLR) and novel object recognition (NOR) tests were performed to evaluate the recognition memory. The levels of Kv4 channels were assessed by qRT-PCR and western blotting. The effects of SI and SI + EE on the excitability of pyramidal neurons in the mPFC were measured using whole-cell recording. We found that SI led to a reduction in the excitability of pyramidal neurons. Specifically, we have identified that the reduction in the firing activity of pyramidal neurons resulted from alterations in the function and expression of Kv4.2 channels. Furthermore, EE regulated Kv4.2 channels, normalized the activity of pyramidal neurons, and restored the behavioral deficits following SI. Thus, the roles of Kv4.2 channels in excitability of pyramidal neurons suggest that the Kv4.2 channels present a promising therapeutic target for recognition memory impairment.

Determination of single-molecule transport activity of OATP2B1 by measuring the number of transporter molecules using electrophysiological approach

Transporter-mediated clearance is determined by two factors, its single-molecule clearance, and expression level. However, no reliable method has been developed to evaluate them separately. This study aimed to develop a reliable method for evaluating the single-molecule activity of membrane transporters, such as organic anion transporting polypeptide (OATP) 2B1. HEK293 cells that co-expressed large conductance calcium-activated potassium (BK) channel and OATP2B1 were established and used for the following experiments. i) BK channel-mediated whole-cell conductance was measured using patch-clamp technique and divided by its unitary conductance to estimate the number of channels on plasma membrane (QI). ii) Using plasma membrane fraction, quantitative targeted absolute proteomics determined the stoichiometric ratio (ρ) of OATP2B1 to BK channel. iii) The uptake of estrone 3-sulfate was evaluated to calculate the Michaelis constant and uptake clearance (CL) per cell. Single-molecule clearance (CLint) was calculated by dividing CL by QI·ρ. QI and ρ values were estimated to be 916 and 2.16, respectively, yielding CLint of 5.23 fL/min/molecule. We successfully developed a novel method to reliably measure the single-molecule activity of a transporter, which could be used to evaluate the influences of factors such as genetic variations and post-translational modifications on the intrinsic activity of transporters.

Short-term Effects of Cadmium Exposure on Blood Pressure and Vascular Function in Wistar Rats

Chronic cadmium exposure is known to be associated with vascular changes and increased blood pressure, but its short-term effects on the cardiovascular system remain poorly understood. This study aimed to investigate the pressoric and vascular effects of a 7-day exposure to CdCl2 in Wistar rats. The rats were divided in control group (Ct), which received tap water, and the Cd group, which received a 100 mg/L CdCl2 solution via drinking water for 7 days. We analyzed body weight, plasma Cadmium concentration, systolic blood pressure (SBP), and vascular responses. Despite relatively low plasma Cadmium concentration, the Cd group exhibited elevated SBP and increased contractile response to phenylephrine. Endothelium removal and NOS inhibition increased contractions in both groups. In the Cd group's aorta, we observed enhanced levels of phospho-eNOS (Ser1177) and basal NO release. Cd group showed reduced Catalase expression and increased basal release of H2O2, with catalase reducing the contractile response. In arteries pre-contracted with phenylephrine, Cd group showed impaired endothelium-dependent (Acetylcholine) and independent (sodium nitroprussiate-SNP) relaxation responses. However, responses to SNP were similar after pre-contraction with KCl in both groups. These data suggest early effects of Cadmium on blood pressure and aortic function, indicating impaired H2O2-scavenging by catalase. Increased H2O2 due to Cadmium exposure might explain heightened responses to phenylephrine and weakened relaxation responses mediated by the NO-K+-channels pathway. Our findings shed light on Cadmium's short-term impact on the cardiovascular system, providing insights into potential mechanisms underlying its effects on blood pressure regulation and vascular function.

Molecular mechanisms and therapeutic potential: A narrative review

Potassium channels (K+-channels) selectively control the passive flow of potassium ions across biological membranes and thereby also regulate membrane excitability. Genetic variants affecting many of the human K+-channels are well known causes of Mendelian disorders within cardiology, neurology, and endocrinology. K+-channels are also primary targets of many natural toxins from poisonous organisms and drugs used within cardiology and metabolism. As genetic tools are improving and larger clinical samples are being investigated, the spectrum of clinical phenotypes implicated in K+-channels dysfunction is rapidly expanding, notably within immunology, neurosciences, and metabolism. K+-channels that previously were considered to be expressed in only a few organs and to have discrete physiological functions, have recently been found in multiple tissues and with new, unexpected functions. The pleiotropic functions and patterns of expression of K+-channels may provide additional therapeutic opportunities, along with new emerging challenges from off-target effects. Here we review the functions and therapeutic potential of K+-channels, with an emphasis on the nervous system, roles in neuropsychiatric disorders and their involvement in other organ systems and diseases.

Insights into Leishmania donovani potassium channel family and their biological functions

Leishmania donovani is the causative organism for visceral leishmaniasis. Although this parasite was discovered over a century ago, nothing is known about role of potassium channels in L. donovani. Potassium channels are known for their crucial roles in cellular functions in other organisms. Recently the presence of a calcium-activated potassium channel in L. donovani was reported which prompted us to look for other proteins which could be potassium channels and to investigate their possible physiological roles. Twenty sequences were identified in L. donovani genome and subjected to estimation of physio-chemical properties, motif analysis, localization prediction and transmembrane domain analysis. Structural predictions were also done. The channels were majorly α-helical and predominantly localized in cell membrane and lysosomes. The signature selectivity filter of potassium channel was present in all the sequences. In addition to the conventional potassium channel activity, they were associated with gene ontology terms for mitotic cell cycle, cell death, modulation by virus of host process, cell motility etc. The entire study indicates the presence of potassium channel families in L. donovani which may have involvement in several cellular pathways. Further investigations on these putative potassium channels are needed to elucidate their roles in Leishmania.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03692-y.

New insights on mode of action of vasorelaxant activity of simvastatin

Simvastatin is a semisynthetic inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and is used extensively to treat atherosclerotic cardiovascular disease. Apart from the lipid-lowering effect, simvastatin has been documented to offer impressive vasorelaxant activity. However, the mechanism associated with this vasorelaxant activity has yet not been substantially explored. Thus, the present study has aimed to elucidate the mechanism(s) associated with simvastatin-induced vasorelaxation using an established rat aortic ring model. The results from the study depicted that simvastatin caused significant relaxation in aortic rings pre-contracted with phenylephrine and potassium chloride (KCl). The vasorelaxant effect of simvastatin was attenuated by methylene blue (sGC-dependent cyclic guanosine monophosphate (cGMP) inhibitor), N^G-nitro-L-arginine methyl ester (L-NAME; NO synthase inhibitor), 4-aminopyridine (K_v blocker), glibenclamide (K_ATP blocker), and barium chloride (K_ir blocker). In addition, the vasorelaxant effect of simvastatin was slightly reduced by PD123319 (angiotensin II type 2 receptor (AT₂R) antagonist). However, indomethacin (COX inhibitor), 1H-[1,2,4]Ox adiazolol [4,3-α]quinoxalin-1-one (ODQ; selective soluble guanylate cyclase (sGC) inhibitor), losartan (angiotensin II type 1 receptor (AT₁R) antagonist), atropine (muscarinic receptor blocker), and tetraethyl ammonium (TEA; K_Ca blocker) did not affect the vasorelaxant effect of simvastatin. Furthermore, simvastatin was found to attenuate the release of calcium (Ca²⁺) from intracellular stores in the presence of ruthenium red (ryanodine receptor, RyR inhibitor) and extracellular stores via nifedipine (voltage-operated Ca²⁺ channels, VOCC blocker) and SK&F96365 (receptor-operated Ca²⁺ channel, ROCC blocker). Thus, it can be concluded that the vasorelaxant effect of simvastatin involves NO/cGMP pathways, AT₂R receptors, Ca²⁺ channels, and K⁺ channels.

Potassium channels, tumorigenesis and targeted drugs

Potassium channels play an important role in human physiological function. Recently, various molecular mechanisms have implicated abnormal functioning of potassium channels in the proliferation, migration, invasion, apoptosis, and cancer stem cell phenotype formation. Potassium channels also mediate the association of tumor cells with the tumor microenvironment. Meanwhile, potassium channels are important targets for cancer chemotherapy. A variety of drugs exert anti-cancer effects by modulating potassium channels in tumor cells. Therefore, there is a need to understand how potassium channels participate in tumor development and progression, which could reveal new, novel targets for cancer diagnosis and treatment. This review summarizes the roles of voltage-gated potassium channels, calcium-activated potassium channels, inwardly rectifying potassium channels, and two-pore domain potassium channels in tumorigenesis and the underlying mechanism of potassium channel-targeted drugs. Therefore, the study lays the foundation for rational and effective drug design and individualized clinical therapeutics.

Bacterial lipopolysaccharide hyperpolarizes the membrane potential and is antagonized by the K2p channel blocker doxapram

Exposure of Drosophila skeletal muscle to bacterial lipopolysaccharides (LPS) rapidly and transiently hyperpolarizes membrane potential. However, the mechanism responsible for hyperpolarization remains unclear. The resting membrane potential of the cells is maintained through multiple mechanisms. This study investigated the possibility of LPS activating calcium-activated potassium channels (K_Ca) and/or K2p channels. 2-Aminoethyl diphenylborinate (2-APB), blocks uptake of Ca²⁺ into the endoplasmic reticulum (ER); thus, limiting release from ryanodine-sensitive internal stores to reduce the function of K_Ca channels. Exposure to 2-APB produces waves of hyperpolarization even during desensitization of the response to LPS and in the presence of doxapram. This finding in this study suggests that doxapram blocked the acid-sensitive K2p tandem-pore channel subtype known in mammals. Doxapram blocked LPS-induced hyperpolarization and depolarized the muscles as well as induced motor neurons to produce evoked excitatory junction potentials (EJPs). This was induced by depolarizing motor neurons, similar to the increase in extracellular K⁺ concentration. The hyperpolarizing effect of LPS was not blocked by decreased extracellular Ca²⁺or the presence of Cd²⁺. LPS appears to transiently activate doxapram sensitive K2p channels independently of K_Ca channels in hyperpolarizing the muscle. Septicemia induced by gram-negative bacteria results in an increase in inflammatory cytokines, primarily induced by bacterial LPS. Currently, blockers of LPS receptors in mammals are unknown; further research on doxapram and other K2p channels is warranted. (220 words).

Targeting Ion Channels For The Treatment Of GLIOMA

BACKGROUND

Glioma refers to the most aggressive tumor in the central nervous system that starts from support cells or glial cells. The glial cell is the most common cell type in the CNS, and they insulate, surround, as well as feed, oxygen, and nutrition to the neurons. Seizures, headaches, irritability, vision difficulties, and weakness are some of the symptoms. Targeting ion channels is particularly helpful when it comes to glioma treatment because of their substantial activity in glioma genesis through multiple pathways.

OBJECTIVE

In this study, we explore how distinct ion channels can be targeted for glioma treatment and summarize the pathogenic ion channels activity in gliomas.

RECENT FINDINGS

Current research found several side effects such as bone marrow suppression, alopecia, insomnia, and cognitive impairments for presently done chemotherapy. The involvement of research on ion channels in the regulation of cellular biology and towards improvements of glioma have expanded recognition of their innovative roles.

CONCLUSION

Present review article has expanded knowledge of ion channels as therapeutic targets and detailed cellular mechanisms in the roles of ion channels in gliomas pathogenesis.

Co-dependent regulation of p-BRAF and potassium channel KCNMA1 levels drives glioma progression

BRAF mutations have been found in gliomas which exhibit abnormal electrophysiological activities, implying their potential links with the ion channel functions. In this study, we identified the Drosophila potassium channel, Slowpoke (Slo), the ortholog of human KCNMA1, as a critical factor involved in dRaf^GOF glioma progression. Slo was upregulated in dRaf^GOF glioma. Knockdown of slo led to decreases in dRaf^GOF levels, glioma cell proliferation, and tumor-related phenotypes. Overexpression of slo in glial cells elevated dRaf expression and promoted cell proliferation. Similar mutual regulations of p-BRAF and KCNMA1 levels were then recapitulated in human glioma cells with the BRAF mutation. Elevated p-BRAF and KCNMA1 were also observed in HEK293T cells upon the treatment of 20 mM KCl, which causes membrane depolarization. Knockdown KCNMA1 in these cells led to a further decrease in cell viability. Based on these results, we conclude that the levels of p-BRAF and KCNMA1 are co-dependent and mutually regulated. We propose that, in depolarized glioma cells with BRAF mutations, high KCNMA1 levels act to repolarize membrane potential and facilitate cell growth. Our study provides a new strategy to antagonize the progression of gliomas as induced by BRAF mutations.

Potassium channelopathies associated with epilepsy-related syndromes and directions for therapeutic intervention

A number of mutations to members of several CNS potassium (K) channel families have been identified which result in rare forms of neonatal onset epilepsy, or syndromes of which one prominent characteristic is a form of epilepsy. Benign Familial Neonatal Convulsions or Seizures (BFNC or BFNS), also referred to as Self-Limited Familial Neonatal Epilepsy (SeLNE), results from mutations in 2 members of the K_V7 family (KCNQ) of K channels; while generally self-resolving by about 15 weeks of age, these mutations significantly increase the probability of generalized seizure disorders in the adult, in some cases they result in more severe developmental syndromes. Epilepsy of Infancy with Migrating Focal Seizures (EIMSF), or Migrating Partial Seizures of Infancy (MMPSI), is a rare severe form of epilepsy linked primarily to gain of function mutations in a member of the sodium-dependent K channel family, KCNT1 or SLACK. Finally, KCNMA1 channelopathies, including Liang-Wang syndrome (LIWAS), are rare combinations of neurological symptoms including seizure, movement abnormalities, delayed development and intellectual disabilities, with Liang-Wang syndrome an extremely serious polymalformative syndrome with a number of neurological sequelae including epilepsy. These are caused by mutations in the pore-forming subunit of the large-conductance calcium-activated K channel (BK channel) KCNMA1. The identification of these rare but significant channelopathies has resulted in a resurgence of interest in their treatment by direct pharmacological or genetic modulation. We will briefly review the genetics, biophysics and pharmacology of these K channels, their linkage with the 3 syndromes described above, and efforts to more effectively target these syndromes.

14-3-3 proteins regulate cullin 7-mediated Eag1 degradation

BACKGROUND

Mutations in the human gene encoding the neuron-specific Eag1 (K_V10.1; KCNH1) potassium channel are linked to congenital neurodevelopmental diseases. Disease-causing mutant Eag1 channels manifest aberrant gating function and defective protein homeostasis. Both the E3 ubiquitin ligase cullin 7 (Cul7) and the small acid protein 14-3-3 serve as binding partners of Eag1. Cul7 mediates proteasomal and lysosomal degradation of Eag1 protein, whereas over-expression of 14-3-3 notably reduces Eag1 channel activity. It remains unclear whether 14-3-3 may also contribute to Eag1 protein homeostasis.

RESULTS

In human cell line and native rat neurons, disruptions of endogenous 14-3-3 function with the peptide inhibitor difopein or specific RNA interference up-regulated Eag1 protein level in a transcription-independent manner. Difopein hindered Eag1 protein ubiquitination at the endoplasmic reticulum and the plasma membrane, effectively promoting the stability of both immature and mature Eag1 proteins. Suppression of endogenous 14-3-3 function also reduced excitotoxicity-associated Eag1 degradation in neurons. Difopein diminished Cul7-mediated Eag1 degradation, and Cul7 knock-down abolished the effect of difopein on Eag1. Inhibition of endogenous 14-3-3 function substantially perturbed the interaction of Eag1 with Cul7. Further structural analyses suggested that the intracellular Per-Arnt-Sim (PAS) domain and cyclic nucleotide-binding homology domain (CNBHD) of Eag1 are essential for the regulatory effect of 14-3-3 proteins. Significantly, suppression of endogenous 14-3-3 function reduced Cul7-mediated degradation of disease-associated Eag1 mutant proteins.

CONCLUSION

Overall these results highlight a chaperone-like role of endogenous 14-3-3 proteins in regulating Eag1 protein homeostasis, as well as a therapeutic potential of 14-3-3 modulators in correcting defective protein expression of disease-causing Eag1 mutants.

Identification of KCNK1 as a potential prognostic biomarker and therapeutic target of breast cancer

BACKGROUND

Breast cancer is the most common malignant cancer and is the second most common cause of cancer-related deaths among females worldwide. Thus, it warrants the urgent development of new therapeutic targets and strategies. Potassium channels are aberrantly expressed in various tumors and are related to tumor progression. However, studies on potassium channels in breast cancer remain limited.

METHOD

First, The Cancer Genome Atlas (TCGA) and Gene Set Enrichment Analysis (GSEA) were used to screen the differentially expressed potassium channels in breast cancer. Several other databases were utilized for further data analysis and visualization, including Gene Expression Profiling Interactive Analysis 2 (GEPIA2), Human Protein Atlas (HPA), GeneMANIA, Tumor Immune Estimation Resource 2 (TIMER2), Catalog of Somatic Mutations in Cancer (COSMIC), cBioPortal, and UCSC Xena tool. Besides, cell proliferation was detected by cell counting kit-8 (CCK8) and 5-Ethynyl-20-deoxyuridine (EdU), and cell migration was detected by wound healing and Transwell assays after knocking down KCNK1. Furthermore, the effect of KCNK1 knockdown on the sensitivity of breast cancer cells to paclitaxel was also evaluated.

RESULT

KCNK1 was overexpressed in breast cancer. Higher KCNK1 expression predicted an unfavorable prognosis. Moreover, the abnormal expression of KCNK1 was attributed to promoter hypomethylation of KCNK1 in breast cancer. Besides, cell proliferation and migration were significantly inhibited post-KCNK1 silencing, while KCNK1 knockdown significantly increased breast cancer cell sensitivity to paclitaxel.

CONCLUSION

Taken together, our findings demonstrated that KCNK1 is a potential prognostic biomarker and therapeutic target of breast cancer. Thus, targeting KCNK1 might help synergize with paclitaxel function in breast cancer treatment.

Lysosomal Potassium Channels

Lysosomes are acidic membrane-bound organelles that use hydrolytic enzymes to break down material through pathways such as endocytosis, phagocytosis, mitophagy, and autophagy. To function properly, intralysosomal environments are strictly controlled by a set of integral membrane proteins such as ion channels and transporters. Potassium ion (K⁺) channels are a large and diverse family of membrane proteins that control K⁺ flux across both the plasma membrane and intracellular membranes. In the plasma membrane, they are essential in both excitable and non-excitable cells for the control of membrane potential and cell signaling. However, our understanding of intracellular K⁺ channels is very limited. In this review, we summarize the recent development in studies of K⁺ channels in the lysosome. We focus on their characterization, potential roles in maintaining lysosomal membrane potential and lysosomal function, and pathological implications.

Intravenous Semaphorin 3A Administration Maintains Cardiac Contractility and Improves Electrical Remodeling in a Mouse Model of Isoproterenol-Induced Heart Failure

The effects of recombinant semaphorin 3A (Sema3A) on myocardial contractility and electrical remodeling in mice with isoproterenol (ISP) -induced heart failure were investigated.C57BL/6J mice intraperitoneally received ISP (480 mg/kg/day, ISP group; n = 24) or saline (control group; n = 31) for 14 days. Twenty-one ISP-treated mice received 0.5 mg/kg Sema3A intravenously on days 7 and 11 (ISP+Sema3A group). The sympathetic nervous system was activated upon ISP treatment, but was reduced upon Sema3A administration. Greater myocardial tissue fibrosis was observed in the ISP group than in the control group. However, fibrosis was not significantly different between the ISP+Sema3A and control groups. Fractional shortening of the left ventricle was lower in the ISP group than in the control group and was restored in the ISP+Sema3A group (control, 53 ± 8%; ISP, 37 ± 7%; ISP+Sema3A, 48 ± 3%; P < 0.05). Monophasic action potential duration at 20% repolarization (MAPD₂₀) was prolonged in the ISP group (compared to control group), but this was reversed upon Sema3A administration (control, 29 ± 3 ms; ISP, 35 ± 6 ms; ISP+Sema3A, 29 ± 3 ms; P < 0.05). qPCR revealed Kv4.3, KChIP2, and SERCA2 downregulation in the ISP group and upregulation in the ISP+Sema3A group; however, Western blotting revealed similar changes only for Kv4.3 (P < 0.05).Intravenous Sema3A may maintain myocardial contractility by suppressing the sympathetic innervation of the myocardium and reducing myocardial tissue damage, in addition to restoring MAPD via Kv4.3 upregulation.

The effects of doxapram (blocker of K2p channels) on resting membrane potential and synaptic transmission at the Drosophila neuromuscular junction

The resting membrane potential of most cells is maintained by potassium K2p channels. The pharmacological profile and distribution of various K2p channel subtypes in organisms are still being investigated. The Drosophila genome contains 11 subtypes; however, their function and expression profiles have not yet been determined. Doxapram is clinically used to enhance respiration in humans and blocks the acid-sensitive K2p TASK subtype in mammals. The resting membrane potential of larval Drosophila muscle and synaptic transmission at the neuromuscular junction are pH sensitive. The present study investigated the effects of doxapram on membrane potential and synaptic transmission using intracellular recordings of larval Drosophila muscles. Doxapram (1 mM and 10 mM) depolarizes the muscle and appears to depolarize motor neurons, causing an increase in the frequency of spontaneous quantal events and evoked excitatory junction potentials. Verapamil (1 and 10 mM) paralleled the action of doxapram. These changes were matched by an extracellular increase in KCl (50 mM) and blocked by Cd²⁺. It is assumed that the motor nerve depolarizes to open voltage-gated Ca²⁺ channels in presynaptic nerve terminals because of exposure to doxapram. These findings are significant for building models to better understand the function of pharmacological agents that affect K2p channels and how K2p channels contribute to the physiology of tissues. Drosophila offers a genetically amenable model that can alter the tissue-specific expression of K2p channel subtypes to simulate known human diseases related to this family of channels.

Rosa damascena Miller essential oil relaxes rat trachea via KV channels, KATP channels, and BKCa channels

AIM/OBJECTIVE

This study aimed to investigate the effects of Rosa damascena Miller essential oil on rat tracheal smooth muscle contractility and the hypothesis that voltage-gated potassium (K_V) channels, ATP-sensitive potassium (K_ATP) channels, and large-conductance calcium-activated potassium (BK_Ca) channels may have roles in these effects.

METHODS

Isometric contraction-relaxation responses of tracheal rings were measured with an isolated tissue bath model. The steady contraction was induced with both 10^-5 M ACh and 60 mM KCl, and then the concentration-dependent responses of rose oil (0.1-100 µg/mL) were examined. The time-matched control (double distilled water) group was also formed. To evaluate the role of K_V, K_ATP, and BK_Ca channels, tracheal rings were incubated with 4-AP (K_V channel blocker), glibenclamide (K_ATP channel blocker), TEA (BK_Ca channel blocker), and iberiotoxin (selective BK_Ca channel blocker). Also, a vehicle control group was formed for dimethyl sulfoxide (DMSO).

RESULTS

Rose oil exerted the relaxant effects in tracheal rings pre-contracted with both ACh and KCl at concentrations of 1, 10, and 100 µg/mL (p < 0.05). Besides, K_V channel blocker 4-AP, K_ATP channel blocker glibenclamide, and BK_Ca channel blockers TEA and iberiotoxin incubations significantly inhibited the rose oil-induced relaxation responses (p < 0.05). However, incubation of tissues with DMSO, glibenclamide solvent, for 10 min did not cause a significant change in the relaxation responses to rose oil (p > 0.05).

CONCLUSIONS

In conclusion, the first physiological findings were obtained regarding the functional relaxant effects of rose essential oil in rat trachea. The findings showed that rose oil induces bronchorelaxation in a concentration-dependent manner. Besides, this study is the first to report that rose oil-mediated bronchodilation responses are associated with the activity of K_V, K_ATP, and BK_Ca channels. These results suggest that rose oil might be a useful agent in the treatment of abnormal bronchoconstriction-related diseases such as asthma and chronic obstructive pulmonary disease.

The Effect of 40-Hz White LED Therapy on Structure-Function of Brain Mitochondrial ATP-Sensitive Ca-Activated Large-Conductance Potassium Channel in Amyloid Beta Toxicity

Photobiomodulation therapy has become the focus of medical research in many areas such as Alzheimer's disease (AD), because of its modulatory effect on cellular processes through light energy absorption via photoreceptors/chromophores located in the mitochondria. However, there are still many questions around the underlying mechanisms. This study was carried out to unravel whether the function-structure of ATP-sensitive mitoBK_Ca channels, as crucial components for maintenance of mitochondrial homeostasis, can be altered subsequent to light therapy in AD. Induction of Aβ neurotoxicity in male Wistar rats was done by intracerebroventricular injection of Aβ1-42. After a week, light-treated rats were exposed to 40-Hz white light LEDs, 15 min for 7 days. Electrophysiological properties of mitoBK_Ca channel were investigated using a channel incorporated into the bilayer lipid membrane, and mitoBK_Ca-β2 subunit expression was determined using western blot analysis in Aβ-induced toxicity and light-treated rats. Our results describe that conductance and open probability (Po) of mitoBK_Ca channel decreased significantly and was accompanied by a Po curve rightward shift in mitochondrial preparation in Aβ-induced toxicity rats. We also showed a significant reduction in expression of mitoBK_Ca-β2 subunit, which is partly responsible for a leftward shift in BK_Ca Po curve in low calcium status. Interestingly, we provided evidence of a significant improvement in channel conductance and Po after light therapy. We also found that light therapy improved mitoBK_Ca-β2 subunit expression, increasing it close to saline group. The current study explains a light therapy improvement in brain mitoBK_Ca channel function in the Aβ-induced neurotoxicity rat model, an effect that can be linked to increased expression of β2 subunit.

Potassium Channel KCNH1 Activating Variants Cause Altered Functional and Morphological Ciliogenesis

The primary cilium is a non-motile sensory organelle that extends from the surface of most vertebrate cells and transduces signals regulating proliferation, differentiation, and migration. Primary cilia dysfunctions have been observed in cancer and in a group of heterogeneous disorders called ciliopathies, characterized by renal and liver cysts, skeleton and limb abnormalities, retinal degeneration, intellectual disability, ataxia, and heart disease and, recently, in autism spectrum disorder, schizophrenia, and epilepsy. The potassium voltage-gated channel subfamily H member 1 (KCNH1) gene encodes a member of the EAG (ether-à-go-go) family, which controls potassium flux regulating resting membrane potential in both excitable and non-excitable cells and is involved in intracellular signaling, cell proliferation, and tumorigenesis. KCNH1 missense variants have been associated with syndromic neurodevelopmental disorders, including Zimmermann-Laband syndrome 1 (ZLS1, MIM #135500), Temple-Baraitser syndrome (TMBTS, MIM #611816), and, recently, with milder phenotypes as epilepsy. In this work, we provide evidence that KCNH1 localizes at the base of the cilium in pre-ciliary vesicles and ciliary pocket of human dermal fibroblasts and retinal pigment epithelial (hTERT RPE1) cells and that the pathogenic missense variants (L352V and R330Q; NP_002229.1) perturb cilia morphology, assembly/disassembly, and Sonic Hedgehog signaling, disclosing a multifaceted role of the protein. The study of KCNH1 localization, its functions related to primary cilia, and the alterations introduced by mutations in ciliogenesis, cell cycle coordination, cilium morphology, and cilia signaling pathways could help elucidate the molecular mechanisms underlying neurological phenotypes and neurodevelopmental disorders not considered as classical ciliopathies but for which a significant role of primary cilia is emerging.

Predicting the functional effects of voltage-gated potassium channel missense variants with multi-task learning

BACKGROUND

Variants in genes encoding voltage-gated potassium channels are associated with a broad spectrum of neurological diseases including epilepsy, ataxia, and intellectual disability. Knowledge of the resulting functional changes, characterized as overall ion channel gain- or loss-of-function, is essential to guide clinical management including precision medicine therapies. However, for an increasing number of variants, little to no experimental data is available. New tools are needed to evaluate variant functional effects.

METHODS

We catalogued a comprehensive dataset of 959 functional experiments across 19 voltage-gated potassium channels, leveraging data from 782 unique disease-associated and synthetic variants. We used these data to train a taxonomy-based multi-task learning support vector machine (MTL-SVM), and compared performance to several baseline methods.

FINDINGS

MTL-SVM maintains channel family structure during model training, improving overall predictive performance (mean balanced accuracy 0·718 ± 0·041, AU-ROC 0·761 ± 0·063) over baseline (mean balanced accuracy 0·620 ± 0·045, AU-ROC 0·711 ± 0·022). We can obtain meaningful predictions even for channels with few known variants (KCNC1, KCNQ5).

INTERPRETATION

Our model enables functional variant prediction for voltage-gated potassium channels. It may assist in tailoring current and future precision therapies for the increasing number of patients with ion channel disorders.

FUNDING

This work was supported by intramural funding of the Medical Faculty, University of Tuebingen (PATE F.1315137.1), the Federal Ministry for Education and Research (Treat-ION, 01GM1907A/B/G/H) and the German Research Foundation (FOR-2715, Le1030/16-2, He8155/1-2).

KNa1.1 gain-of-function preferentially dampens excitability of murine parvalbumin-positive interneurons

KCNT1 encodes the sodium-activated potassium channel K_Na1.1, expressed preferentially in the frontal cortex, hippocampus, cerebellum, and brainstem. Pathogenic missense variants in KCNT1 are associated with intractable epilepsy, namely epilepsy of infancy with migrating focal seizures (EIMFS), and sleep-related hypermotor epilepsy (SHE). In vitro studies of pathogenic KCNT1 variants support predominantly a gain-of-function molecular mechanism, yet how these variants behave in a neuron or ultimately drive formation of an epileptogenic circuit is an important and timely question. Using CRISPR/Cas9 gene editing, we introduced a gain-of-function variant into the endogenous mouse Kcnt1 gene. Compared to wild-type (WT) littermates, heterozygous and homozygous knock-in mice displayed greater seizure susceptibility to the chemoconvulsants kainate and pentylenetetrazole (PTZ), but not to flurothyl. Using acute slice electrophysiology in heterozygous and homozygous Kcnt1 knock-in and WT littermates, we demonstrated that CA1 hippocampal pyramidal neurons exhibit greater amplitude of miniature inhibitory postsynaptic currents in mutant mice with no difference in frequency, suggesting greater inhibitory tone associated with the Kcnt1 mutation. To address alterations in GABAergic signaling, we bred Kcnt1 knock-in mice to a parvalbumin-tdTomato reporter line, and found that parvalbumin-expressing (PV+) interneurons failed to fire repetitively with large amplitude current injections and were more prone to depolarization block. These alterations in firing can be recapitulated by direct application of the K_Na1.1 channel activator loxapine in WT but are occluded in knock-in littermates, supporting a direct channel gain-of-function mechanism. Taken together, these results suggest that K_Na1.1 gain-of-function dampens interneuron excitability to a greater extent than it impacts pyramidal neuron excitability, driving seizure susceptibility in a mouse model of KCNT1-associated epilepsy.

Kv5.1 antibody in epilepsy patients with unknown etiology

BACKGROUND

Neuronal autoantibodies and favorable response to immunosuppressive treatment have been described in patients with chronic epilepsy of unknown cause, suggesting autoimmune etiology. Our aim was to identify novel epilepsy-specific autoantibodies reactive with neuronal surface antigens.

METHODS

Sera of 172 epilepsy patients with unknown cause and 30 healthy controls were screened with indirect immunofluorescence to identify IgG reacting with primary rat neuronal cultures. Putative target autoantigens were investigated with immunoprecipitation (IP) and liquid chromatography-mass/mass spectrometry (LC-MS/MS) studies using SH-SY5Y cells. Validation of LC-MS/MS results was carried out by IP and immunocytochemistry assays.

RESULTS

Antibodies to neuronal cell surface antigens were detected in 18 epilepsy patients. LC-MS/MS analysis identified voltage-gated potassium channel modifier subfamily F member 1 (KCNF1, Kv5.1) as the single common cell surface antigen in 4 patients with Lennox-Gastaut syndrome (n = 2), focal epilepsy of unknown cause (n = 1) and mesial temporal lobe epilepsy with hippocampal sclerosis (n = 1). These patients had the common features of early seizure onset and treatment-resistance. IP assays and co-localization (serum IgG and commercial Kv5.1-antibody) studies done with non-fixed Kv5.1-transfected HEK293 cells and primary neuronal cultures confirmed the presence of Kv5.1-antibody in 4 epilepsy patients identified by LC-MS/MS. Similar findings were not obtained by sera of other patients with epilepsy, patients with autoimmune encephalitis and healthy controls.

CONCLUSION

The herein described novel neuronal surface antibody to Kv5.1 appears to be associated with treatment-resistant epilepsy of unknown cause. Exact clinical and pathogenic significance of this antibody remains to be elucidated.

Physiological Functions, Biophysical Properties, and Regulation of KCNQ1 (KV7.1) Potassium Channels

KCNQ1 (K_V7.1) K⁺ channels are expressed in multiple tissues, including the heart, pancreas, colon, and inner ear. The gene encoding the KCNQ1 protein was discovered by a positional cloning effort to determine the genetic basis of long QT syndrome, an inherited ventricular arrhythmia that can cause sudden death. Mutations in KCNQ1 can also cause other types of arrhythmia (i.e., short QT syndrome, atrial fibrillation) and the gene may also have a role in diabetes and certain cancers. KCNQ1 α-subunits can partner with accessory β-subunits (KCNE1-KCNE5) to form K⁺-selective channels that have divergent biophysical properties. In the heart, KCNQ1 α-subunits coassemble with KCNE1 β-subunits to form channels that conduct I_Ks, a very slowly activating delayed rectifier K⁺ current. K_V7.1 channels are highly regulated by PIP₂, calmodulin, and phosphorylation, and rich pharmacology includes blockers and gating modulators. Recent biophysical studies and a cryo-EM structure of the KCNQ1-calmodulin complex have provided new insights into K_V7.1 channel function, and how interactions between KCNQ1 and KCNE subunits alter the gating properties of heteromultimeric channels.

Genome Sequencing in a Genetically Elusive Multi-Generational Long QT Syndrome Pedigree Identifies a Novel LQT2-Causative Deeply Intronic KCNH2 Variant

BACKGROUND

Most of long QT syndrome (LQTS) stems from pathogenic variants in KCNQ1, KCNH2, or SCN5A. However, ∼10-20% of LQTS index cases remain genotype-negative.

OBJECTIVE

Here, we identified and characterized functionally a novel LQTS genetic substrate in a multi-generational, "genotype-negative" LQTS pedigree.

METHODS

The patient was a 40-year-old female with a history of syncope, seizures, ventricular fibrillation, and a family history of LQTS and sudden death. Commercial genetic testing of all LQTS-causative genes was negative. Genome sequencing was performed on 6 affected family members. Patient-specific and CRISPR/Cas9 "gene-corrected" isogenic control induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated.

RESULTS

No ultra-rare, nonsynonymous heterozygous variants co-segregated among the 6 LQTS phenotype-positive individuals. Instead, a deep intronic KCNH2 variant (c.3331-316G>T) was present in all affected individuals. RT-PCR analysis of patient-specific iPSC-CM-derived RNA revealed that c.3331-316G>T creates a novel 89 base-pair exon that results in a frame-shift variant (p.S1112Pfs*171). The action potential duration (APD₉₀) was significantly longer in p.S1112Pfs*171-iPSC-CMs (602.4 ± 12.2 ms, n=70) compared to isogenic control iPSC-CMs (425.7 ± 9.3 ms, n=61, p<0.0001). Further, the field potential duration (FPD) was significantly longer in p.S1112Pfs*171-iPSC-CMs (358.9 ± 7.7 ms, n=65) compared to isogenic control iPSC-CMs (282.2 ± 10.8 ms, n=51, p<0.0001).

CONCLUSIONS

A novel deep intronic KCNH2 variant was identified in a multi-generational, genetically elusive LQTS pedigree. The iPSC-CMs establish that the variant is the monogenetic cause for this family's LQTS. Deep intronic variants within the two most common LQTS-susceptibility genes should be considered in patients with seemingly, genetically elusive LQTS.

Fu rescues hypoxia-induced potassium channel inhibition in rat pulmonary artery smooth muscle cells

ETHNOPHARMACOLOGICAL RELEVANCE

Previous studies have shown that the active fraction of Rhodiola tangutica (Maxim.) S.H. Fu (ACRT) dilates pulmonary arteries and thwarts pulmonary artery remodelling. The dilatation effect of ACRT on pulmonary artery vascular rings could be reduced by potassium (K⁺) channel blockers. However the exact mechanisms of ACRT on ion channels are still unclear.

AIM OF THE STUDY

This study aimed to investigate whether the effect of ACRT on K⁺ channels inhibits cell proliferation after pulmonary artery smooth muscle cells (PASMCs) are exposed to hypoxia.

MATERIALS AND METHODS

The whole-cell patch-clamp method was used to clarify the effect of ACRT on the K⁺ current (I_K) of rat PASMCs exposed to hypoxia. The mRNA and protein expression levels were detected using real-time quantitative polymerase chain reaction (RT-qPCR) and western blotting, respectively. The intracellular calcium (Ca²⁺) concentration ([Ca²⁺]_i) values in rat PASMCs were detected by laser scanning confocal microscopy. The cell cycle and cell proliferation were assessed using flow cytometry analysis and CCK-8 and EdU assays.

RESULTS

ACRT pretreatment alleviated the inhibition of I_K induced by hypoxia in rat PASMCs. Compared with hypoxia, ACRT upregulated voltage-dependent K⁺ channel (K_v) 1.5 and big-conductance calcium-activated K⁺ channel (BK_Ca) mRNA and protein expression and downregulated voltage-dependent Ca²⁺ channel (Ca_v) 1.2 mRNA and protein expression. ACRT decreased [Ca²⁺]_i, inhibited the promotion of cyclin D1 and proliferating cell nuclear antigen (PCNA) expression, and prevented the proliferation of rat PASMCs exposed to hypoxia.

CONCLUSION

In conclusion, the present study demonstrated that ACRT plays a key role in restoring ion channel function and then inhibiting the proliferation of PASMCs under hypoxia, ACRT has preventive and therapeutic potential in hypoxic pulmonary hypertension.

Impaired Kv7 channel activity in the central amygdala contributes to elevated sympathetic outflow in hypertension

AIMS

Elevated sympathetic outflow is associated with primary hypertension. However, the mechanisms involved in heightened sympathetic outflow in hypertension are unclear. The central amygdala (CeA) regulates autonomic components of emotions through projections to the brainstem. The neuronal Kv7 channel is a non-inactivating voltage-dependent K+ channel encoded by KCNQ2/3 genes involved in stabilizing the neuronal membrane potential and regulating neuronal excitability. In this study, we investigated if altered Kv7 channel activity in the CeA contributes to heightened sympathetic outflow in hypertension.

METHODS AND RESULTS

The mRNA and protein expression levels of Kv7.2/Kv7.3 in the CeA were significantly reduced in spontaneously hypertensive rats (SHRs) compared with Wistar-Kyoto (WKY) rats. Lowering blood pressure with coeliac ganglionectomy in SHRs did not alter Kv7.2 and Kv7.3 channel expression levels in the CeA. Fluospheres were injected into the rostral ventrolateral medulla (RVLM) to retrogradely label CeA neurons projecting to the RVLM (CeA-RVLM neurons). Kv7 channel currents recorded from CeA-RVLM neurons in brain slices were much smaller in SHRs than in WKY rats. Furthermore, the basal firing activity of CeA-RVLM neurons was significantly greater in SHRs than in WKY rats. Bath application of specific Kv7 channel blocker 10, 10-bis (4-pyridinylmethyl)-9(10H)-anthracnose (XE-991) increased the excitability of CeA-RVLM neurons in WKY rats, but not in SHRs. Microinjection of XE-991 into the CeA increased arterial blood pressure (ABP) and renal sympathetic nerve activity (RSNA), while microinjection of Kv7 channel opener QO-58 decreased ABP and RSNA, in anaesthetized WKY rats but not SHRs.

CONCLUSIONS

Our findings suggest that diminished Kv7 channel activity in the CeA contributes to elevated sympathetic outflow in primary hypertension. This novel information provides new mechanistic insight into the pathogenesis of neurogenic hypertension.

Potassium and the kidney: a reciprocal relationship with clinical relevance

By controlling urinary potassium excretion, the kidneys play a key role in maintaining whole-body potassium homeostasis. Conversely, low urinary potassium excretion (as a proxy for insufficient dietary intake) is increasingly recognized as a risk factor for the progression of kidney disease. Thus, there is a reciprocal relationship between potassium and the kidney: the kidney regulates potassium balance but potassium also affects kidney function. This review explores this relationship by discussing new insights into kidney potassium handling derived from recently characterized tubulopathies and studies on sexual dimorphism. These insights reveal a central but non-exclusive role for the distal convoluted tubule in sensing potassium and subsequently modifying the activity of the sodium-chloride cotransporter. This is another example of reciprocity: activation of the sodium-chloride cotransporter not only reduces distal sodium delivery and therefore potassium secretion but also increases salt sensitivity. This mechanism helps explain the well-known relationship between dietary potassium and blood pressure. Remarkably, in children, blood pressure is related to dietary potassium but not sodium intake. To explore how potassium deficiency can cause kidney injury, we review the mechanisms of hypokalemic nephropathy and discuss if these mechanisms may explain the association between low dietary potassium intake and adverse kidney outcomes. We discuss if potassium should be repleted in patients with kidney disease and what role dietary potassium plays in the risk of hyperkalemia. Supported by data and physiology, we reach the conclusion that we should view potassium not only as a potentially dangerous cation but also as a companion in the battle against kidney disease.

Structures of Gating Intermediates in a K+ channel

Regulation of ion conduction through the pore of a K⁺ channel takes place through the coordinated action of the activation gate at the bundle crossing of the inner helices and the inactivation gate located at the selectivity filter. The mechanism of allosteric coupling of these gates is of key interest. Here we report new insights into this allosteric coupling mechanism from studies on a W67F mutant of the KcsA channel. W67 is in the pore helix and is highly conserved in K⁺ channels. The KcsA W67F channel shows severely reduced inactivation and an enhanced rate of activation. We use continuous wave EPR spectroscopy to establish that the KcsA W67F channel shows an altered pH dependence of activation. Structural studies on the W67F channel provide the structures of two intermediate states: a pre- open state and a pre-inactivated state of the KcsA channel. These structures highlight key nodes in the allosteric pathway. The structure of the KcsA W67F channel with the activation gate open shows altered ion occupancy at the second ion binding site (S2) in the selectivity filter. This finding in combination with previous studies strongly support a requirement for ion occupancy at the S2 site for the channel to inactivate.

Cell death-inducing cytotoxicity in truncated KCNQ4 variants associated with DFNA2 hearing loss

KCNQ4 encodes the homotetrameric voltage-dependent potassium ion channel Kv7.4, and is the causative gene for autosomal dominant nonsyndromic sensorineural hearing loss, DFNA2. Dominant-negative inhibition accounts for the observed dominant inheritance of many DFNA2-associated KCNQ4 variants. In addition, haploinsufficiency has been presumed as the pathological mechanism for truncated Kv7.4 variants lacking the C-terminal tetramerization region, as they are unlikely to exert a dominant-negative inhibitory effect. Such truncated Kv7.4 variants should result in relatively mild hearing loss when heterozygous; however, this is not always the case. In this study, we characterized Kv7.4^Q71fs (c.211delC), Kv7.4^W242X (c.725G>A) and Kv7.4^A349fs (c.1044_1051del8) in heterologous expression systems and found that expression of these truncated Kv7.4 variants induced cell death. We also found similar cell death-inducing cytotoxic effects in truncated Kv7.1 (KCNQ1) variants, suggesting that the generality of our findings could account for the dominant inheritance of many, if not most, truncated Kv7 variants. Moreover, we found that the application of autophagy inducers can ameliorate the cytotoxicity, providing a novel insight for the development of alternative therapeutic strategies for Kv7.4 variants.

Summary: Expression of truncated KCNQ4 variants lacking the C-terminal tetramerization domain results in cell-death inducing cytotoxicity, providing novel insight into the development of alternative therapeutic strategies for DFNA2 hearing loss.

Adiponectin and the regulation of gastric content volume in the newborn rat

BACKGROUND

Oral intake is dependent on the gastric ability to accommodate the food bolus. Comparatively, neonates have a smaller gastric capacity than adults and this may limit the volume of their milk intake. Yet, we previously reported that the newborn rat gastric milk volume is greatest after birth and, when normalized to body weight, decreases with postnatal age. Such age-dependent changes are not the result of intake differences, but greater gastric accommodation and reduced emptying rate.

AIM

Hypothesizing that breastmilk-derived adiponectin is the factor regulating gastric accommodation in neonates, we comparatively evaluated its effects on the rat fundic muscle tone at different postnatal ages.

METHODS

In freshly dispersed smooth muscle cells (SMC), we measured the adiponectin effect on the carbachol-induced length changes.

RESULTS

Adiponectin significantly reduced the carbachol-stimulated SMC shortening independently of age. In the presence of the inhibitor iberiotoxin, the adiponectin effect on SMC shortening was suppressed, suggesting that it is mediated via large-conductance Ca²⁺ sensitive K⁺ channel activation. Lastly, we comparatively measured the newborn rat gastric milk curd adiponectin content in one- and two-week-old rats and found a 50% lower value in the latter.

CONCLUSION

Adiponectin, a major component of breastmilk, downregulates fundic smooth muscle contraction potential, thus facilitating gastric volume accommodation. This rodent’s adaptive response maximizes breastmilk intake volume after birth.

The functions of repressor element 1-silencing transcription factor in models of epileptogenesis and post-ischemia

Epilepsy is a debilitating neurological disorder characterised by recurrent seizures for which 30% of patients are refractory to current treatments. The genetic and molecular aetiologies behind epilepsy are under investigation with the goal of developing new epilepsy medications. The transcriptional repressor REST (Repressor Element 1-Silencing Transcription factor) is a focus of interest as it is consistently upregulated in epilepsy patients and following brain insult in animal models of epilepsy and ischemia. This review analyses data from different epilepsy models and discusses the contribution of REST to epileptogenesis. We propose that in healthy brains REST acts in a protective manner to homeostatically downregulate increases in excitability, to protect against seizure through downregulation of BDNF (Brain-Derived Neurotrophic Factor) and its receptor, TrkB (Tropomyosin receptor kinase B). However, in epilepsy patients and post-seizure, REST may increase to a larger degree, which allows downregulation of the glutamate receptor subunit GluR2. This leads to AMPA glutamate receptors lacking GluR2 subunits, which have increased permeability to Ca²⁺, causing excitotoxicity, cell death and seizure. This concept highlights therapeutic potential of REST modulation through gene therapy in epilepsy patients.

Potassium and Chloride Ion Channels in Cancer: A Novel Paradigm for Cancer Therapeutics

Cancer is a collection of diseases caused by specific changes at the genomic level that support cell proliferation indefinitely. Traditionally, ion channels are known to control a variety of cellular processes including electrical signal generation and transmission, secretion, and contraction by controlling ionic gradients. However, recent studies had brought to light important facts on ion channels in cancer biology.In this review we discuss the mechanism linking potassium or chloride ion channel activity to biochemical pathways controlling proliferation in cancer cells and the potential advantages of targeting ion channels as an anticancer therapeutic option.

Comparison of K+ Channel Families

K⁺ channels enable potassium to flow across the membrane with great selectivity. There are four K⁺ channel families: voltage-gated K (K_v), calcium-activated (K_Ca), inwardly rectifying K (K_ir), and two-pore domain potassium (K_2P) channels. All four K⁺ channels are formed by subunits assembling into a classic tetrameric (4x1P = 4P for the K_v, K_Ca, and K_ir channels) or tetramer-like (2x2P = 4P for the K_2P channels) architecture. These subunits can either be the same (homomers) or different (heteromers), conferring great diversity to these channels. They share a highly conserved selectivity filter within the pore but show different gating mechanisms adapted for their function. K⁺ channels play essential roles in controlling neuronal excitability by shaping action potentials, influencing the resting membrane potential, and responding to diverse physicochemical stimuli, such as a voltage change (K_v), intracellular calcium oscillations (K_Ca), cellular mediators (K_ir), or temperature (K_2P).

Pharmacological Approaches to Studying Potassium Channels

In this review, we consider the pharmacology of potassium channels from the perspective of these channels as therapeutic targets. Firstly, we describe the three main families of potassium channels in humans and disease states where they are implicated. Secondly, we describe the existing therapeutic agents which act on potassium channels and outline why these channels represent an under-exploited therapeutic target with potential for future drug development. Thirdly, we consider the evidence desired in order to embark on a drug discovery programme targeting a particular potassium channel. We have chosen two "case studies": activators of the two-pore domain potassium (K_2P) channel TREK-2 (K_2P10.1), for the treatment of pain and inhibitors of the voltage-gated potassium channel K_V1.3, for use in autoimmune diseases such as multiple sclerosis. We describe the evidence base to suggest why these are viable therapeutic targets. Finally, we detail the main technical approaches available to characterise the pharmacology of potassium channels and identify novel regulatory compounds. We draw particular attention to the Comprehensive in vitro Proarrhythmia Assay initiative (CiPA, https://cipaproject.org ) project for cardiac safety, as an example of what might be both desirable and possible in the future, for ion channel regulator discovery projects.

Human iPSC-derived cardiomyocytes and pyridyl-phenyl mexiletine analogs

In the United States, approximately one million individuals are hospitalized every year for arrhythmias, making arrhythmias one of the top causes of healthcare expenditures. Mexiletine is currently used as an antiarrhythmic drug but has limitations. The purpose of this work was to use normal and Long QT syndrome Type 3 (LQTS3) patient-derived human induced pluripotent stem cell (iPSC)-derived cardiomyocytes to identify an analog of mexiletine with superior drug-like properties. Compared to racemic mexiletine, medicinal chemistry optimization of substituted racemic pyridyl phenyl mexiletine analogs resulted in a more potent sodium channel inhibitor with greater selectivity for the sodium over the potassium channel and for late over peak sodium current.

A small-molecule Psora-4 acts as a caloric restriction mimetic to promote longevity in C. elegans

In populations around the world, the fraction of humans aged 65 and above is increasing at an unprecedented rate. Aging is the main risk factor for the most important degenerative diseases and this demographic shift poses significant social, economic, and medical challenges. Pharmacological interventions directly targeting mechanisms of aging are an emerging strategy to delay or prevent age-dependent diseases. Successful application of this approach has the potential to yield dramatic health, social, and economic benefits. Psora-4 is an inhibitor of the voltage-gated potassium channel, Kv1.3, that has previously been shown to increase longevity and health span in the nematode Caenorhabditis elegans (C. elegans). Our recent discovery that Psora-4 lifespan benefits in C. elegans are synergistic with those of several other lifespan-extending drugs has motivated us to investigate further the mechanism by which Psora-4 extends lifespan. Here, we report that Psora-4 increases the production of free radicals and modulates genes related to stress response and that its effect intersects closely with the target set of caloric restriction (CR) genes, suggesting that it, in part, acts as CR mimetic. This effect may be related to the role of potassium channels in energy metabolism. Our discovery of a potassium channel blocker as a CR mimetic suggests a novel avenue for mimicking CR and extending a healthy lifespan.

Therapeutic Antibodies Targeting Potassium Ion Channels

Monoclonal antibodies combine specificity and high affinity binding with excellent pharmacokinetic properties and are rapidly being developed for a wide range of drug targets including clinically important potassium ion channels. Nonetheless, while therapeutic antibodies come with great promise, K⁺ channels represent particularly difficult targets for biologics development for a variety of reasons that include their dynamic structures and relatively small extracellular loops, their high degree of sequence conservation (leading to immune tolerance), and their generally low-level expression in vivo. The process is made all the more difficult when large numbers of antibody candidates must be screened for a given target, or when lead candidates fail to cross-react with orthologous channels in animal disease models due to their highly selective binding properties. While the number of antibodies targeting potassium channels in preclinical or clinical development is still modest, significant advances in the areas of protein expression and antibody screening are converging to open the field to an avalanche of new drugs. Here, the opportunities and constraints associated with the discovery of antibodies against K⁺ channels are discussed, with an emphasis on novel technologies that are opening the field to exciting new possibilities for biologics development.

Pharmacology of A-Type K+ Channels

Transient outward potassium currents were first described nearly 60 years ago, since then major strides have been made in understanding their molecular basis and physiological roles. From the large family of voltage-gated potassium channels members of 3 subfamilies can produce such fast-inactivating A-type potassium currents. Each subfamily gives rise to currents with distinct biophysical properties and pharmacological profiles and a simple workflow is provided to aid the identification of channels mediating A-type currents in native cells. Their unique properties and regulation enable A-type K⁺ channels to perform varied roles in excitable cells including repolarisation of the cardiac action potential, controlling spike and synaptic timing, regulating dendritic integration and long-term potentiation as well as being a locus of neural plasticity.

Functional testing for variant prioritization in a family with long QT syndrome

Next-generation sequencing platforms are being increasingly applied in clinical genetic settings for evaluation of families with suspected heritable disease. These platforms potentially improve the diagnostic yield beyond that of disease-specific targeted gene panels, but also increase the number of rare or novel genetic variants that may confound precise diagnostics. Here, we describe a functional testing approach used to interpret the results of whole exome sequencing (WES) in a family presenting with syncope and sudden death. One individual had a prolonged QT interval on electrocardiogram (ECG) and carried a diagnosis of long QT syndrome (LQTS), but a second individual did not meet criteria for LQTS. Filtering WES results for uncommon variants with arrhythmia association identified four for further analyses. In silico analyses indicated that two of these variants, KCNH2 p.(Cys555Arg) and KCNQ1 p.(Arg293Cys), were likely to be causal in this family's LQTS. We subsequently performed functional characterization of these variants in a heterologous expression system. The expression of KCNQ1-Arg293Cys did not show a deleterious phenotype but KCNH2-Cys555Arg demonstrated a loss-of-function phenotype that was partially dominant. Our stepwise approach identified a precise genetic etiology in this family, which resulted in the establishment of a LQTS diagnosis in the second individual as well as an additional asymptomatic family member, enabling personalized clinical management. Given its ability to aid in the diagnosis, the application of functional characterization should be considered as a value adjunct to in silico analyses of WES.

Fluorescent toxins as ion channel activity sensors

Voltage gated ion channels (VGICs) shape the electrical character of cells by undergoing structural changes in response to membrane depolarization. High-resolution techniques have provided a wealth of data on individual VGIC structures, but the conformational changes of endogenous channels in live cell membranes have remained unexplored. Here, we describe methods for imaging structural changes of voltage-gated K⁺ channels in living cells, using peptidyl toxins labeled with fluorophores that report specific protein conformations. These Endogenous Voltage-sensor Activity Probes (EVAPs) enable study of both VGIC allostery and function in the context of endogenous live-cell membranes under different physiological states. In this chapter, we describe methods for the synthesis, imaging, and analysis of dynamic EVAPs, which can report K⁺ channel activity in complex tissue preparations via 2-photon excitation microscopy, and environment-sensitive EVAPs, which report voltage-dependent conformational changes at the VGIC-toxin interface. The methods here present the utility of current EVAPs and lay the groundwork for the development of other probes that act by similar mechanisms. EVAPs can be correlated with electrophysiology, offering insight into the molecular details of endogenous channel function and allostery in live cells. This enables investigation of conformational changes of channels in their native, functional states, putting structures and models into a context of live-cell membranes. The expansive array of state-dependent ligands and optical probes should enable probes more generally for investigating the molecular motions of endogenous proteins.