From the 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.
Hemidesmosomes, including anchoring filaments and fibrils, are
adhesion complexes mediating firm attachment of epithelial basal cells
to the underlying basement membrane, thereby mechanically linking the
cytoplasmic keratin filament network to collagen fibrils and other
extracellular matrix components of the connective tissue (1-3). In the
past decade, several hemidesmosomal constituents have been identified
and it has been possible to elucidate the cellular regulation of
hemidesmosomes at the molecular level.
In the process of epidermal organization and differentiation, cells
dynamically regulate their hemidesmosomes during attachment to and
detachment from the basement membrane. In most cases, this involves new
formation of hemidesmosomes, but, during cell movement in wound healing
and differentiation in stratification, hemidesmosomes must become
detached from the basement membrane, with separation of hemidesmosomal
transmembrane proteins from their ligands being an important step.
However, the underlying mechanism is still poorly understood.
There are at least three hemidesmosomal transmembrane polypeptides,
i.e. integrin BP180, a 180-kDa bullous pemphigoid antigen, is a type II transmembrane
glycoprotein with a collagenous carboxyl-terminal extracellular domain
and a noncollagenous amino-terminal cytoplasmic domain (7, 8, 20, 21).
The extracellular domain is interrupted by non-triple-helical
sequences, consisting of 15 separate triple-helical stretches in the
human form and 13 in the mouse. Autoantibodies to BP180 are thought to
play a crucial role in skin blistering in patients with bullous
pemphigoid, herpes gestationis, and cicatricial pemphigoid (22-24).
This is supported by findings with a passive transfer model, featuring
injection of rabbit polyclonal antibodies against a pathogenic epitope
of BP180 into neonatal mice (25). In addition, mutations in the BP180
gene have been found in cases of generalized atrophic benign
epidermolysis bullosa, which is characterized by universal alopecia and
atrophy of the skin (26-29). However, while these results suggest an
important role for BP180 in cell to extracellular matrix adhesion, in
contrast to integrin In our preliminary studies, we have detected a novel 120-kDa
hemidesmosomal component, specifically recognized by a monoclonal antibody (mAb)1 termed 1337 (30). The component was found to be present primarily in stratified
epithelia by immunofluorescence microscopy. The fact that it was lost
by collagenase treatment, suggesting a collagen nature is the rationale
for the present examination of the relationship between the 120-kDa
polypeptide and BP180. We present evidence here that the 120-kDa
polypeptide is indeed derived from BP180 and that, both in tissue and
in culture, cells can sever this extracellular portion of the molecule
from their surfaces. The biological and clinical significance of this
finding is also considered.
Antibodies--
Mouse mAbs against hemidesmosomal proteins were
prepared by immunizing mice with the hemidesmosome fraction isolated
from bovine corneal epithelial cells, as described previously (31). mAb
233, mAb 1D1, mAb D20, and mAb R223 are against the extracellular part,
and mAb 1A8c and mAb 1A6 are against the cytoplasmic part of BP180
(Fig. 1) (21). Epitopes of mAb 233 and
mAb 1D1 are mapped on the carboxyl-terminal half, and that of mAb D20
is mapped on the amino-terminal half of the extracellular
part.2 mAb 1E5 binds
specifically to BP230. mAb 855, mAb 310, and mAb 617 target the
extracellular part and mAb 1A3 targets the cytoplasmic part of the
integrin Unit of Biosystems,
Department of Anatomy,
Department of Dermatology,
Kurume University School of Medicine, Fukuoka 830, §§ Department of Dermatology, Gifu University
School of Medicine, Gifu 500, and ¶¶ National Institute for
Basic Biology, Okazaki 444, Japan
ABSTRACT
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
INTRODUCTION
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
6 and
4 subunits
(4-6) and BP180, which is also called type XVII collagen (7, 8). The
integrin
6
4, an adhesion molecule whose
ligands are laminins (9-11), especially laminin-5, plays an essential
role in assembly and functioning of hemidesmosomes (11, 12). Mutations
of the
4 integrin gene have been described in some forms
of junctional epidermolysis bullosa with pyloric atresia (13, 14) and
animals with targeted
6 integrin or
4
mutations show similar, but even more severe deficiencies (15-17).
Recently, Hopkinson et al. (18) have shown that integrin
6 subunit interacts with BP180. On the other hand, Borradori et al. (19) have reported the interaction between cytoplasmic domain of
4 and that of BP180. These results
suggest that the
6
4 complex plays a major
role in epidermal cell-basement membrane adhesion.
6
4, our
understanding of the molecular biology of BP180 is quite limited, and
even its ligand(s) is unknown.
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
4 subunit. For immunofluorescence microscopy, hybridoma supernatants of each mAb were diluted to 50 times their antibody titers, i.e. 50-fold the critical concentration to
detect the antigen in skin basement membrane zone (BMZ) by
immunofluorescence microscopy.
View larger version (19K):
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Fig. 1.
Schematic diagram of human BP180, with the
locations of epitopes for mAbs. Arrows indicate the portions
including epitopes for mAbs. The epitope of mAb 1A8c is mapped on the
cytoplasmic part, and those of mAb 233 and 1D1 are mapped on the
carboxyl-terminal half of the extracellular part of BP180 (Ref. 21; Y. Hirako and K. Owaribe, unpublished data). In this study, the epitope of
mAb 1D1 is localized within the most carboxyl-terminal 20-kDa portion,
which is not recognized by mAb 233. Recently, Borradori et
al. (19) have suggested the probable location of the epitope for
mAb 1A8c within the region of amino acids 113-201 amino acids of
BP180. Cyto, TM, and Ext are the
cytoplasmic, transmembrane, and extracellular portions of BP180,
respectively. The schematic domain organization of human BP180 is based
on the primary structural analysis performed by Giudice et
al. (7).
Cells and Cell Cultures--
BMGE+H and BMGEH cells, mammary
gland epithelial cell lines derived from a lactating bovine udder, were
the kind gift of Dr. W. W. Franke of the German Cancer Research
Center (Heidelberg, Germany) (33). The DJM-1 cell is an isolated cell
line from human skin squamous cell carcinoma (34). A431 cells of an
epidermal cell carcinoma cell line derived from human vulva were
obtained from the Japanese Cancer Research Resources Bank (Tokyo,
Japan). Cell culture conditions were as described previously (35).
Immunofluorescence Microscopy--
Freshly prepared tissues were
snap-frozen in isopentane precooled in liquid nitrogen. Sections from
frozen specimens were cut at 5-6 µm with a cryostat, mounted on
glass slides, air-dried, and fixed in 100% acetone at 20 °C for
10 min. In some cases the sections were treated with 0.5% Triton X-100
or 1.5 M KCl in phosphate-buffered saline before fixation.
Cells grown on glass coverslips were fixed with acetone. These fixed
sections and cells were processed for immunofluorescence staining as
described previously (35).
Fractions Prepared from Cultured Cells-- Cells were rinsed in phosphate-buffered saline, scraped from dishes, and homogenized on ice in a lysis buffer (50 mM NaCl, 5 mM EDTA, 20 mM Tris-HCl (pH 7.4)) containing 1 mM phenylmethylsulfonyl fluoride (PMSF), 5 µg/ml leupeptin, and 5 µg/ml pepstatin A. After extraction for 30 min on ice, cells were centrifuged at 30,000 × g for 30 min, and the resultant supernatants were collected as cytosolic fractions. Then residual pellets were resuspended and extracted in a low salt buffer solution (150 mM NaCl, 5 mM EDTA, 20 mM Tris-HCl (pH 7.4)) containing nonionic detergent 0.5% Triton X-100, 1 mM PMSF, 5 µg/ml leupeptin, and 5 µg/ml pepstatin A, and were centrifuged to yield Triton X-100-soluble membrane-bound (supernatant) and Triton X-100-insoluble (pellet) fractions. Cytoskeletal fractions were recovered from pellets by resuspension of Triton X-100-insoluble fractions in a high salt buffer solution (1.5 M KCl, 5 mM EDTA, 20 mM Tris-HCl (pH 7.4)) containing 0.5% Triton X-100.
Conditioned medium collected from 3-day cultures was centrifuged at 1,000 rpm for 10 min to remove unattached cells and 5 mM EDTA, 1 mM PMSF, 5 µg/ml leupeptin, and 0.1 mM N-ethylmaleimide were added. Proteins were precipitated by the addition of saturated ammonium sulfate solution (half the volume of the medium). Precipitates were resuspended in TBS containing 1 mM EDTA as the medium fractions. In some experiments, the spent medium was precipitated first with 50% ammonium sulfate for concentration, and subsequently the precipitant dissolved in TBS containing 1 mM EDTA was precipitated with 33% ammonium sulfate. For immunoprecipitation, the resuspension buffer for the medium fractions was changed to a low salt buffer containing 0.5% Triton X-100, to equalize the buffer conditions for the Triton X-100-soluble membrane-bound fractions.Isolation of Hemidesmosomes-- Hemidesmosomes were isolated from bovine corneal epithelial cells, as described previously (31).
Collagenase Digestion-- Collagenase from Clostridium histolyticum (Amano Pharmaceutical, Nagoya, Japan) was added at 50 Mandle units/ml final concentration to the HD fraction isolated from corneas of 30 bovine eyes. After incubation for 60 min at 37 °C, the enzymatic reaction was stopped by the addition of 2× SDS sample buffer.
Electrophoresis and Immunoblotting-- SDS-PAGE was performed according to the method of Laemmli with a slight modification (36). Immunoblotting was performed using SDS-PAGE and subsequent electrophoretic transfer onto nitrocellulose sheets using a semidry system as described previously (37).
Immunoprecipitation--
12 ml of Triton X-100-soluble fraction
and 1.2 ml of medium fraction were prepared from three 15-cm dishes of
BMGE+H cells as described above. The prepared fractions were divided
into four aliquots. Each aliquot was supplemented with 3 ml of
hybridoma supernatant, incubated for 1 h at room temperature,
mixed with 400 µl of second antibodies conjugated to agarose beads
(American Qualex International Inc., La Mirada, CA), and incubated for
an additional 1 h. The agarose beads were washed with the same
washing buffers used for the immunoblotting. After removal of the
supernatant by centrifugation, aliquots were added to 150 µl of 4×
SDS sample buffer (4% SDS, 125 mM Tris-HCl (pH 6.8), 20%
glycerin). The agarose beads were then suspended and boiled for 2 min.
After the addition of -mercaptoethanol, the polypeptides were
separated by SDS-PAGE and subsequently transferred electrophoretically
onto nitrocellulose sheets. The sheets were incubated with 1% BSA in
TBS (pH 8.0) containing 0.05% Tween 20 (Tween-TBS) and subsequently
with biotinylated mAb 233 for 90 min. After washing with Tween-TBS,
they were incubated with alkaline phosphatase-labeled avidin for 60 min, followed by visualization with nitro blue tetrazolium chloride and
5-bromo-4-chloro-3-indolyl phosphate.
Immunoaffinity Purification of BP180-- Ascites fluid of mAb 233 and mAb 1A8c were purified with an AmpureTM PA kit (Amersham, Tokyo, Japan) and the immunoglobulin fractions were coupled to Affi-Gel 10 (Bio-Rad, Tokyo, Japan). The Triton X-100-soluble membrane-bound fraction of DJM-1 cells prepared from 10 dishes (15 cm diameter) was applied to an mAb 1A8c immunoaffinity column (2 ml) and washed with a washing buffer (20 mM Tris-HCl (pH 7.5), 0.6 M NaCl, 5 mM MgCl2, 1 mM ATP, 0.1% Triton X-100). The medium fraction prepared from 500 ml of conditioned cultured medium of DJM-1 cells was applied to an mAb 233 immunoaffinity column (2 ml) and washed with a washing buffer (20 mM Tris-HCl (pH 7.5), 1 M NaCl, 0.1% Triton X-100). Bound antigens were eluted with 3 M NaSCN containing 0.1% Triton X-100, and the eluted fractions were dialyzed against a low salt buffer containing 0.1% Triton X-100. The purity of the eluted fractions was assessed by SDS-PAGE using silver staining and immunoblotting.
Low Angle Rotary-shadowing Electron
Microscopy--
Affinity-purified samples were mixed with 50%
glycerol, and samples were sprayed, using an air brush, onto freshly
cleaved mica. The droplets on the mica were dried at room temperature in a vacuum at 108 mmHg in newly developed freeze-etch
equipment (Hitachi HR7000) for 10 min. Dried specimens were
rotary-shadowed with platinum using an electron gun positioned at
2.5° to the mica surface, and then coated with a film of carbon
generated by an electron gun positioned at 90° to the mica surface.
The replica was floated on distilled water and collected on a grid
covered with Formvar film. The specimens were examined with a JEM 100CX
electron microscope.
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RESULTS |
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Identification of the mAb 1337 Antigen-- The mAb 1337 is a monoclonal antibody obtained by immunizing a mouse with the HD fraction, and stained BMZ of bovine epidermis in immunofluorescence microscopy. Immunoblot analysis of the HD fraction showed that the antibody recognized a 120-kDa polypeptide as described previously in our preliminary report (Fig. 2) (30). To confirm the previous results and further characterize the antigen, the HD fraction was treated with collagenase and examined by immunoblotting (Fig. 2). The 120-kDa band was completely lost by collagenase digestion as reported previously, showing that the polypeptide has a collagenous domain(s) as BP180 do. Hence, we investigated the 120-kDa polypeptide with special reference to BP180, a hemidesmosomal transmembrane collagen. As shown in Fig. 2, immunoblotting with mAbs against extracellular parts of BP180 showed the electrophoretic mobility of the antigen to be equal to that of the smallest polypeptide among a group of proteolytic fragments appearing in the HD fraction. On two-dimensional SDS-PAGE and subsequent immunoblotting, the antigen and the BP180 fragment showed an identical spot (data not shown). From these results, we conclude that mAb 1337 recognizes primarily the 120-kDa fragment of BP180 on immunoblotting.
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Immunofluorescence Microscopy of Bovine Skin and Cultured Cells-- To examine whether the mAb 1337 specifically recognizes the 120-kDa fragment of BP180 by immunofluorescence microscopy, the staining pattern of mAb 1337 was compared with those of mAbs against BP180. The titers among the mAbs were equalized to allow comparative observations. The mAb 1337 clearly stained the BMZ of bovine skin, as did the other mAbs to BP180, but, in contrast, not the lateral surfaces of the basal cells (Fig. 3, A-D). Thus, mAb 1337 did not recognize Triton X-100-soluble BP180, which is free from the cytoskeleton and not incorporated into HDs (35). Immunofluorescence microscopy of BMGE+H cells and primary cultured bovine conjunctival epithelial cells (data not shown) showed that mAb 1337 did not detect HDs (Fig. 4), pointing to a lack of binding to insoluble BP180. These results are quite different from those with other typical anti-BP180 mAbs, and suggest that mAb 1337 does not recognize intact BP180 in either soluble or insoluble states. Therefore, its staining pattern observed in bovine skin indicates the presence of the 120-kDa fragment. When frozen sections of bovine skin were treated with 0.5% Triton X-100 or 1.5 M KCl and stained with mAb 1337, the immunofluorescent staining was essentially the same as in control sections, showing that the 120-kDa fragments are largely insolubilized in the tissue (Fig. 3, E-H).
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Detection of 120-kDa Fragments in Culture Medium-- Immunofluorescence microscopy with mAb 1337 showed the antigenic fragment to be lacking or present at only a very low level on BMGE+H cells (Fig. 4). If the fragment does not have a transmembrane domain nor cytoplasmic part, diffusion into the culture medium would be expected. To examine this possibility, spent media from BMGE+H cells were analyzed by immunoblotting using mAb 1D1, a monoclonal antibody against the extracellular part of BP180, and a specific band of 120 kDa was detected (Fig. 5A). This polypeptide was concentrated in the fraction precipitated with 33% ammonium sulfate, and was also recognized by mAb 1337 and mAb 233, but not by mAb 1A8c, which recognizes the cytoplasmic part of BP180 (Fig. 5A). The electrophoretic mobility of the 120-kDa polypeptide proved equal to that of the fragment from the HD fraction. Therefore, the polypeptide in the spent medium was concluded to be the 120-kDa fragment of the HD fraction. The 120-kDa polypeptide appeared prone to degradation to a 100-kDa polypeptide when the spent medium was precipitated first with 50% ammonium sulfate for concentration, and consequently the precipitant dissolved in TBS containing 1 mM EDTA was precipitated with 33% ammonium sulfate. The 100-kDa polypeptide was recognized by mAb 1337 and 233, but not by mAb 1D1, the epitope of which appeared to be removed (Fig. 5B).
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120-kDa Fragment Is Present as a Trimer Form-- Trimer formation by the 120-kDa fragment was confirmed by immunoblotting (Fig. 7). When the membrane-bound fraction in SDS-PAGE sample solution was not boiled, a 600-kDa component was detected for BP180. The apparent molecular mass is equal to that of the chemically cross-linked trimers of BP180 described previously (35). Therefore, in the non-boiled condition, BP180 molecules exist in a trimer form, probably due to the collagen links. In the non-boiled condition, the 120-kDa fragment in the medium fraction was detected as a 350-kDa component. Since this molecular mass is consistent with a trimer of the fragment, it is concluded that the fragment exists as trimer form. mAb 1337 recognized the fragment in both monomer and trimer forms, but did not recognize the intact 180-kDa polypeptide.
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Specificity of mAb 1337-- The immunoreactivity of mAb 1337 against the 120-kDa fragment and the intact BP180 molecule in their native soluble forms was examined by immunoprecipitation. Immunoprecipitants were also analyzed by immunoblotting with mAb 233. While mAb 233 precipitated both the intact molecule and the fragment, mAb 1337 precipitated only the latter (Fig. 8A). The result demonstrated that mAb 1337 specifically recognizes the fragment, offering an explanation for the unique staining pattern observed on immunofluorescence, and supports the idea that the fragment actually exists in tissues. The recognition of the mAb 1337 immunoprecipitant with mAb 233 clearly demonstrated that the 120-kDa fragment is identical to the extracellular part of BP180.
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Purification of the 120-kDa Fragment and Determination of Its Morphology-- 120-kDa fragments were purified from the medium fraction by immunoaffinity column chromatography using mAb 233 (Fig. 9). Intact BP180 molecules were purified from the Triton X-100-soluble membrane-bound fraction by immunoaffinity column chromatography using mAb 1A8c. Fractions were prepared from DJM-1 cells, a human skin squamous carcinoma cell line. The purity of the eluted fractions was assessed by SDS-PAGE using silver staining (Fig. 9, lanes 1 and 4). The eluted fractions from the mAb 233 column showed the 120-kDa band, recognized by mAb 233 (Fig. 9, lane 2) and other monoclonal antibodies (mAb 1D1, mAb D20, and mAb R223) that binding to the extracellular part of BP180 (data not shown). The eluted fraction of the mAb 1A8c column demonstrated the 180-kDa band, and, on immunoblotting with mAb 1A8c, an additional 60-kDa band was detected (Fig. 9, lane 5). As described above, the 60-kDa polypeptide appears to be the remnant BP180 after removal of its 120-kDa extracellular fragment. To identify the cleavage site more precisely, the 120-kDa fragment and the 60-kDa fragment were immunoblotted with the polyclonal antibody against the human NC16A domain comprising the 76-amino acid stretches positioned between the transmembrane domain and the first extracellular collagenous domain. The 120-kDa fragment was recognized by the polyclonal antibody, while the 60-kDa fragment was hardly detected (Fig. 9, lanes 3 and 6). This demonstrates most of the NC16A domain to be within the 120-kDa fragment. Therefore, the cleavage site(s) is localized within the NC16A domain near the cell membrane. 100-kDa fragments, degradation products of 120-kDa fragments, were also purified using the mAb 233 column and found to be recognized by the polyclonal antibody against the NC16A domain. Thus, this fragment is produced by cleavage at the carboxyl-terminal side of the 120-kDa fragment (Fig. 9, lane 9). The 100-kDa fragment was recognized by mAbs 233 (Fig. 9, lane 8), D20, and R223, but not by mAb 1D1 (data not shown). Amino-terminal sequencing of the purified 120-kDa fragment failed, probably due to the low amount of the protein.
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DISCUSSION |
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The present study demonstrated the existence of a 120-kDa extracellular fragment of BP180, which can be further degraded to a 100-kDa form, in culture medium of keratinocytes and skin BMZ, suggesting that the cells cleave the BP180 molecule at their surfaces. The mAb 1337 was found to be very useful to distinguish the fragment from intact BP180, apparently recognizing a unique epitope that is exposed or formed by the cleavage. We could not determine the cleavage site exactly, but it is located in the NC16A domain near to the cell membrane. The epitope may be present in the NC16A domain, which is probably more affected by the cleavage than other stable collagenous parts.
Rotary shadowing of intact BP180 molecules confirmed our previous observation, i.e. that the molecule consists of a globular head, a central rod and a flexible tail (35). It is clear from the rotary-shadowed images of the 120-kDa fragments together with biochemical results that they have no globular head, showing that the globular head corresponds to the amino-terminal cytoplasmic part of the molecule as we speculated previously.
The immunofluorescent staining pattern of mAb 1337, showing the
distribution of the 120-kDa fragments, differed between tissues and
cultured cells. The fragments were localized exclusively in the BMZ in
skin, whereas, in cultured cells, they are detected in the culture
medium. Since the staining of skin BMZ with mAb 1337 proved resistant
to 0.5% Triton X-100 or 1.5 M KCl treatments, the
fragments in the tissue appear to be insoluble, whereas the intact
BP180 molecules present in the lateral surfaces of basal cells are
solubilized by 0.5% Triton X-100 treatment (35). The insolubility of
the fragments in the tissue is probably due to their anchorage to or
interaction with other molecule(s). Since the fragments lack the
transmembrane domain and seem to be no longer anchored to the cell body
directly, they are probably fixed extracellularly. Hemidesmosomal
transmembrane proteins such as integrin
6
4 and BP180 are candidates for the
extracellular anchorage of the fragment. In fact, Hopkinson et
al. (18) have shown that truncated BP180 lacking the entire
collagen domain, in other words consisting of only the cytoplasmic and
non-collagenous extracellular domains, is incorporated into
hemidesmosomes and associates with integrin
6 subunit
extracellularly. However, these candidates cannot explain the absence
of the 120-kDa fragment on the basal sides of cultured cells, because
they do have hemidesmosomes including integrin
6
4 and BP180. Another possibility is that
an unidentified extracellular ligand(s) of BP180 exists in tissue but
not in cultured cells.
The presence of the 120-kDa fragment in skin suggests that the cleavage has some biological meaning. In wound healing or stratification, hemidesmosomes must be detached from the basement membrane, with transmembrane proteins becoming separated from their ligands.
Like other integrins, 6
4 appears highly
regulative and its recycling has been shown during cell locomotion
(38). During epithelial wound healing of cornea, the integrin
6
4 redistributes from their location
within hemidesmosomes to more evenly in the basal cell membrane, and
this event occurs without any measurable change in the synthesis of
6
4 and with no indication of proteolytic cleavage of either
6 or
4 chain (39).
Recent studies have demonstrated that the ligand ligation of integrin
6
4 causes tyrosine phosphorylation on its
cytoplasmic domain (40). Considering these results, it is reasonable to
suppose that integrin
6
4 controls ligand
detachment by some structural change in the cytoplasmic domain.
Our structural analyses of BP180 have demonstrated collagenous trimer
formation (35). From the molecular dimension, we suggested that BP180
is a major component of anchoring filaments and warranting attention as
a target molecule of cicatricial pemphigoid, responsible for an
autoimmune subepidermal blistering disease with scar formation. Recently, Masunaga et al. (41) have demonstrated that the
extracellular domain of BP180 extends to the lamina densa of skin BMZ
by immunoelectron microscopy. Considering the stable collagenous
extracellular rod of BP180, it might be expected that BP180 is not as
regulative as integrin 6
4. Thus if
structural change in the cytoplasmic domain cannot cause detachment of
its extracellular domain from its ligand, a capacity for cleavage would
be necessary to allow differentiation and movement away from the
basement membrane. Kitajima et al. (42, 43) observed
disappearance of BP180 in DJM-1 cells during rearrangement of
hemidesmosomes induced by the treatment with
12-O-tetradecanoylphorbol-13-acetate. However, the
possibility that the fragment plays a more direct role in cell-matrix
adhesion distinct from the intact molecule, cannot be excluded. The
fact that significant amounts of the 60-kDa polypeptide could be
detected on immunoblotting of cell extracts using mAb 1A8c but not 233 or 1D1, is suggestive of constitutive cleavage and removal of the
120-kDa extracellular part rather than generation of alternatively
spliced products of BP180 gene.
Although this is highly speculative, cleavage of the extracellular part of BP180 might have some clinical, in addition to biological, significance. Thus alteration in BP180 fragments might cause autoimmune diseases like bullous pemphigoid and other skin blistering disorders. Linear IgA bullous dermatosis (LAD) is an autoimmune subepidermal blistering disease characterized by linear deposits of IgA autoantibodies at the dermal-epidermal basement membrane. Immunoblot analyses of patients' sera have demonstrated that the target antigens are heterogeneous, although a 97-kDa antigen is most frequently detected (44, 45). Recent studies have shown that the autoantibodies recognize a 97-kDa and a 120-kDa polypeptide in skin extracts and a 120-kDa form in keratinocyte culture medium (46). The available data suggest that the 97-kDa peptide is a degradation rather than a specifically processed product. Immunoelectron microscopy has indicated that the 97-kDa polypeptide is localized to lamina lucida, suggesting a novel anchoring filament protein (47). Very recently, Pas et al. (48) have found that both BP IgG and LAD IgA recognize the same 120-kDa protein in keratinocyte conditioned medium and in cell extracts. They also showed that the 120-kDa protein is glycosylated and collagenous, and lacking in culture medium and in skin from patients with generalized atrophic benign epidermolysis bullosa. These results suggest close relationship between LAD antigen and BP180, though LAD sera have so far not been found to recognize BP180.
We have no direct evidence concerning the relationship between the LAD antigen(s) and the 120-kDa BP180 fragment at present. However, further characterization of the latter appears warranted to clarify the molecular nature and function of BP180, under physiological and pathological conditions.
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ACKNOWLEDGEMENTS |
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We thank Prof. H. Hotani and Dr. Y. Nishizawa (Nagoya University, Nagoya, Japan) for encouragement.
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FOOTNOTES |
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* This work was supported by grants from the Ministry of Education, Science, and Culture of Japan.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ To whom correspondence should be addressed: Unit of Biosystems, Graduate School of Human Informatics, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan. Tel.: 81-52-789-4777; Fax: 81-52-789-4818.
1 The abbreviations used are: mAb, monoclonal antibody; TBS, Tris-buffered saline; PAGE, polyacrylamide gel electrophoresis; BSA, bovine serum albumin; PMSF, phenylmethylsulfonyl fluoride; BMZ, basement membrane zone; LAD, linear IgA bullous dermatosis; HD, hemidesmosome.
2 Y. Hirako and K. Owaribe, unpublished data.
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REFERENCES |
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