An inwardly rectifying K+ channel in bovine parotid acinar cells: possible involvement of Kir2.1

ATP-sensitive inward rectifier potassium channels regulate secretion of pro-feeding salivary proteins in the lone star tick (Amblyomma americanum)

Understanding the physiological and molecular regulation of tick feeding is necessary for developing intervention strategies to curb disease transmission by ticks. Pharmacological activation of ATP-gated inward rectifier potassium (KATP) channels reduced fluid secretion from isolated salivary gland and blood feeding in the lone star tick, Amblyomma americanum, yet the temporal expression pattern of KATP channel proteins remained unknown. KATP channels were highly expressed in type II and III acini in off-host stage and early feeding phase ticks, yet expression was reduced in later stages of feeding. We next assessed KATP channel regulation of the secreted proteome of tick saliva. LC-MS/MS analysis identified 40 differentially secreted tick saliva proteins after exposure to KATP activators or inhibitors. Secretion of previously validated tick saliva proteins that promote tick feeding, AV422, AAS27, and AAS41 were significantly reduced by upwards of 8 log units in ticks exposed to KATP channel activators when compared to untreated ticks. Importantly, activation of KATP channels inhibited tick feeding and vice versa for KATP channel inhibitors. Data indicate KATP channels regulate tick feeding biology by controlling secretion of pro-feeding proteins that are essential during early feeding phases, which provides insights into physiological and molecular regulation of tick feeding behavior.

Investigation of novel de novo KCNC2 variants causing severe developmental and early-onset epileptic encephalopathy

Purpose The voltage-gated potassium channel K_v3.2, encoded by KCNC2, facilitates fast-spiking GABAergic interneurons to fire action potentials at high frequencies. It is pivotal to maintaining excitation/inhibition balance in mammalian brains. This study identified two novel de novo KCNC2 variants, p.Pro470Ser (P470S) and p.Phe382Leu (F382L), in patients with early onset developmental and epileptic encephalopathy (DEE). Methods To examine the molecular basis of DEE, we studied the functional characteristics of variant channels using patch-clamp techniques and computational modeling. Results Whole-cell patch clamp recordings from infected HEK293 cells revealed that channel activation and deactivation kinetics strongly decreased in both K_v3.2 P470S and F382L variant channels. This decrease also occurred in K_v3.2 p.Val471Leu (V471L) channels, known to be associated with DEE. In addition, K_v3.2 F382L and V471L variants exhibited a significant increase in channel conductance and a ∼20 mV negative shift in the threshold for voltage-dependent activation. Simulations of model GABAergic interneurons revealed that all variants decreased neuronal firing frequency. Thus, the variants' net loss-of-function effects disinhibited neural networks. Conclusion Our findings provide compelling evidence supporting the role of KCNC2 as a disease-causing gene in human neurodevelopmental delay and epilepsy.

Inward rectifier potassium (Kir) channels mediate salivary gland function and blood feeding in the lone star tick, Amblyomma americanum

Background

Tick feeding causes extreme morbidity and mortality to humans through transmission of pathogens and causes severe economic losses to the agricultural industry by reducing livestock yield. Salivary gland secretions are essential for tick feeding and thus, reducing or preventing saliva secretions into the vertebrate host is likely to reduce feeding and hinder pathogen life cycles. Unfortunately, the membrane physiology of tick salivary glands is underexplored and this gap in knowledge limits the development of novel therapeutics for inducing cessation of tick feeding.

Methodology

We studied the influence of inward rectifier potassium (Kir) channel subtypes to the functional capacity of the isolated tick salivary gland through the use of a modified Ramsay assay. The secreted saliva was subsequently used for quantification of the elemental composition of the secreted saliva after the glands were exposed to K⁺ channel modulators as a measure of osmoregulatory capacity. Lastly, changes to blood feeding behavior and mortality were measured with the use of a membrane feeding system.

Principal findings

In this study, we characterized the fundamental role of Kir channel subtypes in tick salivary gland function and provide evidence that pharmacological inhibition of these ion channels reduces the secretory activity of the Amblyomma americanum salivary gland. The reduced secretory capacity of the salivary gland was directly correlated with a dramatic reduction of blood ingestion during feeding. Further, exposure to small-molecule modulators of Kir channel subtypes induced mortality to ticks that is likely resultant from an altered osmoregulatory capacity.

Conclusions

Our data contribute to understanding of tick salivary gland function and could guide future campaigns aiming to develop chemical or reverse vaccinology technologies to reduce the worldwide burden of tick feeding and tick-vectored pathogens.

Tick feeding results in negative health and economic consequences worldwide and there has been continued interest in the development of products with novel mechanisms of action for control of tick populations. Kir channels have been shown to be a significant ion conductance pathway in arthropods and are critical for proper functioning of multiple biological processes. Previous work on insect Kir channels has focused on their physiological roles in renal system of mosquitoes and the data suggest that these channels represent a viable pathway to induce renal failure that leads to mortality. Based on the functional and cellular similarities of arthropod salivary glands and Malpighian tubules, we hypothesized that Kir channels constitute a critical conductance pathway within arthropod salivary glands and inhibition of this pathway will preclude feeding. Data presented in this study show that pharmacological modulators of Kir channels elicited a significant reduction in the fluid and ion secretory activity of tick salivary glands that resulted in reduced feeding, altered osmoregulation, and lead to mortality. These data could guide the future development of novel acaricides, RNAi, or genetically modified ticks to mitigate health and economic damages resulting from their feeding. Further, these data indicate a conserved function of Kir channels within multiple tissues of taxonomically diverse organisms, such as ticks and humans.

BPA Directly Decreases GnRH Neuronal Activity via Noncanonical Pathway

Peripheral feedback of gonadal estrogen to the hypothalamus is critical for reproduction. Bisphenol A (BPA), an environmental pollutant with estrogenic actions, can disrupt this feedback and lead to infertility in both humans and animals. GnRH neurons are essential for reproduction, serving as an important link between brain, pituitary, and gonads. Because GnRH neurons express several receptors that bind estrogen, they are potential targets for endocrine disruptors. However, to date, direct effects of BPA on GnRH neurons have not been shown. This study investigated the effects of BPA on GnRH neuronal activity using an explant model in which large numbers of primary GnRH neurons are maintained and express many of the receptors found in vivo. Because oscillations in intracellular calcium have been shown to correlate with electrical activity in GnRH neurons, calcium imaging was used to assay the effects of BPA. Exposure to 50μM BPA significantly decreased GnRH calcium activity. Blockage of γ-aminobutyric acid ergic and glutamatergic input did not abrogate the inhibitory BPA effect, suggesting direct regulation of GnRH neurons by BPA. In addition to estrogen receptor-β, single-cell RT-PCR analysis confirmed that GnRH neurons express G protein-coupled receptor 30 (G protein-coupled estrogen receptor 1) and estrogen-related receptor-γ, all potential targets for BPA. Perturbation studies of the signaling pathway revealed that the BPA-mediated inhibition of GnRH neuronal activity occurred independent of estrogen receptors, GPER, or estrogen-related receptor-γ, via a noncanonical pathway. These results provide the first evidence of a direct effect of BPA on GnRH neurons.

Functional expression of a Kir2.1-like inwardly rectifying potassium channel in mouse mammary secretory cells

K(+) channels in mammary secretory (MS) cells are believed to play a role in transcellular electrolyte transport and thus determining ionic composition of the aqueous phase of milk. However, direct evidence for specific K(+) channel activity in native MS cells is lacking at the single-cell level. Here, we show for the first time that an inwardly rectifying K(+) (Kir) channel is functionally expressed in fully differentiated MS cells that were freshly isolated from the mammary gland of lactating mice. Using the standard whole cell patch-clamp technique, we found that mouse MS cells consistently displayed a K(+) current, whose electrophysiological properties are similar to those previously reported for Kir2.x channels, particularly Kir2.1: 1) current-voltage relationship with strong inward rectification, 2) slope conductance approximately proportional to the square root of external K(+) concentration, 3) voltage- and time-dependent and high-affinity block by external Ba(2+), and 4) voltage-dependent inhibition by external Cs(+). Accordingly, RT-PCR analysis revealed the gene expression of Kir2.1, but not Kir2.2, Kir2.3, and Kir2.4, in lactating mouse mammary gland, and immunohistochemical staining showed Kir2.1 protein expression in the secretory cells. Cell-attached patch recordings from MS cells revealed that a 31-pS K(+) channel with strong inward rectification was likely active at the resting membrane potential. Collectively, the present work demonstrates that a functional Kir2.1-like channel is expressed in lactating mouse MS cells. We propose that the channel might be involved, at least in part, in secretion and/or preservation of ionic components of milk stored into the lumen of these cells.

IRBIT reduces the apparent affinity for intracellular Mg²⁺ in inhibition of the electrogenic Na⁺-HCO₃⁻ cotransporter NBCe1-B

The electrogenic Na(+)-HCO(3)(-) cotransporter NBCe1-B can be regulated by intracellular Mg(2+) (Mg(2+)(i)). We previously reported that under whole-cell voltage-clamp conditions, bovine NBCe1-B (bNBCe1-B) currents heterologously expressed in mammalian cells are strongly inhibited by Mg(2+)(i), and the inhibition is likely mediated by electrostatic interaction and relieved by truncation of the cytosolic NBCe1-B specific N-terminal region. Intriguingly, NBCe1-B-like currents natively expressed in bovine parotid acinar (BPA) cells are much less sensitive to Mg(2+)(i) inhibition than bNBCe1-B currents. Here, we hypothesized that this apparent discrepancy may involve IRBIT, a previously identified NBCe1-B-interacting protein. RT-PCR, Western blot and immunofluorescence confocal microscopy revealed that IRBIT was not only expressed in the cytosol, but also colocalized with NBCe1-B in the region of plasma membranes of BPA cells. IRBIT was coimmunoprecipitated with NBCe1-B by an anti-NBCe1 antibody in bovine parotid cell lysate. Whole-cell patch-clamp experiments showed that coexpression of IRBIT lowered the Mg(2+)(i) sensitivity of bNBCe1-B currents stably expressed in HEK293 cells. Collectively, these results suggest that IRBIT may reduce the apparent affinity for Mg(2+)(i) in inhibition of NBCe1-B activity in mammalian cells.

Channels and transporters in salivary glands

According to the two-stage hypothesis, primary saliva, a NaCl-rich plasma-like isotonic fluid is secreted by salivary acinar cells and its ionic composition becomes modified in the duct system. The ducts secrete K(+) and HCO (3) (-) and reabsorb Na(+) and Cl(-) without any water movement, thus establishing a hypotonic final saliva. Salivary secretion depends on the coordinated action of several channels and transporters localized in the apical and basolateral membrane of acinar and duct cells. Early functional studies in perfused glands, followed by the molecular cloning of several transport proteins and the subsequent analysis of mutant mice, have greatly contributed to our understanding of salivary fluid and the electrolyte secretion process. With a few exceptions, most of the key channels and transporters involved in salivary secretion have now been identified and characterized. However, the picture that has emerged from all these studies is one of a complex molecular network characterized by redundancy for several transport proteins, compensatory mechanisms, and adaptive changes in health and disease. Current research is directed to the molecular interactions between the determinants and the ways in which they are regulated by extracellular signals and intracellular mediators. This review focuses on the functionally and molecularly best-characterized channels and transporters that are considered to be involved in transepithelial fluid and electrolyte transport in salivary glands.

The electrogenic Na+-HCO3- cotransporter NBCe1-B is regulated by intracellular Mg2+

NBCe1-B, a major splice variant of the electrogenic Na+--HCO3- cotransporter (NBCe1) fulfills basic cellular functions including regulation of intracellular pH and epithelial HCO3- secretion. However, its cellular regulatory mechanism still remains elusive. Here, we provide evidence for the first time that NBCe1-B activity can be controlled by intracellular Mg2+ (Mg2+(i)), the physiologically most abundant intracellular divalent cation. Using the whole-cell patch-clamp technique, we found that recombinant NBCe1-B currents expressed in HEK293 and NIH3T3 cells were inhibited voltage-independently by Mg2+(i) in a concentration-dependent manner (K(i) approximately 0.01 mM). The Mg2+(i) inhibition was partially relieved by truncation of the NBCe1-B specific N-terminal region (K(i) approximately 0.3 mM), and was also observed for native electrogenic Na+--HCO3- cotransporter current in bovine parotid acinar cells that endogenously express NBCe1-B (K(i) approximately 1 mM). These results suggest that Mg2+ may be a cytosolic factor that limits intrinsic cotransport activity of NBCe1-B in mammalian cells.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Low-affinity spermine block mediating outward currents through Kir2.1 and Kir2.2 inward rectifier potassium channels

The outward component of the strong inward rectifier K(+) current (I(Kir)) plays a pivotal role in polarizing the membranes of excitable and non-excitable cells and is regulated by voltage-dependent channel block by internal cations. Using the Kir2.1 channel, we previously showed that a small fraction of the conductance susceptible only to a low-affinity mode of block likely carries a large portion of the outward current. To further examine the relevance of the low-affinity block to outward I(Kir) and to explore its molecular mechanism, we studied the block of the Kir2.1 and Kir2.2 channels by spermine, which is the principal Kir2 channel blocker. Current-voltage relations of outward Kir2.2 currents showed a peak, a plateau and two peaks in the presence of 10, 1 and 0.1 microm spermine, respectively, which was explained by the presence of two conductances that differ in their susceptibility to spermine block. When the current-voltage relations showed one peak, like those of native I(Kir), outward Kir2.2 currents were mediated mostly by the conductance susceptible to the low-affinity block. They also flowed in a narrower range than the corresponding Kir2.1 currents, because of 3- to 4-fold greater susceptibility to the low-affinity block than in Kir2.1. Reducing external [K(+)] shifted the voltage dependences of both the high- and low-affinity block of Kir2.1 in parallel with the shift in the reversal potential, confirming the importance of the low-affinity block in mediating outward I(Kir). When Kir2.1 mutants known to have reduced sensitivity to internal blockers were examined, the D172N mutation in the transmembrane pore region made almost all of the conductance susceptible only to low-affinity block, while the E224G mutation in the cytoplasmic pore region reduced the sensitivity to low-affinity block without markedly altering that to the high-affinity block or the high/low conductance ratio. The effects of these mutations support the hypothesis that Kir2 channels exist in two states having different susceptibilities to internal cationic blockers.

Electrophysiological characterization of native Na+-HCO3- cotransporter current in bovine parotid acinar cells

Using patch-clamp and molecular biological techniques, we identified and characterized membrane currents most likely generated by an electrogenic Na+-HCO3- cotransporter (NBCe) in acutely dissociated bovine parotid acinar (BPA) cells. When BPA cells were dialysed with a N-methyl-D-glucamine (NMDG)-glutamate-rich pipette solution, switching a Na-glutamate-rich, nominally HCO3--free bath solution to the one containing 25 mM HCO3-, but not Cl-, elicited a whole-cell current with a linear current-voltage relation. The HCO3- evoked current was abolished by total replacement of extracellular Na+ (Na+o) with NMDG or by 0.5 mM 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS), and was only partially supported by Li+o, but not by K+o, Cs+o, and cholineo. The reversal potential shift of DIDS (0.5 mM)-sensitive current induced by a change of [Na+]o corresponded to an apparent coupling ratio of HCO3- to Na+ of 2. RT-PCR analysis showed the presence of transcripts of NBCe1-B, but not NBCe1-A in BPA cells. Electrophysiological and pharmacological properties of whole-cell currents recorded from HEK293 cells transfected with the NBCe1-B, which was cloned from BPA cells resembled those of the native currents. Non-invasive measurements of membrane potential changes in the cell-attached patch configuration indicated that an NBCe activity is present in intact unstimulated BPA cells bathed in a 25 mM HCO3--containing solution. Collectively, these results not only suggest that an NBCe is present, functional and may be mediated, at least in part, by NBCe1-B in BPA cells, but also provide the first electrophysiological characterization of transport properties of NBCe expressed in native exocrine glands.

Functional Consequences of Polyamine Synthesis Inhibition by l-α-Difluoromethylornithine (DFMO)

L-Alpha-difluoromethylornithine (DFMO) is a chemopreventive agent for colon cancer in clinical trials. Yet, the drug produces an across-frequency elevation of the hearing threshold, suggesting that DFMO may affect a common trait along the cochlear spiral. The mechanism for the ototoxic effects of DFMO remains uncertain. The cochlear duct is exclusively endowed with endocochlear potential (EP). EP is a requisite for normal sound transduction, as it provides the electromotive force that determines the magnitude of the receptor potential of hair cells. EP is generated by the high throughput of K(+) across cells of the stria vascularis, conferred partly by the activity of Kir4.1 channels. Here, we show that the ototoxicity of DFMO may be mediated by alteration of the inward rectification of Kir4.1 channels, resulting in a marked reduction in EP. These findings are surprising given that the present model for EP generation asserts that Kir4.1 confers the outflow of K(+) in the stria vascularis. We have proposed an alternative model. These findings should also enable the rational design of new pharmaceuticals devoid of the untoward effect of DFMO.

An inwardly rectifying potassium channel in apical membrane of Calu-3 cells

Patch clamp methods and reverse transcription-polymerase chain reaction (RT-PCR) were used to characterize an apical K+ channel in Calu-3 cells, a widely used model of human airway gland serous cells. In cell-attached and excised apical membrane patches, we found an inwardly rectifying K+ channel (Kir). The permeability ratio was PNa/PK = 0.058. In 30 patches with both cystic fibrosis transmembrane conductance regulator and Kir present, we observed 79 cystic fibrosis transmembrane conductance regulator and 58 Kir channels. The average chord conductance was 24.4 +/- 0.5 pS (n = 11), between 0 and -200 mV, and was 9.6 +/- 0.7 pS (n = 8), between 0 and 50 mV; these magnitudes and their ratio of approximately 2.5 are most similar to values for rectifying K+ channels of the Kir4.x subfamilies. We attempted to amplify transcripts for Kir4.1, Kir4.2, and Kir5.1; of these only Kir4.2 was present in Calu-3 lysates. The channel was only weakly activated by ATP and was relatively insensitive to internal pH. External Cs+ and Ba2+ blocked the channel with Kd values in the millimolar range. Quantitative modeling of Cl- secreting epithelia suggests that secretion rates will be highest and luminal K+ will rise to 16-28 mm if 11-25% of the total cellular K+ conductance is placed in the apical membrane (Cook, D. I., and Young, J. A. (1989) J. Membr. Biol. 110, 139-146). Thus, we hypothesize that the K+ channel described here optimizes the rate of secretion and is involved in K+ recycling for the recently proposed apical H+ -K+ -ATPase in Calu-3 cells.

Electrophysiological and molecular characterization of the inward rectifier in juxtaglomerular cells from rat kidney

Renin, the key element of the renin-angiotensin-aldosterone system, is mainly produced by and stored in the juxtaglomerular cells in the kidney. These cells are situated in the media of the afferent arteriole close to the vessel pole and can transform into smooth muscle cells and vice versa. In this study, the electrophysiological properties and the molecular identity of the K+ channels responsible for the resting membrane potential (approximately -60 mV) of the juxtaglomerular cells were examined. In order to increase the number of juxtaglomerular cells, afferent arterioles from NaCl-depleted rats were used, and > 90% of the afferent arterioles were renin positive at the distal end of the arteriole. Whole-cell and cell-attached single-channel patch-clamp experiments showed that juxtaglomerular cells are endowed with a strongly inwardly rectifying K+ channel (Kir). The channel was highly sensitive to inhibition by Ba2+ (inhibition constant 37 microM at 0 mV), but relatively insensitive to Cs+ and, with 142 mM K+ in the pipette, had a single-channel conductance of 31.5 pS. Immunocytochemical studies showed the presence of Kir2.1 but no signal for Kir2.2 in the media of the afferent arteriole. In PCR analyses using isolated juxtaglomerular cells, the mRNA for Kir2.1 and Kir2.2 was detected. Collectively, the results show that Kir2.1 is the dominant component of the channel. The current carried by these channels plays a decisive role in setting the membrane potential of juxtaglomerular cells.