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

Synthesis of amphiphilic seleninic acid derivatives with considerable activity against cellular membranes and certain pathogenic microbes

Selenium compounds play a major role in Biology, where they are often associated with pronounced antioxidant activity or toxicity. Whilst most selenium compounds are not necessarily hazardous, their often selective cytotoxicity is interesting from a biochemical and pharmaceutical perspective. We have synthesized a series of amphiphilic molecules which combine a hydrophilic seleninic acid head group - which at the same time serves as thiol-specific warhead - with a hydrophobic tail. These molecules possess a surface activity similar to the one of SDS, yet their biological activity seems to exceed by far the one of a simple surfactant (e.g. SDS) or seleninic acid (e.g. phenyl seleninic acid). Such compounds effectively haemolyse Red Blood Cells and exhibit pronounced activity against Saccharomyces cerevisiae. From a chemical perspective, the seleninic warheads are likely to attack crucial cysteine proteins of the cellular thiolstat.

N-ethylmaleimide activates a Cl(-)-independent component of K(+) flux in mouse erythrocytes

The K-Cl cotransporters (KCCs) of mouse erythrocytes exhibit higher basal activity than those of human erythrocytes, but are similarly activated by cell swelling, by hypertonic urea, and by staurosporine. However, the dramatic stimulation of human erythroid KCCs by N-ethylmaleimide (NEM) is obscured in mouse erythrocytes by a prominent NEM-stimulated K(+) efflux that lacks Cl(-)-dependence. The NEM-sensitivity of Cl(-)-independent K(+) efflux of mouse erythrocytes is lower than that of KCC. The genetically engineered absence of the K-Cl cotransporters KCC3 and KCC1 from mouse erythrocytes does not modify Cl(-)-independent K(+) efflux. Mouse erythrocytes genetically devoid of the Gardos channel KCNN4 show increased NEM-sensitivity of both Cl(-)-independent K(+) efflux and K-Cl cotransport. The increased NEM-sensitivity and stimulation magnitude of Cl(-)-independent K(+) efflux in mouse erythrocytes expressing transgenic hypersickling human hemoglobin SAD (HbSAD) are independent of the presence of KCC3 and KCC1, but absence of KCNN4 reduces the stimulatory effect of HbSAD. NEM-stimulated Cl(-)-independent K(+) efflux of mouse red cells is insensitive to ouabain and bumetanide, but partially inhibited by chloroquine, barium, and amiloride. The NEM-stimulated activity is modestly reduced at pH6.0 but not significantly altered at pH8.0, and is abolished at 0°C. Although the molecular identity of this little-studied K(+) efflux pathway of mouse erythrocytes remains unknown, its potential role in the pathophysiology of sickle red cell dehydration will be important for the extrapolation of studies in mouse models of sickle cell disease to our understanding of humans with sickle cell anemia.

The conductance of red blood cells from sickle cell patients: ion selectivity and inhibitors

The abnormally high cation permeability in red blood cells (RBCs) from patients with sickle cell disease (SCD) occupies a central role in pathogenesis. Sickle RBC properties are notably heterogeneous, however, thus limiting conventional flux techniques that necessarily average out the behaviour of millions of cells. Here we use the whole-cell patch configuration to characterise the permeability of single RBCs from patients with SCD in more detail. A non-specific cation conductance was reversibly induced upon deoxygenation and was permeable to both univalent (Na+, K+, Rb+) and also divalent (Ca2+, Mg2+) cations. It was sensitive to the tarantula spider toxin GsMTx-4. Mn2+ caused partial, reversible inhibition. The aromatic aldehyde o-vanillin also irreversibly inhibited the deoxygenation-induced conductance, partially at 1mM and almost completely at 5mM. Nifedipine, amiloride and ethylisopropylamiloride were ineffective. In oxygenated RBCs, the current was pH sensitive showing a marked increase as pH fell from 7.4 to 6, with no change apparent when pH was raised from 7.4 to 8. The effects of acidification and deoxygenation together were not additive. Many features of this deoxygenation-induced conductance (non-specificity for cations, permeability toCa2+ andMg2+, pH sensitivity, reversibility, partial inhibition by DIDS and Mn2+) are shared with the flux pathway sometimes referred to as Psickle. Sensitivity to GsMTx-4 indicates its possible identity as a stretch-activated channel. Sensitivity to o-vanillin implies that activation requires HbS polymerisation but since the conductance was observed in whole-cell patches, results suggest that bulk intracellular Hb is not involved; rather a membrane-bound subfraction is responsible for channel activation. The ability to record P(sickle)-like activity in single RBCs will facilitate further studies and eventual molecular identification of the pathway involved.

Inhibition of KCC2 in mouse spinal cord neurons leads to hypersensitivity to thermal stimulation

BACKGROUND

KCC2, a neuronal-specific K-Cl cotransporter, is involved in pain perception physiology through its effects on postsynaptic inhibition in spinal cord neurons. We injected a newly identified, highly potent and selective inhibitor of KCC2 (D4), an inactive structural variant (D4.14), and the Na-K-2Cl cotransporter (NKCC1) inhibitor, bumetanide, into the intrathecal space of mice to measure their effect on heat-evoked nociceptive responses.

METHODS

Commercially available intrathecal catheters were modified and surgically placed into 2 cohorts of 10 mice. After recovery from the procedure, the mice were injected with D4, D4.14, and bumetanide through this catheter. Nociceptive measurements (hotplate assay, tail flick assay) were performed after injection of each of the test drugs and compared with vehicle controls.

RESULTS

Two mice in each cohort were omitted because of postprocedure complications. There was a statistically significant decrease (P < 0.01) in withdrawal latency after injection of the active KCC2 inhibitor but not after injection of the inactive compound (P = 0.78), as measured by hotplate assay at 55°C. Injection of bumetanide significantly increased withdrawal latency (P = 0.02) at the same temperature. These results were confirmed using tail flick assays performed at 49°C.

CONCLUSIONS

Inhibition of KCC2 by D4 led to decreased heat-evoked withdrawal latency in mice, as measured by hotplate and tail flick assays, whereas inhibition of NKCC1 by bumetanide resulted in increased response latencies to heat stimuli as measured by both of these nociceptive tests.

Cryohydrocytosis: increased activity of cation carriers in red cells from a patient with a band 3 mutation

BACKGROUND

Cryohydrocytosis is an inherited dominant hemolytic anemia characterized by mutations in a transmembrane segment of the anion exchanger (band 3 protein). Transfection experiments performed in Xenopus oocytes suggested that these mutations may convert the anion exchanger into a non-selective cation channel. The present study was performed to characterize so far unexplored ion transport pathways that may render erythrocytes of a single cryohydrocytosis patient cation-leaky.

DESIGN AND METHODS

Cold-induced changes in cell volume were monitored using ektacytometry and density gradient centrifugation. Kinetics, temperature and inhibitor-dependence of the cation and water movements in the cryohydrocytosis patient's erythrocytes were studied using radioactive tracers and flame photometry. Response of the membrane potential of the patient's erythrocyte membrane to the presence of ionophores and blockers of anion and cation channels was assessed.

RESULTS

In the cold, the cryohydrocytosis patient's erythrocytes swelled in KCl-containing, but not in NaCl-containing or KNO(3)-containing media indicating that volume changes were mediated by an anion-coupled cation transporter. In NaCl-containing medium the net HOE-642-sensitive Na(+)/K(+) exchange prevailed, whereas in KCl-containing medium swelling was mediated by a chloride-dependent K(+) uptake. Unidirectional K(+) influx measurements showed that the patient's cells have abnormally high activities of the cation-proton exchanger and the K(+),Cl(-) co-transporter, which can account for the observed net movements of cations. Finally, neither chloride nor cation conductance in the patient's erythrocytes differed from that of healthy donors. Conclusions These results suggest that cross-talk between the mutated band 3 and other transporters might increase the cation permeability in cryohydrocytosis.

Differential Effect of HOE642 on Two Separate Monovalent Cation Transporters in the Human Red Cell Membrane

Residual K(+) fluxes in red blood cells can be stimulated in conditions of low ionic strength. Previous studies have identified both the non-selective, voltage-dependent cation (NSVDC) channel and the K(+)(Na(+))/H(+) exchanger as candidate pathways mediating this effect, although it is possible that these pathways represent different modes of operation of a single system. In the present study the effects of HOE642, recently characterised as an inhibitor of the K(+)(Na(+))/H(+) exchanger, on NSVDC has been determined to clarify this question. Radioisotope flux measurements and conductance determinations showed that HOE642 exerted differential effects on the NSVDC channel and the K(+)(Na(+))/H(+) exchanger, confirming that the salt loss observed in low ionic strength solutions represents contributions from at least two independent ion transport pathways. The findings are discussed in the context of red blood cell apoptosis (eryptosis) and disease.

Effect of Peroxynitrite on Passive K+ Transport in Human Red Blood Cells

Peroxynitrite is generated in vivo by the reaction between nitric oxide, from endothelial and other cells, and the superoxide anion. It is therefore pertinent to examine its effects on the membrane permeability of red blood cells. Treatment of human red blood cells with peroxynitrite (nominally 1 mM) markedly stimulated passive K+ permeability. The main effect was on a Cl(-)-independent K+ pathway, which remains unidentified. Although K+-Cl- cotransport (KCC) was stimulated, this was dependent on saline composition, being inhibited by physiological levels of glucose (IC50 4 mM), and also by sucrose and MOPS. Effects on the Cl(-)-independent K+ pathway were less dependent on saline composition, and were not inhibited by amiloride, ethylisopropylamiloride, dimethylamiloride or gadolinium. Na+-K+-2Cl- cotransporter was inhibited whilst there was little effect on the Gardos channel (Ca2+-activated K+ channel). Peroxynitrite was markedly more effective in oxygenated cells than deoxygenated ones. Treatment with peroxynitrite per se did not affect initial cell volume. Anisotonic swelling modestly increased the Cl(-)-independent K+ influx, but did not affect peroxynitrite-stimulated KCC. Decreasing extracellular pH from 7.4 to 7.2 or 7.0 increased KCC stimulation, whilst the Cl(-)-independent component of K+ transport was lowest at pH 7.2. Finally, protein phosphatase inhibition with calyculin A (100 nM) inhibited KCC, implying that, as with other KCC stimuli, peroxynitrite acts via decreased protein phosphorylation; pre-treatment with calyculin A also inhibited the Cl(-)-independent component of K+ transport. These findings are relevant to the actions of peroxynitrite in vivo.