γ-Aminobutyric Acid Transporter Mutation GAT1 (S295L) Substantially Impairs Neurogenesis in Dentate Gyrus

<b>Background</b>: GABAergic signaling plays a crucial role in modulating neuronal proliferation, migration, and the formation of neural network connections. The termination of GABA transmission primarily occurs through the action of GABA transporter 1 (GAT1), encoded by the <i>SLC6A1</i> gene. Multiple <i>SLC6A1</i> mutations have been implicated in neurodevelopmental disorders, but their effects on the nervous system are unclear. <b>Methods</b>: We estimated the expression pattern of the GAT1 (S295L) protein using the <i>Slc6a1^S295L/S295L</i> mouse model via RT-PCR, Western blotting, and confocal immunofluorescence. The effect of GAT1 (S295L) on hippocampal neurogenesis was investigated by neuronal marker staining (Sox2, Tbr2, NeuroD1, DCX, NeuN) and BrdU label experiments. The dendritic complexity was mapped through Sholl analysis. RNA-Seq was utilized to explore the signaling pathways and molecules associated with neurodevelopmental disorders. <b>Results</b>: We detected a remarkable decline in the quantity of type-2b intermediate progenitor cells, neuroblasts, and immature neurons in the dentate gyrus (DG) of <i>Slc6a1^S295L/S295L</i> mice at 4 weeks. These abnormalities were exacerbated in adulthood, as evidenced by compromised dendritic length and height as well as the complexity of immature neurons. Immunofluorescence staining showed the abnormal aggregation of GAT1 (S295L) protein in neurons. RNA-seq analysis identified pathways associated with neurodevelopment, neurological disorders, protein homeostasis, and neuronutrition. The neurotrophin <i>Bdnf</i> decreased at all ages in the <i>Slc6a1^S295L/S295L</i> mice. <b>Conclusions</b>: Our data provide new evidence that GAT1 (S295L) causes impaired neurogenesis in the DG. GAT1 mutation not only disrupts GABA homeostasis but also impairs the neurotrophic support necessary for normal hippocampal development, which may be one of the factors contributing to impaired neurogenesis.