Liver kinase B1 (LKB1) in murine erythroid progenitors modulates erythropoietin setpoint in association with maturation control

Erythropoiesis is a tightly regulated process. It is stimulated by decreased oxygen in circulation, which leads to the secretion of the hormone erythropoietin (Epo) by the kidneys. An additional layer of control involves the coordinated sensing and use of nutrients. Much cellular machinery contributes to sensing and responding to nutrient status in cells, and one key participant is the kinase LKB1. The current study examines the role of LKB1 in erythropoiesis using a murine in vivo and ex vivo conditional knockout system. In vivo analysis showed erythroid loss of LKB1 to be associated with a robust increase in serum Epo and mild reticulocytosis. Despite these abnormalities, no evidence of anemia or hemolysis was found. Further characterization using an ex vivo progenitor culture assay demonstrated accelerated erythroid maturation in the LKB1-deficient cells. Based on pharmacologic evidence, this phenotype appeared to result from impaired AMP-activated protein kinase (AMPK) signaling downstream of LKB1. These findings reveal a role for LKB1 in fine-tuning Epo-driven erythropoiesis in association with maturational control.

Relieving Dyrk1a repression of MKL1 confers an adult-like phenotype to human infantile megakaryocytes

Infantile (fetal and neonatal) megakaryocytes (Mks) have a distinct phenotype consisting of hyperproliferation, limited morphogenesis, and low platelet production capacity. These properties contribute to clinical problems that include thrombocytopenia in neonates, delayed platelet engraftment in recipients of cord blood stem cell transplants, and inefficient ex vivo platelet production from pluripotent stem cell–derived Mks. The infantile phenotype results from deficiency of the actin-regulated coactivator, MKL1, which programs cytoskeletal changes driving morphogenesis. As a strategy to complement this molecular defect, we screened pathways with the potential to affect MKL1 function and found that DYRK1A inhibition dramatically enhanced Mk morphogenesis in vitro and in vivo. Dyrk1 inhibitors rescued enlargement, polyploidization, and thrombopoiesis in human neonatal Mks. Mks derived from induced pluripotent stem cells responded in a similar manner. Progenitors undergoing Dyrk1 inhibition demonstrated filamentous actin assembly, MKL1 nuclear translocation, and modulation of MKL1 target genes. Loss-of-function studies confirmed MKL1 involvement in this morphogenetic pathway. Expression of Ablim2, a stabilizer of filamentous actin, increased with Dyrk1 inhibition, and Ablim2 knockdown abrogated the actin, MKL1, and morphogenetic responses to Dyrk1 inhibition. These results delineate a pharmacologically tractable morphogenetic pathway whose manipulation may alleviate clinical problems associated with the limited thrombopoietic capacity of infantile Mks.

RUNX3 levels in human hematopoietic progenitors are regulated by aging and dictate erythroid-myeloid balance

Healthy bone marrow progenitors yield a co-ordinated balance of hematopoietic lineages. This balance shifts with aging toward enhanced granulopoiesis with diminished erythropoiesis and lymphopoiesis, changes which likely contribute to the development of bone marrow disorders in the elderly. In this study, RUNX3 was identified as a hematopoietic stem and progenitor cell factor whose levels decline with aging in humans and mice. This decline is exaggerated in hematopoietic stem and progenitor cells from subjects diagnosed with unexplained anemia of the elderly. Hematopoietic stem cells from elderly unexplained anemia patients had diminished erythroid but unaffected granulocytic colony forming potential. Knockdown studies revealed human hematopoietic stem and progenitor cells to be strongly influenced by RUNX3 levels, with modest deficiencies abrogating erythroid differentiation at multiple steps while retaining capacity for granulopoiesis. Transcriptome profiling indicated control by RUNX3 of key erythroid transcription factors, including KLF1 and GATA1 These findings thus implicate RUNX3 as a participant in hematopoietic stem and progenitor cell aging, and a key determinant of erythroid-myeloid lineage balance.

Neonatal expression of RNA-binding protein IGF2BP3 regulates the human fetal-adult megakaryocyte transition

Hematopoietic transitions that accompany fetal development, such as erythroid globin chain switching, play important roles in normal physiology and disease development. In the megakaryocyte lineage, human fetal progenitors do not execute the adult morphogenesis program of enlargement, polyploidization, and proplatelet formation. Although these defects decline with gestational stage, they remain sufficiently severe at birth to predispose newborns to thrombocytopenia. These defects may also contribute to inferior platelet recovery after cord blood stem cell transplantation and may underlie inefficient platelet production by megakaryocytes derived from pluripotent stem cells. In this study, comparison of neonatal versus adult human progenitors has identified a blockade in the specialized positive transcription elongation factor b (P-TEFb) activation mechanism that is known to drive adult megakaryocyte morphogenesis. This blockade resulted from neonatal-specific expression of an oncofetal RNA-binding protein, IGF2BP3, which prevented the destabilization of the nuclear RNA 7SK, a process normally associated with adult megakaryocytic P-TEFb activation. Knockdown of IGF2BP3 sufficed to confer both phenotypic and molecular features of adult-type cells on neonatal megakaryocytes. Pharmacologic inhibition of IGF2BP3 expression via bromodomain and extraterminal domain (BET) inhibition also elicited adult features in neonatal megakaryocytes. These results identify IGF2BP3 as a human ontogenic master switch that restricts megakaryocyte development by modulating a lineage-specific P-TEFb activation mechanism, revealing potential strategies toward enhancing platelet production.

Calpain 2 activation of P-TEFb drives megakaryocyte morphogenesis and is disrupted by leukemogenic GATA1 mutation

Megakaryocyte morphogenesis employs a "hypertrophy-like" developmental program that is dependent on P-TEFb kinase activation and cytoskeletal remodeling. P-TEFb activation classically occurs by a feedback-regulated process of signal-induced, reversible release of active Cdk9-cyclin T modules from large, inactive 7SK small nuclear ribonucleoprotein particle (snRNP) complexes. Here, we have identified an alternative pathway of irreversible P-TEFb activation in megakaryopoiesis that is mediated by dissolution of the 7SK snRNP complex. In this pathway, calpain 2 cleavage of the core 7SK snRNP component MePCE promoted P-TEFb release and consequent upregulation of a cohort of cytoskeleton remodeling factors, including α-actinin-1. In a subset of human megakaryocytic leukemias, the transcription factor GATA1 undergoes truncating mutation (GATA1s). Here, we linked the GATA1s mutation to defects in megakaryocytic upregulation of calpain 2 and of P-TEFb-dependent cytoskeletal remodeling factors. Restoring calpain 2 expression in GATA1s mutant megakaryocytes rescued normal development, implicating this morphogenetic pathway as a target in human leukemogenesis.

Cyclic AMP signaling inhibits megakaryocytic differentiation by targeting transcription factor 3 (E2A) cyclin-dependent kinase inhibitor 1A (CDKN1A) transcriptional axis

Signaling via the intracellular second messenger cyclic AMP (cAMP) has long been implicated in the repression of megakaryocytic differentiation. However, the mechanisms by which cAMP signaling impairs megakaryopoiesis have never been elucidated. In a human CD34(+) cell culture model, we show that the adenylyl cyclase agonist forskolin inhibits megakaryocytic differentiation in a protein kinase A-dependent manner. Using this system to screen for downstream effectors, we identified the transcription factor E2A as a key target in a novel repressive signaling pathway. Specifically, forskolin acting through protein kinase A-induced E2A down-regulation and enforced expression of E2A overrode the inhibitory effects of forskolin on megakaryopoiesis. The dependence of megakaryopoiesis on critical thresholds of E2A expression was confirmed in vivo in haploinsufficient mice and ex vivo using shRNA knockdown in human progenitors. Using a variety of approaches, we further identified p21 (encoded by CDKN1A) as a functionally important megakaryopoietic regulator residing downstream of E2A. These results thus implicate the E2A-CDKN1A transcriptional axis in the control of megakaryopoiesis and reveal the lineage-selective inhibition of this axis as a likely mechanistic basis for the inhibitory effects of cAMP signaling.

Regulation of RUNX1 transcriptional function by GATA-1

Runt-related transcription factor 1 (RUNX1) and GATA-1 are both transcription factors known to play essential roles in hematopoiesis. Genetic alterations of each are associated with abnormal platelet development, as well as predisposition to leukemia. In addition, in vitro and animal studies indicate that both factors are involved in megakaryopoiesis. We and others have previously shown that RUNX1 and GATA-1 physically interact and cooperate in the activation of megakaryocytic promoters such as alpha IIb integrin and glycoprotein Ibalpha. Moreover, transcriptional cooperation of RUNX1 with GATA-1 is conserved back to Drosophila in which RUNX1 and GATA-1 homologs cooperate in crystal cell development. In this article, we will review the molecular and functional significance of the transcriptional cross talk between RUNX1 and GATA-1. In particular, we will elaborate on recent data which suggest that GATA-1 targets RUNX1 for modification, in particular phosphorylation by cyclin-dependent kinases. Furthermore, targeting of RUNX1 by GATA-1 for phosphorylation may convert RUNX1 from a repressor to an activator. This is a potential mechanism of transcriptional cooperation and may be an essential step in megakaryocytic differentiation.

Oncogenic pathways of AML1-ETO in acute myeloid leukemia: multifaceted manipulation of marrow maturation

The leukemic fusion protein AML1-ETO occurs frequently in human acute myeloid leukemia (AML) and has received much attention over the past decade. An initial model for its pathogenetic effects emphasized the conversion of a hematopoietic transcriptional activator, RUNX1 (or AML1), into a leukemogenic repressor which blocked myeloid differentiation at the level of target gene regulation. This view has been absorbed into a larger picture of AML1-ETO pathogenesis, encompassing dysregulation of hematopoietic stem cell homeostasis at several mechanistic levels. Recent reports have highlighted a multifaceted capacity of AML1-ETO directly to inhibit key hematopoietic transcription factors that function as tumor suppressors at several nodal points during hematopoietic differentiation. A new model is presented in which AML1-ETO coordinates expansion of the stem cell compartment with diminished lineage commitment and with genome instability.

Erythroid inhibition by the leukemic fusion AML1-ETO is associated with impaired acetylation of the major erythroid transcription factor GATA-1

Human acute myeloid leukemias with the t(8;21) translocation express the AML1-ETO fusion protein in the hematopoietic stem cell compartment and show impairment in erythroid differentiation. This clinical finding is reproduced in multiple murine and cell culture model systems in which AML1-ETO specifically interferes with erythroid maturation. Using purified normal human early hematopoietic progenitor cells, we find that AML1-ETO impedes the earliest discernable steps of erythroid lineage commitment. Correspondingly, GATA-1, a central transcriptional regulator of erythroid differentiation, undergoes repression by AML1-ETO in a nonconventional histone deacetylase-independent manner. In particular, GATA-1 acetylation by its transcriptional coactivator, p300/CBP, a critical regulatory step in programming erythroid development, is efficiently blocked by AML1-ETO. Fusion of a heterologous E1A coactivator recruitment module to GATA-1 overrides the inhibitory effects of AML1-ETO on GATA-1 acetylation and transactivation. Furthermore, the E1A-GATA-1 fusion, but not wild-type GATA-1, rescues erythroid lineage commitment in primary human progenitors expressing AML1-ETO. These results ascribe a novel repressive mechanism to AML1-ETO, blockade of GATA-1 acetylation, which correlates with its inhibitory effects on primary erythroid lineage commitment.

AML-1-ETO-Mediated erythroid inhibition: new paradigms for differentiation blockade by a leukemic fusion protein

The chromosomal translocation t(8;21), generating the AML1-ETO fusion protein, is frequently associated with French-American-British (FAB) type M2 acute myeloid leukemia (AML). t(8;21) fuses the runt domain from the hematopoietic transcription factor RUNX1 with almost the entire transcriptional repressor ETO. AML1-ETO inhibits normal definitive hematopoiesis and blocks erythroid differentiation. Several mechanistic models for the role of AML1-ETO in leukemia development have emerged over the last decade. Most of these models have emphasized the capacity of the fusion protein to redirect repressive cofactors, such as histone deacetylases (HDACs) and DNA methyltransferases (DNMTs), to RUNX target genes, thereby reversing the hematopoietic transcriptional program activated by wild-type RUNX1a phenomenon referred to collectively in this review as the "classical" corepressor model. Because erythropoiesis occurs in a RUNX-independent manner, this dominant-negative "classical" model cannot explain the prominent repression of red-cell development by AML1-ETO. This review will consider the clinical and mechanistic significance of erythroid inhibition by AML1-ETO. Additional models to account for this mysterious oncogenic function are proposed.

Jun blockade of erythropoiesis: role for repression of GATA-1 by HERP2

Although Jun upregulation and activation have been established as critical to oncogenesis, the relevant downstream pathways remain incompletely characterized. In this study, we found that c-Jun blocks erythroid differentiation in primary human hematopoietic progenitors and, correspondingly, that Jun factors block transcriptional activation by GATA-1, the central regulator of erythroid differentiation. Mutagenesis of c-Jun suggested that its repression of GATA-1 occurs through a transcriptional mechanism involving activation of downstream genes. We identified the hairy-enhancer-of-split-related factor HERP2 as a novel gene upregulated by c-Jun. HERP2 showed physical interaction with GATA-1 and repressed GATA-1 transcriptional activation. Furthermore, transduction of HERP2 into primary human hematopoietic progenitors inhibited erythroid differentiation. These results thus define a novel regulatory pathway linking the transcription factors c-Jun, HERP2, and GATA-1. Furthermore, these results establish a connection between the Notch signaling pathway, of which the HERP factors are a critical component, and the GATA family, which participates in programming of cellular differentiation.

RUNX1 and GATA-1 coexpression and cooperation in megakaryocytic differentiation

Megakaryocytic and erythroid lineages derive from a common bipotential progenitor and share many transcription factors, most prominently factors of the GATA zinc-finger family. Little is known about transcription factors unique to the megakaryocytic lineage that might program divergence from the erythroid pathway. To identify such factors, we used the K562 system in which megakaryocyte lineage commitment is dependent on sustained extracellular regulatory kinase (ERK) activation and is inhibited by stromal cell contact. During megakaryocytic induction in this system, the myeloid transcription factor RUNX1 underwent up-regulation, dependent on ERK signaling and inhibitable by stromal cell contact. Immunostaining of healthy human bone marrow confirmed a strong expression of RUNX1 and its cofactor, core-binding factor beta (CBFbeta), in megakaryocytes and a minimal expression in erythroblasts. In primary human hematopoietic progenitor cultures, RUNX1 and CBFbeta up-regulation preceded megakaryocytic differentiation, and down-regulation of these factors preceded erythroid differentiation. Functional studies showed cooperation among RUNX1, CBFbeta, and GATA-1 in the activation of a megakaryocytic promoter. By contrast, the RUNX1-ETO leukemic fusion protein potently repressed GATA-1-mediated transactivation. These functional interactions correlated with physical interactions observed between GATA-1 and RUNX1 factors. Enforced RUNX1 expression in K562 cells enhanced the induction of the megakaryocytic integrin proteins alphaIIb and alpha2. These results suggest that RUNX1 may participate in the programming of megakaryocytic lineage commitment through functional and physical interactions with GATA transcription factors. By contrast, RUNX1-ETO inhibition of GATA function may constitute a potential mechanism for the blockade of erythroid and megakaryocytic differentiation seen in leukemias with t(8;21).

Characterization and V(H) sequences of human monoclonal anti-F(ab')(2) autoantibodies from normals and Sjögren's syndrome patients

To investigate the genetic background of anti-F(ab')(2) autoantibodies and the mechanism behind their production we have analyzed 10 human monoclonal antibodies directed against IgG F(ab')(2) and IgG Fab. They were all derived from peripheral blood by the EBV/hybridoma technique. Eight were from three healthy individuals and two from two patients with primary Sjögren's syndrome (pSS). They react with epitopes on distinct regions of IgG, including epitopes present on or near the hinge of IgG, epitopes on the Fd gamma, and an antigenic determinant(s) present on lambda light chains. These determinants are either exposed on the intact IgG molecule or revealed following pepsin or papain digestion. The V(H) germline gene repertoire used is diverse and with considerable overlap with that used by rheumatoid factors (RF). The two IgG antibodies from normals are extensively mutated (13 and 24 mutations/V(H)), but with a replacement to silent mutation ratio in the CDR(H)1 + 2 of only 3.7. The IgM antibodies from normals are also heavily mutated (mean 10 mutations/V(H)). This suggests that anti-F(ab')(2) from normals are generated by an antigen-driven somatic hypermutation mechanism. In contrast, the two IgM antibodies from pSS are virtually unmutated in both V(H) and V(L). Together with published data of pSS RF and anti-Ro 52-kDa sequences (1-3), this suggests that there is an expanded population of naïve B cells with autoantibody specificities in the peripheral blood of pSS patients.

Light chain variable (VL) sequences of rheumatoid factors (RF) in patients with primary Sjögren's syndrome (pSS): moderate contribution of somatic hypermutation

We have characterized and sequenced the variable (V) region genes of the light (L) chains of 10 immunoglobulin (IgM) rheumatoid factor (RF) monoclonal antibodies (MoAb) derived by the hybridoma/Epstein-Barr virus (EBV) technique from the peripheral blood of patients with primary Sjögren's syndrome (pSS). Six out of 10 RFs used lambda (lambda) L chains, while four RFs used kappa (kappa) L chains. Five out of the six lambda RFs were encoded by Vlambda3 gene segments, the sixth one was encoded by a Vlambda1 gene segment. This preferential utilization of the Vlambda3 family genes suggests selective expansion of the B cell in pSS. Three of the kappa RFs used Vkappa3 gene segments, while the fourth used a Vkappa2 gene segment. Half of the RFs were found as unmutated copies of their closest germline (GL) gene. Interestingly these RFs were previously shown to use heavy (H) chains in GL gene configuration. Three RFs have very few mutations (2-3) and only two RFs have substantial numbers of mutations (6 and 11). This also correlated with the number of mutations in the respective H chains. In contrast to RFs in normal and RA these results further suggest the somatic mutation to be of moderate importance in the generation of RF from the peripheral blood of pSS patients.

Immunoglobulin variable genes and epitope recognition of human monoclonal anti-Ro 52-kd in primary Sjögren's syndrome

OBJECTIVE

To clone and characterize human anti-Ro/SSA autoantibodies from a patient with primary Sjögren's syndrome (pSS).

METHODS

Monoclonal antibodies (mAb) were raised from the peripheral blood of a patient with pSS using Epstein-Barr virus transformation and a hybridoma technique. Specificity was determined using cell extracts, recombinant Ro 52-kd, Ro 60-kd, and La proteins as well as Ro 52-kd peptides in enzyme-linked immunosorbent assay (ELISA) and Western blot. The immunofluorescence pattern was analyzed using cultured human and mouse cell lines. Complementary DNA was amplified by polymerase chain reaction, and Ig variable (V)-region genes were directly sequenced.

RESULTS

Two human anti-Ro 52-kd mAb of IgM isotype, denoted SG1 and SG3, were cloned from the peripheral blood of a patient with pSS. The 2 mAb reacted with the Ro 52-kd antigen in cell extracts of human cell lines and mouse cell lines, and with purified human recombinant Ro 52-kd protein in ELISA and Western blot. SG1 reacted specifically with 1 peptide, amino acids 136-156, of the Ro 52-kd protein, and SG3 was mapped to react with a recombinant fragment representing amino acids 136-292. Immunofluorescence studies revealed cytoplasmic staining with both mAb. Both were encoded by V(H)3-family genes. SG1 was highly homologous to the DP-77 germ-line gene, with 2 replacement mutations and 1 silent. It utilized the DPL-11 germ-line gene from the Vlambda2-family gene, with 1 silent mutation. SG3 was 100% homologous to the DP-47 germ-line gene, combined with a Vkappa1-family gene that was 100% homologous to the A30 germ-line gene.

CONCLUSION

Two human mAb were demonstrated to be specific for the Ro 52-kd protein and to be directed against 2 different epitopes, 1 linear and 1 conformation-dependent, within a region previously described to be immunodominant. Somatic hypermutation appeared to be of minor importance in generating these 2 specificities.

Susceptibility to periportal (Symmers) fibrosis in human schistosoma mansoni infections: evidence that intensity and duration of infection, gender, and inherited factors are critical in disease progression

Lethal disease in Schistosoma mansoni infections is mostly due to portal hypertension caused by hepatic periportal fibrosis. To evaluate the factors that may determine severe disease, livers and spleens were examined by ultrasound in a Sudanese population living in a village where S. mansoni is endemic. Early (FI), moderate (FII), or advanced (FIII) fibrosis was observed in 58%, 9%, and 3% of the population, respectively. Although FI affected 50%-70% of the children and adolescents, FII prevalence was low in subjects </=20 years old but increased sharply (45%-58%) in men 21-30 years old and was associated with the highest infections. Portal and splenic vein diameters were increased in one-third of persons with FII and in almost all with FIII disease. Severe disease, FII or FIII with portal hypertension, affected 6% of the population, was associated with splenomegaly, occurred mostly in adult men, and was clustered in a few pedigrees. These observations suggest that infection intensity and duration, gender-related factors, and inherited factors are important in fibrosis development.

Rheumatoid factors in primary Sjögren's syndrome (pSS) use diverse VH region genes, the majority of which show no evidence of somatic hypermutation

Rheumatoid factor (RF) is the most common autoantibody found in patients with Sjögren's syndrome (SS). To study the genetic origin and the mechanisms acting behind its generation we have characterized and sequenced the immunoglobulin VH genes used by 10 IgM RF MoAbs derived from peripheral blood of six female patients with pSS. We compared the structure of the RF immunoglobulin VH genes with those obtained previously from rheumatoid arthritis (RA) patients and healthy immunized donors (HID). VH1 and VH4 were each used by four RF clones, one clone was encoded by VH3 family gene and one by VH2 family gene. This distribution frequency was different from that observed in RA, where VH3 was the dominant family, followed by VH1. Eight different germ-line (GL) genes encoded the clones and all of these genes were seen previously in RA and/or HID RF. Five clones rearranged to JH6, four rearranged to JH4 and one to JH5, in contrast to RF from RA and HID, where JH4 was most frequently used. D segment use and CDR3 structure were diverse. Interestingly, three out of four VH4 clones used the GL gene DP-79 that was seen frequently in RA RF. The degree of somatic mutation in the pSS RF was very much lower than seen in RA and HID RF. All the pSS RF clones except three were in or very close to GL configuration. This indicates that there is little role for somatic hypermutation and a germinal centre reaction in the generation of RF from peripheral blood in pSS.