Desmoglein 4 is expressed in highly differentiated keratinocytes and trichocytes in human epidermis and hair follicle

Genome-wide association study in alopecia areata implicates both innate and adaptive immunity

Alopecia areata (AA) is among the most highly prevalent human autoimmune diseases, leading to disfiguring hair loss due to the collapse of immune privilege of the hair follicle and subsequent autoimmune attack. The genetic basis of AA is largely unknown. We undertook a genome-wide association study (GWAS) in a sample of 1,054 cases and 3,278 controls and identified 139 single nucleotide polymorphisms that are significantly associated with AA (P <or= 5 x 10(-7)). Here we show an association with genomic regions containing several genes controlling the activation and proliferation of regulatory T cells (T(reg) cells), cytotoxic T lymphocyte-associated antigen 4 (CTLA4), interleukin (IL)-2/IL-21, IL-2 receptor A (IL-2RA; CD25) and Eos (also known as Ikaros family zinc finger 4; IKZF4), as well as the human leukocyte antigen (HLA) region. We also find association evidence for regions containing genes expressed in the hair follicle itself (PRDX5 and STX17). A region of strong association resides within the ULBP (cytomegalovirus UL16-binding protein) gene cluster on chromosome 6q25.1, encoding activating ligands of the natural killer cell receptor NKG2D that have not previously been implicated in an autoimmune disease. By probing the role of ULBP3 in disease pathogenesis, we also show that its expression in lesional scalp from patients with AA is markedly upregulated in the hair follicle dermal sheath during active disease. This study provides evidence for the involvement of both innate and acquired immunity in the pathogenesis of AA. We have defined the genetic underpinnings of AA, placing it within the context of shared pathways among autoimmune diseases, and implicating a novel disease mechanism, the upregulation of ULBP ligands, in triggering autoimmunity.

Plakophilin-1 localizes to the nucleus and interacts with single-stranded DNA

Plakophilins (Pkp-1, -2, and -3) comprise a family of armadillo repeat-containing proteins first identified as desmosomal plaque components, in which they link desmoplakin to the desmosomal cadherins. In addition to their role in desmosomal cell-cell adhesion, Pkps also localize to the nucleus, where they perform unknown functions. Of the three Pkps, Pkp-1 is most readily detected in the nucleus, where it is localized to the nucleoplasm. Pkp chimeras containing the Pkp-1 head domain and Pkp-3 armadillo repeat domain were localized to the nucleus in A431 cells, whereas Pkp chimeras containing the Pkp-3 head domain and Pkp-1 armadillo repeat domain localized to the desmosome and the cytosol. DNAse I digestion of chromatin in cultured cells results in loss of nuclear Pkp-1, suggesting that Pkp-1 associates specifically with nuclear components. In addition, in vitro assays revealed that the amino-terminal head domains of Pkps-1 and -2 were sufficient to bind single-stranded DNA. Induction of DNA damage induced a partial redistribution of Pkp-1 protein to the nucleolus, and depletion of Pkp-1 resulted in increased survival in response to DNA damage. These data suggest that in addition to mediating desmosome assembly, the nuclear pool of Pkp can influence cell survival by interactions with DNA.

APCDD1 is a novel Wnt inhibitor mutated in hereditary hypotrichosis simplex

Hereditary hypotrichosis simplex is a rare autosomal dominant form of hair loss characterized by hair follicle miniaturization. Using genetic linkage analysis, we mapped a new locus for the disease to chromosome 18p11.22, and identified a mutation (Leu9Arg) in the adenomatosis polyposis down-regulated 1 (APCDD1) gene in three families. We show that APCDD1 is a membrane-bound glycoprotein that is abundantly expressed in human hair follicles, and can interact in vitro with WNT3A and LRP5-two essential components of Wnt signalling. Functional studies show that APCDD1 inhibits Wnt signalling in a cell-autonomous manner and functions upstream of beta-catenin. Moreover, APCDD1 represses activation of Wnt reporters and target genes, and inhibits the biological effects of Wnt signalling during both the generation of neurons from progenitors in the developing chick nervous system, and axis specification in Xenopus laevis embryos. The mutation Leu9Arg is located in the signal peptide of APCDD1, and perturbs its translational processing from the endoplasmic reticulum to the plasma membrane. APCDD1(L9R) probably functions in a dominant-negative manner to inhibit the stability and membrane localization of the wild-type protein. These findings describe a novel inhibitor of the Wnt signalling pathway with an essential role in human hair growth. As APCDD1 is expressed in a broad repertoire of cell types, our findings indicate that APCDD1 may regulate a diversity of biological processes controlled by Wnt signalling.

Discovering the molecular components of intercellular junctions--a historical view

The organization of metazoa is based on the formation of tissues and on tissue-typical functions and these in turn are based on cell-cell connecting structures. In vertebrates, four major forms of cell junctions have been classified and the molecular composition of which has been elucidated in the past three decades: Desmosomes, which connect epithelial and some other cell types, and the almost ubiquitous adherens junctions are based on closely cis-packed glycoproteins, cadherins, which are associated head-to-head with those of the hemi-junction domain of an adjacent cell, whereas their cytoplasmic regions assemble sizable plaques of special proteins anchoring cytoskeletal filaments. In contrast, the tight junctions (TJs) and gap junctions (GJs) are formed by tetraspan proteins (claudins and occludins, or connexins) arranged head-to-head as TJ seal bands or as paracrystalline connexin channels, allowing intercellular exchange of small molecules. The by and large parallel discoveries of the junction protein families are reported.

Desmoglein 4 is regulated by transcription factors implicated in hair shaft differentiation

The hair fiber is made of specialized keratinocytes, known as trichocytes, that primarily express hair keratins, which are cemented by a multitude of keratin-associated proteins (KAPs). The hair keratins form the intermediate filament cytoskeleton of the trichocytes, which are linked to abundant cell-cell adhesion junctions, called desmosomes. Desmoglein 4 (DSG4) is the major desmosomal cadherin expressed in the hair shaft cortex where the hair keratins are highly expressed. In humans, mutations affecting either the hair keratins or DSG4 lead to beaded hair phenotypes with features of monilethrix. In this work, we postulated that the regulatory pathways governing the expression of hair shaft components, such as hair keratins and DSG4, are shared. Therefore, we studied the transcriptional regulation of DSG4 by transcription factors/pathways that are known regulators of hair keratin or KAP expression. We show that HOXC13, LEF1 and FOXN1 repress DSG4 transcription and provide in vitro and in vivo evidence correlating the Notch pathway with the activation and/or maintenance of DSG4 expression in the hair follicle.

Desmoglein 1-dependent suppression of EGFR signaling promotes epidermal differentiation and morphogenesis

Dsg1 (desmoglein 1) is a member of the cadherin family of Ca²⁺-dependent cell adhesion molecules that is first expressed in the epidermis as keratinocytes transit out of the basal layer and becomes concentrated in the uppermost cell layers of this stratified epithelium. In this study, we show that Dsg1 is not only required for maintaining epidermal tissue integrity in the superficial layers but also supports keratinocyte differentiation and suprabasal morphogenesis. Dsg1 lacking N-terminal ectodomain residues required for adhesion remained capable of promoting keratinocyte differentiation. Moreover, this capability did not depend on cytodomain interactions with the armadillo protein plakoglobin or coexpression of its companion suprabasal cadherin, Dsc1 (desmocollin 1). Instead, Dsg1 was required for suppression of epidermal growth factor receptor–Erk1/2 (extracellular signal-regulated kinase 1/2) signaling, thereby facilitating keratinocyte progression through a terminal differentiation program. In addition to serving as a rigid anchor between adjacent cells, this study implicates desmosomal cadherins as key components of a signaling axis governing epithelial morphogenesis.

Smad4-dependent desmoglein-4 expression contributes to hair follicle integrity

We have previously shown that keratinocyte-specific deletion of Smad4, a TGFbeta/Activin/BMP signaling mediator, results in a progressive alopecia. To further assess the molecular mechanisms of Smad4 loss-mediated alopecia, we examined expression levels of key molecules associated with hair follicle differentiation in Smad4-deleted skin. Among them, Desmoglein 4 (Dsg4) was down-regulated in Smad4-deleted skin prior to the onset of hair follicle abnormalities with gradual depletion coinciding with hair follicle degeneration. Chromatin immunoprecipitation (ChIP) assay showed that Smad4, together with the BMP mediators Smad1 and Smad5, but not the TGFbeta/Activin mediators Smad2 or Smad3, bound to the Smad Binding Element (SBE) of the Dsg4 promoter. A Dsg4 reporter assay revealed that Smad4 was required for the maximal transactivation of Dsg4 in cooperation with Smad1 and Smad5. Mutating the SBE of the Dsg4 promoter abrogated Smad4 transactivation of Dsg4. Furthermore, BMP ligands, but not ligands of TGFbeta and Activin, induced endogenous Dsg4 expression. Our data demonstrate that in the presence of Smad4, BMP signaling participated in transcriptional regulation of Dsg4. Thus, Smad4 loss-associated Dsg4 depletion contributed, at least in part, to hair follicles degeneration in Smad4 deficient skin.

Staphylococcal exfoliative toxins: “Molecular scissors” of bacteria that attack the cutaneous defense barrier in mammals

Bullous impetigo and its generalized form, staphylococcal scalded-skin syndrome (SSSS), are highly contagious, blistering skin diseases caused by Staphylococcus aureus infection. Virulent strains of the bacteria produce exfoliative toxins (ETs) that cause the loss of keratinocyte cell-cell adhesion in the superficial epidermis. Recent studies have indicated that the three isoforms of ETs, i.e., ETA, ETB, and ETD, are glutamate-specific serine proteases that specifically and efficiently cleave a single peptide bond in the extracellular region of human and mouse desmoglein 1 (Dsg1), a desmosomal intercellular adhesion molecule. In addition, four isoforms of S. hyicus exfoliative toxin, ExhA, ExhB, ExhC, and ExhD, cleave swine Dsg1, resulting in skin exfoliation similar to that observed in pigs with exudative epidermitis. In this review, we describe recent advances in our knowledge of the mechanisms of action of staphylococcal exfoliative toxins, which act as "molecular scissors" to facilitate percutaneous bacterial invasion of mammalian skin by cleavage of keratinocyte cell-cell adhesion molecules. The species-specificity of staphylococcal exfoliative toxins to cleave Dsg1 in certain mammalian species is discussed.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

Isolation of pathogenic monoclonal anti-desmoglein 1 human antibodies by phage display of pemphigus foliaceus autoantibodies

Pemphigus foliaceus (PF) is a blistering disease caused by autoantibodies to desmoglein 1 (Dsg1) that cause loss of epidermal cell adhesion. To better understand PF pathophysiology, we used phage display to isolate anti-Dsg1 mAbs as single-chain variable fragments (scFvs) from a PF patient. Initial panning of the library isolated only non-pathogenic scFvs. We then used these scFvs to block non-pathogenic epitopes and were able to isolate two unique scFvs, each of which caused typical PF blisters in mice or human epidermis models, showing that a single mAb can disrupt Dsg1 function to cause disease. Both pathogenic scFvs bound conformational epitopes in the N terminus of Dsg1. Other PF sera showed a major antibody response against the same or nearby epitopes defined by these pathogenic scFvs. Finally, we showed restriction of the heavy-chain gene usage of all anti-Dsg1 clones to only five genes, which determined their immunological properties despite promiscuous light-chain gene usage. These mAbs will be useful for studying Dsg1 function and mechanisms of blister formation in PF and for developing targeted therapies and tools to monitor disease activity.

Broken hearts, woolly hair, and tattered skin: when desmosomal adhesion goes awry

Desmosomal cadherins constitute the adhesive core of desmosomes. Different desmosomal cadherins are differentially expressed in a tissue-specific as well as differentiation-dependent manner. The skin and the heart are two examples of tissues whose vital functions require the ability to endure mechanical stress, and therefore, rely on the integrity of desmosomal adhesion. When this adhesion is compromised via mutations in genes encoding desmosomal cadherins or associated plaque proteins, both tissues can suffer the consequences. Open questions revolve around whether the resulting phenotypes are solely because of physical disruption of cell adhesion or whether these events are coupled with signaling mechanisms that influence many additional cellular processes. In this review, we focus on new developments in desmosomal adhesion with an emphasis on the skin, hair, and heart.

Hair follicle-specific keratins and their diseases

The human keratin family comprises 54 members, 28 type I and 26 type II. Out of the 28 type I keratins, 17 are epithelial and 11 are hair keratins. Similarly, the 26 type II members comprise 20 epithelial and 6 hair keratins. As, however, 9 out of the 37 epithelial keratins are specifically expressed in the hair follicle, the total number of hair follicle-specific keratins (26) almost equals that of those expressed in the various forms of epithelia (28). Up to now, more than half of the latter have been found to be involved in inherited diseases, with mutated type I and type II members being roughly equally causal. In contrast, out of the 26 hair follicle-specific keratins only 5 have, at present, been associated with inherited hair disorders, while one keratin merely acts as a risk factor. In addition, all hair follicle-specific keratins involved in pathologies are type II keratins. Here we provide a detailed description of the respective hair diseases which are either due to mutations in hair keratins (monilethrix, ectodermal dysplasia of hair and nail type) or hair follicle-specific epithelial keratins (two mouse models, RCO3 and Ca(Rin) as well as pseudofolliculitis barbae).

Discriminating roles of desmosomal cadherins: Beyond desmosomal adhesion

The desmosomal cadherins, which include desmogleins and desmocollins, are Ca(2+)-dependent adhesion molecules that cooperate to make up the adhesive core of intercellular junctions known as desmosomes. The roles of desmosomal cadherins in epidermal integrity and as targets in human cutaneous disease have been well established. However, the molecular basis of these disorders is still poorly understood, due in part to a lack of fundamental knowledge about the organization of the adhesive interface and molecular machinery that dictates the proper presentation of desmogleins and desmocollins on the cell surface. Further, the diversity of the desmosomal cadherin family, and their individualized expression patterns within complex tissues, suggests that these adhesion molecules may have differentiation-specific functions that transcend their roles in intercellular adhesion. Here we will review the most recent data from our own group and others that are beginning to unveil the diverse properties and functions of this complex family of adhesion molecules.

A potential role for multiple tissue kallikrein serine proteases in epidermal desquamation

Desquamation of the stratum corneum is a serine protease-dependent process. Two members of the human tissue kallikrein (KLK) family of (chymo)tryptic-like serine proteases, KLK5 and KLK7, are implicated in desquamation by digestion of (corneo)desmosomes and inhibition by desquamation-related serine protease inhibitors (SPIs). However, the epidermal localization and specificity of additional KLKs also supports a role for these enzymes in desquamation. This study aims to delineate the probable contribution of KLK1, KLK5, KLK6, KLK13, and KLK14 to desquamation by examining their interactions, in vitro, with: 1) colocalized SPI, lympho-epithelial Kazal-type-related inhibitor (LEKTI, four recombinant fragments containing inhibitory domains 1-6 (rLEKTI(1-6)), domains 6-8 and partial domain 9 (rLEKTI(6-9')), domains 9-12 (rLEKTI(9-12)), and domains 12-15 (rLEKTI(12-15)), secretory leukocyte protease inhibitor, and elafin and 2) their ability to digest the (corneo)desmosomal cadherin, desmoglein 1. KLK1 was not inhibited by any SPI tested. KLK5, KLK6, KLK13, and KLK14 were potently inhibited by rLEKTI(1-6), rLEKTI(6-9'), and rLEKTI(9-12) with Ki values in the range of 2.3-28.4 nm, 6.1-221 nm, and 2.7-416 nm for each respective fragment. Only KLK5 was inhibited by rLEKTI(12-15) (Ki = 21.8 nm). No KLK was inhibited by secretory leukocyte protease inhibitor or elafin. Apart from KLK13, all KLKs digested the ectodomain of desmoglein 1 within cadherin repeats, Ca2+ binding sites, or in the juxtamembrane region. Our study indicates that multiple KLKs may participate in desquamation through cleavage of desmoglein 1 and regulation by LEKTI. These findings may have clinical implications for the treatment of skin disorders in which KLK activity is elevated.

Non-pathogenic anti-desmoglein 3 IgG autoantibodies in Fogo Selvagem

The endemic form of pemphigus foliaceus, fogo selvagem, is caused by IgG autoantibodies directed against desmoglein 1 (Dsg1). Hilario-Vargas and his colleagues describe a high prevalence of IgG autoantibodies against Dsg3, the target antigen of pemphigus vulgaris, in a Brazilian population where fogo selvagem is endemic, although those patients do not develop any apparent clinical phenotype of pemphigus vulgaris.

Mutations in the desmoglein 4 gene underlie localized autosomal recessive hypotrichosis with monilethrix hairs and congenital scalp erosions

Localized autosomal recessive hypotrichosis (LAH) is a recently defined disorder characterized by fragile, short, sparse hairs on the scalp, trunk, and extremities. Mutations in desmoglein 4 (DSG4), a novel member of the desmosomal cadherin family that is expressed in the hair follicle as well as the suprabasal epidermis, have been found to underlie LAH. Thus far, the allelic series includes a recurrent intragenic deletion identified in affected Pakastani kindreds and a missense mutation detected in an Iraqi family. We report three siblings of Iraqi and Iranian origin with LAH that presented with congenital scalp erosions and monilethrix-like hairs, features that have not been previously described in this disorder. Follicular hyperkeratotic papules and marked pruritus were also prominent clinical findings. Novel compound heterozygous DSG4 mutations, including a splice-site mutation and a missense mutation that disrupts a conserved calcium-binding site in the extracellular (EC)2-EC3 interface, were found to underlie the disease in this family. These observations broaden the phenotypic and genotypic spectrum of LAH, further illustrating the consequences of DSG4 dysfunction on epidermal and hair shaft integrity.