Wnt5a causes ROR1 to complex and activate cortactin to enhance migration of chronic lymphocytic leukemia cells

Chronic lymphocytic leukemia cells (CLL) migrate between the blood and lymphoid tissues in response to chemokines. Such migration requires structured cytoskeletal-actin polymerization, which may involve the protein cortactin. We discovered that treatment of CLL cells with Wnt5a causes Receptor tyosin kinase-like orphan receptor 1 (ROR1) to bind cortactin, which undergoes tyrosine phosphorylation at Y421, recruits ARHGEF1, and activates RhoA, thereby enhancing leukemia-cell migration; such effects could be inhibited by cirmtuzumab, a humanized mAb specific for ROR1. We transfected the CLL-cell-line MEC1 with either full-length ROR1 or various mutant forms of ROR1 to examine the structural features required for binding cortactin. We found that the proline-rich domain (PRD) was necessary for ROR1 to recruit cortactin. We generated MEC1 cells that each expressed a mutant form of ROR1 with a single amino-acid substitution of alanine (A) for proline (P) in potential SH3-binding sites in the ROR1-PRD at positions 784, 808, 826, 841, or 850. In contrast to wild-type ROR1, or other ROR1^P=>A mutants, ROR1^P(841)A failed to complex with cortactin or ARHGEF1 in response to Wnt5a. Moreover, Wnt5a could not induce MEC1-ROR1^P(841)A to phosphorylate cortactin or enhance CLL-cell F-actin polymerization. Taken together, these studies show that cortactin plays an important role in ROR1-dependent Wnt5a-enhanced CLL-cell migration.