Serine/threonine protein phosphatases and regulation of K-Cl cotransport in human erythrocytes

The biogenesis of potassium transporters: implications of disease-associated mutations

The concentration of intracellular and extracellular potassium is tightly regulated due to the action of various ion transporters, channels, and pumps, which reside primarily in the kidney. Yet, potassium transporters and cotransporters play vital roles in all organs and cell types. Perhaps not surprisingly, defects in the biogenesis, function, and/or regulation of these proteins are linked to range of catastrophic human diseases, but to date, few drugs have been approved to treat these maladies. In this review, we discuss the structure, function, and activity of a group of potassium-chloride cotransporters, the KCCs, as well as the related sodium-potassium-chloride cotransporters, the NKCCs. Diseases associated with each of the four KCCs and two NKCCs are also discussed. Particular emphasis is placed on how these complex membrane proteins fold and mature in the endoplasmic reticulum, how non-native forms of the cotransporters are destroyed in the cell, and which cellular factors oversee their maturation and transport to the cell surface. When known, we also outline how the levels and activities of each cotransporter are regulated. Open questions in the field and avenues for future investigations are further outlined.

Oral Antiviral Defense: Saliva- and Beverage-like Hypotonicity Dynamically Regulate Formation of Membraneless Biomolecular Condensates of Antiviral Human MxA in Oral Epithelial Cells

The oral mucosa represents a defensive barrier between the external environment and the rest of the body. Oral mucosal cells are constantly bathed in hypotonic saliva (normally one-third tonicity compared to plasma) and are repeatedly exposed to environmental stresses of tonicity, temperature, and pH by the drinks we imbibe (e.g., hypotonic: water, tea, and coffee; hypertonic: assorted fruit juices, and red wines). In the mouth, the broad-spectrum antiviral mediator MxA (a dynamin-family large GTPase) is constitutively expressed in healthy periodontal tissues and induced by Type III interferons (e.g., IFN-λ1/IL-29). Endogenously induced human MxA and exogenously expressed human GFP-MxA formed membraneless biomolecular condensates in the cytoplasm of oral carcinoma cells (OECM1 cell line). These condensates likely represent storage granules in equilibrium with antivirally active dispersed MxA. Remarkably, cytoplasmic MxA condensates were exquisitely sensitive sensors of hypotonicity-the condensates in oral epithelium disassembled within 1-2 min of exposure of cells to saliva-like one-third hypotonicity, and spontaneously reassembled in the next 4-7 min. Water, tea, and coffee enhanced this disassembly. Fluorescence changes in OECM1 cells preloaded with calcein-AM (a reporter of cytosolic "macromolecular crowding") confirmed that this process involved macromolecular uncrowding and subsequent recrowding secondary to changes in cell volume. However, hypertonicity had little effect on MxA condensates. The spontaneous reassembly of GFP-MxA condensates in oral epithelial cells, even under continuous saliva-like hypotonicity, was slowed by the protein-phosphatase-inhibitor cyclosporin A (CsA) and by the K-channel-blocker tetraethylammonium chloride (TEA); this is suggestive of the involvement of the volume-sensitive WNK kinase-protein phosphatase (PTP)-K-Cl cotransporter (KCC) pathway in the regulated volume decrease (RVD) during condensate reassembly in oral cells. The present study identifies a novel subcellular consequence of hypotonic stress in oral epithelial cells, in terms of the rapid and dynamic changes in the structure of one class of phase-separated biomolecular condensates in the cytoplasm-the antiviral MxA condensates. More generally, the data raise the possibility that hypotonicity-driven stresses likely affect other intracellular functions involving liquid-liquid phase separation (LLPS) in cells of the oral mucosa.

Signaling mechanisms in red blood cells: A view through the protein phosphorylation and deformability

Intracellular signaling mechanisms in red blood cells (RBCs) involve various protein kinases and phosphatases and enable rapid adaptive responses to hypoxia, metabolic requirements, oxidative stress, or shear stress by regulating the physiological properties of the cell. Protein phosphorylation is a ubiquitous mechanism for intracellular signal transduction, volume regulation, and cytoskeletal organization in RBCs. Spectrin-based cytoskeleton connects integral membrane proteins, band 3 and glycophorin C to junctional proteins, ankyrin and Protein 4.1. Phosphorylation leads to a conformational change in the protein structure, weakening the interactions between proteins in the cytoskeletal network that confers a more flexible nature for the RBC membrane. The structural organization of the membrane and the cytoskeleton determines RBC deformability that allows cells to change their ability to deform under shear stress to pass through narrow capillaries. The shear stress sensing mechanisms and oxygenation-deoxygenation transitions regulate cell volume and mechanical properties of the membrane through the activation of ion transporters and specific phosphorylation events mediated by signal transduction. In this review, we summarize the roles of Protein kinase C, cAMP-Protein kinase A, cGMP-nitric oxide, RhoGTPase, and MAP/ERK pathways in the modulation of RBC deformability in both healthy and disease states. We emphasize that targeting signaling elements may be a therapeutic strategy for the treatment of hemoglobinopathies or channelopathies. We expect the present review will provide additional insights into RBC responses to shear stress and hypoxia via signaling mechanisms and shed light on the current and novel treatment options for pathophysiological conditions.

SLC12A cryo-EM: analysis of relevant ion binding sites, structural domains, and amino acids

The solute carrier family 12A (SLC12A) superfamily of membrane transporters modulates the movement of cations coupled with chloride across the membrane. In doing so, these cotransporters are involved in numerous aspects of human physiology: cell volume regulation, ion homeostasis, blood pressure regulation, and neurological action potential via intracellular chloride concentration modulation. Their physiological characterization has been largely studied; however, understanding the mechanics of their function and the relevance of structural domains or specific amino acids has been a pending task. In recent years, single-particle cryogenic electron microscopy (cryo-EM) has been successfully applied to members of the SLC12A family including all K+:Cl- cotransporters (KCCs), Na+:K+:2Cl- cotransporter NKCC1, and recently Na+:Cl- cotransporter (NCC); revealing structural elements that play key roles in their function. The present review analyzes the data provided by these cryo-EM reports focusing on structural domains and specific amino acids involved in ion binding, domain interactions, and other important SCL12A structural elements. A comparison of cryo-EM data from NKCC1 and KCCs is presented in the light of the two recent NCC cryo-EM studies, to propose insight into structural elements that might also be found in NCC and are necessary for its proper function. In the final sections, the importance of key coordination residues for substrate specificity and their implication on various pathophysiological conditions and genetic disorders is reviewed, as this could provide the basis to correlate structural elements with the development of novel and selective treatments, as well as mechanistic insight into the function and regulation of cation-coupled chloride cotransporters (CCCs).

Osmoregulation and the Hypothalamic Supraoptic Nucleus: From Genes to Functions

Due to the relatively high permeability to water of the plasma membrane, water tends to equilibrate its chemical potential gradient between the intra and extracellular compartments. Because of this, changes in osmolality of the extracellular fluid are accompanied by changes in the cell volume. Therefore, osmoregulatory mechanisms have evolved to keep the tonicity of the extracellular compartment within strict limits. This review focuses on the following aspects of osmoregulation: 1) the general problems in adjusting the "milieu interieur" to challenges imposed by water imbalance, with emphasis on conceptual aspects of osmosis and cell volume regulation; 2) osmosensation and the hypothalamic supraoptic nucleus (SON), starting with analysis of the electrophysiological responses of the magnocellular neurosecretory cells (MNCs) involved in the osmoreception phenomenon; 3) transcriptomic plasticity of SON during sustained hyperosmolality, to pinpoint the genes coding membrane channels and transporters already shown to participate in the osmosensation and new candidates that may have their role further investigated in this process, with emphasis on those expressed in the MNCs, discussing the relationships of hydration state, gene expression, and MNCs electrical activity; and 4) somatodendritic release of neuropeptides in relation to osmoregulation. Finally, we expect that by stressing the relationship between gene expression and the electrical activity of MNCs, studies about the newly discovered plastic-regulated genes that code channels and transporters in the SON may emerge.

Exercise decreases PP2A-specific reversible thiol oxidation in human erythrocytes: Implications for redox biomarkers

New readily accessible systemic redox biomarkers are needed to understand the biological roles reactive oxygen species (ROS) play in humans because overtly flawed, technically fraught, and unspecific assays severely hamper translational progress. The antibody-linked oxi-state assay (ALISA) makes it possible to develop valid ROS-sensitive target-specific protein thiol redox state biomarkers in a readily accessible microplate format. Here, we used a maximal exercise bout to disrupt redox homeostasis in a physiologically meaningful way to determine whether the catalytic core of the serine/threonine protein phosphatase PP2A is a candidate systemic redox biomarker in human erythrocytes. We reasoned that: constitutive oxidative stress (e.g., haemoglobin autoxidation) would sensitise erythrocytes to disrupted ion homeostasis as manifested by increased oxidation of the ion regulatory phosphatase PP2A. Unexpectedly, an acute bout of maximal exercise lasting ~16 min decreased PP2A-specific reversible thiol oxidation (redox ratio, rest: 0.46; exercise: 0.33) without changing PP2A content (rest: 193 pg/ml; exercise: 191 pg/ml). The need for only 3-4 μl of sample to perform ALISA means PP2A-specific reversible thiol oxidation is a capillary-fingertip blood-compatible candidate redox biomarker. Consistent with biologically meaningful redox regulation, thiol reductant-inducible PP2A activity was significantly greater (+10%) at rest compared to exercise. We establish a route to developing new readily measurable protein thiol redox biomarkers for understanding the biological roles ROS play in humans.

Does Plasma Inhibit the Activity of KCl Cotransport in Red Cells From LK Sheep?

Red cells from LK sheep represent an important paradigm for control of KCl cotransport activity, as well as being important to sheep erythroid function. A previous report (Godart et al., 1997) suggested that autologous plasma markedly inhibits red cell KCC activity and identified the presence of the bicarbonate/CO₂ buffer system as the probable cause. Findings were restricted, however, to red cells from patients with sickle cell disease (SCD) swollen anisotonically and carried out at a very high O₂ tension (c.700 mmHg). It was therefore important to investigate the generality of the effect described and whether it was also relevant to the two main stimuli for KCC activity encountered most often by circulating red cells in vivo - low pH in active muscle beds during exercise and high urea concentrations in the renal medulla during antidiuresis. Results confirm that inhibition was significant in response to anisotonic swelling with KCC activity in MOPS-buffered saline (MBS) vs. bicarbonate-buffered saline (BBS) and in MBS vs. plasma both reduced (by about 25 and 50%, respectively). By contrast, however, inhibition was absent at low pH and in high concentrations of urea. These findings suggest therefore that red cell KCC activity represents an important membrane permeability in vivo in red cells suspended in plasma. They are relevant, in particular, to sheep red cells, and may also be important by extension to those of other species and to the abnormal red cells found in human patients with SCD.

Genetic disruption of KCC cotransporters in a mouse model of thalassemia intermedia

β-thalassemia (β-Thal) is caused by defective β-globin production leading to globin chain imbalance, aggregation of free alpha chain in developing erythroblasts, reticulocytes, and mature circulating red blood cells. The hypochromic thalassemic red cells exhibit increased cell dehydration in association with elevated K+ leak and increased K-Cl cotransport activity, each of which has been linked to globin chain imbalance and related oxidative stress. We therefore tested the effect of genetic inactivation of K-Cl cotransporters KCC1 and KCC3 in a mouse model of β-thalassemia intermedia. In the absence of these transporters, the anemia of β-Thal mice was ameliorated, in association with increased MCV and reductions in CHCM and hyperdense cells, as well as in spleen size. The resting K+ content of β-Thal red cells was greatly increased, and Thal-associated splenomegaly slightly decreased. Lack of KCC1 and KCC3 activity in Thal red cells reduced red cell density and improved β-Thal-associated osmotic fragility. We conclude that genetic inactivation of K-Cl cotransport can reverse red cell dehydration and partially attenuate the hematologic phenotype in a mouse model of β-thalassemia.

The role of WNK in modulation of KCl cotransport activity in red cells from normal individuals and patients with sickle cell anaemia

Abnormal activity of red cell KCl cotransport (KCC) is involved in pathogenesis of sickle cell anaemia (SCA). KCC-mediated solute loss causes shrinkage, concentrates HbS, and promotes HbS polymerisation. Red cell KCC also responds to various stimuli including pH, volume, urea, and oxygen tension, and regulation involves protein phosphorylation. The main aim of this study was to investigate the role of the WNK/SPAK/OSR1 pathway in sickle cells. The pan WNK inhibitor WNK463 stimulated KCC with an EC₅₀ of 10.9 ± 1.1 nM and 7.9 ± 1.2 nM in sickle and normal red cells, respectively. SPAK/OSR1 inhibitors had little effect. The action of WNK463 was not additive with other kinase inhibitors (staurosporine and N-ethylmaleimide). Its effects were largely abrogated by pre-treatment with the phosphatase inhibitor calyculin A. WNK463 also reduced the effects of physiological KCC stimuli (pH, volume, urea) and abolished any response of KCC to changes in oxygen tension. Finally, although protein kinases have been implicated in regulation of phosphatidylserine exposure, WNK463 had no effect. Findings indicate a predominant role for WNKs in control of KCC in sickle cells but an apparent absence of downstream involvement of SPAK/OSR1. A more complete understanding of the mechanisms will inform pathogenesis whilst manipulation of WNK activity represents a potential therapeutic approach.

The signaling role for chloride in the bidirectional communication between neurons and astrocytes

It is well known that the electrical signaling in neuronal networks is modulated by chloride (Cl^-) fluxes via the inhibitory GABA_A and glycine receptors. Here, we discuss the putative contribution of Cl^- fluxes and intracellular Cl^- to other forms of information transfer in the CNS, namely the bidirectional communication between neurons and astrocytes. The manuscript (i) summarizes the generic functions of Cl^- in cellular physiology, (ii) recaps molecular identities and properties of Cl^- transporters and channels in neurons and astrocytes, and (iii) analyzes emerging studies implicating Cl^- in the modulation of neuroglial communication. The existing literature suggests that neurons can alter astrocytic Cl^- levels in a number of ways; via (a) the release of neurotransmitters and activation of glial transporters that have intrinsic Cl^- conductance, (b) the metabotropic receptor-driven changes in activity of the electroneutral cation-Cl^- cotransporter NKCC1, and (c) the transient, activity-dependent changes in glial cell volume which open the volume-regulated Cl^-/anion channel VRAC. Reciprocally, astrocytes are thought to alter neuronal [Cl^-]_i through either (a) VRAC-mediated release of the inhibitory gliotransmitters, GABA and taurine, which open neuronal GABA_A and glycine receptor/Cl^- channels, or (b) the gliotransmitter-driven stimulation of NKCC1. The most important recent developments in this area are the identification of the molecular composition and functional heterogeneity of brain VRAC channels, and the discovery of a new cytosolic [Cl^-] sensor - the Wnk family protein kinases. With new work in the field, our understanding of the role of Cl^- in information processing within the CNS is expected to be significantly updated.

Sickle cell dehydration: Pathophysiology and therapeutic applications

Cell dehydration is a distinguishing characteristic of sickle cell disease and an important contributor to disease pathophysiology. Due to the unique dependence of Hb S polymerization on cellular Hb S concentration, cell dehydration promotes polymerization and sickling. In double heterozygosis for Hb S and C (SC disease) dehydration is the determining factor in disease pathophysiology. Three major ion transport pathways are involved in sickle cell dehydration: the K-Cl cotransport (KCC), the Gardos channel (KCNN4) and Psickle, the polymerization induced membrane permeability, most likely mediated by the mechano-sensitive ion channel PIEZO1. Each of these pathways exhibit unique characteristics in regulation by oxygen tension, intracellular and extracellular environment, and functional expression in reticulocytes and mature red cells. The unique dependence of K-Cl cotransport on intracellular Mg and the abnormal reduction of erythrocyte Mg content in SS and SC cells had led to clinical studies assessing the effect of oral Mg supplementation. Inhibition of Gardos channel by clotrimazole and senicapoc has led to Phase 1,2,3 trials in patients with sickle cell disease. While none of these studies has resulted in the approval of a novel therapy for SS disease, they have highlighted the key role played by these pathways in disease pathophysiology.

Phosphoregulation of K+ -Cl- cotransporters during cell swelling: Novel insights

The K+ -Cl- cotransporters (KCCs) belong to the cation-Cl- cotransporter family and consist of four isoforms and many splice variants. Their main role is to promote electroneutral efflux of K+ and Cl- ions across the surface of many cell types and, thereby, to regulate intracellular ion concentration, cell volume, and epithelial salt movement. These transport systems are induced by an increase in cell volume and are less active at lower intracellular [Cl- ] (Cli ), but the mechanisms at play are still ill-defined. In this work, we have exploited the Xenopus laevis expression system to study the role of lysine-deficient protein kinases (WNKs), protein phosphatases 1 (PP1s), and SPS1-related proline/alanine-rich kinase (SPAK) in KCC4 regulation during cell swelling. We have found that WNK4 and PP1 regulate KCC4 activity as part of a common signaling module, but that they do not exert their effects through SPAK or carrier dephosphorylation. We have also found that the phosphatases at play include PP1α and PP1γ1, but that WNK4 acts directly on the PP1s instead of the opposite. Unexpectedly, however, both cell swelling and a T926A substitution in the C-terminus of full-length KCC4 led to higher levels of heterologous K+ -Cl- cotransport and overall carrier phosphorylation. These results imply that the response to cell swelling must also involve allosteric-sensitive kinase-dependent phosphoacceptor sites in KCC4. They are thus partially inconsistent with previous models of KCC regulation.

Protein phosphatase 2A regulatory subunit B56β modulates erythroid differentiation

Anemia due to attenuated erythroid terminal differentiation is one of the most common hematological disorders occurring at all stages of life. We previously demonstrated that catalytic subunit α of protein phosphatase 2A (PP2Acα) modulates fetal liver erythropoiesis. However the corresponding PP2A regulatory subunit in this process remains unknown. In this study, we report that chemical inhibition of PP2A activity with okadaic acid impairs hemin-induced erythroid differentiation. Interestingly, B56 family member B56β is the only regulatory subunit whose expression is induced by both erythropoietin in fetal liver cells and hemin in erythroleukemia K562 cells. Finally, knockdown of B56β attenuates hemin-induced K562 erythroid differentiation. Collectively, our data identify B56β as the potential functional regulatory subunit of PP2A in erythroid differentiation, shedding light on new target for precise modulation of PP2A activity for treatment of anemia and related diseases.

Kinase-KCC2 coupling: Cl- rheostasis, disease susceptibility, therapeutic target

The intracellular concentration of Cl(-) ([Cl(-)]i) in neurons is a highly regulated variable that is established and modulated by the finely tuned activity of the KCC2 cotransporter. Despite the importance of KCC2 for neurophysiology and its role in multiple neuropsychiatric diseases, our knowledge of the transporter's regulatory mechanisms is incomplete. Recent studies suggest that the phosphorylation state of KCC2 at specific residues in its cytoplasmic COOH terminus, such as Ser940 and Thr906/Thr1007, encodes discrete levels of transporter activity that elicit graded changes in neuronal Cl(-) extrusion to modulate the strength of synaptic inhibition via Cl(-)-permeable GABAA receptors. In this review, we propose that the functional and physical coupling of KCC2 to Cl(-)-sensitive kinase(s), such as the WNK1-SPAK kinase complex, constitutes a molecular "rheostat" that regulates [Cl(-)]i and thereby influences the functional plasticity of GABA. The rapid reversibility of (de)phosphorylation facilitates regulatory precision, and multisite phosphorylation allows for the control of KCC2 activity by different inputs via distinct or partially overlapping upstream signaling cascades that may become more or less important depending on the physiological context. While this adaptation mechanism is highly suited to maintaining homeostasis, its adjustable set points may render it vulnerable to perturbation and dysregulation. Finally, we suggest that pharmacological modulation of this kinase-KCC2 rheostat might be a particularly efficacious strategy to enhance Cl(-) extrusion and therapeutically restore GABA inhibition.

Activation of the erythroid K-Cl cotransporter Kcc1 enhances sickle cell disease pathology in a humanized mouse model

We used an N-ethyl-N-nitrosurea-based forward genetic screen in mice to identify new genes and alleles that regulate erythropoiesis. Here, we describe a mouse line expressing an activated form of the K-Cl cotransporter Slc12a4 (Kcc1), which results in a semi-dominant microcytosis of red cells. A missense mutation from methionine to lysine in the cytoplasmic tail of Kcc1 impairs phosphorylation of adjacent threonines required for inhibiting cotransporter activity. We bred Kcc1(M935K) mutant mice with a humanized mouse model of sickle cell disease to directly explore the relevance of the reported increase in KCC activity in disease pathogenesis. We show that a single mutant allele of Kcc1 induces widespread sickling and tissue damage, leading to premature death. This mouse model reveals important new insights into the regulation of K-Cl cotransporters and provides in vivo evidence that increased KCC activity worsened end-organ damage and diminished survival in sickle cell disease.

Identification of the WNK-SPAK/OSR1 signaling pathway in rodent and human lenses

PURPOSE

To identify whether the kinases that regulate the activity of cation chloride cotransporters (CCC) in other tissues are also expressed in rat and human lenses.

METHODS

The expression of with-no-lysine kinase (WNK 1, 3, 4), oxidative stress response kinase 1 (OSR1), and Ste20-like proline alanine rich kinase (SPAK) were determined at either the transcript or protein levels in the rat and human lenses by reverse-transcriptase PCR and/or Western blotting, respectively. Selected kinases were regionally and subcellularly characterized in rat and human lenses. The transparency, wet weight, and tissue morphology of lenses extracted from SPAK knock-out animals was compared with wild-type lenses.

RESULTS

WNK 1, 3, 4, SPAK, and OSR1 were identified at the transcript level in rat lenses and WNK1, 4, SPAK, and OSR1 expression confirmed at the protein level in both rat and human lenses. SPAK and OSR1 were found to associate with membranes as peripheral proteins and exhibited distinct subcellular and region-specific expression profiles throughout the lens. No significant difference in the wet weight of SPAK knock-out lenses was detected relative to wild-type lenses. However, SPAK knock-out lenses showed an increased susceptibility to opacification.

CONCLUSIONS

Our results show that the WNK 1, 3, 4, OSR1, and SPAK signaling system known to play a role in regulating the phosphorylation status, and hence activity of the CCCs in other tissues, is also present in the rat and human lenses. The increased susceptibility of SPAK lenses to opacification suggests that disruption of this signaling pathway may compromise the ability of the lens to control its volume, and its ability to maintain its transparency.

Calpain-1 knockout reveals broad effects on erythrocyte deformability and physiology

Pharmacological inhibitors of cysteine proteases have provided useful insights into the regulation of calpain activity in erythrocytes. However, the precise biological function of calpain activity in erythrocytes remains poorly understood. Erythrocytes express calpain-1, an isoform regulated by calpastatin, the endogenous inhibitor of calpains. In the present study, we investigated the function of calpain-1 in mature erythrocytes using our calpain-1-null [KO (knockout)] mouse model. The calpain-1 gene deletion results in improved erythrocyte deformability without any measurable effect on erythrocyte lifespan in vivo. The calcium-induced sphero-echinocyte shape transition is compromised in the KO erythrocytes. Erythrocyte membrane proteins ankyrin, band 3, protein 4.1R, adducin and dematin are degraded in the calcium-loaded normal erythrocytes but not in the KO erythrocytes. In contrast, the integrity of spectrin and its state of phosphorylation are not affected in the calcium-loaded erythrocytes of either genotype. To assess the functional consequences of attenuated cytoskeletal remodelling in the KO erythrocytes, the activity of major membrane transporters was measured. The activity of the K+-Cl- co-transporter and the Gardos channel was significantly reduced in the KO erythrocytes. Similarly, the basal activity of the calcium pump was reduced in the absence of calmodulin in the KO erythrocyte membrane. Interestingly, the calmodulin-stimulated calcium pump activity was significantly elevated in the KO erythrocytes, implying a wider range of pump regulation by calcium and calmodulin. Taken together, and with the atomic force microscopy of the skeletal network, the results of the present study provide the first evidence for the physiological function of calpain-1 in erythrocytes with therapeutic implications for calcium imbalance pathologies such as sickle cell disease.

Ablation of the Kell/Xk complex alters erythrocyte divalent cation homeostasis

XK is a putative transporter of unknown function that is ubiquitously expressed and linked through disulfide bonds to Kell protein, an endothelin-3 (ET-3)-converting enzyme. We generated three knockout (KO) mice that lacked either Xk, Kell or both proteins and characterized erythrocyte cation levels, transport and hematological parameters. Absence of Xk or Kell was accompanied by changes in erythrocyte K(+), Mg(2+), Na(+) and Ca(2+) transport that were associated with changes in mean cellular volume and corpuscular hemoglobin concentration mean. Baseline Ca(2+)-ATPase activity was undetected in erythrocytes from all three mouse types but was restored upon pre-incubation with ET-3. Consistent with these alterations in Ca(2+) handling, we observed increased Gardos channel activity in Kel and Xk KO mice. In addition Kel deletion was associated with increased Mg(2+) permeability while Xk deletion blocked Na/Mg exchanger activity. Our results provide evidence that cellular divalent cation regulation is functionally coupled to the Kell/XK system in erythrocytes and loss of this complex may contribute to acanthocytosis formation in McLeod syndrome.

Protein phosphatase 2A catalytic subunit α (PP2Acα) maintains survival of committed erythroid cells in fetal liver erythropoiesis through the STAT5 pathway

Suppression of programmed cell death is critical for the final maturation of red blood cells and depends largely on the anti-apoptotic effects of EpoR-STAT5-Bcl-x(L) signaling. As the major eukaryotic serine/threonine phosphatase, protein phosphatase 2A (PP2A) regulates multiple cellular processes, including apoptosis. However, whether PP2A plays a role in preventing erythroid cells from undergoing apoptosis remains to be elucidated. We conditionally inactivated the catalytic subunit α of PP2A (PP2Acα), which is the predominant form of PP2Ac, during early embryonic hematopoiesis. Loss of PP2Acα in hematopoietic cells perturbed definitive erythropoiesis characterized by fetal liver atrophy, reduced Ter119(+) cell number, abnormal expression patterns of molecular markers, less colony formation, and a reduction in definitive globin expression. Levels of erythropoiesis-promoting cytokines and initial seeding with hematopoietic progenitors remained unchanged in PP2Acα(TKO) fetal livers. We noted impaired expansion of the fetal erythroid compartment, which was associated with increased apoptosis of committed erythroid cells. Mechanistically, PP2Acα depletion markedly reduced Tyr(694) phosphorylation of STAT5 and expression of Bcl-x(L). Unexpectedly, PP2Acα-deficient embryos did not manifest any early embryonic vascular defects. Collectively, these data provide direct loss-of-function evidence demonstrating the importance of PP2Acα for the survival of committed erythroid cells during fetal liver erythropoiesis.

Altered phosphorylation of cytoskeleton proteins in sickle red blood cells: the role of protein kinase C, Rac GTPases, and reactive oxygen species

The small Rho GTPases Rac1 and Rac2 regulate actin structures and mediate reactive oxygen species (ROS) production via NADPH oxidase in a variety of cells. We have demonstrated that deficiency of Rac1 and Rac2 GTPases in mice disrupts the normal hexagonal organization of the RBC cytoskeleton and reduces erythrocyte deformability. This is associated with increased phosphorylation of adducin at Ser-724, (corresponding to Ser-726 in human erythrocytes), a domain target of protein kinase C (PKC). PKC phosphorylates adducin and leads to decreased F-actin capping and dissociation of spectrin from actin, implicating a significant role of such phosphorylation in cytoskeletal remodeling. We evaluated adducin phosphorylation in erythrocytes from patients with sickle cell disease and found it consistently increased at Ser-726. In addition, ROS concentration is elevated in sickle erythrocytes by 150-250% compared to erythrocytes from normal control individuals. Here, we review previous studies demonstrating that altered phosphorylation of erythrocyte cytoskeletal proteins and increased ROS production result in disruption of cytoskeleton stability in healthy and sickle cell erythrocytes. We discuss in particular the known and potential roles of protein kinase C and the Rac GTPases in these two processes.

Regulation of K-Cl cotransport in erythrocytes of frog Rana temporaria by commonly used protein kinase and protein phosphatase inhibitors

Recently (Agalakova and Gusev in J Comp Physiol 179:443-450, 2009), we demonstrated that the activity of K-Cl cotransport (KCC) in frog red blood cells is inhibited under stimulation of protein kinase C (PKC) with phorbol ester PMA (12-myristate-13-acetate). Present work was performed to uncover possible implication of protein kinases and protein phosphatases (PPs) in the regulation of baseline and volume-dependent KCC activity in these cells. K+ influx was estimated as 86Rb uptake by the cells in isotonic or hypotonic media in the presence of ouabain, K+ efflux was determined as the difference between K+ loss by the cells incubated in parallel in isotonic or hypotonic K(+)-free Cl(-)- and NO(3)(-)-media. Swelling of the cells in hypotonic medium was accompanied by approximately 50% activation of Cl-dependent K+ influx and efflux. Protein tyrosine kinase (PTK) inhibitor genistein (0.1 mM) stably and considerably (up to 89%) suppressed both baseline and volume-dependent KCC activity in each direction. Other PTK blockers (tyrphostin 23 and quercetin) had no influence on KCC activity in frog erythrocytes. PKC inhibitor chelerythrine (20 microM) and both PP inhibitors, fluoride (5 mM) and okadaic acid (1 microM), reduced KCC activity by 25-70%. Neither basal nor swelling-activated KCC in frog erythrocytes was affected by PKC inhibitor staurosporine (1 microM). Based on the previous and present results, we can suggest that the main role in the maintenance of basal and volume-dependent KCC activity in frog erythrocytes belongs to PTKs and PPs, whereas PKC is a negative regulator of this ion system.

Current knowledge about the functional roles of phosphorylative changes of membrane proteins in normal and diseased red cells

With the advent of proteomic techniques the number of known post-translational modifications (PTMs) affecting red cell membrane proteins is rapidly growing but the understanding of their role under physiological and pathological conditions is incompletely established. The wide range of hereditary diseases affecting different red cell membrane functions and the membrane modifications induced by malaria parasite intracellular growth represent a unique opportunity to study PTMs in response to variable cellular stresses. In the present review, some of the major areas of interest in red cell membrane research have been considered as modifications of erythrocyte deformability and maintenance of the surface area, membrane transport alterations, and removal of diseased and senescent red cells. In all mentioned research areas the functional roles of PTMs are prevalently restricted to the phosphorylative changes of the more abundant membrane proteins. The insufficient information about the PTMs occurring in a large majority of the red membrane proteins and the general lack of mass spectrometry data evidence the need of new comprehensive, proteomic approaches to improve the understanding of the red cell membrane physiology.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Adenosine receptors and the heart: role in regulation of coronary blood flow and cardiac electrophysiology

Adenosine is an autacoid that plays a critical role in regulating cardiac function, including heart rate, contractility, and coronary flow. In this chapter, current knowledge of the functions and mechanisms of action of coronary flow regulation and electrophysiology will be discussed. Currently, there are four known adenosine receptor (AR) subtypes, namely A(1), A(2A), A(2B), and A(3). All four subtypes are known to regulate coronary flow. In general, A(2A)AR is the predominant receptor subtype responsible for coronary blood flow regulation, which dilates coronary arteries in both an endothelial-dependent and -independent manner. The roles of other ARs and their mechanisms of action will also be discussed. The increasing popularity of gene-modified models with targeted deletion or overexpression of a single AR subtype has helped to elucidate the roles of each receptor subtype. Combining pharmacologic tools with targeted gene deletion of individual AR subtypes has proven invaluable for discriminating the vascular effects unique to the activation of each AR subtype. Adenosine exerts its cardiac electrophysiologic effects mainly through the activation of A(1)AR. This receptor mediates direct as well as indirect effects of adenosine (i.e., anti-beta-adrenergic effects). In supraventricular tissues (atrial myocytes, sinuatrial node and atriovetricular node), adenosine exerts both direct and indirect effects, while it exerts only indirect effects in the ventricle. Adenosine exerts a negative chronotropic effect by suppressing the automaticity of cardiac pacemakers, and a negative dromotropic effect through inhibition of AV-nodal conduction. These effects of adenosine constitute the rationale for its use as a diagnostic and therapeutic agent. In recent years, efforts have been made to develop A(1)R-selective agonists as drug candidates that do not induce vasodilation, which is considered an undesirable effect in the clinical setting.

Adenosine receptors and the kidney

The autacoid, adenosine, is present in the normoxic kidney and generated in the cytosol as well as at extracellular sites. The rate of adenosine formation is enhanced when the rate of ATP hydrolysis prevails over the rate of ATP synthesis during increased tubular transport work or during oxygen deficiency. Extracellular adenosine acts on adenosine receptor subtypes (A(1), A(2A), A(2B), and A(3)) in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate by constricting afferent arterioles, especially in superficial nephrons, and thus lowers the salt load and transport work of the kidney consistent with the concept of metabolic control of organ function. In contrast, it leads to vasodilation in the deep cortex and the semihypoxic medulla, and exerts differential effects on NaCl transport along the tubular and collecting duct system. These vascular and tubular effects point to a prominent role of adenosine and its receptors in the intrarenal metabolic regulation of kidney function, and, together with its role in inflammatory processes, form the basis for potential therapeutic approaches in radiocontrast media-induced acute renal failure, ischemia reperfusion injury, and in patients with cardiorenal failure.

K-Cl cotransport function and its potential contribution to cardiovascular disease

K-Cl cotransport is the coupled electroneutral movement of K and Cl ions carried out by at least four protein isoforms, KCC1-4. These transporters belong to the SLC12A family of coupled cotransporters and, due to their multiple functions, play an important role in the maintenance of cellular homeostasis. Significant information exists on the overall function of these transporters, but less is known about the role of the specific isoforms. Most functional studies were done on K-Cl cotransport fluxes without knowing the molecular details, and only recently attention has been paid to the isoforms and their individual contribution to the fluxes. This review summarizes briefly and updates the information on the overall functions of this transporter, and offers some ideas on its potential contribution to the pathophysiological basis of cardiovascular disease. By virtue of its properties and the cellular ionic distribution, K-Cl cotransport participates in volume regulation of the nucleated and some enucleated cells studied thus far. One of the hallmarks in cardiovascular disease is the inability of the organism to maintain water and electrolyte balance in effectors and/or target tissues. Oxidative stress is another compounding factor in cardiovascular disease and of great significance in our modern life styles. Several functions of the transporter are modulated by oxidative stress, which in turn may cause the transporter to operate in either "overdrive" with the purpose to counteract homeostatic changes, or not to respond at all, again setting the stage for pathological changes leading to cardiovascular disease. Intracellular Mg, a second messenger, acts as an inhibitor of K-Cl cotransport and plays a crucial role in regulating the activity of protein kinases and phosphatases, which, in turn, regulate a myriad of cellular functions. Although the role of Mg in cardiovascular disease has been dealt with for several decades, this chapter is evolving nowadays at a faster pace and the relationships between Mg, K-Cl cotransport, and cardiovascular disease is an area that awaits further experimentation. We envision that further studies on the role of K-Cl cotransport, and ideally on its specific isoforms, in mammalian cells will add missing links and help to understand the cellular mechanisms involved in the pathophysiology of cardiovascular disease.

Urea stimulation of KCl cotransport induces abnormal volume reduction in sickle reticulocytes

KCl cotransport (KCC) activity contributes to pathologic dehydration in sickle (SS) red blood cells (RBCs). KCC activation by urea was measured in SS and normal (AA) RBCs as Cl-dependent Rb influx. KCC-mediated volume reduction was assessed by measuring reticulocyte cellular hemoglobin concentration (CHC) cytometrically. Urea activated KCC fluxes in fresh RBCs to levels seen in swollen cells, although SS RBCs required lower urea concentrations than did normal (AA) RBCs. Little additional KCC stimulation by urea occurred in swollen AA or SS RBCs. The pH dependence of KCC in "euvolemic" SS RBCs treated with urea was similar to that in swollen cells. Urea triggered volume reduction in SS and AA reticulocytes, establishing a higher CHC. Volume reduction was Cl dependent and was limited by the KCC inhibitor, dihydro-indenyl-oxyalkanoic acid. Final CHC depended on urea concentration, but not on initial CHC. Under all activation conditions, volume reduction was exaggerated in SS reticulocytes and produced higher CHCs than in AA reticulocytes. The sulfhydryl-reducing agent, dithiothreitol, normalized the sensitivity of KCC activation to urea in SS RBCs and mitigated the urea-stimulated volume decrease in SS reticulocytes, suggesting that the dysfunctional activity of KCC in SS RBCs was due in part to reversible sulfhydryl oxidation.

Identification of key functional domains in the C terminus of the K+-Cl- cotransporters

The K+-Cl- cotransporter (KCC) isoforms constitute a functionally heterogeneous group of ion carriers. Emerging evidence suggests that the C terminus (Ct) of these proteins is important in conveying isoform-specific traits and that it may harbor interacting sites for 4beta-phorbol 12-myristate 13-acetate (PMA)-induced effectors. In this study, we have generated KCC2-KCC4 chimeras to identify key functional domains in the Ct of these carriers and single point mutations to determine whether canonical protein kinase C sites underlie KCC2-specific behaviors. Functional characterization of wild-type (wt) and mutant carriers in Xenopus laevis oocytes showed for the first time that the KCCs do not exhibit similar sensitivities to changes in osmolality and that this distinguishing feature as well as differences in transport activity under both hypotonic and isotonic conditions are in part determined by the residue composition of the distal Ct. At the same time, several mutations in this domain and in the proximal Ct of the KCCs were found to generate allosteric-like effects, suggesting that the regions analyzed are important in defining conformational ensembles and that isoform-specific structural configurations could thus account for variant functional traits as well. Characterization of the other mutants in this work showed that KCC2 is not inhibited by PMA through phosphorylation of its canonical protein kinase C sites. Intriguingly, however, the substitutions N728S and S940A were seen to alter the PMA effect paradoxically, suggesting again that allosteric changes in the Ct are important determinants of transport activity and, furthermore, that the structural configuration of this domain can convey specific functional traits by defining the accessibility of cotransporter sites to regulatory intermediates such as PMA-induced effectors.

Regulation of K-Cl cotransport by protein phosphatase 1alpha in mouse erythrocytes

The K-Cl cotransport (KCC) is an electroneutral-gradient-driven-membrane transport system, which is involved in regulation of red cell volume. Although the regulatory cascade of KCC is largely unknown, a signaling pathway involving phosphatases and kinases has been proposed. Here, we investigated the expression and the activity of protein phosphatase 1(PP-1) isoforms in mouse red cells, focusing on two models of abnormally activated KCC: mice genetically lacking the two Src-family tyrosine kinases, Hck and Fgr, (hck-/-fgr-/-) and the SAD transgenic sickle-cell-mice. The PP-1alpha, PP-1gamma, PP-1delta isoforms were expressed at similar levels in wild-type, hck-/-fgr-/- and SAD mouse erythrocytes and in each case were predominantly localized to cytoplasm. The PP-1alpha activity was significantly higher in both membrane and cytosol fractions of hck-/-fgr-/- and of SAD erythrocytes than in those of wild-type red cells, suggesting PP-1alpha as a target of the Hck and Fgr kinases. The PP2, a specific inhibitor of Src-family kinase, significantly increased KCC activity in wild-type mouse red cells, but failed to modify the already increased KCC activity in SAD erythrocytes. The lag-time for activation of KCC was considerably reduced in both hck-/-fgr-/- and SAD erythrocytes, suggesting that the rate limiting activation steps in both strains are freed from their tonic inhibition. Sulfhydryl reduction by dithiothreitol (DTT) lowered KCC activity only in SAD red cells, but did not affect the PP2-treated erythrocytes. These data suggest up-regulation of KCC in SAD red cells is mainly secondary to oxidative damage, which most likely reduces or removes the tonic KCC inhibition resulting from PP-1alpha activity controlled in turn by Src-family kinases.

NH2-terminal heterogeneity in the KCC3 K+-Cl− cotransporter

The SLC12A6 gene encoding the K+-Cl− cotransporter KCC3 is expressed in multiple tissues, including kidney. Here, we report the molecular characterization of several NH2-terminal isoforms of human and mouse KCC3, along with intrarenal localization and functional characterization in Xenopus laevis oocytes. Two major isoforms, KCC3a and KCC3b, are generated by transcriptional initiation 5′ of two distinct first coding exons. Northern blot analysis of mouse tissues indicates that KCC3b expression is particularly robust in the kidney, which also expresses KCC3a. Western blotting of mouse tissue using an exon 3-specific antibody reveals that the kidney is also unique in expressing immunoreactive protein of a lower mass, suggestive evidence that the shorter KCC3b protein predominates in kidney. Immunofluorescence reveals basolateral expression of KCC3 protein along the entire length of the proximal tubule, in both the mouse and rat. Removal of the 15-residue exon 2 by alternative splicing generates the KCC3a-x2M and KCC3b-x2M isoforms; other splicing events at an alternative acceptor site within exon 1a generate the KCC3a-S isoform, which is 60 residues shorter than KCC3a. This variation in sequence of NH2-terminal cytoplasmic domains occurs proximal to a stretch of highly conserved residues and affects the content of putative phosphorylation sites. Kinetic characterization of KCC3a in X. laevis oocytes reveals apparent Kms for Rb+ and Cl− of 10.7 ± 2.5 and 7.3 ± 1.2 mM, respectively, with an anion selectivity of Br− > Cl− > PO4 = I− = SCN− = gluconate. All five NH2-terminal isoforms are activated by cell swelling (hypotonic conditions), with no activity under isotonic conditions. Although the isoforms do not differ in the osmotic set point of swelling activation, this activation is more rapid for the KCC3a-x2M and KCC3a-S proteins. In summary, there is significant NH2-terminal heterogeneity of KCC3, with particularly robust expression of KCC3b in the kidney. Basolateral swelling-activated K+-Cl− cotransport mediated by KCC3 likely functions in cell volume regulation during the transepithelial transport of both salt and solutes by the proximal tubule.

Protein phosphatase 1alpha is tyrosine-phosphorylated and inactivated by peroxynitrite in erythrocytes through the src family kinase fgr

Protein serine/threonine phosphorylation is a significant component of the intracellular signal that together with tyrosine phosphorylation regulates several processes, including cell-cycle progression, muscle contraction, transcription, and neuronal signaling. Cross-talk between phosphoserine/threonine- and phosphotyrosine-mediated pathways is not yet well understood. In this study we found that peroxynitrite, a physiological oxidant formed by the fast radical-radical reaction between the nitric oxide and the superoxide anion, induced tyrosine phosphorylation of the serine/threonine protein phosphatase 1alpha (PP1alpha) in human erythrocytes through activation of src family kinases. We have previously shown in mouse red cells that upregulation of the src kinase fgr phosphorylates PP1alpha, acting as an upstream negative regulator of PP1alpha, and downregulates K-Cl cotransport. Here we found that PP1alpha is a selective substrate of peroxynitrite-activated fgr and that tyrosine phosphorylation of PP1alpha corresponds to an inhibition of its enzymatic activity. Despite fgr activation and PP1alpha downregulation, peroxynitrite stimulated in a dose-dependent fashion the function of the K-Cl cotransporter. In an attempt to understand the mechanism of K-Cl cotransport activation, we found that the effect of peroxynitrite is completely reversed by dithriothreitol, suggesting that peroxynitrite acts as an oxidizing agent by an SH-dependent and PP1alpha-independent mechanism. These findings highlight a novel function of peroxynitrite in regulating the intracellular signal transduction pathways involving serine/threonine phosphorylation and the functional role of proteins that are targets of these phosphatases.

Regulation of K-Cl cotransport: from function to genes

This review intends to summarize the vast literature on K-Cl cotransport (COT) regulation from a functional and genetic viewpoint. Special attention has been given to the signaling pathways involved in the transporter's regulation found in several tissues and cell types, and more specifically, in vascular smooth muscle cells (VSMCs). The number of publications on K-Cl COT has been steadily increasing since its discovery at the beginning of the 1980s, with red blood cells (RBCs) from different species (human, sheep, dog, rabbit, guinea pig, turkey, duck, frog, rat, mouse, fish, and lamprey) being the most studied model. Other tissues/cell types under study are brain, kidney, epithelia, muscle/smooth muscle, tumor cells, heart, liver, insect cells, endothelial cells, bone, platelets, thymocytes and Leishmania donovani. One of the salient properties of K-Cl-COT is its activation by cell swelling and its participation in the recovery of cell volume, a process known as regulatory volume decrease (RVD). Activation by thiol modification with N-ethylmaleimide (NEM) has spawned investigations on the redox dependence of K-Cl COT, and is used as a positive control for the operation of the system in many tissues and cells. The most accepted model of K-Cl COT regulation proposes protein kinases and phosphatases linked in a chain of phosphorylation/dephosphorylation events. More recent studies include regulatory pathways involving the phosphatidyl inositol/protein kinase C (PKC)-mediated pathway for regulation by lithium (Li) in low-K sheep red blood cells (LK SRBCs), and the nitric oxide (NO)/cGMP/protein kinase G (PKG) pathway as well as the platelet-derived growth factor (PDGF)-mediated mechanism in VSMCs. Studies on VSM transfected cells containing the PKG catalytic domain demonstrated the participation of this enzyme in K-Cl COT regulation. Commonly used vasodilators activate K-Cl COT in a dose-dependent manner through the NO/cGMP/PKG pathway. Interaction between the cotransporter and the cytoskeleton appears to depend on the cellular origin and experimental conditions. Pathophysiologically, K-Cl COT is altered in sickle cell anemia and neuropathies, and it has also been proposed to play a role in blood pressure control. Four closely related human genes code for KCCs (KCC1-4). Although considerable information is accumulating on tissue distribution, function and pathologies associated with the different isoforms, little is known about the genetic regulation of the KCC genes in terms of transcriptional and post-transcriptional regulation. A few reports indicate that the NO/cGMP/PKG signaling pathway regulates KCC1 and KCC3 mRNA expression in VSMCs at the post-transcriptional level. However, the detailed mechanisms of post-transcriptional regulation of KCC genes and of regulation of KCC2 and KCC4 mRNA expression are unknown. The K-Cl COT field is expected to expand further over the next decades, as new isoforms and/or regulatory pathways are discovered and its implication in health and disease is revealed.

Inhibitors of the K+(Na+)/H+ exchanger of human red blood cells

The effect of substances as possible inhibitors of the K+(Na+)/H+ exchanger in the human red cell membrane has been tested on the (ouabain+bumetanide+EGTA)-resistant K+ influx in both physiological (HIS) and low ionic strength (LIS) solution with tracer kinetic methods. It is demonstrated that high concentrations of quinacrine (1 mM) and chloroquine (2 mM) inhibit the residual K+ influx in LIS solution to 60% and 85%, respectively, but activate it in HIS solution. Thus, chloroquine suppressed the 10-fold LIS-induced activation of the flux nearly completely. Amiloride derivatives were able to inhibit the K+ influx in both HIS and LIS solution. EIPA (75 microM) reduced the flux by about 20% and 55% in HIS and LIS solution, respectively. Newly developed drugs (HOE 642, 1 mM; HOE 694, 0.5 mM) designed to inhibit Na+/H+ exchanger isoforms showed an inhibition of the residual K+ influx of 40% and 33% in HIS and 65% and 44% in LIS solution, respectively, without haemolysis. The inhibitory effect of HOE 642 persisted in HIS (24%) and LIS (48%) solutions when Cl- was replaced by CH3SO4-. The K(+)-Cl- cotransport inhibitor DIOA (100 microM) stimulated the residual K+ influx in both solutions. It is, therefore, concluded that the K(+)-Cl- cotransporter does not contribute to the residual K+ influx both in HIS and LIS media. Okadaic acid decreased the residual K+ influx by 40% and 25% in HIS and LIS solution, respectively, showing that the residual K+ influx is affected by phosphatases like other ion transport pathways. The results show that the residual K+ influx can be decreased further by inhibiting the K+(Na+)/H+ exchanger. It remains still unclear to what extent the K+(Na+)/H+ exchanger is inhibited by the different substances used. However, the ground state membrane permeability for K+ is much smaller than assumed so far.

Distinct properties of functional KCC2 expression in immature mouse hippocampal neurons in culture and in acute slices

A hallmark in the development of GABAergic neurotransmission is the switch in GABA(A)-mediated responses from depolarizing to hyperpolarizing. This occurs due to a gradual decrease in the intracellular concentration of chloride caused by the functional expression of the neuron-specific K-Cl cotransporter KCC2. Whether a mere increase in the amount of KCC2 protein is the rate-limiting step in vivo, or a further activation of the otherwise nonfunctional cotransporter is required, is not clear. Imposing a fixed Cl(-) load via patch pipette we measured the resultant somato-dendritic gradients in reversal potential of GABAergic currents to determine the time course of functional maturation of KCC2-mediated Cl(-) extrusion in two preparations: cultured mouse hippocampal neurons plated at embryonic day 17 and CA1 pyramidal cells in acute slices. We found that in immature neurons in both preparations the gradient is initially small or not detectable. It undergoes an abrupt increase at around days 13-14 in culture, while a more gradual increase occurs between postnatal days 5-14 in slices. Consistent with the presence of a nonfunctional form of KCC2 in immature hippocampal neurons grown in culture, application of the broad-spectrum kinase inhibitor staurosporine produces a rapid and potent up-regulation of KCC2 function in these cultured neurons, but not in neonatal slices. Taken together with our previously published data, these results indicate that the functional activity of KCC2 in vivo parallels the developmental expression of the protein, whereas cultured neurons require an additional activation step (mimicked by staurosporine) for KCC2 to become functional.

Ion transport pathology in the mechanism of sickle cell dehydration

Polymers of deoxyhemoglobin S deform sickle cell anemia red blood cells into sickle shapes, leading to the formation of dense, dehydrated red blood cells with a markedly shortened life-span. Nearly four decades of intense research in many laboratories has led to a mechanistic understanding of the complex events leading from sickling-induced permeabilization of the red cell membrane to small cations, to the generation of the heterogeneity of age and hydration condition of circulating sickle cells. This review follows chronologically the major experimental findings and the evolution of guiding ideas for research in this field. Predictions derived from mathematical models of red cell and reticulocyte homeostasis led to the formulation of an alternative to prevailing gradualist views: a multitrack dehydration model based on interactive influences between the red cell anion exchanger and two K(+) transporters, the Gardos channel (hSK4, hIK1) and the K-Cl cotransporter (KCC), with differential effects dependent on red cell age and variability of KCC expression among reticulocytes. The experimental tests of the model predictions and the amply supportive results are discussed. The review concludes with a brief survey of the therapeutic strategies aimed at preventing sickle cell dehydration and with an analysis of the main open questions in the field.

KCl cotransport mediates abnormal sulfhydryl-dependent volume regulation in sickle reticulocytes

KCl cotransport (KCC) activation by cell swelling and pH was compared in sickle (SS) and normal (AA) red blood cells (RBCs). KCC fluxes had the same relationship to mean corpuscular hemoglobin concentration (MCHC) in SS and AA RBCs when normalized to the maximal volume-stimulated (VSmax) flux (MCHC < 270 g/L [27 g/dL]). Acid-stimulated (pH 6.9) KCC flux in SS RBCs was 60% to 70% of VSmax KCC versus 20% in AA RBCs. Density gradients were used to track changes in reticulocyte MCHC during KCC-mediated regulatory volume decrease (RVD). Swelling to MCHC of 260 g/L (26 g/dL) produced Cl-dependent RVD that resulted in higher MCHC in SS than AA reticulocytes. In acid pH, RVD was also greater in SS than AA reticulocytes. Sulfhydryl reduction by dithiothreitol (DTT) lowered VSmax KCC flux in AA and SS RBCs by one third but did not alter swelling-induced RVD. DTT lowered acid-activated KCC in SS RBCs by 50% and diminished acid-induced RVD in SS reticulocytes. Thus, swelling activation of KCC is normal in SS RBCs but KCC-mediated RVD produces higher MCHC in SS than AA reticulocytes. Acid activation of KCC is exaggerated in SS RBCs and causes dehydration in SS reticulocytes. KCC response to acid stimulation was mitigated by DTT, suggesting that it arises from sulfhydryl oxidation.

Calcium handling in afferent arterioles

The cytosolic intracellular calcium concentration ([Ca(2+)](i)) is a major determining factor in the vascular smooth muscle tone. In the afferent arteriole it has been shown that agonists utilizing G-protein coupled receptors recruit Ca(2+) via release from intracellular stores and entry via pathways in the plasma membrane. The relative importances of entry vs. mobilization seem to differ between different agonists, species and preparations. The entry pathway might include different types of voltage sensitive Ca(2+) channels located in the plasmalemma such as dihydropyridine sensitive L-type channels, T-type channels and P/Q channels. A role for non-voltage sensitive entry pathways has also been suggested. The importance of voltage sensitive Ca(2+) channels in the control of the tone of the afferent arteriole (and thus in the control of renal function and whole body control of extracellular fluid volume and blood pressure) sheds light on the control of the membrane potential of afferent arteriolar smooth muscle cells. Thus, K(+) and Cl(-) channels are of importance in their role as major determinants of membrane potential. Some studies suggest a role for calcium-activated chloride (Cl(Ca)) channels in the renal vasoconstriction elicited by agonists. Other investigators have found evidence for several types of K(+) channels in the regulation of the afferent arteriolar tone. The available literature in this field regarding afferent arterioles is, however, relatively sparse and not conclusive. This review is an attempt to summarize the results obtained by others and ourselves in the field of agonist induced afferent arteriolar Ca(2+) recruitment, with special emphasis on the control of voltage sensitive Ca(2+) entry. Outline of the Manuscript: This manuscript is structured as follows: it begins with an introduction where the general role for [Ca(2+)](i) as a key factor in the regulation of the tone of vascular smooth muscles (VSMC) is detailed. In this section there is an emphasis is on observations that could be attributed to afferent arteriolar function. We then investigate the literature and describe our results regarding the relative roles for Ca(2+) entry and intracellular release in afferent arterioles in response to vasoactive agents, with the focus on noradrenalin (NA) and angiotensin II (Ang II). Finally, we examine the role of ion channels (i.e. K(+) and Cl(-) channels) for the membrane potential, and thus activation of voltage sensitive Ca(2+) channels.

Regulation of Na+/Mg2+ antiport in rat erythrocytes

In rat erythrocytes, the regulation of Na+/Mg2+ antiport by protein kinases (PKs), protein phosphatases (PPs), intracellular Mg2+, ATP and Cl- was investigated. In untreated erythrocytes, Na+/Mg2+ antiport was slightly inhibited by the PK inhibitor staurosporine, slightly stimulated by the PP inhibitor calyculin A and strongly stimulated by vanadate. PMA stimulated Na+/Mg2+ antiport. This effect was completely inhibited by staurosporine and partially inhibited by the PKC inhibitors Ro-31-8425 and BIM I. Participation of other PKs such as PKA, the MAPK cascade, PTK, CK I, CK II, CAM II-K, PI 3-K, and MLCK was excluded by use of inhibitors. Na+/Mg2+ antiport in rat erythrocytes can thus be stimulated by PKCalpha. In non-Mg2+ -loaded erythrocytes, ATP depletion reduced Mg2+ efflux and PMA stimulation in NaCl medium. A drastic activation of Na+/Mg2+ antiport was induced by Mg2+ loading which was not further stimulated by PMA. Staurosporine, Ro-31-8425, BIM I and calyculin A did not inhibit Na+/Mg2+ antiport of Mg2+ -loaded cells. Obviously, at high [Mg2+]i Na+/Mg2+ antiport is maximally stimulated. PKCalpha or PPs are not involved in stimulation by intracellular Mg2+. ATP depletion of Mg2+ -loaded erythrocytes reduced Mg2+ efflux and the affinity of Mg2+ binding sites of the Na+/Mg2+ antiporter to Mg2+. In non-Mg2+ -loaded erythrocytes Na+/Mg2+ antiport essentially depends on Cl-. Mg2+ -loaded erythrocytes were less sensitive to the activation of Na+/Mg2+ antiport by [Cl-]i.

Sickle cell disease: from membrane pathophysiology to novel therapies for prevention of erythrocyte dehydration

Sickle cell anemia is characterized by the presence of dense dehydrated erythrocytes that have lost most of their K content. Due to the unique dependence of Hb S polymerization on intracellular Hb S concentration, preventing this dehydration should markedly reduce polymerization. The erythrocyte intermediate conductance Ca-activated K channel (hSK4 or KCNN4), first described by Gardos, has been shown to be a major pathway for sickle cell dehydration. Studies with the imidazole antimycotic clotrimazole have shown reduction of sickle cell dehydration in vivo in a small number of patients with sickle cell disease; dose-limiting gastrointestinal and liver toxicities were observed. Based on the chemical structure of clotrimazole metabolites, a novel Gardos channel inhibitor, ICA-17043, has been developed. It has shown substantial activity both in vitro and in vivo in transgenic sickle mice. ICA-17043 is currently in phase 2 human trials. Another potential therapeutic target is the K-Cl cotransport. When sickle erythrocytes are exposed to relatively acidic conditions, they undergo cell shrinkage via activation of this pathway. K-Cl cotransport can be blocked by increasing the abnormally low erythrocyte Mg content of sickle erythrocytes. Oral Mg supplementation has been shown to reduce sickle cell dehydration in vivo in transgenic sickle mice and in patients in two separate clinical trials. Oral Mg pidolate is being tested in clinical trials in homozygous sickle cell disease and in Hb S/HbC (SC) disease, either as a single agent or in combination with hydroxyurea. The ongoing trials will determine the clinical effectiveness of therapies aimed at preventing sickle erythrocyte dehydration.

Adenosine Induces Vasoconstriction through Gi-Dependent Activation of Phospholipase C in Isolated Perfused Afferent Arterioles of Mice

ABSTRACT. Adenosine induces vasoconstriction of renal afferent arterioles through activation of A1 adenosine receptors (A1AR). A1AR are directly coupled to Gi/Go, resulting in inhibition of adenylate cyclase, but the contribution of this signaling pathway to smooth muscle cell activation is unclear. In perfused afferent arterioles from mouse kidney, adenosine and the A1 agonistN6-cyclohexyladenosine, when added to the bath, caused constriction in the concentration range of 10−9to 10−6M (mean diameter: control, 8.8 ± 0.3 μm; adenosine at 10−6M, 2.8 ± 0.5 μm). Adenosine-induced vasoconstriction was stable for up to 30 min and was most pronounced in the most distal part of the afferent arterioles. Adenosine did not cause vasoconstriction in arterioles from A1AR−/− mice. Pretreatment with pertussis toxin (PTX) (400 ng/ml) for 2 h blocked the vasoconstricting action of adenosine orN6-cyclohexyladenosine. PTX pretreatment did not affect the constriction response to KCl, whereas the angiotensin II dose-response relationship was shifted rightward. Reverse transcription-PCR revealed expression of Gi but not Go in kidney cortex and preglomerular vessels. The phospholipase C inhibitor U73122 (4 μM) blocked the constriction responses to both adenosine and angiotensin II. In contrast, the adenylate cyclase inhibitor SQ22536 (10 μM) and the protein kinase A antagonist KT5720 (0.1 and 1 μM) did not induce significant vasoconstriction of afferent arterioles. It is concluded that the constriction response to adenosine in afferent arterioles is mediated by A1AR coupled to a PTX-sensitive Gi protein and subsequent activation of phospholipase C, presumably through βγ subunits released from Gαi. E-mail: jurgens@intra.niddk.nih.gov

Regulation of K-Cl cotransport during reticulocyte maturation and erythrocyte aging in normal and sickle erythrocytes

The age/density-dependent decrease in K-Cl cotransport (KCC), PP1 and PP2A activities in normal and sickle human erythrocytes, and the effect of urea, a known KCC activator, were studied using discontinuous, isotonic gradients. In normal erythrocytes, the densest fraction (d approximately 33.4 g/dl) has only about approximately 5% of the KCC and 4% of the membrane (mb)-PP1 activities of the least-dense fraction (d approximately 24.7 g/dl). In sickle and normal erythrocytes, density-dependent decreases for mb-PP1 activity were similar (d50% 28.1 +/- 0.4 vs. 27.2 +/- 0.2 g/dl, respectively), whereas those for KCC activity were not (d50% 31.4 +/- 0.9 vs. 26.8 +/- 0.3 g/dl, respectively, P = 0.004). Excluding the 10% least-dense cells, a very tight correlation exists between KCC and mb-PP1 activities in normal (r2 = 0.995) and sickle erythrocytes (r2 = 0.93), but at comparable mb-PP1 activities, KCC activity is higher in sickle erythrocytes, suggesting a defective, mb-PP1-independent KCC regulation. In normal, least-dense but not in densest cells, urea stimulates KCC (two- to fourfold) and moderately increases mb-PP1 (20-40%). Thus mb-PP1 appears to mediate part of urea-stimulated KCC activity.

Regulation of K-Cl cotransport by Syk and Src protein tyrosine kinases in deoxygenated sickle cells

Protein tyrosine kinases (PTK) of the Src family are thought to suppress K-Cl cotransport (KCC) activity via negative regulation of protein phosphatases. However, some PTK inhibitors reduce KCC activity, suggesting opposite regulation by different PTK families. We have reported previously that deoxygenation of sickle cells stimulates KCC and activates Syk (a Syk family PTK), but not Lyn (an Src family PTK). In this study the same results were obtained when PTK activities were measured under the conditions used to measure KCC activity and which prevent any change in intracellular [Mg(2+)]. Methyl-2,5-dihydroxycinnamate (DHC), a PTK inhibitor, was more selective for Syk than Lyn, while staurosporine (ST), a broad-specificity protein kinase inhibitor, inhibited Lyn more than Syk. Deoxygenation or 4-amino-5-(4-chlorophenyl)-7-( t-butyl)pyrazolo[3,4- d] pyrimidine (pp2, a specific Src inhibitor) stimulated KCC independently. These effects were not additive and were inhibited by DHC. In contrast, ST-induced KCC activation was resistant to DHC, suggesting a different pathway of activation. Overall, these data indicate that Syk activity is required for KCC activation, either induced by deoxygenation of sickle cells, or mediated by Src inhibition in oxygenated cells, and that Syk and Src PTKs exert opposing and interconnected regulatory effects on the activity of the transporter.

The K-Cl cotransporter in the lobster stretch receptor neurone--a kinetic analysis

Experiments were performed to define quantitatively the substrate (K(+) and Cl(-)) dependence of the transport function (production of equally large and oppositely directed K(+)and Cl(-) flows/currents) of an earlier (Theander et al., 1999) identified electroneutral K-Cl cotransporter in the slowly adapting stretch receptor neurone of the European lobster. The experiments were based on microelectrode techniques. This allowed us to perform steady-state measurements of the so-called "instantaneous" current-voltage relationships (around a holding voltage of -65 mV after a blockage of the cell's action potential and hyperpolarization-activated currents) and intracellular ion concentrations at various settings of the extracellular K(+) and Cl(-) concentrations. From the results, we could then define steady-state values of all of the cell's non-KCl cotransporter K(+) and Cl(-) currents. Finally, the negative sums of the inferred non-KCl cotransporter K(+) and Cl(-) currents could be taken as equivalents of the K-Cl cotransporter's K(+) and Cl(-) currents for the reason that, in steady state, all membrane currents add up to zero. For the cotransporter currents, thus inferred for a range from 2.5/410.5 to 40.0/448.0 mM external K(+)/Cl(-), we found that their absolute values increased in a nonlinear fashion from about 5 nA cell(-1) at the lowest, to about 20 nA cell(-1) at the highest external K(+)/Cl(-) concentrations. Formally, this relationship could be reproduced by a Hill function-based enzyme kinetic expression simulating inward and outward transmembrane electroneutral ion transports. Following insertion of this expression into a comprehensive model of electrical membrane functions and intracellular solute and solvent control in the lobster stretch receptor neurone, the model predictions suggested that the K-Cl cotransporter does play an important role in (a) keeping intracellular Cl(-) low for a proper function of the cell's inhibitory system, and (b) enabling rapid transmembrane K(+) shifts that provide for a stabilization of the cell's membrane voltage and membrane excitability in cases of varying extracellular K(+) concentrations. The model predictions gave, however, no clear evidence that the K-Cl cotransporter is critically involved in the cell's volume regulation in conditions of varying extracellular osmolalities.

Human and murine phenotypes associated with defects in cation-chloride cotransport

The diuretic-sensitive cotransport of cations with chloride is mediated by the cation-chloride cotransporters, a large gene family encompassing a total of seven Na-Cl, Na-K-2Cl, and K-Cl cotransporters, in addition to two related transporters of unknown function. The cation-chloride cotransporters perform a wide variety of physiological roles and differ dramatically in patterns of tissue expression and cellular localization. The renal-specific Na-Cl cotransporter (NCC) and Na-K-2Cl cotransporter (NKCC2) are involved in Gitelman and Bartter syndrome, respectively, autosomal recessive forms of metabolic alkalosis. The associated phenotypes due to loss-of-function mutations in NCC and NKCC2 are consistent, in part, with their functional roles in the distal convoluted tubule and thick ascending limb, respectively. Other cation-chloride cotransporters are positional candidates for Mendelian human disorders, and the K-Cl cotransporter KCC3, in particular, may be involved in degenerative peripheral neuropathies linked to chromosome 15q14. The characterization of mice with both spontaneous and targeted mutations of several cation-chloride cotransporters has also yielded significant insight into the physiological and pathophysiological roles of several members of the gene family. These studies implicate the Na-K-2Cl cotransporter NKCC1 in hearing, salivation, pain perception, spermatogenesis, and the control of extracellular fluid volume. Targeted deletion of the neuronal-specific K-Cl cotransporter KCC2 generates mice with a profound seizure disorder and confirms the central role of this transporter in modulating neuronal excitability. Finally, the comparison of human and murine phenotypes associated with loss-of-function mutations in cation-chloride cotransporters indicates important differences in physiology of the two species and provides an important opportunity for detailed physiological and morphological analysis of the tissues involved.

Time-dependent autoregulation of renal blood flow in conscious rats

Response of renal vasculature to changes in renal perfusion pressure (RPP) involves mechanisms with different frequency characteristics. Autoregulation of renal blood flow is mediated by a rapid myogenic response and a slower tubuloglomerular feedback mechanism. In 25 male conscious rats, ramp-shaped changes in RPP were induced to quantify dynamic properties of autoregulation. Decremental RPP ramps immediately followed by incremental ramps were made for four different rates of change, ranging from 0.118 to 1.056 mmHg/s. Renal blood flow and cortical and medullary fluxes were assessed, and the corresponding relative conductance values were calculated continuously. During RPP decrements, conductance increased. With increasing rate of change of RPP decrements, maximum conductance increased from 10% to 80%, as compared with control. This response, which indicates the magnitude of autoregulation, was more pronounced in cortical versus medullary vasculature. Pressure at maximum conductance decreased with increasing rate of change of RPP decrements from 88 to 72 mmHg. During RPP increments, dependence of maximum conductance changes on the rate of change was enhanced (-20 to 110% of control). Thus, a hysteresis-like asymmetry between RPP decrements and increments, a resetting of autoregulation, was observed, which in direction and magnitude depended on the rate of change and duration of RPP changes. In conclusion, renal vascular responses to changes in RPP are highly dependent on the dynamics of the error signal. Furthermore, the method presented allows differentiated stimulation of various static and dynamic components of pressure-flow relationship and, thus, a direct assessment of the magnitudes and operating pressure range of active mechanisms of pressure-flow relationships.