Conditional deletion of gremlin causes a transient increase in bone formation and bone mass

Aberrant epithelial GREM1 expression initiates colonic tumorigenesis from cells outside the stem cell niche

Hereditary mixed polyposis syndrome (HMPS) is characterized by the development of mixed-morphology colorectal tumors and is caused by a 40-kb genetic duplication that results in aberrant epithelial expression of the gene encoding mesenchymal bone morphogenetic protein antagonist, GREM1. Here we use HMPS tissue and a mouse model of the disease to show that epithelial GREM1 disrupts homeostatic intestinal morphogen gradients, altering cell fate that is normally determined by position along the vertical epithelial axis. This promotes the persistence and/or reacquisition of stem cell properties in Lgr5-negative progenitor cells that have exited the stem cell niche. These cells form ectopic crypts, proliferate, accumulate somatic mutations and can initiate intestinal neoplasia, indicating that the crypt base stem cell is not the sole cell of origin of colorectal cancer. Furthermore, we show that epithelial expression of GREM1 also occurs in traditional serrated adenomas, sporadic premalignant lesions with a hitherto unknown pathogenesis, and these lesions can be considered the sporadic equivalents of HMPS polyps.

Gremlin, a bone morphogenetic protein antagonist, is a crucial angiogenic factor in pituitary adenoma

Gremlin is an antagonist of bone morphogenetic protein (BMP) and a major driving force in skeletal modeling in the fetal stage. Several recent reports have shown that Gremlin is also involved in angiogenesis of lung cancer and diabetic retinopathy. The purpose of this study was to investigate the role of Gremlin in tumor angiogenesis in pituitary adenoma. Double fluorescence immunohistochemistry of Gremlin and CD34 was performed in pituitary adenoma tissues obtained during transsphenoidal surgery in 45 cases (7 PRLoma, 17 GHoma, 2 ACTHoma, and 2 TSHoma). Gremlin and microvascular density (MVD) were detected by double-immunofluorescence microscopy in CD34-positive vessels from tissue microarray analysis of 60 cases of pituitary adenomas (6 PRLoma, 23 GHoma, 22 NFoma, 5 ACTHoma, and 4 TSHoma). In tissue microarray analysis, MVD was significantly correlated with an increased Gremlin level (linear regression: P < 0.005,  r (2) = 0.4958). In contrast, Gremlin expression showed no correlation with tumor subtype or Knosp score. The high level of expression of Gremlin in pituitary adenoma tissue with many CD34-positive vessels and the strong coherence of these regions indicate that Gremlin is associated with angiogenesis in pituitary adenoma cells.

High-throughput screening of mouse gene knockouts identifies established and novel skeletal phenotypes

Screening gene function in vivo is a powerful approach to discover novel drug targets. We present high-throughput screening (HTS) data for 3 762 distinct global gene knockout (KO) mouse lines with viable adult homozygous mice generated using either gene-trap or homologous recombination technologies. Bone mass was determined from DEXA scans of male and female mice at 14 weeks of age and by microCT analyses of bones from male mice at 16 weeks of age. Wild-type (WT) cagemates/littermates were examined for each gene KO. Lethality was observed in an additional 850 KO lines. Since primary HTS are susceptible to false positive findings, additional cohorts of mice from KO lines with intriguing HTS bone data were examined. Aging, ovariectomy, histomorphometry and bone strength studies were performed and possible non-skeletal phenotypes were explored. Together, these screens identified multiple genes affecting bone mass: 23 previously reported genes (Calcr, Cebpb, Crtap, Dcstamp, Dkk1, Duoxa2, Enpp1, Fgf23, Kiss1/Kiss1r, Kl (Klotho), Lrp5, Mstn, Neo1, Npr2, Ostm1, Postn, Sfrp4, Slc30a5, Slc39a13, Sost, Sumf1, Src, Wnt10b), five novel genes extensively characterized (Cldn18, Fam20c, Lrrk1, Sgpl1, Wnt16), five novel genes with preliminary characterization (Agpat2, Rassf5, Slc10a7, Slc26a7, Slc30a10) and three novel undisclosed genes coding for potential osteoporosis drug targets.

Roles of bone morphogenetic protein signaling in osteosarcoma

PURPOSE

Since the original extraction of bone morphogenetic proteins (BMPs) from bovine bone, research interest and clinical use has increased exponentially. With this, a concomitant analysis of BMP expression in bone tumours has been performed. BMP ligands, receptors, and signaling activity have been observed in diverse benign and malignant bone tumours. However, the reported expression, function, and importance of BMPs in bone tumours, and specifically osteosarcomas, have been far from uniform. This review highlights recent advances in understanding the role of BMP signaling in osteosarcoma biology, focusing on the sometimes divergent findings by various researchers and the challenges inherent in the study of osteosarcoma.

METHODS

We performed a literature review of all studies examining BMP signaling in osteosarcoma.

RESULTS

Overall, multiple BMP ligands and receptors are expressed in most osteosarcoma cell lines and subtypes, although BMP signaling may be reduced in comparison with benign bone-forming tumours. Studies suggest that osteosarcomas with different lineages of differentiation may have differential expression of BMP ligands. Although significant disagreement in the literature exists, the presence of BMP signaling in osteosarcoma may impart a worse prognosis. On the cellular level, BMP signaling appears to mediate promigratory effects in osteosarcoma and chondrosarcoma cell types, possibly via interaction and activation of Integrin β1.

CONCLUSIONS

BMP signaling has clear biologic importance in osteosarcoma, although it is not yet fully understood. Future questions for study include assessing the utility of BMP signaling in prognostication of osteosarcoma and the potential modulation of BMP signaling for inhibition of osteosarcomagenesis, growth and invasion.

Follistatin interacts with Noggin in the development of the axial skeleton

When compared to single mutants for Follistatin or Noggin, we find that double mutants display a dramatic further reduction in trunk cartilage formation, particularly in the vertebral bodies and proximal ribs. Consistent with these observations, expression of the early sclerotome markers Pax1 and Uncx is diminished in Noggin;Follistatin compound mutants. In contrast, Sim1 expression expands medially in double mutants. As the onset of Follistatin expression coincides with sclerotome specification, we argue that the effect of Follistatin occurs after sclerotome induction. We hypothesize that Follistatin aids in maintaining proper somite size, and consequently sclerotome progenitor numbers, by preventing paraxial mesoderm from adopting an intermediate/lateral plate mesodermal fate in the Noggin-deficient state.

A polymorphic enhancer near GREM1 influences bowel cancer risk through differential CDX2 and TCF7L2 binding

A rare germline duplication upstream of the bone morphogenetic protein antagonist GREM1 causes a Mendelian-dominant predisposition to colorectal cancer (CRC). The underlying disease mechanism is strong, ectopic GREM1 overexpression in the intestinal epithelium. Here, we confirm that a common GREM1 polymorphism, rs16969681, is also associated with CRC susceptibility, conferring ∼20% differential risk in the general population. We hypothesized the underlying cause to be moderate differences in GREM1 expression. We showed that rs16969681 lies in a region of active chromatin with allele- and tissue-specific enhancer activity. The CRC high-risk allele was associated with stronger gene expression, and higher Grem1 mRNA levels increased the intestinal tumor burden in Apc(Min) mice. The intestine-specific transcription factor CDX2 and Wnt effector TCF7L2 bound near rs16969681, with significantly higher affinity for the risk allele, and CDX2 overexpression in CDX2/GREM1-negative cells caused re-expression of GREM1. rs16969681 influences CRC risk through effects on Wnt-driven GREM1 expression in colorectal tumors.

Deconvoluting the ontogeny of hematopoietic stem cells

Two different models describe the development of definitive hematopoiesis and hematopoietic stem cells (HSCs). In one of these, the visceral yolk sac serves as a starting point of relatively lengthy developmental process culminating in the fetal liver hematopoiesis. In another, the origin of adult hematopoiesis is split between the yolk sac and the dorsal aorta, which has a peculiar capacity to generate definitive HSCs. Despite a large amount of experimental data consistent with the latter view, it becomes increasingly unsustainable in the light of recent cell tracing studies. Moreover, analysis of the published studies supporting the aorta-centered version uncovers significant caveats in standard experimental approach and argumentation. As a result, the theory cannot offer feasible cellular mechanisms of the HSC emergence. This review summarizes key efforts to discern the developmental pathway of the adult-type HSCs and attempts to put forward a hypothesis on the inflammatory mechanisms of hematopoietic ontogenesis.

MicroRNA miR-378 promotes BMP2-induced osteogenic differentiation of mesenchymal progenitor cells

Background

MicroRNAs (miRNAs) are a family of small, non-coding single-stranded RNA molecules involved in post-transcriptional regulation of gene expression. As such, they are believed to play a role in regulating the step-wise changes in gene expression patterns that occur during cell fate specification of multipotent stem cells. Here, we have studied whether terminal differentiation of C2C12 myoblasts is indeed controlled by lineage-specific changes in miRNA expression.

Results

Using a previously generated RNA polymerase II (Pol-II) ChIP-on-chip dataset, we show differential Pol-II occupancy at the promoter regions of six miRNAs during C2C12 myogenic versus BMP2-induced osteogenic differentiation. Overexpression of one of these miRNAs, miR-378, enhances Alp activity, calcium deposition and mRNA expression of osteogenic marker genes in the presence of BMP2.

Conclusions

Our results demonstrate a previously unknown role for miR-378 in promoting BMP2-induced osteogenic differentiation.

Expression of Potential Regulatory Genes in Abdominal Adipose Tissue of Broiler Chickens during Early Development

The identities of genes that underlie population variation in adipose tissue development in farm animals are poorly understood. Previous studies in our laboratory have suggested that increased fat tissue involves the expression modulation of an array of genes in broiler chickens. Of special interest are eight genes, FGFR3, EPHB2, IGFBP2, GREM1, TNC, COL3A1, ACBD7, and SCD. To understand their expression regulation and response to dietary manipulation, we investigated their mRNA levels after dietary manipulation during early development. Chickens were fed either a recommended standard or a high caloric diet from hatch to eight weeks of age (WOA). The high caloric diet markedly affected bodyweight of the broiler birds. mRNA levels of the eight genes in the abdominal adipose tissue were assayed at 2, 4, 6, and 8 WOA using RT-qPCR. Results indicate that (1) FGFR3 mRNA level was affected significantly by diet, age, and diet:age interaction; (2) COL3A mRNA level was repressed by high caloric diet; (3) mRNA levels of EPHB2, ACBD7, and SCD were affected by age; (4) mRNA level of TNC was modulated by age:diet interaction; (5) changes in GREM1 and IGFBP2 mRNA levels were not statistically different.

Ribonucleic acid interference induced gene knockdown

Despite major advances in periodontal regeneration over the past three decades, complete regeneration of the lost periodontium on a regular and predictable basis in humans has still remained elusive. The identification of stem cells in the periodontal ligament together with the growing concept of tissue engineering has opened new vistas in periodontal regenerative medicine. In this regard, ribonucleic acid interference (RNAi) opens a new gate way for a novel RNA based approach in periodontal management. This paper aims to summarize the current opinion on the mechanisms underlying RNAi, in vitro and in vivo existing applications in the dental research, which could lead to their future use in periodontal regeneration.

GREM1, FRZB and DKK1 mRNA levels correlate with osteoarthritis and are regulated by osteoarthritis-associated factors

Introduction

Osteoarthritis is, at least in a subset of patients, associated with hypertrophic differentiation of articular chondrocytes. Recently, we identified the bone morphogenetic protein (BMP) and wingless-type MMTV integration site (WNT) signaling antagonists Gremlin 1 (GREM1), frizzled-related protein (FRZB) and dickkopf 1 homolog (Xenopus laevis) (DKK1) as articular cartilage’s natural brakes of hypertrophic differentiation. In this study, we investigated whether factors implicated in osteoarthritis or regulation of chondrocyte hypertrophy influence GREM1, FRZB and DKK1 expression levels.

Methods

GREM1, FRZB and DKK1 mRNA levels were studied in articular cartilage from healthy preadolescents and healthy adults as well as in preserved and degrading osteoarthritic cartilage from the same osteoarthritic joint by quantitative PCR. Subsequently, we exposed human articular chondrocytes to WNT, BMP, IL-1β, Indian hedgehog, parathyroid hormone-related peptide, mechanical loading, different medium tonicities or distinct oxygen levels and investigated GREM1, FRZB and DKK1 expression levels using a time-course analysis.

Results

GREM1, FRZB and DKK1 mRNA expression were strongly decreased in osteoarthritis. Moreover, this downregulation is stronger in degrading cartilage compared with macroscopically preserved cartilage from the same osteoarthritic joint. WNT, BMP, IL-1β signaling and mechanical loading regulated GREM1, FRZB and DKK1 mRNA levels. Indian hedgehog, parathyroid hormone-related peptide and tonicity influenced the mRNA levels of at least one antagonist, while oxygen levels did not demonstrate any statistically significant effect. Interestingly, BMP and WNT signaling upregulated the expression of each other’s antagonists.

Conclusions

Together, the current study demonstrates an inverse correlation between osteoarthritis and GREM1, FRZB and DKK1 gene expression in cartilage and provides insight into the underlying transcriptional regulation. Furthermore, we show that BMP and WNT signaling are linked in a negative feedback loop, which might prove essential in articular cartilage homeostasis by balancing BMP and WNT activity.

Review of Signaling Pathways Governing MSC Osteogenic and Adipogenic Differentiation

Mesenchymal stem cells (MSC) are multipotent cells, functioning as precursors to a variety of cell types including adipocytes, osteoblasts, and chondrocytes. Between osteogenic and adipogenic lineage commitment and differentiation, a theoretical inverse relationship exists, such that differentiation towards an osteoblast phenotype occurs at the expense of an adipocytic phenotype. This balance is regulated by numerous, intersecting signaling pathways that converge on the regulation of two main transcription factors: peroxisome proliferator-activated receptor- γ (PPAR γ ) and Runt-related transcription factor 2 (Runx2). These two transcription factors, PPAR γ and Runx2, are generally regarded as the master regulators of adipogenesis and osteogenesis. This review will summarize signaling pathways that govern MSC fate towards osteogenic or adipocytic differentiation. A number of signaling pathways follow the inverse balance between osteogenic and adipogenic differentiation and are generally proosteogenic/antiadipogenic stimuli. These include β -catenin dependent Wnt signaling, Hedgehog signaling, and NELL-1 signaling. However, other signaling pathways exhibit more context-dependent effects on adipogenic and osteogenic differentiation. These include bone morphogenic protein (BMP) signaling and insulin growth factor (IGF) signaling, which display both proosteogenic and proadipogenic effects. In summary, understanding those factors that govern osteogenic versus adipogenic MSC differentiation has significant implications in diverse areas of human health, from obesity to osteoporosis to regenerative medicine.

Recent developments in bone anabolic therapy for osteoporosis

Osteoporosis is a disorder in which there is a net bone loss and microarchitectural deterioration with an increased risk of bone fracture because of uncoupling of bone formation and bone resorption. The treatment of osteoporosis aims to inhibit bone resorption by osteoclasts and/or promote bone formation by osteoblasts. However, most of the current approaches for treating osteoporosis focus on inhibiting bone resorption. As the only US FDA-approved anabolic agent, the recombinant human parathyroid hormone is recommended for consecutive 2-year period treatment in a clinical setting. Therefore, it is highly desirable to identify novel bone anabolic agents or approaches for osteoporosis treatment. In this review, the authors introduce a new bone anabolic therapy by means of RNAi strategy. Specifically, the authors also discuss the current status and perspectives for RNAi as a novel anabolic approach in the treatment of osteoporosis.

BMP2 differentially regulates the expression of Gremlin1 and Gremlin2, the negative regulators of BMP function, during osteoblast differentiation

Bone morphogenetic proteins (BMPs) control the expressions of many genes involved in bone formation. On the basis of our hypothesis that BMP2 stimulation-regulated gene expression plays a critical role in osteoblast differentiation, we performed genome-wide screening of messenger RNA from BMP2-treated and -untreated C2C12 cells using a DNA microarray technique. We found that the expressions of Gremlin1 and Gremlin2, which are known BMP antagonists, were bidirectionally regulated by BMP2. Gremlin1 was down-regulated by BMP2, while Gremlin2 was up-regulated in both time- and dose-dependent manners. Ablation of Gremlin1 or Gremlin2 enhanced osteoblast differentiation induced by BMP2. On the other hand, treatment with recombinant Gremlin1 inhibited BMP2-induced osteoblast differentiation. Furthermore, treatment with Smad4 siRNA and the p38 MAPK inhibitor SB203580 suppressed BMP2-induced Gremlin2 gene expression. The differential regulation of Gremlin1 and Gremlin2 gene expressions by BMP2 may explain the critical function of these genes during osteoblast differentiation.

Co-expression of BMPs and BMP-inhibitors in human fractures and non-unions

Bone morphogenetic proteins (BMPs) are increasingly being used clinically to enhance fracture repair and healing of non-unions. However, the potential efficacy of supraphysiological dosing for clinical results warrants further clarification of the BMP signaling pathway in human fracture healing. As BMP signaling can be fine-tuned at numerous levels, the role of BMP-inhibitors has become a major focus. The aim of the present study was to document co-expression of BMPs, pSmad 1/5/8, and BMP-inhibitors in human fracture callus and human non-unions. Using human tissue of fracture callus (n=14) and non-unions (n=4) we documented expression of BMPs (BMP2, BMP3 and BMP7), pSmad 1/5/8 and the BMP-inhibitors noggin, gremlin, chordin, Smad-6, Smad-7 and BAMBI. Co-expression of pSmad 1/5/8, BMPs and BMP-inhibitors was noted in the osteoblasts of fracture callus as well as of non-unions. Expression of BMP-inhibitors was generally stronger in non-unions than in fracture callus. The most pertinent differences were noted in the cartilaginous tissue components. Expression of BMP2 in chondrocytes was markedly decreased in non-unions compared to fracture callus and that of BMP7 was almost completely absent. Expression of BMP-inhibitors was almost the same in osteoblasts, chondrocytes and fibroblasts of fracture callus and well as in non-unions. Interestingly, although BMP ligands were present in the chondrocytes and fibroblasts of non-unions, they did not co-express pSmad 1/5/8 suggesting that BMP signaling may have been inhibited at some point before Smad 1/5/8 phosphorylation. These results suggest co-expression of BMP, pSmad 1/5/8 and BMP-inhibitors occurs in human fracture callus as well as non-unions but the relative expression of BMPs vs. BMP-inhibitors was different between these two tissue types. In contrast to our expectations, the expression of BMP inhibitors was comparable between fracture callus and non-unions, whereas the expression of BMPs was notably lower in the cartilaginous component of the non-unions in comparison to fracture callus. Based on these results, we believe that aberrations in the BMP-signaling pathway in the cartilaginous component of fracture healing could influence clinical fracture healing. An imbalance between the local presence of BMP and BMP-inhibitors may switch the direction towards healing or non-healing of a fracture.

The "bone morphogenic proteins" pathways in bone and joint diseases: translational perspectives from physiopathology to therapeutic targets

A large body of evidence supports an important role of bone morphogenic proteins (BMPs) pathways in skeletal development in the embryo. BMPs are also involved in skeletal homeostasis and diseases in the adult. They were first identified as major bone anabolic agents and recent advances indicate that they also regulate osteoclastogenesis and joint components via multiple cross-talks with other signaling pathways. This review attempts to integrate these data in the pathogenesis of bone and joints diseases, such as osteoporosis, fracture healing, osteoarthritis, inflammatory arthritis, or bone metastasis. The use of recombinant BMPs in bone tissue engineering and in the treatment of skeletal diseases, or future therapeutic strategies targeting BMPs signal and its regulators, will be discussed based on these considerations.

Conditional inactivation of noggin in the postnatal skeleton causes osteopenia

Noggin is an antagonist of bone morphogenetic proteins (BMP), and its overexpression causes suppressed osteoblastogenesis and osteopenia. Global inactivation of Noggin results in severe developmental defects and prenatal lethality, but the consequences of the conditional inactivation of Noggin on the postnatal skeleton are not known. To study the function of noggin in osteoblasts, we generated tissue-specific null Noggin mice by mating Noggin conditional mice, where the Noggin allele is flanked by loxP sequences, with mice expressing the Cre recombinase under the control of the osteocalcin promoter (Oc-Cre). Noggin conditional null mice exhibited decreased weight, shortened femoral length, and generalized osteopenia. Bone histomorphometric and microarchitectural analyses of distal femurs revealed decreased bone volume due to a reduced number of trabeculae in 1- and 3-month-old Noggin conditional null mice. Vertebral microarchitecture confirmed the osteopenia observed in Noggin conditional null mice. Osteoclast number was increased in 1-month-old male Noggin conditional null mice, and bone formation was increased in 3-month-old mice, but female mice did not exhibit increased bone remodeling. In conclusion, Noggin inactivation causes osteopenia, suggesting that BMP in excess have a detrimental effect on bone or that noggin has a BMP-independent role in skeletal homeostasis.

Gremlin1 is required for skeletal development and postnatal skeletal homeostasis

Gremlin is an antagonist of bone morphogenetic proteins, and its overexpression causes suppressed osteoblastogenesis and osteopenia. Inactivation of Grem1 results in severe developmental defects, but the consequences of the global inactivation of Grem1 on the postnatal skeleton are not known. To study the function of gremlin, Grem1 was inactivated by homologous recombination, and mice were maintained in a C57BL/6/FVB mixed genetic background due to embryonic and neonatal lethality in the uniform C57BL/6 background. Grem1 null mice exhibited developmental skeletal abnormalities, leading to incomplete formation of metatarsal bones and of fore limbs and hind limbs. Grem1 null mice exhibited decreased weight and body fat and shortened femoral length. Bone histomorphometric and microarchitectural analyses of distal femurs revealed decreased bone volume and increased bone formation in 1-month-old Grem1 null mice. Trabecular femoral bone volume was restored in older Grem1 null female mice, and to a lesser extent in male mice. Vertebral microarchitecture confirmed the osteopenia observed in 1-month-old Grem1 null mice and demonstrated recovery of trabecular bone in older female, but not in older male Grem1 null mice, which exhibited persistent vertebral osteopenia. In conclusion, Grem1 is not only necessary for skeletal development, but also for postnatal skeletal homeostasis; its inactivation causes osteopenia, which is partially reversed in a spatial, temporal, and sex-dependent manner due to an increase in bone formation.

Loss of gremlin delays primordial follicle assembly but does not affect female fertility in mice

The transforming growth factor beta (TGFB) protein family is renowned for its diverse roles in developmental biology including reproduction. Gremlin is a member of the differential screening-selected gene aberrative in neuroblastoma (DAN)/cerberus family of bone morphogenetic protein (BMP) antagonists. Recent studies on gremlin focus on its involvement in embryonic skeletal, lung, and kidney development. To define the role of gremlin (Grem1) in female reproduction, we analyzed postnatal folliculogenesis using global and conditional knockout (cKO) mice for gremlin. Grem1(-/-) mice die within 48 h after birth, and ovaries collected from neonatal Grem1(-/-) mice demonstrated reduced oocyte numbers and delayed primordial follicle development. Transplanting Grem1(-/-) neonatal ovaries showed that folliculogenesis proceeded to large antral follicle stage, but Grem1(-/-) ovaries contained corpora lutea-like structures not found in control-transplanted ovaries. However, Grem1 cKO mice had comparable fertility to control mice. These data suggest that gremlin plays a previously uncharacterized role in the regulation of oocyte numbers and the timing of primordial follicle development, but either it is not required for later folliculogenesis or its loss is possibly compensated by other BMP antagonists.

Nephroblastoma overexpressed (Nov) induces gremlin in ST-2 stromal cell lines by post-transcriptional mechanisms

Nephroblastoma overexpressed (Nov) inhibits osteoblastogenesis in part because it binds bone morphogenetic protein (BMP)-2. In the present study, we investigated whether Nov regulated the expression of the BMP antagonist gremlin. Overexpression of Nov increased gremlin mRNA levels in ST-2 cells, and its downregulation by RNA interference decreased gremlin mRNA. Nov did not affect Grem1 transcription, but prolonged the half-life of gremlin mRNA in ST-2 cells, demonstrating that Nov acts by post-transcriptional mechanisms. This was confirmed by demonstrating that downregulation of Nov destabilizes gremlin transcripts. To assess whether the 3'-untranslated region (UTR) of gremlin mRNA mediated the effect of Nov, the decay of a chimeric cfos gremlin 3'-UTR construct was compared to that of cfos in ST-2 cells. The presence of the gremlin 3'-UTR prolonged the half-life of cfos and was responsible for the effect of Nov. To examine the binding of the gremlin 3'-UTR to ribonucleoproteins, radiolabeled gremlin RNA fragments were incubated with cytosolic extracts from Nov overexpressing and control cells. RNA electrophoretic mobility analysis revealed that Nov enhanced the binding of cytosolic proteins to the fragments spanning the 3'-UTR of gremlin between bases 1,358-1,557 and 1,158-1,357 from the transcriptional start. Mutations of AU-rich elements in these two RNA fragments prevented the formation of RNA-protein complexes induced by Nov. Nov did not alter the binding of cytosolic extracts to sequences present in the 5'-UTR or coding region of gremlin. In conclusion, Nov stabilizes gremlin transcripts, and this effect is possibly mediated by AU-rich elements present in the 3'-UTR of gremlin.

Cooperative activity of noggin and gremlin 1 in axial skeleton development

Inductive signals from adjacent tissues initiate differentiation within the somite. In this study, we used mouse embryos mutant for the BMP antagonists noggin (Nog) and gremlin 1 (Grem1) to characterize the effects of BMP signaling on the specification of the sclerotome. We confirmed reduction of Pax1 and Pax9 expression in Nog mutants, but found that Nog;Grem1 double mutants completely fail to initiate sclerotome development. Furthermore, Nog mutants that also lack one allele of Grem1 exhibit a dramatic reduction in axial skeleton relative to animals mutant for Nog alone. By contrast, Pax3, Myf5 and Lbx1 expression indicates that dermomyotome induction occurs in Nog;Grem1 double mutants. Neither conditional Bmpr1a mutation nor treatment with the BMP type I receptor inhibitor dorsomorphin expands sclerotome marker expression, suggesting that BMP antagonists do not have an instructive function in sclerotome specification. Instead, we hypothesize that Nog- and Grem1-mediated inhibition of BMP is permissive for hedgehog (Hh) signal-mediated sclerotome specification. In support of this model, we found that culturing Nog;Grem1 double-mutant embryos with dorsomorphin restores sclerotome, whereas Pax1 expression in smoothened (Smo) mutants is not rescued, suggesting that inhibition of BMP is insufficient to induce sclerotome in the absence of Hh signaling. Confirming the dominant inhibitory effect of BMP signaling, Pax1 expression cannot be rescued in Nog;Grem1 double mutants by forced activation of Smo. We conclude that Nog and Grem1 cooperate to maintain a BMP signaling-free zone that is a crucial prerequisite for Hh-mediated sclerotome induction.

Animal models of typical heterotopic ossification

Heterotopic ossification (HO) is the formation of marrow-containing bone outside of the normal skeleton. Acquired HO following traumatic events is a common and costly clinical complication. In contrast, hereditary HO is rarer, progressive, and life-threatening. Substantial effort has been directed towards understanding the mechanisms underlying HO and finding efficient treatments. However, one crucial limiting factor has been the lack of relevant animal models. This article reviews the major currently available animal models, summarizes some of the insights gained from these studies, and discusses the potential future challenges and directions in HO research.

Bone morphogenetic protein and growth differentiation factor cytokine families and their protein antagonists

The BMPs (bone morphogenetic proteins) and the GDFs (growth and differentiation factors) together form a single family of cystine-knot cytokines, sharing the characteristic fold of the TGFbeta (transforming growth factor-beta) superfamily. Besides the ability to induce bone formation, which gave the BMPs their name, the BMP/GDFs display morphogenetic activities in the development of a wide range of tissues. BMP/GDF homo- and hetero-dimers interact with combinations of type I and type II receptor dimers to produce multiple possible signalling complexes, leading to the activation of one of two competing sets of SMAD transcription factors. BMP/GDFs have highly specific and localized functions. These are regulated in a number of ways, including the developmental restriction of BMP/GDF expression and through the secretion of several specific BMP antagonist proteins that bind with high affinity to the cytokines. Curiously, a number of these antagonists are also members of the TGF-beta superfamily. Finally a number of both the BMP/GDFs and their antagonists interact with the heparan sulphate side chains of cell-surface and extracellular-matrix proteoglycans.

Update in new anabolic therapies for osteoporosis

Skeletal anabolic agents enhance bone formation, which is determined by the number and function of osteoblasts. Cell number is controlled by factors that regulate the replication, differentiation, and death of cells of the osteoblastic lineage, whereas cell function is controlled by signals acting on the mature osteoblast. Bone morphogenetic proteins (BMP) and Wnt induce the differentiation of mesenchymal cells toward osteoblasts, and IGF-I enhances the function of mature osteoblasts. The activity of BMP, Wnt, and IGF-I is controlled by proteins that, by binding to the growth factor or to its receptors, can antagonize its effects. Changes in the expression or binding affinity of these extracellular antagonists can be associated with increased or decreased bone formation and bone mass. Novel approaches to anabolic therapies for osteoporosis may include the use of factors with anabolic properties, or the neutralization of a growth factor antagonist. Selected approaches include the use of neutralizing antibodies to Wnt antagonists, the enhancement of BMP signaling by proteasome inhibitors, or the use of activin soluble receptors, IGF-I, or PTH analogs. An anabolic agent needs to be targeted specifically to the skeleton to avoid unwanted nonskeletal effects and ensure safety. Clinical trials are being conducted to test the long-term effectiveness and safety of novel bone anabolic agents.

Future approaches in periodontal regeneration: gene therapy, stem cells, and RNA interference

Periodontal disease is a major public health issue and the development of effective therapies to treat the disease and regenerate periodontal tissue is an important goal of today's medicine. This article highlights recent scientific advancements in gene therapy, stem cell biology, and RNA interference with the intent of identifying their potential in periodontal tissue regeneration. Results from basic research, preclinical, and clinical studies indicate that these fields of research may soon contribute to more effective regenerative therapies for periodontal disease.

Transforming growth factor-beta isoforms and the induction of bone formation: implications for reconstructive craniofacial surgery

Craniofacial skeletal reconstruction remains a challenging problem despite major molecular and surgical developments in the understanding of bone formation by induction. The induction of bone formation has been a critical topic of research across the planet. The bone induction principle identified important cues for tissue engineering of bone, namely, osteogenic soluble molecular signals, the bone morphogenetic and osteogenic proteins, and insoluble signals or substrata including biomimetic bioactive matrices and responding stem cells. In primates, and in primates only, the osteogenic soluble molecular signals that initiate the induction of bone formation additionally include the 3 mammalian transforming growth factor-beta (TGF-beta) isoforms, members of the TGF-beta supergene family. The mammalian TGF-beta isoforms, when implanted in the rectus abdominis muscle of the nonhuman primate Papio ursinus, induce rapid and substantial endochondral bone formation resulting in large corticalized ossicles by day 30 after heterotopic implantation; in calvarial defects of the same nonhuman primates, identical or higher doses of the TGF-beta protein do not induce bone formation because of the overexpression of Smad-6 and Smad-7, gene product inhibitors of the TGF-beta signaling pathway. The addition of minced fragments of autogenous rectus abdominis muscle partially restores the osteoinductive activity of the human TGF-beta3 isoform resulting in the induction of bone formation in the treated calvarial defects. Recombinant human TGF-beta3 delivered by Matrigel matrix and implanted in class II and III furcation defects of mandibular molars of P. ursinus induce periodontal tissue regeneration. The addition of minced fragments of autogenous rectus abdominis muscle significantly enhances cementogenesis. This review highlights the induction of bone formation by the osteogenic proteins of the TGF-beta superfamily in the nonhuman primate P. ursinus and reviews combinatorial applications of myoblastic/myogenic stem cell-based therapeutics for bone induction and morphogenesis. The recruitment of myoendothelial cells is also discussed in the light of the intrinsic and spontaneous induction of bone formation by smart biomaterial matrices that induce bone differentiation in heterotopic extraskeletal sites of P. ursinus without the exogenous application of the osteogenic soluble molecular signals of the TGF-beta superfamily.

Growth factor control of bone mass

Bone formation is determined by the number and function of osteoblasts. Cell number is governed by factors that regulate the replication and differentiation of pre-osteoblasts and factors that regulate osteoblastic cell death. Cell function is controlled by signals acting on the mature osteoblast. Platelet-derived and fibroblast growth factors are bone cell mitogens. Bone morphogenetic proteins (BMPs) and Wnt induce the differentiation of mesenchymal cells toward osteoblasts, and insulin-like growth factor (IGF)-I stimulates the function of mature osteoblasts and prevents their death. The activity of BMP, Wnt, and IGF-I is modulated by extracellular antagonists or binding proteins. Changes in growth factor synthesis and activity may play a role in the pathogenesis of selected forms of osteoporosis, and alterations in the expression or binding of the extracellular antagonists can be associated with changes in bone mass. Current approaches to bone anabolic therapies for osteoporosis include the administration of a growth factor, such as IGF-I, or the neutralization of an antagonist. Ideally, the targeting of an anabolic agent should be specific to bone to preclude non-skeletal unwanted side effects. Clinical trials are needed to determine the long-term effectiveness and safety of novel anabolic agents for the management of osteoporosis.

Allelic depletion of grem1 attenuates diabetic kidney disease

OBJECTIVE

Gremlin (grem1) is an antagonist of the bone morphogenetic protein family that plays a key role in limb bud development and kidney formation. There is a growing appreciation that altered grem1 expression may regulate the homeostatic constraints on damage responses in diseases such as diabetic nephropathy.

RESEARCH DESIGN AND METHODS

Here we explored whether knockout mice heterozygous for grem1 gene deletion (grem1(+/-)) exhibit protection from the progression of diabetic kidney disease in a streptozotocin-induced model of type 1 diabetes.

RESULTS

A marked elevation in grem1 expression was detected in the kidneys and particularly in kidney tubules of diabetic wild-type mice compared with those of littermate controls. In contrast, diabetic grem1(+/-) mice displayed a significant attenuation in grem1 expression at 6 months of diabetes compared with that in age- and sex-matched wild-type controls. Whereas the onset and induction of diabetes were similar between grem1(+/-) and wild-type mice, several indicators of diabetes-associated kidney damage such as increased glomerular basement membrane thickening and microalbuminuria were attenuated in grem1(+/-) mice compared with those in wild-type controls. Markers of renal damage such as fibronectin and connective tissue growth factor were elevated in diabetic wild-type but not in grem1(+/-) kidneys. Levels of pSmad1/5/8 decreased in wild-type but not in grem1(+/-) diabetic kidneys, suggesting that bone morphogenetic protein signaling may be maintained in the absence of grem1.

CONCLUSIONS

These data identify grem1 as a potential modifier of renal injury in the context of diabetic kidney disease.

Effect of the methoxychlor metabolite HPTE on the rat ovarian granulosa cell transcriptome in vitro

Ovarian granulosa cells play a central role in steroidogenesis, which is critical for female reproduction. Follicle-stimulating hormone (FSH) promotes cyclic adenosine monophosphate (cAMP)-mediated signaling to regulate granulosa cell steroidogenesis. We have shown previously that 2,2-bis-(p-hydroxyphenyl)-1,1,1-trichloroethane (HPTE) inhibits FSH- and dibutyryl cAMP-stimulated steroidogenesis and affects the messenger RNA levels of steroidogenic pathway enzymes in rat granulosa cells. However, HPTE showed a differential effect in FSH- and cAMP-stimulated cells in that HPTE more completely blocked FSH- when compared to cAMP-driven steroidogenesis. The objective of this study was to analyze the effects of HPTE on global gene expression profiles in untreated granulosa cells and those challenged with FSH or cAMP. Granulosa cells from immature rats were cultured with 0, 1, 5, or 10 microM HPTE in the presence or absence of either 3 ng FSH/ml or 1mM cAMP for 48 h. Total RNA was isolated for real-time quantitative PCR and microarray analysis using the GeneChip Rat Genome 230 2.0 and ArrayAssist Microarray Suite. An investigation of changes in gene expression across all HPTE treatments showed that HPTE altered more genes in FSH- (approximately 670 genes) than in cAMP-stimulated cells (approximately 366 genes). Analysis confirmed that HPTE more effectively inhibited FSH- than cAMP-induced steroid pathway gene expression and steroidogenesis. Furthermore, expression patterns of novel genes regulating signal transduction, transport, cell cycle, adhesion, differentiation, motility and growth, apoptosis, development, and metabolism were all altered by HPTE. This study further established that HPTE exerts differential effects within the granulosa cell steroidogenic pathway and revealed that these effects include broader changes in gene expression.

Quantitative trait loci, genes, and polymorphisms that regulate bone mineral density in mouse

This is an in silico analysis of data available from genome-wide scans. Through analysis of QTL, genes and polymorphisms that regulate BMD, we identified 82 BMD QTL, 191 BMD-associated (BMDA) genes, and 83 genes containing known BMD-associated polymorphisms (BMDAP). The catalogue of all BMDA/BMDAP genes and relevant literatures are provided. In total, there are substantially more BMDA/BMDAP genes in regions of the genome where QTL have been identified than in non-QTL regions. Among 191 BMDA genes and 83 BMDAP genes, 133 and 58 are localized in QTL regions, respectively. The difference was still noticeable for the chromosome distribution of these genes between QTL and non-QTL regions. These results have allowed us to generate an integrative profile of QTL, genes, polymorphisms that determine BMD. These data could facilitate more rapid and comprehensive identification of causal genes underlying the determination of BMD in mouse and provide new insights into how BMD is regulated in humans.

Antisense makes sense in engineered regenerative medicine

The use of antisense strategies such as ribozymes, oligodeoxynucleotides (ODNs) and small interfering RNA (siRNA) in gene therapy, in conjunction with the use of stem cells and tissue engineering, has opened up possibilities in curing degenerative diseases and injuries to non-regenerating organs and tissues. With their unique ability to down-regulate or silence gene expression, antisense oligonucleotides are uniquely suited in turning down the production of pathogenic or undesirable proteins and cytokines. Here, we review the antisense strategies and their applications in regenerative medicine with a focus on their efficacies in promoting cell viability, regulating cell functionalities as well as shaping an optimal microenvironment for therapeutic purposes.

Activation of the ERK pathway in osteoblastic cells, role of gremlin and BMP-2

Gremlin is a glycoprotein that binds and antagonizes the actions of bone morphogenetic proteins (BMPs) -2, -4, and -7. Gremlin appears to activate the extracellular regulated kinase (ERK) pathway in endothelial and tumor cells, and as a consequence to have direct cellular effects. To determine whether gremlin has direct effects in osteoblasts, independent of its BMP binding activity, we examined its effects in ST-2 murine stromal cell lines and in primary cultures of murine calvarial osteoblasts. Gremlin did not activate Signaling mothers against decapentaplegic (Smad), and suppressed the BMP-2 induced Smad 1/5/8 phosphorylation and the transactivation of the BMP/Smad reporter construct 12xSBE-Oc-pGL3, confirming its BMPs antagonizing activity. Neither gremlin nor BMP-2 induced ERK 1/2 activation in ST-2 cells or calvarial osteoblasts. Moreover, slight changes in culture conditions induced the phosphorylation of ERK independent from BMP or gremlin exposure. In conclusion, gremlin inhibits BMP-2 signaling and activity, and does not have independent actions on ERK signaling in osteoblasts. Consequently, gremlin activity in osteoblasts can be attributed only to its BMP antagonizing effects.