Anti-apoptotic Mcl-1 is essential for the development and sustained growth of acute myeloid leukemia.

Acute myeloid leukemia (AML) frequently relapses after initial treatment. Drug resistance in AML has been attributed to high levels of the anti-apoptotic Bcl-2 family members Bcl-x(L) and Mcl-1. Here we report that removal of Mcl-1, but not loss or pharmacological blockade of Bcl-x(L), Bcl-2, or Bcl-w, caused the death of transformed AML and could cure disease in AML-afflicted mice. Enforced expression of selective inhibitors of prosurvival Bcl-2 family members revealed that Mcl-1 is critical for survival of human AML cells. Thus, targeting of Mcl-1 or regulators of its expression may be a useful strategy for the treatment of AML.

Fli-1 regulates the DN2 to DN3 thymocyte transition and promotes γδ T-cell commitment by enhancing TCR signal strength.

Friend leukemia integration 1 (Fli-1) is a member of the Ets transcription factor family and is expressed during T-cell development; however, the role Fli-1 plays in early T-cell differentiation has not been elucidated. In this report, we demonstrate that in mouse, Fli-1 overexpression retards the CD4(-) CD8(-) double-negative (DN) to CD4(+) CD8(+) double-positive (DP) transition by deregulating normal DN thymocyte development. Specifically, Fli-1 expression moderates the DN2 and DN3 developmental transitions. We further show that Fli-1 overexpression partially mimics strong TCR signals in developing DN thymocytes and thereby enhances γδ T-cell development. Conversely, Fli-1 knockdown by small hairpin RNA reverses the lineage bias from γδ T cells and directs DN cells to the αß lineage by attenuating TCR signaling. Therefore, Fli-1 plays a critical role in both the DN2 to DN3 transition and αß/γδ lineage commitment.

Removal of myeloid cytokines from the cellular environment enhances T-cell development in vitro.

The majority of T-cell development occurs in the thymus. Thymic epithelial cells are specialized cells that express NOTCH ligands and secrete specific cytokines required for normal T-cell lymphopoiesis. It has been demonstrated that OP9 cells derived from macrophage colony-stimulating factor (M-CSF)-deficient mice can support T-cell development when transduced with a NOTCH ligand, Delta-like 1 (Dll1). In this report, we have tested CSF-deficient mouse fibroblasts transduced with Dll1 for their ability to support T-cell differentiation. The data provided here demonstrate that CSF-deficient fibroblasts expressing DLL1 can support T-cell development. Indeed, co-cultures with these fibroblasts produced more T-cell progenitors compared with OP9-DL1 cultures. Addition of myeloid cytokines to OP9-DL1 co-cultures significantly inhibited T-cell development while CSF-deficient DLL1(+) fibroblasts retained partial T-cell differentiation. Taken together, these data imply that their lack of myeloid cytokines allows DLL1(+) fibroblasts to more efficiently generate T-cells. Development of this fibroblast system suggests that there is potential for generating human T-cell precursors via co-culture with human fibroblasts expressing DLL1 or DLL4. These T-cell precursors could be used for treating immunodeficient patients.

ATM substrate Chk2-interacting Zn2+ finger (ASCIZ) Is a bi-functional transcriptional activator and feedback sensor in the regulation of dynein light chain (DYNLL1) expression.

The highly conserved DYNLL1 (LC8) protein was originally discovered as a light chain of the dynein motor complex, but is increasingly emerging as a sequence-specific regulator of protein dimerization with hundreds of targets and wide-ranging cellular functions. Despite its important roles, DYNLL1's own regulation remains poorly understood. Here we identify ASCIZ (ATMIN/ZNF822), an essential Zn(2+) finger protein with dual roles in the DNA base damage response and as a developmental transcription factor, as a conserved regulator of Dynll1 gene expression. DYNLL1 levels are reduced by ∼10-fold in the absence of ASCIZ in human, mouse and chicken cells. ASCIZ binds directly to the Dynll1 promoter and regulates its activity in a Zn(2+) finger-dependent manner. DYNLL1 protein in turn interacts with ten binding sites in the ASCIZ transcription activation domain, and high DYNLL1 levels inhibit the transcriptional activity of ASCIZ. In addition, DYNLL1 was also required for DNA damage-induced ASCIZ focus formation. The dual ability of ASCIZ to activate Dynll1 gene expression and to sense free DYNLL1 protein levels enables a simple dynamic feedback loop to adjust DYNLL1 levels to cellular needs. The ASCIZ-DYNLL1 feedback loop represents a novel mechanism for auto-regulation of gene expression, where the gene product directly inhibits the transcriptional activator while bound at its own promoter.

Identification of Flt3⁺CD150⁻ myeloid progenitors in adult mouse bone marrow that harbor T lymphoid developmental potential.

Common myeloid progenitors (CMPs) were first identified as progenitors that were restricted to myeloid and erythroid lineages. However, it was recently demonstrated that expression of both lymphoid- and myeloid-related genes could be detected in myeloid progenitors. Furthermore, these progenitors were able to give rise to T and B lymphocytes, in addition to myeloid cells. Yet, it was not known whether these progenitors were multipotent at the clonogenic level or there existed heterogeneity within these progenitors with different lineage potential. Here we report that previously defined CMPs possess T-lineage potential, and that this is exclusively found in the Flt3(+)CD150(-) subset of CMPs at the clonal level. In contrast, we did not detect B-lineage potential in CMP subsets. Therefore, these Flt3(+)CD150(-) myeloid progenitors were T/myeloid potent. Yet, Flt3(+)CD150(-) myeloid progenitors are not likely to efficiently traffic to the thymus and contribute to thymopoiesis under normal conditions because of the lack of CCR7 and CCR9 expression. Interestingly, both Flt3(+)CD150(-) and Flt3(-)CD150(-) myeloid progenitors are susceptible to Notch1-mediated T-cell acute lymphoblastic leukemia (T-ALL). Hence, gain-of-function Notch1 mutations occurring in developing myeloid progenitors, in addition to known T-lineage progenitors, could lead to T-ALL oncogenesis.

Erythropoietin couples erythropoiesis, B-lymphopoiesis, and bone homeostasis within the bone marrow microenvironment.

Erythropoietin (Epo) has been used in the treatment of anemia resulting from numerous etiologies, including renal disease and cancer. However, its effects are controversial and the expression pattern of the Epo receptor (Epo-R) is debated. Using in vivo lineage tracing, we document that within the hematopoietic and mesenchymal lineage, expression of Epo-R is essentially restricted to erythroid lineage cells. As expected, adult mice treated with a clinically relevant dose of Epo had expanded erythropoiesis because of amplification of committed erythroid precursors. Surprisingly, we also found that Epo induced a rapid 26% loss of the trabecular bone volume and impaired B-lymphopoiesis within the bone marrow microenvironment. Despite the loss of trabecular bone, hematopoietic stem cell populations were unaffected. Inhibition of the osteoclast activity with bisphosphonate therapy blocked the Epo-induced bone loss. Intriguingly, bisphosphonate treatment also reduced the magnitude of the erythroid response to Epo. These data demonstrate a previously unrecognized in vivo regulatory network coordinating erythropoiesis, B-lymphopoiesis, and skeletal homeostasis. Importantly, these findings may be relevant to the clinical application of Epo.

Asymmetric cell division of T cells upon antigen presentation uses multiple conserved mechanisms.

Asymmetric cell division is a potential means by which cell fate choices during an immune response are orchestrated. Defining the molecular mechanisms that underlie asymmetric division of T cells is paramount for determining the role of this process in the generation of effector and memory T cell subsets. In other cell types, asymmetric cell division is regulated by conserved polarity protein complexes that control the localization of cell fate determinants and spindle orientation during division. We have developed a tractable, in vitro model of naive CD8(+) T cells undergoing initial division while attached to dendritic cells during Ag presentation to investigate whether similar mechanisms might regulate asymmetric division of T cells. Using this system, we show that direct interactions with APCs provide the cue for polarization of T cells. Interestingly, the immunological synapse disseminates before division even though the T cells retain contact with the APC. The cue from the APC is translated into polarization of cell fate determinants via the polarity network of the Par3 and Scribble complexes, and orientation of the mitotic spindle during division is orchestrated by the partner of inscuteable/G protein complex. These findings suggest that T cells have selectively adapted a number of evolutionarily conserved mechanisms to generate diversity through asymmetric cell division.

The earliest step in B lineage differentiation from common lymphoid progenitors is critically dependent upon interleukin 7.

Little is known about the signals that promote early B lineage differentiation from common lymphoid progenitors (CLPs). Using a stromal-free culture system, we show that interleukin (IL)-7 is sufficient to promote the in vitro differentiation of CLPs into B220(+) CD19(+) B lineage progenitors. Consistent with current models of early B cell development, surface expression of B220 was initiated before CD19 and was accompanied by the loss of T lineage potential. To address whether IL-7 receptor (R) activity is essential for early B lineage development in vivo, we examined the frequencies of CLPs and downstream pre-pro- and pro-B cells in adult mice lacking either the alpha chain or the common gamma chain (gamma(c)) of the IL-7R. The data indicate that although gamma(c)(-/-) mice have normal frequencies of CLPs, both gamma(c)(-/-) and IL-7R(alpha)(-/-) mice lack detectable numbers of all downstream early B lineage precursors, including pre-pro-B cells. These findings challenge previous notions regarding the point in B cell development affected by the loss of IL-7R signaling and suggest that IL-7 plays a key and requisite role during the earliest phases of B cell development.

An in vivo functional genetic screen for suppressors of the Rag1-/- T-cell defect.

Functional genetic screens on mutant backgrounds have been successfully used in lower organisms to investigate biological processes. However, few identical screens have been performed in mice. Recombinase activating gene-1 deficient (Rag1-/-) mice have a severe T-cell developmental block owing to lack of rearrangement of their T-cell receptor (TCR) genes. Using a retroviral cDNA library derived from wild-type embryonic thymocytes we performed a suppressor screen in Rag1-/- hematopoietic cells and recovered TCRbeta. This is the first demonstration that targeted genetic screens are feasible using transduced primary cells in vivo. Consequently, this technique can be used to interrogate multiple blood lineages using diverse hematopoietic mouse mutants.

Perturbed thymopoiesis in vitro in the absence of suppressor of cytokine signalling 1 and 3.

Cytokine signals are central to the differentiation of thymocytes and their stepwise progression through defined developmental stages. The intensity and duration of cytokine signals are regulated by the suppressor of cytokine signalling (SOCS) proteins. A clear role for SOCS1 during the later stages of thymopoiesis has been established, but little is known about its role during early thymopoiesis, nor the function of its closest relative, SOCS3. Here, we find that both SOCS1 and SOCS3 are expressed during early thymopoiesis, with expression coincident during the double negative (DN)2 and DN3 stages. We examined thymocyte differentiation in vitro by co-culture of SOCS-deficient bone marrow cells with OP9 cells expressing the Notch ligand Delta-like1 (OP9-DL1). Cells lacking SOCS1 were retarded at the DN3:DN4 transition and appeared unable to differentiate into double positive (DP) thymocytes. Cells lacking both SOCS1 and SOCS3 were more severely affected, and displayed an earlier block in T cell differentiation at DN2, the stage at which expression of SOCS1 and SOCS3 coincides. This indicates that, in addition to their specific roles, SOCS1 and SOCS3 share overlapping roles during thymopoiesis. This is the first demonstration of functional redundancy within the SOCS family, and has uncovered a vital role for SOCS1 and SOCS3 during two important checkpoints in early T cell development.

Overexpression of stem cell associated ALDH1A1, a target of the leukemogenic transcription factor TLX1/HOX11, inhibits lymphopoiesis and promotes myelopoiesis in murine hematopoietic progenitors.

TLX1/HOX11 is an oncogenic transcription factor in human T-cell leukemia, however, the molecular basis for its transforming activity has remained elusive. The ALDH1A1 gene, whose product participates in retinoic acid synthesis, was previously identified as a TLX1-responsive gene. Here, we confirm regulation of ALDH1A1 transcription by TLX1 and show that ALDH1A1 can profoundly perturb murine hematopoiesis by promoting myeloid differentiation at the expense of lymphopoiesis. Together, these data demonstrate that ALDH1A1 plays a key role in normal hematopoiesis, and confirm ALDH1A1 as a TLX1 transcriptional target that may contribute to the ability of this homeoprotein to alter cell fate and induce tumor growth.

Deciphering the role of Notch signaling in lymphopoiesis.

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

Src family kinases and their role in hematological malignancies.

The Src family protein tyrosine kinases (SFKs) are non-receptor intracellular kinases that have important roles in both hematopoiesis and leukemogenesis. The derangement of their expression or activation has been demonstrated to contribute to hematological malignancies. This review first examines the mechanisms of SFK overexpression and hyperactivation, emphasizing the dysregulation of the upstream modulators. Subsequently, the role of SFK up-regulation in the initiation, progression and therapy resistance of many hematological malignancies is also analyzed. The presented evidence endeavors to highlight the influence of SFK up-regulation on an extensive number of hematological malignancies and the need to consider them as candidates in targeted anticancer therapy.

Asymmetric cell division during T cell development controls downstream fate.

During mammalian T cell development, the requirement for expansion of many individual T cell clones, rather than merely expansion of the entire T cell population, suggests a possible role for asymmetric cell division (ACD). We show that ACD of developing T cells controls cell fate through differential inheritance of cell fate determinants Numb and α-Adaptin. ACD occurs specifically during the ß-selection stage of T cell development, and subsequent divisions are predominantly symmetric. ACD is controlled by interaction with stromal cells and chemokine receptor signaling and uses a conserved network of polarity regulators. The disruption of polarity by deletion of the polarity regulator, Scribble, or the altered inheritance of fate determinants impacts subsequent fate decisions to influence the numbers of DN4 cells arising after the ß-selection checkpoint. These findings indicate that ACD enables the thymic microenvironment to orchestrate fate decisions related to differentiation and self-renewal.

The Rothmund-Thomson syndrome helicase RECQL4 is essential for hematopoiesis.

Mutations within the gene encoding the DNA helicase RECQL4 underlie the autosomal recessive cancer-predisposition disorder Rothmund-Thomson syndrome, though it is unclear how these mutations lead to disease. Here, we demonstrated that somatic deletion of Recql4 causes a rapid bone marrow failure in mice that involves cells from across the myeloid, lymphoid, and, most profoundly, erythroid lineages. Apoptosis was markedly elevated in multipotent progenitors lacking RECQL4 compared with WT cells. While the stem cell compartment was relatively spared in RECQL4-deficent mice, HSCs from these animals were not transplantable and even selected against. The requirement for RECQL4 was intrinsic in hematopoietic cells, and loss of RECQL4 in these cells was associated with increased replicative DNA damage and failed cell-cycle progression. Concurrent deletion of p53, which rescues loss of function in animals lacking the related helicase BLM, did not rescue BM phenotypes in RECQL4-deficient animals. In contrast, hematopoietic defects in cells from Recql4Δ/Δ mice were fully rescued by a RECQL4 variant without RecQ helicase activity, demonstrating that RECQL4 maintains hematopoiesis independently of helicase activity. Together, our data indicate that RECQL4 participates in DNA replication rather than genome stability and identify RECQL4 as a regulator of hematopoiesis with a nonredundant role compared with other RecQ helicases.

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.

The Zinc-finger protein ASCIZ regulates B cell development via DYNLL1 and Bim.

Developing B lymphocytes expressing defective or autoreactive pre-B or B cell receptors (BCRs) are eliminated by programmed cell death, but how the balance between death and survival signals is regulated to prevent immunodeficiency and autoimmunity remains incompletely understood. In this study, we show that absence of the essential ATM (ataxia telangiectasia mutated) substrate Chk2-interacting Zn(2+)-finger protein (ASCIZ; also known as ATMIN/ZNF822), a protein with dual functions in the DNA damage response and as a transcription factor, leads to progressive cell loss from the pre-B stage onwards and severely diminished splenic B cell numbers in mice. This lymphopenia cannot be suppressed by deletion of p53 or complementation with a prearranged BCR, indicating that it is not caused by impaired DNA damage responses or defective V(D)J recombination. Instead, ASCIZ-deficient B cell precursors contain highly reduced levels of DYNLL1 (dynein light chain 1; LC8), a recently identified transcriptional target of ASCIZ, and normal B cell development can be restored by ectopic Dynll1 expression. Remarkably, the B cell lymphopenia in the absence of ASCIZ can also be fully suppressed by deletion of the proapoptotic DYNLL1 target Bim. Our findings demonstrate a key role for ASCIZ in regulating the survival of developing B cells by activating DYNLL1 expression, which may then modulate Bim-dependent apoptosis.

Complete diabetes protection despite delayed thymic tolerance in NOD8.3 TCR transgenic mice due to antigen-induced extrathymic deletion of T cells.

Prevention of autoimmunity requires the elimination of self-reactive T cells during their development in the thymus and maturation in the periphery. Transgenic NOD mice that overexpress islet-specific glucose 6 phosphatase catalytic subunit-related protein (IGRP) in antigen-presenting cells (NOD-IGRP mice) have no IGRP-specific T cells. To study the relative contribution of central and peripheral tolerance mechanisms to deletion of antigen-specific T cells, we crossed NOD-IGRP mice to highly diabetogenic IGRP206-214 T-cell receptor transgenic mice (NOD8.3 mice) and studied the frequency and function of IGRP-specific T cells in the thymus and periphery. Peripheral tolerance was extremely efficient and completely protected NOD-IGRP/NOD8.3 mice from diabetes. Peripheral tolerance was characterized by activation of T cells in peripheral lymphoid tissue where IGRP was expressed followed by activation-induced cell death. Thymectomy showed that thymic output of IGRP-specific transgenic T cells compensated for peripheral deletion to maintain peripheral T-cell numbers. Central tolerance was undetectable until 10 weeks and complete by 15 weeks. These in vivo data indicate that peripheral tolerance alone can protect NOD8.3 mice from autoimmune diabetes and that profound changes in T-cell repertoire can follow subtle changes in thymic antigen presentation.

Taking HSCs down a Notch in leukemia.

The Notch signaling pathway is activated in the majority of T cell acute lymphoblastic leukemias (T-ALL). Adding to the complexity of Notch signaling in hematopoiesis, recently in Nature, Klinakis et al. (2011) demonstrate a tumor-suppressor function for the Notch pathway in myeloid malignancy.

Hematopoietic AMPK ß1 reduces mouse adipose tissue macrophage inflammation and insulin resistance in obesity.

Individuals who are obese are frequently insulin resistant, putting them at increased risk of developing type 2 diabetes and its associated adverse health conditions. The accumulation in adipose tissue of macrophages in an inflammatory state is a hallmark of obesity-induced insulin resistance. Here, we reveal a role for AMPK ß1 in protecting macrophages from inflammation under high lipid exposure. Genetic deletion of the AMPK ß1 subunit in mice (referred to herein as ß1(-/-) mice) reduced macrophage AMPK activity, acetyl-CoA carboxylase phosphorylation, and mitochondrial content, resulting in reduced rates of fatty acid oxidation. ß1(-/-) macrophages displayed increased levels of diacylglycerol and markers of inflammation, effects that were reproduced in WT macrophages by inhibiting fatty acid oxidation and, conversely, prevented by pharmacological activation of AMPK ß1-containing complexes. The effect of AMPK ß1 loss in macrophages was tested in vivo by transplantation of bone marrow from WT or ß1(-/-) mice into WT recipients. When challenged with a high-fat diet, mice that received ß1(-/-) bone marrow displayed enhanced adipose tissue macrophage inflammation and liver insulin resistance compared with animals that received WT bone marrow. Thus, activation of AMPK ß1 and increasing fatty acid oxidation in macrophages may represent a new therapeutic approach for the treatment of insulin resistance.