On GABAergic interneuron diversity and maturation

GABAergic interneurons provide finely, distinct styles of inhibition granted by their unique targeting preferences, molecular profiles, and morphological silhouettes. A central quest of my thesis was to explore what constitutes such diversity, touching upon how this diversity is reached and preserved during development and what maintains distinct functional features later on. This is a compilation of my overview on a vibrant, fast-paced research field that still holds several unresolved questions.

In Paper I, we used large-scale single-cell RNA sequencing to dissect the cellular com-position of the mouse somatosensory cortex and hippocampal CA1 region by identifying the distinct molecular subclasses of cells forming these brain regions. In particular, we unveiled a previously undescribed inhibitory interneuron labeled by transcription factor Pax6, which was further confirmed with immunohistochemistry, electrophysiology and morphological reconstructions. In Paper II, we characterized all striatal neuronal populations and compared them to their cortical counterparts using single-cell RNA sequencing. An important finding was that the typical parvalbumin-expressing neurons are part of a larger group of neurons expressing a novel marker, Pthlh, and that they exhibit a continuum of electrophysiological properties correlated with the expression of parvalbumin. Furthermore, cortical and striatal parvalbumin-expressing neurons show significant transcriptomic and electrophysiological differences.

In Paper III, we show that cortical somatostatin-expressing interneurons need the transcription factor Sox6 to maintain their subtype identity, specifically during migration. Using a combination of mouse genetics, single-cell RNA-sequencing, and electrophysiology we show that, while in controls the somatostatin-expressing class comprises nine molecularly distinct neuronal subtypes, the Sox6-mutant cortex contained only three molecular subtypes, without any significant somatostatin-cell loss. In Paper IV, we utilized conditional knockout strategies to remove Sox6 in parvalbumin-expressing interneurons at different postnatal stages. Our data revealed that class of interneurons relies on postnatal expression of Sox6 for the growth and maintenance of their axonal boutons and synaptic function until adulthood.

Altogether, the studies included in this thesis shine light on what GABAergic interneuron diversity encompasses, highlighting the particular role a transcription factor in maintenance of subtype identity (in somatostatin neurons) or particular functional features (in parvalbumin neurons).

List of scientific papers

I. Amit Zeisel, Ana B. Muñoz-Manchado, Simone Codeluppi, Peter Lonnerberg, Gioele La Manno, Anna Juréus, Sueli Marques, Hermany Munguba, Liqun He, Christer Betsholtz, Charlotte Rolny, Gonçalo Castelo-Branco, Jens Hjerling-Leffler, Sten Linnarsson. Cell types in the mouse cortex and hippocampus revealed by single-cell RNA-seq. Science. 2015, volume 347, 1139-1142.
https://doi.org/10.1126/science.aaa1934

II. Ana B. Muñoz-Manchado, Carolina Bengtsson Gonzales, Amit Zeisel, Hermany Munguba, Bo Bekkouche, Nathan G. Skene, Peter Lonnerberg, Jesper Ryge, Kenneth D. Harris, Sten Linnarsson, Jens Hjerling-Leffler. Diversity of Interneurons in the Dorsal Striatum Revealed by Single-Cell RNA Sequencing and PatchSeq. Cell Reports. 2018, vol. 24, 2179-90.
https://doi.org/10.1016/j.celrep.2018.07.053

III. Hermany Munguba, Kasra Nikouei, Hannah Hochgerner, Polina Oberst, Alexandra Kouznetsova, Jesper Ryge, Renata Batista-Brito, Ana B. Muñoz-Manchado, Jennie Close, Sten Linnarsson, Jens Hjerling-Leffler. Transcriptional maintenance of cortical somatostatin interneuron subtype identity during migration. [Submitted]

IV. Hermany Munguba, Bidisha Chattopadhyaya, Stephan Nilsson, Josianne N. Carriço, Polina Oberst, Ilya Kruglikov, Renata Batista-Brito, Ana Belen Muñoz-Manchado, Michael Wegner, Bernardo Rudy, Gordon Fishell, Graziella di Cristo, Jens Hjerling-Leffler. Sox6 regulates cortical parvalbumin-expressing neurons' synaptic function until adulthood. [Manuscript]