Isolation and characterisation of quercitrin as a potent anti-sickle cell anaemia agent from Alchornea cordifolia.

Alchornea cordifolia MÜll. Arg. (commonly known as Christmas Bush) has been used traditionally in Africa to treat sickle cell anaemia (a recessive disease, arising from the S haemoglobin [Hb] allele) but the active compounds are yet to be characterised. Herein we describe the use of sequential fractionation coupled with in vitro anti-sickling assays to purify the active component. Sickling was induced in HbSS genotype blood samples using sodium metabisulphite (Na2S2O5) or incubation in 100 % N2. Methanol extracts of A. cordifolia leaves and its sub-fractions showed >70 % suppression of HbSS erythrocyte sickling. Purified compound demonstrated 87.2 ± 2.39 % significant anti-sickling activity and 93.1 ± 2.69 % erythrocyte sickling-inhibition at 0.4 mg/mL. Nuclear magnetic resonance (NMR) spectra and high-resolution mass spectroscopy identified it as quercitrin (quercetin 3-rhamnoside). Purified quercitrin also inhibited the polymerisation of isolated HbS and stabilized sickle erythrocytes membranes. Metabolomic comparisons of blood samples using flow-infusion electrospray-high resolution mass spectrometry indicated that quercitrin could convert HbSS erythrocyte metabolomes to be similar to HbAA. Sickling was associated with changes in anti-oxidants, anaerobic bioenergy, and arachidonic acid metabolism, all of which were reversed by quercitrin. The findings described could inform efforts directed to the development of an anti-sickling drug or quality control assessments of A. cordifolia preparations.

Renal complications of sickle cell disease

The nephropathy and renal complications of sickle cell disease are associated with various events such as hypoxic or ischemic conditions and reperfusion injury. Erythrocyte sickling occurs following these events and renal medullary acidosis.

P-selectin deficiency promotes liver senescence in sickle cell disease mice

Sickle cell disease (SCD) is caused by a homozygous mutation in the β-globin gene, which leads to erythrocyte sickling, vaso-occlusion, and intense hemolysis. P-selectin inhibition has been shown to prevent vaso-occlusive events in SCD patients, however, the chronic effect of P-selectin inhibition in SCD remains to be determined. Here, we used quantitative liver intravital microscopy in our recently generated P-selectin deficient SCD mice to show that chronic P-selectin deficiency attenuates liver ischemia, but fails to prevent hepatobiliary injury. Remarkably, we find that this failure in resolution of hepatobiliary injury in P-selectin deficient SCD mice is associated with the increase in cellular senescence and reduced epithelial cell proliferation in the liver. These findings highlight the importance to investigate the long-term effects of chronic P-selectin inhibition therapy on liver pathophysiology in SCD patients.

VZHE-039, a novel antisickling agent that prevents erythrocyte sickling under both hypoxic and anoxic conditions

Sickle cell disease (SCD) results from a hemoglobin (Hb) mutation βGlu6 → βVal6 that changes normal Hb (HbA) into sickle Hb (HbS). Under hypoxia, HbS polymerizes into rigid fibers, causing red blood cells (RBCs) to sickle; leading to numerous adverse pathological effects. The RBC sickling is made worse by the low oxygen (O2) affinity of HbS, due to elevated intra-RBC concentrations of the natural Hb effector, 2,3-diphosphoglycerate. This has prompted the development of Hb modifiers, such as aromatic aldehydes, with the intent of increasing Hb affinity for O2 with subsequent prevention of RBC sickling. One such molecule, Voxelotor was recently approved by U.S. FDA to treat SCD. Here we report results of a novel aromatic aldehyde, VZHE-039, that mimics both the O2-dependent and O2-independent antisickling properties of fetal hemoglobin. The latter mechanism of action—as elucidated through crystallographic and biological studies—is likely due to disruption of key intermolecular contacts necessary for stable HbS polymer formation. This dual antisickling mechanism, in addition to VZHE-039 metabolic stability, has translated into significantly enhanced and sustained pharmacologic activities. Finally, VZHE-039 showed no significant inhibition of several CYPs, demonstrated efficient RBC partitioning and high membrane permeability, and is not an efflux transporter (P-gp) substrate.

An Investigation of Structure-Activity Relationships of Azolylacryloyl Derivatives Yielded Potent and Long-Acting Hemoglobin Modulators for Reversing Erythrocyte Sickling

Aromatic aldehydes that bind to sickle hemoglobin (HbS) to increase the protein oxygen affinity and/or directly inhibit HbS polymer formation to prevent the pathological hypoxia-induced HbS polymerization and the subsequent erythrocyte sickling have for several years been studied for the treatment of sickle cell disease (SCD). With the exception of Voxelotor, which was recently approved by the U.S. Food and Drug Administration (FDA) to treat the disease, several other promising antisickling aromatic aldehydes have not fared well in the clinic because of metabolic instability of the aldehyde moiety, which is critical for the pharmacologic activity of these compounds. Over the years, our group has rationally developed analogs of aromatic aldehydes that incorporate a stable Michael addition reactive center that we hypothesized would form covalent interactions with Hb to increase the protein affinity for oxygen and prevent erythrocyte sickling. Although, these compounds have proven to be metabolically stable, unfortunately they showed weak to no antisickling activity. In this study, through additional targeted modifications of our lead Michael addition compounds, we have discovered other novel antisickling agents. These compounds, designated MMA, bind to the α-globin and/or β-globin to increase Hb affinity for oxygen and concomitantly inhibit erythrocyte sickling with significantly enhanced and sustained pharmacologic activities in vitro.

Non-hematopoietic deficiency of proprotein convertase subtilisin/kexin type 9 deficiency leads to more severe anemia in a murine model of sickle cell disease

Proprotein convertase subtilisin/kexin type 9 (PCSK9) deficiency leads to lower cholesterol and is associated with reduced vascular complications in the general population. Cholesterol lowering may also have beneficial effects in sickle cell disease (SCD). The objective of this study was to determine effects of PCSK9 deficiency in a mouse model of SCD. Bone marrow transplantation (BMT) was performed from donor SCD mice to wild-type, PCSK9-deficient, and LDLR-deficient recipients to generate SCD controls (Pcsk9+/+, SCDbmt) with preserved PCSK9 status, SCD mice with deficiency of PCSK9 (Pcsk9−/−, SCDbmt), and SCD mice with deficiency of LDLR (Ldlr−/−, SCDbmt). Although cholesterol levels were lower in Pcsk9−/−, SCDbmt mice compared to Pcsk9+/+, SCDbmt mice, anemia was more severe in Pcsk9−/−, SCDbmt mice. Increased reticulocytosis, enhanced ex vivo erythrocyte sickling, and increased erythrocyte phosphatidylserine exposure was also observed. Livers, spleens, and kidneys contained increased iron in Pcsk9−/−, SCDbmt mice compared to Pcsk9+/+, SCDbmt mice consistent with greater hemolysis. SCD mice with deficiency of LDLR (Ldlr−/−, SCDbmt mice) had similar anemia as Ldlr+/+, SCDbmt mice despite higher serum cholesterol. In conclusion, deficiency of PCSK9 is associated with worsened anemia in SCD mice due to increased hemolysis. These findings may have implications for lipid-lowering strategies in patients with SCD, as well as for potential novel modifiers of anemia severity.

Quantitative prediction of erythrocyte sickling for the development of advanced sickle cell therapies

Sickle cell disease is induced by a mutation that converts normal adult hemoglobin to sickle hemoglobin (HbS) and engenders intracellular polymerization of deoxy-HbS and erythrocyte sickling. Development of anti-sickling therapies requires quantitative understanding of HbS polymerization kinetics under organ-specific conditions, which are difficult to assess with existing experimental techniques. Thus, we developed a kinetic model based on the classical nucleation theory to examine the effectiveness of potential anti-sickling drug candidates. We validated this model by comparing its predictability against prior in vivo and in vitro experimental results. We used the model to quantify the efficacy of sickling inhibitors and obtain results consistent with recent screening assays. Global sensitivity analysis on the kinetic parameters in the model revealed that the solubility, nucleation rate prefactor, and oxygen affinity are quantities that dictate HbS polymerization. This finding provides quantitative guidelines for the discovery of intracellular processes to be targeted by sickling inhibitors.

The genetic basis and evolution of red blood cell sickling in deer

Crescent-shaped red blood cells, the hallmark of sickle cell disease, present a striking departure from the biconcave disc shape normally found in mammals. Characterized by increased mechanical fragility, sickled cells promote haemolytic anaemia and vaso-occlusions and contribute directly to disease in humans. Remarkably, a similar sickle-shaped morphology has been observed in erythrocytes from several deer species, without pathological consequences. The genetic basis of erythrocyte sickling in deer, however, remains unknown, limiting the utility of deer as comparative models for sickling. Here, we determine the sequences of human β-globin orthologs in 15 deer species and identify a set of co-evolving, structurally related residues that distinguish sickling from non-sickling deer. Protein structural modelling indicates a sickling mechanism distinct from human sickle cell disease, coordinated by a derived valine (E22V) in the second alpha helix of the β-globin protein. The evolutionary history of deer β-globins is characterized by incomplete lineage sorting, episodes of gene conversion between adult and foetal β-globin paralogs, and the presence of a trans-species polymorphism that is best explained by long-term balancing selection, suggesting that sickling in deer is adaptive. Our results reveal structural and evolutionary parallels and differences in erythrocyte sickling between human and deer, with implications for understanding the ecological regimes and molecular architectures that favour the evolution of this dramatic change in erythrocyte shape.