Type I lissencephaly, a neuronal migration disorder characterized by cognitive disability and refractory epilepsy, is often caused by heterozygous mutations in the LIS1 gene. Histopathologies of malformation-associated epilepsies have been well described, but it remains unclear whether hyperexcitability is attributable to disruptions in neuronal organization or abnormal circuit function. Here, we examined the effect of LIS1 deficiency on excitatory synaptic function in the dentate gyrus of hippocampus, a region believed to serve critical roles in seizure generation and learning and memory. Mice with heterozygous deletion of LIS1 exhibited robust granule cell layer dispersion, and adult-born granule cells labeled with enhanced green fluorescent protein were abnormally positioned in the molecular layer, hilus, and granule cell layer. In whole-cell patch-clamp recordings, reduced LIS1 function was associated with greater excitatory synaptic input to mature granule cells that was consistent with enhanced release probability at glutamatergic synapses. Adult-born granule cells that were ectopically positioned in the molecular layer displayed a more rapid functional maturation and integration into the synaptic network compared with newborn granule cells located in the hilus or granule cell layer or in wild-type controls. In a conditional knock-out mouse, induced LIS1 deficiency in adulthood also enhanced the excitatory input to granule cells in the absence of neuronal disorganization. These findings indicate that disruption of LIS1 has direct effects on excitatory synaptic transmission independent of laminar disorganization, and the ectopic position of adult-born granule cells within a malformed dentate gyrus critically influences their functional maturation and integration.

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