A Novel Desmosomal COP9 Signalosome Complex in Epidermal Differentiation

Najor, Nicole

Abstract

Desmosomes are intercellular adhesive junctions, which are required for integrity of tissues that experience mechanical stress, such as the epidermis. Their mechanical functions are facilitated by the linkage of the intermediate filament cytoskeleton to transmembrane desmosomal cadherins through a complex of cytoplasmic proteins in the armadillo and plakin families. It is known that disrupting desmosome structure and function through gene mutations, bacterial toxins or autoantibodies results in severe blistering disorders and keratodermas that have limited treatment options. However, the extent to which impairment of adhesion versus newly emerging signaling functions of desmosomes contributes to disease pathogenesis is unknown. It was recently shown that the cytoplasmic domain of the desmosomal cadherin Desmoglein 1 (Dsg1), which is first expressed as cells commit to differentiate, promotes epidermal differentiation by inhibiting EGFR/MAPK signaling. Towards elucidating the signaling role of the Dsg1 cytoplasmic tail in differentiation, a yeast two hybrid screen was performed. Among the putative binding partners identified by the screen was the COP9 signalosome subunit 3, Cops3. COP9 is complex of proteins, comprising eight subunits (Cops1-Cops8). This complex removes Nedd8 stabilizing protein modifications from cullin-RING ligases, to regulate activation cycles of cullin-RING ligases (which promote ubiquitination and degradation of proteins). The Nedd8 protein has also been shown to modify membrane receptors (e.g. EGFR), which can promote receptor stabilization. Ectopic expression of Dsg1 is known to promote epidermal differentiation through the suppression of EGFR signaling, but it is unable to do so in Cops3-deficient cells. This observation suggests that Dsg1 requires Cops3 to inhibit EGFR/MAPK and promote epidermal differentiation. Further, biochemical analyses revealed that Cops3 associates with both Dsg1 and the desmosomal plakin protein Desmoplakin (Dp), and that cullin family members (Cul3) associate with Dp. These data suggest that there is more than one interface for the desmosome to scaffold the COP9 signalosome for functioning. Based on these observations, it is hypothesized that the desmosome acts as a scaffold for the COP9 signalosome to de-neddylate cullins and EGFR, and promotes epidermal differentiation by inhibition of EGFR signaling.
Aim1 will define the association between the COP9 signalosome and the desmosome, and the extent to which desmosome molecules affect COP9-dependent de-neddylation activity.
Aim2 will determine how Cops3 assists in Dsg1- dependent differentiation by suppressing EGFR/MAPK signaling in epidermal keratinocytes. This project will provide insight into the functions of a novel desmosome-COP9 signaling complex and its role in epidermal differentiation. Understanding how desmosomes integrate mechanical and intracellular signaling will help identify new biological pathways as potential targets for therapeutics to treat the severe skin disorders associated with desmosomal impairment.

Public Health Relevance

The desmosome is a protein complex that is essential for adhering skin cells together. Mutations in genes that code for desmosome proteins cause debilitating skin disorders, but how desmosomes control communication pathways within the cell is poorly understood. This project will investigate the structure and function of a novel interaction between the desmosome and the COP9 signalosome (a complex that degrades proteins), which presents new opportunities to develop innovative therapeutics to treat desmosomal skin disorders.