ApoE peptide and apoE isoforms affect phosphorylation of tau in primary neurons. Primary neurons were treated with control buffer ("C") or 2 uM EP (panel A), E2 (B), E3 (C) and E4 (D) for two hours. Cells were lysed in RIPA buffer (n = 4) and cell proteins (20 ug/lane) were separated by SDS-PAGE. Relative levels of tau epitopes were determined by immunoblotting with the indicated antibodies: anti-5E2, a measure of total tau; Tau 1, a measure of nonphosphorylated tau; pS396, AT8 and AT270, measures of phospho-tau epitopes. EP, E2, E3 resulted in increased Tau-1 immunoreactivity and decreased phosphorylation of tau at several sites. Quantification of these blots revealed that the apoE peptide induced a 115% increase in levels of unphosphorylated tau and a 70–90% decrease in phospho-tau proteins (panel E). Similar experiments with EP were conducted, but cells were extracted either RIPA buffer and sequentially in the stronger SDS buffer (panel F) or directly in the SDS buffer (panel G). A similar increase in non-phosphorylated tau and a decrease in phosphorylated tau were observed, but the levels of total tau were increased by EP treatment, in contrast to the decrease seen in panels A-D.