Expression of the HFE allelic variant H63D in SH-SY5Y cells affects tau phosphorylation at serine residues

A number of genetic association studies have appeared that address HFE gene variants in neurodegenerative disorders. However, the cellular impact of HFE in the nervous system has received little attention. To begin to address the role of the HFE allelic variants on cellular events associated with neurodegeneration, we examined the hypothesis that HFE polymorphisms are associated with alterations in tau phosphorylation in a human neuroblastoma cell line (SH-SY5Y). The results show that in a cell culture model, the H63D allele is associated with increased tau phosphorylation. The mechanisms responsible for these changes appear related to increased glycogen synthase kinase (GSK)-3β activity. GSK-3β activity is up-regulated in the cells expressing H63D HFE and can be modified by the addition of iron or treatment with an iron chelator in SH-SY5Y cells expressing wild-type HFE. Oxidative stress, also associated with elevated cellular iron, is associated with increased tau phosphorylation at the same sites as seen in H63D cells and treatment with Trolox, an anti-oxidant, lowered tau phosphorylation. These results suggest H63D HFE increases tau phosphorylation via GSK-3β activity and iron-mediated oxidative stress.     

The H63D HFE gene variant promotes activation of the intrinsic apoptotic pathway via mitochondria dysfunction following β‐amyloid peptide exposure

Numerous epidemiological studies suggest that the expression of the HFE allelic variant H63D may be a risk factor or genetic modifier for Alzheimer's disease (AD). The H63D variant alters cellular iron homeostasis and increases baseline oxidative stress. The elevated cellular stress milieu, we have proposed, may alter cellular responses to genetic and environmental determinants of AD. Accumulation of β‐amyloid peptides (Aβ) is one of the most prominent pathogenic characteristics of AD. Several studies have demonstrated that Aβ can induce neuronal cell death through apoptosis. In this study, we provide evidence that an Aβ_25–35 fragment, which contains the cytotoxic sequence of the amyloid peptide, activates the intrinsic apoptotic pathway in SH‐SY5Y human neuroblastoma cells expressing the HFE allelic variant H63D to a greater extent than in cells with wild‐type (WT) HFE. Specifically, Aβ_25–35 peptide exposure significantly induced Bax translocation from the cytosol to the mitochondria in H63D‐expressing cells compared with WT cells. This translocation was associated with increased cytochrome c release from mitochondria and an increase in active caspase‐9 and caspase‐3 activity in H63D cells. Consequently, there is increased apoptosis in cells expressing the H63D variant as opposed to cells expressing WT HFE. We also found increased amyloid precursor protein (APP) and Aβ_1–42 peptide in the mitochondrial compartment as well as increased mitochondrial stress in H63D‐expressing cells compared with WT. These findings support our hypothesis that the presence of the HFE H63D allele enables factors that trigger neurodegenerative processes associated with AD and predisposes cells to cytotoxcity. © 2010 Wiley‐Liss, Inc.