Human Plasma and Recombinant Hemopexins: Heme Binding Revisited

In-depth structure-function profiling of the complex formation between clotting factor VIII and heme

BACKGROUND AND AIMS

Blood disorders, such as sickle cell disease, and other clinical conditions are often accompanied by intravascular hemolytic events along with the development of severe coagulopathies. Hemolysis, in turn, leads to the accumulation of Fe(II/III)-protoporphyrin IX (heme) in the intravascular compartment, which can trigger a variety of proinflammatory and prothrombotic reactions. As such, heme binding to the blood coagulation proteins factor VIII (FVIII), fibrinogen, and activated protein C with functional consequences has been demonstrated earlier.

METHODS

We herein present an in-depth characterization of the FVIII-heme interaction at the molecular level and its (patho-)physiological relevance through the application of biochemical, biophysical, structural biology, bioinformatic, and diagnostic tools.

RESULTS

FVIII has a great heme-binding capacity with seven heme molecules associating with the protein. The respective binding sites were identified by investigating heme binding to FVIII-derived peptides in combination with molecular docking and dynamic simulation studies of the complex as well as cryo-electron microscopy, revealing three high-affinity and four moderate heme-binding motifs (HBMs). Furthermore, the relevance of the FVIII-heme complex formation was characterized in physiologically relevant assay systems, revealing a ~ 50 % inhibition of the FVIII cofactor activity even in the protein-rich environment of blood plasma.

CONCLUSION

Our study provides not only novel molecular insights into the FVIII-heme interaction and its physiological relevance, but also strongly suggests the reduction of the intrinsic pathway and the accentuation of the final clotting step (by, for example, fibrinogen crosslinking) in hemolytic conditions as well as a future perspective in the context of FVIII substitution therapy of hemorrhagic events in hemophilia A patients.

Hemopexin reverses activation of lung eIF2α and decreases mitochondrial injury in chlorine-exposed mice

We assessed the mechanisms by which nonencapsulated heme, released in the plasma of mice after exposure to chlorine (Cl2) gas, resulted in the initiation and propagation of acute lung injury. We exposed adult male and female C57BL/6 mice to Cl2 (500 ppm for 30 min), returned them to room air, and injected them intramuscularly with either human hemopexin (hHPX; 5 µg/g BW in 50-µL saline) or vehicle at 1 h post-exposure. Upon return to room air, Cl2-exposed mice, injected with vehicle, developed respiratory acidosis, increased concentrations of plasma proteins in the alveolar space, lung mitochondrial DNA injury, increased levels of free plasma heme, and major alterations of their lung proteome. hHPX injection mice mitigated the onset and development of lung and mitochondrial injury and the increase of plasma heme, reversed the Cl2-induced changes in 83 of 237 proteins in the lung proteome at 24 h post-exposure, and improved survival at 15 days post-exposure. Systems biology analysis of the lung global proteomics data showed that hHPX reversed changes in a number of key pathways including elF2 signaling, verified by Western blotting measurements. Recombinant human hemopexin, generated in tobacco plants, injected at 1 h post-Cl2 exposure, was equally effective in reversing acute lung and mtDNA injury. The results of this study offer new insights as to the mechanisms by which exposure to Cl2 results in acute lung injury and the therapeutic effects of hemopexin.NEW & NOTEWORTHY Herein, we demonstrate that exposure of mice to chlorine gas causes significant changes in the lung proteome 24 h post-exposure. Systems biology analysis of the proteomic data is consistent with damage to mitochondria and activation of eIF2, the master regulator of transcription and protein translation. Post-exposure injection of hemopexin, which scavenges free heme, attenuated mtDNA injury, eIF2α phosphorylation, decreased lung injury, and increased survival.

Double-edged functions of hemopexin in hematological related diseases: from basic mechanisms to clinical application

It is now understood that hemolysis and the subsequent release of heme into circulation play a critical role in driving the progression of various diseases. Hemopexin (HPX), a heme-binding protein with the highest affinity for heme in plasma, serves as an effective antagonist against heme toxicity resulting from severe acute or chronic hemolysis. In the present study, changes in HPX concentration were characterized at different stages of hemolytic diseases, underscoring its potential as a biomarker for assessing disease progression and prognosis. In many heme overload-driven conditions, such as sickle cell disease, transfusion-induced hemolysis, and sepsis, endogenous HPX levels are often insufficient to provide protection. Consequently, there is growing interest in developing HPX therapeutics to mitigate toxic heme exposure. Strategies include HPX supplementation when endogenous levels are depleted and enhancing HPX's functionality through modifications, offering a potent defense against heme toxicity. It is worth noting that HPX may also exert deleterious effects under certain circumstances. This review aims to provide a comprehensive overview of HPX's roles in the progression and prognosis of hematological diseases. It highlights HPX-based clinical therapies for different hematological disorders, discusses advancements in HPX production and modification technologies, and offers a theoretical basis for the clinical application of HPX.

Hemopexin Reverses Activation of Lung eIF2a and Decreases Mitochondrial Injury in Chlorine Exposed Mice

We assessed the mechanisms by which non-encapsulated heme, released in the plasma of mice post exposure to chlorine (Cl 2 ) gas, resulted in the initiation and propagation of acute lung injury. We exposed adult C57BL/6 male and female to Cl 2 (500 ppm for 30 min) in environmental chambers and returned them to room air and injected them intramuscularly with a single dose of human hemopexin (hHPX; 5 µg/ g BW), the most efficient scavenger of heme, 30-60 min post exposure. Concentrations of hHPX in plasma of air and Cl 2 exposed mice were 9081±900 vs. 1879± 293 at 6 h and 2966±463 vs. 1555±250 at 50 h post injection (ng/ml; X±1 SEM=3; p<0.01). Cl 2 exposed mice developed progressive acute lung injury post exposure characterized by increased concentrations of plasma heme, marked inflammatory response, respiratory acidosis and increased concentrations of plasma proteins in the alveolar space. Injection of hHPX decreased the onset of acute lung injury at 24 h post exposure; mean survival, for the saline and hHPX groups were 40 vs. 80% (P<0.001) at 15 d post exposure. Non-supervised global proteomics analysis of mouse lungs at 24 h post exposure, revealed the upregulation of 92 and downregulation of 145 lung proteins. Injection of hHPX at one h post exposure moderated the Cl 2 induced changes in eighty-three of these 237 lung proteins. System biology analysis of the global proteomics data showed that hHPX reversed changes in mitochondrial dysfunction and elF2 and integrin signaling. Western blot analysis of lung tissue showed significant increase of phosphorylated elF2 at 24 h post exposure in vehicle treated mice but normal levels in those injected with hHPX. Similarly, RT-PCR analysis of lung tissue showed that hHPX reversed the onset of mtDNA lesions. A form of recombinant human hemopexin generated in tobacco plants was equally effective in reversing acute lung and mtDNA injury. The results of this study offer new insights as to the mechanisms by which exposure to Cl 2 results in acute lung injury and to the therapeutic effects of hemopexin.

Engineering Therapeutics to Detoxify Hemoglobin, Heme, and Iron

Hemolysis (i.e., red blood cell lysis) can increase circulatory levels of cell-free hemoglobin (Hb) and its degradation by-products, namely heme (h) and iron (Fe). Under homeostasis, minor increases in these three hemolytic by-products (Hb/h/Fe) are rapidly scavenged and cleared by natural plasma proteins. Under certain pathophysiological conditions, scavenging systems become overwhelmed, leading to the accumulation of Hb/h/Fe in the circulation. Unfortunately, these species cause various side effects such as vasoconstriction, hypertension, and oxidative organ damage. Therefore, various therapeutics strategies are in development, ranging from supplementation with depleted plasma scavenger proteins to engineered biomimetic protein constructs capable of scavenging multiple hemolytic species. In this review, we briefly describe hemolysis and the characteristics of the major plasma-derived protein scavengers of Hb/h/Fe. Finally, we present novel engineering approaches designed to address the toxicity of these hemolytic by-products.

Genomic organization and gene evolution of two warm temperature acclimation proteins (Wap65s) of Micropterus salmoides and their responses to temperature and bacterial/viral infections

Warm temperature acclimation-related 65-kDa proteins (Wap65s) are fish plasma acute-phase glycoproteins homologous to hemopexin with high affinity and clearance for heme. The study characterized Mswap65-1 and Mswap65-2 genes in Micropterus salmoides. Structural analysis showed MsWap65s contained conserved heme-binding sites. MsWap65-1 had a chloride-binding site similar to hemopexin, while MsWap65-2 had an additional calcium-binding site. Phylogenetic and Ka/Ks analysis showed that fish Wap65s were evolutionarily conserved and underwent strong purifying selection. Functional divergence analysis indicated that fish Wap65-2 retained the putative function of ancestral Wap65, while Wap65-1 underwent neofunctional differentiation. QPCR showed Mswap65s were predominantly expressed in liver, but prolonged hyperthermy inhibited Mswap65-2 expression. Mswap65-2 expression was up-regulated in liver and spleen after Nocardia seriolae infection, while Mswap65-1 was down-regulated. MsWap65-2 may be associated with pathogenesis and play potential role in pathogen resistance. LMBV infection resulted in both significant downregulation of Mswap65s were both significantly down-regulated, with differences observed between sexes. We speculated the immune system might suppress expression after viral infection. Exogenous rMsWap65s were prepared, and injection of rMsWap65s alleviated phenylhydrazine-induced hemolysis and inhibited increases in heme, complement C3 and inflammatory symptoms. Our results contribute to an advanced understanding of the functions and mechanisms of MsWap65s in stress resistance.

Hemolysis, free hemoglobin toxicity, and scavenger protein therapeutics

During hemolysis, erythrophagocytes dispose damaged red blood cells. This prevents the extracellular release of hemoglobin, detoxifies heme, and recycles iron in a linked metabolic pathway. Complementary to this process, haptoglobin and hemopexin scavenge and shuttle the red blood cell toxins hemoglobin and heme to cellular clearance. Pathological hemolysis outpaces macrophage capacity and scavenger synthesis across a diversity of diseases. This imbalance leads to hemoglobin-driven disease progression. To meet a void in treatment options, scavenger protein-based therapeutics are in clinical development.

A Model Peptide Reveals Insights into the Interaction of Human Hemopexin with Heme

Under hemolytic conditions, toxic heme is scavenged by hemopexin. Recently, the heme-binding properties of hemopexin have been reassessed, which revealed a KD of ~ 0.32 nM as well as a stoichiometry of one to two heme molecules binding to hemopexin. A 66mer hemopexin-derived peptide that spans over three heme-binding motifs was used to verify the earlier suggested heme-recruiting mechanism. Herein, we employed spectroscopic and computational methods to substantiate the hypothesis of more than one heme molecule binding to hemopexin and to analyze the heme-binding mode. Both, hemopexin and the 66mer peptide, were found to bind heme in mixed penta- and hexacoordinated states, which strongly indicates that heme binding follows distinct criteria and increases rigidity of the peptide-heme complex. Additional in silico molecular dynamics simulations support these experimental findings and, thus, contribute to our understanding of the molecular basis of the heme-hemopexin interaction. This analysis provides further details for consideration of hemopexin in biomedical applications.

Tracking Heme-Protein Interactions in Healthy and Pathological Human Serum in Native Conditions by Miniaturized FFF-Multidetection

The interaction of heme with blood serum proteins plays an important role in many physiological and pathological processes involving enzyme activity, gene expression and cell proliferation. The mechanisms underlying these interactions are; however, not yet fully understood. New analytical methods able to investigate protein-heme binding in native, biologically representative conditions are thus required. In this work, we present a method based on miniaturized, hollow-fiber flow field-flow fractionation with multiple spectrophotometric and light-scattering detection for size separation of high-abundance serum proteins and selective detection of heme-bound subpopulations. Heme is found to mainly interact with serum albumin, whereas a low amount also binds to other proteins such as IgM. The ability to bind heme in physiological conditions is also investigated for individual serum proteins. IgG is found unable to bind heme at clinically relevant concentrations. The proposed method allows separation, quantitation, and mass/size characterization of serum high-abundance proteins, providing information of heme-protein complex stability and preferred heme-clearing pathways. The same approach could be in perspective extended to the investigation of specific heme-antibody binding, and to further studies involving other molecules of pharmaceutical/clinical interest.

Proteomics Disclose the Potential of Gingival Crevicular Fluid (GCF) as a Source of Biomarkers for Severe Periodontitis

Periodontal disease is a widespread disorder comprising gingivitis, a mild early gum inflammation, and periodontitis, a more severe multifactorial inflammatory disease that, if left untreated, can lead to the gradual destruction of the tooth-supporting apparatus. To date, effective etiopathogenetic models fully explaining the clinical features of periodontal disease are not available. Obviously, a better understanding of periodontal disease could facilitate its diagnosis and improve its treatment. The purpose of this study was to employ a proteomic approach to analyze the gingival crevicular fluid (GCF) of patients with severe periodontitis, in search of potential biomarkers. GCF samples, collected from both periodontally healthy sites (H-GCF) and the periodontal pocket (D-GCF), were subjected to a comparison analysis using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). A total of 26 significantly different proteins, 14 up-regulated and 12 down-regulated in D-GCF vs. H-GCF, were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The main expressed proteins were inflammatory molecules, immune responders, and host enzymes. Most of these proteins were functionally connected using the STRING analysis database. Once validated in a large scale-study, these proteins could represent a cluster of promising biomarkers capable of making a valuable contribution for a better assessment of periodontitis.