Dally regulates Dpp morphogen gradient formation by stabilizing Dpp on the cell surface

The glypican Dally is required in the niche for the maintenance of germline stem cells and short-range BMP signaling in the Drosophila ovary

The Drosophila ovary is an excellent system with which to study germline stem cell (GSC) biology. Two or three female GSCs are maintained in a structure called a niche at the anterior tip of the ovary. The somatic niche cells surrounding the GSCs include terminal filament cells, cap cells and escort stem cells. Mounting evidence has demonstrated that BMP-like morphogens are the immediate upstream signals to promote GSC fate by preventing the expression of Bam, a key differentiation factor. In contrast to their morphogenic long-range action in imaginal epithelia, BMP molecules in the ovarian niche specify GSC fate at single-cell resolution. How this steep gradient of BMP response is achieved remains elusive. In this study, we found that the glypican Dally is essential for maintaining GSC identity. Dally is highly expressed in cap cells. Cell-specific Dally-RNAi, mutant clonal analysis and cell-specific rescue of the GSC-loss phenotype suggest that Dally acts in the cap cells adjacent to the GSCs. We confirmed that Dally facilitated BMP signaling in GSCs by examining its downstream targets in various dally mutants. Conversely, when we overexpressed Dally in somatic cells outside the niche, we increased the number of GSC-like cells apparently by expanding the pro-GSC microenvironment. Furthermore, in a genetic setting we revealed a BMP-sensitivity distinction between germline and somatic cells, namely that Dally is required for short-range BMP signaling in germline but not in somatic cells. We propose that Dally ensures high-level BMP signaling in the ovarian niche and thus female GSC determination.

The Drosophila DPP signal is produced by cleavage of its proprotein at evolutionary diversified furin-recognition sites

Maturation of bone morphogenetic proteins (BMPs) requires cleavage of their precursor proteins by furin-type proprotein convertases. Here, we find that cleavage sites of the BMP2/4/decapentaplegic (DPP) subfamily have been evolutionary diversified and can be categorized into 4 different types. Cnidaria BMP2/4/DPP is considered to be a prototype containing only 1 furin site. Bilateria BMP2/4/DPP acquired an additional cleavage site with either the combination of minimal-optimal or optimal-optimal furin sites. DPPs belonging to Diptera, such as Drosophila and mosquito, and Lepidoptera of silkworm contain a third cleavage site between the 2 optimal furin sites. We studied how the 3 furin sites (FSI-III) of Drosophila DPP coordinate maturation of ligands and contribute to signals in vivo. Combining mutational analysis of furin-recognition sites and RNAi experiments, we found that the Drosophila DPP precursor is initially cleaved at an upstream furin-recognition site (FSII), with consequent cleavages at 2 furin sites (FSI and FSIII). Both Dfurin1 and Dfurin2 are involved in the processing of DPP proproteins. Biochemical and genetic analyses using cleavage mutants of DPP suggest the first cleavage at FSII to be critical and sufficient for long-range DPP signaling. Our data suggest that the Drosophila DPP precursor is cleaved in a different manner from vertebrate BMP4 even though they are functional orthologs. This indicates that the furin-cleavage sites in BMP2/4/DPP precursors are tolerant to mutations acquired through evolution and have adapted to different systems in diversified species.

'Big frog, small frog'--maintaining proportions in embryonic development: delivered on 2 July 2008 at the 33rd FEBS Congress in Athens, Greece

We discuss mechanisms that enable the scaling of pattern with size during the development of multicellular organisms. Recently, we analyzed scaling in the context of the early Xenopus embryo, focusing on the determination of the dorsal-ventral axis by a gradient of BMP activation. The ability of this system to withstand extreme perturbation was exemplified in classical experiments performed by Hans Spemann in the early 20th century. Quantitative analysis revealed that patterning is governed by a noncanonical 'shuttling-based' mechanism, and defined the feedback enabling the scaling of pattern with size. Robust scaling is due to molecular implementation of an integral-feedback controller, which adjusts the width of the BMP morphogen gradient with the size of the system. We present an 'expansion-repression' feedback topology which generalizes this concept for a wider range of patterning systems, providing a general, and potentially widely applicable model for the robust scaling of morphogen gradients with size.

Glypicans

The glypican family of cell-surface heparan sulfate proteoglycans modulate the actions of many developmentally important signal proteins.

Glypicans are heparan sulfate proteoglycans that are bound to the outer surface of the plasma membrane by a glycosyl-phosphatidylinositol anchor. Homologs of glypicans are found throughout the Eumetazoa. There are six family members in mammals (GPC1 to GPC6). Glypicans can be released from the cell surface by a lipase called Notum, and most of them are subjected to endoproteolytic cleavage by furin-like convertases. In vivo evidence published so far indicates that the main function of membrane-attached glypicans is to regulate the signaling of Wnts, Hedgehogs, fibroblast growth factors and bone morphogenetic proteins (BMPs). Depending on the context, glypicans may have a stimulatory or inhibitory activity on signaling. In the case of Wnt, it has been proposed that the stimulatory mechanism is based on the ability of glypicans to facilitate and/or stabilize the interaction of Wnts with their signaling receptors, the Frizzled proteins. On the other hand, GPC3 has recently been reported to inhibit Hedgehog protein signaling during development by competing with Patched, the Hedgehog receptor, for Hedgehog binding. Surprisingly, the regulatory activity of glypicans in the Wnt, Hedgehog and BMP signaling pathways is only partially dependent on the heparan sulfate chains.

The Decapentaplegic morphogen gradient: a precise definition

Two key processes are in the basis of morphogenesis: the spatial allocation of cell types in fields of naïve cells and the regulation of growth. Both are controlled by morphogens, which activate target genes in the growing tissue in a concentration-dependent manner. Thus the morphogen model is an intrinsically quantitative concept. However, quantitative studies were performed only in recent years on two morphogens: Bicoid and Decapentaplegic. This review covers quantitative aspects of the formation and precision of the Decapentaplegic morphogen gradient. The morphogen gradient concept is transitioning from a soft definition to a precise idea of what the gradient could really do.