Desmosomes and their autoimmune pathologies

ARTICLE

Desmosomes are major cell-cell adhesive structures assembled by keratinocytes and other epithelial cells experiencing mechanical stress [1]. In keratinocytes, desmosomes are present at all epidermal layers, though during differentiation they undergo important ultrastructural changes and vary in number and dimensions. Few at the apical surface of basal cells, they increase in number in the spinous and granular layers and decrease in the stratum corneum [2]. Desmosomes consist of two symmetrical cytoplasmic plaques belonging to two adjacent cells and of a central core region (the desmoglea). The desmoglea is 30 nm thick and is sandwiched between two desmosomal plaques [1, 3] (Fig. 1). The desmosomal plaque, just inside the cell membrane, is an electron-dense proteinaceous 14-20 nm thick region, which includes an outer (submembranous), very dense portion and an inner, less dense portion. Bundles of intermediate filaments of keratin converging from the cytoplasm insert tangentially into the latter part [1, 3].

The basic function of the cutaneous desmosome is to guarantee the epidermal integrity by attaching the cytoskeleton of a cell to the cytoskeleton of the adjacent cell [1, 4]. Because the keratins of the cytoskeleton do not cross the cell membrane, there must be dedicated proteins to mediate the attachment. In fact, the intermediate filaments of keratin link to particular plaque proteins, the plakins, which in turn link to transmembrane proteins, the desmosomal cadherins. These last proteins hook into the desmosomal cadherins-plakin-keratin complex of the adjacent cell, completing the anchorage of both cells and creating a transcellular network of intermediate filaments that is critical for epidermal integrity [5, 6].

Desmosomes, furthermore, are important not only because they play a role in epithelial morphogenesis and differentiation [1, 2, 6], but also because their proteins are major antigens in autoimmune bullous diseases, namely in pemphigus. In pemphigus, specific antibodies cause desmosomal damage and the consequent loss of normal cell-cell adhesion (acantholysis) resulting in the formation of intraepidermal clefts [7]. The intraepidermal and clinical sites of such clefts vary, however, and different types of pemphigus are currently recognized. Such differences may be explained by the different antibody profile and expression sites of desmosomal antigens (Table I).

The transmembrane desmosomal cadherins are glycoproteins and include desmogleins and desmocollins. Plaque proteins are non-glycosylated proteins and include four proteins of the plakin family, namely desmoplakins, plectin, envoplakin and periplakin, and two armadillo-like proteins, namely plakoglobin and plakophilin [1, 8, 9].

Transmembrane desmosomal cadherins

Location

Desmogleins and desmocollins form the desmoglea and are integrated into the desmosomal plaque. Desmoglein 3 extends across the entire plaque, beyond both desmocollins [10].

Biochemistry

Desmogleins and desmocollins are members of the cadherin superfamily [8]. They are calcium-dependent proteins that contain a glycosylated extracellular amino-terminal region, which mediates cell-cell adhesion, a single transmembrane spanning domain, and an intracellular carboxy-terminal region which is larger in desmogleins than in desmocollins [11, 12]. Both desmogleins and desmocollins have three different isoforms (desmoglein 1, 2, 3 and desmocollin 1, 2, 3) [8].

Desmoglein 1 and desmoglein3 have molecular weights of 160 and 130 kDa respectively [8]. Desmocollin1 and desmocollin2 have a molecular weight of 115 and 107 kDa respectively. They differ in size because of different degrees of glycosylation and phosphorylation [12]. Alternative splicing from a single gene and from the same mRNA increases desmocollin diversity producing long (desmocollin "a") and short (desmocollin "b") isoforms that differ in their intracellular domains [13]. Only the cytoplasmic tail of the long-splice form "a" interacts with desmoplakins and plakoglobin [12], while the shorter "b" form may be spatially separated from them [10].

In both desmogleins, the immunogen epitopes are located in the extracellular domains [14] and are conformation- and calcium-dependent [15].

Tissue distribution

Genes encoding for the different isoforms of desmogleins and desmocollins are expressed in a tissue-specific and differentiation-dependent manner, individual desmosomes containing more than one isoform [16]. Desmoglein2 and desmocollin2 are the most widely expressed desmosomal cadherins, being present in the myocardium, in the simple epithelia and in the epidermis. In the epidermis, the expression of desmocollin2 appears to be stronger in the lower cell layers, while desmoglein2 is exclusively expressed in the basal layer. The expression of desmocollin1 and, particularly, desmoglein1 increases instead from basal to suprabasal layers. Desmocollin3 is widely expressed in the epidermis and in other stratifying epithelia as well. Desmoglein3 expression is limited to the basal and suprabasal layers in the epidermis while it is expressed in all cell layers in the mucosa [16, 17].

Function

Desmosomal cadherins are known to mediate calcium-dependent cell-cell adhesion through homophilic interactions of their extracellular domain [8]. In other words, they are usually considered to interact with the homologous cadherins of the adjacent cells. Probably, however, neither the desmocollin nor the desmoglein extracellular domains alone are able to mediate strong adhesion [18]. Some type of heterodimer formation may therefore be required between the extracellular domains of desmogleins on one side and the extracellular domains of desmocollins on the adjacent cell [19].

The intracellular region of the desmosomal cadherins binds plakoglobin, plakophilin and desmoplakin, and, in addition to the adhesive function, seems to play an important role in the assembly of the desmosomal plaque [1].

Role in autoimmune bullous diseases

Desmoglein3 and desmoglein1 are traditionally considered the targets of pathogenic autoantibodies in pemphigus vulgaris and foliaceus respectively [20]. Between 25 to 60% of pemphigus vulgaris sera, however, also recognize desmoglein1 [14, 15, 20-23] and such antibodies appear to be pathogenic [24]. In addition, there are pemphigus foliaceus sera that also react against desmoglein3 [25]. When present together, antibodies to desmoglein3 appear first in pemphigus vulgaris patients with only mucous membrane lesions, while the anti-desmoglein1 reactivity may develop later in the course of the disease, when skin is involved [21]. This concept will be extensively discussed in the next paragraphs.

Desmoglein1 and desmoglein3 are recognized also by pemphigus herpetiformis [26] and paraneoplastic pemphigus [27] sera. Desmoglein3 is an autoantigen also in IgA pemphigus, intraepidermal neutrophilic type [28].

Desmocollin1 is recognized by IgA pemphigus, subcorneal pustular dermatosis type sera [29]. Both desmocollins are also recognized by some sera from pemphigus foliaceus and pemphigus vulgaris [22, 30].

Plaque proteins

Plakin family

Plakins share a common structure formed by three regions: an amino-terminal globular domain, which is believed to govern the association of these proteins with the desmosomal plaque, a central coiled-coil rod domain, which most likely mediates homodimerization, and a carboxy-terminal globular domain, which plays a role in the attachment of intermediate filaments of keratin to the desmosome [31].

Plakins are involved in the anchorage of intermediate filaments of keratin to the plasma membrane and to the cornified envelope, a layer of transglutaminase-crosslinked proteins which is assembled beneath the plasma membrane in terminally differentiating epidermal keratinocytes [32]. They thus contribute to the keratinocyte structural integrity [31].

The plakin family includes desmoplakins, plectin, envoplakin and periplakin.

Desmoplakins

Location

Desmoplakins are the most abundant desmosomal components and are located in the innermost portion of the plaque [33]. The amino terminus of desmoplakin lies within the outer dense plaque and the carboxy terminus some 40 nm distant in the zone of intermediate filament attachment [10].

Biochemistry

There are two splice variants, desmoplakin1 and desmoplakin2, which weigh 250 and 210 kDa respectively. Their structure and amino-acidic sequence are known, desmoplakin2 having a shorter rod domain [33].

Functions

Desmoplakins play a critical role in linking intermediate filaments of keratin to the desmosome and to the plasma membrane. They also contribute to desmosome assembly and stabilization [6, 33]. The binding of desmoplakins to the intermediate filaments is probably very tight and critical to the mechanical integrity of the epidermis [1, 6].

In the desmosomal plaque, desmoplakins link to plakoglobin, plakophilins and plectin, and tend to arrange in dimers or higher order aggregates [6, 10, 34]. Besides, desmoplakin amino-terminal domain appears to be linked to the cytoplasmic tail of the desmosomal cadherins directly or through plakoglobin and plakophilins [6, 34]. Thus, desmoplakins play an important role in coupling intermediate filaments to desmosomal cadherins, thereby integrating the intermediate filaments networks between adjacent cells [1].

Plectin

Location

Plectin is a protein of the desmosomal plaque, where it lies deeper than desmoplakins and is associated with various types of cytoskeletal components and/or filaments including intermediate filaments. It is also present in the hemidesmosomal plaque [35].

Biochemistry

Plectin is a 300 kDa protein whose overall structural organization is very similar to that of desmoplakin [35].

Function

Plectin cooperates with desmoplakins to couple intermediate filaments with desmosomes [1]. Although it plays an auxiliary role compared to desmoplakins, it appears to be important in maintaining the structural integrity of the skin [36].

Envoplakin and periplakin

Location

Envoplakin and periplakin co-localize with desmoplakins in the inner plaque region of the desmosome. They are components of the cornified envelope [32, 37].

Biochemistry

Envoplakin and periplakin are plakin proteins of 210 kDa and 195 kDa, respectively. Probably they tend to form heterodimers or homodimers assembled into higher order complexes [37].

Function

Envoplakin and periplakin anchor the desmosomes and intermediate filaments to the cornified envelope, possibly in conjunction with desmoplakins [37]. The ability of envoplakin and periplakin to directly bind to intermediate filaments and to mediate filament-membrane interactions remains to be demonstrated, however [1].

Role in autoimmune bullous diseases

Plakin proteins have been identified as targets in paraneoplastic pemphigus [38-40].

Armadillo-like proteins

These proteins have both structural and signaling functions. They are characterized by a central domain that is composed of a series of 42-45 amino acid repeats, the so called arm repeats, involved in protein-protein interactions. They include plakoglobin and plakophilins [41].

Plakoglobin

Location

Plakoglobin is located in the desmosomal plaque and also in the cytoplasmic plaque of adherens junctions [42].

Biochemistry

Plakoglobin is a non-glycosylated protein of 83 kDa. The central repeat region of plakoglobin is highly basic and binds with high affinity with the acidic amino-acids of the cytoplasmic region of the desmosomal cadherins. The complex between plakoglobin and desmoglein1 is formed by assembling six plakoglobin molecules for each desmoglein tail. By contrast, only one molecule of plakoglobin is needed for each desmocollin molecule [1].

Function

Plakoglobin has both adhesive and signaling functions and a crucial role in desmosome assembly and/or stability [42].

In the desmosomal plaque, plakoglobin links to the cytoplasmic tails of the desmosomal cadherins [42], more strongly to desmogleins than to desmocollins [43]. Plakoglobin also links to the amino-terminal domain of desmoplakins [34]. Like desmoplakins and plakophilin1, plakoglobin associates with intermediate filaments, although in a much weaker way [6]. However, its role in linking the desmosomal cadherins to the intermediate filaments cytoskeleton [1] remains an important one.

Plakophilins

There are three isoforms, plakophilin1, plakophilin2 and plakophilin3.

Location

Plakophilins are localized both in the desmosomal plaque and in the nucleus of epithelial cells [44]. In vitro they have been shown to bind intermediate filaments, but they are localized very close to the plasma membrane, rather than in the region where the intermediate filaments insert into the desmosomal plaque [10]. In the epidermis, the spinous layer is prominently immunostained by anti-plakophilin 1 antibodies, whereas the basal cell layer is only weakly stained and the stratum corneum is entirely unstained [44].

Biochemistry

The molecular weights of the three isoforms are 75, 100 and 87 kDa respectively. Plakophilin1 is a cytokeratin-binding basic protein, originally described as "band 6 protein" [45].

Function

Their dual intracellular location suggests their involvement both in desmosome-dependent adhesion and in signaling pathways [46]. They may play a role in linking desmosomal cadherins to the intermediate filament cytoskeleton [46].

In vitro, plakophilin1 binds to desmoplakins and desmocollin1, and, to a lesser extent, to desmoglein1 [6]. It anchors intermediate filaments directly and more strongly than plakoglobin [6]. As epidermal cells differentiate, plakophilin 1 is added as a molecular reinforcement to the desmosomal plaque, enhancing the anchorage of intermediate filaments to the desmosomal plaque and partially accounting for the increase in number and stability of desmosomes in suprabasal cells [6].

Role in autoimmune bullous diseases

At present, no antibodies are known to be directed to armadillo-like proteins.

The desmosomal proteins in pemphigus

Role in producing acantholysis

Earlier studies suggested that pemphigus antibodies cause blisters indirectly by inducing the release of proteases, such as plasminogen activator, from keratinocytes [47]. More recent studies suggests that pemphigus autoantibodies may directly inhibit the adhesive function of desmogleins [4].

Role in localizing acantholysis

As mentioned before, the site of blister formation depends on the anti-desmoglein antibody profile and on tissue distribution of desmogleins. The co-expression of desmoglein1 and desmoglein3 in keratinocytes protects against the damage induced by antibodies directed to one of them alone. In the oral mucosa, where desmoglein3 is highly expressed and desmoglein1 is not sufficient to compensate for loss of desmoglein3-mediated adhesion, anti-desmoglein3 antibodies alone are sufficient to cause blistering as it occurs in early pemphigus vulgaris [21]. In pemphigus foliaceus, however, anti-desmoglein1 antibodies alone are not able to cause mucosal lesions [48]. When the function of mucosal desmoglein1 is damaged by its specific autoantibody, desmoglein3 compensates its function preventing acantholysis. In the superficial epidermis, where desmoglein3 is less expressed, anti-desmoglein1 antibodies alone suffice to cause acantholysis [49]. Later in the course of pemphigus vulgaris, when patients' sera contain both anti-desmoglein3 and anti-desmoglein1 antibodies, the function of both desmogleins is compromised and blisters occur in both skin and mucous membranes [21, 49].

The cytoplasmic location of plakins makes it unlikely for anti-plakin autoantibodies to initiate blistering in paraneoplastic pemphigus. The mechanism may be more complex. Antibodies directed against superficial antigens, such as anti-170 kDa [50] or anti-desmoglein3 antibodies [51], probably play a role of primer. They damage the cell membrane favoring the induction of anti-plakin autoantibodies, which might then penetrate the cells and inhibit the plakin functions. The induction of an autoimmune response to normally "sequestered" autoantigens, such as plakins, is probably important in determining the course and duration of the disease explaining the peculiar differences in clinical presentation and severity of paraneoplastic pemphigus with respect to pemphigus vulgaris [5].

Antiplakin antibodies in paraneoplastic pemphigus may develop as an extreme form of the process dubbed "epitope spreading" [52] which will be discussed in the next paragraph.

Desmosomal proteins as specific immune target

Although it is commonly accepted that each type of pemphigus has its own antigenic targets, it is becoming increasingly clear that certain autoantibodies are not restricted to just one form of pemphigus, but are directed against multiple intracellular and extracellular desmosomal proteins, whose pathogenic role remains to be elucidated. Some of these autoantibodies may represent an innocent marker of the breakdown of B-cell tolerance against components of keratinocyte junctions. Pemphigus, in other words, would not escape the rule that governs most organ-specific autoimmune diseases in which several antigens of the same tissue become targets of the autoimmune response in consequence of the "epitope spreading" phenomenon [52]. Autoimmune responses, therefore, may spread to other epitopes distinct from and non-cross-reactive with the disease-inducing ones and belonging or not to the same proteins of the same tissue. The "epitope spreading" concept also applies to situations in which the tissue damage from a primary inflammatory process unmasks a previously "hidden" antigen, which in turn may elicit a secondary autoimmune response [52].

The autoimmune response in pemphigus vulgaris and foliaceus, therefore, is likely to be more heterogeneous than we thought before. Even desmocollins [22, 23, 30], periplakin [Ghohestani R, unpublished data] and a plaque antigen of 180-190 kDa recognized by the human monoclonal antibody F12 [53] have been found to be involved. The plaque antigen of 180-190 kDa seems to be peculiar to pemphigus foliaceus, endemic in Tunisia among young women of rural areas [54].

The involvement of a particular protein, furthermore, does not always result in the same clinical event, namely acantholytic blisters. In pemphigus herpetiformis, for example, circulating autoantibodies recognize both desmoglein1 and desmoglein3 but acantholysis rarely occurs, differently from what happens in pemphigus foliaceus and pemphigus vulgaris [26]. They rather induce spongiosis with eosinophil infiltration, possibly recognizing different epitopes on desmogleins and being thus unable to inhibit the desmogleins' adhesive function [26].

The class of auto-antibody may also matter. IgA pemphigus includes two distinct disorders with different histological features and different IgA deposition patterns in the epidermis, a subcorneal pustular dermatosis type and an intraepidermal neutrophilic infiltration type. The epidermal antigens recognized by IgA have been recently demonstrated to be desmocollin1 in the subcorneal pustular dermatosis type [29] and desmoglein3 in the intraepidermal neutrophilic infiltration type [28]. The intraepidermal neutrophilic infiltration type, therefore, might be the IgA counterpart of IgG-mediated pemphigus vulgaris. Circulating IgA are likely to have a pathogenic role in the induction of acantholysis [55], but why desmoglein3, which is expressed in all layers of the mucosa and lacks in the upper layers of epidermis, is the target protein of a disease confined to the subcorneal region and without mucosal lesions remains unclear.

The number and complexity of target proteins should also be considered. Patients with paraneoplastic pemphigus, for example, develop characteristic autoantibodies against numerous antigens, including a diagnostic antigen complex with relative molecular weights of 250, 230, 210, 190 and 170 kDa. Most of such proteins have now been identified as members of the plakin family [5, 27]. The 250 kDa antigen has been identified as desmoplakin1 [38]; the 230 kDa antigen is BPAg1, the major plaque protein of the hemidesmosome and also a target antigen in bullous pemphigoid [5]; the 210 kDa antigen initially identified as desmocollin2 by immunoprecipitation [38] has been subsequently recognized to be a doublet, the slower migrating band being desmoplakin2 and the faster one envoplakin [40, 56]. The 190 kDa antigen was recently shown to be periplakin [40], whereas the 170 kDa antigen is a transmembrane protein found only by immunoprecipitation and remains to be identified [50]. Even plectin [39] and, recently, desmoglein3 and desmoglein1 [27, 51] have been included among antigens. Oral lesions are a constant feature of paraneoplastic pemphigus, and desmoglein3 antibodies are present in virtually all cases. Antibodies against desmoglein1 are present in about 2/3 of cases and may contribute to skin blistering, which occurs in some cases, but not in all [5].

It is now clear that some autoantibodies from paraneoplastic pemphigus sera are pathogenic, and not merely an epiphenomenon [5]. As for the pathogenic mechanism, two hypotheses have been proposed. First, the tumor antigens would be similar to desmosomal proteins and the anti-tumor immune response would cross-react with normal desmosomal proteins, causing the mucocutaneous disease [38]. In fact, desmoplakins are known to be expressed in thymomas and in Castleman's tumor, which may be associated with paraneoplastic pemphigus, and desmosomes and desmosome-like junctions are anomalously produced by tumors that are not expected to possess them. Most patients with paraneoplastic pemphigus, however, have lymphomas or chronic leukemias of B-cell origin, which do not naturally produce desmosomes or express desmoplakins [38]. Secondly, autoimmunity would be due to a dysregulated cytokine production by the tumor cells, IL-6 for example [57] which is known to promote B-cell differentiation and to drive immunoglobulin production.

CONCLUSION

In conclusion, understanding the complexity of the biochemical structure of desmosome has been one of the major advances in the recent years and has improved our knowledge of the basic pathological phenomena of autoimmune bullous diseases. As it usually occurs in biology, knowledge progress has also made it possible to understand how much to be understood is still ahead.

Article accepted on 3/1/00

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