Pilot Project: Fibulin-1 maintenance of the Fgf8-dependent proximal mandibular genetic program
The overall goal of this project is to determine the role of fibulin-1 in regulating Fgf8 signaling in the proximal region of branchial arch 1 during critical periods of mandibular development.
The extracellular matrix protein, fibulin-1 (Fbln1), has been shown to be critical for the development of craniofacial structures. The spectrum of malformations observed in Fbln1 null mice are consistent with Fbln1 having a role in the growth and differentiation of cranial neural crest cells (CNCs) which contribute to the morphogenesis of structure derived from the first branchial arch (BA1) e.g., the mandible. Pathways key to the growth and differentiation of CNCs have not been fully established, however a number of genes including fibroblast growth factor 8 (Fgf8) are known to be critical. The fact that mice deficient in Fbln1 share some overlapping mandibular malformations with mice deficient in Fgf8, together with evidence that Fbln1 binds Fgf8 has lead us to hypothesize that Fbln1 may be involved in promoting Fgf8 signaling in BA1. A key aspect of Fgf8 signaling in BA1 is the proximal activation of Fgf8 response genes. One such gene is the homeodomain transcription factor, Barx1 which is critical for mandibular formation and activates Fbln1 expression. We have found that Fbln1-deficient mice show reduced Barx1 expression. Furthermore, we have identified two potential Barx1 binding sites in the Fbln1 gene. In light of this information, we propose a novel regulatory model in which Fbln1 promotes Fgf8-FgfR signaling in CNCs that leads to the expression of Barx1 and the transcriptional activation of the Fbln1 gene, thus creating a positive feedback loop. We speculate that this positive feedback system promotes the proliferation and differentiation of CNCs within BA1. To test this model there are three specific aims: 1) to determine whether Fbln1 deficiency leads to suppression of Fgf8 signaling in the BA1, 2) to determine whether Fbln1 deficiency leads to suppression of Fgf8 response genes required for mandibular morphogenesis, and 3) to determine if Fbln1 is regulated transcriptionally by Barx1.