A nuclear actin function regulates neuronal motility by serum response factor-dependent gene transcription

NGLY1 mutations cause protein aggregation in human neurons

Biallelic mutations in the gene that encodes the enzyme N-glycanase 1 (NGLY1) cause a rare disease with multi-symptomatic features including developmental delay, intellectual disability, neuropathy, and seizures. NGLY1's activity in human neural cells is currently not well understood. To understand how NGLY1 gene loss leads to the specific phenotypes of NGLY1 deficiency, we employed direct conversion of NGLY1 patient-derived induced pluripotent stem cells (iPSCs) to functional cortical neurons. Transcriptomic, proteomic, and functional studies of iPSC-derived neurons lacking NGLY1 function revealed several major cellular processes that were altered, including protein aggregate-clearing functionality, mitochondrial homeostasis, and synaptic dysfunctions. These phenotypes were rescued by introduction of a functional NGLY1 gene and were observed in iPSC-derived mature neurons but not astrocytes. Finally, laser capture microscopy followed by mass spectrometry provided detailed characterization of the composition of protein aggregates specific to NGLY1-deficient neurons. Future studies will harness this knowledge for therapeutic development.

Regulation of actin filament assembly and disassembly in growth cone motility and axon guidance

Directed outgrowth of axons is fundamental for the establishment of neuronal networks. Axon outgrowth is guided by growth cones, highly motile structures enriched in filamentous actin (F-actin) located at the axons' distal tips. Growth cones exploit F-actin-based protrusions to scan the environment for guidance cues, and they contain the sensory apparatus to translate guidance cue information into intracellular signaling cascades. These cascades act upstream of actin-binding proteins (ABP) and thereby control assembly and disassembly of F-actin. Spatiotemporally controlled F-actin dis-/assembly in growth cones steers the axon towards attractants and away from repellents, and it thereby navigates the axon through the developing nervous system. Hence, ABP that control F-actin dynamics emerged as critical regulators of neuronal network formation. In the present review article, we will summarize and discuss current knowledge of the mechanisms that control remodeling of the actin cytoskeleton in growth cones, focusing on recent progress in the field. Further, we will introduce tools and techniques that allow to study actin regulatory mechanism in growth cones.

Regulation of nuclear actin levels and MRTF/SRF target gene expression during PC6.3 cell differentiation

Actin has important functions in both cytoplasm and nucleus of the cell, with active nuclear transport mechanisms maintaining the cellular actin balance. Nuclear actin levels are subject to regulation during many cellular processes from cell differentiation to cancer. Here we show that nuclear actin levels increase upon differentiation of PC6.3 cells towards neuron-like cells. Photobleaching experiments demonstrate that this increase is due to decreased nuclear export of actin during cell differentiation. Increased nuclear actin levels lead to decreased nuclear localization of MRTF-A, a well-established transcription cofactor of SRF. In line with MRTF-A localization, transcriptomics analysis reveals that MRTF/SRF target gene expression is first transiently activated, but then substantially downregulated during PC6.3 cell differentiation. This study therefore describes a novel cellular context, where regulation of nuclear actin is utilized to tune MRTF/SRF target gene expression during cell differentiation.

Arc Regulates Transcription of Genes for Plasticity, Excitability and Alzheimer’s Disease

The immediate early gene Arc is a master regulator of synaptic function and a critical determinant of memory consolidation. Here, we show that Arc interacts with dynamic chromatin and closely associates with histone markers for active enhancers and transcription in cultured rat hippocampal neurons. Both these histone modifications, H3K27Ac and H3K9Ac, have recently been shown to be upregulated in late-onset Alzheimer’s disease (AD). When Arc induction by pharmacological network activation was prevented using a short hairpin RNA, the expression profile was altered for over 1900 genes, which included genes associated with synaptic function, neuronal plasticity, intrinsic excitability, and signalling pathways. Interestingly, about 100 Arc-dependent genes are associated with the pathophysiology of AD. When endogenous Arc expression was induced in HEK293T cells, the transcription of many neuronal genes was increased, suggesting that Arc can control expression in the absence of activated signalling pathways. Taken together, these data establish Arc as a master regulator of neuronal activity-dependent gene expression and suggest that it plays a significant role in the pathophysiology of AD.

MICAL2 Facilitates Gastric Cancer Cell Migration via MRTF-A-Mediated CDC42 Activation

Aims and Hypothesis: Cell migration is driven by the reorganization of the actin cytoskeleton. Although MICAL2 is known to mediate the oxidation of actin filaments to regulate F-actin dynamics, relatively few studies have investigated the potential role of MICAL2 during cancer cell migration. Methods: The migratory ability of gastric cancer cells was measured by wound healing and transwell assays. The relationship between MICAL2 expression and MRTF-A nuclear localization was analyzed using gene overexpression and knockdown strategies. The production of reactive oxygen species (ROS) was evaluated by DCFH-DA staining. mRNA and protein levels of MMP9 were measured using qPCR and immunoblotting analysis. The activities of CDC42 and RhoA were assessed using pulldown assays. Results: Depletion of MICAL2 markedly reduced gastric cancer cell migration. Mechanistically, silencing of MICAL2 inhibited the nuclear translocation of MRTF-A in response to EGF and serum stimulation, whereas the contents of MRTF-A remained unchanged. Further analysis showed that silencing of MICAL2 decreased the activation of CDC42 as well as mRNA and protein levels of MMP9. Ectopic expression of MICAL2 augmented MRTF-A levels in the nucleus, and promoted the activation of CDC42, MMP9 expression, and gastric cancer cell migration. Moreover, silencing of MRTF-A inhibited the CDC42 activation induced by overexpression of MICAL2. In addition, MICAL2-induced ROS generation contributed to the effect exerted by MICAL2 on MRTF-A nuclear translocation. Conclusion: Together, these results provide evidence that MICAL2 facilitates gastric cancer cell migration via positive regulation of nuclear translocation of MRTF-A and subsequent CDC42 activation and MMP9 expression.

MRTFA: A critical protein in normal and malignant hematopoiesis and beyond

Myocardin-related transcription factor A (MRTFA) is a coactivator of serum response factor, a transcription factor that participates in several critical cellular functions including cell growth and apoptosis. MRTFA couples transcriptional regulation to actin cytoskeleton dynamics, and the transcriptional targets of the MRTFA–serum response factor complex include genes encoding cytoskeletal proteins as well as immediate early genes. Previous work has shown that MRTFA promotes the differentiation of many cell types, including various types of muscle cells and hematopoietic cells, and MRTFA's interactions with other protein partners broaden its cellular roles. However, despite being first identified as part of the recurrent t(1;22) chromosomal translocation in acute megakaryoblastic leukemia, the mechanisms by which MRTFA functions in malignant hematopoiesis have yet to be defined. In this review, we provide an in-depth examination of the structure, regulation, and known functions of MRTFA with a focus on hematopoiesis. We conclude by identifying areas of study that merit further investigation.

Emerging roles of cytoskeletal proteins in regulating gene expression and genome organization during differentiation

In the eukaryotic cell nucleus, cytoskeletal proteins are emerging as essential players in nuclear function. In particular, actin regulates chromatin as part of ATP-dependent chromatin remodeling complexes, it modulates transcription and it is incorporated into nascent ribonucleoprotein complexes, accompanying them from the site of transcription to polyribosomes. The nuclear actin pool is undistinguishable from the cytoplasmic one in terms of its ability to undergo polymerization and it has also been implicated in the dynamics of chromatin, regulating heterochromatin segregation at the nuclear lamina and maintaining heterochromatin levels in the nuclear interiors. One of the next frontiers is, therefore, to determine a possible involvement of nuclear actin in the functional architecture of the cell nucleus by regulating the hierarchical organization of chromatin and, thus, genome organization. Here, we discuss the repertoire of these potential actin functions and how they are likely to play a role in the context of cellular differentiation.

Lysosomal Hydrolase Cathepsin D Non-proteolytically Modulates Dendritic Morphology in Drosophila

The main lysosomal protease cathepsin D (cathD) is essential for maintaining tissue homeostasis via its degradative function, and its loss leads to ceroid accumulation in the mammalian nervous system, which results in progressive neurodegeneration. Increasing evidence implies non-proteolytic roles of cathD in regulating various biological processes such as apoptosis, cell proliferation, and migration. Along these lines, we here showed that cathD is required for modulating dendritic architecture in the nervous system independent of its traditional degradative function. Upon cathD depletion, class I and class III arborization (da) neurons in Drosophila larvae exhibited aberrant dendritic morphology, including over-branching, aberrant turning, and elongation defects. Re-introduction of wild-type cathD or its proteolytically-inactive mutant dramatically abolished these morphological defects. Moreover, cathD knockdown also led to dendritic defects in the adult mushroom bodies, suggesting that cathD-mediated processes are required in both the peripheral and central nervous systems. Taken together, our results demonstrate a critical role of cathD in shaping dendritic architecture independent of its proteolytic function.

Indirect visualization of endogenous nuclear actin by correlative light and electron microscopy (CLEM) using an actin-directed chromobody

Actin fulfills important cytoplasmic but also nuclear functions in eukaryotic cells. In the nucleus, actin modulates gene expression and chromatin remodeling. Monomeric (G-actin) and polymerized actin (F-actin) have been analyzed by fluorescence microscopy in the nucleus; however, the resolution at the ultrastructural level has not been investigated in great detail. We provide a first documentation of nuclear actin in mouse fibroblasts by electron microscopy (EM). For this, we employed correlative light and electron microscopy on the same section using actin-directed nanobodies recognizing endogenous monomeric and polymeric actin proteins (so-called nuclear Actin-chromobody-GFP; nAC-GFP). Indeed, using this strategy, we could identify actin proteins present in the nucleus. Here, immunogold-labeled actin proteins were spread throughout the entire nucleoplasm. Of note, nuclear actin was complementarily localized to DAPI-positive areas, the latter marking preferentially transcriptionally inactive heterochromatin. Since actin aggregates in rod structures upon cell stress including neurodegeneration, we analyzed nuclear actin at the ultrastructural level after DMSO or UV-mediated cell damage. In those cells the ratio between cytoplasmic and nuclear gold-labeled actin proteins was altered compared to untreated control cells. In summary, this EM analysis (i) confirmed the presence of endogenous nuclear actin at ultrastructural resolution, (ii) revealed the actin abundance in less chromatin-dense regions potentially reflecting more transcriptionally active euchromatin rather than transcriptionally inactive heterochromatin and (iii) showed an altered abundance of actin-associated gold particles upon cell stress.

Withdrawal from repeated morphine administration augments expression of the RhoA network in the nucleus accumbens to control synaptic structure

The nucleus accumbens (NAc) is a critical brain reward region that mediates the rewarding effects of drugs of abuse, including those of morphine and other opiates. Drugs of abuse induce widespread alterations in gene transcription and dendritic spine morphology in medium spiny neurons (MSNs) of the NAc that ultimately influence NAc excitability and hence reward-related behavioral responses. Growing evidence indicates that within the NAc small GTPases are common intracellular targets of drugs of abuse where these molecules regulate drug-mediated transcriptional and spine morphogenic effects. The RhoA small GTPase is among the most well-characterized members of the Ras superfamily of small GTPases, and recent work highlights an important role for hippocampal RhoA in morphine-facilitated reward behavior. Despite this, it remains unclear how RhoA pathway signaling in the NAc is affected by withdrawal from morphine. To investigate this question, using subcellular fractionation and subsequent protein profiling we examined the expression of key components of the RhoA pathway in NAc nuclear, cytoplasmic, and synaptosomal compartments during multiple withdrawal periods from repeated morphine administration. Furthermore, using in vivo viral-mediated gene transfer, we determined the consequences of revealed RhoA pathway alterations on NAc MSN dendritic spine morphology. Our findings reveal an important role for RhoA signaling cascades in mediating the effects of long-term morphine withdrawal on NAc MSN dendritic spine elimination.

OPEN PRACTICES

Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/.

Child with cerebral malformations and epilepsy

PURPOSE

Baraitser-Winter cerebrofrontofacial syndrome (BWCFF) is a rare autosomal dominant genetic disorder involving multiple organ systems and primarily characterized by structural brain abnormalities and a distinctive facial appearance.

METHODS

To study the clinical characteristics, gene types and seizures of BWCFF. The natural history, clinical data and peripheral blood sample were collected in the child and his patients. To screen the β-actin gene (ACTB) of a newly diagnosed child, hoping to find the gene mutation.

RESULTS

The child had left ptosis, ocular hypertelorism, arched eyebrows, only 30% of the left ear hearing, a slight hypotonia, normal muscle strength, walking instability. The seizures were difficult to control with antiepileptic drugs and presented some degree of psychomotor development delay. Genetic screening showed De Novo in ACTB gene (c.484A> G, p.Thr162Ala). Parents did not detect related gene mutations.

CONCLUSIONS

Patients with typical facial features and cerebral cortical malformations associated with refractory epilepsy should be highly suspected BWCFF. Patients are advised to carry out genetic screening to confirm the diagnosis.

The dendritic spine morphogenic effects of repeated cocaine use occur through the regulation of serum response factor signaling

The nucleus accumbens (NAc) is a primary brain reward region composed predominantly of medium spiny neurons (MSNs). In response to early withdrawal from repeated cocaine administration, de novo dendritic spine formation occurs in NAc MSNs. Much evidence indicates that this new spine formation facilitates the rewarding properties of cocaine. Early withdrawal from repeated cocaine also produces dramatic alterations in the transcriptome of NAc MSNs, but how such alterations influence cocaine's effects on dendritic spine formation remain unclear. Studies in non-neuronal cells indicate that actin cytoskeletal regulatory pathways in nuclei have a direct role in the regulation of gene transcription in part by controlling the access of co-activators to their transcription factor partners. In particular, actin state dictates the interaction between the serum response factor (SRF) transcription factor and one of its principal co-activators, MAL. Here we show that cocaine induces alterations in nuclear F-actin signaling pathways in the NAc with associated changes in the nuclear subcellular localization of SRF and MAL. Using in vivo optogenetics, the brain region-specific inputs to the NAc that mediate these nuclear changes are investigated. Finally, we demonstrate that regulated SRF expression, in turn, is critical for the effects of cocaine on dendritic spine formation and for cocaine-mediated behavioral sensitization. Collectively, these findings reveal a mechanism by which nuclear-based changes influence the structure of NAc MSNs in response to cocaine.

Baraitser-Winter cerebrofrontofacial syndrome

Baraitser-Winter cerebrofrontofacial syndrome (BWCFF) (BRWS; MIM #243310, 614583) is a rare developmental disorder affecting multiple organ systems. It is characterised by intellectual disability (mild to severe) and distinctive facial appearance (metopic ridging/trigonocephaly, bilateral ptosis, hypertelorism). The additional presence of cortical malformations (pachygyria/lissencephaly) and ocular colobomata are also suggestive of this syndrome. Other features include moderate short stature, contractures, congenital cardiac disease and genitourinary malformations. BWCFF is caused by missense mutations in the cytoplasmic beta- and gamma-actin genes ACTB and ACTG1. We provide an overview of the clinical characteristics (including some novel findings in four recently diagnosed patients), diagnosis, management, mutation spectrum and genetic counselling.

Unravelling a new mechanism linking actin polymerization and gene transcription

In the recent years, the role of actin and actin-binding proteins in gene transcription has received considerable attention. Nuclear monomeric and polymerized actin and several actin binding proteins have been detected in the mammalian cell nucleus, although their roles in transcription are just beginning to emerge. Our group recently reported that the actin-binding protein Filamin A interacts with the transcriptional coactivator MKL1 to link actin polymerization with transcriptional activity of Serum Response Factor. Here we summarize the regulation and function of MKL1, and highlight this novel mechanism of MKL1 regulation through binding to Filamin A and its implications for cell migration.

Filamin A interacts with the coactivator MKL1 to promote the activity of the transcription factor SRF and cell migration

Megakaryoblastic leukemia 1 (MKL1) is a coactivator of serum response factor (SRF) that promotes the expression of genes associated with cell proliferation, motility, adhesion, and differentiation-processes that also involve dynamic cytoskeletal changes in the cell. MKL1 is inactive when bound to monomeric globular actin (G-actin), but signals that activate the small guanosine triphosphatase RhoA cause actin polymerization and MKL1 dissociation from G-actin. We found a new mechanism of MKL1 activation that is mediated through its binding to filamin A (FLNA), a protein that binds filamentous actin (F-actin). The interaction of FLNA and MKL1 was required for the expression of MKL1 target genes in primary fibroblasts, melanoma, mammary and hepatocellular carcinoma cells. We identified the regions of interaction between MKL1 and FLNA, and cells expressing an MKL1 mutant that was unable to bind FLNA exhibited impaired cell migration and reduced expression of MKL1-SRF target genes. Induction and repression of MKL1-SRF target genes correlated with increased or decreased MKL1-FLNA interaction, respectively. Lysophosphatidic acid-induced RhoA activation in primary human fibroblasts promoted the association of endogenous MKL1 with FLNA, whereas exposure to an actin polymerization inhibitor dissociated MKL1 from FLNA and decreased MKL1-SRF target gene expression in melanoma cells. Thus, FLNA functions as a positive cellular transducer linking actin polymerization to MKL1-SRF activity, counteracting the known repressive complex of MKL1 and monomeric G-actin.

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.

Nuclear actin and myocardin-related transcription factors control disuse muscle atrophy through regulation of Srf activity

Skeletal muscle atrophy is a debilitating process that is associated with a wide variety of conditions including inactivity, disease and aging. Here, we demonstrate that the actin, myocardin-related transcription factors and serum response factor (actin-Mrtf-Srf) pathway is specifically downregulated in the muscle atrophy that is induced through disuse in mice. We show in vivo that the abolition of mechanical signals leads to the rapid accumulation of G-actin in myonuclei and the export of the Srf coactivator Mrtf-A, resulting in a decrease of Mrtf-Srf-dependent transcription that contributes to atrophy. We demonstrate that inhibition of the actin-Mrtf-Srf axis through overexpression of nuclear non-polymerizable actin, through pharmacological inhibition of Mrtf-Srf and through muscle-specific Srf deletion worsens denervation-induced atrophy. Conversely, maintenance of high levels of activity of Srf or Mrtfs in denervated muscle, through overexpression of constitutively active derivatives, counteracts atrophy. Altogether, our data provide new mechanistic insights into the control of muscle mass upon disuse atrophy by the actin-Mrtf-Srf pathway, highlighting Srf as a key mediator of mechanotransduction in muscle.

Nuclear actin filaments recruit cofilin and actin-related protein 3, and their formation is connected with a mitotic block

Although actin monomers polymerize into filaments in the cytoplasm, the form of actin in the nucleus remains elusive. We searched for the form and function of β-actin fused to nuclear localization signal and to enhanced yellow fluorescent protein (EN-actin). Our results reveal that EN-actin is either dispersed in the nucleoplasm (homogenous EN-actin) or forms bundled filaments in the nucleus (EN-actin filaments). Formation of such filaments was not connected with increased EN-actin levels. Among numerous actin-binding proteins tested, only cofilin is recruited to the EN-actin filaments. Overexpression of EN-actin causes increase in the nuclear levels of actin-related protein 3 (Arp3). Although Arp3, a member of actin nucleation complex Arp2/3, is responsible for EN-actin filament nucleation and bundling, the way cofilin affects nuclear EN-actin filaments dynamics is not clear. While cells with homogenous EN-actin maintained unaffected mitosis during which EN-actin re-localizes to the plasma membrane, generation of nuclear EN-actin filaments severely decreases cell proliferation and interferes with mitotic progress. The introduction of EN-actin manifests in two mitotic-inborn defects—formation of binucleic cells and generation of micronuclei—suggesting that cells suffer aberrant cytokinesis and/or impaired chromosomal segregation. In interphase, nuclear EN-actin filaments passed through chromatin region, but do not co-localize with either chromatin remodeling complexes or RNA polymerases I and II. Surprisingly presence of EN-actin filaments was connected with increase in the overall transcription levels in the S-phase by yet unknown mechanism. Taken together, EN-actin can form filaments in the nucleus which affect important cellular processes such as transcription and mitosis.

SRF phosphorylation by glycogen synthase kinase-3 promotes axon growth in hippocampal neurons

The growth of axons is an intricately regulated process involving intracellular signaling cascades and gene transcription. We had previously shown that the stimulus-dependent transcription factor, serum response factor (SRF), plays a critical role in regulating axon growth in the mammalian brain. However, the molecular mechanisms underlying SRF-dependent axon growth remains unknown. Here we report that SRF is phosphorylated and activated by GSK-3 to promote axon outgrowth in mouse hippocampal neurons. GSK-3 binds to and directly phosphorylates SRF on a highly conserved serine residue. This serine phosphorylation is necessary for SRF activity and for its interaction with MKL-family cofactors, MKL1 and MKL2, but not with TCF-family cofactor, ELK-1. Axonal growth deficits caused by GSK-3 inhibition could be rescued by expression of a constitutively active SRF. The SRF target gene and actin-binding protein, vinculin, is sufficient to overcome the axonal growth deficits of SRF-deficient and GSK-3-inhibited neurons. Furthermore, short hairpin RNA-mediated knockdown of vinculin also attenuated axonal growth. Thus, our findings reveal a novel phosphorylation and activation of SRF by GSK-3 that is critical for SRF-dependent axon growth in mammalian central neurons.

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

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

The transcription factor serum response factor stimulates axon regeneration through cytoplasmic localization and cofilin interaction

Axonal injury generates growth inert retraction bulbs with dynamic cytoskeletal properties that are severely compromised. Conversion of "frozen" retraction bulbs into actively progressing growth cones is a major aim in axon regeneration. Here we report that murine serum response factor (SRF), a gene regulator linked to the actin cytoskeleton, modulates growth cone actin dynamics during axon regeneration. In regeneration-competent facial motoneurons, Srf deletion inhibited axonal regeneration. In wild-type mice after nerve injury, SRF translocated from the nucleus to the cytoplasm, suggesting a cytoplasmic SRF function in axonal regeneration. Indeed, adenoviral overexpression of cytoplasmic SRF (SRF-ΔNLS-GFP) stimulated axonal sprouting and facial nerve regeneration in vivo. In primary central and peripheral neurons, SRF-ΔNLS-GFP stimulated neurite outgrowth, branch formation, and growth cone morphology. Furthermore, we uncovered a link between SRF and the actin-severing factor cofilin during axonal regeneration in vivo. Facial nerve axotomy increased the total cofilin abundance and also nuclear localization of phosphorylated cofilin in a subpopulation of lesioned motoneurons. This cytoplasmic-to-nucleus translocation of P-cofilin upon axotomy was reduced in motoneurons expressing SRF-ΔNLS-GFP. Finally, we demonstrate that cytoplasmic SRF and cofilin formed a reciprocal regulatory unit. Overexpression of cytoplasmic SRF reduced cofilin phosphorylation and vice versa: overexpression of cofilin inhibited SRF phosphorylation. Therefore, a regulatory loop consisting of SRF and cofilin might take part in reactivating actin dynamics in growth-inert retraction bulbs and facilitating axon regeneration.

Complex network analysis of CA3 transcriptome reveals pathogenic and compensatory pathways in refractory temporal lobe epilepsy

We previously described - studying transcriptional signatures of hippocampal CA3 explants - that febrile (FS) and afebrile (NFS) forms of refractory mesial temporal lobe epilepsy constitute two distinct genomic phenotypes. That network analysis was based on a limited number (hundreds) of differentially expressed genes (DE networks) among a large set of valid transcripts (close to two tens of thousands). Here we developed a methodology for complex network visualization (3D) and analysis that allows the categorization of network nodes according to distinct hierarchical levels of gene-gene connections (node degree) and of interconnection between node neighbors (concentric node degree). Hubs are highly connected nodes, VIPs have low node degree but connect only with hubs, and high-hubs have VIP status and high overall number of connections. Studying the whole set of CA3 valid transcripts we: i) obtained complete transcriptional networks (CO) for FS and NFS phenotypic groups; ii) examined how CO and DE networks are related; iii) characterized genomic and molecular mechanisms underlying FS and NFS phenotypes, identifying potential novel targets for therapeutic interventions. We found that: i) DE hubs and VIPs are evenly distributed inside the CO networks; ii) most DE hubs and VIPs are related to synaptic transmission and neuronal excitability whereas most CO hubs, VIPs and high hubs are related to neuronal differentiation, homeostasis and neuroprotection, indicating compensatory mechanisms. Complex network visualization and analysis is a useful tool for systems biology approaches to multifactorial diseases. Network centrality observed for hubs, VIPs and high hubs of CO networks, is consistent with the network disease model, where a group of nodes whose perturbation leads to a disease phenotype occupies a central position in the network. Conceivably, the chance for exerting therapeutic effects through the modulation of particular genes will be higher if these genes are highly interconnected in transcriptional networks.

Analysis of nuclear actin by overexpression of wild-type and actin mutant proteins

Compared to the cytoplasmic F-actin abundance in cells, nuclear F-actin levels are generally quite low. However, nuclear actin is present in certain cell types including oocytes and under certain cellular conditions including stress or serum stimulation. Currently, the architecture and polymerization status of nuclear actin networks has not been analyzed in great detail. In this study, we investigated the architecture and functions of such nuclear actin networks. We generated nuclear actin polymers by overexpression of actin proteins fused to a nuclear localization signal (NLS). Raising nuclear abundance of a NLS wild-type actin, we observed phalloidin- and LifeAct-positive actin bundles forming a nuclear cytoskeletal network consisting of curved F-actin. In contrast, a polymer-stabilizing actin mutant (NLS-G15S-actin) deficient in interacting with the actin-binding protein cofilin generated a nuclear actin network reminiscent of straight stress fiber-like microfilaments in the cytoplasm. We provide a first electron microscopic description of such nuclear actin polymers suggesting bundling of actin filaments. Employing different cell types from various species including neurons, we show that the morphology of and potential to generate nuclear actin are conserved. Finally, we demonstrate that nuclear actin affects cell function including morphology, serum response factor-mediated gene expression, and herpes simplex virus infection. Our data suggest that actin is able to form filamentous structures inside the nucleus, which share architectural and functional similarities with the cytoplasmic F-actin.

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.

Intranuclear rods myopathy with autonomic dysfunction

Intranuclear rods myopathy (IRM), a variant of nemaline myopathy (NM), is characterized by rod structure in the myonuclei. Patients with IRM present with similar symptoms to those of severe infantile-type NM but have worse outcome. Several extramuscular manifestations have been reported in NM but no dysautonomia. We herein report a 2-year-old girl with IRM and a heterozygous mutation, c.430C>T (p.L144F) in ACTA1. During the infancy, the patient showed severe diaphoresis and facial flushing. Arrhythmia and hypertension with the precipitating factors of feeding, defecation, and urination were observed. Sympathetic antagonist was prescribed and showed some effectiveness. Our report may widen the clinical spectrum of IRM. It also reminds clinicians that autonomic dysfunction may occur in patients with IRM or other actinopathies and appropriate treatment may be necessary.

Myocardin related transcription factors are required for coordinated cell cycle progression

Myocardin related transcription factors A and B (MRTFs) activate serum response factor-driven transcription in response to Rho signaling and changes in actin dynamics. Myocardin and MRTFs have been implicated in anti-proliferative effects on a range of cell types. The precise mechanisms, however, remained elusive. We employed double knockdown of MRTF-A and MRTF-B in NIH 3T3 fibroblasts to evaluate its effects on cell cycle progression and proliferation. We show that transient depletion of MRTFs conveys a modest anti-proliferative effect and impinges on normal cell cycle progression, resulting in significantly shortened G 1 phase and slightly extended S and G 2 phase under normal growth conditions. Under serum-starved conditions we observed aberrant entry into the S and G 2 phases without subsequent cell division. This was accompanied by downregulation of cyclin-CDK inhibitors p27Kip1, p18Ink4c and 19Ink4d as well as upregulation of p21Waf1 and cyclin D1. Extended knockdown led to increased formation of micronuclei, while cells stably depleted of MRTFs tend to become aneuploid and polyploid. Thus, MRTFs are required for accurate cell cycle progression and maintenance of genomic stability in fibroblast cells.

Serum Response Factor (SRF)-cofilin-actin signaling axis modulates mitochondrial dynamics

Aberrant mitochondrial function, morphology, and transport are main features of neurodegenerative diseases. To date, mitochondrial transport within neurons is thought to rely mainly on microtubules, whereas actin might mediate short-range movements and mitochondrial anchoring. Here, we analyzed the impact of actin on neuronal mitochondrial size and localization. F-actin enhanced mitochondrial size and mitochondrial number in neurites and growth cones. In contrast, raising G-actin resulted in mitochondrial fragmentation and decreased mitochondrial abundance. Cellular F-actin/G-actin levels also regulate serum response factor (SRF)-mediated gene regulation, suggesting a possible link between SRF and mitochondrial dynamics. Indeed, SRF-deficient neurons display neurodegenerative hallmarks of mitochondria, including disrupted morphology, fragmentation, and impaired mitochondrial motility, as well as ATP energy metabolism. Conversely, constitutively active SRF-VP16 induced formation of mitochondrial networks and rescued huntingtin (HTT)-impaired mitochondrial dynamics. Finally, SRF and actin dynamics are connected via the actin severing protein cofilin and its slingshot phosphatase to modulate neuronal mitochondrial dynamics. In summary, our data suggest that the SRF-cofilin-actin signaling axis modulates neuronal mitochondrial function.

A critical cell-intrinsic role for serum response factor in glial specification in the CNS

Astrocytes and oligodendrocytes play crucial roles in nearly every facet of nervous system development and function, including neuronal migration, synaptogenesis, synaptic plasticity, and myelination. Previous studies have widely characterized the signaling pathways important for astrocyte differentiation and unveiled a number of transcription factors that guide oligodendrocyte differentiation in the CNS. However, the identities of the transcription factors critical for astrocyte specification in the brain remain unknown. Here we show that deletion of the stimulus-dependent transcription factor, serum response factor (SRF), in neural precursor cells (NPCs) (Srf-Nestin-cKO) results in nearly 60% loss in astrocytes and 50% loss in oligodendrocyte precursors at birth. Cultured SRF-deficient NPCs exhibited normal growth rate and capacity to self-renew. However, SRF-deficient NPCs generated fewer astrocytes and oligodendrocytes in response to several lineage-specific differentiation factors. These deficits in glial differentiation were rescued by ectopic expression of wild-type SRF in SRF-deficient NPCs. Interestingly, ectopic expression of a constitutively active SRF (SRF-VP16) in NPCs augmented astrocyte differentiation in the presence of pro-astrocytic factors. However, SRF-VP16 expression in NPCs had an inhibitory effect on oligodendrocyte differentiation. In contrast, mice carrying conditional deletion of SRF in developing forebrain neurons (Srf-NEX-cKO) did not exhibit any deficits in astrocytes in the brain. Together, our observations suggest that SRF plays a critical cell-autonomous role in NPCs to regulate astrocyte and oligodendrocyte specification in vivo and in vitro.

A requirement for polymerized actin in DNA double-strand break repair

Nuclear actin is involved in several nuclear processes from chromatin remodeling to transcription. Here we examined the requirement for actin polymerization in DNA double-strand break repair. Double-strand breaks are considered the most dangerous type of DNA lesion. Double-strand break repair consists of a complex set of events that are tightly regulated. Failure at any step can have catastrophic consequences such as genomic instability, oncogenesis or cell death. Many proteins involved in this repair process have been identified and their roles characterized. We discovered that some DNA double-strand break repair factors are capable of associating with polymeric actin in vitro and specifically, that purified Ku70/80 interacts with polymerized actin under these conditions. We find that the disruption of polymeric actin inhibits DNA double strand break repair both in vitro and in vivo. Introduction of nuclear targeted mutant actin that cannot polymerize, or the depolymerization of endogenous actin filaments by the addition of cytochalasin D, alters the retention of Ku80 at sites of DNA damage in live cells. Our results suggest that polymeric actin is required for proper DNA double-strand break repair and may function through the stabilization of the Ku heterodimer at the DNA damage site.

Serum response factor modulates neuron survival during peripheral axon injury

Background

The transcription factor SRF (serum response factor) mediates neuronal survival in vitro. However, data available so far suggest that SRF is largely dispensable for neuron survival during physiological brain function.

Findings

Here, we demonstrate that upon neuronal injury, that is facial nerve transection, constitutively-active SRF-VP16 enhances motorneuron survival. SRF-VP16 suppressed active caspase 3 abundance in vitro and enhanced neuron survival upon camptothecin induced apoptosis. Following nerve fiber injury in vitro, SRF-VP16 improved survival of neurons and re-growth of severed neurites. Further, SRF-VP16 enhanced immune responses (that is microglia and T cell activation) associated with neuronal injury in vivo. Genome-wide transcriptomics identified target genes associated with axonal injury and modulated by SRF-VP16.

Conclusion

In sum, this is a first report describing a neuronal injury-related survival function for SRF.

Serum response factor is required for cortical axon growth but is dispensable for neurogenesis and neocortical lamination

Previous studies have shown that neuron-specific deletion of serum response factor (SRF) results in deficits in tangential cell migration, guidance-dependent circuit assembly, activity-dependent gene expression, and synaptic plasticity in the hippocampus. Furthermore, SRF deletion in mouse embryonic stem cells causes cell death in vitro. However, the requirement of SRF for early neuronal development including neural stem cell homeostasis, neurogenesis, and axonal innervations remains unknown. Here, we report that SRF is critical for development of major axonal tracts in the forebrain. Conditional mutant mice lacking SRF in neural progenitor cells (Srf-Nestin-cKO) exhibit striking deficits in cortical axonal projections including corticostriatal, corticospinal, and corticothalamic tracts, and they show a variable loss of the corpus callosum. Neurogenesis and interneuron specification occur normally in the absence of SRF and the deficits in axonal projections were not due to a decrease or loss in cell numbers. Radial migration of neurons and neocortical lamination were also not affected. No aberrant cell death was observed during development, whereas there was an increase in the number of proliferative cells in the ventricular zone from embryonic day 14 to day 18. Similar axonal tract deficits were also observed in mutant mice lacking SRF in the developing excitatory neurons of neocortex and hippocampus (Srf-NEX-cKO). Together, these findings suggest distinct roles for SRF during neuronal development; SRF is specifically required in a cell-autonomous manner for axonal tract development but is dispensable for cell survival, neurogenesis, neocortical lamination, and neuronal differentiation.

Ephrin-A5 suppresses neurotrophin evoked neuronal motility, ERK activation and gene expression

During brain development, growth cones respond to attractive and repulsive axon guidance cues. How growth cones integrate guidance instructions is poorly understood. Here, we demonstrate a link between BDNF (brain derived neurotrophic factor), promoting axonal branching and ephrin-A5, mediating axonal repulsion via Eph receptor tyrosine kinase activation. BDNF enhanced growth cone filopodial dynamics and neurite branching of primary neurons. We show that ephrin-A5 antagonized this BDNF-evoked neuronal motility. BDNF increased ERK phosphorylation (P-ERK) and nuclear ERK entry. Ephrin-A5 suppressed BDNF-induced ERK activity and might sequester P-ERK in the cytoplasm. Neurotrophins are well established stimulators of a neuronal immediate early gene (IEG) response. This is confirmed in this study by e.g. c-fos, Egr1 and Arc upregulation upon BDNF application. This BDNF-evoked IEG response required the transcription factor SRF (serum response factor). Notably, ephrin-A5 suppressed a BDNF-evoked neuronal IEG response, suggesting a role of Eph receptors in modulating gene expression. In opposite to IEGs, long-term ephrin-A5 application induced cytoskeletal gene expression of tropomyosin and actinin. To uncover specific Eph receptors mediating ephrin-As impact on neurotrophin signaling, EphA7 deficient mice were analyzed. In EphA7 deficient neurons alterations in growth cone morphology were observed. However, ephrin-A5 still counteracted neurotrophin signaling suggesting that EphA7 is not required for ephrin and BDNF crosstalk. In sum, our data suggest an interaction of ephrin-As and neurotrophin signaling pathways converging at ERK signaling and nuclear gene activity. As ephrins are involved in development and function of many organs, such modulation of receptor tyrosine kinase signaling and gene expression by Ephs might not be limited to the nervous system.

Actin: from structural plasticity to functional diversity

This article addresses the multiple activities of actin. Starting out with the history of actin's discovery, purification and structure, it emphasizes the close relation between structure and function. In this context, we also point to unconventional actin conformations. Their existence in living cells is not yet well documented, however, they seem to play a special role in the supramolecular patterning that underlies some of the physiological functions of actin. Conceivably, such conformations may contribute to actin's diverse activities in the nucleus that are poorly understood so far.

Degeneration of the mouse retina upon dysregulated activity of serum response factor

PURPOSE

Our aim was to generate and phenotypically characterize a transgenic mouse line expressing a constitutively active variant of the transcription regulatory protein serum response factor (SRF), namely the SRF-VP16 protein. This new mouse strain has been registered under the designation Gt(ROSA)26Sor(tm1(SRF-VP16)Antu). We found phenotypic changes upon ectopic expression of SRF-VP16, especially in the mouse retina.

METHODS

Using homologous recombination, we integrated an SRF-VP16 conditional (i.e., "flox-STOP" repressed) expression transgene into the Rosa26 locus of murine embryonic stem (ES) cells. These engineered ES cells were used to derive the Gt(ROSA)26Sor(tm1(SRF-VP16)Antu) mouse strain. Semiquantitative real-time PCR was used to determine expression of the SRF-VP16 transgene at the mRNA level, both in young (P20 and P30) and adult (six months old) Gt(ROSA)26Sor(tm1(SRF-VP16)Antu) mice. We also investigated the transcript levels of endogenous Srf and several SRF target genes. Retinal function was tested by electroretinography in both young and adult mice. Morphological abnormalities could be visualized by hematoxylin and eosin staining of sectioned, paraffin-embedded eye tissue samples. Scanning-laser ophthalmoscopy was used to investigate retinal vascularization and degeneration in adult mice.

RESULTS

We show that the SRF-VP16 mRNA is expressed to a low but significant degree in the retinas of young and adult animals of the Gt(ROSA)26Sor(tm1(SRF-VP16)Antu) mouse strain, even in the absence of Cre-mediated deletion of the "flox-STOP" cassette. In the retinas of these transgenic mice, endogenous Srf displays elevated transcript levels. Ectopic retinal expression of constitutively active SRF-VP16 is correlated with the malfunction of retinal neurons in both heterozygous and homozygous animals of both age groups (P20 and adult). Additionally, mislamination of retinal cell layers and cellular rosette formations are found in retinas of both heterozygous and homozygous animals of young age. In homozygous individuals, however, the cellular rosettes are more widespread over the fundus. At adult age, retinas both from animals that are heterozygous and homozygous for the floxSTOP/SRF-VP16 transgene display severe degeneration, mainly of the photoreceptor cell layer. Wild-type age-matched littermates, however, do not show any degeneration. The severity of the observed effects correlates with dosage of the transgene.

CONCLUSIONS

This is the first report suggesting an influence of the transcription factor SRF on the development and function of the murine retina. Ectopic SRF-VP16 mRNA expression in the retinas of young animals is correlated with photoreceptor layer mislamination and impaired retinal function. At an advanced age of six months, degenerative processes are detected in SRF-VP16 transgenic retinas accompanied by impaired retinal function. The Gt(ROSA)26Sor(tm1(SRF-VP16)Antu) mouse strain represents a genetic SRF gain-of-function mouse model that will complement the current SRF loss-of-function models. It promises to provide new insight into the hitherto poorly defined role of SRF in retinal development and function, including potential contributions to ophthalmologic disorders. Furthermore, using conditional Cre-mediated activation of SRF-VP16, the described mouse strain will enable assessment of the impact of dysregulated SRF activity on the physiologic functions of various other organs.

Signaling in migrating neurons: from molecules to networks

During prenatal and postnatal development of the mammalian brain, new neurons are generated by precursor cells that are located in the germinal zones. Subsequently newborn neurons migrate to their destined location in the brain. On the migrational route immature neurons interact via a series of recognition molecules with a plethora of extracellular cues. Stimuli that are conveyed by extracellular cues are translated into complex intracellular signaling networks that eventually enable neuronal migration. In this Focused Review we discuss signaling networks underlying neuronal migration emphasizing molecules and pathways that appear to be neuron-specific.

The growth cone cytoskeleton in axon outgrowth and guidance

Axon outgrowth and guidance to the proper target requires the coordination of filamentous (F)-actin and microtubules (MTs), the dynamic cytoskeletal polymers that promote shape change and locomotion. Over the past two decades, our knowledge of the many guidance cues, receptors, and downstream signaling cascades involved in neuronal outgrowth and guidance has increased dramatically. Less is known, however, about how those cascades of information converge and direct appropriate remodeling and interaction of cytoskeletal polymers, the ultimate effectors of movement and guidance. During development, much of the communication that occurs between environmental guidance cues and the cytoskeleton takes place at the growing tip of the axon, the neuronal growth cone. Several articles on this topic focus on the "input" to the growth cone, the myriad of receptor types, and their corresponding cognate ligands. Others investigate the signaling cascades initiated by receptors and propagated by second messenger pathways (i.e., kinases, phosphatases, GTPases). Ultimately, this plethora of information converges on proteins that associate directly with the actin and microtubule cytoskeletons. The role of these cytoskeletal-associated proteins, as well as the cytoskeleton itself in axon outgrowth and guidance, is the subject of this article.

STK35L1 associates with nuclear actin and regulates cell cycle and migration of endothelial cells

BACKGROUND

Migration and proliferation of vascular endothelial cells are essential for repair of injured endothelium and angiogenesis. Cyclins, cyclin-dependent kinases (CDKs), and cyclin-dependent kinase inhibitors play an important role in vascular tissue injury and wound healing. Previous studies suggest a link between the cell cycle and cell migration: cells present in the G(1) phase have the highest potential to migrate. The molecular mechanism linking these two processes is not understood.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we explored the function of STK35L1, a novel Ser/Thr kinase, localized in the nucleus and nucleolus of endothelial cells. Molecular biological analysis identified a bipartite nuclear localization signal, and nucleolar localization sequences in the N-terminal part of STK35L1. Nuclear actin was identified as a novel binding partner of STK35L1. A class III PDZ binding domains motif was identified in STK35L1 that mediated its interaction with actin. Depletion of STK35L1 by siRNA lead to an accelerated G(1) to S phase transition after serum-stimulation of endothelial cells indicating an inhibitory role of the kinase in G(1) to S phase progression. Cell cycle specific genes array analysis revealed that one gene was prominently downregulated (8.8 fold) in STK35L1 silenced cells: CDKN2A alpha transcript, which codes for p16(INK4a) leading to G(1) arrest by inhibition of CDK4/6. Moreover in endothelial cells seeded on Matrigel, STK35L1 expression was rapidly upregulated, and silencing of STK35L1 drastically inhibited endothelial sprouting that is required for angiogenesis. Furthermore, STK35L1 depletion profoundly impaired endothelial cell migration in two wound healing assays.

CONCLUSION/SIGNIFICANCE

The results indicate that by regulating CDKN2A and inhibiting G1- to S-phase transition STK35L1 may act as a central kinase linking the cell cycle and migration of endothelial cells. The interaction of STK35L1 with nuclear actin might be critical in the regulation of these fundamental endothelial functions.

The essential function for serum response factor in T-cell development reflects its specific coupling to extracellular signal-regulated kinase signaling

Serum response factor (SRF) recruits members of two families of signal-regulated coactivators, the extracellular signal-regulated kinase (ERK)-regulated ternary complex factors (TCFs) and the actin-regulated myocardin-related transcription factors (MRTFs), to its target genes through its DNA-binding domain. Whether coactivator association is required for SRF function in vivo and whether particular SRF functions reflect specific coupling to one or the other signal pathway have remained largely unexplored. We show that SRF is essential for thymocyte positive selection and thymic T(reg) and NK T-cell development but dispensable for early thymocyte development and negative selection. Expression of wild-type SRF, or mutants lacking the N-terminal phosphorylation sites or C-terminal transcriptional activation domain, restores positive selection in SRF null thymocytes. In contrast, SRF.V194E, which cannot recruit TCF or MRTF family members, is inactive, although it is recruited to target genes. Fusion of a TCF C-terminal activation domain to SRF.V194E effectively restores ERK-dependent single-positive (SP) thymocyte development. The resulting SP thymocytes exhibit normal surface marker expression and proliferation following T-cell receptor cross-linking. Thus, ERK signaling through the TCF pathway to SRF is necessary and sufficient for SRF function in thymocyte positive selection.

Actin on DNA-an ancient and dynamic relationship

In the cytoplasm of eukaryotic cells the coordinated assembly of actin filaments drives essential cell biological processes, such as cell migration. The discovery of prokaryotic actin homologues, as well as the appreciation of the existence of nuclear actin, have expanded the scope by which the actin family is utilized in different cell types. In bacteria, actin has been implicated in DNA movement tasks, while the connection with the RNA polymerase machinery appears to exist in both prokaryotes and eukaryotes. Within the nucleus, actin has further been shown to play a role in chromatin remodeling and RNA processing, possibly acting to link these to transcription, thereby facilitating the gene expression process. The molecular mechanism by which actin exerts these newly discovered functions is still unclear, because while polymer formation seems to be required in bacteria, these species lack conventional actin-binding proteins to regulate the process. Furthermore, although the nucleus contains a plethora of actin-regulating factors, the polymerization status of actin within this compartment still remains unclear. General theme, however, seems to be actin's ability to interact with numerous binding partners. A common feature to the novel modes of actin utilization is the connection between actin and DNA, and here we aim to review the recent literature to explore how this connection is exploited in different contexts.

Actin-mediated gene expression in neurons: the MRTF-SRF connection

The traditional view of cellular actin is a rather autarkic cytoskeletal framework function confined to the cytoplasm. However, there is now evidence that alterations in actin dynamics are sensed by the nucleus and subsequently modulate gene expression. In communicating with the nucleus, cytoplasmic, and most likely also nucleus-resident actin, provides a further (gene) regulatory loop to cell motility. A transcription module composed of MRTF (myocardin-related transcription factor) and SRF (serum response factor) emerges as prime target of such actin signaling. Here, I focus on the nervous system, where the actin-MRTF-SRF entity governs multiple aspects of neuronal motility.

Involvement of the serum response factor coactivator megakaryoblastic leukemia (MKL) in the activin-regulated dendritic complexity of rat cortical neurons

Dynamic changes in neuronal morphology and transcriptional regulation play crucial roles in the neuronal network and function. Accumulating evidence suggests that the megakaryoblastic leukemia (MKL) family members, which function not only as actin-binding proteins but also as serum response factor (SRF) transcriptional coactivators, regulate neuronal morphology. However, the extracellular ligands and signaling pathways, which activate MKL-mediated morphological changes in neurons, remain unresolved. Here, we demonstrate that in addition to MKL1, MKL2, highly enriched in the forebrain, strongly contributes to the dendritic complexity, and this process is triggered by stimulation with activin, a member of the transforming growth factor β (TGF-β) superfamily. Activin promoted dendritic complexity in a SRF- and MKL-dependent manner without drastically affecting MKL localization and protein levels. In contrast, activin promoted the nuclear export of suppressor of cancer cell invasion (SCAI), which is a corepressor for SRF and MKL. Furthermore, overexpression of SCAI blocked activin-induced SRF transcriptional responses and dendritic complexity. Collectively, these results strongly suggest that activin-SCAI-MKL signaling is a novel pathway that regulates the dendritic morphology of rat cortical neurons by excluding SCAI from the nucleus and activating MKL/SRF-mediated gene expression.

SRF binding to SRE 6.9 in the Arc promoter is essential for LTD in cultured Purkinje cells

It has been suggested that gene expression and protein synthesis are required for both long-term memory consolidation and late phases of long-term potentiation and long-term depression (LTD). The necessary genes and the specific transcription factor binding sites in their promoters remain unknown. We found that inhibition of the transcription factor SRF or its cofactor MAL blocked the late phase of LTD in mouse cultured cerebellar Purkinje cells, as did deletion of the immediate early gene Arc. Using neuronal bacterial artificial chromosome (BAC) transfection, we found that, in Arc-/- cells transfected with a wild-type Arc BAC, late-phase LTD was rescued. However, mutation of one SRF-binding site in the Arc promoter (SRE 6.9) blocked this rescue. Co-transfection of wild-type Arc and SRF engineered to bind mutated SRE 6.9 restored late-phase LTD in Arc-/-, SRE 6.9 mutant BAC cells. Thus, SRF binding to SRE 6.9 in the Arc promoter is required for the late phase of cerebellar LTD.

Myocardin-related transcription factors regulate the Cdk5/Pctaire1 kinase cascade to control neurite outgrowth, neuronal migration and brain development

Numerous motile cell functions depend on signaling from the cytoskeleton to the nucleus. Myocardin-related transcription factors (MRTFs) translocate to the nucleus in response to actin polymerization and cooperate with serum response factor (Srf) to regulate the expression of genes encoding actin and other components of the cytoskeleton. Here, we show that MRTF-A (Mkl1) and MRTF-B (Mkl2) redundantly control neuronal migration and neurite outgrowth during mouse brain development. Conditional deletion of the genes encoding these Srf coactivators disrupts the formation of multiple brain structures, reflecting a failure in neuronal actin polymerization and cytoskeletal assembly. These abnormalities were accompanied by dysregulation of the actin-severing protein gelsolin and Pctaire1 (Cdk16) kinase, which cooperates with Cdk5 to initiate a kinase cascade that governs cytoskeletal rearrangements essential for neuron migration and neurite outgrowth. Thus, the MRTF/Srf partnership interlinks two key signaling pathways that control actin treadmilling and neuronal maturation, thereby fulfilling a regulatory loop that couples cytoskeletal dynamics to nuclear gene transcription during brain development.

Linking actin dynamics and gene transcription to drive cellular motile functions

Numerous physiological and pathological stimuli promote the rearrangement of the actin cytoskeleton, thereby modulating cellular motile functions. Although it seems intuitively obvious that cell motility requires coordinated protein biosynthesis, until recently the linkage between cytoskeletal actin dynamics and correlated gene activities remained unknown. This knowledge gap was filled in part by the discovery that globular actin polymerization liberates myocardin-related transcription factor (MRTF) cofactors, thereby inducing the nuclear transcription factor serum response factor (SRF) to modulate the expression of genes encoding structural and regulatory effectors of actin dynamics. This insight stimulated research to better understand the actin-MRTF-SRF circuit and to identify alternative mechanisms that link cytoskeletal dynamics and genome activity.

Serum response factor regulates hippocampal lamination and dendrite development and is connected with reelin signaling

During brain development, neurons and their nerve fibers are often segregated in specific layers. The hippocampus is a well-suited model system to study lamination in health and aberrant cell/fiber lamination associated with neurological disorders. SRF (serum response factor), a transcription factor, regulates synaptic-activity-induced immediate-early gene (IEG) induction and cytoskeleton-based neuronal motility. Using early postnatal conditional SRF ablation, we uncovered distorted hippocampal lamination, including malpositioning of granule cell neurons and disruption of layer-restricted termination of commissural-associational and mossy fiber axons. Besides axons, dendrite branching and spine morphogenesis in Srf mutants were impaired, offering a first morphological basis for SRF's reported role in learning and memory. Srf mutants resemble mice lacking components of the reelin signaling cascade, a fundamental signaling entity in brain lamination. Our data indicate that reelin signaling and SRF-mediated gene transcription might be connected: reelin induces IEG and cytoskeletal genes in an SRF-dependent manner. Further, reelin-induced neurite motility is blocked in Srf mutants and constitutively active SRF rescues impaired neurite extension in reeler mouse mutants in vitro. In sum, data provided in this report show that SRF contributes to hippocampal layer and nerve fiber organization and point at a link between Srf gene transcription and reelin signaling.