RESEARCH INTERESTS

1. How does the JAK/STAT pathway regulate stem cells numbers? Regulating the number of stem cells is a primary mechanism by which homeostasis is maintained and oncogenesis is prevented. Stem cells divide to produce daughter cells that renew the stem cell pool or that regenerate tissue by differentiating. The choice between self-renewal and differentiation must be tightly controlled as increasing the stem cell pool provides a condition for oncogenesis. Tumors have cancer stem cells that self-renew and establish new tumors at low numbers. One of the critical regulators of stem cell numbers in mammals is the JAK/STAT pathway. Furthermore, dominant-active mutations in jak and stat genes cause cancer, and Stat3 is a target for therapeutic intervention since its ablation blocks the growth of human cancer cells. Despite these compelling observations, the mechanisms utilized by this pathway to regulate stem cell numbers in mammals have not yet been elucidated, in part due to the redundancy of 4 jak and 7 stat genes.

Drosophila provides an ideal system to study how JAK/STAT signaling regulates stem cell numbers, as this function is conserved in several Drosophila tissues, including testis and eye. Unlike the redundancy of the mammalian system, Drosophila has only one jak and one stat gene (called stat92E), which allows facile in vivo analysis. Despite these advantages, nothing is known mechanistically about how this pathway controls stem cell populations in Drosophila. Previous work has shown that over-expression of the cytokine Unpaired, which activates JAK/STAT signaling, leads to an expansion of stem/progenitor cells in the eye and testis. We find that these overgrowths depend on activation of Stat92E within stem cells. Our current hypothesis is that Stat92E must regulate three distinct processes in stem cells in order to regulate their numbers: it must increase cellular mass and accelerate cell cycle progression and, after mitosis, promote self-renewal in some daughter cells. Since Stat92E is a transcription factor, discrete Stat92E target genes should mediate its effects on these processes. We have identified several genes with human homologs that may lie directly downstream of Stat92E and may regulate self-renewal, cellular growth and cell cycle in stem cells. The following are current research projects in the lab:

            Identify stem cell self-renewal target genes of Stat92E. Stat92E controls the self-renewal of stem cells in the testis. We have used micro-arrays to identify new JAK/STAT target genes. One of these – chinmo – is directly regulated by Stat92E and is required for self-renewal of testis stem cells. Having defined a Stat92E-Chinmo linear pathway, we are now using genetic approaches to assess whether other potential Stat92E target genes act between Stat92E and Chinmo to promote stem cell self-renewal.

            Characterize the role of the Stat92E in the cellular growth of stem cells.Cellular growth (or mass accumulation) is controlled by the rate of protein synthesis. Our data suggest that activated Stat92E regulates this process at the level of ribosome biogenesis. We are using genetic approaches and FACS analysis to test this hypothesis. Furthermore, we are assessing if Stat92E regulates cellular growth through dmyc, the sole Drosophila homolog of the Myc proto-oncogene and key growth regulator.

            Determine the targets of Stat92E in stem cell proliferation. Activation of JAK/STAT signaling leads to increased proliferation. Our data suggest that Stat92E regulates the G1/S and G2 cell cycle checkpoints. We are currently using genetic approaches to determine if Stat92E regulates cell cycle regulators, like Cyclin E and Cdc25, which control cell cycle progression. We are also assessing the roles of potential Stat92E target genes identified by our micro-array in regulating proliferation of stem cells.

2. How does the JAK/STAT pathway control pattern formation? We were the first to identify genes regulated by the JAK/STAT pathway that control patterning in Drosophila imaginal discs, which give rise to adult structures. In fact, our studies have revealed that the repression of the wingless (wg) gene, which encodes a Wnt-1 homolog, by activated Stat92E is a broad mechanism utilized during pattern formation in imaginal discs. For example, we showed that Stat92E promotes the formation of the eye field by autonomously repressing wg (Ekas, 2006). In addition, we recently reported that the JAK/STAT pathway regulates proximal-distal patterning in Drosophila limbs, in part through Stat92E’s autonomous repression of wg (Ayala-Camargo, 2007). To extend these studies, we are currently studying how the JAK/STAT pathway regulates pattern formation in the developing wing. Our preliminary data show that Stat92E is required for formation of the wing hinge, which attaches the wing blade to the body wall.

            Characterize how Stat92E regulates development of the wing hinge. Previous work has shown that the Zn finger transcription factor Teashirt (Tsh) and the TALE-class homeodomain Homothorax (Hth) are also required for the development of the hinge. We are using genetic and biochemical approaches to determine the relationship between Stat92E, Tsh and Hth. Furthermore, we are assessing whether several factors expressed specifically in the hinge, including Vein (Vn, an EGF ligand), Zfh-2 (a Zn finger homeodomain transcription factor), Dachsous (Ds, an atypical Cadherin), and Msh (a homeodomain protein), are regulated directly by JAK/STAT signaling. Since Tsh, Hth, Vn, Zfh-2, Ds and Msh are conserved in higher organisms, our studies are likely to provide valuable insights to conserved regulatory interactions between these proteins.