Three-dimensional analysis and visualization of myofibrillogenesis in adult cardiomyocytes by confocal microscopy

3D imaging of the early embryonic chicken heart with focused ion beam scanning electron microscopy

Early embryonic heart development is a period of dynamic growth and remodeling, with rapid changes occurring at the tissue, cell, and subcellular levels. A detailed understanding of the events that establish the components of the heart wall has been hampered by a lack of methodologies for three-dimensional (3D), high-resolution imaging. Focused ion beam scanning electron microscopy (FIB-SEM) is a novel technology for imaging 3D tissue volumes at the subcellular level. FIB-SEM alternates between imaging the block face with a scanning electron beam and milling away thin sections of tissue with a FIB, allowing for collection and analysis of 3D data. FIB-SEM was used to image the three layers of the day 4 chicken embryo heart: myocardium, cardiac jelly, and endocardium. Individual images obtained with FIB-SEM were comparable in quality and resolution to those obtained with transmission electron microscopy. Up to 1,100 serial images were obtained in 4 nm increments at 4.88 nm resolution, and image stacks were aligned to create volumes 800-1,500 μm3 in size. Segmentation of organelles revealed their organization and distinct volume fractions between cardiac wall layers. We conclude that FIB-SEM is a powerful modality for 3D subcellular imaging of the embryonic heart wall.

Three-dimensional refractive index reconstruction with quantitative phase tomography

Optical tomography based on quantitative phase microscopy is used to determine nondestructively and with high spatial resolution the three-dimensional (3D) refractive index distributions within optical fiber devices. After obtaining a series of phase images of the fiber as it is rotated around its longitudinal axis at regularly-spaced angular positions, filtered backprojection is used to reconstruct a 3D map of the refractive index. The 3D refractive index distribution of the join region between two fusion spliced optical fibers is reconstructed with accuracy better than 10(-3).

In vivo confocal laser scanning microscopy and micropuncture in intact rat

BACKGROUND

Intravital microscopy theoretically provides the optimal conditions for studying specific organ functions. However, the application of microscopy in intact organs in vivo has been limited so far due to technical difficulties. The purpose of this study was to establish a method of in vivo confocal laser scanning microscopy (CLSM) for the study of endocytosis in proximal tubules of intact kidney.

METHODS

The left kidney of rats placed on a modified microscope stage was exposed and stabilized in a thermostatically controlled cup. The stage was then attached to an upright confocal microscope. Surface proximal tubules were microinfused with fluorescent albumin or transferrin. Single or time-series images of microinfused proximal tubules were recorded in reflection and/or fluorescence mode.

RESULTS

The stability of the kidney and the resolution of images were sufficient to visualize intracellular vesicles. Albumin and transferrin were initially observed at the brush border, then later internalized by proximal tubules and accumulated in lysosomes over a time period of 15 min. Furthermore, fusion of vesicles was observed in time-lapse images.

CONCLUSION

These results show that in vivo CLSM in intact kidney may be an excellent method to evaluate proximal tubular endocytosis and ligand trafficking.

Insulin resistance in adult cardiomyocytes undergoing dedifferentiation: role of GLUT4 expression and translocation

Myocardium undergoing remodeling in vivo exhibits insulin resistance that has been attributed to a shift from the insulin-sensitive glucose transporter GLUT4 to the fetal, less insulin-sensitive, isoform GLUT1. To elucidate the role of altered GLUT4 expression in myocardial insulin resistance, glucose uptake and the expression of the glucose transporter isoforms GLUT4 and GLUT1 were measured in adult rat cardiomyocytes (ARC). ARC in culture spontaneously undergo dedifferentiation, hypertrophy-like spreading, and return to a fetal-like gene expression pattern. Insulin stimulation of 2-deoxy-D-glucose uptake was completely abolished on day 2 and 3 of culture and recovered thereafter. Although GLUT4 protein level was reduced, the time-course of unresponsiveness to insulin did not correlate with altered expression of GLUT1 and GLUT4. However, translocation of GLUT4 to the sarcolemma in response to insulin was completely abolished during transient insulin resistance. Insulin-mediated phosphorylation of Akt was not reduced, indicating that activation of phosphatidylinositol 3-kinase (PI3K) was preserved. On the other hand, total and phosphorylated Cbl was reduced during insulin resistance, suggesting that activation of Cbl/CAP is essential for insulin-mediated GLUT4 translocation, in addition to activation of PI3K. Pharmacological inhibition of contraction in insulin-sensitive ARC reduced insulin sensitivity and lowered phosphorylated Cbl. The results suggest that transient insulin resistance in ARC is related to impairment of GLUT4 translocation. A defect in the PI3K-independent insulin signaling pathway involving Cbl seems to contribute to reduced insulin responsiveness and may be related to contractile arrest.

Cytoskeletal modulation of electrical and mechanical activity in cardiac myocytes

The cardiac myocyte has an intracellular scaffold, the cytoskeleton, which has been implicated in several cardiac pathologies including hypertrophy and failure. In this review we describe the role that the cytoskeleton plays in modulating both the electrical activity (through ion channels and exchangers) and mechanical (or contractile) activity of the adult heart. We focus on the 3 components of the cytoskeleton, actin microfilaments, microtubules, and desmin filaments. The limited visual data available suggest that the subsarcolemmal actin cytoskeleton is sparse in the adult myocyte. Selective disruption of cytoskeletal actin by pharmacological tools has yet to be verified in the adult cell, yet evidence exists for modulation of several ionic currents, including I(CaL), I(Na), I(KATP), I(SAC) by actin microfilaments. Microtubules exist as a dense network throughout the adult cardiac cell, and their structure, architecture, kinetics and pharmacological manipulation are well described. Both polymerised and free tubulin are functionally significant. Microtubule proliferation reduces contraction by impeding sarcomeric motion; modulation of sarcoplasmic reticulum Ca(2+) release may also be involved in this effect. The lack of effect of microtubule disruption on cardiac contractility in adult myocytes, and the concentration-dependent modulation of the rate of contraction by the disruptor nocodazole in neonatal myocytes, support the existence of functionally distinct microtubule populations. We address the controversy regarding the stimulation of the beta-adrenergic signalling pathway by free tubulin. Work with mice lacking desmin has demonstrated the importance of intermediate filaments to normal cardiac function, but the precise role that desmin plays in the electrical and mechanical activity of cardiac muscle has yet to be determined.

Three-dimensional imaging of extracellular matrix and extracellular matrix-cell interactions

In summary, noninvasive and nondestructive imaging modalities such as reflection and autofluorescence can readily be used in conjunction with the 3-D optical sectioning capabilities of confocal and multiphoton microscopy to investigate biological processes within living systems. The elimination of specimen fixation and extensive processing reduces the possibility of structural artifacts and facilitates repeat observations within a single sample. Therefore, information representing up to four dimensions (x, y, z, and time) can be readily collected and reconstructed for purposes of visualization and/or quantitative analysis. An advantage of using the techniques described in this chapter is the possibility of performing quantitative measurement of cell size, surface area, volume, depth (in matrix), orientation, receptor density, as well as fluorescence-based indicators of phenotype and function. At present, we are effectively utilizing these techniques to study collagen fibrillogenesis and ECM assembly, structural aspects of ECM-based biomaterials, as well as cell interactions within 3-D matrices (e.g., migration). New insights provided by these techniques regarding ECM and ECM-cell signaling will further the understanding of tissue structure and function and contribute to the development of new and improved strategies for tissue repair, replacement, and maintenance.

Cytochalasin D reduces Ca2+ sensitivity and maximum tension via interactions with myofilaments in skinned rat cardiac myocytes

The F-actin disrupter cytochalasin D depresses cardiac contractility, an effect previously ascribed to the interaction of cytochalasin D with cytoskeletal actin. We have investigated the possibility that this negative inotropic effect is due to the interaction of cytochalasin D with sarcomeric actin of the thin filament. Confocal images of Triton X-100-skinned myocytes incubated with a fluorescent conjugate of cytochalasin D revealed a longitudinally striated pattern of binding, consistent with a myofibrillar rather than cytoskeletal structure.Tension-pCa relationships were determined at sarcomere lengths (SLs) of 2.0 and 2.3 [mu]m following 2 min incubation with 1 [mu]M cytochalasin D. Cytochalasin D significantly reduced the pCa for half-maximal activation (pCa50) at both SLs. The shift in pCa50 was significantly greater at a SL of 2.3 [mu]m compared with that at a SL of 2.0 [mu]m. Cytochalasin D had no effect on the Hill co-efficient at either SL. Cytochalasin D significantly reduced the maximum tension at both SLs. We suggest that the length-dependent decrease in myofilament Ca2+ sensitivity in response to cytochalasin D is due to a decrease in the affinity of troponin C for Ca2+. Cytochalasin D has been used for many years as the agent of choice for disruption of cytoskeletal actin. However, we have demonstrated for the first time an interaction of cytochalasin D with sarcomeric actin of the thin filament, which can account for the effects of cytochalasin D on cardiac contractility.

The Drosophila projectin mutant, bentD, has reduced stretch activation and altered indirect flight muscle kinetics

Projectin is a ca. 900 kDa protein that is a member of the titin protein superfamily. In skeletal muscle titins are involved in the longitudinal reinforcement of the sarcomere by connecting the Z-band to the M-line. In insect indirect flight muscle (IFM), projectin is believed to form the connecting filaments that link the Z-band to the thick filaments and is responsible for the high relaxed stiffness found in this muscle type. The Drosophila mutant bentD (btD) has been shown to have a breakpoint close to the carboxy-terminal kinase domain of the projectin sequence. Homozygotes for btD are embryonic lethal but heterozygotes (btD/+) are viable. Here we show that btD/+ flies have normal flight ability and a slightly elevated wing beat frequency (btD/+ 223+/-13 Hz; +/+ 203+/-5 Hz, mean +/- SD; P < 0.01). Electron microscopy of btD/+ IFM show normal ultrastructure but skinned fiber mechanics show reduced stretch activation and oscillatory work. Although btD/+ IFM power output was at wild-type levels, maximum power was achieved at a higher frequency of applied length perturbation (btD/+ 151+/-6 Hz; +/+ 102+/-14 Hz; P < 0.01). Results were interpreted in the context of a viscoelastic model of the sarcomere and indicate altered cross-bridge kinetics of the power-producing step. These results show that the btD mutation reduces oscillatory work in a way consistent with the proposed role of the connecting filaments in the stretch activation response of IFM.

Modulation of sarcomere organization during embryonic stem cell-derived cardiomyocyte differentiation

Myofibrillogenesis - sarcomeres - mouse embryonic stem cells - cardiomyocytes - beta1 integrin Mouse embryonic stem (ES) cells, when cultivated as embryoid bodies, differentiate in vitro into cardiomyocytes of ventricle-, atrium- and pacemaker-like cell types characterized by developmentally controlled expression of cardiac-specific genes, structural proteins and ion channels. Using this model system, we show here, (I) that during cardiac myofibrillogenesis sarcomeric proteins are organized in a developmentally regulated manner following the order: titin (Z-disk), alpha-actinin, myomesin, titin (M-band), myosin heavy chain, alpha-actin, cardiac troponin T and M-protein, recapitulating the sarcomeric organization in the chicken embryonal heart in vivo. Our data support the view that the formation of I-Z-I complexes is developmentally delayed with respect to A-band assembly. We show (2) that the process of cardiogenic differentiation in vitro is influenced by medium components: Using a culture medium supplemented with glucose, amino acids, vitamins and selenium ions, we were able to increase the efficiency of cardiac differentiation of wild-type, as well as of beta1 integrin-deficient (beta1-/-) ES cells, and to improve the degree of organization of sarcomeric structures in wild-type and in beta1-/- cardiac cells. The data demonstrate the plasticity of cardiogenesis during the differentiation of wild-type and of genetically modified ES cells.

Epitope tagging

Epitope tagging is a recombinant DNA method by which a protein encoded by a cloned gene is made immunoreactive to a known antibody. This review discusses the major advantages and limitations of epitope tagging and describes a number of recent applications. Major areas of application include monitoring protein expression, localizing proteins at the cellular and subcellular levels, and protein purification, as well as the analysis of protein topology, dynamics and interactions. Recently the method has also found use in transgenic and gene therapy studies and in the emerging fields of functional genomics and proteomics.

Studies on the structure of sperm heads of Eledone cirrhosa by means of CLSM linked to bioimage-oriented devices

We have used a confocal laser scanning optical microscope imaging device and a bioimage-oriented workstation equipped for augmented reality to study the helical sperm head of the octopus Eledone cirrhosa. This approach allows us to study different complex organisational motifs due to the spatial arrangement of linear helical structures. We consider this helical specimen an enlarged copy of one of the most important biostructures governing cell functioning such as chromatin-DNA. Moreover, this very same sample is made of highly compacted chromatin that can be studied at higher resolution, i.e., by means of scanning force microscopy. Fluorescence optical sectioning has been used to enter the spatial organisation. Three-dimensional images of single, twisted, and folded fibers are shown.