Desmosomes from a structural perspective

Defining domain-specific orientational order in the desmosomal cadherins

Desmosomes are large, macromolecular protein assemblies that mechanically couple the intermediate filament cytoskeleton to sites of cadherin-mediated cell adhesion, thereby providing structural integrity to tissues that routinely experience large forces. Proper desmosomal adhesion is necessary for the normal development and maintenance of vertebrate tissues, such as epithelia and cardiac muscle, while dysfunction can lead to severe disease of the heart and skin. Therefore, it is important to understand the relationship between desmosomal adhesion and the architecture of the molecules that form the adhesive interface, the desmosomal cadherins (DCs). However, desmosomes are embedded in two plasma membranes and are linked to the cytoskeletal networks of two cells, imposing extreme difficulty on traditional structural studies of DC architecture, which have yielded conflicting results. Consequently, the relationship between DC architecture and adhesive function remains unclear. To overcome these challenges, we utilized excitation-resolved fluorescence polarization microscopy to quantify the orientational order of the extracellular and intracellular domains of three DC isoforms: desmoglein 2, desmocollin 2, and desmoglein 3. We found that DC ectodomains were significantly more ordered than their cytoplasmic counterparts, indicating a drastic difference in DC architecture between opposing sides of the plasma membrane. This difference was conserved among all DCs tested, suggesting that it may be an important feature of desmosomal architecture. Moreover, our findings suggest that the organization of DC ectodomains is predominantly the result of extracellular adhesive interactions. We employed azimuthal orientation mapping to show that DC ectodomains are arranged with rotational symmetry about the membrane normal. Finally, we performed a series of mathematical simulations to test the feasibility of a recently proposed antiparallel arrangement of DC ectodomains, finding that it is supported by our experimental data. Importantly, the strategies employed here have the potential to elucidate molecular mechanisms for diseases that result from defective desmosome architecture.

Triptonide is a reversible non-hormonal male contraceptive agent in mice and non-human primates

There are no non-hormonal male contraceptives currently on the market despite decades of efforts toward the development of "male pills". Here, we report that triptonide, a natural compound purified from the Chinese herb Tripterygium Wilfordii Hook F displays reversible male contraceptive effects in both mice and monkeys. Single daily oral doses of triptonide induces deformed sperm with minimal or no forward motility (close to 100% penetrance) and consequently male infertility in 3-4 and 5-6 weeks in mice and cynomolgus monkeys, respectively. Male fertility is regained in ~4-6 weeks after cessation of triptonide intake in both species. Either short- or long-term triptonide treatment causes no discernable systematic toxic side effects based on histological examination of vital organs in mice and hematological and serum biochemical analyses in monkeys. Triptonide appears to target junction plakoglobin and disrupts its interactions with SPEM1 during spermiogenesis. Our data further prove that targeting late spermiogenesis represents an effective strategy for developing non-hormonal male contraceptives.

Mechanotransduction and Adrenergic Stimulation in Arrhythmogenic Cardiomyopathy: An Overview of in vitro and in vivo Models

Arrhythmogenic Cardiomyopathy (AC) is a rare inherited heart disease, manifesting with progressive myocardium degeneration and dysfunction, and life-threatening arrhythmic events that lead to sudden cardiac death. Despite genetic determinants, most of AC patients admitted to hospital are athletes or very physically active people, implying the existence of other disease-causing factors. It is recognized that AC phenotypes are enhanced and triggered by strenuous physical activity, while excessive mechanical stretch and load, and repetitive adrenergic stimulation are mechanisms influencing disease penetrance. Different approaches have been undertaken to recapitulate and study both mechanotransduction and adrenergic signaling in AC, including the use of in vitro cellular and tissue models, and the development of in vivo models (particularly rodents but more recently also zebrafish). However, it remains challenging to reproduce mechanical load stimuli and physical activity in laboratory experimental settings. Thus, more work to drive the innovation of advanced AC models is needed to recapitulate these subtle physiological influences. Here, we review the state-of-the-art in this field both in clinical and laboratory-based modeling scenarios. Specific attention will be focused on highlighting gaps in the knowledge and how they may be resolved by utilizing novel research methodology.

CAR-T Cell Therapy-An Overview of Targets in Gastric Cancer

Gastric cancer (GC) is one of the most commonly diagnosed malignancies and, unfortunately, still has a high mortality rate. Recent research points to CAR-T immunotherapy as a promising treatment for this disease. Using genetically engineered T cells designed to target a previously selected antigen, researchers are able to harness the natural anti-tumor activity of T cells. For therapy to be successful, however, it is essential to choose antigens that are present on tumor cells but not on healthy cells. In this review, we present an overview of the most important targets for CAR-T therapy in the context of GC, including their biologic function and therapeutic application. A number of clinical studies point to the following as important markers in GC: human epidermal growth factor receptor 2, carcinoembryonic antigen, mucin 1, epithelial cell adhesion molecule, claudin 18.2, mesothelin, natural-killer receptor group 2 member D, and folate receptor 1. Although these markers have been met with some success, the search for new and improved targets continues. Key among these novel biomarkers are the B7H6 ligand, actin-related protein 2/3 (ARP 2/3), neuropilin-1 (NRP-1), desmocollin 2 (DSC2), anion exchanger 1 (AF1), and cancer-related antigens CA-72-4 and CA-19-9.

Distinct proteomic profiles in monozygotic twins discordant for ischaemic stroke

Stroke is a common disorder with significant morbidity and mortality, and complex aetiology involving both environmental and genetic risk factors. Although some of the major risk factors for stoke, such as smoking and hypertension, are well-documented, the underlying genetic and detailed molecular mechanisms remain elusive. Exploring the relevant biochemical pathways may contribute to the clinical diagnosis of stroke and shed light on its aetiology. A comparative proteomic analysis of blood serum of a pair of monozygotic (MZ) twins discordant for ischaemic stroke (IS) was performed using a label-free quantitative proteomics approach. To overcome the limit of reproducibility in the serum preparation, two separate runs were performed, each consisting of three technical replicates per sample. Biological processes associated with proteins differentially expressed between the twins were explored with gene ontology (GO) classification using the functional analysis tool g:Profiler. ANOVA test performed in Progenesis LC-MS identified 179 (run 1) and 209 (run 2) proteins as differentially expressed between the affected and unaffected twin (p < 0.05). Furthermore, the level of serum fibulin 1, an extracellular matrix protein associated with arterial stiffness, was on average 13.37-fold higher in the affected twin. Each dataset was then analysed independently, and the proteins were classified according to GO terms. The categories overrepresented in the affected twin predominantly corresponded to stroke-relevant processes, including wound healing, blood coagulation and haemostasis, with a high proportion of the proteins overexpressed in the affected twin associated with these terms. By contrast, in the unaffected twin diagnosed with atopic dermatitis, there were increased levels of keratin proteins and GO terms associated with skin development. The identification of cellular pathways enriched in IS as well as the upregulation of fibulin 1 sheds new light on the underlying disease-causing mechanisms at the molecular level. Our findings of distinct proteomic signatures associated with IS and atopic dermatitis suggest proteomic profiling could be used as a general approach for improved diagnostic, prognostic and therapeutic strategies.

Alterations of protein expression of phospholamban, ZASP and plakoglobin in human atria in subgroups of seniors

The mature mammalian myocardium contains composite junctions (areae compositae) that comprise proteins of adherens junctions as well as desmosomes. Mutations or deficiency of many of these proteins are linked to heart failure and/or arrhythmogenic cardiomyopathy in patients. We firstly wanted to address the question whether the expression of these proteins shows an age-dependent alteration in the atrium of the human heart. Right atrial biopsies, obtained from patients undergoing routine bypass surgery for coronary heart disease were subjected to immunohistology and/or western blotting for the plaque proteins plakoglobin (γ-catenin) and plakophilin 2. Moreover, the Z-band protein cypher 1 (Cypher/ZASP) and calcium handling proteins of the sarcoplasmic reticulum (SR) like phospholamban, SERCA and calsequestrin were analyzed. We noted expression of plakoglobin, plakophilin 2 and Cypher/ZASP in these atrial preparations on western blotting and/or immunohistochemistry. There was an increase of Cypher/ZASP expression with age. The present data extend our knowledge on the expression of anchoring proteins and SR regulatory proteins in the atrium of the human heart and indicate an age-dependent variation in protein expression. It is tempting to speculate that increased expression of Cypher/ZASP may contribute to mechanical changes in the aging human myocardium.

Trade-offs in aging lung diseases: a review on shared but opposite genetic risk variants in idiopathic pulmonary fibrosis, lung cancer and chronic obstructive pulmonary disease

PURPOSE OF REVIEW

The process of aging involves biological changes that increases susceptibility for disease. In the aging lung disease IPF, GWAS studies identified genes associated with risk for disease. Recently, several of these genes were also found to be involved in risk for COPD or lung cancer. This review describes GWAS-derived risk genes for IPF that overlap with risk genes for lung cancer or COPD.

RECENT FINDINGS

Risk genes that overlap between aging lung diseases, include FAM13A, DSP and TERT. Most interestingly, disease predisposing alleles for IPF are opposite to those for COPD or lung cancer. Studies show that the alleles are associated with differential gene expression and with physiological traits in the general population. The opposite allelic effect sizes suggest the presence of trade-offs in the aging lung. For TERT, the trade-off involves cellular senescence versus proliferation and repair. For FAM13A and DSP, trade-offs may involve protection from noxious gases or tissue integrity.

SUMMARY

The overlap in risk genes in aging lung diseases provides evidence that processes associated with FAM13A, DSP and TERT are important for healthy aging. The opposite effect size of the disease risk alleles may represent trade-offs, for which a model involving an apicobasal gene expression gradient is presented.

Plakophilin3 increases desmosome assembly, size and stability by increasing expression of desmocollin2

Previous reports show that the desmosomal plaque protein plakophilin3 (PKP3) is essential for desmosome formation. Here, we report that PKP3 over-expression decreases calcium dependency for de novo desmosome formation and makes existing cell-cell adhesion junctions more resilient in low calcium medium due to an increase in desmocollin2 expression. PKP3 overexpression increases the stability of other desmosomal proteins independently of the increase in DSC2 levels and regulates desmosome formation and stability by a multimodal mechanism affecting transcription, protein stability and cell border localization of desmosomal proteins.

Molecular organization of the desmosome as revealed by direct stochastic optical reconstruction microscopy

Desmosomes are macromolecular junctions responsible for providing strong cell-cell adhesion. Because of their size and molecular complexity, the precise ultrastructural organization of desmosomes is challenging to study. Here, we used direct stochastic optical reconstruction microscopy (dSTORM) to resolve individual plaque pairs for inner and outer dense plaque proteins. Analysis methods based on desmosomal mirror symmetry were developed to measure plaque-to-plaque distances and create an integrated map. We quantified the organization of desmoglein 3, plakoglobin and desmoplakin (N-terminal, rod and C-terminal domains) in primary human keratinocytes. Longer desmosome lengths correlated with increasing plaque-to-plaque distance, suggesting that desmoplakin is arranged with its long axis at an angle within the plaque. We next examined whether plaque organization changed in different adhesive states. Plaque-to-plaque distance for the desmoplakin rod and C-terminal domains decreased in PKP-1-mediated hyperadhesive desmosomes, suggesting that protein reorganization correlates with function. Finally, in human epidermis we found a difference in plaque-to-plaque distance for the desmoplakin C-terminal domain, but not the desmoplakin rod domain or plakoglobin, between basal and suprabasal cells. Our data reveal the molecular organization of desmosomes in cultured keratinocytes and skin as defined by dSTORM.

Claws, Disorder, and Conformational Dynamics of the C-Terminal Region of Human Desmoplakin

Multicellular organisms consist of cells that interact via elaborate adhesion complexes. Desmosomes are membrane-associated adhesion complexes that mechanically tether the cytoskeletal intermediate filaments (IFs) between two adjacent cells, creating a network of tough connections in tissues such as skin and heart. Desmoplakin (DP) is the key desmosomal protein that binds IFs, and the DP·IF association poses a quandary: desmoplakin must stably and tightly bind IFs to maintain the structural integrity of the desmosome. Yet, newly synthesized DP must traffic along the cytoskeleton to the site of nascent desmosome assembly without "sticking" to the IF network, implying weak or transient DP···IF contacts. Recent work reveals that these contacts are modulated by post-translational modifications (PTMs) in DP's C-terminal tail (DPCTT). Using molecular dynamics simulations, we have elucidated the structural basis of these PTM-induced effects. Our simulations, nearing 2 μs in aggregate, indicate that phosphorylation of S2849 induces an "arginine claw" in desmoplakin's C-terminal tail. If a key arginine, R2834, is methylated, the DPCTT preferentially samples conformations that are geometrically well-suited as substrates for processive phosphorylation by the cognate kinase GSK3. We suggest that DPCTT is a molecular switch that modulates, via its conformational dynamics, DP's overall efficacy as a substrate for GSK3. Finally, we show that the fluctuating DPCTT can contact other parts of DP, suggesting a competitive binding mechanism for the modulation of DP···IF interactions.

HoxA13 Regulates Phenotype Regionalization of Human Pregnant Myometrium

CONTEXT

Bipedalism separates humans from most other animal species, but results in significant physiologic challenges, particularly with respect to the maintenance of pregnancy and induction of parturition. A contracted lower uterine segment (LUS) and a relaxed uterine fundal myometrium (FUN) during pregnancy are required to prevent pressure on the cervix from the fetal head due to gravity. With the onset of labor, this regionalization of myometrial function must be reversed, allowing descent of the fetus, dilation of the cervix, and expulsion of the fetus through the birth canal. However, the molecular mechanisms remain unclear.

OBJECTIVE AND DESIGN

This study sought to identify phenotypic regionalization of LUS and FUN during pregnancy, RNA sequencing was performed to analyze the human myometrial transcriptome. Real-time PCR and immunoblotting were applied to validate sequencing results. Cell contraction/adhesion assays and gene microarrays were used to study the cellular functions of the identified genes.

RESULTS

Homeobox A13 (HoxA13), prostacyclin synthase (PTGIS), and periostin (POSTN) genes are more highly expressed in LUS than FUN of nonlaboring, but not laboring, myometrial cells at term. HoxA13 up-regulates transcription of PTGIS and POSTN genes. Elevated HoxA13 expression enhances myometrial cell contractility and cell-cell adhesion. Gene microarray studies show that HoxA13-regulated genes are associated with immune response, gap junction/cell adhesion, and pregnancy.

CONCLUSION

The LUS expresses higher levels of HoxA13, PTGIS, and POSTN, and is more contractile than the FUN at term prior to labor. This pregnancy-maintaining regionalization of myometrial function may be mediated by HoxA13.

Clinicopathologic and molecular characteristics of gastric cancer showing gastric and intestinal mucin phenotype

Gastric cancer (GC), one of the most common human cancers, can be classified into gastric or intestinal phenotype according to mucin expression. TP53 mutation, allelic deletion of the APC gene and nuclear staining of β-catenin are frequently detected in the intestinal phenotype of GC, whereas CDH1 gene mutation, microsatellite instability and DNA hypermethylation of MLH1 are common events in the gastric phenotype of GC. Our Serial Analysis of Gene Expression (SAGE) and Escherichia coli ampicillin secretion trap (CAST) analyses revealed that CDH17, REG4, OLFM4, HOXA10, DSC2, TSPAN8 and TM9SF3 are upregulated in GC and that CLDN18 is downregulated in GC. Expression of CDH17, REG4, HOXA10 and DSC2 and downregulation of CLDN18 are observed in the intestinal phenotype of GC. In contrast, OLFM4 is expressed in the gastric phenotype of GC. Expression of TSPAN8, TM9SF3 and HER2 are not associated with either gastric or intestinal phenotypes. Ectopic CDX2 expression plays a key function in the GC intestinal phenotype. MUC2, CDH17, REG4, DSC2 and ABCB1 are direct targets of CDX2. Importantly, these genes encode transmembrane/secretory proteins, indicating that the microenvironment as well as cancer cells are also different between gastric and intestinal phenotypes of GC.

Desmosomes in acquired disease

Desmosomes are cell-cell junctions that mediate adhesion and couple the intermediate filament cytoskeleton to sites of cell-cell contact. This architectural arrangement integrates adhesion and cytoskeletal elements of adjacent cells. The importance of this robust adhesion system is evident in numerous human diseases, both inherited and acquired, which occur when desmosome function is compromised. This review focuses on autoimmune and infectious diseases that impair desmosome function. In addition, we discuss emerging evidence that desmosomal genes are often misregulated in cancer. The emphasis of our discussion is placed on the way in which human diseases can inform our understanding of basic desmosome biology and in turn, the means by which fundamental advances in the cell biology of desmosomes might lead to new treatments for acquired diseases of the desmosome.

Assembly of the cardiac intercalated disk during pre- and postnatal development of the human heart

BACKGROUND

In cardiac muscle, the intercalated disk (ID) at the longitudinal cell-edges of cardiomyocytes provides as a macromolecular infrastructure that integrates mechanical and electrical coupling within the heart. Pathophysiological disturbance in composition of this complex is well known to trigger cardiac arrhythmias and pump failure. The mechanisms underlying assembly of this important cellular domain in human heart is currently unknown.

METHODS

We collected 18 specimens from individuals that died from non-cardiovascular causes. Age of the specimens ranged from a gestational age of 15 weeks through 11 years postnatal. Immunohistochemical labeling was performed against proteins comprising desmosomes, adherens junctions, the cardiac sodium channel and gap junctions to visualize spatiotemporal alterations in subcellular location of the proteins.

RESULTS

Changes in spatiotemporal localization of the adherens junction proteins (N-cadherin and ZO-1) and desmosomal proteins (plakoglobin, desmoplakin and plakophilin-2) were identical in all subsequent ages studied. After an initial period of diffuse and lateral labelling, all proteins were fully localized in the ID at approximately 1 year after birth. Nav1.5 that composes the cardiac sodium channel and the gap junction protein Cx43 follow a similar pattern but their arrival in the ID is detected at (much) later stages (two years for Nav1.5 and seven years for Cx43, respectively).

CONCLUSION

Our data on developmental maturation of the ID in human heart indicate that generation of the mechanical junctions at the ID precedes that of the electrical junctions with a significant difference in time. In addition arrival of the electrical junctions (Nav1.5 and Cx43) is not uniform since sodium channels localize much earlier than gap junction channels.

Plasma membrane--cortical cytoskeleton interactions: a cell biology approach with biophysical considerations

From a biophysical standpoint, the interface between the cell membrane and the cytoskeleton is an intriguing site where a "two-dimensional fluid" interacts with an exceedingly complex three-dimensional protein meshwork. The membrane is a key regulator of the cytoskeleton, which not only provides docking sites for cytoskeletal elements through transmembrane proteins, lipid binding-based, and electrostatic interactions, but also serves as the source of the signaling events and molecules that control cytoskeletal organization and remolding. Conversely, the cytoskeleton is a key determinant of the biophysical and biochemical properties of the membrane, including its shape, tension, movement, composition, as well as the mobility, partitioning, and recycling of its constituents. From a cell biological standpoint, the membrane-cytoskeleton interplay underlies--as a central executor and/or regulator--a multitude of complex processes including chemical and mechanical signal transduction, motility/migration, endo-/exo-/phagocytosis, and other forms of membrane traffic, cell-cell, and cell-matrix adhesion. The aim of this article is to provide an overview of the tight structural and functional coupling between the membrane and the cytoskeleton. As biophysical approaches, both theoretical and experimental, proved to be instrumental for our understanding of the membrane/cytoskeleton interplay, this review will "oscillate" between the cell biological phenomena and the corresponding biophysical principles and considerations. After describing the types of connections between the membrane and the cytoskeleton, we will focus on a few key physical parameters and processes (force generation, curvature, tension, and surface charge) and will discuss how these contribute to a variety of fundamental cell biological functions.

Silencing of desmoplakin decreases connexin43/Nav1.5 expression and sodium current in HL‑1 cardiomyocytes

Desmosomes and gap junctions are situated in the intercalated disks of cardiac muscle and maintain the integrity of mechanical coupling and electrical impulse conduction between cells. The desmosomal plakin protein, desmoplakin (DSP), also plays a crucial role in the stability of these interconnected components as well as gap junction connexin proteins. In addition to cell‑to‑cell junctions, other molecules, including voltage‑gated sodium channels (Nav1.5) are present in the intercalated disk and support the contraction of cardiac muscle. Mutations in genes encoding desmosome proteins may result in fatal arrhythmias, including arrhythmogenic right ventricular cardiomyopathy (ARVC). Therefore, the aim of the present study was to determine whether the presence of DSP is necessary for the normal function and localization of gap junction protein connexin43 (Cx43) and Nav1.5. To examine this hypothesis, RNA interference was utilized to knock down the expression of DSP in HL‑1 cells and the content, distribution and function of Cx43 and Nav1.5 was assessed. Western blotting and flow cytometry experiments revealed that Cx43 and Nav1.5 expression decreased following DSP silencing. In addition, immunofluorescence studies demonstrated that a loss of DSP expression led to an abnormal distribution of Cx43 and Nav1.5, while scrape‑loading dye/transfer revealed a decrease in dye transfer in DSP siRNA‑treated cells. The sodium current was also recorded by the whole‑cell patch clamp technique. The results indicated that DSP suppression decreased sodium current and slowed conduction velocity in cultured cells. The present study indicates that impaired mechanical coupling largely affects electrical synchrony, further uncovering the pathogenesis of ARVC.

Insights into the role of cell-cell junctions in physiology and disease

Contacting cells establish different classes of intricate structures at the cell-cell junctions. These structures are of increasing research interest as they regulate a broad variety of processes in development and disease. Further, in vitro studies are revealing that various cell-cell interaction proteins are involved not only in cell-cell processes but also in many additional aspects of physiology, such as migration and apoptosis. This chapter reviews the basic classification of cell-cell junctional structures and some of their representative proteins. Their roles in development and disease are briefly outlined, followed by a section on contemporary methods for probing cell-cell interactions and some recent developments. This chapter concludes with a few suggestions for potential research directions to further develop this promising area of study.

Regulation of spermiogenesis, spermiation and blood-testis barrier dynamics: novel insights from studies on Eps8 and Arp3

Spermiogenesis in the mammalian testis is the most critical post-meiotic developmental event occurring during spermatogenesis in which haploid spermatids undergo extensive cellular, molecular and morphological changes to form spermatozoa. Spermatozoa are then released from the seminiferous epithelium at spermiation. At the same time, the BTB (blood-testis barrier) undergoes restructuring to facilitate the transit of preleptotene spermatocytes from the basal to the apical compartment. Thus meiotic divisions take place behind the BTB in the apical compartment to form spermatids. These germ cells enter spermiogenesis to transform into elongating spermatids and then into spermatozoa to replace those that were released in the previous cycle. However, the mole-cular regulators that control spermiogenesis, in particular the dynamic changes that occur at the Sertoli cell-spermatid interface and at the BTB, are not entirely known. This is largely due to the lack of suitable animal models which can be used to study these events. During the course of our investigation to develop adjudin [1-(2,4-dichlorobenzyl)-1H-indazole-3-carbohydrazide] as a potential male contraceptive, this drug was shown to 'accelerate' spermiation by inducing the release of premature spermatids from the epithelium. Using this model, we have identified several molecules that are crucial in regulating the actin filament network and the unique adhesion protein complex at the Sertoli cell-spermatid interface known as the apical ES (ectoplasmic specialization). In the present review, we critically evaluate these and other findings in the literature as they relate to the restricted temporal and spatial expression of two actin regulatory proteins, namely Eps8 (epidermal growth factor receptor pathway substrate 8) and Arp3 (actin-related protein 3), which regulate these events.

The role of intracellular protein O-glycosylation in cell adhesion and disease

Post-translational protein modification, including phosphorylation, is generally quick and reversible, facilitating rapid biologic adjustments to altered cellular physiologic demands. In addition to protein phosphorylation, other post-translational modifications have been identified. Intracellular protein O-glycosylation, the addition of the simple sugar O-linked N-acetylglucosamine (O-GlcNAc) to serine/threonine residues, is a relatively recently identified post-translational modification that has added to the complexity by which protein function is regulated. Two intracellular enzymes, O-GlcNAc transferase and O-GlcNAcase, catalyze the addition and removal, respectively, of O-GlcNAc to serine and threonine side-chain hydroxyl groups. Numerous proteins, including enzymes, transcription factors, receptors and structural proteins have been shown to be modified by intracellular O-glycosylation. In this review, the mechanism and relevance of O-GlcNAc protein modification are discussed in the context of cell adhesion and several representative diseases.

Arrhythmogenic right ventricular cardiomyopathy plakophilin-2 mutations disrupt desmosome assembly and stability

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is characterized by life-threatening ventricular arrhythmias and fibrofatty replacement of the cardiac tissue. Desmosomes are prominent cell-cell junctions found in a variety of tissues that resist mechanical stress, including the heart, and recruit the intermediate filament cytoskeleton to sites of cell-cell contact. Mutations in several desmosomal components including plakophilin-2 have been identified in ARVC patients; however, the molecular interactions disrupted by plakophilin-2 mutations are currently unknown. To understand the pathological basis of ARVC, the authors analyzed desmosome assembly and stability in epithelial cell lines expressing mutants of plakophilin-2 found in ARVC patients. Mutant plakophilin-2 proteins were unable to disrupt established desmosomes when expressed in an E-cadherin-expressing epithelial cell model; however, they were unable to initiate de novo assembly of desmosomes in an N-cadherin-expressing epithelial cell model. These studies expand our understanding of desmosome assembly and dynamics.

Quantitative mass spectrometry catalogues Salmonella pathogenicity island-2 effectors and identifies their cognate host binding partners

Gram-negative bacterial pathogens have developed specialized secretion systems to transfer bacterial proteins directly into host cells. These bacterial effectors are central to virulence and reprogram host cell processes to favor bacterial survival, colonization, and proliferation. Knowing the complete set of effectors encoded by a particular pathogen is the key to understanding bacterial disease. In addition, the identification of the molecular assemblies that these effectors engage once inside the host cell is critical to determining the mechanism of action of each effector. In this work we used stable isotope labeling of amino acids in cell culture (SILAC), a powerful quantitative proteomics technique, to identify the proteins secreted by the Salmonella pathogenicity island-2 type three secretion system (SPI-2 T3SS) and to characterize the host interaction partners of SPI-2 effectors. We confirmed many of the known SPI-2 effectors and were able to identify several novel substrate candidates of this secretion system. We verified previously published host protein-effector binding pairs and obtained 11 novel interactions, three of which were investigated further and confirmed by reciprocal co-immunoprecipitation. The host cell interaction partners identified here suggest that Salmonella SPI-2 effectors target, in a concerted fashion, cellular processes such as cell attachment and cell cycle control that are underappreciated in the context of infection. The technology outlined in this study is specific and sensitive and serves as a robust tool for the identification of effectors and their host targets that is readily amenable to the study of other bacterial pathogens.

The biology of the desmosome-like junction a versatile anchoring junction and signal transducer in the seminiferous epithelium

Mammalian spermatogenesis, a complex process that involves the movement of developing germ cells across the seminiferous epithelium, entails extensive restructuring of Sertoli-Sertoli and Sertoli-germ cell junctions. Presently, it is not entirely clear how zygotene spermatocytes gain entry into the adluminal compartment of the seminiferous epithelium, which is sealed off from the systemic circulation by the Sertoli cell component of the blood-testis barrier, without compromising barrier integrity. To begin to address this question, it is critical that we first have a good understanding of the biology and the regulation of different types of Sertoli-Sertoli and Sertoli-germ cell junctions in the testis. Supported by recent studies in the field, we discuss how crosstalk between different types of junctions contributes to their restructuring during germ cell movement across the blood-testis barrier. We place special emphasis on the emerging role of desmosome-like junctions as signal transducers during germ cell movement across the seminiferous epithelium.

The calcium ATPase SERCA2 regulates desmoplakin dynamics and intercellular adhesive strength through modulation of PKC&alpha; signaling

Darier's disease (DD) is an inherited autosomal-dominant skin disorder characterized histologically by loss of adhesion between keratinocytes. DD is typically caused by mutations in sarcoendoplasmic reticulum Ca(2+)-ATPase isoform 2 (SERCA2), a major regulator of intracellular Ca(2+) homeostasis in the skin. However, a defined role for SERCA2 in regulating intercellular adhesion remains poorly understood. We found that diminution of SERCA2 function by pharmacological inhibition or siRNA silencing in multiple human epidermal-derived cell lines was sufficient to disrupt desmosome assembly and weaken intercellular adhesive strength. Specifically, SERCA2-deficient cells exhibited up to a 60% reduction in border translocation of desmoplakin (DP), the desmosomal cytolinker protein necessary for intermediate filament (IF) anchorage to sites of robust cell-cell adhesion. In addition, loss of SERCA2 impaired the membrane translocation of protein kinase C α (PKCα), a known regulator of DP-IF association and desmosome assembly, to the plasma membrane by up to 70%. Exogenous activation of PKCα in SERCA2-deficient cells was sufficient to rescue the defective DP localization, desmosome assembly, and intercellular adhesive strength to levels comparable to controls. Our findings indicate that SERCA2-deficiency is sufficient to impede desmosome assembly and weaken intercellular adhesive strength via a PKCα-dependent mechanism, implicating SERCA2 as a novel regulator of PKCα signaling.

Loss of PI3Kγ enhances cAMP-dependent MMP remodeling of the myocardial N-cadherin adhesion complexes and extracellular matrix in response to early biomechanical stress

RATIONALE

Mechanotransduction and the response to biomechanical stress is a fundamental response in heart disease. Loss of phosphoinositide 3-kinase (PI3K)γ, the isoform linked to G protein-coupled receptor signaling, results in increased myocardial contractility, but the response to pressure overload is controversial.

OBJECTIVE

To characterize molecular and cellular responses of the PI3Kγ knockout (KO) mice to biomechanical stress.

METHODS AND RESULTS

In response to pressure overload, PI3KγKO mice deteriorated at an accelerated rate compared with wild-type mice despite increased basal myocardial contractility. These functional responses were associated with compromised phosphorylation of Akt and GSK-3α. In contrast, isolated single cardiomyocytes from banded PI3KγKO mice maintained their hypercontractility, suggesting compromised interaction with the extracellular matrix as the primary defect in the banded PI3KγKO mice. β-Adrenergic stimulation increased cAMP levels with increased phosphorylation of CREB, leading to increased expression of cAMP-responsive matrix metalloproteinases (MMPs), MMP2, MT1-MMP, and MMP13 in cardiomyocytes and cardiofibroblasts. Loss of PI3Kγ resulted in increased cAMP levels with increased expression of MMP2, MT1-MMP, and MMP13 and increased MMP2 activation and collagenase activity in response to biomechanical stress. Selective loss of N-cadherin from the adhesion complexes in the PI3KγKO mice resulted in reduced cell adhesion. The β-blocker propranolol prevented the upregulation of MMPs, whereas MMP inhibition prevented the adverse remodeling with both therapies, preventing the functional deterioration in banded PI3KγKO mice. In banded wild-type mice, long-term propranolol prevented the adverse remodeling and systolic dysfunction with preservation of the N-cadherin levels.

CONCLUSIONS

The enhanced propensity to develop heart failure in the PI3KγKO mice is attributable to a cAMP-dependent upregulation of MMP expression and activity and disorganization of the N-cadherin/β-catenin cell adhesion complex. β-Blocker therapy prevents these changes thereby providing a novel mechanism of action for these drugs.

Comparison of two dimensional electrophoresis mouse colon proteomes before and after knocking out Aquaporin 8

Aquaporin (AQP) family plays a fundamental role in transmembrane water and small solutes movement. Within this family, aquaporin 8 (AQP8), showed to be widely distributed in the digestive system especially colon. To investigate the possible protein alterations involved in AQP8 regulation and trafficking, we extensively compared between wild type and AQP8 knockout mouse colon using semi-quantitative fluorescence- stained two dimensional gel electrophoresis (2-DE) coupled with nano LC-Ms/Ms. Our analysis revealed identification and regulation of 21 proteins, most notably, actin-related family which suggests its possible involvement in regulating AQP8 secretory vesicles migration to be integrated as a cell membrane protein.

Exploring the Nature of Desmosomal Cadherin Associations in 3D

Desmosomes are a complex assembly of protein molecules that mediate adhesion between adjacent cells. Desmosome composition is well established and spatial relationships between components have been identified. Intercellular cell-cell adhesion is created by the interaction of extracellular domains of desmosomal cadherins, namely, desmocollins and desmogleins. High-resolution methods have provided insight into the structural interactions between cadherins. However, there is a lack of understanding about the architecture of the intact desmosomes and the physical principles behind their adhesive strength are unclear. Electron Tomography (ET) studies have offered three-dimensional visual data of desmosomal cadherin associations at molecular resolution. This review discusses the merits of two cadherin association models represented using ET. We discuss the possible role of sample preparation on the structural differences seen between models and the possibility of adaptive changes in the structure as a direct consequence of mechanical stress and stratification.

Transcription profiling provides insights into gene pathways involved in horn and scurs development in cattle

BACKGROUND

Two types of horns are evident in cattle - fixed horns attached to the skull and a variation called scurs, which refers to small loosely attached horns. Cattle lacking horns are referred to as polled. Although both the Poll and Scurs loci have been mapped to BTA1 and 19 respectively, the underlying genetic basis of these phenotypes is unknown, and so far, no candidate genes regulating these developmental processes have been described. This study is the first reported attempt at transcript profiling to identify genes and pathways contributing to horn and scurs development in Brahman cattle, relative to polled counterparts.

RESULTS

Expression patterns in polled, horned and scurs tissues were obtained using the Agilent 44 k bovine array. The most notable feature when comparing transcriptional profiles of developing horn tissues against polled was the down regulation of genes coding for elements of the cadherin junction as well as those involved in epidermal development. We hypothesize this as a key event involved in keratinocyte migration and subsequent horn development. In the polled-scurs comparison, the most prevalent differentially expressed transcripts code for genes involved in extracellular matrix remodelling, which were up regulated in scurs tissues relative to polled.

CONCLUSION

For this first time we describe networks of genes involved in horn and scurs development. Interestingly, we did not observe differential expression in any of the genes present on the fine mapped region of BTA1 known to contain the Poll locus.

Animal models of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathies are a heterogeneous group of pathological conditions that give rise to myocardial dysfunction with an increased risk for atrial or ventricular arrhythmias. Inherited defects in cardiomyocyte proteins in the sarcomeric contractile apparatus, the cytoskeleton and desmosomal cell-cell contact junctions are becoming recognized increasingly as major causes of sudden cardiac death in the general population. Animal models have been developed for the systematic dissection of the genetic pathways involved in the pathogenesis of arrhythmogenic cardiomyopathies. This review presents an overview of current animal models for arrhythmogenic right ventricular cardiomyopathy (ARVC), hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) associated with cardiac arrhythmias and sudden cardiac death.

Discovering the molecular components of intercellular junctions--a historical view

The organization of metazoa is based on the formation of tissues and on tissue-typical functions and these in turn are based on cell-cell connecting structures. In vertebrates, four major forms of cell junctions have been classified and the molecular composition of which has been elucidated in the past three decades: Desmosomes, which connect epithelial and some other cell types, and the almost ubiquitous adherens junctions are based on closely cis-packed glycoproteins, cadherins, which are associated head-to-head with those of the hemi-junction domain of an adjacent cell, whereas their cytoplasmic regions assemble sizable plaques of special proteins anchoring cytoskeletal filaments. In contrast, the tight junctions (TJs) and gap junctions (GJs) are formed by tetraspan proteins (claudins and occludins, or connexins) arranged head-to-head as TJ seal bands or as paracrystalline connexin channels, allowing intercellular exchange of small molecules. The by and large parallel discoveries of the junction protein families are reported.

Myocyte necrosis underlies progressive myocardial dystrophy in mouse dsg2-related arrhythmogenic right ventricular cardiomyopathy

Mutations in the cardiac desmosomal protein desmoglein-2 (DSG2) are associated with arrhythmogenic right ventricular cardiomyopathy (ARVC). We studied the explanted heart of a proband carrying the DSG2-N266S mutation as well as transgenic mice (Tg-NS) with cardiac overexpression of the mouse equivalent of this mutation, N271S-dsg2, with the aim of investigating the pathophysiological mechanisms involved. Transgenic mice recapitulated the clinical features of ARVC, including sudden death at young age, spontaneous ventricular arrhythmias, cardiac dysfunction, and biventricular dilatation and aneurysms. Investigation of transgenic lines with different levels of transgene expression attested to a dose-dependent dominant-negative effect of the mutation. We demonstrate for the first time that myocyte necrosis is the key initiator of myocardial injury, triggering progressive myocardial damage, including an inflammatory response and massive calcification within the myocardium, followed by injury repair with fibrous tissue replacement, and myocardial atrophy. These observations were supported by findings in the explanted heart from the patient. Insight into mechanisms initiating myocardial damage in ARVC is a prerequisite to the future development of new therapies aimed at delaying onset or progression of the disease.

Mutations of plakophilin-2 in Chinese with arrhythmogenic right ventricular dysplasia/cardiomyopathy

Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is an inherited heart muscle disease associated with increased risks of sudden death, particularly in young, otherwise healthy, patients. The pathologic features are progressive myocardial atrophy and fibrofatty replacement. Plakophilin-2 (PKP2) is reported as the most common ARVD/C-causing gene in Western countries. In this study we aimed to determine the prevalence of PKP2 mutations in Chinese patients with ARVD/C and their phenotype characteristics. Genotype and phenotype were investigated in a cohort of 18 unrelated Chinese patients with a clinical diagnosis of ARVD/C. Direct sequencing of PKP2 led to the identification of 5 novel heterozygous mutations (R158K, Q211X, L419S, A793D, and N852fsX930) in 39% of patients (7 of 18) with ARVD/C. Among them, N852fsX930 was found in 3 unrelated young patients who presented with symptomatic ventricular tachyarrhythmia. Nevertheless, no significant difference could be detected between patients with ARVD/C with (n = 7) and without (n = 11) PKP2 mutations with regard to the phenotype characteristics and clinical outcomes. Decreased penetrance was prominent in family members. In conclusion, 5 novel PKP2 mutations were identified in a cohort of symptomatic Chinese patients with ARVD/C. N852fsX930 appeared to be a hot-spot mutation in which patients presented with a severe ARVD/C phenotype, and 2/3 had early onset of arrhythmic events. No significant difference was found in phenotype characteristics between patients with ARVD/C with and without PKP2 mutations. The decreased penetrance indicated that an ARVD/C diagnosis cannot solely rely on genotyping results.

Expression of N-terminal truncated desmoglein 3 (deltaNDg3) in epidermis and its role in keratinocyte differentiation

During a search for keratinocyte differentiation-related genes, we obtained a cDNA fragment from the 5'-untranslated region of a previously identified splicing variant of desmoglein 3 (Dg3). This transcript encodes a protein of 282 amino acids, which corresponds to the N-terminal truncated intracellular domain of Dg3 (deltaNDg3). Northern blot analysis detected a 4.6-kb transcript matching the predicted size of deltaNDg3 mRNA, and Western blot analysis with an antibody raised against the Dg3 C-terminus (H-145) detected a 31-kDa protein. Increased deltaNDg3 expression was observed in differentiating keratinocytes by RT-PCR and Western blot analysis, suggesting that deltaNDg3 is indeed a differentiation-related gene product. In immunohistochemical studies of normal and pathologic tissues, H-145 antibody detected the protein in the cytoplasm of suprabasal layer cells, whereas an antibody directed against the N-terminal region of Dg3 (AF1720) reacted with a membrane protein in the basal layer. In addition, deltaNDg3 transcript and protein were upregulated in psoriatic epidermis, and protein expression appeared to increase in epidermal tumors including Bowen's disease and squamous cell carcinoma. Moreover, overexpression of deltaNDg3 led to increased migration and weakening of cell adhesion. These results suggest that deltaNDg3 have a role in keratinocyte differentiation, and that may be related with tumorigenesis of epithelial origin.

Mutations in the desmoglein 1 gene in five Pakistani families with striate palmoplantar keratoderma

BACKGROUND

Striate palmoplantar keratoderma (SPPK; OMIM #148700) is a rare autosomal dominant genodermatosis characterized by linear hyperkeratosis on the digits and hyperkeratosis on the palms and soles. SPPK is known to be caused by heterozygous mutations in either the desmoglein 1 (DSG1), desmoplakin (DSP), or keratin 1 (KRT1) genes.

OBJECTIVE

To define the molecular basis of SPPK in five Pakistani families showing a clear autosomal dominant inheritance pattern of SPPK.

METHODS

Based on previous reports of DSG1 mutations in SPPK, we performed direct sequencing of the DSG1 gene of all five families.

RESULTS

Mutation analysis resulted in the identification of one recurrent mutation (p.R26X) and four novel mutations (c.Ivs4-2A>G, c.515C>T, c.Ivs9-3C>G, and c.1399delA) in the DSG1 gene. Each mutation is predicted to cause haploinsufficiency of DSG1 protein.

CONCLUSION

The results of our study further underscore the significance of the desmoglein gene family in diseases of epidermal integrity.

E7 oncoprotein of novel human papillomavirus type 108 lacking the E6 gene induces dysplasia in organotypic keratinocyte cultures

The genome organization of the novel human papillomavirus type 108 (HPV108), isolated from a low-grade cervical lesion, deviates from those of other HPVs in lacking an E6 gene. The three related HPV types HPV103, HPV108, and HPV101 were isolated from cervicovaginal cells taken from normal genital mucosa (HPV103) and low-grade (HPV108) and high-grade cervical (HPV101) intraepithelial neoplasia (Z. Chen, M. Schiffman, R. Herrero, R. DeSalle, and R. D. Burk, Virology 360:447-453, 2007, and this report). Their unusual genome organization, against the background of considerable phylogenetic distance from the other HPV types usually associated with lesions of the genital tract, prompted us to investigate whether HPV108 E7 per se is sufficient to induce the above-mentioned clinical lesions. Expression of HPV108 E7 in organotypic keratinocyte cultures increases proliferation and apoptosis, focal nuclear polymorphism, and polychromasia. This is associated with irregular intra- and extracellular lipid accumulation and loss of the epithelial barrier. These alterations are linked to HPV108 E7 binding to pRb and inducing its decrease, an increase in PCNA expression, and BrdU incorporation, as well as increased p53 and p21(CIP1) protein levels. A delay in keratin K10 expression, increased expression of keratins K14 and K16, and loss of the corneal proteins involucrin and loricrin have also been noted. These modifications are suggestive of infection by a high-risk papillomavirus.

Feeder-free monolayer cultures of human embryonic stem cells express an epithelial plasma membrane protein profile

Human embryonic stem cells (hESCs) are often cocultured on mitotically inactive fibroblast feeder cells to maintain their undifferentiated state. Under these growth conditions, hESCs form multilayered colonies of morphologically heterogeneous cells surrounded by flattened mesenchymal cells. In contrast, hESCs grown in feeder cell-conditioned medium on Matrigel instead tend to grow as monolayers with uniform morphology. Using mass spectrometry and immunofluorescence microscopy, we showed that hESCs under these conditions primarily express proteins belonging to epithelium-related cell-cell adhesion complexes, including adherens junctions, tight junctions, desmosomes, and gap junctions. This indicates that monolayers of hESCs cultured under feeder-free conditions retain a homogeneous epithelial phenotype similar to that of the upper central cell layer of colonies maintained on feeder cells. Notably, feeder-free hESCs also coexpressed vimentin, which is usually associated with mesenchyme, suggesting that these cells may have undergone epithelium-to-mesenchyme transitions, indicating differentiation. However, if grown on a "soft" substrate (Hydrogel), intracellular vimentin levels were substantially reduced. Moreover, when hESCs were transferred back to feeder cells, expression of vimentin was again absent from the epithelial cell population. These results imply that on tissue culture substrates, vimentin expression is most likely a stress-induced response, unrelated to differentiation. Disclosure of potential conflicts of interest is found at the end of this article.

Egfl7 knockdown causes defects in the extension and junctional arrangements of endothelial cells during zebrafish vasculogenesis

The endothelial cell (EC) -specific secreted protein EGFL7 is important for tubulogenesis in newly forming blood vessels. We studied its role in vascular tube formation by a quantitative ultrastructural analysis of Egfl7-knockdown zebrafish embryos. At 24 hours postfertilization, the endothelia of dorsal aorta (DA) and posterior cardinal vein (PCV) were correctly anchored to the hypochord and endoderm, respectively, but failed to expand into the vascular area. This resulted in vessels with reduced or split lumen and open sheets of ECs. Concomitantly, the organization of hematopoietic cells-identified by the presence of previously undescribed membrane tubules-between DA and PCV, and within the vessels, was severely disturbed. Strikingly, ectopic cell junctions occurred across the obstructed vessel lumen, on the luminal EC surfaces, which in control conditions never display junctions of any kind. These data suggest that Egfl7 provides ECs with a cue for their extension into the vascular area and in establishing EC cell polarity.