Tsukushi cooperates with VG1 to induce primitive streak and Hensen's node formation in the chick embryo

Proteoglycans and Diseases of Soft Tissues

Proteoglycans consist of protein cores to which at least one glycosaminoglycan chain is attached. They play important roles in the physiology and biomechanical function of tendons, ligaments, cardiovascular system, and other systems through their involvement in regulation of assembly and maintenance of extracellular matrix, and through their participation in cell proliferation together with growth factors. They can be divided into two main groups, small and large proteoglycans. The small proteoglycans are also known as small leucine-rich proteoglycans (SLRPs) which are encoded by 18 genes and are further subclassified into Classes I-V. Several members of Class I and II, such as decorin and biglycan from Class I, and Class II fibromodulin and lumican, are known to regulate collagen fibrillogenesis. Decorin limits the diameter of collagen fibrils during fibrillogenesis. The function of biglycan in fibrillogenesis is similar to that of decorin. Though biomechanical function of tendon is compromised in decorin-deficient mice, decorin can substitute for lack of biglycan in biglycan-deficient mice. New data also indicate an important role for biglycan in disorders of the cardiovascular system, including aortic valve stenosis and aortic dissection. Two members of the Class II of SLRPs, fibromodulin and lumican bind to the same site within the collagen molecule and can substitute for each other in fibromodulin- or lumican-deficient mice.Aggrecan and versican are the major representatives of the large proteoglycans. Though they are mainly found in the cartilage where they provide resilience and toughness, they are present also in tensile portions of tendons and, in slightly different biochemical form in fibrocartilage. Degradation by aggrecanase is responsible for the appearance of different forms of aggrecan and versican in different parts of the tendon where these cleaved forms play different roles. In addition, they are important components of the ventricularis of cardiac valves. Mutations in the gene for versican or in the gene for elastin (which binds to versican ) lead to severe disruptions of normal developmental of the heart at least in mice.

Novel roles of Tsukushi in signaling pathways and multiple disease processes

Tsukushi (TSK), a newly identified hepatokine, is a member of the small leucine-rich proteoglycans (SLRPs) family. TSK was originally isolated and identified in the lens of the chicken. Preliminary research on TSK has focused on its role in various physiological processes such as growth and development, wound healing, and cartilage formation. In recent years, the role of TSK in regulating cell signaling pathways, cell proliferation, and differentiation has been studied. In addition, the research has gradually expanded to the fields of glycolipid metabolism and energy balance. This article briefly reviews the role of TSK in the physiological and pathological process.

Tsukushi is essential for the formation of the posterior semicircular canal that detects gait performance

Tsukushi is a small, leucine-rich repeat proteoglycan that interacts with and regulates essential cellular signaling cascades in the chick retina and murine subventricular zone, hippocampus, dermal hair follicles, and the cochlea. However, its function in the vestibules of the inner ear remains unknown. Here, we investigated the function of Tsukushi in the vestibules and found that Tsukushi deficiency in mice resulted in defects in posterior semicircular canal formation in the vestibules, but did not lead to vestibular hair cell loss. Furthermore, Tsukushi accumulated in the non-prosensory and prosensory regions during the embryonic and postnatal developmental stages. The downregulation of Tsukushi altered the expression of key genes driving vestibule differentiation in the non-prosensory regions. Our results indicate that Tsukushi interacts with Wnt2b, bone morphogenetic protein 4, fibroblast growth factor 10, and netrin 1, thereby controlling semicircular canal formation. Therefore, Tsukushi may be an essential component of the molecular pathways regulating vestibular development.

Sequence features, structure, ligand interaction, and diseases in small leucine rich repeat proteoglycans

Small leucine rich repeat proteoglycans (SLRPs) are a group of active components of the extracellular matrix in all tissues. SLRPs bind to collagens and regulate collagen fibril growth and fibril organization. SLRPs also interact with various cytokines and extracellular compounds, which lead to various biological functions such cell adhesion and signaling, proliferation, and differentiation. Mutations in SLRP genes are associated with human diseases. Now crystal structures of five SLRPs are available. We describe some features of amino acid sequence and structures of SLRPs. We also review ligand interactions and then discuss the interaction surfaces. Furthermore, we map mutations associated with human diseases and discuss possible effects on structures by the mutations.

Two Secreted Proteoglycans, Activators of Urothelial Cell-Cell Adhesion, Negatively Contribute to Bladder Cancer Initiation and Progression

Osteomodulin (OMD) and proline/arginine-rich end leucine repeat protein (PRELP) are secreted extracellular matrix proteins belonging to the small leucine-rich proteoglycans family. We found that OMD and PRELP were specifically expressed in umbrella cells in bladder epithelia, and their expression levels were dramatically downregulated in all bladder cancers from very early stages and various epithelial cancers. Our in vitro studies including gene expression profiling using bladder cancer cell lines revealed that OMD or PRELP application suppressed the cancer progression by inhibiting TGF-β and EGF pathways, which reversed epithelial-mesenchymal transition (EMT), activated cell-cell adhesion, and inhibited various oncogenic pathways. Furthermore, the overexpression of OMD in bladder cancer cells strongly inhibited the anchorage-independent growth and tumorigenicity in mouse xenograft studies. On the other hand, we found that in the bladder epithelia, the knockout mice of OMD and/or PRELP gene caused partial EMT and a loss of tight junctions of the umbrella cells and resulted in formation of a bladder carcinoma in situ-like structure by spontaneous breakdowns of the umbrella cell layer. Furthermore, the ontological analysis of the expression profiling of an OMD knockout mouse bladder demonstrated very high similarity with those obtained from human bladder cancers. Our data indicate that OMD and PRELP are endogenous inhibitors of cancer initiation and progression by controlling EMT. OMD and/or PRELP may have potential for the treatment of bladder cancer.

Tsukushi is essential for the development of the inner ear

Tsukushi (TSK)—a small, secreted, leucine-rich-repeat proteoglycan—interacts with and regulates essential cellular signaling cascades. However, its functions in the mouse inner ear are unknown. In this study, measurement of auditory brainstem responses, fluorescence microscopy, and scanning electron microscopy revealed that TSK deficiency in mice resulted in the formation of abnormal stereocilia in the inner hair cells and hearing loss but not in the loss of these cells. TSK accumulated in nonprosensory regions during early embryonic stages and in both nonprosensory and prosensory regions in late embryonic stages. In adult mice, TSK was localized in the organ of Corti, spiral ganglion cells, and the stria vascularis. Moreover, loss of TSK caused dynamic changes in the expression of key genes that drive the differentiation of the inner hair cells in prosensory regions. Finally, our results revealed that TSK interacted with Sox2 and BMP4 to control stereocilia formation in the inner hair cells. Hence, TSK appears to be an essential component of the molecular pathways that regulate inner ear development.

Movements of chick gastrulation

In birds as in all amniotes, the site of gastrulation is a midline structure, the primitive streak. This appears as cells in the one cell-thick epiblast undergo epithelial-to-mesenchymal transition to ingress and form definitive mesoderm and endoderm. Global movements involving tens of thousands of cells in the embryonic epiblast precede gastrulation. They position the primitive streak precursors from a marginal position (equivalent to the situation in anamniotes) along the future antero-posterior axis (typical for amniotes). These epithelial movements continue in modified form during gastrulation, when they are accompanied by collective movements of different class in the forming mesoderm and endoderm. Here I discuss the nature of these collective cell movements shaping the embryo, their interplay with signaling events controlling fate specification and significance in an evolutionary perspective.

The hepatokine Tsukushi is released in response to NAFLD and impacts cholesterol homeostasis

Nonalcoholic fatty liver disease (NAFLD) prevails in obesity and is linked to several health complications including dyslipidemia and atherosclerosis. How exactly NAFLD induces atherogenic dyslipidemia to promote cardiovascular diseases is still elusive. Here, we identify Tsukushi (TSK) as a hepatokine induced in response to NAFLD. We show that both endoplasmic reticulum stress and inflammation promote the expression and release of TSK in mice. In humans, hepatic TSK expression is also associated with steatosis, and its circulating levels are markedly increased in patients suffering from acetaminophen-induced acute liver failure (ALF), a condition linked to severe hepatic inflammation. In these patients, elevated blood TSK levels were associated with decreased transplant-free survival at hospital discharge, suggesting that TSK could have a prognostic significance. Gain- and loss-of-function studies in mice revealed that TSK impacts systemic cholesterol homeostasis. TSK reduces circulating HDL cholesterol, lowers cholesterol efflux capacity, and decreases cholesterol-to-bile acid conversion in the liver. Our data identify the hepatokine TSK as a blood biomarker of liver stress that could link NAFLD to the development of atherogenic dyslipidemia and atherosclerosis.

Small leucine rich proteoglycans in host immunity and renal diseases

The small leucine rich proteoglycans (SLRPs), structurally consisting of protein cores and various glycosaminoglycan side chains, are grouped into five classes based on common structural and functional properties. Besides being an important structural component of extracellular matrix (ECM), SLRPs have been implicated in the complex network of signal transduction and host immune responses. The focus of this review is on SLRPs in host immunity. Because host immunity plays an important part in the pathogenesis of renal diseases, the role of SLRPs in this set of diseases will also be discussed.

CCN2/CTGF binds the small leucine rich proteoglycan protein Tsukushi

Extracellular molecules coordinate the multiple signaling pathways spatiotemporally to exchange information between cells during development. Understanding the regulation of these signal molecule-dependent pathways elucidates the mechanism of intercellular crosstalks. CCN2/CTGF is one of the CCN family members that binds BMP2, fibronectin, aggrecan, FGFR2 - regulating cartilage and bone formation, angiogenesis, wound repair etc. Tsukushi (TSK), which belongs to the Small Leucine-Rich Proteoglycan (SLRP) family, binds nodal/Vg1/TGF-β1, BMP4/chordin, Delta, FGF8, Frizzled4, and is involved in the early body formation, bone growth, wound healing, retinal stem cell regulation etc. These two secreted molecules are expressed in similar tissues and involved in several biological events by functioning as extracellular signaling modulators. Here, we examine the molecular interaction between CCN2 and TSK biochemically. Co-precipitation assay and Surface Plasmon Resonance measurement showed their direct binding with the Kd value 15.3 nM. Further, the Solid-phase Binding Assay indicated that TSK binds to IGFBP and CT domains of CCN2. Our data suggest that CCN2 and TSK exert their function together in the body formation.

Involvement of Tsukushi in diverse developmental processes

Tsukushi (TSK) is a small signaling molecule which takes part in different developmental processes of multiple vertebrate organisms. The diverse activity of TSK depends on its ability to bind various intermediate molecules from different major signaling pathways. Interactions of TSK with BMP, FGF, TGF-β and Wnt pathways have already been confirmed. In this review, we will introduce the latest information regarding the involvement of TSK in developmental events. We suggest a fine tuning role for TSK in multiple signaling cascades. Also, we recommend further studies on the developmental role of TSK to fully reveal its potential.

The role of Tsukushi (TSK), a small leucine-rich repeat proteoglycan, in bone growth

INTRODUCTION

Endochondral ossification is one of a key process for bone maturation. Tsukushi (TSK) is a novel member of the secreted small leucine-rich repeat proteoglycan (SLRP) family. SLRPs localize to skeletal regions and play significant roles during whole phases of bone development. Although prior evidence suggests that TSK may be involved in the regulation of bone formation, its role in skeletal development has not yet been elucidated.

METHODS

In the present study, we examined TSK's function during bone growth by comparing skeletal growth of TSK deficient (TSK-/-) mice and wild type (WT) mice. And an in vitro experiment using siRNA transfection of a chondrogenic cell line was performed.

RESULTS

TSK-/- mice exhibited decreased weight and short stature at 3 weeks of age due to decreased longitudinal bone growth coupled with low bone mass. Furthermore, an in vitro experiment using siRNA transfection into a chondrogenic cell line revealed that decreased TSK expression induced down-regulation of key chondrogenic marker gene expression and up-regulation of mid-to-late chondrogenic markers gene expression.

CONCLUSIONS

Our results reveal that TSK regulates bone elongation and bone mass by modulating growth plate chondrocyte function and consequently, overall body size.

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.

Proteoglycan form and function: A comprehensive nomenclature of proteoglycans

We provide a comprehensive classification of the proteoglycan gene families and respective protein cores. This updated nomenclature is based on three criteria: Cellular and subcellular location, overall gene/protein homology, and the utilization of specific protein modules within their respective protein cores. These three signatures were utilized to design four major classes of proteoglycans with distinct forms and functions: the intracellular, cell-surface, pericellular and extracellular proteoglycans. The proposed nomenclature encompasses forty-three distinct proteoglycan-encoding genes and many alternatively-spliced variants. The biological functions of these four proteoglycan families are critically assessed in development, cancer and angiogenesis, and in various acquired and genetic diseases where their expression is aberrant.

The biology of small leucine-rich proteoglycans in bone pathophysiology

The class of small leucine-rich proteoglycans (SLRPs) is a family of homologous proteoglycans harboring relatively small (36-42 kDa) protein cores compared with the larger cartilage and mesenchymal proteoglycans. SLRPs have been localized to most skeletal regions, with specific roles designated during all phases of bone formation, including periods relating to cell proliferation, organic matrix deposition, remodeling, and mineral deposition. This is mediated by key signaling pathways regulating the osteogenic program, including the activities of TGF-β, bone morphogenetic protein, Wnt, and NF-κB, which influence both the number of available osteogenic precursors and their subsequent development, differentiation, and function. On the other hand, SLRP depletion is correlated with degenerative diseases such as osteoporosis and ectopic bone formation. This minireview will focus on the SLRP roles in bone physiology and pathology.

Stromal expression of decorin, Semaphorin6D, SPARC, Sprouty1 and Tsukushi in developing prostate and decreased levels of decorin in prostate cancer

Background and Aim

During prostate development, mesenchymal-epithelial interactions regulate organ growth and differentiation. In adult prostate, stromal-epithelial interactions are important for tissue homeostasis and also play a significant role in prostate cancer. In this study we have identified molecules that show a mesenchymal expression pattern in the developing prostate, and one of these showed reduced expression in prostate cancer stroma.

Methodology and Principal Findings

Five candidate molecules identified by transcript profiling of developmental prostate mesenchyme were selected using a wholemount in situ hybridisation screen and studied Decorin (Dcn), Semaphorin6D (Sema6D), SPARC/Osteonectin (SPARC), Sprouty1 (Spry-1) and Tsukushi (Tsku). Expression in rat tissues was evaluated using wholemount in situ hybridisation (postnatal day (P) 0.5) and immunohistochemistry (embryonic day (E) E17.5, E19.5; P0.5; P6; 28 & adult). Four candidates (Decorin, SPARC, Spry-1, Tsukushi) were immunolocalised in human foetal prostate (weeks 14, 16, 19) and expression of Decorin was evaluated on a human prostate cancer tissue microarray. In embryonic and perinatal rats Decorin, Semaphorin6D, SPARC, Spry-1 and Tsukushi were expressed with varying distribution patterns throughout the mesenchyme at E17.5, E19.5, P0.5 and P6.5. In P28 and adult prostates there was either a decrease in the expression (Semaphorin6D) or a switch to epithelial expression of SPARC, and Spry-1, whereas Decorin and Tsukushi were specific to mesenchyme/stroma at all ages. Expression of Decorin, SPARC, Spry-1 and Tsukushi in human foetal prostates paralleled that in rat. Decorin showed mesenchymal and stromal-specific expression at all ages and was further examined in prostate cancer, where stromal expression was significantly reduced compared with non-malignant prostate.

Conclusion and Significance

We describe the spatio-temporal expression of Decorin, Semaphorin6D, SPARC, Spry-1 and Tsukushi in developing prostate and observed similar mesenchymal expression patterns in rat and human. Additionally, Decorin showed reduced expression in prostate cancer stroma compared to non-malignant prostate stroma.

Tsukushi functions as a Wnt signaling inhibitor by competing with Wnt2b for binding to transmembrane protein Frizzled4

The Wnt signaling pathway is essential for the development of diverse tissues during embryogenesis. Signal transduction is activated by the binding of Wnt proteins to the type I receptor low-density lipoprotein receptor-related protein 5/6 and the seven-pass transmembrane protein Frizzled (Fzd), which contains a Wnt-binding site in the form of a cysteine-rich domain. Known extracellular antagonists of the Wnt signaling pathway can be subdivided into two broad classes depending on whether they bind primarily to Wnt or to low-density lipoprotein receptor-related protein 5/6. We show that the secreted protein Tsukushi (TSK) functions as a Wnt signaling inhibitor by binding directly to the cysteine-rich domain of Fzd4 with an affinity of 2.3 × 10(-10) M and competing with Wnt2b. In the developing chick eye, TSK is expressed in the ciliary/iris epithelium, whereas Wnt2b is expressed in the adjacent anterior rim of the optic vesicle, where it controls the differentiation of peripheral eye structures, such as the ciliary body and iris. TSK overexpression effectively antagonizes Wnt2b signaling in chicken embryonic retinal cells both in vivo and in vitro and represses Wnt-dependent specification of peripheral eye fates. Conversely, targeted inactivation of the TSK gene in mice causes expansion of the ciliary body and up-regulation of Wnt2b and Fzd4 expression in the developing peripheral eye. Thus, we uncover a crucial role for TSK as a Wnt signaling inhibitor that regulates peripheral eye formation.

The matricellular functions of small leucine-rich proteoglycans (SLRPs)

The small leucine-rich proteoglycans (SLRPs) are biologically active components of the extracellular matrix (ECM), consisting of a protein core with leucine rich-repeat (LRR) motifs covalently linked to glycosaminoglycan (GAG) side chains. The diversity in composition resulting from the various combinations of protein cores substituted with one or more GAG chains along with their pericellular localization enables SLRPs to interact with a host of different cell surface receptors, cytokines, growth factors, and other ECM components, leading to modulation of cellular functions. SLRPs are capable of binding to: (i) different types of collagens, thereby regulating fibril assembly, organization, and degradation; (ii) Toll-like receptors (TLRs), complement C1q, and tumor necrosis factor-alpha (TNFalpha), regulating innate immunity and inflammation; (iii) epidermal growth factor receptor (EGF-R), insulin-like growth factor receptor (IGF-IR), and c-Met, influencing cellular proliferation, survival, adhesion, migration, tumor growth and metastasis as well as synthesis of other ECM components; (iv) low-density lipoprotein receptor-related protein (LRP-1) and TGF-beta, modulating cytokine activity and fibrogenesis; and (v) growth factors such as bone morphogenic protein (BMP-4) and Wnt-I-induced secreted protein-1 (WISP-1), controlling cell proliferation and differentiation. Thus, the ability of SLRPs, as ECM components, to directly or indirectly regulate cell-matrix crosstalk, resulting in the modulation of various biological processes, aptly qualifies these compounds as matricellular proteins.

Comparative analysis of gene regulation by the transcription factor PPARalpha between mouse and human

Background

Studies in mice have shown that PPARα is an important regulator of hepatic lipid metabolism and the acute phase response. However, little information is available on the role of PPARα in human liver. Here we set out to compare the function of PPARα in mouse and human hepatocytes via analysis of target gene regulation.

Methodology/Principal Findings

Primary hepatocytes from 6 human and 6 mouse donors were treated with PPARα agonist Wy14643 and gene expression profiling was performed using Affymetrix GeneChips followed by a systems biology analysis. Baseline PPARα expression was similar in human and mouse hepatocytes. Depending on species and time of exposure, Wy14643 significantly induced the expression of 362–672 genes. Surprisingly minor overlap was observed between the Wy14643-regulated genes from mouse and human, although more substantial overlap was observed at the pathway level. Xenobiotics metabolism and apolipoprotein synthesis were specifically regulated by PPARα in human hepatocytes, whereas glycolysis-gluconeogenesis was regulated specifically in mouse hepatocytes. Most of the genes commonly regulated in mouse and human were involved in lipid metabolism and many represented known PPARα targets, including CPT1A, HMGCS2, FABP1, ACSL1, and ADFP. Several genes were identified that were specifically induced by PPARα in human (MBL2, ALAS1, CYP1A1, TSKU) or mouse (Fbp2, lgals4, Cd36, Ucp2, Pxmp4). Furthermore, several putative novel PPARα targets were identified that were commonly regulated in both species, including CREB3L3, KLF10, KLF11 and MAP3K8.

Conclusions/Significance

Our results suggest that PPARα activation has a major impact on gene regulation in human hepatocytes. Importantly, the role of PPARα as master regulator of hepatic lipid metabolism is generally well-conserved between mouse and human. Overall, however, PPARα regulates a mostly divergent set of genes in mouse and human hepatocytes.

Role of CCN, a vertebrate specific gene family, in development

The CCN family of genes constitutes six members of small secreted cysteine rich proteins, which exists only in vertebrates. The major members of CCN are CCN1 (Cyr61), CCN2 (CTGF), and CCN3 (Nov). CCN4, CCN5, and CCN6 were formerly reported to be in the Wisp family, but they are now integrated into CCN due to the resemblance of their four principal modules: insulin like growth factor binding protein, von Willebrand factor type C, thrombospondin type 1, and carboxy-terminal domain. CCNs show a wide and highly variable expression pattern in adult and in embryonic tissues, but most studies have focused on their principal role in osteo/chondrogenesis and vasculo/angiogenesis from the aspect of migration, growth, and differentiation of mesenchymal cells. CCN proteins simultaneously integrate and modulate the signals of integrins, bone morphogenetic protein, vascular endothelial growth factor, Wnt, and Notch by direct binding. However, the priority in the use of the signals is different depending on the cell status. Even the equivalent counterparts show a difference in signal usage among species. It may be that the evolution of the CCN family continues to keep pace with vertebrate evolution itself.

Biological functions of the small leucine-rich proteoglycans: from genetics to signal transduction

The small leucine-rich proteoglycan (SLRP) family has significantly expanded in the past decade to now encompass five discrete classes, grouped by common structural and functional properties. Some of these gene products are not classical proteoglycans, whereas others have new and unique features. In addition to being structural proteins, SLRPs constitute a network of signal regulation: being mostly extracellular, they are upstream of multiple signaling cascades. They affect intracellular phosphorylation, a major conduit of information for cellular responses, and modulate distinct pathways, including those driven by bone morphogenetic protein/transforming growth factor beta superfamily members, receptor tyrosine kinases such as ErbB family members and the insulin-like growth factor I receptor, and Toll-like receptors. The wealth of mechanistic insights into the molecular and cellular functions of SLRPs has revealed both the sophistication of this family of regulatory proteins and the challenges that remain in uncovering the totality of their functions. This review is focused on novel biological functions of SLRPs with special emphasis on their protein cores, newly described genetic diseases, and signaling events in which SLRPs play key functions.

Tsukushi modulates Xnr2, FGF and BMP signaling: regulation of Xenopus germ layer formation

Background

Cell-cell communication is essential in tissue patterning. In early amphibian development, mesoderm is formed in the blastula-stage embryo through inductive interactions in which vegetal cells act on overlying equatorial cells. Members of the TGF-β family such as activin B, Vg1, derrière and Xenopus nodal-related proteins (Xnrs) are candidate mesoderm inducing factors, with further activity to induce endoderm of the vegetal region. TGF-β-like ligands, including BMP, are also responsible for patterning of germ layers. In addition, FGF signaling is essential for mesoderm formation whereas FGF signal inhibition has been implicated in endoderm induction. Clearly, several signaling pathways are coordinated to produce an appropriate developmental output; although intracellular crosstalk is known to integrate multiple pathways, relatively little is known about extracellular coordination.

Methodology/Principal Findings

Here, we show that Xenopus Tsukushi (X-TSK), a member of the secreted small leucine rich repeat proteoglycan (SLRP) family, is expressed in ectoderm, endoderm, and the organizer during early development. We have previously reported that X-TSK binds to and inhibits BMP signaling in cooperation with chordin. We now demonstrate two novel interactions: X-TSK binds to and inhibits signaling by FGF8b, in addition to binding to and enhancement of Xnr2 signaling. This signal integration by X-TSK at the extracellular level has an important role in germ layer formation and patterning. Vegetally localized X-TSK potentiates endoderm formation through coordination of BMP, FGF and Xnr2 signaling. In contrast, X-TSK inhibition of FGF-MAPK signaling blocks ventrolateral mesoderm formation, while BMP inhibition enhances organizer formation. These actions of X-TSK are reliant upon its expression in endoderm and dorsal mesoderm, with relative exclusion from ventrolateral mesoderm, in a pattern shaped by FGF signals.

Conclusions/Significance

Based on our observations, we propose a novel mechanism by which X-TSK refines the field of positional information by integration of multiple pathways in the extracellular space.