BackgroundHistidine-rich glycoprotein (HRG) regulates coagulation through its ability to bind and neutralize heparins. HRG associates with Zn2+ to stimulate HRG–heparin complex formation. Under normal conditions, the majority of plasma Zn2+ associates with human serum albumin (HSA). However, free fatty acids (FFAs) allosterically disrupt Zn2+ binding to HSA. Thus, high levels of circulating FFAs, as are associated with diabetes, obesity, and cancer, may increase the proportion of plasma Zn2+ associated with HRG, contributing to an increased risk of thrombotic disease.ObjectivesTo characterize Zn2+ binding by HRG, examine the influence that FFAs have on Zn2+ binding by HSA, and establish whether FFA-mediated displacement of Zn2+ from HSA may influence HRG–heparin complex formation.MethodsZn2+ binding to HRG and to HSA in the presence of different FFA (myristate) concentrations were examined by isothermal titration calorimetry (ITC) and the formation of HRG–heparin complexes in the presence of different Zn2+ concentrations by both ITC and ELISA.Results and conclusionsWe found that HRG possesses 10 Zn2+ sites (K′ = 1.63 × 105) and that cumulative binding of FFA to HSA perturbed its ability to bind Zn2+. Also Zn2+ binding was shown to increase the affinity with which HRG interacts with unfractionated heparins, but had no effect on its interaction with low molecular weight heparin (˜ 6850 Da). [Correction added on 1 December 2014, after first online publication: In the preceding sentence, “6850 kDa” was corrected to “6850 Da”.] Speciation modeling of plasma Zn2+ based on the data obtained suggests that FFA-mediated displacement of Zn2+ from serum albumin would be likely to contribute to the development of thrombotic complications in individuals with high plasma FFA levels.

Circulatory transport of the essential nutrient zinc primarily occurs through its binding to serum albumin. Here, we present the first crystal structures of mammalian albumins in complex with zinc. These structures, together with accompanying zinc binding data, allow identification of key zinc transport sites on human and equine albumins.

Fatty acid levels in the blood are dynamic and chronic elevation of plasma fatty acid levels is associated with some metabolic disorders such as cardiovascular disease and diabetes. Since the binding of Zn(2+) to albumin is important for the control of circulatory/cellular Zn(2+) dynamics, this relationship is likely to have important physiological and pathological implications. This article is part of a Special Issue entitled Serum Albumin.

Advanced glycation end products (AGEs) are implicated in the pathology of Alzheimer's disease (AD), as they induce neurodegeneration following interaction with the receptor for AGE (RAGE). This study aimed to establish a mechanistic link between AGE-RAGE signaling and AD pathology. AGE-induced changes in the neuro2a proteome were monitored by SWATH-MS. Western blotting and cell-based reporter assays were used to investigate AGE-RAGE regulated APP processing and tau phosphorylation in primary cortical neurons. Selected protein expression was validated in brain samples affected by AD. The AGE-RAGE axis altered proteome included increased expression of cathepsin B and asparagine endopeptidase (AEP), which mediated an increase in Aβ formation and tau phosphorylation, respectively. Elevated cathepsin B, AEP, RAGE, and pTau levels were found in human AD brain, coincident with enhanced AGEs. This study demonstrates that the AGE-RAGE axis regulates Aβ formation and tau phosphorylation via increased cathepsin B and AEP, providing a new molecular link between AGEs and AD pathology.

Key Points• The x-ray crystal structure of the N2 domain from HRG at 1.93Å resolution is presented.• The structure reveals an S-glutathionyl adduct at Cys185, which has implications for angiogenic regulation.Histidine-rich glycoprotein (HRG) is a plasma protein consisting of 6 distinct functional domains and is an important regulator of key cardiovascular processes, including angiogenesis and coagulation. The protein is composed of 2 N-terminal domains (N1 and N2), 2 proline-rich regions (PRR1 and PRR2) that flank a histidine-rich region (HRR), and a C-terminal domain. To date, structural information of HRG has largely come from sequence analysis and spectroscopic studies. It is thought that an HRG fragment containing the HRR, released via plasmin-mediated cleavage, acts as a negative regulator of angiogenesis in vivo. However, its release also requires cleavage of a disulphide bond suggesting that its activity is mediated by a redox process. Here, we present a 1.93Å resolution crystal structure of the N2 domain of serum-purified rabbit HRG. The structure confirms that the N2 domain, which along with the N1 domain, forms an important molecular interaction site on HRG, possesses a cystatin-like fold composed of a 5-stranded antiparallel b-sheet wrapped around a 5-turn a-helix. A native N-linked glycosylation site was identified at Asn184. Moreover, the structure reveals the presence of an S-glutathionyl adduct at Cys185, which has implications for the redox-mediated release of the antiangiogenic cleavage product from HRG. (Blood. 2014;123(12):1948-1955 Introduction Histidine-rich glycoprotein (HRG) is a plasma protein that regulates angiogenesis, coagulation, and immune function in vertebrates. Human HRG has an approximate mass of 70 kDa and is present in plasma at low micromolar concentrations (ca ;1.5 mM). The protein is arranged into 6 domains: 2 N-terminal domains (N1 and N2), a central histidine-rich region (HRR) flanked by 2 proline rich regions (PRR1 and PRR2), and a C-terminal domain (C). The N1, N2, and C domains are highly conserved between species, as is an arrangement of 6 disulfide bridges (Figure 1). In plasma, HRG binds to and regulates the function of a diverse variety of targets that include fibrinogen, plasminogen, thrombospondin, immunoglobulin G (IgG), complement factors, and heparin as well as cell-surface molecules such as Fcg receptors and heparan sulfate.1-9 HRG binds divalent metal cations within the HRR. 10 In particular, Zn 21 is known to bind this region and can modulate HRG activity by altering the protein's affinity for other targets. 11HRG is heavily glycosylated; the human protein has 6 putative N-linked glycosylation sites.11 The histidine-and proline-rich regions are predicted to be intrinsically disordered but N1, N2, and the C-terminal domains are likely to have ordered structures. The N1 and N2 domains share a high degree of sequence similarity to members of the cystatin superfamily of cysteine protease inhibitors.12 Type 1 cystatins (also known as stefins) are characterized b...