Marcel Wehrli

Assistant Professor - Dept of Cell and Developmental Biology

Cell communication and the role of Wnt signaling in Cancer

Elucidating the Wnt signal transduction mechanism using fruit flies. Secreted molecules of the Wnt family repeatedly provide key signals for cell fate determination and organ formation during development, as well as for stem cell maintenance in the adult.  In addition to its involvement in many other diseases, dysregulation of Wnt signaling

• causes most cases of colorectal cancer and brain tumors
• is central to defects in the proper maintenance of bone mass, resulting in osteoporosis or irregularly increased bone mass
• causes birth defects
• results in loss of stem cells in adults

Most of what we know about Wnt signaling comes from work in the fruit fly, Drosophila melanogaster.  Some reasons for this are 

• the Wnt signaling mechanism is virtually identical between us and flies
• powerful genetic techniques allow us to functionally identify missing components in the pathway
• a huge number of tools were first developed in flies and are at our disposal to figure out how precisely the mechanism works; this makes the analysis fast
• flies are a developmental model system that allow us to analyze protein function in vivo (i.e. proteins can be eliminated, manipulated and expressed in their normal place)
• Flies are much simpler organisms than vertebrates and have proved to be a much faster and more precise system to analyze question of cell communication than vertebrates

Our goal is to understand how cells communicate with each other using the Wnt signaling pathway. In spite of its significance to development and disease, the precise mechanism of Wnt signal transduction remains elusive.  Using Drosophila as a model system, we aim to identify the canonical (Wnt/beta-catenin) signaling mechanism and define key interactions of pathway components that then may serve as drug targets for the treatment of Wnt pathway dysregulation, particularly for cancer and osteoporosis.

Our approach exploits the combination of genetics and molecular biology in Drosophila, which allows a uniquely targeted approach to obtain answers directly where it matters, i.e. in vivo.  Ideally, we design molecularly defined situations that can be directly tested in vivo, e.g. through protein fusion.  In such a situation, we obtain in a single experiment information on function in vivo and physical interactions of proteins that, in other systems, would have to be obtained using different approaches (e.g. biochemistry and overexpression studies) with all their associated problems.

Figure 2. In our model Arrow functions both in signal initiation and signal amplification, which are two separable steps in the transduction mechanism. As a receptor subunit, Arrow (Arr) initiates the cytoplasmic cascade together with Frizzleds (Fz) by activating Dishevelled (Dsh). A fusion protein (Fz2-Arr) can execute this step even in the absence of Wg ligand. The signal transduced to Axin induces this scaffold protein to translocate to the cell membrane, where it interacts with Arrow. In this second step, Arrow inactivates Axin and targets it for degradation.  This second function of Arrow serves to amplify the Wg signal (it may be an obligatory step in the pathway) and we propose that such signal amplification is particularly relevant in situations where little Wnt is present but a signal must be transduced, such as in the shallow part of a Wnt gradient. While Fz2-Arr activated receptor mimics the initiation step but is deficient in amplifying the signal, another Arrow-derived construct (tor^DArr) exclusively and efficiently amplifies the signal.  Thus, Fz2-Arr and tor^DArr demonstrate two separate functions of wild type Arrow.  Co-expressed Fz2-Arr and tor^DArr can largely replace Arrow function, reconstituting the system (Baig-Lewis et al., 2007).

Our contributions (so far)

• identified Arrow/LRP as a Wnt pathway component (Wehrli et al. 2001)
• functionally showed that Arrow functions as a receptor subunit (Tolwinski, Wehrli et al., 2003)
• designed a constitutively activated receptor by fusion of the two receptor subunits Fz2 and Arrow (Tolwinski, Wehrli et al., 2003)
• identified the scaffold protein Axin as a binding partner of Arrow (Tolwinski, Wehrli et al., 2003)
• showed in vivo that Arrow has a double function in the Wnt pathway in that it first functions as a receptor subunit in the initiation of the signaling cascade and subsequently serves to potentiate this signal through inactivation of Axin.  This Arrow-Axin interaction appears particularly relevant to long-range signaling in a Wnt gradient (Fig.2,3; Baig-Lewis et al., 2007)
• a systematic analysis of the scaffold protein Axin, using designed Axin mutant proteins, revealed a surprisingly robust assembly of the multiprotein complex in vivo.  This was the first such analysis in a higher eukaryote and demonstrates that the complete loss of a binding site on Axin can be largely compensated for by secondary interaction between binding partners.  This predicts that single point mutations will only mildly compromise Axin function and, similarly, might have only mild effects if such mutations are present in Axin’s binding partners (Fig. 4; in preparation)

Figure 4. Axin complex assembly is robust in vivo. Schematic representation of a protein complex assembled around Axin, which includes APC, Shaggy kinase (Sgg) and the fly beta-catenin, Armadillo (Arm). Interactions between APC, Sgg and Arm, become critically important if a binding site on Axin is lost, for example if the APC binding site RGS is deleted (∆RGS).  Red bars show known interactions.

Potential Rotation Projects:

• examining novel Wg pathway components
• investigating how Wg signaling changes the location of pathway components using a confocal microscope
• precisely timed disruption of Wg signal transduction using engineered temperature-sensitive components
• examining how microRNAs modulate Wnt signaling
• analyzing how Axin protein complex stability in vivo can be manipulated to exploit it for the treatment of cancer
• identifying specific kinase isoforms relevant to Wg signaling using GFP traps

Lab members:

Collaborators (local):

                        Jan Christian, Professor
                        Shailaja Sopory, Ph.D., Postdoc
                        Naz Erdeniz, Research Assistant Professor

Publications

To contact Dr. Wehrli directly: wehrlim@ohsu.edu