Harald Bär

Intermediate filaments: from cell architecture to nanomechanics

Intermediate filaments (IFs) constitute a major structural element of animal cells. They build two distinct systems, one in the nucleus and one in the cytoplasm. In both cases, their major function is assumed to be that of a mechanical stress absorber and an integrating device for the entire cytoskeleton. In line with this, recent disease mutations in human IF proteins indicate that the nanomechanical properties of cell-type-specific IFs are central to the pathogenesis of diseases as diverse as muscular dystrophy and premature ageing. However, the analysis of these various diseases suggests that IFs also have an important role in cell-type-specific physiological functions.

Desmin and Vimentin Intermediate Filament Networks: Their Viscoelastic Properties Investigated by Mechanical Rheometry

We have investigated the viscoelastic properties of the cytoplasmic intermediate filament (IF) proteins desmin and vimentin. Mechanical measurements were supported by time-dependent electron microscopy studies of the assembly process under similar conditions. Network formation starts within 2 min, but it takes more than 30 min until equilibrium mechanical network strength is reached. Filament bundling is more pronounced for desmin than for vimentin. Desmin filaments (persistence length l(p) approximately 900 nm) are stiffer than vimentin filaments (l(p) approximately 400 nm), but both IFs are much more flexible than microfilaments. The concentration dependence of the plateau modulus G(0) approximately c(alpha) is much weaker than predicted theoretically for networks of semiflexible filaments. This is more pronounced for vimentin (alpha=0.47) than for desmin (alpha=0.70). Both networks exhibit strain stiffening at large shear deformations. At the transition from linear to nonlinear viscoelastic response, only desmin shows characteristics of nonaffine network deformation. Strain stiffening and the maximum modulus occur at strain amplitudes about an order of magnitude larger than those for microfilaments. This is probably attributable to axial slippage within the tetramer building blocks of the IFs. Network deformation beyond a critical strain gamma(max) results in irreversible damage. Strain stiffening sets in at lower concentrations, is more pronounced, and is less sensitive to ionic strength for desmin than for vimentin. Hence, desmin exhibits strain stiffening even at low-salt concentrations, which is not observed for vimentin, and we conclude that the strength of electrostatic repulsion compared to the strength of attractive interactions forming the network junctions is significantly weaker for desmin than for vimentin filaments. These findings indicate that both IFs exhibit distinct mechanical properties that are adapted to their respective cellular surroundings [i.e., myocytes (desmin) and fibroblasts (vimentin)].

Upregulation of embryonic transcription factors in right ventricular hypertrophy

Abstract.Increased ventricular expression of genes encoding for various structural and contractile proteins has been reported in cardiac hypertrophy. Mechanisms leading to this altered gene expression are only partly understood. Recently, various transcription factors (TF), among them GATA-4, Nkx-2.5/Csx, MEF-2 and the HAND family (eHAND and dHAND), and their role in embryonic cardiac development have been described. These transcription factors are known to have binding sites to promotor regions of many genes known to be regulated in hypertrophy of adult ventricular myocardium.We investigated the temporal and spatial expression pattern of these transcription factors in a rat model of acute pressure-overload of the right ventricle, induced by banding (coarctation) of the pulmonary artery. Expression of GATA-4, Nkx-2.5/Csx, MEF-2 and dHAND protein was found to increase in the right ventricle after the banding procedure as determined by immunohistochemistry and western blotting. A marker of the onset of cardiac hypertrophy was expression of ANP protein. We conclude that TF known to regulate embryonic heart development are involved in the adaptational response of adult ventricular myocardium to pressure overload.

Smoothelin is an indicator of reversible phenotype modulation of smooth muscle cells in balloon-injured rat carotid arteries

Abstract Restenosis is the major obstacle interfering with a successful long-term outcome of balloon angioplasty. Neointima formation following endothelial injury is the result of phenotype modulation and proliferation of smooth muscle cells (SMC). To characterize these time-dependent changes, a rat balloon injury model of carotid artery restenosis was assessed. We applied monoclonal antibodies recognizing desmin, sm-α-actin and smoothelin, a novel marker specific for the differentiated phenotype of SMC.Neointima formation could be seen from day 7 after injury onwards. During early phases, the number of smoothelin-positive cells in the media was decreased compared with uninjured controls. Smoothelin staining was absent in the neointima during formation. Increased levels of smoothelin in both media and neointima were observed at days 28 and 56, correlating with a decrease in proliferation as assessed by Ki-67 antigen staining. No such changes were observed for desmin and sm-α-actin.Following balloon injury, SMC in both the media and the neointima underwent an early, reversible dedifferentiation, followed by proliferation. The novel SMC-specific marker protein smoothelin can be used to monitor this SMC (de)differentiation in neointima and media. These findings support the pivotal role of SMC phenotype modulation in neointima formation and restenosis.

Incomplete nonsense-mediated decay of mutant lamin A/C mRNA provokes dilated cardiomyopathy and ventricular tachycardia

We have identified a family in which several members died of sudden cardiac death or suffer from dilated cardiomyopathy (DCM) and rhythm disturbances. Mutation screening revealed co-segregation of a novel nonsense mutation (pR321X) in the lamin A gene, LMNA, with the disease. Lamin A, and its smaller splice form lamin C are nuclear intermediate filament proteins forming a major part of the lamina, which is underlying the inner nuclear membrane. They are involved in the organization of heterochromatin and both in DNA replication and transcription. Recently, an increasing number of missense mutations in LMNA have been discovered to cause various types of rare diseases. Here, we investigated the causal role of the new nonsense mutation for the disease. Quantification of wild type and mutant lamin A mRNA from explanted myocardial tissue and cultured fibroblasts revealed an up to 30-fold reduction in the relative amount of the mutant transcript indicating that its synthesis was massively down-regulated by nonsense-mediated mRNA decay (NMD). Correspondingly, we did not detect the mutant truncated lamin A by Western blot analysis in extracts of patient fibroblasts and cardiac muscle tissue. Both wild type lamin A and C were present, however, in normal quantities. The immunohistochemical analyses of patient tissues revealed a normal distribution of lamin A/C and of major inner nuclear membrane proteins such as emerin and the lamin B receptor. Moreover, both chromatin distribution and nuclear shape were normal. However, over-expression of truncated lamin A in HeLa cells by transient transfection caused major disturbances of lamin A organization within both the nucleoplasm and the cytoplasm. In addition, after treatment of patient fibroblasts with the proteasome inhibitor epoxomicin, mutant truncated lamin A was detected in relatively high levels by Western blotting demonstrating that it is synthesized in these cells. Therefore, we conclude that NMD is not sufficient to completely prevent the expression of truncated lamin A and that even trace amounts of it may negatively interfere with structural and/or regulatory functions of lamin A/C eventually leading to the development of DCM and rhythm disturbances.

Disease mutations in the “head” domain of the extra-sarcomeric protein desmin distinctly alter its assembly and network-forming properties

The intermediate filament protein desmin generates an extra-sarcomeric network in myocytes. Mutations in the desmin gene cause myofibrillar myopathy characterized by desmin-positive aggregates and myofibrillar dissolution. Past analysis revealed that the non-α-helical amino-terminal “head” domain of desmin is a vital coordinator of protein assembly. We have now characterized assembly and network-forming properties of five recently discovered myopathy-causing mutations residing in this domain. In vitro analyses with recombinant proteins show that two mutant variants residing in a conserved nonapeptide motif “SSYRRTFGG”—Ser13Phe and Arg16Cys—interfere with assembly by forming filamentous aggregates. Consistent with in vitro data, both mutant proteins are unable to generate a bona fide filament system in cells lacking an intermediate filament cytoskeleton. In cells expressing vimentin or desmin, both mutants firstly fail to integrate into the endogenous filament network and secondly severely affect its cellular localization. The other three mutations—Ser2Iso, Ser46Phe, and Ser46Tyr—influence in vitro filament properties less severely, but in vivo, Ser46Phe and Ser46Tyr impair de novo filament formation. These effects of the “head” mutant proteins on endogenous intermediate filament system and their competition for binding to cellular anchoring structures might explain part of the molecular mechanism that causes disease.

Prevalence of asymptomatic mitral valve malformations

IntroductionCongenital abnormalities of the mitral valve are considered to be very rare cardiac anomalies. In particular, more severe malformations, such as the complete absence of either aortic (anterior) or mural (posterior) mitral leaflet, are usually considered to be incompatible with life. Ebstein-like malformation of the mitral valve is an extremely rare form of mitral valve deformity hitherto unreported in an asymptomatic adult patient.Materials and MethodsThe detection of such a malformation prompted us to perform a prospective analysis of 26,484 consecutive comprehensive 2D-echocardiographic examinations, conducted at our tertiary care university hospital between April 2007 and July 2008, with regard to the presence of malformations of the mitral valvular apparatus.ResultsIn total, we found three cases of hypoplastic or even absent functional mural valve leaflets. All were diagnosed in adult patients who attended our outpatient department and were surprisingly asymptomatic regarding this finding. From our patient cohort, we calculate an actual prevalence of asymptomatic hypoplasia of the mitral valve of 1:8,800.ConclusionsOur findings broaden the spectrum of known mitral valve pathologies. The comparatively high prevalence of this malformation in our preselected patient cohort might indicate that this particular malformation has so far been under-diagnosed. In the context of this observation, both embryological development of the atrioventricular (AV) valves and recent functional insights into mitral valve physiology gained by mitral valve reconstructive surgery are discussed.

Phosphorylation of cytokeratin 8 and 18 in human vascular smooth muscle cells of atherosclerotic lesions and umbilical cord vessels.

Abstract Expression of cytokeratins (CK) is considered a hallmark of the state of epithelial differentiation. CK also occur in certain vascular smooth muscle cells (VSMC), inferring an association with a less differentiated phenotype. Recently, CK posttranslational modification was shown to occur in epithelial cells in stress, mitosis or apoptosis.The aim of this study was to determine potential CK phosphorylation patterns in human VSMC. Tissue samples of normal peripheral and coronary arteries, atherosclerotic lesions and umbilical cord vessels were evaluated by immunofluorescence microscopy applying antibodies specific for cytokeratins 8 and 18, specific cytokeratin phosphorylation sites, Ki-67-antigen as a proliferation marker and nick end labeling (TUNEL) to detect apoptosis.All samples contained cytokeratin-positive VSMC but diverse phosphorylation patterns. The C-terminal serine 431 of cytokeratin 8 (CK8Ser-431) was phosphorylated in the vast majority of CK-expressing VSMC of coronary artery lesions. Only a subset of these cells demonstrated phosphorylation of CK18Ser-33 or, to an even lesser extent, CK8Ser-73. DNA fragmentation occurred predominantly in samples containing cells with phosphorylated CK8Ser-431 domains. In contrast, occluded peripheral lesions exhibited little or no phosphorylation. Neonatal VSMC in umbilical cord vessels contain abundant phosphorylated CK domains, again predominantly CK8Ser-431, but also CK18Ser-33. Again, only single cells were found to be proliferating or to contain DNA fragmentation.Thus, abundant CK phosphorylation in VSMC of atherosclerotic lesions suggests a specific functional response to cell stress and a possible relation to apoptosis.

Cardiac α-actin in smooth muscle cells: detection in umbilical cord vessels and in atherosclerotic lesions

Abstract Phenotypic modulation of smooth muscle cells (SMC) is a key event during the development of atherosclerotic and restenotic lesions. During this process, the composition of the cytoskeleton is substantially altered, with changes predominantly in actin expression reflecting a shift from smooth muscle α-actin to the non-muscle β-isoform. We now demonstrate that yet another actin isoform, cardiac α-actin, is synthesized, de novo, in SMC of various atherosclerotic lesions. Using a highly specific monoclonal antibody against cardiac α-actin, we analyzed and compared the accumulation of this actin isoform in diverse SMC by immunofluorescence microscopy and immunoblotting. As expect, cardiac α-actin was present in human myocardium but not in healthy SMC of adult aorta, coronary arteries, trabeculae of the spleen, colon, stomach or skeletal muscle. Interestingly, the presence of cardiac α-actin was detected in umbilical cord vessels, human myometrium, in atherosclerotic coronary lesions and atherosclerotic lesions from peripheral vascular disease. The distribution of cardiac α-actin often paralleled that of cytokeratins 8 and 18, intermediate filament proteins typically found in dedifferentiated SMC. Taken together, the data presented here illustrate the expression of cardiac α-actin to be limited to either fetal vessels or those vessels or tissue having suffered damage or atrophy, outside its ‘native’ environment in the heart. The demonstration of cardiac α-actin in SMC of umbilical cord vessels and in atherosclerotic lesions but not in apparently healthy vessels supports the notion that SMC in atherosclerotic lesions exhibit a dedifferentiated phenotype.

Biventricular involvement in transient apical ballooning syndrome.

Transient apical ballooning syndrome (Takotsubo cardiomyopathy) is an acute cardiac syndrome mimicking ST-elevation myocardial infarction. It is characterized by ventricular wall motion abnormalities confined to the apical regions of the left ventricle. Here we describe an 80-year old woman presenting with acute shortness of breath. Echocardiography demonstrated left and right ventricular apical akinesia and basal hyperkinesia. Cardiac catheterisation disclosed minimal atherosclerotic changes of the coronary arteries. Both symptoms and echocardiographic findings resolved completely within one week.