cAMP enhances BMP2-signaling through PKA and MKP1-dependent mechanisms

Protein tyrosine phosphatases in skeletal development and diseases

Skeletal development and homeostasis in mammals are modulated by finely coordinated processes of migration, proliferation, differentiation, and death of skeletogenic cells originating from the mesoderm and neural crest. Numerous molecular mechanisms are involved in these regulatory processes, one of which is protein posttranslational modifications, particularly protein tyrosine phosphorylation (PYP). PYP occurs mainly through the action of protein tyrosine kinases (PTKs), modifying protein enzymatic activity, changing its cellular localization, and aiding in the assembly or disassembly of protein signaling complexes. Under physiological conditions, PYP is balanced by the coordinated action of PTKs and protein tyrosine phosphatases (PTPs). Dysregulation of PYP can cause genetic, metabolic, developmental, and oncogenic skeletal diseases. Although PYP is a reversible biochemical process, in contrast to PTKs, little is known about how this equilibrium is modulated by PTPs in the skeletal system. Whole-genome sequencing has revealed a large and diverse superfamily of PTP genes (over 100 members) in humans, which can be further divided into cysteine (Cys)-, aspartic acid (Asp)-, and histidine (His)-based PTPs. Here, we review current knowledge about the functions and regulatory mechanisms of 28 PTPs involved in skeletal development and diseases; 27 of them belong to class I and II Cys-based PTPs, and the other is an Asp-based PTP. Recent progress in analyzing animal models that harbor various mutations in these PTPs and future research directions are also discussed. Our literature review indicates that PTPs are as crucial as PTKs in supporting skeletal development and homeostasis.

Effect of N-Vinyl-2-Pyrrolidone (NVP), a Bromodomain-Binding Small Chemical, on Osteoblast and Osteoclast Differentiation and Its Potential Application for Bone Regeneration

The human skeleton is a dynamic and remarkably organized organ system that provides mechanical support and performs a variety of additional functions. Bone tissue undergoes constant remodeling; an essential process to adapt architecture/resistance to growth and mechanical needs, but also to repair fractures and micro-damages. Despite bone's ability to heal spontaneously, certain situations require an additional stimulation of bone regeneration, such as non-union fractures or after tumor resection. Among the growth factors used to increase bone regeneration, bone morphogenetic protein-2 (BMP2) is certainly the best described and studied. If clinically used in high quantities, BMP2 is associated with various adverse events, including fibrosis, overshooting bone formation, induction of inflammation and swelling. In previous studies, we have shown that it was possible to reduce BMP2 doses significantly, by increasing the response and sensitivity to it with small molecules called "BMP2 enhancers". In the present study, we investigated the effect of N-Vinyl-2-pyrrolidone (NVP) on osteoblast and osteoclast differentiation in vitro and guided bone regeneration in vivo. We showed that NVP increases BMP2-induced osteoblast differentiation and decreases RANKL-induced osteoclast differentiation in a dose-dependent manner. Moreover, in a rabbit calvarial defect model, the histomorphometric analysis revealed that bony bridging and bony regenerated area achieved with NVP-loaded poly (lactic-co-glycolic acid (PLGA) membranes were significantly higher compared to unloaded membranes. Taken together, our results suggest that NVP sensitizes BMP2-dependent pathways, enhances BMP2 effect, and inhibits osteoclast differentiation. Thus, NVP could prove useful as "osteopromotive substance" in situations where a high rate of bone regeneration is required, and in the management of bone diseases associated with excessive bone resorption, like osteoporosis.

The Effects of Stably Tethered BMP-2 on MC3T3-E1 Preosteoblasts Encapsulated in a PEG Hydrogel

Bone morphogenetic protein-2 (BMP-2) is a clinically used osteoinductive growth factor. With a short half-life and side effects, alternative delivery approaches are needed. This work examines thiolation of BMP-2 for chemical attachment to a poly(ethylene glycol) hydrogel using thiol-norbornene click chemistry. BMP-2 retained bioactivity post-thiolation and was successfully tethered into the hydrogel. To assess tethered BMP-2 on osteogenesis, MC3T3-E1 preosteoblasts were encapsulated in matrix metalloproteinase (MMP)-sensitive hydrogels containing RGD and either no BMP-2, soluble BMP-2 (5 nM), or tethered BMP-2 (40-200 nM) and cultured in a chemically defined medium containing dexamethasone for 7 days. The hydrogel culture supported MC3T3-E1 osteogenesis regardless of BMP-2 presentation, but tethered BMP-2 augmented the osteogenic response, leading to significant increases in osteomarkers, Bglap and Ibsp. The ratio, Ibsp-to-Dmp1, highlighted differences in the extent of differentiation, revealing that without BMP-2, MC3T3-E1 cells showed a higher expression of Dmp1 (low ratio), but an equivalent expression with tethered BMP-2 and more abundant bone sialoprotein. In addition, this work identified that dexamethasone contributed to Ibsp expression but not Bglap or Dmp1 and confirmed that tethered BMP-2 induced the BMP canonical signaling pathway. This work presents an effective method for the modification and incorporation of BMP-2 into hydrogels to enhance osteogenesis.

The SPOM-adapted IVM system improves in vitro production of bovine embryos

This study aimed to test the effects of an IVM SPOM adaptation (SPOM-adapted IVM) on the production, total number of cells (TNC), apoptosis, and cryotolerance (post-warming survival and cytoskeleton actin integrity) of bovine IVP embryos. Two experiments were conducted with two experimental groups based on IVM treatment: A control group (TCM 199 without FCS) and an SPOM-adapted group (TCM 199 with forskolin and IBMX in pre-IVM and IVM with cilostamide). The first experiment evaluated embryo in vitro production, TNC, and apoptosis rate on D9 of development. In the second experiment, embryos were vitrified/warmed at D7 (control fresh and vitrified; SPOM-adapted fresh and vitrified) and assessed regarding post-warming survival rates and cytoskeleton actin integrity. Statistical analysis was performed using GraphPad INSTAT software at a significance level of 5%. An increase (p < 0.05) in blastocyst production was observed in the SPOM-adapted group comparing to the control group. There was no difference (p > 0.05) in the TNC or apoptosis rate between the groups. Regarding cryopreservation, no differences were found (p > 0.05) in actin integrity or post-warming survival rates between the vitrified groups. In both vitrified groups, we observed a significantly lower uninjured pattern of actin integrity compared to the fresh groups (p < 0.05). We conclude that the SPOM-adapted IVM system is beneficial for blastocyst production and does not affect the quality and cryotolerance of the produced embryos.

Enhancement of local bone formation on titanium implants in osteoporotic rats by biomimetic multilayered structures containing parathyroid hormone (PTH)-related protein

Osteoporosis is a severe health problem causing bone fragility and consequent fracture. Titanium (Ti) implants, used in patients with osteoporotic fractures, are prone to failure because of the decreased bone mass and strength. Therefore, it is of utmost importance to fabricate implants possessing osteogenic properties to improve implant osseointegration. To improve the long-term survival rate of Ti implants in osteoporotic patients, hyaluronic acid/ϵ-polylysine multilayers containing the parathyroid hormone (PTH)-related protein (PTHrP) were deposited on Ti implants by a layer-by-layer (LBL) electro assembly technique. The murine pre-osteoblast cell line MC3T3-E1, possessing a high potential of osteoblast differentiation, was used to evaluate the osteo-inductive effects of Ti-LBL-PTHrP in vitro. In addition, the performance of the Ti (Ti-LBL-PTHrP) implant was evaluated in vivo in a femoral intramedullary implantation in Sprague Dawley rats. The Ti-LBL-PTHrP implant regulated the release of the loaded PTHrP to increase bone formation in the early stage of implantation. The in vitro results revealed that cells on Ti-LBL-PTHrP did not show any evident proliferation, but a high level of alkaline phosphatase activity and osteoblast-related protein expression was found, compared to the uncoated Ti group (p < 0.05). In addition, in vivo micro-CT and histological analysis demonstrated that the Ti-LBL-PTHrP implants could significantly promote the formation and remodeling of new bone in osteoporotic rats at 14 d after implantation. Overall, this study established a profound and straightforward methodology for the manufacture of biofunctional Ti implants for the treatment of osteoporosis.

Bmp2 regulates Serpinb6b expression via cAMP/PKA/Wnt4 pathway during uterine decidualization

Serpinb6b is a novel member of Serpinb family and found in germ and somatic cells of mouse gonads, but its physiological function in uterine decidualization remains unclear. The present study revealed that abundant Serpinb6b was noted in decidual cells, and advanced the proliferation and differentiation of stromal cells, indicating a creative role of Serpinb6b in uterine decidualization. Further analysis found that Serpinb6b modulated the expression of Mmp2 and Mmp9. Meanwhile, Serpinb6b was identified as a target of Bmp2 regulation in stromal differentiation. Treatment with rBmp2 resulted in an accumulation of intracellular cAMP level whose function in this differentiation program was mediated by Serpinb6b. Addition of PKA inhibitor H89 impeded the Bmp2 induction of Serpinb6b, whereas 8-Br-cAMP rescued the defect of Serpinb6b expression elicited by Bmp2 knock-down. Attenuation of Serpinb6b greatly reduced the induction of constitutive Wnt4 activation on stromal cell differentiation. By contrast, overexpression of Serpinb6b prevented this inhibition of differentiation process by Wnt4 siRNA. Moreover, blockage of Wnt4 abrogated the up-regulation of cAMP on Serpinb6b. Collectively, Serpinb6b mediates uterine decidualization via Mmp2/9 in response to Bmp2/cAMP/PKA/Wnt4 pathway.

Mitochondrial function in immature bovine oocytes is improved by an increase of cellular cyclic AMP

Although in vitro maturation (IVM) of oocytes is important for assisted reproduction, the rate of development of embryos from IVM oocytes is lower than from their in vivo counterparts. It has been shown that an artificial increase of intracellular cAMP before culture significantly improves oocyte developmental competence in cattle and mice. Here, we revealed that forskolin and 3-isobutyl-1-methylxanthine treatment of prophase-stage oocytes induced the expression of genes required for glycolysis, fatty acid degradation, and the mitochondrial electron transport system and improved mitochondrial functions and ATP levels in oocytes without involving nuclear maturation. We propose the existence of a comprehensive energy-supply system in oocytes under follicle-stimulating hormone stimulation as a potential explanation of how oocytes acquire developmental competence.

Role of cAMP modulator supplementations during oocyte in vitro maturation in domestic animals

Cyclic adenosine monophosphate (cAMP) is an important molecule in signal transduction within the cell, functioning as a second cell messenger of gonadotrophin stimulation. The concentration of cAMP in cumulus-oocyte complexes (COCs) is known to be controlled through modulation of its synthesis by adenylyl cyclase (AC) and by degradation through the cyclic nucleotide phosphodiesterase (PDE) enzymes. One of the main obstacles for in vitro embryo production is the optimization of reproduction processes that occur in oocyte maturation. The function of cAMP is important in maintaining meiotic arrest in mammalian oocytes. When the oocyte is physically removed from the antral follicle for in vitro maturation (IVM), intra-oocyte cAMP concentrations decrease and spontaneous meiotic resumption begins, due to the depletion of inhibitory factors from the follicle. In many studies, relatively greater cAMP concentrations before IVM has been reported to improve oocyte competence, leading to subsequent benefits in embryonic development in different species. There, therefore, has been an increase in oocyte cAMP concentrations with several treatments and different approaches, such as invasive AC, stimulators of AC activity, PDE inhibitors, and cAMP analogs. The aim of this review is to comprehensively evaluate and provide data related to (i) the use of cAMP modulators during IVM and the effects on completion of meiosis and cytoplasmic reorganization, which are required for development of oocytes with the capacity to contribute to fertilization and subsequent embryonic development; and (ii) the main cAMP modulators and the effects when used in oocyte IVM.

N,N Dimethylacetamide a drug excipient that acts as bromodomain ligand for osteoporosis treatment

N,N-Dimethylacetamide (DMA) is a water-miscible solvent, FDA approved as excipient and therefore widely used as drug-delivery vehicle. As such, DMA should be devoid of any bioactivity. Here we report that DMA is epigenetically active since it binds bromodomains and inhibits osteoclastogenesis and inflammation. Moreover, DMA enhances bone regeneration in vivo. Therefore, our in vivo and in vitro data reveal DMA's potential as an anti-osteoporotic agent via the inhibition of osteoclast mediated bone resorption and enhanced bone regeneration. Our results highlight the potential therapeutic benefits of DMA and the need for reconsideration of previous reports where DMA was used as an 'inactive' drug-delivery vehicle.

Language deficits in schizophrenia and autism as related oscillatory connectomopathies: An evolutionary account

Schizophrenia (SZ) and autism spectrum disorders (ASD) are characterised by marked language deficits, but it is not clear how these arise from gene mutations associated with the disorders. Our goal is to narrow the gap between SZ and ASD and, ultimately, give support to the view that they represent abnormal (but related) ontogenetic itineraries for the human faculty of language. We will focus on the distinctive oscillatory profiles of the SZ and ASD brains, in turn using these insights to refine our understanding of how the brain implements linguistic computations by exploring a novel model of linguistic feature-set composition. We will argue that brain rhythms constitute the best route to interpreting language deficits in both conditions and mapping them to neural dysfunction and risk alleles of the genes. Importantly, candidate genes for SZ and ASD are overrepresented among the gene sets believed to be important for language evolution. This translational effort may help develop an understanding of the aetiology of SZ and ASD and their high prevalence among modern populations.

Mitogen-activated protein kinase phosphatase-1: function and regulation in bone and related tissues

In this review, we have highlighted work that has clearly demonstrated that mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1), a negative regulator of MAPKs, is an important signaling mediator in bone, muscle, and fat tissue homeostasis and differentiation. Further, we examined recent studies with particular focus on MKP-1 overexpression or deletion and its impact on tissues connected to bone. We also summarized regulation of MKP-1 by known skeletal regulators like parathyroid hormone (PTH)/PTH-related peptide (PTHrP) and bone morphogenic proteins. MKP-1's integration into the pathophysiological state of osteoporosis, osteoarthritis, rheumatoid arthritis, obesity, and muscular dystrophy are examined to emphasize possible involvement of MKP-1 both at the molecular level and in disease complications such as sarcopenia- or diabetes-related osteoporosis. We predict that understanding the mechanism of MKP-1-mediated signaling in bone-muscle-fat crosstalk will be a key in coordinating their activities and developing therapeutics to improve clinical outcomes for diseases associated with advanced age.

Transcriptomic analysis of cyclic AMP response in bovine cumulus cells

Acquisition of oocyte developmental competence needs to be understood to improve clinical outcomes of assisted reproduction. The stimulation of cumulus cell concentration of cyclic adenosine 3'5'-monophosphate (cAMP) by pharmacological agents during in vitro maturation (IVM) participates in improvement of oocyte quality. However, precise coordination and downstream targets of cAMP signaling in cumulus cells are largely unknown. We have previously demonstrated better embryo development after cAMP stimulation for first 6 h during IVM. Using this model, we investigated cAMP signaling in cumulus cells through in vitro culture of cumulus-oocyte complexes (COCs) in the presence of cAMP raising agents: forskolin, IBMX, and dipyridamole (here called FID treatment). Transcriptomic analysis of cumulus cells indicated that FID-induced differentially expressed transcripts were implicated in cumulus expansion, steroidogenesis, cell metabolism, and oocyte competence. Functional genomic analysis revealed that protein kinase-A (PKA), extracellular signal regulated kinases (ERK1/2), and calcium (Ca(2+)) pathways as key regulators of FID signaling. Inhibition of PKA (H89) in FID-supplemented COCs or substitution of FID with calcium ionophore (A23187) demonstrated that FID activated primarily the PKA pathway which inhibited ERK1/2 phosphorylation and was upstream of calcium signaling. Furthermore, inhibition of ERK1/2 phosphorylation by FID supported a regulation by dual specific phosphatase (DUSP1) via PKA. Our findings imply that cAMP (FID) regulates cell metabolism, steroidogenesis, intracellular signaling and cumulus expansion through PKA which modulates these functions through optimization of ERK1/2 phosphorylation and coordination of calcium signaling. These findings have implications for development of new strategies for improving oocyte in vitro maturation leading to better developmental competence.

Protein kinase inhibitor γ reciprocally regulates osteoblast and adipocyte differentiation by downregulating leukemia inhibitory factor

The protein kinase inhibitor (Pki) gene family inactivates nuclear protein kinase A (PKA) and terminates PKA-induced gene expression. We previously showed that Pkig is the primary family member expressed in osteoblasts and that Pkig knockdown increases the effects of parathyroid hormone and isoproterenol on PKA activation, gene expression, and inhibition of apoptosis. Here, we determined whether endogenous levels of Pkig regulate osteoblast differentiation. Pkig is the primary family member in murine embryonic fibroblasts (MEFs), murine marrow-derived mesenchymal stem cells, and human mesenchymal stem cells. Pkig deletion increased forskolin-dependent nuclear PKA activation and gene expression and Pkig deletion or knockdown increased osteoblast differentiation. PKA signaling is known to stimulate adipogenesis; however, adipogenesis and osteogenesis are often reciprocally regulated. We found that the reciprocal regulation predominates over the direct effects of PKA since adipogenesis was decreased by Pkig deletion or knockdown. Pkig deletion or knockdown also simultaneously increased osteogenesis and decreased adipogenesis in mixed osteogenic/adipogenic medium. Pkig deletion increased PKA-induced expression of leukemia inhibitory factor (Lif) mRNA and LIF protein. LIF neutralizing antibodies inhibited the effects on osteogenesis and adipogenesis of either Pkig deletion in MEFs or PKIγ knockdown in both murine and human mesenchymal stem cells. Collectively, our results show that endogenous levels of Pkig reciprocally regulate osteoblast and adipocyte differentiation and that this reciprocal regulation is mediated in part by LIF. Stem Cells 2013;31:2789-2799.

Globularity and language-readiness: generating new predictions by expanding the set of genes of interest

This study builds on the hypothesis put forth in Boeckx and Benítez-Burraco (2014), according to which the developmental changes expressed at the levels of brain morphology and neural connectivity that resulted in a more globular braincase in our species were crucial to understand the origins of our language-ready brain. Specifically, this paper explores the links between two well-known 'language-related' genes like FOXP2 and ROBO1 implicated in vocal learning and the initial set of genes of interest put forth in Boeckx and Benítez-Burraco (2014), with RUNX2 as focal point. Relying on the existing literature, we uncover potential molecular links that could be of interest to future experimental inquiries into the biological foundations of language and the testing of our initial hypothesis. Our discussion could also be relevant for clinical linguistics and for the interpretation of results from paleogenomics.

Recombinant biglycan promotes bone morphogenetic protein-induced osteogenesis

The aim of this study was to determine the effects of glutathione-S-transferase-fused recombinant biglycan (GST-BGN) on craniofacial bone regeneration. We recently demonstrated a positive effect of tissue-derived BGN on bone morphogenetic protein 2 (BMP-2) function, which is exerted likely via the BGN core protein. Here, we investigated the effects of GST-BGN lacking any posttranslational modifications on BMP-2 function in vitro and in vivo. In the C2C12 cell culture system, BMP-2-induced Smad 1/5/8 phosphorylation and alkaline phosphatase activity were both enhanced by the addition of GST-BGN. For the in vivo effect, we employed a Sprague-Dawley rat mandible defect model utilizing 1 µg (optimal) or 0.1 µg (suboptimal) of BMP-2 combined with 0, 2, 4, or 8 µg of GST-BGN. At 2 weeks post-surgery, newly formed bone was evaluated by microcomputed tomography and histologic analyses. The results revealed that the greatest amounts of bone within the defect were formed in the groups of suboptimal BMP-2 combined with 4 or 8 µg of GST-BGN. Also, bone was well organized versus that formed by the optimal dose of BMP. These results indicate that recombinant BGN is an efficient substrate to promote low-dose BMP-induced osteogenesis.

MicroRNA-214 suppresses osteogenic differentiation of C2C12 myoblast cells by targeting Osterix

Osterix (Osx) is an osteoblast-specific transcription factor that is essential for osteoblast differentiation and bone formation. Osx-null mice, which exhibit a complete absence of bone formation and arrested osteoblast differentiation, die immediately after birth. However, our understanding of the regulatory mechanism of Osx expression remains poor. MicroRNAs (miRNAs) are a class of small non-coding RNAs that play pivotal roles in diverse biological processes, including the development, differentiation, proliferation, survival, and oncogenesis of cells and organisms. In this study, we aimed to investigate the impact of miRNAs on Osx expression. Bioinformatic analyses predicted that miR-214 would be a potential regulator of Osx. The direct binding of miR-214 to the Osx 3' untranslated region (3' UTR) was demonstrated by a luciferase reporter assay using a construct containing the Osx 3' UTR. Deletion mutant construction revealed that the Osx 3' UTR contained two miR-214 binding sites. MiR-214 expression was inversely correlated with Osx expression in Saos-2 and U2OS cells. The forced expression of miR-214 in Saos-2 cells led to a reduction in the level of Osx protein. Moreover, the role of miR-214 in the osteogenic differentiation of C2C12 cells was investigated. We found that the osteogenic differentiation of C2C12 cells was enhanced by the downregulation of miR-214 expression, as measured by increased alkaline phosphatase activity and matrix mineralization. Taken together, these results indicate that miR-214 is a novel regulator of Osx, and that it plays an important role in the osteogenic differentiation of C2C12 cells as a suppressor.

Differentiation of mesenchymal stem cells to osteoblasts and chondrocytes: a focus on adenosine receptors

Skeletogenesis, either during development, post-injury or for maintenance, is a carefully coordinated process reliant on the appropriate differentiation of mesenchymal stem cells. Some well described, as well as a new regulator of this process (adenosine receptors), are alike in that they signal via cyclic-AMP (cAMP). This review highlights the known contribution of cAMP signalling to mesenchymal stem cell differentiation to osteoblasts and to chondrocytes. Focus has been given to how these regulators influence the commitment of the osteochondroprogenitor to these separate lineages.

Different roles of GNAS and cAMP signaling during early and late stages of osteogenic differentiation

Progressive osseous heteroplasia (POH) and fibrous dysplasia (FD) are genetic diseases of bone formation at opposite ends of the osteogenic spectrum: imperfect osteogenesis of the skeleton occurs in FD, while heterotopic ossification in skin, subcutaneous fat, and skeletal muscle forms in POH. POH is caused by heterozygous inactivating germline mutations in GNAS, which encodes G-protein subunits regulating the cAMP pathway, while FD is caused by GNAS somatic activating mutations. We used pluripotent mouse ES cells to examine the effects of Gnas dysregulation on osteoblast differentiation. At the earliest stages of osteogenesis, Gnas transcripts Gsα, XLαs and 1A are expressed at low levels and cAMP levels are also low. Inhibition of cAMP signaling (as in POH) by 2',5'-dideoxyadenosine enhanced osteoblast differentiation while conversely, increased cAMP signaling (as in FD), induced by forskolin, inhibited osteoblast differentiation. Notably, increased cAMP was inhibitory for osteogenesis only at early stages after osteogenic induction. Expression of osteogenic and adipogenic markers showed that increased cAMP enhanced adipogenesis and impaired osteoblast differentiation even in the presence of osteogenic factors, supporting cAMP as a critical regulator of osteoblast and adipocyte lineage commitment. Furthermore, increased cAMP signaling decreased BMP pathway signaling, indicating that G protein-cAMP pathway activation (as in FD) inhibits osteoblast differentiation, at least in part by blocking the BMP-Smad pathway, and suggesting that GNAS inactivation as occurs in POH enhances osteoblast differentiation, at least in part by stimulating BMP signaling. These data support that differences in cAMP levels during early stages of cell differentiation regulate cell fate decisions. Supporting information available online at http:/www.thieme-connect.de/ejournals/toc/hmr.

A2B adenosine receptor promotes mesenchymal stem cell differentiation to osteoblasts and bone formation in vivo

The differentiation of osteoblasts from their precursors, mesenchymal stem cells, is an important component of bone homeostasis as well as fracture healing. The A2B adenosine receptor (A2BAR) is a Gα(s)/α(q)-protein-coupled receptor that signals via cAMP. cAMP-mediated signaling has been demonstrated to regulate the differentiation of mesenchymal stem cells (MSCs) into various skeletal tissue lineages. Here, we studied the role of this receptor in the differentiation of MSCs to osteoblasts. In vitro differentiation of bone marrow-derived MSCs from A2BAR KO mice resulted in lower expression of osteoblast differentiation transcription factors and the development of fewer mineralized nodules, as compared with WT mice. The mechanism of effect involves, at least partially, cAMP as indicated by experiments involving activation of the A2BAR or addition of a cAMP analog during differentiation. Intriguingly, in vivo, microcomputed tomography analysis of adult femurs showed lower bone density in A2BAR KO mice as compared with WT. Furthermore, A2BAR KO mice display a delay in normal fracture physiology with lower expression of osteoblast differentiation genes. Thus, our study identified the A2BAR as a new regulator of osteoblast differentiation, bone formation, and fracture repair.

Dorsomorphin stimulates neurite outgrowth in PC12 cells via activation of a protein kinase A-dependent MEK-ERK1/2 signaling pathway

In this study, we investigated the effect of dorsomorphin, a selective inhibitor of bone morphogenetic protein (BMP) signaling, on rat PC12 pheochromocytoma cell differentiation. PC12 cells can be induced to differentiate into neuron-like cells possessing elongated neurites by nerve growth factor, BMP2, and other inducers. Cells were incubated with BMP2 and/or dorsomorphin, and the extent of neurite outgrowth was evaluated. Unexpectedly, BMP2-mediated neuritogenesis was not inhibited by co-treatment with dorsomorphin. We also found that treatment with dorsomorphin alone, but not another BMP signaling inhibitor, LDN-193189, induced neurite outgrowth in PC12 cells. To further understand the mechanism of action of dorsomorphin, the effects of this drug on intracellular signaling were investigated using the following signaling inhibitors: the ERK kinase (MEK) inhibitor U0126; the tropomyosin-related kinase A inhibitor GW441756; and the protein kinase A (PKA) inhibitor H89. Dorsomorphin induced rapid and sustained ERK1/2 activation; however, dorsomorphin-mediated ERK1/2 activation and neuritogenesis were robustly inhibited in the presence of U0126 or H89, but not GW441756. These findings suggest that dorsomorphin has the potential to induce neuritogenesis in PC12 cells, a response that requires the activation of PKA-dependent MEK-ERK1/2 signaling.

Ascl1-induced neuronal differentiation of P19 cells requires expression of a specific inhibitor protein of cyclic AMP-dependent protein kinase

cAMP-dependent protein kinase (PKA) plays a critical role in nervous system development by modulating sonic hedgehog and bone morphogenetic protein signaling. In the current studies, P19 embryonic carcinoma cells were neuronally differentiated by expression of the proneural basic helix-loop-helix transcription factor Ascl1. After expression of Ascl1, but prior to expression of neuronal markers such as microtubule associated protein 2 and neuronal β-tubulin, P19 cells demonstrated a large, transient increase in both mRNA and protein for the endogenous protein kinase inhibitor (PKI)β. PKIβ-targeted shRNA constructs both reduced the levels of PKIβ expression and blocked the neuronal differentiation of P19 cells. This inhibition of differentiation was rescued by transfection of a shRNA-resistant expression vector for the PKIβ protein, and this rescue required the PKA-specific inhibitory sequence of the PKIβ protein. PKIβ played a very specific role in the Ascl1-mediated differentiation process as other PKI isoforms were unable to rescue the deficit conferred by shRNA-mediated knockdown of PKIβ. Our results define a novel requirement for PKIβ and its inhibition of PKA during neuronal differentiation of P19 cells.

Transcriptional regulation of BMP2 expression by the PTH-CREB signaling pathway in osteoblasts

Intermittent application of parathyroid hormone (PTH) has well established anabolic effects on bone mass in rodents and humans. Although transcriptional mechanisms responsible for these effects are not fully understood, it is recognized that transcriptional factor cAMP response element binding protein (CREB) mediates PTH signaling in osteoblasts, and that there is a communication between the PTH-CREB pathway and the BMP2 signaling pathway, which is important for osteoblast differentiation and bone formations. These findings, in conjunction with putative cAMP response elements (CREs) in the BMP2 promoter, led us to hypothesize that the PTH-CREB pathway could be a positive regulator of BMP2 transcription in osteoblasts. To test this hypothesis, we first demonstrated that PTH signaling activated CREB by phosphorylation in osteoblasts, and that both PTH and CREB were capable of promoting osteoblastic differentiation of primary mouse osteoblast cells and multiple rodent osteoblast cell lines. Importantly, we found that the PTH-CREB signaling pathway functioned as an effective activator of BMP2 expression, as pharmacologic and genetic modulation of PTH-CREB activity significantly affected BMP2 expression levels in these cells. Lastly, through multiple promoter assays, including promoter reporter deletion, mutation, chromatin immunoprecipitation (ChIP), and electrophoretic mobility shift assay (EMSA), we identified a specific CRE in the BMP2 promoter which is responsible for CREB transactivation of the BMP2 gene in osteoblasts. Together, these results demonstrate that the anabolic function of PTH signaling in bone is mediated, at least in part, by CREB transactivation of BMP2 expression in osteoblasts.

Inhibition of osteoclast differentiation and bone resorption by N-methylpyrrolidone

Regulation of RANKL (receptor activator of nuclear factor κB ligand)-induced osteoclast differentiation is of current interest in the development of antiresorptive agents. Osteoclasts are multinucleated cells that play a crucial role in bone resorption. In this study, we investigated the effects of N-methylpyrrolidone (NMP) on the regulation of RANKL-induced osteoclastogenesis. NMP inhibited RANKL-induced tartrate-resistant acid phosphatase activity and the formation of tartrate-resistant acid phosphatase-positive multinucleated cells. The RANKL-induced expression of NFATc1 (nuclear factor of activated T cells, cytoplasmic 1) and c-Fos, which are key transcription factors for osteoclastogenesis, was also reduced by treatment with NMP. Furthermore, NMP induced disruption of the actin rings and decreased the mRNAs of cathepsin K and MMP-9 (matrix metalloproteinase-9), both involved in bone resorption. Taken together, these results suggest that NMP inhibits osteoclast differentiation and attenuates bone resorption. Therefore, NMP could prove useful for the treatment of osteoporosis or other bone diseases associated with excessive bone resorption.

The small molecule PKA-specific cyclic AMP analogue as an inducer of osteoblast-like cells differentiation and mineralization

Osteoblastic differentiation is an important landmark for bone formation, bone repair and regeneration; however, it is a very complex process controlled by different signalling mechanisms. Several groups have reported that the cyclic adenosine monophosphate (cAMP) signalling system is responsible for regulating osteoblast cell differentiation. Nonetheless, to date, the principle role of the cAMP molecules related to this process remains controversial. Moreover, the underlying cAMP-dependent signalling cascade governing the osteoblastic differentiation has not been clarified. In this study we investigated the roles of the cAMP-dependent protein kinase A (PKA) signalling in proliferation, differentiation and mineralization of osteoblast-like MC3T3-E1 cells, using the PKA-specific small molecule cAMP analogue, 6-Bnz-cAMP, at 100 µM. Alkaline phosphatase (ALP) activity, runt transcription factor 2 (Runx2), osteopontin (OPN) and osteocalcin (OCN) protein expressions were used as osteoblast-specific markers to demonstrate osteoblastic differentiation. Further, calcium measurement of the extracellular matrix was employed as the hallmark of matrix mineralization or calcification. We report here that activation of PKA by the small molecule 6-Bnz-cAMP induces osteoblastic differentiation and matrix mineralization of osteoblast-like MC3T3-E1 cells. Moreover, 6-Bnz-cAMP does not induce cytotoxicity to the cells, as revealed by our cell proliferation studies. Therefore, based on these findings, we propose that the PKA-specific small molecule 6-Bnz-cAMP may serve as a novel bone-inducing growth factor for repairing and regenerating bone tissues during bone-regenerative engineering.