LRP1 regulates Schwann cell mobility by its effect on the activity of the GTPases, Rac and Rho

After peripheral nerve injury, Schwann cell survival and migration are essential for successful nerve regeneration. The low-density lipoprotein receptor-related protein (LRP1) is robustly expressed by Schwann cells only after injury. LRP1 functions in endocytosis and in cell signaling, directly, in response to ligand-binding, and indirectly, by regulating levels of other cell-signaling receptors. Because LRP1 increases the activity of phosphatidyl-inositol 3-kinase (PI3K) and Akt, Schwann cell survival is promoted. However, it has been shown that activation of PI3K may also promote cell migration, thus, we hypothesized that LRP1 may orchestrate many aspects of the Schwann cell response to injury, including the transition from an immobile to a mobile phenotype. When the LRP1 antagonist, receptor-associated protein (RAP) was injected into axotomized nerves, activation of PI3K was inhibited at times corresponding to Schwann cell migration in vivo. When LRP1 was silenced in primary cultures of rat Schwann cells or when Schwann cells were treated with the LRP1 antagonist, receptor-associated protein (RAP), cell migration on fibronectin-coated surfaces was decreased by greater than 85% (p<0.01). This result was explained by inhibiting PI3K activation and altered activity of the small GTPases, Rac and RhoA. In LRP1 gene-silenced cells, GTP-loaded or activated Rac1 was substantially decreased whereas GTP-loaded RhoA was increased. Within one hour of plating, the morphology of LRP1 gene-silenced Schwann cells demonstrated greater number of focal adhesions and stress fiber formation. These morphological changes permitted stronger attachment to fibronectin, rendering the cells immobile. Indeed, Schwann cell adhesion to fibronectin was increased 4-fold (p<0.01) in LRP1 gene-silenced cells, compared with control cells that were transfected with non-targeting siRNA. Cell adhesion also was increased by RAP. Blocking Rho activation with the specific pharmacological inhibitor, Y-27632, in LRP-1 genesilenced Schwann cells, decreased adhesion and increased migration, confirming an important role for RhoA in the control of Schwann cell motility. The ability of LRP1 to promote the transition of Schwann cells from an immobile to a mobile phenotype supports a model in which LRP-1 regulates diverse aspects of the Schwann cell response to PNS injury.