Haploinsufficiency of the essential gene Rps12 causes defects in erythropoiesis and hematopoietic stem cell maintenance.

Ribosomal protein (Rp) gene haploinsufficiency can result in Diamond-Blackfan Anemia (DBA), characterized by defective erythropoiesis and skeletal defects. Some mouse Rp mutations recapitulate DBA phenotypes, although others lack erythropoietic or skeletal defects. We generated a conditional knockout mouse to partially delete Rps12. Homozygous Rps12 deletion resulted in embryonic lethality. Mice inheriting the Rps12KO/+ genotype had growth and morphological defects, pancytopenia, and impaired erythropoiesis. A striking reduction in hematopoietic stem cells (HSCs) and progenitors in the bone marrow (BM) was associated with decreased ability to repopulate the blood system after competitive and non-competitive BM transplantation. Rps12KO/+ lost HSC quiescence, experienced ERK and MTOR activation, and increased global translation in HSC and progenitors. Post-natal heterozygous deletion of Rps12 in hematopoietic cells using Tal1-Cre-ERT also resulted in pancytopenia with decreased HSC numbers. However, post-natal Cre-ERT induction led to reduced translation in HSCs and progenitors, suggesting that this is the most direct consequence of Rps12 haploinsufficiency in hematopoietic cells. Thus, RpS12 has a strong requirement in HSC function, in addition to erythropoiesis.

Unraveling the Link between Class 1A PI3-Kinase, Autophagy, and Myelodysplasia.

Myelodysplastic syndrome (MDS) is a clonal malignancy that develops from hematopoietic stem cells (HSCs), but the underlying mechanisms of MDS initiation are not well understood. The phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) pathway is often dysregulated in MDS. To investigate how PI3K inactivation affects HSC function, we generated a mouse model in which three Class IA PI3K genes were deleted in hematopoietic cells. Surprisingly, PI3K deficiency caused cytopenias, reduced survival, and multilineage dysplasia with chromosomal abnormalities, consistent with MDS initiation. PI3K-deficient HSCs had impaired autophagy, and pharmacologic treatment with autophagy-inducing reagents improved HSC differentiation. Furthermore, a similar autophagic degradation defect was observed in MDS patient HSCs. Therefore, our study uncovered a crucial protective role for Class IA PI3K in maintaining autophagic flux in HSCs to preserve the balance between self-renewal and differentiation.

PI3-kinase deletion promotes myelodysplasia by dysregulating autophagy in hematopoietic stem cells.

Myelodysplastic syndrome (MDS) is a clonal malignancy arising in hematopoietic stem cells (HSCs). The mechanisms of MDS initiation in HSCs are still poorly understood. The phosphatidylinositol 3-kinase (PI3K)/AKT pathway is frequently activated in acute myeloid leukemia, but in MDS, PI3K/AKT is often down-regulated. To determine whether PI3K down-regulation can perturb HSC function, we generated a triple knockout (TKO) mouse model with Pik3ca, Pik3cb, and Pik3cd deletion in hematopoietic cells. Unexpectedly, PI3K deficiency caused cytopenias, decreased survival, and multilineage dysplasia with chromosomal abnormalities, consistent with MDS initiation. TKO HSCs exhibit impaired autophagy, and pharmacologic autophagy induction improved HSC differentiation. Using intracellular LC3 and P62 flow cytometry and transmission electron microscopy, we also observed abnormal autophagic degradation in patient MDS HSCs. Therefore, we have uncovered an important protective role for PI3K in maintaining autophagic flux in HSCs to preserve the balance between self-renewal and differentiation and to prevent MDS initiation.

Crizotinib Has Preclinical Efficacy in Philadelphia-Negative Myeloproliferative Neoplasms.

PURPOSE: The Philadelphia chromosome-negative myeloproliferative neoplasms (MPN) polycythemia vera, essential thrombocythemia, and primary myelofibrosis are characterized by JAK/STAT pathway activation. JAK inhibitors are approved for MPN treatment, but persistence has been observed, due to JAK/STAT reactivation. EXPERIMENTAL DESIGN: Using MPN patient samples, JAK2-mutated cell lines, and MPN mouse models, we examined both the efficacy and mechanism by which crizotinib, the ALK/MET/RON/ROS1 inhibitor approved for the treatment of non-small cell lung cancer, alters MPN cell proliferation and JAK/STAT activation. RESULTS: We found that crizotinib suppresses proliferation and activation of JAK/STAT signaling, and decreases the disease burden in the JAK2V617F mouse model of MPN. Furthermore, we found that crizotinib could overcome JAK inhibitor persistence to ruxolitinib. Interestingly, phosphorylation of the crizotinib target RON kinase was enhanced in ruxolitinib-persistent cells. We show that phospho-JAK2 and phospho-RON can physically interact to sustain JAK/STAT signaling, and that the combination of crizotinib and ruxolitinib disrupts this interaction. Furthermore, RON knockdown suppresses proliferation and activation of JAK/STAT signaling in JAK2-mutated cells, and RON deletion in a JAK2V617F mouse MPN model decreases the disease burden. We also observed RON hyperactivation in MPN patient cells, suggesting that RON may be an important target of crizotinib in MPN. CONCLUSIONS: In summary, we demonstrate that crizotinib has preclinical efficacy in MPN patient cells, JAK2-mutated cell lines, and a JAK2-mutated mouse model, and that the combination of crizotinib with JAK inhibitors suppresses JAK inhibitor persistence. Our work suggests that crizotinib should be investigated for the treatment of patients with MPN.

Signaling Pathways in Leukemic Stem Cells.

Hematopoietic stem cells (HSCs) and leukemic stem cells (LSCs) utilize many of the same signaling pathways for their maintenance and survival. In this review, we will focus on several signaling pathways whose roles have been extensively studied in both HSCs and LSCs. Our main focus will be on the PI3K/AKT/mTOR pathway and several of its regulators and downstream effectors. We will also discuss several other signaling pathways of particular importance in LSCs, including the WNT/ß-catenin pathway, the NOTCH pathway, and the TGFß pathway. For each of these pathways, we will emphasize differences in how these pathways operate in LSCs, compared to their function in HSCs, to highlight opportunities for the specific therapeutic targeting of LSCs. We will also highlight areas of crosstalk between multiple signaling pathways that may affect LSC function.

PI3 kinase alpha and delta promote hematopoietic stem cell activation.

Many cytokines and chemokines that are important for hematopoiesis activate the PI3K signaling pathway. Because this pathway is frequently mutated and activated in cancer, PI3K inhibitors have been developed for the treatment of several malignancies, and are now being tested in the clinic in combination with chemotherapy. However, the role of PI3K in adult hematopoietic stem cells (HSCs), particularly during hematopoietic stress, is still unclear. We previously showed that the individual PI3K catalytic isoforms P110α or P110ß have dispensable roles in HSC function, suggesting redundancy between PI3K isoforms in HSCs. We now demonstrate that simultaneous deletion of P110α and P110δ in double knockout (DKO) HSCs uncovers their redundant requirement in HSC cycling after 5-fluorouracil (5-FU) chemotherapy administration. In contrast, DKO HSCs are still able to exit quiescence in response to other stress stimuli, such as LPS. We found that DKO HSCs and progenitors have impaired sensing of inflammatory signals ex vivo, and that levels of IL1-ß and MIG are higher in the bone marrow after LPS than after 5-FU administration. Furthermore, exogenous in vivo administration of IL1-ß can induce cell cycle entry of DKO HSCs. Our findings have important clinical implications for the use of PI3K inhibitors in combination with chemotherapy.

A Non-Cell-Autonomous Role of BEC-1/BECN1/Beclin1 in Coordinating Cell-Cycle Progression and Stem Cell Proliferation during Germline Development.

The decision of stem cells to proliferate and differentiate is finely controlled. The Caenorhabditis elegans germline provides a tractable system for studying the mechanisms that control stem cell proliferation and homeostasis [1-4]. Autophagy is a conserved cellular recycling process crucial for cellular homeostasis in many different contexts [5], but its function in germline stem cell proliferation remains poorly understood. Here, we describe a function for autophagy in germline stem cell proliferation. We found that autophagy genes such as bec-1/BECN1/Beclin1, atg-16.2/ATG16L, atg-18/WIPI1/2, and atg-7/ATG7 are required for the late larval expansion of germline stem cell progenitors in the C. elegans gonad. We further show that BEC-1/BECN1/Beclin1 acts independently of the GLP-1/Notch or DAF-7/TGF-ß pathways but together with the DAF-2/insulin IGF-1 receptor (IIR) signaling pathway to promote germline stem cell proliferation. Similar to DAF-2/IIR, BEC-1/BECN1/Beclin1, ATG-18/WIPI1/2, and ATG-16.2/ATG16L all promote cell-cycle progression and are negatively regulated by the phosphatase and tensin homolog DAF-18/PTEN. However, whereas BEC-1/BECN1/Beclin1 acts through the transcriptional regulator SKN-1/Nrf1, ATG-18/WIPI1/2 and ATG-16.2/ATG16L exert their function through the DAF-16/FOXO transcription factor. In contrast, ATG-7 functions in concert with the DAF-7/TGF-ß pathway to promote germline proliferation and is not required for cell-cycle progression. Finally, we report that BEC-1/BECN1/Beclin1 functions non-cell-autonomously to facilitate cell-cycle progression and stem cell proliferation. Our findings demonstrate a novel non-autonomous role for BEC-1/BECN1/Beclin1 in the control of stem cell proliferation and cell-cycle progression, which may have implications for the understanding and development of therapies against malignant cell growth in the future.

Autophagy genes are required for normal lipid levels in C. elegans.

Autophagy is a cellular catabolic process in which various cytosolic components are degraded. For example, autophagy can mediate lipolysis of neutral lipid droplets. In contrast, we here report that autophagy is required to facilitate normal levels of neutral lipids in C. elegans. Specifically, by using multiple methods to detect lipid droplets including CARS microscopy, we observed that mutants in the gene bec- 1 (VPS30/ATG6/BECN1), a key regulator of autophagy, failed to store substantial neutral lipids in their intestines during development. Moreover, loss of bec-1 resulted in a decline in lipid levels in daf-2 [insulin/IGF-1 receptor (IIR) ortholog] mutants and in germline-less glp-1/Notch animals, both previously recognized to accumulate neutral lipids and have increased autophagy levels. Similarly, inhibition of additional autophagy genes, including unc-51/ULK1/ATG1 and lgg-1/ATG8/MAP1LC3A/LC3 during development, led to a reduction in lipid content. Importantly, the decrease in fat accumulation observed in animals with reduced autophagy did not appear to be due to a change in food uptake or defecation. Taken together, these observations suggest a broader role for autophagy in lipid remodeling in C. elegans.

Modulation of TCRß surface expression during TCR revision.

TCR revision is a tolerance mechanism by which self-reactive TCRs expressed by mature CD4(+) peripheral T cells are replaced by receptors encoded by genes generated by post-thymic DNA rearrangement. The downmodulation of surface TCR expression initiates TCR revision, and serves as a likely trigger for the induction of the recombinase machinery. We show here in a Vß5 transgenic mouse model system that downregulation of the self-reactive transgene-encoded TCR is not maintained by transgene loss or diminished transcription or translation. The downregulation of surface TCR expression likely occurs in two stages, only one of which requires tolerogen expression.

Cutting Edge: Rag deletion in peripheral T cells blocks TCR revision.

Mature CD4(+)Vbeta5(+) T cells that recognize a peripherally expressed endogenous superantigen are tolerized either by deletion or TCR revision. In Vbeta5 transgenic mice, this latter tolerance pathway results in the appearance of CD4(+)Vbeta5(-)TCRbeta(+) T cells, coinciding with Rag1, Rag2, and TdT expression and the accumulation of V(beta)-DJ(beta) recombination intermediates in peripheral CD4(+) T cells. Because postthymic RAG-dependent TCR rearrangement has remained controversial, we sought to definitively determine whether TCR revision is an extrathymic process that occurs in mature peripheral T cells. We show in this study that Rag deletion in post-positive selection T cells in Vbeta5 transgenic mice blocks TCR revision in vivo and that mature peripheral T cells sorted to remove cells bearing endogenous TCRbeta-chains can express newly generated TCRbeta molecules in adoptive hosts. These findings unambiguously demonstrate postthymic, RAG-dependent TCR rearrangement and define TCR revision as a tolerance pathway that targets mature peripheral CD4(+) T cells.

Cutting edge: two distinct motifs within the Fas ligand tail regulate Fas ligand-mediated costimulation.

The cytoplasmic domain of Fas ligand is sufficient to costimulate CD8(+) T cells by driving Fas ligand recruitment into lipid rafts and association with select Src homology 3-containing proteins, activating PI3K and MAPK pathways, mediating nuclear translocation of the transcription factors NFAT and AP-1, and enhancing IFN-gamma production and Ag-specific CD8(+) T cell proliferation. We now show that Fas ligand molecules lacking amino acids 45-54 in the proline-rich region of the cytoplasmic domain fail to costimulate but serve as effective death inducers. Death induction and costimulation by Fas ligand are therefore clearly separable functions. Further, upon Fas ligand-mediated costimulation, casein kinase I phosphorylates Fas ligand, in which two conserved casein kinase I binding sites regulate NFAT activation and costimulation. These results help resolve how one molecule can serve as a double-edged immunomodulator by directing discrete biological consequences.

The cytoplasmic domain of Fas ligand costimulates TCR signals.

Productive T cell activation generally requires costimulation in addition to a signal delivered through the TCR. Although FasL is well-characterized for its capacity to deliver a death signal through Fas, this TNF family member can also transmit a reverse signal to enhance Ag-driven T cell proliferation. In this study, we define this reverse signal through FasL as costimulation by showing it requires TCR coengagement and is CD28 independent. We demonstrate that FasL-mediated costimulation drives FasL recruitment into lipid rafts and association with select Src homology 3 (SH3)-containing proteins. We further show that the proline-rich intracellular domain of FasL is sufficient to costimulate by enhancing the phosphorylation of Akt, ERK1/2, JNK, and FasL itself, by activating the transcription factors NFAT and AP-1, and by enhancing IFN-gamma production. These results elucidate the pathway of costimulation through the death inducer FasL, and comprise the first mechanistic analysis of a newly emerging group of costimulators, the TNF family.