The Actin-Severing Protein Cofilin Is Downstream Of Neuregulin Signaling, Is Regulated By The Tumor Suppressor Merlin, And Is Essential For Schwann Cell Myelination

Myelination is a complex process requiring coordination of directional motility and an increase in Schwann cell (SC) size to generate a multi-lamellar myelin sheath. Regulation of actin dynamics during myelination is poorly understood. However, it is known that myelin thickness is related to the abundance of neuregulin1-type III (NRG) expressed on the axon surface. NRG binding to ErbB2/3 receptors on the Schwann cell surface initiates signaling cascades necessary for myelination. We identify cofilin1, an actin depolymerizing and severing protein, as a downstream target of NRG-ErbB2/3 signaling in rat SC. A five minute exposure of SCs to NRG triggers phosphorylation of ErbB2 with concomitant dephosphorylation, and activation, of cofilin, and its upstream regulators, LIM domain kinase (LIMK) and Slingshot-1 phosphatase (SSH). This leads to cofilin activation and recruitment to the leading edge of the SC plasma membrane. These changes are associated with rapid plasma membrane expansion yielding a 35–50% increase in SC size within 30 minutes of NRG1 exposure. Cofilin1-deficient SCs increase phosphorylation of ErbB2, ERK, focal adhesion kinase, and paxillin in response to NRG, but fail to increase in size possibly due to stabilization of unusually long focal adhesions. Cofilin1-deficient SCs co-cultured with sensory neurons fail to elaborate myelin. Ultrastructural analysis reveals that they unsuccessfully segregate or engage axons and form only patchy basal lamina. After 48 hours of co-culturing with neurons, cofilin-deficient SCs fail to align and elongate on axons and often adhere to the underlying substrate rather than to axons. We show that the Neurofibromatosis Type II (NF2) tumor suppressor, merlin, is an upstream regulator of cofilin1, and that merlin knockdown in Schwann cells inhibits their elaboration of normal myelin sheaths in vitro. Merlin-deficient SCs form shorter myelin segments in DRG neuron/SC co-cultures. Merlin-deficient Schwann cells have increased levels of both active Rac (Rac-GTP) and F-actin indicative of a stable actin cytoskeleton. Surprisingly merlin-deficient Schwann cells fail to dephosphorylate and activate cofilin1 in response to NRG stimulation. Inhibition of LIMK restores the ability of merlin-deficient SCs to activate cofilin in response to NRG. In developing rat sciatic nerve, merlin becomes hyper-phosphorylated at S518 during the time of peak myelin formation. During this time, cofilin is localized to the inner mesaxon, and subsequently to Schmidt-Lanterman incisures in mature myelin. This study: 1) identifies cofilin and its upstream regulators, LIMK and SSH, as end targets of a NRG-ErbB2/3 signaling pathway in Schwann cells, 2) demonstrates that cofilin modulates actin dynamics in Schwann cells allowing for motility needed to effectively engage and myelinate axons, 3) shows that merlin regulates NRG-ErbB2/3-cofilin-actin signaling during SC myelination to determine the myelin segment length