Caryn Navarro, Ph.D.
Assistant Professor, Biomedical Genetics, Boston University School of Medicine
Post-doctoral fellow, Skirball Institute/NYU Medical Center
Ph.D. Biochemistry, The University of Connecticut
B.S. Biochemistry, Indiana University
Molecular motor activity and the microtubule network are important for many biological processes such as cell division, organelle positioning, intracellular transport, vesicle sorting and intracellular pathogen targeting. The dynein motor complex, which travels along the microtubule network carrying cargos of various sorts in a minus end directed fashion, plays a major role in all of these processes. An understanding of how the dynein complex recognizes its cargos, how directionality of movement arises and what cellular processes regulate complex movement will provide insight into the nature of diseases such as cancer and neurodegeneration.
Our research uses the Drosophila ovary to understand the mechanisms of dynein directed molecular transport and how intracellular transport is affected by mutations in piRNA (piwi associated small RNA molecules) pathway components. Drosophila oogenesis provides a good model system to address these questions since directional transport is important for the establishment of oocyte fate and polarity, and many of the genes important for this process are conserved between Drosophila and higher eukaryotes. Furthermore, these processes can be visualized in vivo, and the well-established genetics of Drosophila makes this system easy to manipulate.
Biochemical and Genetic Identification of Dynein Complex Associated Molecules. Early in Drosophila oogenesis a cluster of 16 interconnected cells forms from four rounds of incomplete cell division. RNA and protein molecules are transported to one of the cells with four connections from the other 15 supporting cells. The cell that accumulates the majority of these molecules will become the oocyte. This directional transport is microtubule and dynein dependent. Several female sterile mutations have phenotypes that resemble mutations in dynein complex members. Microtubule based transport to the oocyte does not occur in ovaries from these mutant mothers. One of the genes associated with the mutations is egalitarian (egl). Previously, we identified the dynein motor associated protein, Dynein Light Chain (Dlc), as an Egl interactor. This interaction provided the first physical link between a molecule involved in directed localization to the oocyte and a core motor component. This interaction proved to be important for the maintenance of oocyte fate (see picture below).
In addition to using biochemical approaches to finding dynein motor complex interactors we have used a genetic approach. We have carried out a mutagenesis screen to identify genes which, when mutant, affect molecular transport during oogenesis. We identified 11 complementation groups, which likely contain novel oogenesis genes. Using both biochemical and genetic approaches we are working to identify additional dynein complex components. The identification of these molecules could have implications in cancer and neurodegenerative disease research.
Altered Regulation of the Dynein Motor Complex in the piRNA pathway mutant germ-line. The piRNA pathway is responsible for silencing selfish elements such as retrotransposons and repetitive elements in the Drosophila germ-line, independently of many known mi/siRNA components. We have found that the silencing of these retroelements is important for proper dynein movement both to and within the oocyte. Increased levels of retrotransposon RNAs activate a DNA surveillance checkpoint, possibly due to a high level of unrepaired double strand breaks. Activation of the checkpoint leads to ribonucleotide particle aggregation (see figure). These aggregates resemble aggresomes, which form in response to a high level of unfolded proteins or upon viral infection, and contain members of the dynein motor complex, such as Dynein Heavy Chain, Egalitarian and Bicaudal-D (BicD), as well as retrotransposon and embryonic patterning RNA molecules. We think these aggresome like structures are sites of retrotransposon degradation. Retrotransposon silencing within the germ-line is important to maintain genomic integrity, which is vital for the survival of future generations. We are interested in understanding how these aggregates from in response to the DNA damage checkpoint. In addition we are further characterizing these aggregates in terms of what molecules they contain and what cellular function they serve. We are currently employing genetic, biochemical and cell biological methods to answer these questions. Additionally, since dynein-dependent aggregates are associated with many neurodegenerative diseases such as Alzheimer’s and ALS it will be interesting to see how similar the mechanisms of aggregation are between the two systems.
Morris, J*., Navarro, C*., and Lehmann, R. (2003) Identification and Analysis of Mutations in bob, Doa, and Eight New Genes Required for Oocyte Specification and Development in Drosophila melanogaster. Genetics 164(4) 1435-1446.
Navarro, C*., Puthalakath H*., Adams, JM., Strasser, A., and Lehmann, R. (2004) Egalitarian Binds Dynein Light Chain to Establish Oocyte Polarity and Maintain Oocyte Fate. Nature Cell Biology. 6(5) 427-435.
Navarro, C., Bullock, SL., and Lehmann, R. (2009) Altered Dynein-dependent transport in piRNA pathway mutants. Proc Natl Acad Sci U.S.A. 106(24) 9691-6
Morris, J., Lehmann, R. and Navarro, C. (2000) PARallels in Axis Formation. Science 288,1759-60.
Navarro, C., Lehmann, R., and Morris, J. (2001) Oogenesis: Setting one Sister Above the Rest. Curr Biol. 11(5):R162-5.