Cloning of type XVII collagen. Complementary and genomic DNA sequences of mouse 180-kilodalton bullous pemphigoid antigen (BPAG2) predict an interrupted collagenous domain, a transmembrane segment, and unusual features in the 5‘-end of the gene and the 3‘-untranslated region of the mRNA

The extracellular domain of BPAG2 has a loop structure in the carboxy terminal flexible tail in vivo

The 180 kDa bullous pemphigoid antigen is a hemidesmosome-associated transmembranous protein with a molecule length estimated to be 190-230 nm, which is much longer than the transverse length of the lamina lucida and lamina densa. The purpose of this study was to clarify the precise in vivo structure of the 180 kDa bullous pemphigoid antigen in normal human skin. We used three monoclonal antibodies directed to (i) the intracellular globular head of the 180 kDa bullous pemphigoid antigen, (ii) the mid-portion of the flexible tail of the antigen, corresponding approximately to amino acids 1000-1320, and (iii) the carboxyl terminal end, corresponding approximately to amino acids 1320-1500 of the antigen. Using low temperature postembedding immunoelectron microscopy, we quantitated the distribution of immunogold labeling of these monoclonal antibodies in normal human skin. The results showed that the monoclonal antibodies (i) bound to the intracellular portion of the hemidesmosome at a mean distance of 20 nm from the plasma membrane, (ii) bound to the lamina densa beneath the hemidesmosome at a mean distance of 65 nm from the plasma membrane, and (iii) bound to the lamina densa-lamina lucida interface at a mean distance of 39 nm from the plasma membrane. Considering the reported size of the 180 kDa bullous pemphigoid antigen, our results indicate that the extracellular domain of the antigen has at least one loop structure in the lamina densa in vivo. This unique structure of the antigen is thought to contribute to dermo- epidermal adhesion by intertwining with other basement membrane components.

A short sequence in the N-terminal region is required for the trimerization of type XIII collagen and is conserved in other collagenous transmembrane proteins

The recombinant transmembrane protein type XIII collagen is shown to reside on the plasma membrane of insect cells in a 'type II' orientation. Expressions of deletion constructs showed that sequences important for the association of three alpha1(XIII) chains reside in their N- rather than C-terminal portion. In particular, a deletion of residues 63-83 immediately adjacent to the transmembrane domain abolished the formation of disulfide-bonded trimers. The results imply that nucleation of the type XIII collagen triple helix occurs at the N-terminal region and that triple helix formation proceeds from the N- to the C-terminus, in opposite orientation to that of the fibrillar collagens. Interestingly, a sequence homologous to the deleted residues was found at the same plasma membrane-adjacent location in other collagenous transmembrane proteins, suggesting that it may be a conserved association domain. The type XIII collagen was secreted into insect cell medium in low amounts, but this secretion was markedly enhanced when the cytosolic portion was lacking. The cleavage occurred in the non-collagenous NC1 domain after four arginines and was inhibited by a furin protease inhibitor.

Hemizygosity for a glycine substitution in collagen XVII: unfolding and degradation of the ectodomain

Defects of collagen XVII, a keratinocyte adhesion protein, are associated with epidermal detachment in junctional epidermolysis bullosa. Although some missense mutations in the collagen XVII gene COL17A1 have been described, the molecular mechanisms leading to disease have remained elusive in these cases. Here we assessed the biologic consequences of a missense mutation by studying the folding and stability of wild-type and mutated recombinant collagen XVII domains. The mutation occurred in a junctional epidermolysis bullosa patient who was compound heterozygous for the novel glycine substitution mutation G633D and the novel nonsense mutation R145X. Collagen XVII mRNA was significantly reduced, indicating nonsense-mediated mRNA degradation and hemizygosity of the patient for the G633D substitution. As glycine residues within the collagen triple helices are important for stable conformation, the thermal stability of the wild-type and mutated eukaryotic recombinant Col15 domain of collagen XVII was assessed. The stability of the mutated fragment was clearly reduced. The midpoint of the helix-to-coil transition, Tm, was 5 degrees C lower than that of wild-type rCol15, indicating abnormal triple-helix folding and susceptibility to proteolysis. Consistently, immunoassays demonstrated reduced amounts of the full-length collagen XVII and absence of the soluble ectodomain in keratinocyte cultures, and lack of the ectodomain from the junctional epidermolysis bullosa skin. These observations show that the glycine substitution G633D in collagen XVII causes abnormal folding and susceptibility to degradation, and thus perturbs the physiologic adhesive functions of collagen XVII in the skin.

Production of the entire extracellular domain of BP180 (type XVII collagen) by baculovirus expression

Bullous pemphigoid (BP) is an acquired autoimmune skin disease, and its target antigens are a 230 kDa plaque protein (BP230) and a 180 kDa transmembrane protein with interrupted collagenous domains (BP180, type XVII collagen), which localize at the hemidesmosome. In this study we have attempted to express the entire extracellular domain of BP180 (rBP180EC) as a secreted protein by baculovirus expression. Seventy out of 83 BP sera (84.4%) showed positive reactivity against rBP180EC by immunoblot analysis, and 56 out of 83 BP sera (67.5%) were positive against rBP180EC by ELISA. These figures were comparable with those when a bacterial recombinant protein encoding the NC16a domain of BP180 (rNC16a) was used as an antigen source. Reactivity of BP sera against rBP180EC by ELISA was completely abolished or significantly reduced by immunocompetition with rNC16a in 11 out of 14 BP sera tested, while the reactivity was not altered in the rest of the three sera. These findings indicate that the NC16a domain represents the major epitopes on the extracellular domain of BP180, although there are some other minor epitopes outside of NC16a which are uniquely expressed by rBP180EC. rBP180EC will be useful to develop a diagnostic tool for BP as well as to dissect a molecular role for BP180 in interactions of keratinocytes with epidermal basement membrane.

A disease-associated glycine substitution in BP180 (type XVII collagen) leads to a local destabilization of the major collagen triple helix

BP180 is a homotrimeric transmembrane protein with a carboxy-terminal ectodomain that forms an interrupted collagen triple helix. Null type mutations in the BP180 gene produce a recessive subepidermal blistering disease, non-Herlitz junctional epidermolysis bullosa. Like the null mutations, a glycine substitution (G627V) within the longest BP180 collagenous domain (COL15) is also associated with the recessive skin disease; however, unlike the null mutations, this glycine substitution appears to act in a dominant fashion to give rise to a novel form of random pitting dental enamel hypoplasia. The dominant effects of this mutation were thought to be due to alterations in the assembly and/or stability of this BP180 collagenous region. To further investigate this issue, a structural analysis was performed on recombinant forms of the wild type and G627V mutant BP180 ectodomain. Both proteins were found to form collagen-like triple helices with very similar Stokes radii and melting temperatures and exhibited very similar rates of synthesis, secretion and turn-over. Tryptic digestion analysis revealed that the mutant G627V-sec180e contains an additional highly sensitive proteolytic site that maps within the region of the mutation. Thus, the disease-associated G627V mutation in BP180 does not grossly alter protein structure, but causes a local destabilization of the triple-helix that exposes sensitive residues to the in vitro effects of trypsin and possibly affects its structure-function in vivo.

Posttranslational processing and differential glycosylation of Tractin, an Ig-superfamily member involved in regulation of axonal outgrowth

Tractin is a novel member of the Ig-superfamily which has a highly unusual structure. It contains six Ig domains, four FNIII-like domains, an acidic domain, 12 repeats of a novel proline- and glycine-rich motif with sequence similarity to collagen, a transmembrane domain, and an intracellular tail with an ankyrin and a PDZ domain binding motif. By generating domain-specific antibodies, we show that Tractin is proteolytically processed at two cleavage sites, one located in the third FNIII domain, and a second located just proximal to the transmembrane domain resulting in the formation of four fragments. The most NH(2)-terminal fragment which is glycosylated with the Lan3-2, Lan4-2, and Laz2-369 glycoepitopes is secreted, and we present evidence which supports a model in which the remaining fragments combine to form a secreted homodimer as well as a transmembrane heterodimer. The extracellular domain of the dimers is mostly made up of the collagen-like PG/YG-repeat domain but also contains 11/2 FNIII domain and the acidic domain. The collagen-like PG/YG-repeat domain could be selectively digested by collagenase and we show by yeast two-hybrid analysis that the intracellular domain of Tractin can interact with ankyrin. Thus, the transmembrane heterodimer of Tractin constitutes a novel protein domain configuration where sequence that has properties similar to that of extracellular matrix molecules is directly linked to the cytoskeleton through interactions with ankyrin.

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.

Formation of hemidesmosome-like structures in the absence of ligand binding by the (alpha)6(beta)4 integrin requires binding of HD1/plectin to the cytoplasmic domain of the (beta)4 integrin subunit

Hemidesmosomes are adhesion structures that mediate anchorage of epithelial cells to the underlying basement membrane. We have previously shown that the (alpha)6(beta)4 integrin can induce the assembly of these multi-protein structures independent of binding to its ligand laminin-5 (ligand-independent formation of hemidesmosomes). Our results suggested a role for HD1/plectin, which binds to the cytoplasmic domain of the (beta)4 integrin subunit, in controlling the clustering of hemidesmosomal components at the basal side of the cell. Using keratinocytes derived from patients lacking HD1/plectin, we now show that ligand-independent formation of hemidesmosomal clusters indeed requires HD1/plectin, in contrast to the ligand-dependent assembly of hemidesmosomes. No clustering of the (alpha)6(beta)4 integrin, or of the bullous pemphigoid antigens BP180 and BP230, was seen when HD1/plectin-deficient keratinocytes were plated on fibronectin or type IV collagen. In (β)4-deficient keratinocytes, expression of an interleukin 2 receptor (IL2R) transmembrane chimera containing the (beta)4 cytoplasmic tail with the mutation R1281W, which abrogates HD1/plectin binding, resulted in a diffuse distribution of the chimeric receptor. In contrast, a (beta)4(R1281W) mutant that can associate with (alpha)6 and bind ligand, was found to be directed to the basal surface of the cells, at sites where laminin-5 was deposited. In addition, this mutant induced clustering of BP180 and BP230 at these sites. Together, these results show that the formation of hemidesmosomes requires binding of either ligand or HD1/plectin to the (beta)4 integrin subunit. Intriguingly, we found that IL2R/(beta)4 chimeras become localized in pre-existing hemidesmosomes of HD1/plectin-deficient keratinocytes, and that this localization requires a domain in the (beta)4 cytoplasmic tail that is also required for HD1/plectin binding (residues 1115–1356). Because this part of (beta)4 lacks the BP180 binding site, and since we show in this study that it is unable to interact with the same part on another (beta)4 molecule, we suggest that the chimera becomes incorporated into hemidesmosomes of HD1/plectin-deficient keratinocytes by interacting with an as yet unidentified hemidesmosomal component.

The shed ectodomain of collagen XVII/BP180 is targeted by autoantibodies in different blistering skin diseases

Collagen XVII/BP180, an epidermal adhesion molecule, exists as a full-length transmembrane protein and as a soluble 120-kd ectodomain that is shed from the keratinocyte surface by furin-mediated proteolysis. Despite a number of studies on autoantibody targets in blistering skin diseases, it has remained unclear whether the physiologically shed ectodomain of collagen XVII plays a role as an autoantigen. Here we isolated the authentic, soluble form of human collagen XVII and showed that it is an autoantigen recognized by IgG and IgA autoantibodies in different blistering skin diseases and is, in some cases, the preferential target. The ectodomain was isolated from the epidermis, keratinocyte media, amniotic fluid, and pemphigoid blister fluid, and autoantibodies affinity-purified with this ectodomain bound to the proximal surface of the epidermis in normal skin but not in collagen XVII-deficient skin. The antibody reactivity was not dependent on the native conformation or the N-glycosylation of the soluble ectodomain, but was abolished by collagenase treatment. Sera of 81 patients with a clinically active blistering skin disease were reacted with full-length collagen XVII, the authentic soluble ectodomain, and recombinant fragments. In bullous and cicatricial pemphigoid, IgG reactive with full-length collagen XVII also recognized the soluble ectodomain. In linear IgA dermatosis and chronic bullous dermatosis of childhood, IgA targeted the soluble ectodomain more efficiently than the full-length protein. The use of recombinant fragments demonstrated that epitopes were present in several noncollagenous and collagenous subdomains of the molecule, and that a significant portion of the sera targeted Col15 domain, a hitherto unrecognized epitope region.

Molecular cloning of canine bullous pemphigoid antigen 2 cDNA and immunomapping of NC16A domain by canine bullous pemphigoid autoantibodies

The autoantibody-mediated subepidermal blistering skin disease bullous pemphigoid affects both humans and dogs. We previously demonstrated that canine bullous pemphigoid patient's autoantibodies targeted skin basement membrane component and a 180-kDa keratinocyte protein. We extend our works to partially isolate the cDNA encoding canine bullous pemphigoid antigen 2 (BPAg2, BP180). Total RNA extracted from a papillomavirus-immortalized canine keratinocyte cell line and a cultured canine squamous carcinoma cell line SCC 2/88 were used to isolate fragments of cDNA encoding BPAg2 by reverse transcription-PCR and 5'-rapid amplification of cDNA end. The isolated sequence included the 5'-untranslated region, the entire intracellular, transmembranous, and extracellular NC16A autoantigenic domains, plus a small segment of the collagenous domain. Sequence analyses of the isolated cDNA showed 87 and 85% identities between canine and human at the nucleotide sequence and at the deduced amino acid sequence levels, respectively. The canine BPAg2 sequence was confirmed by a rabbit antibody raised against a 18-amino acid peptide deduced from the canine NC16A nucleotide sequence. Autoantibodies from canine bullous pemphigoid patients' sera recognized epitopes within the human NC16A domain. The cloning of the cDNA encoding this disease-associated protein may allow us to develop a canine model in dissecting the immunopathologic mechanism underlying bullous pemphigoid.

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.

A critical role for neutrophil elastase in experimental bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune skin disease characterized by subepidermal blisters and autoantibodies against 2 hemidesmosome-associated proteins, BP180 and BP230. The immunopathologic features of BP can be reproduced in mice by passive transfer of anti-BP180 antibodies. Lesion formation in this animal model depends upon complement activation and neutrophil recruitment. In the present study, we investigated the role of neutrophil elastase (NE) in antibody-induced blister formation in experimental BP. Abnormally high levels of caseinolytic activity, consistent with NE, were detected in extracts of lesional skin and blister fluid of mice injected with anti-BP180 IgG. The pathogenic anti-BP180 IgG failed to induce subepidermal blistering in NE-null (NE(-/-)) mutant mice. NE(-/-) mice reconstituted with neutrophils from wild-type mice became susceptible to experimental BP. Wild-type mice given NE inhibitors (alpha1-proteinase inhibitor and Me-O-Suc-Ala-Ala-Pro-Val-CH(2)Cl), but not mice given cathepsin G/chymase inhibitors (alpha1-antichymotrypsin or Z-Gly-Leu-Phe-CH(2)Cl), were resistant to the pathogenic activity of anti-BP180 antibodies. Incubation of murine skin with NE induced BP-like epidermal-dermal detachment. Finally, NE cleaved BP180 in vitro and in vivo. These results implicate NE directly in the dermal-epidermal cleavage induced by anti-BP180 antibodies in the experimental BP model.

Two type XVII collagen (BP180) mRNA transcripts in human keratinocytes: a long and a short form

We have analysed BP180 mRNA expression in normal human keratinocytes. Here we report the presence in normal keratinocytes of two COL17A1 transcripts which differ by 0.6 kb in length. Both mRNAs hybridized on Northern blot with probes directed to sequences encoding intracellular and extracellular fragments of BP180. By BLAST homology search alignments we extended the 3' untranslated region (3'UTR) of the known BP180 mRNA sequence by 877 bases to completion. Three of 20 cDNAs identified by BLAST searches contained a 610 bp deletion in this new 3'UTR sequence. Northern blot analysis with a probe complementary to this deleted sequence showed binding only to the larger mRNA. The deletion of 610 nucleotides in the smaller mRNA was verified by reverse transcription-PCR and sequencing. Genomic PCR showed the new sequence to be an extension of exon 56 of the COL17A1 gene which suggests that the second mRNA is generated by differential splicing. In normal keratinocytes the level of the smaller transcript was 5-15% of that of the larger transcript whereas in a squamous cell carcinoma cell line this ratio was reversed, the smaller mRNA being three times more abundant than the larger mRNA. The biological significance of this newly identified transcript in protein synthesis and tissue expression or in cell differentiation, proliferation or adhesion is as yet unknown.

Compound heterozygosity for novel splice site mutations in the BPAG2/COL17A1 gene underlies generalized atrophic benign epidermolysis bullosa

Generalized atrophic benign epidermolysis bullosa, GABEB (OMIM# 226650), is a nonlethal variant of epidermolysis bullosa with autosomal recessive inheritance pattern. The pathogenesis of this disorder can be caused by mutations affecting two different gene/protein systems. Most of the mutations have been identified in the BPAG2/COL17A1 gene encoding a hemidesmosomal transmembrane protein, the 180 kDa bullous pemphigoid antigen (BP180), also known as type XVII collagen. The minority of the mutations are localized in the LAMB3 gene encoding the beta3 polypeptide of laminin 5. In In this study we describe a GABEB patient who showed absent expression of BP180 in the cultured keratinocytes as well as in the skin. The patient was a compound heterozygote for two different splice site mutations, 3053-1G-->C and 3871+1G-->C, affecting the extra-cellular domain of the protein. These mutations resulted in multiple aberrant splice variants, three of them causing premature termination codons for translation. This case, dealing with out-of-frame splice site mutations in BPAG2/COL17A1, attests to the molecular heterogeneity of GABEB.

Myotactin, a novel hypodermal protein involved in muscle-cell adhesion in Caenorhabditis elegans

In C. elegans, assembly of hypodermal hemidesmosome-like structures called fibrous organelles is temporally and spatially coordinated with the assembly of the muscle contractile apparatus, suggesting that signals are exchanged between these cell types to position fibrous organelles correctly. Myotactin, a protein recognized by monoclonal antibody MH46, is a candidate for such a signaling molecule. The antigen, although expressed by hypodermis, first reflects the pattern of muscle elements and only later reflects the pattern of fibrous organelles. Confocal microscopy shows that in adult worms myotactin and fibrous organelles show coincident localization. Further, cell ablation studies show the bodywall muscle cells are necessary for normal myotactin distribution. To investigate myotactin's role in muscle-hypodermal signaling, we characterized the myotactin locus molecularly and genetically. Myotactin is a novel transmembrane protein of approximately 500 kd. The extracellular domain contains at least 32 fibronectin type III repeats and the cytoplasmic domain contains unique sequence. In mutants lacking myotactin, muscle cells detach when embryonic muscle contraction begins. Later in development, fibrous organelles become delocalized and are not restricted to regions of the hypodermis previously contacted by muscle. These results suggest myotactin helps maintain the association between the muscle contractile apparatus and hypodermal fibrous organelles.

Autoantibodies in lichen planus pemphigoides react with a novel epitope within the C-terminal NC16A domain of BP180

Lichen planus pemphigoides is an autoimmune subepidermal blistering disease. The finding of immunoglobulin G antibodies directed against the basement membrane zone differentiates it from bullous lichen planus. The aim of this study was to identify the target antigen of lichen planus pemphigoides autoantibodies. Sera from lichen planus pemphigoides patients (n = 4) stained the epidermal side of NaCl-split human skin in a pattern indistinguishable from that produced by bullous pemphigoid sera. In bullous pemphigoid, the autoimmune response is directed against BP180, a hemidesmosomal transmembrane collagenous glycoprotein. We previously demonstrated that bullous pemphigoid sera predominantly react with a set of four epitopes (MCW-0 through MCW-3) clustered within a 45 amino acid stretch of the major noncollagenous extracellular domain (NC16A) of BP180. By immunoblotting and enzyme-linked immunosorbent assay, lichen planus pemphigoides sera were also strongly reactive with recombinant bullous pemphigoid 180 NC16A. The lichen planus pemphigoides epitopes were further mapped using a series of overlapping recombinant segments of the NC16A domain. All lichen planus pemphigoides sera reacted with amino acids 46-59 of domain NC16A, a protein segment that was previously shown to be unreactive with bullous pemphigoid sera. Two lichen planus pemphigoides sera, in addition, reacted with the immunodominant antigenic region associated with bullous pemphigoid. In conclusion, there are now five bullous diseases that are associated with an autoimmune response to BP180: bullous pemphigoid; pemphigoid/herpes gestationis; cicatricial pemphigoid; linear immunoglobulin A disease; and lichen planus pemphigoides. In addition, we have identified a novel epitope within the BP180 NC16A domain, designated MCW-4, that appears to be uniquely recognized by sera from patients with lichen planus pemphigoides.

Hereditary skin diseases of hemidesmosomes

Studies of hereditary blistering skin diseases (epidermolysis bullosa) and targeted gene mutation experiments in knockout mice have greatly improved our understanding of hemidesmosomes and their associated structures in the cytoskeleton and basement membrane of the skin and mucous membranes. At least 10 molecules are recruited in hemidesmosome complexes, where they interact in a complex way. Hemidesmosomes are not simple adhesion devices, but also transduce signals for cell spreading, cell proliferation and basement membrane organisation. The dynamics of a hemidesmosome raises the metaphor of a self-assembling suspension bridge which evokes activities on both sides of the river. This review summarises our current knowledge of the molecular pathology of hemidesmosomes caused by hereditary skin disease or gene targeting experiment.

Acquired skin disease of hemidesmosomes

The hemidesmosome is a membrane-associated supramolecular dermal epidermal complex linking the cytoskeleton of the basal keratinocyte to structures within the papillary dermis. Different components of this complex have been identified as autoantigens in autoimmune bullous skin diseases. Some of the autoantigens have been characterized at the molecular level. Little is known, however, about the factors that initiate the production of autoantibodies. By histopathology, acquired skin diseases of hemidesmosomes show subepidermal blisters and by direct immunofluorescence, linear deposits of IgG, C3 or IgA at the dermal epidermal junction. Bullous pemphigoid (BP) is the most common acquired disease of hemidesmosomes. Two proteins, BP180 and BP230, have been identified as primary targets of autoantibodies in BP. In addition, pemphigoid/herpes gestationis, lichen planus pemphigoides, cicatricial pemphigoid and linear IgA disease are characterized by an immune response to BP180. Laminin 5 is another well-characterized anchoring filament-lamina densa component of hemidesmosomes. Patients with autoantibodies to laminin 5 show the clinical phenotype of cicatricial pemphigoid. Other acquired skin diseases of the hemidesmosomes reveal autoantibodies to a plectin-like protein, the beta4 subunit of alpha6beta4 integrin, uncein and a not yet characterized 168 kDa protein. Recently, diseases with autoantibodies to 105 and 200 kDa proteins of the lower lamina lucida have been reported. The association of these autoantigens with hemidesmosomes still needs to be demonstrated. Finally, anchoring fibrils associate with the dermal epidermal anchoring complex. The major structural component of anchoring fibrils is type VII collagen, the autoantigen of epidermolysis bullosa acquisita.

Corneal cell proteins and ocular surface pathology

The cornea is a transparent and avascular tissue that functions as the major refractive structure for the eye. A wide variety of growth factors, chemokines, cytokines and their receptors are synthesized by corneal epithelial and stromal cells, and are found in tears. These molecules function in corneal wound healing and in inflammatory responses. Proteoglycans and glycoproteins are essential for normal corneal function, both at the air-epithelial interface and within the extracellular matrix. The ocular MUC mucins may play roles in forming the mucus layer of the tear film, in regulating tear film spread, and in inhibiting the adhesion of pathogens to the ocular surface. Lumican, keratocan and mimecan are the major keratan sulfate proteoglycans of the corneal stroma. They are essential, along with other proteoglycans and interfibrillar proteins, including collagens type VI and XII, for the maintenance of corneal transparency. Corneal epithelial cells interact with a specialized extracellular matrix structure, the basement membrane, composed of a specific subset of collagen type IV and laminin isoforms in addition to ubiquitous extracellular matrix molecules. Matrix metalloprotein-ases have been identified in normal corneal tissue and cells and may play a role in the development of ulcerative corneal diseases. Changes in extracellular matrix molecule localization and synthesis have been noted in other types of corneal diseases as well, including bullous keratopathy and keratoconus.

180-kD bullous pemphigoid antigen/type XVII collagen: tissue-specific expression and molecular interactions with keratin 18

The 180-kD bullous pemphigoid antigen (BPAG2) is a hemidesmosomal transmembrane protein, also known as type XVII collagen. In this study, potential interactions of BPAG2 with other proteins expressed in epidermal keratinocytes were explored by yeast two-hybrid system using the amino-terminal intracellular domain of BPAG2 as a bait. Several independent interacting clones encoding keratin 18 (K18) were identified when the keratinocyte cDNA library, cloned into the yeast two-hybrid activation domain vector, was screened. The peptide sequence responsible for the interaction of BPAG2 was restricted to amino acids 15-25, and substitution of a valine residue in the middle of this sequence by a proline (V23P) by site-directed mutagenesis abolished the interaction. Further examination of the K18 sequences by restricted cDNA constructs in yeast two-hybrid system identified a carboxyl-terminal segment corresponding to helix 2B domain as critical for BPAG2 binding. The interaction of BPAG2/K18 was confirmed by an in vitro protein-protein interaction assay, which also confirmed that normal human keratinocytes express K18 in culture. The tissue specific expression of BPAG2 was first examined using a multi-tissue RNA blot. Human multiple tissue cDNA panels representing a variety of adult and fetal tissues as well as tumor cells were used as PCR-templates to study the expression patterns of both BPAG2 and K18. The results demonstrated significant level of expression of BPAG2, besides in epidermal keratinocytes, also in a variety of tissues with predominant epithelial component, such as mammary, salivary and thyroid glands, colon, prostate, testis, placenta, and adult and fetal thymus, as well as in colon, pancreatic and prostatic adenocarcinoma cell lines, and an ovarian carcinoma. As expected, K18 transcript is present in liver, pancreas, colon, placenta, and in fetal kidney. Collectively, the results suggest that BPAG2 has a relatively broad tissue distribution including specialized and simple epithelia, and that within the tissues such as colon and placenta, BPAG2 may have direct interactions with K18, a keratin characteristically expressed in a simple epithelia.

Biology and function of hemidesmosomes

Hemidesmosomes are cell-substratum adhesion sites that connect the extracellular matrix to the keratin cytoskeleton. Our knowledge of the function of these structures has greatly increased as a result of studies on patients with aberrant expression of hemidesmosome components and studies using targeted inactivation of mouse genes encoding these components. Insight into the formation of hemidesmosomes, as well as into protein-protein interactions that occur in these junctional complexes, has recently been gained by in vitro cell transfections, blot overlay and yeast two-hybrid assays. In addition, recent results indicate that the alpha6 beta4 integrin is involved in the transduction of signals that are induced by the extracellular matrix and which modulate processes as diverse as cell proliferation, differentiation, apoptosis, migration and tissue morphogenesis. Thus it seems that hemidesmosomes do not merely maintain dermo-epidermal adhesion and tissue integrity, but that they are also implicated in intracellular signaling. Here we discuss recently published data on the biology and function of hemidesmosomes.

Mutation analysis and molecular genetics of epidermolysis bullosa

Cutaneous basement membrane zone (BMZ) consists of a number of attachment structures that are critical for stable association of the epidermis to the underlying dermis. These include hemidesmosomes, anchoring filaments and anchoring fibrils which form an interconnecting network extending from the intracellular milieu of basal keratinocytes across the dermal-epidermal basement membrane to the underlying dermis. Aberrations in this network structure, e.g. due to genetic lesions in the corresponding genes, can result in fragility of the skin at the level of the cutaneous BMZ. The prototype of such diseases is epidermolysis bullosa (EB), a heterogeneous group of genodermatoses characterized by fragility and blistering of the skin, often associated with extracutaneous manifestations, and inherited either in an autosomal dominant or autosomal recessive manner. Based on constellations of the phenotypic manifestations, severity of the disease, and the level of tissue separation within the cutaneous BMZ, EB has been divided into clinically distinct subcategories, including the simplex, hemidesmosomal, junctional and dystrophic variants. Elucidation of BMZ gene/protein systems and development of mutation detection strategies have allowed identification of mutations in 10 different BMZ genes which can explain the clinical heterogeneity of EB. These include mutations in the type VII collagen gene (COL7A1) in the dystrophic (severely scarring) forms of EB; mutations in the laminin 5 genes (LAMA3, LAMB3 and LAMC2) in a lethal (Herlitz) variant of junctional EB; aberrations in the type XVII collagen gene (COL17A1) in non-lethal forms of junctional EB; mutations in the alpha6 and beta4 integrin genes in a distinct hemidesmosomal variant of EB with congenital pyloric atresia; and mutations in the plectin gene (PLEC1) in a form of EB associated with late-onset muscular dystrophy. Identification of mutations in these gene/protein systems attests to their critical importance in the overall stability of the cutaneous BMZ. Furthermore, elucidation of mutations in different variants of EB has direct clinical applications in terms of refined classification, improved genetic counseling, and development of DNA-based prenatal testing in families with EB.

Laminins of the dermo-epidermal junction

Laminins are the most abundant structural non-collagenous glycoproteins ubiquitously present in basement membranes. They are multidomain molecules constituting a family of possibly more than 50 members. Some members such as laminins 5, 6 and 10 are specific of the basal lamina present under stratified epithelia. Although only few intact laminin isoforms have been purified from cultivated cells or tissues, genetic engineering has opened the way for a rapid development of laminin structural biology. Moreover, the phenotypes resulting from gene targeting in mouse or from laminin defects in acquired or inherited human diseases highlight the pivotal role of laminins in morphogenesis, development, and physiology. Indeed, the laminins display a remarkable repertoire of functions, most importantly as structural elements forming a network throughout the basement membrane to which other collagenous or non-collagenous glycoproteins and proteoglycans attach. Furthermore, they are signaling molecules providing adjacent cells with diverse information by interacting with cell surface components.

SMAD3/4-dependent transcriptional activation of the human type VII collagen gene (COL7A1) promoter by transforming growth factor beta

The human type VII collagen gene (COL7A1) recently has been identified as an immediate-early response gene for transforming growth factor beta (TGF-beta)/SMAD signaling pathway. In this study, by using MDA-MB-468 SMAD4-/- breast carcinoma cells, we demonstrate that expression of SMAD4 is an absolute requirement for SMAD-mediated promoter activity. We also demonstrate that the SMAD binding sequence (SBS) representing the TGF-beta response element in the region -496/-444 of the COL7A1 promoter functions as an enhancer in the context of a heterologous promoter. Electrophoretic mobility-shift assays with nuclear extracts from COS-1 cells transfected with expression vectors for SMADs 1-5 indicate that SMAD3 forms a complex with a migration similar to that of the endogenous TGF-beta-specific complex observed in fibroblast extracts. Electrophoretic mobility-shift assays using recombinant glutathione S-transferase-SMAD fusion proteins indicate that both SMAD4 and C-terminally truncated SMAD3, but not SMAD2, can bind the COL7A1 SBS. Coexpression of SMAD3 and SMAD4 in COS-1 cells leads to the formation of two complexes: a DNA/protein complex containing SMAD3 alone and another slower-migrating complex containing both SMAD3 and SMAD4, the latter complex not being detected in fibroblasts. Maximal transactivation of COL7A1 SBS-driven promoters in either MDA-MB-468 carcinoma cells or fibroblasts requires concomitant overexpression of SMAD3 and SMAD4. These data may represent the first identification of a functional homomeric SMAD3 complex regulating a human gene.

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.

Hemidesmosomes and their unique transmembrane protein BP180

Hemidesmosomes are adhesion complexes responsible for linking keratin intermediate filaments of stratified and complex epithelia to components of the extracellular matrix such as collagen fibrils. Over the past several years, it has become clear that there are at least five hemidesmosomal proteins, including HD1/plectin and BP230 as cytoplasmic plaque proteins and integrin alpha6beta4 and BP180 as transmembrane proteins. Among them, BP180 is unique as a transmembrane protein because of its collagenous extracellular domain. Recent biochemical and ultrastructural analyses have revealed its molecular configuration and nature as a major component of anchoring filaments connecting hemidesmosomes to the basement membrane. These results indicate that BP180 is a new type of adhesion receptor. In addition to biochemical analyses of these hemidesmosomal proteins, recent studies on patients with inherited skin blistering diseases and on knockout mice have demonstrated roles in hemidesmosome formation and stabilization, as well as unexpected, novel functions.

Two forms of collagen XVII in keratinocytes. A full-length transmembrane protein and a soluble ectodomain

The cDNA sequence of human collagen XVII predicts an unusual type II transmembrane protein, but a biochemical characterization of this structure has not been accomplished yet. Using domain-specific antibodies against recombinant collagen XVII fragments, we identified two molecular forms of the collagen in human skin and epithelial cells. Full-length collagen XVII appeared as a homotrimeric transmembrane molecule of three 180-kDa alpha1(XVII) chains. The globular intracellular domain was disulfide-linked, and the N-glycosylated extracellular domain of three 120-kDa polypeptides was triple-helical at physiological temperatures. A second, soluble form of collagen XVII in keratinocyte culture media was recognized with antibodies to the ectodomain, but not the endodomain. The soluble form exhibited molecular properties of the collagen XVII ectodomain: a triple-helical, N-glycosylated molecule of three 120-kDa polypeptides. Northern blot analysis with probes spanning either the distal 5'or the distal 3' end of the collagen XVII cDNA revealed an identical 6-kb mRNA, suggesting that both the 180- and 120-kDa polypeptides were translated from the same mRNA, and that the 120-kDa polypeptide was generated post-translationally. In concert, keratinocytes harboring a homozygous nonsense mutation in the COL17A1 gene synthesized neither the 180-kDa alpha1(XVII) chain nor the 120-kDa polypeptide. Finally, treatment of normal keratinocytes with a synthetic inhibitor of furin proprotein convertases, decanoyl-RVKR-chloromethyl ketone, prevented the generation of the 120-kDa polypeptide. These data strongly suggest that the soluble 120-kDa polypeptide represents a specifically cleaved ectodomain of collagen XVII, generated through furin-mediated proteolytic processing. Thus, collagen XVII is not only an unusual type II transmembrane collagen, but the first collagen with a specifically processed, soluble triple-helical ectodomain.

Supercoiled protein motifs: the collagen triple-helix and the alpha-helical coiled coil

The collagen triple-helix and the alpha-helical coiled coil represent the two basic supercoiled multistranded protein motifs. Originally they were characterized in fibrous proteins, but have been found more recently in a number of other proteins containing rod-shaped domains. Coiled-coil domains are responsible for the oligomerization of proteins, as well as other specific functions, while the triple-helix domains associate to form supramolecular structures and bind a variety of ligands. Both structures were originally solved by fiber diffraction, and recent crystallographic studies on small proteins and peptide models have confirmed the structure and provided molecular details. The differences in the molecular conformations of these two motifs and the interactions stabilizing these conformations are discussed. The molecular structures of both motifs constrain the amino acid sequence to recognizable patterns, requiring the (Gly-X-Y)n repeating sequence for the collagen triple-helix and a less stringent heptad repeat requirement (h-x-x-h-x-x-x)n for the coiled-coil domains, where h represents hydrophobic residues. The features and roles of these supercoiled domains in proteins are considered when they are found adjacent in the same protein.

Novel homozygous and compound heterozygous COL17A1 mutations associated with junctional epidermolysis bullosa

Junctional epidermolysis bullosa is a heritable, heterogeneous blistering skin disease with mechanically induced dermal-epidermal separation, mild skin atrophy, nail dystrophy, and alopecia. Four unrelated junctional epidermolysis bullosa families with different phenotypes were investigated here and four novel mutations associated with the disease were identified. Patients 1, 2, and 3 had generalized atrophic benign epidermolysis bullosa, with nonscarring blistering and varying degree of alopecia. Patient 4 had the localisata variant of junctional epidermolysis bullosa, with predominantly acral blistering and normal hair. All patients had mutations in the COL17A1 gene encoding collagen XVII, a hemidesmosomal transmembrane protein. Patients 1 and 2 carried homozygous deletions 520delAG and 2965delG, respectively. Patient 3 was compound heterozygous for a missense and a deletion mutation (G539E and 2666delTT), and patient 4 was heterozygous for a known mutation R1226X. The deletions led to premature termination codons and to drastically reduced collagen XVII mRNA and protein levels, consistent with the absence of the collagen in generalized atrophic benign epidermolysis bullosa skin. The missense mutation G539E allowed synthesis of immunoreactive collagen XVII in keratinocytes, but prevented its secretion, thus causing lack of the protein in the skin. The data suggest that different COL17A1 mutations and their combinations can result in a spectrum of biologic and clinical phenotypes of not only generalized atrophic benign epidermolysis bullosa, but also localized junctional epidermolysis bullosa.

Gelatinase B-deficient mice are resistant to experimental bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease characterized by deposition of autoantibodies at the basement membrane zone. In an experimental BP model in mice, the subepidermal blistering is mediated by antibodies directed against the hemidesmosomal protein BP180 (collagen XVII, BPAG2), and depends on complement activation and neutrophil infiltration. Gelatinase B is present in BP blister fluid and can cleave BP180. In this study we investigated the role of gelatinase B in the immunopathogenesis of experimental BP using mice containing targeted disruption of the gelatinase B (MMP-9, 92 kD gelatinase) gene. Gelatinase B-deficient mice were resistant to the blistering effect of intracutaneous anti-mBP180 antibodies, although these mice showed deposition of autoantibodies at the basement membrane zone and neutrophil recruitment to the skin comparable to that observed in the control mice. Interleukin 8 given intradermally concomitantly with pathogenic anti-mBP180 elicited a significant neutrophil recruitment into the skin in gelatinase B-deficient mice, but blistering did not occur. However, gelatinase B-deficient mice reconstituted with neutrophils from normal mice developed blistering in response to anti-mBP180 antibodies. These results implicate neutrophil-derived gelatinase B in the pathogenesis of experimental BP and might lead to novel therapeutic strategies for BP.

Hemidesmosome formation is initiated by the beta4 integrin subunit, requires complex formation of beta4 and HD1/plectin, and involves a direct interaction between beta4 and the bullous pemphigoid antigen 180

Hemidesmosomes (HDs) are stable anchoring structures that mediate the link between the intermediate filament cytoskeleton and the cell substratum. We investigated the contribution of various segments of the beta4 integrin cytoplasmic domain in the formation of HDs in transient transfection studies using immortalized keratinocytes derived from an epidermolysis bullosa patient deficient in beta4 expression. We found that the expression of wild-type beta4 restored the ability of the beta4-deficient cells to form HDs and that distinct domains in the NH2- and COOH-terminal regions of the beta4 cytoplasmic domain are required for the localization of HD1/plectin and the bullous pemphigoid antigens 180 (BP180) and 230 (BP230) in these HDs. The tyrosine activation motif located in the connecting segment (CS) of the beta4 cytoplasmic domain was dispensable for HD formation, although it may be involved in the efficient localization of BP180. Using the yeast two-hybrid system, we could demonstrate a direct interaction between beta4 and BP180 which involves sequences within the COOH-terminal part of the CS and the third fibronectin type III (FNIII) repeat. Immunoprecipitation studies using COS-7 cells transfected with cDNAs for alpha6 and beta4 and a mutant BP180 which lacks the collagenous extracellular domain confirmed the interaction of beta4 with BP180. Nevertheless, beta4 mutants which contained the BP180-binding region, but lacked sequences required for the localization of HD1/plectin, failed to localize BP180 in HDs. Additional yeast two- hybrid assays indicated that the 85 COOH-terminal residues of beta4 can interact with the first NH2-terminal pair of FNIII repeats and the CS, suggesting that the cytoplasmic domain of beta4 is folded back upon itself. Unfolding of the cytoplasmic domain may be part of a mechanism by which the interaction of beta4 with other hemidesmosomal components, e.g., BP180, is regulated.

Type XIII collagen is identified as a plasma membrane protein

The complete primary structure of the mouse type XIII collagen chain was determined by cDNA cloning. Comparison of the mouse amino acid sequences with the previously determined human sequences revealed a high identity of 90%. Surprisingly, the mouse cDNAs extended further in the 5' direction than the previously identified human clones. The 5' sequences contained a new in-frame ATG codon for translation initiation which resulted in elongation of the N-terminal noncollagenous domain by 81 residues. These N-terminal sequences lack a typical signal sequence but include a highly hydrophobic segment that clearly fulfills the criteria for a transmembrane domain. The sequence data thus unexpectedly suggested that type XIII collagen may be located on the plasma membrane, with a short cytosolic N-terminal portion and a long collagenous extracellular portion. These sequence data prompted us to generate antipeptide antibodies against type XIII collagen in order to study the protein and its subcellular location. Western blotting of human tumor HT-1080 cell extract revealed bands of over 180 kDa. These appeared to represent disulfide-bonded multimeric polypeptide forms that resolved upon reduction into 85-95-kDa bands that are likely to represent a mixture of splice forms of monomeric type XIII collagen chains. These chains were shown to contain the predicted N-terminal extension and thus also the putative transmembrane segment. Immunoprecipitation of biotinylated type XIII collagen from surface-labeled HT-1080 cells, subcellular fractionation, and immunofluorescence staining were used to demonstrate that type XIII collagen molecules are indeed located in the plasma membranes of these cells.

Ligand-independent role of the beta 4 integrin subunit in the formation of hemidesmosomes

Recently, we have shown that a region within the beta4 cytoplasmic domain, encompassing the second fibronectin type III (FNIII) repeat and the first 27 amino acids of the connecting segment, is critical for the localization of alpha6 beta4 in hemidesmosomes. In addition, this region was shown to regulate the distribution of HD1/plectin in transfected cells. In order to investigate the function of the beta4 extracellular and cytoplasmic domains in the assembly and integrity of hemidesmosomes, we have constructed chimeric receptors consisting of the extracellular and transmembrane domains of the interleukin 2 receptor (IL2R), fused to different parts of the beta4 cytoplasmic domain. These chimeras are expressed as single subunits at the plasma membrane. The results show that the first and the second FNIII repeat, together with the first part of the connecting segment (in total a stretch of 241 amino acids spanning amino acids 1,115 to 1,356) are both essential and sufficient for the localization of beta4 in pre-existing hemidesmosomes. Moreover, expression of the IL2R/beta4 chimeric constructs in COS-7 and CHO cells, which do not express alpha6 beta4 or the bullous pemphigoid (BP) antigens but do express HD1/plectin, revealed that the stretch of 241 amino acids is sufficient for inducing the formation of type II hemidesmosomes. Expression of the IL2R/beta4 chimeras in a keratinocyte cell line derived from a patient lacking beta4 expression, showed that amino acids 1,115 to 1,356 can also induce the formation of type I hemidesmosomes. We further demonstrate that type I and II hemidesmosomes can also be formed upon adhesion of alpha6 beta4-expressing cells to fibronectin. These findings establish that the beta4 extracellular domain is not essential for the induction of hemidesmosome assembly. Moreover, they demonstrate that binding of alpha6 beta4 to ligand, and heterodimerization of alpha6 with beta4, are not required for hemidesmosome formation. This indicates that the assembly of hemidesmosomes can be regulated from within the cell.

Structure and assembly of hemidesmosomes

The hemidesmosome is a complex junction containing many proteins. The keratin cytoskeleton attaches to its cytoplasmic plaque, while its transmembrane elements interact with components of the extracellular matrix. Hemidesmosome assembly involves recruitment of alpha 6 beta 4 integrin heterodimers, as well as cytoskeletal elements and cytoskeleton-associated proteins to the cell surface. In our cell culture models, these phenomena appear to be triggered by laminin-5 in the extracellular matrix. Cell interaction with laminin-5 apparently induces both phosphorylation and dephosphorylation of subunits of alpha 6 beta 4 integrin. There is emerging evidence that such events are necessary for subsequent cytoskeleton anchorage to the hemidesmosome cytoplasmic plaque. Once assembled, the hemidesmosome plays an essential role in maintaining firm epithelial adhesion to the basement membrane, with hemidesmosome disruption being a hallmark of certain devastating blistering diseases. However, the hemidesmosome is more than just a stable anchor, as it may also be the site of signal transduction, mediated by its alpha 6 beta 4 integrin component. This review discusses our current knowledge of the structure and assembly of the hemidesmosome.

Cleavage of BP180, a 180-kDa bullous pemphigoid antigen, yields a 120-kDa collagenous extracellular polypeptide

The hemidesmosome (HD) is a cell-to-substrate adhesion apparatus found in stratified and complex epithelia. One of the putative cell-matrix adhesion molecules present in the HD is the 180-kDa bullous pemphigoid antigen (BP180), also termed type XVII collagen. In our previous study, using a monoclonal antibody (mAb) 1337, we have detected a 120-kDa collagenase-sensitive polypeptide in the HD fraction (Uematsu, J. and Owaribe, K. (1993) Cell Struct. Funct. 18, 588 (abstr.)). The present study was undertaken to assess the relation of the 120-kDa polypeptide to this BP180. Immunofluorescence microscopy of bovine skin revealed the basement membrane zone of skin to be stained clearly with mAb 1337, whereas the lateral surfaces of basal cells, which were decorated by typical antibodies against BP180, were not. The antibody did not detect HDs in cultured cells but rather in the culture medium. These results indicate a localization of mAb 1337 antigen distinct from BP180. However, the same polypeptide was also recognized by monoclonal antibodies to the extracellular but not the cytoplasmic part of BP180, and found to react with a polyclonal antibody against the non-collagenous 16A domain of BP180. Therefore, the polypeptide was identified as an extracellular fragment of BP180. mAb 1337 immunoprecipitated the 120-kDa fragment from the medium, but not the 180-kDa molecule of BP180 extracted from cultured cells, indicating that the antibody specifically recognizes the fragment. The mAb 1337 apparently recognizes a unique epitope that is exposed or formed by the cleavage. Hence, the staining pattern observed for bovine skin demonstrated the presence of the 120-kDa extracellular fragment. Rotary shadow electron microscopy of affinity-purified 120-kDa fragments demonstrated that they have the unique molecular shape consisting of a central rod and a flexible tail, without the globular head that is present in the BP180 molecule. From these results, we conclude that mAb 1337 shows unique epitope specificity, recognizing only the 120-kDa extracellular fragment of BP180, which is constitutively cleaved on the cell surface as a 120-kDa fragment both in in vivo and in vitro.

Hemidesmosomal variants of epidermolysis bullosa. Mutations in the alpha6beta4 integrin and the 180-kD bullous pemphigoid antigen/type XVII collagen genes

Epidermolysis bullosa (EB), a heterogeneous group of genodermatoses, is characterized by fragility and blistering of the skin, associated with characteristic extracutaneous manifestations. Based on clinical severity, constellation of the phenotypic manifestations, and the level of tissue separation within the cutaneous basement membrane zone, EB has been divided into distinct subcategories. Traditionally, these include the simplex, junctional and dystrophic variants of EB. Recent attention has been drawn to variants of EB demonstrating tissue separation at the level of hemidesmosomes, ultrastructurally recognizable adhesion complexes within the cutaneous basement membrane zone. Clinically, these hemidesmosomal variants manifest either as generalized atrophic benign epidermolysis bullosa (GABEB), EB with pyloric atresia, or EB with late-onset muscular dystrophy. Elucidation of basement membrane zone components by molecular cloning and development of mutation detection strategies have revealed that the hemidesmosomal variants of EB result from mutations in the genes encoding the subunit polypeptides of the 180-kD bullous pemphigoid antigen/type XVII collagen, the alpha6beta4 integrin, or plectin, respectively. Collectively, these data add to the understanding of the molecular complexity of the cutaneous basement membrane zone in EB, as attested by the fact that mutations in 10 different genes can underlie different variants of EB. Elucidation of mutations in different forms of EB has direct application to genetic counseling and DNA-based prenatal testing in families with EB.

Role of the bullous pemphigoid antigen 180 (BP180) in the assembly of hemidesmosomes and cell adhesion--reexpression of BP180 in generalized atrophic benign epidermolysis bullosa keratinocytes

Bullous pemphigoid antigen 180 (BP180) is a transmembrane component of hemidesmosomes (HD), cell-substrate attachment complexes in stratified and complex epithelia. To determine the role of BP180 in the assembly of HD and cell adhesion, using SV40 virions we have immortalized BP180-deficient keratinocytes derived from a patient with the inherited skin blistering disorder generalized atrophic benign epidermolysis bullosa (GABEB). The GABEB keratinocytes form HD-like structures, which contain alpha 6 beta 4 integrin and HD1/plectin, but not the bullous pemphigoid antigen 230 (BP230). The expression of integrin subunits by GABEB keratinocytes was comparable to that of an immortalized normal human keratinocyte cell line (NHK), except for alpha 6 and beta 4, which were less strongly expressed in GABEB cells. In short-term adhesion assays, both GABEB keratinocytes and NHK bound strongly and to a similar extent to laminin-1, laminin-5, fibronectin, and type IV and V collagens, which suggests that BP180 is not involved in promoting the initial adhesion to these ligands. Transfection of GABEB keratinocytes with cDNAs for wild-type or a mutant of BP180 lacking the collagenous extracellular domain resulted in the expression of recombinant BP180 proteins that were correctly polarized at the basal cell surface together with alpha 6 beta 4. In addition, restored synthesis of BP180 affected the subcellular localization of BP230, which was no longer diffusely distributed in the cytoplasm, but was found in HD-like structures. In contrast, a BP180 mutant with a 36-amino-acid deletion from the amino terminus of the cytoplasmic domain failed to localize to HD-like structures. These results demonstrate that a region within the cytoplasmic domain of BP180 is essential for its localization into HD and that BP180 may play a critical role in coordinating the subcellular distribution of BP230.

A highly sensitive enzyme-linked immunosorbent assay for the detection of circulating anti-BP180 autoantibodies in patients with bullous pemphigoid

The BP180 antigen, a component of the epidermal anchoring complex, has been identified as one of the major antigenic targets of autoantibodies associated with the blistering skin disease, bullous pemphigoid. Our research group has recently demonstrated that reactivity of bullous pemphigoid autoantibodies to the BP180 ectodomain is almost entirely restricted to a set of four antigenic sites clustered within the membrane-proximal noncollagenous stretch (NC16A). Using a passive transfer mouse model, antibodies to the corresponding noncollagenous region of murine BP180 were shown to trigger an inflammatory subepidermal blistering disease that closely mimics bullous pemphigoid. We now report the development of an enzyme-linked immunoabsorbent assay system that is extremely sensitive in detecting disease-specific autoantibodies in the sera of bullous pemphigoid patients. The target antigen in this assay is a recombinant form of the BP180 NC16A domain that contains all four of the well-defined bullous pemphigoid-associated antigenic sites. Of 50 randomly selected bullous pemphigoid sera tested, 47 (94%) were positive in this assay, whereas no specific reactivity was detected in any of the 107 controls. Interestingly, all three of the bullous pemphigoid sera that were negative in this assay had been obtained from patients who were already undergoing treatment. The NC16A enzyme-linked immunosorbent assay is more sensitive than any of the standard techniques for detecting circulating bullous pemphigoid autoantibodies, including other enzyme-linked immunosorbent assays, immunoblotting, and indirect immunofluorescence. Finally, the NC16A enzyme-linked immunosorbent assay provides immunologic information that cannot be obtained from direct immunofluorescence studies of skin biopsies, and that may well be relevant in the diagnosis and treatment of bullous pemphigoid.

Tight clustering of extracellular BP180 epitopes recognized by bullous pemphigoid autoantibodies

Bullous pemphigoid is a blistering skin disease associated with autoantibodies against the BP180 antigen, a transmembrane component of the hemidesmosome. Anti-BP180 antibodies have been demonstrated to be pathogenic in a passive transfer mouse model. One extracellular site on human BP180 (MCW-1) was previously shown to be recognized by 50-60% of bullous pemphigoid sera. To facilitate the identification of additional autoantibody-reactive epitopes, recombinant forms of the BP180 ectodomain were generated using both bacterial and mammalian expression systems. One recombinant protein, sec180e, that was expressed in COS-1 cells and that contained the entire BP180 ectodomain, provided us with a tool to detect conformational epitopes. Bullous pemphigoid sera immunoadsorbed against the major noncollagenous NC16A domain no longer reacted with sec180e, indicating that autoantibody reactivity to the BP180 ectodomain is restricted to the NC16A region. Immunoblot analysis of bullous pemphigoid sera immunoadsorbed with a series of recombinant NC16A peptides revealed the presence of three novel autoantigenic sites that, along with the MCW-1 epitope, are clustered within the N-terminal 45 amino acid stretch of NC16A. All 15 bullous pemphigoid sera tested reacted with a recombinant protein containing this BP180 segment. No disease-associated epitopes were detectable within the remaining 28 amino acids of NC16A. Thus, bullous pemphigoid patient autoantibodies react with a set of epitopes on the BP180 ectodomain that are highly clustered. This autoantibody-reactive region on human BP180 shows overlap with the corresponding murine BP180 site that is targeted by antibodies that are pathogenic in the mouse model of bullous pemphigoid. These findings suggest new directions for the development of diagnostic and therapeutic tools for this disease.

Epidermolysis bullosa: a spectrum of clinical phenotypes explained by molecular heterogeneity

Great progress has recently been made in understanding the molecular basis of various heritable skin diseases. A prototype of such conditions is epidermolysis bullosa (EB), a heterogeneous group of mechano-bullous disorders characterized by fragility of the skin and other specialized epithelia. Blistering of the skin in EB results either from fragility of epidermal cells or from defective attachment of the epidermis to the underlying dermis, because of genetic lesions within molecules of the basement-membrane zone at the dermal-epidermal junction. Distinct mutations have been discovered in ten different genes encoding the structural components within this layer. The combinations and the types of mutations, as well as their positions in the altered gene products, collectively reflect the phenotypic variability observed in this group of heritable skin diseases.

Beta2-microglobulin-deficient mice are resistant to bullous pemphigoid

Recent understanding of the mechanism of immunoglobulin G (IgG) catabolism has yielded new insight into antibody-mediated diseases. We proposed that beta2-microglobulin (beta2m)-deficient mice have been protected from systemic lupus erythematosis (SLE)-like syndromes because they lack the beta2m-associated IgG protection receptor (FcRn) and therefore catabolize IgG, including pathogenic IgG autoantibodies, considerably more rapidly than normal mice. Such an hypothesis would predict that beta2m-deficient mice would also be resistant to experimental bullous pemphigoid, a disease with a pathogenesis thought to be much simpler than SLE, being the result of antibody directed toward a pathogenic epitope on the epidermal hemidesmosome that anchors basal keratinocytes to the basement membrane. To test this hypothesis, we administered pathogenic rabbit antibody directed toward the hemidesmosome to beta2m-deficient mice and to normal control mice, both intraperitoneally and intradermally, and assessed the mice clinically, histologically, and immunologically for manifestations of skin disease. We found that the beta2m-deficient mice were protected when the antibody was given intraperitoneally whereas intradermal administration resulted in blisters only slightly less severe than those seen in normal mice. These data would indicate that autoantibody-mediated inflammation might be prevented or controlled by appropriate modulation of FcRn function.

The extracellular domain of BPAG2 localizes to anchoring filaments and its carboxyl terminus extends to the lamina densa of normal human epidermal basement membrane

Bullous pemphigoid antigen 2 (BPAG2) is a 180 kDa type II transmembrane protein associated with hemidesmosomes (HDs) in basal keratinocytes. To better understand how BPAG2 promotes keratinocyte adhesion to epidermal basement membrane (BM), purified IgG against a baculovirus-encoded recombinant was used to localize its carboxyl terminus in human skin by immunogold electron microscopy (IEM). A 2.1-kb BPAG2 cDNA encoding the distal extracellular domain and carboxyl terminus of BPAG2 was used in a baculovirus expression system to create virus that produced a 70-kDa recombinant form of BPAG2 (BV4). BV4 was purified, characterized, and used to raise high-titer specific rabbit IgG. Purified anti-BV4 IgG bound the epidermal side of 1 M NaCl split skin and bound only BPAG2 on immunoblots containing extracts of human keratinocytes. In IEM studies of pre- and post-embedded skin, the distal ectodomain of BPAG2 localized beneath HDs in basal keratinocytes; there was no evidence of BPAG2 beneath melanocytes. Anti-BV4 IgG extensively bound anchoring filaments on the epidermal side of 1 M NaCl split skin; this staining extended along anchoring filaments to their ends. In post-embedded skin, the carboxyl terminus of BPAG2 was localized within the lamina densa, 41 nm (mean of 400 determinations) beneath plasma membranes of basal keratinocytes. BPAG2 thus extends from the intracellular HD plaque of basal keratinocytes to the lamina densa of human epidermal BM.

Three novel homozygous point mutations and a new polymorphism in the COL17A1 gene: relation to biological and clinical phenotypes of junctional epidermolysis bullosa

Junctional epidermolysis bullosa (JEB) is a clinically and biologically heterogeneous genodermatosis, characterized by trauma-induced blistering and healing without scarring but sometimes with skin atrophy. We investigated three unrelated patients with different JEB phenotypes. Patients 1 and 2 had generalized atrophic benign epidermolysis bullosa (GABEB), with features including skin atrophy and alopecia. Patient 3 had the localisata variant of JEB, with predominantly acral blistering and normal hair. All patients carried novel homozygous point mutations (Q1016X, R1226X, and R1303Q) in the COL17A1 gene encoding collagen XVII, a hemidesmosomal transmembrane component; and, therefore, not only GABEB but also the localisata JEB can be a collagen XVII disorder. The nonsense mutations led to drastically reduced collagen XVII mRNA and protein levels. In contrast, the missense mutation allowed expression of abnormal collagen XVII, and epidermal extracts from that patient contained polypeptides of normal size, as well as larger aggregates. The homozygous nonsense mutations in the COL17A1 gene were consistent with the absence of the collagen from the skin and with the GABEB phenotype, whereas homozygosity for the missense mutation resulted in expression of aberrant collagen XVII and, clinically, in localisata JEB.

Bullous pemphigoid and cicatricial pemphigoid autoantibodies react with ultrastructurally separable epitopes on the BP180 ectodomain: evidence that BP180 spans the lamina lucida

The BP180 antigen is a hemidesmosomal glycoprotein that is recognized by autoantibodies associated with three autoimmune disorders, bullous pemphigoid (BP), herpes gestationis (HG), and cicatricial pemphigoid (CP). BP and HG sera have been shown to recognize a common extracellular site located near the membrane-spanning domain of this protein, whereas CP sera react predominantly with a distinct site near the C terminus. In the current study, the main immunogenic sites on the BP180 ectodomain were ultrastructurally localized using six BP sera, four CP sera, and two rabbit antisera. The immunolocalization pattern of BP sera was largely restricted to the upper lamina lucida region immediately subjacent to the epidermal hemidesmosome and closely resembled that of a rabbit antiserum directed against the NC16A (membrane-proximal) domain of BP180. CP sera, on the other hand, exhibited a lower lamina lucida/lamina densa labeling pattern that was strikingly similar to that of rabbit antibodies to the BP180 C-terminal region. Finally, antibodies to the BP180 C-terminal region co-localized with an anti-laminin-5 antibody in the anchoring filament zone. These findings strongly suggest that the BP180 extracellular domain exists in an extended conformation, with the C terminus of this protein projecting into the lamina densa. These data support the hypothesis that BP180 contributes to the structure and function of the anchoring filaments. Differences in the ultrastructural mapping of BP and CP autoantibodies appear to correlate with epitope mapping data, which, together, may help to explain the clinical heterogeneity observed in this group of bullous disorders.

Premature termination codons are present on both alleles of the bullous pemphigoid antigen 2/type XVII collagen gene in five Austrian families with generalized atrophic benign epidermolysis bullosa

Patients with generalized atrophic benign epidermolysis bullosa (GABEB), an inherited subepidermal blistering disease, often have no immunologically detectable bullous pemphigoid antigen 2 (BPAG2) in their epidermal basement membrane. Recently, we analyzed the BPAG2 gene (GenBank no. M91669) in an Austrian family with GABEB and identified a homozygous deletion mutation, 4003delTC, that results in a downstream premature termination codon (PTC). This mutation has now been identified in additional descendants, suggesting transmission of this mutant allele through at least six generations. Screening of four other Austrian GABEB families revealed that affected members were homozygous for 4003delTC in two cases and heterozygous in two others. In the latter, mutational analysis identified two novel nonsense mutations, Q1403X and G803X, that were confirmed by restriction endonuclease digestions. Thus, PTCs on both alleles of BPAG2 are present in all of these GABEB families. Immunoprecipitation and northern blot studies of cultured keratinocytes from homozygous GABEB patients show that 4003delTC results in undetectable levels of BPAG2 protein and mRNA-findings consistent with the process of nonsense-mediated mRNA decay. Incubating keratinocytes with cycloheximide increased BPAG2 mRNA to a level detectable by northern analysis. When the latter was used in reverse transcription-PCR studies, the mutation was demonstrated, suggesting that cycloheximide may allow mutational analysis in cases where low transcript levels have previously thwarted RT-PCR studies. These findings account for the absence of BPAG2 in GABEB patients and attest to the importance of this protein in adhesion of epidermis to epidermal basement membrane.