Peter J. Koch

Explanations for the clinical and microscopic localization of lesions in pemphigus foliaceus and vulgaris

Patients with pemphigus foliaceus (PF) have blisters on skin, but not mucous membranes, whereas patients with pemphigus vulgaris (PV) develop blisters on mucous membranes and/or skin. PF and PV blisters are due to loss of keratinocyte cell-cell adhesion in the superficial and deep epidermis, respectively. PF autoantibodies are directed against desmoglein (Dsg) 1; PV autoantibodies bind Dsg3 or both Dsg3 and Dsg1. In this study, we test the hypothesis that coexpression of Dsg1 and Dsg3 in keratinocytes protects against pathology due to antibody-induced dysfunction of either one alone. Using passive transfer of pemphigus IgG to normal and DSG3(null) neonatal mice, we show that in the areas of epidermis and mucous membrane that coexpress Dsg1 and Dsg3, antibodies against either desmoglein alone do not cause spontaneous blisters, but antibodies against both do. In areas (such as superficial epidermis of normal mice) where Dsg1 without Dsg3 is expressed, anti-Dsg1 antibodies alone can cause blisters. Thus, the anti-desmoglein antibody profiles in pemphigus sera and the normal tissue distributions of Dsg1 and Dsg3 determine the sites of blister formation. These studies suggest that pemphigus autoantibodies inhibit the adhesive function of desmoglein proteins, and demonstrate that either Dsg1 or Dsg3 alone is sufficient to maintain keratinocyte adhesion.

Loss of desmocollin 3 in mice leads to epidermal blistering

Desmocollin 3 (DSC3) belongs to a subfamily of cadherins and is a major component of desmosomes in keratinocytes of stratified epithelia, such as the epidermis. Based on its amino acid sequence homology to classical cadherins, such as E-cadherin, it has been postulated that DSC3 functions as a cell-adhesion molecule. To test this hypothesis, we assessed the function of DSC3 in the development and maintenance of stratified epithelia, in particular the epidermis and hair follicles. Using a conditional null allele, we show that loss of Dsc3 function in the epidermis causes impaired cell–cell adhesion, leading to intra-epidermal blistering and telogen hair loss. Furthermore, the lesions in Dsc3-null skin resemble those observed in individuals with pemphigus vulgaris (PV), indicating that impaired Dsc3 function could be a potential cause of PV-like inherited or acquired skin blistering diseases.

In Vivo Function of Desmosomes

Desmosomes are morphologically and biochemically defined cell‐cell junctions that are required for maintaining the mechanical integrity of skin and the heart in adult mammals. Furthermore, since mice with null mutations in desmosomal plaque proteins (plakoglobin and desmoplakin) die in utero, it is also evident that desmosomes are indispensable for normal embryonic development. This review focuses on the role of desmosomes in vivo. We will summarize the effects of mutations in desmosomal genes on pre‐ and post‐embryonic development of mouse and man and discuss recent findings relating to the specific role of desmosomal cadherins in skin differentiation and homeostasis.

Desmocollin 3 is required for pre-implantation development of the mouse embryo.

Desmocollin 3 (Dsc3) is a transmembrane glycoprotein that belongs to the cadherin family of cell adhesion receptors. Together with desmoglein(s), it forms the transmembrane core of desmosomes, a multiprotein complex involved in cell adhesion, organization of the cytoskeleton, cell sorting and cell signaling. Previous reports have suggested that Dsc3 synthesis is largely restricted to stratified epithelia, and that it plays a role in the proper differentiation of these tissues during mammalian embryonic development. To test these hypotheses, we generated Dsc3-null mice. Unexpectedly, homozygous mutants show a pre-implantation lethal phenotype. In fact, most mutants die even before mature desmosomes are formed in the embryo, suggesting a new and unexpected role of Dsc3 during early development.

Assessment of Splice Variant-Specific Functions of Desmocollin 1 in the Skin

ABSTRACT Desmocollin 1 (Dsc1) is part of a desmosomal cell adhesion receptor formed in terminally differentiating keratinocytes of stratified epithelia. The dsc1 gene encodes two proteins (Dsc1a and Dsc1b) that differ only with respect to their COOH-terminal cytoplasmic amino acid sequences. On the basis of in vitro experiments, it is thought that the Dsc1a variant is essential for assembly of the desmosomal plaque, a structure that connects desmosomes to the intermediate filament cytoskeleton of epithelial cells. We have generated mice that synthesize a truncated Dsc1 receptor that lacks both the Dsc1a- and Dsc1b-specific COOH-terminal domains. This mutant transmembrane receptor, which does not bind the common desmosomal plaque proteins plakoglobin and plakophilin 1, is integrated into functional desmosomes. Interestingly, our mutant mice did not show the epidermal fragility previously observed in dsc1-null mice. This suggests that neither the Dsc1a- nor the Dsc1b-specific COOH-terminal cytoplasmic domain is required for establishing and maintaining desmosomal adhesion. However, a comparison of our mutants with dsc1-null mice suggests that the Dsc1 extracellular domain is necessary to maintain structural integrity of the skin.

An unexpected role for keratin 10 end domains in susceptibility to skin cancer

Keratin 10 (K10) is a type I keratin that is expressed in post-mitotic suprabasal keratinocytes of the skin. Based on cell culture experiments and transgenic mouse studies, it has been proposed that K10 suppresses cell proliferation and tumor formation in the skin. Furthermore, the ability of K10 to suppress cell proliferation was mapped to its unique N- and C-terminal protein domains. In the present study, we modified the endogenous keratin 14 (K14) gene of mice using a knock-in approach to encode a chimeric keratin that consists of the K14 rod domain fused to the K10 head and tail domains (K1014chim). This transgene was expressed in the basal layer of the epidermis and the outer root sheath of hair follicles. Unexpectedly, we found that the K10 end domains had no effect on basal keratinocyte proliferation in vivo. Moreover, when subjected to a chemical skin carcinogenesis protocol, papilloma formation in mutant mice was accelerated instead of being inhibited. Our data suggest that the increased tumor susceptibility of K1014chim mice is in part due to a suppression of apoptosis in mutant keratinocytes. Our results support the notion that intermediate filaments, in addition to their function as cytoskeletal components, affect tumor susceptibility of epithelial cells.

An adult passive transfer mouse model to study desmoglein 3 signaling in pemphigus vulgaris

Evidence has accumulated that changes in intracellular signaling downstream of desmoglein 3 (Dsg3) may play a significant role in epithelial blistering in the autoimmune disease pemphigus vulgaris (PV). Currently, most studies on PV involve passive transfer of pathogenic antibodies into neonatal mice which have not finalized epidermal morphogenesis, and do not permit analysis of mature hair follicles (HFs) and stem cell niches. To investigate Dsg3 antibody-induced signaling in the adult epidermis at defined stages of the HF cycle, we here developed a model with passive transfer of the monospecific pathogenic Dsg3 antibody AK23 into adult 8-week-old C57Bl/6J mice. Validated using histopathological and molecular methods, we found that this model faithfully recapitulates major features described in PV patients and PV models. Two hours after AK23 transfer we observed widening of intercellular spaces between desmosomes and EGFR activation, followed by increased Myc expression and epidermal hyperproliferation, desmosomal Dsg3 depletion and predominant blistering in HFs and oral mucosa. These data confirm that the adult passive transfer mouse model is ideally suited for detailed studies of Dsg3 antibody-mediated signaling in adult skin, providing the basis for investigations on novel keratinocyte-specific therapeutic strategies.

The Desmosomal Plaque Proteins of the Plakophilin Family

Three related proteins of the plakophilin family (PKP1_3) have been identified as junctional proteins that are essential for the formation and stabilization of desmosomal cell contacts. Failure of PKP expression can have fatal effects on desmosomal adhesion, leading to abnormal tissue and organ development. Thus, loss of functional PKP 1 in humans leads to ectodermal dysplasia/skin fragility (EDSF) syndrome, a genodermatosis with severe blistering of the epidermis as well as abnormal keratinocytes differentiation. Mutations in the human PKP 2 gene have been linked to severe heart abnormalities that lead to arrhythmogenic right ventricular cardiomyopathy (ARVC). In the past few years it has been shown that junctional adhesion is not the only function of PKPs. These proteins have been implicated in cell signaling, organization of the cytoskeleton, and control of protein biosynthesis under specific cellular circumstances. Clearly, PKPs are more than just cell adhesion proteins. In this paper we will give an overview of our current knowledge on the very distinct roles of plakophilins in the cell.

Desmosomes: just cell adhesion or is there more?

Desmosomes are cell adhesion structures (junctions) that are particularly abundant in cells derived from the ectodermal lineages. These junctions are required to maintain the integrity of organs subjected to mechanical stress, in particular the skin and the heart. This conclusion is partially based on tissue fragility phenotypes observed in mice with null mutations in certain desmosomal genes. Furthermore, patients have been identified that develop severe skin disorders, and even fatal heart diseases, due to impaired desmosome function. Nevertheless, desmosomes are more than cellular glue. New evidence suggests that these junctions can transmit signals from the extracellular environment to the nucleus, for example by controling the cytoplasmic pool of transcriptional co-factors that belong to the armadillo family of desmosomal proteins (i.e. plakoglobin, plakophilins). Understanding the signaling properties of desmosomes will provide new insights into developmental processes such as skin and skin appendage development. Furthermore, there is evidence to suggest that abnormal signaling through these junctions contributes to the symptoms of certain skin and heart diseases.

Plakoglobin as a Regulator of Desmocollin Gene Expression

Desmosomes are cell adhesion junctions required for the normal development and maintenance of mammalian tissues and organs such as the skin, skin appendages and the heart. The goal of the present study was to investigate how desmocollins (DSC), transmembrane components of desmosomes, are regulated at the transcriptional level. We hypothesized that differential expression of the Dsc2 and Dsc3 genes is a prerequisite for normal development of skin appendages. We demonstrate that plakoglobin (Pg) in conjunction with Lef-1 differentially regulates the proximal promoters of these two genes. Specifically, we found that Lef-1 acts as a switch activating Dsc2 and repressing Dsc3 in the presence of Pg. Interestingly, we also determined that NFκB pathway components, down-stream effectors of the Eda/EDAR signaling cascade, can activate Dsc2 expression. We hypothesize that Lef-1 and Eda/EDAR/NFκB signaling contribute to a shift in Dsc isoform expression from Dsc3 to Dsc2 in placode keratinocytes. It is tempting to speculate that this shift is required for invasive growth of placode keratinocytes into the dermis, a crucial step in skin appendage formation.