Clusterin shortens the incubation and alters the histopathology of bovine spongiform encephalopathy in mice

Regulated Proteolysis Induces Aberrant Phase Transition of Biomolecular Condensates into Aggregates: A Protective Role for the Chaperone Clusterin

Several proteins associated with neurodegenerative diseases, such as the mammalian prion protein (PrP), undergo liquid-liquid phase separation (LLPS), which led to the hypothesis that condensates represent precursors in the formation of neurotoxic protein aggregates. However, the mechanisms that trigger aberrant phase separation are incompletely understood. In prion diseases, protease-resistant and infectious amyloid fibrils are composed of N-terminally truncated PrP, termed C2-PrP. C2-PrP is generated by regulated proteolysis (β-cleavage) of the cellular prion protein (PrPC) specifically upon prion infection, suggesting that C2-PrP is a misfolding-prone substrate for the propagation of prions. Here we developed a novel assay to investigate the role of both LLPS and β-cleavage in the formation of C2-PrP aggregates. We show that β-cleavage induces the formation of C2-PrP aggregates, but only when full-length PrP had formed biomolecular condensates via LLPS before proteolysis. In contrast, C2-PrP remains soluble after β-cleavage of non-phase-separated PrP. To investigate whether extracellular molecular chaperones modulate LLPS of PrP and/or misfolding of C2-PrP, we focused on Clusterin. Clusterin does not inhibit LLPS of full-length PrP, however, it prevents aggregation of C2-PrP after β-cleavage of phase-separated PrP. Furthermore, Clusterin interferes with the in vitro amplification of infectious human prions isolated from Creutzfeldt-Jakob disease patients. Our study revealed that regulated proteolysis triggers aberrant phase transition of biomolecular condensates into aggregates and identified Clusterin as a component of the extracellular quality control pathway to prevent the formation and propagation of pathogenic PrP conformers.

Extracellular protein homeostasis in neurodegenerative diseases

The protein homeostasis (proteostasis) system encompasses the cellular processes that regulate protein synthesis, folding, concentration, trafficking and degradation. In the case of intracellular proteostasis, the identity and nature of these processes have been extensively studied and are relatively well known. By contrast, the mechanisms of extracellular proteostasis are yet to be fully elucidated, although evidence is accumulating that their age-related progressive impairment might contribute to neuronal death in neurodegenerative diseases. Constitutively secreted extracellular chaperones are emerging as key players in processes that operate to protect neurons and other brain cells by neutralizing the toxicity of extracellular protein aggregates and promoting their safe clearance and disposal. Growing evidence indicates that these extracellular chaperones exert multiple effects to promote cell viability and protect neurons against pathologies arising from the misfolding and aggregation of proteins in the synaptic space and interstitial fluid. In this Review, we outline the current knowledge of the mechanisms of extracellular proteostasis linked to neurodegenerative diseases, and we examine the latest understanding of key molecules and processes that protect the brain from the pathological consequences of extracellular protein aggregation and proteotoxicity. Finally, we contemplate possible therapeutic opportunities for neurodegenerative diseases on the basis of this emerging knowledge.

Genetic Factors in Mammalian Prion Diseases

Mammalian prion diseases are a group of neurodegenerative conditions caused by infection of the central nervous system with proteinaceous agents called prions, including sporadic, variant, and iatrogenic Creutzfeldt-Jakob disease; kuru; inherited prion disease; sheep scrapie; bovine spongiform encephalopathy; and chronic wasting disease. Prions are composed of misfolded and multimeric forms of the normal cellular prion protein (PrP). Prion diseases require host expression of the prion protein gene (PRNP) and a range of other cellular functions to support their propagation and toxicity. Inherited forms of prion disease are caused by mutation of PRNP, whereas acquired and sporadically occurring mammalian prion diseases are controlled by powerful genetic risk and modifying factors. Whereas some PrP amino acid variants cause the disease, others confer protection, dramatically altered incubation times, or changes in the clinical phenotype. Multiple mechanisms, including interference with homotypic protein interactions and the selection of the permissible prion strains in a host, play a role. Several non-PRNP factors have now been uncovered that provide insights into pathways of disease susceptibility or neurotoxicity.

Improvement of neuronal cell survival by astrocyte-derived exosomes under hypoxic and ischemic conditions depends on prion protein

Prion protein (PrP) protects neural cells against oxidative stress, hypoxia, ischemia, and hypoglycemia. In the present study we confirm that cultured PrP-deficient neurons are more sensitive to oxidative stress than wild-type neurons and present the novel findings that wild-type, but not PrP-deficient astrocytes protect wild-type cerebellar neurons against oxidative stress and that exosomes released from stressed wild-type, but not from stressed PrP-deficient astrocytes reduce neuronal cell death induced by oxidative stress. We show that neuroprotection by exosomes of stressed astrocytes depends on exosomal PrP but not on neuronal PrP and that astrocyte-derived exosomal PrP enters into neurons, suggesting neuronal uptake of astrocyte-derived exosomes. Upon exposure of wild-type astrocytes to hypoxic or ischemic conditions PrP levels in exosomes were increased. By mass spectrometry and Western blot analysis, we detected increased levels of 37/67 kDa laminin receptor, apolipoprotein E and the ribosomal proteins S3 and P0, and decreased levels of clusterin/apolipoprotein J in exosomes from wild-type astrocytes exposed to oxygen/glucose deprivation relative to exosomes from astrocytes maintained under normoxic conditions. The levels of these proteins were not altered in exosomes from stressed PrP-deficient astrocytes relative to unstressed PrP-deficient astrocytes. These results indicate that PrP in astrocytes is a sensor for oxidative stress and mediates beneficial cellular responses, e.g. release of exosomes carrying PrP and other molecules, resulting in improved survival of neurons under hypoxic and ischemic conditions.

Analysis of the hippocampal proteome in ME7 prion disease reveals a predominant astrocytic signature and highlights the brain-restricted production of clusterin in chronic neurodegeneration

Prion diseases are characterized by accumulation of misfolded protein, gliosis, synaptic dysfunction, and ultimately neuronal loss. This sequence, mirroring key features of Alzheimer disease, is modeled well in ME7 prion disease. We used iTRAQ^TM/mass spectrometry to compare the hippocampal proteome in control and late-stage ME7 animals. The observed changes associated with reactive glia highlighted some specific proteins that dominate the proteome in late-stage disease. Four of the up-regulated proteins (GFAP, high affinity glutamate transporter (EAAT-2), apo-J (Clusterin), and peroxiredoxin-6) are selectively expressed in astrocytes, but astrocyte proliferation does not contribute to their up-regulation. The known functional role of these proteins suggests this response acts against protein misfolding, excitotoxicity, and neurotoxic reactive oxygen species. A recent convergence of genome-wide association studies and the peripheral measurement of circulating levels of acute phase proteins have focused attention on Clusterin as a modifier of late-stage Alzheimer disease and a biomarker for advanced neurodegeneration. Since ME7 animals allow independent measurement of acute phase proteins in the brain and circulation, we extended our investigation to address whether changes in the brain proteome are detectable in blood. We found no difference in the circulating levels of Clusterin in late-stage prion disease when animals will show behavioral decline, accumulation of misfolded protein, and dramatic synaptic and neuronal loss. This does not preclude an important role of Clusterin in late-stage disease, but it cautions against the assumption that brain levels provide a surrogate peripheral measure for the progression of brain degeneration.

Non-secreted clusterin isoforms are translated in rare amounts from distinct human mRNA variants and do not affect Bax-mediated apoptosis or the NF-κB signaling pathway

Clusterin, also known as apolipoprotein J, is expressed from a variety of tissues and implicated in pathological disorders such as neurodegenerative diseases, ischemia and cancer. In contrast to secretory clusterin (sCLU), which acts as an extracellular chaperone, the synthesis, subcellular localization and function(s) of intracellular CLU isoforms is currently a matter of intense discussion. By investigating human CLU mRNAs we here unravel mechanisms leading to the synthesis of distinct CLU protein isoforms and analyze their subcellular localization and their impact on apoptosis and on NF-κB-activity. Quantitative PCR-analyses revealed the expression of four different stress-inducible CLU mRNA variants in non-cancer and cancer cell lines. In all cell lines variant 1 represents the most abundant mRNA, whereas all other variants collectively account for no more than 0.34% of total CLU mRNA, even under stressed conditions. Overexpression of CLU cDNAs combined with in vitro mutagenesis revealed distinct translational start sites including a so far uncharacterized non-canonical CUG start codon. We show that all exon 2-containing mRNAs encode sCLU and at least three non-glycosylated intracellular isoforms, CLU_1‑449, CLU_21‑449 and CLU_34‑449, which all reside in the cytosol of unstressed and stressed HEK‑293 cells. The latter is the only form expressed from an alternatively spliced mRNA variant lacking exon 2. Functional analysis revealed that none of these cytosolic CLU forms modulate caspase-mediated intrinsic apoptosis or significantly affects TNF-α-induced NF-κB-activity. Therefore our data challenge some of the current ideas regarding the physiological functions of CLU isoforms in pathologies.

Analysis of clusterin glycoforms in the urine of BSE-infected Fleckvieh-Simmental cows

Currently approved tests for bovine spongiform encephalopathy (BSE) monitoring in cattle are based on the detection of the disease-related isoform of the prion protein in brain tissue and consequently are only suitable for postmortem diagnosis. Previously, to meet the demand for an antemortem test based on a matrix that would permit easy access and repeated sampling, two-dimensional differential gel electrophoresis (2D-DIGE) was used to perform an unbiased screen of bovine urine. Data demonstrated the altered abundance of particular isoforms of the multifunctional glycoprotein clusterin in urine samples obtained from BSE-infected and age-matched Fleckvieh-Simmental cattle. Unfortunately, the use of particular isoforms of a relatively abundant urine protein such as clusterin for diagnosis faces many of the same challenges encountered in tests based on PrP(d) detection. In both instances the specific detection of the marker protein is complicated by the high background levels of proteins with identical amino acid sequences, but lacking the disease-specific posttranslational modifications or configuration. The goal of the current study was to define the distinguishing characteristics of the clusterin isoforms observed. Biochemical and mass spectrometry analyses in combination with the generation of bovine clusterin subunit-specific antibodies enabled us to demonstrate that it was β-subunits of clusterin possessing N-linked glycans of complex structure that exhibited differential abundance in response to BSE infection. The charateristic highly glycosylated clusterin β-subunit was detectable as early as 16 mo post infection (mpi) by one-dimensional (1D) Western blot analysis of urine obtained from BSE-infected cattle.

Genetics of prion disease

Prion diseases or transmissible spongiform encephalopathies (TSEs) are neurodegenerative disorders of humans and animals for which there are no effective treatments or cure. They include Creutzfeldt-Jakob disease (CJD) in humans and sheep scrapie, bovine spongiform encephalopathy (BSE) and chronic wasting disease (CWD) in cervids. The prion protein (PrP) is central to the disease process. An abnormal form of PrP is generally considered to be the sole or principal component of the infectious agent and a multimeric isomer (PrP(Sc)) is deposited in affected brains. Inherited prion diseases are caused by over 30 mutations in the prion protein gene (PRNP) and common polymorphisms can have a considerable affect on susceptibility and phenotype. Susceptibility and incubation time are also partly determined by other (non-PRNP) genetic modifiers. Understanding how these other genes modify prion diseases may lead to insights into biological mechanisms. Several approaches including human genome wide association studies (GWAS), mouse mapping and differential expression studies are now revealing some of these genes which include RARB (retinoic acid receptor beta), the E3 ubiquitin ligase HECTD2 and SPRN (Shadoo, shadow of prion protein gene).

The retinoic acid receptor beta (Rarb) region of Mmu14 is associated with prion disease incubation time in mouse

In neurodegenerative conditions such as Alzheimer's and prion disease it has been shown that host genetic background can have a significant effect on susceptibility. Indeed, human genome-wide association studies (GWAS) have implicated several candidate genes. Understanding such genetic susceptibility is relevant to risks of developing variant CJD (vCJD) in populations exposed to bovine spongiform encephalopathy (BSE) and understanding mechanisms of neurodegeneration. In mice, aspects of prion disease susceptibility can be modelled by examining the incubation period following experimental inoculation. Quantitative trait linkage studies have already identified multiple candidate genes; however, it is also possible to take an individual candidate gene approach. Rarb and Stmn2 were selected as candidates based on the known association with vCJD. Because of the increasing overlap described between prion and Alzheimer's diseases we also chose Clu, Picalm and Cr1, which were identified as part of Alzheimer's disease GWAS. Clusterin (Clu) was considered to be of particular interest as it has already been implicated in prion disease. Approximately 1,000 heterogeneous stock (HS) mice were inoculated intra-cerebrally with Chandler/RML prions and incubation times were recorded. Candidate genes were evaluated by sequencing the whole transcript including exon-intron boundaries and potential promoters in the parental lines of the HS mice. Representative SNPs were genotyped in the HS mice. No SNPs were identified in Cr1 and no statistical association with incubation time was seen for Clu (P = 0.96) and Picalm (P = 0.91). Significant associations were seen for both Stmn2 (P = 0.04) and Rarb (P = 0.0005), however, this was only highly significant for Rarb. This data provides significant further support for a role for the Rarb region of Mmu14 and Stmn2 in prion disease.

Gene expression profile of quinacrine-cured prion-infected mouse neuronal cells

Prion diseases are transmissible fatal neurodegenerative diseases of humans and animals, characterised by the presence of an abnormal isoform (scrapie prion protein; PrP(Sc)) of the endogenous cellular prion protein (PrP(C)). The pathological mechanisms at the basis of prion diseases remain elusive, although the accumulation of PrP(Sc) has been linked to neurodegeneration. Different genomic approaches have been applied to carry out large-scale expression analysis in prion-infected brains and cell lines, in order to define factors potentially involved in pathogenesis. However, the general lack of overlap between the genes found in these studies prompted us to carry an analysis of gene expression using an alternative approach. Specifically, in order to avoid the complexities of shifting gene expression in a heterogeneous cell population, we used a single clone of GT1 cells that was de novo infected with mouse prion-infected brain homogenate and then treated with quinacrine to clear PrP(Sc). By comparing the gene expression profiles of about 15 000 genes in quinacrine-cured and not cured prion-infected GT1 cells, we investigated the influence of the presence or the absence of PrP(Sc). By real-time PCR, we confirmed that the gene encoding for laminin was down-regulated as a consequence of the elimination of PrP(Sc) by the quinacrine treatment. Thus, we speculate that this protein could be a specific candidate for further analysis of its role in prion infection and pathogenesis.

Prions: protein only or something more? Overview of potential prion cofactors

Transmissible spongiform encephalopathies (TSEs) in humans and animals are attributed to protein-only infectious agents, called prions. Prions have been proposed to arise from the conformational conversion of the cellular protein PrP(C) into a misfolded form (e.g., PrP(Sc) for scrapie), which precipitates into aggregates and fibrils. It has been proposed that the conversion process is triggered by the interaction of the infectious form (PrP(Sc)) with the cellular form (PrP(C)) or might result from a mutation in the gene for PrP(C). However, until recently, all efforts to reproduce this process in vitro had failed, suggesting that host factors are necessary for prion replication. In this review we discuss recent findings such as the cellular factors that might be involved in the conformational conversion of prion proteins and the potential mechanisms by which they could operate.

Clusterin expression in follicular dendritic cells associated with prion protein accumulation

Peripheral accumulation of abnormal prion protein (PrP) in variant Creutzfeldt-Jakob disease and some animal models of transmissible spongiform encephalopathies (TSEs) may occur in the lymphoreticular system. Within the lymphoid tissues, abnormal PrP accumulation occurs on follicular dendritic cells (FDCs). Clusterin (apolipoprotein J) has been recognized as one of the molecules associated with PrP in TSEs, and clusterin expression is increased in the central nervous system where abnormal PrP deposition has occurred. We therefore examined peripheral clusterin expression in the context of PrP accumulation on FDCs in a range of human and experimental TSEs. PrP was detected immunohistochemically on tissue sections using a novel highly sensitive method involving detergent autoclaving pretreatment. A dendritic network pattern of clusterin immunoreactivity in lymphoid follicles was observed in association with the abnormal PrP on FDCs. The increased clusterin immunoreactivity appeared to correlate with the extent of PrP deposition, irrespective of the pathogen strains, host mouse strains or various immune modifications. The observed co-localization and correlative expression of these proteins suggested that clusterin might be directly associated with abnormal PrP. Indeed, clusterin immunoreactivity in association with PrP was retained after FDC depletion. Together these data suggest that clusterin may act as a chaperone-like molecule for PrP and play an important role in TSE pathogenesis.

Clusterin is not essential for androgen-regulated involution and regeneration of the normal mouse prostate

BACKGROUND

Inhibition of clusterin expression has been shown to enhance the sensitivity of prostate cancer cells to chemo and hormone therapy. Clusterin antisense oligonucleotides (ASOs) are currently in phase II human clinical trials for treatment of hormone refractory prostate cancer. However, the role of clusterin in androgen-regulated involution and regeneration of the normal prostate gland has not been established.

METHODS

Prostate involution and regeneration was examined in clusterin-deficient mice undergoing up to three cycles of androgen withdrawal and restoration.

RESULTS

Surprisingly, clusterin deficiency did not affect the apoptotic index, and the temporal biochemical and morphological changes associated with involution and regeneration of the normal adult prostate following multiple rounds of androgen withdrawal and replacement.

CONCLUSION

Clusterin is not critical for normal prostate development or androgen-regulated involution and regrowth of the mouse prostate gland, suggesting that clusterin may have distinct functions in malignant versus normal prostatic epithelial cells.

The epididymal soluble prion protein forms a high-molecular-mass complex in association with hydrophobic proteins

We have shown previously that a 'soluble' form of PrP (prion protein), not associated with membranous vesicles, exists in the male reproductive fluid [Ecroyd, Sarradin, Dacheux and Gatti (2004) Biol. Reprod. 71, 993-1001]. Attempts to purify this 'soluble' PrP indicated that it behaves like a high-molecular-mass complex of more than 350 kDa and always co-purified with the same set of proteins. The main associated proteins were sequenced by MS and were found to match to clusterin (apolipoprotein J), BPI (bacterial permeability-increasing protein), carboxylesterase-like urinary excreted protein (cauxin), beta-mannosidase and beta-galactosidase. Immunoblotting and enzymatic assay confirmed the presence of clusterin and a cauxin-like protein and showed that a 17 kDa hydrophobic epididymal protein was also associated with this complex. These associated proteins were not separated by a high ionic strength treatment but were by 2-mercaptoethanol, probably due to its action on reducing disulphide bonds that maintain the interaction of components of the complex. Our results suggest that the associated PrP retains its GPI (glycosylphosphatidylinositol) anchor, in contrast with brain-derived PrP, and that it is resistant to cleavage by phosphatidylinositol-specific phospholipase C. Based on these results, the identity of the associated proteins and the overall biochemical properties of this protein ensemble, we suggest that 'soluble' PrP can form protein complexes that are maintained by hydrophobic interactions, in a similar manner to lipoprotein vesicles or micellar complexes.

Gene expression correlates of neurofibrillary tangles in Alzheimer's disease

Neurofibrillary tangles (NFT) constitute one of the cardinal histopathological features of Alzheimer's disease (AD). To explore in vivo molecular processes involved in the development of NFTs, we compared gene expression profiles of NFT-bearing entorhinal cortex neurons from 19 AD patients, adjacent non-NFT-bearing entorhinal cortex neurons from the same patients, and non-NFT-bearing entorhinal cortex neurons from 14 non-demented, histopathologically normal controls (ND). Of the differentially expressed genes, 225 showed progressively increased expression (AD NFT neurons > AD non-NFT neurons > ND non-NFT neurons) or progressively decreased expression (AD NFT neurons < AD non-NFT neurons < ND non-NFT neurons), raising the possibility that they may be related to the early stages of NFT formation. Immunohistochemical studies confirmed that many of the implicated proteins are dysregulated and preferentially localized to NFTs, including apolipoprotein J, interleukin-1 receptor-associated kinase 1, tissue inhibitor of metalloproteinase 3, and casein kinase 2, beta. Functional validation studies are underway to determine which candidate genes may be causally related to NFT neuropathology, thus providing therapeutic targets for the treatment of AD.

Cell line dependent RNA expression profiles of prion-infected mouse neuronal cells

The overall impact of prion disease on gene expression is not well characterized. We have carried out a large-scale expression analysis of specific cell types commonly employed in studies of prion disease. Neuroblastoma cells (N2a) and hypothalamic neuronal cells (GT1) can be persistently infected with mouse-adapted scrapie prions, the latter demonstrating cytopathologic effects associated with prion neuropathology. Exploiting a mouse DNA microarray containing approximately 21,000 spotted cDNAs, we have identified several hundred differentially expressed sequences in the two cell lines when infected with prion strain RML. ScN2a and ScGT1 cells demonstrate unique changes in RNA profiles and both differ from the reported changes in human microglia and prion-infected brain studies albeit with some overlap. In addition, several of the identified changes are shared in common with other neurodegenerative diseases such as Alzheimer's disease. The results illustrate that prion infection differs in effect depending on cell type, which could be exploited for diagnostic or therapeutic intervention.

Time of transforming growth factor beta 1 inoculation alters the incubation of BSE in mice

Groups of 20 C57BL/6J mice (10 males and 10 females) were given BSE strain 301C i.p. and subsequently given 2 microg recombinant human TGFbeta1 s.c. at single or multiple times. There was a significant positive correlation between the day of TGFbeta1 administration and incubation time; the later TGFbeta1 was administered after BSE inoculation the longer the incubation time became. The administration of TGFbeta1 at any time point did not significantly alter the distribution or severity of pathology. The effects of TGFbeta1 on BSE pathogenesis appears to be dependent upon its time of administration; early administration shortens the incubation time and late administration lengthens the incubation time.