Effect of apolipoprotein AII on the interaction of apolipoprotein E with beta-amyloid: some apo(E-AII) complexes inhibit the internalization of beta-amyloid in cultures of neuroblastoma cells

Plasma apolipoprotein E levels, isoform composition, and dimer profile in relation to plasma lipids in racially diverse patients with Alzheimer's disease and mild cognitive impairment

BACKGROUND

The APOEε4-promoted risk of Alzheimer's disease (AD) is lower in Black/African-Americans (B/AAs), compared to non-Hispanic whites (NHWs). Previous studies reported lower plasma apolipoprotein E (apoE) levels in NHW APOEε4-carriers compared to non-carriers, and low plasma apoE levels were directly associated with an increased risk of AD and all dementia. We further showed that APOEε3/ε3 AD patients exhibited reduced plasma apoE dimers compared to corresponding control subjects. Whether plasma apoE levels and apoE dimer formation differ between races/ethnicities and therefore may help explain AD risk racial disparity remains to be elucidated.

METHODS

Using mass spectrometry, we determined total plasma apoE and apoE isoform levels in a cohort of B/AAs (n = 58) and NHWs (n = 67) including subjects with normal cognition (B/AA: n = 25, NHW: n = 28), mild cognitive impairment (MCI) (B/AA: n = 24, NHW: n = 24), or AD dementia (B/AA: n = 9, NHW: n = 15). Additionally, we used non-reducing western blot analysis to assess the distribution of plasma apoE into monomers/disulfide-linked dimers. Plasma total apoE, apoE isoform levels, and % apoE monomers/dimers were assessed for correlations with cognition, cerebrospinal fluid (CSF) AD biomarkers, sTREM2, neurofilament light protein (NfL), and plasma lipids.

RESULTS

Plasma apoE was predominantly monomeric in both racial groups and the monomer/dimer distribution was not affected by disease status, or correlated with CSF AD biomarkers, but associated with plasma lipids. Plasma total apoE levels were not related to disease status and only in the NHW subjects we observed lower plasma apoE levels in the APOEε4/ε4-carriers. Total plasma apoE levels were 13% higher in B/AA compared to NHW APOEε4/ε4 subjects and associated with plasma high-density lipoprotein (HDL) in NHW subjects but with low-density lipoprotein levels (LDL) in the B/AA subjects. Higher plasma apoE4 levels, exclusively in APOEε3/ε4 B/AA subjects, were linked to higher plasma total cholesterol and LDL levels. In the controls, NHWs and B/AAs exhibited opposite associations between plasma apoE and CSF t-tau.

CONCLUSIONS

The previously reported lower APOEε4-promoted risk of AD in B/AA subjects may be associated with differences in plasma apoE levels and lipoprotein association. Whether differences in plasma apoE levels between races/ethnicities result from altered APOEε4 expression or turnover, needs further elucidation.

Aggregation Limiting Cell-Penetrating Peptides Derived from Protein Signal Sequences

Alzheimer's disease (AD) is the most common neurodegenerative disease (ND) and the leading cause of dementia. It is characterized by non-linear, genetic-driven pathophysiological dynamics with high heterogeneity in the biological alterations and the causes of the disease. One of the hallmarks of the AD is the progression of plaques of aggregated amyloid-β (Aβ) or neurofibrillary tangles of Tau. Currently there is no efficient treatment for the AD. Nevertheless, several breakthroughs in revealing the mechanisms behind progression of the AD have led to the discovery of possible therapeutic targets. Some of these include the reduction in inflammation in the brain, and, although highly debated, limiting of the aggregation of the Aβ. In this work we show that similarly to the Neural cell adhesion molecule 1 (NCAM1) signal sequence, other Aβ interacting protein sequences, especially derived from Transthyretin, can be used successfully to reduce or target the amyloid aggregation/aggregates in vitro. The modified signal peptides with cell-penetrating properties reduce the Aβ aggregation and are predicted to have anti-inflammatory properties. Furthermore, we show that by expressing the Aβ-EGFP fusion protein, we can efficiently assess the potential for reduction in aggregation, and the CPP properties of peptides in mammalian cells.

Plasma high-density lipoprotein cargo is altered in Alzheimer's disease and is associated with regional brain volume

Cholesterol levels have been repeatedly linked to Alzheimer's Disease (AD), suggesting that high levels could be detrimental, but this effect is likely attributed to Low-Density Lipoprotein (LDL) cholesterol. On the other hand, High-Density Lipoproteins (HDL) cholesterol levels have been associated with reduced brain amyloidosis and improved cognitive function. However, recent findings have suggested that HDL-functionality, which depends upon the HDL-cargo proteins associated with HDL, rather than HDL levels, appears to be the key factor, suggesting a quality over quantity status. In this report, we have assessed the HDL-cargo (Cholesterol, ApoA-I, ApoA-II, ApoC-I, ApoC-III, ApoD, ApoE, ApoH, ApoJ, CRP, and SAA) in stable healthy control (HC), healthy controls who will convert to MCI/AD (HC-Conv) and AD patients (AD). Compared to HC we observed an increased cholesterol/ApoA-I ratio in AD and HC-Conv, as well as an increased ApoD/ApoA-I ratio and a decreased ApoA-II/ApoA-I ratio in AD. Higher cholesterol/ApoA-I ratio was also associated with lower cortical grey matter volume and higher ventricular volume, while higher ApoA-II/ApoA-I and ApoJ/ApoA-I ratios were associated with greater cortical grey matter volume (and for ApoA-II also with greater hippocampal volume) and smaller ventricular volume. Additionally, in a clinical status-independent manner, the ApoE/ApoA-I ratio was significantly lower in APOE ε4 carriers and lowest in APOE ε4 homozygous. Together, these data indicate that in AD patients the composition of HDL is altered, which may affect HDL functionality, and such changes are associated with altered regional brain volumetric data.

Extracellular protein components of amyloid plaques and their roles in Alzheimer's disease pathology

Alzheimer's disease (AD) is pathologically defined by the presence of fibrillar amyloid β (Aβ) peptide in extracellular senile plaques and tau filaments in intracellular neurofibrillary tangles. Extensive research has focused on understanding the assembly mechanisms and neurotoxic effects of Aβ during the last decades but still we only have a brief understanding of the disease associated biological processes. This review highlights the many other constituents that, beside Aβ, are accumulated in the plaques, with the focus on extracellular proteins. All living organisms rely on a delicate network of protein functionality. Deposition of significant amounts of certain proteins in insoluble inclusions will unquestionably lead to disturbances in the network, which may contribute to AD and copathology. This paper provide a comprehensive overview of extracellular proteins that have been shown to interact with Aβ and a discussion of their potential roles in AD pathology. Methods that can expand the knowledge about how the proteins are incorporated in plaques are described. Top-down methods to analyze post-mortem tissue and bottom-up approaches with the potential to provide molecular insights on the organization of plaque-like particles are compared. Finally, a network analysis of Aβ-interacting partners with enriched functional and structural key words is presented.

The redox status of cysteine thiol residues of apolipoprotein E impacts on its lipid interactions

Redox-mediated modulation of cysteine (Cys) thiols has roles in various pathophysiological functions. We recently found that formation of disulfide-linked complexes of apolipoprotein (apo) E3 prevented apoE3 from irreversible oxidation. In this report, the influence of modification of Cys thiols in apoE2 and apoE3 on interactions with lipids was investigated. The apoE redox status was examined by a band-shift assay using a maleimide compound, and interactions with lipids were evaluated by a kinetic assay using dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and non-denaturing polyacrylamide gel electrophoresis. A reduction in DMPC clearance activity of apoE2 and apoE3 but not apoE4 was observed. Although hydrogen peroxide-induced oxidation decreased the clearance activity of the isoforms, apoE2 showed the greatest residual activity. Both Cys thiol masking and dimerization decreased the activity of apoE2 and apoE3 but not apoE4. In contrast, apoAII preincubation markedly increased the activity (apoE2 > apoE3 > apoE4), in accordance with the formation of apoE-AII and apoAII-E2-AII complexes. ApoAII preincubation also reduced the particle size of apoE-DMPC liposome complexes, especially for apoE2. Redox-mediated modification of Cys thiols of apoE2 or apoE3, especially disulfide bond formation with apoAII, affects lipid metabolism and consequently may be responsible for the diverse isoform specificity of apoE.

Plasma Apolipoprotein E Monomer and Dimer Profile and Relevance to Alzheimer's Disease

The APOEɛ4 gene variant is the strongest genetic risk factor for Alzheimer's disease (AD), whereas APOEɛ3 conventionally is considered as 'risk neutral' although APOEɛ3-carriers also develop AD. Previous studies have shown that the apolipoprotein E3 (apoE3) isoform occurs as monomers, homodimers and heterodimers with apolipoprotein A-II in human body fluids and brain tissue, but the relevance of a plasma apoE3 monomer/dimer profile to AD is unknown. Here we assessed the distribution of monomers, homodimers and heterodimers in plasma from control subjects and patients with mild cognitive impairment (MCI) and AD with either a homozygous APOEɛ3 (n = 31 control subjects, and n = 14 MCI versus n = 5 AD patients) or APOEɛ4 genotype (n = 1 control subject, n = 21 MCI and n = 7 AD patients). Total plasma apoE levels were lower in APOEɛ4-carriers and overall correlated significantly to CSF Aβ42, p(Thr181)-tau and t-tau levels. Apolipoprotein E dimers were only observed in the APOEɛ3-carriers and associated with total plasma apoE levels, negatively correlated to apoE monomers, but were unrelated to plasma homocysteine levels. Importantly, the APOEɛ3-carrying AD patients versus controls exhibited a significant decrease in apoE homodimers (17.8±9.6% versus 26.7±6.3%, p = 0.025) paralleled by an increase in apoE monomers (67.8±18.3% versus 48.5±11.2%, p = 0.008). In the controls, apoE monomers and heterodimers were significantly associated with plasma triglycerides; the apoE heterodimers were also associated with levels of high-density lipoprotein cholesterol. The physiological relevance of apoE dimer formation needs to be further investigated, though the distribution of apoE in monomers and dimers appears to be of relevance to AD in APOEɛ3 subjects.

Metals and cholesterol: two sides of the same coin in Alzheimer's disease pathology

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease. It begins years prior to the onset of clinical symptoms, such as memory loss and cognitive decline. Pathological hallmarks of AD include the accumulation of β-amyloid in plaques and hyperphosphorylated tau in neurofibrillary tangles. Copper, iron, and zinc are abnormally accumulated and distributed in the aging brain. These metal ions can adversely contribute to the progression of AD. Dysregulation of cholesterol metabolism has also been implicated in the development of AD pathology. To date, large bodies of research have been carried out independently to elucidate the role of metals or cholesterol on AD pathology. Interestingly, metals and cholesterol affect parallel molecular and biochemical pathways involved in AD pathology. The possible links between metal dyshomeostasis and altered brain cholesterol metabolism in AD are reviewed.

Soluble apoE/Aβ complex: mechanism and therapeutic target for APOE4-induced AD risk

The APOE4 allele of apolipoprotein E (apoE) is the greatest genetic risk factor for Alzheimer's disease (AD) compared to APOE2 and APOE3. Amyloid-β (Aβ), particularly in a soluble oligomeric form (oAβ), is considered a proximal cause of neurodegeneration in AD. Emerging data indicate that levels of soluble oAβ are increased with APOE4, providing a potential mechanism of APOE4-induced AD risk. However, the pathway(s) by which apoE4 may increase oAβ levels are unclear and the subject of continued inquiry. In this editorial review, we present the hypothesis that apoE isoform-specific interactions with Aβ, namely apoE/Aβ complex, modulate Aβ levels. Specifically, we propose that compared to apoE3, apoE4-containing lipoproteins are less lipidated, leading to less stable apoE4/Aβ complexes, resulting in reduced apoE4/Aβ levels and increased accumulation, particularly of oAβ. Evidence that support or counter this argument, as well as the therapeutic significance of this pathway to neurodegeneration, are discussed.

Levels of soluble apolipoprotein E/amyloid-β (Aβ) complex are reduced and oligomeric Aβ increased with APOE4 and Alzheimer disease in a transgenic mouse model and human samples

Human apolipoprotein E (apoE) isoforms may differentially modulate amyloid-β (Aβ) levels. Evidence suggests physical interactions between apoE and Aβ are partially responsible for these functional effects. However, the apoE/Aβ complex is not a single static structure; rather, it is defined by detection methods. Thus, literature results are inconsistent and difficult to interpret. An ELISA was developed to measure soluble apoE/Aβ in a single, quantitative method and was used to address the hypothesis that reduced levels of soluble apoE/Aβ and an increase in soluble Aβ, specifically oligomeric Aβ (oAβ), are associated with APOE4 and AD. Previously, soluble Aβ42 and oAβ levels were greater with APOE4 compared with APOE2/APOE3 in hippocampal homogenates from EFAD transgenic mice (expressing five familial AD mutations and human apoE isoforms). In this study, soluble apoE/Aβ levels were lower in E4FAD mice compared with E2FAD and E3FAD mice, thus providing evidence that apoE/Aβ levels isoform-specifically modulate soluble oAβ clearance. Similar results were observed in soluble preparations of human cortical synaptosomes; apoE/Aβ levels were lower in AD patients compared with controls and lower with APOE4 in the AD cohort. In human CSF, apoE/Aβ levels were also lower in AD patients and with APOE4 in the AD cohort. Importantly, although total Aβ42 levels decreased in AD patients compared with controls, oAβ levels increased and were greater with APOE4 in the AD cohort. Overall, apoE isoform-specific formation of soluble apoE/Aβ modulates oAβ levels, suggesting a basis for APOE4-induced AD risk and a mechanistic approach to AD biomarkers.

Preferential interactions between ApoE-containing lipoproteins and Aβ revealed by a detection method that combines size exclusion chromatography with non-reducing gel-shift

The association between apolipoprotein E (apoE) and amyloid-β peptide (Aβ) may significantly impact the function of both proteins, thus affecting the etiology of Alzheimer's disease (AD). However, apoE/Aβ interactions remain fundamentally defined by the stringency of the detection method. Here we use size exclusion chromatography (SEC) as a non-stringent approach to the detection of apoE/Aβ interactions in solution, specifically apoE and both endogenous and exogenous Aβ from plasma, CSF and astrocyte conditioned media. By SEC analysis, Aβ association with plasma and CNS lipoproteins is apoE-dependent. While endogenous Aβ elutes to specific human plasma lipoproteins distinct from those containing apoE, it is the apoE-containing lipoproteins that absorb excess amounts of exogenous Aβ40. In human CSF, apoE, endogenous Aβ and phospholipid elute in an almost identical profile, as do apoE, exogenous Aβ and phospholipid from astrocyte conditioned media. Combining SEC fractionation with subsequent analysis for SDS-stable apoE/Aβ complex reveals that apoE-containing astrocyte lipoproteins exhibit the most robust interactions with Aβ. Thus, standardization of the methods for detecting apoE/Aβ complex is necessary to determine its functional significance in the neuropathology characteristic of AD. Importantly, a systematic understanding of the role of apoE-containing plasma and CNS lipoproteins in Aβ homeostasis could potentially contribute to identifying a plasma biomarker currently over-looked because it has multiple components.

Apolipoprotein-E forms dimers in human frontal cortex and hippocampus

Background

Apolipoprotein-E (apoE) plays important roles in neurobiology and the apoE4 isoform increases risk for Alzheimer's disease (AD). ApoE3 and apoE2 are known to form disulphide-linked dimers in plasma and cerebrospinal fluid whereas apoE4 cannot form these dimers as it lacks a cysteine residue. Previous in vitro research indicates dimerisation of apoE3 has a significant impact on its functions related to cholesterol homeostasis and amyloid-beta peptide degradation. The possible occurrence of apoE dimers in cortical tissues has not been examined and was therefore assessed. Human frontal cortex and hippocampus from control and AD post-mortem samples were homogenised and analysed for apoE by western blotting under both reducing and non-reducing conditions.

Results

In apoE3 homozygous samples, ~12% of apoE was present as a homodimer and ~2% was detected as a 43 kDa heterodimer. The level of dimerisation was not significantly different when control and AD samples were compared. As expected, these dimerised forms of apoE were not detected in apoE4 homozygous samples but were detected in apoE3/4 heterozygotes at a level approximately 60% lower than seen in the apoE3 homozygous samples. Similar apoE3 dimers were also detected in lysates of SK-N-SH neuroblastoma cells and in freshly prepared rabbit brain homogenates. The addition of the thiol trapping agent, iodoacetamide, to block reactive thiols during both human and rabbit brain sample homogenisation and processing did not reduce the amount of apoE homodimer recovered. These data indicate that the apoE dimers we detected in the human brain are not likely to be post-mortem artefacts.

Conclusion

The identification of disulphide-linked apoE dimers in human cortical and hippocampal tissues represents a distinct structural difference between the apoE3 and apoE4 isoforms that may have functional consequences.

The role of apolipoprotein E in Alzheimer's disease

The epsilon4 allele of apolipoprotein E (APOE) is the major genetic risk factor for Alzheimer's disease (AD). Although there have been numerous studies attempting to elucidate the underlying mechanism for this increased risk, how apoE4 influences AD onset and progression has yet to be proven. However, prevailing evidence suggests that the differential effects of apoE isoforms on Abeta aggregation and clearance play the major role in AD pathogenesis. Other potential mechanisms, such as the differential modulation of neurotoxicity and tau phosphorylation by apoE isoforms as well as its role in synaptic plasticity and neuroinflammation, have not been ruled out. Inconsistent results among studies have made it difficult to define whether the APOE epsilon4 allele represents a gain of toxic function, a loss of neuroprotective function, or both. Therapeutic strategies based on apoE propose to reduce the toxic effects of apoE4 or to restore the physiological, protective functions of apoE.

Differentially expressed cortical genes contribute to perivascular deposition in transgenic mice with inducible neuron‐specific expression of TGF‐β1

In the brain the expression of transforming growth factor beta1 (TGF-beta1) is involved both in neuroprotective and neurodegenerative processes. Recently, we have established a transgenic mouse model with inducible neuron-specific expression of TGF-beta1 based on the tetracycline-regulated gene expression system. A long-term expression of TGF-beta1 results in persisting perivascular thioflavin-positive depositions, which did not disappear even though the transgene synthesis was repressed completely by administration of doxycycline. Formation and composition of these depositions are hardly elucidated. The aim of this study was to identify TGF-beta1 responding genes potentially participating in forming these depositions. To address this problem we have compared the cortical mRNA expression pattern of TGF-beta1 expressing mice with mice impeded to express the transgenic protein using oligonucleotide microarray analysis. Differential gene expression was further characterized by quantitative real-time reverse transcription-polymerase chain reaction including animals, where the long-lasting TGF-beta1 expression was repressed. While no change of amyloid precursor protein RNA expression level was detected, various genes strongly involved in calcium homeostasis, tissue mineralization or vascular calcification were identified differentially expressed. It is suggested, that these genes might contribute to the perivascular depositions in the TGF-beta1 expressing mice.

Novel mass spectrometric immunoassays for the rapid structural characterization of plasma apolipoproteins

Novel mass spectrometric immunoassays (MSIAs) for the isolation and structural characterization of plasma apolipoprotein A-I (apoA-I), apoA-II, and apoE have been developed. The assays combine selective isolation of apolipoprotein species via affinity capture with mass-specific detection using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. In application, plasma (from 50 microl of whole blood drawn from individuals, using finger lancet) was addressed with affinity pipette tips derivatized with antibodies toward the specific apolipoprotein. The time required for each assay was approximately 15 min, less if assays on multiple individuals were performed in parallel. In a brief study of five individuals, several recently reported apoA-II variants were identified and observed consistently in all individuals. Additionally, the apoE phenotype of E3/E3 was observed in three of the individuals, and E2/E3 and E3/E4 observed in the remaining two individuals, the latter of whom suffers from Alzheimer's disease. Overall, the MSIA approach offers a rapid, sensitive, and highly accurate means of profiling apolipoproteins from small volumes of plasma.