Msx2-interacting nuclear target protein (Mint) deficiency reveals negative regulation of early thymocyte differentiation by Notch/RBP-J signaling

SPOC domain proteins in health and disease

Since it was first described >20 yr ago, the SPOC domain (Spen paralog and ortholog C-terminal domain) has been identified in many proteins all across eukaryotic species. SPOC-containing proteins regulate gene expression on various levels ranging from transcription to RNA processing, modification, export, and stability, as well as X-chromosome inactivation. Their manifold roles in controlling transcriptional output implicate them in a plethora of developmental processes, and their misregulation is often associated with cancer. Here, we provide an overview of the biophysical properties of the SPOC domain and its interaction with phosphorylated binding partners, the phylogenetic origin of SPOC domain proteins, the diverse functions of mammalian SPOC proteins and their homologs, the mechanisms by which they regulate differentiation and development, and their roles in cancer.

Multiple Mechanisms of NOTCH1 Activation in Chronic Lymphocytic Leukemia: NOTCH1 Mutations and Beyond

Simple Summary

Mutations of the NOTCH1 gene are a validated prognostic marker in chronic lymphocytic leukemia and a potential predictive marker for anti-CD20-based therapies. At present, the most frequent pathological alteration of the NOTCH1 gene is due to somatic genetic mutations, which have a multifaceted functional impact. However, beside NOTCH1 mutations, other factors may lead to activation of the NOTCH1 pathway, and these include mutations of FBXW7, MED12, SPEN, SF3B1 as well as other B-cell pathways. Understanding the preferential strategies though which CLL cells hijack NOTCH1 signaling may present important clues for designing targeted treatment strategies for the management of CLL.

Abstract

The Notch signaling pathway plays a fundamental role for the terminal differentiation of multiple cell types, including B and T lymphocytes. The Notch receptors are transmembrane proteins that, upon ligand engagement, undergo multiple processing steps that ultimately release their intracytoplasmic portion. The activated protein ultimately operates as a nuclear transcriptional co-factor, whose stability is finely regulated. The Notch pathway has gained growing attention in chronic lymphocytic leukemia (CLL) because of the high rate of somatic mutations of the NOTCH1 gene. In CLL, NOTCH1 mutations represent a validated prognostic marker and a potential predictive marker for anti-CD20-based therapies, as pathological alterations of the Notch pathway can provide significant growth and survival advantage to neoplastic clone. However, beside NOTCH1 mutation, other events have been demonstrated to perturb the Notch pathway, namely somatic mutations of upstream, or even apparently unrelated, proteins such as FBXW7, MED12, SPEN, SF3B1, as well as physiological signals from other pathways such as the B-cell receptor. Here we review these mechanisms of activation of the NOTCH1 pathway in the context of CLL; the resulting picture highlights how multiple different mechanisms, that might occur under specific genomic, phenotypic and microenvironmental contexts, ultimately result in the same search for proliferative and survival advantages (through activation of MYC), as well as immune escape and therapy evasion (from anti-CD20 biological therapies). Understanding the preferential strategies through which CLL cells hijack NOTCH1 signaling may present important clues for designing targeted treatment strategies for the management of CLL.

The developmental origins of Notch-driven intrahepatic bile duct disorders

The Notch signaling pathway is an evolutionarily conserved mechanism of cell-cell communication that mediates cellular proliferation, cell fate specification, and maintenance of stem and progenitor cell populations. In the vertebrate liver, an absence of Notch signaling results in failure to form bile ducts, a complex tubular network that radiates throughout the liver, which, in healthy individuals, transports bile from the liver into the bowel. Loss of a functional biliary network through congenital malformations during development results in cholestasis and necessitates liver transplantation. Here, we examine to what extent Notch signaling is necessary throughout embryonic life to initiate the proliferation and specification of biliary cells and concentrate on the animal and human models that have been used to define how perturbations in this signaling pathway result in developmental liver disorders.

Chromatin Regulator SPEN/SHARP in X Inactivation and Disease

Simple Summary

Carcinogenesis is a multistep process involving not only the activation of oncogenes and disabling tumor suppressor genes, but also epigenetic modulation of gene expression. X chromosome inactivation (XCI) is a paradigm to study heterochromatin formation and maintenance. The double dosage of X chromosomal genes in female mammals is incompatible with early development. XCI is an excellent model system for understanding the establishment of facultative heterochromatin initiated by the expression of a 17,000 nt long non-coding RNA, known as Xinactivespecifictranscript (Xist), on the X chromosome. This review focuses on the molecular mechanisms of how epigenetic modulators act in a step-wise manner to establish facultative heterochromatin, and we put these in the context of cancer biology and disease. An in depth understanding of XCI will allow a better characterization of particular types of cancer and hopefully facilitate the development of novel epigenetic therapies.

Abstract

Enzymes, such as histone methyltransferases and demethylases, histone acetyltransferases and deacetylases, and DNA methyltransferases are known as epigenetic modifiers that are often implicated in tumorigenesis and disease. One of the best-studied chromatin-based mechanism is X chromosome inactivation (XCI), a process that establishes facultative heterochromatin on only one X chromosome in females and establishes the right dosage of gene expression. The specificity factor for this process is the long non-coding RNA Xinactivespecifictranscript (Xist), which is upregulated from one X chromosome in female cells. Subsequently, Xist is bound by the corepressor SHARP/SPEN, recruiting and/or activating histone deacetylases (HDACs), leading to the loss of active chromatin marks such as H3K27ac. In addition, polycomb complexes PRC1 and PRC2 establish wide-spread accumulation of H3K27me3 and H2AK119ub1 chromatin marks. The lack of active marks and establishment of repressive marks set the stage for DNA methyltransferases (DNMTs) to stably silence the X chromosome. Here, we will review the recent advances in understanding the molecular mechanisms of how heterochromatin formation is established and put this into the context of carcinogenesis and disease.

T cell costimulation, checkpoint inhibitors and anti-tumor therapy

The hallmarks of the adaptive immune response are specificity and memory. The cellular response is mediated by T cells which express cell surface T cell receptors (TCRs) that recognize peptide antigens in complex with major histocompatibility complex (MHC) molecules on antigen presenting cells (APCs). However, binding of cognate TCRs with MHC-peptide complexes alone (signal 1) does not trigger optimal T cell activation. In addition to signal 1, the binding of positive and negative costimulatory receptors to their ligands modulates T cell activation. This complex signaling network prevents aberrant activation of T cells. CD28 is the main positive costimulatory receptor on naı¨ve T cells; upon activation, CTLA4 is induced but reduces T cell activation. Further studies led to the identification of additional negative costimulatory receptors known as checkpoints, e.g. PD1. This review chronicles the basic studies in T cell costimulation that led to the discovery of checkpoint inhibitors, i.e. antibodies to negative costimulatory receptors (e.g. CTLA4 and PD1) which reduce tumor growth. This discovery has been recognized with the award of the 2018 Nobel prize in Physiology/Medicine. This review highlights the structural and functional roles of costimulatory receptors, the mechanisms by which checkpoint inhibitors work, the challenges encountered and future prospects.

Spen links RNA-mediated endogenous retrovirus silencing and X chromosome inactivation

The Xist lncRNA mediates X chromosome inactivation (XCI). Here we show that Spen, an Xist-binding repressor protein essential for XCI , binds to ancient retroviral RNA, performing a surveillance role to recruit chromatin silencing machinery to these parasitic loci. Spen loss activates a subset of endogenous retroviral (ERV) elements in mouse embryonic stem cells, with gain of chromatin accessibility, active histone modifications, and ERV RNA transcription. Spen binds directly to ERV RNAs that show structural similarity to the A-repeat of Xist, a region critical for Xist-mediated gene silencing. ERV RNA and Xist A-repeat bind the RRM domains of Spen in a competitive manner. Insertion of an ERV into an A-repeat deficient Xist rescues binding of Xist RNA to Spen and results in strictly local gene silencing in cis. These results suggest that Xist may coopt transposable element RNA-protein interactions to repurpose powerful antiviral chromatin silencing machinery for sex chromosome dosage compensation.

Structural and Functional Studies of the RBPJ-SHARP Complex Reveal a Conserved Corepressor Binding Site

Notch is a conserved signaling pathway that is essential for metazoan development and homeostasis; dysregulated signaling underlies the pathophysiology of numerous human diseases. Receptor-ligand interactions result in gene expression changes, which are regulated by the transcription factor RBPJ. RBPJ forms a complex with the intracellular domain of the Notch receptor and the coactivator Mastermind to activate transcription, but it can also function as a repressor by interacting with corepressor proteins. Here, we determine the structure of RBPJ bound to the corepressor SHARP and DNA, revealing its mode of binding to RBPJ. We tested structure-based mutants in biophysical and biochemical-cellular as-says to characterize the role of RBPJ as a repressor, clearly demonstrating that RBPJ mutants deficient for SHARP binding are incapable of repressing transcription of genes responsive to Notch signaling in cells. Altogether, our structure-function studies provide significant insights into the repressor function of RBPJ.

Yuan et al. determine the X-ray structure of the corepressor SHARP bound to RBPJ, the nuclear effector of the Notch pathway. The structure-function analysis provides insights into corepressor binding to RBPJ and how RBPJ functions as a repressor of transcription of Notch target genes.

The Canonical Notch Signaling Pathway: Structural and Biochemical Insights into Shape, Sugar, and Force

The Notch signaling pathway relies on a proteolytic cascade to release its transcriptionally active intracellular domain, on force to unfold a protective domain and permit proteolysis, on extracellular domain glycosylation to tune the forces exerted by endocytosed ligands, and on a motley crew of nuclear proteins, chromatin modifiers, ubiquitin ligases, and a few kinases to regulate activity and half-life. Herein we provide a review of recent molecular insights into how Notch signals are triggered and how cell shape affects these events, and we use the new insights to illuminate a few perplexing observations.

An autonomous metabolic role for Spen

Preventing obesity requires a precise balance between deposition into and mobilization from fat stores, but regulatory mechanisms are incompletely understood. Drosophila Split ends (Spen) is the founding member of a conserved family of RNA-binding proteins involved in transcriptional regulation and frequently mutated in human cancers. We find that manipulating Spen expression alters larval fat levels in a cell-autonomous manner. Spen-depleted larvae had defects in energy liberation from stores, including starvation sensitivity and major changes in the levels of metabolic enzymes and metabolites, particularly those involved in β-oxidation. Spenito, a small Spen family member, counteracted Spen function in fat regulation. Finally, mouse Spen and Spenito transcript levels scaled directly with body fat in vivo, suggesting a conserved role in fat liberation and catabolism. This study demonstrates that Spen is a key regulator of energy balance and provides a molecular context to understand the metabolic defects that arise from Spen dysfunction.

All animals need energy to fuel development and survive as adults. Excess energy stored as fat provides a means to endure periods when external energy is unavailable, but there is a delicate balance between accumulating sufficient fat stores and becoming obese. While the enzymes that mediate energy deposition into and mobilization from fat stores are well studied, the complex upstream regulatory pathways have not been fully worked out. We report here that two members of a conserved family of RNA-binding proteins, Spen and Nito, operate in fat storage cells in fruit fly larvae to control the expression of genes that mediate energy liberation from fat stores. Manipulating Spen or Spenito function grossly perturbs larval energy metabolism, including imbalances in the amounts of stored fats, key metabolites, and metabolic enzymes, and resulting in defects in survival under starvation conditions. Interestingly, Nito opposes Spen functions, indicative of a regulatory mechanism that helps keep energy balance in check. We find that the mouse homologs of Spen and Nito, which were known to regulate gene expression in other pathways, respond similarly to changes in body fat induced by a high-fat diet, suggesting that the balancing effect of these two proteins also prevents mammalian obesity.

Structure and Function of the Su(H)-Hairless Repressor Complex, the Major Antagonist of Notch Signaling in Drosophila melanogaster

Notch is a conserved signaling pathway that specifies cell fates in metazoans. Receptor-ligand interactions induce changes in gene expression, which is regulated by the transcription factor CBF1/Su(H)/Lag-1 (CSL). CSL interacts with coregulators to repress and activate transcription from Notch target genes. While the molecular details of the activator complex are relatively well understood, the structure-function of CSL-mediated repressor complexes is poorly defined. In Drosophila, the antagonist Hairless directly binds Su(H) (the fly CSL ortholog) to repress transcription from Notch targets. Here, we determine the X-ray structure of the Su(H)-Hairless complex bound to DNA. Hairless binding produces a large conformational change in Su(H) by interacting with residues in the hydrophobic core of Su(H), illustrating the structural plasticity of CSL molecules to interact with different binding partners. Based on the structure, we designed mutants in Hairless and Su(H) that affect binding, but do not affect formation of the activator complex. These mutants were validated in vitro by isothermal titration calorimetry and yeast two- and three-hybrid assays. Moreover, these mutants allowed us to solely characterize the repressor function of Su(H) in vivo.

The transcription factor CSL regulates gene expression in response to Notch pathway signaling. The X-ray structure of the complex between the fruit fly version of CSL, Su(H), and its antagonist, Hairless, reveals a novel binding mode and unanticipated structural plasticity.

Notch signaling is a form of cell-to-cell communication, in which extracellular receptor-ligand interactions ultimately result in changes in gene expression. The Notch pathway is highly conserved from the model organism Drosophila melanogaster to humans. When mutations occur within Notch pathway components, this often leads to human disease, such as certain types of cancers and birth defects. Transcription of Notch target genes is regulated by the transcription factor CSL (for CBF1/RBP-J in mammals, Su(H) in Drosophila, and Lag-1 in Caenorhabditis elegans). CSL functions as both a transcriptional activator and repressor by forming complexes with coactivator and corepressor proteins, respectively. Here we determine the high-resolution X-ray structure of Su(H) (the fly CSL ortholog) in complex with the corepressor Hairless, which is the major antagonist of Notch signaling in Drosophila. The structure unexpectedly reveals that Hairless binding results in a dramatic conformational change in Su(H). In parallel, we designed mutations in Su(H) and Hairless based on our structure and showed that these mutants are defective in complex formation in vitro and display functional deficiencies in in vivo assays. Taken together, our work provides significant molecular insights into how CSL functions as a transcriptional repressor in the Notch pathway.

A phospho-dependent mechanism involving NCoR and KMT2D controls a permissive chromatin state at Notch target genes

The transcriptional shift from repression to activation of target genes is crucial for the fidelity of Notch responses through incompletely understood mechanisms that likely involve chromatin-based control. To activate silenced genes, repressive chromatin marks are removed and active marks must be acquired. Histone H3 lysine-4 (H3K4) demethylases are key chromatin modifiers that establish the repressive chromatin state at Notch target genes. However, the counteracting histone methyltransferase required for the active chromatin state remained elusive. Here, we show that the RBP-J interacting factor SHARP is not only able to interact with the NCoR corepressor complex, but also with the H3K4 methyltransferase KMT2D coactivator complex. KMT2D and NCoR compete for the C-terminal SPOC-domain of SHARP. We reveal that the SPOC-domain exclusively binds to phosphorylated NCoR. The balance between NCoR and KMT2D binding is shifted upon mutating the phosphorylation sites of NCoR or upon inhibition of the NCoR kinase CK2β. Furthermore, we show that the homologs of SHARP and KMT2D in Drosophila also physically interact and control Notch-mediated functions in vivo. Together, our findings reveal how signaling can fine-tune a committed chromatin state by phosphorylation of a pivotal chromatin-modifier.

Developmental gene networks: a triathlon on the course to T cell identity

Cells acquire their ultimate identities by activating combinations of transcription factors that initiate and sustain expression of the appropriate cell type-specific genes. T cell development depends on the progression of progenitor cells through three major phases, each of which is associated with distinct transcription factor ensembles that control the recruitment of these cells to the thymus, their proliferation, lineage commitment and responsiveness to T cell receptor signals, all before the allocation of cells to particular effector programmes. All three phases are essential for proper T cell development, as are the mechanisms that determine the boundaries between each phase. Cells that fail to shut off one set of regulators before the next gene network phase is activated are predisposed to leukaemic transformation.

AF1q/MLLT11 regulates the emergence of human prothymocytes through cooperative interaction with the Notch signaling pathway

The mechanisms regulating the emergence of BM prothymocytes remain poorly characterized. Genome-wide transcriptome analyses looking for genes expressed in human prothymocytes led to the identification of AF1q/MLLT11 as a candidate gene conceivably involved in this process. Analysis of AF1q protein subcellular localization and intracellular trafficking showed that despite pronounced karyophily, it was subjected to constitutive nuclear export followed by ubiquitin-mediated degradation in the centrosomal area. Using in vitro assays based on either forced expression or shRNA-mediated silencing of AF1q, we provide evidence that the protein promotes T- over B-cell differentiation in multipotent hematopoietic progenitors. At the molecular level, AF1q confers to multipotent progenitors an increased susceptibility to Delta-like/Notch-mediated signaling. Consistent with these findings, enforced AF1q expression in humanized mice fosters the emergence of BM CD34(+)CD7(+) prothymocytes, enhances subsequent thymus colonization, and accelerates intrathymic T-cell development. In contrast, AF1q silencing provokes a global shift of BM lymphopoiesis toward the B-cell lineage, hinders prothymocyte development, inhibits thymus colonization, and leads to intrathymic accumulation of B cells. Our results indicate that AF1q cooperates with the Notch signaling pathway to foster the emergence of BM prothymocytes and drive subsequent intrathymic specification toward the T-cell lineage.

Disturbed tooth germ development in the absence of MINT in the cultured mouse mandibular explants

The Msx2-interacting nuclear target protein (MINT) is a nuclear matrix protein that regulates the development of many tissues. However, little is known regarding the role of MINT in tooth development. In this study, we prepared polyclonal antibodies against MINT, and found that that MINT was expressed in different cells at each stage of tooth germ development by immunohistochemistry. The role of MINT in tooth development was further illustrated by the misshapen and severely hypoplastic tooth organ in the cultured mandibular explants of MINT deficient mice. From the initiation to cap stage, the differences between mutants and wild-type molars were more and more distinguished histologically. In the MINT-deficient mandibular explants, the tooth germ was reduced in the overall size and lacked enamel knot, with abnormal dental lamina and collapsed stellate reticulum. Furthermore, the BrdU incorporation experiment showed that the proliferation activity was significantly reduced in MINT-deficient dental epithelium. Our results suggest that MINT plays an important role in tooth development, in particular, epithelial morphogenesis.

Transcriptional repression in the Notch pathway: thermodynamic characterization of CSL-MINT (Msx2-interacting nuclear target protein) complexes

The Notch pathway is a conserved cell-to-cell signaling mechanism that mediates cell fate decisions in metazoans. Canonical signaling results in changes in gene expression, which is regulated by the nuclear effector of the pathway CSL (CBF1/RBP-J, Su(H), Lag-1). CSL is a DNA binding protein that functions as either a repressor or an activator of transcription, depending upon whether it is complexed by transcriptional corepressor or coactivator proteins, respectively. In stark contrast to CSL-coactivator complexes, e.g. the transcriptionally active CSL-Notch-Mastermind ternary complex, the structure and function of CSL-corepressor complexes are poorly understood. The corepressor MINT (Msx2-interacting nuclear target protein) has been shown in vivo to antagonize Notch signaling and shown in vitro to biochemically interact with CSL; however, the molecular details of this interaction are only partially defined. Here, we provide a quantitative thermodynamic binding analysis of CSL-MINT complexes. Using isothermal titration calorimetry, we demonstrate that MINT forms a high affinity complex with CSL, and we also delineate the domains of MINT and CSL that are necessary and sufficient for complex formation. Moreover, we show in cultured cells that this region of MINT can inhibit Notch signaling in transcriptional reporter assays. Taken together, our results provide functional insights into how CSL is converted from a repressor to an activator of transcription.

Notch-independent functions of CSL

Notch-dependent CSL transcription complexes control essential biological processes such as cell proliferation, differentiation, and cell-fate decisions in diverse developmental systems. The orthologous proteins CBF1/Rbpj (mammalian), Su(H) (Drosophila), and Lag-1 (Caenorhabditis elegans) compose the CSL family of sequence-specific DNA-binding transcription factors. The CSL proteins are best known for their role in canonical Notch signaling. However, CSL factors also form transcription complexes that can function independent of Notch signaling and include repression and activation of target gene transcription. Because the different complexes share CSL as a DNA-binding subunit, they can control overlapping sets of genes; but they can also control distinct sets when partnered with tissue-specific cofactors that restrict DNA-sequence recognition or stability of the DNA-bound complex. The Notch-independent functions of CSL and the processes they regulate will be reviewed here with a particular emphasis on the tissue-specific CSL-activator complex with the bHLH factor Ptf1a.

Notch regulation of early thymocyte development

Notch signaling plays multiple roles in T cell development. Following thymic entry, Notch signals are required to specify the T cell fate from a multipotent hematopoietic progenitor. At subsequent steps in early T cell development, Notch provides important differentiation, survival, proliferation and metabolic signals. This review focuses on the multiple functions of Notch in early T cell development, from T cell specification in the thymus through beta selection.

Eliciting the T cell fate with Notch

Multipotent progenitors arrive at the thymus via the blood. Constraining the non-T cell fates of these progenitors while promoting the T cell fate is a major task of the thymus. Notch appears to be the initial trigger for a developmental program that eventually results in T cell lineage commitment. Several downstream targets of Notch are known, but the specific roles of each are poorly understood. A greater understanding of how Notch and other thymic signals direct progenitors to a T cell fate could be useful for translational work. For example, such work could eventually allow for the generation of fully competent T cells in vitro that could supplement the waning T cell numbers and function in the elderly and boost T cell-mediated immunity in patients with immunodeficiency and after stem cell transplantation.

Notch target gene deregulation and maintenance of the leukemogenic phenotype do not require RBP-J kappa in Ikaros null mice

Ikaros and Notch are transcriptional regulators essential for normal T cell development. Aberrant activation of Notch target genes is observed in Ikaros-deficient thymocytes as well as leukemia cell lines. However, it is not known whether Notch deregulation plays a preferential or obligatory role in the leukemia that arise in Ikaros null (Ik(-/-)) mice. To answer this question, the expression of the DNA-binding Notch target gene activator RBP-Jkappa was abrogated in Ik(-/-) double-positive thymocytes. This was accomplished through conditional inactivation using CD4-Cre transgenic mice containing floxed RBP-Jkappa alleles (RBPJ(fl/fl)). Ik(-/-) x RBPJ(fl/fl) x CD4-Cre(+) transgenic mice develop clonal T cell populations in the thymus that escape to the periphery, with similar kinetics and penetrance as their CD4-Cre(-) counterparts. The clonal populations do not display increased RBP-Jkappa expression compared with nontransformed thymocytes, suggesting there is no selection for clones that have not fully deleted RBP-Jkappa. However, RBPJ-deficient clonal populations do not expand as aggressively as their RBPJ-sufficient counterparts, suggesting a qualitative role for deregulated Notch target gene activation in the leukemogenic process. Finally, these studies show that RBP-Jkappa plays no role in Notch target gene repression in double-positive thymocytes but rather that it is Ikaros that is required for the repression of these genes at this critical stage of T cell development.

Runx2 trans-activation mediated by the MSX2-interacting nuclear target requires homeodomain interacting protein kinase-3

Runt-related transcription factor 2 (Runx2) and muscle segment homeobox homolog 2-interacting nuclear target (MINT) (Spen homolog) are transcriptional regulators critical for mammalian development. MINT enhances Runx2 activation of osteocalcin (OC) fibroblast growth factor (FGF) response element in an FGF2-dependent fashion in C3H10T1/2 cells. Although the MINT N-terminal RNA recognition motif domain contributes, the muscle segment homeobox homolog 2-interacting domain is sufficient for Runx2 activation. Intriguingly, Runx1 cannot replace Runx2 in this assay. To better understand this Runx2 signaling cascade, we performed structure-function analysis of the Runx2-MINT trans-activation relationship. Systematic truncation and domain swapping in Runx1:Runx2 chimeras identified that the unique Runx2 activation domain 3 (AD3), encompassed by residues 316-421, conveys MINT+FGF2 trans-activation in transfection assays. Ala mutagenesis of Runx2 Ser/Thr residues identified that S301 and T326 in AD3 are necessary for full MINT+FGF2 trans-activation. Conversely, phosphomimetic Asp substitution of these AD3 Ser/Thr residues enhanced activation by MINT. Adjacent Pro residues implicated regulation by a proline-directed protein kinase (PDPK). Systematic screening with PDPK inhibitors identified that the casein kinase-2/homeodomain-interacting protein kinase (HIPK)/dual specificity tyrosine phosphorylation regulated kinase inhibitor 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT), but not ERK, c-Jun N-terminal kinase, p38MAPK, or other casein kinase-2 inhibitors, abrogated Runx2-, MINT-, and FGF2-activation. Systematic small interfering RNA-mediated silencing of DMAT-inhibited PDPKs revealed that HIPK3 depletion reduced MINT+FGF2-dependent activation of Runx2. HIPK3 and Runx2 coprecipitate after in vitro transcription-translation, and recombinant HIPK3 recognizes Runx2 AD3 as kinase substrate. Furthermore, DMAT treatment and HIPK3 RNAi inhibited MINT+FGF2 activation of Runx2 AD3, and nuclear HIPK3 colocalized with MINT. HIPK3 antisense oligodeoxynucleotide selectively reduced Runx2 protein accumulation and OC gene expression in C3H10T1/2 cells. Thus, HIPK3 participates in MINT+FGF2 regulation of Runx2 AD3 activity and controls Runx2-dependent OC expression.

Mechanistic insights into Notch receptor signaling from structural and biochemical studies

Notch proteins are the receptors in a highly conserved signal transduction system used to communicate signals between cells that contact each other. Studies investigating structure-function relationships in Notch signaling have gained substantial momentum in recent years. Here, we summarize the current understanding of the molecular logic of Notch signal transduction, emphasizing structural and biochemical studies of Notch receptors, their ligands, and complexes of intracellular Notch proteins with their target transcription factors. Recent advances in the structure-based modulation of Notch-signaling activity are also discussed.

Two opposing roles of RBP-J in Notch signaling

RBP-J/Su(H)/Lag1, the main transcriptional mediator of Notch signaling, binds DNA with the consensus sequence YRTGDGAD. Notch target genes can be controlled by two opposing activities of RBP-J. The interaction of the Notch intracellular domain with RBP-J induces a weak transcriptional activation and requires an additional tissue-specific transcriptional activator such as bHLH proteins or GATA to mediate strong target gene expression. For example, during Drosophila sensory organ precursor (SOP) cell development, proneural bHLH interacts with Da, a Drosophila orthologue of E2A, to form a tissue-specific activator of Su(H), the Drosophila orthologue of RBP-J. This complex and Su(H) act synergistically to promote the epidermal cell fate. In contrast, a complex of Su(H) with Hairless, a Drosophila functional homologue of MINT, has transcriptional repression activity that promotes SOP differentiation to neurons. Recent conditional loss-of-function studies demonstrated that transcriptional networks involving RBP-J, MINT, and E2A are conserved in mammalian cell differentiation, including multiple steps of lymphocyte development, and probably also in neuronal maturation in adult neurogenesis. During neurogenesis, Notch-RBP-J signaling was thought historically to be involved mainly in the maintenance of undifferentiated neural progenitors. However, the identification of a tissue-specific transcriptional activator of RBP-J-Notch has revealed new roles of RBP-J in the promotion of neuronal maturation. Finally, the Notch-independent function of RBP-J was recently discovered and will be reviewed here.

The contribution of Notch1 to nephron segmentation in the developing kidney is revealed in a sensitized Notch2 background and can be augmented by reducing Mint dosage

We previously determined that Notch2, and not Notch1, was required for forming proximal nephron segments. The dominance of Notch2 may be conserved in humans, since Notch2 mutations occur in Alagille syndrome (ALGS) 2 patients, which includes renal complications. To test whether mutations in Notch1 could increase the severity of renal complications in ALGS, we inactivated conditional Notch1 and Notch2 alleles in mice using a Six2-GFP::Cre. This BAC transgene is expressed mosaically in renal epithelial progenitors but uniformly in cells exiting the progenitor pool to undergo mesenchymal-to-epithelial transition. Although delaying Notch2 inactivation had a marginal effect on nephron numbers, it created a sensitized background in which the inactivation of Notch1 severely compromised nephron formation, function, and survival. These and additional observations indicate that Notch1 in concert with Notch2 contributes to the morphogenesis of renal vesicles into S-shaped bodies in a RBP-J-dependent manner. A significant implication is that elevating Notch1 activity could improve renal functions in ALGS2 patients. As proof of principle, we determined that conditional inactivation of Mint, an inhibitor of Notch-RBP-J interaction, resulted in a moderate rescue of Notch2 null kidneys, implying that temporal blockage of Notch signaling inhibitors downstream of receptor activation may have therapeutic benefits for ALGS patients.

NK-like homeodomain proteins activate NOTCH3-signaling in leukemic T-cells

Background

Homeodomain proteins control fundamental cellular processes in development and in cancer if deregulated. Three members of the NK-like subfamily of homeobox genes (NKLs), TLX1, TLX3 and NKX2-5, are implicated in T-cell acute lymphoblastic leukemia (T-ALL). They are activated by particular chromosomal aberrations. However, their precise function in leukemogenesis is still unclear. Here we screened further NKLs in 24 T-ALL cell lines and identified the common expression of MSX2. The subsequent aim of this study was to analyze the role of MSX2 in T-cell differentiation which may be disturbed by oncogenic NKLs.

Methods

Specific gene activity was examined by quantitative real-time PCR, and globally by expression profiling. Proteins were analyzed by western blot, immuno-cytology and immuno-precipitation. For overexpression studies cell lines were transduced by lentiviruses.

Results

Quantification of MSX2 mRNA in primary hematopoietic cells demonstrated higher levels in CD34+ stem cells as compared to peripheral blood cells and mature CD3+ T-cells. Furthermore, analysis of MSX2 expression levels in T-cell lines after treatment with core thymic factors confirmed their involvement in regulation. These results indicated that MSX2 represents an hematopoietic NKL family member which is downregulated during T-cell development and may functionally substituted by oncogenic NKLs. For functional analysis JURKAT cells were lentivirally transduced, overexpressing either MSX2 or oncogenic TLX1 and NKX2-5, respectively. These cells displayed transcriptional activation of NOTCH3-signaling, including NOTCH3 and HEY1 as analyzed by gene expression profiling and quantitative RT-PCR, and consistently attenuated sensitivity to gamma-secretase inhibitor as analyzed by MTT-assays. Furthermore, in addition to MSX2, both TLX1 and NKX2-5 proteins interacted with NOTCH-pathway repressors, SPEN/MINT/SHARP and TLE1/GRG1, representing a potential mechanism for (de)regulation. Finally, elevated expression of NOTCH3 and HEY1 was detected in primary TLX1/3 positive T-ALL cells corresponding to the cell line data.

Conclusion

Identification and analysis of MSX2 in hematopoietic cells implicates a modulatory role via NOTCH3-signaling in early T-cell differentiation. Our data suggest that reduction of NOTCH3-signaling by physiological downregulation of MSX2 expression during T-cell development is abrogated by ectopic expression of oncogenic NKLs, substituting MSX2 function.

The canonical Notch signaling pathway: unfolding the activation mechanism

Notch signaling regulates many aspects of metazoan development and tissue renewal. Accordingly, the misregulation or loss of Notch signaling underlies a wide range of human disorders, from developmental syndromes to adult-onset diseases and cancer. Notch signaling is remarkably robust in most tissues even though each Notch molecule is irreversibly activated by proteolysis and signals only once without amplification by secondary messenger cascades. In this Review, we highlight recent studies in Notch signaling that reveal new molecular details about the regulation of ligand-mediated receptor activation, receptor proteolysis, and target selection.

NKAP is a transcriptional repressor of notch signaling and is required for T cell development

T cell development depends on the coordinated interplay between receptor signaling and transcriptional regulation. Through a genetic complementation screen a transcriptional repressor, NKAP, was identified. NKAP associated with the histone deacetylase HDAC3 and was shown to be part of a DNA-binding complex, as demonstrated by chromatin immunoprecipitation. NKAP also associated with the Notch corepressor complex. The expression of NKAP during T cell development inversely correlated with the expression of Notch target genes, implying that NKAP may modulate Notch-mediated transcription. To examine the function of NKAP in T cell development, we ablated NKAP by Lck(cre). Loss of NKAP blocked development of alphabeta but not gammadelta T cells, and Nkap(fl/o)Lck(cre) DP T cells expressed 8- to 20-fold higher amounts of Hes1, Deltex1, and CD25 mRNA. Thus, NKAP functions as a transcriptional repressor, acting on Notch target genes, and is required for alphabeta T cell development.

The OTT-MAL fusion oncogene activates RBPJ-mediated transcription and induces acute megakaryoblastic leukemia in a knockin mouse model

Acute megakaryoblastic leukemia (AMKL) is a form of acute myeloid leukemia (AML) associated with a poor prognosis. The genetics and pathophysiology of AMKL are not well understood. We generated a knockin mouse model of the one twenty-two-megakaryocytic acute leukemia (OTT-MAL) fusion oncogene that results from the t(1;22)(p13;q13) translocation specifically associated with a subtype of pediatric AMKL. We report here that OTT-MAL expression deregulated transcriptional activity of the canonical Notch signaling pathway transcription factor recombination signal binding protein for immunoglobulin kappa J region (RBPJ) and caused abnormal fetal megakaryopoiesis. Furthermore, cooperation between OTT-MAL and an activating mutation of the thrombopoietin receptor myeloproliferative leukemia virus oncogene (MPL) efficiently induced a short-latency AMKL that recapitulated all the features of human AMKL, including megakaryoblast hyperproliferation and maturation block, thrombocytopenia, organomegaly, and extensive fibrosis. Our results establish that concomitant activation of RBPJ (Notch signaling) and MPL (cytokine signaling) transforms cells of the megakaryocytic lineage and suggest that specific targeting of these pathways could be of therapeutic value for human AMKL.

Ikaros regulates Notch target gene expression in developing thymocytes

Both Ikaros and Notch are essential for normal T cell development. Collaborative mutations causing a reduction in Ikaros activity and an increase in Notch activation promote T cell leukemogenesis. Although the molecular mechanisms of this cooperation have been studied, its consequences in thymocyte development remain unexplored. In this study, we show that Ikaros regulates expression of a subset of Notch target genes, including Hes1, Deltex1, pTa, Gata3, and Runx1, in both Ikaros null T cell leukemia lines and Ikaros null primary thymocytes. In Ikaros null leukemia cells, Notch deregulation occurs at both the level of Notch receptor cleavage and expression of Notch target genes, because re-expression of Ikaros in these cells down-regulates Notch target gene expression without affecting levels of intracellular cleaved Notch. In addition, abnormal expression of Notch target genes is observed in Ikaros null double-positive thymocytes, in the absence of detectable intracellular cleaved Notch. Finally, we show that this role of Ikaros is specific to double-positive and single-positive thymocytes because derepression of Notch target gene expression is not observed in Ikaros null double-negative thymocytes or lineage-depleted bone marrow. Thus, in this study, we provide evidence that Ikaros and Notch play opposing roles in regulation of a subset of Notch target genes and that this role is restricted to developing thymocytes where Ikaros is required to appropriately regulate the Notch program as they progress through T cell development.

Ikaros represses the transcriptional response to Notch signaling in T-cell development

Notch activity is essential for early T-cell differentiation, but aberrant activity induces T-cell transformation. Thus, Notch target genes must be efficiently silenced in cells where Notch activity is no longer required. How these genes are repressed remains poorly understood. We report here that the Ikaros transcription factor plays a crucial role in repressing the transcriptional response to Notch signaling in T-cell development. Using the Notch target gene Hes-1 as a model, we show that Ikaros and RBP-Jkappa, the transcriptional mediator of Notch signaling, compete for binding to two elements in the Hes-1 promoter in immature thymocytes. This antagonistic interaction likely occurs at the CD4(-) CD8(-) CD3(-) double-negative 4 (DN4) stage, where Ikaros levels and binding to the Hes-1 promoter increase sharply and wild-type thymocytes lose their capacity to transcribe Hes-1 upon Notch stimulation. Nonresponsiveness to Notch signaling requires Ikaros, as Ikaros-deficient DN4 and CD4(+) CD8(+) double-positive (DP) cells remain competent to express Hes-1 after Notch activation. Further, Hes-1 promoter sequences from Ikaros-deficient DP cells show reduced trimethylated H3K27, a modification associated with silent chromatin. These results indicate that Ikaros functions as a transcriptional checkpoint to repress Notch target gene expression in T cells.

Competition and collaboration: GATA-3, PU.1, and Notch signaling in early T-cell fate determination

T-cell precursors remain developmentally plastic for multiple cell generations after entering the thymus, preserving access to developmental alternatives of macrophage, dendritic-cell, and even mast-cell fates. The underlying regulatory basis of this plasticity is that early T-cell differentiation depends on transcription factors which can also promote alternative developmental programs. Interfactor competition, together with environmental signals, keep these diversions under control. Here the pathways leading to several lineage alternatives for early pro-T-cells are reviewed, with close focus on the mechanisms of action of three vital factors, GATA-3, PU.1, and Notch-Delta signals, whose counterbalance appears to be essential for T-cell specification.

Notch and presenilin regulate cellular expansion and cytokine secretion but cannot instruct Th1/Th2 fate acquisition

Recent reports suggested that Delta1, 4 and Jagged1, 2 possessed the ability to instruct CD4⁺ T cell into selection of Th1 or Th2 fates, respectively, although the underlying mechanism endowing the cleaved Notch receptor with memory of ligand involved in its activation remains elusive. To examine this, we prepared artificial antigen-presenting cells expressing either DLL1 or Jag1. Although both ligands were efficient in inducing Notch2 cleavage and activation in CD4⁺ T or reporter cells, the presence of Lunatic Fringe in CD4⁺ T cells inhibited Jag1 activation of Notch1 receptor. Neither ligand could induce Th1 or Th2 fate choice independently of cytokines or redirect cytokine-driven Th1 or Th2 development. Instead, we find that Notch ligands only augment cytokine production during T cell differentiation in the presence of polarizing IL-12 and IL-4. Moreover, the differentiation choices of naïve CD4⁺ T cells lacking γ-secretase, RBP-J, or both in response to polarizing cytokines revealed that neither presenilin proteins nor RBP-J were required for cytokine-induced Th1/Th2 fate selection. However, presenilins facilitate cellular proliferation and cytokine secretion in an RBP-J (and thus, Notch) independent manner. The controversies surrounding the role of Notch and presenilins in Th1/Th2 polarization may reflect their role as genetic modifiers of T-helper cells differentiation.

ETO, but not leukemogenic fusion protein AML1/ETO, augments RBP-Jkappa/SHARP-mediated repression of notch target genes

Notch is a transmembrane receptor that determines cell fates and pattern formation in all animal species. After specific ligand binding, the intracellular part of Notch is cleaved off and translocates to the nucleus, where it targets the DNA binding protein RBP-Jkappa. In the absence of Notch, RBP-Jkappa represses Notch target genes by recruiting a corepressor complex. We and others have previously identified SHARP as one component of this complex. Here, we show that the corepressor ETO as well as the leukemogenic fusion protein AML1/ETO directly interacts with SHARP, that ETO is part of the endogenous RBP-Jkappa-containing corepressor complex, and that ETO is found at Notch target gene promoters. In functional assays, corepressor ETO, but not AML1/ETO, augments SHARP-mediated repression in an histone deacetylase-dependent manner. Furthermore, either the knockdown of ETO or the overexpression of AML1/ETO activates Notch target genes. Therefore, we propose that AML1/ETO can disturb the normal, repressive function of ETO at Notch target genes. This activating (or derepressing) effect of AML1/ETO may contribute to its oncogenic potential in myeloid leukemia.

Launching the T-cell-lineage developmental programme

Multipotent blood progenitor cells enter the thymus and begin a protracted differentiation process in which they gradually acquire T-cell characteristics while shedding their legacy of developmental plasticity. Notch signalling and basic helix-loop-helix E-protein transcription factors collaborate repeatedly to trigger and sustain this process throughout the period leading up to T-cell lineage commitment. Nevertheless, the process is discontinuous with separately regulated steps that demand roles for additional collaborating factors. This Review discusses new evidence on the coordination of specification and commitment in the early T-cell pathway; effects of microenvironmental signals; the inheritance of stem-cell regulatory factors; and the ensemble of transcription factors that modulate the effects of Notch and E proteins, to distinguish individual stages and to polarize T-cell-lineage fate determination.

The Thymus as an Inductive Site for T Lymphopoiesis

Like all hematopoietic cells, T lymphocytes are derived from bone-marrow-resident stem cells. However, whereas most blood lineages are generated within the marrow, the majority of T cell development occurs in a specialized organ, the thymus. This distinction underscores the unique capacity of the thymic microenvironment to support T lineage restriction and differentiation. Although the identity of many of the contributing thymus-derived signals is well established and rooted in highly conserved pathways involving Notch, morphogenetic, and protein tyrosine kinase signals, the manner in which the ensuing cascades are integrated to orchestrate the underlying processes of T cell development remains under investigation. This review focuses on the current definition of the early stages of T cell lymphopoiesis, with an emphasis on the nature of thymus-derived signals delivered to T cell progenitors that support the commitment and differentiation of T cells toward the alphabeta and gammadelta T cell lineages.

Split ends antagonizes the Notch and potentiates the EGFR signaling pathways during Drosophila eye development

The Notch and Epidermal Growth Factor Receptor (EGFR) signaling pathways interact cooperatively and antagonistically to regulate many aspects of Drosophila development, including the eye. How output from these two signaling networks is fine-tuned to achieve the precise balance needed for specific inductive interactions and patterning events remains an open and important question. Previously, we reported that the gene split ends (spen) functions within or parallel to the EGFR pathway during midline glial cell development in the embryonic central nervous system. Here, we report that the cellular defects caused by loss of spen function in the developing eye imaginal disc place spen as both an antagonist of the Notch pathway and a positive contributor to EGFR signaling during retinal cell differentiation. Specifically, loss of spen results in broadened expression of Scabrous, ectopic activation of Notch signaling, and a corresponding reduction in Atonal expression at the morphogenetic furrow. Consistent with Spen's role in antagonizing Notch signaling, reduction of spen levels is sufficient to suppress Notch-dependent phenotypes. At least in part due to loss of Spen-dependent down-regulation of Notch signaling, loss of spen also dampens EGFR signaling as evidenced by reduced activity of MAP kinase (MAPK). This reduced MAPK activity in turn leads to a failure to limit expression of the EGFR pathway antagonist and the ETS-domain transcriptional repressor Yan and to a corresponding loss of cell fate specification in spen mutant ommatidia. We propose that Spen plays a role in modulating output from the Notch and EGFR pathways to ensure appropriate patterning during eye development.

Regulation of lymphocyte development by Notch signaling

Notch molecules are well conserved from Drosophila melanogaster to mammals and regulate a broad spectrum of various cell lineage commitment processes. Recent studies using inhibitors, transgenic mice and conditional loss-of-function approaches have demonstrated essential roles for Notch signaling in the differentiation of thymocytes and peripheral T cells, as well as B cells. Here we highlight parallels in the developmental regulation of mammalian lymphocytes and the D. melanogaster nervous system through Notch cooperation with the transcriptional regulators RBP-J (Su(H)), MINT (Hairless) and E2A (Ac-Sc-Da).

Generation of a conditional knockout allele for mammalian Spen protein Mint/SHARP

The Spen protein family is found in worms, flies, and mammals, and is implicated in diverse biological processes from embryogenesis to aging. Spen proteins have three N-terminal RNA recognition motifs and a C-terminal SPOC domain. The mammalian Spen proteins Mint and its human ortholog SHARP interact with the Notch-signaling mediator RBP-J as well as Msx2 and several unliganded nuclear hormone receptors, and impart transcription-repressing activity to these molecules by recruiting corepressors through the SPOC domain. Despite these in vitro findings, Mint/SHARP's physiological role is largely unknown, because Mint germline knockouts are embryonic lethal. To analyze Mint/SHARP function in postnatal mice, we created Mint-floxed mice that allow the Cre/loxP-mediated conditional knockout of Mint. We analyzed Mint and RBP-J epistasis during Notch-dependent splenic B-lymphocyte development, and found that Mint suppresses Notch signaling through RBP-J. In addition, Mint deficiency caused severe hypoplasia in postnatal brain, suggesting it may regulate neuronal cell survival.