Pvr is a receptor tyrosine kinase (RTK) that functions in Drosophila testis cyst stem cells. Kenneth Hammer, Kelli Johnson, Judy Leatherman. Biological Sciences, University of Northern Colorado, Greeley, CO.

   The Drosophila testis niche is an excellent model for studying adult stem cells. This niche is composed of a group of somatic cells called the hub, surrounded by two stem cell populations, germline stem cells (GSCs) and cyst stem cells (CySCs). Extracellular signal-regulated kinase (ERK) signaling activated by epidermal growth factor receptor (EGFR) promotes differentiation in cyst cells, leading to differentiation of neighboring germ cells (Kiger, et al., 2000; Tran, et al., 2000). However, dpERK accumulates in both CySCs and cyst cells, and it is not understood how differentiation is prevented in CySCs with ERK signaling. CySCs still accumulate dpERK upon EGFR ligand inhibition, suggesting another functional RTK (Schulz et al. 2002). We show that Pvr (PDGF-VEGF receptor) is an RTK expressed in the testis, and its ligand Pvf1 is restricted to the hub, suggesting Pvr is active in CySCs. Dominant negative cyst lineage Pvr showed modest CySC loss, while constitutive Pvr promoted proliferation of cyst lineage cells away from the hub. Interestingly, constitutive Pvr did not promote differentiation of normal GSCs, while constitutive Ras alone did promote rapid GSC differentiation. Thus, active Pvr promotes some stem-like characteristics, suggesting that Pvr signaling is distinct from ERK signaling alone. This distinction may explain why CySCs with dpERK accumulation do not differentiate. We are also investigating if CySC-restricted Zfh-1 may influence RTK signaling. Zfh-1 is a member of the ZEB family, which promote epithelial to mesenchymal transition. ZEB factors impart resistance to EGFR-induced senescence and EGFR inhibitors, and Zfh-1 was identified in an RNAi screen as an RTK modifier (Brabletz and Brabletz, 2010, Friedman and Perrimon, 2006). In a preliminary approach, we are characterizing Zfh-1s effect on ERK signaling in cultured cells. We found that stimulated S2 cells accumulated high levels of dpERK, and Zfh-1 knockdown slightly inhibited dpERK accumulation. Future work will clarify whether Zfh-1 differentially regulates EGFR versus Pvr pathway activation.