Acts as a coreceptor with members of the frizzled family of seven-transmembrane spanning receptors to transduce signal by Wnt proteins. Activates the canonical Wnt signaling pathway that controls cell fate determination and self-renewal during embryonic development and adult tissue regeneration (PubMed:11956231). In particular, may play an important role in the development of the posterior patterning of the epiblast during gastrulation (PubMed:15142971). During bone development, regulates osteoblast proliferation and differentiation thus determining bone mass (PubMed:11956231). Mechanistically, the formation of the signaling complex between Wnt ligand, frizzled receptor and LRP5 coreceptor promotes the recruitment of AXIN1 to LRP5, stabilizing beta-catenin/CTNNB1 and activating TCF/LEF-mediated transcriptional programs (By similarity). Acts as a coreceptor for non-Wnt proteins, such as norrin/NDP. Binding of norrin/NDP to frizzled 4/FZD4-LRP5 receptor complex triggers beta-catenin/CTNNB1-dependent signaling known to be required for retinal vascular development (By similarity). Plays a role in controlling postnatal vascular regression in retina via macrophage-induced endothelial cell apoptosis (PubMed:11956231). Homodimer; disulfide-linked. Forms phosphorylated oligomer aggregates on Wnt-signaling (PubMed:12581525). Component of a WNT-signaling complex that contains a WNT protein, a FZD protein and LRP5 or LRP6. Interacts with FZD8; the interaction is formed on WNT-binding and signaling. Interacts (via the phosphorylated PPPSP motif domains) with AXIN1; the interaction prevents inhibition of beta-catenin phosphorylation and signaling and is enhanced in the presence of GSK3B and WNT1 or WNT3A. Interacts (via beta-propeller regions 3 and 4) with DKK1; the interaction, enhanced by MESD and/or KREMEN, inhibits beta-catenin signaling by preventing GSK3-mediated phosphorylation of the PPPSP motifs and subsequent, AXIN1 binding. Interacts with CSNK1E. Interacts with SOST; the interaction antagonizes canonical Wnt signaling. Interacts with APCDD1 (By similarity). Interacts with MESD; the interaction prevents the formation of LRP5 aggregates, targets LRP5 to the plasma membrane and, when complexed with KREMEN2, increases DKK1 binding (PubMed:12581525). Interacts with CAPRIN2 (By similarity). Chaperoned to the plasma membrane by MESD. A number of isoforms are produced. Widely expressed, with the highest expression levels in liver, heart, and lung and the lowest levels in brain and spleen. Expressed in early embryo throughout the ectoderm at 6.5 dpc and in visceral endoderm overlying the extraembryonic ectoderm at 7.5 dpc. Not present in the mesoderm nor in endoderm emerging from the primitive streak (PubMed:15142971). Expressed in differentiating osteoblasts that contribute to the lateral membranous part of clavicle at embryonic day 13.5. Expressed in osteoblasts lining the bony trabeculae of the humerus at embryonic day 16.5. Expressed in osteoblasts on both surfaces of the temporal bone at embryonic day 17.5 (PubMed:11719191). Phosphorylation of cytoplasmic PPPSP motifs regulates the signal transduction of the Wnt signaling pathway through acting as a docking site for AXIN1. Mice exhibit decreased osteoblast proliferation, developing low bone mass postnatally. Also display persistent embryonic eye vascularization due to a failure of macrophage-induced endothelial cell apoptosis. Mutant animals exhibit a loss of middle phalanx ossification at 18.5 dpc. LRP5 and LRP6 double null mutants are more severely affected. Embryos arrest prior to mid-gestation. Belongs to the LDLR family.