[P179]: Met signalling modulates cortical pyramidal cell dendritic development in vivo

During embryonic brain development, fibroblast growth factors (FGFs) exert pleiotropic effects via their receptors (FGFRs). We have previously shown that mice carrying the kinase domain K644E mutation in Fgfr3 (EIIa; Fgfr3, “mutant”) have enlarged brains caused by an increase in cell proliferation and a decrease in apoptosis (Inglis-Broadgate et al., 2005). The mechanisms by which these changes are elicited remain unidentified. In this study, we investigated the temporal and spatial regulatory mechanisms of progenitor proliferation in the Fgfr3 mutant mouse cortex. Concomitant with our previous finding, in vivo BrdU labelling study showed that cell proliferation was higher in the Fgfr3 mutant cortex compared with wild type at each developmental stage studied during E11.5–E13.5 (10–46%). In accordance to the expression of Fgfr3 in a rostral-low caudalhigh gradient, the difference in proliferation between the Fgfr3 mutant and wild type cortices was the greatest in the caudal cortex (41% and 46% increase, at E12.5 and E13.5, respectively). In addition, in vitro BrdU assay showed that inhibition of MEK by U0126 reduced the proliferation rate of the mutant cortical progenitors to the level observed in the wild type at E11.5, indicating that increase in cell proliferation caused by Fgfr3 is MAPK pathway-dependent. Western blotting confirmed that activation of pErk1/2 was greater in the Fgfr3 mutant forebrains compared to wild type at E11.5. Interestingly, up-regulation of MAPK inhibitor Sprouty was observed in the mutant forebrains at E14.5, while its expression was similar at E11.5. We suggest that the negative feedback by Sprouty may lead to a decrease in MAPK signalling in the Fgfr3 mutant forebrains at E14.5, attenuating the further effect of the mutation at the later stage. We suggest that temporal activation of MAPK and Sprouty regulate cell proliferation caused by the Fgfr3 mutation during development.