Intermediate filament-plasma membrane interactions

The investigation of detection and sensing mechanism of spicy substance based on human TRPV1 channel protein-cell membrane biosensor

The transient receptor potential vanilloid 1 (TRPV1) is a key target for the spicy taste sensor and analgesic drug development. However, the human TRPV1-associated signaling remains to be obscure. In this study, we overexpressed human TRPV1 (hTRPV1) in HEK293T cells and explored its signaling activated by spicy substances. A cell membrane biosensor was constructed by using the cells highly expressed hTRPV1 through a layer-by-layer assembly. Our results showed that the activation constants by capsaicin, allicin and sanshool, the active components of chili pepper, garlic and mountain pepper, were K_{a, capsaicin} = 3.5206 × 10^-16 mol/L, K_{a, allicin} = 5.0227 × 10^-15 mol/L, K_{a, sanshool} = 1.7832 × 10^-15 mol/L. Obviously, the order of the sensitivity mediated by hTRPV1 was capsaicin > sanshool > allicin. The affinity values of the three spicy substances with hTRPV1 analyzed by molecular docking simulation also displayed the same law. Most importantly, some amide bonds and their similar groups and even benzene rings of spicy compounds were fund to be critical in the spicy sensing process. In addition, Glu570 in the active pocket of hTRPV1 plays an important role in identifying spicy substances. The elucidation of the detailed mechanism mediated by hTRPV1 in spicy sensing will lay a theoretical foundation to design rational strategies for screening of potential analgesics.

Keratinocyte cytoskeletal roles in cell sheet engineering

Background

There is an increasing need to understand cell-cell interactions for cell and tissue engineering purposes, such as optimizing cell sheet constructs, as well as for examining adhesion defect diseases. For cell-sheet engineering, one major obstacle to sheet function is that cell sheets in suspension are fragile and, over time, will contract. While the role of the cytoskeleton in maintaining the structure and adhesion of cells cultured on a rigid substrate is well-characterized, a systematic examination of the role played by different components of the cytoskeleton in regulating cell sheet contraction and cohesion in the absence of a substrate has been lacking.

Results

In this study, keratinocytes were cultured until confluent and cell sheets were generated using dispase to remove the influence of the substrate. The effects of disrupting actin, microtubules or intermediate filaments on cell-cell interactions were assessed by measuring cell sheet cohesion and contraction. Keratin intermediate filament disruption caused comparable effects on cell sheet cohesion and contraction, when compared to actin or microtubule disruption. Interfering with actomyosin contraction demonstrated that interfering with cell contraction can also diminish cell cohesion.

Conclusions

All components of the cytoskeleton are involved in maintaining cell sheet cohesion and contraction, although not to the same extent. These findings demonstrate that substrate-free cell sheet biomechanical properties are dependent on the integrity of the cytoskeleton network.

Keratins mediate localization of hemidesmosomes and repress cell motility

The keratin (K)-hemidesmosome (HD) interaction is crucial for cell-matrix adhesion and migration in several epithelia, including the epidermis. Mutations in constituent proteins cause severe blistering skin disorders by disrupting the adhesion complex. Despite extensive studies, the role of keratins in HD assembly and maintenance is only partially understood. Here we address this issue in keratinocytes in which all keratins are depleted by genome engineering. Unexpectedly, such keratinocytes maintain many characteristics of their normal counterparts. However, the absence of the entire keratin cytoskeleton leads to loss of plectin from the hemidesmosomal plaque and scattering of the HD transmembrane core along the basement membrane zone. To investigate the functional consequences, we performed migration and adhesion assays. These revealed that, in the absence of keratins, keratinocytes adhere much faster to extracellular matrix substrates and migrate approximately two times faster compared with wild-type cells. Reexpression of the single keratin pair K5 and K14 fully reversed the above phenotype. Our data uncover a role of keratins, which to our knowledge is previously unreported, in the maintenance of HDs upstream of plectin, with implications for epidermal homeostasis and pathogenesis. They support the view that the downregulation of keratins observed during epithelial-mesenchymal transition supports the migratory and invasive behavior of tumor cells.

The keratin-binding protein Albatross regulates polarization of epithelial cells

The keratin intermediate filament network is abundant in epithelial cells, but its function in the establishment and maintenance of cell polarity is unclear. Here, we show that Albatross complexes with Par3 to regulate formation of the apical junctional complex (AJC) and maintain lateral membrane identity. In nonpolarized epithelial cells, Albatross localizes with keratin filaments, whereas in polarized epithelial cells, Albatross is primarily localized in the vicinity of the AJC. Knockdown of Albatross in polarized cells causes a disappearance of key components of the AJC at cell-cell borders and keratin filament reorganization. Lateral proteins E-cadherin and desmoglein 2 were mislocalized even on the apical side. Although Albatross promotes localization of Par3 to the AJC, Par3 and ezrin are still retained at the apical surface in Albatross knockdown cells, which retain intact microvilli. Analysis of keratin-deficient epithelial cells revealed that keratins are required to stabilize the Albatross protein, thus promoting the formation of AJC. We propose that keratins and the keratin-binding protein Albatross are important for epithelial cell polarization.

Release of integrin macroaggregates as a mechanism of rear detachment during keratinocyte migration

Cell-substrate adhesion can be mediated by the relatively short-lived focal complexes and focal adhesions and by the more stable hemidesmosomes. During cell migration both types of cell-substrate adhesions must be disrupted allowing the cell rear to detach. Rear detachment has been described to be accompanied by membrane ripping and the loss of cellular material in a variety of cell types including fibroblasts and chondrocytes, but also in fast moving cells such as keratinocytes. Here we show that migrating keratinocytes leave behind "migration tracks" of cellular remnants that can be classified due to their size, distribution and molecular composition. Type I macroaggregates appeared as spherical and tubular structures with a diameter of about 50-100 nm that were arranged like "pearls on a string". These structures apparently derived from fragmentation of long tubular extensions, the retracting fibers, at the cell rear and contained high amounts of beta1 integrin and different alpha integrins that are components of fibronectin and laminin receptors in migrating keratinocytes usually found in focal adhesions. Type II macroaggregates were recognized as spherical structures with a diameter of about 30 - 50 nm that were arranged in clusters scattered over the gaps between type I, macroaggregates. In contrast to type I type II macroaggregates contained high amounts of beta4 integrin and seemed to derive from former hemidesmosomes. Both types of macroaggregates were completely membrane covered, impermeable compartments devoid of cytosolic proteins. Our observations strongly support the concept that the release of macroaggregates represents a distinct cellular mechanism of rear detachment based on the loss of adhesive receptors embedded in membrane-covered cellular remnants.

A dynamic podosome-like structure of epithelial cells

Focal contacts and hemidesmosomes are cell-matrix adhesion structures of cultured epithelial cells. While focal contacts link the extracellular matrix to microfilaments, hemidesmosomes make connections with intermediate filaments. We have analyzed hemidesmosome assembly in 804G carcinoma cells. Our data show that hemidesmosomes are organized around a core of actin filaments that appears early during cell adhesion. These actin structures look similar to podosomes described in cells of mesenchymal origin. These podosome-like structures are distinct from focal contacts and specifically contain Arp3 (Arp2/3 complex), cortactin, dynamin, gelsolin, N-WASP, VASP, Grb2 and src-like kinase(s). The integrin alpha3beta1 is localized circularly around F-actin cores and co-distributes with paxillin, vinculin, and zyxin. We also show that the maintenance of the actin core and hemidesmosomes is dependent on actin polymerization, src-family kinases, and Grb2, but not on microtubules. Video microscopy analysis reveals that assembly of hemidesmosomes is preceded by recruitment of beta4 integrin subunit to the actin core before its positioning at hemidesmosomes. When 804G cells are induced to migrate, actin cores as well as hemidesmosomes disappear and beta4 integrin subunit becomes co-localized with dynamic actin at leading edges. We show that podosome-like structures are not unique to cells of mesenchymal origin, but also appear in epithelial cells, where they seem to be related to basement membrane adhesion.

Assembly of desmosomal cadherins into desmosomes is isoform dependent

Desmosomes are intercellular adhesive junctions that exhibit cell- and differentiation-specific differences in their molecular composition. In complex epithelia, desmosomes contain multiple representatives of the desmosomal cadherin family, which includes three desmogleins and three desmocollins. Rules governing the assembly of desmosomal cadherin isoforms into desmosomes of different cell types are unknown. Here we compared the assembly properties of desmoglein 2 (Dsg2) and desmocollin 2 (Dsc2), which are widely expressed, with Dsg1 and Dsc1, which are expressed in the differentiated layers of complex epithelia, by introducing myc-tagged forms into simple and squamous epithelial cells that do not express Dsg1 or Dsc1. Dsc2.myc and Dsg2.myc assembled efficiently into desmosomes in every cell type in spite of significant shifts in the stoichiometric relationship between desmogleins and desmocollins. In contrast, Dsc1a.myc, Dsc1b.myc, and Dsg1.myc did not stably incorporate into desmosomes in any line. Coexpression of Dsc1a.myc or Dsc1b.myc and Dsg1.myc did not lead to their colocalization and failed to enhance incorporation of either cadherin into desmosomes. Dsg1.myc, but not Dsc1a, Dsc1b, disrupted desmosome assembly in a cell-type-specific manner, and disruption correlated with the recruitment of Dsg1.myc, but not Dsc1a or Dsc1b, into a Triton-insoluble pool. The plakoglobin:E-cadherin ratio decreased in Dsg1-expressing cells with disrupted desmosomes, but a decrease was also observed in a Dsc1a line. Thus, a modest reduction of plakoglobin associated with E-cadherin is apparently not sufficient to disrupt desmosome assembly. Our results demonstrate that desmosome assembly tolerates large shifts in cadherin stoichiometry, but is sensitive to isoform-specific differences exhibited by desmogleins and desmocollins.

Cable-piliated Burkholderia cepacia binds to cytokeratin 13 of epithelial cells

Although the Burkholderia cepacia complex consists of several genomovars, one highly transmissible strain of B. cepacia has been isolated from the sputa of cystic fibrosis (CF) patients throughout the United Kingdom and Canada. This strain expresses surface cable (Cbl) pili and is thought to be the major strain associated with the fatal "cepacia syndrome." In the present report we characterize the specific 55-kDa buccal epithelial cell (BEC) protein that binds cable pilus-positive B. cepacia. N-terminal sequences of CNBr-generated internal peptides identified the protein as cytokeratin 13 (CK13). Western blots of BEC extracts probed with a specific monoclonal antibody to CK13 confirmed the identification. Mixed epidermal cytokeratins (which contain CK13), cytokeratin extract from BEC (which consists essentially of CK13 and CK4), and a polyclonal antibody to mixed cytokeratins inhibited B. cepacia binding to CK13 blots and to normal human bronchial epithelial (NHBE) cells. Preabsorption of the antikeratin antibody with the BEC cytokeratin fraction reversed the inhibitory effect of the antibody. A cytokeratin mixture lacking CK13 was ineffective as an inhibitor of binding. Colocalization of CK13 and B. cepacia by confocal microscopy demonstrated that intact nonpermeabilized NHBE cells express small amounts of surface CK13 and bind Cbl-positive B. cepacia in the same location. Binding to intact NHBE cells was dependent on bacterial concentration and was saturable, whereas a Cbl-negative isolate exhibited negligible binding. These findings raise the possibility that surface-accessible CK13 in respiratory epithelia may be a biologically relevant target for the binding of cable piliated B. cepacia.

Regulation of the type II hemidesmosomal plaque assembly in intestinal epithelial cells

Hemidesmosomes (HDs) are cellular junctions that anchor epithelial cells to the extracellular matrix (ECM) and are associated morphologically with the cytoskeleton. Hemidesmosomal molecular components include two proteins involved in linking intermediate filaments, HD1/plectin and BP230, and two transmembrane proteins, BP180 and the alpha6beta4 integrin, a laminin receptor. In cells lacking BP230 and BP180, HD1/plectin still associates with alpha6beta4 integrin, forming HD-like structures, called type II HDs. In the present study, we used an intestinal epithelial cell line that expresses HD1/plectin and the alpha6beta4 integrin to investigate the regulation of assembly of these proteins in type II HDs. These compounds were found to be clustered at sites of cell-ECM contact and their polarized localization was influenced by either cell confluency or extracellular matrix deposition. Conventional and immunoelectron microscopy showed that HD1/plectin and the beta4 integrin subunit are colocalized in an adhesion structure. Using cytoskeleton-disrupting drugs and confocal microscopy, we demonstrated that type II HDs are made up of numerous individual plaques whose assembly into a cluster requires actin filaments, but not microtubules.

Characterization of a tight molecular complex between integrin alpha 6 beta 4 and laminin-5 extracellular matrix

In many adult epithelia, e.g., epidermis or intestine, adhesion of epithelial cells to basement membrane requires the integrin alpha6 beta4 and laminin-5 (Ln-5). In the absence of one or the other, extensive blistering and exfoliation occur. While alpha6 beta4 was reported to be a receptor for Ln-5, this interaction is poorly understood. We characterize complexes between alpha6 beta4 and Ln-5 in cell-free preparations of extracellular matrix (ECM) from the epithelial cell line, 804G. By microsequencing, Ln-5 and alpha6 beta4 were the major proteins in this ECM and were likely engaged in receptor/ligand complexes because, by immunofluorescence, alpha6 beta4 was colocalized with Ln-5 both in cell monolayers and in cell-free ECM preparations, but they disappeared after preincubation of the monolayers with alpha6 beta4 or Ln-5 function-blocking antibodies. The alpha6 beta4/Ln-5 complexes were resistant to dissociation by extreme pH, urea, chaotropes, eDTA, non-ionic detergents, and b-mercaptoethanol. They were only dissociated by strong anionic detergents, e.g., 1% SDS, suggesting receptor/ligand interactions based on high affinity or avidity. We propose that these alpha6 beta4/Ln-5 complexes may provide links between plasma membrane and basement membrane that resist mechanical stress and support epithelial integrity.

Contributions of extracellular and intracellular domains of full length and chimeric cadherin molecules to junction assembly in epithelial cells

The integrity of cell-cell junctions in epithelial cells depends on functional interactions of both extracellular and intracellular domains of cadherins with other junction proteins. To examine the roles of the different domains of E-cadherin and desmoglein in epithelial junctions, we stably expressed full length desmoglein 1 and chimeras of E-cadherin and desmoglein 1 in A431 epithelial cells. Full length desmoglein 1 was able to incorporate into or disrupt endogenous desmosomes depending on expression level. Each of the chimeric cadherin molecules exhibited distinct localization patterns at the cell surface. A chimera of the desmoglein 1 extracellular domain and the E-cadherin intracellular domain was distributed diffusely at the cell surface while the reverse chimera, comprising the E-cadherin extracellular domain and the desmoglein 1 intracellular domain, localized in large, sometimes contiguous patches at cell-cell interfaces. Nevertheless, both constructs disrupted desmosome assembly. Expression of constructs containing the desmoglein 1 cytoplasmic domain resulted in approximately a 3-fold decrease in E-cadherin bound to plakoglobin and a 5- to 10-fold reduction in the steady-state levels of the endogenous desmosomal cadherins, desmoglein 2 and desmocollin 2, possibly contributing to the dominant negative effect of the desmoglein 1 tail. In addition, biochemical analysis of protein complexes in the stable lines revealed novel in vivo protein interactions. Complexes containing beta-catenin and desmoglein 1 were identified in cells expressing constructs containing the desmoglein 1 tail. Furthermore, interactions were identified between endogenous E-cadherin and the chimera containing the E-cadherin extracellular domain and the desmoglein 1 intracellular domain providing in vivo evidence for previously predicted lateral interactions of E-cadherin extracellular domains.

Monoclonal antibody P-31 recognizes a novel intermediate filament-associated protein (p250) in rat podocytes

The visceral glomerular epithelial cells (GECs) or podocytes of the renal glomerulus constitute a highly specialized epithelium. To study the nature of podocytes, we established mouse monoclonal antibodies against GEC. Clone P-31 reacted exclusively with the cytoplasm of GEC by immunofluorescence. Immunoblot analysis with P-31 showed that a single band of 250 kDa was detectable in a glomerular lysate. The 250-kDa polypeptide (p250) was recovered from Triton X-100-insoluble fractions of isolated glomeruli, suggesting that this molecule is associated with the cytoskeleton. Immunogold staining with P-31 demonstrated that the gold particles were located at the intersections of vimentin-type intermediate filaments of podocytes. In developing kidney, this protein first appeared in immature GECs during the S-shaped body stage. In puromycin aminonucleoside nephrosis, p250 was dramatically increased in glomeruli where enhanced desmin expression was observed in GECs. These results indicate that p250 is a novel intermediate filament-associated protein and plays a role in the organization of the intermediate filament network in both normal and diseased conditions.

Splice variant-specific interaction of the NMDA receptor subunit NR1 with neuronal intermediate filaments

NMDA receptors are excitatory neurotransmitter receptors critical for synaptic plasticity and neuronal development in the mammalian brain. These receptors are found highly concentrated in the postsynaptic membrane of glutamatergic synapses. To investigate the molecular mechanisms underlying NMDA receptor localization, we used the yeast two-hybrid system to identify proteins expressed in the brain that interact with the NMDA receptor subunit NR1. Here we report that the 68 kDa neurofilament subunit NF-L directly interacts with the NR1 subunit. This interaction occurs between the cytoplasmic C-terminal domain of NR1 and the rod domain of NF-L. However, NR1 splice variants lacking the first C-terminal exon cassette (C1) failed to associate with NF-L. Immunogold electron microscopy revealed a preferential localization of NR1 at the ends of in vitro-assembled neurofilaments. Overexpression of C1 cassette-containing NR1 constructs in fibroblast cells disrupted the assembly of recombinant neurofilaments. In addition, NR1 and NF-L cofractionated in detergent-treated rat brain synaptic plasma membranes. Furthermore, NR1 and NF-L colocalize in the dendrites and growth cones of cultured hippocampal neurons. These results demonstrate the splice variant-specific association of NR1 with neurofilaments and suggest a possible mechanism for anchoring or localizing NMDA receptors in the neuronal plasma membrane.

Two-hybrid analysis reveals fundamental differences in direct interactions between desmoplakin and cell type-specific intermediate filaments

Desmosomes are cell junctions that act as sites of strong intercellular adhesion and also serve to anchor the intermediate filament (IF) cytoskeleton to the plasma membrane of a variety of cell types. Previous studies demonstrated that the COOH terminus of the desmosomal plaque protein, desmoplakin (DP), is required for the association of DP with IF networks in cultured cells and that this domain interacts directly with type II epidermal keratin polypeptides in vitro. However, these studies left open the question of how desmosomes might anchor other IF types known to associate with these junctions. In this report we used yeast two-hybrid and in vitro dot blot assays to further examine the requirements for direct interactions between desmoplakin and various IF types. Our results confirm the ability of the DP COOH terminus (DPCT) to interact with at least two regions of the head domain of the type II epidermal keratin K1 and also demonstrate that DPCT can interact with the type III IF family members, vimentin and desmin, as well as simple epithelial keratins. Unlike the situation for type II epidermal keratins, the interaction between DPCT and simple epithelial keratins appears to depend on heterodimerization of the type I and II keratin polypeptides, since both are required to detect an interaction. Furthermore, although the interaction between DPCT and K1 requires the keratin head domain, deletion of this domain from the simple epithelial keratins does not compromise interaction with DPCT. The interaction between DPCT and type III or simple epithelial keratins also appeared to be less robust than that between DPCT and K1. In the case of K8/K18, however, the interaction as assessed by yeast two-hybrid assays increased 9-fold when a serine located in a protein kinase A consensus phosphorylation site 23 residues from the end of DP was altered to a glycine. Taken together, these data indicate that DP interacts directly with different IF types in specific ways.

Laminin-5 and modulation of keratin cytoskeleton arrangement in FG pancreatic carcinoma cells: involvement of IFAP300 and evidence that laminin-5/cell interactions correlate with a dephosphorylation of alpha 6A integrin

Under normal culture conditions, epithelial cells of the FG line, derived from a pancreatic tumor, characteristically grow in mounds and fail to flatten efficiently onto their substrate. In such cells, keratin intermediate filaments (IFs) are concentrated in the perinuclear region. Furthermore, the IF associated protein, IFAP300, primarily localizes along these keratin bundles. Additionally, alpha 6 beta 4 integrin heterodimers localize in streaks or spots towards the edges of cells while alpha 3 beta 1 integrin is predominantly at cell-cell surfaces. Neither show any obvious interaction with IF. Remarkably, upon plating FG cells into medium containing soluble rat laminin-5, FG cells rapidly adhere and spread onto their substrate. Moreover, FG cells "capture" rat laminin-5 and place it basally in circles or arcs at areas of cell-substrate interaction. Double label immunofluorescence microscopy reveals colocalization of IFAP300 as well as alpha 6 beta 4 and alpha 3 beta 1 integrin with the polarized laminin-5. Concomitantly, alpha 6 integrin undergoes dephosphorylation on serine residue 1041. Laminin-5-induced rapid adhesion can be blocked by antibodies against the alpha 3 integrin subunit. In contrast, while alpha 6 integrin antibodies do not block laminin-5-induced rapid adhesion, they prevent FG cells from assuming an epithelial-like morphology. Keratin IF bundles associate with IFAP300-alpha 6 beta 4/alpha 3 beta 1 integrin complexes along the cell-substratum-attached surface of FG cells coincubated in laminin-5-containing medium. Coprecipitation results suggest that in these complexes, IFAP300 may associate with the alpha 6 beta 4 integrin heterodimer. Based on our results and published evidence that IFAP300 binds keratin in vitro [Skalli et al., 1994; J. Cell Biol. 125:159-170], we propose that laminin-5/FG cell interaction results in a novel integrin dephosphorylation event, which subsequently induces IFAP300 association with alpha 6 beta 4 integrin. IFAP300 then mediates the interaction of IFs with the cell surface via the alpha 6 beta 4 integrin heterodimer.

Quantitative analysis of cadherin-catenin-actin reorganization during development of cell-cell adhesion

Epithelial cell-cell adhesion requires interactions between opposing extracellular domains of E-cadherin, and among the cytoplasmic domain of E-cadherin, catenins, and actin cytoskeleton. Little is known about how the cadherin-catenin-actin complex is assembled upon cell-cell contact, or how these complexes initiate and strengthen adhesion. We have used time-lapse differential interference contrast (DIC) imaging to observe the development of cell-cell contacts, and quantitative retrospective immunocytochemistry to measure recruitment of proteins to those contacts. We show that E-cadherin, alpha-catenin, and beta-catenin, but not plakoglobin, coassemble into Triton X-100 insoluble (TX-insoluble) structures at cell-cell contacts with kinetics similar to those for strengthening of E-cadherin-mediated cell adhesion (Angres, B., A. Barth, and W.J. Nelson. 1996. J. Cell Biol. 134:549-557). TX-insoluble E-cadherin, alpha-catenin, and beta-catenin colocalize along cell-cell contacts in spatially discrete micro-domains which we designate "puncta," and the relative amounts of each protein in each punctum increase proportionally. As the length of the contact increases, the number of puncta increases proportionally along the contact and each punctum is associated with a bundle of actin filaments. These results indicate that localized clustering of E-cadherin/catenin complexes into puncta and their association with actin is involved in initiating cell contacts. Subsequently, the spatial ordering of additional puncta along the contact may be involved in zippering membranes together, resulting in rapid strengthening of adhesion.

Plectin and human genetic disorders of the skin and muscle. The paradigm of epidermolysis bullosa with muscular dystrophy

Recent progress in understanding the molecular organization of the cutaneous basement membrane zone (BMZ) has revealed an intricate network of structural proteins necessary for stable association of the epidermis to the underlying dermis. Molecular genetics of the cutaneous BMZ has also revealed that defects in as many as nine distinct genes within the dermal-epidermal junction which result in different forms of epidermolysis bullosa (EB), a group of heritable mechano-bullous disorders. We have recently demonstrated that a variant of EB associated with late-onset development of muscular dystrophy (EB-MD, MIM no. 226670) results from mutations in the gene encoding plectin (PLEC1), a cytoskeleton associated attachment protein present in the hemidesmosomal inner plaque and the sarcolemma of the muscle. Consequently, mutations in this multi-functional gene/protein system can result in phenotypic manifestations of EB-MD both in the skin and the muscle. In this overview, we will summarize the domain organization of plectin and the structure of the corresponding gene (PLEC1), as well as the genetic basis of EB-MD in families studied thus far. Elucidation of the molecular basis of this subtype of EB adds to our understanding of the structural and functional complexity of the cutaneous BMZ.

Plectin deficiency results in muscular dystrophy with epidermolysis bullosa

We report that mutation in the gene for plectin, a cytoskeleton-membrane anchorage protein, is a cause of autosomal recessive muscular dystrophy associated with skin blistering (epidermolysis bullosa simplex). The evidence comes from absence of plectin by antibody staining in affected individuals from four families, supportive genetic analysis (localization of the human plectin gene to chromosome 8q24.13-qter and evidence for disease segregation with markers in this region) and finally the identification of a homozygous frameshift mutation detected in plectin cDNA. Absence of the large multifunctional cytoskeleton protein plectin can simultaneously account for structural failure in both muscle and skin.

Loss of plectin causes epidermolysis bullosa with muscular dystrophy: cDNA cloning and genomic organization

Plectin is a widely expressed high molecular weight protein that is involved in cytoskeleton-membrane attachment in epithelial cells, muscle, and other tissues. The human autosomal recessive disorder epidermolysis bullosa with muscular dystrophy (MD-EBS) shows epidermal blister formation at the level of the hemidesmosome and is associated with a myopathy of unknown etiology. Here, plectin was found to be absent in skin and cultured keratinocytes from an MD-EBS patient by immunofluorescence and immunoprecipitation, suggesting that plectin is a candidate gene/protein system for MD-EBS mutation. The 14800-bp human plectin cDNA was cloned and sequenced. The predicted 518-kD polypeptide has homology to the actin-binding domain of the dystrophin family at the amino terminus, a central rod domain, and homology to the intermediate filament-associated protein desmoplakin at the carboxyl terminus. The corresponding human gene (PLEC1), consisting of 33 exons spanning >26 kb of genomic DNA was cloned, sequenced, and mapped to chromosomal band 8q24. Homozygosity by descent was observed in the consanguineous MD-EBS family with intragenic plectin polymorphisms. Direct sequencing of PCR-amplified plectin cDNA from the patient's keratinocytes revealed a homozygous 8-bp deletion in exon 32 causing a frameshift and a premature termination codon 42 bp downstream. The clinically unaffected parents of the proband were found to be heterozygous carriers of the mutation. These results establish the molecular basis of MD-EBS in this family and clearly demonstrate the important structural role for plectin in cytoskeleton-membrane adherence in both skin and muscle.

Hemidesmosome ontogeny in digit skin of the human fetus

Hemidesmosomes are junctional complexes involved in the attachment of epidermal basal keratinocytes to the basement membrane. To try to understand better the sequence of events in the morphogenesis of hemidesmosomes, we undertook an ultrastructural analysis of hemidesmosome formation in fetal and neonatal digit skin. Hemidesmosomes, defined as membrane-associated densities or plaques, were counted and scored for three morphological characteristics: (1) the presence of a sub-basal dense plate, (2) association with anchoring filaments within the lamina lucida and (3) contacts with intermediate filaments. No hemidesmosomes were evident at 7 weeks' gestational age. Between 9 and 15 weeks the number of hemidesmosomes increased by about fourfold (from 20.6 +/- 3.8 (SD) to 95.5 +/- 8.4 per 40 micro m of basal cell plasma membrane; P < 0.01). The association of hemidesmosomes with intermediate filaments and anchoring filaments also increased after 15 weeks (P < 0.05). Early attachment plaques first appeared as triangular focal densities on the basal plasma membrane with the appearance of sub-basal dense plates, which later became both larger and more electron dense. By 15 weeks, an inner plaque could be distinguished from the outer plaque, which coincided with a closer association with intermediate filaments. Hemidesmosomes appeared fully developed by 15 weeks' gestation. This study illustrates the structural relationship of hemidesmosomes to both intra- and extracellular filaments, suggesting close functional interactions. The complexity of the hemidesmosome plaque is also revealed early during development.

Anchorage mediated by integrin alpha6beta4 to laminin 5 (epiligrin) regulates tyrosine phosphorylation of a membrane-associated 80-kD protein

Detachment of basal keratinocytes from basement membrane signals a differentiation cascade. Two integrin receptors alpha6beta4 and alpha3beta1 mediate adhesion to laminin 5 (epiligrin), a major extracellular matrix protein in the basement membrane of epidermis. By establishing a low temperature adhesion system at 4 degrees C, we were able to examine the exclusive role of alpha6beta4 in adhesion of human foreskin keratinocyte (HFK) and the colon carcinoma cell LS123. We identified a novel 80-kD membrane-associated protein (p80) that is tyrosine phosphorylated in response to dissociation of alpha6beta4 from laminin 5. The specificity of p80 phosphorylation for laminin 5 and alpha6beta4 was illustrated by the lack of regulation of p80 phosphorylation on collagen, fibronectin, or poly-L-lysine surfaces. We showed that blocking of alpha3beta1 function using inhibitory mAbs, low temperature, or cytochalasin D diminished tyrosine phosphorylation of focal adhesion kinase but not p80 phosphorylation. Therefore, under our assay conditions, p80 phosphorylation is regulated by alpha6beta4, while motility via alpha3beta1 causes phosphorylation of focal adhesion kinase. Consistent with a linkage between p80 dephosphorylation and alpha6beta4 anchorage to laminin 5, we found that phosphatase inhibitor sodium vanadate, which blocked the p80 dephosphorylation, prevented the alpha6beta4-dependent cell anchorage to laminin 5 at 4degreesC. In contrast, adhesion at 37 degrees C via alpha3beta1 was unaffected. Furthermore, by in vitro kinase assay, we identified a kinase activity for p80 phosphorylation in suspended HFKs but not in attached cells. The kinase activity, alpha6beta4, and its associated adhesion structure stable anchoring contacts were all cofractionated in the Triton-insoluble cell fraction that lacks alpha3beta1. Thus, regulation of p80 phosphorylation, through the activities of p80 kinase and phosphatase, correlates with alpha6beta4-SAC anchorage to laminin 5 at 4 degrees C in epithelial cells of the skin and intestine. Transmembrane signaling through p80 is an early tyrosine phosphorylation event responsive to and possibly required for anchorage to laminin 5 by HFK and LS123 epithelial cells.

Rapid spreading and mature hemidesmosome formation in HaCaT keratinocytes induced by incubation with soluble laminin-5r

HaCaT cells, an immortalized keratinocyte line, incubated in plastic wells in the presence of conditioned medium from 804G cells adhered and spread rapidly in less than 30 min. In contrast, cells plated in fibroblast or keratinocyte conditioned medium adhered poorly and remained rounded at 30 min. Immunodepletion of 804G conditioned medium with polyclonal antisera to laminin-5r, but not control antisera, abolished rapid cell spreading. Electron microscopy of HaCaT cells spread by incubation in 804G conditioned medium, but not control medium, revealed mature hemidesmosomes after 24 h. Rapid spreading was also observed in wells precoated with 804G conditioned medium or 804G cell-deposited matrix, but not with fibronectin, vitronectin, or laminin-1. Immunoblotting of 804G conditioned medium with anti-laminin-5r antibodies unveiled polypeptides of 150, 140, 135, and 100 kDa, identical by electrophoretic mobility to immunoreactive polypeptides in 804G deposited matrix. Our results suggest that addition of laminin-5r in a soluble form is sufficient to promote rapid spreading and hemidesmosome assembly in keratinocytes. The mechanism of soluble laminin-5r action may include efficient surface "priming" for cell adhesion. Soluble laminin-5r may have a physiologic role in morphogenesis and repair of the epidermis and may be of use for therapeutic applications.

Transforming growth factor-alpha abrogates glucocorticoid-stimulated tight junction formation and growth suppression in rat mammary epithelial tumor cells

The glucocorticoid and transforming growth factor-alpha (TGF-alpha) regulation of growth and cell-cell contact was investigated in the Con8 mammary epithelial tumor cell line derived from a 7,12-dimethylbenz(alpha)anthracene-induced rat mammary adenocarcinoma. In Con8 cell monolayers cultured on permeable filter supports, the synthetic glucocorticoid, dexamethasone, coordinately suppressed [3H]thymidine incorporation, stimulated monolayer transepithelial electrical resistance (TER), and decreased the paracellular leakage of [3H]inulin or [14C]mannitol across the monolayer. These processes dose dependently correlated with glucocorticoid receptor occupancy and function. Constitutive production of TGF-alpha in transfected cells or exogenous treatment with TGF-alpha prevented the glucocorticoid growth suppression response and disrupted tight junction formation without affecting glucocorticoid responsiveness. Treatment with hydroxyurea or araC demonstrated that de novo DNA synthesis is not a requirement for the growth factor disruption of tight junctions. Immunofluorescence analysis revealed that the ZO-1 tight junction protein is localized exclusively at the cell periphery in dexamethasone-treated cells and that TGF-alpha caused-ZO-1 to relocalize from the cell periphery back to a cytoplasmic compartment. Taken together, our results demonstrate that glucocorticoids can coordinately regulate growth inhibition and cell-cell contact of mammary tumor cells and that TGF-alpha, can override both effects of glucocorticoids. These results have uncovered a novel functional "cross-talk" between glucocorticoids and TGF-alpha which potentially regulates the proliferation and differentiation of mammary epithelial cells.

Junctional epidermolysis bullosis: defects in expression of epiligrin/nicein/kalinin and integrin beta 4 that inhibit hemidesmosome formation

Junctional epidermolysis bullosis (JEB) is a heterogeneous inherited blistering disorder of human epithelial basement membranes (BMs). Characteristically, the epidermis detaches from the BM between the basal cells and the lamina lucida due to reduced numbers of hemidesmosomes (HDs). Attempts to identify a candidate gene for JEB led to the characterization of nicein, a protein complex in normal BMs that is absent from BMs of patients with JEB gravis. In independent research, two related BM glycoproteins, epiligrin and kalinin, were identified as functional adhesion components of HDs. Epiligrin was characterized as a BM ligand for basal cell adhesion via integrins alpha 3 beta 1 in focal adhesions and alpha 6 beta 4 in HDs. Kalinin was characterized as an adhesive ligand and a component of anchoring filaments. Recent antibody and sequence studies on epiligrin/nicein/kalinin have identified limited homologies with laminin. Ongoing studies in multiple laboratories seek to identify mutations in one or more of the three subunits of epiligrin that are causal in JEB gravis. Consistent with the genetic heterogeneity of JEB, we have identified a patient with a variant form of JEB that is associated with pyloric atresia. This patient has negligible HDs, normal epiligrin, but reduced expression of integrin beta 4. A defect in the beta 4 expression may define a subset of JEB cases that also present with pyloric atresia. These results testify to the dual requirements for epiligrin in the BM and integrin beta 4 in the plasma membrane in regulating function of HDs in epithelium.

Increased expression of keratin 16 causes anomalies in cytoarchitecture and keratinization in transgenic mouse skin

Injury to epidermis and other stratified epithelia triggers profound but transient changes in the pattern of keratin expression. In postmitotic cells located at the wound edge, a strong induction of K6, K16, and K17 synthesis occurs at the expense of the keratins produced under the normal situation. The functional significance of these alterations in keratin expression is not known. Here, we report that overexpression of a wild-type human K16 gene in a tissue-specific fashion in transgenic mice causes aberrant keratinization of the hair follicle outer root sheath and proximal epidermis, and it leads to hyperproliferation and increased thickness of the living layers (acanthosis), as well as cornified layers (hyperkeratosis). The pathogenesis of lesions in transgenic mouse skin begins with a reorganization of keratin filaments in postmitotic keratinocytes, and it progresses in a transgene level-dependent fashion to include disruption of keratinocyte cytoarchitecture and structural alterations in desmosomes at the cell surface. No evidence of cell lysis could be found at the ultrastructural level. These results demonstrate that the disruption of the normal keratin profile caused by increased K16 expression interferes with the program of terminal differentiation in outer root sheath and epidermis. They further suggest that when present at sufficiently high intracellular levels, K16, along with K6 and K17, appear capable of inducing a reorganization of keratin filaments in the cytoplasm of skin epithelial cells.

Band 6 protein, a major constituent of desmosomes from stratified epithelia, is a novel member of the armadillo multigene family

Desmosomes are intercellular adhering junctions characteristic of epithelial cells. Several constitutive proteins--desmoplakin, plakoglobin and the transmembrane glycoproteins desmoglein and desmocollin--have been identified as fundamental constituents of desmosomes in all tissues. A number of additional and cell type-specific constituents also contribute to desmosomal plaque formation. Among these proteins is the band 6 polypeptide (B6P). This positively charged, non-glycosylated protein is a major constituent of the plaque in stratified and complex glandular epithelia. Using an overlay assay we show that purified keratins bind in vitro to B6P. Thus B6P may play a role in ordering intermediate filament networks of adjacent epithelial cells. To characterize the structure of B6P in the desmosome we have isolated cDNA clones representing the entire coding sequence. The predicted amino acid sequence of human B6P shows strong sequence homology with a murine p120 protein, which is a substrate of protein tyrosine kinase receptors and of p60v-src. P120 and B6P show amino-terminal domains differing distinctly in length and sequence. These are followed in both proteins by 460 residues that display a series of imperfect repeats corresponding to the repeats in the cadherin binding proteins armadillo, plakoglobin and beta-catenin. Over this repeat region B6P and p120 share 33% sequence identity (54% similarity). These sequence characteristics define B6P as a novel member of the armadillo multigene family and raise the question of whether the structural proteins B6P, plakoglobin, beta-catenin and armadillo share some function. Since armadillo, plakoglobin, beta-catenin and p120 seem involved in signal transduction this may also hold for B6P. The amino-terminal region of B6P (residues 1 to 263) shows no significant homology to any known protein sequence. It may therefore be involved in unique functions of B6P.

Identification of a second protein product of the gene encoding a human epidermal autoantigen

A 230 kDa polypeptide component of the hemidesmosome, an epithelial-cell-connective-tissue attachment device, is thought to be involved in cytoskeleton-cell-surface anchorage. This 230 kDa polypeptide is recognized by bullous pemphigoid auto-antibodies and for this reason is generally termed the bullous pemphigoid antigen (BPA). We have identified two distinct mRNA products of the single BPA gene by RACE (rapid amplification of cDNA ends)/PCR techniques. The first of these mRNAs encodes the 230 kDa protein component of the hemidesmosome. A second mRNA lacks over 1800 bases that encode the C-terminus of the 230 kDa protein. We have raised antibodies against a peptide specific to the predicted protein product of this second mRNA. To our surprise this antibody recognizes a protein that migrates at 280 kDa on SDS/PAGE of extracts of a variety of human epidermal cell lines that also express the 230 kDa BPA. Moreover, we have confirmed the co-expression of the 230 and 280 kDa polypeptides in these cells by immunoblotting analyses using a monoclonal antibody preparation directed against a polypeptide encoded by sequence common to both mRNAs transcribed from the BPA gene. Intriguingly, in one non-epidermal tumour line (a pancreatic cell line termed FG), the 280 kDa polypeptide appears to be the only product of the BPA gene. Furthermore, in FG cells the 280 kDa protein is found in association with the intermediate filament cytoskeleton. We discuss our results in relation to control of BPA gene expression and with regard to potential functions of the domains of the protein products of the BPA gene.

IFAP 300 is common to desmosomes and hemidesmosomes and is a possible linker of intermediate filaments to these junctions

The distribution of IFAP 300, a protein previously characterized as cross-linking vimentin intermediate filaments (IF), has been investigated in epithelial cells. In frozen sections of bovine tongue epithelium the staining obtained with IFAP 300 antibodies is concentrated in the peripheral cytoplasm of keratinocytes, including the entire peripheral region of basal cells. Further immunofluorescence studies reveal that in primary cultures of mouse keratinocytes the distribution of IFAP 300 is similar to that of the desmosomal protein desmoplakin. In rat bladder carcinoma 804G cells the staining pattern of IFAP 300 antibodies coincides with that obtained with antibodies against the hemidesmosomal protein BP 230. By immunogold electron microscopy IFAP 300 is mainly located at sites where IF appear to attach to desmosomes and hemidesmosomes. Morphometric analyses of the distribution of the gold particles show that IFAP 300 overlaps with desmoplakin and BP 230, but also that it extends deeper into the cytoplasm than these latter two proteins. The staining reaction seen in epithelial cells by immunofluorescence and immunogold is specific for IFAP 300 as shown by immunoblotting. Immunoblotting also reveals that IFAP 300 is present in both cell-free preparations of desmosomes and hemidesmosomes. These morphological and biochemical results are intriguing since, in recent years, the proteins appearing in these two types of junctions have been found to be different. One possible exception is plectin, a protein that has been suggested to be very similar to IFAP 300. However, we show here that IFAP 300 differs from plectin in several respects, including differences at the primary sequence level. We also show that purified IFAP 300 pellets with in vitro polymerized IF prepared from desmosome-associated keratins under conditions in which IFAP 300 alone is not sedimentable. This indicates that IFAP 300 can associate with keratin IF. These data, taken together with the immunogold results, suggest that IFAP 300 functions in epithelial cells as a linker protein connecting IF to desmosomes as well as to hemidesmosomes, possibly through structurally related proteins such as desmoplakin and BP 230, respectively.

Identification of a keratin-associated protein that localizes to a membrane compartment

We describe the characterization of an acidic glycoprotein (molecular mass approximately 85 kDa) that associates with keratin intermediate filaments of 'simple'-type epithelia. Using a number of anti-keratin monoclonal antibodies, the 85 kDa glycoprotein was identified by co-immunoprecipitation with keratin polypeptides 8 and 18 (K8/18) from the human colonic epithelial cell line HT29 and several other epithelial cell lines. This Keratin-Associated Protein (termed KAP85) was readily detected after in vitro galactosylation of K8/18 immunoprecipitates obtained from mitosis-arrested cells. Its solubilization and detection were dependent on the detergent used, and it was barely detected after in vitro galactosylation of asynchronously growing G0/G1-phase cells. Its poor in vitro galactosylation in G0/G1-phase cells is likely a reflection of the lack of available terminal N-acetylglucosamine residues, since it can be labelled to a similar extent in G0/G1- and G2/M-phase cells using NaIO4/NaB3H4. Glycosidase digestion showed that KAP85 contains high mannose and complex oligosaccharides. Fractionation of total cellular K8/18 into soluble and cytoskeletal insoluble pools showed that KAP85 associates exclusively with the cytoskeletal K8/18 pool. Subcellular fractionation showed that KAP85 co-localizes with a plasma-membrane-enriched fraction that includes the transferrin receptor and KS-1 antigen. Our results demonstrate in vitro evidence of a membrane-associated glycoprotein (KAP85) which may serve as an attachment site for filamentous K8/18.

Purification of the 230-kD bullous pemphigoid antigen (BP230) from bovine tongue mucosa: structural analyses and assessment of BP230 tissue distribution using a new monoclonal antibody

In the epidermis the autoantigen BP230 is a component of the hemidesmosomal plaque. We have developed a procedure for the isolation of BP230 from bovine tongue mucosa using chromatographic means. The identity of the isolated protein was confirmed by its recognition by bullous pemphigoid autoantibodies. A monoclonal antibody (MoAb230), generated against the purified protein, localizes to the region of the plaque of the hemidesmosome with which keratin bundles interact. Furthermore, the tissue distribution of BP230, assessed using MoAb230, suggests that BP230 or an immunologically related protein is a component of all hemidesmosomes. Ultrastructural analyses of the BP230 preparation reveal that the BP230 molecules assemble into macromolecular aggregates. The few images of individual intact molecules that we have observed in platinum replicas of rotary shadowed BP230 preparations suggest that BP230 is an elongate rod-shaped molecule. This is consistent with predictions based on the primary sequence of BP230 deduced from BP230 cDNAs reported by others. We discuss our results in relation to the potential function of BP230. Isolation of BP230 should now allow more rigorous biochemical analyses of potential protein-protein interactions of BP230 in the hemidesmosome.

Functional analysis of desmoplakin domains: specification of the interaction with keratin versus vimentin intermediate filament networks

We previously demonstrated that truncated desmoplakin I (DP I) molecules containing the carboxyl terminus specifically coalign with and disrupt both keratin and vimentin intermediate filament (IF) networks when overexpressed in tissue culture cells (Stappenbeck, T. S., and K. J. Green. J. Cell Biol. 116:1197-1209). These experiments suggested that the DP carboxyl-terminal domain is involved either directly or indirectly in linking IF with the desmosome. Using a similar approach, we have now investigated the behavior of ectopically expressed full-length DP I in cultured cells. In addition, we have further dissected the functional sequences in the carboxyl terminus of DP I that facilitate the interaction with IF networks. Transient transfection of a clone encoding full-length DP I into COS-7 cells produced protein that appeared in some cells to associate with desmosomes and in others to coalign with and disrupt IF. Deletion of the carboxyl terminus from this clone resulted in protein that still appeared capable of associating with desmosomes but not interacting with IF networks. As the amino terminus appeared to be dispensable for IF interaction, we made finer deletions in the carboxyl terminus of DP based on blocks of sequence similarity with the related molecules bullous pemphigoid antigen and plectin. We found a sequence at the very carboxyl terminus of DP that was necessary for coalignment with and disruption of keratin IF but not vimentin IF. Furthermore, the coalignment of specific DP proteins along keratin IF but not vimentin IF was correlated with resistance to extraction by Triton. The striking uncoupling resulting from the deletion of specific DP sequences suggests that the carboxyl terminus of DP interacts differentially with keratin and vimentin IF networks.

Genetic skin diseases caused by mutations in keratin intermediate filaments

Keratin intermediate filaments are the major differentiation products of epithelial cells such as the epidermis. The filaments are highly dynamic entities involved in the maintenance of the structural integrity of both the individual cells and the entire tissue. Recent biochemical studies suggest that the keratin proteins overlap each other in several key locations when packed together in filaments. Interestingly, mutations that introduce inappropriate amino acid substitutions in at least some of these overlap regions cause defective keratin filaments that result in at least three classes of autosomal dominant skin disease.

The matrix secreted by 804G cells contains laminin-related components that participate in hemidesmosome assembly in vitro

Hemidesmosomes are important adhesion devices found in epithelial cells. They connect the intermediate filament cytoskeleton network with components of the basement membrane zone. 804G cells are an unusual epithelial cell line, since they form bona fide hemidesmosomes when plated on glass or plastic. In this study we tested an hypothesis: that this ability is a consequence of an extracellular component produced by the 804G cells. As probes for our study we generated a rabbit antiserum (J18) and monoclonal antibodies against components of urea-solubilized 804G matrix. Antibodies in the J18 serum recognize major lectin-binding polypeptides of 150, 140 and 135 kDa in the 804G matrix. A monoclonal antibody (5C5) that shows reactivity with the 150 and 135 kDa polypeptides in western immunoblots immunoprecipitates all three molecular mass species, indicating that these polypeptides are part of a matrix complex. Moreover, one, at least, of these matrix elements is immunologically related to laminin, since J18 antibodies selected on fusion protein fragments of a newly characterized laminin variant, laminin B2t (Kallunki et al., J. Cell Biol., 119, 679–694, 1992), react with the 140 kDa polypeptide component of the 804G cell matrix. To undertake functional analyses of 804G matrix, cells of the human epidermal carcinoma line SCC12, which do not assemble bona fide hemidesmosomes in vitro, were cultured on 804G matrix for 24 h and then analysed by confocal immunofluorescence and electron microscopy. In SCC12 cells maintained on 804G cell matrix, hemidesmosomal antigens localize in a distinctive leopard spot pattern that mirrors the distribution of 804G matrix elements. Furthermore, ultrastructural analysis reveals that the 804G cell matrix supports the formation of ‘mature’ hemidesmosomes by SCC12 cells. Thus 804G cell matrix is a remarkable tool for hemidesmosome studies and it will now be of great importance to determine the exact composition of the 804G matrix, especially its structural and antigenic relationship to laminins.

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.

Method of cell handling affects leakiness of cell surface labeling and detection of intracellular keratins

Keratins are a subgroup of cytoskeletal intermediate filament proteins found in most epithelial cells. Some reports have suggested that keratins may be found on the cell surface as well as their well-accepted cytoskeletal location. A major part of the evidence in the interpretation of cell surface expression of keratins is cell surface radioiodination. Here we show that lactoperoxidase-catalyzed iodination of colonic and breast tissue culture cells results in radiolabeling of the keratins when cells are manipulated. No labeling of keratins is detected when cells are labeled directly on the tissue culture dish. A similar result was obtained when intact cells were biotinylated using water-soluble sulfo-NHS-biotin. Partitioning of the keratins to a soluble and an insoluble pool after "cell surface" 125I-labeling showed that both pools became iodinated. Indirect immunofluorescence showed that binding of a panel of anti-keratin antibodies to intact epithelial cells occurs only on the cells that are more adherent, which are the cells that require longer manipulation to remove from the tissue culture dish. Taken together, our results indicate that the reported expression of cell surface keratins in some cells likely reflects intracellular keratins. In addition, the method of epithelial cell handling can dramatically alter the leakiness of cell surface iodination techniques.

Binding of nucleic acids to intermediate filaments of the vimentin type and their effects on filament formation and stability

Guanine-rich polynucleotides such as poly(dG), oligo(dG)12-18 or poly(rG) were shown to exert a strong inhibitory effect on vimentin filament assembly and also to cause disintegration of preformed filaments in vitro. Gold-labeled oligo(dG)25 was preferentially localized at the physical ends of the aggregation and disaggregation products and at sites along filaments with a basic periodicity of 22.7 nm. Similar effects were observed with heat-denatured eukaryotic nuclear DNA or total rRNA, although these nucleic acids could affect filament formation and structure only at ionic strengths lower than physiological. However, whenever filaments were formed or stayed intact, they appeared associated with the nucleic acids. These electron microscopic observations were corroborated by sucrose gradient analysis of complexes obtained from preformed vimentin filaments and radioactively labeled heteroduplexes. Among the duplexes of the DNA type, particularly poly(dG).poly(dC), and, of those of the RNA type, preferentially poly(rA).poly(rU), were carried by the filaments with high efficiency into the pellet fraction. Single-stranded 18S and 28S rRNA interacted only weakly with vimentin filaments. Nevertheless, in a mechanically undisturbed environment, vimentin filaments could be densely decorated with intact 40S and 60S ribosomal subunits as revealed by electron microscopy. These results indicate that, in contrast to single-stranded nucleic acids with their compact random coil configuration, double-stranded nucleic acids with their elongated and flexible shape have the capability to stably interact with the helically arranged, surface-exposed amino-terminal polypeptide chains of vimentin filaments. Such interactions might be of physiological relevance in regard to the transport and positioning of nucleic acids and nucleoprotein particles in the various compartments of eukaryotic cells. Conversely, nucleic acids might be capable of affecting the cytoplasmic organization of vimentin filament networks through their filament-destabilizing potentials.

Laminins and other strange proteins

Laminins are large multidomain proteins of the extracellular matrix (ECM) with important functions in the development and maintenance of cellular organization and supramolecular structure, in particular in basement membranes. Each molecule is composed of three polypeptide chains, A (300-400 kDa) and B1 and B2 (180-200 kDa), which together form the characteristic cross-shaped laminin structure with three short arms and one long arm. Many different domains have been identified in laminin by sequence analysis, structural investigations, and functional studies. Each short arm is formed by homologous N-terminal portions of one of the three chains. Structurally, each short arm contains two or three globular domains which are connected by rows of manyfold-repeated Cys-rich "EGF-like" domains. In all three chains this region is followed by a long heptad repeat region similar to those found in many alpha-helical coiled-coil proteins. These parts of the three laminin chains constitute a triple-stranded coiled-coil domain, which forms the extended rodlike structure of the long arm. This is the only domain in the protein which is made up of more than one chain and consequently serves the function of chain assembly. The two B chains are terminated by the coiled-coil domain, but the A chain contains an additional C-terminal segment which accounts for five globular domains located at the tip of the long arm. Several important functions of laminin have been assigned to individual domains in either the short arms or terminal regions of the long arm.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytoskeleton--plasma membrane interactions

Proteins at the boundary between the cytoskeleton and the plasma membrane control cell shape, delimit specialized membrane domains, and stabilize attachments to other cells and to the substrate. These proteins also regulate cell locomotion and cytoplasmic responses to growth factors and other external stimuli. This diversity of cellular functions is matched by the large number of biochemical mechanisms that mediate the connections between membrane proteins and the underlying cytoskeleton, the so-called membrane skeleton. General organizational themes are beginning to emerge from examination of this biochemical diversity.

Identification of an epithelial protein related to the desmosome and intermediate filament network

Using a mAb, referred to as 08L, we have identified a protein, of M(r) approximately 140,000, associated with desmosomes of epithelial cells. The 08L antibody stained the intracellular side of lateral cell margins of monolayer epithelial cells but did not stain cell margins free of cell contact. Immunoelectron microscopy revealed that the 08L antigen was localized to the cytosolic surface of the desmosomal plaque near points of intermediate filament convergence with apparently little staining of the desmosomal plaque proper. Western blots revealed the 08L antigen to be a protein, of M(r) approximately 140,000, found in the Triton-X 100 insoluble pellet. High salt-containing buffers extracted the 08L antigen from the insoluble material. Examination of the assembly of 08L to the desmosome complex, in cells grown in low confluent culture or in calcium-switch assays, by double immunofluorescence with 08L and anti-desmoplakin antibody, revealed that 08L was recruited to morphologically identifiable desmosomes. 08L antigen may exist in a cytosolic pool prior to assembly to the cell surface. The solubility of 08L in low calcium and normal calcium conditions, however, was similar. 08L association to the desmosome was correlated with increased organization of the intermediate filament network. We suggest that the 08L antigen may be involved in the organization and stabilization of the desmosome-IF complexes of epithelia.

Cytoplasmic domain of the 180-kD bullous pemphigoid antigen, a hemidesmosomal component: molecular and cell biologic characterization

Using a serum sample of a bullous pemphigoid (BP) patient we have isolated a cDNA clone encoding a portion of a 180-kD polypeptide component of the hemidesmosome, the "BP180 autoantigen." The identity of the clone was confirmed by the generation of a fusion protein antibody that recognizes BP180 in both a basal epithelial cell extract of bovine tongue and extracts of human epidermal cells. Immunoelectron microscopy indicates that the 588-bp cDNA encodes a cytoplasmic fragment of BP180. Furthermore, the wide species reactivity of the fusion protein suggests that this portion of BP180 is highly conserved. In cultured human epidermal cells processed for confocal immunofluorescence microscopy, the fusion protein antibody generates a punctate cell substrate-associated staining pattern that is similar to that seen using BP230 antibodies. Using the original BP180 cDNA we have now isolated additional cDNA clones encoding approximately 1800bp of BP180 the 3' sequence of which overlaps with the sequence detailed in Guidice et al (J Clin Invest 87:734-738, 1991). Secondary structural analyses have been undertaken on the predicted amino acids encoded by the 1800bp. These suggest that the collagen-like sequences of BP180 described by Guidice et al (ibid.) are separated by a putative transmembrane region from the domain of BP180 recognized by our fusion protein antibody. Indeed, BP180 appears to belong to a relatively rare group of proteins in which the N-terminus is located in the cytoplasm and the C-terminus is extracellular. We detail some preliminary biochemical experiments in support of this hypothesis. We discuss possible functions of BP180 and BP230 in the hemidesmosome.

The vertebrate adhesive junction proteins beta-catenin and plakoglobin and the Drosophila segment polarity gene armadillo form a multigene family with similar properties

Three proteins identified by quite different criteria in three different systems, the Drosophila segment polarity gene armadillo, the human desmosomal protein plakoglobin, and the Xenopus E-cadherin-associated protein beta-catenin, share amino acid sequence similarity. These findings raise questions about the relationship among the three molecules and their roles in different cell-cell adhesive junctions. We have found that antibodies against the Drosophila segment polarity gene armadillo cross react with a conserved vertebrate protein. This protein is membrane associated, probably via its interaction with a cadherin-like molecule. This cross-reacting protein is the cadherin-associated protein beta-catenin. Using anti-armadillo and antiplakoglobin antibodies, it was shown that beta-catenin and plakoglobin are distinct molecules, which can coexist in the same cell type. Plakoglobin interacts with the desmosomal glycoprotein desmoglein I, and weakly with E-cadherin. Although beta-catenin interacts tightly with E-cadherin, it does not seem to be associated with either desmoglein I or with isolated desmosomes. Anti-armadillo antibodies have been further used to determine the intracellular localization of beta-catenin, and to examine its tissue distribution. The implications of these results for the structure and function of different cell-cell adhesive junctions are discussed.