Pfizer Fellow 2005
Jasper Weinberg, Carnegie Mellon University
Mentor: Dr. Brooke McCartney
Adenomatous Polyposis Coli 2 May Act with Diaphanous and Zeste White 3 in Organizing the Cytoskeleton in Early Drosophila Development
In the syncytial embryo, there are 13 rounds of synchronous nuclear division without cytokinesis accompanied by cyclic reorganizations of actin, from caps over interphase nuclei to rings and furrows around dividing nuclei. Defects in organizing actin into furrows between the dividing nuclei can result in nuclear collisions and improper mitosis which lead to nuclei detaching from the cortex and falling into the embryo. In the Wnt/Wingless signaling pathway, Adenomatous polyposis coli 2 (APC2), Zeste White 3 (Zw3), and Axin act together in a complex to promote the destruction of Armadillo. In addition, we have proposed that a complex of APC2, Zw3 and Armadillo could function to tether mitotic spindles to the actin cortex in early embryos. However, our analysis of embryos null for APC2 suggests that APC2 may also have a role in organizing actin into rings and pseudocleavage furrows. To test this hypothesis, I observed actin dynamics in live wild type and mutant embryos expressing a fluorescent actin tag using a spinning disk confocal microscope. By analyzing time-lapse movies of actin dynamics in syncytial embryos, I found that mutant embryos exhibited actin defects including small rings, indicative of nuclear loss, absence of cortical actin and incomplete rings. This suggests a role for APC2 in actin organization or stability. To understand how APC2 influences these processes, I examined embryos reduced for Zw3 in a background null for APC2, which had more severe actin defects, suggesting that Zw3 may be acting with APC2 in this process. The formin Diaphanous (Dia) has a mutant phenotype in syncytial embryos similar to APC2 and has been shown to interact with APC1 in mammalian systems. Embryos reduced for Dia and mutant for APC2 have a more severe phenotype then APC2 mutants alone, indicating that Dia and APC2 may be acting in a complex with Zw3 to organize or stabilize actin. My observations of a possible genetic interaction between APC2 and Dia is consistent with other data from the lab showing that APC2 and Dia colocalize in and coimmunoprecipitate from syncytial embryos. In humans, mutations in APC1 are linked to inherited colon cancers. One way that loss of APC1 could promote cancer is by disrupting mitosis due to the loss of a cytoskeletal organizer function. Therefore, understanding APC2's role in cytoskeletal organization may be important for understanding how APC1 mutations promote cancer in humans.