Loss of the Heparan Sulfate Proteoglycan Glypican5 facilitates long-range Shh signaling.

As a morphogen, Sonic Hedgehog (Shh) mediates signaling at a distance from its sites of synthesis. After secretion, Shh must traverse a distance through the extracellular matrix (ECM) to reach the target cells and activate the Hh response. Extracellular matrix proteins, in particular the heparan sulfate proteoglycans (HSPGs) of the glypican family have both negative and positive effects on Shh signaling, all attributed to their ability to bind Shh. Using mouse embryonic stem cell-derived mosaic tissues with compartments that lack the glycosyltransferases Exostosin1 (Ext1) and Exostosin2 (Ext2), or the HSPG core protein Glypican5, we show that Shh accumulates around its source cells when they are surrounded by cells that have a mutated extracellular matrix. This accumulation of Shh is correlated with an increased non-cell autonomous Shh response. Our results support a model in which Shh presented on the cell surface accumulates at or near ECM that lacks HSPGs, possibly due to the absence of these Shh sequestering molecules. SIGNIFICANCE STATEMENT: Sonic Hedgehog (Shh) is a signaling molecule that despite its association with molecules in the extracellular matrix signals over several cell diameters. The authors used mosaic neural organoids comprised of genetically distinct cells to assess the requirement of heparan sulfate proteoglycans exclusively in the Shh producing cells, the Shh-transporting cells and the cells responding to Shh. The results show that Shh transport, but the Shh response is not inhibited by Heparan Sulfate modified Glypican5, a component of the extracellular matrix. The facilitated Shh transport observed by Glypican5 null cells provides an elegant explanation for why Glypican5 is a tumor suppressor. © AlphaMed Press 2019.