FELLOWSHIP for Michael Weaver: Strn3 is a novel Rac1 effector in Schwann cells

Project Summary / Abstract During peripheral nervous system (PNS) development, Schwann cells (SCs) undergo extensive Rac1- dependent cytoskeletal reorganization as they insert cytoplasmic extensions into axon bundles to radially sort, ensheath, and myelinate individual axons. Similarly, following peripheral nerve injury there is extensive Rac1- dependent actin polymerization around specialized regions known as Schmidt-Lantermann incisures (SLIs) as SCs differentiate into a repair phenotype2. The Feltri laboratory previously demonstrated that Rac1 activation in SCs is driven by engagement of a6ß1 integrin with laminins, and that this is essential for peripheral nerve development4. Using a proteomic screen, we then identified Striatin-3 (Strn3) as a novel candidate Rac1 interactor in peripheral nerves. Striatin proteins (Strn1/3/4) function as core scaffolding components of STriatin- Interacting Phosphatase And Kinase (STRIPAK) complexes. STRIPAK complex members include the upstream Hippo pathway kinases Mst1/2 and the phosphatase PP2A-C, which dephosphorylates Mst1/2 to deactivate the Hippo pathway, permitting nuclear translocation of the mechanosensitive transcriptional co- activators Yap/Taz26,27,28. The Feltri laboratory previously demonstrated that Yap/Taz activity in SCs is also critical for myelinated peripheral nerve development25. Initial in vitro data suggest that knockdown of Strn3 in SCs impairs their ability to adhere to various substrates, decreases proliferation, and disrupts their association with axons. I then generated a mouse model with Strn3 specifically deleted in SCs (Strn3SCKO) and demonstrated early radial sorting and hypomyelination defects. Additionally, Strn3 null SCs isolated from these animals display reduced elongation and process extension on a laminin substrate. I thus hypothesize that Strn3 is required in SCs for peripheral nerve development and injury response via interaction with Rac1 and STRIPAK-mediated regulation of the Hippo pathway. To investigate this hypothesis, Strn3SCKO mice will be further characterized by morphological, functional, and electrophysiological measures during development and after injury. Cell and biochemical experiments in tissues and isolated Strn3 null SCs will determine if Strn3 indeed regulates SCs by interacting with Rac1, the STRIPAK complex, and the Hippo pathway. The long-term goal of this project is to understand the functions of Strn3 and the STRIPAK complex during peripheral nerve development and repair to advance the field of basic neuroscience and to identify new therapeutic targets with the potential to alleviate the burden of disease for patients suffering from demyelinating neuropathies and traumatic peripheral nerve injuries. This project will be integrated with a training plan that emphasizes scientific communication, problem solving, critical thinking, technical skills, and clinical experience. Research training will take place at the Hunter James Kelly Research Institute, an interdisciplinary research center devoted to understanding and treating diseases of myelin.