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APOLIPOPROTEIN E SPECIES ARE ALTERED IN THE CEREBROSPINAL FLUID OF PATIENTS WITH ALZHEIMER’S DISEASE
Abstract
Aims
The majority of Alzheimer’s disease (AD) cases develop sporadically, although different genetic factors have been linked to the illness. The most studied is the APOE genotype, whose ε4 allele increases the likelihood of developing AD, reducing the age of onset. Many studies have analyzed AD biomarkers in APOE-stratified cerebrospinal fluid (CSF) collections, but few addressed whether the apolipoprotein E (apoE) protein is altered in AD.
Methods
ApoE protein levels from CSF samples of individuals with varying disease states (control and AD) and APOE genotype (ε3/3, ε3/4, ε4/4) from 2 independent cohorts were studied via SDS-PAGE (reducing and non-reducing conditions) and native-PAGE.
Results
We found that apoE is present in CSF as ~36 kDa species, but also at 34 kDa, probably representing immature glycoforms. Despite the absolute amount of CSF-apoE protein levels being significantly higher in AD patients, the ratio of mature/immature apoE glycoforms was lower compared to controls. Furthermore, a high molecular weight SDS-PAGE-resistant apoE band of ~100 kDa appeared almost exclusively in AD patients. ApoE3 and apoE2 form disulphide-linked dimers in CSF, and apoE homodimers have been reported at ~100 kDa (as opposed to the predicted ~70 kDa), whereas apoE4 cannot form these dimers as it lacks a cysteine residue. We corroborated the presence of these species under non-reducing conditions, although we also observed the presence of 100 kDa apoE β-mercaptoethanol-resistant species in AD samples, including apoE4 carriers. Results were corroborated in two independent cohorts.
Conclusions
In conclusion, the study shows that apoE protein levels and conformational state appear to be affected in AD CSF.