Induction of clusterin in the immature brain following a hypoxic-ischemic injury

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.

Brain ischemia with Alzheimer phenotype dysregulates Alzheimer's disease-related proteins

There are evidences for the influence of Alzheimer's proteins on postischemic brain injury. We present here an overview of the published evidence underpinning the relationships between β-amyloid peptide, hyperphosphorylated tau protein, presenilins, apolipoproteins, secretases and neuronal survival/death decisions after ischemia and development of postischemic dementia. The interactions of above molecules and their influence and contribution to final ischemic brain degeneration resulting in dementia of Alzheimer phenotype are reviewed. Generation and deposition of β-amyloid peptide and tau protein pathology are essential factors involved in Alzheimer's disease development as well as in postischemic brain dementia. Postischemic injuries demonstrate that ischemia may stimulate pathological amyloid precursor protein processing by upregulation of β- and γ-secretases and therefore are capable of establishing a vicious cycle. Functional postischemic brain recovery is always delayed and incomplete by an injury-related increase in the amount of the neurotoxic C-terminal of amyloid precursor protein and β-amyloid peptide. Finally, we present here the concept that Alzheimer's proteins can contribute to and/or precipitate postischemic brain neurodegeneration including dementia with Alzheimer's phenotype.

Sporadic Alzheimer's disease begins as episodes of brain ischemia and ischemically dysregulated Alzheimer's disease genes

The study of sporadic Alzheimer’s disease etiology, now more than ever, needs an infusion of new concepts. Despite ongoing interest in Alzheimer’s disease, the basis of this entity is not yet clear. At present, the best-established and accepted “culprit” in Alzheimer’s disease pathology by most scientists is the amyloid, as the main molecular factor responsible for neurodegeneration in this disease. Abnormal upregulation of amyloid production or a disturbed clearance mechanism may lead to pathological accumulation of amyloid in brain according to the “amyloid hypothesis.” We will critically review these observations and highlight inconsistencies between the predictions of the “amyloid hypothesis” and the published data. There is still controversy over the role of amyloid in the pathological process. A question arises whether amyloid is responsible for the neurodegeneration or if it accumulates because of the neurodegeneration. Recent evidence suggests that the pathophysiology and neuropathology of Alzheimer’s disease comprises more than amyloid accumulation, tau protein pathology and finally brain atrophy with dementia. Nowadays, a handful of researchers share a newly emerged view that the ischemic episodes of brain best describe the pathogenic cascade, which eventually leads to neuronal loss, especially in hippocampus, with amyloid accumulation, tau protein pathology and irreversible dementia of Alzheimer type. The most persuasive evidences come from investigations of ischemically damaged brains of patients and from experimental ischemic brain studies that mimic Alzheimer-type dementia. This review attempts to depict what we know and do not know about the triggering factor of the Alzheimer’s disease, focusing on the possibility that the initial pathological trigger involves ischemic episodes and ischemia-induced gene dysregulation. The resulting brain ischemia dysregulates additionally expression of amyloid precursor protein and amyloid-processing enzyme genes that, in addition, ultimately compromise brain functions, leading over time to the complex alterations that characterize advanced sporadic Alzheimer’s disease. The identification of the genes involved in Alzheimer’s disease induced by ischemia will enable to further define the events leading to sporadic Alzheimer’s disease-related abnormalities. Additionally, knowledge gained from the above investigations should facilitate the elaboration of the effective treatment and/or prevention of Alzheimer’s disease.

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.

New insight on the mechanisms of epileptogenesis in the developing brain

The incidence of epilepsy is at its highest in childhood and seizures can persist for a lifetime. As brain tissue from pediatric patients with epilepsy is rarely available, the analysis of molecular and cellular changes during epileptogenesis, which could serve as targets for treatment approaches, has to rely largely on the analysis of tissue from animal models. However, these data have to be analyzed in the context of the developmental stage when the insult occurs. Here we review the current status of the available animal models, the molecular analysis done in these models, as well as treatment attempts to prevent epileptogenesis in the immature brain. Considering that epilepsy is one of the major childhood neurological diseases, it is remarkable how little is known on epileptogenesis in the immature brain at a molecular level. It is a true challenge for the future to expand the armamentarium of clinically relevant animal models, and systematic analysis of molecular and cellular data to enhance the probability of developing syndrome specific antiepileptogenic treatments and biomarkers for acquired pediatric epileptogenesis.

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.

Latent factor analysis to discover pathway-associated putative segmental aneuploidies in human cancers

Tumor microenvironmental stresses, such as hypoxia and lactic acidosis, play important roles in tumor progression. Although gene signatures reflecting the influence of these stresses are powerful approaches to link expression with phenotypes, they do not fully reflect the complexity of human cancers. Here, we describe the use of latent factor models to further dissect the stress gene signatures in a breast cancer expression dataset. The genes in these latent factors are coordinately expressed in tumors and depict distinct, interacting components of the biological processes. The genes in several latent factors are highly enriched in chromosomal locations. When these factors are analyzed in independent datasets with gene expression and array CGH data, the expression values of these factors are highly correlated with copy number alterations (CNAs) of the corresponding BAC clones in both the cell lines and tumors. Therefore, variation in the expression of these pathway-associated factors is at least partially caused by variation in gene dosage and CNAs among breast cancers. We have also found the expression of two latent factors without any chromosomal enrichment is highly associated with 12q CNA, likely an instance of “trans”-variations in which CNA leads to the variations in gene expression outside of the CNA region. In addition, we have found that factor 26 (1q CNA) is negatively correlated with HIF-1α protein and hypoxia pathways in breast tumors and cell lines. This agrees with, and for the first time links, known good prognosis associated with both a low hypoxia signature and the presence of CNA in this region. Taken together, these results suggest the possibility that tumor segmental aneuploidy makes significant contributions to variation in the lactic acidosis/hypoxia gene signatures in human cancers and demonstrate that latent factor analysis is a powerful means to uncover such a linkage.

Gene signatures are a powerful tool to investigate biological processes in human cancer. However, it is clear that these gene signatures do not fully reflect the complexity of human cancer. Here we demonstrate how a latent factor model can improve the in vivo relevance of these pathway-associated gene signatures by dissecting them into co-regulated transcriptional components which better represent the structure in human cancer. We use this approach to analyze hypoxia and lactic acidosis gene signatures to identify latent factors that represent distinct, interacting components of the various biological processes which are in the initial gene signatures but poorly dissected. Some factors are clustered in small chromosomal regions and their expression values are highly correlated with their DNA copy number in both cancer cell lines and human tumors. Therefore, the gene dosage at the DNA levels may explain the differences in gene expression. Several factors contain genes which are known to directly modulate the hypoxia response and allow us to generate testable hypotheses regarding particular copy number changes and hypoxia signatures. Therefore, the use of latent factor analysis is a powerful means to identify pathway-associated changes in the DNA copy number and gene dosage.

Alzheimer's mechanisms in ischemic brain degeneration

There is increasing evidence for influence of Alzheimer's proteins and neuropathology on ischemic brain injury. This review investigates the relationships between beta-amyloid peptide, apolipoproteins, presenilins, tau protein, alpha-synuclein, inflammation factors, and neuronal survival/death decisions in brain following ischemic episode. The interactions of these molecules and influence on beta-amyloid peptide synthesis and contribution to ischemic brain degeneration and finally to dementia are reviewed. Generation and deposition of beta-amyloid peptide and tau protein pathology are important key players involved in mechanisms in ischemic neurodegeneration as well as in Alzheimer's disease. Current evidence suggests that inflammatory process represents next component, which significantly contribute to degeneration progression. Although inflammation was initially thought to arise secondary to ischemic neurodegeneration, recent studies present that inflammatory mediators may stimulate amyloid precursor protein metabolism by upregulation of beta-secretase and therefore are able to establish a vicious cycle. Functional brain recovery after ischemic lesion was delayed and incomplete by an injury-related increase in the amount of the neurotoxic C-terminal of amyloid precursor protein and beta-amyloid peptide. Moreover, ischemic neurodegeneration is strongly accelerated with aging, too. New therapeutic alternatives targeting these proteins and repairing related neuronal changes are under development for the treatment of ischemic brain consequences including memory loss prevention.

Clusterin increases post-ischemic damages in organotypic hippocampal slice cultures

Clusterin or apolipoprotein J is a heterodimeric glycoprotein which is known to be increased during tissue involution in response to hormonal changes or injury and under circumstances leading to apoptosis. Previous studies in wild-type (WT) and clusterin-null (Clu-/-) mice indicated a protective role of clusterin over-expression in astrocytes lasting up to 90 days post-ischemia. However, in in vitro and in vivo models of neonatal hypoxia-ischemia, clusterin exacerbates necrotic cell death. We developed recombinant forms of clusterin and examined their effect on propidium iodide uptake, neuronal and synaptic markers as well as electrophysiological recordings in hippocampal slice cultures from Clu-/- and WT mice subjected to oxygen-glucose deprivation (OGD). WT mice displayed a marked up-regulation of clusterin associated with electrophysiological deficits and dramatic increase of propidium iodide uptake 5 days post-OGD. Immunocytochemical and western blot analyses revealed a substantial decrease of neuronal nuclei and synaptophysin immunoreactivity that predominated in WT mice. These findings contrasted with the relative post-OGD resistance of Clu-/- mice. The addition of biologically active recombinant forms of human clusterin for 24 h post-OGD led to the abolishment of the ischemic tolerance in Clu-/- slices. This deleterious effect of clusterin was reverted by the concomitant administration of the NMDA receptor antagonist, d-2-amino-5-phosphonopentanoate. The present data indicate that in an in vitro model of ischemia characterized by the predominance of NMDA-mediated cell death, clusterin exerts a negative effect on the structural integrity and functionality of hippocampal neurons.

Clusterin expression during fetal and postnatal CNS development in mouse

Clusterin (or apolipoprotein J) is a widely distributed multifunctional glycoprotein involved in CNS plasticity and post-traumatic remodeling. Using biochemical and morphological approaches, we investigated the clusterin ontogeny in the CNS of wild-type (WT) mice and explored developmental consequences of clusterin gene knock-out in clusterin null (Clu-/-) mice. A punctiform expression of clusterin mRNA was detected through the hypothalamic region, neocortex and hippocampus at embryonic stages E14/E15. From embryonic stage E16 to the first week of the postnatal life, the vast majority of CNS neurons expressed low levels of clusterin mRNA. In contrast, a very strong hybridizing signal mainly localized in pontobulbar and spinal cord motor nuclei was observed from the end of the first postnatal week to adulthood. Astrocytes expressing clusterin mRNA were often detected through the hippocampus and neocortex in neonatal mice. Real-time polymerase chain amplification and clusterin-immunoreactivity dot-blot analyses indicated that clusterin levels paralleled mRNA expression. Comparative analyses between WT and Clu-/- mice during postnatal development showed no significant differences in brain weight, neuronal, synaptic and astrocyte markers as well myelin basic protein expression. However, quantitative estimation of large motor neuron populations in the facial nucleus revealed a significant deficit in motor cells (-16%) in Clu-/- compared with WT mice. Our data suggest that clusterin expression is already present in fetal life mainly in subcortical structures. Although the lack of this protein does not significantly alter basic aspects of the CNS development, it may have a negative impact on neuronal development in certain motor nuclei.

Stress-Induced Retrotranslocation of Clusterin/ApoJ into the Cytosol

Clusterin is a usually secreted glycoprotein with chaperone properties. Recently, it has been suggested that clusterin isoforms reside in the nuclear and cytosolic compartments of human cell types, where they can influence various cellular programs including DNA repair, transcription and apoptosis. Several mechanisms have been proposed to explain this atypical location, including alternative transcription initiation and alternative splicing. However, none of these have been unequivocally established as occurring in live cells. Here we provide direct experimental evidence that in live intact cells, under certain stress conditions, clusterin can evade the secretion pathway and reach the cytosol. This was demonstrated using several complementary approaches. Flow cytometry and selective permeabilization of U251 cell membranes with digitonin allowed detection of cytosolic clusterin in stressed U251 cells. In addition, a stringent enzymatic assay reliant upon the exclusively cytosolic deubiquitinase enzymes confirmed that clusterin synthesized with its hydrophobic secretion signal sequence can reach the cytosol of U251 cells. The retrotranslocation of clusterin is likely to occur through a mechanism similar to the endoplasmic reticulum (ER)-associated protein degradation pathway and involves passage through the Golgi apparatus. We also report that the ER-associated ubiquitin ligase Hrd1/synoviolin can interact with, and ubiquitinate clusterin. The possible biological functions of these novel behaviours of clusterin are discussed.

Sustained astrocytic clusterin expression improves remodeling after brain ischemia

Clusterin is a glycoprotein highly expressed in response to tissue injury. Using clusterin-deficient (Clu-/-) mice, we investigated the role of clusterin after permanent middle cerebral artery occlusion (MCAO). In wild-type (WT) mice, clusterin mRNA displayed a sustained increase in the peri-infarct area from 14 to 30 days post-MCAO. Clusterin transcript was still present up to 90 days post-ischemia in astrocytes surrounding the core infarct. Western blot analysis also revealed an increase of clusterin in the ischemic hemisphere of WT mice, which culminates up to 30 days post-MCAO. Concomitantly, a worse structural restoration and higher number of GFAP-reactive astrocytes in the vicinity of the infarct scar were observed in Clu-/- as compared to WT mice. These findings go beyond previous data supporting a neuroprotective role of clusterin in early ischemic events in that they demonstrate that this glycoprotein plays a central role in the remodeling of ischemic damage.

Up-regulation of the clusterin gene after proteotoxic stress: implication of HSF1-HSF2 heterocomplexes

Clusterin is a secreted protein chaperone up-regulated in several pathologies, including cancer and neurodegenerative diseases. The present study shows that accumulation of aberrant proteins, caused by the proteasome inhibitor MG132 or the incorporation of the amino acid analogue AZC (L-azetidine-2-carboxylic acid), increased both clusterin protein and mRNA levels in the human glial cell line U-251 MG. Consistently, MG132 treatment was capable of stimulating a 1.3 kb clusterin gene promoter. Promoter deletion and mutation studies revealed a critical MG132-responsive region between -218 and -106 bp, which contains a particular heat-shock element, named CLE for 'clusterin element'. Gel mobility-shift assays demonstrated that MG132 and AZC treatments induced the formation of a protein complex that bound to CLE. As shown by supershift and chromatin-immunoprecipitation experiments, CLE is bound by HSF1 (heat-shock factor 1) and HSF2 upon proteasome inhibition. Furthermore, co-immunoprecipitation assays indicated that these two transcription factors interact. Gel-filtration analyses revealed that the HSF1-HSF2 heterocomplexes bound to CLE after proteasome inhibition have the same apparent mass as HSF1 homotrimers after heat shock, suggesting that HSF1 and HSF2 could heterotrimerize. Therefore these studies indicate that the clusterin is a good candidate to be part of a cellular defence mechanism against neurodegenerative diseases associated with misfolded protein accumulation or decrease in proteasome activity.

Traumatic brain injury induces biphasic upregulation of ApoE and ApoJ protein in rats

Apolipoproteins play an important role in cell repair and have been found to increase shortly after traumatic brain injury (TBI). In addition, apolipoproteins reduce amyloid-beta (Abeta) accumulation in models of Alzheimer's disease. Considering that TBI induces progressive neurodegeneration including Abeta accumulation, we explored potential long-term changes in the gene and protein expression of apolipoproteins E and J (ApoE and J) over 6 months after injury. Anesthetized male Sprague-Dawley rats were subjected to parasagittal fluid-percussion brain injury and their brains were evaluated at 2, 4, 7, 14 days, and 1 and 6 months after TBI. In situ hybridization, Western blot, and immunohistochemical analysis demonstrated that although there was a prolonged upregulation in both the gene expression and protein concentration of ApoE and J after injury, these responses were uncoupled. Upregulation of ApoE and J mRNA expression lasted from 4 days to 1 month after injury. In contrast, a biphasic increase in protein concentration and number of immunoreactive cells for ApoE and ApoJ was observed, initially peaking at 2 days (i.e., before increased mRNA expression), returning to baseline by 2 weeks and then gradually increasing through 6 months postinjury. In addition, ApoE and J were found to colocalize with Abeta accumulation in neurons and astrocytes at 1-6 months after injury. Collectively, these data suggest that ApoE and J play a role in the acute sequelae of brain trauma and reemerge long after the initial insult, potentially to modulate progressive neurodegenerative changes.

Differential effects of clusterin/apolipoprotein J on cellular growth and survival

The secreted clusterin/apolipoprotein J (CLU) protein form is a ubiquitously expressed heterodimeric glycoprotein which is differentially regulated in many severe physiological disturbance states including cell death, ageing, cancer progression, and various neurological diseases. Despite extensive efforts CLU function remains an enigma, the main cause being the intriguingly distinct and usually opposed functions in various cell types and tissues. In the current report we investigated the effects of CLU on cellular growth and survival in three human osteosarcoma (OS) cell lines, namely KH OS, Sa OS, and U-2 OS that express very low, moderate, and high endogenous steady-state CLU amounts, respectively. We found that exposure of these established OS cell lines or primary OS cells to genotoxic stress results in CLU gene induction at distinct levels that correlate negatively to CLU endogenous amounts. Following CLU-forced overexpression by means of an artificial transgene, we found that although extracellular CLU inhibits cell death in all three OS cell lines, intracellular CLU has different effects on cellular proliferation and survival in these cell lines. Transgenic KH OS cell lines adapted to moderate intracellular CLU levels were growth-retarded and became resistant to genotoxic and oxidative stress. In contrast, transgenic Sa OS and U2 OS cell lines adapted to high intracellular CLU amounts were sensitive to genotoxic and oxidative stress. In these two cell lines, the proapoptotic CLU function could be rescued by caspase inhibition. To monitor the immediate effects of heterologous CLU overexpression prior to cell adaptation, we performed transient transfections in all three OS cell lines. We found that induction of high intracellular CLU amounts increases spontaneous apoptosis in KH OS cells and reduces DNA synthesis in all three cell lines assayed. On the basis of these novel findings we propose that although extracellular CLU as well as intracellular CLU at low/moderate levels is cytoprotective, CLU may become highly cytostatic and/or cytotoxic if it accumulates intracellularly in high amounts either by direct synthesis or by uptake from the extracellular milieu.

Functional analysis of clusterin/apolipoprotein J in cellular death induced by severe genotoxic stress

Clusterin/apolipoprotein J (CLU) is a secreted heterodimeric glycoprotein that is reportedly upregulated during tumorigenesis, as well as during cell injury or death. Despite extensive efforts, CLU function during cellular death remains largely elusive. We are using as a model system to study CLU function three human osteosarcoma (OS) cell lines, namely, Sa OS, KH OS, and U-2 OS cells, induced to die after exposure to severe genotoxic stress mediated by the chemotherapeutic drug doxorubicin (DXR). We initially applied small interfering RNA (siRNA)-mediated specific knockdown of the CLU protein in OS cells. In all three cell lines, CLU knockdown resulted in increased sensitization to DXR-induced apoptosis. Supportively, moderate levels of forced transgene-mediated CLU stable overexpression in KH OS cells could rescue them from DXR-mediated apoptosis. In contrast, stable overexpression of high CLU levels in Sa OS and U-2 OS cells augmented apoptosis induced by cell exposure to severe DXR-mediated genotoxic stress. In summary, our data provide evidence that, although CLU is essential for cellular homeostasis, it may become highly cytotoxic in certain cellular contexts when it accumulates in high amounts intracellularly either by direct synthesis or by uptake from the extracellular milieu.

Increased clusterin expression in old but not young adult S100B transgenic mice: evidence of neuropathological aging in a model of Down Syndrome

S100B is a calcium-binding protein, localized to astroglial cells, which has a variety of neurotrophic functions, including roles in serotonergic neuronal growth, synaptogenesis dendritic branching and apoptosis. In humans, the gene for S100B is found on chromosome 21, within what is considered the obligate region for Down Syndrome (DS) and levels of S100B are increased in brain of both DS and Alzheimer's Disease (AD). We have been characterizing a transgenic mouse overexpressing this protein and have previously found evidence of pathological changes in brains of the mice. In the current study, we have examined the expression of clusterin, a protein expressed in aging neurons, in the mice at two ages. Our findings show increased clusterin expression in the aged S100B mice compared to their CD-1 controls, a finding we have interpreted as further evidence of pathological brain aging.

Intracellular clusterin causes juxtanuclear aggregate formation and mitochondrial alteration

Clusterin is a puzzling protein upregulated in many diseased tissues, presented as either a survival or a death protein. The role of clusterin might depend on the final maturation and localization of the protein, which can be secreted or reside inside cells, either after in situ synthesis or uptake of extracellular clusterin. We studied the biological effects of intracellular clusterin and observed that clusterin forms containing the alpha-chain region strongly accumulated in an ubiquitinated form in juxtanuclear aggregates meeting the main criterions of aggresomes and leading to profound alterations of the mitochondrial network. The viability of cells transfected by intracellular forms of clusterin was improved by overexpression of Bcl-2, and caspase inhibition was capable of rescuing cells expressing clusterin, which presented an altered mitochondrial permeability. We propose that, although it might be an inherently pro-survival and anti-apoptotic protein expressed by cells under stress in an attempt to protect themselves, clusterin can become highly cytotoxic when accumulated in the intracellular compartment. This activity might reconcile the opposite purported influences of clusterin on cell survival and explain how clusterin can be causally involved in neurodegeneration.

Delayed, but prolonged increases in astrocytic clusterin (ApoJ) mRNA expression following acute cortical spreading depression in the rat: evidence for a role of clusterin in ischemic tolerance

Clusterin is a sulfated glycoprotein produced by neurons and by resting and activated astrocytes that has several putative functions, including protective responses to brain injury. Cortical spreading depression (CSD) is a powerful yet largely benign stimulus that acutely is capable of providing long-lasting ischemic tolerance. The current study investigated possible alterations in expression of clusterin mRNA in the cerebral cortex of the rat at various times after unilateral CSD. Using semiquantitative in situ hybridization histochemistry, significant increases (30-100%; P< or =0.05) in clusterin mRNA were detected in layers I-III and IV-VI of the ipsilateral cortex at 1, 2, 7 and 14 (layers I-III only) days after CSD. Transcript levels in the ipsilateral cortex were again equivalent to contralateral (control) levels at 28 days after CSD. These molecular anatomical studies also revealed that both neurons and nonneuronal cells (presumed reactive astrocytes) increased their expression of clusterin mRNA following CSD. Notably the time-course of increases in clusterin mRNA after CSD (1-14 days) overlaps that during which CSD reportedly provides neuroprotection against subsequent cerebral ischemia. These findings along with other evidence suggest that increased clusterin production and secretion, particularly by astrocytes, could be neuroprotective-perhaps via one or more of its putative actions that include inhibition of complement activation and cytolysis, effects on chemotaxis and apoptosis, and actions as an anti-stress protein chaperone.

Adrenalectomy-induced cell death in the dentate gyrus: further characterisation using TUNEL and effects of the Ginkgo biloba extract, EGb 761, and ginkgolide B

This study investigated the potential neuroprotective effects of the Ginkgo biloba extract, EGb-761, and ginkgolide B, on adrenalectomy (ADX)-induced cell death in the dentate gyrus (DG). Adrenalectomised, sham surgery-treated, and naive controls received either EGb-761 (25, 50, or 100 mg/kg), 0.9% saline vehicle control, ginkgolide B (10 or 25 mg/kg), or a polyethylene glycol vehicle control, i.p, daily for 6 days postsurgery. Cell death in the DG was determined by in situ labelling of DNA fragments, using the TUNEL method; sections were counterstained with hematoxylin. Radioimmunoassay was used to confirm a decrease in plasma corticosterone (CORT) after ADX. TUNEL-positive granule cells were observed in the DG at 1 week, but not at 24 h, post-ADX. The rate of granule cell death at this time was highest in the suprapyramidal blade and increased in a crest tip and a rostrotemporal gradient. Whereas CORT replacement completely prevented the occurrence of TUNEL-positive granule cells, EGb-761 and ginkgolide B did not, at any of the doses used. These results suggest that these drugs may not have substantial neuroprotective effects in the ADX model of neurodegeneration.

Clusterin contributes to caspase-3-independent brain injury following neonatal hypoxia-ischemia

Clusterin, also known as apolipoprotein J, is a ubiquitously expressed molecule thought to influence a variety of processes including cell death. In the brain, it accumulates in dying neurons following seizures and hypoxic-ischemic (H-I) injury. Despite this, in vivo evidence that clusterin directly influences cell death is lacking. Following neonatal H-I brain injury in mice (a model of cerebral palsy), there was evidence of apoptotic changes (neuronal caspase-3 activation), as well as accumulation of clusterin in dying neurons. Clusterin-deficient mice had 50% less brain injury following neonatal H-I. Surprisingly, the absence of clusterin had no effect on caspase-3 activation, and clusterin accumulation and caspase-3 activation did not colocalize to the same cells. Studies with cultured cortical neurons demonstrated that exogenous purified astrocyte-secreted clusterin exacerbated oxygen/glucose-deprivation-induced necrotic death. These results indicate that clusterin may be a new therapeutic target to modulate non-caspase-dependent neuronal death following acute brain injury.

Clusterin upregulation following rubrospinal tract lesion in the adult rat

We have examined the expression of the multifunctional protein clusterin in the axotomized red nucleus, at the lesion site in the lateral funiculus of C3, as well as along the Wallerian degeneration in the lateral funiculus of T1. There was a marked increase in clusterin-immunoreactivity (IR) and clusterin mRNA in red nucleus nerve cell bodies. An early, transient occurrence of large, heavily clusterin-IR globules were found in axons in the spinal cord at the lesion site in C3 as well as a marked upregulation of mRNA for clusterin, presumably associated with reactive astrocytes and oligodendrocytes from 1 to 4 weeks postoperatively. Clusterin-IR and its mRNA were markedly increased in the zone of Wallerian degeneration at T1, where some strongly expressing cells were identified as oligodendrocytes. Taken together with previous changes in clusterin expression following peripheral nerve and dorsal root injury, we suggest that this protein is involved in regenerative as well as degenerative neural responses.

Neuronal death and survival in two models of hypoxic-ischemic brain damage

Two unilateral hypoxic-ischemia (HI) models (moderate and severe) in immature rat brain have been used to investigate the role of various transcription factors and related proteins in delayed neuronal death and survival. The moderate HI model results in an apoptotic-like neuronal death in selectively vulnerable regions of the brain while the more severe HI injury consistently produces widespread necrosis resulting in infarction, with some necrosis resistant cell populations showing evidence of an apoptotic type death. In susceptible regions undergoing an apoptotic-like death there was not only a prolonged induction of the immediate early genes, c-jun, c-fos and nur77, but also of possible target genes amyloid precursor protein (APP751) and CPP32. In contrast, increased levels of BDNF, phosphorylated CREB and PGHS-2 were found in cells resistant to the moderate HI insult suggesting that these proteins either alone or in combination may be of importance in the process of neuroprotection. An additional feature of both the moderate and severe brain insults was the rapid activation and/or proliferation of glial cells (microglia and astrocytes) in and around the site of damage. The glial response following HI was associated with an upregulation of both the CCAAT-enhancer binding protein alpha (microglia only) and NFkappaB transcription factors.

Activity and injury-dependent expression of inducible transcription factors, growth factors and apoptosis-related genes within the central nervous system

This review primarily discusses work that has been performed in our laboratories and that of our direct collaborators and therefore does not represent an exhaustive review of the current literature. Our aim is to further discuss the role that gene expression plays in neuronal plasticity and pathology. In the first part of this review we examine activity-dependent changes in the expression of inducible transcription factors (ITFs) and neurotrophins with long-term potentiation (LTP) and kindling. This work has identified particular ITFs (Krox-20 and Krox-24) and neurotrophin systems (particularly the brain-derived neurotrophic factor (BDNF)/tyrosine receptor kinase-B, Trk-B system) that may be involved in stabilizing long-lasting LTP (i.e. LTP3). We also show that changes in the expression of other ITFs (Fos, Jun-D and Krox-20) and the BDNF/trkB neurotrophin system may play a central role in the development of hippocampal kindling, an animal model of human temporal lobe epilepsy. In the next part of this review we examine changes in gene expression after neuronal injuries (ischemia, prolonged seizure activity and focal brain injury) and after nerve transection (axotomy). We identify apoptosis-related genes (p53, c-Jun, Bax) whose delayed expression selectively increases in degenerating neurons, further suggesting that some forms of neuronal death may involve apoptosis. Moreover, since overexpression of the tumour-suppressor gene p53 induces apoptosis in a wide variety of dividing cell types we speculate that it may perform the same function in post-mitotic neurons following brain injuries. Additionally, we show that neuronal injury is associated with rapid, transient, activity-dependent expression of neurotrophins (BDNF and activinA) in neurons, contrasting with a delayed and more persistent injury-induced expression of certain growth factors (IGF-1 and TGFbeta) in glia. In this section we also describe results linking ITFs and neurotrophic factor expression. Firstly, we show that while BDNF and trkB are induced as immediate-early genes following injury, the injury-induced expression of activinA and trkC may be regulated by ITFs. We also discuss whether loss of retrograde transport of neurotrophic factors such as nerve growth factor following nerve transection triggers the selective and prolonged expression of c-Jun in axotomized neurons and whether c-Jun is responsible for regeneration or degeneration of these axotomized neurons. In the last section we further examine the role that gene expression may play in memory formation, epileptogenesis and neuronal degeneration, lastly speculating whether the expression of various growth factors after brain injury represents an endogenous neuroprotective response of the brain to injury. Here we discuss our results which show that pharmacological enhancement of this response with exogenous application of IGF-1 or TGF-beta reduces neuronal loss after brain injury.

ATF-2 phosphorylation in apoptotic neuronal death

Activating transcription factor (ATF-2) is a basic region-leucine zipper transcription factor that can mediate a diverse range of transcriptional responses including those generated by various forms of cellular stress. Activation of ATF-2 in response to these stimuli requires post-translational modification, in particular the phosphorylation of Thr69 and Thr71. To investigate whether ATF-2 activation also has a role in neuronal apoptosis, immunocytochemistry using a phospho-specific ATF-2 (Thr71) antibody was carried out in the 21 day old rat brain following a unilateral hypoxic-ischemic (HI) insult and PC12 cells cultured in the presence of okadaic acid. In both models a dramatic increase in phosphorylated ATF-2 was found within cells undergoing apoptosis.

Prostaglandin H synthase-2 and cytosolic phospholipase A2 in the hypoxic-ischemic brain: role in neuronal death or survival?

The breakdown of membrane phospholipids and subsequent arachidonic acid metabolism to prostanoids is a well-documented brain response to cerebral ischemia. To further elucidate the components of this signal transduction pathway, immunocytochemistry was used to determine the levels of two potentially important enzymes, cytosolic phospholipase A2 (cPLA2) and prostaglandin H synthase-2 (PGHS-2), in the immature rat brain following moderate unilateral hypoxic-ischemia (HI). The CA1 pyramidal cells of the hippocampus which undergo delayed neuronal death on the injured side following HI demonstrated a significant induction of PGHS-2 immunoreactivity 48 h post-insult. However, a consistent increase in PGHS-2 was also evident in the resistant dentate granule cells at an earlier time point. Although PGHS-2 is present in both susceptible and resistant cell populations following HI, the possibility remains that divergence further down-stream in the pathway is responsible for selective vulnerability. In contrast to the neuronal PGHS-2 expression, cPLA2 immunoreactivity appears to be of glial origin with increases in and around the CAI-2 pyramidal cell layer at the 72-168-h time points. These results suggest that prostanoids are likely to serve important roles in HI brain damage and repair in infant brain.

Gene expression induced by cerebral ischemia: an apoptotic perspective

The flow of new information on gene expression related to apoptosis has been relentless in the last several years. This has also been the case with respect to gene expression after cerebral ischemia. Many of genes associated with an apoptotic mode of cell death have now been studied in the context of experimental cerebral ischemia from the immediate early genes through modulating genes such as bcl-2 to genes in the final execution phase such as interleukin-1β converting enzyme (ICE)-related proteases. It was impossible to adequately cite all primary reports on these subjects. However, many excellent reviews have appeared in the last year, which together, cover all these areas of interest. In this review, we have elected to cite only reports published since January 1996 and use an extensive collection of reviews (indicated in italics) to guide the reader to the earlier literature. Our intent is to provide the reader with a timely and useful analysis of the current state of the art. It is hoped that this approach does not cause offense with our colleagues whose contributions before 1996 laid the foundation for much of this work.