PACSIN1-BINDING TO LRP1, A PRO SURVIVAL RECEPTOR IN SCHWANN CELLS, TRANSACTIVATES TRKC VIA A SRC FAMILY KINASE-DEPENDENT PATHWAY

Introduction: Low-density lipoprotein receptor-related protein (LRP1) binds numerous ligands, some of which transactivate receptor tyro- sine kinases (RTKs) via Src family kinases (SFKs) in neurons. Using affinity precipitation and LC-MS/MS, we identified PACSIN1 as an intracellular neuronal protein released from injured axons, which binds to LRP1. Binding of ligands to LRP1 may support Schwann cell (SC) reprogramming into the Repair Phenotype. The goal of this study was to determine whether PACSIN1 activates cell-signaling via LRP1 and thus, may trigger SC reprogramming in PNS injury. Methods: Injured rat sciatic nerve extracts were isolated using an approach that does not cause de novo cell lysis and affinity- precipitated with Fc-fusion proteins containing the ligand-binding domains of LRP1 (CCR2 and CCR4). By this approach, PACSIN1 was identified as an LRP1 ligand. The cell-signaling pathway activated by purified PACSIN1 was characterized in primary cultured SCs. The effects of Lrp1 silencing and the SFK inhibitor, PP2, on PACSIN1-induced ERK1/2, C-Jun, and TrkC phosphorylation were examined. Results: Purified PACSIN1 caused phosphorylation of TrkC and ERK1/2 in a concentration- and time-dependent manner. Silencing Lrp1 expression blocked the effects of PACSIN1 on TrkC and ERK1/2 phosphorylation without affecting the activity of neuro-trophin-3, a direct TrkC ligand. PP2 also blocked PACSIN1-induced ERK1/2 and TrkC phosphorylation in SCs, indicating an essential role for SFKs. PACSIN1 activated c-Jun, a key factor in the SC Repair Program in an SFK- and LRP1-dependent manner. Conclusions: PACSIN1 activates cell-signaling in SCs via LRP1. The effects of PACSIN1 on TrkC phosphorylation demonstrate that RTK transactivation is conserved in the LRP1 signaling pathway across dif- ferent nervous system cells. The specific RTK targeted in SCs (TrkC) is novel. Activation of ERK1/2 and c-Jun by PACSIN1 suggests a major role for PACSIN1 as an initiator of the SC Repair Program in PNS injury.