Quantitative fl uorescence imaging reveals point of release for lipoproteins during LDLR-dependent uptake

The LDL receptor (LDLR) supports effi cient uptake of both LDL and VLDL remnants by binding lipoprotein at the cell surface, internalizing lipoprotein through coated pits, and releasing lipoprotein in endocytic compartments before returning to the surface for further rounds of uptake. While many aspects of lipoprotein binding and receptor entry are well understood, it is less clear where, when, and how the LDLR releases lipoprotein. To address these questions, the current study employed quantitative fl uorescence imaging to visualize the uptake and endosomal processing of LDL and the VLDL remnant-VLDL. We fi nd that lipoprotein release is rapid, with most release occurring prior to entry of lipoprotein into early endosomes. Published biochemical studies have identifi ed two mechanisms of lipoprotein release: one that involves the-propeller module of the LDLR and a second that is independent of this module. Quantitative imaging comparing uptake supported by the normal LDLR or by an LDLR variant incapable of-propeller-dependent release shows that the-propellerindependent process is suffi cient for release for both lipoproteins but that the-propeller process accelerates both LDL and-VLDL release. Together these fi ndings defi ne where, when, and how lipoprotein release occurs and provide a generalizable methodology for visualizing endocytic handling in situ.