Continuous transport of a small fraction of plasma membrane cholesterol to endoplasmic reticulum regulates total cellular cholesterol

摘要

Cells are surrounded by a plasma membrane made mostly from oily molecules known as lipids. One of these lipids, called cholesterol, is essential for keeping this membrane stable. Cholesterol is partly produced within the cells at a specialized structure called the endoplasmic reticulum, and partly imported from the blood surrounding the cell. In the blood, cholesterol is shielded inside particles called low-density lipoprotein (or LDL for short), which is taken into the cell and then sent to another structure called the lysosome. Inside the cell, cholesterol that is freshly produced in the endoplasmic reticulum or freshly imported into the lysosome, must be moved to the plasma membrane, where most of the cholesterol is located. Cholesterol levels are regulated by a ‘control machinery’ of proteins located in the endoplasmic reticulum. To keep the cholesterol levels constant, the endoplasmic reticulum needs to be in continual communication with the plasma membrane. However, the mechanisms by which cholesterol is transported between membranes are still poorly understood. Here, Infante and Radhakrishnan report a new tool to study how cholesterol is transported in human and hamster cells. The tool, which is based on part of a bacterial protein, traps cholesterol in the plasma membrane and prevents it from moving to the endoplasmic reticulum, and thus from updating the control machinery about cholesterol levels. From this inhibition, it is inferred that a stream of cholesterol constantly travels from the plasma membrane back to endoplasmic reticulum. This way, proteins in the endoplasmic reticulum can monitor the cholesterol levels in the plasma membrane in real-time. The endoplasmic reticulum responded rapidly even to small declines in cholesterol levels by activating genes that increase cholesterol production or the amount of cholesterol imported via the LDL pathway. Further work showed that cholesterol derived from LDL travels from the lysosome directly to the plasma membrane to maintain optimal cholesterol levels. It then moves to the endoplasmic reticulum to signal that cholesterol levels in the cell have been satisfied. The findings and tools described in this study will help to further investigate the mechanisms underlying the transport of cholesterol between the different membranes and structures in a cell. A next step will be to see if the mechanisms that apply to distribution of imported cholesterol from lysosomes, also apply to the cholesterol produced in the endoplasmic reticulum.