Delay time of hemoglobin S polymerization prevents most cells from sickling in vivo

A framework of computer vision-enhanced microfluidic approach for automated assessment of the transient sickling kinetics in sickle red blood cells

The occurrence of vaso-occlusive crisis greatly depends on the competition between the sickling delay time and the transit time of individual sickle cells, i.e., red blood cells (RBCs) from sickle cell disease (SCD) patients, while they are traversing the circulatory system. Many drugs for treating SCD work by inhibiting the polymerization of sickle hemoglobin (HbS), effectively delaying the sickling process in sickle cells (SS RBCs). Most previous studies on screening anti-sickling drugs, such as voxelotor, rely on in vitro testing of sickling characteristics, often conducted under prolonged deoxygenation for up to 1 hour. However, since the microcirculation of RBCs typically takes less than 1 minute, the results of these studies may be less accurate and less relevant for in vitro-in vivo correlation. In our current study, we introduce a computer vision-enhanced microfluidic framework designed to automatically capture the transient sickling kinetics of SS RBCs within a 1-min timeframe. Our study has successfully detected differences in the transient sickling kinetics between vehicle control and voxelotor-treated SS RBCs. This approach has the potential for broader applications in screening anti-sickling therapies.

Clinical and laboratory factors associated with splenic dysfunction among patients with sickle cell disease in a malaria endemic region

BACKGROUND

Although loss of splenic function is the expected natural course for individuals with sickle cell disease (SCD), factors such as high HbF and coexistence of alpha thalassemia may ameliorate this process. We evaluated factors associated with two surrogate markers of spleen dysfunction, namely Howell-Jolly bodies (HJBs) and argyrophilic inclusion (AI) red cell counts, among patients with SCD.

METHODS

Cross-sectional data of 182 patients with SCD (median age 11 y; 1-45 y) and 102 normal controls (median age 12 y; 1-32 y) were evaluated. Blood tests including full blood count, serum chemistry and high-performance liquid chromatography were performed. The HJB and AI red cell counts were performed on peripheral blood smears.

RESULTS

The percentages of HJB and AI red cells rose significantly with increasing age in the SCD group. On regression analysis, the frequency of HJB red cells associated positively with mean corpuscular hemoglobin (MCH) (β=0.289; p=0.001) and negatively with HbF (β=-0.259; p=0.002). The AI red cell counts also associated positively with MCH (β=0.321; P=0.001) and negatively with HbF (β=-0.242; p=0.020).

CONCLUSIONS

Data from this study indicate that the negative association of HbF with both markers of splenic dysfunction among our patients with SCD residing in a malaria endemic region is similar to findings elsewhere of its ameliorating effect on splenic dysfunction.

Drug discovery by a basic research scientist

I was fortunate to do my military service during the Vietnam era as a medical officer at the National Institutes of Health (NIH) in Bethesda, Maryland. My first research at NIH was concerned with making a variety of optical measurements on nucleic acid bases and proteins, including single crystal spectra in linearly polarized light and near infrared circular dichroism, interpreting the spectra using molecular orbital and crystal field theories. What I do now is drug discovery, a field at the opposite end of the scientific spectrum. This article gives a brief account of my transition from spectroscopy to sickle cell hemoglobin polymerization to protein folding to drug discovery for treating sickle cell disease. My lab recently developed a high throughput assay to screen the 12,657 compounds of the California Institute of Biomedical Research ReFrame drug repurposing library. This is a precious library because the compounds have either been FDA approved or have been tested in clinical trials. Since the 1970s numerous agents have been reported in the literature to inhibit HbS polymerization and/or sickling with only one successful drug, hydroxyurea, and another of dubious value, voxelotor, even though it has been approved by the FDA. Our screen has discovered 106 anti-sickling agents in the ReFrame compound library. We estimate that as many as 21 of these compounds could become oral drugs for treating sickle cell disease because they inhibit at concentrations typical of the free concentrations of oral drugs in human serum.

Phenotypic screening of the ReFRAME drug repurposing library to discover new drugs for treating sickle cell disease

Stem cell transplantation and genetic therapies offer potential cures for patients with sickle cell disease (SCD), but these options require advanced medical facilities and are expensive. Consequently, these treatments will not be available for many years to the majority of patients suffering from this disease. What is urgently needed now is an inexpensive oral drug in addition to hydroxyurea, the only drug approved by the FDA that inhibits sickle-hemoglobin polymerization. Here, we report the results of the first phase of our phenotypic screen of the 12,657 compounds of the Scripps ReFRAME drug repurposing library using a recently developed high-throughput assay to measure sickling times following deoxygenation to 0% oxygen of red cells from sickle trait individuals. The ReFRAME library is a very important collection because the compounds are either FDA-approved drugs or have been tested in clinical trials. From dose-response measurements, 106 of the 12,657 compounds exhibit statistically significant antisickling at concentrations ranging from 31 nM to 10 μM. Compounds that inhibit sickling of trait cells are also effective with SCD cells. As many as 21 of the 106 antisickling compounds emerge as potential drugs. This estimate is based on a comparison of inhibitory concentrations with free concentrations of oral drugs in human serum. Moreover, the expected therapeutic potential for each level of inhibition can be predicted from measurements of sickling times for cells from individuals with sickle syndromes of varying severity. Our results should motivate others to develop one or more of these 106 compounds into drugs for treating SCD.

Intravenous Fluid Administration and the Risk of Adverse Outcomes in Sickle Cell Disease Patients Hospitalized for Vaso-Occlusive Crisis

Vaso-occlusive crisis (VOC) is the leading cause of hospitalization in sickle cell disease (SCD). Intravenous fluid (IVF) administration is the usual practice during VOC episodes to slow the sickling process. In the absence of an evidence-based, clear-cut consensus on the optimal choice, route, and rate of fluid administration, there has been a wide variability in the practice of IVF administration in the treatment of VOC. However, there are growing concerns about the safety of this practice. This systematic review summarized the current evidence on the risk of negative outcomes in SCD patients treated for VOC with IVFs. A database search of Medline/PubMed, EMBASE, Scopus, Web of Science, CINAHL, Wiley Cochrane Library, Clinicaltrials.gov, and conference proceedings of the European Hematology Association (EHA) and American Society of Hematology (ASH) were performed. The results were presented using narrative analysis of quantitative data. Of the 2,821 identified records, a total of three eligible retrospective cohort studies with a total demographic population of 549 SCD patients were included in this review. Normal saline, a frequently used IVF for VOC may be associated with adverse outcomes such as poor pain control and volume overload. Volume overload, new oxygen requirement, acute chest syndrome, and acute kidney injury are potential adverse outcomes of inappropriate IVF use in VOC. There is limited evidence supporting the current practice of IVF use in VOC. Randomized controlled trials are required to fully clarify the place and safety of IVF in the management of VOC.

Extracellular Vesicles in Sickle Cell Disease: A Promising Tool

Sickle cell disease (SCD) is the most common hemoglobinopathy worldwide. It is characterized by an impairment of shear stress-mediated vasodilation, a pro-coagulant, and a pro-adhesive state orchestrated among others by the depletion of the vasodilator nitric oxide, by the increased phosphatidylserine exposure and tissue factor expression, and by the increased interactions of erythrocytes with endothelial cells that mediate the overexpression of adhesion molecules such as VCAM-1, respectively. Extracellular vesicles (EVs) have been shown to be novel actors involved in SCD pathophysiological processes. Medium-sized EVs, also called microparticles, which exhibit increased plasma levels in this pathology, were shown to induce the activation of endothelial cells, thereby increasing neutrophil adhesion, a key process potentially leading to the main complication associated with SCD, vaso-occlusive crises (VOCs). Small-sized EVs, also named exosomes, which have also been reported to be overrepresented in SCD, were shown to potentiate interactions between erythrocytes and platelets, and to trigger endothelial monolayer disruption, two processes also known to favor the occurrence of VOCs. In this review we provide an overview of the current knowledge about EVs concentration and role in SCD.

The oral ferroportin inhibitor vamifeport improves hemodynamics in a mouse model of sickle cell disease

Sickle cell disease (SCD) is an inherited hemolytic anemia caused by a single point mutation in the β-globin gene of hemoglobin that leads to synthesis of sickle hemoglobin (HbS) in red blood cells (RBCs). HbS polymerizes in hypoxic conditions, leading to intravascular hemolysis, release of free hemoglobin and heme, and increased adhesion of blood cells to the endothelial vasculature, which causes painful vaso-occlusion and organ damage. HbS polymerization kinetics are strongly dependent on the intracellular HbS concentration; a relatively small reduction in cellular HbS concentration may prevent HbS polymerization and its sequelae. We hypothesized that iron restriction via blocking ferroportin, the unique iron transporter in mammals, might reduce HbS concentration in RBCs, thereby decreasing hemolysis, improving blood flow, and preventing vaso-occlusive events. Indeed, vamifeport (also known as VIT-2763), a clinical-stage oral ferroportin inhibitor, reduced hemolysis markers in the Townes model of SCD. The RBC indices of vamifeport-treated male and female Townes mice exhibited changes attributable to iron-restricted erythropoiesis: decreased corpuscular hemoglobin concentration mean and mean corpuscular volume, as well as increased hypochromic and microcytic RBC fractions. Furthermore, vamifeport reduced plasma soluble VCAM-1 concentrations, which suggests lowered vascular inflammation. Accordingly, intravital video microscopy of fluorescently labeled blood cells in the microvasculature of Townes mice treated with vamifeport revealed diminished adhesion to the endothelium and improved hemodynamics. These preclinical data provide a strong proof-of-concept for vamifeport in the Townes model of SCD and support further development of this compound as a potential novel therapy in SCD.

Mitapivat increases ATP and decreases oxidative stress and erythrocyte mitochondria retention in a SCD mouse model

Polymerization of deoxygenated sickle hemoglobin (HbS) leads to erythrocyte sickling. Enhancing activity of the erythrocyte glycolytic pathway has anti-sickling potential as this reduces 2,3-diphosphoglycerate (2,3-DPG) and increases ATP, factors that decrease HbS polymerization and improve erythrocyte membrane integrity. These factors can be modulated by mitapivat, which activates erythrocyte pyruvate kinase (PKR) and improves sickling kinetics in SCD patients. We investigated mechanisms by which mitapivat may impact SCD by examining its effects in the Townes SCD mouse model. Control (HbAA) and sickle (HbSS) mice were treated with mitapivat or vehicle. Surprisingly, HbSS had higher PKR protein, higher ATP, and lower 2,3-DPG levels, compared to HbAA mice, in contrast with humans with SCD, in whom 2,3-DPG is elevated compared to healthy subjects. Despite our inability to investigate 2,3-DPG-mediated sickling and hemoglobin effects, mitapivat yielded potential benefits in HbSS mice. Mitapivat further increased ATP without significantly changing 2,3-DPG or hemoglobin levels, and decreased levels of leukocytosis, erythrocyte oxidative stress, and the percentage of erythrocytes that retained mitochondria in HbSS mice. These data suggest that, even though Townes HbSS mice have increased PKR activity, further activation of PKR with mitapivat yields potentially beneficial effects that are independent of changes in sickling or hemoglobin levels.

A Patient With Hemoglobin SC Disease and Acute Ischemic Stroke Presenting With Altered Mental Status

A 57-year-old woman with a history of hypertension, diabetes mellitus, obesity, asthma, and hemoglobin SC disease presented to the emergency department by her home health aide after she was found having altered mental status. According to her home health aide, the patient was responding with "Ok" to her questions for more than a day. The hemoglobin on admission was 8.5 g/dL. A magnetic resonance imaging (MRI) without contrast of the brain showed acute cortical infarcts superimposed on the old infarct zone. The patient received 1 unit of packed red blood cells and a session of exchange transfusion, in addition to aspirin, clopidogrel, and atorvastatin during the hospital stay. When a patient known to have sickle cell disease presents with acute neurological deficits, the first consideration is usually acute ischemic stroke due to vaso-occlusion in the cerebral vessels. However, it is essential to not overlook other potential causes of acute neurological deficits.

Extracellular Vesicles in Sickle Cell Disease: Plasma Concentration, Blood Cell Types Origin Distribution and Biological Properties

Prototype of monogenic disorder, sickle cell disease (SCD) is caused by a unique single mutation in the β-globin gene, leading to the production of the abnormal hemoglobin S (HbS). HbS polymerization in deoxygenated condition induces the sickling of red blood cells (RBCs), which become less deformable and more fragile, and thus prone to lysis. In addition to anemia, SCD patients may exhibit a plethora of clinical manifestations ranging from acute complications such as the frequent and debilitating painful vaso-occlusive crisis to chronic end organ damages. Several interrelated pathophysiological processes have been described, including impaired blood rheology, increased blood cell adhesion, coagulation, inflammation and enhanced oxidative stress among others. During the last two decades, it has been shown that extracellular vesicles (EVs), defined as cell-derived anucleated particles delimited by a lipid bilayer, and comprising small EVs (sEVs) and medium/large EVs (m/lEVs); are not only biomarkers but also subcellular actors in SCD pathophysiology. Plasma concentration of m/lEVs, originated mainly from RBCs and platelets (PLTs) but also from the other blood cell types, is higher in SCD patients than in healthy controls. The concentration and the density of externalized phosphatidylserine of those released from RBCs may vary according to clinical status (crisis vs. steady state) and treatment (hydroxyurea). Besides their procoagulant properties initially described, RBC-m/lEVs may promote inflammation through their effects on monocytes/macrophages and endothelial cells. Although less intensely studied, sEVs plasma concentration is increased in SCD and these EVs may cause endothelial damages. In addition, sEVs released from activated PLTs trigger PLT-neutrophil aggregation involved in lung vaso-occlusion in sickle mice. Altogether, these data clearly indicate that EVs are both biomarkers and bio-effectors in SCD, which deserve further studies.

Impact of hemoglobin biophysical studies on molecular pathogenesis and drug therapy for sickle cell disease

Basic research on hemoglobin has been essential for understanding the origin and treatment of many hematological disorders due to abnormal hemoglobins. The most important of the hemoglobinopathies is sickle cell disease - Linus Pauling's "molecular disease" that gave birth to molecular medicine. In this review, I will describe the contributions of basic biophysical research on normal and sickle cell hemoglobin (HbS) to understanding the molecular pathogenesis of the disease and providing the conceptual basis for the various approaches to drug therapy that target HbS polymerization. Most prominent among these are the experimental results on the solubility of HbS as a function of oxygen saturation explained by the allosteric model of Monod, Wyman, and Changeux and the Gill-Wyman thermodynamic linkage relation between solubility and oxygen binding, the solubility of mixtures of HbS with normal or fetal hemoglobin explained by Minton's thermodynamic model, and the highly unusual kinetics of HbS polymerization explained by a novel double nucleation mechanism that also accounts for the aggregation kinetics of the Alzheimer's peptide. The HbS polymerization kinetics are of great importance to understanding the pathophysiology and clinical course, as well as guiding drug development for treating this common and severe disease. The article focuses primarily on experimental and theoretical results from my lab, so it is not a comprehensive review of the subject.

Treatment of sickle cell disease by increasing oxygen affinity of hemoglobin

The issue of treating sickle cell disease with drugs that increase hemoglobin oxygen affinity has come to the fore with the FDA approval in 2019 of voxelotor, the only anti-sickling drug approved since hydroxyurea in 1998. Voxelotor reduces sickling by increasing the concentration of the non-polymerizing, high oxygen affinity R (oxy) conformation of HbS. Treatment of sickle cell patients with voxelotor increases Hb levels and decreases indicators of hemolysis, but with no indication as yet that it reduces the frequency of pain episodes. Here we use the allosteric model of Monod, Wyman, and Changeux to simulate whole blood oxygen dissociation curves and red cell sickling in the absence and presence of voxelotor under the in vivo conditions of rapid oxygen pressure decreases. Our modeling agrees with experiments using a new robust assay, which shows the very large, expected decrease in sickling from the drug. The modeling indicates, however, that the increase in oxygen delivery from reduced sickling is largely offset by the increase in oxygen affinity. The net result is that the drug increases overall oxygen delivery only at the very lowest oxygen pressures. Reduction of sickling does, however, mitigate against red cell damage and explains the observed decrease in hemolysis. More importantly, our modeling of in vivo oxygen dissociation, sickling, and oxygen delivery suggests that drugs that increase fetal hemoglobin or decrease MCHC, should be more therapeutically effective than drugs that increase oxygen affinity.

MWC allosteric model explains unusual hemoglobin-oxygen binding curves from sickle cell drug binding

An oxygen-affinity-modifying drug, voxelotor, has very recently been approved by the FDA for treatment of sickle cell disease. The proposed mechanism of action is by preferential binding of the drug to the R quaternary conformation, which cannot copolymerize with the T conformation to form sickle fibers. Here, we report widely different oxygen dissociation and oxygen association curves for normal blood in the presence of voxelotor and interpret the results in terms of the allosteric model of Monod, Wyman, and Changeux with the addition of drug binding. The model does remarkably well in quantitatively explaining a complex data set with just the addition of drug binding and dissociation rates for the R and T conformations. Whereas slow dissociation of the drug from R results in time-independent dissociation curves, the changing association curves result from slow dissociation of the drug from T, as well as extremely slow binding of the drug to T. By calculating true equilibrium curves from the model parameters, we show that there would be a smaller decrease in oxygen delivery from the left shift in the dissociation curve caused by drug binding if drug binding and dissociation for both R and T were rapid. Our application of the Monod, Wyman, and Changeux model demonstrates once more its enormous power in explaining many different kinds of experimental results for hemoglobin. It should also be helpful in analyzing oxygen binding and in vivo delivery in future investigations of oxygen-affinity-modifying drugs for sickle cell disease.

Voxelotor: alteration of sickle cell disease pathophysiology by a first-in-class polymerization inhibitor

Sickle cell disease, despite its recognition as a severely debilitating genetic condition affecting hundreds of thousands of neonates throughout the world each year, was not a target of pharmaceutical research focus for most of its 100-year existence in the medical consciousness. This has changed in recent years as many novel therapeutics are currently under investigation, with three new disease-modifying drugs achieving FDA approval in the last 4 years. One of these drugs, voxelotor, is especially encouraging as an inhibitor of sickling for its ability to safely improve the chronic hemolytic anemia of sickle cell disease. This was demonstrated during all clinical phases of investigation by an average improvement in hemoglobin of greater than 1 g/dL, as well as statistically significant improvements in established markers of hemolysis. While anemia itself represents a potential cause of morbidity, it is more importantly a marker of the hemolysis known to cause the long-term vascular and organ damage that makes sickle cell disease so debilitating and frequently fatal early in life. Given the recency of the approval, there has not been sufficient long-term follow-up to demonstrate improvement in the chronic sequelae of sickle cell disease as a result of voxelotor-induced improvements in hemolytic anemia. There is hope, however, based on the experience with hydroxyurea improving morbidity and mortality via reductions in sickling and improved rheology, that voxelotor may have similar long-term benefits by positively manipulating the kinetics of hemoglobin polymerization. This review aims to summarize the targeted pathobiology of sickle cell disease, the mechanism of action of voxelotor, and the safety and efficacy data from preclinical to late clinical stage investigations of this long-awaited medication, in the hopes of better informing the decision-making process behind prescribing or not prescribing it for patients in need of intervention.

Hydroxyurea improves nitric oxide bioavailability in humanized sickle cell mice

Despite advancements in disease management, sickle cell nephropathy, a major contributor to mortality and morbidity in patients, has limited therapeutic options. Previous studies indicate hydroxyurea, a commonly prescribed therapy for sickle cell disease (SCD), can reduce renal injury in SCD but the mechanisms are uncertain. Because SCD is associated with reduced nitric oxide (NO) bioavailability, we hypothesized that hydroxyurea treatment would improve NO bioavailability in the humanized sickle cell mouse. Humanized male 12-wk-old sickle (HbSS) and genetic control (HbAA) mice were treated with hydroxyurea or regular tap water for 2 wk before renal and systemic NO bioavailability as well as renal injury were assessed. Untreated HbSS mice exhibited increased proteinuria, elevated plasma endothelin-1 (ET-1), and reduced urine concentrating ability compared with HbAA mice. Hydroxyurea reduced proteinuria and plasma ET-1 levels in HbSS mice. Untreated HbSS mice had reduced plasma nitrite and elevated plasma arginase concentrations compared with HbAA mice. Hydroxyurea treatment augmented plasma nitrite and attenuated plasma arginase in HbSS mice. Renal vessels isolated from HbSS mice also had elevated nitric oxide synthase 3 (NOS3) and arginase 2 expression compared with untreated HbAA mice. Hydroxyurea treatment did not alter renal vascular NOS3, however, renal vascular arginase 2 expression was significantly reduced. These data support the hypothesis that hydroxyurea treatment augments renal and systemic NO bioavailability by reducing arginase activity as a potential mechanism for the improvement on renal injury seen in SCD mice.

Microvascular thrombosis: experimental and clinical implications

A significant amount of clinical and research interest in thrombosis is focused on large vessels (eg, stroke, myocardial infarction, deep venous thrombosis, etc.); however, thrombosis is often present in the microcirculation in a variety of significant human diseases, such as disseminated intravascular coagulation, thrombotic microangiopathy, sickle cell disease, and others. Further, microvascular thrombosis has recently been demonstrated in patients with COVID-19, and has been proposed to mediate the pathogenesis of organ injury in this disease. In many of these conditions, microvascular thrombosis is accompanied by inflammation, an association referred to as thromboinflammation. In this review, we discuss endogenous regulatory mechanisms that prevent thrombosis in the microcirculation, experimental approaches to induce microvascular thrombi, and clinical conditions associated with microvascular thrombosis. A greater understanding of the links between inflammation and thrombosis in the microcirculation is anticipated to provide optimal therapeutic targets for patients with diseases accompanied by microvascular thrombosis.

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.

Allosteric control of hemoglobin S fiber formation by oxygen and its relation to the pathophysiology of sickle cell disease

The pathology of sickle cell disease is caused by polymerization of the abnormal hemoglobin S upon deoxygenation in the tissues to form fibers in red cells, causing them to deform and occlude the circulation. Drugs that allosterically shift the quaternary equilibrium from the polymerizing T quaternary structure to the nonpolymerizing R quaternary structure are now being developed. Here we update our understanding on the allosteric control of fiber formation at equilibrium by showing how the simplest extension of the classic quaternary two-state allosteric model of Monod, Wyman, and Changeux to include tertiary conformational changes provides a better quantitative description. We also show that if fiber formation is at equilibrium in vivo, the vast majority of cells in most tissues would contain fibers, indicating that it is unlikely that the disease would be survivable once the nonpolymerizing fetal hemoglobin has been replaced by adult hemoglobin S at about 1 y after birth. Calculations of sickling times, based on a recently discovered universal relation between the delay time prior to fiber formation and supersaturation, show that in vivo fiber formation is very far from equilibrium. Our analysis indicates that patients survive because the delay period allows the majority of cells to escape the small vessels of the tissues before fibers form. The enormous sensitivity of the duration of the delay period to intracellular hemoglobin composition also explains why sickle trait, the heterozygous condition, and the compound heterozygous condition of hemoglobin S with pancellular hereditary persistence of fetal hemoglobin are both relatively benign conditions.

Haptoglobin Therapeutics and Compartmentalization of Cell-Free Hemoglobin Toxicity

Hemolysis and accumulation of cell-free hemoglobin (Hb) in the circulation or in confined tissue compartments such as the subarachnoid space is an important driver of disease. Haptoglobin is the Hb binding and clearance protein in human plasma and an efficient antagonist of Hb toxicity resulting from physiological red blood cell turnover. However, endogenous concentrations of haptoglobin are insufficient to provide protection against Hb-driven disease processes in conditions such as sickle cell anemia, sepsis, transfusion reactions, medical-device associated hemolysis, or after a subarachnoid hemorrhage. As a result, there is increasing interest in developing haptoglobin therapeutics to target 'toxic' cell-free Hb exposures. Here, we discuss key concepts of Hb toxicity and provide a perspective on the use of haptoglobin as a therapeutic protein.

Hemoglobin S polymerization and sickle cell disease: A retrospective on the occasion of the 70th anniversary of Pauling's Science paper

70 years ago, Linus Pauling, the legendary genius of 20^th century chemistry, published his famous work on the molecular cause of sickle cell disease, a paper that gave birth to what is now called molecular medicine. In this paper, Pauling left important questions unanswered that have motivated an enormous amount of scientific and clinical research since then. This retrospective discusses the basic science studies that have answered those questions directly related to the kinetics and thermodynamics of hemoglobin S polymerization.

High-throughput assessment of hemoglobin polymer in single red blood cells from sickle cell patients under controlled oxygen tension

Sickle cell disease (SCD) is caused by a variant hemoglobin molecule that polymerizes inside red blood cells (RBCs) in reduced oxygen tension. Treatment development has been slow for this typically severe disease, but there is current optimism for curative gene transfer strategies to induce expression of fetal hemoglobin or other nonsickling hemoglobin isoforms. All SCD morbidity and mortality arise directly or indirectly from polymer formation in individual RBCs. Identifying patients at highest risk of complications and treatment candidates with the greatest curative potential therefore requires determining the amount of polymer in individual RBCs under controlled oxygen. Here, we report a semiquantitative measurement of hemoglobin polymer in single RBCs as a function of oxygen. The method takes advantage of the reduced oxygen affinity of hemoglobin polymer to infer polymer content for thousands of RBCs from their overall oxygen saturation. The method enables approaches for SCD treatment development and precision medicine.

Antisickling Drugs Targeting βCys93 Reduce Iron Oxidation and Oxidative Changes in Sickle Cell Hemoglobin

Sickle cell disease is a genetic blood disorder caused by a single point mutation in the β globin gene where glutamic acid is replaced by valine at the sixth position of the β chain of hemoglobin (Hb). At low oxygen tension, the polymerization of deoxyHbS into fibers occurs in red blood cells (RBCs) leading to an impaired blood vessel transit. Sickle cell hemoglobin (HbS), when oxidized with hydrogen peroxide (H₂O₂), stays longer in a highly oxidizing ferryl (Fe⁴⁺) form causing irreversible oxidation of βCys93 to a destabilizing cysteic acid. We have previously reported that an antisickling drug can be designed to bind specifically to βCys93 and effectively protect against its irreversible oxidation by H₂O₂. Here, we report oxygen dissociation, oxidation, and polymerization kinetic reactions for four antisickling drugs (under different preclinical/clinical developmental stages) that either site-specifically target βCys93 or other sites on the HbS molecule. Molecules that specifically bind to or modify βCys93, such as 4,4′-di(1,2,3-triazolyl) disulfide (TD-3) and hydroxyurea (HU) were contrasted with molecules that target other sites on Hb including 5-hydroxymethyl-2-furfural (5-HMF) and L-glutamine. All reagents induced a left shift in the oxygen dissociation curve (ODC) except L-glutamine. In the presence of H₂O₂ (2.5:1, H₂O₂:heme), both TD-3 and HU reduced the ferryl heme by 22 and 37%, respectively, which corresponded to a 3- to 2-fold reduction in the levels of βCys93 oxidation as verified by mass spectrometry. Increases in the delay times prior to polymerization of HbS under hypoxia were in the following order: TD-3 > HU > 5-HMF = L-glutamine. Designing antisickling agents that can specifically target βCys93 may provide a dual antioxidant and antisickling therapeutic benefits in treating this disease.

Faster Sickling Kinetics and Sickle Cell Shape Evolution during Repeated Deoxygenation and Oxygenation Cycles

Kinetics of cell sickling and morphological change have been recognized as important parameters that are correlated closely with altered blood rheology and vasoocclusion in microcirculation. A microfluidic transient hypoxia assay was developed to create repeated hypoxia-normoxia cycles for real time observation of repetitive sickling and unsickling of freely suspended red blood cells (RBCs) from sickle cell disease patients. Cell sickling behavior and kinetics were found to be influenced by its previous sickling-unsickling processes accumulatively, where those sickled RBCs that had a history of sickling in a previous hypoxia cycle would sickle again in subsequent hypoxia/sickling cycles and the collective sickling kinetics became progressively faster (with reduced delay time and higher sickled fraction versus deoxygenation time). Individual sickled RBCs would sickle into drastically different shapes randomly in subsequent hypoxia/sickling cycles, however, the collective shape distribution retained similar characteristics. These observations indicate a gradual worsening trend in sickling kinetics over repeated hypoxia cycles, as well as a relatively stable collective shape characteristics within a limited number of hypoxia-normoxia cycles.

Insight into the complex pathophysiology of sickle cell anaemia and possible treatment

Sickle cell anaemia (SCA) is the consequence of abnormal haemoglobin production due to an inherited point mutation in the β-globin gene. The resulting haemoglobin tetramer is poorly soluble when deoxygenated, and when this is prolonged, intracellular gelation of sickle haemoglobin occurs, followed by haemoglobin polymerisation. If many cycles of sickling and unsickling occur, the red cell membrane will be disrupted leading to haemolysis and vaso-occlusive events. Recent studies have also shown that leucocyte adhesion molecules and nitric oxide (NO) depletion are involved in endothelial damage. New insights in SCA pathophysiology and vascular biology have shown that cell-derived microparticle (MP) generation is also involved in the vaso-occlusion. Endothelial damage is perpetuated by impaired production or increased consumption of protective modulators such as protein C, protein S and NO. New therapeutic interventions should address these aspects of SCA pathogenesis. To date, the only US-FDA-approved therapy to prevent painful vaso-occulsive episodes is hydroxyurea that reduces haemoglobin polymerisation in sickle cells by increasing the production of foetal haemoglobin and L-glutamine. However, several new drugs have been tested in the last years in randomised clinical trials. We here report an update on the current status of knowledge on SCA.

Red blood cell mechanical sensitivity improves in patients with sickle cell disease undergoing chronic transfusion after prolonged, subhemolytic shear exposure

BACKGROUND

Sickle cell disease (SCD) is a genetically inherited hemoglobinopathy in which deoxygenated hemoglobin S polymerizes, leading to stiff red blood cells (RBCs) and inefficient microcirculatory blood flow. Transfusion therapy acts as primary and secondary prevention of ischemic stroke in SCD. Whether blood transfusion alters the mechanical sensitivity (MS) of RBCs to prolonged subhemolytic shear stress (shear) is unknown. We hypothesized that individuals with SCD undergoing chronic blood transfusion would have improved sensitivity to shear, compared with patients not undergoing transfusion therapy.

STUDY DESIGN AND METHODS

Blood suspensions from individuals with SCD not receiving (n = 15) and receiving (n = 15) chronic simple transfusion were conditioned to shear (1, 4, 16, 32, and 64 Pa) for various durations (1, 4, 16, 32, and 64 sec), and then deformability of RBCs was immediately measured. Healthy young controls (n = 15) were included for reference. A surface mesh was interpolated using the data to determine the effect of blood transfusion on MS of RBCs.

RESULTS

There was impaired RBC deformability to prolonged supraphysiologic shear in both SCD groups; however, MS improved in transfused patients when exposed to prolonged physiologic shear. Furthermore, in the transfused patients with SCD, the threshold above which subhemolytic damage occurs was similar to controls.

CONCLUSION

We found that chronic transfusion therapy normalizes the MS threshold above which RBC subhemolytic damage occurs after prolonged shear exposure in SCD. An important and novel finding in transfused patients with SCD was the improvement in RBC deformability in response to prolonged shear exposure over the physiologic range.

Sickle Cell Anemia and Its Phenotypes

In the 100 years since sickle cell anemia (SCA) was first described in the medical literature, studies of its molecular and pathophysiological basis have been at the vanguard of scientific discovery. By contrast, the translation of such knowledge into treatments that improve the lives of those affected has been much too slow. Recent years, however, have seen major advances on several fronts. A more detailed understanding of the switch from fetal to adult hemoglobin and the identification of regulators such as BCL11A provide hope that these findings will be translated into genomic-based approaches to the therapeutic reactivation of hemoglobin F production in patients with SCA. Meanwhile, an unprecedented number of new drugs aimed at both the treatment and prevention of end-organ damage are now in the pipeline, outcomes from potentially curative treatments such as allogeneic hematopoietic stem cell transplantation are improving, and great strides are being made in gene therapy, where methods employing both antisickling β-globin lentiviral vectors and gene editing are now entering clinical trials. Encouragingly, after a century of neglect, the profile of the vast majority of those with SCA in Africa and India is also finally improving.

A microfluidic platform to study the effects of vascular architecture and oxygen gradients on sickle blood flow

Our goal was to develop a model of the microvasculature that would allow us to quantify changes in the rheology of sickle blood as it traverses the varying vessel sizes and oxygen tensions in the microcirculation. We designed and implemented a microfluidic model of the microcirculation that comprises a branching microvascular network and physiologic oxygen gradients. We used computational modeling to determine the parameters necessary to generate stable, linear gradients in our devices. Sickle blood from six unique patients was perfused through the microvascular network and subjected to varying oxygen gradients while we observed and quantified blood flow. We found that all sickle blood samples fully occluded the microvascular network when deoxygenated, and we observed that sickle blood could cause vaso-occlusions under physiologic oxygen gradients during the microvascular transit time. The number of occlusions observed under five unique oxygen gradients varied among the patient samples, but we generally observed that the number of occlusions decreased with increasing inlet oxygen tension. The model system we have developed is a valuable tool to address fundamental questions about where in the circulation sickle-cell vaso-occlusions are most likely to occur and to test new therapies.

GBT1118, a compound that increases the oxygen affinity of hemoglobin, improves survival in murine hypoxic acute lung injury

Acute respiratory distress syndrome (ARDS) is characterized by lung inflammation and pulmonary edema, leading to arterial hypoxemia and death if the hypoxemia is severe. Strategies to correct hypoxemia have the potential to improve clinical outcomes in ARDS. The goal of this study was to evaluate the potential of hemoglobin modification as a novel therapy for ARDS-induced hypoxemia. The therapeutic effect of two different doses of GBT1118, a compound that increases the oxygen affinity of hemoglobin, was evaluated in a murine model of acute lung injury induced by intratracheal LPS instillation 24 h before exposure to 5% or 10% hypoxia ( n = 8-15 per group). As expected, administration of GBT1118 to mice significantly increased the oxygen affinity of hemoglobin. Compared with mice receiving vehicle control, mice treated with GBT1118 had significantly lower mortality after LPS + 5% hypoxia (47% with vehicle vs. 22% with low-dose GBT1118, 13% with high-dose GBT1118, P = 0.032 by log rank) and had reduced severity of illness. Mice treated with GBT1118 showed a sustained significant increase in SpO₂ over 4 h of hypoxia exposure. Treatment with GBT1118 did not alter alveolar-capillary permeability, bronchoalveolar lavage (BAL) inflammatory cell counts, or BAL concentrations of IL-1β, TNF-α, or macrophage inflammatory protein-1α. High-dose GBT1118 did not affect histological lung injury but did decrease tissue hypoxia as measured intensity of pimonidazole (Hypoxyprobe) staining in liver ( P = 0.043) and kidney ( P = 0.043). We concluded that increasing the oxygen affinity of hemoglobin using GBT1118 may be a novel therapy for treating hypoxemia associated with acute lung injury. NEW & NOTEWORTHY In this study, we show that GBT1118, a compound that increases hemoglobin affinity for oxygen, improves survival and oxygen saturation in a two-hit lung injury model of intratracheal LPS without causing tissue hypoxia. Modulation of hemoglobin oxygen affinity represents a novel therapeutic approach to treatment of acute lung injury and acute respiratory distress syndrome, conditions characterized by hypoxemia.

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.

Extracellular fluid tonicity impacts sickle red blood cell deformability and adhesion

Abnormal sickle red blood cell (sRBC) biomechanics, including pathological deformability and adhesion, correlate with clinical severity in sickle cell disease (SCD). Clinical intravenous fluids (IVFs) of various tonicities are often used during treatment of vaso-occlusive pain episodes (VOE), the major cause of morbidity in SCD. However, evidence-based guidelines are lacking, and there is no consensus regarding which IVFs to use during VOE. Further, it is unknown how altering extracellular fluid tonicity with IVFs affects sRBC biomechanics in the microcirculation, where vaso-occlusion takes place. Here, we report how altering extracellular fluid tonicity with admixtures of clinical IVFs affects sRBC biomechanical properties by leveraging novel in vitro microfluidic models of the microcirculation, including 1 capable of deoxygenating the sRBC environment to monitor changes in microchannel occlusion risk and an "endothelialized" microvascular model that measures alterations in sRBC/endothelium adhesion under postcapillary venular conditions. Admixtures with higher tonicities (sodium = 141 mEq/L) affected sRBC biomechanics by decreasing sRBC deformability, increasing sRBC occlusion under normoxic and hypoxic conditions, and increasing sRBC adhesion in our microfluidic human microvasculature models. Admixtures with excessive hypotonicity (sodium = 103 mEq/L), in contrast, decreased sRBC adhesion, but overswelling prolonged sRBC transit times in capillary-sized microchannels. Admixtures with intermediate tonicities (sodium = 111-122 mEq/L) resulted in optimal changes in sRBC biomechanics, thereby reducing the risk for vaso-occlusion in our models. These results have significant translational implications for patients with SCD and warrant a large-scale prospective clinical study addressing optimal IVF management during VOE in SCD.

Treating sickle cell disease by targeting HbS polymerization

Although the root cause of sickle cell disease is the polymerization of hemoglobin S (HbS) to form fibers that make red cells less flexible, most drugs currently being assessed in clinical trials are targeting the downstream sequelae of this primary event. Less attention has been devoted to investigation of the multiple ways in which fiber formation can be inhibited. In this article, we describe the molecular rationale for 5 distinct approaches to inhibiting polymerization and also discuss progress with the few antipolymerization drugs currently in clinical trials.

Patient-specific modeling of individual sickle cell behavior under transient hypoxia

Sickle cell disease (SCD) is a highly complex genetic blood disorder in which red blood cells (RBC) exhibit heterogeneous morphology changes and decreased deformability. We employ a kinetic model for cell morphological sickling that invokes parameters derived from patient-specific data. This model is used to investigate the dynamics of individual sickle cells in a capillary-like microenvironment in order to address various mechanisms associated with SCD. We show that all RBCs, both hypoxia-unaffected and hypoxia-affected ones, regularly pass through microgates under oxygenated state. However, the hypoxia-affected cells undergo sickling which significantly alters cell dynamics. In particular, the dense and rigid sickle RBCs are obstructed thereby clogging blood flow while the less dense and deformable ones are capable of circumnavigating dead (trapped) cells ahead of them by choosing a serpentine path. Informed by recent experiments involving microfluidics that provide in vitro quantitative information on cell dynamics under transient hypoxia conditions, we have performed detailed computational simulations of alterations to cell behavior in response to morphological changes and membrane stiffening. Our model reveals that SCD exhibits substantial heterogeneity even within a particular density-fractionated subpopulation. These findings provide unique insights into how individual sickle cells move through capillaries under transient hypoxic conditions, and offer novel possibilities for designing effective therapeutic interventions for SCD.

Sickle cell disease is a genetic blood disease that causes vaso-occlusive pain crises. Here, we investigate the individual sickle cell behavior under controlled hypoxic conditions through patient-specific predictive computational simulations that are informed by companion microfluidic experiments. We identify the different dynamic behavior between individual sickle RBCs and normal ones in microfluidic flow, and analyze the hypoxia-induced alteration in individual cell behavior and single-cell capillary obstruction under physiological conditions.

Kinetic assay shows that increasing red cell volume could be a treatment for sickle cell disease

Although it has been known for more than 60 years that the cause of sickle cell disease is polymerization of a hemoglobin mutant, hydroxyurea is the only drug approved for treatment by the US Food and Drug Administration. This drug, however, is only partially successful, and the discovery of additional drugs that inhibit fiber formation has been hampered by the lack of a sensitive and quantitative cellular assay. Here, we describe such a method in a 96-well plate format that is based on laser-induced polymerization in sickle trait cells and robust, automated image analysis to detect the precise time at which fibers distort ("sickle") the cells. With this kinetic method, we show that small increases in cell volume to reduce the hemoglobin concentration can result in therapeutic increases in the delay time prior to fiber formation. We also show that, of the two drugs (AES103 and GBT440) in clinical trials that inhibit polymerization by increasing oxygen affinity, one of them (GBT440) also inhibits sickling in the absence of oxygen by two additional mechanisms.

Knowledge insufficient: the management of haemoglobin SC disease

Although haemoglobin SC (HbSC) accounts for 30% of sickle cell disease (SCD) in the United States and United Kingdom, evidence-based guidelines for genotype specific management are lacking. The unique pathology of HbSC disease is complex, characterized by erythrocyte dehydration, intracellular sickling and increased blood viscosity. The evaluation and treatment of patients with HbSC is largely inferred from studies of SCD consisting mostly of haemoglobin SS (HbSS) patients. These studies are underpowered to allow definitive conclusions about HbSC. We review the pathophysiology of HbSC disease, including known and potential differences between HbSS and HbSC, and highlight knowledge gaps in HbSC disease management. Clinical and translational research is needed to develop targeted treatments and to validate management recommendations for efficacy, safety and impact on quality of life for people with HbSC.

Biomechanics and biorheology of red blood cells in sickle cell anemia

Sickle cell anemia (SCA) is an inherited blood disorder that causes painful crises due to vaso-occlusion of small blood vessels. The primary cause of the clinical phenotype of SCA is the intracellular polymerization of sickle hemoglobin resulting in sickling of red blood cells (RBCs) in deoxygenated conditions. In this review, we discuss the biomechanical and biorheological characteristics of sickle RBCs and sickle blood as well as their implications toward a better understanding of the pathophysiology and pathogenesis of SCA. Additionally, we highlight the adhesive heterogeneity of RBCs in SCA and their specific contribution to vaso-occlusive crisis.

GBT440 Inhibits Sickling of Sickle Cell Trait Blood Under In Vitro Conditions Mimicking Strenuous Exercise

In sickle cell trait (SCT), hemoglobin A (HbA) and S (HbS) are co-expressed in each red blood cell (RBC). While homozygous expression of HbS (HbSS) leads to polymerization and sickling of RBCs resulting in sickle cell disease (SCD) characterized by hemolytic anemia, painful vaso-occlusive episodes and shortened life-span, SCT is considered a benign condition usually with minor or no complications related to sickling. However, physical activities that cause increased tissue oxygen demand, dehydration and/or metabolic acidosis leads to increased HbS polymerization and life-threatening complications including death. We report that GBT440, an agent being developed for the treatment of SCD, increases the affinity of oxygen for Hb and inhibits in vitro polymerization of a mixture of HbS and HbA that simulates SCT blood. Moreover, GBT440 prevents sickling of SCT blood under in vitro conditions mimicking strenuous exercise with hypoxia, dehydration and acidosis. Together, our results indicate that GBT440 may have the potential to protect SCT individuals from sickling-related complications during conditions that favor HbS polymerization.

GBT440 increases haemoglobin oxygen affinity, reduces sickling and prolongs RBC half-life in a murine model of sickle cell disease

A major driver of the pathophysiology of sickle cell disease (SCD) is polymerization of deoxygenated haemoglobin S (HbS), which leads to sickling and destruction of red blood cells (RBCs) and end-organ damage. Pharmacologically increasing the proportion of oxygenated HbS in RBCs may inhibit polymerization, prevent sickling and provide long term disease modification. We report that GBT440, a small molecule which binds to the N-terminal α chain of Hb, increases HbS affinity for oxygen, delays in vitro HbS polymerization and prevents sickling of RBCs. Moreover, in a murine model of SCD, GBT440 extends the half-life of RBCs, reduces reticulocyte counts and prevents ex vivo RBC sickling. Importantly, oral dosing of GBT440 in animals demonstrates suitability for once daily dosing in humans and a highly selective partitioning into RBCs, which is a key therapeutic safety attribute. Thus, GBT440 has the potential for clinical use as a disease-modifying agent in sickle cell patients.

VLA-4 blockade by natalizumab inhibits sickle reticulocyte and leucocyte adhesion during simulated blood flow

Very Late Antigen-4 (VLA-4, α4β1-integrin, ITGA4) orchestrates cell-cell and cell-endothelium adhesion. Given the proposed role of VLA-4 in sickle cell disease (SCD) pathophysiology, we evaluated the ability of the VLA-4 blocking antibody natalizumab to inhibit SCD blood cell adhesion. Natalizumab recognized surface VLA-4 on leucocytes and reticulocytes in whole blood from SCD subjects. SCD reticulocytes were positive for VLA-4, while VLA-4 staining of non-SCD reticulocytes was undetectable. Titrations with natalizumab revealed the presence of saturable levels of VLA-4 on both SCD reticulocytes and leucocytes similar to healthy subject leucocytes. Under physiological flow conditions, the adhesion of SCD whole blood cells and isolated SCD leucocytes to immobilized vascular cell adhesion molecule 1 (VCAM-1) was blocked by natalizumab in a dose-dependent manner, which correlated with cell surface receptor binding. Natalizumab also inhibited >50% of whole blood cell binding to TNF-α activated human umbilical vein endothelial cell monolayers under physiological flow at clinically relevant concentrations (10 to 100 μg/ml). This indicates that VLA-4 is the dominant receptor that drives SCD reticulocyte and mononuclear cell adhesion to VCAM-1 and that the VLA-4 adhesion to VCAM-1 is a significant contributor to SCD blood cell adhesion to endothelium. Thus, VLA-4 blockade may be beneficial in sickle cell disease.

Universality of supersaturation in protein-fiber formation

The thermodynamics and kinetics of the aggregation of sickle-cell hemoglobin into fibers have been studied in great detail under a wide range of solution conditions. The stability of the fiber is measured by the solubility; the kinetics is characterized by a delay before the appearance of fibers. A review of data in the literature shows that there is no correlation of the delay time with fiber stability and only a weak correlation with the initial protein concentration. There is, however, a striking collapse of all the data onto a single universal curve when the delay time is plotted versus the supersaturation, which is the ratio of the initial protein concentration to the solubility, expressed as activities. Collapse onto the same universal curve is also obtained when using delay times calculated from the double-nucleation theoretical model.

Microfluidic study of enhanced deposition of sickle cells at acute corners

Sickle cell anemia is a blood disorder, known to affect the microcirculation and is characterized by painful vaso-occlusive crises in deep tissues. During the last three decades, many scenarios based on the enhanced adhesive properties of the membrane of sickle red blood cells have been proposed, all related to a final decrease in vessels lumen by cells accumulation on the vascular walls. Up to now, none of these scenarios considered the possible role played by the geometry of the flow on deposition. The question of the exact locations of occlusive events at the microcirculatory scale remains open. Here, using microfluidic devices where both geometry and oxygen levels can be controlled, we show that the flow of a suspension of sickle red blood cells around an acute corner of a triangular pillar or of a bifurcation, leads to the enhanced deposition and aggregation of cells. Thanks to our devices, we follow the growth of these aggregates in time and show that their length does not depend on oxygenation levels; instead, we find that their morphology changes dramatically to filamentous structures when using autologous plasma as a suspending fluid. We finally discuss the possible role played by such aggregates in vaso-occlusive events.

The delay time in sickle cell disease after 40 years: A paradigm assessed

Sickle hemoglobin polymerization commences with a striking latency period, called a "delay time" followed by abrupt polymer formation. The delay time is exceedingly concentration dependent. This discovery (40 years ago) led to the "kinetic hypothesis," that is, that the pathophysiology was related to the relationship between the delay time and the capillary transit. The delay time is well described by a double-nucleation mechanism of polymer formation. In macroscopic volumes, the delay time is highly reproducible, but in small volumes such as erythrocytes, under certain conditions, the intrinsic delay time can be augmented by a stochastic delay owing to random waiting times for the first nucleus to form. This lengthens the average delay and adds further protection from vaso-occlusion. When oxygen removal is not sudden, the growth of polymers after the delay time is limited by the rate of oxygen removal, further lengthening the time before occlusion may occur. This is important if some polymers have remained in the cell after pulmonary transit as their presence otherwise would obliterate any delay. The difficulty of deforming a cell once polymerized rationalizes the "two-step" model of vaso-occlusion in which a postcapillary adhesion event is followed by a sickling logjam. The delay time that is required is therefore generalized to be the delay time for an erythrocyte to move beyond regions in the venuoles where adherent cells have reduced the available lumen. The measurements of delay times correlate well with the severity of sickling syndromes. They also correlate with the improvements owing to the administration of hydroxyurea.

Kinetics of sickle cell biorheology and implications for painful vasoocclusive crisis

We developed a microfluidics-based model to quantify cell-level processes modulating the pathophysiology of sickle cell disease (SCD). This in vitro model enabled quantitative investigations of the kinetics of cell sickling, unsickling, and cell rheology. We created short-term and long-term hypoxic conditions to simulate normal and retarded transit scenarios in microvasculature. Using blood samples from 25 SCD patients with sickle hemoglobin (HbS) levels varying from 64 to 90.1%, we investigated how cell biophysical alterations during blood flow correlated with hematological parameters, HbS level, and hydroxyurea (HU) therapy. From these measurements, we identified two severe cases of SCD that were also independently validated as severe from a genotype-based disease severity classification. These results point to the potential of this method as a diagnostic indicator of disease severity. In addition, we investigated the role of cell density in the kinetics of cell sickling. We observed an effect of HU therapy mainly in relatively dense cell populations, and that the sickled fraction increased with cell density. These results lend support to the possibility that the microfluidic platform developed here offers a unique and quantitative approach to assess the kinetic, rheological, and hematological factors involved in vasoocclusive events associated with SCD and to develop alternative diagnostic tools for disease severity to supplement other methods. Such insights may also lead to a better understanding of the pathogenic basis and mechanism of drug response in SCD.

Limb, tooth, beak: three modes of development and evolutionary innovation of form

The standard model of evolutionary change of form, deriving from Darwin's theory via the Modern Synthesis, assumes a gradualistic reshaping of anatomical structures, with major changes only occurring by many cycles of natural selection for marginal adaptive advantage. This model, with its assertion that a single mechanism underlies both micro- and macroevolutionary change, contains an implicit notion of development which is only applicable in some cases. Here we compare the embryological processes that shape the vertebrate limb bud, the mammalian tooth and the avian beak. The implied notion of development in the standard evolutionary picture is met only in the case of the vertebrate limb, a single-primordium organ with morphostatic shaping, in which cells rearrange in response to signalling centres which are essentially unchanged by cell movement. In the case of the tooth, a single-primordium organ with morphodynamic shaping in which the strengths and relationships between signalling centres is influenced by the cell and tissue movements they induce, and the beak, in which the final form is influenced by the collision and rearrangement of multiple tissue primordia, abrupt appearance of qualitatively different forms (i.e. morphological novelties) can occur with small changes in system parameters induced by a genetic change, or by an environmental factor whose effects can be subsequently canalized genetically. Bringing developmental mechanisms and, specifically, the material properties of tissues as excitable media into the evolutionary picture, demonstrates that gradualistic change for incremental adaptive advantage is only one of the possible modes of morphological evolution.

National trends in hospitalizations for sickle cell disease in the United States following the FDA approval of hydroxyurea, 1998-2008

BACKGROUND

Patients with sickle cell disease (SCD) can suffer frequent hospital admissions for painful vasoocclusive crises. Hydroxyurea was approved by the FDA in 1998 to decrease the morbidity of SCD, but nationwide hospitalizations for SCD in the United States since 1998 have not been evaluated. We hypothesized that the availability of hydroxyurea for SCD would be associated with a decrease in hospitalizations for SCD over time.

OBJECTIVE

To assess trends in hospitalization and length-of-stay in hospital for SCD in the United States, 1998 through 2008.

RESEARCH DESIGN

Retrospective cohort study of SCD-related hospital discharges in the Nationwide Inpatient Sample of US hospital discharges.

SUBJECTS

All discharges in the Nationwide Inpatient Sample associated with a principal diagnosis of SCD in blacks, 1998 through 2008.

MEASURES

Trends in hospitalization rates and average length-of-stay in hospital for SCD.

RESULTS

We found 216 (95% confidence interval, 173.3-258.7) SCD-related hospitalizations per 100,000 US blacks in 1998 and 178.4 (95% confidence interval, 144.2-212.5) in 2008, but no consistent yearly decrease, 1998 through 2008 (P=0.30). Conversely, the length-of-stay in hospital in 1998 was 5.38 days and in 2008 was 5.18 days, an absolute change of 0.2 days and a downward trend that was statistically significant.

CONCLUSIONS

Between 1998 and 2008, there was not a steady decrease in hospitalization rates for the population of SCD in the United States. On the contrary, there was a decline in length-of-stay in hospital over this time. Hydroxyurea underuse is well documented. Efforts to increase hydroxyurea use may help to reduce hospitalization rates.

Red blood cell vesiculation in hereditary hemolytic anemia

Hereditary hemolytic anemia encompasses a heterogeneous group of anemias characterized by decreased red blood cell survival because of inherited membrane, enzyme, or hemoglobin disorders. Affected red blood cells are more fragile, less deformable, and more susceptible to shear stress and oxidative damage, and show increased vesiculation. Red blood cells, as essentially all cells, constitutively release phospholipid extracellular vesicles in vivo and in vitro in a process known as vesiculation. These extracellular vesicles comprise a heterogeneous group of vesicles of different sizes and intracellular origins. They are described in literature as exosomes if they originate from multi-vesicular bodies, or as microvesicles when formed by a one-step budding process directly from the plasma membrane. Extracellular vesicles contain a multitude of bioactive molecules that are implicated in intercellular communication and in different biological and pathophysiological processes. Mature red blood cells release in principle only microvesicles. In hereditary hemolytic anemias, the underlying molecular defect affects and determines red blood cell vesiculation, resulting in shedding microvesicles of different compositions and concentrations. Despite extensive research into red blood cell biochemistry and physiology, little is known about red cell deformability and vesiculation in hereditary hemolytic anemias, and the associated pathophysiological role is incompletely assessed. In this review, we discuss recent progress in understanding extracellular vesicles biology, with focus on red blood cell vesiculation. Also, we review recent scientific findings on the molecular defects of hereditary hemolytic anemias, and their correlation with red blood cell deformability and vesiculation. Integrating bio-analytical findings on abnormalities of red blood cells and their microvesicles will be critical for a better understanding of the pathophysiology of hereditary hemolytic anemias.

Nucleation of sickle hemoglobin mixed with hemoglobin A: experimental and theoretical studies of hybrid-forming mixtures

Sickle hemoglobin (HbS) is a point mutation of the two β subunits in normal Hb (HbA) that leads to nucleated polymerization and accompanying pathology. We measured the rates of homogeneous and heterogeneous nucleation of HbS in the presence of up to 50% HbA under conditions in which hybrid HbAS molecules will also form. The replacement of 50% of HbS by HbA slows polymerization by factors of ∼100 in the physiological range, which is substantially less than previously thought. To provide a theoretical description of these data, we extended the double nucleation model for HbS polymerization to conditions in which hybridized mixtures are present. Measurements of homogeneous nucleation and the theory agree only when at least one of the molecules in the nucleus is not a hybrid. We attribute this to the necessary presence in the nucleus of a molecule that utilizes both β-subunit mutation sites in intermolecular contacts, whereas the remaining molecules engage only one of the mutation sites. Heterogeneous nucleation appears to require an even greater number of nonhybrid molecules, presumably because of the need for the nucleus to attach to the polymer as well as to form internal bonds. These results also provide insights into the pathophysiology of sickle cell disease, including the occasional severe events that strike persons in whom both HbS and HbA are expressed, a condition known as sickle trait. The studies reported here are necessary for understanding physiologically relevant polymerization in the presence of ligands as well as therapeutically relevant copolymerizing inhibitors.