Milk is a complex fluid capable of sustaining the total nutrition of the human infant for 6 months or longer, as well as providing protection against infection and common childhood illnesses during the critical early postnatal period. Milk lipids provide the majority of the calories required for neonatal growth, in addition to being the primary mechanism for transferring fat-soluble vitamins to infants and the source of essential fatty acids needed for neonatal membrane synthesis and the synthesis of eicosanoids and other bioactive lipid signalling molecules. To meet caloric and nutritional demands of newborns, a unique apocrine mechanism evolved in mammals to efficiently secrete large quantities of lipid into milk. Evidence from humans and animal models suggest that apocrine lipid secretion is also important for initiating and sustaining lactation, and that interference with this process increases the risk of lactation failure. Work from our laboratories indicates that apocrine lipid secretion involves three mechanistically distinct steps: (1) formation of specialized ?docking? contacts between CLD and the apical plasma membrane (APM); (2) membrane envelopment of docked CLD, driven in part by Golgi derived secretory vesicles; and (3) release of membrane enveloped CLD into the luminal space as trilaminar membrane coated structures, referred to as milk fat globules (MFG), by an apocrine scission process. We previously identified interactions between butyrophilin (Btn), a mammary gland specific transmembrane protein, the cytoplasmic redox enzyme, xanthine oxidoreductase (XOR), and the CLD coat protein, perilipin-2 (Plin2) as important for apocrine lipid secretion in mice. Importantly, we have demonstrated that genetic deletion of either of these proteins interferes with CLD secretion, and impairs or prevents lactation. The goals of this proposal are: (1) to investigate the mechanism of apocrine lipid secretion and test the hypothesis that prolactin and oxytocin independently regulate discrete steps of this process; (2) define how apocrine lipid secretion affects apical membrane properties, secretory vesicle trafficking, lactogenesis and lactation success; and (3) to define critical molecular determinants of interactions between XOR, Btn and Plin2 that mediate apocrine lipid secretion. The proposed studies take advantage of novel XOR, Btn and Plin2 knockout mouse models developed in our laboratories, quantitative high resolution imaging, and innovative physiological and biochemical analyses and cell culture models of apocrine lipid secretion to accomplish these goals.
Breastfeeding provides neonates with essential nutrients for growth and development, and protection against postnatal and long-term health risks including morbidities associated with bacterial and viral infections, childhood obesity, and type-2 diabetes. Comparatively few mothers in the United States, particularly those who are overweight or obese, successfully breastfeed their children for the recommended period of time due to failure to either initiate or maintain lactation. Interference with milk lipid secretion is linked to impaired milk secretion and lactation failure in humans and animal models. This project uses novel transgenic mouse models to investigate how milk lipid secretion regulates lactation success and to delineate the molecular mechanisms that regulate this function. We anticipate that greater understanding the basic mechanisms controlling milk lipid secretion as proposed in this application will fill conceptual and experimental gaps that pose barriers to understanding biological mechanisms controlling of lactation success in humans.
