Modification of Actin Fibers Changes the Electrical Phenotype of Cardiac Myofibroblasts

Background: Slow conduction and ectopic activity are major determinants of cardiac arrhythmogenesis. Both of these conditions can be elicited by myofibroblasts (MFBs) following establishment of heterocellular gap junctional coupling with cardiomyocytes. MFBs appear during structural remodeling of the heart and are characterized by the expression of α-smooth muscle actin (α-SMA) containing stress fibers. In this study, we investigated whether pharmacological interference with the actin cytoskeleton affects myofibroblast arrhythmogeneicity. Methods: Experiments were performed with patterned growth strands of neonatal rat ventricular cardiomyocytes coated with cardiac MFBs. Impulse conduction velocity (θ) and maximal upstroke velocities of propagated action potentials (dV/dtmax), expressed as % action potential amplitude change (%APA) per ms, were measured optically using voltage sensitive dyes. Actin was destabilized by latrunculin B (LtB) and cytochalasin D and stabilized with jasplakinolide. Data are given as mean ± S.D. (n = 5-22). Single cell electrophysiology was assessed using standard patch-clamp techniques. Results: As revealed by immunocytochemistry, exposure of MFBs to LtB (0.01-10 μmol/L) profoundly disrupted stress fibers which led to drastic changes in cell morphology with MFBs assuming an astrocyte-like shape. In control cardiomyocyte strands (no MFB coat), LtB had negligible effects on θ and dV/dtmax. In contrast, LtB applied to MFB-coated strands increased θ dose-dependently from 197 ± 35 mm/s to 344 ± 26 mm/s and dV/dtmax from 38 ± 5 to 78 ± 3% APA/ms, i.e., to values virtually identical to those of cardiomyocyte control strands (339 ± 24 mm/s; 77 ± 3% APA/ms). Highly similar results were obtained when exposing the preparations to cytochalasin D. In contrast, stabilization of actin with increasing concentrations of jasplakinolide exerted no significant effects on impulse conduction characteristics in MFB-coated strands. Whole-cell patch-clamp experiments showed that LtB hyperpolarized MFBs from -25 mV to -50 mV, thus limiting their depolarizing effect on cardiomyocytes which was shown before to cause arrhythmogenic slow conduction and ectopic activity. Conclusion: Pharmacological interference with the actin cytoskeleton of cardiac MFBs affects their electrophysiological phenotype to such an extent that they loose their detrimental effects on cardiomyocyte electrophysiology. This result might form a basis for the development of therapeutic strategies aimed at limiting the arrhythmogenic potential of MFBs.

PDZ domain-binding motif regulates cardiomyocyte compartment-specific NaV1.5 channel expression and function

BACKGROUND Sodium channel NaV1.5 underlies cardiac excitability and conduction. The last 3 residues of NaV1.5 (Ser-Ile-Val) constitute a PDZ domain-binding motif that interacts with PDZ proteins such as syntrophins and SAP97 at different locations within the cardiomyocyte, thus defining distinct pools of NaV1.5 multiprotein complexes. Here, we explored the in vivo and clinical impact of this motif through characterization of mutant mice and genetic screening of patients. METHODS AND RESULTS To investigate in vivo the regulatory role of this motif, we generated knock-in mice lacking the SIV domain (ΔSIV). ΔSIV mice displayed reduced NaV1.5 expression and sodium current (INa), specifically at the lateral myocyte membrane, whereas NaV1.5 expression and INa at the intercalated disks were unaffected. Optical mapping of ΔSIV hearts revealed that ventricular conduction velocity was preferentially decreased in the transversal direction to myocardial fiber orientation, leading to increased anisotropy of ventricular conduction. Internalization of wild-type and ΔSIV channels was unchanged in HEK293 cells. However, the proteasome inhibitor MG132 rescued ΔSIV INa, suggesting that the SIV motif is important for regulation of NaV1.5 degradation. A missense mutation within the SIV motif (p.V2016M) was identified in a patient with Brugada syndrome. The mutation decreased NaV1.5 cell surface expression and INa when expressed in HEK293 cells. CONCLUSIONS Our results demonstrate the in vivo significance of the PDZ domain-binding motif in the correct expression of NaV1.5 at the lateral cardiomyocyte membrane and underline the functional role of lateral NaV1.5 in ventricular conduction. Furthermore, we reveal a clinical relevance of the SIV motif in cardiac disease.

Mice with disrupted GM2/GD2 synthase gene lack complex gangliosides but exhibit only subtle defects in their nervous system.

Gangliosides, sialic acid-containing glycosphingolipids, are abundant in the vertebrate (mammalian) nervous system. Their composition is spatially and developmentally regulated, and gangliosides have been widely believed to lay essential roles in establishment of the nervous system, especially in neuritogenesis and synaptogenesis. However, this has never been tested directly. Here we report the generation of mice with a disrupted beta 1,4-N-acetylgalactosaminyltransferase (GM2/GD2 synthase; EC 2.4.1.92) gene. The mice lacked all complex gangliosides. Nevertheless, they did not show any major histological defects in their nervous systems or in gross behavior. Just a slight reduction in the neural conduction velocity from the tibial nerve to the somatosensory cortex, but not to the lumbar spine, was detected. These findings suggest that complex gangliosides are required in neuronal functions but not in the morphogenesis and organogenesis of the brain. The higher levels of GM3 and GD3 expressed in the brains of these mutant mice may be able to compensate for the lack of complex gangliosides.

Biopacemaker acceleration without increased synchronization by chronic exposure to phorbol myristate acetate

L'activité électrique du coeur est initiée par la génération spontanée de potentiels d'action venant des cellules pacemaker du noeud sinusal (SN). Toute dysfonction au niveau de cette région entraîne une instabilité électrique du coeur. La majorité des patients souffrant d'un noeud sinusal déficient nécessitent l'implantation chirurgicale d'un pacemaker électronique; cependant, les limitations de cette approche incitent à la recherche d'une alternative thérapeutique. La base moléculaire des courants ioniques jouant un rôle crucial dans l'activité du noeud sinusal sont de plus en plus connues. Une composante importante de l'activité des cellules pacemakers semble être le canal HCN, responsable du courant pacemaker If. Le facteur T-box 3 (Tbx3), un facteur de transcription conservé durant le processus de l'évolution, est nécessaire au développement du système de conduction cardiaque. De précédentes études ont démontré que dans différentes lignées cellulaires le Phorbol 12-myristate 13-acetate (PMA) active l'expression du gène codant Tbx3 via des réactions en cascade partant de la protéine kinase C (PKC). L'objectif principal de cette étude est de tester si le PMA peut augmenter la fréquence et la synchronisation de l'activité spontanée du pacemaker biologique en culture. Plus précisément, nous avons étudié les effets de l'exposition chronique au PMA sur l'expression du facteur de transcription Tbx3, sur HCN4 et l'activité spontanée chez des monocouches de culture de myocytes ventriculaires de rats néonataux (MVRN). Nos résultats démontrent que le PMA augmente significativement le facteur transcription de Tbx3 et l'expression ARNm de HCN4, favorisant ainsi l'augmentation du rythme et de la stabilité de l'activité autonome. De plus, une diminution significative de la vitesse de conduction a été relevée et est attribuée à la diminution du couplage intercellulaire. La diminution de la vitesse de conduction pourrait expliquer l'effet négatif du PMA sur la synchronisation de l'activité autonome du pacemaker biologique. Ces résultats ont été confirmés par un modèle mathématique multicellulaire suggérant que des fréquences et résistances intercellulaires plus élevée pourraient induire une activité plus stable et moins synchrone. Cette étude amène de nouvelles connaissances très importantes destinées à la production d'un pacemaker biologique efficient et robuste.

Neuromuscular efficiency of the sternocleidomastoid and anterior scalene muscles in patients with chronic neck pain

Purpose: This study compared the neuromuscular efficiency (NME) of the sternocleidomastoid (SCM) and anterior scalene (AS) muscles between 20 chronic neck pain patients and 20 asymptomatic controls. Method: Myoelectric signals were recorded from the sternal head of SCM and the AS muscles as subjects performed sub-maximal isometric cervical flexion contractions at 25 and 50% of the maximum voluntary contraction (MVC). The NME was calculated as the ratio between MVC and the corresponding average rectified value of the EMG signal. Ultrasonography was used to measure subcutaneous tissue thickness over the SCM and AS to ensure that differences did not exist between groups. Results: For both the SCM and AS muscles, NME was shown to be significantly reduced in patients with neck pain at 25% MVC (p < 0.05). Subcutaneous tissue thickness over the SCM and AS muscles was not different between groups. Conclusions: Reduced NME in the superficial cervical flexor muscles in patients with neck pain may be a measurable altered muscle strategy for dysfunction in other muscles. This aberrant pattern of muscle activation appears to be most evident under conditions of low load. NME, when measured at 25% MVC, may be a useful objective measure for future investigation of muscle dysfunction in patients with neck pain.

Neck flexor muscle fatigue is side specific in patients with unilateral neck pain

Despite the evidence of greater fatigability of the cervical flexor muscles in neck pain patients, the effect of unilaterality of neck pain on muscle fatigue has not been investigated. This study compared myoelectric manifestations of sternocleidomastoid (SCM) and anterior scalene (AS) muscle fatigue between the painful and non-painful sides in patients with chronic unilateral neck pain. Myoelectric signals were recorded from the sternal head of SCM and the AS muscles bilaterally during sub-maximal isometric cervical flexion contractions at 25% and 50% of the maximum voluntary contraction (MVC). The time course of the mean power frequency, average rectified value and conduction velocity of the electromyographic signals were calculated to quantify myoelectric manifestations of muscle fatigue. Results revealed greater estimates of the initial value and slope of the mean frequency for both the SCM and AS muscles on the side of the patient's neck pain at 25% and 50% of MVC. These results indicate greater myoelectric manifestations of muscle fatigue of the superficial cervical flexor muscles ipsilateral to the side of pain. This suggests a specificity of the effect of pain on muscle function and hence the need for specificity of therapeutic exercise in the management of neck pain patients. (C) 2003 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Ltd. All rights reserved.

The concurrent validity and test-retest reliability of a visuospatial working memory task

To determine whether the visuospatial n-back working memory task is a reliable and valid measure of cognitive processes believed to underlie intelligence, this study compared the reaction times and accuracy of perforniance of 70 participants, with performance on the Multidimensional Aptitude Battery (MAB). Testing was conducted over two sessions separated by 1 week. Participants completed the MAB during the second test session. Moderate testretest reliability for percentage accuracy scores was found across the four levels of the n-back task, whilst reaction times were highly reliable. Furthermore, participants' performance on the MAB was negatively correlated with accuracy of performance at the easier levels of the n-back task and positively correlated with accuracy of performance at the harder task levels. These findings confirm previous research examining the cognitive basis of intelligence, and suggest that intelligence is the product of faster speed of information processing, as well as superior working memory capacity. (C) 2004 Elsevier Inc. All rights reserved.

Muscle fiber and motor unit behavior in the longest human skeletal muscle

The sartorius muscle is the longest muscle in the human body. It is strap-like, up to 600 mm in length, and contains five to seven neurovascular compartments, each with a neuromuscular endplate zone. Some of its fibers terminate intrafascicularly, whereas others may run the full length of the muscle. To assess the location and timing of activation within motor units of this long muscle, we recorded electromyographic potentials from multiple intramuscular electrodes along sartorius muscle during steady voluntary contraction and analyzed their activity with spike-triggered averaging from a needle electrode inserted near the proximal end of the muscle. Approximately 30% of sartorius motor units included muscle fibers that ran the full length of the muscle, conducting action potentials at 3.9 +/- 0.1 m/s. Most motor units were innervated within a single muscle endplate zone that was not necessarily near the midpoint of the fiber. As a consequence, action potentials reached the distal end of a unit as late as 100 ms after initiation at an endplate zone. Thus, contractile activity is not synchronized along the length of single sartorius fibers. We postulate that lateral transmission of force from fiber to endomysium and a wide distribution of motor unit endplates along the muscle are critical for the efficient transmission of force from sarcomere to tendon and for the prevention of muscle injury caused by overextension of inactive regions of muscle fibers.

An endurance-strength training regime is effective in reducing myoelectric manifestations of cervical flexor muscle fatigue in females with chronic neck pain

Objective: The purpose of this study was to investigate whether an endurance-strength training program is effective in reducing myoelectric manifestations of sternocleidomastoid (SCM) and anterior scalene (AS) muscle fatigue which have been found to be greater in people with chronic neck pain. Methods: Fifty-eight female patients with chronic non-severe neck pain were randomized into one of two 6-week exercise intervention groups: an endurance-strength training regime for the cervical flexor muscles or a referent exercise intervention involving low load retraining of the cranio-cervical flexor muscles. The primary outcomes were a change in maximum voluntary contraction (MVC) force and change of the initial value and rate of change of the mean frequency, average rectified value and conduction velocity detected from the SCM and AS muscles during sub-maximal isometric cervical flexion contractions at 50, 25 and 10% MVC. Results: At the 7th week follow-up assessment, the endurance-strength training group revealed a significant increase in MVC force and a reduction in the estimates of the initial value and rate of change of the mean frequency for both the SCM and AS muscles (P < 0.05). Both exercise groups reported a reduced average intensity of neck pain and reduced neck disability index score (P < 0.05). Conclusions: An endurance-strength exercise regime for the cervical flexor muscles is effective in reducing myoelectric manifestations of superficial cervical flexor muscle fatigue as well as increasing cervical flexion strength in a group of patients with chronic non-severe neck pain. Significance: Provision of load to challenge the neck flexor muscles is required to reduce the fatigability of the SCM and AS muscles in people with neck pain. Improvements in cervical muscle strength and reduced fatigability may be responsible for the reported efficacy with this type of exercise program. (c) 2006 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All fights reserved.

Dynamics of a dilute sheared inelastic fluid. I. Hydrodynamic modes and velocity correlation functions

The hydrodynamic modes and the velocity autocorrelation functions for a dilute sheared inelastic fluid are analyzed using an expansion in the parameter epsilon=(1-e)(1/2), where e is the coefficient of restitution. It is shown that the hydrodynamic modes for a sheared inelastic fluid are very different from those for an elastic fluid in the long-wave limit, since energy is not a conserved variable when the wavelength of perturbations is larger than the ``conduction length.'' In an inelastic fluid under shear, there are three coupled modes, the mass and the momenta in the plane of shear, which have a decay rate proportional to k(2/3) in the limit k -> 0, if the wave vector has a component along the flow direction. When the wave vector is aligned along the gradient-vorticity plane, we find that the scaling of the growth rate is similar to that for an elastic fluid. The Fourier transforms of the velocity autocorrelation functions are calculated for a steady shear flow correct to leading order in an expansion in epsilon. The time dependence of the autocorrelation function in the long-time limit is obtained by estimating the integral of the Fourier transform over wave number space. It is found that the autocorrelation functions for the velocity in the flow and gradient directions decay proportional to t(-5/2) in two dimensions and t(-15/4) in three dimensions. In the vorticity direction, the decay of the autocorrelation function is proportional to t(-3) in two dimensions and t(-7/2) in three dimensions.

Information Preservation Modeling of Rayleigh-Benard Transition from Thermal Conduction to Convection

Onset and evolution of the Rayleigh-Benard (R-B) convection are investigated using the Information Preservation (IP) method. The information velocity and temperature are updated using the Octant Flux Splitting (OFS) model developed by Masters & Ye based on the Maxwell transport equation suggested by Sun & Boyd. Statistical noise inherent in particle approaches such as the direct simulation Monte Carlo (DSMC) method is effectively reduced by the IP method, and therefore the evolutions from an initial quiescent fluid to a final steady state are shown clearly. An interesting phenomenon is observed: when the Rayleigh number (Ra) exceeds its critical value, there exists an obvious incubation stage. During the incubation stage, the vortex structure clearly appears and evolves, whereas the Nusselt number (Nu) of the lower plate is close to unity. After the incubation stage, the vortex velocity and Nu rapidly increase, and the flow field quickly reaches a steady, convective state. A relation of Nu to Ra given by IP agrees with those given by DSMC, the classical theory and experimental data.

Axonal velocity distributions in neural feld equations

By modelling the average activity of large neuronal populations, continuum mean field models (MFMs) have become an increasingly important theoretical tool for understanding the emergent activity of cortical tissue. In order to be computationally tractable, long-range propagation of activity in MFMs is often approximated with partial differential equations (PDEs). However, PDE approximations in current use correspond to underlying axonal velocity distributions incompatible with experimental measurements. In order to rectify this deficiency, we here introduce novel propagation PDEs that give rise to smooth unimodal distributions of axonal conduction velocities. We also argue that velocities estimated from fibre diameters in slice and from latency measurements, respectively, relate quite differently to such distributions, a significant point for any phenomenological description. Our PDEs are then successfully fit to fibre diameter data from human corpus callosum and rat subcortical white matter. This allows for the first time to simulate long-range conduction in the mammalian brain with realistic, convenient PDEs. Furthermore, the obtained results suggest that the propagation of activity in rat and human differs significantly beyond mere scaling. The dynamical consequences of our new formulation are investigated in the context of a well known neural field model. On the basis of Turing instability analyses, we conclude that pattern formation is more easily initiated using our more realistic propagator. By increasing characteristic conduction velocities, a smooth transition can occur from self-sustaining bulk oscillations to travelling waves of various wavelengths, which may influence axonal growth during development. Our analytic results are also corroborated numerically using simulations on a large spatial grid. Thus we provide here a comprehensive analysis of empirically constrained activity propagation in the context of MFMs, which will allow more realistic studies of mammalian brain activity in the future.

Heat conduction in a chain of harmonic and anharmonic oscillators under the presence of an energy conserving noise

Reproducing Fourier's law of heat conduction from a microscopic stochastic model is a long standing challenge in statistical physics. As was shown by Rieder, Lebowitz and Lieb many years ago, a chain of harmonically coupled oscillators connected to two heat baths at different temperatures does not reproduce the diffusive behaviour of Fourier's law, but instead a ballistic one with an infinite thermal conductivity. Since then, there has been a substantial effort from the scientific community in identifying the key mechanism necessary to reproduce such diffusivity, which usually revolved around anharmonicity and the effect of impurities. Recently, it was shown by Dhar, Venkateshan and Lebowitz that Fourier's law can be recovered by introducing an energy conserving noise, whose role is to simulate the elastic collisions between the atoms and other microscopic degrees of freedom, which one would expect to be present in a real solid. For a one-dimensional chain this is accomplished numerically by randomly flipping - under the framework of a Poisson process with a variable “rate of collisions" - the sign of the velocity of an oscillator. In this poster we present Langevin simulations of a one-dimensional chain of oscillators coupled to two heat baths at different temperatures. We consider both harmonic and anharmonic (quartic) interactions, which are studied with and without the energy conserving noise. With these results we are able to map in detail how the heat conductivity k is influenced by both anharmonicity and the energy conserving noise. We also present a detailed analysis of the behaviour of k as a function of the size of the system and the rate of collisions, which includes a finite-size scaling method that enables us to extract the relevant critical exponents. Finally, we show that for harmonic chains, k is independent of temperature, both with and without the noise. Conversely, for anharmonic chains we find that k increases roughly linearly with the temperature of a given reservoir, while keeping the temperature difference fixed.

Variable Na(v)1.5 protein expression from the wild-type allele correlates with the penetrance of cardiac conduction disease in the Scn5a(+/-) mouse model

BACKGROUND: Loss-of-function mutations in SCN5A, the gene encoding Na(v)1.5 Na+ channel, are associated with inherited cardiac conduction defects and Brugada syndrome, which both exhibit variable phenotypic penetrance of conduction defects. We investigated the mechanisms of this heterogeneity in a mouse model with heterozygous targeted disruption of Scn5a (Scn5a(+/-) mice) and compared our results to those obtained in patients with loss-of-function mutations in SCN5A. METHODOLOGY/PRINCIPAL FINDINGS: Based on ECG, 10-week-old Scn5a(+/-) mice were divided into 2 subgroups, one displaying severe ventricular conduction defects (QRS interval>18 ms) and one a mild phenotype (QRS< or = 18 ms; QRS in wild-type littermates: 10-18 ms). Phenotypic difference persisted with aging. At 10 weeks, the Na+ channel blocker ajmaline prolonged QRS interval similarly in both groups of Scn5a(+/-) mice. In contrast, in old mice (>53 weeks), ajmaline effect was larger in the severely affected subgroup. These data matched the clinical observations on patients with SCN5A loss-of-function mutations with either severe or mild conduction defects. Ventricular tachycardia developed in 5/10 old severely affected Scn5a(+/-) mice but not in mildly affected ones. Correspondingly, symptomatic SCN5A-mutated Brugada patients had more severe conduction defects than asymptomatic patients. Old severely affected Scn5a(+/-) mice but not mildly affected ones showed extensive cardiac fibrosis. Mildly affected Scn5a(+/-) mice had similar Na(v)1.5 mRNA but higher Na(v)1.5 protein expression, and moderately larger I(Na) current than severely affected Scn5a(+/-) mice. As a consequence, action potential upstroke velocity was more decreased in severely affected Scn5a(+/-) mice than in mildly affected ones. CONCLUSIONS: Scn5a(+/-) mice show similar phenotypic heterogeneity as SCN5A-mutated patients. In Scn5a(+/-) mice, phenotype severity correlates with wild-type Na(v)1.5 protein expression.

Bone conduction in Thiel-embalmed cadaver heads

INTRODUCTION Sound can reach the inner ear via at least two different pathways: air conduction and bone conduction (BC). BC hearing is used clinically for diagnostic purposes and for BC hearing aids. Research on the motion of the human middle ear in response to BC stimulation is typically conducted using cadaver models. We evaluated middle ear motion of Thiel-embalmed whole-head specimens in terms of linearity, reproducibility, and consistency with the reported middle ear motion of living subjects, fresh cadaveric temporal bones, and whole-heads embalmed with a Non-Thiel solution of salts. METHODS We used laser Doppler vibrometry to measure the displacement of the skull, the umbo, the cochlear promontory, the stapes, and the round window in seven ears from four human whole-head specimens embalmed according to Thiel's method. The ears were stimulated with a Baha(®) implanted behind the auricle. RESULTS The Thiel model shows promontory velocity similar to that reported in the literature for whole-heads embalmed with a Non-Thiel solution of salts (0- to 7-dB difference). The Thiel heads' relative velocity of the stapes with respect to the promontory was similar to that of fresh cadaver temporal bones (0- to 4-dB difference). The velocity of the umbo was comparable in Thiel-embalmed heads and living subjects (0- to 10-dB difference). The skull and all middle ear elements measured responded linearly to different stimulation levels, with an average difference less than 1 dB. The variability of repeated measurements for both short- (2 h; 4 dB) and long-term (4-16 weeks; 6 dB) repetitions in the same ear, and the difference between the two ears of the same donor (approximately 10 dB) were lower than the inter-individual difference (up to 25 dB). CONCLUSION Thiel-embalmed human whole-head specimens can be used as an alternative model for the study of human middle ear mechanics secondary to BC stimulation. At some frequencies, differences from living subjects must be considered.