The p38α MAPK positively regulates osteoblast function and postnatal bone acquisition

Bone continuously remodels throughout life by coordinated actions of osteoclasts and osteoblasts. Abnormalities in either osteoclast or osteoblast functions lead to bone disorders. The p38 MAPK pathway has been shown to be essential in controlling osteoblast differentiation and skeletogenesis. Although p38α is the most abundant p38 member in osteoblasts, its specific individual contribution in regulating postnatal osteoblast activity and bone metabolism is unknown. To elucidate the specific role of p38α in regulating osteoblast function and bone homeostasis, we generated mice lacking p38α in differentiated osteoblasts. Osteoblast-specific p38a knockout mice were of normal weight and size. Despite non-significant bone alterations until 5weeks of age, mutant mice demonstrated significant and progressive decrease in bone mineral density from that age. Adult mice deficient in p38a in osteoblasts displayed a striking reduction in cancellous bone volume at both axial and appendicular skeletal sites. At 6months of age, trabecular bone volume was reduced by 62% in those mice. Mutant mice also exhibited progressive decrease in cortical thickness of long bones. These abnormalities correlated with decreased endocortical and trabecular bone formation rate and reduced expressions of type 1 collagen, alkaline phosphatase, osteopontin and osteocalcin whereas bone resorption and osteoclasts remained unaffected. Finally, osteoblasts lacking p38α showed impaired marker gene expressions and defective mineralization in vitro. These findings indicate that p38α is an essential positive regulator of osteoblast function and postnatal bone formation in viv

Origine, caractérisation et rôles des vésicules matricielles dans la minéralisation physiologique et pathologique.

Matrix vesicles (MVs) are involved in the initiation of mineralization in tissues undergoing physiological and pathological calcification. Pyrophosphate (PPi) has a dual effect on mineralization: a source of phosphate (Pi) to sustain hydroxyapatite (HA) formation and an inhibitor of HA growth. We found that formation of HA was optimal when the Pi/PPi molar ratio was above 140, while calcium pyrophosphate dihydrate, identified in osteoarthritis was exclusively produced by MVs when the ratio was below 6. Proteomic analysis and lipid compositions on both MVs and microvilli from osteoblast-like Saos-2 cells, revealed that MVs have an endoplasmic reticular origin and characteristic lipids and proteins as in lipid rafts. Finally, we demonstrated that MV release from microvilli is caused by the concomitant actions of actin-depolymerizing and contractile motor proteins. Proteins involved in MV biogenesis could be new therapeutic targets to prevent pathological calcification.Les vésicules matricielles (VM) sont impliquées dans l'initiation des minéralisations physiologique ou pathologique. Le pyrophosphate (PPi) est une source de phosphate (Pi) pour maintenir la formation d'hydroxyapatite (HA) mais aussi un inhibiteur de la croissance de ces minéraux. Nous avons montré que la formation d'HA était optimale lorsque le rapport molaire Pi/PPi était supérieur à 140, tandis que du calcium pyrophosphate dihydraté, marqueur de l'arthrose, était exclusivement formé lorsque ce rapport était inférieur à 6. Des analyses protéomiques et en compositions lipidiques sur les VM et les microvillosités des cellules Saos-2 ont révélé que les VM étaient formées dans le réticulum endoplasmique et qu'elles possèdent des lipides et protéines caractéristiques de radeaux lipidiques. Finalement, nous avons montré que les VM sont libérées à partir des microvillosités grâce aux actions coordonnées de protéines dépolymérisant l'actine et de protéines contractiles. Les protéines impliquées dans la biogenèse des VM peuvent être des nouvelles cibles thérapeutiques pour prévenir des calcifications pathologiques

Origine, caractérisation et rôles des vésicules matricielles dans la minéralisation physiologique et pathologique.

Matrix vesicles (MVs) are involved in the initiation of mineralization in tissues undergoing physiological and pathological calcification. Pyrophosphate (PPi) has a dual effect on mineralization: a source of phosphate (Pi) to sustain hydroxyapatite (HA) formation and an inhibitor of HA growth. We found that formation of HA was optimal when the Pi/PPi molar ratio was above 140, while calcium pyrophosphate dihydrate, identified in osteoarthritis was exclusively produced by MVs when the ratio was below 6. Proteomic analysis and lipid compositions on both MVs and microvilli from osteoblast-like Saos-2 cells, revealed that MVs have an endoplasmic reticular origin and characteristic lipids and proteins as in lipid rafts. Finally, we demonstrated that MV release from microvilli is caused by the concomitant actions of actin-depolymerizing and contractile motor proteins. Proteins involved in MV biogenesis could be new therapeutic targets to prevent pathological calcification.Les vésicules matricielles (VM) sont impliquées dans l'initiation des minéralisations physiologique ou pathologique. Le pyrophosphate (PPi) est une source de phosphate (Pi) pour maintenir la formation d'hydroxyapatite (HA) mais aussi un inhibiteur de la croissance de ces minéraux. Nous avons montré que la formation d'HA était optimale lorsque le rapport molaire Pi/PPi était supérieur à 140, tandis que du calcium pyrophosphate dihydraté, marqueur de l'arthrose, était exclusivement formé lorsque ce rapport était inférieur à 6. Des analyses protéomiques et en compositions lipidiques sur les VM et les microvillosités des cellules Saos-2 ont révélé que les VM étaient formées dans le réticulum endoplasmique et qu'elles possèdent des lipides et protéines caractéristiques de radeaux lipidiques. Finalement, nous avons montré que les VM sont libérées à partir des microvillosités grâce aux actions coordonnées de protéines dépolymérisant l'actine et de protéines contractiles. Les protéines impliquées dans la biogenèse des VM peuvent être des nouvelles cibles thérapeutiques pour prévenir des calcifications pathologiques

Sclerostin inhibits osteoblast differentiation without affecting BMP2/SMAD1/5 or Wnt3a/β-catenin signaling but through activation of platelet-derived growth factor receptor signaling in vitro

Sclerostin inhibits bone formation mostly by antagonizing LRP5/6, thus inhibiting Wnt signaling. However, experiments with genetically modified mouse models suggest that a significant part of sclerostin-mediated inhibition of bone formation is due to interactions with other binding partners. The objective of the present work was to identify signaling pathways affected by sclerostin in relation with its inhibitory action on osteogenic differentiation of C3H10T1/2 cells, MC3T3-E1 cells and primary osteoblasts. Sclerostin inhibited BMP2-induced osteoblast differentiation without altering SMAD1/5 phosphorylation and transcriptional activity. Moreover, sclerostin prevented Wnt3a-mediated osteoblastogenesis without affecting LRP5/6 phosphorylation or β-catenin transcriptional activity. In addition, sclerostin inhibited mineralization promoted by GSK3 inhibition, which mimics canonical Wnt signaling without activation of LRP5/6, suggesting that sclerostin can prevent osteoblast differentiation without antagonizing LRP5/6. Finally, we found that sclerostin could activate platelet-derived growth factor receptor (PDGFR) and its downstream signaling pathways PLCγ, PKC, Akt and ERK1/2. PDGFR inhibition could reverse sclerostin-mediated inhibitory activity on BMP2-induced osteoblast differentiation. Therefore, our data suggest that sclerostin can activate PDGFR signaling by itself, and this functional interaction may be involved in the negative effect of sclerostin on osteoblast differentiation

Activation of FGF receptors is a new mechanism by which strontium ranelate induces osteoblastic cell growth

Strontium ranelate (SrRan) is an anti-osteoporotic treatment that reduces the risk of vertebral and hip fractures. Recent in vitro studies suggest that the effect of strontium ranelate on osteoblastic cell growth likely involves two processes including activation of the calcium sensing receptor (CaSR) and a yet undefined mechanism. In the present study, we investigated the CaSR-independent molecular mechanism by which SrRan stimulates osteoblast growth

Ablation of p38α MAPK Signaling in Osteoblast Lineage Cells Protects Mice From Bone Loss Induced by Estrogen Deficiency

Estrogen deficiency causes bone loss by increasing the number of bone-resorbing osteoclasts. Selective p38α MAPK inhibitors prevent bone-wasting effects of estrogen withdrawal but implicated mechanisms remain to be identified. Here, we show that inactivation of the p38α-encoding gene in osteoblast lineage cells with the use of an osteocalcin-cre transgene protects mice from ovariectomy-induced bone loss (a murine model of postmenopausal osteoporosis). Ovariectomy fails to induce bone loss, increase bone resorption, and stimulate receptor activator of nuclear factor κB ligand and IL-6 expression in mice lacking p38α in osteoblasts and osteocytes. Finally, TNFα or IL-1, which are osteoclastogenic cytokines overproduced in the bone marrow under estrogen deficiency, can activate p38α signaling in osteoblasts, but those cytokines cannot enhance Rankl and Il6 expressions or increase osteoclast formation in p38a-deficient osteoblast cultures. These findings demonstrate that p38α MAPK signaling in osteoblast lineage cells mediates ovariectomy-induced bone loss by up-regulating receptor activator of nuclear factor κB ligand and IL-6 production.</jats:p

Focus on the p38 MAPK signaling pathway in bone development and maintenance

The p38 mitogen-activated protein kinase (MAPK) signaling pathway can be activated in response to a wide range of extracellular signals. As a consequence, it can generate many different biological effects that depend on the stimulus and on the activated cell type. Therefore, this pathway has been found to regulate many aspects of tissue development and homeostasis. Recent work with the aid of genetically modified mice has highlighted the physiological functions of this pathway in skeletogenesis and postnatal bone maintenance. In this review, emphasis is given to the roles of the p38 MAPK pathway in chondrocyte, osteoblast and osteoclast biology. In particular, we describe the molecular mechanisms of p38 MAPK activation and downstream targets. The requirement of this pathway in physiological bone development and homeostasis is demonstrated by the ability of p38 MAPK to regulate master transcription factors controlling geneses and functions of chondrocytes, osteoblasts and osteoclasts