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Wnt/β-catenin Pathway Functions by Inactivation of Tcf7l1 Protein

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posted on 20.06.2014, 00:00 authored by Chun-I Wu
The canonical Wnt/β-catenin signaling pathway classically functions through the activation of target genes by Tcf/Lef–β-catenin complexes. Each Tcf/Lef in mammals is believed to function as a switch, going from a transcriptional repressor in the absence of Wnt stabilized β-catenin to a transcriptional activator in the presence of Wnt-stabilized β-catenin. I determined that mouse Tcf7l1 (formerly named Tcf3) mediates a distinct and new form of regulation of the pathway, which is independent of transactivator activity of Tcf7l1-β-catenin complexes. Instead of switching Tcf7l1 into a transactivator, Wnt/β-catenin reduces Tcf7l1 protein levels, which relieves repression of target genes, such as Lef1. Experiments using mouse embryonic stem cells show that recombinant Wnt3a, GSK3-inhibition, or simply increasing β-catenin levels were all sufficient to reduce Tcf7l1 protein independent of mRNA levels. The effect did not occur in Tcf7l1ΔN/ΔN ES cells, which lack nine Tcf7l1 residues necessary for binding to β-catenin. Thus, β-catenin binding to Tcf7l1 is necessary and sufficient for destabilizing Tcf7l1 protein in stem cells. We ablated the Tcf7l1-β-catenin interaction by generating a mouse Tcf7l1ΔN knockin allele and show Tcf7l1ΔN blocks Wnt-reduction of Tcf7l1, inhibits Lef1 expression, and causes peri-natal lethality in Tcf7l1ΔN/ΔN mice. Interestingly, Tcf7l1-/ΔN mice developed into viable and ostensibly normal adult mice, demonstrating that reducing the amount of Tcf7l1 replaced the requirement for Tcf7l1-β-catenin interaction. These findings impact understanding of the mechanisms whereby Wnt/β-catenin mediates its effects, especially in Tcf7l1-expressing stem cells and tumor cells.

History

Advisor

Merrill, Bradley

Department

Biochemistry and Molecular Genetics

Degree Grantor

University of Illinois at Chicago

Degree Level

Doctoral

Committee Member

Benevolenskaya, Elizaveta Frolov, Maxim Lau, Lester Schmidt, Jennifer

Submitted date

2014-05

Language

en

Issue date

20/06/2014