Secondary loss of miR-3607 reduced cortical progenitor amplification during rodent evolution

The evolutionary expansion and folding of the mammalian cerebral cortex resulted from amplification of progenitor cells during embryonic development. This process was reversed in the rodent lineage after splitting from primates, leading to smaller and smooth brains. Genetic mechanisms underlying this secondary loss in rodent evolution remain unknown. We show that microRNA miR-3607 is expressed embryonically in the large cortex of primates and ferret, distant from the primate-rodent lineage, but not in mouse. Experimental expression of miR-3607 in embryonic mouse cortex led to increased Wnt/beta-catenin signaling, amplification of radial glia cells (RGCs), and expansion of the ventricular zone (VZ), via blocking the beta-catenin inhibitor APC (adenomatous polyposis coli). Accordingly, loss of endogenous miR-3607 in ferret reduced RGC proliferation, while overexpression in human cerebral organoids promoted VZ expansion. Our results identify a gene selected for secondary loss during mammalian evolution to limit RGC amplification and, potentially, cortex size in rodents.

Automated summary

The expansion and folding of mammalian cerebral cortex is a result of amplification of progenitor cells during embryonic development, and this process was reversed in rodents. It was found that microRNA miR-3607 is expressed in the cortex of primates and ferret but not in mice, and its experimental expression in mice led to increased signaling and amplification of radial glia cells, as well as expansion of the ventricular zone.