Life and times of a cellular bleb

Blebs are spherical cellular protrusions that occur in many physiological situations. Two distinct phases make up the life of a bleb, each of which have their own biology and physics: expansion, which lasts ∼30 s, and retraction, which lasts ∼2 min. We investigate these phases using optical microscopy and simple theoretical concepts, seeking information on blebbing itself, and on cytomechanics in general. We show that bleb nucleation depends on pressure, membrane-cortex adhesion energy, and membrane tension, and test this experimentally. Bleb growth occurs through a combination of bulk flow of lipids and delamination from the cell cortex via the formation and propagation of tears. In extreme cases, this can give rise to a traveling wave around the cell periphery, known as “circus movement.” When growth stalls, an actin cortex reforms under the bleb membrane, and retraction starts, driven by myosin-II. Using flicker spectroscopy, we find that retracting blebs are fivefold more rigid than expanding blebs, an increase entirely explained by the properties of the newly formed cortical actin mesh. Finally, using artificially nucleated blebs as pressure sensors, we show that cells rounded up in mitosis possess a substantial intracellular pressure.     

Differences in cortical actin structure and dynamics document that different types of blebs are formed by distinct mechanisms

Bleb formation has been studied by specifically targeting major factors controlling this process, such as microtubule disassembly, local actin depolymerization, and increased pressure. At least two different types of blebs (types 1 and 2) formed by different mechanisms and possibly a third type (type 3) can be documented at the front of living polarized cells expressing green fluorescent protein-actin and/or in fixed and stained cells. Type 1 blebs (membrane/cortex dissociation blebs) formed by dissociation of the plasma membrane from cortical actin develop cytoplasmic actin layers associated with restriction rings. They can be induced by the microtubule-disassembling agent colchicine. Type 2 blebs (cortical actin disassembly blebs) form after disassembly of the cortical actin layer in the presence of latrunculin A. Restriction rings without a cytoplasmic actin layer occur in a transition zone between the intact cortical actin layer of the cell body and the compromised actin layer of the bleb. Evidence for a third type of bleb (type 3), showing an intact cortical actin layer but no cytoplasmic actin layer and no recognizable relationship between the actin cytoskeleton and the restriction ring, has been obtained by passive cell deformation in micropipettes, which increases pressure. Repolymerization of the cortical actin layer does not necessarily result in bleb retraction. Once formed, restriction rings do not narrow, suggesting that they result from isometric contraction. A simplified classification scheme has been developed to relate the type of bleb to specific signals or cell functions. Its application shows that spontaneously blebbing cells form almost exclusively type 1 blebs.     

Blebbing of Dictyostelium cells in response to chemoattractant

Stimulation of Dictyostelium cells with a high uniform concentration of the chemoattractant cyclic-AMP induces a series of morphological changes, including cell rounding and subsequent extension of pseudopodia in random directions. Here we report that cyclic-AMP also elicits blebs and analyse their mechanism of formation. The surface area and volume of cells remain constant during blebbing indicating that blebs form by the redistribution of cytoplasm and plasma membrane rather than the exocytosis of internal membrane coupled to a swelling of the cell. Blebbing occurs immediately after a rapid rise and fall in submembraneous F-actin, but the blebs themselves contain little F-actin as they expand. A mutant with a partially inactivated Arp2/3 complex has a greatly reduced rise in F-actin content, yet shows a large increase in blebbing. This suggests that bleb formation is not enhanced by the preceding actin dynamics, but is actually inhibited by them. In contrast, cells that lack myosin-II completely fail to bleb. We conclude that bleb expansion is likely to be driven by hydrostatic pressure produced by cortical contraction involving myosin-II. As blebs are induced by chemoattractant, we speculate that hydrostatic pressure is one of the forces driving pseudopod extension during movement up a gradient of cyclic-AMP.     

Intracellular Pressure Dynamics in Blebbing Cells

Blebs are pressure-driven protrusions that play an important role in cell migration, particularly in three-dimensional environments. A bleb is initiated when the cytoskeleton detaches from the cell membrane, resulting in the pressure-driven flow of cytosol toward the area of detachment and local expansion of the cell membrane. Recent experiments involving blebbing cells have led to conflicting hypotheses regarding the timescale of intracellular pressure propagation. The interpretation of one set of experiments supports a poroelastic model of the cytoplasm that leads to slow pressure equilibration when compared to the timescale of bleb expansion. A different study concludes that pressure equilibrates faster than the timescale of bleb expansion. To address this discrepancy, a dynamic computational model of the cell was developed that includes mechanics of and the interactions among the cytoplasm, the actin cortex, the cell membrane, and the cytoskeleton. The model results quantify the relationship among cytoplasmic rheology, pressure, and bleb expansion dynamics, and provide a more detailed picture of intracellular pressure dynamics. This study shows the elastic response of the cytoplasm relieves pressure and limits bleb size, and that both permeability and elasticity of the cytoplasm determine bleb expansion time. Our model with a poroelastic cytoplasm shows that pressure disturbances from bleb initiation propagate faster than the timescale of bleb expansion and that pressure equilibrates slower than the timescale of bleb expansion. The multiple timescales in intracellular pressure dynamics explain the apparent discrepancy in the interpretation of experimental results.     

Cellular blebs: pressure-driven,axisymmetric, membrane protrusions

Blebs are cellular protrusions that are used by cells for multiple purposes including locomotion. A mechanical model for the problem of pressure-driven blebs based on force and moment balances of an axisymmetric shell model is proposed. The formation of a bleb is initiated by weakening the shell over a small region, and the deformation of the cellular membrane from the cortex is obtained during inflation. However, simply weakening the shell leads to an area increase of more than 4 %, which is physically unrealistic. Thus, the model is extended to include a reconfiguration process that allows large blebs to form with small increases in area. It is observed that both geometric and biomechanical constraints are important in this process. In particular, it is shown that although blebs are driven by a pressure difference across the cellular membrane, it is not the limiting factor in determining bleb size.     

Ultrastructure of blebbing and phagocytosis of blebs by hyperplastic thyroid epithelial cells in vivo

In addition to pseudopods, somewhat pleomorphic blebs were consistently found protruding from the apical surfaces of hyperplastic rat thyroid epithelial cells into the follicular lumens in vivo. Many blebs were knobby, roughly hemispherical protrusions, with a diameter of 2-3 mum. Such blebs had a densely packed microfilamentous core and contained numerous apparent ribosomes. They were morphologically similar to blebs that have been observed in a variety of cultured cells. Other blebs were larger, more elongate, and less knobby, but had a similar ultrastructural organization. Blebs of all sizes appeared to be phagocytosed on some occasions by nearby epithelial cells. The phagocytic process involved partial engulfment of the bleb by a typical epithelial pseudopod, followed by an apparent pinching-off process, presumably resulting in the separation of the bleb from its cells or origin. The pinching-off process was associated with a band of approx. 6-nm diameter microfilaments that developed within the pseudopod cytoplasm surrounding the base of the bleb and is postulated to function as a contractile ring. The finding of blebbing is an intact tissue in vivo indicates that this phenomenon is not restricted to cultured cells, and thus tends to extend the significance of in vitro observations of the process. In relation to their occurrence in the hyperplastic thyroid gland in vivo, possible interconversions are considered between different types of blebs, and between blebs and pseudopods.     

Electric Field Exposure Triggers and Guides Formation of Pseudopod-Like Blebs in U937 Monocytes

We describe a new phenomenon of anodotropic pseudopod-like blebbing in U937 cells stimulated by nanosecond pulsed electric field (nsPEF). In contrast to “regular,” round-shaped blebs, which are often seen in response to cell damage, pseudopod-like blebs (PLBs) formed as longitudinal membrane protrusions toward anode. PLB length could exceed the cell diameter in 2 min of exposure to 60-ns, 10-kV/cm pulses delivered at 10–20 Hz. Both PLBs and round-shaped nsPEF-induced blebs could be efficiently inhibited by partial isosmotic replacement of bath NaCl for a larger solute (sucrose), thereby pointing to the colloid-osmotic water uptake as the principal driving force for bleb formation. In contrast to round-shaped blebs, PLBs retracted within several minutes after exposure. Cells treated with 1 nM of the actin polymerization blocker cytochalasin D were unable to form PLBs and instead produced stationary, spherical blebs with no elongation or retraction capacity. Live cell fluorescent actin tagging showed that during elongation actin promptly entered the PLB interior, forming bleb cortex and scaffold, which was not seen in stationary blebs. Overall, PLB formation was governed by both passive (physicochemical) effects of membrane permeabilization and active cytoskeleton assembly in the living cell. To a certain extent, PLB mimics the membrane extension in the process of cell migration and can be employed as a nonchemical model for studies of cytomechanics, membrane–cytoskeleton interaction and cell motility.     

Dia-Interacting Protein (DIP) Imposes Migratory Plasticity in mDia2-Dependent Tumor Cells in Three-Dimensional Matrices

Tumor cells rely upon membrane pliancy to escape primary lesions and invade secondary metastatic sites. This process relies upon localized assembly and disassembly cycles of F-actin that support and underlie the plasma membrane. Dynamic actin generates both spear-like and bleb structures respectively characterizing mesenchymal and amoeboid motility programs utilized by metastatic cells in three-dimensional matrices. The molecular mechanism and physiological trigger(s) driving membrane plasticity are poorly understood. mDia formins are F-actin assembly factors directing membrane pliancy in motile cells. mDia2 is functionally coupled with its binding partner DIP, regulating cortical actin and inducing membrane blebbing in amoeboid cells. Here we show that mDia2 and DIP co-tether to nascent blebs and this linkage is required for bleb formation. DIP controls mesenchymal/amoeboid cell interconvertability, while CXCL12 induces assembly of mDia2:DIP complexes to bleb cortices in 3D matrices. These results demonstrate how DIP-directed mDia2-dependent F-actin dynamics regulate morphological plasticity in motile cancer cells.     

Immunocytochemical Identification and Localization of Active and Inactive alpha-Amylase and Pullulanase in Cells of Clostridium thermosulfurogenes EM1

Clostridium thermosulfurogenes EM1 formed blebs, i.e., protrusions still in contact with the cytoplasmic membrane, that originated from the cytoplasmic membrane during growth in batch culture and continuous culture. They could be observed squeezed between the cell wall and cytoplasmic membrane in cells with seemingly intact wall layers (surface layer and peptidoglycan layer) as well as in cells with wall layers in different states of degradation caused by phosphate limitation or high dilution rates. Blebs were found to turn into membrane vesicles by constriction in cases when the cell wall was heavily degraded. Bleb and vesicle formation was also observed in the absence of substrates that induce alpha-amylase and pullulanase synthesis. No correlations existed between bleb formation and the presence of active enzyme. Similar blebs could also be observed in a number of other gram-positive bacteria not producing these enzymes, but they were not observed in gram-negative bacteria. For immunoelectron-microscopic localization of alpha-amylase and pullulanase in C. thermosulfurogenes EM1, two different antisera were applied. One was raised against the enzymes isolated from the culture fluid; the other was produced against a peptide synthesized, as a defined epitope, in analogy to the N-terminal amino acid sequence (21 amino acids) of the native extracellular alpha-amylase. By using these antisera, alpha-amylase and pullulanase were localized at the cell periphery in samples taken from continuous culture or batch culture. In samples prepared for electron microscopy by freeze substitution followed by ultrathin sectioning, blebs could be seen, and the immunolabel pinpointing alpha-amylase enzyme particles was seen not only randomly distributed in the cell periphery, but also lining the surface of the cytoplasmic membrane and the blebs. Cells exhibiting high or virtually no enzyme activity were labeled similarly with both antisera. This finding strongly suggests that alpha-amylase and pullulanase may occur in both active and inactive forms, depending on growth conditions.     

Phosphorylation of actin-binding protein (ABP-280; filamin) by tyrosine kinase p56lck modulates actin filament cross-linking

Actin-binding protein (ABP-280; filamin) is a phosphoprotein present in the periphery of the cytoplasm where it can cross-link actin filaments, associate with lipid membranes, and bind to membrane surface receptors. Given its function and localization in the cell, we decided to investigate the possibility of whether it serves as substrate for p56lck, a lymphocyte-specific member of the src family of protein tyrosine kinases associated with cell surface glycoproteins. The interaction of p56lck with membrane glycoproteins is important for cell development and functional activation. Here, we show that purified p56lck interacts and catalyzes in vitro kinase reactions. Tyrosine phosphorylation by p56lck is restricted to a single peptide of labeled ABP-280 shown by protease digest. The addition of phorbol ester to cells results in the inhibition of phosphorylation of ABP-280 by p56lck. These results show a decrease in phosphorylation suggesting conformationally induced regulation. Dynamic light scattering confirmed increased actin filament cross-linking due to phosphorylation of ABP-280 by p56lck.     

Blebs lead the way: how to migrate without lamellipodia

Blebs are spherical membrane protrusions that are produced by contractions of the actomyosin cortex. Blebs are often considered to be a hallmark of apoptosis; however, blebs are also frequently observed during cytokinesis and during migration in three-dimensional cultures and in vivo. For tumour cells and a number of embryonic cells, blebbing migration seems to be a common alternative to the more extensively studied lamellipodium-based motility. We argue that blebs should be promoted to a more prominent place in the world of cellular protrusions.     

Identification of the cardiac ryanodine receptor channel in membrane blebs of sarcoplasmic reticulum

Blebs of the sarcoplasmic reticulum (SR) membrane of heart muscle cells were generated after saponin perforation of the plasma membrane followed by complete hypercontraction of the cell. Although characteristic proteins of the plasma membrane, namely the beta1-adrenoreceptor and Galphai, were stained by monoclonal antibodies in the hypercontracted cells, these proteins could not be detected in the adjacent blebs. Monoclonal antibodies to the cardiac ryanodine receptor (RyR2), calsequestrin and SERCA2 bound at different amounts to surface components of the blebs and to components of the hypercontracted cells. From the immunofluorescence signals we conclude that the blebs are mainly constituted of corbular and junctional SR membrane, and only to a lesser extent of network SR membrane. Deconvolution microscopy revealed that the membrane location of RyR2, calsequestrin and SERCA2 in the bleb is comparable to native SR membrane. At the bleb membrane giga-ohm seals could be obtained and patches could be excised in a way that single-channel currents could be measured, although these are not completely identified.     

Peripheral hyaline blebs (podosomes) of macrophages

The plasmalemma and hyaline ectoplasm together constitute the sensory and motor organ of macrophages. The purpose of this study was to isolate this cell fraction in order to analyze it biochemically and functionally. Brief sonification of warmed rabbit lung macrophages caused release of heterodisperse hyaline blebs and filopodia, which were easily collected by differential centrifugation. Viewed in the electron microscope, these structures consisted of membrane-bounded sacs principally containing actin filaments. Some contained secondary lysosomes. They were enriched threefold over whole cell homogenates in specific adenylate cyclase activity and in trichloroacetic-acid-precipitable (125)I when derived from cells labeled with 125(I) by means of a lactoperoxidase-catalyzed reaction. These markers were found to have identical isopycnic densitites when macrophage homogenates were subjected to sedimentation in a focusing sucrose density gradient system, and these markers had densities distinct from those of other cytoplasmic organelles. These markers were therefore assumed to be associated with macrophage plasma membranes. The specific β- glucuronidase activity of the bleb fraction was similar to that of homogenates, but the blebs had considerably lower specific succinic dehydrogenase activity and RNA content, and DNA was undetectable. Electrophoresis of blebs solubilized in sodium dodecyl sulfate on polyacrylamide gels revealed polypeptides co-migrating with macrophage actin-binding protein, myosin, and actin; blebs also had EDTA-activated adenosine triphosphatase activity characteristic of myosin. The concentrations of actin-binding protein and myosin were higher in blebs than in cells or cytoplasmic extracts, whereas actin concentrations were similar (relative to extracts) or only slightly greater (than in cells). Blebs and intact cells had high lactate dehydrogenase activities in the presence but not the absence of Triton X-100. Blebs and cells oxidased 1-[(14)C]glucose, and the rate of glucose oxidation was increased substantially in the presence of latex beads. We conclude that intact sacs of plasmalemma encasing contractile proteins and cytoplasmic enzymes can be isolated from macrophages. They are enriched in myosin and actin-binding protein, indicating that the contractile apparatus is regulated in the cell periphery. These structures have the capacity to respond to environmental signals. We suggest the name "podosomes" for them because of their resemblance to macrophage pseudopodia. We propose that podosome formation results from rapid dissolution of the cortical gel when the membrane is in an actively extended configuration.     

Dynamic modes of the cortical actomyosin gel during cell locomotion and division

Tight regulation of the contractility of the actomyosin cortex is essential for proper cell locomotion and division. Enhanced contractility leads, for example, to aberrations in the positioning of the mitotic spindle or to anomalous migration modes that allow tumor cells to escape anti-dissemination treatments. Spherical membrane protrusions called blebs occasionally appear during cell migration, cell division or apoptosis. We have shown that the cortex ruptures at sites where actomyosin cortical contractility is increased, leading to the formation of blebs. Here, we propose that bleb formation, which releases cortical tension, can be used as a reporter of cortical contractility. We go on to analyze the implications of spontaneous cortical contractile behaviors on cell locomotion and division and we particularly emphasize that variations in actomyosin contractility can account for a variety of migration modes.     

Cytoskeletal Protein ABP-280 Directs the Intracellular Trafficking of Furin and Modulates Proprotein Processing in the Endocytic Pathway

Furin catalyzes the proteolytic maturation of many proproteins within the trans-Golgi network (TGN)/endosomal system. Furin's cytosolic domain (cd) directs both the compartmentalization to and transit between its manifold processing compartments (i.e., TGN/biosynthetic pathway, cell surface, and endosomes). Here we report the identification of the first furin cd sorting protein, ABP-280 (nonmuscle filamin), an actin gelation protein. The furin cd was used as bait in a yeast two-hybrid screen to identify ABP-280 as a furin-binding protein. Binding analyses in vitro and coimmunoprecipitation studies in vivo showed that furin and ABP-280 interact directly and that ABP-280 tethers furin molecules to the cell surface. Quantitative analysis of both ABP-280-deficient and genetically replete cells showed that ABP-280 modulates the rate of internalization of furin but not of the transferrin receptor, a cycling receptor. However, although ABP-280 directs the rate of furin internalization, the efficiency of sorting of the endoprotease from the cell surface to early endosomes is independent of expression of ABP-280. By contrast, efficient sorting of furin from early endosomes to the TGN requires expression of ABP-280. In addition, ABP-280 is also required for the correct localization of late endosomes (dextran bead uptake) and lysosomes (LAMP-1 staining), demonstrating a pleiotropic role for this actin binding protein in the organization of cellular compartments and directing protein traffic. Finally, and consistent with the trafficking studies on furin, we showed that ABP-280 modulates the processing of furin substrates in the endocytic but not the biosynthetic pathways. The novel roles of ABP-280 and the cytoskeleton in the sorting of furin in the TGN/ endosomal system and the formation of proprotein processing compartments are discussed.     

A reduced 1D stochastic model of bleb-driven cell migration

Blebs are pressure-driven protrusions that have been observed in cells undergoing apoptosis, cytokinesis, or migration, including tumor cells that use blebs to escape their organs of origin. Here, we present a minimal 1D model of bleb-driven cell motion that combines a simple mechanical model with turnover kinetics of the actin cortex and adhesions between the membrane and the cortex. The deterministic version of this model is used to study the properties of individual blebbing events. We further introduce stochastic turnover of the adhesions, which allows for spontaneous initiation of repeated blebbing events, thus leading to sustained cell travel. We explore how the main parameters of the system control the properties of the blebbing events and the speed of cell travel. Finally, we derive a further simplification by deriving a Langevin approximation to this stochastic model.     

Microtubule-Mediated Inositol Lipid Signaling Plays Critical Roles in Regulation of Blebbing

Cells migrate by extending pseudopods such as lamellipodia and blebs. Although the signals leading to lamellipodia extension have been extensively investigated, those for bleb extension remain unclear. Here, we investigated signals for blebbing in Dictyostelium cells using a newly developed assay to induce blebbing. When cells were cut into two pieces with a microneedle, the anucleate fragments vigorously extended blebs. This assay enabled us to induce blebbing reproducibly, and analyses of knockout mutants and specific inhibitors identified candidate molecules that regulate blebbing. Blebs were also induced in anucleate fragments of leukocytes, indicating that this assay is generally applicable to animal cells. After cutting, microtubules in the anucleate fragments promptly depolymerized, followed by the extension of blebs. Furthermore, when intact cells were treated with a microtubule inhibitor, they frequently extended blebs. The depolymerization of microtubules induced the delocalization of inositol lipid phosphatidylinositol 3,4,5-trisphosphate from the cell membrane. PI3 kinase-null cells frequently extended blebs, whereas PTEN-null cells extended fewer blebs. From these observations, we propose a model in which microtubules play a critical role in bleb regulation via inositol lipid metabolism.     

Phosphorylation of filamin (ABP-280) regulates the binding to the lipid membrane, integrin, and actin

Actin-binding protein (ABP-280; filamin) is a phosphoprotein present in the periphery of the cytoplasm, where it can cross-link actin filaments, associate with lipid membranes, and bind to membrane surface receptors. Given its function and localization in the cell, the hypothesis that it serves as a substrate for p56lck, a lymphocyte-specific member of the src family of protein tyrosine kinases associated with cell surface glycoproteins is considered. The results suggest conformationally-induced regulation of filamin (ABP-280).     

Immunocytochemical Identification and Localization of Active and Inactive α-Amylase and Pullulanase in Cells of Clostridium thermosulfurogenes EM1

Clostridium thermosulfurogenes EM1 formed blebs, i.e., protrusions still in contact with the cytoplasmic membrane, that originated from the cytoplasmic membrane during growth in batch culture and continuous culture. They could be observed squeezed between the cell wall and cytoplasmic membrane in cells with seemingly intact wall layers (surface layer and peptidoglycan layer) as well as in cells with wall layers in different states of degradation caused by phosphate limitation or high dilution rates. Blebs were found to turn into membrane vesicles by constriction in cases when the cell wall was heavily degraded. Bleb and vesicle formation was also observed in the absence of substrates that induce α-amylase and pullulanase synthesis. No correlations existed between bleb formation and the presence of active enzyme. Similar blebs could also be observed in a number of other gram-positive bacteria not producing these enzymes, but they were not observed in gram-negative bacteria. For immunoelectron-microscopic localization of α-amylase and pullulanase in C. thermosulfurogenes EM1, two different antisera were applied. One was raised against the enzymes isolated from the culture fluid; the other was produced against a peptide synthesized, as a defined epitope, in analogy to the N-terminal amino acid sequence (21 amino acids) of the native extracellular α-amylase. By using these antisera, α-amylase and pullulanase were localized at the cell periphery in samples taken from continuous culture or batch culture. In samples prepared for electron microscopy by freeze substitution followed by ultrathin sectioning, blebs could be seen, and the immunolabel pinpointing α-amylase enzyme particles was seen not only randomly distributed in the cell periphery, but also lining the surface of the cytoplasmic membrane and the blebs. Cells exhibiting high or virtually no enzyme activity were labeled similarly with both antisera. This finding strongly suggests that α-amylase and pullulanase may occur in both active and inactive forms, depending on growth conditions.     

Embryonic corneal epithelial interaction with exogenous laminin and basal lamina is F-actin dependent

Between the third and sixth day of embryonic development, the avian corneal epithelium produces both a basal lamina and the primary corneal stroma composed of 20 orthogonally arranged layers of collagen fibrils. If the epithelium is removed by enzyme treatment from the basal lamina and stroma, the basal cell surface extends cell processes (blebs) which contain disorganized actin filaments and the epithelium decreases production of collagen. When placed on extracellular matrix or on Millipore filters in media containing soluble matrix molecules, the epithelium retracts the blebs, forms an organized basal actin cortical mat, and doubles its production of collagen. In the current investigation, we provide evidence for the hypothesis that organization of the RER by the actin cytoskeleton mediates this stimulation of collagen production. Laminin-treated epithelia and epithelia isolated with the basal lamina intact were treated with an actin-disrupting drug, cytochalasin D. Actin aggregates occur throughout the epithelium, the RER becomes disorganized, and the increase in collagen production expected to result from addition of laminin does not take place. Morphometrical analysis of the distribution of RER in the basal compartment of control and cytochalasin-treated epithelia shows that the decrease in collagen production is accompanied by displacement of the RER from the basal area of the cells, suggesting that attachment of RER to the intact actin cytoskeleton is essential to maintenance of normal RER organization and function. We also found that laminin-mediated bleb retraction requires intact actin microfilaments, whereas bleb extension does not, and that nocodazole does not inhibit bleb extension or retraction.