FGF4 and FGF8 Comprise the Wavefront Activity That Controls Somitogenesis
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FGF4 and FGF8 comprise the wavefront activity that controls somitogenesis L. A. Naiche, Nakisha Holder1, and Mark Lewandoski2 Genetics of Vertebrate Development Section, National Cancer Institute, Frederick, MD 21701 Edited by Gail Martin, University of California, San Francisco, CA, and approved January 26, 2011 (received for review May 29, 2010) Somites form along the embryonic axis by sequential segmenta- required for somitogenesis (7). Furthermore, mice homozygous for tion from the presomitic mesoderm (PSM) and differentiate into null mutations in other FGF ligand genes (Fgf3, Fgf5, Fgf15, Fgf17, the segmented vertebral column as well as other unsegmented and Fgf18) expressed in the PSM also show no early somitogenesis tissues. Somites are thought to form via the intersection of two defects, or die before somitogenesis (Fgf4)(7–9). These results activities known as the clock and the wavefront. Previous work suggest that if FGFs have any function in somitogenesis, loss of has suggested that fibroblast growth factor (FGF) activity may be individual ligands is compensated for via genetic redundancy. the wavefront signal, which maintains the PSM in an undifferen- To circumvent the problem of redundancy, the gene encoding tiated state. However, it is unclear which (if any) of the FGFs FGF receptor 1 (Fgfr1), the only FGF receptor known in the PSM (10, 11), was conditionally deleted (11, 12). These studies dem- expressed in the PSM comprise this activity, as removal of any one fi onstrate a strong effect of FGF signaling on controlling clock gene gene is insuf cient to disrupt early somitogenesis. Here we show expression and in positioning the determination front. However, that when both Fgf4 and Fgf8 are deleted in the PSM, expression FGF signaling was retained during anterior somite formation and of most PSM genes is absent, including cycling genes, WNT path- progressively lost as somitogenesis proceeded. Perhaps due to this fi way genes, and markers of undifferentiated PSM. Signi cantly, gradual loss, premature differentiation of the PSM, a key pre- markers of nascent somite cell fate expand throughout the PSM, dicted outcome of the loss of wavefront activity, was not observed. demonstrating the premature differentiation of this entire tissue, WNT signaling has also been proposed to contribute to a highly unusual phenotype indicative of the loss of wavefront wavefront activity (13, 14). Manipulation of canonical WNT activity. When WNT signaling is restored in mutants, PSM progen- signaling also causes corresponding shifts in the determination BIOLOGY itor markers are partially restored but premature differentiation of front (13, 15, 16). However, in WNT loss-of-function embryos, DEVELOPMENTAL the PSM still occurs, demonstrating that FGF signaling operates FGF signaling is also affected. Thus, the observed somitogenesis independently of WNT signaling. This study provides genetic evi- defects may be primarily due to loss of WNT signaling or to dence that FGFs are the wavefront signal and identifies the spe- secondary effects on FGF signaling. cific FGF ligands that encode this activity. Furthermore, these data To understand the actual role of FGF signaling in somito- show that FGF action maintains WNT signaling, and that both genesis, we initiated a comprehensive strategy to inactivate all signaling pathways are required in parallel to maintain PSM pro- combinations of FGF ligand genes expressed in the PSM. We genitor tissue. report here that inactivation of both Fgf4 and Fgf8 in the PSM results in dramatic shifts in the determination front and the genetic redundancy | Spry | T-Cre | axis extension | paraxial mesoderm premature differentiation of the PSM. Moreover, the restoration of WNT signaling to these mutants does not restore the de- termination front. Our data demonstrate that these two FGFs central problem in developmental biology is the identifi- constitute the proposed wavefront activity that maintains the A cation of molecular signals that control the acquisition PSM in an undifferentiated state. of differentiated cell fates. In the vertebrate embryo, a critical differentiation event is the formation of bilaterally paired epi- Results thelialized segments, called somites, from the presomitic meso- We have inactivated Fgf8 pairwise with other Fgfs expressed in derm (PSM). Somites give rise to the vertebral column as well as the PSM. Null alleles were used where possible. To bypass early skeletal muscle and the dermal skin layers (1). Defects in somi- lethality, conditional alleles of Fgf4 and Fgf8 (8, 17) were used in togenesis cause common human developmental disorders such as combination with the pan-mesodermal T-Cre line (7), which abnormal vertebrae segmentation and scoliosis (2). Somite seg- recombines throughout the PSM before the onset of somito- mentation occurs in an anterior-to-posterior progression as the genesis (Fig. S1). Compound mutant embryos lacking Fgf8 and embryo elongates, with a new somite emerging periodically from either Fgf3, Fgf5,orFgf17 showed no obvious synergistic defects the PSM approximately every 2 h in the mouse. in somitogenesis. However, compound Fgf8 and Fgf4 mutants The prevailing hypothesis for somitogenesis posits two inter- have a severe defect in somitogenesis, and we here report our secting activities in the PSM, known as the clock and the wave- analysis in detail. front (3). Clock activity is characterized by oscillating gene expression that cycles in tandem with somite emergence. The Normal Somitogenesis with One Functional Allele of Either Fgf4 or wavefront is an activity gradient, strongest at the caudal end of Fgf8. Embryos in which both Fgf4 and Fgf8 had been ablated in fl fl the embryo, which maintains the undifferentiated PSM; below the PSM (T-Cre; Fgf4 ox/null; Fgf8 ox/null; hereafter “FGF mu- a critical wavefront threshold, called the determination front, tants”) died prenatally (Fig. 1A). Embryos with one or more anterior PSM cells differentiate (1). If cells pass the deter- mination front when clock genes are at a certain point in their cycle, segmentation occurs, resulting in a somite boundary. Whereas numerous genes in the Notch, WNT, and FGF path- Author contributions: L.A.N., N.H., and M.L. designed research; L.A.N., N.H., and M.L. fi performed research; L.A.N., N.H., and M.L. analyzed data; and L.A.N. and M.L. wrote ways have been identi ed as oscillating clock genes, wavefront the paper. identity has been more controversial. Initially, Fgf8 was proposed to fl encode wavefront activity because experimental manipulation of The authors declare no con ict of interest. FGF8 levels caused corresponding shifts in the position of the de- This article is a PNAS Direct Submission. termination front in cultured chick and zebrafish embryos (4, 5). 1Present address: Johns Hopkins Technology Transfer, Baltimore, MD 21201-3894. Moreover, manipulation of downstream components implicated 2To whom correspondence should be addressed. E-mail: [email protected]. FGF signaling in determination front positioning (6). However, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. mouse embryos lacking Fgf8 in the PSM revealed that Fgf8 is not 1073/pnas.1007417108/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1007417108 PNAS Early Edition | 1of6 Downloaded by guest on October 2, 2021 functional alleles of either Fgf4 or Fgf8 (hereafter “control em- chyury and Shh on its anterior surface (Fig. S2), and variably also bryos”) were viable to birth (Fig. 1A). Skeletal preparations expressed somite markers (Fig. 2 E–J). This suggests that this showed changes in vertebral numbers and homeotic trans- protrusion is formed by the elongating notochord, but may also formations of vertebral identity in some control genotypes contain some paraxial mesoderm. (Table S1) but otherwise normal vertebral development. Despite Normally, somites form in a linear array (Fig. 2C). By contrast, previous observations showing that chemical inhibition of FGF FGF mutants form misshapen somite-like epithelial conden- signaling reduces PSM length (4, 5), no difference was seen sations that overlap in the dorsal–ventral axis due to abnormal between control genotypes in the length or extension rate of the segmentation planes (Fig. 2D). Markers of anterior (Tbx18) and PSM (Fig. 1 B and C). Collectively, these data demonstrate that posterior (Uncx4.1, Dll1) somite compartments are disorganized – a single copy of either Fgf4 or Fgf8 is sufficient for somitogenesis. in FGF mutants, with no clear stripes observed (Fig. 2 E J). These data suggest that whereas a somitogenic program initiates Loss of Fgf4 and Fgf8 Ablates FGF Signaling in the PSM. We exam- in FGF mutant embryos, the resulting somites are malformed ined the expression of the FGF-responsive genes (18) Etv4, and disorganized. Spry1, Spry2, and Spry4 in FGF mutant embryos. Expression of We investigated whether these disordered somites might form these target genes was lost in the primitive streak of gastrulae at from cells that had not undergone Cre-mediated recombination. bud stages (19) (Fig. 1 D–I) as well as the PSM during early Examination of R26R-lacZ, a reporter of Cre activity, indicated that all cells contributing to FGF mutant somite-like tissue had somitogenesis (Fig. S1). Some FGF signaling presumably oc- indeed undergone Cre recombination and presumably excised curred in streak-stage embryos because it is required for gas- Fgf4 and Fgf8 (Fig. 2 C and D). These data, together with the trulation onset (8), but these results show that FGF signaling was early loss of FGF-responsive genes (Fig. 1 D–I), indicate that ablated substantially before the initiation of somitogenesis. This these structures form in the absence of FGF signaling. demonstrates that FGF4 and FGF8 are the critical ligands for FGF signaling in the PSM. Loss of FGF Signaling Results in Loss of PSM Markers and Cycling Genes. To determine the origin of the somite defects, we in- Loss of Fgf4 and Fgf8 Results in Axis Truncation and Abnormal vestigated the expression of genes known to be important in Somitogenesis.