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Electron Tomography Revels that Milk Lipids Originate from Endoplasmic Reticulum Domains with Novel Structural Features

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Abstract

Lipid droplets (LD) are dynamically-regulated organelles that originate from the endoplasmic reticulum (ER), and function in the storage, trafficking and metabolism of neutral lipids. In mammary epithelial cells (MEC) of lactating animals, intact LD are secreted intact into milk to form milk lipids by a novel apocrine mechanism. The secretion of intact LD and the relatively large amounts of lipid secreted by lactating MEC increase demands on the cellular processes responsible for lipid synthesis and LD formation. As yet these processes are poorly defined due to limited understanding of LD-ER interactions. To overcome these limitations, we used rapid-freezing and freeze-substitution methods in conjunction with 3D electron tomography and high resolution immunolocalization to define interactions between LD with ER in MEC of pregnant and lactating rats. Using these approaches, we identified distinct ER domains that contribute to lipid droplet formation and stabilization and which possess unique features previously unrecognized or not fully appreciated. Our results show nascent lipid droplets within the ER lumen and the association of both forming and mature droplets with structurally unique regions of ER cisternae, characterized by the presence of perilipin-2, a protein implicated in lipid droplet formation, and enzymes involved in lipid synthesis. These data demonstrate that milk lipids originate from LD-ER domains with novel structural features and suggest a mechanism for initial droplet formation in the ER lumen and subsequent maturation of the droplets in association with ER cisternae.

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Abbreviations

Acat1:

Acyl-CoA cholesterol acyltransferase-1

Dgat:

Acyl-CoA diacylglycerol acyltransferase

EM:

Electron microscopy

ER:

Endoplasmic reticulum

LD:

Lipid droplets

MEC:

Mammary epithelial cell

MFG:

Milk fat globules

NR:

Nile red

Plin2:

Perilipin-2

P5:

Pregnancy day 5

P10:

Pregnancy day 10

PDI:

Protein disulfide isomerase

3D:

Three-dimensional

2D:

Two-dimensional

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Acknowledgements

We thank Dr. Robert Farese Jr., Harvard School of Public Health for the antibodies against Dgat1 and Dgat2 and Dr. T.Y. Chang of Dartmouth Medical School for the antibodies Acat1. This work was supported by NIH grants PO1GM61306 (KEH); 5PO1HD038129 (KEH); 2PO1HD038129 (JLM); and 2R01HD45965 and R01HD093729 (JLM).

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Author notes

  1. Mark S. Ladinsky

    Present address: Division of Biology, California Institute of Technology, Pasadena, CA, USA

  2. Gonzalo A. Mardones

    Present address: Instituto de Fisiologia, Universidad Austral de Chile, Valdiva, Chile

Authors and Affiliations

  1. Boulder Laboratory for 3D Electron Microscopy of Cells, University of Colorado, Boulder, CO, 80309, USA

    Mark S. Ladinsky

  2. Department of Cell & Developmental Biology, University of Colorado Anschutz Medical Campus, Aurora, CO, 80045, USA

    Gonzalo A. Mardones & Kathryn E. Howell

  3. Department of Pathology, University of Colorado Anschutz Medical Campus, Aurora, CO, 80045, USA

    David J. Orlicky

  4. Division of Reproductive Sciences, University of Colorado Anschutz Medical Campus, 12700 E. 19th Ave., Aurora, CO, 80045, USA

    James L. McManaman

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  1. Mark S. Ladinsky
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Correspondence to James L. McManaman.

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Ladinsky, M.S., Mardones, G.A., Orlicky, D.J. et al. Electron Tomography Revels that Milk Lipids Originate from Endoplasmic Reticulum Domains with Novel Structural Features. J Mammary Gland Biol Neoplasia 24, 293–304 (2019). https://doi.org/10.1007/s10911-019-09438-y

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