title: Deep-apical tubules: dynamic lipid-raft microdomains in the brush-border region of enterocytes.
abstract: The brush border of small intestinal enterocytes is highly enriched in cholesterol- and glycosphingolipid-containing membrane microdomains, commonly termed as lipid 'rafts'. Functionally, transcytosis of IgA and exocytosis of newly made brush-border proteins in enterocytes occur through apical lipid raft-containing compartments, but little is otherwise known about these raft microdomains. We therefore studied in closer detail apical lipid-raft compartments in enterocytes by immunogold electron microscopy and biochemical analyses. Novel membrane structures, deep-apical tubules, were visualized by the non-permeable surface marker Ruthenium Red in the brush-border region of the cells. The surface-connected tubules were labelled by antibodies to caveolin-1 and the glycolipid asialo G(M1), and they were sensitive to cholesterol depletion by methyl-beta-cyclodextrin, indicating the presence of raft microdomains. Deep-apical tubules were positioned close to the actin rootlets of adjacent microvilli in the terminal web region, which had a diameter of 50-100 nm, and penetrated up to 1 microm into the cytoplasm. Markers for transcytosis, IgA and the polymeric immunoglobulin receptor, as well as the resident brush-border enzyme aminopeptidase N, were present in these deep-apical tubules. We propose that deep-apical tubules are a specialized lipid-raft microdomain in the brush-border region functioning as a hub in membrane trafficking at the brush border. In addition, the sensitivity to cholesterol depletion suggests that deep-apical tubules function as a cell-surface membrane reservoir for cholesterol and for rapid adaptive changes in the size of microvilli at the brush border.
authors.affiliation: Department of Medical Biochemistry and Genetics, The Panum Institute, University of Copenhagen, Blegdamsvej 3, DK-2200 Copenhagen N, Denmark.
authors.forename: Gert H
authors.initials: GH
authors.lastname: Hansen
authors.forename: Jens
authors.initials: J
authors.lastname: Pedersen
authors.forename: Lise-Lotte
authors.initials: LL
authors.lastname: Niels-Christiansen
authors.forename: Lissi
authors.initials: L
authors.lastname: Immerdal
authors.forename: E Michael
authors.initials: EM
authors.lastname: Danielsen
mesh_terms: D000818:Animals; D051242:Caveolin 1; D022461:Caveolins / Q000032:analysis; D002458:Cell Fractionation / Q000379:methods; D002784:Cholesterol / Q000378:metabolism; D007413:Intestinal Mucosa / Q000648:ultrastructure*; D007421:Intestine, Small / Q000648:ultrastructure; D008563:Membrane Lipids / Q000378:metabolism; D021962:Membrane Microdomains / Q000648:ultrastructure*; D008871:Microvilli / Q000648:ultrastructure*; D013552:Swine
chemical_list: D051242:Caveolin 1; D022461:Caveolins; D008563:Membrane Lipids; D002784:Cholesterol