A Central Role for the Armadillo Protein Plakoglobin in the Autoimmune Disease Pemphigus Vulgaris

Abstract

In pemphigus vulgaris (PV), autoantibody binding to desmoglein (Dsg) 3 induces loss of intercellular adhesion in skin and mucous membranes. Two hypotheses are currently favored to explain the underlying molecular mechanisms: (a) disruption of adhesion through steric hindrance, and (b) interference of desmosomal cadherin-bound antibody with intracellular events, which we speculated to involve plakoglobin. To investigate the second hypothesis we established keratinocyte cultures from plakoglobin knockout (PG−/−) embryos and PG+/+ control mice. Although both cell types exhibited desmosomal cadherin-mediated adhesion during calcium-induced differentiation and bound PV immunoglobin (IgG) at their cell surface, only PG+/+ keratinocytes responded with keratin retraction and loss of adhesion. When full-length plakoglobin was reintroduced into PG−/− cells, responsiveness to PV IgG was restored. Moreover, in these cells like in PG+/+ keratinocytes, PV IgG binding severely affected the linear distribution of plakoglobin at the plasma membrane. Taken together, the establishment of an in vitro model using PG+/+ and PG−/− keratinocytes allowed us (a) to exclude the steric hindrance only hypothesis, and (b) to demonstrate for the first time that plakoglobin plays a central role in PV, a finding that will provide a novel direction for investigations of the molecular mechanisms leading to PV, and on the function of plakoglobin in differentiating keratinocytes.

Desmosome Signaling

“…Keratinocytes from plakoglobin knock-out mice form desmosome-like structures that appear to mediate cell-cell adhesion in monolayer cultures but are resistant to PV IgG-mediated keratin filament retraction (11). This observation suggests that mechanisms in addition to steric hindrance may be required for PV IgG to mediate the loss of keratinocyte cell-cell adhesion and provides additional support for the hypothesis that signaling plays a role.…”

“…Proposed mechanisms for PV IgG-induced keratinocyte detachment include (a) proteinase activation, (b) steric hindrance, and (c) activation of transmembrane signaling that down-regulates cell-cell adhesion (7)(8)(9)(10)(11). Previous work has suggested that PV IgG may activate intracellular signaling events; however, the precise nature and biological consequences of these events and their relationship to the mechanism of PV IgG-induced acantholysis remain unknown (7,8).…”

Plakoglobin suppresses keratinocyte motility through both cell–cell adhesion-dependent and -independent mechanisms

“…The average areas were compared by using Student's t test. Previous studies (5) demonstrated that loss of PG expression caused severe skin fragility in PG knockout mice and decreased adhesion in keratinocytes established from the skin of these mice (8). Despite the lack of PG in PGϪ͞Ϫ cells, the protein level and cellular localization of the closely related ␤-catenin were comparable in keratinocyte cultures established from the skin of PG knockout mice (PGϪ͞Ϫ) and heterozygote control mice (PGϩ͞Ϫ) (Fig.…”

“…PG's critical role in maintaining tissue integrity is underscored by skin and heart dysfunction in PG-null mice and patients with pathogenic homozygous mutations (4)(5)(6)(7). In addition, keratinocyte cultures established from PG-null mice exhibit reduced adhesion in a mechanical dissociation assay (8).…”

Peptides Targeting the Desmoglein 3 Adhesive Interface Prevent Autoantibody-induced Acantholysis in Pemphigus

“…Dispase-based Keratinocyte Dissociation Assay-The assay was performed as described in the literature with the following modifications (17,18). HaCaT cells were seeded on 12-well plates and grown to confluence.…”