Redox regulation of the actin cytoskeleton and its role in the vascular system

Abstract

The actin cytoskeleton is critical for form and function of vascular cells, serving mechanical, organizational and signaling roles. Because many cytoskeletal proteins are sensitive to reactive oxygen species, redox regulation has emerged as a pivotal modulator of the actin cytoskeleton and its associated proteins. Here, we summarize work implicating oxidants in altering actin cytoskeletal proteins and focus on how these alterations affect cell migration, proliferation and contraction of vascular cells. Finally, we discuss the role of oxidative modification of the actin cytoskeleton in vivo and highlight its importance for vascular diseases.

Keywords: Actin cytoskeleton; Redox regulation; Vascular disease.

Figure 1. The major mechanisms of protein oxidative modifications

Left panel indicates the conventional model of reversible oxidative modifications of protein on cysteine thiol groups. These reversible modifications of sulfenic acid residues (-SOH) include formation of glutathione disulfide (GSSG), intra- or extra-molecular disulfide bonds (RS-SR′), S-glutathionylated proteins (R-SSG) and S-nitrosylation (SNO). The modifications can be reversed by, for example, thioredoxin (Trx) and/or glutathione (GSH). When levels of ROS increase, sulfenic acids undergo further oxidation to sulfinic (SO₂H) and/or sulfonic acid (SO₃H), which are irreversible. These and two other major irreversible oxidative modifications of proteins (tyrosine nitration and carbonylation) are shown on the right panel.

Figure 2. The actin cytoskeleton signaling network controlling cell motility and its redox regulation

Cell migration consists of cycles of lamellipodia formation, focal adhesion assembly at the leading edge, contraction of the cell body and de-adhesion and retraction at the rear edge. The signaling pathways that have been implicated in cell adhesion and migration are shown., including cell division control protein 42 homolog, Cdc42; Wiskott-Aldrich syndrome protein, WASP; WASp family verprolin-homologous protein, WAVE; actin-related protein 2/3, ARP2/3; protein kinase C, PKC; Rho-associated protein kinase, ROCK; LIM domain kinase, LIMK; myosin light chain kinase, MLCK; p21-activated kinase, PAK; protein tyrosine phosphatases, slingshot-1L phosphatase, SSH1L, low molecular weight PTPs, LMW-PTPs, myosin light chain phosphatase, MLCP; Rho GTPases and guanine nucleotide exchange factors (GEFs); phospholipase C β/γ (PLC β/γ); phosphatidylinositol (4,5)-bisphosphate (PIP₂); inositol 1,4,5-trisphosphate (IP₃); diacylglycerol (DAG). In this diagram, directly oxidized proteins are indicated by bold in red.

Figure 3. The actin cytoskeleton signaling network controlling cell contraction and its redox regulation

Cell contraction is induced when agonists such as norepinephrine or angiotensin II bind to receptors and activate phosphoinositide-specific-phospholipase C (PLC) to catalyze the formation of inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG) from phosphatidylinositol (4,5)-bisphosphate (PIP₂). Meanwhile, Ca²⁺ influx induced by voltage-gated Ca²⁺ channels (LTCC) along with inositol 1,4,5-trisphosphate receptor (IP₃R) activation inducing release of Ca²⁺ from the endoplasmic reticulum, promotes Ca²⁺ /calmodulin (CaM) activation of the actin-myosin complex. Decreased intracellular Ca²⁺ concentration achieved by inactivation of LTCC, activation of Ca²⁺ reuptake by the sarco-/endoplasmic reticulum Ca²⁺ -ATPase (SERCA), and activation of Ca²⁺ extrusion by the sodium-calcium exchanger (NCX) and plasma membrane Ca²⁺-ATPase (PMCA) results in cell relaxation by reducing Ca²⁺ and disrupting actin-myosin interaction. These processes are also regulated by kinases (calmodulin-dependent protein kinase II, CaMKII; Rho-associated protein kinase, ROCK; myosin light chain kinase, MLCK; protein kinase C, PKC; protein kinase A, PKA; protein kinase G, PKG) and phosphatases (myosin light chain phosphatase, MLCP), Rho GTPases and Guanine Nucleotide Exchange Factors (GEFs). In this diagram, directly oxidized proteins are indicated by bold in red.

Figure 4. The actin cytoskeleton signaling network controlling cell division and its redox regulation

The ratio of globular to filamentous actin within a cell regulates transcription of anti-proliferative genes; cell rounding at mitosis onset; mitotic spindle orientation and function; and contractile ring formation/cytokinesis completion. These processes are further regulated by transcriptional regulators (serum response factor, SRF; myocardin-related transcription factor, MRTFA), actin regulatory proteins (diaphanous-related formin-1, mDia; Cofilin; cell division control protein 42 homolog, Cdc42; Wiskott-Aldrich syndrome protein, WASP; WASp family verprolin-homologous protein, WAVE; actin-related protein 2/3, ARP2/3), kinases (Rho-associated protein kinase, ROCK; LIM domain kinase, LIMK; myosin light chain kinase, MLCK; citron kinase, Citron-K; p21-activated kinase, PAK), phosphatases (myosin light chain phosphatase, MLCP), Rho GTPases, guanine nucleotide exchange factors (Rho guanine nucleotide exchange factor 2, GEFH1; epithelial cell transforming sequence #2, Ect2), and GTPase activating proteins, of which many can be directly oxidized to regulate their function. In this diagram, directly oxidized proteins are indicated by bold in red.