Abstracts

Rationale: Tau is an intracellular protein known to undergo increased phosphorylation (hyperphosphorylation) and subsequent neurotoxic aggregation in Alzheimer’s Disease (AD). It is unclear whether tau undergoes similar changes in animal model or human epilepsy. Previously (Concepcion et al., 2023) we used the rat pilocarpine model of epilepsy and found that at tau loci recognized by the AT8 antibody and known to be hyperphosphorylated in AD, there was a 45% reduction in phosphorylation in hippocampus where seizures arise. There was no change in AT8 levels in the somatosensory cortex, outside of the seizure onset zone. These findings suggest that tau in the epileptogenic hippocampus is not hyperphosphorylated at the canonical AD AT8 loci but instead is dephosphorylated. However, tau has many phosphosites, not all associated with AD pathogenesis. We surveyed all tau phosphosites using mass spectrometry to determine whether tau is net dephosphorylated or hyperphosphorylated in epilepsy, both in an animal model and in humans.


Methods: We used the pilocarpine rat model of temporal lobe epilepsy (TLE) to study changes in tau phosphorylation in the hippocampus 4 months post-status epilepticus, and in hippocampal and neocortical tissue resected from patients undergoing surgery for refractory epilepsy.

Results: We were able to detect phosphorylation at 43 tau phosphosites in rat hippocampus. Significant changes in phosphorylation state in chronic epilepsy compared to age-matched naïve animals were identified at T58 (32% increase), T167 (13% decrease), S189 (28% decrease), T203 (22% decrease) and S253 (39% decrease). We also identified 32 tau methylation sites but found no significant changes in methylation state in epilepsy. These data show that specific tau phosphosites are dysregulated in an animal model of chronic epilepsy, with 4 of the 5 sites showing dephosphorylation, not hyperphosphorylation as seen in AD. We then used mass spectrometry to analyze epileptogenic brain tissue resected from patients with refractory epilepsy, and compared levels of tau phosphorylation to data from our animal model. We were able to detect 39 tau phosphosites in human brain, with 30 orthologous to rat tau. Comparing to those phosphosites undergoing phosphorylation change in our animal model of TLE, we found similar levels of tau phosphorylation in human tissue, including: S198 (9.0% in human and 9.9% at rat S189), T212 (3.2% human and 6.7% at rat T203), and S262 (10.2% and 10.2% S253). (Note T167 has no human ortholog.)

Conclusions: These data indicate that tau in the epileptogenic hippocampus in an animal is net dephosphorylated rather than hyperphosphorylated as in AD. Overall tau phosphorylation is highly conserved in rat and human brain, and that sites significantly dysregulated in rat epilepsy are phosphorylated at similar levels in human epilepsy. This suggests that tau in human epileptogenic tissue is also net dephosphorylated, which differs from tau in the AD brain. Further study is needed to understand the functional impacts of tau dephosphorylation and how these changes affect neuronal excitability in chronic epilepsy.


Funding: NIH NS050229, AES Seed Grant