Abstract
The late endosomes/endo‐lysosomes of vertebrates contain an atypical phospholipid, lysobisphosphatidic acid (LBPA) (also termed bis[monoacylglycero]phosphate [BMP]), which is not detected elsewhere in the cell. LBPA is abundant in the membrane system present in the lumen of this compartment, including intralumenal vesicles (ILVs). In this review, the current knowledge on LBPA and LBPA‐containing membranes will be summarized, and their role in the control of endosomal cholesterol will be outlined. Some speculations will also be made on how this system may be overwhelmed in the cholesterol storage disorder Niemann‐Pick C. Then, the roles of intralumenal membranes in endo‐lysosomal dynamics and functions will be discussed in broader terms. Likewise, the mechanisms that drive the biogenesis of intralumenal membranes, including ESCRTs, will also be discussed, as well as their diverse composition and fate, including degradation in lysosomes and secretion as exosomes. This review will also discuss how intralumenal membranes are hijacked by pathogenic agents during intoxication and infection, and what is the biochemical composition and function of the intra‐endosomal lumenal milieu. Finally, this review will allude to the size limitations imposed on intralumenal vesicle functions and speculate on the possible role of LBPA as calcium chelator in the acidic calcium stores of endo‐lysosomes.
Highlights
The late endosomes/endo-lysosomes of vertebrates contain an atypical phospholipid, lysobisphosphatidic acid (LBPA), which is not detected elsewhere in the cell
Influenza A virus (IAV) infection depends on VPS4, as well as ubiquitination,[214] the SPOPL/Cullin-3 ubiquitin ligase complex and its target EPS15.215,216 the precise role of LBPA, ALIX and endosomal sorting complexes required for transport (ESCRT) in infection or intoxication remains to be elucidated, it has been proposed that anthrax,[71,207] VSV67,68 and Japanese encephalitis and yellow fever flaviviruses[217] may hijack intralumenal vesicles (ILVs) so that toxin or nucleic acid be delivered to the cytoplasm by ILV back-fusion with the limiting membrane.[29,30,218]
The privileged and secluded environment of ILVs, bathed into the late endosome/endo-lysosome lumen, is fully disconnected from all cytosolic machineries that drive signaling or protein and lipid sorting, and the fate of ILVs cannot rely on these established mechanisms
Summary
Funding information LipidX from SystemsX.ch; National Center of Competence in Research Chemical Biology; Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Grant/ Award Number: 31003A_159479. Peer Review The peer review history for this article is available at https://publons.com/publon/10. Peer Review The peer review history for this article is available at https://publons.com/publon/10. 1111/tra.12715/
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