Molecular link in the sequential induction of the Spemann organizer: direct activation of the cerberus gene by Xlim-1, Xotx2, Mix.1, and Siamois, immediately downstream from Nodal and Wnt signaling.
To elucidate the molecular basis of organizer functions in Xenopus, we sought the target genes of the LIM homeodomain protein Xlim-1, which is one of the organizer-specific transcriptional activators. We found that an activated form of Xlim-1, Xlim-1/3m, initiates ectopic expression of the head-inducing organizer factor gene cerberus in animal caps. Thus, we analyzed the cerberus promoter using reporter assays. We show that three consecutive TAAT motifs of the homeodomain-binding sites between positions -141 and -118, collectively designated the "3xTAAT element," are crucial for the response of the cerberus promoter to Xlim-1/3m, and for its activation in the dorsal region of the embryo. Because cooperative activation of the cerberus promoter by Xnr1 and Xwnt8 also requires the 3xTAAT element, we focused on homeodomain transcriptional activators downstream from either Nodal or Wnt signaling. We found that wild-type Xlim-1 synergistically activates the cerberus promoter with Mix.1 and Siamois through the 3xTAAT element, and this synergy requires the LIM domains of Xlim-1. In contrast, Xotx2 acts synergistically with Mix.1 and Siamois through the TAATCT sequence at -95. Electrophoretic mobility shift assays revealed that Xlim-1, Siamois, and Mix.1 are likely to bind as a complex, in a LIM domain-dependent manner, to the region containing the 3xTAAT element. These data suggest that cerberus is a direct target for Xlim-1, Mix.1, Siamois, and Xotx2. Therefore, we propose a model for the molecular link in the inductive sequence from the formation of the organizer to anterior neural induction.
Yamamoto
Shinji
S
Department of Biological Sciences, Graduate School of Science, University of Tokyo, Hongo 7-3-1, 113-0033, Bunkyo-ku, Tokyo, Japan.
Hikasa
Hiroki
H
Ono
Hirofumi
H
Taira
Masanori
M
D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D018398:Homeodomain Proteins / Q000378:metabolism*; D036341:Intercellular Signaling Peptides and Proteins; D060850:LIM-Homeodomain Proteins; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D055457:Nodal Protein; D051857:Otx Transcription Factors; D011401:Promoter Regions, Genetic; D011506:Proteins / Q000235:genetics / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D015534:Trans-Activators / Q000378:metabolism*; D014157:Transcription Factors; D016212:Transforming Growth Factor beta / Q000378:metabolism; D051153:Wnt Proteins; D014981:Xenopus; D029867:Xenopus Proteins; D029961:Zebrafish Proteins
D018398:Homeodomain Proteins; D036341:Intercellular Signaling Peptides and Proteins; D060850:LIM-Homeodomain Proteins; C543964:Lhx1 protein, Xenopus; C100368:Mix1 protein, Xenopus; D009419:Nerve Tissue Proteins; D055457:Nodal Protein; D051857:Otx Transcription Factors; C497661:Otx2 protein, Xenopus; D011506:Proteins; D011518:Proto-Oncogene Proteins; C093133:SIA1 protein, Xenopus; D015534:Trans-Activators; D014157:Transcription Factors; D016212:Transforming Growth Factor beta; D051153:Wnt Proteins; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; C100866:cer1 protein, Xenopus