Project summary/abstract Both Schwann cells and neurons are essential for the formation of myelinated nerve fibers in the peripheral nervous system (PNS). In addition to the electrical insulation of axons by myelin sheaths, Schwann cells assemble nodes of Ranvier, the excitable axonal domains required for rapid and efficient action potential propagation. Furthermore, Schwann cells actively modulate PNS degeneration and regeneration. However, the cellular and molecular mechanisms of how these structures are formed, maintained, and disrupted in disease conditions remain poorly understood. This critical gap in knowledge limits the field’s ability to manipulate the Schwann cells and neurons for treatment of PNS diseases and injuries. The overall objective of this application is to identify a critical molecular mechanism in the process of PNS myelinated nerve formation and injury. The prior studies and preliminary data provided here have identified activation of calpains, calcium-dependent intracellular cysteine proteases, and elevation of calpain-2 levels are involved in these processes. The constitutive knockout of calpain-2 in mice results in embryonic lethality before myelin is formed, further emphasizing the importance of calpain-2 in development of multiple organs including nervous system. The central hypothesis is that calpain-2 modulates formation and injury of peripheral myelinated nerves. To begin to test this hypothesis, this application will generate conditional knockout mice lacking calpain-2 in myelinating Schwann cells (Aim 1) or in sensory neurons (Aim 2). Specific aim one will test the hypothesis that Schwann cell calpain-2 mediates PNS myelination. Specific aim two will test the hypothesis that axonal calpain-2 modulates PNS node of Ranvier structures. Myelin and nodal structures in PNS, nerve conduction along PNS myelinated axons, and animal behavior will be examined during postnatal development and in adult conditional knockout and control mice. This application is conceptually innovative, in that we propose that calpain-2 is a key modulator of myelinated nerve formation in PNS. This application will generate novel calpain-2 conditional knockout mice, which will be used in future studies to determine Schwann cell-specific or neuron-specific roles of calpain-2 in PNS injury and repair. The proposed research is significant, because completion of the aims will identify calpain-2 as a potential modulator of myelinated nerve formation and remodeling in the PNS. Furthermore, future research, utilizing conditional knockout mice generated in this application, is expected to uncover cell type-specific roles of calpain-2 in PNS injuries and will substantially impact future translational research aimed at the development of novel therapies for a wide variety of PNS diseases and injuries.