Intermediate filaments in monkey kidney TC7 cells: focal centers and interrelationship with other cytoskeletal systems

Dissection of keratin dynamics: different contributions of the actin and microtubule systems

It has only recently been recognized that intermediate filaments (IFs) and their assembly intermediates are highly motile cytoskeletal components with cell-type- and isotype-specific characteristics. To elucidate the cell-type-independent contribution of actin filaments and microtubules to these motile properties, fluorescent epithelial IF keratin polypeptides were introduced into non-epithelial, adrenal cortex-derived SW13 cells. Time-lapse fluorescence microscopy of stably transfected SW13 cell lines synthesizing fluorescent human keratin 8 and 18 chimeras HK8-CFP and HK18-YFP revealed extended filament networks that are entirely composed of transgene products and exhibit the same dynamic features as keratin systems in epithelial cells. Detailed analyses identified two distinct types of keratin motility: (I) Slow (approximately 0.23 microm/min), inward-directed, continuous transport of keratin filament precursor particles from the plasma membrane towards the cell interior, which is most pronounced in lamellipodia. (II) Fast (approximately 17 microm/min), bidirectional and intermittent transport of keratin particles in axonal-type cell processes. Disruption of actin filaments inhibited type I motility while type II motility remained. Conversely, microtubule disruption inhibited transport mode II while mode I continued. Combining the two treatments resulted in a complete block of keratin motility. We therefore conclude that keratin motility relies both on intact actin filaments and microtubules and is not dependent on epithelium-specific cellular factors.

Identification of novel principles of keratin filament network turnover in living cells

It is generally assumed that turnover of the keratin filament system occurs by exchange of subunits along its entire length throughout the cytoplasm. We now present evidence that a circumscribed submembranous compartment is actually the main site for network replenishment. This conclusion is based on the following observations in living cells synthesizing fluorescent keratin polypeptides: 1) Small keratin granules originate in close proximity to the plasma membrane and move toward the cell center in a continuous motion while elongating into flexible rod-like fragments that fuse with each other and integrate into the peripheral KF network. 2) Recurrence of fluorescence after photobleaching is first seen in the cell periphery where keratin filaments are born that translocate subsequently as part of the network toward the cell center. 3) Partial keratin network reformation after orthovanadate-induced disruption is restricted to a distinct peripheral zone in which either keratin granules or keratin filaments are transiently formed. These findings extend earlier investigations of mitotic cells in which de novo keratin network formation was shown to originate from the cell cortex. Taken together, our results demonstrate that the keratin filament system is not homogeneous but is organized into temporally and spatially distinct subdomains. Furthermore, the cortical localization of the regulatory cues for keratin filament turnover provides an ideal way to adjust the epithelial cytoskeleton to dynamic cellular processes.

Epidermolysis bullosa simplex-type mutations alter the dynamics of the keratin cytoskeleton and reveal a contribution of actin to the transport of keratin subunits

Dominant keratin mutations cause epidermolysis bullosa simplex by transforming keratin (K) filaments into aggregates. As a first step toward understanding the properties of mutant keratins in vivo, we stably transfected epithelial cells with an enhanced yellow fluorescent protein-tagged K14R125C mutant. K14R125C became localized as aggregates in the cell periphery and incorporated into perinuclear keratin filaments. Unexpectedly, keratin aggregates were in dynamic equilibrium with soluble subunits at a half-life time of <15 min, whereas filaments were extremely static. Therefore, this dominant-negative mutation acts by altering cytoskeletal dynamics and solubility. Unlike previously postulated, the dominance of mutations is limited and strictly depends on the ratio of mutant to wild-type protein. In support, K14R125C-specific RNA interference experiments resulted in a rapid disintegration of aggregates and restored normal filaments. Most importantly, live cell inhibitor studies revealed that the granules are transported from the cell periphery inwards in an actin-, but not microtubule-based manner. The peripheral granule zone may define a region in which keratin precursors are incorporated into existing filaments. Collectively, our data have uncovered the transient nature of keratin aggregates in cells and offer a rationale for the treatment of epidermolysis bullosa simplex by using short interfering RNAs.

A new look at the cellular scaffold by embedment-free electron microscopy method

The basic scaffold of most cells is afforded by the cytoskeleton (comprising microfilaments, intermediate filaments and the microtubules). The conventional methods of electron microscopy fail to visualize filamentous cell structure. They can show only these filaments lying at the section surface. Heavy metal staining (I), and the optical properties of the resins used for embedding are similar to those of proteins hence most proteinaceous structures remain unresolved and the cytoplasm seems to be quite homogenous (II). Aldehyde fixation could cross-link proteins and lead to the emergence of artificial structures (III). These limitations may be overcome by the use of the embedment-free electron microscopy (EF-EM). This technique present cellular scaffold as a purified, isolated, three-dimensional network with various thickness of filaments. Our study on the dynamic aspect of cellular scaffold indicate that the thickness and arrangement of filaments depend on cell type and both physiological or pathological environments. Thank also to the adaptation of immunocytochemistry to EF-EM it was possible to understand the nuclear matrix and cytomatrix structure in relation to function. Thus, combination these methods revealed findings suggesting the nuclear homing of proapoptotic proteins and their association with intermediate filaments.

De novo formation of cytokeratin filament networks originates from the cell cortex in A-431 cells

Of the three major cytoskeletal filament systems, the intermediate filaments are the least understood. Since they differ fundamentally from the actin- and microtubule-based networks by their lack of polarity, it has remained a mystery how and where these principally endless filaments are formed. Using a recently established epithelial cell system in which fluorescently labeled intermediate filaments of the cytokeratin type can be monitored in living cells, we address these issues. By multidimensional time-lapse fluorescence microscopy, we examine de novo intermediate filament network formation from non-filamentous material at the end of mitosis and show that it mirrors disassembly. It is demonstrated that filament formation is initiated from the cell cortex without focal preference after cytokinesis. Furthermore, it is shown that this process is dependent on energy, on the integrity of the actin filament network and the microtubule system, and that it can be inhibited by the tyrosine phosphatase inhibitor pervanadate. Based on these observations, a two-step working model is proposed involving (1) interactions within the planar cortical layer acting as an organizing center forming a two-dimensional network and (2) subsequent radial dynamics facilitating the formation of a mature three-dimensional network.

Insights into the dynamic properties of keratin intermediate filaments in living epithelial cells

The properties of keratin intermediate filaments (IFs) have been studied after transfection with green fluorescent protein (GFP)-tagged K18 and/or K8 (type I/II IF proteins). GFP-K8 and -K18 become incorporated into tonofibrils, which are comprised of bundles of keratin IFs. These tonofibrils exhibit a remarkably wide range of motile and dynamic activities. Fluorescence recovery after photobleaching (FRAP) analyses show that they recover their fluorescence slowly with a recovery t(1/2) of approximately 100 min. The movements of bleach zones during recovery show that closely spaced tonofibrils (<1 microm apart) often move at different rates and in different directions. Individual tonofibrils frequently change their shapes, and in some cases these changes appear as propagated waveforms along their long axes. In addition, short fibrils, termed keratin squiggles, are seen at the cell periphery where they move mainly towards the cell center. The motile properties of keratin IFs are also compared with those of type III IFs (vimentin) in PtK2 cells. Intriguingly, the dynamic properties of keratin tonofibrils and squiggles are dramatically different from those of vimentin fibrils and squiggles within the same cytoplasmic regions. This suggests that there are different factors regulating the dynamic properties of different types of IFs within the same cytoplasmic regions.

Changes in the ultrastructure of cytoskeleton and nuclear matrix during HaCaT keratinocyte differentiation

The cellular scaffold that comprises nuclear matrix and cytoskeleton provides mechanical support for the cell and plays a crucial role in motility, cellular signaling, regulation of gene transcription and DNA replication. In this study we examined the structure of cytoskeleton and nuclear matrix in the keratinocyte cell line HaCaT using a recently developed technique, embedment-free electron microscopy. With this method the three-dimensional structure of cellular scaffold is visualized in the cells extracted from soluble proteins and the chromatin. In actively proliferating cells the cytoskeleton appeared to consist of a continuous meshwork of 10--15 nm filaments with a smaller amount of thin (5 nm) and ultrathin (1--2 nm) filaments. In contrast to what could be expected from earlier immunofluorescence and electron microscopy studies, the cytoskeleton in HaCaT keratinocytes did not consist of superposed autonomous networks of different filaments but was a highly integrated, continuous structure filling whole cytoplasmic territory. Moreover, cytoskeletons of adjacent cells were in a direct physical contact. Nuclear matrix consisted of globular ribonucleoprotein aggregates attached to the meshwork of 20--40 nm filaments. Nuclear envelope was firmly fastened to the cytoskeleton. In keratinocytes induced to differentiation by calcium switch both the cytoskeleton and nuclear matrix were drastically rearranged and comprised a monomorphic, dense and regular meshwork of 10--15 nm filaments. Our data underscore the fact that in HaCaT keratinocyte monolayer in vitro, and probably also in the epidermis in vivo, the nuclear matrices and the cytoskeletons of adjacent cells seemed to form a continuous, highly ordered structure which is rapidly rearranged during cell differentiation. This feature may be crucial for the understanding of how the signal initiated by, e.g. mechanical forces generated through the cell--cell and cell--matrix interaction is transmitted to the nucleus producing ultimately changes in the pattern of gene expression.

Detection of cytokeratin dynamics by time-lapse fluorescence microscopy in living cells

To monitor the desmosome-anchored cytokeratin network in living cells fusion protein HK13-EGFP consisting of human cytokeratin 13 and the enhanced green fluorescent protein was stably expressed in vulvar carcinoma-derived A-431 cells. It is shown for A-431 subclone AK13-1 that HK13-EGFP emits strong fluorescence in fixed and living cells, being part of an extended cytoplasmic intermediate filament network that is indistinguishable from that of parent A-431 cells. Biochemical, immunological and ultrastructural analyses demonstrate that HK13-EGFP behaves identically to the endogenous cytokeratin 13 and is therefore a reliable in vivo tag for this polypeptide and the structures formed by it. Time-lapse fluorescence microscopy reveals that the cytokeratin 13-containing network is in constant motion, resulting in continuous restructuring occurring in single and migratory cells, as well as in desmosome-anchored cells. Two major types of movement are distinguished: (i) oscillations of mostly long filaments, and (ii) an inward-directed flow of fluorescence originating as diffuse material at the cell periphery and moving in the form of dots and thin filaments toward the deeper cytoplasm where it coalesces with other filaments and filament bundles. Both movements are energy dependent and can be inhibited by nocodazole, but not by cytochalasin D. Finally, disassembly and reformation of cytokeratin filament networks are documented in dividing cells revealing distinct and rapidly occurring stages of cytokeratin organisation and distribution.

Nonuniform microtubular polarity established by CHO1/MKLP1 motor protein is necessary for process formation of podocytes

Podocytes are unique cells that are decisively involved in glomerular filtration. They are equipped with a complex process system consisting of major processes and foot processes whose function is insufficiently understood (Mundel, P., and W. Kriz. 1995. Anat. Embryol. 192:385-397). The major processes of podocytes contain a microtubular cytoskeleton. Taking advantage of a recently established cell culture system for podocytes with preserved ability to form processes (Mundel, P., J. Reiser, A. Zúñiga Mejía Borja, H. Pavenstädt, G.R. Davidson, W. Kriz, and R. Zeller. 1997b. Exp. Cell Res. 36:248-258), we studied the functional significance of the microtubular system in major processes. The following data were obtained: (a) Microtubules (MTs) in podocytes show a nonuniform polarity as revealed by hook-decoration. (b) CHO1/ MKLP1, a kinesin-like motor protein, is associated with MTs in podocytes. (c) Treatment of differentiating podocytes with CHO1/MKLP1 antisense oligonucleotides abolished the formation of processes and the nonuniform polarity of MTs. (d) During the recovery from taxol treatment, taxol-stabilized (nocodazole- resistant) MT fragments were distributed in the cell periphery along newly assembled nocodazole-sensitive MTs. A similar distribution pattern of CHO1/MKLP1 was found under these circumstances, indicating its association with MTs. (e) In the recovery phase after complete depolymerization, MTs reassembled exclusively at centrosomes. Taken together, these findings lead to the conclusion that the nonuniform MT polarity in podocytes established by CHO1/MKLP1 is necessary for process formation.

In vitro efficacy of anti-glial fibrillary acidic protein monoclonal antibodies against human malignant glioma cell lines

Our studies have confirmed the presence of large concentrations of various intermediate filament proteins (IFPs) in glioma tissue compared to normal brain. This avenue of research was extended to assess the anti-proliferative activity of anti-intermediate filament protein monoclonal antibodies (anti-IFP mAbs) against human glioma cells. In this study, anti-proliferative activity of glial fibrillary acidic protein monoclonal antibodies (anti-GFAP mAbs) has been tested in vitro, using glioma cell lines prepared and established from freshly resected brain tumors. One anaplastic astrocytoma (AA), two glioblastoma multiforme (GB1 and GB2) cell lines and three anti-GFAP mAbs (B12C4, B12B4 and B6C6, all IgG1, kappa) were used. Immunofluorescence study indicated the ability of anti-GFAP mAbs to recognize the cell surface of glioma cells and the inhibition study showed that mAb B12B4 inhibited the proliferation of GB1 (96%), GB2 (85%) and AA (93%) at a concentration of 3.2 x 10(-10) M. mAb B12C4 inhibited the proliferation of GB1 (95%), GB2 (86%) and AA (94%) at a concentration of 3.26 x 10(-10) M and mAb B6C6 inhibited the proliferation of GB1 (75%), GB2 (75%) and AA (91%) at a concentration of 2.074 x 10(-10) M. Thymidine release assay demonstrated the cytolytic activities of anti-GFAP mAbs towards these glioma cell lines, and this observation was confirmed by dye exclusion, which indicated the lysis of glioma cells after anti-GFAP mAbs treatment. Anti-GFAP mAbs had little effect (< or = 20%) on normal human lymphocyte, liver and intestine cell lines. These results look promising for radioimaging and immunotherapy of human gliomas.

Integration of intermediate filaments into cellular organelles

The intermediate filaments represent core components of the cytoskeleton and are known to interact with several membranous organelles. Classic examples of this are the attachment of keratin filaments to the desmosomes and the association of the lamin filament meshwork with the inner nuclear membrane. At this point, the molecular mechanisms by which the filaments link to membranes are not clearly understood. However, since a substantial body of information has been amassed, the time is now ripe for comparing notes and formulating working hypotheses. With this objective in mind, we review here pioneering studies on this subject, together with work that has appeared more recently in the literature.

Association of vimentin intermediate filaments with the centrosome

SW-13 cells that lack cytoplasmic intermediate filaments (IFs) were stably transfected with a human vimentin cDNA expression vector. Isolated subclones displayed two prevalent patterns of vimentin distribution as observed by indirect immuno-localization: (1) cytoplasmic filaments characteristic of a vimentin IF network; and (2) a distinct, juxtanuclear focus with limited filamentous extensions. Comparative analysis of two subclones that uniquely segregated these patterns of vimentin organization indicated that vimentin accumulated as a perinuclear focus in cells that expressed a 4-fold lower level of the protein. The observed variation in cellular organization was not due to detectable differences in vimentin protein modification, as determined by two-dimensional gel analysis. Increasing the amount of vimentin in a low expressing clone by a secondary transfection with human or mouse vimentin cDNA resulted in well-dispersed, cytoplasmic filaments, suggesting that the distinct juxtanuclear organization of vimentin arose due to lower cellular vimentin levels. Employing anti-gamma-tubulin and anti-vimentin antibodies, dual immunofluorescence together with confocal microscopy revealed that the juxtanuclear focus of vimentin was located in the centrosomal region. Electron microscopy showed a spheroidal, filamentous structure with at least some filaments closely associated with the pericentriolar material (PCM). Because vimentin IF organization is at least partially dependent on microtubules, the effects of nocodazole and taxol on perinuclear vimentin foci were examined. Neither drug affected the juxtanuclear localization of foci, although taxol (10 microM, 5 hours) caused a release of pericentriolar gamma-tubulin from the nuclear region in 50–60% of the cells. These studies indicate that lower, in vivo, levels of vimentin fail to form extended IFs but rather are organized as a perinuclear aggregate. Moreover, the PCM of the centrosome appears to possess attachment sites for vimentin IFs.

Adenovirus inhibition of cell translation facilitates release of virus particles and enhances degradation of the cytokeratin network

Infection of animal cells by a number of viruses generally results in an array of metabolic defects, including inhibition of host DNA, RNA, and protein synthesis, and morphological alterations known as cytopathic effects. For adenovirus infection there is a profound loss of cell structural integrity and a marked inhibition of host protein synthesis, the latter generally assumed necessary to enhance virus production. We examined the purpose of viral inhibition of cell translation and found that it was related in part to cytopathic wasting of infected cells. We show that viral shutoff of host translation promotes destruction of the intermediate filament network, particularly cytokeratins which are proteolysed at keratins K7 and K18 by the adenovirus late-acting L3 23-kDa proteinase. We found that if adenovirus is prevented from inhibiting cell translation, the intermediate filament network remains relatively intact, keratin proteins are still synthesized, and cells possess an almost normal morphological appearance and lyse poorly, reducing the release of nascent virus particles by several hundredfold. Remarkably, in tissue culture cells the accumulation of late viral structural proteins is only marginally reduced if host translation shutoff does not occur. Thus, a surprising major function for adenovirus inhibition of cellular protein synthesis is to enhance impairment of cellular structural integrity, facilitating cell lysis and release of progeny adenovirus particles.

Colchicine-sensitive and colchicine-insensitive intermediate filament systems distinguished by a new intermediate filament-associated protein, IFAP-70/280 kD

A monoclonal antibody was produced, using as antigen a BHK-21 cytoskeletal preparation enriched in intermediate filaments (IF) and their associated proteins. This antibody reacted exclusively with a reproducible set of 70-280 kD polypeptides present in minor quantities in this preparation, as detected by immunoblot analysis. Based upon several criteria, this immunologically related group of polypeptides was designated as IFAP-70/280 kD (IF-Associated Protein): (1) it co-isolated with IF in vitro, (2) it co-localized (by both immunofluorescence and immunoelectron microscopy) with IF in situ in all stages of cell spreading, and (3) it segregated in vitro with the 54/55 kD (desmin/vimentin) structural IF subunit proteins of BHK cells through two cycles of in vitro disassembly/assembly. Immunogold labeling further localized IFAP-70/280 kD to regions of parallel or loosely bundled IF in situ, suggesting a role in regulating the supramolecular organization of IF. When this monoclonal antibody was used for double-label immunofluorescence observations of colchicine-treated BHK cells, it demonstrated the presence of colchicine-sensitive and colchicine-insensitive IF. Anti-IFAP-70/280 kD localized entirely to the drug-induced juxtanuclear IF cap, while a polyclonal antibody directed against the desmin/vimentin structural IF subunits and the previously characterized monoclonal anti-IFAP-300 kD [Yang et al., 1985; J. Cell Biol. 100:620] localized to both the juxtanuclear IF cap and a colchicine-insensitive IF network peripheral to the cap in the same cells. The colchicine-insensitive IF pattern often exhibited similarities to that observed for the actin-based stress fiber system, suggesting that stress fiber association may be an additional factor in IF organization.

An immunofluorescence study of the calcium-induced coordinated reorganization of microfilaments, keratin intermediate filaments, and microtubules in cultured human epidermal keratinocytes

Indirect immunofluorescence microscopy has been used to investigate the coordinated reorganization of microtubules, microfilaments, and keratin intermediate filaments in cultured human epidermal keratinocytes following a switch from low-Ca++ (0.15 mM) medium to high-Ca++ (1.05 mM) medium. A dramatic reorganization occurs concurrently in the three major cytoskeletal components shortly after the calcium switch. The most prominent features are the alignment of keratin filaments at the plasma membranes of apposed cells, the induction of microfilament rings, the restriction of microtubules to the area within the boundaries of the microfilament rings, and the alignment of actin bundles at cell borders. Additional changes are observed in terminally differentiated cells. This is the first report that describes simultaneous changes in the organization of the three major cytoskeletal components of epidermal keratinocytes. Cytochalasin D and demecolcine (colcemid) studies were performed to determine whether the organization of microtubules, microfilaments, and keratin filaments, as well as the calcium-induced reorganization of these cytoskeletal elements, may be dependent on the existence of structural relationships between them. These studies demonstrate that the disruption of microfilaments results in the formation of a latticelike keratin network, with a close association of actin and keratin being maintained. The formation of keratin filament alignments occurs even in the absence of intact microfilaments. In addition, it was found that the Ca(++)-induced reorganization of microfilaments and keratin filaments is not dependent on an intact microtubule network. Furthermore, the reorganization of actin into concentric rings can be dissociated from changes in the organization of keratin filaments.

Changes in the organization and antigenic determinants of intermediate filaments of rat hepatocytes after infusion of cytochalasin B in vivo

The changes in cytokeratin intermedial filaments (IFs) after cytochalasin B (CB) infusion of rat liver in vivo were studied by light and electron microscopy, immunofluorescent staining (IMF), and immunoelectron microscopy (IEM). The CB treatment caused a change in the IFs at the cell border associated with a change in the distribution of microfilaments. The IFs at the cell border were partially disrupted. Actin aggregates were localized at points where IFs had condensed together. The pericanalicular sheath was intact but very dilated. These results indicated that the CB treatment caused an irregular distribution of the microfilaments at the cell periphery but spared the actin at the bile canaliculus. Cytokeratin staining by IMF was markedly decreased or absent; however, IEM clearly showed the presence of nonstaining IFs after CB treatment. These results indicated that the antigenic determinant of normal cytokeratin IFs became masked after CB treatment. The results indicate that F-actin disassembly induced by CB affects both the organization and conformation of cytokeratins associated with loss of integrity of the plasma membrane and vesicular uptake of plasma proteins by hepatocytes.

The relationship between intermediate filaments and microfilaments before and during the formation of desmosomes and adherens-type junctions in mouse epidermal keratinocytes

Actin, keratin, vinculin and desmoplakin organization were studied in primary mouse keratinocytes before and during Ca2+-induced cell contact formation. Double-label fluorescence shows that in cells cultured in low Ca2+ medium, keratin-containing intermediate filament bundles (IFB) and desmoplakin-containing spots are both concentrated towards the cell center in a region bounded by a series of concentric microfilament bundles (MFB). Within 5-30 min after raising Ca2+ levels, a discontinuous actin/vinculin-rich, submembranous zone of fluorescence appears at cell-cell interfaces. This zone is usually associated with short, perpendicular MFB, which become wider and longer with time. Later, IFB and the desmoplakin spots are seen aligned along the perpendicular MFB as they become redistributed to cell-cell interfaces where desmosomes form. Ultrastructural analysis confirms that before the Ca2+ switch, IFB and desmosomal components are found predominantly within the perimeter defined by the outermost of the concentric MFB. Individual IF often splay out, becoming interwoven into these MFB in the region of cell-substrate contact. In the first 30 min after the Ca2+ switch, areas of submembranous dense material (identified as adherens junctions), which are associated with the perpendicular MFB, can be seen at newly formed cell-cell contact sites. By 1-2 h, IFB-desmosomal component complexes are aligned with the perpendicular MFB as the complexes become redistributed to cell-cell interfaces. Cytochalasin D treatment causes the redistribution of actin into numerous patches; keratin-containing IFB undergo a concomitant redistribution, forming foci that coincide with the actin-containing aggregates. These results are consistent with an IF-MF association before and during desmosome formation in the primary mouse epidermal keratinocyte culture system, and with the temporal and spatial coordination of desmosome and adherens junction formation.

Cytochalasin B-induced redistribution of cytokeratin filaments in PtK1 cells

Indirect immunofluorescence demonstrated a dramatic reorganization of cytokeratin filaments produced by cytochalasin B (CB) treatment of PtK1 cells. Much of the normal cytokeratin network became arranged into a latticework consisting of bundles of cytokeratin filaments that radiated from, and interconnected, distinct foci. Electron microscopy showed foci to be dense granular regions through which bundles of cytokeratin filaments looped. Composition of the foci included actin, myosin, and alpha-actinin, as shown by labeling with rhodamine phalloidin or specific antisera. Simultaneous treatment with CB and colchicine was not required for lattice formation, but did produce more extensive development than did CB alone. In cells treated only with CB, the microtubule network remained intact, even in regions of extensive lattice formation. These results contrast sharply with those of Knapp et al (J. Cell Biol. 97:1788 [1983b]), who found lattice formation dependent upon simultaneous CB and colchicine treatment. Time-course and dose-response studies of CB treatment showed lattice formation to follow disruption of stress fibers and the concentration of actin into distinct patches that marked the location of lattice foci. Overall results suggest a structural association between microfilaments and cytokeratin filaments that produces the lattice pattern upon CB-induced disruption of stress fibers. Lattice formation was not limited to a specific cell-cycle stage, since G1, G2, and M cells displayed the lattice. Treatment of cells with dihydro-CB and experiments with enucleated cells showed that lattice formation was dependent upon neither the inhibition of sugar transport nor the nuclear extrusion effects of CB.

Reorganization of keratin intermediate filaments by the drug-induced disruption of microfilaments in cultured human keratinocytes

It has been shown to date that a combined treatment with microtubule and microfilament inhibitors alters the cytoskeletal organization of keratin intermediate filaments in cultured HeLa, fetal mouse epidermal, and epithelial PtK2 cells, although neither of these inhibitors alone is able to do so. In the present study, we found that disruption of microfilaments with cytochalasin B induced a remarkable reorganization of keratin filaments in cultured human keratinocytes, while disruption of microtubules with colchicine did not affect keratin filaments. Keratin filament organization in the presence of cytochalasin B demonstrated a network of connecting star-like knots or foci. These foci coincided with actin aggregates that were formed by depolymerization of actin filaments as studied by double immunofluorescence using antiactin and antikeratin antibodies. Under these conditions, no change in microtubule arrangement was observed. Our observations suggest that the stability and architecture of keratin filament organization may be supported with the microfilament rather than the microtubule cytoskeleton in cultured human keratinocytes.

Effects of dimethylsulfoxide (DMSO), nocodazole, and taxol on mast cell histamine secretion

Nocodazole depolymerized microtubules and increased the number of microfilaments, and dimethylsulfoxide increased the number of microfilaments. Both drugs inhibited compound 48/80-induced histamine release from rat mast cells. Taxol, which increased the number of microtubules, had no effect on histamine release. These observations support the view that microtubules may not be directly involved in secretion, but apparently an increased number of microfilaments is associated with a decreased capacity of the mast cells for histamine release. We suggest that microfilaments have to be depolymerized or rearranged before secretion can take place.

Effects of mechanical tension on protrusive activity and microfilament and intermediate filament organization in an epidermal epithelium moving in culture

Mechanical tension influences tissue morphogenesis and the synthetic, mitotic, and motile behavior of cells. To determine the effects of tension on epithelial motility and cytoskeletal organization, small, motile clusters of epidermal cells were artificially extended with a micromanipulated needle. Protrusive activity perpendicular to the axis of tension was dramatically suppressed. To determine the ultrastructural basis for this phenomenon, cells whose exact locomotive behavior was recorded cinemicrographically were examined by transmission electron microscopy. In untensed, forward-moving lamellar protrusions, microfilaments appear disorganized and anisotropically oriented. But in cytoplasm held under tension by micromanipulation or by the locomotive activity of other cells within the epithelium, microfilaments are aligned parallel to the tension. In non-spreading regions of the epithelial margin, microfilaments lie in tight bundles parallel to apparent lines of tension. Thus, it appears that tension causes alignment of microfilaments. In contrast, intermediate filaments are excluded from motile protrusions, being confined to the thicker, more central part of the cell. They roughly follow the contours of the cell, but are not aligned relative to tension even when microfilaments in the same cell are. This suggests that the organization of intermediate filaments is relatively resistant to physical distortion and the intermediate filaments may act as passive structural support within the cell. The alignment of microfilaments under tension suggests a mechanism by which tension suppresses protrusive activity: microfilaments aligned by forces exerted through filament-surface or filament-filament interconnections cannot reorient against such force and so cannot easily extend protrusions in directions not parallel to tension.

Relationship between intermediate filaments and microfilaments in cultured fibroblasts: evidence for common foci during cell spreading

Spreading and fully spread chick embryo fibroblasts (CEF) were examined by double-label fluorescence microscopy using the actin-specific probe rhodamine-phalloidin and an antibody directed against CEF intermediate filaments (IF). During midspreading, a striking relationship became discernible: statistical analysis showed that approximately half of the cell population exhibited one or more phase-dense, phalloidin-binding nodules that appeared to act as foci from which IF diverged. Coincidence between actin-containing structures and IF was not limited to these centers; IF could also frequently be seen running in close parallel arrays with stress fibers. Ultrastructural analysis confirmed the presence of non-membrane-bound out-pocketings along the length of stress fibers from which 10-nm IF diverged. These structures varied in size and shape, and displayed a dense, fine fibrillar appearance. IF and microfilaments (MF) were distinguished by size and by decoration of MF with myosin subfragment-1. Other IF-MF interactions were seen in cells of all stages: IF were observed to loop through stress fibers, most frequently at the cell margins. In colchicine-treated cells, IF became redistributed into cables that often ran parallel and appeared to merge with stress fibers. Cytochalasin D-treated CEF exhibited loose aggregates of actin-containing material that appeared to be associated with IF. These results suggest the possibility of an interaction between actin-containing structures and IF, particularly during cell spreading in cultured fibroblasts.