Structural characterization of clusterin-chaperone client protein complexes

Identification of novel endoplasmic reticulum-related genes and their association with immune cell infiltration in major depressive disorder

BACKGROUND

Several lines of evidence point to an interaction between genetic predisposition and environmental factors in the onset of major depressive disorder (MDD). This study is aimed to investigate the pathogenesis of MDD by identifying key biomarkers, associated immune infiltration using bioinformatic analysis and human postmortem sample.

METHODS

The Gene Expression Omnibus (GEO) database of GSE98793 was adopted to identify hub genes linked to endoplasmic reticulum (ER) stress-related genes (ERGs) in MDD. Another GEO database of GSE76826 was employed to validate the novel target associated with ERGs and immune infiltration in MDD. Moreover, human postmortem sample from MDD patients was utilized to confirm the differential expression analysis of hub genes.

RESULTS

We discovered 12 ER stress-related differentially expressed genes (ERDEGs). A LASSO Cox regression analysis helped construct a diagnostic model for these ERDEGs, incorporating immune infiltration analysis revealed that three hub genes (ERLIN1, SEC61B, and USP13) show the significant and consistent expression differences between the two groups. Western blot analysis of postmortem brain samples indicated notably higher expression levels of ERLIN1 and SEC61B in the MDD group, with USP13 also tending to increase compared to control group.

LIMITATIONS

The utilization of the MDD gene chip in this analysis was sourced from the GEO database, which possesses a restricted number of pertinent gene chip samples.

CONCLUSIONS

These findings indicate that ERDEGs especially including ERLIN1, SEC61B, and USP13 associated the infiltration of immune cells may be potential diagnostic indicators for MDD.

Complement C7 and clusterin form a complex in circulation

Introduction

The complement system is part of innate immunity and is comprised of an intricate network of proteins that are vital for host defense and host homeostasis. A distinct mechanism by which complement defends against invading pathogens is through the membrane attack complex (MAC), a lytic structure that forms on target surfaces. The MAC is made up of several complement components, and one indispensable component of the MAC is C7. The role of C7 in MAC assembly is well documented, however, inherent characteristics of C7 are yet to be investigated.

Methods

To shed light on the molecular characteristics of C7, we examined the properties of serum-purified C7 acquired using polyclonal and novel monoclonal antibodies. The properties of serum‑purified C7 were investigated through a series of proteolytic analyses, encompassing Western blot and mass spectrometry. The nature of C7 protein-protein interactions were further examined by a novel enzyme-linked immunosorbent assay (ELISA), as well as size‑exclusion chromatography.

Results

Protein analyses showcased an association between C7 and clusterin, an inhibitory complement regulator. The distinct association between C7 and clusterin was also demonstrated in serum-purified clusterin. Further assessment revealed that a complex between C7 and clusterin (C7-CLU) was detected. The C7-CLU complex was also identified in healthy serum and plasma donors, highlighting the presence of the complex in circulation.

Discussion

Clusterin is known to dissociate the MAC structure by binding to polymerized C9, nevertheless, here we show clusterin binding to the native form of a terminal complement protein in vivo. The presented data reveal that C7 exhibits characteristics beyond that of MAC assembly, instigating further investigation of the effector role that the C7-CLU complex plays in the complement cascade.

Method to Assess the Intracellular Fate and Bioavailability of Clusterin Using Live Cell Confocal Microscopy Imaging

Clusterin, also known as apolipoprotein J, is an ATP-independent holdase chaperone protein. Clusterin is involved in various functions including protein quality control and lipid transport. Though clusterin is secreted upon stress, the intracellular fate of clusterin after a stress response is not well understood. The protocol described here utilizes clusterin tagged to fluorescent proteins like green fluorescent protein and red fluorescent protein to understand the intracellular fate of clusterin.

Circadian Regulation of Apolipoproteins in the Brain: Implications in Lipid Metabolism and Disease

The circadian rhythm is a 24 h internal clock within the body that regulates various factors, including sleep, body temperature, and hormone secretion. Circadian rhythm disruption is an important risk factor for many diseases including neurodegenerative illnesses. The central and peripheral oscillators' circadian clock network controls the circadian rhythm in mammals. The clock genes govern the central clock in the suprachiasmatic nucleus (SCN) of the brain. One function of the circadian clock is regulating lipid metabolism. However, investigations of the circadian regulation of lipid metabolism-associated apolipoprotein genes in the brain are lacking. This review summarizes the rhythmic expression of clock genes and lipid metabolism-associated apolipoprotein genes within the SCN in Mus musculus. Nine of the twenty apolipoprotein genes identified from searching the published database (SCNseq and CircaDB) are highly expressed in the SCN. Most apolipoprotein genes (ApoE, ApoC1, apoA1, ApoH, ApoM, and Cln) show rhythmic expression in the brain in mice and thus might be regulated by the master clock. Therefore, this review summarizes studies on lipid-associated apolipoprotein genes in the SCN and other brain locations, to understand how apolipoproteins associated with perturbed cerebral lipid metabolism cause multiple brain diseases and disorders. This review describes recent advancements in research, explores current questions, and identifies directions for future research.

A characteristic N-glycopeptide signature associated with diabetic cognitive impairment identified in a longitudinal cohort study

BACKGROUND

Identifying a biomarker for the decline in cognitive function in patients with diabetes is important. Therefore, we aimed to identify the N-glycopeptides on plasma proteins associated with diabetic cognitive impairment in participants in a longitudinal study using N-glycoproteomics.

METHODS

We used samples from the 3-year SONIC (Septuagenarians, Octogenarians, Nonagenarians Investigation with Centenarians) longitudinal cohort study of older Japanese people in the general population. First, we placed the participants with diabetes into two groups: those that did or did not have cognitive decline over a 6-year period. Next, their plasma protein profiles were compared between baseline and the 6-year time point using two-dimensional fluorescence difference gel electrophoresis. Finally, an N-glycoproteomic study of the focused proteins was performed using an enrichment technique and liquid chromatography-tandem mass spectrometry.

RESULTS

Approximately 500 N-glycopeptides, derived from 18 proteins, were identified in each sample, from among which we identified the N-glycopeptides that were associated with diabetic cognitive impairment using multivariate analysis. We found that N-glycopeptides with sialylated tri- or tetra-antennary glycans on alpha-2-macroglobulin, clusterin, serum paraoxonase/arylesterase 1, and haptoglobin were less abundant, whereas 3-sialylated tri-antennary N-glycopeptides on serotransferrin were more abundant.

CONCLUSION

N-glycopeptides with sialylated multi-antennary glycans comprise a characteristic signature associated with diabetic cognitive impairment.

GENERAL SIGNIFICANCE

The characterized N-glycopeptides represent potential biomarker candidates for diabetic cognitive impairment.

In silico structural analysis of secretory clusterin to assess pathogenicity of mutations identified in the evolutionarily conserved regions

Clusterin (CLU) is a secreted glycoprotein, heterodimeric in nature, and is expressed in a wide variety of tissues and body fluids such as serum and plasma. CLU has also been known to be a promising biomarker for cell death, malignancy, cancer progression, and resistance development. However, the lack of a CLU crystal structure obstructs understanding the possible role of reported mutations on the structure, and the subsequent effects on downstream signaling pathways and cancer progression. Considering the importance of crystal structure, a model structure of the pre-secretory isoform of CLU was built to predict the effect of mutations at the molecular level. Ab initio model was built using RaptorX, and loop refinement and energy minimization were carried out with ModLoop, ModRefiner, and GalaxyWeb servers. The cancer associated mutational spectra of CLU was retrieved from the cBioPortal server and 117 unique missense mutations were identified. Evolutionarily conserved regions and pathogenicity of mutations identified in CLU were analyzed using ConSurf and Rhapsody, respectively. Furthermore, sequence and structure-based mutational analysis were carried out with iSTABLE, DynaMut and PremPS servers. Molecular dynamics simulations were carried out with GROMACS for 50 ns to determine the stability of the wild type and mutant protein structures. A dynamically stable model structure of pre-secretory CLU (psCLU) which has high concurrence with the sequence based secondary structure predictions has been explored. Changes in the intra-atomic interactions and folding pattern between wild type and mutant structures were observed. To our conclusion, eleven mutations with the highest structural and functional significance have been predicted to have pathogenic and deleterious effects.Communicated by Ramaswamy H. Sarma.

Recombinant Human Clusterin Seals Damage to the Ocular Surface Barrier in a Mouse Model of Ophthalmic Preservative-Induced Epitheliopathy

There is a significant unmet need for therapeutics to treat ocular surface barrier damage, also called epitheliopathy, due to dry eye and related diseases. We recently reported that the natural tear glycoprotein CLU (clusterin), a molecular chaperone and matrix metalloproteinase inhibitor, seals and heals epitheliopathy in mice subjected to desiccating stress in a model of aqueous-deficient/evaporative dry eye. Here we investigated CLU sealing using a second model with features of ophthalmic preservative-induced dry eye. The ocular surface was stressed by topical application of the ophthalmic preservative benzalkonium chloride (BAC). Then eyes were treated with CLU and sealing was evaluated immediately by quantification of clinical dye uptake. A commercial recombinant form of human CLU (rhCLU), as well as an rhCLU form produced in our laboratory, designed to be compatible with U.S. Food and Drug Administration guidelines on current Good Manufacturing Practices (cGMP), were as effective as natural plasma-derived human CLU (pCLU) in sealing the damaged ocular surface barrier. In contrast, two other proteins found in tears: TIMP1 and LCN1 (tear lipocalin), exhibited no sealing activity. The efficacy and selectivity of rhCLU for sealing of the damaged ocular surface epithelial barrier suggests that it could be of therapeutic value in treating BAC-induced epitheliopathy and related diseases.

Plasma proteome profiling of healthy individuals across the life span in a Sicilian cohort with long-lived individuals

The study of healthy human aging is important for shedding light on the molecular mechanisms behind aging to promote well-being and to possibly predict and/or avoid the development of age-related disorders such as atherosclerosis and diabetes. Herein, we have employed an untargeted mass spectrometry-based approach to study age-related protein changes in a healthy Sicilian plasma cohort including long-lived individuals. This approach confirmed some of the previously known proteins correlated with age including fibulin-1, dystroglycan, and gamma-glutamyl hydrolase. Furthermore, our findings include novel proteins that correlate with age and/or with location and uric acid, which could represent a unique signature for healthy aging.

A Novel Role for Plasminogen Activator Inhibitor Type-2 as a Hypochlorite-Resistant Serine Protease Inhibitor and Holdase Chaperone

Plasminogen activator inhibitor type-2 (PAI-2), a member of the serpin family, is dramatically upregulated during pregnancy and in response to inflammation. Although PAI-2 exists in glycosylated and non-glycosylated forms in vivo, the majority of in vitro studies of PAI-2 have exclusively involved the intracellular non-glycosylated form. This study shows that exposure to inflammation-associated hypochlorite induces the oligomerisation of PAI-2 via a mechanism involving dityrosine formation. Compared to plasminogen activator inhibitor type-1 (PAI-1), both forms of PAI-2 are more resistant to hypochlorite-induced inactivation of its protease inhibitory activity. Holdase-type extracellular chaperone activity plays a putative non-canonical role for PAI-2. Our data demonstrate that glycosylated PAI-2 more efficiently inhibits the aggregation of Alzheimer’s disease and preeclampsia-associated amyloid beta peptide (Aβ), compared to non-glycosylated PAI-2 in vitro. However, hypochlorite-induced modification of non-glycosylated PAI-2 dramatically enhances its holdase activity by promoting the formation of very high-molecular-mass chaperone-active PAI-2 oligomers. Both PAI-2 forms protect against Aβ-induced cytotoxicity in the SH-SY5Y neuroblastoma cell line in vitro. In the villous placenta, PAI-2 is localised primarily to syncytiotrophoblast with wide interpersonal variation in women with preeclampsia and in gestational-age-matched controls. Although intracellular PAI-2 and Aβ staining localised to different placental cell types, some PAI-2 co-localised with Aβ in the extracellular plaque-like aggregated deposits abundant in preeclamptic placenta. Thus, PAI-2 potentially contributes to controlling aberrant fibrinolysis and the accumulation of misfolded proteins in states characterised by oxidative and proteostasis stress, such as in Alzheimer’s disease and preeclampsia.

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.

Identification of Clusterin as a Major ABri- and ADan-Binding Protein Using Affinity Chromatography

Affinity chromatography has, for many years, been at the research forefront as one of the simplest although highly versatile techniques capable of identifying biologically relevant protein-protein interactions. In the field of amyloid disorders, the use of ligands immobilized to a variety of affinity matrices was the method of choice to individualize proteins with affinity for soluble circulating forms of amyloid subunits. The methodology has also played an important role in the identification of proteins that interact with different amyloidogenic peptides and, as a result, are capable of modulating their physiological and pathological functions by altering solubility, aggregation propensity, and fibril formation proclivity. Along this line, classical studies conducted in the field of Alzheimer's disease (AD) identified clusterin as a major binding protein to both circulating soluble Aβ as well as to the brain deposited counterpart. The affinity chromatography-based approach employed herein, individualized clusterin as the major protein capable of binding the amyloid subunits associated with familial British and Danish dementias, two non-Aβ neurodegenerative conditions also exhibiting cerebral amyloid deposition and sharing striking similarities to AD. The data demonstrate that clusterin binding ability to amyloid molecules is not restricted to Aβ, suggesting a modulating effect on the aggregation/fibrillization propensity of the amyloidogenic peptides that is consistent with its known chaperone activity.

Neuroserpin and transthyretin are extracellular chaperones that preferentially inhibit amyloid formation

Neuroserpin is a secreted protease inhibitor known to inhibit amyloid formation by the Alzheimer’s beta peptide (Aβ). To test whether this effect was constrained to Aβ, we used a range of in vitro assays to demonstrate that neuroserpin inhibits amyloid formation by several different proteins and protects against the associated cytotoxicity but, unlike other known chaperones, has a poor ability to inhibit amorphous protein aggregation. Collectively, these results suggest that neuroserpin has an unusual chaperone selectivity for intermediates on the amyloid-forming pathway. Bioinformatics analyses identified a highly conserved 14-residue region containing an α helix shared between neuroserpin and the thyroxine-transport protein transthyretin, and we subsequently demonstrated that transthyretin also preferentially inhibits amyloid formation. Last, we used rationally designed neuroserpin mutants to demonstrate a direct involvement of the conserved 14-mer region in its chaperone activity. Identification of this conserved region may prove useful in the future design of anti-amyloid reagents.

The interactions of molecular chaperones with client proteins: why are they so weak?

The major classes of molecular chaperones have highly variable sequences, sizes, and shapes, yet they all bind to unfolded proteins, limit their aggregation, and assist in their folding. Despite the central importance of this process to protein homeostasis, it has not been clear exactly how chaperones guide this process or whether the diverse families of chaperones use similar mechanisms. For the first time, recent advances in NMR spectroscopy have enabled detailed studies of how unfolded, "client" proteins interact with both ATP-dependent and ATP-independent classes of chaperones. Here, we review examples from four distinct chaperones, Spy, Trigger Factor, DnaK, and HscA-HscB, highlighting the similarities and differences between their mechanisms. One striking similarity is that the chaperones all bind weakly to their clients, such that the chaperone-client interactions are readily outcompeted by stronger, intra- and intermolecular contacts in the folded state. Thus, the relatively weak affinity of these interactions seems to provide directionality to the folding process. However, there are also key differences, especially in the details of how the chaperones release clients and how ATP cycling impacts that process. For example, Spy releases clients in a largely folded state, while clients seem to be unfolded upon release from Trigger Factor or DnaK. Together, these studies are beginning to uncover the similarities and differences in how chaperones use weak interactions to guide protein folding.

Association of clusterin with the BRI2-derived amyloid molecules ABri and ADan

Familial British and Danish dementias (FBD and FDD) share striking neuropathological similarities with Alzheimer's disease (AD), including intraneuronal neurofibrillary tangles as well as parenchymal and vascular amyloid deposits. Multiple amyloid associated proteins with still controversial role in amyloidogenesis colocalize with the structurally different amyloid peptides ABri in FBD, ADan in FDD, and Aβ in AD. Genetic variants and plasma levels of one of these associated proteins, clusterin, have been identified as risk factors for AD. Clusterin is known to bind soluble Aβ in biological fluids, facilitate its brain clearance, and prevent its aggregation. The current work identifies clusterin as the major ABri- and ADan-binding protein and provides insight into the biochemical mechanisms leading to the association of clusterin with ABri and ADan deposits. Mirroring findings in AD, the studies corroborate clusterin co-localization with cerebral parenchymal and vascular amyloid deposits in both disorders. Ligand affinity chromatography with downstream Western blot and amino acid sequence analyses unequivocally identified clusterin as the major ABri- and ADan-binding plasma protein. ELISA highlighted a specific saturable binding of clusterin to ABri and ADan with low nanomolar Kd values within the same range as those previously demonstrated for the clusterin-Aβ interaction. Consistent with its chaperone activity, thioflavin T binding assays clearly showed a modulatory effect of clusterin on ABri and ADan aggregation/fibrillization properties. Our findings, together with the known multifunctional activity of clusterin and its modulatory activity on the complex cellular pathways leading to oxidative stress, mitochondrial dysfunction, and the induction of cell death mechanisms - all known pathogenic features of these protein folding disorders - suggests the likelihood of a more complex role and a translational potential for the apolipoprotein in the amelioration/prevention of these pathogenic mechanisms.

DIPPER, a spatiotemporal proteomics atlas of human intervertebral discs for exploring ageing and degeneration dynamics

The spatiotemporal proteome of the intervertebral disc (IVD) underpins its integrity and function. We present DIPPER, a deep and comprehensive IVD proteomic resource comprising 94 genome-wide profiles from 17 individuals. To begin with, protein modules defining key directional trends spanning the lateral and anteroposterior axes were derived from high-resolution spatial proteomes of intact young cadaveric lumbar IVDs. They revealed novel region-specific profiles of regulatory activities and displayed potential paths of deconstruction in the level- and location-matched aged cadaveric discs. Machine learning methods predicted a ‘hydration matrisome’ that connects extracellular matrix with MRI intensity. Importantly, the static proteome used as point-references can be integrated with dynamic proteome (SILAC/degradome) and transcriptome data from multiple clinical samples, enhancing robustness and clinical relevance. The data, findings, and methodology, available on a web interface (http://www.sbms.hku.hk/dclab/DIPPER/), will be valuable references in the field of IVD biology and proteomic analytics.

The backbone of vertebrate animals consists of a series of bones called vertebrae that are joined together by disc-like structures that allow the back to move and distribute forces to protect it during daily activities. It is common for these intervertebral discs to degenerate with age, resulting in back pain and severely reducing quality of life.

The mechanical features of intervertebral discs are the result of their proteins. These include extracellular matrix proteins, which form the external scaffolding that binds cells together in a tissue, and signaling proteins, which allow cells to communicate. However, how the levels of different proteins in each region of the disc vary with time has not been fully examined.

To establish how protein composition changes with age, Tam, Chen et al. quantified the protein levels and gene activity (which leads to protein production) of intervertebral discs from young and old deceased individuals. They found that the position of different mixtures of proteins in the intervertebral disc changes with age, and that young people have high levels of extracellular matrix proteins and signaling proteins. Levels of these proteins decreased as people got older, as did the amount of proteins produced.

To determine which region of the intervertebral disc different proteins were in, Tam, Chen et al. also performed magnetic resonance imaging (MRI) of the samples to correlate image intensity (which represents water content) with the corresponding protein signature. The data obtained provides a high-quality map of how the location of different proteins changes with age, and is available online under the name DIPPER. This database is an informative resource for research into skeletal biology, and it will likely advance the understanding of intervertebral disc degeneration in humans and animals, potentially leading to the development of new treatment strategies for this condition.

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.

Therapeutic Potential of the Molecular Chaperone and Matrix Metalloproteinase Inhibitor Clusterin for Dry Eye

Evidence is presented herein supporting the potential of the natural homeostatic glycoprotein CLU (clusterin) as a novel therapeutic for the treatment of dry eye. This idea began with the demonstration that matrix metalloproteinase MMP9 is required for damage to the ocular surface in mouse dry eye. Damage was characterized by degradation of OCLN (occludin), a known substrate of MMP9 and a key component of the paracellular barrier. Following up on this finding, a yeast two-hybrid screen was conducted using MMP9 as the bait to identify other proteins involved. CLU emerged as a strong interacting protein that inhibits the enzymatic activity of MMP9. Previously characterized as a molecular chaperone, CLU is expressed prominently by epithelia at fluid-tissue interfaces and secreted into bodily fluids, where it protects cells and tissues against damaging stress. It was demonstrated that CLU also protects the ocular surface in mouse dry eye when applied topically to replace the natural protein depleted from the dysfunctional tears. CLU is similarly depleted from tears in human dry eye. The most novel and interesting finding was that CLU binds selectively to the damaged ocular surface. In this position, CLU protects against epithelial cell death and barrier proteolysis, and dampens the autoimmune response, while the apical epithelial cell layer is renewed. When present at high enough concentration, CLU also blocks staining by vital dyes used clinically to diagnose dry eye. None of the current therapeutics have this combination of properties to "protect, seal, and heal". Future work will be directed towards human clinical trials to investigate the therapeutic promise of CLU.

Glycosylation of Serum Clusterin in Wild-Type Transthyretin-Associated (ATTRwt) Amyloidosis: A Study of Disease-Associated Compositional Features Using Mass Spectrometry Analyses

Wild-type transthyretin-associated (ATTRwt) amyloidosis is an age-related disease that causes heart failure in older adults. This disease frequently features cardiac amyloid fibril deposits that originate from dissociation of the tetrameric protein, transthyretin (TTR). Unlike hereditary TTR (ATTRm) amyloidosis, where amino acid replacements destabilize the native protein, in ATTRwt amyloidosis, amyloid-forming TTR lacks protein sequence alterations. The initiating cause of fibril formation in ATTRwt amyloidosis is unclear, and thus, it seems plausible that other factors are involved in TTR misfolding and unregulated accumulation of wild-type TTR fibrils. We believe that clusterin (CLU, UniProtKB P10909), a plasma circulating glycoprotein, plays a role in the pathobiology of ATTRwt amyloidosis. Previously, we have suggested a role for CLU in ATTRwt amyloidosis based on our studies showing that (1) CLU codeposits with non-native TTR in amyloid fibrils from ATTRwt cardiac tissue, (2) CLU interacts only with non-native (monomeric and aggregated) forms of TTR, and (3) CLU serum levels in patients with ATTRwt are significantly lower compared to healthy controls. In the present study, we provide comprehensive detail of compositional findings from mass spectrometry analyses of amino acid and glycan content of CLU purified from ATTRwt and control sera. The characterization of oligosaccharide content in serum CLU derived from patients with ATTRwt amyloidosis is novel data. Moreover, results comparing CLU oligosaccharide variations between patient and healthy controls are original and provide further evidence for the role of CLU in ATTRwt pathobiology, possibly linked to disease-specific structural features that limit the chaperoning capacity of CLU.

Novel nucleolar localization of clusterin and its associated functions in human oral cancers: An in vitro and in silico analysis

Clusterin (CLU), a multifunctional chaperonic glycoprotein associated with diverse cellular functions has been shown to act as an oncogene or tumour suppressor gene in different cancers, implying a dual role in tumorigenesis. Here, we investigated the expression of CLU isoforms, their subcellular localization and functional significance in oral cancer cells. Significant downregulation of secretory CLU (sCLU) transcripts was observed in oral cancer cell lines and tumours versus normal cells while the nuclear CLU (nCLU) transcripts were undetectable. We demonstrated for the first time the nucleolar localization of sCLU, its response to different nucleolar stresses and association with cajal bodies post nucleolar stress. Functionally, knockdown of CLU revealed its negative association with ribosome biogenesis implying a possible tumour suppressor like role in oral cancers. Further, loss of sCLU in these cells also resulted in altered nuclear morphology and shrunken tubulin filaments. In addition, the levels of nucleolar Nucleophosmin 1(NPM1) and Fibrillarin, known to regulate nuclear morphology were downregulated indicating a possible role of sCLU in their stabilization. Further, an in silico docking approach to gain insights into the interaction of sCLU with nucleolar proteins NPM1, Fibrillarin, UBF and Nucleolin, revealed the involvement of a conserved region comprising of amino acid residues 140-155 of sCLU β-chain, specifically via the Phe152 residue in hydrophobic interactions with these client nucleolar proteins indicating a possible stabilizing or regulatory role of sCLU. SIGNIFICANCE OF THE STUDY: This is the first study to demonstrate the nucleolar localization of sCLU and its associated functions in oral cancer cells. Downregulation of sCLU in oral cancer tissues and cell lines, and its negative association with ribogenesis suggest its tumour suppressor like role in oral cancers. The possible role of sCLU in stabilization or regulation of different nucleolar proteins thereby impacting their functions is also implicated.

Heparan sulfate is a clearance receptor for aberrant extracellular proteins

The accumulation of aberrant proteins leads to various neurodegenerative disorders. Mammalian cells contain several intracellular protein degradation systems, including autophagy and proteasomal systems, that selectively remove aberrant intracellular proteins. Although mammals contain not only intracellular but also extracellular proteins, the mechanism underlying the quality control of aberrant extracellular proteins is poorly understood. Here, using a novel quantitative fluorescence assay and genome-wide CRISPR screening, we identified the receptor-mediated degradation pathway by which misfolded extracellular proteins are selectively captured by the extracellular chaperone Clusterin and undergo endocytosis via the cell surface heparan sulfate (HS) receptor. Biochemical analyses revealed that positively charged residues on Clusterin electrostatically interact with negatively charged HS. Furthermore, the Clusterin-HS pathway facilitates the degradation of amyloid β peptide and diverse leaked cytosolic proteins in extracellular space. Our results identify a novel protein quality control system for preserving extracellular proteostasis and highlight its role in preventing diseases associated with aberrant extracellular proteins.

Heparan sulfate and clusterin: Cleaning squad for extracellular protein degradation

The mechanisms of quality control for extracellular proteins are still poorly understood. In this issue, Itakura et al. (2020. J. Cell. Biol.https://doi.org/10.1083/jcb.201911126) show that upon binding to misfolded proteins, the extracellular chaperone clusterin is internalized via the heparan sulfate receptor to undergo lysosomal degradation.

Galectins as Regulators of Corneal Inflammation

The cornea is a transparent avascular tissue on the anterior segment of the eye responsible for providing refractive power and forming a protective barrier against the external environment. Infectious and inflammatory conditions can compromise the structure of the cornea, leading to visual impairment and blindness. Galectins are a group of β-galactoside-binding proteins expressed by immune and non-immune cells that play pivotal roles in innate and adaptive immunity. In this brief review, we discuss how different members of this family of proteins affect both pro-inflammatory and anti-inflammatory responses in the cornea, particularly in the context of infection, transplantation and wound healing. We further describe recent research showing beneficial effects of galectin-targeted therapy in corneal diseases.

Single-Molecule Characterization of the Interactions between Extracellular Chaperones and Toxic α-Synuclein Oligomers

The aberrant aggregation of α-synuclein is associated with several human diseases, collectively termed the α-synucleinopathies, which includes Parkinson’s disease. The progression of these diseases is, in part, mediated by extracellular α-synuclein oligomers that may exert effects through several mechanisms, including prion-like transfer, direct cytotoxicity, and pro-inflammatory actions. In this study, we show that two abundant extracellular chaperones, clusterin and α₂-macroglobulin, directly bind to exposed hydrophobic regions on the surface of α-synuclein oligomers. Using single-molecule fluorescence techniques, we found that clusterin, unlike α₂-macroglobulin, exhibits differential binding to α-synuclein oligomers that may be related to structural differences between two previously described forms of αS oligomers. The binding of both chaperones reduces the ability of the oligomers to permeabilize lipid membranes and prevents an oligomer-induced increase in ROS production in cultured neuronal cells. Taken together, these data suggest a neuroprotective role for extracellular chaperones in suppressing the toxicity associated with α-synuclein oligomers.

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Two extracellular chaperones directly bind to α-synuclein oligomers

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The binding is mediated by hydrophobicity on the oligomer surface

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Bound chaperones significantly attenuate the toxicity of α-synuclein oligomers

Effects of in vivo conditions on amyloid aggregation

One of the grand challenges of biophysical chemistry is to understand the principles that govern protein misfolding and aggregation, which is a highly complex process that is sensitive to initial conditions, operates on a huge range of length- and timescales, and has products that range from protein dimers to macroscopic amyloid fibrils. Aberrant aggregation is associated with more than 25 diseases, which include Alzheimer's, Parkinson's, Huntington's, and type II diabetes. Amyloid aggregation has been extensively studied in the test tube, therefore under conditions that are far from physiological relevance. Hence, there is dire need to extend these investigations to in vivo conditions where amyloid formation is affected by a myriad of biochemical interactions. As a hallmark of neurodegenerative diseases, these interactions need to be understood in detail to develop novel therapeutic interventions, as millions of people globally suffer from neurodegenerative disorders and type II diabetes. The aim of this review is to document the progress in the research on amyloid formation from a physicochemical perspective with a special focus on the physiological factors influencing the aggregation of the amyloid-β peptide, the islet amyloid polypeptide, α-synuclein, and the hungingtin protein.

Development of AD-Like Pathology in Skeletal Muscle

Effective therapeutic strategy against Alzheimer's disease (AD) requires early detection of AD; however, clinical diagnosis of Alzheimer's disease (AD) is not precise and a definitive diagnosis of AD is only possible via postmortem examination for AD pathological hallmarks including senile plaques composed of Aβ and neuro fibrillary tangles composed of phosphorylated tau. Although a variety of biomarker has been developed and used in clinical setting, none of them robustly predicts subsequent clinical course of AD. Thus, it is essential to identify new biomarkers that may facilitate the diagnosis of early stages of AD, prediction of subsequent clinical course, and development of new therapeutic strategies. Given that pathological hallmarks of AD including Aβaccumulation and the presence of phosphorylated tau are also detected in peripheral tissues, AD is considered a systemic disease. Without the protection of blood-brain barrier, systemic factors can affect peripheral tissues much earlier than neurons in brain. Here, we will discuss the development of AD-like pathology in skeletal muscle and the potential use of skeletal muscle biopsy (examination for Aβaccumulation and phosphorylated tau) as a biomarker for AD.

Targeting Alzheimer's disease with gene and cell therapies

Alzheimer's disease (AD) causes dementia in both young and old people affecting more than 40 million people worldwide. The two neuropathological hallmarks of the disease, amyloid beta (Aβ) plaques and neurofibrillary tangles consisting of protein tau are considered the major contributors to the disease. However, a more complete picture reveals significant neurodegeneration and decreased cell survival, neuroinflammation, changes in protein and energy homeostasis and alterations in lipid and cholesterol metabolism. In addition, gene and cell therapies for severe neurodegenerative disorders have recently improved technically in terms of safety and efficiency and have translated to the clinic showing encouraging results. Here, we review broadly current data within the field for potential targets that could modify AD through gene and cell therapy strategies. We envision that not only Aβ will be targeted in a disease-modifying treatment strategy but rather that a combination of treatments, possibly at different intervention times may prove beneficial in curing this devastating disease. These include decreased tau pathology, neuronal growth factors to support neurons and modulation of neuroinflammation for an appropriate immune response. Furthermore, cell based therapies may represent potential strategies in the future.

A Role for Clusterin in Exfoliation Syndrome and Exfoliation Glaucoma?

The multifunctional protein clusterin (CLU) is a secreted glycoprotein ubiquitously expressed throughout the body, including in the eye. Its primary function is to act as an extracellular molecular chaperone, preventing the precipitation and aggregation of misfolded extracellular proteins. Clusterin is commonly identified at fluid-tissue interfaces, and has been identified in most body fluids. It is a component of exfoliation material, and CLU mRNA is reduced in eyes with exfoliation syndrome compared with controls. SNPs located in the CLU genomic region have been associated with Alzheimer disease (AD) at the genome-wide level and several CLU SNPs located in an apparent regulatory region have been nominally associated with XFS/XFG in Caucasians with European ancestry and in south Indians. Interestingly, clusterin associates with altered elastic fibers in human photoaged skin and prevents UV-induced elastin aggregation in vitro. In light of the known geographic risk factors for XFS/XFG, which could include UV light, investigations of CLU-geographic interactions could be of interest. Future studies investigating rare CLU variation and other complex interactions including gene-gene interactions in XFS/XFG cases and controls may also be fruitful. Although CLU has been considered as a therapeutic target in AD, cancer and dry eye, a role for clusterin in XFS/XFG needs to be better defined before therapeutic approaches involving CLU can be entertained.

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.

Amorphous protein aggregates stimulate plasminogen activation, leading to release of cytotoxic fragments that are clients for extracellular chaperones

The misfolding of proteins and their accumulation in extracellular tissue compartments as insoluble amyloid or amorphous protein aggregates are a hallmark feature of many debilitating protein deposition diseases such as Alzheimer's disease, prion diseases, and type II diabetes. The plasminogen activation system is best known as an extracellular fibrinolytic system but was previously reported to also be capable of degrading amyloid fibrils. Here we show that amorphous protein aggregates interact with tissue-type plasminogen activator and plasminogen, via an exposed lysine-dependent mechanism, to efficiently generate plasmin. The insoluble aggregate-bound plasmin is shielded from inhibition by α₂-antiplasmin and degrades amorphous protein aggregates to release smaller, soluble but relatively hydrophobic fragments of protein (plasmin-generated protein fragments (PGPFs)) that are cytotoxic. In vitro, both endothelial and microglial cells bound and internalized PGPFs before trafficking them to lysosomes. Clusterin and α₂-macroglobulin bound to PGPFs to significantly ameliorate their toxicity. On the basis of these findings, we hypothesize that, as part of the in vivo extracellular proteostasis system, the plasminogen activation system may work synergistically with extracellular chaperones to safely clear large and otherwise pathological protein aggregates from the body.

Clusterin: full-length protein and one of its chains show opposing effects on cellular lipid accumulation

Proteins, made up of either single or multiple chains, are designed to carry out specific biological functions. We found an interesting example of a two-chain protein where administration of one of its chains leads to a diametrically opposite outcome than that reported for the full-length protein. Clusterin is a highly glycosylated protein consisting of two chains, α- and β-clusterin. We have investigated the conformational features, cellular localization, lipid accumulation, in vivo effects and histological changes upon administration of recombinant individual chains of clusterin. We demonstrate that recombinant α- and β-chains exhibit structural and functional differences and differ in their sub-cellular localization. Full-length clusterin is known to lower lipid levels. In contrast, we find that β-chain-treated cells accumulate 2-fold more lipid than controls. Interestingly, α-chain-treated cells do not show such increase. Rabbits injected with β-chain, but not α-chain, show ~40% increase in weight, with adipocyte hypertrophy, liver and kidney steatosis. Many, sometimes contrasting, roles are ascribed to clusterin in obesity, metabolic syndrome and related conditions. Our findings of differential localization and activities of individual chains of clusterin should help in understanding better the roles of clusterin in metabolism.

Effect of molecular chaperones on aberrant protein oligomers in vitro: super-versus sub-stoichiometric chaperone concentrations

Living systems protect themselves from aberrant proteins by a network of chaperones. We have tested in vitro the effects of different concentrations, ranging from 0 to 16 μm, of two molecular chaperones, namely αB-crystallin and clusterin, and an engineered monomeric variant of transthyretin (M-TTR), on the morphology and cytotoxicity of preformed toxic oligomers of HypF-N, which represent a useful model of misfolded protein aggregates. Using atomic force microscopy imaging and static light scattering analysis, all were found to bind HypF-N oligomers and increase the size of the aggregates, to an extent that correlates with chaperone concentration. SDS-PAGE profiles have shown that the large aggregates were predominantly composed of the HypF-N protein. ANS fluorescence measurements show that the chaperone-induced clustering of HypF-N oligomers does not change the overall solvent exposure of hydrophobic residues on the surface of the oligomers. αB-crystallin, clusterin and M-TTR can diminish the cytotoxic effects of the HypF-N oligomers at all chaperone concentration, as demonstrated by MTT reduction and Ca2+ influx measurements. The observation that the protective effect is primarily at all concentrations of chaperones, both when the increase in HypF-N aggregate size is minimal and large, emphasizes the efficiency and versatility of these protein molecules.

Differential Effects of apoE4 and Activation of ABCA1 on Brain and Plasma Lipoproteins

Apolipoprotein E4 (apoE4), the leading genetic risk factor for Alzheimer's disease (AD), is less lipidated compared to the most common and AD-benign allele, apoE3. We have recently shown that i.p. injections of the ATP-binding cassette A1 (ABCA1) agonist peptide CS-6253 to apoE mice reverse the hypolipidation of apoE4 and the associated brain pathology and behavioral deficits. While in the brain apoE is the main cholesterol transporter, in the periphery apoE and apoA-I both serve as the major cholesterol transporters. We presently investigated the extent to which apoE genotype and CS-6253 treatment to apoE3 and apoE4-targeted replacement mice affects the plasma levels and lipid particle distribution of apoE, and those of plasma and brain apoA-I and apoJ. This revealed that plasma levels of apoE4 were lower and eluted faster following FPLC than plasma apoE3. Treatment with CS-6253 increased the levels of plasma apoE4 and rendered the elution profile of apoE4 similar to that of apoE3. Similarly, the levels of plasma apoA-I were lower in the apoE4 mice compared to apoE3 mice, and this effect was partially reversed by CS-6253. Conversely, the levels of apoA-I in the brain which were higher in the apoE4 mice, were unaffected by CS-6253. The plasma levels of apoJ were higher in apoE4 mice than apoE3 mice and this effect was abolished by CS-6253. Similar but less pronounced effects were obtained in the brain. In conclusion, these results suggest that apoE4 affects the levels of apoA-I and apoJ and that the anti-apoE4 beneficial effects of CS-6253 may be related to both central and peripheral mechanisms.

Kidney Disease and the Nexus of Chronic Kidney Disease and Acute Kidney Injury: The Role of Novel Biomarkers as Early and Accurate Diagnostics

Chronic kidney disease (CKD) and acute kidney injury (AKI) are interconnected and the presence of one is a risk for the other. CKD is an important predictor of AKI after exposure to nephrotoxic drugs or major surgery, whereas persistent or repetitive injury could result in the progression of CKD. This brings new perspectives to the diagnosis and monitoring of kidney diseases highlighting the need for a panel of kidney-specific biomarkers that reflect functional as well as structural damage and recovery, predict potential risk and provide prognosis. This article discusses the kidney-specific biomarkers, symmetric dimethylarginine (SDMA), clusterin, cystatin B, and inosine.

Central Nervous System Lipoproteins: ApoE and Regulation of Cholesterol Metabolism

ApoE on high-density lipoproteins is primarily responsible for lipid transport and cholesterol homeostasis in the central nervous system (CNS). Normally produced mostly by astrocytes, apoE is also produced under neuropathologic conditions by neurons. ApoE on high-density lipoproteins is critical in redistributing cholesterol and phospholipids for membrane repair and remodeling. The 3 main structural isoforms differ in their effectiveness. Unlike apoE2 and apoE3, apoE4 has markedly altered CNS metabolism, is associated with Alzheimer disease and other neurodegenerative disorders, and is expressed at lower levels in brain and cerebrospinal fluid. ApoE4-expressing cultured astrocytes and neurons have reduced cholesterol and phospholipid secretion, decreased lipid-binding capacity, and increased intracellular degradation. Two structural features are responsible for apoE4 dysfunction: domain interaction, in which arginine-61 interacts ionically with glutamic acid-255, and a less stable conformation than apoE3 and apoE2. Blocking domain interaction by gene targeting (replacing arginine-61 with threonine) or by small-molecule structure correctors increases CNS apoE4 levels and lipid-binding capacity and decreases intracellular degradation. Small molecules (drugs) that disrupt domain interaction, so-called structure correctors, could prevent the apoE4-associated neuropathology by blocking the formation of neurotoxic fragments. Understanding how to modulate CNS cholesterol transport and metabolism is providing important insights into CNS health and disease.

Clusterin facilitates apoptotic cell clearance and prevents apoptotic cell-induced autoimmune responses

Clusterin (Clu), an extracellular chaperone, exhibits characteristics of soluble innate immunity receptors, as assessed by its ability to bind some bacteria strains. In this study, we report that Clu also binds specifically to late apoptotic cells but not to live, early apoptotic, or necrotic cells. Histones, which accumulate on blebs during the apoptotic process, represent privileged Clu-binding motifs at the surface of late apoptotic cells. As a consequence, Clu potentiates, both in vitro and in vivo, the phagocytosis of late apoptotic cells by macrophages. Moreover, the increased phagocytosis of late apoptotic cells induced by Clu favors the presentation and cross-presentation of apoptotic cell-associated antigens. Finally, we observed that, in a model of apoptotic cell-induced autoimmunity, and relative to control mice, Clu^−/− mice develop symptoms of autoimmunity, including the generation of anti-dsDNA antibodies, deposition of immunoglobulins and complement components within kidneys, and splenomegaly. These results identify Clu as a new molecule partner involved in apoptotic cell efferocytosis and suggest a protective role for Clu in inflammation and autoimmune diseases.

Clusterin Binds to Aβ1-42 Oligomers with High Affinity and Interferes with Peptide Aggregation by Inhibiting Primary and Secondary Nucleation

The aggregation of amyloid β protein (Aβ) is a fundamental pathogenic mechanism leading to the neuronal damage present in Alzheimer disease, and soluble Aβ oligomers are thought to be a major toxic culprit. Thus, better knowledge and specific targeting of the pathways that lead to these noxious species may result in valuable therapeutic strategies. We characterized some effects of the molecular chaperone clusterin, providing new and more detailed evidence of its potential neuroprotective effects. Using a classical thioflavin T assay, we observed a dose-dependent inhibition of the aggregation process. The global analysis of time courses under different conditions demonstrated that clusterin has no effect on the elongation rate but mainly interferes with the nucleation processes (both primary and secondary), reducing the number of nuclei available for further fibril growth. Then, using a recently developed immunoassay based on surface plasmon resonance, we obtained direct evidence of a high-affinity (KD= 1 nm) interaction of clusterin with biologically relevant Aβ1-42oligomers, selectively captured on the sensor chip. Moreover, with the same technology, we observed that substoichiometric concentrations of clusterin prevent oligomer interaction with the antibody 4G8, suggesting that the chaperone shields hydrophobic residues exposed on the oligomeric assemblies. Finally, we found that preincubation with clusterin antagonizes the toxic effects of Aβ1-42oligomers, as evaluated in a recently developedin vivomodel inCaenorhabditis elegans.These data substantiate the interaction of clusterin with biologically active regions exposed on nuclei/oligomers of Aβ1-42, providing a molecular basis for the neuroprotective effects of the chaperone.

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.

Inhibition of intracellular clusterin attenuates cell death in nephropathic cystinosis

Nephropathic cystinosis, characterized by accumulation of cystine in the lysosomes, is caused by mutations in CTNS. The molecular and cellular mechanisms underlying proximal tubular dysfunction and progressive renal failure in nephropathic cystinosis are largely unclear, and increasing evidence supports the notion that cystine accumulation alone is not responsible for the end organ injury in cystinosis. We previously identified clusterin as potentially involved in nephropathic cystinosis. Here, we studied the expression of clusterin in renal proximal tubular epithelial cells obtained from patients with nephropathic cystinosis. The cytoprotective secretory form of clusterin, as evaluated by Western blot analysis, was low or absent in cystinosis cells compared with normal primary cells. Confocal microscopy revealed elevated levels of intracellular clusterin in cystinosis cells. Clusterin in cystinosis cells localized to the nucleus and cytoplasm and showed a filamentous and punctate aggresome-like pattern compared with diffuse cytoplasmic staining in normal cells. In kidney biopsy samples from patients with nephropathic cystinosis, clusterin protein expression was mainly limited to the proximal tubular cells. Furthermore, expression of clusterin overlapped with the expression of apoptotic proteins (apoptosis-inducing factor and cleaved caspase-3) and autophagy proteins (LC3 II and p62). Silencing of the clusterin gene resulted in a significant increase in cell viability and attenuation of apoptosis in cystinosis cells. Results of this study identify clusterin as a pivotal factor in the cell injury mechanism of nephropathic cystinosis and provide evidence linking cellular stress and injury to Fanconi syndrome and progressive renal injury in nephropathic cystinosis.

The chaperone protein clusterin may serve as a cerebrospinal fluid biomarker for chronic spinal cord disorders in the dog

Chronic spinal cord dysfunction occurs in dogs as a consequence of diverse aetiologies, including long-standing spinal cord compression and insidious neurodegenerative conditions. One such neurodegenerative condition is canine degenerative myelopathy (DM), which clinically is a challenge to differentiate from other chronic spinal cord conditions. Although the clinical diagnosis of DM can be strengthened by the identification of the Sod1 mutations that are observed in affected dogs, genetic analysis alone is insufficient to provide a definitive diagnosis. There is a requirement to identify biomarkers that can differentiate conditions with a similar clinical presentation, thus facilitating patient diagnostic and management strategies. A comparison of the cerebrospinal fluid (CSF) protein gel electrophoresis profile between idiopathic epilepsy (IE) and DM identified a protein band that was more prominent in DM. This band was subsequently found to contain a multifunctional protein clusterin (apolipoprotein J) that is protective against endoplasmic reticulum (ER) stress-mediated apoptosis, oxidative stress, and also serves as an extracellular chaperone influencing protein aggregation. Western blot analysis of CSF clusterin confirmed elevated levels in DM compared to IE (p < 0.05). Analysis of spinal cord tissue from DM and control material found that clusterin expression was evident in neurons and that the clusterin mRNA levels from tissue extracts were elevated in DM compared to the control. The plasma clusterin levels was comparable between these groups. However, a comparison of clusterin CSF levels in a number of neurological conditions found that clusterin was elevated in both DM and chronic intervertebral disc disease (cIVDD) but not in meningoencephalitis and IE. These findings indicate that clusterin may potentially serve as a marker for chronic spinal cord disease in the dog; however, additional markers are required to differentiate DM from a concurrent condition such as cIVDD.

Expression and purification of chaperone-active recombinant clusterin

Clusterin was the first described secreted mammalian chaperone and is implicated as being a key player in both intra- and extracellular proteostasis. Its unique combination of structural features and biological chaperone activity has, however, previously made it very challenging to express and purify the protein in a correctly processed and chaperone-active form. While there are multiple reports in the literature describing the use of recombinant clusterin, all of these reports suffer from one or more of the following shortcomings: details of the methods used to produce the protein are poorly described, the product is incompletely (if at all) characterised, and purity (if shown) is in many cases inadequate. The current report provides the first well validated method to economically produce pure chaperone-active recombinant clusterin. The method was developed after trialling expression in cultured bacterial, yeast, insect and mammalian cells, and involves the expression of recombinant clusterin from stably transfected HEK293 cells in protein-free medium. The product is expressed at between 7.5 and 10 µg/ml of culture, and is readily purified by a combination of immunoaffinity, cation exchange and size exclusion chromatography. The purified product was shown to be glycosylated, correctly proteolytically cleaved into α- and β-subunits, and have chaperone activity similar to that of human plasma clusterin. This new method creates the opportunity to use mutagenesis and metabolic labelling approaches in future studies to delineate functionally important sites within clusterin, and also provides a theoretically unlimited supply of recombinant clusterin which may in the future find applications in the development of therapeutics.

Non-enzymatic post-translational protein modifications and proteostasis network deregulation in carcinogenesis

Organisms are constantly challenged by stressors and thus the maintenance of biomolecules functionality is essential for the assurance of cellular homeostasis. Proteins carry out the vast majority of cellular functions by mostly participating in multimeric protein assemblies that operate as protein machines. Cells have evolved a complex proteome quality control network for the rescue, when possible, or the degradation of damaged polypeptides. Nevertheless, despite these proteostasis ensuring mechanisms, new protein synthesis, and the replication-mediated dilution of proteome damage in mitotic cells, the gradual accumulation of stressors during aging (or due to lifestyle) results in increasingly damaged proteome. Non-enzymatic post-translational protein modifications mostly arise by unbalanced redox homeostasis and/or high glucose levels and may cause disruption of proteostasis as they can alter protein function. This outcome may then increase genomic instability due to reduced fidelity in processes like DNA replication or repair. Herein, we present a synopsis of the major non-enzymatic post-translation protein modifications and of the proteostasis network deregulation in carcinogenesis. We propose that activation of the proteostasis ensuring mechanisms in premalignant cells has tumor-preventive effects, whereas considering that over-activation of these mechanisms represents a hallmark of advanced tumors, their inhibition provides a strategy for the development of anti-tumor therapies. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

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.

Xeroprotectants for the stabilization of biomaterials

With the advancement of science and technology, it is crucial to have effective preservation methods for the stable long-term storage of biological material (biomaterials). As an alternative to cryopreservation, various techniques have been developed, which are based on the survival mechanism of anhydrobiotic organisms. In this sense, it has been found that the synthesis of xeroprotectants can effectively stabilize biomaterials in a dry state. The most widely studied xeroprotectant is trehalose, which has excellent properties for the stabilization of certain proteins, bacteria, and biological membranes. There have also been attempts to apply trehalose to the stabilization of eukaryotic cells but without conclusive results. Consequently, a xeroprotectant or method that is useful for the stable drying of a particular biomaterial might not necessarily be suitable for another one. This article provides an overview of recent advances in the use of new techniques to stabilize biomaterials and compare xeroprotectants with other more standard methods.

Phase I/II trial of custirsen (OGX-011), an inhibitor of clusterin, in combination with a gemcitabine and platinum regimen in patients with previously untreated advanced non-small cell lung cancer

PURPOSE

Clusterin (CLU), an antiapoptotic, stress-associated protein, confers resistance to therapy when overexpressed. This trial tested custirsen (OGX-011), an inhibitor of CLU protein production, combined with gemcitabine/platinum in patients with advanced non-small cell lung cancer (NSCLC).

PATIENTS AND METHODS

This was a single-arm, multicenter, phase I/II study in chemotherapy-naive stage IIIB/IV NSCLC. Custirsen was infused during a loading dose period and weekly in combination with gemcitabine (1250 mg/m) on days 1 and 8 and with cisplatin (75 mg/m) or carboplatin (area under the curve 5) on day 1 of each 21-day cycle. Ten patients were treated in a phase I lead-in and 71 in the phase II component. The primary efficacy endpoint was response rate, with exploratory analyses of other efficacy outcomes and biomarker relationships.

RESULTS

Eighty-one patients received custirsen and were included in the primary analysis. The median age was 61 years; 82% had stage IV disease. Overall response was 25 of 81 (31%; 95% confidence interval 21-42). The 1- and 2-year survivals were 54 and 30%, respectively. Toxicity of the combination was not appreciably different from what is reported for gemcitabine/platinum combinations. Custirsen treatment decreased serum CLU levels in 95% of patients evaluated. Patients who achieved a minimum median CLU level for the population of ≤38 μg/ml during treatment had a median survival of 27.1 compared with 16.1 months for patients who did not (p = 0.02).

CONCLUSION

Based on the above results, a randomized phase 3 trial to evaluate the survival benefit of custirsen in patients with NSCLC is warranted.