Effects of ion channels on proliferation in cultured human cardiac fibroblasts

Exploring the Impact of BKCa Channel Function in Cellular Membranes on Cardiac Electrical Activity

This review paper delves into the current body of evidence, offering a thorough analysis of the impact of large-conductance Ca2+-activated K+ (BKCa or BK) channels on the electrical dynamics of the heart. Alterations in the activity of BKCa channels, responsible for the generation of the overall magnitude of Ca2+-activated K+ current at the whole-cell level, occur through allosteric mechanisms. The collaborative interplay between membrane depolarization and heightened intracellular Ca2+ ion concentrations collectively contribute to the activation of BKCa channels. Although fully developed mammalian cardiac cells do not exhibit functional expression of these ion channels, evidence suggests their presence in cardiac fibroblasts that surround and potentially establish close connections with neighboring cardiac cells. When cardiac cells form close associations with fibroblasts, the high single-ion conductance of these channels, approximately ranging from 150 to 250 pS, can result in the random depolarization of the adjacent cardiac cell membranes. While cardiac fibroblasts are typically electrically non-excitable, their prevalence within heart tissue increases, particularly in the context of aging myocardial infarction or atrial fibrillation. This augmented presence of BKCa channels' conductance holds the potential to amplify the excitability of cardiac cell membranes through effective electrical coupling between fibroblasts and cardiomyocytes. In this scenario, this heightened excitability may contribute to the onset of cardiac arrhythmias. Moreover, it is worth noting that the substances influencing the activity of these BKCa channels might influence cardiac electrical activity as well. Taken together, the BKCa channel activity residing in cardiac fibroblasts may contribute to cardiac electrical function occurring in vivo.

Progress on role of ion channels of cardiac fibroblasts in fibrosis

Cardiac fibrosis is defined as excessive deposition of extracellular matrix (ECM) in pathological conditions. Cardiac fibroblasts (CFs) activated by injury or inflammation differentiate into myofibroblasts (MFs) with secretory and contractile functions. In the fibrotic heart, MFs produce ECM which is composed mainly of collagen and is initially involved in maintaining tissue integrity. However, persistent fibrosis disrupts the coordination of excitatory contractile coupling, leading to systolic and diastolic dysfunction, and ultimately heart failure. Numerous studies have demonstrated that both voltage- and non-voltage-gated ion channels alter intracellular ion levels and cellular activity, contributing to myofibroblast proliferation, contraction, and secretory function. However, an effective treatment strategy for myocardial fibrosis has not been established. Therefore, this review describes the progress made in research related to transient receptor potential (TRP) channels, Piezo1, Ca²⁺ release-activated Ca²⁺ (CRAC) channels, voltage-gated Ca²⁺ channels (VGCCs), sodium channels, and potassium channels in myocardial fibroblasts with the aim of providing new ideas for treating myocardial fibrosis.

Heterogeneity and Remodeling of Ion Currents in Cultured Right Atrial Fibroblasts From Patients With Sinus Rhythm or Atrial Fibrillation

Cardiac fibroblasts express multiple voltage-dependent ion channels. Even though fibroblasts do not generate action potentials, they may influence cardiac electrophysiology by electrical coupling via gap junctions with cardiomyocytes, and through fibrosis. Here, we investigate the electrophysiological phenotype of cultured fibroblasts from right atrial appendage tissue of patients with sinus rhythm (SR) or atrial fibrillation (AF). Using the patch-clamp technique in whole-cell mode, we observed steady-state outward currents exhibiting either no rectification or inward and/or outward rectification. The distributions of current patterns between fibroblasts from SR and AF patients were not significantly different. In response to depolarizing voltage pulses, we measured transient outward currents with fast and slow activation kinetics, an outward background current, and an inward current with a potential-dependence resembling that of L-type Ca²⁺ channels. In cell-attached patch-clamp mode, large amplitude, paxilline-sensitive single channel openings were found in ≈65% of SR and ∼38% of AF fibroblasts, suggesting the presence of “big conductance Ca²⁺-activated K⁺ (BK_Ca)” channels. The open probability of BK_Ca was significantly lower in AF than in SR fibroblasts. When cultured in the presence of paxilline, the shape of fibroblasts became wider and less spindle-like. Our data confirm previous findings on cardiac fibroblast electrophysiology and extend them by illustrating differential channel expression in human atrial fibroblasts from SR and AF tissue.

Swelling-activated ClC-3 activity regulates prostaglandin E2 release in human OUMS-27 chondrocytes

Articular chondrocytes are exposed to dynamic osmotic environments during normal joint loading, and thus, require effective volume regulatory mechanisms. A regulatory volume decrease (RVD) is one of the mechanisms for protecting chondrocytes from swelling and damage. Swelling-activated Cl^- currents (I_Cl,swell) are responsible for the RVD, but the molecular identity in chondrocytes is largely unknown. In this study, we reveal that in human OUMS-27 chondrocytes, I_Cl,swell can be elicited by hypoosmotic stimulation (180 mOsm) and be inhibited by classical Cl^- channel blockers, 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) and niflumic acid, and be attenuated by siRNA knockdown of ClC-3. Our molecular analyses revealed that ClC-3A is expressed as a major splice variant in both human articular chondrocytes and OUMS-27 cells. The onset and early phase of RVD following hypoosmotic stress in OUMS-27 cells were affected by DIDS and ClC-3 knockdown. Hypoosmotic stimulation caused Ca²⁺ influx and subsequent release of prostaglandin E₂ (PGE₂) in OUMS-27 cells, and both of these responses were reduced by DIDS and ClC-3 knockdown. These results strongly suggest that ClC-3 is responsible for I_Cl,swell and RVD under the hypoosmotic environments. It is likely that ClC-3 is associated with the pathogenesis of cartilage degenerative diseases including osteoarthritis via PGE₂ release.

A Novel Calcium-Activated Potassium Channel Controls Membrane Potential and Intracellular pH in Trypanosoma cruzi

Trypanosoma cruzi develops in environments where nutrient availability, osmolarity, ionic concentrations, and pH undergo significant changes. The ability to adapt and respond to such conditions determines the survival and successful transmission of T. cruzi. Ion channels play fundamental roles in controlling physiological parameters that ensure cell homeostasis by rapidly triggering compensatory mechanisms. Combining molecular, cellular and electrophysiological approaches we have identified and characterized the expression and function of a novel calcium-activated potassium channel (TcCAKC). This channel resides in the plasma membrane of all 3 life stages of T. cruzi and shares structural features with other potassium channels. We expressed TcCAKC in Xenopus laevis oocytes and established its biophysical properties by two-electrode voltage clamp. Oocytes expressing TcCAKC showed a significant increase in inward currents after addition of calcium ionophore ionomycin or thapsigargin. These responses were abolished by EGTA suggesting that TcCAKC activation is dependent of extracellular calcium. This activation causes an increase in current and a negative shift in reversal potential that is blocked by barium. As predicted, a single point mutation in the selectivity filter (Y313A) completely abolished the activity of the channels, confirming its potassium selective nature. We have generated knockout parasites deleting one or both alleles of TcCAKC. These parasite strains showed impaired growth, decreased production of trypomastigotes and slower intracellular replication, pointing to an important role of TcCAKC in regulating infectivity. To understand the cellular mechanisms underlying these phenotypic defects, we used fluorescent probes to evaluate intracellular membrane potential, pH, and intracellular calcium. Epimastigotes lacking the channel had significantly lower cytosolic calcium, hyperpolarization, changes in intracellular pH, and increased rate of proton extrusion. These results are in agreement with previous reports indicating that, in trypanosomatids, membrane potential and intracellular pH maintenance are linked. Our work shows TcCAKC is a novel potassium channel that contributes to homeostatic regulation of important physiological processes in T. cruzi and provides new avenues to explore the potential of ion channels as targets for drug development against protozoan parasites.

BK channel overexpression on plasma membrane of fibroblasts from Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is an extremely rare genetic disorder wherein symptoms resembling aspects of aging are manifested at a very early age. It is a genetic condition that occurs due to a de novo mutation in the LMNA gene encoding for the nuclear structural protein lamin A. The lamin family of proteins are thought to be involved in nuclear stability, chromatin structure and gene expression and this leads to heavy effects on the regulation and functionality of the cell machinery. The functional role of the large-conductance calcium-activated potassium channels (BK_Ca) is still unclear, but has been recently described a strong relationship with their membrane expression, progerin nuclear levels and the ageing process. In this study, we found that: i) the outward potassium membrane current amplitude and the fluorescence intensity of the BK_Ca channel probe showed higher values in human dermal fibroblast obtained from patients affected by HGPS if compared to that from healthy young subjects; ii) this result appears to correlate with a basic cellular activity such as the replicative boost. We suggest that studying the HGPS also from the electrophysiological point of view might reveal new clues about the normal process of aging.

The TRPA1 Channel in the Cardiovascular System: Promising Features and Challenges

The transient receptor potential ankyrin 1 (TRPA1) channel is a calcium-permeable nonselective cation channel in the plasma membrane that belongs to the transient receptor potential (TRP) channel superfamily. Recent studies have suggested that the TRPA1 channel plays an essential role in the development and progression of several cardiovascular conditions, such as atherosclerosis, heart failure, myocardial ischemia-reperfusion injury, myocardial fibrosis, arrhythmia, vasodilation, and hypertension. Activation of the TRPA1 channel has a protective effect against the development of atherosclerosis. Furthermore, TRPA1 channel activation elicits peripheral vasodilation and induces a biphasic blood pressure response. However, loss of channel expression or blockade of its activation suppressed heart failure, myocardial ischemia-reperfusion injury, myocardial fibrosis, and arrhythmia. In this paper, we review recent research progress on the TRPA1 channel and discuss its potential role in the cardiovascular system.

Ca2+ Signaling in Cardiac Fibroblasts and Fibrosis-Associated Heart Diseases

Cardiac fibrosis is the excessive deposition of extracellular matrix proteins by cardiac fibroblasts and myofibroblasts, and is a hallmark feature of most heart diseases, including arrhythmia, hypertrophy, and heart failure. This maladaptive process occurs in response to a variety of stimuli, including myocardial injury, inflammation, and mechanical overload. There are multiple signaling pathways and various cell types that influence the fibrogenesis cascade. Fibroblasts and myofibroblasts are central effectors. Although it is clear that Ca2+ signaling plays a vital role in this pathological process, what contributes to Ca2+ signaling in fibroblasts and myofibroblasts is still not wholly understood, chiefly because of the large and diverse number of receptors, transporters, and ion channels that influence intracellular Ca2+ signaling. Intracellular Ca2+ signals are generated by Ca2+ release from intracellular Ca2+ stores and by Ca2+ entry through a multitude of Ca2+-permeable ion channels in the plasma membrane. Over the past decade, the transient receptor potential (TRP) channels have emerged as one of the most important families of ion channels mediating Ca2+ signaling in cardiac fibroblasts. TRP channels are a superfamily of non-voltage-gated, Ca2+-permeable non-selective cation channels. Their ability to respond to various stimulating cues makes TRP channels effective sensors of the many different pathophysiological events that stimulate cardiac fibrogenesis. This review focuses on the mechanisms of Ca2+ signaling in fibroblast differentiation and fibrosis-associated heart diseases and will highlight recent advances in the understanding of the roles that TRP and other Ca2+-permeable channels play in cardiac fibrosis.

Institution of localized high-frequency electrical stimulation targeting early myocardial infarction: Effects on left ventricle function and geometry

BACKGROUND

Although strategies have focused on myocardial salvage/regeneration in the context of an acute coronary syndrome and a myocardial infarction (MI), interventions targeting the formed MI region and altering the course of the post-MI remodeling process have not been as well studied. This study tested the hypothesis that localized high-frequency stimulation instituted within a formed MI region using low-amplitude electrical pulses would favorably change the trajectory of changes in left ventricle geometry and function.

METHODS

At 7 days following MI induction, pigs were randomized for localized high-frequency stimulation (n = 5, 240 bpm, 0.8 V, and 0.05 ms pulses) or unstimulated (n = 6). Left ventricle geometry and function were measured at baseline (pre-MI) and at 7, 14, 21, and 28 days post-MI using echocardiography. MI size at 28 days post-MI was determined by histochemical staining and planimetry.

RESULTS

At 7 days post-MI and before randomization to localized high-frequency stimulation, left ventricular ejection fraction and end-diastolic volume was equivalent. However, when compared with 7-day post-MI values, left ventricle end-diastolic volume increased in a time-dependent manner in the MI unstimulated group, but the relative increase in left ventricle end-diastolic volume was reduced in the MI localized high-frequency stimulation group. For example, by 28 days post-MI, left ventricle end-diastolic volume increased by 32% in the MI unstimulated group but only by 12% in the MI localized high-frequency stimulation group (P < .05). Whereas left ventricular ejection fraction appeared unchanged between MI groups, estimates of pulmonary capillary wedge pressure, a marker of adverse left ventricle performance and progression to failure, increased by 62% in the MI unstimulated group and actually decreased by 17% in the MI localized high-frequency stimulation group when compared with 7-day post-MI values (P < .05). MI size was equivalent between the MI groups, indicative of no difference in the extent of absolute myocardial injury.

CONCLUSIONS

The unique findings from this study are 2-fold. First, targeting the MI region following the resolution of the acute event using a localized stimulation approach is feasible. Second, localized stimulation modified a key parameter of adverse post-MI remodeling (dilation) and progression to heart failure. These findings demonstrate that the MI region itself is a modifiable tissue and responsive to localized electrical stimulation.

Effects of Nitric Oxide on Voltage-Gated K⁺ Currents in Human Cardiac Fibroblasts through the Protein Kinase G and Protein Kinase A Pathways but Not through S-Nitrosylation

This study investigated the expression of voltage-gated K⁺ (K_V) channels in human cardiac fibroblasts (HCFs), and the effect of nitric oxide (NO) on the K_V currents, and the underlying phosphorylation mechanisms. In reverse transcription polymerase chain reaction, two types of K_V channels were detected in HCFs: delayed rectifier K⁺ channel and transient outward K⁺ channel. In whole-cell patch-clamp technique, delayed rectifier K⁺ current (I_K) exhibited fast activation and slow inactivation, while transient outward K⁺ current (I_to) showed fast activation and inactivation kinetics. Both currents were blocked by 4-aminopyridine. An NO donor, S-nitroso-N-acetylpenicillamine (SNAP), increased the amplitude of I_K in a concentration-dependent manner with an EC₅₀ value of 26.4 µM, but did not affect I_to. The stimulating effect of SNAP on I_K was blocked by pretreatment with 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or by KT5823. 8-bromo-cyclic GMP stimulated the I_K. The stimulating effect of SNAP on I_K was also blocked by pretreatment with KT5720 or by SQ22536. Forskolin and 8-bromo-cyclic AMP each stimulated I_K. On the other hand, the stimulating effect of SNAP on I_K was not blocked by pretreatment of N-ethylmaleimide or by DL-dithiothreitol. Our data suggest that NO enhances I_K, but not I_to, among K_V currents of HCFs, and the stimulating effect of NO on I_K is through the PKG and PKA pathways, not through S-nitrosylation.

Cardiac fibroblasts : Active players in (atrial) electrophysiology?

Fibrotic areas in cardiac muscle-be it in ventricular or atrial tissue-are considered as obstacles for conduction of the excitatory wave and can therefore facilitate re-entry, which may contribute to the sustenance of cardiac arrhythmias. Persistence of one of the most frequent arrhythmias, atrial fibrillation (AF), is accompanied by enhanced atrial fibrosis. Any kind of myocardial perturbation, whether via mechanical stress or ischemic damage, inflammation, or irregular and high-frequency electrical activity, activates fibroblasts. This leads to the secretion of paracrine factors and extracellular matrix proteins, especially collagen, and to the differentiation of fibroblasts into myofibroblasts. Excessive collagen production is the hallmark of fibrosis and impairs regular impulse propagation. In addition, direct electrical coupling between cardiomyocytes and nonmyocytes, such as fibroblasts and macrophages, via gap junctions affects conduction. Although fibroblasts are not electrically excitable, they express functional ion channels, in particular K⁺ channels and mechanosensitive channels, some of which could be involved in tissue remodeling. Here, we briefly review these aspects with special reference to AF.

Functional BKCa channel in human resident cardiac stem cells expressing W8B2

Recently, a new population of resident cardiac stem cells (CSCs) positive for the W8B2 marker has been identified. These CSCs are considered to be an ideal cellular source to repair myocardial damage after infarction. However, the electrophysiological profile of these cells has not been characterized yet. We first establish the conditions of isolation and expansion of W8B2⁺ CSCs from human heart biopsies using a magnetic sorting system followed by flow cytometry cell sorting. These cells display a spindle-shaped morphology, are highly proliferative, and possess self-renewal capacity demonstrated by their ability to form colonies. Besides, W8B2⁺ CSCs are positive for mesenchymal markers but negative for hematopoietic and endothelial ones. RT-qPCR and immunostaining experiments show that W8B2⁺ CSCs express some early cardiac-specific transcription factors but lack the expression of cardiac-specific structural genes. Using patch clamp in the whole-cell configuration, we show for the first time the electrophysiological signature of BKCa current in these cells. Accordingly, RT-PCR and western blotting analysis confirmed the presence of BKCa at both mRNA and protein levels in W8B2⁺ CSCs. Interestingly, BKCa channel inhibition by paxilline decreased cell proliferation in a concentration-dependent manner and halted cell cycle progression at the G0/G1 phase. The inhibition of BKCa also decreased the self-renewal capacity but did not affect migration of W8B2⁺ CSCs. Taken together, our results are consistent with an important role of BKCa channels in cell cycle progression and self-renewal in human cardiac stem cells.

TGF-β1 (Transforming Growth Factor-β1) Plays a Pivotal Role in Cardiac Myofibroblast Arrhythmogenicity

BACKGROUND

TGF-β₁ (transforming growth factor-β₁) importantly contributes to cardiac fibrosis by controlling differentiation, migration, and collagen secretion of cardiac myofibroblasts. It is still elusive, however, to which extent TGF-β₁ alters the electrophysiological phenotype of myofibroblasts and cardiomyocytes and whether it affects proarrhythmic myofibroblast-cardiomyocyte crosstalk observed in vitro.

METHODS AND RESULTS

Patch-clamp recordings of cultured neonatal rat ventricular myofibroblasts revealed that TGF-β₁, applied for 24 to 48 hours at clinically relevant concentrations (≤2.5 ng/mL), causes substantial membrane depolarization concomitant with a several-fold increase of transmembrane currents. Transcriptome analysis revealed TGF-β₁-dependent changes in 29 of 63 ion channel/pump/connexin transcripts, indicating a pleiotropic effect on the electrical phenotype of myofibroblasts. Whereas not affecting cardiomyocyte membrane potentials and cardiomyocyte-cardiomyocyte gap junctional coupling, TGF-β₁ depolarized cardiomyocytes coupled to myofibroblasts by ≈20 mV and increased gap junctional coupling between myofibroblasts and cardiomyocytes >5-fold as reflected by elevated connexin 43 and consortin transcripts. TGF-β₁-dependent cardiomyocyte depolarization resulted from electrotonic crosstalk with myofibroblasts as demonstrated by immediate normalization of cardiomyocyte electrophysiology after targeted disruption of coupled myofibroblasts and by cessation of ectopic activity of cardiomyocytes coupled to myofibroblasts during pharmacological gap junctional uncoupling. In cardiac fibrosis models exhibiting slow conduction and ectopic activity, block of TGF-β₁ signaling completely abolished both arrhythmogenic conditions.

CONCLUSIONS

TGF-β₁ profoundly alters the electrophysiological phenotype of cardiac myofibroblasts. Apart from possibly contributing to the control of cell function in general, the changes proved to be pivotal for proarrhythmic myofibroblast-cardiomyocyte crosstalk in vitro, which suggests that TGF-β₁ may play a potentially important role in arrhythmogenesis of the fibrotic heart.

Altered physiological functions and ion currents in atrial fibroblasts from patients with chronic atrial fibrillation

The contribution of human atrial fibroblasts to cardiac physiology and pathophysiology is poorly understood. Fibroblasts may contribute to arrhythmogenesis through fibrosis, or by directly altering electrical activity in cardiomyocytes. The objective of our study was to uncover phenotypic differences between cells from patients in sinus rhythm (SR) and chronic atrial fibrillation (AF), with special emphasis on electrophysiological properties. We isolated fibroblasts from human right atrial tissue for patch-clamp experiments, proliferation, migration, and differentiation assays, and gene expression profiling. In culture, proliferation and migration of AF fibroblasts were strongly impaired but differentiation into myofibroblasts was increased. This was associated with a higher number of AF fibroblasts expressing functional Nav1.5 channels. Strikingly Na(+) currents were considerably larger in AF cells. Blocking Na(+) channels in culture with tetrodotoxin did not affect proliferation, migration, or differentiation in neither SR nor AF cells. While freshly isolated fibroblasts showed mostly weak rectifier currents, fibroblasts in culture developed outward rectifier K(+) currents of similar amplitude between the SR and AF groups. Adding the K(+) channel blockers tetraethylammonium and 4-aminopyridin in culture reduced current amplitude and inhibited proliferation in the SR group only. Analysis of gene expression revealed significant differences between SR and AF in genes encoding for ion channels, collagen, growth factors, connexins, and cadherins. In conclusion, this study shows that under AF conditions atrial fibroblasts undergo phenotypic changes that are revealed in culture. Future experiments should be performed in situ to understand the nature of those changes and whether they affect cardiac electrical activity.

Effects of BKCa and Kir2.1 Channels on Cell Cycling Progression and Migration in Human Cardiac c-kit+ Progenitor Cells

Our previous study demonstrated that a large-conductance Ca2+-activated K+ current (BKCa), a voltage-gated TTX-sensitive sodium current (INa.TTX), and an inward rectifier K+ current (IKir) were heterogeneously present in most of human cardiac c-kit+ progenitor cells. The present study was designed to investigate the effects of these ion channels on cell cycling progression and migration of human cardiac c-kit+ progenitor cells with approaches of cell proliferation and mobility assays, siRNA, RT-PCR, Western blots, flow cytometry analysis, etc. It was found that inhibition of BKCa with paxilline, but not INa.TTX with tetrodotoxin, decreased both cell proliferation and migration. Inhibition of IKir with Ba2+ had no effect on cell proliferation, while enhanced cell mobility. Silencing KCa.1.1 reduced cell proliferation by accumulating the cells at G0/G1 phase and decreased cell mobility. Interestingly, silencing Kir2.1 increased the cell migration without affecting cell cycling progression. These results demonstrate the novel information that blockade or silence of BKCa channels, but not INa.TTX channels, decreases cell cycling progression and mobility, whereas inhibition of Kir2.1 channels increases cell mobility without affecting cell cycling progression in human cardiac c-kit+ progenitor cells.

Disease and region-related cardiac fibroblast potassium current variations and potential functional significance

AIMS

Fibroblasts, which play an important role in cardiac function/dysfunction, including arrhythmogenesis, have voltage-dependent (Kv) currents of unknown importance. Here, we assessed the differential expression of Kv currents between atrial and ventricular fibroblasts from control dogs and dogs with an atrial arrhythmogenic substrate caused by congestive heart failure (CHF).

METHODS AND RESULTS

Left atrial (LA) and ventricular (LV) fibroblasts were freshly isolated from control and CHF dogs (2-week ventricular tachypacing, 240 bpm). Kv currents were measured with whole-cell voltage-clamp, mRNA by quantitative polymerase chain reaction (qPCR) and fibroblast proliferation by (3)H-thymidine incorporation. Robust voltage-dependent tetraethylammonium (TEA)-sensitive K(+) currents (IC50 ∼1 mM) were recorded. The morphologies and TEA responses of LA and LV fibroblast Kv currents were similar. LV fibroblast Kv-current densities were significantly greater than LA, and Kv-current densities were significantly less in CHF than control. The mRNA expression of Kv-channel subunits Kv1.5 and Kv4.3 was less in LA vs. LV fibroblasts and was down-regulated in CHF, consistent with K(+)-current recordings. Ca(2+)-dependent K(+)-channel subunit (KCa1.1) mRNA and currents were less expressed in LV vs. LA fibroblasts. Inhibiting LA fibroblast K(+) current with 1 mmol/L of TEA or KCa1.1 current with paxilline increased proliferation.

CONCLUSIONS

Fibroblast Kv-current expression is smaller in CHF vs. control, as well as LA vs. LV. KCa1.1 current is greater in LA vs. LV. Suppressing Kv current with TEA enhances fibroblast proliferation, suggesting that Kv current might act to check fibroblast proliferation and that reduced Kv current in CHF may contribute to fibrosis. Fibroblast Kv-current remodelling may play a role in the atrial fibrillation (AF) substrate; modulating fibroblast K(+) channels may present a novel strategy to prevent fibrosis and AF.

BKCa and hEag1 channels regulate cell proliferation and differentiation in human bone marrow-derived mesenchymal stem cells

Human bone marrow-derived mesenchymal stem cells (MSCs) serve as a reservoir for the continuous renewal of various mesenchymal tissues; however, cellular physiology of ion channels is not fully understood. The present study investigated potential roles of large-conductance Ca(2+) -activated potassium (BKCa ) channels and ether-à-go-go potassium (hEag1 or Kv10.1) channels in regulating cell proliferation and differentiation in human MSCs. We found that inhibition of BKCa with paxilline or hEag1 with astemizole, or knockdown of BKCa with shRNAs targeting KCa1.1 or hEag1 channels with shRNAs targeting KCNH1 arrested the cells at G0/G1 phase. In addition, silencing BKCa or hEag1 channels significantly reduced adipogenic differentiation with decrease of lipid accumulation and expression of the adipocyte marker PPARγ, and decreased osteogenic differentiation with reduction of mineral precipitation and osteocalcin. These effects were accompanied with a reduced cyclin D1, cyclin E, p-ERK1/2, and p-Akt. Our results demonstrate that BKCa and hEag1 channels not only regulate cell proliferation, but also participate in the adipogenic and osteogenic differentiations in human MSCs, which indicates that BKCa and hEag1 channels may be essential in maintaining bone marrow physiological function and bone regeneration.

Hydrogen sulphide suppresses human atrial fibroblast proliferation and transformation to myofibroblasts

Cardiac fibroblasts are crucial in pathophysiology of the myocardium whereby their aberrant proliferation has significant impact on cardiac function. Hydrogen sulphide (H₂S) is a gaseous modulator of potassium channels on cardiomyocytes and has been reported to attenuate cardiac fibrosis. Yet, the mechanism of H₂S in modulating proliferation of cardiac fibroblasts remains poorly understood. We hypothesized that H₂S inhibits proliferative response of atrial fibroblasts through modulation of potassium channels. Biophysical property of potassium channels in human atrial fibroblasts was examined by whole-cell patch clamp technique and their cellular proliferation in response to H₂S was assessed by BrdU assay. Large conductance Ca²⁺-activated K⁺ current (BK_Ca), transient outward K⁺ current (I_to) and inwardly rectifying K⁺ current (IK_ir) were found in human atrial fibroblasts. Current density of BK_Ca (IC₅₀ = 69.4 μM; n = 6), I_to (IC₅₀ = 55.1 μM; n = 6) and IK_ir (IC₅₀ = 78.9 μM; n = 6) was significantly decreased (P < 0.05) by acute exposure to NaHS (a H₂S donor) in atrial fibroblasts. Furthermore, NaHS (100–500 μM) inhibited fibroblast proliferation induced by transforming growth factor-β1 (TGF-β1; 1 ng/ml), Ang II (100 nM) or 20% FBS. Pre-conditioning of fibroblasts with NaHS decreased basal expression of Kv4.3 (encode I_to), but not KCa1.1 (encode BK_Ca) and Kir2.1 (encode IK_ir). Furthermore, H₂S significantly attenuated TGF-β1–stimulated Kv4.3 and α-smooth muscle actin expression, which coincided with its inhibition of TGF-β–induced myofibroblast transformation. Our results show that H₂S attenuates atrial fibroblast proliferation via suppression of K⁺ channel activity and moderates their differentiation towards myofibroblasts.

Cell proliferation, potassium channels, polyamines and their interactions: a mini review

Polyamines, which are obligatory molecules involved in cell cycling and proliferation, are subject to a change in their free intracellular concentrations during the cell cycle. Potassium (K(+)) channels are also considered, but less well recognized, to be necessary for cell proliferation by either hyperpolarizing or depolarizing cells during the cell cycle. A block of polyamine synthesis as well as block or knockout of K(+) channels can halt cell proliferation. K(+) channels like BK (maxi calcium (Ca(2+))-activated K(+)), Kir (inward rectifier), M-type K(+)-and TASK (two-pore domain K(+)) channels or the delayed rectifier K(+) channels are modulated in their electrical properties by polyamines. Polyamines are most effective in blocking these channels when applied to the intracellular face of these channels except for TASK channels where they act only from the extracellular side. Quinidine, a general K(+) channel blocker, was found to reduce putrescine concentrations, to block the ornithine decarboxylase and halt cell proliferation. From these results, the question arises if there is an interaction between polyamines, K(+) channels and proliferation. It might be speculated that a decrease of intracellular polyamines allows more K(+) channels to be active, thus inducing hyperpolarization, while an increase of the polyamine concentration may block K(+) channel activity leading to depolarization of the membrane potential. On the other hand, a block or a deletion of K(+) channels may cause a decrease of the polyamine concentration in cells. More research is needed to test these hypotheses.

Angiotensin II upregulates K(Ca)3.1 channels and stimulates cell proliferation in rat cardiac fibroblasts

The proliferation of cardiac fibroblasts is implicated in the pathogenesis of myocardial remodeling and fibrosis. Intermediate-conductance calcium-activated K⁺ channels (K(Ca)3.1 channels) have important roles in cell proliferation. However, it is unknown whether angiotensin II (Ang II), a potent profibrotic molecule, would regulate K(Ca)3.1 channels in cardiac fibroblasts and participate in cell proliferation. In the present study, we investigated whether K(Ca)3.1 channels were regulated by Ang II, and how the channel activity mediated cell proliferation in cultured adult rat cardiac fibroblasts using electrophysiology and biochemical approaches. It was found that mRNA, protein, and current density of K(Ca)3.1 channels were greatly enhanced in cultured cardiac fibroblasts treated with 1 μM Ang II, and the effects were countered by the angiotensin type 1 receptor (AT₁R) blocker losartan, the p38-MAPK inhibitor SB203580, the ERK1/2 inhibitor PD98059, and the PI3K/Akt inhibitor LY294002. Ang II stimulated cell proliferation and the effect was antagonized by the K(Ca)3.1 blocker TRAM-34 and siRNA targeting K(Ca)3.1. In addition, Ang II-induced increase of K(Ca)3.1 expression was attenuated by transfection of activator protein-1 (AP-1) decoy oligodeoxynucleotides. These results demonstrate for the first time that Ang II stimulates cell proliferation mediated by upregulating K(Ca)3.1 channels via interacting with the AT₁R and activating AP-1 complex through ERK1/2, p38-MAPK and PI3K/Akt signaling pathways in cultured adult rat cardiac fibroblasts.

Adenosine-5'-triphosphate up-regulates proliferation of human cardiac fibroblasts

BACKGROUND AND PURPOSE

ATP is a potent signalling molecule that regulates biological activities including increasing or decreasing proliferation in different types of cells. The aim of the present study was to investigate how ATP regulates the proliferation of human cardiac fibroblasts.

EXPERIMENTAL APPROACH

Reverse transcription (RT)-PCR, Western blot analysis, cell proliferation and migration assays were employed to investigate the effects of ATP on human adult ventricular fibroblasts.

KEY RESULTS

ATP increased cell proliferation in a concentration-dependent manner. Similarly, the P2X receptor agonist α,β-methylene ATP and P2Y receptor agonist ATP-γS also up-regulated cell proliferation. The P2 receptor antagonists suramin and reactive blue-2 prevented the ATP-induced increase in proliferation and RT-PCR and Western blot analysis revealed that mRNAs of P2X(4/7) and P2Y(2) are abundant in cardiac fibroblasts. ATP increased phosphorylated PKB (Akt) and ERK1/2 levels; an effect antagonized by suramin, reactive blue-2, the PI3-kinase inhibitor, wortmannin, PKB inhibitor, API-2, and MAPK inhibitor, PD98059. These kinase inhibitors also prevented the ATP-induced increase in proliferation. In addition, ATP enhanced the progression of cells from the G0/G1 phase to the S phase by increasing the expression of proteins for cyclin D1 and cyclin E. Silencing the P2X(4/7) and P2Y(2) receptors with siRNA targeting the corresponding receptor diminished ATP-stimulated proliferation and migration of the cardiac fibroblasts.

CONCLUSION AND IMPLICATION

ATP activates P2X(4/7) and P2Y(2) receptors and up-regulates the proliferation of human cardiac fibroblasts by promoting cell cycling progression. It also increases the migration of these cells. These effects of ATP may be involved in cardiac remodelling of injured hearts.

Evidence for functional expression of TRPM7 channels in human atrial myocytes

Transient receptor potential melastatin-7 (TRPM7) channels have been recently reported in human atrial fibroblasts and are believed to mediate fibrogenesis in human atrial fibrillation. The present study investigates whether TRPM7 channels are expressed in human atrial myocytes using whole-cell patch voltage-clamp, RT-PCR and Western blotting analysis. It was found that a gradually activated TRPM7-like current was recorded with a K⁺- and Mg²⁺-free pipette solution in human atrial myocytes. The current was enhanced by removing extracellular Ca²⁺ and Mg²⁺, and the current increase could be inhibited by Ni²⁺ or Ba²⁺. The TRPM7-like current was potentiated by acidic pH and inhibited by La³⁺ and 2-aminoethoxydiphenyl borate. In addition, Ca²⁺-activated TRPM4-like current was recorded in human atrial myocytes with the addition of the Ca²⁺ ionophore A23187 in bath solution. RT-PCR and Western immunoblot analysis revealed that in addition to TRPM4, TRPM7 channel current, mRNA and protein expression were evident in human atrial myocytes. Interestingly, TRPM7 channel protein, but not TRPM4 channel protein, was significantly increased in human atrial specimens from the patients with atrial fibrillation. Our results demonstrate for the first time that functional TRPM7 channels are present in human atrial myocytes, and the channel expression is upregulated in the atria with atrial fibrillation.

Hydrogen peroxide enhanced Ca(2+)-activated BK currents and promoted cell injury in human dermal fibroblasts

AIMS

Recent studies have shown that dermal fibroblasts possess multiple types of voltage-dependent K(+) channels, and the activation of these channels induces apoptosis. In the present study, we aimed to investigate whether hydrogen peroxide (H(2)O(2)), an oxidative stress inducer, could modulate these channels or induce human dermal fibroblasts injury.

MAIN METHODS

The effects of H(2)O(2) on K(+) currents were studied using a whole-cell recording. Intracellular PKC levels were measured with a direct human PKC enzyme immunoassay kit. Cell viability was assessed using PI staining and apoptotic nuclei were detected with TdT-mediated digoxigenin-dUTP nick-end labelling assay (TUNEL) assay.

KEY FINDINGS

Treatment of cells with 100μM H(2)O(2) resulted in a partially reversible increase in non-inactivating outward K(+) currents and an alteration in the steady-state activation property of the channels. The H(2)O(2)-induced increase in K(+) currents was mimicked by a PKC activator, and was blocked by the PKC inhibitor or the large conductance Ca(2+)-activited K(+) (BK) channel blockers. The intracellular PKC levels were significantly enhanced by H(2)O(2) treatment in a concentration-dependent manner. After exposure to H(2)O(2), evaluation of fibroblasts survival rate and damaged cell number with TUNEL-positive nuclei revealed an increased cell injury. Blocking the K(+) channels with blockers significantly decreased the H(2)O(2)-induced human dermal fibroblasts injury.

SIGNIFICANCE

Our results revealed that H(2)O(2) could enhance BK currents by PKC pathway. Increased K(+) currents might be related to H(2)O(2)-induced human dermal fibroblasts injury. The results reported here contribute to our understanding of the mechanism underlying H(2)O(2)-induced human dermal fibroblasts injury.