Glypican4 mediates Wnt transport between germ layers via signaling filopodia

Glypicans influence signaling pathways by regulating morphogen trafficking and reception. However, the underlying mechanisms in vertebrates are poorly understood. In zebrafish, Glypican 4 (Gpc4) is required for convergence and extension (C&E) of both the mesoderm and endoderm. Here we show that transgenic expression of GFP-Gpc4 in the endoderm of gpc4 mutants rescues C&E defects in all germ layers. The rescue of mesoderm was likely mediated by Wnt5b and Wnt11f2, and depended on signaling filopodia rather than on cleavage of the Gpc4 GPI anchor. Gpc4 bound Wnt5b and regulated formation of the filopodia that transport Wnt5b to neighboring cells. Blocking signaling filopodia that extend from endodermal cells suppressed this rescue. Thus, endodermal signaling filopodia that expressed GFP-Gpc4 transported Wnt5b, and likely Wnt11f2, to other germ layers, rescuing the C&E defects caused by a gpc4 deficiency. Our study reveals a new mechanism that could explain in vivo morphogen distribution involving Gpc4.