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Humans and other great apes stand out among mammals for the big size and advanced folding of their brains' cerebral cortex. This cortex expansion during evolution has been linked to the acquisition of higher cognitive abilities and results from gene regulatory changes during fetal development. New research in The EMBO Journal implicates the growth factor EPIREGULIN and the epigenetic regulation of its expression in the evolutionary expansion of the cerebral cortex. A new study sheds light on the mechanisms of cerebral cortex expansion during evolution. [ABSTRACT FROM AUTHOR]

Structural integrity and cellular homeostasis of the embryonic stem cell niche are critical for normal tissue development. In the telencephalic neuroepithelium, this is controlled in part by cell adhesion molecules and regulators of progenitor cell lineage, but the specific orchestration of these processes remains unknown. Here, we studied the role of microRNAs in the embryonic telencephalon as key regulators of gene expression. By using the early recombiner Rx‐Cre mouse, we identify novel and critical roles of miRNAs in early brain development, demonstrating they are essential to preserve the cellular homeostasis and structural integrity of the telencephalic neuroepithelium. We show that Rx‐Cre;DicerF/F mouse embryos have a severe disruption of the telencephalic apical junction belt, followed by invagination of the ventricular surface and formation of hyperproliferative rosettes. Transcriptome analyses and functional experiments in vivo show that these defects result from upregulation of Irs2 upon loss of let‐7 miRNAs in an apoptosis‐independent manner. Our results reveal an unprecedented relevance of miRNAs in early forebrain development, with potential mechanistic implications in pediatric brain cancer. Synopsis: Dicer1‐dependent let‐7 microRNAs are critical to repress p53 pathway activity and Irs2 expression levels in the early telencephalic neuroepithelium, which otherwise enters into hyperproliferation and loss of adhesion of progenitor cells, leading to the formation of true rosettes reminiscent of pediatric cancers. Loss of miRNAs in the early embryonic telencephalon of Rx‐Cre;Dicer1flox mutant mice produces massive transient apoptosis of progenitor cells, loss of adherens junctions and increased proliferation, leading to the formation of true hyperproliferative rosettes.Rx‐Dicer mutants have a significant loss of let‐7 family microRNAs, and increased p53 signaling and Irs2 expression. Loss of p53 in Rx‐Dicer mutants rescues their phenotype in a partially‐penetrant manner.Increased Irs2 expression or blockade of let‐7 activity in early wild‐type embryos, induce rosette formation in the rostral telencephalon, rescued by overexpression of let‐7 and downregulation of Irs‐2, respectively.Let‐7 miRNAs downregulate the amplification of human cortical progenitor cells by repressing Irs2 expression, consistent with a role of this signaling axis in pediatric cancers with true rosettes. [ABSTRACT FROM AUTHOR]

One of the most prominent features of the human brain is the fabulous size of the cerebral cortex and its intricate folding. Cortical folding takes place during embryonic development and is important to optimize the functional organization and wiring of the brain, as well as to allow fitting a large cortex in a limited cranial volume. Pathological alterations in size or folding of the human cortex lead to severe intellectual disability and intractable epilepsy. Hence, cortical expansion and folding are viewed as key processes in mammalian brain development and evolution, ultimately leading to increased intellectual performance and, eventually, to the emergence of human cognition. Here, we provide an overview and discuss some of the most significant advances in our understanding of cortical expansion and folding over the last decades. These include discoveries in multiple and diverse disciplines, from cellular and molecular mechanisms regulating cortical development and neurogenesis, genetic mechanisms defining the patterns of cortical folds, the biomechanics of cortical growth and buckling, lessons from human disease, and how genetic evolution steered cortical size and folding during mammalian evolution . [ABSTRACT FROM AUTHOR]

Gyrencephalic species develop folds in the cerebral cortex in a stereotypic manner, but the genetic mechanisms underlying this patterning process are unknown. We present a large-scale transcriptomic analysis of individual germinal layers in the developing cortex of the gyrencephalic ferret, comparing between regions prospective of fold and fissure. We find unique transcriptional signatures in each germinal compartment, where thousands of genes are differentially expressed between regions, including ~80% of genes mutated in human cortical malformations. These regional differences emerge from the existence of discrete domains of gene expression, which occur at multiple locations across the developing cortex of ferret and human, but not the lissencephalic mouse. Complex expression patterns emerge late during development and map the eventual location of folds or fissures. Protomaps of gene expression within germinal layers may contribute to define cortical folds or functional areas, but our findings demonstrate that they distinguish the development of gyrencephalic cortices. [ABSTRACT FROM AUTHOR]

Size and folding of the cerebral cortex increased massively during mammalian evolution leading to the current diversity of brain morphologies. Various subtypes of neural stem and progenitor cells have been proposed to contribute differently in regulating thickness or folding of the cerebral cortex during development, but their specific roles have not been demonstrated. We report that the controlled expansion of unipotent basal progenitors in mouse embryos led to megalencephaly, with increased surface area of the cerebral cortex, but not to cortical folding. In contrast, expansion of multipotent basal progenitors in the naturally gyrencephalic ferret was sufficient to drive the formation of additional folds and fissures. In both models, changes occurred while preserving a structurally normal, six-layered cortex. Our results are the first experimental demonstration of specific and distinct roles for basal progenitor subtypes in regulating cerebral cortex size and folding during development underlying the superior intellectual capability acquired by higher mammals during evolution. [ABSTRACT FROM AUTHOR]