Role of Physico-Chemical and Cellular Conditions on the Bone Repair Potential of Plastically Compressed Collagen Hydrogels

Since their first description nearly 20 years ago, dense collagen hydrogels obtained by plastic compression have become popular scaffolds in tissue engineering. In particular, when seeded with dental pulp stem cells, they have demonstrated a great in vivo potential in cranial bone repair. Here, we investigated how physico-chemical and cell-seeding conditions could influence the formation and in vitro mineralization of these cellularized scaffolds. A qualitative assessment demonstrated that the gel stability before and after compression was highly sensitive to the conditions of fibrillogenesis, especially initial acid acetic and buffer concentrations. Gels with similar rheological properties but different fibrillar structures that exhibited different stabilities when used for the 3D culture of Stem cells from Human Exfoliated Deciduous teeth (SHEDs) could be prepared. Finally, in our optimal physico-chemical conditions, mineralization could be achieved only using human dental pulp stem cells (hDPSCs) at a high cell density. These results highlight the key role of fibrillogenic conditions and cell type/density on the bone repair potential of cell-laden plastically compressed collagen hydrogels.

Multiple congenital malformations arise from somatic mosaicism for constitutively active Pik3ca signaling

Recurrent missense mutations of the PIK3CA oncogene are among the most frequent drivers of human cancers. These often lead to constitutive activation of its product p110α, a phosphatidylinositol 3-kinase (PI3K) catalytic subunit. In addition to causing a broad range of cancers, the H1047R mutation is also found in affected tissues of a distinct set of congenital tumors and malformations. Collectively termed PIK3CA-related disorders (PRDs), these lead to overgrowth of brain, adipose, connective and musculoskeletal tissues and/or blood and lymphatic vessel components. Vascular malformations are frequently observed in PRD, due to cell-autonomous activation of PI3K signaling within endothelial cells. These, like most muscle, connective tissue and bone, are derived from the embryonic mesoderm. However, important organ systems affected in PRDs are neuroectodermal derivatives. To further examine their development, we drove the most common post-zygotic activating mutation of Pik3ca in neural crest and related embryonic lineages. Outcomes included macrocephaly, cleft secondary palate and more subtle skull anomalies. Surprisingly, Pik3ca-mutant subpopulations of neural crest origin were also associated with widespread cephalic vascular anomalies. Mesectodermal neural crest is a major source of non-endothelial connective tissue in the head, but not the body. To examine the response of vascular connective tissues of the body to constitutive Pik3ca activity during development, we expressed the mutation by way of an Egr2 (Krox20) Cre driver. Lineage tracing led us to observe new lineages that had normally once expressed Krox20 and that may be co-opted in pathogenesis, including vascular pericytes and perimysial fibroblasts. Finally, Schwann cell precursors having transcribed either Krox20 or Sox10 and induced to express constitutively active PI3K were associated with vascular and other tumors. These murine phenotypes may aid discovery of new candidate human PRDs affecting craniofacial and vascular smooth muscle development as well as the reciprocal paracrine signaling mechanisms leading to tissue overgrowth.

Dental pulp stem cells as a promising model to study imprinting diseases

Parental imprinting is an epigenetic process leading to monoallelic expression of certain genes depending on their parental origin. Imprinting diseases are characterized by growth and metabolic issues starting from birth to adulthood. They are mainly due to methylation defects in imprinting control region that drive the abnormal expression of imprinted genes. We currently lack relevant animal or cellular models to unravel the pathophysiology of growth failure in these diseases. We aimed to characterize the methylation of imprinting regions in dental pulp stem cells and during their differentiation in osteogenic cells (involved in growth regulation) to assess the interest of this cells in modeling imprinting diseases. We collected dental pulp stem cells from five controls and four patients (three with Silver-Russell syndrome and one with Beckwith-Wiedemann syndrome). Methylation analysis of imprinting control regions involved in these syndromes showed a normal profile in controls and the imprinting defect in patients. These results were maintained in dental pulp stem cells cultured under osteogenic conditions. Furthermore, we confirmed the same pattern in six other loci involved in imprinting diseases in humans. We also confirmed monoallelic expression of H19 (an imprinted gene) in controls and its biallelic expression in one patient. Extensive imprinting control regions methylation analysis shows the strong potential of dental pulp stem cells in modeling imprinting diseases, in which imprinting regions are preserved in culture and during osteogenic differentiation. This will allow to perform in vitro functional and therapeutic tests in cells derived from dental pulp stem cells and generate other cell-types.

Microvascular maturation by mesenchymal stem cells in vitro improves blood perfusion in implanted tissue constructs

Blood perfusion of grafted tissue constructs is a hindrance to the success of stem cell-based therapies by limiting cell survival and tissue regeneration. Implantation of a pre-vascularized network engineered in vitro has thus emerged as a promising strategy for promoting blood supply deep into the construct, relying on inosculation with the host vasculature. We aimed to fabricate in vitro tissue constructs with mature microvascular networks, displaying perivascular recruitment and basement membrane, taking advantage of the angiogenic properties of dental pulp stem cells and self-assembly of endothelial cells into capillaries. Using digital scanned light-sheet microscopy, we characterized the generation of dense microvascular networks in collagen hydrogels and established parameters for quantification of perivascular recruitment. We also performed original time-lapse analysis of stem cell recruitment. These experiments demonstrated that perivascular recruitment of dental pulp stem cells is driven by PDGF-BB. Recruited stem cells participated in deposition of vascular basement membrane and vessel maturation. Mature microvascular networks thus generated were then compared to those lacking perivascular coverage generated using stem cell conditioned medium. Implantation in athymic nude mice demonstrated that in vitro maturation of microvascular networks improved blood perfusion and cell survival within the construct. Taken together, these data demonstrate the strong potential of in vitro production of mature microvasculature for improving cell-based therapies.

Caspase-12 Is Present During Craniofacial Development and Participates in Regulation of Osteogenic Markers

Caspases are evolutionary conserved proteases traditionally known as participating in apoptosis and inflammation but recently discovered also in association with other processes such as proliferation or differentiation. This investigation focuses on caspase-12, ranked among inflammatory caspases but displaying other, not yet defined functions. A screening analysis pointed to statistically significant (P < 0.001) increase in expression of caspase-12 in a decisive period of mandibular bone formation when the original mesenchymal condensation turns into vascularized bone tissue. Immunofluorescence analysis confirmed the presence of caspase-12 protein in osteoblasts. Therefore, the osteoblastic cell line MC3T3-E1 was challenged to investigate any impact of caspase-12 on the osteogenic pathways. Pharmacological inhibition of caspase-12 in MC3T3-E1 cells caused a statistically significant decrease in expression of some major osteogenic genes, including those for alkaline phosphatase, osteocalcin and Phex. This downregulation was further confirmed by an alkaline phosphatase activity assay and by a siRNA inhibition approach. Altogether, this study demonstrates caspase-12 expression and points to its unknown physiological engagement in bone cells during the course of craniofacial development.

Priming Dental Pulp Stem Cells from Human Exfoliated Deciduous Teeth with Fibroblast Growth Factor-2 Enhances Mineralization Within Tissue-Engineered Constructs Implanted in Craniofacial Bone Defects

The craniofacial area is prone to trauma or pathologies often resulting in large bone damages. One potential treatment option is the grafting of a tissue-engineered construct seeded with adult mesenchymal stem cells (MSCs). The dental pulp appears as a relevant source of MSCs, as dental pulp stem cells display strong osteogenic properties and are efficient at bone formation and repair. Fibroblast growth factor-2 (FGF-2) and/or hypoxia primings were shown to boost the angiogenesis potential of dental pulp stem cells from human exfoliated deciduous teeth (SHED). Based on these findings, we hypothesized here that these primings would also improve bone formation in the context of craniofacial bone repair. We found that both hypoxic and FGF-2 primings enhanced SHED proliferation and osteogenic differentiation into plastically compressed collagen hydrogels, with a much stronger effect observed with the FGF-2 priming. After implantation in immunodeficient mice, the tissue-engineered constructs seeded with FGF-2 primed SHED mediated faster intramembranous bone formation into critical size calvarial defects than the other groups (no priming and hypoxia priming). The results of this study highlight the interest of FGF-2 priming in tissue engineering for craniofacial bone repair. Stem Cells Translational Medicine 2019;8:844&857.

Growth-dependent phenotype in FasL-deficient mandibular/alveolar bone

FASL (CD178) is known for its role in triggering apoptosis, mostly in relation with immune cells but additional functions have been reported more recently, including those in bone development. Examination of postnatal FasL-deficient mice (gld) showed an increased bone deposition in adult mice when compared with wild types. However, a different phenotype was observed prenatally, when the gld bone was underdeveloped. The aim of the following investigation was to evaluate this indication for an growth-dependent bone phenotype of gld mice and to search for the 'switch point'. This study focused on the mandibular/alveolar bone as an important structure for tooth anchorage. In vivo micro-computed tomography (CT) analysis was performed at different stages during the first month (6, 12 and 24 days) of postnatal bone development. In 6-day-old gld mice, a decrease in bone volume/tissue volume (BV/TV), trabecular thickness and trabecular number was revealed. In contrast, the 12-day-old gld mice showed an increased BV/TV and trabecular thickness in the alveolar bone. The same observation applied for bone status in 24-day-old gld mice. Therefore, changes in the bone phenotype occurred between day 6 and 12 of the postnatal development. The switch point is likely related to the changing proportion of bone cells at these stages of development, when the number of osteocytes increases. Indeed, the immunohistochemical analysis of FASL localized this protein in osteoblasts, whereas osteocytes were mostly negative at examined stages. The impact of FASL particularly on osteoblasts would agree with an earlier in vivo observed effect of FASL deficiency on expression of Mmp2, typical for osteoblasts, in the gld mandibular/alveolar bone. Notably, an age-dependent bone phenotype was reported in Mmp2-deficient mice.

Early angiogenesis detected by PET imaging with 64Cu-NODAGA-RGD is predictive of bone critical defect repair

Therapies using stem cells may be applicable to all fields of regenerative medicine, including craniomaxillofacial surgery. Dental pulp stem cells (DPSCs) have demonstrated in vitro and in vivo osteogenic and proangiogenic properties. The aim of the study was to evaluate whether early angiogenesis investigated by nuclear imaging can predict bone formation within a mouse critical bone defect. Two symmetrical calvarial critical-sized defects were created. Defects were left empty or filled with i) DPSC-containing dense collagen scaffold, ii) 5% hypoxia-primed DPSC-containing dense collagen scaffold, iii) acellular dense collagen scaffold, or iv) left empty. Early angiogenesis assessed by PET using ⁶⁴Cu-NODAGA-RGD as a tracer was found to be correlated with bone formation determined by micro-CT within the defects from day 30, and to be correlated to the late calcium apposition observed at day 90 using ¹⁸F-Na PET. These results suggest that nuclear imaging of angiogenesis, a technique applicable in clinical practice, is a promising approach for early prediction of bone grafting outcome, thus potentially allowing to anticipate alternative regenerative strategies. STATEMENT OF SIGNIFICANCE: Bone defects are a major concern in medicine. As life expectancy increases, the number of bone lesions grows, and occurring complications lead to a delay or even lack of consolidation. Therefore, to be able to predict healing or the absence of scarring at early times would be very interesting. This would not "waste time" for the patient. We report here that early nuclear imaging of angiogenesis, using ⁶⁴Cu-NODAGA-RGD as a tracer, associated with nuclear imaging of mineralization, using ¹⁸F-Na as a tracer, is correlated to late bone healing objectivized by classical histology and microtomography. This nuclear imaging represents a promising approach for early prediction of bone grafting outcome in clinical practice, thus potentially allowing to anticipate alternative regenerative strategies.

Coupling Activation of Pro-Apoptotic Caspases With Autophagy in the Meckel´s Cartilage

Mammalian Meckel´s cartilage is a temporary structure associated with mandible development. Notably, its elimination is not executed by apoptosis, and autophagy was suggested as the major mechanism. Simultaneous reports point to pro-apoptotic caspases as novel participants in autophagic pathways in general. The aim of this research was to find out whether activation of pro-apoptotic caspases (-2, -3, -6, -7, -8 and -9) was associated with autophagy of the Meckel´s cartilage chondrocytes. Active caspases were examined in serial histological sections of mouse mandible using immunodetection and were correlated with incidence of autophagy based on Beclin-1 expression. Caspase-2 and caspase-8 were found in Beclin-1 positive regions, whereas caspase-3, -6, -7 and -9 were not present. Caspase-8 was further correlated with Fas/FasL and HIF-1alpha, potential triggers for its activation. Some Fas and FasL positivity was observed in the chondrocytes but caspase-8 activation was found also in FasL deficient cartilage. HIF-1alpha was abundantly present in the hypertrophic chondrocytes. Taken together, caspase-8 activation in the Meckel´s cartilage was demonstrated for the first time. Caspase-8 and caspase-2 were the only pro-apoptotic caspases detected in the Beclin-1 positive segment of the cartilage. Activation of caspase-8 appears FasL/Fas independent but may be switched on by HIF-1alpha.

FasL Modulates Expression of Mmp2 in Osteoblasts

FasL is a well-known actor in the apoptotic pathways but recent reports have pointed to its important novel roles beyond cell death, as observed also for bone cells. This is supported by non-apoptotic appearance of FasL during osteogenesis and by significant bone alterations unrelated to apoptosis in FasL deficient (gld) mice. The molecular mechanism behind this novel role has not yet been revealed. In this report, intramembranous bone, where osteoblasts differentiate directly from mesenchymal precursors without intermediary chondrogenic step, was investigated. Mouse mandibular bone surrounding the first lower molar was used as a model. The stage where a complex set of bone cells (osteoblasts, osteocytes, osteoclasts) is first present during development was selected for an initial examination. Immunohistochemical staining detected FasL in non-apoptotic cells at this stage. Further, FasL deficient vs. wild type samples subjected to osteogenic PCR Array analysis displayed a significantly decreased expression of Mmp2 in gld bone. To examine the possibility of this novel FasL–Mmp2 relationship, intramembranous bone-derived osteoblastic cells (MC3T3-E1) were treated with anti-FasL antibody or rmFasL. Indeed, the FasL neutralization caused a decreased expression of Mmp2 and rmFasL added to the cells resulted in the opposite effect. Since Mmp2^-/- mice display age-dependent alterations in the intramembranous bone, early stages of gld mandibular bone were examined and age-dependent phenotype was confirmed also in gld mice. Taken together, the present in vivo and in vitro findings point to a new non-apoptotic function of FasL in bone development associated with Mmp2 expression.

Phosphorylated and Non-phosphorylated Leucine Rich Amelogenin Peptide Differentially Affect Ameloblast Mineralization

The Leucine Rich Amelogenin Peptide (LRAP) is a product of alternative splicing of the amelogenin gene. As full length amelogenin, LRAP has been shown, in precipitation experiments, to regulate hydroxyapatite (HAP) crystal formation depending on its phosphorylation status. However, very few studies have questioned the impact of its phosphorylation status on enamel mineralization in biological models. Therefore, we have analyzed the effect of phosphorylated (+P) or non-phosphorylated (−P) LRAP on enamel formation in ameloblast-like cell lines and ex vivo cultures of murine postnatal day 1 molar germs. To this end, the mineral formed was analyzed by micro-computed tomography, Field Emission Scanning Electron Microscopy, Transmission Electron Microscopy, Selected Area Electon Diffraction imaging. Amelogenin gene transcription was evaluated by qPCR analysis. Our data show that, in both cells and germ cultures, LRAP is able to induce an up-regulation of amelogenin transcription independently of its phosphorylation status. Mineral formation is promoted by LRAP(+P) in all models, while LRAP(–P) essentially affects HAP crystal formation through an increase in crystal length and organization in ameloblast-like cells. Altogether, these data suggest a differential effect of LRAP depending on its phosphorylation status and on the ameloblast stage at the time of treatment. Therefore, LRAP isoforms can be envisioned as potential candidates for treatment of enamel lesions or defects and their action should be further evaluated in pathological models.

Phosphate induces formation of matrix vesicles during odontoblast-initiated mineralization in vitro

Mineralization is a process of deposition of calcium phosphate crystals within a fibrous extracellular matrix (ECM). In mineralizing tissues, such as dentin, bone and hypertrophic cartilage, this process is initiated by a specific population of extracellular vesicles (EV), called matrix vesicles (MV). Although it has been proposed that MV are formed by shedding of the plasma membrane, the cellular and molecular mechanisms regulating formation of mineralization-competent MV are not fully elucidated. In these studies, 17IIA11, ST2, and MC3T3-E1 osteogenic cell lines were used to determine how formation of MV is regulated during initiation of the mineralization process. In addition, the molecular composition of MV secreted by 17IIA11 cells and exosomes from blood and B16-F10 melanoma cell line was compared to identify the molecular characteristics distinguishing MV from other EV. Western blot analyses demonstrated that MV released from 17IIA11 cells are characterized by high levels of proteins engaged in calcium and phosphate regulation, but do not express the exosomal markers CD81 and HSP70. Furthermore, we uncovered that the molecular composition of MV released by 17IIA11 cells changes upon exposure to the classical inducers of osteogenic differentiation, namely ascorbic acid and phosphate. Specifically, lysosomal proteins Lamp1 and Lamp2a were only detected in MV secreted by cells stimulated with osteogenic factors. Quantitative nanoparticle tracking analyses of MV secreted by osteogenic cells determined that standard osteogenic factors stimulate MV secretion and that phosphate is the main driver of their secretion. On the molecular level, phosphate-induced MV secretion is mediated through activation of extracellular signal-regulated kinases Erk1/2 and is accompanied by re-organization of filamentous actin. In summary, we determined that mineralization-competent MV are distinct from exosomes, and we identified a new role of phosphate in the process of ECM mineralization. These data provide novel insights into the mechanisms of MV formation during initiation of the mineralization process.

Priming Dental Pulp Stem Cells With Fibroblast Growth Factor-2 Increases Angiogenesis of Implanted Tissue-Engineered Constructs Through Hepatocyte Growth Factor and Vascular Endothelial Growth Factor Secretion

Tissue engineering strategies based on implanting cellularized biomaterials are promising therapeutic approaches for the reconstruction of large tissue defects. A major hurdle for the reliable establishment of such therapeutic approaches is the lack of rapid blood perfusion of the tissue construct to provide oxygen and nutrients. Numerous sources of mesenchymal stem cells (MSCs) displaying angiogenic potential have been characterized in the past years, including the adult dental pulp. Establishment of efficient strategies for improving angiogenesis in tissue constructs is nevertheless still an important challenge. Hypoxia was proposed as a priming treatment owing to its capacity to enhance the angiogenic potential of stem cells through vascular endothelial growth factor (VEGF) release. The present study aimed to characterize additional key factors regulating the angiogenic capacity of such MSCs, namely, dental pulp stem cells derived from deciduous teeth (SHED). We identified fibroblast growth factor-2 (FGF-2) as a potent inducer of the release of VEGF and hepatocyte growth factor (HGF) by SHED. We found that FGF-2 limited hypoxia-induced downregulation of HGF release. Using three-dimensional culture models of angiogenesis, we demonstrated that VEGF and HGF were both responsible for the high angiogenic potential of SHED through direct targeting of endothelial cells. In addition, FGF-2 treatment increased the fraction of Stro-1+/CD146+ progenitor cells. We then applied in vitro FGF-2 priming to SHED before encapsulation in hydrogels and in vivo subcutaneous implantation. Our results showed that FGF-2 priming is more efficient than hypoxia at increasing SHED-induced vascularization compared with nonprimed controls. Altogether, these data demonstrate that FGF-2 priming enhances the angiogenic potential of SHED through the secretion of both HGF and VEGF.

EMMPRIN/CD147 deficiency disturbs ameloblast-odontoblast cross-talk and delays enamel mineralization

Tooth development is regulated by a series of reciprocal inductive signaling between the dental epithelium and mesenchyme, which culminates with the formation of dentin and enamel. EMMPRIN/CD147 is an Extracellular Matrix MetalloPRoteinase (MMP) INducer that mediates epithelial-mesenchymal interactions in cancer and other pathological processes and is expressed in developing teeth. Here we used EMMPRIN knockout (KO) mice to determine the functional role of EMMPRIN on dental tissue formation. We report a delay in enamel deposition and formation that is clearly distinguishable in the growing incisor and associated with a significant reduction of MMP-3 and MMP-20 expression in tooth germs of KO mice. Insufficient basement membrane degradation is evidenced by a persistent laminin immunostaining, resulting in a delay of both odontoblast and ameloblast differentiation. Consequently, enamel volume and thickness are decreased in adult mutant teeth but enamel maturation and tooth morphology are normal, as shown by micro-computed tomographic (micro-CT), nanoindentation, and scanning electron microscope analyses. In addition, the dentino-enamel junction appears as a rough calcified layer of approximately 10±5μm thick (mean±SD) in both molars and growing incisors of KO adult mice. These results indicate that EMMPRIN is involved in the epithelial-mesenchymal cross-talk during tooth development by regulating the expression of MMPs. The mild tooth phenotype observed in EMMPRIN KO mice suggests that the direct effect of EMMPRIN may be limited to a short time window, comprised between basement membrane degradation allowing direct cell contact and calcified matrix deposition.

Dual role of the Trps1 transcription factor in dentin mineralization

TRPS1 (tricho-rhino-phalangeal syndrome) is a unique GATA-type transcription factor that acts as a transcriptional repressor. TRPS1 deficiency and dysregulated TRPS1 expression result in skeletal and dental abnormalities implicating TRPS1 in endochondral bone formation and tooth development. Moreover, patients with tricho-rhino-phalangeal syndrome frequently present with low bone mass indicating TRPS1 involvement in bone homeostasis. In addition, our previous data demonstrated accelerated mineralization of the perichondrium in Trps1 mutant mice and impaired dentin mineralization in Col1a1-Trps1 transgenic mice, implicating Trps1 in the mineralization process. To understand the role of Trps1 in the differentiation and function of cells producing mineralized matrix, we used a preodontoblastic cell line as a model of dentin mineralization. We generated both Trps1-deficient and Trps1-overexpressing stable cell lines and analyzed the progression of mineralization by alkaline phosphatase and alizarin red staining. As predicted, based on our previous in vivo data, delayed and decreased mineralization of Trps1-overexpressing odontoblastic cells was observed when compared with control cells. This was associated with down-regulation of genes regulating phosphate homeostasis. Interestingly, Trps1-deficient cells lost the ability to mineralize and demonstrated decreased expression of several genes critical for initiating the mineralization process, including Alpl and Phospho1. Based on these data, we have concluded that Trps1 serves two critical and context-dependent functions in odontoblast-regulated mineralization as follows: 1) Trps1 is required for odontoblast maturation by supporting expression of genes crucial for initiating the mineralization process, and 2) Trps1 represses the function of mature cells and, consequently, restricts the extent of extracellular matrix mineralization.

Pulp cell tracking by radionuclide imaging for dental tissue engineering

Pulp engineering with dental mesenchymal stem cells is a promising therapy for injured teeth. An important point is to determine the fate of implanted cells in the pulp over time and particularly during the early phase following implantation. Indeed, the potential engraftment of the implanted cells in other organs has to be assessed, in particular, to evaluate the risk of inducing ectopic mineralization. In this study, our aim was to follow by nuclear imaging the radiolabeled pulp cells after implantation in the rat emptied pulp chamber. For that purpose, indium-111-oxine (¹¹¹In-oxine)-labeled rat pulp cells were added to polymerizing type I collagen hydrogel to obtain a pulp equivalent. This scaffold was implanted in the emptied pulp chamber space in the upper first rat molar. Labeled cells were then tracked during 3 weeks by helical single-photon emission computed tomography (SPECT)/computed tomography performed on a dual modality dedicated small animal camera. Negative controls were performed using lysed radiolabeled cells obtained in a hypotonic solution. In vitro data indicated that ¹¹¹In-oxine labeling did not affect cell viability and proliferation. In vivo experiments allowed a noninvasive longitudinal follow-up of implanted living cells for at least 3 weeks and indicated that SPECT signal intensity was related to implanted cell integrity. Notably, there was no detectable systemic release of implanted cells from the tooth. In addition, histological analysis of the samples showed mitotically active fibroblastic cells as well as neoangiogenesis and nervous fibers in pulp equivalents seeded with entire cells, whereas pulp equivalents prepared from lysed cells were devoid of cell colonization. In conclusion, our study demonstrates that efficient labeling of pulp cells can be achieved and, for the first time, that these cells can be followed up after implantation in the tooth by nuclear imaging. Furthermore, it appears that grafted cells retained the label and are viable to follow the repair process. This technique is expected to be of major interest for monitoring implanted cells in innovative therapies for injured teeth.

ES cells-derived ectomesenchymal cells for tooth engineering

Recent progresses in stem cell biology and tissue engineering allow considering the possible development of new therapies for compensating the dental tissue losses associated with traumas, pathologies or ageing. The possibility of generating a tooth by mimicking development through reassociations between dental epithelial cells and ectomesenchymal cells derived from the neural crest (NC) has been demonstrated in the mouse. In the search of cell sources to be used for a human transfer, pluripotent stem cells could represent a good alternative. Our study thus focuses on obtaining, ectomesenchymal cells from pluripotent ES cells, capable of promoting tooth histomorphogenesis, when reassociated with a competent dental epithelium. To this end, two ES differentiation protocols, using cyclopamine or a combination of FGF2 and BMP4, have been developed and tested for their capacity to generate such cells. The differentiated ES cells were characterized by quantitative RT-PCR. Both protocols led the cells to acquire in 10 days a mesenchymal-like cell morphology. Rapidly after induction, the cells loose their expression of pluripotent genes while sequentially activating typical NC specifiers. However, the kinetics of gene activation differed between the 2 protocols. Interestingly, Twist, a gene whose expression in the NC is associated with a commitment towards an ectomesenchymal fate, is only activated under the influence of FGF2 and BMP4. Reassociation experiments with a competent epithelium will allow testing the odontogenic potential of the differentiated ES cells. These experiments performed in the mouse system should allow defining a strategy for obtaining odontogenic competent human cells.

MEPE-derived ASARM peptide inhibits odontogenic differentiation of dental pulp stem cells and impairs mineralization in tooth models of X-linked hypophosphatemia

Mutations in PHEX (phosphate-regulating gene with homologies to endopeptidases on the X-chromosome) cause X-linked familial hypophosphatemic rickets (XLH), a disorder having severe bone and tooth dentin mineralization defects. The absence of functional PHEX leads to abnormal accumulation of ASARM (acidic serine- and aspartate-rich motif) peptide − a substrate for PHEX and a strong inhibitor of mineralization − derived from MEPE (matrix extracellular phosphoglycoprotein) and other matrix proteins. MEPE-derived ASARM peptide accumulates in tooth dentin of XLH patients where it may impair dentinogenesis. Here, we investigated the effects of ASARM peptides in vitro and in vivo on odontoblast differentiation and matrix mineralization. Dental pulp stem cells from human exfoliated deciduous teeth (SHEDs) were seeded into a 3D collagen scaffold, and induced towards odontogenic differentiation. Cultures were treated with synthetic ASARM peptides (phosphorylated and nonphosphorylated) derived from the human MEPE sequence. Phosphorylated ASARM peptide inhibited SHED differentiation in vitro, with no mineralized nodule formation, decreased odontoblast marker expression, and upregulated MEPE expression. Phosphorylated ASARM peptide implanted in a rat molar pulp injury model impaired reparative dentin formation and mineralization, with increased MEPE immunohistochemical staining. In conclusion, using complementary models to study tooth dentin defects observed in XLH, we demonstrate that the MEPE-derived ASARM peptide inhibits both odontogenic differentiation and matrix mineralization, while increasing MEPE expression. These results contribute to a partial mechanistic explanation of XLH pathogenesis: direct inhibition of mineralization by ASARM peptide leads to the mineralization defects in XLH teeth. This process appears to be positively reinforced by the increased MEPE expression induced by ASARM. The MEPE-ASARM system can therefore be considered as a potential therapeutic target.

Concomitant multipotent and unipotent dental pulp progenitors and their respective contribution to mineralised tissue formation

Upon in vitro induction or in vivo implantation, the stem cells of the dental pulp display hallmarks of odontoblastic, osteogenic, adipogenic or neuronal cells. However, whether these phenotypes result from genuine multipotent cells or from coexistence of distinct progenitors is still an open question. Furthermore, determining whether a single cell-derived progenitor is capable of undergoing a differentiation cascade leading to tissue repair in situ is important for the development of cell therapy strategies. Three clonal pulp precursor cell lines (A4, C5, H8), established from embryonic ED18 first molars of mouse transgenic for a recombinant plasmid adeno-SV40, were induced to differentiate towards the odonto/osteogenic, chondrogenic or adipogenic programme. Expression of phenotypic markers of each lineage was evaluated by RT-PCR, histochemistry or immunocytochemistry. The clones were implanted into mandibular incisors or calvaria of adult mice. The A4 clone was capable of being recruited towards at least 3 mesodermal lineages in vitro and of contributing to dentin-like or bone formation, in vivo, thus behaving as a multipotent cell. In contrast, the C5 and H8 clones displayed a more restricted potential. Flow cytometric analysis revealed that isolated monopotent and multipotent clones could be distinguished by a differential expression of CD90. Altogether, isolation of these clonal lines allowed demonstrating the coexistence of multipotential and restricted-lineage progenitors in the mouse pulp. These cells may further permit unravelling specificities of the different types of pulp progenitors, hence facilitating the development of cell-based therapies of the dental pulp or other cranio-facial tissues.

Pulpal progenitors and dentin repair

Mesenchymal stem cells are present in the dental pulp. They have been shown to contribute to dentin-like tissue formation in vitro and to participate in bone repair after a mandibular lesion. However, their capacity to contribute efficiently to reparative dentin formation after pulp lesion has never been explored. After pulp exposure, we have identified proliferative cells within 3 zones. In the crown, zone I is near the cavity, and zone II corresponds to the isthmus between the mesial and central pulp. In the root, zone III, near the apex, at a distance from the inflammatory site, contains mitotic stromal cells which may represent a source of progenitor cells. Stem-cell-based strategies are promising treatments for tissue injury in dentistry. Our experiments focused on (1) location of stem cells induced to leave their quiescent state early after pulp injury and (2) implantation of pulp progenitors, a substitute for classic endodontic treatments, paving the way for pulp stem-cell-based therapies.

Pulpal progenitors and dentin repair

Mesenchymal stem cells are present in the dental pulp. They have been shown to contribute to dentin-like tissue formation in vitro and to participate in bone repair after a mandibular lesion. However, their capacity to contribute efficiently to reparative dentin formation after pulp lesion has never been explored. After pulp exposure, we have identified proliferative cells within 3 zones. In the crown, zone I is near the cavity, and zone II corresponds to the isthmus between the mesial and central pulp. In the root, zone III, near the apex, at a distance from the inflammatory site, contains mitotic stromal cells which may represent a source of progenitor cells. Stem-cell-based strategies are promising treatments for tissue injury in dentistry. Our experiments focused on (1) location of stem cells induced to leave their quiescent state early after pulp injury and (2) implantation of pulp progenitors, a substitute for classic endodontic treatments, paving the way for pulp stem-cell-based therapies.

Tooth engineering: searching for dental mesenchymal cells sources

The implantation of cultured re-associations between embryonic dental mesenchymal cells and epithelial cells from mouse molars at embryonic day 14 (ED14) allowed making full teeth with crown, root, periodontal ligament fibers, and bone. Although representing valuable tools to set up methodologies embryonic cells are not easily available. This work thus aimed to replace the embryonic cells by dental mesenchymal cell lines or cultured expanded embryonic cells, and to test their ability to mediate tooth development in vitro when re-associated with a competent dental epithelium. Histology, immunostaining and RT-PCR allowed getting complementary sets of results. Two different immortalized cell lines from ED18 dental mesenchyme failed in mediating tooth formation. The potentialities of embryonic dental mesenchymal cells decreased from ED14 to ED16 and were lost at ED18. This is likely related to a change in the mesenchymal cell phenotype and/or populations during development. Attempts to cultivate ED14 or ED16 embryonic dental mesenchymal cells prior to re-association led to the loss of their ability to support tooth development. This was accompanied by a down-regulation of Fgf3 transcription. Supplementation of the culture medium with FGF2 allowed restoring Fgf3 expression, but not the ability of mesenchymal cells to engage in tooth formation. Altogether, these observations suggest that a competent cell population exists in the dental mesenchyme at ED14, progressively decreases during development, and cannot as such be maintained in vitro. This study evidenced the need for specific conditions to maintain the ability of dental mesenchymal cells to initiate whole tooth formation, when re-associated with an odontogenic epithelium. Efforts to improve the culture conditions will have to be combined with attempts to characterize the competent cells within the dental mesenchyme.

DMP1 C-terminal mutant mice recapture the human ARHR tooth phenotype

DMP1 mutations in autosomal recessive hypophosphatemic rickets (ARHR) patients and mice lacking Dmp1 display an overlapping pathophysiology, such as hypophosphatemia. However, subtle differences exist between the mouse model and human ARHR patients. These differences could be due to a species specificity of human versus mouse, or it may be that the mutant DMP1 in humans maintains partial function of DMP1. In this study we report a deformed tooth phenotype in a human DMP1 deletion mutation case. Unexpectedly, the deletion of nucleotides 1484 to 1490 (c.1484_1490delCTATCAC, delMut, resulting in replacement of the last 18 residues with 33 random amino acids) showed a severe dentin and enamel defect similar to a dentinogenesis imperfecta (DI) III-like phenotype. To address the molecular mechanism behind this phenotype, we generated delMut transgenic mice with the endogenous Dmp1 gene removed. These mutant mice did not recapture the abnormal phenotype observed in the human patient but displayed a mild rachitic tooth phenotype in comparison with that in the Dmp1-null mice, suggesting that the DI III-like phenotype may be due to an as-yet-undetermined acquired gene modifier. The mechanism studies showed that the mutant fragment maintains partial function of DMP1 such as stimulating MAP kinase signaling in vitro. Last, the in vitro and in vivo data support a role of odontoblasts in the control of fibroblast growth factor 23 (FGF-23) regulation during early postnatal development, although this regulation on Pi homeostasis is likely limited.

Serotonergic 5-HT(2B) receptor controls tissue-nonspecific alkaline phosphatase activity in osteoblasts via eicosanoids and phosphatidylinositol-specific phospholipase C

In previous studies, we observed that mice knocked out for the serotonin-2B receptor (5-HT(2B)R) show defects in bone homeostasis. The present work focuses on the downstream targets relaying the anabolic function of this receptor in osteoblasts. A functional link between the 5-HT(2B)R and the activity of the tissue-nonspecific alkaline phosphatase (TNAP) is established using the C1 osteoprogenitor cell line. During C1 osteogenic differentiation, both 5-HT(2B)R and TNAP mRNA translations are delayed with respect to extracellular matrix deposition. Once the receptor is expressed, it constitutively controls TNAP activity at a post-translational level along the overall period of mineral deposition. Indeed, pharmacological inhibition of the 5-HT(2B)R intrinsic activity or shRNA-mediated 5-HT(2B)R knockdown prevents TNAP activation, but not its mRNA translation. In contrast, agonist stimulation of the receptor further increases TNAP activity during the initial mineralization phase. Building upon our previous observations that the 5-HT(2B)R couples with the phospholipase A2 pathway and prostaglandin production at the beginning of mineral deposition, we show that the 5-HT(2B)R controls leukotriene synthesis via phospholipase A2 at the terminal stages of C1 differentiation. These two 5-HT(2B)R-dependent eicosanoid productions delineate distinct time windows of TNAP regulation during the osteogenic program. Finally, prostaglandins or leukotrienes are shown to relay the post-translational activation of TNAP via stimulation of the phosphatidylinositol-specific phospholipase C. In agreement with the above findings, primary calvarial osteoblasts from 5-HT(2B)R-null mice exhibit defects in TNAP activity.

Autoregulatory loop of Msx1 expression involving its antisense transcripts

The Msx1 homeogene plays an important role in epithelial-mesenchymal interactions leading organogenesis. Msx1 gene is submitted to bidirectional transcription generating a long non-coding antisense (AS) RNA potentially involved in Msx1 expression regulation. RT-Q-PCR and RNA-FISH studies indicated that transient overexpression of the Msx1 AS transcript in 705IC5 mouse odontoblasts decreased the abundance of endogenous Msx1 S mRNA at the post-transcriptional level. Conversely, Msx1 overexpression increased the AS RNA level probably by activating AS transcription. In vivo mapping by RT-PCR evidenced both Msx1 RNAs in all adult mouse tissues tested raising the issue of Msx1 function during adulthood. The expression patterns of the two RNAs were similar, confirming the tight S/AS relationship. In particular, both Msx1 mRNAs and Msx1 protein were similarly distributed in eyes, and were found in regions with a common ectodermic origin and in cells potentially involved in regeneration. In conclusion, we report that Msx1 S RNA is negatively controlled by its AS RNA at a post-transcriptional level, and that the AS RNA is retrocontrolled positively by Msx1. The tight link between Msx1 S and AS RNAs constitutes a regulatory loop resulting in a fine-tuned expression of Msx1 which appears to be significant for adult homeostasis.

Inflammatory and immunological aspects of dental pulp repair

The repair of dental pulp by direct capping with calcium hydroxide or by implantation of bioactive extracellular matrix (ECM) molecules implies a cascade of four steps: a moderate inflammation, the commitment of adult reserve stem cells, their proliferation and terminal differentiation. The link between the initial inflammation and cell commitment is not yet well established but appears as a potential key factor in the reparative process. Either the release of cytokines due to inflammatory events activates resident stem (progenitor) cells, or inflammatory cells or pulp fibroblasts undergo a phenotypic conversion into osteoblast/odontoblast-like progenitors implicated in reparative dentin formation. Activation of antigen-presenting dendritic cells by mild inflammatory processes may also promote osteoblast/odontoblast-like differentiation and expression of ECM molecules implicated in mineralization. Recognition of bacteria by specific odontoblast and fibroblast membrane receptors triggers an inflammatory and immune response within the pulp tissue that would also modulate the repair process.

An In vivo Model for Short-Term Evaluation of the Implantation Effects of Biomolecules or Stem Cells in the Dental Pulp

The continuously growing rodent incisor is a widely used model to investigate odontogenesis and mineralized tissue formation. This study focused on evaluating the mouse mandibular incisor as an experimental biological tool for analyzing in vivo the capacity of odontoblast-like progenitors or bioactive molecules to contribute to reparative dentinogenesis. We describe here a surgical procedure allowing direct access to the forming part of the incisor dental pulp Amelogenin peptide A+4 adsorbed on agarose beads, or dental pulp progenitor cells were implanted in the pulp following this procedure. After 10 days A+4 induced the formation of an osteodentin occluding almost the totality of the pulp compartment. Implantation of progenitor cells leads to formation of islets of osteodentin-like structures located centrally in the pulp. These pilot studies validate the incisor as an experimental model to test the capacity of progenitor cells or bioactive molecules to induce the formation of reparative dentin.

Matricellular molecules and odontoblast progenitors as tools for dentin repair and regeneration

This review summarizes the in vivo experiments carried out by our group after implantation of bioactive molecules (matricellular molecules) into the exposed pulp of the first maxillary molar of the rat or the mandibular incisor of rats and mice. We describe the cascade of recruitment, proliferation and terminal differentiation of cells involved in the formation of reparative dentin. Cloned immortalized odontoblast progenitors were also implanted in the incisors and in vitro studies aimed at revealing the signaling pathways leading from undifferentiated progenitors to fully differentiated polarized cells. Together, these experimental approaches pave the way for controlled dentin regenerative processes and repair.

Short-term effects of amelogenin gene splice products A+4 and A-4 implanted in the exposed rat molar pulp

In order to study the short-time effects of two bioactive low-molecular amelogenins A+4 and A-4, half-moon cavities were prepared in the mesial aspect of the first maxillary molars, and after pulp exposure, agarose beads alone (controls) or beads soaked in A+4 or A-4 (experimental) were implanted into the pulp. After 1, 3 or 7 days, the rats were killed and the teeth studied by immunohistochemistry. Cell proliferation was studied by PCNA labeling, positive at 3 days, but decreasing at day 7 for A+4, whilst constantly high between 3 and 7 days for A-4. The differentiation toward the osteo/odontoblast lineage shown by RP59 labeling was more apparent for A-4 compared with A+4. Osteopontin-positive cells were alike at days 3 and 7 for A-4. In contrast, for A+4, the weak labeling detected at day 3 became stronger at day 7. Dentin sialoprotein (DSP), an in vivo odontoblast marker, was not detectable until day 7 where a few cells became DSP positive after A-4 stimulation, but not for A+4. These results suggest that A +/- 4 promote the proliferation of some pulp cells. Some of them further differentiate into osteoblast-like progenitors, the effects being more precocious for A-4 (day 3) compared with A+4 (day 7). The present data suggest that A +/- 4 promote early recruitment of osteogenic progenitors, and evidence functional differences between A+4 and A-4.

Noxp20 and Noxp70, two new markers of early neuronal differentiation, detected in teratocarcinoma-derived neuroectodermic precursor cells

The murine 1C11 cell line, derived from F9 pluripotent teratocarcinoma cells, exhibits features of a bipotential neuronal precursor as it converts into serotonergic or catecholaminergic neurons under appropriate induction. In order to point out molecular markers expressed in this early neuroectodermic commitment, we used a cDNA subtractive hybridization method. The 105 different isolated cDNAs represented 75 known genes, expressed sequence tags (EST) or genomic fragments. A majority of known proteins encoded by these sequences are involved in cellular mobility or migration. We characterized two sequences showing identities with ESTs and we called them Noxp20 and Noxp70. The Noxp20 transcript encodes a putative protein with a predicted caspase recruitment domain and the Noxp70 transcript encodes a putative protein displaying a Zn-finger domain. Consistent with their roles in neuronal cell development, in situ hybridization showed that Noxp20 and Noxp70 are over-expressed in brain. At embryonic days 12 and 15, Noxp20 is strongly expressed in the ventricular and intermediate zones of the brain and of the spinal cord. At embryonic day 15, Noxp70 was found to be strongly expressed in the ventricular zone around the telencephalic ventricle, and to a lower extent in the thalamus and hypothalamus. At post-natal day 10, Noxp20 mRNA was detected in the dentate gyrus, the hippocampus, the cerebellum and the olfactory bulb.

Early in vivo and in vitro effects of amelogenin gene splice products on pulp cells

Recombinant amelogenin gene splice products A+4 and A-4, implanted in the pulp, induce the recruitment, proliferation, and differentiation of reparative cells. Our aim was to investigate the precocious events occurring in the pulp 1 d and 3 d after implantation of agarose beads alone or loaded with A+4 or A-4. Proliferation and cell recruitment towards an odonto/osteogenic phenotype were visualized by detection of the proliferation cell nuclear antigen (PCNA) and RP59. After implantation of beads alone or loaded with A+4, at day 3, pulp cells were moderately immunopositive for osteopontin (OP), whereas labeling was strongly positive upon treatment with A-4. Dentin sialoprotein (DSP) labeling was not detectable. Parallel in vitro studies were carried out on odontoblastic and mesenchymal progenitor cells in order to evaluate the effect of the amelogenin peptides on the expression of a series of marker genes involved in the odontoblastic/osteogenic/chondrogenic differentiation pathways. Altogether, our results suggest that the 'signaling' effects of the amelogenin peptides A+4 and A-4 may differ according to the type of target cells, their stage of differentiation, the time of treatment, and the type of amelogenin peptide (A+4 or A-4).

The impact of bioactive molecules to stimulate tooth repair and regeneration as part of restorative dentistry

After implantation in the exposed pulp, some molecules of the den-tin extracellular matrix induce the formation of a reparative dentinal bridge in the coronal pulp. In some cases, total occlusion of the root canal also is observed. This is the case for bone sialoprotein, bone morphogenetic protein-7, Dentonin (a fragment from matrix extracellular phosphoglycoprotein), and two small amelogenin gene splice products (A+4 and A-4). Cells implicated in the reparative process are recruited, proliferate, and differentiate into osteoblast-like and odontoblast-like cells. The same results may be obtained by direct implantation of odontoblast progenitor cell into the pulp.

Stepwise control of osteogenic differentiation by 5-HT(2B) receptor signaling: nitric oxide production and phospholipase A2 activation

During development, antagonists of 5-HT(2) receptor subtypes cause morphological defects of mesodermal and neural crest derivatives including the craniofacial skeleton. We used an inducible mesoblastic cell line, C1, able to fully convert into osteocytes within 12 days, to assess the involvement of 5-HT(2) receptors during osteogenic differentiation. On day 5 of the osteogenic program, immediately before matrix mineralization, the cells selectively implement 5-HT(2B) receptors (5-HT(2B)R) which remain functional until terminal differentiation. In 5-HT-depleted medium, the receptor exhibits a constitutive activity leading to basal nitric oxide (NO) release and phospholipase A2 (PLA2)-dependent arachidonic acid (AA) production. Blockade of this intrinsic activity affects the efficiency of mineralization by decreasing calcium incorporation within the matrix by 40%. Optimal bone matrix mineralization involves both NO and PLA2 signaling pathways. Moreover, between day 5 and day 10, at the beginning of mineral deposition, the 5-HT(2B)R promotes prostaglandin E2 production through AA-dependent cyclooxygenase (COX) activation. From day 10 onwards, when C1 osteoblasts undergo conversion into osteocyte-like cells, COX activity is quenched. Altogether these observations indicate that the 5-HT(2B)R contributes in an autocrine manner to osteogenic differentiation and highlight a switch in the downstream targets of the receptor at the terminal stage of the program. Finally, in addition to its autocrine function, the 5-HT(2B)R responds to 5-HT by increasing NO production and AA release. These findings raise concern regarding the use of 5-HT(2B)R-related drugs that may interfere with bone metabolism in physiological or pathological situations.

New cellular models for tracking the odontoblast phenotype

Odontoblasts and osteoblasts differ functionally and histologically. Because of their close relationship, mesenchymal cells derived from teeth and bone are difficult to distinguish ex vivo. Indeed, the main non-collagenous components of the odontoblastic extracellular matrix, dentin sialoprotein (DSP) or dentin matrix protein 1 (DMP1), have also been detected in osteoblasts. The need to develop cellular models of odontoblast differentiation and to identify markers specific for the odontoblast lineage, has led us to establish clonal cell lines from tooth germs of day 18 mouse embryos transgenic for an adenovirus-SV40 recombinant plasmid. In this study, we analyzed the phenotypes of three independent clones by RT-PCR and Western blot. These clones synthesised DSP, DMP1 and other extracellular matrix proteins typical of the odontoblast and are therefore likely to be derived from the pulp. Transcripts encoding a set of homeobox proteins involved in craniofacial development, such as Pax9, Msx1, Cbfa1, Dlx2 and 5 were also expressed albeit at a different level. These features of the pulpal clones are shared by the C1 mesodermal cells that are capable of differentiating along osteogenic, chondrogenic or adipogenic lineages In contrast, transcripts for two LIM-domain homeobox family genes (Lhx6 and Lhx7) were only detected in the dental clones. Since these genes are preferentially expressed in the mesenchyme of the developing tooth, this suggests that our transgenic-derived cell lines retain intrinsic properties of odontoblastic cells. They may help to characterise genes specifying the odontoblast phenotype and the signalling pathways underlying odontoblast differentiation.

Paracrine and autocrine signals promoting full chondrogenic differentiation of a mesoblastic cell line

The pluripotent mesoblastic C1 cell line was used under serum-free culture conditions to investigate how paracrine and autocrine signals cooperate to drive chondrogenesis. Sequential addition of two systemic hormones, dexamethasone and triiodothyronine, permits full chondrogenic differentiation. The cell intrinsic activation of the BMP signaling pathway and Sox9 expression occurring on mesoblastic condensation is insufficient for recruitment of the progenitors. Dexamethasone-dependent Sox9 upregulation is essential for chondrogenesis.

INTRODUCTION

Differentiation of lineage stem cells relies on cell autonomous regulations modulated by external signals. We used the pluripotent mesoblastic C1 cell line under serum-free culture conditions to investigate how paracrine and autocrine signals cooperate to induce differentiation of a precursor clone along the chondrogenic lineage.

MATERIALS AND METHODS

C1 cells, cultured as aggregates, were induced toward chondrogenesis by addition of 10(-7) M dexamethasone in serum-free medium. After 30 days, dexamethasone was replaced by 10 nM triiodothyronine to promote final hypertrophic conversion. Mature and hypertrophic phenotypes were characterized by immunocytochemistry using specific antibodies against types II and X collagens, respectively. Type II collagen, bone morphogenetic proteins (BMPs), BMP receptors, Smads, and Sox9 expression were monitored by reverse transcriptase-polymerase chain reaction (RT-PCR), Northern blot, and/or Western blot analysis.

RESULTS AND CONCLUSIONS

Once C1 cells have formed nodules, sequential addition of two systemic hormones is sufficient to promote full chondrogenic differentiation. In response to dexamethasone, nearly 100% of the C1 precursors engage in chondrogenesis and convert within 30 days into mature chondrocytes, which triggers a typical cartilage matrix. On day 25, a switch in type II procollagen mRNA splicing acted as a limiting step in the acquisition of the mature chondrocyte phenotype. On day 30, substitution of dexamethasone with triiodothyronine triggers the final differentiation into hypertrophic chondrocytes within a further 15 days. The chondrogenic process is supported by intrinsic expression of Sox9 and BMP family genes. Similarly to the in vivo situation, activation of Sox9 expression and the BMP signaling pathway occurred on mesoblastic condensation. After induction, BMP-activated Smad nuclear translocation persisted throughout the process until the onset of hypertrophy. After dexamethasone addition, Sox9 expression was upregulated. Dexamethasone withdrawal reversed the increase in Sox9 expression and stopped differentiation. Thus, Sox9 seems to be a downstream mediator of dexamethasone action.

Overexpression of murine small heat shock protein HSP25 interferes with chondrocyte differentiation and decreases cell adhesion

Although multiple functions for the small heat shock protein HSP25 have been proposed, its specific role during developmental and differentiation processes is not known. Cartilage is one of the tissues in which HSP25 is specifically and highly expressed during development. C1 cells, able to form aggregates in vitro, can be induced to differentiate into chondrocytes. In this study, we generated two stable transfected clones overexpressing HSP25 at two different levels. Cell morphology and growth rate were modified in both clones, although the actin content and distribution did not seem to be altered. Overexpressing clones had more difficulties in coalescing, leading to smaller aggregates and they did not differentiate into chondrocytes. Subsequently, these aggregates tended to dissociate into loose masses of dying cells. The strength of all these effects was directly correlated to the level of HSP25 overexpression. These data suggest that overexpressing HSP25 decreases cellular adhesion and interferes with chondrocyte differentiation.

Lineage-dependent collagen expression and assembly during osteogenic or chondrogenic differentiation of a mesoblastic cell line

The mesoblastic clone, C1, behaves as a tripotential progenitor able to self-renew and to differentiate toward osteogenesis, chondrogenesis, or adipogenesis in response to specific inducers. In this study, expression and deposition by the C1 cells of essential components of the extracellular matrix, collagens type I, II, III, V, XI, VI, IX, and X were followed along the osteogenic and chondrogenic pathways, through biochemical, immunochemical, and electron microscopy analyses. Implementation of each program involves profiles of collagen synthesis and matrix assembly close to those documented in vivo. Depending on the applied inducers, cells adopt a defined identity and, controls acting at transcriptional and posttranslational levels adapt the set of deposited collagens to one particular cell fate. Osteogenic C1 cells selectively build a type I collagen matrix also containing type III, V, and XI collagens but selectively exclude type II collagen. Chondrogenic C1 cells first elaborate a type II collagen network and then acquire hypertrophic chondrocyte properties while assembling a type X collagen matrix as in the growth plate. This study provides an example of how a mesoblastic cell line can develop, in vitro, each of its genetic programs up to terminal differentiation. Intrinsic factors and time-dependent cell-matrix interactions might, as in vivo, underline the implementation of an entire differentiation program.

Controlled conversion of an immortalized mesodermal progenitor cell towards osteogenic, chondrogenic, or adipogenic pathways

The teratocarcinoma-derived C1 clone behaves as a mesodermal tripotential progenitor cell whose choice of fate, either osteoblast, chondroblast, or adipoblast, is strictly dependent on the spatial organization of the cells and the nature of the induction. In the absence of cell contact before the addition of inducers, the C1 cells maintain a stable undifferentiated phenotype while expressing potential regulators of embryonic mesodermal stem cell fate such a M-twist and Id1. Upon establishment of cell contacts before the induction of differentiation, the early genes characteristic of the three fates become expressed. In the presence of beta glycerophosphate and ascorbate, provided the cells have formed aggregates, 95% of the C1 cells mineralize with a kinetics of gene expression close to that of osteoblasts (Poliard, A., D. Lamblin, P. J. Marie, M. H. Buc, and O. Kellerman. 1993. J. Cell Sci. 106:503-512). With 10(-6)M dexamethasone, 80% of the same aggregates differentiate into foci of chondroblast-like cells. The kinetics of expression of the genes encoding type II, IX, X, and XI collagens, aggrecan and link protein during the conversion toward cartilage hypertrophy resembles that accompanying in vivo chondrogenesis. The synergistic action of dexamethasone and insulin convert most confluent C1 cells into functional adipocytes and induce a pattern of gene expression close to that reported for adipoblast cell lines. The C1 clone with its capacity to differentiate along three alternative pathways with high frequency, therefore appears as a valid in vitro model for deciphering the molecular basis of mesoblast ontogeny.

Commitment of the teratocarcinoma-derived mesodermal clone C1 towards terminal osteogenic differentiation

The mesodermal clone C1 was derived from the multipotent embryonal carcinoma 1003 cell line transformed with the plasmid pK4 carrying SV40 oncogenes under the control of the adenovirus E1A promoter. We have shown that the C1 clone becomes committed to the osteogenic pathway when cultured in aggregates in the presence of mediators of the osteogenic differentiation. To further validate C1 as a model with which to study osteogenesis in vitro the kinetics of its differentiation was studied, focusing on the histology of the aggregates and on the expression of a set of genes corresponding to representative bone matrix proteins. The presence of ascorbic acid and beta- glycerophosphate specifically leads to mineralization in almost 100% of the aggregates. Transcription of the above genes, silent in exponentially growing cells, specifically occurred with the establishment of cell-cell contacts independently of the presence of ascorbic acid and inorganic phosphate. The latter, however, were absolutely required for matrix deposition and mineralization. In their presence, one observed an overall decline in type I collagen and alkaline phosphatase transcripts while osteocalcin and osteopontin transcripts preferentially accumulated in cells lining the mineralizing foci. Concomitantly, type I collagen and osteocalcin became extracellularly deposited. The osteogenic differentiation of C1 occurred while cells were still proliferating. The C1 clone thus behaves as a mesodermal stem cell, becoming committed to the osteogenic pathway upon: firstly, establishment of cellular contacts; and secondly, addition of ascorbate and beta-glycerophosphate. It therefore appears to be a promising in vitro system for deciphering the molecular basis of osteoblast ontogeny.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of the rat alpha-fetoprotein gene expression in liver. Both the promoter region and an enhancer element are liver-specific and negatively modulated by dexamethasone

Cis-acting elements involved in the control of rat alpha-fetoprotein gene expression in the liver and its modulation by glucocorticoid hormones were detected after transfection of chloramphenicol acetyltransferase constructs and their transient expression into two hepatoma cell lines. The proximal promoter region (-324 to -15) was found to contain all the information necessary for tissue-specific expression. It is also involved in the negative gene modulation by glucocorticoids and includes an activating regulatory domain allowing efficient expression in the HepG2 cells. Three regions within 7 kilobase pairs of the 5' extragenic sequences are capable of stimulating the chloramphenicol acetyltransferase activity driven by the alpha-fetoprotein promoter sequence. One of these regions, at about -2.5 kilobase pairs, contains a short indivisible 170-base pair DNA element that fulfills all the criteria of a tissue-specific enhancer, i.e. orientation and position independence, as well as cell-specific stimulation of gene expression driven by a homologous or heterologous promoter. The enhancing properties of this element are totally abolished by glucocorticoids. DNase I footprinting experiments indicate that several rat liver nuclear proteins interact with this enhancer element.

A common liver-specific factor binds to the rat albumin and alpha-foetoprotein promoters in vitro and acts as a positive trans-acting factor in vivo

Rat liver nuclear extracts were tested for the presence of factors which might be common to the transcriptional regulation of both the albumin and alpha-foetoprotein genes. Gel shift assay showed the formation of three complexes (I, II and III) with the albumin probe. Two of them (I and III) could be displaced by the alpha-foetoprotein promoter. Analysis of nuclear extracts from liver, kidney, spleen and brain and competition experiments using several oligonucleotides covering regions from the albumin and alpha-foetoprotein promoters showed that complex III results from the binding of the ubiquitous nuclear factor 1, while complex II involves a CCAAT-box-binding protein also detected in brain and spleen extracts. Complex I is formed upon binding of a liver-specific factor to a proximal element of the rat albumin promoter. This factor also binds to a similar sequence in the alpha-foetoprotein promoter and is closely related to the hepatocyte nuclear factor 1, as shown by competition experiments using an oligonucleotide covering its target sequence on the beta-fibrinogen promoter. Transfection competition experiments indicated that, in vivo, this factor acts as a positive trans-acting element in the expression of both the rat albumin and alpha-foetoprotein genes.

Alpha fetoprotein and albumin gene transcripts are detected in distinct cell populations of the brain and kidney of the developing rat

We report the cellular localization of alpha-fetoprotein (AFP) and albumin (ALB) gene transcripts in rat kidney and brain as detected by in situ hybridization on tissue sections with [35S]-labelled alpha-fetoprotein and albumin cDNA probes. Both types of mRNA were present in distinct cell populations of the developing kidney and brain. In the kidney, both gene transcripts were distributed over all developing tubular cells in the 20-day-old fetus. During the first 3 weeks of life, a gradual decrease in the expression of AFP and ALB mRNA was apparent, the rate of decrease being greater on proximal tubules than on the other tubular cells. From the 4th week onwards, a weak signal for both mRNAs persisted in the majority of the tubular cells. In the brain, all neuronal cells expressed both genes. Transcript cellular distribution was mainly cytoplasmic during fetal and early postnatal life and became predominantly nuclear at 3, 4 and 5 weeks, suggesting that posttranscriptional mechanisms are involved in the control of AFP and ALB gene expression at these stages. In the adult brain no significant signal was recorded thereafter. Coexpression of AFP and ALB transcripts by specific cell types, together with their gradual disappearance concomitant with postnatal organ maturation, suggests a possible role for these proteins in terminal differentiation processes of tubular and neuronal cells.