Changes to just three genetic letters among billions contributed to the evolution and development of the mammalian motor sensory circuits and laid the groundwork for the defining characteristics of the human brain, Yale University researchers report.
Yale researchers report that a tiny change in the genome helped shape an essential neural pathway unique to mammals. Published in the May 31 issue of the journal Nature, the study identifies a small regulatory DNA element that plays a key role in the formation of the corticospinal system—the neural circuitry that links the cerebral cortex, the conscious and planning center of the brain, with the brainstem and spinal cord. This direct cortical projection enables fine, skilled movements that underlie human abilities such as tool use, complex hand movements and aspects of speech.
The corticospinal circuit is a mammal-specific evolutionary advance, yet until now the genetic mechanisms guiding its emergence were not well understood. The Yale team shows that evolution of this circuitry can hinge on minute changes in non-coding DNA—regions that do not encode proteins but control when and where protein-coding genes are expressed during development.
Most mammalian genomes contain roughly 22,000 protein-coding genes, but the timing and location of those genes’ activity are largely controlled by surrounding non-coding sequences known as cis-regulatory elements. These elements act as switches that help orchestrate developmental programs shaping cell identity, connectivity and function across tissues and organs. Small mutations within these regulatory sequences can therefore have outsized effects on anatomy and behavior by altering gene expression patterns during critical developmental windows.
In this work, Sungbo Shim, the study’s first author, together with colleagues in Nenad Šestan’s laboratory, identified a specific cis-regulatory region they named E4. E4 governs the dynamic activity of the gene Fezf2, a gene previously implicated in guiding cortical neurons to form long-range projections. By modulating Fezf2 expression, E4 helps direct the differentiation and axonal targeting of cortical neurons that give rise to the corticospinal projection.
Crucially, E4 is conserved across mammalian species but differs subtly from related sequences in non-mammalian craniates, a pattern consistent with a role in the emergence and refinement of mammalian brain features. The study shows that tiny sequence differences within E4 alter how that regulatory element is read by specific transcription factors—proteins that bind DNA and control gene expression. In particular, members of the SOX family of transcription factors, including SOX4, SOX11 and SOX5, cooperate to activate or repress E4 at distinct times during embryonic development, shaping the window of Fezf2 expression and thereby sculpting corticospinal circuit formation.
The findings highlight how small genetic changes in non-coding regulatory DNA can rewire developmental programs and contribute to the evolution of lineage-specific neural circuits. By connecting a discrete regulatory element to a well-characterized developmental gene and to a major mammalian neural pathway, the study provides a clearer molecular link between genotype and the evolution of complex neural traits.
Beyond evolutionary significance, these insights improve understanding of the genetic control of neural circuit assembly. Better knowledge of cis-regulatory mechanisms and transcriptional networks that build motor-sensory connectivity may inform future research into developmental disorders that disrupt cortical projection neuron identity or connectivity, and could provide new entry points for investigating how conserved genes produce species-specific neural architectures.
Notes about this neuroscience research article
Other Yale-affiliated authors on the paper include Kenneth Y. Kwan and Mingfeng Li. Primary funding for the research came from the National Institutes of Health and March of Dimes.
Written by Bill Hathaway. Contact: Bill Hathaway, Yale University.
Original research: “Cis-regulatory control of corticospinal system development and evolution” by Sungbo Shim, Kenneth Y. Kwan, Mingfeng Li, Veronique Lefebvre & Nenad Šestan, published in Nature, May 2012 (doi:10.1038/nature11094).