Abstracts

Rationale: Focal cortical dysplasia type II (FCDII) is a neurodevelopmental disorder caused by brain somatic variants in the mTOR pathway, leading to focal cortical malformations and intractable epilepsy. Expression of FCDII-associated mTOR variants in mouse cortical neurons leads to disrupted translational control and aberrant translation of genes enriched in clusters of epileptic seizures, cytomegalic neurons, and dyslamination. Knockdown of eIF4E, the rate-limiting factor in translational initiation, prevents epilepsy and related pathologies in mice, supporting an essential role for eIF4E in FCDII pathogenesis. The functions of eIF4E are regulated through two known mechanisms: inhibitory binding by 4E-BPs, downstream of mTOR, and phosphorylation at the S209 residue by MNK kinases, downstream of ERK and p38 MAPK. The two modes of eIF4E regulation occur in response to different stimuli and are thought to modulate different mRNA translation. We previously showed that 4E-BP activity is essential to seizure generation in a mouse model of FCDII, but the role of the MNK-eIF4E axis is unknown. In this study, we examined whether MNK-mediated eIF4E phosphorylation is altered in FCDII mice.

Methods: To model FCDII in mice, we expressed the mTOR^S2215Y gain-of-function variant in a subset of developing cortical pyramidal neurons by in utero electroporation. We specifically targeted expression in neurons destined to layer 2/3 in the medial prefrontal cortex. Brain sections were collected on postnatal day 21 for histological analysis of neuronal morphology and placement and immunofluorescent staining for phosphorylated eIF4E at S209. Another cohort of 7- to 10-week-old mice were monitored for seizure activity by video-EEG recording.

Results: Mice expressing mTOR^S2215Y exhibited key FCDII neuropathology, including neuronal enlargement and misplacement in the deeper cortical layers, and spontaneous recurrent seizures (mean frequency of 7.9 seizures/day). Increased staining for phosphorylated eIF4E was detected within mTOR^S2215Y-expressing cortical neurons compared to controls. The increase in eIF4E phosphorylation was detected in the electroporated cortex, but not in the non-electroporated contralateral cortex, suggesting that the observed eIF4E hyperphosphorylation was not a global change due to seizures but a result of cell-specific mTOR^S2215Y expression.

Conclusions: In summary, our study demonstrates the presence of dysregulated MNK-eIF4E signaling in the mTOR^S2215Y mouse model of FCDII. These findings suggest that some pathology-contributing genes in FCDII may be translationally regulated by the MNK-eIF4E axis. Ongoing studies are testing the effects of targeting MNK-eIF4E signaling for epilepsy treatment.

Funding: None