Cross-Talk between Adherens Junctions and Desmosomes Depends on Plakoglobin |
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| Authors: | Jani E Lewis James K Wahl III Kristin M Sass Pamela J Jensen Keith R Johnson Margaret J Wheelock |
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| Affiliation: | *Department of Biology, University of Toledo, Toledo, Ohio 43606; and ‡Department of Dermatology, University of Pennsylvania, Philadelphia, Pennsylvania 19104 |
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| Abstract: | Squamous epithelial cells have both adherens junctions and desmosomes. The ability of these cells to organize the desmosomal proteins into a functional structure depends upon their ability first to organize an adherens junction. Since the adherens junction and the desmosome are separate structures with different molecular make up, it is not immediately obvious why formation of an adherens junction is a prerequisite for the formation of a desmosome. The adherens junction is composed of a transmembrane classical cadherin (E-cadherin and/or P-cadherin in squamous epithelial cells) linked to either β-catenin or plakoglobin, which is linked to α-catenin, which is linked to the actin cytoskeleton. The desmosome is composed of transmembrane proteins of the broad cadherin family (desmogleins and desmocollins) that are linked to the intermediate filament cytoskeleton, presumably through plakoglobin and desmoplakin. To begin to study the role of adherens junctions in the assembly of desmosomes, we produced an epithelial cell line that does not express classical cadherins and hence is unable to organize desmosomes, even though it retains the requisite desmosomal components. Transfection of E-cadherin and/or P-cadherin into this cell line did not restore the ability to organize desmosomes; however, overexpression of plakoglobin, along with E-cadherin, did permit desmosome organization. These data suggest that plakoglobin, which is the only known common component to both adherens junctions and desmosomes, must be linked to E-cadherin in the adherens junction before the cell can begin to assemble desmosomal components at regions of cell–cell contact. Although adherens junctions can form in the absence of plakoglobin, making use only of β-catenin, such junctions cannot support the formation of desmosomes. Thus, we speculate that plakoglobin plays a signaling role in desmosome organization.Squamous epithelial cells typically contain two prominent types of cell–cell junctions: the adherens junction and the desmosome. The adherens junction is an intercellular adhesion complex that is composed of a transmembrane protein (a classical cadherin) and numerous cytoplasmic proteins (α-catenin, β-catenin and plakoglobin, vinculin and α-actinin; for reviews see Takeichi, 1990; Geiger and Ayalon, 1992). The cadherins are directly responsible for adhesive interactions via a Ca2+-dependent, homotypic mechanism, i.e., in the presence of sufficient Ca2+, cadherin on one cell binds to an identical molecule on an adjacent cell. The desmosome, also an intercellular adhesion complex, is composed of at least two different transmembrane proteins (desmoglein and desmocollin) as well as several cytoplasmic proteins, including desmoplakins and plakoglobin (Koch and Franke, 1994). The transmembrane components of the desmosome are members of the broadly defined cadherin family and also require Ca2+ for adhesive activity. However, decisive experimental evidence for homophilic or heterophilic interactions between desmosomal cadherins via their extracellular domains has not yet been presented (Koch and Franke, 1994; Kowalczyk et al., 1996). While members of the cadherin family constitute the transmembrane portion of both adherens junctions and desmosomes, the different classes of cadherins are linked to different cytoskeletal elements by the cytoplasmic components of each junction. Specifically, the classical cadherins are linked to actin filaments and the desmosomal cadherins to intermediate filaments.The organization of the proteins within the adherens junction is well understood (for reviews see Kemler, 1993; Cowin, 1994; Wheelock et al., 1996). Specifically, the intracellular domain of cadherin interacts directly with either plakoglobin or β-catenin, which in turns binds to α-catenin (Jou et al., 1995; Sacco et al., 1995). α-Catenin interacts with α-actinin and actin filaments, thereby linking the cadherin/ catenin complex to the cytoskeleton (Knudsen et al., 1995; Rimm et al., 1995). Cadherin/catenin complexes include either plakoglobin or β-catenin but not both (Näthke et al., 1994). The importance of the classical cadherins to the formation of adherens junctions and desmosomes has been demonstrated. Keratinocytes maintained in medium with low Ca2+ (i.e., 30 μM) grow as a monolayer and do not exhibit adherens junctions or desmosomes; however, elevation of Ca2+ concentration induces the rapid formation of adherens junctions followed by the formation of desmosomes (Hennings et al., 1980; Tsao et al., 1982; Boyce and Ham, 1983; Hennings and Holbrook, 1983; O''Keefe et al., 1987; Wheelock and Jensen, 1992; Hodivala and Watt, 1994; Lewis et al., 1994). Simultaneous blocking with functionperturbing antibodies against the two classical cadherins (E- and P-cadherin) found in keratinocytes inhibits not only Ca2+-induced adherens junction formation but also severely limits desmosome formation (Lewis et al., 1994; Jensen et al., 1996). Consistent with these findings, expression of a dominant-negative cadherin by keratinocytes results in decreased E-cadherin expression and delayed assembly of desmosomes (Fujimori and Takeicki, 1993; Amagai, et al., 1995). These data suggest some form of cross-talk between the proteins of the adherens junction and those of the desmosome. One candidate protein that might mediate such cross-talk is plakoglobin, since it is the only known common component of both junctions.Plakoglobin is found to be associated with the cytoplasmic domains of both the classical cadherins and the desmosomal cadherins. Despite the high degree of identity between plakoglobin and β-catenin (65% at the amino acid level; Fouquet et al., 1992), β-catenin only associates with the classical cadherins and not with the desmosomal cadherins. In the adherens junction, plakoglobin and β-catenin have at least one common function, i.e., the linking of cadherin to α-catenin and thus to actin. However, there is emerging evidence that other functions of these two proteins are not identical. For example, in a study by Navarro et al. (1993), E-cadherin transfected into a spindle cell carcinoma was shown to associate with α- and β-catenin, but not with the low levels of endogenous plakoglobin. The transfected cells did not revert to a more epithelial morphology in spite of the presence of functional E-cadherin, and the authors suggested that the lack of plakoglobin may have prevented such morphological reversion.In the present study, we have tested the hypothesis that plakoglobin, through its interaction with E- or P-cadherin, serves as a regulatory molecule for desmosome organization. Even though plakoglobin is not an essential structural component of the adherens junction (Sacco et al., 1995), our data indicate that plakoglobin can function as a regulator of desmosome formation only when it is associated with a classical cadherin. Thus, we propose that plakoglobin has at least two functions: (a) as a structural component of the adherens junction and the desmosome and (b) as a signaling molecule that regulates communication between the adherens junction and the desmosome. |
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