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Wnt Signal Transduction Networks: From Biology to Medicine
Summary: Randall Moon studies the signal transduction pathways that are activated by the Wnt family of secreted ligands and that regulate cell proliferation, cell fate, and cell behavior in development, and stem and progenitor cells in adults. His first goal is to identify the broad functions of Wnt signaling in vertebrates. His second goal is to elucidate the molecular mechanisms by which Wnts transduce signals and to understand how cells respond to these signals. His third goal is to identify how Wnt signaling is involved in diseases and acute injury.
Wnts are members of a conserved family of secreted proteins that activate multiple receptor-mediated signal transduction pathways during embryonic development and in adults. The best-understood pathway, the Wnt/β-catenin network, stabilizes β-catenin, resulting in its accumulation in the nucleus, where it regulates transcription. There are also one or more noncanonical Wnt networks that signal through different mechanisms. Our lab is focused on identifying the roles of Wnt signaling networks, dissecting the molecular and cellular mechanisms by which this signaling occurs, and applying this knowledge to illuminate how Wnt signaling participates in disease processes and in responses to acute injury.
Goal 1: The Roles of Wnt Signaling
Wnt/β-catenin signaling participates in a wide range of processes in both embryos and adults. We have recently investigated the roles of Wnt signaling in regeneration in adult vertebrates, beginning with regeneration of the tail fin of adult zebrafish (Figure 1). We found that there is a striking requirement for Wnt/β-catenin signaling for the tail fin to regenerate after injury. Moreover, activating β-catenin signaling accelerates the rate of regeneration of the tail fin. Looking in other regenerating tissues, we similarly found that regeneration of the adult zebrafish heart, the Xenopus tadpole hindlimb, and the vertebrate liver (in collaboration with Leonard Zon, HHMI, Children's Hospital Boston), depends on Wnt/β-catenin signaling. We have also established that a β-catenin–independent noncanonical Wnt network antagonizes the regeneration of the tail fin, and does so by antagonizing β-catenin signaling. Given that regenerative processes generally involve stem and progenitor cells, we have expanded our research to investigate Wnt signaling in human stem and progenitor cells.
Goal 2: The Mechanisms of Wnt Signaling
Advances in screening technologies have greatly facilitated our elucidation of the complex networks of proteins that regulate β-catenin signaling. For example, in collaboration with Rosetta/Merck, we used pools of small interfering RNAs (siRNAs) directed against more than 30,000 target RNAs to ask whether reduction of any of the target RNAs would modulate β-catenin signaling. This high-throughput robotic screen led to the identification of numerous new components of the Wnt/β-catenin signaling network.
We have also been using antibodies to known components of both the Wnt/β-catenin and noncanonical Wnt networks to affinity purify complexes of interacting proteins from cell lysates. We then use mass spectrometry to determine whether any unknown proteins are found in the complex. Integrating the proteomic data (which define complexes of proteins but do not reveal protein function) with the siRNA data (which identify functional genes but not how the encoded proteins interact to form a signaling network) allows us to greatly accelerate the process of defining the Wnt signaling networks (Figure 2).
A third screening approach has been to test diverse small molecules for their ability to activate or inhibit a β-catenin–responsive reporter in cultured cells and to integrate the results with siRNA screens. This has enabled us to identify Bruton's tyrosine kinase (BTK) as a negative regulator of Wnt/β-catenin signaling. Thus we use the full spectrum of available technologies to identify new components of Wnt signaling networks and to identify their mechanisms of action.
Goal 3: From Injury and Disease Toward Therapies
Although we often use zebrafish, mice, and cultured mammalian cells to dissect the functions and mechanisms of Wnt signaling, we do so with the goal of leveraging our insights to better understand how Wnt signaling may be involved in diseases and in responses to chronic injury, and to contribute to the development of new therapies for these conditions. For example, in previous collaborative projects we have shown that mutations in a Wnt receptor are linked to a retinal disease, and that an allele of low-density lipoprotein receptor-related protein 6 (LRP6), a Wnt coreceptor, is linked to Alzheimer's disease in a specific subgroup of patients. More recently we have assisted Li-Huei Tsai (HHMI, Massachusetts Institute of Technology) in demonstrating that the disrupted in schizophrenia 1 (DISC1) gene modulates β-catenin signaling, and we have collaborated with Philip Horner (University of Washington) to demonstrate that β-catenin signaling is transiently activated in proliferating neural progenitor cells in the brain following traumatic brain injury.
We have also been using our insights into Wnt biology to better understand cancer. For example, we have shown that the WTX gene, a tumor suppressor that is mutated in 30 percent of Wilm's kidney tumors, is a negative regulator of β-catenin stability. More recently, in collaboration with David Rimm (Yale University) and Andy Chien (University of Washington), we found that elevated β-catenin signaling unexpectedly correlates with improved patient survival in melanoma and that it promotes decreased cell proliferation and increased expression of differentiation markers in melanoma cells. These results are in stark contrast to colorectal cancer, where elevated β-catenin correlates with a poorer prognosis. As activating mutations in β-catenin have also been reported to correlate with increased survival of patients with medulloblastoma, it appears that β-catenin plays different roles in different cancers.
To provide tools for investigating Wnt signaling and to provide candidate molecules for drug development, we have conducted small-molecule screens (some in collaboration with Stephen Haggarty of the Broad Institute and Massachusetts General Hospital) looking for modulators of β-catenin signaling. This led us to recently identify the FDA-approved drug riluzole as being able to synergize with low levels of Wnt to activate β-catenin signaling. Moreover, riluzole mimics the ability of Wnt-3a to decrease cell proliferation in vitro and in mouse models of melanoma. Riluzole has been under independent investigation in clinical trials for treating melanoma, although it had not been linked to regulation of β-catenin. Our ongoing unpublished work is unexpectedly shedding new light on the mechanisms of action of investigational drugs for treating this disease.
In summary, our laboratory is focused on identifying the functions and mechanisms of action of Wnt signaling networks in vertebrates. We use these insights to better understand the roles of Wnt signaling in human diseases and to contribute to the discovery and understanding of new therapies.
Grants from the National Institutes of Health and awards from the Alzheimer's Association and the Department of Defense provided partial support for some of these projects.
As of May 30, 2012
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