Abnormal permeability pathways in human red blood cells

Deformation-dependent polydimethylsiloxane permeability measured using osmotic microactuators

In soft solids, large deformations significantly alter molecular structure and device geometry, which can impact other properties. In the case of mass transport, an interplay between flux and mechanical deformation results. Here we demonstrate a platform for the simultaneous characterization of mechano-permselectivity using the (slow) transport of water through polydimethylsiloxane (PDMS) as a challenging test case. The platform uses micron-sized, cylindrical, NaCl solution-filled PDMS chambers encapsulated by selectively-permeable PDMS thin film membranes. When placed in a high chemical potential environment (high water potential) the osmotic pressure difference between the chamber and environment induces water to flow through the PDMS membrane into the chamber, resulting in membrane bulging. A model combining membrane flux and nonlinear elasticity captures the time-dependent response well, but only when a deformation-dependent permeability is used. Notably, the permeability of water through PDMS decreases by nearly an order of magnitude, from 2 × 10-12 to 5 × 10-13 m2 s-1, due primarily to its thickness decreasing by nearly an order of magnitude as the average biaxial stretch increases from 1 to 2.75.

Rapid degradation of protein tyrosine phosphatase 1B in sickle cells: Possible contribution to sickle cell membrane weakening

Although both protein tyrosine phosphatases and kinases are constitutively active in healthy human red blood cells (RBCs), the preponderance of phosphatase activities maintains the membrane proteins in a predominantly unphosphorylated state. We report here that unlike healthy RBCs, proteins in sickle cells are heavily tyrosine phosphorylated, raising the question regarding the mechanism underpinning this tyrosine phosphorylation. Upon investigating possible causes, we observe that protein tyrosine phosphatase 1B (PTP1B), the major erythrocyte tyrosine phosphatase, is largely digested to a lower molecular weight fragment in sickle cells. We further find that the resulting truncated form of PTP1B is significantly less active than its intact counterpart, probably accounting for the intense tyrosine phosphorylation of Band 3 in sickle erythrocytes. Because this tyrosine phosphorylation of Band 3 promotes erythrocyte membrane weakening that causes release of both membrane vesicles and cell free hemoglobin that in turn initiates vaso-occlusive events, we conclude that cleavage of PTP1B could contribute to the symptoms of sickle cell disease. We further posit that methods to inhibit proteolysis of PTP1B could mitigate symptoms of the disease.

Flow adhesion of whole blood to P-selectin: a prognostic biomarker for vaso-occlusive crisis in sickle cell disease

Blood cell adhesion to P-selectin and vascular cell adhesion molecule-1 (VCAM-1) contributes to the pathophysiology of vaso-occlusion crisis (VOC) events in individuals with sickle cell disease (SCD). We evaluated the use of standardized flow adhesion biomarkers in a six-month, 35-subjects longitudinal study (ELIPSIS). Flow adhesion of whole blood on P-selectin (FA-WB-Psel) and VCAM1 (FA-WB-VCAM), and of isolated white blood cells on P-selectin (FA-WBC-Psel) and VCAM-1 (FA-WBC-VCAM) were elevated on VOC days compared with non-VOC days, but only FA-WB-Psel reached statistical significance (P = 0·015). Optimal cut-off values were established with Cox regression models for FA-WB-Psel [46 cells/mm²; hazard ratio (HR): 2·3; 95% confidence interval (CI):1·4-4·0; P = 0·01] and FA-WB-VCAM (408 cells/mm², HR:1·8; 95% CI: 0·9-3·45; P = 0·01). A combined (FA-WB-Psel and FA-WB-VCAM) multimarker risk score was also significantly (P = 0·0006) correlated with VOC risk that was two-fold higher for intermediate and 5·64-fold higher for high score. The concordance (C)-index for the multimarker score was 0·63 in the six-month period (95% CI: 0·56-0·70), indicating a better ability to distinguish patient risk of VOC, compared to individual biomarkers FA-WB-VCAM (C-index: 0·57; 95% CI: 0·49-0·65) or FA-WB-Psel (C-index: 0·58; 95% CI: 0·53-0·62). The presented multimarker score can be used to risk-stratify individuals with SCD during their steady state into low, intermediate, and high-risk strata for self-reported VOCs. Such risk stratification could help focus healthcare resources more efficiently to maintiain health, personalize treatment selection to each patient's individual needs, and potentially reduce healthcare costs.

Inhibition of Band 3 tyrosine phosphorylation: a new mechanism for treatment of sickle cell disease

Many hypotheses have been proposed to explain how a glutamate to valine substitution in sickle haemoglobin (HbS) can cause sickle cell disease (SCD). We propose and document a new mechanism in which elevated tyrosine phosphorylation of Band 3 initiates sequelae that cause vaso-occlusion and the symptoms of SCD. In this mechanism, denaturation of HbS and release of heme generate intracellular oxidants which cause inhibition of erythrocyte tyrosine phosphatases, thus permitting constitutive tyrosine phosphorylation of Band 3. This phosphorylation in turn induces dissociation of the spectrin-actin cytoskeleton from the membrane, leading to membrane weakening, discharge of membrane-derived microparticles (which initiate the coagulation cascade) and release of cell-free HbS (which consumes nitric oxide) and activates the endothelium to express adhesion receptors). These processes promote vaso-occlusive events which cause SCD. We further show that inhibitors of Syk tyrosine kinase block Band 3 tyrosine phosphorylation, prevent release of cell-free Hb, inhibit discharge of membrane-derived microparticles, increase sickle cell deformability, reduce sickle cell adhesion to human endothelial cells, and enhance sickle cell flow through microcapillaries. In view of reports that imatinib (a Syk inhibitor) successfully treats symptoms of sickle cell disease, we suggest that Syk tyrosine kinase inhibitors warrant repurposing as potential treatments for SCD.

Dehydrated hereditary stomatocytosis: clinical perspectives

Dehydrated hereditary stomatocytosis (DHSt) is a nonimmune congenital hemolytic disorder characterized by red blood cell (RBC) dehydration and lysis. It has been a recognized diagnostic entity for almost 50 years, and autosomal dominant inheritance has long been suspected, but it was not until 2011 that the first genetic alterations were identified. The current study reviews 73 articles published during 1971–2019 and focuses on clinical perspectives of the disease. All but one of the published clinical data in DHSt were either single case reports or case series. From these, it can be seen that patients with DHSt often have fully or partially compensated hemolysis with few symptoms. Despite this, iron overload is an almost universal finding even in patients without or with only sporadic blood transfusions, and this may lead to organ dysfunction. Other severe complications, such as thrombosis and perinatal fluid effusions unrelated to fetal hemoglobin concentration, may also occur. No specific treatment for symptomatic hemolysis exists, and splenectomy should be avoided as it seems to aggravate the risk of thrombosis. Recently, treatment with senicapoc has shown activity against RBC dehydration in vitro; however, it is not known if this translates into relevant clinical effects. In conclusion, despite recent advances in the understanding of pathophysiology in DHSt, options for clinical management have not improved. Entering data into international registries has the potential to fill gaps in knowledge and eventually care of these rare patients.

In vitro reconstitution of eukaryotic ion channels using droplet interface bilayers

The ability to routinely study eukaryotic ion channels in a synthetic lipid environment would have a major impact on our understanding of how different lipids influence ion channel function. Here, we describe a straightforward, detergent-free method for the in vitro reconstitution of eukaryotic ion channels and ionotropic receptors into droplet interface bilayers and measure their electrical activity at both the macroscopic and single-channel level. We explore the general applicability of this method by reconstitution of channels from a wide range of sources including recombinant cell lines and native tissues, as well as preparations that are difficult to study by conventional methods including erythrocytes and mitochondria.

Effects of disodium cromoglycate on cationic exchange of deoxygenated sickle cells

In the present work, we explored the way in which cromoglycate, a drug used to treat allergies acts on ion movements in sickle cells. Cells were either slowly deoxygenated by overnight exposure to nitrogen or acutely deoxygenated by exposure to metabisulfite, a strong reducing agent which induces sickling of red blood sickle cells. Flushing the cells with nitrogen increased the intracellular concentration of Na(+) and decreased the intracellular concentration of K(+) and the sum of the concentrations of the two cations. One hundred nM cromoglycate inhibited the decrease of intracellular K(+) and the increase of intracellular Na(+) induced by deoxygenation (n=17). Metabisulfite (100mM) increased the intracellular concentration of Ca(2+) (measured by Fura Red) (n=15) and the shape of the cells (measured by light scattering) (n=9). One μM cromoglycate partially inhibited these two responses. In conclusion, cromoglycate partially inhibits abnormal K(+) loss, Ca(2+) entry pathways or Ca(2+) channels opened by cell deoxygenation and ensuing membrane modifications and prevents cell sickling.

Determinants of erythrocyte hydration

PURPOSE OF REVIEW

Maintenance of cellular water and solute homeostasis is critical for survival of the erythrocyte. Inherited or acquired disorders that perturb this homeostasis jeopardize the erythrocyte, leading to its premature destruction. This study reviews recent progress in our understanding the determinants of erythrocyte hydration and its related disorders.

RECENT FINDINGS

The molecular and genetic bases of primary disorders of erythrocyte hydration are poorly understood. Recent studies have implicated roles for the anion transporter, SLC4A1, and the Rh-associated glycoprotein, RhAG. The most common secondary disorder associated with perturbed hydration of the erythrocyte is sickle cell disease, in which dehydration contributes to disease pathology and clinical complications. Advances in understanding the mechanisms regulating erythrocyte solute and water content, particularly associated with KCl cotransport and Gardos channel activation, have revealed novel signaling mechanisms controlling erythrocyte hydration. These signaling pathways may provide innovative strategies to prevent erythrocyte dehydration in sickle cell disease.

SUMMARY

Clinical, translational and biologic studies all contribute to our knowledge of erythrocyte hydration. Understanding the mechanisms controlling erythrocyte water and solute homeostasis will serve as a paradigm for other cells and may reveal new therapeutic targets for disease prevention and treatment.

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.