Sensing stretch is fundamental

Synchrony of sarcomeric movement regulates left ventricular pump function in the in vivo beating mouse heart

Sarcomeric contraction in cardiomyocytes serves as the basis for the heart's pump functions. It has generally been considered that in cardiac muscle as well as in skeletal muscle, sarcomeres equally contribute to myofibrillar dynamics in myocytes at varying loads by producing similar levels of active and passive force. In the present study, we expressed α-actinin-AcGFP in Z-disks to analyze dynamic behaviors of sequentially connected individual sarcomeres along a myofibril in a left ventricular (LV) myocyte of the in vivo beating mouse heart. To quantify the magnitude of the contribution of individual sarcomeres to myofibrillar dynamics, we introduced the novel parameter "contribution index" (CI) to measure the synchrony in movements between a sarcomere and a myofibril (from -1 [complete asynchrony] to 1 [complete synchrony]). First, CI varied markedly between sarcomeres, with an average value of ∼0.3 during normal systole. Second, when the movements between adjacent sarcomeres were asynchronous (CI < 0), a sarcomere and the ones next to the adjacent sarcomeres and farther away moved in synchrony (CI > 0) along a myofibril. Third, when difference in LV pressure in diastole and systole (ΔLVP) was lowered to <10 mm Hg, diastolic sarcomere length increased. Under depressed conditions, the movements between adjacent sarcomeres were in marked asynchrony (CI, -0.3 to -0.4), and, as a result, average CI was linearly decreased in association with a decrease in ΔLVP. These findings suggest that in the left ventricle of the in vivo beating mouse heart, (1) sarcomeres heterogeneously contribute to myofibrillar dynamics due to an imbalance of active and passive force between neighboring sarcomeres, (2) the force imbalance is pronounced under depressed conditions coupled with a marked increase in passive force and the ensuing tug-of-war between sarcomeres, and (3) sarcomere synchrony via the distal intersarcomere interaction regulates the heart's pump function in coordination with myofibrillar contractility.

Real-Time In Vivo Imaging of Mouse Left Ventricle Reveals Fluctuating Movements of the Intercalated Discs

Myocardial contraction is initiated by action potential propagation through the conduction system of the heart. It has been thought that connexin 43 in the gap junctions (GJ) within the intercalated disc (ID) provides direct electric connectivity between cardiomyocytes (electronic conduction). However, recent studies challenge this view by providing evidence that the mechanosensitive cardiac sodium channels Na_v1.5 localized in perinexii at the GJ edge play an important role in spreading action potentials between neighboring cells (ephaptic conduction). In the present study, we performed real-time confocal imaging of the CellMask-stained ID in the living mouse heart in vivo. We found that the ID structure was not rigid. Instead, we observed marked flexing of the ID during propagation of contraction from cell to cell. The variation in ID length was between ~30 and ~42 μm (i.e., magnitude of change, ~30%). In contrast, tracking of α-actinin-AcGFP revealed a comparatively small change in the lateral dimension of the transitional junction near the ID (i.e., magnitude of change, ~20%). The present findings suggest that, when the heart is at work, mechanostress across the perinexii may activate Na_v1.5 by promoting ephaptic conduction in coordination with electronic conduction, and, thereby, efficiently transmitting excitation-contraction coupling between cardiomyocytes.

Time- and Ventricular-Specific Expression Profiles of Genes Encoding Z-Disk Proteins in Pressure Overload Model of Left Ventricular Hypertrophy

Mechanotransduction is an essential mechanism of transforming external mechanical stimulus to biochemical response. In cardiomyocytes mechanotransduction plays an important role in contraction, stretch sensing and homeostasis regulation. One of the major mechanosensitive area in cardiomyocytes, the Z-disk, consists of numbers of structural and signaling proteins, that may undergo conformational or gene expression changes under pathological stress conditions. In present study we examined a rat model of pressure overload cardiac hypertrophy validated by echocardiographic and histopathological examinations. We revealed, that during hypertrophy progression expression of several genes encoding Z-disk proteins (Actn2, Ldb3, Cmya5, Nebl) is different at early and late points of cardiac remodeling. Moreover, expression patterns of several genes are opposite in myocardium of overloaded left ventricle and hemodynamically unaffected right ventricle, and expression profiles in interventricular septum are more similar to right ventricle. Additionally, we revealed inconsistencies between mRNA and protein level changes of Actn2, one of the major structural Z-disk element. All these findings point out, that investigated Z-disk proteins participate in pathological stress adaptation through undergoing the gene expression changes, and suggest the novel important role of hypertrophic response modulation during different stages of cardiac remodeling.

The Src homology and collagen A (ShcA) adaptor protein is required for the spatial organization of the costamere/Z-disk network during heart development

Src homology and collagen A (ShcA) is an adaptor protein that binds to tyrosine kinase receptors. Its germ line deletion is embryonic lethal with abnormal cardiovascular system formation, and its role in cardiovascular development is unknown. To investigate its functional role in cardiovascular development in mice, ShcA was deleted in cardiomyocytes and vascular smooth muscle cells by crossing ShcA flox mice with SM22a-Cre transgenic mice. Conditional mutant mice developed signs of severe dilated cardiomyopathy, myocardial infarctions, and premature death. No evidence of a vascular contribution to the phenotype was observed. Histological analysis of the heart revealed aberrant sarcomeric Z-disk and M-band structures, and misalignments of T-tubules with Z-disks. We find that not only the ErbB3/Neuregulin signaling pathway but also the baroreceptor reflex response, which have been functionally associated, are altered in the mutant mice. We further demonstrate that ShcA interacts with Caveolin-1 and the costameric protein plasma membrane Ca(2+)/calmodulin-dependent ATPase (PMCA), and that its deletion leads to abnormal dystrophin signaling. Collectively, these results demonstrate that ShcA interacts with crucial proteins and pathways that link Z-disk and costamere.

ACTN3 genotype and modulation of skeletal muscle response to exercise in human subjects

α-Actinin-3 is a Z-disc protein expressed only in type II muscle fibers. A polymorphism in the ACTN3 gene (R577X) results in lack of α-actinin-3 in XX genotype. The prevalence of the mutated X-allele is lower among power/sprint oriented athletes compared with controls, indicating that the lack of α-actinin-3 is detrimental in these sports, but a mechanistic link has not been established. Results from Actn3-knockout (KO) mouse model suggest that α-actinin-3 may affect muscle mass and muscle glycogen levels. In the present investigation we examined muscle fiber type composition, cross-sectional fiber area (CSA), and muscle glycogen levels at baseline in 143 human subjects with different ACTN3 genotypes. In addition, hypertrophy signaling and glycogen utilization in response to sprint exercise were studied in a subset of subjects. Glycogen utilization was analyzed in separate pools of type I and type II fibers. No differences in fiber type composition, CSA, or muscle glycogen levels were observed at baseline across the ACTN3 genotypes. However, the sprint exercise-induced increase in phosphorylation of mTOR and p70S6k was smaller in XX than in RR+RX (P = 0.03 and P = 0.01, respectively), indicating a less pronounced activation of hypertrophy signaling in XX. Glycogen utilization during sprint exercise varied across ACTN3 genotypes in type II fibers (P = 0.03) but not in type I fibers (P = 0.38). The present results are in accordance with findings from the KO mice and reinforce the hypothesis that ACTN3 genotype-associated differences in muscle mass and glycogen utilization provide a mechanistic explanation for the modulation of human performance by the ACTN3 genotype.

Age-dependent changes in contractile function and passive elastic properties of myocardium from mice lacking muscle LIM protein (MLP)

AIMS

Muscle LIM protein (MLP) null mice are often used as a model for human dilated cardiomyopathy. So far, little is known about the time course and pathomechanisms leading to the development of the adult phenotype.

METHODS AND RESULTS

We systematically analysed the contractile phenotype, myofilament calcium (Ca(2)(+)) responsiveness, passive myocardial mechanics, histology, and mRNA expression in mice aged 4 and 12 weeks. In 4-week-old animals, there was no significant difference in the force-frequency relationship (FFR) and catecholamine response of intact isolated papillary muscles between wild-type (WT) and MLP null myocardium. In 12-week-old animals, WT myocardium exhibited a significantly positive FFR, while that of MLP null mice was significantly negative, and the inotropic response to catecholamines was significantly reduced in MLP null mice. This time course of decline in contractile function was confirmed in vivo by echocardiography. Whereas at 4 weeks of age MLP null mice and WT littermates showed similar levels of SERCA2a (sarcoplasmic reticulum Ca(2+) ATPase) expression, the expression was significantly lower in 12-week-old MLP null mice compared with littermate controls. Myofilament Ca(2)(+) responsiveness was not affected by the lack of MLP, irrespective of age. Whereas in 4-week-old animals MLP null myocardium showed a trend to an increased compliance compared with the WT, myocardium of 12-week-old MLP null mice was significantly less compliant than WT myocardium. Parallel to the decrease in compliance there was an increase in fibrosis in the MLP null animals.

CONCLUSION

Our data suggest that MLP deficiency does not primarily influence myocardial contractility. A lack of MLP leads to an age-dependent impairment of excitation-contraction coupling with resulting contractile dysfunction and secondary fibrosis.

Placental growth factor regulates cardiac adaptation and hypertrophy through a paracrine mechanism

RATIONALE

Paracrine growth factor-mediated crosstalk between cardiac myocytes and nonmyocytes in the heart is critical for programming adaptive cardiac hypertrophy in which myocyte size, capillary density, and the extracellular matrix function coordinately.

OBJECTIVE

To examine the role that placental growth factor (PGF) plays in the heart as a paracrine regulator of cardiac adaptation to stress stimulation.

METHODS AND RESULTS

PGF is induced in the heart after pressure-overload stimulation, where it is expressed in both myocytes and nonmyocytes. We generated cardiac-specific and adult inducible PGF-overexpressing transgenic mice and analyzed Pgf(-/-) mice to examine the role that this factor plays in cardiac disease and paracrine signaling. Although PGF transgenic mice did not have a baseline phenotype or a change in capillary density, they did exhibit a greater cardiac hypertrophic response, a greater increase in capillary density, and increased fibroblast content in the heart in response to pressure-overload stimulation. PGF transgenic mice showed a more adaptive type of cardiac growth that was protective against signs of failure with pressure overload and neuroendocrine stimulation. Antithetically, Pgf(-/-) mice rapidly died of heart failure within 1 week of pressure overload, they showed an inability to upregulate angiogenesis, and they showed significantly less fibroblast activity in the heart. Mechanistically, we show that PGF does not have a direct effect on cardiomyocytes but works through endothelial cells and fibroblasts by inducing capillary growth and fibroblast proliferation, which secondarily support greater cardiac hypertrophy through intermediate paracrine growth factors such as interleukin-6.

CONCLUSIONS

PGF is a secreted factor that supports hypertrophy and cardiac function during pressure overload by affecting endothelial cells and fibroblasts that in turn stimulate and support the myocytes through additional paracrine factors.

Deletion of Drosophila muscle LIM protein decreases flight muscle stiffness and power generation

Muscle LIM protein (MLP) can be found at the Z-disk of sarcomeres where it is hypothesized to be involved in sensing muscle stretch. Loss of murine MLP results in dilated cardiomyopathy, and mutations in human MLP lead to cardiac hypertrophy, indicating a critical role for MLP in maintaining normal cardiac function. Loss of MLP in Drosophila (mlp84B) also leads to muscle dysfunction, providing a model system to examine MLP's mechanism of action. Mlp84B-null flies that survive to adulthood are not able to fly or beat their wings. Transgenic expression of the mlp84B gene in the Mlp84B-null background rescues flight ability and restores wing beating ability. Mechanical analysis of skinned flight muscle fibers showed a 30% decrease in oscillatory power production and a slight increase in the frequency at which maximum power is generated for fibers lacking Mlp84B compared with rescued fibers. Mlp84B-null muscle fibers displayed a 25% decrease in passive, active, and rigor stiffness compared with rescued fibers, but no significant decrease in isometric tension generation was observed. Muscle ultrastructure of Mlp84B-null muscle fibers is grossly normal; however, the null fibers have a slight decrease, 11%, in thick filament number per unit cross-sectional area. Our data indicate that MLP contributes to muscle stiffness and is necessary for maximum work and power generation.

Blood flow dynamics of one cardiac cycle and relationship to mechanotransduction and trabeculation during heart looping

Analyses of form-function relationships during heart looping are directly related to technological advances. Recent advances in four-dimensional optical coherence tomography (OCT) permit observations of cardiac dynamics at high-speed acquisition rates and high resolution. Real-time observation of the avian stage 13 looping heart reveals that interactions between the endocardial and myocardial compartments are more complex than previously depicted. Here we applied four-dimensional OCT to elucidate the relationships of the endocardium, myocardium, and cardiac jelly compartments in a single cardiac cycle during looping. Six cardiac levels along the longitudinal heart tube were each analyzed at 15 time points from diastole to systole. Using image analyses, the organization of mechanotransducing molecules, fibronectin, tenascin C, α-tubulin, and nonmuscle myosin II was correlated with specific cardiac regions defined by OCT data. Optical coherence microscopy helped to visualize details of cardiac architectural development in the embryonic mouse heart. Throughout the cardiac cycle, the endocardium was consistently oriented between the midline of the ventral floor of the foregut and the outer curvature of the myocardial wall, with multiple endocardial folds allowing high-volume capacities during filling. The cardiac area fractional shortening is much higher than previously published. The in vivo profile captured by OCT revealed an interaction of the looping heart with the extra-embryonic splanchnopleural membrane providing outside-in information. In summary, the combined dynamic and imaging data show the developing structural capacity to accommodate increasing flow and the mechanotransducing networks that organize to effectively facilitate formation of the trabeculated four-chambered heart.

Cardiac Z-disc signaling network

During the last 15 years, the perception of the cardiac z-disc has undergone substantial changes. Initially viewed as a structural component at the lateral boundaries of the sarcomere, the cardiac z-disc has increasingly become recognized as a nodal point in cardiomyocyte signal transduction and disease. This minireview thus focuses on novel components and recent developments in z-disc biology and their role in cardiac signaling and disease.

Fibronectin increases the force production of mouse papillary muscles via α5β1 integrin

The extracellular matrix (ECM) protein-integrin-cytoskeleton axis plays a central role as a mechanotransducing protein assemblage in many cell types. However, how the process of mechanotransduction and the mechanically generated signals arising from this axis affect myofilament function in cardiac muscle are not completely understood. We hypothesize that ECM proteins can regulate cardiac function through integrin binding, and thereby alter the intracellular calcium concentration ([Ca(2+)](i)) and/or modulate myofilament activation processes. Force measurements made in mouse papillary muscle demonstrated that in the presence of the soluble form of the ECM protein, fibronectin (FN), active force was increased significantly by 40% at 1 Hz, 54% at 2 Hz, 35% at 5 Hz and 16% at 9 Hz stimulation frequencies. Furthermore, increased active force in the presence of FN was associated with 12-33% increase in [Ca(2+)](i) and 20-50% increase in active force per unit Ca(2+). A function blocking antibody for α5 integrin prevented the effects of the FN on the changes in force and [Ca(2+)](i), whereas a function blocking α3 integrin antibody did not reverse the effects of FN. The effects of FN were reversed by an L-type Ca(2+) channel blocker, verapamil or PKA inhibitor. Freshly isolated cardiomyocytes exhibited a 39% increase in contraction force and a 36% increase in L-type Ca(2+) current in the presence of FN. Fibers treated with FN showed a significant increase in the phosphorylation of phospholamban; however, the phosphorylation of troponin I was unchanged. These results demonstrate that FN acts via α5β1 integrin to increase force production in myocardium and that this effect is partly mediated by increases in [Ca(2+)](i) and Ca(2+) sensitivity, PKA activation and phosphorylation of phospholamban.

Comparative biomechanics of thick filaments and thin filaments with functional consequences for muscle contraction

The scaffold of striated muscle is predominantly comprised of myosin and actin polymers known as thick filaments and thin filaments, respectively. The roles these filaments play in muscle contraction are well known, but the extent to which variations in filament mechanical properties influence muscle function is not fully understood. Here we review information on the material properties of thick filaments, thin filaments, and their primary constituents; we also discuss ways in which mechanical properties of filaments impact muscle performance.

Are biological sensors modulated by their structural scaffolds? The role of the structural muscle proteins alpha-actinin-2 and alpha-actinin-3 as modulators of biological sensors

Biological sensors and their ability to detect and respond to change in the cellular environment can be modulated by protein scaffolds acting within their interaction network. The skeletal muscle alpha-actinins have been considered as primarily structural scaffold proteins. However, deficiency of alpha-actinin-3 due to a common null polymorphism results in predominantly metabolic changes in skeletal muscle function. In this review, we explore the range of phenotypes associated with alpha-actinin-3 deficiency, and draw supporting evidence from known interaction partners for its role as a scaffold which acts to modulate biological sensors that result in changes in muscle metabolism and structure.

Cardiomyocyte contractile status is associated with differences in fibronectin and integrin interactions

Integrins link the extracellular matrix (ECM) with the intracellular cytoskeleton and other cell adhesion-associated signaling proteins to function as mechanotransducers. However, direct quantitative measurements of the cardiomyocyte mechanical state and its relationship to the interactions between specific ECM proteins and integrins are lacking. The purpose of this study was to characterize the interactions between the ECM protein fibronectin (FN) and integrins in cardiomyocytes and to test the hypothesis that these interactions would vary during contraction and relaxation states in cardiomyocytes. Using atomic force microscopy, we quantified the unbinding force (adhesion force) and adhesion probability between integrins and FN and correlated these measurements with the contractile state as indexed by cell stiffness on freshly isolated mouse cardiomyocytes. Experiments were performed in normal physiological (control), high-K(+) (tonically contracted), or low-Ca(2+) (fully relaxed) solutions. Under control conditions, the initial peak of adhesion force between FN and myocyte alpha(3)beta(1)- and/or alpha(5)beta(1)-integrins was 39.6 +/- 1.3 pN. The binding specificity between FN and alpha(3)beta(1)- and alpha(5)beta(1)-integrins was verified by using monoclonal antibodies against alpha(3)-, alpha(5)-, alpha(3) + alpha(5)-, or beta(1)-integrin subunits, which inhibited binding by 48%, 65%, 70%, or 75%, respectively. Cytochalasin D or 2,3-butanedione monoxime (BDM), to disrupt the actin cytoskeleton or block myofilament function, respectively, significantly decreased the cell stiffness; however, the adhesion force and binding probability were not altered. Tonic contraction with high-K(+) solution increased total cell adhesion (1.2-fold) and cell stiffness (27.5-fold) compared with fully relaxed cells with low-Ca(2+) solution. However, it could be partially prevented by high-K(+) bath solution containing BDM, which suppresses contraction by inhibiting the actin-myosin interactions. Thus, our results demonstrate that integrin binding to FN is modulated by the contractile state of cardiac myocytes.

Functional muscle analysis of the Tcap knockout mouse

Autosomal recessive limb-girdle muscular dystrophy type 2G (LGMD2G) is an adult-onset myopathy characterized by distal lower limb weakness, calf hypertrophy and progressive decline in ambulation. The disease is caused by mutations in Tcap, a z-disc protein of skeletal muscle, although the precise mechanisms resulting in clinical symptoms are unknown. To provide a model for preclinical trials and for mechanistic studies, we generated knockout (KO) mice carrying a null mutation in the Tcap gene. Here we present the first report of a Tcap KO mouse model for LGMD2G and the results of an investigation into the effects of Tcap deficiency on skeletal muscle function in 4- and 12-month-old mice. Muscle histology of Tcap-null mice revealed abnormal myofiber size variation with central nucleation, similar to findings in the muscles of LGMD2G patients. An analysis of a Tcap binding protein, myostatin, showed that deletion of Tcap was accompanied by increased protein levels of myostatin. Our Tcap-null mice exhibited a decline in the ability to maintain balance on a rotating rod, relative to wild-type controls. No differences were detected in force or fatigue assays of isolated extensor digitorum longus (EDL) and soleus (SOL) muscles. Finally, a mechanical investigation of EDL and SOL indicated an increase in muscle stiffness in KO animals. We are the first to establish a viable KO mouse model of Tcap deficiency and our model mice demonstrate a dystrophic phenotype comparable to humans with LGMD2G.

The vertebrate muscle Z-disc: sarcomere anchor for structure and signalling

The Z-disc, appearing as a fine dense line forming sarcomere boundaries in striated muscles, when studied in detail reveals crosslinked filament arrays that transmit tension and house myriads of proteins with diverse functions. At the Z-disc the barbed ends of the antiparallel actin filaments from adjoining sarcomeres interdigitate and are crosslinked primarily by layers of α-actinin. The Z-disc is therefore the site of polarity reversal of the actin filaments, as needed to interact with the bipolar myosin filaments in successive sarcomeres. The layers of α-actinin determine the Z-disc width: fast fibres have narrow (~30–50 nm) Z-discs and slow and cardiac fibres have wide (~100 nm) Z-discs. Comprehensive reviews on the roles of the numerous proteins located at the Z-disc in signalling and disease have been published; the aim here is different, namely to review the advances in structural aspects of the Z-disc.

Atrial fibrosis and atrial fibrillation: the role of the TGF-β1 signaling pathway

BACKGROUND

Atrial fibrosis concurs with chronic atrial fibrillation (AF), a phenomenon that contributes to the resistance to restore and maintain sinus rhythm (SR). Fibrogenesis represents a complex process in which the transforming growth factor-β1 (TGF-β1) pathway may play a major role, e.g. in the setting of myocardial infarction. The present study addresses the potential contribution of the TGF-β1 signaling pathway to atrial fibrosis in patients with AF.

METHODS AND RESULTS

Right atrial appendages of 163 patients were excised during heart surgery and grouped according to rhythm (SR vs. AF) and AF duration. Five groups were defined: SR, paroxysmal/chronic persistent AF (<6 months), chronic permanent AF (CAF) of 7-24 months, 25-60 months, and >60 months duration. Collagen content of atria, determined morphometrically, revealed a steady and significant increase in patients with SR (14.6±8.9%) up to patients with CAF of >60 months (28.1±7.1%). Likewise, expression of TGF-β1 mRNA and protein, TGF-β-receptor-II protein, profibrotic phospho-Smad-2 and -4 proteins increased. However, the TGF-β(1) effect appeared to decline with increasing AF duration, characterized by a decrease in TGF-β-receptor-I protein, increases of TGF-β inhibiting Smad-7 protein and a reduction of ph-Smad-2.

CONCLUSIONS

Human atrial fibrogenesis in patients with atrial fibrillation is accompanied by a biphasic response, an early increase and later loss of responsiveness to TGF-β(1). It appears that fibrosis progresses despite compensatory changes in the TGF-β-signaling pathway. The sequential changes in the contribution of different profibrotic processes during the establishment of AF may offer the opportunity to selectively interfere with the atrial remodeling process at different stages.

Age-related atrial fibrosis

Many age-related diseases are associated with, and may be promoted by, cardiac fibrosis. Transforming growth factor (TGF)-beta, hypoxia-induced factor (HIF), and the matrix metalloproteinase (MMP) system have been implicated in fibrogenesis. Thus, we investigated whether age is related to these systems and to atrial fibrosis. Right atrial appendages (RAA) obtained during heart surgery (n = 115) were grouped according to patients' age (<50 years, 51-60 years, 61-70 years, or >70 years). Echocardiographic ejection fractions (EF) and fibrosis using Sirius-red-stained histological sections were determined. TGF-beta was determined by quantitative RT-PCR and hypoxia-related factors [HIF1 alpha, the vascular endothelial growth factor (VEGF)-receptor, CD34 (a surrogate marker for microvessel density), the factor inhibiting HIF (FIH), and prolyl hydroxylase 3 (PHD 3)] were detected by immunostaining. MMP-2 and -9 activity were determined zymographically, and mRNA levels of their common tissue inhibitor TIMP-1 were determined by RT-PCR. Younger patients (<50 years) had significantly less fibrosis (10.1% +/- 4.4% vs 16.6% +/- 8.3%) than older individuals (>70 years). While HIF1 alpha, FIH, the VEGF-receptor, and CD34 were significantly elevated in the young, TGF-beta and PHD3 were suppressed in these patients. MMP-2 and -9 activity was found to be higher while TIMP-1 levels were lower in older patients. Statistical analysis proved age to be the only factor influencing fibrogenesis. With increasing age, RAAs develop significantly more fibrosis. An increase of fibrotic and decrease of hypoxic signalling and microvessel density, coupled with differential expression of MMPs and TIMP-1 favouring fibrosis may have helped promote atrial fibrogenesis.

Atrial fibrillation is associated with cardiac hypoxia

BACKGROUND

Atrial fibrillation (AF), the most common human arrhythmia, is responsible for substantial morbidity and mortality and may be promoted by selective atrial ischemia and atrial fibrosis. Consequently, we investigated markers for hypoxia and angiogenesis in AF.

METHODS

Right atrial appendages (n=158) were grouped according to heart rhythm [sinus rhythm (SR) or AF]. The degree of fibrosis and microvessel density of all patients were determined morphometrically using Sirius-Red- and CD34/CD105-stained sections, respectively. Next, sections (n=77) underwent immunostaining to detect hypoxia- and angiogenesis-related proteins [hypoxia-inducible factor (HIF)1 alpha, HIF2 alpha, vascular endothelial growth factor (VEGF), VEGF receptor 2 (KDR), phosphorylated KDR (pKDR), carboanhydrase IX, platelet-derived growth factor] and the apoptosis-related B-cell lymphoma 2 protein.

RESULTS

Fibrosis progressed significantly from 14.7+/-0.8% (SR) to 22.3+/-1.4% (AF). While the positive cytoplasmic staining of HIF1 alpha, HIF2 alpha, VEGF, KDR, and pKDR rose significantly from SR to AF, their nuclear fractions fell (only pKDR significantly). The median CD34/CD105-positive microvessel size increased significantly from SR to AF.

CONCLUSIONS

AF is closely associated with an atrial up-regulation of hypoxic and angiogenic markers. Whether this is cause, effect, or co-phenomenon of fibrosis remains to be investigated. It is conceivable that fibrosis might lead to an increased O(2) diffusion distance and thus induce ischemic signaling, which, in turn, leads to angiogenesis.

Cellular nonmuscle myosins NMHC-IIA and NMHC-IIB and vertebrate heart looping

Flectin, a protein previously described to be expressed in a left-dominant manner in the embryonic chick heart during looping, is a member of the nonmuscle myosin II (NMHC-II) protein class. During looping, both NMHC-IIA and NMHC-IIB are expressed in the mouse heart on embryonic day 9.5. The patterns of localization of NMHC-IIB, rather than NMHC-IIA in the mouse looping heart and in neural crest cells, are equivalent to what we reported previously for flectin. Expression of full-length human NMHC-IIA and -IIB in 10 T1/2 cells demonstrated that flectin antibody recognizes both isoforms. Electron microscopy revealed that flectin antibody localizes in short cardiomyocyte cell processes extending from the basal layer of the cardiomyocytes into the cardiac jelly. Flectin antibody also recognizes stress fibrils in the cardiac jelly in the mouse and chick heart; while NMHC-IIB antibody does not. Abnormally looping hearts of the Nodal(Delta 600) homozygous mouse embryos show decreased NMHC-IIB expression on both the mRNA and protein levels. These results document the characterization of flectin and extend the importance of NMHC-II and the cytoskeletal actomyosin complex to the mammalian heart and cardiac looping.

Geometric control of tissue morphogenesis

Morphogenesis is the dynamic and regulated change in tissue form that leads to creation of the body plan and development of mature organs. Research over the past several decades has uncovered a multitude of genetic factors required for morphogenesis in animals. The behaviors of individual cells within a developing tissue are determined by combining these genetic signals with information from the surrounding microenvironment. At any point in time, the local microenvironment is influenced by macroscale tissue geometry, which sculpts long range signals by affecting gradients of morphogens and mechanical stresses. The geometry of a tissue thus acts as both a template and instructive cue for further morphogenesis.

MURC, a muscle-restricted coiled-coil protein that modulates the Rho/ROCK pathway, induces cardiac dysfunction and conduction disturbance

We identified a novel muscle-restricted putative coiled-coil protein, MURC, which is evolutionarily conserved from frog to human. MURC was localized to the cytoplasm with accumulation in the Z-line of the sarcomere in the murine adult heart. MURC mRNA expression in the heart increased during the developmental process from the embryonic stage to adulthood. In response to pressure overload, MURC mRNA expression increased in the hypertrophied heart. Using the yeast two-hybrid system, we identified the serum deprivation response (SDPR) protein, a phosphatidylserine-binding protein, as a MURC-binding protein. MURC induced activation of the RhoA/ROCK pathway, which modulated serum response factor-mediated atrial natriuretic peptide (ANP) expression and myofibrillar organization. SDPR augmented MURC-induced transactivation of the ANP promoter in cardiomyocytes, and RNA interference of SDPR attenuated the action of MURC on the ANP promoter. Transgenic mice expressing cardiac-specific MURC (Tg-MURC) exhibited cardiac contractile dysfunction and atrioventricular (AV) conduction disturbances with atrial chamber enlargement, reduced thickness of the ventricular wall, and interstitial fibrosis. Spontaneous episodes of atrial fibrillation and AV block were observed in Tg-MURC mice. These findings indicate that MURC modulates RhoA signaling and that MURC plays an important role in the development of cardiac dysfunction and conduction disturbance with increased vulnerability to atrial arrhythmias.

Early incorporation of obscurin into nascent sarcomeres: implication for myofibril assembly during cardiac myogenesis

Obscurin is a recently identified giant multidomain muscle protein whose functions remain poorly understood. The goal of this study was to investigate the process of assembly of obscurin into nascent sarcomeres during the transition from non-striated myofibril precursors to striated structure of differentiating myofibrils in cell cultures of neonatal rat cardiac myocytes. Double immunofluorescent labeling and high resolution confocal microscopy demonstrated intense incorporation of obscurin in the areas of transition from non-striated to striated regions on the tips of developing myofibrils and at the sites of lateral fusion of nascent sarcomere bundles. We found that obscurin rapidly and precisely accumulated in the middle of the A-band regions of the terminal newly assembled half-sarcomeres in the zones of transition from the continuous, non-striated pattern of sarcomeric alpha-actinin distribution to cross-striated structure of laterally expanding nascent Z-discs. The striated pattern of obscurin typically ended at these points. This occurred before the assembly of morphologically differentiated terminal Z-discs of the assembling sarcomeres on the tips of growing myofibrils. The presence of obscurin in the areas of the terminal Z-discs of each new sarcomere was detected at the same time or shortly after complete assembly of sarcomeric structure. Many non-striated fibers with very low concentration of obscurin were already immunopositive for sarcomeric actin and myosin. This suggests that obscurin may serve for organization and alignment of myofilaments into the striated pattern. The comparison of obscurin and titin localization in these areas showed that obscurin assembly into the A-bands occurred soon after or concomitantly with incorporation of titin. Electron microscopy of growing myofibrils demonstrated intense formation and integration of myosin filaments into the "open" half-assembled sarcomeres in the areas of the terminal Z-I structures and at the lateral surfaces of newly formed, terminally located nascent sarcomeres. This process progressed before the assembly of the second-formed, terminal Z-discs of new sarcomeres and before the development of ultrastructurally detectable mature M-lines that define the completion of myofibril assembly, which supports the data of immunocytochemical study. Abundant non-aligned sarcomeres in immature myofibrils located on the growing tips were spatially separated and underwent the transition to the registered, aligned pattern. The sarcoplasmic reticulum, the organelle known to interact with obscurin, assembled around each new sarcomere. These results suggest that obscurin is directly involved in the proper positioning and alignment of myofilaments within nascent sarcomeres and in the establishment of the registered pattern of newly assembled myofibrils and the sarcoplasmic reticulum at advanced stages of myofibrillogenesis.

Principal strain changes precede ventricular wall thinning during transition to heart failure in a mouse model of dilated cardiomyopathy

This study was performed to elucidate the relation between in vivo measurements of two-dimensional principal strains and the progression of left ventricle (LV) wall thinning during development of dilated cardiomyopathy in the protein kinase C-ε (PKC-ε) transgenic (TG) overexpressing mouse heart. Principal two-dimensional strains, E1 and E2, were determined in the LV wall of the anesthetized mouse using cardiac MRI tagging at 14.1 T. PKC-ε TG provided a model of pure dilated cardiomyopathy without evidence of hypertrophy (PKC-ε TG, n = 6). Ejection fraction, wall thickness, and principal strains were determined at 1-mo intervals in hearts of PKC-ε TG vs. age-matched, nontransgenic mice (NTG, n = 5) from age 6 to 13 mo. Through the study, PKC-ε TG displayed lower ejection fraction than NTG. At 7 mo, average principal strain E1 in PKC-ε TG hearts was lower compared with NTG (PKC-ε TG = 0.14 ± 0.03, NTG = 0.19 ± 0.03, P < 0.05). The greatest reductions in regional E1 occurred in the lateral segments. The principal strain E2 did not change significantly in either group. At 9 mo, LV wall thinning occurred in PKC-ε TG mice ( P < 0.01 vs. 8 mo) to 21% below values in NTG ( P < 0.001). Average E1 strain diverged between PKC-ε TG and NTG hearts by 25–43%. These E1 changes preceded LV wall thinning and predated the eventual transition from a compensated circumstance to the dilated phenotype. The findings indicate a near step function in E1 depression that precedes the onset of LV wall thinning and suggest E1 as a prognostic indicator of dilated cardiomyopathy.

Protein kinase A, Ca2+/calmodulin-dependent kinase II, and calcineurin regulate the intracellular trafficking of myopodin between the Z-disc and the nucleus of cardiac myocytes

Spatial and temporal resolution of intracellular signaling can be achieved by compartmentalizing transduction units. Myopodin is a dual-compartment, actin-bundling protein that shuttles between the nucleus and the Z-disc of myocytes in a differentiation- and stress-dependent fashion. Importin alpha binding and nuclear import of myopodin are regulated by serine/threonine phosphorylation-dependent binding of myopodin to 14-3-3. Here we show that in the heart myopodin forms a Z-disc signaling complex with alpha-actinin, calcineurin, Ca2+/calmodulin-dependent kinase II (CaMKII), muscle-specific A-kinase anchoring protein, and myomegalin. Phosphorylation of myopodin by protein kinase A (PKA) or CaMKII mediates 14-3-3 binding and nuclear import in myoblasts. Dephosphorylation of myopodin by calcineurin abrogates 14-3-3beta binding. Activation of PKA or inhibition of calcineurin in adult cardiac myocytes releases myopodin from the Z-disc and induces its nuclear import. The identification of myopodin as a direct target of PKA, CaMKII, and calcineurin defines a novel intracellular signaling pathway whereby changes in Z-disc dynamics may translate into compartmentalized signal transduction in the heart.

Engineering the follicle microenvironment

In vitro ovarian follicle culture provides a tool to investigate folliculogenesis, and may one day provide women with fertility-preservation options. The application of tissue engineering principles to ovarian follicle maturation may enable the creation of controllable microenvironments that will coordinate the growth of the multiple cellular compartments within the follicle. Three-dimensional culture systems can preserve follicle architecture, thereby maintaining critical cell-cell and cell-matrix signaling lost in traditional two-dimensional attached follicle culture systems. Maintaining the follicular structure while manipulating the biochemical and mechanical environment will enable the development of controllable systems to investigate the fundamental biological principles underlying follicle maturation. This review describes recent advances in ovarian follicle culture, and highlights the tissue engineering principles that may be applied to follicle culture, with the ultimate objective of germline preservation for females facing premature infertility.

Altered expression of early cardiac marker genes in circulating cells of patients with hypertrophic cardiomyopathy

BACKGROUND

Early cardiac marker genes, such as cardiac-specific homeobox (Csx/Nkx2.5), myocardin, homeodomain only protein, GATA4, and myocyte enhancer factor 2C, are thought to participate in cardiomyocyte differentiation and to contribute to heart hypertrophy in animal models. In this study, we investigated whether the expression of early cardiac genes is altered in the peripheral blood of patients with hypertrophic cardiomyopathy.

METHODS

Peripheral blood mononuclear cells were isolated from 30 consecutive hypertrophic cardiomyopathy patients and 20 healthy controls, and gene expression was determined by quantitative real-time reverse transcription-polymerase chain reaction.

RESULTS

Csx/Nkx2.5, myocardin, and GATA4 expressions were significantly higher in hypertrophic cardiomyopathy patients by 5.14+/-0.89 (P<.001), 1.65+/-0.21 (P<.05), and 2.04+/-0.41 (P<.04) times, respectively, while homeodomain only protein showed a fourfold decrease in expression (P<.02) compared to controls. In addition, expression of the differentiation-specific marker genes beta-myosin heavy chain and smooth muscle myosin heavy chain was significantly higher in hypertrophic cardiomyopathy patients by 3.72+/-0.82 (P<.02) and 2.57+/-0.72 (P<.05) times, respectively, compared to controls. Myocyte enhancer factor 2C expression was not different between patients and controls. Furthermore, increased expression of GATA4, myocardin, and beta-myosin heavy chain positively correlated with increased left ventricular mass.

CONCLUSIONS

In conclusion, we found altered expressions of early cardiac marker genes and differentiation-specific marker genes in peripheral blood mononuclear cells of hypertrophic cardiomyopathy patients compared to control individuals, possibly reflecting changes in response to disease.

Two-state model of acto-myosin attachment-detachment predicts C-process of sinusoidal analysis

The force response of activated striated muscle to length perturbations includes the so-called C-process, which has been considered the frequency domain representation of the fast single-exponential force decay after a length step (phases 1 and 2). The underlying molecular mechanisms of this phenomenon, however, are still the subject of various hypotheses. In this study, we derived analytical expressions and created a corresponding computer model to describe the consequences of independent acto-myosin cross-bridges characterized solely by 1), intermittent periods of attachment (t(att)) and detachment (t(det)), whose values are stochastically governed by independent probability density functions; and 2), a finite Hookian stiffness (k(stiff)) effective only during periods of attachment. The computer-simulated force response of 20,000 (N) cross-bridges making up a half-sarcomere (F(hs)(t)) to sinusoidal length perturbations (L(hs)(t)) was predicted by the analytical expression in the frequency domain, (F(hs)(omega)/L(hs)(omega))=(t(att)/t(cycle))Nk(stiff)(iomega/(t(att)(-1)+iomega)), where t(att) = mean value of t(att), t(cycle) = mean value of t(att) + t(det), k(stiff) = mean stiffness, and omega = 2pi x frequency of perturbation. The simulated force response due to a length step (L(hs)) was furthermore predicted by the analytical expression in the time domain, F(hs)(t)=(t(att)/t(cycle))Nk(stiff)L(hs)e(-t/t(att)). The forms of these analytically derived expressions are consistent with expressions historically used to describe these specific characteristics of a force response and suggest that the cycling of acto-myosin cross-bridges and their associated stiffnesses are responsible for the C-process and for phases 1 and 2. The rate constant 2pic, i.e., the frequency parameter of the historically defined C-process, is shown here to be equal to t(att)(-1). Experimental results from activated cardiac muscle examined at different temperatures and containing predominately alpha- or beta-myosin heavy chain isoforms were found to be consistent with the above interpretation.

Losartan prevents stretch-induced electrical remodeling in cultured atrial neonatal myocytes

Atrial fibrillation (AF) is the most frequent arrhythmia found in clinical practice. In recent studies, a decrease in the development or recurrence of AF was found in hypertensive patients treated with angiotensin-converting enzyme inhibitors or angiotensin receptor-blocking agents. Hypertension is related to an increased wall tension in the atria, resulting in increased stretch of the individual myocyte, which is one of the major stimuli for the remodeling process. In the present study, we used a model of cultured atrial neonatal rat cardiomyocytes under conditions of stretch to provide insight into the mechanisms of the preventive effect of the angiotensin receptor-blocking agent losartan against AF on a molecular level. Stretch significantly increased protein-to-DNA ratio and atrial natriuretic factor mRNA expression, indicating hypertrophy. Expression of genes encoding for the inward rectifier K(+) current (I(K1)), Kir 2.1, and Kir 2.3, as well as the gene encoding for the ultrarapid delayed rectifier K(+) current (I(Kur)), Kv 1.5, was significantly increased. In contrast, mRNA expression of Kv 4.2 was significantly reduced in stretched myocytes. Alterations of gene expression correlated with the corresponding current densities: I(K1) and I(Kur) densities were significantly increased in stretched myocytes, whereas transient outward K(+) current (I(to)) density was reduced. These alterations resulted in a significant abbreviation of the action potential duration. Losartan (1 microM) prevented stretch-induced increases in the protein-to-DNA ratio and atrial natriuretic peptide mRNA expression in stretched myocytes. Concomitantly, losartan attenuated stretch-induced alterations in I(K1), I(Kur), and I(to) density and gene expression. This prevented the stretch-induced abbreviation of action potential duration. Prevention of stretch-induced electrical remodeling might contribute to the clinical effects of losartan against AF.

Developmental changes in rat cardiac titin/connectin: transitions in normal animals and in mutants with a delayed pattern of isoform transition

Rat cardiac titin undergoes developmental changes in isoform expression during the period from late embryonic through the first 20-25 days of life. At least five size classes of titin isoforms have been identified using SDS agarose gel electrophoresis. The longest normal isoform is expressed in the embryonic stages, and it is progressively replaced with increasingly smaller versions. The isoform switching is consistent with changes in resting tension from lower values in one-day neonates to higher levels in adult myocytes. Considerable micro-heterogeneity in alternative splicing patterns also was found, particularly in the N2BA PEVK region of human, rat, and dog ventricle. A rat mutation has been identified in which the embryonic-neonatal titin isoform transitions are markedly delayed. These mutant animals may prove useful for examining the role of titin in stretch-activated signal transduction and in the Frank-Starling relationship.

Cardiac dysfunction and heart failure are associated with abnormalities in the subcellular distribution and amounts of oligomeric muscle LIM protein

Prolonged hemodynamic overload results in cardiac hypertrophy and failure with detrimental changes in myocardial gene expression and morphology. Cysteine-rich protein 3 or muscle LIM protein (MLP) is thought to be a mechanosensor in cardiac myocytes. Therefore, the subcellular location of MLP may have functional implications in health and disease. Our hypothesis is that MLP becomes mislocalized after prolonged overload, resulting in impaired mechanosensing in cardiac myocytes. Using the techniques of biochemical subcellular fractionation and immunocytochemistry, we found MLP exhibits oligomerization in the membrane and cytoskeleton of cultured cardiac rat neonatal myocytes. Nuclear MLP was always monomeric. MLP translocated to the nucleolus in response to 10% cyclic stretch at 1 Hz for 48 h. This was associated with a threefold increase in S6 ribosomal protein ( P < 0.01; n = 3 cultures). Adenoviral overexpression of MLP also resulted in a twofold increase in S6 protein, suggesting that MLP can activate ribosomal protein synthesis in the nucleolus. In ventricles from aortic-banded and myocardially infarcted rat hearts, nuclear MLP increased by twofold ( P < 0.01; n = 7) along with a significant decrease in the nonnuclear oligomeric fraction. The ratio of nuclear to nonnuclear MLP increased threefold in both groups ( P < 0.01; n = 7). In failing human hearts, there was almost a complete loss of oligomeric MLP. Using a flag-tagged adenoviral MLP, we demonstrate that the COOH terminus is required for oligomerization and that this is a precursor to stretch sensing and subsequent nuclear translocation. Therefore, reduced oligomeric MLP in the costamere and cytoskeleton may contribute to impaired mechanosensing in heart failure.

Obstructive hypertrophic cardiomyopathy is associated with reduced expression of vinculin in the intercalated disc

Mutations in the cardiac-specific insert of vinculin, metavinculin, rarely cause hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). Subsequently, a missense mutation in the ubiquitously expressed vinculin was discovered in a patient with obstructive HCM. Microscopic examination of both myectomy specimens from patients bearing genetic defects in metavinculin and vinculin showed a marked reduction of vinculin/metavinculin expression in the intercalated disc, but normal expression in the Z-disc. Given that distinct functional domains were altered by the metavinculin and vinculin mutations, we hypothesized that the intercalated disc-specific reduction of vinculin may stem from left ventricular tract obstruction in general rather than rarely observed perturbations in VCL-encoded vinculin. To test this hypothesis, we examined the localization of vinculin/metavinculin in hypertrophied human heart tissue from patients with cardiovascular conditions associated with obstruction and hemodynamic overload using an immunohistochemistry approach. Tissue specimens derived from patients with obstructive HCM and aortic stenosis (AS) showed a universal defect of vinculin/metavinculin expression in the intercalated disc but preserved expression in the cardiac Z-disc, whereas tissue specimens derived from patients with either DCM, hypertensive heart disease (HTN), or pulmonary hypertension (PHTN) exhibited normal expression of vinculin/metavinculin in both the Z- and the intercalated disc despite being associated with hypertrophy. Results of this study suggest that cardiac hypertrophy may be associated with different expression of the marker vinculin/metavinculin depending on the underlying pathophysiology; hemodynamic overload may not affect the localization whereas obstructive disease substantially reduces the expression of vinculin preferentially in the intercalated disc.

Acceleration of stretch activation in murine myocardium due to phosphorylation of myosin regulatory light chain

The regulatory light chains (RLCs) of vertebrate muscle myosins bind to the neck region of the heavy chain domain and are thought to play important structural roles in force transmission between the cross-bridge head and thick filament backbone. In vertebrate striated muscles, the RLCs are reversibly phosphorylated by a specific myosin light chain kinase (MLCK), and while phosphorylation has been shown to accelerate the kinetics of force development in skeletal muscle, the effects of RLC phosphorylation in cardiac muscle are not well understood. Here, we assessed the effects of RLC phosphorylation on force, and the kinetics of force development in myocardium was isolated in the presence of 2,3-butanedione monoxime (BDM) to dephosphorylate RLC, subsequently skinned, and then treated with MLCK to phosphorylate RLC. Since RLC phosphorylation may be an important determinant of stretch activation in myocardium, we recorded the force responses of skinned myocardium to sudden stretches of 1% of muscle length both before and after treatment with MLCK. MLCK increased RLC phosphorylation, increased the Ca(2+) sensitivity of isometric force, reduced the steepness of the force-pCa relationship, and increased both Ca(2+)-activated and Ca(2+)-independent force. Sudden stretch of myocardium during an otherwise isometric contraction resulted in a concomitant increase in force that quickly decayed to a minimum and was followed by a delayed redevelopment of force, i.e., stretch activation, to levels greater than pre-stretch force. MLCK had profound effects on the stretch activation responses during maximal and submaximal activations: the amplitude and rate of force decay after stretch were significantly reduced, and the rate of delayed force recovery was accelerated and its amplitude reduced. These data show that RLC phosphorylation increases force and the rate of cross-bridge recruitment in murine myocardium, which would increase power generation in vivo and thereby enhance systolic function.

Sarcomeric protein mutations in dilated cardiomyopathy

This review aims to provide a concise summary of the DCM associated mutations identified in the proteins of the sarcomere and cytoskeleton, and discuss the reported effects of the mutations, as determined by functional studies, and in relation to the known structure of the protein affected. The mechanisms by which single missense mutations in the proteins of the sarcomere can lead to similar diseases as those caused by mutations in the proteins of the sarcolemma and cytoskeleton, are still unknown. However, a wide variety of mutations being associated with DCM suggests a complex mechanism shared by the proteins affected. The DCM mutations reviewed here are those of the beta-myosin heavy chain (beta-MHC), myosin binding protein-C (MyBP-C), actin, alpha- tropomyosin (Tm), troponin T (TnT), troponin I (TnI), troponin C (TnC), of the sarcomere, and titin, T-cap, desmin, vinculin, and muscle LIM protein (MLP) of the cytoskeleton.

Kinetics and energetics of the crossbridge cycle

Myosin heads interacting with actin filaments, a process fueled by MgATP and regulated by calcium, powers the pump-like action of the human heart. Hydrolysis of MgATP, the competition between MgATP, its products of hydrolysis, and actin for binding to myosin, and the sequence of shifting affinities in that competition, constitute the central mechanism of muscular contraction. The force, work, and power produced during the cardiac cycle stems from an isomerization of the myosin head that is closely associated with strong binding of myosin to actin and release of phosphate. While fluctuations of intracellular [Ca2+] bound to troponin and related shifts in tropomyosin on the thin filaments regulate the number of crossbridges on a beat-to-beat basis, the oscillatory work produced is augmented by a delayed force response to stretch that develops during diastole. This stretch-activated myogenic response is facilitated by specialized myofilament structures, including actin-binding portions of the myosin essential light chain and myosin binding protein C, which are thought to guide and orient the myosin head or enhance thin filament activation. Phosphorylation of the myosin regulatory light chain, myosin binding protein C, and troponin T also assist in this regard. Animal models show isoform shifts in myosin and other myofibrillar proteins have major effects on power output, but isoform shifts in human myocardium are modest at best and are therefore likely to play only a minor role in modulating crossbridge kinetics compared to disease-related post-translational modifications of the contractile proteins and to changes in their chemical environment.

Embryonic stem cells utilize reactive oxygen species as transducers of mechanical strain-induced cardiovascular differentiation

Growing stem cells are subjected to mechanical forces, which may initiate differentiation programs. Mechanical strain stimulated cardiovascular differentiation of mouse embryonic stem (ES) cells as evaluated by quantification of contracting cardiac foci and capillary areas, respectively. Mechanical strain rapidly elevated intracellular reactive oxygen species (ROS). After 24 h up-regulation of NADPH oxidase subunits p22-phox, p47-phox, p67-phox, and Nox-4 as well as Nox-1 and Nox-4 mRNA was observed. In parallel, mechanical strain increased hypoxia-inducible factor-1alpha (HIF-1alpha) and vascular endothelial growth factor (VEGF) mRNA and protein as well as MEF2C and GATA-4 mRNA, which are involved in cardiovascular development. Furthermore, phosphorylation of extracellular-regulated kinase 1,2 (ERK1,2), p38, and c-jun N-terminal kinase (c-Jun NH2-terminal kinase (JNK)) was observed. Stimulation of cardiovascular commitment, HIF-1alpha, VEGF, and MEF2C expression as well as MAPK activation were abolished by free radical scavengers, whereas GATA-4 expression was increased. Cardiomyogenesis was inhibited by the p38 inhibitor SB203580, the ERK1,2 inhibitor UO126, and the JNK inhibitor SP600125. Vasculogenesis/angiogenesis was blunted following inhibition of ERK1,2 and JNK, whereas p38 inhibition was ineffective. Our data outline a role of ROS as mechanotransducing molecules in mechanical strain-stimulated cardiovascular differentiation of ES cells, and point toward a microenvironment of elevated ROS required for signaling cascades initiating cardiovascular differentiation programs.

Ablation of cardiac myosin-binding protein-C accelerates stretch activation in murine skinned myocardium

Cardiac myosin binding protein-C (cMyBP-C) is a thick filament accessory protein that binds tightly to myosin, but despite evidence that mutations in the cMyBP-C gene comprise a frequent cause of hypertrophic cardiomyopathy, relatively little is known about the role(s) of cMyBP-C in myocardium. Based on earlier studies demonstrating the potential importance of stretch activation in cardiac contraction, we examined the effects of cMyBP-C on the stretch activation responses of skinned ventricular preparations from wild-type (WT) and homozygous cMyBP-C knockout mice (cMyBP-C(-/-)) previously developed in our laboratory. Sudden stretch of skinned myocardium during maximal or submaximal Ca2+ activations resulted in an instantaneous increase in force that quickly decayed to a minimum and was followed by a delayed redevelopment of force (ie, stretch activation) to levels greater than prestretch force. Ablation of cMyBP-C dramatically altered the stretch activation response, ie, the rates of force decay and delayed force transient were accelerated compared with WT myocardium. These results suggest that cMyBP-C normally constrains the spatial position of myosin cross-bridges, which, in turn, limits both the rate and extent of interaction of cross-bridges with actin. We propose that ablation of cMyBP-C removes this constraint, increases the likelihood of cross-bridge binding to actin, and speeds the rate of delayed force development following stretch. Regardless of the specific mechanism, acceleration of cross-bridge cycling in cMyBP-C(-/-) myocardium could account for the abbreviation of systolic ejection in this mouse as a direct consequence of premature stretch activation of ventricular myocardium.

Muscle costameric protein, Chisel/Smpx, associates with focal adhesion complexes and modulates cell spreading in vitro via a Rac1/p38 pathway

The murine X-linked gene Chisel (Csl/Smpx) encodes a 9-kDa protein that associates in heart and skeletal muscle cells with the costameric cytoskeleton, implicated in maintaining muscle integrity and responses to biomechanical stress. After expression in C2C12 myoblasts, MYC epitope-tagged Csl co-localized with actin networks at peripheral membranes, and with focal adhesion proteins vinculin, paxillin, integrin beta1, and the small GTPase Rac1. Csl could be co-immunoprecipitated with vinculin from extracts of C2C12 cells and native muscle. MYC-Csl induced cell spreading and lamellipodia formation in C2C12 cells at the expense of filopodia, suggestive of modulation of Rac1 activity. Lamellipodia formation was indeed Rac1-dependent, and in MYC-Csl cells replated on fibronectin, Rac1 activity was increased relative to controls. Expression of MYC-Csl led to an increased association between vinculin and p34, a subunit of the Arp2/3 actin nucleation complex, a Rac1-dependent event. Induced cell spreading was also dependent upon p38 kinases that act downstream of Rac1 to control the actin capping activity of heat shock protein 27. Our data suggest that Csl localizes to the costameric cytoskeleton of muscle cells through an association with focal adhesion proteins, where it may participate in regulation of cytoskeletal dynamics through the Rac1-p38 pathway.

The Z-disc proteins myotilin and FATZ-1 interact with each other and are connected to the sarcolemma via muscle-specific filamins

Myotilin and the calsarcin family member FATZ-1 (also called calsarcin-2 or myozenin-1) are recently discovered sarcomeric proteins implicated in the assembly and stabilization of the Z-discs in skeletal muscle. The essential role of myotilin in skeletal muscle is attested by the observation that certain forms of myofibrillar myopathy and limb girdle muscular dystrophy are caused by mutations in the human myotilin gene. Here we show by transfection, biochemical and/or yeast two-hybrid assay that: (1) myotilin is able to interact with the C-terminal region of FATZ-1 and that the N- or C-terminal truncations of myotilin abrogate binding; (2) myotilin can also interact with another calsarcin member, FATZ-2 (calsarcin-1, myozenin-2); (3) myotilin and FATZ-1 bind not only to the C-terminal region of filamin-C containing the Ig repeats 19-24, but also to the other two filamins, filamin-A and filamin-B, as well as the newly identified filamin-Bvar-1variant; (4) the binding of myotilin to filamin-C involves binding sites in its N-terminal region, whereas FATZ-1 associates with filamin-C via sequences within either its N- or C-terminal region; and finally, (5) the C-terminal region of filamin-C like filamin-B and filamin-Bvar-1, shows binding activity with the β1A integrin subunit. Our findings further dissect the molecular interactions within the Z-disc that are essential for its organization, and provide evidence for a novel connection between Z-disc proteins and the sarcolemma via filamins and β1 integrins. These data shed new light on the complex organization of the Z-disc that is highly relevant to understanding muscular dystrophies.

Mechanical signals regulating extracellular matrix gene expression in fibroblasts

Mechanical forces are essential for connective tissue homeostasis. The extracellular matrix (ECM) plays a key role in the transmission of forces generated by the organism (e.g. muscle contraction) and externally applied (e.g. gravity). The expression of specific ECM proteins such as collagens and tenascin-C, as well as of matrix metalloproteinases, involved in their turnover, is influenced by mechanical stimuli. The precise mechanisms by which mechanical strains are translated into chemical signals and lead to differential gene expression are however not fully understood. Cell-matrix adhesion sites are good candidates for hosting a "mechanosensory switch", as they transmit forces from the ECM to the cytoskeleton and vice versa by physically linking the cytoskeleton to the ECM. Integrins, transmembrane proteins located to these adhesion sites, have been shown to trigger a set of internal signaling cascades after mechanical stimulation. We have shown that the expression level of tenascin-C directly correlates with externally applied mechanical stress, as well as with RhoA/RhoA-dependent kinase-mediated cytoskeletal tension. Presumably other genes are regulated in a similar manner. The changes in ECM composition and mechanical properties derived from mechanical stress are relevant in medical intervention after ligament and tendon injury.

RhoA/ROCK signaling is critical to FAK activation by cyclic stretch in cardiac myocytes

Focal adhesion kinase (FAK) has been shown to be activated in cardiac myocytes exposed to mechanical stress. However, details of how mechanical stimuli induce FAK activation are unknown. We investigated whether signaling events mediated by the RhoA/Rho-associated coiled coil-containing kinase (ROCK) pathway are involved in regulation of stretch-induced FAK phosphorylation at Tyr(397) in neonatal rat ventricular myocytes (NRVMs). Immunostaining showed that RhoA localized to regions of myofilaments alternated with phalloidin (actin) staining. The results of coimmunoprecipitation assays indicated that FAK and RhoA are associated in nonstretched NRVMs, but cyclic stretch significantly reduced the amount of RhoA recovered from anti-FAK immunoprecipitates. Cyclic stretch induced rapid and sustained (up to 2 h) increases in phosphorylation of FAK at Tyr(397) and ERK1/2 at Thr(202)/Tyr(204). Blockade of RhoA/ROCK signaling by pharmacological inhibitors of RhoA (Clostridium botulinum C3 exoenzyme) or ROCK (Y-27632, 10 micromol/l, 1 h) markedly attenuated stretch-induced FAK and ERK1/2 phosphorylation. Similar effects were observed in cells treated with the inhibitor of actin polymerization cytochalasin D. Transfection of NRVMs with RhoA antisense oligonucleotide attenuated stretch-induced FAK and ERK1/2 phosphorylation and expression of beta-myosin heavy chain mRNA. Similar results were seen in cells transfected with FAK antisense oligonucleotide. These findings demonstrate that RhoA/ROCK signaling plays a crucial role in stretch-induced FAK phosphorylation, presumably by coordinating upstream events operationally linked to the actin cytoskeleton.

Mechanical force-activated phospholipase D is mediated by Galpha12/13-Rho and calmodulin-dependent kinase in renal epithelial cells

The renal glomerulus, the site of plasma ultrafiltration, is exposed to mechanical force in vivo arising from capillary blood pressure and fluid flow. Studies of cultured podocytes demonstrate that they respond to stretch by altering the structure of the actin cytoskeleton, but the mechanisms by which physical force triggers this architectural change and the signaling pathways that lead to generation of second messengers are not defined. In the present study, we found that in renal epithelial cells [podocytes and Madin-Darby canine kidney (MDCK) cells], application of mechanical force to the cell surface through fibronectin-coated ferric beads and exposure of the cells to magnetic force lead to Rho translocation and actin cytoskeleton reorganization. This application of force recruited Rho and filamentous actin (F-actin) to bead loci and subsequently stimulated phospholipase D (PLD), a downstream effector of Rho. Using MDCK cells that stably express regulators of G protein-signaling (RGS) proteins [RGS4 attenuates Galpha(i) and Galpha(q), and the p115RhoGEF-RGS domain (p115-RGS) attenuates Galpha(12/13)] to define the signaling pathway, we found that mechanical force induced Galpha(12/13)-Rho activation and increased F-actin to stimulate PLD activity. The activation can be partially prevented by the C(3) exoenzyme. Pretreatment of the cells with chemical inhibitors of several kinases showed that calmodulin-dependent kinase is also involved in stretch-induced PLD activation by a separate pathway. Taken together, our data demonstrate that in cultured podocytes and MDCK cells, mechanical force leads to actin cytoskeleton reorganization and PLD activation. The signaling pathways for PLD activation involve Galpha(12/13)/Rho/F-actin and calmodulin-dependent kinase.