Actin Bundles Dynamics and Architecture

Cells use the actin cytoskeleton for many of their functions, including their division, adhesion, mechanosensing, endo- and phagocytosis, migration, and invasion. Actin bundles are the main constituent of actin-rich structures involved in these processes. An ever-increasing number of proteins that crosslink actin into bundles or regulate their morphology is being identified in cells. With recent advances in high-resolution microscopy and imaging techniques, the complex process of bundles formation and the multiple forms of physiological bundles are beginning to be better understood. Here, we review the physiochemical and biological properties of four families of highly conserved and abundant actin-bundling proteins, namely, α-actinin, fimbrin/plastin, fascin, and espin. We describe the similarities and differences between these proteins, their role in the formation of physiological actin bundles, and their properties-both related and unrelated to their bundling abilities. We also review some aspects of the general mechanism of actin bundles formation, which are known from the available information on the activity of the key actin partners involved in this process.

Cortical Proteins Associated With Cognitive Resilience in Community-Dwelling Older Persons

Importance

Identifying genes and proteins for cognitive resilience (ie, targets that may be associated with slowing or preventing cognitive decline regardless of the presence, number, or combination of common neuropathologic conditions) provides a complementary approach to developing novel therapeutics for the treatment and prevention of Alzheimer disease and related dementias.

Objective

To identify proteins associated with cognitive resilience via a proteome-wide association study of the human dorsolateral prefrontal cortex.

Design, Setting, and Participants

This study used data from 391 community-dwelling older persons who participated in the Religious Orders Study and the Rush Memory and Aging Project. The Religious Orders Study began enrollment January 1, 1994, and the Rush Memory and Aging Project began enrollment September 1, 1997, and data were collected and analyzed through October 23, 2019.

Exposures

Participants had undergone annual detailed clinical examinations, postmortem evaluations, and tandem mass tag proteomics analyses.

Main Outcomes and Measures

The outcome of cognitive resilience was defined as a longitudinal change in cognition over time after controlling for common age-related neuropathologic indices, including Alzheimer disease, Lewy bodies, transactive response DNA-binding protein 43, hippocampal sclerosis, infarcts, and vessel diseases. More than 8000 high abundance proteins were quantified from frozen dorsolateral prefrontal cortex tissue using tandem mass tag and liquid chromatography-mass spectrometry.

Results

There were 391 participants (273 women); their mean (SD) age was 79.7 (6.7) years at baseline and 89.2 (6.5) years at death. Eight cortical proteins were identified in association with cognitive resilience: a higher level of NRN1 (estimate, 0.140; SE, 0.024; P = 7.35 × 10-9), ACTN4 (estimate, 0.321; SE, 0.065; P = 9.94 × 10-7), EPHX4 (estimate, 0.198; SE, 0.042; P = 2.13 × 10-6), RPH3A (estimate, 0.148; SE, 0.031; P = 2.58 × 10-6), SGTB (estimate, 0.211; SE, 0.045; P = 3.28 × 10-6), CPLX1 (estimate, 0.136; SE, 0.029; P = 4.06 × 10-6), and SH3GL1 (estimate, 0.179; SE, 0.039; P = 4.21 × 10-6) and a lower level of UBA1 (estimate, -0.366; SE, 0.076; P = 1.43 × 10-6) were associated with greater resilience.

Conclusions and Relevance

These protein signals may represent novel targets for the maintenance of cognition in old age.

Intra- and intermuscular variations of postmortem protein degradation for PMI estimation

In recent years, protein decomposition has become of increasing interest for the use in forensic estimation of the postmortem interval (PMI). Especially skeletal muscle tissue has proven to be a prime target tissue, among other reasons, due to its large abundance in the human body. In this regard, it is important to know whether there are any intra- and intermuscular differences in the behavior of protein degradation. Thus, samples from different locations within several skeletal muscles as well as from cardiac and smooth muscle tissue samples were collected from three autopsy cases with varying degree of decomposition. Samples were analyzed by SDS-PAGE and Western blotting and compared for protein degradation patterns. Intramuscular variations turned out to be minimal and without major influence for the use of the method. Observed intermuscular differences provide possibilities for future improvement of the precision and temporal application range. The results of this study show the strengths and current limitations of protein degradation-based PMI estimation and provide a deeper understanding of intraindividual postmortem protein degradation processes.

Housekeeping proteins: How useful are they in skeletal muscle diabetes studies and muscle hypertrophy models?

The use of Western blot analysis is of great importance in research, and the measurement of housekeeping proteins is commonly used for loading controls. However, Ponceau S staining has been shown to be an alternative to analysis of housekeeping protein levels as loading controls in some conditions. In the current study, housekeeping protein levels were measured in skeletal muscle hypertrophy and streptozotocin-induced diabetes experimental models. The following housekeeping proteins were investigated: glyceraldehyde-3-phosphate dehydrogenase (GAPDH), β-actin, α-tubulin, γ-tubulin, and α-actinin. Evidence is presented that Ponceau S is more reliable than housekeeping protein levels for specific protein quantifications in Western blot analysis.

The alternatively spliced actinin-4 variant as a prognostic marker for metastasis in small-cell lung cancer

BACKGROUND

The alternatively spliced actinin-4 variant (ACTN4va) is expressed in small-cell lung cancer (SCLC) and is thought to be a potential diagnostic marker. However, ACTN4va expression has not been examined in transbronchial biopsy specimens.

MATERIALS AND METHODS

We retrospectively examined the relationship between ACTN4va expression, clinical factors and survival in 104 consecutive newly-diagnosed SCLC patients.

RESULTS

Of the 104 screened cases, 83 (median age=69 years; transbronchial biopsy, 71) were included in our study. Survival was significantly different in the group with no distant metastasis (1996 vs. 422 days, respectively; p=0.000115) but was not significantly different with regard to ACTN4va expression in the group with distant metastasis (293 vs. 254 days, respectively; p=0.678).

CONCLUSION

ACTN4va expression was identifiable in small biopsy samples. ACTN4va expression was also significantly related to distant metastasis and could stratify SCLC patients according to prognosis.

Possible role of non-muscle alpha-actinins in muscle cell mechanosensitivity

The main hypothesis suggested that changes in the external mechanical load would lead to different deformations of the submembranous cytoskeleton and, as a result, dissociation of different proteins from its structure (induced by increased/decreased mechanical stress). The study subjects were fibers of the soleus muscle and cardiomyocytes of Wistar rats. Changes in external mechanical conditions were reconstructed by means of antiorthostatic suspension of the animals by their tails for 6, 12, 18, 24 and 72 hours. Transversal stiffness was measured by atomic force microscopy imaging; beta-, gamma-actin, alpha-actinin 1 and alpha-actinin 4 levels in membranous and cytoplasmic fractions were quantified by Western blot analysis; expression rates of the corresponding genes were studied using RT-PCR.

RESULTS

In 6 hours, alpha-actinin 1 and alpha-actinin 4 levels decreased in the membranous fraction of proteins of cardiomyocytes and soleus muscle fibers, respectively, but increased in the cytoplasmic fraction of the abovementioned cells. After 6-12 hours of suspension, the expression rates of beta-, gamma-actin, alpha-actinin 1 and alpha-actinin 4 were elevated in the soleus muscle fibers, but the alpha-actinin 1 expression rate returned to the reference level in 72 hours. After 18-24 hours, the expression rates of beta-actin and alpha-actinin 4 increased in cardiomyocytes, while the alpha-actinin 1 expression rate decreased in soleus muscle fibers. After 12 hours, the beta- and gamma-actin content dropped in the membranous fraction and increased in the cytoplasmic protein fractions from both cardiomyocytes and soleus muscle fibers. The stiffness of both cell types decreased after the same period of time. Further, during the unloading period the concentration of nonmuscle actin and different isoforms of alpha-actinins increased in the membranous fraction from cardiomyocytes. At the same time, the concentration of the abovementioned proteins decreased in the soleus muscle fibers.

Differential expressions of BMPR1α, ACTN4α and FABP7 in Hirschsprung disease

Hirschsprung disease (HSCR) is characterized by the absence of intramural ganglion cells in the nerve plexuses of the distal gut. Recent studies have shown that the bone morphogenetic protein receptor-type IA (BMPR1α), actinin-alpha 4 (ACTN4α) and fatty acid binding protein 7 (FABP7) play important roles in the differentiation and development of neurons. The aganglionic (stenotic) and the ganglionic (normal) colon segment tissues of 60 HSCR patients were collected to investigate the expression pattern of BMPR1α, ACTININ-4α and FABP7 using RT-PCR, quantitative real-time RT-PCR (qRT-PCR) and immunohistochemical staining. The mRNA and protein expressions of BMPR1α and ACTN4α were higher in the stenotic colon segment tissue than those in the normal colon segment tissue. However, the mRNA and protein expressions of FABP7 were lower in the stenotic colon segment tissue than those in the normal colon segment tissue. The study in HSCR patients, findings in mRNA and protein alterations to expecting provide more information to in order to find some clue for the pathomechanism of HSCR disease.

[Expression of nephrin, podocin and a-actinine-4 in renal tissue of patients with proteinuria]

INTRODUCTION

Slit diaphragm and/or podocyte's cytoskeleton alterations are related to proteinuria and nephrotic syndrome. In our population, focal and segmental glomerulosclerosis causing nephrotic syndrome is the more frequent biopsy demonstrated glomerulopathy. Our aim was search for alterations in some slit diaphragm-associated proteins in patients with nephrotic range proteinuria.

METHODS

Renal tissue from 40 patients with nephrotic range proteinuria, 10 patients with non-nephrotic proteinuria, 3 with isolated hematuria, and 10 samples of normal renal tissue (deceased donors) were studied, by indirect immunofluorescence, for expression of nephrin, podocin, and alpha-actinin-4.

RESULTS

Expression of these proteins was lineal, homogeneous, in the glomerular capillary walls in normal renal tissue and in patients with isolated hematuria. In nephrotic proteinuria this normal appearance was altered and immunostaining showed a fine granular appearance. In 18 cases (45%) of patients with nephrotic proteinuria and 3 cases (30%) of patients with non-nephrotic proteinuria there was loss of at least one of these proteins (p = 0.49). These alterations were found in the diverse glomerulopathies more frequently causing nephrotic syndrome.

CONCLUSIONS

In nephrotic range proteinuria redistribution or loss of expression of slit diaphragm-associated proteins is very frequent. In many of our cases this fact could be more a consequence than a cause of proteinuria. These alterations can be also evidenced in patients with non-nephrotic proteinuria.

Movement of stress fibers away from focal adhesions identifies focal adhesions as sites of stress fiber assembly in stationary cells

Force generated in contractile actin filament bundles (stress fibers-SFs) is transmitted to the extracellular matrix (ECM) via linker proteins and transmembrane integrins at focal adhesions (FAs). Though it has long been known that actin is rapidly exchanged in FAs, the connection between SFs and FAs has not been studied in detail. We introduced fiduciary marks on SFs by expressing GFP-palladin or GFP-alpha-actinin-1, which are both FA and dense body proteins, and by pattern bleaching of GFP-actin. Following fiduciary marks on SFs over time by time-lapse fluorescence microscopy, we detected assembly of SFs at FAs in stationary cells resulting in movement of SFs away from FAs with a velocity of 0.2-0.4 microm/min. Visualization of FAs in GFP-palladin/DsRed-paxillin double transfected cells showed that SF elongation was not accompanied by a change in FA length. SF elongation at FAs depended on actin polymerization and force as demonstrated by inhibitors of actin polymerization (cytochalasin D, jasplakinolide) and inhibitors of myosin-dependent contraction (blebbistatin, Y-27632), respectively. Our finding of SF assembly at FAs has important implications for SF formation, force transmission, and tension distribution within the actin cytoskeletal network of stationary cells.

Clusterin isoforms differentially affect growth and motility of prostate cells: possible implications in prostate tumorigenesis

Besides a fully processed, secreted form of clusterin (sCLU), an alternative proapoptotic form of the protein targeting the nucleus (nCLU) was recently described. The possible differential roles played by the two clusterin forms in growth and motility of nonmalignant and malignant prostate cells are investigated here. sCLU or nCLU was transiently transfected in both androgen-independent prostate cancer cells (PC3 and DU 145) and immortalized prostate epithelial cells (PNT1A, a nontumoral control). Then, cell growth, motility, and cytoskeleton organization were studied. We found that (a) in PNT1A cells, both sCLU and nCLU significantly decreased cell proliferation and motility; (b) in PC3 and DU 145 cancer cells, only nCLU inhibited cell growth and migration, with sCLU being ineffective; and (c) the antimotility effect of nCLU was accompanied by a dramatic dismantling of the actin cytoskeleton. Moreover, transfection with "full-length" CLU cDNA produced both sCLU and nCLU in nonmalignant PNT1A cells, whereas only sCLU was found in cancer cells. Thus, CLU gene expression might play a crucial role in prostate tumorigenesis by exerting differential biological effects on normal versus tumor cells through differential processing of CLU isoforms in the two cell systems. We also found that nCLU binds to alpha-actinin, a key protein for the regulation of actin cytoskeleton, and that nCLU and alpha-actinin colocalize in the cytoplasm. Thus, the antimotility activity of nCLU and its ability to cause dismantling of the actin cytoskeleton seem to be mediated by its binding to alpha-actinin.

Actin filament organization of foot processes in vertebrate glomerular podocytes

We investigated the actin filament organization and immunolocalization of actin-binding proteins (alpha-actinin and cortactin) in the podocyte foot processes of eight vertebrate species (lamprey, carp, newt, frog, gecko, turtle, quail, and rat). Three types of actin cytoskeleton were found in these foot processes. (1) A cortical actin network with cortactin filling the space between the plasma membrane and the other actin cytoskeletons described below was found in all of the species examined here. The data indicated that the cortical actin network was the minimal essential actin cytoskeleton for the formation and maintenance of the foot processes in vertebrate podocytes. (2) An actin bundle with alpha-actinin existing along the longitudinal axis of foot process above the level of slit diaphragms was only observed in quail and rat. (3) An actin fascicle consisting of much fewer numbers of actin filaments than that of the actin bundle was observed in the species other than quail and rat, but at various frequencies. These findings suggest that the actin bundle is an additional actin cytoskeleton reflecting a functional state peculiar to quail and rat glomeruli. Considering the higher intraglomerular pressure and the extremely thin filtration barrier in birds and mammals, the foot processes probably mainly protect the thinner filtration barrier from the higher internal pressure occurring in quail and rat glomeruli. Therefore, we consider that the actin bundle plays a crucial role in the mechanical protection of the filtration barrier. Moreover, the actin fascicle may be a potential precursor of the actin bundle.

Linker region of nebulin family members plays an important role in targeting these molecules to cellular structures

The nebulin family of actin-binding proteins plays an essential role in cytoskeletal dynamics and actin filament stability. All of the family members are modular proteins with their key defining structural feature being the presence of the 35-residue nebulin modules. The family members now include nebulin, nebulette, N-RAP, LASP-1, and LIM-nebulette. Nebulin and nebulette are associated with the thin filament/Z-line junction of striated muscle. LASP-1 and LIM-nebulette are found within focal adhesions, and N-RAP is associated with muscle cellular junctions. Although much investigation has focused on the role of the interactions between nebulin modules and actin, each of these proteins contains other domains that are essential for their cellular targeting and functions. The serine-rich linker region of nebulette has previously been shown to serve just such a purpose by targeting the association of the nebulin modules to the cardiac Z-line in cultured cardiomyocytes. In this report, we analyze the targeting functions of the homologous regions of LASP-1 and LIM-nebulette in their incorporation into focal adhesions. We have found that the linker region of LASP-1 is indeed important for its cellular localization and that the shortened linker region of LIM-nebulette drives the association of nebulin modules to focal adhesions.

Label-free Quantitative Proteomics Using Large Peptide Data Sets Generated by Nanoflow Liquid Chromatography and Mass Spectrometry

We developed an integrated platform consisting of machinery and software modules that can apply vast amounts of data generated by nanoflow LC-MS to differential protein expression analyses. Unlabeled protein samples were completely digested with modified trypsin and separated by low speed (200 nl/min) one-dimensional HPLC. Mass spectra were obtained every 1 s by using the survey mode of a hybrid Q-TOF mass spectrometer and displayed in a two-dimensional plane with m/z values along the x axis, and retention time was displayed along the y axis. The time jitter of nano-LC was adjusted using newly developed software based on a dynamic programming algorithm. The comprehensiveness (60,000-160,000 peaks above the predetermined threshold detectable in 60-microg cell protein samples), reproducibility (average coefficient of variance of 0.35-0.39 and correlation coefficient of over 0.92 between duplicates), and accurate quantification with a wide dynamic range (over 10(3)) of our platform warrant its application to various types of experimental and translational proteomics.

Probing the role of septins in cardiomyocytes

Heart growth in the embryo is achieved by division of differentiated cardiomyocytes. Around birth, cardiomyocytes stop dividing and heart growth occurs only by volume increase of the individual cells. Cardiomyocytes seem to lose their capacity for cytokinesis at this developmental stage. Septins are GTP-binding proteins that have been shown to be involved in cytokinesis from yeast to vertebrates. We wanted to determine whether septin expression patterns can be correlated to the cessation of cytokinesis during heart development. We found significant levels of expression only for SEPT2, SEPT6, SEPT7 and SEPT9 in heart, in a developmentally regulated fashion, with high levels in the embryonic heart, downregulation around birth and no detectable expression in the adult. In dividing embryonic cardiomyocytes, all septins localize to the cleavage furrow. We used drugs to probe for the functional interactions of SEPT2 in dividing embryonic cardiomyocytes. Differences in the effects on subcellular septin localization in cardiomyocytes were observed, depending whether a Rho kinase (ROCK) inhibitor was used or whether actin and myosin were targeted directly. Our data show a tight correlation of high levels of septin expression and the ability to undergo cytokinesis in cardiomyocytes. In addition, we were able to dissect the different contributions of ROCK signaling and the actomyosin cytoskeleton to septin localization to the contractile ring using cardiomyocytes as an experimental system.

Stress fibers are generated by two distinct actin assembly mechanisms in motile cells

Stress fibers play a central role in adhesion, motility, and morphogenesis of eukaryotic cells, but the mechanism of how these and other contractile actomyosin structures are generated is not known. By analyzing stress fiber assembly pathways using live cell microscopy, we revealed that these structures are generated by two distinct mechanisms. Dorsal stress fibers, which are connected to the substrate via a focal adhesion at one end, are assembled through formin (mDia1/DRF1)–driven actin polymerization at focal adhesions. In contrast, transverse arcs, which are not directly anchored to substrate, are generated by endwise annealing of myosin bundles and Arp2/3-nucleated actin bundles at the lamella. Remarkably, dorsal stress fibers and transverse arcs can be converted to ventral stress fibers anchored to focal adhesions at both ends. Fluorescence recovery after photobleaching analysis revealed that actin filament cross-linking in stress fibers is highly dynamic, suggesting that the rapid association–dissociation kinetics of cross-linkers may be essential for the formation and contractility of stress fibers. Based on these data, we propose a general model for assembly and maintenance of contractile actin structures in cells.

Insulin-dependent interactions of proteins with GLUT4 revealed through stable isotope labeling by amino acids in cell culture (SILAC)

The insulin-regulated glucose transporter (GLUT4) translocates to the plasma membrane in response to insulin in order to facilitate the postprandial uptake of glucose into fat and muscle cells. While early insulin receptor signaling steps leading to this translocation are well defined, the integration of signaling and regulation of GLUT4 traffic remains elusive. Several lines of evidence suggest an important role for the actin cytoskeleton and for protein-protein interactions in regulating GLUT4 localization by insulin. Here, we applied stable isotope labeling by amino acids in cell culture (SILAC) to identify proteins that interact with GLUT4 in an insulin-regulated manner. Myc-tagged GLUT4 (GLUT4myc) stably expressed in L6 myotubes was immunoprecipitated via the myc epitope from total membranes isolated from basal and insulin-stimulated cells grown in medium containing normal isotopic abundance leucine or deuterated leucine, respectively. Proteins coprecipitating with GLUT4myc were analyzed by liquid chromatography/ tandem mass spectrometry. Of 603 proteins quantified, 36 displayed an insulin-dependent change of their interaction with GLUT4myc of more than 1.5-fold in either direction. Several cytoskeleton-related proteins were elevated in immunoprecipates from insulin-treated cells, whereas components of the ubiquitin-proteasome degradation system were generally reduced. Proteins participating in vesicle traffic also displayed insulin-regulated association. Of cytoskeleton-related proteins, alpha-actinin-4 recovery in GLUT4 immunoprecipitates rose in response to insulin 2.1 +/- 0.5-fold by SILAC and 2.9 +/- 0.8-fold by immunoblotting. Insulin caused GLUT4 and alpha-actinin-4 co-localization as revealed by confocal immunofluorescence microscopy. We conclude that insulin elicits changes in interactions between diverse proteins and GLUT4, and that cytoskeletal proteins, notably alpha-actinin-4, associate with the transporter, potentially to facilitate its routing to the plasma membrane.

Granulocyte colony-stimulating factor and stem cell factor improve endogenous repair after myocardial infarction

OBJECTIVE

The aims of this study were, first, to determine if granulocyte colony-stimulating factor (G-CSF) and stem cell factor (SCF) improved left ventricular function in the setting of a reperfusion model of myocardial infarction (MI) and, second, to evaluate the effects of G-CSF/SCF on cellular repair and, in particular, the fate of bone marrow cells homing to the site of tissue injury.

METHODS

MI was induced in mice by transient ligation of the left descending coronary artery. G-CSF/SCF were administered for 5 days after MI. Cardiac function was assessed 28 days after MI. The effect of G-CSF/SCF on the cellular composition of the infarct region was assessed by immunohistochemistry. MI was performed in mice reconstituted with bone marrow cells expressing DsRed to track the fate of bone marrow-derived cells within the infarct region.

RESULTS

G-CSF/SCF-treated mice had significantly improved left ventricular (LV) function as determined by LV developed pressure, LV+/-dp/dt(max/min), and LV end-diastolic pressure. G-CSF alone produced similar improvements in cardiac function. These improvements in LV function were associated with 70% more blood vessels and a doubling of cells expressing cardiomyocyte-specific transcription factors GATA-4, Nkx2.5 and alpha-actinin cells within the infarct zone. Cells within the infarct expressing stromal-derived factor also increased by 200%. To elucidate the origin of these cells, bone marrow chimeras, where hematopoietic cells expressed the fluorescent marker DsRed, were treated with G-CSF/SCF after MI. Bone marrow-derived, DsRed-expressing cells in the infarct region of G-CSF/SCF-treated chimeras increased by an average of 12-fold; however, the vast majority of DsRed cells expressed the hematopoietic-specific marker CD45 but not blood vessel or cardiomyocyte markers.

CONCLUSIONS

G-CSF/SCF therapy improved cardiac function when delivered after MI, increasing the number of blood vessels and cells of cardiomyogenic lineage. However, these cells were of myocardial rather than bone marrow origin.

Tensile properties of single stress fibers isolated from cultured vascular smooth muscle cells

Stress fibers (SFs), a contractile bundle of actin filaments, play a critical role in mechanotransduction in adherent cells; yet, the mechanical properties of SFs are poorly understood. Here, we measured tensile properties of single SFs by in vitro manipulation with cantilevers. SFs were isolated from cultured vascular smooth muscle cells with a combination of low ionic-strength extraction and detergent extraction and were stretched until breaking. The breaking force and the Young's modulus (assuming that SFs were isotropic) were, on average, 377 nN and 1.45 MPa, which were approximately 600-fold greater and three orders of magnitude lower, respectively, than those of actin filaments reported previously. Strain-induced stiffening was observed in the force-strain curve. We also found that the extracted SFs shortened to approximately 80% of the original length in an ATP-independent manner after they were dislodged from the substrate, suggesting that SFs had preexisting strain in the cytoplasm. The force required for stretching the single SFs from the zero-stress length back to the original length was approximately 10 nN, which was comparable with the traction force level applied by adherent cells at single adhesion sites to maintain cell integrity. These results suggest that SFs can bear intracellular stresses that may affect overall cell mechanical properties and will impact interpretation of intracellular stress distribution and force-transmission mechanism in adherent cells.

Targeting of a novel Ca+2/calmodulin-dependent protein kinase II is essential for extracellular signal-regulated kinase-mediated signaling in differentiated smooth muscle cells

Subcellular targeting of kinases controls their activation and access to substrates. Although Ca2+/calmodulin-dependent protein kinase II (CaMKII) is known to regulate differentiated smooth muscle cell (dSMC) contractility, the importance of targeting in this regulation is not clear. The present study investigated the function in dSMCs of a novel variant of the gamma isoform of CaMKII that contains a potential targeting sequence in its association domain (CaMKIIgamma G-2). Antisense knockdown of CaMKIIgamma G-2 inhibited extracellular signal-related kinase (ERK) activation, myosin phosphorylation, and contractile force in dSMCs. Confocal colocalization analysis revealed that in unstimulated dSMCs CaMKIIgamma G-2 is bound to a cytoskeletal scaffold consisting of interconnected vimentin intermediate filaments and cytosolic dense bodies. On activation with a depolarizing stimulus, CaMKIIgamma G-2 is released into the cytosol and subsequently targeted to cortical dense plaques. Comparison of phosphorylation and translocation time courses indicates that, after CaMKIIgamma G-2 activation, and before CaMKIIgamma G-2 translocation, vimentin is phosphorylated at a CaMKII-specific site. Differential centrifugation demonstrated that phosphorylation of vimentin in dSMCs is not sufficient to cause its disassembly, in contrast to results in cultured cells. Loading dSMCs with a decoy peptide containing the polyproline sequence within the association domain of CaMKIIgamma G-2 inhibited targeting. Furthermore, prevention of CaMKIIgamma G-2 targeting led to significant inhibition of ERK activation as well as contractility. Thus, for the first time, this study demonstrates the importance of CaMKII targeting in dSMC signaling and identifies a novel targeting function for the association domain in addition to its known role in oligomerization.

Drosophila non-muscle alpha-actinin is localized in nurse cell actin bundles and ring canals, but is not required for fertility

The single copy Drosophila alpha-actinin gene is alternatively spliced to generate three different isoforms that are expressed in larval muscle, adult muscle and non-muscle cells, respectively. We have generated novel alpha-actinin alleles, which specifically remove the non-muscle isoform. Homozygous mutant flies are viable and fertile with no obvious defects. Using a monoclonal antibody that recognizes all three splice variants, we compared alpha-actinin distribution in wild type and mutant embryos and ovaries. We found that non-muscle alpha-actinin was present in young embryos and in the embryonic central nervous system. In ovaries, non-muscle alpha-actinin was localized in the nurse cell subcortical cytoskeleton, cytoplasmic actin cables and ring canals. In the mutant, alpha-actinin expression remained in muscle tissues, but also in a subpopulation of epithelial cells in both embryos and ovaries. This suggests that various populations of non-muscle cells regulate alpha-actinin expression in different ways. We also show that ectopically expressed adult muscle-specific alpha-actinin localizes to all F-actin containing structures in the nurse cells in the absence of endogenous non-muscle alpha-actinin.

The effect of calcium ions on subcellular localization of aldolase-FBPase complex in skeletal muscle

In skeletal muscles, FBPase-aldolase complex is located on alpha-actinin of the Z-line. In the present paper, we show evidence that stability of the complex is regulated by calcium ions. Real time interaction analysis, confocal microscopy and the protein exchange method have revealed that elevated calcium concentration decreases association constant of FBPase-aldolase and FBPase-alpha-actinin complex, causes fast dissociation of FBPase from the Z-line and slow accumulation of aldolase within the I-band and M-line. Therefore, the release of Ca2+ during muscle contraction might result, simultaneously, in the inhibition of glyconeogenesis and in the acceleration of glycolysis.

Osteoblast alignment, elongation and migration on grooved polystyrene surfaces patterned by Langmuir–Blodgett lithography

Topographically patterned surfaces are known to influence cellular behavior in a controllable manner. However, the relatively large surface areas (several cm2) required for many biomaterial applications are beyond the practical limits of traditional lithography. Langmuir-Blodgett lithography, a recently developed method, was used to fabricate regularly spaced grooves of different depths (50 and 150 nm) with a periodicity of 500 nm over several square centimeter on silicon surfaces. These topographies were transferred into polystyrene surfaces by means of nanoimprinting. Primary osteoblasts were cultured on the patterned polymer surfaces. They were observed to align, elongate and migrate parallel to the grooves. The combination of Langmuir-Blodgett lithography with nanoimprinting enables the fabrication of large, nanostructured surface areas on a wide spectrum of different biomaterials. Osteoblasts show a significant anisotropic behavior to these surfaces, which can enhance cell settlement on the surface or be used to direct tissue generation on the biomaterial interface.

Morphological, electrophysiological and coupling characteristics of bone marrow-derived mononuclear cells?an in vitro-model

OBJECTIVE

Transplantation of bone marrow-derived mononuclear cells (BMC) may be suitable to prevent myocardial remodeling and improve left ventricular function after myocardial infarction. However, it is unknown whether or not cardiomyocytes and BMCs can form functioning cell-to-cell coupling and develop adequate electrophysiological properties.

METHODS

BMCs were isolated from minipig leg bones, treated with 5-azacytidine (10 microM) for 24 h, cultured for 7 days and labeled with a fluoroscopic dye (DIL). BMCs were cocultured with spontaneously beating cardiomyocytes of neonatal rats. On days 4, 7 and 14 cocultured cells were analyzed. Immunhistochemistry (Connexin 43, alpha-actinin) was used to assess cardiomyogenic differentiation. Action potential characteristics were recorded in whole cell patch clamp mode and to investigate intercellular communication a second gap junction permeable fluoroscopic dye was brought into BMCs by microinjection (Lucifer yellow, LY).

RESULTS

From day 7 in coculture BMCs beated synchronously with neonatal rat cardiomyocytes. On day 14, 55.9% of BMCs expressed alpha-actinin and 98.3% were positive for gap junction protein connexin 43. BMC action potential duration (APD90) was mean 11.1 ms with dV/dt(max) of 26.8 V/s and similar to atrial cardiac type. However, microinjection of LY revealed only little dye transfer into adjacent rat cardiomyocytes.

CONCLUSIONS

Cocultured BMCs have the potential for early expression of muscle specific proteins in about 60% after 14 days and for cardiac gap junction proteins. Synchronous beating indicates an effective electromechanical coupling. In this heterologous setting we could prove only weak metabolic coupling.

Improvement of an in vitro stem cell assay for developmental toxicity: the use of molecular endpoints in the embryonic stem cell test

The embryonic stem cell test (EST) takes advantage of the potential of murine embryonic stem (ES) cells to differentiate in culture to test embryotoxicity in vitro. The EST represents a reliable, scientifically validated in vitro system for the classification of compounds according to their teratogenic potential based on the morphological analysis of beating cardiomyocytes in embryoid body (EB) outgrowths compared to cytotoxic effects on undifferentiated murine ES cells and differentiated 3T3 fibroblasts. In order to identify more objective endpoints of differentiation other than the microscopic evaluation of "beating areas" and to adapt the EST to applications in high-throughput screening systems we improved and expanded the EST protocol by establishing molecular endpoints of differentiation. The quantitative expression of sarcomeric myosin heavy chain (MHC) and alpha-actinin genes under the influence of test compounds was studied employing intracellular flow cytometry. Strong embryotoxicants exerted a dose-dependent effect on both the expression levels of MHC and alpha-actinin and the differentiation into beating cardiomyocytes. Furthermore, quantitative FACS (fluorescence-activating cell sorting) analysis showed the same sensitivity for the classification of substances as the conventional endpoint but allowed a significant reduction of the test period. Within 7 days, maximal expression of sarcomeric marker proteins was observed. Our findings indicate that structural proteins of the sarcomere apparatus, alpha-actinin and myosin heavy chain (MHC), seem to be promising candidates to predict developmental toxicity in vivo from in vitro data. Thus, the improved EST holds promise as a new predictive screen for risk assessment with respect to developmental toxicity using stem cell technology and technological advances in the field of gene expression analysis.

Expression of alpha-actinin-4 in acquired human nephrotic syndrome: a quantitative immunoelectron microscopy study

BACKGROUND

The molecular basis for the alteration of glomerular podocyte phenotype in nephrotic syndrome probably involves adaptive changes of the actin cytoskeleton. alpha-Actinin-4 is an actin cross-linking protein that also interacts with intra- and intercellular adhesion molecules and elements of the transmembrane signal transduction pathway and is implicated in nephrotic syndrome by animal models and human genetic studies.

METHODS

We have performed the first quantitative immunoelectron microscopy study of alpha-actinin-4 expression in humans, analysing 12 cases of minimal change nephrosis (MCNS) and 16 cases of idiopathic membranous nephropathy (MGN), and comparing this with expression in normal tissue from seven nephrectomies (Nx). alpha-Actinin-4 was visualized by immunogold labelling of plastic-embedded whole glomerular cross-sections, and analysed using LUCIA software.

RESULTS

Despite podocyte effacement, alpha-actinin-4 expression (group mean+/-1 SD) in MCNS was similar to that seen in normal Nx podocytes. In contrast, alpha-actinin-4 expression in MGN was significantly higher than in MCNS or Nx (P<0.001). Furthermore, in MGN cases showing a segmental deposition, expression of alpha-actinin-4 was significantly higher only in those capillary loop segments containing deposits, whereas in those segments without deposits expression was unchanged compared with that seen in Nx (P<0.001). alpha-Actinin-4 expression was higher in MGN cases where subepithelial deposits abutted podocyte cytoplasm, and lower in disease stages where deposits were contained within the glomerular basement membrane or were being reabsorbed.

CONCLUSIONS

Elevated alpha-actinin-4 expression is observed only in MGN, and only in areas of subepithelial deposits. Further investigation into the cause of this may reveal insights into the pathogenesis of acquired nephrosis.

Cytoskeletal structure and recovery in single human cardiac myocytes

BACKGROUND

Mechanical support of the failing human heart with a left ventricular assist device (LVAD) normalizes many components of myocyte structure and function. We hypothesized that recovery of the cytoskeleton, a major site of mechanotransduction in cardiac myocytes, is crucial for sustained improvement of myocardial function. We therefore measured the effects of LVAD support on 4 cytoskeletal proteins in single human heart cells.

METHODS

Myocytes were isolated from non-failing (NF), hypertrophied (H), failing (F) and LVAD-supported failing (L) human hearts. Protein quantitation was performed using Western blot analysis and cellular distribution was determined by immunolabeling and confocal microscopy.

RESULTS

alpha-actinin did not differ in cells from H or F as compared with NF, and L had no effect. Vinculin was not quantitatively different in H or F vs NF, but localization at the intercalated disks was significantly decreased in H and absent in F, and this pattern was consistently reversed in L. Desmin protein was significantly increased in F vs NF, both in quantity and distribution, and these increases were reversed in L. beta-tubulin was increasingly polymerized in H and F, and the hyperpolymerization was reversed in L.

CONCLUSIONS

On the level of the single cardiomyocyte, major proteins of the cytoskeleton are significantly altered in hypertrophied and failing human hearts. These alterations are reversed by mechanical unloading with an LVAD, suggesting that the cytoskeleton is not the limiting factor in determining full cardiac recovery.

Simultaneous stretching and contraction of stress fibers in vivo

To study the dynamics of stress fiber components in cultured fibroblasts, we expressed alpha-actinin and the myosin II regulatory myosin light chain (MLC) as fusion proteins with green fluorescent protein. Myosin activation was stimulated by treatment with calyculin A, a serine/threonine phosphatase inhibitor that elevates MLC phosphorylation, or with LPA, another agent that ultimately stimulates phosphorylation of MLC via a RhoA-mediated pathway. The resulting contraction caused stress fiber shortening and allowed observation of changes in the spacing of stress fiber components. We have observed that stress fibers, unlike muscle myofibrils, do not contract uniformly along their lengths. Although peripheral regions shortened, more central regions stretched. We detected higher levels of MLC and phosphorylated MLC in the peripheral region of stress fibers. Fluorescence recovery after photobleaching revealed more rapid exchange of myosin and alpha-actinin in the middle of stress fibers, compared with the periphery. Surprisingly, the widths of the myosin and alpha-actinin bands in stress fibers also varied in different regions. In the periphery, the banding patterns for both proteins were shorter, whereas in central regions, where stretching occurred, the bands were wider.

[Relationship between podocyte molecule's distribution and expression, foot process morphology and proteinuria]

OBJECTIVE

To investigate the association between nephrin, podocin and alpha-actinin of the glomerular podocyte molecules, the morphometric change of podocyte foot process and the development of proteinuria.

METHODS

Puromycin aminonucleoside (PAN) nephrosis was established. Immunofluorescence staining, image analysis and real time quantitative PCR were employed to study the distribution and quantitation of glomerular expression of nephrin, podocin and alpha-actinin. Morphometric methods were applied to evaluate the morphology change of podocyte foot processes under electron microscopy.

RESULTS

(1) Before the onset of proteinuria, 2 days after PAN injection, the podocyte foot process became swollen nephrin and podocin staining were changed into discontinuous pattern accompanied by the decrease of podocin staining intensity. The foot process became more swollen on day 5,and podocin intensity continued to decrease. Meanwhile, nephrin decreased significantly both in protein intensity and at mRNA level. (2) When heavy proteinuria [(130.8+/-30.7) mg/d, P=0.02] occurred, complete effacement of podocyte foot processes was revealed; both podocin and nephrin staining intensity decreased dramatically(P<0.01), and no linear staining could be observed; nephrin and podocin mRNA gained back. (3) During the recovery of proteinuria, the foot process morphology recovered stepwise; both the intensity of nephrin and podocin increased. When proteinuria disappeared, the podocyte foot process reappeared; podocin immunofluorescence intensity recovered whereas nephrin did not; the intensity of alpha-actinin increased significantly and the distribution changed too. (4) Podocyte foot process volume density correlated negatively with nephrin(r(nephrin)=-0.78, P=0.000 1) and podocin immunofluorescence intensity(r(podocin)=-0.76, P=0.000 1), respectively.

CONCLUSION

Before the onset of proteinuria, the first response of podocyte is the swollen foot process, the distribution change of nephrin and podocin and the decreased podocin intensity. There was close relationship between nephrin, podocin protein level and distribution pattern with the development of proteinuria and podocyte foot process effacement, whereas no major role was found for mRNA of nephrin and podocin. Our results suggest that proteinuria might occur after series of events of nephrin and podocin distribution change, their protein and mRNA expression level decrease, and morphology change of podocyte foot process.

Terminal differentiation of osteoblasts to osteocytes is accompanied by dramatic changes in the distribution of actin-binding proteins

Immunofluorescence staining of actin-binding proteins in osteoblasts and osteocytes was performed. alpha-Actinin, myosin, and tropomyosin showed similar organization in both osteoblastic stress fibers and osteocyte processes. However, fimbrin, villin, filamin, and spectrin showed dramatic differences in distribution between osteoblasts and osteocytes. This study suggested that terminal differentiation of osteoblasts to osteocytes is accompanied by highly dramatic changes in the distribution of actin-binding proteins.

INTRODUCTION

We previously reported that osteocyte shape is dependent on actin filaments. To analyze the terminal differentiation from osteoblasts to osteocytes, we investigated the actin-binding proteins, which are the control elements in the dynamic organization of the actin cytoskeleton.

MATERIALS AND METHODS

We used primary chick osteocytes and osteoblasts, the phenotypes of which were confirmed by use of OB7.3, a chick osteocyte-specific monoclonal antibody and by detection of alkaline phosphatase activity, respectively. Immunofluorescence staining was performed for visualizing actin-binding proteins. Furthermore, we applied shear stress at 12 dyns/cm2 to the cells and compared the changes in fimbrin distribution.

RESULTS

Immunofluorescence staining of fimbrin and alpha-actinin showed their presence in the processes of osteocytes, with especially strong signals of fimbrin at the sites of divarication of the processes. Anti-villin was reactive with the osteocyte cytoplasm but not with the processes. Interestingly, anti-villin immunoreactivity was much stronger in osteocytes than in osteoblasts. Filamin was localized along the stress fibers of osteoblasts but was seen only in those in the proximal base of osteocyte processes. Myosin and tropomyosin were found to have a similar pattern in both stress fibers of osteoblasts and osteocyte processes. The difference in the distribution of anti-spectrin staining was highly dramatic. Osteoblasts immunostained with anti-spectrin showed punctate signals on their cytoplasmic membranes, whereas anti-spectrin in osteocytes detected a filamentous organization; and the spectrin was totally colocalized with actin from the distal portion of the cytoplasmic processes to the cell center. In osteoblasts, shear stress induced recruitment of fimbrin to the end of stress fibers. However, fimbrin in the osteocyte processes did not change its localization.

CONCLUSION

We found that terminal differentiation of osteoblasts to osteocytes was accompanied by highly dramatic changes in the distribution of actin-binding proteins, changes of which may affect cellular function.

[A novel mutation of NPHS2 identified in a Chinese family with steroid-resistant nephrotic syndrome]

OBJECTIVE

Autosomal recessive steroid-resistant nephrotic syndrome (SRNS) is a subgroup of familial nephrotic syndrome. A causative gene has been identified, that is NPHS2, in chromosome 1q25-31, which encodes podocin. This study aimed to detect NPHS2 mutation in a Chinese family with SRNS.

METHODS

Renal biopsy was performed on the proband and her sibling for routine histologic and immunohistochemical investigation and electron microscopic examination. The expressions of podocin, nephrin, alpha-actinin and WT1 in glomeruli of the proband were detected by indirect immunofluorescence. Peripheral blood samples were collected for genetic analysis from the proband and her parents, and 53 adults with normal urinalysis. Genomic DNA was isolated from peripheral blood leucocytes. Eight exons of NPHS2 were amplified by polymerase chain reaction. Mutational analysis was performed using denaturing high-performance liquid chromatography (DHPLC) and DNA fragments with aberrant elution profiles of both strands revealed by DHPLC were re-amplified and sequenced directly.

RESULTS

The histologic findings on kidney biopsies were focal segmental glomerulosclerosis. In controls, the distribution of staining with P35, rabbit against a human podocin recombinant protein (amino acids 135 - 383 = all the C-terminal part of the protein downstream the transmembrane domain), and P21, rabbit against a human podocin recombinant protein (amino acids 15 - 89 = all the N-terminal part of the protein upstream the transmembrane domain) showed a linear pattern along glomerular capillary walls on glomeruli, and the fluorescent intensity of the staining with P35 was intensely positive. The fluorescent intensity of the staining with P21 was positive. In the proband, the distribution of the staining with P35 showed uneven and nonlinear, and the fluorescent intensity of the staining with P35 was weakly positive. The staining with P21 was negative. The area, location, distribution and fluorescent intensity of the staining with nephrin, alpha-actinin and WT1 on glomeruli of the proband were the same as those in the controls. The DHPLC elution profiles of exon 4 of NPHS2 from the proband and her parent were aberrant. The chromatograms by sequencing detected in the exon 4 of NPHS2 showed a composite heterozygous mutation of both 467_468insT and 503G > A in the proband, a heterozygous mutation of 503G > A in her father, and a heterozygous mutation of 467_468insT in her mother, respectively.

CONCLUSION

The study demonstrated for the first time a novel mutation, 503G > A, of NPHS2 in Chinese kindred with autosomal recessive SRNS. A significantly decreased or negative expression was also revealed in glomeruli of the proband stained with two kinds of anti-podocin antibodies.

Actin and alpha-actinin dynamics in the adhesion and motility of EPEC and EHEC on host cells

Two pathogenic Escherichia coli, Enteropathogenic E. coli (EPEC) and Enterohemorrhagic E. coli (EHEC), adhere to the outside of host cells and induce cytoskeletal rearrangements leading to the formation of membrane-encased pedestals comprised of actin filaments and other associated proteins beneath the bacteria. The structure of the pedestals induced by the two pathogens appears similar, although those induced by EHEC are shorter in length. Fluorescence Recovery After Photobleaching (FRAP) was used to determine potential differences of actin polymerization in EPEC and EHEC induced pedestals in cultured PtK2 cells expressing either Green or Yellow Fluorescent Protein (GFP or YFP) fused to actin or alpha-actinin. When all the fluorescent actin in a pedestal on EPEC-infected cells was photobleached, fluorescence recovery first occurred directly beneath the bacterium in a band that grew wider at a rate of one micron/minute. Consistently observed in all EPEC-induced pedestals, whether they were stationary, lengthening, or translocating, the rate of actin polymerization that occurred at the pedestal tip was approximately 1 mum/min. Overall, a much slower rate of actin polymerization was measured in long EHEC-induced pedestals. In contrast to the dynamics of GFP-actin, recovery of GFP-alpha-actinin fluorescence was not polarized, with the actin cross-linking protein exchanging all the length of the EPEC/EHEC induced pedestals. Surprisingly, the depolymerization and retrograde flow of pedestal actin, as well as pedestal translocations, were inhibited reversibly by either 2,3-butanedione monoxime (BDM) or by a combination of sodium azide and 2-deoxy D-glucose, leading to an increase in the lengths of the pedestals. A simple physical model was developed to describe elongation and translocation of EPEC/EHEC pedestals in terms of actin polymerization and depolymerization dynamics.

Postnatal changes in caldesmon expression and localization in cardiac myocytes

Caldesmon is a heat-stable protein found in both muscle and non-muscle tissue. It binds to a number of contractile and cytoskeletal proteins and may be involved in regulating acto-myosin interaction in smooth muscle cells and/or the assembly of microfilaments in muscle and non-muscle cells. We have shown previously that caldesmon is localized at the Z-lines in adult cardiac myocytes and that both the low- and high-molecular-weight forms (/-caldesmon and h-caldesmon, respectively) are present in atrial and ventricular myocytes. Here we examined the expression of caldesmon and its localization in freshly isolated cardiac myocytes during postnatal development and when these myocytes were grown in culture. We found that /-caldesmon is expressed in both neonatal and adult rat ventricular myocytes. The expression of h-caldesmon, however, was not detected in myocytes from newborn animals but increased during the first 2 weeks of postnatal development. Caldesmon was generally not co-localized with alpha-actinin at the Z-lines in neonatal myocytes but became increasingly more so during the first 2 weeks of postnatal development. When myocytes from 5- and 10-day-old rats were grown in primary culture, h-caldesmon expression decreased and caldesmon could not be detected at the Z-lines in the cultured cells. These results indicate that caldesmon plays a role at the Z-lines in adult cardiac myocytes; however, its localization at the Z-lines is not necessary for the prenatal development that occurs at these sites or for the establishment of a contractile phenotype in cultured cardiac myocytes.

Ca2+ transients of cardiomyocytes from senescent mice peak late and decay slowly

Ventricular myocytes were isolated from either young (2 months, "young myocytes") or senescent (20-26 months, "senescent myocytes") mice. Ca2+ transients were evoked by 40ms voltage-clamp pulses depolarising at 0.4, 1, 2, 4 or 8Hz. At 8Hz, Ca2+ transients from senescent cells peaked later (39ms versus 23ms) to smaller systolic [Ca2+](c) (667nM versus 1110nM) and decayed at slower rate (16s(-1) versus 33s(-1)) to higher end-diastolic [Ca2+](c) (411nM versus 220nM) than those from young myocytes. These differences were less pronounced at lower frequencies of pulsing and could not be explained by differences of the time integral of Ca2+ inward current. Since concentrations of SERCA2a and SERCA2b proteins were similar in young and senescent cells, slow rate of Ca2+ decay and high diastolic [Ca2+]c are explained on the assumption that the usual Ca2+ stimulation of SERCA2 activity is attenuated in senescent cells. The prolonged time-to-peak [Ca2+]c is discussed to result from insufficient SR Ca2+ filling by SERCA2 and, in context with confocal images, from a shift of the SERCA2b distribution to the subsarcolemmal space. The age-related changes of the Ca2+ transients are discussed to cause systolic and diastolic failure if senescent mouse hearts beat at high frequencies.

S100A7 (psoriasin) interacts with epidermal fatty acid binding protein and localizes in focal adhesion-like structures in cultured keratinocytes

S100 proteins are calcium-responsive signaling proteins that are overexpressed in cancer and inflammatory diseases. They act by forming complexes with target proteins to modify target protein function. Identifying S100 intracellular distribution, site of action, and protein targets are important goals. S100A7 (psoriasin) is an important member of this family that is markedly overexpressed in psoriatic keratinocytes; however, its role in disease progression is poorly understood. In this study, we express S100A7 in normal keratinocytes as a means to study S100A7 function. We show that S100A7 is present in the cytosol and in BiP/GRP78-positive (endoplasmic reticulum) tubular structures. When cells are challenged with elevated intracellular calcium, cytoplasmic S100A7 redistributes to alpha-actinin- and paxillin-positive peripheral structures that contact the substrate surface. Epidermal fatty acid binding protein is also overexpressed in psoriasis, and is a putative target of S100A7 in keratinocytes. To study this interaction, we coexpressed S100A7 and epidermal fatty acid binding protein. These studies indicate that S100A7 and epidermal fatty acid binding protein colocalize in the cytoplasm in untreated cultures, and localize in peripheral structures in response to calcium challenge. In addition, S100A7 expression appears to stabilize epidermal fatty acid binding protein level, and vice versa. Moreover, the proteins can be coprecipitated in the presence of bifunctional cross-linker, suggesting that they are part of a common complex. The colocalization with alpha-actinin and paxillin suggests that S100A7 and epidermal fatty acid binding protein colocalize in focal adhesion-like structures following calcium treatment.

The small heat shock protein (HSP) 20 is dynamically associated with the actin cross-linking protein actinin

BACKGROUND

The heat shock-related protein (HSP) 20 is associated with actin and modulates smooth-muscle relaxation. We hypothesized that HSP20 mediates vasorelaxation via dynamic interactions with cytoskeletal proteins, such as actin, or actin binding proteins, such as alpha-actinin.

METHODS

Physiological responses of strips of bovine carotid artery were analyzed with a muscle bath. In other experiments, the arteries were homogenized, and imunoprecipitations were performed. Immunohistochemistry with anti-HSP20 and anti-actinin antibodies was used to determine co-localization of the two proteins.

RESULTS

Bovine carotid arteries contracted in response to serotonin and rapidly relaxed in response to forskolin. HSP20 co-immunoprecipitated with both actin and alpha-actinin, but not with HSP27 or paxillin. Immunostaining with HSP20 and alpha-actinin antibodies demonstrated that HSP20 and alpha-actinin co-localized. The amount of HSP20 that immunoprecipitated with alpha -actinin was markedly diminished in muscles that were treated with the vasorelaxant forskolin.

CONCLUSIONS

HSP20 is associated with both actin and alpha-actinin. Activation of cyclic nucleotide-dependent signaling pathways leads to increases in the phosphorylation of HSP20 and a decrease in the association of HSP20 with alpha-actinin. These data suggest that phosphorylation of HSP20 may lead to relaxation of vascular smooth muscles through a dynamic association with cytoskeletal elements.

The human adult cardiomyocyte phenotype

AIM

Determination of the phenotype of adult human atrial and ventricular myocytes based on gene expression and morphology.

METHODS

Atrial and ventricular cardiomyocytes were obtained from patients undergoing cardiac surgery using a modified isolation procedure. Myocytes were isolated and cultured with or without serum. The relative cell attachment promoting efficiency of several reagents was evaluated and compared. Morphological changes during long-term culture were assessed with phase contrast microscopy, morphometric analysis and immunocytochemistry or RT-PCR of sarcomeric markers including alpha-actinin, myosin light chain-2 (MLC-2) and the adhesion molecule, cadherin.

RESULTS

The isolation method produced viable rod-shaped atrial (16.6+/-6.0%, mean+/-S.E.; n=5) and ventricular cells (22.4+/-8.0%, mean+/-S.E.; n=5) in addition to significant numbers of apoptotic and necrotic cells. Cell dedifferentiation was characterized by the loss of sarcomeric structure, condensation and extrusion of sarcomeric proteins. Cells cultured with low serum recovered and assumed a flattened, spread form with two distinct morphologies apparent. Type I cells were large, had extensive sarcolemmal spreading, with stress fibers and nascent myofibrils, whilst type II cells appeared smaller, with more mature myofibril organisation and focal adhesions.

CONCLUSION

Characterization of the redifferentiation capabilities of cultured adult cardiac myocytes in culture, provides an important system for comparing cardiomyocytes differentiating from human stem cells and provides the basis for an in vitro transplantation model to study interaction and communication between primary adult and stem cell-derived cardiomyocytes.

P19 embryonal carcinoma cells: a suitable model system for cardiac electrophysiological differentiation at the molecular and functional level

OBJECTIVE

Murine P19 embryonal carcinoma (EC) cells can differentiate into spontaneously beating cardiomyocytes in vitro and have revealed important insight into the early molecular processes of cardiomyocyte differentiation. We assessed the suitability of the P19 cell model for studying cardiac ion channel regulation at the molecular and functional level.

METHODS

P19 cells were induced to differentiate towards cardiomyocytes. mRNAs for cardiac markers and ion channels were determined by RT-PCR at six timepoints during the differentiation process. Action potentials and individual ion currents were measured by whole cell patch clamp.

RESULTS

Ion channel mRNA expression of several channels is temporally regulated during differentiation, while others show little or no regulation. L-type calcium and transient outward channels are expressed from very early on, while sodium and delayed and inward rectifier channels are upregulated at somewhat later stages during differentiation, which mirrors the in vivo murine cardiomyocyte differentiation during embryogenesis. Spontaneous cardiomyocyte action potentials exhibit a low upstroke velocity, which often can be enhanced by hyperpolarizing the cells, hence activating thusfar dormant ion channels to contribute to the action potential upstroke. Action potential duration decreases considerably during the differentiation of spontaneously beating cells. In late stages, non-beating myocytes can be found which only generate action potentials upon electrical stimulation. Their shape is comparable to neonatal/juvenile ventricular mouse myocytes in culture. Finally, we show that P19-derived cardiomyocytes display a very complete set of functional ion channels.

CONCLUSION

P19 cells represent a powerful model to study the regulation of myocardial electrophysiological differentiation at the molecular and functional level.

Ascorbic acid enhances differentiation of embryonic stem cells into cardiac myocytes

BACKGROUND

Embryonic stem (ES) cells are capable of self-renewal and differentiation into cellular derivatives of all 3 germ layers. In appropriate culture conditions, ES cells can differentiate into specialized cells, including cardiac myocytes, but the efficiency is typically low and the process is incompletely understood.

METHODS AND RESULTS

We evaluated a chemical library for its potential to induce cardiac differentiation of ES cells in the absence of embryoid body formation. Using ES cells stably transfected with cardiac-specific alpha-cardiac myosin heavy chain (MHC) promoter-driven enhanced green fluorescent protein (EGFP), 880 compounds approved for human use were screened for their ability to induce cardiac differentiation. Treatment with ascorbic acid, also known as vitamin C, markedly increased the number of EGFP-positive cells, which displayed spontaneous and rhythmic contractile activity and stained positively for sarcomeric myosin and alpha-actinin. Furthermore, ascorbic acid induced the expression of cardiac genes, including GATA4, alpha-MHC, and beta-MHC in untransfected ES cells in a developmentally controlled manner. This effect of ascorbic acid on cardiac differentiation was not mimicked by the other antioxidants such as N-acetylcysteine, Tiron, or vitamin E.

CONCLUSIONS

Ascorbic acid induces cardiac differentiation in ES cells. This study demonstrates the potential for chemically modifying the cardiac differentiation program of ES cells.

Transformation of adult mesenchymal stem cells isolated from the fatty tissue into cardiomyocytes

BACKGROUND

Myocardial infarction results in the death of cardiomyocytes, which are replaced by scar tissue. Cardiomyocytes cannot regenerate because they are terminally differentiated. Mesenchymal cells are pluripotent cells, which have the potential to differentiate to specialized tissues under appropriate stimuli. The aim of this study was to direct differentiation of the adult mesenchymal stem cells isolated from fatty tissue into cardiomyocytes using 5-azacytidine.

METHODS

Adult mesenchymal stem cells were isolated from the fatty tissue of New Zealand White rabbits and cultured in RPMI medium. Second-passaged mesenchymal cells were treated with various concentrations of 5-azacytidine and incubated for different intervals of time. The cells were plated in six-well dishes at 500, 5,000, and 50,000 cells/well. These cells were treated with 1-, 3-, 6-, 9-, and 12-micromol/L concentrations of 5-azacytidine and incubated for 12, 24, 48, and 72 hours. Later, the medium was replaced with fresh medium and incubated in a CO2 incubator. The medium was changed once at 3 to 4 days. At 2 months, the cells were fixed with 0.4% glutaraldehyde for 2 hours and later washed with phosphate-buffered saline. The transformed cells were subjected to immunostaining for the myosin heavy chain, alpha actinin, and troponin-I.

RESULTS

After treatment with 5-azacytidine, the adult mesenchymal stem cells were transformed into cardiomyocytes. At 1 week, some cells showed binucleation and extended cytoplasmic processes with adjacent cells. At 2 weeks, 20% to 30% of the cells increased in size and formed a ball-like appearance. At 3 weeks, these cells began to beat spontaneously in culture when observed under phase contrast microscope. Immunostaining of the transformed cells for myosin heavy chain, alpha actinin, and troponin-I was positive. The differentiated cells maintained the phenotype and did not dedifferentiate up to 2 months after treatment with 5-azacytidine.

CONCLUSIONS

These observations confirm that adult mesenchymal stem cells isolated from fatty tissue can be chemically transformed into cardiomyocytes. This can potentially be a source of autologous cells for myocardial repair.

The deficiency of PIP2 5-phosphatase in Lowe syndrome affects actin polymerization

Lowe syndrome is a rare X-linked disorder characterized by bilateral congenital cataracts, renal Fanconi syndrome, and mental retardation. Lowe syndrome results from mutations in the OCRL1 gene, which encodes a phosphatidylinositol 4,5 bisphosphate 5-phosphatase located in the trans-Golgi network. As a first step in identifying the link between ocrl1 deficiency and the clinical disorder, we have identified a reproducible cellular abnormality of the actin cytoskeleton in fibroblasts from patients with Lowe syndrome. The cellular abnormality is characterized by a decrease in long actin stress fibers, enhanced sensitivity to actin depolymerizing agents, and an increase in punctate F-actin staining in a distinctly anomalous distribution in the center of the cell. We also demonstrate an abnormal distribution of two actin-binding proteins, gelsolin and alpha-actinin, proteins regulated by both PIP(2) and Ca(+2) that would be expected to be altered in Lowe cells. Actin polymerization plays a key role in the formation, maintenance, and proper function of tight junctions and adherens junctions, which have been demonstrated to be critical in renal proximal tubule function, and in the differentiation of the lens. These findings point to a general mechanism to explain how this PIP(2) 5-phosphatase deficiency might produce the Lowe syndrome phenotype.

Telethonin protein expression in neuromuscular disorders

Telethonin is a 19-kDa sarcomeric protein, localized to the Z-disc of skeletal and cardiac muscles. Mutations in the telethonin gene cause limb-girdle muscular dystrophy type 2G (LGMD2G). We investigated the sarcomeric integrity of muscle fibers in LGMD2G patients, through double immunofluorescence analysis for telethonin with three sarcomeric proteins: titin, alpha-actinin-2, and myotilin and observed the typical cross striation pattern, suggesting that the Z-line of the sarcomere is apparently preserved, despite the absence of telethonin. Ultrastructural analysis confirmed the integrity of the sarcomeric architecture. The possible interaction of telethonin with other proteins responsible for several forms of neuromuscular disorders was also analyzed. Telethonin was clearly present in the rods in nemaline myopathy (NM) muscle fibers, confirming its localization to the Z-line of the sarcomere. Muscle from patients with absent telethonin showed normal expression for the proteins dystrophin, sarcoglycans, dysferlin, and calpain-3. Additionally, telethonin showed normal localization in muscle biopsies from patients with LGMD2A, LGMD2B, sarcoglycanopathies, and Duchenne muscular dystrophy (DMD). Therefore, the primary deficiency of calpain-3, dysferlin, sarcoglycans, and dystrophin do not seem to alter telethonin expression.

Typical nemaline bodies presenting in a patient with polymyositis

A 66-year-old woman presented with a progressive myopathy affecting the proximal limbs and unusual pathological findings of nemaline bodies on muscle biopsy. Histological examination demonstrated that the bodies were mainly located in the subsarcolemmal region of atrophic fibers, exhibited strong immunoreactivity with antibodies to both alpha-actinin and m-actin, and had a typical lattice-like appearance at higher magnification on electron microscopy. These findings were the same as those for nemaline myopathy. The patient responded to steroid therapy, but relapse occurred after steroid was discontinued. Given the clinical criteria of polymyositis, we believe that the occurrence of nemaline bodies in our patient should be interpreted primarily as an epiphenomenon of primary myopathy.

Identification of lipids as the main component of skeletal muscle Z-discs

The existence of lipids in Z-discs of the vertebrate skeletal muscle was suggested by the results of microscopy using rhodamine 6G reagent, a fluorescent probe. After removal of lipids by treatment of myofibrils with 0.5% Trition X-100, intact configurations of Z-filaments composed of alpha-actinin were clearly visible under an electron microscope. These findings indicated that lipids were amorphous-matrix materials of the Z-disc. Lipids were proved to be the main component of Z-discs by their extraction from I-Z-I brushes with a mixture of methanol and chloroform and by analysing them. The total amount of lipids in Z-discs exceeded that of alpha-actinin and varied from 2.4 to 7.1 g per 100 g of myofibrillar proteins depending on the phenotype of skeletal muscle. The sum of the amounts of lipids and alpha-actinin was 4.5 g per 100 g of myofibrillar proteins in chicken pectoralis profundus muscle (fast type), while it was 10.1 g in chicken soleus muscle (slow type). Lipid classes were phospholipids, triacylglycerols, cholesterol and free fatty acids. Since the lipids extracted from I-Z-I brushes were hardly contaminated with those from the sarcoplasmic reticulum and mitochondria, their amounts and classes were considered to be characteristic of Z-discs. The lipids probably cement neighbouring Z-filaments electrostatically, reinforce the Z-disc structure, and play an important role in the force transmission of skeletal muscle myofibrils.

Dedifferentiation of atrial myocytes during atrial fibrillation: role of fibroblast proliferation in vitro

OBJECTIVES

Severe myocyte alterations, characterized by enlarged myocytes and myolysis, is observed in fibrillating and dilated atria and contributes to atrial fibrillation. The aim of this study was to determine the nature of this cellular remodeling process and factors involved in its regulation.

METHODS

In vivo, contractile proteins were studied in 24 human right atrial specimens by means of immunohistochemical techniques. In an attempt to reproduce in vitro the myocyte remodeling and to study its regulation, human atrial myocytes were cultured (n=27) and analyzed immunocytochemically; intracellular Ca(2+) transients (Ca(i)-tr) in response to electrical stimulation were monitored using Fura-2/AM.

RESULTS

In diseased specimens, sarcomeres, seen at the periphery of myolytic myocytes, stained positively with antibodies against sarcomeric proteins of the Z-band (alpha-actinin and titin epitope T12) but not with antibodies against titin epitope T11 (I-band) or desmin (intermediate filament). beta-myosin heavy chain (MHC) and smooth muscle alpha-actin, two proteins of the fetal program, were re-expressed. In culture, diseased myocytes also showed myolysis and glycogen accumulation; their sarcomeres stained positively with anti-alpha-actinin, anti-T12, anti-beta-MHC and anti-smooth muscle alpha-actin but not with anti-titin T11 or anti-desmin antibodies. At confluence, myocytes regained a normal sarcomeric apparatus and were excitable, as shown by electrical Ca(i)-tr triggering. This redifferentiation process was inhibited by fibroblast proliferation.

CONCLUSION

In diseased atria, myolytic myocytes are in a dedifferentiated state resembling that of immature muscle cells. In vitro, fibroblast proliferation prevents the reversibility of this cellular alteration.

Myocyte redistribution of GRK2 and GRK5 in hypertensive, heart-failure-prone rats

Gprotein-coupled receptor kinases (GRKs) are known to be involved in the development of cardiac hypertrophy. Their exact role and subcellular distribution during cardiac hypertrophy and failure remain to be elucidated. We examined expression and subcellular distribution of GRK2 and GRK5 in the left ventricle of female spontaneously hypertensive heart failure (SHHF) rats at 6 months of age using Western blots and fluorescent confocal microscopy. GRK2 was expressed mainly in the Triton X-100 soluble fraction in the left ventricle with similar expression levels between SHHF and age-matched Wistar-Kyoto (WKY) rats. GRK2 had a striated pattern which colocalized with sarcomeric alpha-actinin and G protein in both SHHF and WKY rat myocytes and specifically accumulated in the intercalated disks of myocytes from SHHF but not WKY rats. GRK5 was expressed in both the Triton X-100 soluble fraction and Triton X-100 insoluble fraction in the left ventricle with similar expression levels between SHHF and WKY rats. GRK5 distributed diffusely in the cytoplasm in both SHHF and WKY rat myocytes and specifically accumulated in the nucleus of myocytes from SHHF but not WKY rats. GRK5 colocalized with coilin, the major component of the nuclear substructure involved in RNA synthesis and processing. The results suggest different roles for GRK2 and GRK5 in G-protein signaling and RNA biogenesis. Subcellular redistribution of GRK2 and GRK5 may be involved in cardiac hypertrophy resulting from chronic hypertension.

Evaluating dot and Western blots using image analysis and pixel quantification of electronic images

Inexpensive computer imaging technology was used to assess levels of insulin-like growth factor-I (IGF-I) on dot blots (DB) and alpha-Actinin on Western blots (WB). In the first procedure, known IGF-I samples were dotted on nitrocellulose membranes using a vacuum manifold. After the DB were developed and dried, the images were digitized using an HP Deskscan II flat bed scanner, exported into Image-Pro Plus and analyzed by taking the combined mean of 45 degrees and 135 degrees sample lines drawn through each dot. Dot blots corresponding to a linear concentration range from 10 to 300 ng IGF-I were assessed by this method. In the second procedure, WB were scanned with a ScanJet 3c flat bed scanner and their backgrounds were clarified using Image-Pro Plus. A second image analysis program, Alpha Imager 2000, was then used to define the boundaries of protein bands, assess pixel number and density, and to obtain final numerical data for quantifying alpha-Actinin on the WB. Collectively, the results of these two studies suggest that specific proteins may be evaluated by using relatively inexpensive image analysis software systems via pixel quantification of electronic images.

Heterogeneity of myocardial sleeve morphology and gap junctions in canine superior vena cava

BACKGROUND

The myocardial sleeve of the superior vena cava (SVC) has been identified as a potential initiating focus in atrial fibrillation, but information on cell-to-cell linkage at this site is lacking.

METHODS AND RESULTS

We examined the SVC in 8 dogs by immunoconfocal and electron microscopy. Cardiomyocytes outlined with vinculin and bearing striations positive for alpha-actinin are found in the proximal segment of the SVC. These cells, grouped in bundles of various orientations according to location, extend cephalically as far as 3 cm from the right atrium (RA)-SVC junction. Comparison between the junctional level and the level 2 cm distal shows that the myocardial layer in the latter is thinner and not as compact and is composed of longer cells (87.3+/-15.7 versus 71.6+/-14.4 micrometer, P<0.01). Gap junctions made of connexin43 (Cx43), Cx40, and Cx45 are aggregated mainly at the intercalated disks, and colocalization of connexins is a common feature throughout the myocardial sleeve. Areas of atypical expression exist, however, characterized by a center of abundant Cx43 labels surrounded by a periphery of scattered tiny Cx40-labeled spots. Although in the ventral subluminal compact myocardial layer, individual cells at both levels are surrounded by similar numbers of cells, the number of aggregation of labeled gap junctions at the distal level is less (2.3+/-0.6 versus 3.7+/-0.9, P<0.01). In addition, electron-microscopic examination demonstrates that the gap junctions at the distal level are smaller in size (0.37+/-0.30 versus 0.55+/-0.34 micrometer, P<0.01).

CONCLUSIONS

The myocardial sleeve in the canine SVC is a heterogeneous structure, which could potentially form a substrate for heterogeneity of electrical coupling.

Plant Golgi-associated vesicles contain a novel alpha-actinin-like protein

By using Western blotting, immunofluorescence and immunogold labeling, a novel alpha-actinin-like protein was found in pollen and pollen tubes of Lilium davidii, a model system for cytoskeleton and Golgi apparatus study of plant. As measured by Western blotting, the molecular mass of the a-actinin-like protein was about 80 kDa. Under confocal laser scanning microscopy after immunofluorescence labeling, the distribution of the alpha-actinin-like protein appeared punctated in the cytoplasm of the pollen and pollen tubes. When double labeled, the protein was co-localized with Golgi 58K protein. In addition, some fraction of the alpha-actinin-like protein was found to co-distribute with F-actin bundles in the pollen tubes. Additional studies with immuno-gold labeling and transmission electron microscopy revealed that the alpha-actinin-like protein bound mainly to the membranes of Golgi-associated vesicles. When the pollen tubes were treated with Brefeldin A (BFA), the a-actinin-like proteins were dispersed into the cytoplasm, and the growth of pollen tubes was inhibited. After BFA was removed, the protein was reversibly recovered on the Golgi apparatus. These results suggest that the novel alpha-actinin-like protein is a BFA-sensitive protein on the membranes of Golgi-associated vesicles, and may participate in Golgi-associated vesicles budding and/or sorting, together with actin microfilaments.

CD2AP and p130Cas localize to different F-actin structures in podocytes

Mice lacking the 80-kDa CD2-associated protein (CD2AP) develop progressive renal failure that starts soon after birth with proteinuria and foot process effacement by unknown mechanisms. CD2AP has been identified and cloned independently by virtue of its interaction with the T cell protein CD2 and with the docking protein p130Cas. In the present study we examined the localization of CD2AP and p130Cas in the mouse glomerulus and in cultured podocytes. In glomeruli, CD2AP and p130Cas immunofluorescence were observed in podocytes, where they colocalized with F-actin in foot processes. In addition, p130Cas was strongly expressed in mesangial cells. Immunoelectron microscopy demonstrated that CD2AP was present in podocyte foot processes without a prevailing localization. In cultured podocytes, p130Cas was enriched at sites of focal adhesions, where it colocalized like vinculin with F-actin at stress fiber ends. In contrast, CD2AP colocalized with F-actin at the leading edge of lamellipodia and in small spots, which were unevenly distributed in the cytoplasm. The spot-shaped F-actin structures were also stained by antibodies against the actin nucleation Arp2/3 complex and cortactin, both contributing to dynamic actin assembly. Moreover, CD2AP spots in cultured podocytes were in close spatial association with actinin-4, but not actinin-1. Our results suggest that CD2AP and p130Cas, which both colocalize with F-actin in podocytes in situ, possess different functions. Whereas p130Cas is found in focal adhesions, CD2AP seems to be involved in the regulation of highly dynamic F-actin structures in podocyte foot processes.

Muc-1, integrin, and osteopontin expression during the implantation cascade in sheep

The extracellular matrix protein osteopontin (OPN) is a component of histotroph that increases in uterine flushings from pregnant ewes during the peri-implantation period and is localized on the apical surfaces of the uterine luminal epithelium (LE) and conceptus trophectoderm (Tr). The potential involvement of OPN in the implantation adhesion cascade in sheep was investigated by examining temporal, spatial, and potential functional relationships between OPN, Muc-1, and integrin subunits during the estrous cycle and early pregnancy. Immunoreactive Muc-1 was highly expressed at the apical surfaces of uterine luminal (LE) and glandular epithelium (GE) in both cycling and pregnant ewes but was decreased dramatically on LE by Day 9 and was nearly undetectable by Day 17 of pregnancy when intimate contact between LE and Tr begins. In contrast, integrin subunits alpha(v), alpha(4), alpha(5), beta(1), beta(3), and beta(5) were constitutively expressed on conceptus Tr and at the apical surface of uterine LE and GE in both cyclic and early pregnant ewes. The apical expression of these subunits could contribute to the apical assembly of several OPN receptors including the alpha(v)beta(3), alpha(v)beta(1), alpha(v)beta(5), alpha(4)beta(1), and alpha(5)beta(1) heterodimers on endometrial LE and GE, and conceptus Tr in sheep. Functional analysis of potential OPN interactions with conceptus and endometrial integrins was performed on LE and Tr cells in vitro using beads coated with OPN, poly-L-lysine, or recombinant OPN in which the Arg-Gly-Asp sequence was replaced with RGE or RAD. Transmembrane accumulation of talin or alpha-actinin at the apical surface of uterine LE and conceptus Tr cells in contact with OPN-coated beads revealed functional integrin activation and cytoskeletal reorganization in response to OPN binding. These results provide a physiological framework for the role of OPN, a potential mediator of implantation in sheep, as a bridge between integrin heterodimers expressed by Tr and uterine LE responsible for adhesion for initial conceptus attachment.