MicroRNAs and exosomes in human milk

Virtually all living cells produce and secrete extracellular vesicles into the extracellular space. Among the various classes of extracellular vesicles, exosomes (~100 nm diameter) are of particular interest because of their unique role in cell-to-cell communication. This type of communication is facilitated by the transfer of regulatory cargos such as microRNAs from donor cells to adjacent or distant recipient cells. Evidence suggests that exosomes and their microRNA cargos are derived not only from endogenous synthesis but may also be absorbed in the gastrointestinal tract from milk, and microRNAs in milk exosomes alter gene expression in recipient cells. Studies of exosomes and microRNA cargos in human milk and infant nutrition are just beginning to emerge. Preliminary evidence, discussed in this chapter, suggests that exosomes found in milk and consumed from the diet accumulate primarily in the brain and liver in mice and change microbial communities in the murine rectum. In addition, consumption of a diet depleted of milk-borne exosomes and their microRNA cargos causes a loss in spatial learning and memory, alters purine metabolism, and might impair immune function compared with exosomes and microRNA-sufficient controls in murine models. Much remains to be discovered about milk-borne exosomes and their biological functions.