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The thermal motion of stiff filaments in a crowded environment is highly constrained and anisotropic; it underlies the behavior of such disparate systems as polymer materials, nanocomposites, and the ceil cytoskeleton. Despite decades of theoretical study, the fundamental dynamics of such systems remains a mystery. Using near-infrared video microscopy, we studied the thermal diffusion of individual single-walled carbon nanotubes (SWNTs) confined in porous agarose networks. We found that even a small bending flexibility of SWNTs strongly enhances their motion: The rotational diffusion constant is proportional to the filament-bending compliance and is independent of the network pore size. The interplay between crowding and thermal bending implies that the notion of a filament's stiffness depends on its confinement. Moreover, the mobility of SWNTs and other inclusions can be controlled by tailoring their stiffness. 10.1126/science.1197321

In skinned myocardium, cyclic AMP--dependent protein kinase A (PKA)-catalyzed phosphorylation of cardiac myosin--binding protein C (cMyBP-C) and cardiac troponin I (cTnI) is associated with a reduction in the [Ca.sup.2+] responsiveness of myofilaments and an acceleration in the kinetics of cross-bridge cycling, although the respective contribution of these two proteins remains controversial. To further examine the relative roles that cTnI and cMyBP-C phosphorylation play in altering myocardial function, we determined the [Ca.sup.2+] sensitivity of force ([pCa.sub.50]) and the activation dependence of the rate of force redevelopment ([k.sub.tr]) in control and PKA-treated mouse myocardium (isolated in the presence of 2,3-butanedione monoxime) expressing: (a) phosphorylatable cTnI and cMyBP-C (wild type [WT]), (b) phosphorylatable cTnI on a cMyBP-C--null background ([cMyBP-C.sup.-/-]), (c) nonphosphorylatable cTnI with [serines.sup.23/24/43/45] and [threonine.sup.144] mutated to alanines ([cTnI.sub.Ala5]), and (d) nonphosphorylatable cTnI on a cMyBP-C-null background ([cTnI.sub.Ala5]/[cMyBP-C.sup.-/-]). Here, PKA treatment decreased [pCa.sub.50] in WT, [cTnI.sub.Ala5], and [cMyBP-C.sup.-/-] myocardium by 0.13, 0.08, and 0.09 pCa units, respectively, but had no effect in [cTnI.sub.Ala5]/[cMyBP-C.sup.-/-] myocardium. In WT and [cTnI.sub.Ala5] myocardium, PKA treatment also increased [k.sub.tr] at submaximal levels of activation; however, PKA treatment did not have an effect on [k.sub.tr] in [cMyBP-C.sup.-/-] or [cTnI.sub.Ala5]/[cMyBP-C.sup.-/-] myocardium. In addition, reconstitution of [cTnI.sub.Ala5]/[cMyBP-C.sup.-/-] myocardium with recombinant cMyBP-C restored the effects of PKA treatment on [pCa.sub.50] and [k.sub.tr] reported in [cTnI.sub.Ala5] myocardium. Collectively, these results indicate that the attenuation in myofilament force response to PKA occurs as a result of both cTnI and cMyBP-C phosphorylation, and that the reduction in [pCa.sub.50] mediated by cMyBP-C phosphorylation most likely arises from an accelerated cross-bridge cycling kinetics partly as a result of an increased rate constant of cross-bridge detachment. 10.1085/jgp.201010448

The Arp2/3 complex polymerizes new actin filaments from the sides of existing filaments, forming Y-branched networks that are critical for actin-mediated force generation. Binding of the Arp2/3 complex to the sides of actin filaments is therefore central to its actin-nucleating and branching activities. Although a model of the Arp2/3 complex in filament branches has been proposed based on electron microscopy, this model has not been validated using independent approaches, and the functional importance of predicted actin-binding residues has not been extensively tested. Using a combination of molecular dynamics and protein-protein docking simulations, we derived an independent structural model of the interaction between two subunits of the Arp2/3 complex that are key to actin binding, ARPC2 and ARPC4, and the side of an actin filament. This model agreed remarkably well with the previous results from electron microscopy. Complementary mutagenesis experiments revealed numerous residues in ARPC2 and ARPC4 that were required for the biochemical activity of the entire complex. Functionally critical residues clustered together and defined a surface that was predicted by protein-protein docking to be buried in the interaction with actin. Moreover, key residues at this interface were crucial for actin nucleation and Y-branching, high-affinity F-actin binding, and Y-branch stability, demonstrating that the affinity of Arp2/3 complex for F actin independently modulates branch formation and stability. Our results highlight the utility of combining computational and experimental approaches to study protein-protein interactions and provide a basis for further elucidating the role of F-actin binding in Arp2/3 complex activation and function. cytoskeleton | actin branching doi/ 10.1073/pnas.0911668107

The goal of this study was to develop a system to experiment with sarcomeres mechanically isolated from skeletal muscles. Single myofibrils from rabbit psoas were transferred into a temperature-controlled (22[degrees]C or 15[degrees]C) experimental chamber, and sarcomeres were isolated using precalibrated glass microneedles that were pierced externally, adjacent to the Z-lines. The force produced during activation was measured by tracking the displacement of the microneedles, and the sarcomere and half-sarcomere changes were measured by continuously tracking the Z-lines and A-bands position during the experiments. Sarcomeres produced a stress (force/cross-sectional area) of 112.75 [+ or -] 4.96 nN/[micro][m.sup.2] (15[degrees]C) and 128.47 [+ or -] 5.58 nN/[micro][m.sup.2] (22[degrees]C) at lengths between 2.0 [micro]m and 2.4 [micro]m. The descending limb was fitted with linear regression for length between 2.4 [micro]m and 3.5 [micro]m, which provided an abscissa extrapolating to 3.87 [micro]m. The force-length relation was remarkably similar to a theoretical curve based on the degree of filament overlap. During sarcomere activation, we tracked the distance between the center of the A-band and the Z-lines. At lengths below 1.6 [micro]m, movements of A-band were not detected. A-band movements increased with length to achieve a maximum displacement of 59.40 [+ or -] 10.1 nm from the center at 2.0 [micro]m-2.4 [micro]m. A-band displacement decreased linearly in sarcomere lengths between 2.6 [micro]m and 3.6 [micro]m. A technique for monitoring force and length in single sarcomeres isolated from myofibrils represents a reliable technique to evaluate contractile mechanisms at the most basic, intact level of muscle organization, opening the possibility to clarify long-standing issues in the field of muscle contraction. cross-bridges; force-length relation; myosin-actin interaction doi: 10.1152/ajpcell.00233.2009.

Neuron connectivity and correct neural function largely depend on axonal integrity. Neurofilaments (NFs) constitute the main cytoskeletal network maintaining the structural integrity of neurons and exhibit dynamic changes during axonal and dendritic growth. However, the mechanisms underlying axonal development and maintenance remain poorly understood. Here, we identify that multisynthetase complex p43 (MSC p43) is essential for NF assembly and axon maintenance. The MSC p43 protein was predominantly expressed in central neurons and interacted with NF light subunit in vivo. Mice lacking MSC p43 exhibited axon degeneration in motor neurons, defective neuromuscular junctions, muscular atrophy, and motor dysfunction. Furthermore, MSC p43 depletion in mice caused disorganization of the axonal NF network. Mechanistically, MSC p43 is required for maintaining normal phosphorylation levels of NFs. Thus, MSC p43 is indispensable in maintaining axonal integrity. Its dysfunction may underlie the NF disorganization and axon degeneration associated with motor neuron degenerative diseases. assembly | axon degeneration | multisynthetase complex | neurofilament

Myofibrillar proteins must be removed from the myofibril before they can be turned over metabolically in functioning muscle cells. It is uncertain how this removal is accomplished without disruption of the contractile function of the myofibril. It has been proposed that the calpains could remove the outer layer of filaments from myofibrils as a first step in myofibrillar protein turnover. Several studies have found that myofilaments can be removed from myofibrils by trituration in the presence of ATP. These easily releasable myofilaments (ERMs) were proposed to be intermediates in myofibrillar protein turnover. It was unclear, however, whether the ERMs were an identifable entity in muscle or whether additional trituration would remove more myofilaments until the myofibril was gone and whether calpains could release ERMs from intact myofibrils. The present study shows that few ERMs could be obtained from the residue after the first removal of ERMs, and the yield of ERMs from well-washed myofibrils was reduced, probably because some ERMs had been removed by the washing process. Mild calpain treatment of myofibrils released filaments that had a polypeptide composition and were ultrastructurally similar to ERMs. The yield of calpain-released ERMs was two- to threefold greater than the normal yield. Hence, ERMs are an identifiable entity in myofibrils, and calpain releases filaments that are similar to ERMs. The role of ERMs in myofibrillar protein turnover is unclear, because only filaments on the surface of the myofibril would turn over, and changes in myofibrillar protein isoforms during development could not occur via the ERM mechanism. calpain; muscle; proteasome

Bipolar elongation of filaments of the bacterial actin homolog ParM drives movement of newly replicated plasmid DNA to opposite pores of a bacterial cell. We used a combination of vitreous sectioning and electron cryotomography to study this DNA partitioning system directly in native, frozen cells. The diffraction patterns from overexpressed ParM bundles in electron cryotomographic reconstructions were used to unambiguously identify ParM filaments in Escherichia coil cells. Using a low--copy number plasmid encoding components required for partitioning, we observed small bundles of three to five intracellular ParM filaments that were situated dose to the edge of the nucleoid. We propose that this may indicate the capture of plasmid DNA within the periphery of this loosely defined, chromosome-containing region.

In human cardiomyopathy, anatomical abnormalities such as hypertrophy and fibrosis contribute to the risk of ventricular arrhythmias and sudden death. Here we have shown that increased myofilament [Ca.sup.2+] sensitivity, also a common feature in both inherited and acquired human cardiomyopathies, created arrhythmia susceptibility in mice, even in the absence of anatomical abnormalities. In mice expressing troponin T mutants that cause hypertrophic cardiomyopathy in humans, the risk of developing ventricular tachycardia was directly proportional to the degree of [Ca.sup.2+] sensitization caused by the troponin T mutation. Arrhythmia susceptibility was reproduced with the [Ca.sup.2+]-sensitizing agent EMD 57033 and prevented by myofilament [Ca.sup.2+] desensitization with blebbistatin. [Ca.sup.2+] sensitization markedly changed the shape of ventricular action potentials, resulting in shorter effective refractory periods, greater beat-to-beat variability of action potential durations, and increased dispersion of ventricular conduction velocities at fast heart rates. Together these effects created an arrhythmogenic substrate. Thus, myofilament [Ca.sup.2+] sensitization represents a heretofore unrecognized arrhythmia mechanism. The protective effect of blebbistatin provides what we believe to be the first direct evidence that reduction of [Ca.sup.2+] sensitivity in myofilaments is antiarrhythmic and might be beneficial to individuals with hypertrophic cardiomyopathy., Introduction Sudden cardiac death (SCD) as a result of cardiac arrhythmias is responsible for 10% of all deaths in the United States (1). The study of monogenetic SCD syndromes has [...]

The creation of a completely new family of S-adenosyl-L-homocystein hydrolase (AHCY) proteins with new functions by the application of the inositol 1,4,5-triphosphate receptor (IRBIT) domain is discussed. The IRBIT domain is shown to target the AHCY-like proteins towards the cytoplasmic targets.

Increased signaling by [G.sub.i]-coupled receptors has been implicated in dilated cardiomyopathy. To investigate the mechanisms, we used transgenic mice that develop dilated cardiomyopathy after conditional expression of a cardiac-targeted [G.sub.i]-coupled receptor (Ro1). Activation of [G.sub.i] signaling by the Ro1 agonist spiradoline caused decreased cellular cAMP levels and bradycardia in Langendorff-perfused hearts. However, acute termination of Ro1 signaling with the antagonist norbinaltorphimine did not reverse the Ro1-induced contractile dysfunction, indicating that Ro1 cardiomyopathy was not due to acute effects of receptor signaling. Early after initiation of Ro1 expression, there was a 40% reduction in the abundance of the sarcoplasmic reticulum [Ca.sup.2+]-ATPase (P < 0.05); thereafter, there was progressive impairment of both [Ca.sup.2+] handling and force development assessed with ventricular trabeculae. Six weeks after initiation of Ro1 expression, systolic [Ca.sup.2+] concentration was reduced to 0.61 [+ or -] 0.08 vs. 0.91 [+ or -] 0.07 [micro]M for control (n = 6-8; P < 0.05), diastolic [Ca.sup.2+] concentration was elevated to 0.41 [+ or -] 0.07 vs. 0.23 [+ or -] 0.06 [micro]M for control (n = 6-8; P < 0.01), and the decline phase of the [Ca.sup.2+] transient (time from peak to 50% decline) was slowed to 0.25 [+ or -] 0.02 s vs. 0.13 [+ or -] 0.02 s for control (n = 6-8; P < 0.01). Early after initiation of Ro1 expression, there was a ninefold elevation of matrix metalloproteinase-2 (P < 0.01), which is known to cause myofilament injury. Consistent with this, 6 wk after initiation of Ro1 expression, [Ca.sup.2+]-saturated myofilament force in skinned trabeculae was reduced to 21 [+ or -] 2 vs. 38 [+ or -] 0.1 mN/[mm.sup.2] for controls (n = 3; P < 0.01). Furthermore, electron micrographs revealed extensive myofilament damage. These findings may have implications for some forms of human heart failure in which increased activity of [G.sub.i]-coupled receptors leads to impaired [Ca.sup.2+] handling and myofilament injury, contributing to impaired ventricular pump function and heart failure. matrix metalloproteinase-2; sarco(endo)plasmic reticulum [Ca.sup.2+]-ATPase; contraction; [kappa]-opioid receptor; conditional expression

Lamin A and C (A/C) are type V intermediate filaments that form the nuclear lamina. Lamin A/C mutations lead to reduced expression of lamin A/C and diverse phenotypes such as familial cardiomyopathies and accelerated aging syndromes. Normal aging is associated with reduced expression of lamin A/C in osteoblasts and dermal fibroblasts but has never been assessed in cardiomyocytes. Our objective was to compare the expression of lamin A/C in cardiomyocytes of old (24 too) versus young (4 mo) C57B1/6J mice using a well-validated mouse model of aging. Lamin B1 was used as a control. Immunohistochemical and immunofluorescence analyses showed reduced expression of lamin A/C in cardiomyocyte nuclei of old mice (proportion of nuclei expressing lamin A/C, 9% vs. 62%, P < 0.001). Lamin A/C distribution was scattered peripherally and perinuclear in old mice, whereas it was homogeneous throughout the nuclei in young mice. Western blot analyses confirmed reduced expression of lamin A/C in nuclear extracts of old mice (ratio of lamin A/C to B1, 0.6 vs. 1.2, P < 0.01). Echocardiographic studies showed increased left ventricular wall thickness with preserved cavity size (concentric remodeling), increased left ventricular mass, and a slight reduction in fractional shortening in old mice. This is the first study to show that normal aging is associated with reduced expression and altered distribution of lamin A/C in nuclei of cardiomyocytes. aging; laminopathy; nucleus; cardiomyopathy

The increase in [Ca.sup.2+] sensitivity of isometric force development along with sarcomere length (SL) is considered as the basis of the Frank-Starling law of the heart, possibly involving the regulation of cross-bridge turnover kinetics. Therefore, the [Ca.sup.2+] dependencies of isometric force production and of the cross-bridge-sensitive rate constant of force redevelopment ([k.sub.tr]) were determined at different SLs (1.9 and 2.3 [micro]m) in isolated human, murine, and porcine permeabilized cardiomyocytes, [k.sub.tr] was also determined in the presence of 10 mM inorganic phosphate ([P.sub.i]), which interfered with the force-generating cross-bridge transitions. The increases in [Ca.sup.2+] sensitivities of force with SL were very similar in human, murine, and porcine cardiomyocytes ([DELTA]p[Ca.sub.50]: ~0.11). [k.sub.tr] was higher (P < 0.05) in mice than in humans or pigs at all [Ca.sup.2+] concentrations ([[Ca.sup.2+]]) [maximum [k.sub.tr] ([k.sub.tr,max]) at a SL of 1.9 [micro]m and pCa 4.75: 1.33 [+ or -] 0.11, 7.44 [+ or -] 0.15, and 1.02 [+ or -] 0.05 [s.sup.-1], in humans, mice, and pigs, respectively] but [k.sub.tr] did not depend on SL in any species. Moreover, when the [k.sub.tr] values for each species were expressed relative to their respective maxima, similar [Ca.sup.2+] dependencies were obtained. Ten millimolar [P.sub.i] decreased force to ~60-65% and left [DELTA]p[Ca.sub.50] unaltered in all three species. [P.sub.i] increased [k.sub.tr,max] by a factor of ~1.6 in humans and pigs and by a factor of ~3 in mice, independent of SL. In conclusion, species differences exert a major influence on [k.sub.tr], but SL does not appear to modulate the cross-bridge turnover rates in human, murine, and porcine hearts. mouse; pig; heart; skinned muscle; myofilament length-dependent activation; rate of tension redevelopment; calcium doi:10.1152/ajpregu.00537.2006

Our published studies show that the distribution of the ANG II type 1 (A[T.sub.1]) receptor (A[T.sub.1]R), expressed as a enhanced yellow fluorescent fusion (YFP) protein (A[T.sub.1]R/EYFP), is altered upon cellular treatment with ANG II or coexpression with intracellular ANG II. A[T.sub.1]R accumulates in nuclei of cells only in the presence of ANG II. Several transmembrane receptors are known to accumulate in nuclei, some as holoreceptors and others as cleaved receptor products. The present study was designed to determine whether the A[T.sub.1]R is cleaved before nuclear transport. A plasmid encoding a rat A[T.sub.1]R labeled at the amino terminus with enhanced cyan fluorescent protein (CFP) and at the carboxy terminus with EYFP was employed. Image analyses of this protein in COS-7 cells, CCF-STTG1 glial cells, and A10 vascular smooth muscle cells show the two fluorescent moieties to be largely spatially colocalized in untreated cells. ANG II treatment, however, leads to a separation of the fluorescent moieties with yellow fluorescence accumulating in more than 30% of cellular nuclei. Immunoblot analyses of extracts and conditioned media from transfected cells indicate that the CFP domain fused to the extracellular amino-terminal A[T.sub.1]R domain is cleaved from the membrane and that the YFP domain, together with the intracellular cytoplasmic carboxy terminus of the A[T.sub.1]R, is also cleaved from the membrane-bound receptor. The carboxy terminus of the A[T.sub.1]R is essential for cleavage; cleavage does not occur in protein deleted with respect to this region. Overexpressed native A[T.sub.1]R (nonfusion) is also cleaved; the intracellular 6-kDa cytoplasmic domain product accumulates to a significantly higher level with ANG II treatment. nuclear angiotensin II type 1 receptor; intracrine; intracellular

Parkinson's disease (PD), a late-onset condition characterized by dysfunction and loss of dopaminergic neurons in the substantia nigra, has both sporadic and neurotoxic forms. Neurotoxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and its metabolite 1-methyl-4-phenylpyridinium (MPP+) induce PD symptoms and recapitulate major pathological hallmarks of PD in human and animal models. Both sporadic and MPP+-induced forms of PD proceed through a 'dying-back' pattern of neuronal degeneration in affected neurons, characterized by early loss of synaptic terminals and axonopathy. However, axonal and synaptic-specific effects of MPP+ are poorly understood. Using isolated squid axoplasm, we show that MPP+ produces significant alterations in fast axonal transport (FAT) through activation of a caspase and a previously undescribed protein kinase C (PKC[delta]) isoform. Specifically, MPP+ increased cytoplasmic dynein-dependent retrograde FAT and reduced kinesin-1-mediated anterograde FAT. Significantly, MPP+ effects were independent of both nuclear activities and ATP production. Consistent with its effects on FAT, MPP+ injection in presynaptic domains led to a dramatic reduction in the number of membranous profiles. Changes in availability of synaptic and neurotrophin-signaling components represent axonal and synaptic-specific effects of MPP+ that would produce a dying-back pathology. Our results identify a critical neuronal process affected by MPP+ and suggest that alterations in vesicle trafficking represent a primary event in PD pathogenesis. We propose that PD and other neurodegenerative diseases exhibiting dying-back neuropathology represent a previously undescribed category of neurological diseases characterized by dysfunction of vesicle transport and associated with the loss of synaptic function. cytoplasmic dynein | kinesin | Parkinson's disease | synaptic vesicle | caspase

Compared to sham-operated controls, myofilaments from hearts of ovariectomized (OVX) rats demonstrate an increase in [Ca.sup.2+] sensitivity with no change in maximum tension (Wattanapermpool J and Reiser PJ. Am J Physiol 277: H467-H473, 1999). To test the significance of this modification in intact cells, we compared intracellular [Ca.sup.2+] transients and shortening of ventricular myocytes isolated from sham and 10-wk OVX rats. There was a decrease in the peak [Ca.sup.2+] transient with prolonged 50% decay time in OVX cardiac myocytes without changes in the resting intracellular [Ca.sup.2+] concentration. Percent cell shortening was also depressed, and relaxation was prolonged in cardiac myocytes from OVX rats compared with shams. Ovariectomy induced a sensitization of the myofilaments to [Ca.sup.2+]. Hypercapnic acidosis suppressed the shortening of OVX myocytes to a lesser extent than that detected in shams. Moreover, a larger compensatory increase in % cell shortening was obtained in OVX myocytes during prolonged acidosis. The elevated compensation in cell shortening was related to a higher amount of increase in the amplitude of the [Ca.sup.2+] transient in OVX myocytes. However, these differences in [Ca.sup.2+] transients and %cell shortening were no longer evident in the presence of 1 [micro]M cariporide, a specific inhibitor of [Na.sup.+]/[H.sup.+] exchanger type 1 ([NHE.sub.1]). Our results indicate that deprivation of female sex hormones modulates the intracellular [Ca.sup.2+] concentration in cardiac myocytes, possibly via an increased [NHE.sub.1] activity, which may act in concert with [Ca.sup.2+] hypersensitivity of myofilament activation as a determinant of sex differences in cardiac function. heart; estrogen; sarcomeric proteins; sodium; proton exchanger

Recent studies have identified sex differences in heart function that may affect the risk of developing heart failure. We hypothesized that there are fundamental differences in calcium (Ca) regulation in cardiac myocytes of males and premenopausal females. Isometric force transients (n = 45) were measured at various stimulation frequencies to define the force frequency responses (FFR) (0.5, 1.0, 1.5, and 2.0 Hz) during either changes in bath Ca ([[Ca].sub.o]) (1.0, 1.75, 3.5, and 7.0 mM) or length-tension (20, 40, 60, 80, and 100% [L.sub.max]) in right ventricle trabeculae from normal male (MT) and premenopausal female (FT) cats. Force-Ca measurements were also obtained in chemically skinned trabeculae. Under basal conditions (0.5 Hz, 1.75 mM Ca, 80% [L.sub.max]) both MT and FT achieved similar developed forces (DF) (MT 11 [+ or -] 1, FT = 10 [+ or -] 1 mN/[mm.sup.2]). At low rates and lengths, there is no sex difference. At higher preloads and rates, there is a separation in DF in MT and FT. At basal [[Ca].sub.o] both MT and FT exhibited positive FFR (2.0 Hz, 1.75 mM Ca: MT 38 [+ or -] 3, FT 21 [+ or -] 4 mN/[mm.sup.2]); however, at higher [[Ca].sub.o], MT achieved greater DF (2.0 Hz, 7.0 mM Ca: MT 40 [+ or -] 3 and FT = 24 [+ or -] 4 mN/[mm.sup.2]). We detected no sex difference in myofilament Ca sensitivity at a sarcomere length of 2.1 [micro]m. However, rapid cooling contractures indicated greater sarcoplasmic reticulum (SR) Ca load in MT at higher frequencies. Despite virtually identical contractile performance under basal conditions, significant sex differences emerge under conditions of increased physiological stress. Given the lack of sex differences in myofilament Ca sensitivity, these studies suggest fundamental sex differences in cellular Ca regulation to achieve contractile reserve, with myocardium from males exhibiting higher SR Ca load. myocardium; force-frequency response; length-tension relationship; myofilament calcium sensitivity; sarcoplasmic reticulum calcium load

Abstract: A novel 120 kDa actin-binding protein (ApABP-F1) was found in Amoeba proteus. It was distributed throughout the cytoplasm, mainly in the subplasma membrane and perinuclear-nuclear areas, enriched in actin. [...]

Changes in interfilament lattice spacing have been proposed as the mechanism underlying myofilament length-dependent activation. Much of the evidence to support this theory has come from experiments in which high-molecular-weight compounds, such as dextran, were used to osmotically shrink the myofilament lattice. However, whether interfilament spacing directly affects myofilament calcium sensitivity ([EC.sub.50]) has not been established. In this study, skinned isolated rat myocardium was osmotically compressed over a wide range (Dextran T500; 0-6%), and [EC.sub.50] was correlated to both interfilament spacing and [I.sub.1,1]/[I.sub.1,0] intensity ratio. The latter two parameters were determined by X-ray diffraction in a separate group of skinned muscles. Osmotic compression induced a marked reduction in myofilament lattice spacing, concomitant with increases in both [EC.sub.50] and [I.sub.1,1]/[I.sub.1,0] intensity ratio. However, interfilament spacing was not well correlated with [EC.sub.50] ([r.sup.2] = 0.78). A much better and deterministic relationship was observed between [EC.sub.50] and the [I.sub.1,1]/[I.sub.1,0] intensity ratio ([r.sup.2] = 0.99), albeit with a marked discontinuity at low levels of dextran compression; that is, a small amount of external osmotic compression (0.38 kPa, corresponding to 1% Dextran T500) produced a stepwise increase in the [I.sub.1,1]/[I.sub.1,0] ratio concomitant with a stepwise decrease in [EC.sub.50]. These parameters then remained stable over a wide range of further applied osmotic compression (up to 6% dextran). These findings provide support for a 'switch-like' activation mechanism within the cardiac sarcomere that is highly sensitive to changes in external osmotic pressure. skinned muscle; trabeculae; X-ray diffraction; dextran; osmotic pressure; myofilament length-dependent activation; regulation

The ends of striated muscle tropomyosin (TM) are integral for thin filament cooperativity, determining the cooperative unit size and regulating the affinity of TM for actin. We hypothesized that altering the [alpha]-TM carboxy terminal overlap end to the [beta]-TM counterpart would affect the amino-terminal association, which would alter the end-to-end interactions of TM molecules in the thin filament regulatory strand and affect the mechanisms of cardiac muscle contraction. To test this hypothesis, we generated transgenic (TG) mouse lines that express a mutant form of a-TM in which the first 275 residues are from [alpha]-TM and the last nine amino acids are from [beta]-TM ([alpha]-TM9aaA[DELTA][beta]). Molecular analyses show that endogenous [alpha]-TM mRNA and protein are nearly completely replaced with [alpha]-TM9aaA[DELTA][beta]. Working heart preparations data show that the rates of contraction and relaxation are reduced in [alpha]-TM9aa[DELTA][beta] hearts. Left ventricular pressure and time to peak pressure are also reduced (-12% and -13%, respectively). The ratio of maximum to minimum first derivatives of change in left ventricular systolic pressure with respect to time (ratio of +dP/dt to -dP/dt, respectively) is increased, but [tau] is not changed significantly. Force-intracellular calcium concentration ([[[Ca.sup.2+]].sub.i]) measurements from intact papillary fibers demonstrate that [alpha]-TM9aa[DELTA][beta] TG fibers produce less force per given ([[[Ca.sup.2+]].sub.i]) compared with nontransgenic fibers. Taken together, the data demonstrate that the rate of contraction is primarily affected in TM TG hearts. Protein docking studies show that in the mutant molecule, the overall carbon backbone is perturbed about 1.5 [Angstrom], indicating that end-to-end interactions are altered. These results demonstrate that the localized flexibility present in the coiled-coil structures of TM isoforms is different, and that plays an important role in interacting with neighboring thin filament regulatory proteins and with differentially modulating the myofilament activation processes. force-calcium; thin filament; force-frequency; myofilament activation doi:10.1152/ajpheart.00688.2005

Cardiac contraction-relaxation coupling is determined by both the free intracellular calcium concentration ([[[Ca.sup.2+] ].sub.i]) and myofilament properties. We set out to develop a technique where we could assess these parameters (twitch and steady-state force [[[Ca.sup.2+] ].sub.i]) under near physiological conditions. Bis-fura-2 was iontophorically introduced into ultrathin rat trabeculae preparations to monitor the [[[Ca.sup.2+] ].sub.i], and steady-state contractures were achieved by using a modified Krebs-Henseleit solution containing high [K.sup.+]. During [K.sup.+] contractures, the very slow changes in [[[Ca.sup.2+] ].sub.i] and force development were in equilibrium and allowed for the construction of a steady-state, force-[[[Ca.sup.2+] ].sub.i] relationship. Twitch contractions before and after this myofilament calcium sensitivity assessment were unaltered, and this protocol could be repeated several times. For the first time, this novel protocol allows us to measure myofilament calcium sensitivity under physiological temperature. Not only do the data so obtained allow us to assess myofilament calcium sensitivity, the data also will allow us, ill the same preparation under nearly identical conditions, to compare the dynamic to the steady-state, force-calcium relationship. To test whether the steady-state relationship between force and calcium in our novel protocol reproduces expected changes, we determined this relationship in the presence of isoproterenol and under acidosis and alkalosis. As expected, [beta]-adrenergic stimulation resulted in an increase of calcium amplitude and twitch force and a desensitization of the myofilaments as indicated by a rightward shift of the obtained steady-state, force-calcium relationship. An increase in pH shifted the curve leftward, whereas a decrease in pH resulted in the expected rightward shift. rat: intracellular calcium concentration: [beta]-adrenergic stimulation

We have previously demonstrated that intermittent high-altitude (IHA) hypoxia significantly attenuates ischemia-reperfusion (I/R) injury-induced excessive increase in resting intracellular [Ca.sup.2+] concentrations ([[[Ca.sup.2+]].sub.i]. Because the sarcoplasmic reticulum (SR) and [Na.sup.+]/[Ca.sup.2+] exchanger (NCX) play crucial roles in regulating [[[Ca.sup.2+]].sub.i] and both are dysfunctional during I/R, we tested the hypothesis that IHA hypoxia may prevent I/R-induced [Ca.sup.2+] overload by maintaining [Ca.sup.2+] homeostasis via SR and NCX mechanisms. We thus determined the dynamics of [Ca.sup.2+] transients and cell shortening during preischemia and I/R injury in ventricular cardiomyocytes from normoxic and IHA hypoxic rats. IHA hypoxia did not affect the preischemic dynamics of [Ca.sup.2+] transients and cell shortening, but it significantly suppressed the I/R-induced increase in resting [[[Ca.sup.2+]].sub.i] levels and attenuated the depression of the [Ca.sup.2+] transients and cell shortening during reperfusion. Moreover, IHA hypoxia significantly attenuated I/R-induced depression of the protein contents of SR [Ca.sup.2+] release channels and/or ryanodine receptors (RyRs) and SR [Ca.sup.2+] pump ATPase (SERCA2) and SR [Ca.sup.2+] release and uptake. In addition, a delayed decay rate time constant of [Ca.sup.2+] transients and cell shortening of [Ca.sup.2+] transients observed during ischemia was accompanied by markedly inhibited NCX currents, which were prevented by IHA hypoxia. These findings indicate that IHA hypoxia may preserve [Ca.sup.2+] homeostasis and contraction by preserving RyRs and SERCA2 proteins as well as NCX activity during I/R. intracellular [Ca.sup.2+] concentration; [Ca.sup.2+] transients; [Ca.sup.2+] transporters; myofilament [Ca.sup.2+] sensitivity

[[alpha].sub.M][[beta].sub.2] integrins mediate phagocytosis of opsonized particles in a process controlled by RhoA, Rho kinase, myosin II, Arp2/3, and actin polymerization. [[alpha].sub.M][[beta].sub.2], Rho, Arp2/3, and F-actin accumulate underneath bound particles; however, the mechanism regulating Rho function during [[alpha].sub.M][[beta].sub.2]-mediated phagocytosis is poorly understood. We report that the binding of C3bi-opsonized sheep red blood cells (RBCs) to [[alpha].sub.M][[beta].sub.2] increases Rho-GTP, but not Rac-GTP, levels. Deletion of the cytoplasmic domain of [[beta].sub.2], but not of [[alpha].sub.M], abolished Rho recruitment and activation, as well as phagocytic uptake. Interestingly, a 16-amino acid (aa) region in the membrane-proximal half of the [[beta].sub.2] cytoplasmic domain was necessary for activating Rho. Three COOH-terminal residues (aa 758-760) were essential for [[beta].sub.2]-induced accumulation of Rho at complement receptor 3 (CR3) phagosomes. Activation of Rho was necessary, but not sufficient, for its stable recruitment underneath bound particles or for uptake. However, recruitment of active Rho was sufficient for phagocytosis. Our data shed light on the mechanism of outside-in signaling, from ligated integrins to the activation of Rho GTPase signaling.

[beta]-Actin, once thought to be an exclusively cytoplasmic protein, is now known to have important functions within the nucleus. Nuclear [beta]-actin associates with and functions in chromatin remodeling complexes, ribonucleic acid polymerase complexes, and at least some ribonucleoproteins. Proteins involved in regulating actin polymerization are also found in the interphase nucleus. We define the dynamic properties of nuclear actin molecules using fluorescence recovery after photobleaching. Our results indicate that actin and actin-containing complexes are reduced in their mobility through the nucleoplasm diffusing at ~0.5 [micro][m.sup.2] [s.sup.-1]. We also observed that ~20% of the total nuclear actin pool has properties of polymeric actin that turns over rapidly. This pool could be detected in endogenous nuclear actin by using fluorescent polymeric actin binding proteins and was sensitive to drugs that alter actin polymerization. Our results validate previous reports of polymeric forms of nuclear actin observed in fixed specimens and reveal that these polymeric forms are very dynamic.

Decreased [Ca.sup.2+] responsiveness of the myofilaments underlies myocardial stunning. Given that cross-bridge cycling is a major determinant of myofilament behavior, we quantified cross-bridge cycling rate in stunned myocardium. After stabilization, rat hearts were subjected to 20 min of no-flow global ischemia and 30 min of reperfusion at 37[degrees]C. Control hearts were perfused continuously at 37[degrees]C for 60 min. Trabeculae were dissected and chemically skinned with 1% Triton X-100. The muscles were then activated with solutions of varied [Ca.sup.2+] concentration ([[Ca.sup.2+]]). Force-[[Ca.sup.2+]] relations, rate of force redevelopment after release ([k.sub.tr]), muscle stiffness ([k.sub.m]), and myofilament ATP consumption were determined. Maximal [Ca.sup.2+]-activated force ([F.sub.max]) was depressed in stunned myocardium (49 [+ or -] 5 vs. 82 [+ or -] 5 mN/[mm.sup.2], P < 0.01). Western immunoblotting showed degradation of troponin I in stunned myocardium. The [k.sub.tr] at [F.sub.max] was significantly increased in stunned muscles (19.82 [+ or -] 2.74 vs. 13.19 + 0.96 [s.sup.-1], 22[degrees]C, P < 0.01; 7.49 [+ or -] 0.52 vs. 5.81 [+ or -] 0.54 [s.sup.-1], 10[degrees]C, P < 0.05). The ratio of km measured at 100 Hz over that at 1 Hz, during [F.sub.max], is lower in stunned muscles (8.22 [+ or -] 1.56 vs. 12.94 [+ or -] 0.71, P < 0.05). In comparison with [k.sub.m] at rigor, [k.sub.m] at [F.sub.max] is significantly lower in the stunned group (78.82 [+ or -] 6.11 vs. 93.27 [+ or -] 3.03%, P < 0.05). Myofilament ATP consumption at [F.sub.max] did not change in stunned muscles (5,901 [+ or -] 952 vs. 5,596 [+ or -] 972 pmol*[micro][l.sup-1]*[min.sup.-1], p = 0.49). These results show that cross-bridge cycling is increased in stunned myocardium. Such increases are likely the result of increased transition rate from force-generating states to non-force-generating states. Thus stunned myocardium still maintains ATP consumption in spite of lower force development, rationalizing the long-standing paradox of decreased force but unchanged oxygen consumption in the postischemic heart. cross-bridge cycling; contraction; myocardial stunning

We previously reported that glucosamine and hyperglycemia attenuate the response of cardiomyocytes to inositol 1,4,5-trisphosphate-generating agonists such as ANG II. This appears to be related to an increase in flux through the hexosamine biosynthesis pathway (HBP) and decreased [Ca.sup.2+] entry into the cells; however, a direct link between HBP and intracellular [Ca.sup.2+] homeostasis has not been established. Therefore, using neonatal rat ventricular myocytes, we investigated the relationship between glucosamine treatment; the concentration of UDP-N-acetylglucosamine (UDP-GlcNAc), an end product of the HBP; and the level of protein O-linked N-acetylglucosamine (O-GIcNAc) on ANG II-mediated changes in intracellular free [Ca.sup.2+] concentration ([Ca2 +]). We found that glucosamine blocked ANG II-induced [[[Ca.sup.2+]].sub.i] increase and that this phenomenon was associated with a significant increase in UDP-GlcNAc and O-GlcNAc levels. O-(2-acetamido-2-deoxy-D-glucopyranosylidene)-amino-N-phenylcarbamate, an inhibitor of OGlcNAcase that increased O-GlcNAc levels without changing UDPG1cNAc concentrations, mimicked the effect of glucosamine on the ANG II-induced increase in [[[Ca.sup.2+]].sub.i]. An inhibitor of O-GlcNActransferase, alloxan, prevented the glucosamine-induced increase in O-GlcNAc but not the increase in UDP-GlcNAc; however, alloxan abrogated the inhibition of the ANG II-induced increase in [[[Ca.sup.2+]].sub.i]. These data support the notion that changes in O-GlcNAc levels mediated via increased HBP flux may be involved in the regulation of [[[Ca.sup.2+]].sub.i] homeostasis in the heart. hypertrophy; left ventricle; calcium channels; calcium signaling

A major development in smooth muscle research in recent years is the recognition that the myofilament lattice of the muscle is malleable. The malleability appears to stem from plastic rearrangement of contractile and cytoskeletal filaments in response to stress and strain exerted on the muscle cell, and it allows the muscle to adapt to a wide range of cell lengths and maintain optimal contractility. Although much is still poorly understood, we have begun to comprehend some of the basic mechanisms underlying the assembly and disassembly of contractile and cytoskeletal filaments in smooth muscle during the process of adaptation to large changes in cell geometry. One factor that likely facilitates the plastic length adaptation is the ability of myosin filaments to form and dissolve at the right place and the right time within the myofilament lattice. It is proposed herein that formation of myosin filaments in vivo is aided by the various filament-stabilizing proteins, such as caldesmon, and that the thick filament length is determined by the dimension of the actin filament lattice. It is still an open question as to how the dimension of the dynamic filament lattice is regulated. In light of the new perspective of malleable myofilament lattice in smooth muscle, the roles of many smooth muscle proteins could be assigned or reassigned in the context of plastic reorganization of the contractile apparatus and cytoskeleton. contraction mechanism; length adaptation; thick filament; plasticity; contractile unit

Myofilament dysfunction is a common point of convergence for many forms of heart failure. Recently, we showed that cardiac overexpression of PKC[epsilon] initially depresses myofilament activity and then leads to a progression of changes characteristic of human heart failure. Here, we examined the effects of PKC[epsilon] on contractile reserve, Starling mechanism, and myofilament activation in this model of end-stage dilated cardiomyopathy. Pressure-volume loop analysis and echocardiography showed that the PKC[epsilon] mice have markedly compromised systolic function and increased end-diastolic volumes. Dobutamine challenge resulted in a small increase in contractility in PKC[epsilon] mice but failed to enhance cardiac output. The PKC[epsilon] mice showed a normal length-dependent tension development in skinned cardiac muscle preparations, although Frank-Starling mechanism appeared to be compromised in the intact animal. Simultaneous measurement of tension and ATPase demonstrated that the maximum tension and ATPase were markedly lower in the PKC[epsilon] mice at any length or [Ca.sup.2+] concentration. However, the tension cost was also lower indicating less energy expenditure. We conclude 1) that prolonged overexpression of PKC[epsilon] ultimately leads to a dilated cardiomyopathy marked by exhausted contractile reserve, 2) that PKC[epsilon] does not compromise the Frank-Starling mechanism at the myofilament level, and 3) that the Starling curve excursion is limited by the inotropic state of the heart. These results reflect the significance of the primary myofilament contractilopathy induced by phosphorylation and imply a role for PKC[epsilon]-mediated phosphorylation in myofilament physiology and the pathophysiology of decompensated cardiac failure. pressure-volume loops; cardiac myofilaments

Our objectives were to identify the relative contributions of [[[Ca.sup.2+]].sub.i] and myofilament [Ca.sup.2+] sensitivity in the pulmonary venous smooth muscle (PVSM) contractile response to the thromboxane [A.sub.2] mimetic U-46619 and to assess the roles of PKC, tyrosine kinases (TK), and Rho-kinase (ROK) in that response. We tested the hypothesis that U-46619-induced contraction in PVSM is mediated by both increases in [[[Ca.sup.2+]].sub.i] and myofilament [Ca.sup.2+] sensitivity and that the PKC, TK, and ROK signaling pathways are involved. Isometric tension was measured in isolated endothelium-denuded (E-) canine pulmonary venous (PV) tings. In addition, [[[Ca.sup.2+]].sub.i] and tension were simultaneously measured in fura-2-1oaded E- PVSM strips. U-46619 (0.1 nM-1 [micro]M) caused dose-dependent (P < 0.001) contraction in PV tings. U-46619 contraction was attenuated by inhibitors of L-type voltage-operated [Ca.sup.2+] channels (nifedipine, P < 0.001), inositol 1,4,5-trisphosphate-mediated [Ca.sup.2+] release (2-aminoethoxydiphenylborate, P < 0.001), PKC (bisindolylmaleimide I, P < 0.001), TK (tyrphostin A-47, P = 0.014), and ROK (Y-27632, P = 0.008). In PV strips, U-46619 contraction was associated with increases in [[[Ca.sup.2+]].sub.i] and myofilament [Ca.sup.2+] sensitivity. Both [Ca.sup.2+] influx and release mediated the early transient increase in [[[Ca.sup.2+]].sub.i], whereas the late sustained increase in [[[Ca.sup.2+]].sub.i] only involved [Ca.sup.2+] influx. Inhibition of both PKC and ROK (P = 0.006 and P = 0.002, respectively), but not TK, attenuated the U-46619-induced increase in myofilament [Ca.sup.2+] sensitivity. These results suggest that U-46619 contraction is mediated by [Ca.sup.2+] influx, [Ca.sup.2+] release, and increased myofilament [Ca.sup.2+] sensitivity. The PKC, TK, and ROK signaling pathways are involved in U-46619 contraction. vein; [Ca.sup.2+] homeostasis; myofilament [Ca.sup.2+] sensitivity

Organs and tissues adapt to acute or chronic mechanical stress by remodeling their actin cytoskeletons. Cells that are stimulated by cyclic stretch or shear stress in vitro undergo bimodal cytoskeletal responses that include rapid reinforcement and gradual reorientation of actin stress fibers; however, the mechanism by which cells respond to mechanical cues has been obscure. We report that the application of either unidirectional cyclic stretch or shear stress to cells results in robust mobilization of zyxin from focal adhesions to actin filaments, whereas many other focal adhesion proteins and zyxin family members remain at focal adhesions. Mechanical stress also induces the rapid zyxin-dependent mobilization of vasodilator-stimulated phosphoprotein from focal adhesions to actin filaments. Thickening of actin stress fibers reflects a cellular adaptation to mechanical stress; this cytoskeletal reinforcement coincides with zyxin mobilization and is abrogated in zyxin-null cells. Our findings identify zyxin as a mechanosensitive protein and provide mechanistic insight into how cells respond to mechanical cues.

The hypothesis that regulation of nucleocytoplasmic shuttling is a means of driving differentiation, using spermatogenesis as a model is explored. The nuclear proteins and nucleocytoplasmic transport machinery is consolidated and their functional linkages are highlighted not only as a means of regulating gene transcription and hence cell fate.

The action of cytochalasins, actin-disrupting agents on human Kv1.5 channel (hKv1.5) stably expressed in [Ltk.sup.-] cells was investigated using the whole cell patch-clamp technique. Cytochalasin B inhibited hKv1.5 currents rapidly and reversibly at +60 mV in a concentration-dependent manner with an I[C.sub.50] of 4.2 [micro]M. Cytochalasin A, which has a structure very similar to cytochalasin B, inhibited hKv1.5 (I[C.sub.50] of 1.4 [micro]M at +60 mV). Pretreatment with other actin filament disruptors cytochalasin D and cytochalasin J, and an actin filament stabilizing agent phalloidin had no effect on the cytochalasin B-induced inhibition of hKv1.5 currents. Cytochalasin B accelerated the decay rate of inactivation for the hKv1.5 currents. Cytochalasin B-induced inhibition of the hKv1.5 channels was voltage dependent with a steep increase over the voltage range of the channel's opening. However, the inhibition exhibited voltage independence over the voltage range in which channels are fully activated. Cytochalasin B produced no significant effect on the steady-state activation or inactivation curves. The rate constants for association and dissociation of cytochalasin B were 3.7 [micro]M/s and 7.5 [s.sup.-1], respectively. Cytochalasin B produced a use-dependent inhibition of hKv1.5 current that was consistent with the slow recovery from inactivation in the presence of the drug. Cytochalasin B (10 [micro]M) also inhibited an ultrarapid delayed rectifier [K.sup.+] current ([I.sub.K,ur]) in human atrial myocytes. These results indicate that cytochalasin B primarily blocks activated hKv1.5 channels and endogenous [I.sub.K,ur] in a cytoskeleton-independent manner as an open-channel blocker. voltage-gated [K.sup.+] channel; heart, open channel block

Recent studies have shown that the transport of microtubules (MTs) and neurofilaments (NFs) within the axon is rapid, infrequent, asynchronous, and bidirectional. Here, we used RNA interference to investigate the role of cytoplasmic dynein in powering these transport events. To reveal transport of MTs and NFs, we expressed EGFP-tagged tubulin or NF proteins in cultured rat sympathetic neurons and performed live-cell imaging of the fluorescent cytoskeletal elements in photobleached regions of the axon. The occurrence of anterograde MT and retrograde NF movements was significantly diminished in neurons that had been depleted of dynein heavy chain, whereas the occurrence of retrograde MT and anterograde NF movements was unaffected. These results support a cargo model for NF transport and a sliding filament model for MT transport.

The evidence supporting two leading models of neurofilaments (NF) and microtubules (MT) organization in neurites such as cross-bridging and polymer-brush-based repulsion is reviewed. The side arms of neurofilaments and the projection domains of microtubule-associated proteins are highly unstructured and exert long-range repulsive forces that are largely entropic in axons.

Some rod-shaped bacteria, including Escherichia coil, exhibit cell filamentation without septum formation under high-hydrostatic-pressure conditions, indicating that the cell-division process is affected by hydrostatic pressure. The effects of elevated pressure on FtsZ-ring formation in E. coli cells were examined using indirect immunofluorescence microscopy. Elevated pressure of 40 MPa completely inhibited colony formation of E. coli cells under the cultivation conditions used, and the cells exhibited obviously filamentous shapes. In the elongated cells, normal cell-division processes appeared to be inhibited, because no FtsZ rings were observed by indirect immunofluorescent staining. In addition, it was observed that hydrostatic pressure dissociated the E. coli FtsZ polymers in vitro. These results suggest that high hydrostatic pressure directly affects cell survival and morphology through the dissociation of the cytoskeletal frameworks.

Neurofilaments are essential for acquisition of normal axonal calibers. Several lines of evidence have suggested that neurofilament-dependent structuring of axoplasm arises through an 'outside-in' signaling cascade originating from myelinating cells. Implicated as targets in this cascade are the highly phosphorylated KSP domains of neurofilament subunits NF-H and NF-M. These are nearly stoichiometrically phosphorylated in myelinated internodes where radial axonal growth takes place, but not in the smaller, unmyelinated nodes. Gene replacement has now been used to produce mice expressing normal levels of the three neurofilament subunits, but which are deleted in the known phosphorylation sites within either NF-M or within both NF-M and NF-H. This has revealed that the tail domain of NF-M, with seven KSP motifs, is an essential target for the myelination-dependent outside-in signaling cascade that determines axonal caliber and conduction velocity of motor axons.

Phosphorylation of neurofilaments has long been considered to regulate their axonal transport rate and, in doing so, to provide stability to mature axons. Interpretation of data recently obtained following C-terminal deletion experiments has prompted a challenge to this hypothesis. We present evidence that these deletion studies remain consistent with, rather than refute, a role for C-terminal phosphorylation in regulation of neurofilament axonal transport.

Neurofilaments possess side arms that comprise the carboxy-terminal domains of neurofilament middle and heavy chains (NFM and NFH); that of NFH is heavily phosphorylated in axons. Here, we demonstrate that phosphorylation of NFH side arms is a mechanism for regulating transport of neurofilaments through axons. Mutants in which known NFH phosphorylation sites were mutated to preclude phosphorylation or mimic permanent phosphorylation display altered rates of transport in a bulk transport assay. Similarly, application of roscovitine, an inhibitor of the NFH side arm kinase Cdk5/p35, accelerates neurofilament transport. Analyses of neurofilament movement in transfected living neurons demonstrated that a mutant mimicking permanent phosphorylation spent a higher proportion of time pausing than one that could not be phosphorylated. Thus, phosphorylation of NFH slows neurofilament transport, and this is due to increased pausing in neurofilament movement.

To test the hypothesis that fast anterograde molecular motor proteins power the slow axonal transport of neurofilaments (NFs), we used homologous recombination to generate mice lacking the neuronal-specific conventional kinesin heavy chain, KIF5A. Because null KIF5A mutants die immediately after birth, a synapsin-promoted Cre-recombinase transgene was used to direct inactivation of KIF5A in neurons postnatally. Three fourths of such mutant mice exhibited seizures and death at around 3 wk of age; the remaining animals survived to 3 mo or longer. In young mutant animals, fast axonal transport appeared to be intact, but NF-H, as well as NF-M and NF-L, accumulated in the cell bodies of peripheral sensory neurons accompanied by a reduction in sensory axon caliber. Older animals also developed age-dependent sensory neuron degeneration, an accumulation of NF subunits in cell bodies and a reduction in axons, loss of large caliber axons, and hind limb paralysis. These data support the hypothesis that a conventional kinesin plays a role in the microtubule-dependent slow axonal transport of at least one cargo, the NF proteins.

Three-dimensional rotors, or scroll waves, provide essential insight into the activity of excitable media. They also are a suspected cause in the formation and maintenance of ventricular fibrillation, whose lethality is well known. It is therefore of considerable interest to find out what configurations can be adopted by such pathologies. A scroll's behavior is embodied in its organizing center or filament, a largely quiescent tube about which the scroll rotates. Predicting filament shape has normally required computer-intensive simulations of the whole scroll in time. We have found a fast and robust principle that yields the prediction for stationary filaments on a purely geometrical basis, blind to the reaction parameters of the medium. The procedure is to calculate the filament shape as a minimal path. We work in singly diffusive media whose diffusivity tensor--and no other feature--varies spatially. Mathematical and numerical evidence is presented for the proposition that a stable filament is a geodesic in a three-dimensional space whose metric is given by the inverse diffusivity tensor of the medium. Away from the boundaries, a stable filament is unaffected by the reaction parameters. The algorithmic aspects of this work are subsidiary to our main purpose of drawing attention to the universal and unexpectedly exact fit of an elementary geodesic principle within reaction-diffusion theories.

Nonmuscle myosin-II is a key motor protein that drives cell shape change and cell movement. Here, we analyze the function of nonmuscle myosin-II during Drosophila embryonic myogenesis. We find that nonmuscle myosin-II and the adhesion molecule, PS2 integrin, colocalize at the developing muscle termini. In the paradigm emerging from cultured fibroblasts, nonmuscle actomyosin-II contractility, mediated by the small GTPase Rho, is required to cluster integrins at focal adhesions. In direct opposition to this model, we find that neither nonmuscle myosin-II nor RhoA appear to function in PS2 clustering. Instead, PS2 integrin is required for the maintenance of nonmuscle myosin-II localization and we show that the cytoplasmic tail of the [[beta].sub.PS] integrin subunit is capable of mediating this PS2 integrin function. We show that embryos that lack zygotic expression of nonmuscle myosin-II fail to form striated myofibrils. In keeping with this, we demonstrate that a PS2 mutant that specifically disrupts myofibril formation is unable to mediate proper localization of nonmuscle myosin-II at the muscle termini. In contrast, embryos that lack RhoA function do generate striated muscles. Finally, we find that nonmuscle myosin-II localizes to the Z-line in mature larval muscle. We suggest that nonmuscle myosin-II functions at the muscle termini and the Z-line as an actin crosslinker and acts to maintain the structural integrity of the sarcomere. Key Words: Drosophila; nonmuscle myosin-II; integrins; myofibrils; RhoA GTPase.

A novel filamentous bacterium, Bacillus sp., strain NAF001, and its forming of endospores and budding cells are discussed. The strain was isolated from suspended water of a domestic wastewater treatment tank. It formed long filamentous trichome and produced endospores and formed heat-resistant spore-like resting cells.

This study documents age-related changes in the immunoreactivity of the medium-molecular weight subunit of neurofilaments in sensory and motor neurons in the brainstem of the cat. In old age, there was a clear decrease in immunoreactivity in the following brainstem sensory and motor nuclei: sensory trigeminal, gracile, cuneate, and facial motor. Only a few neuronal perikarya and dendrites were labeled in these nuclei in old cats; moreover, when present, the labeling was weak. In contrast, in adult cats, these nuclei contained intensely stained neuronal perikarya and dendrites. In other sensory and motor nuclei of the brainstem, there was an obvious age-related increase in the immunoreactivity of the medium-molecular weight subunit of neurofilaments in the perikarya. Despite different patterns of age-related alterations in immunoreactivity within perikarya and dendrites in distinct brainstem regions, most sensory and motor axons in old cats were smaller than those in adult cats. A decrease in the medium-molecular weight neurofilament subunit in the dendrites may be the basis for the dendritic atrophy that has been shown to occur in sensory nuclei in old animals. The decrease in axonal size is likely to be one of the causes of the decrease in axonal conduction velocity, in these neurons, that was reported in our previous studies.