Notch signaling in lymphocyte development

Promotion effect of apical tooth germ cell-conditioned medium on osteoblastic differentiation of periodontal ligament stem cells through regulating miR-146a-5p

BACKGROUND

MicroRNAs (miRNAs) play an important role in gene regulation that controls stem cells differentiation. Periodontal ligament stem cells (PDLSCs) could differentiate into osteo-/cementoblast-like cells that secretes cementum-like matrix both in vitro and in vivo. Whether miRNAs play key roles in osteoblastic differentiation of PDLSCs triggered by a special microenviroment remains elusive. In this study, we aimed to investigate potential miRNA expression changes in osteoblastic differentiation of PDLSCs by the induction of apical tooth germ cell-conditioned medium (APTG-CM).

METHODS AND RESULTS

First, we analyzed the ability of APTG-CM to osteogenically differentiate PDLSCs. The results exhibited an enhanced mineralization ability, higher ALP activity and increased expression of osteogenic genes in APTG-CM-induced PDLSCs. Second, we used miRNA sequencing to analyze the miRNA expression profile of PDLSCs derived from three donors under 21-day induction or non-induction of APTG-CM. MiR-146a-5p was found to be up-regulated miRNA in induced PDLSCs and validated by RT-qPCR. Third, we used lentivirus-up/down system to verify the role of miR-146a-5p in the regulation of osteoblastic differentiation of PDLSCs.

CONCLUSIONS

In conclusion, our results demonstrated that miR-146a-5p was involved in the promotion effect of APTG-CM on osteoblastic differentiation of PDLSCs, and suggested that miR-146a-5p might be a novel way in deciding the direction of PDLSCs differentiation.

Notch signaling differentially regulates the cell fate of early endocrine precursor cells and their maturing descendants in the mouse pancreas and intestine

Notch signaling inhibits differentiation of endocrine cells in the pancreas and intestine. In a number of cases, the observed inhibition occurred with Notch activation in multipotential cells, prior to the initiation of endocrine differentiation. It has not been established how direct activation of Notch in endocrine precursor cells affects their subsequent cell fate. Using conditional activation of Notch in cells expressing Neurogenin3 or NeuroD1, we examined the effects of Notch in both organs, on cell fate of early endocrine precursors and maturing endocrine-restricted cells, respectively. Notch did not preclude the differentiation of a limited number of endocrine cells in either organ when activated in Ngn3(+) precursor cells. In addition, in the pancreas most Ngn3(+) cells adopted a duct but not acinar cell fate; whereas in intestinal Ngn3(+) cells, Notch favored enterocyte and goblet cell fates, while selecting against endocrine and Paneth cell differentiation. A small fraction of NeuroD1(+) cells in the pancreas retain plasticity to respond to Notch, giving rise to intraislet ductules as well as cells with no detectable pancreatic lineage markers that appear to have limited ultrastructural features of both endocrine and duct cells. These results suggest that Notch directly regulates cell fate decisions in multipotential early endocrine precursor cells. Some maturing endocrine-restricted NeuroD1(+) cells in the pancreas switch to the duct lineage in response to Notch, indicating previously unappreciated plasticity at such a late stage of endocrine differentiation.

Blockage of Notch1 signaling modulates the T-helper (Th)1/Th2 cell balance in chronic hepatitis B patients

AIM

Chronic hepatitis B virus (HBV) infection is thought to involve the imbalance of T-helper (Th)1/Th2 cells. Many procedures found Notch signaling involved the proliferation and differentiation of T lymphocytes during development and peripheral functions. The aim of this study was to discover the effect of blockage of Notch1 signaling to Th cells and the mechanisms involved in chronic hepatitis B patients.

METHODS

CD4(+) T cells from hepatitis B patients and healthy volunteers were isolated, and Notch1 expression of CD4(+) T cells was determined by reverse transcription polymerase chain reaction (RT-PCR). Blockage of Notch1 signaling of peripheral blood mononuclear cells (PBMC) from chronic hepatitis B patients, Th1- and Th2-type cytokines were assayed by enzyme-linked immunosorbent assay and levels of T-bet, GATA-3 mRNA were measured by RT-PCR.

RESULTS

Notch1 expression of CD4(+) T cells from chronic hepatitis B patients was upregulated, on the contrary to that from acute hepatitis B patients and healthy volunteers. Blockage of Notch1 signaling can strongly inhibit the production of Th2-type cytokines and the expression of GATA-3; the production of Th1-type cytokines and the expression of T-bet, however, were enhanced.

CONCLUSION

Blockage of Notch1 signaling could regulate the immune balance of Th1/Th2 in chronic hepatitis B patients, which may be mediated partly by regulating transcription factors T-bet and GATA-3.

The thymus microenvironment in regulating thymocyte differentiation

The thymus plays a crucial role in the development of T lymphocytes by providing an inductive microenvironment in which committed progenitors undergo proliferation, T-cell receptor gene rearrangements and thymocyte differentiate into mature T cells. The thymus microenvironment forms a complex network of interaction that comprises non lymphoid cells (e.g., thymic epithelial cells, TEC), cytokines, chemokines, extracellular matrix elements (ECM), matrix metalloproteinases and other soluble proteins. The thymic epithelial meshwork is the major component of the thymic microenvironment, both morphologically and phenotypically limiting heterogeneous regions in thymic lobules and fulfilling an important role during specific stages of T-cell maturation. The process starts when bone marrow-derived lymphocyte precursors arrive at the outer cortical region of the thymic gland and begin to mature into functional T lymphocytes that will finally exit the thymus and populate the peripheral lymphoid organs. During their journey inside the thymus, thymocytes must interact with stromal cells (and their soluble products) and extracellular matrix proteins to receive appropriate signals for survival, proliferation and differentiation. The crucial components of the thymus microenvironment, and their complex interactions during the T-cell maturation process are summarized here with the objective of contributing to a better understanding of the function of the thymus, as well as assisting in the search for new therapeutic approaches to improve the immune response in various pathological conditions.

Genetics and visceral leishmaniasis: of mice and man

Ninety per cent of the 500,000 annual new cases of visceral leishmaniasis (VL) occur in India/Bangladesh/Nepal, Sudan and Brazil. Importantly, 80-90% of human infections are sub-clinical or asymptomatic, usually associated with strong cell-mediated immunity. Understanding the environmental and genetic risk factors that determine why two people with the same exposure to infection differ in susceptibility could provide important leads for improved therapies. Recent research using candidate gene association analysis and genome-wide linkage studies (GWLS) in collections of families from Sudan, Brazil and India have identified a number of genes/regions related both to environmental risk factors (e.g. iron), as well as genes that determine type 1 vs. type 2 cellular immune responses. However, until now all of the allelic association studies carried out have been underpowered to find genes of small effect sizes (odds ratios or OR < 2), and GWLS using multicase pedigrees have only been powered to find single major genes, or at best oligogenic control. The accumulation of large DNA banks from India and Brazil now makes it possible to undertake genome-wide association studies (GWAS), which are ongoing as part of phase 2 of the Wellcome Trust Case Control Consortium. Data from this analysis should seed research into novel genes and mechanisms that influence susceptibility to VL.

Apical tooth germ cell-conditioned medium enhances the differentiation of periodontal ligament stem cells into cementum/periodontal ligament-like tissues

BACKGROUND AND OBJECTIVE

Limitations of current periodontal regeneration modalities in both predictability and extent of healing response, especially on new cementum and attachment formation, underscore the importance of restoring or providing a microenvironment that is capable of promoting the differentiation of periodontal ligament stem cells (PDLSCs) towards cementoblast-like cells and the formation of cementum/periodontal ligament-like tissues. The aim of this study was to investigate the biological effect of conditioned medium from developing apical tooth germ cells (APTG-CM) on the differentiation and cementogenesis of PDLSCs both in vitro and in vivo.

MATERIAL AND METHODS

Using the limiting dilution technique, single-colony-derived human PDLSCs were isolated and expanded to obtain homogeneous populations of PDLSCs. Morphological appearance, cell cycle analysis, bromodeoxyuridine incorporation, alkaline phosphatase (ALP) activity, mineralization behavior, gene expression of cementoblast phenotype and in vivo differentiation capacities of PDLSCs co-cultured with APTG-CM were evaluated.

RESULTS

The induced PDLSCs exhibited several characteristics of cementoblast lineages, as indicated by the morphological changes, increased proliferation, high ALP activity, and the expression of cementum-related genes and calcified nodule formation in vitro. When transplanted into immunocompromised mice, the induced PDLSCs showed tissue-regenerative capacity to produce cementum/periodontal ligament-like structures, characterized by a layer of cementum-like mineralized tissues and associated periodontal ligament-like collagen fibers connecting with the newly formed cementum-like deposits, whereas control, untreated PDLSCs transplants mainly formed connective tissues.

CONCLUSION

Our findings suggest that APTG-CM is able to provide a cementogenic microenvironment and induce differentiation of PDLSCs along the cementoblastic lineage. This has important implications for periodontal engineering.

Ikaros directly represses the notch target gene Hes1 in a leukemia T cell line: implications for CD4 regulation

Ikaros and Notch1, two regulators of gene transcription, are critically important at many stages of T cell development. Deregulation of Ikaros and Notch activities cooperate to promote T cell leukemogenesis, providing evidence that they function in converging pathways in developing T cells. In this report, a mechanism for Ikaros:Notch cooperativity is described, revealing a non-redundant role for Ikaros in regulating expression of the Notch target gene Hes1 in a leukemia T cell line. We provide evidence that Ikaros directly represses Hes1 in concert with the transcriptional repressor, RBP-Jkappa, allowing for cross-talk between Notch and Ikaros that impacts regulation of CD4 expression. Taken together, these data describe a potential mechanism for Ikaros' function during T cell development and define Ikaros as an obligate repressor of Hes1.

Identification of novel genes and transcription factors involved in spleen, thymus and immunological development and function

We constructed and analyzed six serial analysis of gene expression (SAGE) libraries to identify genes with previously uncharacterized roles in spleen or thymus development. A total of 625 070 tags were sequenced from the three spleen (embryonic day (E)15.5, E16.5 and adult) and three thymus (E15.5, E18.5 and adult) libraries. These tags corresponded to 83 182 tag types, which mapped unambiguously to 36 133 different genes. Genes over-represented in these libraries, compared to 115 mouse SAGE libraries (www.mouseatlas.org), included genes of known and unknown immunological or developmental relevance. The expression profiles of 11 genes with unknown roles in spleen and thymus development were validated using reverse transcription-qPCR. We further characterized the expression of one of these candidates, RIKEN cDNA 9230105E10 that encodes a murine homolog of Trim5alpha, in numerous adult tissues and immune cell types. In addition, we demonstrate that transcript levels are upregulated in response to TLR stimulation of plasmacytoid dendritic cells and macrophages. This work provides the first evidence of regulated and cell type-specific expression of this gene. In addition, these observations suggest that the SAGE libraries provide an important resource for further investigations into the molecular mechanisms regulating spleen and thymus organogenesis, as well as the development of immunological competence.

Altered T cell differentiation and Notch signaling induced by the ectopic expression of keratin K10 in the epithelial cells of the thymus

Transgenic mice expressing hK10 under the keratin K5 promoter display several alterations in the epidermis including decreased cell proliferation, and reduced susceptibility to tumor development. Given that K5 promoter is also active in the epithelial cells of the thymus, we explored the possible alterations of the thymus because of K10 transgene expression. We found severe thymic alterations, which affect not only the thymic epithelial cells (TEC), but also thymocytes. We observed altered architecture and premature thymus involution in the transgenic mice associated with increased apoptosis and reduced proliferation of the thymocytes. Interestingly, prior to the development of this detrimental phenotype, thymocytes of the transgenic mice also displayed altered differentiation, which is aggravated later on. Molecular characterization of this phenotype indicated that Akt activity is reduced in TEC, but not in thymocytes. In addition, we also observed altered expression of Notch family members and some of their ligands both in TEC and T cells. This produces reduced Notch activity in TEC but increased Notch activity in thymocytes, which is detectable prior to the disruption of the thymic architecture. In addition, we also detect altered Notch expression in the epidermis of bK5hK10 transgenic mice. Collectively the present data indicate that keratin K10 may induce severe alterations not only in a cell autonomous manner, but also in neighboring cells by the modulation of signals involved in cell-cell interactions.

Impairment of thymocyte development by dominant-negative Kuzbanian (ADAM-10) is rescued by the Notch ligand, delta-1

Although Notch plays a crucial role in T cell development, regulation of Notch signaling in the thymus is not well understood. Kuzbanian, an ADAM protease, has been implicated in the cleavage of both Notch receptors and the Notch ligand, Delta. In this study we show that the expression of a dominant-negative form of Kuzbanian (dnKuz) leads to reduced TCRbeta expression in double-negative thymocytes and to a partial block between the double-negative to double-positive stages of development. These defects were rescued by overexpression of Delta-1 on thymocytes. Mixed chimeras showed a cell-autonomous block by dnKuz, but non-cell-autonomous rescue by Delta-1. This suggests that dnKuz impairs Notch signaling in receiving cells, and increasing Delta-1 on sending cells overcomes this defect. Interestingly, the expression of an activated form of Notch-1 rescued some, but not all, the defects in dnKuz Tg mice. Our data suggest that multiple Notch-dependent steps in early thymocyte development require Kuzbanian, but differ in the involvement of other Notch signaling components.

Epithelial differentiation in pancreatic development and neoplasia: new niches for nestin and Notch

Changes in epithelial differentiation represent a characteristic early feature of human pancreatic cancer. Recent work suggests that many of these changes may reflect a pathologic recapitulation of developmental events. Reflecting this principle, metaplastic and neoplastic pancreatic epithelium appear to share many features in common with embryonic pancreatic epithelium, including reactivation of the Notch signaling pathway. In this review, we summarize recent studies involving regulation of epithelial differentiation in both embryonic and adult pancreas and highlight the role of Notch in regulating an exocrine progenitor pool.

Notch signaling in lymphocyte development and function

Over the past few years, the crucial role of Notch signaling in multiple stages of T-cell development has become apparent. Recent studies have helped to define more precisely the functions and components of the Notch signaling pathway in T-cell development, including during the T versus B fate decision and in early CD4(-)CD8(-)double-negative thymocytes. In addition, new evidence points to a requirement for Notch2 in the development of marginal zone B cells. Finally, recent studies have provided our first glimpse into the complex and paradoxical roles of Notch signaling in the activation and differentiation of mature T cells.

Notch inhibits Ptf1 function and acinar cell differentiation in developing mouse and zebrafish pancreas

Notch signaling regulates cell fate decisions in a variety of adult and embryonic tissues, and represents a characteristic feature of exocrine pancreatic cancer. In developing mouse pancreas, targeted inactivation of Notch pathway components has defined a role for Notch in regulating early endocrine differentiation, but has been less informative with respect to a possible role for Notch in regulating subsequent exocrine differentiation events. Here, we show that activated Notch and Notch target genes actively repress completion of an acinar cell differentiation program in developing mouse and zebrafish pancreas. In developing mouse pancreas, the Notch target gene Hes1 is co-expressed with Ptf1-P48 in exocrine precursor cells, but not in differentiated amylase-positive acinar cells. Using lentiviral delivery systems to induce ectopic Notch pathway activation in explant cultures of E10.5 mouse dorsal pancreatic buds, we found that both Hes1 and Notch1-IC repress acinar cell differentiation, but not Ptf1-P48 expression, in a cell-autonomous manner. Ectopic Notch activation also delays acinar cell differentiation in developing zebrafish pancreas. Further evidence of a role for endogenous Notch in regulating exocrine pancreatic differentiation was provided by examination of zebrafish embryos with homozygous mindbomb mutations, in which Notch signaling is disrupted. mindbomb-deficient embryos display accelerated differentiation of exocrine pancreas relative to wild-type clutchmate controls. A similar phenotype was induced by expression of a dominant-negative Suppressor of Hairless [Su(H)] construct, confirming that Notch actively represses acinar cell differentiation during zebrafish pancreatic development. Using transient transfection assays involving a Ptf1-responsive reporter gene, we further demonstrate that Notch and Notch/Su(H) target genes directly inhibit Ptf1 activity, independent of changes in expression of Ptf1 component proteins. These results define a normal inhibitory role for Notch in the regulation of exocrine pancreatic differentiation.

Functional diversity ofnotchfamily genes in fetal lung development

In Drosophila, developmental signaling via the transmembrane Notch receptor modulates branching morphogenesis and neuronal differentiation. To determine whether the notch gene family can regulate mammalian organogenesis, including neuroendocrine cell differentiation, we evaluated developing murine lung. After demonstrating gene expression for notch-1, notch-2, notch-3, and the Notch ligands jagged-1 and jagged-2 in embryonic mouse lung, we tested whether altering expression of these genes can modulate branching morphogenesis. Branching of embryonic day (E) 11.5 lung buds increased when they were treated with notch-1 antisense oligodeoxynucleotides in culture compared with the corresponding sense controls, whereas notch-2, notch-3, jagged-1, or jagged-2 antisense oligos had no significant effect. To assess cell differentiation, we immunostained lung bud cultures for the neural/neuroendocrine marker PGP9.5. Antisense to notch-1 or jagged-1 markedly increased numbers of PGP9.5-positive neuroendocrine cells alone without affecting neural tissue, whereas only neural tissue was promoted by notch-3 antisense in culture. There was no significant effect on cell proliferation or apoptosis in these antisense experiments. Cumulatively, these observations suggest that interactions between distinct Notch family members can have diverse tissue-specific regulatory functions during development, arguing against simple functional redundancy.

Membrane-bound Delta-4 Notch ligand reduces the proliferative activity of primitive human hematopoietic CD34+CD38low cells while maintaining their LTC-IC potential

To examine the role of the Notch ligand Delta-4 on hematopoietic stem cells, human CD34+CD38low cord blood cells were cocultured on S17 cells transduced with transmembrane Delta-4 (mbD4/S17) or an empty vector (C/S17). By the end of a 3-week culture, mbD4/S17 induced a 25-fold reduction in nucleated cell production, as compared to C/S17, by maintaining a higher proportion of cells in G0/G1 phase. A specific retention of a high proportion of CD34+ cells throughout the culture was observed with mbD4/S17, contrary to C/S17. Although mbD4/S17 promoted expansion of cells with the phenotype of committed lymphoid precursors (CD34+CD7+CD45RA+), these cells still retained their myeloid differentiation potential. mbD4/S17 maintained a higher LTC-IC frequency in output CD34+ cells, compared to C/S17, as in the subsets of cells having completed the same number of divisions on mbD4/S17. A Delta4-Fc protein (extracellular part of human Delta4 fused to Fc human IgG1 portion), immobilized on plastic, also reduced cell production and retained the LTC-IC potential. Transplantation of cells grown on mbD4/S17 into NOD/SCID mice showed no significant enhancement of the long-term repopulating ability. Thus, Delta4 appears to inhibit hematopoietic stem cell proliferation, in association with the maintenance of short-term lymphoid and myeloid repopulation capacity.

Notch signaling as a therapeutic target in cancer: a new approach to the development of cell fate modifying agents

Notch signaling controls cell fate decisions including during development and stem cell renewal and differentiation in many postnatal tissues. Increasing evidence suggests that the Notch signaling network is frequently deregulated in human malignancies and that genetic or pharmacological manipulation of Notch signaling is a novel potential strategy for the treatment of human neoplasms. This review article summarizes the most recent preclinical and clinical evidence linking Notch signaling to cancer, delineates questions that remain unanswered and explores potential biopharmacological strategies to manipulate Notch signaling in vivo.

Delta1-Notch3 Interactions Bias the Functional Differentiation of Activated CD4+ T Cells

Following activation by antigen, naive CD4+ T helper precursor cells execute distinct genetic programs that result in their differentiation toward the type 1 or type 2 helper T cell (Th1 or Th2) phenotype. Although the differentiation and function of these Th subsets has been well studied, little is known about the contribution to these differentiation events of cell surface receptors other than those for soluble cytokines, such as IL-12 or IL-4. Here, we provide direct evidence that the Delta1 interaction with Notch3 on CD4+ T cells transduces signals, promoting development toward the Th1 phenotype. The positive role of Notch signaling in effector cell differentiation was dose dependent, with high levels of stimulation resulting in reduced T cell activation. Our data revealed a clear contribution of Notch pathways to Th1 versus Th2 fate decisions, while also providing insight into another mechanism for inhibition of CD4+ T cell activation.

Notch signaling augments T cell responsiveness by enhancing CD25 expression

Notch receptors signal through a highly conserved pathway to influence cell fate decisions. Notch1 is required for T lineage commitment; however, a role for Notch signaling has not been clearly defined for the peripheral T cell response. Notch gene expression is induced, and Notch1 is activated in primary CD4(+) T cells following specific peptide-Ag stimulation. Notch activity contributes to the peripheral T cell response, as inhibition of endogenous Notch activation decreases the proliferation of activated T cells in a manner associated with the diminished production of IL-2 and the expression of the high affinity IL-2R (CD25). Conversely, forced expression of a constitutively active Notch1 in primary T cells results in increased surface expression of CD25, and renders these cells more sensitive to both cognate Ag and IL-2, as measured by cell division. These data suggest an important role for Notch signaling during CD4(+) T cell responses, which operates through augmenting a positive feedback loop involving IL-2 and its high affinity receptor.

Loss of Ly-6A.2 expression on immature developing T cells in the thymus is necessary for their normal growth and generation of the Vbeta T-cell repertoire

Stage-specific expression of a number of cell-surface and signaling proteins is critical for normal development of T cells in the thymus. Equally important may be the loss of expression/signaling of developmentally regulated proteins for proper transitioning of developing T cells into thymic subsets. Ly-6A.2 exhibits a regulated pattern of expression on T cells maturing in the thymus, and dysregulating its expression results in arrest of developing T cells within the CD3-CD4-CD8- triple negative (TN) stage where the normal expression of Ly-6A.2 is extinguished. To further characterize the mechanisms underlying this block, we examined whether cell signaling and/or cell adhesion properties of the Ly-6A.2 molecule influenced the block in T-cell development. Analysis of bone marrow chimeras generated by injecting CFSE-labeled Ly-6A.2 transgenic bone marrow cells into irradiated syngeneic non-transgenic mice revealed normal trafficking of developing T cells from the cortex into the medulla. Production of LAT but not p56lck was diminished in CD4-CD8- DN cells from Ly-6A.2 dysregulated mice when compared with control littermates. Dysregulated expression of Ly-6A.2 did not suppress endogenous TCR-Vbeta expression. Finally, dysregulated expression of Ly-6A.2 enhanced apoptosis of an immature CD4+CD8+ (DP) subset of developing cells and altered the selected TCR-Vbeta repertoire. Taken together, these observations indicate that the termination of Ly-6A.2 expression and signaling within the CD4-CD8-CD3- subset of developing T cells is an important checkpoint during normal thymic development.

Notch signaling in lymphopoiesis

Notch signaling regulates many cell fate decisions during development of multi-cellular organisms. Signals initiated by Notch influence a wide variety of processes that include lineage specification, cell survival and proliferation, and border formation. During development of the immune system, Notch has been shown to influence the fate of both hematopoietic stem cells (HSCs) and committed progenitors. Notch appears to play an especially important role in the development of cells that mediate acquired immunity where Notch influences multiple aspects of T and B cell development. In this review, we will focus on the potential functions of Notch signaling during lymphoid development.

Microenvironmental regulation of Notch signalling in T cell development

T cells develop in the thymus from blood-borne progenitors derived from haematopoietic tissues. Amongst the mechanisms by which stromal cells in thymic and prethymic tissues influence lymphoid progenitors, recent attention has focussed on the importance of Notch signalling in early T cell development. Here, we review evidence that developing T cells and their progenitors receive signals through Notch receptors as a result of interactions with Notch ligands expressed by stromal cells. In particular, we focus on the role of Notch ligand-expressing stromal cells in regulating key control points during pre- and intrathymic phases of T cell development.

B-cell commitment: deciding on the players

Considerable progress has been made over the past few years in determining the key molecular players that regulate the commitment and development of early B-lineage cells in mouse bone marrow (transcription factors, cytokines and their receptors). However, confusion remains as to the precise stages, surface phenotypes and lineage restrictions that are observed early in B-cell development. Increasingly powerful flow cytometry analysis of genetically altered animals (knockouts and transgenics) combined with sensitive functional assays may provide an answer, a "scorecard" for the cell stages.

Root or crown: a developmental choice orchestrated by the differential regulation of the epithelial stem cell niche in the tooth of two rodent species

The rodent incisor grows continuously throughout its lifetime. The epithelial stem cell niche is located at the apical end of the tooth and its progeny gives rise to the ameloblasts that form the hard enamel. Previously, mesenchymal FGF10 was shown to support the niche, in conjunction with epithelial Notch signaling. Here we show that in a different continuously growing tooth type, the molar of the sibling vole, a similar regulatory system is in place. Moreover, the identical expression pattern of Bmp4 compared to Fgf10 suggests that BMP4 could also be involved in the regulation of the epithelial stem cell niche. Notch and FGF10 signaling is mainly absent in the mouse molar, which stops growing and develops roots. The regulation of the epithelial stem cell niche seems to be flexible allowing for the existence of different tooth types, such as continuously growing teeth, and high and low crowned molars.

The Ras/MAPK cascade and the control of positive selection

Immature double positive (DP) thymocytes bearing a T cell receptor (TCR) that interacts with self-major histocompatibility complex (MHC) molecules receive signals that induce either their differentiation (positive selection) or apoptosis (negative selection). Furthermore, those cells that are positively selected develop into two different lineages, CD4 or CD8, depending on whether their TCRs bind to MHC class II or I, respectively. Positive selection therefore involves rescue from the default fate (death), lineage commitment, and progression to the single positive (SP) stage. These are probably temporally distinct events that may require both unique and overlapping signals. Work in the past several years has started to unravel the signaling networks that control these processes. One of the first pathways identified as important for positive selection was Ras and its downstream effector, the Erk mitogen-activated protein kinase (MAPK) cascade. In this review we examine the factors that connect the TCR to the Ras/Erk cascade in DP thymocytes, as well as what we know about the downstream effectors of the Ras/Erk cascade important for positive selection. We also consider the possible role of this cascade in CD4/CD8 lineage development, and the possible interactions of the Ras/Erk cascade with Notch during these cell fate determination processes.

Transcriptional specificity of human SWI/SNF BRG1 and BRM chromatin remodeling complexes

Mammalian SWI/SNF chromatin remodeling complexes are involved in critical aspects of cellular growth and genomic stability. Each complex contains one of two highly homologous ATPases, BRG1 and BRM, yet little is known about their specialized functions. We show that BRG1and BRM associate with different promoters during cellular proliferation and differentiation, and in response to specific signaling pathways by preferential interaction with certain classes of transcription factors. BRG1 binds to zinc finger proteins through a unique N-terminal domain that is not present in BRM. BRM interacts with two ankyrin repeat proteins that are critical components of Notch signal transduction. Thus, BRG1 and BRM complexes may direct distinct cellular processes by recruitment to specific promoters through protein-protein interactions that are unique to each ATPase.

Notch2 protein distribution in human teeth under normal and pathological conditions

Notch signaling is essential for the appropriate differentiation of many cell types during development and, furthermore, is implicated in a variety of human diseases. Previous studies have shown that although the Notch1, -2, and -3 receptors are expressed in developing and injured rodent teeth, Notch2 expression was predominant after a lesion. To pursue the role of the Notch pathway in tooth development and disease, we have analyzed the expression of the Notch2 protein in embryonic and adult wounded human teeth. During the earlier stages of tooth development, the Notch2 protein was expressed in the epithelium, but was absent from proliferating cells of the inner enamel epithelium. At more advanced stages, Notch2 was expressed in the enamel-producing ameloblasts, while it was absent in mesenchyme-derived odontoblasts that synthesize the dentin matrix. Although Notch2 was not expressed in the pulp of adult intact teeth, it was reexpressed during dentin repair processes in odontoblasts and subodontoblastic cells. Transforming growth factor beta-1, which stimulates odontoblast differentiation and hard tissue formation after dental injury, downregulated Notch2 expression in cultured human dental slices, in vitro. These observations are consistent with the notion that Notch signaling is an important element in dental physiological and pathogenic conditions.

Controlling CD4 gene expression during T cell lineage commitment

T cell lineage commitment as the double-positive (DP) thymocyte matures into the single-positive (SP) T cell requires the irreversible repression or maintenance of CD4 gene expression. Signals transmitted from the T cell antigen receptor (TCR) during thymic selection are believed to be linked to the transcriptional regulation of the CD4 gene; thus, a study of the factors that control CD4 gene expression may lead to further insight into the molecular mechanisms that drive T cell development. This review discusses the work conducted to date to identify and characterize the transcriptional control elements in the CD4 locus and the factors that mediate their function. From these studies, it is clear that the molecular mechanisms controlling CD4 gene expression are very complex and are controlled by many different signals as the thymocyte develops.

Positive selection by the pre-TCR yields mature CD8+ T cells

It has been of much interest whether there is functional redundancy between the constitutively signaling pre-Talpha/TCRbeta (pre-TCR) and ligated TCRalphabeta complexes, which independently operate the two distinct checkpoints during thymocyte development, i.e., the pre-TCR involved in beta-selection at the CD4(-)CD8(-) double-negative stage and the TCRalphabeta being crucial for positive/negative selection at the CD4(+)CD8(+) double-positive stage. We found that the pre-TCR expressed on double-positive cells in TCRalpha-deficient (TCRalpha(-/-)) mice produced a small number of mature CD8(+) T cells. Surprisingly, when pre-Talpha was overexpressed, resulting in augmentation of pre-TCR expression, there was a striking increase of the CD8(+) T cells. In addition, even in the absence of up-regulation of pre-TCR expression, a similar increase of CD8(+) T cells was also observed in TCRalpha(-/-) mice overexpressing Egr-1, which lowers the threshold of signal strength required for positive selection. In sharp contrast, the CD8(+) T cells drastically decreased in the absence of pre-Talpha on a TCRalpha(-/-) background. Thus, the pre-TCR appears to functionally promote positive selection of CD8(+) T cells. The biased production of CD8(+) T cells via the pre-TCR might also support the potential involvement of signal strength in CD4/CD8 lineage commitment.

The role of notch in promoting glial and neural stem cell fates

The Notch signaling pathway has long been known to influence cell fate in the developing nervous system. However, this pathway has generally been thought to inhibit the specification of certain cell types in favor of others, or to simply maintain a progenitor pool. Recently, this view has been challenged by numerous studies suggesting that Notch may play an instructive role in promoting glial development. This work has inspired a new look at the role of Notch signaling in specifying cell fate. It has also prompted further consideration of the emerging view that in some contexts glia may be multipotent progenitors. This review examines the role of Notch during gliogenesis in both fruit flies and vertebrates, as well as evidence in vertebrates that some glia may be stem cells.

Active form of Notch imposes T cell fate in human progenitor cells

The crucial role of Notch signaling in cell fate decisions in hematopoietic lineage and T lymphocyte development has been well established in mice. Overexpression of the intracellular domain of Notch mediates signal transduction of the protein. By retroviral transduction of this constitutively active truncated intracellular domain in human CD34+ umbilical cord blood progenitor cells, we were able to show that, in coculture with the stromal MS-5 cell line, depending on the cytokines added, the differentiation toward CD19+ B lymphocytes was blocked, the differentiation toward CD14+ monocytes was inhibited, and the differentiation toward CD56+ NK cells was favored. The number of CD7+cyCD3+ cells, a phenotype similar to T/NK progenitor cells, was also markedly increased. In fetal thymus organ culture, transduced CD34+ progenitor cells from umbilical cord blood cells or from thymus consistently generated more TCR-gammadelta T cells, whereas the other T cell subpopulations were largely unaffected. Interestingly, when injected in vivo in SCID-nonobese diabetic mice, the transduced cells generated ectopically human CD4+CD8+ TCR-alphabeta cells in the bone marrow, cells that are normally only present in the thymus, and lacked B cell differentiation potential. Our results show unequivocally that, in human, Notch signaling inhibits the monocyte and B cell fate, promotes the T cell fate, and alters the normal T cell differentiation pathway compatible with a pretumoral state.

Notch signaling in hematopoiesis and early lymphocyte development

Notch signals regulate multiple cell fate decisions during metazoan development. During hematopoiesis, Notch affects both hematopoietic stem cells and committed progenitors. In hematopoietic stem cells, Notch signaling has the propensity to expand the stem cells, promote their self-renewal, and influence their survival. In committed progenitors, Notch signaling plays a key role in determining lymphoid cell fates. This review focuses on recent developments to understand the role of Notch signaling in early events in hematopoiesis.

A reassessment of the effect of activated Notch1 on CD4 and CD8 T cell development

The Notch signaling pathway plays an important role in the early steps of T cell development and in the generation of T cell tumors, but its role in the CD4 vs CD8 lineage decision is controversial. Notch1 is not essential for CD4 or CD8 T cell development; however, there are suggestions that multiple Notch family members may act in a redundant fashion during thymic development. In theory, expressing a constitutively activated form of Notch in CD4(+)CD8(+) thymocytes could provide clues about the normal role of Notch in developing CD4 and CD8 T cells. Unfortunately, two different studies of transgenic mice expressing activated forms of Notch1 (Notch1IC) led to conflicting conclusions. In this study, we re-examine the effect of the two Notch1IC transgenes on thymocyte development. We find that both Notch1IC transgenic lines display a decrease in CD4 single positive (SP) thymocytes and a corresponding increase in CD8 SP thymocytes. The enhanced development of CD8 SP thymocytes is dependent on either class I or II MHC. Thus, data from two different Notch1IC transgenic lines indicate that Notch activity promotes CD8 and inhibits CD4 SP development. We suggest that the discrepancies in previous reports of Notch1IC transgenic mice are due to differences in the propensity of the two different transgenic lines to develop tumors.

Mastermind mediates chromatin-specific transcription and turnover of the Notch enhancer complex

Signaling through the Notch pathway activates the proteolytic release of the Notch intracellular domain (ICD), a dedicated transcriptional coactivator of CSL enhancer-binding proteins. Here we show that chromatin-dependent transactivation by the recombinant Notch ICD-CBF1 enhancer complex in vitro requires an additional coactivator, Mastermind (MAM). MAM provides two activation domains necessary for Notch signaling in mammalian cells and in Xenopus embryos. We show that the central MAM activation domain (TAD1) recruits CBP/p300 to promote nucleosome acetylation at Notch enhancers and activate transcription in vitro. We also find that MAM expression induces phosphorylation and relocalization of endogenous CBP/p300 proteins to nuclear foci in vivo. Moreover, we show that coexpression with MAM and CBF1 strongly enhances phosphorylation and proteolytic turnover of the Notch ICD in vivo. Enhanced phosphorylation of the ICD and p300 requires a glutamine-rich region of MAM (TAD2) that is essential for Notch transcription in vivo. Thus MAM may function as a timer to couple transcription activation with disassembly of the Notch enhancer complex on chromatin.

Transcriptional control of B-cell development

Significant progress has recently been made in our understanding of how transcription factors such as PU.1, Notch1, E2A, EBF, Pax5, Bcl6, Blimp1 and XBP1 control different developmental decisions during the onset and terminal phase of B-lymphopoiesis. One emerging theme is that negative regulatory networks play an important role in suppressing alternative gene programs and their corresponding cell fates throughout B-cell development.

Deciphering the role of Notch signaling in lymphopoiesis

Components of the Notch signaling pathway are expressed during multiple stages of lymphoid development. Consistent with its function during invertebrate development, Notch signaling is proposed to have a central role in lymphoid cell-fate specification. Recent studies show that Notch signaling is a proximal event in T-cell commitment from a common lymphoid progenitor. The role of Notch at later stages of lymphoid development is controversial, but recent data suggest models that may help clarify observations. Current studies suggest that Notch activity is cell-context dependent and interactions between Notch and other environmental receptors are integrated during cell-fate decisions. Furthermore, the requirement for precise regulation of Notch activity is evident from human and murine neoplasms in which dysregulated Notch signaling leads to T-cell leukemia. Future studies that identify the stages of lymphoid development where Notch signaling is physiologically active and the exact targets of Notch signaling that are relevant to lymphopoiesis should significantly improve our understanding of Notch function in T- and B-cell development.

An invitation to T and more: notch signaling in lymphopoiesis

Cell fate decisions in metazoans are regulated by Notch signals. During lymphoid development, Notch influences a series of cell fate decisions involving multipotent progenitors. This review focuses on current views and lingering uncertainties about Notch function in lymphoid cells.

Notch1 perturbation of hemopoiesis involves non-cell- autonomous modifications

To study the effects of Notch on hemopoiesis we used a bone marrow transduction/transplantation model and compared the transduced and nontransduced populations in reconstituted mice. While cells expressing a constitutively active form of murine Notch1 (Notch1IC) completely lacked B cells, a profound suppression of the B lineage was also seen in the nontransduced compartment. Experiments performed with retroviral supernatants of varying titers showed that the perturbations of B cell development among the nontransduced population correlated with the percentage of Notch1IC-transduced cells inoculated into the mice. The myeloid lineage of the Notch1IC-transplanted mice was altered as well, and this also affected the nontransduced population that had features of excessive maturation. To explore the basis of these non-cell-autonomous modifications we prepared conditioned medium from ex vivo cultures of Notch1IC-transplanted mice bone marrow and showed that it inhibited B cell maturation and promoted myeloid differentiation in a dose-dependent manner. Finally, we found that the T cell leukemia/lymphomas that occur in Notch1IC-transplanted mice were accompanied by abnormal maturation of nontransduced T cells in the bone marrow. These findings indicate that modifications of neighboring cells through non-cell-autonomous modifications take part in multiple facets of the activity of Notch on hemopoiesis.

Direct induction of T lymphocyte-specific gene expression by the mammalian Notch signaling pathway

The Notch signaling pathway regulates the commitment and early development of T lymphocytes. We studied Notch-mediated induction of the pre-T cell receptor alpha (pTa) gene, a T-cell-specific transcriptional target of Notch. The pTa enhancer was activated by Notch signaling and contained binding sites for its nuclear effector, CSL. Mutation of the CSL-binding sites abolished enhancer induction by Notch and delayed the up-regulation of pTa transgene expression during T cell lineage commitment. These results show a direct mechanism of stage- and tissue-specific gene induction by the mammalian Notch/CSL signaling pathway.