The chaperone Clusterin in neurodegeneration−friend or foe?

Fibrillar protein aggregates are the pathological hallmark of a group of age‐dependent neurodegenerative conditions, including Alzheimer's and Parkinson's disease. Aggregates of the microtubule‐associated protein Tau are observed in Alzheimer's disease and primary tauopathies. Tau pathology propagates from cell to cell in a prion‐like process that is likely subject to modulation by extracellular chaperones such as Clusterin. We recently reported that Clusterin delayed Tau fibril formation but enhanced the activity of Tau oligomers to seed aggregation of endogenous Tau in a cellular model. In contrast, Clusterin inhibited the propagation of α‐Synuclein aggregates associated with Parkinson's disease. These findings raise the possibility of a mechanistic link between Clusterin upregulation observed in Alzheimer's disease and the progression of Tau pathology. Here we review the diverse functions of Clusterin in the pathogenesis of neurodegenerative diseases, focusing on evidence that Clusterin may act either as a suppressor or enhancer of pathology.


INTRODUCTION
The formation of protein aggregates within and around neurons is a signature of age-dependent neurodegenerative diseases (NDs) and dementias.Insoluble fibrillar (amyloid-like) deposits together with soluble, oligomeric aggregate species are considered major toxic agents driving pathology. [1,2]The aggregates consist of specific disease proteins as the main component: α-Synuclein in Parkinson's disease (PD) Abbreviations: AD, Alzheimer's disease; Aβ, amyloid-β; ApoER2, apolipoprotein E receptor 2; ApoJ, apolipoprotein J; CNS, central nervous system; CSF, cerebrospinal fluid; HSPG, heparan sulfate proteoglycan; iClu, intracellular Clusterin; iPSC, induced pluripotent stem cells; KO, Knock out; LOAD, late-onset Alzheimer's disease; LRP, low density lipoprotein receptor-related protein; ND, neurodegenerative disease; PD, Parkinson's disease; psClu, F I G U R E 1 Role of chaperones in amyloid protein aggregation and prion-like, transcellular aggregate propagation.Native proteins in the donor cell (left) unfold or misfold, populating aggregation-prone states.Primary nucleation of amyloid fibril formation may involve oligomer formation.Oligomers are also generated at the surface of preexistent fibrils through secondary nucleation.Intracellular molecular chaperones (green) interfere with amyloid aggregation at different stages, by preventing misfolding, oligomerization, primary and secondary nucleation, fibrilization and fibril elongation.Chaperones may bind to oligomers or fibrils neutralizing their interactive surfaces (green thin arrows), suppressing aggregate toxicity.Chaperones can also promote fibril fragmentation, forming seeds that can further propagate and template aggregation (red thin arrow).Transcellular aggregate propagation may involve the release of aggregates by donor cells directly into the extracellular space, secretion in exosomes or ectosomes, or transport through intercellular nanotubes.Free seed material in the extracellular space is substrate of extracellular chaperones such as Clusterin, with different outcomes: Clusterin may neutralize seeds of α-Synuclein (α-Syn), but stabilize seeds of Tau. [14]ggregate seeds can be internalized from the extracellular space by recipient cells (right) through endocytosis, possibly in complex with chaperone. [14]Aggregate seeds may damage endolysosomal membranes and escape to the cytosol to induce aggregation of endogenous, native protein.This templating process may be interfered with by intracellular chaperones The prion-like spreading of pathological aggregates involves the transport of seed aggregates between cells, either through tubular intercellular connections, by secretion of seeds in exosome vesicles or upon release into the extracellular space and uptake by recipient cells [3,8] (Figure 1).[11][12] Thus, it is plausible that aggregate propagation is subject to modulation by extracellular chaperones and quality control factors. [13]Using a cellular model of Tau aggregate propagation, we recently found that the abundant extracellular chaperone Clusterin, while delaying Tau fibril formation, markedly enhanced Tau aggregate seeding by stabilizing highly potent, soluble seed species. [14]The pathophysiological relevance of these findings remains to be established, but given its frequent upregulation in AD, [15][16][17] Clusterin may conceivably contribute to promoting Tau pathology.
Here we review possible roles of the extracellular chaperone Clusterin in the pathogenesis and progression of neurodegeneration with a focus on AD and tauopathies.We discuss the effects of Clusterin on protein aggregation and toxicity, as well as its functions in aggregate clearance by glial cells and in suppressing neuroinflammation.As proposed previously, [18][19][20] Clusterin appears to be a Janus-faced chaper-one, acting either as a suppressor or enhancer of pathology, dependent on specific disease context.

CLUSTERIN, AN UNUSUAL CHAPERONE
Clusterin (also known as ApoJ), encoded by the CLU gene, is a ubiquitously expressed extracellular chaperone and apolipoprotein in all vertebrates.][23] The name Clusterin derives from its identification as a cell-aggregating factor in ram rete testis. [24]Clusterin is translated as a precursor protein of 449 amino acids containing a 22 amino acid signal peptide that is cleaved during translocation into the endoplasmic reticulum (ER) (Figure 2A).
Once in the oxidizing environment of the ER, formation of five disulfide bonds followed by N-glycosylation generates pre-secretory Clusterin (psClu).psClu is then transferred to the Golgi apparatus where it is further processed and cleaved by a furin-like protease resulting in two chains (α and β) of ̴ 35 kDa, which remain disulfide-linked. [19,25]e mature heterodimeric Clusterin is then secreted to the extracellular space, with glycans comprising ̴ 30% of its mass [19] (Figure 2A).Subsequently, psClu is transferred to the Golgi apparatus where the N-glycans are further processed and psClu is cleaved by a furin-like protease, resulting in two chains of similar size (α and β chains, light and dark green, respectively), which remain linked by the disulfide bonds.The mature glycosylated heterodimeric Clusterin is then secreted to the extracellular space.Numbers represent amino acid positions.Modified from ref. [14]  (B) Predicted 3D-structure of Clusterin [27,28] (https://alphafold.ebi.ac.uk/entry/P10909).The structural model predicted with AlphaFold2 is shown in ribbon representation in rainbow colors.The signal peptide is not represented.N-and C-termini are indicated.Disulfide bonds are represented as spheres (silver, 1-5), N-glycosylation sites (N-Glyc) as sticks (magenta) and the Furin-like protease cleavage site in white.(C) Surface conservation of the predicted Clusterin structure.The similarity score was calculated with the program ESPript [140] based on the alignment of 10 representative Clu sequences and is shown as a color gradient from magenta (invariant residue) to cyan (no conservation).Mainly structurally important residues appear to be conserved tendency of the protein to self-associate. [24,26]AlphaFold2 [27,28] predicts an elongated, mostly α-helical structure of psClu, in which the two chains are linked via five disulfide bridges in a globular domain at one end of a central, mixed anti-parallel coiled-coil bundle (Figure 2B).
The regions containing the cysteines involved in disulfide bond formation are well conserved, while other parts of the protein are more variable (PFAM number PF01093) [29] (Figure 2C).The predicted structure agrees with the experimentally determined disulfide topology, and all asparagine residues known to be glycosylated [30] are exposed to the solvent.Consistently, the furin cleavage site maps to a long accessible loop segment.
Clusterin is an ATP-independent chaperone with functional properties of a ''holdase'' , similar to so-called small heat shock proteins (sHSP). [31]Holdase chaperones bind and stabilize folding intermediates and misfolded proteins against aggregation, but do not actively promote refolding.][34][35][36][37] Clusterin has been proposed to interact with client proteins via an as yet undefined ''molten globule''-like domain(s). [38]In addition to its chaperone capacity, Clusterin functions in sperm maturation, [39] cell differentiation, [40] regulation of cell death and survival mechanisms, [41] and as an anti-inflammatory inhibitor of the complement system. [42,43]Moreover, it is often overlooked that Clusterin is an apolipoprotein (ApoJ) that has been identified in plasma high-density lipoprotein particles, suggesting a role in lipid and cholesterol metabolism. [44]Indeed, together with ApoE, Clusterin is one of the major apolipoproteins in the brain parenchyma, but its role in lipid metabolism in the central nervous system (CNS) is not well understood. [45]Although Clusterin lipidation status does not seem to affect amyloid binding, it may modify the affinity of Clusterin for cell surface receptors involved in uptake. [46,47]][50] An increase in iClu levels has been observed in neurons upon exposure to Aβ oligomers and has been suggested to play a role in mediating Aβ toxicity. [51]The biogenesis and regulation of iClu has mainly been studied in certain cancer cells where it is abundant, [41] but its biogenesis is not well understood.Although alternative splicing and alternative initiation codons have been implicated, the mRNA species for these Clusterin isoforms are of very low abundance.Rather, iClu appears to be generated predominantly by retrotranslocation from the ER or Golgi apparatus to the cytosol under stress conditions.The differences in size and glycosylation pattern found in iClu species are therefore likely to reflect different maturation stages along the secretory pathway. [50]It is possible that prematurely retro-translocated iClu retains chaperone activity, at least partially, contributing to intracellular proteostasis. [26,34,50]usterin expression is regulated by hormones, growth factors, and cytokines.The CLU promoter contains multiple transcription factor motifs, including a heat shock element from the cytosolic heat shock response. [52,53]In addition, CLU is also regulated epigenetically by DNA methylation and histone deacetylation, and by micro-RNAs. [53]e wide range of ascribed functions and its complex regulation make Clusterin a puzzling and enigmatic player in neurodegeneration.

CLUSTERIN IN ALZHEIMER'S DISEASE
54][55][56][57][58][59][60][61][62][63][64] AD is the most common cause of dementia, characterized by two neuropathological hallmarks: the deposition of extracellular amyloid plaques mainly composed of Aβ and the formation of intracellular neurofibrillary tangles of the microtubule-associated protein Tau. [65]][68] Indeed, the CLU gene ranks third among the genetic risk factors for late-onset AD (LOAD), with genome wide association studies having identified several single nucleotide polymorphisms (SNPs) linked to AD. [18,[69][70][71] While some rare, non-synonymous mutations have been suggested to affect Clusterin secretion, [72] other variants may affect CLU alternative splicing [73] and regulatory elements, [74][75][76] with complex effects on Clusterin expression.LOAD risk variants of CLU have been associated with either unchanged, [16,17] increased [73,75] or decreased [22,77] Clusterin levels in the brain, plasma or CSF of AD patients when compared to AD patients with a normal CLU gene.Despite this complexity, there is agreement that the LOAD risk variants of CLU are associated with increased Aβ deposition [78] and accelerated cognitive decline. [79,80]terestingly, CLU variants have also been linked to changes in brain connectivity and structure in healthy individuals, effects that could precede clinical phenotypes. [81,82]Critical insights into the mechanism by which CLU variants promote LOAD may be gained using patientderived induced pluripotent stem cells (iPSCs) that can be differentiated into neurons and other brain cells.
Importantly, Clusterin levels are often increased in the brain, CSF and plasma of AD patients independent of the presence of CLU variants.[17] Local Clusterin expression has been observed to be associated with regional Aβ deposition [83,84] and possibly with Tau pathology. [84]Remarkably, in healthy middle-aged adults, a high level of plasma Clusterin is associated with a lower volume of the entorhinal cortex, a brain region that atrophies early in AD, suggesting that plasma Clusterin may serve as a biomarker for preclinical AD. [85] However, these findings leave open the question whether elevated Clusterin levels are a consequence of pathology or a promoting factor.Both the chaperone function of Clusterin in modulating amyloid aggregation, toxicity and clearance, as well as its anti-inflammatory effect have the capacity to modulate neurodegenerative pathology.

EFFECTS OF CLUSTERIN ON Aβ AND TAU AGGREGATION
Clusterin has been detected in association with various disease aggregates. [13]While its colocalization with Aβ deposits in the extracellular space has been studied extensively, [66,67] the physiological consequences of these interactions are not well understood.60] On the other hand, CLU knock out (KO) mouse models of AD displayed a reduction in oligomeric Aβ aggregates and plaques, [20,61] especially at early stages of pathogenesis, [62] pointing to a possible pro-amyloidogenic role of Clusterin.However, it has not been ruled out that the lower oligomer concentration resulted from compensatory effects in response to the CLU KO.Indeed, upregulation of multiple pathways related to neurodegeneration has been reported in iPSC derived CLU KO neurons [51] and in a CLU KO mouse model. [86]yloid fibrils form through a process of nucleation-dependent polymerization [2,87] (Figure 1).Various intermediate aggregate species have been characterized, including structurally ill-defined soluble oligomers and prefibrillar species. [2,87]A key question with particular relevance in disease is to determine which of these species exert direct cellular toxicity and/or propagate the pathological conformation as seeds in a prion-like manner.While soluble oligomers are widely considered highly interactive and cytotoxic, insoluble aggregates contribute to pathology by sequestering key cellular proteins and physically displacing organelle structures. [2,88]Chaperones, including Clusterin, can act at different stages of the aggregation pathway, thereby modulating the levels of aggregate species and their toxicity.
They may interfere with primary nucleation by binding to misfolded monomers or small oligomers, or inhibit fibril elongation by blocking fibril ends.Chaperones may also block secondary nucleation, a process in which oligomer formation is catalyzed on the surface of preformed fibrils [89,90] (Figure 1).Prevention of aggregation is generally cell-protective: binding of chaperones may shield exposed hydrophobic surfaces of oligomeric or prefibrillar aggregate species, thereby impeding their ability to engage in aberrant interactions with key cellular factors or disrupt cellular membranes. [37,91,92]However, chaperone intervention in the aggregation pathway, either by binding monomers, intermediates or mature fibrils, may also shift the dynamic equilibrium between aggregation intermediates, potentially promoting the accumulation of toxic species or stabilizing seeding-competent aggregates.
Clusterin has been shown to inhibit primary and secondary nucleation of Aβ oligomers, as well as fibril elongation, [32,54,57] thereby reducing toxicity. [46,58,59,63]On the other hand, Clusterin has also been reported to promote the formation of rare toxic oligomers [56,64] and to enhance fibril formation when present at a very low molar ratio relative to its substrate [32] (Figure 3).
Less is known about the potential role of Clusterin in Tau aggregation and toxicity, despite the fact that Tau pathology strongly correlates with the severity of AD. [93] Tau is a microtubule-associated protein encoded by the MAPT gene that functions in maintaining microtubule stability.Six different isoforms of Tau, generated by alternative splicing, are expressed in the human CNS.They differ in the number of N-terminal 29 amino acid inserts (0N, 1N, or 2N) and of microtubulebinding repeat domains (3R or 4R). [94]Accumulation of hyperphosphorylated Tau in neurons facilitates the formation of fibrillar aggregates associated with AD and tauopathies (so-called neurofibrillary tangles and neuropil threads).Specific tauopathies are linked with different fibril conformations in which the repeat domains and the 10 to 13 amino acids following them adopt distinct amyloid folds. [95]In healthy  Clusterin levels are elevated in AD, facilitating its interaction with these aggregates.Aβ-Clusterin complexes may be internalized by brain cells via receptor-mediated endocytosis using potential Clusterin receptors, including LRP1, HSPGs, VLDLR, ApoER2, Plexin A4 (neurons, astrocytes and microglia), LRP2 (neurons and astrocytes), and TREM2 (microglia), followed by lysosomal degradation.This function of Clusterin is mainly beneficial, but may also facilitate the uptake of potentially toxic Aβ oligomers that Clusterin is unable to neutralize. [20,32,56,61,62,64]Tau aggregates may be stabilized by Clusterin in a seeding-competent state. [14]These complexes may also be internalized in the same way as Aβ complexes by receptor-mediated endocytosis using Clusterin receptors.In addition, glial cells take up aggregates by phagocytosis.When the lysosomal system is overwhelmed, degradation of Tau-Clusterin complexes becomes inefficient.Tau seeds may escape from endocytic vesicles and template aggregation of native Tau in neurons. [14]Incomplete digestion of aggregates by glial cells can lead to their secretion via exosomes, promoting spreading.In contrast, Clusterin neutralizes α-Synuclein seeds and therefore, α-Synuclein-Clusterin complexes are unable to template aggregation of endogenous α-Synuclein [14] cells, quality control machineries including chaperones HSP40, HSP70, HSP90, and sHSPs normally function in preventing Tau aggregation, but these mechanisms apparently fail in disease. [96]usterin has been shown to interfere with Tau aggregation in vitro by extending the lag phase of fibril formation and slowing fibril elongation. [14,35,36]However, as Tau aggregation is an intracellular process, it would be unlikely to be affected by secreted Clusterin.
Yet intracellular Clusterin, accumulating under stress conditions, could have a role in modulating Tau aggregation, consistent with a recent study reporting aggravated Tau pathology in CLU KO mice. [35]gardless of a possible direct effect on aggregation of intracellular Tau, we have recently made the surprising observation that Clusterin can bind and stabilize Tau oligomers competent in seeding aggregates of endogenous Tau upon uptake by neurons and cells in culture. [14]is effect was specific to Tau, as Clusterin neutralized aggregate seeds of α-Synuclein.
For Clusterin acting as a possible enhancer of the prion-like spreading of Tau pathology, Tau aggregates have to be accessible in the extracellular space.Tau and other neurodegenerative disease proteins may reach the extracellular environment via release from dying cells or upon active secretion by neurons, which may occur through a transsynaptic mechanism [8] or in a manner facilitated by chaperones, such as the HSP40 protein DnaJC5. [97,98]Indeed, seeding-competent Tau species have been detected in the CSF of AD patients [10,11] and Clusterin binding to Tau in patient brain has been reported. [68,99,100]Of note, the concentrations of Clusterin used in the in vitro experiments 15211878, 2022, 7, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/bies.202100287,Wiley Online Library on [11/06/2024].See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions)on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License demonstrating stabilization of Tau aggregate seeds [14] were substantially higher (in the micromolar range) than those in CSF (nanomolar) and thus further studies using CSF samples from AD patients will be useful in assessing the effect of Clusterin on Tau aggregation and seeding under more physiologically relevant conditions.

ROLE OF CLUSTERIN IN AGGREGATE CLEARANCE
Soluble extracellular waste, including oligomeric and prefibrillar amyloid species, is removed from the brain by various clearance systems.
Extracellular proteins such as Aβ can be degraded by extracellular proteases or internalized by glial cells or neurons, followed by degradation via the lysosomal pathway.In addition, clearance of Aβ and possibly Tau through the blood brain barrier is an important mechanism to prevent aggregate accumulation in the brain. [101]ial cells, such as microglia and astrocytes, have a pivotal role in brain homeostasis, supporting neuronal function and survival.They are also key regulators of inflammation in the CNS, a condition often associated with neurodegenerative pathologies (see below).
Reactive microglia and astrocytes are located near Aβ plaques and Tau inclusions [102] and are thought to mediate aggregate clearance through phagocytosis as well as fluid-phase and receptor mediated endocytosis, while neurons internalize oligomers and fibrillar species of Aβ and Tau only through endocytosis [103][104][105][106][107][108][109] (Figure 3).Notably, a fraction of internalized aggregates may escape from the endolysosomal pathway to the cytoplasm where they can act as seeds in templating aggregation of endogenous native protein [14,110,111] (Figure 1 and 3).][114][115][116][117][118] The role of glial cells in aggregate propagation could relate to failed attempts at degradation, as incomplete degradation of aggregates has been suggested to promote spreading. [103,114]usterin binds to extracellular aggregates and promotes their clearance via receptor mediated endocytosis. [14,99,119,120]Accordingly, high levels of Clusterin have been suggested to be beneficial in PD, [14,37,121] consistent with findings that Clusterin efficiently interferes with aggregate seeding of α-Synuclein and its toxic effects [14,37] (Figure 3).In contrast, a fraction of Clusterin-Tau complexes was found to escape from endosomes upon uptake by HEK cells and cultured neurons to induce the aggregation of endogenous Tau [14] (Figure 3).The mechanism of Clusterin internalization is not yet clear.Multiple potential receptors expressed on brain cells have been implicated in mediating Clusterin uptake, including scavenger receptors, [119] heparan sulfate proteoglycans (HSPGs), [120] apolipoprotein E receptor 2 (ApoER2), very low density lipoprotein receptor (VLDLR), [122] triggering receptor expressed on myeloid cells 2 (TREM2), [47] Plexin A4, [123] and low density lipoprotein receptor-related proteins 1 and 2 (LRP1 and LRP2) [124][125][126][127] (Figure 3).TREM2 and PLXNA4 (encoding Plexin A4) are both also risk factors for LOAD. [69,128]TREM2 variants linked to LOAD present with impaired binding and uptake of Clusterin-Aβ complexes, suggesting a protective role of TREM2 in Aβ clearance via Clusterin. [47]Recent research has identified LRP1 as an endocytic receptor for Tau uptake. [108,129]Interestingly, while LRP1 was shown to efficiently bind and internalize monomeric Tau for lysosomal degradation, it promoted seeding of endogenous Tau aggregation by uptake of pathological Tau forms. [129]Stabilization of Tau seeds by Clusterin may conceivably exacerbate this effect.
In summary, receptor mediated-endocytosis of Clusterin-client protein complexes by neurons and glial cells may effectively clear α-Synuclein and Aβ aggregates, but in the case of Tau may be associated with the detrimental side effect of promoting aggregate propagation.

FUNCTION OF CLUSTERIN IN NEUROINFLAMMATION
While the neuroinflammatory response contributes to homeostasis maintenance in the brain, a turning point in AD pathology is the transition from the physiological role of inflammation to chronic, maladaptive activation triggered by Aβ and Tau aggregation. [130]Several risk genes for LOAD, including CLU, are involved in regulating the immune response, providing support for the critical role of neuroinflammation in AD pathogenesis. [131]An upregulation of inflammation-related genes in the brain is generally observed during normal aging, consistent with age being the primary risk factor for developing AD. [130]croglia and astrocytes are key mediators of neuroinflammation in the CNS.These cells undergo transcriptional, morphological, and functional changes and release pro-or anti-inflammatory cytokines in response to external stimuli, such as the presence of protein aggregates. [132]Clusterin is mainly expressed in the brain by astrocytes [133] (Figure 3) and its expression is positively regulated by several cytokines, including the anti-inflammatory TGF-β [134] and the pro-inflammatory IL-1β, [135] which are secreted by activated microglia. [132]Thus, inflammation is likely one of the triggers of Clusterin overexpression in AD.Interestingly, Clusterin in turn seems to directly activate microglia, which would result in a positive feedback loop contributing to maintaining the chronic state of microglial activation observed in AD. [136] Interestingly, activated microglia appear to be more efficient in Tau internalization and promoting spreading than quiescent microglia, [105,113] and microglial activation correlates with propagation of Tau pathology. [137]usterin exerts anti-inflammatory effects mainly by suppressing complement activation. [42,43]Because synapse pruning by astrocytes and microglia during development involves the complement system, [138] upregulation of complement proteins in AD has been associated with synapse loss and cognitive decline. [139]Accordingly, inhibition of the complement system by Clusterin may be neuroprotective.
In support of this interpretation, astroglial overexpression of Clusterin rescued synapse loss in CLU KO mice and reduced Aβ pathology and synaptic deficits in the 5x familial AD (5xFAD) mouse model. [60]ile similar basic mechanisms underlie the cellular pathology of several neurodegenerative diseases associated with protein aggregation, the specific effects of Clusterin-whether protective or potentially harmful-may vary.The general trend of Clusterin to be elevated

CONCLUSIONS
The abundant extracellular chaperone Clusterin has become of major interest in recent years, especially due to its association with AD, a connection that remains incompletely understood.Clusterin is upregulated in AD and several other neurodegenerative diseases where it colocalizes with the pathognomonic amyloid deposits.Clusterin modulates disease mechanism in a complex manner, including aggregation prevention, promoting aggregate clearance and anti-inflammatory effects.However, these protective functions may eventually fail, for instance when clearance mechanisms are overwhelmed, then possibly allowing undesired activities of Clusterin in stabilizing seedingcompetent aggregates to come to the fore.Our recent findings from cell culture models indicate that while Clusterin efficiently interferes with α-Synuclein aggregation and aggregate propagation, it can potentiate the seeding-activity of Tau aggregates, enhancing the conversion of endogenous Tau into toxic aggregates upon uptake of Clusterinbound Tau seeds by recipient cells (Figure 3).These results in combination with previous findings support the view that Clusterin is a Janusfaced chaperone, [18][19][20] having both beneficial functions in suppressing aggregation and potentially detrimental activities in promoting aggregate pathology, dependent on specific disease context.Future studies employing organoids and animal models will be required to define the effect of Clusterin on Tau aggregate spreading in disease.These experimental models will also help to understand the contribution of different brain cell types, such as glial cells, to aggregate spreading.
Finally, cellular and animal models combining Aβ plaques and Tau tangles would provide most relevant insight into the complex role of Clusterin in AD.

F I G U R E 2
Biogenesis of Clusterin and predicted structure.(A) Clusterin is synthesized as a precursor protein of 449 amino acids containing a 22 amino acid signal peptide (brown) that is cleaved during translocation into the endoplasmic reticulum (ER).Once in the ER, N-glycosylation (gray circles) and formation of five intramolecular disulfide bonds (black lines) is thought to occur, resulting in pre-secretory Clu (psClu).

F I G U R E 3
Differential effects of Clusterin on Aβ, Tau, and α-Synuclein (α-Syn) aggregates.Clusterin (CLU, red, mainly secreted by astrocytes) may interact with Aβ oligomers and deposits and with Tau and α-Synuclein aggregates that have been released into the extracellular space.