Mutations in human PMP22 cause the most common forms of inherited peripheral nerve diseases, which include hereditary neuropathy with liability to pressure palsies (HNPP) with heterozygous deletion of PMP22. Patients with HNPP presents with focal sensory loss and muscle weakness that are related to the conduction block. We have discovered that PMP22 deficiency impairs action potential propagation in the absence of segmental demyelination. This conduction defect is likely resulted from abnormally increased permeability of myelin through disruption of myelin junctions. In the present study, we will investigate how junction complexes are disrupted through their abnormal maintenance in PMP22 deficient nerves. Aim 1: Test the hypothesis that PMP22 deficiency increases actin polymerization at myelin junctions, leading to removal of myelin junction protein complexes. Our preliminary study has demonstrated a remarkable increase of F-actin overlaps with the areas that show disruption of junction protein complexes. We will utilize genetically labeled F-actin mouse model to track dynamics of F-actin formation. We will further test how PMP22 deficiency activates the actin polymerization. Aim 2: Removal of p21-activated kinase (PAK1) reverses the increase of F-actin in PMP22 deficient nerves, and thereby prevents myelin junction complexes from being disrupted. Our preliminary results show an abnormal increase of PAK1 activity in Pmp22+/- nerves. Ablation of Pak1 in Pmp22+/- mice reverses abnormal myelin permeability. We will use the double knockout mice (Pmp22+/- /Pak1-/-) to determine whether removal of PAK1 suppresses the formation of F-actin, and dislocation of junction complexes in HNPP mouse model. Moreover, utilizing our newly produced transgenic mouse expressing PAK-inhibiting peptides under the control of inducible Schwann cell-specific cre, we will determine the temporal requirement in rescuing junction abnormalities via PAK inhibition. Aim 3: Systemic administration of PAK1 inhibitor reverses the phenotype in the HNPP mouse model. A successful reversal of abnormal myelin permeability by deletion of Pak1 in Pmp22+/- mice strongly justifies a therapeutic trial in Pmp22+/- mice using PAK1 inhibitors. Taken together, these three aims will further substantiate the pathogenic roles of myelin junction disruption and abnormal actin polymerization in HNPP. We are optimistic that a therapy will likely be developed through this study. Because myelin junctions are essential complexes that seal myelin, results derived from this study may have broad applications in other myelin diseases.