p63 control of desmosome gene expression and adhesion is compromised in AEC syndrome

Ankyloblepharon, ectodermal defects, cleft lip/palate (AEC) syndrome is a rare autosomal dominant disorder caused by mutations in the p63 gene, essential for embryonic development of stratified epithelia. The most severe cutaneous manifestation of this disorder is the long-lasting skin fragility associated with severe skin erosions after birth. Using a knock-in mouse model for AEC syndrome, we found that skin fragility was associated with microscopic blistering between the basal and suprabasal compartments of the epidermis and reduced desmosomal contacts. Expression of desmosomal cadherins and desmoplakin was strongly reduced in AEC mutant keratinocytes and in newborn epidermis. A similar impairment in desmosome gene expression was observed in human keratinocytes isolated from AEC patients, in p63-depleted keratinocytes and in p63 null embryonic skin, indicating that p63 mutations causative of AEC syndrome have a dominant-negative effect on the wild-type p63 protein. Among the desmosomal components, desmocollin 3, desmoplakin and desmoglein 1 were the most significantly reduced by mutant p63 both at the RNA and protein levels. Chromatin immunoprecipitation experiments and transactivation assays revealed that p63 controls these genes at the transcriptional level. Consistent with reduced desmosome function, AEC mutant and p63-deficient keratinocytes had an impaired ability to withstand mechanical stress, which was alleviated by epidermal growth factor receptor inhibitors known to stabilize desmosomes. Our study reveals that p63 is a crucial regulator of a subset of desmosomal genes and that this function is impaired in AEC syndrome. Reduced mechanical strength resulting from p63 mutations can be alleviated pharmacologically by increasing desmosome adhesion with possible therapeutic implications.

Beyond cell adhesion: the role of armadillo proteins in the heart

Plakoglobin (PG, γ-Catenin, JUP), a member of the armadillo protein family and close homolog of β-catenin, functions to link cell surface cadherin molecules with the cytoskeleton. PG is the only junctional component found in both desmosomes and adherens junctions and thus plays a critical role in the regulation of cell-cell adhesion. Similar to β-catenin, PG is able to interact with components of the Wnt signaling pathway and directly affect gene expression by binding with LEF/TCF transcription factors. In addition, it has been proposed that PG functions primarily as a competitive inhibitor of β-catenin transcriptional activity by sequestering LEF/TCF. Compared to β-catenin, the contribution of PG as a transcriptional regulator in either physiological or pathological conditions is poorly understood. There is increasing clinical interest in PG as both a structural protein as well as a signaling molecule as mutations have been identified in the human PG gene that cause Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC) and cutaneous syndromes. This review will discuss the connection between altered cell adhesion and gene expression and its contribution to disease pathogenesis.

Desmosomal genodermatoses

Desmosomes are intercellular junctions that contribute to cell-cell adhesion, signalling, development and differentiation in various tissues, including the skin. Composed of a network of transmembranous and intracellular plaque proteins, pathogenic autosomal dominant or recessive mutations have been reported in 10 different desmosomal genes, resulting in a spectrum of phenotypes variably affecting skin, hair and heart. This review summarizes the molecular pathology and phenotypes that predominantly affect the skin/hair. Recent desmosomal genodermatoses described include lethal congenital epidermolysis bullosa (plakoglobin), cardiomyopathy with alopecia and palmoplantar keratoderma (plakoglobin), hypotrichosis with scalp vesicles (desmocollin 3), and generalized peeling skin disease (corneodesmosin). Understanding the range of clinical phenotypes in combination with knowledge of the inherent desmosome gene mutation(s) is helpful in managing and counselling patients, as well as providing insight into the biological function of specific components of desmosomes in skin and other tissues.

Lack of plakoglobin in epidermis leads to keratoderma

Loss-of-function mutation of Jup has been associated with Naxos disease, which is characterized by arrhythmogenic cardiomyopathy and the cutaneous disorder palmoplantar keratoderma. Previously, we have shown that genetic ablation of Jup in cardiomyocytes in mice leads to arrhythmogenic cardiomyopathy similar to Naxos disease in humans. Currently, to determine the pathogenesis of Naxos disease-associated keratoderma, we generated Jup mutant mice by inactivating Jup restrictively in keratinocytes. Jup mutant mice largely recapitulated the clinical features of human palmoplantar keratoderma: overcornification and thickening of the epidermis. Jup mutant mice also suffered skin ulceration and inflammation. Cell apoptosis and proliferation were significantly elevated in Jup mutant epidermis. Ultrastructural analyses revealed the disruption of the assembly of desmosomes and adherens junctions in Jup mutant epidermis. We also demonstrated the compensational increase in β-catenin at Jup mutant cell-cell junctions without altering its signaling activities. Our findings provide important insights for understanding the pathogenesis of human palmoplantar keratoderma.

Cell-cell connectivity: desmosomes and disease

Cell-cell connectivity is an absolute requirement for the correct functioning of cells, tissues and entire organisms. At the level of the individual cell, direct cell-cell adherence and communication is mediated by the intercellular junction complexes: desmosomes, adherens, tight and gap junctions. A broad spectrum of inherited, infectious and auto-immune diseases can affect the proper function of intercellular junctions and result in either diseases affecting specific individual tissues or widespread syndromic conditions. A particularly diverse group of diseases result from direct or indirect disruption of desmosomes--a consequence of their importance in tissue integrity, their extensive distribution, complex structure, and the wide variety of functions their components accomplish. As a consequence, disruption of desmosomal assembly, structure or integrity disrupts not only their intercellular adhesive function but also their functions in cell communication and regulation, leading to such diverse pathologies as cardiomyopathy, epidermal and mucosal blistering, palmoplantar keratoderma, woolly hair, keratosis, epidermolysis bullosa, ectodermal dysplasia and alopecia. Here, as well as describing the importance of the other intercellular junctions, we focus primarily on the desmosome, its structure and its role in disease. We will examine the various pathologies that result from impairment of desmosome function and thereby demonstrate the importance of desmosomes to tissues and to the organism as a whole.