The paradox was explained when the group showed that Vax activated expression of a molecular decoy, namely a stump of Tcf7l2 protein missing its front end, the part required for it to activate gene expression. When bound to DNA the fragment would instead recruit repressor proteins to silence dorsalizing signals. "Cells expressing the inhibitor would be blind to Wnt signaling," says Lemke.
Full-length Tcf7l2 proteins pair with an activator called -catenin. But truncated Tcf7l2 lacks the -catenin interaction region, short-circuiting its function. Scientists call such interfering proteins "dominant negatives."
Tomas Vacik, Ph.D., a postdoctoral fellow in the Lemke lab and the study's first author, re-evaluated gene expression patterns in mice the lab had engineered to lack Vax2. "We found that Vax2 was necessary for expression of a group of Wnt antagonists in the mouse eye, including dominant negative Tcf7l2" he says.
Bioinformatics analysis of the Tcf7l2 DNA sequence bound by Vax revealed another surprise. Approximately 700 base pairs, or nucleotides, of the mouse genome around the Vax binding site in Tcf7l2 showed an astonishing 99% identity between mouse, humans, and chickens and 85% identity with fish, a conservation Lemke calls, "exceptional in the extreme."
"This means that over several million years of evolution, Mother Nature says you can't change a single nucleotide, " he says. "That tells you straightway that this DNA sequence fulfills a very important regulatory function."
Database searches revealed that truncated Tcf7l2 is expressed in human brain cells, and the group's own analysis revealed sim
|Contact: Andy Hoang|