Xbves is a regulator of epithelial movement during early Xenopus laevis development

Defective BVES-mediated feedback control of cAMP in muscular dystrophy

Biological processes incorporate feedback mechanisms to enable positive and/or negative regulation. cAMP is an important second messenger involved in many aspects of muscle biology. However, the feedback mechanisms for the cAMP signaling control in skeletal muscle are largely unknown. Here we show that blood vessel epicardial substance (BVES) is a negative regulator of adenylyl cyclase 9 (ADCY9)-mediated cAMP signaling involved in maintaining muscle mass and function. BVES deletion in mice reduces muscle mass and impairs muscle performance, whereas virally delivered BVES expressed in Bves-deficient skeletal muscle reverses these defects. BVES interacts with and negatively regulates ADCY9's activity. Disruption of BVES-mediated control of cAMP signaling leads to an increased protein kinase A (PKA) signaling cascade, thereby promoting FoxO-mediated ubiquitin proteasome degradation and autophagy initiation. Our study reveals that BVES functions as a negative feedback regulator of ADCY9-cAMP signaling in skeletal muscle, playing an important role in maintaining muscle homeostasis.

Bves maintains vascular smooth muscle cell contractile phenotype and protects against transplant vasculopathy via Dusp1-dependent p38MAPK and ERK1/2 signaling

BACKGROUND AND AIMS

Vascular smooth muscle cell (VSMC) plasticity is tightly associated with the pathological process of vasculopathy. Blood vessel epicardial substance (Bves) has emerged as an important regulator of intracardiac vasculogenesis and organ homeostasis. However, the involvement and role of Bves in VSMC plasticity and neointimal lesion development remain unclear.

METHODS

We used an in vivo rat model of graft arteriosclerosis and in vitro PDGF-treated VSMCs and identified the novel VSMC contractile phenotype-related gene Bves using a transcriptomic analysis and literature search. In vitro knockdown and overexpression approaches were used to investigate the mechanisms underlying VSMC phenotypic plasticity. In vivo, VSMC-specific Bves overexpression in rat aortic grafts was generated to assess the physiological function of Bves in neointimal lesion development.

RESULTS

Here, we found that Bves expression was negatively regulated in aortic allografts in vivo and PDGF-treated VSMCs in vitro. The genetic knockdown of Bves dramatically inhibited, whereas Bves overexpression markedly promoted, the VSMC contractile phenotype. Furthermore, RNA sequencing unraveled a positive correlation between Bves and dual-specificity protein phosphatase 1 (Dusp1) expression in VSMCs. We found that Bves knockdown restrained Dusp1 expression, but enhanced p38MAPK and ERK1/2 activation, resulting in the loss of the VSMC contractile phenotype. In vivo, an analysis of a rat graft model confirmed that VSMC-specific Bves and Dusp1 overexpression in aortic allografts significantly attenuated neointimal lesion formation.

CONCLUSIONS

Bves maintains the VSMC contractile phenotype through Dusp1-dependent p38MAPK and ERK1/2 signaling, and protects against neointimal formation, underscoring the important role of Bves in preventing transplant vasculopathy.

Ten years of research on the role of BVES/ POPDC1 in human disease: a review

Since the blood vessel epicardial substance or Popeye domain-containing protein 1 (BVES/POPDC1) was first identified in the developing heart by two independent laboratories in 1999, an increasing number of studies have investigated the structure, function, and related diseases of BVES/POPDC1. During the first 10 years following the discovery of BVES/POPDC1, studies focused mainly on its structure, expression patterns, and functions. Based on these studies, further investigations conducted over the previous decade examined the role of BVES/POPDC1 in human diseases, such as colitis, heart diseases, and human cancers. This review provides an overview of the structure and expression of BVES/POPDC1, mainly focusing on its potential role and mechanism through which it is involved in human cancers.

New kids on the block: The Popeye domain containing (POPDC) protein family acting as a novel class of cAMP effector proteins in striated muscle

The cyclic 3′,5′-adenosine monophosphate (cAMP) signalling pathway constitutes an ancient signal transduction pathway present in prokaryotes and eukaryotes. Previously, it was thought that in eukaryotes three effector proteins mediate cAMP signalling, namely protein kinase A (PKA), exchange factor directly activated by cAMP (EPAC) and the cyclic-nucleotide gated channels. However, recently a novel family of cAMP effector proteins emerged and was termed the Popeye domain containing (POPDC) family, which consists of three members POPDC1, POPDC2 and POPDC3. POPDC proteins are transmembrane proteins, which are abundantly present in striated and smooth muscle cells. POPDC proteins bind cAMP with high affinity comparable to PKA. Presently, their biochemical activity is poorly understood. However, mutational analysis in animal models as well as the disease phenotype observed in patients carrying missense mutations suggests that POPDC proteins are acting by modulating membrane trafficking of interacting proteins. In this review, we will describe the current knowledge about this gene family and also outline the apparent gaps in our understanding of their role in cAMP signalling and beyond.

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Popeye domain containing (POPDC) proteins are novel class of cAMP effector proteins.

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POPDC proteins control membrane trafficking of interacting proteins.

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POPDC proteins play a role in cardiac pacemaking and atrioventricular conduction.

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Mutations of POPDC genes are causing muscular dystrophy.

Popdc1/Bves Functions in the Preservation of Cardiomyocyte Viability While Affecting Rac1 Activity and Bnip3 Expression

The Popeye domain containing1, also called Bves (Popdc1/Bves), is a transmembrane protein that functions in muscle regeneration, heart rate regulation, hypoxia tolerance, and ischemia preconditioning. The expression of Popdc1/Bves is elevated in cardiomyocytes maintained in serum free defined medium. We hypothesized that Popdc1/Bves is important for cardiomyocyte survival under the stress of serum deprivation and investigated the mechanisms involved. A deficit in Popdc1/Bves, achieved by siRNA-mediated gene silencing, results in cardiomyocyte injury and death, upregulation of the pro-apoptotic protein Bcl-2/adenovirus E1B 19-kDa interacting protein3 (Bnip3), as well as reduction in Rac1-GTPase activity and in Akt phosphorylation. Combined Popdc1/Bves and Bnip3 silencing attenuated cell injury and prevented Bnip3 upregulation induced by the silencing of Popdc1/Bves alone. Chromatin immunoprecipitation indicated an increased binding of the transcription factor FoxO3 to the Bnip3 promoter although augmentation of FoxO3 in the nuclei was not detected. By contrast, the transcription factor NFκB was excluded from the nuclei of Popdc1/Bves deficient cardiomyocytes and exhibited decreased binding to the Bnip3 promoter. The data indicates that Popdc1/Bves plays a role in the preservation of cardiomyocyte viability under serum deficiency through the alteration of Rac1 activity and the regulation of Bnip3 expression by FoxO3 and NFκB transcription factors pointing to Popdc1/Bves as a potential target to enhance heart protection. J. Cell. Biochem. 118: 1505-1517, 2017. © 2016 Wiley Periodicals, Inc.

The Popeye domain containing protein family--A novel class of cAMP effectors with important functions in multiple tissues

Popeye domain containing (Popdc) proteins are a unique family, which combine several different properties and functions in a surprisingly complex fashion. They are expressed in multiple tissues and cell types, present in several subcellular compartments, interact with different classes of proteins, and are associated with a variety of physiological and pathophysiological processes. Moreover, Popdc proteins bind the second messenger cAMP with high affinity and it is thought that they act as a novel class of cAMP effector proteins. Here, we will review the most important findings about the Popdc family, which accumulated since its discovery about 15 years ago. We will be focussing on Popdc protein interaction and function in striated muscle tissue. However, as a full picture only emerges if all aspects are taken into account, we will also describe what is currently known about the role of Popdc proteins in epithelial cells and in various types of cancer, and discuss these findings with regard to their relevance for cardiac and skeletal muscle.

Netrin-1 promotes cell migration and invasion by down-regulation of BVES expression in human hepatocellular carcinoma

The axon guidance cues netrin-1 has been reported to be associated with cancer progression in various types of human cancers. However, the underlying molecular mechanism of netrin-1-mediated metastasis remains obscure. In this study, we found that overexpression of netrin-1 promoted HCC cell migration and invasion as determined by transwell assay and 3D cell culture assay. However, netrin-1 knockdown inhibited these processes. Further investigation indicated that netrin-1 decreased the expression of Blood Vessel Epicardial Substance (BVES), which was down-regulated in HCC. Interestingly, LY294002, a special inhibitor to PI3K/AKT signaling which was determined as a downstream pathway of netrin-1, restored the reduction in BVES caused by netrin-1. In addition, BVES exhibited an opposite effect on HCC cell metastasis to that of netrin-1. Importantly, up-regulating BVES expression significantly attenuated netrin-1-enhanced migration and invasion, whereas silencing BVES expression rescued the metastatic phenotype in netrin-1 knockdown cells. Moreover, netrin-1 expression was negatively correlated with BVES in HCC tissues and cell lines with different metastatic potential. Taken together, these results reveal that netrin-1 promotes HCC cell metastasis by regulating BVES expression via AKT activation.

The Popeye domain containing genes: essential elements in heart rate control

The Popeye domain containing (Popdc) gene family displays preferential expression in skeletal muscle and heart. Only recently a significant gain in the understanding of the function of Popdc genes in the heart has been obtained. The Popdc genes encode membrane proteins harboring an evolutionary conserved Popeye domain, which functions as a binding domain for cyclic adenosine monophosphate (cAMP). Popdc proteins interact with the two-pore channel TREK-1 and enhance its current. This protein interaction is modulated by cAMP. Null mutations of members of the Popdc gene family in zebrafish and mouse are associated with severe cardiac arrhythmia phenotypes. While in zebrafish an atrioventricular block was prevalent, in mouse a stress-induced sinus bradycardia was observed, which was due to the presence of sinus pauses. Moreover, the phenotype develops in an age-dependent manner, being absent in the young animal and becoming increasingly severe, as the animals grow older. This phenotype is reminiscent of the sick sinus syndrome (SSS), which affects mostly the elderly and is characterized by the poor ability of the cardiac pacemaker to adapt the heart rate to the physiological demand. While being a prevalent disease, which is responsible for a large fraction of pacemaker implantations in Western countries, SSS is poorly understood at the molecular level. It is therefore expected that the study of the molecular basis of the stress-induced bradycardia in Popdc mice will shed new light on the etiology of pacemaker disease.

Bves and NDRG4 regulate directional epicardial cell migration through autocrine extracellular matrix deposition

The Bves and NDRG4 proteins interact to regulate directional cell movement by mediating cell surface fusion of internalized fibronectin for resecretion. This provides the first evidence of Bves/NDRG4 protein function within subcellular trafficking pathways and explains how the Bves complex diversely influences development, cancer, and repair.

Directional cell movement is universally required for tissue morphogenesis. Although it is known that cell/matrix interactions are essential for directional movement in heart development, the mechanisms governing these interactions require elucidation. Here we demonstrate that a novel protein/protein interaction between blood vessel epicardial substance (Bves) and N-myc downstream regulated gene 4 (NDRG4) is critical for regulation of epicardial cell directional movement, as disruption of this interaction randomizes migratory patterns. Our studies show that Bves/NDRG4 interaction is required for trafficking of internalized fibronectin through the “autocrine extracellular matrix (ECM) deposition” fibronectin recycling pathway. Of importance, we demonstrate that Bves/NDRG4-mediated fibronectin recycling is indeed essential for epicardial cell directional movement, thus linking these two cell processes. Finally, total internal reflectance fluorescence microscopy shows that Bves/NDRG4 interaction is required for fusion of recycling endosomes with the basal cell surface, providing a molecular mechanism of motility substrate delivery that regulates cell directional movement. This is the first evidence of a molecular function for Bves and NDRG4 proteins within broader subcellular trafficking paradigms. These data identify novel regulators of a critical vesicle-docking step required for autocrine ECM deposition and explain how Bves facilitates cell-microenvironment interactions in the regulation of epicardial cell–directed movement.

Popeye domain-containing proteins and stress-mediated modulation of cardiac pacemaking

An intricate network of ion channels and pumps are involved in generating a diastolic pacemaker potential, which is transmitted to the working myocardium with the help of the cardiac conduction system. The principles of cardiac pacemaking are reasonably well understood, however, the mechanism by which the heart increases its beating frequency in response to adrenergic stimulation has not been fully worked out. The Popeye domain-containing (Popdc) genes encode plasma membrane-localized proteins that are able to bind cAMP with high affinity; mice with null mutations in Popdc1 or 2 have a stress-induced pacemaker dysfunction. The phenotype in both mutants develops in an age-dependent manner and thus may model pacemaker dysfunction in man, as well as provide novel mechanistic insights into the process of pacemaker adaptation to stress.

Reduced Popdc3 expression correlates with high risk and poor survival in patients with gastric cancer

AIM

To investigate the expression of Popeye domain containing 3 (Popdc3) and its correlation with clinicopathological features and prognosis of gastric cancer.

METHODS

The method of immunohistochemistry was used to investigate the expression of Popdc3 in 306 cases of human gastric cancer and 84 noncancerous gastric tissues. Simultaneously, the relationship between Popdc3 expression and the survival of the patients was retrospectively analyzed.

RESULTS

Popdc3 was detected in 72 (85.71%) of 84 human nontumor mucosa. High expression of Popdc3 protein was detected in 78 (25.49%) of 306 human gastric cancer cases, and low expression was detected in 228 (74.51%). Low expression of Popdc3 correlated with depth of invasion (P < 0.0001), regional lymph nodes (P < 0.0001) and distant metastasis (P = 0.02), and tumor, nodes, metastasis (TNM) stages (P < 0.0001). On multivariate analysis, only the patient's gender, regional lymph node metastasis, distant metastasis, TNM stages, and the expression of Popdc3 were independent prognostic factors in patients with gastric cancer. The Kaplan-Meier plot showed that low Popdc3 expression had a much more significant effect on the survival of those patients with early-stage tumors (χ² = 104.741, P < 0.0001), with a > 51.9% reduction in the three-year survival compared with high Popdc3 expression. In late stages, the difference was also significant (χ² = 5.930, P = 0.015), with a 32.6% reduction in the three-year survival.

CONCLUSION

Reduced expression of Popdc3 may play a significant role in the carcinogenesis and progression of gastric cancer. Popdc3 may be an independent prognostic factor.

Popeye domain containing proteins are essential for stress-mediated modulation of cardiac pacemaking in mice

Cardiac pacemaker cells create rhythmic pulses that control heart rate; pacemaker dysfunction is a prevalent disorder in the elderly, but little is known about the underlying molecular causes. Popeye domain containing (Popdc) genes encode membrane proteins with high expression levels in cardiac myocytes and specifically in the cardiac pacemaking and conduction system. Here, we report the phenotypic analysis of mice deficient in Popdc1 or Popdc2. ECG analysis revealed severe sinus node dysfunction when freely roaming mutant animals were subjected to physical or mental stress. In both mutants, bradyarrhythmia developed in an age-dependent manner. Furthermore, we found that the conserved Popeye domain functioned as a high-affinity cAMP-binding site. Popdc proteins interacted with the potassium channel TREK-1, which led to increased cell surface expression and enhanced current density, both of which were negatively modulated by cAMP. These data indicate that Popdc proteins have an important regulatory function in heart rate dynamics that is mediated, at least in part, through cAMP binding. Mice with mutant Popdc1 and Popdc2 alleles are therefore useful models for the dissection of the mechanisms causing pacemaker dysfunction and could aid in the development of strategies for therapeutic intervention.

The Popeye domain containing 2 (popdc2) gene in zebrafish is required for heart and skeletal muscle development

The Popeye domain containing (Popdc) genes encode a family of transmembrane proteins with an evolutionary conserved Popeye domain. These genes are abundantly expressed in striated muscle tissue, however their function is not well understood. In this study we have investigated the role of the popdc2 gene in zebrafish. Popdc2 transcripts were detected in the embryonic myocardium and transiently in the craniofacial and tail musculature. Morpholino oligonucleotide-mediated knockdown of popdc2 resulted in aberrant development of skeletal muscle and heart. Muscle segments in the trunk were irregularly shaped and craniofacial muscles were severely reduced or even missing. In the heart, pericardial edema was prevalent in the morphants and heart chambers were elongated and looping was abnormal. These pathologies in muscle and heart were alleviated after reducing the morpholino concentration. However the heart still was abnormal displaying cardiac arrhythmia at later stages of development. Optical recordings of cardiac contractility revealed irregular ventricular contractions with a 2:1, or 3:1 atrial/ventricular conduction ratio, which caused a significant reduction in heart frequency. Recordings of calcium transients with high spatiotemporal resolution using a transgenic calcium indicator line (Tg(cmlc2:gCaMP)(s878)) and SPIM microscopy confirmed the presence of a severe arrhythmia phenotype. Our results identify popdc2 as a gene important for striated muscle differentiation and cardiac morphogenesis. In addition it is required for the development of the cardiac conduction system.

BVES regulates EMT in human corneal and colon cancer cells and is silenced via promoter methylation in human colorectal carcinoma

The acquisition of a mesenchymal phenotype is a critical step in the metastatic progression of epithelial carcinomas. Adherens junctions (AJs) are required for suppressing this epithelial-mesenchymal transition (EMT) but less is known about the role of tight junctions (TJs) in this process. Here, we investigated the functions of blood vessel epicardial substance (BVES, also known as POPDC1 and POP1), an integral membrane protein that regulates TJ formation. BVES was found to be underexpressed in all stages of human colorectal carcinoma (CRC) and in adenomatous polyps, indicating its suppression occurs early in transformation. Similarly, the majority of CRC cell lines tested exhibited decreased BVES expression and promoter DNA hypermethylation, a modification associated with transcriptional silencing. Treatment with a DNA-demethylating agent restored BVES expression in CRC cell lines, indicating that methylation represses BVES expression. Reexpression of BVES in CRC cell lines promoted an epithelial phenotype, featuring decreased proliferation, migration, invasion, and anchorage-independent growth; impaired growth of an orthotopic xenograft; and blocked metastasis. Conversely, interfering with BVES function by expressing a dominant-negative mutant in human corneal epithelial cells induced mesenchymal features. These biological outcomes were associated with changes in AJ and TJ composition and related signaling. Therefore, BVES prevents EMT, and its epigenetic silencing may be an important step in promoting EMT programs during colon carcinogenesis.

BVES regulates EMT in human corneal and colon cancer cells and is silenced via promoter methylation in human colorectal carcinoma

The acquisition of a mesenchymal phenotype is a critical step in the metastatic progression of epithelial carcinomas. Adherens junctions (AJs) are required for suppressing this epithelial-mesenchymal transition (EMT) but less is known about the role of tight junctions (TJs) in this process. Here, we investigated the functions of blood vessel epicardial substance (BVES, also known as POPDC1 and POP1), an integral membrane protein that regulates TJ formation. BVES was found to be underexpressed in all stages of human colorectal carcinoma (CRC) and in adenomatous polyps, indicating its suppression occurs early in transformation. Similarly, the majority of CRC cell lines tested exhibited decreased BVES expression and promoter DNA hypermethylation, a modification associated with transcriptional silencing. Treatment with a DNA-demethylating agent restored BVES expression in CRC cell lines, indicating that methylation represses BVES expression. Reexpression of BVES in CRC cell lines promoted an epithelial phenotype, featuring decreased proliferation, migration, invasion, and anchorage-independent growth; impaired growth of an orthotopic xenograft; and blocked metastasis. Conversely, interfering with BVES function by expressing a dominant-negative mutant in human corneal epithelial cells induced mesenchymal features. These biological outcomes were associated with changes in AJ and TJ composition and related signaling. Therefore, BVES prevents EMT, and its epigenetic silencing may be an important step in promoting EMT programs during colon carcinogenesis.

Bves modulates tight junction associated signaling

Blood vessel epicardial substance (Bves) is a transmembrane adhesion protein that regulates tight junction (TJ) formation in a variety of epithelia. The role of TJs within epithelium extends beyond the mechanical properties. They have been shown to play a direct role in regulation of RhoA and ZONAB/DbpA, a y-box transcription factor. We hypothesize that Bves can modulate RhoA activation and ZONAB/DbpA activity through its regulatory effect on TJ formation. Immortalized human corneal epithelial (HCE) cells were stably transfected with Flag-tagged full length chicken Bves (w-Bves) or C-terminus truncated Bves (t-Bves). We found that stably transfected w-Bves and t-Bves were interacting with endogenous human Bves. However, interaction with t-Bves appeared to disrupt cell membrane localization of endogenous Bves and interaction with ZO-1. w-Bves cells exhibited increased TJ function reflected by increased trans-epithelial electrical resistance, while t-Bves cells lost TJ protein immunolocalization at cell-cell contacts and exhibited decreased trans-epithelial electrical resistance. In parental HCE and w-Bves cells ZONAB/DbpA and GEF-H1 were seen at cell borders in the same pattern as ZO-1. However, expression of t-Bves led to decreased membrane localization of both ZONAB/DbpA and GEF-H1. t-Bves cells had increased RhoA activity, as indicated by a significant 30% increase in FRET activity compared to parental HCE cells. ZONAB/DbpA transcriptional activity, assessed using a luciferase reporter probe, was increased in t-Bves cells. These studies demonstrate that Bves expression and localization can regulate RhoA and ZONAB/DbpA activity.

Identification of a novel Bves function: regulation of vesicular transport

Blood vessel/epicardial substance (Bves) is a transmembrane protein that influences cell adhesion and motility through unknown mechanisms. We have discovered that Bves directly interacts with VAMP3, a SNARE protein that facilitates vesicular transport and specifically recycles transferrin and beta-1-integrin. Two independent assays document that cells expressing a mutated form of Bves are severely impaired in the recycling of these molecules, a phenotype consistent with disruption of VAMP3 function. Using Morpholino knockdown in Xenopus laevis, we demonstrate that elimination of Bves function specifically inhibits transferrin receptor recycling, and results in gastrulation defects previously reported with impaired integrin-dependent cell movements. Kymographic analysis of Bves-depleted primary and cultured cells reveals severe impairment of cell spreading and adhesion on fibronectin, indicative of disruption of integrin-mediated adhesion. Taken together, these data demonstrate that Bves interacts with VAMP3 and facilitates receptor recycling both in vitro and during early development. Thus, this study establishes a newly identified role for Bves in vesicular transport and reveals a novel, broadly applied mechanism governing SNARE protein function.

Inhibition of RhoA signaling with increased Bves in trabecular meshwork cells

PURPOSE

Blood vessel epicardial substance (Bves) is a novel adhesion molecule that regulates tight junction (TJ) formation. TJs also modulate RhoA signaling, which has been implicated in outflow regulation. Given that Bves has been reported in multiple ocular tissues, the authors hypothesize that Bves plays a role in the regulation of RhoA signaling in trabecular meshwork (TM) cells.

METHODS

Human TM cell lines NTM-5 and NTM-5 transfected to overexpress Bves (NTM-w) were evaluated for TJ formation, and levels of occludin, cingulin, and ZO-1 protein were compared. Assays of TJ function were carried out using diffusion of sodium fluorescein and transcellular electrical resistance (TER). Levels of activated RhoA were measured using FRET probes, and phosphorylation of myosin light chain (MLC-p), a downstream target of RhoA, was assessed by Western blot analysis.

RESULTS

Overexpression of Bves led to increased TJ formation in NTM-5 cells. Increased TJ formation was confirmed by increased occludin, cingulin, and ZO-1 protein. Functionally, NTM-w cells showed decreased permeability and increased TER compared with NTM-5 cells, consistent with increased TJ formation. NTM-w cells also exhibited decreased levels of active RhoA and lower levels of MLC-p than did NTM-5 cells. These findings support a TJ role in RhoA signaling.

CONCLUSIONS

Increased Bves in TM cells leads to increased TJ formation with decreased RhoA activation and decreased MLC-p. This is the first report of a regulatory pathway upstream of RhoA in TM cells. In TM tissue, RhoA has been implicated in outflow regulation; thus, Bves may be a key regulatory molecule in aqueous outflow.

Bves: ten years after

Bves was discovered in 1999 by two independent laboratories using screens to identify novel genes that were highly expressed in the developing heart (Reese et al., 1999; Andree et al., 2000). As an evolutionarily conserved transmembrane protein, Bves is postulated to play a role in cell adhesion and cell motility. In studies of Bves protein disruption, there have been multiple phenotypes, but few molecular mechanisms have been advanced to explain the underlying cause of these phenotypes. As the molecular function of Bves protein begins to be uncovered, it is now time to review the literature to examine the significance of this work and future directions of study. This review summarizes the literature on this unique protein and explores new and exciting data that support emerging themes on its molecular function.

Regulated adhesion as a driving force of gastrulation movements

Recent data have reinforced the fundamental role of regulated cell adhesion as a force that drives morphogenesis during gastrulation. As we discuss, cell adhesion is required for all modes of gastrulation movements in all organisms. It can even be instructive in nature, but it must be tightly and dynamically regulated. The picture that emerges from the recent findings that we review here is that different modes of gastrulation movements use the same principles of adhesion regulation, while adhesion molecules themselves coordinate the intra- and extracellular changes required for directed cell locomotion.

Cell adhesion in amphibian gastrulation

The amphibian gastrula can be regarded as a single coherent tissue which folds and distorts itself in a reproducible pattern to establish the embryonic germ layers. It is held together by cadherins which provide the flexible adhesion required for the massive cell rearrangements that accompany gastrulation. Cadherin expression and adhesiveness increase as one goes from the vegetal cell mass through the anterior mesendoderm to the chordamesoderm, and then decrease again slightly in the ectoderm. Together with a basic random component of cell motility, this flexible, differentially expressed adhesiveness generates surface and interfacial tension effects which, in principle, can exert strong forces. However, conclusive evidence for an in vivo role of differential adhesion-related effects in gastrula morphogenesis is still lacking. The most important morphogenetic process in the amphibian gastrula seems to be intercellular migration, where cells crawl actively across each other's surface. The crucial aspect of this process is that cell motility is globally oriented, leading for example to mediolateral intercalation of bipolar cells during convergent extension of the chordamesoderm or to the directional migration of unipolar cells during translocation of the anterior mesendoderm on the ectodermal blastocoel roof. During these movements, the boundary between ectoderm and mesoderm is maintained by a tissue separation process.

Expression pattern of Popdc2 during mouse embryogenesis and in the adult

The Popdc2 gene is a member of the Popeye domain containing gene family encoding membrane proteins with prominent expression in striated and smooth muscle tissue. After introducing a LacZ reporter gene into the Popdc2 locus, expression was studied during embryonic development and postnatal life. At embryonic day (E) 7.5, expression was present in cardiac and extraembryonic mesoderm. At E10.5, expression was found in heart, somites, and mesothelial cells lining the coelom. At E12.5, expression was present in the coelomic mesothelium, pericardial and myocardial layer of the heart, skeletal muscle, bladder, gut, and umbilical vessels. Postnatal expression was found in cardiac and skeletal muscle and in the smooth muscle layer of colon, rectum, and bladder. In the stomach, Popdc2 was exclusively present in the pyloric epithelium. In conclusion, Popdc2 is expressed in various muscle and nonmuscle cell types during embryonic development and in postnatal life.

Bves directly interacts with GEFT, and controls cell shape and movement through regulation of Rac1/Cdc42 activity

Bves is an integral membrane protein with no determined function and no homology to proteins outside of the Popdc family. It is widely expressed throughout development in myriad organisms. Here, we demonstrate an interaction between Bves and guanine nucleotide exchange factor T (GEFT), a GEF for Rho-family GTPases. This interaction represents the first identification of any protein that has a direct physical interaction with any member of the Popdc family. Bves and GEFT are shown to colocalize in adult skeletal muscle. We also demonstrate that exogenous expression of Bves reduces Rac1 and Cdc42 activity levels while not affecting levels of active RhoA. Consistent with a repression of Rac1 and Cdc42 activity, we show changes in speed of cell locomotion and cell roundness also result from exogenous expression of Bves. Modulation of Rho-family GTPase signaling by Bves would be highly consistent with previously described phenotypes occurring upon disruption of Bves function in a wide variety of model systems. Therefore, we propose Bves as a novel regulator of the Rac1 and Cdc42 signaling cascades.

Identification of a novel intracellular interaction domain essential for Bves function

While Blood vessel epicardial substance (Bves) confers adhesive properties, the molecular mechanism of regulating this activity is unknown. No predicted functional motifs in this highly conserved integral membrane protein, other than the transmembrane domain, have been identified. Here, we report for the first time that Bves interacts with itself through an intracellular interaction domain that is essential for its intercellular adhesion activity. Glutathion-S-transferase (GST) pull-down and SPOTs analyses mapped this domain to amino acids 268-274 in the intracellular C-terminus. Site-directed mutagenesis revealed that lysines 272 and 273 are essential for homodimerization and cell adhesion. Human corneal cells transfected with wild-type Bves trafficked the protein to the cell surface, assembled junction complexes and formed epithelial sheets. In contrast, cells expressing Bves mutated at these positions did not form continuous epithelial sheets or maintain junctional proteins such as ZO-1 and E-cadherin at the membrane. A dramatic reduction in transepithelial electrical resistance was also observed indicating a functional loss of tight junctions. Importantly, expression of mutated Bves in epithelial cells promoted the transformation of cells from an epithelial to a mesenchymal phenotype. This study is the first to demonstrate the essential nature of any domain within Bves for maintenance of epithelial phenotype and function.

Characterization of Bves expression during mouse development using newly generated immunoreagents

Bves (blood vessel/epicardial substance) is a transmembrane protein postulated to play a role in cell-cell interaction/adhesion. It was independently isolated by two groups as a gene product highly enriched in the developing heart. Disagreement exists about its expression during development. Most notably, the expression of Bves in non-muscle cells is disputed. Determining the expression profile of Bves is a critical initial step preceding the characterization of protein function in development and in the adult. We have generated new monoclonal antibodies against mouse Bves and used these immunoreagents to elucidate Bves expression in development. As expected, we detect Bves in myocytes of the developing heart throughout development. In addition, skeletal and smooth muscle cells including those of the coronary system express Bves. Finally, specific, but not all, epithelial derivatives of the three germ layers are stained positively with these monoclonal antibodies. Protein expression in cultured epithelial and muscle cell lines corroborate our in vivo findings. Taken together, these results demonstrate the expression of Bves in a wide range of epithelial and muscle cells during mouse embryogenesis and indicate a broad function for this protein in development, and show that these newly generated reagents will be invaluable in further investigation of Bves.

Bves, a member of the Popeye domain-containing gene family

Bves was discovered through subtractive screens designed to identify heart-enriched transcripts. Bves is a transmembrane protein that possesses a highly conserved structure among species of the animal kingdom. Various approaches have been used to elucidate the expression pattern of Bves mRNA and protein as well as its function in developing and mature organisms. Emerging evidence indicates that this protein is present in muscle and epithelia of developing embryos and the adult. In vitro functional studies predict a role in cell-cell interaction and/or adhesion. In vivo analysis of protein function is very limited at present, but recent work in Xenopus supports the importance of Bves in epithelial integrity. Presented in this review is a compilation of published findings concerning Bves gene and protein characteristics, expression patterns in embryos and cells, and functional significance as determined thus far. Presently, the literature supports a hypothesis that Bves is essential to the junctional architecture of muscle and epithelial cell types. Although there remain aspects of Bves structure, expression, and function that are not completely resolved, now is an appropriate time to summarize current knowledge about this protein, the remaining questions, and what its potential role in development might be. This review will serve as a departure point for others who become interested in the study of this highly conserved protein.