E2A proteins enforce a proliferation checkpoint in developing thymocytes

The thymocyte-specific RNA-binding protein Arpp21 provides TCR repertoire diversity by binding to the 3'-UTR and promoting Rag1 mRNA expression

The regulation of thymocyte development by RNA-binding proteins (RBPs) is largely unexplored. We identify 642 RBPs in the thymus and focus on Arpp21, which shows selective and dynamic expression in early thymocytes. Arpp21 is downregulated in response to T cell receptor (TCR) and Ca2+ signals. Downregulation requires Stim1/Stim2 and CaMK4 expression and involves Arpp21 protein phosphorylation, polyubiquitination and proteasomal degradation. Arpp21 directly binds RNA through its R3H domain, with a preference for uridine-rich motifs, promoting the expression of target mRNAs. Analysis of the Arpp21-bound transcriptome reveals strong interactions with the Rag1 3'-UTR. Arpp21-deficient thymocytes show reduced Rag1 expression, delayed TCR rearrangement and a less diverse TCR repertoire. This phenotype is recapitulated in Rag1 3'-UTR mutant mice harboring a deletion of the Arpp21 response region. These findings show how thymocyte-specific Arpp21 promotes Rag1 expression to enable TCR repertoire diversity until signals from the TCR terminate Arpp21 and Rag1 activities.

The many faces of the zinc finger protein 335 in brain development and immune system

Zinc finger protein 335 (ZNF335) plays a crucial role in the methylation and, consequently, regulates the expression of a specific set of genes. Variants of the ZNF335 gene have been identified as risk factors for microcephaly in a variety of populations worldwide. Meanwhile, ZNF335 has also been identified as an essential regulator of T-cell development. However, an in-depth understanding of the role of ZNF335 in brain development and T cell maturation is still lacking. In this review, we summarize current knowledge of the molecular mechanisms underlying the involvement of ZNF335 in neuronal and T cell development across a wide range of pre-clinical, post-mortem, ex vivo, in vivo, and clinical studies. We also review the current limitations regarding the study of the pathophysiological functions of ZNF335. Finally, we hypothesize a potential role for ZNF335 in brain disorders and discuss the rationale of targeting ZNF335 as a therapeutic strategy for preventing brain disorders.

Multi-modular structure of the gene regulatory network for specification and commitment of murine T cells

T cells develop from multipotent progenitors by a gradual process dependent on intrathymic Notch signaling and coupled with extensive proliferation. The stages leading them to T-cell lineage commitment are well characterized by single-cell and bulk RNA analyses of sorted populations and by direct measurements of precursor-product relationships. This process depends not only on Notch signaling but also on multiple transcription factors, some associated with stemness and multipotency, some with alternative lineages, and others associated with T-cell fate. These factors interact in opposing or semi-independent T cell gene regulatory network (GRN) subcircuits that are increasingly well defined. A newly comprehensive picture of this network has emerged. Importantly, because key factors in the GRN can bind to markedly different genomic sites at one stage than they do at other stages, the genes they significantly regulate are also stage-specific. Global transcriptome analyses of perturbations have revealed an underlying modular structure to the T-cell commitment GRN, separating decisions to lose "stem-ness" from decisions to block alternative fates. Finally, the updated network sheds light on the intimate relationship between the T-cell program, which depends on the thymus, and the innate lymphoid cell (ILC) program, which does not.

Loss of Zfp335 triggers cGAS/STING-dependent apoptosis of post-β selection thymocytes

Production of a functional peripheral T cell compartment typically involves massive expansion of the bone marrow progenitors that seed the thymus. There are two main phases of expansion during T cell development, following T lineage commitment of double-negative (DN) 2 cells and after successful rearrangement and selection for functional TCRβ chains in DN3 thymocytes, which promotes the transition of DN4 cells to the DP stage. The signals driving the expansion of DN2 thymocytes are well studied. However, factors regulating the proliferation and survival of DN4 cells remain poorly understood. Here, we uncover an unexpected link between the transcription factor Zfp335 and control of cGAS/STING-dependent cell death in post-β-selection DN4 thymocytes. Zfp335 controls survival by sustaining expression of Ankle2, which suppresses cGAS/STING-dependent cell death. Together, this study identifies Zfp335 as a key transcription factor regulating the survival of proliferating post-β-selection thymocytes and demonstrates a key role for the cGAS/STING pathway in driving apoptosis of developing T cells.

Transcriptional dynamics and epigenetic regulation of E and ID protein encoding genes during human T cell development

T cells are generated from hematopoietic stem cells through a highly organized developmental process, in which stage-specific molecular events drive maturation towards αβ and γδ T cells. Although many of the mechanisms that control αβ- and γδ-lineage differentiation are shared between human and mouse, important differences have also been observed. Here, we studied the regulatory dynamics of the E and ID protein encoding genes during pediatric human T cell development by evaluating changes in chromatin accessibility, histone modifications and bulk and single cell gene expression. We profiled patterns of ID/E protein activity and identified up- and downstream regulators and targets, respectively. In addition, we compared transcription of E and ID protein encoding genes in human versus mouse to predict both shared and unique activities in these species, and in prenatal versus pediatric human T cell differentiation to identify regulatory changes during development. This analysis showed a putative involvement of TCF3/E2A in the development of γδ T cells. In contrast, in αβ T cell precursors a pivotal pre-TCR-driven population with high ID gene expression and low predicted E protein activity was identified. Finally, in prenatal but not postnatal thymocytes, high HEB/TCF12 levels were found to counteract high ID levels to sustain thymic development. In summary, we uncovered novel insights in the regulation of E and ID proteins on a cross-species and cross-developmental level.

Helix-Loop-Helix Proteins in Adaptive Immune Development

The E/ID protein axis is instrumental for defining the developmental progression and functions of hematopoietic cells. The E proteins are dimeric transcription factors that activate gene expression programs and coordinate changes in chromatin organization. Id proteins are antagonists of E protein activity. Relative levels of E/Id proteins are modulated throughout hematopoietic development to enable the progression of hematopoietic stem cells into multiple adaptive and innate immune lineages including natural killer cells, B cells and T cells. In early progenitors, the E proteins promote commitment to the T and B cell lineages by orchestrating lineage specific programs of gene expression and regulating VDJ recombination of antigen receptor loci. In mature B cells, the E/Id protein axis functions to promote class switch recombination and somatic hypermutation. E protein activity further regulates differentiation into distinct CD4+ and CD8+ T cells subsets and instructs mature T cell immune responses. In this review, we discuss how the E/Id proteins define the adaptive immune system lineages, focusing on their role in directing developmental gene programs.

The perception and response of T cells to a changing environment are based on the law of initial value

αβ T cells are critical components of the adaptive immune system and are capable of inducing sterilizing immunity after pathogen infection and eliminating transformed tumor cells. The development and function of T cells are controlled through the T cell antigen receptor, which recognizes peptides displayed on major histocompatibility complex (MHC) molecules. Here, we review how T cells generate the ability to recognize self-peptide-bound MHC molecules and use signals derived from these interactions to instruct cellular development, activation thresholds, and functional specialization in the steady state and during immune responses. We argue that the basic tenants of T cell development and function follow Weber-Fetcher's law of just noticeable differences and Wilder's law of initial value. Together, these laws argue that the ability of a system to respond and the quality of that response are scalable to the basal state of that system. Manifestation of these laws in T cells generates clone-specific activation thresholds that are based on perceivable differences between homeostasis and pathogen encounter (self versus nonself discrimination), as well as poised states for subsequent differentiation into specific effector cell lineages.

Monoallelic Heb/Tcf12 Deletion Reduces the Requirement for NOTCH1 Hyperactivation in T-Cell Acute Lymphoblastic Leukemia

Early T-cell development is precisely controlled by E proteins, that indistinguishably include HEB/TCF12 and E2A/TCF3 transcription factors, together with NOTCH1 and pre-T cell receptor (TCR) signalling. Importantly, perturbations of early T-cell regulatory networks are implicated in leukemogenesis. NOTCH1 gain of function mutations invariably lead to T-cell acute lymphoblastic leukemia (T-ALL), whereas inhibition of E proteins accelerates leukemogenesis. Thus, NOTCH1, pre-TCR, E2A and HEB functions are intertwined, but how these pathways contribute individually or synergistically to leukemogenesis remain to be documented. To directly address these questions, we leveraged Cd3e-deficient mice in which pre-TCR signaling and progression through β-selection is abrogated to dissect and decouple the roles of pre-TCR, NOTCH1, E2A and HEB in SCL/TAL1-induced T-ALL, via the use of Notch1 gain of function transgenic (Notch1ICtg) and Tcf12+/- or Tcf3+/- heterozygote mice. As a result, we now provide evidence that both HEB and E2A restrain cell proliferation at the β-selection checkpoint while the clonal expansion of SCL-LMO1-induced pre-leukemic stem cells in T-ALL is uniquely dependent on Tcf12 gene dosage. At the molecular level, HEB protein levels are decreased via proteasomal degradation at the leukemic stage, pointing to a reversible loss of function mechanism. Moreover, in SCL-LMO1-induced T-ALL, loss of one Tcf12 allele is sufficient to bypass pre-TCR signaling which is required for Notch1 gain of function mutations and for progression to T-ALL. In contrast, Tcf12 monoallelic deletion does not accelerate Notch1IC-induced T-ALL, indicating that Tcf12 and Notch1 operate in the same pathway. Finally, we identify a tumor suppressor gene set downstream of HEB, exhibiting significantly lower expression levels in pediatric T-ALL compared to B-ALL and brain cancer samples, the three most frequent pediatric cancers. In summary, our results indicate a tumor suppressor function of HEB/TCF12 in T-ALL to mitigate cell proliferation controlled by NOTCH1 in pre-leukemic stem cells and prevent NOTCH1-driven progression to T-ALL.

The Evolution of Rag Gene Enhancers and Transcription Factor E and Id Proteins in the Adaptive Immune System

Adaptive immunity relies on the V(D)J DNA recombination of immunoglobulin (Ig) and T cell receptor (TCR) genes, which enables the recognition of highly diverse antigens and the elicitation of antigen-specific immune responses. This process is mediated by recombination-activating gene (Rag) 1 and Rag2 (Rag1/2), whose expression is strictly controlled in a cell type-specific manner; the expression of Rag1/2 genes represents a hallmark of lymphoid lineage commitment. Although Rag genes are known to be evolutionally conserved among jawed vertebrates, how Rag genes are regulated by lineage-specific transcription factors (TFs) and how their regulatory system evolved among vertebrates have not been fully elucidated. Here, we reviewed the current body of knowledge concerning the cis-regulatory elements (CREs) of Rag genes and the evolution of the basic helix-loop-helix TF E protein regulating Rag gene CREs, as well as the evolution of the antagonist of this protein, the Id protein. This may help to understand how the adaptive immune system develops along with the evolution of responsible TFs and enhancers.

Programming for T-lymphocyte fates: modularity and mechanisms

In this review, Rothenburg discusses the gene regulatory network and chromatin-based kinetic constraints that determine activities of transcription factors in the primary establishment of T-cell identity.

T-cell development in mammals is a model for lineage choice and differentiation from multipotent stem cells. Although T-cell fate choice is promoted by signaling in the thymus through one dominant pathway, the Notch pathway, it entails a complex set of gene regulatory network and chromatin state changes even before the cells begin to express their signature feature, the clonal-specific T-cell receptors (TCRs) for antigen. This review distinguishes three developmental modules for T-cell development, which correspond to cell type specification, TCR expression and selection, and the assignment of cells to different effector types. The first is based on transcriptional regulatory network events, the second is dominated by somatic gene rearrangement and mutation and cell selection, and the third corresponds to establishing a poised state of latent regulator priming through an unknown mechanism. Interestingly, in different lineages, the third module can be deployed at variable times relative to the completion of the first two modules. This review focuses on the gene regulatory network and chromatin-based kinetic constraints that determine activities of transcription factors TCF1, GATA3, PU.1, Bcl11b, Runx1, and E proteins in the primary establishment of T-cell identity.

An integrated transcriptional switch at the β-selection checkpoint determines T cell survival, development and leukaemogenesis

In T cell development, a pivotal decision-making stage, termed β-selection, integrates a TCRβ checkpoint to coordinate survival, proliferation and differentiation to an αβ T cell. Here, we review how transcriptional regulation coordinates fate determination in early T cell development to enable β-selection. Errors in this transcription control can trigger T cell acute lymphoblastic leukaemia. We describe how the β-selection checkpoint goes awry in leukaemic transformation.

Helix-loop-helix proteins and the advent of cellular diversity: 30 years of discovery

In this review from Murre, the evolution of HLH genes, the structures of HLH domains, and the elaborate activities of HLH proteins in multicellular life are discussed.

Helix–loop–helix (HLH) proteins are dimeric transcription factors that control lineage- and developmental-specific gene programs. Genes encoding for HLH proteins arose in unicellular organisms >600 million years ago and then duplicated and diversified from ancestral genes across the metazoan and plant kingdoms to establish multicellularity. Hundreds of HLH proteins have been identified with diverse functions in a wide variety of cell types. HLH proteins orchestrate lineage specification, commitment, self-renewal, proliferation, differentiation, and homing. HLH proteins also regulate circadian clocks, protect against hypoxic stress, promote antigen receptor locus assembly, and program transdifferentiation. HLH proteins deposit or erase epigenetic marks, activate noncoding transcription, and sequester chromatin remodelers across the chromatin landscape to dictate enhancer–promoter communication and somatic recombination. Here the evolution of HLH genes, the structures of HLH domains, and the elaborate activities of HLH proteins in multicellular life are discussed.

Paradoxical role of Id proteins in regulating tumorigenic potential of lymphoid cells

A family of transcription factors known as Id proteins, or inhibitor of DNA binding and differentiation, is capable of regulating cell proliferation, survival and differentiation, and is often upregulated in multiple types of tumors. Due to their ability to promote self-renewal, Id proteins have been considered as oncogenes, and potential therapeutic targets in cancer models. On the contrary, certain Id proteins are reported to act as tumor suppressors in the development of Burkitt's lymphoma in humans, and hepatosplenic and innate-like T cell lymphomas in mice. The contexts and mechanisms by which Id proteins can serve in such contradictory roles to determine tumor outcomes are still not well understood. In this review, we explore the roles of Id proteins in lymphocyte development and tumorigenesis, particularly with respect to inhibition of their canonical DNA binding partners known as E proteins. Transcriptional regulation by E proteins, and their antagonism by Id proteins, act as gatekeepers to ensure appropriate lymphocyte development at key checkpoints. We re-examine the derailment of these regulatory mechanisms in lymphocytes that facilitate tumor development. These mechanistic insights can allow better appreciation of the context-dependent roles of Id proteins in cancers and improve considerations for therapy.

Cytokines, Transcription Factors, and the Initiation of T-Cell Development

Multipotent blood progenitor cells migrate into the thymus and initiate the T-cell differentiation program. T-cell progenitor cells gradually acquire T-cell characteristics while shedding their multipotentiality for alternative fates. This process is supported by extracellular signaling molecules, including Notch ligands and cytokines, provided by the thymic microenvironment. T-cell development is associated with dynamic change of gene regulatory networks of transcription factors, which interact with these environmental signals. Together with Notch or pre-T-cell-receptor (TCR) signaling, cytokines always control proliferation, survival, and differentiation of early T cells, but little is known regarding their cross talk with transcription factors. However, recent results suggest ways that cytokines expressed in distinct intrathymic niches can specifically modulate key transcription factors. This review discusses how stage-specific roles of cytokines and transcription factors can jointly guide development of early T cells.

Phosphorylation Regulates Id2 Degradation and Mediates the Proliferation of Neural Precursor Cells

Inhibitor of DNA binding proteins (Id1-Id4) function to inhibit differentiation and promote proliferation of many different cell types. Among the Id family members, Id2 has been most extensively studied in the central nervous system (CNS). Id2 contributes to cultured neural precursor cell (NPC) proliferation as well as to the proliferation of CNS tumors such as glioblastoma that are likely to arise from NPC-like cells. We identified three phosphorylation sites near the N-terminus of Id2 in NPCs. To interrogate the importance of Id2 phosphorylation, Id2(-/-) NPCs were modified to express wild type (WT) Id2 or an Id2 mutant protein that could not be phosphorylated at the identified sites. We observed that NPCs expressing this mutant lacking phosphorylation near the N-terminus had higher steady-state levels of Id2 when compared to NPCs expressing WT Id2. This elevated level was the result of a longer half-life and reduced proteasome-mediated degradation. Moreover, NPCs expressing constitutively de-phosphorylated Id2 proliferated more rapidly than NPCs expressing WT Id2, a finding consistent with the well-characterized function of Id2 in driving proliferation. Observing that phosphorylation of Id2 modulates the degradation of this important cell-cycle regulator, we sought to identify a phosphatase that would stabilize Id2 enhancing its activity in NPCs and extended our analysis to include human glioblastoma-derived stem cells (GSCs). We found that expression of the phosphatase PP2A altered Id2 levels. Our findings suggest that inhibition of PP2A may be a novel strategy to regulate the proliferation of normal NPCs and malignant GSCs by decreasing Id2 levels. Stem Cells 2016;34:1321-1331.

Transcription factors and target genes of pre-TCR signaling

Almost 30 years ago pioneering work by the laboratories of Harald von Boehmer and Susumo Tonegawa provided the first indications that developing thymocytes could assemble a functional TCRβ chain-containing receptor complex, the pre-TCR, before TCRα expression. The discovery and study of the pre-TCR complex revealed paradigms of signaling pathways in control of cell survival and proliferation, and culminated in the recognition of the multifunctional nature of this receptor. As a receptor integrated in a dynamic developmental process, the pre-TCR must be viewed not only in the light of the biological outcomes it promotes, but also in context with those molecular processes that drive its expression in thymocytes. This review article focuses on transcription factors and target genes activated by the pre-TCR to drive its different outcomes.

ShcA regulates thymocyte proliferation through specific transcription factors and a c-Abl-dependent signaling axis

Signaling via the pre-T-cell receptor (pre-TCR), along with associated signals from Notch and chemokine receptors, regulates the β-selection checkpoint that operates on CD4(-) CD8(-) doubly negative (DN) thymocytes. Since many hematopoietic malignancies arise at the immature developmental stages of lymphocytes, understanding the signal integration and how specific signaling molecules and distal transcription factors regulate cellular outcomes is of importance. Here, a series of molecular and genetic approaches revealed that the ShcA adapter protein critically influences proliferation and differentiation during β-selection. We found that ShcA functions downstream of the pre-TCR and p56(Lck) and show that ShcA is important for extracellular signal-regulated kinase (ERK)-dependent upregulation of transcription factors early growth factor 1 (Egr1) and Egr3 in immature thymocytes and, in turn, of the expression and function of the Id3 and E2A helix-loop-helix (HLH) proteins. ShcA also contributes to pre-TCR-mediated induction of c-Myc and additional cell cycle regulators. Moreover, using an unbiased Saccharomyces cerevisiae (yeast) screen, we identified c-Abl as a binding partner of phosphorylated ShcA and demonstrated the relevance of the ShcA-c-Abl interaction in immature thymocytes. Collectively, these data identify multiple modes by which ShcA can fine-tune the development of early thymocytes, including a previously unappreciated ShcA-c-Abl axis that regulates thymocyte proliferation.

Regulation of CD4 and CD8 coreceptor expression and CD4 versus CD8 lineage decisions

During blood cell development, hematopoietic stem cells generate diverse mature populations via several rounds of binary fate decisions. At each bifurcation, precursors adopt one fate and inactivate the alternative fate either stochastically or in response to extrinsic stimuli and stably maintain the selected fates. Studying of these processes would contribute to better understanding of etiology of immunodeficiency and leukemia, which are caused by abnormal gene regulation during the development of hematopoietic cells. The CD4(+) helper versus CD8(+) cytotoxic T-cell fate decision serves as an excellent model to study binary fate decision processes. These two cell types are derived from common precursors in the thymus. Positive selection of their TCRs by self-peptide presented on either MHC class I or class II triggers their fate decisions along with mutually exclusive retention and silencing of two coreceptors, CD4 and CD8. In the past few decades, extensive effort has been made to understand the T-cell fate decision processes by studying regulation of genes encoding the coreceptors and selection processes. These studies have identified several key transcription factors and gene regulatory networks. In this chapter, I will discuss recent advances in our understanding of the binary cell fate decision processes of T cells.

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.

Lmo2 induces hematopoietic stem cell-like features in T-cell progenitor cells prior to leukemia

LIM domain only 2 (Lmo2) is frequently deregulated in sporadic and gene therapy-induced acute T-cell lymphoblastic leukemia (T-ALL) where its overexpression is an important initiating mutational event. In transgenic and retroviral mouse models, Lmo2 expression can be enforced in multiple hematopoietic lineages but leukemia only arises from T cells. These data suggest that Lmo2 confers clonal growth advantage in T-cell progenitors. We analyzed proliferation, differentiation, and cell death in CD2-Lmo2 transgenic thymic progenitor cells to understand the cellular effects of enforced Lmo2 expression. Most impressively, Lmo2 transgenic T-cell progenitor cells were blocked in differentiation, quiescent, and immortalized in vitro on OP9-DL1 stromal cells. These cellular effects were concordant with a transcriptional signature in Lmo2 transgenic T-cell progenitor cells that is also present in hematopoietic stem cells (HSCs) and early T-cell precursor ALL. These results are significant in light of the crucial role of Lmo2 in the maintenance of the HSC. The cellular effects and transcriptional effects have implications for LMO2-dependent leukemogenesis and the treatment of LMO2-induced T-ALL.

The bHLH factors extramacrochaetae and daughterless control cell cycle in Drosophila imaginal discs through the transcriptional regulation of the Cdc25 phosphatase string

One of the major issues in developmental biology is about having a better understanding of the mechanisms that regulate organ growth. Identifying these mechanisms is essential to understand the development processes that occur both in physiological and pathological conditions, such as cancer. The E protein family of basic helix-loop helix (bHLH) transcription factors, and their inhibitors the Id proteins, regulate cell proliferation in metazoans. This notion is further supported because the activity of these factors is frequently deregulated in cancerous cells. The E protein orthologue Daughterless (Da) and the Id orthologue Extramacrochaetae (Emc) are the only members of these classes of bHLH proteins in Drosophila. Although these factors are involved in controlling proliferation, the mechanism underlying this regulatory activity is poorly understood. Through a genetic analysis, we show that during the development of epithelial cells in the imaginal discs, the G2/M transition, and hence cell proliferation, is controlled by Emc via Da. In eukaryotic cells, the main activator of this transition is the Cdc25 phosphatase, string. Our genetic analyses reveal that the ectopic expression of string in cells with reduced levels of Emc or high levels of Da is sufficient to rescue the proliferative defects seen in these mutant cells. Moreover, we present evidence demonstrating a role of Da as a transcriptional repressor of string. Taken together, these findings define a mechanism through which Emc controls cell proliferation by regulating the activity of Da, which transcriptionally represses string.

Precise control of cell proliferation is critical for normal development and tissue homeostasis. Members of the inhibitor of differentiation (Id) family of helix-loop-helix (HLH) proteins are key regulators that coordinate the balance between cell division and differentiation. These proteins exert this function in part by combining with ubiquitously expressed bHLH transcription factors (E proteins), preventing these transcription factors from forming functional hetero- or homodimeric DNA binding complexes. Deregulation of the activity of Id proteins frequently leads to tumour formation. The Daughterless (Da) and Extramacrochaetae (Emc) proteins are the only members of the E and Id families in Drosophila, yet their role in the control of cell proliferation has not been determined. In this study, we show that the elimination of emc or the ectopic expression of da arrests cells in the G2 phase of the cell cycle. Moreover, we demonstrate that emc controls cell proliferation via Da, which acts as a transcriptional repressor of the Cdc25 phosphatase string. These results provide an important insight into the mechanisms through which Id and E protein interactions control cell cycle progression and therefore how the disruption of the function of Id proteins can induce oncogenic transformation.

The ID proteins: master regulators of cancer stem cells and tumour aggressiveness

Inhibitor of DNA binding (ID) proteins are transcriptional regulators that control the timing of cell fate determination and differentiation in stem and progenitor cells during normal development and adult life. ID genes are frequently deregulated in many types of human neoplasms, and they endow cancer cells with biological features that are hijacked from normal stem cells. The ability of ID proteins to function as central 'hubs' for the coordination of multiple cancer hallmarks has established these transcriptional regulators as therapeutic targets and biomarkers in specific types of human tumours.

E2A predicts prognosis of colorectal cancer patients and regulates cancer cell growth by targeting miR-320a

BACKGROUND AND OBJECTIVE

Transcriptional factor E2A is crucial for the normal development and differentiation of B and T lymphocytes. Dysregulation of E2A leads to leukemia and tumorigenesis of some solid tumors. The expression and clinical significance of E2A as well as its role in colorectal cancer (CRC) are still unknown. This study aims to assess E2A expression in CRC tissues, evaluate its prognosis value, and investigate its role in colon cancer cell growth.

METHODS

E2A expression in CRC tissues and normal mucosa was detected by immunohistochemical staining; Kaplan-Meier survival curve and Cox regression model were used to evaluate the prognostic value of E2A. Lentivirus was used to construct E2A stably knocked-down cells. MTT assay was employed to detect cell proliferation change; cell cycle was analyzed by flow cytometry; and chromatin immunoprecipitation (ChIP) assay was used to validate the predicted binding target of E2A.

RESULTS

Expression of E2A was lower in CRC tissues than normal mucosa; low E2A expression correlated with advanced TNM stage and larger tumor size, and predicted poor prognosis of CRC patients. E2A knockdown resulted in increased cell proliferation rate and cell cycle acceleration. ChIP assay showed miR-320a was a direct target of E2A and upregulation of miR-320a in E2A downregulated cells could reverse cell proliferation and cell cycle changes caused by E2A deficiency.

CONCLUSIONS

E2A is an independent prognostic factor for CRC patients and targets miR-320a to regulate cell proliferation of colon cancer cells.

Identification of TMEM131L as a novel regulator of thymocyte proliferation in humans

In this study, we identify transmembrane protein 131-like (TMEM131L) as a novel regulator of thymocyte proliferation and demonstrate that it corresponds to a not as yet reported inhibitor of Wnt signaling. Short hairpin RNA-mediated silencing of TMEM131L in human CD34(+) hematopoietic progenitors, which were then grafted in NOD-SCID/IL-2rγ(null) mice, resulted in both thymocyte hyperproliferation and multiple pre- and post-β-selection intrathymic developmental defects. Consistent with deregulated Wnt signaling, TMEM131L-deficient thymocytes expressed Wnt target genes at abnormally high levels, and they displayed both constitutive phosphorylation of Wnt coreceptor LRP6 and β-catenin intranuclear accumulation. Using T cell factor reporter assays, we found that membrane-associated TMEM131L inhibited canonical Wnt/β-catenin signaling at the LRP6 coreceptor level. Whereas membrane-associated TMEM131L did not affect LRP6 expression under basal conditions, it triggered lysosome-dependent degradation of its active phosphorylated form following Wnt activation. Genetic mapping showed that phosphorylated LRP6 degradation did not depend on TMEM131L cytoplasmic part but rather on a conserved extracellular domain proximal to the membrane. Collectively, these data indicate that, during thymopoiesis, stage-specific surface translocation of TMEM131L may regulate immature single-positive thymocyte proliferation arrest by acting through mixed Wnt-dependent and -independent mechanisms.

Fli-1 overexpression in hematopoietic progenitors deregulates T cell development and induces pre-T cell lymphoblastic leukaemia/lymphoma

The Ets transcription factor Fli-1 is preferentially expressed in hematopoietic tissues and cells, including immature T cells, but the role of Fli-1 in T cell development has not been closely examined. To address this we retrovirally overexpressed Fli-1 in various in vitro and in vivo settings and analysed its effect on T cell development. We found that Fli-1 overexpression perturbed the DN to DP transition and inhibited CD4 development whilst enhancing CD8 development both in vitro and in vivo. Surprisingly, Fli-1 overexpression in vivo eventuated in development of pre-T cell lymphoblastic leukaemia/lymphoma (pre-T LBL). Known Fli-1 target genes such as the pro-survival Bcl-2 family members were not found to be upregulated. In contrast, we found increased NOTCH1 expression in all Fli-1 T cells and detected Notch1 mutations in all tumours. These data show a novel function for Fli-1 in T cell development and leukaemogenesis and provide a new mouse model of pre-T LBL to identify treatment options that target the Fli-1 and Notch1 signalling pathways.

E2A transcription factors limit expression of Gata3 to facilitate T lymphocyte lineage commitment

The E2A transcription factors promote the development of thymus-seeding cells, but it remains unknown whether these proteins play a role in T lymphocyte lineage specification or commitment. Here, we showed that E2A proteins were required to promote T-lymphocyte commitment from DN2 thymocytes and to extinguish their potential for alternative fates. E2A proteins functioned in DN2 cells to limit expression of Gata3, which encodes an essential T-lymphocyte transcription factor whose ectopic expression can arrest T-cell differentiation. Genetic, or small interfering RNA-mediated, reduction of Gata3 rescued T-cell differentiation in the absence of E2A and restricted the development of alternative lineages by limiting the expanded self-renewal potential in E2A−/− DN2 cells. Our data support a novel paradigm in lymphocyte lineage commitment in which the E2A proteins are necessary to limit the expression of an essential lineage specification and commitment factor to restrain self-renewal and to prevent an arrest in differentiation.

Regulatory mechanisms of thymus and T cell development

The thymus is a central hematopoietic organ which produces mature T lymphocytes with diverse antigen specificity. During development, the thymus primordium is derived from the third pharyngeal endodermal pouch, and then differentiates into cortical and medullary thymic epithelial cells (TECs). TECs represent the primary functional cell type that forms the unique thymic epithelial microenvironment which is essential for intrathymic T-cell development, including positive selection, negative selection and emigration out of the thymus. Our understanding of thymopoiesis has been greatly advanced by using several important animal models. This review will describe progress on the molecular mechanisms involved in thymus and T cell development with particular focus on the signaling and transcription factors involved in this process in mouse and zebrafish.

Transcription factor network regulating CD(+)CD8(+) thymocyte survival

More than 80% of thymocytes are CD4(+)CD8(+) double positive (DP) cells subject to positive/ negative selection. The lifespan of DP thymocytes is critical in shaping the peripheral T-cell repertoire essential for mounting immune responses against foreign, but not self, antigens. During T-cell maturation, if the first round of T-cell receptor (TCR) α chain rearrangement fails to generate a productive T-cell receptor, DP cells start another round of α chain rearrangement until positive selection or cell death intervenes. Thus, the lifespan of DP cells determines how many rounds of α chain rearrangement can be carried out, and influences the likelihood of completing positive selection. The antiapoptotic protein Bcl-x(L) is the ultimate effector regulating DP cell survival, and several transcription factors critical for T-cell development, such as TCF-1, E proteins, c-Myb, and RORγt, regulate DP survival via a Bcl-x(L)-dependent pathway. However, the relationship between these transcription factors in this process is largely unclear. Recent results are revealing an interactive network among these critical factors during regulation of DP thymocyte survival. This review will discuss how these transcription factors potentially work together to control DP thymocyte survival that is critical for successful completion of T-cell development.

Transcriptional control of T-cell development

T lymphocytes, which are central players in orchestrating immune responses, consist of several subtypes with distinct functions. The thymus is an organ where hematopoietic progenitors undergo sequential developmental processes to give rise to this variety of T-cell subsets with diverse antigen specificity. In the periphery, naive T cells further differentiate into effector cells upon encountering antigens. There are several developmental checkpoints during T-cell development, where regulation by a combination of transcription factors imprints specific functional properties on precursors. The transcription factors E2A, GATA-binding protein 3 (Gata3) and RUNT-related transcription factor (Runx) are involved at various stages in the differentiation of double-negative thymocytes and in β-selection, as are transcription factors from the Notch signaling pathway; other transcription factors such as B-cell lymphoma/leukemia 11b (Bcl11b), myeloblastosis viral oncogene homolog (Myb) and inhibitor of DNA binding 3 (Id3) are involved at specific stages. Differentiation of T cells into helper versus cytotoxic cells involves not only antagonistic interplay between Runx and T(h) inducing POZ-Kruppel factor (ThPOK) but also complex interactions between MAZR, Gata3 and Myb in the activation and silencing of genes such as Cd4 and Cd8 as well as the gene that encodes ThPOK itself. A wide range of well-defined transcription factors, including signal transducer and activator of transcriptions (STATs), T-bet, Gata3, nuclear factor of activated T cell (NFAT), adaptor-related protein complex 1 (AP-1) and nuclear factor κB (NF-κB), are known to shape T(h)1/T(h)2 differentiation. Runx and Gata3 also operate in this process, as do c-Maf and recombining binding protein for immunoglobulin Jκ region (RBP-J) and the chromatin-reorganizing protein special AT-rich sequence-binding protein 1 (SATB1). In this review, we briefly discuss how T-cell characteristics are acquired and become divergent from the point of view of transcriptional regulation.

Stage-specific functions of E-proteins at the β-selection and T-cell receptor checkpoints during thymocyte development

The E-protein transcription factors E2A and HEB function in a lineage- and stage-specific manner to orchestrate many critical events throughout lymphocyte development. The function of E-proteins in both B- and T-lymphocyte development has been extensively studied through the use of single-gene knockout animals. Unlike B cells, which rely primarily on E2A alone, T cells are regulated by the combinatorial expression of both E2A and HEB. Therefore, many of the roles of E-proteins during T-cell development may be masked in single-gene knockout studies due to the compensatory function of E2A and HEB. More recently, our laboratory has established double-conditional knockout models to eliminate both E2A and HEB in a stage-specific manner throughout T-cell development. These models, in combination with other complimentary genetic approaches, have identified new E-protein functions at each of the two major T-cell developmental checkpoints. Here, we will discuss how E-proteins function to regulate the expression of T-cell receptor components and cell cycle at the β-selection checkpoint, and how they control positive selection, survival, and lineage-specific gene expression at the subsequent T-cell receptor checkpoint.

Transcription factor AP4 modulates reversible and epigenetic silencing of the Cd4 gene

CD4 coreceptor expression is negatively regulated through activity of the Cd4 silencer in CD4(-)CD8(-) double-negative (DN) thymocytes and CD8(+) cytotoxic lineage T cells. Whereas Cd4 silencing is reversed during transition from DN to CD4(+)CD8(+) double-positive stages, it is maintained through heritable epigenetic processes following its establishment in mature CD8(+) T cells. We previously demonstrated that the Runx family of transcription factors is required for Cd4 silencing both in DN thymocytes and CD8(+) T cells. However, additional factors that cooperate with Runx proteins in the process of Cd4 silencing remain unknown. To identify collaborating factors, we used microarray and RNAi-based approaches and found the basic helix-loop-helix ZIP transcription factor AP4 to have an important role in Cd4 regulation. AP4 interacts with Runx1 in cells in which Cd4 is silenced, and is required for Cd4 silencing in immature DN thymocytes through binding to the proximal enhancer. Furthermore, although AP4-deficient CD8(+) T cells appeared to normally down-regulate CD4 expression, AP4 deficiency significantly increased the frequency of CD4-expressing effector/memory CD8(+) T cells in mice harboring point mutations in the Cd4 silencer. Our results suggest that AP4 contributes to Cd4 silencing both in DN and CD8(+) T cells by enforcing checkpoints for appropriate timing of CD4 expression and its epigenetic silencing.

αβ versus γδ fate choice: counting the T-cell lineages at the branch point

Both αβ and γδ T cells develop in the thymus from a common progenitor. Historically distinguished by their T-cell receptor (TCR), these lineages are now defined on the basis of distinct molecular programs. Intriguingly, in many transgenic and knockout systems these programs are mismatched with the TCR type, leading to the development of γδ lineage cells driven by αβTCR and vice versa. These puzzling observations were recently explained by the demonstration that TCR signal strength, rather than TCR type per se, instructs lineage fate, with stronger TCR signal favoring γδ and weaker signal favoring αβ lineage fates. These studies also highlighted the ERK (extracellular signal regulated kinase)-Egr (early growth response)-Id3 (inhibitor of differentiation 3) axis as a potential molecular switch downstream of TCR that determines lineage choice. Indeed, removal of Id3 was sufficient to redirect TCRγδ transgenic cells to the αβ lineage, even in the presence of strong TCR signal. However, in TCR non-transgenic Id3 knockout mice the overall number of γδ lineage cells was increased due to an outgrowth of a Vγ1Vδ6.3 subset, suggesting that not all γδ T cells depend on this molecular switch for lineage commitment. Thus, the γδ lineage may in fact be a collection of two or more lineages not sharing a common molecular program and thus equipollent to the αβ lineage. TCR signaling is not the only factor that is required for development of αβ and γδ lineage cells; other pathways, such as signaling from Notch and CXCR4 receptors, cooperate with the TCR in this process.

E proteins and the regulation of early lymphocyte development

Lymphopoiesis generates mature B, T, and NK lymphocytes from hematopoietic stem cells via a series of increasingly restricted developmental intermediates. The transcriptional networks that regulate these fate choices are composed of both common and lineage-specific components, which combine to create a cellular context that informs the developmental response to external signals. E proteins are an important factor during lymphopoiesis, and E2A in particular is required for normal T- and B-cell development. Although the other E proteins, HEB and E2-2, are expressed during lymphopoiesis and can compensate for some of E2A's activity, E2A proteins have non-redundant functions during early T-cell development and at multiple checkpoints throughout B lymphopoiesis. More recently, a role for E2A has been demonstrated in the generation of lymphoid-primed multipotent progenitors and shown to favor their specification toward lymphoid over myeloid lineages. This review summarizes both our current understanding of the wide-ranging functions of E proteins during the development of adaptive lymphocytes and the novel functions of E2A in orchestrating a lymphoid-biased cellular context in early multipotent progenitors.

Notch Signaling in T-Cell Development and T-ALL

The Notch signaling pathway is an evolutionarily conserved cell signaling system present in most multicellular organisms, as it controls cell fate specification by regulating cell proliferation, differentiation, apoptosis, and survival. Regulation of the Notch signaling pathway can be achieved at multiple levels. Notch proteins are involved in lineage fate decisions in a variety of tissues in various species. Notch is essential for T lineage cell differentiation including T versus B and αβ versus γδ lineage specification. In this paper, we discuss Notch signaling in normal T-cell maturation and differentiation as well as in T-cell acute lymphoblastic lymphoma/leukemia.

Towards a molecular understanding of the differential signals regulating alphabeta/gammadelta T lineage choice

While insights into the molecular processes that specify adoption of the alphabeta and gammadelta fates are beginning to emerge, the basis for control of specification remains highly controversial. This review highlights the current models attempting to explain T lineage commitment. Recent observations support the hypothesis that the T cell receptor (TCR) provides instructive cues through differences in TCR signaling intensity and/or longevity. Accordingly, we review evidence addressing the importance of differences in signal strength/longevity, how signals differing in intensity/longevity may be generated, and finally how such signals modulate the activity of downstream effectors to promote the opposing developmental fates.

A global network of transcription factors, involving E2A, EBF1 and Foxo1, that orchestrates B cell fate

It is now established that the transcription factors E2A, EBF1 and Foxo1 play critical roles in B cell development. Here we show that E2A and EBF1 bound regulatory elements present in the Foxo1 locus. E2A and EBF1 as well as E2A and Foxo1, in turn, were wired together by a vast spectrum of cis-regulatory codes. These associations were dynamic during developmental progression. Occupancy by the E2A isoform, E47, directly elevated the abundance as well as the pattern of histone H3K4 monomethylation across putative enhancer regions. Finally, the pro-B cell epigenome was divided into clusters of loci that show E2A, EBF and Foxo1 occupancy. From this analysis a global network consisting of transcriptional regulators, signaling and survival factors, was constructed that we propose orchestrates the B cell fate.

Bcl11b heterozygosity promotes clonal expansion and differentiation arrest of thymocytes in gamma-irradiated mice

Bcl11b encodes a zinc-finger transcription factor and functions as a haploinsufficient tumor suppressor gene. Bcl11b(KO/KO) mice exhibit differentiation arrest of thymocytes during beta-selection as has been observed with other mouse models involving knockouts of genes in the Wnt/beta-catenin signaling pathway. Recurrent chromosomal rearrangement at the BCL11B locus occurs in human T-cell leukemias, but it is not clear how such rearrangement would contribute to lymphomagenesis. To address this issue, we studied clonal cell growth, cell number, and differentiation of thymocytes in Bcl11b(KO/+) mice at different time points following gamma-irradiation. Analysis of D-J rearrangement at the T cell receptor beta-chain (TCRbeta) locus and cell surface markers by flow cytometry revealed two distinct populations of clonally growing thymocytes. In one population, thymocytes share a common D-J rearrangement but retain the capacity to differentiate. In contrast, thymocytes in the second population have lost their ability to differentiate. Since the capacity to self renew and differentiate into multiple cell lineages are fundamental properties of adult stem cells, the differentiation competent population of thymocytes that we have isolated could potentially function as cancer stem cells. We also demonstrate increased expression of beta-catenin, a well-known oncogenic protein, in Bcl11b(KO/+) thymocytes. Collectively, the Bcl11b(KO/+) genotype contributes to clonal expansion and differentiation arrest in part through an increase in the level of beta-catenin.

gammadelta and alphabeta T cell lineage choice: resolution by a stronger sense of being

A common bipotent thymocyte precursor gives rise to both lineages of T cells, alphabeta and gammadelta. However, the cell intrinsic and extrinsic factors that influence alphabeta- versus gammadelta-lineage bifurcation remain controversial. gammadelta T cells play a unique and vital role in host defense, from maintaining integrity at epithelial and mucosal barriers to their newly defined role as an important innate source of interleukin-17. Although a T cell receptor (TCR)-independent fate choice may take place, emerging data supports a model in which the differential signaling capacity of alphabeta and gammadeltaTCRs play an instructional role in specifying lineage fate, with strength of signal measured by the amount of ERK/MAPK pathway activation. Here we discuss how the interplay between intrinsic TCR signals and cell extrinsic signals provided by Notch and TCR ligands help to assign and support a final lineage fate decision.

Inhibitor of DNA binding 3 limits development of murine slam-associated adaptor protein-dependent "innate" gammadelta T cells

BACKGROUND

Id3 is a dominant antagonist of E protein transcription factor activity that is induced by signals emanating from the alphabeta and gammadelta T cell receptor (TCR). Mice lacking Id3 were previously shown to have subtle defects in positive and negative selection of TCRalphabeta+ T lymphocytes. More recently, Id3(-/-) mice on a C57BL/6 background were shown to have a dramatic expansion of gammadelta T cells.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report that mice lacking Id3 have reduced thymocyte numbers but increased production of gammadelta T cells that express a Vgamma1.1+Vdelta6.3+ receptor with restricted junctional diversity. These Vgamma1.1+Vdelta6.3+ T cells have multiple characteristics associated with "innate" lymphocytes such as natural killer T (NKT) cells including an activated phenotype, expression of the transcription factor PLZF, and rapid production of IFNg and interleukin-4. Moreover, like other "innate" lymphocyte populations, development of Id3(-/-) Vgamma1.1+Vdelta6.3+ T cells requires the signaling adapter protein SAP.

CONCLUSIONS

Our data provide novel insight into the requirements for development of Vgamma1.1+Vdelta6.3+ T cells and indicate a role for Id3 in repressing the response of "innate" gammadelta T cells to SAP-mediated expansion or survival.

An essential role for the transcription factor HEB in thymocyte survival, Tcra rearrangement and the development of natural killer T cells

E proteins are basic helix-loop-helix transcription factors that regulate many key aspects of lymphocyte development. Thymocytes express multiple E proteins that are thought to provide cooperative and compensatory functions crucial for T cell differentiation. Contrary to that, we report here that the E protein HEB was uniquely required at the CD4(+)CD8(+) double-positive (DP) stage of T cell development. Thymocytes lacking HEB showed impaired survival, failed to make rearrangements of variable-alpha (V(alpha)) segments to distal joining-alpha (J(alpha)) segments in the gene encoding the T cell antigen receptor alpha-chain (Tcra) and had a profound, intrinsic block in the development of invariant natural killer T cells (iNKT cells) at their earliest progenitor stage. Thus, our results show that HEB is a specific and essential factor in T cell development and in the generation of the iNKT cell lineage, defining a unique role for HEB in the regulation of lymphocyte maturation.

Alphabeta versus gammadelta lineage choice at the first TCR-controlled checkpoint

Alphabeta and gammadelta T cells develop in the thymus from a common precursor. Although lineages initially were defined by the type of TCR they express, it soon became clear that the TCR type per se does not play a deterministic role in the lineage decision, since in various transgenic and knockout models, as well as in a small fraction of cells in wt mice, the TCRgammadelta can drive the differentiation of alphabeta lineage cells and the TCRalphabeta can drive differentiation of gammadelta lineage cells. Thus until recently it was unclear what determines lineage choice and at which stage the two lineages diverge. Recent observations suggest that TCR signal strength determines lineage fate and that lineage choice is made at or shortly after the first TCR-controlled checkpoint. While it is clear that the decision between alphabeta and gammadelta lineages is made at the first TCR-controlled checkpoint and the alphabeta sublineages split off later, it is less clear whether gammadelta sublineages divert already at the first TCR-controlled checkpoint or later. Recent experiments support the former view.

Early T cell differentiation lessons from T-cell acute lymphoblastic leukemia

T cells develop from bone marrow-derived self-renewing hematopoietic stem cells (HSC). Upon entering the thymus, these cells undergo progressive commitment and differentiation driven by the thymic stroma and the pre-T cell receptor (pre-TCR). These processes are disrupted in T-cell acute lymphoblastic leukemia (T-ALL). More than 70% of recurring chromosomal rearrangements in T-ALL activate the expression of oncogenic transcription factors, belonging mostly to three families, basic helix-loop-helix (bHLH), homeobox (HOX), and c-MYB. This prevalence is indicative of their importance in the T lineage, and their dominant mechanisms of transformation. For example, bHLH oncoproteins inhibit E2A and HEB, revealing their tumor suppressor function in the thymus. The induction of T-ALL, nonetheless, requires collaboration with constitutive NOTCH1 signaling and the pre-TCR, as well as loss-of-function mutations for CDKN2A and PTEN. Significantly, NOTCH1, the pre-TCR pathway, and E2A/HEB proteins control critical checkpoints and branchpoints in early thymocyte development whereas several oncogenic transcription factors, HOXA9, c-MYB, SCL, and LYL-1 control HSC self-renewal. Together, these genetic lesions alter key regulatory processes in the cell, favoring self-renewal and subvert the normal control of thymocyte homeostasis.

Marked induction of the helix-loop-helix protein Id3 promotes the gammadelta T cell fate and renders their functional maturation Notch independent

alphabeta and gammadelta T cells arise from a common thymocyte progenitor during development in the thymus. Emerging evidence suggests that the pre-T cell receptor (pre-TCR) and gammadelta T cell receptor (gammadeltaTCR) play instructional roles in specifying the alphabeta and gammadelta T-lineage fates, respectively. Nevertheless, the signaling pathways differentially engaged to specify fate and promote the development of these lineages remain poorly understood. Here, we show that differential activation of the extracellular signal-related kinase (ERK)-early growth response gene (Egr)-inhibitor of DNA binding 3 (Id3) pathway plays a defining role in this process. In particular, Id3 expression served to regulate adoption of the gammadelta fate. Moreover, Id3 was both necessary and sufficient to enable gammadelta-lineage cells to differentiate independently of Notch signaling and become competent IFNgamma-producing effectors. Taken together, these findings identify Id3 as a central player that controls both adoption of the gammadelta fate and its maturation in the thymus.

Developmental trajectories in early hematopoiesis

The paths that hematopoietic stem cells take to develop from multipotent, self-renewing cells into committed lymphocytes has been a topic of debate for some time. During early hematopoiesis, multiple branchpoints have been described in which progeny cells segregate into cell lineages with distinct developmental potentials. In this issue of Genes & Development, Inlay and colleagues (pp. 2376-2381) identify novel intermediate stages through which hematopoietic progenitor cells travel.

Transcriptional regulator Id2 controls survival of hepatic NKT cells

Natural killer T cells expressing an invariant T-cell receptor (iNKT) regulate activation of both innate and adaptive immunity in many contexts. iNKT cells accumulate in the liver and rapidly produce prodigious amounts of numerous cytokines upon activation, impacting the immune response to viral infection, immunosurveillance for malignant cells, and liver regeneration. However, little is known about the factors controlling iNKT homeostasis, survival and hepatic localization. Here, we report that the absence of the transcriptional regulator Id2 resulted in a severe, intrinsic defect in the accumulation of hepatic iNKT cells. Id2-deficient iNKT cells showed increased cell death in the liver, although migration and functional activity were not impaired in comparison to Id2-expressing iNKT cells. Id2-deficient iNKT cells exhibited diminished expression of CXCR6, a critical determinant of iNKT cell accumulation in the liver, and of the anti-apoptotic molecules bcl-2 and bcl-X(L), compared to Id2-sufficient iNKT cells. Furthermore, survival and accumulation of iNKT cells lacking Id2 expression was rescued by deficiency in bim, a key pro-apoptotic molecule. Thus, Id2 was necessary to establish a hepatic iNKT cell population, defining a role for Id2 and implicating the Id targets, E protein transcription factors, in the regulation of iNKT cell homeostasis.

Distinct roles for E12 and E47 in B cell specification and the sequential rearrangement of immunoglobulin light chain loci

The E2A gene products, E12 and E47, are critical regulators of B cell development. However, it remains elusive whether E12 and E47 have overlapping and/or distinct functions during B lymphopoiesis. We have generated mice deficient for either E12 or E47 and examined their roles in B cell maturation. We show that E47 is essential for developmental progression at the prepro–B cell stage, whereas E12 is dispensable for early B cell development, commitment, and maintenance. In contrast, both E12 and E47 play critical roles in pre–B and immature B cells to promote immunoglobulin λ (Igλ) germline transcription as well as Igλ VJ gene rearrangement. Furthermore, we show that E12 as well as E47 is required to promote receptor editing upon exposure to self-antigen. We demonstrate that increasing levels of E12 and E47 act to induce Igλ germline transcription, promote trimethylated lysine 4 on histone 3 (H3) as well as H3 acetylation across the Jλ region, and activate Igλ VJ gene rearrangement. We propose that in the pre–B and immature B cell compartments, gradients of E12 and E47 activities are established to mechanistically regulate the sequential rearrangement of the Ig light chain genes.

Pre-TCR-induced beta-catenin facilitates traversal through beta-selection

Pre-TCR induced signals regulate development of the alphabeta TCR lineage cells at the beta-selection checkpoint. We have previously shown that conditional deletion of beta-catenin, a central mediator of Wnt-beta-catenin-T cell factor signaling pathway, impairs traversal through the beta-selection checkpoint. We now provide a molecular basis for the impairment. We demonstrate that pre-TCR signals specifically stabilize beta-catenin in CD4-CD8- double negative thymocytes during beta-selection. Pre-TCR induced Erk activity was required to stabilize beta-catenin. Enforced expression of stabilized beta-catenin was sufficient to mediate aspects of beta-selection including sustained expression of early growth response (Egr) genes. Consistently, deletion of beta-catenin reduced induction of Egr gene expression by the pre-TCR signal and blocked efficient beta-selection. Thus, we demonstrate that pre-TCR induced beta-catenin sustains expression of Egr genes that facilitate traversal through the beta-selection checkpoint.

E2A proteins maintain the hematopoietic stem cell pool and promote the maturation of myelolymphoid and myeloerythroid progenitors

Hematopoiesis is a tightly controlled process maintained by a small pool of hematopoietic stem cells (HSCs). Here, we demonstrate that the LT-HSC, MPP, premegakaryocytic/erythroid, Pre CFU-E, Pre GM, MkP, and granulocyte-macrophage compartments were all significantly reduced in E2A-deficient bone marrow. Despite a severe depletion of erythroid progenitors, the erythrocyte and megakaryocyte compartments were equivalent in E2A-deficient bone marrow as compared with wild-type mice. E2A-deficient HSCs also failed to efficiently maintain the HSC pool on serial transplantation, and we demonstrate that the E2A proteins regulate cell cycle progression of HSCs by regulating the expression of p21(Cip1), p27(Kip1), and the thrombopoietin receptor, known regulators of HSC self-renewal activity. Based on these observations, we propose that the E2A proteins promote the developmental progression of the entire spectrum of early hematopoietic progenitors and to suppress an erythroid specific program of gene expression in alternative cell lineages. Last, the data mechanistically link E2A, cell cycle regulators, and the maintenance of the HSC pool in a common pathway.

E47 controls the developmental integrity and cell cycle quiescence of multipotential hematopoietic progenitors

Little is known about the transcriptional regulators that control the proliferation of multipotent bone marrow progenitors. Understanding the mechanisms that restrict proliferation is of significant interest since the loss of cell cycle integrity can be associated with hematopoietic exhaustion, bone marrow failure, or even oncogenic transformation. Herein, we show that multipotent LSKs (lineage(-)Sca(high)c-kit(+)) from E47-deficient mice exhibit a striking hyperproliferation associated with a loss of cell cycle quiescence and increased susceptibility to in vivo challenge with a mitotoxic drug. Total LSKs contain long-term self-renewing hematopoietic stem cells and downstream multipotential progenitors (MPPs) that possess very limited or no self-renewal ability. Within total LSKs, we found specific developmental and functional deficits in the MPP subset. E47 knockout mice have grossly normal numbers of self-renewing hematopoietic stem cells but a 50-70% reduction in nonrenewing MPPs and downstream lineage-restricted populations. The residual MPPs in E47 knockout mice fail to fully up-regulate flk2 or initiate V(D)J recombination, hallmarks of normal lymphoid lineage progression. Consistent with the loss of normal cell cycle restraints, we show that E47-deficient LSKs have a 50% decrease in p21, a cell cycle inhibitor and known regulator of LSK proliferation. Moreover, enforced expression studies identify p21 as an E47 target gene in primary bone marrow LSKs. Thus, E47 appears to regulate the developmental and functional integrity of early hematopoietic subsets in part through effects on p21-mediated cell cycle quiescence.

Ebf1-mediated down-regulation of Id2 and Id3 is essential for specification of the B cell lineage

Gene knockout experiments in mice have suggested a hierarchical model of early B cell commitment wherein E2A proteins (E47 and E12) activate early B cell factor (Ebf1), which in turn activates expression of the B cell commitment factor, Pax5. In IL-7 receptor alpha (IL-7Ralpha) knockout mice, B cell development is blocked before B-lineage commitment at the prepro-B cell stage in adult animals. In IL-7Ralpha(-/-) prepro-B cells, E47 is expressed and yet is insufficient to transcriptionally activate the putative downstream target gene, Ebf1. In this study, we show that further increases of E47 expression in IL-7Ralpha(-/-) prepro-B cells fails to activate Ebf1, but rather leads to a dramatic induction of the E2A inhibitory factors, Id2 and Id3. In contrast, enforced expression of Ebf1 in IL-7Ralpha(-/-) bone marrow potently down-regulates Id2 and Id3 mRNA expression and restores B cell differentiation in vivo. Down-regulation of both Id2 and Id3 during B cell specification is essential in that overexpression of either Id2 or Id3 in wild-type bone marrow blocks B cell specification at the prepro-B cell stage. Collectively, these studies suggest a model where Ebf1 induction specifies the B cell fate by dramatically increasing activity of E47 at the posttranslational level.