Exosomes

Endosomal pathways from Danzer et al. (here) © 2012 Danzer et al.

Exosomes are tiny vesicles (50-100 nm) released from living cells. They represent a hitherto overlooked route of intercellular communication. Exosomes were the focus of a symposium at the CTR Annual Meeting at Cambridge earlier this month.

The precursors to exosomes are intralumenal vesicles (ILVs), which are found in multivesicular bodies (MVBs) or "late endosomes." ILVs are formed by invagination of the outer membrane of the MVB. Thus initially they contain cytosol. However, the ILV outer membrane can be restructured by a scramblase and both proteins and lipids can be sorted into the ILVs before they are released as exosomes. Recently the focus has been on the direction of microRNAs into exosomes. The argument convincingly presented at Cambridge was that exosomes once released act as carriers to take proteins, lipids and RNAs to target cells. Their contents are released to the target cell following fusion of the exosome with its plasma membrane or uptake of the complete exosome. One of the speakers at Cambridge was Michel Record whose previous review (here) can be recommended. 

Multivesicular bodies are a prominent feature of the guinea placenta (here). They are associated with a system capable of taking up maternal proteins and transporting them to the fetal circulation. The potential role of exosomes in this context deserves to be explored. Could they be involved in the transfer of intact maternal antibodies that confers passive immunity on the fetus?