Redox modification of nuclear actin by MICAL-2 regulates SRF signaling

Cell (Impact Factor: 32.24). 01/2014; 156(3). DOI: 10.1016/j.cell.2013.12.035
Source: PubMed


The serum response factor (SRF) binds to coactivators, such as myocardin-related transcription factor-A (MRTF-A), and mediates gene transcription elicited by diverse signaling pathways. SRF/MRTF-A-dependent gene transcription is activated when nuclear MRTF-A levels increase, enabling the formation of transcriptionally active SRF/MRTF-A complexes. The level of nuclear MRTF-A is regulated by nuclear G-actin, which binds to MRTF-A and promotes its nuclear export. However, pathways that regulate nuclear actin levels are poorly understood. Here, we show that MICAL-2, an atypical actin-regulatory protein, mediates SRF/MRTF-A-dependent gene transcription elicited by nerve growth factor and serum. MICAL-2 induces redox-dependent depolymerization of nuclear actin, which decreases nuclear G-actin and increases MRTF-A in the nucleus. Furthermore, we show that MICAL-2 is a target of CCG-1423, a small molecule inhibitor of SRF/MRTF-A-dependent transcription that exhibits efficacy in various preclinical disease models. These data identify redox modification of nuclear actin as a regulatory switch that mediates SRF/MRTF-A-dependent gene transcription.

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    • "These short-lived filaments appear to promote activity of the transcriptional co-activator MRTF by depleting monomeric actin from the nucleus. Serum stimulation also activates the actin-severing protein MICAL-2, which reversibly oxidizes actin monomers, rendering them incapable of inhibiting MRTF-dependent transcription (Lundquist et al., 2014). Environmental stresses also promote actin assembly in somatic cell nuclei. "
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    ABSTRACT: Actin filaments assemble inside the nucleus in response to multiple cellular perturbations, including heat shock, protein misfolding, integrin engagement, and serum stimulation. We find that DNA damage also generates nuclear actin filaments-detectable by phalloidin and live-cell actin probes-with three characteristic morphologies: (i) long, nucleoplasmic filaments; (ii) short, nucleolus-associated filaments; and (iii) dense, nucleoplasmic clusters. This DNA damage-induced nuclear actin assembly requires two biologically and physically linked nucleation factors: formin-2 and Spire-1/Spire-2. Formin-2 accumulates in the nucleus after DNA damage, and depletion of either formin-2 or actin's nuclear import factor, importin-9, increases the number of DNA double-strand breaks (DSBs), linking nuclear actin filaments to efficient DSB clearance. Nuclear actin filaments are also required for nuclear oxidation induced by acute genotoxic stress. Our results reveal a previously unknown role for nuclear actin filaments in DNA repair and identify the molecular mechanisms creating these nuclear filaments.
    eLife Sciences 08/2015; 4. DOI:10.7554/eLife.07735 · 9.32 Impact Factor
    • "The serum response results in the interaction of SRF protein with the b-actin promoter, through the disassembly of the MAL-actin interaction, resulting also in the assembly of cytoplasmic b-actin filaments (F-actin) from the now-available Gactin monomers. Interestingly, in recent years, the role of actin has been demonstrated in the regulation of gene expression via the nuclear pool of the actin protein (Hendzel et al., 1999; Huet et al., 2012; Jockusch et al., 2006; Lundquist et al., 2014; McDonald et al., 2006; Khanna et al., 2014; Treisman, 2013). Specifically in SRF signaling, G-actin in a mutant nonpolymerizing form, or as NLS-actin, negatively regulates SRF (Posern et al., 2002). "
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    ABSTRACT: The transcriptional response of β-actin to extra-cellular stimuli is a paradigm for transcription factor complex assembly and regulation. Serum induction leads to a precisely timed pulse of β-actin transcription in the cell population. Actin protein is proposed to be involved in this response, but it is not known whether cellular actin levels affect nuclear β-actin transcription. We perturbed the levels of key signaling factors and examined the effect on the induced transcriptional pulse by following endogenous β-actin alleles in single living cells. Lowering serum response factor (SRF) protein levels leads to loss of pulse integrity, whereas reducing actin protein levels reveals positive feedback regulation, resulting in elevated gene activation and a prolonged transcriptional response. Thus, transcriptional pulse fidelity requires regulated amounts of signaling proteins, and perturbations in factor levels eliminate the physiological response, resulting in either tuning down or exaggeration of the transcriptional pulse. Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.
    Cell Reports 04/2015; 11(3). DOI:10.1016/j.celrep.2015.03.039 · 8.36 Impact Factor
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    • "Disruption of the Rho pathway with Rho kinase (ROCK) inhibitors has reversed myofibroblast differentiation in vitro and fibrosis in several animal models (Buhl et al., 1995; Masszi et al., 2003; Zhao et al., 2007; Akhmetshina et al., 2008; Sandbo et al., 2011; Small, 2012; Zhou et al., 2013). We recently identified in high-throughput screens, a compound, CCG-1423, that blocks MRTF nuclear localization by interfering with the regulation of intranuclear actin polymerization mediated by microtubule-associated mono-oxygenase, calponin and LIM domain–containing 2 (MICAL-2) (Evelyn et al., 2007; Lundquist et al., 2014). CCG-1423 is more effective than ROCK inhibitors in reducing SRF-mediated transcription (Evelyn et al., 2007). "
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    ABSTRACT: Systemic sclerosis (SSc) or scleroderma, like many fibrotic disorders, lacks effective therapies. Current trials focus on anti-inflammatory drugs or targeted approaches aimed at one of the many receptor mechanisms initiating fibrosis. In light of evidence that a myocardin-related transcription factor (MRTF) and serum response factor (SRF)-regulated gene transcriptional program induced by Rho GTPases is essential for myofibroblast activation, we explore the hypothesis that inhibitors of this pathway may represent novel antifibrotics. MRTF-SRF-regulated genes show spontaneously increased expression in primary dermal fibroblasts from patients with diffuse cutaneous SSc. A novel small-molecule inhibitor of MRTF/SRF-regulated transcription (CCG-203971) inhibits expression of connective tissue growth factor (CTGF), alpha-smooth muscle actin (α-SMA), and collagen 1 (COL1A2) in both SSc fibroblasts and in LPA- and transforming growth factor β (TGFβ)-stimulated fibroblasts. In vivo treatment with CCG-203971 also prevented bleomycin-induced skin thickening and collagen deposition. Thus targeting the MRTF/SRF gene transcription pathway could provide an efficacious new approach to therapy for SSc and other fibrotic disorders.
    Journal of Pharmacology and Experimental Therapeutics 04/2014; 349(3). DOI:10.1124/jpet.114.213520 · 3.97 Impact Factor
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