Development and function of GABAergic neurons in health and in a predisposition model of schizophrenia

Abstract

In this thesis, I present four studies which examine i) the role of transcript factor Sox6 in the development and maintenance of cortical Sst interneurons, ii) the involvement of cortical neurogliaform cells (NGFCs) in the 22q11.2 deletion syndrome (22q11.2DS) mouse model, iii) the expression of Bcl11b in GABAergic interneurons, and iv) a comparative analysis of single-nucleus RNA sequencing (snRNA-seq) methods on human postmortem brain tissue.

In Paper I, we demonstrated that transcription factor Sox6 is crucial for the maintenance of Sst interneurons' subtype identity during migration to the cortex, a function that is temporally regulated and intrinsic to the neurons. Despite Sox6's downregulation not affecting the specification, migration nor maturation processes of Sst interneurons, it plays a significant role in maintaining their preacquired subtype identity. After network integration the subtype maintenance is no longer dependent on Sox6 expression.

The 22q11.2DS, associated with the highest known genetic predisposition for schizophrenia, disrupts bottom-up (thalamocortical) and top-down (corticocortical) signal integration, as seen in reduced mismatch negativity. Paper II highlighted the potential role of cortical NGFCs in this integration in the 22q11.2DS mouse model. Using a novel Mia-Cre mouse line targeting Mia-expressing NGFCs (Mia-NGFCs), we found reduced excitatory inputs from both thalamus and anterior cingulate cortex to layer I Mia-NGFCs in 22q11.2DS mouse primary visual cortex. Electrophysiological and single-cell RNA sequencing (scRNA-seq) evidence indicating this reduction is due to postsynaptic dysregulation in the Mia-NGFCs.

In Paper III, we revealed high expression of Bcl11b across various GABAergic interneurons in the mouse somatosensory cortex which puts its specificity as a marker for layer V-VI subcortical projecting pyramidal neurons into question. In fact, we show that 40% of layer V Bcl11b-positive cells were GABAergic interneurons, and scRNA-seq showed higher Bcl11b expression in interneurons than in layer V-VI pyramidal neurons, especially within the Htr3a-positive/Vipnegative interneuron group (putative NGFCs).

Finally in Paper IV, we benchmarked four snRNA-seq methods on human postmortem forebrain tissue, consisting mainly of striatal GABAergic neurons, aiming to guide method selection for future studies. Despite variations in performance metrics, all methods yielded comparable results, emphasizing the importance of considering practical factors such as tissue quality and data requirements when choosing among them.

Overall, our research provides new insights into the intricate mechanisms underlying GABAergic neuronal development and function in health and in a predisposition model of schizophrenia while offering practical guidance for methodological choices.