Centrin plays an essential role in microtubule severing during flagellar excision in Chlamydomonas reinhardtii

Previously, we reported that flagellar excision in Chlamydomonas reinhardtii is mediated by an active process whereby microtubules are severed at select sites within the flagellar-basal body transition zone (Sanders, M. A., and J. L. Salisbury. 1989. J. Cell Biol. 108:1751-1760). At the time of flagellar excision, stellate fibers of the transition zone contract and displace the microtubule doublets of the axoneme inward. The resulting shear force and torsional load generated during inward displacement leads to microtubule severing immediately distal to the central cylinder of the transition zone. In this study, we have used a detergent-extracted cell model of Chlamydomonas that allows direct experimental access to the molecular machinery responsible for microtubule severing without the impediment of the plasma membrane. We present four independent lines of experimental evidence for the essential involvement of centrin-based stellate fibers of the transition zone in the process of flagellar excision: (a) Detergent-extracted cell models excise their flagella in response to elevated, yet physiological, levels of free calcium. (b) Extraction of cell models with buffers containing the divalent cation chelator EDTA leads to the disassembly of centrin-based fibers and to the disruption of transition zone stellate fiber structure. This treatment results in a complete loss of flagellar excision competence. (c) Three separate anti-centrin monoclonal antibody preparations, which localize to the stellate fibers of the transition zone, specifically inhibit contraction of the stellate fibers and block calcium-induced flagellar excision, while control antibodies have no inhibitory effect. Finally, (d) cells of the centrin mutant vfl-2 (Taillon, B., S. Adler, J. Suhan, and J. Jarvik. 1992. J. Cell Biol. 119:1613-1624) fail to actively excise their flagella following pH shock in living cells or calcium treatment of detergent-extracted cell models. Taken together, these observations demonstrate that centrin-based fiber contraction plays a fundamental role in microtubule severing at the time of flagellar excision in Chlamydomonas.

Involvement of profilin in the actin-based motility of L. monocytogenes in cells and in cell-free extracts

Within hours of Listeria monocytogenes infection, host cell actin filaments form a dense cloud around the intracytoplasmic bacteria and then rearrange to form a polarized comet tail that is associated with moving bacteria. We have devised a cell-free extract system capable of faithfully reconstituting L. monocytogenes motility, and we have used this system to demonstrate that profilin, a host actin monomer-binding protein, is necessary for bacterial actin-based motility. We find that extracts from which profilin has been depleted do not support comet tail formation or bacterial motility. In extracts and host cells, profilin is localized to the back half of the surface of motile L. monocytogenes, the site of actin filament assembly in the tail. This association is not observed with L. monocytogenes mutants that do not express the ActA protein, a bacterial gene product necessary for motility and virulence. Profilin also fails to bind L. monocytogenes grown outside of host cytoplasm, suggesting that at least one other host cell factor is required for this association.

[The role of oxytalan fibers in the stomatognathic system. A bibliographic review]

The authors review the oxytalan fibers, that belong to the elastic system fibers of the connective tissue. They are observed in many structures of the stomatognathic system, particularly in the periodontal ligament and TMJ disc.

Simulation study on the effect of external vibration on left ventricular function: potential indicator of LV tolerance to the sudden reduction of myocardial contractility

In a previous study the authors reported that external mechanical vibration applied to the left ventricular (LV) epicardium induces contractility-dependent depression in LV pressure, stroke volume and stroke work. It was suggested that this depression may be caused by the direct effect of external vibration on contractile protein. In another paper in this issue, it is proved that LV function with various myocardial contractilities and the actual process of deterioration in heart failure are well simulated in the model proposed by Beyar and Sideman, after some modifications have been made. In the study reported here it is assumed that an external mechanical vibration induces sudden reduction in myocardial active stress in the model of Beyar and Sideman; in this way the contractility-dependent effect of external vibration on LV function has been simulated. The results of this simulation support the suggestion that external mechanical vibration directly affects contractile protein and reduces LV function, and it is further suggested that the reduction of LV function induced by external vibration reflects the reserve or tolerance capacity of LV to a sudden reduction of myocardial contractility.

Pure thoughts with impure proteins: permeabilized cell models of organelle motility

Permeabilized cell models provide an experimental middle ground wherein the in vitro properties of mechanochemical proteins can be reconciled with the physical and topological constraints of the intact cell. Several well-studied examples of organelle motility are described here, including the actin-based cytoplasmic streaming of Characean algae, the microtubule-based aggregation and dispersion of pigment granules in chromatophores and the saltatory movements of vesicles along microtubules in fibroblasts and macrophages. The permeabilized models developed for these systems have helped to integrate observations in vivo with in vitro assays of motor proteins.

Intracellular calcium concentration during low-frequency fatigue in isolated single fibers of mouse skeletal muscle

Low-frequency fatigue is a form of muscle fatigue that follows intense muscle activity and is characterized by reduced tetanic tension at low frequencies of stimulation while tetanic tension at high stimulus frequencies is close to normal. The present experiments were performed on isolated single fibers of mouse in which tension and intracellular calcium concentration ([Ca2+]i) were measured. Fatigue was produced by intermittent short tetani continued until tension had declined to 30% of control. Comparison of low- (30- and 50-Hz) and high- (100-Hz) frequency tetani under control conditions and after 30 min of recovery from fatigue showed that low-frequency fatigue was present. During low-frequency fatigue, tetanic [Ca2+]i was substantially reduced at all stimulus frequencies but there was no change in Ca2+ sensitivity or maximum Ca(2+)-activated tension. One possible cause of the reduced tetanic [Ca2+]i is failure of conduction of the action potential in the T tubule, leading to reduced [Ca2+]i in the center of the fiber. However, imaging of [Ca2+]i across the fiber during low-frequency fatigue did not show any such gradient, suggesting that Ca2+ release is uniform across the fiber. Another possible mechanism is that changes in the Ca2+ pumping ability of the sarcoplasmic reticulum might affect tetanic [Ca2+]i. Measurements of the sarcoplasmic reticulum pump function showed a small slowing of Ca2+ uptake rate during low-frequency fatigue, which is unlikely to cause the reduced tetanic [Ca2+]i. In conclusion, the immediate cause of low-frequency fatigue appears to be a reduced tetanic [Ca2+]i, which is probably a consequence of a reduced Ca2+ release from the sarcoplasmic reticulum.

Influence of ionic strength on contractile force and energy consumption of skinned fibers from mammalian and crustacean striated muscle

Increased ionic strength decreases maximal calcium-activated force (Fmax) of skinned muscle fibers via mechanisms that are incompletely understood. In detergent-skinned fibers from either rabbit (psoas) or lobster (leg or abdomen), Fmax in KCl-containing solutions was less than in potassium methanesulfonate (KMeSO3), which we showed previously was the least deleterious salt for adjusting ionic strength. In either salt, lobster fibers were considerably less sensitive to elevated ionic strength than rabbit fibers. Trimethylamine N-oxide (TMAO, a zwitterionic osmolyte found in high concentration in cells of salt-tolerant animals) increased Fmax, especially in high KCl solutions. In this regard, TMAO was more effective than a variety of other natural or synthetic zwitterions. In rabbit fibers, increasing ionic strength decreases Fmax but has little effect on contractile ATPase rate measured simultaneously using a linked-enzyme assay. Thus high salt increases the tension-cost of contraction (i.e. ratio ATPase/Fmax). At both high and low salt, TMAO decreases tension-cost. Given a simple two-state model of the cross-bridge cycle, these data indicate that ionic strength and TMAO affect the apparent detachment rate constant. High ionic strength KCl solutions extract myosin heavy- and light-chains, and troponin C from rabbit fibers. This extraction is virtually abolished by TMAO. Natural zwitterions, such as TMAO, have been shown to protect proteins against destabilization by high salt or other denaturatants. Our data indicate that, even in the best of salts, destabilization of the actomyosin complex may play a role in the effect of high ionic strength on the contractile process.

Excitation-contraction coupling and contractile protein function in failing and nonfailing human myocardium

Isometric force, heat output, and aequorin light emission were measured in isolated muscle strips from nonfailing human hearts and from hearts with endstage failing dilated cardiomyopathy (37 degrees C; 30-180 beats per minute (bpm)). In nonfailing myocardium, peak twitch tension increased with higher rates of stimulation, whereas the force-frequency relation was inverse in the failing myocardium. At 60 bpm and at higher rates of stimulation, peak twitch tension was reduced significantly in the failing myocardium. Myothermal measurements, performed at 60 bpm, indicated that the number of crossbridge interactions and the amount of calcium cycling are reduced significantly in the failing myocardium. Furthermore, aequorin light transients indicated that the inverse force-frequency relation in failing myocardium results from altered calcium cycling; with increasing rates of stimulation aequorin light emission increased continuously in the nonfailing and decreased continuously in the failing myocardium. The data suggest that impaired myocardial performance in failing human myocardium may result primarily from disturbed excitation-contraction coupling processes with a reduced amount of calcium cycling and, thus, a decreased activation of contractile proteins.

Alteration of contractile function and excitation-contraction coupling in dilated cardiomyopathy

Myocardial failure in dilated cardiomyopathy may result from subcellular alterations in contractile protein function, excitation-contraction coupling processes, or recovery metabolism. We used isometric force and heat measurements to quantitatively investigate these subcellular systems in intact left ventricular muscle strips from nonfailing human hearts (n = 14) and from hearts with end-stage failing dilated cardiomyopathy (n = 13). In the failing myocardium, peak isometric twitch tension, maximum rate of tension rise, and maximum rate of relaxation were reduced by 46% (p = 0.013), 51% (p = 0.003), and 46% (p = 0.018), respectively (37 degrees C, 60 beats per minute). Tension-dependent heat, reflecting the number of crossbridge interactions during the isometric twitch, was reduced by 61% in the failing myocardium (p = 0.006). In terms of the individual crossbridge cycle, the average crossbridge force-time integral was increased by 33% (p = 0.04) in the failing myocardium. In the nonfailing myocardium, the crossbridge force-time integral was positively correlated with the patient's age (r = 0.86, p less than 0.02), whereas there was no significant correlation with age in the failing group. The amount and rate of excitation-contraction coupling-related heat evolution (tension-independent heat) were reduced by 69% (p = 0.24) and 71% (p = 0.028), respectively, in the failing myocardium, reflecting a considerable decrease in the amount of calcium released and in the rate of calcium removal. The efficiency of the metabolic recovery process, as assessed by the ratio of initial heat to total activity-related heat, was similar in failing and nonfailing myocardium (0.54 +/- 0.03 versus 0.50 +/- 0.02, p = 0.23).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of tissue expansion on dermal fibroblast contraction

Tissue expansion alters the function of skin cells. We studied the effects of expansion on the contractile function of dermal fibroblasts using an in vitro model, the fibroblast-populated collagen lattice (FPCL). Spherical expanders were placed dorsally in 30 Sprague-Dawley rats; one-half were serially inflated. One, 2, and 4 weeks later, 5 rats from each group were killed. Fibroblasts were cultured from dermis overlying the expanders and mixed with collagen, medium, and serum in petri dishes to form FPCL. Fibroblasts from 5 rats that had not undergone surgery were cultured to make control FPCL. Contraction was assessed by measuring the areas of the FPCL. At 48 hours, FPCL containing expanded fibroblasts had contracted significantly less than those containing unexpanded or control fibroblasts. Four weeks of expansion resulted in less contraction than 1 or 2 weeks. Tissue expansion inhibits the in vitro contractile function of dermal fibroblasts in the rat in a time-related fashion.

Accelerated nonmuscle contraction after subarachnoid hemorrhage: culture and characterization of myofibroblasts from human cerebral arteries in vasospasm

Cell culture lines from human cerebral arteries showing vasospasm after subarachnoid hemorrhage were established from three autopsy cases. Each culture line showed the ultrastructural characteristics of myofibroblasts. Decreased alpha-actin antigenicity, demonstrated using the anti-smooth muscle cell alpha-actin antibody, was observed in cultured cells possessing abundant F-actin. When incorporated into the three-dimensional collagen matrix in vitro, the cultured cells compacted the collagen lattice at a rate equivalent to that of human dermal fibroblasts. Lattice compaction was significantly accelerated by cerebrospinal fluid taken from patients with symptomatic vasospasm. Compaction was completely inhibited by the addition of 10(-6) mol/L verapamil or 100 U/mL heparin. Neither nimodipine (10(-5) mol/L) nor nicardipine (10(-5) mol/L) inhibited compaction, and endothelin (10(-6) mol/L) and potassium chloride (40 mmol/L) had no effect. The morphological change of cells in the collagen lattice suggests that both verapamil and heparin affect cellular motility, filopodial protrusion, and cell attachment. These data suggest that myofibroblasts in human cerebral arteries differ from medial smooth muscle cells and can generate a force rearranging the proliferated collagen matrix present after subarachnoid hemorrhage. This reorganization can contribute to, or be responsible for, sustained vasoconstriction. Consequently, current treatment for vasospasm may need to be reevaluated to include the nonmuscle components in the vessel wall.

[Contractile elements in proliferative retinal diseases]

Contraction of epiretinal membranes remains the leading cause of failure in retinal surgery. The mechanism of contraction of cellular units is as yet unknown. We have used immunohistochemical methods to demonstrate the intracellular localization of the proteins actin, myosin, tropomyosin and vinculin, which are thought to be responsible for cellular contraction, in 26 surgically obtained epiretinal traction membranes from patients with traumatic (n = 12) and idiopathic (n = 7) proliferative vitreoretinopathy, and proliferative diabetic retinopathy (n = 7). The use of cell marker proteins for glial cells (GFAP) and retinal pigment epithelium (cytokeratin) demonstrates that the prevalence of contractile components is independent of cellular construction. We suggest that an intracellular mechanism is responsible for the contraction of epiretinal membranes in proliferative vitreoretinal disorders.

Sympathetic modulation of the cardiac myocyte phenotype: studies with a cell-culture model of myocardial hypertrophy

Myocardial hypertrophy is the common endpoint of many cardiovascular stimuli such as hypertension, myocardial infarction, valvular disease, and congestive failure. Catecholamines have long been implicated in the pathogenesis of myocardial hypertrophy, however, it is very difficult to sort out catecholamine mechanisms in vivo. We have developed a cell-culture model which excludes hemodynamic effects and allows the assignment of receptor specificity to catecholamine effects. Utilizing this system, we have shown that stimulation of the alpha 1 adrenergic receptor leads to the development of myocardial hypertrophy and results in the selective up-regulation of the fetal/neonatal mRNAs encoding skeletal alpha-actin and beta-MHC, a pattern similar to that seen with hypertrophy in-vivo. Utilizing a co-transfection assay, we have also obtained data that suggest that the beta-PKC isozyme is in a pathway regulating transcription of the beta-MHC isogene. Beta adrenergic stimulation of the cultured cardiac myocytes also results in a modest degree of hypertrophy, however, this effect may be dependent upon myocyte contractile activity and may involve, at least in part, the non-muscle cells present in the culture system.

The invasin protein of Yersinia enterocolitica: internalization of invasin-bearing bacteria by eukaryotic cells is associated with reorganization of the cytoskeleton

Yersinia enterocolitica, a facultative intracellular pathogen of mammals, readily enters (i.e., invades) cultured eukaryotic cells, a process that can be conferred by the cloned inv locus of the species. We have studied the mechanism by which the product of inv, a microbial outer membrane protein termed "invasin," mediates the internalization of bacteria by HEp-2 cells and chicken embryo fibroblasts. Invasin-bearing bacteria initially bound the filopodia and the leading edges of cultured cells. Multiple points of contact between the bacterial surface and the surface of the cell ensued and led to the internalization of the bacterium within an endocytic vacuole; the same multi-step process could be induced by an inert particle coated with invasin-containing membranes. Both adherence and internalization were blocked by an antisera directed against the beta 1 integrin cell-adherence molecule. Ultrastructural studies of detergent-insoluble cytoskeletons from infected cells and immunofluorescence microscopy of phalloidin-labeled cells showed alterations in the structure of the cytoskeleton during the internalization process including the accumulation of polymerized actin around entering bacteria. Bacterial entry was prevented by cytochalasin D indicating that the internalization process requires actin microfilament function. Possible linkages between beta 1 containing integrins and the cytoskeleton were examined during the internalization process through the use of protein-specific antibodies and immunofluorescence microscopy. Like actin, the actin-associated proteins filamin, talin and the beta 1 integrin subunit were also found to accumulate around entering bacteria. These findings suggest that the invasin-mediated internalization process is associated with cytoskeletal reorganization.

Contractile protein function in failing and nonfailing human myocardium

Isometric heat and force measurements were used to relate mechanical performance to function of contractile proteins in muscle strips from failing and nonfailing human hearts (37 degrees C, 60 beats per minute). Compared to control myocardium, crossbridge behavior was altered in myocardium from hearts with end-stage failing dilated and ischemic cardiomyopathy, resulting in increased crossbridge force-time integral by 33% and 36%, respectively. Peak isometric twitch tension was reduced significantly by 46% in muscle strips from hearts with dilated cardiomyopathy. In myocardium from hearts with ischemic cardiomyopathy peak isometric twitch tension was comparable to values from nonfailing hearts. Including all three types of myocardium, there was a close correlation between the number of crossbridge interactions during the isometric twitch (tension-dependent heat) and peak twitch tension (r = 0.88; p less than 0.001). Compared to control, in failing myocardium from dilated cardiomyopathic hearts, tension-independent heat (calcium cycling) was significantly reduced. This indicates that in dilated cardiomyopathy reduced peak twitch tension results from decreased calcium activation of contractile proteins with reduced number of crossbridge interactions during the isometric twitch. In ischemic cardiomyopathy mechanisms different from those observed in dilated cardiomyopathy seem to be involved in the development of heart failure.

Replacement of three troponin components with cardiac troponin components within single glycerinated skeletal muscle fibers

The tension of single glycerinated rabbit skeletal muscle fiber was desensitized to a Ca(2+)-concentration after treatment with an excessive amount of bovine cardiac troponin T and reached a level of about 70% of the maximum tension of the untreated fiber. A SDS-gel electrophoretic examination indicated that troponin C.I.T complex in the fiber was replaced with the added cardiac troponin T. The Ca(2+)-sensitivity of the tension of the troponin T-treated fiber was then recovered by the addition of bovine cardiac troponins I and C. The rabbit skeletal muscle fiber thus hybridized with bovine cardiac troponin C.I.T showed the same cooperativity of Ca(2+)-activation as the cardiac muscle.