Fig. 7.

Reciprocal relationship between lysosomal cholesterol and NPC1-regulated MCS. A schematic representation of cholesterol transport at NPC1-regulated lysosomal MCSs. In the presence of cholesterol in the endocytic pathway, lysosomal NPC1 interacts with Gramd1b on the ER to tether MCSs that mediate the transport of cholesterol from the endosome to the ER for esterification by ACAT. In the absence of functional NPC2, luminal cholesterol fails to be delivered to the limiting membrane for egress at NPC1-Gramd1b tethered MCS and therefore accumulates in the lysosome. In the absence of functional NPC1, NPC1-tethered lysosome-ER contact sites are lost and cholesterol accumulates in the lysosome. Under these conditions STARD3 mediates extended lysosome contact sites with mitochondria. However, when the lysosome is artificially tethered to the ER by expression of an ORP1L mutant with its sterol-binding domain deleted (ORP1L-ΔORD), cholesterol transport to the ER is restored