Progressive loss of PAX6, TBR2, NEUROD and TBR1 mRNA gradients correlates with translocation of EMX2 to the cortical plate during human cortical development

Abstract

The transcription factors Emx2 and Pax6 are expressed in the proliferating zones of the developing rodent neocortex, and gradients of expression interact in specifying caudal and rostral identities. Pax6 is also involved in corticoneurogenesis, being expressed by radial glial progenitors that give rise to cells that also sequentially express Tbr2, NeuroD and Tbr1, genes temporally downstream of Pax6. In this study, using in situ hybridization, we analysed the expression of EMX2, PAX6, TBR2, NEUROD and TBR1 mRNA in the developing human cortex between 8 and 12 postconceptional weeks (PCW). EMX2 mRNA was expressed in the ventricular (VZ) and subventricular zones (SVZ), but also in the cortical plate, unlike in the rodent. However, gradients of expression were similar to that of the rodent at all ages studied. PAX6 mRNA expression was limited to the VZ and SVZ. At 8 PCW, PAX6 was highly expressed rostrally but less so caudally, as has been seen in the rodent, however this gradient disappeared early in corticogenesis, by 9 PCW. There was less restricted compartment-specific expression of TBR2, NEUROD and TBR1 mRNA than in the rodent, where the gradients of expression were similar to that of PAX6 prior to 9 PCW. The gradient disappeared for TBR2 by 10 PCW, and for NEUROD and TBR1 by 12 PCW. These data support recent reports that EMX2 but not PAX6 is more directly involved in arealization, highlighting that analysis of human development allows better spatio-temporal resolution than studies in rodents.

Investigating gradients of gene expression involved in early human cortical development

“…These findings indicate that at 8 PCW (CS23), and probably before, different development mechanisms may be occurring, resulting in the differing gene expression profiles. This observation is supported by previous studies in human that demonstrate that a developmental switch may occur between 8 and 9 PCW (CS23 and F1) resulting in the deregulation of the PAX6 (high-anterior ⁄ lateral, low-posterior ⁄ medial) gradient, which is maintained in rodents throughout corticogenesis (Manuel & Price, 2005;Bayatti et al 2008b). Recently, evidence has accumulated that the Pax6 gradient in rodents is not directly involved in regionalization processes (Manuel et al 2007;Pinon et al 2008), as over-expression of Pax6 in ).…”

“…Short-term dissociated human foetal cell cultures were initiated according to an established rodent protocol with minor Compiled by comparing studies reported in O' Leary et al (2007) and Bayatti et al (2008b). CP, cortical plate; SVZ, subventricular zone; VZ, ventricular zone.…”

“…Quality control was assessed using the AFFY package for BIOCON-DUCTOR (http://www.bioconductor.org; RNA degradation plots and PM intensity charts) and GENESPRING GX 7.3 (box plots; Agilent Technologies, South Queensferry, UK) software, and GENESPRING GX 7.3 was used for expression level analysis. Expression data for each probe was normalized by GC-RMA, and gene expression levels are presented as a fold change with a threshold of 1.75-fold (this threshold was chosen, as EMX2 was measured at 1.77-fold and had previously been confirmed to exhibit a gradient on the anterior-posterior axis by in situ hybridization in tissue sections (see Bayatti et al 2008b)). Gene ontologies were analysed using the GO Ontology Browser within the GENE-SPRING GX analysis platform (Agilent Technologies).…”

Unravelling the development of the visual cortex: implications for plasticity and repair

“…For example, Pax6 null mice have a larger domain associated with vision and a smaller motor cortex. In the human neocortex, the gradient expression of Pax6 and Emx2 along the rostrocaudal neuraxis has been studied in embryonic and fetal development (Bayatti et al 2008b, Ip et al 2010, with spatiotemporal changes being observed. Interestingly, Pax6 was found to be expressed in cells in the outer subventricular zone of the human and non-human primate neocortex (Bayatti et al 2008b;Fish et al 2008;Mo & Zecevic, 2008), unlike the rodent where it is expressed by radial glial cells in the ventricular zone (Costa et al 2008).…”

“…In the human neocortex, the gradient expression of Pax6 and Emx2 along the rostrocaudal neuraxis has been studied in embryonic and fetal development (Bayatti et al 2008b, Ip et al 2010, with spatiotemporal changes being observed. Interestingly, Pax6 was found to be expressed in cells in the outer subventricular zone of the human and non-human primate neocortex (Bayatti et al 2008b;Fish et al 2008;Mo & Zecevic, 2008), unlike the rodent where it is expressed by radial glial cells in the ventricular zone (Costa et al 2008). Therefore, although these homeobox transcription factors may not be directly responsible for the arealization of the visual cortical areas or specific neocortical areas, in general it is likely that they have played an important role in the expansion of the neocortical sheet, allowing for additional neocortical fields to evolve.…”