Ultrastructural quantitation of desmosome and differentiation-related keratinocyte membrane antigen

Chondroitin/dermatan sulphate proteoglycan, desmosealin, showing affinity to desmosomes

Objective

Desmosomes are the most prominent interkeratinocyte junctions. The correct barrier function of stratified epithelia such as epidermis depends on their expression. During epidermal differentiation, the molecular composition of desmosomes evolves and so do their physical and chemical properties. Desquamation of corneocytes at the surface of the stratum corneum depends on an orderly degradation of desmosomes by endogenous enzymes. This process may be regulated by glycosylated molecules. We focused on the detection and characterization of potentially implicated players bearing ‘sugar’ characteristics.

Methods

Using an original monoclonal antibody and biochemical methods, we partially characterized a proteoglycan of the exclusively chondroitin/dermatan sulphate type, secreted into the interkeratinocyte spaces, that is incorporated into the extracellular parts of desmosomes in quantities proportional to the degree of cell differentiation, as visualized with immuno-electron microscopy.

Results

This antigen, that we named desmosealin, displays biochemical and immunocytochemical characteristics that clearly differentiate it from known desmosomal elements. Unlike so far described epidermal proteoglycans, which belong to the heparan sulphate family, desmosealin displays chondroitin/dermatan sulphate glycosaminoglycan chains. It can be detected within the extracellular ‘cores’ of desmosomes in the upper viable epidermal layers and in corneodesmosomes from the lowermost part of the stratum corneum.

Conclusion

Extensive integration of proteoglycans into the extracellular parts of desmosomes at the late stages of keratinocyte maturation is likely of functional importance. Given its biochemical profile, its pattern of expression in the epidermis and its desmosomal localization, desmosealin may emerge as a key element in the regulation of desmosome processing, epidermal cohesion and formation of a functional epidermal barrier.

Multimerization is required for antigen binding activity of an engineered IgM/IgG chimeric antibody recognizing a skin-related antigen

Monoclonal antibodies offer great tools for research. We encountered a potentially useful mouse IgM monoclonal antibody whose antigen is expressed in normal skin but lost in human skin cancer. Because IgM is difficult to work with and the antigen was unknown, we decided to convert the IgM (µ) to IgG (γ) version. After cDNA for the antibody was obtained by RACE PCR, we made a series of molecules with different combinations of IgM and IgG domains. Whereas V_H-Cµ1-Cµ2-Cγ3 and V_H-Cµ1-Cµ2-Hinge-Cγ2-Cγ3 functionally bound to the antigen, V_H-Cγ1-Hinge-Cγ2-Cγ3, V_H-Cµ1-Hinge-Cγ2-Cγ3, and V_H-Cµ1-Cµ2-Cγ2-Cγ3 did not. Gel filtration analyses revealed that the functional molecules tend to form multimers and the multimeric forms retained antigen binding activity. Furthermore, the mutation of amino acid residue p.309Q > C of mouse IgG and addition of IgM tailpiece to the C-terminus of the molecules induced multimer formation, dramatically enhanced antibody functionality and all non-functional molecules became strongly functional. The functional molecules could be bound by protein A/protein G and other IgG specific reagents and therefore should be useful for further characterization of the antigen. Our study revealed that multimerization of converted IgM is functionally important for antigen binding activity of engineered IgM/IgG chimeric antibodies.

Human epidermal desmosome-enriched tissue fractions for analytical and prospective studies

Here, we report a method, adapted to the human epidermis, allowing isolation of desmosomes in small tissue fractions. The methods previously developed for animal skin did not work efficiently with human tissue. Enrichment of desmosomes was performed by the association of two incubation steps in acidic solutions containing detergent NP-40 at two different concentrations followed by a sonication step. The suspension was centrifuged twice: first to remove the heavy cell fragments and then at 16000 g on a discontinuous sucrose gradient. A desmosome-enriched fraction (DsF) was collected at the 30-50% sucrose interface. We demonstrate by immunoelectron microscopy and by western blotting that the central part of the desmosome structure is preserved as well as the antigenicity of its components. Our approach, allowing a significant enrichment of the cell fractions containing desmosomes, can be used to immunize animals and create new antibodies directed against desmosomal components. Using this strategy, new and so far poorly studied molecules incorporated into the desmosome cores could be targeted more easily.

Epithelial cell membrane specific, novel monoclonal antibody KA-1

In order to develop a novel monoclonal antibody reacting with the cell membrane protein which could be a receptor for cell growth, differentiation or an adhesion molecule, keratinocytes from psoriatic epidermis and A-431 cell line were used to immunize BALB/c mice. A monoclonal antibody, KA-1 was developed. KA-1 reacts with the cell membranes of the whole epidermis with the exception of the stratum corneum cells and the basal cell membrane in contact with the basement membrane. In electron microscopy, the antigenic site was located on the surface of the microvilli of the cultured keratinocytes, especially at the contact area with another villi, the cell membrane or collagen. The antigenic protein was a 44-kDa protein which was different from E-cadherin in both molecular weight and cross-reactivity.

Structural and lipid biochemical correlates of the epidermal permeability barrier

As reviewed in this article, the stratum corneum must now be accorded the respect due to a structurally heterogeneous tissue possessing a selected array of enzymatic activity. The sequestration of lipids to intercellular domains and their organization into a unique multilamellar system have broad implications for permeability barrier function, water retention, desquamation, and percutaneous drug delivery. Yet, the functions and organization of specific lipid species in this membrane system are still unknown. Certain novel insights have resulted from comparative studies in avians and marine mammals. Further elucidation of the molecular architecture and interactions of lipid and nonlipid components of the stratum corneum intercellular domains will be a prerequisite for a comprehensive understanding of stratum corneum function.

Desmosomes, corneosomes and desquamation. An ultrastructural study of adult pig epidermis

We recently developed a pig skin model to determine the role of corneosomes (modified desmosomes in the stratum corneum) and extracellular lipids in desquamation. The present study provides control morphometric data on the morphological changes in desmosomes and corneosomes leading to desquamation in adult pig epidermis in vivo. The extracellular space within desmosomes gradually widened from the basal to the granular layer, and decreased slightly in the stratum corneum. Mid-dense line broadening, and increased electron density of the distal light layers, coincided with membrane-coating granule extrusion in the outer granular layer. Corneocyte attachment correlated with corneosome distribution. Compactum packing was relatively tight and corneosomes were numerous. Cohesion was mainly peripheral in the disjunctum, and corneosomes were restricted to corneocyte edges. Adhesion had a tongue-and-groove appearance with corneosomes riveting corneocyte peripheries into a lipped groove on adjoining cells. Cells shed by peeling radially towards the lipped groove, and corneosomes decreased from lower to upper disjunctum. Corneosome breakdown commenced with an electron lucent band forming between the plug and lipid envelope. The plug was then unzipped from the lipid envelope and degraded. Corneosomes did not form squamosomes.

Re-expression of disease-characteristic features of non-bullous congenital ichthyosiform erythroderma (CIE) after grafting of the pathological keratinocyte cultures to athymic mice

Epidermal keratinocytes separated from skin lesions of non-bullous congenital ichthyosiform erythroderma were investigated in an attempt at experimental reproduction of this keratinization disorder. In vitro studies on growth and differentiation of pathological keratinocytes isolated from the influence of the host's dermal and humoral components were performed using the immersed epidermal cell culture technique. Ten to 25-day-old confluent and stratified cultures were examined by means of photonic and electron microscopy, and stained with various differentiation markers for indirect immunofluorescence studies. The cultured epidermis showed low-grade differentiation and no clear-cut evenly distributed signs of the original disease. Grafting on congenitally athymic nude mice allowed further differentiation of the epidermal sheets and re-expression of the histologic and ultrastructural features of non-bullous congenital ichthyosiform erythroderma. Thus, the purely epidermal origin of this particular form of autosomal recessive ichthyosis could be confirmed. Large amounts of pathological keratinocytes generated from small skin biopsies may be used for experimental purposes after grafting on several athymic animals.

Growth and differentiation of human epidermal cultures used as auto- and allografts in humans

Human keratinocytes from small skin specimens were grown on mouse 3T3 cell feeder layers into epidermal sheets free from Langerhans cells and MHC class II antigen. These were found to be suitable for the permanent coverage of wounds when used as autografts or allografts. We report here the ultrastructural differentiation of this cultured epidermis after grafting onto autologous or allogeneic recipients. The cultured epidermis was a thin but multilayered Malpighian epithelium composed of keratinocytes at different stages of differentiation. The dermo-epidermal basement membrane was newly synthesized during the first few days following transplantation onto de-epidermized wounds. The analysis of keratins and examination of various keratinocyte membrane antigens by immunofluorescence indicated that full terminal epithelial differentiation was only achieved after in vivo transplantation of the cultured epidermis. Langerhans cells, absent in cultures, progressively colonized the grafts, while melanocytes, not detectable in sections of the cultures, were identified among the keratinocytes 2 weeks after grafting.