Differential effects of ERK and p38 signaling in BMP-2 stimulated hypertrophy of cultured chick sternal chondrocytes

Osteogenic induction of asiatic acid derivatives in human periodontal ligament stem cells

Asiatic acid (AA) and asiaticoside, pentacyclic triterpenoid compounds derived from Centella asiatica, are known for their biological effects in promoting type I collagen synthesis and inducing osteogenesis of stem cells. However, their applications in regenerative medicine are limited due to their low potency and poor aqueous solubility. This work aimed to evaluate the osteogenic induction activity of AA derivatives in human periodontal ligament stem cells (hPDLSCs) in vitro. Four compounds were synthesised, namely 501, 502, 503, and 506. AA was used as the control. The 502 exhibited low water solubility, while the 506 compound showed the highest. The cytotoxicity analysis demonstrated that 503 caused significant deterioration in cell viability, while other derivatives showed no harmful effect on hPDLSCs. The dimethyl aminopropyl amine derivative of AA, compound 506, demonstrated a relatively high potency in inducing osteogenic differentiation. An elevated mRNA expression of osteogenic-related genes, BMP2, WNT3A, ALP, OSX and IBSP was observed with 506. Additionally, the expression of BMP-2 protein was enhanced with increasing dose of 506, and the effect was pronounced when the Erk signalling molecule was inhibited. The 506 derivative was proposed for the promotion of osteogenic differentiation in hPDLSCs by upregulating BMP2 via the Erk signalling pathway. The 506 molecule showed promise in bone tissue regeneration.

Update on the effects of microgravity on the musculoskeletal system

With the reignited push for manned spaceflight and the development of companies focused on commercializing spaceflight, increased human ventures into space are inevitable. However, this venture would not be without risk. The lower gravitational force, known as microgravity, that would be experienced during spaceflight significantly disrupts many physiological systems. One of the most notably affected systems is the musculoskeletal system, where exposure to microgravity causes both bone and skeletal muscle loss, both of which have significant clinical implications. In this review, we focus on recent advancements in our understanding of how exposure to microgravity affects the musculoskeletal system. We will focus on the catabolic effects microgravity exposure has on both bone and skeletal muscle cells, as well as their respective progenitor stem cells. Additionally, we report on the mechanisms that underlie bone and muscle tissue loss resulting from exposure to microgravity and then discuss current countermeasures being evaluated. We reveal the gaps in the current knowledge and expound upon how current research is filling these gaps while also identifying new avenues of study as we continue to pursue manned spaceflight.

Bone Cells Differentiation: How CFTR Mutations May Rule the Game of Stem Cells Commitment?

Cystic fibrosis (CF)-related bone disease has emerged as a significant comorbidity of CF and is characterized by decreased bone formation and increased bone resorption. Both osteoblast and osteoclast differentiations are impacted by cystic fibrosis transmembrane conductance regulator (CFTR) mutations. The defect of CFTR chloride channel or the loss of CFTRs ability to interact with other proteins affect several signaling pathways involved in stem cell differentiation and the commitment of these cells toward bone lineages. Specifically, TGF-, nuclear factor-kappa B (NF-B), PI3K/AKT, and MAPK/ERK signaling are disturbed by CFTR mutations, thus perturbing stem cell differentiation. High inflammation in patients changes myeloid lineage secretion, affecting both myeloid and mesenchymal differentiation. In osteoblast, Wnt signaling is impacted, resulting in consequences for both bone formation and resorption. Finally, CFTR could also have a direct role in osteoclasts resorptive function. In this review, we summarize the existing literature on the role of CFTR mutations on the commitment of induced pluripotent stem cells to bone cells.

Crosstalk between neuropeptides SP and CGRP in regulation of BMP2-induced bone differentiation

AIM OF THE STUDY

The peripheral nervous system is involved in regulation of bone metabolism via sensory and sympathetic innervation. Substance P (SP) and calcitonin gene-related peptide (CGRP) are two sensory neuropeptides that have been associated with regulation of osteogenic differentiation. However, the interaction between SP and CGRP both with each other and the bone morphogenetic protein 2 (BMP2) in regulation of osteogenic differentiation has not been studied. Therefore, the aim of this study was to investigate the interaction between SP and CGRP on BMP2-induced bone differentiation using model progenitor cells.

MATERIALS AND METHODS

C2C12 myoblasts and MC3T3 pre-osteoblasts were treated with SP and CGRP, both individually and in combination, in the presence of BMP2. The effects of the neuropeptides on BMP2-induced osteogenic differentiation were assessed by measuring alkaline phosphatase (ALP) activity, mineralization, and expression of osteogenic markers.

RESULTS

Both SP and CGRP enhanced BMP2 signaling, Runx2 mRNA expression, as well as mineralization in vitro. Co-stimulation with SP and CGRP resulted in down-regulation of BMP2-induced bone differentiation, suggesting potential crosstalk between the two neuropeptides in regulation of BMP2 signaling.

CONCLUSIONS

Based on the results shown here, CGRP can mitigate augmenting effects of SP on BMP2 signaling and the three pathways potentially converge on Runx2 to regulate BMP2-induced bone differentiation.

Vitamin C-linker-conjugated tripeptide AHK stimulates BMP-2-induced osteogenic differentiation of mouse myoblast C2C12 cells

Vitamin C-linker-conjugated Ala-His-Lys tripeptide (Vit C-AHK) is a derivative of Vitamin C-conjugated tripeptides, which were originally developed as a component of a product for collagen synthesis enhancement or human dermal fibroblast growth. Here, we investigated the effect of Vit C-AHK on bone morphogenetic protein (BMP)-2-induced osteoblast differentiation in a cell culture model. Vit C-AHK enhanced proliferation of C2C12 cells and induction of BMP-2-induced alkaline phosphatase, a typical marker of osteoblast differentiation. Vit C-AHK also stimulated the phosphorylation and translocation of Smad1/5/8 to the nucleus and phosphorylation of mitogen-activated protein kinases (MAPKs) including ERK1/2 and p38. In addition, Vit C-AHK enhanced the BMP-2-induced mRNA expression of osteoblast differentiation-related genes such as ALP, BMP-2, Osteocalcin, and Runx2. Our results suggest that Vit C-AHK exerts an enhancing effect on osteoblast proliferation and differentiation through activation of Smad1/5/8 and MAPK ERK1/2 and p38 signaling and without significant cytotoxicity. These results provide important data for the development of peptide-based bone-regenerative agents and treatment of bone-related disorders.

A Receptor Tyrosine Kinase Inhibitor, Dovitinib (TKI-258), Enhances BMP-2-Induced Osteoblast Differentiation In Vitro

Dovitinib (TKI258) is a small molecule multi-kinase inhibitor currently in clinical phase I/II/III development for the treatment of various types of cancers. This drug has a safe and effective pharmacokinetic/pharmacodynamic profile. Although dovitinib can bind several kinases at nanomolar concentrations, there are no reports relating to osteoporosis or osteoblast differentiation. Herein, we investigated the effect of dovitinib on human recombinant bone morphogenetic protein (BMP)-2-induced osteoblast differentiation in a cell culture model. Dovitinib enhanced the BMP-2-induced alkaline phosphatase (ALP) induction, which is a representative marker of osteoblast differentiation. Dovitinib also stimulated the translocation of phosphorylated Smad1/5/8 into the nucleus and phosphorylation of mitogen-activated protein kinases, including ERK1/2 and p38. In addition, the mRNA expression of BMP-4, BMP-7, ALP, and OCN increased with dovitinib treatment. Our results suggest that dovitinib has a potent stimulating effect on BMP-2-induced osteoblast differentiation and this existing drug has potential for repositioning in the treatment of bone-related disorders.

Hypoxia potentiates the BMP-2 driven COL2A1 stimulation in human articular chondrocytes via p38 MAPK

OBJECTIVE

Since the biological effect of cartilage mediators is generally studied in a non-physiologic environment of 21% O2, we investigated the effects of a chronic hypoxia on the capability of articular chondrocytes to respond to one anabolic stimulation.

DESIGN

Human Articular Chondrocytes (HACs) were cultured under hypoxia and stimulated with the chondrogenic growth factor BMP-2. The phenotype of the chondrocytes was studied by RT-PCR, and the cartilage-specific type II collagen production and deposition were also examined by western immunoblot and immunofluorescence. The Bone Morphogenetic protein (BMP) signalling pathway was also analysed.

RESULTS

BMP-2 is much more efficient to stimulate the expression of the cartilage-specific gene COL2A1 by HACs when cultured under hypoxia (1%O2) compared to normoxia (21%O2). Analysis of the BMP-activated signalling shows that the Smad pathway is inhibited under hypoxia, whereas p38 MAPK is activated, and is involved in a synergy between hypoxia and BMP signalling, thus contributing to the enhanced anabolic response.

CONCLUSIONS

Our study shows that hypoxia interplays with a chondrogenic factor and enhances the overall anabolic activity of the HACs. Alternatively to Hypoxia-Inducible Factor (HIF) signalling, and through a cross-talk with the BMP signalling which involves the p38 pathway, hypoxic stimulation markedly increases the capability of chondrocytes to produce the cartilage-specific type II collagen. Therefore our study provides new evidences of the multilayered effects of hypoxia in the anabolic functions of chondrocytes. This understanding may help promoting the anabolic function of articular chondrocytes, and thus improving their manipulation for cell therapy.

Low-Intensity Pulsed Ultrasound Affects Chondrocyte Extracellular Matrix Production via an Integrin-Mediated p38 MAPK Signaling Pathway

Although low-intensity pulsed ultrasound (LIPUS) regulates p38 mitogen-activated protein kinase (MAPK) and promotes cartilage repair in osteoarthritis, the role of integrin-mediated p38 MAPK in the effect of LIPUS on extracellular matrix (ECM) production of normal and OA chondrocytes remains unknown. The aim of this study was to investigate whether LIPUS affects ECM production in normal and OA rabbit chondrocytes through an integrin-p38 signaling pathway. A rabbit model of OA was established by anterior cruciate ligament transection, and chondrocytes were isolated from normal or OA cartilage and cultured in vitro. Chondrocytes were treated with LIPUS and then pre-incubated with the integrin inhibitor GRGDSP or the p38 inhibitor SB203580. Expression of type II collagen, MMP-13, integrin β1, p38 and phosphorylated p38 was assessed by Western blot analysis. We found that type II collagen and integrin β1 were upregulated (p < 0.05), whereas MMP-13 was downregulated (p < 0.05) in normal and OA chondrocytes. Furthermore, phosphorylated p38 was upregulated (p < 0.05) in normal chondrocytes, but downregulated (p < 0.05) in OA chondrocytes after LIPUS stimulation. Pre-incubation of chondrocytes with the integrin inhibitor disrupted the effects of LIPUS on normal and OA chondrocytes. Pre-incubation of chrondocytes with the p38 inhibitor reduced the effects of LIPUS on normal chondrocytes, but had no impact on OA chondrocytes. Our findings suggest that the integrin-p38 MAPK signaling pathway plays an important role in LIPUS-mediated ECM production in chondrocytes.

The high affinity ALK1-ligand BMP9 induces a hypertrophy-like state in chondrocytes that is antagonized by TGFβ1

OBJECTIVE

In osteoarthritic cartilage, expression of the receptor ALK1 correlates with markers of deleterious chondrocyte hypertrophy. Recently, bone morphogenetic protein 9 (BMP9) was identified as a high affinity ligand for ALK1. Therefore, we studied if BMP9 signaling results in expression of hypertrophy markers in chondrocytes. Furthermore, because transforming growth factorß1 (TGFβ1) is a well known anti-hypertrophic factor, the interaction between BMP9 and TGFβ1 signaling was also studied.

DESIGN

Primary chondrocytes were isolated from bovine cartilage and stimulated with BMP9 and/or TGFβ1 to measure intracellular signaling via pSmads with the use of Western blot. Expression of Smad-responsive genes or hypertrophy-marker genes was measured using qPCR. To confirm observations on TGFβ/Smad3 responsive genes, a Smad3-dependent CAGA12-luc transcriptional reporter assay was performed in the chondrocyte G6 cell line.

RESULTS

In primary chondrocytes, BMP9 potently induced phosphorylation of Smad1/5 and Smad2 to a lesser extent. BMP9-induced Smad1/5 phosphorylation was rapidly (2 h) reflected in gene expression, whereas Smad2 phosphorylation was not. Remarkably, BMP9 and TGFβ1 dose-dependently synergized on Smad2 phosphorylation, and showed an additive effect on expression of Smad3-dependent genes like bSerpine1 after 24 h. The activation of the TGFβ/Smad3 signaling cascade was confirmed using the CAGA12-luc transcriptional reporter. BMP9 selectively induced bAlpl and bColX expression, which are considered early markers of cellular hypertrophy, but this was potently antagonized by addition of a low dose of TGFβ1.

CONCLUSIONS

This study shows that in vitro in chondrocytes, BMP9 potently induces pSmad1/5 and a chondrocyte hypertrophy-like state, which is potently blocked by TGFβ1. This observation underlines the importance of TGFβ1 in maintenance of chondrocyte phenotype.

P38 mitogen-activated protein kinase promotes dedifferentiation of primary articular chondrocytes in monolayer culture

Articular cartilage is an avascular tissue with poor regenerative capacity following injury, a contributing factor to joint degenerative disease. Cell-based therapies for cartilage tissue regeneration have rapidly advanced; however, expansion of autologous chondrocytes in vitro using standard methods causes 'dedifferentiation' into fibroblastic cells. Mitogen-activated protein kinase (MAPK) signalling is crucial for chondrocyte metabolism and matrix production, and changes in MAPK signals can affect the phenotype of cultured cells. We investigated the effects of inhibition of MAPK signalling on chondrocyte dedifferentiation during monolayer culture. Blockade of extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) signalling caused a significant increase in cartilage gene expression, however, also caused up-regulation of fibrotic gene expression. Inhibition of p38 MAPK (p38) caused a significant up-regulation of collagen type II while suppressing collagen type I expression. P38 inhibition also resulted in consistently more organized secretion of collagen type II protein deposits on cell culture surfaces. Follow-on pellet culture of treated cells revealed that MAPK inhibition reduced cell migration from the pellet. ERK and JNK inhibition caused more collagen type I accumulation in pellets versus controls while p38 inhibition strongly promoted collagen type II accumulation with no effect on collagen type I. Blockade of all three MAPKs caused increased GAG content in pellets. These results indicate a role for MAPK signalling in chondrocyte phenotype loss during monolayer culture, with a strong contribution from p38 signalling. Thus, blockade of p38 enhances chondrocyte phenotype in monolayer culture and may promote more efficient cartilage tissue regeneration for cell-based therapies.

Repair of large animal partial-thickness cartilage defects through intraarticular injection of matrix-rejuvenated synovium-derived stem cells

Cartilage defects have a limited ability to self-heal. Stem cell treatment is a promising approach; however, replicative senescence is a challenge to acquiring large-quantity and high-quality stem cells for cartilage regeneration. Synovium-derived stem cells (SDSCs) are a tissue-specific stem cell for cartilage regeneration. Our recent findings suggest that decellularized stem cell matrix (DSCM) can rejuvenate expanded SDSCs in cell proliferation and chondrogenic potential. In this study, we were investigating (1) whether transforming growth factor (TGF)-β1 and TGF-β3 played a similar role in chondrogenic induction of SDSCs after expansion on either DSCM or plastic flasks (plastic), and (2) whether DSCM-expanded SDSCs had an enhanced capacity in repairing partial-thickness cartilage defects in a minipig model. SDSCs were isolated from synovium in two 3-month-old pigs and DSCM was prepared using SDSCs. Passage 2 SDSCs were expanded on either DSCM or plastic for one passage. The expanded cells were evaluated for cell morphology, chondrogenic capacity, and related mechanisms. TGF-β1 and TGF-β3 were compared for their role in chondrogenesis of SDSCs after expansion on either DSCM or plastic. The chondrogenic induction medium without TGF-β served as a control. In 13 minipigs, we intraarticularly injected DSCM- or plastic-expanded SDSCs or saline into knee partial-thickness cartilage defects and assessed their repair using histology and immunohistochemistry. We found DSCM-expanded SDSCs were small, had a fibroblast-like shape, and grew quickly in a three-dimensional format with concomitant up-regulation of phosphocyclin D1 and TGF-β receptor II. Plastic-expanded SDSCs exhibited higher mRNA levels of chondrogenic markers when incubated with TGF-β3, while DSCM-expanded SDSCs displayed comparable chondrogenic potential when treated with either TGF-β isotype. In the minipig model, DSCM-expanded SDSCs were better than plastic-expanded SDSCs in enhancing collagen II and sulfated glycosaminoglycan expression in repair of partial-thickness cartilage defects, but both groups were superior to the saline control group. Our observations suggested that DSCM is a promising cell expansion system that can promote cell proliferation and enhance expanded cell chondrogenic potential in vitro and in vivo. Our approach could lead to a tissue-specific cell expansion system providing large-quantity and high-quality stem cells for the treatment of cartilage defects.

Initiation of BMP2 signaling in domains on the plasma membrane

Bone morphogenetic protein 2 (BMP2) is a potent growth factor crucial for cell fate determination. It directs the differentiation of mesenchymal stem cells into osteoblasts, chondrocytes, adipocytes, and myocytes. Initiation of BMP2 signaling pathways occurs at the cell surface through type I and type II serine/threonine kinases housed in specific membrane domains such as caveolae enriched in the caveolin-1 beta isoform (CAV1β, caveolae) and clathrin-coated pits (CCPs). In order for BMP2 to initiate Smad signaling it must bind to its receptors on the plasma membrane resulting in the phosphorylation of the BMP type Ia receptor (BMPRIa) followed by activation of Smad signaling. The current model suggests that the canonical BMP signaling pathway, Smad, occurs in CCPs. However, several recent studies suggested Smad signaling may occur outside of CCPs. Here, we determined; (i) The location of BMP2 binding to receptors localized in caveolae, CCPs, or outside of these domains using AFM and confocal microscopy. (ii) The location of phosphorylation of BMPRIa on the plasma membrane using membrane fractionation, and (iii) the effect of down regulation of caveolae on Smad signaling. Our data indicate that BMP2 binds with highest force to BMP receptors (BMPRs) localized in caveolae. BMPRIa is phosphorylated in caveolae and the disruption of caveolae-inhibited Smad signaling in the presence of BMP2. This suggests caveolae are necessary for the initiation of Smad signaling. We propose an extension of the current model of BMP2 signaling, in which the initiation of Smad signaling is mediated by BMPRs in caveolae.

Proliferative and mineralogenic effects of insulin, IGF-1, and vanadate in fish osteoblast-like cells

Fish have recently been recognized as a suitable model and a promising alternative to mammalian systems to study skeletogenesis. In this regard, several fish bone-derived cell lines have been developed and are being used to investigate mechanisms associated with insulin-like action of vanadium on extracellular matrix (ECM) mineralization. Although proliferative and mineralogenic effects of vanadate, insulin-like growth factor 1 (IGF-1), and insulin have recently been evaluated in a fish prechondrocyte cell line, no data are available in fish bone-forming cells, the osteoblasts. Using fish preosteoblast cells, we showed that IGF-1, but not insulin or vanadate, stimulated cell proliferation through the mitogen-activated protein kinase (MAPK) pathway, while both IGF-1 and vanadate inhibited cell differentiation/ECM mineralization through the same mechanism. Our data also indicated that the phosphatidyl inositol-3 kinase (PI-3K) pathway stimulates differentiation/ECM mineralization in osteoblasts and could represent a way to balance MAPK pathway action. The comparison of these new data obtained in fish with those available in mammals clearly evidenced a conservation of regulatory mechanisms among vertebrate bone-derived systems, although different players are involved.

Osteoblastic differentiation and stress response of human mesenchymal stem cells exposed to alternating current electric fields

BACKGROUND

Electric fields are integral to many biological events, from maintaining cellular homeostasis to embryonic development to healing. The application of electric fields offers substantial therapeutic potential, while optimal dosing regimens and the underlying mechanisms responsible for the positive clinical impact are poorly understood.

METHODS

The purpose of this study was to track the differentiation profile and stress response of human bone marrow derived mesenchymal stem cells (hMSCs) undergoing osteogenic differentiation during exposure to a 20 mV/cm, 60 kHz electric field. Morphological and biochemical changes were imaged using endogenous two-photon excited fluorescence (TPEF) and quantitatively assessed through eccentricity calculations and extraction of the redox ratio from NADH, FAD and lipofuscin contributions. Real time reverse transcriptase-polymerase chain reactions (RT-PCR) were used to track osteogenic differentiation markers, namely alkaline phosphatase (ALP) and collagen type 1 (col1), and stress response markers, such as heat shock protein 27 (hsp27) and heat shock protein 70 (hsp70). Comparisons of collagen deposition between the stimulated hMSCs and controls were examined through second harmonic generation (SHG) imaging.

RESULTS

Quantitative differences in cell morphology, as described through an eccentricity ratio, were found on days 2 and days 5 (p < 0.05) in samples exposed to the electric field. A delayed but two fold increase in ALP and col1 transcript was detected by week 2 (p < 0.05) in differentiating hMSCs exposed to an electric field in comparison to the nonstimulated controls. Upregulation in stress marker, hsp27, and type 1 collagen deposition were correlated with this response. Increases in NADH, FAD, and lipofuscin were traced in the stimulation group during the first week of field exposure with differences statistically significant on day 10 (p < 0.05). Changes in hsp27 expression correlate well with changes in lipofuscin detected in the stimulation group, suggesting a connection with oxidative stress. Both differentiation factors and electrical stimulation improved hMSC differentiation potential to bone based on calcium deposition on day 28.

CONCLUSIONS

Electrical stimulation is a useful tool to improve hMSC osteogenic differentiation, while heat shock proteins may reveal underlying mechanisms, and optical non-invasive imaging may be used to monitor the induced morphological and biochemical changes.

Bone morphogenetic proteins: a critical review

Bone Morphogenetic Proteins (BMPs) are potent growth factors belonging to the Transforming Growth Factor Beta superfamily. To date over 20 members have been identified in humans with varying functions during processes such as embryogenesis, skeletal formation, hematopoiesis and neurogenesis. Though their functions have been identified, less is known regarding levels of regulation at the extracellular matrix, membrane surface, and receptor activation. Further, current models of activation lack the integration of these regulatory mechanisms. This review focuses on the different levels of regulation, ranging from the release of BMPs into the extracellular components to receptor activation for different BMPs. It also highlights areas in research that is lacking or contradictory.

Calcium-mediated stress kinase activation by DMP1 promotes osteoblast differentiation

Calcium signaling and calcium transport play a key role during osteoblast differentiation and bone formation. Here, we demonstrate that DMP1 mediated calcium signaling, and its downstream effectors play an essential role in the differentiation of preosteoblasts to fully functional osteoblasts. DMP1, a key regulatory bone matrix protein, can be endocytosed by preosteoblasts, triggering a rise in cytosolic levels of calcium that initiates a series of downstream events leading to cellular stress. These events include release of store-operated calcium that facilitates the activation of stress-induced p38 MAPK leading to osteoblast differentiation. However, chelation of intracellular calcium and inhibition of the p38 signaling pathway by specific pharmacological inhibitors and dominant negative plasmid suppressed this activation. Interestingly, activated p38 MAPK can translocate to the nucleus to phosphorylate transcription factors that coordinate the expression of downstream target genes such as Runx 2, a key modulator of osteoblast differentiation. These studies suggest a novel paradigm by which DMP1-mediated release of intracellular calcium activates p38 MAPK signaling cascade to regulate gene expression and osteoblast differentiation.

[Roles of TGF-b superfamily in the genesis, development and maintenance of cartilage]

The transforming growth factor beta (TGF-beta) superfamily is composed of TGF-beta subfamily and bone morphogenetic protein (BMP) subfamily. The ligands, ligand antagonists, receptors and intracellular transductors that engage in the TGF-beta superfamily signaling pathway play their unique roles during endochondral ossification via regulating the lineage differentiation, proliferation, maturation, apoptosis and mineralization of chondrocytes. BMP signaling dominates chondro-genesis through initiating the chondrocytic commitment of mesenchymal cells and maintaining the chondrocytic phenotype. During the development of growth plate, BMP signaling promotes the maturation of chondrocytes to facilitate ossification, whereas TGF-beta signaling inhibits the hypertrophic differentiation to preserve adequate chondrocytes within the growth plate. Both TGF-beta signaling and BMP signaling are indispensable for the maintenance and repair of articular cartilage. Therefore, it indicates that TGF-beta superfamily may function essentially all throughout the development of skeletons.

BMP canonical Smad signaling through Smad1 and Smad5 is required for endochondral bone formation

Bone morphogenetic protein (BMP) signaling is required for endochondral bone formation. However, whether or not the effects of BMPs are mediated via canonical Smad pathways or through noncanonical pathways is unknown. In this study we have determined the role of receptor Smads 1, 5 and 8 in chondrogenesis. Deletion of individual Smads results in viable and fertile mice. Combined loss of Smads 1, 5 and 8, however, results in severe chondrodysplasia. Smad1/5(CKO) (cartilage-specific knockout) mutant mice are nearly identical to Smad1/5(CKO);Smad8(-/-) mutants, indicating that Smads 1 and 5 have overlapping functions and are more important than Smad8 in cartilage. The Smad1/5(CKO) phenotype is more severe than that of Smad4(CKO) mice, challenging the dogma, at least in chondrocytes, that Smad4 is required to mediate Smad signaling through BMP pathways. The chondrodysplasia in Smad1/5(CKO) mice is accompanied by imbalances in cross-talk between the BMP, FGF and Ihh/PTHrP pathways. We show that Ihh is a direct target of BMP pathways in chondrocytes, and that FGF exerts antagonistic effects on Ihh expression. Finally, we tested whether FGF exerts its antagonistic effects directly through Smad linker phosphorylation. The results support the alternative conclusion that the effects of FGFs on BMP signaling are indirect in vivo.

The PI3K pathway regulates endochondral bone growth through control of hypertrophic chondrocyte differentiation

Background

The majority of our bones develop through the process of endochondral ossification that involves chondrocyte proliferation and hypertrophic differentiation in the cartilage growth plate. A large number of growth factors and hormones have been implicated in the regulation of growth plate biology, however, less is known about the intracellular signaling pathways involved. PI3K/Akt has been identified as a major regulator of cellular proliferation, differentiation and death in multiple cell types.

Results and Discussion

Employing an organ culture system of embryonic mouse tibiae and LY294002, a pharmacological inhibitor of PI3K, we show that inhibition of the pathway results in significant growth reduction, demonstrating that PI3K is required for normal endochondral bone growth in vitro. PI3K inhibition reduces the length of the proliferating and particularly of the hypertrophic zone. Studies with organ cultures and primary chondrocytes in micromass culture show delayed hypertrophic differentiation of chondrocytes and increased apoptosis in the presence of LY294002. Surprisingly, PI3K inhibition had no strong effect on IGF1-induced bone growth, but partially blocked the anabolic effects of C-type natriuretic peptide.

Conclusion

Our data demonstrate an essential role of PI3K signaling in chondrocyte differentiation and as a consequence of this, in the endochondral bone growth process.

A comparison of the involvement of p38, ERK1/2 and PI3K in growth factor-induced chondrogenic differentiation of mesenchymal stem cells

Adult mesenchymal stem cells (MSCs) are under investigation as an alternative cell source for the engineering of cartilage tissue in three-dimensional (3D) scaffolds. However, little is known about the intracellular mechanisms involved in the chondrogenic differentiation of MSCs. This study investigated the signaling pathways evoked by TGF-beta1 and IGF-1 that mediated chondrogenic differentiation in adult rat bone-marrow derived MSCs in (i) monolayer on plastic and (ii) a 3D collagen-GAG scaffold. The data demonstrated involvement of the p38 pathway, but not ERK1/2 or PI3K in TGF-beta1-induced chondrogenic differentiation in monolayer. Similarly, when the MSCs were seeded onto a collagen-GAG scaffold and treated with TGF-beta1, the chondrogenic differentiation was dependent upon p38. In contrast, IGF-1-induced chondrogenic differentiation in monolayer involved p38, ERK1/2, as well as PI3K. The phosphorylation of Akt occurred downstream of PI3K and phospho-Akt was found to accumulate in the nucleus of IGF-1-treated cells. When MSCs were seeded onto the collagen-GAG scaffold and exposed to IGF-1, PI3K was required for chondrogenesis. These findings highlight the respective and differential involvement of p38, ERK1/2 and PI3K in growth factor-induced chondrogenesis of MSCs and demonstrates that intracellular signaling pathways are similar when differentiation is stimulated in a 2D or 3D environment.

Alkaline phosphatase reactivity in the vagina and uterovaginal junction sperm-storage tubules of turkeys in egg production: implications for sperm storage

1. Currently there remains contradictory information on the localisation and possible role of alkaline phosphatase (AP) in the chicken and Japanese quail oviducts. 2. Using turkeys with a hard-shelled egg in their uteri, vaginal and uterovaginal junction mucosae were stretched and fixed as whole mounts prior to the histochemical localisation of AP activity. 3. Scattered AP reactive cells were observed in the vaginal and uterovaginal junction surface epithelia and intense AP reactivity of the sperm-storage tubule (SST) epithelium, localised to its apical border. 4. We suggest that such AP reactivity in hens in egg production may reflect cell differentiation and proliferation in the vagina and SST and possibly a mechanism for the transfer of lipid from the SST epithelia to resident sperm.

Hyperosmotic stress-induced apoptotic signaling pathways in chondrocytes

Articular chondrocytes have a well-developed osmoregulatory system that enables cells to survive in a constantly changing osmotic environment. However, osmotic loading exceeding that occurring under physiological conditions severely compromises chondrocyte function and leads to degenerative changes. The aim of the present study was to investigate the form of cell death and changes in apoptotic signaling pathways under hyperosmotic stress using a primary chondrocyte culture. Cell viability and apoptosis assays performed with annexin V and propidium iodide staining showed that a highly hyperosmotic medium (600 mOsm) severely reduced chondrocyte viability and led mainly to apoptotic cell death, while elevating osmotic pressure within the physiological range caused no changes compared to isosmotic conditions. Western blot analysis revealed that a 600 mOsm hyperosmotic environment induced the activation of proapoptotic members of the mitogen-activated protein kinase family such as c-Jun N-terminal kinase (JNK) and p38, and led to an increased level of extracellular signal regulated kinase (ERK1/2). Hyperosmotic stress also induced the activation of caspase-3. In summary, our results show that hyperosmotic stress leads to mainly apoptotic cell death via the involvement of proapoptotic signaling pathways in a primary chondrocyte culture.

BMP signaling in the cartilage growth plate

Transforming growth factor-beta (TGF-beta) superfamily members play diverse roles in all aspects of cartilage development and maintenance. It is well established that TGF-betas and bone morphogenetic proteins (BMPs) play distinct roles in the growth plate. This chapter discusses key experiments and experimental approaches that have revealed these roles, and progress toward the identification of previously unsuspected roles. Current understanding of the mechanisms by which different TGF-beta and BMP pathways exert their functions is discussed. Finally attempts to utilize this information to promote cartilage regeneration, and important issues for future research, are outlined.

The BMP signaling and in vivo bone formation

Bone morphogenetic proteins (BMPs) are multi-functional growth factors that belong to the transforming growth factor beta (TGFbeta) superfamily. The roles of BMPs in embryonic development and cellular functions in postnatal and adult animals have been extensively studied in recent years. Signal transduction studies have revealed that Smads 1, 5 and 8 are the immediate downstream molecules of BMP receptors and play a central role in BMP signal transduction. Studies from transgenic and knockout mice and from animals and humans with naturally occurring mutations in BMPs and their signaling molecules have shown that BMP signaling plays critical roles in bone and cartilage development and postnatal bone formation. BMP activities are regulated at different molecular levels. Tissue-specific knockout of a specific BMP ligand, a subtype of BMP receptors or a specific signaling molecule is required to further determine the specific role of a BMP ligand, receptor or signaling molecule in a particular tissue.