Cortical interneurons: fit for function and fit to function? Evidence from development and evolution

Abstract

Cortical inhibitory interneurons form a broad spectrum of subtypes. This diversity suggests a division of labor, in which each cell type supports a distinct function. In the present era of optimisation-based algorithms, it is tempting to speculate that these functions were the evolutionary or developmental driving force for the spectrum of interneurons we see in the mature mammalian brain. In this study, we evaluated this hypothesis using the two most common interneuron types, parvalbumin (PV) and somatostatin (SST) expressing cells, as examples. PV and SST interneurons control the activity in the cell bodies and the apical dendrites of excitatory pyramidal cells, respectively, due to a combination of anatomical and synaptic properties. But was this compartment-specific inhibition indeed the function for which PV and SST cells originally evolved? Does the compartmental structure of pyramidal cells shape the diversification of PV and SST interneurons over development? To address these questions, we reviewed and reanalyzed publicly available data on the development and evolution of PV and SST interneurons on one hand, and pyramidal cell morphology on the other. These data speak against the idea that the compartment structure of pyramidal cells drove the diversification into PV and SST interneurons. In particular, pyramidal cells mature late, while interneurons are likely committed to a particular fate (PV vs. SST) during early development. Moreover, comparative anatomy and single cell RNA-sequencing data indicate that PV and SST cells, but not the compartment structure of pyramidal cells, existed in the last common ancestor of mammals and reptiles. Specifically, turtle and songbird SST cells also express the Elfn1 and Cbln4 genes that are thought to play a role in compartment-specific inhibition in mammals. PV and SST cells therefore evolved and developed the properties that allow them to provide compartment-specific inhibition before there was selective pressure for this function. This suggest that interneuron diversity originally resulted from a different evolutionary driving force and was only later co-opted for the compartment-specific inhibition it seems to serve in mammals today. Future experiments could further test this idea using our computational reconstruction of ancestral Elfn1 protein sequences.

Keywords: development; evolution; inhibition; interneuron; microcircuits; neural morphology; pyramidal cell dendrites; single cell RNA seq.

Figure 1

Do development and evolution fit interneurons to function? (A) The connectivity and short-term plasticity (STP) of PV and SST positive interneurons seem adapted to the morphology and electrophysiology of pyramidal cells, as highlighted by an optimization-based model (B). In this model, optimizing interneuron parameters to provide compartment- (soma/dendrite) specific inhibition causes interneurons to diversify into two groups that resemble PV and SST interneurons in their connectivity and short-term plasticity (Keijser and Sprekeler, 2022). (C) The existence of PV and SST subtypes might therefore result from an developmental or evolutionary tuning of interneurons based on pyramidal properties. (D) This predicts that immature (or ancestral) circuits contain bursting pyramidal neurons and undiversified interneurons. Development (or evolution) then diversifies the interneurons into PV and SST subtypes. PC activity and short-term plasticity simulated with models from Tsodyks et al. (1998), Naud et al. (2013), and Naud and Sprekeler (2018), respectively. Animal silhouettes from https://beta.phylopic.org/.

Figure 2

Elfn1 expression correlates with short-term facilitation in mammals. (A) UMAP (McInnes et al., 2018) plot of mouse interneurons colored by subclass (left) and Elfn1 expression (right). The two interneuron types—SST and VIP interneurons—known to receive facilitating synapses both express Elfn1. (B) Violin plot of Elfn1 expression by subclass. CP10K: counts per 10 thousand. (C, D) As (A, B), but for human interneurons. Data from Tasic et al. (2018) (A, B), and Bakken et al. (2021) (C, D).

Figure 3

Cbln4 is expressed in a subset of mammalian SST interneurons. (A) UMAP plot of mouse and human interneurons, colored by their expression of Cbln4, a gene that instructs synapse formation onto pyramidal dendrites in mice (Favuzzi et al., 2019). Cbln4 is expressed in certain mouse and human interneuron subtypes, including SST cells. (B) UMAP of SST cells, clustered into subgroups. (C) Cbln4 is expressed in clusters 0, 3, and 12, which also express marker genes Tac1, Calb2, and Etv1 (D), respectively. (E) A subset of human Sst cells also express Cbln4. Data from Tasic et al. (2018) (A–D) and Bakken et al. (2021) (E).

Figure 4

Evolutionary conservation of GABAergic cell types. (A) Phylogenetic approach. (B) Pearson correlation between average RNA expression in clusters of songbird and mouse interneurons. Correlations between GABAergic neurons are typically larger. (C) UMAP plots of integrated gene expression data for GABAergic and glutamatergic neurons. GABAergic neurons first cluster by developmental origin (MGE vs. CGE, see Box 1) and then by species. Mouse data from Tasic et al. (2018), songbird data and correlation analysis from Colquitt et al. (2021).

Figure 5

Evolutionary conservation of Elfn1 expression. (A) UMAP plot showing overexpression of Elfn1 in SST-like and VIP-like interneurons in the turtle forebrain. Data from Tosches et al. (2018). (B) Violin plots of Elfn1 expression for each of the clusters. (C, D) As (A, B), but for zebra finch neurons. Data from Colquitt et al. (2021).

Figure 6

Cbln4 expression in non-mammalian species. Cbln4 is expressed in certain subtypes of turtle SST neurons, but not in songbird SST neurons. Data from Tosches et al. (2018) and Colquitt et al. (2021).

Figure 7

Evolutionary divergence of projection neuron morphology. Both turtle and mammalian projection neurons have a pyramidal morphology, but only mammalian pyramidal neurons have a single apical dendrite. Songbird projection neurons have a stellate, not pyramidal morphology. Turtle and mammalian neurons adapted from Larkum et al. (2008) (published under a Creative Commons License https://creativecommons.org/licenses/by-nc-sa/3.0/). Songbird neuron adapted from Kornfeld et al. (2017) (published under a Creative Commons License https://creativecommons.org/licenses/by/4.0/).

Figure 8

Phylogenetic inference of interneuron and pyramidal evolution. (A) Mice, humans, songbirds and turtles all have PV and SST interneurons. The most likely explanation for these similarities is that the interneuron types were already present in the last common ancestor of these lineages. (B) Only mammalian glutamateric neurons are known to exhibit dendritic plateau potentials that can elicit burst firing. Other lineages probably lack this trait. The most likely explanation is that dendritic bursting evolved only once, in the mammalian lineage.

Figure 9

Reconstruction of ancestral Elfn1 protein. (A) Species-tree showing the phylogenetic relationships of the species whose Elfn1 homologs were used to reconstruct the Elfn1 protein of the amniote ancestor. (B) Domain structure of mouse Elfn1 (Dolan et al., ; Dunn et al., 2018). LRR, leucine-rich repeat; CT, C-terminal domain; FN3, fibronectin type 3 domain; TM, transmembrane domain. (C) Per-site conservation across the tree shown in (A), computed as the fraction of extant species that share the mouse amino acid at a given site. Dashed lines correspond to gaps. Mean conservation: 0.746. (D) Posterior probability of ancestral protein. Gray: most likely (ML) sequence, red: 2nd most likely. Dashed line: cutoff for using the 2nd most likely base in “altAll” sequence. Mean posterior: 0.986. (E) Multiple sequence alignment of protein domains shown in (A). Only the first two LRRs are shown for space reasons. Dots indicate identity to mouse site, dashes indicate gaps.