Phosphatidylserine transporters ORP5 and ORP8 control cholesterol trafficking from the plasma membrane to the endoplasmic reticulum

TL;DR

This study reveals how cholesterol in the cell's outer membrane helps move another lipid, phosphatidylserine, from the cell's interior to the outer membrane, which in turn helps transport cholesterol back to the cell's interior.

Contribution

The study identifies a novel mechanism where cholesterol regulates phosphatidylserine transport via ORP5/8 and PI(4,5)P2, which in turn facilitates cholesterol trafficking back to the ER.

Findings

Excess cholesterol in the plasma membrane increases PI(4,5)P2 levels, recruiting ORP5/8 to deliver phosphatidylserine from the ER to the PM.

Phosphatidylserine delivery to the PM recruits GRAMD1b, a cholesterol transporter that moves cholesterol from the PM to the ER.

ORP5 interacts with GRAMD1b, enhancing GRAMD1b recruitment to the PM and facilitating cholesterol trafficking.

Abstract

Phosphatidylserine (PS), the most abundant negatively charged phospholipid in mammalian cells, is made in the endoplasmic reticulum (ER) but concentrated in the plasma membrane (PM). Similarly, cellular cholesterol is synthesized in the ER, yet enriched in the PM. Recently, PS has been shown to govern the transport of low-density lipoprotein-derived cholesterol from the PM to the ER. Here, we investigated how cholesterol regulates the delivery of PS from the ER to PM by the lipid-transfer proteins, ORP5 and ORP8. Adding exogenous cholesterol markedly increased the level of PI(4,5)P2 on the PM, which recruited ORP5/8 to promote the delivery of PS to the PM from the ER. Similar results were also obtained when the level of PM cholesterol was increased upon sphingomyelinase treatment. The increased delivery of PS to the PM helps recruit GRAMD1b, a cholesterol carrier transporting cholesterol from the PM to the ER. Importantly, we show ORP5 interacts with GRAMD1b, and this interaction further facilitates the recruitment of GRAMD1b to the PM. Our results thus unveil a new mechanism by which excess PM cholesterol promotes its own trafficking to the ER via PI(4,5)P2 and ORP5/8. Our results also provide fundamental new insights into how two major lipid species, PS and cholesterol, can impact each other's cellular homeostasis.

Figures