A novel kinin‐kallikrein pathway regulates vertebrate mouth formation

The mouth, the initial opening between the gut and outside of the embryo, forms from juxtaposed ectoderm and endoderm at the extreme anterior of the embryo. In Xenopus embryos, mouth formation correlates with multiple steps, including cell death, dissolution of the basement membrane, cell layer thinning and intercalation, and cell sheet perforation. From an expression microarray screen we identified three members of the kinin‐kallikrein pathway as highly expressed in the developing mouth region. These are carboxypeptidase‐N, kininogen and neural nitric‐oxide synthase. This pathway acts in adults as a regulator of inflammation and blood pressure, but has not previously been described in embryos. In adults, the pathway converges on production of the signaling molecule nitric oxide (NO). Using antisense oligonucleotides, we show that all three members of this putative embryonic pathway are essential for mouth formation. Consistent with activity of the signaling pathway, all loss of function phenotypes can be rescued by application of exogenous nitric oxide (NO), and by injection of predicted peptide ligands. We further show that endogenous NO is produced in ectodermal tissue and has a role in regulating cell layering during mouth formation. We suggest that the kinin‐kallikrein pathway is active in the embryo and essential for craniofacial development. Grant Funding Source : AAA