Induced morphological changes in isolated microvilli: regulation of membrane topology in vitro by submembranous microfilaments.

We have investigated the effects of treating isolated microvilli with 1 mM ATP, 5 mM CaCl2, and 25 mM MgCl2 in terms of the morphological changes induced and the solubilization of cytoskeletal proteins. Neither ATP nor MgCl2 treatment alone induced significant morphological changes, despite some solubilization of cytoskeletal proteins; yet combined ATP and MgCl2 treatment resulted in "beading" of the membrane and some loss of membrane from the basal end of the structures. CaCl2 treatment resulted in a very regular beading of the microvillus membrane together with partial disassembly and solubilization of the core. These changes could be induced by free Ca++ in the micromolar range, which therefore suggests that they may be of physiological importance. Induced morphological changes are discussed in terms of the effects of these treatments on the microvillus cytoskeleton.