LRP-1 Regulates Schwann Cell Motility by its Effects of the GTPases, Rac1 and Rho

Low-density lipoprotein receptor-related protein (LRP-1) functions in endocytosis and in cell signaling directly (binding signaling adapter proteins) or indirectly (by regulating levels of other cell signaling receptors). In the peripheral nervous system (PNS), LRP-1 is robustly expressed by Schwann cells after injury and is essential for Schwann cell survival. Schwann cell migration and cell-cell interactions are key mechanisms involved in peripheral nerve regeneration as well as in development. The pathways and mechanisms involved in these processes are only partially understood. Recently, we showed that LRP-1 signaling is critical for migration of Schwann cells. When LRP-1 was silenced the basal rate of Schwann cell migration was inhibited by greater than 85%. Herein, we demonstrate that balance between two small GTPases, well known to control the arrangement of actin cytoskeleton, were shifted in LRP-1 gene silenced Schwann cells. LRP-1 gene silencing was associated with decreased levels of activated GTP-bound Rac1, that was a direct result of impaired phosphatidylinosiol 3-kinase signaling. However, there were increased levels of activated GTP-bound RhoA. Consequently, larger and greater numbers of focal contacts were observed in LRP-1 silenced cells compared to control siRNA cells. These morphological changes permitted stronger attachment to extracellular matrix, rendering the cells immobile. Indeed, Schwann cell adhesion to fibronectin was increased 4-fold in LRP-1 gene silenced cells compared to control siRNA cells. Cell adhesion was also increased by addition of the LRP-1 antagonist, receptor-associated protein (RAP) into Schwann cell cultures. Blocking Rho activation with a specific pharmacological inhibitor, Y-27632 in LRP-1 silenced or GST-RAP treated Schwann cells decreased adhesion and increased migration on fibronectin. In contrast, LRP-1 silenced Schwann cells did not inhibit activated GTP-bound Rac 1 on vitronectin; consistent with no changes in morphology, adhesion and/or migration properties compared to control cells. The vitronectin receptor, UPAR, dependent cell signaling (increased PI3K and Rac 1 activity) was activated in LRP-1 silenced cells on vitronectin, thus facilitating a compensatory mobile phenotype. We also showed that PI3K is activated selectively in injured sciatic nerve by a mechanism involving LRP-1. These results establish the importance of LRP-1 in regulating Schwann cell motility after peripheral nerve injury through a mechanism involving Rac1 and RhoA activation.