PACSIN1, an intracellular neuronal protein, activates LRP1 dependent cell signaling in Schwann Cells

ABSTRACT OF THE THESISPACSIN1, an intracellular neuronal protein, activates LRP1 dependent cell signaling in Schwann CellsbyMark Carter as listed on UC San Diego official Academic RecordsMaster of Science in BiologyUniversity of California San Diego, 2019Professor Wendy Campana, ChairPeripheral nerves have the unique ability to regenerate following injury, as opposed to nerves in the central nervous system (CNS) which do not spontaneously regenerate. The ability of peripheral nerves to regenerate is due to the remarkable and versatile principle glia cell of the peripheral nervous system (PNS), the Schwann cell (SC). SCs have the ability to detect injury in peripheral nerves, such as the dorsal root ganglion (DRG), and transform phenotypically to facilitate nerve repair. Downstream cell-signaling pathways and changes in gene expression that drive SC phenotypic transformation in injury have been previously described as well as SC receptors that detect injury. LDL receptor-related protein (LRP1) is a receptor for numerous ligands, including protein components of degenerated myelin. After injury, LRP1 is dramatically upregulated in SCs and functions with the N-methyl-D-aspartate receptor (NMDA-R) to trigger cell signaling in response to ligands. Using a capture tech¬nology and LC-MS/MS to identify novel LRP1 ligands in the injured nerve, we identified Protein kinase C and casein kinase substrate in neurons protein 1, PACSIN1, as a LRP1ligand. Not much is known of PACSIN1 as it pertains to the PNS injury. Herein, I demonstrate, for the first time, that PACSIN 1 is specifically localized in the cell bodies and axons of both large and small sensory neurons in the dorsal root ganglia (DRGs), confirming neuronal specificity. I hypothesized that after injury PACSIN1 is released by dorsal root ganglia (DRG) neurons and binds to Schwann cell LRP1 receptor. Indeed, when rhPACSIN was added to primary SC cultures, LRP1-dependent cell signaling was activated. PACSIN1 robustly activated c-Jun, the SC repair protein, and ERK1/2 in SCs within 10 minutes. This response was blocked when LRP1 was genetically silenced or when the NMDA-Receptor was pharmacologically antagonized with MK801. These data further confirm that the NMDAR is an active co-receptor for LRP1 in SCs. Finally, I demonstrate, for the first time, that expression of PACSIN1 in DRGs is significantly decreased by 5 days after sciatic nerve injury, suggesting that PACSIN1 may play a role in SC signaling in early stages after injury. We conclude that the Schwann cell LRP1 receptor is indeed responsive to intracellular neuronal proteins released at injury. Our results support a model in which SC LRP1 serves as a major injury-induced receptor in the peripheral nervous system