| The cause of Alzheimer's disease (AD) appears to be very complex. Several hypotheses of AD pathogenesis have been tested in this decade, but the mechanisms of AD remain unclear. Many studies revealed apolipoprotein-E (ApoE) isoforms may influence the age of onset and progression of AD. The systemic function of ApoE is transporting and redistributing lipids within organs and tissues. More recent studies showed that ApoE and ApoE receptors not only play roles in cholesterol metabolism, but also in signal transduction. The very low density lipoprotein receptor (VLDLRP), apolipoprotein-E receptor 2 (apoER2), and LDL receptor-related protein (LRP) have been reported to participate in signal transduction pathways in the brain.; Three common isomers of ApoE are found in the human population, apolipoprotein-E2, -E3, and -E4. Clinical studies show APO-E4 is a risk factor for developing AD and some evidence indicates that APO-E2 is associated with a beneficial effect. The goal of this project is to compare the influence of APO-E4 and APO-E2 on gene expression and possible cellular pathways involved in the pathogenesis of AD. We assayed frontal cortices of transgenic mice carrying the human APOE genes (hApo-E4 and hApo-E2) by microarrays, real-time PCR, and Western blots. Referring to results of microarray data (n=3, pooled RNA), we chose five upregulated genes in 24-month-old hApo-E4 transgenic mice for real-time polymerase chain reaction (RT-PCR) examination: parkin, septin 3, transforming growth factor beta3 (Tgfb3), synaptotagmin 1 (Syt1), and mitogen-activated protein kinase 8 interacting protein 2 (JIP2). Unlike the microarray data, RT-PCR showed no significant differences in expression of the genes between 24 month hAPO-E4 and age-matched hAPO-E2 transgenic mice. However, Parkin gene expression in old-aged (24-month-old) mice is significantly different compared to young (8-month-old) mice in the strains tested, including hAPO-E2 transgenic, hAPO-E4 transgenic, wild type C57 B/6J, and mouse APOE knock-out (KO) mice.; Western blots showed that Parkin protein levels were higher in old C57 B/6J and hAPO-E4 mice than in young animals, but were higher in young rather than old APOE KO and hAPO-E2 animals. Parkin protein maintains similar level from 8-month old to 24-month old APOE KO and hAPO-E2 animals. Parkin levels were comparable in mice of each strain at 16 and 24 months; therefore, differences are due to the amount of protein present during younger ages. Parkin is an E3 ubiquitin ligase, a critical component of the ubiquitin proteasome system which plays a major role in protein homeostasis of the intracellular system. Perhaps a more active proteasome system at a younger age would prevent build-up of misfolded protein and subsequent late-onset neurodegeneration diseases.; This study is the first to show three isoforms of JIP2, 89-kD, 76-kD, and 42-kD, were detected in mice and the 42-kD isoform was the most abundant in our Western blot. The level of the 89-kD isoform was consistent at three ages with no significant differences among all lines of mice. JIP2 acts as a scaffold protein interacting with mixed-lineage protein kinase (MLK), mitogen-activated protein kinase kinase 7 (MKK7), and c-Jun NH2-termial kinase (JNK) in the JNK signaling pathway. Four isoforms in human JIP2 were found from different spliced mRNAs, but only one mRNA sequence was previously reported for JIP2 translation in the mouse so far. JIP2 also can bind APP and kinein, therefore, JIP2 may potentially be involved APP processing and AD pathogenesis. |
