Inactivation of vimentin in satellite glial cells affects dorsal root ganglion intermediate filament expression and neuronal axon growth in vitro.

Peripheral nerve trauma and regeneration are complex events, and little is known concerning how occurrences in the distal stump affect the cell body's response to injury. Intermediate filament (IF) proteins underpin cellular architecture and take part in nerve cell proliferation, differentiation and axon regeneration, but their role in these processes is not yet fully understood. The present study aimed to investigate the regulation and interrelationship of major neural IFs in adult dorsal root ganglion (DRG) neurons and satellite glial cells (SGCs) following sciatic nerve injury. We demonstrated that the expression of neural IFs in DRG neurons and SGCs after axotomy depends on vimentin activity. In intact DRGs, synemin M and peripherin proteins are detected in small neurons while neurofilament L (NFL) and synemin L characterize large neurons. Both neuronal populations are surrounded by vimentin positive- and glial fibrillary acidic protein (GFAP)-negative SGCs. In response to axotomy, synemin M and peripherin were upregulated in large wild-type DRG neurons and, to a lesser extent, in vim-/- and synm-/- DRG neurons, suggesting the role for these IFs in axon regeneration. However, an increase in the number of NFL-positive small neurons was observed in vim-/- mice, accompanied by a decrease of peripherin-positive small neurons. These findings suggest that vimentin is required for injury-induced neuronal IF remodeling. We further show that vimentin is also indispensable for nerve injury-induced GFAP upregulation in perineuronal SGCs and that inactivation of vimentin and synemin appears to accelerate the rate of DRG neurite regeneration at early stages in vitro.

Synemin-related skeletal and cardiac myopathies: an overview of pathogenic variants.

This review analyzes data concerning patients with cardiomyopathies or skeletal myopathies associated with a variation in the intermediate filament (IF) synemin gene (SYNM), also referred to as desmuslin (DMN). Molecular studies demonstrate that synemin copolymerizes with desmin and vimentin IF and interacts with vinculin, α-actinin, α-dystrobrevin, dystrophin, talin, and zyxin. It has been found that synemin is an A-kinase-anchoring protein (AKAP) that anchors protein kinase A (PKA) and modulates the PKA-dependent phosphorylation of several cytoskeletal substrates such as desmin. Because several IF proteins, including desmin, have been implicated in human genetic disorders such as dominant or recessive congenital and adult-onset myopathy, synemin becomes a significant candidate for cardiac and skeletal myopathies of unknown etiology. Because SYNM is a new candidate gene that displays numerous sequence polymorphisms, in this review, we summarize the genetic and clinical literature about SYNM mutations. Protein-changing variants (missense, frameshifts, nonsense) were further evaluated based on structural modifications and amino acid interactions. We present in silico modeling of helical salt-bridges between residues to evaluate the impact of the synemin networks crucial to interactions with cytoskeletal proteins. Finally, a discussion is featured regarding certain variants that may contribute to the disease state.

Synemin acts as a regulator of signalling molecules during skeletal muscle hypertrophy.

Synemin, a type IV intermediate filament (IF) protein, forms a bridge between IFs and cellular membranes. As an A-kinase-anchoring protein, it also provides temporal and spatial targeting of protein kinase A (PKA). However, little is known about its functional roles in either process. To better understand its functions in muscle tissue, we generated synemin-deficient (Synm(-) (/-)) mice. Synm(-) (/-) mice displayed normal development and fertility but showed a mild degeneration and regeneration phenotype in myofibres and defects in sarcolemma membranes. Following mechanical overload, Synm(-) (/-) mice muscles showed a higher hypertrophic capacity with increased maximal force and fatigue resistance compared with control mice. At the molecular level, increased remodelling capacity was accompanied by decreased myostatin (also known as GDF8) and atrogin (also known as FBXO32) expression, and increased follistatin expression. Furthermore, the activity of muscle-mass control molecules (the PKA RIIα subunit, p70S6K and CREB1) was increased in mutant mice. Finally, analysis of muscle satellite cell behaviour suggested that the absence of synemin could affect the balance between self-renewal and differentiation of these cells. Taken together, our results show that synemin is necessary to maintain membrane integrity and regulates signalling molecules during muscle hypertrophy.

Posttranslational modifications of desmin and their implication in biological processes and pathologies.

Desmin, the muscle-specific intermediate filament, is involved in myofibrillar myopathies, dilated cardiomyopathy and muscle wasting. Desmin is the target of posttranslational modifications (PTMs) such as phosphorylation, ADP-ribosylation and ubiquitylation as well as nonenzymatic modifications such as glycation, oxidation and nitration. Several PTM target residues and their corresponding modifying enzymes have been discovered in human and nonhuman desmin. The major effect of phosphorylation and ADP-ribosylation is the disassembly of desmin filaments, while ubiquitylation of desmin leads to its degradation. The regulation of the desmin filament network by phosphorylation and ADP-ribosylation was found to be implicated in several major biological processes such as myogenesis, myoblast fusion, muscle contraction, muscle atrophy, cell division and possibly desmin interactions with its binding partners. Phosphorylation of desmin is also implicated in many forms of desmin-related myopathies (desminopathies). In this review, we summarize the findings on desmin PTMs and their implication in biological processes and pathologies, and discuss the current knowledge on the regulation of the desmin network by PTMs. We conclude that the desmin filament network can be seen as an intricate scaffold for muscle cell structure and biological processes and that its dynamics can be affected by PTMs. There are now precise tools to investigate PTMs and visualize cellular structures that have been underexploited in the study of desminopathies. Future studies should focus on these aspects.

Desmin regulates airway smooth muscle hypertrophy through early growth-responsive protein-1 and microRNA-26a.

Bronchial biopsies of asthmatic patients show a negative correlation desmin expression in airway smooth muscle cell (ASMC) and airway hyperresponsiveness. We previously showed that desmin is an intracellular load-bearing protein, which influences airway compliance, lung recoil, and airway contractile responsiveness (Shardonofsky, F. R., Capetanaki, Y., and Boriek, A. M. (2006) Am. J. Physiol. Lung Cell. Mol. Physiol. 290, L890-L896). These results suggest that desmin may play an important role in ASMC homeostasis. Here, we report that ASMCs of desmin null mice (ASMCs(Des-/-)) show hypertrophy and up-regulation microRNA-26a (miR-26a). Knockdown of miR-26a in ASMCs(Des-/-) inhibits hypertrophy, whereas enforced expression of miR-26a in ASMCs(Des+/+) induces hypertrophy. We identify that Egr1 (early growth responsive protein-1) activates miR-26a promoter via enhanced phosphorylation of Erk1/2 in ASMCs(Des-/-). We show glycogen synthase kinase-3ß (GSK-3ß) as a target gene of miR-26a. Moreover, induction of ASMCs(Des-/-) hypertrophy by the Erk-1/2/Egr-1/miR-26a/GSK-3ß pathway is consistent in human recombinant ASMCs, which stably suppresses 90% endogenous desmin expression. Overall, our data demonstrate a novel role for desmin as an anti-hypertrophic protein necessary for ASMC homeostasis and identifies desmin as a novel regulator of microRNA.

Desmin mutations in the terminal consensus motif prevent synemin-desmin heteropolymer filament assembly.

Disorganization of the desmin network is associated with cardiac and skeletal myopathies characterized by accumulation of desmin-containing aggregates in the cells. Multiple associations of intermediate filament proteins form a network to increase mechanical and functional stability. Synemin is a desmin-associated type VI intermediate filament protein. Neither its impact on desmin network nor how it integrates into desmin filament is yet elucidated. To gain more insight into the molecular basis of these processes, we coexpressed synemin with different desmin mutants in ex vivo models. The screening of fourteen desmin mutants showed that synemin with desmin mutants revealed two behaviors. Firstly, synemin was co-localized in desmin aggregates and its coexpression decreased the number of cells containing aggregates. Secondly, synemin was excluded from the aggregates, then synemin had no effect on desmin network organization. Among fourteen desmin mutants, there were only three mutants, p.E401K, p.R406W and p.E413K, in which synemin was not found in aggregates. This behavior was correlated to the abnormal salt-bridges of desmin-dimer as seen in silico constructs. Moreover, desmin constructs in silico and published results in literature have predicted that the salt-bridges absence in the desmin filament building prevent longitudinal annealing and/or radial compaction. These results suggest that the state of desmin-filament assembly is crucial for synemin anchorage and consequently might involve mechanical and functional stability of the cytoskeletal network.

Vimentin: Regulation and pathogenesis.

Vimentin, an abundant cytoplasmic intermediate filament protein, is recognized for its important role in stabilizing intracellular structure. Vimentin has been recognized for its mechanical role in cell plasticity and stress absorbers. Additionally, the functions of vimentin, similar to all other cytoplasmic intermediate filaments, are correlated to its ability to interact with cellular components responsible for signaling as well as kinases, therefore exerting control on gene regulatory networks. Moreover, several studies reveal a novel form of vimentin present at the surface of the plasma membrane or released in the extracellular environment in different physiological and pathological conditions. Based on data pertaining to vimentin's location outside of the cell, novel functions have emerged. The vimentin promoter is complex and appears to be controlled by a combination of positive and negative regulatory elements. In this review, we first present the involvement of these regulatory elements as well as epigenetic regulation of vimentin in different physiological and pathological contexts, including cell growth, cell differentiation, cancer, epithelial to mesenchymal transition and viral infection. Furthermore, this review also analyzes the secretion of vimentin, its presence at the cell surface, the role of extracellular vimentin as a specific marker, its function as a receptor for the von Willebrand factor as well as the entry of viruses, requirements for pathogen invasion, transcellular migration, and the immune response. Finally, a discussion is featured regarding the delocalization of vimentin that may contribute to diseases and disorders.

Dynamic expression of synemin isoforms in mouse embryonic stem cells and neural derivatives.

BACKGROUND: Intermediate filaments (IFs) are major components of the mammalian cytoskeleton and expressed in cell-type-specific patterns. Morphological changes during cell differentiation are linked to IF network remodeling. However, little is known concerning the presence and the role of IFs in embryonic stem (ES) cells and during their differentiation. RESULTS: We have examined the expression profile of synemin isoforms in mouse pluripotent ES cells and during their neural differentiation induced by retinoic acid. Using RT-PCR, Western blotting and immunostaining, we show that synemin M is present at both mRNA and protein levels in undifferentiated ES cells as early as pluripotency factor Oct-3/4 and IF keratin 8. Synemin H was produced only in neural precursors when neural differentiation started, concurrently with synemin M, nestin and glial fibrillary acidic protein. However, both synemin H and M were restricted to the progenitor line during the neural differentiation program. Our in vivo analysis also confirmed the expression of synemins H/M in multipotent neural stem cells in the subventricular zone of the adult brain, a neurogenic germinal niche of the mice. Knocking down synemin in ES cells by shRNA lentiviral particles transduction has no influence on expression of Oct4, Nanog and SOX2, but decreased keratin 8 expression. CONCLUSIONS: Our study shows a developmental stage specific regulation of synemin isoforms in ES cells and its neural derivatives. These findings represent the first evidence that synemins could potentially be useful markers for distinguishing multipotent ES cells from undifferentiated neural stem cells and more committed progenitor cells.

Synemin interacts with the LIM domain protein zyxin and is essential for cell adhesion and migration.

Synemin is a unique cytoplasmic intermediate filament protein for which there is limited understanding of its exact cellular functions. The single human synemin gene encodes at least two splice variants named alpha-synemin and beta-synemin, with the larger alpha-synemin containing an additional 312 amino acid insert within the C-terminal tail domain. We report herein that, by using the entire tail domain of the smaller beta-synemin as the bait in a yeast two-hybrid screen of a human skeletal muscle cDNA library, the LIM domain protein zyxin was identified as an interaction partner for human synemin. The synemin binding site in human zyxin was subsequently mapped to the C-terminal three tandem LIM-domain repeats, whereas the binding site for zyxin within beta-synemin is within the C-terminal 332 amino acid region (SNbetaTII) at the end of the long tail domain. Transient expression of SNbetaTII within mammalian cells markedly reduced zyxin protein level, blocked localization of zyxin at focal adhesion sites and resulted in decreased cell adhesion and increased motility. Knockdown of synemin expression with siRNAs within mammalian cells resulted in significantly compromised cell adhesion and cell motility. Our results suggest that synemin participates in focal adhesion dynamics and is essential for cell adhesion and migration.

Structural and dynamic changes of the serum response element and the core domain of serum response factor induced by their association.

Transcriptional activity of serum response factor (SRF) is dependent on its binding to the CC(A/T)(6)GG box (CArG box) of serum response element (SRE). By Raman spectroscopy, we carried out a comparative analysis, in solution, of the complexes obtained from the association of core-SRF with 20-mer SREs bearing wild-type and mutated c-fos CArG boxes. In case of association with the wild type c-fos CArG box, the complex does not bring out the expected Raman signature of a stable bending of the targeted SRE but keeps a bend-linear conformer oligonucleotide interconversion. The linear conformer population is larger than that of free oligonucleotide. In the core-SRF moiety of the wild-type complex a large spectral change associated with the CO-groups from Asp and/or Glu residues shows that their ionization states and the strength of their interactions decrease as compared to those of mutated non-specific complexes. Structural constraints evidenced on the free core-SRF are released in the wild-type complex and environmental heterogeneities appear in the vicinity of Tyr residues, due to higher water molecule access. The H-bonding configuration of one Tyr OH-group, in average, changes with a net transfer from H-bond acceptor character to a combined donor and acceptor character. A charge repartition distributed on both core-SRF and targeted SRE stabilizes the specific complex, allowing the two partners to experience a variety of conformations.

Synemin isoforms during mouse development: multiplicity of partners in vascular and neuronal systems.

The intermediate filament (IF) synemin gene encodes three IF proteins (H 180, M 150, L 41 kDa isoforms) with overlapping distributions. In the present study we analysed the mRNA and protein expression of each isoform in developing mouse embryos. Synemin M mRNA was present as early as E5 with vimentin and nestin. Synemin H was found later at E9 in the nervous system and mesodermic derivatives concomitantly with angiogenesis, somitogenesis and the migration of neural crest cells. Synemin L appeared later in neurons at E15. Furthermore, the synemin isoforms required different IF partners depending on the cell type to form filamentous structures. In endothelial cells, synemin H/M were found associated with vimentin and were absent in vimentin-null mice. In neurons of the peripheral nervous system of E15 embryos, synemin H/M or L were co-expressed with neurofilament, peripherin and internexin. In adult mice, our data support the existence of different subpopulations of neurons within the dorsal root ganglia: one composed of small neurons containing synemin H/M and peripherin, and another composed of large neurons containing synemin L and neurofilaments. Axons devoid of neurofilaments from mutant mice (NFHLacZ) showed an absence of the L isoform but contained H/M isoforms with peripherin.

Vimentin as a target for the treatment of COVID-19.

We and others propose vimentin as a possible cellular target for the treatment of COVID-19. This innovative idea is so recent that it requires further attention and debate. The significant role played by vimentin in virus-induced infection however is well established: (1) vimentin has been reported as a co-receptor and/or attachment site for SARS-CoV; (2) vimentin is involved in viral replication in cells; (3) vimentin plays a fundamental role in both the viral infection and the consequent explosive immune-inflammatory response and (4) a lower vimentin expression is associated with the inhibition of epithelial to mesenchymal transition and fibrosis. Moreover, the absence of vimentin in mice makes them resistant to lung injury. Since vimentin has a twofold role in the disease, not only being involved in the viral infection but also in the associated life-threatening lung inflammation, the use of vimentin-targeted drugs may offer a synergistic advantage as compared with other treatments not targeting vimentin. Consequently, we speculate here that drugs which decrease the expression of vimentin can be used for the treatment of patients with COVID-19 and advise that several Food and Drug Administration-approved drugs be immediately tested in clinical trials against SARS-CoV-2, thus broadening therapeutic options for this type of viral infection.

Human alpha-synemin interacts directly with vinculin and metavinculin.

Synemin is a very large, unique member of the IF (intermediate filament) protein superfamily. Association of synemin with the major IF proteins, desmin and/or vimentin, within muscle cells forms heteropolymeric IFs. We have previously identified interactions of avian synemin with alpha-actinin and vinculin. Avian synemin, however, is expressed as only one form, whereas human synemin is expressed as two major splice variants, namely alpha- and beta-synemins. The larger alpha-synemin contains an additional 312-amino-acid insert (termed SNTIII) located near the end of the long C-terminal tail domain. Whether alpha- and beta-synemins have different cellular functions is unclear. In the present study we show, by in vitro protein-protein interaction assays, that SNTIII interacts directly with both vinculin and metavinculin. Furthermore, SNTIII interacts with vinculin in vivo, and this association is promoted by PtdIns(4,5)P(2). SNTIII also specifically co-localizes with vinculin within focal adhesions when transiently expressed in mammalian cells. In contrast, other regions of synemin show distinct localization patterns in comparison with those of SNTIII, without labelling focal adhesions. Our results indicate that alpha-synemin, but not beta-synemin, interacts with both vinculin and metavinculin, thereby linking the heteropolymeric IFs to adhesion-type junctions, such as the costameres located within human striated muscle cells.

Effects of the selective inhibition of proteasome caspase-like activity by CLi a derivative of nor-cerpegin in dystrophic mdx mice.

Previous studies have shown that proteasome inhibition can have beneficial effects in dystrophic mouse models. In this study, we have investigated the effects of a new selective proteasome inhibitor, CLi, a strong caspase-like inhibitor of the 20S proteasome, on skeletal and cardiac muscle functions of mdx mice. In the first series of experiments, five-month-old male mdx mice (n = 34) were treated with 2 different doses (20 and 100 µg/kg) of CLi and in the second series of experiments, five-month-old female mdx (n = 19) and wild-type (n = 24) mice were treated with 20 µg/kg CLi and Velcade (1 mg/kg) for 1-month. All animals were treadmill exercised twice a week to worsen the dystrophic features. In the first series of experiments, our results demonstrated that 20 µg/kg CLi did not significantly increase absolute and specific maximal forces in skeletal muscle from male mdx mice. Moreover, the higher susceptibility to contraction induced skeletal muscle injury was worsened by 100 µg/kg CLi since the force drop following lengthening contractions was increased with this high dose. Furthermore, we found no differences in the mRNA levels of the molecular markers implicated in dystrophic features. Concerning cardiac function, CLi had no effect on left ventricular function since ejection and shortening fractions were unchanged in male mdx mice. Similarly, CLi did not modify the expression of genes implicated in cardiac remodeling. In the second series of experiments, our results demonstrated an improvement in absolute and specific maximal forces by CLi, whereas Velcade only increased specific maximal force in female mdx mice. In addition, exercise tolerance was not improved by CLi. Taken together, our results show that CLi treatment can only improve maximal force production in exercised female mdx mice without affecting either exercice tolerance capacity or cardiac function. In conclusion, selective inhibition of caspase-like activity of proteasome with CLi has no compelling beneficial effect in dystrophic mdx mice.

Self-assembly incompetence of synemin is related to the property of its head and rod domains.

The mechanisms regulating the intermediate filament (IF) protein assembly are complex and not yet fully understood. All vertebrate cytoplasmic IF proteins have a central alpha-helical rod domain flanked by variable head and tail domains. The IF protein synemin cannot homopolymerize to form filament networks; it needs an appropriate copolymerization partner. To elucidate the roles of the vimentin head domain, the TAAL motif in the 2A region, and the TYRKLLEGEE motif in the 2B region of the rod domain in synemin filament formation, we have prepared a series of synemin constructs by site-directed mutagenesis and chimeric synemins having the vimentin head domain. The assembly properties of synemin constructs were assessed by the immunofluorescence of transient transfection into cultured SW13 cells without endogenous IFs. Our data showed that the formation of a filamentous network required at least the vimentin-like head domain and both the 2A and 2B regions of the rod domain.

Mosaic inactivation of the serum response factor gene in the myocardium induces focal lesions and heart failure.

BACKGROUND AND AIMS: Regional alterations in ventricular mechanical functions are a primary determinant for the risk of myocardial injuries in various cardiomyopathies. The serum response factor (SRF) is a transcription factor regulating contractile and cytoskeletal genes and may play an important role in the remodelling of myocardium at the cellular level. METHODS: Using Desmin-Cre transgenic mice, we generated a model of mosaic inactivation of a floxed-Srf allele in the heart to analyze the consequence of regional alterations of SRF-mediated functions in the myocardium. RESULTS: Two types of cardiomyocytes co-existed in the Desmin-Cre:Sf/Sf mice. Cardiomyocytes lacking SRF became thin and elongated while cardiomyocytes containing SRF became hypertrophic. Several physiological contractile genes were down-regulated while skeletal alpha-actin was induced in SRF positive area only. Mutants developed heart failure associated with the presence of focal lesions in the myocardium, and died before month 11. CONCLUSIONS: Juxtaposition of functional SRF wild-type and failing SRF mutant cardiomyocytes generates deleterious heterogeneity in the myocardium. Our results show that SRF contributes to the capacity of cardiomyocytes to remodel their shape and contractile functions in response to their local environment; suggesting that it may play a role in pathologies involving regional alterations of ventricular mechanics in the heart.

C-->G base mutations in the CArG box of c-fos serum response element alter its bending flexibility. Consequences for core-SRF recognition.

By binding to the CArG box sequence, the serum response factor (SRF) activates several muscle-specific genes, as well as genes that respond to mitogens. The core domain of the SRF (core-SRF) binds as a dimer to the CArG box C-5C-4A-3T-2A-1T+1T+2A+3G+4G+5 of the c-fos serum response element (SREfos). However, previous studies using 20-mer DNAs have shown that the binding stoichiometry of core-SRF is significantly altered by mutations C-5-->G (SREGfos) and C-5C-4-->GG (SREGGfos) of the CArG box [A Huet, A Parlakian, M-C Arnaud, J-M Glandières, P Valat, S Fermandjian, D Paulin, B Alpert & C Zentz (2005) FEBS J272, 3105-3119]. To understand these effects, we carried out a comparative analysis of the three 20-mer DNAs SREfos, SREGfos and SREGGfos in aqueous solution. Their CD spectra were of the B-DNA type with small differences generated by variations in the mutual arrangement of the base pairs. Analysis by singular value decomposition of a set of Raman spectra recorded as a function of temperature, revealed a premelting transition associated with a conformational shift in the DNA double helices from a bent to a linear form. Time-resolved fluorescence anisotropy shows that the fluorescein reporter linked to the oligonucleotide 5'-ends experiences twisting motions of the double helices related to the interconversion between bent and linear conformers. The three SREs present various bent populations submitted, however, to particular internal dynamics, decisive for the mutual adjustment of binding partners and therefore specific complex formation.

Intermediate filament-like protein syncoilin in normal and myopathic striated muscle.

The intermediate filament-like protein syncoilin is a member of the dystrophin protein complex, and links the complex to the cytoskeleton through binding alpha-dystrobrevin and desmin in muscle. Here, we identify further sites of syncoilin location in normal muscle: at the perinuclear space, myotendinous junction, and enrichment in the sarcolemma and sarcoplasm of oxidative muscle fibers in mice. To understand the importance of the dystrophin protein complex-syncoilin-cytoskeletal link and its implication to disease, we analyzed syncoilin in mice null for alpha-dystrobrevin (adbn-/-) and desmin (des-/-). Syncoilin was upregulated in dystrophic muscles of adbn-/- mice, without alteration in its subcellular location. In des-/- mice, syncoilin was severely reduced in skeletal muscle; lost from sarcomeric Z-lines and neuromuscular junctions, and redistributed from the sub-sarcolemmal cytoskeleton to the cytoplasm. The data show that absence of alpha-dystrobrevin or desmin leads to dynamic changes in syncoilin that may compensate for, or participate in, different muscle myopathies.

Targeted inactivation of serum response factor in the developing heart results in myocardial defects and embryonic lethality.

Serum response factor (SRF) is at the confluence of multiple signaling pathways controlling the transcription of immediate-early response genes and muscle-specific genes. There are active SRF target sequences in more than 50 genes expressed in the three muscle lineages including normal and diseased hearts. However, the role of SRF in heart formation has not been addressed in vivo thus far due to the early requirement of SRF for mesoderm formation. We have generated a conditional mutant of SRF by using Cre-LoxP strategy that will be extremely useful to study the role of SRF in embryonic and postnatal cardiac functions, as well as in other tissues. This report shows that heart-specific deletion of SRF in the embryo by using a new beta MHC-Cre transgenic mouse line results in lethal cardiac defects between embryonic day 10.5 (E10.5) and E13.5, as evidenced by abnormally thin myocardium, dilated cardiac chambers, poor trabeculation, and a disorganized interventricular septum. At E9.5, we found a marked reduction in the expression of essential regulators of heart development, including Nkx2.5, GATA4, myocardin, and the SRF target gene c-fos prior to overt maldevelopment. We conclude that SRF is crucial for cardiac differentiation and maturation, acting as a global regulator of multiple developmental genes.

Temporally controlled onset of dilated cardiomyopathy through disruption of the SRF gene in adult heart.

BACKGROUND: Serum response factor (SRF) is a cardiac transcription factor involved in cell growth and differentiation. We have shown, using the Cre/loxP system, that cardiac-specific disruption of SRF gene in the embryonic heart results in lethal cardiac defects. The role of SRF in adult heart is unknown. METHODS AND RESULTS: We disrupted SRF in the adult heart using a heart-specific tamoxifen-inducible Cre recombinase. This disruption led to impaired left ventricular function with reduced contractility, subsequently progressing to dilated cardiomyopathy, as demonstrated by serial echocardiography, including tissue Doppler imaging. The cytoarchitecture of cardiomyocytes was altered in the intercalated disks. All mutant mice died from heart failure 10 weeks after treatment. These functional and structural defects were preceded by early alterations in the cardiac gene expression program: major decreases in mRNA levels for cardiac alpha-actin, muscle creatine kinase, and calcium-handling genes. CONCLUSIONS: SRF is crucial for adult cardiac function and integrity. We suggest that the rapid progression to heart failure in SRF mutant mice results primarily from decreased expression of proteins involved in force generation and transmission, low levels of polymerized actin, and changes in cytoarchitecture, without hypertrophic compensation. These cardiac-specific SRF-deficient mice have the morphological and clinical features of acquired dilated cardiomyopathy in humans and may therefore be used as an inducible model of this disorder.