Abstracts

Rationale: Infantile spasms (IS) is a debilitating epilepsy syndrome in which childhood spasms progress to seizures in adulthood. Interneuron (IN) loss and dysfunction is a common feature in human and mouse models of IS. Interestingly, 11 out of 15 known IS risk genes have potential links to the β-catenin (β-cat)/Wnt signaling pathway. To investigate whether aberrant β-cat signaling contributes to IS, we utilized the APC conditional knockout (cKO) mouse model, in which the APC gene is removed in excitatory forebrain neurons (CaMKII-Cre). APC is part of the β-cat destruction complex, so its loss leads to increased β-cat levels. APC cKOs have behavioral spasms at postnatal day 9 (P9) and 80% of adult APC cKO animals develop spontaneous electroclinical seizures. APC cKOs have increased glutamatergic synaptic excitation, but how this affects IN maturation is unknown. Therefore, we investigated if IN networks are disrupted during cortical development in APC cKO mice.

Methods: To identify developing cortical INs, APC cKO mice were crossed with G42 mice, in which GFP is expressed in immature parvalbumin (PV+) Ins, or Lxh6-GFP mice, in which GFP is expressed in PV+ and somatostatin INs. We measured cell density of INs in somatosensory cortex of APC cKO and wildtype (WT) mice using confocal imaging. Because cortical INs undergo cell death during early postnatal life, we quantified apoptotic INs using a cleaved caspase-3 antibody. Finally, we examined the function of developing IN networks using whole-cell patch-clamp recordings in cortex of WT and cKO mice.

Results: Immature PV+ IN density was decreased at the time of spasms (P9), but not later in life. Consistent with this, Caspase-3 positive INs were significantly more abundant at P9, and significantly decreased at P14, in APC cKOs. Interestingly, both developing and mature PV+ INs receive increased excitation. Surprisingly, the inhibitory output of PV+ INs, as quantified by recording inhibitory synaptic currents in L5 pyramidal cells, was strongly decreased. This results in a net increase in excitation and decrease in inhibition in cortical circuits in APC cKOs.

Conclusions: Our findings show that increased β-cat signaling during development in APC cKOs disrupts GABAergic IN maturation, survival, and function. Developmental apoptosis is shifted to an earlier developmental timepoint, PV+ INs receive aberrant excitation, and inhibitory synaptic output is significantly reduced in APC cKOs. Overall, both developing and mature GABAergic IN networks are disrupted in the APC cKO which may contribute to cortical circuit hyperexcitability, spasms, seizures, and other IS-like phenotypes. Future studies will determine if reducing aberrant β-cat levels or restoring GABAergic IN activity can restore normal inhibitory network function and reduce IS-like phenotypes.

Funding: Please list any funding that was received in support of this abstract.: AES Predoctoral Fellowship.