Production of functional human fetal hemoglobin in Nicotiana benthamiana for development of hemoglobin-based oxygen carriers

Hemoglobin-based oxygen carriers have long been pursued to meet clinical needs by using native hemoglobin (Hb) from human or animal blood, or recombinantly produced Hb, but the development has been impeded by safety and toxicity issues. Herewith we report the successful production of human fetal hemoglobin (HbF) in Nicotiana benthamiana through Agrobacterium tumefaciens-mediated transient expression. HbF is a heterotetrameric protein composed of two identical α- and two identical γ-subunits, held together by hydrophobic interactions, hydrogen bonds, and salt bridges. In our study, the α- and γ-subunits of HbF were fused in order to stabilize the α-subunits and facilitate balanced expression of α- and γ-subunits in N. benthamiana. Efficient extraction and purification methods enabled production of the recombinantly fused endotoxin-free HbF (rfHbF) in high quantity and quality. The transiently expressed rfHbF protein was identified by SDS-PAGE, Western blot and liquid chromatography-tandem mass spectrometry analyses. The purified rfHbF possessed structural and functional properties similar to native HbF, which were confirmed by biophysical, biochemical, and in vivo animal studies. The results demonstrate a high potential of plant expression systems in producing Hb products for use as blood substitutes.

Fetal hemoglobin is much less prone to DNA cleavage compared to the adult protein

Hemoglobin (Hb) is well protected inside the red blood cells (RBCs). Upon hemolysis and when free in circulation, Hb can be involved in a range of radical generating reactions and may thereby attack several different biomolecules. In this study, we have examined the potential damaging effects of cell-free Hb on plasmid DNA (pDNA). Hb induced cleavage of supercoiled pDNA (sc pDNA) which was proportional to the concentration of Hb applied. Almost 70% of sc pDNA was converted to open circular or linear DNA using 10 µM of Hb in 12 h. Hb can be present in several different forms. The oxy (HbO₂) and met forms are most reactive, while the carboxy-protein shows only low hydrolytic activity. Hemoglobin A (HbA) could easily induce complete pDNA cleavage while fetal hemoglobin (HbF) was three-fold less reactive. By inserting, a redox active cysteine residue on the surface of the alpha chain of HbF by site-directed mutagenesis, the DNA cleavage reaction was enhanced by 82%. Reactive oxygen species were not directly involved in the reaction since addition of superoxide dismutase and catalase did not prevent pDNA cleavage. The reactivity of Hb with pDNA can rather be associated with the formation of protein based radicals.

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Hemoglobin induced plasmid DNA cleavage in the absence of hydrogen peroxide.

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Fetal hemoglobin was three-fold less reactive compared to the adult protein on plasmid DNA.

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Insertion of a cysteine residue in the alpha chain enhanced the DNA cleavage reaction by 82%.

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Protein based radicals are associated with the DNA cleavage activity of hemoglobin.

Trapping of human hemoglobin by haptoglobin: molecular mechanisms and clinical applications

SIGNIFICANCE

Haptoglobin (Hp) is an abundant plasma protein controlling the fate of hemoglobin (Hb) released from red blood cells after intravascular hemolysis. The complex formed between Hp and Hb is extraordinary strong, and once formed, this protein-protein association can be considered irreversible.

RECENT ADVANCES

A model of the Hp-Hb complex has been generated and the first steps toward understanding the mechanism behind the shielding effects of Hp have been taken. The clinical potential of the complex for modulating inflammatory reactions and for functioning as an Hb-based oxygen carrier have been described.

CRITICAL ISSUES

The three-dimensional structure of the Hp-Hb complex is unknown. Moreover, Hp is not a homogeneous protein. There are two common alleles at the Hp genetic locus denoted Hp1 and Hp2, which when analyzed on the protein levels result in differences between their physiological behavior, particularly in their shielding against Hb-driven oxidative stress. Additional cysteine residues on the α-subunit allow Hp2 to form a variety of native multimers, which influence the biophysical and biological properties of Hp. The multimeric conformations, in turn, also modulate the glycosylation patterns of Hp by steric hindrance.

FUTURE DIRECTIONS

A detailed analysis of the influence of Hp glycosylation will be instrumental to generate a deeper understanding of its biological function. Several pathological conditions also modify the glycan compositions allowing Hp to be potentially used as a marker protein for these disorders.