Abstracts

Rationale: Tau is an intracellular protein known to undergo an increase in phosphorylation (hyperphosphorylation) and subsequent neuro-toxic aggregation in Alzheimer’s disease. Genetic deletion of tau in animal models of epilepsy reduces seizure frequency, suggesting that tau expression exerts a pro-epileptic action. In post-mortem studies of brain tissue from patients with refractory temporal lobe epilepsy using immunohistochemical methods, it has been unclear whether tau expression and phosphorylation is altered in the hippocampus where seizures originate. Our aim is to investigate total tau expression and phosphorylation at the S202/T205 human locus, known to be hyperphosphorylated in Alzheimer’s disease, in a rat model of temporal lobe epilepsy.

Methods: Chronically epileptic rats were generated by pilocarpine-induced status epilepticus (SE). Either whole hippocampi or hippocampal subregions CA1 & CA3 were collected at two time points of chronic epilepsy: 6-9 weeks post-SE and 14-17 weeks post-SE. Western blotting involving both anti-3R tau antibody (Ab) and anti-4R tau Ab identified tau isoforms. Tau-5 Ab and the AT8 Ab measured total tau expression and phosphorylation at the S202/T205 human locus, respectively. Immunohistochemistry involving the AT8 Ab was used to determine localization of S202/T205 dually phosphorylated tau within the CA1 and CA3 hippocampal subregions of naïve rats.

Results: When co-staining with the anti-3R and anti-4R tau Abs in lysates from the entire hippocampal formation, four out of six tau isoforms (0N3R, 0N4R, 1N4R, and 2N4R) were readily observed, while the other two isoforms, 1N3R and 2N3R, were present only at low levels. In the entire hippocampus at 6-9 weeks post-SE, we observed modestly reduced total tau levels (20.6 ± 3.10% lower than naïve controls; p=< 0.001 but no significant reduction of S202/T205 phosphorylation levels. However, in whole hippocampi from 14-17 weeks post-SE rats, total tau expression was unchanged, but there was a significant reduction in S202/T205 phosphorylation levels (45.0 ± 7.57% lower than naïve controls; p< 0.001). Both CA3 and CA1 showed loss of S202/T205 phosphorylation in chronic epilepsy, with CA3 showing larger changes than CA1. Unlike in CA1, area CA3 showed loss of total tau expression in both periods of epilepsy. Immunohistochemical analysis showed S202/T205 tau phosphorylation was most strongly observed in the cell body layers of areas CA1 and CA3, and also in the mossy fiber projections to CA3, with less expression in the dendritic regions of pyramidal neurons.