In plants and invertebrates, long RNAs and silencing small (s)RNAs move between cells and over long distances to exert regulatory functions in remote tissues. Recenfly, micro-vesicles secreted in mammalian body fluids and in the extra-cellular space were found to contain RNA;these, moreover, might be internalized in recipient cells. sRNA effector proteins called Argonautes (AGOs) were also detected in serum. These findings have spurred speculaflons that mammalian extracellular RNAs bound to lipids and/or effector proteins might have biological functions following their uptake by target cells/tissues. Little, if any, experimental evidence presently supports this idea, however. Our laboratory has pioneered research on non-cell autonomous RNAi and was among the first to establish a link between RNA silencing, endomembranes and vesicle trafficking in mammalian cells. Recently, we discovered that human and bovine milk contains physiological amounts of regulatory micro (mi)RNAs concentrated, notably, in milk fat globules (MFG), the source of fat for newborns. Moreover, we also detected high amounts of speciflc AGOs in MFG and other milk fractions, suggesting that AGO-bound miRNA, and perhaps other RNAs, might be functionally transferred into the new born gut epithelium and, perhaps distant organs, during feeding. Taking advantage of the formidable collection of mouse and cell-based genetic tools available from the McManus and Hunter laboratories, we propose here to use mammalian milk -an easily accessible and abundant body bluid- as a paradigm to investigate (i) the mechanisms underpinning miRNA secretion from the mammary gland, (ii) the protein and lipid environment of milk-contained miRNAs, including possibly novel milk-associated AGO partners, (iii) the mechanisms underpinning milk miRNA uptake by the gut epithelium, and (iv) if milk-borne miRNAs transferred from mother to pup during feeding effectively display biological activity in mouse newborns. Drawing from this knowledge and on clear precedents of trans-kingdom RNAi transfer between plants and parasific invertebrates, we will finally address, using our unique plant-mammal dual expertise, if dietary RNA derived from crops can be functionally taken up by the gut epithelium of plant-fed mice.
A clear demonstrafion that mammalian miRNAs and other RNAs can be effectively secreted, packaged, transported and functionally internalized into target cells or organs would radically change our current views on genetic regulafion and cell-to-cell commmunicafion in mammals. In the context of milk, and more generally, dietary food intake, such findings would have major implicafions in the fields of nutrifion, gastrointestinal diseases inclnrilng colon cancer and r^hrnnic inflammation, and immunity.
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