Targeted proteolysis of plectin isoform 1a accounts for hemidesmosome dysfunction in mice mimicking the dominant skin blistering disease EBS-Ogna

Mechanics of cell sheets: plectin as an integrator of cytoskeletal networks

Epithelia are multicellular sheets that form barriers defining the internal and external environments. The constant stresses acting at this interface require that epithelial sheets are mechanically robust and provide a selective barrier to the hostile exterior. These properties are mediated by cellular junctions which are physically linked with heavily crosslinked cytoskeletal networks. Such hardwiring is facilitated by plakins, a family of giant modular proteins which serve as 'molecular bridges' between different cytoskeletal filaments and multiprotein adhesion complexes. Dysfunction of cytoskeletal crosslinking compromises epithelial biomechanics and structural integrity. Subsequent loss of barrier function leads to disturbed tissue homeostasis and pathological consequences such as skin blistering or intestinal inflammation. In this article, we highlight the importance of the cytolinker protein plectin for the functional organization of epithelial cytoskeletal networks. In particular, we focus on the ability of plectin to act as an integrator of the epithelial cytoarchitecture that defines the biomechanics of the whole tissue. Finally, we also discuss the role of cytoskeletal crosslinking in emerging aspects of epithelial mechanobiology that are critical for the maintenance of epithelial homeostasis.

Role of plectin and its interacting molecules in cancer

Plectin, as the cytolinker and scaffolding protein, are widely expressed and abundant in many tissues, and has involved in various cellular activities contributing to tumorigenesis, such as cell adhesion, migration, and signal transduction. Due to the specific expression and differential localization of plectin in cancer, most researchers focus on the role of plectin in cancer, and it has emerged as a potent driver of malignant hallmarks in many human cancers, which provides the possibility for plectin to be widely used as a biomarker and therapeutic target in the early diagnosis and targeted drug delivery of the disease. However, there is still a lack of systematic review on the interaction molecules and mechanism of plectin. Herein, we summarized the structure, expression and function of plectin, and mainly focused on recent studies on the functional and physical interactions between plectin and its interacting molecules, shedding light on the potential of targeting plectin for cancer therapy.

The Versatility of Plectin in Cancer: A Pan-Cancer Analysis on Potential Diagnostic and Prognostic Impacts of Plectin Isoforms

Plectin, encoded by PLEC, is a cytoskeletal and scaffold protein with a number of unique isoforms that act on various cellular functions such as cell adhesion, signal transduction, cancer cell invasion, and migration. While plectin has been shown to display high expression and mislocalization in tumor cells, our knowledge of the biological significance of plectin and its isoforms in tumorigenesis remain limited. In this study, we first performed pathway enrichment analysis to identify cancer hallmark proteins associated with plectin. Then, a pan-cancer analysis was performed using RNA-seq data collected from the Cancer Genome Atlas (TCGA) to detect the mRNA expression levels of PLEC and its transcript isoforms, and the prognostic as well as diagnostic significance of the transcript isoforms was evaluated considering cancer stages. We show here that several tissue specific PLEC isoforms are dysregulated in different cancer types and stages but not the expression of PLEC. Among them, PLEC 1d and PLEC 1f are potential biomarker candidates and call for further translational and personalized medicine research. This study makes a contribution as a stride to unravel the molecular mechanisms underpinning plectin isoforms in cancer development and progression by revealing the potent plectin isoforms in different stages of cancer as potential early cancer detection biomarkers. Importantly, uncovering how plectin isoforms guide malignancy and particular cancer types by comprehensive functional studies might open new avenues toward novel cancer therapeutics.

Mutation update: The spectra of PLEC sequence variants and related plectinopathies

Plectin, encoded by PLEC, is a cytoskeletal linker of intermediate filaments expressed in many cell types. Plectin consists of three main domains that determine its functionality: the N-terminal domain, the Rod domain, and the C-terminal domain. Molecular defects of PLEC correlating with the functional aspects lead to a group of rare heritable disorders, plectinopathies. These multisystem disorders include an autosomal dominant form of epidermolysis bullosa simplex (EBS-Ogna), limb-girdle muscular dystrophy (LGMD), aplasia cutis congenita (ACC), and an autosomal recessive form of EBS, which may associate with muscular dystrophy (EBS-MD), pyloric atresia (EBS-PA), and/or congenital myasthenic syndrome (EBS-MyS). In this study, genotyping of over 600 Iranian patients with epidermolysis bullosa by next-generation sequencing identified 15 patients with disease-causing PLEC variants. This mutation update analyzes the clinical spectrum of PLEC in our cohort and in the literature and demonstrates the relationship between PLEC genotype and phenotypic manifestations. This study has integrated our seven novel PLEC variants and phenotypic findings with previously published data totaling 116 variants to provide the most complete overview of pathogenic PLEC variants and related disorders.

Plectin-mediated cytoskeletal crosstalk controls cell tension and cohesion in epithelial sheets

The coordinated interplay of cytoskeletal networks critically determines tissue biomechanics and structural integrity. Here, we show that plectin, a major intermediate filament-based cytolinker protein, orchestrates cortical cytoskeletal networks in epithelial sheets to support intercellular junctions. By combining CRISPR/Cas9-based gene editing and pharmacological inhibition, we demonstrate that in an F-actin-dependent context, plectin is essential for the formation of the circumferential keratin rim, organization of radial keratin spokes, and desmosomal patterning. In the absence of plectin-mediated cytoskeletal cross-linking, the aberrant keratin-desmosome (DSM)-network feeds back to the actin cytoskeleton, which results in elevated actomyosin contractility. Also, by complementing a predictive mechanical model with Förster resonance energy transfer-based tension sensors, we provide evidence that in the absence of cytoskeletal cross-linking, major intercellular junctions (adherens junctions and DSMs) are under intrinsically generated tensile stress. Defective cytoarchitecture and tensional disequilibrium result in reduced intercellular cohesion, associated with general destabilization of plectin-deficient sheets upon mechanical stress.

Keratins as an Inflammation Trigger Point in Epidermolysis Bullosa Simplex

Epidermolysis bullosa simplex (EBS) is a group of inherited keratinopathies that, in most cases, arise due to mutations in keratins and lead to intraepidermal ruptures. The cellular pathology of most EBS subtypes is associated with the fragility of the intermediate filament network, cytolysis of the basal layer of the epidermis, or attenuation of hemidesmosomal/desmosomal components. Mutations in keratins 5/14 or in other genes that encode associated proteins induce structural disarrangements of different strengths depending on their locations in the genes. Keratin aggregates display impaired dynamics of assembly and diminished solubility and appear to be the trigger for endoplasmic reticulum (ER) stress upon being phosphorylated by MAPKs. Global changes in cellular signaling mainly occur in cases of severe dominant EBS mutations. The spectrum of changes initiated by phosphorylation includes the inhibition of proteasome degradation, TNF-α signaling activation, deregulated proliferation, abnormal cell migration, and impaired adherence of keratinocytes. ER stress also leads to the release of proinflammatory danger-associated molecular pattern (DAMP) molecules, which enhance avalanche-like inflammation. Many instances of positive feedback in the course of cellular stress and the development of sterile inflammation led to systemic chronic inflammation in EBS. This highlights the role of keratin in the maintenance of epidermal and immune homeostasis.

Plectin in Skin Fragility Disorders

Plectin is a multi-faceted, 500 kDa-large protein, which due to its expression in different isoforms and distinct organs acts diversely as a cytoskeletal crosslinker and signaling scaffold. It functions as a mediator of keratinocyte mechanical stability in the skin, primarily through linking intermediate filaments to hemidesmosomes. Skin fragility may occur through the presence of mutations in the gene encoding for plectin, PLEC, or through the presence of autoantibodies against the molecule. Below, we review the cutaneous manifestations of plectinopathies as well as their systemic involvement in specific disease subtypes. We summarize the known roles of plectin in keratinocytes and fibroblasts and provide an outlook on future perspectives for plectin-associated skin disorders.

Identifying Plectin Isoform Functions through Animal Models

Plectin, a high-molecular-weight cytoskeletal linker protein, binds with high affinity to intermediate filaments of all types and connects them to junctional complexes, organelles, and inner membrane systems. In addition, it interacts with actomyosin structures and microtubules. As a multifunctional protein, plectin has been implicated in several multisystemic diseases, the most common of which is epidermolysis bullosa simplex with muscular dystrophy (EBS-MD). A great part of our knowledge about plectin’s functional diversity has been gained through the analysis of a unique collection of transgenic mice that includes a full (null) knockout (KO), several tissue-restricted and isoform-specific KOs, three double KOs, and two knock-in lines. The key molecular features and pathological phenotypes of these mice will be discussed in this review. In summary, the analysis of the different genetic models indicated that a functional plectin is required for the proper function of striated and simple epithelia, cardiac and skeletal muscle, the neuromuscular junction, and the vascular endothelium, recapitulating the symptoms of humans carrying plectin mutations. The plectin-null line showed severe skin and muscle phenotypes reflecting the importance of plectin for hemidesmosome and sarcomere integrity; whereas the ablation of individual isoforms caused a specific phenotype in myofibers, basal keratinocytes, or neurons. Tissue-restricted ablation of plectin rendered the targeted cells less resilient to mechanical stress. Studies based on animal models other than the mouse, such as zebrafish and C. elegans, will be discussed as well.

Regulation of hemidesmosome dynamics and cell signaling by integrin α6β4

Hemidesmosomes (HDs) are specialized multiprotein complexes that connect the keratin cytoskeleton of epithelial cells to the extracellular matrix (ECM). In the skin, these complexes provide stable adhesion of basal keratinocytes to the underlying basement membrane. Integrin α6β4 is a receptor for laminins and plays a vital role in mediating cell adhesion by initiating the assembly of HDs. In addition, α6β4 has been implicated in signal transduction events that regulate diverse cellular processes, including proliferation and survival. In this Review, we detail the role of α6β4 in HD assembly and beyond, and we discuss the molecular mechanisms that regulate its function.

Plectin-Mediated Intermediate Filament Functions: Why Isoforms Matter

This essay focuses on the role of plectin and its various isoforms in mediating intermediate filament (IF) network functions. It is based on previous studies that provided comprehensive evidence for a concept where plectin acts as an IF recruiter, and plectin-mediated IF networking and anchoring are key elements in IF function execution. Here, plectin’s global role as modulator of IF functionality is viewed from different perspectives, including the mechanical stabilization of IF networks and their docking platforms, contribution to cellular viscoelasticity and mechanotransduction, compartmentalization and control of the actomyosin machinery, connections to the microtubule system, and mechanisms and specificity of isoform targeting. Arguments for IF networks and plectin acting as mutually dependent partners are also given. Lastly, a working model is presented that describes a unifying mechanism underlying how plectin–IF networks mechanically control and propagate actomyosin-generated forces, affect microtubule dynamics, and contribute to mechanotransduction.

Plectin ensures intestinal epithelial integrity and protects colon against colitis

Plectin, a highly versatile cytolinker protein, provides tissues with mechanical stability through the integration of intermediate filaments (IFs) with cell junctions. Here, we hypothesize that plectin-controlled cytoarchitecture is a critical determinant of the intestinal barrier function and homeostasis. Mice lacking plectin in an intestinal epithelial cell (IEC; Ple^ΔIEC) spontaneously developed colitis characterized by extensive detachment of IECs from the basement membrane (BM), increased intestinal permeability, and inflammatory lesions. Moreover, plectin expression was reduced in the colons of ulcerative colitis (UC) patients and negatively correlated with the severity of colitis. Mechanistically, plectin deficiency in IECs led to aberrant keratin filament (KF) network organization and the formation of dysfunctional hemidesmosomes (HDs) and intercellular junctions. In addition, the hemidesmosomal α6β4 integrin (Itg) receptor showed attenuated association with KFs, and protein profiling revealed prominent downregulation of junctional constituents. Consistent with the effects of plectin loss in the intestinal epithelium, plectin-deficient IECs exhibited remarkably reduced mechanical stability and limited adhesion capacity in vitro. Feeding mice with a low-residue liquid diet that reduced mechanical stress and antibiotic treatment successfully mitigated epithelial damage in the Ple^ΔIEC colon.

Vimentin Intermediate Filaments as Potential Target for Cancer Treatment

Intermediate filaments constitute the third component of the cellular skeleton. Unlike actin and microtubule cytoskeletons, the intermediate filaments are composed of a wide variety of structurally related proteins showing distinct expression patterns in tissues and cell types. Changes in the expression patterns of intermediate filaments are often associated with cancer progression; in particular with phenotypes leading to increased cellular migration and invasion. In this review we will describe the role of vimentin intermediate filaments in cancer cell migration, cell adhesion structures, and metastasis formation. The potential for targeting vimentin in cancer treatment and the development of drugs targeting vimentin will be reviewed.

Expression of Pork Plectin during Postmortem Aging

The current study investigated the distribution and degradation of pork plectin during postmortem aging. Longissimus thoracis (LT) muscles from 12 pig carcasses were vacuum-packaged and aged at 4 °C for 0 h, 6 h, 12 h, 1 day, 3 days, 7 days, and 13 days. Immunofluorescence analysis showed that pork plectin was distributed in a honeycomb-like pattern in the cross section and a regularly striated pattern in the longitudinal section. However, plectin was found preferentially expressed in fibers that were stained with high anti-fast-MyHC. Double immunostaining revealed the colocalization of plectin and desmin in the cytoplasm and beneath the sarcolemma. Western blot analysis showed that the amount of intact plectin was rapidly reduced during the early postmortem aging (P < 0.05) and almost disappeared at day 3. The degraded 240 kDa plectin accumulated fast and was further cleaved after 3 days of aging (P < 0.05). The plectin degradation could be significantly blocked by calpain inhibitor MDL-28170 rather than caspase-3 inhibitor Ac-DEVD-CHO (P < 0.05). Double immunostaining of μ-calpain and plectin showed a large amount of overlap at 0 h and 3 days of postmortem. Accordingly, these findings showed that plectin was preferentially expressed in fast muscle fiber and regularly distributed along with desmin at the strategic cellular sites. Plectin suffered a prominent and prompt degradation during postmortem aging, which might be attributed to μ-calpain.

No major role for rare plectin variants in arrhythmogenic right ventricular cardiomyopathy

Aims

Likely pathogenic/pathogenic variants in genes encoding desmosomal proteins play an important role in the pathophysiology of arrhythmogenic right ventricular cardiomyopathy (ARVC). However, for a substantial proportion of ARVC patients, the genetic substrate remains unknown. We hypothesized that plectin, a cytolinker protein encoded by the PLEC gene, could play a role in ARVC because it has been proposed to link the desmosomal protein desmoplakin to the cytoskeleton and therefore has a potential function in the desmosomal structure.

Methods

We screened PLEC in 359 ARVC patients and compared the frequency of rare coding PLEC variants (minor allele frequency [MAF] <0.001) between patients and controls. To assess the frequency of rare variants in the control population, we evaluated the rare coding variants (MAF <0.001) found in the European cohort of the Exome Aggregation Database. We further evaluated plectin localization by immunofluorescence in a subset of patients with and without a PLEC variant.

Results

Forty ARVC patients carried one or more rare PLEC variants (11%, 40/359). However, rare variants also seem to occur frequently in the control population (18%, 4754/26197 individuals). Nor did we find a difference in the prevalence of rare PLEC variants in ARVC patients with or without a desmosomal likely pathogenic/pathogenic variant (14% versus 8%, respectively). However, immunofluorescence analysis did show decreased plectin junctional localization in myocardial tissue from 5 ARVC patients with PLEC variants.

Conclusions

Although PLEC has been hypothesized as a promising candidate gene for ARVC, our current study did not show an enrichment of rare PLEC variants in ARVC patients compared to controls and therefore does not support a major role for PLEC in this disorder. Although rare PLEC variants were associated with abnormal localization in cardiac tissue, the confluence of data does not support a role for plectin abnormalities in ARVC development.

Complexes of α6β4 integrin and vimentin act as signaling hubs to regulate epithelial cell migration

We find that clusters of β4 integrin, organized into distinct puncta, localize along vimentin filaments within lamellipodia at the cell edge of A549 cells, as assessed by interferometric photoactivated localization microscopy. Moreover, puncta and vimentin filaments exhibit a dynamic interplay in live cells, as viewed by structured-illumination microscopy, with β4 integrin puncta that associate with vimentin persisting for longer than those that do not. Interestingly, in A549 cells β4 integrin regulates vimentin cytoskeleton organization. When β4 integrin is knocked down there is a loss of vimentin filaments from lamellipodia. However, in these conditions, vimentin filaments instead concentrate around the nucleus. Although β4 integrin organization is unaffected in vimentin-deficient A549 cells, such cells move in a less-directed fashion and exhibit reduced Rac1 activity, mimicking the phenotype of β4 integrin-deficient A549 cells. Moreover, in vimentin-deficient cells, Rac1 fails to cluster at sites enriched in α6β4 integrin heterodimers. The aberrant motility of both β4 integrin and vimentin-deficient cells is rescued by expression of active Rac1, leading us to propose that complexes of β4 integrin and vimentin act as signaling hubs, regulating cell motility behavior.

Plectin controls biliary tree architecture and stability in cholestasis

BACKGROUND & AIMS

Plectin, a highly versatile cytolinker protein, controls intermediate filament cytoarchitecture and cellular stress response. In the present study, we investigate the role of plectin in the liver under basal conditions and in experimental cholestasis.

METHODS

We generated liver-specific plectin knockout (Ple^Δalb) mice and analyzed them using two cholestatic liver injury models: bile duct ligation (BDL) and 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) feeding. Primary hepatocytes and a cholangiocyte cell line were used to address the impact of plectin on keratin filament organization and stability in vitro.

RESULTS

Plectin deficiency in hepatocytes and biliary epithelial cells led to aberrant keratin filament network organization, biliary tree malformations, and collapse of bile ducts and ductules. Further, plectin ablation significantly aggravated biliary damage upon cholestatic challenge. Coincidently, we observed a significant expansion of A6-positive progenitor cells in Ple^Δalb livers. After BDL, plectin-deficient bile ducts were prominently dilated with more frequent ruptures corresponding to an increased number of bile infarcts. In addition, more abundant keratin aggregates indicated less stable keratin filaments in Ple^Δalb hepatocytes. A transmission electron microscopy analysis revealed a compromised tight junction formation in plectin-deficient biliary epithelial cells. In addition, protein profiling showed increased expression of the adherens junction protein E-Cadherin, and inefficient upregulation of the desmosomal protein desmoplakin in response to BDL. In vitro analyses revealed a higher susceptibility of plectin-deficient keratin networks to stress-induced collapse, paralleled by elevated activation of p38 MAP kinase.

CONCLUSION

Our study shows that by maintaining proper keratin network cytoarchitecture and biliary epithelial stability, plectin plays a critical role in protecting the liver from stress elicited by cholestasis.

LAY SUMMARY

Plectin is a cytolinker protein capable of interconnecting all three cytoskeletal filament systems and linking them to plasma membrane-bound junctional complexes. In liver, the plectin-controlled cytoskeleton mechanically stabilizes epithelial cells and provides them with the capacity to adapt to increased bile pressure under cholestasis.

Advances in understanding the molecular basis of skin fragility

Skin fragility refers to a large group of conditions in which the ability of the skin to provide protection against trivial mechanical trauma is diminished, resulting in the formation of blisters, erosions, wounds, or scars. Acquired and physiological skin fragility is common; genetic disorders are rare but give insight into the molecular mechanisms ensuring skin stability. The paradigm is represented by inherited epidermolysis bullosa. This review is focused on recent advances in understanding the molecular basis of genetic skin fragility, including emerging concepts, controversies, unanswered questions, and opinions of the author. In spite of the advanced knowledge on the genetic causes of skin fragility, the molecular pathology is still expanding. Open questions in understanding the molecular basis of genetic skin fragility are the following: what are the causes of phenotypes which remain genetically unsolved, and what are the molecular modifiers which might explain phenotypic differences among individuals with similar mutations?  New mutational mechanisms and new genes have recently been discovered and are briefly described here. Comprehensive next-generation sequencing-based genetic testing improved mutation detection and facilitated the identification of the genetic basis of unclear and new phenotypes. Characterization of the biochemical and cell biological consequences of the genetic variants is challenging and laborious but may represent the basis for personalized therapeutic approaches. Molecular modifiers of skin fragility have been uncovered in particular animal and genetic models but not in larger cohorts of patients. This scientific progress is the basis for revisions of the epidermolysis bullosa classification and for innovative therapeutic approaches designed for this intractable condition.

An Arf6- and caveolae-dependent pathway links hemidesmosome remodeling and mechanoresponse

Hemidesmosomes are epithelial-specific cell-matrix adhesions stably anchoring the intracellular keratin network to the extracellular matrix and providing mechanical resilience to epithelia. The small GTPase Arf6 and caveolae are essential for their remodeling, notably in response to external mechanical cues.

Hemidesmosomes (HDs) are epithelial-specific cell–matrix adhesions that stably anchor the intracellular keratin network to the extracellular matrix. Although their main role is to protect the epithelial sheet from external mechanical strain, how HDs respond to mechanical stress remains poorly understood. Here we identify a pathway essential for HD remodeling and outline its role with respect to α6β4 integrin recycling. We find that α6β4 integrin chains localize to the plasma membrane, caveolae, and ADP-ribosylation factor-6+ (Arf6+) endocytic compartments. Based on fluorescence recovery after photobleaching and endocytosis assays, integrin recycling between both sites requires the small GTPase Arf6 but neither caveolin1 (Cav1) nor Cavin1. Strikingly, when keratinocytes are stretched or hypo-osmotically shocked, α6β4 integrin accumulates at cell edges, whereas Cav1 disappears from it. This process, which is isotropic relative to the orientation of stretch, depends on Arf6, Cav1, and Cavin1. We propose that mechanically induced HD growth involves the isotropic flattening of caveolae (known for their mechanical buffering role) associated with integrin diffusion and turnover.

Structural proteins of the dermal-epidermal junction targeted by autoantibodies in pemphigoid diseases

The dermal-epidermal junction consists of a network of several interacting structural proteins that strengthen adhesion and mediate signalling events. This structural network consists of hemidesmosomal-anchoring filament complexes connecting the basal keratinocytes to the basement membrane. The anchoring filaments in turn interact with the anchoring fibrils to attach the basement membrane to the underlying dermis. Several of these structural proteins are recognized by autoantibodies in pemphigoid diseases, a heterogeneous group of clinically and immunopathologically diverse entities. Targeted proteins include the two intracellular plakins, plectin isoform 1a and BP230 (also called bullous pemphigoid antigen (BPAG) 1 isoform e (BPAG1e)). Plectin 1a and BP230 are connected to the intermediate filaments and to the cell surface receptor α6β4 integrin, which in turn is connected to laminin 332, a component of the anchoring filaments. Further essential adhesion proteins are BP180, a transmembrane protein, laminin γ1 and type VII collagen. Latter protein is the major constituent of the anchoring fibrils. Mutations in the corresponding genes of these adhesion molecules lead to inherited epidermolysis bullosa emphasizing the importance of these proteins for the integrity of the dermal-epidermal junction. This review will provide an overview on the structure and function of the proteins situated in the dermal-epidermal junction targeted by autoantibodies.

Vimentin-mediated regulation of cell motility through modulation of beta4 integrin protein levels in oral tumor derived cells

Vimentin expression correlates well with migratory and invasive potential of the carcinoma cells. The molecular mechanism by which vimentin regulates cell motility is not yet clear. Here, we addressed this issue by depleting vimentin in oral squamous cell carcinoma derived cell line. Vimentin knockdown cells showed enhanced adhesion and spreading to laminin-5. However, we found that they were less invasive as compared to the vector control cells. In addition, signaling associated with adhesion behavior of the cell was increased in vimentin knockdown clones. These findings suggest that the normal function of β4 integrin as mechanical adhesive device is enhanced upon vimentin downregulation. As a proof of principle, the compromised invasive potential of vimentin depleted cells could be rescued upon blocking with β4 integrin adhesion-blocking (ASC-8) antibody or downregulation of β4 integrin in vimentin knockdown background. Interestingly, plectin which associates with α6β4 integrin in the hemidesmosomes, was also found to be upregulated in vimentin knockdown clones. Furthermore, experiments on lysosome and proteasome inhibition revealed that perhaps vimentin regulates the turnover of β4 integrin and plectin. Moreover, an inverse association was observed between vimentin expression and β4 integrin in oral squamous cell carcinoma (OSCC). Collectively, our results show a novel role of vimentin in modulating cell motility by destabilizing β4 integrin-mediated adhesive interactions. Further, vimentin-β4 integrin together may prove to be useful markers for prognostication of human oral cancer.

The cytolinker plectin regulates nuclear mechanotransduction in keratinocytes

The transmission of mechanical forces to the nucleus is important for intracellular positioning, mitosis and cell motility, yet the contribution of specific components of the cytoskeleton to nuclear mechanotransduction remains unclear. In this study, we examine how crosstalk between the cytolinker plectin and F-actin controls keratin network organisation and the 3D nuclear morphology of keratinocytes. Using micro-patterned surfaces to precisely manipulate cell shape, we find that cell adhesion and spreading regulate the size and shape of the nucleus. Disruption of the keratin cytoskeleton through loss of plectin facilitated greater nuclear deformation, which depended on acto-myosin contractility. Nuclear morphology did not depend on direct linkage of the keratin cytoskeleton with the nuclear membrane, rather loss of plectin reduced keratin filament density around the nucleus. We further demonstrate that keratinocytes have abnormal nuclear morphologies in the epidermis of plectin-deficient, epidermolysis bullosa simplex patients. Taken together, our data demonstrate that plectin is an essential regulator of nuclear morphology in vitro and in vivo and protects the nucleus from mechanical deformation.

Epidermal Basement Membrane in Health and Disease

Skin, as the organ protecting the individual from environmental aggressions, constantly meets external insults and is dependent on mechanical toughness for its preserved function. Accordingly, the epidermal basement membrane (BM) zone has adapted to enforce tissue integrity. It harbors anchoring structures created through unique organization of common BM components and expression of proteins exclusive to the epidermal BM zone. Evidence for the importance of its correct assembly and the nonredundancy of its components for skin integrity is apparent from the multiple skin blistering disorders caused by mutations in genes coding for proteins associated with the epidermal BM and from autoimmune disorders in which autoantibodies target these molecules. However, it has become clear that these proteins not only provide mechanical support but are also critically involved in tissue homeostasis, repair, and regeneration. In this chapter, we provide an overview of the unique organization and components of the epidermal BM. A special focus will be given to its function during regeneration, and in inherited and acquired diseases.

Molecular architecture and function of the hemidesmosome

Hemidesmosomes are multiprotein complexes that facilitate the stable adhesion of basal epithelial cells to the underlying basement membrane. The mechanical stability of hemidesmosomes relies on multiple interactions of a few protein components that form a membrane-embedded tightly-ordered complex. The core of this complex is provided by integrin α6β4 and P1a, an isoform of the cytoskeletal linker protein plectin that is specifically associated with hemidesmosomes. Integrin α6β4 binds to the extracellular matrix protein laminin-332, whereas P1a forms a bridge to the cytoplasmic keratin intermediate filament network. Other important components are BPAG1e, the epithelial isoform of bullous pemphigoid antigen 1, BPAG2, a collagen-type transmembrane protein and CD151. Inherited or acquired diseases in which essential components of the hemidesmosome are missing or structurally altered result in tissue fragility and blistering. Modulation of hemidesmosome function is of crucial importance for a variety of biological processes, such as terminal differentiation of basal keratinocytes and keratinocyte migration during wound healing and carcinoma invasion. Here, we review the molecular characteristics of the proteins that make up the hemidesmosome core structure and summarize the current knowledge about how their assembly and turnover are regulated by transcriptional and post-translational mechanisms.

Functional and Genetic Analysis of Plectin in Skin and Muscle

Plectin is a large cytoskeletal linker protein with a multitude of functions affecting various cellular processes. It is expressed as several different isoforms from a highly complex gene. Both, this transcript diversity (mainly caused by short 5'-sequences contained in alternative first exons) and the size (>500 kDa) of the resulting proteins, present considerable challenges to plectin researchers. In this chapter, we will consider these problems and offer advice on how to tackle them best. As plectin has been studied most extensively in skin and muscle, we will focus on these types of tissues and describe some selected methods in detail. Foremost, however, we aim to give the readers some good pointers to available tools and into the existing literature.

The rod domain is not essential for the function of plectin in maintaining tissue integrity

Plectin is a cytoskeletal linker protein that consists of a central rod domain connecting two globular domains. Rodless plectin is able to functionally compensate for the loss of full-length plectin in mice and, like full-length plectin, is able to form dimers.

Epidermolysis bullosa simplex associated with late-onset muscular dystrophy (EBS-MD) is an autosomal recessive disorder resulting from mutations in the plectin gene. The majority of these mutations occur within the large exon 31 encoding the central rod domain and leave the production of a low-level rodless plectin splice variant unaffected. To investigate the function of the rod domain, we generated rodless plectin mice through conditional deletion of exon 31. Rodless plectin mice develop normally without signs of skin blistering or muscular dystrophy. Plectin localization and hemidesmosome organization are unaffected in rodless plectin mice. However, superresolution microscopy revealed a closer juxtaposition of the C-terminus of plectin to the integrin β4 subunit in rodless plectin keratinocytes. Wound healing occurred slightly faster in rodless plectin mice than in wild-type mice, and keratinocytes migration was increased in the absence of the rod domain. The faster migration of rodless plectin keratinocytes is not due to altered biochemical properties because, like full-length plectin, rodless plectin is a dimeric protein. Our data demonstrate that rodless plectin can functionally compensate for the loss of full-length plectin in mice. Thus the low expression level of plectin rather than the absence of the rod domain dictates the development of EBS-MD.

Structural insights into Ca2+-calmodulin regulation of Plectin 1a-integrin β4 interaction in hemidesmosomes

The mechanical stability of epithelial cells, which protect organisms from harmful external factors, is maintained by hemidesmosomes via the interaction between plectin 1a (P1a) and integrin α6β4. Binding of calcium-calmodulin (Ca(2+)-CaM) to P1a together with phosphorylation of integrin β4 disrupts this complex, resulting in disassembly of hemidesmosomes. We present structures of the P1a actin binding domain either in complex with the N-ter lobe of Ca(2+)-CaM or with the first pair of integrin β4 fibronectin domains. Ca(2+)-CaM binds to the N-ter isoform-specific tail of P1a in a unique manner, via its N-ter lobe in an extended conformation. Structural, cell biology, and biochemical studies suggest the following model: binding of Ca(2+)-CaM to an intrinsically disordered N-ter segment of plectin converts it to an α helix, which repositions calmodulin to displace integrin β4 by steric repulsion. This model could serve as a blueprint for studies aimed at understanding how Ca(2+)-CaM or EF-hand motifs regulate F-actin-based cytoskeleton.

Mutation in exon 1a of PLEC, leading to disruption of plectin isoform 1a, causes autosomal-recessive skin-only epidermolysis bullosa simplex

PLEC, the gene encoding the cytolinker protein plectin, has eight tissue-specific isoforms in humans, arising by alternate splicing of the first exon. To date, all PLEC mutations that cause epidermolysis bullosa simplex (EBS) were found in exons common to all isoforms. Due to the ubiquitous presence of plectin in mammalian tissues, EBS from recessive plectin mutations is always associated with extracutaneous involvement including muscular dystrophy, pyloric atresia and cardiomyopathy. We studied a consanguineous family with sisters having isolated blistering suggesting EBS. Skin disease started with foot blisters at walking age and became generalized at puberty while sparing mucous membranes. DNA sequencing revealed a homozygous nonsense mutation (c.46C>T; p.Arg16X) in the first exon of the plectin variant encoding plectin isoform 1a (P1a). Immunofluorescence antigen mapping, transmission electron microscopy, western blot analysis and qRT-PCR were performed on patient skin and cultured keratinocytes, control myocardium and striated muscle samples. We found hypoplastic hemidesmosomes and intra-epidermal 'pseudo-junctional' cleavage fitting EBS. Screening for cardiomyopathy and muscle dystrophy showed no abnormalities. We report the first cases of autosomal-recessive EBS from P1a deficiency affecting skin, while mucous membranes, heart and muscle are spared. The dominant expression of the P1a isoform in epidermal basal cell layer and cultured keratinocytes suggests that mutations in the first exon of isoform 1a cause skin-only EBS without extracutaneous involvement. Our study characterizes yet another of the eight isoforms of plectin and adds a tissue-specific phenotype to the spectrum of 'plectinopathies' produced by mutations of unique first exons of this gene.

UV exposure modulates hemidesmosome plasticity, contributing to long-term pigmentation in human skin

Human skin colour, ie pigmentation, differs widely among individuals, as do their responses to various types of ultraviolet radiation (UV) and their risks of skin cancer. In some individuals, UV-induced pigmentation persists for months to years in a phenomenon termed long-lasting pigmentation (LLP). It is unclear whether LLP is an indicator of potential risk for skin cancer. LLP seems to have similar features to other forms of hyperpigmentation, eg solar lentigines or age spots, which are clinical markers of photodamage and risk factors for precancerous lesions. To investigate what UV-induced molecular changes may persist in individuals with LLP, clinical specimens from non-sunburn-inducing repeated UV exposures (UVA, UVB or UVA + UVB) at 4 months post-exposure (short-term LLP) were evaluated by microarray analysis and dataset mining. Validated targets were further evaluated in clinical specimens from six healthy individuals (three LLP+ and three LLP-) followed for more than 9 months (long-term LLP) who initially received a single sunburn-inducing UVA + UVB exposure. The results support a UV-induced hyperpigmentation model in which basal keratinocytes have an impaired ability to remove melanin that leads to a compensatory mechanism by neighbouring keratinocytes with increased proliferative capacity to maintain skin homeostasis. The attenuated expression of SOX7 and other hemidesmosomal components (integrin α6β4 and plectin) leads to increased melanosome uptake by keratinocytes and points to a spatial regulation within the epidermis. The reduced density of hemidesmosomes provides supporting evidence for plasticity at the epidermal-dermal junction. Altered hemidesmosome plasticity, and the sustained nature of LLP, may be mediated by the role of SOX7 in basal keratinocytes. The long-term sustained subtle changes detected are modest, but sufficient to create dramatic visual differences in skin colour. These results suggest that the hyperpigmentation phenomenon leading to increased interdigitation develops in order to maintain normal skin homeostasis in individuals with LLP.

Intermediate filaments in cell migration and invasion: the unusual suspects

Cell migration is a multistep process which relies on the coordination of cytoskeletal structures in space and time. While the roles of actin and microtubules have been investigated in great details, the lack of inhibitors and visualizing tools and the large number of proteins forming intermediate filaments (IFs) have delayed the characterization of IF functions during migration. However, a large body of evidence has progressively pointed to changes in IF composition as an important parameter in the regulation of cell migratory properties both during development and tumor invasion. More recent in-depth analyses show that IFs are dynamically reorganized to participate, together with microfilaments and microtubules, to the key steps leading to cell migration.

Molecular architecture and function of the hemidesmosome

Hemidesmosomes are multiprotein complexes that facilitate the stable adhesion of basal epithelial cells to the underlying basement membrane. The mechanical stability of hemidesmosomes relies on multiple interactions of a few protein components that form a membrane-embedded tightly-ordered complex. The core of this complex is provided by integrin α6β4 and P1a, an isoform of the cytoskeletal linker protein plectin that is specifically associated with hemidesmosomes. Integrin α6β4 binds to the extracellular matrix protein laminin-332, whereas P1a forms a bridge to the cytoplasmic keratin intermediate filament network. Other important components are BPAG1e, the epithelial isoform of bullous pemphigoid antigen 1, BPAG2, a collagen-type transmembrane protein and CD151. Inherited or acquired diseases in which essential components of the hemidesmosome are missing or structurally altered result in tissue fragility and blistering. Modulation of hemidesmosome function is of crucial importance for a variety of biological processes, such as terminal differentiation of basal keratinocytes and keratinocyte migration during wound healing and carcinoma invasion. Here, we review the molecular characteristics of the proteins that make up the hemidesmosome core structure and summarize the current knowledge about how their assembly and turnover are regulated by transcriptional and post-translational mechanisms.

Plectin-related skin diseases

Plectin has been characterized as a linker protein that is expressed in many cell types and is distinctive in various isoforms in the N-terminus and around the rod domain due to complicated alternative splicing of PLEC, the gene encoding plectin. Plectin deficiency causes autosomal recessive epidermolysis bullosa simplex (EBS) with involvement of the skin and other organs, such as muscle and gastrointestinal tract, depending on the expression pattern of the defective protein. In addition, a point mutation in the rod domain of plectin leads to autosomal dominant EBS, called as EBS-Ogna. Plectin can be targeted by circulating autoantibodies in subepidermal autoimmune blistering diseases. This review summarizes plectin-related skin diseases, from congenital to autoimmune disorders.

Networking and anchoring through plectin: a key to IF functionality and mechanotransduction

Intermediate filaments (IFs) are involved in multiple cellular processes that are essential for the maintenance of cell and tissue integrity as well as response and adaption to stress. Mainly through pathological manifestations in patients and the analysis of genetic mouse models, it became evident that cytolinker proteins of the plakin protein family are essential for many of the functions ascribed to IFs. As discussed in this review, one of them, plectin, affects the assembly properties, interaction potential, compartmentalization, and linkage properties of IFs, making it to a key player for IF functionality. The far reaching consequences of IFs not being well-connected for skin and muscular integrity, migration, and mechanotransduction are highlighted.

Interaction of plectin with keratins 5 and 14: dependence on several plectin domains and keratin quaternary structure

Plectin, a cytolinker of the plakin family, anchors the intermediate filament (IF) network formed by keratins 5 and 14 (K5/K14) to hemidesmosomes, junctional adhesion complexes in basal keratinocytes. Genetic alterations of these proteins cause epidermolysis bullosa simplex (EBS) characterized by disturbed cytoarchitecture and cell fragility. The mechanisms through which mutations located after the documented plectin IF-binding site, composed of the plakin-repeat domain (PRD) B5 and the linker, as well as mutations in K5 or K14, lead to EBS remain unclear. We investigated the interaction of plectin C terminus, encompassing four domains, the PRD B5, the linker, the PRD C, and the C extremity, with K5/K14 using different approaches, including a rapid and sensitive fluorescent protein-binding assay, based on enhanced green fluorescent protein-tagged proteins (FluoBACE). Our results demonstrate that all four plectin C-terminal domains contribute to its association with K5/K14 and act synergistically to ensure efficient IF binding. The plectin C terminus predominantly interacted with the K5/K14 coil 1 domain and bound more extensively to K5/K14 filaments compared with monomeric keratins or IF assembly intermediates. These findings indicate a multimodular association of plectin with K5/K14 filaments and give insights into the molecular basis of EBS associated with pathogenic mutations in plectin, K5, or K14 genes.

The genetics of skin fragility

Genetic skin fragility manifests with diminished resistance of the skin and mucous membranes to external mechanical forces and with skin blistering, erosions, and painful wounds as clinical features. Skin fragility disorders, collectively called epidermolysis bullosa, are caused by mutations in 18 distinct genes that encode proteins involved in epidermal integrity and dermal-epidermal adhesion. The genetic spectrum, along with environmental and genetic modifiers, creates a large number of clinical phenotypes, spanning from minor localized lesions to severe generalized blistering, secondary skin cancer, or early demise resulting from extensive loss of the epidermis. Laboratory investigations of skin fragility have greatly augmented our understanding of genotype-phenotype correlations in epidermolysis bullosa and have also advanced skin biology in general. Current translational research concentrates on the development of biologically valid treatments with therapeutic genes, cells, proteins, or small-molecule compounds in preclinical settings or human pilot trials.

Plakins, a versatile family of cytolinkers: roles in skin integrity and in human diseases

The plakin family consists of giant proteins involved in the cross-linking and organization of the cytoskeleton and adhesion complexes. They further modulate several fundamental biological processes, such as cell adhesion, migration, and polarization or signaling pathways. Inherited and acquired defects of plakins in humans and in animal models potentially lead to dramatic manifestations in the skin, striated muscles, and/or nervous system. These observations unequivocally demonstrate the key role of plakins in the maintenance of tissue integrity. Here we review the characteristics of the mammalian plakin members BPAG1 (bullous pemphigoid antigen 1), desmoplakin, plectin, envoplakin, epiplakin, MACF1 (microtubule-actin cross-linking factor 1), and periplakin, highlighting their role in skin homeostasis and diseases.

Epidermolysis bullosa simplex with PLEC mutations: new phenotypes and new mutations

BACKGROUND

Genetic mutations in the plectin gene (PLEC) cause autosomal recessive forms of epidermolysis bullosa simplex (EBS) associated with either muscular dystrophy (EBS-MD) or pyloric atresia (EBS-PA). Phenotype-genotype analysis has suggested that EBS-MD is due mostly to genetic mutations affecting the central rod domain of plectin, and EBS-PA to mutations outside this domain.

OBJECTIVES

This study aimed to describe new phenotypes of patients with EBS-MD and EBS-PA, to identify novel PLEC mutations and to establish genotype-phenotype correlations.

METHODS

Seven patients with a suspicion of EBS linked to PLEC mutations were included. A standardized clinical questionnaire was sent to the physicians in charge of each patient. Immunofluorescence studies of skin biopsies followed by molecular analysis of PLEC were performed in all patients.

RESULTS

We report the first case of nonlethal EBS-PA improving with age, the first multisystemic involvement in a patient with lethal EBS-PA, and the first patients with EBS-MD with involvement of either the bladder or oesophagus. Eleven novel PLEC mutations are also reported.

CONCLUSIONS

Our results confirm that EBS-PA is linked to mutations in the distal exons 1-30 and 32 of PLEC. Long-term survival is possible, with skin improvement, but a delayed onset of MD is probable. While EBS-MD is linked to PLEC mutations in all exons, in most cases one of the mutations affects exon 31. The precocity of MD seems to be linked to the type and localization of the PLEC mutation(s), but no correlation with mucosal involvement has been found.

Phosphorylation of serine 4,642 in the C-terminus of plectin by MNK2 and PKA modulates its interaction with intermediate filaments

Plectin is a versatile cytolinker of the plakin family conferring cell resilience to mechanical stress in stratified epithelia and muscles. It acts as a critical organizer of the cytoskeletal system by tethering various intermediate filament (IF) networks through its C-terminal IF-binding domain (IFBD). Mutations affecting the IFBD cause devastating human diseases. Here, we show that serine 4642, which is located in the extreme C-terminus of plectin, is phosphorylated in different cell lines. Phosphorylation of S4642 decreased the ability of plectin IFBD to associate with various IFs, as assessed by immunofluorescence microscopy and cell fractionation studies, as well as in yeast two-hybrid assays. Plectin phosphorylated at S4642 was reduced at sites of IF network anchorage along cell-substrate contacts in both skin and cultured keratinocytes. Treatment of SK-MEL-2 and HeLa cells with okadaic acid increased plectin S4642 phosphorylation, suggesting that protein phosphatase 2A dephosphorylates this residue. Moreover, plectin S4642 phosphorylation was enhanced after cell treatment with EGF, phorbol ester, sorbitol and 8-bromo-cyclic AMP, as well as during wound healing and protease-mediated cell detachment. Using selective protein kinase inhibitors, we identified two different kinases that modulate the phosphorylation of plectin S4642 in HeLa cells: MNK2, which is downstream of the ERK1/2-dependent MAPK cascade, and PKA. Our study indicates that phosphorylation of S4642 has an important regulatory role in the interaction of plectin with IFs and identifies a novel link between MNK2 and the cytoskeleton.

Stabilization of the dystroglycan complex in Cajal bands of myelinating Schwann cells through plectin-mediated anchorage to vimentin filaments

Previous studies have unmasked plectin, a uniquely versatile intermediate filament-associated cytolinker protein, to be essential for skin and skeletal muscle integrity. Different sets of isoforms of the protein were found to stabilize cells mechanically, regulate cytoskeletal dynamics, and serve as a scaffolding platform for signaling molecules. Here, we investigated whether a similar scenario prevails in myelinating Schwann cells. Using isoform-specific antibodies, the two plectin variants predominantly expressed in the cytoplasmic compartment (Cajal bands) of Schwann cells were identified as plectin (P)1 and P1c. Coimmunoprecipitation and immunolocalization experiments revealed complex formation of Cajal band plectin with β-dystroglycan, the core component of the dystrophin glycoprotein complex that in Schwann cells is crucial for the compartmentalization and stabilization of the myelin sheath. To study the functional implications of Schwann cell-specific plectin-β-dystroglycan interaction, we generated conditional (Schwann cell-restricted) plectin knockout mice. Ablation of plectin in myelinating Schwann cells (SCs) was found not to affect myelin sheath formation but to abrogate the tight association of the dystroglycan complex with the intermediate filament cytoskeleton. We show that the disruption of this association leads to the destabilization of the dystroglycan complex combined with increased myelin sheath deformations observed in the peripheral nerve during ageing of the animal.

Plectin-intermediate filament partnership in skin, skeletal muscle, and peripheral nerve

Plectin is a large, 500-kDa, intermediate filament (IF)-associated protein. It acts as a cytoskeletal crosslinker and signaling scaffold, affecting mechanical as well as dynamic properties of the cytoskeleton. As a member of the plakin family of cytolinker proteins, plectin has a multidomain structure that is responsible for its vast binding portfolio. It not only binds to all types of IFs, actin filaments and microtubules, but also to transmembrane receptors, proteins of the subplasma membrane protein skeleton, components of the nuclear envelope, and several kinases with known roles in migration, proliferation, and energy metabolism of cells. Due to alternative splicing, plectin is expressed as various isoforms with differing N-terminal heads that dictate their differential subcellular targeting. Through specific interactions with other proteins at their target sites and their ability to bind to all types of IFs, plectin molecules provide strategically located IF anchorage sites within the cytoplasm of cells. In this review, we will present an overview of the structural features and functional properties of plectin and discuss recent progress in defining the role of its isoforms in stress-prone tissues and the implicated diseases, with focus on skin, skeletal muscle, and Schwann cells of peripheral nerve.

Intermediate filament-associated cytolinker plectin 1c destabilizes microtubules in keratinocytes

The transition of microtubules (MTs) from an assembled to a disassembled state plays an essential role in several cellular functions. While MT dynamics are often linked to those of actin filaments, little is known about whether intermediate filaments (IFs) have an influence on MT dynamics. We show here that plectin 1c (P1c), one of the multiple isoforms of the IF-associated cytolinker protein plectin, acts as an MT destabilizer. We found that MTs in P1c-deficient (P1c(-/-)) keratinocytes are more resistant toward nocodazole-induced disassembly and display increased acetylation. In addition, live imaging of MTs in P1c(-/-), as well as in plectin-null, cells revealed decreased MT dynamics. Increased MT stability due to P1c deficiency led to changes in cell shape, increased velocity but loss of directionality of migration, smaller-sized focal adhesions, higher glucose uptake, and mitotic spindle aberrations combined with reduced growth rates of cells. On the basis of ex vivo and in vitro experimental approaches, we suggest a mechanism for MT destabilization in which isoform-specific binding of P1c to MTs antagonizes the MT-stabilizing and assembly-promoting function of MT-associated proteins through an inhibitory function exerted by plectin's SH3 domain. Our results open new perspectives on cytolinker-coordinated IF-MT interaction and its physiological significance.

The many faces of plectin and plectinopathies: pathology and mechanisms

Plectin, a giant multifunctional cytolinker protein, plays a crucial role in stabilizing and orchestrating intermediate filament networks in cells. Mutations in the human plectin gene result in multiple diseases manifesting with muscular dystrophy, skin blistering, and signs of neuropathy. The most common disease caused by plectin deficiency is epidermolysis bullosa simplex (EBS)-MD, a rare autosomal-recessive skin blistering disorder with late-onset muscular dystrophy. EBS-MD patients and plectin-deficient mice display pathologic desmin-positive protein aggregates, degenerated myofibrils, and mitochondrial abnormalities, the hallmarks of myofibrillar myopathies. In addition to EBS-MD, plectin mutations have been shown to cause EBS-MD with a myasthenic syndrome, limb-girdle muscular dystrophy type 2Q, EBS with pyloric atresia, and EBS-Ogna. This review focuses on clinical and pathological manifestations of these plectinopathies. It addresses especially plectin's role in skeletal muscle, where a loss of muscle fiber integrity and profound changes of myofiber cytoarchitecture are observed in its absence. Furthermore, the highly complex genetic and molecular structure of plectin is discussed; a high number of differentially spliced exons give rise to a variety of different isoforms, which fulfill distinct functions in different cell types and tissues. Plectin's abilities to act as a dynamic organizer of intermediate filament networks and to interact with a multitude of different interaction partners are the basis for its function as a scaffolding platform for proteins involved in signaling. Finally, the article addresses a series of genetically manipulated mouse lines that were generated to serve as powerful models to study functional and molecular consequences of plectin gene defects.

Genetic pathways in disorders of epidermal differentiation

More than 100 human genetic skin diseases, impacting over 20% of the population, are characterized by disrupted epidermal differentiation. A significant proportion of the 90 genes identified in these disorders to date are concentrated within several functional pathways, suggesting the emergence of organizing themes in epidermal differentiation. Among these are the Notch, transforming growth factor β (TGFβ), IκB kinase (IKK), Ras/mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K), p63, and Wnt signaling pathways, as well as core biological processes mediating calcium homeostasis, tissue integrity, cornification, and lipid biogenesis. Here, we review recent results supporting the central role of these pathways in epidermal differentiation, highlighting the integration of genetic information with functional studies to illuminate the biological actions of these pathways in humans as well as to guide development of future therapeutics to correct their dysfunction.

Cytoplasmic plaque formation in hemidesmosome development is dependent on SoxF transcription factor function

Hemidesmosomes are composed of intricate networks of proteins, that are an essential attachment apparatus for the integrity of epithelial tissue. Disruption leads to blistering diseases such as epidermolysis bullosa. Members of the Sox gene family show dynamic and diverse expression patterns during development and mutation analyses in humans and mice provide evidence that they play a remarkable variety of roles in development and human disease. Previous studies have established that the mouse mutant ragged-opossum (Ra^op) expresses a dominant-negative form of the SOX18 transcription factor that interferes with the function of wild type SOX18 and of the related SOXF-subgroup proteins SOX7 and −17. Here we show that skin and oral mucosa in homozygous Ra^op mice display extensive detachment of epithelium from the underlying mesenchymal tissue, caused by tearing of epithelial cells just above the plasma membrane due to hemidesmosome disruption. In addition, several hemidesmosome proteins expression were found to be dysregulated in the Ra^op mice. Our data suggest that SOXF transcription factors play a role in regulating formation of cytoplasmic plaque protein assembly, and that disrupted SOXF function results in epidermolysis bullosa-like skin phenotypes.