Mechanistic basis of desmosome-targeted diseases

Abstract

Desmosomes are dynamic junctions between cells that maintain the structural integrity of skin and heart tissues by withstanding shear forces. Mutations in component genes cause life-threatening conditions including arrhythmogenic right ventricular cardiomyopathy, and desmosomal proteins are targeted by pathogenic autoantibodies in skin blistering diseases such as pemphigus. Here, we review a set of newly discovered pathogenic alterations and discuss the structural repercussions of debilitating mutations on desmosomal proteins. The architectures of native desmosomal assemblies have been visualized by cryo-electron microscopy and cryo-electron tomography, and the network of protein domain interactions is becoming apparent. Plakophilin and desmoplakin mutations have been discovered to alter binding interfaces, structures, and stabilities of folded domains that have been resolved by X-ray crystallography and NMR spectroscopy. The flexibility within desmoplakin has been revealed by small-angle X-ray scattering and fluorescence assays, explaining how mechanical stresses are accommodated. These studies have shown that the structural and functional consequences of desmosomal mutations can now begin to be understood at multiple levels of spatial and temporal resolution. This review discusses the recent structural insights and raises the possibility of using modeling for mechanism-based diagnosis of how deleterious mutations alter the integrity of solid tissues.

Keywords: ARVC; DIFC; EC; EGFR; IA; ICS; Lef; NMD; PPAR; PRD; SH3; SR; Src homology 3; T-cell factor; Tcf; arrhythmogenic right ventricular cardiomyopathy; desmoplakin; desmosomal cadherin; desmosome; desmosome–intermediate filament complex; epidermal growth factor receptor; extracellular cadherin; iPSC; induced pluripotent stem cell; intracellular anchor; intracellular cadherin-typical sequence; lymphoid enhancer factor; nonsense-mediated RNA decay; peroxisome proliferator-activated receptor; plakin repeat domain; plakoglobin; spectrin repeat.

Graphical abstract

Fig. 1

Architecture of the cardiac desmosome. The approximate locations of the core proteins are shown, including the structures of DSG2's EC1 domain (green ribbon) and the arm repeat domains of plakoglobin (blue) and PKP2 (purple). Also shown are crystal structures of the first four SRs of the desmoplakin plakin domain (SR3–6) and PRDs B and C. The unstructured DSC2 and DSG2 C-termini are shown as wavy lines, as is the protease-sensitive hinge between the long and short arms of the desmoplakin plakin domain . The N-termini of the proteins are labeled and their respective binding sites are juxtaposed. Both homophilic and heterophilic interactions between desmosomal cadherins may take place in the extracellular space, but for simplicity, only homophilic interactions are shown.

Fig. 2

Desmosomal cadherin structures. The DSG2 EC1 domain solution structure [Protein Data Bank (PDB) code: 2YQG] is shown as a ribbon, as is a homology-derived model of DSC2's EC1 (based on the crystal structure of the corresponding region of the mouse N-cadherin extracellular domain) (PDB code: 3Q2W). Point mutations that are known to be pathogenic are shown as copper (buried residues) or green (surface-exposed residues). Alignment of the EC1 sequences of DSG2, DSC2, and mouse N-cadherin indicates that mutated residues are all highly conserved, supporting their critical functional or structural roles.

Fig. 3

Plakoglobin structure and locations of ARVC mutations. Crystal structure of the plakoglobin arm domain (PDB code: 3IFQ) (left) and electrostatic potential of surface-exposed residues (right). In the ribbon diagram, point mutations that are known to be pathogenic are shown as either copper (buried) or green (surface exposed). Although plakoglobin exhibits comparatively fewer pathogenic mutations than other desmosomal proteins, they are balanced between exposed and buried positions across the domain, with consequences on protein stability and binding properties that require further examination. In the electrostatic potential map, blue and red represent positively and negatively charged regions, respectively. The surface shows the charged E-cadherin binding groove, which is also proposed to bind to desmoglein and desmocollin . Amino acids proposed to mediate desmoglein and desmocollin binding are highlighted in purple .

Fig. 4

Plakophilin 2 structure and locations of ARVC mutations. A sequence homology-based model of the entire PKP2 arm domain is shown based on a fragment of the PKP2 arm domain (PDB code: 3TT9) and the PKP1 arm repeat domain (PDB code 1XM9) as calculated by the PHYRE server . Point mutations that are known to be pathogenic are shown as either copper (buried) or green (surface exposed). PKP2 exhibits the most numerous and diverse pathogenic mutations found in any desmosomal protein. This includes a mutation “hotspot” involving residues N613 and S615 as well as the newly identified L611R and L614P mutations that are linked to ARVC and are clustered within the hydrophobic core of the PKP2 arm domain.

Fig. 5

Desmoplakin structures and locations of ARVC mutations. Crystal structures of desmoplakin (a) SRs 3-6 (PDB code: 3R6N), (b) PRD-B (PDB code: 1LM7), and (c) PRD-C (PDB code: 1LM5) are depicted as ribbon diagrams. Point mutations that are known to be pathogenic are shown as either copper (buried) or green (surface exposed). For PRDs B and C, electrostatic potential maps are also shown with blue and red representing positively and negatively charged groups, respectively. Both PRDs B and C possess a conserved basic groove, which it is speculated may represent intermediate filament binding sites . Amino acid residues that are proposed to interact with vimentin are indicated in purple. Also depicted are the newly identified and as of yet unpublished mutations of R222L and H618P.