|dc.description.abstract||Fibroblast growth factors (FGFs) play an essential role in development and patterning of the vertebrate embryo. Despite extensive literature documenting the diverse roles of FGF signalling during craniofacial development, comparatively little is known about the specific downstream effectors through which FGFs influence gene expression. A previous study in our laboratory reported exogenous FGF elicited differential chondrogenic responses in frontonasal and mandibular mesenchyme (Bobick et al., 2007). Pea3 transcription factors are crucial components of the downstream effector pathway through which FGFs influence gene expression (Raible and Brand, 2001). Therefore, the purpose of my research was to examine whether differences in pea3, erm, and er81 gene expression profiles underlie the distinct responses of the frontonasal and mandibular mesenchyme cells to FGF.
The present study demonstrates that FGF2 treatment differentially affects chondrogenesis in micromass cultures of frontonasal and mandibular mesenchyme isolated from stage 24/25 chick embryos. Whereas FGF2 inhibited chondrogenesis in frontonasal mesenchyme cultures, it had no effect on micromass cultures of mandibular mesenchyme. RT-qPCR and RNA dot blot analyses demonstrated that mRNA transcripts for the pea3, erm, and er81 genes are expressed by mesenchyme cells of both frontonasal and mandibular processes of stage 24/25 and stage 28/29 chick embryos. In addition, western blot data demonstrated expression of the Pea3 and Er81 proteins in micromass and explant cultures of stage 24/25 frontonasal and mandibular mesenchyme.
The expression profiles of Pea3 genes were similar between the frontonasal and mandibular facial primordia prior to treatment with exogenous FGF2. However, these expression profiles were differentially altered in response to FGF2 exposure in both explant and micromass cultures. Specifically, whereas FGF2 treatment upregulated pea3 mRNA levels in explants of frontonasal mesenchyme, it had no effect on pea3 expression in mandibular explants. In micromass cultures, exogenous FGF2 elevated levels of pea3 transcripts in both frontonasal and mandibular mesenchyme. However, FGF2 treatment elevated er81 expression in frontonasal, but not mandibular mesenchyme. Conversely, exogenous FGF2 elevated erm mRNA levels in mandibular, but not frontonasal mesenchyme.
Micromass cultures of mandibular mesenchyme from stage 28/29 chick embryos exhibited significantly lower levels of pea3 expression than cultures of stage 24/25 mandibular mesenchyme. This stage-dependent change correlated with a reduction in the ability of the mandibular cells to undergo spontaneous chondrogenic differentiation in micromass culture. In contrast, no stage-dependent changes in pea3 expression were observed in frontonasal mesenchyme cultures.
Collectively, my data indicate that the expression profiles of pea3, erm, and er81 in frontonasal and mandibular mesenchyme become distinct only after exposure to exogenous FGF. This raises the possibility that the differences in Pea3 transcription factor expression patterns that arise in response to FGF stimuli may subsequently lead to distinct chondrogenic responses in the two facial mesenchyme populations.||en_US