Clusterin (SGP-2): a multifunctional glycoprotein with regional expression in astrocytes and neurons of the adult rat brain

Neuroprotective astroglial response to neural damage and its relevance to affective disorders

Astrocytes not only support neuronal function with essential roles in synaptic neurotransmission, action potential propagation, metabolic support, or neuroplastic and developmental adaptations. They also respond to damage or dysfunction in surrounding neurons and oligodendrocytes by releasing neurotrophic factors and other molecules that increase the survival of the supported cells or contribute to mechanisms of structural and molecular restoration. The neuroprotective responsiveness of astrocytes is based on their ability to sense signals of degeneration, metabolic jeopardy and structural damage, and on their aptitude to locally deliver specific molecules to remedy threats to the molecular and structural features of their cellular partners. To the extent that neuronal and other glial cell disturbances are known to occur in affective disorders, astrocyte responsiveness to those disturbances may help to better understand the roles astrocytes play in affective disorders. The astrocytic sensing apparatus supporting those responses involves receptors for neurotransmitters, purines, cell adhesion molecules and growth factors. Astrocytes also share with the immune system the capacity of responding to cytokines released upon neuronal damage. In addition, in responses to specific signals astrocytes release unique factors such as clusterin or humanin that have been shown to exert potent neuroprotective effects. Astrocytes integrate the signals above to further deliver structural lipids, removing toxic metabolites, stabilizing the osmotic environment, normalizing neurotransmitters, providing anti-oxidant protection, facilitating synaptogenesis and acting as barriers to contain varied deleterious signals, some of which have been described in brain regions relevant to affective disorders and related animal models. Since various of the injurious signals that activate astrocytes have been implicated in different aspects of the etiopathology of affective disorders, particularly in relation to the diagnosis of depression, potentiating the corresponding astrocyte neuroprotective responses may provide additional opportunities to improve or complement available pharmacological and behavioral therapies for affective disorders.

Astrocytic response mediated by the CLU risk allele inhibits OPC proliferation and myelination in a human iPSC model

The C allele of rs11136000 variant in the clusterin (CLU) gene represents the third strongest known genetic risk factor for late-onset Alzheimer's disease. However, whether this single-nucleotide polymorphism (SNP) is functional and what the underlying mechanisms are remain unclear. In this study, the CLU rs11136000 SNP is identified as a functional variant by a small-scale CRISPR-Cas9 screen. Astrocytes derived from isogenic induced pluripotent stem cells (iPSCs) carrying the "C" or "T" allele of the CLU rs11136000 SNP exhibit different CLU expression levels. TAR DNA-binding protein-43 (TDP-43) preferentially binds to the "C" allele to promote CLU expression and exacerbate inflammation. The interferon response and CXCL10 expression are elevated in cytokine-treated C/C astrocytes, leading to inhibition of oligodendrocyte progenitor cell (OPC) proliferation and myelination. Accordingly, elevated CLU and CXCL10 but reduced myelin basic protein (MBP) expression are detected in human brains of C/C carriers. Our study uncovers a mechanism underlying reduced white matter integrity observed in the CLU rs11136000 risk "C" allele carriers.

Association of circulating markers with cognitive decline after radiation therapy for brain metastasis

BACKGROUND

A recent phase III trial (NCT01372774) comparing use of stereotactic radiosurgery [SRS] versus whole-brain radiation therapy [WBRT] after surgical resection of a single brain metastasis revealed that declines in cognitive function were more common with WBRT than with SRS. A secondary endpoint in that trial, and the primary objective in this secondary analysis, was to identify baseline biomarkers associated with cognitive impairment after either form of radiotherapy for brain metastasis. Here we report our findings on APOE genotype and serum levels of associated proteins and their association with radiation-induced neurocognitive decline.

METHODS

In this retrospective analysis of prospectively collected samples from a completed randomized clinical trial, patients provided blood samples every 3 months that were tested by genotyping and enzyme-linked immunosorbent assay, and results were analyzed in association with cognitive impairment.

RESULTS

The APOE genotype was not associated with neurocognitive impairment at 3 months. However, low serum levels of ApoJ, ApoE, or ApoA protein (all P < .01) and higher amyloid beta (Aβ 1-42) levels (P = .048) at baseline indicated a greater likelihood of neurocognitive decline at 3 months after SRS, whereas lower ApoJ levels were associated with decline after WBRT (P = .014).

CONCLUSIONS

Patients with these pretreatment serum markers should be counseled about radiation-related neurocognitive decline.

Physical exercise modulates the microglial complement pathway in mice to relieve cortical circuitry deficits induced by mutant human TDP-43

The aggregation of TAR DNA binding protein 43 kDa (TDP-43) is related to different neurodegenerative diseases, which leads to microglial activation and neuronal loss. The molecular mechanism driving neuronal death by reactive microglia, however, has not been completely resolved. In this study, we generated a mouse model by overexpressing mutant human TDP-43 (M337V) in the primary motor cortex, leading to prominent motor-learning deficits. In vivo 2-photon imaging shows an active approach of microglia toward parvalbumin interneurons, resulting in disrupted cortical excitatory-inhibitory balance. Proteomics studies suggest that activation of the complement pathway induces microglial activity. To develop an early interventional strategy, treadmill exercise successfully prevents the deterioration of motor dysfunction under enhanced adipocytic release of clusterin to block the complement pathway. These results demonstrate a previously unrecognized pathway by which TDP-43 induces cortical deficits and provide additional insights for the mechanistic explanation of exercise training in disease intervention.

Axonal injury is detected by βAPP immunohistochemistry in rapid death from head injury following road traffic collision

The accumulation of βAPP caused by axonal injury is an active energy-dependent process thought to require blood circulation; therefore, it is closely related to the post-injury survival time. Currently, the earliest reported time at which axonal injury can be detected in post-mortem traumatic brain injury (TBI) tissue by βAPP (Beta Amyloid Precursor Protein) immunohistochemistry is 35 min. The aim of this study is to investigate whether βAPP staining for axonal injury can be detected in patients who died rapidly after TBI in road traffic collision (RTC), in a period of less than 30 min.

We retrospectively studied thirty-seven patients (group 1) died very rapidly at the scene; evidenced by forensic assessment of injuries short survival, four patients died after a survival period of between 31 min and 12 h (group 2) and eight patients between 2 and 31 days (group 3). The brains were comprehensively examined and sampled at the time of the autopsy, and βAPP immunohistochemistry carried out on sections from a number of brain areas.

βAPP immunoreactivity was demonstrated in 35/37 brains in group 1, albeit with a low frequency and in a variable pattern, and with more intensity and frequency in all brains of group 2 and 7/8 brains from group 3, compared with no similar βAPP immunoreactivity in the control group. The results suggest axonal injury can be detected in those who died rapidly after RTC in a period of less than 30 min, which can help in the diagnosis of severe TBI with short survival time.

Lipoproteins in the Central Nervous System: From Biology to Pathobiology

The brain, as one of the most lipid-rich organs, heavily relies on lipid transport and distribution to maintain homeostasis and neuronal function. Lipid transport mediated by lipoprotein particles, which are complex structures composed of apolipoproteins and lipids, has been thoroughly characterized in the periphery. Although lipoproteins in the central nervous system (CNS) were reported over half a century ago, the identification of APOE4 as the strongest genetic risk factor for Alzheimer's disease has accelerated investigation of the biology and pathobiology of lipoproteins in the CNS. This review provides an overview of the different components of lipoprotein particles, in particular apolipoproteins, and their involvements in both physiological functions and pathological mechanisms in the CNS. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Genetic Insights into the Impact of Complement in Alzheimer's Disease

The presence of complement activation products at sites of pathology in post-mortem Alzheimer's disease (AD) brains is well known. Recent evidence from genome-wide association studies (GWAS), combined with the demonstration that complement activation is pivotal in synapse loss in AD, strongly implicates complement in disease aetiology. Genetic variations in complement genes are widespread. While most variants individually have only minor effects on complement homeostasis, the combined effects of variants in multiple complement genes, referred to as the "complotype", can have major effects. In some diseases, the complotype highlights specific parts of the complement pathway involved in disease, thereby pointing towards a mechanism; however, this is not the case with AD. Here we review the complement GWAS hits; CR1 encoding complement receptor 1 (CR1), CLU encoding clusterin, and a suggestive association of C1S encoding the enzyme C1s, and discuss difficulties in attributing the AD association in these genes to complement function. A better understanding of complement genetics in AD might facilitate predictive genetic screening tests and enable the development of simple diagnostic tools and guide the future use of anti-complement drugs, of which several are currently in development for central nervous system disorders.

SerpinA3N attenuates ischemic stroke injury by reducing apoptosis and neuroinflammation

Objective

To assess the effect of serine protein inhibitor A3N (serpinA3N) in ischemic stroke and to explore its mechanism of action.

Methods

Mouse ischemic stroke model was induced by transient middle cerebral artery occlusion followed by reperfusion. The expression pattern of serpinA3N was assessed using immunofluorescence, Western blot analysis, and real‐time quantitative PCR. An adeno‐associated virus (AAV) and recombinant serpinA3N were administered. Additionally, co‐immunoprecipitation‐mass spectrometry and immunofluorescence co‐staining were used to identify protein interactions.

Results

SerpinA3N was upregulated in astrocytes and neurons within the ischemic penumbra after stroke in the acute phase. The expression of serpinA3N gradually increased 6 h after reperfusion, peaked on the day 2–3, and then decreased by day 7. Overexpression of serpinA3N by AAV significantly reduced the infarct size and improved motor function, associated with alleviated inflammation and oxidative stress. SerpinA3N treatment also reduced apoptosis both in vivo and in vitro. Co‐immunoprecipitation‐mass spectrometry and Western blotting revealed that clusterin interacts with serpinA3N, and Akt‐mTOR pathway members were upregulated by serpinA3N both in vivo and in vitro.

Conclusions

SerpinA3N is expressed in astrocytes and penumbra neurons after stroke in mice. It reduces brain damage possibly via interacting with clusterin and inhibiting neuronal apoptosis and neuroinflammation.

Untangling huge literature to disinter genetic underpinnings of Alzheimer's Disease: A systematic review and meta-analysis

Drug discovery for Alzheimer's Disease (AD) is channeled towards unravelling key disease specific drug targets/genes to predict promising therapeutic candidates. Though enormous literature on AD genetics is available, there exists dearth in data pertinent to drug targets and crucial pathological pathways intertwined in disease progression. Further, the research findings revealing genetic associations failed to demonstrate consistency across different studies. This scenario prompted us to initiate a systematic review and meta-analysis with an aim of unearthing significant genetic hallmarks of AD. Initially, a Boolean search strategy was developed to retrieve case-control studies from PubMed, Cochrane, ProQuest, Europe PMC, grey literature and HuGE navigator. Subsequently, certain inclusion and exclusion criteria were framed to shortlist the relevant studies. These studies were later critically appraised using New Castle Ottawa Scale and Q-Genie followed by data extraction. Later, meta-analysis was performed only for those Single Nucleotide Polymorphisms (SNPs) which were evaluated in at least two different ethnicities from two different reports. Among, 204,351 studies retrieved, 820 met our eligibility criteria and 117 were processed for systematic review after critical appraisal. Ultimately, meta-analysis was performed for 23 SNPs associated with 15 genes which revealed significant associations of rs3865444 (CD33), rs7561528 (BIN1) and rs1801133 (MTHFR) with AD risk.

The Cerebellar Dopaminergic System

In the central nervous system (CNS), dopamine (DA) is involved in motor and cognitive functions. Although the cerebellum is not been considered an elective dopaminergic region, studies attributed to it a critical role in dopamine deficit-related neurological and psychiatric disorders [e.g., Parkinson's disease (PD) and schizophrenia (SCZ)]. Data on the cerebellar dopaminergic neuronal system are still lacking. Nevertheless, biochemical studies detected in the mammalians cerebellum high dopamine levels, while chemical neuroanatomy studies revealed the presence of midbrain dopaminergic afferents to the cerebellum as well as wide distribution of the dopaminergic receptor subtypes (DRD1-DRD5). The present review summarizes the data on the cerebellar dopaminergic system including its involvement in associative and projective circuits. Furthermore, this study also briefly discusses the role of the cerebellar dopaminergic system in some neurologic and psychiatric disorders and suggests its potential involvement as a target in pharmacologic and non-pharmacologic treatments.

Extracellular clusterin limits the uptake of α-synuclein fibrils by murine and human astrocytes

The progressive neuropathological damage seen in Parkinson's disease (PD) is thought to be related to the spreading of aggregated forms of α-synuclein. Clearance of extracellular α-synuclein released by degenerating neurons may be therefore a key mechanism to control the concentration of α-synuclein in the extracellular space. Several molecular chaperones control misfolded protein accumulation in the extracellular compartment. Among these, clusterin, a glycoprotein associated with Alzheimer's disease, binds α-synuclein aggregated species and is present in Lewy bodies, intraneuronal aggregates mainly composed by fibrillary α-synuclein. In this study, using murine primary astrocytes with clusterin genetic deletion, human-induced pluripotent stem cell (iPSC)-derived astrocytes with clusterin silencing and two animal models relevant for PD we explore how clusterin affects the clearance of α-synuclein aggregates by astrocytes. Our findings showed that astrocytes take up α-synuclein preformed fibrils (pffs) through dynamin-dependent endocytosis and that clusterin levels are modulated in the culture media of cells upon α-synuclein pffs exposure. Specifically, we found that clusterin interacts with α-synuclein pffs in the extracellular compartment and the clusterin/α-synuclein complex can be internalized by astrocytes. Mechanistically, using clusterin knock-out primary astrocytes and clusterin knock-down hiPSC-derived astrocytes we observed that clusterin limits the uptake of α-synuclein pffs by cells. Interestingly, we detected increased levels of clusterin in the adeno-associated virus- and the α-synuclein pffs- injected mouse model, suggesting a crucial role of this chaperone in the pathogenesis of PD. Overall, our observations indicate that clusterin can limit the uptake of extracellular α-synuclein aggregates by astrocytes and, hence, contribute to the spreading of Parkinson pathology.

The role of complement in brain injury following intracerebral hemorrhage: A review

Intracerebral hemorrhage (ICH) is a significant cause of death and disability and current treatment is limited to supportive measures to reduce brain edema and secondary hematoma expansion. Current evidence suggests that the complement cascade is activated early after hemorrhage and contributes to brain edema/injury in multiple ways. The aim of this review is to summarize the most recent literature about the role of the complement cascade after ICH. Primary literature demonstrating complement mediated brain edema and neurologic injury through the membrane attack complex (MAC) as well as C3a and C5a are reviewed. Further, attenuation of brain edema and improved functional outcomes are demonstrated after inhibition of specific components of the complement cascade. Conversely, complement also plays a significant role in neurologic recovery after ICH and in other neurologic disorders. We conclude that the role of complement after ICH is complex. Understanding the role of complement after ICH is essential and may elucidate possible interventions to reduce brain edema and injury.

Clusterin secreted from astrocyte promotes excitatory synaptic transmission and ameliorates Alzheimer's disease neuropathology

Background

Genome-wide association studies have established clusterin (CLU) as a genetic modifier for late-onset Alzheimer’s disease (AD). Both protective and risk alleles have been identified which may be associated with its expression levels. However, the physiological function of clusterin in the central nervous system remains largely unknown.

Methods

We examined Clu expression in mouse brains by immunohistochemistry and high-resolution imaging. We performed electrophysiological recordings and morphological analysis of dendritic spines in wild-type and Clu knockout mice. We tested synaptic function of astrocytic Clu using neuron-glia co-cultures and by AAV-mediated astroglial Clu expression in vivo. Finally, we investigated the role of astrocytic Clu on synaptic properties and amyloid pathology in 5xFAD transgenic mouse model of AD.

Results

We show that astrocyte secreted Clu co-localizes with presynaptic puncta of excitatory neurons. Loss of Clu led to impaired presynaptic function and reduced spine density in vivo. Neurons co-cultured with Clu-overexpressing astrocytes or treated with conditioned media from HEK293 cells transfected with Clu displayed enhanced excitatory neurotransmission. AAV-mediated astroglial Clu expression promoted excitatory neurotransmission in wild-type mice and rescued synaptic deficits in Clu knockout mice. Overexpression of Clu in the astrocytes of 5xFAD mice led to reduced Aβ pathology and fully rescued the synaptic deficits.

Conclusion

We identify Clu as an astrocyte-derived synaptogenic and anti-amyloid factor; the combination of these activities may influence the progression of late-onset AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00426-7.

Cell Culture Experiments Reveal that High S100B and Clusterin Levels may Convey Hypoxia-tolerance to the Hooded Seal (Cystophora cristata) Brain

While the brain of most mammals suffers from irreversible damage after only short periods of low oxygen levels (hypoxia), marine mammals are excellent breath-hold divers that have adapted to hypoxia. In addition to physiological adaptations, such as large oxygen storing capacity and strict oxygen economy during diving, the neurons of the deep-diving hooded seal (Cystophora cristata) have an intrinsic tolerance to hypoxia. We aim to understand the molecular basis of this neuronal hypoxia tolerance. Previously, transcriptomics of the cortex of the hooded seal have revealed remarkably high expression levels of S100B and clusterin (apolipoprotein J) when compared to the ferret, a non-diving carnivore. Both genes have much-debated roles in hypoxia and oxidative stress. Here, we evaluated the effects of S100B and of two isoforms of clusterin (soluble and nucleus clusterin) on the survival, metabolic activity and the amount of reactive oxygen species (ROS) in HN33 neuronal mouse cells exposed to hypoxia and oxidative stress. S100B and soluble clusterin had neuroprotective effects, with reduced ROS-levels and retention of normoxic energy status of cells during both stress conditions. The protective effects of nucleus clusterin were restricted to hypoxia. S100B and clusterin showed purifying selection in marine and terrestrial mammals, indicating a functional conservation across species. Immunofluorescence revealed identical cellular distributions of S100B and clusterin in mice, ferrets and hooded seals, further supporting the functional conservation. Taken together, our data suggest that the neuroprotective effects of all three proteins are exclusively facilitated by their increased expression in the brain of the hooded seal.

Plasma Clusterin as a Potential Link Between Diabetes and Alzheimer Disease

OBJECTIVE

Plasma clusterin, a promising biomarker of Alzheimer disease (AD), has been associated with diabetes mellitus (DM). However, clusterin has not been investigated considering a relationship with both DM and AD. In this study, we aimed to investigate the individual and interactive relationships of plasma clusterin levels with both diseases.

DESIGN

Cross-sectional observation study.

METHODS

We classified participants by the severity of cognitive (normal cognition, mild cognitive impairment [MCI], and AD) and metabolic (healthy control, prediabetes, and DM) impairments. We evaluated the cognitive and metabolic functions of the participants with neuropsychological assessments, brain magnetic resonance imaging, and various blood tests, to explore potential relationships with clusterin.

RESULTS

Plasma clusterin levels were higher in participants with AD and metabolic impairment (prediabetes and DM). A two-way ANCOVA revealed no synergistic, but an additive effect of AD and DM on clusterin. Clusterin was negatively correlated with cognitive scores. It was also associated with metabolic status indicated by glycated hemoglobin A1c (HbA1c), the Homeostatic Model Assessment for Insulin Resistance index, and fasting C-peptide. It showed correlations between medial temporal atrophy and periventricular white matter lesions, indicating neurodegeneration and microvascular insufficiency, respectively. Further mediation analysis to understand the triadic relationship between clusterin, AD, and DM revealed that the association between DM and AD was significant when clusterin is considered as a mediator of their relationship.

CONCLUSIONS

Clusterin is a promising biomarker of DM as well as of AD. Additionally, our data suggest that clusterin may have a role in linking DM with AD as a potential mediator.

Pulsed SILAM Reveals In Vivo Dynamics of Murine Brain Protein Translation

Identification of proteins that are synthesized de novo in response to specific microenvironmental cues is critical for understanding molecular mechanisms that underpin vital physiological processes and pathologies. Here, we report that a brief period of SILAM (Stable Isotope Labeling of Mammals) diet enables the determination of biological functions corresponding to actively translating proteins in the mouse brain. Our results demonstrate that the synapse, dendrite, and myelin sheath are highly active neuronal structures that display rapid protein synthesis, producing key mediators of chemical signaling as well as nutrient sensing, lipid metabolism, and amyloid precursor protein processing/stability. Together, these findings confirm that protein metabolic activity varies significantly between brain functional units in vivo. Our data indicate that pulsed SILAM approaches can unravel complex protein expression dynamics in the murine brain and identify active synthetic pathways and associated functions that are likely impaired in neurodegenerative diseases.

Brain clusterin protein isoforms and mitochondrial localization

Clusterin (CLU), or apolipoprotein J (ApoJ), is the third most predominant genetic risk factor associated with late-onset Alzheimer’s disease (LOAD). In this study, we use multiple rodent and human brain tissue and neural cell models to demonstrate that CLU is expressed as multiple isoforms that have distinct cellular or subcellular localizations in the brain. Of particular significance, we identify a non-glycosylated 45 kDa CLU isoform (mitoCLU) that is localized to the mitochondrial matrix and expressed in both rodent and human neurons and astrocytes. In addition, we show that rodent mitoCLU is translated from a non-canonical CUG (Leu) start site in Exon 3, a site that coincides with an AUG (Met) in human CLU. Last, we reveal that mitoCLU is present at the gene and protein level in the currently available CLU^–/– mouse model. Collectively, these data provide foundational knowledge that is integral in elucidating the relationship between CLU and the development of LOAD.

α-synuclein-lipoprotein interactions and elevated ApoE level in cerebrospinal fluid from Parkinson's disease patients

Two of the most important issues in Parkinson’s disease (PD) research are the identification of mechanisms underlying α-synuclein cell-to-cell transfer in the nervous system and the discovery of early diagnostic biomarkers. Both of these issues are addressed in our current manuscript. Using multiple approaches, we present that α-synuclein interacts with lipoproteins within human cerebrospinal fluid and can be taken up by cells in such a state. Moreover, using cerebrospinal fluid samples from 3 large and independent cohorts of patients, we demonstrate that apolipoprotein E is elevated in early, not yet medicated, patients with PD. Finally, using postmortem brain tissue, we provide preliminary histological evidence that apolipoprotein E is enriched in a subpopulation of dopaminergic neurons of human substantia nigra.

The progressive accumulation, aggregation, and spread of α-synuclein (αSN) are common hallmarks of Parkinson’s disease (PD) pathology. Moreover, numerous proteins interact with αSN species, influencing its toxicity in the brain. In the present study, we extended analyses of αSN-interacting proteins to cerebrospinal fluid (CSF). Using coimmunoprecipitation, followed by mass spectrometry, we found that αSN colocalize with apolipoproteins on lipoprotein vesicles. We confirmed these interactions using several methods, including the enrichment of lipoproteins with a recombinant αSN, and the subsequent uptake of prepared vesicles by human dopaminergic neuronal-like cells. Further, we report an increased level of ApoE in CSF from early PD patients compared with matched controls in 3 independent cohorts. Moreover, in contrast to controls, we observed the presence of ApoE-positive neuromelanin-containing dopaminergic neurons in substantia nigra of PD patients. In conclusion, the cooccurrence of αSN on lipoprotein vesicles, and their uptake by dopaminergic neurons along with an increase of ApoE in early PD, proposes a mechanism(s) for αSN spreading in the extracellular milieu of PD.

Cholesterol 24-Hydroxylation by CYP46A1: Benefits of Modulation for Brain Diseases

Cholesterol 24-hydroxylation is the major mechanism for cholesterol removal from the brain and the reaction catalyzed by cytochrome P450 46A1 (CYP46A1), a CNS-specific enzyme. This review describes CYP46A1 in the context of cholesterol homeostasis in the brain and summarizes available experimental data on CYP46A1 association with different neurologic diseases, including the mechanisms by which changes in the CYP46A1 activity in the brain could be beneficial for these diseases. The modulation of CYP46A1 activity by genetic and pharmacologic means is also presented along with a brief synopsis of the two clinical trials that evaluate CYP46A1 as a therapeutic target for Alzheimer's disease as well as Dravet and Lennox-Gastaut syndromes.

Knockout of apolipoprotein A-I decreases parenchymal and vascular β-amyloid pathology in the Tg2576 mouse model of Alzheimer's disease

AIMS

Apolipoprotein A-I (apoA-I), the principal apolipoprotein associated with high-density lipoproteins in the periphery, is also found at high concentrations in the cerebrospinal fluid. Previous studies have reported either no impact or vascular-specific effects of apoA-I knockout (KO) on β-amyloid (Aβ) pathology. However, the putative mechanism(s) by which apoA-I may influence Aβ deposition is unknown.

METHODS

We evaluated the effect of apoA-I deletion on Aβ pathology, Aβ production and clearance from the brain in the Tg2576 mouse model of Alzheimer's disease (AD).

RESULTS

Contrary to previous reports, deletion of the APOA1 gene significantly reduced concentrations of insoluble Aβ40 and Aβ42 and reduced plaque load in both the parenchyma and blood vessels of apoA-I KO × Tg2576 mice compared to Tg2576 animals. This was not due to decreased Aβ production or alterations in Aβ species. Levels of soluble clusterin/apoJ were significantly higher in neurons of apoA-I KO mice compared to both wildtype (WT) and apoA-I KO × Tg2576 mice. In addition, clearance of Aβ along intramural periarterial drainage pathways was significantly higher in apoA-I KO mice compared to WT animals.

CONCLUSION

These data suggest that deletion of apoA-I is associated with increased clearance of Aβ and reduced parenchymal and vascular Aβ pathology in the Tg2576 model. These results suggest that peripheral dyslipidaemia can modulate the expression of apolipoproteins in the brain and may influence Aβ clearance and aggregation in AD.

Clusterin in Alzheimer's Disease: Mechanisms, Genetics, and Lessons From Other Pathologies

Clusterin (CLU) or APOJ is a multifunctional glycoprotein that has been implicated in several physiological and pathological states, including Alzheimer's disease (AD). With a prominent extracellular chaperone function, additional roles have been discussed for clusterin, including lipid transport and immune modulation, and it is involved in pathways common to several diseases such as cell death and survival, oxidative stress, and proteotoxic stress. Although clusterin is normally a secreted protein, it has also been found intracellularly under certain stress conditions. Multiple hypotheses have been proposed regarding the origin of intracellular clusterin, including specific biogenic processes leading to alternative transcripts and protein isoforms, but these lines of research are incomplete and contradictory. Current consensus is that intracellular clusterin is most likely to have exited the secretory pathway at some point or to have re-entered the cell after secretion. Clusterin's relationship with amyloid beta (Aβ) has been of great interest to the AD field, including clusterin's apparent role in altering Aβ aggregation and/or clearance. Additionally, clusterin has been more recently identified as a mediator of Aβ toxicity, as evidenced by the neuroprotective effect of CLU knockdown and knockout in rodent and human iPSC-derived neurons. CLU is also the third most significant genetic risk factor for late onset AD and several variants have been identified in CLU. Although the exact contribution of these variants to altered AD risk is unclear, some have been linked to altered CLU expression at both mRNA and protein levels, altered cognitive and memory function, and altered brain structure. The apparent complexity of clusterin's biogenesis, the lack of clarity over the origin of the intracellular clusterin species, and the number of pathophysiological functions attributed to clusterin have all contributed to the challenge of understanding the role of clusterin in AD pathophysiology. Here, we highlight clusterin's relevance to AD by discussing the evidence linking clusterin to AD, as well as drawing parallels on how the role of clusterin in other diseases and pathways may help us understand its biological function(s) in association with AD.

Glial activation and inflammation along the Alzheimer's disease continuum

Background

Neuronal and glial cell interaction is essential for synaptic homeostasis and may be affected in Alzheimer’s disease (AD). We measured cerebrospinal fluid (CSF) neuronal and glia markers along the AD continuum, to reveal putative protective or harmful stage-dependent patterns of activation.

Methods

We included healthy controls (n = 36) and Aβ-positive (Aβ+) cases (as defined by pathological CSF amyloid beta 1-42 (Aβ42)) with either subjective cognitive decline (SCD, n = 19), mild cognitive impairment (MCI, n = 39), or AD dementia (n = 27). The following CSF markers were measured: a microglial activation marker—soluble triggering receptor expressed on myeloid cells 2 (sTREM2), a marker of microglial inflammatory reaction—monocyte chemoattractant protein-1 (MCP-1), two astroglial activation markers—chitinase-3-like protein 1 (YKL-40) and clusterin, a neuron-microglia communication marker—fractalkine, and the CSF AD biomarkers (Aβ42, phosphorylated tau (P-tau), total tau (T-tau)). Using ANOVA with planned comparisons, or Kruskal-Wallis tests with Dunn’s pairwise comparisons, CSF levels were compared between clinical groups and between stages of biomarker severity using CSF biomarkers for classification based on amyloid pathology (A), tau pathology (T), and neurodegeneration (N) giving rise to the A/T/N score.

Results

Compared to healthy controls, sTREM2 was increased in SCD (p < .01), MCI (p < .05), and AD dementia cases (p < .001) and increased in AD dementia compared to MCI cases (p < .05). MCP-1 was increased in MCI (p < .05) and AD dementia compared to both healthy controls (p < .001) and SCD cases (p < .01). YKL-40 was increased in dementia compared to healthy controls (p < .01) and MCI (p < .05). All of the CSF activation markers were increased in subjects with pathological CSF T-tau (A+T−N+ and A+T+N+), compared to subjects without neurodegeneration (A−T−N− and A+T−N−).

Discussion

Microglial activation as indicated by increased sTREM2 is present already at the preclinical SCD stage; increased MCP-1 and astroglial activation markers (YKL-40 and clusterin) were noted only at the MCI and AD dementia stages, respectively, and in Aβ+ cases (A+) with pathological T-tau (N+). Possible different effects of early and later glial activation need to be explored.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1399-2) contains supplementary material, which is available to authorized users.

Loss of clusterin shifts amyloid deposition to the cerebrovasculature via disruption of perivascular drainage pathways

Alzheimer's disease (AD) is characterized by amyloid-β (Aβ) peptide deposition in brain parenchyma as plaques and in cerebral blood vessels as cerebral amyloid angiopathy (CAA). CAA deposition leads to several clinical complications, including intracerebral hemorrhage. The underlying molecular mechanisms that regulate plaque and CAA deposition in the vast majority of sporadic AD patients remain unclear. The clusterin (CLU) gene is genetically associated with AD and CLU has been shown to alter aggregation, toxicity, and blood-brain barrier transport of Aβ, suggesting it might play a key role in regulating the balance between Aβ deposition and clearance in both brain and blood vessels. Here, we investigated the effect of CLU on Aβ pathology using the amyloid precursor protein/presenilin 1 (APP/PS1) mouse model of AD amyloidosis on a Clu^+/+ or Clu^-/- background. We found a marked decrease in plaque deposition in the brain parenchyma but an equally striking increase in CAA within the cerebrovasculature of APP/PS1;Clu^-/- mice. Surprisingly, despite the several-fold increase in CAA levels, APP/PS1;Clu^-/- mice had significantly less hemorrhage and inflammation. Mice lacking CLU had impaired clearance of Aβ in vivo and exogenously added CLU significantly prevented Aβ binding to isolated vessels ex vivo. These findings suggest that in the absence of CLU, Aβ clearance shifts to perivascular drainage pathways, resulting in fewer parenchymal plaques but more CAA because of loss of CLU chaperone activity, complicating the potential therapeutic targeting of CLU for AD.

Clusterin expression is upregulated following acute head injury and localizes to astrocytes in old head injury

There is mounting evidence linking traumatic brain injury (TBI) to neurodegeneration. Clusterin (apolipoprotein J or ApoJ) is a complement inhibitor that appears to have a neuroprotective effect in response to tissue damage and has been reported to be upregulated in Alzheimer's disease. Here we investigated the time course and cellular expression pattern of clusterin in human TBI. Tissue from 32 patients with TBI of varying survival times (from under 30 min to 10 months) was examined using immunohistochemistry for clusterin alongside other markers of neurodegeneration and neuroinflammation. TBI cases were compared to ischemic brain damage, Alzheimer's disease and controls. Double immunofluorescence was carried out in order to examine cellular expression. Clusterin was initially expressed in an axonal location less than 30 min following TBI and increased in intensity and the frequency of deposits with increasing survival time up to 24 h, after which it appeared to reduce in intensity but was still evident several weeks after injury. Clusterin was first evident in astrocytes after 45 min, being increasingly seen up to 48 h but remaining intense in TBI cases with long survival times. Our results suggest clusterin plays a role in modulating the inflammatory response of acute and chronic TBI and that it is a useful marker for TBI, particularly in cases with short survival times. Its prominent accumulation in astrocytes, alongside a mounting inflammatory response and activation of microglial cells supports a potential role in the neurodegenerative changes that occur as a result of TBI.

Elevated Expression of the Cerebrospinal Fluid Disease Markers Chromogranin A and Clusterin in Astrocytes of Multiple Sclerosis White Matter Lesions

Using proteomics, we previously identified chromogranin A (CgA) and clusterin (CLU) as disease-related proteins in the cerebrospinal fluid (CSF) of patients with multiple sclerosis (MS). CgA and CLU are involved in cell survival and are implicated in neurodegenerative disorders and may also have roles in MS pathophysiology. We investigated CgA and CLU expression in lesions and nonlesional regions in postmortem brains of MS patients and controls and in the brains of marmosets with experimental autoimmune encephalomyelitis. By quantitative PCR, mRNA levels of CgA and CLU were elevated in white matter but not in grey matter of MS patients. In situ analyses showed greater expression of CgA and CLU in white matter lesions than in normal-appearing regions in MS patients and in the marmosets, primarily in or adjacent to perivascular spaces and inflammatory infiltrates. Both proteins were expressed by glial fibrillary acidic protein-positive astrocytes. CgA was more localized in astrocytic processes and endfeet surrounding blood vessels and was abundant in the superficial glia limitans and ependyma, 2 CSF-brain borders. Increased expression of CgA and CLU in reactive astrocytes in MS white matter lesions supports a role for these molecules as neuro-inflammatory mediators and their potential as CSF markers of active pathological processes in MS patients.

Apo J and Apo D: Complementary or Antagonistic Roles in Alzheimer's Disease?

Apolipoprotein D (Apo D) and Apolipoprotein J (Apo J) are among the only nine apolipoproteins synthesized in the nervous system. Apart from development, these apolipoproteins are implicated in the normal aging process as well as in different neuropathologies as Alzheimer's disease (AD), where a neuroprotective role has been postulated. Different authors have proposed that Apo D and Apo J could be biomarkers for AD but as far as we know, there are no studies about the relationship between them as well as their expression pattern along the progression of the disease. In this paper, using double immunohistochemistry techniques, we have demonstrated that Apo D is mainly located in glial cells while Apo J expression preferentially occurs in neurons; both proteins are also present in AD diffuse and mature senile plaques but without signal overlap. In addition, we have observed that Apo J and Apo D immunostaining shows a positive correlation with the progression of the disease and the Braak's stages. These results suggest complementary and cell-dependent neuroprotective roles for each apolipoprotein during AD progress.

Reduced secreted clusterin as a mechanism for Alzheimer-associated CLU mutations

BACKGROUND

The clusterin (CLU) gene has been identified as an important risk locus for Alzheimer's disease (AD). Although the actual risk-increasing polymorphisms at this locus remain to be identified, we previously observed an increased frequency of rare non-synonymous mutations and small insertion-deletions of CLU in AD patients, which specifically clustered in the β-chain domain of CLU. Nonetheless the pathogenic nature of these variants remained unclear. Here we report a novel non-synonymous CLU mutation (p.I360N) in a Belgian Alzheimer patient and have explored the pathogenic nature of this and 10 additional CLU mutations on protein localization and secretion in vitro using immunocytochemistry, immunodetection and ELISAs.

RESULTS

Three patient-specific CLU mutations in the β-chain (p.I303NfsX13, p.R338W and p.I360N) caused an alteration of the subcellular CLU localization and diminished CLU transport through the secretory pathway, indicative of possible degradation mechanisms. For these mutations, significantly reduced CLU intensity was observed in the Golgi while almost all CLU protein was exclusively present in the endoplasmic reticulum. This was further confirmed by diminished CLU secretion in HEK293T and HEK293 FLp-In cell lines.

CONCLUSIONS

Our data lend further support to the contribution of rare coding CLU mutations in the pathogenesis of neurodegenerative diseases. Functional analyses suggest reduced secretion of the CLU protein as the mode of action for three of the examined CLU mutations. One of those is a frameshift mutation leading to a loss of secreted protein, and the other two mutations are amino acid substitutions in the disulfide bridge region, possibly interfering with heterodimerization of the α- and β-chain of CLU.

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.

Expression of the protein chaperone, clusterin, in spinal cord cells constitutively and following cellular stress, and upregulation by treatment with Hsp90 inhibitor

Clusterin, a protein chaperone found at high levels in physiological fluids, is expressed in nervous tissue and upregulated in several neurological diseases. To assess relevance to amyotrophic lateral sclerosis (ALS) and other motor neuron disorders, clusterin expression was evaluated using long-term dissociated cultures of murine spinal cord and SOD1(G93A) transgenic mice, a model of familial ALS. Motor neurons and astrocytes constitutively expressed nuclear and cytoplasmic forms of clusterin, and secreted clusterin accumulated in culture media. Although clusterin can be stress inducible, heat shock failed to increase levels in these neural cell compartments despite robust upregulation of stress-inducible Hsp70 (HspA1) in non-neuronal cells. In common with HSPs, clusterin was upregulated by treatment with the Hsp90 inhibitor, geldanamycin, and thus could contribute to the neuroprotection previously identified for such compounds in disease models. Clusterin expression was not altered in cultured motor neurons expressing SOD1(G93A) by gene transfer or in presymptomatic SOD1(G93A) transgenic mice; however, clusterin immunolabeling was weakly increased in lumbar spinal cord of overtly symptomatic mice. More striking, mutant SOD1 inclusions, a pathological hallmark, were strongly labeled by anti-clusterin. Since secreted, as well as intracellular, mutant SOD1 contributes to toxicity, the extracellular chaperoning property of clusterin could be important for folding and clearance of SOD1 and other misfolded proteins in the extracellular space. Evaluation of chaperone-based therapies should include evaluation of clusterin as well as HSPs, using experimental models that replicate the control mechanisms operant in the cells and tissue of interest.

Development- and activity-dependent expression of clusterin in the mouse olfactory bulb

Clusterin, a protein involved in many biological processes, is expressed broadly in the central nervous system, but its functions remain largely unknown. As preparations for elucidating some possible functions, we examined the spatiotemporal expression patterns of clusterin in the mouse olfactory bulb at different developmental stages and under different neuronal activity levels. Our results revealed a dynamic expression of the protein during development. Clusterin signal was seemingly diffuse during the early stages of development, shifted to the cell somas later and then predominantly to the axons of projection neurons in the adult stage, with a transition point at approximately postnatal day 18. The effects of olfactory deficits on the clusterin expression level in an anosmic mouse model were neuron-specific: the signals increased remarkably from faint to strong in olfactory sensory neurons, reduced considerably from moderate/strong to faint in the centrifugal projection neurons, decreased moderately from moderate to faint in the local bulbar projection neurons, and remained intense in long-distance bulbar projection neurons. These results showed that clusterin expression is modulated dynamically during development and by sensory activity. These findings deepen our understanding of this broadly expressed protein.

Potential roles of zinc in the pathophysiology and treatment of major depressive disorder

Incomplete response to monoaminergic antidepressants in major depressive disorder (MDD), and the phenomenon of neuroprogression, suggests a need for additional pathophysiological markers and pharmacological targets. Neuronal zinc is concentrated exclusively within glutamatergic neurons, acting as an allosteric modulator of the N-methyl D-aspartate and other receptors that regulate excitatory neurotransmission and neuroplasticity. Zinc-containing neurons form extensive associational circuitry throughout the cortex, amygdala and hippocampus, which subserve mood regulation and cognitive functions. In animal models of depression, zinc is reduced in these circuits, zinc treatment has antidepressant-like effects and dietary zinc insufficiency induces depressive behaviors. Clinically, serum zinc is lower in MDD, which may constitute a state-marker of illness and a risk factor for treatment-resistance. Marginal zinc deficiency in MDD may relate to multiple putative mechanisms underlying core symptomatology and neuroprogression (e.g. immune dysfunction, monoamine metabolism, stress response dysregulation, oxidative/nitrosative stress, neurotrophic deficits, transcriptional/epigenetic regulation of neural networks). Initial randomized trials suggest a benefit of zinc supplementation. In summary, molecular and animal behavioral data support the clinical significance of zinc in the setting of MDD.

Retrospective Case-Control Study of Apolipoprotein J/Clusterin Protein Expression in Early Liveborn Neonatal Deaths with and without Pontosubicular Necrosis

Aims. Our objective was to examine Apo J protein expression in a total of 27 early liveborn neonatal deaths (less than 7 days of age) selected from the Scottish Perinatal Study (gestation of 25–42 weeks) comparing a group with histological pontosubicular necrosis (PSN) (n = 12) to a control group lacking PSN (n = 15). Methods. Using immunohistochemistry we evaluated postmortem pons and hippocampus from patients with PSN versus controls. Results. In the group with PSN, 11/12 (92%) cases showed positive Apo J neurones in the hippocampus/pons compared with 6/15 (40%) cases without PSN (P = 0.014, odds ratio 27.5, 95% confidence interval 2.881–262.48, using exact logistic regression)—independent of gestation, presence or absence of clinical asphyxia, duration of labour, or postnatal age. Clinical asphyxia was present in 10/15 (67%) without PSN compared with 11/12 (92%) with PSN. Neuronal Apo J positivity was present in 15/21 (71%) of clinically asphyxiated cases compared with 2/6 (33%) of the cases with no evidence of clinical asphyxia (P = 0.154, odds ratio 5, 95% confidence interval 0.71 to 34.94). Conclusions. Apo J neuronal protein expression is significantly increased in cases with PSN compared to cases without PSN—independent of gestation, presence of clinical asphyxia, duration of labour, or postnatal age.

Clusterin in Alzheimer's disease

Clusterin, also known as apolipoprotein J, is a ubiquitous multifunctional glycoprotein. Following its identification in 1983, clusterin was found to be clearly increased in Alzheimer's disease (AD). Later research demonstrated that clusterin could bind amyloid-beta (Abeta) peptides and prevent fibril formation, a hallmark of AD pathology. In addition to preventing excessive inflammation, intracellular clusterin was found to reduce apoptosis and oxidative stress. Although early studies were inconclusive, two recent large-scale genome-wide association studies (GWAS) independently identified variants within the clusterin gene as risk factors for developing AD. This review focuses on the characteristics of clusterin and possible mechanisms of its relationship to AD.

Quantitative temporal proteomic analysis of human embryonic stem cell differentiation into oligodendrocyte progenitor cells

Oligodendrocytes (OLs) are glial cells of the central nervous system, which produce myelin. Cultured OLs provide immense therapeutic opportunities for treating a variety of neurological conditions. One of the most promising sources for such therapies is human embryonic stem cells (ESCs) as well as providing a model to study human OL development. For these purposes, an investigation of proteome level changes is critical for understanding the process of OL differentiation. In this report, an iTRAQ-based quantitative proteomic approach was used to study multiple steps during OL differentiation including neural progenitor cells, glial progenitor cells and oligodendrocyte progenitor cells (OPCs) compared to undifferentiated ESCs. Using a 1% false discovery rate cutoff, ∼3145 proteins were quantitated and several demonstrated progressive stage-specific expression. Proteins such as transferrin, neural cell adhesion molecule 1, apolipoprotein E and wingless-related MMTV integration site 5A showed increased expression from the neural progenitor cell to the OPC stage. Several proteins that have demonstrated evidence or been suspected in OL maturation were also found upregulated in OPCs including fatty acid-binding protein 4, THBS1, bone morphogenetic protein 1, CRYAB, transferrin, tenascin C, COL3A1, TGFBI and EPB41L3. Thus, by providing the first extensive proteomic profiling of human ESC differentiation into OPCs, this study provides many novel proteins that are potentially involved in OL development.

Cholesterol metabolism in neurons and astrocytes

Cells in the mammalian body must accurately maintain their content of cholesterol, which is an essential membrane component and precursor for vital signalling molecules. Outside the brain, cholesterol homeostasis is guaranteed by a lipoprotein shuttle between the liver, intestine and other organs via the blood circulation. Cells inside the brain are cut off from this circuit by the blood-brain barrier and must regulate their cholesterol content in a different manner. Here, we review how this is accomplished by neurons and astrocytes, two cell types of the central nervous system, whose cooperation is essential for normal brain development and function. The key observation is a remarkable cell-specific distribution of proteins that mediate different steps of cholesterol metabolism. This form of metabolic compartmentalization identifies astrocytes as net producers of cholesterol and neurons as consumers with unique means to prevent cholesterol overload. The idea that cholesterol turnover in neurons depends on close cooperation with astrocytes raises new questions that need to be addressed by new experimental approaches to monitor and manipulate cholesterol homeostasis in a cell-specific manner. We conclude that an understanding of cholesterol metabolism in the brain and its role in disease requires a close look at individual cell types.

The conserved clusterin gene is expressed in the developing choroid plexus under the regulation of notch but not IGF signaling in zebrafish

Recent genome-wide association studies have implicated the clusterin gene in the etiology of Alzheimer's disease. The expression and function of clusterin in the developing brain, however, is poorly understood. In this study, we have characterized the zebrafish clusterin gene and determined its structural conservation, developmental expression, and physiological regulation. The structure of the zebrafish clusterin gene and protein is similar to its human orthologue. Biochemical assays show that zebrafish Clusterin is a secreted protein that cannot bind IGFs. In adult zebrafish, clusterin mRNA is detected in many tissues. In early development, clusterin mRNA becomes detectable at 12 h postfertilization, and its levels gradually increase thereafter. In situ hybridization analysis indicates that clusterin mRNA is specifically expressed in the developing diencephalic and myelencephalic choroid plexus. Among various stresses tested, heat shock, but not hypoxic or ionic stresses, increases the levels of clusterin mRNA. Inhibition of the IGF-I receptor-mediated signaling or overexpression of IGF ligands did not change clusterin mRNA levels. In comparison, inhibition or targeted knockdown of Notch signaling significantly increased clusterin mRNA expression in choroid plexus. These results suggest that clusterin is a marker of choroid plexus in zebrafish, and its expression in the developing choroid plexus is under the regulation of Notch but not IGF signaling.

Clusterin secreted by astrocytes enhances neuronal differentiation from human neural precursor cells

Neuronal differentiation from expanded human ventral mesencephalic neural precursor cells (NPCs) is very limited. Astrocytes are known to secrete neurotrophic factors, and so in order to enhance neuronal survival from NPCs, we tested the effect of regional astrocyte-conditioned medium (ACM) from the rat cortex, hippocampus and midbrain on this process. Human NPC's were expanded in FGF-2 before differentiation for 1 or 4 weeks in ACM. The results show that ACM from the hippocampus and midbrain increase the number of neurons from expanded human NPCs, an effect that was not observed with cortical ACM. In addition, both hippocampal and midbrain ACM increased the number and length of phosphorylated neurofilaments. MALDI-TOF analysis used to determine differences in media revealed that although all three regional ACMs had cystatin C, α-2 macroglobulin, extracellular matrix glycoprotein and vimentin, only hippocampal and midbrain ACM also contained clusterin, which when immunodepleted from midbrain ACM eliminated the observed effects on neuronal differentiation. Furthermore, clusterin is a highly glycosylated protein that has no effect on cell proliferation but decreases apoptotic nuclei and causes a sustained increase in phosphorylated extracellular signal-regulated kinase, implicating its role in cell survival and differentiation. These findings further reveal differential effects of regional astrocytes on NPC behavior and identify clusterin as an important mediator of NPC-derived neuronal survival and differentiation.

The regulation of the CNS innate immune response is vital for the restoration of tissue homeostasis (repair) after acute brain injury: a brief review

Neurons and glia respond to acute injury by participating in the CNS innate immune response. This involves the recognition and clearance of "not self " pathogens and "altered self " apoptotic cells. Phagocytic receptors (CD14, CD36, TLR-4) clear "not self" pathogens; neurons and glia express "death signals" to initiate apoptosis in T cells.The complement opsonins C1q, C3, and iC3b facilitate the clearance of apoptotic cells by interacting with CR3 and CR4 receptors. Apoptotic cells are also cleared by the scavenger receptors CD14, Prs-R, TREM expressed by glia. Serpins also expressed by glia counter the neurotoxic effects of thrombin and other systemic proteins that gain entry to the CNS following injury. Complement pathway and T cell activation are both regulated by complement regulatory proteins expressed by glia and neurons. CD200 and CD47 are NIRegs expressed by neurons as "don't eat me" signals and they inhibit microglial activity preventing host cell attack. Neural stem cells regulate T cell activation, increase the Treg population, and suppress proinflammatory cytokine expression. Stem cells also interact with the chemoattractants C3a, C5a, SDF-1, and thrombin to promote stem cell migration into damaged tissue to support tissue homeostasis.

Apolipoproteins in the brain: implications for neurological and psychiatric disorders

The brain is the most lipid-rich organ in the body and, owing to the impermeable nature of the blood-brain barrier, lipid and lipoprotein metabolism within this organ is distinct from the rest of the body. Apolipoproteins play a well-established role in the transport and metabolism of lipids within the CNS; however, evidence is emerging that they also fulfill a number of functions that extend beyond lipid transport and are critical for healthy brain function. The importance of apolipoproteins in brain physiology is highlighted by genetic studies, where apolipoprotein gene polymorphisms have been identified as risk factors for several neurological diseases. Furthermore, the expression of brain apolipoproteins is significantly altered in several brain disorders. The purpose of this article is to provide an up-to-date assessment of the major apolipoproteins found in the brain (ApoE, ApoJ, ApoD and ApoA-I), covering their proposed roles and the factors influencing their level of expression. Particular emphasis is placed on associations with neurological and psychiatric disorders.

Implication of complement system and its regulators in Alzheimer's disease

Alzheimer’s disease (AD) is an age-related neurodegenerative disease that affects approximately 24 million people worldwide. A number of different risk factors have been implicated in AD, however, neuritic (amyloid) plaques are considered as one of the defining risk factors and pathological hallmarks of the disease. Complement proteins are integral components of amyloid plaques and cerebral vascular amyloid in Alzheimer brains. They can be found at the earliest stages of amyloid deposition and their activation coincides with the clinical expression of Alzheimer's dementia. This review emphasizes on the dual key roles of complement system and complement regulators (CRegs) in disease pathology and progression. The particular focus of this review is on currently evolving strategies for design of complement inhibitors that might aid therapy by restoring the fine balance between activated components of complement system, thus improving the cognitive performance of patients. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of AD.

The role of clusterin (CLU) in malignant transformation and drug resistance in breast carcinomas

Breast cancer is the main cause of cancer-related death among women in Western countries. Current research is focused on identifying antiapoptotic proteins which could be a possible target for novel chemotherapeutic drugs. Secretory clusterin (sCLU) is an extracellular chaperone that has been functionally implicated in DNA repair, cell-cycle regulation, apoptotic cell death and tumorigenesis. The implication of sCLU in carcinogenesis and the progression of breast carcinomas make it an interesting gene, worthy of investigation. It has been reported to present powerful antiapoptotic activity and to perform a prosurvival function with most therapeutic treatments for breast cancer. This review summarizes our current understanding of the role of CLU in tumorigenesis, progression, and response to treatment in breast carcinomas.

Neuronal death in Alzheimer's disease and therapeutic opportunities

Alzheimer’s disease (AD) is an age-related neurodegenerative disease that affects approximately 24 million people worldwide. A number of different risk factors have been implicated in AD; however, neuritic (amyloid) plaques are considered as one of the defining risk factors and pathological hallmarks of the disease. In the past decade, enormous efforts have been devoted to understand the genetics and molecular pathogenesis leading to neuronal death in AD, which has been transferred into extensive experimental approaches aimed at reversing disease progression. Modern medicine is facing an increasing number of treatments available for vascular and neurodegenerative brain diseases, but no causal or neuroprotective treatment has yet been established. Almost all neurological conditions are characterized by progressive neuronal dysfunction, which, regardless of the pathogenetic mechanism, finally leads to neuronal death. The particular emphasis of this review is on risk factors and mechanisms resulting in neuronal loss in AD and current and prospective opportunities for therapeutic interventions. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of AD.

Clusterin: a forgotten player in Alzheimer's disease

Clusterin, also known as apolipoprotein J, is a versatile chaperone molecule which contains several amphipathic and coiled-coil alpha-helices, typical characteristics of small heat shock proteins. In addition, clusterin has three large intrinsic disordered regions, so-called molten globule domains, which can stabilize stressed protein structures. Twenty years ago, it was demonstrated that the expression of clusterin was clearly increased in Alzheimer's disease (AD). Later it was observed that clusterin can bind amyloid-beta peptides and prevent their fibrillization. Clusterin is also involved in the clearance of amyloid-beta peptides and fibrils by binding to megalin receptors and enhancing their endocytosis within glial cells. Clusterin is a complement inhibitor and can suppress complement activation observed in AD. Clusterin is also present in lipoprotein particles and regulates cholesterol and lipid metabolism of brain which is disturbed in AD. Clusterin is a stress-induced chaperone which is normally secreted but in conditions of cellular stress, it can be transported to cytoplasm where it can bind to Bax protein and inhibit neuronal apoptosis. Clusterin can also bind to Smad2/3 proteins and potentiate the neuroprotective TGFbeta signaling. An alternative splicing can produce a variant isoform of clusterin which can be translocated to nuclei where it induces apoptosis. The role of nuclear clusterin in AD needs to be elucidated. We will review here the extensive literature linking clusterin to AD and examine the recent progress in clusterin research with the respect to AD pathology. Though clusterin can be viewed as a multipotent guardian of brain, it is unable to prevent the progressive neuropathology in chronic AD.

The Role of Clusterin inIn VitroIschemia of Human Retinal Endothelial Cells

Clusterin has been reported to be up-regulated in diverse pathophysiological stresses, but its role is controversial. In this study, we investigated the role of clusterin under in vitro ischemia of human retinal endothelial cells (HRECs). When HRECs were exposed to oxygen-glucose deprivation (OGD), clusterin expression increased, whereas von Willebrand factor (vWF), occludin, and zonula occludens (ZO-1) markedly decreased. Interestingly, loss of tight junction proteins and death of HRECs in OGD conditions were restored by clusterin treatment. Our results suggest that the enhanced clusterin in OGD conditions may play a protective role against ischemia-induced tight junction protein loss and HRECs death.