Research Description
Highlights of Major Accomplishments
Major Results
1. Regulation of b-catenin degradation by b-Trcp
2. Association between b-catenin and presenilin-1
3. p300/CBP function in xenopus embryonic layer formation and neural induction
1. Regulation of b-catenin degradation by b-Trcp
b-catenin is a key component in the Wnt signal transduction pathway. Regulation of b-catenin stability is essential for Wnt signaling during development and oncogenesis. It is known that serine/threonine phosphorylation of b-catenin by the Axin-GSK-3b complex targets b-catenin for the ubiquitination-proteosome degradation pathway, and mutations at critical phospho-serine/threonine residues stabilize b-catenin and cause human cancers. During Xenopus early embryogenesis, dorsal-specific accumulation of b-catenin regulated by GSK-3b is responsible for establishing the dorsoventral polarity. Despite the significance of regulation of b-catenin degradation in Wnt signaling during development and tumor formation, the mechanism by which b-catenin phosphorylation leads to its degradation was unknown. We now have compelling evidence that b-Trcp, an F-box/ WD40-repeat protein, plays a central role in coupling phosphorylated b-catenin for degradation. b-Trcp is the Xenopus homologue of the Drosophila silmb gene product, which is genetically implicated in both Wingless (Drosophila Wnt-1) and Hedgehog signaling. We found that b-Trcp specifically binds b-catenin only if b-catenin is phosphorylated by GSK-3b. b-Trcp binds b-catenin via the WD40-repeat domain, and binds Skp1, an essential component of the ubiquitination apparatus, via the F-box motif, suggesting that b-Trcp brings phosphorylated b-catenin into the ubiquitination machinery. Interestingly, b-catenin harboring mutations at the critical phospho-serine/threonine residues escapes recognition by b-Trcp, providing a molecular explanation for why these mutations cause b-catenin accumulation that leads to cancer. Finally, we demonstrated that inhibition of the endogenous b-Trcp function by a dominant negative mutant form of b-Trcp stabilizes b-catenin, activates Wnt/b-catenin signaling and induces axis formation in Xenopus embryos. Therefore, b-Trcp plays a central role in recruiting phosphorylated b-catenin for degradation and in axial patterning of the Xenopus embryo.
2. Association between b-Catenin and presenilin-1
In a collaboration with Dr. Bruce Yankner’s lab in this MRRC, we discovered that the human presenilin-1 protein, whose mutations are responsible for the majority of the early-onset Alzheimer’s disease, is complexed with b-catenin. This complex formation increases b-catenin stability. Pathogenic mutations in the presenilin-1 gene reduce the ability of presenilin-1 to stabilize b-catenin, and lead to increased degradation of b-catenin in the brains of transgenic mice. We also found that b-catenin protein levels are markedly reduced in the brains of Alzheimer’s disease patients with presenilin-1 mutations. Finally, we demonstrated that attenuation of b-catenin signaling enhances the neuronal apoptosis induced by amyloid-b protein, a critical process in neuronal degeneration in Alzheimer’s disease. Thus, b-catenin protein and its regulation may play a role in the pathogenesis of Alzheimer’s disease.
3. p300/CBP function in xenopus embryonic layer formation and neural induction
p300/CBP is a transcriptional co-activator for a plethora of transcription factors and plays critical roles in varieties of signal transduction pathways. In an effort to investigate whether p300/CBP participates in b-catenin mediated transcriptional activation in Wnt signal transduction, we found that p300/CBP plays an essential role in the specification of neural versus non-neural development during early Xenopus embryogenesis (p300/CBP does not appear to play any role in Wnt signaling in Xenopus embryos). We found that inhibition of p300/CBP function in the Xenopus embryo abolishes non-neural germ layer formation, and strikingly, initiates neural induction and primary neurogenesis in the entire embryo. Thus, ectoderm, mesoderm and endoderm all become neuralized in the absence of p300/CBP function. Different from the prediction of the classic "two-step" model of neural induction, the observed neuralization is achieved in the absence of any anterior or posterior gene expression, suggesting that neural fate activation and anterior patterning may represent distinct molecular events. We further demonstrated that the neuralizing and anteriorizing activities of chordin and noggin are separable properties of these neural inducers. This study reveals that all embryonic cells possess intrinsic neuralizing capability and that p300/CBP function is essential for embryonic germ layer formation and neural fate suppression during vertebrate embryogenesis.
