Niemann-Pick type C proteins promote microautophagy by expanding raft-like membrane domains in the yeast vacuole

Abstract

Niemann-Pick type C is a storage disease caused by dysfunction of NPC proteins, which transport cholesterol from the lumen of lysosomes to the limiting membrane of that compartment. Using freeze fracture electron microscopy, we show here that the yeast NPC orthologs, Ncr1p and Npc2p, are essential for formation and expansion of raft-like domains in the vacuolar (lysosome) membrane, both in stationary phase and in acute nitrogen starvation. Moreover, the expanded raft-like domains engulf lipid droplets by a microautophagic mechanism. We also found that the multivesicular body pathway plays a crucial role in microautophagy in acute nitrogen starvation by delivering sterol to the vacuole. These data show that NPC proteins promote microautophagy in stationary phase and under nitrogen starvation conditions, likely by increasing sterol in the limiting membrane of the vacuole.

DOI: http://dx.doi.org/10.7554/eLife.25960.001

eLife digest

Niemann-Pick disease type C is a human disease that is characterized by severe neurological symptoms. The brains of children with this disease develop more slowly and adult patients have difficulty walking, coordinating movements, speaking clearly and they develop dementia. The disease is caused by mutations in two proteins called NPC1 and NPC2, which are normally needed to move lipid molecules, especially cholesterol, between different compartments within a cell.

Lysosomes are compartments within human cells that act as recycling points for many nutrients, including lipid molecules. The membranes surrounding lysosomes can bend to form pouches that can engulf the materials to be recycled. However, it was not clear exactly how this process, also known as microautophagy, happens and whether the NPC1 and NPC2 are involved.

Here Tsuji et al. used a method called freeze fracture to study microautophagy in yeast under an electron microscope. For the experiments, the yeast cells were exposed to conditions that prevented them from dividing to mimic human nerve cells in the brain. The experiments show that NPC1 and NPC2 play important roles in creating and enlarging areas in the lysosome’s membranes that are rich in a lipid molecule called ergosterol, which yeast uses in the same way as animals use cholesterol. These areas, also known as rafts, promoted microautophagy of lipid storage compartments in the yeast cells, ensuring the healthy recycling of nutrients. Furthermore, when the normal version of NPC2 was replaced with a mutated form of NPC2 that could not bind to ergosterol, the yeast cells were less able to form ergosterol-rich rafts.

These findings indicate that the symptoms of Niemann-Pick type C may be due, at least in part, to defects in the formation of cholesterol-rich lipid rafts in the membranes of lysosomes. Future experiments may investigate whether this microautophagy process, which depends on the NPC proteins in yeast, is exactly the same in human cells.

DOI: http://dx.doi.org/10.7554/eLife.25960.002