Xenopus paraxial protocadherin has signaling functions and is involved in tissue separation

Planar cell polarity in kidney development and disease

Planar cell polarity (PCP) describes the coordinated polarization of tissue cells in a direction that is orthogonal to their apical/basal axis. In the last several years, studies in flies and vertebrates have defined evolutionarily conserved pathways that establish and maintain PCP in various cellular contexts. Defective responses to the polarizing signal(s) have deleterious effects on the development and repair of a wide variety of organs/tissues. In this review, we cover the known and hypothesized roles for PCP in the metanephric kidney. We highlight the similarities and differences in PCP establishment in this organ compared with flies, especially the role of Wnt signaling in this process. Finally, we present a model whereby the signal(s) that organizes PCP in the kidney epithelium, at least in part, comes from the adjacent stromal fibroblasts.

PAPC and the Wnt5a/Ror2 pathway control the invagination of the otic placode in Xenopus

BACKGROUND

Paraxial protocadherin (PAPC) plays a crucial role in morphogenetic movements during gastrulation and somitogenesis in mouse, zebrafish and Xenopus. PAPC influences cell-cell adhesion mediated by C-Cadherin. A putative direct adhesion activity of PAPC is discussed. PAPC also promotes cell elongation, tissue separation and coordinates cell mass movements. In these processes the signaling function of PAPC in activating RhoA/JNK and supporting Wnt-11/PCP by binding to frizzled 7 (fz7) is important.

RESULTS

Here we demonstrate by loss of function experiments in Xenopus embryos that PAPC regulates another type of morphogenetic movement, the invagination of the ear placode. Knockdown of PAPC by antisense morpholinos results in deformation of the otic vesicle without altering otocyst marker expression. Depletion of PAPC could be rescued by full-length PAPC, constitutive active RhoA and by the closely related PCNS but not by classical cadherins. Also the cytoplasmic deletion mutant M-PAPC, which influences cell adhesion, does not rescue the PAPC knockdown. Interestingly, depletion of Wnt5a or Ror2 which are also expressed in the otocyst phenocopies the PAPC morphant phenotype.

CONCLUSIONS

PAPC signaling via RhoA and Wnt5a/Ror2 activity are required to keep cells aligned in apical-basal orientation during invagination of the ear placode. Since neither the cytoplasmic deletion mutant M-PAPC nor a classical cadherin is able to rescue loss of PAPC we suggest that the signaling function of the protocadherin rather than its role as modulator of cell-cell adhesion is required during invagination of the ear placode.

Protocadherin-12 cleavage is a regulated process mediated by ADAM10 protein: evidence of shedding up-regulation in pre-eclampsia

Protocadherins are a group of transmembrane proteins with homophilic binding activity, members of the cadherin superfamily. Apart from their role in adhesion, the cellular functions of protocadherins are essentially unknown. Protocadherin (PCDH)12 was previously identified in invasive trophoblasts and endothelial and mesangial cells in the mouse. Invalidation studies revealed that the protein was required for optimal placental development. In this article, we show that its human homolog is abundantly expressed in various trophoblast subtypes of the human placenta and at lower levels in endothelial cells. We demonstrate that PCDH12 is shed at high rates in vitro. The shedding mechanism depends on ADAM10 and results in reduced cellular adhesion in a cell migration assay. PCDH12 is subsequently cleaved by the γ-secretase complex, and its cytoplasmic domain is rapidly degraded by the proteasome. PCDH12 shedding is regulated by interlinked intracellular pathways, including those involving protein kinase C, PI3K, and cAMP, that either increase or inhibit cleavage. In endothelial cells, VEGF, prostaglandin E(2), or histamine regulates PCDH12 shedding. The extracellular domain of PCDH12 was also detected in human serum and urine, thus providing evidence of PCDH12 shedding in vivo. Importantly, we observed an increase in circulating PCDH12 in pregnant women who later developed a pre-eclampsia, a frequent pregnancy syndrome and a major cause of maternal and fetal morbidity and mortality. In conclusion, we speculate that, like in mice, PCDH12 may play an important role in human placental development and that proteolytic cleavage in response to external factors, such as cytokines and pathological settings, regulates its activity.

EphrinB/EphB signaling controls embryonic germ layer separation by contact-induced cell detachment

BACKGROUND

The primordial organization of the metazoan body is achieved during gastrulation by the establishment of the germ layers. Adhesion differences between ectoderm, mesoderm, and endoderm cells could in principle be sufficient to maintain germ layer integrity and prevent intermixing. However, in organisms as diverse as fly, fish, or amphibian, the ectoderm-mesoderm boundary not only keeps these germ layers separated, but the ectoderm also serves as substratum for mesoderm migration, and the boundary must be compatible with repeated cell attachment and detachment.

PRINCIPAL FINDINGS

We show that localized detachment resulting from contact-induced signals at the boundary is at the core of ectoderm-mesoderm segregation. Cells alternate between adhesion and detachment, and detachment requires ephrinB/EphB signaling. Multiple ephrinB ligands and EphB receptors are expressed on each side of the boundary, and tissue separation depends on forward signaling across the boundary in both directions, involving partially redundant ligands and receptors and activation of Rac and RhoA.

CONCLUSION

This mechanism differs from a simple differential adhesion process of germ layer formation. Instead, it involves localized responses to signals exchanged at the tissue boundary and an attachment/detachment cycle which allows for cell migration across a cellular substratum.

Protocadherin-19 and N-cadherin interact to control cell movements during anterior neurulation

The protocadherins comprise the largest subgroup within the cadherin superfamily, yet their cellular and developmental functions are not well understood. In this study, we demonstrate that pcdh 19 (protocadherin 19) acts synergistically with n-cadherin (ncad) during anterior neurulation in zebrafish. In addition, Pcdh 19 and Ncad interact directly, forming a protein-protein complex both in vitro and in vivo. Although both molecules are required for calcium-dependent adhesion in a zebrafish cell line, the extracellular domain of Pcdh 19 does not exhibit adhesive activity, suggesting that the involvement of Pcdh 19 in cell adhesion is indirect. Quantitative analysis of in vivo two-photon time-lapse image sequences reveals that loss of either pcdh 19 or ncad impairs cell movements during neurulation, disrupting both the directedness of cell movements and the coherence of movements among neighboring cells. Our results suggest that Pcdh 19 and Ncad function together to regulate cell adhesion and to mediate morphogenetic movements during brain development.

The tumor-associated EpCAM regulates morphogenetic movements through intracellular signaling

Epithelial cell adhesion molecule (EpCAM) is best known as a tumor-associated protein highly expressed in carcinomas. The function of this cell surface protein during embryonic development and its potential role in cancer are still poorly understood. We identified EpCAM in a gain-of-function screen for inducers of abnormal tissue mixing during gastrulation. Elevated EpCAM levels in either the ectoderm or the mesoderm confer "invasive" properties to cells in both populations. We found that this phenotype represents an "overstimulation" of an essential activity of EpCAM in controlling cell movements during embryonic development. Surprisingly, this property is independent of the putative adhesive function of EpCAM, and rather relies on a novel signaling function that operates through down-regulation of PKC activity. We show that inhibition of novel PKCs accounts entirely for the invasive phenotype induced by abnormally high levels of EpCAM as well as for its normal function in regulating cell rearrangement during early development.

Junctional music that the nucleus hears: cell-cell contact signaling and the modulation of gene activity

Cell-cell junctions continue to capture the interest of cell and developmental biologists, with an emerging area being the molecular means by which junctional signals relate to gene activity in the nucleus. Although complexities often arise in determining the direct versus indirect nature of such signal transduction, it is clear that such pathways are essential for the function of tissues and that alterations may contribute to many pathological outcomes. This review assesses a variety of cell-cell junction-to-nuclear signaling pathways, and outlines interesting areas for further study.

Planar cell polarity: keeping hairs straight is not so simple

The fur on a cat's back, the scales on a fish, or the bristles on a fly are all beautifully organized, with a high degree of polarization in their surface organization. Great progress has been made in understanding how individual cell polarity is established, but our understanding of how cells coordinate their polarity in forming coherent tissues is still fragmentary. The organization of cells in the plane of the epithelium is known as planar cell polarity (PCP), and studies in the past decade have delineated a genetic pathway for the control of PCP. This review will first briefly review data from the Drosophila field, where PCP was first identified and genetically characterized, and then explore how vertebrate tissues become polarized during development.

xGit2 and xRhoGAP 11A regulate convergent extension and tissue separation in Xenopus gastrulation

In a microarray-based screen for genes that are involved in tissue separation downstream of Paraxial Protocadherin (PAPC) and Frizzled-7 (Fz7)-mediated signaling we identified xGit2 and xRhoGAP 11A, two GTPase-activating proteins (GAP) for small GTPases. xGit2 and xRhoGAP 11A are expressed in the dorsal ectoderm, and their transcription is downregulated in the involuting dorsal mesoderm by PAPC and Fz7. Overexpression of xGit2 and xRhoGAP 11A inhibits Rho activity and impairs convergent extension movements as well as tissue separation behaviour. We propose that Rho activity in the involuting mesoderm is enhanced through inhibition of xGit2 and xRhoGAP 11A transcription by PAPC and Fz7. By this mechanism xRhoGAP 11A and xGit2 are restricted to the dorsal ectoderm, while Rho signaling is inhibited.

Biology and physics of cell shape changes in development

Together with cell growth, division and death, changes in cell shape are of central importance for tissue morphogenesis during development. Cell shape is the product of a cell's material and active properties balanced by external forces. Control of cell shape, therefore, relies on both tight regulation of intracellular mechanics and the cell's physical interaction with its environment. In this review, we first discuss the biological and physical mechanisms of cell shape control. We next examine a number of developmental processes in which cell shape change - either individually or in a coordinated manner - drives embryonic morphogenesis and discuss how cell shape is controlled in these processes. Finally, we emphasize that cell shape control during tissue morphogenesis can only be fully understood by using a combination of cellular, molecular, developmental and biophysical approaches.

A protocadherin-cadherin-FLRT3 complex controls cell adhesion and morphogenesis

Background

Paraxial protocadherin (PAPC) and fibronectin leucine-rich domain transmembrane protein-3 (FLRT3) are induced by TGFβ signaling in Xenopus embryos and both regulate morphogenesis by inhibiting C-cadherin mediated cell adhesion.

Principal Findings

We have investigated the functional and physical relationships between PAPC, FLRT3, and C-cadherin. Although neither PAPC nor FLRT3 are required for each other to regulate C-cadherin adhesion, they do interact functionally and physically, and they form a complex with cadherins. By itself PAPC reduces cell adhesion physiologically to induce cell sorting, while FLRT3 disrupts adhesion excessively to cause cell dissociation. However, when expressed together PAPC limits the cell dissociating and tissue disrupting activity of FLRT3 to make it effective in physiological cell sorting. PAPC counteracts FLRT3 function by inhibiting the recruitment of the GTPase RND1 to the FLRT3 cytoplasmic domain.

Conclusions/Significance

PAPC and FLRT3 form a functional complex with cadherins and PAPC functions as a molecular “governor” to maintain FLRT3 activity at the optimal level for physiological regulation of C-cadherin adhesion, cell sorting, and morphogenesis.

Detection of activated Rho in fixed Xenopus tissue

Small GTPases of the Rho family are important modulators of the cytoskeleton and regulate morphogenetic cell movements during embryonic development. In the Xenopus embryo, Rho signaling contributes to the regulation of convergent extension (CE) movements in gastrula and neurula stages as well as to tissue separation (TS). Here we describe a method that allows the detection of activated (GTP-bound) Rho in fixed Xenopus tissue. The assay makes use of a fusion protein of Rhotekin and Green-Fluorescent-Protein (RBD-GFP), which is produced in bacteria and can be purified biochemically. This technique allows a temporal and spatial analysis of Rho signaling in the developing embryo. Developmental Dynamics 238:1407-1411, 2009. (c) 2009 Wiley-Liss, Inc.

Molecular basis of morphogenesis during vertebrate gastrulation

Gastrulation is a crucial step in early embryogenesis. During gastrulation, a set of morphogenetic processes takes place leading to the establishment of the basic body plan and formation of primary germ layers. A rich body of knowledge about these morphogenetic processes has been accumulated over decades. The understanding of the molecular mechanism that controls the complex cell movement and inductive processes during gastrulation remains a challenge. Substantial progress has been made recently to identify and characterize pathways and molecules implicated in the modulation of morphogenesis during vertebrate gastrulation. Here, we summarize recent findings in the analysis of signaling pathways implicated in gastrulation movements, with the aim to generalize the basic molecular principles of vertebrate morphogenesis.

Evolutionary origins of blastoporal expression and organizer activity of the vertebrate gastrula organizer gene lhx1 and its ancient metazoan paralog lhx3

Expression of the LIM homeobox gene lhx1 (lim1) is specific to the vertebrate gastrula organizer. Lhx1 functions as a transcriptional regulatory core protein to exert `organizer' activity in Xenopus embryos. Its ancient paralog, lhx3 (lim3),is expressed around the blastopore in amphioxus and ascidian, but not vertebrate, gastrulae. These two genes are thus implicated in organizer evolution, and we addressed the evolutionary origins of their blastoporal expression and organizer activity. Gene expression analysis of organisms ranging from cnidarians to chordates suggests that blastoporal expression has its evolutionary root in or before the ancestral eumetazoan for lhx1,but possibly in the ancestral chordate for lhx3, and that in the ascidian lineage, blastoporal expression of lhx1 ceased, whereas endodermal expression of lhx3 has persisted. Analysis of organizer activity using Xenopus embryos suggests that a co-factor of LIM homeodomain proteins, Ldb, has a conserved function in eumetazoans to activate Lhx1, but that Lhx1 acquired organizer activity in the bilaterian lineage,Lhx3 acquired organizer activity in the deuterostome lineage and ascidian Lhx3 acquired a specific transactivation domain to confer organizer activity on this molecule. Knockdown analysis using cnidarian embryos suggests that Lhx1 is required for chordin expression in the blastoporal region. These data suggest that Lhx1 has been playing fundamental roles in the blastoporal region since the ancestral eumetazoan arose, that it contributed as an`original organizer gene' to the evolution of the vertebrate gastrula organizer, and that Lhx3 could be involved in the establishment of organizer gene networks.

Frizzled-7-dependent tissue separation in the Xenopus gastrula

Formation of tissue boundaries can be studied in a simple, inexpensive system, the Xenopus gastrula. Here, the internalized mesoderm and endoderm are separated from the ectodermal blastocoel roof by Brachet's cleft. Non-canonical Wnt signaling mediated by the Wnt receptor, Xfz-7, is essential for this tissue separation event. The function of Wnt pathway components and other factors in tissue separation at Brachet's cleft can be tested in a blastocoel roof assay. Small pieces of mesoderm or endoderm are placed on large blastocoel roof explants, and it is observed whether these test explants remain on the surface of their in vivo substratum, or sink into it.

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.

Remodeling of the adherens junctions during morphogenesis

Morphogenesis of epithelial tissues involves various forms of reshaping of cell layers, such as invagination or bending, convergent extension, and epithelial-mesenchymal transition. At the cellular level, these processes include changes in the shape, position, and assembly pattern of cells. During such morphogenetic processes, epithelial sheets in general maintain their multicellular architecture, implying that they must engage the mechanisms to change the spatial relationship with their neighbors without disrupting the junctions. A major junctional structure in epithelial tissues is the "adherens junction," which is composed of cadherin adhesion receptors and associated proteins including F-actin. The adherens junctions are required for the firm associations between cells, as disruption of them causes disorganization of the epithelial architecture. The adherens junctions, however, appear to be a dynamic entity, allowing the rearrangement of cells within cell sheets. This dynamic nature of the adherens junctions seems to be supported by various mechanisms, such as the interactions of cadherins with actin cytoskeleton, endocytosis and recycling of cadherins, and the cooperation of cadherins with other adhesion receptors. In this chapter, we provide an overview of these mechanisms analyzed in vitro and in vivo.

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.

Xenopus ADAM19 is involved in neural, neural crest and muscle development

ADAM19 is a member of the meltrin subfamily of ADAM metalloproteases. In Xenopus, ADAM19 is present as a maternal transcript. Zygotic expression starts during gastrulation and is apparent in the dorsal blastopore lip. ADAM19 expression through neurulation and tailbud formation becomes enriched in dorsal structures such as the neural tube, the notochord and the somites. Using morpholino knock-down, we show that a reduction of ADAM19 protein in gastrula stage embryos results in a decrease of Brachyury expression in the notochord concomitant with an increase in the dorsal markers, Goosecoid and Chordin. These changes in gene expression are accompanied by a decrease in phosphorylated AKT, a downstream target of the EGF signaling pathway, and occur while the blastopore closes at the same rate as the control embryos. During neurulation and tailbud formation, ADAM19 knock-down induces a reduction of the neural markers N-tubulin and NRP1 but not Sox2. In the somitic mesoderm, the expression of MLC is also decreased while MyoD is not. ADAM19 knockdown also reduces neural crest markers prior to cell migration. Neural crest induction is also decreased in embryos treated with an EGF receptor inhibitor suggesting that this pathway is necessary for neural crest cell induction. Using targeted knock-down of ADAM19 we show that the reduction of neural and neural crest markers is cell autonomous and that the migration if the cranial neural crest is perturbed. We further show that ADAM19 protein reduction affects somite organization, reduces 12-101 expression and perturbs fibronectin localization at the intersomitic boundary.

Molecular profile of endothelial invasion of three-dimensional collagen matrices: insights into angiogenic sprout induction in wound healing

Sprouting angiogenesis is a multistep process consisting of basement membrane degradation, endothelial cell (EC) activation, proliferation, invasion, lumen formation, and sprout stabilization. Such complexity is consistent with a requirement for orchestration of individual gene expression alongside multiple signaling pathways. To better understand the mechanisms that direct the transformation of adherent ECs on the surface of collagen matrices to develop multicellular invading sprouts, we analyzed differential gene expression with time using a defined in vitro model of EC invasion driven by the combination of sphingosine-1-phosphate, basic FGF, and VEGF. Gene expression changes were confirmed by real-time PCR and Western blot analyses. A cohort of cell adhesion molecule genes involved in adherens junction and cell-extracellular matrix (ECM) interactions were upregulated, whereas a set of genes associated with tight junctions were downregulated. Numerous genes encoding ECM proteins and proteases were induced, indicating that biosynthesis and remodeling of ECM is indispensable for sprouting angiogenesis. Knockdown of a highly upregulated gene, a disintegrin and metalloproteinase with thrombospondin-type repeats-1 (ADAMTS1), decreased invasion responses, confirming a role for ADAMTS1 in mediating EC invasion. Furthermore, differential expression of multiple members of the Wnt and Notch pathways was observed. Functional experiments indicated that inhibition and activation of the Notch signaling pathway stimulated and inhibited EC invasion responses, respectively. This study has enhanced the molecular road map of gene expression changes that occur during endothelial invasion and highlighted the utility of three-dimensional models to study EC morphogenesis.

Deciphering animal development through proteomics: requirements and prospects

In recent years proteomic techniques have started to become very useful tools in a variety of model systems of developmental biology. Applications cover many different aspects of development, including the characterization of changes in the proteome during early embryonic stages. During early animal development the embryo becomes patterned through the temporally and spatially controlled activation of distinct sets of genes. Patterning information is then translated, from gastrulation onwards, into regional specific morphogenetic cell and tissue movements that give the embryo its characteristic shape. On the molecular level, patterning is the outcome of intercellular communication via signaling molecules and the local activation or repression of transcription factors. Genetic approaches have been used very successfully to elucidate the processes behind these events. Morphogenetic movements, on the other hand, have to be orchestrated through regional changes in the mechanical properties of cells. The molecular mechanisms that govern these changes have remained much more elusive, at least in part due to the fact that they are more under translational/posttranslational control than patterning events. However, recent studies indicate that proteomic approaches can provide the means to finally unravel the mechanisms that link patterning to the generation of embryonic form. To intensify research in this direction will require close collaboration between proteome scientists and developmental researchers. It is with this aim in mind that we first give an outline of the classical questions of patterning and morphogenesis. We then summarize the proteomic approaches that have been applied in developmental model systems and describe the pioneering studies that have been done to study morphogenesis. Finally we discuss current and future strategies that will allow characterizing the changes in the embryonic proteome and ultimately lead to a deeper understanding of the cellular mechanisms that govern the generation of embryonic form.

Asymmetric homotypic interactions of the atypical cadherin flamingo mediate intercellular polarity signaling

Acquisition of planar cell polarity (PCP) in epithelia involves intercellular communication, during which cells align their polarity with that of their neighbors. The transmembrane proteins Frizzled (Fz) and Van Gogh (Vang) are essential components of the intercellular communication mechanism, as loss of either strongly perturbs the polarity of neighboring cells. How Fz and Vang communicate polarity information between neighboring cells is poorly understood. The atypical cadherin, Flamingo (Fmi), is implicated in this process, yet whether Fmi acts permissively as a scaffold or instructively as a signal is unclear. Here, we provide evidence that Fmi functions instructively to mediate Fz-Vang intercellular signal relay, recruiting Fz and Vang to opposite sides of cell boundaries. We propose that two functional forms of Fmi, one of which is induced by and physically interacts with Fz, bind each other to create cadherin homodimers that signal bidirectionally and asymmetrically, instructing unequal responses in adjacent cell membranes to establish molecular asymmetry.

Activin/nodal signaling modulates XPAPC expression during Xenopus gastrulation

Gastrulation is the first obligatory morphogenesis during vertebrate development, by which the body plan is established. Nodal signaling is a key player in many developmental processes, including gastrulation. XPAPC has been found to exert its biological function through modifying the adhesion property of cells and interacting with other several important molecules in embryos. In this report, we show that nodal signaling is necessary and sufficient for XPAPC expression during Xenopus gastrulation. Furthermore, we isolated 4.8 kb upstream DNA sequence of Xenopus XPAPC, and proved that this 4.8-kb genomic contig is sufficient to recapitulate the expression pattern of XPAPC from gastrula to tail bud stage. Transgene and ChIP assays indicate that Activin/nodal signaling participates in regulation of XPAPC expression through a Smad binding element within the XPAPC promoter. Concomitant investigation suggests that the canonical Wnt pathway-activated XPAPC expression requires nodal signaling.

Xenopus Paraxial Protocadherin regulates morphogenesis by antagonizing Sprouty

Xenopus Paraxial Protocadherin (xPAPC) has signaling functions that are essential for convergent extension (CE) movements and tissue separation during gastrulation. PAPC modulates components of the planar cell polarity (PCP) pathway, but it is not clear how PAPC is connected to beta-catenin-independent Wnt-signaling. By yeast two-hybrid screen, we found that the intracellular domain of PAPC interacts with Sprouty (Spry), an inhibitor of CE movements. Upon binding to PAPC, Spry function is inhibited and PCP signaling is enhanced. Our data indicate that PAPC promotes gastrulation movements by sequestration of Spry and reveal a novel mechanism by which protocadherins modulate beta-catenin-independent Wnt-signaling.

PCDH8, the human homolog of PAPC, is a candidate tumor suppressor of breast cancer

Carcinoma is an altered state of tissue differentiation in which epithelial cells no longer respond to cues that keep them in their proper position. A break down in these cues has disastrous consequences not only in cancer but also in embryonic development when cells of various lineages must organize into discrete entities to form a body plan. Paraxial protocadherin (PAPC) is an adhesion protein with six cadherin repeats that organizes the formation and polarity of developing cellular structures in frog, fish and mouse embryos. Here we show that protocadherin-8 (PCDH8), the human ortholog of PAPC, is inactivated through either mutation or epigenetic silencing in a high fraction of breast carcinomas. Loss of PCDH8 expression is associated with loss of heterozygosity, partial promoter methylation, and increased proliferation. Complementation of mutant tumor cell line HCC2218 with wild-type PCDH8 inhibited its growth. Two tumor mutants, E146K and R343H, were defective for inhibition of cell growth and migration. Surprisingly, the E146K mutant transformed the human mammary epithelial cell line MCF10A and sustained the expression of cyclin D1 and MYC without epidermal growth factor. We propose that loss of PCDH8 promotes oncogenesis in epithelial human cancers by disrupting cell-cell communication dedicated to tissue organization and repression of mitogenic signaling.

IRE1beta is required for mesoderm formation in Xenopus embryos

IRE1 is an atypical serine/threonine kinase transmembrane protein with RNase activity. In the unfolded protein response (UPR), they function as proximal sensor of the unfolded proteins in the endoplasmic reticulum (ER). Upon activation by ER stress, IRE1 performs an unconventional cytoplasmic splicing of XBP1 pre-mRNA and thus allows the synthesis of active XBP1, which activates UPR target genes to restore the homeostasis of the ER. IRE1/XBP1 signaling is hence essential for UPR but its function during embryogenesis is yet unknown. The transcripts of the two isoforms of IRE1 in Xenopus, xIRE1alpha and xIRE1beta are differentially expressed during embryogenesis. We found that xIRE1beta is sufficient for cytoplasmic splicing of xXBP1 pre-mRNA. Although gain of xIRE1beta function had no significant effect on Xenopus embryogenesis, overexpression of both, xIRE1beta and xXBP1 pre-mRNA, inhibits activin A induced mesoderm formation, suggesting that an enhanced activity of the IRE1/XBP1 pathway represses mesoderm formation. Surprisingly, while loss of XBP1 function promotes mesoderm formation, the loss of IRE1beta function led to a reduction of mesoderm formation, probably by action of IRE1 being different from the IRE1/XBP1 pathway. Therefore, both gain and loss of function studies demonstrate that IRE1 is required for mesoderm formation in Xenopus embryos.

Cadherin-mediated adhesion regulates posterior body formation

BACKGROUND

The anterior-posterior axis of the vertebrate embryo undergoes a dramatic elongation during early development. Convergence and extension of the mesoderm, occurring during gastrulation, initiates the narrowing and lengthening of the embryo. However the lengthening of the axis continues during post-gastrula stages in the tailbud region, and is thought to involve convergent extension movements as well as other cell behaviors specific to posterior regions.

RESULTS

We demonstrate here, using a semi-dominant N-cadherin allele, that members of the classical cadherin subfamily of cell-cell adhesion molecules are required for tailbud elongation in the zebrafish. In vivo imaging of cell behaviors suggests that the extension of posterior axial mesodermal cells is impaired in embryos that carry the semi-dominant N-cadherin allele. This defect most likely results from a general loss of cell-cell adhesion in the tailbud region. Consistent with these observations, N-cadherin is expressed throughout the tailbud during post-gastrulation stages. In addition, we show that N-cadherin interacts synergistically with vang-like 2, a member of the non-canonical Wnt signaling/planar cell polarity pathway, to mediate tail morphogenesis.

CONCLUSION

We provide the first evidence here that N-cadherin and other members of the classical cadherin subfamily function in parallel with the planar cell polarity pathway to shape the posterior axis during post-gastrulation stages. These findings further highlight the central role that adhesion molecules play in the cellular rearrangements that drive morphogenesis in vertebrates and identify classical cadherins as major contributors to tail development.

Wnt-5A/Ror2 regulate expression of XPAPC through an alternative noncanonical signaling pathway

XWnt-5A, a member of the nontransforming Wnt-5A class of Wnt ligands, is required for convergent extension movements in Xenopus embryos. XWnt-5A knockdown phenocopies paraxial protocadherin (XPAPC) loss of function: involuted mesodermal cells fail to align mediolaterally, which results in aberrant movements and a selective inhibition of constriction. XWnt-5A depletion was rescued by coinjection of XPAPC RNA, indicating that XWnt-5A acts upstream of XPAPC. XWnt-5A, but not XWnt-11, stimulates XPAPC expression independent of the canonical Wnt/beta-catenin pathway. We show that transcriptional regulation of XPAPC by XWnt-5A requires the receptor tyrosine kinase Ror2. XWnt-5A/Xror2 signal through PI3 kinase and cdc42 to activate the JNK signaling cascade with the transcription factors ATF2 and c-jun. The Wnt-5A/Ror2 pathway represents an alternative, distinct branch of noncanonical Wnt signaling that controls gene expression and is required in the regulation of convergent extension movements in Xenopus gastrulation.

Retinoic acid-inducible G protein-coupled receptors bind to frizzled receptors and may activate non-canonical Wnt signaling

Frizzled (Fz) seven-pass transmembrane receptors are Wnt receptors and function in a variety of developmental pathways. Here we identify retinoic acid-inducible gene-1, 2, 3, and 4 (RAIG1, 2, 3, and 4) as potential Fz binding proteins. RAIG proteins are seven-pass transmembrane receptors, and Xenopus RAIG2, 3, and 4 are expressed in early gastrula. XRAIG2 can activate small GTPases, such as RhoA, Rac, and Cdc42, and c-jun N-terminal kinase, thus exhibit activities that overlap with non-canonical Wnt/Fz signaling. Injection of XRAIG2 mRNA into Xenopus embryo causes a severe shortened and bent body axis due to defective gastrulation movements, reminiscent of abnormal non-canonical Wnt signaling. XRAIG2 affects convergent extension in activin-treated animal caps, which can be partially rescued by co-injection of a dominant-negative form of Cdc42. In zebrafish embryo, XRAIG2 also causes Ca(2+) flux, one of the consequences of non-canonical Wnt signaling. These results suggest a possible crosstalk/integration between Wnt/Frizzled and RAIG signal transduction pathways.

ANR5, an FGF target gene product, regulates gastrulation in Xenopus

Gastrulation is a morphogenetic process in which tightly coordinated cell and tissue movements establish the three germ layers (ectoderm, mesoderm, and endoderm) to define the anterior-to-posterior embryonic organization [1]. To elicit this movement, cells modulate membrane protrusions and undergo dynamic cell interactions. Here we report that ankyrin repeats domain protein 5 (xANR5), a novel FGF target gene product, regulates cell-protrusion formation and tissue separation, a process that develops the boundary between the ectoderm and mesoderm [2, 3], during Xenopus gastrulation. Loss of xANR5 function by antisense morpholino oligonucleotide (MO) caused a short trunk and spina bifida without affecting mesodermal gene expressions. xANR5-MO also blocked elongation of activin-treated animal caps (ACs) and tissue separation. The dorsal cells of xANR5-MO-injected embryos exhibited markedly reduced membrane protrusions, which could be restored by coinjecting active Rho. Active Rho also rescued the xANR5-MO-inhibited tissue separation. We further demonstrated that xANR5 interacted physically and functionally with paraxial protocadherin (PAPC), which has known functions in cell-sorting behavior, tissue separation, and gastrulation cell movements [4-6], to regulate early morphogenesis. Our findings reveal for the first time that xANR5 acts through Rho to regulate gastrulation and is an important cytoplasmic partner of PAPC, whose cytoplasmic partner was previously unknown.

Gene function in mouse embryogenesis: get set for gastrulation

During early mouse embryogenesis, temporal and spatial regulation of gene expression and cell signalling influences lineage specification, embryonic polarity, the patterning of tissue progenitors and the morphogenetic movement of cells and tissues. Uniquely in mammals, the extraembryonic tissues are the source of signals for lineage specification and tissue patterning. Here we discuss recent discoveries about the lead up to gastrulation, including early manifestations of asymmetry, coordination of cell and tissue movements and the interactions of transcription factors and signalling activity for lineage allocation and germ-layer specification.

Cadherins in development: cell adhesion, sorting, and tissue morphogenesis

Tissue morphogenesis during development is dependent on activities of the cadherin family of cell-cell adhesion proteins that includes classical cadherins, protocadherins, and atypical cadherins (Fat, Dachsous, and Flamingo). The extracellular domain of cadherins contains characteristic repeats that regulate homophilic and heterophilic interactions during adhesion and cell sorting. Although cadherins may have originated to facilitate mechanical cell-cell adhesion, they have evolved to function in many other aspects of morphogenesis. These additional roles rely on cadherin interactions with a wide range of binding partners that modify their expression and adhesion activity by local regulation of the actin cytoskeleton and diverse signaling pathways. Here we examine how different members of the cadherin family act in different developmental contexts, and discuss the mechanisms involved.

Wnt11 controls cell contact persistence by local accumulation of Frizzled 7 at the plasma membrane

Wnt11 is a key signal, determining cell polarization and migration during vertebrate gastrulation. It is known that Wnt11 functionally interacts with several signaling components, the homologues of which control planar cell polarity in Drosophila melanogaster. Although in D. melanogaster these components are thought to polarize cells by asymmetrically localizing at the plasma membrane, it is not yet clear whether their subcellular localization plays a similarly important role in vertebrates. We show that in zebrafish embryonic cells, Wnt11 locally functions at the plasma membrane by accumulating its receptor, Frizzled 7, on adjacent sites of cell contacts. Wnt11-induced Frizzled 7 accumulations recruit the intracellular Wnt signaling mediator Dishevelled, as well as Wnt11 itself, and locally increase cell contact persistence. This increase in cell contact persistence is mediated by the local interaction of Wnt11, Frizzled 7, and the atypical cadherin Flamingo at the plasma membrane, and it does not require the activity of further downstream effectors of Wnt11 signaling, such as RhoA and Rok2. We propose that Wnt11, by interacting with Frizzled 7 and Flamingo, modulates local cell contact persistence to coordinate cell movements during gastrulation.

Ca2+ signaling and early embryonic patterning during the Blastula and Gastrula Periods of Zebrafish and Xenopus development

It has been proposed that Ca(2+) signaling, in the form of pulses, waves and steady gradients, may play a crucial role in key pattern forming events during early vertebrate development [L.F. Jaffe, Organization of early development by calcium patterns, BioEssays 21 (1999) 657-667; M.J. Berridge, P. Lipp, M.D. Bootman, The versatility and universality of calcium signaling, Nat. Rev. Mol. Cell Biol. 1 (2000) 11-21; S.E. Webb, A.L. Miller, Calcium signalling during embryonic development, Nat. Rev. Mol. Cell Biol. 4 (2003) 539-551]. With reference to the embryos of zebrafish (Danio rerio) and the frog, Xenopus laevis, we review the Ca(2+) signals reported during the Blastula and Gastrula Periods. This developmental window encompasses the major pattern forming events of epiboly, involution, and convergent extension, which result in the establishment of the basic germ layers and body axes [C.B. Kimmel, W.W. Ballard, S.R. Kimmel, B. Ullmann, T.F. Schilling, Stages of embryonic development of the zebrafish, Dev. Dyn. 203 (1995) 253-310]. Data will be presented to support the suggestion that propagating waves (both long and short range) of Ca(2+) release, followed by sequestration, may play a crucial role in: (1) Coordinating cell movements during these pattern forming events and (2) Contributing to the establishment of the basic embryonic axes, as well as (3) Helping to define the morphological boundaries of specific tissue domains and embryonic structures, including future organ anlagen [E. Gilland, A.L. Miller, E. Karplus, R. Baker, S.E. Webb, Imaging of multicellular large-scale rhythmic calcium waves during zebrafish gastrulation, Proc. Natl. Acad. Sci. USA 96 (1999) 157-161; J.B. Wallingford, A.J. Ewald, R.M. Harland, S.E. Fraser, Calcium signaling during convergent extension in Xenopus, Curr. Biol. 11 (2001) 652-661]. The various potential targets of these Ca(2+) transients will also be discussed, as well as how they might integrate with other known pattern forming pathways known to modulate early developmental events (such as the Wnt/Ca(2+)pathway; [T.A. Westfall, B. Hjertos, D.C. Slusarski, Requirement for intracellular calcium modulation in zebrafish dorsal-ventral patterning, Dev. Biol. 259 (2003) 380-391]).

Tbx16 cooperates with Wnt11 in assembling the zebrafish organizer

The organizer, the signaling center that specifies vertebrate axial polarity and the nervous system, is a dorsal midline mesodermal domain in the gastrula that will form prechordal plate and anterior notochord. We show that in zebrafish the organizer is not a single domain when it first arises in the nascent mesoderm at the onset of gastrulation. Rather, in the presumptive prechordal plate region, the organizer is subdivided into two side-by-side cellular fields. Within minutes, concurrent medial and anterior cellular movements merge, or 'coalesce', the two fields to form the well-known singular midline field. Coalescence forms a symmetrical domain because the cell movements on the left and right sides initiate simultaneously and occur synchronously. However, in embryos with reduced function of the T-box transcription factor Tbx16 (Spadetail) or its genetic target paraxial protocadherin (Papc), synchrony is lost, coalesence is disrupted, and the midline domain is misshaped. Furthermore, with combined loss of Tbx16 and Wnt11 (Silberblick), coalesence is essentially absent. Possibly as a consequence, both the anterior movement of presumptive prechordal plate and organizer function, as assayed by eye-field separation, are disrupted. Our findings thus reveal that Tbx16, in combination with Wnt11, are critical components not only in morphogenesis but also in initial assembly of the organizer.

Paraxial protocadherin mediates cell sorting and tissue morphogenesis by regulating C-cadherin adhesion activity

Little is known about how protocadherins function in cell adhesion and tissue development. Paraxial protocadherin (PAPC) controls cell sorting and morphogenetic movements in the Xenopus laevis embryo. We find that PAPC mediates these functions by down-regulating the adhesion activity of C-cadherin. Expression of exogenous C-cadherin reverses PAPC-induced cell sorting and gastrulation defects. Moreover, loss of endogenous PAPC results in elevated C-cadherin adhesion activity in the dorsal mesoderm and interferes with the normal blastopore closure, a defect that can be rescued by a dominant-negative C-cadherin mutant. Importantly, activin induces PAPC expression, and PAPC is required for activin-induced regulation of C-cadherin adhesion activity and explant morphogenesis. Signaling through Frizzled-7 is not required for PAPC regulation of C-cadherin, suggesting that C-cadherin regulation and Frizzled-7 signaling are two distinct branches of the PAPC pathway that induce morphogenetic movements. Thus, spatial regulation of classical cadherin adhesive function by local expression of a protocadherin is a novel mechanism for controlling cell sorting and tissue morphogenesis.

Expression analysis of IGFBP-rP10, IGFBP-like and Mig30 in early Xenopus development

To date, five members of the insulin-like growth factor-binding protein (IGFBP) superfamily have been described in Xenopus laevis. Here, we report the isolation of two new IGFBPs: xIGFBP-rP10, and xIGFBP-like. The proteins share the same domain architecture, and together with Mig30, form a subgroup within the IGFBP superfamily. Temporal expression analysis shows that they are expressed differentially during early development. xIGFBP-rP10 is continuously expressed, whereas Mig30 expression peaks during gastrulation. IGFBP-like is expressed from neurulation onward. The three genes have characteristic spatial expression domains, which overlap in some regions. Both xIGFBP-rP10 and Mig30 are expressed on the dorsal side of the embryo during gastrulation. Later, xIGFBP-rP10 is expressed in the notochord, the floor plate, the somites, and the fin. xIGFBP-like expression is seen primarily in the developing central nervous system and overlaps with Mig30 expression at the end of neurulation in the developing somites and in tail bud stages in the eyes.

PCNS: a novel protocadherin required for cranial neural crest migration and somite morphogenesis in Xenopus

Protocadherins (Pcdhs), a major subfamily of cadherins, play an important role in specific intercellular interactions in development. These molecules are characterized by their unique extracellular domain (EC) with more than 5 cadherin-like repeats, a transmembrane domain (TM) and a variable cytoplasmic domain. PCNS (Protocadherin in Neural crest and Somites), a novel Pcdh in Xenopus, is initially expressed in the mesoderm during gastrulation, followed by expression in the cranial neural crest (CNC) and somites. PCNS has 65% amino acid identity to Xenopus paraxial protocadherin (PAPC) and 42-49% amino acid identity to Pcdh 8 in human, mouse, and zebrafish genomes. Overexpression of PCNS resulted in gastrulation failure but conferred little if any specific adhesion on ectodermal cells. Loss of function accomplished independently with two non-overlapping antisense morpholino oligonucleotides resulted in failure of CNC migration, leading to severe defects in the craniofacial skeleton. Somites and axial muscles also failed to undergo normal morphogenesis in these embryos. Thus, PCNS has essential functions in these two important developmental processes in Xenopus.

The role of Paraxial Protocadherin in Xenopus otic placode development

Vertebrate inner ear develops from its rudiment, otic placode, which later forms otic vesicle and gives rise to tissues comprising the entire inner ear. Although several signaling molecules have been identified as candidates responsible for inner ear specification and patterning, many details remain elusive. Here, we report that Paraxial Protocadherin (PAPC) is required for otic vesicle formation in Xenopus embryos. PAPC is expressed strictly in presumptive otic placode and later in otic vesicle during inner ear morphogenesis. Knockdown of PAPC by dominant-negative PAPC results in the failure of otic vesicle formation and the loss of early inner ear markers Sox9 and Tbx2, suggesting the requirement of PAPC in the early stage of otic vesicle development. However, PAPC alone is not sufficient to induce otic placode formation.

Establishment and maintenance of planar epithelial cell polarity by asymmetric cadherin bridges: A computer model

Animal scales, hairs, feathers, and cilia are oriented due to cell polarization in the epithelial plane. Genes involved have been identified, but the signal and mechanism remain unknown. In Drosophila wing polarization, the action of a gradient of Frizzled activity is widely assumed; and cell-cell signalling by cadherins such as Flamingo surely plays a major role. We present a computer model where reading the Frizzled gradient occurs through biased, feedback-reinforced formation of Flamingo-based asymmetric intercellular complexes. Through these complexes neighboring cells are able to compare their Frizzled activity levels. Our computations are highly noise-resistant and reproduce both wild-type and all known mutant wing phenotypes; other phenotypes are predicted. The model puts stringent limits on a Frizzled activation signal, which should exhibit unusual properties: (1) the extracellular Frizzled signalling gradient should be counterdirectional--decreasing from proximal (P) to distal (D), whereas during polarization, the intracellular Frizzled gradient builds up from P to D; (2) the external gradient should be relatively weak and short-lived, lest it prevent inversion of intracellular Frizzled. These features, largely independent of model details, may provide useful clues for future experimental efforts.

Regulatory targets for transcription factor AP2 in Xenopus embryos

The transcription factor AP2 (TFAP2) has an important role in regulating gene expression in both epidermis and neural crest cells. In order to further characterize these functions we have used a hormone inducible TFAP2alpha fusion protein in a Xenopus animal cap assay to identify downstream targets of this factor. The most common pattern comprised genes predominantly expressed in the epidermis. A second group was expressed at high levels in the neural crest, but all of these were also expressed in the epidermis as well as in other tissues in which TFAP2alpha has not been detected, suggesting modular control involving both TFAP2-dependent and TFAP2-independent components. In addition, a few strongly induced genes did not overlap at all in expression pattern with that of TFAP2alpha in the early embryo, and were also activated precociously in the experimentally manipulated ectoderm, and thus likely represent inappropriate regulatory interactions. A final group was identified that were repressed by TFAP2alpha and were expressed in the neural plate. These results provide further support for the importance of TFAP2alpha in ectoderm development, and also highlight the molecular linkage between the epidermis and neural crest in the Xenopus embryo.

New quantitative trait loci that regulate wound healing in an intercross progeny from DBA/1J and 129 x 1/SvJ inbred strains of mice

Wound healing/regeneration mouse models are few, and studies performed have mainly utilized crosses between MRL/MPJ (a good healer) and SJL/J (a poor healer) or MRL/lpr (a good healer) and C57BL/6J (a poor healer). Wound healing is a complex trait with many genes involved in the expression of the phenotype. Based on data from previous studies that common and additional quantitative trait loci (QTL) were identified using different crosses of inbred strains of mice for various complex traits, we hypothesized that a new cross would identify common and additional QTL, unique modes of inheritance, and interacting loci, which are responsible for variation in susceptibility to fast wound healing. In this study, we crossed DBA/1J (DBA, a good healer) and 129/SvJ (129, a poor healer) and performed a genome-wide scan using 492 (DBA x 129) F2 mice and 98 markers to identify QTL that regulate wound healing/regeneration. Four QTL on chromosomes 1, 4, 12, and 18 were identified which contributed toward wound healing in F2 mice and accounted for 17.1% of the phenotypic variation in ear punch healing. Surprisingly, locus interactions contributed to 55.7% of the phenotype variation in ear punch healing. In conclusion, we have identified novel QTL and shown that minor interacting loci contribute significantly to wound healing in DBA x 129 mice cross.

Organising cells into tissues: new roles for cell adhesion molecules in planar cell polarity

Planar cell polarity (PCP) is the coordinated organization of cells within the plane of the epithelium, first described in Drosophila. A Frizzled signalling pathway dedicated to PCP (the non-canonical Frizzled pathway) acts through Dishevelled and small G proteins, as does the classical Wnt pathway, but then diverges downstream of Dishevelled. Recent studies have demonstrated a crucial role for several atypical cadherin molecules (Fat, Dachsous and Flamingo) in controlling PCP signalling. Recent work has also indicated that the first sign of PCP during development is the polarized localization of PCP proteins (Frizzled, Flamingo, Dishevelled, etc). Exciting new data reveal that this PCP pathway is conserved to man.

beta-Catenin controls cell sorting at the notochord-somite boundary independently of cadherin-mediated adhesion

In Xenopus laevis, patterning of the trunk mesoderm into the dorsal notochord and lateral somites depends on differential regulation of Wnt-beta-catenin signaling. To study the cellular requirements for the physical separation of these tissues, we manipulated beta-catenin activity in individual cells that were scattered within the trunk mesoderm. We found that high activity led to efficient cell sorting from the notochord to the somites, whereas reduced activity led to sorting in the opposite direction. Analysis of individual cells overexpressing beta-catenin revealed that these cells were unable to establish stable contacts with notochord cells but could freely cross the boundary to integrate within the somitic tissue. Interference with cadherin-mediated adhesion disrupted tissue architecture, but it did not affect sorting and boundary formation. Based on these results, we propose that the boundary itself is the result of cell-autonomous changes in contact behavior that do not rely on differences in absolute levels of adhesion.

Cross-regulation of Wnt signaling and cell adhesion

The complex cross-regulation between Wnt signaling, cell-cell adhesion, and cell-matrix adhesion has revealed a number of regulatory components important in development and cancer progression. In the following, we would like to highlight and summarize some of the steps where pathways converge or diverge in regulating Wnt activity, matrix-induced pathways, and cell adhesion. We would like to focus on the involvement of heparan sulfate proteoglycan-rich proteins (HSPGs), integrin-mediated outside-in signaling, and cadherin-mediated cell-cell adhesion on Wnt pathways and the transcriptional regulation of extracellular matrix components and cell adhesion molecules by Wnt signaling.