Cadherin flexibility provides a key difference between desmosomes and adherens junctions.

Desmosomes and adherens junctions are intercellular adhesive structures essential for the development and integrity of vertebrate tissue, including the epidermis and heart. Their cell adhesion molecules are cadherins: type 1 cadherins in adherens junctions and desmosomal cadherins in desmosomes. A fundamental difference is that desmosomes have a highly ordered structure in their extracellular region and exhibit calcium-independent hyperadhesion, whereas adherens junctions appear to lack such ordered arrays, and their adhesion is always calcium-dependent. We present here the structure of the entire ectodomain of desmosomal cadherin desmoglein 2 (Dsg2), using a combination of small-angle X-ray scattering, electron microscopy, and solution-based biophysical techniques. This structure reveals that the ectodomain of Dsg2 is flexible even in the calcium-bound state and, on average, is shorter than the type 1 cadherin crystal structures. The Dsg2 structure has an excellent fit with the electron tomography reconstructions of human desmosomes. This fit suggests an arrangement in which desmosomal cadherins form trans interactions but are too far apart to interact in cis, in agreement with previously reported observations. Cadherin flexibility may be key to explaining the plasticity of desmosomes that maintain tissue integrity in their hyperadhesive form, but can adopt a weaker, calcium-dependent adhesion during wound healing and early development.

Membrane-impermeable cross-linking provides evidence for homophilic, isoform-specific binding of desmosomal cadherins in epithelial cells.

Desmosomes and adherens junctions are cadherin-based protein complexes responsible for cell-cell adhesion of epithelial cells. Type 1 cadherins of adherens junctions show specific homophilic adhesion that plays a major role in developmental tissue segregation. The desmosomal cadherins, desmocollin and desmoglein, occur as several different isoforms with overlapping expression in some tissues where different isoforms are located in the same desmosomes. Although adhesive binding of desmosomal cadherins has been investigated in a variety of ways, their interaction in desmosome-forming epithelial cells has not been studied. Here, using extracellular homobifunctional cross-linking, we provide evidence for homophilic and isoform-specific binding between the Dsc2, Dsc3, Dsg2, and Dsg3 isoforms in HaCaT keratinocytes and show that it represents trans interaction. Furthermore, the cross-linked adducts are present in the detergent-insoluble fraction, and electron microscopy shows that extracellular cross-linking probably occurs in desmosomes. We found no evidence for either heterophilic or cis interaction, but neither can be completely excluded by our data. Mutation of amino acid residues Trp-2 and Ala-80 that are important for trans interaction in classical cadherin adhesive binding abolished Dsc2 binding, indicating that these residues are also involved in desmosomal adhesion. These interactions of desmosomal cadherins may be of key importance for their ordered arrangement within desmosomes that we believe is essential for desmosomal adhesive strength and the maintenance of tissue integrity.

Desmosomal cadherins.

New evidence from blocking desmosomal adhesion with anti-adhesion peptides reveals a role for desmosomes in cell positioning in morphogenesis. Desmosomal adhesion is necessary for the stability of adherens junctions in epithelial cell sheets. Knockout and mis-expression of desmosomal cadherins in mice suggests that they may function directly or indirectly in regulating epidermal differentiation. Protein kinase C signalling and tyrosine phosphorylation appear to regulate desmosomal adhesion. There are new insights into the role of desmosomal cadherins in autoimmune, infectious and genetic disease.

Positive response of a recurrent keloid scar to topical methyl aminolevulinate-photodynamic therapy.

A 36-year-old Caucasian female of Iranian origin presented with a persistently raised dermal lesion under her chin, confirmed histologically to be a keloid scar. There was a 4-year history of a negative response to a range of conventional treatments including topical silicone gel sheets, steroid creams, steroid injections and surgical excision. In view of treatment failure and an in vitro study indicating a positive effect of photodynamic therapy (PDT)on keloid fibroblasts, we treated our patient's lesion with five sessions of methyl aminolevulinate photodynamic therapy (MAL-PDT) over a period of 5 months. Following this treatment regime, her keloid scar had considerably reduced in size and become flattened.The surface of the keloid also became smooth, with attenuation in erythema at the margin as well as an improvement in the colour of the scar, which was better matched to the surrounding skin. There was no recurrence at 1-year follow-up and this treatment resulted in an overall acceptable cosmetic outcome. This case report presents PDT as a potential treatment option for persistent keloid lesions unresponsive to conventional scar modulation therapies and suggests a need for further research in this area.

Pervanadate stabilizes desmosomes.

Desmosomes are intercellular junctions responsible for strong cell-cell adhesion in epithelia and cardiac muscle. Numerous studies have shown that the other major type of epithelial cell adhesion, the adherens junction, is destabilized by src-induced tyrosine phosphorylation of two of its principal components, E-cadherin and beta-catenin. Here we show that treatment of epithelial cells with the potent tyrosine phosphatase inhibitor sodium pervanadate causes tyrosine phosphorylation of the major desmosomal components desmoglein 2 and plakoglobin in both the non-ionic detergent soluble and insoluble cell fractions and, surprisingly, stabilizes desmosomal adhesion, inducing the hyper-adhesive form normally found in tissues and confluent cell sheets. Taken together with the few other studies on desmosomes these results suggest that the effects of tyrosine phosphorylation on desmosomal adhesion are complex.

Desmosomal adhesion: structural basis, molecular mechanism and regulation (Review).

Desmosomes are adhesive intercellular junctions of epithelia and cardiac muscle. They have an essential function in maintaining the integrity of tissues, which is compromised in human genetic and autoimmune disease that targets desmosomal components. Recent evidence (1) suggests new roles for the function of desmosomal adhesion in tissue morphogenesis, (2) gives new insights into the molecular mechanism of adhesion, (3) indicates that the desmosomal adhesion molecules, desmocollin and desmoglein, may contribute to the regulation of epidermal diffentiation, and (4) shows that the affinity of desmosomal adhesion is regulated by protein kinase C.

Cicatricial pemphigoid with circulating IgA and IgG autoantibodies to the central portion of the BP180 ectodomain: beneficial effect of adjuvant therapy with high-dose intravenous immunoglobulin.

Cicatricial pemphigoid (CP) is an autoimmune subepidermal blistering disease characterized by deposits of IgG, IgA, or C3 at the cutaneous basement membrane zone. CP may present with considerable variation regarding age, morphology of lesions, and mucosal involvement, which may heal with or without scarring. We describe a patient with CP who presented with circulating IgA and IgG autoantibodies to the epidermal side of salt-split human skin. By immunoblot analysis, the patient's IgA reacted with the soluble ectodomain of BP180 (LAD-1). This reactivity was mainly directed to the central portion of the BP180 ectodomain, a site that, to date, has not been described as the target of IgA autoantibodies. Different immunosuppressive treatment regimens including steroids and mycophenolate mofetil did not control this patient's disease, and severe scarring of the conjunctivae occurred with impairment of vision. Addition of adjuvant intravenous immunoglobulin (1 g/kg body weight on 2 consecutive days) every 4 weeks led to a dramatic improvement of conjunctivitis and gingivitis. Clinical improvement correlated with the serum's IgA immunoblot reactivity against LAD-1. Further studies on a larger number of patients with CP should try to correlate the specificity of autoantibodies in CP with the response to certain therapeutic regimens.

Pemphigoid gestationis with predominant involvement of oral mucous membranes and IgA autoantibodies targeting the C-terminus of BP180.

Pemphigoid gestationis (PG) is an autoimmune pregnancy-associated subepidermal blistering disease. It usually affects skin and, rarely, mucous membranes. In the vast majority of patients with PG, the autoimmune response is directed to the membrane-proximal NC16A domain of the 180-kd bullous pemphigoid (BP) antigen (BP180) and is mediated by IgG1 and IgG3 autoantibodies. We report the case of a patient with PG associated with extensive lesions on oral mucous membranes. Immunoblotting studies demonstrated the presence of circulating IgA autoantibodies in the patient's serum that were exclusively directed to a 49 amino acid stretch on the C-terminal portion of the BP180 ectodomain located 800 amino acids downstream from NC16A. This C-terminal stretch of BP180 has previously been demonstrated to localize to the lamina lucida/lamina densa interface and to be recognized by IgG and IgA antibodies in a subgroup of patients with cicatricial pemphigoid as well as by IgG autoantibodies in some BP sera. Our patient's lesions healed without scarring within 6 weeks after delivery of a healthy child. The findings in this patient extend the clinical and immunopathologic spectrum of PG.