Embryonic dorsal-ventral signaling: secreted frizzled-related proteins as inhibitors of tolloid proteinases

Double assurance in the induction of axial development by egg dorsal determinants in Xenopus embryos

We recently reported that microinjection of Xenopus nodal-related (xnr) mRNAs into β-catenin-depleted Xenopus embryos rescued a complete dorsal axis. Xnrs mediate the signal of the Nieuwkoop center that induces the Spemann-Mangold organizer in the overlying mesoderm, a process inhibited by the Nodal antagonist Cerberus-short (CerS). However, β-catenin also induces a second signaling center in the dorsal prospective ectoderm, designated the Blastula Chordin and Noggin Expression (BCNE) center, in which the homeobox gene siamois (sia) plays a major role. In this study, we asked whether the Xnrs and Sia depend on each other or function on parallel pathways. Expression of both genes induced β-catenin-depleted embryos to form complete axes with heads and eyes via the activation of similar sets of downstream organizer-specific genes. Xnrs did not activate siamois, and, conversely, Sia did not activate xnrs, although both were induced by β-catenin stabilization. Depletion with morpholinos revealed a robust role for the downstream target Chordin. Remarkably, Chordin depletion prevented all ectopic effects resulting from microinjection of the mRNA encoding the maternal cytoplasmic determinant Huluwa, including the radial expansion of brain tissue and the ectopic expression of the ventral gene sizzled. The main conclusion was that the BCNE and Nieuwkoop centers provide a double assurance mechanism for axial formation by independently activating similar downstream transcriptional target gene repertoires. We suggest that Siamois likely evolved from an ancestral Mix-type homeodomain protein called Sebox as a Xenopus-specific adaptation for the rapid differentiation of the anterior neural plate in the ectoderm.

Dissecting the mystery of embryonic scaling: The Scalers Hypothesis and its confirmation in sea urchin embryos

Embryonic scaling, the ability of embryos to regulate their spatial structure in proportion to size, remains a fascinating yet poorly studied problem in developmental biology. First described in sea urchin embryos by Hans Driesch, this phenomenon is now recognized as a striking example of how living organisms use non-equilibrium self-organization, based on reaction-diffusion (RD) systems, to generate pattern-determining morphogen concentration gradients that scale with size. Although specific molecular mechanisms for scaling such gradients have been described in some cases, a general approach for the targeted identification of such mechanisms had not been developed until recently. In search of a solution, we hypothesized the obligatory participation in scaling mechanisms of special genes, which we named "scalers." We supposed that these genes share two critical features: their expression is sensitive to embryo size, and their protein products determine the scale of morphogen concentration gradients. As proof of principle, we recently identified scalers by detecting differentially expressed genes in wild-type and half-size Xenopus laevis gastrula embryos. Furthermore, we described a mechanism by which one of the identified scalers, the gene encoding Metalloproteinase 3 (Mmp3), regulates the scaling of gradients of the morphogenic protein Bmp and its antagonists, Chordin and Noggin1/2. In the present work, we have made an important theoretical generalization of the Scalers Hypothesis by proving a statement regarding the obligatory presence of scalers in closed RD systems generating morphogen concentration gradients. Furthermore, through a systematic analysis of all known types of embryonic scaling models based on RD systems, we demonstrate that scalers are present in all known types of such models, either explicitly or implicitly. Finally, to test the universality of the Scalers Hypothesis, we applied our method to identify scalers that adjust Bmp/Chordin gradients to the size of the sea urchin embryo, Strongylocentrotus droebachiensis. Our results show that at least two members of the gene cluster encoding astacin metalloproteinases of the Span family, namely bp10 and Span, exhibit properties characteristic of scalers. Namely, their expression levels increase significantly in half-size embryos, and their protein products specifically degrade Chordin. Additionally, we found that the loss of function of bp10 and span leads to a narrowing of the dorsal domain of the Bmp signaling nuclear effector, pSmad1/5. These findings not only validate the Scalers Hypothesis but also uncover a novel mechanism by which Span proteinases fine-tune Chordin and Bmp morphogen concentration gradients in sea urchins. Thus, the Scalers Hypothesis and the approach to targeted search for such genes developed on its basis open up promising avenues for future research into scaling mechanisms in various biological systems.

Molecular analysis of a self-organizing signaling pathway for Xenopus axial patterning from egg to tailbud

Xenopus embryos provide a favorable material to dissect the sequential steps that lead to dorsal-ventral (D-V) and anterior-posterior (A-P) cell differentiation. Here, we analyze the signaling pathways involved in this process using loss-of-function and gain-of-function approaches. The initial step was provided by Hwa, a transmembrane protein that robustly activates early β-catenin signaling when microinjected into the ventral side of the embryo leading to complete twinned axes. The following step was the activation of Xenopus Nodal-related growth factors, which could rescue the depletion of β-catenin and were themselves blocked by the extracellular Nodal antagonists Cerberus-Short and Lefty. During gastrulation, the Spemann-Mangold organizer secretes a cocktail of growth factor antagonists, of which the BMP antagonists Chordin and Noggin could rescue simultaneously D-V and A-P tissues in β-catenin-depleted embryos. Surprisingly, this rescue occurred in the absence of any β-catenin transcriptional activity as measured by β-catenin activated Luciferase reporters. The Wnt antagonist Dickkopf (Dkk1) strongly synergized with the early Hwa signal by inhibiting late Wnt signals. Depletion of Sizzled (Szl), an antagonist of the Tolloid chordinase, was epistatic over the Hwa and Dkk1 synergy. BMP4 mRNA injection blocked Hwa-induced ectopic axes, and Dkk1 inhibited BMP signaling late, but not early, during gastrulation. Several unexpected findings were made, e.g., well-patterned complete embryonic axes are induced by Chordin or Nodal in β-catenin knockdown embryos, dorsalization by Lithium chloride (LiCl) is mediated by Nodals, Dkk1 exerts its anteriorizing and dorsalizing effects by regulating late BMP signaling, and the Dkk1 phenotype requires Szl.

Sizzled (Frzb3) physically interacts with noncanonical Wnt ligands to inhibit gastrulation cell movement

The coordinated movement of germ layer progenitor cells reaches its peak at the dorsal side, where the Bmp signaling gradient is low, and minimum at the ventral side, where the Bmp gradient is high. This dynamic cell movement is regulated by the interplay of various signaling pathways. The noncanonical Wnt signaling cascade serves as a pivotal regulator of convergence and extension cell movement, facilitated by the activation of small GTPases such as Rho, Rab, and Rac. However, the underlying cause of limited cell movement at the ventral side remains elusive. To explore the functional role of a key regulator in constraining gastrulation cell movement at the ventral side, we investigated the Bmp4-direct target gene, sizzled (szl), to assess its potential role in inhibiting noncanonical Wnt signaling. In our current study, we demonstrated that ectopic expression of szl led to gastrulation defects in a dose-dependent manner without altering cell fate specification. Overexpression of szl resulted in decreased elongation of Activin-treated animal cap and Keller explants. Furthermore, our immunoprecipitation assay unveiled the physical interaction of Szl with noncanonical Wnt ligand proteins (Wnt5 and Wnt11). Additionally, the activation of small GTPases involved in Wnt signaling mediation (RhoA and Rac1) was diminished upon szl overexpression. In summary, our findings suggest that Bmp4 signaling negatively modulates cell movement from the ventral side of the embryo by inducing szl expression during early Xenopus gastrulation.

Highly conserved and extremely evolvable: BMP signalling in secondary axis patterning of Cnidaria and Bilateria

Bilateria encompass the vast majority of the animal phyla. As the name states, they are bilaterally symmetric, that is with a morphologically clear main body axis connecting their anterior and posterior ends, a second axis running between their dorsal and ventral surfaces, and with a left side being roughly a mirror image of their right side. Bone morphogenetic protein (BMP) signalling has widely conserved functions in the formation and patterning of the second, dorso-ventral (DV) body axis, albeit to different extents in different bilaterian species. Whilst initial findings in the fruit fly Drosophila and the frog Xenopus highlighted similarities amongst these evolutionarily very distant species, more recent analyses featuring other models revealed considerable diversity in the mechanisms underlying dorsoventral patterning. In fact, as phylogenetic sampling becomes broader, we find that this axis patterning system is so evolvable that even its core components can be deployed differently or lost in different model organisms. In this review, we will try to highlight the diversity of ways by which BMP signalling controls bilaterality in different animals, some of which do not belong to Bilateria. Future research combining functional analyses and modelling is bound to give us some understanding as to where the limits to the extent of the evolvability of BMP-dependent axial patterning may lie.

Inhibition of the serine protease HtrA1 by SerpinE2 suggests an extracellular proteolytic pathway in the control of neural crest migration

We previously showed that SerpinE2 and the serine protease HtrA1 modulate fibroblast growth factor (FGF) signaling in germ layer specification and head-to-tail development of Xenopus embryos. Here, we present an extracellular proteolytic mechanism involving this serpin-protease system in the developing neural crest (NC). Knockdown of SerpinE2 by injected antisense morpholino oligonucleotides did not affect the specification of NC progenitors but instead inhibited the migration of NC cells, causing defects in dorsal fin, melanocyte, and craniofacial cartilage formation. Similarly, overexpression of the HtrA1 protease impaired NC cell migration and the formation of NC-derived structures. The phenotype of SerpinE2 knockdown was overcome by concomitant downregulation of HtrA1, indicating that SerpinE2 stimulates NC migration by inhibiting endogenous HtrA1 activity. SerpinE2 binds to HtrA1, and the HtrA1 protease triggers degradation of the cell surface proteoglycan Syndecan-4 (Sdc4). Microinjection of Sdc4 mRNA partially rescued NC migration defects induced by both HtrA1 upregulation and SerpinE2 downregulation. These epistatic experiments suggest a proteolytic pathway by a double inhibition mechanism.

SerpinE2 ┤HtrA1 protease ┤Syndecan-4 → NC cell migration.

Identification of PCPE-2 as the endogenous specific inhibitor of human BMP-1/tolloid-like proteinases

BMP-1/tolloid-like proteinases (BTPs) are major players in tissue morphogenesis, growth and repair. They act by promoting the deposition of structural extracellular matrix proteins and by controlling the activity of matricellular proteins and TGF-β superfamily growth factors. They have also been implicated in several pathological conditions such as fibrosis, cancer, metabolic disorders and bone diseases. Despite this broad range of pathophysiological functions, the putative existence of a specific endogenous inhibitor capable of controlling their activities could never be confirmed. Here, we show that procollagen C-proteinase enhancer-2 (PCPE-2), a protein previously reported to bind fibrillar collagens and to promote their BTP-dependent maturation, is primarily a potent and specific inhibitor of BTPs which can counteract their proteolytic activities through direct binding. PCPE-2 therefore differs from the cognate PCPE-1 protein and extends the possibilities to fine-tune BTP activities, both in physiological conditions and in therapeutic settings.

The developmental gene Chordin is amplified and expressed in human cancers

Chordin (CHRD) is a secreted protein important in early development, yet a role for CHRD in human disease has not been identified. In this study we investigated CHRD in cancer and normal adult tissues using the wealth of genome-wide data available in public databases. We found that Chordin is amplified in the DNA of specific cancers such as lung squamous cell and others, although copy number variation did not strictly correlate with higher mRNA expression. In some cancers, such as renal and stomach carcinomas, increased CHRD expression significantly correlated with poor survival. In normal adult human tissues, CHRD mRNA was highest in hepatocytes. Crossveinless-2/BMPER, a component of the Chordin morphogenetic pathway expressed at the opposite side in embryos, was expressed in liver stellate cells. This raises the intriguing possibility that a BMP gradient might be established in the extracellular matrix of the space of Disse that surrounds portal sinusoid capillaries.

Dynamics of the secreted frizzled related protein Sizzled and potential implications for binding to bone morphogenetic protein-1 (BMP-1)

Sizzled (Szl) is both a secreted frizzled related protein (sFRP) and a naturally occurring inhibitor of the zinc metalloproteinase bone morphogenetic protein-1 (BMP-1), a key regulator of extracellular matrix assembly and growth factor activation. Here we present a new crystal structure for Szl which differs from that previously reported by a large scale (90°) hinge rotation between its cysteine-rich and netrin-like domains. We also present results of a molecular docking analysis showing interactions likely to be involved in the inhibition of BMP-1 activity by Szl. When compared with known structures of BMP-1 in complex with small molecule inhibitors, this reveals features that may be helpful in the design of new inhibitors to prevent the excessive accumulation of extracellular matrix that is the hallmark of fibrotic diseases.

Evolution and loss of ß-catenin and TCF-dependent axis specification in insects

Mechanisms and evolution of primary axis specification in insects are discussed in the context of the roles of ß-catenin and TCF in polarizing metazoan embryos. Three hypotheses are presented. First, insects with sequential segmentation and posterior growth use cell-autonomous mechanisms for establishing embryo polarity via the nuclear ratio of ß-catenin and TCF. Second, TCF homologs establish competence for anterior specification. Third, the evolution of simultaneous segmentation mechanisms, also known as long-germ development, resulted in primary axis specification mechanisms that are independent of ß-catenin but reliant on TCF, a condition that preceded the frequent replacement of anterior determinants in long germ insects.

BMP Signaling: Lighting up the Way for Embryonic Dorsoventral Patterning

One of the most significant events during early embryonic development is the establishment of a basic embryonic body plan, which is defined by anteroposterior, dorsoventral (DV), and left-right axes. It is well-known that the morphogen gradient created by BMP signaling activity is crucial for DV axis patterning across a diverse set of vertebrates. The regulation of BMP signaling during DV patterning has been strongly conserved across evolution. This is a remarkable regulatory and evolutionary feat, as the BMP gradient has been maintained despite the tremendous variation in embryonic size and shape across species. Interestingly, the embryonic DV axis exhibits robust stability, even in face of variations in BMP signaling. Multiple lines of genetic, molecular, and embryological evidence have suggested that numerous BMP signaling components and their attendant regulators act in concert to shape the developing DV axis. In this review, we summarize the current knowledge of the function and regulation of BMP signaling in DV patterning. Throughout, we focus specifically on popular model animals, such as Xenopus and zebrafish, highlighting the similarities and differences of the regulatory networks between species. We also review recent advances regarding the molecular nature of DV patterning, including the initiation of the DV axis, the formation of the BMP gradient, and the regulatory molecular mechanisms behind BMP signaling during the establishment of the DV axis. Collectively, this review will help clarify our current understanding of the molecular nature of DV axis formation.

Targeted search for scaling genes reveals matrixmetalloproteinase 3 as a scaler of the dorsal-ventral pattern in Xenopus laevis embryos

How embryos scale patterning according to size is still not fully understood. Through in silico screening and analysis of reaction-diffusion systems that could be responsible for scaling, we predicted the existence of genes whose expression is sensitive to embryo size and which regulate the scaling of embryonic patterning. To find these scalers, we identified genes with strongly altered expression in half-size Xenopus laevis embryos compared with full-size siblings at the gastrula stage. Among found genes, we investigated the role of matrix metalloproteinase-3 (mmp3), which was most strongly downregulated in half-size embryos. We show that Mmp3 scales dorsal-ventral patterning by degrading the slowly diffusing embryonic inducers Noggin1 and Noggin2 but preventing cleavage of the more rapidly diffusing inducer Chordin via degradation of a Tolloid-type proteinase. In addition to unraveling the mechanism underlying the scaling of dorsal-ventral patterning, this work provides proof of principal for scalers identification in embryos of other species.

SerpinB13 antibodies promote β cell development and resistance to type 1 diabetes

Pancreatic endocrine cell development is dependent on the rescue of the neurogenin3 (Ngn3) transcription factor from repression by Notch. The signals that prevent Notch signaling, thereby allowing the formation of pancreatic endocrine cells, remain unclear. We show that inhibiting serpinB13, a cathepsin L (CatL) protease inhibitor expressed in the pancreatic epithelium, caused in vitro and in vivo cleavage of the extracellular domain of Notch1. This was followed by a twofold increase in the Ngn3⁺ progenitor cell population and enhanced conversion of these cells to express insulin. Conversely, both recombinant serpinB13 protein and CatL deficiency down-regulated pancreatic Ngn3⁺ cell output. Mouse embryonic exposure to inhibitory anti-serpinB13 antibody resulted in increased islet cell mass and improved outcomes in streptozotocin-induced diabetes at 8 weeks of age. Moreover, anti-serpinB13 autoantibodies stimulated Ngn3⁺ endocrine progenitor formation in the pancreas and were associated with delayed progression to type 1 diabetes (T1D) in children. These data demonstrate long-term impact of serpinB13 activity on islet biology and suggest that promoting protease activity by blocking this serpin may have prophylactic potential in T1D.

Pinhead antagonizes Admp to promote notochord formation

Dorsoventral patterning of a vertebrate embryo critically depends on the activity of Smad1 that mediates signaling by BMP proteins, anti-dorsalizing morphogenetic protein (Admp), and their antagonists. Pinhead (Pnhd), a cystine-knot-containing secreted protein, is expressed in the ventrolateral mesoderm during Xenopus gastrulation; however, its molecular targets and signaling mechanisms have not been fully elucidated. Our mass spectrometry-based screen of the gastrula secretome identified Admp as Pnhd-associated protein. We show that Pnhd binds Admp and inhibits its ventralizing activity by reducing Smad1 phosphorylation and its transcriptional targets. Importantly, Pnhd depletion further increased phospho-Smad1 levels in the presence of Admp. Furthermore, Pnhd synergized with Chordin and a truncated BMP4 receptor in the induction of notochord markers in ectoderm cells, and Pnhd-depleted embryos displayed notochord defects. Our findings suggest that Pnhd binds and inactivates Admp to promote notochord development. We propose that the interaction between Admp and Pnhd refines Smad1 activity gradients during vertebrate gastrulation.

Proteolytic Restriction of Chordin Range Underlies BMP Gradient Formation

A fundamental question in developmental biology is how morphogens, such as bone morphogenetic protein (BMP), form precise signaling gradients to impart positional and functional identity to the cells of the early embryo. We combine rigorous mutant analyses with quantitative immunofluorescence to determine that the proteases Bmp1a and Tolloid spatially restrict the BMP antagonist Chordin in dorsoventral (DV) axial patterning of the early zebrafish gastrula. We show that maternally deposited Bmp1a plays an unexpected and non-redundant role in establishing the BMP signaling gradient, while the Bmp1a/Tolloid antagonist Sizzled is surprisingly dispensable. Combining computational modeling and in vivo analyses with an immobile Chordin construct, we demonstrate that long-range Chordin diffusion is not necessary for BMP gradient formation and DV patterning. Our data do not support a counter-gradient of Chordin and instead favor a Chordin sink, established by Bmp1a and Tolloid, as the primary mechanism that drives BMP gradient formation.

The BMP morphogen generates a precise signaling gradient during axial patterning. In the zebrafish embryo, Tuazon et al. find that proteases Bmp1a/Tolloid are key to this process, preventing the long-range diffusion of the BMP antagonist, Chordin. By regionally restricting Chordin, Bmp1a/Tolloid establish the signaling sink that drives BMP gradient formation.

Diversity and robustness of bone morphogenetic protein pattern formation

Pattern formation by bone morphogenetic proteins (BMPs) demonstrates remarkable plasticity and utility in several contexts, such as early embryonic development, tissue patterning and the maintenance of stem cell niches. BMPs pattern tissues over many temporal and spatial scales: BMP gradients as short as 1-2 cell diameters maintain the stem cell niche of the Drosophila germarium over a 24-h cycle, and BMP gradients of several hundred microns establish dorsal-ventral tissue specification in Drosophila, zebrafish and Xenopus embryos in timescales between 30 min and several hours. The mechanisms that shape BMP signaling gradients are also incredibly diverse. Although ligand diffusion plays a dominant role in forming the gradient, a cast of diffusible and non-diffusible regulators modulate gradient formation and confer robustness, including scale invariance and adaptability to perturbations in gene expression and growth. In this Review, we document the diverse ways that BMP gradients are formed and refined, and we identify the core principles that they share to achieve reliable performance.

Apcdd1 is a dual BMP/Wnt inhibitor in the developing nervous system and skin

Animal development and homeostasis depend on precise temporal and spatial intercellular signaling. Components shared between signaling pathways, generally thought to decrease specificity, paradoxically can also provide a solution to pathway coordination. Here we show that the Bone Morphogenetic Protein (BMP) and Wnt signaling pathways share Apcdd1 as a common inhibitor and that Apcdd1 is a taxon-restricted gene with novel domains and signaling functions. Previously, we showed that Apcdd1 inhibits Wnt signaling (Shimomura et al., 2010), here we find that Apcdd1 potently inhibits BMP signaling in body axis formation and neural differentiation in chicken, frog, zebrafish. Furthermore, we find that Apcdd1 has an evolutionarily novel protein domain. Our results from experiments and modeling suggest that Apcdd1 may coordinate the outputs of two signaling pathways that are central to animal development and human disease.

BMP-1 disrupts cell adhesion and enhances TGF-β activation through cleavage of the matricellular protein thrombospondin-1

Bone morphogenetic protein 1 (BMP-1) is an important metalloproteinase that synchronizes growth factor activation with extracellular matrix assembly during morphogenesis and tissue repair. The mechanisms by which BMP-1 exerts these effects are highly context dependent. Because BMP-1 overexpression induces marked phenotypic changes in two human cell lines (HT1080 and 293-EBNA cells), we investigated how BMP-1 simultaneously affects cell-matrix interactions and growth factor activity in these cells. Increasing BMP-1 led to a loss of cell adhesion that depended on the matricellular glycoprotein thrombospondin-1 (TSP-1). BMP-1 cleaved TSP-1 between the VWFC/procollagen-like domain and the type 1 repeats that mediate several key TSP-1 functions. This cleavage induced the release of TSP-1 C-terminal domains from the extracellular matrix and abolished its previously described multisite cooperative interactions with heparan sulfate proteoglycans and CD36 on HT1080 cells. In addition, BMP-1-dependent proteolysis potentiated the TSP-1-mediated activation of latent transforming growth factor-β (TGF-β), leading to increased signaling through the canonical SMAD pathway. In primary human corneal stromal cells (keratocytes), endogenous BMP-1 cleaved TSP-1, and the addition of exogenous BMP-1 enhanced cleavage, but this had no substantial effect on cell adhesion. Instead, processed TSP-1 promoted the differentiation of keratocytes into myofibroblasts and stimulated production of the myofibroblast marker α-SMA, consistent with the presence of processed TSP-1 in human corneal scars. Our results indicate that BMP-1 can both trigger the disruption of cell adhesion and stimulate TGF-β signaling in TSP-1-rich microenvironments, which has important potential consequences for wound healing and tumor progression.

Maternal contributions to gastrulation in zebrafish

Gastrulation is a critical early morphogenetic process of animal development, during which the three germ layers; mesoderm, endoderm and ectoderm, are rearranged by internalization movements. Concurrent epiboly movements spread and thin the germ layers while convergence and extension movements shape them into an anteroposteriorly elongated body with head, trunk, tail and organ rudiments. In zebrafish, gastrulation follows the proliferative and inductive events that establish the embryonic and extraembryonic tissues and the embryonic axis. Specification of these tissues and embryonic axes are controlled by the maternal gene products deposited in the egg. These early maternally controlled processes need to generate sufficient cell numbers and establish the embryonic polarity to ensure normal gastrulation. Subsequently, after activation of the zygotic genome, the zygotic gene products govern mesoderm and endoderm induction and germ layer patterning. Gastrulation is initiated during the maternal-to-zygotic transition, a process that entails both activation of the zygotic genome and downregulation of the maternal transcripts. Genomic studies indicate that gastrulation is largely controlled by the zygotic genome. Nonetheless, genetic studies that investigate the relative contributions of maternal and zygotic gene function by comparing zygotic, maternal and maternal zygotic mutant phenotypes, reveal significant contribution of maternal gene products, transcripts and/or proteins, that persist through gastrulation, to the control of gastrulation movements. Therefore, in zebrafish, the maternally expressed gene products not only set the stage for, but they also actively participate in gastrulation morphogenesis.

Sfrp1 Modulates Cell-signaling Events Underlying Telencephalic Patterning, Growth and Differentiation

The mammalian dorsal telencephalic neuroepithelium develops-from medial to lateral-into the choroid plaque, cortical hem, hippocampal primordium and isocortex under the influence of Bmp, Wnt and Notch signaling. Correct telencephalic development requires a tight coordination of the extent/duration of these signals, but the identification of possible molecular coordinators is still limited. Here, we postulated that Secreted Frizzled Related Protein 1 (Sfrp1), a multifunctional regulator of Bmp, Wnt and Notch signaling strongly expressed during early telencephalic development, may represent 1 of such molecules. We report that in E10.5-E12.5 Sfrp1-/- embryos, the hem and hippocampal domains are reduced in size whereas the prospective neocortex is medially extended. These changes are associated with a significant reduction of the medio-lateral telencephalic expression of Axin2, a read-out of Wnt/βcatenin signaling activation. Furthermore, in the absence of Sfrp1, Notch signaling is increased, cortical progenitor cell cycle is shorter, with expanded progenitor pools and enhanced generation of early-born neurons. Hence, in postnatal Sfrp1-/- animals the anterior hippocampus is reduced and the neocortex is shorter in the antero-posterior and medio-lateral axis but is thicker. We propose that, by controlling Wnt and Notch signaling in opposite directions, Sfrp1 promotes hippocampal patterning and balances medio-lateral and antero-posterior cortex expansion.

Multiple Roles of sFRP2 in Cardiac Development and Cardiovascular Disease

The Wnt signaling pathway plays important roles in organ development and disease processes. Secreted frizzled-related protein 2 (sFRP2), a vital molecule of Wnt signaling, can regulate cardiac development and cardiovascular disease. Recent studies have suggested that sFRP2 is not only an antagonist of the canonical Wnt signaling pathway, but also has a more complex relationship in myocardial fibrosis, angiogenesis, cardiac hypertrophy and cardiac regeneration. Here, we review the role of sFRP2 and Wnt signaling in cardiac development and cardiovascular disease.

The BMP ligand Pinhead together with Admp supports the robustness of embryonic patterning

Vertebrate embryonic dorsoventral axis is robustly stable in the face of variations in bone morphogenetic protein (BMP) signaling. However, the molecular mechanism behind this robustness remains uncharacterized. In this study, we show that zebrafish Pinhead, together with Admp, plays an important compensatory role in ensuring the robustness of axial patterning through fine-tuning of BMP signaling. pinhead encodes a BMP-like ligand expressed in the ventrolateral margin of the early gastrula. Transcription of pinhead and admp is under opposing regulation, where pinhead depletion results in a compensatory increase in admp transcription and vice versa, leading to normal axis formation in pinhead or admp mutants. Expression of pinhead and admp is directly repressed by the BMP/Smad pathway. When BMP signals were inhibited or excessively activated, pinhead/admp expression changed accordingly, allowing for self-regulation. Thus, this study reveals a negative feedback loop between BMP signaling and pinhead/admp that effectively stabilizes embryonic patterning by buffering against fluctuations in BMP signaling.

The C-terminal region of human plasma fetuin-B is dispensable for the raised-elephant-trunk mechanism of inhibition of astacin metallopeptidases

Human fetuin-B plays a key physiological role in human fertility through its inhibitory action on ovastacin, a member of the astacin family of metallopeptidases. The inhibitor consists of tandem cystatin-like domains (CY1 and CY2), which are connected by a linker containing a "CPDCP-trunk" and followed by a C-terminal region (CTR) void of regular secondary structure. Here, we solved the crystal structure of the complex of the inhibitor with archetypal astacin from crayfish, which is a useful model of human ovastacin. Two hairpins from CY2, the linker, and the tip of the "legumain-binding loop" of CY1 inhibit crayfish astacin following the "raised-elephant-trunk mechanism" recently reported for mouse fetuin-B. This inhibition is exerted by blocking active-site cleft sub-sites upstream and downstream of the catalytic zinc ion, but not those flanking the scissile bond. However, contrary to the mouse complex, which was obtained with fetuin-B nicked at a single site but otherwise intact, most of the CTR was proteolytically removed during crystallization of the human complex. Moreover, the two complexes present in the crystallographic asymmetric unit diverged in the relative arrangement of CY1 and CY2, while the two complexes found for the mouse complex crystal structure were equivalent. Biochemical studies in vitro confirmed the differential cleavage susceptibility of human and mouse fetuin-B in front of crayfish astacin and revealed that the cleaved human inhibitor blocks crayfish astacin and human meprin α and β only slightly less potently than the intact variant. Therefore, the CTR of animal fetuin-B orthologs may have a function in maintaining a particular relative orientation of CY1 and CY2 that nonetheless is dispensable for peptidase inhibition.

Transcriptome analysis of regeneration during Xenopus laevis experimental twinning

Animal embryos have the remarkable property of self-organization. Over 125 years ago, Hans Driesch separated the two blastomeres of sea urchin embryos and obtained twins, in what was the foundation of experimental embryology. Since then, embryonic twinning has been obtained experimentally in many animals. In a recent study, we developed bisection methods that generate identical twins reliably from Xenopus blastula embryos. In the present study, we have investigated the transcriptome of regenerating half-embryos after sagittal and dorsal-ventral (D-V) bisections. Individual embryos were operated at midblastula (stage 8) with an eyelash hair and cultured until early gastrula (stage 10.5) or late gastrula (stage 12) and the transcriptome of both halves were analyzed by RNA-seq. Since many genes are activated by wound healing in Xenopus embryos, we resorted to stringent sequence analyses and identified genes up-regulated in identical twins but not in either dorsal or ventral fragments. At early gastrula, cell division-related transcripts such as histones were elevated, whereas at late gastrula, pluripotency genes (such as sox2) and germ layer determination genes (such as eomesodermin, ripply2 and activin receptor ACVRI) were identified. Among the down-regulated transcripts, sizzled, a regulator of Chordin stability, was prominent. These findings are consistent with a model in which cell division is required to heal damage, while maintaining pluripotency to allow formation of the organizer with a displacement of 90 ⁰ from its original site. The extensive transcriptomic data presented here provides a valuable resource for data mining of gene expression during early vertebrate development.

Structure of mammalian plasma fetuin-B and its mechanism of selective metallopeptidase inhibition

Mammalian fetuin-A and fetuin-B are abundant serum proteins with pleiotropic functions. Fetuin-B is a highly selective and potent inhibitor of metallo-peptidases (MPs) of the astacin family, which includes ovastacin in mammals. By inhibiting ovastacin, fetuin-B is essential for female fertility. The crystal structure of fetuin-B was determined unbound and in complex with archetypal astacin, and it was found that the inhibitor has tandem cystatin-type modules (CY1 and CY2). They are connected by an exposed linker with a rigid, disulfide-linked 'CPDCP-trunk', and are followed by a C-terminal region (CTR) with little regular secondary structure. The CPDCP-trunk and a hairpin of CY2 form a bipartite wedge, which slots into the active-site cleft of the MP. These elements occupy the nonprimed and primed sides of the cleft, respectively, but spare the specificity pocket so that the inhibitor is not cleaved. The aspartate in the trunk blocks the catalytic zinc of astacin, while the CY2 hairpin binds through a QWVXGP motif. The CY1 module assists in structural integrity and the CTR is not involved in inhibition, as verified by in vitro studies using a cohort of mutants and variants. Overall, the inhibition conforms to a novel 'raised-elephant-trunk' mechanism for MPs, which is reminiscent of single-domain cystatins that target cysteine peptidases. Over 200 sequences from vertebrates have been annotated as fetuin-B, underpinning its ubiquity and physiological relevance; accordingly, sequences with conserved CPDCP- and QWVXGP-derived motifs have been found from mammals to cartilaginous fishes. Thus, the raised-elephant-trunk mechanism is likely to be generally valid for the inhibition of astacins by orthologs of fetuin-B.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Pitx1 regulates cement gland development in Xenopus laevis through activation of transcriptional targets and inhibition of BMP signaling

The cement gland in Xenopus laevis has long been used as a model to study the interplay of cell signaling and transcription factors during embryogenesis. It has been shown that an intermediate level of Bone Morphogenetic Protein (BMP) signaling is essential for cement gland formation. In addition, several transcription factors have been linked to cement gland development. One of these, the homeodomain-containing protein Pitx1, can generate ectopic cement gland formation; however, the mechanisms underlying this process remain obscure. We report here, for the first time, a requirement for Pitx proteins in cement gland formation, in vivo: knockdown of both pitx1 and the closely related pitx2c inhibit endogenous cement gland formation. Pitx1 transcriptionally activates cement gland differentiation genes through both direct and indirect mechanisms, and functions as a transcriptional activator to inhibit BMP signaling. This inhibition, required for the expression of pitx genes, is partially mediated by Pitx1-dependent follistatin expression. Complete suppression of BMP signaling inhibits induction of cement gland markers by Pitx1; furthermore, we find that Pitx1 physically interacts with Smad1, an intracellular transducer of BMP signaling. We propose a model of cement gland formation in which Pitx1 limits local BMP signaling within the cement gland primordium, and recruits Smad1 to activate direct downstream targets.

Matricellular proteins in cancer: a focus on secreted Frizzled-related proteins

Tumours are complex entities, wherein cancer cells interact with myriad soluble, insoluble and cell associated factors. These microenvironmental mediators regulate tumour growth, progression and metastasis, and are produced by cancer cells and by stromal components such as fibroblast, adipocytes and immune cells. Through their ability to bind to extracellular matrix proteins, cell surface receptors and growth factors, matricellular proteins enable a dynamic reciprocity between cancer cells and their microenvironment. Hence, matricellular proteins play a critical role in tumour progression by regulating where and when cancer cells are exposed to key growth factors and regulatory proteins. Recent studies suggest that, in addition to altering Wingless (Wnt) signalling, certain members of the Secreted Frizzled Related Protein (sFRP) family are matricellular in nature. In this review, we outline the importance of matricellular proteins in cancer, and discuss how sFRPs may function to both inhibit and promote cancer progression in a context-dependent manner. By considering the matricellular functionality of sFRPs, we may better understand their apparently paradoxical roles in cancers.

ADMP controls the size of Spemann's organizer through a network of self-regulating expansion-restriction signals

BACKGROUND

The bone morphogenetic protein (BMP) signaling gradient is central for dorsoventral patterning in amphibian embryos. This gradient is established through the interaction of several BMPs and BMP antagonists and modulators, some secreted by Spemann's organizer, a cluster of cells coordinating embryonic development. Anti-dorsalizing morphogenetic protein (ADMP), a BMP-like transforming growth factor beta ligand, negatively affects the formation of the organizer, although it is robustly expressed within the organizer itself. Previously, we proposed that this apparent discrepancy may be important for the ability of ADMP to scale the BMP gradient with embryo size, but how this is achieved is unclear.

RESULTS

Here we report that ADMP acts in the establishment of the organizer via temporally and mechanistically distinct signals. At the onset of gastrulation, ADMP is required to establish normal organizer-specific gene expression domains, thus displaying a dorsal, organizer-promoting function. The organizer-restricting, BMP-like function of ADMP becomes apparent slightly later, from mid-gastrula. The organizer-promoting signal of ADMP is mediated by the activin A type I receptor, ACVR1 (also known as activin receptor-like kinase-2, ALK2). ALK2 is expressed in the organizer and is required for organizer establishment. The anti-organizer function of ADMP is mediated by ACVRL1 (ALK1), a putative ADMP receptor expressed in the lateral regions flanking the organizer that blocks expansion of the organizer. Truncated ALK1 prevents the organizer-restricting effects of ADMP overexpression, suggesting a ligand-receptor interaction. We also present a mathematical model of the regulatory network controlling the size of the organizer.

CONCLUSIONS

We show that the opposed, organizer-promoting and organizer-restricting roles of ADMP are mediated by different receptors. A self-regulating network is proposed in which ADMP functions early through ALK2 to expand its own expression domain, the organizer, and later functions through ALK1 to restrict this domain. These effects are dependent on ADMP concentration, timing, and the spatial localization of the two receptors. This self-regulating temporal switch may control the size of the organizer and the genes expressed within in response to genetic and external stimuli during gastrulation.

Roles of two types of heparan sulfate clusters in Wnt distribution and signaling in Xenopus

Wnt proteins direct embryonic patterning, but the regulatory basis of their distribution and signal reception remain unclear. Here, we show that endogenous Wnt8 protein is distributed in a graded manner in Xenopus embryo and accumulated on the cell surface in a punctate manner in association with “N-sulfo-rich heparan sulfate (HS),” not with “N-acetyl-rich HS”. These two types of HS are differentially clustered by attaching to different glypicans as core proteins. N-sulfo-rich HS is frequently internalized and associated with the signaling vesicle, known as the Frizzled/Wnt/LRP6 signalosome, in the presence of Wnt8. Conversely, N-acetyl-rich HS is rarely internalized and accumulates Frzb, a secreted Wnt antagonist. Upon interaction with Frzb, Wnt8 associates with N-acetyl-rich HS, suggesting that N-acetyl-rich HS supports Frzb-mediated antagonism by sequestering Wnt8 from N-sulfo-rich HS. Thus, these two types of HS clusters may constitute a cellular platform for the distribution and signaling of Wnt8.

Wnt proteins mediate embryonic development but how protein localization and patterning is regulated is unclear. Here, the authors show that distinct structures with different heparan sulfate modifications (‘N-sulfo-rich’ and ‘N-acetyl-rich’) regulate cellular localization and signal transduction of Wnt8 in Xenopus.

Generation of animal form by the Chordin/Tolloid/BMP gradient: 100 years after D'Arcy Thompson

The classic book "On Growth and Form" by naturalist D'Arcy Thompson was published 100 years ago. To celebrate this landmark, we present experiments in the Xenopus embryo that provide a framework for understanding how simple, quantitative transformations of a morphogen gradient might have affected evolution and morphological diversity of organisms. D'Arcy Thompson proposed that different morphologies might be generated by modifying physical parameters in an underlying system of Cartesian coordinates that pre-existed in Nature and arose during evolutionary history. Chordin is a BMP antagonist secreted by the Spemann organizer located on the dorsal side of the gastrula. Chordin generates a morphogen gradient as first proposed by mathematician Alan Turing. The rate-limiting step of this dorsal-ventral (D-V) morphogen is the degradation of Chordin by the Tolloid metalloproteinase in the ventral side. Chordin is expressed at gastrula on the dorsal side where BMP signaling is low, while at the opposite side peak levels of BMP signaling are reached. In fishes, amphibians, reptiles and birds, high BMP signaling in the ventral region induces transcription of a secreted inhibitor of Tolloid called Sizzled. By depleting Sizzled exclusively in the ventral half of the embryo we were able to expand the ventro-posterior region in an otherwise normal embryo. Conversely, ventral depletion of Tolloid, which stabilizes Chordin, decreased ventral and tail structures, phenocopying the tolloid zebrafish mutation. We explain how historical constraints recorded in the language of DNA become subject to the universal laws of physics when an ancestral reaction-diffusion morphogen gradient dictates form.

TGF-β Family Signaling in Neural and Neuronal Differentiation, Development, and Function

Signaling by the transforming growth factor β (TGF-β) family is necessary for proper neural development and function throughout life. Sequential waves of activation, inhibition, and reactivation of TGF-β family members regulate numerous elements of the nervous system from the earliest stages of embryogenesis through adulthood. This review discusses the expression, regulation, and function of TGF-β family members in the central nervous system at various developmental stages, beginning with induction and patterning of the nervous system to their importance in the adult as modulators of inflammatory response and involvement in degenerative diseases.

Xenopus as a model organism to study heterotrimeric G-protein pathway during collective cell migration of neural crest

Collective cell migration is essential in many fundamental aspects of normal development, like morphogenesis, organ formation, wound healing, and immune responses, as well as in the etiology of severe pathologies, like cancer metastasis. In spite of the huge amount of data accumulated on cell migration, such a complex process involves many molecular actors, some of which still remain to be functionally characterized. One of these signals is the heterotrimeric G-protein pathway that has been studied mainly in gastrulation movements. Recently we have reported that Ric-8A, a GEF for Gα proteins, plays an important role in neural crest migration in Xenopus development. Xenopus neural crest cells, a highly migratory embryonic cell population induced at the border of the neural plate that migrates extensively in order to differentiate in other tissues during development, have become a good model to understand the dynamics that regulate cell migration. In this review, we aim to provide sufficient evidence supporting how useful Xenopus model with its different tools, such as explants and transplants, paired with improved in vivo imaging techniques, will allow us to tackle the multiple signaling mechanisms involved in neural crest cell migration.

Scaling of pattern formations and morphogen gradients

The concentration gradient of morphogens provides positional information for an embryo and plays a pivotal role in pattern formation of tissues during the developmental processes. Morphogen-dependent pattern formations show robustness despite various perturbations. Although tissues usually grow and dynamically change their size during histogenesis, proper patterns are formed without the influence of size variations. Furthermore, even when the blastula embryo of Xenopus laevis is bisected into dorsal and ventral halves, the dorsal half of the embryo leads to proportionally patterned half-sized embryos. This robustness of pattern formation despite size variations is termed as scaling. In this review, I focused on the morphogen-dependent dorsal-ventral axis formation in Xenopus and described how morphogens form a proper gradient shape according to the embryo size.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

Pluripotent stem cells as a model for embryonic patterning: From signaling dynamics to spatial organization in a dish

In vivo studies have identified the signaling pathways and transcription factors involved in patterning the vertebrate embryo, but much remains unknown about how these are organized in space and time to orchestrate embryogenesis. Recently, embryonic stem cells have been established as a platform for studying spatial pattern formation and differentiation dynamics in the early mammalian embryo. The ease of observing and manipulating stem cell systems promises to fill gaps in our understanding of developmental dynamics and identify aspects that are uniquely human. Developmental Dynamics 245:976-990, 2016. © 2016 Wiley Periodicals, Inc.

The role of chordin fragments generated by partial tolloid cleavage in regulating BMP activity

Chordin-mediated regulation of bone morphogenetic protein (BMP) family growth factors is essential in early embryogenesis and adult homoeostasis. Chordin binds to BMPs through cysteine-rich von Willebrand factor type C (vWC) homology domains and blocks them from interacting with their cell surface receptors. These domains also self-associate and enable chordin to target related proteins to fine-tune BMP regulation. The chordin–BMP inhibitory complex is strengthened by the secreted glycoprotein twisted gastrulation (Tsg); however, inhibition is relieved by cleavage of chordin at two specific sites by tolloid family metalloproteases. As Tsg enhances this cleavage process, it serves a dual role as both promoter and inhibitor of BMP signalling. Recent developments in chordin research suggest that rather than simply being by-products, the cleavage fragments of chordin continue to play a role in BMP regulation. In particular, chordin cleavage at the C-terminus potentiates its anti-BMP activity in a type-specific manner.

Agonists and Antagonists of TGF-β Family Ligands

The discovery of the transforming growth factor β (TGF-β) family ligands and the realization that their bioactivities need to be tightly controlled temporally and spatially led to intensive research that has identified a multitude of extracellular modulators of TGF-β family ligands, uncovered their functions in developmental and pathophysiological processes, defined the mechanisms of their activities, and explored potential modulator-based therapeutic applications in treating human diseases. These studies revealed a diverse repertoire of extracellular and membrane-associated molecules that are capable of modulating TGF-β family signals via control of ligand availability, processing, ligand-receptor interaction, and receptor activation. These molecules include not only soluble ligand-binding proteins that were conventionally considered as agonists and antagonists of TGF-β family of growth factors, but also extracellular matrix (ECM) proteins and proteoglycans that can serve as "sink" and control storage and release of both the TGF-β family ligands and their regulators. This extensive network of soluble and ECM modulators helps to ensure dynamic and cell-specific control of TGF-β family signals. This article reviews our knowledge of extracellular modulation of TGF-β growth factors by diverse proteins and their molecular mechanisms to regulate TGF-β family signaling.

Mammalian tolloid proteinases: role in growth factor signalling

Tolloid proteinases are essential for tissue patterning and extracellular matrix assembly. The members of the family differ in their substrate specificity and activity, despite sharing similar domain organization. The mechanisms underlying substrate specificity and activity are complex, with variation between family members, and depend on both multimerization and substrate interaction. In addition, enhancers, such as Twisted gastrulation (Tsg), promote cleavage of tolloid substrate, chordin, to regulate growth factor signalling. Although Tsg and mammalian tolloid (mTLD) are involved in chordin cleavage, no interaction has been detected between them, suggesting Tsg induces a change in chordin to increase susceptibility to cleavage. All members of the tolloid family bind the N terminus of latent TGFβ‐binding protein‐1, providing support for their role in TGFβ signalling.

Syndecan4 coordinates Wnt/JNK and BMP signaling to regulate foregut progenitor development

Temporally and spatially dynamic Wnt and BMP signals are essential to pattern foregut endoderm progenitors that give rise to the liver, pancreas and lungs, but how these two signaling pathways are coordinated in the extracellular space is unknown. Here we identify the transmembrane heparan sulphate proteoglycan Syndecan-4 (Sdc4), as a key regulator of both non-canonical Wnt and BMP signaling in the Xenopus foregut. Foregut-specific Sdc4 depletion results in a disrupted Fibronectin (Fn1) matrix, reduced cell adhesion, and failure to maintain foregut gene expression ultimately leading to foregut organ hypoplasia. Sdc4 is required to maintain robust Wnt/JNK and BMP/Smad1 signaling in the hhex+ foregut progenitors. Pathway analysis suggests that Sdc4 functionally interacts with Fzd7 to promote Wnt/JNK signaling, which maintains foregut identity and cell adhesion. In addition, the Sdc4 ectodomain is required to support Fn1 matrix assembly, which is essential for the robust BMP signaling that promotes foregut gene expression. This work sheds lights on how the extracellular matrix can coordinate different signaling pathways during organogenesis.

Osteogenesis induced by frizzled-related protein (FRZB) is linked to the netrin-like domain

Abnormal Wnt signaling is associated with bone mass disorders. Frizzled-related protein (FRZB, also known as secreted frizzled-related protein-3 (SFRP3)) is a Wnt modulator that contains an amino-terminal cysteine-rich domain (CRD) and a carboxy-terminal Netrin-like (NTN) motif. Frzb(-/-) mice show increased cortical thickness. However, the direct effect of FRZB on osteogenic differentiation and the involvement of the structural domains herein are not fully understood. In this study, we observed that stable overexpression of Frzb in MC3T3-E1 cells increased calcium deposition and osteoblast markers compared with control. Western blot analysis showed that the increased osteogenesis was associated with reduced canonical, but increased non-canonical Wnt signaling. On the contrary, loss of Frzb induced the opposite effects on osteogenesis and Wnt signaling. To translationally validate the positive effects of FRZB on primary human cells, we treated human periosteal and human bone marrow stromal cells with conditioned medium from MC3T3-E1 cells overexpressing Frzb and observed an increase in Alizarin red staining. We further studied the effect of the domains. FrzbNTN overexpression induced similar effects on osteogenesis as full-length Frzb, whereas FrzbCRD overexpressing cells mimicked loss of Frzb experiments. The CRD is considered as the Wnt binding domain, but the NTN domain also has important effects on bone biology. FRZB and other SFRPs or their specific domains may hold surprising potential as therapeutics for bone and joint disorders considering that excess of SFRPs has effects that are not expected under physiological, endogenous expression conditions.

sFRP4-dependent Wnt signal modulation is critical for bone remodeling during postnatal development and age-related bone loss

sFRP4 is an extracellular Wnt antagonist that fine-tunes its signal activity by direct binding to Wnts. Bone fragility under oxidative stress by diabetes and aging is partly related to the suppression of the Wnt signal through upregulated sFRP4. Here, to explore the functions of sFRP4 as a balancer molecule in bone development and remodeling, we analyzed the sFRP4 knock-in mouse strain. X-gal and immunohistochemically stained signals in sFRP4-LacZ heterozygous mice were detectable in restricted areas, mostly in osteoblasts and osteoclasts, of the femoral diaphysis after neonatal and postnatal stages. Histological and μCT analyses showed increased trabecular bone mass with alteration of the Wnt signal and osteogenic activity in sFRP4 mutants; this augmented the effect of the buildup of trabecular bone during the ageing period. Our results indicate that sFRP4 plays a critical role in bone development and remodeling by regulating osteoblasts and osteoclasts, and that its functional loss prevents age-related bone loss in the trabecular bone area. These findings imply that sFRP4 functions as a key potential endogenous balancer of the Wnt signaling pathway by efficiently having direct influence on both bone formation and bone absorption during skeletal bone development and maintenance through remodeling.

Sperm is epigenetically programmed to regulate gene transcription in embryos

For a long time, it has been assumed that the only role of sperm at fertilization is to introduce the male genome into the egg. Recently, ideas have emerged that the epigenetic state of the sperm nucleus could influence transcription in the embryo. However, conflicting reports have challenged the existence of epigenetic marks on sperm genes, and there are no functional tests supporting the role of sperm epigenetic marking on embryonic gene expression. Here, we show that sperm is epigenetically programmed to regulate embryonic gene expression. By comparing the development of sperm- and spermatid-derived frog embryos, we show that the programming of sperm for successful development relates to its ability to regulate transcription of a set of developmentally important genes. During spermatid maturation into sperm, these genes lose H3K4me2/3 and retain H3K27me3 marks. Experimental removal of these epigenetic marks at fertilization de-regulates gene expression in the resulting embryos in a paternal chromatin-dependent manner. This demonstrates that epigenetic instructions delivered by the sperm at fertilization are required for correct regulation of gene expression in the future embryos. The epigenetic mechanisms of developmental programming revealed here are likely to relate to the mechanisms involved in transgenerational transmission of acquired traits. Understanding how parental experience can influence development of the progeny has broad potential for improving human health.

Genome-wide analysis of dorsal and ventral transcriptomes of the Xenopus laevis gastrula

RNA sequencing has allowed high-throughput screening of differential gene expression in many tissues and organisms. Xenopus laevis is a classical embryological and cell-free extract model system, but its genomic sequence had been lacking due to difficulties arising from allotetraploidy. There is currently much excitement surrounding the release of the completed X. laevis genome (version 9.1) by the Joint Genome Institute (JGI), which provides a platform for genome-wide studies. Here we present a deep RNA-seq dataset of transcripts expressed in dorsal and ventral lips of the early Xenopus gastrula embryo using the new genomic information, which was further annotated by blast searches against the human proteome. Overall, our findings confirm previous results from differential screenings using other methods that uncovered classical dorsal genes such as Chordin, Noggin and Cerberus, as well as ventral genes such as Sizzled, Ventx, Wnt8 and Bambi. Complete transcriptome-wide tables of mRNAs suitable for data mining are presented, which include many novel dorsal- and ventral-specific genes. RNA-seq was very quantitative and reproducible, and allowed us to define dorsal and ventral signatures useful for gene set expression analyses (GSEA). As an example of a new gene, we present here data on an organizer-specific secreted protein tyrosine kinase known as Pkdcc (protein kinase domain containing, cytoplasmic) or Vlk (vertebrate lonesome kinase). Overexpression experiments indicate that Pkdcc can act as a negative regulator of Wnt/ β-catenin signaling independently of its kinase activity. We conclude that RNA-Seq in combination with the X. laevis complete genome now available provides a powerful tool for unraveling cell-cell signaling pathways during embryonic induction.

Wnt signaling in the heart fields: Variations on a common theme

Wnt signaling plays an essential role in development and differentiation. Heart development is initiated with the induction of precardiac mesoderm requiring the tightly and spatially controlled regulation of canonical and noncanonical Wnt signaling pathways. The role of Wnt signaling in subsequent development of the heart fields is to a large extent unclear. We will discuss the role of Wnt signaling in the development of the arterial and venous pole of the heart, highlighting the dual roles of Wnt signaling with respect to its time- and dosage-dependent effects and the balance between the canonical and noncanonical signaling. Canonical signaling appears to be involved in retaining the cardiac precursors in a proliferative and precursor state, whereas noncanonical signaling promotes their differentiation. Thereafter, both canonical and noncanonical signaling regulate specific steps in differentiation of the cardiac compartments. Because heart development is a contiguous, rather than a sequential, process, analyses tend only to show a single timeframe of development. The repetitive alternating and reciprocal effect of canonical and noncanonical signaling is lost when studied in homogenates. Without the simultaneous in vivo visualization of the different Wnt signaling pathways, the mechanism of Wnt signaling in heart development remains elusive.