Phosphorylation of the desmoplakin COOH terminus negatively regulates its interaction with keratin intermediate filament networks

Breathing new life into the study of COPD with genes identified from genome-wide association studies

COPD is a major cause of morbidity and mortality globally. While the significance of environmental exposures in disease pathogenesis is well established, the functional contribution of genetic factors has only in recent years drawn attention. Notably, many genes associated with COPD risk are also linked with lung function. Because reduced lung function precedes COPD onset, this association is consistent with the possibility that derangements leading to COPD could arise during lung development. In this review, we summarise the role of leading genes (HHIP, FAM13A, DSP, AGER and TGFB2) identified by genome-wide association studies in lung development and COPD. Because many COPD genome-wide association study genes are enriched in lung epithelial cells, we focus on the role of these genes in the lung epithelium in development, homeostasis and injury.

PP2A-B55alpha controls keratinocyte adhesion through dephosphorylation of the Desmoplakin C-terminus

Critical for the maintenance of epidermal integrity and function are attachments between intermediate filaments (IF) and intercellular junctions called desmosomes. The desmosomal cytoplasmic plaque protein desmoplakin (DP) is essential for anchoring IF to the junction. DP-IF interactions are regulated by a phospho-regulatory motif within the DP C-terminus controlling keratinocyte intercellular adhesion. Here we identify the protein phosphatase 2A (PP2A)-B55α holoenzyme as the major serine/threonine phosphatase regulating DP's C-terminus and consequent intercellular adhesion. Using a combination of chemical and genetic approaches, we show that the PP2A-B55α holoenzyme interacts with DP at intercellular membranes in 2D- and 3D- epidermal models and human skin samples. Our experiments demonstrate that PP2A-B55α regulates the phosphorylation status of junctional DP and is required for maintaining strong desmosome-mediated intercellular adhesion. These data identify PP2A-B55α as part of a regulatory module capable of tuning intercellular adhesion strength and a candidate disease target in desmosome-related disorders of the skin and heart.

A truncating variant altering the extreme C-terminal region of desmoplakin (DSP) suggests the crucial functional role of the region: a case report study

BACKGROUND

Homozygous truncating mutations located in the C-terminal region of the desmoplakin gene (DSP) are known to mainly cause Carvajal syndrome, an autosomal recessive syndromic form of arrhythmogenic cardiomyopathy with an extra-cardiac cutaneous phenotype.

CASE PRESENTATION

Here we describe a female proband with a documented arrhythmogenic left ventricular cardiomyopathy and a syncopal episode at the age of 13, who was found homozygous for the novel DSP variant: NM_004415.4:c.8586delC, p.(Ser2863Hisfs*20) at the extreme C-terminal region of the protein, just 8 amino acids upstream the stop codon. She did not have any of the typical dermatological symptoms that characterize Carvajal syndrome. Her brother had died suddenly at the age of 18 during exercise and was found homozygous for the same variant at the post-mortem, while their parents were heterozygous. The region of origin of both parents was the same geographic area of Greece, but they were not aware of any common ancestor. Detailed clinical examination revealed that the mother displayed a mild arrhythmic phenotype, while the father was asymptomatic.

CONCLUSION

These observations pinpoint to a significant functional role of the extreme C-terminal tail of the protein.

Towards a Better Understanding of Genotype-Phenotype Correlations and Therapeutic Targets for Cardiocutaneous Genes: The Importance of Functional Studies above Prediction

Genetic variants in gene-encoding proteins involved in cell−cell connecting structures, such as desmosomes and gap junctions, may cause a skin and/or cardiac phenotype, of which the combination is called cardiocutaneous syndrome. The cardiac phenotype is characterized by cardiomyopathy and/or arrhythmias, while the skin particularly displays phenotypes such as keratoderma, hair abnormalities and skin fragility. The reported variants associated with cardiocutaneous syndrome, in genes DSP, JUP, DSC2, KLHL24, GJA1, are classified by interpretation guidelines from the American College of Medical Genetics and Genomics. The genotype−phenotype correlation, however, remains poorly understood. By providing an overview of variants that are assessed for a functional protein pathology, we show that this number (n = 115) is low compared to the number of variants that are assessed by in silico algorithms (>5000). As expected, there is a mismatch between the prediction of variant pathogenicity and the prediction of the functional effect compared to the real functional evidence. Aiding to improve genotype−phenotype correlations, we separate variants into ‘protein reducing’ or ‘altered protein’ variants and provide general conclusions about the skin and heart phenotype involved. We conclude by stipulating that adequate prognoses can only be given, and targeted therapies can only be designed, upon full knowledge of the protein pathology through functional investigation.

CRISPR interference interrogation of COPD GWAS genes reveals the functional significance of desmoplakin in iPSC-derived alveolar epithelial cells

Genome-wide association studies (GWAS) have identified dozens of loci associated with chronic obstructive pulmonary disease (COPD) susceptibility; however, the function of associated genes in the cell type(s) affected in disease remains poorly understood, partly due to a lack of cell models that recapitulate human alveolar biology. Here, we apply CRISPR interference to interrogate the function of nine genes implicated in COPD by GWAS in induced pluripotent stem cell-derived type 2 alveolar epithelial cells (iAT2s). We find that multiple genes implicated by GWAS affect iAT2 function, including differentiation potential, maturation, and/or proliferation. Detailed characterization of the GWAS gene DSP demonstrates that it regulates iAT2 cell-cell junctions, proliferation, mitochondrial function, and response to cigarette smoke-induced injury. Our approach thus elucidates the biological function, as well as disease-relevant consequences of dysfunction, of genes implicated in COPD by GWAS in type 2 alveolar epithelial cells.

Structure and regulation of desmosomes in intercalated discs: Lessons from epithelia

For electromechanical coupling of cardiomyocytes, intercalated discs (ICDs) are pivotal as highly specialized intercellular contact areas. ICD consists of adhesive contacts, such as desmosomes and adherens junctions (AJs) that are partially intermingled and thereby form an area composita to provide mechanical strength, as well as gap junctions (GJ) and sodium channels for excitation propagation. In contrast, in epithelia, mixed junctions with features of desmosomes and AJs are regarded as transitory primarily during the formation of desmosomes. The anatomy of desmosomes is defined by a typical ultrastructure with dense intracellular plaques anchoring the cadherin-type adhesion molecules to the intermediate filament cytoskeleton. Desmosomal diseases characterized by impaired adhesive and signalling functions of desmosomal contacts lead to arrhythmogenic cardiomyopathy when affecting cardiomyocytes and cause pemphigus when manifesting in keratinocytes or present as cardiocutaneous syndromes when both cell types are targeted by the disease, which underscores the high biomedical relevance of these cell contacts. Therefore, comparative analyses regarding the structure and regulation of desmosomal contacts in cardiomyocytes and epithelial cells are helpful to better understand disease pathogenesis. In this brief review, we describe the structural properties of ICD compared to epithelial desmosomes and suggest that mechanisms regulating adhesion may at least in part be comparable. Also, we discuss whether phenomena such as hyperadhesion or the bidirectional regulation of desmosomes to serve as signalling hubs in epithelial cells may also be relevant for ICD.

Activation of the receptor tyrosine kinase RET improves long-term hematopoietic stem cell outgrowth and potency

Expansion of human hematopoietic stem cells (HSCs) is a rapidly advancing field showing great promise for clinical applications. Recent evidence has implicated the nervous system and glial family ligands (GFLs) as potential drivers of hematopoietic survival and self-renewal in the bone marrow niche; how to apply this process to HSC maintenance and expansion has yet to be explored. We show a role for the GFL receptor, RET, at the cell surface of HSCs in mediating sustained cellular growth, resistance to stress, and improved cell survival throughout in vitro expansion. HSCs treated with the key RET ligand/coreceptor complex, glial-derived neurotrophic factor and its coreceptor, exhibit improved progenitor function at primary transplantation and improved long-term HSC function at secondary transplantation. Finally, we show that RET drives a multifaceted intracellular signaling pathway, including key signaling intermediates protein kinase B, extracellular signal-regulated kinase 1/2, NF-κB, and p53, responsible for a wide range of cellular and genetic responses that improve cell growth and survival under culture conditions.

The keratin-desmosome scaffold: pivotal role of desmosomes for keratin network morphogenesis

Desmosome-anchored keratin intermediate filaments (KFs) are essential for epithelial coherence. Yet, desmosomal KF attachment and network organization are still unexplored in vivo. We, therefore, monitored KF network morphogenesis in fluorescent keratin 8 knock-in murine embryos revealing keratin enrichment at newly formed desmosomes followed by KF formation, KF elongation and KF fusion. To examine details of this process and its coupling to desmosome formation, we studied fluorescent keratin and desmosomal protein reporter dynamics in the periphery of expanding HaCaT keratinocyte colonies. Less than 3 min after the start of desmosomal proteins clustering non-filamentous keratin enriched at these sites followed by KF formation and elongation. Subsequently, desmosome-anchored KFs merged into stable bundles generating a rim-and-spokes system consisting of subcortical KFs connecting desmosomes to each other and radial KFs connecting desmosomes to the cytoplasmic KF network. We conclude that desmosomes are organizing centers for the KF cytoskeleton with a hitherto unknown nucleation capacity.

Protein Phosphatase 2A: More Than a Passenger in the Regulation of Epithelial Cell-Cell Junctions

Cell–cell adhesion plays a key role in the maintenance of the epithelial barrier and apicobasal cell polarity, which is crucial for homeostasis. Disruption of cell–cell adhesion is a hallmark of numerous pathological conditions, including invasive carcinomas. Adhesion between apposing cells is primarily regulated by three types of junctional structures: desmosomes, adherens junctions, and tight junctions. Cell junctional structures are highly regulated multiprotein complexes that also serve as signaling platforms to control epithelial cell function. The biogenesis, integrity, and stability of cell junctions is controlled by complex regulatory interactions with cytoskeletal and polarity proteins, as well as modulation of key component proteins by phosphorylation/dephosphorylation processes. Not surprisingly, many essential signaling molecules, including protein Ser/Thr phosphatase 2A (PP2A) are associated with intercellular junctions. Here, we examine how major PP2A enzymes regulate epithelial cell–cell junctions, either directly by associating with and dephosphorylating component proteins, or indirectly by affecting signaling pathways that control junctional integrity and cytoskeletal dynamics. PP2A deregulation has severe consequences on the stability and functionality of these structures, and disruption of cell–cell adhesion and cell polarity likely contribute to the link between PP2A dysfunction and human carcinomas.

Mechanical loading of desmosomes depends on the magnitude and orientation of external stress

Desmosomes are intercellular adhesion complexes that connect the intermediate filament cytoskeletons of neighboring cells, and are essential for the mechanical integrity of mammalian tissues. Mutations in desmosomal proteins cause severe human pathologies including epithelial blistering and heart muscle dysfunction. However, direct evidence for their load-bearing nature is lacking. Here we develop Förster resonance energy transfer (FRET)-based tension sensors to measure the forces experienced by desmoplakin, an obligate desmosomal protein that links the desmosomal plaque to intermediate filaments. Our experiments reveal that desmoplakin does not experience significant tension under most conditions, but instead becomes mechanically loaded when cells are exposed to external mechanical stresses. Stress-induced loading of desmoplakin is transient and sensitive to the magnitude and orientation of the applied tissue deformation, consistent with a stress absorbing function for desmosomes that is distinct from previously analyzed cell adhesion complexes.

Desmosomes are intercellular adhesion complexes that connect the intermediate filament cytoskeletons of neighboring cells but direct evidence for their load-bearing nature is lacking. Here the authors develop FRET-based tension sensors to measure the forces experienced by desmoplakin and infer that desmosomes become mechanically loaded when cells are exposed to external mechanical stresses.

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.

Development of a Novel Green Fluorescent Protein-Based Binding Assay to Study the Association of Plakins with Intermediate Filament Proteins

Protein-protein interactions are fundamental for most biological processes, such as the formation of cellular structures and enzymatic complexes or in signaling pathways. The identification and characterization of protein-protein interactions are therefore essential for understanding the mechanisms and regulation of biological systems. The organization and dynamics of the cytoskeleton, as well as its anchorage to specific sites in the plasma membrane and organelles, are regulated by the plakins. These structurally related proteins anchor different cytoskeletal networks to each other and/or to other cellular structures. The association of several plakins with intermediate filaments (IFs) is critical for maintenance of the cytoarchitecture. Pathogenic mutations in the genes encoding different plakins can lead to dramatic manifestations, occurring principally in the skin, striated muscle, and/or nervous system, due to cytoskeletal disorganization resulting in abnormal cell fragility. Nevertheless, it is still unclear how plakins bind to IFs, although some general rules are slowly emerging. We here describe in detail a recently developed protein-protein fluorescence binding assay, based on the production of recombinant proteins tagged with green fluorescent protein (GFP) and their use as fluid-phase fluorescent ligands on immobilized IF proteins. Using this method, we have been able to assess the ability of C-terminal regions of GFP-tagged plakin proteins to bind to distinct IF proteins and IF domains. This simple and sensitive technique, which is expected to facilitate further studies in this area, can also be potentially employed for any kind of protein-protein interaction studies.

GSK3- and PRMT-1-dependent modifications of desmoplakin control desmoplakin-cytoskeleton dynamics

Phosphorylation and methylation of desmoplakin are required for proper junction assembly and adhesion strengthening, and inhibition of these modifications might contribute to skin and heart diseases.

Intermediate filament (IF) attachment to intercellular junctions is required for skin and heart integrity, but how the strength and dynamics of this attachment are modulated during normal and pathological remodeling is poorly understood. We show that glycogen synthase kinase 3 (GSK3) and protein arginine methyltransferase 1 (PRMT-1) cooperate to orchestrate a series of posttranslational modifications on the IF-anchoring protein desmoplakin (DP) that play an essential role in coordinating cytoskeletal dynamics and cellular adhesion. Front-end electron transfer dissociation mass spectrometry analyses of DP revealed six novel serine phosphorylation sites dependent on GSK3 signaling and four novel arginine methylation sites including R2834, the mutation of which has been associated with arrhythmogenic cardiomyopathy (AC). Inhibition of GSK3 or PRMT-1 or overexpression of the AC-associated mutant R2834H enhanced DP–IF associations and delayed junction assembly. R2834H blocked the GSK3 phosphorylation cascade and reduced DP–GSK3 interactions in cultured keratinocytes and in the hearts of transgenic R2834H DP mice. Interference with this regulatory machinery may contribute to skin and heart diseases.

Plakophilin 3 mediates Rap1-dependent desmosome assembly and adherens junction maturation

Desmosomal Armadillo family member Pkp3 is established as a coordinator of desmosome and adherens junction assembly and maturation through its physical and functional association with Rap1. It thus functions in a manner distinct from the closely related Pkp2.

The pathways driving desmosome and adherens junction assembly are temporally and spatially coordinated, but how they are functionally coupled is poorly understood. Here we show that the Armadillo protein plakophilin 3 (Pkp3) mediates both desmosome assembly and E-cadherin maturation through Rap1 GTPase, thus functioning in a manner distinct from the closely related plakophilin 2 (Pkp2). Whereas Pkp2 and Pkp3 share the ability to mediate the initial phase of desmoplakin (DP) accumulation at sites of cell–cell contact, they play distinct roles in later steps: Pkp3 is required for assembly of a cytoplasmic population of DP-enriched junction precursors, whereas Pkp2 is required for transfer of the precursors to the membrane. Moreover, Pkp3 forms a complex with Rap1 GTPase, promoting its activation and facilitating desmosome assembly. We show further that Pkp3 deficiency causes disruption of an E-cadherin/Rap1 complex required for adherens junction sealing. These findings reveal Pkp3 as a coordinator of desmosome and adherens junction assembly and maturation through its functional association with Rap1.

In VivoFunction of Desmosomes

Desmosomes are morphologically and biochemically defined cell-cell junctions that are required for maintaining the mechanical integrity of skin and the heart in adult mammals. Furthermore, since mice with null mutations in desmosomal plaque proteins (plakoglobin and desmoplakin) die in utero, it is also evident that desmosomes are indispensable for normal embryonic development. This review focuses on the role of desmosomes in vivo. We will summarize the effects of mutations in desmosomal genes on pre- and post-embryonic development of mouse and man and discuss recent findings relating to the specific role of desmosomal cadherins in skin differentiation and homeostasis.

Beyond expectations: novel insights into epidermal keratin function and regulation

The epidermis is a stratified epithelium that relies on its cytoskeleton and cell junctions to protect the body against mechanical injury, dehydration, and infections. Keratin intermediate filament proteins are involved in many of these functions by forming cell-specific cytoskeletal scaffolds crucial for the maintenance of cell and tissue integrity. In response to various stresses, the expression and organization of keratins are altered at transcriptional and posttranslational levels to restore tissue homeostasis. Failure to restore tissue homeostasis in the presence of keratin gene mutations results in acute and chronic skin disorders for which currently no rational therapies are available. Here, we review the recent progress on the role of keratins in cytoarchitecture, adhesion, signaling, and inflammation. By focusing on epidermal keratins, we illustrate the contribution of keratin isotypes to differentiated epithelial functions.

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.

Mechanistic basis of desmosome-targeted diseases

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

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.

Keratins control intercellular adhesion involving PKC-α-mediated desmoplakin phosphorylation

Keratins limit PKC-α phosphorylation activity and desmosome turnover to ensure the stability of epithelial intracellular adhesion.

Maintenance of epithelial cell adhesion is crucial for epidermal morphogenesis and homeostasis and relies predominantly on the interaction of keratins with desmosomes. Although the importance of desmosomes to epidermal coherence and keratin organization is well established, the significance of keratins in desmosome organization has not been fully resolved. Here, we report that keratinocytes lacking all keratins show elevated, PKC-α–mediated desmoplakin phosphorylation and subsequent destabilization of desmosomes. We find that PKC-α activity is regulated by Rack1–keratin interaction. Without keratins, desmosomes assemble but are endocytosed at accelerated rates, rendering epithelial sheets highly susceptible to mechanical stress. Re-expression of the keratin pair K5/14, inhibition of PKC-α activity, or blocking of endocytosis reconstituted both desmosome localization at the plasma membrane and epithelial adhesion. Our findings identify a hitherto unknown mechanism by which keratins control intercellular adhesion, with potential implications for tumor invasion and keratinopathies, settings in which diminished cell adhesion facilitates tissue fragility and neoplastic growth.

New model of action for mood stabilizers: phosphoproteome from rat pre-frontal cortex synaptoneurosomal preparations

Background

Mitochondrial short and long-range movements are necessary to generate the energy needed for synaptic signaling and plasticity. Therefore, an effective mechanism to transport and anchor mitochondria to pre- and post-synaptic terminals is as important as functional mitochondria in neuronal firing. Mitochondrial movement range is regulated by phosphorylation of cytoskeletal and motor proteins in addition to changes in mitochondrial membrane potential. Movement direction is regulated by serotonin and dopamine levels. However, data on mitochondrial movement defects and their involvement in defective signaling and neuroplasticity in relationship with mood disorders is scarce. We have previously reported the effects of lithium, valproate and a new antipsychotic, paliperidone on protein expression levels at the synaptic level.

Hypothesis

Mitochondrial function defects have recently been implicated in schizophrenia and bipolar disorder. We postulate that mood stabilizer treatment has a profound effect on mitochondrial function, synaptic plasticity, mitochondrial migration and direction of movement.

Methods

Synaptoneurosomal preparations from rat pre-frontal cortex were obtained after 28 daily intraperitoneal injections of lithium, valproate and paliperidone. Phosphorylated proteins were identified using 2D-DIGE and nano LC-ESI tandem mass spectrometry.

Results

Lithium, valproate and paliperidone had a substantial and common effect on the phosphorylation state of specific actin, tubulin and myosin isoforms as well as other proteins associated with neurofilaments. Furthermore, different subunits from complex III and V of the electron transfer chain were heavily phosphorylated by treatment with these drugs indicating selective phosphorylation.

Conclusions

Mood stabilizers have an effect on mitochondrial function, mitochondrial movement and the direction of this movement. The implications of these findings will contribute to novel insights regarding clinical treatment and the mode of action of these drugs.

The molecular composition and function of desmosomes

Desmosomes are intercellular adhesive junctions that are particularly prominent in tissues experiencing mechanical stress, such as the heart and epidermis. Whereas the related adherens junction links actin to calcium-dependent adhesion molecules known as classical cadherins, desmosomes link intermediate filaments (IF) to the related subfamily of desmosomal cadherins. By tethering these stress-bearing cytoskeletal filaments to the plasma membrane, desmosomes serve as integrators of the IF cytoskeleton throughout a tissue. Recent evidence suggests that IF attachment in turn strengthens desmosomal adhesion. This collaborative arrangement results in formation of a supracellular network, which is critical for imparting mechanical integrity to tissues. Diseases and animal models targeting desmosomal components highlight the importance of desmosomes in development and tissue integrity, while the downregulation of individual protein components in cancer metastasis and wound healing suggests their importance in cell homeostasis. This chapter will provide an update on desmosome composition, function, and regulation, and will also discuss recent work which raises the possibility that desmosome proteins do more than play a structural role in tissues where they reside.

Plectin interacts with the rod domain of type III intermediate filament proteins desmin and vimentin

Plectin is a versatile cytolinker protein critically involved in the organization of the cytoskeletal filamentous system. The muscle-specific intermediate filament (IF) protein desmin, which progressively replaces vimentin during differentiation of myoblasts, is one of the important binding partners of plectin in mature muscle. Defects of either plectin or desmin cause muscular dystrophies. By cell transfection studies, yeast two-hybrid, overlay and pull-down assays for binding analysis, we have characterized the functionally important sequences for the interaction of plectin with desmin and vimentin. The association of plectin with both desmin and vimentin predominantly depended on its fifth plakin repeat domain and downstream linker region. Conversely, the interaction of desmin and vimentin with plectin required sequences contained within the segments 1A-2A of their central coiled-coil rod domain. This study furthers our knowledge of the interaction between plectin and IF proteins important for maintenance of cytoarchitecture in skeletal muscle. Moreover, binding of plectin to the conserved rod domain of IF proteins could well explain its broad interaction with most types of IFs.

The calcium ATPase SERCA2 regulates desmoplakin dynamics and intercellular adhesive strength through modulation of PKCα 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.

Desmosomes: adhesive strength and signalling in health and disease

Desmosomes are intercellular junctions whose primary function is strong intercellular adhesion, known as hyperadhesion. In the present review, we discuss how their structure appears to support this function as well as how they are assembled and down-regulated. Desmosomal components also have signalling functions that are important in tissue development and remodelling. Their adhesive and signalling functions are both compromised in genetic and autoimmune diseases that affect the heart, skin and mucous membranes. We conclude that much work is required on structure–function relationships within desmosomes in vivo and on how they participate in signalling processes to enhance our knowledge of tissue homoeostasis and human disease.

Identification and characterization of DSPIa, a novel isoform of human desmoplakin

Desmoplakin is a ubiquitous component of desmosomes and desmosome-like structures, such as the cardiomyocyte area composita. Two major isoforms, desmoplakin I (DSPI) and desmoplakin II (DSPII) are encoded by alternative mRNA transcripts differentially spliced from the same gene. The resulting proteins are identical in amino acid sequence with the exception that DSPII contains only one third of the central alpha-helical rod domain present in DSPI. Here we describe a novel minor isoform of desmoplakin that is also produced by alternative splicing of the desmoplakin gene and that we name desmoplakin Ia (DSPIa). DSPIa is an alternatively spliced DSPI mRNA with a unique splice donor site that is 90% homologous to and downstream of the DSPII specific donor. The resulting DSPIa mRNA is in-frame and encodes a protein that has a central alpha-helical rod domain of intermediate size and that is 156 amino acids larger than DSPII and 443 amino acids smaller than DSPI. We demonstrate, through recombinant expression and short interfering RNA knockdown, that the DSPIa protein is readily detectable, albeit at substantially lower levels than the dominant isoforms, DSPI and DSPII. DSPIa mRNA has a similar tissue distribution to that of DSPI and of DSPII.

A homozygous nonsense mutation in the human desmocollin-3 (DSC3) gene underlies hereditary hypotrichosis and recurrent skin vesicles

Desmosomes are the major players in epidermis and cardiac muscles and contribute to intercellular binding and maintenance of tissue integrity. Two important constituents of desmosomes are transmembrane cadherins named desmogleins and desmocollins. The critical role of these desmosomal proteins in epithelial integrity has been illustrated by their disruption in mouse models and human diseases. In the present study, we have investigated a large family from Afghanistan in which four individuals are affected with hereditary hypotrichosis and the appearance of recurrent skin vesicle formation. All four affected individuals showed sparse and fragile hair on scalp, as well as absent eyebrows and eyelashes. Vesicles filled with thin, watery fluid were observed on the affected individuals' scalps and on most of the skin covering their bodies. A scalp-skin biopsy of an affected individual showed mild hair-follicle plugging. Candidate-gene-based homozygosity linkage mapping assigned the disease locus to 8.30 cM (8.51 Mbp) on chromosome 18q12.1. A maximum multipoint LOD score of 3.30 (theta = 0.00) was obtained at marker D18S877. Sequence analysis of four desmoglein and three desmocollin genes, contained within the linkage interval, revealed a homozygous nonsense mutation (c.2129T>G [p.Leu710X]) in exon-14 of the desmocollin-3 (DSC3) gene.

Plakoglobin is required for effective intermediate filament anchorage to desmosomes

Desmosomes are adhesive junctions that provide mechanical coupling between cells. Plakoglobin (PG) is a major component of the intracellular plaque that serves to connect transmembrane elements to the cytoskeleton. We have used electron tomography and immunolabeling to investigate the consequences of PG knockout on the molecular architecture of the intracellular plaque in cultured keratinocytes. Although knockout keratinocytes form substantial numbers of desmosome-like junctions and have a relatively normal intercellular distribution of desmosomal cadherins, their cytoplasmic plaques are sparse and anchoring of intermediate filaments is defective. In the knockout, beta-catenin appears to substitute for PG in the clustering of cadherins, but is unable to recruit normal levels of plakophilin-1 and desmoplakin to the plaque. By comparing tomograms of wild type and knockout desmosomes, we have assigned particular densities to desmoplakin and described their interaction with intermediate filaments. Desmoplakin molecules are more extended in wild type than knockout desmosomes, as if intermediate filament connections produced tension within the plaque. On the basis of our observations, we propose a particular assembly sequence, beginning with cadherin clustering within the plasma membrane, followed by recruitment of plakophilin and desmoplakin to the plaque, and ending with anchoring of intermediate filaments, which represents the key to adhesive strength.

Plakophilin 2: a critical scaffold for PKC alpha that regulates intercellular junction assembly

Plakophilins (PKPs) are armadillo family members related to the classical cadherin-associated protein p120^ctn. PKPs localize to the cytoplasmic plaque of intercellular junctions and participate in linking the intermediate filament (IF)-binding protein desmoplakin (DP) to desmosomal cadherins. In response to cell–cell contact, PKP2 associates with DP in plaque precursors that form in the cytoplasm and translocate to nascent desmosomes. Here, we provide evidence that PKP2 governs DP assembly dynamics by scaffolding a DP–PKP2–protein kinase Cα (PKCα) complex, which is disrupted by PKP2 knockdown. The behavior of a phosphorylation-deficient DP mutant that associates more tightly with IF is mimicked by PKP2 and PKCα knockdown and PKC pharmacological inhibition, all of which impair junction assembly. PKP2 knockdown is accompanied by increased phosphorylation of PKC substrates, raising the possibility that global alterations in PKC signaling may contribute to pathogenesis of congenital defects caused by PKP2 deficiency.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

Desmosomes: new perspectives on a classic

Desmosomes are highly specialized anchoring junctions that link intermediate filaments to sites of intercellular adhesion, thus facilitating the formation of a supracellular scaffolding that distributes mechanical forces throughout a tissue. These junctions are thus particularly important for maintaining the integrity of tissues that endure physical stress, such as the epidermis and myocardium. The importance of the classic mechanical functions of desmosomal constituents is underscored by pathologies reported in animal models and an ever-expanding list of human mutations that target both desmosomal cadherins and their associated cytoskeletal anchoring proteins. However, the notion that desmosomes are static structures that exist simply to glue cells together belies their susceptibility to remodeling in response to environmental cues and their important tissue-specific roles in cell behavior and signaling. Here, we review the molecular blueprint of the desmosome and models for assembling its protein components to form an adhesive interface and the desmosomal plaque. We also discuss emerging evidence of supra-adhesive roles for desmosomal proteins in regulating tissue morphogenesis and homeostasis. Finally, we highlight the dynamic nature of these adhesive organelles, examining mechanisms in health and disease for modulating adhesive strength and stability of desmosomes.

Desmosome structure, composition and function

Desmosomes are intercellular junctions of epithelia and cardiac muscle. They resist mechanical stress because they adopt a strongly adhesive state in which they are said to be hyper-adhesive and which distinguishes them from other intercellular junctions; desmosomes are specialised for strong adhesion and their failure can result in diseases of the skin and heart. They are also dynamic structures whose adhesiveness can switch between high and low affinity adhesive states during processes such as embryonic development and wound healing, the switching being signalled by protein kinase C. Desmosomes may also act as signalling centres, regulating the availability of signalling molecules and thereby participating in fundamental processes such as cell proliferation, differentiation and morphogenesis. Here we consider the structure, composition and function of desmosomes, and their role in embryonic development and disease.

Proteomic analysis of a membrane skeleton fraction from human liver

The cytoskeleton networks around liver cell cortex can resist Triton extraction and co-pellet with their tightly associated integral membrane proteins, forming assemblies called "membrane skeletons". Despite their important roles in determining cell shape and in signal transduction pathways, the membrane skeletons of human liver cells are uncharacterized to a great extent. In the present work, we prepared a membrane skeleton fraction by Triton extraction of human liver plasma membranes and then separated its protein components by 2-D gels. We optimized the detergent used for protein solubilization and found that 2% ASB-14 allowed the best recovery of membrane skeleton proteins. By analyzing the protein spots with MALDI-TOF and MALDI-TOF-TOF MS, we identified 104 nonredundant proteins, wherein 38 were cytoskeletal proteins that were further classified into several groups, including proteins in fodrin-based meshworks, adhesion proteins (proteins involved in adherens junctions, focal adhesions, desmosomes, hemidesmosomes and tight junctions), proteins that regulate F-actin dynamics, motor proteins, and some other cytoskeletal proteins. To the best of our knowledge, this is one of the largest data sets of membrane skeleton proteins to date. All the results suggested that the liver cells had complex actin- and cytokeratin-based membrane skeletons. This work provided a representative 2-DE map of membrane skeletons from human normal liver, for the purpose of helping to elucidate the composition and function of the membrane skeletons.

Plakins in development and disease

Plakins are large multi-domain molecules that have various functions to link cytoskeletal elements together and to connect them to junctional complexes. Plakins were first identified in epithelial cells where they were found to connect the intermediate filaments to desmosomes and hemidesmosomes [Ruhrberg, C., and Watt, F.M. (1997). The plakin family: versatile organizers of cytoskeletal architecture. Curr Opin Genet Dev 7, 392-397.]. They were subsequently found to be important for the integrity of muscle cells. Most recently, they have been found in the nervous system, where their functions appear to be more complex, including cross-linking of microtubules (MTs) and actin filaments [Leung, C.L., Zheng, M., Prater, S.M., and Liem, R.K. (2001). The BPAG1 locus: Alternative splicing produces multiple isoforms with distinct cytoskeletal linker domains, including predominant isoforms in neurons and muscles. J Cell Biol 154, 691-697., Leung, C.L., Sun, D., Zheng, M., Knowles, D.R., and Liem, R.K. (1999). Microtubule actin cross-linking factor (MACF): a hybrid of dystonin and dystrophin that can interact with the actin and microtubule cytoskeletons. J Cell Biol 147, 1275-1286.]. These plakins have also indicated their relationship to the spectrin superfamily of proteins and the plakins appear to be evolutionarily related to the spectrins, but have diverged to perform different specialized functions. In invertebrates, a single plakin is present in both Drosophila melanogaster and Caenorhabditis elegans, which resemble the more complex plakins found in mammals [Roper, K., Gregory, S.L., and Brown, N.H. (2002). The 'spectraplakins': cytoskeletal giants with characteristics of both spectrin and plakin families. J Cell Sci 115, 4215-4225.]. In contrast, there are seven plakins found in mammals and most of them have alternatively spliced forms leading to a very complex group of proteins with potential tissue specific functions [Jefferson, J.J., Leung, C.L., and Liem, R.K. (2004). Plakins: goliaths that link cell junctions and the cytoskeleton. Nat Rev Mol Cell Biol 5, 542-553.]. In this review, we will first describe the plakins, desmoplakin, plectin, envoplakin and periplakin and then describe two other mammalian plakins, Bullous pemphigoid antigen 1 (BPAG1) and microtubule actin cross-linking factor 1 (MACF1), that are expressed in multiple isoforms in different tissues. We will also describe the relationship of these two proteins to the invertebrate plakins, shortstop (shot) in Drosophila and VAB-10 in C. elegans. Finally, we will describe an unusual mammalian plakin, called epiplakin.

Desmoglein Versus Non-desmoglein Signaling in Pemphigus Acantholysis

Although it is accepted that pemphigus antibody binding to keratinocytes (KCs) evokes an array of intracellular biochemical events resulting in cell detachment and death, the triggering events remain obscure. It has been postulated that the binding of pemphigus vulgaris IgG (PVIgG) to KCs induces "desmosomal" signaling. Because in contrast to integrins and classical cadherins, desmoglein (Dsg) molecules are not known to elicit intracellular signaling, and because PV patients also produce non-Dsg autoantibodies, we investigated the roles of both Dsg and non-desmoglein PV antigens. The time course studies of KCs treated with PVIgG demonstrated that the activity of Src peaked at 30 min, EGF receptor kinase (EGFRK) at 60 min, and p38 MAPK at 240 min. The Src inhibitor PP2 decreased EGFRK and p38 activities by approximately 45 and 30%, respectively, indicating that in addition to Src, PVIgG evokes other triggering events. The shrinkage of KCs (cell volume reduction) became significant at 120 min, keratin aggregation at 240 min, and an increase of TUNEL positivity at 360 min. Pretreatment of KCs with PP2 blocked PVIgG-dependent cell shrinkage and keratin aggregation by approximately 50% and TUNEL positivity by approximately 25%. The p38 MAPK inhibitor PD169316 inhibited these effects by approximately 15, 20, and 70%, respectively. Transfection of KCs with small interfering RNAs that silenced expression of Dsg1 and/or Dsg3 proteins, blocked approximately 50% of p38 MAPK activity but did not significantly alter the PVIgG-dependent rise in Src and EGFRK activities. These results indicate that activation of p38 MAPK is a late signaling step associated with collapse of the cytoskeleton and disassembly of desmosomes caused by upstream events involving Src and EGFRK. Therefore, the early acantholytic events are triggered by non-Dsg antibodies.

New insights into the molecular basis of desmoplakinand desmin-related cardiomyopathies

Desmosomes are intercellular adhesive complexes that anchor the intermediate filament cytoskeleton to the cell membrane in epithelia and cardiac muscle cells. The desmosomal component desmoplakin plays a key role in tethering various intermediate filament networks through its C-terminal plakin repeat domain. To gain better insight into the cytoskeletal organization of cardiomyocytes, we investigated the association of desmoplakin with desmin by cell transfection, yeast two-hybrid, and/or in vitro binding assays. The results indicate that the association of desmoplakin with desmin depends on sequences within the linker region and C-terminal extremity of desmoplakin, where the B and C subdomains contribute to efficient binding; a potentially phosphorylatable serine residue in the C-terminal extremity of desmoplakin affects its association with desmin; the interaction of desmoplakin with non-filamentous desmin requires sequences contained within the desmin C-terminal rod portion and tail domain in yeast, whereas in in vitro binding studies the desmin tail is dispensable for association; and mutations in either the C-terminus of desmoplakin or the desmin tail linked to inherited cardiomyopathy seem to impair desmoplakindesmin interaction. These studies increase our understanding of desmoplakin-intermediate filament interactions, which are important for maintenance of cytoarchitecture in cardiomyocytes, and give new insights into the molecular basis of desmoplakin- and desmin-related human diseases.

Keratin 8 modulation of desmoplakin deposition at desmosomes in hepatocytes

Keratins, the intermediate filament proteins of epithelial cells, connect to desmosomes, the cell-cell adhesion structures at the surface membrane. The building elements of desmosomes include desmoglein and desmocollin, which provide the actual cell adhesive properties, and desmoplakins, which anchor the keratin intermediate filaments to desmosomes. In the work reported here, we address the role of keratin 8 in modulating desmoplakin deposition at surface membrane in mouse hepatocytes. The experimental approach is based on the use of keratin 8- and keratin 18-null mouse hepatocytes as cell models. In wild-type mouse hepatocytes, desmoplakin is aligned with desmoglein and keratin 8 at the surface membrane. In keratin 8-null hepatocytes, the intermediate filament loss leads to alterations in desmoplakin distribution at the surface membrane, but not of desmoglein. Intriguingly, a significant proportion of keratin 18-null hepatocytes express keratin 8 at the surface membrane, associated with a proper desmoplakin alignment with desmoglein at desmosomes. A Triton treatment of the monolayer reveals that most of the desmoplakin present in either wild-type, keratin 8- or keratin 18-null hepatocytes is insoluble. Deletion analysis of keratin 8 further suggests that the recovery of desmoplakin alignment requires the keratin 8 rod domain. In addition, similarly to other works revealing a key role of desmoplakin phosphorylation on its interaction with intermediate filaments, we find that the phosphorylation status of the keratin 8 head domain affects desmoplakin distribution at desmosomes. Together, the data indicate that a proper alignment/deposition of desmoplakin with keratins and desmoglein in hepatocytes requires keratin 8, through a reciprocal phosphoserine-dependent process.

Desmoplakin assembly dynamics in four dimensions: multiple phases differentially regulated by intermediate filaments and actin

The intermediate filament (IF)-binding protein desmoplakin (DP) is essential for desmosome function and tissue integrity, but its role in junction assembly is poorly understood. Using time-lapse imaging, we show that cell-cell contact triggers three temporally overlapping phases of DP-GFP dynamics: (1) the de novo appearance of punctate fluorescence at new contact zones after as little as 3 min; (2) the coalescence of DP and the armadillo protein plakophilin 2 into discrete cytoplasmic particles after as little as 15 min; and (3) the cytochalasin-sensitive translocation of cytoplasmic particles to maturing borders, with kinetics ranging from 0.002 to 0.04 microm/s. DP mutants that abrogate or enhance association with IFs exhibit delayed incorporation into junctions, altering particle trajectory or increasing particle pause times, respectively. Our data are consistent with the idea that DP assembles into nascent junctions from both diffusible and particulate pools in a temporally overlapping series of events triggered by cell-cell contact and regulated by actin and DP-IF interactions.

Characterization of human epiplakin: RNAi-mediated epiplakin depletion leads to the disruption of keratin and vimentin IF networks

Epiplakin is a member of the plakin family with multiple copies of the plakin repeat domain (PRD). We studied the subcellular distribution and interactions of human epiplakin by immunostaining, overlay assays and RNAi knockdown. Epiplakin decorated the keratin intermediate filaments (IF) network and partially that of vimentin. In the binding assays, the repeat unit (PRD plus linker) showed strong binding and preferentially associated with assembled IF over keratin monomers. Epiplakin knockdown revealed disruption of IF networks in simple epithelial but not in epidermal cells. In rescue experiments, the repeat unit was necessary to prevent the collapse of IF networks in transient knockdown; however, it could only partially restore the keratin but not the vimentin IF network in stably knocked down HeLa cells. We suggest that epiplakin is a cytolinker involved in maintaining the integrity of IF networks in simple epithelial cells. Furthermore, we observed an increase of epiplakin expression in keratinocytes after the calcium switch, suggesting the involvement of epiplakin in the process of keratinocyte differentiation.

Intermediate filament associated proteins

Intermediate filament associated proteins (IFAPs) coordinate interactions between intermediate filaments (IFs) and other cytoskeletal elements and organelles, including membrane-associated junctions such as desmosomes and hemidesmosomes in epithelial cells, costameres in striated muscle, and intercalated discs in cardiac muscle. IFAPs thus serve as critical connecting links in the IF scaffolding that organizes the cytoplasm and confers mechanical stability to cells and tissues. However, in recent years it has become apparent that IFAPs are not limited to structural crosslinkers and bundlers but also include chaperones, enzymes, adapters, and receptors. IF networks can therefore be considered scaffolding upon which associated proteins are organized and regulated to control metabolic activities and maintain cell homeostasis.

Synergistic control of keratinocyte adhesion through muscarinic and nicotinic acetylcholine receptor subtypes

The biological mechanisms involved in initiating, coordinating, and ultimately terminating cell-cell adhesion in the stratified epithelium are not well understood at present. This study was designed to elucidate the roles of the muscarinic M3, the nicotinic alpha3, and the mixed muscarinic-nicotinic alpha9 acetylcholine receptors in physiologic control of keratinocyte adhesion. Both muscarinic and nicotinic antagonists caused keratinocyte detachment and reversibly increased the permeability of keratinocyte monolayers, indicative of the involvement of both muscarinic and nicotinic pathways in the cholinergic control of keratinocyte adhesion. Since phosphorylation of adhesion proteins plays an important role in rapid assembly and disassembly of intercellular junctions, we measured muscarinic and nicotinic effects on phosphorylation of keratinocyte adhesion molecules. The phosphorylation levels of E-cadherin, beta-catenin, and gamma-catenin increased following pharmacological blockage of muscarinic receptors. Long-term blocking of alpha3, alpha9, and M3 receptor signaling pathways with antisense oligonucleotides resulted in cell-cell detachment and changes in the expression levels of E-cadherin, beta-catenin, and gamma-catenin in cultured human keratinocytes. Simultaneous inhibition of several receptor subtypes with a mixture of antisense oligonucleotides produced intensified abnormalities with cell adhesion. Moreover, altered cell-cell adhesion was found in the stratified epithelium of alpha3, alpha9, and M3 receptor knockout mice. Keratinocytes from these mice exhibited abnormal expression of adhesion molecules at both the protein and the mRNA levels. Thus, our data indicate that the alpha3, alpha9, and M3 acetylcholine receptors play key roles in regulating in a synergistic mode keratinocyte adhesion, most probably by modulating cadherin and catenin levels and activities. These findings may aid in the development of novel methods useful for the treatment of skin adhesion diseases and tumor metastasis.

Database on monoclonal antibodies to Cytokeratins

Cytokeratins (CK) are being extensively used as diagnostic markers for various malignancies and other diseases, including human oral precancer and cancer, due to their tissue specific expression. CK are epithelia specific intermediate filament (IF) proteins, which are expressed in a differentiation dependent and tissue specific manner. There are about 30 polypeptides of CK expressed by different human epithelia. Each type of epithelium expresses about 4-6 polypeptides. CK polypeptides share many common epitopes, due to which the antibodies developed against CK tend to cross react. Therefore, a large number of monoclonal and polyclonal antibodies have been developed to distinguish among these proteins. Many of these antibodies are not only monospecific but are also epitope specific. These antibodies are being used in pathology laboratories for routine diagnosis using immunohistochemistry. A number of fixatives are used for fixation of tissue sections prior to the use of these antibodies. Sometimes, this leads in epitope masking. Hence, it becomes necessary to use a battery of monoclonal antibodies (MAb) for accurate diagnosis. Apart from the use of these antibodies in diagnostics, they are also being used in basic research for the study of CK function and their interactions with associated proteins and membrane proteins. In the present communication an effort has been made to make a comprehensive list of MAb to CK giving information like cross-reactivity, epitope specificity, various fixatives used, etc. along with the source of the antibodies, which will provide useful information to the users.

New approaches to the treatment of pemphigus

In pemphigus vulgaris, treatment with systemic glucocorticosteroids is life saving; it may, however, cause severe side effects, including death. A patient with pemphigus vulgaris and myasthenia gravis was treated for approximately five years with the cholinomimetic Mestinon (pyridostigmine bromide), Imuran (azathioprine), and a topical corticosteroid gel before the need to introduce systemic glucocorticosteroids. Because activation of keratinocyte acetylcholine receptors also has been shown to abolish pemphigus IgG-induced acantholysis in cultured keratinocyte monolayers, a clinical trial of Mestinon was initiated in patients with active pemphigus vulgaris, pemphigus foliaceus, and paraneoplastic autoimmune multiorgan syndrome (also known as paraneoplastic pemphigus). First results indicate that nonsteroidal treatment of pemphigus is possible. Mestinon may be used to slow down progression of the disease and to treat mild cases with chronic lesions on limited areas. Stimulation of the keratinocyte- acetylcholine axis may lead to a therapeutic effect through any of the following mechanisms: (1) stimulating keratinocyte cell-to-cell attachment; (2) accelerating reepithelialization; and (3) competing with the disease-causing pemphigus antibodies, preventing them from attachment to keratinocytes. Glucocorticosteroids and various types of steroid-sparing drugs used to treat pemphigus exhibit cholinergic side effects, including effects on expression and function of keratinocyte adhesion molecules, that are very similar to those produced by the cholinomimetic drugs. Further elucidation of the mechanisms underlying therapeutic efficacy of antiacantholytics may shed light on the immunopharmacological mechanisms of pemphigus antibody-induced acantholysis.