Vimentin filaments in spreading,randomly locomoting,and f-met-leu-phe-treated neutrophils

Summary Human neutrophils contain intermediate filaments of the vimentin type. A cytoskeletal preparation, produced by high-salt and Triton X-100 extraction of human neutrophils, reveals a major band at 57000 M _r that comigrates with 3T3 cell vimentin on one-dimensional gels. Two-dimensional gel electrophoresis of whole neutrophils illustrates the presence of vimentin but not desminor keratin-filament subunits. The presence of vimentin in neutrophils is also shown by its specific staining with avian vimentin antiserum by two-dimensional gel immunoautoradiography. Indirect immunofluorescence studies show that vimentin antiserum labels an area on one side of the nucleus in spreading neutrophils. This bright area appears as a loose knot of vimentin filaments; a few filaments may radiate from the knot. In contrast to spreading neutrophils, those undergoing random locomotion contain a fine network of filaments that are located in the cytoplasm between the nucleus and the trailing end of the cell. Similarly, in chemoattractant-treated neutrophils, vimentin filaments are bundled in the uropod. Transmission electron microscopy of human neutrophil monolayers confirms the intracellular distribution of intermediate filaments as shown by immunofluorescence in spreading and randomly locomoting cells.     

Intracellular elasticity and viscosity in the body, leading, and trailing regions of locomoting neutrophils

Toinvestigate the mechanisms underlying pseudopod protrusion inlocomoting neutrophils, we measured the intracellular stiffness andviscosity in the leading region, main body, and trailingregion from displacements of oscillating intracellulargranules driven with an optical trap. Experiments were done in controlconditions and after treatment with cytochalasin D or nocodazole. Wefound 1) in the body and trailingregion, the granules divided into a "fixed" population (too stiffto measure) and a "free" population (easily oscillated; fixedfraction 65%, free fraction 35%). By contrast, the fixed fraction inthe leading region was <5%. 2) Inthe body and trailing region, there was no difference in stiffness orviscosity, but both were sharply lower in the leading region (respectively, 20-fold and 5-fold).3) Neither cytochalasin D nornocodazole caused a decrease in stiffness, but both treatments markedlyreduced the fixed fraction in the body and trailing region to <20%and <40%, respectively. These observations suggest a discrete lattice structure in the body and trailing region and suggest that thedeveloping pseudopod has a core that is more fluidlike, in thesense of a much lower viscosity and an almost total loss of stiffness.This is consistent with the contraction/solation hypothesis ofpseudopodial formation.

     

Regional differences in arrhythmogenesis

JGP study shows that the subendocardium is more susceptible to spontaneous Ca²⁺ release events that can initiate arrhythmias, and this may be reduced by local CaMKII inhibition.

Calcium release and uptake must be carefully controlled in cardiomyocytes to ensure that the heart maintains a regular beat, and spontaneous Ca²⁺ release (SCR) from the sarcoplasmic reticulum—due to leaky ryanodine receptors, for example—can trigger lethal ventricular arrhythmias. In this issue of JGP, Dries et al. demonstrate that the subendocardial layer of the ventricular wall is particularly susceptible to arrhythmogenic SCR, and that this could potentially be treated by local inhibition of calcium/calmodulin-dependent kinase II (CaMKII; 1).Using living myocardial slices, Eef Dries (left), Cesare Terracciano (center), and colleagues show that, following injury, the subendocardial layer of the rat ventricular wall is more susceptible than the subepicardial layer to arrhythmogenic SCR events. High-resolution Ca²⁺ imaging of the subendocardium shows the increased number of SCRs (green dots) in the region bordering the injured tissue. The frequency of SCRs and ectopic contractions can be reduced by CaMKII inhibition.SCRs have been extensively studied in isolated cardiomyocytes, but arrhythmias are multicellular events (2) in which the behavior of individual cells is influenced by their interactions with neighboring cells and the extracellular matrix. “In addition, myocardial electrophysiology changes at different depths of the ventricular wall, and the vast majority of studies do not account for this transmurality,” explains Cesare Terracciano, a professor at the National Heart and Lung Institute, Imperial College London.Terracciano’s group has pioneered the use of living myocardial slices prepared from different layers of the ventricular wall to study regional differences in the electrical and mechanical properties of healthy hearts (3,4). However, it is unclear how these differences are impacted by injury or disease and whether this leaves some layers of the heart wall more susceptible to SCRs and arrhythmogenesis.Terracciano and colleagues, including first author Eef Dries, therefore prepared myocardial slices from different layers of the rat ventricular wall and subjected them to cryoinjury (1). Structural remodeling—in the form of reduced T-tubule density—was similar in both subendocardial and subepicardial slices after injury, but only subendocardial slices showed an increase in spontaneous, arrhythmic contractions.Dries et al. used a fluorescent Ca²⁺ indicator and high-resolution imaging to examine Ca²⁺ signaling in the “border zone” surrounding the cryoinjury, as this region has been implicated in triggering arrhythmias following myocardial infarction. “Intriguingly, and only in subendocardial slices after injury, we observed a reduction in the amplitude of calcium transients that also became slower to decline, changes that are hallmarks of heart failure,” Terracciano says. “SCR events were more frequent and more closely distributed when we cryoinjured the slices but, again, only in the subendocardium.”The clustering of multiple SCRs in both space and time makes them more likely to trigger an ectopic contraction. One possibility is that the open probability of ryanodine receptors is increased in subendocardial slices. This could be caused by enhanced CaMKII-mediated phosphorylation of ryanodine receptors and, indeed, Dries et al. found that, after cryoinjury, receptor phosphorylation is increased in subendocardial, but not subepicardial, slices (1).Accordingly, Terracciano and colleagues found that the CaMKII inhibitor AIP reduced the frequency of SCRs and spontaneous contractions in cryoinjured subendocardial slices. In contrast, AIP had no effect on injured subepicardial slices or on normal, healthy cardiac tissue. CaMKII inhibitors have been proposed as potential therapies for cardiac arrhythmias, but their use has so far been limited by off-target effects. Dries et al.’s results suggest that targeting CaMKII inhibitors to specific regions of the ventricular wall (using localized gene therapy, for example) could greatly improve their efficacy.“A picture is emerging that subendocardial slices are more susceptible to arrhythmogenic stimuli, and this can be important for understanding and treating arrhythmias,” Terracciano says. He now plans to study injured myocardial slices over longer time periods and investigate the molecular changes underlying the enhanced arrhythmogenic susceptibility of the subendocardium, as well as testing localized gene therapy approaches in animal models of disease.     

Forces exerted by locomoting cells

The movement of cells along substrata is a complex phenomenon involving cell extension and retraction, and cell-substratum adhesion. Knowledge is beginning to accumulate about the forces required for cell protrusion and retraction. Both of these processes also require traction to be exerted on the substratum, and new assays for these forces are under development. This review briefly discusses the forces that locomotory cells exert, in terms of both recent force measurements and possible mechanisms for their generation.     

Traction forces generated by locomoting keratocytes

Traction forces produced by moving fibroblasts have been observed as distortions in flexible substrata including wrinkling of thin, silicone rubber films. Traction forces generated by fibroblast lamellae were thought to represent the forces required to move the cell forwards. However, traction forces could not be detected with faster moving cell types such as leukocytes and growth cones (Harris, A. K., D. Stopak, and P. Wild. 1981. Nature (Lond.). 290:249-251). We have developed a new assay in which traction forces produced by rapidly locomoting fish keratocytes can be detected by the two-dimensional displacements of small beads embedded in the plane of an elastic substratum. Traction forces were not detected at the rapidly extending front edge of the cell. Instead the largest traction forces were exerted perpendicular to the left and right cell margins. The maximum traction forces exerted by keratocytes were estimated to be approximately 2 x 10(-8) N. The pattern of traction forces can be related to the locomotion of a single keratocyte in terms of lamellar contractility and area of close cell- substratum contact.     

Regional differences in neural crest morphogenesis

Neural crest cells are pluripotent cells that emerge from the neural epithelium, migrate extensively and differentiate into numerous derivatives, including neurons, glial cells, pigment cells and connective tissue. Major questions concerning their morphogenesis include: (1) what establishes the pathways of migration? And (2), what controls the final destination and differentiation of various neural crest subpopulations? These questions will be addressed in this Review. Neural crest cells from the trunk level have been explored most extensively. Studies show that melanoblasts are specified shortly after they depart from the neural tube and this specification directs their migration into the dorsolateral pathway. We also consider other reports that present strong evidence for ventrally migrating neural crest cells being similarly fate restricted. Cranial neural crest cells have been less analyzed in this regard but the preponderance of evidence indicates that either the cranial neural crest cells are not fate-restricted or are extremely plastic in their developmental capability and that specification does not control pathfinding. Thus, the guidance mechanisms that control cranial neural crest migration and their behavior vary significantly from the trunk.The vagal neural crest arises at the axial level between the cranial and trunk neural crest and represents a transitional cell population between the head and trunk neural crest. We summarize new data to support this claim. In particular, we show that: (1) the vagal-level neural crest cells exhibit modest developmental bias; (2) there are differences in the migratory behavior between the anterior and the posterior vagal neural crest cells reminiscent of the cranial and the trunk neural crest, respectively and (3) the vagal neural crest cells take the dorsolateral pathway to the pharyngeal arches and the heart, but take the ventral pathway to the peripheral nervous system and the gut. However, these pathways are not rigidly specified because of prior fate restriction. Understanding the molecular, cellular and behavioral differences between these three populations of neural crest cells will be of enormous assistance when trying to understand the evolution of the neck.Key words: neural crest, morphogenesis, cell migration, chicken embryo, fate restriction, vagal neural crest, pathways     

Regional differences in neural crest morphogenesis

Neural crest cells are pluripotent cells that emerge from the neural epithelium, migrate extensively, and differentiate into numerous derivatives, including neurons, glial cells, pigment cells and connective tissue. Major questions concerning their morphogenesis include: 1) what establishes the pathways of migration and 2) what controls the final destination and differentiation of various neural crest subpopulations. These questions will be addressed in this review. Neural crest cells from the trunk level have been explored most extensively. Studies show that melanoblasts are specified shortly after they depart from the neural tube, and this specification directs their migration into the dorsolateral pathway. We also consider other reports that present strong evidence for ventrally migrating neural crest cells being similarly fate restricted. Cranial neural crest cells have been less analyzed in this regard but the preponderance of evidence indicates that either the cranial neural crest cells are not fate-restricted, or are extremely plastic in their developmental capability and that specification does not control pathfinding. Thus, the guidance mechanisms that control cranial neural crest migration and their behavior vary significantly from the trunk. The vagal neural crest arises at the axial level between the cranial and trunk neural crest and represents a transitional cell population between the head and trunk neural crest. We summarize new data to support this claim. In particular, we show that: 1) the vagal-level neural crest cells exhibit modest developmental bias; 2) there are differences in the migratory behavior between the anterior and the posterior vagal neural crest cells reminiscent of the cranial and the trunk neural crest, respectively; 3) the vagal neural crest cells take the dorsolateral pathway to the pharyngeal arches and the heart, but the ventral pathway to the peripheral nervous system and the gut. However, these pathways are not rigidly specified because of prior fate restriction. Understanding the molecular, cellular and behavioral differences between these three populations of neural crest cells will be of enormous assistance when trying to understand the evolution of the neck.     

Regional aortic differences in atherosclerosis susceptibility

In spontaneously atherosclerosis-susceptible White Carneau pigeons intimal cushions are noted consistently at the coeliac branch of aorta at birth. While these cushions do not progress into atherosclerotic lesions, the area across from the cushion (so called "lesion area") develop a classic atherosclerotic plaque by three years of age. In order to explain this regional aortic susceptibility to atherosclerosis, cholesterol and cholesteryl ester concentrations and prostaglandin biosynthesis in the two aortic regions were examined. It was found that the concentration of free and esterified cholesterol was higher in the intimal cushion area. Examination of the formation of various prostaglandins from C14-arachidonic acid indicates a striking increase in PGE2 synthesis in the lesion area with no difference in the formation of 6-keto PGF1 alpha (stable product of PGI2). These studies suggest that one of the earliest changes noted in the "lesion area" that differs from the intimal cushion is the enhanced formation of PGE2.     

Fc and C3bi receptors and the differentiation antigen BH2-Ag are randomly distributed in the plasma membrane of locomoting neutrophils

Reports from several laboratories suggest that neutrophils arrested during locomotion preferentially bind immune complexes at the front of the cell. Such asymmetry of binding has been interpreted as indicating an active modulation of phagocytic receptors to the anterior of the cell. To investigate this further, we have used digital analysis of fluorescence images to determine the binding patterns of mAbs directed against the Fc receptors, the receptors for the C3bi fragment of C3, and a neutrophil-specific antigen. We found that all three proteins are distributed nearly identically along the length of migrating neutrophils, and their distribution very closely parallels the anterior to posterior distribution of the plasma membrane. The use of mAbs offered an important advantage in that the binding of antireceptor antibodies, unlike the binding of ligands, should be independent of potential changes in the affinity of the receptors. We conclude that the anterior distribution of the phagocytic receptors in the plasma membrane of locomoting neutrophils parallels the overall increase in membrane area at the front of a migrating cell and that specific translocation of phagocytic receptors does not occur.     

Widespread state-dependent shifts in cerebellar activity in locomoting mice

Excitatory drive enters the cerebellum via mossy fibers, which activate granule cells, and climbing fibers, which activate Purkinje cell dendrites. Until now, the coordinated regulation of these pathways has gone unmonitored in spatially resolved neuronal ensembles, especially in awake animals. We imaged cerebellar activity using functional two-photon microscopy and extracellular recording in awake mice locomoting on an air-cushioned spherical treadmill. We recorded from putative granule cells, molecular layer interneurons, and Purkinje cell dendrites in zone A of lobule IV/V, representing sensation and movement from trunk and limbs. Locomotion was associated with widespread increased activity in granule cells and interneurons, consistent with an increase in mossy fiber drive. At the same time, dendrites of different Purkinje cells showed increased co-activation, reflecting increased synchrony of climbing fiber activity. In resting animals, aversive stimuli triggered increased activity in granule cells and interneurons, as well as increased Purkinje cell co-activation that was strongest for neighboring dendrites and decreased smoothly as a function of mediolateral distance. In contrast with anesthetized recordings, no 1-10 Hz oscillations in climbing fiber activity were evident. Once locomotion began, responses to external stimuli in all three cell types were strongly suppressed. Thus climbing and mossy fiber representations can shift together within a fraction of a second, reflecting in turn either movement-associated activity or external stimuli.     

Redundancy of lamellipodia in locomoting Walker carcinosarcoma cells

Locomoting metazoan cells usually form lamellipodia at the leading front and it is widely accepted that lamellipodia are required for locomotion. In this case, suppression of lamellipodia must stop locomotion. However, the experiments show that lamellipodia are redundant for locomotion of Walker carcinosarcoma cells. Low latrunculin A concentrations (10(-7) M) transform polarised locomoting cells with lamellipodia into cells without morphologically recognisable protrusions showing an increased speed of locomotion and a reduced amount of cellular F-actin. Whereas untreated cells show a fairly linear distribution of F-actin along the plasma membrane, cells lacking morphologically recognizable protrusions at the front show modifications at the front consisting in an irregular distribution of F-actin with formation of small or large patches of F-actin alternating with small or large gaps in the F-actin layer. This is associated with a reduced resistance to deformation pressure at the front of the cell. High concentrations of latrunculin A (>10(-7) M) compromising contraction at the rear stop locomotion, suggesting that cortical contraction is important for locomotion to occur in these cells. The results are consistent with the view that actin polymerization is important for formation of lamellipodia but they are not compatible with the view that lamellipodia are essential for locomotion of Walker carcinosarcoma cells. A unifying hypothesis for the formation of different types of protrusions is proposed.     

Functional differences in human neutrophils isolated pre- and post-prandially.

Activated polymorphonuclear leukocytes have been associated with neoplasia, atherogenesis and reperfusion injury. Since some of these conditions are also correlated with dietary fat, we examined the functional characteristics of leukocytes isolated from subjects before and after consumption of a lipid-rich meal. There was up to 2-fold greater superoxide generation in response to agonists in leukocytes obtained post-prandially; the maximum increase was observed about 4 h after eating and followed the peak (2-4 h) in serum triglycerides. Neutrophils isolated post-prandially also exhibited impaired chemotaxis and defective bacterial killing, but normal phagocytosis. These findings provide a new variable that should be considered in studies of leukocytes.     

Regional metabolic differences in the rat diaphragm

This study characterized the biochemical properties of the rat diaphragm by measuring the activities of selected citric acid cycle and glycolytic enzymes. The diaphragm was removed from 10 female Sprague-Dawley rats (180 days old) and dissected into five discrete anatomic regions: crural (region 1), left posterior costal (region 2), left anterior costal (region 3), right anterior costal (region 4), and right posterior costal (region 5). Sections were assayed for total protein concentration and the activities of succinate dehydrogenase (SDH) and lactate dehydrogenase (LDH). The SDH activity in the crural region was approximately 18% lower (P less than 0.05) than that in any costal region. Furthermore, protein concentration was significantly lower (P less than 0.05) in the crural region compared with all costal regions. In contrast, costal regions 2-5 did not significantly differ from each other in protein concentration or SDH activity. LDH activity did not differ significantly (P greater than 0.05) between regions. Finally, the LDH-to-SDH activity ratio was significantly higher (P less than 0.05) in the crural diaphragm compared with all costal regions. We conclude that the crural region of the rat diaphragm is significantly lower in oxidative capacity than all the costal regions. Investigators who use a rodent model to study diaphragmatic function and plasticity should consider the oxidative heterogeneity of the diaphragm when designing experiments.     

Regional differences in neutrophil margination in dog lungs

We investigated the relationship between polymorphonuclear leukocyte (PMN) retention and erythrocyte (RBC) velocity in the lungs of mongrel dogs. Regional velocity was estimated by measuring regional RBC transit times and was correlated with the retention of PMN found in the same lung sample 10 min after the injection of a bolus of labeled cells. Data from the whole lung showed that the total number of cells marginated in the pulmonary vasculature was 2.4 times as great as the number present in the circulation and that this pool turned over at a rate of 1%/s. The regional data showed increased retention, indicating slower PMN turnover in the upper lung regions, which have longer transit times and therefore slower blood velocities than the RBC is attributed to a greater discrepancy between PMN and RBC is attributed to a greater discrepancy between PMN and capillary size and the fact that PMN are less deformable than RBC. The large number of capillary segments present in the lung allows neutrophils to move more slowly while RBC stream around them. We conclude that there are approximately 2.5 times as many PMNs marginated in the lung as there are in the total circulating blood volume of the dog and that the pulmonary marginated pool turns over at approximately 1%/s with slower turnover in the upper compared with the lower regions of the lung.     

Regional differences in abdominal pressure swings in dogs

The pressure swings under the costal (Pcos) and crural diaphragms (Pcru) and between the intestinal loops (Pint) were compared with the swings in gastric pressure (Pga) in 13 supine anesthetized dogs. Pcos, Pcru, and Pint were measured with air-filled latex balloons in eight dogs and saline-filled catheters in five. Pga was measured with an air-filled balloon in all dogs. During quiet breathing differences were often present, the directions of which were variable from animal to animal. During mechanical ventilation, all pressures increased, but both Pcos and Pcru increased more than Pga, whereas only a small change was observed in Pint. During bilateral stimulation of the costal diaphragm, Pcos invariably increased more than Pga and Pint, whereas almost no change was observed in Pcru. During bilateral stimulation of the crural diaphragm, Pcru invariably increased more than Pga, Pint, and Pcos. During abdominal muscle stimulation as during external abdominal compression, Pint always increased more than Pcos and Pcru. During lower rib cage compression, Pga, Pcos, and Pcru increased more than Pint. During sternocleidomastoid stimulation, all pressure swings were negative, but the change in Pint was always smaller than in Pcos, Pcru, or Pga. Inhomogeneities observed with balloons and saline-filled catheters were similar. After the abdomen was filled with 2 liters of saline all pressure swings became much more homogeneous.