title: Xanthine oxidoreductase mediates membrane docking of milk-fat droplets but is not essential for apocrine lipid secretion.
abstract: Xanthine oxidoreductase (XOR) modulates milk lipid secretion and lactation initiation. XOR is required for butyrophilin1a1 clustering in the membrane during milk lipid secretion. XOR mediates apical membrane reorganization during milk lipid secretion. Loss of XOR delays milk fat globule secretion. XOR loss alters the proteome of milk fat globules.
  
  
  
  Apocrine secretion is utilized by epithelial cells of exocrine glands. These cells bud off membrane-bound particles into the lumen of the gland, losing a portion of the cytoplasm in the secretion product. The lactating mammary gland secretes milk lipid by this mechanism, and xanthine oxidoreductase (XOR) has long been thought to be functionally important. We generated mammary-specific XOR knockout (MGKO) mice, expecting lactation to fail. Histology of the knockout glands showed very large lipid droplets enclosed in the mammary alveolar cells, but milk analysis showed that these large globules were secreted. Butyrophilin, a membrane protein known to bind to XOR, was clustered at the point of contact of the cytoplasmic lipid droplet with the apical plasma membrane, in the wild-type gland but not in the knockout, suggesting that XOR mediates 'docking' to this membrane. Secreted milk fat globules were isolated from mouse milk of wild-type and XOR MGKO dams, and subjected to LC-MS/MS for analysis of protein component. Proteomic results showed that loss of XOR leads to an increase in cytoplasmic, cytoskeletal, Golgi apparatus and lipid metabolism proteins associated with the secreted milk fat globule. Association of XOR with the lipid droplet results in membrane docking and more efficient retention of cytoplasmic components by the secretory cell. Loss of XOR then results in a reversion to a more rudimentary, less efficient, apocrine secretion mechanism, but does not prevent milk fat globule secretion.
authors.affiliation: Division of Reproductive Sciences, Department of Obstetrics and Gynecology, School of Medicine, University of Colorado Anschutz Medical Center, Aurora, CO, 80045, USA. jenifer.monks@ucdenver.edu.
authors.forename: Jenifer
authors.identifier: 0000-0002-7990-3389
authors.initials: J
authors.lastname: Monks
authors.affiliation: Biochemistry Department, School of Medicine, University of Colorado Anschutz Medical Center, Aurora, CO, 80045, USA.
authors.forename: Monika
authors.initials: M
authors.lastname: Dzieciatkowska
authors.affiliation: Division of Reproductive Sciences, Department of Obstetrics and Gynecology, School of Medicine, University of Colorado Anschutz Medical Center, Aurora, CO, 80045, USA.
authors.forename: Elise S
authors.initials: ES
authors.lastname: Bales
authors.affiliation: Department of Pathology, School of Medicine, University of Colorado Anschutz Medical Center, Aurora, CO, 80045, USA.
authors.forename: David J
authors.initials: DJ
authors.lastname: Orlicky
authors.forename: Richard M
authors.initials: RM
authors.lastname: Wright
authors.affiliation: Division of Reproductive Sciences, Department of Obstetrics and Gynecology, School of Medicine, University of Colorado Anschutz Medical Center, Aurora, CO, 80045, USA.
authors.forename: James L
authors.initials: JL
authors.lastname: McManaman
mesh_terms: D000818:Animals; D001050:Apocrine Glands / Q000378:metabolism* / Q000502:physiology*; D000071797:Butyrophilins / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D004847:Epithelial Cells / Q000378:metabolism; D005260:Female; D006017:Glycolipids / Q000378:metabolism*; D006023:Glycoproteins / Q000378:metabolism*; D006801:Humans; D007774:Lactation / Q000378:metabolism; D066292:Lipid Droplets; D050356:Lipid Metabolism / Q000502:physiology*; D008055:Lipids / Q000502:physiology; D042361:Mammary Glands, Human / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008892:Milk / Q000378:metabolism*; D040901:Proteomics / Q000379:methods; D014968:Xanthine Dehydrogenase / Q000378:metabolism*
chemical_list: D000071797:Butyrophilins; D006017:Glycolipids; D006023:Glycoproteins; D008055:Lipids; D008565:Membrane Proteins; C110367:milk fat globule; D014968:Xanthine Dehydrogenase
keywords: LC_MS/MS; Xanthine Oxidoreductase; apocrine secretion; lactation; milk fat globule; proteomics