Smad9 is a new type of transcriptional regulator in bone morphogenetic protein signaling

Smad1, Smad5 and Smad9 (also known as Smad8) are activated by phosphorylation by bone morphogenetic protein (BMP)-bound type I receptor kinases. We examined the role of Smad1, Smad5, and Smad9 by creating constitutively active forms (Smad(DVD)). Transcriptional activity of Smad9(DVD) was lower than that of Smad1(DVD) or Smad5(DVD), even though all three Smad(DVD)s associated with Smad4 and bound to the target DNA. The linker region of Smad9 was sufficient to reduce transcriptional activity. Smad9 expression was increased by the activation of BMP signaling, similar to that of inhibitory Smads (I-Smads), and Smad9 reduced BMP activity. In contrast to I-Smads, however, Smad9 did not inhibit the type I receptor kinase and suppressed the constitutively active Smad1(DVD). Smad9 formed complexes with Smad1 and bound to DNA but suppressed the transcription of the target gene. Taken together, our findings suggest that Smad9 is a new type of transcriptional regulator in BMP signaling.

Dual roles of smad proteins in the conversion from myoblasts to osteoblastic cells by bone morphogenetic proteins

Bone morphogenetic proteins (BMPs) induce ectopic bone formation in muscle tissue in vivo and convert myoblasts such that they differentiate into osteoblastic cells in vitro. We report here that constitutively active Smad1 induced osteoblastic differentiation of C2C12 myoblasts in cooperation with Smad4 or Runx2. In floxed Smad4 mice-derived cells, Smad4 ablation partially suppressed BMP-4-induced osteoblast differentiation. In contrast, the BMP-4-induced inhibition of myogenesis was lost by Smad4 ablation and restored by Smad4 overexpression. A nuclear zinc finger protein, E4F1, was identified as a possible component of the Smad4 complex that suppresses myogenic differentiation in response to BMP signaling. In the presence of Smad4, E4F1 stimulated the expression of Ids. Taken together, these findings suggest that the Smad signaling pathway may play a dual role in the BMP-induced conversion of myoblasts to osteoblastic cells.

Inhibition of BMP2-induced bone formation by the p65 subunit of NF-κB via an interaction with Smad4

Bone morphogenic proteins (BMPs) stimulate bone formation in vivo and osteoblast differentiation in vitro via a Smad signaling pathway. Recent findings revealed that the activation of nuclear factor-κB (NF-κB) inhibits BMP-induced osteoblast differentiation. Here, we show that NF-κB inhibits BMP signaling by directly targeting the Smad pathway. A selective inhibitor of the classic NF-κB pathway, BAY11-770682, enhanced BMP2-induced ectopic bone formation in vivo. In mouse embryonic fibroblasts (MEFs) prepared from mice deficient in p65, the main subunit of NF-κB, BMP2, induced osteoblastic differentiation via the Smad complex to a greater extent than that in wild-type MEFs. In p65(-/-) MEFs, the BMP2-activated Smad complex bound much more stably to the target element than that in wild-type MEFs without affecting the phosphorylation levels of Smad1/5/8. Overexpression of p65 inhibited BMP2 activity by decreasing the DNA binding of the Smad complex. The C-terminal region, including the TA2 domain, of p65 was essential for inhibiting the BMP-Smad pathway. The C-terminal TA2 domain of p65 associated with the MH1 domain of Smad4 but not Smad1. Taken together, our results suggest that p65 inhibits BMP signaling by blocking the DNA binding of the Smad complex via an interaction with Smad4. Our study also suggests that targeting the association between p65 and Smad4 may help to promote bone regeneration in the treatment of bone diseases.

Specification of osteoblast cell fate by canonical Wnt signaling requires Bmp2

Enhanced BMP or canonical Wnt (cWnt) signaling are therapeutic strategies employed to enhance bone formation and fracture repair, but the mechanisms each pathway utilizes to specify cell fate of bone-forming osteoblasts remain poorly understood. Among all BMPs expressed in bone, we find that singular deficiency of Bmp2 blocks the ability of cWnt signaling to specify osteoblasts from limb bud or bone marrow progenitors. When exposed to cWnts, Bmp2-deficient cells fail to progress through the Runx2/Osx1 checkpoint and thus do not upregulate multiple genes controlling mineral metabolism in osteoblasts. Cells lacking Bmp2 after induction of Osx1 differentiate normally in response to cWnts, suggesting that pre-Osx1⁺ osteoprogenitors are an essential source and a target of BMP2. Our analysis furthermore reveals Grainyhead-like 3 (Grhl3) as a transcription factor in the osteoblast gene regulatory network induced during bone development and bone repair, which acts upstream of Osx1 in a BMP2-dependent manner. The Runx2/Osx1 transition therefore receives crucial regulatory inputs from BMP2 that are not compensated for by cWnt signaling, and this is mediated at least in part by induction and activation of Grhl3.

Selective inhibition of acyl-CoA:cholesterol acyltransferase 2 isozyme by flavasperone and sterigmatocystin from Aspergillus species

Five known fungal metabolites, aurasperone A, aurasperone D, averufanin, flavasperone and sterigmatocystin, were isolated from the culture broths of Aspergillus species as inhibitors of acyl-CoA:cholesterol acyltransferase (ACAT) in the cell-based assay using ACAT1- and ACAT2-expressing CHO cells. These compounds share a similar polycyclic skeleton. Among them, flavasperone and sterigmatocystin, having an angular skeleton, showed selective inhibition toward ACAT2 isozyme, while the others having a linear one had no selectivity in inhibition.

Effects of FKBP12 and type II BMP receptors on signal transduction by ALK2 activating mutations associated with genetic disorders

Various substitution mutations in ALK2, a transmembrane serine/threonine kinase receptor for bone morphogenetic proteins (BMPs), have been identified in patients with genetic disorders such as fibrodysplasia ossificans progressiva (FOP), diffuse intrinsic pontine glioma (DIPG) and heart defects. In this study, we characterized the ALK2 mutants R258G, G328V and F246Y, which were identified in patients with severe FOP, DIPG and unusual hereditary skeletal dysplasia, respectively. Both R258G and G328V were gain-of-function mutations, but F246Y was equivalent to wild-type ALK2. We also examined the effect of the suppressor FKBP12 on the signal transduction of a further 14 ALK2 mutations associated with FOP and/or DIPG. To varying extents FKBP12 over-expression suppressed the basal signaling induced by thirteen of the ALK2 mutants, whereas PF197-8L was uniquely resistant. In the PF197-8L mutant, the modelled ALK2 residue L197 induced a steric clash with the D36 residue in FKBP12 and dissociated their interaction. The co-expression of BMP type II receptors or stimulation with ligands relieved the suppression by FKBP12 by disrupting the interaction between mutant ALK2 and FKBP12. Taken together, FKBP12 binds to and suppresses mutant ALK2 proteins associated with FOP and DIPG, except for PF197-8L.

TLE3, transducing-like enhancer of split 3, suppresses osteoblast differentiation of bone marrow stromal cells

In senile osteoporosis the balance of adipogenesis and osteoblastogenesis in bone marrow stromal cells (BMSCs) is disrupted so that adipogenesis is increased with respect to osteoblastogenesis, and as a result, bone mass is decreased. While the molecular mechanisms controlling the balance between osteoblastogenesis and adipogenesis are of great interest, the exact nature of the signals regulating this process remains to be determined. In general, adipogenesis is a reciprocal relationship with osteoblastogenesis in BMSCs. Recently transducin-like enhancer of split 3 (TLE3), was reported to enhance adipogenesis in pre adipocytes. However, the effect of TLE3 on osteoblast differentiation of BMSCs is completely unknown. Here we report that TLE3 not only enhances adipocyte differentiation in BMSCs but also suppresses osteoblast differentiation. Firstly we examined the expression and localization of TLE3. We found that TLE3 is expressed in the nucleus of bone marrow stromal cells and that over-expression of TLE3 induced adipocyte differentiation and suppressed ALP activity induced by treatment with BMP2 in these cells. In contrast, adipocyte differentiation was decreased and ALP activity increased when endogenous TLE3 was knocked down by shRNA in BMSCs. To examine the mechanism by which TLE3 is able to suppress osteoblast differentiation, we focused on Runx2, a transcription factor essential for osteoblast differentiation. We found that TLE3 strongly suppressed ALP activity and OSE2-luciferase activity induced by Runx2 and this repression of Runx2 by TLE3 occurs via HDACs because treatment with TSA, a class I and II HDAC inhibitor, rescued this repression. In conclusion, we identify TLE3 as a suppressor of BMSC differentiation in osteoblast lineage cells in vitro. Our data suggest that TLE3 activity may be a key in balancing adipocyte and osteoblast differentiation in the adult bone marrow microenvironment.

Suppression of BMP-Smad signaling axis-induced osteoblastic differentiation by small C-terminal domain phosphatase 1, a Smad phosphatase

Bone morphogenetic proteins (BMPs) induce osteoblastic differentiation in myogenic cells via the phosphorylation of Smads. Two types of Smad phosphatases--small C-terminal domain phosphatase 1 (SCP1) and protein phosphatase magnesium-dependent 1A--have been shown to inhibit BMP activity. Here, we report that SCP1 inhibits the osteoblastic differentiation induced by BMP-4, a constitutively active BMP receptor, and a constitutively active form of Smad1. The phosphatase activity of SCP1 was required for this suppression, and the knockdown of SCP1 in myoblasts stimulated the osteoblastic differentiation induced by BMP signaling. In contrast to protein phosphatase magnesium-dependent 1A, SCP1 did not reduce the protein levels of Smad1 and failed to suppress expression of the Id1, Id2, and Id3 genes. Runx2-induced osteoblastic differentiation was suppressed by SCP1 without affecting the transcriptional activity or phosphorylation levels of Runx2. Taken together, these findings suggest that SCP1 may inhibit the osteoblastic differentiation induced by the BMP-Smad axis via Runx2 by suppressing downstream effector(s).

Reactivation of a developmental Bmp2 signaling center is required for therapeutic control of the murine periosteal niche

Two decades after signals controlling bone length were discovered, the endogenous ligands determining bone width remain unknown. We show that postnatal establishment of normal bone width in mice, as mediated by bone-forming activity of the periosteum, requires BMP signaling at the innermost layer of the periosteal niche. This developmental signaling center becomes quiescent during adult life. Its reactivation however, is necessary for periosteal growth, enhanced bone strength, and accelerated fracture repair in response to bone-anabolic therapies used in clinical orthopedic settings. Although many BMPs are expressed in bone, periosteal BMP signaling and bone formation require only Bmp2 in the Prx1-Cre lineage. Mechanistically, BMP2 functions downstream of Lrp5/6 pathway to activate a conserved regulatory element upstream of Sp7 via recruitment of Smad1 and Grhl3. Consistent with our findings, human variants of BMP2 and GRHL3 are associated with increased risk of fractures.

Synthesis and structure-activity relationship of pyripyropene A derivatives as potent and selective acyl-CoA:cholesterol acyltransferase 2 (ACAT2) inhibitors: part 1

In an effort to develop potent and selective inhibitors toward ACAT2, structure-activity relationship studies were carried out using derivatives based on pyripyropene A (PPPA, 1). We have successfully developed novel PPPA derivatives with a 7-O-substituted benzoyl substituent that significantly exhibit more potent ACAT2 inhibitory activity and higher ACAT2 isozyme selectivity than 1.

A New Subtype of Multiple Synostoses Syndrome Is Caused by a Mutation in GDF6 That Decreases Its Sensitivity to Noggin and Enhances Its Potency as a BMP Signal

Growth and differentiation factors (GDFs) are secreted signaling molecules within the BMP family that have critical roles in joint morphogenesis during skeletal development in mice and humans. Using genetic data obtained from a six-generation Chinese family, we identified a missense variant in GDF6 (NP_001001557.1; p.Y444N) that fully segregates with a novel autosomal dominant synostoses (SYNS) phenotype, which we designate as SYNS4. Affected individuals display bilateral wrist and ankle deformities at birth and progressive conductive deafness after age 40 years. We find that the Y444N variant affects a highly conserved residue of GDF6 in a region critical for binding of GDF6 to its receptor(s) and to the BMP antagonist NOG, and show that this mutant GDF6 is a more potent stimulator of the canonical BMP signaling pathway compared with wild-type GDF6. Further, we determine that the enhanced BMP activity exhibited by mutant GDF6 is attributable to resistance to NOG-mediated antagonism. Collectively, our findings indicate that increased BMP signaling owing to a GDF6 gain-of-function mutation is responsible for loss of joint formation and profound functional impairment in patients with SYNS4. More broadly, our study highlights the delicate balance of BMP signaling required for proper joint morphogenesis and reinforces the critical role of BMP signaling in skeletal development.

Mutant activin-like kinase 2 in fibrodysplasia ossificans progressiva are activated via T203 by BMP type II receptors

Fibrodysplasia ossificans progressiva (FOP) is a genetic disorder characterized by progressive heterotopic ossification in soft tissues, such as the skeletal muscles. FOP has been shown to be caused by gain-of-function mutations in activin receptor-like kinase (ALK)-2, which is a type I receptor for bone morphogenetic proteins (BMPs). In the present study, we examined the molecular mechanisms that underlie the activation of intracellular signaling by mutant ALK2. Mutant ALK2 from FOP patients enhanced the activation of intracellular signaling by type II BMP receptors, such as BMPR-II and activin receptor, type II B, whereas that from heart disease patients did not. This enhancement was dependent on the kinase activity of the type II receptors. Substitution mutations at all nine serine and threonine residues in the ALK2 glycine- and serine-rich domain simultaneously inhibited this enhancement by the type II receptors. Of the nine serine and threonine residues in ALK2, T203 was found to be critical for the enhancement by type II receptors. The T203 residue was conserved in all of the BMP type I receptors, and these residues were essential for intracellular signal transduction in response to ligand stimulation. The phosphorylation levels of the mutant ALK2 related to FOP were higher than those of wild-type ALK2 and were further increased by the presence of type II receptors. The phosphorylation levels of ALK2 were greatly reduced in mutants carrying a mutation at T203, even in the presence of type II receptors. These findings suggest that the mutant ALK2 related to FOP is enhanced by BMP type II receptors via the T203-regulated phosphorylation of ALK2.

Synthesis and structure-activity relationship of pyripyropene A derivatives as potent and selective acyl-CoA:cholesterol acyltransferase 2 (ACAT2) inhibitors: part 3

In an effort to develop potent and selective inhibitors toward ACAT2, structure-activity relationship studies were carried out using derivatives based on pyripyropene A (PPPA, 1). In particular, we investigated the possibility of introducing appropriate 1,11-O-benzylidene and 7-O-substituted benzoyl moieties into PPPA (1). The new o-substituted benzylidene derivatives showed higher selectivity for ACAT2 than PPPA (1). Among them, 1,11-O-o-methylbenzylidene-7-O-p-cyanobenzoyl PPPA derivative 7q and 1,11-O-o,o-dimethylbenzylidene-7-O-p-cyanobenzoyl PPPA derivative 7z proved to be potent ACAT2 inhibitors with unprecedented high isozyme selectivity.

Expression of TLE3 by bone marrow stromal cells is regulated by canonical Wnt signaling

Transducing-like enhancer of split 3 (TLE3), one of the Groucho/TLE family members, targets Runx2 transcription and suppresses osteoblast differentiation in bone marrow stromal cells (BMSCs). Here, we identify Wnt responsive elements of the TLE3 promoter region through comparative genomic and functional analyses and show that expression of TLE3 is increased by Wnt signaling, which is important for osteoblast differentiation. We also demonstrated that TLE3 is able to suppress canonical Wnt signaling in BMSCs. Taken together, our data suggest that induction of TLE3 by Wnt signaling is part of a negative feedback loop active during osteoblast differentiation.

Selectivity of pyripyropene derivatives in inhibition toward acyl-CoA:cholesterol acyltransferase 2 isozyme

Selectivity of 96 semisynthetic derivatives prepared from fungal pyripyropene A, originally isolated as a potent inhibitor of acyl-CoA:cholesterol acyltransferase (ACAT), toward ACAT1 and ACAT2 isozymes was investigated in the cell-based assay using ACAT1- and ACAT2-expressing CHO cells. Eighteen derivatives including PR-71 (7-O-isocaproyl derivative) showed much more potent ACAT2 inhibition (IC50: 6.0 to 62 nM) than pyripyropene A (IC50: 70 nM). Among them, however, natural pyripyropene A showed the highest selectivity toward ACAT2 with a selectivity index (SI) of >1000, followed by PR-71 (SI, 667).

Identification of a novel bone morphogenetic protein (BMP)-inducible transcript, BMP-inducible transcript-1, by utilizing the conserved BMP-responsive elements in the Id genes

Bone morphogenetic proteins (BMPs) inhibit myogenesis and induce osteoblastic differentiation in myoblasts. They also induce the transcription of several common genes, such as Id1, Id2 and Id3, in various cell types. We have reported that a GC-rich element in the Id1 gene functions as a BMP-responsive element (BRE) that is regulated by Smads. In this study, we analyzed and identified BREs in the 5'-flanking regions of the mouse Id2 and Id3 genes. The core GGCGCC sequence was conserved among the BREs in the Id1, Id2 and Id3 genes and was essential for the response to BMP signaling via Smads. We found a novel BRE on mouse chromosome 13 at position 47,723,740-47,723,768 by searching for conserved sequences containing the Id1 BRE. This potential BRE was found in the 5'-flanking region of a novel gene that produces a non-coding transcript, termed BMP-inducible transcript-1 (BIT-1), and this element regulated the expression of this gene in response to BMP signaling. We found that BIT-1 is expressed in BMP target tissues such as the testis, brain, kidney and cartilage. These findings suggest that the transcriptional induction of the Ids, BIT-1 and additional novel genes containing the conserved BRE sequence may play an important role in the regulation of the differentiation and/or function of target cells in response to BMPs.

Identification and functional analysis of Zranb2 as a novel Smad-binding protein that suppresses BMP signaling

Smads 1/5/8 transduce the major intracellular signaling of bone morphogenetic proteins (BMPs). In the present study, we analyzed Smad1-binding proteins in HEK293T cells using a proteomic technique and identified the protein, zinc-finger, RAN-binding domain-containing protein 2 (ZRANB2). Zranb2 interacted strongly with Smad1, Smad5, and Smad8 and weakly with Smad4. The overexpression of Zranb2 inhibited BMP activities in C2C12 myoblasts in vitro, and the injection of Zranb2 mRNA into zebrafish embryos induced weak dorsalization. Deletion analyses of Zranb2 indicated that the serine/arginine-rich (SR) domain and the glutamine-rich domain were required for the inhibition of BMP activity and the interaction with Smad1, respectively. Zranb2 was found to be localized in the nucleus; however, the SR domain-deleted mutant localized to the cytoplasm. The knockdown of endogenous Zranb2 in C2C12 cells enhanced BMP activity. Zranb2 suppressed Smad transcriptional activity without affecting Smad phosphorylation, nuclear localization, or DNA binding. Taken together, these findings suggested that Zranb2 is a novel BMP suppressor that forms a complex with Smads in the nucleus.

Scopranones with Two Atypical Scooplike Moieties Produced by Streptomyces sp. BYK-11038

Three new compounds, designated scopranones A-C, were isolated from the culture broth of a soil isolate, Streptomyces sp. BYK-11038, and shown to be inhibitors of bone morphogenetic protein (BMP) induced alkaline phosphatase activity in a BMP receptor mutant cell line. The structures were elucidated using NMR and other spectral data. The scopranones have an unusual structure with two atypical scooplike moieties linked at the tails to form part of a unique 3-furanone ring.