Article

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

ABSTRACT

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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    • "A s an elusive problem, nuclear actin assembly in somatic cells has been explored for a long time. Over the past few years, researchers have started to reveal the mystery of nuclear actin with the advantage of novel probes for visualizing nuclear filamentous actin (F-actin) [Baarlink et al., 2013; Lundquist et al., 2014; Belin et al., 2013]. One wellknown probe is Lifeact, a 17-amino-acid peptide derived from actin-binding protein Abp-140 in Saccharomyces cerevisiae , which has been widely used for monitoring F-actin in both fixed and living cells [Riedl et al., 2008, 2010; Phng et al., 2013; Dyachok et al., 2014]. "
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    ABSTRACT: Nuclear actin assembly in somatic cells has been an enigma for a long time. Recently, with the advancement of novel F-actin probes, researchers have started to uncover this mystery. In this study, we investigated the actin dynamics in somatic cell nuclei using two probes: Lifeact and Utr230. Surprisingly, we observed that both Lifeact and Utr230 significantly interfered with actin dynamics in cell nuclei. Moreover, these two probes induced distinct patterns of nuclear actin assembly. While Lifeact induced filamentous actin assembly in cell nuclei, Utr230 led to various patterns of actin aggregates, including fibers, small puncta, and large patches. Moreover, the interference of actin dynamics by Lifeact was limited to nuclear actin, while Utr230 induced actin aggregation in both the nucleus and cytoplasm. Using time-lapse microscopy, we found that Lifeact-induced actin fibers remained steady over hours of observation, indicating a deficiency of nuclear F-actin reorganization. These results suggest that Lifeact and Utr230 both interfere with nuclear actin dynamics but with distinct mechanisms. This is an important finding for research on nuclear actin assembly and highlights the potential value of these two probes for exploring the native mechanisms underlying nuclear actin dynamics which appear to be altered in the presence of these probes. This article is protected by copyright. All rights reserved.
    Full-text · Article · Nov 2015 · Cytoskeleton
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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: eLife digest In animals, plants, and other eukaryotic organisms, a cell's DNA is contained within a structure called the nucleus, which separates it from the rest of the interior of the cell. Filaments of a protein called actin are normally found outside the nucleus, where they help give the cell its overall shape and organize its contents. However, these filaments can sometimes form inside the nucleus in response to a sudden increase in heat or another type of stress. However, it was not clear what role these actin filaments play in the nucleus because it was difficult to distinguish them from the actin filaments that form in other parts of the cell. Researchers have recently developed new techniques to study actin filaments inside the nuclei of live cells under a microscope, using fluorescent protein tags. Here, Belin et al.—including some of the researchers involved in the previous work—used this technique to investigate whether DNA damage causes actin filaments to form in the nuclei of human cells. The experiments show that DNA damage does indeed lead to the formation of actin filaments in the nucleus. In a structure within the nucleus called the nucleolus, the actin filaments are short. However, in the rest of the nucleus, the actin forms long filaments and dense clusters. Cells that contained lower levels of actin were less able to repair their DNA than normal cells. Belin et al. also identified three proteins—called Formin-2, Spire-1, and Spire-2—that assemble the actin filaments in the nucleus. These proteins are also required to make actin filaments in other parts of the cell. The experiments show that the level of Formin-2 increases in the nucleus after DNA damage, and that the DNA of cells lacking this protein is more severely damaged. Belin et al.'s findings reveal a new role for actin in the repair of DNA and the next challenge is to understand the details of how this works. DOI: http://dx.doi.org/10.7554/eLife.07735.002
    Full-text · Article · Aug 2015 · eLife Sciences
    • "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.
    No preview · Article · Apr 2015 · Cell Reports
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