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Regulation of cerebral cortex size and folding by expansion of basal progenitors.

ABSTRACT: 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.

SUBMITTER: Nonaka-Kinoshita M

PROVIDER: S-EPMC3926188 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Our brains accommodate a largely folded cerebral cortex that associates to our advanced brain functions. Several theories have been proposed to explain the cortical folding process, reasoning the mechanical forces as neuronal tension in underlying layers, cellular expansion and glial progenitor’s diversity in the OSVZ; but the mechanistic insights and underlying genomics changes causing the appearance of cortical folds is still illusive. Importantly, no studies have attempted to comprehensively characterize the gene regulatory networks underlying cortical folding during development. Here, by using ferret as a model system, we have compared the transcriptomes of germinal layers between sulci and gyri, of different cortical areas and across two developmental stages (E30 & E34), to achieve a comprehensive understanding of the spatio-temporal dynamics of gene expression of cortical progenitors. Furthermore, we have also characterized the epigenetic landscape of germinal layers at E30 and E34, a critical period for their development, to correlate changes in chromatin at promoter and enhancer regions with the observed changes in gene expression. Our preliminary results indicate towards a clear transformational axis of gene regulation between germinal layers. By performing motif analysis of differentially expressed gene (DEGs), we reveal transcription factors which might have a critical role in determining cortical folding. The genes targeted by these factors belong to important pathways implicated in proliferation and neurogenesis. We highlight potential candidates in cortical folding through functional validation studies and acetylation (H3K27ac) levels for these genes correlate with their expression state. Importantly, these are critical for cell adhesion, migration and proliferation processes in-line with previous studies stating that proliferative divisions cause neocortical expansions. Our findings will have strong impact on the clinical interventions for neurological disorders relating to cortical malformation, while at the same time enhancing our understanding of molecular circuitry underlying gyrencephalic brain development and folding.

Project description:Altered macroscopic anatomical characteristics of the cerebral cortex have been identified in individuals affected by various neurodevelopmental disorders. However, the cellular developmental mechanisms that give rise to these abnormalities are not understood. Previously, advances in image reconstruction of diffusion magnetic resonance imaging (MRI) have made possible high-resolution in utero measurements of water diffusion anisotropy in the fetal brain. Here, diffusion anisotropy within the developing fetal cerebral cortex is longitudinally characterized in the rhesus macaque, focusing on gestation day (G85) through G135 of the 165 d term. Additionally, for subsets of animals characterized at G90 and G135, immunohistochemical staining was performed, and 3D structure tensor analyses were used to identify the cellular processes that most closely parallel changes in water diffusion anisotropy with cerebral cortical maturation. Strong correlations were found between maturation of dendritic arbors on the cellular level and the loss of diffusion anisotropy with cortical development. In turn, diffusion anisotropy changes were strongly associated both regionally and temporally with cortical folding. Notably, the regional and temporal dependence of diffusion anisotropy and folding were distinct from the patterns observed for cerebral cortical surface area expansion. These findings strengthen the link proposed in previous studies between cellular-level changes in dendrite morphology and noninvasive diffusion MRI measurements of the developing cerebral cortex and support the possibility that, in gyroencephalic species, structural differentiation within the cortex is coupled to the formation of gyri and sulci.SIGNIFICANCE STATEMENT Abnormal brain morphology has been found in populations with neurodevelopmental disorders. However, the mechanisms linking cellular level and macroscopic maturation are poorly understood, even in normal brains. This study contributes new understanding to this subject using serial in utero MRI measurements of rhesus macaque fetuses, from which macroscopic and cellular information can be derived. We found that morphological differentiation of dendrites was strongly associated both regionally and temporally with folding of the cerebral cortex. Interestingly, parallel associations were not observed with cortical surface area expansion. These findings support the possibility that perturbed morphological differentiation of cells within the cortex may underlie abnormal macroscopic characteristics of individuals affected by neurodevelopmental disorders.

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Regulation of cerebral cortex size and folding by expansion of basal progenitors.

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