Functional and molecular characterization of multiple K-Cl cotransporter isoforms in corneal epithelial cells

KCCs, NKCCs, and NCC: Potential targets for cardiovascular therapeutics? A comprehensive review of cell and region specific expression and function

Cardiovascular diseases, the leading life-threatening conditions, involve cardiac arrhythmia, coronary artery disease, myocardial infarction, heart failure, cardiomyopathy, and heart valve disease that are associated with the altered functioning of cation-chloride cotransporters. The decreased number of cation-chloride cotransporters leads to reduced reactivity to adrenergic stimulation. The KCC family is crucial for numerous physiological processes including cell proliferation and invasion, regulation of membrane trafficking, maintaining ionic and osmotic homeostasis, erythrocyte swelling, dendritic spine formation, maturation of postsynaptic GABAergic inhibition, and inhibitory/excitatory signaling in neural tracts. KCC2 maintains intracellular chlorine homeostasis and opposes β-adrenergic stimulation-induced Cl- influx to prevent arrhythmogenesis. KCC3-inactivated cardiac tissue shows increased vascular resistance, aortic distensibility, heart size and weight (i.e. hypertrophic cardiomyopathy). Due to KCC4's high affinity for K+, it plays a vital role in cardiac ischemia with increased extracellular K+. The NKCC and NCC families play a vital role in the regulation of saliva volume, establishing the potassium-rich endolymph in the cochlea, sodium uptake in astrocytes, inhibiting myogenic response in microcirculatory beds, regulation of smooth muscle tone in resistance vessels, and blood pressure. NKCC1 regulates chlorine homeostasis and knocking it out impairs cardiomyocyte depolarization and cardiac contractility as well as impairs depolarization and contractility of vascular smooth muscle rings in the aorta. The activation of NCC in vascular cells promotes the formation of the abdominal aortic aneurysm. This narrative review provides a deep insight into the structure and function of KCCs, NKCCs, and NCC in human physiology and cardiac pathobiology. Also, it provides cell-specific (21 cell types) and region-specific (6 regions) expression of KCC1, KCC2, KCC3, KCC4, NKCC1, NKCC2, and NCC in heart.

Ions, the Movement of Water and the Apoptotic Volume Decrease

The movement of water across the cell membrane is a natural biological process that occurs during growth, cell division, and cell death. Many cells are known to regulate changes in their cell volume through inherent compensatory regulatory mechanisms. Cells can sense an increase or decrease in their cell volume, and compensate through mechanisms known as a regulatory volume increase (RVI) or decrease (RVD) response, respectively. The transport of sodium, potassium along with other ions and osmolytes allows the movement of water in and out of the cell. These compensatory volume regulatory mechanisms maintain a cell at near constant volume. A hallmark of the physiological cell death process known as apoptosis is the loss of cell volume or cell shrinkage. This loss of cell volume is in stark contrast to what occurs during the accidental cell death process known as necrosis. During necrosis, cells swell or gain water, eventually resulting in cell lysis. Thus, whether a cell gains or loses water after injury is a defining feature of the specific mode of cell death. Cell shrinkage or the loss of cell volume during apoptosis has been termed apoptotic volume decrease or AVD. Over the years, this distinguishing feature of apoptosis has been largely ignored and thought to be a passive occurrence or simply a consequence of the cell death process. However, studies on AVD have defined an underlying movement of ions that result in not only the loss of cell volume, but also the activation and execution of the apoptotic process. This review explores the role ions play in controlling not only the movement of water, but the regulation of apoptosis. We will focus on what is known about specific ion channels and transporters identified to be involved in AVD, and how the movement of ions and water change the intracellular environment leading to stages of cell shrinkage and associated apoptotic characteristics. Finally, we will discuss these concepts as they apply to different cell types such as neurons, cardiomyocytes, and corneal epithelial cells.

Kinetic studies of K-Cl cotransport in cultured rat vascular smooth muscle cells

During aging, and development of atherosclerosis and cardiovascular disease (CVD), aortic vascular smooth muscle cells (VSMCs) transition from healthy contractile to diseased synthetic phenotypes. K-Cl cotransport (KCC) maintains cell volume and ion homeostasis in growth and differentiation, and hence is important for VSMC proliferation and migration. Therefore, KCC activity may play a role in the contractile-to-synthetic VSMC phenotypic transition. Early, medium, and late synthetic passage VSMCs were tested for specific cytoskeletal protein expression. KCC-mediated ouabain- and bumetanide-insensitive Rb+ (a K+ congener) influx was determined as Cl--dependent Rb+ influx at different external Rb+ and Cl- ion concentrations, [Rb+]o and [Cl-]o. Expressions of the cytoskeletal proteins α-actin, vimentin, and desmin fell from early through late synthetic VSMCs. KCC kinetic parameters, such as maximum velocity ( Vm), and apparent Cl- and Rb+ affinities ( Km), were calculated with Lineweaver-Burk, Hanes-Woolf, and Hill approximations. Vm values of both Rb+- and Cl--dependent influxes were of equal magnitude, commensurate with a KCC stoichiometry of unity, and rose threefold from early to late synthetic VSMCs. Hill coefficients for Rb+ and Cl- correlated with cell passage number, suggesting increased KCC ligand cooperativity. However, Km values for [Cl-]o were strikingly bimodal with 60-80 mM in early, ~20-30 mM in medium, and 60 mM in late passage cells. In contrast, Km values for [Rb+]o remained steady at ~17 mM. Since total KCC isoform expression was similar with cell passage, structure/function changes of the KCC signalosome may accompany the transition of aortic VSMCs from a healthy to a diseased phenotype.

The Zn2+-sensing receptor, ZnR/GPR39, upregulates colonocytic Cl- absorption, via basolateral KCC1, and reduces fluid loss

Administration of zinc, as a complement to oral rehydration solutions, effectively diminishes duration and severity of diarrhea, but it is not known whether it merely fulfills a nutritional deficiency, or if zinc has a direct role of regulating solute absorption. We show that Zn2+ acts via a specific receptor, ZnR/GPR39, to reduce fluid loss. Intestinal fluid secretion triggered by cholera toxin (CTx) was lower in WT mice compared to ZnR/GPR39 KO. In the absence of dietary Zn2+ we observed similar fluid accumulation in WT and ZnR/GPR39 KO mice, indicating that Zn2+ and ZnR/GPR39 are both required for a beneficial effect of Zn2+ in diarrhea. In primary colonocytes and in Caco-2 colonocytic cells, activation of ZnR/GPR39 enhanced Cl- transport, a critical factor in diarrhea, by upregulating K+/Cl- cotransporter (KCC1) activity. Importantly, we show basolateral expression of KCC1 in mouse and human colonocytes, thus identifying a novel Cl- absorption pathway. Finally, inhibition of KCC-dependent Cl- transport enhanced CTx-induced fluid loss. Altogether, our data indicate that Zn2+ acting via ZnR/GPR39 has a direct role in controlling Cl- absorption via upregulation of basolateral KCC1 in the colon. Moreover, colonocytic ZnR/GPR39 and KCC1 reduce water loss during diarrhea and may therefore serve as effective drug targets.

Dependence of regulatory volume decrease on transient receptor potential vanilloid 4 (TRPV4) expression in human corneal epithelial cells

TRPV4 is a non-selective cation channel with moderate calcium permeability, which is activated by exposure to hypotonicity. Such a stress induces regulatory volume decrease (RVD) behavior in human corneal epithelial cells (HCEC). We hypothesize that TRPV4 channel mediates RVD in HCEC. Immunohistochemistry revealed centrally and superficially concentrated TRPV4 localization in the corneal tissue. Immunocytochemical and fluorescence activated cell sorter (FACS) analyses identified TRPV4 membrane surface and cytosolic expression. RT-PCR and Western blot analyses identified TRPV4 gene and protein expression in HCEC, respectively. In addition, 4alpha-PDD or a 50% hypotonic medium induced up to threefold transient intracellular Ca2+ ([Ca2+]i) increases. Following TRPV4 siRNA HCEC transfection, its protein expression level declined by 64%, which abrogated these [Ca2+]i transients. Similarly, exposure to either ruthenium red or Ca(2+)-free Ringer's solution also eliminated this response. In these transfected cells, RVD declined by 51% whereas in the non-transfected counterpart, ruthenium red and Ca(2+)-free solution inhibited RVD by 54 and 64%, respectively. In contrast, capsazepine, a TRPV1 antagonist, failed to suppress [Ca2+]i transients and RVD. TRPV4 activation contributes to RVD since declines in TRPV4 expression and activity are associated with suppression of this response. In conclusion, there is TRPV4 functional expression in HCEC.

TRPV4 mediates hypotonicity-induced ATP release by the thick ascending limb

Extracellular ATP is an autocrine/paracrine factor that regulates renal function. Transient receptor potential vanilloid (TRPV) 4 is a cation channel that mediates release of autocrine/paracrine factors by acting as an osmosensor. The renal medulla, and therefore the thick ascending limb, is exposed to osmotic stress. We hypothesize that reduced osmolality stimulates ATP release from the thick ascending limb via transient receptor potential vanilloid (TRPV) 4 activation. We measured ATP release by medullary thick ascending limb suspensions after reducing bath osmolality from 350 to 323 mosmol/kgH2O, using the luciferin-luciferase assay. Decreasing osmolality stimulated ATP release compared with control (38.9+/-7.2 vs. 2.4+/-1.0 pmol/mg protein; n=6, P<0.01). To examine the role of TRPV4, we used 1) Ca-free solutions, 2) a TRPV4 inhibitor, 3) small interfering (si) RNA against TRPV4, and 4) a TRPV4 activator. Removal of Ca completely blocked osmolality-induced ATP release (42.2+/-5.9 vs. 2.6+/-1.5 pmol/mg protein; n=6, P<0.01). In the presence of the TRPV4-selective inhibitor ruthenium red, osmolality-induced ATP release was blocked by 73% (56.4+/-19.9 vs. 8.8+/-2.3 pmol/mg protein; n=6; P<0.03). In vivo treatment of thick ascending limbs with siRNA against TRPV4 decreased osmolality-induced ATP release by 62% (31.5+/-3.4 vs. 12.4+/-1.1 pmol/mg protein; n=6; P<0.01), while reducing TRPV4 expression by 74% compared with the nontreated kidney. Treatment with scrambled siRNA did not affect TRPV4 expression and/or osmolality-induced ATP release. Finally, in the absence of changes in osmolality, the specific TRPV4 agonist 4alpha-PDD increased ATP release (3.6+/-0.9 vs. 25.4+/-7.4 pmol/mg protein; n=6; P<0.04). We concluded that decreases in osmolality stimulate ATP release by thick ascending limbs and this effect is mediated by TRPV4 activation.

Differential dependence of regulatory volume decrease behavior in rabbit corneal epithelial cells on MAPK superfamily activation

We characterized the dependence of hypotonicity-induced regulatory volume decrease (RVD) responses on mitogen-activated protein kinase (MAPK) pathway signaling in SV40-immortalized rabbit corneal epithelial cells (RCEC). Following calcein-AM loading, RVD was monitored using a microplate fluorescence reader. Western blot analysis determined MAPK activation. After 30 min, the RVD response restored the relative cell volume to nearly isotonic values, whereas it was inhibited when cells were bathed either in a Cl- -free solution or with the Cl- -channel inhibitors: 5-nitro-2-(3-phenylpropylamino)benzoic acid or niflumic acid. Similar declines occurred with either a high-K+ (20 mM) supplemented solution or the K+ channel inhibitor 4-aminopyridine. Activation of extracellular signal-regulated kinase (ERK), p38, and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) was time and tonicity-dependent. Stimulation of ERK and SAPK/JNK was maximized earlier than that of p38. Activation of ERK and SAPK/JNK was insensitive to Cl- and K+ channel inhibitors, whereas inhibition with either PD98059 or SP600125, respectively, blocked RVD. However, inhibition of p38 with SB203580had no effect on RVD. Suppression of RVD instead blocked p38 activation. Differences in the dependence of RVD activation on Erk1/2 and p38 signaling were validated in dominant negative (d/n)-Erk1 and d/n-p38 cells. Volume-sensitive Cl- and K+ channel activation contributes, in concert, to RVD in RCEC. Therefore, swelling-induced ERK and SAPK/JNK stimulation precedes Cl- and K+ channel activation, whereas p38 activation occurs as a consequence of RVD.