Abstracts

Rationale: Infantile spasms (IS) is a devastating epilepsy syndrome in which childhood spasms can progress to epileptic seizures and neurodevelopmental deficits 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, which is a known regulator of IN proliferation. To investigate the mechanisms contributing to IS, we utilized the APC conditional knockout (cKO) mouse model, in which the Adenomatous polyposis coli (APC) gene is removed in excitatory forebrain neurons (CaMKII-Cre). Since APC is part of the β-cat destruction complex, its loss leads to abnormal increases in β-cat levels. APC cKOs experience behavioral spasms at postnatal day 9 (P9) and approximately 80% of adult APC cKO animals develop spontaneous electroclinical seizures. Based on the human links between IS, IN-opathies, and β-cat signaling, we investigated if IN networks are disrupted during cortical development in APC cKO mice. Methods: APC cKO mice were crossed with G42 mice, in which GFP is expressed in PV+ INs, to examine early postnatal IN maturation. We measured cell density of GFP+ INs in somatosensory cortex of APC cKO and wildtype (WT) mice at P3, P9, and adulthood using IHC and confocal imaging. We immunolabeled PV INs, to determine if GFP+ cells accurately represent PV+ INs throughout development and to account for potential reporter-based issues. We also examined β-cat levels in GFP+ cells and layer 5 pyramidal neurons to determine if elevating β-cat in excitatory neurons led to changes in β-cat in GABAergic INs. Because cortical INs undergo cell death during early postnatal life, we examined the presence of apoptotic cells in the cortex using a cleaved caspase-3 antibody and examined colocalization between caspase-3 and GFP+ INs. Finally, we examined the presence of Cre recombinase using IHC to determine if cortical INs had Cre expression during development. Results: There was a significant decrease in GFP+ cell density in APC cKO mice at P3 and P9 in the somatosensory cortex compared to WT littermates. In adult mice, GFP+ cell densities were not significantly different between cKO and WT. PV-immunolabeled cells were not significantly different between cKO and WT in adult. We also found that there was caspase-3 colocalization with GFP+ INs at P3 and P9 in both cKO and WT. Interestingly, in WT animals we found abundant capspase-3 staining in GFP+ IN, while in APC cKOs the GFP+ cells that were present in the cortex tended to be caspase-3 negative. Conclusions: APC cKO mice showed a significant deficit in immature PV INs at P9 in the somatosensory cortex, as identified with G42 GFP labeling. Interestingly, the density of GFP+ cells decreased between P9 and adult in the WT cortex, while GFP+ cell density was already low at P9 in APC cKOs. This suggests premature IN loss in the cortex of APC cKOs. β-cat and Cre immunostaining in cKOs occured predominantly in excitatory neurons, suggesting that alterations in INs occur due to changes in excitation. In fact, preliminary data from the lab suggests that parvalbumin (PV+) INs in APC cKOs receive more excitatory input during early development. This is consistent with published studies showing that neuronal activity regulates IN development and survival in the cortex. Future studies will focus on whether this early developmental loss of GABAergic INs contributes to IS-like phenotypes in APC cKO mice. Funding: NINDS R01 NS100706