A unified theory of the control of actin and myosin in nonmuscle movements

Apoptotic cells initiate endothelial cell sprouting via electrostatic signaling

Angiogenesis, the development of new blood vessels from preexisting vessels, is crucial to tissue growth, repair, and maintenance. This process begins with the formation of endothelial cell sprouts followed by the proliferation and migration of neighboring endothelial cells along the preformed extensions. The initiating event and mechanism of sprouting is not known. We show that the phenotypic expression of negatively charged membrane surface in apoptotic cells initiates the formation of directional endothelial cell sprouts that extend toward the dying cells by a mechanism that involves endothelial cell membrane hyperpolarization and cytoskeleton reorganization but is independent of diffusible molecules.

The fundamental motor of the human neutrophil is not random: evidence for local non-Markov movement in neutrophils

The search for a fundamental mechano-chemical process that results in net cell motion has led investigators to fit neutrophil tracking data to well described physical models in hopes of understanding the functional form of the driving force. The Ornstein-Uhlenbeck (OU) equation for mean square displacement describes a locally persistent and globally random process and is often used as a starting point for analysis of neutrophil displacements. Based upon the apparently close fit of neutrophil tracking data to this equation and the nature of its derivation, biologists have inferred that the motor of the neutrophil is best represented as a random process. However, 24 of 37 neutrophil paths that we investigated preferentially display programmatic rather than Markov short term correlations between displacements or turn angles. These correlations reflect a bimodal rather than a uniform distribution of subpath correlations in the two variables, and are strongly sampling rate-dependent. Significant periodic components of neutrophil shape change are also detected at the same time scale using either Fourier or elliptical Fourier transform-based descriptors of the neutrophil perimeter. Oscillations in neutrophil velocity have the same period. Taken together, these data suggest a nonstochastic, and perhaps periodic, component to the process driving neutrophil movement.

Anchorage of secretion-competent dense granules on the plasma membrane of bovine platelets in the absence of secretory stimulation

The ultrastructural changes in electropermeabilized bovine platelets that accompany the Ca2(+)-induced secretion of serotonin were investigated in ultra-thin sections of chemically fixed cells. Such preparations permitted us to study both the localization of and the structures associated with serotonin-containing dense granules. Localization of dense granules within cells was examined by measuring the shortest distances between the granular membranes and the plasma membrane. About 40% of total granules were located close to the plasma membrane at an average distance of 10.8 +/- 1.6 nm. 71% of the total number of granules were localized at a similar average distance of 12.5 +/- 2.7 nm in intact platelets. The percentage of granules apposed to the plasma membrane corresponded closely to the percentage of total serotonin that was maximally secreted after stimulation of the permeabilized (38 +/- 4.9%) and the intact platelets (72 +/- 3.6%). Furthermore, the percentage of granules anchored to the membrane, but not of those in other regions of permeabilized cells, decreased markedly when cells were stimulated for 30 s by extracellularly added Ca2+. The decrease in the numbers of granules in the vicinity of the plasma membrane corresponded to approximately 22% of the total number of dense granules that were used for measurements of the distances between the two membranes and corresponded roughly to the overall decrease (15%) in the average number of the granules per cell. Most dense granules were found to be associated with meshwork structures of microfilaments. Upon secretory stimulation, nonfilamentous, amorphous structures found between the plasma membrane and the apposed granules formed a bridge-like structure that connected both membranes without any obvious accompanying changes in the microfilament structures. These results suggest that the dense granules that are susceptible to secretory stimulation are anchored to the plasma membrane before stimulation, and that the formation of the bridge-like structure may participate in the Ca2(+)-regulated exocytosis.

Phalloidin inhibits cortical granule exocytosis and ooplasmic segregation in loach eggs

Injections of phalloidin under the surface of loach eggs, followed by activation of the eggs in tap water, result in local inhibition of cortical granule (CG) exocytosis. Light and electron microscopy revealed that in the region where exocytosis is inhibited the thickness of the microfilamentous cortex (MC) separating CGs from the plasma membrane (PM) is increased significantly, and many CGs are detached and have moved away from the MC. Injections of phalloidin also inhibit ooplasmic segregation in fertilized eggs. The experiments suggest that in intact eggs the MC represents a physical barrier to CG exocytosis, and that interactions of the MC with the PM and CGs are crucial for the retention of CGs near the sites of fusion.

The cortical microfilament system of lymphoblasts displays a periodic oscillatory activity in the absence of microtubules: implications for cell polarity

For an understanding of the role of microtubules in the definition of cell polarity, we have studied the cell surface motility of human lymphoblasts (KE37 cell line) using video microscopy, time-lapse photography, and immunofluorescent localization of F-actin and myosin. Polarized cell surface motility occurs in association with a constriction ring which forms on the centrosome side of the cell: the cytoplasm flows from the ring zone towards membrane veils which keep protruding in the same general direction. This association is ensured by microtubules: in their absence the ring is conspicuous and moves periodically back and forth across the cell, while a protrusion of membrane occurs alternately at each end of the cell when the ring is at the other. This oscillatory activity is correlated with a striking redistribution of myosin towards a cortical localization and appears to be due to the alternate flow of cortical myosin associated with the ring and to the periodic assembly of actin coupled with membrane protrusion. The ring cycle involves the progressive recruitment of myosin from a polar accumulation, or cap, its transportation across the cell and its accumulation in a new cap at the other end of the cell, suggesting an assembly-disassembly process. Inhibition of actin assembly induces, on the other hand, a dramatic microtubule-dependent cell elongation with definite polarity, likely to involve the interaction of microtubules with the cell cortex. We conclude that the polarized cell surface motility in KE37 cells is based on the periodic oscillatory activity of the actin system: a myosin-powered equatorial contraction and an actin-based membrane protrusion are concerted at the cell level and occur at opposite ends of the cell in absence of microtubules. This defines a polarity which reverses periodically as the ring moves across the cell. Microtubules impose a stable cell polarity by suppressing the ring movement. A permanent association of the myosin-powered contraction and the membrane protrusion is established which results in the unidirectional activity of the actin system. Microtubules exert their effect by controlling the recruitment of cytoplasmic myosin into the cortex, probably through their direct interaction with the cortical microfilament system.

Why are so many biological systems periodic?

The ubiquity of oscillations in biological systems is well established. Oscillations are observed in all types of organisms from the simplest to the most complex. Periods can range from fractions of a second to months or years. From time to time, it has been suggested that many biological oscillations are the result of the breakdown of effective self-regulation. The opposite view is defended here. It is argued that most periodic behavior is not pathological but rather constitutes the normal operation for these systems. They are present because they confer positive functional advantages for the organism. The advantages fall into five general categories: temporal organization, spatial organization, prediction of repetitive events, efficiency and precision of control.

Fluorescent localization of calcium at sites of cell attachment and spreading

Chlorotetracycline (CTC), a calcium-chelating fluorescent probe identifies and localizes calcium when applied to cultured neural crest cells. The fluorescence occurs at specific membrane regions involved in cell attachment and spreading as well as with the formation of cytoskeletal stress fibers (actin microfilament bundles). The observed CTC reaction indicates that calcium sequestration and the development of these membrane-cytoskeletal features share a temporal and spatial pattern. Thus, the selective availability of calcium may be an important determinant of cell morphogenesis.

The use of chlorotetracycline to localize calcium in neural crest cells

Cultured amphibian neural crest cells in the early phases of migration (1-2 days) extend broad, clear cytoplasmic processes known as lobopodia. These regions exhibit rapid changes in the contraction-relaxation cycle and in substratum adhesion. Because of the putative role of Ca2+ in both cytoskeletal function and cell attachment, its distribution in these cells was studied. Chlorotetracycline (CTC), the Ca2+-chelating fluorescent probe, was used to investigate whether there is a temporal and spatial localization of Ca2+ in cell regions concerned with movement and attachment. Differentiated neural crest cells (7 days), which contain an abundance of actin stress fibers, were also studied to determine whether these cytoskeletal elements were associated with Ca2+. The results with CTC indicate that in early migratory cells regions of membrane extension and protrusion contain higher levels of Ca2+ and that the actin stress fiber system of differentiating neural crest cells (7 days) is associated with Ca2+. It was determined that using the CTC in the fixative produced the same labeling profile as when live cells were labeled with the CTC. Use of CTC in the fixative is a decided advantage over treatment of cells with CTC while alive since it avoids the photooxidative damage which normally accompanies fluorescent observations of CTC on living cells.

On the mechanisms of cytokinesis in animal cells

We present a model that attempts to explain some aspects of cytokinesis in animal cells. We propose two separate phases of cytokinesis. The first is not dependent on the presence of the mitotic apparatus and involves a general activation of cortical contractile elements resulting in the development of a surface tension. In the second phase the asters of the mitotic apparatus interact and modulate the activities of the tension generating elements in the cortex to produce gradients of surface tension with the highest values being at the equator. Tension generating elements are assumed to be free to move in the plane of the cortex so that they will consequently move up the gradient of tension and accumulate as an equatorial belt of oriented elements i.e. the contractile ring. The model was simulated on a computer and is capable of reproducing some of the wide variety of cleavage configurations that are observed.

Stimulation of lymphocyte receptor capping by the ionophore monensin

The carboxylic ionophore monensin has a biphasic effect on antibody-induced Thy-1 cap formation. At higher concentrations, 5 X 10(-6)-5 X 10(-5) M monesin causes a significant inhibition of receptor capping similar to that previously found with the Ca2+ selective ionophore A23187. At lower concentrations, 5 X 10(-8)-5 X 10(-7) M capping is stimulated. It is concluded that capping at lower ionophore concentrations is a specific response to the ability of monensin to induce a rise in intracellular Na+, which indirectly elevates intracellular Ca2+ activity. This in turn activates the contractile machinery required for the aggregation of surface receptors into capped structures. At higher concentrations monensin acts as a nonspecific detergent, which causes detrimental structural alterations in some of the membrane components involved in the capping process.

Hydrodynamic models of viscous coupling between motile myosin and endoplasm in characean algae

Cytoplasmic streaming in characean algae is thought to be driven by interaction between stationary subcortical actin bundles and motile endoplasmic myosin. Implicit in this mechanism is a requirement for some form of coupling to transfer motive force from the moving myosin to the endoplasm. Three models of viscous coupling between myosin and endoplasm are presented here, and the hydrodynamic feasibility of each model is analyzed. The results show that individual myosinlike molecules moving along the actin bundles at reasonable velocities cannot exert enough viscous pull on the endoplasm to account for the observed streaming. Attachment of myosin to small spherical organelles improves viscous coupling to the endoplasm, but results for this model show that streaming can be generated only if the myosin-spheres move along the actin bundles in a virtual solid line at about twice the streaming velocity. In the third model, myosin is incorporated into a fibrous or membranous network or gel extending into the endoplasm. This network is pulled forward as the attached myosin slides along the actin bundles. Using network dimensions estimated from published micrographs of characean endoplasm, the results show that this system can easily generate the observed cytoplasmic streaming.

In vitro reconstitution of chromaffin granule-cytoskeleton interactions: ionic factors influencing the association of F-actin with purified chromaffin granule membranes

Chromaffin granules are the secretory vesicles directly involved in exocytosis of catecholamines, enkephalins, and other components from adrenal medullary cells. The granules occupy a large portion of the cytoplasmic volume and thus may interact extensively with cytoskeletal elements such as actin. Indeed, using both sedimentation techniques and falling ball viscometry [Fowler et al: J Cell Biol 88: 388, 1981] to measure actin binding by membranes, we were able to show that chromaffin granules bind F-actin via a protein site on the membrane, and that these interactions are reversibly inhibited by raising the free calcium ion concentration to micromolar levels ([Ca++]free for half-maximal inhibition approximately 2.6 x 10(-7)M)[Fowler and Pollard: Nature 295:336, 1982]. Here, we show that F-actin-chromaffin granule interactions are unaffected by changes in pH between about pH 6.4 and 7.4 but are about 50% inhibited by raising the pH from 7.5 to 8.0. They are also 50% inhibited by increasing the KCl concentration to about 200 mM but are not significantly affected by increasing concentrations of K-glutamate up to 500 mM or by varying the MgCl2 concentration between 0 and 6 mM. The interactions between chromaffin granule membranes and F-actin are also reduced in the presence of ATP, AMP-PNP, or free pyrophosphate; cAMP and AMP are without effect. The ability of chromaffin granule membranes to interact with F-actin under conditions that may approximate the resting intracellular environment (neutral pH, low KCl, 1-2 mM MgCl2, 1 mM ATP, [Ca++]free less than 10(-7)M, 30 degrees C) suggests that these interactions may partially reconstitute naturally occurring associations between chromaffin granules and the cytoskeleton. Further, regulation of chromaffin granule membrane-actin interactions by ionic factors (pH, calcium, chloride ions, nucleotides) that vary intracellularly leads us to propose that associations between actin and the chromaffin granule membrane could influence the location and dislocation of these organelles in the cytoplasm.

The effects of pharmacologic agents on immune-complex-induced redistribution of B-lymphocyte Fc receptors

The effects of pharmacologic agents on the immune-complex-induced redistribution of B-lymphocyte Fc receptors (and, as a control, the anti-Ig induced redistribution of surface Ig) were examined. Immune-complex-induced capping of B-cell Fc receptors was moderately to markedly inhibited by the combination of colchicine and cytochalasin B, the Ca++ ionophore A23187, and the local anaesthetic lidocaine but was only slightly inhibited by cytochalasin B alone and was not inhibited by colchicine alone. Inhibition of capping was not due to the inhibition of binding of immune complexes to the B-lymphocytes or to decreased cell viability since these effects were absent. Preformed immune complex-Fc receptor caps were disrupted by A23187, lidocaine, and the combination of colchicine and cytochalasin B, but not by either colchicine or cytochalasin B alone. The effects of the pharmacologic agents were similar for Fc receptors and surface Ig in all cases. These results suggest that ligand bound Fc receptors are affected by cytoskeletal structures and that the ligand-induced redistribution of two distinct B lymphocyte surface receptors (fc receptors and surface Ig) occurs by similar or identical mechanisms.

Effect of colchicine on cell membrane and on biopterin transport in Crithidia fasciculata

Incorporation of 14C-labeled biopterin into Crithidia fasciculata was inhibited by 1 mM colchicine or lumicolchicine. These substance do not penetrate the cell membrane, hence they cannot interact with the subpellicular microtubules. In view of this, interference of colchicine with biopterin transport must occur on the outer surface of the cell membrane. Binding of colchicine to Crithidia was not temperature-dependent and did not exhibit saturation kinetics. These facts exclude a binding as in the case of tubulin, or similar proteins which may be present in the membrane. The results suggest an inhibition reflecting steric hindrance of the biopterin carrier system.

Feedback quenching as a means of effectively increasing the period of biochemical and biological oscillators

A cellular oscillator can be expected to modify the levels of other constituents. In turn, some of these are likely to modulate the behaviour of the oscillating system. Under appropriate conditions this feedback can temporarily quench the periodicity. By such means the frequency of the oscillation can be effectively reduced by a factor of ten or more.

Phylogenetic affinities between eukaryotic cells and a thermophilic mycoplasma

Thermoplasma acidophilum, a thermophilic mycoplasma, has several unusual features suggesting a possible relationship to eukaryotic cells. One feature is a histone-like protein that is associated with the DNA, condensing it into subunits similar to those in eukaryotic chromatin. A second feature is an association of cytoplasmic proteins that resembles eukaryotic actin and myosin. These two components are widely distributed in different groups of eukaryotic cells, but are typically lacking in prokaryotic cells. Furthermore, T. acidophilum lacks cytochromes and respires by enzymes that apparently are not coupled to oxidative phosphorylation. This primitive type of respiration resembles that of microbodies, another feature which is represented in the cytoplasm of all groups of eukaryotic cells. Furthermore, since T. acidophilum lacks a cell wall and appears to have a primitive correlate of endocytosis, it would appear to be mechanically capable of acquiring a symbiotic mitochondrion. Thus, our observations are consistent with the symbiotic hypothesis for the origin of eukaryotic cells. We suggest that an organism similar to T. acidophilum was the host cell for the original symbiosis, becoming the nucleus and cytoplasm of modern eukaryotic cells.

Biosynthetic changes in myosin heavy subunit during myogenesis in culture

In primary culture of chick embryo muscle cells myosin synthesis is detected in mononucleated cells and increased at the onset of fusion with a maximal increment of 20-fold per plate in differentiated myotube. The possibility that the myosin synthetized by duplicating myoblast could be different from that present in post-mitotic myoblast and myotube was evaluated by investigating the regulation of its synthesis and the turnover of the molecule. Following Actinomycin D treatment (0.05 microgram/ml, 8 h), myosin synthesis is partially affected (about 50% inhibition) in pre-fusion myoblast while the synthesis is more sensitive to the drug at the onset of fusion (80% inhibition). With the progress of the differentiative stage the half-life of the molecule increases from 30 h in duplicating myoblasts to 200 h in fibers. The half-life of myosin synthetized by duplicating myoblasts in the explanted embryonic muscle, is 12 h. These data show different features of myosin heavy chains related to specific stages of differentiation and suggest the possibility that modulative changes of the molecule could induce its functional maturation during myogenesis.

Ultrastructure of pericytes in mouse heart

The pericytes of mouse myocardium are extensively branched cells that form an incomplete layer around the endothelium of capillaries and postcapillary venules. The membranes of pericytes and endothelial cells are connected by specialized junctions. Microtubules, intermediate (10-nm) filaments and microfilaments are oriented within circumferentially-arranged cytoplasmic processes of pericytes so as partially to encircle the endothelial cylinder. The intracellular organization of these myocardial pericytes suggests that they are smooth muscle-like cells which may be capable of influencing microvascular dynamics in the heart.

Transport of sodium, potassium, and calcium across rabbit polymorphonuclear leukocyte membranes. Effect of chemotactic factor

The transport properties of the rabbit peritoneal polymorphonuclear leukocyte (PMN) plasma membrane to Na+, K+, and Ca2+ have been characterized. The use of a silicone oil centrifugation technique provided a rapid and reliable method for measuring ion fluxes in these cells. Na+ and K+ movements across PMN membranes were found to be rapid. The value for the unifirectional steady-state fluxes (in meq/liter cell X min) were of the order of 3.0 for Na+ and 7.4 for K+. Ouabian inhibited both K+ influx and Na+ efflux, the latter being also dependent on the presence of extracellular potassium. The rate constant (in min-1) for 45Ca influx was found to be .05 and that for 45Ca efflux .04. The synthetic chemotactic factor formyl-methionyl-leucyl-phenylalanine (FMLP) was found to affect the fluxes of Na+, K+, and Ca2+ at concentrations as low as 10(-10)M. FMLP induced a large and rapid increase in the permeability of the PMN plasma membrane to 22Na. Smaller and delayed enhancements of 42K influx and 22Na efflux were also noted. Some evidence that the latter findings are a consequence of the increased 22Na influx is presented. 45Ca influx and efflux were also stimulated by FMLP. In the presence of 0.25 mM extracellular calcium, FMLP induced an increase in the steady-state level of cell-associated 45Ca. In the presence of .01 mM extracellular calcium, however, a transient decrease in the steady-state level of cell-associated 45Ca was induced by FMLP. The curves relating the concentration of FMLP to its effects on cation fluxes are very similar to those found for its enhancement of migration.

Microfilaments in Naegleria fowleri amoebae

Examination by electron microscopy has revealed 2 types of microfilament in the cytoplasm of 3 strains of axenically grown Naegleria fowleri amoebae. Thin, actin-like microfilaments 5-7 nm in diameter are randomly oriented in the nonmotile amoebae, and are concentrated near the plasma membrane. In the actively motile amoebae these microfilaments aggregate to form colateral bundles in close proximity to the plasma membrane. Thick, myosin-like microfilaments 17-19 nm in diameter also occur in the amoebae cytoplasm. The significance of these 2 kinds of microfilament in amoeboid motion is discussed.

Involvement of vesicle coat material in casein secretion and surface regeneration

The ultrastructure of the apical zone of lactating rat mammary epithelial cells was studied with emphasis on vesicle coat structures. Typical 40-60 nm ID "coated vesicles" were abundant, frequently associated with the internal filamentous plasma membrane coat or in direct continuity with secretory vesicles (SV) or plasma membrane proper. Bristle coats partially or totally covered membranes of secretory vesicles identified by their casein micelle content. This coat survived SV isolation. Exocytotic fusion of SV membranes and release of the casein micelles was observed. Frequently, regularly arranged bristle coat structures were identified in those regions of the plasma membrane that were involved in exocytotic processes. Both coated and uncoated surfaces of the casein-containing vesicles, as well as typical "coated vesicles", were frequently associated with microtubules and/or microfilaments. We suggest that coat materials of vesicles are related or identical to components of the internal coat of the surface membrane and that new plasma membrane and associated internal coat is produced concomitantly by fusion and integration of bristle coat moieties. Postexocytotic association of secreted casein micelles with the cell surface, mediated by finely filamentous extensions, provided a marker for the integrated vesicle membrane. An arrangement of SV with the inner surface of the plasma membrane is described which is characterized by regularly spaced, heabily stained membrane to membrane cross-bridges (pre-exocytotic attachment plaques). Such membrane-interconnecting elements may represent a form of coat structure important to recognition and interaction of membrane surfaces.

Control of chemotaxis in Physarum polycephalum

Plasmodia migrate towards those situations which increase the frequency of their alternations in streaming, and away from those which decrease the frequency. Therefore peristalsis-like waves in Physarum move in the direction opposite from the net movement of the organism. The mechanism is fundamentally related to other known types of chemotaxis.