Exosome-Derived MicroRNAs of Human Milk and Their Effects on Infant Health and Development

Abstract

Multiple biologically active components of human milk support infant growth, health and development. Milk provides a wide spectrum of mammary epithelial cell-derived extracellular vesicles (MEVs) for the infant. Although the whole spectrum of MEVs appears to be of functional importance for the growing infant, the majority of recent studies report on the MEV subfraction of milk exosomes (MEX) and their miRNA cargo, which are in the focus of this review. MEX and the dominant miRNA-148a play a key role in intestinal maturation, barrier function and suppression of nuclear factor-κB (NF-κB) signaling and may thus be helpful for the prevention and treatment of necrotizing enterocolitis. MEX and their miRNAs reach the systemic circulation and may impact epigenetic programming of various organs including the liver, thymus, brain, pancreatic islets, beige, brown and white adipose tissue as well as bones. Translational evidence indicates that MEX and their miRNAs control the expression of global cellular regulators such as DNA methyltransferase 1-which is important for the up-regulation of developmental genes including insulin, insulin-like growth factor-1, α-synuclein and forkhead box P3-and receptor-interacting protein 140, which is important for the regulation of multiple nuclear receptors. MEX-derived miRNA-148a and miRNA-30b may stimulate the expression of uncoupling protein 1, the key inducer of thermogenesis converting white into beige/brown adipose tissue. MEX have to be considered as signalosomes derived from the maternal lactation genome emitted to promote growth, maturation, immunological and metabolic programming of the offspring. Deeper insights into milk's molecular biology allow the conclusion that infants are both "breast-fed" and "breast-programmed". In this regard, MEX miRNA-deficient artificial formula is not an adequate substitute for breastfeeding, the birthright of all mammals.

Keywords: DNA methyltransferase 1; adipogenesis; immune tolerance; intestinal maturation; milk exosome; milk miRNAs; necrotizing enterocolitis; nuclear factor-κB; receptor-interacting protein 140; systemic milk effects.

Figure 1

Illustration of a human milk exosome (MEX). The bilayer membrane is important for MEX resistance against the harsh conditions in the gastrointestinal tract. hsa-miRNA-148a-3p is the dominant miRNA of MEX. Note, MEX also contain transforming growth factor-β (TGF-β). Tetraspanins are CD9, CD63, CD81 and CD83.

Figure 2

Synopsis of intestinal effects of milk exosomes (MEX). MEX increase intestinal epithelial cell proliferation, goblet cell proliferation and activity and enhance the activity and viability of intestinal stem cells with up-regulation of the stem cell marker leucine-rich-repeat-containing G-protein-coupled receptor 5 (Lgr5). MEX support the formation of the mucus barrier and increase the production of mucin 2 (MUC2), and exert anti-inflammatory activities via the suppression of nuclear factor κB signaling, tumor necrosis factor-α (TNF-α), toll-like receptor 4 (TLR4), myeloperoxidase (MPO) and interleukin 6 (IL-6). Furthermore, MEX support the anti-microbial barrier via up-regulation of the anti-bacterial lectin regenerating islet-derived 3γ (RegIIIγ) and induce the expression of tight junction (TJ) proteins zonula occludens 1, claudin and occludin. Furthermore, MEX directly interact with bacteria of the gut microbiome.

Figure 3

Anti-inflammatory actions of miRNA-148a and miRNA-22 on nuclear factor κB signaling. miRNA-148a via suppression of DNA methyltransferase 1 (DNMT1) enhances the expression of FOXP3, which is a negative regulator of nuclear factor κB. miRNA-148a directly targets calcium/calmodulin-dependent protein kinase IIα (CaMKIIα), which phosphorylates CARD-containing MAGUK protein 1 (CARMA1) involved in the activation of IκB kinase α (IKKα) and IκB kinase β (IKKβ), Notably, miRNA-148a directly targets IKKα and IKKβ, thereby enhancing the inhibitory effect of IκB on NF-κB. In addition, miRNA-148a targets the interleukin 6 (IL-6) signal transducer gp130. miRNA-22, which is highly expressed in preterm MEX, targets nuclear receptor co-activator 1 (NCOA1) and cystein-rich protein 61 (CYR61), which both activate NF-κB. miRNA-30b via targeting RIP140 suppresses IL-6 expression. MEX-derived miRNAs thus provide anti-inflammatory signaling.

Figure 4

Potential impact of milk exosome-derived miRNA-148a and miRNA-30b on the expression of uncoupling protein 1 (UCP1). miRNA-148a inhibits DNA methyltransferase 1 (DNMT1) and miRNA-30b directly targets the mRNA of NRIP1 encoding receptor-interacting protein 140 (RIP140), activating the enhancer/promoter of UCP1 and increasing UCP1 expression, which drives thermogenesis and the conversion of white adipose tissue (WAT) into brown adipose tissue (BAT).