Kinetics of milk lipid droplet transport, growth, and secretion revealed by intravital imaging: lipid droplet release is intermittently stimulated by oxytocin.
The lipid droplet (LD) fraction of milk has attracted special attention because it supplies preformed lipids for neonatal development, and the assembled LDs are secreted by a unique apocrine mechanism. Because many aspects of this key process remain uncharacterized, we developed a facile method for the intravital imaging of mammary cells in transgenic mice that express fluorescently tagged marker proteins. Using these techniques, we describe the first kinetic analysis of LD growth and secretion at peak lactation in real time. LD transit from basal to apical regions was slow (0-2 μm/min) and frequently intermittent. Droplets grew by the fusion of preexisting droplets, with no restriction on the size of fusogenic partners. Most droplet expansion took several hours and occurred in apical nucleation centers, either close to or in association with the apical surface. Droplets even continued to expand as they were emerging from the cell. Contrary to expectations, LDs attached to the apical plasma membrane but still associated with the cytoplasm were released after oxytocin-mediated contraction of the myoepithelium. Thus milk LD secretion is an intermittently regulated process. This novel procedure will have broad application for investigating trafficking events within the mammary epithelium in real time.
Laboratory of Cellular and Molecular Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892.|Intracellular Membrane Trafficking Section, National Institute of Craniofacial and Dental Research, National Institutes of Health, Bethesda, MD 20892.
Andrius
A
Masedunskas
Laboratory of Cellular and Molecular Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892.|Intracellular Membrane Trafficking Section, National Institute of Craniofacial and Dental Research, National Institutes of Health, Bethesda, MD 20892.
Yun
Y
Chen
Laboratory of Cellular and Molecular Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892.|Intracellular Membrane Trafficking Section, National Institute of Craniofacial and Dental Research, National Institutes of Health, Bethesda, MD 20892.
Rebecca
R
Stussman
Laboratory of Cellular and Molecular Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892.|Intracellular Membrane Trafficking Section, National Institute of Craniofacial and Dental Research, National Institutes of Health, Bethesda, MD 20892.
Roberto
R
Weigert
Intracellular Membrane Trafficking Section, National Institute of Craniofacial and Dental Research, National Institutes of Health, Bethesda, MD 20892 imather@umd.edu.|Intracellular Membrane Trafficking Section, National Institute of Craniofacial and Dental Research, National Institutes of Health, Bethesda, MD 20892.|Department of Animal and Avian Sciences, University of Maryland, College Park, MD 20742.
Ian H
IH
Mather
D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D004847:Epithelial Cells / Q000378:metabolism; D005260:Female; D000069416:Intravital Microscopy / Q000379:methods*; D007700:Kinetics; D007774:Lactation / Q000378:metabolism / Q000502:physiology; D066292:Lipid Droplets / Q000378:metabolism* / Q000502:physiology*; D050356:Lipid Metabolism; D008055:Lipids; D008321:Mammary Glands, Animal / Q000378:metabolism; D051379:Mice; D008892:Milk; D010121:Oxytocin / Q000378:metabolism
D008055:Lipids; D010121:Oxytocin