• RME Events: Proteins and Vesicle Formation
• RME--Uptake of Cholesterol: The Beginning
• CURL Formation & Lysosome Digestion
• RME & Cholesterolemia
• Iron Uptake
• Endocytosis is a Complex Process

Cholesterol is synthesized in the liver and is ingested in food. To get into cells, cholesterol which is insoluble in water (the aqueous phase of blood), must be transported by carriers. LDL (low-density lipoprotein) particles serve this function as transport vesicles for cells. Cells destined to take up cholesterol possess surface receptors for the LDL particle. The early events of LDL-particle receptor mediated endocytosis are the same as those described above. But here we take the process through the full sequence of events:

• Receptor Binding & Activation: that LDL receptor binds to Apo-B protein on the LDL particle
• Coated Pit Formation: Clathrin forms cage around forming endosome
• Clathrin-Coated Vesicle Budding
• Uncoating of the Vesicle
• Early Endosome associates with other vesicles
• Formation of CURL (Compartment for Uncoupling of Ligand and Receptor) or Late Endosome
• Recycling of the Receptor to the cell surface
• Fusion of Transport Vesicle with Lysosome
• Digestion of the LDL to Release Cholesterol

The CURL endosome was discussed in our lecture on protein targeting. Here we describe it in relation to LDL.

• CURL = Compartment for Uncoupling of Receptor & Ligand
• pH drops to acidic (pH 5)
• Conformational change in Receptor releases LDL
• Receptor recycles to cell membrane
• Late Endosome fuses with lysosomal vesicles
• LDL is degraded; Esterases digest esters Cholesterol is released into the cytoplasm

• Familial Hypercholesterolemia (FH): high levels of blood cholesterol and other characteristics
• Leads to an increase in blood LDL (cholesterol)
• Risk of Atherosclerosis & Heart Disease
• Atherosclerosis: buildup of cholesterol deposits lead to plaques & clog arteries
• Contributes to heart attacks at early age
• One human mutation is due to a defect in LDL receptor (e.g., in adapter binding site; can't form coated pit for LDL uptake) which causes the buildup of LDL particles at the cell surface leading to plaque formation

The following diagram shows how a single amino acid change in the receptor results in the inability of the receptor-ligand complex for form a clathrin coat. This prevents uptake of the complex leaving it at the cell surface. Continual build-up of the complexes leads to plaque formation.

Iron uptake is another example of receptor mediated endocytosis but with a twist: both the receptor and ligand recycle to the surface.

• Receptor binds ferrotransferrin
• RME leads to Receptor & ligand in CURL endosome
• Drop in pH releases Fe3+ which diffuses into the cytoplasm
• Receptor & Apotransferrin stay bound & recycle to surface
• At surface pH is neutral causing Apotransferrin release into bloodstream

There are other forms of receptor mediated endocytosis. For example, transcytosis of IgG immunoglobulins of mother's milk across the intestinal epithelium of newborn infants. There are also other forms of endoctyosis. In the lecture on cell membrane structure, we mentioned membrane lipid rafts and specialized caveolae. There is evidence that, among other things, caveolae are involved in cholesterol uptake and organizing some signaling components. The caveolae may themselves lead to endocytosis or they may be a prelude to coated pit formation before endocytosis. Inside the cell the various endocytotic vesicles may become part of the same early and late endosomes (Review: Miaczynska & Zerial, 2002. Exp. Cell Research 272: 8-14). Thus, like many other subjects we've discussed there is still much to learn in the dynamic process of endocytosis.