Xenopus Kielin: A dorsalizing factor containing multiple chordin-type repeats secreted from the embryonic midline

The midline tissues are important inductive centers of early vertebrate emb ryos. By signal peptide selection screening, we isolated a secreted factor, Kielin, which contains multiple cys-rich repeats similar to those in chord in (Chd). Expression of Kielin starts at midgastrula stages in the notochor d and is detected in the floor plate of neurula embryos. Kielin is induced in mesoderm and in ectoderm by nodal-related genes. Chd is sufficient to ac tivate Kielin expression in mesoderm whereas Shh or HNF-3 beta in addition to Chd is required for induction in ectoderm, Kielin has a distinct biologi cal activity from that of Chd. injection of Kielin mRNA causes dorsalizatio n of ventral marginal zone explants and expansion of MyoD expression in neu rula embryos. Unlike Chd, Kielin does not efficiently induce neural differe ntiation of animal cap ectoderm. suggesting that the activity of Kielin is not simply caused by BMP4 blockade. Kielin is a signaling molecule that med iates inductive activities of the embryonic midline.