Binding of the periplakin linker requires vimentin acidic residues D176 and E187

Missense Mutations in Desmoplakin Plakin Repeat Domains Have Dramatic Effects on Domain Structure and Function

Plakin repeat domains (PRDs) are globular modules that mediate the interaction of plakin proteins with the intermediate filament (IF) cytoskeleton. These associations are vital for maintaining tissue integrity in cardiac muscle and epithelial tissues. PRDs are subject to mutations that give rise to cardiomyopathies such as arrhythmogenic right ventricular cardiomyopathy, characterised by ventricular arrhythmias and associated with an increased risk of sudden heart failure, and skin blistering diseases. Herein, we have examined the functional and structural effects of 12 disease-linked missense mutations, identified from the human gene mutation database, on the PRDs of the desmosomal protein desmoplakin. Five mutations (G2056R and E2193K in PRD-A, G2338R and G2375R in PRD-B and G2647D in PRD-C) rendered their respective PRD proteins either fully or partially insoluble following expression in bacterial cells. Each of the residues affected are conserved across plakin family members, inferring a crucial role in maintaining the structural integrity of the PRD. In transfected HeLa cells, the mutation G2375R adversely affected the targeting of a desmoplakin C-terminal construct containing all three PRDs to vimentin IFs. The deletion of PRD-B and PRD-C from the construct compromised its targeting to vimentin. Bioinformatic and structural modelling approaches provided multiple mechanisms by which the disease-causing mutations could potentially destabilise PRD structure and compromise cytoskeletal linkages. Overall, our data highlight potential molecular mechanisms underlying pathogenic missense mutations and could pave the way for informing novel curative interventions targeting cardiomyopathies and skin blistering disorders.

State-of-the-art review of human autoimmune blistering diseases (AIBD)

Autoimmune blistering diseases (AIBDs) are a heterogenous group of skin conditions, broadly classified into two categories depending on the location of blister formation: intraepidermal blistering in the pemphigus group and subepidermal blistering in the pemphigoid group. Although AIBDs occur in both humans and animals, the arsenal of data for human AIBDs far exceeds those of their animal counterpart. Therefore, the main purpose of this review is to highlight existing knowledge, and recent advances in the diagnosis and management of AIBDs in humans - to serve as a road map for veterinary dermatologists. AREAS COVERED: Recent findings include complement-independent pathways in the pathogenesis of bullous pemphigoid, as well as the role of desmoglein and desmocollin autoantibodies in inducing acantholysis. Systemic glucocorticoids are the mainstay of treatment for AIBDs in humans, yet their long-term use is associated with severe adverse effects and complications, thereby limiting their use. Therefore, researchers have been exploring new and safer alternative therapeutic options for human AIBDs such as anti-CD20 monoclonal antibodies (Rituximab), Bruton's tyrosine kinase inhibitors (BTKi) and neonatal Fc receptor (FcRn) blockers. EXPERT OPINION: Randomised controlled trial (RCT) level evidence show that Rituximab and short-course GC regimes are more effective and safer than traditional GC treatment for human AIBDs. FcRn blockers such as SYNT001 have shown positive results in preliminary phase 2 clinical trials for treatment of human pemphigus; further trials are required. Rilzabrutinib (PRN1008), an orally administered BTKi, has recently completed phase 2 trials in pemphigus and is in a phase 3 RCT in humans.

Desmosomal protein structure and function and the impact of disease-causing mutations

In this graphical review we focus on the structural characteristics of desmosomal proteins, their interactions with each other and with the intermediate filament cytoskeleton. The wealth of structural information that is now available allows predictions to be made about the pathogenic effect of disease-causing mutations. We have selected representative examples of missense mutations that are buried, semi-buried or surface exposed, and demonstrate how such variants could affect the structural fold of desmosomal proteins that are expressed in the heart. We explain how such alterations could compromise desmosomal adhesion, resulting in life threatening diseases including arrhythmogenic right ventricular cardiomyopathy.

Recent insight into intermediate filament structure

Intermediate filaments (IFs) are key players in multiple cellular processes throughout human tissues. Their biochemical and structural properties are important for understanding filament assembly mechanisms, for interactions between IFs and binding partners, and for developing pharmacological agents that target IFs. IF proteins share a conserved coiled-coil central-rod domain flanked by variable N-terminal 'head' and C-terminal 'tail' domains. There have been several recent advances in our understanding of IF structure from the study of keratins, glial fibrillary acidic protein, and lamin. These include discoveries of (i) a knob-pocket tetramer assembly mechanism in coil 1B; (ii) a lamin-specific coil 1B insert providing a one-half superhelix turn; (iii) helical, yet flexible, linkers within the rod domain; and (iv) the identification of coil 2B residues required for mature filament assembly. Furthermore, the head and tail domains of some IFs contain low-complexity aromatic-rich kinked segments, and structures of IFs with binding partners show electrostatic surfaces are a major contributor to complex formation. These new data advance the connection between IF structure, pathologic mutations, and clinical diseases in humans.

Molecular mechanism of intermediate filament recognition by plakin proteins

The plakin family of cytolinkers interacts with intermediate filaments (IFs) through plakin repeat domain (PRD) and linker modules. Recent structure/function studies have established the molecular basis of envoplakin-PRD and periplakin-linker interactions with vimentin. Both plakin modules share a broad basic groove which recognizes acidic rod elements on IFs, a mechanism that is applicable to other plakin family members. This review postulates a universal IF engagement mechanism that illuminates the specific effects of pathogenic mutations associated with diseases including arrhythmogenic right ventricular cardiomyopathy, and reveals how diverse plakin proteins offer tailored IF tethering to ensure stable, dynamic and regulated cellular structures.