Emergence of cortical interneuron diversity: a long game and its new player

The mammalian neocortex, which controls sensory perception, motor behaviour and cognition, relies on complex networks involving both excitatory glutamatergic pyramidal neurons and inhibitory GABAergic interneurons. Dissecting the diverse identities of these neurons and classifying them into categories of different granularities represent an important milestone towards understanding the contribution of each type of neuron to cortical function.

Prof. Kessaris first presented their early efforts in understanding the diversity of interneurons. Interneurons are a very heterogenous neuronal population in terms of molecular identities, electrical properties, morphologies and participation to neuronal networks.

While the main method for identifying interneuron types is through histological characterisation of marked expression, recent single-cell RNA sequencing techniques allow for a more global and unbiased study of interneuron types and subtypes. The first single cell transcriptomics study in adult mice by Tasic et al. 2016, identified more than 20 types of GABAergic interneurons in the visual cortex, later complemented by information on the structure and function of these neurons. Gouwens et al., 2020 performed patch sequencing together with morphological reconstruction and identified 28 types of interneurons in the mouse visual cortex with matching transcriptomic, electrophysiological and morphological properties.

However, one of the main questions yet to be addressed is:

- How are the identities of different interneuron subtypes acquired during development?

In this context, the Kessaris lab revealed a novel mechanism regulating the emergence of cortical interneuron diversity in their latest research paper (Asgarian et al., 2022).

The embryonic medial ganglionic eminence (MGE) serves a major structure that generate cortical interneurons through a cascade involving transcription factor LHX6 (Liodis et al., 2007). Prof. Kessaris’ group investigated genetic programmes driving MGE-derived cortical interneuron development and identified enriched expression of the transcriptional co-factor Mtg8 in the Lhx6+ MGE-lineage. In situ hybridization confirmed Mtg8 was expressed in the MGE at early embryonic stages, and its expression gradually diminished in prenatal cortical interneurons.

To investigate the roles of Mtg8 during MGE-derived interneurons development, the Kessaris lab generated Mtg8 mouse mutants. Interestingly, Mtg8 mutant embryos showed impaired distribution of cortical Sst+ interneurons despite a normal distribution of Lhx6+ MGE cells.

However, Mtg8 is widely expressed in the prenatal telencephalon. Does Mtg8 cell-autonomously function to regulate SST+ interneuron development? To address this question, Prof. Kessaris’ group distinctively labelled Mtg8 mutant and wildtype MGE cells at embryonic day 13.5 and transplanted labelled cells into the cortices of neonatal mice. Immunohistochemistry analysis in adult cortex showed decreased proportion of SST+ interneuron from the mutant lineage, supporting a cell-autonomous impairment of SST+ cortical interneuron development.

The transcription factor LHX6 is indispensable for the normal development of MGE-derived cortical interneurons, including the SST+ population (Liodis et al., 2007). Interestingly, a hypomorphic Lhx6 allele leads to a selective reduction in SST+ cortical interneurons (Neves et al., 2013). The Sst+ interneuron phenotype observed in the Mtg8 mutants was similar to that found in Lhx6 hypomorphic mice. RNA sequencing from Mtg8 and Lhx6 mutant MGE-derived cortical interneurons showed a small set of genes, including Sst and Npy, differentially expressed in both mutants, suggesting they may represent common downstream targets of MTG8 and LHX6. MTG8 is a transcriptional co-factor and does not bind DNA or regulate transcription on its own. Intriguingly, MTG8 and LHX6 co-immunoprecipitated from cells
transfected with both proteins, suggesting that they could physically interact to regulate target gene expression.

Finally, they tested whether MTG8 and LHX6 are sufficient to specify SST-NPY interneuron fate. Although overexpression of either Mtg8 or Lhx6 alone in the embryonic cortex failed to up-regulate Sst or Npy, coexpression of both Mtg8 and Lhx6 robustly increased Npy expression. In addition, overexpression of Mtg8 in the entire MGE-lineage promoted the SST-NPY fate. Altogether, these data indicate that MTG8 and LHX6 act synergistically to specify SST-NPY fate within the MGE.

In conclusion, Prof. Kessaris’ research illustrates a novel molecular mechanism driving the developmentof MGE-derived SST-NPY cortical interneurons. The work highlights that, beyond transcriptomic networks, protein-protein interaction networks may hold the key to cortical interneuron diversity specification.

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References

Asgarian, Z. et al. (2022). MTG8 interacts with LHX6 to specify cortical interneuron subtype identity. Nat Commun.

Gouwens, N. et al. (2020). Integrated Morphoelectric and Transcriptomic Classification of Cortical GABAergic Cells. Cell

Liodis, P. et al. (2007). Lhx6 Activity Is Required for the Normal Migration and Specification of CorticalInterneuron Subtypes. J Neurosci.

Neves, G. et al. (2013). The LIM Homeodomain Protein Lhx6 Regulates Maturation of Interneurons and Network Excitability in the Mammalian Cortex. Cerebral Cortex.

Tasic, B. et al. (2016). Adult mouse cortical cell taxonomy revealed by single cell transcriptomics. Nat Neurosci.