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

Deciphering the Contribution of BP230 Autoantibodies in Bullous Pemphigoid

Bullous pemphigoid (BP) is a subepidermal autoimmune blistering disease predominantly affecting elderly patients and carries significant morbidity and mortality. Patients typically suffer from severe itch with eczematous lesions, urticarial plaques, and/or tense blisters. BP is characterized by the presence of circulating autoantibodies against two components of the hemidesmosome, BP180 and BP230. The transmembrane BP180, also known as type XVII collagen or BPAG2, represents the primary pathogenic autoantigen in BP, whereas the intracellular BP230 autoantigen is thought to play a minor role in disease pathogenesis. Although experimental data exist suggesting that anti-BP230 antibodies are secondarily formed following initial tissue damage mediated by antibodies targeting extracellular antigenic regions of BP180, there is emerging evidence that anti-BP230 IgG autoantibodies alone directly contribute to tissue damage. It has been further claimed that a subset of patients has a milder variant of BP driven solely by anti-BP230 autoantibodies. Furthermore, the presence of anti-BP230 autoantibodies might correlate with distinct clinical features. This review summarizes the current understanding of the role of BP230 and anti-BP230 antibodies in BP pathogenesis.

Autoantibodies to BPAG1e Trigger Experimental Bullous Pemphigoid in Mice

Bullous pemphigoid (BP) is an autoimmune blistering disease that targets the hemidesmosomal proteins BP180 and BP230/BPAG1e. Whereas the role of anti-BP180 antibodies has been extensively characterized, the pathogenicity of anti-BPAG1e antibodies remains unclear. The purpose of this study is to elucidate the role of antibodies to BPAG1e in the experimental bullous pemphigoid models. We generated Bpag1 conditional knockout mice, where the knockout of Bpag1 is restricted to keratin 5-expressing epithelial cells. Bpag1 conditional knockout mice were immunized with the C-terminal portion of BPAG1e, and the splenocytes were injected into Rag2^-/- mice intravenously. The recipient mice presented with erosion on the feet and tails. Microscopic examination showed subepidermal blisters and a linear deposition of IgG at the dermal-epidermal junction. To assess the potential role of trauma on BP development, we inflicted surface wounds on the dorsum of the Rag2^-/- recipient mice after adoptive transfer. The wounded Rag2^-/- mice had increased morbidity and severity of BP-like symptoms. Moreover, the depletion of B cells from splenocytes abolished a subepidermal blistering phenotype in vivo. These findings demonstrate that antibodies to BPAG1e might play a pathogenic role in causing subepidermal blistering, and external factors, including trauma, might be a trigger for BP development.

The spectraplakin Dystonin antagonizes YAP activity and suppresses tumourigenesis

Aberrant expression of the Spectraplakin Dystonin (DST) has been observed in various cancers, including those of the breast. However, little is known about its role in carcinogenesis. In this report, we demonstrate that Dystonin is a candidate tumour suppressor in breast cancer and provide an underlying molecular mechanism. We show that in MCF10A cells, Dystonin is necessary to restrain cell growth, anchorage-independent growth, self-renewal properties and resistance to doxorubicin. Strikingly, while Dystonin maintains focal adhesion integrity, promotes cell spreading and cell-substratum adhesion, it prevents Zyxin accumulation, stabilizes LATS and restricts YAP activation. Moreover, treating DST-depleted MCF10A cells with the YAP inhibitor Verteporfin prevents their growth. In vivo, the Drosophila Dystonin Short stop also restricts tissue growth by limiting Yorkie activity. As the two Dystonin isoforms BPAG1eA and BPAG1e are necessary to inhibit the acquisition of transformed features and are both downregulated in breast tumour samples and in MCF10A cells with conditional induction of the Src proto-oncogene, they could function as the predominant Dystonin tumour suppressor variants in breast epithelial cells. Thus, their loss could deem as promising prognostic biomarkers for breast cancer.

BPAG1 in muscles: Structure and function in skeletal, cardiac and smooth muscle

BPAG1, also known as Dystonin or BP230, belongs to the plakin family of proteins, which has multiple cytoskeleton-binding domains. Several BPAG1 isoforms are produced by a single BPAG1 genomic locus using different promoters and exons. For example, BPAG1a, BPAG1b, and BPAG1e are predominantly expressed in the nervous system, muscle, and skin, respectively. Among BPAG1 isoforms, BPAG1e is well studied because it was first identified as an autoantigen in patients with bullous pemphigoid, an autoimmune skin disease. BPAG1e is a component of hemidesmosomes, the adhesion complexes that promote dermal-epidermal cohesion. In the nervous system, the role of BPAG1a is also well studied because disruption of BPAG1a results in a phenotype identical to that of Dystonia musculorum (dt) mutants, which show progressive motor disorder. However, the expression and function of BPAG1 in muscles is not well studied. The aim of this review is to provide an overview of and highlight some recent findings on the expression and function of BPAG1 in muscles, which can assist future studies designed to delineate the role and regulation of BPAG1 in the dt mouse phenotype and in human hereditary sensory and autonomic neuropathy type 6 (HSAN6).

Extracellular cleavage of collagen XVII is essential for correct cutaneous basement membrane formation

In skin, basal keratinocytes in the epidermis are tightly attached to the underlying dermis by the basement membrane (BM). The correct expression of hemidesmosomal and extracellular matrix (ECM) proteins is essential for BM formation, and the null-expression of one molecule may induce blistering diseases associated with immature BM formation in humans. However, little is known about the significance of post-translational processing of hemidesmosomal or ECM proteins in BM formation. Here we show that the C-terminal cleavage of hemidesmosomal transmembrane collagen XVII (COL17) is essential for correct BM formation. The homozygous p.R1303Q mutation in COL17 induces BM duplication and blistering in humans. Although laminin 332, a major ECM protein, interacts with COL17 around p.R1303, the mutation leaves the binding of both molecules unchanged. Instead, the mutation hampers the physiological C-terminal cleavage of COL17 in the ECM. Consequently, non-cleaved COL17 ectodomain remnants induce the aberrant deposition of laminin 332 in the ECM, which is thought to be the major pathogenesis of the BM duplication that results from this mutation. As an example of impaired cleavage of COL17, this study shows that regulated processing of hemidesmosomal proteins is essential for correct BM organization in skin.

Blistering disease: insight from the hemidesmosome and other components of the dermal-epidermal junction

The hemidesmosome is a specialized transmembrane complex that mediates the binding of epithelial cells to the underlying basement membrane. In the skin, this multiprotein structure can be regarded as the chief adhesion unit at the site of the dermal-epidermal junction. Focal adhesions are additional specialized attachment structures located between hemidesmosomes. The integrity of the skin relies on well-assembled and functional hemidesmosomes and focal adhesions (also known as integrin adhesomes). However, if these adhesion structures are impaired, e.g., as a result of circulating autoantibodies or inherited genetic mutations, the mechanical strength of the skin is compromised, leading to blistering and/or tissue inflammation. A particular clinical presentation emerges subject to the molecule that is targeted. None of these junctional complexes are simply compounds of adhesion molecules; they also play a significant role in signalling pathways involved in the differentiation and migration of epithelial cells such as during wound healing and in tumour invasion. We summarize current knowledge about hereditary and acquired blistering diseases emerging from pathologies of the hemidesmosome and its neighbouring proteins as components of the dermal-epidermal junction.

Genomic analysis of Sleeping Beauty transposon integration in human somatic cells

The Sleeping Beauty (SB) transposon is a non-viral integrating vector system with proven efficacy for gene transfer and functional genomics. However, integration efficiency is negatively affected by the length of the transposon. To optimize the SB transposon machinery, the inverted repeats and the transposase gene underwent several modifications, resulting in the generation of the hyperactive SB100X transposase and of the high-capacity “sandwich” (SA) transposon. In this study, we report a side-by-side comparison of the SA and the widely used T2 arrangement of transposon vectors carrying increasing DNA cargoes, up to 18 kb. Clonal analysis of SA integrants in human epithelial cells and in immortalized keratinocytes demonstrates stability and integrity of the transposon independently from the cargo size and copy number-dependent expression of the cargo cassette. A genome-wide analysis of unambiguously mapped SA integrations in keratinocytes showed an almost random distribution, with an overrepresentation in repetitive elements (satellite, LINE and small RNAs) compared to a library representing insertions of the first-generation transposon vector and to gammaretroviral and lentiviral libraries. The SA transposon/SB100X integrating system therefore shows important features as a system for delivering large gene constructs for gene therapy applications.

Targeted gene addition in human epithelial stem cells by zinc-finger nuclease-mediated homologous recombination

Preclinical and clinical studies showed that autologous transplantation of epidermis derived from genetically modified epithelial stem cells (EpSCs) leads to long-term correction of inherited skin adhesion defects. These studies were based on potentially genotoxic retroviral vectors. We developed an alternative gene transfer strategy aimed at targeting a "safe harbor" locus, the adeno-associated virus integration site 1 (AAVS1), by zinc-finger nuclease (ZFN)-induced homologous recombination (HR). Delivery of AAVS1-specific ZFNs and a GFP-expressing HR cassette by integration-defective lentiviral (LV) vectors (IDLVs) or adenoviral (Ad) vectors resulted in targeted gene addition with an efficiency of > 20% in a human keratinocyte cell line, > 10% in immortalized keratinocytes, and < 1% in primary keratinocytes. Deep sequencing of the AAVS1 locus showed that ZFN-induced double-strand breaks are mostly repaired by nonhomologous end joining (NHEJ) in primary cells, indicating that poor induction of the HR-dependent DNA repair pathway may be a significant limitation for targeted gene integration. Skin equivalents derived from unselected keratinocyte cultures coinfected with a GFP-IDLV and a ZFN-Ad vector were grafted onto immunodeficient mice. GFP-positive clones were observed in all grafts up to 18 weeks post-transplantation. By histological and molecular analysis, we were able to demonstrate highly efficient targeting of the AAVS1 locus in human repopulating EpSCs.

Demonstration of epitope-spreading phenomena in bullous pemphigoid: results of a prospective multicenter study

Bullous pemphigoid (BP), the most common autoimmune subepidermal bullous disease, is associated with an autoantibody response to BP180 and BP230, two components of junctional adhesion complexes in human skin promoting dermo-epidermal cohesion. Retrospective analyses demonstrated that these autoantigens harbor several epitopes targeted by autoaggressive B and T cells. The aim of this prospective multicenter study was to assess the evolution of IgG autoantibodies in 35 BP patients over a 12-month observation period. Epitope-spreading (ES) events were detected in 17 of 35 BP patients (49%). They preferentially occurred in an early stage of the disease and were significantly related to disease severity at diagnosis. Moreover, in three patients, spreading of IgG reactivity to intracellular epitopes of BP180 and BP230 was preceded by recognition of the BP180 ectodomain. Finally, IgG reactivity with extracellular epitopes of BP180 and intracellular epitopes of BP230 correlated with the severity of BP in disease course. These findings support the idea that IgG recognition of the BP180 ectodomain is an early and crucial event in BP disease, followed by variable intra- and intermolecular ES events, which likely shape the individual course of BP.

Molecular organization of the basement membrane zone

The dermal-epidermal basement membrane is a complex assembly of proteins that provide adhesion and regulate many important processes such as development, wound healing, and cancer progression. This contribution focuses on the structure and function of individual components of the basement membrane, how they assemble together, and how they participate in human tissues and diseases, with an emphasis on skin involvement. Understanding the composition and structure of the basement membrane provides insight into the pathophysiology of inherited blistering disorders, such as epidermolysis bullosa, and acquired bullous diseases, such as the pemphigoid group of autoimmune diseases and epidermolysis bullosa acquisita.

Lysophosphatidic Acid Upregulates Laminin-332 Expression during A431 Cell Colony Dispersal

Lysophosphatidic acid (LPA) is a bioactive phospholipid that affects various biological functions, such as cell proliferation, migration, survival, wound healing, and tumor invasion through LPA receptors. Previously, we reported that LPA induces A431 colony dispersal, accompanied by disruption of cell-cell contacts and cell migration. However, it remains unclear how LPA affects cell migration and gene expression during A431 colony dispersal. In this paper, we performed cDNA microarray analysis to investigate this question by comparing gene expression between untreated and LPA-treated A431 cells. Interestingly, these results revealed that LPA treatment upregulates several TGF-β1 target genes, including laminin-332 (Ln-332) components (α3, β3, and γ2 chains). Western blot analysis also showed that LPA increased phosphorylation of Smad2, an event that is carried out by TGF-β1 interactions. Among the genes upregulated, we further addressed the role of Ln-332. Real-time PCR analysis confirmed the transcriptional upregulation of all α3, β3, and γ2 chains of Ln-332 by LPA, corresponding to the protein level increases revealed by western blot. Further, the addition of anti-Ln-332 antibody prevented LPA-treated A431 colonies from dispersing. Taken together, our results suggest that LPA-induced Ln-332 plays a significant role in migration of individual cells from A431 colonies.

Epitope mapping of function-blocking monoclonal antibody CM6 suggests a "weak" integrin binding site on the laminin-332 LG2 domain

Laminin-332 (Ln-332) is an extracellular matrix molecule that regulates cell adhesion, spreading, and migration by interaction with cell surface receptors such as alpha3beta1 and alpha6beta4. Previously, we developed a function-blocking monoclonal antibody against rat Ln-332, CM6, which blocks hemidesmosome assembly induced by Ln-332-alpha6beta4 interactions. However, the location of its epitope on Ln-332 has remained unclear. In this study, we show that the CM6 epitope is located on the laminin G-like (LG)2 module of the Ln-332 alpha3 chain. To specify the residues involved in this epitope, we produced a series of GST-fused alpha3 LG2 mutant proteins in which rat-specific acids were replaced with human acids by a site-directed mutagenesis strategy. CM6 reactivity against these proteins showed that CM6 binds to the (1089)NERSVR(1094) sequence of rat Ln-332 LG2 module. In a structural model, this sequence maps to an LG2 loop sequence that is exposed to solvent according to predictions, consistent with its accessibility to antibody. CM6 inhibits integrin-dependent cell adhesion on Ln-332 and inhibits cell spreading on both Ln-332 and recombinant LG2 (rLG2; but not rLG3), suggesting the presence of an alpha3beta1 binding site on LG2. However, we were unable to show that rLG2 supports adhesion in standard assays, suggesting that LG2 may contain a "weak" integrin binding site, only detectable in spreading assays that do not require washes. These results, together with our previous findings, indicate that binding sites for alpha3beta1 and alpha6beta4 are closely spaced in the Ln-332 LG domains where they regulate alternative cell functions, namely adhesion/migration or hemidesmosome anchoring.

Localisation of bullous pemphigoid antigen 180 (BP180) in cultured human keratinocytes: functionally relevant modification by calcium

The expression of BP180 has previously been demonstrated to be influenced by both calcium (Ca(2+)) concentration and binding of anti-BP180-antibodies in cultured keratinocytes of the skin squamous cell carcinoma line DJM-1. Here, BP180 expression was studied in cultured normal human epidermal keratinocytes by confocal laser scanning microscopy. We exploited an experimental system, in which BP180 was previously shown to mediate, upon incubation with anti-BP180 antibodies, a specific signal-transducing event that leads to the release of inflammatory mediators, such as IL-8 from cultured normal human epidermal keratinocytes (NHEK). We found that without addition of BP180-specific IgG, BP180 is predominantly expressed on the cell surface irrespective of the Ca(2+) concentration. In contrast, cell surface BP180 was greatly reduced in NHEK kept in high Ca(2+) medium after incubation with BP180-specific IgG for 12 h compared to low Ca(2+) medium. This effect was seen with antibodies to both N- and C-terminal fragments of the BP180 ectodomain, respectively. In addition, a slightly higher BP180 expression was found in NHEK cultured in low compared to high Ca(2+) medium by Western blotting. Interestingly, in contrast to NHEK kept under low Ca(2+ )conditions, in NHEK grown in high Ca(2+) medium, no elevated levels of IL-8 were released after treatment of cells with anti-BP180 IgG compared to normal IgG. Our data indicate that the Ca(2+)-modulated expression of BP180 is functionally relevant. This finding sheds further light on the complex pathomechanism in blister formation of BP180-related autoimmune blistering skin diseases.

Number 1Epithelial biology

The oral mucous membrane has features similar to skin but also differs in several ways. This paper reviews the aspects of epithelial biology necessary for an understanding of the vesiculoerosive disorders.

Bullous Pemphigoid

Bullous pemphigoid (BP) is a chronic, autoimmune, blistering disease observed primarily in the elderly population. Several clinical variants have been described, including classic (bullous), localised, nodular, vegetating, erythrodermic, erosive, childhood and drug-induced forms. Autoantibodies target the BP230 and BP180 antigens, located in the hemidesmosomal complex of the skin basement membrane zone. Subsequent complement activation recruits chemical and cellular immune mediators to the skin, ultimately resulting in blister formation. Both autoantibodies and complement may be detected by various immunofluorescent, immune electron microscopy and molecular biology techniques. Recent trials suggest that potent topical corticosteroids should be considered as first-line therapy. Tetracycline with or without nicotinamide may benefit a subset of patients with mild BP. Oral corticosteroids should rarely exceed 0.75 mg/kg/day and corticosteroid-sparing agents may be useful for recalcitrant disease.

Hemidesmosomes: molecular organization and their importance for cell adhesion and disease

In the skin, basal epithelial cells constantly divide to renew the epidermis. The newly formed epithelial cells then differentiate in a process called keratinization, ultimately leading to the death of these cells and a pile-up of cell material containing vast amounts of keratins. The basal keratinocytes in skin are attached to their underlying basement membrane via specialized adhesion complexes termed hemidesmosomes (HDs). These complexes ascertain stable adhesion of the epidermis to the dermis, and mutations in components of these complexes often result in tissue fragility and blistering of the skin. In this review, we will describe the various hemidesmosomal proteins in detail as well as, briefly, the protein families to which they belong. Specifically, we will report the protein-protein interactions involved in the assembly of hemidesmosomes and their molecular organization. Some signaling pathways involving primarily the alpha6beta4 integrin will be discussed, since they appear to profoundly modulate the assembly and function of hemidesmosomes. Furthermore, the importance of these hemidesmosomal components for the maintenance of tissue homeostasis and their involvement in various clinical disorders will be emphasized. Finally, we will present a model for the assembly of HDs, based on our present knowledge.

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.

Generation and characterization of epidermolysis bullosa simplex cell lines: scratch assays show faster migration with disruptive keratin mutations

BACKGROUND

Epidermolysis bullosa simplex (EBS) is an inherited skin fragility disorder caused by mutations in keratin intermediate filament proteins. While discoveries of these mutations have increased understanding of the role of keratins and other intermediate filaments in epithelial tissues, progress towards the development of therapy for these disorders is much slower.

OBJECTIVES

Cell culture model systems that display these structural defects are needed for analysis of the cellular consequences of the mutations and to enable possible therapeutic strategies to be developed. Our aim was to generate immortalized cell lines as such model systems for the study of EBS.

METHODS

We generated a series of stable cell lines expressing EBS-associated keratin mutations, by immortalizing keratinocytes from EBS-affected skin biopsies with either simian virus 40 (SV40) T antigen or human papillomavirus 16 (HPV16) E6/E7, and assessed their keratin expression (by immunofluorescence), proliferation rates and migratory behaviour (in outgrowth and scratch wound assays).

RESULTS

Clonal immortalized keratinocyte cell lines KEB-1, KEB-2, KEB-3 (using SV40 T antigen) and KEB-4, KEB-7 and NEB-1 (using HPV16 E6/E7) were established. These include two lines from a single individual with Weber-Cockayne EBS (i.e. KEB-3 and KEB-4, mutation K14 V270M), and three cell lines from a second family, two from siblings carrying the same mutation (KEB-1, KEB-2 lines from Dowling-Meara EBS, mutation K5 E475G) and one from an unaffected relative (NEB-1). The sixth cell line (KEB-7), with a previously unreported severe mutation (K14 R125P), was the only one to show keratin aggregates in resting conditions. Despite variations in the immortalization procedure, there was no significant difference between cell lines in keratin expression, outgrowth capabilities or response to transient heat shock. However, cell migration, as measured by speed of scratch wound closure, was significantly faster in cells with severe EBS mutations.

CONCLUSIONS

These cell lines provide useful culture systems in which to assess aspects of EBS-induced cell changes. The faster migration after scratch wounding of the EBS keratinocytes may be a consequence of the known upregulation of stress-activated kinase pathways in these cells.

Plectin tethers desmin intermediate filaments onto subsarcolemmal dense plaques containing dystrophin and vinculin

Plectin is a versatile cytoskeletal linker protein that preferentially localizes at interfaces between intermediate filaments and the plasma membrane in muscle, epithelial cells, and other tissues. Its deficiency causes muscular dystrophy with epidermolysis bullosa simplex. To better understand the functional roles of plectin beneath the sarcolemma of skeletal muscles and to gain some insights into the underlying mechanism of plectin-deficient muscular dystrophy, we studied in vivo structural and molecular relationships of plectin to subsarcolemmal cytoskeletal components, such as desmin, dystrophin, and vinculin, in rat skeletal muscles. Immunogold electron microscopy revealed that plectin fine threads tethered desmin intermediate filaments onto subsarcolemmal dense plaques overlying Z-lines and I-bands. These dense plaques were found to contain dystrophin and vinculin, and thus may be the structural basis of costameres. The in vivo association of plectin with desmin, (meta-)vinculin, dystrophin, and actin was demonstrated by immunoprecipitation experiments. Treatment of plectin immunoprecipitates with gelsolin reduced actin, dystrophin, and (meta-)vinculin but not desmin, implicating that subsarcolemmal actin could partly mediate the interaction between plectin and dystrophin or (meta-)vinculin. Altogether, our data suggest that plectin, along with desmin intermediate filaments, might serve a vital structural role in the stabilization of the subsarcolemmal cytoskeleton.

Analysis of the interactions between BP180, BP230, plectin and the integrin alpha6beta4 important for hemidesmosome assembly

Hemidesmosomes (HDs) are multi-protein complexes that promote stable adhesion of epithelial cells to the underlying extracellular matrix. We assessed the interactions between different hemidesmosomal components with each other, mapped the binding sites and studied the importance of these interactions for HD assembly in yeast two-hybrid and cell-transfection assays. The results show that: (1) bullous pemphigoid antigen (BP) 180 binds not only to BP230, but also to plectin. The interactions between these proteins are facilitated by the Y subdomain in the N-terminal plakin domain of BP230 and plectin, and residues 145-230 of the cytoplasmic domain of BP180; (2) different, but overlapping, sequences on BP180 mediate binding to beta4, which, in turn associates with BP180 via its third fibronectin type III repeat; (3) sequences in the N-terminal extremity of BP230 mediate its binding to beta4, which requires the C-terminal end of the connecting segment up to the fourth FNIII repeat of the beta4 subunit. (4) Finally, cell-transfection studies showed that the localization of BP230 into hemidesmosome-like structures depends on its Z-Y subdomains as well as on the availability of BP180. By having further uncovered interactions between various hemidesmosomal components, mapped the involved binding sites and dissected a hierarchy of interactions relevant for their topogenic fate, our findings give novel insights into the molecular organization of hemidesmosomes.

Epidermolysis bullosa: new and emerging trends

Epidermolysis bullosa is a family of inherited blistering skin disorders characterized by blister formation in response to mechanical trauma. Major types of epidermolysis bullosa include epidermolysis bullosa simplex, hemidesmosomal epidermolysis bullosa, junctional epidermolysis bullosa, and dystrophic epidermolysis bullosa. Current treatment for epidermolysis bullosa consists of supportive care for skin and other organ systems and entails a combination of wound management, infection support for chronic wounds, surgical management as needed, nutritional support, and preventative screening for squamous cell carcinoma in recessive dystrophic epidermolysis bullosa. The regimen must be tailored specifically to the severity and extent of skin and systemic involvement in each case. Recent studies have identified specific protein and genetic abnormalities for most epidermolysis bullosa subtypes. These new advancements in the understanding of molecular pathophysiology have provided much of the basis for current efforts to develop effective gene and protein therapy for epidermolysis bullosa.

Different phenotypes in human prostate cancer: alpha6 or alpha3 integrin in cell-extracellular adhesion sites

The distribution of alpha6/alpha3 integrin in adhesion complexes at the basal membrane in human normal and cancer prostate glands was analyzed in 135 biopsies from 61 patients. The levels of the polarized alpha6/alpha3 integrin expression at the basal membrane of prostate tumor glands were determined by quantitative immunohistochemistry. The alpha6/alpha3 integrin expression was compared with Gleason sum score, pathological stage, and preoperative serum prostate-specific antigen (PSA). The associations were assessed by statistical methods. Eighty percent of the tumors expressed the alpha6 or alpha3 integrin and 20% was integrin-negative. Gleason sum score, but not serum PSA, was associated with the integrin expression. Low Gleason sum score correlated with increased integrin expression, high Gleason sum score with low and negative integrin expression. Three prostate tumor phenotypes were distinguished based on differential integrin expression. Type I coexpressed both alpha6 and alpha3 subunits, type II exclusively expressed alpha6 integrin, and type III expressed alpha3 integrin only. Fifteen cases were further examined for the codistribution of vinculin, paxillin, and CD 151 on frozen serial sections using confocal laser scanning microscopy. The alpha6/alpha3 integrins, CD151, paxillin, and vinculin were present within normal glands. In prostate carcinoma, alpha6 integrin was colocalized with CD 151, but not with vinculin or paxillin. In tumor phenotype I, the alpha6 subunit did not colocalize with the alpha subunit indicating the existence of two different adhesion complexes. Human prostate tumors display on their cell surface the alpha6beta1 and/or alpha3beta1 integrins. Three tumor phenotypes associated with two different adhesion complexes were identified, suggesting a reorganization of cell adhesion structures in prostate cancer.

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.

Cutaneous gene transfer and therapy: the present and the future

The easy accessibility of the skin as a therapeutic target provides an exciting potential for this organ for the development of gene therapy protocols for cutaneous diseases and a variety of metabolic disorders. Thus far, full phenotypic reversion of a diseased phenotype has been achieved in vivo for junctional epidermolysis bullosa and X-linked or lamellar ichthyosis and in vitro for xeroderma pigmentosum. These recessive skin diseases are characterized by skin blistering, abnormalities in epidermal differentiation and increased development of skin cancers, respectively. Corrective gene delivery at both molecular and functional levels was achieved by transduction of cultured skin cells using retroviral vectors carrying the specific curative cDNA. These positive results should prompt clinical trials based on transplantation of artificial epithelia reconstructed ex vivo using genetically modified keratinocytes. Promising results have also been obtained in phenotypic reversion of cells isolated from patients suffering from a number of metabolic diseases such as gyrate atrophy, familial hypercholesterolemia or phenylketonuria. In these diseases transplantation of autologous artificial epithelia expressing the transgenes of interest or direct transfer of the DNA to the skin represents a potential therapeutic approach for the systemic delivery of active molecules. Successful cutaneous gene therapy trials, however, require development of protocols for efficient gene transfer to epidermal stem cells, and information about the host immune response to the recombinant polypeptides produced by the implanted keratinocytes. The availability of spontaneous animal models for genodermatoses will validate the gene therapy approach in preclinical trials.

The tetraspan molecule CD151, a novel constituent of hemidesmosomes, associates with the integrin alpha6beta4 and may regulate the spatial organization of hemidesmosomes

CD151 is a cell surface protein that belongs to the tetraspan superfamily. It associates with other tetraspan molecules and certain integrins to form large complexes at the cell surface. CD151 is expressed by a variety of epithelia and mesenchymal cells. We demonstrate here that in human skin CD151 is codistributed with alpha3beta1 and alpha6beta4 at the basolateral surface of basal keratinocytes. Immunoelectron microscopy showed that CD151 is concentrated in hemidesmosomes. By immunoprecipitation from transfected K562 cells, we established that CD151 associates with alpha3beta1 and alpha6beta4. In beta4-deficient pyloric atresia associated with junctional epidermolysis bullosa (PA-JEB) keratinocytes, CD151 and alpha3beta1 are clustered together at the basal cell surface in association with patches of laminin-5. Focal adhesions are present at the periphery of these clusters, connected with actin filaments, and they contain both CD151 and alpha3beta1. Transient transfection studies of PA-JEB cells with beta4 revealed that the integrin alpha6beta4 becomes incorporated into the alpha3beta1-CD151 clusters where it induces the formation of hemidesmosomes. As a result, the amount of alpha3beta1 in the clusters diminishes and the protein becomes restricted to the peripheral focal adhesions. Furthermore, CD151 becomes predominantly associated with alpha6beta4 in hemidesmosomes, whereas its codistribution with alpha3beta1 in focal adhesions becomes partial. The localization of alpha6beta4 in the pre-hemidesmosomal clusters is accompanied by a strong upregulation of CD151, which is at least partly due to increased cell surface expression. Using beta4 chimeras containing the extracellular and transmembrane domain of the IL-2 receptor and the cytoplasmic domain of beta4, we found that for recruitment of CD151 into hemidesmosomes, the beta4 subunit must be associated with alpha6, confirming that integrins associate with tetraspans via their alpha subunits. CD151 is the only tetraspan identified in hemidesmosomal structures. Others, such as CD9 and CD81, remain diffusely distributed at the cell surface. In conclusion, we show that CD151 is a major component of (pre)-hemidesmosomal structures and that its recruitment into hemidesmosomes is regulated by the integrin alpha6beta4. We suggest that CD151 plays a role in the formation and stability of hemidesmosomes by providing a framework for the spatial organization of the different hemidesmosomal components.

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.

Re-epithelialization rate and protein expression in the suction-induced wound model: comparison between intact blisters, open wounds and calcipotriol-pretreated open wounds

We have investigated re-epithelialization following induction of suction blisters in humans in intact blisters, open wounds, i.e. blister roofs removed immediately after blister induction, and calcipotriol-pretreated open wounds. Intact blisters simulate blister healing in bullous disease, while open wounds simulate re-epithelialization during wound healing. Re-epithelialization was clearly faster in open wounds than in intact blisters, and was not affected by calcipotriol pretreatment. Bullous pemphigoid antigen 2 (BP180), bullous pemphigoid antigen 1 (BP230), plectin/hemidesmosomal 1 protein (HD1), laminin 5, laminin alpha5, laminin beta1, type VII collagen, tenascin-C, beta4, alphavbeta5, alpha5 and alpha9 integrins were studied in intact blisters and open wounds by immunohistochemistry. Hemidesmosomal plaque proteins BP230 and plectin/HD1, which connect the keratin cytoskeleton to the hemidesmosome, appeared earlier at the leading edge in intact blisters than in open wounds. Band-like immunostaining in the basement membrane for laminin 5, alpha5 and beta1 chains was continuous in blister bases, but partially discontinuous in open wound bases. The other antigens studied showed similar expression in intact blisters and open wounds. BP180, BP230, plectin/HD1, beta4 integrin, laminin 5 and tenascin-C expression were further studied in calcipotriol-pretreated open wounds. Calcipotriol did not affect the expression of these antigens. The immunohistochemical results suggest that the keratin cytoskeleton is linked to the basal plasma membrane of migrating basal cells via BP230 and plectin/HD1 earlier in the more slowly re-epithelializing blisters than in open wounds. An intact laminin sheath may inhibit keratinocyte migration in intact blisters.

Collagen XVII is destabilized by a glycine substitution mutation in the cell adhesion domain Col15

Collagen XVII is a hemidesmosomal transmembrane molecule important for epithelial adhesion in the skin. It exists in two forms, as a full-length protein and as a soluble ectodomain that is shed from the keratinocyte surface by furin-mediated proteolysis. To obtain information on the conformation and the functions of this unusual collagen, its largest collagenous domain, Col15, was expressed in a eukaryotic episomal expression system and purified by DEAE and fast protein liquid- Mono S chromatography. The protein was triple-helical (T(m) of 26.5 degrees C) when produced in cultures containing ascorbic acid. When the vitamin supply was limited, the 4-hydroxyproline content was reduced from 74 to 9%, which, in turn, resulted in a drastic reduction of the stability of the triple helix. The glycine substitution mutation G627V associated with junctional epidermolysis bullosa, a human blistering skin disease, also had a striking effect on thermal stability of rCol15 causing partial unfolding already at 4 degrees C. Col15 promoted cell adhesion of epithelial and fibroblastic cell lines with a beta1 integrin-mediated mechanism. In concert with this, in acquired autoimmune blistering skin diseases, circulating IgG and IgA autoantibodies were found to target rCol15r.

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.

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.

Binding of integrin alpha6beta4 to plectin prevents plectin association with F-actin but does not interfere with intermediate filament binding

Hemidesmosomes are stable adhesion complexes in basal epithelial cells that provide a link between the intermediate filament network and the extracellular matrix. We have investigated the recruitment of plectin into hemidesmosomes by the alpha6beta4 integrin and have shown that the cytoplasmic domain of the beta4 subunit associates with an NH(2)-terminal fragment of plectin that contains the actin-binding domain (ABD). When expressed in immortalized plectin-deficient keratinocytes from human patients with epidermol- ysis bullosa (EB) simplex with muscular dystrophy (MD-EBS), this fragment is colocalized with alpha6beta4 in basal hemidesmosome-like clusters or associated with F-actin in stress fibers or focal contacts. We used a yeast two-hybrid binding assay in combination with an in vitro dot blot overlay assay to demonstrate that beta4 interacts directly with plectin, and identified a major plectin-binding site on the second fibronectin type III repeat of the beta4 cytoplasmic domain. Mapping of the beta4 and actin-binding sites on plectin showed that the binding sites overlap and are both located in the plectin ABD. Using an in vitro competition assay, we could show that beta4 can compete out the plectin ABD fragment from its association with F-actin. The ability of beta4 to prevent binding of F-actin to plectin explains why F-actin has never been found in association with hemidesmosomes, and provides a molecular mechanism for a switch in plectin localization from actin filaments to basal intermediate filament-anchoring hemidesmosomes when beta4 is expressed. Finally, by mapping of the COOH-terminally located binding site for several different intermediate filament proteins on plectin using yeast two-hybrid assays and cell transfection experiments with MD-EBS keratinocytes, we confirm that plectin interacts with different cytoskeletal networks.

Update on inherited bullous dermatoses

Bullous diseases are becoming increasingly better understood owing to the active research which has taken place in this field over the past decade. Advances in understanding of bullous disease pathophysiology is translating into clinical applications for diagnosis and therapy that will greatly enhance the quality of care bullous disease patients may receive now and in the future. This review focuses on the progress which has been achieved in inherited bullous dermatoses.

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.

Structure and function of hemidesmosomes: more than simple adhesion complexes

The attachment of cells to the extracellular matrix is of crucial importance in the maintenance of tissue structure and integrity. In stratified epithelia such as in skin as well as in other complex epithelia multiprotein complexes called hemidesmosomes are involved in promoting the adhesion of epithelial cells to the underlying basement membrane. In the past few years our understanding of the role of hemidesmosomes has improved considerably. Their importance has become apparent in clinical conditions, in which absence or defects of hemidesmosomal proteins result in devastating blistering diseases of the skin. Molecular genetic studies have increased our knowledge of the function of the various components of hemidesmosomes and enabled the characterization of protein-protein interactions involved in their assembly. It has become clear that the alpha6beta4 integrin, a major component of hemidesmosomes, is able to transduce signals from the extracellular matrix to the interior of the cell, that critically modulate the organization of the cytoskeleton, proliferation, apoptosis, and differentiation. Nevertheless, our knowledge of the mechanisms regulating the functional state of hemidesmosomes and, hence, the dynamics of cell adhesion, a process of crucial importance in development, wound healing or tumor invasion, remains limited. The aims of this review are to highlight the recent progresses of our knowledge on the organization and assembly of hemidesmosomes, their involvement in signaling pathways as well as their participation in clinical pathologic conditions.

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.

IgG autoantibodies from bullous pemphigoid (BP) patients bind antigenic sites on both the extracellular and the intracellular domains of the BP antigen 180

Bullous pemphigoid (BP) and gestational pemphigoid (PG) are subepidermal blistering disorders associated with autoantibodies directed against two components of hemidesmosomes: the BP antigen 180 (BP180) and the BP antigen 230 (BP230). Autoantibodies against the extracellular domain (ECD) of BP180 are thought to play an initiatory role in subepidermal blister formation. To characterize the targeted antigenic sites on BP180, we have assessed the reactivity of sera from BP and PG patients against eukaryotic recombinant proteins encompassing various portions of the ECD and the intracellular domain (ICD) of BP180. Twenty-two of 22 (100%) BP sera that immunoblotted BP180 in keratinocyte extracts, bound a mutant form consisting of the entire ECD of BP180, whereas only three of these 22 sera (14%) reacted against the ECD of BP180 lacking the NC16A membrane proximal region. Thirteen out of the 22 (59%) BP sera recognized the ICD of BP180. Circulating IgG from a representative BP patient that was affinity purified against the ECD of BP180 did not bind the ICD when reblotted, indicating that there was no antigenic cross-reactivity between the ECD and the ICD of BP180. Reactivity against the ICD of BP180 was further ascertained by immunofluorescence microscopy studies showing that nine of the 22 (41%) BP sera stained COS-7 cells expressing the ICD of BP180. Using deletion mutants of the ICD of BP180, the majority of the sera was found to recognize the central region of the ICD of BP180. Specifically, an immunodominant region was localized to an 87-amino acid segment located towards the NH2-terminus of BP180. In contrast to BP sera, five of six (83%) PG sera contained IgG that recognized exclusively the NC16A region, whereas none bound to the ICD of BP180. Together, the results indicate that in BP, autoantibody reactivity to BP180 is not exclusively restricted to the NC16A region, but that additional antigenic determinants exist on the ICD of BP180. The observed heterogeneous immune response against BP180 might reflect intramolecular epitope spreading. Because the ICD ofBP180 harbors functionally important regions, it is possible that autoantibodies against the ICD of BP180 have pathogenic significance for the progression of the disease.

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.

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.