Lipid Raft Disruption Selectively Blocks the Cell‐Signaling Activity of LRP1 in Neuron‐Like Cells

LDL receptor‐related protein‐1 (LRP1) is an endocytic and phagocytic receptor. In neurons and neuron‐like cells, binding and endocytosis of specific ligands is coupled to robust cell‐signaling responses. The majority of the LRP1, present at the cell‐surface in most cells, localizes to clathrin‐coated pits. However, in some cells, LRP1 also local‐izes to lipid rafts. We previously demonstrated that, in PC12 and N2A neuron‐like cells, tissue‐type plasminogen activator (tPA) and activated α 2 ‐macroglobulin (α 2 M*) phos‐phorylate TrkA and activate ERK1/2 in a response that requires LRP1 and the NMDA Receptor. We have now shown that in PC12 and N2A cells, LRP1 is partially localized to lipid rafts. In these cells, both enzymatically‐inactive tPA and α 2 M* acti‐vated ERK1/2, confirming our earlier result. When PC12 or N2A cells were treated with methyl‐β‐cyclodextrin (MβCD), to disrupt lipid rafts, cell‐signaling in response to tPA and α 2 M* was blocked. MβCD had no effect on the total cellular abundance of LRP1, as deter‐mined by immunoblot analysis or on the cell‐surface abundance of LRP1, as deter‐mined by measuring the binding of radioiodinated α 2 M* at 4º C. MβCD also failed to af‐fect endocytosis of radioiodinated α 2 M* at 37º C. The selective effects of MβCD on LRP1‐initiated cell‐signaling was apparent in neuritogenesis experiments in PC12 cells; when lipid rafts were disrupted, tPA and α 2 M* failed to induce neurite outgrowth, which was observed in control cultures. Our studies suggest that LRP1 signaling is initiated in lipid rafts. The fact that LRP1 localizes to lipid rafts only in some cell types may partially explain why its activity in cell‐signaling is cell type‐specific.