Molecular and Genetic Studies of the Cerebral Folate System in Human Aging & Alzheimer’s Brain

Abstract

Background: Recent publications highlight the need for new directions in the search for cause, aetiology and effective treatments for Alzheimer’s disease (AD). Metabolic disorders, and specifically folate metabolism, have been identified as areas of potential interest in neurological conditions, and an investigation of the cerebral folate system in normal and AD human brain tissues was therefore carried out. Methods: Post-mortem human brain tissue and matched cerebrospinal fluid (CSF) samples were provided by the Manchester Brain Bank. Western and dot blots, to measure folate-related proteins and metabolites were performed on CSF and tissue lysates. Immunohistochemistry (IHC) for folate-related proteins and metabolites was performed on formalin-fixed, cryoprotected frozen sections of cerebral cortex. Nutrigenomic analysis of folate related genes was carried out to identify single nucleotide polymorphisms (SNPs) and correlate to physiological changes in folate metabolism. Results: A decrease in CSF folate metabolism was measured including in 10-formyl tetrahydrofolate dehydrogenase (FDH, ALDH1L1), a critical folate enzyme. In tissue, a switch in pathway of folate supply was found in AD compared to normal. The main folate carrier, folate receptor alpha (FOLR1), switched from FDH-positive astrocytes in normal, to glial fibrillary acidic protein (GFAP)-positive astrocytes in the AD cortex which was correlated with hypermethylation of neurones. All folate enzymes were reduced in the cortex, reflecting changes in the CSF except FDH which, although the most reduced of the proteins in the CSF, was raised in the tissue. In addition, a novel SNP in methylene tetrahydrofolate dehydrogenase 1 (MTHFD1) was correlated with AD. This was found to be correlated with an increase in glutathione in tissue, while in individuals without this SNP, there was an increase in MTHFD1. Conclusions: These results suggest that in the AD brain, FOLR1 enters the cortex from the CSF via GFAP-positive astrocytes, rather than FDH-positive astrocytes seen in normal brain. Folate is then delivered directly to neurones for hypermethylation. Moreover, there is a significant association of an SNP in MTHFD1 with AD that is reflected in a change in folate metabolism with an increase in tissue glutathione, while in normal there is no increase in this metabolite but there is an increase in MTHFD1