Dynamics of the mitochondrial reticulum in live cells using Fourier imaging correlation spectroscopy and digital video microscopy

Mitochondrial dynamics and respiration within cells with increased open pore cytoskeletal meshes

The cytoskeletal architecture directly affects the morphology, motility, and tensional homeostasis of the cell. In addition, the cytoskeleton is important for mitosis, intracellular traffic, organelle motility, and even cellular respiration. The organelle responsible for a majority of the energy conversion for the cell, the mitochondrion, has a dependence on the cytoskeleton for mobility and function. In previous studies, we established that cytoskeletal inhibitors altered the movement of the mitochondria, their morphology, and their respiration in human dermal fibroblasts. Here, we use this protocol to investigate applicability of power law diffusion to describe mitochondrial locomotion, assessment of rates of fission and fusion in healthy and diseased cells, and differences in mitochondria locomotion in more open networks either in response to cytoskeletal destabilizers or by cell line. We found that mitochondria within fibrosarcoma cells and within fibroblast cells treated with an actin-destabilizing toxin resulted in increased net travel, increased average velocity, and increased diffusion of mitochondria when compared to control fibroblasts. Although the mitochondria within the fibrosarcoma travel further than mitochondria within their healthy counterparts, fibroblasts, the dependence on mitochondria for respiration is much lower with higher rates ofhydrogen peroxide production and was confirmed using the OROBOROS O2K. We also found that rates of fission and fusion of the mitochondria equilibrate despite significant alteration of the cytoskeleton. Rates ranged from 15% to 25%, where the highest rates were observed within the fibrosarcoma cell line. This result is interesting because the fibrosarcoma cell line does not have increased respiration metrics including when compared to fibroblast. Mitochondria travel further, faster, and have an increase in percent mitochondria splitting or joining while not dependent on the mitochondria for a majority of its energy production. This study illustrates the complex interaction between mitochondrial movement and respiration through the disruption of the cytoskeleton.

Summary: We assessed the effect of cytoskeletal inhibitors and the use of a calcium ionophore as an additional stressor on mitochondrial motility and bioenergetic function in fibroblasts and fibrosarcoma cells.

Novel mitochondrial extensions provide evidence for a link between microtubule-directed movement and mitochondrial fission

Mitochondrial dynamics play an important role in a large number of cellular processes. Previously, we reported that treatment of mammalian cells with the cysteine-alkylators, N-ethylmaleimide and ethacrynic acid, induced rapid mitochondrial fusion forming a large reticulum approximately 30 min after treatment. Here, we further investigated this phenomenon using a number of techniques including live-cell confocal microscopy. In live cells, drug-induced fusion coincided with a cessation of fast mitochondrial movement which was dependent on microtubules. During this loss of movement, thin mitochondrial tubules extending from mitochondria were also observed, which we refer to as 'mitochondrial extensions'. The formation of these mitochondrial extensions, which were not observed in untreated cells, depended on microtubules and was abolished by pretreatment with nocodazole. In this study, we provide evidence that these extensions result from of a block in mitochondrial fission combined with continued application of motile force by microtubule-dependent motor complexes. Our observations strongly suggest the existence of a link between microtubule-based mitochondrial trafficking and mitochondrial fission.

Control of nuclear centration in the C. elegans zygote by receptor-independent Galpha signaling and myosin II

Mitotic spindle positioning in the Caenorhabditis elegans zygote involves microtubule-dependent pulling forces applied to centrosomes. In this study, we investigate the role of actomyosin in centration, the movement of the nucleus-centrosome complex (NCC) to the cell center. We find that the rate of wild-type centration depends equally on the nonmuscle myosin II NMY-2 and the Galpha proteins GOA-1/GPA-16. In centration- defective let-99(-) mutant zygotes, GOA-1/GPA-16 and NMY-2 act abnormally to oppose centration. This suggests that LET-99 determines the direction of a force on the NCC that is promoted by Galpha signaling and actomyosin. During wild-type centration, NMY-2-GFP aggregates anterior to the NCC tend to move further anterior, suggesting that actomyosin contraction could pull the NCC. In GOA-1/GPA-16-depleted zygotes, NMY-2 aggregate displacement is reduced and largely randomized, whereas in a let-99(-) mutant, NMY-2 aggregates tend to make large posterior displacements. These results suggest that Galpha signaling and LET-99 control centration by regulating polarized actomyosin contraction.

Stress fluctuations and motion of cytoskeletal-bound markers

Cytoskeletal (CSK) dynamics such as remodeling and reorganization can be studied by tracking the spontaneous motion of CSK-bound particles. Particle motion is thought to be driven by local, ATP-dependent intracellular force fluctuations due to polymerization processes and motor proteins, and to be impeded by a viscoelastic, metastable cytoskeletal network. The mechanisms that link particle motion to force fluctuations and the CSK dynamics remain unclear. We report simultaneous measurements of the spontaneous motion of CSK-bound particles and of cellular force fluctuations. Cellular force fluctuations were measured by tracking fluorescent markers embedded in an elastic polyacrylamide hydrogel substrate that served as an extracellular matrix (ECM). The motion of CSK-bound particles and markers embedded in the ECM showed both persistence and superdiffusive behavior. Moreover, the movements of CSK-bound beads were temporally and spatially correlated with force fluctuations in the ECM. The findings suggest that the spontaneous motion of CSK-bound beads is driven not by random, local stress fluctuations within a viscoelastic continuum or cage, but rather by stress fluctuations within a tensed and constantly remodeling CSK network that transmits stresses over considerable distances to the ECM.

Mitochondrial dynamics in chondrocytes and their connection to the mechanical properties of the cytoplasm

BACKGROUND

The motion and redistribution of intracellular organelles is a fundamental process in cells. Organelle motion is a complex phenomenon that depends on a large number of variables including the shape of the organelle, the type of motors with which the organelles are associated, and the mechanical properties of the cytoplasm. This paper presents a study that characterizes the diffusive motion of mitochondria in chondrocytes seeded in agarose constructs and what this implies about the mechanical properties of the cytoplasm.

METHOD OF APPROACH

Images showing mitochondrial motion in individual cells at 30 s intervals for 15 min were captured with a confocal microscope. Digital image correlation was used to quantify the motion of the mitochondria, and the mean square displacement (MSD) was calculated. Statistical tools for testing whether the characteristic motion of mitochondria varied throughout the cell were developed. Calculations based on statistical mechanics were used to establish connections between the measured MSDs and the mechanical nature of the cytoplasm.

RESULTS

The average MSD of the mitochondria varied with time according to a power law with the power term greater than 1, indicating that mitochondrial motion can be viewed as a combination of diffusion and directional motion. Statistical analysis revealed that the motion of the mitochondria was not uniform throughout the cell, and that the diffusion coefficient may vary by over 50%, indicating intracellular heterogeneity. High correlations were found between movements of mitochondria when they were less than 2 microm apart. The correlation is probably due to viscoelastic properties of the cytoplasm. Theoretical analysis based on statistical mechanics suggests that directed diffusion can only occur in a material that behaves like a fluid on large time scales.

CONCLUSIONS

The study shows that mitochondria in different regions of the cell experience different characteristic motions. This suggests that the cytoplasm is a heterogeneous viscoelastic material. The study provides new insight into the motion of mitochondria in chondrocytes and its connection with the mechanical properties of the cytoplasm.

Translational diffusion of fluorescent proteins by molecular fourier imaging correlation spectroscopy

The ability to noninvasively observe translational diffusion of proteins and protein complexes is important to many biophysical problems. We report high signal/noise (>or=250) measurements of the translational diffusion in viscous solution of the fluorescent protein, DsRed. This is carried out using a new technique: molecular Fourier imaging correlation spectroscopy (M-FICS). M-FICS is an interferometric method that detects a collective Fourier component of the fluctuating density of a small population of fluorescent molecules, and provides information about the distribution of molecular diffusivities. A theoretical analysis is presented that expresses the detected signal fluctuations in terms of the relevant time-correlation functions for molecular translational diffusion. Furthermore, the role played by optical orientational degrees of freedom is established. We report Fickian self-diffusion of the DsRed tetramer at short timescales. The long-time deviation of our data from Fickian behavior is used to determine the variance of the distribution of the protein self-diffusion coefficient. We compare our results to the expected outcomes for 1), a bi-disperse distribution of protein species, and 2), dynamic disorder of the host solvent.

k-Space image correlation spectroscopy: a method for accurate transport measurements independent of fluorophore photophysics

We present the theory and application of reciprocal space image correlation spectroscopy (kICS). This technique measures the number density, diffusion coefficient, and velocity of fluorescently labeled macromolecules in a cell membrane imaged on a confocal, two-photon, or total internal reflection fluorescence microscope. In contrast to r-space correlation techniques, we show kICS can recover accurate dynamics even in the presence of complex fluorophore photobleaching and/or "blinking". Furthermore, these quantities can be calculated without nonlinear curve fitting, or any knowledge of the beam radius of the exciting laser. The number densities calculated by kICS are less sensitive to spatial inhomogeneity of the fluorophore distribution than densities measured using image correlation spectroscopy. We use simulations as a proof-of-principle to show that number densities and transport coefficients can be extracted using this technique. We present calibration measurements with fluorescent microspheres imaged on a confocal microscope, which recover Stokes-Einstein diffusion coefficients, and flow velocities that agree with single particle tracking measurements. We also show the application of kICS to measurements of the transport dynamics of alpha5-integrin/enhanced green fluorescent protein constructs in a transfected CHO cell imaged on a total internal reflection fluorescence microscope using charge-coupled device area detection.

Direct measurement of relative and collective diffusion in a dilute binary colloidal suspension

Experimental characterization of the dynamics of multicomponent fluids is a problem of general importance to the field of complex fluids. We demonstrate a new experimental approach, termed two-color Fourier imaging correlation spectroscopy, which allows direct measurement of the partial dynamic structure factors, S(11)(k,tau), S(22)(k,tau), and S(12)(k,tau), where 1, 2 label the component species of a binary colloidal suspension. Linear combinations of the partial dynamic structure factors yield the characteristic time-correlation functions of the binary fluid. These are the correlation functions of concentration fluctuations S(CC)(k,tau), number density fluctuations S(NN)(k,tau), and cross-correlations between number density and concentration fluctuations S(NC)(k,tau). Test measurements are performed on a dilute symmetric mixture of fluorescently labeled 0.5 and 1.0 microm polystyrene spheres. From these data, we determine generalized collective and relative diffusion coefficients, and compare them to the predictions for an ideal mixture of noninteracting particles.

Monitoring live cell viability: Comparative study of fluorescence, oblique incidence reflection and phase contrast microscopy imaging techniques

The relative performances of fluorescence, oblique incidence reflection and phase contrast imaging techniques have been studied for the purpose of monitoring long-term cellular activity and cell viability of several types of normal and cancerous cells in cultures. Time-lapse movies of live cell imaging of untagged and green fluorescent protein (GFP) tagged cell lines are presented. Oblique incidence reflection microscopy is the simplest and least expensive method to implement, appears to be the least phototoxic to cells, and is recommended for use in long-term optical monitoring of cell viability.

Altered mitochondrial structure and motion dynamics in living cells with energy metabolism defects revealed by real time microscope imaging

Using the real time microscope (RTM), a system applying new developments in light microscopy, we documented the spatial and temporal dynamics of mitochondrial behavior in human cultured skin fibroblasts. Without the use of stains or probes, we resolved fibroblast mitochondria as dark slender filaments of approximately 0.2 m wide and up to 10 m long, as well as a few smaller ovoid forms. In the living cell, the three most common mitochondrial movements were: (1) small oscillatory movements; (2) larger movements including filament extension, retraction, and branching as well as combinations of these actions; and (3) whole transit movements of single mitochondrial filaments. Skin fibroblasts from patients with mitochondrial complex I deficiency and normal fibroblasts during incubation with rotenone, or antimycin A, contained higher proportions of mitochondria in the swollen filamentous forms, nodal filaments, and ovoid forms rather than the slender filamentous forms in normal cells. Interestingly, decreased motility was observed with the more ovoid mitochondrial forms compared to the filamentous forms. We conclude that mitochondrial morphology and dynamic motion are strongly associated with changes in mitochondrial energy metabolism. Images documenting our observations are presented both at single time points and as QuickTime videos.

Metabolic consequences of functional complexes of mitochondria, myofibrils and sarcoplasmic reticulum in muscle cells

Regulation of mitochondrial respiration both by endogenous and exogenous ADP in the cells in situ was studied in isolated and permeabilized cardiomyocytes, permeabilized cardiac fibers and 'ghost' fibers (all with a diameter of 10-20 micro m) at different (0-3 micro moll(-1)) free Ca(2+) concentrations in the medium. In all these preparations, the apparent K(m) of mitochondrial respiration for exogenous ADP at free Ca(2+) concentrations of 0-0.1 micro moll(-1) was very high, in the range of 250-350 micro moll(-1), in contrast to isolated mitochondria in vitro (apparent K(m) for ADP is approximately 20 micro moll(-1)). An increase in the free Ca(2+) concentration (up to 3 micro moll(-1), which is within physiological range), resulted in a very significant decrease of the apparent K(m) value to 20-30 micro moll(-1), a decrease of V(max) of respiration in permeabilized intact fibers and a strong contraction of sarcomeres. In ghost cardiac fibers, from which myosin was extracted but mitochondria were intact, neither the high apparent K(m) for ADP (300-350 micro moll(-1)) nor V(max) of respiration changed in the range of free Ca(2+) concentration studied, and no sarcomere contraction was observed. The exogenous-ADP-trapping system (pyruvate kinase + phosphoenolpyruvate) inhibited endogenous-ADP-supported respiration in permeabilized cells by no more than 40%, and this inhibition was reversed by creatine due to activation of mitochondrial creatine kinase. These results are taken to show strong structural associations (functional complexes) among mitochondria, sarcomeres and sarcoplasmic reticulum. Inside these complexes, mitochondrial functional state is controlled by channeling of ADP, mostly via energy- and phosphoryl-transfer networks, and apparently depends on the state of sarcomere structures.

Chronology of cellular alterations during 7-ketocholesterol-induced cell death on A7R5 rat smooth muscle cells: analysis by time lapse-video microscopy and conventional fluorescence microscopy

BACKGROUND

Time-lapse video microscopy was used to determine whether mitochondrial and nuclear changes (decrease in mitochondrial transmembrane potential, condensation, and/or fragmentation of the nuclei, morphologic features typical of apoptosis) occurring during 7-ketocholesterol-induced cell death on A7R5 rat smooth muscle cells took place before or after the loss of cell adhesion. In addition, changes in actin organization were followed by conventional fluorescence microscopy.

METHODS

Morphologic, functional, and spatial changes at the mitochondrial level were investigated with 3,3'-dihexyloxacarbocyanine iodide and/or MitoTracker Red, and nuclear morphology was characterized by staining with Hoechst 33342. Actin fibers, which are major components of the filament network of the cytoskeleton, were visualized with phalloidin linked to fluorescein. The numbers of adherent and nonadherent cells were determined by cell counting.

RESULTS

7-Ketocholesterol-induced cell death was associated with a rapid alteration of actin fibers, a loss of intercellular junctions, and cell shape modifications. Analysis of mitochondrial transmembrane potential showed successively a hyperpolarization and a more or less pronounced progressive decrease followed by a dramatic drop associated with an increase in Hoechst 33342 staining, reflecting chromatin condensation and morphologic changes in the nuclei.

CONCLUSIONS

During cell death induced by 7-ketocholesterol in A7R5 rat smooth muscle cells, the different methods of microscopy allowed us to establish that alterations of actin fibers and mitochondrial dysfunctions occurred before condensation and/or fragmentation of the nuclei, which preceded the loss of cell adhesion.

Cytoskeletal-assisted dynamics of the mitochondrial reticulum in living cells

Subcellular organelle dynamics are strongly influenced by interactions with cytoskeletal filaments and their associated motor proteins, and lead to complex multiexponential relaxations that occur over a wide range of spatial and temporal scales. Here we report spatio-temporal measurements of the fluctuations of the mitochondrial reticulum in osteosarcoma cells by using Fourier imaging correlation spectroscopy, over time and distance scales of 10(-2) to 10(3) s and 0.5-2.5 microm. We show that the method allows a more complete description of mitochondrial dynamics, through the time- and length-scale-dependent collective diffusion coefficient D(k,tau), than available by other means. Addition of either nocodazole to disrupt microtubules or cytochalasin D to disassemble microfilaments simplifies the intermediate scattering function. When both drugs are used, the reticulum morphology of mitochondria is retained even though the cytoskeletal elements have been de-polymerized. The dynamics of the organelle are then primarily diffusive and can be modeled as a collection of friction points interconnected by elastic springs. This study quantitatively characterizes organelle dynamics in terms of collective cytoskeletal interactions in living cells.

Involvement of conventional kinesin in glucose-stimulated secretory granule movements and exocytosis in clonal pancreatic beta-cells

Recruitment of secretory vesicles to the cell surface is essential for the sustained secretion of insulin in response to glucose. At present, the molecular motors involved in this movement, and the mechanisms whereby they may be regulated, are undefined. To investigate the role of kinesin family members, we labelled densecore vesicles in clonal beta-cells using an adenovirally expressed, vesicle-targeted green fluorescent protein (phogrin.EGFP), and employed immunoadsorption to obtain highly purified insulin-containing vesicles. Whereas several kinesin family members were expressed in this cell type, only conventional kinesin heavy chain (KHC) was detected in vesicle preparations. Expression of a dominant-negative KHC motor domain (KHC(mut)) blocked all vesicular movements with velocity >0.4 micro m second(-1), which demonstrates that kinesin activity was essential for vesicle motility in live beta-cells. Moreover, expression of KHC(mut) strongly inhibited the sustained, but not acute, stimulation of secretion by glucose. Finally, vesicle movement was stimulated by ATP dose-dependently in permeabilized cells, which suggests that glucose-induced increases in cytosolic [ATP] mediate the effects of the sugar in vivo, by enhancing kinesin activity. These data therefore provide evidence for a novel mechanism whereby glucose may enhance insulin release.

Quantitative proteomics: the copy number of pyruvate dehydrogenase is more than 10(2)-fold lower than that of complex III in human mitochondria

Pyruvate dehydrogenase (PDH) and complex III are two key protein complexes in mitochondrial metabolic activity. Using a novel quantitative Western blotting method, we find that PDH and complex III exist at a steady-state ratio of 1:100, 1:128 and 1:202 in HeLa cell extracts, fibroblast mitochondria and heart tissue mitochondria, respectively. This difference in stoichiometry is reflected in the immunogold labeling intensities of the two complexes and by the much more sparse distribution of PDH in fluorescence microscopy. In Rho0 fibroblasts there is a 64% reduction of complex III but the concentration of PDH remains the same as wild-type.

A replicating module as the unit of mitochondrial structure and functioning

The mitochondrion within human cells in tissue culture is pleomorphic and highly dynamic. The organelle mass can exist as thousands of small ovoids or as one continuous reticulum. In either state, the mitochondrial mass is in constant thermal motion, as well as moving in approximately 0.8-microm jumps that are determined by, and related to, attachments with cytoskeletal elements. Many protein complexes, such as the pyruvate dehydrogenase (PDH) complex and DNA containing nucleoids, are dispersed through the mass and as though fixed by attachments to membranes, such that they can become distributed to all of the individual small ovoid mitochondria when the reticulum becomes fragmented. This leads us to propose that a replicating module is the repeating unit of mitochondrial structure. Studies to examine heterogeneity of functioning within the organelle mass are briefly reviewed.

Gene therapy for pyruvate dehydrogenase E1alpha deficiency using recombinant adeno-associated virus 2 (rAAV2) vectors

To determine the feasibility of gene transfer to correct defects in the E1alpha subunit of the pyruvate dehydrogenase (PDH) complex (PDC), we constructed rAAV vectors that expressed PDH E1alpha, either alone or with a green fluorescent protein tag, from a hybrid cytomegalovirus (CMV) enhancer/chicken beta-actin (CB) promoter. These vectors were functional in vitro, as judged by increased expression of mRNA in vector-transduced deficient cell lines and correction of the biochemical defect in PDH activity in these cells. Approximately 30% of wild-type levels of PDH activity were restored under conditions with which only about 15% of cells were transduced. These same vectors were then used in vivo to transduce neurons within the rat striatum. Gene transfer, expression, and translocation into mitochondria were observed, without any obvious untoward effects. In vivo vector-mediated PDH expression persisted for at least 1 year after injection, indicating the stability of gene transfer. These studies provide the basis for future efforts to develop a recombinant AAV (rAAV)-based gene therapy approach for the correction of PDC deficiency.

Synergistic movements of Ca(2+) and Bax in cells undergoing apoptosis

Apoptosis is a physiological counterbalance to mitosis and plays important roles in tissue development and homeostasis. Cytosolic Ca(2+) has been implicated as a proapoptotic second messenger involved in both triggering apoptosis and regulating cell death-specific enzymes. A critical early event in apoptosis is associated with the redistribution of Bax from cytosol to mitochondria and endoplasmic reticulum (ER) membranes; however, the molecular mechanism of Bax translocation and its relationship to Ca(2+) is largely unknown. Here we provide functional evidence for a synergistic interaction between the movements of intracellular Ca(2+) and cytosolic Bax in the induction of apoptosis. Overexpression of Bax in cultured cells causes a loss of ER Ca(2+) content. Depletion of ER Ca(2+) through activation of the ryanodine receptor enhances the participation of Bax into the mitochondrial membrane. Neither Bax translocation nor Bax-induced apoptosis is affected by buffering of cytosolic Ca(2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, suggesting that depletion of ER Ca(2+) rather than elevation of cytosolic Ca(2+) is the signal for cell apoptosis. This dynamic interplay of Ca(2+) and Bax movements may serve as an amplifying factor in the initial signaling steps of apoptosis.

Structure and dynamics of fluorescently labeled complex fluids by fourier imaging correlation spectroscopy

We present a method of Fourier imaging correlation spectroscopy (FICS) that performs phase-sensitive measurements of modulated optical signals from fluorescently labeled complex fluids. FICS experiments probe the time-dependent trajectory of a spatial Fourier component of the fluid particle density at a specified wave number k, and provide a direct route to the intermediate scattering function. The FICS approach overcomes signal sensitivity problems associated with dynamic light scattering, while offering a means to acquire time-dependent information about spatial distributions of fluorescent particles, superior in efficiency to direct imaging methods. We describe the instrumental setup necessary to perform FICS experiments, and outline the theory that establishes the connection between FICS observables and statistical mechanical quantities describing liquid state dynamics. Test measurements on monolayer suspensions of rhodamine labeled polystyrene spheres are detailed.