Chaperoning erythropoiesis

Targeting Protein Degradation Pathways in Tumors: Focusing on their Role in Hematological Malignancies

Cells must maintain their proteome homeostasis by balancing protein synthesis and degradation. This is facilitated by evolutionarily-conserved processes, including the unfolded protein response and the proteasome-based system of protein clearance, autophagy, and chaperone-mediated autophagy. In some hematological malignancies, including acute myeloid leukemia, misfolding or aggregation of the wild-type p53 tumor-suppressor renders cells unable to undergo apoptosis, even with an intact p53 DNA sequence. Moreover, blocking the proteasome pathway triggers lymphoma cell apoptosis. Extensive studies have led to the development of proteasome inhibitors, which have advanced into drugs (such as bortezomib) used in the treatment of certain hematological tumors, including multiple myeloma. New therapeutic options have been studied making use of the so-called proteolysis-targeting chimeras (PROTACs), that bind desired proteins with a linker that connects them to an E3 ubiquitin ligase, resulting in proteasomal-targeted degradation. This review examines the mechanisms of protein degradation in the cells of the hematopoietic system, explains the role of dysfunctional protein degradation in the pathogenesis of hematological malignancies, and discusses the current and future advances of therapies targeting these pathways, based on an extensive search of the articles and conference proceedings from 2005 to April 2022.

Molecular and cellular mechanisms that regulate human erythropoiesis

To enable effective oxygen transport, ∼200 billion red blood cells (RBCs) need to be produced every day in the bone marrow through the fine-tuned process of erythropoiesis. Erythropoiesis is regulated at multiple levels to ensure that defective RBC maturation or overproduction can be avoided. Here, we provide an overview of different layers of this control, ranging from cytokine signaling mechanisms that enable extrinsic regulation of RBC production to intrinsic transcriptional pathways necessary for effective erythropoiesis. Recent studies have also elucidated the importance of posttranscriptional regulation and highlighted additional gatekeeping mechanisms necessary for effective erythropoiesis. We additionally discuss the insights gained by studying human genetic variation affecting erythropoiesis and highlight the discovery of BCL11A as a regulator of hemoglobin switching through genetic studies. Finally, we provide an outlook of how our ability to measure multiple facets of this process at single-cell resolution, while accounting for the impact of human variation, will continue to refine our knowledge of erythropoiesis and how this process is perturbed in disease. As we learn more about this intricate and important process, additional opportunities to modulate erythropoiesis for therapeutic purposes will undoubtedly emerge.

The role of globins in cardiovascular physiology

Globin proteins exist in every cell type of the vasculature, from erythrocytes to endothelial cells, vascular smooth muscle cells, and peripheral nerve cells. Many globin subtypes are also expressed in muscle tissues (including cardiac and skeletal muscle), in other organ-specific cell types, and in cells of the central nervous system. The ability of each of these globins to interact with molecular oxygen (O₂) and nitric oxide (NO) is preserved across these contexts. Endothelial α-globin is an example of extra-erythrocytic globin expression. Other globins, including myoglobin, cytoglobin, and neuroglobin are observed in other vascular tissues. Myoglobin is observed primarily in skeletal muscle and smooth muscle cells surrounding the aorta or other large arteries. Cytoglobin is found in vascular smooth muscle but can also be expressed in non-vascular cell types, especially in oxidative stress conditions after ischemic insult. Neuroglobin was first observed in neuronal cells, and its expression appears to be restricted mainly to the central and peripheral nervous systems. Brain and central nervous system neurons expressing neuroglobin are positioned close to many arteries within the brain parenchyma and can control smooth muscle contraction and, thus, tissue perfusion and vascular reactivity. Overall, reactions between NO and globin heme-iron contribute to vascular homeostasis by regulating vasodilatory NO signals and scaveging reactive species in cells of the mammalian vascular system. Here, we discuss how globin proteins affect vascular physiology with a focus on NO biology, and offer perspectives for future study of these functions.

The Hsp70 chaperone system: distinct roles in erythrocyte formation and maintenance

Erythropoiesis is a tightly regulated cell differentiation process in which specialized oxygen- and carbon dioxide-carrying red blood cells are generated in vertebrates. Extensive reorganization and depletion of the erythroblast proteome leading to the deterioration of general cellular protein quality control pathways and rapid hemoglobin biogenesis rates could generate misfolded/aggregated proteins and trigger proteotoxic stresses during erythropoiesis. Such cytotoxic conditions could prevent proper cell differentiation resulting in premature apoptosis of erythroblasts (ineffective erythropoiesis). The heat shock protein 70 (Hsp70) molecular chaperone system supports a plethora of functions that help maintain cellular protein homeostasis (proteostasis) and promote red blood cell differentiation and survival. Recent findings show that abnormalities in the expression, localization and function of the members of this chaperone system are linked to ineffective erythropoiesis in multiple hematological diseases in humans. In this review, we present latest advances in our understanding of the distinct functions of this chaperone system in differentiating erythroblasts and terminally differentiated mature erythrocytes. We present new insights into the protein repair-only function(s) of the Hsp70 system, perhaps to minimize protein degradation in mature erythrocytes to warrant their optimal function and survival in the vasculature under healthy conditions. The work also discusses the modulatory roles of this chaperone system in a wide range of hematological diseases and the therapeutic gain of targeting Hsp70.

Stage Specific Expression Pattern of Alpha-Hemoglobin-Stabilizing-Protein (AHSP) Portrayed in Erythroblast Chronology

During erythropoiesis, the molecular chaperone alpha-hemoglobin-stabilizing protein (AHSP) sequesters free alpha-hemoglobin (αHb) and prevents precipitation of excess αHb. While AHSP is linked to hereditary anemia, the pattern of expression during specific erythroblast stages is poorly understood. We investigated gene and protein expressions of AHSP throughout progressive maturation stages of erythroblasts in biphasic cultures of blood and bone marrow samples from healthy donors. Differentiating erythroblasts were periodically subjected to flow cytometry, Amnis imaging and RT-qPCR analyses. We made parallel in vivo validations from naive murine bone marrow cells. Percentages of AHSP⁺ erythroblasts, protein expressions and AHSP gene expressions are negligible on culture day 6 (CFU-Es) and progressively increases from culture days 8-12 (peaks on day 12) and declines on day 14. Notably, sub-cellular location of AHSP is both in the cytoplasm and nucleus in the early erythroblasts while in the late stages of maturation AHSP is found predominantly in the nucleus, being expelled with it during enucleation. As both human bone marrow and peripheral blood mononuclear cells (PBMC) derived erythroblasts demonstrated similar expression patterns, sampling of erythroblasts from day 11 cultures could portray erythroblast chronology and provide optimum representative stage specific expression patterns. PBMCs may be suitable for comparison studies of AHSP expression in pathologic erythropoiesi.

Smad2/3-pathway ligand trap luspatercept enhances erythroid differentiation in murine β-thalassaemia by increasing GATA-1 availability

In β‐thalassaemia, anaemia results from ineffective erythropoiesis characterized by inhibition of late‐stage erythroid differentiation. We earlier used luspatercept and RAP‐536 protein traps for certain Smad2/3‐pathway ligands to implicate Smad2/3‐pathway overactivation in dysregulated erythroid differentiation associated with murine β‐thalassaemia and myelodysplasia. Importantly, luspatercept alleviates anaemia and has been shown to reduce transfusion burden in patients with β‐thalassaemia or myelodysplasia. Here, we investigated the molecular mechanisms underlying luspatercept action and pSmad2/3‐mediated inhibition of erythroid differentiation. In murine erythroleukemic (MEL) cells in vitro, ligand‐mediated overactivation of the Smad2/3 pathway reduced nuclear levels of GATA‐1 (GATA‐binding factor‐1) and its transcriptional activator TIF1γ (transcription intermediary factor 1γ), increased levels of reactive oxygen species, reduced cell viability and haemoglobin levels, and inhibited erythroid differentiation. Co‐treatment with luspatercept in MEL cells partially or completely restored each of these. In β‐thalassaemic mice, RAP‐536 up‐regulated Gata1 and its target gene signature in erythroid precursors determined by transcriptional profiling and gene set enrichment analysis, restored nuclear levels of GATA‐1 in erythroid precursors, and nuclear distribution of TIF1γ in erythroblasts. Bone marrow cells from β‐thalassaemic mice treated with luspatercept also exhibited restored nuclear availability of GATA‐1 ex vivo. Our results implicate GATA‐1, and likely TIF1γ, as key mediators of luspatercept/RAP‐536 action in alleviating ineffective erythropoiesis.

Heat shock protein 60 regulates yolk sac erythropoiesis in mice

The yolk sac is the first site of blood-cell production during embryonic development in both murine and human. Heat shock proteins (HSPs), including HSP70 and HSP27, have been shown to play regulatory roles during erythropoiesis. However, it remains unknown whether HSP60, a molecular chaperone that resides mainly in mitochondria, could also regulate early erythropoiesis. In this study, we used Tie2-Cre to deactivate the Hspd1 gene in both hematopoietic and vascular endothelial cells, and found that Tie2-Cre⁺Hspd1^f/f (HSP60^CKO) mice were embryonic lethal between the embryonic day 10.5 (E10.5) and E11.5, exhibiting growth retardation, anemia, and vascular defects. Of these, anemia was observed first, independently of vascular and growth phenotypes. Reduced numbers of erythrocytes, as well as an increase in cell apoptosis, were found in the HSP60^CKO yolk sac as early as E9.0, indicating that deletion of HSP60 led to abnormality in yolk sac erythropoiesis. Deletion of HSP60 was also able to reduce mitochondrial membrane potential and the expression of the voltage-dependent anion channel (VDAC) in yolk sac erythrocytes. Furthermore, cyclosporine A (CsA), which is a well-recognized modulator in regulating the opening of the mitochondrial permeability transition pore (mPTP) by interacting with Cyclophilin D (CypD), could significantly decrease cell apoptosis and partially restore VDAC expression in mutant yolk sac erythrocytes. Taken together, we demonstrated an essential role of HSP60 in regulating yolk sac cell survival partially via a mPTP-dependent mechanism.

Regulation of globin-heme balance in Diamond-Blackfan anemia by HSP70/GATA1

Diamond-Blackfan anemia (DBA) is a congenital erythroblastopenia that is characterized by a blockade in erythroid differentiation related to impaired ribosome biogenesis. DBA phenotype and genotype are highly heterogeneous. We have previously identified 2 in vitro erythroid cell growth phenotypes for primary CD34⁺ cells from DBA patients and following short hairpin RNA knockdown of RPS19, RPL5, and RPL11 expression in normal human CD34⁺ cells. The haploinsufficient RPS19 in vitro phenotype is less severe than that of 2 other ribosomal protein (RP) mutant genes. We further documented that proteasomal degradation of HSP70, the chaperone of GATA1, is a major contributor to the defect in erythroid proliferation, delayed erythroid differentiation, increased apoptosis, and decreased globin expression, which are all features of the RPL5 or RPL11 DBA phenotype. In the present study, we explored the hypothesis that an imbalance between globin and heme synthesis may be involved in pure red cell aplasia of DBA. We identified disequilibrium between the globin chain and the heme synthesis in erythroid cells of DBA patients. This imbalance led to accumulation of excess free heme and increased reactive oxygen species production that was more pronounced in cells of the RPL5 or RPL11 phenotype. Strikingly, rescue experiments with wild-type HSP70 restored GATA1 expression levels, increased globin synthesis thereby reducing free heme excess and resulting in decreased apoptosis of DBA erythroid cells. These results demonstrate the involvement of heme in DBA pathophysiology and a major role of HSP70 in the control of balanced heme/globin synthesis.

Quantitative proteomics of plasma vesicles identify novel biomarkers for hemoglobin E/β-thalassemic patients

Hemoglobin E (HbE)/β-thalassemia has a wide spectrum of clinical manifestations that cannot be explained purely by its genetic background. Circulating extracellular vesicles (EVs) are one factor that likely contributes to disease severity. This study has explored the differences in protein composition and quantity between EVs from HbE/β-thalassemic patients and healthy individuals. We used tandem mass tag labeling mass spectrometry to analyze the EV proteins isolated from the plasma of 15 patients compared with the controls. To reduce biological variation between individuals, the EV proteins isolated from randomly assigned groups of 5 HbE/β-thalassemic patients were pooled and compared with 5 pooled age- and sex-matched controls in 3 separate experiments. Alpha hemoglobin-stabilizing protein had the highest fold increase. Catalase, superoxide dismutase, T-complex proteins, heat shock proteins, transferrin receptor, ferritin, and cathepsin S were also upregulated in thalassemic circulating EVs. Importantly, haptoglobin and hemopexin were consistently reduced in patients' EVs across all data sets, in keeping with the existing hemolysis that occurs in thalassemia. The proteomic data analysis of EV samples isolated from 6 individual HbE/β-thalassemic patients and western blotting results corroborated these findings. In conclusion, we have successfully identified consistent alterations of protein quantity between EVs from HbE/β-thalassemic and healthy individuals. This work highlights haptoglobin, hemopexin, and cathepsin S as potential clinically relevant biomarkers for levels of hemolysis and inflammation. Monitoring of these plasma proteins could help in the clinical management of thalassemia.

The Chaperones Involved in Hemoglobin Synthesis Take the Spotlight: Analysis of AHSP in the Argentinean Population and Review of the Literature

Hemoglobin (Hb) synthesis is a complex, well-coordinated process that requires molecular chaperones. These intervene in different steps: regulating epigenetic mechanisms necessary for the adequate expression of the α- and β-globin clusters, binding the nascent peptides and helping them acquire their native structure, preventing oxidative damage by free globin chains and preventing the cleavage of essential erythroid transcription factors. This study analyzed the distribution of the single nucleotide polymorphism (SNP) rs4296276 in intron 1 of the α-globin chaperone α Hb-stabilizing protein (AHSP) in the Argentinean population. The risk allele was found in thalassemia patients who exhibited more severe phenotypes than expected. Future studies may help establish the role of these chaperones as modifiers in pathological states with globin chain imbalance, such as thalassemia.

The intricate role of selenium and selenoproteins in erythropoiesis

Selenium (Se) is incorporated as the 21st amino acid selenocysteine (Sec) into the growing polypeptide chain of proteins involved in redox gatekeeper functions. Erythropoiesis presents a particular problem to redox regulation as the presence of iron, heme, and unpaired globin chains lead to high levels of free radical-mediated oxidative stress, which are detrimental to erythroid development and can lead to anemia. Under homeostatic conditions, bone marrow erythropoiesis produces sufficient erythrocytes to maintain homeostasis. In contrast, anemic stress induces an alternative pathway, stress erythropoiesis, which rapidly produces new erythrocytes at extramedullary sites, such as spleen, to alleviate anemia. Previous studies suggest that dietary Se protects erythrocytes from such oxidative damage and the absence of selenoproteins causes hemolysis of erythrocytes due to oxidative stress. Furthermore, Se deficiency or lack of selenoproteins severely impairs stress erythropoiesis exacerbating the anemia in rodent models and human patients. Interestingly, erythroid progenitors develop in close proximity with macrophages in structures referred to as erythroblastic islands (EBIs), where macrophage expression of selenoproteins appears to be critical for the expression of heme transporters to facilitate export of heme from macrophage stores to the developing erythroid cells. Here we review the role of Se and selenoproteins in the intrinsic development of erythroid cells in addition to their role in the development of the erythropoietic niche that supports the functional role of EBIs in erythroid expansion and maturation in the spleen during recovery from anemia.

Hsp90 chaperones hemoglobin maturation in erythroid and nonerythroid cells

Maturation of adult (α2β2) and fetal hemoglobin (α2γ2) tetramers requires that heme be incorporated into each globin. While hemoglobin alpha (Hb-α) relies on a specific erythroid chaperone (alpha Hb-stabilizing protein, AHSP), the other chaperones that may help mature the partner globins (Hb-γ or Hb-β) in erythroid cells, or may enable nonerythroid cells to express mature Hb, are unknown. We investigated a role for heat-shock protein 90 (hsp90) in Hb maturation in erythroid precursor cells that naturally express Hb-α with either Hb-γ (K562 and HiDEP-1 cells) or Hb-β (HUDEP-2) and in nonerythroid cell lines that either endogenously express Hb-αβ (RAW and A549) or that we transfected to express the globins. We found the following: (i) AHSP and hsp90 associate with distinct globin partners in their immature heme-free states (AHSP with apo-Hbα, and hsp90 with apo-Hbβ or Hb-γ) and that hsp90 does not associate with mature Hb. (ii) Hsp90 stabilizes the apo-globins and helps to drive their heme insertion reactions, as judged by pharmacologic hsp90 inhibition or by coexpression of an ATP-ase defective hsp90. (iii) In nonerythroid cells, heme insertion into all globins became hsp90-dependent, which may explain how mixed Hb tetramers can mature in cells that do not express AHSP. Together, our findings uncover a process in which hsp90 first binds to immature, heme-free Hb-γ or Hb-β, drives their heme insertion process, and then dissociates to allow their heterotetramer formation with Hb-α. Thus, in driving heme insertion, hsp90 works in concert with AHSP to generate functional Hb tetramers during erythropoiesis.

Unravelling pathways downstream Sox6 induction in K562 erythroid cells by proteomic analysis

The Sox6 transcription factor is crucial for terminal maturation of definitive red blood cells. Sox6-null mouse fetuses present misshapen and nucleated erythrocytes, due to impaired actin assembly and cytoskeleton stability. These defects are accompanied with a reduced survival of Sox6^−/− red blood cells, resulting in a compensated anemia. Sox6-overexpression in K562 cells and in human primary ex vivo erythroid cultures enhances erythroid differentiation and leads to hemoglobinization, the hallmark of erythroid maturation. To obtain an overview on processes downstream to Sox6 expression, we performed a differential proteomic analysis on human erythroid K562 cells overexpressing Sox6. Sox6-overexpression induces dysregulation of 64 proteins, involved in cytoskeleton remodeling and in protein synthesis, folding and trafficking, key processes for erythroid maturation. Moreover, 43 out of 64 genes encoding for differentially expressed proteins contain within their proximal regulatory regions sites that are bound by SOX6 according to ENCODE ChIP-seq datasets and are possible direct SOX6 targets. SAR1B, one of the most induced proteins upon Sox6 overexpression, shares a conserved regulatory module, composed by a double SOX6 binding site and a GATA1 consensus, with the adjacent SEC24 A gene. Since both genes encode for COPII components, this element could concur to the coordinated expression of these proteins during erythropoiesis.

Molecular basis of β thalassemia and potential therapeutic targets

The remarkable phenotypic diversity of β thalassemia that range from severe anemia and transfusion-dependency, to a clinically asymptomatic state exemplifies how a spectrum of disease severity can be generated in single gene disorders. While the genetic basis for β thalassemia, and how severity of the anemia could be modified at different levels of its pathophysiology have been well documented, therapy remains largely supportive with bone marrow transplant being the only cure. Identification of the genetic variants modifying fetal hemoglobin (HbF) production in combination with α globin genotype provide some prediction of disease severity for β thalassemia but generation of a personalized genetic risk score to inform prognosis and guide management requires a larger panel of genetic modifiers yet to be discovered. Nonetheless, genetic studies have been successful in characterizing the key variants and pathways involved in HbF regulation, providing new therapeutic targets for HbF reactivation. BCL11A has been established as a quantitative repressor, and progress has been made in manipulating its expression using genomic and gene-editing approaches for therapeutic benefits. Recent discoveries and understanding in the mechanisms associated with ineffective and abnormal erythropoiesis have also provided additional therapeutic targets, a couple of which are currently being tested in clinical trials.

Mechanism of Human Apohemoglobin Unfolding

Removal of heme from human hemoglobin (Hb) results in formation of an apoglobin heterodimer. Titration of this apodimer with guanidine hydrochloride (GdnHCl) leads to biphasic unfolding curves indicating two distinct steps. Initially, the heme pocket unfolds and generates a dimeric intermediate in which ∼50% of the original helicity is lost, but the α₁β₁ interface is still intact. At higher GdnHCl concentrations, this intermediate dissociates into unfolded monomers. This structural interpretation was verified by comparing GdnHCl titrations for adult human hemoglobin A (HbA), recombinant fetal human hemoglobin (HbF), recombinant Hb cross-linked with a single glycine linker between the α chains, and recombinant Hbs with apolar heme pocket mutations that markedly stabilize native conformations in both subunits. The first phase of apoHb unfolding is independent of protein concentration, little affected by genetic cross-linking, but significantly shifted toward higher GdnHCl concentrations by the stabilizing distal pocket mutations. The second phase depends on protein concentration and is shifted to higher GdnHCl concentrations by genetic cross-linking. This model for apoHb unfolding allowed us to quantitate subtle differences in stability between apoHbA and apoHbF, which suggest that the β and γ heme pockets have similar stabilities, whereas the α₁γ₁ interface is more resistant to dissociation than the α₁β₁ interface.

Genetic Basis and Genetic Modifiers of β-Thalassemia and Sickle Cell Disease

β-thalassemia and sickle cell disease (SCD) are prototypical Mendelian single gene disorders, both caused by mutations affecting the adult β-globin gene. Despite the apparent genetic simplicity, both disorders display a remarkable spectrum of phenotypic severity and share two major genetic modifiers-α-globin genotype and innate ability to produce fetal hemoglobin (HbF, α₂γ₂).This article provides an overview of the genetic basis for SCD and β-thalassemia, and genetic modifiers identified through phenotype correlation studies. Identification of the genetic variants modifying HbF production in combination with α-globin genotype provide some prediction of disease severity for β-thalassemia and SCD but generation of a personalized genetic risk score to inform prognosis and guide management requires a larger panel of genetic modifiers yet to be discovered.Nonetheless, genetic studies have been successful in characterizing some of the key variants and pathways involved in HbF regulation, providing new therapeutic targets for HbF reactivation.

Platelet proteomics in thalassemia: Factors responsible for hypercoagulation

PURPOSE

Thalassemias can be defined as a group with inherited hemolytic anemia due to differential expressions of α and β globin genes. Hemoglobin E combined with β thalassemia (HbEβ) creates high oxidative stress in platelets producing different degrees of pathophysiological severity. Numerous cases of thalassemia have been reported with thromboembolic complications due to the hypercoagulable state, the mechanism underlying that is not yet well understood.

EXPERIMENTAL DESIGN

We have used 2DE and DIGE coupled with MALDI TOF/TOF-based MS identification and characterization of altered proteins in both splenectomized and nonsplenectomized HbEβ and β thalassemia to investigate the factors responsible for hypercoagulation.

RESULTS

The study revealed elevated levels of chaperones like HSP70, protein disulfide isomerase; oxidative stress proteins like peroxiredoxin2 and superoxide dismutase1 along with high ROS levels. Upregulation of translation initiation factor 5a observed in thalassemia is a novel finding and plays a protective role toward cell survival under oxidative stress.

CONCLUSIONS AND CLINICAL RELEVANCE

The altered levels of chaperones and oxidative stress proteins indicate toward regulation of integrin binding and platelet activation under oxidative stress. Altogether, this comparative proteomics study of platelets in thalassemia could provide an insight into better understanding of the pathophysiology of the disease.

[Role of alpha-hemoglobin molecular chaperone in the hemoglobin formation and clinical expression of some hemoglobinopathies]

Alpha-hemoglobin stabilizing protein (AHSP), described as a chaperone of alpha-hemoglobin (α-Hb), is synthesized at a high concentration in the erythroid precursors. AHSP specifically recognizes the G and H helices of α-Hb and forms a stable complex with free α-Hb until its association with the partner β-subunits. Unlike the free β-Hb which are soluble and form homologous tetramers, freshly synthesized α-Hb chains are highly unstable molecular species which precipitate and generate reactive oxygen species within the erythrocyte precursors of the bone marrow leading to apoptosis and ineffective erythropoiesis. AHSP protects the free α-Hb chains in maintaining it in the soluble state. In this review, we report data from the literature and our laboratory concerning the key role of AHSP in the biosynthesis of Hb and its possible involvement in some disorders of the red blood cell as well as the hemoglobinopathies and we discuss its use as a prognostic tool in thalassemia syndromes.

Review: Beta-thalassemia and molecular chaperones

Thalassemia is known as a diverse single gene disorder, which is prevalent worldwide. The molecular chaperones are set of proteins that help in two important processes while protein synthesis and degradation include folding or unfolding and assembly or disassembly, thereby helping in cell homeostasis. This review recaps current knowledge regarding the role of molecular chaperones in thalassemia, with a focus on beta thalassemia.

Role of α-globin H helix in the building of tetrameric human hemoglobin: interaction with α-hemoglobin stabilizing protein (AHSP) and heme molecule

Alpha-Hemoglobin Stabilizing Protein (AHSP) binds to α-hemoglobin (α-Hb) or α-globin and maintains it in a soluble state until its association with the β-Hb chain partner to form Hb tetramers. AHSP specifically recognizes the G and H helices of α-Hb. To investigate the degree of interaction of the various regions of the α-globin H helix with AHSP, this interface was studied by stepwise elimination of regions of the α-globin H helix: five truncated α-Hbs α-Hb1-138, α-Hb1-134, α-Hb1-126, α-Hb1-123, α-Hb1-117 were co-expressed with AHSP as two glutathione-S-transferase (GST) fusion proteins. SDS-PAGE and Western Blot analysis revealed that the level of expression of each truncated α-Hb was similar to that of the wild type α-Hb except the shortest protein α-Hb1-117 which displayed a decreased expression. While truncated GST-α-Hb1-138 and GST-α-Hb1-134 were normally soluble; the shorter globins GST-α-Hb1-126 and GST-α-Hb1-117 were obtained in very low quantities, and the truncated GST-α-Hb1-123 provided the least material. Absorbance and fluorescence studies of complexes showed that the truncated α-Hb1-134 and shorter forms led to modified absorption spectra together with an increased fluorescence emission. This attests that shortening the H helix leads to a lower affinity of the α-globin for the heme. Upon addition of β-Hb, the increase in fluorescence indicates the replacement of AHSP by β-Hb. The CO binding kinetics of different truncated AHSP^WT/α-Hb complexes showed that these Hbs were not functionally normal in terms of the allosteric transition. The N-terminal part of the H helix is primordial for interaction with AHSP and C-terminal part for interaction with heme, both features being required for stability of α-globin chain.

Simply red: A novel spectrophotometric erythroid proliferation assay as a tool for erythropoiesis and erythrotoxicity studies

Most mammalian cell proliferation assays rely on manual or automated cell counting or the assessment of metabolic activity in colorimetric assays, with the former being either labor and time intensive or expensive and the latter being multistep procedures requiring the addition of several reagents. The proliferation of erythroid cells from hematopoietic stem cells and their differentiation into mature red blood cells is characterized by the accumulation of large amounts of hemoglobin. Hemoglobin concentrations are easily quantifiable using spectrophotometric methods due to the specific absorbance peak of the molecule’s heme moiety between 400 and 420 nm. Erythroid proliferation can therefore be readily assessed using spectrophotometric measurement in this range. We have used this feature of erythroid cells to develop a simple erythroid proliferation assay that is minimally labor/time- and reagent-intensive and could easily be automated for use in high-throughput screening. Such an assay can be a valuable tool for investigations into hematological disorders where erythropoiesis is dysregulated, i.e., either inhibited or enhanced, into the development of anemia as a side-effect of primary diseases such as parasitic infections and into cyto-(particularly erythro-) toxicity of chemical agents or drugs.

An activin receptor IIA ligand trap corrects ineffective erythropoiesis in β-thalassemia

The pathophysiology of ineffective erythropoiesis in β-thalassemia is poorly understood. We report that RAP-011, an activin receptor IIA (ActRIIA) ligand trap, improved ineffective erythropoiesis, corrected anemia and limited iron overload in a mouse model of β-thalassemia intermedia. Expression of growth differentiation factor 11 (GDF11), an ActRIIA ligand, was increased in splenic erythroblasts from thalassemic mice and in erythroblasts and sera from subjects with β-thalassemia. Inactivation of GDF11 decreased oxidative stress and the amount of α-globin membrane precipitates, resulting in increased terminal erythroid differentiation. Abnormal GDF11 expression was dependent on reactive oxygen species, suggesting the existence of an autocrine amplification loop in β-thalassemia. GDF11 inactivation also corrected the abnormal ratio of immature/mature erythroblasts by inducing apoptosis of immature erythroblasts through the Fas-Fas ligand pathway. Taken together, these observations suggest that ActRIIA ligand traps may have therapeutic relevance in β-thalassemia by suppressing the deleterious effects of GDF11, a cytokine which blocks terminal erythroid maturation through an autocrine amplification loop involving oxidative stress and α-globin precipitation.

Ineffective erythropoiesis in β -thalassemia

In humans, β-thalassemia dyserythropoiesis is characterized by expansion of early erythroid precursors and erythroid progenitors and then ineffective erythropoiesis. This ineffective erythropoiesis is defined as a suboptimal production of mature erythrocytes originating from a proliferating pool of immature erythroblasts. It is characterized by (1) accelerated erythroid differentiation, (2) maturation blockade at the polychromatophilic stage, and (3) death of erythroid precursors. Despite extensive knowledge of molecular defects causing β-thalassemia, less is known about the mechanisms responsible for ineffective erythropoiesis. In this paper, we will focus on the underlying mechanisms leading to premature death of thalassemic erythroid precursors in the bone marrow.

Genetic association studies in β-hemoglobinopathies

Characterization of the molecular basis of the β-thalassemias and sickle cell disease (SCD) clearly showed that individuals with the same β-globin genotypes can have extremely diverse clinical severity. Two key modifiers, an innate ability to produce fetal hemoglobin and coinheritance of α-thalassemia, both derived from family and population studies, affect the pathophysiology of both disorders at the primary level. In the past 2 decades, scientific research had applied genetic approaches to identify additional genetic modifiers. The review summarizes recent genetic studies and key genetic modifiers identified and traces the story of fetal hemoglobin genetics, which has led to an emerging network of globin gene regulation. The discoveries have provided insights on new targets for therapeutic intervention and raise possibilities of developing fetal hemoglobin predictive diagnostics for predicting disease severity in the newborn and for integration into prenatal diagnosis to better inform genetic counseling.

α-Hemoglobin stabilizing protein (AHSP) markedly decreases the redox potential and reactivity of α-subunits of human HbA with hydrogen peroxide

α-Hemoglobin stabilizing protein (AHSP) is a molecular chaperone that binds monomeric α-subunits of human hemoglobin A (HbA) and modulates heme iron oxidation and subunit folding states. Although AHSP·αHb complexes autoxidize more rapidly than HbA, the redox mechanisms appear to be similar. Both metHbA and isolated met-β-subunits undergo further oxidation in the presence of hydrogen peroxide (H(2)O(2)) to form ferryl heme species. Surprisingly, much lower levels of H(2)O(2)-induced ferryl heme are produced by free met-α-subunits as compared with met-β-subunits, and no ferryl heme is detected in H(2)O(2)-treated AHSP·met-α-complex at pH values from 5.0 to 9.0 at 23 °C. Ferryl heme species were similarly not detected in AHSP·met-α Pro-30 mutants known to exhibit different rates of autoxidation and hemin loss. EPR data suggest that protein-based radicals associated with the ferryl oxidation state exist within HbA α- and β-subunits. In contrast, treatment of free α-subunits with H(2)O(2) yields much smaller radical signals, and no radicals are detected when H(2)O(2) is added to AHSP·α-complexes. AHSP binding also dramatically reduces the redox potential of α-subunits, from +40 to -78 mV in 1 m glycine buffer, pH 6.0, at 8 °C, demonstrating independently that AHSP has a much higher affinity for Fe(III) versus Fe(II) α-subunits. Hexacoordination in the AHSP·met-α complex markedly decreases the rate of the initial H(2)O(2) reaction with iron and thus provides α-subunits protection against damaging oxidative reactions.

Pathophysiology and Clinical Manifestations of the β-Thalassemias

The β-thalassemia syndromes reflect deficient or absent β-globin synthesis usually owing to a mutation in the β-globin locus. The relative excess of α-globin results in the formation of insoluble aggregates leading to ineffective erythropoiesis and shortened red cell survival. A relatively high capacity for fetal hemoglobin synthesis is a major genetic modifier of disease severity, with polymorphisms in other genes also having a significant role. Iron overload secondary to enhanced absorption and red cell transfusions causes an increase in liver iron and in various other tissues, leading to endocrine and cardiac dysfunction. Modern chelation regimens are effective in removing iron and preserving or restoring organ function.

Integrated protein quality-control pathways regulate free α-globin in murine β-thalassemia

Cells remove unstable polypeptides through protein quality-control (PQC) pathways such as ubiquitin-mediated proteolysis and autophagy. In the present study, we investigated how these pathways are used in β-thalassemia, a common hemoglobinopathy in which β-globin gene mutations cause the accumulation and precipitation of cytotoxic α-globin subunits. In β-thalassemic erythrocyte precursors, free α-globin was polyubiquitinated and degraded by the proteasome. These cells exhibited enhanced proteasome activity, and transcriptional profiling revealed coordinated induction of most proteasome subunits that was mediated by the stress-response transcription factor Nrf1. In isolated thalassemic cells, short-term proteasome inhibition blocked the degradation of free α-globin. In contrast, prolonged in vivo treatment of β-thalassemic mice with the proteasome inhibitor bortezomib did not enhance the accumulation of free α-globin. Rather, systemic proteasome inhibition activated compensatory proteotoxic stress-response mechanisms, including autophagy, which cooperated with ubiquitin-mediated proteolysis to degrade free α-globin in erythroid cells. Our findings show that multiple interregulated PQC responses degrade excess α-globin. Therefore, β-thalassemia fits into the broader framework of protein-aggregation disorders that use PQC pathways as cell-protective mechanisms.

Kinetics of α-globin binding to α-hemoglobin stabilizing protein (AHSP) indicate preferential stabilization of hemichrome folding intermediate

Human α-hemoglobin stabilizing protein (AHSP) is a conserved mammalian erythroid protein that facilitates the production of Hemoglobin A by stabilizing free α-globin. AHSP rapidly binds to ferrous α with association (k'(AHSP)) and dissociation (k(AHSP)) rate constants of ≈10 μm(-1) s(-1) and 0.2 s(-1), respectively, at pH 7.4 at 22 °C. A small slow phase was observed when AHSP binds to excess ferrous αCO. This slow phase appears to be due to cis to trans prolyl isomerization of the Asp(29)-Pro(30) peptide bond in wild-type AHSP because it was absent when αCO was mixed with P30A and P30W AHSP, which are fixed in the trans conformation. This slow phase was also absent when met(Fe(3+))-α reacted with wild-type AHSP, suggesting that met-α is capable of rapidly binding to either Pro(30) conformer. Both wild-type and Pro(30)-substituted AHSPs drive the formation of a met-α hemichrome conformation following binding to either met- or oxy(Fe(2+))-α. The dissociation rate of the met-α·AHSP complex (k(AHSP) ≈ 0.002 s(-1)) is ∼100-fold slower than that for ferrous α·AHSP complexes, resulting in a much higher affinity of AHSP for met-α. Thus, in vivo, AHSP acts as a molecular chaperone by rapidly binding and stabilizing met-α hemichrome folding intermediates. The low rate of met-α dissociation also allows AHSP to have a quality control function by kinetically trapping ferric α and preventing its incorporation into less stable mixed valence Hemoglobin A tetramers. Reduction of AHSP-bound met-α allows more rapid release to β subunits to form stable fully, reduced hemoglobin dimers and tetramers.

Insights into hemoglobin assembly through in vivo mutagenesis of α-hemoglobin stabilizing protein

α-Hemoglobin stabilizing protein (AHSP) is believed to facilitate adult Hemoglobin A assembly and protect against toxic free α-globin subunits. Recombinant AHSP binds multiple forms of free α-globin to stabilize their structures and inhibit precipitation. However, AHSP also stimulates autooxidation of αO(2) subunit and its rapid conversion to a partially unfolded bishistidyl hemichrome structure. To investigate these biochemical properties, we altered the evolutionarily conserved AHSP proline 30 in recombinantly expressed proteins and introduced identical mutations into the endogenous murine Ahsp gene. In vitro, the P30W AHSP variant bound oxygenated α chains with 30-fold increased affinity. Both P30W and P30A mutant proteins also caused decreased rates of αO(2) autooxidation as compared with wild-type AHSP. Despite these abnormalities, mice harboring P30A or P30W Ahsp mutations exhibited no detectable defects in erythropoiesis at steady state or during induced stresses. Further biochemical studies revealed that the AHSP P30A and P30W substitutions had minimal effects on AHSP interactions with ferric α subunits. Together, our findings indicate that the ability of AHSP to stabilize nascent α chain folding intermediates prior to hemin reduction and incorporation into adult Hemoglobin A is physiologically more important than AHSP interactions with ferrous αO(2) subunits.

Transcriptome profiling and sequencing of differentiated human hematopoietic stem cells reveal lineage-specific expression and alternative splicing of genes

Hematopoietic differentiation is strictly regulated by complex network of transcription factors that are controlled by ligands binding to cell surface receptors. Disruptions of the intricate sequences of transcriptional activation and suppression of multiple genes cause hematological diseases, such as leukemias, myelodysplastic syndromes, or myeloproliferative syndromes. From a clinical standpoint, deciphering the pattern of gene expression during hematopoiesis may help unravel disease-specific mechanisms in hematopoietic malignancies. Herein, we describe a human in vitro hematopoietic model system where lineage-specific differentiation of CD34(+) cells was accomplished using specific cytokines. Microarray and RNAseq-based whole transcriptome and exome analysis was performed on the differentiated erythropoietic, granulopoietic, and megakaryopoietic cells to delineate changes in expression of whole transcripts and exons. Analysis on the Human 1.0 ST exon arrays indicated differential expression of 172 genes (P < 0.0000001) and significant alternate splicing of 86 genes during differentiation. Pathway analysis identified these genes to be involved in Rac/RhoA signaling, Wnt/B-catenin signaling and alanine/aspartate metabolism. Comparison of the microarray data to next generation RNAseq analysis during erythroid differentiation demonstrated a high degree of correlation in gene (R = 0.72) and exon (R = 0.62) expression. Our data provide a molecular portrait of events that regulate differentiation of hematopoietic cells. Knowledge of molecular processes by which the cells acquire their cell-specific fate would be beneficial in developing cell-based therapies for human diseases.

Oxidative stress modulates heme synthesis and induces peroxiredoxin-2 as a novel cytoprotective response in β-thalassemic erythropoiesis

BACKGROUND

β-thalassemic syndromes are inherited red cell disorders characterized by severe ineffective erythropoiesis and increased levels of reactive oxygen species whose contribution to β-thalassemic anemia is only partially understood.

DESIGN AND METHODS

We studied erythroid precursors from normal and β-thalassemic peripheral CD34(+) cells in two-phase liquid culture by proteomic, reverse transcriptase polymerase chain reaction and immunoblot analyses. We measured intracellular reactive oxygen species, heme levels and the activity of δ-aminolevulinate-synthase-2. We exposed normal cells and K562 cells with silenced peroxiredoxin-2 to H(2)O(2) and generated a recombinant peroxiredoxin-2 for kinetic measurements in the presence of H(2)O(2) or hemin.

RESULTS

In β-thalassemia the increased production of reactive oxygen species was associated with down-regulation of heme oxygenase-1 and biliverdin reductase and up-regulation of peroxiredoxin-2. In agreement with these observations in β-thalassemic cells we found decreased heme levels related to significantly reduced activity of the first enzyme of the heme pathway, δ-aminolevulinate synthase-2 without differences in its expression. We demonstrated that the activity of recombinant δ-aminolevulinate synthase-2 is inhibited by both reactive oxygen species and hemin as a protective mechanism in β-thalassemic cells. We then addressed the question of the protective role of peroxiredoxin-2 in erythropoiesis by exposing normal cells to oxidative stress and silencing peroxiredoxin-2 in human erythroleukemia K562 cells. We found that peroxiredoxin-2 expression is up-regulated in response to oxidative stress and required for K562 cells to survive oxidative stress. We then showed that peroxiredoxin-2 binds heme in erythroid precursors with high affinity, suggesting a possible multifunctional cytoprotective role of peroxiredoxin-2 in β-thalassemia.

CONCLUSIONS

In β-thalassemic erythroid cells the reduction of δ-aminolevulinate synthase-2 activity and the increased expression of peroxiredoxin-2 might represent two novel stress-response protective systems.

Protein quality control during erythropoiesis and hemoglobin synthesis

Erythrocytes must regulate hemoglobin synthesis to limit the toxicities of unstable free globin chain subunits. This regulation is particularly relevant in β-thalassemia, in which β-globin deficiency causes accumulation of free α-globin, which forms intracellular precipitates that destroy erythroid precursors. Experimental evidence accumulated over more than 40 years indicates that erythroid cells can neutralize moderate amounts of free α-globin through generalized protein quality control mechanisms, including molecular chaperones, the ubiquitin-proteasome system, and autophagy. In many ways, β-thalassemia resembles protein aggregation disorders of the nervous system, liver, and other tissues, which occur when levels of unstable proteins overwhelm cellular compensatory mechanisms. Information gained from studies of nonerythroid protein aggregation disorders may be exploited to further understand and perhaps treat β-thalassemia.

Knockdown of Hspa9, a del(5q31.2) gene, results in a decrease in hematopoietic progenitors in mice

Heterozygous deletions spanning chromosome 5q31.2 occur frequently in the myelodysplastic syndromes (MDS) and are highly associated with progression to acute myeloid leukemia (AML) when p53 is mutated. Mutagenesis screens in zebrafish and mice identified Hspa9 as a del(5q31.2) candidate gene that may contribute to MDS and AML pathogenesis, respectively. To test whether HSPA9 haploinsufficiency recapitulates the features of ineffective hematopoiesis observed in MDS, we knocked down the expression of HSPA9 in primary human hematopoietic cells and in a murine bone marrow-transplantation model using lentivirally mediated gene silencing. Knockdown of HSPA9 in human cells significantly delayed the maturation of erythroid precursors, but not myeloid or megakaryocytic precursors, and suppressed cell growth by 6-fold secondary to an increase in apoptosis and a decrease in the cycling of cells compared with control cells. Erythroid precursors, B lymphocytes, and the bone marrow progenitors c-kit(+)/lineage(-)/Sca-1(+) (KLS) and megakaryocyte/erythrocyte progenitor (MEP) were significantly reduced in a murine Hspa9-knockdown model. These abnormalities suggest that cooperating gene mutations are necessary for del(5q31.2) MDS cells to gain clonal dominance in the bone marrow. Our results demonstrate that Hspa9 haploinsufficiency alters the hematopoietic progenitor pool in mice and contributes to abnormal hematopoiesis.

HSP27 controls GATA-1 protein level during erythroid cell differentiation

Heat shock protein 27 (HSP27) is a chaperone whose cellular expression increases in response to various stresses and protects the cell either by inhibiting apoptotic cell death or by promoting the ubiquitination and proteasomal degradation of specific proteins. Here, we show that globin transcription factor 1 (GATA-1) is a client protein of HSP27. In 2 models of erythroid differentiation; that is, in the human erythroleukemia cell line, K562 induced to differentiate into erythroid cells on hemin exposure and CD34(+) human cells ex vivo driven to erythroid differentiation in liquid culture, depletion of HSP27 provokes an accumulation of GATA-1 and impairs terminal maturation. More specifically, we demonstrate that, in the late stages of the erythroid differentiation program, HSP27 is phosphorylated in a p38-dependent manner, enters the nucleus, binds to GATA-1, and induces its ubiquitination and proteasomal degradation, provided that the transcription factor is acetylated. We conclude that HSP27 plays a role in the fine-tuning of terminal erythroid differentiation through regulation of GATA-1 content and activity.

The role of alpha-hemoglobin stabilizing protein in redox chemistry, denaturation, and hemoglobin assembly

Hemoglobin biosynthesis in erythrocyte precursors involves several steps. The correct ratios and concentrations of normal alpha (alpha) and beta (beta) globin proteins must be expressed; apoproteins must be folded correctly; heme must be synthesized and incorporated into these globins rapidly; and the individual alpha and beta subunits must be rapidly and correctly assembled into heterotetramers. These events occur on a large scale in vivo, and dysregulation causes serious clinical disorders such as thalassemia syndromes. Recent work has implicated a conserved erythroid protein known as Alpha-Hemoglobin Stabilizing Protein (AHSP) as a participant in these events. Current evidence suggests that AHSP enhances alpha subunit stability and diminishes its participation in harmful redox chemistry. There is also evidence that AHSP facilitates one or more early-stage post-translational hemoglobin biosynthetic events. In this review, recent experimental results are discussed in light of several current models describing globin subunit folding, heme uptake, assembly, and denaturation during hemoglobin synthesis. Particular attention is devoted to molecular interactions with AHSP that relate to alpha chain oxidation and the ability of alpha chains to associate with partner beta chains.

The alpha-hemoglobin stabilizing protein and expression of unstable alpha-Hb variants

OBJECTIVES

To determine the role of the alpha-hemoglobin stabilizing protein (AHSP) in the clinical expression of alpha-hemoglobin (alpha-Hb) variants described as unstable, ten alpha chain variants have been studied with their chaperone. AHSP specifically binds free alpha-Hb to form a soluble heterodimer until it is replaced by the beta-Hb partner. In this way, AHSP prevents the precipitation of free alpha chains which might damage the membrane of erythrocyte. AHSP specifically recognizes the G and H helices of alpha-Hb that are also involved in the alpha1beta1 dimer interface. AHSP may act as a modifier in alpha-thalassemias and lead to the thalassemic phenotypes observed in certain unstable alpha-Hb variants previously considered unstable. The different abnormalities of the alpha chain were located either in the G helix: Hb Bronovo alpha103(G10)His-->Leu, Hb Sallanches alpha104(G11)Cys-->Tyr, Hb Oegstgeest alpha104(G11)Cys-->Ser, Hb Bleuland alpha108(G15)Thr-->Asn, Hb Suan Dok alpha109(G16)Leu-->Arg and as yet undescribed alpha109(G16)Leu-->Gln, in the GH corner: Hb Foggia alpha117(GH5)Phe-->Ser, or in the H helix: Hb Groene Hart alpha119(H2)Pro-->Ser, Hb Diamant alpha119(H2)Pro-->Leu, Hb Utrecht alpha129(H12)Leu-->Pro.

DESIGN AND METHODS

These different mutated alpha-Hb were co-expressed with their chaperone AHSP as a fusion protein with glutathione S-transferase (GST) and analyzed by SDS-PAGE.

RESULTS

In all cases the proteins were normally synthesized in bacteria as shown by an expression level of mutated GST-alpha-Hbs similar to that observed for normal GST-alpha-Hb. In contrast, the recovered quantities of purified mutated GST-alpha-Hbs associated with AHSP are highly variable. An extreme case is GST-alpha-Hb(Utrecht) which was only found at trace levels.

CONCLUSION

One can assume that different mechanisms may be responsible for the amount of abnormal Hb recovered, such as a highly unstable alpha chain or an impaired formation of the complex AHSP/alpha-Hb or a modification of the alphabeta dimer formation.