In other studies, Li et al. (2012) showed that the relationship between gliogenesis and MEK function is symmetric and cell autonomous. Symmetry was demonstrated by using in utero electroporation to transduce a constitutively active mutant of Mek1 (caMek1) into WT radial progenitors. The caMek1 studies showed that MEK loss of function impairs gliogenesis, while MEK gain of function promotes gliogenesis. Cell autonomy was demonstrated by mosaic loss of function in slice cultures. Here EGFP-marked Cre plasmids were electroporated into E15.5 radial progenitors followed by organotypic cortical

slice cultures for 4 days. The Cre-transduced progenitors were markedly less likely to become astrocytes than vector CHIR99021 controls. So how do MEKs regulate the neuron/glia switch? As indicated in Figure 1, the purpose of the RAF/MEK/ERK signaling pathway is to regulate gene expression. Microarray data sets for E18.5 cortices from WT and the NestinCre knockout

mice were interrogated for Mek-responsive transcription factors. A strong candidate emerged in the form of the Ets transcription factor family member Erm (aka ETV5). In situ hybridization studies show intense Erm expression in the WT ventricular zone at E14.5–E18.5. In GDC0449 Mek null brains, expression of Erm is profoundly reduced in the ventricular zone. These correlative observations were followed by rescue experiments in NestinCre-ablated Mek null brains. Because these mice die at early postnatal stages, Erm expression vectors were electroporated ex vivo. The cortices were then dissociated and challenged with the astrogenic growth factor CNTF. The data showed that expression PAK6 of Erm rescues CNTF-induced formation of astrocytes in Mek null mutant cultures. Conversely, a dominant-negative Erm mutation blocks formation of astrocytes in response to the constitutively active caMek1. The observations of Li et al. (2012) resonate within an emerging body of data on neurofibromatosis type 1 (NF1) and sporadic

low-grade astrocytomas in children. The NF1 gene encodes neurofibromin, a RAS GTPase that converts the GTP-bound active form of RAS proteins to the inactive, guanosine diphosphate (GDP)-bound form ( Scheffzek et al., 1997). As indicated in Figure 1, loss-of-function NF1 leads to hyperactivation of the RAF/MEK/ERK pathway and is associated with neurological diseases—notably low-grade astrocytomas. Studies by Gutmann and his colleagues demonstrate that NF1 inactivation promotes astroglial differentiation ( Dasgupta and Gutmann, 2005). Moreover, deleting floxed Nf1 in neural progenitors during early embryonic stages leads to a dramatic increase in the glia cell population in the brain ( Hegedus et al., 2007)—a phenotype quite similar to that of the caMek1/hGFAP mice described by Li et al. (2012).