Neuronal low-density lipoprotein receptor-related protein 1 binds and endocytoses prion fibrils via receptor cluster 4.

For infectious prion protein (designated PrP(Sc)) to act as a template to convert normal cellular protein (PrP(C)) to its distinctive pathogenic conformation, the two forms of prion protein (PrP) must interact closely. The neuronal receptor that rapidly endocytoses PrP(C) is the low-density lipoprotein receptor-related protein 1 (LRP1). We show here that on sensory neurons LRP1 is also the receptor that binds and rapidly endocytoses smaller oligomeric forms of infectious prion fibrils, and recombinant PrP fibrils. Although LRP1 binds two molecules of most ligands independently to its receptor clusters 2 and 4, PrP(C) and PrP(Sc) fibrils bind only to receptor cluster 4. PrP(Sc) fibrils out-compete PrP(C) for internalization. When endocytosed, PrP(Sc) fibrils are routed to lysosomes, rather than recycled to the cell surface with PrP(C). Thus, although LRP1 binds both forms of PrP, it traffics them to separate fates within sensory neurons. The binding of both to ligand cluster 4 should enable genetic modification of PrP binding without disrupting other roles of LRP1 essential to neuronal viability and function, thereby enabling in vivo analysis of the role of this interaction in controlling both prion and LRP1 biology.

LRP1 controls biosynthetic and endocytic trafficking of neuronal prion protein.

The trafficking of normal cellular prion protein (PrPC) is believed to control its conversion to the altered conformation (designated PrPSc) associated with prion disease. Although anchored to the membrane by means of glycosylphosphatidylinositol (GPI), PrPC on neurons is rapidly and constitutively endocytosed by means of coated pits, a property dependent upon basic amino acids at its N-terminus. Here, we show that low-density lipoprotein receptor-related protein 1 (LRP1), which binds to multiple ligands through basic motifs, associates with PrPC during its endocytosis and is functionally required for this process. Moreover, sustained inhibition of LRP1 levels by siRNA leads to the accumulation of PrPC in biosynthetic compartments, with a concomitant lowering of surface PrPC, suggesting that LRP1 expedites the trafficking of PrPC to the neuronal surface. PrPC and LRP1 can be co-immunoprecipitated from the endoplasmic reticulum in normal neurons. The N-terminal domain of PrPC binds to purified human LRP1 with nanomolar affinity, even in the presence of 1 muM of the LRP-specific chaperone, receptor-associated protein (RAP). Taken together, these data argue that LRP1 controls both the surface, and biosynthetic, trafficking of PrPC in neurons.

Traffic of prion protein between different compartments on the neuronal surface, and the propagation of prion disease.

The key mechanism in prion disease is the conversion of cellular prion protein into an altered, pathogenic conformation, in which cellular mechanisms play a poorly understood role. Both forms of prion protein are lipid-anchored and reside in rafts that appear to protect the native conformation against conversion. Neurons rapidly traffic their cellular prion protein out of its lipid rafts to be endocytosed via coated pits before recycling back to the cell surface. It is argued in this review that understanding the mechanism of this trafficking holds the key to understanding the cellular role in the conformational conversion of prion protein.