Identification of human plasma proteins as major clients for the extracellular chaperone clusterin

Copper in Cancer: from transition metal to potential target

In recent years, with the continuous in-depth exploration of the molecular mechanisms of tumorigenesis, numerous potential new targets for cancer treatment have been identified, some of which have been further developed in clinical practice and have produced positive outcomes. Notably, researchers' initial motivation for studying copper metabolism in cancer stems from the fact that copper is a necessary trace element for organisms and is closely connected to body growth and metabolism. Moreover, over the past few decades, considerable progress has been made in understanding the molecular processes and correlations between copper and cancer. Certain achievements have been made in the development and use of relevant clinical medications. The concept of "cuproptosis," a novel concept that differs from previous forms of cell death, was first proposed by a group of scientists last year, offering fresh perspectives on the targeting capabilities of copper in the treatment of cancer. In this review, we introduced the fundamental physiological functions of copper, the key components of copper metabolism, and a summary of the current research contributions on the connection between copper and cancer. In addition, the development of new copper-based nanomaterials and their associated mechanisms of action are discussed. Finally, we described how the susceptibility of cancer cells to this metallic nutrition could be leveraged to further improve the existing cancer treatment paradigm in the new setting.

Impaired Extracellular Proteostasis in Patients with Heart Failure

BACKGROUND

Proteostasis impairment and the consequent increase of amyloid burden in the myocardium have been associated with heart failure (HF) development and poor prognosis. A better knowledge of the protein aggregation process in biofluids could assist the development and monitoring of tailored interventions.

AIM

To compare the proteostasis status and protein's secondary structures in plasma samples of patients with HF with preserved ejection fraction (HFpEF), patients with HF with reduced ejection fraction (HFrEF), and age-matched individuals.

METHODS

A total of 42 participants were enrolled in 3 groups: 14 patients with HFpEF, 14 patients with HFrEF, and 14 age-matched individuals. Proteostasis-related markers were analyzed by immunoblotting techniques. Fourier Transform Infrared (FTIR) Spectroscopy in Attenuated Total Reflectance (ATR) was applied to assess changes in the protein's conformational profile.

RESULTS

Patients with HFrEF showed an elevated concentration of oligomeric proteic species and reduced clusterin levels. ATR-FTIR spectroscopy coupled with multivariate analysis allowed the discrimination of HF patients from age-matched individuals in the protein amide I absorption region (1700-1600 cm^-1), reflecting changes in protein conformation, with a sensitivity of 73 and a specificity of 81%. Further analysis of FTIR spectra showed significantly reduced random coils levels in both HF phenotypes. Also, compared to the age-matched group, the levels of structures related to fibril formation were significantly increased in patients with HFrEF, whereas the β-turns were significantly increased in patients with HFpEF.

CONCLUSION

Both HF phenotypes showed a compromised extracellular proteostasis and different protein conformational changes, suggesting a less efficient protein quality control system.

HDL Accessory Proteins in Parkinson’s Disease—Focusing on Clusterin (Apolipoprotein J) in Regard to Its Involvement in Pathology and Diagnostics—A Review

Parkinson’s disease (PD)—a neurodegenerative disorder (NDD) characterized by progressive destruction of dopaminergic neurons within the substantia nigra of the brain—is associated with the formation of Lewy bodies containing mainly α-synuclein. HDL-related proteins such as paraoxonase 1 and apolipoproteins A1, E, D, and J are implicated in NDDs, including PD. Apolipoprotein J (ApoJ, clusterin) is a ubiquitous, multifunctional protein; besides its engagement in lipid transport, it modulates a variety of other processes such as immune system functionality and cellular death signaling. Furthermore, being an extracellular chaperone, ApoJ interacts with proteins associated with NDD pathogenesis (amyloid β, tau, and α-synuclein), thus modulating their properties. In this review, the association of clusterin with PD is delineated, with respect to its putative involvement in the pathological mechanism and its application in PD prognosis/diagnosis.

Clusterin, paraoxonase 1, and myeloperoxidase alterations induce high-density lipoproteins dysfunction and contribute to peripheral artery disease; aggravation by type 2 diabetes mellitus

Peripheral artery disease (PAD) is an atherosclerotic disorder affecting arteries of the lower limbs, the major risk factors including dyslipidemia and diabetes mellitus (DM). We aimed to identify alterations of the proteins in high-density lipoproteins (HDL) associated with HDL dysfunction in PAD patients. HDL₂ and HDL₃ were isolated from plasma of PAD patients with/without DM (PAD-DM/PAD) and healthy subjects (N). Apolipoprotein AI (ApoAI), ApoAII, ApoCIII, clusterin (CLU), paraoxonase 1 (PON1), myeloperoxidase (MPO), and ceruloplasmin (CP) were measured in HDL₂ /HDL₃ and plasma. Oxidation and glycation of the analyzed proteins were assessed as malondialdehyde-protein adducts (MDA) and advanced glycation end-products (AGE), respectively. The anti-inflammatory effect of HDL₃ was estimated as its potential to reduce monocyte adhesion to tumor necrosis factor α-activated endothelial cells. We show that in PAD patients compared to N subjects: (i) HDL₂ presented increased levels of MDA-PON1, AGE-PON1, AGE-ApoAI, ApoAII, ApoCIII, and CP levels, and decreased PON1 levels; (ii) HDL₃ had increased levels of MDA- and AGE-CLU and -ApoAI, MDA-PON1, ApoCIII, CLU, MPO, CP, and reduced PON1 levels. All these alterations were exacerbated by DM. These changes were more pronounced in HDL₃ , which had reduced anti-inflammatory potential in PAD and became pro-inflammatory in PAD-DM. In PAD patients' plasma, CLU levels and MPO specific activity increased, while PON1 specific activity decreased. In conclusion, HDL function is altered in PAD patients due to multiple modifications of associated proteins that are aggravated by DM. Plasma CLU, MPO, and PON1 could constitute indicators of HDL dysfunction and contribute to risk stratification in PAD patients.

Novel ACE2 protein interactions relevant to COVID-19 predicted by evolutionary rate correlations

Angiotensin-converting enzyme 2 (ACE2) is the cell receptor that the coronavirus SARS-CoV-2 binds to and uses to enter and infect human cells. COVID-19, the pandemic disease caused by the coronavirus, involves diverse pathologies beyond those of a respiratory disease, including micro-thrombosis (micro-clotting), cytokine storms, and inflammatory responses affecting many organ systems. Longer-term chronic illness can persist for many months, often well after the pathogen is no longer detected. A better understanding of the proteins that ACE2 interacts with can reveal information relevant to these disease manifestations and possible avenues for treatment. We have undertaken an approach to predict candidate ACE2 interacting proteins which uses evolutionary inference to identify a set of mammalian proteins that “coevolve” with ACE2. The approach, called evolutionary rate correlation (ERC), detects proteins that show highly correlated evolutionary rates during mammalian evolution. Such proteins are candidates for biological interactions with the ACE2 receptor. The approach has uncovered a number of key ACE2 protein interactions of potential relevance to COVID-19 pathologies. Some proteins have previously been reported to be associated with severe COVID-19, but are not currently known to interact with ACE2, while additional predicted novel ACE2 interactors are of potential relevance to the disease. Using reciprocal rankings of protein ERCs, we have identified strongly interconnected ACE2 associated protein networks relevant to COVID-19 pathologies. ACE2 has clear connections to coagulation pathway proteins, such as Coagulation Factor V and fibrinogen components FGA, FGB, and FGG, the latter possibly mediated through ACE2 connections to Clusterin (which clears misfolded extracellular proteins) and GPR141 (whose functions are relatively unknown). ACE2 also connects to proteins involved in cytokine signaling and immune response (e.g. XCR1, IFNAR2 and TLR8), and to Androgen Receptor (AR). The ERC prescreening approach has elucidated possible functions for relatively uncharacterized proteins and possible new functions for well-characterized ones. Suggestions are made for the validation of ERC-predicted ACE2 protein interactions. We propose that ACE2 has novel protein interactions that are disrupted during SARS-CoV-2 infection, contributing to the spectrum of COVID-19 pathologies.

Plasma Proteomic Analysis in Morquio A Disease

Mucopolysaccharidosis type IVA (MPS IVA) is a lysosomal disease caused by mutations in the gene encoding the enzymeN-acetylgalactosamine-6-sulfate sulfatase (GALNS), and is characterized by systemic skeletal dysplasia due to excessive storage of keratan sulfate (KS) and chondroitin-6-sulfate in chondrocytes. Although improvements in the activity of daily living and endurance tests have been achieved with enzyme replacement therapy (ERT) with recombinant human GALNS, recovery of bone lesions and bone growth in MPS IVA has not been demonstrated to date. Moreover, no correlation has been described between therapeutic efficacy and urine levels of KS, which accumulates in MPS IVA patients. The objective of this study was to assess the validity of potential biomarkers proposed by other authors and to identify new biomarkers. To identify candidate biomarkers of this disease, we analyzed plasma samples from healthy controls (n=6) and from untreated (n=8) and ERT-treated (n=5, sampled before and after treatment) MPS IVA patients using both qualitative and quantitative proteomics analyses. The qualitative proteomics approach analyzed the proteomic profile of the different study groups. In the quantitative analysis, we identified/quantified 215 proteins after comparing healthy control untreated, ERT-treated MPSIVA patients. We selected a group of proteins that were dysregulated in MPS IVA patients. We identified four potential protein biomarkers, all of which may influence bone and cartilage metabolism: fetuin-A, vitronectin, alpha-1antitrypsin, and clusterin. Further studies of cartilage and bone samples from MPS IVA patients will be required to verify the validity of these proteins as potential biomarkers of MPS IVA.

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.

Clusterin Deficiency Predisposes C57BL/6j Mice to Cationic Bovine Serum Albumin-Induced Glomerular Inflammation

Background

Membranous nephropathy (MN) is a specific entity of glomerulonephritis, and its glomerular inflammation is characterized by the deposition of immune complexes in the glomerular basement membrane and proteinuria. However, the molecular mechanisms underlying the glomerular inflammation of MN are not fully understood. This study was designed to investigate the role of clusterin (CLU) in the development of MN using a mouse model of cationic bovine serum albumin (cBSA)-induced MN.

Methods

Both wild-type C57BL/6j (WT) and CLU-knockout C57BL/6j (CLU-KO) mice were immunized with cBSA. The kidney function was determined by the levels of serum creatinine (SCr), blood urea nitrogen (BUN) and urinary protein. MN and glomerular deposits of CLU, complement C3 and immunoglobulins (Igs) were determined by histological analyses. Serum proteins were analyzed by the enzyme-linked immunosorbent assay, Western blot and liquid chromatography-mass spectrometry.

Results

Here, we showed that after cBSA immunization, SCr and proteinuria were increased in CLU-KO mice but not in WT mice. Similarly, severe glomerular atrophy and mesangial expansion along with C3 deposit were only found in the kidneys of CLU-KO mice but not in WT mice. However, there were no differences of serum IgG and complement 3 levels between CLU-KO and WT mice. In the serum of WT mice, CLU bound to anti-cBSA IgG, complements (eg, C8), proteinase/protease inhibitors and antioxidative proteins to form a complex, and incubation with WT serum reduced the complement-dependent lysis of podocytes in cultures.

Conclusion

Our data suggest that a CLU deficiency induces cBSA-initiated glomerular inflammation of MN in a disease-resistant strain of mice, suggesting an anti-glomerular inflammatory function of CLU in the resistance to MN development. This function may be at least in part due to the formation of CLU-anti-cBSA Igs complex that prevents glomerular inflammation or injury in the disease-resistant mice.

Preliminary assessment of serum clusterin as a potential biomarker for canine lymphoma

Clusterin (CLU), also known as apolipoprotein J, is a widely expressed, heterodimeric, glycoprotein, important in tumourigenesis, apoptosis and immunoregulation. In humans, CLU expression has been associated with anaplastic large cell and Hodgkin's lymphoma. In this study, serum CLU levels in dogs with multicentric lymphoma (MLSA) were compared with healthy control dogs, using both western blot and enzyme-linked immunosorbent assay (ELISA). Western blot confirmed the presence of CLU in dog sera at the predicted molecular weight and the relative levels detected correlated with the levels detected by ELISA. CLU level analysis by ELISA found treatment naïve dogs with MLSA had a significantly (P < .001) lower serum CLU level compared with healthy controls. However, there was no significant difference between MLSA dogs prior to treatment and in complete remission. The wide variation in serum CLU levels may limit its potential as a single candidate biomarker for MLSA, although any prognostic predictive value of serum CLU concentrations has yet to be assessed.

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.

Clusterin in the mouse epididymis: possible roles in sperm maturation and capacitation

Clusterin (CLU) is known as an extracellular chaperone for proteins under stress, thus preventing them from aggregation and precipitation. We showed herein that CLU, expressed by principal cells of the mouse caput epididymis, was present in high amounts in the lumen. In the cauda epididymis, CLU bound tightly to the sperm head surface and its amount on total sperm was similar to that in the bathing luminal fluid. In both immotile and motile caudal epididymal sperm, CLU was localized over the entire sperm head except at the convex ridge, although in the motile sperm population, the CLU immunofluorescence pattern was distinctively mottled with a lower intensity. However, when motile sperm became capacitated, CLU was relocalized to the head hook region, with immunofluorescence intensity being higher than that on the non-capacitated counterparts. Under a slightly acidic pH of the epididymal lumen, CLU may chaperone some luminal proteins and deliver them onto the sperm surface. Immunoprecipitation of epididymal fluid proteins indicated that CLU interacted with SED1, an important egg-binding protein present in a high amount in the epididymal lumen. In a number of non-capacitated sperm, fractions of SED1 and CLU co-localized, but after capacitation, SED1 and CLU dissociated from one another. While CLU moved to the sperm head hook, SED1 translocated to the head convex ridge, the egg-binding site. Overall, CLU localization patterns can serve as biomarkers of immotile sperm, and non-capacitated and capacitated sperm in mice. The chaperone role of CLU may also be important for sperm maturation and capacitation.

How schistosomes alter the human serum proteome

Schistosomes are intravascular parasitic worms that cause the debilitating disease schistosomiasis. To better understand how these long-lived parasites may subvert host immune and hemostatic capabilities, we examine here the impact of adult Schistosoma mansoni worms on the human serum proteome. Normal human serum (150μl) was incubated at 37°C for one hour either in the presence or absence of adult worms (∼50 pairs). Thereafter parasites were removed, serum samples were labeled and their proteins resolved for comparative analysis by 2D-Differential in-Gel Electrophoresis (2D-DIGE). Several differences were noted between the two samples. Twenty protein spots were recovered and identified following trypsin digestion and mass spectroscopy. Strikingly, most of these (11/20) are associated with the complement system and include complement components C3, C4, factor B, complement factor H related protein 1 and clusterin. Western blot analysis confirms the impact of the worms on C3, C4 and factor B in serum. The data suggest that schistosomes engage complement but can rapidly degrade selected complement proteins which may help explain the worm's refractoriness towards complement-mediated attack. Other serum proteins identified as being impinged upon by schistosomes are alpha 2 macroglobulin, alpha 1 anti-chymotrypsin, actin cytoplasmic 2, serum amyloid A-4, protein DDX26B, hemoglobin subunit B and serum albumin. While the molecular nature of the interaction between these proteins and schistosomes is not known, possible roles for some of them in hemostasis, immune evasion and in the host response to serum stress are suggested.

Altered levels of blood proteins in Alzheimer's disease longitudinal study: Results from Australian Imaging Biomarkers Lifestyle Study of Ageing cohort

INTRODUCTION

A blood-based biomarker panel to identify individuals with preclinical Alzheimer's disease (AD) would be an inexpensive and accessible first step for routine testing.

METHODS

We analyzed 14 biomarkers that have previously been linked to AD in the Australian Imaging Biomarkers lifestyle longitudinal study of aging cohort.

RESULTS

Levels of apolipoprotein J (apoJ) were higher in AD individuals compared with healthy controls at baseline and 18 months (P = .0003) and chemokine-309 (I-309) were increased in AD patients compared to mild cognitive impaired individuals over 36 months (P = .0008).

DISCUSSION

These data suggest that apoJ may have potential in the context of use (COU) of AD diagnostics, I-309 may be specifically useful in the COU of identifying individuals at greatest risk for progressing toward AD. This work takes an initial step toward identifying blood biomarkers with potential use in the diagnosis and prognosis of AD and should be validated across other prospective cohorts.

Fluorinated Photodynamic Therapy Device Tips and their Resistance to Fouling for In Vivo Sensitizer Release

We describe progress on a one-step photodynamic therapy (PDT) technique that is simple: device tip delivery of sensitizer, oxygen and light simultaneously. Control is essential for their delivery to target sites to generate singlet oxygen. One potential problem is the silica device tip may suffer from biomaterial fouling and the pace of sensitizer photorelease is slowed. Here, we have used biomaterial (e.g. proteins, cells, etc.) from SQ20B head and neck tumors and whole blood for an assessment of fouling of the silica tips by adsorption. It was shown that by exchanging the native silica tip for a fluorinated tip, a better nonstick property led to an increased sensitizer output by ~10%. The fluorinated tip gave a sigmoidal photorelease where singlet oxygen is stabilized to physical quenching, whereas the native silica tip with unprotected SiO-H groups gave a slower (pseudolinear) photorelease. A further benefit from fluorinated silica is that 15% less biomaterial adheres to its surface compared to native silica based on a bicinchoninic acid assay (BCA) and X-ray photoelectron spectroscopy (XPS) measurements. We discuss how the fluorination of the device tip increases biofouling resistance and can contribute to a new pointsource PDT tool.

LDL electronegativity index: a potential novel index for predicting cardiovascular disease

High cardiovascular risk conditions are frequently associated with altered plasma lipoprotein profile, such as elevated low-density lipoprotein (LDL) and LDL cholesterol and decreased high-density lipoprotein. There is, however, accumulating evidence that specific subclasses of LDL may play an important role in cardiovascular disease development, and their relative concentration can be regarded as a more relevant risk factor. LDL particles undergo multiple modifications in plasma that can lead to the increase of their negative charge. The resulting electronegative LDL [LDL(–)] subfraction has been demonstrated to be especially atherogenic, and became a subject of numerous recent studies. In this review, we discuss the physicochemical properties of LDL(–), methods of its detection, atherogenic activity, and relevance of the LDL electronegativity index as a potential independent predictor of cardiovascular risk.

Fucosylated clusterin in semen promotes the uptake of stress-damaged proteins by dendritic cells via DC-SIGN

STUDY QUESTION

Could seminal plasma clusterin play a role in the uptake of stress-damaged proteins by dendritic cells?

SUMMARY ANSWER

Seminal plasma clusterin, but not serum clusterin, promotes the uptake of stress-damaged proteins by dendritic cells via DC-SIGN.

WHAT IS KNOWN ALREADY

Clusterin is one of the major extracellular chaperones. It interacts with a variety of stressed proteins to prevent their aggregation, guiding them for receptor-mediated endocytosis and intracellular degradation. The concentration of clusterin in semen is almost 20-fold higher than that found in serum, raising the question about the role of seminal plasma clusterin in reproduction. No previous studies have analyzed whether seminal plasma clusterin has chaperone activity. We have previously shown that seminal plasma clusterin, but not serum clusterin, expresses an extreme abundance of fucosylated glycans. These motifs enable seminal plasma clusterin to bind DC-SIGN with very high affinity.

STUDY DESIGN, SIZE, DURATION

In vitro experiments were performed to evaluate the ability of seminal plasma clusterin to inhibit the precipitation of stressed proteins, promoting their uptake by dendritic cells via DC-SIGN (a C-type lectin receptor selectively expressed on dendritic cells (DC)). Moreover, the ability of seminal plasma clusterin to modulate the phenotype and function of DCs was also assessed.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Clusterin was purified from human semen and human serum. Catalase, bovine serum albumin, glutathione S-transferase, and normal human serum were stressed and the ability of seminal plasma clusterin to prevent the precipitation of these proteins, guiding them to DC-SIGN expressed by DCs, was evaluated using a fluorescence-activated cell sorter (FACS). Endocytosis of stressed proteins was analyzed by confocal microscopy and the ability of seminal plasma clusterin-treated DCs to stimulate the proliferation of CD25+FOXP3+CD4+ T cells was also evaluated by FACS.

MAIN RESULTS AND THE ROLE OF CHANCE

Seminal plasma clusterin interacts with stressed proteins, inhibits their aggregation (P < 0.01) and efficiently targets them to dendritic cells via DC-SIGN (P < 0.01). DCs efficiently endocytosed clusterin-client complexes and sorted them to degradative compartments involved in antigen processing and presentation. Moreover, we also found that the interaction of seminal plasma clusterin with DC-SIGN did not change the phenotype of DCs, but stimulates their ability to induce the expansion of CD25+FOXP3+CD4+ T lymphocytes (P < 0.05 versus control).

LIMITATIONS, REASONS FOR CAUTION

All the experiments were performed in vitro; hence the relevance of our observations should be validated in vivo.

WIDER IMPLICATIONS OF THE FINDINGS

Our results suggest that by inducing the endocytosis of stress-damaged proteins by DCs via DC-SIGN, seminal plasma clusterin might promote a tolerogenic response to male antigens, thereby contributing to female tolerance to seminal antigens.

STUDY FUNDING/COMPETING INTERESTS

The present research was supported by the Consejo Nacional de Investigaciones Científicas y Técnicas, the Buenos Aires University School of Medicine, and the Agencia Nacional de Promoción Científica y Tecnológica (Argentina). The authors have no conflicts of interest to declare.

Cooperative stabilization of transthyretin by clusterin and diflunisal

The circulating protein transthyretin (TTR) can unfold, oligomerize, and form highly structured amyloid fibrils that are deposited in tissues, causing organ damage and disease. This pathogenic process is caused by a heritable TTR point mutation in cases of familial TTR-related amyloidosis or wild-type TTR in cases of age-associated amyloidosis (previously called senile systemic amyloidosis). The TTR amyloid cascade is hypothesized to begin with the dissociation of the TTR native tetrameric structure into folded but unstable monomeric TTR subunits. Unfolding of monomeric TTR initiates an oligomerization process leading to aggregation and fibril formation. Numerous proteostatic mechanisms for regulating the TTR amyloid cascade exist. Extracellular chaperones provide an innate defense against misfolded proteins. Clusterin (CLU), a plasma protein, has the capacity to recognize exposed hydrophobic regions of misfolded proteins, shielding them from aggregation. We have previously demonstrated that CLU is associated with the amyloid fibrils in cardiac tissues from patients with TTR amyloidosis. In this study, we have used tetrameric and monomeric TTR structural variants to determine the ability of CLU to inhibit TTR amyloid fibril formation. Using circular dichroism spectroscopy, we determined that CLU preferentially stabilizes monomeric TTR and generates increasingly stable conformations under acid stress. Moreover, studies using surface plasmon resonance showed a direct interaction of CLU with high-molecular weight TTR oligomers. The interactions of CLU with monomeric and aggregated TTR proceed in a cooperative manner in the presence of diflunisal, a small molecule drug used to stabilize TTR tetramers.

Hypochlorite-induced structural modifications enhance the chaperone activity of human α2-macroglobulin

Hypochlorite, an oxidant generated in vivo by the innate immune system, kills invading pathogens largely by inducing the misfolding of microbial proteins. Concomitantly, the nonspecific activity of hypochlorite also damages host proteins, and the accumulation of damaged (misfolded) proteins is implicated in the pathology of a variety of debilitating human disorders (e.g., Alzheimer's disease, atherosclerosis, and arthritis). It is well-known that cells respond to oxidative stress by up-regulating proteostasis machinery, but the direct activation of mammalian chaperones by hypochlorite has not, to our knowledge, been previously reported. In this study, we show that hypochlorite-induced modifications of human α2-macroglobulin (α2M) markedly increase its chaperone activity by generating species, particularly dimers formed by dissociation of the native tetramer, which have enhanced surface hydrophobicity. Moreover, dimeric α2M is generated in whole-blood plasma in the presence of physiologically relevant amounts of hypochlorite. The chaperone activity of hypochlorite-modified α2M involves the formation of stable soluble complexes with misfolded client proteins, including heat-denatured enzymes, oxidized fibrinogen, oxidized LDL, and native or oxidized amyloid β-peptide (Aβ1-42). Here, we show that hypochlorite-modified α2M delivers its misfolded cargo to lipoprotein receptors on macrophages and reduces Aβ1-42 neurotoxicity. Our results support the conclusion that α2M is a specialized chaperone that prevents the extracellular accumulation of misfolded and potentially pathogenic proteins, particularly during innate immune system activity.

The molecular chaperone apolipoprotein J/clusterin as a sensor of oxidative stress: implications in therapeutic approaches - a mini-review

BACKGROUND

Organisms are constantly exposed to physiological and environmental stresses and therefore require an efficient surveillance of genome and proteome quality in order to prevent disruption of homeostasis. Central to the intra- and extracellular proteome surveillance system are the molecular chaperones that contribute to both proteome maintenance and clearance. The conventional protein product of the apolipoprotein J/clusterin (CLU) gene is a heterodimeric secreted glycoprotein (also termed as sCLU) with a ubiquitous expression in human tissues. CLU exerts a small heat shock protein-like stress-induced chaperone activity and has been functionally implicated in numerous physiological processes as well as in ageing and most age-related diseases including tumorigenesis, neurodegeneration, and cardiovascular and metabolic syndromes.

OBJECTIVE

The CLU gene is differentially regulated by a wide variety of stimuli due to the combined presence of many distinct regulatory elements in its promoter that make it an extremely sensitive cellular biosensor of environmental and/or oxidative stress. Downstream to CLU gene induction, the CLU protein seems to actively intervene in pathological states of increased oxidative injury due to its chaperone-related property to inhibit protein aggregation and precipitation (a main feature of oxidant injury), as well as due to its reported distribution in both extra- and, most likely, intracellular compartments.

CONCLUSION

On the basis of these findings, CLU has emerged as a unique regulator of cellular proteostasis. Nevertheless, it seemingly exerts a dual function in pathology. For instance, in normal cells and during early phases of carcinogenesis, CLU may inhibit tumor progression as it contributes to suppression of proteotoxic stress. In advanced neoplasia, however, it may offer a significant survival advantage in the tumor by suppressing many therapeutic stressors and enhancing metastasis. This review will critically present a synopsis of recent novel findings that relate to the function of this amazing molecule and support the notion that CLU is a biosensor of oxidative injury; a common link between ageing and all pathologies where CLU has been implicated. Potential future perspectives, implications and opportunities for translational research and the development of new therapies will be discussed.

Acute phase proteins are major clients for the chaperone action of α₂-macroglobulin in human plasma

Extracellular protein misfolding is implicated in many age-related diseases including Alzheimer's disease, macular degeneration and arthritis. In this study, putative endogenous clients for the chaperone activity of α₂-macroglobulin (α₂M) were identified after human plasma was subjected to physiologically relevant sheer stress at 37 °C for 10 days. Western blot analysis showed that four major acute phase proteins: ceruloplasmin, fibrinogen, α₁-acid glycoprotein and complement component 3, preferentially co-purified with α₂M after plasma was stressed. Furthermore, the formation of complexes between α₂M and these putative chaperone clients, detected by sandwich ELISA, was shown to be enhanced in response to stress. These results support the hypothesis that α₂M plays an important role in extracellular proteostasis by sequestering misfolded proteins and targeting them for disposal, particularly during acute phase reactions.

Electronegative low-density lipoprotein. A link between apolipoprotein B misfolding, lipoprotein aggregation and proteoglycan binding

PURPOSE OF REVIEW

Subendothelial retention of lipoproteins is considered the first step in the development of atherosclerosis, but the molecular mechanisms involved are poorly understood. Recent findings on the atherogenic properties of a minor electronegative fraction of LDL (LDL(-)) could contribute to a better understanding of this process.

RECENT FINDINGS

Circular dichroism, Trp-fluorescence and two-dimensional nuclear magnetic resonance have shown that apolipoprotein B (apoB) in LDL(-) has an abnormal, misfolded conformation. Immunochemical analysis revealed a different conformation, mainly in the N-terminal and C-terminal extremes. These alterations contribute to the high susceptibility to aggregation of LDL(-). Moreover, LDL(-) can seed the aggregation of native LDL, suggesting an amyloidogenic character that has been attributed to the amphipathic helix cluster in the α2-domain. A phospholipase C (PLC)-like activity associated to LDL(-) seems to play a major role in the LDL(-)-induced aggregation. The aggregation of LDL(-) increases its binding to proteoglycans because of the abnormal conformation of the N-terminal extreme of apoB.

SUMMARY

LDL(-) could play a relevant role in atherogenesis by acting as a priming factor that stimulates lipoprotein aggregation. This process, which appears to be mediated by a PLC-like activity intrinsic to LDL(-), increases the binding of LDL to proteoglycans and could promote subendothelial retention of these lipoproteins.

Serum albumin prevents protein aggregation and amyloid formation and retains chaperone-like activity in the presence of physiological ligands

Although serum albumin has an established function as a transport protein, evidence is emerging that serum albumin may also have a role as a molecular chaperone. Using established techniques to characterize chaperone interactions, this study demonstrates that bovine serum albumin: 1) preferentially binds stressed over unstressed client proteins; 2) forms stable, soluble, high molecular weight complexes with stressed client proteins; 3) reduces the aggregation of client proteins when it is present at physiological levels; and 4) inhibits amyloid formation by both WT and L55P transthyretin. Although the antiaggregatory effect of serum albumin is maintained in the presence of physiological levels of Ca(2+) and Cu(2+), the presence of free fatty acids significantly alters this activity: stabilizing serum albumin at normal levels but diminishing chaperone-like activity at high concentrations. Moreover, here it is shown that depletion of albumin from human plasma leads to a significant increase in aggregation under physiologically relevant heat and shear stresses. This study demonstrates that serum albumin possesses chaperone-like properties and that this activity is maintained under a number of physiologically relevant conditions.

Clusterin facilitates in vivo clearance of extracellular misfolded proteins

The extracellular deposition of misfolded proteins is a characteristic of many debilitating age-related disorders. However, little is known about the specific mechanisms that act to suppress this process in vivo. Clusterin (CLU) is an extracellular chaperone that forms stable and soluble complexes with misfolded client proteins. Here we explore the fate of complexes formed between CLU and misfolded proteins both in vitro and in a living organism. We show that proteins injected into rats are cleared more rapidly from circulation when complexed with CLU as a result of their more efficient localization to the liver and that this clearance is delayed by pre-injection with the scavenger receptor inhibitor fucoidan. The CLU-client complexes were found to bind preferentially, in a fucoidan-inhibitable manner, to human peripheral blood monocytes and isolated rat hepatocytes and in the latter cell type were internalized and targeted to lysosomes for degradation. The data suggest, therefore, that CLU plays a key role in an extracellular proteostasis system that recognizes, keeps soluble, and then rapidly mediates the disposal of misfolded proteins.

Semen clusterin is a novel DC-SIGN ligand

The C-type lectin receptor dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) is an important player in the recognition of pathogens by dendritic cells. A plethora of pathogens including viruses, bacteria, parasites, and fungi are recognized by DC-SIGN through both mannose and fucose-containing glycans expressed on the pathogen surface. In this study, we identified semen clusterin as a novel DC-SIGN ligand. Semen clusterin, but not serum clusterin, expresses an extreme abundance of fucose-containing blood-type Ags such as Le(x) and Le(y), which are both excellent DC-SIGN ligands. These motifs enable semen clusterin to bind DC-SIGN with very high affinity (K(d) 76 nM) and abrogate the binding of HIV-1 to DC-SIGN. Depletion of clusterin from semen samples, however, did not completely prevent the ability of semen to inhibit the capture of HIV-1 by DC-SIGN, supporting that besides clusterin, semen contains other DC-SIGN ligands. Further studies are needed to characterize these ligands and define their contribution to the DC-SIGN-blocking activity mediated by semen. Clusterin is an enigmatic protein involved in a variety of physiologic and pathologic processes including inflammation, atherosclerosis, and cancer. Our results uncover an unexpected heterogeneity in the glycosylation pattern of clusterin and suggest that the expression of high concentrations of fucose-containing glycans enables semen clusterin to display a unique set of biological functions that might affect the early course of sexually transmitted infectious diseases.

Inhibition of beta2-microglobulin amyloid fibril formation by alpha2-macroglobulin

The relationship between various amyloidoses and chaperones is gathering attention. In patients with dialysis-related amyloidosis, α(2)-macroglobulin (α2M), an extracellular chaperone, forms a complex with β(2)-microglobulin (β2-m), a major component of amyloid fibrils, but the molecular mechanisms and biological implications of the complex formation remain unclear. Here, we found that α2M substoichiometrically inhibited the β2-m fibril formation at a neutral pH in the presence of SDS, a model for anionic lipids. Binding analysis showed that the binding affinity between α2M and β2-m in the presence of SDS was higher than that in the absence of SDS. Importantly, SDS dissociated tetrameric α2M into dimers with increased surface hydrophobicity. Western blot analysis revealed that both tetrameric and dimeric α2M interacted with SDS-denatured β2-m. At a physiologically relevant acidic pH and in the presence of heparin, α2M was also dissociated into dimers, and both tetrameric and dimeric α2M interacted with β2-m, resulting in the inhibition of fibril growth reaction. These results suggest that under conditions where native β2-m is denatured, tetrameric α2M is also converted to dimeric form with exposed hydrophobic surfaces to favor the hydrophobic interaction with denatured β2-m, thus dimeric α2M as well as tetrameric α2M may play an important role in controlling β2-m amyloid fibril formation.

Extracellular chaperones

The maintenance of the levels and correct folding state of proteins (proteostasis) is a fundamental prerequisite for life. Life has evolved complex mechanisms to maintain proteostasis and many of these that operate inside cells are now well understood. The same cannot yet be said of corresponding processes in extracellular fluids of the human body, where inappropriate protein aggregation is known to underpin many serious diseases such as Alzheimer's disease, type II diabetes and prion diseases. Recent research has uncovered a growing family of abundant extracellular chaperones in body fluids which appear to selectively bind to exposed regions of hydrophobicity on misfolded proteins to inhibit their toxicity and prevent them from aggregating to form insoluble deposits. These extracellular chaperones are also implicated in clearing the soluble, stabilized misfolded proteins from body fluids via receptor-mediated endocytosis for subsequent lysosomal degradation. Recent work also raises the possibility that extracellular chaperones may play roles in modulating the immune response. Future work will better define the in vivo functions of extracellular chaperones in proteostasis and immunology and pave the way for the development of new treatments for serious diseases.

Evidence for a functional role of the molecular chaperone clusterin in amyloidotic cardiomyopathy

Molecular chaperones, including the extracellular protein clusterin (CLU), play a significant role in maintaining proteostasis; they have a unique capacity to bind and stabilize non-native protein conformations, prevent aggregation, and keep proteins in a soluble folding-competent state. In this study, we investigated amyloid-infiltrated cardiac tissue for the presence of CLU and measured serum levels of CLU in patients with and without amyloidotic cardiomyopathy (CMP). Cardiac tissues containing amyloid deposits composed of either transthyretin (TTR) or Ig light chain from nine patients with amyloidotic CMP were examined for the presence of CLU using immunohistochemical techniques. CLU staining coincided with the extracellular myocardial amyloid deposits in tissues from patients with familial TTR, senile systemic, and Ig light chain amyloidosis. The association of CLU with cardiac amyloid deposits was confirmed by immunogold electron microscopy. Serum concentrations of CLU were measured in familial TTR, senile systemic, and Ig light chain amyloidosis patient groups and compared with both age-matched healthy controls and with patients with CMP unrelated to amyloid disease. Subset analysis of disease cohorts, based on cardiac involvement, indicated that decreased serum CLU concentrations were associated with amyloidotic CMP. Taken together, these results suggest that CLU may play a pathogenetic role in TTR and Ig light chain amyloidoses and amyloidotic CMP.

The effects of nitric oxide-oxidase and putative glutathione-peroxidase activities of ceruloplasmin on the viability of cardiomyocytes exposed to hydrogen peroxide

Ceruloplasmin (CP), a ferroxidase (EC 1.16.3.1) and a scavenger of reactive oxygen species, is an important extracellular antioxidant. Bovine CP indeed protects the isolated heart under ischemia-reperfusion conditions. Human CP has been shown to also exhibit, in vitro, glutathione (GSH)-peroxidase and nitric oxide (NO)-oxidase/S-nitrosating activities. This work tested, using bovine CP, the hypothesis that both activities could provide cytoprotection during oxidative stress induced by hydrogen peroxide (H(2)O(2)), the former activity by consuming H(2)O(2) and the latter by shielding thiols from irreversible oxidation. In acellular assays, bovine CP stimulated the generation of the nitrosating NO(+) species from the NO donors propylaminepropylamine-NONOate (PAPA/NO), S-nitroso-N-acetylpenicillamine, and S-nitrosoglutathione. This NO-oxidase activity S-nitrosated GSH as well as CP itself and was not affected by H(2)O(2). In contrast to human CP, bovine CP consumed H(2)O(2) in an additive rather than synergistic manner in the presence of GSH. A nonenzymatic scavenging of H(2)O(2) could have masked the GSH-peroxidase activity. Cytoprotection was evaluated using neonatal rat cardiomyocytes. CP and PAPA/NO were not protective against the H(2)O(2)-induced loss of viability. In contrast, GSH provided a slight protection that increased more than additively in the presence of CP. This increase was canceled by PAPA/NO. CP's putative GSH-peroxidase activity can thus provide cytoprotection but is possibly affected by the S-nitrosation of a catalytically important cysteine residue.

Expression of metabolic, tissue remodeling, oxidative stress, and inflammatory pathways in mammary tissue during involution in lactating dairy cows

Histological and functional changes associated with involution in the mammary gland are partly regulated by changes in gene expression. At 42 d postpartum, Holstein cows underwent a period of 5 d during which they were milked 1X daily until complete cessation of milking. Percutaneous mammary biopsies (n = 5/time point) were obtained on d 1, 5, 14, and 21 relative to the start of 1X milking for transcript profiling via qPCR of 57 genes associated with metabolism, apoptosis/proliferation, immune response/inflammation, oxidative stress, and tissue remodeling. Not surprisingly, there was clear downregulation of genes associated with milk fat synthesis (FASN, ACACA, CD36, FABP3, SCD) and lipid-related transcription regulation (SREBF1, SREBF2). Similar to milk fat synthesis-related genes, those encoding proteins required for glucose uptake (SLC2A1), casein synthesis (CSN2, CSN3), and lactose synthesis (LALBA) decreased during involution. Unlike metabolic genes, those associated with immune response and inflammation (C3, LTF, SAA3), oxidative stress (GPX1, SOD2), and pro-inflammatory cytokine signaling (SPP1, TNF) increased to peak levels by d 14 from the start of 1X milking. These adaptations appeared to be related with tissue remodeling as indicated by upregulation of proteins encoding matrix proteinases (MMP2), IGFBP3, and transcriptional regulation of apoptosis/cell proliferation (MYC). In contrast, the concerted upregulation of STAT3, TGFB1, and TGFB1R during the first 14 d was suggestive of an activation of these signaling pathways probably as an acute response to regulate differentiation and/or mammary cell survival upon the onset of a marked pro-inflammatory and oxidative stress response induced by the gradual reduction in milk removal. Results suggest a central role of STAT3, MYC, PPARG, SREBF1, and SREBF2 in regulating concerted alterations in metabolic and cell survival mechanisms, which were induced partly via oxidative stressed-triggered inflammation and the decline in metabolic activity.

Extracellular Chaperones

The maintenance of the levels and correct folding state of proteins (proteostasis) is a fundamental prerequisite for life. Life has evolved complex mechanisms to maintain proteostasis and many of these that operate inside cells are now well understood. The same cannot yet be said of corresponding processes in extracellular fluids of the human body, where inappropriate protein aggregation is known to underpin many serious diseases such as Alzheimer's disease, type II diabetes and prion diseases. Recent research has uncovered a growing family of abundant extracellular chaperones in body fluids which appear to selectively bind to exposed regions of hydrophobicity on misfolded proteins to inhibit their toxicity and prevent them from aggregating to form insoluble deposits. These extracellular chaperones are also implicated in clearing the soluble, stabilized misfolded proteins from body fluids via receptor-mediated endocytosis for subsequent lysosomal degradation. Recent work also raises the possibility that extracellular chaperones may play roles in modulating the immune response. Future work will better define the in vivo functions of extracellular chaperones in proteostasis and immunology and pave the way for the development of new treatments for serious diseases.

Levels of plasma ceruloplasmin protein are markedly lower following dietary copper deficiency in rodents

Ceruloplasmin (Cp) is a multicopper oxidase and the most abundant copper binding protein in vertebrate plasma. Loss of function mutations in humans or experimental deletion in mice result in iron overload consistent with a putative ferroxidase function. Prior work suggested plasma may contain multiple ferroxidases. Studies were conducted in Holtzman rats (Rattusnorvegicus), albino mice (Mus musculus), Cp-/- mice, and adult humans (Homo sapiens) to investigate the copper-iron interaction. Dietary copper-deficient (CuD) rats and mice were produced using a modified AIN-76A diet. Results confirmed that o-dianisidine is a better substrate than paraphenylene diamine (PPD) for assessing diamine oxidase activity of Cp. Plasma from CuD rat dams and pups, and CuD and Cp-/- mice contained no detectable Cp diamine oxidase activity. Importantly, no ferroxidase activity was detectable for CuD rats, mice, or Cp-/- mice compared to robust activity for copper-adequate (CuA) rodent controls using western membrane assay. Immunoblot protocols detected major reductions (60-90%) in Cp protein in plasma of CuD rodents but no alteration in liver mRNA levels by qRT-PCR. Data are consistent with apo-Cp being less stable than holo-Cp. Further research is needed to explain normal plasma iron in CuD mice. Reduction in Cp is a sensitive biomarker for copper deficiency.