Abstract:The transcription cofactor MAL is regulated by free actin levels and thus by actin dynamics. MAL, together with its DNA-binding partner, SRF, is required for invasive cell migration and in experimental metastasis. Although MAL/SRF has many targets, we provide genetic evidence in both Drosophila and human cellular models that actin is the key target that must be regulated by MAL/SRF for invasive cell migration. By regulating MAL/SRF activity, actin protein feeds back on production of actin mRNA to ensure suffic… Show more
“…Posern and Treisman have suggested that the cell can sense a drop in G-actin levels and thereby activate transcription accordingly (Posern and Treisman, 2006). A recent study in Drosophila found that actin is the sole target gene controlling invasive migration and is probably an ancestral system that evolved to respond to stimuli that initiate cell migration (Salvany et al, 2014). Along this line of thought, we postulated that a reduction in available actin protein might feed back to the nucleus and trigger an induction in β-actin transcription and in particular should influence the kinetic parameters of the serum response.…”
Section: Discussionmentioning
confidence: 99%
“…Examination of the signaling pathway involving SRF and MTRF-A (also termed megakaryoblastic acute leukemia [MAL] or MKL1) has demonstrated the association of this pathway with the cytoplasmic life cycle of cytoskeletal actin protein (Luxenburg et al, 2011, Miralles et al, 2003, Salvany et al, 2014 and Vartiainen et al, 2007). In many cell types, MAL is predominantly cytoplasmic and can rapidly shuttle in and out of the nucleus (Vartiainen et al, 2007).…”
Summary
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.
“…Posern and Treisman have suggested that the cell can sense a drop in G-actin levels and thereby activate transcription accordingly (Posern and Treisman, 2006). A recent study in Drosophila found that actin is the sole target gene controlling invasive migration and is probably an ancestral system that evolved to respond to stimuli that initiate cell migration (Salvany et al, 2014). Along this line of thought, we postulated that a reduction in available actin protein might feed back to the nucleus and trigger an induction in β-actin transcription and in particular should influence the kinetic parameters of the serum response.…”
Section: Discussionmentioning
confidence: 99%
“…Examination of the signaling pathway involving SRF and MTRF-A (also termed megakaryoblastic acute leukemia [MAL] or MKL1) has demonstrated the association of this pathway with the cytoplasmic life cycle of cytoskeletal actin protein (Luxenburg et al, 2011, Miralles et al, 2003, Salvany et al, 2014 and Vartiainen et al, 2007). In many cell types, MAL is predominantly cytoplasmic and can rapidly shuttle in and out of the nucleus (Vartiainen et al, 2007).…”
Summary
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.
“…To clarify the role of actin in general vs. gene-specific transcriptional regulation, we examined actin-chromatin interactions in Drosophila ovaries, where Mrtf has been shown to regulate Act5C transcription (Salvany et al, 2014). We performed chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) of Mrtf-GFP, actin and Pol II phosphorylated at serine 5 (Pol II S5P) in ovaries of wild type (w 1118 ) and Mrtf mutant (mal-d Δ7 ) flies, where Mrtf expression is abolished (Somogyi and Rorth, 2004), as well as in flies ubiquitously expressing GFP-tagged version of Mrtf (tub mal-d3xGFP) (Salvany et al, 2014) ( Figure 1A). Deletion and overexpression of Mrtf displayed decreased and increased expression of Act5c, respectively ( Figure 1B), and Mrtf bound to promoter and upstream region of the Act5C gene ( Figure 1A,C,D), in agreement with previous studies (Salvany et al, 2014).…”
Section: Actin Is Involved In Transcription Of Act5c Independently Ofmentioning
confidence: 99%
“…Moreover, the genome-wide binding pattern of actin in the context of RNA polymerase II (Pol II) mediated transcription has remained elusive. Importantly, actin itself is one of the target genes for SRF (Salvany et al, 2014), generating a feedback loop, where actin levels are controlled by the actin dynamics cycle. Here we show that chromatin-binding of actin is not dependent on Mrtf transcription factors and that, at the genome-wide level, actin interacts with essentially all transcribed genes in Drosophila ovaries, with a pattern depending on the expression level of the gene.…”
Actin influences gene expression at multiple levels. It regulates the activity of specific transcription factors, such as myocardin-related transcription factor (MRTF), is a component of many chromatin remodelers and linked to transcription by all three RNA polymerases (Pol).However, the molecular mechanisms by which actin participates in the gene-specific vs. general transcription have remained unclear. Here we use chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) in Drosophila ovaries to demonstrate that binding of actin to the Act5C gene is not dependent on the Mrtf transcription cofactor. At the genome-wide level, actin interacts with essentially all transcribed genes and co-occupies most gene promoters together with Pol II. On highly expressed genes, actin and Pol II can be found also on the gene bodies.Manipulation of nuclear transport factors for actin leads to decreased expression of egg shell genes, demonstrating the in vivo relevance of balanced nucleo-cytoplasmic shuttling of actin for transcription.
“…The site at −598 showed significant enrichment following ChIP with anti-Flag antibody compared with controls (IgM ChIP in da>FLAG-SRF and FLAG ChIP in da-GAL4 extracts). A comparison with the Mrtf ChIP-Seq dataset [24] revealed an Mrtf-binding site corresponding to the SRF-binding site at −598, suggesting both SRF and Mrtf are capable of associating at this site. Figure 8.Deterin is a Mrtf/SRF target and suppresses apoptosis during pico -mediated tissue overgrowth.…”
Recent work has implicated the actin cytoskeleton in tissue size control and tumourigenesis, but how changes in actin dynamics contribute to hyperplastic growth is still unclear. Overexpression of Pico, the only Drosophila Mig-10/RIAM/Lamellipodin adapter protein family member, has been linked to tissue overgrowth via its effect on the myocardin-related transcription factor (Mrtf), an F-actin sensor capable of activating serum response factor (SRF). Transcriptional changes induced by acute Mrtf/SRF signalling have been largely linked to actin biosynthesis and cytoskeletal regulation. However, by RNA profiling, we find that the common response to chronic mrtf and pico overexpression in wing discs was upregulation of ribosome protein and mitochondrial genes, which are conserved targets for Mrtf/SRF and are known growth drivers. Consistent with their ability to induce a common transcriptional response and activate SRF signalling in vitro, we found that both pico and mrtf stimulate expression of an SRF-responsive reporter gene in wing discs. In a functional genetic screen, we also identified deterin, which encodes Drosophila Survivin, as a putative Mrtf/SRF target that is necessary for pico-mediated tissue overgrowth by suppressing proliferation-associated cell death. Taken together, our findings raise the possibility that distinct targets of Mrtf/SRF may be transcriptionally induced depending on the duration of upstream signalling.
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