LRP1 signaling occurs in lipid rafts

LDL receptor-related protein-1 (LRP1) is an endocytic receptor for diverse ligands, which also functions in phagocytosis and efferocytosis. In neurons and neuron-like cells, ligand-binding to LRP1 initiates cell signaling. Understanding the function of LRP1 in cell-signaling is important, especially in neurons because LRP1-activated cell-signaling has been implicated in neuronal survival, neurite outgrowth, growth cone navigation, synaptic transmission, and long-term potentiation. In cerebellar granule neurons (CGNs) and neuron-like cell lines, activation of cell-signaling proteins downstream of LRP1, including Src family members (SFKs), ERK1/2, Akt, and RhoA, depends on the assembly of a system of co-receptors that includes the NMDA Receptor, Trk receptors, and/or p75 NTR. Herein, we show that in PC12 and N2a neuron-like cells, LRP1 distributes into lipid rafts and non-raft plasma membrane fractions. When lipid rafts are disrupted, activation of Src family kinases and ERK1/2 by the LRP1 ligands, tissue-type plasminogen activator and activated α2-macroglobulin (α2M*), is blocked. Biological con-sequences of activated LRP1 signaling, including neurite outgrowth and cell growth, also are blocked. The effects of lipid raft disruption on ERK1/2 activation and neurite outgrowth, in response to LRP1 ligands, were replicated in experiments with CGNs. Lipid raft disruption does not affect the total ligand-binding capacity of LRP1 (Bmax), the affinity of LRP1 for ligands (KD values), or its endocytic activity. These latter activities probably represent the function of LRP1 localized to clathrin-coated pits. Our results demonstrate that well-described activities of LRP1 require localization of this receptor to more than one distinct plasma membrane microdomain.