Functional redundancy of Tcf7 and Lef1, transcription
factors mediating the Wnt signaling pathway, during zebrafish fin development
○Gembu Abe, Saori Nagayoshi, Koichi Kawakami
National institute of Genetics, Sizuoka, Japan
We have been developing the enhancer trap method in zebrafish by
using the Tol2 transposable element. We created 73 fish lines that carried
single copy insertions of the enhancer trap construct containing the zebrafish
hsp70 promoter and the GFP gene and expressed GFP in specific cells,
tissues and organs, and showed that the hsp70 promoter is highly capable
of responding to chromosomal enhancers in zebrafish.
One of these fish expressed GFP in the edge of the median fin fold
and the pectoral fin during embryogenesis. In this line, the enhancer trap
construct was integrated in the first exon of the tcf7 gene, which encodes
a transcription factor mediating the Wnt canonical signaling pathway, and
disrupted its function. Homozygous embryos showed shorter and wavy
median and pectoral fins, indicating that Tcf7 plays a role in morphogenesis
of these fins. It has been shown that Wnt signaling is essential for both
limb/fin initiation and AER formation. However, the pectoral fins can still
outgrow in the tcf7 homozygous embryos, and the mutant phenotype was
weaker than that expected from a Wnt signaling deficiency in this region.
In the pectoral fin bud, another member of the Tcf/Lef family, Lef1, is
expressed. We knocked down the Lef1 activity by microinjection of lef1
MO into the tcf7 mutant. In the lef1 tcf7 double knock-down embryos,
only rudimentary pectoral fins were formed and expression of the AER
markers was abolished. Thus, Tcf7 and Lef1 are functionally redundant in
AER formation during fin development.
Even in the lef1 tcf7 double knock-down embryos, we found that
fgf10 is induced in the mesenchyme, and outgrowth of the fin buds was
still observed, indicating that fin initiation occurred. We hypothesized that
another member of the Tcf/Lef family or a transcription factor that does
not belong to this family is involved in mediating Wnt canonical signaling
in this step, and currently testing this hypothesis.