Loss of HS attenuates cell-surface binding of Wnt3a and R-spondin. (A) Flow cytometry analysis of cell-surface binding of Wnt3a-flag on control-transduced (EXT1 wild-type [WT]) or EXT1 KO LME1 cells. (B) Flow cytometry analysis of cell-surface binding of Wnt3a-flag to primary MM cells treated with heparitinase or buffer (control). (C) Flow cytometry analysis of cell-surface binding of R-spondin–his to control empty vector–transduced (control), EXT1 KO, and LGR4 KO LME1 cells. (D) Flow cytometry analysis of cell-surface binding of R-spondin–his on primary MM cells treated with heparitinase or buffer (control). (E) Flow cytometry analysis of Wnt reporter activity in control-transduced (EXT1 WT) or EXT1 KO HMCLs treated with Wnt3a, R-spondin–∆AA, or R-spondin–∆TSP condition medium or their combination. The mean ± standard deviation of 3 independent experiments in triplicate is shown. Not significant (ns) P > .05, *P ≤ .05 using 1-way analysis of variance with Bonferroni correction. (F) Model for syndecan-1 promotes Wnt signaling in MM. In human BM microenvironment, the stromal cells (niche cells) and MM cells secrete Wnt ligands, whereas (pre)osteoblasts (niche cells) produce R-spondins. HS chains decorating syndecan-1 promote autocrine and paracrine Wnt signaling and Wnt-mediated proliferation and survival in MM cells by presenting Wnt ligands and R-spondins.