Aconitase Regulation of Erythropoiesis Correlates with a Novel Licensing Function in Erythropoietin-Induced ERK Signaling

Erythroid development requires the action of erythropoietin (EPO) on committed progenitors to match red cell output to demand. In this process, iron acts as a critical cofactor, with iron deficiency blunting EPO-responsiveness of erythroid progenitors. Aconitase enzymes have recently been identified as possible signal integration elements that couple erythropoiesis with iron availability. In the current study, a regulatory role for aconitase during erythropoiesis was ascertained using a direct inhibitory strategy.In C57BL/6 mice, infusion of an aconitase active-site inhibitor caused a hypoplastic anemia and suppressed responsiveness to hemolytic challenge. In a murine model of polycythemia vera, aconitase inhibition rapidly normalized red cell counts, but did not perturb other lineages. In primary erythroid progenitor cultures, aconitase inhibition impaired proliferation and maturation but had no effect on viability or ATP levels. This inhibition correlated with a blockade in EPO signal transmission specifically via ERK, with preservation of JAK2-STAT5 and Akt activation. Correspondingly, a physical interaction between ERK and mitochondrial aconitase was identified and found to be sensitive to aconitase inhibition.Direct aconitase inhibition interferes with erythropoiesis in vivo and in vitro, confirming a lineage-selective regulatory role involving its enzymatic activity. This inhibition spares metabolic function but impedes EPO-induced ERK signaling and disturbs a newly identified ERK-aconitase physical interaction. We propose a model in which aconitase functions as a licensing factor in ERK-dependent proliferation and differentiation, thereby providing a regulatory input for iron in EPO-dependent erythropoiesis. Directly targeting aconitase may provide an alternative to phlebotomy in the treatment of polycythemia vera

Identification and Characterization of Leukocyte-associated Ig-like Receptor-1 as a Major Anchor Protein of Tyrosine Phosphatase SHP-1 in Hematopoietic Cells

SHP-1, an SH2 domain-containing tyrosine phosphatase, has a crucial role in hematopoiesis. Here we report that SHP-1 is associated with two major tyrosine-phosphorylated proteins in hematopoietic cells treated with the tyrosine phosphatase inhibitor, pervanadate. One of the proteins corresponds to leukocyte-associated Ig-like receptor-1 (LAIR-1), a recently cloned transmembrane protein. Molecular cloning revealed four isoforms of the protein. LAIR-1 is hyper-phosphorylated on tyrosyl residues in cells overexpressing a catalytically inactive mutant form of SHP-1 as well as in pervanadate-treated cells. An antibody against the extracellular domain of the protein also induced its tyrosine phosphorylation. Tyrosine-phosphorylated LAIR-1 specifically interacts with SHP-1 but not with SHP-2, a structurally related tyrosine phosphatase. Using site-specific mutagenesis, we demonstrated that Tyr233 and Tyr263, each embedded in an immunoreceptor tyrosine-based inhibitory motif, are responsible for tyrosine phosphorylation of LAIR-1 and recruitment of SHP-1. Both tyrosyl residues are required for SHP-1 binding. Protein kinases responsible for tyrosine phosphorylation of LAIR-1 may belong to the Src family since PP1, a Src family kinase inhibitor, significantly inhibited its phosphorylation. As a major binding protein of SHP-1 on the plasma membrane, LAIR-1 may play an important role in hematopoietic cell signaling

Molecular Cloning and Characterization of SPAP1, an Inhibitory Receptor

We have cloned a novel cell-surface protein designated SPAP1a for SH2 domain-containing phosphatase anchor protein 1a. SPAP1a belongs to the group of type I transmembrane proteins. Its extracellular domain contains a single immunoglobulin-like domain, and its intracellular segment has two immunoreceptor tyrosine-based inhibition motifs (ITIMs). We also identified two alternatively spliced products that were named SPAP1b and SPAP1c. SPAP1b contains a short intracellular part without ITIMs, while SPAP1c lacks the transmembrane segment and represents a potential soluble protein. Sequence alignment with the genomic database revealed that the SPAP1 gene contains seven exons and is localized at chromosome 1q21. PCR analyses demonstrated that SPAP1a mRNA is specifically expressed in human hematopoietic tissues including spleen, peripheral blood, and bone marrow, and it may be restricted to expression in B cells. Recombinant SPAP1a is tyrosine phosphorylated in cells upon pervanadate stimulation and tyrosine-phosphorylated SPAP1a recruits the SH2 domain containing phosphatase SHP-1, but not SHP-2. As a specific anchor protein of SHP-1, SPAP1a may have an important role in hematopoietic cell signaling

SPAP2, an Ig family receptor containing both ITIMs and ITAMs

This study reports cloning and characterization of SPAP2, a novel transmembrane protein. The extracellular portion of SPAP2 contains six immunoglobulin-like domains and its intracellular segment has two immunoreceptor tyrosine-based activation motifs (ITAMs) and two immunoreceptor tyrosine-based inhibition motifs (ITIMs). We also identified four alternatively spliced products. Sequence alignment with the genomic database revealed that the SPAP2 gene contains 16 exons and is localized at chromosome 1q21. PCR analyses demonstrated that SPAP2 mRNA is expressed in restricted human tissues including the kidney, salivary gland, adrenal gland, uterus, and bone marrow. Tyrosine-phosphorylated SPAP2 is specifically associated with SH2 domain-containing tyrosine kinases Syk and Zap70 and SH2 domain-containing tyrosine phosphatases SHP-1 and SHP-2. Site-specific mutagenesis studies revealed that tyrosyl residues 650 and 662 embedded in the ITIMs are responsible for the binding of Syk and Zap70 while tyrosyl residues 692 and 722 embedded in the ITIMs are involved in interactions with SHP-1 and SHP-2. Finally, recruitment of SHP-1 to the tyrosine-phosphorylated ITIMs led to a marked activation of the enzyme

Generation and characterization of a JAK2V617F-containing erythroleukemia cell line.

The JAK2V617F mutation is found in the majority of patients with myeloproliferative neoplasms (MPNs). Transgenic expression of the mutant gene causes MPN-like phenotypes in mice. We have produced JAK2V617F mice with p53 null background. Some of these mice developed acute erythroleukemia. From one of these mice, we derived a cell line designated J53Z1. J53Z1 cells were stained positive for surface markers CD71 and CD117 but negative for Sca-1, TER-119, CD11b, Gr-1, F4/80, CD11c, CD317, CD4, CD8a, CD3e, B220, CD19, CD41, CD42d, NK-1.1, and FceR1. Real time PCR analyses demonstrated expressions of erythropoietin receptor EpoR, GATA1, and GATA2 in these cells. J53Z1 cells grew rapidly in suspension culture containing fetal bovine serum with a doubling time of ∼18 hours. When transplanted into C57Bl/6 mice, J53Z1 cells induced acute erythroleukemia with massive infiltration of tumor cells in the spleen and liver. J53Z1 cells were responsive to stimulation with erythropoietin and stem cell factor and were selectively inhibited by JAK2 inhibitors which induced apoptosis of the cells. Together, J53Z1 cells belong to the erythroid lineage, and they may be useful for studying the role of JAK2V617F in proliferation and differentiation of erythroid cells and for identifying potential therapeutic drugs targeting JAK2

Structure analysis of the receptor binding of 2019-nCoV

2019-nCoV is a newly identified coronavirus with high similarity to SARS-CoV. We performed a structural analysis of the receptor binding domain (RBD) of spike glycoprotein responsible for entry of coronaviruses into host cells. The RBDs from the two viruses share 72% identity in amino acid sequences, and molecular simulation reveals highly similar ternary structures. However, 2019-nCoV has a distinct loop with flexible glycyl residues replacing rigid prolyl residues in SARS-CoV. Molecular modeling revealed that 2019-nCoV RBD has a stronger interaction with angiotensin converting enzyme 2 (ACE2). A unique phenylalanine F486 in the flexible loop likely plays a major role because its penetration into a deep hydrophobic pocket in ACE2. ACE2 is widely expressed with conserved primary structures throughout the animal kingdom from fish, amphibians, reptiles, birds, to mammals. Structural analysis suggests that ACE2 from these animals can potentially bind RBD of 2019-nCoV, making them all possible natural hosts for the virus. 2019-nCoV is thought to be transmitted through respiratory droplets. However, since ACE2 is predominantly expressed in intestines, testis, and kidney, fecal-oral and other routes of transmission are also possible. Finally, antibodies and small molecular inhibitors that can block the interaction of ACE2 with RBD should be developed to combat the virus