The LINC complex transmits integrin-dependent tension to the nuclear lamina and represses epidermal differentiation

While the mechanisms by which chemical signals control cell fate have been well studied, the impact of mechanical inputs on cell fate decisions is not well understood. Here, using the well-defined system of keratinocyte differentiation in the skin, we examine whether and how direct force transmission to the nucleus regulates epidermal cell fate. Using a molecular biosensor, we find that tension on the nucleus through linker of nucleoskeleton and cytoskeleton (LINC) complexes requires integrin engagement in undifferentiated epidermal stem cells and is released during differentiation concomitant with decreased tension on A-type lamins. LINC complex ablation in mice reveals that LINC complexes are required to repress epidermal differentiation in vivo and in vitro and influence accessibility of epidermal differentiation genes, suggesting that force transduction from engaged integrins to the nucleus plays a role in maintaining keratinocyte progenitors. This work reveals a direct mechanotransduction pathway capable of relaying adhesion-specific signals to regulate cell fate.

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

Plakin proteins form connections that link the cell membrane to the intermediate filament cytoskeleton. Their interactions are mediated by a highly conserved linker domain through an unresolved mechanism. Here analysis of the human periplakin linker domain structure reveals a bi-lobed module transected by an electropositive groove. Key basic residues within the periplakin groove are vital for co-localization with vimentin in human cells and compromise direct binding which also requires acidic residues D176 and E187 in vimentin. We propose a model whereby basic periplakin linker domain residues recognize acidic vimentin side chains and form a complementary binding groove. The model is shared amongst diverse linker domains and can be used to investigate the effects of pathogenic mutations in the desmoplakin linker associated with arrhythmogenic right ventricular cardiomyopathy. Linker modules either act solely or collaborate with adjacent plakin repeat domains to create strong and adaptable tethering within epithelia and cardiac muscle.

Mammalian Plakins, Giant Cytolinkers: Versatile Biological Functions and Roles in Cancer

Cancer is a highly lethal disease that is characterized by aberrant cell proliferation, migration, and adhesion, which are closely related to the dynamic changes of cytoskeletons and cytoskeletal-adhesion. These will further result in cell invasion and metastasis. Plakins are a family of giant cytolinkers that connect cytoskeletal elements with each other and to junctional complexes. With various isoforms composed of different domain structures, mammalian plakins are broadly expressed in numerous tissues. They play critical roles in many cellular processes, including cell proliferation, migration, adhesion, and signaling transduction. As these cellular processes are key steps in cancer development, mammalian plakins have in recent years attracted more and more attention for their potential roles in cancer. Current evidence shows the importance of mammalian plakins in various human cancers and demonstrates mammalian plakins as potential biomarkers for cancer. Here, we introduce the basic characteristics of mammalian plakins, review the recent advances in understanding their biological functions, and highlight their roles in human cancers, based on studies performed by us and others. This will provide researchers with a comprehensive understanding of mammalian plakins, new insights into the development of cancer, and novel targets for cancer diagnosis and therapy.

Position matters: multiple functions of LINC-dependent chromosome positioning during meiosis

Chromosome positioning is crucial for multiple chromosomal events, including DNA replication, repair, and recombination. The linker of nucleoskeleton and cytoskeleton (LINC) complexes, which consist of conserved nuclear membrane proteins, were shown to control chromosome positioning and facilitate various biological processes by interacting with the cytoskeleton. However, the precise functions and regulation of LINC-dependent chromosome positioning are not fully understood. During meiosis, the LINC complexes induce clustering of telomeres, forming the bouquet chromosome arrangement, which promotes homologous chromosome pairing. In fission yeast, the bouquet forms through LINC-dependent clustering of telomeres at the spindle pole body (SPB, the centrosome equivalent in fungi) and detachment of centromeres from the SPB-localized LINC. It was recently found that, in fission yeast, the bouquet contributes to formation of the spindle and meiotic centromeres, in addition to homologous chromosome pairing, and that centromere detachment is linked to telomere clustering, which is crucial for proper spindle formation. Here, we summarize these findings and show that the bouquet chromosome arrangement also contributes to nuclear fusion during karyogamy. The available evidence suggests that these functions are universal among eukaryotes. The findings demonstrate that LINC-dependent chromosome positioning performs multiple functions and controls non-chromosomal as well as chromosomal events, and that the chromosome positioning is stringently regulated for its functions. Thus, chromosome positioning plays a much broader role and is more strictly regulated than previously thought.

Brain-metastatic triple-negative breast cancer cells regain growth ability by altering gene expression patterns

BACKGROUD/AIM: Triple-negative breast cancer (TNBC) frequently metastasizes to the brain (BrM). However, genes responsible for BrM of TNBC are yet to be identified.

MATERIALS AND METHODS

Gene expression profiling of TNBC and BrM was conducted, and studies with cultured cells in vitro were performed to verify functions of genes identified in these analyses.

RESULTS

According to gene expression analyses of TNBC and BrM, periplakin (PPL) and mitogen-activated protein kinase 13 (MAPK13) were chosen for further investigations. PPL and MAPK13 were highly expressed in TNBC compared to BrM. While silencing of either PPL or MAPK13 in TNBC cells increased cell growth and reduced cell motility, overexpression of either PPL or MAPK13 in BrM cells, retarded growth rates and facilitated cell motility.

CONCLUSION

Gene expression patterns in TNBC and BrM reflect cancer cell growth in regions of metastasis.

Hinged plakin domains provide specialized degrees of articulation in envoplakin, periplakin and desmoplakin

Envoplakin, periplakin and desmoplakin are cytoskeletal proteins that provide structural integrity within the skin and heart by resisting shear forces. Here we reveal the nature of unique hinges within their plakin domains that provides divergent degrees of flexibility between rigid long and short arms composed of spectrin repeats. The range of mobility of the two arms about the hinge is revealed by applying the ensemble optimization method to small-angle X-ray scattering data. Envoplakin and periplakin adopt 'L' shaped conformations exhibiting a 'helicopter propeller'-like mobility about the hinge. By contrast desmoplakin exhibits essentially unrestricted mobility by 'jack-knifing' about the hinge. Thus the diversity of molecular jointing that can occur about plakin hinges includes 'L' shaped bends, 'U' turns and fully extended 'I' orientations between rigid blocks of spectrin repeats. This establishes specialised hinges in plakin domains as a key source of flexibility that may allow sweeping of cellular spaces during assembly of cellular structures and could impart adaptability, so preventing irreversible damage to desmosomes and the cell cytoskeleton upon exposure to mechanical stress.

Microtubule actin crosslinking factor 1 regulates the Balbiani body and animal-vegetal polarity of the zebrafish oocyte

Although of fundamental importance in developmental biology, the genetic basis for the symmetry breaking events that polarize the vertebrate oocyte and egg are largely unknown. In vertebrates, the first morphological asymmetry in the oocyte is the Balbiani body, a highly conserved, transient structure found in vertebrates and invertebrates including Drosophila, Xenopus, human, and mouse. We report the identification of the zebrafish magellan (mgn) mutant, which exhibits a novel enlarged Balbiani body phenotype and a disruption of oocyte polarity. To determine the molecular identity of the mgn gene, we positionally cloned the gene, employing a novel DNA capture method to target region-specific genomic DNA of 600 kb for massively parallel sequencing. Using this technique, we were able to enrich for the genomic region linked to our mutation within one week and then identify the mutation in mgn using massively parallel sequencing. This is one of the first successful uses of genomic DNA enrichment combined with massively parallel sequencing to determine the molecular identity of a gene associated with a mutant phenotype. We anticipate that the combination of these technologies will have wide applicability for the efficient identification of mutant genes in all organisms. We identified the mutation in mgn as a deletion in the coding sequence of the zebrafish microtubule actin crosslinking factor 1 (macf1) gene. macf1 is a member of the highly conserved spectraplakin family of cytoskeletal linker proteins, which play diverse roles in polarized cells such as neurons, muscle cells, and epithelial cells. In mgn mutants, the oocyte nucleus is mislocalized; and the Balbiani body, localized mRNAs, and organelles are absent from the periphery of the oocyte, consistent with a function for macf1 in nuclear anchoring and cortical localization. These data provide the first evidence for a role for spectraplakins in polarization of the vertebrate oocyte and egg.

How the axes of the embryo are established is an important question in developmental biology. In many organisms, the axes of the embryo are established during oogenesis through the generation of a polarized egg. Very little is known regarding the mechanisms of polarity establishment and maintenance in vertebrate oocytes and eggs. We have identified a zebrafish mutant called magellan, which displays a defect in egg polarity. The gene disrupted in the magellan mutant encodes the cytoskeletal linker protein microtubule actin crosslinking factor 1 (macf1). In vertebrates, it can take years to identify the molecular nature of a mutation. We used a new technique to identify the magellan mutation, which allowed us to rapidly isolate genomic DNA linked to the mutation and sequence it. Our results describe an important new function for macf1 in polarizing the oocyte and egg and demonstrate the feasibility of this new technique for the efficient identification of mutations.

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.

Paraneoplastic pemphigus

A paraneoplasticus pemphigus malignus vagy benignus tumorokhoz társuló autoimmun hólyagos megbetegedés diagnosztikus és immunológiai kritériumait 1990-ben fektette le Anhalt . Klinikailag súlyos, fájdalmas, mélyre terjedő bőr- és nyálkahártyatünetek jellemzik. A bőrtünetek polimorf jellegűek, általában dominál a hólyagképződés. Egyes esetekben, az ún. „graft-versus-host-disease” formákban előfordul, hogy kizárólag papulosus, lichenoid bőrtünetek észlelhetők, hólyagképződés nincs, vagy csak később jelenik meg. Elsősorban ezen alcsoport betegeiben a bőr- és nyálkahártyatünetek mellett súlyos dyspnoét okozó pulmonalis érintettség is kialakul, melynek hátterében bronchiolitis obliterans áll. A paraneoplasticus pemphigus diagnosztikájában alapvető fontosságú a bőr/nyálkahártyák direkt immunfluoreszcens, továbbá a szérum indirekt immunhisztológiai és immunoblot vizsgálata. Az eddig azonosított autoantigének döntő többsége a plakincsalád tagja: envoplakin (210 kDa), periplakin (190 kDa), plectin (~500 kDa), desmoplakin I (250 kDa), desmoplakin II (210 kDa), bullosus pemphigoid antigén 1 (230 kDa). A plakinok mellett a desmosomalis cadherinek közé sorolt desmoglein 1 és 3 a bullosus pemphigoid antigén 2 (180 kDa), a desmocollin 2 és 3, továbbá egy még nem azonosított, 170 kDa molekulatömegű transzmembrán fehérje szintén autoantigénje a kórképnek. A paraneoplasticus pemphigus nagy mortalitású kórkép, az esetek több mint 90%-ában halálos kimenetelű. A háttérben álló tumor eltávolítása mellett a bázisterápia továbbra is a nagy dózisú szisztémás szteroidkezelés, melyet citosztatikumok, immunmodulánsok adásával egészítenek ki. Szóba jön ezenkívül plasmapheresis, plazmacsere, photopheresis, nagy dózisú intravénás immunglobulin és anti-CD20 monoklonális antitest (rituximab) adása is.

Periplakin-dependent re-organisation of keratin cytoskeleton and loss of collective migration in keratin-8-downregulated epithelial sheets

Collective migration of epithelial sheets requires maintenance of cell-cell junctions and co-ordination of the movement of the migrating front. We have investigated the role of keratin intermediate filaments and periplakin, a cytoskeletal linker protein, in the migration of simple epithelial cells. Scratch wounding induces bundling of keratins into a cable of tightly packed filaments adjacent to the free wound edge. Keratin re-organisation is preceded by a re-distribution of periplakin away from the free wound edge. Periplakin participates with dynamic changes in the keratin cytoskeleton via its C-terminal linker domain that co-localises with okadaic-acid-treated keratin granules. Stable expression of the periplakin C-terminal domain increases keratin bundling and Ser431 keratin phosphorylation at wound edge resulting in a delay in wound closure. Ablation of periplakin by siRNA inhibits keratin cable formation and impairs wound closure. Knockdown of keratin 8 with siRNA results in (1) a loss of desmoplakin localisation at cell borders, (2) a failure of MCF-7 epithelial sheets to migrate as a collective unit and (3) accelerated wound closure in vimentin-positive HeLa and Panc-1 cell lines. Thus, keratin 8 is required for the maintenance of epithelial integrity during migration and periplakin participates in the re-organisation of keratins in migrating cells.

Structural analysis of the plakin domain of bullous pemphigoid antigen1 (BPAG1) suggests that plakins are members of the spectrin superfamily

Bullous pemphigoid antigen 1 (BPAG1) is a member of the plakin family of proteins. The plakins are multi-domain proteins that have been shown to interact with microtubules, actin filaments and intermediate filaments, as well as proteins found in cellular junctions. These interactions are mediated through different domains on the plakins. The interactions between plakins and components of specialized cell junctions such as desmosomes and hemidesmosomes are mediated through the so-called plakin domain, which is a common feature of the plakins. We report the crystal structure of a stable fragment from BPAG1, residues 226-448, defined by limited proteolysis of the whole plakin domain. The structure, determined by single-wavelength anomalous diffraction phasing from a selenomethionine-substituted crystal at 3.0 A resolution, reveals a tandem pair of triple helical bundles closely related to spectrin repeats. Based on this structure and analysis of sequence conservation, we propose that the architecture of plakin domains is defined by two pairs of spectrin repeats interrupted by a putative Src-Homology 3 (SH3) domain.

Plectin-controlled keratin cytoarchitecture affects MAP kinases involved in cellular stress response and migration

Plectin is a major intermediate filament (IF)-based cytolinker protein that stabilizes cells and tissues mechanically, regulates actin filament dynamics, and serves as a scaffolding platform for signaling molecules. In this study, we show that plectin deficiency is a cause of aberrant keratin cytoskeleton organization caused by a lack of orthogonal IF cross-linking. Keratin networks in plectin-deficient cells were more susceptible to osmotic shock-induced retraction from peripheral areas, and their okadaic acid-induced disruption (paralleled by stress-activated MAP kinase p38 activation) proceeded faster. Basal activities of the MAP kinase Erk1/2 and of the membrane-associated upstream protein kinases c-Src and PKCdelta were significantly elevated, and increased migration rates, as assessed by in vitro wound-closure assays and time-lapse microscopy, were observed. Forced expression of RACK1, which is the plectin-binding receptor protein for activated PKCdelta, in wild-type keratinocytes elevated their migration potential close to that of plectin-null cells. These data establish a link between cytolinker-controlled cytoarchitecture/scaffolding functions of keratin IFs and specific MAP kinase cascades mediating distinct cellular responses.

Inherited disorders of desmosomes

Desmosomes are highly organized intercellular junctions that provide mechanical integrity to tissues by anchoring intermediate filaments to sites of strong adhesion. These cell-cell adhesion junctions are found in skin, heart, lymph nodes and meninges. Over the last 8 years, several naturally occurring human gene mutations in structural components of desmosomes have been reported. These comprise autosomal dominant or recessive mutations in plakophilin 1, plakophilin 2, desmoplakin, plakoglobin, desmoglein 1, desmoglein 4 and corneodesmosin. These discoveries have often highlighted novel or unusual phenotypes, including abnormal skin fragility and differentiation, and developmental anomalies of various ectodermal appendages, especially hair. Some desmosomal gene mutations may also result in cardiac disease, notably cardiomyopathy. This article describes the spectrum of clinical features that may be found in the inherited disorders of desmosomes and highlights the key functions of several of the desmosomal proteins in tissue adhesion and cell biology.

Microtubule actin crosslinking factor 1b: a novel plakin that localizes to the Golgi complex

MACF1 (microtubule actin crosslinking factor), also called ACF7 (actin crosslinking family 7) is a cytoskeletal linker protein that can associate with both actin filaments and microtubules. We have identified a novel alternatively spliced isoform of MACF1. We named this isoform MACF1b and renamed the original isoform MACF1a. MACF1b is identical to MACF1a, except that it has a region containing plakin (or plectin) repeats in the middle of the molecule. MACF1b is ubiquitously expressed in adult tissues with especially high levels in the lung. We studied the subcellular localization of MACF1b proteins in mammalian cell lines. In two lung cell lines, MACF1b was chiefly localized to the Golgi complex. Upon treatments that disrupt the Golgi complex, MACF1b redistributed into the cytosol, but remained co-localized with the dispersed Golgi ministacks. MACF1b proteins can be detected in the enriched Golgi fraction by western blotting. The domain of MACF1b that targets it to the Golgi was found at the N-terminal part of the region that contains the plakin repeats. Reducing the level of MACF1 proteins by small-interfering RNA resulted in the dispersal of the Golgi complex.