Northwestern University Feinberg School of Medicine

Kathleen Green Lab

Desmosomes in Tissue Morphogenesis

Figure 1

The asymmetric distribution of membrane proteins along the apical to basal axis of a simple epithelial cell ensures that epithelial barrier and transport functions are properly regulated. However, apico-basal polarity means something different in a multi-layered epithelium such as the epidermis. This tissue provides essential protection against water loss, mechanical stress, and other environmental insults. These functions require that architectural features be polarized along the entire apical to basal axis (i.e. superficial to deep layers) of the stratified epithelium, not just within an individual cell. How information embedded within this polarized architecture is translated to generate a functional epidermis is poorly understood.

While canonical polarity proteins such as Par3/5 and aPKC have been established to play important roles in the epidermis, the contributions of other highly patterned membrane proteins to tissue polarity are poorly understood. Among the most highly polarized molecules in the epidermis are the desmosomal cadherins (Figure 1). The polarized distribution of these essential cell-cell adhesion molecules is exemplified by the reciprocal pattern of the epidermal desmogleins (Dsgs) 1 and 3: Dsg3 is concentrated in basal proliferating layers, whereas Dsg1 is expressed first in basal keratinocytes and becomes progressively concentrated in suprabasal layers. Our long-term goal is to elucidate how these reciprocally patterned desmosomal cadherins dictate biochemical and structural information in an apico-basally polarized manner to establish a protective but dynamic barrier capable of sensing and responding to diverse mechanical and chemical stimuli. Toward that end, the Green lab is determining trafficking mechanisms contributing to desmoglein patterning and how differentiation-dependent Dsg1-associated protein scaffolds mediate dynamic changes in cytoarchitecture and signaling necessary for tissue morphogenesis and homeostasis.

Figure 1

Figure 2

To study human epidermal morphogenesis in a physiologically relevant setting, we use an organotypic "raft culture" model. In this 3D model of human epidermis we lift monolayers of human primary keratinocytes to an air liquid interface, which triggers cell differentiation and leads to cell stratification in the culture dish (Figure 2).  Further, we have developed methods to carry out long term knock down of specific desmosomal components, thus allowing us to prevent the onset of expression of differentiation-specific desmosomal cadherins. These studies revealed that Dsg1, which is first expressed as cells move out of the proliferating basal layer of the epidermis and is concentrated in the most differentiated layers, is required for the proper differentiation and morphogenesis of these epidermal cultures.

Figure 2

Figure 3

Our ongoing studies are focused on how Dsg1 coordinates intracellular signaling pathways that promote epidermal differentiation and morphogenesis.  As one example, a protein interaction screen uncovered the PDZ domain-containing protein called Erbin as a novel binding partner of the desmoglein cytoplasmic tail.   We showed that Erbin couples Dsg1 to the Erk1/2 signaling pathway, and is required for Dsg1 to suppress MAPK signaling by interfering with Ras-Raf coupling, which involves Shoc2.  Using the raft culture system we showed that this pathway is necessary to promote proper keratinocyte differentiation (Figure 3. R. Harmon et al. 2013. J Clin Invest. 123:1556-70).

Figure 3