November 28
PROGRAM

05

Glycoscience : diversity and integration

Heparan sulfate proteoglycans inthe stem cell niche :Lessons from Drosophila
Hiroshi Nakato, Ph.D.
Associate Professor
Department of Genetics, Cell Biology and Development,
The University of Minnesota, USA

Hiroshi Nakato
ProfileHiroshi Nakato received his Ph.D. degreein Biology from Tokyo Metropolitan University (1993). He has been studying thefunction of heparan sulfate proteoglycans(HSPGs) in development using Drosophila
genetics for more than 15 years. He firstbecame interested in this class of molecules
when he was a postdoctoral fellow in Dr.
Scott Selleck’s lab at the University of Arizona (1993-1995). During his postdoctoral studies in the Selleck lab, he identified
the division abnormally delayed (dally)
gene, which encodes a Drosophila HSPGof the glypican type, as a regulator of cell
division patterning in the developing CNS.
He continued genetic and biochemical
studies of HSPGs and HS modifying enzymes in Drosophila after becoming a faculty member of Tokyo Metropolitan University (1995) and the University of Arizona (2001). Dr. Nakato is currently an Associate Professor in the Department ofGenetics, Cell Biology and Development at
the University of Minnesota (2003-). He
currently serves as a reviewer in the NIH
Intercellular Interactions (ICI) Study Section (2010-2014). He is also the Presidentof the Minneapolis Japanese School, a nonprofit organization supported by the Japanese government. Dr. Nakato's main research interests are elucidating the mechanisms by which HSPGs regulate molecularrecognition events during developmental
processes, such as morphogen gradientformation, stem cell regulation, and woundhealing.

Stem cells are defined by their uniquepotential to self-renew and differentiate
into mature cell types. Stem cells typicallyreside in specialized microenvironments
called ‘niches’, which support their ability
to retain the characteristics of a stem cell.
The molecular basis underlying the regulation of the stem cell niche is poorly understood. For example, the contact between
niche cells and stem cells is a key component of niche control in many stem cell systems, but the molecular foundation of thiscontact is unknown. It is critical to explore
the regulatory networks controlling stem
cell niches for the advancement of cancer
therapies, studies on aging, and ultimately,
for the understanding of a universal mechanism that controls animal development.
We study the stem cell niche in the genetically tractable model organism Drosophila,
in which specific cell type markers as wellas elegant genetic techniques, are available.
We have demonstrated that heparan sulfate proteoglycans (HSPGs) are essential
regulators of the germline stem cell (GSC)
niche in the Drosophila ovary 1). In thefemale GSC niche, Decapentaplegic (Dpp;a Drosophila BMP), regulates the asymmetric division of GSCs. However, themechanism by which this secreted molecule differentially regulates the fate of thetwo daughter cells has been a mystery. We
found that Dally, a Drosophila HSPG of
the glypican type, is specifically expressedin the niche cells contacting the stem cells(red in Figure 1). Dally activates Dpp signaling in trans in a directly contacting
GSC. This trans co-receptor activity of
Dally explains the contact-dependencyof the GSC niche: when a GSC divides,
the daughter cell directly contacting theDally-expressing niche cells remains aGSC (Dpp signaling is ON), but the otherdaughter cell, which has lost contact, will
differentiate (Dpp signaling is OFF) (Figure 1, wild-type). In a dally mutant ovary,
the daughter cell directly contacting theniche cells cannot activate Dpp signaling(OFF), thus GSCs are lost to differentiation (Figure 1, dally mutant). When dally
is ectopically expressed in somatic cells
outside the niche, all germ cells contacting
ectopic Dally are maintained as GSCs byectopic activation of Dpp signaling (ON)(Figure 1, ectopic dally). Thus, the contactdependent signaling by HSPGs provides amechanism to define the physical space ofthe stem cell niche.
To further investigate the molecular mechanisms by which HSPGs regulate contactdependent BMP signaling, we developed
a novel in vitro assay system using Drosophila S2 tissue culture cells and analyzed
HSPG trans co-receptor activity 2). In this“single-cell BMP-HSPG trans signalingassay”, “signal-sending cells” (expressingDally) and “signal-receiving cells” (expressing BMP receptors) are co-cultured. Contact between the sending and receiving
cells is visualized by the split-GFP complementation system, and BMP signal activation in the receiving cells is monitored byphosphorylation of Mad protein. Using
this single-cell assay system, we demonstrated that HSPGs in trans enhance BMP
signaling in S2 cells in a contact-dependent
manner. The previously known mechanism for contact-dependent signaling isFigure 1. Contact-dependent Dpp signaling in the Drosophila germline stem cell niche.

communication mediated by membranebound ligands, such as the ligand proteinsfor the Notch receptor. Our study on thenew function of Dally provides a novel
mechanism to achieve contact-dependent
signaling, in which complementation ofsignaling complex components can spatially restrict the physical field of signaling. Importantly, this system is used ina specific, extremely important in vivo
context: the stem cell niche. I will present
the molecular mechanisms for this novelsignaling system between contacting cells,
revealed by a combination of in vitro andin vivo analyses.
Functional analyses of HSPGs in other
Drosophila stem cell systems are in prog-ress. We found that HSPGs are critical for
proliferation and maintenance of the maleGSCs, neural stem cells in the developing
CNS, and adult midgut stem cells. Thus,
our study highlights the universal and fundamental roles of HSPGs in the stem cellniche.

References1) 	Hayashi Y, Kobayashi S, and Nakato H. Drosophila glypicans regulate thegermline stem cell niche. J. Cell Biol. (187):473-480, 2009.
2) 	Dejima K, Kanai MI, Akiyama T, Levings D, and Nakato H. Novel contact-28

the 20th Anniversary of the Mizutani Foundation for Glycoscience

dependent bone morphogenetic protein (BMP) signaling mediated by heparan sulfate proteoglycans. J. Biol. Chem. (286): 17103-17111, 2011.

Commemorative Symposium

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