| dc.description.abstracteng | Myelinating Schwann cells wrap around peripheral axons and allow fast neural transmission. Peripheral myelin protein of 22 kDa (PMP22) is a transmembrane protein strongly expressed in Schwann cells. Alterations in PMP22 expression lead to motor and sensory peripheral neuropathies Charcot-Marie-Tooth disease 1A (CMT1A, PMP22 overexpression) and hereditary neuropathy with liability to pressure palsies (HNPP, PMP22 deficiency), while the molecular role of PMP22 is largely unknown. PMP22 is shuttled through the secretory pathway via the endoplasmic reticulum (ER) and the Golgi apparatus. Unaltered expression of unfolded protein response (UPR) components in CMT1A rats showed that the primary disease mechanism does not involve misfolding of PMP22 and subsequent ER stress. CMT1A is characterized by early hypermyelination of small axons and hypomyelination of big axons, whereas HNPP mice show a delayed onset of myelination and the formation of redundant myelin loops (tomacula). Previous studies from our group have partly explained the histological phenotype of CMT1A with defects in differentiation and growth regulation as one of the major growth-signaling pathways in myelination, the PI3K/AKT/mTOR pathway, is inversely regulated by PMP22 gene-dosage. Consistently, inhibition of mTOR with Rapamycin reduced tomacula formation and improved the phenotype in HNPP mice. We observed a direct correlation between PMP22 dosage and protein levels of the major inhibitor of the PI3K/AKT/mTOR pathway, Phosphatase and Tensin homolog (PTEN). In line with the notion of PTEN as the link between PMP22 and disturbed growth regulation, reducing PTEN levels either pharmacologically or genetically increased myelination in CMT1A in vitro. Genetic reduction of PTEN in CMT1A Schwann cells in vivo increased the number of myelinated axons and myelin sheath thickness early in development. However, the differentiation defect was not rescued by PTEN reduction, indicating a PTEN independent mechanism. Further, the growth defect was not ameliorated in adult animals. These results suggest a dual role for PMP22 in the timing of myelination. Therefore, PMP22 dosage is crucial in mediating the onset as well as the termination of myelination. In order to explore the functional link between PMP22 and PTEN we investigated the role of Vinculin in the Schwann cell, a potential interaction partner of PMP22 which is known to stabilize PTEN. Similarly to PTEN, Vinculin protein levels are increased in CMT1A and decreased in HNPP. Vinculin is expressed in focal adhesion plaques in primary Schwann cells in vitro and bands of Cajal in vivo. We demonstrated alterations in cell shape and migration of Vinculin overexpressing CMT1A Schwann cells and disrupted Cajal band structure in CMT1A peripheral nerves. Schwann cell specific depletion of Vinculin (VclcKO) did not alter myelin sheath thickness, behavior, electrophysiology or PTEN protein levels. Thus, PMP22 does not stabilize PTEN via Vinculin. However, VclcKO mice showed decreased numbers of Schmidt-Lanterman incisures (SLI) in peripheral nerves. SLI are cytoplasmic channels in the compact myelin internode and numbers are increased in several peripheral neuropathies. Depletion of Vinculin in CMT1A Schwann cells decreased the number of SLIs and deteriorated the phenotype, indicating a role of SLIs for sustaining Schwann cell function in CMT1A. | de |