An Indirect Method of Micromagnetic Structure Estimation in Microwires

The tunable magnetic properties of amorphous ferromagnetic glass-coated microwires make them suitable for a wide range of applications. Accurate knowledge of the micromagnetic structure is highly desirable since it affects almost all magnetic properties. To select an appropriate wire-sample for a specific application, a deeper understanding of the magnetization reversal process is required, because it determines the measurable response (such as induced voltage waveform and its spectrum). However, the experimental observation of micromagnetic structure of micro-scale amorphous objects has strict size limitations. In this work we proposed a novel experimental technique for evaluating the microstructural characteristics of glass-coated microwires. The cross-sectional permeability distribution in the sample was obtained from impedance measurements at different frequencies. This distribution enables estimation of the prevailing anisotropy in the local region of the wire cross-section. The results obtained were compared with the findings of magnetostatic measurements and remanent state analysis. The advantages and limitations of the methods were discussed.

Persistent growth of microtubules at low density

Microtubules (MTs) often form a polarized array with minus ends anchored at the centrosome and plus ends extended toward the cell margins. Plus ends display behavior known as dynamic instability—transitions between rapid shortening and slow growth. It is known that dynamic instability is regulated locally to ensure entry of MTs into nascent areas of the cytoplasm, but details of this regulation remain largely unknown. Here, we test an alternative hypothesis for the local regulation of MT behavior. We used microsurgery to isolate a portion of peripheral cytoplasm from MTs growing from the centrosome, creating cytoplasmic areas locally depleted of MTs. We found that in sparsely populated areas MT plus ends persistently grew or paused but never shortened. In contrast, plus ends that entered regions of cytoplasm densely populated with MTs frequently transitioned to shortening. Persistent growth of MTs in sparsely populated areas could not be explained by a local increase in concentration of free tubulin subunits or elevation of Rac1 activity proposed to enhance MT growth at the cell leading edge during locomotion. These observations suggest the existence of a MT density–dependent mechanism regulating MT dynamics that determines dynamic instability of MTs in densely populated areas of the cytoplasm and persistent growth in sparsely populated areas.

Stimulation of microtubule-based transport by nucleation of microtubules on pigment granules

In Xenopus melanophores, nucleation of microtubules on pigment granules provides a positive feedback loop that enhances their transport to the cell center during pigment aggregation.

Microtubule (MT)-based transport can be regulated through changes in organization of MT transport tracks, but the mechanisms that regulate these changes are poorly understood. In Xenopus melanophores, aggregation of pigment granules in the cell center involves their capture by the tips of MTs growing toward the cell periphery, and granule aggregation signals facilitate capture by increasing the number of growing MT tips. This increase could be explained by stimulation of MT nucleation either on the centrosome or on the aggregate of pigment granules that gradually forms in the cell center. We blocked movement of pigment granules to the cell center and compared the MT-nucleation activity of the centrosome in the same cells in two signaling states. We found that granule aggregation signals did not stimulate MT nucleation on the centrosome but did increase MT nucleation activity of pigment granules. Elevation of MT-nucleation activity correlated with the recruitment to pigment granules of a major component of MT-nucleation templates, γ-tubulin, and was suppressed by γ-tubulin inhibitors. We conclude that generation of new MT transport tracks by concentration of the leading pigment granules provides a positive feedback loop that enhances delivery of trailing granules to the cell center.

Engineered Tug-of-War Between Kinesin and Dynein Controls Direction of Microtubule Based Transport In Vivo

Bidirectional transport of membrane organelles along microtubules (MTs) is driven by plus-end directed kinesins and minus-end directed dynein bound to the same cargo. Activities of opposing MT motors produce bidirectional movement of membrane organelles and cytoplasmic particles along MT transport tracks. Directionality of MT-based transport might be controlled by a protein complex that determines which motor type is active at any given moment of time, or determined by the outcome of a tug-of-war between MT motors dragging cargo organelles in opposite directions. However, evidence in support of each mechanisms of regulation is based mostly on the results of theoretical analyses or indirect experimental data. Here, we test whether the direction of movement of membrane organelles in vivo can be controlled by the tug-of-war between opposing MT motors alone, by attaching a large number of kinesin-1 motors to organelles transported by dynein to minus-ends of MTs. We find that recruitment of kinesin significantly reduces the length and velocity of minus-end-directed dynein-dependent MT runs, leading to a reversal of the overall direction of dynein-driven organelles in vivo. Therefore, in the absence of external regulators tug-of-war between opposing MT motors alone is sufficient to determine the directionality of MT transport in vivo.

Loss of spastin function results in disease-specific axonal defects in human pluripotent stem cell-based models of hereditary spastic paraplegia

Human neuronal models of hereditary spastic paraplegias (HSP) that recapitulate disease-specific axonal pathology hold the key to understanding why certain axons degenerate in patients and to developing therapies. SPG4, the most common form of HSP, is caused by autosomal dominant mutations in the SPAST gene, which encodes the microtubule-severing ATPase spastin. Here, we have generated a human neuronal model of SPG4 by establishing induced pluripotent stem cells (iPSCs) from an SPG4 patient and differentiating these cells into telencephalic glutamatergic neurons. The SPG4 neurons displayed a significant increase in axonal swellings, which stained strongly for mitochondria and tau, indicating the accumulation of axonal transport cargoes. In addition, mitochondrial transport was decreased in SPG4 neurons, revealing that these patient iPSC-derived neurons recapitulate disease-specific axonal phenotypes. Interestingly, spastin protein levels were significantly decreased in SPG4 neurons, supporting a haploinsufficiency mechanism. Furthermore, cortical neurons derived from spastin-knockdown human embryonic stem cells (hESCs) exhibited similar axonal swellings, confirming that the axonal defects can be caused by loss of spastin function. These spastin-knockdown hESCs serve as an additional model for studying HSP. Finally, levels of stabilized acetylated-tubulin were significantly increased in SPG4 neurons. Vinblastine, a microtubule-destabilizing drug, rescued this axonal swelling phenotype in neurons derived from both SPG4 iPSCs and spastin-knockdown hESCs. Thus, this study demonstrates the successful establishment of human pluripotent stem cell-based neuronal models of SPG4, which will be valuable for dissecting the pathogenic cellular mechanisms and screening compounds to rescue the axonal degeneration in HSP.

Regulation of microtubule-based transport by MAP4

Microtubule (MT)-based transport of organelles driven by the opposing MT motors kinesins and dynein is tightly regulated in cells, but the underlying molecular mechanisms remain largely unknown. Here we tested the regulation of MT transport by the ubiquitous protein MAP4 using Xenopus melanophores as an experimental system. In these cells, pigment granules (melanosomes) move along MTs to the cell center (aggregation) or to the periphery (dispersion) by means of cytoplasmic dynein and kinesin-2, respectively. We found that aggregation signals induced phosphorylation of threonine residues in the MT-binding domain of the Xenopus MAP4 (XMAP4), thus decreasing binding of this protein to MTs. Overexpression of XMAP4 inhibited pigment aggregation by shortening dynein-dependent MT runs of melanosomes, whereas removal of XMAP4 from MTs reduced the length of kinesin-2-dependent runs and suppressed pigment dispersion. We hypothesize that binding of XMAP4 to MTs negatively regulates dynein-dependent movement of melanosomes and positively regulates kinesin-2-based movement. Phosphorylation during pigment aggregation reduces binding of XMAP4 to MTs, thus increasing dynein-dependent and decreasing kinesin-2-dependent motility of melanosomes, which stimulates their accumulation in the cell center, whereas dephosphorylation of XMAP4 during dispersion has an opposite effect.

Force-dependent detachment of kinesin-2 biases track switching at cytoskeletal filament intersections

Intracellular trafficking of organelles often involves cytoskeletal track switching. Organelles such as melanosomes are transported by multiple motors including kinesin-2, dynein, and myosin-V, which drive switching between microtubules and actin filaments during dispersion and aggregation. Here, we used optical trapping to determine the unitary and ensemble forces of kinesin-2, and to reconstitute cargo switching at cytoskeletal intersections in a minimal system with kinesin-2 and myosin-V motors bound to beads. Single kinesin-2 motors exerted forces up to ∼5 pN, similar to kinesin-1. However, kinesin-2 motors were more likely to detach at submaximal forces, and the duration of force maintenance was short as compared to kinesin-1. In multimotor assays, force increased with kinesin-2 density but was not affected by the presence of myosin-V. In crossed filament assays, switching frequencies of motor-bound beads were dependent on the starting track. At equal average forces, beads tended to switch from microtubules onto overlying actin filaments consistent with the relatively faster detachment of kinesin-2 at near-maximal forces. Thus, in addition to relative force, switching probability at filament intersections is determined by the dynamics of motor-filament interaction, such as the quick detachment of kinesin-2 under load. This may enable fine-tuning of filament switching in the cell.

Stimulation of the CLIP-170--dependent capture of membrane organelles by microtubules through fine tuning of microtubule assembly dynamics

Cytoplasmic microtubules (MTs) continuously grow and shorten at their free plus ends, a behavior that allows them to capture membrane organelles destined for MT minus end-directed transport. In Xenopus melanophores, the capture of pigment granules (melanosomes) involves the +TIP CLIP-170, which is enriched at growing MT plus ends. Here we used Xenopus melanophores to test whether signals that stimulate minus end MT transport also enhance CLIP-170-dependent binding of melanosomes to MT tips. We found that these signals significantly (>twofold) increased the number of growing MT plus ends and their density at the cell periphery, thereby enhancing the likelihood of interaction with dispersed melanosomes. Computational simulations showed that local and global increases in the density of CLIP-170-decorated MT plus ends could reduce the half-time of melanosome aggregation by ~50%. We conclude that pigment granule aggregation signals in melanophores stimulate MT minus end-directed transport by the increasing number of growing MT plus ends decorated with CLIP-170 and redistributing these ends to more efficiently capture melanosomes throughout the cytoplasm.

CK1 activates minus-end-directed transport of membrane organelles along microtubules

This study shows that the signal transduction pathway responsible for the initiation of minus-end–directed movement of membrane-bounded pigment granules in melanophores involves sequential activation of protein phosphatase 2A and casein kinase 1 and that this activation correlates with increased phosphorylation of the dynein intermediate chain.

Microtubule (MT)-based organelle transport is driven by MT motor proteins that move cargoes toward MT minus-ends clustered in the cell center (dyneins) or plus-ends extended to the periphery (kinesins). Cells are able to rapidly switch the direction of transport in response to external cues, but the signaling events that control switching remain poorly understood. Here, we examined the signaling mechanism responsible for the rapid activation of dynein-dependent MT minus-end–directed pigment granule movement in Xenopus melanophores (pigment aggregation). We found that, along with the previously identified protein phosphatase 2A (PP2A), pigment aggregation signaling also involved casein kinase 1ε (CK1ε), that both enzymes were bound to pigment granules, and that their activities were increased during pigment aggregation. Furthermore we found that CK1ε functioned downstream of PP2A in the pigment aggregation signaling pathway. Finally, we discovered that stimulation of pigment aggregation increased phosphorylation of dynein intermediate chain (DIC) and that this increase was partially suppressed by CK1ε inhibition. We propose that signal transduction during pigment aggregation involves successive activation of PP2A and CK1ε and CK1ε-dependent phosphorylation of DIC, which stimulates dynein motor activity and increases minus-end–directed runs of pigment granules.

Finding the cell center by a balance of dynein and myosin pulling and microtubule pushing: a computational study

The centrosome position in many types of interphase cells is actively maintained in the cell center. Our previous work indicated that the centrosome is kept at the center by pulling force generated by dynein and actin flow produced by myosin contraction and that an unidentified factor that depends on microtubule dynamics destabilizes position of the centrosome. Here, we use modeling to simulate the centrosome positioning based on the idea that the balance of three forces-dyneins pulling along microtubule length, myosin-powered centripetal drag, and microtubules pushing on organelles-is responsible for the centrosome displacement. By comparing numerical predictions with centrosome behavior in wild-type and perturbed interphase cells, we rule out several plausible hypotheses about the nature of the microtubule-based force. We conclude that strong dynein- and weaker myosin-generated forces pull the microtubules inward competing with microtubule plus-ends pushing the microtubule aster outward and that the balance of these forces positions the centrosome at the cell center. The model also predicts that kinesin action could be another outward-pushing force. Simulations demonstrate that the force-balance centering mechanism is robust yet versatile. We use the experimental observations to reverse engineer the characteristic forces and centrosome mobility.

Melanophores for microtubule dynamics and motility assays

Microtubules (MTs) are cytoskeletal structures essential for cell division, locomotion, intracellular transport, and spatial organization of the cytoplasm. In most interphase cells, MTs are organized into a polarized radial array with minus-ends clustered at the centrosome and plus-ends extended to the cell periphery. This array directs transport of organelles driven by MT-based motor proteins that specifically move either to plus- or to minus-ends. Along with using MTs as tracks for cargo, motor proteins can organize MTs into a radial array in the absence of the centrosome. Transport of organelles and motor-dependent radial organization of MTs require MT dynamics, continuous addition and loss of tubulin subunits at minus- and plus-ends. A unique experimental system for studying the role of MT dynamics in these processes is the melanophore, which provides a useful tool for imaging of both dynamic MTs and moving membrane organelles. Melanophores are filled with pigment granules that are synchronously transported by motor proteins in response to hormonal stimuli. The flat shape of the cell and the radial organization of MTs facilitate imaging of dynamic MT plus-ends and monitoring of their interaction with membrane organelles. Microsurgically produced cytoplasmic fragments of melanophores are used to study the centrosome-independent rearrangement of MTs into a radial array. Here we describe the experimental approaches to study the role of MT dynamics in intracellular transport and centrosome-independent MT organization in melanophores. We focus on the preparation of cell cultures, microsurgery and microinjection, fluorescence labeling, and live imaging of MTs.

Investigating sub-spine actin dynamics in rat hippocampal neurons with super-resolution optical imaging

Morphological changes in dendritic spines represent an important mechanism for synaptic plasticity which is postulated to underlie the vital cognitive phenomena of learning and memory. These morphological changes are driven by the dynamic actin cytoskeleton that is present in dendritic spines. The study of actin dynamics in these spines traditionally has been hindered by the small size of the spine. In this study, we utilize a photo-activation localization microscopy (PALM)–based single-molecule tracking technique to analyze F-actin movements with ∼30-nm resolution in cultured hippocampal neurons. We were able to observe the kinematic (physical motion of actin filaments, i.e., retrograde flow) and kinetic (F-actin turn-over) dynamics of F-actin at the single-filament level in dendritic spines. We found that F-actin in dendritic spines exhibits highly heterogeneous kinematic dynamics at the individual filament level, with simultaneous actin flows in both retrograde and anterograde directions. At the ensemble level, movements of filaments integrate into a net retrograde flow of ∼138 nm/min. These results suggest a weakly polarized F-actin network that consists of mostly short filaments in dendritic spines.

CLIP-170-dependent capture of membrane organelles by microtubules initiates minus-end directed transport

Cytoplasmic microtubules (MTs) continuously grow and shorten at free plus ends. During mitosis, this dynamic behavior allows MTs to capture chromosomes to initiate their movement to the spindle poles; however, the role of MT dynamics in capturing organelles for transport in interphase cells has not been demonstrated. Here we use Xenopus melanophores to test the hypothesis that MT dynamics significantly contribute to the efficiency of MT minus-end directed transport of membrane organelles. We demonstrate that initiation of transport of membrane-bounded melanosomes (pigment granules) to the cell center involves their capture by MT plus ends, and that inhibition of MT dynamics or loss of the MT plus-end tracking protein CLIP-170 from MT tips dramatically inhibits pigment aggregation. We conclude that MT dynamics are required for the initiation of MT transport of membrane organelles in interphase cells, and that +TIPs such as CLIP-170 play an important role in this process.

Quantitative phosphoproteomic analysis of T cell receptor signaling reveals system-wide modulation of protein-protein interactions

Protein phosphorylation events during T cell receptor (TCR) signaling control the formation of complexes among proteins proximal to the TCR, the activation of kinase cascades, and the activation of transcription factors; however, the mode and extent of the influence of phosphorylation in coordinating the diverse phenomena associated with T cell activation are unclear. Therefore, we used the human Jurkat T cell leukemia cell line as a model system and performed large-scale quantitative phosphoproteomic analyses of TCR signaling. We identified 10,665 unique phosphorylation sites, of which 696 showed TCR-responsive changes. In addition, we analyzed broad trends in phosphorylation data sets to uncover underlying mechanisms associated with T cell activation. We found that, upon stimulation of the TCR, phosphorylation events extensively targeted protein modules involved in all of the salient phenomena associated with T cell activation: patterning of surface proteins, endocytosis of the TCR, formation of the F-actin cup, inside-out activation of integrins, polarization of microtubules, production of cytokines, and alternative splicing of messenger RNA. Further, case-by-case analysis of TCR-responsive phosphorylation sites on proteins belonging to relevant functional modules together with network analysis allowed us to deduce that serine-threonine (S-T) phosphorylation modulated protein-protein interactions (PPIs) in a system-wide fashion. We also provide experimental support for this inference by showing that phosphorylation of tubulin on six distinct serine residues abrogated PPIs during the assembly of microtubules. We propose that modulation of PPIs by stimulus-dependent changes in S-T phosphorylation state is a widespread phenomenon applicable to many other signaling systems.

Wavelet analysis of the frontal auditory evoked potentials obtained in the passive oddball paradigm in healthy subjects and schizophrenics

The goal of the present study was to apply the oscillatory brain dynamics model to the structural and quantitative analysis of neurocognitive functions considered as a potential marker of schizophrenia. This was achieved in tests of the detection of auditory events deviating in the regular auditory stream (oddball paradigm, MMN effect). It was hypothesized that the post-stimulus peaks of the oscillation power localized in post-stimulus time in the definite EEG oscillators represented neuro-electrical 'events' evoked in the specific neuronal nets characterized by this oscillation frequency band. We suggest that the time-frequency destination of these events related to the activation of the functional neuronal nets could be used for the determination of specific neurocognitive functions. Thus it was an attempt to distinguish the different neuro-functional parts of auditory processing and to compare these results between healthy subjects and patients with schizophrenia. The present results demonstrate the significant difference between the frontal averaged EEG oscillatory dynamics in healthy subjects and patients with schizophrenia related to neurocognitive function marked by the MMN and orienting response N200/P300a.

Actin dynamics is essential for myosin-based transport of membrane organelles

Actin filaments that serve as "rails" for the myosin-based transport of membrane organelles [1-4] continuously turn over by concurrent growth and shortening at the opposite ends [5]. Although it is known that dynamics of actin filaments is essential for many of the actin cytoskeleton functions, the role of such dynamics in myosin-mediated organelle transport was never studied before. Here, we addressed the role of turnover of actin filaments in the myosin-based transport of membrane organelles by treating cells with the drugs that suppress actin-filament dynamics and found that such a suppression significantly inhibited organelle transport along the actin filaments without inhibiting their intracellular distribution or the activity of the myosin motors. We conclude that dynamics of actin filaments is essential for myosin-based transport of membrane organelles and suggest a previously unknown role of actin-filament dynamics in providing the "rails" for continuous organelle movement resulting in the increased distances traveled by membrane organelles along the actin filaments.

The protein kinase A-anchoring protein moesin is bound to pigment granules in melanophores

Major signaling cascades have been shown to play a role in the regulation of intracellular transport of organelles. In Xenopus melanophores, aggregation and dispersion of pigment granules are regulated by the second messenger cyclic AMP through the protein kinase A (PKA) signaling pathway. PKA is bound to pigment granules where it forms complexes with molecular motors involved in pigment transport. Association of PKA with pigment granules occurs through binding to A-kinase-anchoring proteins (AKAPs), whose identity remains largely unknown. In this study, we used mass spectrometry to examine an 80 kDa AKAP detected in preparations of purified pigment granules. We found that tryptic digests of granule protein fractions enriched in the 80 kDa AKAP contained peptides that corresponded to the actin-binding protein moesin, which has been shown to function as an AKAP in mammalian cells. We also found that recombinant Xenopus moesin interacted with PKA in vitro, copurified with pigment granules and bound to pigment granules in cells. Overexpression in melanophores of a mutant moesin lacking conserved PKA-binding domain did not affect aggregation of pigment granules but partially inhibited their dispersion. We conclude that Xenopus moesin is an AKAP whose PKA-scaffolding activity plays a role in the regulation of pigment dispersion in Xenopus melanophores.

Cytoplasmic dynein is involved in the retention of microtubules at the centrosome in interphase cells

Cytoplasmic dynein is known to be involved in the establishment of radial microtubule (MT) arrays. During mitosis, dynein activity is required for tethering of the MTs at the spindle poles. In interphase cells, dynein inhibitors induce loss of radial MT organization; however, the exact role of dynein in the maintenance of MT arrays is unclear. Here, we examined the effect of dynein inhibitors on MT distribution and the centrosome protein composition in cultured fibroblasts. We found that while these inhibitors induced rapid (t(1/2) approximately 20 min) loss of radial MT organization, the levels of key centrosomal proteins or the rates of MT nucleation did not change significantly in dynein-inhibited cells, suggesting that the loss of dynein activity does not affect the structural integrity of the centrosome or its capacity to nucleate MTs. Live observations of the centrosomal activity showed that dynein inhibition enhanced the detachment of MTs from the centrosome. We conclude that the primary role of dynein in the maintenance of a radial MT array in interphase cells consists of retention of MTs at the centrosome and hypothesize that dynein has a role in the MT retention, separate from the delivery to the centrosome of MT-anchoring proteins.

Switching of membrane organelles between cytoskeletal transport systems is determined by regulation of the microtubule-based transport

Intracellular transport of membrane organelles occurs along microtubules (MTs) and actin filaments (AFs). Although transport along each type of the cytoskeletal tracks is well characterized, the switching between the two types of transport is poorly understood because it cannot be observed directly in living cells. To gain insight into the regulation of the switching of membrane organelles between the two major transport systems, we developed a novel approach that combines live cell imaging with computational modeling. Using this approach, we measured the parameters that determine how fast membrane organelles switch back and forth between MTs and AFs (the switching rate constants) and compared these parameters during different signaling states. We show that regulation involves a major change in a single parameter: the transferring rate from AFs onto MTs. This result suggests that MT transport is the defining factor whose regulation determines the choice of the cytoskeletal tracks during the transport of membrane organelles.

Microtubule motor Ncd induces sliding of microtubules in vivo

The mitotic spindle is a microtubule (MT)-based molecular machine that serves for equal segregation of chromosomes during cell division. The formation of the mitotic spindle requires the activity of MT motors, including members of the kinesin-14 family. Although evidence suggests that kinesins-14 act by driving the sliding of MT bundles in different areas of the spindle, such sliding activity had never been demonstrated directly. To test the hypothesis that kinesins-14 can induce MT sliding in living cells, we developed an in vivo assay, which involves overexpression of the kinesin-14 family member Drosophila Ncd in interphase mammalian fibroblasts. We found that green fluorescent protein (GFP)-Ncd colocalized with cytoplasmic MTs, whose distribution was determined by microinjection of Cy3 tubulin into GFP-transfected cells. Ncd overexpression resulted in the formation of MT bundles that exhibited dynamic "looping" behavior never observed in control cells. Photobleaching studies and fluorescence speckle microscopy analysis demonstrated that neighboring MTs in bundles could slide against each other with velocities of 0.1 microm/s, corresponding to the velocities of movement of the recombinant Ncd in in vitro motility assays. Our data, for the first time, demonstrate generation of sliding forces between adjacent MTs by Ncd, and they confirm the proposed roles of kinesins-14 in the mitotic spindle morphogenesis.

Fluorescence microscopy of microtubules in cultured cells

Cytoplasmic microtubules are noncovalent polymers of the protein tubulin. In the cells, the main function of microtubules is to provide tracks for organelle transport. Two experimental approaches based on fluorescence microscopy are commonly used to examine organization of microtubules in mammalian tissue culture cells. The first experimental approach involves indirect immunofluorescence staining of chemically fixed cells with tubulin antibody. Fluorescence microscopy of immunostained specimens allows the examination of the distribution of microtubules in the cytoplasm at the moment of fixation. The second experimental approach involves introduction of tubulin subunits covalently labeled with a fluorochrome into the cytoplasm of living cells. Time-lapse fluorescence microscopy of cells containing labeled tubulin subunits allows to examine changes in the spatial organization of microtubules in the cytoplasm and also to directly observe their behavior. In this chapter, we describe preparation of samples for fluorescence microscopy of microtubules.

Nonlocal Mechanism of Self-Organization and Centering of Microtubule Asters

Fragments of fish melanophore cells can form and center aggregates of pigment granules by dynein-motor-driven transport along a self-organized radial array of microtubules (MTs). We present a quantitative model that describes pigment aggregation and MT-aster self-organization and the subsequent centering of both structures. The model is based on the observations that MTs are immobile and treadmill, while dynein-motor-covered granules have the ability to nucleate MTs. From assumptions based on experimental observations, we derive partial integro-differential equations describing the coupled granule-MT interaction. We use scaling arguments and perturbation theory to study the model in two limiting cases. The model analysis explains the mechanism of aster self-organization as a positive feedback loop between motor aggregation at the MT minus ends and MT nucleation by motors. Furthermore, the centering mechanism is explained by the spontaneous nucleation of MTs throughout the cytosol which acts as a volume sensing tool. Numerical simulations lend additional support to the analysis. The model sheds light on role of polymer dynamics and polymer-motor interactions in cytoskeletal organization.

Intracellular organelle transport: few motors, many signals

Bidirectional microtubule-dependent organelle transport in melanophores is regulated by cAMP through organelle-bound protein kinase A (PKA); however, the mechanisms responsible for this regulation are unknown. A recent study by Gelfand and colleagues demonstrates that, in addition to PKA, transport is regulated by the organelle-bound mitogen-activated protein kinase (MAPK) signaling components ERK and MEK, whose activity is required for bidirectional transport along microtubules. This pathway apparently acts downstream of PKA, suggesting that bidirectional organelle transport is regulated by a hierarchical cascade of signaling pathways.

Centering of a radial microtubule array by translocation along microtubules spontaneously nucleated in the cytoplasm

Positioning of a radial array of microtubules (MTs) in the cell centre is crucial for cytoplasmic organization, but the mechanisms of such centering are difficult to study in intact cells that have pre-formed radial arrays. Here, we use cytoplasmic fragments of melanophores, and cytoplasts of BS-C-1 cells to study MT centering mechanisms. Using live imaging and computer modelling, we show that the MT aster finds a central location in the cytoplasm by moving along spontaneously nucleated non-astral MTs towards a point at which MT nucleation events occur equally on all sides. We hypothesize that similar mechanisms, in the presence of the centrosome, contribute to this centering mechanism and ensure the robustness of cytoplasmic organization.

Multiscale trend analysis of microtubule transport in melanophores

Microtubule-based transport is critical for trafficking of organelles, organization of endomembranes, and mitosis. The driving force for microtubule-based transport is provided by microtubule motors, which move organelles specifically to the plus or minus ends of the microtubules. Motor proteins of opposite polarities are bound to the surface of the same cargo organelle. Transport of organelles along microtubules is discontinuous and involves transitions between movements to plus or minus ends or pauses. Parameters of the movement, such as velocity and length of runs, provide important information about the activity of microtubule motors, but measurement of these parameters is difficult and requires a sophisticated decomposition of the organelle movement trajectories into directional runs and pauses. The existing algorithms are based on establishing threshold values for the length and duration of runs and thus do not allow to distinguish between slow runs and pauses, making the analysis of the organelle transport incomplete. Here we describe a novel algorithm based on multiscale trend analysis for the decomposition of organelle trajectories into plus- or minus-end runs, and pauses. This algorithm is self-adapted to the characteristic durations and velocities of runs, and allows reliable separation of pauses from runs. We apply the proposed algorithm to compare regulation of microtubule transport in fish and Xenopus melanophores and show that the general mechanisms of regulation are similar in the two pigment cell types.

Multiphoton-excited microfabrication in live cells via Rose Bengal cross-linking of cytoplasmic proteins

We demonstrate the use of multiphoton-excited photochemistry to cross-link three-dimensional matrices directly from cytoplasmic proteins in a live cell (starfish oocyte). Fluorescence recovery after photobleaching measurements were used to determine diffusion coefficients inside intracellular cross-linked structures, and it was found that the diffusion was approximately 3 to 4 orders of magnitude slower than in free solution and 2-3 orders of magnitude slower than in cytoplasm and that the value can be tuned by controlling the laser exposure. Complex structures can be fabricated to construct channels and compartments that could be used to isolate cellular processes, and the method should thus be applicable to a broad range of problems in cell biology.

Intracellular actin-based transport: how far you go depends on how often you switch

Intracellular molecular motor-driven transport is essential for such diverse processes as mitosis, neuronal function, and mitochondrial transport. Whereas there have been in vitro studies of how motors function at the single-molecule level, and in vivo studies of the structure of filamentary networks, studies of how the motors effectively use the networks for transportation have been lacking. We investigate how the combined system of myosin-V motors plus actin filaments is used to transport pigment granules in Xenopus melanophores. Experimentally, we characterize both the actin filament network, and how this transport is altered in response to external signals. We then develop a theoretical formalism to explain these changes. We show that cells regulate transport by controlling how often granules switch from one filament to another, rather than by altering individual motor activity at the single-molecule level, or by relying on structural changes in the network.

Imagination of body rotation can induce eye movements

OBJECTIVE

Several studies have shown that spatiotemporal aspects of motion are stored and can be retrieved with the use of vestibular and somatosensory cues. The purpose of this study was to examine whether intentional imagination of body rotation can induce oculomotor activity similar to the typical vestibulo-ocular reflex (VOR).

MATERIAL AND METHODS

Normal subjects without known vestibular and/or oculometric abnormalities were instructed to imagine a sensation of accelerating body rotation in the horizontal plane (rightward or leftward) while sitting in darkness with closed eyes, using only vestibular and somatosensory cues and not imaginary visual cues. Eye movements were recorded during the imagery session and also during a full, routine electronystagmography (ENG) test. All subjects selected for this study showed normal results in the ENG test, and none of them had gaze-evoked or end-point nystagmus.

RESULTS

In response to imaginary rotations, horizontal eye movements were found in 91/121 recordings (75%) in 10 subjects. A typical pattern of nystagmus (0.3-3 Hz, 3-30 degrees /s maximal speed of slow component) was recorded in 53% of mental rightward rotations and 49% of leftward rotations. The fast component was always in the direction of the imaginary rotation (similar to a normal VOR). Other types of eye movement comprised either contralateral eye drift ( approximately 17% of trials) or macro square waves. In 25% of the recordings no definite eye movements could be detected during the mental maneuvers.

CONCLUSION

These mentally induced eye movements seem to be due to a cortical process which can affect the normal input to the brainstem nuclei. A possible mechanism is discussed. This phenomenon may serve as an objective measurement of mental activity, may be used for testing the cognitive resources of patients and can probably be used for enhancing the rehabilitation process after acute vestibular insult.

Cytoplasmic dynein nucleates microtubules to organize them into radial arrays in vivo

Numerous evidence demonstrates that dynein is crucial for organization of microtubules (MTs) into radial arrays, but its exact function in this process is unclear. Here, we studied the role of cytoplasmic dynein in MT radial array formation in the absence of the centrosome. We found that dynein is a potent MT nucleator in vitro and that stimulation of dynein activity in cytoplasmic fragments of melanophores induces nucleation-dependent formation of MT radial array in the absence of the centrosome. This new property of dynein, in combination with its known role as an MT motor that is essential for MT array organization in the absence and presence of the centrosome, makes it a unique molecule whose activity is necessary and sufficient for the formation and maintenance of MT radial arrays in cells.

Computational model of dynein-dependent self-organization of microtubule asters

Polar arrays of microtubules play many important roles in the cell. Normally, such arrays are organized by a centrosome anchoring the minus ends of the microtubules, while the plus ends extend to the cell periphery. However, ensembles of molecular motors and microtubules also demonstrate the ability to self-organize into polar arrays. We use quantitative modeling to analyze the self-organization of microtubule asters and the aggregation of motor-driven pigment granules in fragments of fish melanophore cells. The model is based on the observation that microtubules are immobile and treadmilling, and on the experimental evidence that cytoplasmic dynein motors associated with granules have the ability to nucleate MTs and attenuate their minus-end dynamics. The model explains the observed sequence of events as follows. Initially, pigment granules driven by cytoplasmic dynein motors aggregate to local clusters of microtubule minus ends. The pigment aggregates then nucleate microtubules with plus ends growing toward the fragment boundary, while the minus ends stay transiently in the aggregates. Microtubules emerging from one aggregate compete with any aggregates they encounter leading to the gradual formation of a single aggregate. Simultaneously, a positive feedback mechanism drives the formation of a single MT aster--a single loose aggregate leads to focused MT nucleation and hence a tighter aggregate which stabilizes MT minus ends more effectively leading to aster formation. We translate the model assumptions based on experimental measurements into mathematical equations. The model analysis and computer simulations successfully reproduce the observed pathways of pigment aggregation and microtubule aster self-organization. We test the model predictions by observing the self-organization in fragments of various sizes and in bi-lobed fragments. The model provides stringent constraints on rates and concentrations describing microtubule and motor dynamics, and sheds light on the role of polymer dynamics and polymer-motor interactions in cytoskeletal organization.

Switching between microtubule- and actin-based transport systems in melanophores is controlled by cAMP levels

BACKGROUND

Intracellular transport involves the movement of organelles along microtubules (MTs) or actin filaments (AFs) by means of opposite-polarity MT motors or actin-dependent motors of the myosin family. The correct delivery of organelles to their different destinations involves a precise coordination of the two transport systems. Such coordination could occur through regulation of the densities of the two cytoskeletal systems or through regulation of the activities of the cytoskeletal motors by signaling mechanisms.

RESULTS

To investigate the mechanisms of switching between MT and AF-dependent transport, we examine the influence of the densities of the MT and AF network on pigment transport in fish melanophores. We also change signaling by using activators and inhibitors of Protein Kinase A (PKA). We find that the key parameters characterizing pigment granule transport along MTs do not depend on MT density and are not significantly altered by complete disruption of AFs. In contrast, the kinetics of changes in these parameters correlate with the kinetics of changes in the intracellular levels of cAMP and are affected by the inhibitors of PKA, suggesting the regulation of MT- and AF-dependent motors by cAMP-induced signaling. Furthermore, perturbation of cAMP levels prevents the transfer of pigment granules from MTs onto AFs.

CONCLUSIONS

We conclude that the switching of pigment granules between the two major cytoskeletal systems is independent of the densities of MT or AF but is tightly controlled by signaling events.

The contribution of the time locking of EEG waves to the generation of the auditory P300

OBJECTIVES

To study the time locking of the 'natural' (delta, theta, slow alpha, fast alpha and beta) EEG waves during the generation of the P300 in passive (P300a) and active (P300b) auditory oddball paradigms in order to obtain insights into the generation of the P300 and into the transitions between background and evoked activity.

METHODS

Tone burst stimuli (standard and deviant) were delivered to normal young adult subjects in passive and active oddball paradigms. The time distributions of EEG waves were analyzed in several frequency bands during background and post-stimulus periods.

RESULTS

The ongoing background activity was modified by the deviant stimulus, producing the time locking of the positive delta, theta and alpha EEG deflections in the time range of the P300. This involved prolongation of the positive component of negative-positive wave complexes so that the positive wave was delayed into the time period of the P300. The time locking effects were more prominent in the delta and theta ranges, and differed in frequency components and scalp topography between the two paradigms. Not all deviant stimulus trials contributed deflections to the P300. The contributing trials can be selected, providing the basis for single trial analysis.

CONCLUSIONS

The study of the time locking of the EEG waves in different frequency bands provides improved analysis of the P300 and an approach to single deviant stimulus trial analysis, that in turn can enhance signal-to-noise ratios. The results show that the time reorganization of EEG can be considered in the generation of P300 separately from the amplitude factor.

SIGNIFICANCE

This can lead to improved analysis of normal and abnormal brain function in individual subjects.

Centrosome positioning in interphase cells

The position of the centrosome is actively maintained at the cell center, but the mechanisms of the centering force remain largely unknown. It is known that centrosome positioning requires a radial array of cytoplasmic microtubules (MTs) that can exert pushing or pulling forces involving MT dynamics and the activity of cortical MT motors. It has also been suggested that actomyosin can play a direct or indirect role in this process. To examine the centering mechanisms, we introduced an imbalance of forces acting on the centrosome by local application of an inhibitor of MT assembly (nocodazole), and studied the resulting centrosome displacement. Using this approach in combination with microinjection of function-blocking probes, we found that a MT-dependent dynein pulling force plays a key role in the positioning of the centrosome at the cell center, and that other forces applied to the centrosomal MTs, including actomyosin contractility, can contribute to this process.

Analysis of visual evoked potentials and background electroencephalographic activity in young and elderly subjects

OBJECTIVE

New techniques developed in this laboratory to overcome the loss of information involved in conventional evoked potential averaging are applied here to visual evoked potential (VEP) in young and elderly normal subjects.

METHODS

The techniques are based on statistical descriptions of the times and amplitudes of the electroencephalographic deflections recorded before (background) and after (evoked) a series of pattern reversal visual stimuli.

RESULTS

The elderly had a higher rate of background deflections at all electrode sites, but lower amplitudes at the occipital electrodes. The elderly had a lower rate of deflections during the period of evoked activity compared with the background period. The young had a higher degree of response deflection time locking and amplitude. The mean amplitude of the deflections recorded during the period of evoked activity was significantly greater in amplitude than the background deflections and greater than the amplitudes of the conventionally averaged VEP derived from the same data. Thus the lower amplitude VEPs seen in the elderly are due to their poor time locking and reduced amplification. Only 79% of the stimulus trials contributed deflections to the P1 response component in both young and elderly subjects and 63% to the N1. In young subjects, several of the new response parameters showed the presence of evoked response components that were not apparent in the conventionally averaged VEP derived from the same data.

CONCLUSIONS

The novel methods presented here provide a great deal of additional information that is unavailable when analyzing data using only conventional evoked potential averaging.

A new technique for the analysis of background and evoked EEG activity: time and amplitude distributions of the EEG deflections

OBJECTIVES

The EEG during background activity and that evoked in response to sensory stimuli (evoked potential, EP) has traditionally been studied by averaging and by frequency analysis. These obscure trial to trial variability. A new analysis technique is presented here which leads to single trial analysis and to insight into the mechanisms of EP generation.

METHODS

This technique is based on the identification of the EEG deflections recorded on the scalp before (background) and immediately after visual stimuli. A statistical description of the time and amplitude distributions of these deflections is defined and leads to the differentiation between background and evoked activity.

RESULTS

In response to stimuli, the time and amplitude of ongoing deflections (background) are re-organized (time locking) and amplified, generating the EP. Not all stimulus trials are accompanied by an appropriate response. Separate analysis of those single trials that do contain a response deflection provides information on the exact timing, variability, amplitude, etc., of those EEG deflections which contribute to the EP.

CONCLUSIONS

New EEG analysis techniques are described which provide single trial EP analysis and insight into mechanisms of EP generation.

Self-organization of a radial microtubule array by dynein-dependent nucleation of microtubules

Polarized radial arrays of cytoplasmic microtubules (MTs) with minus ends clustered at the cell center define the organization of the cytoplasm through interaction with microtubule motors bound to membrane organelles or chromosomes. It is generally assumed that the radial organization results from nucleation of MTs at the centrosome. However, radial MT array can also be attained through self-organization that requires the activity of a minus-end-directed MT motor, cytoplasmic dynein. In this study we examine the role of cytoplasmic dynein in the self-organization of a radial MT array in cytoplasmic fragments of fish melanophores lacking the centrosome. After activation of dynein motors bound to membrane-bound organelles, pigment granules, the fragments rapidly form polarized radial arrays of MTs and position pigment aggregates at their centers. We show that rearrangement of MTs in the cytoplasm is achieved through dynein-dependent MT nucleation. The radial pattern is generated by continuous disassembly and reassembly of MTs and concurrent minus-end-directed transport of pigment granules bearing the nucleation sites.

Centrosomal control of microtubule dynamics

In many animal cells, minus ends of microtubules (MTs) are thought to be capped by the centrosome whereas plus ends are free and display dynamic instability. We tested the role of the centrosome by examining MT behavior in cytoplasts from which the centrosome was removed. Cells were injected with Cy3-tubulin to fluorescently label MTs and were enucleated by using a centrifugation procedure. Enucleation resulted in a mixture of cytoplasts containing or lacking the centrosome. Fibroblast (CHO-K1) and epithelial (BSC-1) cells were investigated. In fibroblast cytoplasts containing the centrosome, MTs showed dynamic instability indistinguishable from that in intact cells. In contrast, in cytoplasts lacking the centrosome, MTs treadmilled-shortened at the minus end at about 12 micrometers/min while growing at the plus end at the same rate. The change in behavior of the plus end from dynamic instability to persistent growth correlated with an elevated level of free tubulin subunits (78% in centrosome-free cytoplasts vs. 44% in intact cells) generated by minus-end depolymerization. In contrast to fibroblast cells, in centrosome-free cytoplasts prepared from epithelial cells, MTs displayed dynamic instability at plus ends and relative stability at minus ends presumably because of specific minus-end stability factors distributed throughout the cytoplasm. We suggest that, in fibroblast cells, a minus-end depolymerization mechanism functions to eliminate errors in MT organization and that dynamic instability of MT plus ends is a result of capping of minus ends by the centrosome.

Analysis of the middle latency evoked potentials to angular acceleration impulses in man

The middle latency vestibular evoked potential (ML-VsEP) recorded with scalp electrodes in man in response to impulses of angular acceleration is dominated by a forehead positive peak at about 15 ms and a negative peak at about 20 ms; the peak to peak amplitude of this component is about 30 microV. This is followed by slower, smaller amplitude activity. The latency of this initial peak is similar to the latency of the vestibulo-ocular reflex (VOR) in monkeys. The present study was undertaken to elucidate the possible relation between the ML-VsEPs and VOR. This included recordings from forehead-mastoid electrodes (sites used to record VsEP) and other scalp electrodes and the recording of potentials due to eye movement: the electro-oculogram. Direct recording of eye movements was also conducted using an infra-red reflection device in those experiments in which the head was not moved. The recordings were conducted in man during vestibular stimulation eliciting VsEPs, during voluntary eye movements and during caloric and optokinetic stimulation. These experiments indicated that the 15-20 ms component of the ML-VsEP was not due to movements of the eye (corneo-retinal dipole). The large amplitude 15-20 ms component of the ML-VsEP was similar in general magnitude, waveform, polarity, duration and rise time to the highly synchronous pre-saccadic spike (neural and/or myogenic) which precedes nystagmus and voluntary saccades. It therefore probably represents vestibular-initiated electrical activity in motor units of the extra-ocular muscles which then produce anti-compensatory saccades.

Vertical plane short and middle latency vestibular evoked potentials in humans

In order to determine whether short and middle latency vestibular evoked potentials (VsEPs) can be recorded in humans in response to angular acceleration stimuli in the vertical plane, a drum, head-holder, and stepper motor were designed to deliver upward acceleration impulses of 10,000 degrees/s2 (1.8 degrees displacement) to the human head. Forehead and mastoid electrodes recorded electrical activity that was filtered, differentially amplified, and averaged in short (12.7 milliseconds) and middle (63.5 milliseconds) latency time frames. Control recordings were used to eliminate various types of artifact. Recordings were conducted in 7 normal subjects and in 4 control patients with congenital, profound hearing loss and absence of caloric responses. Short and middle latency VsEPs with high intrasubject and intersubject consistency were recorded in normal subjects and not in control patients. The middle latency responses were larger in amplitude than the short latency responses. The effects of stimulus intensity and repetition rate on VsEP waveform, latency, and amplitude studied. Experiments have shown that the responses are not electrical artifact, nor are they contaminated by auditory, somatosensory, or passive eye movement potentials.