Lack of causal relationship between clusterin expression and photoreceptor apoptosis in light-induced retinal degeneration

The normalizing effects of the CYP46A1 activator efavirenz on retinal sterol levels and risk factors for glaucoma in Apoj-/- mice

Apolipoprotein J (APOJ) is a multifunctional protein with genetic evidence suggesting an association between APOJ polymorphisms and Alzheimer's disease as well as exfoliation glaucoma. Herein we conducted ocular characterizations of Apoj^-/- mice and found that their retinal cholesterol levels were decreased and that this genotype had several risk factors for glaucoma: increased intraocular pressure and cup-to-disk ratio and impaired retinal ganglion cell (RGC) function. The latter was not due to RGC degeneration or activation of retinal Muller cells and microglia/macrophages. There was also a decrease in retinal levels of 24-hydroxycholesterol, a suggested neuroprotectant under glaucomatous conditions and a positive allosteric modulator of N-methyl-D-aspartate receptors mediating the light-evoked response of the RGC. Therefore, Apoj^-/- mice were treated with low-dose efavirenz, an allosteric activator of CYP46A1 which converts cholesterol into 24-hydroxycholesterol. Efavirenz treatment increased retinal cholesterol and 24-hydroxycholesterol levels, normalized intraocular pressure and cup-to-disk ratio, and rescued in part RGC function. Retinal expression of Abcg1 (a cholesterol efflux transporter), Apoa1 (a constituent of lipoprotein particles), and Scarb1 (a lipoprotein particle receptor) was increased in EVF-treated Apoj^-/- mice, indicating increased retinal cholesterol transport on lipoprotein particles. Ocular characterizations of Cyp46a1^-/- mice supported the beneficial efavirenz treatment effects via CYP46A1 activation. The data obtained demonstrate an important APOJ role in retinal cholesterol homeostasis and link this apolipoprotein to the glaucoma risk factors and retinal 24-hydroxycholesterol production by CYP46A1. As the CYP46A1 activator efavirenz is an FDA-approved anti-HIV drug, our studies suggest a new therapeutic approach for treatment of glaucomatous conditions.

Clusterin, other extracellular chaperones, and eye disease

Proteostasis refers to all the processes that maintain the correct expression level, location, folding and turnover of proteins, essential to organismal survival. Both inside cells and in body fluids, molecular chaperones play key roles in maintaining proteostasis. In this article, we focus on clusterin, the first-recognized extracellular mammalian chaperone, and its role in diseases of the eye. Clusterin binds to and inhibits the aggregation of proteins that are misfolded due to mutations or stresses, clears these aggregating proteins from extracellular spaces, and facilitates their degradation. Clusterin exhibits three main homeostatic activities: proteostasis, cytoprotection, and anti-inflammation. The so-called "protein misfolding diseases" are caused by aggregation of misfolded proteins that accumulate pathologically as deposits in tissues; we discuss several such diseases that occur in the eye. Clusterin is typically found in these deposits, which is interpreted to mean that its capacity as a molecular chaperone to maintain proteostasis is overwhelmed in the disease state. Nevertheless, the role of clusterin in diseases involving such deposits needs to be better defined before therapeutic approaches can be entertained. A more straightforward case can be made for therapeutic use of clusterin based on its proteostatic role as a proteinase inhibitor, as well as its cytoprotective and anti-inflammatory properties. It is likely that clusterin works together in this way with other extracellular chaperones to protect the eye from disease, and we discuss several examples. We end this article by predicting future steps that may lead to development of clusterin as a biological drug.

Protective effect of clusterin on rod photoreceptor in rat model of retinitis pigmentosa

Retinitis Pigmentosa (RP) begins with the death of rod photoreceptors and is slowly followed by a gradual loss of cones and a rearrangement of the remaining retinal neurons. Clusterin is a chaperone protein that protects cells and is involved in various pathophysiological stresses, including retinal degeneration. Using a well-established transgenic rat model of RP (rhodopsin S334ter), we investigated the effects of clusterin on rod photoreceptor survival. To investigate the role of clusterin in S334ter-line3 retinas, Voronoi analysis and immunohistochemistry were used to evaluate the geometry of rod distribution. Additionally, immunoblot analysis, Bax activation, STAT3 and Akt phosphorylation were used to evaluate the pathway involved in rod cell protection. In this study, clusterin (10μg/ml) intravitreal treatment produced robust preservation of rod photoreceptors in S334ter-line3 retina. The mean number of rods in 1mm² was significantly greater in clusterin injected RP retinas (postnatal (P) 30, P45, P60, & P75) than in age-matched saline injected RP retinas (P<0.01). Clusterin activated Akt, STAT3 and significantly reduced Bax activity; in addition to inducing phosphorylated STAT3 in Müller cells, which suggests it may indirectly acts on photoreceptors. Thus, clusterin treatment may interferes with mechanisms leading to rod death by suppressing cell death through activation of Akt and STAT3, followed by Bax suppression. Novel insights into the pathway of how clusterin promotes the rod cell survival suggest this treatment may be a potential therapeutic strategy to slow progression of vision loss in human RP.

Clusterin as a therapeutic target

INTRODUCTION

Clusterin (CLU) is a stress-activated, ATP-independent molecular chaperone, normally secreted from cells, that is up-regulated in Alzheimer disease and in many cancers. It plays important roles in protein homeostasis/proteostasis, inhibition of cell death pathways, and modulation of pro-survival signalling and transcriptional networks. Changes in the CLU gene locus are highly associated with Alzheimer disease, and many therapy-resistant cancers over-express CLU. The extensive post-translational processing and heterogeneous oligomerization of CLU have so far prevented any definitive structure determination. This in turn has meant that targeting CLU with small molecule inhibitors is challenging. Therefore, inhibiting CLU at the gene-expression level using siRNA or antisense is a valid approach to inhibit its function. Areas covered: This article reviews recent advances regarding the role of CLU in proteostasis, cellular trafficking, human diseases, and signalling pathways involved in oncogenesis. It addresses the rationale for CLU as a therapeutic target in cancer, and the current status of pre-clinical and clinical studies using CLU antisense inhibitor OGX011. Expert opinion: Discusses challenges facing the therapeutic targeting of CLU including rapid changes in the treatment landscape for prostate cancer with multiple new FDA approved drugs, selection of windows of intervention, and potential side effects when silencing CLU expression.

Clusterin in the eye: An old dog with new tricks at the ocular surface

The multifunctional protein clusterin (CLU) was first described in 1983 as a secreted glycoprotein present in ram rete testis fluid that enhanced aggregation ('clustering') of a variety of cells in vitro. It was also independently discovered in a number of other systems. By the early 1990s, CLU was known under many names and its expression had been demonstrated throughout the body, including in the eye. Its homeostatic activities in proteostasis, cytoprotection, and anti-inflammation have been well documented, however its roles in health and disease are still not well understood. CLU is prominent at fluid-tissue interfaces, and in 1996 it was demonstrated to be the most highly expressed transcript in the human cornea, the protein product being localized to the apical layers of the mucosal epithelia of the cornea and conjunctiva. CLU protein is also present in human tears. Using a preclinical mouse model for desiccating stress that mimics human dry eye disease, the authors recently demonstrated that CLU prevents and ameliorates ocular surface barrier disruption by a remarkable sealing mechanism dependent on attainment of a critical all-or-none concentration in the tears. When the CLU level drops below the critical all-or-none threshold, the barrier becomes vulnerable to desiccating stress. CLU binds selectively to the ocular surface subjected to desiccating stress in vivo, and in vitro to LGALS3 (galectin-3), a key barrier component. Positioned in this way, CLU not only physically seals the ocular surface barrier, but it also protects the barrier cells and prevents further damage to barrier structure. CLU depletion from the ocular surface epithelia is seen in a variety of inflammatory conditions in humans and mice that lead to squamous metaplasia and a keratinized epithelium. This suggests that CLU might have a specific role in maintaining mucosal epithelial differentiation, an idea that can now be tested using the mouse model for desiccating stress. Most excitingly, the new findings suggest that CLU could serve as a novel biotherapeutic for dry eye disease.

The Role of Clusterin inIn VitroIschemia of Human Retinal Endothelial Cells

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

Citicoline reduces upregulated clusterin following kainic acid injection in the rat retina

PURPOSE

To investigate the effects of citicoline on upregulated clusterin and retinal damage induced by kainic acid (KA).

METHODS

KA was injected into the vitreous of rats. Effects of systemic citicoline treatments were estimated by measuring the thickness of the various retinal layers, immunoblotting, and immunohistochemical techniques.

RESULTS

One day after KA injection, the immunoreactivity of clusterin increased significantly. In rats treated with KA plus citicoline, clusterin immunoreactivity was markedly reduced compared to KA-treated rats. Western blot analysis showed that clusterin protein levels were increased in KA-treated rats, but decreased in KA plus citicoline-treated rats. Apoptotic cell death was determined by TUNEL method. Citicoline reduced the expression of clusterin, as well as the expression of TUNEL after KA injection in the rat retina.

CONCLUSION

The increased expression of clusterin following KA injection in the rat retina suggests the presence of neurodegenerative events; citicoline may provide neuroprotection against neuronal cell damage.

Transplantation of the RPE in AMD

The retinal pigment epithelium (RPE) maintains retinal function as the metabolic gatekeeper between photoreceptors (PRs) and the choriocapillaries. The RPE and Bruch's membrane (BM) suffer cumulative damage over lifetime, which is thought to induce age-related macular degeneration (AMD) in susceptible individuals. Unlike palliative pharmacologic treatments, replacement of the RPE has a curative potential for AMD. This article reviews mechanisms leading to RPE dysfunction in aging and AMD, laboratory studies on RPE transplantation, and surgical techniques used in AMD patients. Future strategies using ex vivo steps prior to transplantation, BM prosthetics, and stem cell applications are discussed. The functional peculiarity of the macular region, epigenetic phenomena leading to an age-related shift in protein expression, along with the accumulation of lipofuscin may affect the metabolism in the central RPE. Thickening of BM with age decreases its hydraulic conductivity. Drusen are deposits of extracellular material and formed in part by activation of the alternative complement pathway in individuals carrying a mutant allele of complement factor H. AMD likely represents an umbrella term for a disease entity with multifactorial etiology and manifestations. Presently, a slow progressing (dry) non-neovascular atrophic form and a rapidly blinding neovascular (wet) form are discerned. No therapy is currently available for the former, while RPE transplantation and promising (albeit non-causal) anti-angiogenic therapies are available for the latter. The potential of RPE transplantation was demonstrated in animal models. Rejection of allogeneic homologous transplants in patients focused further studies on autologous sources. In vitro studies elucidated cell adhesion and wound healing mechanisms on aged human BM. Currently, autologous RPE, harvested from the midperiphery, is being transplanted as a cell suspension or a patch of RPE and choroid in AMD patients. These techniques have been evaluated from several groups. Autologous RPE transplants may have the disadvantage of carrying the same genetic information that may have led to AMD manifestation. An intermittent culturing step would allow for in vitro therapy of the RPE, its rejuvenation and prosthesis of BM to improve the success RPE transplants. Recent advances in stem cell biology when combined with lessons learned from studies of RPE transplantation are intriguing future therapeutic modalities for AMD patients.

Mutation screen of the cone-specific gene, CLUL1, in 376 patients with age-related macular degeneration

Clusterin is a secreted glycoprotein expressed ubiquitously in many tissues that appears to function as a molecular chaperone capable of protecting stressed proteins. It is upregulated in many different forms of neurodegeneration and is thought to represent a defense response against neuronal damage. Clusterin has been found to be a common protein identified in drusen preparations isolated from the retina of donor eyes of patients with age-related macular degeneration (AMD), the leading cause of blindness in the elderly population of developed countries. A retina-specific clusterin-like protein (CLUL1) showing nearly 25% identity to clusterin at the protein level was recently cloned and shown to be expressed specifically in cone photoreceptor cells. For these reasons, we investigated CLUL1 as a candidate gene for AMD. A mutation screen of the entire coding region of the CLUL1 gene in 376 unrelated patients with AMD uncovered three sequence variations, one isocoding change and two intronic changes. One intronic change appears significantly less frequent in patients with the more severe forms of AMD than in control subjects, suggesting that this variant may reduce the risk for AMD or may be linked to a nearby variant that may reduce AMD risk. Variant alleles of the CLUL1 gene were found; however, none are considered pathogenic. None of the variants identified are predicted to create or destroy splice donor or acceptor sites based on splice-site prediction software.

Expression of clusterin in Müller cells of the rat retina after pressure-induced ischemia

We have investigated the expression and cellular localization of clusterin in the rat retina following ischemia induced by transiently increasing the intraocular pressure. In the normal retina, weak clusterin immunoreactivity was visible in Müller cell profiles located in the inner nuclear layer. Following ischemia and reperfusion, strong immunoreactivity appeared in Müller cell somata and processes up to 3 days postlesion. Quantitative evaluation by immunoblotting confirmed that clusterin expression continuously increased and showed a peak value at 3 days after ischemic injury (to 1300% of control levels), and then decreased again to 400% of controls at 4 weeks postlesion. Immunocytochemistry using antisera against clusterin or glutamine synthase combined with the TUNEL method or immunocytochemistry using antisera activated caspase 3 and electron microscopy revealed that some clusterin-labeled Müller cells underwent apoptotic cell death. Our findings demonstrate that some Müller cells die by apoptosis, and suggest that clusterin produced and released by Müller cell may play an important role in the pathogenesis of ischemic injury in the rat retina.

Multidestructive pathways triggered in photoreceptor cell death of the rd mouse as determined through gene expression profiling

In the rd/rd mouse, photoreceptor degeneration is due to a mutation of the rod-specific enzyme cGMP phosphodiesterase, resulting in permanently opened cGMP-gated cation channels in the rod outer segment membrane that allow Na(+) and Ca(2+) ions to enter the cell, resulting in possibly toxic levels of Ca(2+). To identify pathways involved in cell death of the rd/rd rods, we evaluated gene expression in the rd/rd and wild type (wt) mouse retina (U74A oligonucleotide arrays (Affymetrix)) over the known time course of photoreceptor degeneration. 181 genes passed the selection criteria (low standard deviation and high correlation between replicates), falling into six clusters. For any given pair of genes, an expression profile correlation distance and a semantic distance (one for each class of gene ontology terms) were established using newly designed software. Gene expression in rd/rd started to deviate from wt by postnatal day 10. The reduction in photoreceptor-specific genes followed the known time course of photoreceptor degeneration. Likewise the increase in transcription factors and apoptosis- and neuroinflammation-specific genes followed the kinetics of the rise in intracellular cGMP in the rod photoreceptors. In addition, genes coding for calcium-binding proteins and those implicated in tissue and vessel remodeling were increased. These results suggest that photoreceptor degeneration in the rd/rd mouse is a process starting with Ca(2+) toxicity followed by secondary insults involving multidestructive pathways such as apoptosis and neuroinflammation, presumably boosting morphological changes. All of these components need to be addressed if rods are to be successfully protected.

Identification of a unique transcript down-regulated in the retina of rainbow trout (Oncorhynchus mykiss) at smoltification

Developmental and physiological changes in the retina of salmonid fishes occur during smoltification, a metamorphic event associated with thyroid hormone that prepares salmon for oceanic migration. These changes include loss of ultraviolet-sensitive (UVS) cone photoreceptors, switching of visual pigments, alterations in thyroid hormone regulation, and associated changes in behavior. This model provides an opportunity to study substantial neuronal development within an established retina. Little is known, however, about how higher order neurons are altered or how retinal gene expression changes during this transition. Here, we have used differential display RT-PCR and RACE-PCR to identify a previously uncharacterized gene transcript in Oncorhynchus mykiss under developmental regulation in the retina during smoltification, rtp12.5. This unique cDNA encodes a putative protein 112 amino acids long similar to a hypothetical human open reading frame located on chromosome 14.Q24.2. Differential expression was confirmed by RNA dot blot and in situ hybridization. We also present O. mykiss sep15 cDNA sequence and describe its expression in the vertebrate retina. Considering the expression pattern within retinal tissue observed by in situ hybridization, rtp12.5 may be under TH regulation and involved in neuronal remodeling of the retina during loss of UVS cones.

Complement activation and inflammatory processes in Drusen formation and age related macular degeneration

Recent studies implicate inflammation and complement mediated attack as early events in drusen biogenesis. The investigations described here sought to determine whether primary sites of complement activation could be identified within drusen substructure, and whether known inhibitors of the terminal pathway of complement are present in drusen and/or retinal pigmented epithelial (RPE) cells that lie in close proximity to drusen. Immunohistochemical examination shows two fluid phase regulators of the terminal pathway, vitronectin (Vn, S-protein) and clusterin (apolipoprotein J), to be present in drusen; Vn also accumulates in the cytoplasm of RPE cells that are closely associated with drusen. The membrane associated complement inhibitor, complement receptor 1, is also localized in drusen, but it is not detected in RPE cells immunohistochemically. In contrast, a second membrane associated complement inhibitor, membrane cofactor protein, is present in drusen associated RPE cells, as well as in small, spherical substructural elements within drusen. These previously unidentified elements also show strong immunoreactivity for proteolytic fragments of complement component C3 that are characteristically deposited at sites of complement activation. It is proposed that these structures represent residual debris from degenerating RPE cells that are the targets of complement attack. It is likely that RPE cell debris entrapped between the RPE monolayer and Bruch's membrane serves as a chronic inflammatory stimulus and a potential nucleation site for drusen formation. Thus, the process of drusen biogenesis may be envisaged as a secondary manifestation of primary RPE pathology that is exacerbated by consequences of local inflammatory processes.

When X-ray-inducible proteins meet DNA double strand break repair

Cellular responses to ionizing radiation (IR) include (a) activation of signal transduction enzymes; (b) stimulation of DNA repair, most notably DNA double strand break (DSB) repair by homologous or nonhomologous recombinatorial pathways; (c) activation of transcription factors and subsequent IR-inducible transcript and protein changes; (d) cell cycle checkpoint delays in G(1), S, and G(2) required for repair or for programmed cell death of severely damaged cells; (e) activation of zymogens needed for programmed cell death (although IR is a poor inducer of such responses in epithelial cells); and (f) stimulation of IR-inducible proteins that may mediate bystander effects influencing signal transduction, DNA repair, angiogenesis, the immune response, late responses to IR, and possibly adaptive survival responses. The overall response to IR depends on the cell's inherent genetic background, as well as its ability to biochemically and genetically respond to IR-induced damage. To improve the anti-tumor efficacy of IR, our knowledge of these pleiotropic responses must improve. The most important process for the survival of a tumor cell following IR is the repair of DNA double strand breaks (DSBs). Using yeast two-hybrid analyses along with other molecular and cellular biology techniques, we cloned transcripts/proteins that are involved in, or presumably affect, nonhomologous DNA double strand break end-joining (NHEJ) repair mediated by the DNA-PK complex. Using Ku70 as bait, we isolated a number of Ku-binding proteins (KUBs). We identified the first X-ray-inducible transcript/protein (xip8, Clusterin (CLU)) that associates with DNA-PK. A nuclear form of CLU (nCLU) prevented DNA-PK-mediated end joining, and stimulated cell death in response to IR or when overexpressed in the absence of IR. Structure-function analyses using molecular and cellular (including green fluorescence-tagged protein trafficking) biology techniques showed that nCLU appears to be an inactive protein residing in the cytoplasm of epithelial cells. Following IR injury, nCLU levels increase and an as yet undefined posttranslational modification appears to alter the protein, exposing nuclear localization sequences (NLSs) and coiled-coil domains. The modified protein translocates to the nucleus and triggers cell death, presumably through its interaction specifically with Ku70. Understanding nCLU responses, as well as the functions of the KUBs, will be important for understanding DSB repair. Knowledge of DSB repair may be used to improve the antitumor efficacy of IR, as well as other chemotherapeutic agents.