FGF10/FGFR2b signaling plays essential roles during in vivo embryonic submandibular salivary gland morphogenesis.
BACKGROUND
Analyses of Fgf10 and Fgfr2b mutant mice, as well as human studies, suggest that FGF10/FGFR2b signaling may play an essential, nonredundant role during embryonic SMG development. To address this question, we have analyzed the SMG phenotype in Fgf10 and Fgfr2b heterozygous and null mutant mice. In addition, although previous studies suggest that the FGF10/FGFR2b and FGF8/FGFR2c signaling pathways are functionally interrelated, little is known about the functional relationship between these two pathways during SMG development. We have designed in vivo and in vitro experiments to address this question.


RESULTS
We analyzed Fgf10 and Fgfr2b heterozygous mutant and null mice and demonstrate dose-dependent SMG phenotypic differences. Hypoplastic SMGs are seen in Fgf10 and Fgfr2b heterozygotes whereas SMG aplasia is seen in Fgf10 and Fgfr2b null embryos. Complementary in vitro studies further indicate that FGF10/FGFR2b signaling regulates SMG epithelial branching and cell proliferation. To delineate the functional relationship between the FGF10/FGFR2b and FGF8/FGFR2c pathways, we compared the SMG phenotype in Fgfr2c+/Delta/Fgf10+/- double heterozygous mice to that seen in wildtype, Fgf10+/- (Fgfr2c+/+/Fgf10+/-) and Fgfr2c+/Delta (Fgfr2c+/Delta/Fgf10+/+) single heterozygous mutant littermates and demonstrate genotype-specific SMG phenotypes. In addition, exogenous FGF8 was able to rescue the abnormal SMG phenotype associated with abrogated FGFR2b signaling in vitro and restore branching to normal levels.


CONCLUSIONS
Our data indicates that FGF10/FGFR2b signaling is essential for the SMG epithelial branching and histodifferentiation, but not earliest initial bud formation. The functional presence of other endogenous signaling pathways could not prevent complete death of embryonic SMG cells in Fgf10 and Fgfr2b null mice. Though we were able to rescue the abnormal phenotype associated with reduced in vitro FGF10/FGFR2b signaling with exogenous FGF8 supplementation, our results indicate that the FGF10/FGFR2b and FGF8/FGFR2c are nonredundant signaling pathways essential for in vivo embryonic SMG development. What remains to be determined is the in vivo functional relationship between the FGF10/FGFR2b signal transduction pathway and other key signaling pathways, and how these pathways are integrated during embryonic SMG development to compose the functional epigenome.
Jaskoll
Tina
T
Laboratory Developmental Genetics, University of Southern California, Los Angeles, CA, USA. tjaskoll@usc.edu
Abichaker
George
G
Witcher
Daniel
D
Sala
Frederic G
FG
Bellusci
Saverio
S
Hajihosseini
Mohammad K
MK
Melnick
Michael
M
D000818:Animals; D049109:Cell Proliferation; D004622:Embryo, Mammalian; D004847:Epithelial Cells / Q000166:cytology; D051526:Fibroblast Growth Factor 10 / Q000235:genetics / Q000502:physiology*; D051524:Fibroblast Growth Factor 8 / Q000502:physiology; D005838:Genotype; D051379:Mice; D018345:Mice, Knockout; D009024:Morphogenesis; D051497:Receptor, Fibroblast Growth Factor, Type 2 / Q000235:genetics / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*; D013363:Submandibular Gland / Q000166:cytology / Q000196:embryology* / Q000254:growth & development
C496463:FGF10 protein, human; C496457:Fgf8 protein, mouse; D051526:Fibroblast Growth Factor 10; D051524:Fibroblast Growth Factor 8; D051497:Receptor, Fibroblast Growth Factor, Type 2