Life-span influences of apoE4 on CNS function

Altered cerebral insulin response in transgenic mice expressing the epsilon-4 allele of the human apolipoprotein E gene

Apolipoprotein E epsilon-4 (APOEε4 or APOE4), an allelic variation of the APOE gene, not only increases the risk of developing the late-onset form of Alzheimer's disease (AD), but also influences the outcome of treatment. Indeed, data from clinical studies show that the beneficial effect of insulin on cognition is blunted in APOE4 carriers. To investigate how APOE impacts insulin response, we assessed the effects of an acute insulin injection in APOE3- and APOE4-targeted replacement mice that respectively express the human APOE3 or APOE4 isoform instead of the endogenous murine ApoE protein. We evaluated cognition, insulin signaling and proteins implicated in Aβ transport and tau phosphorylation in the cortex and brain capillaries. We found that a single acute insulin injection increased Akt pSer473 in APOE4 compared to APOE3 mice (+113% versus +78.5%), indicating that APOE4 carriage potentiates activation of insulin upstream signaling pathway in the brain. Insulin also led to decreased concentrations of the receptor for advanced glycation endproducts (RAGE) in brain capillaries in both groups of mice. Moreover, higher phosphorylation of tau at Ser202, one of the key markers of AD neuropathology, was observed in insulin-injected APOE4 mice (+44%), consistent with findings in human APOE4 carriers (+400% compared to non-carriers). Therefore, our data suggest that APOE4 carriage leads to an increased insulin-induced activation of cerebral Akt pathway, associated with higher AD-like tau neuropathology. Our results provide evidence of altered insulin signaling in APOE4 carriers as well as a possible mechanism to explain the absence of cognitive benefit from insulin therapy in these individuals.

The benefit of docosahexaenoic acid for the adult brain in aging and dementia

A brief overview of the evidence for omega-3 fatty acids and, in particular, of docosahexaenoic acid (DHA), involvement in cognition and in dementia is given. Two studies are presented in this regard in which the key intervention is a DHA supplement. The fist, the MIDAS Study demonstrated that DHA can be of benefit for episodic memory in healthy adults with a mild memory complaint. The second, the ADCS AD trial found no benefit of DHA in the primary outcomes but found an intriguing benefit for cognitive score in ApoE4 negative allele patients. This leads to a consideration of the mechanisms of action and role of ApoE and its modulation by DHA. Given the fundamental role of ApoE in cellular lipid transport and metabolism in the brain and periphery, it is no surprise that ApoE affects n-3 PUFA brain function as well. It remains to be seen to what extent ApoE4 deleterious effect in AD is associated with n-3 PUFA-related cellular mechanisms in the brain and, more specifically, whether ApoE4 directly impairs the transport of DHA into the brain, as has been suggested.

Neuroimaging and APOE genotype: a systematic qualitative review

Apolipoprotein E (APOE) is the major genetic risk factor for late-onset Alzheimer's disease (AD) and has also been implicated in cardiovascular disease, cognitive decline and cognitive changes in healthy ageing. The aim of this paper is to systematically review and critically assess the association between the APOE genotype and structural/functional cerebral changes as evidenced by brain imaging studies. A second aim is to determine whether these observed associations between APOE and the brain reflect changes which are consistent with the progression of AD neurodegenerative changes described in Braak stages. A search of Pubmed, Psycinfo, and Web of Science databases identified 64 articles available for qualitative review. The review found that presence of the APOE epsilon4 allele is associated with (1) hippocampal, amygdalar and entorhinal cortex atrophy, (2) increased brain atrophy, (3) increased white matter hyperintensity volumes and (4) altered cerebral blood flow and glucose metabolism patterns. It is possible that there are critical age ranges when these effects are evident and that the APOE epsilon2 genotype might present a risk. We conclude that structural brain change is associated with the APOE genotype and that it is more salient in younger ageing individuals.