Hominini-Specific Regulation of CBLN2 Increases Prefrontal Synaptogenesis

The similarities and differences between nervous systems of various species result from developmental constraints and specific adaptations. Comparative analyses of the prefrontal cortex (PFC), a region of the cerebral cortex involved in higher-order cognition and complex social behaviors, have identified confirmed and putative human-specific structural and molecular changes. For example, one crucial specialization involves the anterior-posterior gradient in synaptic density, with a disproportionately higher number of dendritic spines specifically in the human PFC compared to other analyzed primates. These changes are likely mediated by divergence in spatio-temporal patterns of gene expression, which are prominent in the mid-fetal human cerebral neocortex. Analyzing developmental human and macaque brain transcriptomic data, we identified a transient PFC- and laminar-specific upregulation of the gene encoding cerebellin 2 (CBLN2), a neurexin (NRXN) and glutamate receptor delta (GRID/GluD)-associated synaptic organizer, in human mid-fetal development coinciding with the initiation of synaptogenesis. Moreover, we show that this difference in expression level and laminar distribution of CBLN2, is due to Hominini-specific deletions affecting SOX5 binding sites within a retinoic acid-responsive CBLN2 enhancer. In situ genetic humanization of the mouse Cbln2 enhancer drives increased and ectopic laminar Cbln2 expression and promotes glutamatergic and GABAergic synaptogenesis specifically in the PFC. These findings identify a putative genetic and molecular basis for the disproportionately increased connectivity in the Hominini PFC and suggest a developmental mechanism linking dysfunction of the NRXN-GRID-CBLN2 complex, to the pathogenesis of neuropsychiatric disorders.