Links between ApoE, brain cholesterol metabolism, tau and amyloid beta-peptide in patients with cognitive impairment

Brain neurons remove the excess of cholesterol via conversion into the more polar 24OHC [(24S)-hydroxycholesterol]. 24OHC acts as a signalling molecule inducing ApoE (apolipoprotein E)-mediated cholesterol efflux from astrocytes, by a direct effect on ApoE transcription, protein synthesis and secretion. In CSF (cerebrospinal fluid) collected form from patients with cognitive impairment (Alzheimer's disease and patients with mild cognitive impairment) the levels of ApoE, tau, p-tau (hyperphosphorylated tau) were significantly increased, together with 24OHC, compared with controls. We also found that the levels of tau and p-tau were significantly correlated with ApoE and 24OHC in the same samples. Such a correlation was not found in control patients. Increased levels of cholesterol in membranes and impairment in brain cholesterol metabolism were found to be involved both in APP (amyloid precursor protein) processing and amyloid beta-peptide deposition and, recently, in tau pathology. The CSF tau levels are considered to be related to the neurodegenerative process in Alzheimer's disease. During neurodegeneration, the cholesterol accumulated in neurons is converted into 24OHC. The release of 24OHC from neurons induces ApoE secretion by astrocytes, and both are related to the intensity of the neurodegenerative process and neuronal injury. ApoE can also be involved in the scavenging of tau from neurons. The direct correlations between ApoE, 24OHC and tau suggest that cholesterol metabolism may be involved in generation of both tau and amyloid beta-peptide and that the ApoE is released by astrocytes in order to counteract this ongoing process.