Alpha-2-Macroglobulin Is Acutely Sensitive to Freezing and Lyophilization: Implications for Structural and Functional Studies

High-density lipoprotein remodeling affects the osmotic properties of plasma in goldfish under critical salinity

To investigate the stress response and physiological adaptations of goldfish (Carassius auratus) to critical salinity (CS) waters, we analyzed high-density lipoprotein (HDL) stoichiometry, stress markers (cortisol, glucose), and plasma osmotic properties (Na+ , osmolality, water content) using ichthyology, biochemistry, and proteomics approaches. After 21 days of exposure to CS, plasma concentrations of cortisol, glucose, and Na+ increased, indicating stress. Total plasma osmolality (Osmtotal ) and osmolality generated by inorganic (Osminorg ) and organic osmolytes (Osmorg ) also increased, the latter by ~2%. We associated the increase of Osmorg with (1) increased metabolite concentration (glucose), (2) dissociation of HDL particles resulting in increased HDL number per unit plasma volume (~1.5-2-fold) and (3) increased HDL osmotic activity. HDL remodeling may be the reason for the redistribution of bound and free water in plasma, which may contribute to water retention in plasma and, at the same time, to hemodynamic disturbances under CS conditions. The study's findings suggest that HDL remodeling is an important mechanism for maintaining osmotic homeostasis in fish, which is consistent with current capillary exchange models in vertebrates.

Alpha-2-macroglobulin prevents platelet aggregation induced by brain-derived neurotrophic factor

The brain-derived neurotrophic factor (BDNF) has been recently shown to have activating effects in isolated platelets. However, BDNF circulates in plasma and a mechanism to preclude constant activation of platelets appears necessary. Hence, we investigated the mechanism regulating BDNF bioavailability in blood. Protein-protein interactions were predicted by molecular docking and validated through immunoprecipitation. Platelet aggregation was assessed using light transmission aggregometry with washed platelets in response to classical agonists or BDNF, in the absence or presence of alpha-2-macroglobulin (α2M), and in platelet-rich plasma. BDNF signaling was assessed with phospho-blots. As little as 25% autologous plasma was sufficient to completely abolish platelet aggregation in response to BDNF. Docking predicted two forms of BDNF binding to native or activated α2M, in parallel and perpendicular arrangements, and the model suggested that the BDNF-α2M complex cannot bind to the high-affinity BDNF receptor, tropomyosin receptor kinase B (TrkB). Experimentally, native and activated α2M formed stable complexes with BDNF preventing BDNF-induced TrkB activation and signal transduction. Both native and activated α2M inhibited BDNF induced-platelet aggregation in a concentration-dependent manner with comparable half-maximal inhibitory concentrations (IC50≈ 125-150 nM). Our study implicates α2M as a physiological regulator of BDNF bioavailability, and as an inhibitor of BDNF-induced platelet activation in blood.

Frozen fresh blood plasma preserves the functionality of native human α2-macroglobulin

Human α₂-macroglobulin (hα₂M) is a large homotetrameric protein involved in the broad inhibition of endopeptidases. Following cleavage within a bait region, hα₂M undergoes stepwise transitions from its native, expanded, highly flexible, active conformation to an induced, compact, triggered conformation. As a consequence, the peptidase is entrapped by an irreversible Venus flytrap mechanism. Given the importance of hα₂M, biochemical studies galore over more than seven decades have attempted to ascertain its role, typically using authentic hα₂M purified from frozen and non-frozen fresh blood plasma, and even outdated plasma. However, hα₂M is sensitive once isolated and purified, and becomes heterogeneous during storage and/or freezing, raising concerns about the functional competence of frozen plasma-derived hα₂M. We therefore used a combination of native and sodium dodecylsulfate polyacrylamide gel electrophoresis, affinity and ion-exchange chromatography, multi-angle laser light scattering after size-exclusion chromatography, free cysteine quantification, and peptidase inhibition assays with endopeptidases of two catalytic classes and three protein substrates, to characterize the biochemical and biophysical properties of hα₂M purified ad hoc either from fresh plasma or frozen fresh plasma after thawing. We found no differences in the molecular or functional properties of the preparations, indicating that protective components in plasma maintain native hα₂M in a functionally competent state despite freezing.

Cryo-EM structures reveal the dynamic transformation of human alpha-2-macroglobulin working as a protease inhibitor

Human alpha-2-macroglobulin is a well-known inhibitor of a broad spectrum of proteases and plays important roles in immunity, inflammation, and infections. Here, we report the cryo-EM structures of human alpha-2-macroglobulin in its native state, induced state transformed by its authentic substrate, human trypsin, and serial intermediate states between the native and fully induced states. These structures exhibit distinct conformations, which reveal the dynamic transformation of alpha-2-macro-globulin that acts as a protease inhibitor. The results shed light on the molecular mechanism of alpha-2-macroglobulin in entrapping substrates.

Alpha-2-macroglobulin in hemostasis and thrombosis: An underestimated old double-edged sword

Antiproteinases such as alpha-2-macroglobulin (A2M) play a role in hemostasis. A2M is highly conserved throughout evolution and is a high molecular weight homo-tetrameric glycoprotein. A2M proteinase inhibitor activity is possible via a unique cage structure leading to proteinase entrapment without direct enzymatic activity inhibition. Following this entrapment, proteinase clearance is possible through A2M binding to the low-density lipoprotein receptor-related protein 1. A2M synthesis is regulated by pro-inflammatory cytokines and increases during several chronic or acute inflammatory diseases and varies with age. For instance, A2M plasma levels are known to be increased in patients with diabetes mellitus, nephrotic syndrome, or sepsis. Concerning hemostasis, A2M can trap many proteinases involved in coagulation and fibrinolysis. Because of its pleiotropic effects A2M can be seen as both anti- and pro-hemostatic. A2M can inhibit thrombin, factor Xa, activated protein C, plasmin, tissue-plasminogen activator, and urokinase. Through its many different functions A2M is generally put apart in the balanced regulation of hemostasis. In addition, the fact that A2M plasma levels are differently regulated during inflammatory-related diseases and that A2M can neutralize cytokines that also modify hemostasis could explain why it is difficult to link common proteins and parameters of hemostasis with the mechanisms of thrombosis in such diseases. Thus, we propose in the present review to summarize known functions of A2M, give a brief overview about diseases, and then to focus on the roles of this antiproteinase in hemostasis and thrombosis.

Quench me if you can: Alpha-2-macroglobulin trypsin complexes enable serum biomarker analysis by MALDI mass spectrometry

One of the main functions of alpha-2-macroglobulin (A2M) in human blood serum is the binding of all classes of protease. It is known that trypsin, after such interaction, possesses modified proteolytic activity. Trypsin first hydrolyzes two bonds in A2M's 'bait region', and the peptide ⁷⁰⁵VGFYESDVMGR⁷¹⁵ is released from A2M. In this work, specifics of the A2M-trypsin interaction were used to determine A2M concentration directly in human blood serum using MALDI mass-spectrometry. Following exogenous addition of trypsin to human blood serum in vitro, the concentration of the VGFYESDVMGR peptide was measured, using its isotopically-labeled analogue (¹⁸O), and A2M concentration was calculated. The optimized mass spectrometric approach was verified using a standard method for A2M concentration determination (ELISA) and the relevant statistical analysis methods. It was also shown that trypsin's modified proteolytic activity in the presence of serum A2M can be used to analyze other serum proteins, including potential biomarkers of pathological processes. Thus, this work describes a promising approach to serum biomarker analysis that can be technically extended in several useful directions.

Hypochlorite-induced aggregation of fibrinogen underlies a novel antioxidant role in blood plasma

Fibrinogen, a major constituent of blood plasma, is highly susceptible to reaction with biological oxidants. It has been proposed that fibrinogen plays a role in antioxidant defence, but oxidation of fibrinogen is also known to disrupt normal blood clotting and is implicated in the pathology of atherosclerosis. In the present study, we show that the biological oxidant hypochlorite promotes the formation of soluble high molecular weight fibrinogen assemblies ≥40 × 10⁶ Da, that do not accumulate when fibrinogen is induced to aggregate by other stresses such as heating or hydroxyl-mediated damage in vitro. Hypochlorite-modified fibrinogen is stable at 37 °C as assessed by precipitation assays, and has reduced susceptibility to iron-induced (hydroxyl-mediated) precipitation compared to native fibrinogen. In contrast to hypochlorite-modified albumin, which is known to be immunostimulatory, hypochlorite-modified fibrinogen does not induce RAW 264.7 (macrophage-like) cells or EOC 13.31 (microglia-like) cells to produce reactive oxygen species or induce cell death. Furthermore, depletion of fibrinogen from human blood plasma increases the immunostimulatory property of blood plasma after it is supplemented with hypochlorite in situ. We propose that reaction of hypochlorite with fibrinogen in blood plasma potentially reduces the accumulation of other hypochlorite-modified species such as immunostimulatory hypochlorite-modified albumin. The latter represent a novel role for fibrinogen in blood plasma antioxidant defence.

Molecular form and concentration of serum α2-macroglobulin in diabetes

α₂-Macroglobulin is a highly abundant serum protein involved in the development of atherosclerosis and cardiac hypertrophy. However, its circulating molecular form and exact concentrations in human health/diseases are not known. Blue native-polyacrylamide gel electrophoresis of human serum was used to confirm the native conformation of α₂-macroglobulin. We created an enzyme-linked immunosorbent assay suitable for quantifying its circulating molecular form and undertook a cross-sectional study to measure its serum levels in 248 patients with diabetes mellitus and 59 healthy volunteers. The predominant circulating molecular form of α₂-macroglobulin was the tetramer, whereas its dimer was detectable in patients with high serum levels of α₂-macroglobulin. The serum α₂-macroglobulin concentration was not associated with glycated hemoglobin or any other glycemic variable as evaluated from 48-h continuous glucose monitoring, but showed close correlation with left ventricular posterior wall thickness, carotid artery intima-media thickness, urinary albumin:creatinine ratio (ACR) and brachial–ankle pulse wave velocity (baPWV). Multivariate analysis revealed only the ACR and baPWV to be independent variables influencing serum levels of α₂-macroglobulin. Thus, an increased ACR and baPWV are associated with higher serum concentrations of α₂-macroglobulin, and the latter may contribute to the mechanism by which albuminuria increases the risk of developing cardiovascular diseases.

Science and art of protein formulation development

Protein pharmaceuticals have become a significant class of marketed drug products and are expected to grow steadily over the next decade. Development of a commercial protein product is, however, a rather complex process. A critical step in this process is formulation development, enabling the final product configuration. A number of challenges still exist in the formulation development process. This review is intended to discuss these challenges, to illustrate the basic formulation development processes, and to compare the options and strategies in practical formulation development.

Human pregnancy zone protein stabilizes misfolded proteins including preeclampsia- and Alzheimer's-associated amyloid beta peptide

Pregnancy is a unique physiological state involving biological stresses that promote protein damage (misfolding) within the maternal body. Currently, little is known regarding how the maternal body copes with elevated protein misfolding in pregnancy. This is important, because the accumulation of misfolded proteins underlies many human disorders, including preeclampsia, a serious complication of pregnancy. In this study, we show that pregnancy zone protein (PZP) efficiently inhibits the aggregation of misfolded proteins, including the amyloid beta peptide, which forms plaques in preeclampsia and in Alzheimer’s disease. We propose that up-regulation of PZP is a major maternal adaptation that helps to maintain protein homeostasis during pregnancy. Moreover, pregnancy-independent up-regulation of PZP indicates that its chaperone function could be broadly important in humans.

Protein misfolding underlies the pathology of a large number of human disorders, many of which are age-related. An exception to this is preeclampsia, a leading cause of pregnancy-associated morbidity and mortality in which misfolded proteins accumulate in body fluids and the placenta. We demonstrate that pregnancy zone protein (PZP), which is dramatically elevated in maternal plasma during pregnancy, efficiently inhibits in vitro the aggregation of misfolded proteins, including the amyloid beta peptide (Aβ) that is implicated in preeclampsia as well as with Alzheimer’s disease. The mechanism by which this inhibition occurs involves the formation of stable complexes between PZP and monomeric Aβ or small soluble Aβ oligomers formed early in the aggregation pathway. The chaperone activity of PZP is more efficient than that of the closely related protein alpha-2-macroglobulin (α₂M), although the chaperone activity of α₂M is enhanced by inducing its dissociation into PZP-like dimers. By immunohistochemistry analysis, PZP is found primarily in extravillous trophoblasts in the placenta. In severe preeclampsia, PZP-positive extravillous trophoblasts are adjacent to extracellular plaques containing Aβ, but PZP is not abundant within extracellular plaques. Our data support the conclusion that the up-regulation of PZP during pregnancy represents a major maternal adaptation that helps to maintain extracellular proteostasis during gestation in humans. We propose that overwhelming or disrupting the chaperone function of PZP could underlie the accumulation of misfolded proteins in vivo. Attempts to characterize extracellular proteostasis in pregnancy will potentially have broad-reaching significance for understanding disease-related protein misfolding.