Regulatory mechanism of smooth muscle contraction studied with gelsolin-treated strips of taenia caeci in guinea pig

Abstract

The potential roles of the regulatory proteins actin, tropomyosin (Tm), and caldesmon (CaD), i.e., the components of the thin filament, in smooth muscle have been extensively studied in several types of smooth muscles. However, controversy remains on the putative physiological significance of these proteins. In this study, we intended to determine the functional roles of Tm and CaD in the regulation of smooth muscle contraction by using a reconstitution system of the thin filaments. At appropriate conditions, the thin (actin) filaments within skinned smooth muscle strips of taenia caeci in guinea pigs could be selectively removed by an actin-severing protein, gelsolin, without irreversible damage to the contractile apparatus, and then the thin filaments were reconstituted with purified components of thin filaments, i.e., actin, Tm, and CaD. We found that the structural remodeling of actin filaments or thin filaments was functionally linked to the Ca(2+)-induced force development and reduction in muscle cross-sectional area (CSA). That is, after the reconstitution of the gelsolin-treated skinned smooth muscle strips with pure actin, the Ca(2+)-dependent force development was partially restored, but the Ca(2+)-induced reduction in CSA occurred once. In contrast, the reconstitution with actin, followed by Tm and CaD, restored not only the force generation but also both its Ca(2+) sensitivity and the reversible Ca(2+)-dependent reduction in CSA. We confirmed that both removal of the thin filaments by gelsolin treatment and reconstitution of the actin (thin) filaments with Tm and CaD caused no significant changes in the level of myosin regulatory light chain phosphorylation. We thus conclude that Tm and CaD are necessary for the full regulation of smooth muscle contraction in addition to the other regulatory systems, including the myosin-linked one.

Mechanics of Vascular Smooth Muscle

“…Rather than the troponin complex, the thin filament-binding protein caldesmon plays the prominent role in the regulatory process (163,507). In smooth muscle, caldesmon and tropomyosin also appear to play a role in Ca 2+ -dependent actomyosin filament reorganization such that filament cross-sectional area is reduced during contraction (293). Caldesmon may also act as a "molecular staple" that increases microfilament stiffness, and thus, force transmission (168).…”

Effects of genistein on β‐catenin signaling and subcellular distribution of actin‐binding proteins in human umbilical CD105‐positive stromal cells

“…A large family of more than 40 Tm isoforms is derived from four highly conserved genes, Tm-1, Tm-2, Tm-3, and Tm-4, via multiple promoters and alternative splicing, and these are associated with different specialized actin microfilaments ensembles in cells (Gunning et al, 2005(Gunning et al, , 2008. Caldesmon (CaD) is a protein that binds to Ca 2þ /calmodulin, Tm, actin, and myosin in cells (Liou et al, 2009). Two isoforms of CaD are produced from a single gene (Humphrey et al, 1992) through alternative splicing; these are the smooth muscle form h-CaD, with a high molecular weight of 130-140 kDa (Sobue Hunter, 2001;Shen et al, 2005;Annerén, 2008).…”