High-density lipoprotein aggregated by oxidation induces degeneration of neuronal cells

The level of 24-hydroxycholesteryl esters decreases in plasma of patients with Parkinson's disease

24-hydroxycholesterol (24OH-C) is synthesized almost exclusively in neurons. This oxysterol is mostly present as ester form in both cerebrospinal fluid and plasma. The enzyme lecithin-cholesterol acyltransferase esterifies 24OH-C in the brain, and the level of 24OH-C esters in cerebrospinal fluid was found to be correlated with the level of 24OH-C esters in plasma. Decreased levels of 24OH-C esters levels were previously found in Alzheimer's disease and Amyotrophic Lateral Sclerosis. This finding was attributed to the inhibitory effect of oxidative stress on lecithin-cholesterol acyltransferase activity in neurodegenerative conditions. Data reported here show that the plasma level of 24OH-C esters is decreased also in Parkinson's disease. ROC analysis identified 69.0% of 24OH-C esterification as the threshold (AUC = 0.98) discriminating patients (N = 19) from healthy subjects (N = 19) with 100% specificity vs controls, 89.5% sensitivity, 94.7% accuracy, and 100% precision. The level of 24OH-C esters was not correlated with UPDRS I or UPDRS III when evaluated at the time of blood sampling. By contrast, it was negatively correlated with UPDRS I (r = -0.4984, p = 0.0299) after one year of follow up. Therefore, this level might represent a novel biomarker of neurodegeneration in Parkinson's disease. The biomarker level is here proposed as a measure to evaluate the severity of disease, as well as to monitor the progression of this pathology.

Increased levels of lipid hydroperoxides in plasma of patients with multiple sclerosis: a relationship with paraoxonase activity

Paraoxonase, an enzyme associated with high density lipoproteins (HDL), plays an important role in the anti-oxidant and anti-inflammatory properties exerted by HDL. Increasing evidence supports a role of free radicals and oxidative stress in the inflammatory processes and in the pathogenesis of multiple sclerosis (MS). The aim of this study was to further investigate the relationship between oxidative damage and MS; therefore we compared the paraoxonase activity and levels of cholesteryl ester hydroperoxides (CE-OOH), as marker of lipid peroxidation, in plasma isolated from healthy subjects (n=89) and from MS patients (n=24) in the early stage disability (EDSSB<3.5). Our results demonstrated for the first time that the activity of paraoxonase in the plasma of MS subjects was significantly lower with respect to controls (p<0.001). Moreover, our results showed a significant increase in the levels of CE-OOH in plasma from MS subjects (p<0.001). CE-OOH are biologically active substances derived from the oxidation of cholesteryl ester localized in the hydrophobic core of plasma lipoproteins (HDL, LDL). Therefore, our study demonstrates alterations of lipoprotein peroxidation in MS and provides further evidence that oxidative stress and impairment of the anti-oxidant system may play a role in MS.

Factors Associated With Age-related Hearing Impairment: A Retrospective Cohort Study

Age-related hearing impairment (ARHI) is a complex degenerative disease in the elderly. As multiple factors interact during the development of ARHI, it is important to elucidate the major influencing factors to understand and prevent ARHI. We aimed to identify risk factors associated with the development of ARHI with a retrospective cohort from 2001 to 2010. The records of the adult subjects over 40 years of age who consecutively underwent a comprehensive health checkup including pure-tone audiometry at the Health Promotion Center were reviewed. During this period, 1560 subjects who underwent pure-tone audiometry more than twice, had no other otologic diseases, and were followed-up more than 2 years were included. A pure-tone average (PTA: 0.5, 1, 2, 4 kHz) was calculated. Development of ARHI was defined as a PTA at follow-up more than 10 dB greater than the baseline PTA. Times to the first development of ARHI were investigated. Overall, 12.7% of subjects developed ARHI within the first 4 years. High blood ionized calcium (hazard ratio [HR] 0.084), albumin (HR 0.239), systolic blood pressure (HR 0.577), thyroid hormone (T3) (HR 0.593), and alpha fetoprotein levels (HR 0.883) were associated with decreased hazard for the development of ARHI. In contrast, high blood high-density lipoprotein (HR 2.105), uric acid (HR 1.684), total protein (HR 1.423), and total bilirubin levels (HR 1.220) were potential risk factors for the development of ARHI. Development of ARHI is common among the aged population, and a variety of factors may interact during this process. The results of this study can be used for counseling of adults at high-risk of developing ARHI with regard to regular audiological check-up.

Peroxidation of lipoproteins in multiple sclerosis

Human plasma low density lipoproteins (LDL) and high density lipoproteins (HDL) are involved in the transport of lipids, modulate membrane lipid composition and regulate signal transduction. HDL-like lipoproteins have been shown also in human cerebrospinal fluid and it has been hypothesized that they could have a role in lipid transport in central nervous system. After synthesis, lipoproteins are susceptible to lipid peroxidation triggered by reactive oxygen species (ROS and RNS) produced by peripheral and brain cells. Aim of the paper has been to review the scientific literature on the role of lipid peroxidation of LDL and HDL in the molecular mechanisms of multiple sclerosis (MS). Several studies have demonstrated a significant increase in lipid peroxidation products in brain, plasma and cerebrospinal fluid of MS patients. The increase of antibodies against ox-LDL in plasma and the presence of ox-LDL in demyelinating plaques in MS brain suggests that the disease is associated with oxidative damage of lipoproteins. The impairment of antioxidant systems or an increase in the production of ROS and RNS could contribute to lipoprotein peroxidation in MS. Oxidized lipoproteins show several alterations of their functions, they are neurotoxic and have pro-inflammatory properties. Therefore lipoprotein lipid peroxidation products could be involved in demyelination and axonal injury in MS.

Oxidized low-density lipoprotein induces apoptotic insults to mouse cerebral endothelial cells via a Bax-mitochondria-caspase protease pathway

Cerebral endothelial cells (CECs) are crucial components of the blood-brain barrier. Oxidized low-density lipoprotein (oxLDL) can induce cell injuries. In this study, we attempted to evaluate the effects of oxLDL on mouse CECs and its possible mechanisms. Mouse CECs were isolated from brain tissues and identified by immunocytochemical staining of vimentin and Factor VIII. oxLDL was prepared from LDL oxidation by copper sulfate. Exposure of mouse CECs to oxLDL decreased cell viability in concentration- and time-dependent manners. oxLDL time-dependently caused shrinkage of cell morphologies. Administration of oxLDL to CECs induced DNA fragmentation in concentration- and time-dependent manners. Analysis of the cell cycle revealed that oxLDL concentration- and time-dependently increased the proportion of CECs which underwent apoptosis. Analysis of confocal microscopy and immunoblot revealed that oxLDL significantly increased cellular and mitochondrial Bax levels as well as the translocation of this proapoptotic protein from the cytoplasm to mitochondria. In parallel with the increase in the levels and translocation of Bax, oxLDL time-dependently decreased the mitochondrial membrane potential. Exposure of mouse CECs to oxLDL decreased the amounts of mitochondrial cytochrome c, but enhanced cytosolic cytochrome c levels. The amounts of intracellular reactive oxygen species were significantly augmented after oxLDL administration. Sequentially, oxLDL increased activities of caspase-9, -3, and -6 in time-dependent manners. Pretreatment with Z-VEID-FMK, an inhibitor of caspase-6, significantly decreased caspase-6 activity and the oxLDL-induced DNA fragmentation and cell apoptosis. This study showed that oxLDL induces apoptotic insults to CECs via signal-transducing events, including enhancing Bax translocation, mitochondrial dysfunction, cytochrome c release, increases in intracellular reactive oxygen species, and cascade activation of caspase-9, -3, and -6. Therefore, oxLDL can damage the blood-brain barrier through induction of CEC apoptosis via a Bax-mitochondria-caspase protease pathway.

Structural modifications of HDL and functional consequences

High density lipoproteins (HDL) are susceptible to structural modifications mediated by various mechanisms including oxidation, glycation, homocysteinylation or enzymatic degradation. Structural alterations of HDL may affect their functional and atheroprotective properties. Oxidants, such as hypochlorous acid, peroxyl radicals, metal ions, peroxynitrite, lipoxygenases and smoke extracts, can alter both surface and core components of HDL. The formation of lipid peroxidation derivatives, such as thiobarbituric acid reactive substances, conjugated dienes, lipid hydroperoxides and aldehydes, is associated with changes of physical properties (fluidity, molecular order) and of apoprotein conformation. Non-enzymatic glycation, generally associated with lipoxidation, leads to form irreversible complexes called advanced glycation end products. These HDL modifications are accompanied with altered biological activities of HDL and associated enzymes, including paraoxonase, CETP and LCAT. Homocysteine-induced modification of HDL is mediated by homocysteine-thiolactone, and can be prevented by a calcium-dependent thiolactonase/paraoxonase. Tyrosylation of HDL induces the formation of dimers and trimers of apo AI, and alters cholesterol efflux. Phospholipases and proteolytic enzymes can also modify HDL lipid and apoprotein structure. HDL modification induces generally the loss of their anti-inflammatory and cytoprotective properties. This could play a role in the pathogenesis of atherosclerosis and neurodegenerative diseases such as Alzheimer's disease.

Diverse fibrillar peptides directly bind the Alzheimer's amyloid precursor protein and amyloid precursor-like protein 2 resulting in cellular accumulation

The Alzheimer's disease Abeta peptide can increase the levels of cell-associated amyloid precursor protein (APP) in vitro. To determine the specificity of this response for Abeta and whether it is related to cytotoxicity, we tested a diverse range of fibrillar peptides including amyloid-beta (Abeta), the fibrillar prion peptides PrP106-126 and PrP178-193 and human islet-cell amylin. All these peptides increased the levels of APP and amyloid precursor-like protein 2 (APLP2) in primary cultures of astrocytes and neurons. Specificity was shown by a lack of change to amyloid precursor-like protein 1, tau-1 and cellular prion protein (PrP(c)) levels. APP and APLP2 levels were elevated only in cultures exposed to fibrillar peptides as assessed by electron microscopy and not in cultures treated with non-fibrillogenic peptide variants or aggregated lipoprotein. We found that PrP106-126 and the non-toxic but fibril-forming PrP178-193 increased APP levels in cultures derived from both wild-type and PrP(c)-deficient mice indicating that fibrillar peptides up-regulate APP through a non-cytotoxic mechanism and irrespective of parental protein expression. Fibrillar PrP106-126 and Abeta peptides bound recombinant APP and APLP2 suggesting the accumulation of these proteins was mediated by direct binding to the fibrillated peptide. This was supported by decreased APP accumulation following extensive washing of the cultures to remove fibrillar aggregates. Pre-incubation of fibrillar peptide with recombinant APP18-146, the putative fibril binding site, also abrogated the accumulation of APP. These findings show that diverse fibrillogenic peptides can induce accumulation of APP and APLP2 and this mechanism could contribute to pathogenesis in neurodegenerative disorders.

Scavenger receptors in neurobiology and neuropathology: their role on microglia and other cells of the nervous system

Scavenger receptor class A (SR-A, CD204), scavenger receptor-BI (SR-BI), and CD36 are cell surface proteins that mediate cell adhesion to, and endocytosis of, various native and pathologically modified substances, and participate in intracellular signaling, lipid metabolism, and host defense against bacterial pathogens. Microglia, Mato cells, astrocytes, cerebral microvascular endothelial cells, cerebral arterial smooth muscle cells, and retinal pigment epithelial cells express one or more of these SR. Expression of SR-A and SR-BI by microglia is developmentally regulated. Neonatal microglia express SR-A and SR-BI, while microglia in normal mouse and human adult brain express neither. Astrocytes in adult brain express SR-BI. In Alzheimer's disease, microglial expression of SR-A is increased. Such findings, and evidence that SR-A and SR-BI mediate adhesion and endocytosis of fibrillar beta-amyloid by microglia and astrocytes, respectively, and that SR-A, SR-BI, and CD36 participate in secretion of reactive oxygen species by microglia, suggest roles for these receptors in homeostasis and neuropathology.

Neuroplasticity in Alzheimer's disease

Ramon y Cajal proclaimed in 1928 that "once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree." (Ramon y Cajal, 1928). In large part, despite the extensive knowledge gained since then, the latter directive has not yet been achieved by 'modern' science. Although we know now that Ramon y Cajal's observation on CNS plasticity is largely true (for lower brain and primary cortical structures), there are mechanisms for recovery from CNS injury. These mechanisms, however, may contribute to the vulnerability to neurodegenerative disease. They may also be exploited therapeutically to help alleviate the suffering from neurodegenerative conditions.

Human, but not bovine, oxidized cerebral spinal fluid lipoproteins disrupt neuronal microtubules

Cerebral spinal fluid (CSF) lipoproteins have become a focus of research since the observation that inheritance of particular alleles of the apolipoprotein E gene affects the risk of Alzheimer's disease (AD). There is evidence of increased lipid peroxidation in CSF lipoproteins from patients with AD, but the biological significance of this observation is not known. A characteristic of the AD brain is a disturbance of the neuronal microtubule organization. We have shown previously that 4-hydroxy-2(E)-nonenal, a major product of lipid peroxidation, causes disruption of neuronal microtubules and therefore tested whether oxidized CSF lipoproteins had the same effect. We exposed Neuro 2A cells to human CSF lipoproteins and analyzed the microtubule organization by immunofluorescence. In vitro oxidized human CSF lipoproteins caused disruption of the microtubule network, while their native (nonoxidized) counterparts did not. Microtubule disruption was observed after short exposures (1 h) and lipoprotein concentrations were present in CSF (20 microg/mL), conditions that did not result in loss of cell viability. Importantly, adult bovine CSF lipoproteins, oxidized under identical conditions, had no effect on the microtubule organization of Neuro 2A cells. Comparison of human and bovine CSF lipoproteins revealed similar oxidation-induced modifications of apolipoproteins E and A-I as analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting. Fatty acid analysis revealed substantially lower amounts of unsaturated fatty acids in bovine CSF lipoproteins, when compared to their human counterparts. Our data therefore indicate that oxidized human CSF lipoproteins are detrimental to neuronal microtubules. This effect is species-specific, since equally oxidized bovine CSF lipoproteins left the neuronal microtubule organization unchanged.

Cerebrospinal fluid lipoproteins in Alzheimer's disease

Interest in cerebrospinal fluid (CSF) lipoproteins has been stimulated by the association of certain alleles of the human apolipoprotein E gene (APOE) with an increased risk of Alzheimer's disease (AD), and because apolipoprotein E (apoE) is one of the major apolipoproteins in CSF. CSF lipoproteins (d < 1.210 g/ml fraction) are distinct from their plasma counterparts, and in AD patients CSF may contain novel particles. The protein concentration of CSF lipoproteins is reduced in AD patients. Moreover, the molecular distribution of apoE- and apoAII-containing apolipoproteins in CSF is dictated by APOE. The lipid composition suggests that CSF lipoproteins from AD patients may have undergone increased free radical-mediated damage; experimental data support the possibility that this may occur both before and after lipoprotein assembly. Finally, human CSF lipoproteins oxidized ex vivo are neurotoxic to neuronal cells in culture and disrupt microtubule structure, an activity not observed with oxidized bovine CSF lipoproteins. CSF lipoproteins may represent a means whereby apoE influences the outcome of free radical-mediated damage to brain.

Oxidized high-density lipoprotein induces neuron death

High-density lipoprotein (HDL) exists within the brain and is highly vulnerable to oxidative modifications. The focus of the present study was to determine the effect of HDL and oxidized HDL (oxHDL) upon neurons, astrocytes, and microglia. Administration of highly oxidized HDL, but not native, minimally, or moderately modified HDL resulted in a dose- and time-dependent increase in oxidative stress and death of cultured rat embryonic neurons. Astrocyte and microglia cultures treated with highly oxidized HDL displayed increased reactive oxygen species formation but no toxicity. Application of oxHDL exacerbated oxidative stress and neuron death induced by beta-amyloid peptide. Studies using pharmacological inhibitors implicate the involvement of calcium and reactive oxygen species in oxHDL-induced neuronal loss. Neural cells expressing increased levels of BCL-2 had decreased levels of oxidative stress and neuron death following exposure to oxHDL. Together, these data demonstrate that oxHDL increases oxidative stress in neurons, astrocytes, and microglia which ultimately culminate in neuron death.

Cerebrospinal fluid lipoproteins are more vulnerable to oxidation in Alzheimer's disease and are neurotoxic when oxidized ex vivo

Brain regional oxidative damage is thought to be a central mechanism in the pathogenesis of Alzheimer's disease (AD). Recent studies of cerebrospinal fluid (CSF) have suggested that increased lipid peroxidation of CSF and CSF lipoproteins also may occur in AD patients. In the present study, we determined the susceptibility of human CSF to ex vivo lipid peroxidation and tested the hypothesis that oxidized CSF lipoproteins may be neurotoxic. Whole CSF or a CSF lipoprotein fraction (d < 1.210 g/mL) was oxidized with 2,2'-azobis(2-amidino-propane)dihydrochloride (AAPH), a hydrophilic free-radical generator. Kinetics of CSF lipid peroxidation were followed by a standard fluorescence product accumulation assay. Oxidation of AD CSF yielded significantly shorter fluorescent lag times than controls, indicating reduced antioxidant capacity. Electrophoretic mobilities of CSF apolipoproteins were specifically reduced upon oxidation of CSF with AAPH, suggesting that lipoproteins are primary targets of CSF lipid peroxidation. Cultured neuronal cells were exposed to physiological concentrations of isolated CSF lipoproteins oxidized with increasing concentrations of AAPH; the resulting neurotoxicity showed a significant linear AAPH concentration-response relationship. These results suggest that oxidized CSF lipoproteins may contribute to the pathogenesis of neurodegeneration in AD.

Hippocampal alterations of apolipoprotein E and D mRNA levels in vivo and in vitro following kainate excitotoxicity

Alteration in the expression of apolipoprotein E (ApoE) and apolipoprotein D (ApoD) genes was evaluated in rat, 7 days following status epilepticus (SE) induced by intra-amygdala injection of kainate (KA), and in organotypic hippocampal cultures, 2 days after a single 1 h exposure to KA. Global polyadenylated RNA (poly A+) steady state, assessing global regulation of mRNA transcription was first measured in cortices and hippocampi from each animal and in the organotypic cultures. No alteration due to KA treatment was observed and individual concentrations of ApoE and ApoD mRNA species were therefore measured and comparative analysis performed. In the cortices of KA-treated animals, ApoE and ApoD mRNA levels did not show statistically significant changes. In contrast, in hippocampi, 7 days after SE, ApoE and ApoD mRNA levels were significantly increased, respectively, by 123 and 138%. This in vivo effect was confirmed in vitro on organotypic cultures, where KA treatment increased ApoE and ApoD mRNA expressions, respectively, by 72 and 61%. These observations indicate that lipidic metabolism is modified in the lesioned structure and suggest an increased traffic of lipids and a need for more ApoE and D in the hippocampus during the period of recovery and restructuration that follows severe seizures.