14-3-3γ-Mediated transport of plakoglobin to the cell border is required for the initiation of desmosome assembly in vitro and in vivo

Desmosomes at a glance

Desmosomes are relatives of ancient cadherin-based junctions, which emerged late in evolution to ensure the structural integrity of vertebrate tissues by coupling the intermediate filament cytoskeleton to cell-cell junctions. Their ability to dynamically counter the contractile forces generated by actin-associated adherens junctions is particularly important in tissues under high mechanical stress, such as the skin and heart. Much more than the simple cellular 'spot welds' depicted in textbooks, desmosomes are in fact dynamic structures that can sense and respond to changes in their mechanical environment and external stressors like ultraviolet light and pathogens. These environmental signals are transmitted intracellularly via desmosome-dependent mechanochemical pathways that drive the physiological processes of morphogenesis and differentiation. This Cell Science at a Glance article and the accompanying poster review desmosome structure and assembly, highlight recent insights into how desmosomes integrate chemical and mechanical signaling in the epidermis, and discuss desmosomes as targets in human disease.

Classical cadherins in the testis: how are they regulated?

Cadherins (CDH) are crucial intercellular adhesion molecules, contributing to morphogenesis and creating tissue barriers by regulating cells' movement, clustering and differentiation. In the testis, classical cadherins such as CDH1, CDH2 and CDH3 are critical to gonadogenesis by promoting the migration and the subsequent clustering of primordial germ cells with somatic cells. While CDH2 is present in both Sertoli and germ cells in rodents, CDH1 is primarily detected in undifferentiated spermatogonia. As for CDH3, its expression is mainly found in germ and pre-Sertoli cells in developing gonads until the establishment of the blood-testis barrier (BTB). This barrier is made of Sertoli cells forming intercellular junctional complexes. The restructuring of the BTB allows the movement of early spermatocytes toward the apical compartment as they differentiate during a process called spermatogenesis. CDH2 is among many junctional proteins participating in this process and is regulated by several pathways. While cytokines promote the disassembly of the BTB by enhancing junctional protein endocytosis for degradation, testosterone facilitates the assembly of the BTB by increasing the recycling of endocytosed junctional proteins. Mitogen-activated protein kinases (MAPKs) are also mediators of the BTB kinetics in many chemically induced damages in the testis. In addition to regulating Sertoli cell functions, follicle stimulating hormone can also regulate the expression of CDH2. In this review, we discuss the current knowledge on regulatory mechanisms of cadherin localisation and expression in the testis.

Kinesins in Mammalian Spermatogenesis and Germ Cell Transport

In mammalian testes, the apical cytoplasm of each Sertoli cell holds up to several dozens of germ cells, especially spermatids that are transported up and down the seminiferous epithelium. The blood-testis barrier (BTB) established by neighboring Sertoli cells in the basal compartment restructures on a regular basis to allow preleptotene/leptotene spermatocytes to pass through. The timely transfer of germ cells and other cellular organelles such as residual bodies, phagosomes, and lysosomes across the epithelium to facilitate spermatogenesis is important and requires the microtubule-based cytoskeleton in Sertoli cells. Kinesins, a superfamily of the microtubule-dependent motor proteins, are abundantly and preferentially expressed in the testis, but their functions are poorly understood. This review summarizes recent findings on kinesins in mammalian spermatogenesis, highlighting their potential role in germ cell traversing through the BTB and the remodeling of Sertoli cell-spermatid junctions to advance spermatid transport. The possibility of kinesins acting as a mediator and/or synchronizer for cell cycle progression, germ cell transit, and junctional rearrangement and turnover is also discussed. We mostly cover findings in rodents, but we also make special remarks regarding humans. We anticipate that this information will provide a framework for future research in the field.

Disruption of desmosome function leads to increased centrosome clustering in 14-3-3γ-knockout cells with supernumerary centrosomes

14-3-3 proteins are conserved, dimeric, acidic proteins that regulate multiple cellular pathways. Loss of either 14-3-3ε or 14-3-3γ leads to centrosome amplification. However, we find that while the knockout of 14-3-3ε leads to multipolar mitoses, the knockout of 14-3-3γ results in centrosome clustering and pseudo-bipolar mitoses. 14-3-3γ knockouts demonstrate compromised desmosome function and a decrease in keratin levels, leading to decreased cell stiffness and an increase in centrosome clustering. Restoration of desmosome function increased multipolar mitoses, whereas knockdown of either plakoglobin or keratin 5 led to decreased cell stiffness and increased pseudo-bipolar mitoses. These results suggest that the ability of the desmosome to anchor keratin filaments maintains cell stiffness, thus inhibiting centrosome clustering, and that phenotypes observed upon 14-3-3 loss reflect the dysregulation of multiple pathways.

14-3-3 targets keratin intermediate filaments to mechanically sensitive cell-cell contacts

Intermediate filament (IF) cytoskeletal networks simultaneously support mechanical integrity and influence signal transduction pathways. Marked remodeling of the keratin IF network accompanies collective cellular morphogenetic movements that occur during early embryonic development in the frog Xenopus laevis. While this reorganization of keratin is initiated by force transduction on cell–cell contacts mediated by C-cadherin, the mechanism by which keratin filament reorganization occurs remains poorly understood. In this work, we demonstrate that 14-3-3 proteins regulate keratin reorganization dynamics in embryonic mesendoderm cells from Xenopus gastrula. 14-3-3 colocalizes with keratin filaments near cell–cell junctions in migrating mesendoderm. Coimmunoprecipitation, mass spectrometry, and bioinformatic analyses indicate 14-3-3 is associated with Keratin 19 (K19) in the whole embryo and, more specifically, mesendoderm tissue. Inhibition of 14-3-3 results in both the decreased exchange of keratin subunits into filaments and blocks keratin filament recruitment toward cell–cell contacts. Synthetically coupling 14-3-3 to K19 through a unique fusion construct conversely induces the localization of this keratin population to the region of cell–cell contacts. Taken together, these findings indicate that 14-3-3 acts on keratin IFs and is involved in their reorganization to sites of cell adhesion.

Beyond cell-cell adhesion: Plakoglobin and the regulation of tumorigenesis and metastasis

Plakoglobin (also known as? -catenin) is a member of the Armadillo family of proteins and a paralog of β -catenin. Plakoglobin is a component of both the adherens junctions and desmosomes, and therefore plays a vital role in the regulation of cell-cell adhesion. Similar to β -catenin, plakoglobin is capable of participating in cell signaling in addition to its role in cell-cell adhesion. In this context, β -catenin has a well-documented oncogenic potential as a component of the Wnt signaling pathway. In contrast, while some studies have suggested a tumor promoting activity of plakoglobin in a cell/malignancy specific context, it generally acts as a tumor/metastasis suppressor. How plakoglobin acts as a growth/metastasis inhibitory protein has remained, until recently, unclear. Recent evidence suggests that plakoglobin may suppress tumorigenesis and metastasis by multiple mechanisms, including the suppression of oncogenic signaling, interactions with various proteins involved in tumorigenesis and metastasis, and the regulation of the expression of genes involved in these processes. This review is primarily focused on various mechanisms by which plakoglobin may inhibit tumorigenesis and metastasis.

14-3-3 proteins regulate desmosomal adhesion via plakophilins

Desmosomes are essential for strong intercellular adhesion and are abundant in tissues exposed to mechanical strain. At the same time, desmosomes need to be dynamic to allow for remodeling of epithelia during differentiation or wound healing. Phosphorylation of desmosomal plaque proteins appears essential for desmosome dynamics. However, the mechanisms how context-dependent post-translational modifications regulate desmosome formation, dynamics or stability are incompletely understood. Here, we show that growth factor signaling regulates the phosphorylation-dependent association of plakophilins 1 and 3 with 14-3-3 protein isoforms and uncover unique and partially antagonistic functions of members of the 14-3-3 family in the regulation of desmosomes. 14-3-3γ associated primarily with cytoplasmic plakophilin 1 phosphorylated at S155 and destabilized intercellular cohesion of keratinocytes by reducing its incorporation into desmosomes. In contrast, stratifin/14-3-3σ interacted preferentially with S285-phosphorylated plakophilin 3 to promote its accumulation at tricellular contact sites, leading to stable desmosomes. Taken together, our study identifies a new layer of regulation of intercellular adhesion by 14-3-3 proteins.

Plakoglobin localization to the cell border restores desmosome function in cells lacking 14-3-3γ

Desmosomes are cell-cell adhesion junctions that anchor intermediate filaments. Loss of 14-3-3γ in HCT116 cells led to defects in desmosome assembly due to a decrease in the transport of Plakoglobin (PG) to the cell border thus disrupting desmosome formation. Desmosome formation in cells lacking 14-3-3γ was restored by artificially localizing PG to the cell border by fusing it to EGFP-f (PG-EGFP-f). These results suggest that a major role of 14-3-3γ in desmosome assembly is to transport PG to the cell border leading to the initiation of desmosome formation.

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.

Development of an efficient screening system to identify novel bone metabolism-related genes using the exchangeable gene trap mutagenesis mouse models

Despite numerous genetic studies on bone metabolism, understanding of the specific mechanisms is lacking. We developed an efficient screening system to identify novel genes involved in bone metabolism using mutant mouse strains registered with the Exchangeable Gene Trap Clones (EGTC) database. From 1278 trap clones in the EGTC database, 52 candidate lines were selected in the first screening, determined based on "EST profile", "X-gal", "Related article", and "Novel gene". For the second screening, bone morphometric analysis, biomechanical strength analysis, bone X-gal staining, etc. were performed on candidate lines. Forty-two male trap lines (80.8%) showed abnormalities with either bone morphometric analysis or biomechanical strength analysis. In the screening process, X-gal staining was significantly efficient (P = 0.0057). As examples, Lbr and Nedd4 trap lines selected using the screening system showed significant bone decrease and fragility, suggesting a relationship with osteoblast differentiation. This screening system using EGTC mouse lines is extremely efficient for identifying novel genes involved in bone metabolism. The gene trap lines identified as abnormal using this screening approach are highly likely to trap important genes for bone metabolism. These selected trap mice will be valuable for use as novel bio-resources in bone research.

14-3-3σ Gene Loss Leads to Activation of the Epithelial to Mesenchymal Transition Due to the Stabilization of c-Jun Protein

Loss of 14-3-3σ has been observed in multiple tumor types; however, the mechanisms by which 14-3-3σ loss leads to tumor progression are not understood. The experiments in this report demonstrate that loss of 14-3-3σ leads to a decrease in the expression of epithelial markers and an increase in the expression of mesenchymal markers, which is indicative of an induction of the epithelial to mesenchymal transition (EMT). The EMT was accompanied by an increase in migration and invasion in the 14-3-3σ(-/-) cells. 14-3-3σ(-/-) cells show increased stabilization of c-Jun, resulting in an increase in the expression of the EMT transcription factor slug. 14-3-3σ induces the ubiquitination and degradation of c-Jun in an FBW7-dependent manner. c-Jun ubiquitination is dependent on the presence of an intact nuclear export pathway as c-Jun is stabilized and localized to the nucleus in the presence of a nuclear export inhibitor. Furthermore, the absence of 14-3-3σ leads to the nuclear accumulation and stabilization of c-Jun, suggesting that 14-3-3σ regulates the subcellular localization of c-Jun. Our results have identified a novel mechanism by which 14-3-3σ maintains the epithelial phenotype by inhibiting EMT and suggest that this property of 14-3-3σ might contribute to its function as a tumor suppressor gene.

Integrate Omics Data and Molecular Dynamics Simulations toward Better Understanding of Human 14-3-3 Interactomes and Better Drugs for Cancer Therapy

The 14-3-3 protein family is among the most extensively studied, yet still largely mysterious protein families in mammals to date. As they are well recognized for their roles in apoptosis, cell cycle regulation, and proliferation in healthy cells, aberrant 14-3-3 expression has unsurprisingly emerged as instrumental in the development of many cancers and in prognosis. Interestingly, while the seven known 14-3-3 isoforms in humans have many similar functions across cell types, evidence of isoform-specific functions and localization has been observed in both healthy and diseased cells. The strikingly high similarity among 14-3-3 isoforms has made it difficult to delineate isoform-specific functions and for isoform-specific targeting. Here, we review our knowledge of 14-3-3 interactome(s) generated by high-throughput techniques, bioinformatics, structural genomics and chemical genomics and point out that integrating the information with molecular dynamics (MD) simulations may bring us new opportunity to the design of isoform-specific inhibitors, which can not only be used as powerful research tools for delineating distinct interactomes of individual 14-3-3 isoforms, but also can serve as potential new anti-cancer drugs that selectively target aberrant 14-3-3 isoform.

Ablation of the 14-3-3gamma Protein Results in Neuronal Migration Delay and Morphological Defects in the Developing Cerebral Cortex

14-3-3 proteins are ubiquitously-expressed and multifunctional proteins. There are seven isoforms in mammals with a high level of homology, suggesting potential functional redundancy. We previously found that two of seven isoforms, 14-3-3epsilon and 14-3-3zeta, are important for brain development, in particular, radial migration of pyramidal neurons in the developing cerebral cortex. In this work, we analyzed the function of another isoform, the protein 14-3-3gamma, with respect to neuronal migration in the developing cortex. We found that in utero 14-3-3gamma-deficiency resulted in delays in neuronal migration as well as morphological defects. Migrating neurons deficient in 14-3-3gamma displayed a thicker leading process stem, and the basal ends of neurons were not able to reach the boundary between the cortical plate and the marginal zone. Consistent with the results obtained from in utero electroporation, time-lapse live imaging of brain slices revealed that the ablation of the 14-3-3gamma proteins in pyramidal neurons slowed down their migration. In addition, the 14-3-3gamma deficient neurons showed morphological abnormalities, including increased multipolar neurons with a thicker leading processes stem during migration. These results indicate that the 14-3-3gamma proteins play an important role in radial migration by regulating the morphology of migrating neurons in the cerebral cortex. The findings underscore the pathological phenotypes of brain development associated with the disruption of different 14-3-3 proteins and will advance the preclinical data regarding disorders caused by neuronal migration defects.

Dynamic tubulation of mitochondria drives mitochondrial network formation

Mitochondria form networks. Formation of mitochondrial networks is important for maintaining mitochondrial DNA integrity and interchanging mitochondrial material, whereas disruption of the mitochondrial network affects mitochondrial functions. According to the current view, mitochondrial networks are formed by fusion of individual mitochondria. Here, we report a new mechanism for formation of mitochondrial networks through KIF5B-mediated dynamic tubulation of mitochondria. We found that KIF5B pulls thin, highly dynamic tubules out of mitochondria. Fusion of these dynamic tubules, which is mediated by mitofusins, gives rise to the mitochondrial network. We further demonstrated that dynamic tubulation and fusion is sufficient for mitochondrial network formation, by reconstituting mitochondrial networks in vitro using purified fusion-competent mitochondria, recombinant KIF5B, and polymerized microtubules. Interestingly, KIF5B only controls network formation in the peripheral zone of the cell, indicating that the mitochondrial network is divided into subzones, which may be constructed by different mechanisms. Our data not only uncover an essential mechanism for mitochondrial network formation, but also reveal that different parts of the mitochondrial network are formed by different mechanisms.