Cells within all types of epithelia are polarized such that they have distinct domains at their lower and upper regions. For example, in simple epithelia, which consist of a single layer of cells, there are specialized protein complexes near their apical surface that are essential for holding cells together, limiting the passage of molecules and ions through the space between cells, and stopping the movement of membrane proteins between the apical and basolateral membranes. These structures coordinate with cytoskeletal networks to generate contractile forces that are essential in regulating tissue growth, shape, movement, and barrier establishment. Multilayer epithelial tissues, like the epidermis of the skin, are also polarized, but across many cell layers. How these patterns emerge in the epidermis and how they regulate normal tissue functions are not fully understood. This study will focus on the cadherin desmoglein 1 (Dsg1), which is a part of the desmosome, a cell-cell adhesive organelle. Dsg1 is a disease target involved in autoimmune, bacterial toxin-mediated, and inherited diseases and is only expressed in multilayer epithelia. The amount of Dsg1 present in each of the layers of the skin is patterned, with very little protein in the lowest layer and increasing amounts toward the outer layers. This suggests that the functional overlay of patterned Dsg1 onto the baseline machinery found in simple epithelia led to new mechanisms to increase tissue complexity. Preliminary studies indicate that there is a region under high tension in the outermost living layers of the epidermis. Moreover, loss of Dsg1 resulted in a shift in the localization of this high-tension region. It is known that skin is under tension, and that tension contributes to the growth of epidermal tissue as well as to the process of wound healing.
In Aim 1 of this proposal, we will use laser ablation and atomic force microscopy to test the role of Dsg1 in regulating epidermal tissue mechanics (tension and stiffness). I hypothesize this occurs through Dsg1 integrating with modulators of the actin cytoskeleton, known regulators of cell forces. Chemical signaling platforms are also patterned in the epidermis, including members of the epidermal growth factor receptor (ErbB) family. The best-known member, epidermal growth factor receptor (EGFR), plays an important role in regulating cell proliferation in the basal layer of the epidermis. However, the functions of other members in the skin are not well known. Preliminary data show that ErbB2 is located in the uppermost living layers of the epidermis, where tight junctions are formed. Tight junctions are an integral part of the epidermal barrier, preventing loss of body fluids and entrance of foreign substances. Dsg1 regulates the total amount and the activity of ErbB2, and together Dsg1 and ErbB2 regulate tight junction proteins.
Aim 2 of this proposal will ascertain the mechanism by which Dsg1 affects ErbB2, and the extent to which these proteins work together to regulate the formation and function of the epidermal barrier. Future work will examine the effects of Dsg1-mediated mechanics on ErbB2 activity and barrier function, linking Aims 1 and 2. I propose that Dsg1 integrates mechanical and chemical signals to control the polarized architecture and function of the epidermis.
Many common skin diseases like atopic dermatitis and psoriasis are characterized by barrier defects and are often present in patterns on patient skin that are consistent with the diseases having an etiology that is affected by mechanics (e.g. in folds versus ridges of the skin). The proposed work will define for the first time how the evolutionarily recent cadherin desmoglein 1 contributes to the establishment of a mechanically and biochemically polarized epidermis necessary for formation of the essential skin barrier. Understanding the underlying mechanisms that regulate barrier function and skin mechanics may enhance our ability to treat skin diseases like atopic dermatitis and psoriasis as well as to promote better wound healing.
