In the bullous autoimmune disease pemphigus vulgaris (PV), autoantibodies directed mainly against desmoglein 1 (Dsg1) and Dsg3 cause loss of desmosomal adhesion. We recently showed that intracellular cAMP increase by the phosphodiesterase 4 inhibitor apremilast was protective in different PV models. Thus, we here analyzed the involvement of the cAMP effector exchange factor directly activated by cAMP1 (Epac1). In Epac1-deficient mice pemphigus antibody-induced blistering was ameliorated in vivo while apremilast had no additional effect. Interestingly, augmented protein levels of Dsg1 and Dsg3 as well as increased Dsg1 mRNA levels and higher numbers of Dsg1- and Dsg3-dependent single-molecule interactions were detected in keratinocytes derived from Epac1-deficient mice. This was paralleled by stronger intercellular adhesion under baseline conditions and prevention of pemphigus autoantibody-induced loss of intercellular adhesion. However, the protective effect of apremilast against loss of intercellular adhesion in response to the pathogenic Dsg3 antibody AK23 was attenuated in Epac1-deficient keratinocytes. Similarly, the Epac1 inhibitor Esi09 protected keratinocytes from pemphigus antibody-induced loss of adhesion. Mechanistically, Epac1 deficiency resulted in lack of apremilast-induced Rap1 activation and phosphorylation of Pg at S665. Taken together, these data indicate that Epac1 is involved in the regulation of baseline and cAMP-mediated stabilization of keratinocyte adhesion.

In pemphigus, autoantibodies against the desmosomal cadherins desmoglein (DSG) DSG1 and DSG3 cause intraepidermal blistering. Recently, we found that increasing cAMP with the phosphodiesterase-4 inhibitor apremilast stabilizes keratinocyte cohesion in pemphigus. This effect is paralleled by phosphorylation of the desmosomal plaque protein plakoglobin (PG) at serine 665 (S665). Here, we investigated the relevance of PG phosphorylation at S665 for stabilization of keratinocyte cohesion and further characterized the underlying mechanisms. Ultrastructural analysis of a recently established PG-S665 phospho-deficient mouse model (PG-S665A) showed diminished keratin insertion. Accordingly, the protective effect of apremilast against pemphigus autoantibody-induced skin blistering was diminished, and apremilast failed to restore alterations of the keratin cytoskeleton in PG-S665A mice. Keratinocytes derived from PG-S665A mice revealed a disorganized keratin cytoskeleton and reduced single-molecule binding strength of DSG3. In line with this, in ex vivo human skin, increased cAMP augmented keratin insertion into desmosomal plaques. Additionally, PG phosphorylated at S665 colocalized with desmoplakin and keratin filaments anchoring to desmosomes and increased cAMP-accelerated assembly of desmosomes. Taken together, phosphorylation of PG at S665 was crucial for protective effects of apremilast in pemphigus and for maintenance of DSG3 binding and keratin filament anchorage to desmosomes.