Adherens Junctions, Desmosomes and Tight Junctions in Epidermal Barrier Function Johanna M

14 The Open Dermatology Journal, 2010, 4, 14-20 Open Access Adherens Junctions, Desmosomes and Tight Junctions in Epidermal Barrier Function Johanna M. Brandner1,§, Marek Haftek*,2,§ and Carien M. Niessen3,§ 1Department of Dermatology and Venerology, University Hospital Hamburg-Eppendorf, Hamburg, Germany 2University of Lyon, EA4169 Normal and Pathological Functions of Skin Barrier, E. Herriot Hospital, Lyon, France 3Department of Dermatology, Center for Molecular Medicine, Cologne Excellence Cluster on Cellular Stress Responses in Aging-Associated Diseases (CECAD), University of Cologne, Germany Abstract: The skin is an indispensable barrier which protects the body from the uncontrolled loss of water and solutes as well as from chemical and physical assaults and the invasion of pathogens. In recent years several studies have suggested an important role of intercellular junctions for the barrier function of the epidermis. In this review we summarize our knowledge of the impact of adherens junctions, (corneo)-desmosomes and tight junctions on barrier function of the skin. Keywords: Cadherins, catenins, claudins, cell polarity, stratum corneum, skin diseases. INTRODUCTION ADHERENS JUNCTIONS The stratifying epidermis of the skin physically separates Adherens junctions are intercellular structures that couple the organism from its environment and serves as its first line intercellular adhesion to the cytoskeleton thereby creating a of structural and functional defense against dehydration, transcellular network that coordinate the behavior of a chemical substances, physical insults and micro-organisms. population of cells. Adherens junctions are dynamic entities The living cell layers of the epidermis are crucial in the and also function as signal platforms that regulate formation and maintenance of the barrier on two different cytoskeletal dynamics and cell polarity. As such, they levels. First, keratinocytes ultimately form the outermost regulate a diverse range of other cellular processes next to protective dead layer of the skin, the cornified layer, through adhesion, such as cell shape, division, growth, apoptosis and a complex spatial and temporal differentiation process. barrier function. The molecular basis of adherens junctions is Impairment of this differentiation results in a reduced formed by two cell adhesion receptor complexes, the stratum corneum (SC) barrier function, as can be seen e.g. in classical cadherin/catenin complex and the nectin/afadin atopic dermatitis [1]. complex, which both can link to the actin cytoskeleton. Classical cadherins are single transmembrane Ca2+- Second, the living cell layers themselves form a barrier dependent cell adhesion molecules that at their cytoplasmic by providing tight mechanical cohesion between the cells of face interact with catenins: P120ctn regulates cadherin cell the same and different epidermal layers. To establish this surface stability and potentially connects adhesion to barrier the viable cells have to connect to each other by intercellular junctions that link intercellular contacts to the regulation of Rho GTPases whereas -catenin links the cadherin to the actin regulator -catenin [2]. Nectins are IgG cytoskeleton, such as tight junctions, (corneo) desmosomes like adhesion molecules that bind the actin binding protein and adherens junctions (Fig. 1). Important examples for the afadin. indispensability of the viable cell layers are the severe outcomes of blistering skin diseases, e.g. epidermolyis Two types of classical cadherins are expressed in the bullosa simplex and pemphigus vulgaris, the first affecting epidermis: P-cadherin (cadherin 3), expressed in the basal the integrity of the cells, the latter the integrity of cell-cell layer mainly around and in hair follicles, and E-cadherin junctions. In the last decades evidence emerged that the (cadherin 1) found in all layers of the epidermis (Fig. 2). It intercellular junctions not only directly contribute to the was recently reported that E-cadherin is a target of auto- viable epidermal barrier but also have important antibodies in pemphigus, a skin blistering disease [3]. contributions to the correct formation and maintenance of the Surprisingly, epidermal specific deletion of E-cadherin in the stratum corneum barrier. This review will focus on the role mouse did result in hair loss but did not cause any blistering, of adherens junctions, (corneo)-desmosomes and tight as was perhaps to be expected for an adhesion molecule like junctions in barrier function of mammalian skin. E-cadherin [4, 5]. In contrast to what was implicated by several in vitro studies, ultrastructural normal desmosomes *Address correspondence to this author at the University of Lyon, EA4169 were present. However, the skin of epidermal E-cadherin Normal and pathological functions of skin barrier, E. Herriot Hospital, knockout mice showed extensive water loss resulting in Lyon, France; Tel: +33 472110292; Fax: 33 472110290; perinatal death [6]. Closer analysis revealed a loss of inside- E-mail: [email protected] out but not outside in barrier function that was associated §All authors contributed equally to the manuscript, are corresponding with impaired in vivo and in vitro tight junction function [6]. authors and are named alphabetically 1874-3722/10 2010 Bentham Open Adherens Junctions, Desmosomes and Tight Junctions The Open Dermatology Journal, 2010, Volume 4 15 results thus suggest that E- and P-cadherin either serve partially differential functions or have a different functional SC overlap in human and mouse. Epidermal inactivation in mice revealed overlapping and Ds specific functions for the cadherin associated catenins. Loss of -catenin in the epidermis confirmed its importance in the Wnt signaling pathway and its role in hair follicle Ds morphogenesis and stem cell regulation. However, no obvious defects in intercellular adhesion and junction TJ formation were observed, most likely because plakoglobin Ds substituted for -catenin in the cadherin complex. Inactivation of p120ctn in the epidermis resulted in reduced Ds adherens junctions and skin inflammation associated with activation of NfB [10]. An almost complete loss of adherens junctions, reduced desmosomes and subsequent TJ skin blistering was observed when -catenin was deleted from the epidermis [11]. Since both P-cadherin and E- A cadherin interact with the catenins, loss of a single cadherin may be insufficient to phenocopy deletion of one of the catenins. This is in agreement with studies in human keratinocytes. These show that a combination of antibodies to E- and P-cadherin inhibited not only the formation of adherens junctions but also desmosomes and interfered with Ds stratification. Recent in vitro and in vivo studies using knockout/knockdown strategies confirmed that classical cadherins are crucial not only for the formation of tight junctions but also regulate desmosome formation [12, 13]. AJ Ds This does not depend on a specific function of E- and P- cadherin but is related to the relative total expression levels of these two classical cadherins together. DESMOSOMES - CORNEODESMOSOMES Desmosomes are “mechanical” junctions, involved B primarily in cell cohesion [14]. They are composed of the desmosomal cadherins, which, similar to the classical cadherins of adherens junctions, are part of the cadherin superfamily. Desmogleins 1-4 and desmocollins 1-3 are found in the human epidermis (Fig. 2). The intracellular ends of desmosomal cadherins are inserted in the molecular Ds network of adaptor proteins forming desmosomal plaques, to which keratin filaments bind. Two families of plaque GJ GJ proteins can be distinguished in desmosomes: i) plakoglobin and plakophilins, which belong to the armadillo repeat Ds protein family that also includes -catenin and p120ctn, and Ds Ds AJ ii) desmoplakin, envoplakin, periplakin and plectin from the plakin family. As keratinocytes move through the epidermal layers, they constantly form and retrieve desmosomes at the cell periphery. During this turnover, the molecules that C compose junctions (even without physical dissociation of the structure) are also constantly replaced. According to the level Fig. (1). Close spatial relationship between adherens junctions (AJ), of keratinocyte differentiation, desmogleins 2 and 3 from the desmosomes (Ds) and tight junctions (TJ) in human epidermis, as lower epidermal compartment are progressively substituted seen with electron microscopy. (A): fetal, (B, C) adult human skin. by desmogleins 1 and 4 in the upper viable epidermal layers. The micrograph of TJ (small arrows pointing to the TJ “kissing In the same way desmocollin 3 is replaced by desmocollin 1 points”) has been taken from [30]. GJ = gap junctions. SC = stratum (Fig. 2). This differentiation –dependent composition of corneum. Bar = 500 nm. desmosomes coincides with the increase of their mechanical Loss of P-cadherin in mice did not reveal any obvious stability. The fact that extracellular calcium levels are phenotypes [7] even though mutations in human P-cadherin elevated in the upper epidermis certainly also contributes to are associated with a hair disorder, hypotrichosis with the desmosome stabilization. In the basal layer, desmosomes juvenile macular dystrophy (OMIM # 601553) [8], and with are infrequent and small; their size and number rise ectodermal dysplasia associated with ectrodactyly and significantly in the spinous layers and decrease subsequently macular dystrophy (EEM; OMIM # 225280) [9]. These in the granular compartment

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