Apontic acts as a JAK/STAT pathway regulator in the Drosophila testis niche. Kathryn A. Bus, Archana Murali, Michelle Starz-Gaiano. University of Maryland Baltimore County, Batimore, MD.

   Production and maintenance of adult stem cells depends upon a complex microenvironment, called a niche. Stem cells provide the basis for all subsequent differentiated tissues. Thus, the dynamics of the niche are complex and although some mechanisms of this environment are established, there are questions that need to be investigated. The Drosophila testis is an excellent model system to study the genetic and molecular interactions needed during stem cell self-renewal and differentiation. The testis niche is comprised of a group of cells, known as hub cells, which are surrounded by germ line stem cells (GSCs) and somatic cyst stem cells (CySCs). Several laboratories have shown that activation of the Janus Kinase/Signal Transducer and Activator of Transcription (JAK/STAT) pathway is necessary for maintaining both types of stem cells. We have shown that Apontic (APT), a transcription factor and STAT signaling feedback inhibitor, is highly expressed in the somatic stem cells of the testis, as well as in early daughter cells. When apt is overexpressed in the soma, there are fewer CySCs, while overexpression in the germ line has no effect. In apt loss of function mutants, we observe more Zfh-1-positive CySCs, and an expanded domain of GSCs. The apt mutant phenotype is distinct from those due to mutations in other STAT targets or regulators, such as socs36e. apt mutant cells display altered adhesion and morphological properties in ovarian follicle cells, and parallel changes may explain delayed CySC differentiation in the testis. Thus, additional CySCs in apt mutants may permit GSC self-renewal by acting as a secondary niche or altering the properties of the microenvironment. We are currently utilizing a mosaic clonal analysis to determine the effects of apt on the two cell populations and how it affects adhesion molecules in the niche environment. Thus, this work supports a new role for a Jak/Stat regulator, APT, in the testis niche and suggests APT is required for the maintenance of the stem cell populations.