Clusterin levels increase during neuronal development

Association between cerebrospinal fluid clusterin and biomarkers of Alzheimer's disease pathology in mild cognitive impairment: a longitudinal cohort study

Background

Clusterin, a glycoprotein implicated in Alzheimer's disease (AD), remains unclear. The objective of this study was to analyze the effect of cerebrospinal fluid (CSF) clusterin in relation to AD biomarkers using a longitudinal cohort of non-demented individuals.

Methods

We gathered a sample comprising 86 individuals under cognition normal (CN) and 134 patients diagnosed with MCI via the Alzheimer's Disease Neuroimaging Initiative (ADNI) database. To investigate the correlation of CSF clusterin with cognitive function and markers of key physiological changes, we employed multiple linear regression and mixed-effect models. We undertook a causal mediation analysis to inspect the mediating influence of CSF clusterin on cognitive abilities.

Results

Pathological characteristics associated with baseline Aβ42, Tau, brain volume, exhibited a correlation with initial CSF clusterin in the general population, Specifically, these correlations were especially prominent in the MCI population; CSF Aβ42 (PCN = 0.001; PMCI = 0.007), T-tau (PCN < 0.001; PMCI < 0.001), and Mid temporal (PCN = 0.033; PMCI = 0.005). Baseline CSF clusterin level was predictive of measurable cognitive shifts in the MCI population, as indicated by MMSE (β = 0.202, p = 0.029), MEM (β = 0.186, p = 0.036), RAVLT immediate recall (β = 0.182, p = 0.038), and EF scores (β = 0.221, p = 0.013). In MCI population, the alterations in brain regions (17.87% of the total effect) mediated the effect of clusterin on cognition. It was found that variables such as age, gender, and presence of APOE ε4 carrier status, influenced some of these connections.

Conclusion

Our investigation underscored a correlation between CSF clusterin concentrations and pivotal AD indicators, while also highlighting clusterin's potential role as a protective factor for cognitive abilities in MCI patients.

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.

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.

Altered levels of blood proteins in Alzheimer's disease longitudinal study: Results from Australian Imaging Biomarkers Lifestyle Study of Ageing cohort

INTRODUCTION

A blood-based biomarker panel to identify individuals with preclinical Alzheimer's disease (AD) would be an inexpensive and accessible first step for routine testing.

METHODS

We analyzed 14 biomarkers that have previously been linked to AD in the Australian Imaging Biomarkers lifestyle longitudinal study of aging cohort.

RESULTS

Levels of apolipoprotein J (apoJ) were higher in AD individuals compared with healthy controls at baseline and 18 months (P = .0003) and chemokine-309 (I-309) were increased in AD patients compared to mild cognitive impaired individuals over 36 months (P = .0008).

DISCUSSION

These data suggest that apoJ may have potential in the context of use (COU) of AD diagnostics, I-309 may be specifically useful in the COU of identifying individuals at greatest risk for progressing toward AD. This work takes an initial step toward identifying blood biomarkers with potential use in the diagnosis and prognosis of AD and should be validated across other prospective cohorts.

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.

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.

Chapter 9: Oxidative stress in malignant progression: The role of Clusterin, a sensitive cellular biosensor of free radicals

Clusterin/Apolipoprotein J (CLU) gene is expressed in most human tissues and encodes for two protein isoforms; a conventional heterodimeric secreted glycoprotein and a truncated nuclear form. CLU has been functionally implicated in several physiological processes as well as in many pathological conditions including ageing, diabetes, atherosclerosis, degenerative diseases, and tumorigenesis. A major link of all these, otherwise unrelated, diseases is that they are characterized by increased oxidative injury due to impaired balance between production and disposal of reactive oxygen or nitrogen species. Besides the aforementioned diseases, CLU gene is differentially regulated by a wide variety of stimuli which may also promote the production of reactive species including cytokines, interleukins, growth factors, heat shock, radiation, oxidants, and chemotherapeutic drugs. Although at low concentration reactive species may contribute to normal cell signaling and homeostasis, at increased amounts they promote genomic instability, chronic inflammation, lipid oxidation, and amorphous aggregation of target proteins predisposing thus cells for carcinogenesis or other age-related disorders. CLU seems to intervene to these processes due to its small heat-shock protein-like chaperone activity being demonstrated by its property to inhibit protein aggregation and precipitation, a main feature of oxidant injury. The combined presence of many potential regulatory elements in the CLU gene promoter, including a Heat-Shock Transcription Factor-1 and an Activator Protein-1 element, indicates that CLU gene is an extremely sensitive cellular biosensor of even minute alterations in the cellular oxidative load. This review focuses on CLU regulation by oxidative injury that is the common molecular link of most, if not all, pathological conditions where CLU has been functionally implicated.

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.

Clusterin expression in non-neoplastic adenohypophyses and pituitary adenomas: cytoplasmic clusterin localization in adenohypophysis is related to aging

Clusterin is a circulating multifunctional glycoprotein produced in several kinds of epithelial and neuronal cells. Clusterin is upregulated during different physiological and pathological states, such as senescence, type-2 diabetes mellitus, Alzheimer disease, and in various neoplasms. Herein, we investigated the immunohistochemical expression of clusterin in non-neoplastic adenohypophysis of human autopsy subjects and pituitary adenomas. We also investigated the association of clusterin increase with age in adenohypophysis of autopsy subjects. Immunohistochemically, clusterin was found positive in the cytoplasm of all adenoma cases, and in the cytoplasm of parenchymal cells, stellate cells, mixed cell follicles and in colloidal material inside of the follicles of non-neoplastic adenohypophysis as well. Clusterin expression in pituitary adenomas was found significantly higher than in non-neoplastic adenohypophyses. In addition, in non-neoplastic adenohypophysis, a significant increase in clusterin expression levels between young (<or=30 years), middle aged (31 to 60 years), and older (>or=61 years) subjects (p < 0.00001, analysis of variance [ANOVA]) was found. In addition to clusterin accumulation, presence of calcification (p < 0.045, ANOVA) and presence of large follicles with colloid accumulation (p < 0.004, ANOVA) were also statistically significant factors related to aging in non-neoplastic adenohypophysis. In conclusion, the present study demonstrated that clusterin expression was found in non-neoplastic adenohypophysis and in upregulated amounts in pituitary adenomas. This study also demonstrated that in non-neoplastic adenohypophyses, increase of clusterin positive cells; histopathological findings of calcification or presence colloidal material accumulation in large follicles were associated with age. To our knowledge, immunohistochemical localization of clusterin in pituitary adenomas was not reported previously.

The role of clusterin in retinal development and free radical damage

AIM

To assess the role of clusterin in retinal vascular development and in free radical damage in vivo and in vitro.

METHODS

The expression of clusterin, von Willebrand factor (vWF), flk-1, heat shock protein 27 (Hsp27) and heat shock protein 70 (Hsp70) was examined in the retinas of developing mice and oxygen-induced retinopathy (OIR) mice by immunofluorescence staining and western blot analysis. Hydrogen peroxide (H(2)O(2))-pretreated human retinal endothelial cells (HREC) and astrocytes were cultured in the presence or absence of exogenous clusterin, and then the cell viability was measured using the MTT assay and DAPI staining.

RESULTS

Clusterin was expressed mainly in the inner retina and co-localised with vWF, an endothelial cell marker. During the mouse developmental process, clusterin expression was decreased, which was similar to the expression of flk-1, vWF and Hsp27. Furthermore, in the OIR model, clusterin expression changed in a similar way to both vWF and Hsp27. Under hypoxic conditions, clusterin expression increased in HREC and astrocytes. In H(2)O(2)-pretreated HREC and astrocytes, clusterin protected against apoptotic cell death.

CONCLUSIONS

These results suggest that clusterin is associated with protection from apoptotic retinal cell death in retinal development and in free radical damage.

Regulation of clusterin/apolipoprotein J, a functional homologue to the small heat shock proteins, by oxidative stress in ageing and age-related diseases

Clusterin/apolipoprotein J (CLU) gene has a nearly ubiquitous expression pattern in human tissues. The two main CLU protein isoforms in human cells include the conventional glycosylated secreted heterodimer (sCLU) and a truncated nuclear form (nCLU). CLU has been implicated in various physiological processes and in many severe physiological disturbance states including ageing, cancer progression, vascular damage, diabetes, kidney and neuron degeneration. Although unrelated in their etiology and clinical manifestation, these diseases represent states of increased oxidative stress, which in turn, promotes amorphous aggregation of target proteins, increased genomic instability and high rates of cellular death. Among the various properties attributed to CLU so far, those mostly investigated and invariably appreciated are its small heat shock proteins-like chaperone activity and its involvement in cell death regulation, which are both directly correlated to the main features of oxidant injury. Moreover, the presence of both a heat shock transcription factor-1 and an activator protein-1 element in the CLU gene promoter indicate that CLU gene can be an extremely sensitive biosensor to reactive oxygen species. This review emphasizes on CLU gene regulation by oxidative stress that is the common link between all pathological conditions where CLU has been implicated.

Age-dependent increase of clusterin in the human pituitary gland

Clusterin is a glycoprotein known to play various physiological roles including complement activity, amyloid binding activity in Alzheimer disease, as well as binding with heat shock proteins and abnormal prions. The present study immunohistochemically investigated the expression of clusterin in the human pituitary gland in subjects of 10-88 years of age (n=173). Causes of death were blunt injury (n=35), sharp injury (n=15), poisoning (n=11), drowning (n=14), fire fatalities (n=28), asphyxiation (n=15), hypothermia (n=7), hyperthermia (n=3), and natural diseases (n=45). Clusterin was detected in mixed cell follicles and the anterior lobar parenchymal cells. The area occupied by cells positive for clusterin were measured, and the ratio to the whole area of the anterior lobe (% clusterin-positive cell area) was estimated. There was a good correlation between the age of the subjects in years and the % clusterin-positive cell area in the anterior lobe of the pituitary gland (r=0.736, P<0.01). Relationships between % clusterin-positive cell and gender, cause of death, and survival time were insignificant. These findings indicate an age-dependent accumulation of clusterin in the pituitary gland, which may be related to the aging of endocrine systems.

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.

Progressive changes in regulation of apolipoproteins E and J in glial cultures during postnatal development and aging

Apolipoprotein (Apo) E and ApoJ are lipid- and cholesterol-carriers in the central nervous system and are implicated in age-related neurodegenerative diseases. The primary source of secreted ApoE and ApoJ (clusterin) in the brain is glia. Regulation of these apolipoproteins in mixed glial cultures from rat cerebral cortex differed most strongly between neonatal- and adult-derived glia. Basal secretion of ApoJ was two-fold greater in neonatal than adult glia. Responses to cytokines also differed by donor age. In adult glia, IL-6 increased ApoE secretion, but slightly decreased ApoJ. Both IL-1 beta and TNFalpha treatments increased ApoJ secretion from adult glia, with little effect on ApoE. In contrast to adult glia, neonatal ApoJ secretion did not respond to IL-1 beta, IL-6, or TNFalpha, and ApoE secretion from neonatal glia was slightly increased by IL-6. These differences may contribute to age-related neuroinflammatory processes, and are pertinent to the general use of neonatal-derived primary glia in models for neurodegenerative disease.

Silencing expression of the clusterin/apolipoprotein j gene in human cancer cells using small interfering RNA induces spontaneous apoptosis, reduced growth ability, and cell sensitization to genotoxic and oxidative stress

Clusterin/Apolipoprotein J (CLU) is a heterodimeric ubiquitously expressed secreted glycoprotein that is implicated in several physiological processes and is differentially expressed in many severe physiological disturbances, including tumor formation and in vivo cancer progression. Despite extensive efforts, clarification of CLU's biological role has been exceptionally difficult and its precise function remains elusive. Short RNA duplexes, referred to as small interfering RNAs (siRNAs), provide a new approach for the elucidation of gene function in human cells. Here, we describe siRNA-mediated CLU gene silencing in osteosarcoma and prostate human cancer cells and illustrate that CLU mRNA is amenable to siRNA-mediated degradation. Our data demonstrate that CLU knockdown in human cancer cells induces significant reduction of cellular growth and higher rates of spontaneous endogenous apoptosis. Moreover, CLU knockdown cancer cells were significantly sensitized to both genotoxic and oxidative stress induced by chemotherapeutic drugs and H(2)O(2), respectively. These effects were more pronounced in cell lines that express high endogenous steady-state levels of the CLU protein and occur through hyperactivation of the cellular apoptotic machinery. Overall, our results reveal that, in the distinct cellular contexts of the osteosarcoma and prostate cancer cells assayed, CLU is a central molecule in cell homeostasis that exerts a cytoprotective function. The described CLU-specific siRNA oligonucleotides that can potently silence CLU gene expression may thus prove valuable agents during antitumor therapy or at other pathological conditions where CLU has been implicated.

Clusterin/apolipoprotein J in human aging and cancer

Clusterin/Apolipoprotein J (ApoJ) is a heterodimeric highly conserved secreted glycoprotein being expressed in a wide variety of tissues and found in all human fluids. Despite being cloned since 1989, no genuine function has been attributed to ApoJ so far. The protein has been reportedly implicated in several diverse physiological processes such as sperm maturation, lipid transportation, complement inhibition, tissue remodeling, membrane recycling, cell-cell and cell-substratum interactions, stabilization of stressed proteins in a folding-competent state and promotion or inhibition of apoptosis. ApoJ gene is differentially regulated by cytokines, growth factors and stress-inducing agents, while another defining prominent and intriguing ApoJ feature is its upregulation in many severe physiological disturbances states and in several neurodegenerative conditions mostly related to advanced aging. Moreover, ApoJ accumulates during the viable growth arrested cellular state of senescence, that is thought to contribute to aging and to tumorigenesis suppression; paradoxically ApoJ is also upregulated in several cases of in vivo cancer progression and tumor formation. This review focuses on the reported data related to ApoJ cell-type and signal specific regulation, function and site of action in normal and cancer cells. We discuss the role of ApoJ during cellular senescence and tumorigenesis, especially under the light of the recently demonstrated various ApoJ intracellular protein forms and their interaction with molecules involved in signal transduction and DNA repair, raising the possibility that its overexpression during cellular senescence might cause a predisposition to cancer.

Upregulation of glial clusterin in brains of patients with AIDs

Since clusterin (CLU) production in reactive astrocytes may be neuroprotective, we examined its distribution in AIDS brains where brain injury and reactive astrocytosis are common. The relative area and number of CLU-positive astrocytes, as well as their percent total of all white matter glia, significantly increased in AIDS brains with and without HIV encephalitis (P<0.05). Proliferation markers were absent. In contrast, the relative area and number of GFAP-positive astrocytes and their percent of all white matter glia, increased in some cases but the mean increases were not significant. Clusterin is sensitive marker of glial reactivity in AIDS brains and its enhanced expression was not dependent on increases in GFAP.

Clusterin (apolipoprotein J) protein levels are increased in hippocampus and in frontal cortex in Alzheimer's disease

We studied the multifunctional protein clusterin (apolipoprotein J, SGP-2, SP-40,40) in brain tissue using quantitative Western blotting and immunohistochemistry. The material included postmortem brains from 19 patients with Alzheimer's disease (AD), 6 with vascular dementia (VAD), and 7 age-matched control subjects. Intense clusterin staining was found in the soma of both neuronal and astroglial cells. In addition, positive staining was found in a portion of senile plaques (SP) in AD brains. Quantitative analysis showed that clusterin levels were significantly increased in AD, both in frontal cortex (150% of the control value, P = 0.002) and in the hippocampus (179% of the control value, P < 0.001), while normal clusterin levels were found in cerebellum (104% of the control value). No significant changes were found in VAD. Within the AD group, there was a significant negative correlation between clusterin levels in hippocampus and severity of dementia (r = -0.40), while no such correlation was found in frontal cortex (r = 0.12). No significant correlations were found between clusterin levels and the number of SP or neurofibrillary tangles. No significant differences in clusterin levels were found in any brain region between AD patients possessing different numbers of the ApoE4 allele. The increased clusterin levels in AD brain, together with the absence of a correlation between SP counts and clusterin levels, and the finding that clusterin is only found in a smaller portion of SP do not suggest a link between clusterin and beta-amyloid dependence. Instead we hypothesize that the increase is part of a regional response in AD brain.

Epidermal growth factor suppresses renal tubular apoptosis following ureteral obstruction

OBJECTIVES

Acute unilateral ureteral obstruction (UUO) results in ipsilateral hydronephrosis characterized by a decrease in epidermal growth factor (EGF) mRNA expression and EGF protein levels in the distal renal tubules. UUO results in programmed cell death with increases in the characteristic markers of apoptosis. To suppress the apoptotic response during UUO, recombinant EGF was administered during renal obstruction and the ensuing molecular and histologic changes were studied.

METHODS

Mature Sprague-Dawley rats underwent left ureteral obstruction and the kidneys were harvested at 24, 48, and 72 hours. Markers of apoptosis included DNA laddering pattern on agarose gel electrophoresis, in situ gap labeling of fragmented DNA for quantitative apoptotic body determination, polyadenylated mRNA expression of SGP-2, and in situ hybridization for sulfated glycoprotein-2 (SGP-2) mRNA. Studies were repeated in rats following administration of 10, 20, and 40 micrograms of subcutaneous recombinant EGF on a daily basis after UUO.

RESULTS

Subcutaneous injection of EGF into unilaterally obstructed rats promotes renal tubular epithelial cell regeneration, as demonstrated by increased cortical mitotic activity. Systemic EGF supplementation in these unilaterally obstructed rats also resulted in a decrease in the intensity of the DNA laddering pattern associated with renal tubular apoptosis. An in situ labeling procedure to identify apoptotic nuclei in the ureterally obstructed kidneys revealed a 50% reduction in apoptosis after EGF administration. Northern blot analysis and in situ hybridization for SGP-2 mRNA or clustering gene product also revealed a decreased expression in the obstructed and EGF-treated renal parenchyma.

CONCLUSIONS

These data suggest that EGF, apart from its known role as a mitogenic substance for renal tubular epithelial cells, is also a critical in vivo renal cell survival factor for the developmentally mature kidney.

Injury-induced synthesis and release of apolipoprotein E and clusterin from rat neural cells

Apolipoproteins in the brain have assumed major clinical importance since it was shown that one of the allelic forms of apolipoprotein E, apoE-4, is a risk factor for Alzheimer's disease. Using tissue culture of embryonic rat spinal cord, we examined the effect of neuronal injury on the up-regulation of two apolipoproteins, apolipoprotein E and clusterin (apoJ). In order to study the influence of neuronal cells, we exploited the specific neurotoxic effect of elevated glutamate on these cells. Overstimulation by excess glutamate induced neuronal degeneration as assessed by morphological and biochemical criteria, notably the activity of choline acetyltransferase, which serves as a marker for cholinergic neurons. High concentrations of glutamate increased mRNA synthesis and the production and secretion of both apolipoprotein E and clusterin protein. Both neuronal cell death and release of the peptides were calcium-dependent and could be blocked by the NMDA receptor antagonist MK-801. Immunohistochemical data revealed the presence of clusterin in both neuronal and non-neuronal cells whereas apolipoprotein E was mainly expressed in non-neuronal cells. The results are suggestive of concerted up-regulation of apolipoprotein E and clusterin when neural cells are subjected to injury.

Complement and clusterin in the injured nervous system

Peripheral nerve injury and neuronal degeneration resulting from toxic ricin induce activation of the classical pathway of complement close to the injured motorneuron perikarya or sensory terminals. In contrast, degeneration of central myelinated fibers is not accompanied by complement expression. The main source of complement in peripheral nerve injury and toxic ricin degeneration appears to be microglia. Brain contusion is associated with complement activation. Some of the complement in this situation may derive from plasma, because the blood-brain barrier is disrupted. Clusterin expression is increased in astrocytes along with their activation in the vicinity of lesioned neurons. In addition, axotomized motorneurons show a marked clusterin upregulation. A relationship between clusterin and cell death is suggested by the prominent aggregation of clusterin in neuronal perikarya destroyed by the effects of toxic ricin, as well as by the neosynthesis of clusterin in apparently degenerating nonneuronal cells, presumed to be oligodendrocytes. Our findings indicate that the expression of complement and clusterin are prominent features of neural degeneration and regeneration, as it is in Alzheimer's disease brains as well. The nerve injury conditions described, therefore, offer attractive experimental models to elucidate the roles of these molecular components in neurodegenerative disorders, thereby providing useful insights into potentially new therapeutic approaches in these conditions.

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

A unilateral hypoxic-ischemic (HI) insult in the 21 day old rat has been used to assess the role of clusterin in nerve cell death. Both clusterin mRNA and protein levels were measured at various time points after moderate (15 min) and severe (60 min) HI insult using in situ hybridisation and immunocytochemistry respectively. The severe HI insult lead primarily to necrotic neuronal death and showed very little if any clusterin mRNA and protein induction on the ligated side of the brain. However, following the moderate HI insult there was a dramatic time-dependent accumulation of clusterin protein in neurons of the CA1-CA2 pyramidal cell layers in the hippocampus and cortical layers 3-5, regions undergoing delayed neuronal death. Clusterin mRNA expression, in contrast to neuronal protein accumulation, appeared to be glial in origin (probably astrocytes) with increases in mRNA in and around the hippocampal fissure and only a weak signal over the CA1-CA2 pyramidal cell layer. These results support the hypothesis that the clusterin protein is synthesised in the astrocytes, secreted and then taken up by dying neurons. Clusterin immunoreactivity and in situ DNA end-labelling performed on the same sections revealed that clusterin was accumulating in neurons destined to die by programmed cell death. However the relative time-courses of DNA fragmentation and clusterin immunoreactivity suggest that clusterin production was a result of the selective delayed neuronal death rather than being involved in the biochemical cascade of events that cause it.

Clusterin: a role in cell survival in the face of apoptosis?

Clusterin is a multifunctional glycoprotein complex found in virtually all body fluids and on the surface of cells lining body cavities. Demonstrated and proposed functions include the transport of lipoproteins, the inhibition of complement-mediated cell lysis and the modulation of cell-cell interactions. On the basis of its elevated expression in apoptotic tissues, it was originally proposed that the protein might be casually involved in apoptosis. Here, we discuss the recent data that, in contrast to the earlier notion, suggest that clusterin expression is not enhanced, but rather is down-regulated in the cells undergoing apoptosis and that its expression in the apoptotic tissue is restricted to the vital neighboring cells. These results led to the proposal that rather than being a cell death gene, clusterin is a cell survival gene, exerting a protective function on the surviving bystander cells.

Clusterin: physiologic and pathophysiologic considerations

Clusterin is a heterodimeric glycoprotein produced by a wide array of tissues and found in most biologic fluids. A number of physiologic functions have been proposed for clusterin based on its distribution and in vitro properties. These include complement regulation, lipid transport, sperm maturation, initiation of apoptosis, endocrine secretion, membrane protection, and promotion of cell interactions. A prominent and defining feature of clusterin is its induction in such disease states as glomerulonephritis, polycystic kidney disease, renal tubular injury, neurodegenerative conditions including Alzheimer's disease, atherosclerosis, and myocardial infarction. The expression of clusterin in these states is puzzling, from the specific molecular species and cellular pathways eliciting such expression, to the roles subserved by clusterin once induced. This review will discuss these physiologic and pathophysiologic aspects of clusterin and speculate on its role in disease.

Spatio-temporal pattern of ocular clusterin mRNA expression in the rd mouse

To define the relationship between progressive photoreceptor degeneration and clusterin upregulation, we analysed the spatio-temporal distribution and level of clusterin mRNA in the retinal degeneration (rd) mouse. Expression was increased in the retinal pigment epithelium and inner retina, but not detected in the photoreceptors. These results indicate that increased clusterin mRNA is not causally involved in apoptotic mechanisms of photoreceptor death but may relate to lipid-recycling or cytoprotective functions.

Selective expression of clusterin (SGP-2) and complement C1qB and C4 during responses to neurotoxins in vivo and in vitro

This study concerns expression of the genes encoding three multifunctional proteins: clusterin and two complement cascade components, C1q and C4. Previous work from this and other laboratories has established that clusterin, Clq and C4 messenger RNAs are elevated during Alzheimer's disease, and in response to deafferenting and excitotoxic brain lesion. This study addresses hippocampal clusterin, ClqB and C4 expression in response to neurotoxins that caused selective neuron death. Kainate, which preferentially kills hippocampal CA3 pyramidal neurons but not dentate gyrus granule neurons induced clusterin immunoreactivity in CA1 and CA3 pyramidal neurons and adjacent astrocytes, but not in dentate gyrus granule neurons. In contrast, colchicine, which preferentially kills the dentate gyrus granule neurons, induced clusterin immunoreactivity in the local neuropil as punctate deposits, but not in the surviving or degenerating dentate gyrus granule neurons. Clusterin messenger RNA was increased in astrocytes. ClqB and C4 messenger RNAs increased within 48 h after kainate injections, particularly in the CA3 pyramidal layer, less in the dentate gyrus-CA4, and less in CA1. Clq immunoreactivity was detected in CA1 pyramidal neurons and also as small punctate deposits in the CA1 region at eight and 14 days after kainate. The increase of both clusterin and ClqB messenger RNAs after kainate injections was blocked by barbiturates that prevented seizures and neurodegeneration. In primary hippocampal neuronal cultures treated with glutamate, a subpopulation of cultured neurons that survived glutamate toxicity also had parallel elevations of clusterin and ClqB messenger RNA. In conclusion, cytotoxins that target selective hippocampal neurons increase the expression of both clusterin and ClqB in vivo and in vitro. These results show that elevations of clusterin messenger RNA or protein can be dissociated from each other and from cell death. These increased messenger RNAs were associated with immunoreactive deposits that differed by cell type and intra- versus extracellular locations. These results suggest that the complement system is involved in brain responses to injury.

Expression of complement C1qB and C4 mRNAs during rat brain development

This study examined the distribution of complement C1qB and C4 mRNAs during rat brain development by northern blot and in situ hybridization. Both C1q and C4 mRNAs were already present at embryonic day 14 (E14) and showed little change in abundance through six weeks postnatal. At E16, C1qB mRNA was present at high abundance in putative microglia/macrophages in cortical marginal and intermediate zones, and hippocampal analge, but not in the neurogenic ventricular or sub-ventricular zones. C4 mRNA had a broadly similar regional distribution, but was present at lower abundance in a larger number of cells, putatively neurons. The distribution pattern for C1qB and C4 mRNAs did not change appreciably as brain development proceeded. The lower prevalence of C mRNAs in neuroepithelial or subventricular zones suggests an inverse relationship of C mRNA to cell proliferation. The frequency of apoptotic nuclear profiles, which was as much as ten-fold higher at P7 vs. E17, did not correlate anatomically with C1qB or C4 mRNA levels. Thus, the widespread distribution and consistent presence of each C mRNA during development argues against a role for C in programmed cell death during brain development. We suggest that C1q and C4 components have novel roles during brain development that may be unrelated to normal cytotoxic actions of the activated classical C cascade.

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

Clusterin (SGP-2) is a newly described glycoprotein associated with several putative functions including responses to brain injury. This study reports the regional and cell type expression of clusterin mRNA and its encoded glycoprotein in the rat brain; a limited comparison was also done with the human brain. Using in situ hybridization combined with immunocytochemistry, we found that astrocytes and neurons may express clusterin mRNA in the normal adult brain. While astrocytes throughout the brain contained clusterin mRNA, there was regional selectivity for neuronal clusterin expression. In the striatum, clusterin mRNA was not detected in neurons. Only a subset of substantia nigra dopaminergic neurons or locus ceruleus noradrenergic neurons (tyrosine hydroxylase immunopositive) contained clusterin mRNA. However, neuronal clusterin mRNA was prevalent in pontine nuclei and in the red nucleus of the midbrain tegmentum. Similarly, clusterin mRNA was prevalent in both rat and human hippocampal neuron-specific enolase immunopositive pyramidal neurons, although rat CA1 neurons had less mRNA than CA2-CA3 neurons. Monotypic primary cell cultures from the neonatal rat showed clusterin mRNA in both neurons and astrocytes, but not in microglia. By immunocytochemistry, no clusterin immunopositive glia were observed in any region of the rat brain, confirming previous studies. However, clusterin immunopositive cells (putative neurons) were observed in the Purkinje cell layer of the cerebellum, medial and interposed cerebellar nuclei, trigeminal motor nucleus, and red nucleus. Finally, in vitro studies suggest that astrocytes, but not neurons, secrete clusterin, which is pertinent to clusterin immunodeposits found after experimental lesioning.

Comparison of clusterin gene expression in normal and dystrophic human retinas

To gain insight into the mechanisms underlying altered clusterin expression in retinal degeneration, the cellular distribution of clusterin mRNA in normal and in retinitis pigmentosa-affected retinas was compared using in situ hybridization. In contrast to the normal retina, where clusterin mRNA is localized in the inner nuclear and ganglion cell layers, a clustered distribution is observed throughout the dystrophic retina. The results indicate an expression of clusterin gene in normal retinal neurons and suggest that its altered regulation in neurodegeneration is not purely a glial cell phenomenon.

Abnormal distribution of retinal clusterin in retinitis pigmentosa

Increased expression of clusterin mRNA is associated with neurodegenerative states, including retinas affected by retinitis pigmentosa (RP). We have investigated the distribution of immunoreactive clusterin in normal and RP-affected retinas. Reactivity at the inner limiting membrane, plexiform layers, and photoreceptors in normal retina accords well with clusterin's postulated role as a membrane protective agent. In RP-affected retina the organized distribution is lost and overall reactivity appears decreased. The changes in this case may reflect increased turnover or removal of clusterin, perhaps via interaction with components of the immune system.