Apolipoprotein E in degenerative processes in the brain, with focus on Alzheimer's disease

Abstract

Apolipoprotein E (apoE) is a constitute of several lipoprotein particles with an important role in lipid metabolism in the brain. ApoE has been suggested to have a function in synaptic remodeling after injury. ApoE exist in three major isoforms, apoE2, E3 and E4. An increased frequency of the apoE4 allele has been associated with Alzheimer´s disease (AD).The overall aim of this project was to elucidate the role of apoE and its potential as a biochemical marker for degenerative processes in the brain. To achieve these purposes the following methods have been developed.ELISA methods for measurements of apoE in cerebrospinal fluid (CSF) have been developed. Two different types of ELISAs were used, in paper I a sandwich ELISA and in paper IV and V a direct ELISA. The direct ELISA showed several advantages (lower coefficient of variation and higher specificity) compared with the sandwich ELISA. Possible confounding factors were evaluated with the direct ELISA. It was found that apoE absorbs into test tube walls and this makes handling of CSF samples critical. However, both ELISA methods revealed lower levels of apoE in CSF in AD patients as compared to controls. ApoE was determined in brain tissue (frontal cortex, temporal cortex, hippocampus and cerebellum). The level of apoE was found to be decreased in the frontal cortex and hippocampus in AD patients compared to controls. The apoE level was not related to the apoE isoform, senile plaque or neurofibrillary tangles counts. Also, immunohistochemistry examination revealed generally less staining of apoE in the AD brain compared to controls. The finding of decreased levels of apoE in both CSF and brain tissue in AD, support the suggestion that analysis of apoE in CSF reflects biochemical changes of apoE in the brain. The reduction might have several explanations, apoE might be absorbed into senile plaque and therefore the levels are decreased. However, this explanation is not supported in our studies, since no correlation was found between SP counts and the apoE levels. Another explanation is that the production of apoE is not induced in AD patients and that the apoE regenerative system is defective. To determine the apoE isoform an isoelectric focusing (IEF) method was established. The frequency of the apoE4 allele was increased in the AD patients compared to controls. In the second paper we examined the frequency of the apoE4 allele in a very old population and its relation to cerebrovascular disorders. In the old population the apoE4 allele was only a risk factor for developing dementia, if white matter lesions also occurred. Thus other factors than the apoE4 allele are important for developing dementia in very old age. In the last paper, patients with repeated samples after acute ischemic stroke were examined, in order to evaluate different biochemical markers for AD in CSF. ApoE in CSF exhibited rather stable levels after the stroke. It is possible that there is a difference between different types of damage but this finding might also question the regenerative role of apoE in the human brain.Finally, different procedures to isolate apoE lipoprotein particles from CSF have been evaluated. The purpose was to characterize apoE and to study the presence of described apoE and b-amyloid complexes. Alzheimer´s disease is associated with apoE alterations quantitative as well as qualitative. These findings support an involvement of apoE in the degenerative process in this disease but the mechanism for this involvement is still unclear and needs further studies