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The aetiology of sickle cell disease is well known, but pathogenesis is complicated and details remain uncertain. A thorough understanding may suggest novel ways for designing more effective therapies. One area of importance, covered here in Nader et al., is the altered cation permeability of sickle cells and how the co-ordinated operation of a number of membrane transport proteins contributes to disease progression, all driven by the initial event of HbS polymerisation. There are echoes here of the cation leaks of hereditary stomatocytosis. Nader et al. propose a central role for PIEZO1, a novel mechanosensitive channel found in red cells, which may be aberrantly activated in sickle cells following HbS polymerisation and which may have potential as a novel target for future chemotherapies. Commentary on: Nader et al. Piezo1 activation augments sickling propensity and the adhesive properties of sickle red blood cells in a calcium-dependent manner. Br J Haematol 2022 (Online ahead of print). doi: 10.1111/bjh.18799.

α-Thalassemia is one of the most important genetic modulators of sickle cell disease (SCD). Both beneficial and detrimental effects have been described previously. We use a 12-year data set on a large cohort of patients with HbSS (n = 411) and HbSC (n = 146) to examine a wide range of these clinical and laboratory associations. Our novel findings are that α-thalassemia strongly reduces erythrocyte potassium chloride co-transporter (KCC) activity in both HbSS and HbSC (p = .035 and p = .00045 respectively), suggesting a novel mechanism through which α-thalassemia induces a milder phenotype by reducing red cell cation loss. This may be particularly important in HbSC where reduction in mean cell hemoglobin concentration is not seen and where KCC activity has previously been found to correlate with disease severity. Additionally, we show that α-thalassemia not only increases hemoglobin in patients with HbSS (p = .0009) but also reduces erythropoietin values (p = .0005), demonstrating a measurable response to improved tissue oxygenation. We confirm the reno-protective effect of α-thalassemia in patients with HbSS, with reduced proteinuria (p = .003) and demonstrate a novel association with increased serum sodium (p = .0004) and reduced serum potassium values (p = 5.74 × 10

Many erythrocyte processes and pathways, including glycolysis, the pentose phosphate pathway (PPP), KCl cotransport, ATP release, Na(+)/K(+)-ATPase activity, ankyrin–band 3 interactions, and nitric oxide (NO) release, are regulated by changes in O(2) pressure that occur as a red blood cell (RBC) transits between the lungs and tissues. The O(2) dependence of glycolysis, PPP, and ankyrin–band 3 interactions (affecting RBC rheology) are controlled by O(2)-dependent competition between deoxyhemoglobin (deoxyHb), but not oxyhemoglobin (oxyHb), and other proteins for band 3. We undertook the present study to determine whether the O(2) dependence of Na(+)/K(+)/2Cl(−) cotransport (catalyzed by Na(+)/K(+)/2Cl(−) cotransporter 1 [NKCC1]) might similarly originate from competition between deoxyHb and a protein involved in NKCC1 regulation for a common binding site on band 3. Using three transgenic mouse strains having mutated deoxyhemoglobin-binding sites on band 3, we found that docking of deoxyhemoglobin at the N terminus of band 3 displaces the protein with no lysine kinase 1 (WNK1) from its overlapping binding site on band 3. This displacement enabled WNK1 to phosphorylate oxidative stress-responsive kinase 1 (OSR1), which, in turn, phosphorylated and activated NKCC1. Under normal solution conditions, the NKCC1 activation increased RBC volume and thereby induced changes in RBC rheology. Because the deoxyhemoglobin-mediated WNK1 displacement from band 3 in this O(2) regulation pathway may also occur in the regulation of other O(2)-regulated ion transporters, we hypothesize that the NKCC1-mediated regulatory mechanism may represent a general pattern of O(2) modulation of ion transporters in erythrocytes.

Red cells from patients with sickle cell anaemia (SCA) contain the abnormal haemoglobin HbS. Under hypoxic conditions, HbS polymerises and causes red cell sickling, a rise in intracellular Ca2+ and exposure of phosphatidylserine (PS). These changes make sickle cells sticky and liable to lodge in the microvasculature, and so reduce their lifespan. The aim of the present work was to investigate how the peculiar conditions found in the renal medulla – hypoxia, acidosis, lactate, hypertonicity and high levels of urea – affect red cell behaviour. Results show that the first four conditions all increased sickling and PS exposure. The presence of urea at levels found in a healthy medulla during antidiuresis, however, markedly reduced sickling and PS exposure and would therefore protect against red cell adherence. Loss of the ability to concentrate urine, which occurs in sickle cell nephropathy would obviate this protective effect and may therefore contribute to pathogenesis.

Sickle cell disease (SCD) presents a significant global health problem. At present there is no effective treatment, with most being supportive for its associated complications such as the vaso-occlusive crises that result from increased cell adhesion. Hypoxic sickle cells have previously shown greater phosphatidylserine (PS) exposure and oxidative damage, as well as being notably "stickier" suggesting that increased cell cohesion and adhesion to the blood vessel endothelium is a possible mechanism for vaso-occlusion. The present work uses the hybrid technique of atomic force microscopy nano-infrared spectroscopy (AFM-IR) to probe changes to the coefficient of friction and C-O IR intensity in SCD on a nanoscale for dried red blood cells (RBCs) fixed under conditions of hypoxia and correlates these observations with adhesive interactions at the membrane. Using functionalised AFM tips, it has been possible to probe adhesive interactions between hydrophilic and hydrophobic moieties exposed at the surface of the dried RBCs fixed under different oxygenation states and for different cell genotypes. The results are consistent with greater PS-exposure and oxidative damage in hypoxic sickle cells, as previously proposed, and also show strong correlation between localised oxidative damage and increased adhesion. A mechanistic explanation involving significant lipid tail disruption as a result of oxidative action, in combination with differing concentrations of externalised PS lipids, is proposed to explain the observed adhesion behaviour of each type of cell.

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.

Studying different sickle cell genotypes may throw light on the pathogenesis of sickle cell disease (SCD). Here, the clinical profile, red cell sickling and K+ permeability in 29 SCD patients (15 patients with severe disease and 14 with a milder form) of HbA/S-Oman genotype were analysed. The super sickling nature of this Hb variant was confirmed. The red cell membrane permeability to K+ was markedly abnormal with elevated activities of Psickle , Gardos channel and KCl cotransporter (KCC). Results were consistent with Ca2+ entry and Mg2+ loss via Psickle stimulating Gardos channel and KCC activities. The abnormal red cell behaviour was similar to that in the commonest genotype of SCD, HbSS, in which the level of mutated Hb is considerably higher. Although activities of all three K+ transporters also correlated with the level of HbS-Oman, there was no association between transport phenotype and disease severity. The super sickling behaviour of HbS-Oman may obviate the need for solute loss and red cell dehydration to encourage Hb polymerisation, required in other SCD genotypes. Disease severity was reduced by concurrent α thalassaemia, as observed in other SCD genotypes, and represents an obvious genetic marker for prognostic tests of severity in young SCD patients of the HbA/S-Oman genotype.

The integration of IR-spectroscopy and Atomic Force Microscopy (AFM) in the technique of Nano-IR circumvents the Abbe diffraction limit that restricts the resolution of other commonly used vibrational spectroscopies and permits chemical characterisation of biological surfaces at previously unattainable spatial resolution. Using this relatively new technique in conjunction with surface specific SFG spectroscopy has enabled the molecular compositions, structures and conformations of surface films to be investigated at nanometre spatial resolution. In this work these complementary techniques have been applied to Red Blood Cells (RBCs) from patients with Sickle Cell Disease (SCD) to elucidate the effects of oxidative stress on the cell lipid bilayer membrane. The HbS mutated form of Haemoglobin present in RBCs of SCD patients polymerises when de-oxygenated causing deformation of the cell from the well-known biconcave disc to sickled morphologies. In this form the cells are found to be in great abundance in sites of cell occlusion which is explained by their observed change in adhesive properties. It has been proposed that oxidative damage to the cell membrane contributes to these adhesive changes and hence to the pathogenesis of SCD. In this work surface-localised enhancement of the C-O(H) vibrational band as well as significant lipid bilayer disordering was found for the RBCs. To elucidate the fundamental changes causing such observations the effect of oxidative stress on a supported lipid bilayer model membrane was investigated. Our two-pronged approach leads to a proposed mechanism for membrane damage under oxidative stress. The analytical techniques used in this study promise to be effective tools for assessing the efficacy of novel antioxidants and antioxidant regimes which will inform their rational use for amelioration of the clinical signs of SCD.

Red cells contain a unique constellation of membrane lipids. Although much is known about regulated protein expression, the regulation of lipid metabolism during erythropoiesis is poorly studied. Here, we show that transcription of PHOSPHO1, a phosphoethanolamine and phosphocholine phosphatase that mediates the hydrolysis of phosphocholine to choline, is strongly upregulated during the terminal stages of erythropoiesis of both human and mouse erythropoiesis, concomitant with increased catabolism of phosphatidylcholine (PC) and phosphocholine as shown by global lipidomic analyses of mouse and human terminal erythropoiesis. Depletion of PHOSPHO1 impaired differentiation of fetal mouse and human erythroblasts, and, in adult mice, depletion impaired phenylhydrazine-induced stress erythropoiesis. Loss of PHOSPHO1 also impaired phosphocholine catabolism in mouse fetal liver progenitors and resulted in accumulation of several lipids; adenosine triphosphate (ATP) production was reduced as a result of decreased oxidative phosphorylation. Glycolysis replaced oxidative phosphorylation in PHOSPHO1-knockout erythroblasts and the increased glycolysis was used for the production of serine or glycine. Our study elucidates the dynamic changes in lipid metabolism during terminal erythropoiesis and reveals the key roles of PC and phosphocholine metabolism in energy balance and amino acid supply.

Red cells from patients with sickle cell anemia (SCA) are under greater oxidative challenge than those from normal individuals. We postulated that oxidants generated by xanthine oxidase (XO) and hypoxanthine (HO) contribute to the pathogenesis of SCA through altering solute permeability. Sickling, activities of the main red cell dehydration pathways (P sickle , Gardos channel, and KCl cotransporter [KCC]), and cell volume were measured at 100, 30, and 0 mmHg O 2 , together with deoxygenation‐induced nonelectrolyte hemolysis. Unexpectedly, XO/HO mixtures had mainly inhibitory effects on sickling, P sickle , and Gardos channel activities, while KCC activity and nonelectrolyte hemolysis were increased. Gardos channel activity was significantly elevated in red cells pharmacologically loaded with Ca 2+ using the ionophore A23187, consistent with an effect on the transport system per se as well as via Ca 2+ entry likely via the P sickle pathway. KCC activity is controlled by several pairs of conjugate protein kinases and phosphatases. Its activity, however, was also stimulated by XO/HO mixtures in red cells pretreated with N ‐ethylmaleimide (NEM), which is thought to prevent regulation via changes in protein phosphorylation, suggesting that the oxidants formed could also have direct effects on this transporter. In the presence of XO/HO, red cell volume was better maintained in deoxygenated red cells. Overall, the most notable effect of XO/HO mixtures was an increase in red cell fragility. These findings increase our understanding of the effects of oxidative challenge in SCA patients and are relevant to the behavior of red cells in vivo.

Summary Phosphatidylserine (PS) exposure increases as red cells age, and is an important signal for the removal of senescent cells from the circulation. PS exposure is elevated in red cells from sickle cell anaemia (SCA) patients and is thought to enhance haemolysis and vaso‐occlusion. Although precise conditions leading to its externalisation are unclear, high intracellular Ca2+ has been implicated. Red cells from SCA patients are also exposed to an increased oxidative challenge, and we postulated that this stimulates PS exposure, through increased Ca2+ levels. We tested four different ways of generating oxidative stress: hypoxanthine and xanthine oxidase, phenazine methosulphate, nitrite and tert‐butyl hydroperoxide, together with thiol modification with N‐ethylmaleimide (NEM), dithiothreitol and hypochlorous acid (HOCl), in red cells permeabilised to Ca2+ using bromo‐A23187. Unexpectedly, our findings showed that the four oxidants significantly reduced Ca2+‐induced PS exposure (by 40–60%) with no appreciable effect on Ca2+ affinity. By contrast, NEM markedly increased PS exposure (by about 400%) and slightly but significantly increased the affinity for Ca2+. Dithiothreitol modestly reduced PS exposure (by 25%) and HOCl had no effect. These findings emphasise the importance of thiol modification for PS exposure in sickle cells but suggest that increased oxidant stress alone is not important.

Studying different sickle cell genotypes may throw light on the pathogenesis of sickle cell disease (SCD). Here, the clinical profile, red cell sickling and K

Aromatic aldehydes like o-vanillin were designed to reduce the complications of sickle cell disease (SCD) by interaction with HbS, to reduce polymerisation and RBC sickling. Present results show that o-vanillin also directly affects RBC membrane permeability. Both the K+-Cl- cotransporter (KCC) and the Ca2+-activated K+ channel (or Gardos channel) were inhibited with IC50 of about 0.3 and 1mM, respectively, with activities almost completely abolished by 5mM. Similar effects were observed in RBCs treated with the thiol reacting reagent N-ethylmaleimide or with the Ca2+ ionophore A23187, to circumvent any action via HbS polymerisation. The deoxygenation-induced cation conductance (sometimes termed Psickle) was partially inhibited, whilst deoxygenation-induced exposure of phosphatidylserine was completely abrogated. Na+/K+ pump activity was also reduced. Notwithstanding, o-vanillin stimulated K+ efflux through an unidentified pathway and resulted in reduction in cell volume (as measured by wet weight-dry weight). These actions are relevant to understanding how aromatic aldehydes may affect RBC membrane permeability per se as well as HbS polymerisation and thereby inform design of compounds most efficacious in ameliorating the complications of SCD.

In this issue of Blood , Chu et al show that oxygen, by changing the conformation of hemoglobin (Hb), modulates many red cell properties by altering its interaction with band 3, the major red cell membrane protein. 1

Hypoxia and acidosis are recognized features of inflammatory arthroses. This study describes the effects of IGF-1 and TGF-β1 on pH regulatory mechanisms in articular cartilage under hypoxic conditions. Acid efflux, reactive oxygen species (ROS), and mitochondrial membrane potential were measured in equine articular chondrocytes isolated in the presence of serum (10% fetal calf serum), IGF-1 (1, 10, 50, 100 ng/ml) or TGF-β1 (0.1, 1, 10 ng/ml) and then exposed to a short-term (3 h) hypoxic insult (1% O2). Serum and 100 ng/ml IGF-1 but not TGF-β1 attenuated hypoxic regulation of pH homeostasis. IGF-1 appeared to act through mitochondrial membrane potential stabilization and maintenance of intracellular ROS levels in very low levels of oxygen. Using protein phosphorylation inhibitors PD98059 (25 µM) and wortmannin (200 nM) and Western blotting, ERK1/2 and PI-3 kinase pathways are important for the effect of IGF-1 downstream to ROS generation in normoxia but only PI-3 kinase is implicated in hypoxia. These results show that oxygen and growth factors interact to regulate pH recovery in articular chondrocytes by modulating intracellular oxygen metabolites. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31: 197–203, 2013

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.

Phosphatidylserine (PS) is usually confined to the inner leaflet of the red blood cell (RBC) membrane. It may become externalised in various conditions, however, notably in RBCs from patients with sickle cell disease (SCD) where exposed PS may contribute to anaemic and ischaemic complications. PS externalisation requires both inhibition of the aminophospholipid translocase (or flippase) and activation of the scramblase. Both may follow from elevation of intracellular Ca 2+ . Flippase inhibition occurs at low [Ca 2+ ] i , about 1 μM, but [Ca 2+ ] i required for scrambling is reported to be much higher (around 100 μM). In this work, FITC-labelled lactadherin and FACS were used to measure externalised PS, with [Ca 2+ ] i altered using bromo-A23187 and EGTA/Ca 2+ mixtures. Two components of Ca 2+ -induced scrambling were apparent, of high (EC 50 1.8 ± 0.3 μM) and low (306 ± 123 μM) affinity, in RBCs from normal individuals and the commonest SCD genotypes, HbSS and HbSC. The high affinity component was lost in the presence of unphysiologically high [Mg 2+ ] but was unaffected by high K + (90 mM) or vanadate (1 mM). The high affinity component accounted for PS scrambling in ≥2/3rd RBCs. It is likely to be most significant in vivo and may be involved in the pathophysiology of SCD or other conditions involving eryptosis.

More than 100 different blood and urine biomarkers have been described in sickle cell disease (SCD), with the number increasing rapidly as analytical techniques develop. Nearly all of these biomarkers are abnormal in the steady state, and become more so during complications. The range of abnormalities demonstrates the multisystem nature of SCD and the complex pathophysiology. Some biomarkers indicate damage to specific organs, such as urine albumin:creatinine ratio in nephropathy, whereas others indicate more systemic processes. Biomarkers have been useful in identifying various interrelated pathological mechanisms, including haemolysis, inflammation, hypercoagulability, oxidative stress, reperfusion injury, vasculopathy and endothelial dysfunction. However, most biomarkers correlate closely with other more routine measurements, and also with each other. It is not clear that any provide specific prognostic or clinical information beyond that given by the simple measurement of haemoglobin concentration. The identification of prognostically validated biomarkers in prospective clinical trials would be useful.

Articular cartilage is an avascular tissue dependent on diffusion mainly from synovial fluid to service its metabolic requirements. Levels of oxygen (O(2)) in the tissue are low, with estimates of between 1 and 6%. Metabolism is largely, if not entirely, glycolytic, with little capacity for oxidative phosphorylation. Notwithstanding, the tissue requires O(2) and consumes it, albeit at low rates. Changes in O(2) tension also have profound effects on chondrocytes affecting phenotype, gene expression, and morphology, as well as response to, and production of, cytokines. Although chondrocytes can survive prolonged anoxia, low O(2) levels have significant metabolic effects, inhibiting glycolysis (the negative Pasteur effect), and also notably matrix production. Why this tissue should respond so markedly to reduction in O(2) tension remains a paradox. Ion homeostasis in articular chondrocytes is also markedly affected by the extracellular matrix in which the cells reside. Recent work has shown that ion homeostasis also responds to changes in O(2) tension, in such a way as to produce significant effects on cell function. For this purpose, O(2) probably acts via alteration in levels of reactive oxygen species. We discuss the possibility that O(2) consumption by this tissue is required to maintain levels of ROS, which are then used physiologically as an intracellular signalling device. This postulate may go some way towards explaining why the tissue is dependent on O(2) and why its removal has such marked effects. Understanding the role of oxygen has implications for disease states in which O(2) or ROS levels may be perturbed.

Summary Objective To examine the effect of O 2 and the role, and source, of reactive oxygen species (ROS) on pH regulation in articular chondrocytes. Methods Cartilage from equine metacarpo/tarsophalangeal joints was digested (collagenase) to isolate chondrocytes and loaded with 2′,7′-bis-2-(carboxyethyl)-5(6)-carboxylfluorescein, a pH-sensitive fluorophore. O 2 tension was maintained using Eschweiler tonometers and a Wosthoff gas mixer. Cells were exposed to agents which alter ROS levels, mitochondrial inhibitors and/or inhibitors of protein phosphorylation. ROS levels were determined by dichlorofluorescein and mitochondrial membrane potential measured using JC-1. Results pH homeostasis was dependent on ROS. Na + /H + exchanger (NHE) activity was inhibited at low O 2 tension (acid efflux reducing from 2.30±0.05 to 1.27±0.11mMmin −1 at 1%). NHE activity correlated with ROS levels ( r 2 =0.65). ROS levels were increased by antimycin A (with levels at 1% O 2 tension increasing from 59±9% of the value at 20% to 87±7%), but reduced by rotenone, myxothiazol and diphenyleneiodonium. Hypoxia induced depolarisation of the mitochondrial membrane potential (with JC-1 red–green fluorescence ratio at 1% O 2 tension decreasing to 40±10% of the value at 20%). The response to changes in O 2 and to antimycin A was inhibited by staurosporine, wortmanin and calyculin A. Conclusion The fall in ROS levels in hypoxia reduces the ability of articular chondrocytes to regulate pH, inhibiting NHE activity via changes in protein phosphorylation. The site of ROS generation is likely to be mitochondrial electron transport chain complex III. These effects are important to understanding normal chondrocyte function and response to altered O 2 tension.

Red cells from patients with sickle cell disease contain HbS rather than the normal HbA (here termed HbS cells). On deoxygenation, HbS cells exhibit a distinctive solute permeability pathway, P(sickle), activated stochastically, and partially inhibited by DIDS and dipyridamole. It is often referred to as a cation channel although its permeability characteristics remain vague and its molecular identity is unknown. We show that, in contrast to normal red cells, a proportion of HbS cells underwent haemolysis when deoxygenated in isosmotic non-electrolyte solutions. Haemolysis was stochastic: cells unlysed after an initial deoxygenation pulse showed lysis when harvested, reoxygenated and subsequently exposed to a second period of deoxygenation. O(2) dependence of haemolysis was similar to that of P(sickle) activation. Haemolysis was accompanied by high rates of sucrose influx, and both haemolysis and sucrose influx were inhibited by DIDS and dipyridamole. Sucrose influx was only detected as ionic strength was reduced below 80 mM. These findings are consistent with the postulate that deoxygenation of HbS cells, under certain conditions, activates a novel non-electrolyte pathway. Their significance lies in understanding the nature of the deoxygenation-induced permeability in HbS cells, together with its relationship with novel pathways induced by a variety of manipulations in normal red cells.

HbSC disease is the second commonest form of sickle cell disease, with poorly understood pathophysiology and few treatments. We studied the role of K-Cl cotransport activity in determining clinical and laboratory features, and investigated its potential role as a biomarker. Samples were collected from 110 patients with HbSC disease and 41 with sickle cell anemia (HbSS). K-Cl cotransport activity was measured in the oxygenated (K-Cl cotransport(100)) and deoxygenated (K-Cl cotransport(0)) states, using radioactive tracer studies. K-Cl cotransport activity was high in HbSC and decreased significantly on deoxygenation. K-Cl cotransport activity correlated significantly and positively with the formation of sickle cells. On multiple regression analysis, K-Cl cotransport increased significantly and independently with increasing reticulocyte count and age. K-Cl cotransport activity was increased in patients who attended hospital with acute pain in 2011 compared to those who did not (K-Cl cotransport(100): mean 3.87 versus 3.20, P=0.009, independent samples T-test; K-Cl cotransport(0): mean 0.96 versus 0.68, P=0.037). On logistic regression only K-Cl cotransport was associated with hospital attendance. Increased K-Cl cotransport activity was associated with the presence of retinopathy, but this effect was confounded by age. This study links variability in a fundamental aspect of cellular pathology with a clinical outcome, suggesting that K-Cl cotransport is central to the pathology of HbSC disease. Increased K-Cl cotransport activity is associated with increasing age, which may be of pathophysiological significance. Effective inhibition of K-Cl cotransport activity is likely to be of therapeutic benefit.

Red blood cells from patients with sickle cell disease (SCD) exhibit increased electrogenic cation permeability, particularly following deoxygenation and hemoglobin (Hb) polymerisation. This cation permeability, termed Psickle, contributes to cellular dehydration and sickling, and its inhibition remains a major goal for SCD treatment. Nevertheless, its characteristics remain poorly defined, its molecular identity is unknown, and effective inhibitors have not been established. Here, patch-clamp methodology was used to record whole-cell currents in single red blood cells from healthy individuals and patients with SCD. Oxygenated normal red blood cells had a low membrane conductance, unaffected by deoxygenation. Oxygenated HbS cells had significantly increased conductance and, on deoxygenation, showed a further rise in membrane conductance. The deoxygenation-induced pathway was variable in magnitude. It had equal permeability to Na+ and K+, but was less permeable to NMDG+ and Cl−. Conductance to Ca2+ was also of a similar magnitude to that of monovalent cations. It was inhibited by DIDS (100 μM), Zn2+ (100 μM), and by Gd3+ (IC50 of approximately 2 μM). It therefore shares some properties with Psickle. These findings represent the first electrical recordings of single HbS cells and will facilitate progress in understanding altered red blood cell cation transport characteristics of SCD.

In red cells from normal individuals (HbA cells), the K+-Cl− cotransporter (KCC) is inactivated by low O2 tension whilst in those from sickle cell patients (HbS cells), it remains fully active. Changes in free intracellular [Mg2+] have been proposed as a mechanism. In HbA cells, KCC activity was stimulated by Mg2+ depletion and inhibited by Mg2+ loading but the effect of O2 was independent of Mg2+. At all [Mg2+]is, the transporter was stimulated in oxygenated cells, minimally active in deoxygenated ones. By contrast, the stimulatory effects of O2 was abolished by inhibitors of protein (de)phosphorylation. HbS cells had elevated KCC activity, which was of similar magnitude in oxygenated and deoxygenated cells, regardless of Mg2+ clamping. In deoxygenated cells, the antisickling agent dimethyl adipimidate inhibited sickling, Psickle and KCC. Results indicate a role for protein phosphorylation in O2 dependence of KCC, with different activities of the relevant enzymes in HbA and HbS cells, probably dependent on Hb polymerisation, but not on [Mg2+]i.

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.

KCC activity in normal human red cells (containing haemoglobin A, HbA, and termed HbA cells) is O2-dependent, being active in oxygenated cells but inactive in deoxygenated ones. The mechanism for O2 dependence is unknown but a role for Hb has been suggested. In this paper, we address two main questions. First, do membrane ghosts prepared from HbA cells retain an O2-sensitive KCC activity? Second, how is the response of KCC to changes in O2 tension altered in sickle cell patients heterozygous for HbS and HbC? We found that substantial Cl(-)-dependent K+ influx, indicative of KCC activity, was present in both pink (5-10% normal Hb complement) and white (no measurable Hb) ghosts when equilibrated with air. KCC responded to deoxygenation in pink ghosts only (86 +/- 10% inhibition, mean+/-S.E.M., n = 3), whilst KCC activity in white ghosts remained high (23 +/- 8% inhibition). Results indicate that pink ghosts retain an O2-dependent KCC activity but that this is lost in white ghosts. Second, HbSC-containing red cells showed sickling (88 +/- 3%) when deoxygenated, together with activation of the deoxygenation-induced cation pathway (Psickle) and the Gardos channel. KCC activity, however, was elevated in oxygenated HbSC cells, but inhibited by deoxygenation. Thus Hb polymerisation and sickling could be dissociated from the abnormal response of KCC to deoxygenation observed in HbS-containing red cells. These preparations provide a useful system with which to study the components involved in O2-sensitive membrane transport and why it is perturbed in certain pathological conditions (such as sickle cell disease and oxidant toxicity).

In this paper, we provide an update on O2-dependent membrane transport in red cells. O2-sensitive membrane transport was compared in nucleated (chicken) and enucleated (human) red cells, to investigate effects on organic (glucose transporter [GLUT]) and inorganic (K(+)-Cl- cotransporter [KCC]/Na(+)-K(+)-2Cl- cotransporter [NKCC]) transporters, to study the response of so-called "housekeeping" transporters (Na+/K+ pump and anion exchanger [AE]) and, finally, to compare O2 sensitivity in normal human red cells with those from sickle cell patients. The Na+/K+ pump showed no change in activity between oxygenated and deoxygenated cells in any of the samples. KCC in normal human red cells had the greatest O2 sensitivity, being stimulated some 20-fold on oxygenation. It was more modestly stimulated by O2 in chicken red cells and HbS cells. By contrast, NKCC was stimulated by deoxygenation in all cases. GLUT showed little response to O2 tension, other than a small stimulation in deoxygenated chicken red cells. Finally, AE1 was stimulated by oxygenation in HbA cells, but this stimulation by O2 was absent in HbS cells and pink ghosts prepared from HbA cells. The significance of these findings is discussed.

1-Chloro-2,4-dinitrobenzene (CDNB), which causes oxidative stress through depletion of reduced glutathione (GSH), increases the passive K+ permeability of red cells. In this paper, we investigated the effects of CDNB (1 mM) on the activities of the K+-Cl- cotransporter (KCC; measured as Cl--dependent K+ influx) and the Gardos channel (taken as clotrimazole-sensitive K+ influx, 5 microM) in human red cells, using 86Rb+ as a K+ congener. 45Ca2+ was used to study passive Ca2+ entry and active Ca2+ efflux via the plasma membrane Ca2+ pump. Both the Gardos channel and KCC were stimulated in both normal and sickle red cells. In sickle cells, stimulation of KCC was similar in oxygenated and deoxygenated cells; that of the Gardos channel was greater in deoxygenated cells. In normal red cells, stimulation of both pathways was greater in oxygenated cells (by 4 +/- 1-fold; all means +/- S.E.M., n = 3). The effects on the Gardos channel were dependent on extracellular Ca2+ and were associated with inhibition of the plasma membrane Ca2+ pump (by 29 +/- 3 %, P < 0.01) and increased Ca2+ sensitivity of the channel (EC50 for [Ca2+]i reduced from 260 +/- 26 to 175 +/- 15 nM; P < 0.05). Cell volume, pHi, ATP levels and passive Ca2+ entry were not affected by CDNB. The effects on KCC were inhibited (93 +/- 6 %) by prior treatment with the protein phosphatase inhibitor calyculin A (100 nM) and were not additive with stimulation by N-ethylmaleimide (1 mM), regardless of the order of addition. These findings are therefore consistent with inhibition of a regulatory protein kinase, although stimulation of the conjugate protein phosphatase(s) may also occur. KCC stimulation was also Ca2+ dependent. These findings are important for understanding how GSH depletion alters membrane permeability and how to protect against red cell dehydration.

The effect of phenazine methosulphate (PMS; 1 mM) on (86Rb+) K+ transport in human red cells was investigated to ascertain its action on the K+-Cl- cotransporter (KCC; defined as the Cl- dependent component of K+ flux measured in the presence of ouabain and bumetanide) and the Ca2+-activated K+ channel (Gardos channel; defined as the clotrimazole, 5 microM, -sensitive K+ flux). In the presence of Ca2+, both transport pathways were stimulated but effects were markedly greater under deoxygenated conditions (5-fold for KCC; 20-fold for the Gardos channel). KCC activation was inhibited by prior treatment with calyculin A (100 nM), implying action via protein dephosphorylation. Activation of the Gardos channel correlated with 28 +/- 3% inhibition of the plasma membrane Ca2+ pump, with maximal activity reduced from 7.7 +/- 1.1 to 2.7 +/- 0.7 micromol.(l cells.h)(-1) (all means +/- S.E.M. for n = 3), and a 3-fold increase in sensitivity of the channel to Ca2+ (EC50 reduced from 437 +/- 156 to 152 +/- 57 nM). Increased availability of NADH in deoxygenated conditions, resulting in increased free radical generation by PMS, may be responsible. We speculate that the similarity of the K+ transport phenotype produced by PMS to that seen in deoxygenated sickle cells is relevant to the pathophysiology of sickle cell disease.

Introduction: Intracellular calcium concentration ([Ca2+]i) is a critical parameter in cellular homeostasis, including articular chondrocytes. Perturbed [Ca2+]i of chondrocytes may be associated with joint disease. The objective of the study was to compare large animal models for investigating Ca2+ homeostasis in chondrocytes. Materials and Methods: The gross anatomy of the metacarpophalangeal joint (MCP) of cattle and sheep was compared, along with the effect of various manoeuvres used to study the mechanisms of Ca2+ homeostasis in chondrocytes from load-bearing areas. The gross anatomy was observed before and after dissection, and internal architecture was examined after sectioning. Cartilage thickness was measured with a digital micrometer. Chondrocyte yield was determined after isolation. Chondrocytes were incubated with Fura-2 and Ca2+i followed in different extracellular conditions. A hypotonic shock (HTS) was used to mimic removal of a load. Results: The results showed that ovids and bovids were skeletally immature and aspects of Ca2+ homeostasis were similar. Ovine chondrocytes had higher resting fluorescence, consistent with elevated resting Ca2+ levels. Results from ion substitution experiments were consistent with a role for Na+/Ca2+ exchange, and swelling-induced Ca2+ enters into the cytoplasm via the plasma membrane and intracellular stores. Conclusions: Ca2+ homeostasis in chondrocytes from both species behaved in a similar manner to HTS and ion substitutions. Differences in resting [Ca2+]i could be associated with species, stage of maturation, or Fura-2 itself and require further investigation. These findings contribute to our understanding of the physiology of articular cartilage in different species, and their potential use as models for studying joint disease in humans.

The heterocyclic aldehyde 5-hydroxymethyl-2-furfural (5HMF) interacts allosterically with the abnormal formof haemoglobin (Hb), HbS, in red blood cells (RBCs) frompatients with sickle cell disease (SCD), thereby increasing oxygen affinity and decreasing HbS polymerization andRBCsickling during hypoxia.We hypothesized that should5HMFalso inhibit the main cation pathways implicated in the dehydration of RBCs from SCD patients - the deoxygenation-induced cation pathway (Psickle), the Ca2+-activated K+ channel (the Gardos channel) and the K+-Cl- cotransporter (KCC) - it would have a synergistic effect in protection against sickling, directly through interactingwith HbS, and indirectly through maintaining hydration and reducing [HbS]. This study was therefore designed to investigate the effects of 5HMF on RBC volume and K+ permeability in vitro. 5HMF markedly reduced the deoxygenation-induced dehydration of RBCs whether in response to maintained deoxygenation or to cyclical deoxygenation/re-oxygenation. 5HMF was found to inhibit Psickle, an effect which correlated with its effects on sickling. Deoxygenation-induced activation of the Gardos channel and exposure of phosphatidylserine were also inhibited, probably indirectly via reduced entry of Ca2+ through the Psickle pathway. Effects of 5HMF on KCC were more modest with a slight inhibition in N-ethylmaleimide (NEM,1 mM)-treated RBCs and stimulation in RBCs untreated with NEM. These findings support the hypothesis that 5HMF may also be beneficial through effects on RBC ion and water homeostasis.

Background: Patients with sickle cell disease (SCD) have the abnormal haemoglobin (Hb) HbS in their red blood cells (RBCs). The complications of SCD may be extensive but are markedly variable between patients. Renal damage (sometimes called sickle cell nephropathy, SCN) is a common finding in about a third of children with SCD, a significant number of whom will proceed to renal failure. As yet, it is not possible to predict which children will develop SCN and who would therefore benefit from earlier and aggressive management. Various biomarkers have been investigated but none have proved efficacious. Changes in RBC membrane permeability are closely linked with polymerisation of HbS and, as such, may represent an important early event in pathogenesis. We postulated that certain RBC characteristics (sickling, K+ transport, PS exposure), which may be inherited independently of the HbS mutation, may correlate with renal pathology, but more importantly, may be identifiable in advance of damage, thereby providing prognostic markers to inform patient management. Participants and Methods: Children (>4 years old) with sickle cell anaemia (SCA) were recruited from the paediatric haematology clinic at King's College Hospital in South London, UK. Patients who had received blood transfusion in the preceding four months or who were on medications known to directly alter RBC cation transport (e.g. dipyridamole and Ca2+ channel blockers) were excluded. Patients on hydroxyurea were included. All patients were in steady state, with no acute symptoms requiring treatment for at least seven days. Clinical and laboratory parameters were collected prospectively at routine clinical attendance. Details of enuresis were collected using specifically developed proforma and patients were divided into two groups: those who stopped wetting their bed before the age of five years and those who were still enuretic after five years of age. RBC permeability was assessed using 86Rb+ as a congener for K+ to measure the activity of main cation transport systems involved in RBC dehydration. The activity of RBC transport pathways and percentage of sickling was measured at 100mmHg, 35mmHg, 15mmHg and 0mmHg. Statistical analysis was conducted using IBM-SPSS, New York. Results: 112 patients of HbSS genotype (SCA) consented to take part in the study. Their clinical profile is summarised Table 1. The percentage of children with enuresis, hypertension and significant albuminuria is shown in Table 2. There was significant correlation between activity of Gardos channel at O2 tensions of 35mmHg, 15mmHg and 0mmHg and enuresis after the age of five years. Psickle activity also showed significant correlation but only in fully deoxygenated RBCs (O2 of 0mmHg.) Table 3. Conclusion In conclusion, the present findings emphasise the significance of a thorough appreciation of RBC permeability to the pathogenesis of SCD. Whilst preliminary, these findings are exciting because they show that the activity of a particular transport pathways, abnormally elevated in SCA patients, are associated with enuresis. These findings suggest that the early identification of increased Gardos and Psickle activity may identify children who would benefit from early treatment for nocturnal enuresis. Disclosures No relevant conflicts of interest to declare.

Introduction: Reduced glutathione is an important antioxidant in red cells whose depletion may contribute to the pathophysiology of sickle cell disease. The current study was designed to examine the availability of reduced glutathione precursors (glutamate, cysteine, glycine and possibly glutamine) together with the activity of the main transport pathways for their uptake (system ASC for cysteine and glycine; system gly for glycine). Materials and methods: Blood samples were obtained from normal (HbAA, HbA cells) and sickle cell disease patients (HbSS, HbS cells); amino acids were measured by HPLC; and transporter activity was measured by radioactive tracer fluxes (using serine and glycine for activity of system ASC; and glycine for that of system gly). Results: Plasma concentrations of cysteine and glycine were increased and concentrations of all amino acids were elevated in HbS cells. The activity of system ASC was increased in HbS cells (both transport capacity and aAnity were elevated for serine transport; transport capacity only for glycine). Activity of system gly was also increased (twofold increase in Vmax for glycine flux), though not significantly. Oxygenation also increased the activity of both transporters in normal and HbS cells. CO prevented deoxy-inhibition of glycine transport. Staurosporine (5 mM) inhibited O2-stimulated glycine transport through system ASC. It also inhibited the absolute magnitude of transport through system gly, but the O2-dependent flux was unaAected. Conclusion: Low reduced glutathione levels in HbS cells were not due to decreased substrate availability and O2 stimulated transport of reduced glutathione percursors in both normal and HbScells,throughamechanismthatislikelytoinvolveHbandpossiblyproteinphosphorylation. The Hematology Journal (2000) 1, 243‐249

The ability to regulate cell volume in the face of perturbation is a commonly observed property of cells (Parker, 1993; Hoffmann & Dunham, 1995). Shrinkage in response to an initial increase in volume is termed regulatory volume decrease (RVD); swelling in response to shrinkage is termed regulatory volume increase (RVI). In the short term, RVI and RVD are carried out by a variety of membrane transport systems, which use pre-existing electrochemical gradients for the dissipative movement of a range of solutes (or osmolytes). Water then passively follows these solutes by osmosis. Volume regulatory capacities, together with the identity of the transport systems, vary with cell type and species. This is well illustrated by examining the pattern of volume regulatory responses found in vertebrate red cells (Cossins & Gibson, 1997), whose ease of procurement and homogeneity have made them a popular choice for the study of such processes. Avian red cells possess a powerful Na+-K+-2Cl− cotransport system (McManus & Schmidt, 1978; Alper et al. 1980; Kregenow, 1981; Palfrey & Greengard, 1981). In many tissues, this cotransporter can carry out RVI, as well as having a variety of other roles including transepithelial transport and possibly extrarenal K+ regulation (McManus & Schmidt, 1978; Chipperfield, 1986; Haas, 1994; Mount et al. 1998). The cotransporter has been cloned (and termed NKCC), and monoclonal antibodies to it are available, facilitating its study (Haas, 1994; Lytle, 1997). As well as responding to volume, the activity of NKCC in avian red cells is altered by a number of other stimuli including deoxygenation, fluoride, intracellular [Mg2+] and [ATP], and β-adrenergic agonists (Palfrey & Greengard, 1981). These stimuli fall into two groups, those which act via cAMP and those which do not. However, the transport protein per se is the final target of protein phosphorylation, possibly at the same residues regardless of the identity of the stimulus, since, in all cases studied, increased activity of the transporter was correlated with its phosphorylation (Lytle, 1997). We have shown previously that oxygen tension (PO2) represents the dominant modulator for an RVD system, K+-Cl− cotransport (termed KCC), in red cells from a number of different species (Cossins & Gibson, 1997). In this case, transport activity is stimulated by high PO2 levels. The response to O2 can be blocked by calyculin A (Cossins et al. 1994; Honess et al. 1996), a specific protein phosphatase inhibitor, indicating that a phosphorylation event is involved. We have also demonstrated that it is possible to ‘clamp’ the activity of KCC using sequential addition first of N-ethylmaleimide (NEM), which acts as a protein kinase inhibitor, and then of calyculin A (Cossins et al. 1994; Honess et al. 1996). Under these conditions, transport activity is locked and unresponsive to the usual stimuli, including PO2, swelling and acid. The responses of RVD systems (e.g. KCC) and RVI systems (e.g. NKCC cotransport) to many experimental manipulations are reciprocal (Parker et al. 1990; Cossins, 1991; Parker, 1994). Thus deoxygenation, which inhibits KCC, has been shown previously to stimulate NKCC in avian red cells (Palfrey & Greengard, 1981). In this report, we demonstrate that deoxygenation, in fact, represents a major stimulus for NKCC, just as oxygenation controls K+-Cl− cotransport, and we investigate its interaction with other stimuli. We also show that a ‘reverse clamp’ could be established with NKCC, treating cells first with calyculin A, then NEM, and again this clamp abolished its ability to respond to any stimulus. Our findings emphasize the reciprocal behaviour of RVD and RVI systems and are relevant to understanding their co-ordinated regulation. A preliminary account of some of these findings has been published previously (Muzyamba et al. 1999).

Phosphatidylserine (PS) exposure in red blood cells (RBCs) from sickle cell disease (SCD) patients is increased compared to levels in normal individuals and may participate in the anaemic and ischaemic complications of SCD. Exposure is increased by deoxygenation and occurs with elevation of intracellular Ca 2+ to low micromolar levels. The Ca2+ entry step has not been defined but a role for the deoxygenation-induced pathway, P sickle, is postulated. Partial Psickle inhibitors 4-acetamido-4′-isothiocyanostilbene-2,2′-disulphonic acid (SITS), 4,4′-dithiocyano-2,2′-stilbene-disulphonic acid (DIDS) and dipyridamole inhibited deoxygenation-induced PS exposure (DIDS IC50, 118 nM). Inhibitors and activators of other pathways (including these stimulated by depolarisation, benzodiazepines, glutamate and stretch) were without effect. Zn2+ and Gd3+ stimulated PS exposure to high levels. In the case of Zn2+, this effect was independent of oxygen (and hence HbS polymerisation and RBC sickling) but required extracellular Ca2+. The effect was completely abolished when Zn2+ (100 μM) was added to RBCs suspended in autologous plasma, implying a requirement of high levels of free Zn2+.

Key points • Red blood cells (RBCs) from the two main genotypes of sickle cell disease (SCD) patients showed haemolysis in deoxygenated isosmotic sucrose solution, to a greater extent than those from sickle trait individuals (HbAS). RBCs from normal individuals (HbAA) did not lyse. • Several treatments reduced haemolysis in RBCs from HbSS patients – cytochalasin B, aromatic aldehydes (o-vanillin and 5-hydroxymethylfurfural), urea, low temperature and high pH. These effects were not due to increased oxygen affinity of haemoglobin. Long-term treatment of patients with hydroxyurea was also associated with reduced levels of haemolysis. • Treatment with aromatic aldehydes and urea reduced sickling of HbSS RBCs in deoxygenated saline solutions. Further, RBCs from sickle trait individuals (HbAS) could be induced to sickle under acidic (pH 6), hypertonic (400 mosmol kg−1) conditions. When these conditions were reproduced in sucrose solutions, HbAS RBCs also showed considerable haemolysis. • Results suggest that haemolysis of HbS-containing RBCs is associated with HbS polymerisation, perhaps damaging the RBC membrane. These findings suggest that haemolysis may be used as a simple, novel technique to diagnose SCD, using the altered membrane permeability of RBCs rather than the presence of HbS per se. As extent of haemolysis varies between individuals and is reduced following hydroxyurea treatment, the method may also be useful prognostically. Abstract Red blood cells (RBCs) from patients with sickle cell disease (SCD) lyse in deoxygenated isosmotic non-electrolyte solutions. Haemolysis has features which suggest that it is linked to activation of the pathway termed Psickle. This pathway is usually described as a non-specific cationic conductance activated by deoxygenation, HbS polymerisation and RBC sickling. The current work addresses the hypothesis that this haemolysis will provide a novel diagnostic and prognostic test for SCD, dependent on the altered properties of the RBC membrane resulting from HbS polymerisation. A simple test represented by this haemolysis assay would be useful especially in less affluent deprived areas of the world where SCD is most prevalent. RBCs from HbSS and most HbSC individuals showed progressive lysis in deoxygenated isosmotic sucrose solution at pH 7.4 to a level greater than that observed with RBCs from HbAS or HbAA individuals. Cytochalasin B prevented haemolysis. Haemolysis was temperature- and pH-dependent. It required near physiological temperatures to occur in deoxygenated sucrose solutions at pH 7.4. At pH 6, haemolysis occurred even in oxygenated samples. Haemolysis was reduced in patients on long-term (>5 months) hydroxyurea treatment. Several manoeuvres which stabilise soluble HbS (aromatic aldehydes o-vanillin or 5-hydroxymethyl, and urea) reduced haemolysis, an effect not due to increased oxygen affinity. Conditions designed to elicit HbS polymerisation in cells from sickle trait patients (deoxygenated hyperosmotic sucrose solutions at pH 6) supported their haemolysis. These findings are consistent with haemolysis requiring HbS polymerisation and support the hypothesis that this may be used as a test for SCD.

Articular chondrocytes live in an unusual and constantly changing physicochemical environment. Due to the structure of the extracellular matrix, adult cartilage is avascular, relatively hypoxic and acidic compared to other tissues (Wilkins et al., 2000). In this challenging environment the maintenance and regulation of extracellular matrix by chondrocytes is dependent on signals received through this milieu (Lai et al., 2002). In joint disease, such as osteoarthritis, the extracellular environment is altered and the cellular physiology of the chondrocyte will change to reflect this, leading to alterations in its key role of regulating matrix turnover and hence contributing to the pathophysiology of joint disease (Goldring 2006). This chapter will discuss the challenges to the chondrocyte and how cellular physiology is affected in both health and disease. We will discuss how the structure of the matrix confers its biomechanical properties to cartilage and how this translates to physiological sensing by the cartilage during static and dynamic loading with particular emphasis on effects on membrane transporters and cell signalling pathways. We will also consider how other features of cartilage in the adult influence the chondrocyte, such as oxygen tension, osmolarity and pH. Finally we will consider the changes that occur in osteoarthritis and how these translate to alterations in cellular physiology and hence matrix integrity, the loss of the which is the key feature of osteoarthritis, and how these events may be new targets for treatment of this condition.

More than 100 different blood and urine biomarkers have been described in sickle cell disease (SCD), with the number increasing rapidly as analytical techniques develop. Nearly all of these biomarkers are abnormal in the steady state, and become more so during complications. The range of abnormalities demonstrates the multisystem nature of SCD and the complex pathophysiology. Some biomarkers indicate damage to specific organs, such as urine albumin:creatinine ratio in nephropathy, whereas others indicate more systemic processes. Biomarkers have been useful in identifying various interrelated pathological mechanisms, including haemolysis, inflammation, hypercoagulability, oxidative stress, reperfusion injury, vasculopathy and endothelial dysfunction. However, most biomarkers correlate closely with other more routine measurements, and also with each other. It is not clear that any provide specific prognostic or clinical information beyond that given by the simple measurement of haemoglobin concentration. The identification of prognostically validated biomarkers in prospective clinical trials would be useful.