Role of megakaryoblastic acute leukemia-1 in ERK1/2-dependent stimulation of serum response factor-driven transcription by BDNF or increased synaptic activity

The actin-binding protein CAP1 represses MRTF-SRF-dependent gene expression in mouse cerebral cortex

Serum response factor (SRF) is an essential transcription factor for brain development and function. Here, we explored how an SRF cofactor, the actin monomer-sensing myocardin-related transcription factor MRTF, is regulated in mouse cortical neurons. We found that MRTF-dependent SRF activity in vitro and in vivo was repressed by cyclase-associated protein CAP1. Inactivation of the actin-binding protein CAP1 reduced the amount of actin monomers in the cytoplasm, which promoted nuclear MRTF translocation and MRTF-SRF activation. This function was independent of cofilin1 and actin-depolymerizing factor, and CAP1 loss of function in cortical neurons was not compensated by endogenous CAP2. Transcriptomic and proteomic analyses of cerebral cortex lysates from wild-type and Cap1 knockout mice supported the role of CAP1 in repressing MRTF-SRF-dependent signaling in vivo. Bioinformatic analysis identified likely MRTF-SRF target genes, which aligned with the transcriptomic and proteomic results. Together with our previous studies that implicated CAP1 in axonal growth cone function as well as the morphology and plasticity of excitatory synapses, our findings establish CAP1 as a crucial actin regulator in the brain relevant for formation of neuronal networks.

Brain-derived neurotrophic factor (BDNF) downregulates mRNA levels of suppressor of cancer cell invasion (SCAI) variants in cortical neurons

Suppressor of cancer cell invasion (SCAI) acts as a transcriptional repressor of serum response factor (SRF)-mediated gene expression by binding to megakaryoblastic leukemia (MKL)/myocardin-related transcription factor (MRTF), which is an SRF transcriptional coactivator. Growing evidence suggests that SCAI is a negative regulator of neuronal morphology, whereas MKL2/MRTFB is a positive regulator. The mRNA expression of SCAI is downregulated during brain development, suggesting that a reduction in SCAI contributes to the reduced suppression of SRF-mediated gene induction, thus increasing dendritic complexity and developing neuronal circuits. In the present study, we hypothesized that brain-derived neurotrophic factor (BDNF), which is important for neuronal plasticity and development, might alter SCAI mRNA levels. We therefore investigated the effects of BDNF on SCAI mRNA levels in primary cultured cortical neurons. Furthermore, because alternative splicing generates several SCAI variants in the brain, we measured SCAI variant mRNA after BDNF stimulation. Both SCAI variant 1 and total SCAI mRNA expression levels were downregulated by BDNF. Moreover, the extracellular signal-regulated protein kinase/mitogen-activated protein kinase (ERK/MAPK) pathway was involved in the BDNF-mediated decrease in SCAI mRNA expression. Our findings provide insights into the molecular mechanism underlying a neurotrophic factor switch for the repressive transcriptional complex that includes SCAI.

SRF and SRF cofactor mRNA expression is differentially regulated by BDNF stimulation in cortical neurons

Serum response factor (SRF) is a transcription factor that plays essential roles in multiple brain functions in concert with SRF cofactors such as ternary complex factor (TCF) and megakaryoblastic leukemia (MKL)/myocardin-related transcription factor (MRTF), which comprises MKL1/MRTFA and MKL2/MRTFB. Here, we stimulated primary cultured rat cortical neurons with brain-derived neurotrophic factor (BDNF) and investigated the levels of SRF and SRF cofactor mRNA expression. We found that SRF mRNA was transiently induced by BDNF, whereas the levels of SRF cofactors were differentially regulated: mRNA expression of Elk1, a TCF family member, and MKL1/MRTFA were unchanged, while in contrast, mRNA expression of MKL2/MRTFB was transiently decreased. Inhibitor experiments revealed that BDNF-mediated alteration in mRNA levels detected in this study was mainly due to the extracellular signal-regulated protein kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway. Collectively, BDNF mediates the reciprocal regulation of SRF and MKL2/MRTFB at the mRNA expression level through ERK/MAPK, which may fine-tune the transcription of SRF target genes in cortical neurons. Accumulating evidence regarding the alteration of SRF and SRF cofactor levels detected in several neurological disorders suggests that the findings of this study might also provide novel insights into valuable therapeutic strategies for the treatment of brain diseases.

SRF in Neurochemistry: Overview of Recent Advances in Research on the Nervous System

Serum response factor (SRF) is a representative transcription factor that plays crucial roles in various biological phenomena by regulating immediate early genes (IEGs) and genes related to cell morphology and motility, among others. Over the years, the signal transduction pathways activating SRF have been clarified and SRF-target genes have been identified. In this overview, we initially briefly summarize the basic biology of SRF and its cofactors, ternary complex factor (TCF) and megakaryoblastic leukemia (MKL)/myocardin-related transcription factor (MRTF). Progress in the generation of nervous system-specific knockout (KO) or genetically modified mice as well as genetic analyses over the last few decades has not only identified novel SRF-target genes but also highlighted the neurochemical importance of SRF and its cofactors. Therefore, here we next present the phenotypes of mice with nervous system-specific KO of SRF or its cofactors by depicting recent findings associated with brain development, plasticity, epilepsy, stress response, and drug addiction, all of which result from function or dysfunction of the SRF axis. Last, we develop a hypothesis regarding the possible involvement of SRF and its cofactors in human neurological disorders including neurodegenerative, psychiatric, and neurodevelopmental diseases. This overview should deepen our understanding, highlight promising future directions for developing novel therapeutic strategies, and lead to illumination of the mechanisms underlying higher brain functions based on neuronal structure and function.

SRF depletion in early life contributes to social interaction deficits in the adulthood

Alterations in social behavior are core symptoms of major developmental neuropsychiatric diseases such as autism spectrum disorders or schizophrenia. Hence, understanding their molecular and cellular underpinnings constitutes the major research task. Dysregulation of the global gene expression program in the developing brain leads to modifications in a number of neuronal connections, synaptic strength and shape, causing unbalanced neuronal plasticity, which may be important substrate in the pathogenesis of neurodevelopmental disorders, contributing to their clinical outcome. Serum response factor (SRF) is a major transcription factor in the brain. The behavioral influence of SRF deletion during neuronal differentiation and maturation has never been studied because previous attempts to knock-out the gene caused premature death. Herein, we generated mice that lacked SRF from early postnatal development to precisely investigate the role of SRF starting in the specific time window before maturation of excitatory synapses that are located on dendritic spine occurs. We show that the time-controlled loss of SRF in neurons alters specific aspects of social behaviors in SRF knock-out mice, and causes deficits in developmental spine maturation at both the structural and functional levels, including downregulated expression of the AMPARs subunits GluA1 and GluA2, and increases the percentage of filopodial/immature dendritic spines. In aggregate, our study uncovers the consequences of postnatal SRF elimination for spine maturation and social interactions revealing novel mechanisms underlying developmental neuropsychiatric diseases.

Regulation of Dendritic Synaptic Morphology and Transcription by the SRF Cofactor MKL/MRTF

Accumulating evidence suggests that the serum response factor (SRF) cofactor megakaryoblastic leukemia (MKL)/myocardin-related transcription factor (MRTF) has critical roles in many physiological and pathological processes in various cell types. MKL/MRTF molecules comprise MKL1/MRTFA and MKL2/MRTFB, which possess actin-binding motifs at the N-terminus, and SRF-binding domains and a transcriptional activation domain (TAD) at the C-terminus. Several studies have reported that, in association with actin rearrangement, MKL/MRTF translocates from the cytoplasm to the nucleus, where it regulates SRF-mediated gene expression and controls cell motility. Therefore, it is important to elucidate the roles of MKL/MRTF in the nervous system with regard to its structural and functional regulation by extracellular stimuli. We demonstrated that MKL/MRTF is highly expressed in the brain, especially the synapses, and is involved in dendritic complexity and dendritic spine maturation. In addition to the positive regulation of dendritic complexity, we identified several MKL/MRTF isoforms that negatively regulate dendritic complexity in cortical neurons. We found that the MKL/MRTF isoforms were expressed differentially during brain development and the impacts of these isoforms on the immediate early genes including Arc/Arg3.1, were different. Here, we review the roles of MKL/MRTF in the nervous system, with a special focus on the MKL/MRTF-mediated fine-tuning of neuronal morphology and gene transcription. In the concluding remarks, we briefly discuss the future perspectives and the possible involvement of MKL/MRTF in neurological disorders such as schizophrenia and autism spectrum disorder.

RhoA: a dubious molecule in cardiac pathophysiology

The Ras homolog gene family member A (RhoA) is the founding member of Rho GTPase superfamily originally studied in cancer cells where it was found to stimulate cell cycle progression and migration. RhoA acts as a master switch control of actin dynamics essential for maintaining cytoarchitecture of a cell. In the last two decades, however, RhoA has been coined and increasingly investigated as an essential molecule involved in signal transduction and regulation of gene transcription thereby affecting physiological functions such as cell division, survival, proliferation and migration. RhoA has been shown to play an important role in cardiac remodeling and cardiomyopathies; underlying mechanisms are however still poorly understood since the results derived from in vitro and in vivo experiments are still inconclusive. Interestingly its role in the development of cardiomyopathies or heart failure remains largely unclear due to anomalies in the current data available that indicate both cardioprotective and deleterious effects. In this review, we aimed to outline the molecular mechanisms of RhoA activation, to give an overview of its regulators, and the probable mechanisms of signal transduction leading to RhoA activation and induction of downstream effector pathways and corresponding cellular responses in cardiac (patho)physiology. Furthermore, we discuss the existing studies assessing the presented results and shedding light on the often-ambiguous data. Overall, we provide an update of the molecular, physiological and pathological functions of RhoA in the heart and its potential in cardiac therapeutics.

Suppression of MKL1 promotes adipocytic differentiation and reduces the proliferation of myxoid liposarcoma cells

Myxoid liposarcoma (MLS) is thought to occur due to defective adipocytic differentiation in mesenchymal stem cells. A promising strategy for MLS treatment is the prevention of sarcomagenesis by promoting the terminal differentiation of MLS cells into adipocytes. Previous studies have reported that the suppression of megakaryoblastic leukemia 1 (MKL1) expression induces adipocytic differentiation in preadipocyte cell lines. The present study aimed to investigate the effects of MKL1 suppression on MLS cells. In the present study, MKL1 knockdown was demonstrated to promote the adipocytic differentiation of an MLS-derived cell line, designated 1955/91, under adipogenic conditions. This suggests that therapeutic targeting of the MKL1-associated molecular pathway has potential as a promising method of MLS treatment. However, the induction of adipogenesis by MKL knockdown was incomplete, and Oil Red O staining indicated that intracellular lipid droplets were only sporadically generated. Conversely, MKL1 knockdown reduced the growth of the MLS cells. As adipocytic differentiation in vitro requires cellular confluence, the decreased growth rate of the MLS cells following MKL1 knockdown could be attributed to the incomplete induction of adipogenesis. Translocated in liposarcoma-CCAAT/enhancer-binding protein homologous protein (TLS-CHOP) is an MLS-specific oncoprotein that is thought to play key roles in sarcomagenesis and the suppression of adipocytic differentiation. However, the results of western blotting analyses suggest that TLS-CHOP has limited effects on MKL1 expression in MLS cells and that MKL1 knockdown hardly affects TLS-CHOP expression. Thus, it is postulated that the inhibitory effect of TLS-CHOP on adipogenesis is not associated with MKL1 expression. However, MKL1 and the molecular pathway involving MKL1 appear to be attractive targets for the differentiation therapy of MLS.

Protein Kinase N Promotes Stress-Induced Cardiac Dysfunction Through Phosphorylation of Myocardin-Related Transcription Factor A and Disruption of Its Interaction With Actin

BACKGROUND

Heart failure is a complex syndrome that results from structural or functional impairment of ventricular filling or blood ejection. Protein phosphorylation is a major and essential intracellular mechanism that mediates various cellular processes in cardiomyocytes in response to extracellular and intracellular signals. The RHOA-associated protein kinase (ROCK/Rho-kinase), an effector regulated by the small GTPase RHOA, causes pathological phosphorylation of proteins, resulting in cardiovascular diseases. RHOA also activates protein kinase N (PKN); however, the role of PKN in cardiovascular diseases remains unclear.

METHODS

To explore the role of PKNs in heart failure, we generated tamoxifen-inducible, cardiomyocyte-specific PKN1- and PKN2-knockout mice by intercrossing the αMHC-CreERT2 line with Pkn1^flox/flox and Pkn2^flox/flox mice and applied a mouse model of transverse aortic constriction- and angiotensin II-induced heart failure. To identify a novel substrate of PKNs, we incubated GST-tagged myocardin-related transcription factor A (MRTFA) with recombinant GST-PKN-catalytic domain or GST-ROCK-catalytic domain in the presence of radiolabeled ATP and detected radioactive GST-MRTFA as phosphorylated MRTFA.

RESULTS

We demonstrated that RHOA activates 2 members of the PKN family of proteins, PKN1 and PKN2, in cardiomyocytes of mice with cardiac dysfunction. Cardiomyocyte-specific deletion of the genes encoding Pkn1 and Pkn2 (cmc-PKN1/2 DKO) did not affect basal heart function but protected mice from pressure overload- and angiotensin II-induced cardiac dysfunction. Furthermore, we identified MRTFA as a novel substrate of PKN1 and PKN2 and found that MRTFA phosphorylation by PKN was considerably more effective than that by ROCK in vitro. We confirmed that endogenous MRTFA phosphorylation in the heart was induced by pressure overload- and angiotensin II-induced cardiac dysfunction in wild-type mice, whereas cmc-PKN1/2 DKO mice suppressed transverse aortic constriction- and angiotensin II-induced phosphorylation of MRTFA. Although RHOA-mediated actin polymerization accelerated MRTFA-induced gene transcription, PKN1 and PKN2 inhibited the interaction of MRTFA with globular actin by phosphorylating MRTFA, causing increased serum response factor-mediated expression of cardiac hypertrophy- and fibrosis-associated genes.

CONCLUSIONS

Our results indicate that PKN1 and PKN2 activation causes cardiac dysfunction and is involved in the transition to heart failure, thus providing unique targets for therapeutic intervention for heart failure.

Loss of serum response factor in mature neurons in the dentate gyrus alters the morphology of dendritic spines and hippocampus-dependent behavioral tasks

Serum response factor (SRF) is a major transcription factor that regulates the expression of several plasticity-associated genes in the brain. Although the developmental expression of SRF in excitatory neurons is crucial for establishing proper hippocampal circuitry, no substantial evidence of its role in unstimulated mature neurons has been provided. The present study used time-controlled, conditional SRF knockout mice and found that the lack of SRF in adult neurons led to decreased actin levels and inactivation of the actin-severing protein cofilin 1 through its increase in phosphorylation at Ser3. The augmentation of cofilin 1 phosphorylation correlated with an alteration of dendritic spine morphology in the dentate gyrus, which was reflected by an increase in the number of spines that clustered into the long-spine category. The changes in spine morphology coincided with a lower amplitude and frequency of miniature excitatory postsynaptic currents. Moreover, SRF knockout animals were hyperactive and exhibited impairments in hippocampus-dependent behaviors, such as digging, marble burying, and nesting. Altogether, our data indicate that the adult deletion of neuronal SRF leads to alterations of spine morphology and function and hippocampus-dependent behaviors. Thus, SRF deletion in adult neurons recapitulates some aspects of morphological, electrophysiological, and behavioral changes that are observed in such psychiatric disorders as schizophrenia and autism spectrum disorders.

Nucleoskeletal regulation of transcription: Actin on MRTF

Myocardin-related transcription factor A (MRTF-A) and serum response factor (SRF) form an essential transcriptional complex that regulates the expression of many cytoskeletal genes in response to dynamic changes in the actin cytoskeleton. The nucleoskeleton, a “dynamic network of networks,” consists of numerous proteins that contribute to nuclear shape and to its various functions, including gene expression. In this review, we will discuss recent work that has identified many nucleoskeletal proteins, such as nuclear lamina and lamina-associated proteins, nuclear actin, and the linker of the cytoskeleton and nucleoskeleton complex as important regulators of MRTF-A/SRF transcriptional activity, especially in the context of mechanical control of transcription.

Impact statement

Regulation of gene expression is a fundamental cellular process that ensures the appropriate response of a cell to its surroundings. Alongside biochemical signals, mechanical cues, such as substrate rigidity, have been recognized as key regulators of gene expression. Nucleoskeletal components play an important role in mechanoresponsive transcription, particularly in controlling the activity of MRTF-A/SRF transcription factors. This ensures that the cell can balance the internal and external mechanical forces by fine-tuning the expression of cytoskeletal genes.

The nature and intensity of mechanical stimulation drive different dynamics of MRTF-A nuclear redistribution after actin remodeling in myoblasts

Serum response factor and its cofactor myocardin-related transcription factor (MRTF) are key elements of muscle-mass adaptation to workload. The transcription of target genes is activated when MRTF is present in the nucleus. The localization of MRTF is controlled by its binding to G-actin. Thus, the pathway can be mechanically activated through the mechanosensitivity of the actin cytoskeleton. The pathway has been widely investigated from a biochemical point of view, but its mechanical activation and the timescales involved are poorly understood. Here, we applied local and global mechanical cues to myoblasts through two custom-built set-ups, magnetic tweezers and stretchable substrates. Both induced nuclear accumulation of MRTF-A. However, the dynamics of the response varied with the nature and level of mechanical stimulation and correlated with the polymerization of different actin sub-structures. Local repeated force induced local actin polymerization and nuclear accumulation of MRTF-A by 30 minutes, whereas a global static strain induced both rapid (minutes) transient nuclear accumulation, associated with the polymerization of an actin cap above the nucleus, and long-term accumulation, with a global increase in polymerized actin. Conversely, high strain induced actin depolymerization at intermediate times, associated with cytoplasmic MRTF accumulation.

Single-molecule imaging of the transcription factor SRF reveals prolonged chromatin-binding kinetics upon cell stimulation

How transcription factors (TFs) activate transcription is a long-standing but still unsolved question. We analyzed serum response factor (SRF), a stimulus-responsive TF mediating immediate early gene (IEG) and cytoskeletal gene expression at single-molecule resolution. Cell stimulation enhanced SRF activity by increasing the number of long chromatin-associated SRF molecules in an oscillating pattern. Further, stimulation enhanced the SRF chromatin residence time, and SRF binding events segregated into three distinct residence time regimes (short, intermediate, and long bound). In summary, our single-molecule imaging study reveals highly dynamic and diverse SRF interactions with DNA. Thus, cell stimulation regulates TF activity by several interconnected mechanisms including nucleus−cytoplasm shuttling, TF phosphorylation, cofactor recruitment, and extension of chromatin residence time and enhancing chromatin-bound TF numbers.

Serum response factor (SRF) mediates immediate early gene (IEG) and cytoskeletal gene expression programs in almost any cell type. So far, SRF transcriptional dynamics have not been investigated at single-molecule resolution. We provide a study of single Halo-tagged SRF molecules in fibroblasts and primary neurons. In both cell types, individual binding events of SRF molecules segregated into three chromatin residence time regimes, short, intermediate, and long binding, indicating a cell type-independent SRF property. The chromatin residence time of the long bound fraction was up to 1 min in quiescent cells and significantly increased upon stimulation. Stimulation also enhanced the long bound SRF fraction at specific timepoints (20 and 60 min) in both cell types. These peaks correlated with activation of the SRF cofactors MRTF-A and MRTF-B (myocardin-related transcription factors). Interference with signaling pathways and cofactors demonstrated modulation of SRF chromatin occupancy by actin signaling, MAP kinases, and MRTFs.

SRF'ing and SAP'ing - the role of MRTF proteins in cell migration

Actin-based cell migration is a fundamental cellular activity that plays a crucial role in a wide range of physiological and pathological processes. An essential feature of the remodeling of actin cytoskeleton during cell motility is the de novo synthesis of factors involved in the regulation of the actin cytoskeleton and cell adhesion in response to growth-factor signaling, and this aspect of cell migration is critically regulated by serum-response factor (SRF)-mediated gene transcription. Myocardin-related transcription factors (MRTFs) are key coactivators of SRF that link actin dynamics to SRF-mediated gene transcription. In this Review, we provide a comprehensive overview of the role of MRTF in both normal and cancer cell migration by discussing its canonical SRF-dependent as well as its recently emerged SRF-independent functions, exerted through its SAP domain, in the context of cell migration. We conclude by highlighting outstanding questions for future research in this field.

Involvement of SRF coactivator MKL2 in BDNF-mediated activation of the synaptic activity-responsive element in the Arc gene

The expression of immediate early genes (IEGs) is thought to be an essential molecular basis of neuronal plasticity for higher brain function. Many IEGs contain serum response element in their transcriptional regulatory regions and their expression is controlled by serum response factor (SRF). SRF is known to play a role in concert with transcriptional cofactors. However, little is known about how SRF cofactors regulate IEG expression during the process of neuronal plasticity. We hypothesized that one of the SRF-regulated neuronal IEGs, activity-regulated cytoskeleton-associated protein (Arc; also termed Arg3.1), is regulated by an SRF coactivator, megakaryoblastic leukemia (MKL). To test this hypothesis, we initially investigated which binding site of the transcription factor or SRF cofactor contributes to brain-derived neurotrophic factor (BDNF)-induced Arc gene transcription in cultured cortical neurons using transfection and reporter assays. We found that BDNF caused robust induction of Arc gene transcription through a cAMP response element, binding site of myocyte enhancer factor 2, and binding site of SRF in an Arc enhancer, the synaptic activity-responsive element (SARE). Regardless of the requirement for the SRF-binding site, the binding site of a ternary complex factor, another SRF cofactor, did not affect BDNF-mediated Arc gene transcription. In contrast, chromatin immunoprecipitation revealed occupation of MKL at the SARE. Furthermore, knockdown of MKL2, but not MKL1, significantly decreased BDNF-mediated activation of the SARE. Taken together, these findings suggest a novel mechanism by which MKL2 controls the Arc SARE in response to BDNF stimulation.

Intravenous Infusion of Mesenchymal Stem Cells Alters Motor Cortex Gene Expression in a Rat Model of Acute Spinal Cord Injury

Recent evidence has demonstrated that remote responses in the brain, as well as local responses in the injured spinal cord, can be induced after spinal cord injury (SCI). Intravenous infusion of mesenchymal stem cells (MSCs) has been shown to provide functional improvements in SCI through local therapeutic mechanisms that provide neuroprotection, stabilization of the blood-spinal cord barrier, remyelination, and axonal sprouting. In the present study, we examined the brain response that might be associated with the functional improvements induced by the infused MSCs after SCI. Genome-wide RNA profiling was performed in the motor cortex of SCI rats at 3 days post-MSC or vehicle infusion. Then, quantitative reverse transcription-polymerase chain reaction (qRT-PCR) data revealed that the "behaviorally-associated differentially expressed genes (DEGs)" were identified by the Pearson's correlation analysis with the behavioral function, suggesting that the "behaviorally-associated DEGs" may be related to the functional recovery after systemic infusion of MSCs in SCI. These results suggested that the infused MSCs alter the gene expression signature in the brain and that these expression changes may contribute to the improved function in SCI.

Synaptic localisation of SRF coactivators, MKL1 and MKL2, and their role in dendritic spine morphology

The megakaryoblastic leukaemia (MKL) family are serum response factor (SRF) coactivators, which are highly expressed in the brain. Accordingly, MKL plays important roles in dendritic morphology, neuronal migration, and brain development. Further, nucleotide substitutions in the MKL1 and MKL2 genes are found in patients with schizophrenia and autism spectrum disorder, respectively. Thus, studies on the precise synaptic localisation and function of MKL in neurons are warranted. In this study, we generated and tested new antibodies that specifically recognise endogenously expressed MKL1 and MKL2 proteins in neurons. Using these reagents, we biochemically and immunocytochemically show that MKL1 and MKL2 are localised at synapses. Furthermore, shRNA experiments revealed that postsynaptic deletion of MKL1 or MKL2 reduced the percentage of mushroom- or stubby-type spines in cultured neurons. Taken together, our findings suggest that MKL1 and MKL2 are present at synapses and involved in dendritic spine maturation. This study may, at least in part, contribute to better understanding of the molecular mechanisms underlying MKL-mediated synaptic plasticity and neurological disorders.

Ribosomal stress and Tp53-mediated neuronal apoptosis in response to capsid protein of the Zika virus

We report here that in rat and human neuroprogenitor cells as well as rat embryonic cortical neurons Zika virus (ZIKV) infection leads to ribosomal stress that is characterized by structural disruption of the nucleolus. The anti-nucleolar effects were most pronounced in postmitotic neurons. Moreover, in the latter system, nucleolar presence of ZIKV capsid protein (ZIKV-C) was associated with ribosomal stress and apoptosis. Deletion of 22 C-terminal residues of ZIKV-C prevented nucleolar localization, ribosomal stress and apoptosis. Consistent with a casual relationship between ZIKV-C-induced ribosomal stress and apoptosis, ZIKV-C-overexpressing neurons were protected by loss-of-function manipulations targeting the ribosomal stress effector Tp53 or knockdown of the ribosomal stress mediator RPL11. Finally, capsid protein of Dengue virus, but not West Nile virus, induced ribosomal stress and apoptosis. Thus, anti-nucleolar and pro-apoptotic effects of protein C are flavivirus-species specific. In the case of ZIKV, capsid protein-mediated ribosomal stress may contribute to neuronal death, neurodevelopmental disruption and microcephaly.

Cellular diversity of the somatosensory cortical map plasticity

Sensory maps are representations of the sensory epithelia in the brain. Despite the intuitive explanatory power behind sensory maps as being neuronal precursors to sensory perception, and sensory cortical plasticity as a neural correlate of perceptual learning, molecular mechanisms that regulate map plasticity are not well understood. Here we perform a meta-analysis of transcriptional and translational changes during altered whisker use to nominate the major molecular correlates of experience-dependent map plasticity in the barrel cortex. We argue that brain plasticity is a systems level response, involving all cell classes, from neuron and glia to non-neuronal cells including endothelia. Using molecular pathway analysis, we further propose a gene regulatory network that could couple activity dependent changes in neurons to adaptive changes in neurovasculature, and finally we show that transcriptional regulations observed in major brain disorders target genes that are modulated by altered sensory experience. Thus, understanding the molecular mechanisms of experience-dependent plasticity of sensory maps might help to unravel the cellular events that shape brain plasticity in health and disease.

SRF modulates seizure occurrence, activity induced gene transcription and hippocampal circuit reorganization in the mouse pilocarpine epilepsy model

A hallmark of temporal lobe epilepsy (TLE) is hippocampal neuronal demise and aberrant mossy fiber sprouting. In addition, unrestrained neuronal activity in TLE patients induces gene expression including immediate early genes (IEGs) such as Fos and Egr1.

We employed the mouse pilocarpine model to analyze the transcription factor (TF) serum response factor (SRF) in epileptogenesis, seizure induced histopathology and IEG induction. SRF is a neuronal activity regulated TF stimulating IEG expression as well as nerve fiber growth and guidance. Adult conditional SRF deficient mice (Srf^CaMKCreERT2) were more refractory to initial status epilepticus (SE) acquisition. Further, SRF deficient mice developed more spontaneous recurrent seizures (SRS). Genome-wide transcriptomic analysis uncovered a requirement of SRF for SE and SRS induced IEG induction (e.g. Fos, Egr1, Arc, Npas4, Btg2, Atf3). SRF was required for epilepsy associated neurodegeneration, mossy fiber sprouting and inflammation. We uncovered MAP kinase signaling as SRF target during epilepsy. Upon SRF ablation, seizure evoked induction of dual specific phosphatases (Dusp5 and Dusp6) was reduced. Lower expression of these negative ERK kinase regulators correlated with altered P-ERK levels in epileptic Srf mutant animals.

Overall, this study uncovered an SRF contribution to several processes of epileptogenesis in the pilocarpine model.

Proapoptotic Requirement of Ribosomal Protein L11 in Ribosomal Stress-Challenged Cortical Neurons

While impaired ribosomal biogenesis is observed in neurodegenerative diseases, its pathogenic contributions are not clear. For instance, it is well established that in rodent neurons, genetic inhibition of RNA-polymerase 1 that transcribes rRNA results in structural disruption of the nucleolus, neuronal apoptosis, and neurodegeneration. However, in most neurodegenerative diseases, nucleolar morphology is unaffected. It is reported here that in primary cortical neurons from newborn rats, inhibition of ribosomal biogenesis by shRNA-mediated knockdowns of several ribosomal proteins including S6, S14, or L4 resulted in p53-mediated apoptosis despite absence of structural disruption of the nucleolus. Conversely, knockdown of the RP L11, which in nonneuronal systems mediates p53 activation downstream of ribosomal stress, protected neurons against inhibition of ribosomal biogenesis but not staurosporine. Moreover, overexpression of L11 enhanced p53-driven transcription and increased neuronal apoptosis. In addition, inhibition of p53, or L11 knockdown, blocked apoptosis in response to the RNA analog 5-fluorouridine which perturbed nucleolar structure, inhibited ribosomal synthesis, and activated p53. Although the DNA double-strand break (DSB) inducer etoposide activated p53, nucleolar structure appeared intact. However, by activating the DNA damage response kinase ATM, etoposide increased 47S pre-rRNA levels, and enhanced nucleolar accumulation of nascent RNA, suggesting slower rRNA processing and/or increased Pol1 activity. In addition, shL11 reduced etoposide-induced apoptosis. Therefore, seemingly normal morphology of the neuronal nucleolus does not exclude presence of ribosomal stress. Conversely, targeting the ribosomal stress-specific signaling mediators including L11 offers a novel approach to uncover neurodegenerative contributions of deregulated ribosomal synthesis as exemplified in DSB-challenged neurons.

Association between MKL1 rs6001946 and schizophrenia in a Han Chinese population

Megakaryoblastic leukemia 1 (MKL1) is highly expressed in the nervous system and plays a potentially principal role in neuronal migration and morphology. A recent study showed that some genetic loci within the MKL1 gene, especially single nucleotide polymorphism (SNP) rs6001946, may increase schizophrenia susceptibility. Here, we sought to determine whether the polymorphism rs6001946 was associated with schizophrenia in a Han Chinese population using the ligase detection reaction-polymerase chain reaction method to genotype SNP rs6001946 in the MKL1 gene. We observed that there was a marginally significant association between SNP rs6001946 and the risk of schizophrenia (P=0.077). Our results indicated that SNP rs6001946 of the MKL1 gene is likely a risk factor for schizophrenia, but the role of SNP rs6001946 in the development of schizophrenia in Han Chinese should be interpreted cautiously. Further studies with larger sample sizes are needed to determine the involvement of the MKL1 polymorphism in schizophrenia susceptibility.

Stress-dependent phosphorylation of myocardin-related transcription factor A (MRTF-A) by the p38(MAPK)/MK2 axis

Myocardin-related transcription factor A (MRTF-A) is a known actin-regulated transcriptional coactivator of serum response factor (SRF). Stimulation of actin polymerization activates MRTF-A by releasing it from G-actin and thus allowing it to bind to and activate SRF. Here, we compared protein phosphorylation in MK2/3-deficient cells rescued or not by ectopic expression of MK2 in two independent phosphoproteomic approaches using anisomycin-treated MEF cells and LPS-stimulated mouse macrophages, respectively. Two MRTF-A sites, Ser³⁵¹ (corresponding to Ser³¹² in human) and Ser³⁷¹ (Ser³³³ in human), showed significantly stronger phosphorylation (12-fold and 6-fold increase) in the cells expressing MK2. MRTF-A is phosphorylated at these sites in a stress-, but not in a mitogen-induced manner, and p38^MAPK/MK2 catalytic activities are indispensable for this phosphorylation. MK2-mediated phosphorylation of MRTF-A at Ser³¹² and Ser³³³ was further confirmed in an in vitro kinase assay and using the phospho-protein kinase-D (PKD)-consensus motif antibody (anti-LXRXXpS/pT), the p38^MAPK inhibitor BIRB-796, MK2/3-deficient cells and MRTF-A phospho-site mutants. Unexpectedly, dimerization, subcellular localization and translocation, interaction with actin, SRF or SMAD3 and transactivating potential of MRTF-A seem to be unaffected by manipulating the p38^MAPK/MK2-dependent phosphorylations. Hence, MRTF-A is stress-dependently phosphorylated by MK2 at Ser³¹² and Ser³³³ with so far undetected functional and physiological consequences.

Phosphorylation acts positively and negatively to regulate MRTF-A subcellular localisation and activity

The myocardin-related transcription factors (MRTF-A and MRTF-B) regulate cytoskeletal genes through their partner transcription factor SRF. The MRTFs bind G-actin, and signal-regulated changes in cellular G-actin concentration control their nuclear accumulation. The MRTFs also undergo Rho- and ERK-dependent phosphorylation, but the function of MRTF phosphorylation, and the elements and signals involved in MRTF-A nuclear export are largely unexplored. We show that Rho-dependent MRTF-A phosphorylation reflects relief from an inhibitory function of nuclear actin. We map multiple sites of serum-induced phosphorylation, most of which are S/T-P motifs and show that S/T-P phosphorylation is required for transcriptional activation. ERK-mediated S98 phosphorylation inhibits assembly of G-actin complexes on the MRTF-A regulatory RPEL domain, promoting nuclear import. In contrast, S33 phosphorylation potentiates the activity of an autonomous Crm1-dependent N-terminal NES, which cooperates with five other NES elements to exclude MRTF-A from the nucleus. Phosphorylation thus plays positive and negative roles in the regulation of MRTF-A.

Nucleolar Enrichment of Brain Proteins with Critical Roles in Human Neurodevelopment

To study nucleolar involvement in brain development, the nuclear and nucleolar proteomes from the rat cerebral cortex at postnatal day 7 were analyzed using LC-MS/iTRAQ methodology. Data of the analysis are available via ProteomeXchange with identifier PXD002188. Among 504 candidate nucleolar proteins, the overrepresented gene ontology terms included such cellular compartmentcategories as "nucleolus", "ribosome" and "chromatin". Consistent with such classification, the most overrepresented functional gene ontology terms were related to RNA metabolism/ribosomal biogenesis, translation, and chromatin organization. Sixteen putative nucleolar proteins were associated with neurodevelopmental phenotypes in humans. Microcephaly and/or cognitive impairment were the most common phenotypic manifestations. Although several such proteins have links to ribosomal biogenesis and/or genomic stability/chromatin structure (e.g. EMG1, RPL10, DKC1, EIF4A3, FLNA, SMC1, ATRX, MCM4, NSD1, LMNA, or CUL4B), others including ADAR, LARP7, GTF2I, or TCF4 have no such connections known. Although neither the Alazami syndrome-associated LARP7nor the Pitt-Hopkins syndrome-associated TCF4 were reported in nucleoli of non-neural cells, in neurons, their nucleolar localization was confirmed by immunostaining. In cultured rat hippocampal neurons, knockdown of LARP7 reduced both perikaryal ribosome content and general protein synthesis. Similar anti-ribosomal/anti-translation effects were observed after knockdown of the ribosomal biogenesis factor EMG1 whose deficiency underlies Bowen-Conradi syndrome. Finally, moderate reduction of ribosome content and general protein synthesis followed overexpression of two Pitt-Hopkins syndrome mutant variants of TCF4. Therefore, dysregulation of ribosomal biogenesis and/or other functions of the nucleolus may disrupt neurodevelopment resulting in such phenotypes as microcephaly and/or cognitive impairment.

Adult Deletion of SRF Increases Epileptogenesis and Decreases Activity-Induced Gene Expression

Although the transcription factor serum response factor (SRF) has been suggested to play a role in activity-dependent gene expression and mediate plasticity-associated structural changes in the hippocampus, no unequivocal evidence has been provided for its role in brain pathology, such as epilepsy. A genome-wide program of activity-induced genes that are regulated by SRF also remains unknown. In the present study, we show that the inducible and conditional deletion of SRF in the adult mouse hippocampus increases the epileptic phenotype in the kainic acid model of epilepsy, reflected by more severe and frequent seizures. Moreover, we observe a robust decrease in activity-induced gene transcription in SRF knockout mice. We characterize the genetic program controlled by SRF in neurons and using functional annotation, we find that SRF target genes are associated with synaptic plasticity and epilepsy. Several of these SRF targets function as regulators of inhibitory or excitatory balance and the structural plasticity of neurons. Interestingly, mutations in those SRF targets have found to be associated with such human neuropsychiatric disorders, as autism and intellectual disability. We also identify novel direct SRF targets in hippocampus: Npas4, Gadd45g, and Zfp36. Altogether, our data indicate that proteins that are highly upregulated by neuronal stimulation, identified in the present study as SRF targets, may function as endogenous protectors against overactivation. Thus, the lack of these effector proteins in SRF knockout animals may lead to uncontrolled excitation and eventually epilepsy.

The transcription factor scleraxis is a critical regulator of cardiac fibroblast phenotype

Background

Resident fibroblasts synthesize the cardiac extracellular matrix, and can undergo phenotype conversion to myofibroblasts to augment matrix production, impairing function and contributing to organ failure. A significant gap in our understanding of the transcriptional regulation of these processes exists. Given the key role of this phenotype conversion in fibrotic disease, the identification of such novel transcriptional regulators may yield new targets for therapies for fibrosis.

Results

Using explanted primary cardiac fibroblasts in gain- and loss-of-function studies, we found that scleraxis critically controls cardiac fibroblast/myofibroblast phenotype by direct transcriptional regulation of myriad genes that effectively define these cells, including extracellular matrix components and α-smooth muscle actin. Scleraxis furthermore potentiated the TGFβ/Smad3 signaling pathway, a key regulator of myofibroblast conversion, by facilitating transcription complex formation. While scleraxis promoted fibroblast to myofibroblast conversion, loss of scleraxis attenuated myofibroblast function and gene expression. These results were confirmed in scleraxis knockout mice, which were cardiac matrix-deficient and lost ~50 % of their complement of cardiac fibroblasts, with evidence of impaired epithelial-to-mesenchymal transition (EMT). Scleraxis directly transactivated several EMT marker genes, and was sufficient to induce mesenchymal/fibroblast phenotype conversion of A549 epithelial cells. Conversely, loss of scleraxis attenuated TGFβ-induced EMT marker expression.

Conclusions

Our results demonstrate that scleraxis is a novel and potent regulator of cellular progression along the continuum culminating in the cardiac myofibroblast phenotype. Scleraxis was both sufficient to drive conversion, and required for full conversion to occur. Scleraxis fulfills this role by direct transcriptional regulation of key target genes, and by facilitating TGFβ/Smad signaling. Given the key role of fibroblast to myofibroblast conversion in fibrotic diseases in the heart and other tissue types, scleraxis may be an important target for therapeutic development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0243-8) contains supplementary material, which is available to authorized users.

LSD1 modulates stress-evoked transcription of immediate early genes and emotional behavior

Behavioral changes in response to stressful stimuli can be controlled via adaptive epigenetic changes in neuronal gene expression. Here we indicate a role for the transcriptional corepressor Lysine-Specific Demethylase 1 (LSD1) and its dominant-negative splicing isoform neuroLSD1, in the modulation of emotional behavior. In mouse hippocampus, we show that LSD1 and neuroLSD1 can interact with transcription factor serum response factor (SRF) and set the chromatin state of SRF-targeted genes early growth response 1 (egr1) and c-fos Deletion or reduction of neuro LSD1 in mutant mice translates into decreased levels of activating histone marks at egr1 and c-fos promoters, dampening their psychosocial stress-induced transcription and resulting in low anxiety-like behavior. Administration of suberoylanilide hydroxamine to neuroLSD1(KO)mice reactivates egr1 and c-fos transcription and restores the behavioral phenotype. These findings indicate that LSD1 is a molecular transducer of stressful stimuli as well as a stress-response modifier. Indeed, LSD1 expression itself is increased acutely at both the transcriptional and splicing levels by psychosocial stress, suggesting that LSD1 is involved in the adaptive response to stress.

The multiple sclerosis drug fingolimod (FTY720) stimulates neuronal gene expression, axonal growth and regeneration

Fingolimod (FTY720) is a new generation oral treatment for multiple sclerosis (MS). So far, FTY720 was mainly considered to target trafficking of immune cells but not brain cells such as neurons. Herein, we analyzed FTY720's potential to directly alter neuronal function. In CNS neurons, we identified a FTY720 governed gene expression response. FTY720 upregulated immediate early genes (IEGs) encoding for neuronal activity associated transcription factors such as c-Fos, FosB, Egr1 and Egr2 and induced actin cytoskeleton associated genes (actin isoforms, tropomyosin, calponin). Stimulation of primary neurons with FTY720 enhanced neurite growth and altered growth cone morphology. In accordance, FTY720 enhanced axon regeneration in mice upon facial nerve axotomy. We identified components of a FTY720 engaged signaling cascade including S1P receptors, G12/13G-proteins, RhoA-GTPases and the transcription factors SRF/MRTF. In summary, we uncovered a broader cellular and therapeutic operation mode of FTY720, suggesting beneficial FTY720 effects also on CNS neurons during MS therapy and for treatment of other neurodegenerative diseases requiring neuroprotective and neurorestorative processes.

Requirement of Neuronal Ribosome Synthesis for Growth and Maintenance of the Dendritic Tree

The nucleolus serves as a principal site of ribosome biogenesis but is also implicated in various non-ribosomal functions, including negative regulation of the pro-apoptotic transcription factor p53. Although disruption of the nucleolus may trigger the p53-dependent neuronal death, neurotoxic consequences of a selective impairment of ribosome production are unclear. Here, we report that in rat forebrain neuronal maturation is associated with a remarkable expansion of ribosomes despite postnatal down-regulation of ribosomal biogenesis. In cultured rat hippocampal neurons, inhibition of the latter process by knockdowns of ribosomal proteins S6, S14, or L4 reduced ribosome content without disrupting nucleolar integrity, cell survival, and signaling responses to the neurotrophin brain-derived neurotrophic factor. Moreover, reduced general protein synthesis and/or formation of RNA stress granules suggested diminished ribosome recruitment to at least some mRNAs. Such a translational insufficiency was accompanied by impairment of brain-derived neurotrophic factor-mediated dendritic growth. Finally, RNA stress granules and smaller dendritic trees were also observed when ribosomal proteins were depleted from neurons with established dendrites. Thus, a robust ribosomal apparatus is required to carry out protein synthesis that supports dendritic growth and maintenance. Consequently, deficits of ribosomal biogenesis may disturb neurodevelopment by reducing neuronal connectivity. Finally, as stress granule formation and dendritic loss occur early in neurodegenerative diseases, disrupted homeostasis of ribosomes may initiate and/or amplify neurodegeneration-associated disconnection of neuronal circuitries.

Myozap Deficiency Promotes Adverse Cardiac Remodeling via Differential Regulation of Mitogen-activated Protein Kinase/Serum-response Factor and β-Catenin/GSK-3β Protein Signaling

The intercalated disc (ID) is a "hot spot" for heart disease, as several ID proteins have been found mutated in cardiomyopathy. Myozap is a recent addition to the list of ID proteins and has been implicated in serum-response factor signaling. To elucidate the cardiac consequences of targeted deletion of myozap in vivo, we generated myozap-null mutant (Mzp(-/-)) mice. Although Mzp(-/-) mice did not exhibit a baseline phenotype, increased biomechanical stress due to pressure overload led to accelerated cardiac hypertrophy, accompanied by "super"-induction of fetal genes, including natriuretic peptides A and B (Nppa/Nppb). Moreover, Mzp(-/-) mice manifested a severe reduction of contractile function, signs of heart failure, and increased mortality. Expression of other ID proteins like N-cadherin, desmoplakin, connexin-43, and ZO-1 was significantly perturbed upon pressure overload, underscored by disorganization of the IDs in Mzp(-/-) mice. Exploration of the molecular causes of enhanced cardiac hypertrophy revealed significant activation of β-catenin/GSK-3β signaling, whereas MAPK and MKL1/serum-response factor pathways were inhibited. In summary, myozap is required for proper adaptation to increased biomechanical stress. In broader terms, our data imply an essential function of the ID in cardiac remodeling beyond a mere structural role and emphasize the need for a better understanding of this molecular structure in the context of heart disease.

A novel inhibitory mechanism of MRTF-A/B on the ICAM-1 gene expression in vascular endothelial cells

The roles of myocardin-related transcription factor A (MRTF-A) and MRTF-B in vascular endothelial cells are not completely understood. Here, we found a novel regulatory mechanism for MRTF-A/B function. MRTF-A/B tend to accumulate in the nucleus in arterial endothelial cells in vivo and human aortic endothelial cells (HAoECs) in vitro. In HAoECs, nuclear localization of MRTF-A/B was not significantly affected by Y27632 or latrunculin B, primarily due to the reduced binding of MRTF-A/B to G-actin and in part, to the low level of MRTF-A phosphorylation by ERK. MRTF-A/B downregulation by serum depletion or transfection of siRNA against MRTF-A and/or MRTF-B induced ICAM-1 expression in HAoECs. It is known that nuclear import of nuclear factor−κB (NF−κB) plays a key role in ICAM-1 gene transcription. However, nuclear accumulation of NF−κB p65 was not observed in MRTF-A/B-depleted HAoECs. Our present findings suggest that MRTF-A/B inhibit ICAM-1 mRNA expression by forming a complex with NF−κB p65 in the nucleus. Conversely, downregulation of MRTF-A/B alleviates this negative regulation without further translocation of NF−κB p65 into the nucleus. These results reveal the novel roles of MRTF-A/B in the homeostasis of vascular endothelium.

Common variants in the MKL1 gene confer risk of schizophrenia

Genome-wide association studies (GWAS) of schizophrenia have identified multiple risk variants with robust association signals for schizophrenia. However, these variants could explain only a small proportion of schizophrenia heritability. Furthermore, the effect size of these risk variants is relatively small (eg, most of them had an OR less than 1.2), suggesting that additional risk variants may be detected when increasing sample size in analysis. Here, we report the identification of a genome-wide significant schizophrenia risk locus at 22q13.1 by combining 2 large-scale schizophrenia cohort studies. Our meta-analysis revealed that 7 single nucleotide polymorphism (SNPs) on chromosome 22q13.1 reached the genome-wide significance level (P < 5.0×10(-8)) in the combined samples (a total of 38441 individuals). Among them, SNP rs6001946 had the most significant association with schizophrenia (P = 2.04×10(-8)). Interestingly, all 7 SNPs are in high linkage disequilibrium and located in the MKL1 gene. Expression analysis showed that MKL1 is highly expressed in human and mouse brains. We further investigated functional links between MKL1 and proteins encoded by other schizophrenia susceptibility genes in the whole human protein interaction network. We found that MKL1 physically interacts with GSK3B, a protein encoded by a well-characterized schizophrenia susceptibility gene. Collectively, our results revealed that genetic variants in MKL1 might confer risk to schizophrenia. Further investigation of the roles of MKL1 in the pathogenesis of schizophrenia is warranted.

CNS axon regeneration inhibitors stimulate an immediate early gene response via MAP kinase-SRF signaling

Background

CNS axon regeneration inhibitors such as Nogo and CSPGs (Chondroitin Sulfate Proteoglycans) are major extrinsic factors limiting outgrowth of severed nerve fibers. However, knowledge on intracellular signaling cascades and gene expression programs activated by these inhibitors in neurons is sparse. Herein we studied intracellular signaling cascades activated by total myelin, Nogo and CSPGs in primary mouse CNS neurons.

Results

Total myelin, Nogo and CSPGs stimulated gene expression activity of the serum response factor (SRF), a central gene regulator of immediate early (IEG) and actin cytoskeletal gene transcription. As demonstrated by pharmacological interference, SRF-mediated IEG activation by myelin, Nogo or CSPGs depended on MAP kinase, to a lesser extent on Rho-GTPase but not on PKA signaling. Stimulation of neurons with all three axon growth inhibitors activated the MAP kinase ERK. In addition to ERK activation, myelin activated the IEG c-Fos, an important checkpoint of neuronal survival vs. apoptosis. Employing Srf deficient neurons revealed that myelin-induced IEG activation requires SRF. This suggests an SRF function in mediating neuronal signaling evoked by axon regeneration associated inhibitors. Besides being a signaling target of axon growth inhibitors, we show that constitutively-active SRF-VP16 can be employed to circumvent neurite growth inhibition imposed by myelin, Nogo and CSPGs.

Conclusion

In sum, our data demonstrate that axon regeneration inhibitors such as Nogo trigger gene expression programs including an SRF-dependent IEG response via MAP kinases and Rho-GTPases.

Calcyclin-binding protein/Siah-1-interacting protein as a regulator of transcriptional responses in brain cells

The calcyclin-binding protein/Siah-1-interacting protein (CacyBP/SIP) is highly expressed in the brain and has been shown to regulate β-catenin-driven transcription in thymocytes. Therefore, we investigated whether CacyBP/SIP plays a role as a transcriptional regulator in brain cells. In brain-derived neurotrophic factor (BDNF)- and forskolin-stimulated rat primary cortical neurons, overexpression of CacyBP/SIP enhanced transcriptional activity of the cAMP-response element (CRE). In addition, overexpressed CacyBP/SIP enhanced BDNF-mediated activation of the nuclear factor of activated T cells (NFAT) but not the serum response element (SRE). These stimulatory effects required an intact C-terminal domain of CacyBP/SIP. Moreover, in C6 rat glioma cells, the overexpressed CacyBP/SIP enhanced activation of CRE and NFAT following forskolin and serum stimulation, respectively. Conversely, knockdown of endogenous CacyBP/SIP reduced activation of CRE and NFAT but not of SRE. Taken together, these results indicate that CacyBP/SIP is a novel regulator of CRE- and NFAT-driven transcription.

Towards a better understanding of cognitive behaviors regulated by gene expression downstream of activity-dependent transcription factors

In the field of molecular and cellular neuroscience, it is not a trivial task to see the forest for the trees, where numerous, and seemingly independent, molecules often work in concert to control critical steps of synaptic plasticity and signalling. Here, we will first summarize our current knowledge on essential activity-dependent transcription factors (TFs) such as CREB, MEF2, Npas4 and SRF, then examine how various transcription cofactors (TcoFs) also contribute to defining the transcriptional outputs during learning and memory. This review finally attempts a provisory synthesis that sheds new light on some of the emerging principles of neuronal circuit dynamics driven by activity-regulated gene transcription to help better understand the intricate relationship between activity-dependent gene expression and cognitive behavior.

The Erk MAP kinase pathway is activated at muscle spindles and is required for induction of the muscle spindle-specific gene Egr3 by neuregulin1

Muscle spindles are sensory receptors composed of specialized muscle fibers, known as intrafusal muscle fibers, along with the endings of sensory neuron axons that innervate these muscle fibers. Formation of muscle spindles requires neuregulin1 (NRG1), which is released by sensory axons, activating ErbB receptors in muscle cells that are contacted. The transcription factor Egr3 is transcriptionally induced by NRG1, which in turn activates various target genes involved in forming intrafusal fibers. We have previously shown that, in cultured muscle cells, NRG1 signaling activates the Egr3 gene through SRF and CREB, which bind to a composite regulatory element, and that NRG1 signaling targets SRF by stimulating nuclear translocation of SRF coactivators myocardin-related transcription factor (MRTF)-A and MRTF-B and targets CREB by phosphorylation. The current studies examined signaling relays that might function in the NRG1 pathway upstream of SRF and CREB. We found that transcriptional induction of Egr3 in response to NRG1 requires the MAP kinase Erk1/2, which acts upstream of CREB to induce its phosphorylation. MRTFs are targeted by the Rho-actin pathway, yet in the absence of Rho-actin signaling, even though MRTFs fail to be translocated to the nucleus, NRG1 induces Egr3 transcription. In mouse muscle in vivo, activation of Erk1/2 is enhanced selectively where muscle spindles are located. These results suggest that Erk1/2 acts in intrafusal fibers of muscle spindles to induce transcription of Egr3 and that Egr3 induction occurs independently of MRTFs and involves Erk1/2 acting on other transcriptional regulatory targets that interact with the SRF-CREB regulatory element.

Identification, expression and characterization of rat isoforms of the serum response factor (SRF) coactivator MKL1

Megakaryoblastic leukemia 1 (MKL1) is a member of the MKL family of serum response factor (SRF) coactivators. Here we have identified three rat MKL1 transcripts: two are homologues of mouse MKL1 transcripts, full-length MKL1 (FLMKL1) and basic, SAP, and coiled-coil domains (BSAC), the third is a novel transcript, MKL1-elongated derivative of yield (MELODY). These rat MKL1 transcripts are differentially expressed in a wide variety of tissues with highest levels in testis and brain. During brain development, these transcripts display differential patterns of expression. The FLMKL1 transcript encodes two isoforms that utilize distinct translation start sites. The longer form possesses three actin-binding RPXXXEL (RPEL) motifs and the shorter form, MKL1met only has two RPEL motifs. All four rat MKL1 isoforms, FLMKL1, BSAC, MKL1met and MELODY increased SRF-mediated transcription, but not CREB-mediated transcription. Accordingly, the differential expression of MKL1 isoforms may help fine-tune gene expression during brain development.

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Megakaryoblastic leukemia 1 (MKL1) is a serum response factor (SRF) coactivator.

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We have identified multiple rat MKL1 isoforms, including a novel one named MELODY.

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Rat MKL1 isoforms are enriched in testis and brain.

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Expression of rat MKL1 isoforms is regulated during brain development.

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All rat MKL1 isoforms act as SRF transcriptional coactivators.

Early phase of plasticity-related gene regulation and SRF dependent transcription in the hippocampus

Hippocampal organotypic cultures are a highly reliable in vitro model for studying neuroplasticity: in this paper, we analyze the early phase of the transcriptional response induced by a 20 µM gabazine treatment (GabT), a GABA-Ar antagonist, by using Affymetrix oligonucleotide microarray, RT-PCR based time-course and chromatin-immuno-precipitation. The transcriptome profiling revealed that the pool of genes up-regulated by GabT, besides being strongly related to the regulation of growth and synaptic transmission, is also endowed with neuro-protective and pro-survival properties. By using RT-PCR, we quantified a time-course of the transient expression for 33 of the highest up-regulated genes, with an average sampling rate of 10 minutes and covering the time interval [10∶90] minutes. The cluster analysis of the time-course disclosed the existence of three different dynamical patterns, one of which proved, in a statistical analysis based on results from previous works, to be significantly related with SRF-dependent regulation (p-value<0.05). The chromatin immunoprecipitation (chip) assay confirmed the rich presence of working CArG boxes in the genes belonging to the latter dynamical pattern and therefore validated the statistical analysis. Furthermore, an in silico analysis of the promoters revealed the presence of additional conserved CArG boxes upstream of the genes Nr4a1 and Rgs2. The chip assay confirmed a significant SRF signal in the Nr4a1 CArG box but not in the Rgs2 CArG box.

Centaurin-α1-Ras-Elk-1 signaling at mitochondria mediates β-amyloid-induced synaptic dysfunction

Alzheimer's disease is thought to be caused by β-amyloid peptide (Aβ)-dependent synaptic dysfunction. However, the signaling pathways connecting Aβ and synaptic dysfunction remain elusive. Here we report that Aβ transiently increases the expression level of centaurin-α1 (CentA1) in neurons, which induces a Ras-dependent association of Elk-1 with mitochondria, leading to mitochondrial and synaptic dysfunction in organotypic hippocampal slices of rats. Downregulation of the CentA1-Ras-Elk-1 pathway restored normal mitochondrial activity, spine structural plasticity, spine density, and the amplitude and frequency of miniature EPSCs in Aβ-treated neurons, whereas upregulation of the pathway was sufficient to decrease spine density. Elevations of CentA1 and association of Elk-1 with mitochondria were also observed in transgenic mice overexpressing a human mutant form of amyloid precursor protein. Therefore, the CentA1-Ras-Elk-1 signaling pathway acts on mitochondria to regulate dendritic spine density and synaptic plasticity in response to Aβ in hippocampal neurons, providing new pharmacological targets for Alzheimer's disease.

BDNF and memory processing

Brain-derived neurotrophic factor has emerged as one of the most important molecules involved in memory. Its wide role in different types of memories that depend on different structures as well as its involvement in distinct memory stages points at BDNF as one likely target to treat cognitive impairments and anxiety-related memory disorders. However, regulation of BDNF expression is very complex as well as its modes of action. Here we describe the latest research carried out on the function of BDNF in memory to illustrate such complexity. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Brain-derived neurotrophic factor induces matrix metalloproteinase 9 expression in neurons via the serum response factor/c-Fos pathway

Brain-derived neurotrophic factor (BDNF) plays a pivotal role in the regulation of the transcription of genes that encode proplasticity proteins. In the present study, we provide evidence that stimulation of rat primary cortical neurons with BDNF upregulates matrix metalloproteinase 9 (MMP-9) mRNA and protein levels and increases enzymatic activity. The BDNF-induced MMP-9 transcription was dependent on extracellular signal-regulated kinase 1/2 (ERK1/2) pathway and c-Fos expression. Overexpression of AP-1 dimers in neurons led to MMP-9 promoter activation, with the most potent being those that contained c-Fos, whereas knockdown of endogenous c-Fos by small hairpin RNA (shRNA) reduced BDNF-mediated MMP-9 transcription. Additionally, mutation of the proximal AP-1 binding site in the MMP-9 promoter inhibited the activation of MMP-9 transcription. BDNF stimulation of neurons induced binding of endogenous c-Fos to the proximal MMP-9 promoter region. Furthermore, as the c-Fos gene is a known target of serum response factor (SRF), we investigated whether SRF contributes to MMP-9 transcription. Inhibition of SRF and its cofactors by either overexpression of dominant negative mutants or shRNA decreased MMP-9 promoter activation. In contrast, MMP-9 transcription was not dependent on CREB activity. Finally, we showed that neuronal activity stimulates MMP-9 transcription in a tyrosine kinase receptor B (TrkB)-dependent manner.

Acrolein-induced inflammatory signaling in vascular smooth muscle cells requires activation of serum response factor (SRF) and NFκB

BACKGROUND

Modulation of inflammatory signaling has been elucidated in several disease models. Acrolein, an environmental pollutant, has been linked to diseases such as atherosclerosis and to the inflammatory process involving nuclear factor κB (NFκB). Serum response factor (SRF), a transcription factor, regulates cell development, differentiation and proliferation through signaling molecules such as extracellular signal-regulated kinase 1/2 (ERK1/2) and CD36. We hypothesized that acrolein toxicity involves SRF in the process of activating NFκB and may involve CD36/ERK1/2.

METHODS

Vascular smooth muscle cells (VSMCs) were exposed to acrolein (0.5 μg/mL) in the presence or absence of 10 nM QNZ (NFκB inhibitor), 300 nM CCG1423 (SRF inhibitor) and 50 μM PD98059 (ERK1/2 inhibitor). Protein and RNA were isolated. Changes in expression were determined by Western blot and polymerase chain reaction (PCR) array.

RESULTS

Subtoxic doses of acrolein increased ERK1/2, SRF and NFκB protein expression, whereas CD36 expression was unchanged. Increase in NFκB expression was accompanied by an increase in activity. ERK1/2 inhibition only blunted SRF expression. SRF inhibition blunted NFκB expression but not that of ERK1/2. CD36 expression was unchanged in the presence of either inhibitor. PCR array analysis indicated up-regulation of nine genes (>4- to 50-fold) and down-regulation of six genes (>4- to 12-fold) involved in inflammatory signaling.

CONCLUSIONS

We propose that SRF is required in acrolein activation of NFκB and is ERK1/2 dependent.

Induction of megakaryocyte differentiation drives nuclear accumulation and transcriptional function of MKL1 via actin polymerization and RhoA activation

How components of the cytoskeleton regulate complex cellular responses is fundamental to understanding cellular function. Megakaryoblast leukemia 1 (MKL1), an activator of serum response factor (SRF) transcriptional activity, promotes muscle, neuron, and megakaryocyte differentiation. In muscle cells, where MKL1 subcellular localization is one mechanism by which cells control SRF activity, MKL1 translocation from the cytoplasm to the nucleus in response to actin polymerization is critical for its function as a transcriptional regulator. MKL1 localization is cell-type specific; it is predominantly cytoplasmic in unstimulated fibroblasts and some muscle cell types and is constitutively nuclear in neuronal cells. In the present study, we report that in megakaryocytes, subcellular localization and regulation of MKL1 is dependent on RhoA activity and actin organization. Induction of megakaryocytic differentiation of human erythroleukemia cells by 12-O-tetradecanoylphorbol-13-acetate and primary megakaryocytes by thrombopoietin promotes MKL1 nuclear localization. This MKL1 localization is blocked by drugs inhibiting RhoA activity or actin polymerization.We also show that nuclear-localized MKL1 activates the transcription of SRF target genes. This report broadens our knowledge of the molecular mechanisms regulating megakaryocyte differentiation.

Serum response factor and cAMP response element binding protein are both required for cocaine induction of ΔFosB

The molecular mechanism underlying induction by cocaine of ΔFosB, a transcription factor important for addiction, remains unknown. Here, we demonstrate a necessary role for two transcription factors, cAMP response element binding protein (CREB) and serum response factor (SRF), in mediating this induction within the mouse nucleus accumbens (NAc), a key brain reward region. CREB and SRF are both activated in NAc by cocaine and bind to the fosB gene promoter. Using viral-mediated Cre recombinase expression in the NAc of single- or double-floxed mice, we show that deletion of both transcription factors from this brain region completely blocks cocaine induction of ΔFosB in NAc, whereas deletion of either factor alone has no effect. Furthermore, deletion of both SRF and CREB from NAc renders animals less sensitive to the rewarding effects of moderate doses of cocaine when tested in the conditioned place preference (CPP) procedure and also blocks locomotor sensitization to higher doses of cocaine. Deletion of CREB alone has the opposite effect and enhances both cocaine CPP and locomotor sensitization. In contrast to ΔFosB induction by cocaine, ΔFosB induction in NAc by chronic social stress, which we have shown previously requires activation of SRF, is unaffected by the deletion of CREB alone. These surprising findings demonstrate the involvement of distinct transcriptional mechanisms in mediating ΔFosB induction within this same brain region by cocaine versus stress. Our results also establish a complex mode of regulation of ΔFosB induction in response to cocaine, which requires the concerted activities of both SRF and CREB.