Exosomes and microRNAs in maternal milk are important for growth and gut health during lactation in murine pups Janos Zempleni. Mahrou Sadri, Fang Zhou University of Nebraska-Lincoln, Department of Nutrition and Health Sciences Background: Human milk contains approximately 2.2x1011 exosomes per mL, which harbor more than 200 microRNAs. Mammals absorb exosomes and microRNAs from milk, and microRNAs regulate approximately 60% of human genes. Studies in transgenic mice suggest that maternal milk exosomes accumulate primarily in the liver, brain and gastrointestinal (GI) mucosa in neonate pups. Objective: Assess whether maternal EVs and their microRNA cargos promote postnatal growth and GI health in neonate mice. Methods: Wild-type (WT) pups were fostered to homozygous Tsg101 knockout (KO) dams (impaired exosome biogenesis), heterozygous Dicer KO dams (loss of microRNA biogenesis) or WT dams (control) from synchronized pregnancies (4 pups/dam). We assessed milk exosome counts and microRNA expression, gut development, barrier function, mRNA expression profile in the jejunum, postnatal weight gain, and milk quality and intake. Statistics: unpaired t-test (Tsg101/Dicer vs. control); P < 0.05. Results: KO of TSG101 and Dicer caused an 80% and 60% decrease of exosomes and microRNAs, respectively, in milk. The loss of milk exosomes and microRNAs led to an up to 20% shorter length of the gut, 20% decrease of villi height and 15% crypt depth, 50% increase in leakiness of the gut (appearance of FITC-dextran in blood), and a 50% loss of postnatal weight gain in pups. Approximately 400 mRNAs were differentially expressed in the jejunums of pups fostered to TSG101 KO dams or WT dams. Nutritional quality of milk and milk intake were not study confounders. Conclusions: Mothers communicate with their offspring through exosomes and microRNAs in milk, and the maternal message plays a role in optimal growth and gut health in neonate mice. Funding: NIFA/USDA 2016-67001-25301 and 2020-67017-30834, NIH P20GM104320, USDA Hatch and W-40022 and Gates Foundation OPP1200494. J.Z. is a consultant for PureTech Health, Inc.