Differential location of different types of intermediate-sized filaments in various tissues of the chicken embryo

Immunolocalization of epidermal differentiation complex proteins reveals distinct molecular compositions of cells that control structure and mechanical properties of avian skin appendages

The epidermal differentiation complex (EDC) is a cluster of genes that encode structural proteins of skin derivatives with variable mechanical performances, from the scales of reptiles and birds to the hard claws and beaks, and to the flexible but resistant corneous material of feathers. Corneous proteins with or without extended beta-regions are produced from avian genomes, and include the largely prevalent corneous beta proteins (CβPs, formerly indicated as beta-keratins), and minor contribution from histidine-rich proteins, trichohyalin-like proteins (scaffoldin), loricrin, and other proteins rich in cysteine or other types of amino acids. The light-microscopic and ultrastructural immunolocalization of major and minor EDC-proteins in avian skin (feather CβPs, EDKM, EDWM, EDMTFH, EDDM, and scaffoldin) suggests that each specific appendage consists of a particular mix of these proteins in addition to the main proteins containing a peculiar beta-region of 34 amino acids, indicated as feather/scale/claw/beak CβPs (fCβPs, sCβPs, cCβPs, bCβPs). This indicates that numerous proteins of the EDC are added to the variable meshwork of intermediate filament keratins to produce avian epidermis with different mechanical and functional properties. Although the specific roles for these proteins are not known they likely make an important contribution to the final material properties of the different skin appendages of birds. The highest number of sauropsid CβPs is found in birds, suggesting a relation to the evolution of feathers, and additional epidermal differentiation proteins have contributed to the evolutionary adaptations of avian skin.

Keratin 18 is an integral part of the intermediate filament network in murine skeletal muscle

Intermediate filaments (IFs) contribute to force transmission, cellular integrity, and signaling in skeletal muscle. We previously identified keratin 19 (Krt19) as a muscle IF protein. We now report the presence of a second type I muscle keratin, Krt18. Krt18 mRNA levels are about half those for Krt19 and only 1:1,000th those for desmin; the protein was nevertheless detectable in immunoblots. Muscle function, measured by maximal isometric force in vivo, was moderately compromised in Krt18-knockout (Krt18-KO) or dominant-negative mutant mice (Krt18 DN), but structure was unaltered. Exogenous Krt18, introduced by electroporation, was localized in a reticulum around the contractile apparatus in wild-type muscle and to a lesser extent in muscle lacking Krt19 or desmin or both proteins. Exogenous Krt19, which was either reticular or aggregated in controls, became reticular more frequently in Krt19-null than in Krt18-null, desmin-null, or double-null muscles. Desmin was assembled into the reticulum normally in all genotypes. Notably, all three IF proteins appeared in overlapping reticular structures. We assessed the effect of Krt18 on susceptibility to injury in vivo by electroporating siRNA into tibialis anterior (TA) muscles of control and Krt19-KO mice and testing 2 wk later. Results showed a 33% strength deficit (reduction in maximal torque after injury) compared with siRNA-treated controls. Conversely, electroporation of siRNA to Krt19 into Krt18-null TA yielded a strength deficit of 18% after injury compared with controls. Our results suggest that Krt18 plays a complementary role to Krt19 in skeletal muscle in both assembling keratin-based filaments and transducing contractile force.

Role of Intermediate Filaments in Vesicular Traffic

Intermediate filaments are an important component of the cellular cytoskeleton. The first established role attributed to intermediate filaments was the mechanical support to cells. However, it is now clear that intermediate filaments have many different roles affecting a variety of other biological functions, such as the organization of microtubules and microfilaments, the regulation of nuclear structure and activity, the control of cell cycle and the regulation of signal transduction pathways. Furthermore, a number of intermediate filament proteins have been involved in the acquisition of tumorigenic properties. Over the last years, a strong involvement of intermediate filament proteins in the regulation of several aspects of intracellular trafficking has strongly emerged. Here, we review the functions of intermediate filaments proteins focusing mainly on the recent knowledge gained from the discovery that intermediate filaments associate with key proteins of the vesicular membrane transport machinery. In particular, we analyze the current understanding of the contribution of intermediate filaments to the endocytic pathway.

The cardiac isoform of α-actin in regenerating and atrophic skeletal muscle, myopathies and rhabdomyomatous tumors: an immunohistochemical study using monoclonal antibodies

The two sarcomeric isoforms of actins, cardiac and skeletal muscle alpha-actin, are highly homologous so that their immunohistochemical distinction is extremely difficult. Taking advantage of monoclonal antibodies distinguishing the two conservative amino acid exchanges near the aminoterminus, we have performed an extended immunohistochemical analysis of the cardiac alpha-actin (CAA) isoform in normal, regenerating, diseased and neoplastic human muscle tissues. Intense and uniform CAA staining is seen in fetal and adult myocardium and in fetal skeletal muscle while adult skeletal muscle is essentially negative, except for muscle spindle myocytes and a few scattered muscle fibres with overall reduced diameter. By contrast, CAA synthesis is markedly induced in regenerating skeletal muscle cells, in Duchenne muscular dystrophy and upon degenerative atrophy. CAA has also been detected in certain vascular and visceral smooth muscle cells. Among tumors, CAA has consistently been seen in rhabdomyosarcomas and rhabdomyomatous cells of nephroblastomas, whereas, smooth muscle tumors have shown only occasional staining. While the synthesis of this actin isoform is less restricted than previously thought, monoclonal antibodies against CAA provide a well-defined, reliable and sensitive diagnostic tool for the definition and detection of aberrant differentiation in diseased skeletal muscle and of striated muscle differentiation in rhabdomyosarcomas.

Structural and Immunocytochemical Characterization of Keratinization in Vertebrate Epidermis and Epidermal Derivatives

This review presents comparative aspects of epidermal keratinization in vertebrates, with emphasis on the evolution of the stratum corneum in land vertebrates. The epidermis of fish does not contain proteins connected with interkeratin matrix and corneous cell envelope formation. Mucus-like material glues loose keratin filaments. In amphibians a cell corneous envelope forms but matrix proteins, aside from mucus/glycoproteins, are scarce or absent. In reptiles, birds, and mammals specific proteins associated with keratin become relevant for the production of a resistant corneous layer. In reptiles some matrix, histidine-rich and sulfur-rich corneous cell envelope proteins are produced in the soft epidermis. In avian soft epidermis low levels of matrix and cornified proteins are present while lipids become abundant. In mammalian keratinocytes, interkeratin proteins, cornified cell envelope proteins, and transglutaminase are present. Topographically localized areas of dermal-epidermal interactions in amniote skin determine the formation of skin derivatives such as scales, feathers, and hairs. New types of keratin and associated proteins are produced in these derivatives. In reptiles and birds beta-keratins form the hard corneous material of scales, claws, beaks, and feathers. In mammals, small sulfur-rich and glycine-tyrosine-rich proteins form the corneous material of hairs, horns, hooves, and claws. Molecular studies on reptilian beta-keratins show they are glycine-rich proteins. They have C- and N-terminal amino acid regions homologous to those of mammalian proteins and a central core with homology to avian scale/feather keratins. These findings suggest that ancient reptiles already possessed some common genes that later diversified to produce some keratin-associated protein in extant reptiles and birds, and others in mammals. The evolution of these small proteins represents the more recent variation of the process of cornification in vertebrates.

Assessing self-renewal and differentiation in human embryonic stem cell lines

Like other cell populations, undifferentiated human embryonic stem cells (hESCs) express a characteristic set of proteins and mRNA that is unique to the cells regardless of culture conditions, number of passages, and methods of propagation. We sought to identify a small set of markers that would serve as a reliable indicator of the balance of undifferentiated and differentiated cells in hESC populations. Markers of undifferentiated cells should be rapidly downregulated as the cells differentiate to form embryoid bodies (EBs), whereas markers that are absent or low during the undifferentiated state but that are induced as hESCs differentiate could be used to assess the presence of differentiated cells in the cultures. In this paper, we describe a list of markers that reliably distinguish undifferentiated and differentiated cells. An initial list of approximately 150 genes was generated by scanning published massively parallel signature sequencing, expressed sequence tag scan, and microarray datasets. From this list, a subset of 109 genes was selected that included 55 candidate markers of undifferentiated cells, 46 markers of hESC derivatives, four germ cell markers, and four trophoblast markers. Expression of these candidate marker genes was analyzed in undifferentiated hESCs and differentiating EB populations in four different lines by immunocytochemistry, reverse transcription-polymer-ase chain reaction (RT-PCR), microarray analysis, and quantitative RT-PCR (qPCR). We show that qPCR, with as few as 12 selected genes, can reliably distinguish differentiated cells from undifferentiated hESC populations.

Histology and Ultrastructure of the Equine Lingual Tonsil. I. Crypt Epithelium and Associated Structures

The microstructural and ultrastructural features of the equine lingual tonsil were studied in five young horses. Located at the root of the tongue it presented an irregular surface with rounded elevations, numerous folds and crypts. Stratified squamous non-keratinized epithelium lining its outer surface was modified by heavy infiltration of lymphoid cells to form reticular epithelium within the crypt. The latter implies a role in initiating and maintaining immune responses to incoming infectious agents and antigens. Lamellated structures resembling Hassall's corpuscle were observed towards the outer surface epithelium. Microplicae were visible by scanning electron microscopy on the surface of both the outer and reticular epithelia. No microvillus cells resembling M cells were observed. The stratum superficiale of the reticular epithelium showed strong affinity for Soybean (SBA), Phosphocarpus tetragonolobus 1 (WBA 1), Ulex europaeus (UEA) and Griffonia simplicifolia 1 isolectin-B4 (GS1-B4). The characteristic lectin binding patterns may be useful for embryological and microbiological investigations. Vimentin filaments were not detected consistent with absence of M cells. Mucus glandular acini in the deeper lamina propria mucosae contained glycogen, acidic, neutral and weakly sulphated mucopolysaccharides. Transmission electron microscopy revealed that the layers of the outer surface and reticular epithelia shared characteristic features except the stratum superficiale, which had nuclei of varying shapes and an abundance of cell organelles. A few mast cells with electron lucent granules and myelinated nerve fibres were localized in the deeper portion.

Immunocytochemical and autoradiographic studies on the process of keratinization in avian epidermis suggests absence of keratohyalin

The process of keratinization in apteric avian epidermis and in scutate scales of some avian species has been studied by autoradiography for histidine and immunohistochemistry for keratins and other epidermal proteins. Acidic or basic alpha-keratins are present in basal, spinosus, and transitional layers, but are not seen in the corneous layer. Keratinization-specific alpha-keratins (AE2-positive) are observed in the corneous layer of apteric epidermis but not in that of scutate scales, which contain mainly beta-keratin. Alpha-keratin bundles accumulate along the plasma membrane of transitional cells of apteric epidermis. In contrast to the situation in scutate scales, in the transitional layer and in the lowermost part of the corneous layer of apteric epidermis, filaggrin-like, loricrin-like, and transglutaminase immunoreactivities are present. The lack of isopeptide bond immunoreactivity suggests that undetectable isopeptide bonds are present in avian keratinocytes. Using immunogold ultrastructural immunocytochemistry a low but localized loricrin-like and, less, filaggrin-like labeling is seen over round-oval granules or vesicles among keratin bundles of upper spinosus and transitional keratinocytes of apteric epidermis. Filaggrin-and loricrin-labeling are absent in alpha-keratin bundles localized along the plasma membrane and in the corneous layer, formerly considered keratohyalin. Using ultrastructural autoradiography for tritiated histidine, occasional trace grains are seen among these alpha-keratin bundles. A different mechanism of redistribution of matrix and corneous cell envelope proteins probably operates in avian keratinocytes as compared to that of mammals. Keratin bundles are compacted around the lipid-core of apteric epidermis keratinocytes, which do not form complex chemico/mechanical-resistant corneous cell envelopes as in mammalian keratinocytes. These observations suggest that low amounts of matrix proteins are present among keratin bundles of avian keratinocytes and that keratohyalin granules are absent.

Tight junction-related structures in the absence of a lumen: Occludin, claudins and tight junction plaque proteins in densely packed cell formations of stratified epithelia and squamous cell carcinomas

Tight junctions (TJs), hallmark structures of one-layered epithelia and of endothelia, are of central biological importance as intramembranous "fences" and as hydrophobic "barriers" between lumina represented by liquid- or gas-filled spaces on the one hand and the mesenchymal space on the other. They have long been thought to be absent from stratified epithelia. Recently, however, constitutive TJ proteins and TJ-related structures have also been identified in squamous stratified epithelia, including the epidermis, where they occur in special positions, most prominently in the uppermost living epidermal cell layer, the stratum granulosum. Much to our surprise, however, we have now also discovered several major TJ proteins (claudins 1 and 4, occludin, cingulin, symplekin, protein ZO-1) and TJ-related structures in specific positions of formations of epithelium-derived tissues that lack any lumen and do not border on luminal or body surfaces. Using immunohistochemistry and electron microscopy we have localized TJ proteins and structures in peripheral cells of the Hassall's corpuscles of human and bovine thymi as well as in specific central formations of tumor nests in squamous cell carcinomas, including the so-called "horn pearls". Such structures have even been found in carcinoma metastases. In carcinomas, they often seem to separate certain tumor regions from others or from stroma. The structural significance and the possible functional relevance of the locally restricted synthesis of TJ proteins and of the formations of TJ-related structures are discussed. It is proposed to include the determination of the presence or absence of such proteins and structures in the diagnostic program of tumor pathology.

Revisiting vimentin expression in early chick development

To identify somite-specific antigens exhibiting novel expression patterns in the developing chick embryo, epithelial somites were isolated and used for intrasplenic immunization. Immunohistochemical screening of chick embryos of various stages focussed our attention on a monoclonal antibody, Som5H5, which stained somitic derivatives, spinal nerves, and neural tubes as well as the costameres of adult skeletal muscle. Western blot and mass spectrometric analysis revealed the Som5H5 antigen to be vimentin, the distribution of which has been well characterized before. In addition to the described vimentin pattern, Som5H5 stained a region in the proximal portion of the developing limb bud. This novel expression domain was confirmed by in situ hybridization using vimentin riboprobes. Signaling molecules (Shh and BMP-2), known to play a role in limb development, did not influence vimentin expression. Thus, no functional or cellular correlate to this vimentin-positive region could be determined.

Sarcolemmal organization in skeletal muscle lacking desmin: evidence for cytokeratins associated with the membrane skeleton at costameres

The sarcolemma of fast-twitch muscle is organized into "costameres," structures that are oriented transversely, over the Z and M lines of nearby myofibrils, and longitudinally, to form a rectilinear lattice. Here we examine the role of desmin, the major intermediate filament protein of muscle in organizing costameres. In control mouse muscle, desmin is enriched at the sarcolemmal domains that lie over nearby Z lines and that also contain beta-spectrin. In tibialis anterior muscle from mice lacking desmin due to homologous recombination, most costameres are lost. In myofibers from desmin -/- quadriceps, by contrast, most costameric structures are stable. Alternatively, Z line domains may be lost, whereas domains oriented longitudinally or lying over M lines are retained. Experiments with pan-specific antibodies to intermediate filament proteins and to cytokeratins suggest that control and desmin -/- muscles express similar levels of cytokeratins. Cytokeratins concentrate at the sarcolemma at all three domains of costameres when the latter are retained in desmin -/- muscle and redistribute with beta-spectrin at the sarcolemma when costameres are lost. Our results suggest that desmin associates with and selectively stabilizes the Z line domains of costameres, but that cytokeratins associate with all three domains of costameres, even in the absence of desmin.

FGF signalling is required for differentiation-induced cytoskeletal reorganisation and formation of actin-based processes by podocytes

To examine the potential role of fibroblast growth factor (FGF) signalling during cell differentiation, we used conditionally immortalised podocyte cells isolated from kidneys of Fgf2 mutant and wild-type mice. Wild-type mouse podocyte cells upregulate FGF2 expression when differentiating in culture, as do maturing podocytes in vivo. Differentiating wild-type mouse podocyte cells undergo an epithelial to mesenchymal-like transition, reorganise their actin cytoskeleton and extend actin-based cellular processes; all of these activities are similar to the activity of podocytes in vivo. Molecular analysis of Fgf2 mutant mouse podocyte cells reveals a general disruption of FGF signalling as expression of Fgf7 and Fgf10 are also downregulated. These FGF mutant mouse podocyte cells in culture fail to activate mesenchymal markers and their post-mitotic differentiation is blocked. Furthermore, mutant mouse podocyte cells in culture fail to reorganise their actin cytoskeleton and form actin-based cellular processes. These studies show that FGF signalling is required by cultured podocytes to undergo the epithelial to mesenchymal-like changes necessary for terminal differentiation. Together with other studies, these results point to a general role for FGF signalling in regulating cell differentiation and formation of actin-based cellular processes during morphogenesis.

Vimentin- and desmin-positive cells in the moulting budgerigar (Melopsittacus undulatus) skin

The distribution of vimentin- and desmin-positive cells in the budgerigar (Melopsittacus undulatus) dermis was investigated by means of immunohistochemical reactivity with the commercially available (Euro-Diagnostics) polyclonal antibodies. The staining pattern for vimentin in the paraffin sections was generally comparable to that in other animal species with regard to endothelial cells, vascular wall cells, muscle cells and fibroblasts. The modified Schwann cells in the inner core of the Herbst corpuscles reacted distinctly with anti-vimentin and anti-desmin. Some connective tissue cells in the superficial dermal layer, in the feather papilla and along the pulp core inside of the regenerating feathers were particularly well stained with anti-vimentin. Fibroblast-like cells of the regenerating feathers, particularly at the base of the pulp, also reacted strongly with anti-desmin. The findings were discussed with regard to references.

Constitutively active mutant of the mitogen-activated protein kinase kinase MEK1 induces epithelial dedifferentiation and growth inhibition in madin-darby canine kidney-C7 cells

Overexpression of a constitutively active mitogen-activated protein kinase kinase (MAPKK or MEK) induces neuronal differentiation in adrenal pheochromocytoma 12 cells but transformation in fibroblasts. In the present study, we used a constitutively active MAPK/extracellular signal-regulated kinase (ERK) kinase 1 (MEK1) mutant to investigate the function of the highly conserved MEK1-ERK2 signaling module in renal epithelial cell differentiation and proliferation. Stable expression of constitutively active MEK1 (CA-MEK1) in epithelial MDCK-C7 cells led to an increased basal and serum-stimulated ERK1 and ERK2 phosphorylation as well as ERK2 activation when compared with mock-transfected cells. In both mock-transfected and CA-MEK1-transfected MDCK-C7 cells, basal and serum-stimulated ERK1 and ERK2 phosphorylation was almost abolished by the synthetic MEK inhibitor PD098059. Increased ERK2 activation due to stable expression of CA-MEK1 in MDCK-C7 cells was associated with epithelial dedifferentiation as shown by both a dramatic alteration in cell morphology and an abolished cytokeratin expression but increased vimentin expression. In addition, we obtained a delayed and reduced serum-stimulated cell proliferation in CA-MEK1-transfected cells (4.6-fold increase in cell number/cm2 after 5 days of serum stimulation) as compared with mock-transfected controls (12.9-fold increase in cell number/cm2 after 5 days). This result was confirmed by flow cytometric DNA analysis showing that stable expression of CA-MEK1 decreased the proportion of MDCK-C7 cells moving from G0/G1 to G2/M as compared with both untransfected and mock-transfected cells. Taken together, our data demonstrate an association of increased basal and serum-stimulated activity of the MEK1-ERK2 signaling module with epithelial dedifferentiation and growth inhibition in MDCK-C7 cells. Thus, the MEK1-ERK2 signaling pathway could act as a negative regulator of epithelial differentiation thereby leading to an attenuation of MDCK-C7 cell proliferation.

Immunohistochemical distribution of keratin proteins in feline tissues

The immunohistochemical distribution pattern of some keratin intermediate filament proteins has been analysed in a wide range of formalin fixed, paraffin embedded feline tissues using one polyclonal and two monoclonal antibodies raised against human keratins by means of the avidin-biotin-peroxidase complex technique. Only the epithelial and mesothelial cells were stained by the three antibodies, but differences in their corresponding staining pattern were noticed. The staining reaction of the polyclonal antibody raised against human skin keratin was found in both stratified and complex epithelia, while that of the monoclonal antibody which recognizes human keratins 8 + 18 + 19 of the Moll catalogue (NCL-5D3) was restricted to some simple epithelia. The staining reaction of the monoclonal antibody which reacts with human keratins 5 + 8 of the Moll catalogue (RCK-102) covered the widest spectrum of feline epithelial tissues analysed, including stratified, complex and simple epithelia. These staining patterns of feline tissues are basically similar with respect to those of corresponding tissues in other mammalian species, although some differences were also noticed and some obvious epithelial tissues were not stained. This study confirms the broad interspecies cross-reactivity of keratin proteins antibodies and demonstrates their capability to differentiate between various types of feline epithelia and some epithelial compartments.

Immunohistochemical distribution of vimentin, desmin, glial fibrillary acidic protein and neurofilament proteins in feline tissues

The immunohistochemical distribution pattern of vimentin, desmin, glial fibrillary acidic protein and neurofilaments intermediate filament proteins has been analyzed in a wide range of formalin fixed, paraffin embedded tissues using polyclonal and monoclonal antibodies raised against non-feline antigens. The vimentin antibody reacted with mesenchymal cells, the desmin antibody with striated and smooth muscle fibres, the glial fibrillary acidic protein antibody with glial cells in the central and peripheral nervous system, and the neurofilament proteins antibody with neuronal cell bodies and processes. In addition, some epithelial cells were vimentin positive, perisinusoidal liver cells were desmin positive, and basal/myoepithelial cells of the mammary gland, and luteinic cells were glial fibrillary acidic protein positive. These staining patterns of feline tissues are basically similar with respect to that of corresponding tissues in other mammalian species for each of the four intermediate filament proteins studied, but some differences have been also noticed. This study confirms the broad interspecies cross-reactivity of intermediate filament proteins antisera and demonstrates their capability to differentiate particular types of feline cells and tissues.

Selected monoclonal antibodies can increase the accuracy of cytodiagnosis of neoplastic effusions of cryptic origin expanded in a short term culture

The use of a selected panel of monoclonal antibodies (MoAb) to tumor associated antigens (TAA) in immunocytochemical (IIC) tests has been shown, in a preliminary study, to be a powerful diagnostic tool for the identification of the primary solid tumor causing metastatic effusion. Despite this improvement in a minority of neoplastic fluids a number of different causes may still determine false negative (FN) immunocytochemical diagnoses. The aim of the present study was to confirm the diagnostic accuracy of this panel of MoAb. This was done by analyzing in IIC tests a larger number of effusions and by evaluating whether the expansion in short term culture of those fluids with an uncertain malignant morphology could provide an adequate cellular substrate for immunocytodiagnosis. The analysis of 314 effusions confirmed the results of the pilot study and demonstrated that the combination of short term culture and immunocytochemical assays can further increase the sensitivity of this novel diagnostic procedure from 84.3% to 95.3%.

Smooth muscle cells of neural crest origin form the aorticopulmonary septum in the avian embryo

Previous studies have shown that the cells of the aorticopulmonary (AP) septum are similar to the smooth muscle cells of the mediae of the great vessels in their common origin from the cardiac neural crest and in their common expression of an elastic extracellular matrix. The purpose of this study was to test the cells of the AP septum for the presence of certain cytoplasmic proteins, especially smooth muscle alpha-actin (SMAA) whose presence is definitive of smooth muscle. A monoclonal antibody against SMAA was applied to normal chicken embryos at 3.5-8 days of incubation and to age-matched embryos from which the cardiac neural crest had been ablated surgically. Antibodies against the intermediate filaments desmin, cytokeratin, and vimentin also were applied. The results showed that the AP septal cells expressed SMAA during the process of septation, days 5-8; but when the cardiac neural crest was ablated and septation was defective, no cells in the conotruncal connective tissue expressed SMAA. None of the intermediate filament proteins were detected in the septum. These results indicate that the AP septal cells are smooth muscle and therefore may be hypothesized to have an active role in septation.

An immunohistochemical study of canine tissues with vimentin, desmin, glial fibrillary acidic protein, and neurofilament antisera

In a wide range of canine tissues the immunoreactivity with commercially available antisera against intermediate filament antigens viz. vimentin, desmin, glial fibrillary acidic protein (GFAP) and neurofilament proteins, was studied. In addition, the results of formalin and Carnoy fixation were compared. Carnoy fixation appeared to result in optimal reactivity for all antisera. Epithelial cells did not react with any of the antisera, with exception of ovarian surface epithelium, which showed staining with the vimentin and desmin antisera. The vimentin antiserum induced staining of several cell types viz. fibroblasts, endothelial cells, chondrocytes, Schwann cells, ependymal cells, astrocytes, Leydig cells, synovial cells, podocytes and some parietal cells of Bowman's capsule. Sertoli cells showed a faint staining reaction. Muscle cells in various tissues reacted with the desmin antiserum. In the kidney a varying number of parietal cells appeared to react as did a restricted number of epithelial cells of proximal tubules and loops of Henle. GFAP reactivity was confined to glial cells, predominantly fibrous astrocytes, Schwann cells and axons. Additionally, some neuronal cell bodies in peripheral ganglia showed staining of varying intensity. Neurofilament staining was restricted to axons and some neurons. The immunoreactivity of canine tissues with these antisera is compared to findings in other species. The results confirm a broad interspecies cross-reactivity of these antisera. They can be used in studying the nature of canine tissues.

Immunohistochemical localization of desmin in the quail ovary. Demonstration of a suspensory apparatus

We have localized desmin in the quail ovary, by the unlabelled antibody peroxidase-antiperoxidase technique, using two monoclonal and one polyclonal antisera. Special attention has been paid to the influence of fixation and of proteolytic pretreatment of sections. It appeared that the immunostaining of desmin largely depends on the nature of the fixative. Carnoy fluid, Bouin's fixative, and a paraformaldehyde-acetic acid fixative preserved the histological structure very efficiently. However, trypsin pretreatment proved to be necessary to unmask the antigenic sites in the ovaries fixed in Bouin's fixative and the paraformaldehyde-acetic acid fixative. Desmin immunoreactivity was detected in the tunica albuginea and the chordae, a number of which surrounding the blood vessels, from the hilus to the thecal surface of the follicles. Small branches of chordae connected them with the tunica albuginea, forming a suspensory apparatus. Desmin was also localized in the smooth-muscle cells of the blood vessels. In the theca, immunoreactivity was detected in the wall of arterioles, of venules, and of capillaries. Further experimental and immunohistochemical research have to be performed to establish if the suspensory apparatus is a myoid tissue.

An epithelium-type cytoskeleton in a glial cell: astrocytes of amphibian optic nerves contain cytokeratin filaments and are connected by desmosomes

In higher vertebrates the cytoskeleton of glial cells, notably astrocytes, is characterized (a) by masses of intermediate filaments (IFs) that contain the hallmark protein of glial differentiation, the glial filament protein (GFP); and (b) by the absence of cytokeratin IFs and IF-anchoring membrane domains of the desmosome type. Here we report that in certain amphibian species (Xenopus laevis, Rana ridibunda, and Pleurodeles waltlii) the astrocytes of the optic nerve contain a completely different type of cytoskeleton. In immunofluorescence microscopy using antibodies specific for different IF and desmosomal proteins, the astrocytes of this nerve are positive for cytokeratins and desmoplakins; by electron microscopy these reactions could be correlated to IF bundles and desmosomes. By gel electrophoresis of cytoskeletal proteins, combined with immunoblotting, we demonstrate the cytokeratinous nature of the major IF proteins of these astroglial cells, comprising at least three major cytokeratins. In this tissue we have not detected a major IF protein that could correspond to GFP. In contrast, cytokeratin IFs and desmosomes have not been detected in the glial cells of brain and spinal cord or in certain peripheral nerves, such as the sciatic nerve. These results provide an example of the formation of a cytokeratin cytoskeleton in the context of a nonepithelial differentiation program. They further show that glial differentiation and functions, commonly correlated with the formation of GFP filaments, are not necessarily dependent on GFP but can also be achieved with structures typical of epithelial differentiation; i.e., cytokeratin IFs and desmosomes. We discuss the cytoskeletal differences of glial cells in different kinds of nerves in the same animal, with special emphasis on the optic nerve of lower vertebrates as a widely studied model system of glial development and nerve regeneration.

Cytokeratin filaments and desmosomes in the epithelioid cells of the perineurial and arachnoidal sheaths of some vertebrate species

Using electron microscopy and immunohistochemistry with a large panel of antibodies to various cytoskeletal proteins we have noted that the single- or multi-layered sheaths of epithelioid cells ("neurothelia") surrounding peripheral nerves (perineurial cells) or structures of the central nervous system, including the optic nerve (arachnoid cells), show remarkable interspecies differences in their cytoskeletal complements. In two anuran amphibia examined (Xenopus laevis, Rana ridibunda), the cells of both forms of neurothelia, i.e., perineurial and arachnoid, are interconnected by true desmosomes and are rich intermediate-sized filaments (IFs) of the cytokeratin type. Among higher vertebrates, a similar situation is found in the bovine and chicken nervous systems, in which the arachnoid cells of the meninges contain desmosomes and IFs of both the cytokeratin (apparently with restricted epitope accessibilities in the chicken) and the vimentin type, whereas the perineurial cells of many nerves contain cytokeratin IFs, often together with vimentin, but no desmosomes. In contrast, in rat arachnoidal and perineurial cells significant reactions have been observed neither for cytokeratins nor for desmosomes. In the human nervous system, cytokeratins and desmosomes have also not been seen in the various perineuria studied whereas desmosomes are frequent in arachnoidal cell layers which are dominated by vimentin IFs and only in certain small regions of the brain contain some additional cytokeratins. The occurrence of cytokeratins in the tissues found positive by immunohistochemistry has been confirmed by gel electrophoresis of cytoskeletal proteins, followed by immunoblotting. Our results emphasize both similarities and differences between the neurothelia on the one hand and epithelia or endothelia on the other, justifying classification as a separate kind of tissue, i.e., neurothelium. The observations of interspecies differences lead to the challenging conclusion that neither desmosomes nor cytokeratins are essential for the basic functions of neurothelial sheaths nor does the specific type of IF protein expressed in these cells appear to matter in this respect. The results are also discussed in relation to the cytoskeletal characteristics of other epithelioid tissues and of human neurothelium-derived tumors.

Keratin staining of canine epithelial tissues by a polyclonal antiserum

The keratin distribution pattern in various canine epithelial tissues has been studied using a commercially available rabbit antiserum, raised against human skin keratins, in an immunoperoxidase staining method with the peroxidase-antiperoxidase complex (PAP). The staining results of two fixation methods were compared. Paraffin sections after fixation in Carnoy's solution showed optimal results, whereas paraffin sections after fixation in 10% buffered formalin resulted in a strongly reduced keratin staining reaction. Keratinizing and non-keratinizing stratified epithelial showed a strong staining reaction. Glandular epithelium and the non-stratified epithelia of internal organs e.g. liver, kidney and intestine did not react with the antiserum. However, glandular ductal epithelium, myoepithelial cells and basal cells in various epithelial tissues showed a positive staining reaction. The results indicate the presence of different keratin types in these canine epithelial tissues. The keratin distribution pattern is compared with the distribution pattern observed in tissues of other species.

Type I collagen gel induces Madin-Darby canine kidney cells to become fusiform in shape and lose apical-basal polarity

In the embryo, epithelia give rise to mesenchyme at specific times and places. Recently, it has been reported (Greenburg, G., and E. D. Hay. 1986. Dev. Biol. 115:363-379; Greenberg, G., and E. D. Hay. 1988. Development (Camb.). 102:605-622) that definitive epithelia can give rise to fibroblast-like cells when suspended within type I collagen gels. We wanted to know whether Madin-Darby canine kidney (MDCK) cells, an epithelial line, can form mesenchyme under similar conditions. Small explants of MDCK cells on basement membrane were suspended within or placed on top of extracellular matrix gels. MDCK cells on basement membrane gel are tall, columnar in shape, and ultrastructurally resemble epithelia transporting fluid and ions. MDCK explants cultured on type I collagen gel give rise to isolated fusiform-shaped cells that migrate over the gel surface. The fusiform cells extend pseudopodia and filopodia, lose cell membrane specializations, and develop an actin cortex around the entire cell. Unlike true mesenchymal cells, which express vimentin and type I collagen, fusiform cells produce both keratin and vimentin, continue to express laminin, and do not turn on type I collagen. Fusiform cells are not apically-basally polarized, but show mesenchymal cell polarity. Influenza hemagglutinin and virus budding localize to the front end or entire cell surface. Na,K-ATPase occurs intracellularly and also symmetrically distributes on the cell surface. Fodrin becomes diffusely distributed along the plasma membrane, ZO-1 cannot be detected, and desmoplakins distribute randomly in the cytoplasm. The loss of epithelial polarity and acquisition of mesenchymal cell polarity and shape by fusiform MDCK cells on type I collagen gel was previously unsuspected. The phenomenon may offer new opportunities for studying cytoplasmic and nuclear mechanisms regulating cell shape and polarity.

Cytoskeletons of retinal pigment epithelial cells: interspecies differences of expression patterns indicate independence of cell function from the specific complement of cytoskeletal proteins

In vertebrate tissue development a given cell differentiation pathway is usually associated with a pattern of expression of a specific set of cytoskeletal proteins, including different intermediate filament (IF) and junctional proteins, which is identical in diverse species. The retinal pigment epithelium (RPE) is a layer of polar cells that have very similar morphological features and practically identical functions in different vertebrate species. However, in biochemical and immunolocalization studies of the cytoskeletal proteins of these cells we have noted remarkable interspecies differences. While chicken RPE cells contain only IFs of the vimentin type and do not possess desmosomes and desmosomal proteins RPE cells of diverse amphibian (Rana ridibunda, Xenopus laevis) and mammalian (rat, guinea pig, rabbit, cow, human) species express cytokeratins 8 and 18 either as their sole IF proteins, or together with vimentin IFs as in guinea pig and a certain subpopulation of bovine RPE cells. Plakoglobin, a plaque protein common to desmosomes and the zonula adhaerens exists in RPE cells of all species, whereas desmoplakin and desmoglein have been identified only in RPE desmosomes of frogs and cows, including bovine RPE cell cultures in which cytokeratins have disappeared and vimentin IFs are the only IFs present. These challenging findings show that neither cytokeratin IFs nor desmosomes are necessary for the establishment and function of a polar epithelial cell layer and that the same basic cellular architecture can be achieved by different programs of expression of cytoskeletal proteins. The differences in the composition of the RPE cytoskeleton further indicate that, at least in this tissue, a specific program of expression of IF and desmosomal proteins is not related to the functions of the RPE cell, which are very similar in the various species.

Transient coexpression of desmin and cytokeratins 8 and 18 in developing myocardial cells of some vertebrate species

During myogenesis the intermediate-sized filament (IF) cytoskeleton is characterized by increasing proportions of desmin. While skeletal and smooth muscle formation occurs in free mesenchymal cells containing vimentin-type IFs, myocardial development starts from a polar epithelium containing cytokeratin IFs and desmosomes. Therefore, we have studied the formation of the epicardium and the myocardium in different vertebrate species, combining light and electron microscopic immunolocalization techniques with gel-electrophoretic analyses of cytoskeletal proteins of microdissected myocardial tissue at differing developmental stages. In this report, we describe results obtained from advanced stages of myocardial differentiation. In all species studied the myocardial cell possess IFs abundant in desmin, often together with smaller amounts of vimentin, and the mesothelial layer of the epicardium contains cytokeratin IFs. However, we have observed remarkable interspecies differences with respect to the occurrence of cytokeratins in embryonic myocardial cells. In fetal human myocardium, from week 10 of pregnancy on, but not in juvenile and adult myocardium, and in chicken myocardium of all stages examined (until several days after hatching) specific immunostaining was seen with certain broad-range cytokeratin antibodies as well as with antibodies specific for cytokeratins 18 (in both species) and 8 (showing significant reaction only in human). This cytokeratin immunoreaction, however, did not appear in IFs extending throughout the cytoplasm or at Z-lines, but was localized in punctate arrays representing aggregates of dense material. The aggregates were often enriched at, but not restricted to, the desmosomal plaques of the intercalated discs. These observations were supported by gel-electrophoretic demonstration of small but significant amounts of cytokeratins 18 (in both species) and 8 (detected only in human) in microdissected myocardial tissue. We also observed cytokeratins in smooth muscle cells of some cardiac blood vessels. In contrast, bovine myocardium of advanced fetal age as well as rat and mouse myocardium (from fetal day 12 on) were negative for cytokeratins with all methods, although epicardial cytokeratin IFs were demonstrable. These observations are discussed in relation to myocardial histogenesis and to general problems of cytokeratin gene expression control in epithelial and nonepithelial cells.

Patterns of expression of trichocytic and epithelial cytokeratins in mammalian tissues. II. Concomitant and mutually exclusive synthesis of trichocytic and epithelial cytokeratins in diverse human and bovine tissues (hair follicle, nail bed and matrix, lingual papilla, thymic reticulum)

The hair-forming cells (trichocytes) and the mature hair contain four major trichocytic cytokeratins from each of the subfamilies, basic (Hb1-4) and acidic (Ha1-4); these are related - but not identical - to the epithelial cytokeratins. Here we show, by biochemical methods and immunofluorescence microscopy using antibodies specific for either epithelial or trichocyte cytokeratins, that the same set of hair-type cytokeratins, including two newly identified minor components, designated Hax (type I) and Hbx (type II), are also expressed in cells forming nails, in the filiform papillae of the dorsal surface of human and bovine tongue, and, most surprisingly, in some cells of the epithelial reticulum of bovine and human thymus. By double-label immunofluorescence microscopy, we also show that the expression of the two subsets of cytokeratins, i.e., the epithelial and the trichocytic ones, is not necessarily mutually exclusive, but that certain cells of hair follicles, nail matrix and bed, lingual papillae, and the nonlymphoid cell system of the thymus contain both trichocytic and certain epithelial cytokeratins. This indicates that these cells coexpress representatives of both kinds of cytokeratin. Implications of these findings with respect to problems of regulatory control of cytokeratin synthesis in tissue development and differentiation, and the possible functional meaning of the occurrence of trichocytic cytokeratins in such histologically diverse tissues, are discussed.

Cytokeratins in certain endothelial and smooth muscle cells of two taxonomically distant vertebrate species, Xenopus laevis and man

Using immunolocalization techniques, electron microscopy, and gel electrophoresis combined with immunoblotting, we have noted remarkable interspecies differences in the expression of cytokeratins in certain nonepithelial cells. In the present study we describe, in two taxonomically distant vertebrate species, the African clawed toad Xenopus laevis and man, endothelial and smooth muscle cells which express cytokeratin intermediate filaments (IFs), in addition to vimentin and/or desmin IFs. In Xenopus, all endothelia seem to produce both vimentin and cytokeratin IFs. As well, certain smooth muscle bundles located in the periphery of the walls of the esophagus and the urinary bladder produce small amounts of cytokeratin IFs in addition to IFs containing vimentin or desmin or both. The amphibian equivalents of human cytokeratins 8 and 18 have been identified in these nonepithelial tissues. In human endothelial cells, immunocytochemical reactions with certain cytokeratin antibodies are restricted to a rare subset of blood vessels. Vessels of this type were first noted in synovial and submucosal tissues, but also occur in some other locations. Cytokeratins have also been detected in certain groups of smooth muscles, such as those present in the walls of some blood vessels in synovial tissue and umbilical cord. Here, the synthesis of low levels of cytokeratins 8 and 18, sometimes with traces of cytokeratin 19, has been demonstrated in smooth muscle cells by colocalization with myogenic marker proteins, such as desmin and/or the smooth-muscle-specific isoform of alpha-actin. Possible reasons for the differences in cytokeratin expression between adjacent endothelia in man, and smooth-muscle structures in both species, as well as biologic and histodiagnostic implications of these findings, are discussed.

Changes in the distribution of intermediate-filament types in Japanese quail embryos during morphogenesis

We examined the distribution of intermediate filaments in early quail embryos in order to determine whether these cytoskeletal proteins play a role in the epithelial-mesenchymal transitions that commonly occur during embryogenesis, e.g., the separation of neural-crest cells from the neural epithelium. The distribution of cytokeratins, vimentin, and desmin was examined in frozen sections of quail embryos at stages during which dramatic reorganizations of tissues take place. All embryonic tissues were found to contain either vimentin or cytokeratins, but the distribution of these cytoskeletal proteins was characteristic neither of the cellular organization (e.g., epithelium vs. mesenchyme) nor of the germ-layer derivation of the tissues. Cytokeratin monoclonal antibodies stained most embryonic epithelia (defined here as being sheet-like tissue with an underlying basement membrane), including epidermis and extraembryonic membranes derived in part from the ectoderm, splanchnopleure and kidney tubules derived from mesoderm, and endoderm. Cytokeratin antibodies did not stain some epithelia, including the neural tube, neural plate, and dermatome/myotome. Whereas the cytokeratin antibodies exclusively stained epithelia, the vimentin antibodies labeled both epithelial (the neural tube, dermatome/myotome, and somatic and splanchnic mesoderm) and mesenchymal tissues (the sclerotome and neural-crest cells), regardless of their germ-layer derivation. In early embryos, antibodies against desmin only stained the myotome and, in 4-day embryos, the heart and mesenchyme around the pharynx. As the distribution of intermediate-filament types did not reflect tissue organization or germ-layer derivation, we propose that the distribution of intermediate filaments in early avian embryos reflects the motile capacity of an embryonic cell and/or the presence of specialized cell junctions, i.e., desmosomes.

Rearrangement of the vimentin cytoskeleton during adipose conversion: formation of an intermediate filament cage around lipid globules

During adipose conversion of murine 3T3-L1 cells, the arrangement of vimentin intermediate filaments (IFs) changes from an extended fibrillar state to a complex cage formation tightly associated with the forming lipid globules. The fully developed cage complex surrounding the lipid globule consists of a monolayer of groups of regularly spaced vimentin IFs that in turn is closely ensheathed by a special endoplasmic reticulum cisterna. The same IF cage is also seen in other adipocytes in culture and in tissues. The specificity of the association of lipid globules with vimentin IFs during adipose conversion is discussed as a special form of compartmentalization supporting adipogenesis and is taken as an example of a possible IF function in relation to a cell differentiation process.

The ultrastructure and immunocytochemistry of renal cell carcinoma

The spectrum of ultrastructural features of a series of primary renal cell adenocarcinomas, including clear cell, granular, oncocytic, and sarcomatoid types, and a group of metastatic renal cell adeno-carcinomas has been studied. The contributions of electron microscopy and immunocytochemistry as adjuncts to light microscopy and histochemistry in the differential diagnosis of metastatic renal cell adenocarcinoma in various anatomic locations are reviewed.

Identification of the conserved, conformation-dependent cytokeratin epitope recognized by monoclonal antibody (lu-5)

The epitope recognized by the murine monoclonal antibody (mAB lu-5) recently described as a formaldehyde-resistant, "pan-epithelial marker" of great value in tumour diagnosis is located on the surface of cytokeratin filaments. It has been preserved during vertebrate evolution from amphibia to man. As this epitope is not reactive after SDS-polyacrylamide gel electrophoresis (SDS-PAGE), the epitope-bearing protein has been identified by a dot-blot antibody binding assay, using purified proteins in which the epitope is reconstituted. We show that the epitope is present in most cytokeratin polypeptides of both the acidic (type I) and basic (type II) subfamily but does not occur in other cytoskeletal proteins. The location of this widespread epitope is discussed with respect to homologies of amino acid sequences of cytokeratins and their conformations.

Immunocytochemical studies of endothelial cells in vivo. I. The presence of desmin only, or of desmin plus vimentin, or vimentin only, in the endothelial cells of different capillaries of the adult chicken

It is currently believed that the intermediate filaments of endothelial cells contain vimentin subunits exclusively. This inference, however, is derived from studies of only a few types of endothelial cells. By double indirect immunofluorescence and immunoelectron microscopy, we have now examined the endothelial cells of the micro- and macrovasculature of a variety of tissues and organs of adult chicken in vivo for their content of desmin and vimentin. Endothelial cells of the peritubular capillary in the renal cortex, the hepatic sinusoid, and the splenic sinusoid were found to contain only desmin; those of the exocrine pancreas capillary contained both desmin and vimentin; and the endothelial cells of the macrovasculatures and of all the other microvasculatures examined, including the vasa recta of the renal medulla, contained only vimentin. Such heterogeneity suggests that different types of adult chicken endothelial cells may have different embryological origins. To the extent that desmin and vimentin intermediate filaments may be functionally distinct, these results also suggest that different capillary endothelial cells may have different functional properties.

Patterns of expression and organization of cytokeratin intermediate filaments

Cytokeratins are a large multigene family comprising two polypeptide types, i.e. acidic (type I) and basic (type II) ones, which are distinguished on the basis of immunological, peptide mapping, mRNA hybridization, and primary amino acid sequence data. The acidic (type I) cytokeratins can be subdivided into at least two different subtypes on the basis of their carboxy-terminal sequences. Considerable interspecies conservation of sequences exists, even extending to the 3'-non-coding mRNA regions. Different pairs of type I and II cytokeratins show different resistance to dissociation in urea. Sequence differences of the type I cytokeratins containing functional domains may be an explanation of the observed preference of co-expression with certain type II cytokeratins. The distribution of the different type I and II cytokeratins in normal epithelia and in carcinomas is differentiation related and can be used for cell typing and identification. The cell type-specific expression of cytokeratin polypeptides is recognized at both the protein and the mRNA level. The building block of cytokeratin IFs is a heterotypic tetramer, consisting of two type I and two type II polypeptides arranged in pairs of laterally aligned coiled coils. This principle of tetrameric organization is thought to be generally applicable to IFs.

Collagens, laminin, fibronectin, and cytoskeletal composition of cells in syngeneic aortal vein grafts in the rat

The supradiaphragmatic vena cava of the rat was transplanted to the abdominal aorta in syngeneic recipients. The cells and the connective tissue matrix of these grafts were studied 3 days to 12 weeks after transplantation by immunofluorescence staining of the cytoskeletal proteins desmin, vimentin, and myosin. The matrix proteins, collagen Types I and III, laminin, fibronectin, and fibrin(ogen) were similarly demonstrated. Blood coagulation Factor VIII was used as a marker for endothelial cells. Inflammatory cells invaded the graft during Week 1, but later on the grafts were crowded with cells containing vimentin. Intimal thickenings developed already after 1 week. Smooth muscle cells containing desmin proliferated in these thickenings. After transplantation collagen Type I increased in the graft, but collagen Type III rather decreased. During the phase of intense proliferation of smooth muscle cells the media contained a rich matrix of diffusely distributed laminin. The laminin of older grafts was confined mostly to the subintimal zone and to the intimal plaques.

Bronchial carcinoid cells contain neural-type intermediate filaments

Monospecific antibodies and indirect immunofluorescent microscopic examination, combined with immunochemical analysis, were used to examine intermediate filaments in four cases of bronchial carcinoid tumors. The results show that carcinoid cells express intermediate filaments of neural type (neurofilaments) but are negative for intermediate filaments of mesenchymal type (vimentin), epithelial type (keratin), muscle type (desmin), and glial type (glial fibrillary acidic protein). Since the expression of intermediate filaments shows a high degree of tissue specificity, the results suggest either derivation of bronchial carcinoid cells from maternal cells displaying neural characteristics or from cells with the capacity to acquire neural properties on neoplastic growth. It is also suggested that antineurofilament antibodies can be used as a useful aid in differential diagnosis of bronchial carcinoids from other pulmonary tumors.

Epidermal Langerhans cells contain intermediate-sized filaments of the vimentin type: an immunocytologic study

In epidermal cell suspensions, prepared from healthy human and murine skin, Langerhans cells (LC) were identified by indirect immunofluorescence microscopy using monoclonal antibodies directed against human T6 and Ia markers, and against a murine Iak determinant, respectively. The cells were double-labeled on cytospin slides with antibodies against a cytoskeletal protein, either vimentin or keratin. In this way it was shown that epidermal LC contain intermediate-sized filaments of the vimentin type.

Distribution of the major histocompatibility complex antigens on different cellular components of human liver

Monoclonal mouse antibodies to the "framework" determinants of the class I and II molecules of the major histocompatibility complex (MHC) were used to demonstrate the presence of the MHC antigens in human liver. First, the localization of these antigens was demonstrated from frozen section histology with indirect FITC immunofluorescence and the cell component(s) binding the mouse antibody were identified by rabbit marker antisera and indirect TRITC immunofluorescence. Second, the antigen expression on the cell surface was analyzed by the Staphylococcus aureus rosette method from cytological cell smears. All antibodies reacted with cells in the liver sinusoids, both with the Kupffer cells and at least partially with the sinusoidal endothelial cells. The same antisera reacted also with the bile duct cells, though weaker, and with some stromal cells in close proximity of the blood vessels. The vascular endothelial cells of hepatic artery, hepatic vein, and portal vein displayed no reaction. Thus human liver differs strikingly from, e.g., human kidney, where the vascular endothelial cells contain large amounts of MHC antigens on the cell surface. This difference may be one explanation to why liver allografts are less promptly rejected than renal allografts in man.

Integration of different keratins into the same filament system after microinjection of mRNA for epidermal keratins into kidney epithelial cells

We have isolated poly (A)+ RNA, highly enriched in keratin mRNA from bovine muzzle epidermis, and injected it into epithelial cells of a different type, i.e., cultured kidney epithelial cells of the same (MDBK) or taxonomically distant (PtK2) species. Both recipient cell lines contain keratin polypeptides that are different from those present in epidermal cells. Using keratin subtype-specific antibodies in immunofluorescence and immunoelectron microscopy, we show that foreign keratin mRNAs when injected into a different type of epithelial cell can recruit polyribosomes and are translated together with the keratin mRNAs of the host cell. Foreign epidermal keratins are excluded from vimentin filaments and other structures but readily coassemble with the endogenous keratins and appear to be integrated into the meshwork of the preexisting kidney-type keratin filaments. Our observations indicate that different sets of keratin polypeptides from the same or different species can coassemble in the living cell into a common filament system. Thus we have developed a procedure that allows experimental alteration of the intermediate filament cytoskeleton within living epithelial cells.

Protein complexes of intermediate-sized filaments: melting of cytokeratin complexes in urea reveals different polypeptide separation characteristics

Subunit complexes of cytokeratin polypeptides from intermediate-sized filaments (IF) of various tissues and cultured cells from rat, cow, and man were solubilized in low-salt buffer containing 4 M urea and exposed to increasing concentrations of urea, followed by urea gradient electrophoresis or two-dimensional gel electrophoresis at different urea concentrations. Correspondingly, cytokeratin polypeptides dissociated in 9.5 or 10 M urea were dialyzed into lower concentrations of urea and allowed to reassociate into specific complexes. It was found that the polypeptide constituents of a given cytokeratin complex dissociate in the form of a rather sharp "melting curve" and that dissociated polypeptides reassociate in the same mode of dependence on urea concentration. The midpoint of melting in urea (Um) is a characteristic property of a given complex of cytokeratin polypeptides. Um values differ markedly between different cytokeratin complexes, ranging from 5.9 to 9.0 M urea. The results also show that cytokeratins do not form complexes with vimentin, another type of IF protein. The data suggest that certain cytokeratin polypeptides are complementary and contain sequences that direct their association into specific complexes forming IF subunits.