Regionalized gene expression during human neocorticogenesis

Abstract

The mammalian neocortical areas arise from graded/regionalized gene expression from the earliest stages of corticogenesis. However, little is known about the establishment of the human motor cortex, despite being a common site of perinatal damage causing cerebral palsy. Affymetrix gene chip analysis of human neocortical tissue from 8-12.5 post-conceptional weeks (PCW) previously discovered genes expressed in gradients along the anterior-posterior human neocortex. Aims: i) Identify putative anterior/posterior molecular determinants of the developing human neocortex. ii) Test the hypothesis that the anterior pole of the human neocortex as an early site of the developing motor cortex generating corticofugal neurones and understand their development. iii) Establish a human in-vitro regionalisation model and study potential regulatory mechanisms of some corticofugal neurone-related genes. Results and Conclusion: Graded expression of some putative anterior/posterior markers was confirmed in the human neocortex by real-time PCR (8-12 PCW). Among them, the corticofugal neurone-related genes (transcription factor CTIP2, axon guidance molecule ROBO1 and its downstream signalling molecule SRGAP1) were up-regulated anteriorly. Their spatiotemporal expression patterns were examined further by tissue in-situ hybridization and immunohistochemistry, and compared with other corticofugal neurone-related transcription factors that showed no/opposite expression gradients (FEZF2 and SOX5) and various laminar-enriched markers previously established in humans (ER81, TBR1, TBR2, GAP43 and Synaptophysin) or validated in the current project (SATB2 and NURR1). Layer V was shown to arise as early as 12 PCW in humans. Prominent medullary pyramids containing corticospinal fibres strongly expressed both ROBO1/SRGAP1 by 14-17 PCW, when a distinct ROBO1/SRGAP1-positive Layer V emerges. Co-expression of the three genes in anterior neocortex might mark the site of the emerging human motor cortex, which is the predominant origin of corticofugal neurones such as corticospinal/corticopontine projection neurones. Dissociated neocortical cell cultures were established from anteriorly-/posteriorly-derived foetal neocortex that maintained regional intrinsic molecular identities. This in-vitro model of regionalization allowed gene regulation study to be performed in which a preliminary investigation of fibroblast growth factor signalling in controlling the expression of corticofugal neurone-related genes was initiated.