Chronic ocular surface disease (OSD) is characterized by corneal epitheliopathy, reduced barrier function and loss of nerves, accompanied by persistent inflammation. Current treatments offer limited relief and there is no approved therapy that promotes neurosensory regeneration in OSD. Here, we tested the therapeutic efficacy of clusterin (CLU), a molecular chaperone and MMP9 inhibitor found in tears, in Thbs1-deficient mice, a preclinical model of autoimmune dry eye associated with Sjögren's disease (SjD). These mice were treated topically at the ocular surface, bilaterally, for 3 weeks with recombinant human CLU (rhCLU) or human plasma-derived CLU (pCLU) eyedrops and compared to standard-of-care 0.1% dexamethasone eyedrops. Treatment with CLU significantly improved corneal barrier integrity, increased corneal nerve density, enhanced the proportion of corneal nerves with immunoreactivity for CGRP and promoted conjunctival goblet cell regeneration. Furthermore, CLU reduced immunoreactivity for ADAM17 in the corneal epithelium and reduced conjunctival Tnfa and Ifng expression, supporting its anti-inflammatory effect. Notably, all these effects were comparable to, or even exceeded, those resulting from treatment with dexamethasone. Based on its efficacy, we introduce CLU as a multifunctional and promising biotherapeutic for a widespread range of ocular inflammatory conditions involving corneal epitheliopathy and nerve loss, including dry eye associated with SjD.

AIMS:The clusterin (CLU) gene is genetically associated with Alzheimer's disease (AD), and CLU levels have been shown to positively correlate with regional Aβ deposition in the brain, including in arteries from cerebral amyloid angiopathy (CAA) patients. CLU has also been shown to alter the aggregation, toxicity and blood-brain barrier transport of amyloid beta (Aβ) and has therefore been suggested to play a key role in regulating the balance between Aβ deposition and clearance in both the brain and cerebral blood vessels. However, it remains unclear whether the role of clusterin in relation to Aβ deposition is protective or pathogenic. The aim of this study was to determine how the presence of clusterin influences the pattern of Aβ deposition in hippocampal cerebral vessels. METHODS:Intrahippocampal injections of fluorescent human recombinant Aβ alone or in combination with human recombinant CLU were carried out in Clu knockout mice. Aβ deposition and aggregate size in arterioles and capillaries were assessed by confocal microscopy. RESULTS:The presence of CLU significantly reduced the size of Aβ deposits in the walls of cerebral arterioles but not in the tissue outside arterioles. There was no significant difference in overall Aβ deposition within cerebral arterioles and capillaries of mice injected with Aβ + CLU versus Aβ alone. CONCLUSIONS:Our findings confirm that CLU directly impacts cerebral vascular Aβ aggregation, the implications of which are particularly relevant to CAA, which is a major cause of cerebral haemorrhage and cognitive decline, particularly in individuals with AD.

We report the results of structural, functional and genetic studies on the CD33 sialic acid- binding receptor that reveal how non-coding variants in CD33 alter risk for Alzheimer's disease (AD). The full-length CD33 M isoform, whose expression is upregulated by non-coding AD-risk alleles, preferentially forms dimers at the cell surface, where they interact with AD-related proteins (clusterin and Aβ). This interaction induces CD33 M inhibitory signalling and downregulates protective microglial functions including phagocytic removal of amyloid plaques. Human brain expression quantitative trait loci (eQTL) and causal mediation analyses confirm that quantitative interactions between CLU and CD33 genotypes modulate AD phenotypes and suggest that genotypes at these loci might be used to personalise future therapeutic approaches. Our work also highlights several other unexpected aspects of CD33 biology, including a soluble shed extracellular fragment of CD33 M and a similar soluble secreted product arising from a truncating mutation in the CD33 extracellular domain (CD33 MΔ4bp ).

The sialic-acid binding immunoglobulin-like lectin 3 receptor (Siglec-3 / CD33) expressed on microglia, regulates immune functions relevant to Alzheimer's disease (AD). Clusterin (CLU) and apolipoprotein E (ApoE) are soluble, sialylated proteins implicated in AD pathogenesis through genetic associations and their interactions with amyloid-beta (Aβ). However, the role of these proteins as potential CD33 ligands remains unclear. This study explores whether CLU and/or ApoE bind CD33 and examines the functional impact of these interactions on Aβ uptake and amyloid plaque clearance. The binding of CD33 to CLU and ApoE was assessed through co-immunoprecipitation using U937 cells (endogenously expressing CD33) and HEK293 cells (expressing exogenous CD33). Quantitative bio-layer interferometry (BLI) and microscale thermophoresis determined binding affinities, focusing on the role of CD33's Arg119 sialic acid binding site. In situ proximity ligation assays (PLA) and co-immunoprecipitation from AD and control human brain lysates validated in vivo interactions. Functional assays examined Aβ uptake and amyloid plaque clearance in monocytes and U937 cells, with or without CLU treatment. The quantitative binding assay revealed that CLU, but not ApoE, was a sialylation-dependent ligand for CD33, binding with high affinity (Kd = 28.9 ± 10.3 nM). Binding required an intact Arg119 residue and dimeric CD33 structure. PLA and co-immunoprecipitation studies demonstrated colocalization of CD33 and CLU on microglia in AD brains, especially near amyloid plaques. Functionally, sialylated CLU inhibited Aβ uptake in monocytes from CD33 “CC” risk allele carriers and reduced amyloid plaque clearance in U937 cells. Desialylated CLU showed no significant effects. Notably, CLU + Aβ oligomers induced stronger CD33 ITIM signaling than CLU alone, enhancing phosphorylation and SHP-1 recruitment. This study identifies CLU as a specific CD33 ligand and highlights its role in modulating microglial functions via CD33 ITIM signaling. Sialylated CLU inhibits Aβ uptake and amyloid plaque clearance, suggesting a potential mechanism underlying microglial dysfunction in AD. These findings underscore the therapeutic potential of targeting the CD33-CLU axis to restore microglial homeostasis and enhance amyloid clearance in AD.

Rationale: Extracellular histones, released into the surrounding environment during extensive cell death, promote inflammation and cell death, and these deleterious roles have been well documented in sepsis. Clusterin (CLU) is a ubiquitous extracellular protein that chaperones misfolded proteins and promotes their removal. Objectives: We investigated whether CLU could protect against the deleterious properties of histones. Methods: We assessed CLU and histone expression in patients with sepsis and evaluated the protective role of CLU against histones in in vitro assays and in vivo models of experimental sepsis. Measurements and Main Results: We show that CLU binds to circulating histones and reduces their inflammatory, thrombotic, and cytotoxic properties. We observed that plasma CLU levels decreased in patients with sepsis and that the decrease was greater and more durable in nonsurvivors than in survivors. Accordingly, CLU deficiency was associated with increased mortality in mouse models of sepsis and endotoxemia. Finally, CLU supplementation improved mouse survival in a sepsis model. Conclusions: This study identifies CLU as a central endogenous histone-neutralizing molecule and suggests that, in pathologies with extensive cell death, CLU supplementation may improve disease tolerance and host survival.

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.

Protein quality control involves many processes that jointly act to regulate the expression, localization, turnover, and degradation of proteins, and has been highlighted in recent studies as critical to the differentiation of stem cells during regeneration. The roles of constitutively secreted extracellular chaperones in neuronal injury and disease are poorly understood. Extracellular chaperones are multifunctional proteins expressed by many cell types, including those of the nervous system, known to facilitate protein quality control processes. These molecules exert pleiotropic effects and have been implicated as playing important protective roles in a variety of stress conditions, including tissue damage, infections, and local tissue inflammation. This article aims to provide a critical review of what is currently known about the functions of extracellular chaperones in neuronal repair and regeneration and highlight future directions for this important research area. We review what is known of four constitutively secreted extracellular chaperones directly implicated in processes of neuronal damage and repair, including transthyretin, clusterin, α2-macroglobulin, and neuroserpin, and propose that investigation into the effects of these and other extracellular chaperones on neuronal repair and regeneration has the potential to yield valuable new therapies.

Amyloid fibril formation by the extracellular protein β2-microglobulin (β2m) and its subsequent accumulation in periarticular tissues have been linked to dialysis-related amyloidosis. A natural variant of human β2m responsible for aggressive systemic amyloidosis contains an aspartate to asparagine mutation at residue 76 (i.e. D76N β2m), which readily forms amyloid fibrils in vitro under physiological conditions. In this study, we examined the role of the extracellular molecular chaperone clusterin in modulating D76N β2m fibril formation in vitro under physiological conditions. The presence of extrinsic charged amino acids modulated D76N β2m fibril formation, implying that electrostatic interactions are involved in the protein’s aggregation. Thioflavin T (ThT) and 1-anilinonaphthalene-8-sulfonate fluorescence assays indicated that clusterin interacts via hydrophobic and electrostatic forces with the monomeric, prefibrillar and fibrillar species of D76N β2m. As a result, clusterin was incorporated into D76N β2m aggregates during the latter’s fibril formation, as indicated by SDS-PAGE of depolymerised fibrils. SYPRO Orange and ThT fluorescence assays suggested that, compared to pure D76N β2m fibrils, those formed in the presence of clusterin are chemically more stable with a reduced ability to act as nucleation seeds. Detailed 15N NMR relaxation studies of mixtures of 15N-labelled β2m with clusterin confirmed that the chaperone interacts transiently and non-specifically with monomeric β2m. Clusterin inhibits both primary and secondary nucleation of D76N β2m fibril formation. In doing so, clusterin binds to D76N β2m fibrils and stabilises them to prevent possible fragmentation. In vivo, the multifaceted chaperone action of clusterin may delay, if not prevent, β2m amyloid proliferation and deposition in tissues.

The protein homeostasis (proteostasis) system consists in a network of processes that governs the synthesis, folding, concentration, trafficking and degradation of proteins.1 The maintenance of protein homeostasis is especially important in extracellular spaces, where protein molecules are constantly exposed to oxidizing agents, large variations in pH and ionic strength, and mechanical and thermal stresses associated with fluid circulation (Figure 1). These environmental conditions can destabilize the native states (normal folded forms) of proteins, which can result in misfolding.2 In turn, protein misfolding may expose aggregation-prone regions normally buried in the native protein interior, leading to self-association into potentially cytotoxic aggregates. In Alzheimer's disease (AD), the amyloid-β (Aβ) peptide aggregates into small oligomers that damage brain cells and later form insoluble fibrillar structures that deposit into large amyloid plaques.3 Extracellular protein homeostasis is tasked with preventing disease by controlling the process of protein misfolding in extracellular fluids, safeguarding against toxic protein aggregates that may arise, and mediating their systematic clearance. This perspective article provides a therapy-focussed commentary to extend a discussion presented in a recent review of extracellular protein homeostasis in neurodegenerative diseases.4 Extracellular chaperones (ECs) are central components of the extracellular protein homeostasis system (Figure 1). ECs are secreted proteins, abundant in body fluids, that specifically interact with extracellular misfolding proteins to inhibit their aggregation, neutralise their toxicity, and facilitate their receptor-mediated endocytosis and subsequent degradation in lysosomes. By virtue of these actions, ECs are emerging as key players in processes that operate to protect the human body from a variety of disease pathologies causally associated with the excessive misfolding and aggregation of specific extracellular proteins (Table 1). The extracellular protein homeostasis network may also include proteolytic systems, which could assist in the clearance of larger extracellular protein deposits (Figure 1). Since the discovery of clusterin (CLU) as the first known mammalian EC just over 20 years ago, about 20 more ECs have been identified.6 Each of these ECs offers the potential to be developed as therapeutic agents to treat the types of human diseases shown in Table 1. In this discussion, we will focus on CLU to illustrate possible translational pathways that other ECs may follow in the future. Like many of the other known ECs, CLU is multifunctional. In addition to its potent chaperone activity and demonstrated roles in the clearance of Aβ and apoptotic cells,7-9 CLU is also an inhibitor of the terminal complement pathway10 and matrix metalloproteases.11 This broad range of activities suggests that CLU is likely to exert pleiotropic anti-inflammatory effects in vivo, which have the potential to be harnessed therapeutically. The first investigation in which this potential has been explored is dry eye disease. In a series of studies in a mouse model, the topical administration of low μg/ml concentrations of CLU in a physiological buffer to the eyes was shown to provide complete protection from dry eye pathology and to rapidly resolve previously induced symptoms.12 Human clinical trials of CLU to treat dry eye pathology are being planned. Similarly, a recent study showed that: (1) CLU-deficient mice were more susceptible to sepsis and endotoxemia, and (2) exogenously administered CLU bound to circulating histones to reduce their inflammatory, thrombotic and cytotoxic properties and improved survival in a mouse model of sepsis. This same study also reported that plasma CLU levels collapsed in human sepsis patients, and proposed that in pathologies with extensive cell death, CLU supplementation may improve disease tolerance and host survival.13 Inflammation is also strongly implicated in the pathology of Alzheimer's disease. Mutations in the CLU gene are one of the most significant risk factors for Alzheimer's disease, and multiple studies implicate CLU as protecting brain cells from toxic Aβ oligomers by neutralising their toxicity and mediating their safe clearance from the brain.7, 14 The brief outline above identifies several important disease scenarios in which natural CLU may prove to have high therapeutic value. As our understanding of the extracellular protein homeostasis system improves,4 one could expect more therapeutic opportunities to emerge. A promising direction exploits the possibility to rationally design functionally enhanced, engineered versions of CLU to provide even greater potency in specific therapeutic applications. CLU is a disulfide-bonded glycoprotein with a relatively complex heterodimeric structure. This made it a challenging molecule to express recombinantly, let alone generate engineered mutants. However, the significant technical challenges were recently overcome, so that it is now possible to express and purify relatively large quantities of wild-type and mutant CLU.15 move the following sentence to the start of the following paragraph CLU is a difficult candidate for analysis by X-ray diffraction, cryo-electron microscopy and nuclear magnetic resonance spectroscopy, and as a result there are currently no published structures available for full-length CLU. Nevertheless, the regions of CLU involved in specific interactions with misfolded proteins, histones, and cell receptors are under active investigation. Once these regions are identified, it will be possible to rationally design engineered versions of CLU with, for example, higher affinity binding to specific disease-relevant ligands (e.g. Aβ, histones). It is already possible to design in silico complementary binding sequences for chosen epitopes in any target protein.16, 17 Therefore, by introducing such sequences into a molecular chaperone it becomes possible to enhance its ability to interact with a target protein. This strategy has been illustrated for the intracellular chaperone Hsp70, where an in silico-designed binding sequence for α-synuclein (a protein implicated in the causation of Parkinson's disease) was grafted onto the Hsp70 scaffold and significantly improved the ability of Hsp70 to inhibit α-synuclein aggregation and toxicity.18 Other approaches to manipulate extracellular protein homeostasis for the purpose of disease therapy are under investigation.4 Therapeutic compounds could be used to: (1) enhance EC function, (2) upregulate EC secretion, (3) stabilise the native fold of extracellular proteins, (4) inhibit extracellular protein aggregation, (5) upregulate EC secretion, (6) enhance extracellular protease activity, or (7) promote cellular uptake for intracellular degradation.4 This list is likely to grow, as this is a new, almost completely untapped field, ripe with opportunities for bold thinkers. The translation of our growing understanding of extracellular protein homeostasis into effective new treatments for diseases associated with protein misfolding and aggregation, many of which are still intractable, is a goal with potentially huge future benefits for global health.

Abstract A new mouse model for the classical subtype of human glioblastoma has been generated using Cre-mediated EGFRvIII overexpression and homozygous p19-ARF deletion (the mouse homolog of human p14-ARF/CDKN2A) in GFAP expressing cells. Transgenic mice develop intraparchenymal and/or leptomeningeal brain lesions with some spinal cord invasion as early as 1 month old and 95% of mice die by 6 months due to hydrocephalus and/or paralysis. Mice with high grade tumors have worse survival and similar features to human classical glioblastoma such as necrosis, high levels of mitosis, and infiltration of tumor cells into normal brain. Immunohistochemical analysis confirms EGFRvIII overexpression and p19-ARF loss in tumor cells, along with patchy positive GFAP and positivity for Olig2, S100β and NeuN, suggesting that tumor cells arise from a progenitor glial cell type. Adherent and neurosphere primary culture of dissociated tumors indicate that tumor cells maintain EGFRvIII expression in culture and are able to generate xenograft tumors by 3 weeks after intracranial injections into NODSCID mice. Xenograft tumors are reminiscent of the primary tumor, with similar histopathological features and immunohistochemical staining. This novel mouse model can be used to study diffuse glioma with a leptomeningeal component.

Previous work suggests that cell stress induces release of the normally secreted chaperone clusterin (CLU) into the cytosol. We analyzed the localization of CLU in healthy and stressed cells, the mechanism of its cytosolic release, and its interactions with cytosolic misfolded proteins. Key results of this study are the following: (1) full-length CLU is released to the cytosol during stress, (2) the CLU N-terminal D1 residue is recognized by the N-end rule pathway and together with the enzyme ATE1 is essential for cytosolic release, (3) CLU can form stable complexes with cytosolic misfolded proteins and direct them to the proteasome and autophagosomes, and (4) cytosolic CLU protects cells from hypoxic stress and the cytosolic overexpression of an aggregation-prone protein. Collectively, the results suggest that enhanced cytosolic release of CLU is a stress response that can inhibit the toxicity of misfolded proteins and facilitate their targeted degradation via both autophagy and the proteasome.

Eighty percent of cerebrospinal fluid leaks (CSF) occur following trauma and complicate 12 to 13% percent of all basilar skull fractures (Prosser, Vender, and Solares, 2011). An endoscopic endonasal approach (EEA) is often the preferred method of repair with greater than 90% success rates (Prosser, Vender, and Solares, 2011). We report a case of a 37-year-old man who presented to our regional level 1 trauma centre with multiple facial injuries. Initial cross-sectional imaging revealed multiple, continuous anterior skull base fractures with associated pneumocephalus. Though initially managed conservatively, the patient represented five days later with unilateral left-sided rhinorrhoea. An endoscopic endonasal repair with a multilayer fat, tensor fascia lata, free mucosal graft, and vascularised local flap reconstruction was undertaken. This case highlights the importance of maintaining a high level of suspicion for delayed CSF leak in traumatic base of skull injury. The EEA enables meticulous dissection and thorough inspection of the skull base, facilitating multilayered repair and reconstruction of defects.

The immune system is essential to protect organisms from internal and external threats. The rapidly acting, non-specific innate immune system includes complement, which initiates an inflammatory cascade and can form pores in the membranes of target cells to induce cell lysis. Regulation of protein homeostasis (proteostasis) is essential for normal cellular and organismal function, and has been implicated in processes controlling immunity and infection. Chaperones are key players in maintaining proteostasis in both the intra- and extracellular environments. Whilst intracellular proteostasis is well-characterised, the role of constitutively secreted extracellular chaperones (ECs) is less well understood. ECs may interact with invading pathogens, and elements of the subsequent immune response, including the complement pathway. Both ECs and complement can influence the progression of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis, as well as other diseases including kidney diseases and diabetes. This review will examine known and recently discovered ECs, and their roles in immunity, with a specific focus on the complement pathway.

Clusterin is a glycoprotein present at high concentrations in many extracellular fluids, including semen. Its increased expression accompanies disorders associated with extracellular amyloid fibril accumulation such as Alzheimer's disease. Clusterin is an extracellular molecular chaperone which prevents the misfolding and amorphous and amyloid fibrillar aggregation of a wide variety of unfolding proteins. In semen, amyloid fibrils formed from a 39-amino acid fragment of prostatic acid phosphatase, termed Semen-derived Enhancer of Virus Infection (SEVI), potentiate HIV infectivity. In this study, clusterin potently inhibited the in vitro formation of SEVI fibrils, along with dissociating them. Furthermore, clusterin reduced the toxicity of SEVI to pheochromocytoma-12 cells. In semen, clusterin may play an important role in preventing SEVI amyloid fibril formation, in dissociating SEVI fibrils and in mitigating their enhancement of HIV infection.

Parkinson's disease is associated with the aberrant aggregation of α-synuclein. Although the causes of this process are still unclear, post-translational modifications of α-synuclein are likely to play a modulatory role. Since α-synuclein is constitutively N-terminally acetylated, we investigated how this post-translational modification alters the aggregation behavior of this protein. By applying a three-pronged aggregation kinetics approach, we observed that N-terminal acetylation results in a reduced rate of lipid-induced aggregation and slows down both elongation and fibril-catalyzed aggregate proliferation. An analysis of the amyloid fibrils produced by the aggregation process revealed different morphologies for the acetylated and non-acetylated forms in both lipid-induced aggregation and seed-induced aggregation assays. In addition, we found that fibrils formed by acetylated α-synuclein exhibit a lower β-sheet content. These findings indicate that N-terminal acetylation of α-synuclein alters its lipid-dependent aggregation behavior, reduces its rate of in vitro aggregation, and affects the structural properties of its fibrillar aggregates.

Proteostasis refers to all the processes that maintain the correct expression level, location, folding and turnover of proteins, essential to organismal survival. Both inside cells and in body fluids, molecular chaperones play key roles in maintaining proteostasis. In this article, we focus on clusterin, the first-recognized extracellular mammalian chaperone, and its role in diseases of the eye. Clusterin binds to and inhibits the aggregation of proteins that are misfolded due to mutations or stresses, clears these aggregating proteins from extracellular spaces, and facilitates their degradation. Clusterin exhibits three main homeostatic activities: proteostasis, cytoprotection, and anti-inflammation. The so-called "protein misfolding diseases” are caused by aggregation of misfolded proteins that accumulate pathologically as deposits in tissues; we discuss several such diseases that occur in the eye. Clusterin is typically found in these deposits, which is interpreted to mean that its capacity as a molecular chaperone to maintain proteostasis is overwhelmed in the disease state. Nevertheless, the role of clusterin in diseases involving such deposits needs to be better defined before therapeutic approaches can be entertained. A more straightforward case can be made for therapeutic use of clusterin based on its proteostatic role as a proteinase inhibitor, as well as its cytoprotective and anti-inflammatory properties. It is likely that clusterin works together in this way with other extracellular chaperones to protect the eye from disease, and we discuss several examples. We end this article by predicting future steps that may lead to development of clusterin as a biological drug.

Proteostasis refers to a delicately tuned balance between the processes of protein synthesis, folding, localization, and the degradation of proteins found inside and outside cells. Our understanding of extracellular proteostasis is rather limited and largely restricted to knowledge of 11 currently established extracellular chaperones (ECs). This review will briefly outline what is known of the established ECs, before moving on to discuss experimental strategies used to identify new members of this growing family, and an examination of a group of putative new ECs identified using one of these approaches. An observation that emerges from an analysis of the expanding number of ECs is that all of these proteins are multifunctional. Strikingly, the armory of activities each possess uniquely suit them as a group to act together at sites of tissue damage, infection, and inflammation to restore homeostasis. Lastly, we highlight outstanding questions to guide future research in this field.