Reactivation of intestinal epithelial cell brush border motility: ATP-dependent contraction via a terminal web contractile ring.

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Abstract
Various models have been put forward suggesting ways in which brush borders from intestinal epithelial cells may be motile. Experiments documenting putative brush border motility have been performed on isolated brush borders and have generated models suggesting microvillar retraction or microvillar rootlet interactions. The reported Ca2+, ATP-induced retraction of microvilli has been shown, instead, to be microvillar dissolution in response to Ca2+ and not active brush border motility. Studies on the reactivation of motility in intact sheets of isolated intestinal epithelium are presented. Whole epithelial sheets were glycerinated, which leaves the brush border and intercellular junctions intact, and then treated with ATP, PPi, ITP, ADP, GTP or .delta.S-ATP. Analysis by video enhanced differential interference-contrast microscopy and thin-section transmission electron microscopy reveals contractions in the terminal web region causing microvilli to be fanned apart in response to ATP and .delta.S-ATP but not in response to ADP, PPi, ITP or GTP. EM reveals that the contractions occur at the level of the intermediate junction in a circumferential constriction which can pull cells completely apart. This constriction occurs in a location occupied by an actin-containing circumferential band of filaments, as demonstrated by S-1 binding, which completely encircles the terminal web at the level of the intermediate junction. Upon contraction, this band becomes denser and thicker. Since myosin, .alpha.-actinin and tropomyosin, in addition to actin, have been localized to this region of the terminal web, it is proposed that the intestinal epithelial cell can be motile via a circumferential terminal web contractile ring analogous to the contractile ring of dividing cells.