Reactivation of Delta-Notch signaling after injury: complementary expression patterns of ligand and receptor in dental pulp

Involvement of the Notch signaling system in alveolar bone resorption

The Notch pathway is an evolutionarily preserved signaling pathway involved in a variety of vital cell functions. Additionally, it is one of the key regulators of inflammation, and controls the differentiation and function of different cells. Moreover, it was found to be involved in skeletal development and bone remodeling process. This review provides an overview of the involvement of the Notch signaling pathway in the pathogenesis of alveolar bone resorption in different forms of pathological conditions such as apical periodontitis, periodontal disease, and peri-implantitis. In vitro and in vivo evidence have confirmed the involvement of Notch signaling in alveolar bone homeostasis. Nonetheless, Notch signaling system, along with complex network of different biomolecules are involved in pathological process of bone resorption in apical periodontitis, periodontitis, and peri-implantitis. In this regard, there is a substantial interest to control the activity of this pathway in the treatment of disorders associated with its dysregulation. This review provides knowledge on Notch signaling and outlines its functions in alveolar bone homeostasis and alveolar bone resorption. Further investigations are needed to determine whether inhibition of the Notch signaling pathways might be beneficial and safe as a novel approach in the treatment of these pathological conditions.

E3 ubiquitin ligase RNF187 promotes growth of spermatogonia via lysine 48-linked polyubiquitination-mediated degradation of KRT36/KRT84

Ubiquitination is the most common post-translational modification and is essential for various cellular regulatory processes. RNF187, which is known as RING domain AP1 coactivator-1, is a member of the RING finger family. RNF187 can promote the proliferation and migration of various tumor cells. However, whether it has a similar role in regulating spermatogonia is not clear. This study explored the role and molecular mechanism of RNF187 in a mouse spermatogonia cell line (GC-1). We found that RNF187 knockdown reduced the proliferation and migration of GC-1 cells and promoted their apoptosis. RNF187 overexpression significantly increased the proliferation and migration of GC-1 cells. In addition, we identified Keratin36/Keratin84 (KRT36/KRT84) as interactors with RNF187 by co-immunoprecipitation and mass spectrometry analyses. RNF187 promoted GC-1 cell growth by degrading KRT36/KRT84 via lysine 48-linked polyubiquitination. Subsequently, we found that KRT36 or KRT84 overexpression significantly attenuated proliferation and migration of RNF187-overexpressing GC-1 cells. In summary, our study explored the involvement of RNF187 in regulating the growth of spermatogonia via lysine 48-linked polyubiquitination-mediated degradation of KRT36/KRT84. This may provide a promising new strategy for treating infertility caused by abnormal spermatogonia development.

Notch signaling in the dynamics of perivascular stem cells and their niches

The Notch signaling pathway is a fundamental regulator of cell fate determination in homeostasis and regeneration. In this work, we aimed to determine how Notch signaling mediates the interactions between perivascular stem cells and their niches in human dental mesenchymal tissues, both in homeostatic and regenerative conditions. By single cell RNA sequencing analysis, we showed that perivascular cells across the dental pulp and periodontal human tissues all express NOTCH3, and that these cells are important for the response to traumatic injuries in vivo in a transgenic mouse model. We further showed that the behavior of perivascular NOTCH3‐expressing stem cells could be modulated by cellular and molecular cues deriving from their microenvironments. Taken together, the present studies, reinforced by single‐cell analysis, reveal the pivotal importance of Notch signaling in the crosstalk between perivascular stem cells and their niches in tissue homeostasis and regeneration.

The Effect of Calcium-Silicate Cements on Reparative Dentinogenesis Following Direct Pulp Capping on Animal Models

Dental pulp vitality is a desideratum for preserving the health and functionality of the tooth. In certain clinical situations that lead to pulp exposure, bioactive agents are used in direct pulp-capping procedures to stimulate the dentin-pulp complex and activate reparative dentinogenesis. Hydraulic calcium-silicate cements, derived from Portland cement, can induce the formation of a new dentin bridge at the interface between the biomaterial and the dental pulp. Odontoblasts are molecularly activated, and, if necessary, undifferentiated stem cells in the dental pulp can differentiate into odontoblasts. An extensive review of literature was conducted on MedLine/PubMed database to evaluate the histological outcomes of direct pulp capping with hydraulic calcium-silicate cements performed on animal models. Overall, irrespective of their physico-chemical properties and the molecular mechanisms involved in pulp healing, the effects of cements on tertiary dentin formation and pulp vitality preservation were positive. Histological examinations showed different degrees of dental pulp inflammatory response and complete/incomplete dentin bridge formation during the pulp healing process at different follow-up periods. Calcium silicate materials have the ability to induce reparative dentinogenesis when applied over exposed pulps, with different behaviors, as related to the animal model used, pulpal inflammatory responses, and quality of dentin bridges.

Differential expression of Notch related genes in dental pulp stem cells and stem cells isolated from apical papilla

Although dental pulp and apical papilla are originated from neural crest cells, these tissues exhibit distinct characteristics. Notch signaling is one of the known signaling pathways regulating stemness and behaviors of stem cells. The aim of this study was to examine Notch signaling related gene expression profile comparing between coronal pulp tissues and apical pulp complex. Results demonstrated that coronal pulp tissue had higher expression levels of various genes in Notch pathway. However, NOTCH2, MAML2, DTX4, and NEDD4 mRNA levels were significantly lower in coronal pulp tissue than those of apical pulp complex. Furthermore, dental pulp stem cells (DPSCs) and stem cells isolated from apical papilla (SCAPs) were isolated and characterized. These two cell types exhibited similar mesenchymal stem cell surface markers. DPSCs expressed higher mRNA levels of NOTCH3, NOTCH4, DLL1, and HES1. In addition, SCAPs demonstrated higher colony formation and cell proliferation than DPSCs. In summary, cells and tissues from dental pulp and apical papilla exhibited the distinct gene expression profile of Notch related genes. This could be of one the signaling participated in control of DPSCs and SCAPs cells behaviors.

Ameloblastomas Exhibit Stem Cell Potential, Possess Neurotrophic Properties, and Establish Connections with Trigeminal Neurons

Ameloblastomas are locally invasive and aggressive odontogenic tumors treated via surgical resection, which results in facial deformity and significant morbidity. Few studies have addressed the cellular and molecular events of ameloblastoma onset and progression, thus hampering the development of non-invasive therapeutic approaches. Tumorigenesis is driven by a plethora of factors, among which innervation has been long neglected. Recent findings have shown that innervation directly promotes tumor progression. On this basis, we investigated the molecular characteristics and neurotrophic properties of human ameloblastomas. Our results showed that ameloblastomas express dental epithelial stem cell markers, as well as components of the Notch signaling pathway, indicating persistence of stemness. We demonstrated that ameloblastomas express classical stem cell markers, exhibit stem cell potential, and form spheres. These tumors express also molecules of the Notch signaling pathway, fundamental for stem cells and their fate. Additionally, we showed that ameloblastomas express the neurotrophic factors NGF and BDNF, as well as their receptors TRKA, TRKB, and P75/NGFR, which are responsible for their innervation by trigeminal axons in vivo. In vitro studies using microfluidic devices showed that ameloblastoma cells attract and form connections with these nerves. Innervation of ameloblastomas might play a key role in the onset of this malignancy and might represent a promising target for non-invasive pharmacological interventions.

Hey1 functions as a positive regulator of odontogenic differentiation in odontoblast‑lineage cells

Substantial evidence has indicated that Notch and bone morphogenetic protein (BMP) signaling may regulate odontoblastic differentiation. Hairy/enhancer‑of‑split related with YRPW motif 1 (Hey1), a downstream target gene of Notch and BMP signaling, is expressed in dental pulp tissues and has been demonstrated to be responsible for osteoblast mineralization. The aim of this study was to investigate the effects of Hey1 on odontoblast differentiation. The results of the study demonstrated that Hey1 expression in odontoblast‑lineage cells (OLCs) was upregulated by stimulation of osteoblastic/odontoblastic differentiation medium containing ascorbic acid, β‑glycerol phosphate and dexamethasone. Furthermore, stable Hey1‑overexpressing cells expressed higher levels of dentin sialophosphoprotein (DSPP) and exhibited higher mineralization capabilities following stimulation by differentiation medium. Furthermore, RNA interference‑mediated knockdown of Hey1 downregulated the expression levels of DSPP in OLCs stimulated by differentiation medium. Taken together, the findings indicate that Hey1 may be a positive regulator of odontoblastic differentiation. The present study broadens the understanding of odontoblast differentiation and biomineralization.

Monitoring Notch Signaling-Associated Activation of Stem Cell Niches within Injured Dental Pulp

Dental pulp stem/progenitor cells guarantee tooth homeostasis, repair and regeneration throughout life. The decision between renewal and differentiation of these cells is influenced by physical and molecular interactions with stromal cells and extracellular matrix molecules forming the specialized microenvironment of dental pulp stem cell niches. Here we study the activation of putative pulp niches after tooth injury through the upregulation of Notch signaling pathway. Notch1, Notch2, and Notch3 molecules were used as markers of dental pulp stem/progenitor cells. Upon dental injury, Notch1 and Notch3 are detected in cells related to vascular structures suggesting a role of these proteins in the activation of specific pulpal perivascular niches. In contrast, a population of Notch2-positive cells that are actively proliferative is observed in the apical part of the pulp. Kinetics of these cells is followed up with a lipophilic DiI labeling, showing that apical pulp cells migrate toward the injury site where dynamic regenerative/repair events occur. The knowledge of the activation and regulation of dental pulp stem/progenitor cells within their niches in pathologic conditions may be helpful for the realization of innovative dental treatments in the near future.

Parallels between Tooth Development and Repair: Conserved Molecular Mechanisms following Carious and Dental Injury

The reparative mechanisms that operate following carious and traumatic dental injury are critical for pulp survival and involve a series of highly conserved processes. It appears that these processes share genetic programs—linked to cytoskeletal organization, cell movement, and differentiation—that occur throughout embryogenesis. Reactionary dentin is secreted by surviving odontoblasts in response to moderate stimuli, leading to an increase in metabolic activity. In severe injury, necrotic odontoblasts are replaced by other pulp cells, which are able to differentiate into odontoblast-like cells and produce a reparative dentin. This complex process requires the collaborative efforts of cells of different lineage. The behavior of each of the contributing cell types during the phases of proliferation, migration, and matrix synthesis as well as details of how growth factors control wound cell activities are beginning to emerge. In this review, we discuss what is known about the molecular mechanisms involved in dental repair.

Odontoblasts: Specialized hard-tissue-forming cells in the dentin-pulp complex

Odontoblasts are specialized cells that produce dentin and exhibit unique morphological characteristics; i.e., they extend cytoplasmic processes into dentinal tubules. While osteoblasts, which are typical hard-tissue-forming cells, are generated from mesenchymal stem cells during normal and pathological bone metabolism, the induction of odontoblasts only occurs once during tooth development, and odontoblasts survive throughout the lives of healthy teeth. During the differentiation of odontoblasts, signaling molecules from the inner enamel epithelium are considered necessary for the differentiation of odontoblast precursors, i.e., peripheral dental papilla cells. If odontoblasts are destroyed by severe external stimuli, such as deep caries, the differentiation of dental pulp stem cells into odontoblast-like cells is induced. Various bioactive molecules, such as non-collagenous proteins, might be involved in this process, although the precise mechanisms responsible for odontoblast differentiation have not been fully elucidated. Recently, our knowledge about the other functional activities of odontoblasts (apart from dentin formation) has increased. For example, it has been suggested that odontoblasts might act as nociceptive receptors, and surveillance cells that detect the invasion of exogenous pathogens. The regeneration of the dentin-pulp complex has recently gained much attention as a promising future treatment modality that could increase the longevity of pulpless teeth. Finally, congenital dentin anomalies, which are concerned with the disturbance of odontoblast functions, are summarized.

Activation and dynamic expression of Notch signalling in dental pulp cells after injury in vitro and in vivo

AIM

To investigate the expression pattern of Notch signalling in odontoblast-like cells stimulated by lipopolysaccharide (LPS) in vitro, and in injured rat dental pulp in vivo.

METHODOLOGY

Mouse odontoblast-like cells (MDPC-23) were exposed to LPS. Expression of Notch-related genes was detected by real-time PCR. A rat pulpitis model was established by mechanical injury and LPS plus mechanical injury was followed by the analysis of expression of Notch2 by immunohistochemical staining. One-way analysis of variance (anova) was performed to examine the effect of differing concentrations of LPS on cell proliferation, and least significant difference test was used for paired comparisons. For independent sample, t-test was performed to compare the expression of Notch signalling genes between LPS group and control group in vitro.

RESULTS

The in vitro study revealed the proliferation of MDPC-23 cells on exposure to 10 ng mL^-1 to 1 μg mL^-1 LPS. Expression of Notch1 and Notch2 was significantly higher in the LPS group than that in the control group on day 1 and day 3 (P ˂ 0.05). The levels of both Delta1 and Jagged1 were higher in the study group than in the control group on day 3 (P = 0.019 and P = 0.034) and day 5 (P ˂ 0.001 and P = 0.046), respectively. In addition, Hes1 levels were significantly higher in the study group than in the control group on day 5 (P = 0.005). The in vivo study demonstrated positive staining for Notch2, both in the mechanical injury (MI) group and in the LPS plus mechanical injury (LMI) group from day 3 to day 7, which showed very weak or absent staining on day 14, thereby demonstrating the dynamic nature of the change.

CONCLUSIONS

Both in vitro and in vivo activation and dynamic expression of Notch signalling in dental pulp cells after injury were found. Notch signalling activation by LPS stimulation or mechanical injury showed a similar pattern in vivo.

Distribution of syndecan-1 protein in developing mouse teeth

Syndecan-1 is a cell surface proteoglycan involved in the regulation of various biological processes such as proliferation, migration, condensation and differentiation of cells, intercellular communication, and morphogenesis. The extracellular domain of syndecan-1 can bind to extracellular matrix components and signaling molecules, while its intracellular domain interacts with cytoskeletal proteins, thus allowing the transfer of information about extracellular environment changes into the cell that consequently affect cellular behavior. Although previous studies have shown syndecan-1 expression during precise stages of tooth development, there is no equivalent study regrouping the expression patterns of syndecan-1 during all stages of odontogenesis. Here we examined the distribution of syndecan-1 protein in embryonic and post-natal developing mouse molars and incisors. Syndecan-1 distribution in mesenchymal tissues such as dental papilla and dental follicle was correlated with proliferating events and its expression was often linked to stem cell niche territories. Syndecan-1 was also expressed in mesenchymal cells that will differentiate into the dentin producing odontoblasts, but not in differentiated functional odontoblasts. In the epithelium, syndecan-1 was detected in all cell layers, by the exception of differentiated ameloblasts that form the enamel. Furthermore, syndecan-1 was expressed in osteoblast precursors and osteoclasts of the alveolar bone that surrounds the developing tooth germs. Taken together these results show the dynamic nature of syndecan-1 expression during odontogenesis and suggest its implication in various processes of tooth development and homeostasis.

Promotion of dentin regeneration via CCN3 modulation on Notch and BMP signaling pathways

Dentin regeneration remains a great challenge in clinic. Dental pulp stem cells (DPSCs) actively contribute to dentinogenesis, which is orchestrated by a spectrum of signaling factors. However, the exact mechanism underlying the reparative dentin regeneration process is largely unknown and the application of DPSCs in the repair of dentin defect is thus limited. Here, using a rat reparative dentin regeneration model, we observed that DPSCs underwent a proliferation phase followed by a differentiation phase after dental injury. A transient elevation of nephroblastoma overexpressed (NOV, or CCN3) expression correlated with this progressive dental tissue restoration process. Further studies revealed that over-expression of CCN3 promoted human DPSCs proliferation via activation of Notch. Moreover, using cocultured cells (DPSCs/CCN3 and DPSCs) in vitro and the cocultured cells-poly (lactic-co-glycolic acid) (PLGA) scaffold complex in vivo, we demonstrated that CCN3 was capable of promoting mineralization in a non-cell autonomous manner through promoting secretion of BMP2. CCN3 can promote dentinogenesis by coordinating proliferation and odontoblastic differentiation of DPSCs via modulating Notch and BMP2 signaling pathways and CCN3 is a promising therapeutic target in dentin tissue engineering.

Notch1 cardioprotection in myocardial ischemia/reperfusion involves reduction of oxidative/nitrative stress

Oxidative/nitrative stress plays an important role in myocardial ischemia/reperfusion (MI/R) injury. Notch1 participates in the regulation of cardiogenesis and cardiac response to hypertrophic stress, but the function of Notch1 signaling in MI/R has not been explored. This study aims to determine the role of Notch1 in MI/R, and investigate whether Notch1 confers cardioprotection. Notch1 specific small interfering RNA (siRNA, 20 μg) or Jagged1 (a Notch ligand, 12 μg) was delivered through intramyocardial injection. 48 h after injection, mice were subjected to 30 min of myocardial ischemia followed by 3 h (for cell apoptosis and oxidative/nitrative stress), 24 h (for infarct size and cardiac function), or 2 weeks (for cardiac fibrosis and function) of reperfusion. Cardiac-specific Notch1 knockdown resulted in significantly aggravated I/R injury, as evidenced by enlarged infarct size, depressed cardiac function, increased myocardial apoptosis and cardiac fibrosis. Downregulation of Notch1 increased expression of inducible NO synthase (iNOS) and gp(91phox), enhanced the production of NO metabolites and superoxide, as well as their cytotoxic reaction product peroxynitrite. Moreover, Notch1 blockade also reduced phosphorylation of endothelial NO synthase (eNOS) and Akt, and increased expression of PTEN, a key phosphatase involved in the regulation of Akt phosphorylation. In addition, activation of Notch1 by Jagged1 or administration of peroxynitrite scavenger reduced production of peroxynitrite and attenuated MI/R injury. These data indicate that Notch1 signaling protects against MI/R injury partly though PTEN/Akt mediated anti-oxidative and anti-nitrative effects.

Regenerative endodontics and tissue engineering: what the future holds?

The work performed by researchers in regenerative endodontics and tissue engineering over the last decades has been superb; however, many questions remain to be answered. The basic biologic mechanisms must be elucidated that will allow the development of dental pulp and dentin in situ. Stress must be placed on the many questions that will lead to the design of effective, safe treatment options and therapies. This article discusses those questions, the answers to which may become the future of regenerative endodontics. The future remains bright, but proper support and patience are required.

Local regeneration of dentin-pulp complex using controlled release of fgf-2 and naturally derived sponge-like scaffolds

Restorative and endodontic procedures have been recently developed in an attempt to preserve the vitality of dental pulp after exposure to external stimuli, such as caries infection or traumatic injury. When damage to dental pulp is reversible, pulp wound healing can proceed, whereas irreversible damage induces pathological changes in dental pulp, eventually requiring its removal. Nonvital teeth lose their defensive abilities and become severely damaged, resulting in extraction. Development of regeneration therapy for the dentin-pulp complex is important to overcome limitations with presently available therapies. Three strategies to regenerate the dentin-pulp complex have been proposed; regeneration of the entire tooth, local regeneration of the dentin-pulp complex from amputated dental pulp, and regeneration of dental pulp from apical dental pulp or periapical tissues. In this paper, we focus on the local regeneration of the dentin-pulp complex by application of exogenous growth factors and scaffolds to amputated dental pulp.

Dental pulp stem cells, niches, and notch signaling in tooth injury

Stem cells guarantee tissue repair and regeneration throughout life. The decision between cell self-renewal and differentiation is influenced by a specialized microenvironment called the 'stem cell niche'. In the tooth, stem cell niches are formed at specific anatomic locations of the dental pulp. The microenvironment of these niches regulates how dental pulp stem cell populations participate in tissue maintenance, repair, and regeneration. Signaling molecules such as Notch proteins are important regulators of stem cell function, with various capacities to induce proliferation or differentiation. Dental injuries often lead to odontoblast apoptosis, which triggers activation of dental pulp stem cells followed by their proliferation, migration, and differentiation into odontoblast-like cells, which elaborate a reparative dentin. Better knowledge of the regulation of dental pulp stem cells within their niches in pathological conditions will aid in the development of novel treatments for dental tissue repair and regeneration.

Notch signalling pathway in tooth development and adult dental cells

Notch signalling is a highly conserved intercellular signal transfer mechanism that includes canonical and non-canonical pathways. It regulates differentiation and proliferation of stem/progenitor cells by means of para-inducing effects. Expression and activation of Notch signalling factors (receptors and ligands) are critical not only for development of the dental germ but also for regeneration of injured tissue associated with mature teeth. Notch signalling plays key roles in differentiation of odontoblasts and osteoblasts, calcification of tooth hard tissue, formation of cusp patterns and generation of tooth roots. After tooth eruption, Notch signalling can also be triggered in dental stem cells of the pulp, where it induces them to differentiate into odontoblasts, thus generating fresh dentine tissue. Other signalling pathways, such as TGFβ, NF-κB, Wnt, Fgf and Shh also interact with Notch signalling during tooth development.

Continuum limits of pattern formation in hexagonal-cell monolayers

Intercellular signalling is key in determining cell fate. In closely packed tissues such as epithelia, juxtacrine signalling is thought to be a mechanism for the generation of fine-grained spatial patterns in cell differentiation commonly observed in early development. Theoretical studies of such signalling processes have shown that negative feedback between receptor activation and ligand production is a robust mechanism for fine-grained pattern generation and that cell shape is an important factor in the resulting pattern type. It has previously been assumed that such patterns can be analysed only with discrete models since significant variation occurs over a lengthscale concomitant with an individual cell; however, considering a generic juxtacrine signalling model in square cells, in O'Dea and King (Math Biosci 231(2):172-185 2011), a systematic method for the derivation of a continuum model capturing such phenomena due to variations in a model parameter associated with signalling feedback strength was presented. Here, we extend this work to derive continuum models of the more complex fine-grained patterning in hexagonal cells, constructing individual models for the generation of patterns from the homogeneous state and for the transition between patterning modes. In addition, by considering patterning behaviour under the influence of simultaneous variation of feedback parameters, we construct a more general continuum representation, capturing the emergence of the patterning bifurcation structure. Comparison with the steady-state and dynamic behaviour of the underlying discrete system is made; in particular, we consider pattern-generating travelling waves and the competition between various stable patterning modes, through which we highlight an important deficiency in the ability of continuum representations to accommodate certain dynamics associated with discrete systems.

Inhibition of Delta1 promotes differentiation of odontoblasts and inhibits proliferation of human dental pulp stem cell in vitro

OBJECTIVE

Dental pulp stem cells (DPSCs) have been receiving more attentions recently as an important biomaterial for tissue engineering. Notch signalling plays a key role in regulating self-renewal and differentiation of a variety of cells. The objective of this study is to investigate the effects of Notch-Delta1 RNA interference (RNAi) on the proliferation and differentiation of human dental pulp stem cells in vitro.

DESIGN

In the present study, we performed gene knockdown of Notch ligand Delta1 in DPSCs using lentivirus-mediated Delta1-RNAi. Changes of proliferation in DPSCs/Delta1-RNAi were examined by cell cycle analysis, Cell viability assay (CCK-8) and Western blot analysis of proliferating cell nuclear antigen (PCNA). Cells were cultured in odontoblast differentiation-inducing medium, and the differentiation of cells was detected with Alkaline phosphatase ALP activity assay, Alizarin red S staining, calcium concentration measurement, and Western blot analysis of Dentine sialophosphoprotein (DSPP).

RESULTS

Lentivirus-mediated Delta1-RNAi stably knocked-down the expression of Delta1 and Notch signalling, and some of DPSCs/Delta1-RNAi displayed changes in morphology or DSPP expression. The growth rate of Delta1-deficient DPSCs was significantly suppressed as compared with wild type DPSCs and control lentivirus vector transfected DPSCs. Furthermore, the differentiating capability of DPSCs/Delta1-RNAi into odontoblasts is much higher than the two control groups.

CONCLUSIONS

Notch signalling plays a crucial role in regulating self-renewal and differentiation in DPSCs. The deficient Notch signalling inhibits the self-renewal capacity of DPSCs and tends to induce DPSCs differentiation under odontoblast differentiation-inducing conditions. These findings suggested that DPSCs/Delta1-RNAi might be applicable to stem cell therapies and tooth tissue engineering.

Serum- and glucocorticoid-inducible kinase 1 (SGK1) controls Notch1 signaling by downregulation of protein stability through Fbw7 ubiquitin ligase

Notch is a transmembrane protein that acts as a transcriptional factor in the Notch signaling pathway for cell survival, cell death and cell differentiation. Notch1 and Fbw7 mutations both lead the activation of the Notch1 pathway and are found in the majority of patients with the leukemia T-ALL. However, little is known about the mechanisms and regulators that are responsible for attenuating the Notch signaling pathway through Fbw7. Here, we report that the serum- and glucocorticoid-inducible protein kinase SGK1 remarkably reduced the protein stability of the active form of Notch1 through Fbw7. The protein level and transcriptional activity of the Notch1 intracellular domain (Notch1-IC) were higher in SGK1-deficient cells than in SGK1 wild-type cells. Notch1-IC was able to form a trimeric complex with Fbw7 and SGK1, thereby SGK1 enhanced the protein degradation of Notch1-IC via a Fbw7-dependent proteasomal pathway. Furthermore, activated SGK1 phosphorylated Fbw7 at serine 227, an effect inducing Notch1-IC protein degradation and ubiquitylation. Moreover, accumulated dexamethasone-induced SGK1 facilitated the degradation of Notch1-IC through phosphorylation of Fbw7. Together our results suggest that SGK1 inhibits the Notch1 signaling pathway via phosphorylation of Fbw7.

Can interaction of materials with the dentin-pulp complex contribute to dentin regeneration?

Understanding outcomes of the interaction between a dental material and tooth tissue is important in terms not only of biocompatibility but also of the potential for the material to modulate the response of the tissue. This interaction is influenced by many factors, including the chemistry of the material and any of its eluted components or degradation products, and the manner in which the tissue responds to these agents. Past studies of this interaction have primarily been aimed at identifying cytotoxic effects. More recently, investigations have focused on specific cellular responses, and in particular, on understanding how the materials themselves actually may contribute to regenerative processes in the tooth. Recent work has demonstrated the solubilization of proteins from dentin exposed to certain materials, such as calcium hydroxide, mineral trioxide aggregate, and acidic solutions that relate to those used in dentin bonding agents, with the subsequent modulation by these proteins of gene expression in odontoblast-like cells. This work suggests that dentin bridge formation under such materials may be stimulated through this process. Thus, there is much merit in examining both how new dental materials can be developed and how more traditional ones can be modified to preferentially stimulate regenerative processes when preferred. This review summarizes current knowledge about the potential beneficial effects derived from the interaction of dental materials with the dentin-pulp complex, as well as potential future developments in this exciting field.

Cell fate determination during tooth development and regeneration

Teeth arise from sequential and reciprocal interactions between the oral epithelium and the underlying cranial neural crest-derived mesenchyme. Their formation involves a precisely orchestrated series of molecular and morphogenetic events, and gives us the opportunity to discover and understand the nature of the signals that direct cell fates and patterning. For that reason, it is important to elucidate how signaling factors work together in a defined number of cells to generate the diverse and precise patterned structures of the mature functional teeth. Over the last decade, substantial research efforts have been directed toward elucidating the molecular mechanisms that control cell fate decisions during tooth development. These efforts have contributed toward the increased knowledge on dental stem cells, and observation of the molecular similarities that exist between tooth development and regeneration.

Notch signaling and Hes labeling in the normal and drug-damaged organ of Corti

During the development of the inner ear, the Notch cell signaling pathway is responsible for the specification of the pro-sensory domain and influences cell fate decisions. It is assumed that Notch signaling ends during maturity and cannot be reinitiated to alter the fate of new or existing cells in the organ of Corti. This is in contrast to non-mammalian species which reinitiate Delta 1-Notch1 signaling in response to trauma in the auditory epithelium, resulting in hair cell regeneration through transdifferentiation and/or mitosis. We report immunohistochemical data and Western protein analysis showing that in the aminoglycoside-damaged guinea pig organ of Corti, there is an increase in proteins involved in Notch activation occurring within 24h of a chemical hair cell lesion. The signaling response is characterized by the increased presence of Jagged1 ligand in pillar and Deiters cells, Notch1 signal in surviving supporting cell nuclei, and the absence of Jagged2 and Delta-like1. The pro-sensory bHLH protein Atoh1 was absent at all time points following an ototoxic lesion, while the repressor bHLH transcription factors Hes1 and Hes5 were detected in surviving supporting cell nuclei in the former inner and outer hair cell areas, respectively. Notch pathway proteins peaked at 2 weeks, decreased at 1 month, and nearly disappeared by 2 months. These results indicate that the mammalian auditory epithelium retains the ability to regulate Notch signaling and Notch-dependent Hes activity in response to cellular trauma and that the signaling is transient. Additionally, since Hes activity antagonizes the transcription of pro-sensory Atoh1, the presence of Hes after a lesion may prohibit the occurrence of transdifferentiation in the surviving supporting cells.

Myocardial induction of nucleostemin in response to postnatal growth and pathological challenge

Stem cell-specific proteins and regulatory pathways that determine self-renewal and differentiation have become of fundamental importance in understanding regenerative and reparative processes in the myocardium. One such regulatory protein, named nucleostemin, has been studied in the context of stem cells and several cancer cell lines, where expression is associated with proliferation and maintenance of a primitive cellular phenotype. We find nucleostemin is present in young myocardium and is also induced following cardiomyopathic injury. Nucleostemin expression in cardiomyocytes is induced by fibroblast growth factor-2 and accumulates in response to Pim-1 kinase activity. Cardiac stem cells also express nucleostemin that is diminished in response to commitment to a differentiated phenotype. Overexpression of nucleostemin in cultured cardiac stem cells increases proliferation while preserving telomere length, providing a mechanistic basis for potential actions of nucleostemin in promotion of cell survival and proliferation as seen in other cell types.

Activation of Notch-mediated protective signaling in the myocardium

The Notch network regulates multiple cellular processes, including cell fate determination, development, differentiation, proliferation, apoptosis, and regeneration. These processes are regulated via Notch-mediated activity that involves hepatocyte growth factor (HGF)/c-Met receptor and phosphatidylinositol 3-kinase/Akt signaling cascades. The impact of HGF on Notch signaling was assessed following myocardial infarction as well as in cultured cardiomyocytes. Notch1 is activated in border zone cardiomyocytes coincident with nuclear c-Met following infarction. Intramyocardial injection of HGF enhances Notch1 and Akt activation in adult mouse myocardium. Corroborating evidence in cultured cardiomyocytes shows treatment with HGF or insulin increases levels of Notch effector Hes1 in immunoblots, whereas overexpression of activated Notch intracellular domain prompts a 3-fold increase in phosphorylated Akt. Infarcted hearts injected with adenoviral vector expressing Notch intracellular domain treatment exhibit improved hemodynamic function in comparison with control mice after 4 weeks, implicating Notch signaling in a cardioprotective role following cardiac injury. These results indicate Notch activation in cardiomyocytes is mediated through c-Met and Akt survival signaling pathways, and Notch1 signaling in turn enhances Akt activity. This mutually supportive crosstalk suggests a positive survival feedback mechanism between Notch and Akt signaling in adult myocardium following injury.

Effects of Lipopolysaccharide on Newly Established Rat Dental Pulp–derived Cell Line with Odontoblastic Properties

To clarify mechanisms of pulp wound healing and regeneration, it is important to establish continuous odontoblast-lineage cell lines. In this study, we established the proliferating pulp progenitor cell lines from dental papilla cells of rat incisor. These cell lines showed high levels of alkaline phosphatase (ALP) activity, expression of Runx2 and dentin sialophosphoprotein (DSPP), and extracellular formation of mineralized nodules. By using the cell line with high expression level of DSPP and the prominent mineral deposition, we examined whether bacterial lipopolysaccharide (LPS) had effects on its odontoblastic properties and found that ALP activity, expression of DSPP and Runx2, and the formation of mineralized nodules were suppressed in LPS dose-dependent manner. These results indicate that our established pulp progenitor cell line exhibits odontoblastic properties, which were suppressed by LPS, suggesting that gram-negative bacterial infection might downregulate the odontoblast function.

Tip60 histone acetyltransferase acts as a negative regulator of Notch1 signaling by means of acetylation

The Notch signaling pathway appears to perform an important function in a wide variety of organisms and cell types. In our present study, we provide evidence that UV irradiation-induced Tip60 proteins reduced Notch1 activity to a marked degree. Accumulated UV irradiation-induced Tip60 suppresses Notch1 transcriptional activity via the dissociation of the Notch1-IC-CSL complex. The binding between endogenous Tip60 and Notch1-IC in UV radiation-exposed cells was verified in this study by coimmunoprecipitation. Interestingly, the physical interaction of Tip60 with Notch1-IC occurs to a more profound degree in the presence of CSL but does not exist in a trimeric complex. Using Notch1-IC and Tip60 deletion mutants, we also determined that the N terminus, which harbors the RAM domain and seven ankyrin repeats of Notch1-IC, interacts with the zinc finger and acetyl coenzyme A domains of Tip60. Furthermore, here we report that Notch1-IC is a direct target of the acetyltransferase activity of Tip60. Collectively, our data suggest that Tip60 is an inhibitor of the Notch1 signaling pathway and that Tip60-dependent acetylation of Notch1-IC may be relevant to the mechanism by which Tip60 suppresses Notch1 signaling.

Influence of extended operation time and of occlusal force on determination of pulpal healing pattern in replanted mouse molars

The mechanism regulating the divergent healing processes following tooth replantation is unclear. This study clarifies the relationship between the healing pattern, the time taken for tooth replantation, and the influence of occlusal force. We investigated the pulpal healing process after tooth replantation by immunohistochemistry for 5-bromo-2'-deoxyuridine and nestin and by histochemistry for tartrate-resistant acid phosphatase. The upper right first molar of 3-week-old mice was extracted and repositioned in the original socket immediately or 30 min to 6 h after the operation. We divided the animals into a non-occluded group in which the lower right first molar was extracted and an occluded group without extraction of the counterpart tooth. In control teeth (upper left first molar), the periphery of the coronal dental pulp showed intense nestin-positive reaction. Tooth replantation weakened the nestin-positive reaction in the pulp tissue. On postoperative days 5-7, tubular dentin formation commenced next to preexisting dentin in which nestin-positive odontoblast-like cells were arranged in successful cases. In other cases, bone-like tissue formation occurred in the pulp chamber until day 14. The ratio of tertiary dentin formation was significantly higher in the non-occluded group. The intentionally prolonged time for the completion of tooth replantation induced bone-like tissue formation, expanded inflammatory reaction, or fibrous tissue formation in pulp tissue. Thus, the lack of a proper oxygenated medium is probably decisive for the survival of odontoblast-lineage cells, and occlusal force during and/or after operation worsens the fate of these cells.

ADAM28 participates in the regulation of tooth development

Disintegrin and metalloprotease (ADAM) proteins are a family of membrane-anchored glycoproteins with diverse functions in fertilisation, development, neurogenesis and protein ectodomain shedding. ADAM28 is a newly discovered member of the ADAM family in humans and murine with autocatalytic activity. Recently, the authors screened ADAM28 genes from patients with congenital hypoplasia of tooth root, and studied the relationship between ADAM28 and tooth development. A polyclonal antibody (pAb) against ADAM28 was preparared, and the expression and localisation of ADAM28 were detected in tooth germ and dental mesenchymal cells. The results indicated that the prokaryotic expression vector pGEX-4T-ADAM28 was constructed successfully. Glutathione S-transferase-ADAM28 fusion protein was generated after inducement by isopropylthio-beta-d-galactoside and isolated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The purified fusion protein was used as an antigen for production of antibody. Western blot and enzyme-linked immunosorbent assay analyses verified that the antibody had a high specificity and titre. Immunohistochemistry and reverse transcriptase-polymerase chain reaction showed that ADAM28 was expressed at each stage of tooth germ development at different levels. Moreover, it was expressed in human dental follicle cells, human dental papilla cells, human dental pulp stem cells, human periodontal ligament cells and human dental cervical loop epithelial cells at transcription level. In conclusion, it is reasonable to suggest that ADAM28 may participate in tooth development and the regulation of odontogenic mesenchymal cells through progressive reciprocal inductive interactions between the epithelium and the mesenchyme.

Smooth Muscle alpha-actin is a direct target of Notch/CSL

Intercellular signaling mediated by Notch receptors is essential for proper cardiovascular development and homeostasis. Notch regulates cell fate decisions that affect proliferation, survival, and differentiation of endothelial and smooth muscle cells. It has been reported that Jagged1-Notch interactions may participate in endocardial cushion formation by inducing endothelial-to-mesenchymal transformation. Here, we show that Notch directly regulates expression of the mesenchymal and smooth muscle cell marker smooth muscle alpha-actin (SMA) in endothelial and vascular smooth muscle cells via activation of its major effector, CSL. Notch/CSL activation induces SMA expression during endothelial-to-mesenchymal transformation, and Notch activation is required for expression of SMA in vascular smooth muscle cells. CSL directly binds a conserved cis element in the SMA promoter, and this consensus sequence is required for Notch-mediated SMA induction. This is the first evidence of the requirement for Notch activation in the regulation of SMA expression.

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.

Defining the caspase-containing apoptotic machinery contributing to cornification in human epidermal equivalents

Whether terminal differentiation/stratum corneum formation of keratinocytes (KCs) represents a form of programmed cell death, utilizing mediators of classical apoptosis, is unclear. Apoptosis, an evolutionarily conserved death process, is comprised of extrinsic and intrinsic pathways, which converge using caspase 3. To define upstream and downstream caspases involved in terminal differentiation, we utilized human epidermal equivalents (EEs). Using submerged cultures comprised of human KCs, EEs were sequentially analyzed before and after being raised to an air/liquid (A/L) interface at 3-24 h intervals. At each time point, EEs were analyzed morphologically and for specific enzyme activity to distinguish different initiator (caspases 1, 2, 8, 9) and effector caspases (3, 6, 7). Terminal differentiation began at 6-8 h, as defined by stratum corneum with loricirin expression and completed at 18-24 h producing an epidermis resembling normal skin. Enzyme activity for caspases 1, 2, 3, 6, 7, 8, and 9 (but not 4, 5) was enhanced (>two-fold nmol/mg/h) at 3-6 h compared with submerged cultures. Processing of caspase 14 occurred at 18 h, and cleaved caspase 14 was increased at 24 h. Activated caspase 3-positive and terminal deoxynucleotidyl transferase-mediated nick end labeling-positive KCs were identified in EEs at 3-6 h corresponding to initiation sites of terminal differentiation. Addition of caspase inhibitors reduced levels of involucrin and loricrin in EEs raised to an A/L interface. We conclude caspases function as important death effectors strategically positioned at intersection of intrinsic and extrinsic pathways in KCs undergoing stratum corneum formation.

Activation of the Notch signaling pathway in response to pulp capping of rat molars

Notch signaling is an evolutionarily conserved pathway that controls the developmental choices made by individual cells. Cells communicate via Notch receptors and their ligands, which direct decisions on the fate of stem cells according to the states of their neighbors. In this study we explored Notch signaling after the pulp capping of adult first upper rat molars. The wound was capped with calcium hydroxide. In situ hybridization revealed an increased expression of Notch signaling genes on day 1, which showed a tendency to decrease on day 3. Notch1 increased in the subodontoblast zone and close to the lesion limited to a few cells. Notch2 increased in pulp stroma surrounded by coronal odontoblasts. Notch1 and, especially, Notch3 expression increased, corresponding to perivascular cell groups. A low increase of ligand expression was observed near the injury with Delta1 expression along the dentin wall and Jagged1 in the stroma. Expression of the downstream target, Hes1, was observed along the lesion and adjacent dentin walls. Hes5 expression was not observed. The results indicate that Notch signaling is activated in response to injury and associated with the differentiation of pulp cells into perivascular cells and odontoblasts. The findings are consistent with the concept that the Notch pathway controls stem cell fate during pulp regeneration.

Analysis of gene and protein expression in healthy and carious tooth pulp with cDNA microarray and two-dimensional gel electrophoresis

Complementary DNA (cDNA) microarray and two-dimensional (2-D) gel electrophoresis, combined with mass spectrometry, enable simultaneous analysis of expression patterns of thousands of genes, but their use in pulp biology has been limited. Here we compared gene and protein expression of pulp tissues from sound and carious human teeth using cDNA microarray and 2-D gel electrophoresis to evaluate their usefulness in pulp biology research and to identify the genes with changes in carious teeth. The cDNA microarray revealed several differentially expressed genes and genes with a high expression in both tissues. These genes have various functions, e.g. effects on vascular and nerve structures, inflammation, and cell differentiation. Variability between cDNA hybridizations indicates that the overall gene expression pattern may vary significantly between individual teeth. The 2-D gel electrophoresis revealed no change between healthy and diseased tissue. The identification of 96 proteins in the pulp tissue revealed none of the gene products with corresponding high/different mRNA expression in cDNA microarray. Interestingly, we detected also a hypothetical protein (putative nucleoside diphosphate kinase), and present therefore the first evidence for the existence of this protein. Even though the methods reveal potentially important gene expression, they may currently have only limited value in in vivo pulp biology research.