Ventral closure, headfold fusion and definitive endoderm migration defects in mouse embryos lacking the fibronectin leucine-rich transmembrane protein FLRT3

Abstract

The three fibronectin leucine-rich repeat transmembrane (FLRT) proteins contain 10 leucine-rich repeats (LRR), a type III fibronectin (FN) domain, followed by the transmembrane region, and a short cytoplasmic tail. XFLRT3, a Nodal/TGFbeta target, regulates cell adhesion and modulates FGF signalling during Xenopus gastrulation. The present study describes the onset and pattern of FLRT1-3 expression in the early mouse embryo. FLRT3 expression is activated in the anterior visceral endoderm (AVE), and during gastrulation appears in anterior streak derivatives namely the node, notochord and the emerging definitive endoderm. To explore FLRT3 function we generated a null allele via gene targeting. Early Nodal activities required for anterior-posterior (A-P) patterning, primitive streak formation and left-right (L-R) axis determination were unperturbed. However, FLRT3 mutant embryos display defects in headfold fusion, definitive endoderm migration and a failure of the lateral edges of the ventral body wall to fuse, leading to cardia bifida. Surprisingly, the mutation has no effect on FGF signalling. Collectively these experiments demonstrate that FLRT3 plays a key role in controlling cell adhesion and tissue morphogenesis in the developing mouse embryo.

Genetic ablation of FLRT3 reveals a novel morphogenetic function for the anterior visceral endoderm in suppressing mesoderm differentiation

“…While we observed similar defects in our FLRT3 −/− mice as described before (Maretto et al 2008), our analysis shows that FLRT3 is required in the VE but not in the DE. Moreover, we show that AVE signaling properties seem intact in the FLRT3 −/− embryos.…”

“…At midgestation, FLRT3 −/− embryos suffered from a range of malformations including anterior neural tube closure defects, asymmetric development of the headfolds, and failure of embryonic turning and ventral body closure, as well as cardia bifida as described previously (Supplemental Fig. S3; Maretto et al 2008). In most severe cases, the embryos developed outside the YS and were extremely growth-retarded (Supplemental Fig.…”

“…To this end, we analyzed the expression pattern of the Nodal inhibitors Cer1 and Lefty1 and of the Wnt inhibitor Dkk1 by in situ hybridization in early to MS stage embryos and found that their levels and spatial distribution was unaffected in FLRT3 −/− embryos ( Fig. 4F-H,K-M, three or more mutant embryos per genotype and probe; see also Maretto et al 2008). Moreover, we were unable to detect an expansion of the expression domains of Wnt3 or Nodal from posterior to anterior region (Fig.…”

“…Genetic deletion of FLRT3 in the mouse resulted in ventral closure and headfold fusion defects suggesting that FLRT3 may be required for tissue fusion (Maretto et al 2008). The observed anterior truncations in a fraction of FLRT3 −/− embryos were suggested to result from a reduction in the number of anterior DE cells (Maretto et al 2008) as described for mutations affecting Nodal signaling (for review, see Lu et al 2001). Surprisingly, the phenotypes caused by loss of FLRT3 expression were not associated with defects in FGF signaling (Maretto et al 2008).…”

“…Conversely, FLRT3 was shown to promote deadhesion of migrating animal cap cells during Xenopus gastrulation through the internalization of C-cadherin (Ogata et al 2007). Genetic deletion of FLRT3 in the mouse resulted in ventral closure and headfold fusion defects suggesting that FLRT3 may be required for tissue fusion (Maretto et al 2008). The observed anterior truncations in a fraction of FLRT3 −/− embryos were suggested to result from a reduction in the number of anterior DE cells (Maretto et al 2008) as described for mutations affecting Nodal signaling (for review, see Lu et al 2001).…”