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

Reconstitution of composite actin and keratin networks in vesicles

Although cytoskeletal networks are interpenetrating and interacting in living cells, very little is understood as to the effect their interaction has on their properties. Here, as a step towards elucidating the synergistic cellular role of these structural proteins, we investigate isolated keratin and actin composites and show how the in vitro network formation of keratin influences the properties of actin networks and vice versa. By encapsulating purified composite networks into vesicles and separating the time scales of network formation we are able to demonstrate that the actin network stabilizes keratin networks by providing an elastic resistance to their collapse in vitro.

Multidimensional Monitoring of Keratin Intermediate Filaments in Cultured Cells and Tissues

Keratin filaments are a hallmark of epithelial differentiation. Their cell type-specific spatial organization and dynamic properties reflect and support epithelial function. To study this interdependency, imaging of fluorescently tagged keratins is a widely used method by which the temporospatial organization and behavior of the keratin intermediate filament network can be analyzed in living cells. Here, we describe methods that have been adapted and optimized to dissect and quantify keratin intermediate filament network dynamics in vital cultured cells and functional tissues.

Dissection of keratin network formation, turnover and reorganization in living murine embryos

Epithelial functions are fundamentally determined by cytoskeletal keratin network organization. However, our understanding of keratin network plasticity is only based on analyses of cultured cells overexpressing fluorescently tagged keratins. In order to learn how keratin network organization is affected by various signals in functional epithelial tissues in vivo, we generated a knock-in mouse that produces fluorescence-tagged keratin 8. Homozygous keratin 8-YFP knock-in mice develop normally and show the expected expression of the fluorescent keratin network both in fixed and in vital tissues. In developing embryos, we observe for the first time de novo keratin network biogenesis in close proximity to desmosomal adhesion sites, keratin turnover in interphase cells and keratin rearrangements in dividing cells at subcellular resolution during formation of the first epithelial tissue. This mouse model will help to further dissect keratin network dynamics in its native tissue context during physiological and also pathological events.

Measuring the regulation of keratin filament network dynamics

The organization of the keratin intermediate filament cytoskeleton is closely linked to epithelial function. To study keratin network plasticity and its regulation at different levels, tools are needed to localize and measure local network dynamics. In this paper, we present image analysis methods designed to determine the speed and direction of keratin filament motion and to identify locations of keratin filament polymerization and depolymerization at subcellular resolution. Using these methods, we have analyzed time-lapse fluorescence recordings of fluorescent keratin 13 in human vulva carcinoma-derived A431 cells. The fluorescent keratins integrated into the endogenous keratin cytoskeleton, and thereby served as reliable markers of keratin dynamics. We found that increased times after seeding correlated with down-regulation of inward-directed keratin filament movement. Bulk flow analyses further revealed that keratin filament polymerization in the cell periphery and keratin depolymerization in the more central cytoplasm were both reduced. Treating these cells and other human keratinocyte-derived cells with EGF reversed all these processes within a few minutes, coinciding with increased keratin phosphorylation. These results highlight the value of the newly developed tools for identifying modulators of keratin filament network dynamics and characterizing their mode of action, which, in turn, contributes to understanding the close link between keratin filament network plasticity and epithelial physiology.

The keratin-filament cycle of assembly and disassembly

Continuous and regulated remodelling of the cytoskeleton is crucial for many basic cell functions. In contrast to actin filaments and microtubules, it is not understood how this is accomplished for the third major cytoskeletal filament system, which consists of intermediate-filament polypeptides. Using time-lapse fluorescence microscopy of living interphase cells, in combination with photobleaching, photoactivation and quantitative fluorescence measurements, we observed that epithelial keratin intermediate filaments constantly release non-filamentous subunits, which are reused in the cell periphery for filament assembly. This cycle is independent of protein biosynthesis. The different stages of the cycle occur in defined cellular subdomains: assembly takes place in the cell periphery and newly formed filaments are constantly transported toward the perinuclear region while disassembly occurs, giving rise to diffusible subunits for another round of peripheral assembly. Remaining juxtanuclear filaments stabilize and encage the nucleus. Our data suggest that the keratin-filament cycle of assembly and disassembly is a major mechanism of intermediate-filament network plasticity, allowing rapid adaptation to specific requirements, notably in migrating cells.

Polycystin-1 is a microtubule-driven desmosome-associated component in polarized epithelial cells

In this study, we have analyzed the expression and localization of polycystin-1 in intestinal epithelial cells, a system lacking primary cilia. Polycystin-1 was found to be expressed in the epithelium of the small intestine during development and levels remained elevated in the adult. Dual-labelling indirect immunofluorescence revealed polycystin-1 at sites of cell-cell contact co-localizing with the desmosomes both in situ as well as in polarized Caco-2/15 cells. In unpolarized cultures of Caco-2/15 cells, polycystin-1 was recruited to the cell surface early during initiation of cell junction assembly. In isolated Caco-2/15 cells and HIEC-6 cell cultures, where junctional complexes are absent, polycystin-1 was found predominantly associated with the cytoskeletal elements of the intermediate filaments and microtubule networks. More precisely, polycystin-1 was seen as brightly labelled puncta decorating the keratin-18 positive filaments as well as the beta-tubulin positive microtubules, which was particularly obvious in the lamellipodia. Treatment with the microtubule-disrupting agent, nocodazole, eliminated the microtubule association of polycystin-1 but did not seem to affect its association with keratin or the desmosomes. Taken together these data suggest that polycystin-1 is involved with the establishment of cell-cell junctions in absorptive intestinal epithelial cells and exploits the microtubule-based machinery in order to be transported to the plasma membrane.

Intermediate filaments in smooth muscle

The intermediate filament (IF) network is one of the three cytoskeletal systems in smooth muscle. The type III IF proteins vimentin and desmin are major constituents of the network in smooth muscle cells and tissues. Lack of vimentin or desmin impairs contractile ability of various smooth muscle preparations, implying their important role for smooth muscle force development. The IF framework has long been viewed as a fixed cytostructure that solely provides mechanical integrity for the cell. However, recent studies suggest that the IF cytoskeleton is dynamic in mammalian cells in response to various external stimulation. In this review, the structure and biological properties of IF proteins in smooth muscle are summarized. The role of IF proteins in the modulation of smooth muscle force development and redistribution/translocation of signaling partners (such as p130 Crk-associated substrate, CAS) is depicted. This review also summarizes our latest understanding on how the IF network may be regulated in smooth muscle.

Intermediate filament assembly: dynamics to disease

Intermediate filament (IF) proteins belong to a large and diverse gene family with broad representation in vertebrate tissues. Although considered the 'toughest' cytoskeletal fibers, studies in cultured cells have revealed that IF can be surprisingly dynamic and highly regulated. This review examines the diversity of IF assembly behaviors, and considers the ideas that IF proteins are co- or post-translationally assembled into oligomeric precursors, which can be delivered to different subcellular compartments by microtubules or actomyosin and associated motor proteins. Their interaction with other cellular elements via IF associated proteins (IFAPs) affects IF dynamics and also results in cellular networks with properties that transcend those of individual components. We end by discussing how mutations leading to defects in IF assembly, network formation or IF-IFAP association compromise in vivo functions of IF as protectors against environmental stress.

p38 MAPK-dependent shaping of the keratin cytoskeleton in cultured cells

Plasticity of the resilient keratin intermediate filament cytoskeleton is an important prerequisite for epithelial tissue homeostasis. Here, the contribution of stress-activated p38 MAPK to keratin network organization was examined in cultured cells. It was observed that phosphorylated p38 colocalized with keratin granules that were rapidly formed in response to orthovanadate. The same p38^p recruitment was noted during mitosis, in various stress situations and in cells producing mutant keratins. In all these situations keratin 8 became phosphorylated on S73, a well-known p38 target site. To demonstrate that p38-dependent keratin phosphorylation determines keratin organization, p38 activity was pharmacologically and genetically modulated: up-regulation induced keratin granule formation, whereas down-regulation prevented keratin filament network disassembly. Furthermore, transient p38 inhibition also inhibited keratin filament precursor formation and mutant keratin granule dissolution. Collectively, the rapid and reversible effects of p38 activity on keratin phosphorylation and organization in diverse physiological, stress, and pathological situations identify p38-dependent signalling as a major intermediate filament–regulating pathway.

The motility and dynamic properties of intermediate filaments and their constituent proteins

Intermediate filament (IF) proteins exist in multiple structural forms within cells including mature IF, short filaments or 'squiggles', and non-filamentous precursors called particles. These forms are interconvertible and their relative abundance is IF type, cell type- and cell cycle stage-dependent. These structures are often associated with molecular motors, such as kinesin and dynein, and are therefore capable of translocating through the cytoplasm along microtubules. The assembly of mature IF from their precursor particles is also coupled to translation. These dynamic properties of IF provide mechanisms for regulating their reorganization and assembly in response to the functional requirements of cells. The recent findings that IF and their precursors are frequently associated with signaling molecules have revealed new functions for IF beyond their more traditional roles as mechanical integrators of cells and tissues.

Structural and regulatory functions of keratins

The diversity of epithelial functions is reflected by the expression of distinct keratin pairs that are responsible to protect epithelial cells against mechanical stress and to act as signaling platforms. The keratin cytoskeleton integrates these functions by forming a supracellular scaffold that connects at desmosomal cell-cell adhesions. Multiple human diseases and murine knockouts in which the integrity of this system is destroyed testify to its importance as a mechanical stabilizer in certain epithelia. Yet, surprisingly little is known about the precise mechanisms responsible for assembly and disease pathology. In addition to these structural aspects of keratin function, experimental evidence accumulating in recent years has led to a much more complex view of the keratin cytoskeleton. Distinct keratins emerge as highly dynamic scaffolds in different settings and contribute to cell size determination, translation control, proliferation, cell type-specific organelle transport, malignant transformation and various stress responses. All of these properties are controlled by highly complex patterns of phosphorylation and molecular associations.

Focal adhesions are hotspots for keratin filament precursor formation

Recent studies showed that keratin filament (KF) formation originates primarily from sites close to the actin-rich cell cortex. To further characterize these sites, we performed multicolor fluorescence imaging of living cells and found drastically increased KF assembly in regions of elevated actin turnover, i.e., in lamellipodia. Abundant KF precursors (KFPs) appeared within these areas at the distal tips of actin stress fibers, moving alongside the stress fibers until their integration into the peripheral KF network. The earliest KFPs were detected next to actin-anchoring focal adhesions (FAs) and were only seen after the establishment of FAs in emerging lamellipodia. Tight spatiotemporal coupling of FAs and KFP formation were not restricted to epithelial cells, but also occurred in nonepithelial cells and cells producing mutant keratins. Finally, interference with FA formation by talin short hairpin RNA led to KFP depletion. Collectively, our results support a major regulatory function of FAs for KF assembly, thereby providing the basis for coordinated shaping of the entire cytoskeleton during cell relocation and rearrangement.

High expression of a cytokeratin-associated protein in many cancers

We have described previously a cDNA library made from membrane-bound polysomal mRNA prepared from breast and prostate cancer cell lines. The library is highly enriched for cDNAs encoding membrane proteins, secreted proteins, and cytokeratins. To characterize this library, 25,277 cDNA clones were sequenced and aligned with various databases; 1,439 clones did not align with known genes. From this set of clones we identified a previously uncharacterized gene encoding a 334-aa protein. Although protein structural motif prediction programs indicate that the gene encodes a membrane protein comprising a signal sequence, a series of leucine-rich repeats, and a single transmembrane domain with a cytoplasmic tail, confocal microscopy of MCF7 breast cancer cells demonstrates that the protein is not directly associated with the plasma membrane or intracellular membranes but instead colocalizes with intermediate filaments and cytokeratins within the cell. Immunofluorescence studies also show that protein expression is increased greatly in mitotic MCF7 cells, and immunohistochemistry demonstrates its expression in human breast cancer cells. Analysis of mRNA levels in 25 different normal tissues by RT-PCR shows that this gene is expressed highly in normal prostate and salivary gland, very weakly in colon, pancreas, and intestine, and not at all in other tissues. RT-PCR studies on human cancer samples show that the RNA is expressed highly in many cancer cell lines and cancer specimens, including 26 of 33 human breast cancers, 3 of 3 prostate cancers, 3 of 3 colon cancers, and 3 of 3 pancreatic cancers. We name the protein CAPC, cytokeratin-associated protein in cancer.

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.

Intermediate filament networks: in vitro and in vivo assembly models

We propose two systems of ordinary differential equations modeling the assembly of intermediate filament networks. The first one describes the in vitro intermediate filament assembly dynamics. The second one deals with the in vivo evolution of cytokeratin, which is the intermediate filament protein expressed by epithelial cells. The in vitro model is then briefly analyzed in a simplified case.

Intermediate filaments mediate cytoskeletal crosstalk

Intermediate filaments, actin-containing microfilaments and microtubules are the three main cytoskeletal systems of vertebrate and many invertebrate cells. Although these systems are composed of distinctly different proteins, they are in constant and intimate communication with one another. Understanding the molecular basis of this cytoskeletal crosstalk is essential for determining the mechanisms that underlie many cell-biological phenomena. Recent studies have revealed that intermediate filaments and their associated proteins are important components in mediating this crosstalk.

Molecular and biophysical characterization of assembly-starter units of human vimentin

We have developed an assembly protocol for the intermediate filament (IF) protein vimentin based on a phosphate buffer system, which enables the dynamic formation of authentic IFs. The advantage of this physiological buffer is that analysis of the subunit interactions by chemical cross-linking of internal lysine residues becomes feasible. By this system, we have analyzed the potential interactions of the coiled-coil rod domains with one another, which are assumed to make a crucial contribution to IF formation and stability. We show that headless vimentin, which dimerizes under low salt conditions, associates into tetramers of the A(22)-type configuration under assembly conditions, indicating that one of the effects of increasing the ionic strength is to favor coil 2-coil 2 interactions. Furthermore, in order to obtain insight into the molecular interactions that occur during the first phase of assembly of full-length vimentin, we employed a temperature-sensitive variant of human vimentin, which is arrested at the "unit-length filament" (ULF) state at room temperature, but starts to elongate upon raising the temperature to 37 degrees C. Most importantly, we demonstrate by cross-linking analysis that ULF formation predominantly involves A(11)-type dimer-dimer interactions. The presence of A(22) and A(12) cross-linking products in mature IFs, however, indicates that major rearrangements do occur during the longitudinal annealing and radial compaction steps of IF assembly.

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.

The dynamic and motile properties of intermediate filaments

For many years, cytoplasmic intermediate filaments (IFs) were considered to be stable cytoskeletal elements contributing primarily to the maintenance of the structural and mechanical integrity of cells. However, recent studies of living cells have revealed that IFs and their precursors possess a remarkably wide array of dynamic and motile properties. These properties are in large part due to interactions with molecular motors such as conventional kinesin, cytoplasmic dynein, and myosin. The association between IFs and motors appears to account for much of the well-documented molecular cross talk between IFs and the other major cytoskeletal elements, microtubules, and actin-containing microfilaments. Furthermore, the associations with molecular motors are also responsible for the high-speed, targeted delivery of nonfilamentous IF protein cargo to specific regions of the cytoplasm where they polymerize into IFs. This review considers the functional implications of the motile properties of IFs and discusses the potential relationships between malfunctions in these motile activities and human diseases.

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.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

Observation of keratin particles showing fast bidirectional movement colocalized with microtubules

Keratin intermediate filament networks were observed in living cultured epithelial cells using the incorporation of fluorescently tagged keratin from a transfected enhanced green fluorescent protein (EGFP) construct. In steady-state conditions EGFP-keratin exists not only as readily detectable intermediate filaments, but also as small particles, of which there are two types: a less mobile population (slow or static S particles) and a highly dynamic one (fast or F particles). The dynamic F particles move around the cell very fast and in a non-random way. Their movement is composed of a series of steps, giving an overall characteristic zig-zag trajectory. The keratin particles are found all over the cell and their movement is aligned with microtubules; treatment of cells with nocodazole has an inhibitory effect on keratin particle movement, suggesting the involvement of microtubule motor proteins. Double-transfection experiments to visualize tubulin and keratin together suggest that the movement of keratin particles can be bidirectional, as particles are seen moving both towards and away from the centrosome area. Using field emission scanning and transmission electron microscopy combined with immunogold labelling, we also detected particulate keratin structures in untransfected epithelial cells, suggesting that keratin particles may be a natural component of keratin filament dynamics in living cells.

Light-induced resistance of the keratin network to the filament-disrupting tyrosine phosphatase inhibitor orthovanadate

Epidermal keratinocytes respond to low-dose light irradiation by inducing signaling cascades that lead to long-term effects on gene transcription thereby protecting cells against damage. In contrast, little is known about immediate light-induced alterations of structural proteins. We have made the intriguing observation that light produces fundamental changes in the properties of the keratin filament system of cultured epidermoid A-431 cells. A short light exposure (1-10 min) causes the keratin cytoskeleton to become immediately resistant to the tyrosine phosphatase inhibitor orthovanadate, which otherwise disrupts the keratin filament network completely in just a few minutes. This protective effect is inducible throughout the entire visible spectrum and is elicited by normal room light (<200 Lux). Exposure of cells to monochromatic light of various wavelengths is therefore equally effective. In addition, the acquisition of orthovanadate resistance has been directly monitored in living cells; a partially disrupted keratin cytoskeleton recovers to a completely filamentous network in half an hour. Finally, the protective light effect is largely reversed in 2 h and can be mimicked by preincubation with the p38 kinase inhibitor SB203580. In contrast, the mitogen-activated protein kinase inhibitor PD98059 and epidermal growth factor inhibit orthovanadate action to a lesser extent. Taken together, these observations suggest a stabilizing function of light on the keratin filament network; this may be of relevance to the treatment of skin diseases with reduced keratin stability.

Induction of rapid and reversible cytokeratin filament network remodeling by inhibition of tyrosine phosphatases

The cytokeratin filament network is intrinsically dynamic, continuously exchanging subunits over its entire surface, while conferring structural stability on epithelial cells. However, it is not known how cytokeratin filaments are remodeled in situations where the network is temporarily and spatially restricted. Using the tyrosine phosphatase inhibitor orthovanadate we observed rapid and reversible restructuring in living cells, which may provide the basis for such dynamics. By examining cells stably expressing fluorescent cytokeratin chimeras, we found that cytokeratin filaments were broken down and then formed into granular aggregates within a few minutes of orthovanadate addition. After drug removal, gradual reincorporation of granules into the filament network was observed for aggregates that were either part of residual filaments or stayed in close apposition to remaining filaments. Even when cytokeratin filaments were no longer detectable, granules with low mobility were still able to reestablish a cytokeratin filament network. This process took less than 30 minutes and occurred at multiple foci throughout the cytoplasm without apparent correlation to alterations in the actin- and tubulin-based systems. Interestingly, the short-lived and rather small orthovanadate-induced cytokeratin granules contained the cytoskeletal crosslinker plectin but lacked the cytokeratin-solubilising 14-3-3 proteins. By contrast, the long-lived and larger cytokeratin aggregates generated after treatment with the serine/threonine phosphatase inhibitor okadaic acid were negative for plectin but positive for 14-3-3 proteins. Taken together, our observations in living orthovanadate-treated interphase cells revealed modes of cytokeratin remodeling that qualify as basic mechanisms capable of rapidly adapting the cytokeratin filament cytoskeleton to specific requirements.

Desmosomes: interconnected calcium-dependent structures of remarkable stability with significant integral membrane protein turnover

Desmosomes are prominent cell adhesion structures that are major stabilizing elements, together with the attached cytoskeletal intermediate filament network, of the cytokeratin type in epithelial tissues. To examine desmosome dynamics in tightly coupled cells and in situations of decreased adhesion, fluorescent desmosomal cadherin desmocollin 2a (Dsc2a) chimeras were stably expressed in human hepatocellular carcinoma-derived PLC cells (clone PDc-13) and in Madin-Darby canine kidney cells (clone MDc-2) for the continuous monitoring of desmosomes in living cells. The hybrid polypeptides integrated specifically and without disturbance into normal-appearing desmosomes that occurred in association with typical cytokeratin filament bundles. Tracking of labeled adhesion sites throughout the cell cycle by time-lapse fluorescence microscopy revealed that they were immobile and that they maintained their structural integrity for long periods of time. Time-space diagrams further showed that desmosomal positioning was tightly controlled, even during pronounced cell shape changes, although the desmosomal arrays extended and contracted, suggesting that they were interconnected by a flexible system with intrinsic elasticity. Double-fluorescence microscopy detecting Dsc2a chimeras together with fluorescent cytokeratin 18 chimeras revealed the association and synchronous movement of labeled desmosomes and fluorescent cytokeratin filaments. Only a minor destabilization of desmosomes was observed during mitosis, demonstrated by increased diffuse plasma membrane fluorescence and the fusion of desmosomes into larger structures. Desmosomes did not disappear completely at any time in any cell, and residual cytokeratin filaments remained in association with adhesion sites throughout cell division. On the other hand, a rapid loss of desmosomes was observed upon calcium depletion, with irreversible uptake of some desmosomal particles. Simultaneously, diffusely distributed desmosomal cadherins were detected in the plasma membrane that retained the competence to nucleate the reformation of desmosomes after the cells were returned to a standard calcium-containing medium. To examine the molecular stability of desmosomes, exchange rates of fluorescent chimeras were determined by fluorescence recovery after photobleaching, thereby identifying considerable Dsc2a turnover with different rates of fluorescence recovery for PDc-13 cells (36±17% recovery after 30 minutes) and MDc-2 cells (60±20% recovery after 30 minutes). Taken together, our observations suggest that desmosomes are pliable structures capable of fine adjustment to functional demands despite their overall structural stability and relative immobility.