In vivo convergence of BMP and MAPK signaling pathways: impact of differential Smad1 phosphorylation on development and homeostasis

Structure, unique biological properties, and mechanisms of action of transforming growth factor β

Transforming growth factor β (TGF-β) is a ubiquitous molecule that is extremely conserved structurally and plays a systemic role in human organism. TGF-β is a homodimeric molecule consisting of two subunits joined through a disulphide bond. In mammals, three genes code for TGF-β1, TGF-β2, and TGF-β3 isoforms of this cytokine with a dominating expression of TGF-β1. Virtually, all normal cells contain TGF-β and its specific receptors. Considering the exceptional role of fine balance played by the TGF-β in anumber of physiological and pathological processes in human body, this cytokine may be proposed for use in medicine as an immunosuppressant in transplantology, wound healing and bone repair. TGFb itself is an important target in oncology. Strategies for blocking members of TGF-β signaling pathway as therapeutic targets have been considered. In this review, signalling mechanisms of TGF-β1 action are addressed, and their role in physiology and pathology with main focus on carcinogenesis are described.

Metabolic regulator ERRγ governs gastric stem cell differentiation into acid-secreting parietal cells

Parietal cells (PCs) produce gastric acid to kill pathogens and aid digestion. Dysregulated PC census is common in disease, yet how PCs differentiate is unclear. Here, we identify the PC progenitors arising from isthmal stem cells, using mouse models and human gastric cells, and show that they preferentially express cell-metabolism regulator and orphan nuclear receptor Estrogen-related receptor gamma (Esrrg, encoding ERRγ). Esrrg expression facilitated the tracking of stepwise molecular, cellular, and ultrastructural stages of PC differentiation. EsrrgP2ACreERT2 lineage tracing revealed that Esrrg expression commits progenitors to differentiate into mature PCs. scRNA-seq indicated the earliest Esrrg+ PC progenitors preferentially express SMAD4 and SP1 transcriptional targets and the GTPases regulating acid-secretion signal transduction. As progenitors matured, ERRγ-dependent metabolic transcripts predominated. Organoid and mouse studies validated the requirement of ERRγ for PC differentiation. Our work chronicles stem cell differentiation along a single lineage in vivo and suggests ERRγ as a therapeutic target for PC-related disorders.

Drosophila Smad2 degradation occurs independently of linker phosphorylations

TGF-β signals are important for proliferation, differentiation, and cell fate determination during embryonic development and tissue homeostasis in adults. Drosophila Activin/TGF-β signals are transduced intracellularly when its transcription factor dSmad2 (also called Smad on X or Smox) is C-terminally phosphorylated by pathway receptors. Recently, it has been shown that receptor-activated dSmad2 undergoes bulk degradation, however, the mechanism of how this occurs is unknown. Here we investigated if two putative linker phosphorylation sites are involved in dSmad2 degradation. We demonstrate that degradation of activated-dSmad2 occurs independently of threonine phosphorylation at linker sites 252 and 277. We also show that dSmad2 degradation is not carried out by cellular proteasomes.

Head organizer: Cerberus and IGF cooperate in brain induction in Xenopus embryos

Neural induction by cell-cell signaling was discovered a century ago by the organizer transplantations of Spemann and Mangold in amphibians. Spemann later found that early dorsal blastopore lips induced heads and late organizers trunk-tail structures. Identifying region-specific organizer signals has been a driving force in the progress of animal biology. Head induction in the absence of trunk is designated archencephalic differentiation. Two specific head inducers, Cerberus and Insulin-like growth factors (IGFs), that induce archencephalic brain but not trunk-tail structures have been described previously. However, whether these two signals interact with each other had not been studied to date and was the purpose of the present investigation. It was found that Cerberus, a multivalent growth factor antagonist that inhibits Nodal, BMP and Wnt signals, strongly cooperated with IGF2, a growth factor that provides a positive signal through tyrosine kinase IGF receptors that activate MAPK and other pathways. The ectopic archencephalic structures induced by the combination of Cerberus and IGF2 are of higher frequency and larger than either one alone. They contain brain, a cyclopic eye and multiple olfactory placodes, without trace of trunk structures such as notochord or somites. A dominant-negative secreted IGF receptor 1 blocked Cerberus activity, indicating that endogenous IGF signals are required for ectopic brain formation. In a sensitized embryonic system, in which embryos were depleted of β-catenin, IGF2 did not by itself induce neural tissue while in combination with Cerberus it greatly enhanced formation of circular brain structures expressing the anterior markers Otx2 and Rx2a, but not spinal cord or notochord markers. The main conclusion of this work is that IGF provides a positive signal initially uniformly expressed throughout the embryo that potentiates the effect of an organizer-specific negative signal mediated by Cerberus. The results are discussed in the context of the history of neural induction.

Myhre syndrome is caused by dominant-negative dysregulation of SMAD4 and other co-factors

Myhre syndrome is a connective tissue disorder characterized by congenital cardiovascular, craniofacial, respiratory, skeletal, and cutaneous anomalies as well as intellectual disability and progressive fibrosis. It is caused by germline variants in the transcriptional co-regulator SMAD4 that localize at two positions within the SMAD4 protein, I500 and R496, with I500 V/T/M variants more commonly identified in individuals with Myhre syndrome. Here we assess the functional impact of SMAD4-I500V variant, identified in two previously unpublished individuals with Myhre syndrome, and provide novel insights into the molecular mechanism of SMAD4-I500V dysfunction. We show that SMAD4-I500V can dimerize, but its transcriptional activity is severely compromised. Our data show that SMAD4-I500V acts dominant-negatively on SMAD4 and on receptor-regulated SMADs, affecting transcription of target genes. Furthermore, SMAD4-I500V impacts the transcription and function of crucial developmental transcription regulator, NKX2-5. Overall, our data reveal a dominant-negative model of disease for SMAD4-I500V where the function of SMAD4 encoded on the remaining allele, and of co-factors, are perturbed by the continued heterodimerization of the variant, leading to dysregulation of TGF and BMP signaling. Our findings not only provide novel insights into the mechanism of Myhre syndrome pathogenesis but also extend the current knowledge of how pathogenic variants in SMAD proteins cause disease.

Epiblast inducers capture mouse trophectoderm stem cells in vitro and pattern blastoids for implantation in utero

The embryo instructs the allocation of cell states to spatially regulate functions. In the blastocyst, patterning of trophoblast (TR) cells ensures successful implantation and placental development. Here, we defined an optimal set of molecules secreted by the epiblast (inducers) that captures in vitro stable, highly self-renewing mouse trophectoderm stem cells (TESCs) resembling the blastocyst stage. When exposed to suboptimal inducers, these stem cells fluctuate to form interconvertible subpopulations with reduced self-renewal and facilitated differentiation, resembling peri-implantation cells, known as TR stem cells (TSCs). TESCs have enhanced capacity to form blastoids that implant more efficiently in utero due to inducers maintaining not only local TR proliferation and self-renewal, but also WNT6/7B secretion that stimulates uterine decidualization. Overall, the epiblast maintains sustained growth and decidualization potential of abutting TR cells, while, as known, distancing imposed by the blastocyst cavity differentiates TR cells for uterus adhesion, thus patterning the essential functions of implantation.

Smad2/3 Activation Regulates Smad1/5/8 Signaling via a Negative Feedback Loop to Inhibit 3T3-L1 Adipogenesis

Adipose tissues (AT) expand in response to energy surplus through adipocyte hypertrophy and hyperplasia. The latter, also known as adipogenesis, is a process by which multipotent precursors differentiate to form mature adipocytes. This process is directed by developmental cues that include members of the TGF-β family. Our goal here was to elucidate, using the 3T3-L1 adipogenesis model, how TGF-β family growth factors and inhibitors regulate adipocyte development. We show that ligands of the Activin and TGF-β families, several ligand traps, and the SMAD1/5/8 signaling inhibitor LDN-193189 profoundly suppressed 3T3-L1 adipogenesis. Strikingly, anti-adipogenic traps and ligands engaged the same mechanism of action involving the simultaneous activation of SMAD2/3 and inhibition of SMAD1/5/8 signaling. This effect was rescued by the SMAD2/3 signaling inhibitor SB-431542. By contrast, although LDN-193189 also suppressed SMAD1/5/8 signaling and adipogenesis, its effect could not be rescued by SB-431542. Collectively, these findings reveal the fundamental role of SMAD1/5/8 for 3T3-L1 adipogenesis, and potentially identify a negative feedback loop that links SMAD2/3 activation with SMAD1/5/8 inhibition in adipogenic precursors.

Hydrostatic mechanical stress regulates growth and maturation of the atrioventricular valve

During valvulogenesis, cytoskeletal, secretory and transcriptional events drive endocardial cushion growth and remodeling into thin fibrous leaflets. Genetic disorders play an important role in understanding valve malformations but only account for a minority of clinical cases. Mechanical forces are ever present, but how they coordinate molecular and cellular decisions remains unclear. In this study, we used osmotic pressure to interrogate how compressive and tensile stresses influence valve growth and shape maturation. We found that compressive stress drives a growth phenotype, whereas tensile stress increases compaction. We identified a mechanically activated switch between valve growth and maturation, by which compression induces cushion growth via BMP-pSMAD1/5, while tension induces maturation via pSer-19-mediated MLC2 contractility. The compressive stress acts through BMP signaling to increase cell proliferation and decrease cell contractility, and MEK-ERK is essential for both compressive stress and BMP mediation of compaction. We further showed that the effects of osmotic stress are conserved through the condensation and elongation stages of development. Together, our results demonstrate that compressive/tensile stress regulation of BMP-pSMAD1/5 and MLC2 contractility orchestrates valve growth and remodeling.

Influence of the TGF-β Superfamily on Osteoclasts/Osteoblasts Balance in Physiological and Pathological Bone Conditions

The balance between bone forming cells (osteoblasts/osteocytes) and bone resorbing cells (osteoclasts) plays a crucial role in tissue homeostasis and bone repair. Several hormones, cytokines, and growth factors-in particular the members of the TGF-β superfamily such as the bone morphogenetic proteins-not only regulate the proliferation, differentiation, and functioning of these cells, but also coordinate the communication between them to ensure an appropriate response. Therefore, this review focuses on TGF-β superfamily and its influence on bone formation and repair, through the regulation of osteoclastogenesis, osteogenic differentiation of stem cells, and osteoblasts/osteoclasts balance. After introducing the main types of bone cells, their differentiation and cooperation during bone remodeling and fracture healing processes are discussed. Then, the TGF-β superfamily, its signaling via canonical and non-canonical pathways, as well as its regulation by Wnt/Notch or microRNAs are described and discussed. Its important role in bone homeostasis, repair, or disease is also highlighted. Finally, the clinical therapeutic uses of members of the TGF-β superfamily and their associated complications are debated.

Expression and possible roles of extracellular signal-related kinases 1-2 (ERK1-2) in mouse primordial germ cell development

In the present work, we described the expression and activity of extracellular signal-related kinases 1-2 (ERK1-2) in mouse primordial germ cells (PGCs) from 8.5–14.5 days post coitum (dpc) and investigated whether these kinases play a role in regulating the various processes of PGC development. Using immunofluorescence and immunoblotting to detect the active phosphorylated form of ERK1-2 (p-ERK1-2), we found that the kinases were present in most proliferating 8.5–10.5 dpc PGCs, low in 11.5 dpc PGCs, and progressively increasing between 12.5–14.5 dpc both in female and male PGCs. In vitro culture experiments showed that inhibiting activation of ERK1-2 with the MEK-specific inhibitor U0126 significantly reduced the growth of 8.5 dpc PGCs in culture but had little effect on 11.5–12.5 dpc PGCs. Moreover, we found that the inhibitor did not affect the adhesion of 11.5 dpc PGCs, but it significantly reduced their motility features onto a cell monolayer. Further, while the ability of female PGCs to begin meiosis was not significantly affected by U0126, their progression through meiotic prophase I was slowed down. Notably, the activity of ERK1-2 was necessary for maintaining the correct expression of oocyte-specific genes crucial for germ cells survival and the formation of primordial follicles.

A most formidable arsenal: genetic technologies for building a better mouse

The mouse is one of the most widely used model organisms for genetic study. The tools available to alter the mouse genome have developed over the preceding decades from forward screens to gene targeting in stem cells to the recent influx of CRISPR approaches. In this review, we first consider the history of mice in genetic study, the development of classic approaches to genome modification, and how such approaches have been used and improved in recent years. We then turn to the recent surge of nuclease-mediated techniques and how they are changing the field of mouse genetics. Finally, we survey common classes of alleles used in mice and discuss how they might be engineered using different methods.

Synergistic stimulating effect of 2-hydroxymelatonin and BMP-4 on osteogenic differentiation in vitro

2-Hydroxymelatonin is a metabolite produced when melatonin 2-hydroxylase catalyzes melatonin. Recent studies have reported the important roles of melatonin in bone metabolism. However, the roles of 2-hydroxymelatonin in bone metabolism remains poorly understood. The purpose of this study is to present evidence of the effect of 2-hydroxymelatonin on osteogenic differentiation in C2C12 cells. In this study, we demonstrated the synergistic stimulating effect of 2-hydroxymelatonin and bone morphogenetic protein (BMP)-4 on osteogenic differentiation in vitro, using alkaline phosphatase (ALP) staining, Alizarin red S (ARS) staining, qPCR, and luciferase reporter assay. The combination of 2-hydroxymelatonin and BMP-4 revealed a synergistic effect on osteogenic differentiation in vitro. This finding provides evidence that optimal concentrations of both 2-hydroxymelatonin and BMP-4 are beneficial for anabolic effects on bone in vitro.

Intracellular Communication among Morphogen Signaling Pathways during Vertebrate Body Plan Formation

During embryonic development in vertebrates, morphogens play an important role in cell fate determination and morphogenesis. Bone morphogenetic proteins (BMPs) belonging to the transforming growth factor-β (TGF-β) family control the dorsal-ventral (DV) patterning of embryos, whereas other morphogens such as fibroblast growth factor (FGF), Wnt family members, and retinoic acid (RA) regulate the formation of the anterior-posterior (AP) axis. Activation of morphogen signaling results in changes in the expression of target genes including transcription factors that direct cell fate along the body axes. To ensure the correct establishment of the body plan, the processes of DV and AP axis formation must be linked and coordinately regulated by a fine-tuning of morphogen signaling. In this review, we focus on the interplay of various intracellular regulatory mechanisms and discuss how communication among morphogen signaling pathways modulates body axis formation in vertebrate embryos.

Homozygous missense variant in BMPR1A resulting in BMPR signaling disruption and syndromic features

Background

The bone morphogenetic protein (BMP) pathway is known to play an imperative role in bone, cartilage, and cardiac tissue formation. Truncating, heterozygous variants, and deletions of one of the essential receptors in this pathway, Bone Morphogenetic Protein Receptor Type1A (BMPR1A), have been associated with autosomal dominant juvenile polyposis. Heterozygous deletions have also been associated with cardiac and minor skeletal anomalies. Populations with atrioventricular septal defects are enriched for rare missense BMPR1A variants.

Methods

We report on a patient with a homozygous missense variant in BMPR1A causing skeletal abnormalities, growth failure a large atrial septal defect, severe subglottic stenosis, laryngomalacia, facial dysmorphisms, and developmental delays.

Results

Functional analysis of this variant shows increased chondrocyte death for cells with the mutated receptor, increased phosphorylated R‐Smads1/5/8, and loss of Sox9 expression mediated by decreased phosphorylation of p38.

Conclusion

This homozygous missense variant in BMPR1A appears to cause a distinct clinical phenotype.

Expression of BMP-4 and Smad1 in patients with Hirschsprung disease and its clinical significance

Expression and clinical significance of bone morphogenetic protein (BMP)-4 and Smad1 in patients with Hirschsprung disease (HD) were investigated. A retrospective analysis of 96 HD patients (experimental group) admitted to Xuzhou Children's Hospital, Xuzhou Medical University from June 2015 to June 2017 was performed. According to the samples, the experimental group was divided into the stenosis group, the transition group and the expansion group. Forty-seven children with colostomy due to intestinal obstruction were selected as the control group. The expression levels of BMP-4 and Smad1 proteins were detected by immunohistochemical staining. The expression levels of BMP-4 and Smad mRNA were detected by real-time quantitative PCR (RT-qPCR), and were quantified and compared. Via immunohistochemistry, BMP-4 and Smad1 proteins were detected in the samples of different parts of HD patients and children with intestinal obstruction. The positive expression levels of BMP-4 and Smad1 proteins in the transition group were decreased compared with those in the expansion and control groups (P<0.05), and the positive expression levels of BMP-4 and Smad1 proteins in the stenosis group were decreased compared with those in the transition, expansion, and control groups (P<0.05). Also, the gene expression levels of BMP-4 and Smad1 in the transition and stenosis groups were successively decreased, and the differences were statistically significant (P<0.05). In conclusion, the expression of BMP-4 and Smad1 in the intestinal plexus of HD lesions was significantly reduced, indicating that BMP-4 and Smad1 are closely related to the occurrence of HD, and it is suspected that they have a certain influence on the intestinal development of congenital digestive tract malformations.

Transcriptomic landscape regulated by the 14 types of bone morphogenetic proteins (BMPs) in lineage commitment and differentiation of mesenchymal stem cells (MSCs)

Mesenchymal stem cells (MSCs) are ubiquitously-existing multipotent progenitors that can self-renew and differentiate into multiple lineages including osteocytes, chondrocytes, adipocytes, tenocytes and myocytes. MSCs represent one of the most commonly-used adult progenitors and serve as excellent progenitor cell models for investigating lineage-specific differentiation regulated by various cellular signaling pathways, such as bone morphogenetic proteins (BMPs). As members of TGFβ superfamily, BMPs play diverse and important roles in development and adult tissues. At least 14 BMPs have been identified in mammals. Different BMPs exert distinct but overlapping biological functions. Through a comprehensive analysis of 14 BMPs in MSCs, we demonstrated that BMP9 is one of the most potent BMPs in inducing osteogenic differentiation of MSCs. Nonetheless, a global mechanistic view of BMP signaling in regulating the proliferation and differentiation of MSCs remains to be fully elucidated. Here, we conducted a comprehensive transcriptomic profiling in the MSCs stimulated by 14 types of BMPs. Hierarchical clustering analysis classifies 14 BMPs into three subclusters: an osteo/chondrogenic/adipogenic cluster, a tenogenic cluster, and BMP3 cluster. We also demonstrate that six BMPs (e.g., BMP2, BMP3, BMP4, BMP7, BMP8, and BMP9) can induce I-Smads effectively, while BMP2, BMP3, BMP4, BMP7, and BMP11 up-regulate Smad-independent MAP kinase pathway. Furthermore, we show that many BMPs can upregulate the expression of the signal mediators of Wnt, Notch and PI3K/AKT/mTOR pathways. While the reported transcriptomic changes need to be further validated, our expression profiling represents the first-of-its-kind to interrogate a comprehensive transcriptomic landscape regulated by the 14 types of BMPs in MSCs.

MAPK and PI3K signaling: At the crossroads of neural crest development

Receptor tyrosine kinase-mediated growth factor signaling is essential for proper formation and development of the neural crest. The many ligands and receptors implicated in these processes signal through relatively few downstream pathways, frequently converging on the MAPK and PI3K pathways. Despite decades of study, there is still considerable uncertainty about where and when these signaling pathways are required and how they elicit particular responses. This review summarizes our current understanding of growth factor-induced MAPK and PI3K signaling in the neural crest.

A time-resolved multi-omic atlas of the developing mouse stomach

The mammalian stomach is structurally highly diverse and its organ functionality critically depends on a normal embryonic development. Although there have been several studies on the morphological changes during stomach development, a system-wide analysis of the underlying molecular changes is lacking. Here, we present a comprehensive, temporal proteome and transcriptome atlas of the mouse stomach at multiple developmental stages. Quantitative analysis of 12,108 gene products allows identifying three distinct phases based on changes in proteins and RNAs and the gain of stomach functions on a longitudinal time scale. The transcriptome indicates functionally important isoforms relevant to development and identifies several functionally unannotated novel splicing junction transcripts that we validate at the peptide level. Importantly, many proteins differentially expressed in stomach development are also significantly overexpressed in diffuse-type gastric cancer. Overall, our study provides a resource to understand stomach development and its connection to gastric cancer tumorigenesis.

Novel Interplay Between Smad1 and Smad3 Phosphorylation via AGE Regulates the Progression of Diabetic Nephropathy

Diabetic nephropathy (DN) is the major cause of end-stage renal failure and is associated with increased morbidity and mortality compared with other causes of renal diseases. We previously found that Smad1 plays a critical role in the development of DN both in vitro and in vivo. However, functional interaction between Smad1 and Smad3 signaling in DN is unclear. Here, we addressed the molecular interplay between Smad1 and Smad3 signaling under a diabetic condition by using Smad3-knockout diabetic mice. Extracellular matrix (ECM) protein overexpression and Smad1 activation were observed in the glomeruli of db/db mice but were suppressed in the glomeruli of Smad3^+/-; db/db mice. Smad3 activation enhanced the phosphorylation of Smad1 C-terminal domain but decreased the phosphorylation of linker domain, thus regulating Smad1 activation in advanced glycation end product-treated mesangial cells (MCs). However, forced phosphorylation of the Smad1 linker domain did not affect Smad3 activation in MCs. Phosphorylation of the Smad1 linker domain increased in Smad3^+/-; db/db mice and probucol-treated db/db mice, which was consistent with the attenuation of ECM overproduction. These results indicate that Smad3 expression and activation or probucol treatment alters Smad1 phosphorylation, thus suggesting new molecular mechanisms underlying DN development and progression.

Compound mutations in Bmpr1a and Tak1 synergize facial deformities via increased cell death

BMP signaling plays a critical role in craniofacial development. Augmentation of BMPR1A signaling through neural crest-specific expression of constitutively active Bmpr1a (caBmpr1a) results in craniofacial deformities in mice. To investigate whether deletion of Tak1 may rescue the craniofacial deformities caused by enhanced Smad-dependent signaling through caBMPR1A, we generated embryos to activate transcription of caBmpr1a transgene and ablate Tak1 in neural crest derivatives at the same time. We found that deformities of the double mutant mice showed more severe than those with each single mutation, including median facial cleft and cleft palate. We found higher levels of cell death in the medial nasal and the lateral nasal processes at E10.5 in association with higher levels of p53 in the double mutant embryos. We also found higher levels of pSmad1/5/9 in the lateral nasal processes at E10.5 in the double mutant embryos. Western analyses revealed that double mutant embryos showed similar degrees of upregulation of pSmad1/5/9 with caBmpr1a or Tak1-cKO embryos while the double mutant embryos showed higher levels of phospho-p38 than caBmpr1a or Tak1-cKO embryos at E17.5, but not at E10.5. It suggested that deletion of Tak1 aggravates the craniofacial deformities of the caBmpr1a mutants by increasing p53 and phospho-p38 at different stage of embryogenesis.

Gonadal soma controls ovarian follicle proliferation through Gsdf in zebrafish

BACKGROUND

Aberrant signaling between germ cells and somatic cells can lead to reproductive disease and depends on diffusible signals, including transforming growth factor-beta (TGFB) -family proteins. The TGFB-family protein Gsdf (gonadal soma derived factor) controls sex determination in some fish and is a candidate for mediating germ cell/soma signaling.

RESULTS

Zebrafish expressed gsdf in somatic cells of bipotential gonads and expression continued in ovarian granulosa cells and testicular Sertoli cells. Homozygous gsdf knockout mutants delayed leaving the bipotential gonad state, but then became a male or a female. Mutant females ovulated a few oocytes, then became sterile, accumulating immature follicles. Female mutants stored excess lipid and down-regulated aromatase, gata4, insulin receptor, estrogen receptor, and genes for lipid metabolism, vitellogenin, and steroid biosynthesis. Mutant females contained less estrogen and more androgen than wild-types. Mutant males were fertile. Genomic analysis suggests that Gsdf, Bmp15, and Gdf9, originated as paralogs in vertebrate genome duplication events.

CONCLUSIONS

In zebrafish, gsdf regulates ovarian follicle maturation and expression of genes for steroid biosynthesis, obesity, diabetes, and female fertility, leading to ovarian and extra-ovarian phenotypes that mimic human polycystic ovarian syndrome (PCOS), suggesting a role for a related TGFB signaling molecule in the etiology of PCOS. Developmental Dynamics 246:925-945, 2017. © 2017 Wiley Periodicals, Inc.

Ventromorphins: A New Class of Small Molecule Activators of the Canonical BMP Signaling Pathway

Here, we describe three new small-molecule activators of BMP signaling found by high throughput screening of a library of ∼600 000 small molecules. Using a cell-based luciferase assay in the BMP4-responsive human cervical carcinoma clonal cell line, C33A-2D2, we identified three compounds with similar chemotypes that each ventralize zebrafish embryos and stimulate increased expression of the BMP target genes, bmp2b and szl. Because these compounds ventralize zebrafish embryos, we have termed them "ventromorphins." As expected for a BMP pathway activator, they induce the differentiation of C2C12 myoblasts to osteoblasts. Affymetrix RNA analysis confirmed the differentiation results and showed that ventromorphins treatment elicits a genetic response similar to BMP4 treatment. Unlike isoliquiritigenin (SJ000286237), a flavone that maximally activates the pathway after 24 h of treatment, all three ventromorphins induced SMAD1/5/8 phosphorylation within 30 min of treatment and achieved peak activity within 1 h, indicating that their responses are consistent with directly activating BMP signaling.

Bone morphogenetic protein-5, a key molecule that mediates differentiation in MC3T3E1 osteoblast cell line

Bone morphogenetic protein-5 (BMP-5) is a member of the TGF receptor-β family with osteoinductive property. However, its physiological role in osteoblast differentiation is not defined. This study highlights the importance of BMP-5 in MC3T3E1 osteoblast differentiation. Pre-osteoblasts exposed to osteogenic media (ascorbic acid, 50 µg/ml and β-glycerophosphate, 10 mM) showed high protein expression of BMP-5 in cell lysates and cell culture supernatants, which peaked during early time-points of differentiation and declined with onset of mineralization. Attenuation of endogenous BMP-5 protein expression by RNA interference downregulated the expression of type I collagen (COLIA1), an early osteoblast differentiation marker but not osteocalcin, a late osteoblast differentiation marker. Further experiments to analyze the cell signaling components revealed that BMP-5 modulates COLIA1 expression via p38-Runx2 axis involving Runx2 (Ser19) phosphorylation. These effects were also observed when recombinant BMP-5 was added to pre-osteoblast cultures reinforcing the fact that BMP-5 is a modulator of COLIA1 expression. We conclude that BMP-5 has stage-specific role to play during MC3T3E1 osteoblast differentiation in part by autocrine p38/Runx2/COLIA1 signaling. © 2017 BioFactors, 43(4):558-566, 2017.

TGF-β Family Signaling in Embryonic and Somatic Stem-Cell Renewal and Differentiation

Soon after the discovery of transforming growth factor-β (TGF-β), seminal work in vertebrate and invertebrate models revealed the TGF-β family to be central regulators of tissue morphogenesis. Members of the TGF-β family direct some of the earliest cell-fate decisions in animal development, coordinate complex organogenesis, and contribute to tissue homeostasis in the adult. Here, we focus on the role of the TGF-β family in mammalian stem-cell biology and discuss its wide and varied activities both in the regulation of pluripotency and in cell-fate commitment.

Posttranslational Regulation of Smads

Transforming growth factor β (TGF-β) family signaling dictates highly complex programs of gene expression responses, which are extensively regulated at multiple levels and vary depending on the physiological context. The formation, activation, and destruction of two major functional complexes in the TGF-β signaling pathway (i.e., the TGF-β receptor complexes and the Smad complexes that act as central mediators of TGF-β signaling) are direct targets for posttranslational regulation. Dysfunction of these complexes often leads or contributes to pathogenesis in cancer and fibrosis and in cardiovascular, and autoimmune diseases. Here we discuss recent insights into the roles of posttranslational modifications in the functions of the receptor-activated Smads in the common Smad4 and inhibitory Smads, and in the control of the physiological responses to TGF-β. It is now evident that these modifications act as decisive factors in defining the intensity and versatility of TGF-β responsiveness. Thus, the characterization of posttranslational modifications of Smads not only sheds light on how TGF-β controls physiological and pathological processes but may also guide us to manipulate the TGF-β responses for therapeutic benefits.

An investigation of BMP-7 mediated alterations to BMP signalling components in human tenocyte-like cells

The incidence of tendon re-tears post-surgery is an ever present complication. It is suggested that the application of biological factors, such as bone morphogenetic protein 7 (BMP-7), can reduce complication rates by promoting tenogenic characteristics in in vitro studies. However, there remains a dearth of information in regards to the mechanisms of BMP-7 signalling in tenocytes. Using primary human tenocyte-like cells (hTLCs) from the supraspinatus tendon the BMP-7 signalling pathway was investigated: induction of the BMP associated Smad pathway and non-Smad pathways (AKT, p38, ERK1/2 and JNK); alterations in gene expression of BMP-7 associated receptors, Smad pathway components, Smad target gene (ID1) and tenogenic marker scleraxis. BMP-7 increases the expression of specific BMP associated receptors, BMPR-Ib and BMPR-II, and Smad8. Additionally, BMP-7 activates significantly Smad1/5/8 and slightly p38 pathways as indicated by an increase in phosphorylation and proven by inhibition experiments, where p-ERK1/2 and p-JNK pathways remain mainly unresponsive. Furthermore, BMP-7 increases the expression of the Smad target gene ID1, and the tendon specific transcription factor scleraxis. The study shows that tenocyte-like cells undergo primarily Smad8 and p38 signalling after BMP-7 stimulation. The up-regulation of tendon related marker genes and matrix proteins such as Smad8/9, scleraxis and collagen I might lead to positive effects of BMP-7 treatment for rotator cuff repair, without significant induction of osteogenic and chondrogenic markers.

BST2 Mediates Osteoblast Differentiation via the BMP2 Signaling Pathway in Human Alveolar-Derived Bone Marrow Stromal Cells

The molecular mechanisms controlling the differentiation of bone marrow stromal stem cells into osteoblasts remain largely unknown. In this study, we investigated whether bone marrow stromal antigen 2 (BST2) influences differentiation toward the osteoblasts lineage. BST2 mRNA expression in human alveolar-derived bone marrow stromal cells (hAD-BMSCs) increased during differentiation into osteoblasts. hAD-BMSCs differentiation into osteoblasts and the mRNA expression of the bone-specific markers alkaline phosphatase, collagen type α 1, bone sialoprotein, osteocalcin, and osterix were reduced by BST2 knockdown using siRNA. Furthermore, BST2 knockdown in hAD-BMSCs resulted in decreased RUNX2 mRNA and protein expression. We hypothesized that BST2 is involved in differentiation of into osteoblasts via the BMP2 signaling pathway. Accordingly, we evaluated the mRNA expression levels of BMP2, BMP receptors (BMPR1 and 2), and the downstream signaling molecules SMAD1, SMAD4, and p-SMAD1/5/8 in BST2 knockdown cells. BMP2 expression following the induction of differentiation was significantly lower in BST2 knockdown cells than in cells treated with a non-targeting control siRNA. Similar results were found for the knockdown of the BMP2 receptor- BMPR1A. We also identified significantly lower expression of SMAD1, SMAD4, and p-SMAD1/5/8 in the BST2 knockdown cells than control cells. Our data provide the first evidence that BST2 is involved in the osteogenic differentiation of bone marrow stromal cells via the regulation of the BMP2 signaling pathway.

Reduced response of human meniscal cells to Osteogenic Protein 1 during osteoarthritis and pro-inflammatory stimulation

OBJECTIVE

Many cell types lose responsiveness to anabolic factors during inflammation and disease. Osteogenic Protein 1 (OP1/BMP7) was evaluated for the ability to enhance extracellular matrix synthesis in healthy and OA meniscus cells. Mechanisms of cell response to OP1 were explored.

DESIGN

Meniscus and cartilage tissues from healthy tissue donors and osteoarthritis (OA) patients undergoing total knee arthroplasties were acquired. Primary cell cultures were stimulated with OP1 and/or inflammatory factors (IL1α, IL1β, or fibronectin fragments (FnF)) and cellular responses were analyzed by RT-qPCR and immunoblots. Frozen section immunohistochemistry (IHC) was conducted to assess OP1 and receptor proteins in normal and OA meniscus.

RESULTS

OP1 treatment of normal meniscus cells resulted in significant, dose-dependent increases in ACAN (aggrecan) and COL2A1, and decreased MMP13 gene transcription, while only ACAN was upregulated (P < 0.01) at the highest dose of OP1 in OA meniscus cells. OP1 induced significantly more ACAN gene transcription in normal meniscus than normal articular cartilage (P = 0.05), and no differences between normal and OA cartilage were detected. Receptor expression and kinetics of canonical signaling activation were similar between normal and OA specimens. Normal meniscus cells treated with inflammatory factors were refractory to OP1 stimulation. Smad1 phosphorylation at an inhibitory site was induced (P = 0.01 for both normal and OA meniscus) by inflammatory cytokine treatment.

CONCLUSIONS

The meniscus demonstrates resistance to OP1 stimulation in OA and in the presence of inflammatory mediators. MAPK-mediated Smad1 linker phosphorylation is a possible mediator of the loss of anabolic extracellular matrix production in the inflammatory cytokine affected meniscus.

Signaling Pathways Involved in Mammalian Sex Determination and Gonad Development

The development of any organ system requires a complex interplay of cellular signals to initiate the differentiation and development of the heterogeneous cell and tissue types required to carry out the organs' functions. In this way, an extracellular stimulus is transmitted to an intracellular target through an array of interacting protein intermediaries, ultimately enabling the target cell to elicit a response. Surprisingly, only a small number of signaling pathways are implicated throughout embryogenesis and are used over and over again. Gonadogenesis is a unique process in that 2 morphologically distinct organs, the testes and ovaries, arise from a common precursor, the bipotential genital ridge. Accordingly, most of the signaling pathways observed throughout embryogenesis also have been shown to be important for mammalian sex determination and gonad development. Here, we review the mechanisms of signal transduction within these pathways and the importance of these pathways throughout mammalian gonad development, mainly concentrating on data obtained in mouse but including other species where appropriate.

Bone morphogenetic protein 4 and retinoic acid trigger bovine VASA homolog expression in differentiating bovine induced pluripotent stem cells

Primordial germ cells (PGCs) are the earliest identifiable and completely committed progenitors of female and male gametes. They are obvious targets for genome editing because they assure the transmission of desirable or introduced traits to future generations. PGCs are established at the earliest stages of embryo development and are difficult to propagate in vitro--two characteristics that pose a problem for their practical application. One alternative method to enrich for PGCs in vitro is to differentiate them from pluripotent stem cells derived from adult tissues. Here, we establish a reporter system for germ cell identification in bovine pluripotent stem cells based on green fluorescent protein expression driven by the minimal essential promoter of the bovine Vasa homolog (BVH) gene, whose regulatory elements were identified by orthologous modelling of regulatory units. We then evaluated the potential of bovine induced pluripotent stem cell (biPSC) lines carrying the reporter construct to differentiate toward the germ cell lineage. Our results showed that biPSCs undergo differentiation as embryoid bodies, and a fraction of the differentiating cells expressed BVH. The rate of differentiation towards BVH-positive cells increased up to tenfold in the presence of bone morphogenetic protein 4 or retinoic acid. Finally, we determined that the expression of key PGC genes, such as BVH or SOX2, can be modified by pre-differentiation cell culture conditions, although this increase is not necessarily mirrored by an increase in the rate of differentiation.

Bone morphogenetic protein-7: Review of signalling and efficacy in fracture healing

Bone morphogenetic proteins (BMPs) are a group of signalling molecules that belong to the transforming growth factor-β superfamily of proteins. Initially identified for their ability to induce bone formation, recent advances in the understanding of cellular and molecular mechanisms regarding BMPs have led to the use of the growth factor to accelerate bone healing. Recent clinical trials have demonstrated that BMPs, BMP-7 in particular, may present an alternative line of treatment other than the gold standard, autogenous bone grafting, in the treatment of fracture nonunion. We performed a literature search in September 2014 of PubMed and Embase using search terms, including "bone morphogenetic proteins", "BMP-7", "non-union", "fracture healing" and "cost-effectiveness", reviewing the efficacy, safety, and cost of treatment of nonunions with BMP-7. The authors further canvassed the reference lists of selected articles and used online search tools, such as Google Scholar. BMP-7 uses both the canonical and noncanonical signalling pathways. The treatment of fracture nonunion with recombinant human BMP-7 (rhBMP-7) has a comparable efficacy with that of autogenous bone grafting with an average union rate of 87% compared with 93% for bone grafting. Furthermore, fewer complications have been described with the use of rhBMP-7 compared with traditional bone grafting. We describe the signalling pathways that BMP-7 uses to exert its effect on bone. In nonunions, rhBMP-7 has been shown to have a similar efficacy to bone grafting with fewer complications.

TGF-β signaling and its role in the regulation of hematopoietic stem cells

Transforming growth factor-betas (TGF-βs) and their family members that include bone morphogenic proteins and activins have been implicated in the regulation of proliferation, hibernation, quiescence and differentiation of hematopoietic stem cells (HSCs). Increasing evidence suggests that the superfamily of TGF-βs play an integral role in the intercellular cross-talk between the stem cells and their microenvironment as well as within the cells at an intracellular level. Active sites of hematopoiesis, such as fetal liver and bone marrow are known to have abundant presence of TGF-β indicating their importance in the maintenance and regulation of hematopoiesis. One of the striking features of TGF-β superfamily is the variety of effects they evoke, contingent on the developing history of the responding cells. In the present review, we discuss the Smad-dependent and Smad-independent TGF-β signaling pathways in order to understand and underscore their role in the regulation of HSCs.

The inhibition effects of insulin on BMP2-induced muscle heterotopic ossification

Bone morphogenetic proteins (BMPs) play an important role in regulating osteoblastic differentiation and bone formation. But the diffuse of BMPs into muscle tissues around bone injury sites often leads to heterotopic ossification, which has been regarded as one of major side-effects of BMP implementation in bone defect patients. It raises great demands for exploring effective methods that preventing BMP-induced heterotopic ossification while not interrupting the osteoinductive activity of BMPs for in situ bone defect repair. Here we found insulin, a positive regulator for bone regeneration, inhibited BMP2-induced muscle heterotopic ossification by suppressing the expression of bone transcription factor Osterix. By analyzing downstream molecules of insulin pathway, we found AKT/mTOR/GSK3 signaling was responsible for the inhibition of insulin on BMP2-induced ossification, and GSK3 inhibitor SB216763 attenuated BMP2-induced muscle heterotopic ossification. The data might shed light on developing effective clinical therapy for inhibiting muscle heterotopic ossification when BMPs were used bone defect repair.

Bone Morphogenetic Proteins: structure, biological function and therapeutic applications

Bone Morphogenetic Proteins (BMPs) are multifunctional secreted cytokines, which belong to the TGF-β superfamily. These glycoproteins act as a disulfide-linked homo- or heterodimers, being potent regulators of bone and cartilage formation and repair, cell proliferation during embryonic development and bone homeostasis in the adult. BMPs are promising molecules for tissue engineering and bone therapy. The present review discusses this family of proteins, their structure and biological function, their therapeutic applications and drawbacks, their effects on mesenchymal stem cells differentiation, and the cell signaling pathways involved in this process.

The basic helix-loop-helix transcription factor Hand1 regulates mouse development as a homodimer

Hand1 is a basic helix-loop-helix transcription factor that is essential for development of the placenta, yolk sac and heart during mouse development. While Hand1 is essential for trophoblast giant cell (TGC) differentiation, its potential heterodimer partners are not co-expressed in TGCs. To test the hypothesis that Hand1 functions as homodimer, we generated knock-in mice in which the Hand1 gene was altered to encode a tethered homodimer (TH). Some Hand1(TH/-) conceptuses in which the only form of Hand1 is Hand1(TH) are viable and fertile, indicating that homodimer Hand1 is sufficient for mouse survival. ~2/3 of Hand1(TH/-) and all Hand1(TH/TH) mice died in utero and displayed severe placental defects and variable cardial and cranial-facial abnormalities, indicating a dosage-dependent effect of Hand1(TH). Meanwhile, expression of the Hand1(TH) protein did not have negative effects on viability or fertility in all Hand1(TH/+) mice. These data imply that Hand1 homodimer plays a dominant role during development and its expression dosage is critical for survival, whereas Hand1 heterodimers can be either dispensable or play a regulatory role to modulate the activity of Hand1 homodimer in vivo.

Differential phosphorylation of Smad1 integrates BMP and neurotrophin pathways through Erk/Dusp in axon development

Sensory axon development requires concerted actions of growth factors for the precise control of axonal outgrowth and target innervation. How developing sensory neurons integrate different cues is poorly understood. We demonstrate here that Smad1 activation is required for neurotrophin-mediated sensory axon growth in vitro and in vivo. Through differential phosphorylation, Smad1 exerts transcriptional selectivity to regulate the expression and activity of Erk1 and Erk2-two key neurotrophin effectors. Specifically, bone morphogenetic proteins (BMPs) signal through carboxy-terminal phosphorylation of Smad1 (pSmad1C) to induce Erk1/2 transcription for enhanced neurotrophin responsiveness. Meanwhile, neurotrophin signaling results in linker phosphorylation of Smad1 (pSmad1L), which in turn upregulates an Erk-specific dual-specificity phosphatase, Dusp6, leading to reduced pErk1/2 and constituting a negative-feedback loop for the prevention of axon overgrowth. Together, the BMP and neurotrophin pathways form a tightly regulated signaling network with a balanced ratio of Erk1/2 and pErk1/2 to direct the precise connections between sensory neurons and peripheral targets.

Dullard/Ctdnep1 modulates WNT signalling activity for the formation of primordial germ cells in the mouse embryo

Dullard/Ctdnep1 is a member of the serine/threonine phosphatase family of the C-terminal domain of eukaryotic RNA polymerase II. Embryos lacking Dullard activity fail to form primordial germ cells (PGCs). In the mouse, the formation of PGCs is influenced by BMP4 and WNT3 activity. Although Dullard is reputed to negatively regulate BMP receptor function, in this study we found mutations in Dullard had no detectable effect on BMP4 and p-Smad activity. Furthermore Dullard mutations did not influence the dosage-dependent inductive effect of Bmp4 in PGC formation. However, Dullard may function as a positive regulator of WNT signalling. Combined loss of one copy each of Dullard and Wnt3 had a synergistic effect on the reduction of PGC numbers in the compound heterozygous embryo. In addition, loss of Dullard function was accompanied by down-regulation of WNT/β-catenin signalling activity and a reduction in the level of Dishevelled 2 (Dvl2). Therefore, Dullard may play a role in the fine-tuning of WNT signalling activity by modulating the expression of ligands/antagonists and the availability of Dvl2 protein during specification of the germ cell lineage.

Reduced bone morphogenetic protein receptor type 1A signaling in neural-crest-derived cells causes facial dysmorphism

Bone morphogenetic protein (BMP) receptor type 1A (BMPR1A) mutations are associated with facial dysmorphism, which is one of the main clinical signs in both juvenile polyposis and chromosome 10q23 deletion syndromes. Craniofacial development requires reciprocal epithelial/neural crest (NC)-derived mesenchymal interactions mediated by signaling factors, such as BMP, in both cell populations. To address the role of mesenchymal BMP signaling in craniofacial development, we generated a conditional knockdown mouse by expressing the dominant-negative Bmpr1a in NC-derived cells expressing the myelin protein zero(Mpz)-Cre transgene. At birth, 100% of the conditional mutant mice had wide-open anterior fontanelles, and 80% of them died because of cleft face and cleft palate soon after birth. The other 20% survived and developed short faces, hypertelorism and calvarial foramina. Analysis of the NC-derived craniofacial mesenchyme of mutant embryos revealed an activation of the P53 apoptosis pathway, downregulation of both c-Myc and Bcl-XL, a normal growth rate but an incomplete expansion of mesenchymal cells. These findings provide genetic evidence indicating that optimal Bmpr1a-mediated signaling is essential for NC-derived mesenchymal cell survival in both normal nasal and frontal bone development and suggest that our model is useful for studying some aspects of the molecular etiology of human craniofacial dysmorphism.

Primordial germ cells in mice

Germ cell development creates totipotency through genetic as well as epigenetic regulation of the genome function. Primordial germ cells (PGCs) are the first germ cell population established during development and are immediate precursors for both the oocytes and spermatogonia. We here summarize recent findings regarding the mechanism of PGC development in mice. We focus on the transcriptional and signaling mechanism for PGC specification, potential pluripotency, and epigenetic reprogramming in PGCs and strategies for the reconstitution of germ cell development using pluripotent stem cells in culture. Continued studies on germ cell development may lead to the generation of totipotency in vitro, which should have a profound influence on biological science as well as on medicine.

Prediction of functional phosphorylation sites by incorporating evolutionary information

Protein phosphorylation is a ubiquitous protein post-translational modification, which plays an important role in cellular signaling systems underlying various physiological and pathological processes. Current in silico methods mainly focused on the prediction of phosphorylation sites, but rare methods considered whether a phosphorylation site is functional or not. Since functional phosphorylation sites are more valuable for further experimental research and a proportion of phosphorylation sites have no direct functional effects, the prediction of functional phosphorylation sites is quite necessary for this research area. Previous studies have shown that functional phosphorylation sites are more conserved than non-functional phosphorylation sites in evolution. Thus, in our method, we developed a web server by integrating existing phosphorylation site prediction methods, as well as both absolute and relative evolutionary conservation scores to predict the most likely functional phosphorylation sites. Using our method, we predicted the most likely functional sites of the human, rat and mouse proteomes and built a database for the predicted sites. By the analysis of overall prediction results, we demonstrated that protein phosphorylation plays an important role in all the enriched KEGG pathways. By the analysis of protein-specific prediction results, we demonstrated the usefulness of our method for individual protein studies. Our method would help to characterize the most likely functional phosphorylation sites for further studies in this research area.

Regulation of TGF-β signal transduction by mono- and deubiquitylation of Smads

Polyubiquitylation leading to proteasomal degradation is a well-established mechanism for regulating TGF-β signal transduction components such as receptors and Smads. Recently, an equally important role was suggested for monoubiquitylation of both Smad4 and receptor-associated Smads that regulates their function without protein degradation. Monoubiquitylation of Smads was discovered following the identification of deubiquitylases required for TGF-β signaling, suggesting that continuous cycles of Smad mono- and deubiquitylation are required for proper TGF-β signal transduction. Here we summarize and discuss recent work on Smad mono- and deubiquitylation.

Generation of multipotent lung and airway progenitors from mouse ESCs and patient-specific cystic fibrosis iPSCs

Deriving lung progenitors from patient-specific pluripotent cells is a key step in producing differentiated lung epithelium for disease modeling and transplantation. By mimicking the signaling events that occur during mouse lung development, we generated murine lung progenitors in a series of discrete steps. Definitive endoderm derived from mouse embryonic stem cells (ESCs) was converted into foregut endoderm, then into replicating Nkx2.1+ lung endoderm, and finally into multipotent embryonic lung progenitor and airway progenitor cells. We demonstrated that precisely-timed BMP, FGF, and WNT signaling are required for NKX2.1 induction. Mouse ESC-derived Nkx2.1+ progenitor cells formed respiratory epithelium (tracheospheres) when transplanted subcutaneously into mice. We then adapted this strategy to produce disease-specific lung progenitor cells from human Cystic Fibrosis induced pluripotent stem cells (iPSCs), creating a platform for dissecting human lung disease. These disease-specific human lung progenitors formed respiratory epithelium when subcutaneously engrafted into immunodeficient mice.

Signaling in cell differentiation and morphogenesis

All the information to make a complete, fully functional living organism is encoded within the genome of the fertilized oocyte. How is this genetic code translated into the vast array of cellular behaviors that unfold during the course of embryonic development, as the zygote slowly morphs into a new organism? Studies over the last 30 years or so have shown that many of these cellular processes are driven by secreted or membrane-bound signaling molecules. Elucidating how the genetic code is translated into instructions or signals during embryogenesis, how signals are generated at the correct time and place and at the appropriate level, and finally, how these instructions are interpreted and put into action, are some of the central questions of developmental biology. Our understanding of the causes of congenital malformations and disease has improved substantially with the rapid advances in our knowledge of signaling pathways and their regulation during development. In this article, I review some of the signaling pathways that play essential roles during embryonic development. These examples show some of the mechanisms used by cells to receive and interpret developmental signals. I also discuss how signaling pathways downstream from these signals are regulated and how they induce specific cellular responses that ultimately affect cell fate and morphogenesis.

Genome-wide mechanisms of Smad binding

A dual role of transforming growth factor β (TGF-β), to both suppress and promote tumor progression and metastasis, has been well established, but its molecular basis has remained elusive. In this review, we focus on Smad proteins, which are central mediators of the signal transduction of TGF-β family members. We describe current knowledge of cell-type-specific binding patterns of Smad proteins and mechanisms of transcriptional regulation, obtained from recent studies on genome-wide binding sites of Smad molecules. We also discuss potential application of the genome-wide analyses for cancer research, which will allow clarification of the complex mechanisms occurring during cancer progression, and the identification of potential biomarkers for future cancer diagnosis, prognosis and therapy.

BMP receptor-activated Smads confer diverse functions during the development of the dorsal spinal cord

Bone Morphogenetic Proteins (BMPs) have multiple activities in the developing spinal cord: they specify the identity of the dorsal-most neuronal populations and then direct the trajectories of dorsal interneuron (dI) 1 commissural axons. How are these activities decoded by dorsal neurons to result in different cellular outcomes? Our previous studies have shown that the diverse functions of the BMPs are mediated by the canonical family of BMP receptors and then regulated by specific inhibitory (I) Smads, which block the activity of a complex of Smad second messengers. However, the extent to which this complex translates the different activities of the BMPs in the spinal cord has remained unresolved. Here, we demonstrate that the receptor-activated (R) Smads, Smad1 and Smad5 play distinct roles mediating the abilities of the BMPs to direct cell fate specification and axon outgrowth. Smad1 and Smad5 occupy spatially distinct compartments within the spinal cord, with Smad5 primarily associated with neural progenitors and Smad1 with differentiated neurons. Consistent with this expression profile, loss of function experiments in mouse embryos reveal that Smad5 is required for the acquisition of dorsal spinal neuron identities whereas Smad1 is critical for the regulation of dI1 axon outgrowth. Thus the R-Smads, like the I-Smads, have discrete roles mediating BMP-dependent cellular processes during spinal interneuron development.

Fibroblast growth factor regulates human neuroectoderm specification through ERK1/2-PARP-1 pathway

Fibroblast growth factor (FGF) signaling and PAX6 transcription are required for neuroectoderm specification of human embryonic stem cells (hESCs). In this study, we asked how FGF signaling leads to PAX6 transcription and neuroectoderm specification from hESCs. Under a chemically defined medium, FGF inhibition blocked phosphorylation of extracellular signal-regulated kinase 1/2 (ERK 1/2) with a significant reduction of PAX6-expressing neuroepithelia, indicating that FGF regulates neural induction through ERK1/2 activation. Activation of FGF-ERK1/2 pathway was necessary for the activity of poly(ADP-ribose) polymerase-1 (PARP-1), a conserved nuclear protein catalyzing polymerization of ADP-ribose units. Pharmacological inhibition and genetic ablation of PARP-1 inhibited neural induction from hESCs, suggesting that FGF-ERK1/2 signal pathway regulates neuroectoderm specification through regulating PARP-1 activity. Furthermore, FGF-ERK1/2-PARP-1 cascade regulated the expression of PAX6, a transcription determinant of human neuroectoderm. Together, we propose that FGF regulates hESC neural specification through the ERK1/2-PARP-1 signaling pathway.

TAK1 promotes BMP4/Smad1 signaling via inhibition of erk MAPK: a new link in the FGF/BMP regulatory network

FGFs and BMPs act in concert to regulate a wide range of processes in vertebrate development. In most cases, FGFs and BMPs have opposing effects, and specific developmental outcomes arise out of a balance between the two growth factors. We and others have previously demonstrated that signaling pathways activated by FGFs and BMPs interact via inhibitory crosstalk. Here we demonstrate a role for the BMP effector TGF-β Activated Kinase 1 (TAK1) in the maintenance of Smad1 activity in Xenopus embryos, via the inhibition of erk MAPK. Up- or downregulation of TAK1 levels produces an inverse alteration in the amount of activated erk MAPK. The inhibition of erk MAPK by TAK1 is mediated by p38 and a corresponding decrease in phosphorylation of MEK. TAK1 morphant embryos show a decrease in the nuclear accumulation of Smad1. Conversely, reduction of erk MAPK activity via overexpression of MAP Kinase Phosphatase1 (MKP1) leads to an increase in nuclear Smad1. Both TAK1 morphant ectoderm and ectoderm treated with FGF show a decrease in the expression of several Smad1-inducible genes. Neural-specific gene expression is inhibited in isolated ectoderm coexpressing noggin and TAK1, suggesting that TAK1 is sufficient to inhibit neural specification. Introduction of TAK1 morpholino oligonucleotide expands the expression of organizer genes, disrupts formation of the boundary between organizer and non-organizer mesoderm, and increases the spatial range of MAPK activation in response to localized FGF. Our results indicate that inhibitory interactions between FGF and BMP4 effector pathways increase the robustness of BMP signaling via a feed-forward mechanism.

Phosphatases in SMAD regulation

SMAD transcription factors are key mediators of the transforming growth factor-beta (TGFß) family of cytokines. Reversible phosphorylation of SMAD proteins plays a key role in regulating their function. Several phosphatases have been proposed to act on SMAD proteins to influence TGFß/BMP signalling. Here we provide an overview of the SMAD regulation by different protein phosphatases and review the evidence supporting each phosphatase as a candidate SMAD-phosphatase.