Human serum albumin presents isoform variants with altered neonatal Fc receptor interactions

Abstract

Human serum albumin (HSA) is the most abundant protein in plasma and presents the particularity, with IgG, to have an extraordinary long serum half‐life conferred by its interaction with the neonatal Fc receptor (FcRn). If the impact of IgG post‐translational modifications (PTMs) on FcRn binding is well documented, it is far less reported for HSA despite numerous PTMs occurring on the protein in plasma. HSA is susceptible to numerous degradation reactions in plasma, because of aging, oxidative stress or liver and pancreas related pathologies. In the present study, we combined FcRn affinity chromatography and mass spectrometry to investigate the impact of HSA PTMs upon FcRn binding. This methodology presents the advantage to distinguish the effect of a single modification from a plasma HSA preparation made of a mixture of different isoforms. Cys34 oxidation, Lys525 glycation, and Leu585 C‐terminal truncation, which are modifications related to several pathological conditions, were demonstrated to act negatively on HSA‐FcRn interaction. The HSA‐FcRn binding alteration generated by these modifications is consistent with their vicinity with the interaction interface of the two proteins. Results were discussed regarding altered half‐life of HSA observed in several disease states and pave the way toward new understandings of the hypoalbuminemia pathogenesis. Significance statement In this study, we investigated the impact of several post‐translational modifications of HSA toward its ability to bind to the neonatal Fc receptor using in vitro affinity chromatography, mass spectrometry, and surface plasmon resonance. Cys34 oxidation, Lys525 glycation, and Leu585 C‐terminal truncation were demonstrated to decrease HSA‐FcRn binding. These modifications occurring in circulating HSA were discussed in relation to several pathologies as well as for the use of HSA as a therapeutic protein.

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“…Although the principal HSA-FcRn binding interaction site is in the N-terminal domain III, domain I, which contains Cys 34 , also plays a role . FcRn affinity chromatography, ion-exchange chromatography, and surface plasmon resonance data demonstrate that modification at Cys 34 does impair FcRn binding . However, the paper from Leblanc et al .…”

“…60 FcRn affinity chromatography, ion-exchange chromatography, and surface plasmon resonance data demonstrate that modification at Cys 34 does impair FcRn binding. 61 However, the paper from Leblanc et al does not report binding affinities of specific HSA adducts, but rather that a plasma-derived fraction of HSA consisting of a variety of HSA isoforms, including non-Cys 34 modifications, exhibited lower FcRn binding affinity (33 μM K D ) than the high-affinity FcRn binding achieved with an HSA standard (20 μM K D ). Additionally, although Cys 34 oxidation (sulfinic acid) was detected in the low-affinity fraction, the predominant low-affinity fraction Cys 34 adduct was cysteinylation.…”

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“…Moreover, one of the major albumin modifications, glycosylation, leads to reduced affinity with FcRn. This, in turn, reduces transcytosis and increases intracellular catabolism, resulting in the excretion of albumin fragments and rapid vascular clearance [75,76]. It should be noted, among the investigated modifications (Lys525, Lys195, and Lys233), only Lys525, which alters the conformation changes in domain III, decreases HSA-FcRn binding.…”

“…Precisely, the loops I and II of DI and DIII nearby Lys466 are in proximity with the receptor. Therefore, the chemical or posttranslational modification in such sites and ligand binding possibly leads to conformation changes, lower FcRn binding, and better albumin degradation in lysosomes; for example, Cys34 (DI domain) oxidation, cysteinylation, or cysteinylglycinylation act negatively on HSA-FcRn interaction [75]. Moreover, one of the major albumin modifications, glycosylation, leads to reduced affinity with FcRn.…”

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“…Glycation of serum albumin is a known modification that occurs in circulation, especially in disease states such as diabetes. Recent studies have shown that the interaction of glycated albumin, particularly at position K525, leads to a reduction in affinity for FcRn (Leblanc et al, 2019). Molecules involving albumin fusions may require a deeper characterization to understand the PK impact in certain clinical populations.…”