Novel Contact-dependent Bone Morphogenetic Protein (BMP) Signaling Mediated by Heparan Sulfate Proteoglycans

Abstract

We previously proposed a model that DALLY, a Drosophila glypican, acts as a trans co-receptor to regulate BMP signaling in the germ line stem cell niche. To investigate the molecular mechanisms of contact-dependent BMP signaling, we developed novel in vitro assay systems to monitor trans signaling using Drosophila S2 cells. Using immunoblot-based as well as single-cell assay systems, we present evidence that Drosophila glypicans indeed enhance BMP signaling in trans in a contactdependent manner in vitro. Our analysis showed that heparan sulfate modification is required for the trans co-receptor activity of DALLY. Two BMP-like molecules, Decapentaplegic (DPP) and Glass bottom boat, can mediate trans signaling through a heparan sulfate proteoglycan co-receptor in S2 cells. The in vitro systems reflect the molecular characteristics of heparan sulfate proteoglycan functions observed previously in vivo, such as ligand specificity and biphasic activity dependent on the ligand dosage. In addition, experiments using a DALLY-coated surface suggested that DALLY regulates DPP signaling in trans by its effect on the stability of DPP protein on the surface of the contacting cells. Our findings provide the molecular foundation for novel contact-dependent signaling, which defines the physical space of the stem cell niche in vivo. Bone morphogenetic proteins (BMPs)2 play critical roles in cell-cell communication during animal development. Remarkably, these molecules mediate both long and short range signaling dependent on context. For example, Decapentaplegic (DPP), a Drosophila homologue of BMPs, acts as a long range morphogen in the developing wing and as a contact-dependent niche factor in the female germ line stem cell (GSC) niche. The molecular basis underlying this differential activity of BMPs is not fully understood.Signaling and distribution of BMPs in a tissue are modulated by a class of carbohydrate-modified molecules, heparan sulfate proteoglycans (HSPGs) (1-3). In addition to BMPs, HSPGs serve as co-receptors for a number of other growth factors and morphogens, including FGF, WNT, and Hedgehog (4). HSPGs generally are thought to regulate growth factor signaling on the surface of the signal-receiving cells (5). It has been reported, however, that HSPGs can regulate signaling in trans from neighboring cells in some cases (6, 7).Recent studies demonstrated that HSPGs are essential regulators of the GSC niche in the Drosophila ovary (8, 9). Although it has been well established that DPP regulates the asymmetric division of a GSC (10), the mechanism by which this secreted molecule differentially regulates two daughter cells has been a mystery. We have found previously that DALLY, a Drosophila HSPG of the glypican type, is expressed specifically in the somatic niche cells (the cap cells) contacting GSCs and is required for GSC maintenance (8). Ectopic dally expression in somatic cells in a wide region of the germarium was sufficient to maintain all the contacting germ line cells as GSC-like undifferentiated cells, thus e...

Morphogen transport

“…Similar to the hindered diffusion model of FGF transport discussed above, transient binding of Dpp to HSPGs might slow the diffusion of Dpp (Panganiban et al, 1990a;Fujise et al, 2003;Belenkaya et al, 2004;Han et al, 2004;Takei et al, 2004;Akiyama et al, 2008;Dejima et al, 2011). Reduced levels of Dpp are observed in HSPG-deficient tissues (Fujise et al, 2003;Belenkaya et al, 2004;Han et al, 2004;Takei et al, 2004;Akiyama et al, 2008), and increased levels of HSPGs in the source (Müller et al, 2012) using a two-dimensional geometry similar to the geometry described by Yu et al (Yu et al, 2009).…”

Relay of retrograde synaptogenic signals through axonal transport of BMP receptors

“…This antibody recognizes Drosophila phosphorylated Mad (Figs 8, 9) (Dejima et al, 2011), but does not recognize phosphorylated dSmad2 (Mike O'Connor, personal communication) (Fig. 9).…”

Adaptive protein divergence of BMP ligands takes place under developmental and evolutionary constraints

“…It has been previously reported that the heparan sulfate proteoglycan (HSPG) Dally impacts BMP signaling in various contexts. Dally plays a role in Dpp gradient formation in the wing imaginal disc by stabilizing Dpp (Akiyama et al, 2008;Dejima et al, 2011) and it increases the signaling of Gbb and Dpp in Drosophila S2 cells (Dejima et al, , is either less efficiently secreted or less stable. The significance was calculated from five independent experiments (n=5).…”