The regulation of signaling pathways by feedback inhibitors has become an emerging theme in the control of pattern formation during development. Nodal and
Lefty proteins belong to divergent subfamilies of the
TGF-beta family. Nodal signals promote mesendoderm induction in vertebrates, and
Lefty proteins antagonize it. In zebrafish,
Squint functions as a long-range Nodal signal during mesoderm induction. We report that the range over which
Squint induces mesoderm is reduced by
Lefty proteins. In contrast, the activity range of the short-range Nodal signal
Cyclops is not regulated by Lefty activity. We present three lines of evidence that
Lefty proteins diminish the range of
Squint signaling by acting not only as antagonists of
Squint autoregulation but also as long-range inhibitors of
Squint activity. First, Lefty can block Nodal signaling at a distance. Second, Lefty regulates the range of
Squint signaling before regulating
squint expression. Third, Lefty restricts the range of
Squint activity in
squint mutant embryos, in which the endogenous gene is not subject to autoregulation. We also find that Lefty restricts the response to both high and low levels of Nodal signaling. These results indicate that
Lefty proteins restrict the activity range of Nodal signals by dampening Nodal signaling in surrounding cells.