Activates the canonical Wnt signaling pathway through FZD4 and LRP5 coreceptor. Plays a central role in retinal vascularization by acting as a ligand for FZD4 that signals via stabilizing beta-catenin (CTNNB1) and activating LEF/TCF-mediated transcriptional programs. Acts in concert with TSPAN12 to activate FZD4 independently of the Wnt-dependent activation of FZD4, suggesting the existence of a Wnt-independent signaling that also promote accumulation the beta-catenin (CTNNB1). May be involved in a pathway that regulates neural cell differentiation and proliferation. Possible role in neuroectodermal cell-cell interaction (By similarity). Homodimer; disulfide-linked. Component of a complex, at least composed of TSPAN12, FZD4, LRP5/6 and norrin (NDP). Binds FZD4 with high affinity. Interacts with LRP6 (via Beta-propellers 1 and 2).