Functional analysis of transcription factor mScrt2 in cortical neurogenesis.

The neocortex, a specific for the mammals layered structure that cover the forebrain, is characterized by an enormous cellular diversity as a result of several coordinated developmental processes that regulate the spatio-temporal generation of multiple neuronal and glial cell types. The origin of most of the cell types during corticogenesis is the pluripotent radial glia progenitors (RGPs) in the ventricular zone. RGPs are also the source of a second type of neurogenic progenitors, called intermediate progenitors (IPs). IPs have a role in the amplification of distinct neuronal fates, generated at particular developmental stages, predominantly influencing the generation and expansion of the upper cortical layer neurons, an evolutionary hallmark of higher species. In order to produce specific cell types in a appropriate number, the cell divisions of cortical progenitors must be precisely controlled to balance progenitor maintenance (proliferation) and neural differentiation. Here, the first functional in-vivo study of the mammalian Scratch2 (mScrt2), a newly identified member of the C2H2 zinc-finger transcriptional factors, is presented. The obtained findings could be summarized in the following: 1. mScrt2 expression is CNS-restricted and specifically found at transition zones that contain proliferating and post-mitotic cells, confined to embryonic and postnatal regions, indicating a role in the promotion of neuronal differentiation. Additionally, evidence is presented for the presence of functionally different mScrt2-transcripts. 2. Conditional over expression of mScrt2.2 in the developing cortex of generated transgenic mice revealed an increase in neuronal differentiation at early stages of neurogenesis and a severe reduction of cortical thickness at postnatal stages, mainly affecting the upper cortical layers. The mechanism behind could be partially determined by quantitative analysis of the mode of cortical progenitor divisions, that trend to undergo neurogenic divisions at the expense of IP generation and amplification, thus causing an early depletion of cortical progenitors and subsequent decrease in the number of cortical neurons. 3. The results suggests a role for mScrt2 in the regulation of radial migration of neurons across the cortex by promoting the multipolar migration type, a mechanism that is assumed as a kind of arrest for a subpopulation of late-born neurons at the border subventricular/intermediate zone. 4. Ectopic mScrt2 expression in Xenopus leavis embryos indicated that primary neurogenesis was influenced in a spatio-temporal specific manner, suggesting a conserved between mouse and frog function in the CNS-development. In conclusion, by promoting the neurogenic mode of RGP-divisions and IPs, the mammalian mScratch2 appears to modulate neuronal differentiation and migration in the developing cortex, with important consequences for the neocortical morphogenesis of the adult brain. The results allow suggesting, that mScrt2 is probably interfering with pro-neural bHLH transcription factors, and is acting in an evolutionary conserved Pax6-dependent genetic pathway, regulating the cortical neurogenesis.