The Role of Fgf Signaling in Vertebrate Development

Division Of Basic Sciences - Nci

Investigators

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Abstract

The long term goal of this project is to understand how an important family of signaling ligands, called Fibroblast Growth Factors (FGFs), control a wide spectrum of cell biological behaviors such as proliferation, cell death, migration, stem cell maintenance and gene expression. In particular we use complex mouse genetics to understand the role of FGF signaling in mesodermal lineages with a special emphasis on extension of the body axis and formation of somites (segmented mesodermal segments that are the building blocks of vertebrate muscle, dermis and vertebral bodies). Our work has made clear that genetic redundancy is an important aspect of this biology; therefore all work in this project emerges from an effort to comprehensively characterize the genetic redundancy of FGF signaling in the mesodermal lineage. Such work is relevant to many cases of cancer where more than one FGF gene may be damaged. To achieve this, we have generated and characterized important Cre mouse lines, which are tools that allow the control of gene expression in the early embryo. These include TCre (expressed in the early emerging nascent mesoderm; see Development. 132: 3859-71. ), TCreERT2 (active in emerging nascent mesoderm at all embryonic stages; see PLoS ONE. 8: e62479) and Tbx4-Cre (expressed in a posterior mesodermal domain that includes the allantois, hindlimb, and external genitalia; see Dev Dyn. 240: 2290-300. doi: 10.1002/dvdy.22731). TCre in particular has had a major impact on the field, being essential in over 50 publications. For example, both TCre and TCreERT2 have important in a collaborative effort to demonstrate that Wnt5a/Ror2 signaling regulates kidney morphogenesis by controlling intermediate mesoderm extension (Hum Mol Genet. 2014 Jul 31. pii: ddu397). Besides providing the mouse genetics community with valuable mouse lines, this project has yielded papers that document our major insights regarding FGF signaling in the early embryo. We published that Fgf8 not required for somitogenesis, although a body of high profile work had placed it in a central position in current models. However, in collaboration with NCI colleague, Alan Perantoni, we demonstrated that Fgf8 was essential for development of the kidney and male reproductive tract (Development. 132: 3859-71, Development. 138: 5369-78). We showed that Fgf8, together with Fgf4, are required for essential aspects of somitogenesis: expression of oscillating gene domains, WNT pathway genes and markers of undifferentiated presomitic mesoderm (Proc Natl Acad Sci U S A. 108: 4018-23). By examining FGF mutants in which we genetically restored WNT signaling, we demonstrated that FGF signaling operates independently of WNT signaling in this process. The functional redundancy that we uncovered has implications for cancer as both FGFs have been found to be aberrantly active in testicular tumors. This past year we have delved deeper into this work and have shown that Fgf4 mutants (but not Fgf8 mutants) display a range of vertebral defects that model a spectrum of human Segmentation Defects of the Vertebrae caused by defective Notch oscillations. An key innovation in this work is our adaption of computational modeling to generate embryonic volumetric subsets of the embryo. We then quantify mRNA levels of key target genes, affected by the loss of Fgf4 signaling, within these volumes (eLife 2020;9:e55608. DOI: https://doi.org/10.7554/eLife.55608). In another recent publication, we demonstrated that the Fgf8 subfamily (Fgf8, Fgf17 and Fgf18) are required for closing the ventral body wall in the vertebrate embryo. Defects in this process are a major class of human birth defect and a significant health burden (Development (2020) 147, dev189506. doi:10.1242/dev.189506). We are continuing to study genetic redundancy in FGF signaling in several aspects of embryonic development. For example, we recently showed that Fgf3 signaling is important in neural tube closure, an important study that concerns one of the most common birth defects, spina bifida (PLoS Genet. 12(5):e1006018; Genesis 54:91-8). We are investigating the role of Fgf4 and Fgf8 in the differentiation of the somite into its derivative lineages (muscle and bone). In the past year we generated and characterized a new mouse line that carries an FGF18 allele that allows conditional gene inactivation and that allows the precise identification of cells and their lineage that express Fgf18 (Dev Dyn. 2019 Sep;248(9):882-893. doi: 10.1002/dvdy.85). Fgf18 functions in the development of several tissues, including the lung, limb bud, palate, skeleton, central nervous system, and hair follicle. Fgf18 functions in the development of several tissues, including the lung, limb bud, palate, skeleton, central nervous system, and hair follicle. Postnatally, FGF18 is being evaluated for pathogenic roles in fibrosis and several types of cancer. Therefore, these mice will be useful to investigate FGF18 function during organogenesis and tissue homeostasis, and to target specific cell lineages at embryonic and postnatal time points.