Further analyses of epitopes for human monoclonal anti-basement membrane zone antibodies produced by stable human hybridoma cell lines constructed with Epstein-Barr virus transformants

Anti-BP180-type mucous membrane pemphigoid immunoglobulin G shows heterogeneity of internalization of BP180/collagen XVII into keratinocyte cytoplasm

Anti-BP180-type mucous membrane pemphigoid (BP180-MMP) is a rare autoimmune subepidermal blistering disease that targets the C terminus of BP180/collagen XVII. Currently, the pathomechanism of BP180-MMP is not well understood. We reported previously that immunoglobulin G (IgG) from patients with bullous pemphigoid (BP) can induce internalization of BP180 via a macropinocytic pathway, which depletes BP180 and weakens epidermal cell-matrix integrity. The purpose of the present study was to elucidate the pathomechanism of BP180-MMP. Immunohistochemistry of biopsy specimens from two patients with BP180-MMP revealed that one patient had BP180 internalization, but the other did not. In live-cell imaging using IgG from patients with BP180-MMP on several keratinocyte cell lines, IgG from only three out of the seven patients was associated with BP180 internalization into the cytoplasm. Our results suggest that IgG from patients with BP180-MMP shows heterogeneity of internalization of BP180. This variability in BP180 internalization in patients with BP or BP180-MMP may lead to differences in clinical presentation.

The rod domain is not essential for the function of plectin in maintaining tissue integrity

Plectin is a cytoskeletal linker protein that consists of a central rod domain connecting two globular domains. Rodless plectin is able to functionally compensate for the loss of full-length plectin in mice and, like full-length plectin, is able to form dimers.

Epidermolysis bullosa simplex associated with late-onset muscular dystrophy (EBS-MD) is an autosomal recessive disorder resulting from mutations in the plectin gene. The majority of these mutations occur within the large exon 31 encoding the central rod domain and leave the production of a low-level rodless plectin splice variant unaffected. To investigate the function of the rod domain, we generated rodless plectin mice through conditional deletion of exon 31. Rodless plectin mice develop normally without signs of skin blistering or muscular dystrophy. Plectin localization and hemidesmosome organization are unaffected in rodless plectin mice. However, superresolution microscopy revealed a closer juxtaposition of the C-terminus of plectin to the integrin β4 subunit in rodless plectin keratinocytes. Wound healing occurred slightly faster in rodless plectin mice than in wild-type mice, and keratinocytes migration was increased in the absence of the rod domain. The faster migration of rodless plectin keratinocytes is not due to altered biochemical properties because, like full-length plectin, rodless plectin is a dimeric protein. Our data demonstrate that rodless plectin can functionally compensate for the loss of full-length plectin in mice. Thus the low expression level of plectin rather than the absence of the rod domain dictates the development of EBS-MD.

Bullous pemphigoid IgG induces BP180 internalization via a macropinocytic pathway

Bullous pemphigoid (BP) is an autoimmune blistering skin disease induced by pathogenic autoantibodies against a type II transmembrane protein (BP180, collagen type XVII, or BPAG2). In animal models, BP180 autoantibody-antigen interaction appears insufficient to develop blisters, but involvement of complement and neutrophils is required. However, cultured keratinocytes treated with BP-IgG exhibit a reduction in the adhesive strength and a loss of expression of BP180, suggesting that the autoantibodies directly affect epidermal cell-extracellular matrix integrity. In this study, we explored the consequences of two distinct epithelial cells treated with BP-IgG, particularly the fate of BP180. First, we followed the distribution of green fluorescent protein-tagged BP180 in an epithelial cell line, 804G, and normal human epidermal keratinocytes after autoantibody clustering. After BP-IgG treatment, the adhesive strength of the cells to their substrate was decreased, and BP180 was internalized in both cell types, together with the early endosomal antigen-1. By using various endocytosis inhibitors and a fluid-uptake assay, we demonstrated that BP-IgG-induced BP180 internalization is mediated via a macropinocytic pathway. Moreover, a macropinocytosis inhibitor rescued a BP-IgG-induced reduction in the adhesive strength of the cells from their substrate. The results of this study suggest that BP180 internalization induced by BP-IgG plays an important role in the initiation of disease pathogenesis.

Actinin-4 in keratinocytes regulates motility via an effect on lamellipodia stability and matrix adhesions

During wound repair, epidermal cells at the edge of an injury establish front-rear polarity through orchestrated changes in their cytoskeleton and adhesion structures. The polarity and directed migration of such cells is determined by the assembly, extension, and stabilization of a lamellipodium. Actinin-4 associates with lamellipodia and has been implicated in regulating lamellipodial structure, function and assembly. To study the functions of actinin-4 in human keratinocytes, we used shRNA to generate knockdown cells and compared their motility behavior and matrix adhesion assembly to scrambled shRNA treated control keratinocytes. Actinin-4 knockdown keratinocytes lack polarity, assemble multiple lamellipodia with a 2× increased area over controls, display reduced activity of the actin remodeling protein cofilin, and fail to migrate in a directional manner. This motility defect is rescued by plating knockdown cells on preformed laminin-332 matrix. In actinin-4-knockdown keratinocytes, focal contact area is increased by 25%, and hemidesmosome proteins are mislocalized. Specifically, α6β4 integrin localizes to large lamellipodial extensions, displays reduced dynamics, and fails to recruit its bullous pemphigoid antigen binding partners. Together, our data indicate a role for actinin-4 in regulating the steering mechanism of keratinocytes via profound effects on their matrix adhesion sites.

Type XVII collagen regulates lamellipod stability, cell motility, and signaling to Rac1 by targeting bullous pemphigoid antigen 1e to alpha6beta4 integrin

Rac1 activity, polarity, lamellipodial dynamics, and directed motility are defective in keratinocytes exhibiting deficiency in β4 integrin or knockdown of the plakin protein Bullous Pemphigoid Antigen 1e (BPAG1e). The activity of Rac, formation of stable lamellipodia, and directed migration are restored in β4 integrin-deficient cells by inducing expression of a truncated form of β4 integrin, which lacks binding sites for BPAG1e and plectin. In these same cells, BPAG1e, the truncated β4 integrin, and type XVII collagen (Col XVII), a transmembrane BPAG1e-binding protein, but not plectin, colocalize along the substratum-attached surface. This finding suggested to us that Col XVII mediates the association of BPAG1e and α6β4 integrin containing the truncated β4 subunit and supports directed migration. To test these possibilities, we knocked down Col XVII expression in keratinocytes expressing both full-length and truncated β4 integrin proteins. Col XVII-knockdown keratinocytes exhibit a loss in BPAG1e-α6β4 integrin interaction, a reduction in lamellipodial stability, an impairment in directional motility, and a decrease in Rac1 activity. These defects are rescued by a mutant Col XVII protein truncated at its carboxyl terminus. In summary, our results suggest that in motile cells Col XVII recruits BPAG1e to α6β4 integrin and is necessary for activation of signaling pathways, motile behavior, and lamellipodial stability.

BPAG1e maintains keratinocyte polarity through beta4 integrin-mediated modulation of Rac1 and cofilin activities

alpha6beta4 integrin, a component of hemidesmosomes, also plays a role in keratinocyte migration via signaling through Rac1 to the actin-severing protein cofilin. Here, we tested the hypothesis that the beta4 integrin-associated plakin protein, bullous pemphigoid antigen 1e (BPAG1e) functions as a scaffold for Rac1/cofilin signal transduction. We generated keratinocyte lines exhibiting a stable knockdown in BPAG1e expression. Knockdown of BPAG1e does not affect expression levels of other hemidesmosomal proteins, nor the amount of beta4 integrin expressed at the cell surface. However, the amount of Rac1 associating with beta4 integrin and the activity of both Rac1 and cofilin are significantly lower in BPAG1e-deficient cells compared with wild-type keratinocytes. In addition, keratinocytes deficient in BPAG1e exhibit loss of front-to-rear polarity and display aberrant motility. These defects are rescued by inducing expression of constitutively active Rac1 or active cofilin. These data indicate that the BPAG1e is required for efficient regulation of keratinocyte polarity and migration by determining the activation of Rac1.

Epiplakin is dispensable for skin barrier function and for integrity of keratin network cytoarchitecture in simple and stratified epithelia

Epiplakin, a giant epithelial protein of >700 kDa, belongs to the plakin family of cytolinker proteins. It represents an atypical family member, however, as it consists entirely of plakin repeat domains but lacks any of the other domains commonly shared by plakins. Hence, its putative function as a cytolinker protein remains to be shown. To investigate epiplakin's biological role, we generated epiplakin-deficient mice by gene targeting in embryonic stem cells. Epiplakin-deficient mice were viable and fertile, without developing any discernible phenotype. Ultrastructurally, their epidermis revealed no differences compared to wild-type littermates, and cornified envelopes isolated from skin showed no alterations in shape or stability. Furthermore, neither embryonal formation nor later function of the epithelial barrier was affected. In primary cultures of epiplakin-deficient keratinocytes, the organization of actin filaments, microtubules, and keratin networks was found to be normal. Similarly, no alterations in keratin network organization were observed in simple epithelia of small intestine and liver or in primary hepatocytes. We conclude that, despite epiplakin's abundant and highly specific expression in stratified and simple epithelia, its absence in mice does not lead to severe skin dysfunctions, nor has it detectable consequences for keratin filament organization and cytoarchitecture of cells.

Enzyme-linked immunosorbent assay using bacterial recombinant proteins of human BP230 as a diagnostic tool for bullous pemphigoid

BACKGROUND

By immunoblot analyses of normal human epidermal extracts, the 230kDa bullous pemphigoid antigen (BP230) is recognized by most bullous pemphigoid (BP) sera. We produced different recombinant glutathione-S-transferase-fusion proteins, which roughly presented N-terminal domain, central rod domain and C-terminal domain of human BP230.

OBJECTIVE

In the present study, we developed an enzyme-linked immunosorbent assay (ELISA) using the recombinant proteins for detection of anti-BP230 IgG antibodies and assessed the usefulness of this assay in conjunction with an anti-BP180 ELISA to establish the diagnosis of BP.

METHODS

Using the bacterial recombinant proteins of N-terminal and C-terminal domains, we developed an ELISA. A receiver-operating-characteristic (ROC) analysis was performed to determine a cut-off value for the BP230 ELISA.

RESULTS

By this BP230 ELISA, 173 (72.4%) of 239 BP sera were positive, while only one (1.1%) of 94 sera from pemphigus vulgaris and pemphigus foliaceus patients was positive and all the 109 normal control sera were negative. Thus, the sensitivity and specificity of the BP230 ELISA were 72.4 and 99.5%, respectively. Interestingly, while 54 (84.4%) of 64 BP sera in active stage and 113 (64.6%) of 175 BP sera in remission were positive in BP180 ELISA, 37 (57.8%) of 64 BP sera in active stage and 136 (77.7%) of 175 BP sera in remission were positive in BP230 ELISA. These results indicate that the titer of anti-BP230 antibodies is not related with disease activity in some BP cases. Most significantly, by combining the results of BP230 ELISA and BP180 ELISA, 232 (97.1%) of 239 BP sera were positive.

CONCLUSION

The combination of BP230 ELISA and BP180 ELISA is the highly sensitive method for the diagnosis of BP.

Molecular consequences of deletion of the cytoplasmic domain of bullous pemphigoid 180 in a patient with predominant features of epidermolysis bullosa simplex

Bullous pemphigoid antigen 2 (BP180; COL17A1) collagen gene mutations typically result in nonlethal junctional epidermolysis bullosa. We have identified a patient, who had phenotypic features of mainly epidermolysis bullosa simplex and evidence for both intraepidermal and junctional blister formation. Mutation analysis disclosed compound heterozygous mutations in the COL17A1 gene, leading to deletion of Ile-18 to Asn-407 from the intracellular domain of BP180, BP180 Delta 18-407. To gain insight into the mechanisms underlying the phenotype, we have investigated the functional consequences of this truncation in BP180. The results demonstrate that: (1) in cultured keratinocytes of the patient, the assembly of hemidesmosomes, and their linkage with intermediate filaments are impaired; (2) BP180 Delta 18-407 is not capable of binding to the hemidesmosomal components BP230, plectin, and the beta 4 subunit of the alpha 6 beta 4 integrin in yeast two-hybrid assays; (3) BP180 Delta 18-407 is recruited into hemidesmosome-like structures in both normal and BP180-deficient transfected keratinocytes when ectopically expressed, suggesting that the extracellular domain of BP180 Delta 18-407 determines its topogenic fate; and, finally (4) the proteolytic shedding of the extracellular domain of BP180 Delta 18-407 is not impaired in transfected COS-7 cells. Collectively, the data demonstrate that the truncation of the intracellular domain of BP180 impairs the organization of hemidesmosomes, affecting both the mechanical stability of basal keratinocytes and dermoepidermal cohesion.

Role of binding of plectin to the integrin beta4 subunit in the assembly of hemidesmosomes

We have previously shown that plectin is recruited into hemidesmosomes through association of its actin-binding domain (ABD) with the first pair of fibronectin type III (FNIII) repeats and a small part of the connecting segment (residues 1328-1355) of the integrin beta4 subunit. Here, we show that two proline residues (P1330 and P1333) in this region of the connecting segment are critical for supporting beta4-mediated recruitment of plectin. Additional binding sites for the plakin domain of plectin on beta4 were identified in biochemical and yeast two-hybrid assays. These sites are located at the end of the connecting segment (residues 1383-1436) and in the region containing the fourth FNIII repeat and the C-tail (residues 1570-1752). However, in cells, these additional binding sites cannot induce the assembly of hemidesmosomes without the interaction of the plectin-ABD with beta4. Because the additional plectin binding sites overlap with sequences that mediate an intramolecular association of the beta4 cytoplasmic domain, we propose that they are not accessible for binding and need to become exposed as the result of the binding of the plectin-ABD to beta4. Furthermore, these additional binding sites might be necessary to position the beta4 cytoplasmic domain for an optimal interaction with other hemidesmosomal components, thereby increasing the efficiency of hemidesmosome assembly.

Immunofluorescence analysis of the basement membrane zone components in human anagen hair follicles

The ultrastructure and constituent components of the basement membrane zone (BMZ) of the interfollicular epidermis have been well characterized. However, little is known about the junctions between dermal papilla and the surrounding epithelial cells of the hair bulb, as well as those junctions between connective tissues and epithelial cells outside the hair follicle. In the present study, immunofluorescence was used to determine the expression of various BMZ components, particularly plectin, BP230, BP180, alpha6beta4 integrin, laminin 5 and type VII collagen, in anagen hair follicles from human scalp. All the BMZ components examined showed essentially the same immunofluorescence staining pattern. Specifically, staining of the upper portion of the hair follicle demonstrated expression of all BMZ components with a labeling intensity similar to that found in the interfollicular epidermis. Staining in the lower portion of the hair follicle, however, was markedly different: all the BMZ components showed a gradual decrease in staining intensity. Particularly, outside the hair bulb, the linear staining was diminished and even discontinuous in some areas. Finally, between dermal papilla and epithelial cells inside the hair bulb, there was a strong immunoreactivity for all the BMZ components except for BP230, which was completely negative. The present study also confirmed a previous reported ultrastructural finding that hemidesmosomes are not apparent in the hair bulb's exterior BMZ nor in the dermal papilla junctions. Instead, peculiar cloudy materials were seen in both the lamina densa and the adjacent epithelium outside the hair bulb. Taken together, the diminished expression of all the BMZ components outside the hair bulb, as well as the complete absence of BP230 at the dermal papilla junction, seem to be responsible for the incomplete ultrastructure of hemidesmosomes in these regions. Furthermore, the results in the present study led us to speculate that the expression of BMZ components inside and outside the hair bulb are markedly decreased in the transient regions of the hair follicle as compared with their expression in the permanent region, signified by the upper portion of the hair follicle. When the hair follicle moves upward in catagen or downward in anagen, the complete structure of the hemidesmosome may stabilize the upper portion to the surrounding connective tissues, while the incomplete hemidesmosome may facilitate the movement of the transient region.

Production of the bullous pemphigoid antigen 230 (BP230) by Saccharomyces cerevisiae and Pichia pastoris

BP230 is a cytoskeletal linker protein of 2649 amino acids originally identified as the target autoantigen in bullous pemphigoid, a potentially devastating autoimmune skin blistering disorder. To better define its function, we sought to generate recombinant forms of BP230 in both Saccharomyces cerevisiae and Pichia pastoris after cloning its entire cDNA. By immunoblot analysis, full-length BP230 was not found in extracts of P. pastoris, whereas minor amounts of degraded BP230 were detected in extracts of S. cerevisiae. In contrast, both S. cerevisiae and P. pastoris were able to produce the 770-amino acid COOH-terminal domain of BP230. Furthermore, the production level of the recombinant BP230 tail in S. cerevisiae was significantly higher than that observed in P. pastoris and that of endogenous BP230 in cultured human keratinocytes. Finally, 12 of 17 (71%) BP sera recognized the recombinant BP230 protein in yeast extracts. Our results indicate that S. cerevisiae occasionally constitutes a better tool for recombinant protein production than P. pastoris. Although both its large size and poor solubility limit production of BP230, the developed yeast system provides cellular fractions enriched in BP230 recombinant proteins that constitute useful tools for the diagnosis of bullous pemphigoid.

Interaction of the bullous pemphigoid antigen 1 (BP230) and desmoplakin with intermediate filaments is mediated by distinct sequences within their COOH terminus

The bullous pemphigoid antigen 1 (BP230) and desmoplakin (DP) are members of the plakin protein family of cytolinkers. Despite their homology, their COOH termini selectively bind distinct intermediate filaments (IFs). We studied sequences within their COOH termini required for their interaction with the epidermal keratins K5/K14, the simple epithelial keratins K8/K18, and type III IF vimentin by yeast three-hybrid, cell transfection, and overlay assays. The results indicate that BP230 interacts with K5/K14 but not with K8/K18 or vimentin via a region encompassing both the B and C subdomains and the COOH extremity, including a COOH-terminal eight-amino-acid stretch. In contrast, the C subdomain with the COOH-terminal extremity of DP interacts with K5/K14 and K8/K18, and its linker region is able to associate with K8/K18 and vimentin. Furthermore, the potential of DP to interact with IF proteins in yeast seems to be regulated by phosphorylation of Ser 2849 within its COOH terminus. Strikingly, BP230 and DP interacted with cytokeratins only when both type I and type II keratins were present. The head and tail domains of K5/K14 keratins were dispensable for their interaction with BP230 or DP. On the basis of our findings, we postulate that (1) the binding specificity of plakins for various IF proteins depends on their linker region between the highly homologous B and C subdomains and their COOH extremity and (2) the association of DP and BP230 with both epidermal and simple keratins is critically affected by the tertiary structure induced by heterodimerization and involves recognition sites located primarily in the rod domain of these keratins.

Hemidesmosome protein dynamics in live epithelial cells

Hemidesmosomes mediate stable anchorage of epithelial cells to laminin-5 in the basement membrane zone and have been likened to spot-welds. Indeed, it has been assumed that hemidesmosomes are not dynamic, at least when compared to other matrix adhesion sites including focal contacts. We tested this notion by monitoring the fate of green fluorescent protein (GFP)-tagged human integrin beta4 subunit (GFP-hbeta4) and GFP-tagged 180-kD human bullous pemphigoid (BP) autoantigen (GFP-BP180) in live cultures of 804G cells that assemble numerous mature hemidesmosomes. In subconfluent 804G cells, both GFP-hbeta4 and GFP-BP180 protein clusters are not stable but assemble into and disassemble out of cat paw-like arrays at a relatively rapid rate. In confluent populations of 804G cells, although some cat paw-like clusters of both GFP-hbeta4 and GFP-BP180 are stable over periods of >60 min, other GFP-hbeta4 and GFP-BP180 protein arrays form and/or disappear during the same time period. Moreover, individual labeled particles show considerable motility in the plane of the membrane. Fluorescence recovery after photobleaching analyses provide a further indication of the dynamics of hemidesmosome proteins. In particular, bleached GFP-hbeta4 protein clusters in confluent cells recover signal within about 30 min, indicating that there is a relatively rapid turnover of hemidesmosome components in protein arrays clustered along the substratum attached surface of a cell. The rate of recovery is dependent on an intact microfilament system. In sharp contrast, bleached GFP-BP180 protein clusters in confluent cells fail to recover signal even when observed for longer than 60 min. To evaluate hemidesmosome protein dynamics in motile cells, we monitored GFP-hbeta4 and GFP-BP180 in 804G cells populating scrape wound sites in vitro. In these migratory cells, which lack mature hemidesmosomes, integrin beta4 subunit and BP180 protein clusters progressively assemble and disassemble into linear and cat-paw arrays. In summary, hemidesmosome protein clusters, like their counterparts in focal contacts, are dynamic. We discuss these results in relation to hemidesmosome functions.

Dynamics of the alpha6beta4 integrin in keratinocytes

The integrin alpha6beta4 has been implicated in two apparently contrasting processes, i.e., the formation of stable adhesions, and cell migration and invasion. To study the dynamic properties of alpha6beta4 in live cells two different beta4-chimeras were stably expressed in beta4-deficient PA-JEB keratinocytes. One chimera consisted of full-length beta4 fused to EGFP at its carboxy terminus (beta4-EGFP). In a second chimera the extracellular part of beta4 was replaced by EGFP (EGFP-beta4), thereby rendering it incapable of associating with alpha6 and thus of binding to laminin-5. Both chimeras induce the formation of hemidesmosome-like structures, which contain plectin and often also BP180 and BP230. During cell migration and division, the beta4-EGFP and EGFP-beta4 hemidesmosomes disappear, and a proportion of the beta4-EGFP, but not of the EGFP-beta4 molecules, become part of retraction fibers, which are occasionally ripped from the cell membrane, thereby leaving "footprints" of the migrating cell. PA-JEB cells expressing beta4-EGFP migrate considerably more slowly than those that express EGFP-beta4. Studies with a beta4-EGFP mutant that is unable to interact with plectin and thus with the cytoskeleton (beta4(R1281W)-EGFP) suggest that the stabilization of the interaction between alpha6beta4 and LN-5, rather than the increased adhesion to LN-5, is responsible for the inhibition of migration. Consistent with this, photobleaching and recovery experiments revealed that the interaction of beta4 with plectin renders the bond between alpha6beta4 and laminin-5 more stable, i.e., beta4-EGFP is less dynamic than beta4(R1281W)-EGFP. On the other hand, when alpha6beta4 is bound to laminin-5, the binding dynamics of beta4 to plectin are increased, i.e., beta4-EGFP is more dynamic than EGFP-beta4. We suggest that the stability of the interaction between alpha6beta4 and laminin-5 is influenced by the clustering of alpha6beta4 through the deposition of laminin-5 underneath the cells. This clustering ultimately determines whether alpha6beta4 will inhibit cell migration or not.

The hemidesmosomal protein bullous pemphigoid antigen 1 and the integrin beta 4 subunit bind to ERBIN. Molecular cloning of multiple alternative splice variants of ERBIN and analysis of their tissue expression

The bullous pemphigoid antigen 1 (eBPAG1) is a constituent of hemidesmosomes (HDs), cell-substrate adhesion complexes in stratified epithelia. Although its COOH terminus interacts with intermediate filaments, its NH(2) terminus is important for its recruitment into HDs. To identify proteins that interact with the NH(2) terminus of human eBPAG1, we performed a yeast two-hybrid screen, which uncovered a protein belonging to the LAP/LERP (for LRR and PDZ domain) protein family with 16 NH(2)-terminal leucine-rich repeats and a COOH-terminal PDZ domain. The gene for this LAP/LERP protein comprises at least 26 exons located on the long arm of chromosome 5. In most human tissues, several transcripts were detected differing in the coding region situated upstream of or within the PDZ domain. One of the encoded variants was found to correspond to the recently described protein ERBIN. In yeast and in vitro binding experiments, ERBIN was shown to interact not only with eBPAG1 but also with the COOH-terminal region of the cytoplasmic domain of the integrin beta4 subunit, another component of HDs. Antibodies raised against the COOH terminus showed that ERBIN is expressed in keratinocytes. In transfected epithelial cells the protein, however, was not localized in HDs but was either diffusely distributed over the cytoplasm or concentrated at the basolateral plasma membrane. Because ERBIN had been shown previously to interact with the transmembrane tyrosine kinase receptor Erb-B2, which in turn associates with the integrin beta4 subunit, we suggest that ERBIN provides a link between HD assembly and Erb-B2 receptor signaling.

Bullous pemphigoid sera react specifically with various domains of BP230, most frequently with C-terminal domain, by immunoblot analyses using bacterial recombinant proteins covering the entire molecule

By immunoblot analyses of normal human epidermal extracts, the 230 kDa bullous pemphigoid antigen (BP230) is recognized by most bullous pemphigoid sera. By polymerase chain reaction using keratinocyte cDNA library as a template, we successfully amplified 3 cDNAs of about 3 kb, which covered whole human BP230 molecule. By inserting the cDNAs into bacterial expression vector pGEX, we prepared 3 different recombinant glutathione-S-transferase-fusion proteins, which roughly presented N-terminal domain, central rod domain and C-terminal domain of BP230. By immunoblotting using these 3 recombinant proteins, we demonstrated that the majority of bullous pemphigoid sera reacted clearly with multiple recombinant proteins of BP230, most frequently with C-terminal domain. We also examined sera of pemphigus vulgaris, pemphigus foliaceus and herpetiform pemphigus that showed BP230-like protein band by immunoblotting of epidermal extracts, as well as paraneoplastic pemphigus, for reactivity with the 3 recombinant proteins. In the study, we found that only very few of these non-bullous pemphigoid sera reacted with some of the recombinant proteins. These results indicate that the BP230 is specifically reacted by bullous pemphigoid sera, and that the immunoblotting using the BP230 recombinant proteins should be a useful tool for the diagnosis of bullous pemphigoid.

Epithelial development and differentiation in the mammary gland is not dependent on alpha 3 or alpha 6 integrin subunits

In the mammary gland, both laminin and integrins have been shown to be required for normal ductal morphogenesis during development in vivo, and for functional differentiation in culture models. Major integrin receptors for laminins in the mammary gland are alpha 3 beta 1, alpha 6 beta 1, and alpha 6 beta 4. However, the specific subunits that contribute to laminin-mediated mammary cell function and development have not been identified. In this study, we use a genetic approach to test the hypothesis that laminin-binding integrins are required for the function of the mammary gland in vivo. Rudiments of embryonic mammary gland were shown to develop in the absence of these integrin subunits. Postnatal development of the mammary gland was studied in integrin null tissue that had been transplanted into the mammary fat pads of syngeneic hosts. In mammary epithelium lacking alpha 6 integrin, the beta 4 subunit was not apparent and hemidesmosome formation was only rudimentary. However, despite this deficiency, normal ductal morphogenesis and branching of the mammary gland occurred and myoepithelial cells were distributed normally with respect to luminal cells. Mammary alveoli devoid of alpha 3 or alpha 6 integrin formed in pregnancy and were histologically and functionally identical to those in wild-type mammary gland. The tissue underwent full morphological differentiation, and the epithelial cells retained the ability to synthesize beta-casein. This work demonstrates that mammary tissue genetically lacking major laminin-binding integrin receptors is still able to develop and function.

IgG autoantibodies from bullous pemphigoid patients recognize multiple antigenic reactive sites located predominantly within the B and C subdomains of the COOH-terminus of BP230

Bullous pemphigoid is a subepidermal bullous disorder characterized by an autoantibody response against the bullous pemphigoid antigen 230 (BP230) and the bullous pemphigoid antigen 180 (BP180), a cytoplasmic component and a transmembrane component, respectively, of hemidesmosomes. Although immunodominant sequences within the extracellular domain of BP180 have been identified, characterization of the antigenic sites on BP230 is still incomplete. To identify autoantibody-reactive sites on BP230 and to examine whether the targeted regions are contained within functionally important domains, recombinant fragments encompassing almost the entire BP230 were used to assess the reactivity of 25 bullous pemphigoid sera by immunoblotting. Our results demonstrate that (i) the region bearing the B and C subdomains of the COOH-terminus of BP230 contains immunodominant sequences recognized by the majority of bullous pemphigoid sera; (ii) additional autoantibody- reactive sites are present over extended regions of the NH2-terminal half of BP230 without evidence for antigenic cross-reactivity between the NH2- and COOH-termini of BP230; and, finally, (iii) autoantibodies reacting with the BP230 tail predominantly belong to the IgG4 and IgG1 subclasses, suggesting that both autoreactive TH2 and autoreactive TH1 cells regulate the autoantibody response to immunodominant sequences of BP230. As the COOH- terminus of BP230 mediates the attachment of keratin intermediate filaments to the hemidesmosomal plaque, whereas its NH2-terminus contains sequences important for its interaction with other constituents of hemidesmosomes, autoantibodies to BP230 might precipitate subepidermal blister formation and perpetuate the disease not only by eliciting an inflammatory reaction but also by interfering with the function of BP230 and thus the stability of hemidesmosomes.

Human bronchial epithelial cells secrete laminin 5, express hemidesmosomal proteins, and assemble hemidesmosomes

Epithelial cells attach to the basement membrane through adhesive contacts between the basal cells of the epithelium and the proteins of the extracellular matrix (ECM). The hemidesmosome (HD) is a specialized cell-ECM contact, that mediates the attachment of the epithelial cell basal surface to the ECM. In bronchial epithelial cells, the protein components that constitute the HD have not been demonstrated. Using immunohistochemical techniques, we determined that normal human bronchial epithelial (NHBE) cells express the HD cell surface integrin alpha6beta4 and produce laminin 5, the ECM protein associated with HDs. Furthermore, expression of the HD-associated structural proteins, bullous pemphigoid antigens 1 (BPAG 1) and 2 (BPAG 2), was demonstrated in NHBE cells by immunofluorescence microscopy and immunoblot analyses. In addition, we confirmed the presence of laminin 5 in the basement membrane (BM) of bronchial epithelial biopsy specimens and of BP230, BP180, and the alpha6beta4 integrin heterodimer at the site of bronchial epithelial cell-ECM interaction in vivo. Finally, using electron microscopy, we were able to demonstrate intact HDs in a glutaraldehyde-fixed NHBE cell monolayer. These findings suggest that bronchial epithelium forms HDs and that the laminin 5-alpha6beta4 integrin interaction may be important in stabilizing epithelial cell adhesion to the BM in the lung.

The N terminus of the transmembrane protein BP180 interacts with the N-terminal domain of BP230, thereby mediating keratin cytoskeleton anchorage to the cell surface at the site of the hemidesmosome

In epidermal cells, the keratin cytoskeleton interacts with the elements in the basement membrane via a multimolecular junction called the hemidesmosome. A major component of the hemidesmosome plaque is the 230-kDa bullous pemphigoid autoantigen (BP230/BPAG1), which connects directly to the keratin-containing intermediate filaments of the cytoskeleton via its C terminus. A second bullous pemphigoid antigen of 180 kDa (BP180/BPAG2) is a type II transmembrane component of the hemidesmosome. Using yeast two-hybrid technology and recombinant proteins, we show that an N-terminal fragment of BP230 can bind directly to an N-terminal fragment of BP180. We have also explored the consequences of expression of the BP230 N terminus in 804G cells that assemble hemidesmosomes in vitro. Unexpectedly, this fragment disrupts the distribution of BP180 in transfected cells but has no apparent impact on the organization of endogenous BP230 and alpha6beta4 integrin. We propose that the BP230 N terminus competes with endogenous BP230 protein for BP180 binding and inhibits incorporation of BP180 into the cell surface at the site of the hemidesmosome. These data provide new insight into those interactions of the molecules of the hemidesmosome that are necessary for its function in integrating epithelial and connective tissue types.

Partial rescue of epithelial phenotype in integrin beta4 null mice by a keratin-5 promoter driven human integrin beta4 transgene

Integrin beta4 null mice exhibit extensive epidermal detachment, reminiscent of the human skin blistering disease junctional epidermolysis bullosa associated with pyloric atresia. Hemidesmosomes, the stable adhesion structures of squamous epithelia, are not formed in the absence of alpha6beta4. Null mutant mice die shortly after birth, but apart from their striking epithelial phenotype, no obvious developmental defects have been observed. To elucidate the cause of death in these mice, we generated transgenic mice with a heterologous construct consisting of the squamous epithelial-specific keratin-5 promoter and a human integrin beta4 subunit cDNA. The transgene was not expressed in the presence of endogenous beta4, probably as a result of competition for a limited pool of alpha6 subunits. In a beta4 null background, however, the transgene was expressed, and its expression pattern followed that of squamous epithelial-specific keratins. These rescued pups appeared healthy and ultrastructural analysis revealed that the interspecies heterodimer alpha6(mouse)/beta4(human) was sufficient to trigger the assembly of hemidesmosomes. After a variable period of up to 48 hours after birth these animals began to exhibit haemorrhages at the plantar and palmar areas. We observed the formation of small blisters and found that the transgene was not detectably expressed in this region, which is devoid of hair follicles. The rescued neonates became increasingly cyanotic and died soon after the onset of this phenomenon. We performed a developmental study of the expression of beta4 in the complete respiratory tract, but we found no correlation between the spatiotemporal distribution of beta4 and the onset of the respiratory insufficiency. It became clear, however, that there was a gradual detachment of squamous epithelia in the oral and nasal cavities which led to obstruction of the respiratory tract, suggesting that in beta4 null and rescued mice, neonatal death was a direct consequence of decreased adhesion properties of hairless squamous epithelia, rather than a developmental defect of the lungs.

Targeted disruption of the LAMA3 gene in mice reveals abnormalities in survival and late stage differentiation of epithelial cells

Laminin 5 regulates anchorage and motility of epithelial cells through integrins alpha6beta4 and alpha3beta1, respectively. We used targeted disruption of the LAMA3 gene, which encodes the alpha3 subunit of laminin 5 and other isoforms, to examine developmental functions that are regulated by adhesion to the basement membrane (BM). In homozygous null animals, profound epithelial abnormalities were detected that resulted in neonatal lethality, consistent with removal of all alpha3-laminin isoforms from epithelial BMs. Alterations in three different cellular functions were identified. First, using a novel tissue adhesion assay, we found that the mutant BM could not induce stable adhesion by integrin alpha6beta4, consistent with the presence of junctional blisters and abnormal hemidesmosomes. In the absence of laminin 5 function, we were able to detect a new ligand for integrin alpha3beta1 in the epidermal BM, suggesting that basal keratinocytes can utilize integrin alpha3beta1 to interact with an alternative ligand. Second, we identified a survival defect in mutant epithelial cells that could be rescued by exogenous laminin 5, collagen, or an antibody against integrin alpha6beta4, suggesting that signaling through beta1 or beta4 integrins is sufficient for survival. Third, we detected abnormalities in ameloblast differentiation in developing mutant incisors indicating that events downstream of adhesion are affected in mutant animals. These results indicate that laminin 5 has an important role in regulating tissue organization, gene expression, and survival of epithelium.

Interaction of BP180 (type XVII collagen) and alpha6 integrin is necessary for stabilization of hemidesmosome structure

The hemidesmosome is a multimolecular complex that integrates the extracellular matrix with the keratin cytoskeleton and that stabilizes epithelial attachment to connective tissue. A 180 kDa protein (BP180, type XVII collagen), first identified by its reactivity with autoantibodies in the serum of patients with a blistering skin disease called bullous pemphigoid (BP), is a transmembrane component of the hemidesmosome with a collagen-like extracellular domain. Here, using recombinantly expressed molecules and the yeast two-hybrid assay, we have identified alpha6 integrin as a BP180-binding partner. The association between specific domains of the BP180 and alpha6 integrin molecules is inhibited by a 14 mer peptide, whose sequence is identical to amino acid residues 506-519 in the noncollagenous region of the ectodomain of the BP180 molecule, as well as by antibodies raised against this peptide. The 14 mer peptide sequence is part of an epitope recognized by autoantibodies that are pathogenic in BP. In vivo, when 804G cells are plated into medium containing the same peptide, they fail to assemble hemidesmosomes. Furthermore, although BP180 and certain cytoplasmic components of the hemidesmosome colocalize in the peptide-treated cells, they are aberrantly distributed and fail to show extensive association with (alpha6beta4 integrin. Taken together, our results indicate that BP180 is a novel transmembrane ligand of the alpha6beta4 integrin heterodimer. In addition, our data provide support for the possibility that BP180 and alpha6 integrin interaction is not only mediated by the BP epitope but is necessary for hemidesmosome formation.

Localization of antigens associated with adherens junctions, desmosomes, and hemidesmosomes during murine molar morphogenesis

Epitheliomesenchymal interactions are known to play a crucial role during odontogenesis. Since epithelial cell-cell and cell-matrix interactions may also be involved in enamel organ histomorphogenesis, we investigated the localization of proteins associated with junctional complexes in mouse and rat first lower molars by indirect immunofluorescence. Adherens junctions were detected using antibodies directed against E-cadherin, beta-catenin, and plakoglobin (gamma-catenin). Desmosomes were localized with antibodies against desmoglein, and hemidesmosomes using antibodies against BP-230 and HD-1 proteins. When the inner dental epithelium differentiates, a decrease of E-cadherin, plakoglobin, and BP-230 is seen. An asymmetric distribution of plakoglobin, desmoglein, and BP-230 between the lateral and medial side of the tooth exists; desmoglein, which was first restricted to the gubernaculum dentis, progressively accumulated in the stellate reticulum, the stratum intermedium, and the basal pole of ameloblasts. The specific temporospatial distributions patterns of these antigens suggests a direct involvement of adherens junctions, desmosomes, and hemidesmosomes in the development of the murine first lower molar.

Targeted inactivation of plectin reveals essential function in maintaining the integrity of skin, muscle, and heart cytoarchitecture

Previous studies suggest that plectin, a versatile cytoskeletal linker protein, has an important role in maintaining the structural integrity of diverse cells and tissues. To establish plectin's function in a living organism, we have disrupted its gene in mice. Plectin (-/-) mice died 2-3 days after birth exhibiting skin blistering caused by degeneration of keratinocytes. Ultrastructurally, hemidesmosomes and desmosomes appeared unaffected. In plectin-deficient mice, however, hemidesmosomes were found to be significantly reduced in number and apparently their mechanical stability was altered. The skin phenotype of these mice was similar to that of patients suffering from epidermolysis bullosa simplex (EBS)-MD, a hereditary skin blistering disease with muscular dystrophy, caused by defects in the plectin gene. In addition, plectin (-/-) mice revealed abnormalities reminiscent of minicore myopathies in skeletal muscle and disintegration of intercalated discs in heart. Our results clearly demonstrate a general role of plectin in the reinforcement of mechanically stressed cells. Plectin (-/-) mice will provide a useful tool for the study of EBS-MD, and possibly other types of plectin-related myopathies involving skeletal and cardiac muscle, in an organism amenable to genetic manipulation.

Laminin-5 and hemidesmosomes: role of the alpha 3 chain subunit in hemidesmosome stability and assembly

Hemidesmosomes are complex macromolecular structures which integrate elements of the extracellular matrix and the cytoskeleton of epithelial cells. To characterize cell-matrix interactions in the hemidesmosome, we have made use of 804G cells which possess the unusual ability to assemble hemidesmosomes in vitro. During the course of our studies, we have raised a set of monoclonal antibodies against rat laminin-5, the major structural element comprising 804G matrix. One of these, termed CM6, recognizes the 150 kDa alpha chain of rat laminin-5 and binds the globular (G) domain of intact laminin-5 molecules as determined by rotary shadowing. CM6 antibodies perturb formed hemidesmosomes in 804G cells. In particular, within 1 hour of incubation of 804G cells with CM6 antibodies, colocalization of laminin-5 and alpha 6 beta 4 integrin is lost and by 2 hours, staining generated by hemidesmosomal antibodies appears primarily cytoplasmic in the perinuclear zone. Ultrastructurally, CM6 antibodies first appear to induce detachment of hemidesmosomes from the underlying matrix. Next, portions of the basal cell surface invaginate to form vesicles whose cytoplasmic-facing surface is coated with hemidesmosomes still associated with keratin intermediate filaments. Anchoring filaments extend into the inside compartment of the vesicles. We have also studied the impact of CM6 antibodies on a model system in which the matrix of 804G cells induces de novo assembly of hemidesmosomes in human keratinocytes. This process involves the plasma membrane reorganization of the hemidesmosome associated integrin alpha 6 beta 4 as well as a redistribution of other hemidesmosome components such as the 230 kDa bullous pemphigoid antigen. Pretreatment of 804G matrix with CM6 antibodies blocks such plasma membrane reorganization of hemidesmosome components and inhibits hemidesmosome formation. Our studies indicate a crucial role for the G domain of the alpha chain of laminin-5 in both nucleation of hemidesmosome assembly as well as maintenance of hemidesmosome structural integrity.

Epithelial detachment due to absence of hemidesmosomes in integrin beta 4 null mice

Integrins are heterodimeric transmembrane glycoproteins which are engaged in a variety of cellular functions, such as adhesion, migration and differentiation1. The integrin alpha 6 beta 4 is expressed on squamous epithelia, on subsets of endothelial cells, immature thymocytes and on Schwann cells and fibroblasts in the peripheral nervous system. In stratified epithelia, alpha 6 beta 4 is concentrated in specialised adhesion structures, called hemidesmosomes, which are implicated in the stable attachment of the basal cells to the underlying basement membrane by connecting the intermediate filaments with the extracellular matrix. The nature of the interactions between the various hemidesmosomal proteins, that lead to the formation of hemidesmosome is poorly understood. To study the contribution of the integrin alpha 6 beta 4 in hemidesmosome formation and their anchoring properties, we inactivated the beta 4 gene in mice by targeted gene disruption. Homozygous beta 4 null mice died shortly after birth and displayed extensive detachment of the epidermis and other squamous epithelia. The dramatically reduced adhesive properties of the skin was accompanied by the absence of hemidesmosomes at the basal surface of keratinocytes. No evidence was found for impaired T-cell development, nor for defects in myelination in the peripheral nervous system.

Molecular genetic studies of a human epidermal autoantigen (the 180-kD bullous pemphigoid antigen/BP180): identification of functionally important sequences within the BP180 molecule and evidence for an interaction between BP180 and alpha 6 integrin

The 180-kD bullous pemphigoid autoantigen (BP180) is a component of the hemidesmosome, a cell-matrix connector. This protein is oriented in a type II fashion in the membrane of the hemidesmosome and is a hybrid collagen (classified as type XVII). We have analyzed the fate of various mutant BP180 molecules transfected into several different cell types. A protein, D1, lacking the collagen-like extracellular domains of BP180 polarizes normally in 804G epithelial cells and colocalizes with other hemidesmosomal components in the plane of the basal cell surface. However, deletion of a stretch of 36 amino acids located at the NH2 terminus of D1 induces an apical polarization of the protein (D1-36N) in the cell surface of 804G cells. Deletion of the 27-amino acid noncollagenous extracellular domain that is located immediately after the membrane spanning domain of BP180 results in a failure of D1-27C protein to codistribute with other hemidesmosomal components despite its basal localization in transfected 804G cells. In FG cells, which lack their own BP180, transfected D1 protein localizes with the alpha 6 beta 4 integrin heterodimer. In HT1080 cells, which do not possess BP180 or beta 4 integrin, D1 protein localizes with alpha 6 beta 1 integrin while both the D1-27C and D1-36N proteins do not. Moreover, D1 protein coprecipitates with alpha 6 integrin from extracts of HT1080 transfectants. Taken together, these results suggest that the NH2-terminal domain of BP180 determines polarization of BP180 while the noncollagenous extracellular domain of BP180 stabilizes its interactions with other hemidesmosomal components, such as alpha 6 integrin. Perturbation of this latter domain by human bullous pemphigoid autoantibodies may explain the loss of epidermal cell-dermis attachment that characterizes the BP disease.

180-kD bullous pemphigoid antigen (BP180) is deficient in generalized atrophic benign epidermolysis bullosa

Generalized atrophic benign epidermolysis bullosa (GABEB) is a form of nonlethal junctional epidermolysis bullosa characterized by universal alopecia and atrophy of the skin. We report a deficiency of the 180-kD bullous pemphigoid antigen in three patients with GABEB from unrelated families. We screened specimens of clinically normal skin from nine junctional epidermolysis bullosa patients (3 GABEB, 4 lethal, 1 cicatricial, 1 pretibial) by immunofluorescence using monoclonal antibodies to the 180-kD and 230-kD bullous pemphigoid antigens (BP180 and BP230). In the skin of the three GABEB patients there was no reactivity with antibodies to BP180, whereas staining for BP230 was normal. In the skin of the other six, non-GABEB patients, included in this study the expression of BP180 and BP230 was normal. Immunoblot analysis of cultured keratinocytes from one of the GABEB patients also failed to detect BP180 antigen, whereas BP230 was present in normal amounts. The deficient expression of BP180 is reflected in the RNA message, as in Northern blot analysis a reduced amount of BP180 transcripts, although of normal length, were detected. Interestingly, in another GABEB patient there were not-involved areas of skin, in which blistering could not be induced by rubbing. Biopsy material from these areas showed interrupted staining for BP180. There was no staining for BP180 in areas of clinically normal but involved skin of this patient. In conclusion, this study reveals that the BP180 antigen is deficient and the BP180 mRNA is reduced in generalized atrophic benign epidermolysis bullosa.

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.