Regulation of Prefrontal Patterning, Connectivity and Synaptogenesis by Retinoic Acid

The prefrontal cortex (PFC) and its reciprocal connections with the mediodorsal thalamus (MD) are crucial for cognitive flexibility and working memory1-4 and are thought to be altered in several disorders such as autism spectrum disorder5, 6 and schizophrenia6-9. While developmental mechanisms governing regional patterning of the rodent cerebral cortex have been characterized10-15, the mechanisms underlying the development of PFC-MD connectivity and the lateral expansion of PFC with distinct granular layer 4 in anthropoid primates16-23 have not been elucidated. Here we report increased concentration of retinoic acid (RA), a signaling molecule involved in brain development and function24, 25 in the prospective PFC areas of human and macaque, compared to mouse, during mid-fetal development, a crucial period for cortical circuit assembly. In addition, we observed the lateral expansion of RA synthesizing enzyme, ALDH1A3, expression in mid-fetal macaque and human frontal cortex, compared to mouse. Furthermore, we found that enrichment of RA signaling is restricted to the prospective PFC by CYP26B1, a gene encoding an RA-catabolizing enzyme upregulated in the mid-fetal motor cortex. Gene deletion in mice revealed that RA signaling through anteriorly upregulated RA receptors, Rxrg and Rarb, and Cyp26b1-dependent catabolism is required for the proper molecular patterning of PFC and motor areas, the expression of the layer 4 marker RORB, intra-PFC synaptogenesis, and the development of reciprocal PFC-MD connectivity. Together, these findings reveal a critical role for RA signaling in PFC development and, potentially, its evolutionary expansion.