Hematopoietic stem cells (HSCs) continuously regenerate the hematologic system, yet few genes regulating this process have been defined. To identify candidate factors involved in differentiation and self-renewal, we have generated an expression database of hematopoietic stem cells and their differentiated progeny, including erythrocytes, granulocytes, monocytes, NK cells, activated and naive T cells, and B cells. Bioinformatic analysis revealed HSCs were more transcriptionally active than their progeny and shared a common activation mechanism with T cells. Each cell type also displayed unique biases in the regulation of particular genetic pathways, with Wnt signaling particularly enhanced in HSCs. We identified approximately 100-400 genes uniquely expressed in each cell type, termed lineage "fingerprints." In overexpression studies, two of these genes, Zfp 105 from the NK cell lineage, and Ets2 from the monocyte lineage, were able to significantly influence differentiation toward their respective lineages, demonstrating the utility of the fingerprints for identifying genes that regulate differentiation.

JNK pathway-associated phosphatase (JKAP, also known as DUSP22 or JSP-1) is a JNK activator. The in vivo role of JKAP in immune regulation remains unclear. Here we report that JKAP directly inactivates Lck by dephosphorylating tyrosine-394 residue during T-cell receptor (TCR) signalling. JKAP-knockout T cells display enhanced cell proliferation and cytokine production. JKAP-knockout mice show enhanced T-cell-mediated immune responses and are more susceptible to experimental autoimmune encephalomyelitis (EAE). In addition, the recipient mice that are adoptively transferred with JKAP-knockout T cells show exacerbated EAE symptoms. Aged JKAP-knockout mice spontaneously develop inflammation and autoimmunity. Thus, our results indicate that JKAP is an important phosphatase that inactivates Lck in the TCR signalling turn-off stage, leading to suppression of T-cell-mediated immunity and autoimmunity.

MAPKs are activated by a wide range of diverse stimuli and are essential for various cellular processes, such as stress responses, apoptosis, proliferation, differentiation, and early embryonic development (1-3). The prototypical MAPK signaling cascade is a three-kinase module, consisting of MAP kinase kinase kinase (MAP3K), MAP kinase kinase (MAP2K), and MAPK (1, 2, 4). The upstream molecules that link the MAPK module to extracellular stimuli include small G proteins and a group of mammalian Ste20-like kinases, including hematopoietic progenitor kinase 1 (HPK1), germinal center kinase (GCK), HKP1/germinal center kinase-like kinase, germinal center kinase-like kinase (GLK), and kinase homologous to Ste20 (KHS), which have been characterized as potential MAP kinase kinase kinase kinases (MAP4Ks) for the JNK pathway (1, 2, 4, 5). Within the three-kinase module, MAPKs are phosphorylated on both threonine and tyrosine residues within their signature sequence TXY motif by a dual specificity protein kinase MAP2K. These motifs include TEY in ERK, TPY in JNK, and TGY in p38. MAP2K are activated by phosphorylation of serine/threonine residues by MAP3Ks (1, 2, 4). In the case of ERK1/2, phosphorylation of the TEY motif also contributes to the dimerization and nuclear translocation of ERK1/2 in addition to mediating its activation (6). The extracellular stimuli-induced activation of MAPKs is transient under many conditions, and it has been well established that protein phosphatases play an essential role in the down-regulation of MAP kinases. A variety of classes of protein phosphatases, including tyrosine-specific protein phosphatases, serine/threonine protein phosphatases, and a family of dual specificity protein phosphatases (DSPs), have been implicated in the negative regulation of MAPKs (7-9). Among them, DSPs are the major group of phosphatases that contribute to the regulated inactivation of MAP kinases by dephosphorylating both phosphotyrosine and phosphothreonine residues within the TXY motif, thus also called MAP kinase phosphatases (MKPs) (7-9). Most MKPs identified so far consist of a conserved catalytic region and an extended regulatory region. However, some MKPs lack this regulatory region, such as VH1 (10) and VH1-related (VHR) phosphatase (11). The regulatory

BackgroundErythropoiesis is a highly regulated and well-characterized developmental process responsible for providing the oxygen transport system of the body. However, few of the mechanisms involved in this process have been elucidated. Checkpoint Kinase 1 (Chk1) is best known for its role in the cell cycle and DNA damage pathways, and it has been shown to play a part in several pathways which when disrupted can lead to anemia.Methodology/Principal FindingsHere, we show that haploinsufficiency of Chk1 results in 30% of mice developing anemia within the first year of life. The anemic Chk1+/− mice exhibit distorted spleen and bone marrow architecture, and abnormal erythroid progenitors. Furthermore, Chk1+/− erythroid progenitors exhibit an increase in spontaneous DNA damage foci and improper contractile actin ring formation resulting in aberrant enucleation during erythropoiesis. A decrease in Chk1 RNA has also been observed in patients with refractory anemia with excess blasts, further supporting a role for Chk1 in clinical anemia.Conclusions/SignificanceClinical trials of Chk1 inhibitors are currently underway to treat cancer, and thus it will be important to track the effects of these drugs on red blood cell development over an extended period. Our results support a role for Chk1 in maintaining the balance between erythroid progenitors and enucleated erythroid cells during differentiation. We show disruptions in Chk1 levels can lead to anemia.

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