Cadherin-mediated adhesion is essential for myofibril continuity across the plasma membrane but not for assembly of the contractile apparatus

Plakophilin-2 truncating variants impair cardiac contractility by disrupting sarcomere stability and organization

Progressive loss of cardiac systolic function in arrhythmogenic cardiomyopathy (ACM) has recently gained attention as an important clinical consideration in managing the disease. However, the mechanisms leading to reduction in cardiac contractility are poorly defined. Here, we use CRISPR gene editing to generate human induced pluripotent stem cells (iPSCs) that harbor plakophilin-2 truncating variants (PKP2tv), the most prevalent ACM-linked mutations. The PKP2tv iPSC–derived cardiomyocytes are shown to have aberrant action potentials and reduced systolic function in cardiac microtissues, recapitulating both the electrical and mechanical pathologies reported in ACM. By combining cell micropatterning with traction force microscopy and live imaging, we found that PKP2tvs impair cardiac tissue contractility by destabilizing cell-cell junctions and in turn disrupting sarcomere stability and organization. These findings highlight the interplay between cell-cell adhesions and sarcomeres required for stabilizing cardiomyocyte structure and function and suggest fundamental pathogenic mechanisms that may be shared among different types of cardiomyopathies.

Desmin Correlated with Cx43 May Facilitate Intercellular Electrical Coupling during Chronic Heart Failure

Desmin is one of five major intermediate filament proteins in cardiomyocytes. Desmin contributes to the maintenance of healthy muscle. The desmin content in cardiomyocytes directly affects the long-term prognosis of patients with heart failure, and lack of desmin leads to myocyte contractile dysfunction. However, the mechanism is elusive. In this study, we measured desmin expression using western blotting and qPCR in the failed hearts of human patients and rats. Our results showed that desmin content was reduced at the protein level in failed hearts and isolated cardiomyocytes. The association of desmin and the gap junction proteins connexin 43 (Cx43) and zonula occludens-1 (ZO-1) was also investigated. Immunoprecipitation assay showed that desmin was associated with Cx43 in cardiomyocytes. To compare the electrical integration of skeletal myoblasts in cocultures with cardiac myocytes, familial amyloid polyneuropathy (FAP) activation rate was found in 33% desmin overexpressing skeletal myoblasts. Desmin not only affected Cx43 and ZO-1 expression but also facilitated the complex of Cx43 and ZO-1 in skeletal myoblasts, which enhanced cell-to-cell electrical coupling of skeletal myoblasts with cardiac myocytes. Desmin has potential as a novel therapeutic target for heart failure. Preservation of desmin may attenuate heart failure.

Adherens junction engagement regulates functional patterning of the cardiac pacemaker cell lineage

Cardiac pacemaker cells (CPCs) rhythmically initiate the electrical impulses that drive heart contraction. CPCs display the highest rate of spontaneous depolarization in the heart despite being subjected to inhibitory electrochemical conditions that should theoretically suppress their activity. While several models have been proposed to explain this apparent paradox, the actual molecular mechanisms that allow CPCs to overcome electrogenic barriers to their function remain poorly understood. Here, we have traced CPC development at single-cell resolution and uncovered a series of cytoarchitectural patterning events that are critical for proper pacemaking. Specifically, our data reveal that CPCs dynamically modulate adherens junction (AJ) engagement to control characteristics including surface area, volume, and gap junctional coupling. This allows CPCs to adopt a structural configuration that supports their overall excitability. Thus, our data have identified a direct role for local cellular mechanics in patterning critical morphological features that are necessary for CPC electrical activity.

Transforming growth factor-β1-induced N-cadherin drives cell-cell communication through connexin43 in osteoblast lineage

Gap junction (GJ) has been indicated to have an intimate correlation with adhesion junction. However, the direct interaction between them partially remains elusive. In the current study, we aimed to elucidate the role of N-cadherin, one of the core components in adhesion junction, in mediating connexin 43, one of the functional constituents in gap junction, via transforming growth factor-β1(TGF-β1) induction in osteoblasts. We first elucidated the expressions of N-cadherin induced by TGF-β1 and also confirmed the upregulation of Cx43, and the enhancement of functional gap junctional intercellular communication (GJIC) triggered by TGF-β1 in both primary osteoblasts and MC3T3 cell line. Colocalization analysis and Co-IP experimentation showed that N-cadherin interacts with Cx43 at the site of cell-cell contact. Knockdown of N-cadherin by siRNA interference decreased the Cx43 expression and abolished the promoting effect of TGF-β1 on Cx43. Functional GJICs in living primary osteoblasts and MC3T3 cell line were also reduced. TGF-β1-induced increase in N-cadherin and Cx43 was via Smad3 activation, whereas knockdown of Smad3 signaling by using siRNA decreased the expressions of both N-cadherin and Cx43. Overall, these data indicate the direct interactions between N-cadherin and Cx43, and reveal the intervention of adhesion junction in functional gap junction in living osteoblasts.

Electrical stimulation applied during differentiation drives the hiPSC-CMs towards a mature cardiac conduction-like cells

Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) resemble fetal cardiomyocytes and electrical stimulation (ES) has been explored to mature the differentiated cells. Here, we hypothesize that ES applied at the beginning of the differentiation process, triggers both differentiation of the hiPSC-CMs into a specialized conduction system (CS) phenotype and cell maturation. We applied ES for 15 days starting on day 0 of the differentiation process and found an increased expression of transcription factors and proteins associated with the development and function of CS including Irx3, Nkx2.5 and contactin 2, Hcn4 and Scn5a, respectively. We also found activation of intercalated disc proteins (Nrap and β-catenin). We detected ES-induced CM maturation as indicated by increased Tnni1 and Tnni3 expression. Confocal micrographs showed a shift towards expression of the gap junction protein connexin 40 in ES hiPSC-CM compared to the more dominant expression of connexin 43 in controls. Finally, analysis of functional parameters revealed that ES hiPSC-CMs exhibited faster action potential (AP) depolarization, longer intracellular Ca2+ transients, and slower AP duration at 90% of repolarization, resembling fast conducting fibers. Altogether, we provided evidence that ES during the differentiation of hiPSC to cardiomyocytes lead to development of cardiac conduction-like cells with more mature cytoarchitecture. Thus, hiPSC-CMs exposed to ES during differentiation can be instrumental to develop CS cells for cardiac disease modelling, screening individual drugs on a precison medicine type platform and support the development of novel therapeutics for arrhythmias.

Tricellular adherens junctions provide a cell surface delivery platform for connexin 26/30 oligomers in the cochlea

In the cochlea, connexins 26 (Cx26) and 30 (Cx30) largely co-assemble into heteromeric gap junctions, which connect adjacent non-sensory epithelial cells. These channels are believed to ensure the rapid removal of K⁺ away from the base of sensory hair cells, resulting in K⁺ recycling back to the endolymph to maintain cochlear homeostasis. Many of the mutations in GJB2 and GJB6, which encode CX26 and CX30, impair the formation of membrane channels and cause autosomal hearing loss in humans. Although recent advances have been made, several important questions remain about connexin trafficking and gap junction biogenesis. Here we show that tricellular adherens junctions present at the crossroad between adjacent gap junction plaques, provide an unexpected cell surface delivery platform for Cx26/Cx30 oligomers. Using an in situ proximity ligation assay, we detected the presence of non-junctional Cx26/Cx30 oligomers within lipid raft-enriched tricellular junction sites. In addition, we observed that cadherin homophilic interactions are critically involved in microtubule-mediated trafficking of Cx26/Cx30 oligomers to the cell surface. Overall, our results unveil an unexpected role for tricellular junctions in the trafficking and assembly of membrane channels.

Tbx20 Induction Promotes Zebrafish Heart Regeneration by Inducing Cardiomyocyte Dedifferentiation and Endocardial Expansion

Heart regeneration requires replenishment of lost cardiomyocytes (CMs) and cells of the endocardial lining. However, the signaling regulation and transcriptional control of myocardial dedifferentiation and endocardial activation are incompletely understood during cardiac regeneration. Here, we report that T-Box Transcription Factor 20 (Tbx20) is induced rapidly in the myocardial wound edge in response to various sources of cardiac damages in zebrafish. Inducing Tbx20 specifically in the adult myocardium promotes injury-induced CM proliferation through CM dedifferentiation, leading to loss of CM cellular contacts and re-expression of cardiac embryonic or fetal gene programs. Unexpectedly, we identify that myocardial Tbx20 induction activates the endocardium at the injury site with enhanced endocardial cell extension and proliferation, where it induces the endocardial Bone morphogenetic protein 6 (Bmp6) signaling. Pharmacologically inactivating endocardial Bmp6 signaling reduces expression of its targets, Id1 and Id2b, attenuating the increased endocardial regeneration in tbx20-overexpressing hearts. Altogether, our study demonstrates that Tbx20 induction promotes adult heart regeneration by inducing cardiomyocyte dedifferentiation as well as non-cell-autonomously enhancing endocardial cell regeneration.

Gap junction-mediated cell-to-cell communication in oral development and oral diseases: a concise review of research progress

Homoeostasis depends on the close connection and intimate molecular exchange between extracellular, intracellular and intercellular networks. Intercellular communication is largely mediated by gap junctions (GJs), a type of specialized membrane contact composed of variable number of channels that enable direct communication between cells by allowing small molecules to pass directly into the cytoplasm of neighbouring cells. Although considerable evidence indicates that gap junctions contribute to the functions of many organs, such as the bone, intestine, kidney, heart, brain and nerve, less is known about their role in oral development and disease. In this review, the current progress in understanding the background of connexins and the functions of gap junctions in oral development and diseases is discussed. The homoeostasis of tooth and periodontal tissues, normal tooth and maxillofacial development, saliva secretion and the integrity of the oral mucosa depend on the proper function of gap junctions. Knowledge of this pattern of cell-cell communication is required for a better understanding of oral diseases. With the ever-increasing understanding of connexins in oral diseases, therapeutic strategies could be developed to target these membrane channels in various oral diseases and maxillofacial dysplasia.

Vinculin anchors contractile actin to the cardiomyocyte adherens junction

The adherens junction (AJ) couples the actin cytoskeletons of neighboring cells to allow mechanical integration and tissue organization. The physiological demands of intercellular adhesion require that the AJ be responsive to dynamic changes in force while maintaining mechanical load. These demands are tested in the heart, where cardiomyocyte AJs must withstand repeated cycles of actomyosin-mediated contractile force. Here we show that force-responsive cardiomyocyte AJs recruit actin-binding ligands to selectively couple actin networks. We employed a panel of N-cadherin-αE-catenin fusion proteins to rebuild AJs with specific actin linkages in N-cadherin-null cardiomyocytes. In this system, vinculin recruitment was required to rescue myofibril integration at nascent contacts. In contrast, loss of vinculin from the AJ disrupted junction morphology and blocked myofibril integration at cell–cell contacts. Our results identify vinculin as a critical link to contractile actomyosin and offer insight to how actin integration at the AJ is regulated to provide stability under mechanical load.

Substrate Stiffness, Cell Anisotropy, and Cell-Cell Contact Contribute to Enhanced Structural and Calcium Handling Properties of Human Embryonic Stem Cell-Derived Cardiomyocytes

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) can be utilized to understand the mechanisms underlying the development and progression of heart disease, as well as to develop better interventions and treatments for this disease. However, these cells are structurally and functionally immature, which undermines some of their adequacy in modeling adult heart tissue. Previous studies with immature cardiomyocytes have shown that altering substrate stiffness, cell anisotropy, and/or cell-cell contact can enhance the contractile and structural maturation of hPSC-CMs. In this study, the structural and calcium handling properties of human embryonic stem cell-derived cardiomyocytes (hESC-CMs) were enhanced by exposure to a downselected combination of these three maturation stimuli. First, hESC-CMs were seeded onto substrates composed of two commercial formulations of polydimethylsiloxane (PDMS), Sylgard 184 and Sylgard 527, whose stiffness ranged from 5 kPa to 101 kPa. Upon analyzing the morphological and calcium transient properties of these cells, it was concluded that a 21 kPa substrate yielded cells with the highest degree of maturation. Next, these PDMS substrates were microcontact-printed with laminin to force the cultured cells into rod-shaped geometries using line patterns that were 12, 18, or 24 μm in width. We found that cells on the 18 and 24 μm pattern widths had structural and functional properties that were superior to those on the 12 μm pattern. The hESC-CMs were then seeded onto these line-stamped surfaces at a density of 500 000 cells per 25-mm-diameter substrate, to enable the formation of cell-cell contacts at their distal ends. We discovered that this combination of culture conditions resulted in cells that were more structurally and functionally mature than those that were only exposed to one or two stimuli. Our results suggest that downselecting a combination of mechanobiological stimuli could prove to be an effective means of maturing hPSC-CMs in vitro.

The N-cadherin interactome in primary cardiomyocytes as defined using quantitative proximity proteomics

The junctional complexes that couple cardiomyocytes must transmit the mechanical forces of contraction while maintaining adhesive homeostasis. The adherens junction (AJ) connects the actomyosin networks of neighboring cardiomyocytes and is required for proper heart function. Yet little is known about the molecular composition of the cardiomyocyte AJ or how it is organized to function under mechanical load. Here, we define the architecture, dynamics and proteome of the cardiomyocyte AJ. Mouse neonatal cardiomyocytes assemble stable AJs along intercellular contacts with organizational and structural hallmarks similar to mature contacts. We combine quantitative mass spectrometry with proximity labeling to identify the N-cadherin (CDH2) interactome. We define over 350 proteins in this interactome, nearly 200 of which are unique to CDH2 and not part of the E-cadherin (CDH1) interactome. CDH2-specific interactors comprise primarily adaptor and adhesion proteins that promote junction specialization. Our results provide novel insight into the cardiomyocyte AJ and offer a proteomic atlas for defining the molecular complexes that regulate cardiomyocyte intercellular adhesion. This article has an associated First Person interview with the first authors of the paper.

Calreticulin Is Required for TGF-β-Induced Epithelial-to-Mesenchymal Transition during Cardiogenesis in Mouse Embryonic Stem Cells

Calreticulin, a multifunctional endoplasmic reticulum resident protein, is required for TGF-β-induced epithelial-to-mesenchymal transition (EMT) and subsequent cardiomyogenesis. Using embryoid bodies (EBs) derived from calreticulin-null and wild-type (WT) embryonic stem cells (ESCs), we show that expression of EMT and cardiac differentiation markers is induced during differentiation of WT EBs. This induction is inhibited in the absence of calreticulin and can be mimicked by inhibiting TGF-β signaling in WT cells. The presence of calreticulin in WT cells permits TGF-β-mediated signaling via AKT/GSK3β and promotes repression of E-cadherin by SNAIL2/SLUG. This is paralleled by induction of N-cadherin in a process known as the cadherin switch. We show that regulated Ca2+ signaling between calreticulin and calcineurin is critical for the unabated TGF-β signaling that is necessary for the exit from pluripotency and the cadherin switch during EMT. Calreticulin is thus a key mediator of TGF-β-induced commencement of cardiomyogenesis in mouse ESCs.

Cdon deficiency causes cardiac remodeling through hyperactivation of WNT/β-catenin signaling

On pathological stress, Wnt signaling is reactivated and induces genes associated with cardiac remodeling and fibrosis. We have previously shown that a cell surface receptor Cdon (cell-adhesion associated, oncogene regulated) suppresses Wnt signaling to promote neuronal differentiation however its role in heart is unknown. Here, we demonstrate a critical role of Cdon in cardiac function and remodeling. Cdon is expressed and predominantly localized at intercalated disk in both mouse and human hearts. Cdon-deficient mice develop cardiac dysfunction including reduced ejection fraction and ECG abnormalities. Cdon^-/- hearts exhibit increased fibrosis and up-regulation of genes associated with cardiac remodeling and fibrosis. Electrical remodeling was demonstrated by up-regulation and mislocalization of the gap junction protein, Connexin 43 (Cx43) in Cdon^-/- hearts. In agreement with altered Cx43 expression, functional analysis both using Cdon^-/- cardiomyocytes and shRNA-mediated knockdown in rat cardiomyocytes shows aberrant gap junction activities. Analysis of the underlying mechanism reveals that Cdon^-/- hearts exhibit hyperactive Wnt signaling as evident by β-catenin accumulation and Axin2 up-regulation. On the other hand, the treatment of rat cardiomyocytes with a Wnt activator TWS119 reduces Cdon levels and aberrant Cx43 activities, similarly to Cdon-deficient cardiomyocytes, suggesting a negative feedback between Cdon and Wnt signaling. Finally, inhibition of Wnt/β-catenin signaling by XAV939, IWP2 or dickkopf (DKK)1 prevented Cdon depletion-induced up-regulation of collagen 1a and Cx43. Taken together, these results demonstrate that Cdon deficiency causes hyperactive Wnt signaling leading to aberrant intercellular coupling and cardiac fibrosis. Cdon exhibits great potential as a target for the treatment of cardiac fibrosis and cardiomyopathy.

Connexins in endothelial barrier function - novel therapeutic targets countering vascular hyperpermeability

Prolonged vascular hyperpermeability is a common feature of many diseases. Vascular hyperpermeability is typically associated with changes in the expression patterns of adherens and tight junction proteins. Here, we focus on the less-appreciated contribution of gap junction proteins (connexins) to basal vascular permeability and endothelial dysfunction. First, we assess the association of connexins with endothelial barrier integrity by introducing tools used in connexin biology and relating the findings to customary readouts in vascular biology. Second, we explore potential mechanistic ties between connexins and junction regulation. Third, we review the role of connexins in microvascular organisation and development, focusing on interactions of the endothelium with mural cells and tissue-specific perivascular cells. Last, we see how connexins contribute to the interactions between the endothelium and components of the immune system, by using neutrophils as an example. Mounting evidence of crosstalk between connexins and other junction proteins suggests that we rethink the way in which different junction components contribute to endothelial barrier function. Given the multiple points of connexin-mediated communication arising from the endothelium, there is great potential for synergism between connexin-targeted inhibitors and existing immune-targeted therapeutics. As more drugs targeting connexins progress through clinical trials, it is hoped that some might prove effective at countering vascular hyperpermeability.

CREG1 Interacts with Sec8 to Promote Cardiomyogenic Differentiation and Cell-Cell Adhesion

Understanding the regulation of cell-cell interactions during the formation of compact myocardial structures is important for achieving true cardiac regeneration through enhancing the integration of stem cell-derived cardiomyocytes into the recipient myocardium. In this study, we found that cellular repressor of E1A-stimulated genes 1 (CREG1) is highly expressed in both embryonic and adult hearts. Gain- and loss-of-function analyses demonstrated that CREG1 is required for differentiation of mouse embryonic stem (ES) cell into cardiomyocytes and the formation of cohesive myocardium-like structures in a cell-autonomous fashion. Furthermore, CREG1 directly interacts with Sec8 of the exocyst complex, which tethers vesicles to the plasma membrane. Site-directed mutagenesis and rescue of CREG1 knockout ES cells showed that CREG1 binding to Sec8 is required for cardiomyocyte differentiation and cohesion. Mechanistically, CREG1, Sec8, and N-cadherin colocalize at intercalated discs in vivo and are enriched at cell-cell junctions in cultured cardiomyocytes. CREG1 overexpression enhances the assembly of adherens and gap junctions. By contrast, its knockout inhibits the Sec8-N-cadherin interaction and induces their degradation. These results suggest that the CREG1 binding to Sec8 enhances the assembly of intercellular junctions and promotes cardiomyogenesis. Stem Cells 2016;34:2648-2660.

Integrin Based Isolation Enables Purification of Murine Lineage Committed Cardiomyocytes

In contrast to mature cardiomyocytes which have limited regenerative capacity, pluripotent stem cells represent a promising source for the generation of new cardiomyocytes. The tendency of pluripotent stem cells to form teratomas and the heterogeneity from various differentiation stages and cardiomyocyte cell sub-types, however, are major obstacles to overcome before this type of therapy could be applied in a clinical setting. Thus, the identification of extracellular markers for specific cardiomyocyte progenitors and mature subpopulations is of particular importance. The delineation of cardiomyocyte surface marker patterns not only serves as a means to derive homogeneous cell populations by FACS, but is also an essential tool to understand cardiac development. By using single-cell expression profiling in early mouse embryonic hearts, we found that a combination of integrin alpha-1, alpha-5, alpha-6 and N-cadherin enables isolation of lineage committed murine cardiomyocytes. Additionally, we were able to separate trabecular cardiomyocytes from solid ventricular myocardium and atrial murine cells. These cells exhibit expected subtype specific phenotype confirmed by electrophysiological analysis. We show that integrin expression can be used for the isolation of living, functional and lineage-specific murine cardiomyocytes.

Calpain-dependent cleavage of N-cadherin is involved in the progression of post-myocardial infarction remodeling

Enzymatic proteolysis by calpains, Ca(2+)-dependent intracellular cysteine proteases, has been implicated in pathological processes such as cellular degeneration or death. Here, we investigated the role of calpain activation in the hearts subjected to myocardial infarction. We produced myocardial infarction in Cast(-/-) mice deficient for calpastatin, the specific endogenous inhibitory protein for calpains, and Cast(+/+) mice. The activity of cardiac calpains in Cast(+/+) mice was not elevated within 1 day but showed a gradual elevation after 7 days following myocardial infarction, which was further pronounced in Cast(-/-) mice. Although the prevalence of cardiomyocyte death was indistinguishable between Cast(-/-) and Cast(+/+) mice, Cast(-/-) mice exhibited profound contractile dysfunction and chamber dilatation and showed a significant reduction in survival rate after myocardial infarction as compared with Cast(+/+) mice. Notably, immunofluorescence revealed that at 28 days after myocardial infarction, calpains were activated in cardiomyocytes exclusively at the border zone and that Cast(-/-) mice showed higher intensity and a broader extent of calpain activation at the border zone than Cast(+/+) mice. In the border zone of Cast(-/-) mice, pronounced activation of calpains was associated with a decrease in N-cadherin expression and up-regulation of molecular markers for cardiac hypertrophy and fibrosis. In cultured rat neonatal cardiomyocytes, calpain activation by treatment with ionomycin induced cleavage of N-cadherin and decreased expression levels of β-catenin and connexin 43, which was attenuated by calpain inhibitor. These results thus demonstrate that activation of calpains disassembles cell-cell adhesion at intercalated discs by degrading N-cadherin and thereby promotes left ventricular remodeling after myocardial infarction.

N-cadherin/catenin complex as a master regulator of intercalated disc function

Intercellular adhesive junctions are essential for maintaining the physical integrity of tissues; this is particularly true for the heart that is under constant mechanical load. The correct functionality of the heart is dependent on the electrical and mechanical coordination of its constituent cardiomyocytes. The intercalated disc (ID) structure located at the termini of the rod-shaped adult cardiomyocyte contains various junctional proteins responsible for the integration of structural information and cell-cell communication. According to the classical description, the ID consists of three distinct junctional complexes: adherens junction (AJ), desmosome (Des), and gap junction (GJ) that work together to mediate mechanical and electrical coupling of cardiomyocytes. However, recent morphological and molecular studies indicate that AJ and Des components are capable of mixing together resulting in a "hybrid adhering junction" or "area composita." This review summarizes recent progress in understanding the in vivo function(s) of AJ components in cardiac homeostasis and disease.

The canonical WNT2 pathway and FSH interact to regulate gap junction assembly in mouse granulosa cells

WNTs are extracellular signaling molecules that exert their actions through receptors of the frizzled (FZD) family. Previous work indicated that WNT2 regulates cell proliferation in mouse granulosa cells acting through CTNNB1 (beta-catenin), a key component in canonical WNT signaling. In other cells, WNT signaling has been shown to regulate expression of connexin43 (CX43), a gap junction protein, as well as gap junction assembly. Since previous work demonstrated that CX43 is also essential in ovarian follicle development, the objective of this study was to determine if WNT2 regulates CX43 expression and/or gap-junctional intercellular communication (GJIC) in granulosa cells. WNT2 knockdown via siRNA markedly reduced CX43 expression and GJIC. CX43 expression, the extent of CX43-containing gap junction membrane, and GJIC were also reduced by CTNNB1 transient knockdown. CTNNB1 is mainly localized to the membranes between granulosa cells but disappeared from this location after WNT2 knockdown. Furthermore, CTNNB1 knockdown interfered with the ability of follicle-stimulating hormone (FSH) to promote the mobilization of CX43 into gap junctions. We propose that the WNT2/CTNNB1 pathway regulates CX43 expression and GJIC in granulosa cells by modulating CTNNB1 stability and localization in adherens junctions, and that this is essential for FSH stimulation of GJIC.

N-cadherin-mediated adhesion and signaling from development to disease: lessons from mice

Of the 20 classical cadherin subtypes identified in mammals, the functions of the two initially identified family members E- (epithelial) and N- (neural) cadherin have been most extensively studied. E- and N-Cadherin have mostly mutually exclusive expression patterns, with E-cadherin expressed primarily in epithelial cells, whereas N-cadherin is found in a variety of cells, including neural, muscle, and mesenchymal cells. N-Cadherin function, in particular, appears to be cell context-dependent, as it can mediate strong cell-cell adhesion in the heart but induces changes in cell behavior in favor of a migratory phenotype in the context of epithelial-mesenchymal transition (EMT). The ability of tumor cells to alter their cadherin expression profile, for example, E- to N-cadherin, is critical for malignant progression. Recent advances in mouse molecular genetics, and specifically tissue-specific knockout and knockin alleles of N-cadherin, have provided some unexpected results. This chapter highlights some of the genetic studies that explored the complex role of N-cadherin in embryonic development and disease.

Neuronal cadherin (NCAD) increases sensory neurite formation and outgrowth on astrocytes

We examined the neurite outgrowth of sensory neurons on astrocytes following the genetic deletion of N-cadherin (NCAD). Deletion abolished immunostaining for NCAD and the other classical cadherins, indicating that NCAD is likely the only classical cadherin expressed by astrocytes. Only 38% of neurons grown on NCAD-deficient astrocytes for 24 h produced neurites, as compared to 74% of neurons grown on NCAD-expressing astrocytes. Of the neurons that produced neurites, those grown on NCAD-deficient astrocytes had a mean total length of 378 μm, as compared to 1093 μm for neurons grown on NCAD-expressing astrocytes. Thus, the loss of NCAD greatly impairs the formation and extension neurites on astrocytes.

Synemin isoforms differentially organize cell junctions and desmin filaments in neonatal cardiomyocytes

Intermediate filaments (IFs) in cardiomyocytes consist primarily of desmin, surround myofibrils at Z disks, and transmit forces from the contracting myofilaments to the cell surface through costameres at the sarcolemma and desmosomes at intercalated disks. Synemin is a type IV IF protein that forms filaments with desmin and also binds α-actinin and vinculin. Here we examine the roles and expression of the α and β forms of synemin in developing rat cardiomyocytes. Quantitative PCR showed low levels of expression for both synemin mRNAs, which peaked at postnatal day 7. Synemin was concentrated at sites of cell-cell adhesion and at Z disks in neonatal cardiomyocytes. Overexpression of the individual isoforms showed that α-synemin preferentially localized to cell-cell junctions, whereas β-synemin was primarily at the level of Z disks. An siRNA targeted to both synemin isoforms reduced protein expression in cardiomyocytes by 70% and resulted in a failure of desmin to align with Z disks and disrupted cell-cell junctions, with no effect on sarcomeric organization. Solubility assays showed that β-synemin was soluble and interacted with sarcomeric α-actinin by coimmunoprecipitation, while α-synemin and desmin were insoluble. We conclude that β-synemin mediates the association of desmin IFs with Z disks, whereas α-synemin stabilizes junctional complexes between cardiomyocytes.

Microtopographical assembly of cardiomyocytes

One of the central challenges in cardiac tissue engineering is the control of the assembly and organization of functional cardiac tissue. Maintenance of a three-dimensional tissue architecture is key to myocardial function in vivo, and a variety of studies hint that provision of topological cues within scaffolds can facilitate the engineering of functional myocardial tissue by promoting this architecture. To explore this possibility in an isolated and well-defined fashion, we have designed scaffolds of polydimethylsiloxane (PDMS) with microtopographic pillars ("micropegs") to provide cells with defined structures with which to interact in three dimensions. We show that these surfaces permit HL-1 cardiomyocytes to grow, form myofibrillar structures and cell-cell adhesions, and beat spontaneously. Additionally, the cells and their nuclei interact with the full length of the micropegs, indicating that the micropegs promote a three-dimensional cytoarchitecture in the context of a two-dimensional scaffold. We also show that the number of cells interacting with a micropeg can be controlled by manipulating incubation time, micropeg spatial arrangement, or micropeg diameter. Western blots reveal that the expression of the junctional markers N-cadherin and connexin 43 is upregulated in the presence of specific arrangements of micropegs, suggesting that micropegs can enhance cardiomyocyte function. Together, these data show that microtopography can be used to provide three-dimensional adhesion and control the assembly of functional cardiac tissue on a two-dimensional surface.

Mechanotransduction: the role of mechanical stress, myocyte shape, and cytoskeletal architecture on cardiac function

Mechanotransduction refers to the conversion of mechanical forces into biochemical or electrical signals that initiate structural and functional remodeling in cells and tissues. The heart is a kinetic organ whose form changes considerably during development and disease, requiring cardiac myocytes to be mechanically durable and capable of fusing a variety of environmental signals on different time scales. During physiological growth, myocytes adaptively remodel to mechanical loads. Pathological stimuli can induce maladaptive remodeling. In both of these conditions, the cytoskeleton plays a pivotal role in both sensing mechanical stress and mediating structural remodeling and functional responses within the myocyte.

Intercellular and extracellular mechanotransduction in cardiac myocytes

Adult cardiomyocytes are terminally differentiated with minimal replicative capacity. Therefore, long-term preservation or enhancement of cardiac function depends on structural adaptation. Myocytes interact with the extracellular matrix, fibroblasts, and vascular cells and with each other (end to end; side to side). We review the current understanding of the mechanical determinants and environmental sensing systems that modulate and regulate myocyte molecular machinery and its structural organization. We feature the design and application of engineered cellular microenvironments to demonstrate the ability of cardiac cells to remodel their cytoskeletal organization and shape, including sarcomere/myofibrillar architectural topography. Cell shape-dependent functions result from complex mechanical interactions between the cytoskeleton architecture and external conditions, be they cell-cell or cell-extracellular matrix (ECM) adhesion contact-mediated. This mechanobiological perspective forms the basis for viewing the cardiomyocyte as a mechanostructural anisotropic continuum, exhibiting constant mechanosensory-driven self-regulated adjustment of the cytoskeleton through tight interplay between its force generation activity and concurrent cytoarchitectural remodeling. The unifying framework guiding this perspective is the observation that these emerging events and properties are initiated by and respond to cytoskeletal reorganization, regulated by cell-cell and cell-ECM adhesion and its corresponding (mutually interactive) signaling machinery. It is important for future studies to elucidate how cross talk between these mechanical signals is coordinated to control myocyte structure and function. Ultimately, understanding how the highly interactive mechanical signaling can give rise to phenotypic changes is critical for targeting the underlying pathways that contribute to cardiac remodeling associated with various forms of dilated and hypertrophic myopathies, myocardial infarction, heart failure, and reverse remodeling.

ZO-1 determines adherens and gap junction localization at intercalated disks

The disruption of the spatial order of electromechanical junctions at myocyte-intercalated disks (ICDs) is a poorly understood characteristic of many cardiac disease states. Here, in vitro and in vivo evidence is provided that zonula occludens-1 (ZO-1) regulates the organization of gap junctions (GJs) and adherens junctions (AJs) at ICDs. We investigated the contribution of ZO-1 to cell-cell junction localization by expressing a dominant-negative ZO-1 construct (DN-ZO-1) in rat ventricular myocytes (VMs). The expression of DN-ZO-1 in cultured neonatal VMs for 72 h reduced the interaction of ZO-1 and N-cadherin, as assayed by colocalization and coimmunoprecipitation, prompting cytoplasmic internalization of AJ and GJ proteins. DN-ZO-1 expression in adult VMs in vivo also reduced N-cadherin colocalization with ZO-1, a phenomenon not observed when the connexin-43 (Cx43)-ZO-1 interaction was disrupted using a mimetic of the ZO-1-binding ligand from Cx43. DN-ZO-1-infected VMs demonstrated large GJs at the ICD periphery and showed a loss of focal ZO-1 concentrations along plaque edges facing the disk interior. Additionally, there was breakdown of the characteristic ICD pattern of small interior and large peripheral GJs. Continuous DN-ZO-1 expression in VMs over postnatal development reduced ICD-associated Cx43 GJs and increased lateralized and cytoplasmic Cx43. We conclude that ZO-1 regulation of GJ localization is via an association with the N-cadherin multiprotein complex and that this is a key determinant of stable localization of both AJs and GJs at the ICD.

Cardiac myocyte remodeling mediated by N-cadherin-dependent mechanosensing

Cell-to-cell adhesions are crucial in maintaining the structural and functional integrity of cardiac cells. Little is known about the mechanosensitivity and mechanotransduction of cell-to-cell interactions. Most studies of cardiac mechanotransduction and myofibrillogenesis have focused on cell-extracellular matrix (ECM)-specific interactions. This study assesses the direct role of intercellular adhesion, specifically that of N-cadherin-mediated mechanotransduction, on the morphology and internal organization of neonatal ventricular cardiac myocytes. The results show that cadherin-mediated cell attachments are capable of eliciting a cytoskeletal network response similar to that of integrin-mediated force response and transmission, affecting myofibrillar organization, myocyte shape, and cortical stiffness. Traction forces mediated by N-cadherin were shown to be comparable to those sustained by ECM. The directional changes in predicted traction forces as a function of imposed loads (gel stiffness) provide the added evidence that N-cadherin is a mechanoresponsive adhesion receptor. Strikingly, the mechanical sensitivity response (gain) in terms of the measured cell-spread area as a function of imposed load (adhesive substrate rigidity) was consistently higher for N-cadherin-coated surfaces compared with ECM protein-coated surfaces. In addition, the cytoskeletal architecture of myocytes on an N-cadherin adhesive microenvironment was characteristically different from that on an ECM environment, suggesting that the two mechanotransductive cell adhesion systems may play both independent and complementary roles in myocyte cytoskeletal spatial organization. These results indicate that cell-to-cell-mediated force perception and transmission are involved in the organization and development of cardiac structure and function.

Rac1-induced connective tissue growth factor regulates connexin 43 and N-cadherin expression in atrial fibrillation

OBJECTIVES

We studied the signal transduction of atrial structural remodeling that contributes to the pathogenesis of atrial fibrillation (AF).

BACKGROUND

Fibrosis is a hallmark of arrhythmogenic structural remodeling, but the underlying molecular mechanisms are incompletely understood.

METHODS

We performed transcriptional profiling of left atrial myocardium from patients with AF and sinus rhythm and applied cultured primary cardiac cells and transgenic mice with overexpression of constitutively active V12Rac1 (RacET) in which AF develops at old age to characterize mediators of the signal transduction of atrial remodeling.

RESULTS

Left atrial myocardium from patients with AF showed a marked up-regulation of connective tissue growth factor (CTGF) expression compared with sinus rhythm patients. This was associated with increased fibrosis, nicotinamide adenine dinucleotide phosphate oxidase, Rac1 and RhoA activity, up-regulation of N-cadherin and connexin 43 (Cx43) expression, and increased angiotensin II tissue concentration. In neonatal rat cardiomyocytes and fibroblasts, a specific small molecule inhibitor of Rac1 or simvastatin completely prevented the angiotensin II-induced up-regulation of CTGF, Cx43, and N-cadherin expression. Transfection with small-inhibiting CTGF ribonucleic acid blocked Cx43 and N-cadherin expression. RacET mice showed up-regulation of CTGF, Cx43, and N-cadherin protein expression. Inhibition of Rac1 by oral statin treatment prevented these effects, identifying Rac1 as a key regulator of CTGF in vivo.

CONCLUSIONS

The data identify CTGF as an important mediator of atrial structural remodeling during AF. Angiotensin II activates CTGF via activation of Rac1 and nicotinamide adenine dinucleotide phosphate oxidase, leading to up-regulation of Cx43, N-cadherin, and interstitial fibrosis and therefore contributing to the signal transduction of atrial structural remodeling.

Embryonic stem cell-derived cardiomyogenesis: a novel role for calreticulin as a regulator

A role for calreticulin, an endoplasmic reticulum (ER)-resident, Ca(2+)-binding chaperone, has recently emerged in the context of cardiomyogenesis. We previously proposed calreticulin to be a novel cardiac fetal gene, because calreticulin knockout causes embryonic lethality in mice as a result of cardiac defects, it is transiently activated during heart development, and heart-targeted overexpression of constitutively active calcineurin in calreticulin-null mice rescues the lethal phenotype. Calreticulin affects Ca(2+) homeostasis and expression of adhesion-related genes. Using cardiomyocytes derived from both calreticulin-null and wild-type embryonic stem (ES) cells, we show here that cardiomyogenesis from calreticulin-null ES cells is accelerated but deregulated, such that the myofibrils of calreticulin-null cardiomyocytes become disorganized and disintegrate with time in culture. We have previously shown that the disorganization of the actin cytoskeleton in calreticulin-null cells may be explained, at least in part, by the downregulation of adhesion proteins, implying that calreticulin ablation causes adhesion-related defects. Here, upon examination of adhesion proteins, we found that vinculin is downregulated in calreticulin-null cardiomyocytes. We also found c-Src activity to be higher in calreticulin-null cardiomyocytes than in wild-type cardiomyocytes, and c-Src activity is affected by both calreticulin and [Ca(2+)]. Finally, we show that calreticulin and calsequestrin, the major Ca(2+) storage proteins of the ER and sarcoplasmic reticulum, respectively, exhibit alternate distributions. This suggests that calreticulin may have a housekeeping role to play in mature cardiomyocytes as well as during cardiomyogenesis. We propose here that calreticulin, an ER Ca(2+) storage protein, is a crucial regulator of cardiomyogenesis whose presence is required for controlled cardiomyocyte development from ES cells.

DE-Cadherin, a Core Component of the Adherens Junction Complex Modifies Subcellular Localization of theDrosophilaGap Junction Protein Innexin2

The Drosophila innexin multigene family of gap junction encoding proteins consists of eight family members whose function in epithelial morphogenesis is mostly unknown. We have recently shown that innexin2 plays a crucial role in the organization of embryonic epithelia. Innexin2 protein accumulates in the epidermis in the apico-lateral membrane domain and colocalizes with core proteins of adherens junctions, such as DE-cadherin and Armadillo, the ss -catenin homolog. Innexin2 localization is altered in both armadillo and DE-cadherin mutants Biochemical interaction studies point to a direct interaction of DE-cadherin and Armadillo with innexin2 suggesting a close link between gap junction and adherens junction biogenesis. We have used the Drosophila Schneider cell tissue culture system to further study the interaction of innexin2 with DE-cadherin. Our results provide evidence that DE-cadherin may be a key component to control trafficking, and localization of Innexin2 to the plasma membrane.

Reciprocal influence of connexins and apical junction proteins on their expressions and functions

Membranes of adjacent cells form intercellular junctional complexes to mechanically anchor neighbour cells (anchoring junctions), to seal the paracellular space and to prevent diffusion of integral proteins within the plasma membrane (tight junctions) and to allow cell-to-cell diffusion of small ions and molecules (gap junctions). These different types of specialised plasma membrane microdomains, sharing common adaptor molecules, particularly zonula occludens proteins, frequently present intermingled relationships where the different proteins co-assemble into macromolecular complexes and their expressions are co-ordinately regulated. Proteins forming gap junction channels (connexins, particularly) and proteins fulfilling cell attachment or forming tight junction strands mutually influence expression and functions of one another.

Protein kinase C-Fyn kinase cascade mediates the oleic acid-induced disassembly of neonatal rat cardiomyocyte adherens junctions

Oleic acid (OA) affects assembly of gap junctions in neonatal cardiomyocytes. Adherens junction (AJ) regulates the stability of gap junction integrity; however, the effect of OA on AJ remains largely unexplored. The distribution of N-cadherin and catenins at cell-cell junction was decreased by OA. OA induced activation of protein kinase C(PKC)-alpha and -epsilon and Src family kinase, and all three kinases were involved in the oleic acid-induced disassembly of the adherens junction, since it was blocked by pretreatment with Gö6976 (a PKCalpha inhibitor), epsilonV1-2 (a PKCepsilon inhibitor), or PP2 (a Src family kinase inhibitor). Src family kinase appeared to be the downstream of PKC-alpha and -epsilon, as blockade of either PKC-alpha or -epsilon activity prevented the OA-induced activation of Src family kinase. Immunoprecipitation analyses showed that OA activated Fyn and Fer. OA promoted the association of p120 catenin/beta-catenin with Fyn and Fer and caused increased tyrosine phosphorylation of p120 catenin and beta-catenin, resulting in decreased binding of the former to N-cadherin and of the latter to alpha-catenin. Pretreatment with PP2 abrogated this OA-induced tyrosine phosphorylation of p120 catenin and beta-catenin and restored the association of N-cadherin with p120 catenin and that of beta-catenin with alpha-catenin. In conclusion, these results show that OA activates the PKC-Fyn signaling pathway, leading to the disassembly of the AJ. Therefore, inhibitors of PKC-alpha/-epsilon and Src family kinase are potential candidates as cardioprotection agents against OA-induced heart injury during ischemia-reperfusion.

RhoA GTPase and F-actin dynamically regulate the permeability of Cx43-made channels in rat cardiac myocytes

Gap junctions are clusters of transmembrane channels allowing a passive diffusion of ions and small molecules between adjacent cells. Connexin43, the main channel-forming protein expressed in ventricular myocytes, can associate with zonula occludens-1, a scaffolding protein linked to the actin cytoskeleton and to signal transduction molecules. The possible influence of Rho GTPases, major regulators of cellular junctions and of the actin cytoskeleton, in the modulation of gap junctional intercellular communication (GJIC) was examined. The activation of RhoA by cytoxic necrotizing factor 1 markedly enhanced GJIC, whereas its specific inhibition by the Clostridium botulinum C3 exoenzyme significantly reduced it. RhoA activity affects GJIC without major cellular redistribution of junctional plaques or changes in the Cx43 phosphorylation pattern. As these GTPases frequently act via the cortical cytoskeleton, the importance of F-actin in the modulation of GJIC was investigated by means of agents interfering with actin polymerization. Cytoskeleton stabilization by phalloidin slowed down the kinetics of channel rundown in the absence of ATP, whereas its disruption by cytochalasin D rapidly and markedly reduced GJIC despite ATP presence. Cytoskeleton stabilization by phalloidin markedly reduced the consequences of RhoA activation or inactivation. This mechanism appears to be the first described capable to both up- or down-regulate GJIC through RhoA activation or, conversely, inhibition. The inhibition of Rho downstream kinase effectors had no effect on GJIC. The present results provide further insight into the gating and regulation of junctional channels and identify a new downstream target for the small G-protein RhoA.

Connexons and cell adhesion: a romantic phase

Recent evidence indicates, that gap junction forming proteins do not only contribute to intercellular communication (Kanno and Saffitz in Cardiovasc Pathol 10:169–177, 2001; Saez et al. in Physiol Rev 83:1359–1400, 2003), ion homeostasis and volume control (Goldberg et al. in J Biol Chem 277:36725–36730, 2002; Saez et al. in Physiol Rev 83:1359–1400, 2003). They also serve biological functions in a mechanical sense, supporting adherent connections between neighbouring cells of epithelial and non-epithelial tissues (Clair et al. in Exp Cell Res 314:1250–1265, 2008; Shaw et al. in Cell 128:547–560, 2007), where they stabilize migratory pathways in the developing central nervous system (Elias et al. in Nature 448:901–907, 2007; Malatesta et al. in Development 127:5253–5263, 2000; Noctor et al. in Nature 409:714–720, 2001; Rakic in Brain Res 33:471–476, 1971; J Comp Neurol 145:61–83 1972; Science 241:170–176, 1988), or mediate polarized movements and directionality of neural crest cells during organogenesis (Kirby and Waldo in Circ Res 77:211–215, 1995; Xu et al. in Development 133:3629–3639, 2006). Since, most data describing adhesive properties of gap junctions delt with connexin 43 (Cx43) (Beardslee et al. in Circ Res 83:629–635, 1998), we will focus our brief review on this isoform.

N-cadherin haploinsufficiency affects cardiac gap junctions and arrhythmic susceptibility

Cardiac-specific deletion of the murine gene (Cdh2) encoding the cell adhesion molecule, N-cadherin, results in disassembly of the intercalated disc (ICD) structure and sudden arrhythmic death. Connexin 43 (Cx43)-containing gap junctions are significantly reduced in the heart after depleting N-cadherin, therefore we hypothesized that animals expressing half the normal levels of N-cadherin would exhibit an intermediate phenotype. We examined the effect of N-cadherin haploinsufficiency on Cx43 expression and susceptibility to induced arrhythmias in mice either wild-type or heterozygous for the Cx43 (Gja1)-null allele. An increase in hypophosphorylated Cx43 accompanied by a modest decrease in total Cx43 protein levels was observed in the N-cadherin heterozygous mice. Consistent with these findings N-cadherin heterozygotes exhibited increased susceptibility to ventricular arrhythmias compared to wild-type mice. Quantitative immunofluorescence microscopy revealed a reduction in size of large Cx43-containing plaques in the N-cadherin heterozygous animals compared to wild-type. Gap junctions were further decreased in number and size in the N-cad/Cx43 compound heterozygous mice with increased arrhythmic susceptibility compared to the single mutants. The scaffold protein, ZO-1, was reduced at the ICD in N-cadherin heterozygous cardiomyocytes providing a possible explanation for the reduction in Cx43 plaque size. These data provide further support for the intimate relationship between N-cadherin and Cx43 in the heart, and suggest that germline mutations in the human N-cadherin (Cdh2) gene may predispose patients to increased risk of cardiac arrhythmias.

Cell mechanics: integrating cell responses to mechanical stimuli

Forces are increasingly recognized as major regulators of cell structure and function, and the mechanical properties of cells are essential to the mechanisms by which cells sense forces, transmit them to the cell interior or to other cells, and transduce them into chemical signals that impact a spectrum of cellular responses. Comparison of the mechanical properties of intact cells with those of the purified cytoskeletal biopolymers that are thought to dominate their elasticity reveal the extent to which the studies of purified systems can account for the mechanical properties of the much more heterogeneous and complex cell. This review summarizes selected aspects of current work on cell mechanics with an emphasis on the structures that are activated in cell-cell contacts, that regulate ion flow across the plasma membrane, and that may sense fluid flow that produces low levels of shear stress.

The area composita of adhering junctions connecting heart muscle cells of vertebrates - IV: coalescence and amalgamation of desmosomal and adhaerens junction components - late processes in mammalian heart development

In the adult mammalian heart, the cardiomyocytes and thus their terminally anchored myofibrillar bundles are connected by large arrays of closely spaced or even fused adhering junctions (AJs), termed "intercalated disks" (IDs). In recent years, the ID complex has attracted special attention as it has become clear that several human hereditary cardiomyopathies are caused by mutations of genes encoding ID marker proteins, in particular some that are also known as constituents of epithelial desmosomes. Previously, we have shown that in the mature myocardial ID the compositional differences between desmosome-like and adhaerens junctions are, by and large, lost and a composite hybrid structure, the area composita, is formed. We now report results from immunofluorescence and (immuno-)electron microscopic studies of heart formation during mouse embryogenesis and postnatal growth and show that the formation of the IDs with extended area composita structures is a late, primarily postnatal process. While up to birth small distinct desmosomes and AJs are resolved as predominant ID structures, areae compositae of increasing sizes and merged marker protein patterns occupy most of the IDs in the mature heart. Differences in the patterns of ID formation and amalgamation of the two ensembles of junction proteins in time and space are also demonstrated. Together with corresponding observations during rat and human heart development our results indicate that ID topogenesis and area composita formation are also late developmental processes in other mammals. We discuss the importance of the ID and the areae compositae in cardiac functions and, consequently, in cardiomyopathies and possible myocardial regeneration processes.

Beneficial effects of statins on the microcirculation during sepsis: the role of nitric oxide

This review describes the laboratory evidence and microvascular mechanisms responsible for the beneficial effects of statins in sepsis. During sepsis, changes occur within the microcirculation including alterations in arteriolar tone influencing blood pressure, adaptations to endothelial cell integrity causing leakage of proteins and macromolecules, and adhesion and migration of leucocytes through the vascular endothelium. Statins are widely used as cholesterol-lowering agents, but appear to have anti-inflammatory actions during sepsis. We have discussed the effects of statins on specific pathological processed within the microcirculation and focused on the role of nitric oxide (NO). The main mechanism by which statins appear to be an effective treatment for sepsis is increased expression of endothelial nitric oxide synthase (eNOS), in conjunction with down-regulation of inducible nitric oxide synthase. Combined, this results in an increase in physiological concentrations of NO, thus restoring endothelial function. Laboratory studies have therefore suggested that enhancement of eNOS activity during sepsis may lead to restoration of microvascular tone, maintenance of microvascular integrity, and inhibition of cell adhesion molecules. However, other mechanisms independent of lipid-lowering effects, including antioxidant activity and alterations in the development of vascular atherosclerosis, may also contribute to the beneficial effects of statins. We have also addressed the influence on the effects of statins of lipid solubility and pre- and pro-phylactic administration.

Microtubule plus-end-tracking proteins target gap junctions directly from the cell interior to adherens junctions

Gap junctions are intercellular channels that connect the cytoplasms of adjacent cells. For gap junctions to properly control organ formation and electrical synchronization in the heart and the brain, connexin-based hemichannels must be correctly targeted to cell-cell borders. While it is generally accepted that gap junctions form via lateral diffusion of hemichannels following microtubule-mediated delivery to the plasma membrane, we provide evidence for direct targeting of hemichannels to cell-cell junctions through a pathway that is dependent on microtubules; through the adherens-junction proteins N-cadherin and beta-catenin; through the microtubule plus-end-tracking protein (+TIP) EB1; and through its interacting protein p150(Glued). Based on live cell microscopy that includes fluorescence recovery after photobleaching (FRAP), total internal reflection fluorescence (TIRF), deconvolution, and siRNA knockdown, we propose that preferential tethering of microtubule plus ends at the adherens junction promotes delivery of connexin hemichannels directly to the cell-cell border. These findings support an unanticipated mechanism for protein delivery to points of cell-cell contact.

Beyond the epithelium: Cadherin function in fibrous connective tissues

In fibrous connective tissues, fibroblasts are organized into syncytia, cellular networks that enable matrix remodeling and that are interconnected by intercellular adherens junctions (AJs). The AJs of fibroblasts are mediated by N-cadherin, a broadly expressed classical cadherin that is critically involved in developmental processes, wound healing and several diseases of mesenchymal tissues. In contrast to E-cadherin-dependent junctions of epithelia, the formation of AJs in fibrous connective tissues is relatively uncharacterized. Work over the last several years has documented an expanding list of molecules which function to regulate N-cadherin mediated junctions such as: Fer, PTP1B, cortactin, calcium, gelsolin, PIP5KIgamma, PIP2, and the Rho family of GTPases. We present an overview on the regulation of N-cadherin-mediated junction formation that highlights recent molecular advances in the field and rationalizes the roles of N-cadherin in connective tissue function.

Ultrastructural analysis of development of myocardium in calreticulin-deficient mice

BACKGROUND

Calreticulin is a Ca2+ binding chaperone of the endoplasmic reticulum which influences gene expression and cell adhesion. The levels of both vinculin and N-cadherin are induced by calreticulin expression, which play important roles in cell adhesiveness. Cardiac development is strictly dependent upon the ability of cells to adhere to their substratum and to communicate with their neighbours.

RESULTS

We show here that the levels of N-cadherin are downregulated in calreticulin-deficient mouse embryonic hearts, which may lead to the disarray and wavy appearance of myofibrils in these mice, which we detected at all investigated stages of cardiac development. Calreticulin wild type mice exhibited straight, thick and abundant myofibrils, which were in stark contrast to the thin, less numerous, disorganized myofibrils of the calreticulin-deficient hearts. Interestingly, these major differences were only detected in the developing ventricles while the atria of both calreticulin phenotypes were similar in appearance at all developmental stages. Glycogen also accumulated in the ventricles of calreticulin-deficient mice, indicating an abnormality in cardiomyocyte metabolism.

CONCLUSION

Calreticulin is temporarily expressed during heart development where it is required for proper myofibrillogenesis. We postulate that calreticulin be considered as a novel cardiac fetal gene.

N-cadherin is a useful marker for the progenitor of cardiomyocytes differentiated from mouse ES cells in serum-free condition

Cardiomyocytes are known to differentiate spontaneously from embryonic stem (ES) cells when they formed aggregates, so called "embryoid bodies", in the presence of serum. In this study, we explored the induction of cardiomyocytes from mouse ES cells in chemically defined serum-free medium by using a mesoderm-inducing factor, BMP4. Comparing the different inductive methods, we found a candidate cell surface marker, N-cadherin, for cardiomyocyte progenitors from ES cells. N-cadherin-positive cells highly expressed cardiogenic markers, Nkx2.5, Tbx5, and Isl1, and showed a high differentiation rate into cardiomyocyte lineage. These results indicate that N-cadherin can be a useful cell surface marker for the progenitors of cardiomyocyte differentiated from ES cells in the serum-free culture.

N-cadherin and keratinocyte growth factor receptor mediate the functional interplay between Ki-RASG12V and p53V143A in promoting pancreatic cell migration, invasion, and tissue architecture disruption

The genetic basis of pancreatic ductal adenocarcinoma, which constitutes the most common type of pancreatic malignancy, involves the sequential activation of oncogenes and inactivation of tumor suppressor genes. Among the pivotal genetic alterations are Ki-RAS oncogene activation and p53 tumor suppressor gene inactivation. We explain that the combination of these genetic events facilitates pancreatic carcinogenesis as revealed in novel three-dimensional cell (spheroid cyst) culture and in vivo subcutaneous and orthotopic xenotransplantation models. N-cadherin, a member of the classic cadherins important in the regulation of cell-cell adhesion, is induced in the presence of Ki-RAS mutation but subsequently downregulated with the acquisition of p53 mutation as revealed by gene microarrays and corroborated by reverse transcription-PCR and Western blotting. N-cadherin modulates the capacity of pancreatic ductal cells to migrate and invade, in part via complex formation with keratinocyte growth factor receptor and neural cell adhesion molecule and in part via interaction with p120-catenin. However, modulation of these complexes by Ki-RAS and p53 leads to enhanced cell migration and invasion. This preferentially induces the downstream effector AKT over mitogen-activated protein kinase to execute changes in cellular behavior. Thus, we are able to define molecules that in part are directly affected by Ki-RAS and p53 during pancreatic ductal carcinogenesis, and this provides a platform for potential new molecularly based therapeutic interventions.

N-cadherin signals through Rac1 determine the localization of connexin 43 in cardiac myocytes

It has been proposed that the formation of gap junction is influenced by adherens junction in cardiac myocytes. To examine whether signals through N-cadherin are involved in the distribution of connexin 43 (Cx43), the distribution of cell-cell adhesion molecules, N-cadherin and Cx43, was analyzed in aligned cardiac myocytes. To induce cell orientation running in parallel to tension direction, neonatal rat cardiac myocytes were plated for 3 hours and exposed to 20% cyclic stretch for 24 hours on silicone dishes. The aligned cells cultured for 0-5 days were immunostained with anti- N-cadherin or anti-Cx43 antibody. After cultivation for 3-5 days, following the accumulation of N-cadherin, Cx43 was localized at the longitudinal cell termini. Adenoviral gene transfer of dominant negative N-cadherin significantly attenuated the localization of Cx43 at the longitudinal cell termini, suggesting that Cx43 localization is regulated downstream of N-cadherin. In the process of Cx43 localization, Rho family proteins, RhoA and Rac1, were activated, but not Cdc42. RhoA and Rac1 activation was inhibited by the transfection of dominant negative N-cadherin, indicating that RhoA and Rac1 were activated by N-cadherin in the oriented cardiac myocytes. The inhibition of Rho family proteins by Rho GDI significantly attenuated the accumulation of Cx43, but not that of N-cadherin. Furthermore, the translocation of Cx43 to longitudinal cell termini was prevented by the inhibition of Rac1, but not RhoA. Collectively, these findings suggest that the localization of Cx43 was determined through the Rac1 pathway downstream of N-cadherin in cardiac myocytes.

Dysregulation of cell adhesion proteins and cardiac arrhythmogenesis

Proper mechanical and electrical coupling of cardiomyocytes is crucial for normal propagation of the electrical impulse throughout the working myocardium. Various proteins on the surface of cardiomyocytes are responsible for the integration of structural information and cell-cell communication. Increasing evidence from diseased myocardium and animal models indicates that alteration in electrical coupling via gap junctions is a critical determinant in the development of an arrhythmogenic substrate. What is less clear is how gap junctions are maintained and regulated in the working myocardium. In this review, we present data from human disease and animal models that support the idea that cell adhesion proteins regulate the stability of the gap junction protein, connexin.