Catenins: keeping cells from getting their signals crossed

Wnt and hedgehog signaling pathways in bone development

Cell-cell signaling is a major strategy that vertebrate embryos employ to coordinately control cell proliferation, differentiation, and survival in many developmental processes. Similar cell signaling pathways also control adult tissue regeneration and repair. We demonstrated in the developing skeletal system that the Wnt/beta-catenin signaling controls the differentiation of progenitor cells into either osteoblasts or chondrocytes. Genetic ablation of beta-catenin in the developing mouse embryo resulted in ectopic formation of chondrocytes at the expense of osteoblast differentiation during both intramembranous and endochondral ossification. Conversely, ectopic upregulation of the canonical Wnt signaling led to suppression of chondrocyte formation and enhanced ossification. As other signaling pathways also play critical roles in controlling skeletal development, to gain a full picture of the molecular regulatory network of skeletal development, we investigated how the Wnt/beta-catenin signaling is integrated with Indian hedgehog (Ihh) signaling in controlling various aspects of skeletal development. We found that Wnt signaling acts downstream of Ihh signaling and is required in osteoblasts after Osterix expression to promote osteoblast maturation during endochondral bone formation. Since similar controlling mechanisms of osteoblast proliferation and differentiation may be employed by adult mesenchymal progenitor cells during fracture repair, these studies suggest that, to enhance fracture repair or bone formation, Ihh signaling needs to be enhanced at early stages, whereas Wnt signaling should be upregulated slightly later in differentiated osteoblasts.

Glycogen synthase kinase 3 (GSK3) in the heart: a point of integration in hypertrophic signalling and a therapeutic target? A critical analysis

Glycogen synthase kinase 3 (GSK3, of which there are two isoforms, GSK3alpha and GSK3beta) was originally characterized in the context of regulation of glycogen metabolism, though it is now known to regulate many other cellular processes. Phosphorylation of GSK3alpha(Ser21) and GSK3beta(Ser9) inhibits their activity. In the heart, emphasis has been placed particularly on GSK3beta, rather than GSK3alpha. Importantly, catalytically-active GSK3 generally restrains gene expression and, in the heart, catalytically-active GSK3 has been implicated in anti-hypertrophic signalling. Inhibition of GSK3 results in changes in the activities of transcription and translation factors in the heart and promotes hypertrophic responses, and it is generally assumed that signal transduction from hypertrophic stimuli to GSK3 passes primarily through protein kinase B/Akt (PKB/Akt). However, recent data suggest that the situation is far more complex. We review evidence pertaining to the role of GSK3 in the myocardium and discuss effects of genetic manipulation of GSK3 activity in vivo. We also discuss the signalling pathways potentially regulating GSK3 activity and propose that, depending on the stimulus, phosphorylation of GSK3 is independent of PKB/Akt. Potential GSK3 substrates studied in relation to myocardial hypertrophy include nuclear factors of activated T cells, beta-catenin, GATA4, myocardin, CREB, and eukaryotic initiation factor 2Bvarepsilon. These and other transcription factor substrates putatively important in the heart are considered. We discuss whether cardiac pathologies could be treated by therapeutic intervention at the GSK3 level but conclude that any intervention would be premature without greater understanding of the precise role of GSK3 in cardiac processes.

Hypothermia blocks beta-catenin degradation after focal ischemia in rats

Dephosphorylated and activated glycogen synthase kinase (GSK) 3beta hyperphosphorylates beta-catenin, leading to its ubiquitin-proteosome-mediated degradation. beta-catenin-knockdown increases while beta-catenin overexpression prevents neuronal death in vitro; in addition, protein levels of beta-catenin are reduced in the brain of Alzheimer's patients. However, whether beta-catenin degradation is involved in stroke-induced brain injury is unknown. Here we studied activities of GSK 3beta and beta-catenin, and the protective effect of moderate hypothermia (30 degrees C) on these activities after focal ischemia in rats. The results of Western blot showed that GSK 3beta was dephosphorylated at 5 and 24 h after stroke in the normothermic (37 degrees C) brain; hypothermia augmented GSK 3beta dephosphorylation. Because hypothermia reduces infarction, these results contradict with previous studies showing that GSK 3beta dephosphorylation worsens neuronal death. Nevertheless, hypothermia blocked degradation of total GSK 3beta protein. Corresponding to GSK 3beta activity in normothermic rats, beta-catenin phosphorylation transiently increased at 5 h in both the ischemic penumbra and core, and the total protein level of beta-catenin degraded after normothermic stroke. Hypothermia did not inhibit beta-catenin phosphorylation, but it blocked beta-catenin degradation in the ischemic penumbra. In conclusion, moderate hypothermia can stabilize beta-catenin, which may contribute to the protective effect of moderate hypothermia.

The p53 transcriptional target gene wnt7b contributes to NGF-inducible neurite outgrowth in neuronal PC12 cells

Differentiation of PC12 cells by nerve growth factor (NGF) is characterized by changes in signal transduction pathways leading to growth arrest and neurite extension. The transcription factor p53, involved in regulating cell cycle and apoptosis, is also activated during PC12 differentiation and contributes to each of these processes but the mechanisms are incompletely understood. NGF signaling stabilizes p53 protein expression, which enables its transcriptional regulation of target genes, including the newly identified target, wnt7b, a member of the wnt family of secreted morphogens. We tested the hypothesis that wnt7b expression is a factor in NGF-dependent neurite outgrowth of differentiating PC12 cells. Wnt7b transcript and protein levels are increased following NGF treatment in a p53-dependent manner, as demonstrated by a reduction in wnt7b protein levels following stable shRNA-mediated silencing of p53. In addition, overexpressed human tp53 was capable of inducing marked wnt7b expression in neuronal PC12 cells but tp53 overexpression did not elevate wnt7b levels in several tested human tumor cell lines. Ectopic wnt7b overexpression was sufficient to rescue neurite outgrowth in NGF-treated p53-silenced PC12 cells, which could be blocked by c-Jun N-terminal kinase (JNK) inhibition with SP600125 and did not involve beta-catenin nuclear translocation. Addition of sFRP1 to differentiation medium inhibited wnt7b-dependent phosphorylation of JNK, demonstrating that wnt7b is secreted and signals through a JNK-dependent mechanism in PC12 cells. We further identify an NGF-inducible subset of wnt receptors that likely supports wnt7b-mediated neurite extension in PC12 cells. In conclusion, wnt7b is a novel p53-regulated neuritogenic factor in PC12 cells that in conjunction with NGF-regulated Fzd expression is involved in p53-dependent neurite outgrowth through noncanonical JNK signaling.

Plakophilin-3-deficient mice develop hair coat abnormalities and are prone to cutaneous inflammation

We generated mice deficient in plakophilin-3 (PKP3), a member of the Armadillo-repeat family and a component of desmosomes and stress granules in epithelial cells. In these mice, several subsets of hair follicles (HFs) had morphological abnormalities, and the majority of awl and auchene hair shafts had fewer medullar air columns. Desmosomes were absent from the basal layer of the outer root sheath of HFs and from the matrix cells that are in contact with dermal papillae. In the basal layer of PKP3-null epidermis, densities of desmosomes and adherens junctions were remarkably altered. Compensatory changes in several junctional proteins were observed. PKP3-null mice housed in conventional facilities were prone to dermatitis. Our animal model provides in vivo evidence that PKP3 plays a critical role in morphogenesis of HFs and shafts and in limiting inflammatory responses in the skin.

Biology and regulation of ectoplasmic specialization, an atypical adherens junction type, in the testis

Anchoring junctions are cell adhesion apparatus present in all epithelia and endothelia. They are found at the cell-cell interface (adherens junction (AJ) and desmosome) and cell-matrix interface (focal contact and hemidesmosome). In this review, we focus our discussion on AJ in particular the dynamic changes and regulation of this junction type in normal epithelia using testis as a model. There are extensive restructuring of AJ (e.g., ectoplasmic specialization, ES, a testis-specific AJ) at the Sertoli-Sertoli cell interface (basal ES) and Sertoli-elongating spermatid interface (apical ES) during the seminiferous epithelial cycle of spermatogenesis to facilitate the migration of developing germ cells across the seminiferous epithelium. Furthermore, recent findings have shown that ES also confers cell orientation and polarity in the seminiferous epithelium, illustrating that some of the functions initially ascribed to tight junctions (TJ), such as conferring cell polarity, are also part of the inherent properties of the AJ (e.g., apical ES) in the testis. The biology and regulation based on recent studies in the testis are of interest to cell biologists in the field, in particular their regulation, which perhaps is applicable to tumorigenesis.

Dual regulation of endothelial junctional permeability

G protein-coupled receptors (GPCRs) of endothelial cells transmit diverse intracellular signals that regulate adherens junction (AJ) permeability. Increased endothelial permeability contributes to pathological processes such as inflammation, atherogenesis, and acute lung injury. Thrombin, a serine protease, and sphingosine-1-phosphate (S1P), a bioactive lipid, regulate endothelial barrier function by activating their respective GPCRs-the protease-activated receptor PAR(1) and the S1P receptor S1P(1)-which initiate intracellular signals that regulate AJ integrity and cytoskeleton organization. The distinct patterns of PAR(1) and S1P(1) signal transduction underlie the functional antagonism between thrombin and S1P. Evidence points to a role for activation of the S1P(1) receptor that is induced by PAR(1)-mediated signaling in the mechanism of AJ reannealing and endothelial barrier repair. Understanding the molecular basis of AJ integrity in the context of inflammation is important in developing novel anti-inflammatory therapeutics. This Review provides a working model for molecular mechanisms for the dual regulation of endothelial barrier function.

Catenins: playing both sides of the synapse

Synapses of the central nervous system (CNS) are specialized cell-cell junctions that mediate intercellular signal transmission from one neuron to another. The directional nature of signal relay requires synaptic contacts to be morphologically asymmetric with distinct protein components, while changes in synaptic communication during neural network formation require synapses to be plastic. Synapse morphology and plasticity require a dynamic actin cytoskeleton. Classical cadherins, which are junctional proteins associated with the actin cytoskeleton, localize to synapses and regulate synaptic adhesion, stability and remodeling. The major intracellular components of cadherin junctions are the catenin proteins, and increasing evidence suggests that cadherin-catenin complexes modulate an array of synaptic processes. Here we review the role of catenins in regulating the development of pre- and postsynaptic compartments and function in synaptic plasticity, with particular focus on their role in regulating the actin cytoskeleton.

Mammalian diaphanous-related formin Dia1 controls the organization of E-cadherin-mediated cell-cell junctions

The formin-homology protein Dia1 is a target of RhoA and a potent activator of nucleation and elongation of actin filaments. Here, we demonstrate that short-hairpin (sh) RNA-mediated downregulation of Dia1 in human MCF7 epithelial cells disrupts adherens junctions, as manifested by the significantly decreased localization of E-cadherin and associated proteins to cell-cell contacts. Expression of mouse Dia1, which is insensitive to the human Dia1-specific shRNA, rescued the junctional integrity. Coexpression of GFP-tagged Dia1 and a constitutively active RhoA mutant, RhoA-V14, resulted in localization of the exogenous GFP-Dia1 to the cell-cell junctions. This localization was accompanied by a strong increase in the width of the adhesion zone and augmentation of the actin, E-cadherin and beta-catenin content of the junctions. A constitutively active Dia1 mutant lacking the N-terminal portion was unable to localize to cell-cell junctions and did not show any junction-strengthening effect. The adherens junction enhancement induced by Dia1 and active RhoA did not require microtubules, but depended on the activity of myosin II. Inhibition of myosin II activity abolished the Dia1-mediated reinforcement of cell-cell junctions and instead induced the formation of numerous actin-rich filopodia at the contact zone. Thus, Dia1 localizes to and controls cadherin-mediated junctions in a RhoA-dependent manner.

Disruption of E-cadherin-mediated adhesion induces a functionally distinct pathway of dendritic cell maturation

The maturation of dendritic cells (DCs) after exposure to microbial products or inflammatory mediators plays a critical role in initiating the immune response. We found that maturation can also occur under steady-state conditions, triggered by alterations in E-cadherin-mediated DC-DC adhesion. Selective disruption of these interactions induced the typical features of DC maturation including the upregulation of costimulatory molecules, MHC class II, and chemokine receptors. These events were triggered at least in part by activation of the beta-catenin pathway. However, unlike maturation induced by microbial products, E-cadherin-stimulated DCs failed to release immunostimulatory cytokines, exhibiting an entirely different transcriptional profile. As a result, E-cadherin-stimulated DCs elicited an entirely different T cell response in vivo, generating T cells with a regulatory as opposed to an effector phenotype. These DCs induced tolerance in vivo and may thus contribute to the elusive steady-state "tolerogenic DCs."

Bench to bedside and back again: molecular mechanisms of alpha-catenin function and roles in tumorigenesis

The cadherin/catenin complex, comprised of E-cadherin, beta-catenin and alpha-catenin, is essential for initiating cell-cell adhesion, establishing cellular polarity and maintaining tissue organization. Disruption or loss of the cadherin/catenin complex is common in cancer. As the primary cell-cell adhesion protein in epithelial cells, E-cadherin has long been studied in cancer progression. Similarly, additional roles for beta-catenin in the Wnt signaling pathway has led to many studies of the role of beta-catenin in cancer. Alpha-catenin, in contrast, has received less attention. However, recent data demonstrate novel functions for alpha-catenin in regulating the actin cytoskeleton and cell-cell adhesion, which when perturbed could contribute to cancer progression. In this review, we use cancer data to evaluate molecular models of alpha-catenin function, from the canonical role of alpha-catenin in cell-cell adhesion to non-canonical roles identified following conditional alpha-catenin deletion. This analysis identifies alpha-catenin as a prognostic factor in cancer progression.

Sebaceous neoplasia and Torre-Muir syndrome

Sebaceous tumours include hyperplasia, adenoma, sebaceoma and carcinoma. Importantly, the latter three are potential markers of Torre-Muir syndrome; the hereditary association of sebaceous neoplasia and internal malignancy, most commonly colorectal carcinoma. The diagnostic features, differential diagnosis, molecular diagnostics and recent advances in pathogenesis of this rare group of tumours are discussed along with Torre-Muir syndrome and recommendations for screening for this important association.

Adherens and tight junctions: structure, function and connections to the actin cytoskeleton

Adherens junctions and Tight junctions comprise two modes of cell-cell adhesion that provide different functions. Both junctional complexes are proposed to associate with the actin cytoskeleton, and formation and maturation of cell-cell contacts involves reorganization of the actin cytoskeleton. Adherens junctions initiate cell-cell contacts, and mediate the maturation and maintenance of the contact. Adherens junctions consist of the transmembrane protein E-cadherin, and intracellular components, p120-catenin, beta-catenin and alpha-catenin. Tight junctions regulate the paracellular pathway for the movement of ions and solutes in-between cells. Tight junctions consist of the transmembrane proteins occludin and claudin, and the cytoplasmic scaffolding proteins ZO-1, -2, and -3. This review discusses the binding interactions of the most studied proteins that occur within each of these two junctional complexes and possible modes of regulation of these interactions, and the different mechanisms that connect and regulate interactions with the actin cytoskeleton.

The locus of evolution: evo devo and the genetics of adaptation

An important tenet of evolutionary developmental biology ("evo devo") is that adaptive mutations affecting morphology are more likely to occur in the cis-regulatory regions than in the protein-coding regions of genes. This argument rests on two claims: (1) the modular nature of cis-regulatory elements largely frees them from deleterious pleiotropic effects, and (2) a growing body of empirical evidence appears to support the predominant role of gene regulatory change in adaptation, especially morphological adaptation. Here we discuss and critique these assertions. We first show that there is no theoretical or empirical basis for the evo devo contention that adaptations involving morphology evolve by genetic mechanisms different from those involving physiology and other traits. In addition, some forms of protein evolution can avoid the negative consequences of pleiotropy, most notably via gene duplication. In light of evo devo claims, we then examine the substantial data on the genetic basis of adaptation from both genome-wide surveys and single-locus studies. Genomic studies lend little support to the cis-regulatory theory: many of these have detected adaptation in protein-coding regions, including transcription factors, whereas few have examined regulatory regions. Turning to single-locus studies, we note that the most widely cited examples of adaptive cis-regulatory mutations focus on trait loss rather than gain, and none have yet pinpointed an evolved regulatory site. In contrast, there are many studies that have both identified structural mutations and functionally verified their contribution to adaptation and speciation. Neither the theoretical arguments nor the data from nature, then, support the claim for a predominance of cis-regulatory mutations in evolution. Although this claim may be true, it is at best premature. Adaptation and speciation probably proceed through a combination of cis-regulatory and structural mutations, with a substantial contribution of the latter.

The targeted overexpression of a Claudin mutant in the epidermis of transgenic mice elicits striking epidermal and hair follicle abnormalities

Skin is one of the largest organs of the body, and is formed during development through a highly orchestrated process involving mesenchymal-epithelial interactions, cell commitment, and terminal differentiation. It protects against microorganism invasion and UV irradiation, inhibits water loss, regulates body temperature, and is an important part of the immune system. Using transgenic mouse technology, we have demonstrated that Claudin (Cldn)-containing tight junctions (TJs) are intricately involved in cell signaling during epidermal differentiation and that an epidermal suprabasal overexpression of Cldn6 results in a perturbed epidermal terminal differentiation program with distinct phenotypic abnormalities. To delineate the role of the Cldn cytoplasmic tail domain in epidermal differentiation, we engineered transgenic mice targeting the overexpression of a Cldn6 cytoplasmic tail-truncation mutant in the epidermis. Transgenic mice were characterized by a lethal barrier dysfunction in addition to the existence of hyperproliferative squamous invaginations/cysts replacing hair follicles. Immunohistochemical analysis revealed an epidermal cytoplasmic accumulation of Cldn6, Cldn11, Cldn12, and Cldn18, downregulation of Cldn1 and aberrant expression of various classical markers of epidermal differentiation; namely the basal keratins as well as K1, involucrin, loricrin, and filaggrin. Collectively these studies suggest an important role for Cldns in epidermal/hair follicle differentiation programs likely involving cross talk to signaling pathways (e.g., Notch) directing cell fate selection and differentiation.

Wnt signaling in polycystic kidney disease

Wnt signaling cascades activate morphogenetic programs that range from cell migration and proliferation to cell fate determination and stem cell renewal. These pathways enable cells to translate environmental cues into the complex cellular programs that are needed to organize tissues and build organs. Wnt signaling is essential for renal development; however, the specific molecular underpinnings involved are poorly understood. Recent research has revealed an unexpected intersection between Wnt signaling and polycystic kidney disease. Some polycystic kidney disease proteins, such as Inversin and Bardet-Biedl syndrome family members, were found to use components of the Wnt signaling cascade to orient cells along a secondary polarity axis within the plane of the epithelium. These spatial cues may be needed to position nascent tubules with a defined geometry.

Scratching the surface of skin development

The epidermis and its appendages develop from a single layer of multipotent embryonic progenitor keratinocytes. Embryonic stem cells receive cues from their environment that instruct them to commit to a particular differentiation programme and generate a stratified epidermis, hair follicles or sebaceous glands. Exciting recent developments have focused on how adult skin epithelia maintain populations of stem cells for use in the natural cycles of hair follicle regeneration and for re-epithelialization in response to wounding.

Molecular regulation of kidney development: is the answer blowing in the Wnt?

Development of the metanephric kidney is a complicated process regulated by reciprocal signals from the ureteric bud and the metanephric mesenchyme that regulate tubule formation and epithelial branching morphogenesis. Over the past several years, several studies have suggested that Wnt signaling is involved in multiple aspects of normal kidney development as well as injury response and cancer progression. We will review these data here.