Abstracts

Rationale: Infantile spasms (IS) is a devastating early-life epilepsy that is characterized by severe flexion/extension spasms, high-amplitude interictal discharges, and developmental delays. If not successfully treated, IS often progresses into other types of epilepsies later in life. The underlying causes of IS are unclear, but human genetics suggest a link between IS-associated genes, cortical interneuron (IN) development, and the Wnt/beta-catenin (beta-cat) signaling pathway. Proper Wnt/beta-cat signaling is crucial for cortical IN maturation and individuals with IS-associated mutations can have decreased density of cortical INs. Utilizing a conditional deletion of APC in CAMKII neurons to increase beta-cat levels, we show that cortical parvalbumin (PV+) INs have increased excitatory input and more complex dendritic morphology during development. Methods: This model uses a CAMKIIcre-driven deletion of adenomatous polyposis coli to delete APC from excitatory neurons (APC cKO), leading to an increase in beta-cat levels in the developing and adult cortex, hippocampus, and striatum. To examine the function of PV+ INs we crossed APC cKO mice with GAD1-eGFP mice, in which ~90% of labeled neurons are PV+. We measured synaptic (e.g. spontaneous and miniature EPSCs) and intrinsic properties of PV+ INs in APC WT and cKO mice at postnatal days 9, 14, and 60. Biocytin was used to fill cells during electrophysiology recordings and 3D reconstruction of dendrites was done using Imaris Software. Beta-cat and GAD expression were quantified with immunohistochemistry. Results: Deletion of APC in CamKII+ excitatory neurons results in behavioral spasms in neonatal mice at post-natal day 9 (P9) and electroclinical seizure activity in adults. At P9, there was a significant increase in excitatory input due to an increase in mEPSC frequency onto PV+ INs in APC cKO mice, as compared to WT littermates, with no significant changes in intrinsic cellular properties. There was no difference in excitatory input onto PV+ INs in APC cKOs, compared to WT mice, at P14. At P60 there was an increase in sEPSCs frequency in APC cKO mice. There was no change in spontaneous or miniature EPSC amplitude at P9, P14, or P60. Reconstruction of PV+ INs shows that neurons from APC cKO mice have more complex dendritic morphology compared to WTs. Upon examination of beta-cat levels in neonatal and adult mice, we found that increased levels of beta-cat do not colocalize with GAD1-eGFP neurons. These data indicate that excitation of PV+ INs and IN morphology is altered in the APC cKO mouse model of infantile spasms. Conclusions: Our research implicates alterations of cortical IN network maturation and signaling in the APC cKO model of IS. These data further support the role of interneuronopathies in IS, because many patients have mutations in genes that contribute to IN survival and maturation. Since beta-cat does not colocalize with PV+ INs, this suggests that electrophysiological and morphological changes are not due to altered beta-cat levels directly in INs, but may result from altered excitatory input which aids in IN survival and function. Future experiments will focus on determining mechanisms within the Wnt/beta-cat signaling pathway that contribute to aberrant PV+ IN function in the APC cKO model of IS. Funding: NINDS R01-NS100706