Amyloid beta-peptide effects on synaptosomes from apolipoprotein E-deficient mice

Apolipoprotein E (apoE) is present in the brain and may contribute to neurophysiologic or neuropathologic events, depending on environmental and genetic influences. Recent studies indicate a role for apoE in synaptic plasticity and maintenance of synaptic membrane symmetry, suggesting that apoE may be involved in regulating synaptic homeostasis. In the present study, cerebrocortical synaptosomes were prepared from transgenic mice lacking apoE (apoE KO) to analyze the possible contribution of apoE toward maintaining homeostasis in synaptosomes. Synaptosomal preparations from apoE KO and wild-type mice exhibited similar basal levels of reactive oxygen species, mitochondrial function, and caspase activity; however, following application of amyloid beta-peptide [Abeta(1-40)], apoE KO synaptosomes displayed increased levels of oxidative stress, mitochondrial dysfunction, and caspase activation compared with synaptosomes from wild-type mice. Synaptosomal membranes from apoE KO mice were more fluid than wild-type synaptosomes and contained higher levels of thiobarbituric acid-reactive substances, consistent with elevated levels of lipid peroxidation occurring in the synapses of apoE KO mice. Together, these data are consistent with a role for apoE in maintaining homeostasis by attenuating oxidative stress, caspase activation, and mitochondrial homeostasis in synapses.     

Apolipoprotein A-I induced amyloidosis

Amyloidosis is characterized by extracellular deposits of protein fibrils with a high content of β-sheets in secondary structure. The protein forms together with proteoglycans amyloid fibrils causing organ damage and serious morbidity. Intact apolipoprotein A-I (apoA-I) is an important protein in lipid metabolism regulating the synthesis and catabolism of high density lipoproteins (HDL). Usually, apoA-I is not associated with amyloidosis. However, four naturally occuring mutant forms of apoA-I are known so far resulting in amyloidosis. The most important feature of all variants is the very similar formation of N-terminal fragments which were found in the amyloid deposits (residues 1–83 to 1–94). The new insights in the understanding of the association of apoA-I with HDL, its metabolism, and its hypothesized structural findings may explain a common mechanism for the genesis of apoA-I induced amyloidosis. Here we summarized the specific features of all known amyloidogenic variants of apoA-I and speculate about its metabolic pathway, which may have general implications for the metabolism of apoA-I.     

Apolipoprotein E Attenuates β-Amyloid-Induced Astrocyte Activation

Abstract: A common feature of Alzheimer's disease pathology is an abundance of activated glia, indicative of an inflammatory reaction in the brain. The relationship between glial activation and neurodegeneration is not known, although several cytokines and inflammatory mediators produced by activated glia have the potential to initiate or exacerbate the progression of neuropathology. As β-amyloid (Aβ) is one of several stimuli that can activate glia, it is important to determine how Aβ-induced glial activation is influenced by other proteins present in the plaque, such as apolipoprotein E (apoE). We examined the effect of native preparations of apoE on activation of rat cortical astrocyte cultures by Aβ1–42. The apoE source was conditioned medium from human embryonic kidney 293 cells stably transfected with human apoE3 or apoE4 cDNA. By morphological criteria, apoE inhibited Aβ-induced astrocyte activation in three experimental paradigms: apoE pretreatment blocked subsequent Aβ-induced activation, Aβ aged in the presence of apoE did not activate astrocytes, and apoE addition to activated astrocytes transiently reversed the activated phenotype. No apoE isoform selectivity was observed. The effect of apoE appears to be specific to Aβ, as apoE did not attenuate cyclic AMP-induced astrocyte activation. These data suggest that apoE may modulate the ability of Aβ to induce inflammatory responses in the brain.     

Novel Large Apolipoprotein E-Containing Lipoproteins of Density 1.006–1.060 g/ml in Human Cerebrospinal Fluid

Abstract: Although the critical role of apolipoprotein E (apoE) allelic variation in Alzheimer's disease and in the outcome of CNS injury is now recognized, the functions of apoE in the CNS remain obscure, particularly with regard to lipid metabolism. We used density gradient ultracentrifugation to identify apoE-containing lipoproteins in human CSF. CSF apoE lipoproteins, previously identified only in the 1.063–1.21 g/ml density range, were also demonstrated in the 1.006–1.060 g/ml density range. Plasma lipoproteins in this density range include low-density lipoprotein and high-density lipoprotein (HDL) subfraction 1 (HDL₁). The novel CSF apoE lipoproteins are designated HDL₁. No immunoreactive apolipoprotein A-I (apo A-I) or B could be identified in the CSF HDL₁ fractions. Large lipoproteins 18.3 ± 6.6 nm in diameter (mean ± SD) in the HDL₁ density range were demonstrated by electron microscopy. Following fast protein liquid chromatography of CSF at physiologic ionic strength, apoE was demonstrated in particles of average size greater than particles containing apoA-I. The largest lipoproteins separated by this technique contained apoE without apoA-I. Thus, the presence of large apoE-containing lipoproteins was confirmed without ultracentrifugation. Interconversion between the more abundant smaller apoE-HDL subfractions 2 and 3 and the novel larger apoE-HDL₁ is postulated to mediate a role in cholesterol redistribution in brain.     

Specific Neurochemical Derangements of Brain Projecting Neurons in Apolipoprotein E-Deficient Mice

Abstract: Apolipoprotein E (apoE)-deficient mice provide a useful system for studying the role of apoE in neuronal maintenance and repair. Previous studies revealed specific memory impairments in these mice that are associated with presynaptic derangements in projecting forebrain cholinergic neurons. In the present study we examined whether dopaminergic, noradrenergic, and serotonergic projecting pathways of apoE-deficient mice are also affected and investigated the mechanisms that render them susceptible. The densities of nerve terminals of forebrain cholinergic projections were monitored histochemically by measurements of acetylcholinesterase activity, whereas those of the dopaminergic nigrostriatal pathway, the noradrenergic locus coeruleus cortical projection, and the raphe-cortical serotonergic tract were measured autoradiographically using radioligands that bind specifically to the respective presynaptic transporters of these neuronal tracts. The results obtained revealed that synaptic densities of cholinergic, noradrenergic, and serotonergic projections in specific brain regions of apoE-deficient mice are markedly lower than those of controls. Furthermore, the extent of presynaptic derangement within each of these tracts was found to be more pronounced the further away the nerve terminal is from its cell body. In contrast, the nerve terminal density of the dopaminergic neurons that project from the substantia nigra to the striatum was unaffected and was similar to that of the controls. The rank order of these presynaptic derangements at comparable distances from the respective cell bodies was found to be septohippocampal cholinergic > nucleus basalis cholinergic > locus coeruleus adrenergic > raphe serotonergic ? nigrostriatal dopaminergic, which interestingly is similar to that observed in Alzheimer's disease. These results suggest that two complementary factors determine the susceptibility of brain projecting neurons to apoE deficiency: pathway-specific differences and the distance of the nerve terminals from their cell body.     

APOA2 ?256T>C polymorphism interacts with saturated fatty acids intake to affect anthropometric and hormonal variables in type 2 diabetic patients

Recent studies have established the interaction between APOA2 −256T>C polymorphism and dietary saturated fatty acids intake in relation to obesity on healthy individuals. In the current study, we investigate the effects of this interaction on anthropometric variables and serum levels of leptin and ghrelin in patients with type 2 diabetes. In this cross-sectional study, 737 patients with type 2 diabetes mellitus (290 males and 447 females) were recruited from diabetes clinics in Tehran. The usual dietary intake of all participants during the last year was obtained by validated semiquantitative food frequency questionnaire. APOA2 genotyping was performed by real-time PCR on genomic DNA. No significant relation was obtained by univariate analysis between anthropometric variables and APOA2 genotypes. However, after adjusting for age, gender, physical activity and total energy intake, we identified a significant interaction between APOA2-saturated fatty acids intake and body mass index (BMI). After adjusting for potential confounders, serum levels of ghrelin in CC genotype patients were significantly higher than T allele carriers (p = 0.03), whereas the case with leptin did not reveal a significant difference. The result of this study confirmed the interaction between APOA2 −256T>C polymorphism and SFAs intake with BMI in type 2 diabetic patients. In fact, homozygous patients for the C allele with high saturated fatty acids intake had higher BMI. The APOA2 −256T>C polymorphism was associated with elevated levels of serum ghrelin.     

Palmitic acid suppresses apolipoprotein M gene expression via the pathway of PPARβ/δ in HepG2 cells

It has been demonstrated that apolipoprotein M (APOM) is a vasculoprotective constituent of high density lipoprotein (HDL), which could be related to the anti-atherosclerotic property of HDL. Investigation of regulation of APOM expression is of important for further exploring its pathophysiological function in vivo. Our previous studies indicated that expression of APOM could be regulated by platelet activating factor (PAF), transforming growth factors (TGF), insulin-like growth factor (IGF), leptin, hyperglycemia and etc., in vivo and/or in vitro. In the present study, we demonstrated that palmitic acid could significantly inhibit APOM gene expression in HepG2 cells. Further study indicated neither PI-3 kinase (PI3K) inhibitor LY294002 nor protein kinase C (PKC) inhibitor GFX could abolish palmitic acid induced down-regulation of APOM expression. In contrast, the peroxisome proliferator-activated receptor beta/delta (PPAR_β/δ) antagonist GSK3787 could totally reverse the palmitic acid-induced down-regulation of APOM expression, which clearly demonstrates that down-regulation of APOM expression induced by palmitic acid is mediated via the PPAR_β/δ pathway.     

The identification of apolipoprotein genes in rare minnow (Gobiocypris rarus) and their expression following perfluorooctanoic acid exposure

Apolipoproteins play important roles in lipid transport and uptake in vertebrates, and they are associated with pathogenesis of many cardiovascular diseases. However, the diverse apolipoproteins in individual fish species have not been extensively characterized. Partial cDNA sequences encoding ApoA-IV, ApoE, ApoM, ApoL, and ApoO, and full-length cDNA sequences encoding ApoA-I were cloned from rare minnow (Gobiocypris rarus). Sequence analysis showed that these genes, as well as fragments of other known apolipoprotein genes (ApoC-I, ApoC-II, ApoB) of rare minnow had a high similarity (91–96%) to their orthologues in the spotted barbel Hemibarbus mylodon (Teleostei:Cypriniformes). The expression of these nine genes and their possible upstream genes, PPARα, PPARγ, and HNF4α, were investigated in rare minnow after subacute exposure to perfluorooctanoic acid (PFOA) for 14 days. Results showed that the expression of mRNA for ApoA-I, ApoC-II, and ApoM was significantly downregulated in all PFOA-treated animals. Only fish receiving the highest dose of PFOA showed downregulation of the expression of ApoA-IV and ApoC-I, while fish treated with 10 mg PFOA/L showed upregulation of expression of ApoE. Expression of ApoB, ApoO, and ApoL was unchanged between control and treated groups. In addition, the expression of PPARα was increased in all dosed fish, while the mRNAs for PPARγ and HNF4α were significantly altered with 30 and 3 mg PFOA/L doses, respectively. Therefore, subacute exposure to PFOA resulted in alteration of expression of apolipoproteins and related genes. These changes in gene expression may further influence lipid metabolism or other physiological functions in fish.     

Differentially-expressed glycoproteins in Locusta migratoria hemolymph infected with Metarhizium anisopliae

Glycoproteins play important roles in insect physiology. Infection with pathogen always results in the differential expression of some glycoproteins, which may be involved in host-pathogen interactions. In this report, differentially-expressed glycoproteins from the hemolymph of locusts infected with Metarhizium anisopliae were analyzed by two-dimensional electrophoresis (2-DE) and PDQuest software. The results showed that 13 spots were differentially expressed, of which nine spots were upregulated and four were downregulated. Using MS/MS with de novo sequencing and NCBI database searches, three upregulated proteins were identified as locust transferrin, apolipoprotein precursor, and hexameric storage protein 3. These proteins have been reported to be involved in the insect innate immune response to microbial challenge. Due to the limited available genome information and protein sequences of locusts, the possible functions of the other 10 differentially-expressed spots remain unknown.     

Stimulation of lipolysis enhances the rate of cholesterol efflux to HDL in adipocytes

Adipose tissue constitutes a major location for cholesterol storage and, as such, it may play a role in the regulation of circulating cholesterol levels. A possible metabolic link between the lipolytic activity of adipocytes and their ability to release cholesterol to reconstituted human high density lipoprotein, HDL, was investigated in 3T3-L1 adipocytes. In the presence of HDL, composed of human apoA-I and phosphatidylcholine, adipocytes release cholesterol in a lipoprotein-dose and time dependent fashion. β-adrenergic activation of the lipolysis promotes a 22% increase in the extent of cholesterol efflux to reconstituted discoidal HDL particles. Activation of lipolysis promotes a rapid decrease in the cholesterol content of the plasma membrane and a concomitant increase in lipid droplet cholesterol. This change is independent of the presence of HDL. Activation of the lipolysis does not affect the levels of ABCA1 and SR-BI. Therefore, the enhancement of cholesterol efflux is not due to the level of plasma membrane cholesterol, or to the levels of the cholesterol transporters ABCA1 and scavenger receptor SR-BI. Brefeldin A did not affect the rate of cholesterol efflux under basal lipolytic conditions, but it abolished the lipolysis-dependent enhancement of cholesterol efflux to HDL. This study suggests that activation of lipolysis is accompanied by an increase in BFA-sensitive vesicular transport that in turn enhances cholesterol efflux to HDL. The study supports a metabolic link between the lipolytic activity of adipocytes and the rate of cellular cholesterol efflux to HDL.