Written by Dominic Javonillo
Picture a human brain. Most individuals would agree on its basic characteristics: a pink, jelly-like glob of putty with wrinkles and folds. Interestingly, these wrinkles and folds actually play an influential role in the cognitive, sensory, and motor abilities of a particular mammalian species. Referred to as cortical folds, these wrinkles form along the brain’s outer shell and make up what is known as the cortex. These folds allow for tight compacting of the brain’s large surface area within the skull’s small volume [1]. Conceptually, one can imagine a glass jar of a certain volume with a small napkin inside. With a small surface area, the napkin can barely cover all sides of the jar while maintaining the jar volume. However, when a larger table cloth replaces the small napkin, its larger surface area must crumple and fold to fit inside the same jar volume as before. Similarly, our cortical folds allow us to fit more brain tissue into our skull for higher order abilities such as cognition.
Unfortunately, the mechanisms underlying cortical folding are largely unknown. To investigate these questions, researchers often look into dysfunctional conditions that may impair folding. For example, researchers can take out or knockout a gene to find any absent features in cortical folding in order to determine whether the knocked out gene is required. However, these explorations have been difficult due to improper animal models for examining cortical folding and insufficient technology to manipulate genetics quickly and efficiently [2]. Fortunately, Shinmyo et al. published a recent study in Cell Reports that appears to overcome these obstacles in identifying a suitable animal model and a necessary gene required in cortical folding.
How did they tackle these issues? What were their findings?
One of the problems facing this study was an insufficient animal model for research. This is because not all mammals exhibit cortical folding. Common animal models for research, such as mice and rats, do not have folds around their brain. Such differences have made it difficult to find cellular mechanisms that actually contribute to the cortical folding. However, Shinmyo et al. utilized ferrets to model developmental folding mechanisms due to having their own cortical folds.
Another issue relied on the development of gene editing technologies that make gene knockout possible. In previous studies, Shinmyo et al. developed a method to effectively manipulate a specific target gene in ferrets [2]. By sending electric shocks into brain cells during development, the research group was able to make the genetic sequence accessible for manipulation inside the cell [2]. To find a possible genetic sequence of interest, the group looked into a brain malformation called classical lissencephaly in which cortical folding is absent. Any genetic problems attributed to this malformation may shed light on the genes required to fold the cortex. Fortunately, previous studies linked classical lissencephaly to a mutation in the molecule cyclin-dependent kinase 5 (CDK5), which suggests a defect in the Cdk5 gene that produces the CDK5 molecule [3]. With Cdk5 as the gene of interest, Shinmyo et al. aimed to manipulate this gene to find any effects in cortical folding.
To sufficiently knockout this gene from the ferret genome, Shinmyo et al. combined their genetic manipulation technique with a recent development in gene editing technology: CRISPR/Cas9 [2]. In this study, CRISPR/Cas9 worked by targeting the DNA sequence of the Cdk5 gene and alerting the Cas9 molecule of which area to cut [4]. So, the group was able to effectively delete and knockout the genetic material to investigate effects in cortical folding without Cdk5. The knockout resulted in a significantly smaller size of cortical folds compared to normal brains with Cdk5. This finding indicates that the Cdk5 molecule in cortical brain cells is required for proper cortical folding [2]. Not only did they confirm the necessity of Cdk5, Shinmyo et al. also found that proper development of the upper cortical layer with the Cdk5 gene was more important for cortical folding than development of the lower cortical layer [2].
Ultimately, the research group introduces an effective animal model to investigate cortical folding through knockout of the Cdk5 gene in upper cortical layers. Combining their own technique to access the genome of these cells, the study also contributes to the growing application of CRISPR/Cas9 in genetic research. Together with a novel animal model and innovative genetic technology, Shinmyo et al. anticipate further studies that uncover specific mechanisms in determining cortical folding and how these molecular pathways can go awry.
References:
1. Sun T., Hevner R.F. 2014. Growth and folding of the mammalian cerebral cortex: from molecules to malformations. Nature Reviews Neuroscience. 15:217-32.
2. Shinmyo Y., et al. 2017. Folding of the Cerebral Cortex Requires Cdk5 in Upper Layer Neurons in Gyrencephalic Mammals. Cell Reports. 20:2131-43.
3. Magen D., et al. 2015. Autosomal recessive lissencephaly with cerebellar hypoplasia is associated with a loss-of-function mutation in CDK5. Human Genetics. 134:305-14.
4. What are genome editing and CRISPR-Cas9?. (2017, November 14). Genetics Home Reference. Retrieved from https://ghr.nlm.nih.gov/primer/genomicresearch/genomeediting