Effects of microcystin‐lr on actin and the actin‐associated proteins α‐actinin and talin in hepatocytes

Abstract

Microcystin-LR (MCLR) is a commonly encountered blue-green algal hepatotoxin and a known inhibitor of cellular protein phosphatase types 1 and 2A. The toxin causes alterations in, and redistribution of, intermediate filaments, microtubules, and actin microfilaments (MFs) in affected cells. In this study, the effect of MCLR on the sequence of alterations in MFs and actin-associated proteins (AAPs) of isolated hepatocytes was examined in an effort to determine whether morphologic changes induced in MFs by microcystins are a result of prior dislocation of AAPs. We studied the effects of MCLR exposure on alpha-actinin and talin, two AAPs that play a role in the orientation of the MFs. Primary hepatocytes were incubated with 10 microns MCLR for 0-64 min. The distribution of actin, alpha-actinin, and talin were examined using fluorescence microscopy. MCLR induced similar changes in the distribution of actin and the AAPs. Actin filament redistribution was first observed after 12 min of MCLR exposure, and was characterized by detachment of MFs from the cell periphery, followed by condensation at distinct focal points and progressive collapse into the interior of affected cells. Changes in alpha-actinin and talin distribution were first observed after 20 min of toxin exposure. The AAPs appeared to detach from focal contacts on the cytoplasmic surface of the plasma membrane, condense into cytoplasmic aggregates, and ultimately collapse into a juxtanuclear bundle. The results of this study indicate that, in hepatocytes exposed to MCLR, the collapse of actin MFs occurs prior to the dislocation of alpha-actinin and talin. Changes in these actin associated proteins are not likely to account for the initial changes in actin MFs.

Protein phosphatase inhibition in normal and keratin 8/18 assembly-incompetent mouse strains supports a functional role of keratin intermediate filaments in preserving hepatocyte integrity

“…Previous studies have established that microcystins induce in both cultured hepatocytes and in the intact rat liver a characteristic reorganization of MFs to a condensed pericellular structure. 8,[35][36][37]52,53 Correspondingly, we show that MFs in mouse liver after MC-LR treatment reorganize to one or several spots in hepatocytes in the centrilobular area, but remain largely intact in the portal tract area. The primary biochemical reason for the F-actin reorganization by MC-LR may be caused by impaired dephosphorylation of some actin-binding proteins.…”

“…The initiating roles of ␣-actinin and talin have, however, been ruled out. 37 Other possible candidates for this type of effect could be IF-associated proteins such as plectin, a phosphoprotein that links IFs and MTs with each other. 9 Some studies have indicated phosphorylation of actin itself, but it is unclear whether this phosphorylation has any consequences in terms of MF organization.…”

“…29,30 In mice, microcystins cause extensive hepatic hemorrhage with disruption of the lobular and sinusoidal liver architecture, leading to rapid death by hemodynamic shock. [31][32][33] Studies with liver cells indicate disruption of cell morphology as a consequence of a remarkable reorganization of IFs, MFs, 8,[34][35][36][37] and MTs. 36,38 However, the cytoskeletal effects in intact mouse liver have not yet been sufficiently assessed.…”

Cytological alterations in isolated hepatocytes from common carp (Cyprinus carpio L.) exposed to microcystin-LR

“…This result showed that the effect of MC-LR on isolated carp hepatocytes was dose-dependent. Runnegar et al (1981), Ghosh et al (1995), and Khan et al (1995) drew similar conclusions from the observations in rat hepatocytes exposed to MC-LR.…”

Effects of microcystin-LR in isolated perfused rat kidney

“…MCLR has been shown to inhibit significantly the catalytic subunits of only two specific types of protein phosphatases, protein phosphatase 1 (pp-1c) and protein phosphatase 2A (pp-2Ac) (5)(6)(7)(8).…”