Modular Splicing is Linked to Evolution in the Synapse-Specificity Molecule Kirrel3

Abstract

Kirrel3 is a cell-adhesion molecule that instructs the formation of specific synapses during brain development in mouse and Kirrel3 variants may be risk factors for autism and intellectual disabilities in humans. Kirrel3 is predicted to undergo alternative splicing but brain isoforms have not been studied. Here, we present the first in-depth characterization of Kirrel3 isoform diversity in brain using targeted, long-read mRNA sequencing of mouse hippocampus. We identified 19 isoforms with predicted transmembrane and secreted forms and show that even rare isoforms generate detectable protein in the brain. We also analyzed publicly-available long-read mRNA databases from human brain tissue and found 11 Kirrel3 isoforms that, similar to mouse, encode transmembrane and secreted forms. In mice and humans, Kirrel3 diversity arises from alternative, independent use of protein-domain coding exons and alternative early translation-stop signals. Intriguingly, the alternatively spliced exons appear at branch points in the chordate phylogenetic tree, including one exon only found in humans and their closest living relatives, the great apes. Together, these results validate a simple pipeline for analyzing isoform diversity in genes with low expression and suggest that Kirrel3 function is fine-tuned by alternative splicing and may play a role in brain evolution. Significance StatementKirrel3 is an important molecule for synapse and circuit formation with gene variants that are associated with neurodevelopmental disorders, yet Kirrel3 function remains largely unknown. Here, we report new isoforms of mouse and human Kirrel3, including secreted and transmembrane forms, that suggest a diverse repertoire of Kirrel3 actions. Importantly, we identified a new Kirrel3 exon only present in humans and the other great apes with potential to play an important role in circuit formation unique to these species.