Deep-apical tubules: dynamic lipid-raft microdomains in the brush-border region of enterocytes

Biochem J. 2003 Jul 1;373(Pt 1):125-32. doi: 10.1042/BJ20030235.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Caveolin 1
  • Caveolins / analysis
  • Cell Fractionation / methods
  • Cholesterol / metabolism
  • Intestinal Mucosa / ultrastructure*
  • Intestine, Small / ultrastructure
  • Membrane Lipids / metabolism
  • Membrane Microdomains / ultrastructure*
  • Microvilli / ultrastructure*
  • Swine


  • Caveolin 1
  • Caveolins
  • Membrane Lipids
  • Cholesterol