Expression of human-specific ARHGAP11B in mice leads to neocortex expansion and increased memory flexibility

Neocortex expansion during human evolution provides a basis for our enhanced cognitive abilities. Yet, which genes implicated in neocortex expansion are actually responsible for higher cognitive abilities is unknown. The expression of human-specific ARHGAP11B in embryonic/foetal mouse, ferret and marmoset neocortex was previously found to promote basal progenitor proliferation, upper-layer neuron generation and neocortex expansion during development, features commonly thought to contribute to increased cognitive abilities. However, a key question is whether this phenotype persists into adulthood and if so, whether cognitive abilities are indeed increased. Here, we generated a transgenic mouse line with physiological ARHGAP11B expression that exhibits increased neocortical size and upper-layer neuron numbers persisting into adulthood. Adult ARHGAP11B-transgenic mice showed altered neurobehaviour, notably increased memory flexibility and a reduced anxiety level. Our data are consistent with the notion that neocortex expansion by ARHGAP11B, a gene implicated in human evolution, underlies some of the altered neurobehavioural features observed in the transgenic mice, such as the increased memory flexibility, a neocortex-associated trait, with implications for the increase in cognitive abilities during human evolution.

Serotonin Receptor 2A Activation Promotes Evolutionarily Relevant Basal Progenitor Proliferation in the Developing Neocortex

Evolutionary expansion of the mammalian neocortex (Ncx) has been linked to increased abundance and proliferative capacity of basal progenitors (BPs) in the subventricular zone during development. BP proliferation is governed by both intrinsic and extrinsic signals, several of which have been identified. However, a role of neurotransmitters, a canonical class of extrinsic signaling molecules, in BP proliferation remains to be established. Here, we show that serotonin (5-HT), via its receptor HTR2A, promotes BP proliferation in an evolutionarily relevant manner. HTR2A is not expressed in embryonic mouse Ncx; accordingly, 5-HT does not increase mouse BP proliferation. However, ectopic HTR2A expression can increase mouse BP proliferation. Conversely, CRISPR/Cas9-mediated knockout of endogenous HTR2A in embryonic ferret Ncx reduces BP proliferation. Pharmacological activation of endogenous HTR2A in fetal human Ncx ex vivo increases BP proliferation via HER2/ERK signaling. Hence, 5-HT emerges as an important extrinsic pro-proliferative signal for BPs, which may have contributed to evolutionary Ncx expansion.