Regulation of Sodium Channel Activity by Capping of Actin Filaments

Abstract

Ion transport in various tissues can be regulated by the cortical actin cytoskeleton. Specifically, involvement of actin dynamics in the regulation of nonvoltage-gated sodium channels has been shown. Herein, inside-out patch clamp experiments were performed to study the effect of the heterodimeric actin capping protein CapZ on sodium channel regulation in leukemia K562 cells. The channels were activated by cytochalasin-induced disruption of actin filaments and inactivated by G-actin under ionic conditions promoting rapid actin polymerization. CapZ had no direct effect on channel activity. However, being added together with G-actin, CapZ prevented actin-induced channel inactivation, and this effect occurred at CapZ/actin molar ratios from 1:5 to 1:100. When actin was allowed to polymerize at the plasma membrane to induce partial channel inactivation, subsequent addition of CapZ restored the channel activity. These results can be explained by CapZ-induced inhibition of further assembly of actin filaments at the plasma membrane due to the modification of actin dynamics by CapZ. No effect on the channel activity was observed in response to F-actin, confirming that the mechanism of channel inactivation does not involve interaction of the channel with preformed filaments. Our data show that actin-capping protein can participate in the cytoskeleton-associated regulation of sodium transport in nonexcitable cells. INTRODUCTIONIon transport mechanisms in various tissues are regulated by the cortical actin cytoskeleton. It has been shown that cytoskeletal elements are responsible for localization and clustering of channel molecules in the plasma membrane (Levina et al., 1994;Smith et al., 1995). Several lines of evidence indicate that ion channels can interact with microfilaments via actin-binding proteins. Ankyrin and spectrin were found to be linked to voltage-gated sodium channels from brain (Srinivasan et al., 1988). In epithelia, ankyrin and fodrin are associated with amiloride-sensitive sodium channels from apical microvilli (Smith et al., 1991) suggesting that actin-binding proteins link sodium channels to the cortical actin filament network, and that this association may sequester channels at apical microvilli and maintain their polarized distribution in renal epithelial cells.Functional characteristics of single channels can be affected by cytoskeleton rearrangements (Janmey, 1998). Disruption of the actin network by cytochalasin has been shown to activate amiloride-sensitive sodium channels in A6 cells (Cantiello et al., 1991), amiloride-insensitive sodium channels in leukemia cells (Negulyaev et al., 1996a), volumeregulated Cl Ϫ channels in epithelial cells of the renal cortical collecting duct (Schwiebert et al., 1994) and also to enhance ATP-sensitive K ϩ channel activity in guinea pig cardiomyocytes (Terzic and Kurachi, 1996). On the other hand, inhibition of K ϩ channels by cytochalasin has been observed in the rat cortical collecting duct (Wang et al., 1994). In neuronal tissues, cytochalasin affects membra...

Non‐hydrolyzable analog of GTP induces activity of Na+ channels via disassembly of cortical actin cytoskeleton

“…1B) approximated by linear regression corresponds to a single channel conductance value of about 11 pS and a reversal potential of about 20 mV; the estimation of relative permeability gives the value P Na /P K of about 3. These parameters are close to those obtained previously for Na þ channels which were activated by the agents disrupting actin cytoskeleton, such as cytochalasins [7,24] and gelsolin [18]. Fig.…”

“…Free ends of actin ¢laments are available for addition or loss of actin subunits depending on the presence or absence of the pool of G-actin in the vicinity of channels, respectively. The evidence for a possible participation of capping proteins in channel regulation in K562 cells was obtained recently [24].…”

Serum- and glucocorticoid-inducible kinase SGK1 regulates reorganization of actin cytoskeleton in mast cells upon degranulation

“…Indeed, different ion channels have been shown to be regulated by cytoskeleton dynamics. Thus, the activity of a Na ϩ channel in human myeloid leukemia K562 cells is extremely sensitive to the state of actin cytoskeleton (19,21,22,30,31). Depolymerization of actin results in Na ϩ channel activation whereas application of G-actin under conditions promoting its rapid polymerization abolishes Na ϩ channel activity (19,21,22).…”

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