Progressive Dysregulation of Tau Phosphorylation in an Animal Model of Temporal Lobe Epilepsy

Tau is an intracellular protein known to undergo hyperphosphorylation and subsequent neuro-toxic aggregation in Alzheimer's disease (AD). Here, tau expression and phosphorylation at three canonical loci known to be hyperphosphorylated in AD (S202/T205, T181, and T231) were studied in the rat pilocarpine status epilepticus (SE) model of temporal lobe epilepsy (TLE). We measured tau expression at two time points of chronic epilepsy: two months and four months post-SE. Both time points parallel human TLE of at least several years. In the whole hippocampal formation at two months post-SE, we observed modestly reduced total tau levels compared to naïve controls, but no significant reduction of S202/T205 phosphorylation levels. In the whole hippocampal formation from four month post-SE rats, total tau expression had reverted to normal, but there was a significant reduction in S202/T205 tau phosphorylation levels that was also seen in CA1 and CA3. No change in phosphorylation was seen at the T181 and T231 tau loci. In somatosensory cortex, outside of the seizure onset zone, no changes in tau expression or phosphorylation were seen at the later time point. We conclude that total tau expression and phosphorylation in an animal model of TLE studied do not show hyperphosphorylation at the three AD canonical tau loci. Instead, the S202/T205 locus showed progressive dephosphorylation. This suggests that changes in tau expression may play a different role in epilepsy than in AD. Further study is needed to understand how these changes in tau may impact neuronal excitability in chronic epilepsy.

HCN Channel Phosphorylation Sites Mapped by Mass Spectrometry in Human Epilepsy Patients and in an Animal Model of Temporal Lobe Epilepsy

Because hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels modulate the excitability of cortical and hippocampal principal neurons, these channels play a key role in the hyperexcitability that occurs during the development of epilepsy after a brain insult, or epileptogenesis. In epileptic rats generated by pilocarpine-induced status epilepticus, HCN channel activity is downregulated by two main mechanisms: a hyperpolarizing shift in gating and a decrease in amplitude of the current mediated by HCN channels, I_h. Because these mechanisms are modulated by various phosphorylation signaling pathways, we hypothesized that phosphorylation changes occur at individual HCN channel amino acid residues (phosphosites) during epileptogenesis. We collected CA1 hippocampal tissue from male Sprague Dawley rats made epileptic by pilocarpine-induced status epilepticus, and age-matched naïve controls. We also included resected human brain tissue containing epileptogenic zones (EZs) where seizures arise for comparison to our chronically epileptic rats. After enrichment for HCN1 and HCN2 isoforms by immunoprecipitation and trypsin in-gel digestion, the samples were analyzed by mass spectrometry. We identified numerous phosphosites from HCN1 and HCN2 channels, representing a novel survey of phosphorylation sites within HCN channels. We found high levels of HCN channel phosphosite homology between humans and rats. We also identified a novel HCN1 channel phosphosite S791, which underwent significantly increased phosphorylation during the chronic epilepsy stage. Heterologous expression of a phosphomimetic mutant, S791D, replicated a hyperpolarizing shift in I_h gating seen in neurons from chronically epileptic rats. These results show that HCN1 channel phosphorylation is altered in epilepsy and may be of pathogenic importance.

Selective hyperactivation of JNK2 in an animal model of temporal lobe epilepsy

c-Jun N-terminal kinases (JNKs) are members of the mitogen-activated protein kinase (MAPK) family and are derived from three genes, Jnk1-3. These kinases are involved in cellular responses to homeostatic insults, such as inflammation and apoptosis. Furthermore, increased JNK expression and activation are associated with debilitating neurodegenerative diseases, including Alzheimer’s and Parkinson’s. We previously reported elevated levels of phosphorylated JNK (pJNK), indicative of JNK hyperactivation, in the CA1 hippocampus of chronically epileptic rats. We also showed that pharmacological inhibition of JNK activity reduced seizure frequency in a dose-dependent fashion (Tai TY et al., Neuroscience, 2017). Building on these observations, the objectives of this current study were to investigate the timeline of JNK activation during epileptogenesis, and to identify the JNK isoform(s) that undergo hyperactivation in the chronic epilepsy stage. Western blotting analysis of CA1 hippocampal homogenates showed JNK hyperactivation only during the chronic phase of epilepsy (6–9 weeks post-status epilepticus), and not in earlier stages of epileptogenesis (1 h, 1 day, and 1 week post-status epilepticus). After enrichment for pJNK by immunoprecipitation, we identified JNK2 as the only significantly hyperactivated JNK isoform, with expression of the 54 kDa pJNK2 variant elevated to a greater extent than the 46 kDa pJNK2 variant. Expression of the total amounts of both JNK2 variants (phosphorylated plus non-phosphorylated) was reduced in epilepsy, however, suggesting that activation of upstream phosphorylation pathways was responsible for JNK2 hyperactivation. Since our prior work demonstrated that pharmacological inhibition of JNK activation had an antiepileptic effect, JNK2 hyperactivation is therefore likely a pathological event that promotes seizure occurrences. This investigation provides evidence that JNK2 is selectively hyperactivated in epilepsy and thus may be a novel and selective antiepileptic target.

Synergistic effect of Bcl-2 and BAG-1 on the prevention of photoreceptor cell death

PURPOSE

Ectopic expression of Bcl-2 in photoreceptors of mice with retinal degenerative disease slows progression of the disease. BAG-1 has previously been shown to augment the inhibitory effect of Bcl-2 on programmed cell death in cultured cell systems. This study was designed to determine whether the coexpression of BAG-1 and Bcl-2 in the photoreceptors of mice with an autosomal dominant form of retinitis pigmentosa (RP) would enhance the protective effect provided by Bcl-2 alone.

METHODS

An expression vector using the 5' regulatory region of the murine opsin gene was used to target the expression of BAG-1 specifically to photoreceptor cells of mice. The BAG-1 transgenic mice were crossed to Bcl-2 transgenics to obtain animals that coexpress the two transgenes in photoreceptor cells. BAG-1/Bcl-2 animals were then crossed to an RP mouse model (a transgenic line overexpressing the S334ter rhodopsin mutant) to assess the effect of coexpression of BAG-1 and Bcl-2 on retinal degeneration. Morphologic analysis was performed on retinas isolated at various times after birth to monitor disease progression.

RESULTS

High levels of BAG-1 expression resulted in retinal degeneration that was not prevented by Bcl-2 expression. However, coexpression of appropriate levels of BAG-1 and Bcl-2 was found to have a profound inhibitory effect on retinal degeneration caused by overexpression of a mutant rhodopsin transgene. Whereas expression of Bcl-2 alone was previously found to delay degeneration of the retina from 2 weeks to approximately 4 weeks of age, coexpression of BAG-1 and Bcl-2 inhibited photoreceptor cell death for as long as 7 to 9 weeks.

CONCLUSIONS

The synergistic effect against photoreceptor cell death produced by the coexpression of Bcl-2 and BAG-1 indicates that these proteins can function in concert to prevent cell death. At the correct dosage, coexpression of Bcl-2 and BAG-1 may serve as a potential means to treat retinal degenerative diseases.