Background

In the evolutionary history of mammals, the dramatic expansion and folding of the cerebral cortex is considered a key event that led to the development of high-level cognitive functions. While rodents like mice have smooth brain surfaces, primates, including humans, have adopted a method of precisely folding the cortex to arrange vast numbers of neurons within the limited space of the skull. Neurobiologists have long hypothesized that this morphological leap stems from differences in the proliferation cycles and division patterns of neural stem cells, but the specific genetic changes that triggered cortical expansion have remained a long-standing mystery.

Previous studies relied primarily on direct comparisons between laboratory mice and the human brain. The evolutionary gap of 90 million years between the two species resulted in thousands of differences in gene expression, imposing structural limitations on the task of isolating only the factors that directly caused cerebral folding. There was a need for a precise comparative analysis system based on model organisms positioned between rodents and primates. The tree shrew, a mammal with a smooth brain and the closest outgroup to primates, has emerged as a key clue for filtering out accumulated noise between the two lineages and capturing the evolutionary transition process.

Key Findings

Researchers established a multi-species comparative genomic analysis of tree shrews, mice, macaques, and humans to track genes directly linked to cortical expansion. Cross-validation of the transcriptomes of neuroepithelial cells and radial glia during corticogenesis across lineages revealed a specific surge in the expression levels of the Cyclin B1 Interacting Protein 1 (CCNB1IP1) gene in the primate lineage. Known as a cell cycle regulator, CCNB1IP1 is a protein that controls the stability and division speed of the cell division machinery.

To identify the cause of this evolutionary change in expression regulation, the researchers analyzed the base sequences of non-coding regulatory regions. They observed that base substitutions within cis-regulatory elements, specifically acquired in the primate lineage, increased transcription factor binding affinity, thereby promoting the transcription of CCNB1IP1 during neurogenesis. Functional verification using cerebral organoids and transgenic models yielded clear results. Expression of primate-specific regulatory sequences and CCNB1IP1 in non-primate cells shortened the cell cycle of neural progenitor cells, causing radial glia to expand outward and inducing cortical folding structures resembling wrinkles. Conversely, inhibiting the expression of this gene in primate organoids resulted in a significant slowdown of cortical surface expansion.

Significance and Outlook

This achievement clearly demonstrates that the massive morphological transformation of cerebral cortical expansion was triggered not by a wholesale reorganization of many genes, but by mutations in the cis-regulatory sequences of key genes governing the cell cycle. This is evaluated as having specified the molecular basis underlying the formation of the complex brain structure in primates from the perspective of a single regulatory axis. Furthermore, it expanded the methodological horizons of evolutionary developmental biology by demonstrating the validity of a research strategy that uses the intermediate taxon, the treeshrew, as a control.

From a clinical perspective, it provides a clue to reinterpret the pathogenic mechanisms of congenital neurodevelopmental disorders such as microcephaly and lissencephaly. It provides a basis for investigating abnormalities in the CCNB1IP1 regulatory network in patient cohorts with cortical malformations resulting from failed neural progenitor cell proliferation. However, limitations remain, as laboratory-grown organoid models cannot perfectly replicate the entire long-term process of neural migration and synapse formation occurring in the actual fetal brain. Follow-up research is required to elucidate the interplay between mechanical forces involved in cortical folding and microenvironmental factors such as angiogenesis.