BMI1 represses Ink4a/Arf and Hox genes to regulate stem cells in the rodent incisor. Nat Cell Biol. 2013;15:846-852. [PMID: 23728424 PMCID: PMC3735916 DOI: 10.1038/ncb2766]

Isolation of Dental Stem Cell-Enriched Populations from Continuously Growing Mouse Incisors

Continuous growth of the rodent incisor is enabled by epithelial and mesenchymal stem cells (ESCs and MSCs) which unceasingly replenish enamel and dentin, respectively, that wear by persistent animal gnawing. Lineage tracing studies have provided evidence that ESCs contribute to all epithelial lineages of the tooth in vivo. Meanwhile, in the mouse incisor, MSCs continuously contribute to odontoblast lineage and tooth growth. However, in vitro manipulation of ESCs has shown little progress, mainly due to lack of appropriate protocol to successfully isolate, culture, expand, and differentiate ESCs in vitro without using the co-culture system. In this chapter we describe the isolation of the Sox2-GFP+ cell population that is highly enriched in ESCs. Isolated cells can be used for various types of analyses, including in vitro culture, single cell-related analyses, etc. Furthermore, we describe ways to obtain populations enriched in the incisor MSCs using FACS sorting of antibody-labeled cells. Easily accessible FACS sorting enables easy and relatively fast isolation of the cells labeled by the fluorescent protein.

Age-related oxidative stress confines damage-responsive Bmi1+ cells to perivascular regions in the murine adult heart

Adult progenitor cells reside in specialized microenvironments which maintain their undifferentiated cell state and trigger regenerative responses following injury. Although these environments are well described in several tissues, the cellular components that comprise the cardiac environment where progenitor cells are located remain unknown. Here we use Bmi1^CreERT and Bmi1^GFP mice as genetic tools to trace cardiac damage-responsive cells throughout the mouse lifespan. In adolescent mice, Bmi1⁺ damage-responsive cells are broadly distributed throughout the myocardium. In adult mice, however, Bmi1⁺ cells are confined predominately in perivascular areas with low levels of reactive oxygen species (ROS) and their number decline in an age-dependent manner. In vitro co-culture experiments with endothelial cells supported a regulatory role of the endothelium in damage-responsive cell behavior. Accordingly, in vivo genetic decrease of ROS levels in adult heart disengaged Bmi1⁺ cells from the cardiovascular network, recapitulating an adolescent-like Bmi1 expression profile. Thus, we identify cardiac perivascular regions as low-stress microenvironments that favor the maintenance of adult damage-responsive cells.

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Bmi1⁺ cardiac damage-responsive cells are sheltered in areas with low ROS levels.

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Aging-related oxidative damage confines cardiac Bmi1⁺ cells to perivascular regions.

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Microvasculature-derived signals regulate adult Bmi1⁺ damage-responsive cell behavior.

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Genetic ROS levels manipulation modifies the percentage and identity of Bmi1⁺ cells.

BMI1 Deficiency Results in Female Infertility by Activating p16/p19 Signaling and Increasing Oxidative Stress

The polycomb repressor B lymphoma Mo-MLV insertion region 1 (BMI1) is a core composition of polycomb repressive complex 1 (PRC1) and contributes to diverse fundamental cellular processes including cell senescence, apoptosis and proliferation. To investigate the role and mechanism of BMI1 in maintaining normal female reproductive function, we compared the differences in reproductive phenotypes between Bmi1-deficient and wild-type female mice. The Bmi1-deficient female mice were then supplemented with N-acetylcysteine in their drinking water to explore whether antioxidant supplementation could improve reproductive dysfunction caused by BMI1 deficiency. The results revealed that Bmi1 deletion resulted in complete infertility in female mice, estrous cycle disorder, and follicular developmental disorders. The reactive oxygen species levels in the ovarian tissue were increased; the ability of antioxidant enzymes was downregulated; the expression levels of p19 and p53 proteins were significantly upregulated. We also found that oocytes derived from Bmi1-deficient mice could not develop into embryos by in vitro fertilization and in vitro culture of embryos. Furthermore, supplementation with the antioxidant NAC not only improved the reproductive defects caused by Bmi1 deletion, but also largely rescued the ability of Bmi1-deficient oocytes to develop into embryos in vitro. These results indicated that cells lacking Bmi1 resulted in female infertility by activating the p16/p19 signaling pathway, increasing oxidative stress and DNA damage, inhibiting granulosa cell proliferation, and inducing granulosa cell apoptosis. Thus, BMI1 may be a novel potential target for the clinical treatment of female infertility.

Revitalising the rudimentary replacement dentition in the mouse

Most mammals have two sets of teeth (diphyodont) - a deciduous dentition replaced by a permanent dentition; however, the mouse possesses only one tooth generation (monophyodont). In diphyodonts, the replacement tooth forms on the lingual side of the first tooth from the successional dental lamina. This lamina expresses the stem/progenitor marker Sox2 and has activated Wnt/β-catenin signalling at its tip. Although the mouse does not replace its teeth, a transient rudimentary successional dental lamina (RSDL) still forms during development. The mouse RSDL houses Sox2-positive cells, but no Wnt/β-catenin signalling. Here, we show that stabilising Wnt/β-catenin signalling in the RSDL in the mouse leads to proliferation of the RSDL and formation of lingually positioned teeth. Although Sox2 has been shown to repress Wnt activity, overexpression of Wnts leads to a downregulation of Sox2, suggesting a negative-feedback loop in the tooth. In the mouse, the first tooth represses the formation of the replacement, and isolation of the RSDL is sufficient to induce formation of a new tooth germ. Our data highlight key mechanisms that may have influenced the evolution of replacement teeth.This article has an associated 'The people behind the papers' interview.

p53 induces senescence through Lamin A/C stabilization-mediated nuclear deformation

p53-mediated cellular senescence has been intensively investigated, because it is important for tumor suppressive function. In addition, p16/INK4A is well known to be critical for cellular senescence. However, detailed molecular mechanism or relevance between p53 and p16-mediated senescence has not been demonstrated yet. Here we show that p53 induces p16 through Lamin A/C stabilization via direct interaction. Stabilized Lamin A/C promotes degradation of BMI-1 and MEL-18 (Polycomb repressor complex 1, PRC1), which sequesters p16 promotor. Increased p53 can reduce BMI-1/MEL-18 and induce p16 expression via Lamin A/C. Elimination of Lamin A/C can abolish p53-induced p16 expression and BMI-1/MEL-18 reduction. As Lamin A/C expression is increased during cell differentiation, this mechanism seems to be very useful for selective induction of senescence in non-stem cells. Our results suggest that Lamin A/C-p53 network is important for p16/INK4A-mediated cellular senescence.

Prominin-1 controls stem cell activation by orchestrating ciliary dynamics

Proper temporal and spatial activation of stem cells relies on highly coordinated cell signaling. The primary cilium is the sensory organelle that is responsible for transmitting extracellular signals into a cell. Primary cilium size, architecture, and assembly-disassembly dynamics are under rigid cell cycle-dependent control. Using mouse incisor tooth epithelia as a model, we show that ciliary dynamics in stem cells require the proper functions of a cholesterol-binding membrane glycoprotein, Prominin-1 (Prom1/CD133), which controls sequential recruitment of ciliary membrane components, histone deacetylase, and transcription factors. Nuclear translocation of Prom1 and these molecules is particularly evident in transit amplifying cells, the immediate derivatives of stem cells. The absence of Prom1 impairs ciliary dynamics and abolishes the growth stimulation effects of sonic hedgehog (SHH) treatment, resulting in the disruption of stem cell quiescence maintenance and activation. We propose that Prom1 is a key regulator ensuring appropriate response of stem cells to extracellular signals, with important implications for development, regeneration, and diseases.

Bmi-1 high-expressing cells enrich cardiac stem/progenitor cells and respond to heart injury

Bmi‐1 gene is well recognized as an oncogene, but has been recently demonstrated to play a role in the self‐renewal of tissue‐specific stem cells. By using Bmi‐1^GFP/+ mice, we investigated the role of Bmi‐1 in cardiac stem/progenitor cells and myocardial repair. RT‐PCR and flow cytometry analysis indicated that the expression of Bmi‐1 was significantly higher in cardiac side population than the main population from CD45⁻Ter119⁻CD31⁻ heart cells. More Sca‐1⁺ cardiac stem/progenitor cells were found in Bmi‐1 GFP^hi subpopulation, and these Bmi‐1 GFP^hi heart cells showed the potential of differentiation into SMM⁺ smooth muscle‐like cells and TnT⁺ cardiomyocyte‐like cells in vitro. The silencing of Bmi‐1 significantly inhibited the proliferation and differentiation of heart cells. Otherwise, myocardial infarction induced a significantly increase (2.7‐folds) of Bmi‐1 GFP^hi population, mainly within the infarction and border zones. These preliminary data suggest that Bmi‐1^hi heart cells are enriched in cardiac stem/progenitor cells and may play a role in myocardial repair.

Polycomb repression regulates Schwann cell proliferation and axon regeneration after nerve injury

The transition of differentiated Schwann cells to support of nerve repair after injury is accompanied by remodeling of the Schwann cell epigenome. The EED-containing polycomb repressive complex 2 (PRC2) catalyzes histone H3K27 methylation and represses key nerve repair genes such as Shh, Gdnf, and Bdnf, and their activation is accompanied by loss of H3K27 methylation. Analysis of nerve injury in mice with a Schwann cell-specific loss of EED showed the reversal of polycomb repression is required and a rate limiting step in the increased transcription of Neuregulin 1 (type I), which is required for efficient remyelination. However, mouse nerves with EED-deficient Schwann cells display slow axonal regeneration with significantly low expression of axon guidance genes, including Sema4f and Cntf. Finally, EED loss causes impaired Schwann cell proliferation after injury with significant induction of the Cdkn2a cell cycle inhibitor gene. Interestingly, PRC2 subunits and CDKN2A are commonly co-mutated in the transition from benign neurofibromas to malignant peripheral nerve sheath tumors (MPNST's). RNA-seq analysis of EED-deficient mice identified PRC2-regulated molecular pathways that may contribute to the transition to malignancy in neurofibromatosis.

Regulation of Mesenchymal Stem to Transit-Amplifying Cell Transition in the Continuously Growing Mouse Incisor

In adult tissues and organs with high turnover rates, the generation of transit-amplifying cell (TAC) populations from self-renewing stem cells drives cell replacement. The role of stem cells is to provide a renewable source of cells that give rise to TACs to provide the cell numbers that are necessary for cell differentiation. Regulation of the formation of TACs is thus fundamental to controlling cell replacement. Here, we analyze the properties of a population of mesenchymal TACs in the continuously growing mouse incisor to identify key components of the molecular regulation that drives proliferation. We show that the polycomb repressive complex 1 acts as a global regulator of the TAC phenotype by its direct action on the expression of key cell-cycle regulatory genes and by regulating Wnt/β-catenin-signaling activity. We also identify an essential requirement for TACs in maintaining mesenchymal stem cells, which is indicative of a positive feedback mechanism.

Effect of Bmi1 over-expression on gene expression in adult and embryonic murine neural stem cells

The ability of isolated neural stem cells (NSCs) to proliferate as neurospheres is indicative of their competence as stem cells, and depends critically on the polycomb group (PcG) member Bmi1: knockdown of Bmi1 results in defective proliferation and self-renewal of isolated NSCs, whereas overexpression of Bmi1 enhances these properties. Here we report genome-wide changes in gene expression in embryonic and adult NSCs (eNSCs and aNSCs) caused by overexpression of Bmi1. We find that genes whose expression is altered by perturbations in Bmi1 levels in NSCs are mostly distinct from those affected in other multipotent stem/progenitor cells, such as those from liver and lung, aside from a small core of common targets that is enriched for genes associated with cell migration and mobility. We also show that genes differing in expression between prospectively isolated quiescent and activated NSCs are not affected by Bmi1 overexpression. In contrast, a comparison of genes showing altered expression upon Bmi1 overexpression in eNSCs and in aNSCs reveals considerable overlap, in spite of their different provenances in the brain and their differing developmental programs.

The Role of Epithelial Stat3 in Amelogenesis during Mouse Incisor Renewal

The aim of this study was to evaluate the role of epithelial signal transducer and activator of transcription 3 (STAT3) in mouse incisor amelogenesis. Since Stat3 is expressed in the epithelial component of developing and adult mouse teeth, we generated and analyzed Krt14Cre/+;Stat3fl/fl mutant mice in which Stat3 was inactivated in epithelia including ameloblast progenitors and ameloblasts, the cells responsible for enamel formation. Histological analysis showed little enamel matrix in mutant incisors compared to controls. Delayed incisor enamel mineralization was demonstrated using micro-computed X-ray tomography analysis and was supported by an increase in the pre-expression distance of enamel-enriched proteins such as amelogenin, ameloblastin, and kallikrein-4. Lastly, scanning electron microscopy analysis showed little enamel mineralization in mutant incisors underneath the mesial root of the 1st molar; however, the micro-architecture of enamel mineralization was similar in the erupted portion of control and mutant incisors. Taken together, our findings demonstrate for the first time that the absence of epithelial Stat3 in mice leads to delayed incisor amelogenesis.

Biology Explaining Tooth Repair and Regeneration: A Mini-Review

The tooth is an intricate composition of precisely patterned, mineralized matrices and soft tissues. Mineralized tissues include enamel (produced by the epithelial cells called ameloblasts), dentin and cementum (produced by mesenchymal cells called odontoblasts and cementoblasts, respectively), and soft tissues, which include the dental pulp and the periodontal ligament along with the invading nerves and blood vessels. It was perceived for a very long time that teeth primarily serve an esthetical function. In recent years, however, the role of healthy teeth, as well as the impact of oral health on general well-being, became more evident. Tooth loss, caused by tooth decay, congenital malformations (tooth agenesis), trauma, periodontal diseases, or age-related changes, is usually replaced by artificial materials which lack many of the important biological characteristics of the natural tooth. Human teeth have very low to almost absent regeneration potential, due to early loss of cell populations with regenerative capacity, namely stem cells. Significant effort has been made in recent decades to identify and characterize tooth stem cells, and to unravel the developmental programs which these cells follow in order to generate a tooth.

Bmi1 Deficient Mice Exhibit Male Infertility

Previous studies have demonstrated that the polycomb repressor Bmi1 is universally expressed in all types of testicular cells and might regulate the spermatogonia proliferation, however, it is unclear whether Bmi1 plays a critical role in maintaining normal male fertility in vivo. To answer this question, we first confirmed that Bmi1 is universally expressed in all types of testicular cells and found that the gene relative expression levels of Bmi1 in testis were the highest relative to other organs. Then we investigated the role of Bmi1 in maintaining normal male fertility using Bmi1 knockout male mouse model. Our results demonstrated that Bmi1 deficiency resulted in totally male infertility with smaller testis, severe oligospermia and sperm malformation. Mechanistically, decreased serum testosterone levels with down-regulating 3βHSD and 17βHSD expression levels, reduced germ cell proliferation, increased germ cell apoptosis with up-regulating p16, p19, p53 and p21 expression levels, increased reactive oxygen species (ROS) and H₂O₂ levels with down-regulating gene expression levels of anti-oxidant enzymes, and increased 8-OHdG and γ.H2AX positive cells in testis were observed in Bmi1 deficient mice compared with wild-type mice. These results indicate that Bmi1 deficiency results in male infertility by reducing testosterone syntheses, increasing oxidative stress and DNA damage, activating p16 and p19 signaling pathway, inhibiting germ cell proliferation and inducing germ cell apoptosis and sperm malformation. Thus, Bmi1 may be a novel and potential target for the clinic treatment of male infertility.

The putative tumor suppressor gene EphA7 is a novel BMI-1 target

Bmi1 was originally identified as a gene that contributes to the development of mouse lymphoma by inhibiting MYC-induced apoptosis through repression of Ink4a and Arf. It codes for the Polycomb group protein BMI-1 and acts primarily as a transcriptional repressor via chromatin modifications. Although it binds to a large number of genomic regions, the direct BMI-1 target genes described so far do not explain the full spectrum of BMI-1-mediated effects. Here we identify the putative tumor suppressor gene EphA7 as a novel direct BMI-1 target in neural cells and lymphocytes. EphA7 silencing has been reported in several different human tumor types including lymphomas, and our data suggest BMI1 overexpression as a novel mechanism leading to EphA7 inactivation via H3K27 trimethylation and DNA methylation.

Oral Methotrexate-related Lymphoproliferative Disease Presenting with Severe Osteonecrosis of the Jaw: A Case Report and Literature Review

Long-term methotrexate (MTX) treatment can cause MTX-related lymphoproliferative disorder (MTX-LPD). We experienced a case of MTX-LPD that was associated with severe osteonecrosis of the jaw mimicking medication-related osteonecrosis of the jaw. The patient was an 81-year-old woman with rheumatoid arthritis (RA) who was treated with MTX and bisphosphonate. After 7 years, she was referred to our department for the assessment of giant ulcer and exposure of the alveolar bone of the left maxilla. Histopathological and immunological analyses confirmed a diagnosis of MTX-LPD. At seven months after the cessation of MTX treatment, the ulcerative and necrotic lesions had markedly decreased in size. A 1-year follow-up examination showed no evidence of recurrence and good RA control.

Bmi1+ Progenitor Cell Dynamics in Murine Cornea During Homeostasis and Wound Healing

The outermost layer of the eye, the cornea, is renewed continuously throughout life. Stem cells of the corneal epithelium reside in the limbus at the corneal periphery and ensure homeostasis of the central epithelium. However, in young mice, homeostasis relies on cells located in the basal layer of the central corneal epithelium. Here, we first studied corneal growth during the transition from newborn to adult and assessed Keratin 19 (Krt19) expression as a hallmark of corneal maturation. Next, we set out to identify a novel marker of murine corneal epithelial progenitor cells before, during and after maturation, and we found that Bmi1 is expressed in the basal epithelium of the central cornea and limbus. Furthermore, we demonstrated that Bmi1+ cells participated in tissue replenishment in the central cornea. These Bmi1+ cells did not maintain homeostasis of the cornea for more than 3 months, reflecting their status as progenitor rather than stem cells. Finally, after injury, Bmi1+ cells fueled homeostatic maintenance, whereas wound closure occurred via epithelial reorganization. Stem Cells 2018;36:562-573.

Pyrroloquinoline quinone plays an important role in rescuing Bmi-1-/- mice induced developmental disorders of teeth and mandible--anti-oxidant effect of pyrroloquinoline quinone

To investigate whether pyrroloquinoline quinine (PQQ) plays an important role in rescuing Bmi-1^-/- mice induced developmental disorders of teeth and mandible by regulating oxidative stress. We fed Bmi-1^-/- mice a diet supplemented with PQQ (BKO+PQQ), BKO mice with normal diet (BKO) and wild type mice with normal diet (WT) as controls. We compared the differences of dental, mandibular phenotype by means of X-ray photography, micro CT scanning and three-dimensional reconstruction, HE staining, histochemistry, immunoistohemistry, TUNEL staining, Western blot and Flow cytometry in three groups of animals. Results showed that BKO+PQQ mice increased morphology of teeth and mandible, decreased X-ray transmittance, and increased bone density compared with BKO mice. Results also showed that the teeth volume and the dentin sialoprotein (DSP) immunopositive areas, the cortical thickness, alveolar bone volume, osteoblast number and activity, and alkaline phosphatase (ALP), Osteocalcin (OCN) and type I collagen (Col 1) were all reduced significantly in BKO mice compared with their wild-type littermates, whereas these parameters were increased significantly in BKO+PQQ mice compared with BKO mice. Our study indicated that, compared BKO mice, PCNA positive cells percentage of mandibular first molar epithelial root sheath area significantly increased in BKO+PQQ mice, and TUNEL positive cells percentage was significantly decreased. Further studies showed that supplemental PQQ played a role in anti-osteoporosis of teeth and mandible by up-regulating anti-oxidant capacity, inhibiting oxidative stress and reducing DNA damage, down-regulating CDKI proteins levels, and decreasing cell apoptosis. This study demonstrated that PQQ played an important role in rescuing mandible osteoporosis and disorder of teeth development in BKO mice by promoting osteoblastic bone formation of mandibular alveolar bone, inhibiting osteoclastic bone resorption, promoting odontoblast cell proliferation of epithelial root sheath area, inhibiting cell apoptosis, scavenging ROS.

The IRX1/HOXA connection: insights into a novel t(4;11)- specific cancer mechanism

One hallmark of MLL-r leukemia is the highly specific gene expression signature indicative for commonly deregulated target genes. An usual read-out for this transcriptional deregulation is the HOXA gene cluster, where upregulated HOXA genes are detected in MLL-r AML and ALL patients. In case of t(4;11) leukemia, this simple picture becomes challenged, because these patients separate into HOXA^hi- and HOXA^lo-patients. HOXA^lo-patients showed a reduced HOXA gene transcription, but instead overexpressed the homeobox gene IRX1. This transcriptional pattern was associated with a higher relapse rate and worse outcome. Here, we demonstrate that IRX1 binds to the MLL-AF4 complex at target gene promotors and counteract its promotor activating function. In addition, IRX1 induces transcription of HOXB4 and EGR family members. HOXB4 is usually a downstream target of c-KIT, WNT and TPO signaling pathways and necessary for maintaining and expanding in hematopoietic stem cells. EGR proteins control a p21-dependent quiescence program for hematopoietic stem cells. Both IRX1-dependend actions may help t(4;11) leukemia cells to establish a stem cell compartment. We also demonstrate that HDACi administration is functionally interfering with IRX1 and MLL-AF4, a finding which could help to improve new treatment options for t(4;11) patients.

Redox-dependent BMI1 activity drives in vivo adult cardiac progenitor cell differentiation

Accumulation of reactive oxygen species (ROS) is associated with several cardiovascular pathologies and with cell cycle exit by neonanatal cardiomyocytes, a key limiting factor in the regenerative capacity of the adult mammalian heart. The polycomb complex component BMI1 is linked to adult progenitors and is an important partner in DNA repair and redox regulation. We found that high BMI1 expression is associated with an adult Sca1⁺ cardiac progenitor sub-population with low ROS levels. In homeostasis, BMI1 repressed cell fate genes, including a cardiogenic differentiation program. Oxidative damage nonetheless modified BMI1 activity in vivo by derepressing canonical target genes in favor of their antioxidant and anticlastogenic functions. This redox-mediated mechanism is not restricted to damage situations, however, and we report ROS-associated differentiation of cardiac progenitors in steady state. These findings demonstrate how redox status influences the cardiac progenitor response, and identify redox-mediated BMI1 regulation with implications in maintenance of cellular identity in vivo.

Plasticity within the niche ensures the maintenance of a Sox2+ stem cell population in the mouse incisor

In mice, the incisors grow throughout the animal's life, and this continuous renewal is driven by dental epithelial and mesenchymal stem cells. Sox2 is a principal marker of the epithelial stem cells that reside in the mouse incisor stem cell niche, called the labial cervical loop, but relatively little is known about the role of the Sox2⁺ stem cell population. In this study, we show that conditional deletion of Sox2 in the embryonic incisor epithelium leads to growth defects and impairment of ameloblast lineage commitment. Deletion of Sox2 specifically in Sox2⁺ cells during incisor renewal revealed cellular plasticity that leads to the relatively rapid restoration of a Sox2-expressing cell population. Furthermore, we show that Lgr5-expressing cells are a subpopulation of dental Sox2⁺ cells that also arise from Sox2⁺ cells during tooth formation. Finally, we show that the embryonic and adult Sox2⁺ populations are regulated by distinct signalling pathways, which is reflected in their distinct transcriptomic signatures. Together, our findings demonstrate that a Sox2⁺ stem cell population can be regenerated from Sox2^- cells, reinforcing its importance for incisor homeostasis.

Meeting report: a hard look at the state of enamel research

The Encouraging Novel Amelogenesis Models and Ex vivo cell Lines (ENAMEL) Development workshop was held on 23 June 2017 at the Bethesda headquarters of the National Institute of Dental and Craniofacial Research (NIDCR). Discussion topics included model organisms, stem cells/cell lines, and tissues/3D cell culture/organoids. Scientists from a number of disciplines, representing institutions from across the United States, gathered to discuss advances in our understanding of enamel, as well as future directions for the field.

Isl1 Controls Patterning and Mineralization of Enamel in the Continuously Renewing Mouse Incisor

Rodents are characterized by continuously renewing incisors whose growth is fueled by epithelial and mesenchymal stem cells housed in the proximal compartments of the tooth. The epithelial stem cells reside in structures known as the labial (toward the lip) and lingual (toward the tongue) cervical loops (laCL and liCL, respectively). An important feature of the rodent incisor is that enamel, the outer, highly mineralized layer, is asymmetrically distributed, because it is normally generated by the laCL but not the liCL. Here, we show that epithelial-specific deletion of the transcription factor Islet1 (Isl1) is sufficient to drive formation of ectopic enamel by the liCL stem cells, and also that it leads to production of altered enamel on the labial surface. Molecular analyses of developing and adult incisors revealed that epithelial deletion of Isl1 affected multiple, major pathways: Bmp (bone morphogenetic protein), Hh (hedgehog), Fgf (fibroblast growth factor), and Notch signaling were upregulated and associated with liCL-generated ectopic enamel; on the labial side, upregulation of Bmp and Fgf signaling, and downregulation of Shh were associated with premature enamel formation. Transcriptome profiling studies identified a suite of differentially regulated genes in developing Isl1 mutant incisors. Our studies demonstrate that ISL1 plays a central role in proper patterning of stem cell-derived enamel in the incisor and indicate that this factor is an important upstream regulator of signaling pathways during tooth development and renewal. © 2017 American Society for Bone and Mineral Research.

Cellular and molecular mechanisms of tooth root development

The tooth root is an integral, functionally important part of our dentition. The formation of a functional root depends on epithelial-mesenchymal interactions and integration of the root with the jaw bone, blood supply and nerve innervations. The root development process therefore offers an attractive model for investigating organogenesis. Understanding how roots develop and how they can be bioengineered is also of great interest in the field of regenerative medicine. Here, we discuss recent advances in understanding the cellular and molecular mechanisms underlying tooth root formation. We review the function of cellular structure and components such as Hertwig's epithelial root sheath, cranial neural crest cells and stem cells residing in developing and adult teeth. We also highlight how complex signaling networks together with multiple transcription factors mediate tissue-tissue interactions that guide root development. Finally, we discuss the possible role of stem cells in establishing the crown-to-root transition, and provide an overview of root malformations and diseases in humans.

DENTAL ENAMEL FORMATION AND IMPLICATIONS FOR ORAL HEALTH AND DISEASE

Dental enamel is the hardest and most mineralized tissue in extinct and extant vertebrate species and provides maximum durability that allows teeth to function as weapons and/or tools as well as for food processing. Enamel development and mineralization is an intricate process tightly regulated by cells of the enamel organ called ameloblasts. These heavily polarized cells form a monolayer around the developing enamel tissue and move as a single forming front in specified directions as they lay down a proteinaceous matrix that serves as a template for crystal growth. Ameloblasts maintain intercellular connections creating a semi-permeable barrier that at one end (basal/proximal) receives nutrients and ions from blood vessels, and at the opposite end (secretory/apical/distal) forms extracellular crystals within specified pH conditions. In this unique environment, ameloblasts orchestrate crystal growth via multiple cellular activities including modulating the transport of minerals and ions, pH regulation, proteolysis, and endocytosis. In many vertebrates, the bulk of the enamel tissue volume is first formed and subsequently mineralized by these same cells as they retransform their morphology and function. Cell death by apoptosis and regression are the fates of many ameloblasts following enamel maturation, and what cells remain of the enamel organ are shed during tooth eruption, or are incorporated into the tooth's epithelial attachment to the oral gingiva. In this review, we examine key aspects of dental enamel formation, from its developmental genesis to the ever-increasing wealth of data on the mechanisms mediating ionic transport, as well as the clinical outcomes resulting from abnormal ameloblast function.

Ras Signaling Regulates Stem Cells and Amelogenesis in the Mouse Incisor

The role of Ras signaling during tooth development is poorly understood. Ras proteins-which are activated by many upstream pathways, including receptor tyrosine kinase cascades-signal through multiple effectors, such as the mitogen-activated protein kinase (MAPK) and PI3K pathways. Here, we utilized the mouse incisor as a model to study how the MAPK and PI3K pathways regulate dental epithelial stem cells and amelogenesis. The rodent incisor-which grows continuously throughout the life of the animal due to the presence of epithelial and mesenchymal stem cells-provides a model for the study of ectodermal organ renewal and regeneration. Utilizing models of Ras dysregulation as well as inhibitors of the MAPK and PI3K pathways, we found that MAPK and PI3K regulate dental epithelial stem cell activity, transit-amplifying cell proliferation, and enamel formation in the mouse incisor.

Resolving stem and progenitor cells in the adult mouse incisor through gene co-expression analysis

Investigations into stem cell-fueled renewal of an organ benefit from an inventory of cell type-specific markers and a deep understanding of the cellular diversity within stem cell niches. Using the adult mouse incisor as a model for a continuously renewing organ, we performed an unbiased analysis of gene co-expression relationships to identify modules of co-expressed genes that represent differentiated cells, transit-amplifying cells, and residents of stem cell niches. Through in vivo lineage tracing, we demonstrated the power of this approach by showing that co-expression module members Lrig1 and Igfbp5 define populations of incisor epithelial and mesenchymal stem cells. We further discovered that two adjacent mesenchymal tissues, the periodontium and dental pulp, are maintained by distinct pools of stem cells. These findings reveal novel mechanisms of incisor renewal and illustrate how gene co-expression analysis of intact biological systems can provide insights into the transcriptional basis of cellular identity.

An FAK-YAP-mTOR Signaling Axis Regulates Stem Cell-Based Tissue Renewal in Mice

Tissue homeostasis requires the production of newly differentiated cells from resident adult stem cells. Central to this process is the expansion of undifferentiated intermediates known as transit-amplifying (TA) cells, but how stem cells are triggered to enter this proliferative TA state remains an important open question. Using the continuously growing mouse incisor as a model of stem cell-based tissue renewal, we found that the transcriptional cofactors YAP and TAZ are required both to maintain TA cell proliferation and to inhibit differentiation. Specifically, we identified a pathway involving activation of integrin α3 in TA cells that signals through an LATS-independent FAK/CDC42/PP1A cascade to control YAP-S397 phosphorylation and nuclear localization. This leads to Rheb expression and potentiates mTOR signaling to drive the proliferation of TA cells. These findings thus reveal a YAP/TAZ signaling mechanism that coordinates stem cell expansion and differentiation during organ renewal.

Activation of Wnt/β-catenin signaling by lithium chloride attenuates d-galactose-induced neurodegeneration in the auditory cortex of a rat model of aging

Degeneration of the central auditory system, which is characterized by reduced understanding of speech and source localization of sounds, is an important cause of age‐related hearing loss (presbycusis). Accumulating evidence has demonstrated that Wnt/β‐catenin signaling plays an essential role in the development of the auditory system but its potential role in presbycusis remains unclear. In this study, we used a rat model of aging, created by chronic systemic exposure to d‐galactose (d‐gal), and explored changes in Wnt/β‐catenin signaling in the auditory cortex. A decrease in Wnt/β‐catenin signaling in the auditory cortex was found in both naturally aging and d‐gal‐mimetic aging rats, as indicated by increased GSK3β activity and decreased β‐catenin activity. Moreover, lithium chloride (Licl), an activator of Wnt signaling pathway, was administered long term to 15‐month‐old d‐gal‐treated rats. Activation of Wnt/β‐catenin signaling by Licl attenuated d‐gal‐induced auditory cortex apoptosis and neurodegeneration. Bmi1, a transcription factor implicated in antiaging and resistance to apoptosis, can be modulated by β‐catenin activity. Here, we showed that the expression of Bmi1 was reduced and the expression of its downstream genes, p16^INK4a, p19^Arf, and p53 were increased in the auditory cortex both of naturally aging and d‐gal‐mimetic aging rats. In addition, Licl significantly increased Bmi1 expression and reduced p16^INK4a, p19^Arf, and p53 expression. Our results indicated that decreased Wnt/β‐catenin signaling might participate in the pathogenesis of central presbycusis through modulating the expression of Bmi1. Wnt/β‐catenin signaling might be used as a potential therapeutic target against presbycusis.

De Novo Genome and Transcriptome Assembly of the Canadian Beaver (Castor canadensis)

The Canadian beaver (Castor canadensis) is the largest indigenous rodent in North America. We report a draft annotated assembly of the beaver genome, the first for a large rodent and the first mammalian genome assembled directly from uncorrected and moderate coverage (< 30 ×) long reads generated by single-molecule sequencing. The genome size is 2.7 Gb estimated by k-mer analysis. We assembled the beaver genome using the new Canu assembler optimized for noisy reads. The resulting assembly was refined using Pilon supported by short reads (80 ×) and checked for accuracy by congruency against an independent short read assembly. We scaffolded the assembly using the exon–gene models derived from 9805 full-length open reading frames (FL-ORFs) constructed from the beaver leukocyte and muscle transcriptomes. The final assembly comprised 22,515 contigs with an N50 of 278,680 bp and an N50-scaffold of 317,558 bp. Maximum contig and scaffold lengths were 3.3 and 4.2 Mb, respectively, with a combined scaffold length representing 92% of the estimated genome size. The completeness and accuracy of the scaffold assembly was demonstrated by the precise exon placement for 91.1% of the 9805 assembled FL-ORFs and 83.1% of the BUSCO (Benchmarking Universal Single-Copy Orthologs) gene set used to assess the quality of genome assemblies. Well-represented were genes involved in dentition and enamel deposition, defining characteristics of rodents with which the beaver is well-endowed. The study provides insights for genome assembly and an important genomics resource for Castoridae and rodent evolutionary biology.

Maternal diabetes modulates dental epithelial stem cells proliferation and self-renewal in offspring through apurinic/apyrimidinicendonuclease 1-mediated DNA methylation

Maternal gestational diabetes mellitus (GDM) has many adverse effects on the development of offspring. Aberrant DNA methylation is a potential mechanism associated with these effects. However, the effects of GDM on tooth development and the underlying mechanisms have not been thoroughly investigated. In the present study, a GDM rat model was established and incisor labial cervical loop tissue and dental epithelial stem cells (DESCs) were harvested from neonates of diabetic and control dams. GDM significantly suppressed incisor enamel formation and DESCs proliferation and self-renewal in offspring. Gene expression profiles showed that Apex1 was significantly downregulated in the offspring of diabetic dams. In vitro, gain and loss of function analyses showed that APEX1 was critical for DESCs proliferation and self-renewal and Oct4 and Nanog regulation via promoter methylation. In vivo, we confirmed that GDM resulted in significant downregulation of Oct4 and Nanog and hypermethylation of their promoters. Moreover, we found that APEX1 modulated DNA methylation by regulating DNMT1 expression through ERK and JNK signalling. In summary, our data suggest that GDM-induced APEX1 downregulation increased DNMT1 expression, thereby inhibiting Oct4 and Nanog expression, through promoter hypermethylation, resulting in suppression of DESCs proliferation and self-renewal, as well as enamel formation.

Polycomb complex protein BMI-1 promotes invasion and metastasis of pancreatic cancer stem cells by activating PI3K/AKT signaling, an ex vivo, in vitro, and in vivo study

Cancer stem cell theory indicates cancer stem cells are the key to promote tumor invasion and metastasis. Studies showed that BMI-1 could promote self-renew, differentiation and tumor formation of CSCs and invasion/metastasis of human cancer. However, whether BMI-1 could regulate invasion and metastasis ability of CSCs is still unclear. In our study, we found that up-regulated expression of BMI-1 was associated with tumor invasion, metastasis and poor survival of pancreatic cancer patients. CD133+ cells were obtained by using magnetic cell sorting and identified of CSCs properties such as self-renew, multi-differentiation and tumor formation ability. Then, we found that BMI-1 expression was up-regulated in pancreatic cancer stem cells. Knockdown of BMI-1 expression attenuated invasion ability of pancreatic cancer stem cells in Transwell system and liver metastasis capacity in nude mice which were injected CSCs through the caudal vein. We are the first to reveal that BMI-1 could promote invasion and metastasis ability of pancreatic cancer stem cells. Finally, we identified that BMI-1 expression activating PI3K/AKT singing pathway by negative regulating PTEN was the main mechanism of promoting invasion and metastasis ability of pancreatic CSCs. In summary, our findings indicate that BMI-1 could be used as the therapeutic target to inhibiting CSCs-mediated pancreatic cancer metastasis.

Cancer stem cell, cytokeratins and epithelial to mesenchymal transition markers expression in oral squamous cell carcinoma derived from ortothopic xenoimplantation of CD44high cells

Oral squamous cell carcinoma (OSCC) is the most prevalent neoplasia of oral cavity worldwide and prognosis remains unchanged in decades. Recently, different authors reported that head and neck squamous cell carcinomas have a subpopulation of tumor initiating cells that apparently correspond to cancer stem cells (CSC) and are also responsible for tumor growth and metastasis. The purpose of the present study was to investigate the microscopic and phenotypic characteristics of OSCC tumors induced after orthotopic xenoimplantation of SCC9^WT cell line and CSC-enriched subpopulation isolated from SCC9 cell line based on high expression of the putative CSC marker CD44. Different numbers of FACS-sorted SCC9 CD44^high and CD44^low cells as well as SCC9^WT (wild type) were transplanted into the tongue of BALB/C nude (NOD/SCID) mice to evaluate their tumorigenic potential. Sixty days post-induction, tumors were morphologically characterized and immunostained for CSC markers (CD44, Nanog and Bmi-1), epithelial-mesenchymal transition (Snail, Slug) and epithelial differentiating cell markers (cytokeratins 4, 13, 15, 17 and 19), as well as E-cadherin and β-catenin. The data presented here shows that SCC9 CD44^high cells have higher ability to form tumors than SCC9 CD44^low cells, even when significantly lower numbers of SCC9 CD44^high cells were transplanted. Immunoassessment of tumors derived from SCC9 CD44^high cells revealed high expression of cytokeratin CK19, β-catenin, E-cadherin and CD44, and negative or low expression of CK17, CK4, CK15, CK13, Nanog, Bmi-1, Snail and Slug. While tumors derived from SCC9^WT showed high expression of CK17, CK19, CD44, Nanog, Bmi-1, Snail and Slug, and negative or low expression of CK4, CK15, CK13, β-catenin and E-cadherin. Thus, SCC9 CD44^high cells were highly tumorigenic, capable of originating heterogeneous tumors and these tumors have a immunohistochemical profile different from those formed by the wild type cell line.

Sox2 and Lef-1 interact with Pitx2 to regulate incisor development and stem cell renewal

Sox2 marks dental epithelial stem cells (DESCs) in both mammals and reptiles, and in this article we demonstrate several Sox2 transcriptional mechanisms that regulate dental stem cell fate and incisor growth. Conditional Sox2 deletion in the oral and dental epithelium results in severe craniofacial defects, including impaired dental stem cell proliferation, arrested incisor development and abnormal molar development. The murine incisor develops initially but is absorbed independently of apoptosis owing to a lack of progenitor cell proliferation and differentiation. Tamoxifen-induced inactivation of Sox2 demonstrates the requirement of Sox2 for maintenance of the DESCs in adult mice. Conditional overexpression of Lef-1 in mice increases DESC proliferation and creates a new labial cervical loop stem cell compartment, which produces rapidly growing long tusk-like incisors, and Lef-1 epithelial overexpression partially rescues the tooth arrest in Sox2 conditional knockout mice. Mechanistically, Pitx2 and Sox2 interact physically and regulate Lef-1, Pitx2 and Sox2 expression during development. Thus, we have uncovered a Pitx2-Sox2-Lef-1 transcriptional mechanism that regulates DESC homeostasis and dental development.

Bmi1 plays an important role in dentin and mandible homeostasis by maintaining redox balance

To explore whether polycomb repressor Bmi1 plays an important role in dentin and mandible development homeostasis by maintaining redox balance, 3-week-old Bmi1 gene knockout (Bmi1^-/-) mice were treated with the antioxidant N-acetylcysteine (NAC) for 2 weeks in their drinking water and phenotypes of the tooth and mandibles were compared with vehicle-treated Bmi1^-/- mice and wild-type mice by radiograph, histochemistry and immunohistochemistry. Alterations of oxidative stress, DNA damage, cell proliferation and cell cycle-related parameters were also examined in mandibles. Results showed that the tooth volume and the dentin sialoprotein immunopositive areas, the cortical thickness, alveolar bone volume, osteoblast number and activity, and mRNA expression levels of Runx2, alkaline phosphatase and type I collagen were all reduced significantly in Bmi1^-/- mice compared with their wild-type littermates, whereas these parameters were increased significantly in NAC-treated Bmi1^-/- mice compared with vehicle-Bmi1^-/- mice, although they were not normalized. The activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) were reduced, DNA damage markers including γ-H2AX and 8-oxoguanine levels were increased, the number of Ki67 positive cells was decreased, whereas protein expression levels of p16, p19, p21, p27 and p53 were up-regulated in mandibles from Bmi1^-/- mice compared with those from wild-type mice; alterations of these antioxidative enzyme activities, DNA damage markers, cell proliferation and cell cycle-related parameters were all partially rescued by the treatment with antioxidant NAC in Bmi1 deficient mice. These results demonstrated that Bmi1 deficiency resulted in defects in dentin and alveolar bone formation, while the treatment with antioxidant could improve these defects obviously. Therefore, our results indicate that Bmi1 plays an important role in stimulating dentin formation and alveolar bone formation by maintaining redox homeostasis, preventing DNA damage and inhibiting cyclin-dependent kinase inhibitors.

Bmi1 Loss in the Organ of Corti Results in p16ink4a Upregulation and Reduced Cell Proliferation of Otic Progenitors In Vitro

The mature mammalian organ of Corti does not regenerate spontaneously after injury, mainly due to the absence of cell proliferation and the depletion of otic progenitors with age. The polycomb gene B lymphoma Mo-MLV insertion region 1 homolog (Bmi1) promotes proliferation and cell cycle progression in several stem cell populations. The cell cycle inhibitor p16^ink4a has been previously identified as a downstream target of Bmi1. In this study, we show that Bmi1 is expressed in the developing inner ear. In the organ of Corti, Bmi1 expression is temporally regulated during embryonic and postnatal development. In contrast, p16^ink4a expression is not detectable during the same period. Bmi1-deficient mice were used to investigate the role of Bmi1 in cochlear development and otosphere generation. In the absence of Bmi1, the postnatal organ of Corti displayed normal morphology at least until the end of the first postnatal week, suggesting that Bmi1 is not required for the embryonic or early postnatal development of the organ of Corti. However, Bmi1 loss resulted in the reduced sphere-forming capacity of the organ of Corti, accompanied by the decreased cell proliferation of otic progenitors in otosphere cultures. This reduced proliferative capacity was associated with the upregulation of p16^ink4ain vitro. Viral vector-mediated overexpression of p16^ink4a in wildtype otosphere cultures significantly reduced the number of generated otospheres in vitro. The findings strongly suggest a role for Bmi1 as a promoter of cell proliferation in otic progenitor cells, potentially through the repression of p16^ink4a.

Joint Development Involves a Continuous Influx of Gdf5-Positive Cells

Synovial joints comprise several tissue types, including articular cartilage, the capsule, and ligaments. All of these compartments are commonly assumed to originate from an early set of Gdf5-expressing progenitors populating the interzone domain. Here, we provide evidence that joints develop through a continuous influx of cells into the interzone, where they contribute differentially to forming joint tissues. Using a knockin Gdf5-CreER(T2) mouse, we show that early labeling of Gdf5-positive interzone cells failed to mark the entire organ. Conversely, multiple Cre activation steps indicated a contribution of these cells to various joint compartments later in development. Spatiotemporal differences between Gdf5 and tdTomato reporter expression support the notion of a continuous recruitment process. Finally, differential contribution of Gdf5-positive cells to various tissues suggests that the spatiotemporal dynamics of Gdf5 expression may instruct lineage divergence. This work supports the influx model of joint development, which may apply to other organogenic processes.

CTCF-mediated Chromatin Loop for the Posterior Hoxc Gene Expression in MEF Cells

Modulation of chromatin structure has been proposed as a molecular mechanism underlying the spatiotemporal collinear expression of Hox genes during development. CCCTC-binding factor (CTCF)-mediated chromatin organization is now recognized as a crucial epigenetic mechanism for transcriptional regulation. Thus, we examined whether CTCF-mediated chromosomal conformation is involved in Hoxc gene expression by comparing wild-type mouse embryonic fibroblast (MEF) cells expressing anterior Hoxc genes with Akt1 null MEFs expressing anterior as well as posterior Hoxc genes. We found that CTCF binding between Hoxc11 and -c12 is important for CTCF-mediated chromosomal loop formation and concomitant posterior Hoxc gene expression. Hypomethylation at this site increased CTCF binding and recapitulated the chromosomal conformation and posterior Hoxc gene expression patterns observed in Akt1 null MEFs. From this work we found that CTCF at the C12|11 does not function as a barrier/boundary, instead let the posterior Hoxc genes switch their interaction from inactive centromeric to active telomeric genomic niche, and concomitant posterior Hoxc gene expression. Although it is not clear whether CTCF affects Hoxc gene expression solely through its looping activity, CTCF-mediated chromatin structural modulation could be an another tier of Hox gene regulation during development. © 2016 IUBMB Life, 68(6):436-444, 2016.

Mesenchymal Wnt/β-Catenin Signaling Controls Epithelial Stem Cell Homeostasis in Teeth by Inhibiting the Antiapoptotic Effect of Fgf10

Continuous growth of rodent incisors relies on epithelial stem cells (SCs) located in the SC niche called labial cervical loop (LaCL). Here, we found a population of apoptotic cells residing in a specific location of the LaCL in mouse incisor. Activated Caspase 3 and Caspase 9, expressed in this location colocalized in part with Lgr5 in putative SCs. The addition of Caspase inhibitors to incisors ex vivo resulted in concentration dependent thickening of LaCL. To examine the role of Wnt signaling in regulation of apoptosis, we exposed the LaCL of postnatal day 2 (P2) mouse incisor ex vivo to BIO, a known activator of Wnt/β-catenin signaling. This resulted in marked thinning of LaCL as well as enhanced apoptosis. We found that Wnt/β-catenin signaling was intensely induced by BIO in the mesenchyme surrounding the LaCL, but, unexpectedly, no β-catenin activity was detected in the LaCL epithelium either before or after BIO treatment. We discovered that the expression of Fgf10, an essential growth factor for incisor epithelial SCs, was dramatically downregulated in the mesenchyme around BIO-treated LaCL, and that exogenous Fgf10 could rescue the thinning of the LaCL caused by BIO. We conclude that the homeostasis of the epithelial SC population in the mouse incisor depends on a proper rate of apoptosis and that this apoptosis is controlled by signals from the mesenchyme surrounding the LaCL. Fgf10 is a key mesenchymal signal limiting apoptosis of incisor epithelial SCs and its expression is negatively regulated by Wnt/β-catenin. Stem Cells 2015;33:1670-1681.

Bmi1 Regulates the Proliferation of Cochlear Supporting Cells Via the Canonical Wnt Signaling Pathway

Cochlear supporting cells (SCs), which include the cochlear progenitor cells, have been shown to be a promising resource for hair cell (HC) regeneration, but the mechanisms underlying the initiation and regulation of postnatal cochlear SC proliferation are not yet fully understood. Bmi1 is a member of the Polycomb protein family and has been reported to regulate the proliferation of stem cells and progenitor cells in multiple organs. In this study, we investigated the role of Bmi1 in regulating SC and progenitor cell proliferation in neonatal mice cochleae. We first showed that knockout of Bmi1 significantly inhibited the proliferation of SCs and Lgr5-positive progenitor cells after neomycin injury in neonatal mice in vitro, and we then showed that Bmi1 deficiency significantly reduced the sphere-forming ability of the organ of Corti and Lgr5-positive progenitor cells in neonatal mice. These results suggested that Bmi1 is required for the initiation of SC and progenitor cell proliferation in neonatal mice. Next, we found that DKK1 expression was significantly upregulated, while beta-catenin and Lgr5 expression were significantly downregulated in neonatal Bmi1^-/- mice compared to wild-type controls. The observation that Bmi1 knockout downregulates Wnt signaling provides compelling evidence that Bmi1 is required for the Wnt signaling pathway. Furthermore, the exogenous Wnt agonist BIO overcame the downregulation of SC proliferation in Bmi1^-/- mice, suggesting that Bmi1 knockout might inhibit the proliferation of SCs via downregulation of the canonical Wnt signaling pathway. Our findings demonstrate that Bmi1 plays an important role in regulating the proliferation of cochlear SCs and Lgr5-positive progenitor cells in neonatal mice through the Wnt signaling pathway, and this suggests that Bmi1 might be a new therapeutic target for HC regeneration.

Inhibition of Notch Signaling During Mouse Incisor Renewal Leads to Enamel Defects

The continuously growing rodent incisor is an emerging model for the study of renewal of mineralized tissues by adult stem cells. Although the Bmp, Fgf, Shh, and Wnt pathways have been studied in this organ previously, relatively little is known about the role of Notch signaling during incisor renewal. Notch signaling components are expressed in enamel-forming ameloblasts and the underlying stratum intermedium (SI), which suggested distinct roles in incisor renewal and enamel mineralization. Here, we injected adult mice with inhibitory antibodies against several components of the Notch pathway. This blockade led to defects in the interaction between ameloblasts and the SI cells, which ultimately affected enamel formation. Furthermore, Notch signaling inhibition led to the downregulation of desmosome-specific proteins such as PERP and desmoplakin, consistent with the importance of desmosomes in the integrity of ameloblast-SI attachment and enamel formation. Together, our data demonstrate that Notch signaling is critical for proper enamel formation during incisor renewal, in part by regulating desmosome-specific components, and that the mouse incisor provides a model system to dissect Jag-Notch signaling mechanisms in the context of mineralized tissue renewal.

Significance of BMI1 and FSCN1 expression in colorectal cancer

BACKGROUND/AIMS

Colorectal cancer (CRC) is the third most common type of cancer in terms of incidence and the fourth in cause of death world-wide, underscoring the need to identify novel biomarkers for early diagnosis, as well as improved disease stratification and treatment choices.

PATIENTS AND METHODS

The Gene Expression Omnibus (GSE21510) and the Cancer Genome Atlas (TCGA) CRC datasets were utilized in the current study. GeneSpring 13.0 was used for normalization and analysis. The log-rank test was used to compare the outcome between expression groups.

RESULT

Significant upregulation of BMI1 (2.3 FC, P = 3.7 × 10-18) and FSCN1 (1.3 FC,P = 4.7 × 10-3) was observed in CRC. High BMI1 expression was associated with reduced overall survival (OS) [Hazard ratio (HR), 1.87; 95% CI. 1.17-3.03; P= 0.009] and reduced disease-free survival (DFS) [HR, 162; 95% CI 1.01-2.63;P = 0.045]. Similarly, high expression of FSCN1 was associated with reduced OS (HR, 2.0; 95% CI, 1.24-3.2; P= 0.0044) and reduced DFS (HR, 1.60; 95% CI, 0.99-2.57;P = 0.055). Importantly, BMI1high/FSCN1high patients experienced the worst OS (HR, 3.17; 95% CI, 1.77-6.15; P= 0.0002) and DFS (HR, 2.34; 95% CI, 1.27-4.67,P = 0.0078). Using pathway analyses, tumors overexpressing BMI1 were enriched in zinc finger proteins and genes involved in DNA binding and regulation of transcription, whereas tumors expressing FSCN1 were enriched in genes involved in cell migration.

CONCLUSION

Our data revealed poor OS and DFS in CRC patients overexpressing BMI1 or FSCN1 and suggest that these two markers in combination may represent superior prognostic marker to either one. Targeting BMI1 and FSCN1 may also provide potential therapeutic opportunity in CRC.

The Perivascular Niche and Self-Renewal of Stem Cells

Postnatal stem cells are typically found in niches that provide signaling cues to maintain their self-renewal and multipotency. While stem cell populations may serve distinct purposes within their tissue of origin, understanding the conserved biology of stem cells and their respective niches provides insights to the behavior of these cells during homeostasis and tissue repair. Here, we discuss perivascular niches of two distinct stem cell populations (i.e., hematopoietic stem cells, mesenchymal stem cells) and explore mechanisms that sustain these stem cells postnatally. We highlight work that demonstrates the impact of cellular crosstalk to stem cell self-renewal and maintenance of functional perivascular niches. We also discuss the importance of the crosstalk within the perivascular niche to the biology of stem cells, and describe the regenerative potential of perivascular cells. We postulate that signaling events that establish and/or stabilize the perivascular niche, particularly through the modulation of self-renewing factors, are key to the long-term success of regenerated tissues.

miR-300 mediates Bmi1 function and regulates differentiation in primitive cardiac progenitors

B lymphoma Mo-MLV insertion region 1 (Bmi1) is a polycomb-family transcriptional factor critical for self-renewal in many adult stem cells and human neoplasia. We sought to identify microRNAs regulated by Bmi1 that could play a role in multipotent cardiac progenitor cell (CPC) decisions. We found that miR-300, a poorly characterized microRNA mapping in the Dlk1-Dio3 microRNA cluster, was positively regulated by Bmi1 in CPCs. Forced expression of miR-300 in CPCs promoted an improved stemness signature with a significant increase in Oct4 levels, a reduction in senescence progression and an enhanced proliferative status via p19 activation and inhibition of p16 accumulation. Endothelial and cardiogenic differentiation were clearly compromised by sustained miR-300 expression. Additionally, RNA and protein analysis revealed a significant reduction in key cardiac transcription factors, including Nkx2.5 and Tbx5. Collectively, these results suggest that some functions attributed to Bmi1 are due to induction of miR-300, which decreases the cardiogenic differentiation potential of multipotent CPCs in vitro and promotes self-renewal.

Cardiac Bmi1(+) cells contribute to myocardial renewal in the murine adult heart

Introduction

The mammalian adult heart maintains a continuous, low cardiomyocyte turnover rate throughout life. Although many cardiac stem cell populations have been studied, the natural source for homeostatic repair has not yet been defined. The Polycomb protein BMI1 is the most representative marker of mouse adult stem cell systems. We have evaluated the relevance and role of cardiac Bmi1⁺ cells in cardiac physiological homeostasis.

Methods

Bmi1^CreER/+;Rosa26^YFP/+ (Bmi1-YFP) mice were used for lineage tracing strategy. After tamoxifen (TM) induction, yellow fluorescent protein (YFP) is expressed under the control of Rosa26 regulatory sequences in Bmi1⁺ cells. These cells and their progeny were tracked by FACS, immunofluorescence and RT-qPCR techniques from 5 days to 1 year.

Results

FACS analysis of non-cardiomyocyte compartment from TM-induced Bmi1-YFP mice showed a Bmi1⁺-expressing cardiac progenitor cell (Bmi1-CPC: B-CPC) population, SCA-1 antigen-positive (95.9 ± 0.4 %) that expresses some stemness-associated genes. B-CPC were also able to differentiate in vitro to the three main cardiac lineages. Pulse-chase analysis showed that B-CPC remained quite stable for extended periods (up to 1 year), which suggests that this Bmi1⁺ population contains cardiac progenitors with substantial self-maintenance potential. Specific immunostaining of Bmi1-YFP hearts serial sections 5 days post-TM induction indicated broad distribution of B-CPC, which were detected in variably sized clusters, although no YFP⁺ cardiomyocytes (CM) were detected at this time. Between 2 to 12 months after TM induction, YFP⁺ CM were clearly identified (3 ± 0.6 % to 6.7 ± 1.3 %) by immunohistochemistry of serial sections and by flow cytometry of total freshly isolated CM. B-CPC also contributed to endothelial and smooth muscle (SM) lineages in vivo.

Conclusions

High Bmi1 expression identifies a non-cardiomyocyte resident cardiac population (B-CPC) that contributes to the main lineages of the heart in vitro and in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0196-9) contains supplementary material, which is available to authorized users.

Developmental Plasticity of Patterned and Regenerating Oral Organs

In many aquatic vertebrates, including bony and cartilaginous fishes, teeth and taste buds colocalize on jaw elements. In these animals, taste buds are renewed continuously throughout life, whereas teeth undergo cycled whole-organ replacement by various means. Recently, studies of cichlid fishes have yielded new insights into the development and regeneration of these dental and sensory oral organs. Tooth and taste bud densities covary positively across species with different feeding strategies, controlled by common regions of the genome and integrated molecular signals. Developing teeth and taste buds share a bipotent epithelium during early patterning stages, from which dental and taste fields are specified. Moreover, these organs share a common epithelial ribbon that supports label-retaining cells during later stages of regeneration. During both patterning and regeneration stages, dental organs can be converted to taste bud fate by manipulation of BMP signaling. These observations highlight a surprising long-term plasticity between dental and sensory organ types. Here, we review these findings and discuss the implications of developmental plasticity that spans the continuum of craniofacial organ patterning and regeneration.

Stem Cells in Tooth Development, Growth, Repair, and Regeneration

Human teeth contain stem cells in all their mesenchymal-derived tissues, which include the pulp, periodontal ligament, and developing roots, in addition to the support tissues such as the alveolar bone. The precise roles of these cells remain poorly understood and most likely involve tissue repair mechanisms but their relative ease of harvesting makes teeth a valuable potential source of mesenchymal stem cells (MSCs) for therapeutic use. These dental MSC populations all appear to have the same developmental origins, being derived from cranial neural crest cells, a population of embryonic stem cells with multipotential properties. In rodents, the incisor teeth grow continuously throughout life, a feature that requires populations of continuously active mesenchymal and epithelial stem cells. The discrete locations of these stem cells in the incisor have rendered them amenable for study and much is being learnt about the general properties of these stem cells for the incisor as a model system. The incisor MSCs appear to be a heterogeneous population consisting of cells from different neural crest-derived tissues. The epithelial stem cells can be traced directly back in development to a Sox10(+) population present at the time of tooth initiation. In this review, we describe the basic biology of dental stem cells, their functions, and potential clinical uses.