Impaired GABAergic regulation and developmental immaturity in interneurons derived from the medial ganglionic eminence in tuberous sclerosis complex

GABAergic interneurons play a critical role in maintaining neural circuit balance, excitation-inhibition regulation, and cognitive function modulation. In tuberous sclerosis complex (TSC), GABAergic neuron dysfunction contributes to disrupted network activity and associated neurological symptoms, assumingly in a cell type specific manner. This study focuses on identifying specific interneuron subpopulations within TSC, emphasizing the unique characteristics of medial ganglionic eminence (MGE)- and caudal ganglionic eminence (CGE)-derived interneurons. Using single-nuclei RNA sequencing in TSC patient material, we identify somatostatin-expressing (SST+) interneurons as a unique and immature subpopulation in TSC. The disrupted maturation of SST+ interneurons may undergo an incomplete switch from excitatory to inhibitory GABAergic signaling during development, resulting in reduced inhibitory properties. Notably, this study reveals markers of immaturity specifically in SST+ interneurons, including an abnormal NKCC1/KCC2 ratio, indicating an imbalance in chloride homeostasis crucial for the postsynaptic consequences of GABAergic signaling as well as the downregulation of GABAA receptor subunits, GABRA1, and upregulation of GABRA2. To better verify the altered maturation in TSC, we performed experiments using the two-electrode voltage clamp technique on Xenopus laevis oocytes. Firstly, we found an altered GABA reversal potential (EGABA) suggesting a less hyperpolarizing GABAergic signaling. Moreover, we also described a rightward shift of GABA receptor affinity in TSC, associated with an upregulation of GABRA2, using both the cytoplasmatic injection of human membrane and the intranuclear injection of human cDNA in the Xenopus oocytes. Further exploration of SST+ interneurons revealed altered localization patterns of SST+ interneurons in TSC brain tissue, concentrated in deeper cortical layers, possibly linked to cortical dyslamination. In conclusion, this study sheds light on the potential contribution of SST+ interneurons to TSC pathophysiology, offering insights for targeted therapeutic interventions.