Current Research

The Drosophila male germline is a well-established stem cell model system, and Drosophila offers powerful genetic tools with the ability to study cellular dynamics in vivo. A defined niche of post-mitotic cells called the hub supports two resident stem cell populations: germline stem cells (GSCs) and somatic cyst stem cells (CySCs). GSCs undergo oriented mitosis to self-renew and to give rise to gonialblasts, which undergo four mitoses with incomplete cytokinesis before entering meiosis and ultimately becoming functional sperm. CySCs also divide to produce CySCs as well as progeny called cyst cells that are post-mitotic and function to envelop and provide cues for developing germ cells. We are currently investigating four areas of stem cell dynamics within this system. First, we are using genetics and expression profiling to identify new factors required for the self-renewal of GSCs and CySCs. Second, we are studying the factors and mechanisms that control competition between stem cells for limited access to niche space. Third, we are investigating the role of conserved metabolism genes in the maintenance and/or differentiation of GSCs and CySCs. Finally, we are using transplantation approaches and in vivo RNAi to determine factors that control the maintenance of both stem cell populations in the testis during aging.

In both invertebrates and vertebrates, competitive interactions between cells during development influence their ability to contribute to the adult. Faster-growing cells called ‘winners’ induce the apoptotic death of neighboring, slower-growing cells called ‘losers’. While Myc regulation, ribosome biogenesis, and Wingless signaling are known to control cell competition, we have recently added JAK/STAT signaling to the list of cell competition regulators. We showed that Drosophila wing imaginal disc cells lacking STAT become losers and are killed by neighboring wild-type cells. In contrast, wing cells with hyperactivated STAT become ‘supercompetitors’ (i.e., winners that cause the apoptotic death of wild-type cells located several cell diameters away). Interestingly, we demonstrated that JAK/STAT-dependent cell competition does not require Myc, Yorkie, Wingless signaling, or ribosome biogenesis. These results strongly suggest STAT, Wingless, and Myc are parallel regulators of cell competition at least in Drosophila and possibly in other organisms as well. As hyperactivated STATs are causal to tumorigenesis and stem cell niche occupancy, our results have therapeutic implications for cancer and regenerative medicine. We are currently engaged in experiments to determine which genes in STAT supercompetitors are required for their competitive advantage.

The JAK/STAT pathway is pleiotropic and evolutionarily conserved in metazoans. The Drosophila pathway has lower genetic complexity with only a single JAK called Hop and a single STAT called Stat92E, compared with four Jaks and seven STATs in mammals. A gain-of-function mutation in the Drosophila JAK HopTum-l was the first to link sustained activation of the JAK/STAT pathway to oncogenesis. This mutation causes an overproliferation of myeloid-like cells resulting in a fly leukemia and lethality. These phenotypes can be suppressed by reducing the genetic dose of Stat92E, suggesting that modulators of Stat92E activity may be linked to oncogenesis and are potential targets for therapeutic intervention in higher organisms. We have generated the only Stat92E gain-of-function protein by removing the first 133 and the last 36 amino acids, Stat92EΔNΔC, which transactivates a STAT transcriptional reporter in S2 cells without stimulation by cytokines or activation of Hop. In addition, clonal misexpression of Stat92EΔNΔC induces melanotic tumors in flies. We are currently performing a whole genome RNAi screen in Drosophila cultured cells for modifiers of STATΔNΔC. We are using genetic techniques in Drosophila to assay if candidate genes enhance or suppress blood cell tumors (melanotic tumors) in vivo caused by clonal misexpression of STATΔNΔC or HopTum-l. Our ultimate goal is to ascertain if the mammalian orthologs of candidate genes impact JAK/STAT signaling and oncogenic potential in human cells from patients with myeloproliferative neoplasms (MPN) and in Jak2V617F MPN murine models.