Xwnt8 directly initiates expression of labial Hox genes.

Hox transcription factors play an essential role in patterning the anteroposterior axis during embryogenesis and exhibit a complex array of spatial and temporal patterns of expression. Their earliest onset of expression in vertebrates is during gastrulation in a temporally collinear sequence in the presomitic/ventrolateral mesoderm, and it is not clear which upstream signal transduction events initiate this expression. Using Xenopus, we present evidence that Xwnt8 is necessary for initiation of this collinear sequence by activating Hox-1 expression in three Hox clusters: hoxd, hoxa, and hoxb. All three labial genes appear to be direct targets of canonical Wnt signaling through Tcf/Lef. In addition, Xwnt8 loss- and gain-of-function leads to indirect regulation of other Hox genes: Hoxb4, Hoxd4, Hoxa7, Hoxc6, and Hoxc8. These findings shed new light on the early role of Wnt8 as well as of a proposed WNT gradient in patterning the Xenopus central nervous system (Kiecker and Niehrs [2001] Development 128:4189-4201). (c) 2009 Wiley-Liss, Inc.