Dynamics of keratin assembly: exogenous type I keratin rapidly associates with type II keratin in vivo

Keratin intermediate filaments (IF) are obligate heteropolymers containing equal amounts of type I and type II keratin. We have previously shown that microinjected biotinylated type I keratin is rapidly incorporated into endogenous bundles of keratin IF (tonofilaments) of PtK2 cells. In this study we show that the earliest steps in the assembly of keratin subunits into tonofilaments involve the extremely rapid formation of discrete aggregates of microinjected keratin. These are seen as fluorescent spots containing both type I and type II keratins within 1 min post-injection as determined by double label immunofluorescence. These observations suggest that endogenous type II keratin subunits can be rapidly mobilized from their endogenous state to form complexes with the injected type I protein. Furthermore, confocal microscopy and immunogold electron microscopy suggest that the type I-type II keratin spots from in close association with the endogenous keratin IF network. When the biotinylated protein is injected at concentrations of 0.3-0.5 mg/ml, the organization of the endogenous network of tonofilaments remains undisturbed during incorporation into tonofilaments. However, microinjection of 1.5-2.0 mg/ml of biotinylated type I results in significant alterations in the organization and assembly state of the endogenous keratin IF network soon after microinjection. The results of this study are consistent with the existence of a state of equilibrium between keratin subunits and polymerized keratin IF in epithelial cells, and provide further proof that IF are dynamic elements of the cytoskeleton of mammalian cells.

Structure, function, and dynamics of keratin intermediate filaments

The recent widespread application of modern methods of structural biology, molecular biology, and molecular genetics has provided a wealth of new information on the structure and function of the KIF of the epidermis. One of the more surprising aspects of this work has been the realization of the dynamic behavior of the KIF in living cells. Perhaps one of the more exciting aspects has been the discovery and understanding of how simple, single-nucleotide-point mutations in the keratin proteins can cause defects in the KIF that in turn cause serious pathology in the epidermis. The serendipitous and coincident nature of these studies shows us how an integrated, multifaceted approach will be necessary to solve further fundamental questions and to devise useful therapeutic approaches for the management of diseases of cornification. I fully expect that these issues will advance rapidly in the near future.

Avian keratoacanthoma (dermal squamous cell carcinoma) in broiler chicken carcasses

Multiple lesions of dermal squamous cell carcinoma are found at a low frequency (0.04%) in the carcasses of young meat-type chickens at slaughter. For this study, affected carcasses (n = 308) were removed from the processing line, and lesions were characterized by size, distribution, and morphology. Carcasses were also sexed and examined for evidence of metastasis. Nodular (n = 297) and ulcerative (n = 1,707) lesions were counted and examined. Most lesions were present in the pectoral, dorsopelvic, and femoral feather tracts. Few lesions (n = 11) were found in wing tracts. Mean diameter was 5.4 mm for ulcerative lesions and 3.1 mm for nodular lesions. Histologic sections of ulcerative (n = 579) and nodular (n = 113) lesions were examined. Small nodular lesions originated from hyperplastic feather follicle epithelium. Nodules contained keratin-filled cysts lined by squamous epithelium that were associated with isolated islands and infiltrating cords of dermal keratinocytes. Loss of surface epithelium resulted in noduloulcerative and ulcerative lesions. Invasion of underlying skeletal muscle and evidence of visceral metastasis were not present, but invasion through elastic laminae and into the subcutis was present in 20.5% (134/654) of the lesions examined. The nodular lesions in these carcasses were morphologically similar to early nodular lesions previously described in live chickens as avian keratoacanthomas. A retrospective study compared selected production parameters and disease condemnations to the prevalence of squamous cell carcinoma in 665 flocks of broiler chickens. There was a positive correlation (P < 0.0001) with the occurrence of airsacculitis but a negative correlation with increased age and condemnations for leukosis (P < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Cytokeratin in lens epithelial cells and its effect on anterior lens capsule opacification

Postoperative lens capsule opacification is a complication of extracapsular cataract extraction. To avoid this problem, we aspirated the epithelial cells lining the internal surface of the capsular bag. Anterior capsule specimens collected three and six years after the first surgical intervention (extracapsular cataract extraction with intercapsular implantation of a poly[methyl methacrylate] intraocular lens) and in vitro studies of epithelial cell cultures revealed the presence of intracellular and extracellular cytokeratin. After several years, the cytokeratin layers were arranged in a pattern analogous to that of liquid crystalline phases and many nonhuman extraocular tissues.

Disease severity correlates with position of keratin point mutations in patients with epidermolysis bullosa simplex

Keratins are the major structural proteins of the epidermis. Recently, it was discovered that point mutations in the epidermal keratins can lead to the blistering skin diseases epidermolysis bullosa simplex (EBS) and epidermolytic hyperkeratosis (EH), involving epidermal cell fragility and rupture upon mechanical stress. In this study, we demonstrate a correlation between disease severity, location of point mutations within the keratin polypeptides, and degree to which these mutations perturb keratin filament structure. Interestingly, of the 11 EBS or EH mutations thus far identified, 6 affect a single highly evolutionarily conserved arginine residue, which, when mutated, markedly perturbs keratin filament structure and keratin network formation. This site also appears to be a hot spot for mutation by CpG methylation and deamination. In the four epidermal keratins, there are several other CpG dinucleotides that exist at codons within the highly conserved ends of the keratin rod. To elucidate why mutations at these sites have not been detected in severe cases of EBS, we engineered 7 of these C-->T transitions in K14 and tested their ability to perturb keratin network formation and keratin filament assembly in vitro. The effects of these mutants on keratin filament network formation were significantly less severe than the EBS/EH arginine mutation, suggesting that the high incidence of mutations of the residue in EBS and EH patients is a result of both a special sensitivity of filament structure to perturbations in this residue and its susceptibility to mutagenesis.

Keratin intermediate filament structure. Crosslinking studies yield quantitative information on molecular dimensions and mechanism of assembly

One of the major obstacles to solving the full three-dimensional structure of keratin intermediate filaments (KIF) is the determination of the exact mode(s) of alignment of nearest-neighbor molecules; this in turn requires precise information of the lengths of the non-alpha-helical linker segments within the coiled-coil alpha-helical heterodimer molecule. In this study, we have induced lysine-lysine and cysteine-cysteine crosslinks between keratin intermediate filament molecules in small assembly-competent oligomers, isolated them and then characterized the natures and locations of the crosslinks. Of more than 100 found, 21 quantitatively major crosslinks were used to obtain the relative axial alignments of rod domain segments by least-squares fitting methods. Three dominant modes of alignment were found. In each case the molecules are antiparallel with the first involving molecules in approximate register (stagger = -0.2 nm), the second involving molecules staggered so as to bring the 1B segments into approximate alignment (stagger = -16.1 nm), and the third involving molecules staggered so as to bring the 2B segments into approximate alignment (stagger = 28.2 nm). In addition, the data enable quantitative estimates to be made for the first time of the lengths of the non-coiled-coil segments (L1 = 2.5 nm, L12 = 1.6 nm, L2 = 0.8 nm), and the total length of the rod domain (46.0 nm). Alignment of molecules according to these parameters permits construction of a two-dimensional surface lattice which displays a 1.6 nm (10 or 11 residue) overlap between similarly directed molecules. Together, the data predict six important overlapping sequence regions that recur about 16 times per 46 nm of filament length. Interestingly, synthetic peptides corresponding to these sequences, singly or in combination, significantly interfere with keratin filament structural integrity. These results thus represent the most significant set of structural constraints for KIF yet available and provide insights into how disease-causing mutations disrupt filaments and their organization in cells.

An ultrastructural study of the effects of topical tretinoin on microcomedones

It has been hypothesized that topical tretinoin prevents inflammatory acne lesions by loosening follicular impactions (microcomedones) and clearing the follicular canal of retained keratin. To lend support to this hypothesis, 15 volunteers applied 0.1% tretinoin cream once daily for 12 weeks to one side of the face and an emollient cream to the other side. During the 12-week treatment period, samples of microcomedones were obtained using the follicular biopsy technique. By 6 weeks, the number of microcomedones was reduced approximately 50% from baseline on the side treated with tretinoin and approximately 80% at the end of the study. Only a minimal reduction in the number of microcomedones was seen on the emollient side, even after 12 weeks of application. Morphologic examination showed a progressive loss of the cohesiveness of the microcomedones and ultrastructural alterations in the epithelial cells treated with tretinoin. The microcomedones changed from well-developed keratinous plugs containing many bacteria to a few wispy layers of keratin containing few bacteria. This study confirms the microcomedolytic activity of tretinoin and provides evidence that this activity is associated with changes in the differentiation of the follicular epithelial cells.

The roles of K5 and K14 head, tail, and R/K L L E G E domains in keratin filament assembly in vitro

Type I and type II keratins form obligatory heterodimers, which self-assemble into 10-nm intermediate filaments (IFs). Like all IF proteins, they have a central alpha-helical rod domain, flanked by nonhelical head and tail domains. The IF rod is more highly conserved than head and tail, and within the rod, the carboxy R/K L L E G E sequence is more highly conserved than most other regions. Mutagenesis studies have shed some light on the roles of the head, tail, and R/K L L E G E sequence in 10-nm filament structure. However, interpretations have often been complicated in part because many of these studies have focused on transfected cells, where filament structure cannot be evaluated. Of the few in vitro assembly studies thus far conducted, comparison of keratin mutants with other IF mutants have often been difficult, due to the obligatory heteropolymeric nature of keratin IFs. In this report, we describe in vitro filament assembly studies on headless, tailless, headless/tailless, and R/K L L E G E truncated mutants of keratin 5 and its partner keratin 14. Using varying conditions of ionic strength and pH, we examine effects of analogous K5 and K14 mutations on the stability of 10-nm filament structure. Using EM, we examine effects of mutations on the ability of subunits/protofibrils to (a) elongate and (b) laterally associate. Our results demonstrate that (a) tails of K5 and K14 are required for filament stabilization; (b) the head of K5, but not of K14, is required for filament elongation and lateral alignments; and (c) the R/K L L E G E domains are required for lateral alignments, but not for filament elongation.

Do the ends justify the mean? Proline mutations at the ends of the keratin coiled-coil rod segment are more disruptive than internal mutations

Intermediate filament (IF) assembly is remarkable, in that it appears to be self-driven by the primary sequence of IF proteins, a family (40- 220 kd) with diverse sequences, but similar secondary structures. Each IF polypeptide has a central 310 amino acid residue alpha-helical rod domain, involved in coiled-coil dinner formation. Two short (approximately 10 amino acid residue) stretches at the ends of this rod are more highly conserved than the rest, although the molecular basis for this is unknown. In addition, the rod is segmented by three short nonhelical linkers of conserved location, but not sequence. To examine the degree to which different conserved helical and nonhelical rod sequences contribute to dimer, tetramer, and higher ordered interactions, we introduced proline mutations in residues throughout the rod of a type I keratin, and we removed existing proline residues from the linker regions. To further probe the role of the rod ends, we introduced more subtle mutations near the COOH-terminus. We examined the consequences of these mutations on (a) IF network formation in vivo, and (b) 10-nm filament assembly in vitro. Surprisingly, all proline mutations located deep in the coiled-coil rod segment showed rather modest effects on filament network formation and 10-nm filament assembly. In addition, removing the existing proline residues was without apparent effect in vivo, and in vitro, these mutants assembled into 10-nm filaments with a tendency to aggregate, but with otherwise normal appearance. The most striking effects on filament network formation and IF assembly were observed with mutations at the very ends of the rod. These data indicate that sequences throughout the rod are not equal with respect to their role in filament network formation and in 10-nm filament assembly. Specifically, while the internal rod segments seem able to tolerate considerable changes in alpha-helical conformation, the conserved ends seem to be essential for creating a very specific structure, in which even small perturbations can lead to loss of IF stability and disruption of normal cellular interactions. These findings have important implications for the disease Epidermolysis Bullosa Simplex, arising from point mutations in keratins K5 or K14.

Ultrastructural changes in acquired perforating dermatosis

We performed ultrastructural studies of skin lesions in seven adults with acquired perforating dermatosis. Three of the patients had diabetes mellitus and two were undergoing hemodialysis. Lesions in an early stage showed exocytosis of inflammatory cells and alteration of elastic fibers. Lesions in an intermediate stage featured discontinuities of the basement membrane and aggregates of electron-dense material lateral to the perforated focus, together with dermal edema, scattered macrophages, and densely aggregated collagen fibers that focally filled the papillary dermis. Later-stage lesions showed fibroblasts in the dermis and degenerated elastic fibers within transepidermal channels. In most cases there was a single large epidermal channel lined by flattened epithelial cells, and containing a variety of cellular and extracellular materials. Small "secondary" channels without abnormal keratinization were also observed within the epidermis. The findings suggest that altered keratinization is limited to the immediate vicinity of well-formed transepidermal channels, and that exocytosis of inflammatory cells and alterations of elastica are early and possibly key changes in lesion development. The unexpected discovery of hair fragments in one case suggests that curled hairs may play a role in the pathogenesis of some cases of acquired perforating dermatosis.

A synthetic peptide representing the consensus sequence motif at the carboxy-terminal end of the rod domain inhibits intermediate filament assembly and disassembles preformed filaments

All intermediate filament (IF) proteins share a highly conserved sequence motif at the COOH-terminal end of their rod domains. We have studied the influence of a 20-residue peptide, representing the consensus motif on filament formation and stability. Addition of the peptide at a 10-20-fold molar excess over keratins K8 plus K18 had a severe effect on subsequent IF assembly. Filaments displayed a rough surface and variable diameters with a substantial amount present in unravelled form. At higher peptide concentration (50-100-fold molar excess), IF formation was completely inhibited and instead only loose aggregates of "globular" particles were formed. The peptide also influenced performed keratin IF in a dose-dependent manner. While a three-fold molar excess was sufficient to cause partial fragmentation of IF, a 50-fold molar excess caused complete disassembly within 5 min. Loosely associated protofibrils, short needlelike IF fragments, and aggregates of globular particles were detected. The motif peptide also caused the disassembly of filaments formed by desmin, a type III IF protein. Peptide concentrations and incubation times required for complete disassembly were somewhat higher than for the filaments containing K8 plus K18. A 50-fold molar excess was sufficient to cause complete disassembly within 1 h. Peptides unrelated in sequence to the motif did not interfere with filament formation or stability even when present for more than 12 h at a 100-fold molar excess. The results suggest that the motif sequence normally binds to a specific acceptor site for which the motif peptide can successfully compete. Taken together with current models of IF structure the results indicate that normal binding of the motif sequence to its acceptor must play an essential role in IF formation, possibly by directing the proper alignment of neighboring tetramers or protofilaments. Finally we show that in vitro formed IF are much more sensitive and dynamic strutures than previously thought.

[Biophysical characterization of the stratum corneum. Relationship between structure and properties]

Studies carried out over the last ten years have determined the structure and chemical composition of the stratum corneum and their significance for biologic function; the stratum corneum (SC) is composed of flat layers linked to each other by desmosome plates. Intercellular spaces are composed mainly of double lipid layers which are responsible for the main part of the barrier function of the SC. This barrier between the body and the environment must be effective despite the deformations and various insults (chemical, physicochemical) to which the SC is subjected. The SC fulfills its barrier function through remarkable physical properties of which most originate in an ability to bind water molecules to protein sites whose presence depends on the humidity of the environment in which the SC was produced. Lastly, the SC exhibits some enzyme activities and consequently can no longer be considered as an inert membrane.

[Changes in the cuticle of a keratin fiber under the action of a low-temperature plasma]

Oxygen and air low-temperature plasma treatment leads to significant changes in fibre cuticle cell membrane (skin flakes). Both lipids and proteins were destroyed. The processes of intensive lipid oxidation resulted from low-temperature plasma action. This factor seems to change critical surface tension, wetting and increasing penetration of dyes through the cuticle.

An immunofluorescence study of the effects of ultraviolet radiation on the organization of microfilaments, keratin intermediate filaments, and microtubules in human keratinocytes

Indirect immunofluorescence microscopy has been used to investigate the ultraviolet (UV) radiation induced disruption of the organization of microfilaments, keratin intermediate filaments, and microtubules in cultured human epidermal keratinocytes. Following irradiation, concurrent changes in the organization of the three major cytoskeletal components were observed in cells incubated under low Ca2+ (0.15 mM) conditions. UV irradiation induced a dose-dependent condensation of keratin filaments into the perinuclear region. This collapse of the keratin network was accompanied by the reorganization of microfilaments into rings and a restricted distribution of microtubules, responses normally elicited by exposure to high Ca2+ (1.05 mM) medium. The UV induced alteration of the keratin network appears to disrupt the interactions between keratin and actin, permitting the reorganization of actin filaments in the absence of Ca2+ stimulation. In addition to the perinuclear condensation of keratin filaments, UV irradiation inhibits the Ca2+ induced formation of keratin alignments at the membrane of apposed cells if UV treatment precedes exposure to high Ca2+ medium. Incubation of keratinocytes in high Ca2+ medium for 24 hours prior to irradiation results in the stabilization of membrane associated keratin alignments and a reduced susceptibility of cytoplasmic keratin filaments to UV induced disruption. Unlike results from investigations with isogenic skin fibroblasts, no UV induced disassembly of microtubules was discernible in irradiated human keratinocytes.

Cytoskeletal changes in hepatocytes induced by Microcystis toxins and their relation to hyperphosphorylation of cell proteins

The heptapeptide toxins produced by the blue-green alga (cyanobacterium) Microcystis aeruginosa are selectively hepatotoxic in mammals. The characteristic post-mortem pathology of the liver is extensive lobular disruption due to sinusoidal breakdown, leakage of blood into the tissue and hepatocyte disintegration. Isolated hepatocytes incubated with toxin show severe structural deformity and surface blebbing. This paper demonstrates the effects of Microcystis toxins on the contraction and aggregation of actin microfilaments, and on the relocation and breakdown of cytokeratin intermediate filaments, in cultured hepatocytes. Earlier work did not show changes in the assembly/disassembly of actin; however, this paper demonstrates the change in cytokeratin from intermediate filaments to distributed granules in the cytoplasm of toxin-affected cells. Acrylamide gel electrophoresis of cytoskeletal fractions from hepatocytes did not show changes in total cytokeratins; however, marked changes in the immunogenicity of cytokeratins at 52 and 58 kDa were seen on toxin exposure of cells. Measurement of 32P-phosphorylation of proteins in toxin-affected cells incubated with [32P]orthophosphate showed a dramatic increase compared to control incubations. This is in agreement with research elsewhere describing phosphatase inhibition in vitro by Microcystis toxins. The data indicate that phosphorylated cytokeratin is a major component of cytoplasmic fraction phosphorylated protein after toxin exposure to hepatocytes. It is concluded that the mechanism of Microcystis toxicity to the hepatocyte is through cytoskeletal damage leading to loss of cell morphology, cell to cell adhesion and finally cellular necrosis. The underlying biochemical lesion is likely to be phosphatase inhibition causing hyperphosphorylation of a number of hepatocyte proteins, including those cytokeratins responsible for microfilament orientation and intermediate filament integrity.

A function for keratins and a common thread among different types of epidermolysis bullosa simplex diseases

Previously we demonstrated that transgenic mice expressing a mutant keratin in the basal layer of their stratified squamous epithelia exhibited a phenotype bearing resemblance to a subclass (Dowling Meara) of a heterogeneous group of human skin disorders known as epidermolysis bullosa simplex (EBS) (Vassar, R., P. A. Coulombe, L. Degenstein, K. Albers, E. Fuchs. 1991. Cell. 64:365-380.). The extent to which subtypes of EBS diseases might be genetically related is unknown, although they all exhibit skin blistering as a consequence of basal cell cytolysis. We have now examined transgenic mice expressing a range of keratin mutants which perturb keratin filament assembly to varying degrees. We have generated phenotypes which include most subtypes of EBS, demonstrating for the first time that at least in mice, these diseases can be generated by different mutations within a single gene. A strong correlation existed between the severity of the disease and the extent to which the keratin filament network was disrupted, implicating perturbations in keratin networks as an essential component of these diseases. Some keratin mutants elicited subtle perturbations, with no signs of the tonofilament clumping typical of Dowling-Meara EBS and our previous transgenic mice. Importantly, basal cell cytolysis still occurred, thereby uncoupling cytolysis from the generation of large, insoluble cytoplasmic protein aggregates. Moreover, cell rupture occurred in a narrowly defined subnuclear zone, and seemed to involve three factors: (a) filament perturbation, (b) the columnar shape of the basal cell, and (c) physical trauma. This work provides the best evidence to date for a structural function of a cytoplasmic intermediate filament network, namely to impart mechanical integrity to the cell in the context of its tissue.

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.

Epidermolysis bullosa simplex (Dowling-Meara type) is a genetic disease characterized by an abnormal keratin-filament network involving keratins K5 and K14

The distribution and morphology of tonofilament (TF) clumps were examined by light and electron microscopy in skin samples from a total of 17 patients with the Dowling-Meara (DM) form of epidermolysis bullosa simplex (EBS). TF clumps extending from the basal to the upper-spinous epidermal layer were seen in all lesional skin samples and in the majority of peri-lesional and non-lesional skin samples. TF clumps were also noted in adnexal epithelia, including outer hair root sheaths, sweat ducts, and sebaceous glands. Cultured keratinocytes from two patients also demonstrated characteristic TF clumps. All these epithelial cells have in common their expression of the keratin pair K5 and K14. Post-embedding immunogold electron microscopy using antibodies to K5, K14, and K10 showed similar expressed keratins in DM-EBS skin from four patients compared with normal skin, with K5 and K14 predominantly in the basal cell layer and K10 in the suprabasal layers. The clumped TF in DM-EBS samples were labeled strongly with anti-K5 and K14 antibodies in the basal and suprabasal layers. In contrast, the suprabasal clumps were only slightly reactive with anti-K10 antibodies and labeling was usually restricted to the periphery of the clumps. We conclude that DM-EBS is associated with an intrinsic abnormality of the keratin-filament network involving the K5 and K14 pair that is likely to result in impaired resistance of basal epidermal cells to external shearing forces, leading to the characteristic intraepidermal blisters. DM-EBS may become the first genetic skin disease to be recognized as having a specific keratin abnormality.

Organization of coiled-coil molecules in native mouse keratin 1/keratin 10 intermediate filaments: evidence for alternating rows of antiparallel in-register and antiparallel staggered molecules

There is considerable diversity of opinion in the literature concerning the organization of two-chain coiled-coil molecules in intermediate filaments. I have reexplored this issue using the limited proteolysis paradigm with native mouse epidermal keratin intermediate filaments (KIF), consisting of keratins 1 and 10. KIF were harvested as cytoskeletal pellets, dissociated into subfilamentous forms at pH 9.8, 9.0, or 2.6, and were subjected to limited proteolytic digestion to recover alpha-helix-enriched particles that derived from the rod domains of the constituent chains, using conditions that do not promote reorganization of the constituent protein chains or coiled-coil molecules. The multichain particles were subjected to physicochemical analyses, amino acid sequencing, and electron microscopy in order to determine their composition, structure, and organization within the intact KIF. The results predict two principal modes of alignment: neighboring molecules may be aligned in register and antiparallel or staggered and antiparallel. From known structural constraints, this permits construction of a two-dimensional surface lattice for KIF which consists of alternating antiparallel rows of in-register and staggered molecules. These data establish the level of hierarchy at which the well-known antiparallelity and staggered features of KIF are introduced. This model supports the proposals of KIF structure based on theoretical considerations of ionic interactions scores (Crewther et al., 1983). When the KIF are dissociated at extremes of pH, this structural model allows for disruption along alternate axes; the in-register antiparallel alignment is seen only when KIF are dissociated at high pH values; below pH 9, only the staggered antiparallel alignment is seen. The process of molecule realignment especially in concentrated urea solutions indicates that the staggered antiparallel alignment is the more thermodynamically stable form in solution.

Analysis of the mechanism of assembly of mouse keratin 1/keratin 10 intermediate filaments in vitro suggests that intermediate filaments are built from multiple oligomeric units rather than a unique tetrameric building block

The question as to whether keratin intermediate filaments (KIF) are built from a unique "building block" consisting of a pair of coiled-coil molecules has been studied by examining the earliest stages of reassembly of mouse K1/K10 KIF in vitro. Particles formed in protein solutions of about 45 micrograms/ml (near or below the critical concentration for assembly) or 0.5-1.65 mg/ml were monitored by turbidity, visualized by electron microscopy, and their structures resolved biochemically using crosslinking, limited proteolysis, and amino acid sequencing. The rate of KIF reassembly in vitro is limited by an initial slow step involving the formation of a three- or four-molecule oligomer. At 2 min, the particles in solution are about 65 nm long and consist of two molecules aligned antiparallel and staggered. A few minutes later, a three- and/or four-molecule species appears that may be the rate-limiting particle(s). It is also 65 nm long, but contains one or two additional molecules aligned in register but antiparallel with respect to one of the molecules on the two-molecule particle. The present data cannot establish whether the rate-limiting particle contains three or four molecules, or in fact consists of a mixture of both. Below the critical concentration for KIF assembly, it exists in solution in rapid exchange with particles containing one and two molecules. In solutions above the critical concentration for assembly, once this oligomer has formed in sufficient quantity, further assembly into KIF occurs rapidly; 90, 110, and 130-nm particles soon appear by apparent addition of a single molecule or oligomers containing two, three, four, or even several molecules. Within about 20 min short KIF about 200-500 nm long appear which later elongate to long (greater than 1 micron) KIF. These data suggest that KIF assembly requires the initial correct alignment of three or four molecules which, once formed, provides a template for further rapid addition of molecules leading to KIF assembly. Furthermore, the data establish that KIF are built from alternating rows of in-register and staggered antiparallel molecules. The present data confirm independently the observations of the previous paper and do not support earlier notions that IF are built from a tetrameric building block consisting of a pair of in-register molecules. Finally, the data suggest that the mechanism of assembly in vitro and the dynamic in vivo assembly-disassembly characteristics of KIF in particular and IF in general are mediated through a variety of small oligomeric species ranging in size from one to several molecules.

Cytokeratin phosphorylation, cytokeratin filament severing and the solubilization of the maternal mRNA Vg1

During meiotic maturation, the cortical cytokeratin filament system of the Xenopus oocyte disappears (Klymkowsky, M. W., and L. A. Maynell. 1989. Dev. Biol. 134:479). Here we demonstrate that this disappearance results from the severing of cytokeratin filaments into a heterogenous population of oligomers, with S- values ranging from 12S and greater. Cytokeratin filament severing correlates with the hyperphosphorylation of the type II cytokeratin of the oocyte. Both the severing of cytokeratin filaments and cytokeratin hyperphosphorylation are reversed by treatment with cycloheximide. These data suggest that fragmentation of cytokeratin filaments is controlled, at least in part, by the phosphorylation of the type II cytokeratin, and that the cytokeratin kinase activity responsible is biosynthetically labile. Cytokeratin filaments have been suggested to anchor the maternal mRNA Vg1 to the vegetal cortex of the oocyte (Pondel, M., and M. L. King. 1988. Proc. Natl. Acad. Sci. USA. 85:7216). By injecting fractions containing active maturation promoting factor or a purified, mutant cyclin protein, we find that the bulk of the Vg1 mRNA in the oocyte can be solubilized under conditions that block the fragmentation of cytokeratin filaments, and that the fragmentation of cytokeratin filaments itself leads to the solubilization of only a minor fraction of the Vg1 mRNA. Thus, at best, cytokeratin filaments directly anchor only a minor fraction of the Vg1 mRNA in the oocyte. Moreover, factors distinct from maturation promoting factor appear to be required for the complete solubilization of Vg1 mRNA during oocyte maturation.

The ultrastructure of the transition zone between specialized cells ("Flügelzellen") of Huxley's layer of the inner root sheath and cells of the outer root sheath of the human hair follicle

We studied biopsy specimens taken from the scalp of four normal volunteers by transmission electron microscopy (TEM). At the suprabulbous level of the follicle, where the cells of Henle's layer are fully keratinized but the cells of Huxley's layer are not, the cytoplasmic processes of Huxley's layer ("Flügelzellen") reach the cells of the outer root sheath through the keratinized cylinder of Henle's layer. This junction between the outer root sheath cells and the "Flügelzellen" has not been previously characterized ultrastructurally. It is peculiar in its total lack of desmosomes, and it may have the features of a gap junction. The function of this structure and its possible role in hair growth is unknown. It may be related to nutrition, differentiation, or both.

Sorting out IF networks: consequences of domain swapping on IF recognition and assembly

Vimentin and keratin are coexpressed in many cells, but they segregate into two distinct intermediate filament (IF) networks. To understand the molecular basis for the sorting out of these IF subunits, we genetically engineered cDNAs encoding hybrid IF proteins composed of part vimentin and part type I keratin. When these cDNAs were transiently expressed in cells containing vimentin, keratin, or both IFs, the hybrid IF proteins all recognized one or the other or both networks. The ability to distinguish networks was dependent upon which segments of IF proteins were present in each construct. Constructs containing sequences encoding either helix 1B or helix 2B seemed to be the most critical in conferring IF recognition. At least for type I keratins, recognition was exerted at the level of dimer formation with wild-type type II keratin, as demonstrated by anion exchange chromatography. Interestingly, despite the fact that swapping of helical domains was not as deleterious to IF structure/function as deletion of helical domains, keratin/vimentin hybrids still caused structural aberrations in one or more of the cytoplasmic IF network. Thus, sequence diversity among IF proteins seems to influence not only coiled-coil but also higher ordered associations leading to 10-nm filament formation and/or IF interactions with other cellular organelles/proteins.

[Immunohistochemical observation on keratin filaments of cultured tumor cells by ABC staining]

Avidin-Biotin Peroxidase complex technique, ABV staining, was employed by using monoclonal anti-keratin antibody HK2 in this study. The organization and dynamics of keratins in both interphase and mitotic T56 and HeLa cells were analysed. We also observed the effects of microtubule (MT) and microfilament (MF) inhibitors, colchicine and cytochalasin B, on the organization of keratin filaments in T56 and HeLa cells. The results showed that a significant alteration in the structural organization and distribution of keratin filaments occurred during mitosis, and an extensive rearrangement of keratin networks of the two cell lines was induced in interphase after the MT and MF were disrupted by combined treatment with the two drugs, colchicine and cytochalasin B; the keratin networks turned into a star-like lattice rapidly within 1-2h. Neither colchicine nor cytochalasin B alone elicited significant organizational change in the keratin networks of the two cell lines.

Vacuole formation and cytokeratin rearrangement of hepatoma cells induced by teleocidin are not associated with down-regulation of protein kinase C

PLC/PRF/5 human hepatoma cells cultured with teleocidin reduced the rate of cell proliferation and were transformed into large cells with many vacuole-like subcellular structures. In these vacuolated cells, the protein content per cell increased without changing the total cellular protein synthesis. Cytokeratin was one of the proteins which increased quantitatively. This intermediate filament formed fibrous network structures throughout the enlarged cytoplasm. The assembly of other cytoskeletal proteins such as actin, tubulin, and vimentin was not altered remarkably, suggesting that teleocidin morphologically transformed the hepatoma cells by changing the assembly of cytokeratin protein selectively. On the other hand, the alterations of cell proliferation, cell morphology, and cytokeratin assembly induced by teleocidin were not associated with either down-regulation of protein kinase C or reduced number of epidermal growth factor receptors. In addition, these teleocidin effects were not mimicked by the protein kinase C agonist 1-oleoyl-2-acetylglycerol or inhibited by the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine. From these results it can be speculated that the morphological transformation and reduced cell proliferation induced by teleocidin may be mediated by still unknown mechanisms unrelated to protein kinase C.

Biological applications of photoelectron imaging: a practical perspective

Photoelectron imaging is finding a promising niche in the study of biological specimens. The features of photoelectron imaging that contribute to its uniqueness for this application are described. Image formation and the major contrast mechanisms of photoelectron microscopy, material contrast and topographical contrast are reviewed and illustrated with examples of photoelectron images of cultured cells and of DNA. General considerations in sample choice and preparation are also presented. Strategies for photoelectron labeling are discussed including the use of immunogold labeling, silver enhancement and cesium-based photocathodes.

Phosphorylation of keratin intermediate filaments by protein kinase C, by calmodulin-dependent protein kinase and by cAMP-dependent protein kinase

Keratins, constituent proteins of intermediate filaments of epithelial cells, are phosphoproteins containing phosphoserine and phosphothreonine. We examined the in vitro phosphorylation of keratin filaments by cAMP-dependent protein kinase, protein kinase C and Ca2+/calmodulin-dependent protein kinase II. When rat liver keratin filaments reconstituted by type I keratin 18 (molecular mass 47 kDa; acidic type) and type II keratin 8 (molecular mass 55 kDa; basic type) in a 1:1 ratio were used as substrates, all the protein kinases phosphorylated both of the constituent proteins to a significant rate and extent, and disassembly of the keratin filament structure occurred. Kinetic analysis suggested that all these protein kinases preferentially phosphorylate keratin 8, compared to keratin 18. The amino acid residues of keratins 8 and 18 phosphorylated by cAMP-dependent protein kinase or protein kinase C were almost exclusively serine, while those phosphorylated by Ca2+/calmodulin-dependent protein kinase II were serine and threonine. Peptide mapping analysis indicated that these protein kinases phosphorylate keratins 8 and 18 in a different manner. These observations gave the way for in vivo studies of the role of phosphorylation in the reorganization of keratin filaments.

Palmoplantar keratoderma with tonotubular keratin

A 61-year-old man with palmoplantar keratoderma with an unusual tonotubular keratin is reported. The histologic findings, genetic transmission, and clinical course were similar to epidermolytic palmoplantar keratoderma (Voerner type), but keratinocytes ultrastructurally displayed a tonotubular cytoskeleton, which has not been previously described, instead of a tonofilamentous one. Electrophoretically, we found no difference in the keratin pattern of normal plantar skin, skin in palmoplantar keratoderma of Voerner, and that of our patient's skin. Therefore the tubular keratin most likely formed as a result of a posttranslational change of keratin polymerization.

Structure and biochemistry of mammalian hard keratin

In this review, the structure and biological formation of hard alpha-keratin are drawn together. The hard keratins comprising wool, hairs, quills, hooves, horns, nails and baleen contain partly alpha-helical polypeptides which show homology with epidermal polypeptides only in the helical regions. These polypeptides (about 32 chains) are organized into intermediate filaments (IFs) of 7.5 nm diameter which are embedded in variable amounts of a matrix of non-helical cystine-rich proteins and glycine-tyrosine-rich proteins. The total number of proteins may exceed 100. In addition keratins contain a variety of lipid components. Wool and hair are produced in follicles in a multistep procedure. In the lower levels of the follicle, IFs without associated matrix are found. Subsequently matrix proteins are laid down between the IFs and further synthesis takes place concurrently. Finally the proteins are insolubilized by the oxidative formation of disulphide bonds. Keratinized fibres shows considerable complexity and diversity in the structural arrangement of IFs and matrix within cortical cells. Typically the IFs show hexagonal packing or give a whorl-like appearance in cross-section.

Cytokeratin patterns of human oral mucosae in histiotypic culture

In a three-dimensional culture model, oral epithelial differentiation was investigated ultrastructurally and biochemically for cytokeratin expression. Epithelia from the hard palate, gingiva and alveolar mucosa grown on freely floating collagen lattices populated with fibroblasts from homotypic origins, and fed with medium containing 10% delipidized fetal calf serum for 21 days before analysis, stratified and differentiated to basal cuboidal cells, polyhydral spinous cells and elongated superficial cells. The epithelium of palatal origin had non-nucleated superficial cells resembling orthokeratinized cells. The upper spinous cells had keratohyalin-like granules. The corresponding cells of gingival and alveolar mucosal origins retained their nuclei and had smaller numbers of keratohyalin-like granules. Basal cell keratins (CK 5 and 14) and those of hyperproliferation (CK 6 and 16) were consistently found in all epithelia. Furthermore, simple epithelial keratins (CK 18 and 19) were variably expressed by cells from different oral origins. In epithelial cells from the alveolar mucosa, CK 13 and 19 formed major bands, which correlates with their expression in vivo. In contrast, these polypeptides were either absent or formed minor bands in extracts of gingival and hard palatal cells. Although in small quantities, keratins of terminal differentiation (CK 1, 2, 10 and 11) were detected in gels prepared from palatal epithelia. This expression correlates with the higher morphological differentiation of these cells in this model. The model is of interest for studies of epithelial differentiation, as the differentiation markers of keratinized epithelia (CK 1 and 10) were expressed by cells from palatal origin, and those of non-keratinized epithelia (CK 4, 13 and 19) were prominent in cells from alveolar mucosal origin.

[A study of cytokeratin in human normal bladder epithelium and bladder carcinoma cell lines]

Normal epithelial and carcinoma cells of human bladder were investigated for the cytokeratin which is one of the intermediate filaments and comprises cytoskeleton using the immunofluorescence method. Carcinoma cell lines used were JTC-30, JTC-32, HUB-41 and T-24. In normal urothelium, keratin fibers were fine and straight with unchanged diameter and distributed regularly in the cytoplasm. By contrast, keratin fibers in bladder carcinoma cells were kinked and changed in diameter and were distributed irregularly in the cytoplasm. The above findings were most obvious in T-24 which formed undifferentiated carcinoma when transplanted to nude mice, and keratin fibers were dominantly located in the perinuclear area. The changes of keratin fibers appeared to be parallel to the grade of histological anaplasia of the tumor formed by implantation of bladder carcinoma cell line cells to nude mice. These observations suggest that the morphology of cytokeratin is a useful indicator for evaluating the grade of malignancy in transitional cell carcinoma.

Ultrastructural characterization of two new human endometrial carcinoma cell lines and normal human endometrial epithelial cells cultured on extracellular matrix

Two new lines of human endometrial carcinoma (HEC) cells, one from an adenocarcinoma and one from a highly metastatic serous papillary carcinoma, were established in culture. Structural and morphologic properties of these cells at early passage were compared with those of cultured normal human endometrial epithelial (NHEE) cells. For these studies, cells were grown on a conventional plastic surface or on an extracellular matrix substrate (Matrigel), and examined by transmission electron microscopy and immunofluorescent light microscopy. The HEC cells appeared morphologically similar on plastic and Matrigel, whereas the NHEE cells showed significantly greater epithelial morphologic differentiation on Matrigel than on plastic. On extracellular matrix, the morphologic differences observed between HEC cells and NHEE cells were primarily of an architectural nature, which may be in part explained by differences between NHEE and HEC cells in the arrangement of actin microfilaments and cytokeratin intermediate filaments. Furthermore, HEC cells displayed extensive networks of vimentin intermediate filaments, which were absent from the NHEE cells. These observations support the hypothesis that architectural deregulation is a prominent feature of endometrial carcinoma, and that cytoskeletal alterations may uncouple HEC cell ultrastructural morphology from the influence of extracellular matrix.

Elucidating the early stages of keratin filament assembly

Because of extraordinarily tight coiled-coil associations of type I and type II keratins, the composition and structure of keratin subunits has been difficult to determine. We report here the use of novel genetic and biochemical methods to explore the early stages of keratin filament assembly. Using bacterially expressed humans K5 and K14, we show that remarkably, these keratins behave as 1:1 complexes even in 9 M urea and in the presence of a reducing agent. Gel filtration chromatography and chemical cross-linking were used to identify heterodimers and heterotetramers as the most stable building blocks of keratin filament assembly. EM suggested that the dimer consists of a coiled-coil of K5 and K14 aligned in register and in parallel fashion, and the tetramer consists of two dimers in antiparallel fashion, without polarity. In 4 M urea, both end-to-end and lateral packing of tetramers occurred, leading to a variety of larger heteromeric complexes. The coexistence of multiple, higher-ordered associations under strongly denaturing conditions suggests that there may not be a serial sequence of events leading to the assembly of keratin intermediate filaments, but rather a number of associations may take place in parallel.

The two-chain coiled-coil molecule of native epidermal keratin intermediate filaments is a type I-type II heterodimer

The composition of the two-chain coiled-coil molecule of murine epidermal keratin intermediate filaments (KIF) containing keratins 1 (type II) and 10 (type I) has been explored using native-type KIF as well as KIF reassembled in vitro from protein dissolved in urea solutions or from mixtures of 3H-labeled and unlabeled purified chains. By use of cross-linking, high resolution polyacrylamide gel electrophoresis and blotting for 3H-labeled keratins or with an anti-mouse keratin 10 antibody, it was found that individual keratin chains form type I or type II homodimers and homotetramers in solution that do not assemble into KIF in vitro. When mixed in urea solutions of 5 M or greater, such homo-oligomers rapidly rearrange into mostly heterodimers and heterotetramers that support filament assembly. On the other hand, prekeratin, isolated from newborn mouse epidermis with 0.1 M sodium citrate buffer, pH 2.6, under conditions that do not dissociate the native coiled-coil molecule, consists exclusively of type I-II heterodimers and heterotetramers. It is necessary to dissolve prekeratin in 8-9.5 M urea for several hours in order to dissociate the native heterodimer molecule and incorporate tracer amounts of a single 3H-labeled keratin chain. These data establish that native KIF consist of heterodimer coiled-coil molecules. Furthermore, heterodimers are much more stable than homodimers and are the favored form of association in solution. However, some homodimers (10-30% of total) always form after dissolution in concentrated urea and can be assimilated into KIF during reassembly in vitro. The isolation of alpha-helix-enriched dimer particles from the 2B region of the rod domains upon limited proteolysis confirmed the presence of mostly heterodimer and some homodimer molecules in reassembled KIF. These properties of keratin chains in urea solutions hereby clarify a number of conflicting reports in the literature concerning the composition of the coiled-coil molecule. The presence of some homo-oligomeric species in reassembled KIF correlates with earlier observations of polymorphism as well as stoichiometry.

Structure and assembly of calf hoof keratin filaments

Keratin filament polypeptides were purified from calf hoof stratum corneum with the aim of studying the in vitro assembly process and determining structural parameters of reconstituted filaments. Anion exchange chromatography was used to obtain the most complete fractionation and identification of the acidic and basic components in the purified polypeptide mixture to date. The reassembly products of the fractionated components were investigated by electron microscopy. Fully reconstituted filaments yield homogeneous solutions, and values of 9.8 nm for the filament diameter and 25 kDa/nm for the mass per unit length (M/L) were obtained by X-ray solution scattering. The structures formed in solution at various stages of filament assembly were not sufficiently homogeneous to be studied by this technique. X-ray diffraction patterns from native stratum corneum display strong maxima at 3.6 and 5.4 nm. Contrary to previous reports, these maxima do not appear to be due to lipids since they are also observed with delipidated rehydrated specimens. A series of weak maxima is also detected in the patterns of dry tissue. The absence of these features in the patterns of reconstituted filaments suggests that, in contrast to some electron microscopic observations, there are no prominent regularities in the structure of calf hoof keratin filaments.

Immunohistochemical and scanning electron microscopic study of cytokeratin distribution in the collecting tubule of the rat kidney

Cytokeratin distribution in the collecting tubules (CTs) of the rat kidney was studied by immunohistochemistry and scanning electron microscopy (SEM). After preparation of sections from formalin-fixed, paraffin-embedded tissue for immunohistochemistry, the remaining tissue was prepared for SEM. The cut surfaces of the tissue were examined by SEM and compared with sections stained with anti-cytokeratin antibody. The immunostained sections revealed positive staining along the entire CTs. However, in addition to diffusely stained cells, unstained and partially stained cells were seen. The latter were not distributed in inner medullary CTs, whereas the diffusely stained cells were observed in cortical, outer medullary and inner medullary CTs. This immunohistochemical heterogeneity of cytokeratin reactivity was prominent in the outer medullary and cortical CTs. From this comparative study of immunostained sections and SEM specimens, it was concluded that the heterogeneously stained cells correspond to intercalated cells, whereas the diffusely stained cells represent most principal cells. These results suggest that the difference in cytokeratin density among CT cells may represent different functional states, at least in intercalated cells.

Differential extraction of keratin subunits and filaments from normal human epidermis

We have investigated keratin interactions in vivo by sequentially extracting water-insoluble proteins from normal human epidermis with increasing concentrations of urea (2, 4, 6, and 9.5 M) and examining each extract by one- and two-dimensional gel electrophoresis, immunoblot analysis using monoclonal anti-keratin antibodies, and EM. The viable layers of normal human epidermis contain keratins K1, K2, K5, K10/11, K14, and K15, which are sequentially expressed during the course of epidermal differentiation. Only keratins K5, K14, and K15, which are synthesized by epidermal basal cells, were solubilized in 2 M urea. Extraction of keratins K1, K2, and K10/11, which are expressed only in differentiating suprabasal cells, required 4-6 M urea. Negative staining of the 2-M urea extract revealed predominantly keratin filament subunits, whereas abundant intermediate-sized filaments were observed in the 4-urea and 6-M urea extracts. These results indicate that in normal human epidermis, keratins K5, K14, and K15 are more soluble than the differentiation-specific keratins K1, K2, and K10/11. This finding suggests that native keratin filaments of different polypeptide composition have differing properties, despite their similar morphology. Furthermore, the observation of stable filaments in 4 and 6 M urea suggests that epidermal keratins K1, K2, and K10/11, which ultimately form the bulk of the protective, nonviable stratum corneum, may comprise filaments that are unusually resistant to denaturation.

The coiled coil of in vitro assembled keratin filaments is a heterodimer of type I and II keratins: use of site-specific mutagenesis and recombinant protein expression

Recombinant DNA technology has been used to analyze the first step in keratin intermediate filament (IF) assembly; i.e., the formation of the double stranded coiled coil. Keratins 8 and 18, lacking cysteine, were subjected to site specific in vitro mutagenesis to change one amino acid in the same relative position of the alpha-helical rod domain of both keratins to a cysteine. The mutations lie at position -36 of the rod in a "d" position of the heptad repeat pattern, and thus air oxidation can introduce a zero-length cystine cross-link. Mutant keratins 8 and 18 purified separately from Escherichia coli readily formed cystine homodimers in 2 M guanidine-HCl, and could be separated from the monomers by gel filtration. Heterodimers with a cystine cross-link were obtained when filaments formed by the two reduced monomers were allowed to oxidize. Subsequent ion exchange chromatography in 8.5 M urea showed that only a single dimer species had formed. Diagonal electrophoresis and reverse phase HPLC identified the dimer as the cystine containing heterodimer. This heterodimer readily assembled again into IF indistinguishable from those obtained from the nonmutant counterparts or from authentic keratins. In contrast, the mixture of cystine-stabilized homodimers formed only large aberrant aggregates. However, when a reducing agent was added, filaments formed again and yielded the heterodimer after oxidation. Thus, the obligatory heteropolymer step in keratin IF assembly seems to occur preferentially at the dimer level and not during tetramer formation. Our results also suggest that keratin I and II homodimers, once formed, are at least in 2 M guanidine-HCl a metastable species as their mixtures convert spontaneously into heterodimers unless the homodimers are stabilized by the cystine cross-link. This previously unexpected property of homodimers explains major discrepancies in the literature on the keratin dimer.

Keratin filaments restrict organelle migration into the forming spindle of newt pneumocytes

When viewed by light microscopy the mitotic spindle of newt pneumocytes appears to assemble in an optically clear area of cytoplasm, virtually devoid of mitochondria and other organelles, which is often much larger than the spindle. This clear area is also frequently larger than the region previously occupied by the nucleus. It forms even in prometaphase cells depleted of microtubules prior to nuclear envelope breakdown by colchicine treatment. Time-lapse video microscopy reveals that as prometaphase proceeds this clear area slowly and progressively collapses around the forming spindle so that it is greatly diminished or nonexistent by the onset of anaphase. The sharply defined nature of the boundary between the clear area and the remaining cytoplasm and the fact that organelles accumulate at its periphery suggest that a structural barrier is present at the boundary that limits organelle migration into the forming spindle. Immunofluorescence and electron microscopy, of cells previously followed in the living state, reveal that the periphery of the clear area contains prominent bundles of keratin filaments but lacks microtubules and actin. From our observations we conclude that keratin filaments form a loosely organized cage that surrounds the forming newt pneumocyte spindle. We propose that this cage functions, in part, to restrict the dispersion of chromosomes during nuclear envelope breakdown and to impede the bulk migration of organelles into the forming spindle.

Unusual patterns of keratin expression in the overlying epidermis of patients with dermatofibromas: biochemical alterations in the epidermis as a consequence of dermal tumors

Dermatofibromas are frequently associated with acanthosis of the overlying epidermis. Using monospecific antisera and cRNA probes, we have examined the pattern of expression of keratin and keratin mRNA in the affected epidermis of patients with these dermal tumors. Our studies reveal several abnormalities in keratin expression within the thickened areas of overlying epidermis. In two of 15 patients, we detected K6 and K16, keratins which are frequently associated with epidermal diseases of hyperproliferation but are not present in normal epidermis. In both cases, K6 and K16 were found in suprabasal layers, similar to that seen for psoriasis and squamous cell carcinomas. Expression of K6 and K16 in skin samples from patients with dermatofibromas seemed to be dependent upon how near was the tumor to the overlying epidermis, and possibly upon the degree of cellularity within the tumor mass. A second aberration in keratin expression, and one which did not appear to be linked to K6/K16 expression, was the altered expression of the basal epidermal keratin K14. Expression of this keratin and its mRNA was variable, often extending into multiple suprabasal layers and including both basal-like and squamous-like cells. In contrast to the expression of K6/K16, aberrant expression of K14 was a relatively frequent event, occurring in greater than 70% of the dermatofibroma skin samples examined. These observations provide the first biochemical evidence in support of previous morphologic studies, indicating that alterations in epidermal differentiation can occur as a consequence of dermal skin tumors.

Sequence of a human keratin 13 specific cDNA encompassing coil 1B through the 3' end

An expression library established in lambda gt11 with cDNA from squamous epithelium of the human upper digestive tract was screened with an antibody raised against keratin 13. A 1.2 kb fragment from the most strongly reacting plaque was sequenced and compared to known type I keratin sequences. The highest degree of homology was detected with the murine 47K type I keratin, which we consider to be the counterpart of human keratin 13. Tryptic peptides of keratin 13 were separated on a HPLC column and one peptide was sequenced. The amino acid sequence obtained supports the identity of the cDNA. An eight codon motif has been tandemly repeated in the C-domain of keratin 13. In spite of substantial divergence by point mutations and deletions, the remaining sequence homologies suggest that the C-domains of both the human keratin 13 and the orthologous murine protein have originated from a common ancestor.

Immunohistochemical spectrum of rhabdomyosarcoma and rhabdomyosarcoma-like tumors. Expression of cytokeratin and the 68-kD neurofilament protein

Twenty-five rhabdomyosarcomas (RMSs), including 12 alveolar and 13 embryonal types, were immunohistochemically studied for the presence of different classes of intermediate filament proteins and muscle actins (MAs). For the most part, formaldehyde-fixed and paraffin-embedded tissue was used in immunostaining. All RMSs showed desmin and MAs, usually in a major portion of tumor cells. The number of MA-positive cells was sometimes higher than that of desmin-positive cells. Vimentin was present in all tumors studied in frozen sections. Eight of 12 alveolar RMSs showed small number of cytokeratin-positive neoplastic cells. Cytokeratin-positive cells were present less commonly in embryonal RMS (3/13 cases). The 68-kD neurofilament protein was found in frozen sections of two embryonal RMSs. The cytokeratin and neurofilament immunostaining could be reproduced by immunofluorescence technique. In addition, we studied three childhood sarcomas, which showed abundant desmin and MA immunostaining but did not conform to the ultrastructural criteria of RMS. Scattered cytokeratin-positive cells were found in two of these tumors, and neurofilaments were found in the two cases for which frozen sections were available. The results show that typical RMS may demonstrate immunohistological pleomorphism with cytokeratin and neurofilament immunoreactivity suggesting the presence of multidirectional differentiation. In addition, there are tumors that by morphology look like RMS and have muscle cell markers but cannot be verified as RMS by electron microscopy; also, these tumors seem to show immunohistological pleomorphism. The presence of nonmyoid markers in RMS should be considered when making immunohistological diagnosis of soft tissue sarcomas.

Liver cell rosettes: structural differences in cholestasis and hepatitis

Cholestatic and hepatitic liver cell rosettes, gland-like formations found respectively in chronic cholestasis and in chronic active hepatitis, represent structural modifications of liver cell plates in response to injury. Differences in cytokeratin expression, ultrastructure and three-dimensional (3-D) configuration have been investigated. Cholestatic rosettes are considered to be a form of biliary metaplasia of hepatocytes, linking with newly-formed bile ductules in adjacent septa and probably providing some protection from injury caused by abnormal bile constituents. Hepatitis rosettes, by contrast, are a form of liver cell regeneration developing in isolated surviving hepatocytes or small groups of hepatocytes within areas of collapse.