Cortical interneuron paucity and altered PTEN/PI3K/AKT pathway in an infant rat model of malformation of cortical development

Abstract

Background: Malformations of cortical development (MCDs) is one of the major causes of intractable epilepsies such as epileptic spasms. Rats with prenatal exposure to methylazoxymethanol (MAM) have been used as a model of malformation of cortical development (MCD) and increased seizure susceptibility to N-methyl-D-aspartate (NMDA) during infancy was also shown in this model. Objective: The aim of this study is to identify the pathologic changes and in vivo neurometabolic alterations in rat model MCD during infancy. Underlying molecular changes were also explored to understand the mechanisms of the enhanced seizure susceptibility during infancy in MCD rat model. Methods: At gestational day 15, two dosages of MAM (15mg/kg IP) were injected to pregnant rats to produce MCD and normal saline to controls. The offspring underwent in vivo magnetic resonance imaging including 1H-MR spectroscopy (1H-MRS), diffusion tensor imaging (DTI), and chemical exchange saturation transfer of glutamate (GluCEST) at postnatal day (P) 15 using a 7T small-animal imaging system. On the same day, those animals were sacrificed and protein expression of the PI3K/AKT/mTOR pathway was measured from the cortex. Another set of prenatally MAM-exposed rats were pretreated with two different dosing regimens of rapamycin (3 or 10mg/kg, P5 to P14 or P9 to P14) and the number and onset of spasms were monitored for 90 minutes after the NMDA (15mg/kg i.p.) injection at P15. Results: At P15, immunofluorescence staining of retrosplenial cortices of rats with MCD revealed decreased cells with neuronal nuclei (NeuN), parvalbumin, and reelin expression. 1H-MRS and GluCEST imaging of retrosplenial cortex showed reduction of glutamate (Glu, p = 0.002), glutamate-plus-glutamine (Glu+Gln, p = 0.017), N-acetylaspartate (NAA, p = 0.002), N-acetylaspartate-plus-N-acetylaspartylglutamate (NAA+NAAG, p = 0.004), and macromolecules (MM) and lipids (Lips, p < 0.05) and the level of GluCEST (%, p < 0.001) in rats with MCD. There was translational deactivation of phosphatase and tensin homolog (PTEN), Akt, FoxO3a, and GSK3β in rats with MCD. Rapamycin pretreatment did not affect the NMDA-triggered spasm susceptibility and there was no significant change of pS6/S6, S6, Rictor, and PI3k expression in cortices of rats with prenatal MAM exposure. Conclusion: In prenatally MAM exposed infant rats, abnormal cortical migration with decreased GABAergic interneurons and comparable in vivo MR imaging characteristics were identified. Prenatal MAM exposure also leads to alteration of PTEN/PI3K/AKT pathway during their infancy, which can be further investigated as the target of MCD-related epilepsy treatment.