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Full Abstracts – STEM CELLS
135
42
robo2
is a JAK-STAT Target that Controls Stem Cell Maintenance in the
Drosophila
Testis Stem Cell Niche.
Rachel R. Stine, Erika L. Matunis. Johns
Hopkins Sch Med, Baltimore, MD.
Adult stem cells are essential for tissue regeneration and must be carefully regulated by signals from their surrounding niche. The
Drosophila
testis
contains a well-characterized stem cell niche consisting of a cluster of somatic hub cells surrounded by germline stem cells (GSCs) and somatic stem cells
called cyst stem cells (CySCs). Although JAK-STAT and BMP signals are known to be secreted from the hub, few other hub signals have been identified.
We find that
slit
, the only ligand for the
Drosophila
Robo/Slit pathway, is expressed specifically in hub cells. Slit signals via the Robo receptors to mediate
axon guidance, cell migration and cell adhesion in
Drosophila
and mammals, but Robo/Slit signaling has not been previously studied in the testis. We show
that the Slit receptor
robo2
is expressed in the somatic cells of the testis apex and that expression of
robo2
depends on JAK-STAT signaling. A luciferase-
based JAK-STAT response assay further indicates that
robo2
is a direct target of Stat92E. Mosaic analysis indicates that
robo2
mutant CySCs are rapidly
lost from the niche; this phenocopies
E-cadherin
mutant CySCs, suggesting that Robo2 may promote CySC adhesion to the niche. Multiple additional
Robo/Slit pathway members are expressed in the testis apex including an additional receptor,
robo
, and several downstream effectors including
Abelson
kinase
. As an important regulator of CySC maintenance, JAK-STAT is known to control genes important for self-renewal. We now show that JAK-STAT
may also regulate CySC adhesion through its newly discovered integration with Robo/Slit signaling. Further investigation will reveal if these two pathways
interact in other tissues where Robo/Slit signaling functions.
43
A model for formation of the follicle stem cell niche in the Drosophila ovary.
Stephanie Vlachos
1
, Ryan Conder
2
, Todd Nystul
3
, Nicholas Harden
1
. 1)
Molec Biol & Biochem, Simon Fraser Univerisity, Burnaby, BC, Canada; 2) Institute of Molecular Biotechnology, Dr. Bohr Gasse 3, 1030, Vienna, Austria;
3) Department of Anatomy, University of California, San Francisco, CA 41943.
The germarium in the Drosophila ovary is a popular structure for the study of stem cell niche formation. In the germarium, two or three germline stem cells
(GSCs) located at the anterior tip in region 1 give rise to germline cysts that move posteriorly and encounter two follicle stem cells (FSCs) on opposite sides
of the germarium at the region 2a/2b border, which encapsulate one cyst at a time in a monolayer of somatic follicle cells. The FSC niche is an example of a
dynamic somatic stem cell niche in which the stem cell microenvironment demonstrates characteristics of both classic and flexible stem cell niches. Using
females mutant for the p21-activated kinase Pak, we show that the FSC niche can encapsulate cysts when mispositioned in the germarium. Furthermore, we
provide evidence that the number of FSC niches is increased in pak mutant germaria, thus enabling the encapsulation of two or even three cysts at a time.
We propose a model for formation of the FSC niche in which the germline cyst determines the position of the niche.
44
Piwi and the Polycomb Group Proteins interact to regulate Drosophila ovarian germline.
Jamy C. Peng, Na Liu, Haifan Lin. Department of Cell
Biology, Yale Stem Cell Center, Yale University School of Medicine, New Haven, CT.
We use
Drosophila
ovarian germline, a classic model system of tissue stem cells, to probe the molecular relationship between Piwi and Polycomb Group
proteins. Piwi and its associated piRNAs epigenetically regulate the niche and the intrinsic mechanisms within stem cells to regulate germline stem cell
division and differentiation. Despite detailed characterization of Piwi functions, its mechanistic action remains elusive. The Polycomb Group proteins are
epigenetic modifiers that regulate key developmental genes to ensure proper stem cell differentiation. Using
Drosophila
genetic assays, we showed that Piwi
and the Polycomb Group complexes PRC1 and PRC2 functionally interact to maintain germline stem cell division and germline development. Biochemical
analyses revealed that Piwi physically associates with PRC2 core subunits, and that Piwi and PRC2 form a molecular complex in ovarian extract. Expression
analysis determined that the Polycomb Group complexes participate in the Piwi-piRNA pathway to regulate retrotransposons, thereby stabilizing germline
genome. Chromatin immunoprecipitation analysis indicated that Piwi binds selected Polycomb Group target sites to inhibit Polycomb Group functions; this
promotes RNA Polymerase II localization at these sites. Comparative epigenomic analysis of wild type and
piwi
mutant ovarian germline indicated that Piwi
specifically impacts Polycomb Group target genes that are regulators of transcription, developmental processes, and metabolism. These data led to our model
that Piwi interacts with Polycomb Group proteins to regulate gene expression profiles required for proper germline stem cell maintenance and
differentiation. Our study integrates Piwi and Polycomb Group proteins into a molecular pathway which regulates germline stem cell maintenance and
differentiation. This work is funded by an NIH NICHD Pioneer Award to HL (DP1 OD006825) and an American Cancer Society Postdoctoral Fellowship to
JP (115878-PF-08-160-01-DDC).
45
Insulin levels control Delta-Notch signaling in the
Drosophila
female germline stem cell niche via the regulation of FOXO on
fringe
.
Sheng-An Yang,
Hwei-Jan Hsu. Institute of Cellular and Organismic Biology, Academia Sinica, Taipei City, Taiwan.
Stem cells reside in a specialized microenvironment, or the niche, which regulates stem cells for their participation in tissue maintenance, regeneration and
repair. A functional niche can provide both physical contact and secreted factors to regulate stem cell retention and self-renewal, while the dysregulated
niche impairs stem cell function. The stem cell niche also plays a role to integrate niche-local and systemic signals that mediate the balanced response of
stem cells to the needs of organisms. How the niche itself is regulated and how these signals are merged in the niche, however, are largely unknown. We
have previously shown that insulin nutrient-sensing signaling directly controls the competence of the
Drosophila
female germline stem cell (GSC) niche to
respond to Notch ligands, and thereby the niche is maintained. Here, we further dissect the molecular and cellular mechanisms underlying these processes.
We show that Notch activity in cap cells (a major component of the niche) is positively or negatively controlled by
fringe
.
fringe
is required for Notch
activation, but excessive
fringe
also disturbs Notch activation. Further,
fringe
is up-regulated by FOXO in cap cells to suppress Notch activation when
insulin signaling is inactivated, and resulting in loss of cap cells, and thus loss of GSCs. Additionally,
fringe
expression is also regulated by mouse
FOXO1
,
suggesting that this regulation may be evolutionally conserved. Finally, we demonstrate that the Notch ligand, Delta, produced within the niche
predominately activates Notch signaling in cap cells. Our results reveal that FOXO-Fringe regulation serves as a bridge to link insulin signaling and Notch
signaling pathways in GSC niche in response to nutrition and highlight complex interactions between niche-local and systemic signals for proper stem cell
niche function.