Rho-actin signaling to the MRTF coactivators dominates the immediate transcriptional response to serum in fibroblasts

Myocardin-related transcription factors are required for cardiac development and function

Myocardin-Related Transcription Factors A and B (MRTF-A and MRTF-B) are highly homologous proteins that function as powerful coactivators of serum response factor (SRF), a ubiquitously expressed transcription factor essential for cardiac development. The SRF/MRTF complex binds to CArG boxes found in the control regions of genes that regulate cytoskeletal dynamics and muscle contraction, among other processes. While SRF is required for heart development and function, the role of MRTFs in the developing or adult heart has not been explored. Through cardiac-specific deletion of MRTF alleles in mice, we show that either MRTF-A or MRTF-B is dispensable for cardiac development and function, whereas deletion of both MRTF-A and MRTF-B causes a spectrum of structural and functional cardiac abnormalities. Defects observed in MRTF-A/B null mice ranged from reduced cardiac contractility and adult onset heart failure to neonatal lethality accompanied by sarcomere disarray. RNA-seq analysis on neonatal hearts identified the most altered pathways in MRTF double knockout hearts as being involved in cytoskeletal organization. Together, these findings demonstrate redundant but essential roles of the MRTFs in maintenance of cardiac structure and function and as indispensible links in cardiac cytoskeletal gene regulatory networks.

The pH-sensing receptor OGR1 improves barrier function of epithelial cells and inhibits migration in an acidic environment

The pH-sensing receptor ovarian cancer G protein-coupled receptor 1 (OGR1; GPR68) is expressed in the gut. Inflammatory bowel disease is typically associated with a decrease in local pH, which may lead to altered epithelial barrier function and subsequent gastrointestinal repair involving epithelial cell adhesion and migration. As the mechanisms underlying the response to pH changes are not well understood, we have investigated OGR1-mediated, pH-dependent signaling pathways in intestinal epithelial cells. Caco-2 cells stably overexpressing OGR1 were created and validated as tools to study OGR1 signaling. Barrier function, migration, and proliferation were measured using electric cell-substrate impedance-sensing technology. Localization of the tight junction proteins zonula occludens protein 1 and occludin and the rearrangement of cytoskeletal actin were examined by confocal microscopy. Paracellular permeability and protein and gene expression analysis using DNA microarrays were performed on filter-grown Caco-2 monolayers. We report that an acidic pH shift from pH 7.8 to 6.6 improved barrier function and stimulated reorganization of filamentous actin with prominent basal stress fiber formation. Cell migration and proliferation during in vitro wound healing were inhibited. Gene expression analysis revealed significant upregulation of genes related to cytoskeleton remodeling, cell adhesion, and growth factor signaling. We conclude that acidic extracellular pH can have a signaling function and impact the physiology of intestinal epithelial cells. The deconstruction of OGR1-dependent signaling may aid our understanding of mucosal inflammation mechanisms.

Myocardin Family Members Drive Formation of Caveolae

Caveolae are membrane organelles that play roles in glucose and lipid metabolism and in vascular function. Formation of caveolae requires caveolins and cavins. The make-up of caveolae and their density is considered to reflect cell-specific transcriptional control mechanisms for caveolins and cavins, but knowledge regarding regulation of caveolae genes is incomplete. Myocardin (MYOCD) and its relative MRTF-A (MKL1) are transcriptional coactivators that control genes which promote smooth muscle differentiation. MRTF-A communicates changes in actin polymerization to nuclear gene transcription. Here we tested if myocardin family proteins control biogenesis of caveolae via activation of caveolin and cavin transcription. Using human coronary artery smooth muscle cells we found that jasplakinolide and latrunculin B (LatB), substances that promote and inhibit actin polymerization, increased and decreased protein levels of caveolins and cavins, respectively. The effect of LatB was associated with reduced mRNA levels for these genes and this was replicated by the MRTF inhibitor CCG-1423 which was non-additive with LatB. Overexpression of myocardin and MRTF-A caused 5-10-fold induction of caveolins whereas cavin-1 and cavin-2 were induced 2-3-fold. PACSIN2 also increased, establishing positive regulation of caveolae genes from three families. Full regulation of CAV1 was retained in its proximal promoter. Knock down of the serum response factor (SRF), which mediates many of the effects of myocardin, decreased cavin-1 but increased caveolin-1 and -2 mRNAs. Viral transduction of myocardin increased the density of caveolae 5-fold in vitro. A decrease of CAV1 was observed concomitant with a decrease of the smooth muscle marker calponin in aortic aneurysms from mice (C57Bl/6) infused with angiotensin II. Human expression data disclosed correlations of MYOCD with CAV1 in a majority of human tissues and in the heart, correlation with MKL2 (MRTF-B) was observed. The myocardin family of transcriptional coactivators therefore drives formation of caveolae and this effect is largely independent of SRF.

MKL1 is an epigenetic modulator of TGF-β induced fibrogenesis

Transforming growth factor (TGF-β) induced activation of portal fibroblast cells serves as a primary cause for liver fibrosis following cholestatic injury. The underlying epigenetic mechanism is not clear. We studied the role of a transcriptional modulator, megakaryoblastic leukemia 1 (MKL1) in this process. We report here that MKL1 deficiency ameliorated BDL-induced liver fibrosis in mice as assessed by histological stainings and expression levels of pro-fibrogenic genes. MKL1 silencing by small interfering RNA (siRNA) abrogated TGF-β induced transactivation of pro-fibrogenic genes in portal fibroblast cells. TGF-β stimulated the binding of MKL1 on the promoters of pro-fibrogenic genes and promoted the interaction between MKL1 and SMAD3. While SMAD3 was necessary for MKL1 occupancy on the gene promoters, MKL1 depletion impaired SMAD3 binding reciprocally. TGF-β treatment induced the accumulation of trimethylated histone H3K4 on the gene promoters by recruiting a methyltransferase complex. Knockdown of individual members of this complex significantly weakened the binding of SMAD3 and down-regulated the activation of portal fibroblast cells. In conclusion, we have identified an epigenetic pathway that dictates TGF-β induced pro-fibrogenic transcription in portal fibroblast thereby providing novel insights for the development of therapeutic solutions to treat liver fibrosis.

Endothelial depletion of murine SRF/MRTF provokes intracerebral hemorrhagic stroke

Intracerebral hemorrhagic stroke and vascular dementia are age- and hypertension-associated manifestations of human cerebral small vessel disease (SVD). Cerebral microvessels are formed by endothelial cells (ECs), which are connected through tight junctions, adherens junctions, and stabilizing basement membrane structures. These endothelial connections ensure both vessel stability and blood-brain barrier (BBB) functions, the latter enabling selective exchange of ions, bioactive molecules, and cells between the bloodstream and brain tissue. Srf(iECKO) mice, permitting conditional EC-specific depletion of the transcription factor Serum Response Factor (SRF), suffer from loss of BBB integrity and intracerebral hemorrhaging. Cerebral microbleeds and larger hemorrhages developed upon postnatal and adult depletion of either SRF or its cofactors Myocardin Related Transcription Factor (MRTF-A/-B), revealing essential requirements of ongoing SRF/MRTF activity for maintenance of cerebral small vessel integrity. In vivo magnetic resonance imaging allowed detection, localization, and time-resolved quantification of BBB permeability and hemorrhage formation in Srf(iECKO) brains. At the molecular level, direct and indirect SRF/MRTF target genes, encoding structural components of tight junctions (Claudins and ZO proteins), adherens junctions (VE-cadherin, α-Actinin), and the basement membrane (Collagen IV), were down-regulated upon SRF depletion. These results identify SRF and its MRTF cofactors as major transcriptional regulators of EC junctional stability, guaranteeing physiological functions of the cerebral microvasculature. We hypothesize that impairments in SRF/MRTF activity contribute to human SVD pathology.

G Protein-Coupled Receptor and RhoA-Stimulated Transcriptional Responses: Links to Inflammation, Differentiation, and Cell Proliferation

The low molecular weight G protein RhoA (rat sarcoma virus homolog family member A) serves as a node for transducing signals through G protein-coupled receptors (GPCRs). Activation of RhoA occurs through coupling of G proteins, most prominently, G12/13, to Rho guanine nucleotide exchange factors. The GPCR ligands that are most efficacious for RhoA activation include thrombin, lysophosphatidic acid, sphingosine-1-phosphate, and thromboxane A2. These ligands also stimulate proliferation, differentiation, and inflammation in a variety of cell and tissues types. The molecular events underlying these responses are the activation of transcription factors, transcriptional coactivators, and downstream gene programs. This review describes the pathways leading from GPCRs and RhoA to the regulation of activator protein-1, NFκB (nuclear factor κ-light-chain-enhancer of activated B cells), myocardin-related transcription factor A, and Yes-associated protein. We also focus on the importance of two prominent downstream transcriptional gene targets, the inflammatory mediator cyclooxygenase 2, and the matricellular protein cysteine-rich angiogenic inducer 61 (CCN1). Finally, we describe the importance of GPCR-induced activation of these pathways in the pathophysiology of cancer, fibrosis, and cardiovascular disease.

MicroRNA Clusters in the Adult Mouse Heart: Age-Associated Changes

The microRNAs and microRNA clusters have been implicated in normal cardiac development and also disease, including cardiac hypertrophy, cardiomyopathy, heart failure, and arrhythmias. Since a microRNA cluster has from two to dozens of microRNAs, the expression of a microRNA cluster could have a substantial impact on its target genes. In the present study, the configuration and distribution of microRNA clusters in the mouse genome were examined at various inter-microRNA distances. Three important microRNA clusters that are significantly impacted during adult cardiac aging, the miR-17-92, miR-106a-363, and miR-106b-25, were also examined in terms of their genomic location, RNA transcript character, sequence homology, and their relationship with the corresponding microRNA families. Multiple microRNAs derived from the three clusters potentially target various protein components of the cdc42-SRF signaling pathway, which regulates cytoskeleton dynamics associated with cardiac structure and function. The data indicate that aging impacted the expression of both guide and passenger strands of the microRNA clusters; nutrient stress also affected the expression of the three microRNA clusters. The miR-17-92, miR-106a-363, and miR-106b-25 clusters are likely to impact the Cdc42-SRF signaling pathway and thereby affect cardiac morphology and function during pathological conditions and the aging process.

Repression of SRF target genes is critical for Myc-dependent apoptosis of epithelial cells

Oncogenic levels of Myc expression sensitize cells to multiple apoptotic stimuli, and this protects long-lived organisms from cancer development. How cells discriminate physiological from supraphysiological levels of Myc is largely unknown. Here, we show that induction of apoptosis by Myc in breast epithelial cells requires association of Myc with Miz1. Gene expression and ChIP-Sequencing experiments show that high levels of Myc invade target sites that lack consensus E-boxes in a complex with Miz1 and repress transcription. Myc/Miz1-repressed genes encode proteins involved in cell adhesion and migration and include several integrins. Promoters of repressed genes are enriched for binding sites of the serum-response factor (SRF). Restoring SRF activity antagonizes Myc repression of SRF target genes, attenuates Myc-induced apoptosis, and reverts a Myc-dependent decrease in Akt phosphorylation and activity, a well-characterized suppressor of Myc-induced apoptosis. We propose that high levels of Myc engage Miz1 in repressive DNA binding complexes and suppress an SRF-dependent transcriptional program that supports survival of epithelial cells.

G Protein-coupled pH-sensing Receptor OGR1 Is a Regulator of Intestinal Inflammation

BACKGROUND

A novel family of proton-sensing G protein-coupled receptors, including OGR1, GPR4, and TDAG8, was identified to be important for physiological pH homeostasis and inflammation. Thus, we determined the function of proton-sensing OGR1 in the intestinal mucosa.

MTEHODS

OGR1 expression in colonic tissues was investigated in controls and patients with IBD. Expression of OGR1 upon cell activation was studied in the Mono Mac 6 (MM6) cell line and primary human and murine monocytes by real-time PCR. Ogr1 knockout mice were crossbred with Il-10 deficient mice and studied for more than 200 days. Microarray profiling was performed using Ogr1 and Ogr1 (WT) residential peritoneal macrophages.

RESULTS

Patients with IBD expressed higher levels of OGR1 in the mucosa than non-IBD controls. Treatment of MM6 cells with TNF, led to significant upregulation of OGR1 expression, which could be reversed by the presence of NF-κB inhibitors. Kaplan-Meier survival analysis showed a significantly delayed onset and progression of rectal prolapse in female Ogr1/Il-10 mice. These mice displayed significantly less rectal prolapses. Upregulation of gene expression, mediated by OGR1, in response to extracellular acidification in mouse macrophages was enriched for inflammation and immune response, actin cytoskeleton, and cell-adhesion gene pathways.

CONCLUSIONS

OGR1 expression is induced in cells of human macrophage lineage and primary human monocytes by TNF. NF-κB inhibition reverses the induction of OGR1 expression by TNF. OGR1 deficiency protects from spontaneous inflammation in the Il-10 knockout model. Our data indicate a pathophysiological role for pH-sensing receptor OGR1 during the pathogenesis of mucosal inflammation.

Extracellular signaling cues for nuclear actin polymerization

Contrary to cytoplasmic actin structures, the biological functions of nuclear actin filaments remain largely enigmatic. Recent progress in the field, however, has determined nuclear actin structures in somatic cells either under steady state conditions or in response to extracellular signaling cues. These actin structures differ in size and shape as well as in their temporal appearance and dynamics. Thus, a picture emerges that suggests that mammalian cells may have different pathways and mechanisms to assemble nuclear actin filaments. Apart from serum- or LPA-triggered nuclear actin polymerization, integrin activation by extracellular matrix interaction was recently implicated in nuclear actin polymerization through the linker of nucleoskeleton and cytoskeleton (LINC) complex. Some of these extracellular cues known so far appear to converge at the level of nuclear formin activity and subsequent regulation of myocardin-related transcription factors. Nevertheless, as the precise signaling events are as yet unknown, the regulation of nuclear actin polymerization may be of significant importance for different cellular functions as well as disease conditions caused by altered nuclear dynamics and architecture.

The short and long of noncoding sequences in the control of vascular cell phenotypes

The two principal cell types of importance for normal vessel wall physiology are smooth muscle cells and endothelial cells. Much progress has been made over the past 20 years in the discovery and function of transcription factors that coordinate proper differentiation of these cells and the maintenance of vascular homeostasis. More recently, the converging fields of bioinformatics, genomics, and next generation sequencing have accelerated discoveries in a number of classes of noncoding sequences, including transcription factor binding sites (TFBS), microRNA genes, and long noncoding RNA genes, each of which mediates vascular cell differentiation through a variety of mechanisms. Alterations in the nucleotide sequence of key TFBS or deviations in transcription of noncoding RNA genes likely have adverse effects on normal vascular cell phenotype and function. Here, the subject of noncoding sequences that influence smooth muscle cell or endothelial cell phenotype will be summarized as will future directions to further advance our understanding of the increasingly complex molecular circuitry governing normal vascular cell differentiation and how such information might be harnessed to combat vascular diseases.

Cellular Levels of Signaling Factors Are Sensed by β-actin Alleles to Modulate Transcriptional Pulse Intensity

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.

Severe myopathy in mice lacking the MEF2/SRF-dependent gene leiomodin-3

Maintenance of skeletal muscle structure and function requires a precise stoichiometry of sarcomeric proteins for proper assembly of the contractile apparatus. Absence of components of the sarcomeric thin filaments causes nemaline myopathy, a lethal congenital muscle disorder associated with aberrant myofiber structure and contractility. Previously, we reported that deficiency of the kelch-like family member 40 (KLHL40) in mice results in nemaline myopathy and destabilization of leiomodin-3 (LMOD3). LMOD3 belongs to a family of tropomodulin-related proteins that promote actin nucleation. Here, we show that deficiency of LMOD3 in mice causes nemaline myopathy. In skeletal muscle, transcription of Lmod3 was controlled by the transcription factors SRF and MEF2. Myocardin-related transcription factors (MRTFs), which function as SRF coactivators, serve as sensors of actin polymerization and are sequestered in the cytoplasm by actin monomers. Conversely, conditions that favor actin polymerization de-repress MRTFs and activate SRF-dependent genes. We demonstrated that the actin nucleator LMOD3, together with its stabilizing partner KLHL40, enhances MRTF-SRF activity. In turn, SRF cooperated with MEF2 to sustain the expression of LMOD3 and other components of the contractile apparatus, thereby establishing a regulatory circuit to maintain skeletal muscle function. These findings provide insight into the molecular basis of the sarcomere assembly and muscle dysfunction associated with nemaline myopathy.

Dysregulated serum response factor triggers formation of hepatocellular carcinoma

The ubiquitously expressed transcriptional regulator serum response factor (SRF) is controlled by both Ras/MAPK (mitogen-activated protein kinase) and Rho/actin signaling pathways, which are frequently activated in hepatocellular carcinoma (HCC). We generated SRF-VP16iHep mice, which conditionally express constitutively active SRF-VP16 in hepatocytes, thereby controlling subsets of both Ras/MAPK- and Rho/actin-stimulated target genes. All SRF-VP16iHep mice develop hyperproliferative liver nodules that progresses to lethal HCC. Some murine (m)HCCs acquire Ctnnb1 mutations equivalent to those in human (h)HCC. The resulting transcript signatures mirror those of a distinct subgroup of hHCCs, with shared activation of oncofetal genes including Igf2, correlating with CpG hypomethylation at the imprinted Igf2/H19 locus.

CONCLUSION

SRF-VP16iHep mHCC reveal convergent Ras/MAPK and Rho/actin signaling as a highly oncogenic driver mechanism for hepatocarcinogenesis. This suggests simultaneous inhibition of Ras/MAPK and Rho/actin signaling as a treatment strategy in hHCC therapy.

MRTF-SRF signaling is required for seeding of HSC/Ps in bone marrow during development

Chemokine signaling is important for the seeding of different sites by hematopoietic stem cells (HSCs) during development. Serum response factor (SRF) controls multiple genes governing adhesion and migration, mainly by recruiting members of the myocardin-related transcription factor (MRTF) family of G-actin-regulated cofactors. We used vav-iCre to inactivate MRTF-SRF signaling early during hematopoietic development. In both Srf- and Mrtf-deleted animals, hematopoiesis in fetal liver and spleen is intact but does not become established in fetal bone marrow. Srf-null HSC progenitor cells (HSC/Ps) fail to effectively engraft in transplantation experiments, exhibiting normal proximal signaling responses to SDF-1, but reduced adhesiveness, F-actin assembly, and reduced motility. Srf-null HSC/Ps fail to polarize in response to SDF-1 and cannot migrate through restrictive membrane pores to SDF-1 or Scf in vitro. Mrtf-null HSC/Ps were also defective in chemotactic responses to SDF-1. Srf-null HSC/Ps exhibit substantial deficits in cytoskeletal gene expression. MRTF-SRF signaling is thus critical for expression of genes required for the response to chemokine signaling during hematopoietic development.

Gravitational Effects on Human Physiology

Physical working capacity decreases with age and also in microgravity. Regardless of age, increased physical activity can always improve the physical adaptability of the body, although the mechanisms of this adaptability are unknown. Physical exercise produces various mechanical stimuli in the body, and these stimuli may be essential for cell survival in organisms. The cytoskeleton plays an important role in maintaining cell shape and tension development, and in various molecular and/or cellular organelles involved in cellular trafficking. Both intra and extracellular stimuli send signals through the cytoskeleton to the nucleus and modulate gene expression via an intrinsic property, namely the "dynamic instability" of cytoskeletal proteins. αB-crystallin is an important chaperone for cytoskeletal proteins in muscle cells. Decreases in the levels of αB-crystallin are specifically associated with a marked decrease in muscle mass (atrophy) in a rat hindlimb suspension model that mimics muscle and bone atrophy that occurs in space and increases with passive stretch. Moreover, immunofluorescence data show complete co-localization of αB-crystallin and the tubulin/microtubule system in myoblast cells. This association was further confirmed in biochemical experiments carried out in vitro showing that αB-crystallin acts as a chaperone for heat-denatured tubulin and prevents microtubule disassembly induced by calcium. Physical activity induces the constitutive expression of αB-crystallin, which helps to maintain the homeostasis of cytoskeleton dynamics in response to gravitational forces. This relationship between chaperone expression levels and regulation of cytoskeletal dynamics observed in slow anti-gravitational muscles as well as in mammalian striated muscles, such as those in the heart, diaphragm and tongue, may have been especially essential for human evolution in particular. Elucidation of the intrinsic properties of the tubulin/microtubule and chaperone αB-crystallin protein complex systems is expected to provide valuable information for high-pressure bioscience and gravity health science.

Mechanosignaling through YAP and TAZ drives fibroblast activation and fibrosis

Pathological fibrosis is driven by a feedback loop in which the fibrotic extracellular matrix is both a cause and consequence of fibroblast activation. However, the molecular mechanisms underlying this process remain poorly understood. Here we identify yes-associated protein (YAP) (homolog of drosophila Yki) and transcriptional coactivator with PDZ-binding motif (TAZ) (also known as Wwtr1), transcriptional effectors of the Hippo pathway, as key matrix stiffness-regulated coordinators of fibroblast activation and matrix synthesis. YAP and TAZ are prominently expressed in fibrotic but not healthy lung tissue, with particularly pronounced nuclear expression of TAZ in spindle-shaped fibroblastic cells. In culture, both YAP and TAZ accumulate in the nuclei of fibroblasts grown on pathologically stiff matrices but not physiologically compliant matrices. Knockdown of YAP and TAZ together in vitro attenuates key fibroblast functions, including matrix synthesis, contraction, and proliferation, and does so exclusively on pathologically stiff matrices. Profibrotic effects of YAP and TAZ operate, in part, through their transcriptional target plasminogen activator inhibitor-1, which is regulated by matrix stiffness independent of transforming growth factor-β signaling. Immortalized fibroblasts conditionally expressing active YAP or TAZ mutant proteins overcome soft matrix limitations on growth and promote fibrosis when adoptively transferred to the murine lung, demonstrating the ability of fibroblast YAP/TAZ activation to drive a profibrotic response in vivo. Together, these results identify YAP and TAZ as mechanoactivated coordinators of the matrix-driven feedback loop that amplifies and sustains fibrosis.

Sensing the local environment: actin architecture and Hippo signalling

The Hippo network is a major conserved growth suppressor pathway that participates in organ size control during development and prevents tumour formation during adult homeostasis. Recent evidence has implicated the actin cytoskeleton as a link between tissue architecture and Hippo signalling. In this review, we will consider the evidence and models proposed for the regulation of Hippo signalling by actin dynamics and structure. We cover aspects of signalling regulation by mechanotransduction, cytoskeletal tethering and the spatial reorganization of signalling components. We also examine the physiological and pathological contexts in which these mechanisms are relevant.

The oncogenic MicroRNA Hsa-miR-155-5p targets the transcription factor ELK3 and links it to the hypoxia response

The molecular response to hypoxia is a critical cellular process implicated in cancer, and a target for drug development. The activity of the major player, HIF1α, is regulated at different levels by various factors, including the transcription factor ELK3. The molecular mechanisms of this intimate connection remain largely unknown. Whilst investigating global ELK3-chromatin interactions, we uncovered an unexpected connection that involves the microRNA hsa-miR-155-5p, a hypoxia-inducible oncomir that targets HIF1α. One of the ELK3 chromatin binding sites, detected by Chromatin Immuno-Precipitation Sequencing (ChIP-seq) of normal Human Umbilical Vein Endothelial Cells (HUVEC), is located at the transcription start site of the MIR155HG genes that expresses hsa-miR-155-5p. We confirmed that ELK3 binds to this promoter by ChIP and quantitative polymerase chain reaction (QPCR). We showed that ELK3 and hsa-miR-155-5p form a double-negative regulatory loop, in that ELK3 depletion induced hsa-miR-155-5p expression and hsa-miR-155-5p expression decreased ELK3 expression at the RNA level through a conserved target sequence in its 3'-UTR. We further showed that the activities of hsa-miR-155-5p and ELK3 are functionally linked. Pathway analysis indicates that both factors are implicated in related processes, including cancer and angiogenesis. Hsa-miR-155-5p expression and ELK3 depletion have similar effects on expression of known ELK3 target genes, and on in-vitro angiogenesis and wound closure. Bioinformatic analysis of cancer RNA-seq data shows that hsa-miR-155-5p and ELK3 expression are significantly anti-correlated, as would be expected from hsa-miR-155-5p targeting ELK3 RNA. Finally, hypoxia (0% oxygen) down-regulates ELK3 mRNA in a microRNA and hsa-miR-155-5p dependent manner. These results tie ELK3 into the hypoxia response pathway through an oncogenic microRNA and into a circuit implicated in the dynamics of the hypoxic response. This crosstalk could be important for the development of new treatments for a range of pathologies.

SRF regulates craniofacial development through selective recruitment of MRTF cofactors by PDGF signaling

Receptor tyrosine kinase signaling is critical for mammalian craniofacial development, but the key downstream transcriptional effectors remain unknown. We demonstrate that serum response factor (SRF) is induced by both platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) signaling in mouse embryonic palatal mesenchyme cells and that Srf neural crest conditional mutants exhibit facial clefting accompanied by proliferation and migration defects. Srf and Pdgfra mutants interact genetically in craniofacial development, but Srf and Fgfr1 mutants do not. This signal specificity is recapitulated at the level of cofactor activation: while both PDGF and FGF target gene promoters show enriched genome-wide overlap with SRF ChIP-seq peaks, PDGF selectively activates a network of MRTF-dependent cytoskeletal genes. Collectively, our results identify a role for SRF in proliferation and migration during craniofacial development and delineate a mechanism of receptor tyrosine kinase specificity mediated through differential cofactor usage, leading to a PDGF-responsive SRF-driven transcriptional program in the midface.

RNase L attenuates mitogen-stimulated gene expression via transcriptional and post-transcriptional mechanisms to limit the proliferative response

The cellular response to mitogens is tightly regulated via transcriptional and post-transcriptional mechanisms to rapidly induce genes that promote proliferation and efficiently attenuate their expression to prevent malignant growth. RNase L is an endoribonuclease that mediates diverse antiproliferative activities, and tristetraprolin (TTP) is a mitogen-induced RNA-binding protein that directs the decay of proliferation-stimulatory mRNAs. In light of their roles as endogenous proliferative constraints, we examined the mechanisms and functional interactions of RNase L and TTP to attenuate a mitogenic response. Mitogen stimulation of RNase L-deficient cells significantly increased TTP transcription and the induction of other mitogen-induced mRNAs. This regulation corresponded with elevated expression of serum-response factor (SRF), a master regulator of mitogen-induced transcription. RNase L destabilized the SRF transcript and formed a complex with SRF mRNA in cells providing a mechanism by which RNase L down-regulates SRF-induced genes. TTP and RNase L proteins interacted in cells suggesting that RNase L is directed to cleave TTP-bound RNAs as a mechanism of substrate specificity. Consistent with their concerted function in RNA turnover, the absence of either RNase L or TTP stabilized SRF mRNA, and a subset of established TTP targets was also regulated by RNase L. RNase L deficiency enhanced mitogen-induced proliferation demonstrating its functional role in limiting the mitogenic response. Our findings support a model of feedback regulation in which RNase L and TTP target SRF mRNA and SRF-induced transcripts. Accordingly, meta-analysis revealed an enrichment of RNase L and TTP targets among SRF-regulated genes suggesting that the RNase L/TTP axis represents a viable target to inhibit SRF-driven proliferation in neoplastic diseases.

Elk3 deficiency causes transient impairment in post-natal retinal vascular development and formation of tortuous arteries in adult murine retinae

Serum Response Factor (SRF) fulfills essential roles in post-natal retinal angiogenesis and adult neovascularization. These functions have been attributed to the recruitment by SRF of the cofactors Myocardin-Related Transcription Factors MRTF-A and -B, but not the Ternary Complex Factors (TCFs) Elk1 and Elk4. The role of the third TCF, Elk3, remained unknown. We generated a new Elk3 knockout mouse line and showed that Elk3 had specific, non-redundant functions in the retinal vasculature. In Elk3(−/−) mice, post-natal retinal angiogenesis was transiently delayed until P8, after which it proceeded normally. Interestingly, tortuous arteries developed in Elk3(−/−) mice from the age of four weeks, and persisted into late adulthood. Tortuous vessels have been observed in human pathologies, e.g. in ROP and FEVR. These human disorders were linked to altered activities of vascular endothelial growth factor (VEGF) in the affected eyes. However, in Elk3(−/−) mice, we did not observe any changes in VEGF or several other potential confounding factors, including mural cell coverage and blood pressure. Instead, concurrent with the post-natal transient delay of radial outgrowth and the formation of adult tortuous arteries, Elk3-dependent effects on the expression of Angiopoietin/Tie-signalling components were observed. Moreover, in vitro microvessel sprouting and microtube formation from P10 and adult aortic ring explants were reduced. Collectively, these results indicate that Elk3 has distinct roles in maintaining retinal artery integrity. The Elk3 knockout mouse is presented as a new animal model to study retinal artery tortuousity in mice and human patients.

Expression of type I collagen and tenascin C is regulated by actin polymerization through MRTF in dedifferentiated chondrocytes

This study examined actin regulation of fibroblast matrix genes in dedifferentiated chondrocytes. We demonstrated that dedifferentiated chondrocytes exhibit increased actin polymerization, nuclear localization of myocardin related transcription factor (MRTF), increased type I collagen (col1) and tenascin C (Tnc) gene expression, and decreased Sox9 gene expression. Induction of actin depolymerization by latrunculin treatment or cell rounding, reduced MRTF nuclear localization, repressed col1 and Tnc expression, and increased Sox9 gene expression in dedifferentiated chondrocytes. Treatment of passaged chondrocytes with MRTF inhibitor repressed col1 and Tnc expression, but did not affect Sox9 expression. Our results show that actin polymerization regulates fibroblast matrix gene expression through MRTF in passaged chondrocytes.

Dual views of SRF: a genomic exposure

Esnault and colleagues (pp. 943-958) take a genomics approach to investigate the role of SRF (serum response factor) in the serum response of fibroblasts. The well-established dual role of SRF with alternative cofactors and responsiveness to two signaling pathways is illustrated at the genome-wide level, yet new insight comes from this global picture.