Synemin may function to directly link muscle cell intermediate filaments to both myofibrillar Z-lines and costameres

Synaptopodin-2 Isoforms Have Specific Binding Partners and Display Distinct, Muscle Cell Type-Specific Expression Patterns

Synaptopodin-2 (SYNPO2) is a protein associated with the Z-disc in striated muscle cells. It interacts with α-actinin and filamin C, playing a role in Z-disc maintenance under stress by chaperone-assisted selective autophagy (CASA). In smooth muscle cells, SYNPO2 is a component of dense bodies. Furthermore, it has been proposed to play a role in tumor cell proliferation and metastasis in many different kinds of cancers. Alternative transcription start sites and alternative splicing predict the expression of six putative SYNPO2 isoforms differing by extended amino- and/or carboxy-termini. Our analyses at mRNA and protein levels revealed differential expression of SYNPO2 isoforms in cardiac, skeletal and smooth muscle cells. We identified synemin, an intermediate filament protein, as a novel binding partner of the PDZ-domain in the amino-terminal extension of the isoforms mainly expressed in cardiac and smooth muscle cells, and demonstrated colocalization of SYNPO2 and synemin in both cell types. A carboxy-terminal extension, mainly expressed in smooth muscle cells, is sufficient for association with dense bodies and interacts with α-actinin. SYNPO2 therefore represents an additional and novel link between intermediate filaments and the Z-discs in cardiomyocytes and dense bodies in smooth muscle cells, respectively. In pathological skeletal muscle samples, we identified SYNPO2 in the central and intermediate zones of target fibers of patients with neurogenic muscular atrophy, and in nemaline bodies. Our findings help to understand distinct functions of individual SYNPO2 isoforms in different muscle tissues, but also in tumor pathology.

Intermediate filament proteins are reliable immunohistological biomarkers to help diagnose multiple tissue-specific diseases

Cytoskeletal networks are proteins that effectively maintain cell integrity and provide mechanical support to cells by actively transmitting mechanical signals. Intermediate filaments, which are from the cytoskeleton family and are 10 nanometres in diameter, are unlike actin and microtubules, which are highly dynamic cytoskeletal elements. Intermediate filaments are flexible at low strain, harden at high strain and resist breaking. For this reason, these filaments fulfil structural functions by providing mechanical support to the cells through their different strain-hardening properties. Intermediate filaments are suitable in that cells both cope with mechanical forces and modulate signal transmission. These filaments are composed of fibrous proteins that exhibit a central α-helical rod domain with a conserved substructure. Intermediate filament proteins are divided into six groups. Type I and type II include acidic and basic keratins, type III, vimentin, desmin, peripheralin and glial fibrillary acidic protein (GFAP), respectively. Type IV intermediate filament group includes neurofilament proteins and a fourth neurofilament subunit, α-internexin proteins. Type V consists of lamins located in the nucleus, and the type VI group consists of lens-specific intermediate filaments, CP49/phakinin and filen. Intermediate filament proteins show specific immunoreactivity in differentiating cells and mature cells of various types. Various carcinomas such as colorectal, urothelial and ovarian, diseases such as chronic pancreatitis, cirrhosis, hepatitis and cataract have been associated with intermediate filaments. Accordingly, this section reviews available immunohistochemical antibodies to intermediate filament proteins. Identification of intermediate filament proteins by methodological methods may contribute to the understanding of complex diseases.

Retinal gliosis and phenotypic diversity of intermediate filament induction and remodeling upon acoustic blast overpressure (ABO) exposure to the rat eye

Traumatic brain injury (TBI) caused by acoustic blast overpressure (ABO) is frequently associated with chronic visual deficits in military personnel and civilians. In this study, we characterized retinal gliotic response in adult male rats following a single ABO exposure directed to one side of the head. Expression of gliosis markers and intermediate filaments was assessed at 48 h and 1 wk post-ABO exposure, in comparison to age-matched non-exposed control retina. In response to a single ABO exposure, type III IF, glial fibrillary acidic protein (GFAP) was variably induced in a subpopulation of retinal Müller glia in ipsilateral eyes. ABO-exposed eyes exhibited radial Müller glial GFAP filament extension through the inner plexiform layer (IPL) and the inner nuclear layer (INL) through the retina in both the nasal quadrant and juxta-optic nerve head (jONH) eye regions at 1 wk post-ABO. We observed an ∼6-fold increase (p ≤ 0.05) in radial glial GFAP immunolabeling in the IPL in both eye regions, in comparison to regionally matched controls. Similarly, GFAP extension through the INL into the outer retina was elevated ∼3-fold, p ≤ 0.05 in the nasal retina, but exhibited wider variability in the jONH retina. In contrast, constitutive type III IF vimentin exhibited greater remodeling in retinal Müller glia through the jONH retina compared to the nasal retina in response to ABO. We observed areas of lateral vimentin remodeling through the Müller glial end-feet, and greater mid-outer retinal radial vimentin IF extension in a subpopulation of glia at 1 wk post-ABO. We also observed a significant increase in total retinal levels of the type III IF desmin in ABO-exposed retina vs. controls (∼1.6-fold, p ≤ 0.01). In addition, ABO-exposure elicited varied glial induction of developmentally regulated type VI family IFs (nestin and synemin) in subpopulations of Müller cells at 48 h and 1 wk post-ABO. We demonstrate that multiple glial phenotypes emerge in the rat retina following a single ABO exposure, rather than a global homogeneous retinal glial response, involving less well characterized IF protein forms which warrant further investigation in the context of ABO-induced retinal gliosis.

Identifying Plectin Isoform Functions through Animal Models

Plectin, a high-molecular-weight cytoskeletal linker protein, binds with high affinity to intermediate filaments of all types and connects them to junctional complexes, organelles, and inner membrane systems. In addition, it interacts with actomyosin structures and microtubules. As a multifunctional protein, plectin has been implicated in several multisystemic diseases, the most common of which is epidermolysis bullosa simplex with muscular dystrophy (EBS-MD). A great part of our knowledge about plectin’s functional diversity has been gained through the analysis of a unique collection of transgenic mice that includes a full (null) knockout (KO), several tissue-restricted and isoform-specific KOs, three double KOs, and two knock-in lines. The key molecular features and pathological phenotypes of these mice will be discussed in this review. In summary, the analysis of the different genetic models indicated that a functional plectin is required for the proper function of striated and simple epithelia, cardiac and skeletal muscle, the neuromuscular junction, and the vascular endothelium, recapitulating the symptoms of humans carrying plectin mutations. The plectin-null line showed severe skin and muscle phenotypes reflecting the importance of plectin for hemidesmosome and sarcomere integrity; whereas the ablation of individual isoforms caused a specific phenotype in myofibers, basal keratinocytes, or neurons. Tissue-restricted ablation of plectin rendered the targeted cells less resilient to mechanical stress. Studies based on animal models other than the mouse, such as zebrafish and C. elegans, will be discussed as well.

PERM1 interacts with the MICOS-MIB complex to connect the mitochondria and sarcolemma via ankyrin B

Skeletal muscle subsarcolemmal mitochondria (SSM) and intermyofibrillar mitochondria subpopulations have distinct metabolic activity and sensitivity, though the mechanisms that localize SSM to peripheral areas of muscle fibers are poorly understood. A protein interaction study and complexome profiling identifies PERM1 interacts with the MICOS-MIB complex. Ablation of Perm1 in mice reduces muscle force, decreases mitochondrial membrane potential and complex I activity, and reduces the numbers of SSM in skeletal muscle. We demonstrate PERM1 interacts with the intracellular adaptor protein ankyrin B (ANKB) that connects the cytoskeleton to the plasma membrane. Moreover, we identify a C-terminal transmembrane helix that anchors PERM1 into the outer mitochondrial membrane. We conclude PERM1 functions in the MICOS-MIB complex and acts as an adapter to connect the mitochondria with the sarcolemma via ANKB.

Absence of Desmin Results in Impaired Adaptive Response to Mechanical Overloading of Skeletal Muscle

Background: Desmin is a muscle-specific protein belonging to the intermediate filament family. Desmin mutations are linked to skeletal muscle defects, including inherited myopathies with severe clinical manifestations. The aim of this study was to examine the role of desmin in skeletal muscle remodeling and performance gain induced by muscle mechanical overloading which mimics resistance training.

Methods: Plantaris muscles were overloaded by surgical ablation of gastrocnemius and soleus muscles. The functional response of plantaris muscle to mechanical overloading in desmin-deficient mice (DesKO, n = 32) was compared to that of control mice (n = 36) after 7-days or 1-month overloading. To elucidate the molecular mechanisms implicated in the observed partial adaptive response of DesKO muscle, we examined the expression levels of genes involved in muscle growth, myogenesis, inflammation and oxidative energetic metabolism. Moreover, ultrastructure and the proteolysis pathway were explored.

Results: Contrary to control, absolute maximal force did not increase in DesKO muscle following 1-month mechanical overloading. Fatigue resistance was also less increased in DesKO as compared to control muscle. Despite impaired functional adaptive response of DesKO mice to mechanical overloading, muscle weight and the number of oxidative MHC2a-positive fibers per cross-section similarly increased in both genotypes after 1-month overloading. However, mechanical overloading-elicited remodeling failed to activate a normal myogenic program after 7-days overloading, resulting in proportionally reduced activation and differentiation of muscle stem cells. Ultrastructural analysis of the plantaris muscle after 1-month overloading revealed muscle fiber damage in DesKO, as indicated by the loss of sarcomere integrity and mitochondrial abnormalities. Moreover, the observed accumulation of autophagosomes and lysosomes in DesKO muscle fibers could indicate a blockage of autophagy. To address this issue, two main proteolysis pathways, the ubiquitin-proteasome system and autophagy, were explored in DesKO and control muscle. Our results suggested an alteration of proteolysis pathways in DesKO muscle in response to mechanical overloading.

Conclusion: Taken together, our results show that mechanical overloading increases the negative impact of the lack of desmin on myofibril organization and mitochondria. Furthermore, our results suggest that under these conditions, the repairing activity of autophagy is disturbed. Consequently, force generation is not improved despite muscle growth, suggesting that desmin is required for a complete response to resistance training in skeletal muscle.

Absence of Desmin in Myofibers of the Zebrafish Extraocular Muscles

Purpose

To study the medial rectus (MR) muscle of zebrafish (Daniorerio) with respect to the pattern of distribution of desmin and its correlation to distinct types of myofibers and motor endplates.

Methods

The MRs of zebrafish were examined using confocal microscopy in whole-mount longitudinal specimens and in cross sections processed for immunohistochemistry with antibodies against desmin, myosin heavy chain isoforms, and innervation markers. Desmin patterns were correlated to major myofiber type and type of innervation. A total of 1382 myofibers in nine MR muscles were analyzed.

Results

Four distinct desmin immunolabeling patterns were found in the zebrafish MRs. Approximately a third of all slow myofibers lacked desmin, representing 8.5% of the total myofiber population. The adult zebrafish MR muscle displayed en grappe, en plaque, and multiterminal en plaque neuromuscular junctions (NMJs) with intricate patterns of desmin immunolabeling.

Conclusions

The MRs of zebrafish showed important similarities with the human extraocular muscles with regard to the pattern of desmin distribution and presence of the major types of NMJs and can be regarded as an adequate model to further study the role of desmin and the implications of heterogeneity in cytoskeletal protein composition.

Translational Relevance

The establishment of a zebrafish model to study the cytoskeleton in muscles that are particularly resistant to muscle disease opens new avenues to understand human myopathies and muscle dystrophies and may provide clues to new therapies.

The Diversity of Intermediate Filaments in Astrocytes

Despite the remarkable complexity of the individual neuron and of neuronal circuits, it has been clear for quite a while that, in order to understand the functioning of the brain, the contribution of other cell types in the brain have to be accounted for. Among glial cells, astrocytes have multiple roles in orchestrating neuronal functions. Their communication with neurons by exchanging signaling molecules and removing molecules from extracellular space takes place at several levels and is governed by different cellular processes, supported by multiple cellular structures, including the cytoskeleton. Intermediate filaments in astrocytes are emerging as important integrators of cellular processes. Astrocytes express five types of intermediate filaments: glial fibrillary acidic protein (GFAP); vimentin; nestin; synemin; lamins. Variability, interactions with different cellular structures and the particular roles of individual intermediate filaments in astrocytes have been studied extensively in the case of GFAP and vimentin, but far less attention has been given to nestin, synemin and lamins. Similarly, the interplay between different types of cytoskeleton and the interaction between the cytoskeleton and membranous structures, which is mediated by cytolinker proteins, are understudied in astrocytes. The present review summarizes the basic properties of astrocytic intermediate filaments and of other cytoskeletal macromolecules, such as cytolinker proteins, and describes the current knowledge of their roles in normal physiological and pathological conditions.

Synemin Redefined: Multiple Binding Partners Results in Multifunctionality

Historically synemin has been studied as an intermediate filament protein. However, synemin also binds the type II regulatory (R) subunit α of protein kinase A (PKA) and protein phosphatase type 2A, thus participating in the PKA and phosphoinositide 3-kinase (PI3K)-Akt and signaling pathways. In addition, recent studies using transgenic mice indicate that a significant function of synemin is its role in signaling pathways in various tissues, including the heart. Recent clinical reports have shown that synemin mutations led to multiple cases of dilated cardiomyopathy. Additionally, a single case of the rare condition ulnar-mammary-like syndrome with left ventricular tachycardia due to a mutation in the synemin gene (SYNM) has been reported. Therefore, this review uses these recent studies to provide a new framework for detailed discussions on synemin tissue distribution, binding partners and synemin in disease. Differences between α- and β-synemin are highlighted. The studies presented here indicate that while synemin does function as an intermediate filament protein, it is unique among this large family of proteins as it is also a regulator of signaling pathways and a crosslinker. Also evident is that the dominant function(s) are isoform-, developmental-, and tissue-specific.

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.

Keratin 18 is an integral part of the intermediate filament network in murine skeletal muscle

Intermediate filaments (IFs) contribute to force transmission, cellular integrity, and signaling in skeletal muscle. We previously identified keratin 19 (Krt19) as a muscle IF protein. We now report the presence of a second type I muscle keratin, Krt18. Krt18 mRNA levels are about half those for Krt19 and only 1:1,000th those for desmin; the protein was nevertheless detectable in immunoblots. Muscle function, measured by maximal isometric force in vivo, was moderately compromised in Krt18-knockout (Krt18-KO) or dominant-negative mutant mice (Krt18 DN), but structure was unaltered. Exogenous Krt18, introduced by electroporation, was localized in a reticulum around the contractile apparatus in wild-type muscle and to a lesser extent in muscle lacking Krt19 or desmin or both proteins. Exogenous Krt19, which was either reticular or aggregated in controls, became reticular more frequently in Krt19-null than in Krt18-null, desmin-null, or double-null muscles. Desmin was assembled into the reticulum normally in all genotypes. Notably, all three IF proteins appeared in overlapping reticular structures. We assessed the effect of Krt18 on susceptibility to injury in vivo by electroporating siRNA into tibialis anterior (TA) muscles of control and Krt19-KO mice and testing 2 wk later. Results showed a 33% strength deficit (reduction in maximal torque after injury) compared with siRNA-treated controls. Conversely, electroporation of siRNA to Krt19 into Krt18-null TA yielded a strength deficit of 18% after injury compared with controls. Our results suggest that Krt18 plays a complementary role to Krt19 in skeletal muscle in both assembling keratin-based filaments and transducing contractile force.

The Cytoskeleton in the Extraocular Muscles of Desmin Knockout Mice

Purpose

To investigate the effect of absence of desmin on the extraocular muscles (EOMs) with focus on the structure and composition of the cytoskeleton.

Methods

The distribution of synemin, syncoilin, plectin, nestin, and dystrophin was evaluated on cross and longitudinal sections of EOMs and limb muscles from 1-year-old desmin knockout mice (desmin-/-) by immunofluorescence. General morphology was evaluated with hematoxylin and eosin while mitochondrial content and distribution were evaluated by succinate dehydrogenase (SDH) and modified Gomori trichrome stainings.

Results

The muscle fibers of the EOMs in desmin-/- mice were remarkably well preserved in contrast to those in the severely affected soleus and the slightly affected gastrocnemius muscles. There were no signs of muscular pathology in the EOMs and all cytoskeletal proteins studied showed a correct location at sarcolemma and Z-discs. However, an increase of SDH staining and mitochondrial aggregates under the sarcolemma was detected.

Conclusions

The structure of the EOMs was well preserved in the absence of desmin. We suggest that desmin is not necessary for correct synemin, syncoilin, plectin, and dystrophin location on the cytoskeleton of EOMs. However, it is needed to maintain an appropriate mitochondrial distribution in both EOMs and limb muscles.

Molecular insights into cardiomyopathies associated with desmin (DES) mutations

Increasing usage of next-generation sequencing techniques pushed during the last decade cardiogenetic diagnostics leading to the identification of a huge number of genetic variants in about 170 genes associated with cardiomyopathies, channelopathies, or syndromes with cardiac involvement. Because of the biochemical and cellular complexity, it is challenging to understand the clinical meaning or even the relevant pathomechanisms of the majority of genetic sequence variants. However, detailed knowledge about the associated molecular pathomechanism is essential for the development of efficient therapeutic strategies in future and genetic counseling. Mutations in DES, encoding the muscle-specific intermediate filament protein desmin, have been identified in different kinds of cardiac and skeletal myopathies. Here, we review the functions of desmin in health and disease with a focus on cardiomyopathies. In addition, we will summarize the genetic and clinical literature about DES mutations and will explain relevant cell and animal models. Moreover, we discuss upcoming perspectives and consequences of novel experimental approaches like genome editing technology, which might open a novel research field contributing to the development of efficient and mutation-specific treatment options.

Rare Case of Ulnar-Mammary-Like Syndrome With Left Ventricular Tachycardia and Lack of TBX3 Mutation

"Heart-hand" type syndromes represent a group of rare congenital conditions that combine cardiac pathology (structural defect or arrhythmic disorder) and limb abnormality. Significant clinical variability and genetic heterogeneity typical for such syndromes complicate correct diagnosis, prognosis, and appropriate genetic counseling of the affected families. By now, only single genes have been unambiguously determined as a genetic cause of heart-hand syndromes and phenotypically similar conditions. In the present study, we report on a 25-year-old Russian female patient with a clinical picture resembling ulnar-mammary syndrome (UMS). Principal clinical manifestations included heart septal fibrosis and non-sustained left ventricular tachycardia combined with fifth finger camptodactyly, hypoplastic breast, abnormal teeth, and mental retardation. Target Sanger sequencing and array-based comparative genome hybridization confirmed the lack of pathogenic mutations and large-scale deletions in TBX3 (12q24.21), the only gene known to be associated with UMS cases to date. Based on the results of whole-exome sequencing, 14 potential candidate variants were identified. Among them, a novel missense variant in SYNM gene (exon 1, c.173C > T, p.A58V), encoding intermediate filament protein synemin was characterized. Until the present, no association between SYNM mutations and congenital clinical syndromes has been reported. At the same time, taking into account synemin tissue-specific expression profiles and available data on abnormal knock-out mice phenotypes, we propose SYNM as a candidate gene contributing to the UMS-like phenotype. Further comprehensive functional studies are required to evaluate possible involvement of SYNM in genesis of complex heart-limb pathology.

Absence of synemin in mice causes structural and functional abnormalities in heart

Cardiomyopathies have been linked to changes in structural proteins, including intermediate filament (IF) proteins located in the cytoskeleton. IFs associate with the contractile machinery and costameres of striated muscle and with intercalated disks in the heart. Synemin is a large IF protein that mediates the association of desmin with Z-disks and stabilizes intercalated disks. It also acts as an A-kinase anchoring protein (AKAP). In murine skeletal muscle, the absence of synemin causes a mild myopathy. Here, we report that the genetic silencing of synemin in mice (synm -/-) causes left ventricular systolic dysfunction at 3months and 12-16months of age, and left ventricular hypertrophy and dilatation at 12-16months of age. Isolated cardiomyocytes showed alterations in calcium handling that indicate defects intrinsic to the heart. Although contractile and costameric proteins remained unchanged in the old synm -/- hearts, we identified alterations in several signaling proteins (PKA-RII, ERK and p70S6K) critical to cardiomyocyte function. Our data suggest that synemin plays an important regulatory role in the heart and that the consequences of its absence are profound.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Role of nestin in glioma invasion

AIM: To determine the role for the intermediate filament protein nestin in glioma invasion.

METHODS: We examined the expression and function of nestin in gliomas (Grades II-IV as defined by the World Health Organization). We determined nestin expression using Immunohistochemical methods. To elucidate nestin’s biological function(s), we reduced mRNA levels by 61% and 87% in two glioblastoma-derived neurosphere lines using short hairpin RNAs and determined the effect of reduced nestin expression on glioma cell proliferation and invasion using MTS and matrigel migration assays, respectively. We also utilized quantitative real time polymerase chain reaction assays to determine the effect of reduced nestin expression on the expression of other markers associated with glioma stem cells and their differentiated progenies.

RESULTS: We found a significant correlation between nestin immunoreactivity and astrocytoma tumor grade, with 36% of grade II, 75% of grade III, and 100% of grade IV tumors expressing significant levels of the protein when assessed using immunohistochemistry. Reduction in nestin expression had no effect on cell growth in culture, but did retard the capacity of one line to migrate in-vitro on matrigel. Interestingly, in the line whose migration was not affected, mRNA levels of a second intermediate filament, synemin (also knowns as desmuslin), were elevated following introduction of shRNA targeting nestin. As synemin was not induced in the line which required nestin for migration, it is a possibility that synemin may compensate for the loss of nestin in this process.

CONCLUSION: Nestin expression is prominent in high-grade astrocytomas. Nestin is not required for cell growth but it may, however, be required for cell motility.

Order and disorder in intermediate filament proteins

Intermediate filaments (IFs), important components of the cytoskeleton, provide a versatile, tunable network of self-assembled proteins. IF proteins contain three distinct domains: an α-helical structured rod domain, flanked by intrinsically disordered head and tail domains. Recent studies demonstrated the functional importance of the disordered domains, which differ in length and amino-acid sequence among the 70 different human IF genes. Here, we investigate the biophysical properties of the disordered domains, and review recent findings on the interactions between them. Our analysis highlights key components governing IF functional roles in the cytoskeleton, where the intrinsically disordered domains dictate protein-protein interactions, supramolecular assembly, and macro-scale order.

Effects of aging, exercise, and disease on force transfer in skeletal muscle

The loss of muscle strength and increased injury rate in aging skeletal muscle has previously been attributed to loss of muscle protein (cross-sectional area) and/or decreased neural activation. However, it is becoming clear that force transfer within and between fibers plays a significant role in this process as well. Force transfer involves a secondary matrix of proteins that align and transmit the force produced by the thick and thin filaments along muscle fibers and out to the extracellular matrix. These specialized networks of cytoskeletal proteins aid in passing force through the muscle and also serve to protect individual fibers from injury. This review discusses the cytoskeleton proteins that have been identified as playing a role in muscle force transmission, both longitudinally and laterally, and where possible highlights how disease, aging, and exercise influence the expression and function of these proteins.

Myopathic changes in murine skeletal muscle lacking synemin

Diseases of striated muscle linked to intermediate filament (IF) proteins are associated with defects in the organization of the contractile apparatus and its links to costameres, which connect the sarcomeres to the cell membrane. Here we study the role in skeletal muscle of synemin, a type IV IF protein, by examining mice null for synemin (synm-null). Synm-null mice have a mild skeletal muscle phenotype. Tibialis anterior (TA) muscles show a significant decrease in mean fiber diameter, a decrease in twitch and tetanic force, and an increase in susceptibility to injury caused by lengthening contractions. Organization of proteins associated with the contractile apparatus and costameres is not significantly altered in the synm-null. Elastimetry of the sarcolemma and associated contractile apparatus in extensor digitorum longus myofibers reveals a reduction in tension consistent with an increase in sarcolemmal deformability. Although fatigue after repeated isometric contractions is more marked in TA muscles of synm-null mice, the ability of the mice to run uphill on a treadmill is similar to controls. Our results suggest that synemin contributes to linkage between costameres and the contractile apparatus and that the absence of synemin results in decreased fiber size and increased sarcolemmal deformability and susceptibility to injury. Thus synemin plays a moderate but distinct role in fast twitch skeletal muscle.

α-Synemin localizes to the M-band of the sarcomere through interaction with the M10 region of titin

α-Synemin contains a unique 312 amino acid insert near the end of its C-terminal tail. Therefore we set out to determine if the insert is a site of protein-protein interaction that regulates the sub-cellular localization of this large isoform of synemin. Yeast-two hybrid analysis indicated that this region is a binding site for the M10 region of titin. This was confirmed with GST pull-down assays. Co-immunoprecipitation of endogenous proteins indicated close association of the two proteins in vivo and immunostaining of cardiomyocytes demonstrated co-localization of the proteins at the M-band of the sarcomere.

Desmin in muscle and associated diseases: beyond the structural function

Desmin is a muscle-specific type III intermediate filament essential for proper muscular structure and function. In human, mutations affecting desmin expression or promoting its aggregation lead to skeletal (desmin-related myopathies), or cardiac (desmin-related cardiomyopathy) phenotypes, or both. Patient muscles display intracellular accumulations of misfolded proteins and desmin-positive insoluble granulofilamentous aggregates, leading to a large spectrum of molecular alterations. Increasing evidence shows that desmin function is not limited to the structural and mechanical integrity of cells. This novel perception is strongly supported by the finding that diseases featuring desmin aggregates cannot be easily associated with mechanical defects, but rather involve desmin filaments in a broader spectrum of functions, such as in organelle positioning and integrity and in signaling. Here, we review desmin functions and related diseases affecting striated muscles. We detail emergent cellular functions of desmin based on reported phenotypes in patients and animal models. We discuss known desmin protein partners and propose an overview of the way that this molecular network could serve as a signal transduction platform necessary for proper muscle function.

Phosphorylation at Y1065 in vinculin mediates actin bundling, cell spreading, and mechanical responses to force

Vinculin is an essential structural adaptor protein that localizes to sites of adhesion and is involved in a number of cell processes including adhesion, spreading, motility, force transduction, and cell survival. The C-terminal vinculin tail domain (Vt) contains the necessary structural components to bind and cross-link actin filaments. Actin binding to Vt induces a conformational change that promotes dimerization through the C-terminal hairpin of Vt and enables actin filament cross-linking. Here we show that Src phosphorylation of Y1065 within the C-terminal hairpin regulates Vt-mediated actin bundling and provide a detailed characterization of Y1065 mutations. Furthermore, we show that phosphorylation at Y1065 plays a role in cell spreading and the response to the application of mechanical force.

Filamentous structures in skeletal muscle: anchors for the subsarcolemmal space

In skeletal muscle fibers, intermediate filaments and actin filaments provide structural support to the myofibrils and the sarcolemma. For many years, it was poorly understood from ultrastructural observations that how these filamentous structures were kept anchored. The present study was conducted to determine the architecture of filamentous anchoring structures in the subsarcolemmal space and the intermyofibrils. The diaphragms (Dp) of adult wild type and mdx mice (mdx is a model for Duchenne muscular dystrophy) were subjected to tension applied perpendicular to the long axis of the muscle fibers, with or without treatment with 1% Triton X-100 or 0.03% saponin. These experiments were conducted to confirm the presence and integrity of the filamentous anchoring structures. Transmission electron microscopy revealed that these structures provide firm transverse connections between the sarcolemma and peripheral myofibrils. Most of the filamentous structures appeared to be inserted into subsarcolemmal densities, forming anchoring connections between the sarcolemma and peripheral myofibrils. In some cases, actin filaments were found to run longitudinally in the subsarcolemmal space to connect to the sarcolemma or in some cases to connect to the intermyofibrils as elongated thin filaments. These filamentous anchoring structures were less common in the mdx Dp. Our data suggest that the transverse and longitudinal filamentous structures form an anchoring system in the subsarcolemmal space and the intermyofibrils.

Genetics of inherited cardiomyopathies

Cardiomyopathies are the most common disorders resulting in heart failure, with dilated cardiomyopathy being responsible for the majority of cases. Other forms of cardiomyopathy, especially hypertrophic forms, are also important causes of heart failure. The mortality rate due to cardiomyopathy in the USA is over 10,000 deaths per year, and the costs associated with heart failure are approximately 200 million US dollars per year in the USA alone. Over the past few years, breakthroughs have occurred in understanding the basic mechanisms of these disorders, potentially enabling clinicians to devise improved diagnostic strategies and therapies. As at least 30 to 40% of cases are inherited, it is now imperative that the genetic basis for these disorders is clearly recognized by caregivers and scientists. However, it has also become clear that these diseases are genetically highly heterogeneous, with multiple genes identified for each of the major forms of cardiomyopathy, and most patients having private mutations. These data suggest that the genetic diagnosis of most patients with cardiomyopathy will be impractical with current technologies. However, there are a few exceptions, such as patients with X-linked cardiomyopathies, with or without the concomitant abnormalities of cyclic neutropenia and 3-methylglutaconic aciduria, or patients with cardiomyopathy associated with conduction disease: these appear to be associated with mutations in a small subset of genes, and can be investigated by certified diagnostic laboratories. This review will summarize current knowledge of the genetics of inherited cardiomyopathies and how findings from research laboratories may be translated into the diagnostic laboratory.

The cell biology of disease: cellular and molecular mechanisms underlying muscular dystrophy

The muscular dystrophies are a group of heterogeneous genetic diseases characterized by progressive degeneration and weakness of skeletal muscle. Since the discovery of the first muscular dystrophy gene encoding dystrophin, a large number of genes have been identified that are involved in various muscle-wasting and neuromuscular disorders. Human genetic studies complemented by animal model systems have substantially contributed to our understanding of the molecular pathomechanisms underlying muscle degeneration. Moreover, these studies have revealed distinct molecular and cellular mechanisms that link genetic mutations to diverse muscle wasting phenotypes.

Nebulin binding impedes mutant desmin filament assembly

Although a clear picture of the in vitro assembly process is established for vimentin, the role of associated partner proteins and their effect on intermediate filament assembly has not been fully examined. This study finds delayed dynamics of desminopathy-linked mutant desmin in myocytes and hindered assembly when associated to nebulin.

Desmin intermediate filaments (DIFs) form an intricate meshwork that organizes myofibers within striated muscle cells. The mechanisms that regulate the association of desmin to sarcomeres and their role in desminopathy are incompletely understood. Here we compare the effect nebulin binding has on the assembly kinetics of desmin and three desminopathy-causing mutant desmin variants carrying mutations in the head, rod, or tail domains of desmin (S46F, E245D, and T453I). These mutants were chosen because the mutated residues are located within the nebulin-binding regions of desmin. We discovered that, although nebulin M160–164 bound to both desmin tetrameric complexes and mature filaments, all three mutants exhibited significantly delayed filament assembly kinetics when bound to nebulin. Correspondingly, all three mutants displayed enhanced binding affinities and capacities for nebulin relative to wild-type desmin. Electron micrographs showed that nebulin associates with elongated normal and mutant DIFs assembled in vitro. Moreover, we measured significantly delayed dynamics for the mutant desmin E245D relative to wild-type desmin in fluorescence recovery after photobleaching in live-cell imaging experiments. We propose a mechanism by which mutant desmin slows desmin remodeling in myocytes by retaining nebulin near the Z-discs. On the basis of these data, we suggest that for some filament-forming desmin mutants, the molecular etiology of desminopathy results from subtle deficiencies in their association with nebulin, a major actin-binding filament protein of striated muscle.

Upregulation of nestin in proximal tubules may participate in cell migration during renal repair

The characteristics of renal tubular progenitor/precursor cells and the role of renal tubule regeneration in the repair of remnant kidneys (RKs) after nephrectomy are not well known. In the present study of a murine model of subtotal nephrectomy, we used immunofluorescence (IF), immunoblot analysis, and in situ hybridization methods to demonstrate that nestin expression was transiently upregulated in tubule cells near the incision edges of RKs. The nestin-positive tubules were immature proximal tubules that colabeled with lotus tetragonolobus agglutinin but not with markers of mature tubules (aquaporin-1, Tamm-Horsfall protein, and aquaporin-2). In addition, many of the nestin-expressing tubule cells were actively proliferative cells, as indicated by colabeling with bromodeoxyuridine. Double-label IF and immunoblot analysis also showed that the upregulation of tubular nestin was associated with enhanced transforming growth factor-β1 (TGF-β1) expression in the incision edge of RKs but not α-smooth muscle actin, which is a marker of fibrosis. In cultured human kidney proximal tubule cells (HKC), immunoblot analysis indicated that TGF-β1 induced nestin expression and loss of E-cadherin expression, suggesting an association of nestin expression and cellular dedifferentiation. Knockdown of nestin expression by a short hairpin RNA-containing plasmid led to decreased migration of HKC cells that were induced by TGF-β1. Taken together, our results suggest that the tubule repair that occurs during the recovery process following nephrectomy may involve TGF-β1-induced nestin expression in immature renal proximal tubule cells and the promotion of renal cell migration.

Synemin promotes AKT-dependent glioblastoma cell proliferation by antagonizing PP2A

Synemin is an intermediate filament protein present in glioblastomas (GBMs) but not in normal brain. In GBM cells synemin interacts with and antagonizes PP2A, which is the phosphatase dephosphorylating Akt. This maintains the phosphorylation status of Akt sites that are substrates for PDPK1 and mTORc2, thereby fostering proliferation.

The intermediate filament protein synemin is present in astrocyte progenitors and glioblastoma cells but not in mature astrocytes. Here we demonstrate a role for synemin in enhancing glioblastoma cell proliferation and clonogenic survival, as synemin RNA interference decreased both behaviors by inducing G1 arrest along with Rb hypophosphorylation and increased protein levels of the G1/S inhibitors p21^Cip1 and p27^Kip1. Akt involvement was demonstrated by decreased phosphorylation of its substrate, p21^Cip1, and reduced Akt catalytic activity and phosphorylation at essential activation sites. Synemin silencing, however, did not affect the activities of PDPK1 and mTOR complex 2, which directly phosphorylate Akt activation sites, but instead enhanced the activity of the major regulator of Akt dephosphorylation, protein phosphatase type 2A (PP2A). This was accompanied by changes in PP2A subcellular distribution resulting in increased physical interactions between PP2A and Akt, as shown by proximity ligation assays (PLAs). PLAs and immunoprecipitation experiments further revealed that synemin and PP2A form a protein complex. In addition, treatment of synemin-silenced cells with the PP2A inhibitor cantharidic acid resulted in proliferation and pAkt and pRb levels similar to those of controls. Collectively these results indicate that synemin positively regulates glioblastoma cell proliferation by helping sequester PP2A away from Akt, thereby favoring Akt activation.

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 isoforms differentially organize cell junctions and desmin filaments in neonatal cardiomyocytes

Intermediate filaments (IFs) in cardiomyocytes consist primarily of desmin, surround myofibrils at Z disks, and transmit forces from the contracting myofilaments to the cell surface through costameres at the sarcolemma and desmosomes at intercalated disks. Synemin is a type IV IF protein that forms filaments with desmin and also binds α-actinin and vinculin. Here we examine the roles and expression of the α and β forms of synemin in developing rat cardiomyocytes. Quantitative PCR showed low levels of expression for both synemin mRNAs, which peaked at postnatal day 7. Synemin was concentrated at sites of cell-cell adhesion and at Z disks in neonatal cardiomyocytes. Overexpression of the individual isoforms showed that α-synemin preferentially localized to cell-cell junctions, whereas β-synemin was primarily at the level of Z disks. An siRNA targeted to both synemin isoforms reduced protein expression in cardiomyocytes by 70% and resulted in a failure of desmin to align with Z disks and disrupted cell-cell junctions, with no effect on sarcomeric organization. Solubility assays showed that β-synemin was soluble and interacted with sarcomeric α-actinin by coimmunoprecipitation, while α-synemin and desmin were insoluble. We conclude that β-synemin mediates the association of desmin IFs with Z disks, whereas α-synemin stabilizes junctional complexes between cardiomyocytes.

Deficiency of α-actinin-3 is associated with increased susceptibility to contraction-induced damage and skeletal muscle remodeling

Sarcomeric α-actinins (α-actinin-2 and -3) are a major component of the Z-disk in skeletal muscle, where they crosslink actin and other structural proteins to maintain an ordered myofibrillar array. Homozygosity for the common null polymorphism (R577X) in ACTN3 results in the absence of fast fiber-specific α-actinin-3 in ∼20% of the general population. α-Actinin-3 deficiency is associated with decreased force generation and is detrimental to sprint and power performance in elite athletes, suggesting that α-actinin-3 is necessary for optimal forceful repetitive muscle contractions. Since Z-disks are the structures most vulnerable to eccentric damage, we sought to examine the effects of α-actinin-3 deficiency on sarcomeric integrity. Actn3 knockout mouse muscle showed significantly increased force deficits following eccentric contraction at 30% stretch, suggesting that α-actinin-3 deficiency results in an increased susceptibility to muscle damage at the extremes of muscle performance. Microarray analyses demonstrated an increase in muscle remodeling genes, which we confirmed at the protein level. The loss of α-actinin-3 and up-regulation of α-actinin-2 resulted in no significant changes to the total pool of sarcomeric α-actinins, suggesting that alterations in fast fiber Z-disk properties may be related to differences in functional protein interactions between α-actinin-2 and α-actinin-3. In support of this, we demonstrated that the Z-disk proteins, ZASP, titin and vinculin preferentially bind to α-actinin-2. Thus, the loss of α-actinin-3 changes the overall protein composition of fast fiber Z-disks and alters their elastic properties, providing a mechanistic explanation for the loss of force generation and increased susceptibility to eccentric damage in α-actinin-3-deficient individuals.

Nestin is a novel target for suppressing pancreatic cancer cell migration, invasion and metastasis

Nestin, is a class VI intermediate filament (IF) that is expressed in 30% of pancreatic ductal adenocarcinoma (PDAC) cases, and its expression in PDAC positively correlates with peripancreatic invasion. An expression vector carrying a short hairpin RNA (shRNA) targeting nestin was stably transfected into PANC-1 and PK-45H human pancreatic cancer cells, which express high nestin levels. Alterations in morphology and alignment of actin filaments and α-tubulin were examined by phase-contrast and immunocytochemistry. Effects on cell growth, migration in scratch and Boyden chamber assays, invasion, cell adhesion, and in vivo growth were determined. Differences in mRNA levels were examined by arrays. Nestin shRNA-transfected cells exhibited decreased nestin expression, a sheet-like appearance with tight cell-cell adhesion, increased expression of filamentous F-actin and E-cadherin, and attenuated migration and invasion, both of which were enhanced following nestin re-expression. Expression of α-tubulin, and in vitro cell growth and adhesion were not altered by nestin down-regulation, whereas hepatic metastases were decreased. Thus, nestin plays important roles in pancreatic cancer cell migration, invasion and metastasis by selectively modulating the expression of actin and cell adhesion molecules, and may therefore be a novel therapeutic target in PDAC.

Synemin down-regulation in human hepatocellular carcinoma does not destabilize cytoskeletons in vivo

Synemin is a large intermediate filament protein that has been identified in all types of muscle cells. It plays a role in human muscle diseases; however, the role of synemin in tumor cell transformation has rarely been investigated. Because hepatocellular carcinoma cells are morphologically different from normal human hepatocytes, we hypothesized that altered synemin expression and cytoskeletal disorganization might underlie this pleomorphic transformation. To test this hypothesis, we studied synemin expression in hepatocellular carcinoma and liver tissues by immunohistochemistry and immunoblotting. In addition, we analyzed the expression level and organization of all cytoskeletal elements after synemin knock-down in human Chang liver cells. Previously we found that plectin knock-down in human Chang liver cells causes a reduction in cytokeratin 18 expression with effects on intermediate filament disorganization and altered cellular morphology. In this study we also compared the effects of synemin knock-down and plectin knock-down on the cytoskeleton expression and organization. The results revealed that synemin expression was down-regulated in human hepatocellular carcinoma compared with normal liver, which is similar to the plectin expression. Surprisingly, the expression of cytoskeletal elements (cytokeratin 18, actin and tubulin) was not influenced by synemin knock-down in human Chang liver cells. The organization of cytoskeletal networks was also unaltered after synemin knock-down. In conclusion, both plectin and synemin are down-regulated in human hepatocellular carcinoma in vivo and transformed human liver cell in vitro. However, the mechanism of cell transformation caused by synemin knock-down is different from that of plectin knock-down. Plectin, but not synemin, knock-down provoked liver cell transformation via suppressing cytokeratin 18 expression and disrupting intermediate filament networks. Synemin knock-down did not influence the cytoskeleton expression and organization of human Chang liver cells.

Vinculin, an adapter protein in control of cell adhesion signalling

Vinculin, discovered in 1979 (Geiger, 1979), is an adapter protein with binding sites for more than 15 proteins. Biochemical and structural analyses have contributed to detailed knowledge about potential binding partners and the understanding of how their binding may be regulated. Despite all this information the molecular basis of how vinculin acts in cells and controls a wide variety of signals remains elusive. This review aims to highlight recent discoveries with an emphasis on how vinculin is involved in the coordination of a network of signals.

Intermediate filament dynamics and breast cancer: aberrant promoter methylation of the Synemin gene is associated with early tumor relapse

Synemin (SYNM) is a type IV intermediate filament that has recently been shown to interact with the LIM domain protein zyxin, thereby possibly modulating cell adhesion and cell motility. Owing to this multiplicity of potential functions relevant to cancer development, we initiated a study to decipher SYNM expression and regulation in benign human breast tissue and breast cancer. Dot blot array analysis showed significant SYNM mRNA downregulation in 86% (n=100, P<0.001) of breast cancers compared with their normal tissue counterparts, a result that was confirmed by real-time PCR analysis (n=36, P<0.0001). Immunohistochemistry analysis showed abundant SYNM protein expression in healthy myoepithelial breast cells, whereas SYNM expression loss was evident in 57% (n=37, P<0.001) of breast cancer specimens. Next, we analyzed methylation of the SYNM promoter to clarify whether the SYNM gene can be silenced by epigenetic means. Indeed, methylation-specific PCR analysis showed tumor-specific SYNM promoter methylation in 27% (n=195) of breast cancers. As expected, SYNM promoter methylation was tightly associated (P<0.0001) with SYNM expression loss. In-depth analysis of the SYNM promoter by pyrosequencing showed extensive CpG methylation of DNA elements supposed to regulate gene transcription. Demethylating treatment of SYNM methylated breast cancer cell lines with 5-aza-2-deoxycytidine clearly reestablished the SYNM expression. Statistical analysis of the patient cohort showed a close association between SYNM promoter methylation and unfavorable recurrence-free survival (hazard ratio=2.941, P=0.0282). Furthermore, SYNM methylation positively correlated with lymph node metastases (P=0.0177) and advanced tumor grade (P=0.0275), suggesting that SYNM methylation is associated with aggressive forms of breast cancer. This is the first study on the epigenetic regulation of the SYNM gene in a cancer entity. We provide first hints that SYNM could represent a novel putative breast tumor suppressor gene that is prone to epigenetic silencing. SYNM promoter methylation may become a useful predictive biomarker to stratify breast cancer patients' risk for tumor relapse.

Dysbindin, syncoilin, and beta-synemin mRNA levels in dystrophic muscles

Progressive muscular dystrophies are genetic diseases with various modes of transmission. Duchenne muscular dystrophy (DMD) is caused by the defect of dystrophin, and Fukuyama congenital muscular dystrophy (FCMD) is caused by an abnormal fukutin gene leading to the glycosylation defect of alpha-dystroglycan. Dystrobrevin is one member of the dystrophin glycoprotein complex and its binding partners include dysbindin, syncoilin, and beta-synemin (desmuslin). Dysbindin is reported to be upregulated at the protein level in mdx mouse muscles, and syncoilin protein is also reported to be upregulated in biopsied muscles with neuromuscular disorders. In the present study we measured mRNA levels of dysbindin, syncoilin, and beta-synemin in biopsied muscles with DMD and FCMD. Upregulation of human dysbindin mRNA was observed in DMD muscles in comparison with normal muscles (p < .05). The differences in human syncoilin and beta-synemin mRNA ratios between DMD and normal muscles were not statistically significant, although upregulation tendency of human syncoilin mRNA was noted in DMD muscles (.05 < p < .1). Furthermore, the differences of human dysbindin, syncoilin, and beta-synemin mRNA ratios between FCMD and normal muscles were not statistically significant. These data provide insight into the pathophysiology of these muscular dystrophies.

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.

Expression of synemin in the mouse spinal cord

beta-Synemin was previously identified as an alpha-dystrobrevin-interacting protein in muscle. To better understand its function in neural tissue, in situ and immunohistochemical analyses were performed to identify where the synemin isoforms are expressed in the spinal cord of C57BL/6 and dystrophin-deficient (mdx) C57BL/10 mice. These analyses show that synemin transcript and its encoded protein colocalize in the anterior horn cells and that no differences in synemin expression were found in nerve tissue from C57BL/6 or mdx mice. The expression of synemin mRNA and protein predominantly in the anterior horn cells suggests that synemin performs an essential function in those cells. Because synemin is more highly expressed in the midbrain and pons, its function in neurological cells was further pursued by identifying coexpressed proteins in cells from those regions of the brain. These results show that neurons that express synemin also express tryptophan hydroxylase-1, a marker of serotoninergic nerve fibers. Muscle Nerve, 2009.

Analysis of skeletal muscle function in the C57BL6/SV129 syncoilin knockout mouse

Syncoilin is a 64-kDa intermediate filament protein expressed in skeletal muscle and enriched at the perinucleus, sarcolemma, and myotendinous and neuromuscular junctions. Due to its pattern of cellular localization and binding partners, syncoilin is an ideal candidate to be both an important structural component of myocytes and a potential mediator of inherited myopathies. Here we present a report of a knockout mouse model for syncoilin and the results of an investigation into the effect of a syncoilin null state on striated muscle function in 6–8-week-old mice. An analysis of proteins known to associate with syncoilin showed that ablation of syncoilin had no effect on absolute expression or spatial localization of desmin or alpha dystrobrevin. Our syncoilin-null animal exhibited no differences in cardiotoxin-induced muscle regeneration, voluntary wheel running, or enforced treadmill exercise capacity, relative to wild-type controls. Finally, a mechanical investigation of isolated soleus and extensor digitorum longus indicated a potential differential reduction in muscle strength and resilience. We are the first to present data identifying an increased susceptibility to muscle damage in response to an extended forced exercise regime in syncoilin-deficient muscle. This study establishes a second viable syncoilin knockout model and highlights the importance of further investigations to determine the role of syncoilin in skeletal muscle.

Plectin 1 links intermediate filaments to costameric sarcolemma through β-synemin, α-dystrobrevin and actin

In skeletal muscles, the sarcolemma is possibly stabilized and protected against contraction-imposed stress by intermediate filaments (IFs) tethered to costameric sarcolemma. Although there is emerging evidence that plectin links IFs to costameres through dystrophin-glycoprotein complexes (DGC), the molecular organization from plectin to costameres still remains unclear. Here, we show that plectin 1, a plectin isoform expressed in skeletal muscle, can interact with β-synemin, actin and a DGC component, α-dystrobrevin, in vitro. Ultrastructurally, β-synemin molecules appear to be incorporated into costameric dense plaques, where they seem to serve as actin-associated proteins rather than IF proteins. In fact, they can bind actin and α-dystrobrevin in vitro. Moreover, in vivo immunoprecipitation analyses demonstrated that β-synemin- and plectin-immune complexes from lysates of muscle light microsomes contained α-dystrobrevin, dystrophin, nonmuscle actin, metavinculin, plectin and β-synemin. These findings suggest a model in which plectin 1 interacts with DGC and integrin complexes directly, or indirectly through nonmuscle actin and β-synemin within costameres. The DGC and integrin complexes would cooperate to stabilize and fortify the sarcolemma by linking the basement membrane to IFs through plectin 1, β-synemin and actin. Besides, the two complexes, together with plectin and IFs, might have their own functions as platforms for distinct signal transduction.

Myofiber integrity depends on desmin network targeting to Z-disks and costameres via distinct plectin isoforms

Dysfunction of plectin, a 500-kD cytolinker protein, leads to skin blistering and muscular dystrophy. Using conditional gene targeting in mice, we show that plectin deficiency results in progressive degenerative alterations in striated muscle, including aggregation and partial loss of intermediate filament (IF) networks, detachment of the contractile apparatus from the sarcolemma, profound changes in myofiber costameric cytoarchitecture, and decreased mitochondrial number and function. Analysis of newly generated plectin isoform-specific knockout mouse models revealed that IF aggregates accumulate in distinct cytoplasmic compartments, depending on which isoform is missing. Our data show that two major plectin isoforms expressed in muscle, plectin 1d and 1f, integrate fibers by specifically targeting and linking desmin IFs to Z-disks and costameres, whereas plectin 1b establishes a linkage to mitochondria. Furthermore, disruption of Z-disk and costamere linkages leads to the pathological condition of epidermolysis bullosa with muscular dystrophy. Our findings establish plectin as the major organizer of desmin IFs in myofibers and provide new insights into plectin- and desmin-related muscular dystrophies.

Intermediate filament protein synemin contributes to the migratory properties of astrocytoma cells by influencing the dynamics of the actin cytoskeleton

We have shown previously that, in astrocytoma cells, synemin is present at the leading edge, an unusual localization for an intermediate filament (IF) protein. Here, we report that synemin down-regulation with specific small hairpin RNAs (shRNAs) sharply decreased the migration of astrocytoma cells. The presence of synemin at the leading edge also correlated with a high migratory potential, as shown by comparing astrocytoma cells to carcinoma cells without synemin at the leading edge. Synemin-silenced astrocytoma cells were smaller and spread more slowly than controls. In addition, synemin silencing reduced proliferation without increasing apoptosis. The adhesion to substratum and distribution of vinculin in focal contacts of synemin-silenced astrocytoma cells were similar to those of controls. Synemin-silenced cells, however, exhibited a reduction in the amount of filamentous (F) -actin and of alpha-actinin, but not of vinculin, associated with F-actin. Altogether, these results demonstrate that synemin is important for the malignant behavior of astrocytoma cells and that it contributes to the high motility of these cells by modulating the dynamics of alpha-actinin and actin.

Identification of a repeated domain within mammalian alpha-synemin that interacts directly with talin

The type VI intermediate filament (IF) protein synemin is a unique member of the IF protein superfamily. Synemin associates with the major type III IF protein desmin forming heteropolymeric intermediate filaments (IFs) within developed mammalian striated muscle cells. These IFs encircle and link all adjacent myofibrils together at their Z-lines, as well as link the Z-lines of the peripheral layer of cellular myofibrils to the costameres located periodically along and subjacent to the sarcolemma. Costameres are multi-protein assemblies enriched in the cytoskeletal proteins vinculin, alpha-actinin, and talin. We report herein a direct interaction of human alpha-synemin with the cytoskeletal protein talin by protein-protein interaction assays. The 312 amino acid insert (SNTIII) present only within alpha-synemin binds to the rod domain of talin in vitro and co-localizes with talin at focal adhesion sites within mammalian muscle cells. Confocal microscopy studies showed that synemin co-localizes with talin within the costameres of human skeletal muscle cells. Analysis of the primary sequences of human alpha- and beta-synemins revealed that SNTIII is composed of seven tandem repeats, each containing a specific Ser/Thr-X-Arg-His/Gln (S/T-X-R-H/Q) motif. Our results suggest human alpha-synemin plays an essential role in linking the heteropolymeric IFs to adherens-type junctions, such as the costameres within mammalian striated muscle cells, via its interaction with talin, thereby helping provide mechanical integration for the muscle cell cytoskeleton.

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.

Roles for the stem cell associated intermediate filament Nestin in prostate cancer migration and metastasis

The intermediate filament protein Nestin identifies stem/progenitor cells in adult tissues, but the function of Nestin is poorly understood. We investigated Nestin expression and function in common lethal cancers. Nestin mRNA was detected in cell lines from small cell lung, and breast cancers, and particularly elevated in cell lines derived from prostate cancer metastases. Whereas the androgen-independent lines PC3, 22RV1, and DU145 all expressed Nestin transcripts under standard culture conditions, the androgen-dependent line LnCaP expressed Nestin only on androgen withdrawal. We confirmed associations of Nestin expression, androgen withdrawal, and metastatic potential by immunohistochemical analysis of samples from 254 prostate cancer patients. Cytoplasmic Nestin protein was readily identifiable in prostate cancer cells from 75% of patients with lethal androgen-independent disease, even in cancer sampled from the prostate itself. However, Nestin expression was undetectable in localized androgen-deprived tumors and in metastases without prior androgen deprivation. To address its function, we reduced Nestin levels with short hairpin RNAs, markedly inhibiting in vitro migration and invasion in prostate cancer cells but leaving cell growth intact. Nestin knockdown also diminished metastases 5-fold compared with controls despite uncompromised tumorigenicity at the site of inoculation. These results specify a function for Nestin in cell motility and identify a novel pathway for prostate cancer metastasis. Activity of this pathway may be selected by the extraprostatic environment or, as supported by our data, may originate within the prostate after androgen deprivation. Further dissection of this novel Nestin migration pathway may lead to strategies to prevent and neutralize metastatic spread.