Large-Conductance Calcium-Activated Potassium Channels in Glomerulus: From Cell Signal Integration to Disease

TAT-Modified Martentoxin Displays Intravenous Antiseizure Activities

Epilepsy is a chronic disease of brain dysfunction, which arises from imbalance between excitatory and inhibitory activities in neural circuits. Previously, we reported that peptide Martentoxin (MarTX), from scorpion Buthus martensii Karsch, displayed antiseizure activities by specifically inhibiting BK(α + β4) channel currents. Injection of MarTX into the hippocampal region of mice significantly alleviated convulsive seizures. However, intravenous injection of MarTX had no antiepileptic efficacy due to the blood-brain barrier (BBB). To address this, here, we designed cell-penetrating peptide TAT-modified MarTX, in which the linker containing three glycines was put between TAT and the N-terminus of MarTX (forming MTX-N-TAT) or between TAT and the C-terminus of MarTX (forming MTX-C-TAT), respectively. We prepared them in a large amount through Escherichia coli overexpression system and then probed their antiseizure activities. Our results indicated that intravenous injection of MTX-C-TAT showed significant therapeutic efficacy of antiseizure. It increased seizure latency, reduced the total seizure duration and the number of seizures at stages 3, 4, and 5, inhibited hippocampal neuronal hyperexcitability, and exhibited neuroprotective effects on hippocampal neurons. These studies implied that MTX-C-TAT displayed intravenous antiseizure activities properly through crossing BBB and would be a potential antiepileptic drug in the future.

Ion channels and channelopathies in glomeruli

An essential step in renal function entails the formation of an ultrafiltrate that is delivered to the renal tubules for subsequent processing. This process, known as glomerular filtration, is controlled by intrinsic regulatory systems and by paracrine, neuronal, and endocrine signals that converge onto glomerular cells. In addition, the characteristics of glomerular fluid flow, such as the glomerular filtration rate and the glomerular filtration fraction, play an important role in determining blood flow to the rest of the kidney. Consequently, disease processes that initially affect glomeruli are the most likely to lead to end-stage kidney failure. The cells that comprise the glomerular filter, especially podocytes and mesangial cells, express many different types of ion channels that regulate intrinsic aspects of cell function and cellular responses to the local environment, such as changes in glomerular capillary pressure. Dysregulation of glomerular ion channels, such as changes in TRPC6, can lead to devastating glomerular diseases, and a number of channels, including TRPC6, TRPC5, and various ionotropic receptors, are promising targets for drug development. This review discusses glomerular structure and glomerular disease processes. It also describes the types of plasma membrane ion channels that have been identified in glomerular cells, the physiological and pathophysiological contexts in which they operate, and the pathways by which they are regulated and dysregulated. The contributions of these channels to glomerular disease processes, such as focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, as well as the development of drugs that target these channels are also discussed.

Development of charybdotoxin Q18F variant as a selective peptide blocker of neuronal BK(α + β4) channel for the treatment of epileptic seizures

Epilepsy is the results from the imbalance between inhibition and excitation in neural circuits, which is mainly treated by some chemical drugs with side effects. Gain-of-function of BK channels or knockout of its β4 subunit associates with spontaneous epilepsy. Currently, few reports were published about the efficacy of BK(α + β4) channel modulators in epilepsy prevention. Charybdotoxin is a non-specific inhibitor of BK and other K+ channels. Here, by nuclear magnetic resonance (NMR) and other biochemical techniques, we found that charybdotoxin might interact with the extracellular loop of human β4 subunit (i.e., hβ4-loop) of BK(α + β4) channel at a molar ratio 4:1 (hβ4-loop vs. charybdotoxin). Charybdotoxin enhanced its ability to prevent K+ current of BK(α + β4 H101Y) channel. The charybdotoxin Q18F variant selectively reduced the neuronal spiking frequency and increased interspike intervals of BK(α + β4) channel by π-π stacking interactions between its residue Phe18 and residue His101 of hβ4-loop. Moreover, intrahippocampal infusion of charybdotoxin Q18F variant significantly increased latency time of seizure, reduced seizure duration and seizure numbers on pentylenetetrazole-induced pre-sensitized rats, inhibited hippocampal hyperexcitability and c-Fos expression, and displayed neuroprotective effects on hippocampal neurons. These results implied that charybdotoxin Q18F variant could be potentially used for intractable epilepsy treatment by therapeutically targeting BK(α + β4) channel.

Glomerular basement membrane deposition of collagen α1(III) in Alport glomeruli by mesangial filopodia injures podocytes via aberrant signaling through DDR1 and integrin α2β1

In Alport mice, activation of the endothelin A receptor (ETA R) in mesangial cells results in sub-endothelial invasion of glomerular capillaries by mesangial filopodia. Filopodia deposit mesangial matrix in the glomerular basement membrane (GBM), including laminin 211 which activates NF-κB, resulting in induction of inflammatory cytokines. Herein we show that collagen α1(III) is also deposited in the GBM. Collagen α1(III) localized to the mesangium in wild-type mice and was found in both the mesangium and the GBM in Alport mice. We show that collagen α1(III) activates discoidin domain receptor family, member 1 (DDR1) receptors both in vitro and in vivo. To elucidate whether collagen α1(III) might cause podocyte injury, cultured murine Alport podocytes were overlaid with recombinant collagen α1(III), or not, for 24 h and RNA was analyzed by RNA sequencing (RNA-seq). These same cells were subjected to siRNA knockdown for integrin α2 or DDR1 and the RNA was analyzed by RNA-seq. Results were validated in vivo using RNA-seq from RNA isolated from wild-type and Alport mouse glomeruli. Numerous genes associated with podocyte injury were up- or down-regulated in both Alport glomeruli and cultured podocytes treated with collagen α1(III), 18 of which have been associated previously with podocyte injury or glomerulonephritis. The data indicate α2β1 integrin/DDR1 co-receptor signaling as the dominant regulatory mechanism. This may explain earlier studies where deletion of either DDR1 or α2β1 integrin in Alport mice ameliorates renal pathology. © 2022 Boys Town National Research Hospital. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

TRPC6 interacted with KCa1.1 channels to regulate the proliferation and apoptosis of glioma cells

Malignant glioma is the most aggressive and deadliest brain malignancy. TRPC6 and K_Ca1.1, two ion channels, have been considered as potential therapeutic targets for malignant glioma treatment. TRPC6, a Ca²⁺-permeable channel, plays a vital role in promoting tumorigenesis and the progression of glioma. K_Ca1.1, a large-conductance Ca²⁺-activated channel, is also involved in growth and migration of glioma. However, the underlying mechanism by which these two ion channels promote glioma progression was unclear. In our study, we found that TRPC6 upregulated the expression of K_Ca1.1, while the immunoprecipitation analysis also showed that TRPC6 interacts with K_Ca1.1 channels in glioma cells. The currents of K_Ca1.1 recorded by the whole-cell patch clamp technique were increased by TRPC6 in glioma cells, suggesting that TRPC6 can provide a Ca²⁺ source for the activation of K_Ca1.1 channels. It was also suggested that TRPC6 regulates the proliferation and apoptosis of glioma cells through K_Ca1.1 channels in vitro. Therefore, C6-bearing glioma rats were established to validate the results in vitro. After the administration of paxilline (a specific inhibitor of K_Ca1.1 channels), TRPC6-dependent growth of glioma was inhibited in vivo. We also found that TRPC6 enhanced co-expression with K_Ca1.1 in glioma. These all suggested that TRPC6/K_Ca1.1 signal plays a role in promoting the growth of glioma. Our results provided new evidence for TRPC6 and K_Ca1.1 as potential targets for glioma treatment.

BKCa Mediates Dysfunction in High Glucose Induced Mesangial Cell Injury via TGF-β1/Smad2/3 Signaling Pathways

OBJECTIVE

To explore the role and mechanism of BK_Ca in diabetic kidney disease.

METHODS

Rat mesangial cells (MCs) HBZY-1 were cultured with high glucose to simulate the high-glucose environment of diabetic kidney disease in vivo. The effects of large conductance calcium-activated potassium channel (BK_Ca) on proliferation, migration, and apoptosis of HBZY-1 cells were observed. The contents of transforming growth factor beta 1 (TGF-β1), Smad2/3, collagen IV (Col IV), and fibronectin (FN) in the extracellular matrix were also observed.

RESULTS

High glucose significantly damaged HBZY-1 cells, which enhanced the ability of cell proliferation, migration, and apoptosis, and increased the secretion of Col IV and FN. Inhibition of BK_Ca and TGF-β1/Smad2/3 signaling pathways can inhibit the proliferation, migration, and apoptosis of HBZY-1 cells and suppress the secretion of Col IV and FN. The effect of excitation is the opposite.

CONCLUSIONS

BK_Ca regulates mesangial cell proliferation, migration, apoptosis, and secretion of Col IV and FN and is associated with TGF-β1/Smad2/3 signaling pathway.

Large-conductance calcium-activated potassium channels mediate lipopolysaccharide-induced activation of murine microglia

Large-conductance calcium-activated potassium (BK) channels are ubiquitously expressed in most cell types where they regulate many cellular, organ, and organismal functions. Although BK currents have been recorded specifically in activated murine and human microglia, it is not yet clear whether and how the function of this channel is related to microglia activation. Here, using patch-clamping, Griess reaction, ELISA, immunocytochemistry, and immunoblotting approaches, we show that specific inhibition of the BK channel with paxilline (10 μm) or siRNA-mediated knockdown of its expression significantly suppresses lipopolysaccharide (LPS)-induced (100 ng/ml) BV-2 and primary mouse microglial cell activation. We found that membrane BK current is activated by LPS at a very early stage through Toll-like receptor 4 (TLR4), leading to nuclear translocation of NF-κB and to production of inflammatory cytokines. Furthermore, we noted that BK channels are also expressed intracellularly, and their nuclear expression significantly increases in late stages of LPS-mediated microglia activation, possibly contributing to production of nitric oxide, tumor necrosis factor-α, and interleukin-6. Of note, a specific TLR4 inhibitor suppressed BK channel expression, whereas an NF-κB inhibitor did not. Taken together, our findings indicate that BK channels participate in both the early and the late stages of LPS-stimulated murine microglia activation involving both membrane-associated and nuclear BK channels.

Induced Pluripotent Stem Cell-Derived Podocyte-Like Cells as Models for Assessing Mechanisms Underlying Heritable Disease Phenotype: Initial Studies Using Two Alport Syndrome Patient Lines Indicate Impaired Potassium Channel Activity

Renal podocyte survival depends upon the dynamic regulation of a complex cell architecture that links the glomerular basement membrane to integrins, ion channels, and receptors. Alport syndrome is a heritable chronic kidney disease where mutations in α3, α4, or α5 collagen genes promote podocyte death. In rodent models of renal failure, activation of the calcium-sensing receptor (CaSR) can protect podocytes from stress-related death. In this study, we assessed CaSR function in podocyte-like cells derived from induced-pluripotent stem cells from two patients with Alport Syndrome (AS1 & AS2) and a renal disease free individual [normal human mesangial cell (NHMC)], as well as a human immortalized podocyte-like (HIP) cell line. Extracellular calcium elicited concentration-dependent elevations of intracellular calcium in all podocyte-like cells. NHMC and HIP, but not AS1 or AS2 podocyte-like cells, also showed acute reductions in intracellular calcium prior to elevation. In NHMC podocyte-like cells this acute reduction was blocked by the large-conductance potassium channel (KCNMA1) inhibitors iberiotoxin (10 nM) and tetraethylammonium (5 mM), as well as the focal adhesion kinase inhibitor PF562271 (N-methyl-N-(3-((2-(2-oxo-2,3-dihydro-1H-indol-5-ylamino)-5-trifluoromethyl-pyrimidin-4-ylamino)-methyl)-pyridin-2-yl)-methanesulfonamide, 10 nM). Quantitative polymerase chain reaction (qPCR) and immunolabeling showed the presence of KCNMA1 transcript and protein in all podocyte-like cells tested. Cultivation of AS1 podocytes on decellularized plates of NHMC podocyte-like cells partially restored acute reductions in intracellular calcium in response to extracellular calcium. We conclude that the AS patient-derived podocyte-like cells used in this study showed dysfunctional integrin signaling and potassium channel function, which may contribute to podocyte death seen in Alport syndrome.

Hypothalamic Ion Channels in Hypertension

Hypertension is a prevalent and major health problem, involving a complex integration of different organ systems, including the central nervous system (CNS). The CNS and the hypothalamus in particular are intricately involved in the pathogenesis of hypertension. In fact, evidence supports altered hypothalamic neuronal activity as a major factor contributing to increased sympathetic drive and increased blood pressure. Several mechanisms have been proposed to contribute to hypothalamic-driven sympathetic activity, including altered ion channel function. Ion channels are critical regulators of neuronal excitability and synaptic function in the brain and, thus, important for blood pressure homeostasis regulation. These include sodium channels, voltage-gated calcium channels, and potassium channels being some of them already identified in hypothalamic neurons. This brief review summarizes the hypothalamic ion channels that may be involved in hypertension, highlighting recent findings that suggest that hypothalamic ion channel modulation can affect the central control of blood pressure and, therefore, suggesting future development of interventional strategies designed to treat hypertension.

Toxic effects of environmental rare earth elements on delayed outward potassium channels and their mechanisms from a microscopic perspective

The wide applications cause a large amount of rare earth elements (REEs) to be released into the environment, and ultimately into the human body through food chain. Toxic effects of REEs on humans have been extensively studied, but their toxic effects and binding targets in cells are not understood. Delayed outward potassium channels (K⁺ channels) are good targets for exogenous substances or clinical drugs. To evaluate cellular toxicities of REEs and clarify toxic mechanisms, the toxicities of REEs on the K⁺ channel and their structural basis were investigated. The results showed that delayed outward potassium channels on the plasma membrane are the targets of REEs acting on living organisms, and the changes in the thermodynamic and kinetic characteristics of the K⁺ channel are the reasons of diseases induced by REEs. Two types of REEs, a light REE La³⁺ and a heavy REE Tb³⁺, displayed different intensity of toxicities on the K⁺ channel, in which the toxicity of Tb³⁺ was stronger than that of La³⁺. More interestingly, in comparison with that of heavy metal Cd²⁺, the cytotoxicities of the light and heavy REEs showed discriminative differences, and the cytotoxicity of Tb³⁺ was higher than that of Cd²⁺, while the cytotoxicity of La³⁺ was lower than that of Cd²⁺. These different cytotoxicities of La³⁺, Tb³⁺ and Cd²⁺ on human resulted from the varying binding abilities of the metals to this channel protein.