Regulation of cytoskeletal structure and contractility in the brush border

Calcium plays a vital part in the regulation of cytoskeletal structure and contractility in the brush border of intestinal epithelial cells. An increased Ca2+ concentration causes a rapid but reversible solation of microvillar core filaments, which is mediated by an actin-'severing' protein of the core (Mr = 95 000), referred to as MV-95 K or villin. Results of recent experiments on the Ca2+-dependent interaction of MV-95 K with actin are summarized, and various functions for this actin-severing protein in the intestinal epithelial cell are discussed. Calcium also regulates contractility in the brush border, as shown by studies of isolated brush borders in vitro from chicken intestine. In the presence of Ca2+ (greater than 1 microM) and ATP (at 37 degrees C) isolated brush borders dramatically contract, over 1-4 min, via an isometric contraction of the terminal web region, similar to that observed by R. Rodewald on neonatal rat brush borders. This contraction is mediated, at least in part, by contraction of the circumferential bundle of actin filaments that are associated with the zonula adherens and may also involve myosin-mediated contractions between adjacent microvillar rootlets. Analysis of Ca2+-dependent phosphorylation of brush border proteins during terminal web contraction demonstrates a simultaneous phosphorylation of the regulatory light chain of brush border myosin. Like contraction, the brush border myosin kinase is activated by free Ca2+ (greater than 1 microM) and is inhibited by trifluoperazine, an inhibitor of calmodulin function. These results demonstrate that the machinery required for both production and regulation of force are integral components of the brush border cytostructure.