Identification Of The Pathway And Mechanism Of Intracellular Cholesterol Transport

Cholesterol is an important structural component and the most abandant sterol molecule of mammalian cells.The molecular formula of cholesterol is C₂₇H₄₆O.It consists of a smaller hydrophilic head,four coupled hydrophobic rigid rings,and a saturated carbon chain tail.As suggested by its structure,cholesterol is a type of rigid and hydrophobic sterol molecule mainly distributed on cellular membrane with a role in regulating membrane fluidity.In fact,cholesterol has many biological functions more than regulating membrane fluidity.It can be used to synthesize sexual hormone.It can participate in bile acid metabolism and affect food absorption and utilization.It can take part in synapse formation and intracellular signal transduction.It can also covalently modify Hedgehog and Smoothened proteins.Cholesterol is distributed in varying concentrations on different organelle membranes.How hydrophobic cholesterol molecules are transported among organelles is a long-lasting question to be resolved.In regard to the post-lysosomal cholesterol transport,the dogma is that low-density lipoprotein-derived cholesterol is delivered from lysosome to the endoplasmic reticulum via the plasma membrane.However,most evidence supporting this view came from studies performed under cholesterol deprivation conditions,where cholesterol level on the plasma membrane was acutely and artificially depleted.Whether and how cholesterol,under physiological conditions,is transported from lysosomes to ER bypassing the plasma membrane are unclear.In Our earlier work?Chu and Liao et al.,2015?,we identified that lysosome-peroxisome membrane contacts mediate cholesterol egress from lysosomes.In the present study,I found peroxisome and endoplasmic reticulum were closely apposed to each other.The interorganellar contact sites were successfully reconstituted in vitro using highly purified peroxisomes and endoplasmic reticulum membranes.Knockdown of peroxisomal PI?4,5?P₂ or E-Syts decreased membrane contacts between peroxisome and endoplasmic reticulum and caused massive cholesterol accumulation in lysosomes.We further demonstrated the transport of ³H-labeled cholesterol from peroxisome or artificial liposomes containing PI?4,5?P₂ to endoplasmic reticulum membranes.Based on these results,We conclude that cholesterol is transported from peroxisome to endoplasmic reticulum via membrane contact mediated by PI?4,5?P₂ and E-Syts.Together with the previous findings on lysosome-peroxisome membrane contacts,we delineate a critical intracellular cholesterol transport pathway of LDL-derived cholesterol along the lysosome-peroxisome-ER axis.