Erythroid Kruppel-like factor (EKLF) coordinates erythroid cell proliferation and hemoglobinization in cell lines derived from EKLF null mice

Krüppel-like factor 1 is a core cardiomyogenic trigger in zebrafish

Cardiac regeneration requires dedifferentiation and proliferation of mature cardiomyocytes, but the mechanisms underlying this plasticity remain unclear. Here, we identify a potent cardiomyogenic role for Krüppel-like factor 1 (Klf1/Eklf), which is induced in adult zebrafish myocardium upon injury. Myocardial inhibition of Klf1 function does not affect heart development, but it severely impairs regeneration. Transient Klf1 activation is sufficient to expand mature myocardium in uninjured hearts. Klf1 directs epigenetic reprogramming of the cardiac transcription factor network, permitting coordinated cardiomyocyte dedifferentiation and proliferation. Myocardial expansion is supported by Klf1-induced rewiring of mitochondrial metabolism from oxidative respiration to anabolic pathways. Our findings establish Klf1 as a core transcriptional regulator of cardiomyocyte renewal in adult zebrafish hearts.

Corrupted DNA-binding specificity and ectopic transcription underpin dominant neomorphic mutations in KLF/SP transcription factors

Background

Mutations in the transcription factor, KLF1, are common within certain populations of the world. Heterozygous missense mutations in KLF1 mostly lead to benign phenotypes, but a heterozygous mutation in a DNA-binding residue (E325K in human) results in severe Congenital Dyserythropoietic Anemia type IV (CDA IV); i.e. an autosomal-dominant disorder characterized by neonatal hemolysis.

Results

To investigate the biochemical and genetic mechanism of CDA IV, we generated murine erythroid cell lines that harbor tamoxifen-inducible (ER™) versions of wild type and mutant KLF1 on a Klf1^−/− genetic background. Nuclear translocation of wild type KLF1 results in terminal erythroid differentiation, whereas mutant KLF1 results in hemolysis without differentiation. The E to K variant binds poorly to the canonical 9 bp recognition motif (NGG-GYG-KGG) genome-wide but binds at high affinity to a corrupted motif (NGG-GRG-KGG). We confirmed altered DNA-binding specificity by quantitative in vitro binding assays of recombinant zinc-finger domains. Our results are consistent with previously reported structural data of KLF-DNA interactions. We employed 4sU-RNA-seq to show that a corrupted transcriptome is a direct consequence of aberrant DNA binding.

Conclusions

Since all KLF/SP family proteins bind DNA in an identical fashion, these results are likely to be generally applicable to mutations in all family members. Importantly, they explain how certain mutations in the DNA-binding domain of transcription factors can generate neomorphic functions that result in autosomal dominant disease.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5805-z) contains supplementary material, which is available to authorized users.

Hi-C detects novel structural variants in HL-60 and HL-60/S4 cell lines

Cancer cell lines often have large structural variants (SVs) that evolve over time. There are many reported differences in large scale SVs between HL-60 and HL-60/S4, two cell lines derived from the same acute myeloid leukemia sample. However, the stability and variability of inter- and intra-chromosomal structural variants between different sources of the same cell line is unknown. Here, we used Hi-C and RNA-seq to identify and compare large SVs in HL-60 and HL-60/S4 cell lines. Comparisons with previously published karyotypes identified novel SVs in both cell lines. Hi-C was used to characterize the known expansion centered on the MYC locus. The MYC expansion was integrated into known locations in HL-60/S4, and a novel location (chr4) in HL-60. The HL-60 cell line has more within-line structural variation than the HL-60/S4 derivative cell line. Collectively we demonstrate the usefulness of Hi-C and with RNA-seq data for the identification and characterization of SVs.

KFL1 Gene Variants in α-Thalassemia Individuals with Increased Fetal Hemoglobin in a Chinese Population

Krüppel-like factor 1 (KLF1) is a pleiotropic erythroid transcription factor that is a regulator of definitive erythropoiesis. The aim of this study was to detect KLF1 gene variants in α-thalassemia (α-thal) carriers with an increased Hb F level in a Chinese population, and determine the changes of hematological parameters as a result of interactions between KLF1 gene mutations and α-thal. Subjects with α-thal and Hb F levels of ≥1.0% were selected for further investigation. Direct sequencing was used to detect KLF1 gene mutations. Hematological parameters of subjects with α-thal and concomitant KLF1 gene mutations and those with α-thal alone were compared. The KLF1 gene variants were detected in 46 of 275 (16.7%) individuals with α-thal and Hb F levels of ≥1.0%. The detection rate of KLF1 gene mutations rose correspondingly when the Hb F level increased. For α⁰-thal carriers, significantly lower mean corpuscular volume (MCV) and mean corpuscular hemoglobin (Hb) (MCH) values were observed in KLF1 gene mutation-positive carriers than that in KLF1 gene mutation-free carriers; conversely, significantly higher Hb A₂ and Hb F levels were observed in the former condition rather than in the latter condition. The results of this study indicate that KLF1 gene variants are common in Chinese subjects with α-thal and increased Hb F levels, and KLF1 gene mutations decreased the red blood cell (RBC) indices in α-thal carriers as that in normal adults.

Natural regulatory mutations elevate the fetal globin gene via disruption of BCL11A or ZBTB7A binding

β-hemoglobinopathies such as sickle cell disease (SCD) and β-thalassemia result from mutations in the adult HBB (β-globin) gene. Reactivating the developmentally silenced fetal HBG1 and HBG2 (γ-globin) genes is a therapeutic goal for treating SCD and β-thalassemia ¹ . Some forms of hereditary persistence of fetal hemoglobin (HPFH), a rare benign condition in which individuals express the γ-globin gene throughout adulthood, are caused by point mutations in the γ-globin gene promoter at regions residing ~115 and 200 bp upstream of the transcription start site. We found that the major fetal globin gene repressors BCL11A and ZBTB7A (also known as LRF) directly bound to the sites at -115 and -200 bp, respectively. Furthermore, introduction of naturally occurring HPFH-associated mutations into erythroid cells by CRISPR-Cas9 disrupted repressor binding and raised γ-globin gene expression. These findings clarify how these HPFH-associated mutations operate and demonstrate that BCL11A and ZBTB7A are major direct repressors of the fetal globin gene.

An Introduction to Erythropoiesis Approaches

Many experimental models have been used to study erythropoiesis. Even prior to the advent of the genetic manipulation of animal models, erythropoiesis was examined in the mouse, chicken, sheep, goat, and rabbit, among other vertebrates. Erythroid cell lines derived from human blood cancers were also very useful, as they could be genetically manipulated more easily than whole animals. Genetic models in the mouse, zebrafish, and frog have provided a plethora of information advancing our understanding of erythropoiesis, and remain gold standards in the field for studies of hemoglobin switching, and experiments to study authentic blood cell development. Mouse and human embryonic stem (ES) and induced pluripotent (iPS) cells can be differentiated to erythroid cells in culture, though their use is somewhat limited by their propensity to express only the embryonic and fetal globin genes. Some very useful cell lines have been developed by manipulating ES or fetal liver erythroid progenitor cells from knockout mouse models. In recent years, our understanding of erythropoiesis has improved, due to the ability to knock down genes in native human hematopoietic stem and progenitor cells derived from umbilical cord blood or bone marrow, and differentiate them ex vivo to the erythroid lineage. These native cells, and cell lines derived from them, are now providing essential information about human erythropoiesis, which is complementary to that obtained from animal studies. This review provides some perspective about the cell and animal models used to study erythropoiesis over the years.

Promiscuous DNA-binding of a mutant zinc finger protein corrupts the transcriptome and diminishes cell viability

The rules of engagement between zinc finger transcription factors and DNA have been partly defined by in vitro DNA-binding and structural studies, but less is known about how these rules apply in vivo. Here, we demonstrate how a missense mutation in the second zinc finger of Krüppel-like factor-1 (KLF1) leads to degenerate DNA-binding specificity in vivo, resulting in ectopic transcription and anemia in the Nan mouse model. We employed ChIP-seq and 4sU-RNA-seq to identify aberrant DNA-binding events genome wide and ectopic transcriptional consequences of this binding. We confirmed novel sequence specificity of the mutant recombinant zinc finger domain by performing biophysical measurements of in vitro DNA-binding affinity. Together, these results shed new light on the mechanisms by which missense mutations in DNA-binding domains of transcription factors can lead to autosomal dominant diseases.

Neomorphic effects of the neonatal anemia (Nan-Eklf) mutation contribute to deficits throughout development

Transcription factor control of cell-specific downstream targets can be significantly altered when the controlling factor is mutated. We show that the semi-dominant neonatal anemia (Nan) mutation in the EKLF/KLF1 transcription factor leads to ectopic expression of proteins that are not normally expressed in the red blood cell, leading to systemic effects that exacerbate the intrinsic anemia in the adult and alter correct development in the early embryo. Even when expressed as a heterozygote, the Nan-EKLF protein accomplishes this by direct binding and aberrant activation of genes encoding secreted factors that exert a negative effect on erythropoiesis and iron use. Our data form the basis for a novel mechanism of physiological deficiency that is relevant to human dyserythropoietic anemia and likely other disease states.

Direct targets of pSTAT5 signalling in erythropoiesis

Erythropoietin (EPO) acts through the dimeric erythropoietin receptor to stimulate proliferation, survival, differentiation and enucleation of erythroid progenitor cells. We undertook two complimentary approaches to find EPO-dependent pSTAT5 target genes in murine erythroid cells: RNA-seq of newly transcribed (4sU-labelled) RNA, and ChIP-seq for pSTAT5 30 minutes after EPO stimulation. We found 302 pSTAT5-occupied sites: ~15% of these reside in promoters while the rest reside within intronic enhancers or intergenic regions, some >100kb from the nearest TSS. The majority of pSTAT5 peaks contain a central palindromic GAS element, TTCYXRGAA. There was significant enrichment for GATA motifs and CACCC-box motifs within the neighbourhood of pSTAT5-bound peaks, and GATA1 and/or KLF1 co-occupancy at many sites. Using 4sU-RNA-seq we determined the EPO-induced transcriptome and validated differentially expressed genes using dynamic CAGE data and qRT-PCR. We identified known direct pSTAT5 target genes such as Bcl2l1, Pim1 and Cish, and many new targets likely to be involved in driving erythroid cell differentiation including those involved in mRNA splicing (Rbm25), epigenetic regulation (Suv420h2), and EpoR turnover (Clint1/EpsinR). Some of these new EpoR-JAK2-pSTAT5 target genes could be used as biomarkers for monitoring disease activity in polycythaemia vera, and for monitoring responses to JAK inhibitors.

Hi-C as a tool for precise detection and characterisation of chromosomal rearrangements and copy number variation in human tumours

Chromosomal rearrangements occur constitutionally in the general population and somatically in the majority of cancers. Detection of balanced rearrangements, such as reciprocal translocations and inversions, is troublesome, which is particularly detrimental in oncology where rearrangements play diagnostic and prognostic roles. Here we describe the use of Hi-C as a tool for detection of both balanced and unbalanced chromosomal rearrangements in primary human tumour samples, with the potential to define chromosome breakpoints to bp resolution. In addition, we show copy number profiles can also be obtained from the same data, all at a significantly lower cost than standard sequencing approaches.

Krüppel-like factors compete for promoters and enhancers to fine-tune transcription

Krüppel-like factors (KLFs) are a family of 17 transcription factors characterized by a conserved DNA-binding domain of three zinc fingers and a variable N-terminal domain responsible for recruiting cofactors. KLFs have diverse functions in stem cell biology, embryo patterning, and tissue homoeostasis. KLF1 and related family members function as transcriptional activators via recruitment of co-activators such as EP300, whereas KLF3 and related members act as transcriptional repressors via recruitment of C-terminal Binding Proteins. KLF1 directly activates the Klf3 gene via an erythroid-specific promoter. Herein, we show KLF1 and KLF3 bind common as well as unique sites within the erythroid cell genome by ChIP-seq. We show KLF3 can displace KLF1 from key erythroid gene promoters and enhancers in vivo. Using 4sU RNA labelling and RNA-seq, we show this competition results in reciprocal transcriptional outputs for >50 important genes. Furthermore, Klf3-/- mice displayed exaggerated recovery from anemic stress and persistent cell cycling consistent with a role for KLF3 in dampening KLF1-driven proliferation. We suggest this study provides a paradigm for how KLFs work in incoherent feed-forward loops or networks to fine-tune transcription and thereby control diverse biological processes such as cell proliferation.

KLF1 directly activates expression of the novel fetal globin repressor ZBTB7A/LRF in erythroid cells

Genes encoding the human β-like hemoglobin proteins undergo a developmental switch from fetal γ-globin to adult β-globin expression around the time of birth. β-hemoglobinopathies, such as sickle-cell disease and β-thalassemia, result from mutations affecting the adult β-globin gene. The only treatment options currently available carry significant adverse effects. Analyses of heritable variations in fetal hemoglobin (HbF) levels have provided evidence that reactivation of the silenced fetal γ-globin genes in adult erythroid cells is a promising therapy. The γ-globin repressor BCL11A has become the major focus, with several studies investigating its regulation and function as a first step to inhibiting its expression or activity. However, a second repression mechanism was recently shown to be mediated by the transcription factor ZBTB7A/LRF, suggesting that understanding the regulation of ZBTB7A may also be useful. Here we show that Krüppel-like factor 1 (KLF1) directly drives expression of ZBTB7A in erythroid cells by binding to its proximal promoter. We have also uncovered an erythroid-specific regulation mechanism, leading to the upregulation of a novel ZBTB7A transcript in the erythroid compartment. The demonstration that ZBTB7A, like BCL11A, is a KLF1 target gene also fits with the observation that reduced KLF1 expression or activity is associated with HbF derepression.

Activation of KLF1 Enhances the Differentiation and Maturation of Red Blood Cells from Human Pluripotent Stem Cells

Blood transfusion is widely used in the clinic but the source of red blood cells (RBCs) is dependent on donors, procedures are susceptible to transfusion-transmitted infections and complications can arise from immunological incompatibility. Clinically-compatible and scalable protocols that allow the production of RBCs from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have been described but progress to translation has been hampered by poor maturation and fragility of the resultant cells. Genetic programming using transcription factors has been used to drive lineage determination and differentiation so we used this approach to assess whether exogenous expression of the Erythroid Krüppel-like factor 1 (EKLF/KLF1) could augment the differentiation and stability of iPSC-derived RBCs. To activate KLF1 at defined time points during later stages of the differentiation process and to avoid transgene silencing that is commonly observed in differentiating pluripotent stem cells, we targeted a tamoxifen-inducible KLF1-ERT2 expression cassette into the AAVS1 locus. Activation of KLF1 at day 10 of the differentiation process when hematopoietic progenitor cells were present, enhanced erythroid commitment and differentiation. Continued culture resulted the appearance of more enucleated cells when KLF1 was activated which is possibly due to their more robust morphology. Globin profiling indicated that these conditions produced embryonic-like erythroid cells. This study demonstrates the successful use of an inducible genetic programing strategy that could be applied to the production of many other cell lineages from human induced pluripotent stem cells with the integration of programming factors into the AAVS1 locus providing a safer and more reproducible route to the clinic. Stem Cells 2017;35:886-897.

Heme-bound iron activates placenta growth factor in erythroid cells via erythroid Krüppel-like factor

In adults with sickle cell disease (SCD), markers of iron burden are associated with excessive production of the angiogenic protein placenta growth factor (PlGF) and high estimated pulmonary artery pressure. Enforced PlGF expression in mice stimulates production of the potent vasoconstrictor endothelin-1, producing pulmonary hypertension. We now demonstrate heme-bound iron (hemin) induces PlGF mRNA >200-fold in a dose- and time-dependent fashion. In murine and human erythroid cells, expression of erythroid Krüppel-like factor (EKLF) precedes PlGF, and its enforced expression in human erythroid progenitor cells induces PlGF mRNA. Hemin-induced expression of PlGF is abolished in EKLF-deficient murine erythroid cells but rescued by conditional expression of EKLF. Chromatin immunoprecipitation reveals that EKLF binds to the PlGF promoter region. SCD patients show higher level expression of both EKLF and PlGF mRNA in circulating blood cells, and markers of iron overload are associated with high PlGF and early mortality. Finally, PlGF association with iron burden generalizes to other human diseases of iron overload. Our results demonstrate a specific mechanistic pathway induced by excess iron that is linked in humans with SCD and in mice to markers of vasculopathy and pulmonary hypertension. These trials were registered at www.clinicaltrials.gov as #NCT00007150, #NCT00023296, #NCT00081523, and #NCT00352430.

Repression of chimeric transcripts emanating from endogenous retrotransposons by a sequence-specific transcription factor

Background

Retroviral elements are pervasively transcribed and dynamically regulated during development. While multiple histone- and DNA-modifying enzymes have broadly been associated with their global silencing, little is known about how the many diverse retroviral families are each selectively recognized.

Results

Here we show that the zinc finger protein Krüppel-like Factor 3 (KLF3) specifically silences transcription from the ORR1A0 long terminal repeat in murine fetal and adult erythroid cells. In the absence of KLF3, we detect widespread transcription from ORR1A0 elements driven by the master erythroid regulator KLF1. In several instances these aberrant transcripts are spliced to downstream genic exons. One such chimeric transcript produces a novel, dominant negative isoform of PU.1 that can induce erythroid differentiation.

Conclusions

We propose that KLF3 ensures the integrity of the murine erythroid transcriptome through the selective repression of a particular retroelement and is likely one of multiple sequence-specific factors that cooperate to achieve global silencing.

Lyn kinase plays important roles in erythroid expansion, maturation and erythropoietin receptor signalling by regulating inhibitory signalling pathways that control survival

In erythroid cells both positive viability signals and feedback inhibitory signalling require the Src family kinase Lyn, influencing cell survival and their ability to differentiate. This illustrates that Lyn is critical for normal erythropoiesis and erythroid cell development.

EKLF-driven PIT1 expression is critical for mouse erythroid maturation in vivo and in vitro

The PIT1/SLC20A1 protein, a well-described sodium/phosphate cotransporter and retrovirus receptor, has been identified recently as a modular of proliferation and apoptosis in vitro. The targeted deletion of the PIT1 gene in mice revealed a lethal phenotype due to severe anemia attributed to defects in liver development. However, the presence of immature erythroid cells associated with impaired maturation of the globin switch led us to investigate the role of PIT1 in hematopoietic development. In the present study, specific deletion of PIT1 in the hematopoietic system and fetal liver transplantation experiments demonstrated that anemia was associated with an erythroid cell- autonomous defect. Moreover, anemia was not due to RBC destruction but rather to maturation defects. Because Erythroid Krüppel-like Factor (EKLF)-knockout mice showed similar maturation defects, we investigated the functional link between PIT1 and EKLF. We demonstrated that EKLF increases PIT1 expression during RBC maturation by binding to its promoter in vivo and that shRNA-driven depletion of either PIT1 or EKLF impairs erythroid maturation of G1E cells in vitro, whereas reexpression of PIT1 in EKLF-depleted G1E cells partially restores erythroid maturation. This is the first demonstration of a physiologic involvement of PIT1 in erythroid maturation in vivo.

Regulation of delta-aminolevulinic acid dehydratase by krüppel-like factor 1

Krüppel-like factor 1(KLF1) is a hematopoietic-specific zinc finger transcription factor essential for erythroid gene expression. In concert with the transacting factor GATA1, KLF1 modulates the coordinate expression of the genes encoding the multi-enzyme heme biosynthetic pathway during erythroid differentiation. To explore the mechanisms underpinning KLF1 action at the gene loci regulating the first 3 steps in this process, we have exploited the K1-ERp erythroid cell line, in which KLF1 translocates rapidly to the nucleus in response to treatment with 4-OH-Tamoxifen (4-OHT). KLF1 acts as a differentiation-independent transcriptional co-regulator of delta-aminolevulinic acid dehydratase (Alad), but not 5-aminolevulinate synthase gene (Alas2) or porphobilinogen deaminase (Pbgd). Similar to its role at the β-globin promoter, KLF1 induces factor recruitment and chromatin changes at the Alad1b promoter in a temporally-specific manner. In contrast to these changes, we observed a distinct mechanism of histone eviction at the Alad1b promoter. Furthermore, KLF1-dependent events were not modulated by GATA1 factor promoter co-occupancy alone. These results not only enhance our understanding of erythroid-specific modulation of heme biosynthetic regulation by KLF1, but provide a model that will facilitate the elucidation of novel KLF1-dependent events at erythroid gene loci that are independent of GATA1 activity.

Large-scale in-vitro expansion of RBCs from hematopoietic stem cells

The quest for RBCs in transfusion medicine has prompted scientists to explore the large-scale expansion of human RBCs from various sources. The successful production of RBCs in the laboratory depends on the selection of potential cell source, optimized culture, bio-physiological parameters, clinically applicable culture media that yields a scalable, contamination-free, non-reactive, non-tumorogenic, stable and functional end product. The expansion protocol considering the in vivo factors involved in homeostasis can generate a cost-effective and readily available cell source for transfusion. This review paper discusses several approaches used to expand RBCs from various sources of stem cells.

The CACCC-binding protein KLF3/BKLF represses a subset of KLF1/EKLF target genes and is required for proper erythroid maturation in vivo

The CACCC-box binding protein erythroid Krüppel-like factor (EKLF/KLF1) is a master regulator that directs the expression of many important erythroid genes. We have previously shown that EKLF drives transcription of the gene for a second KLF, basic Krüppel-like factor, or KLF3. We have now tested the in vivo role of KLF3 in erythroid cells by examining Klf3 knockout mice. KLF3-deficient adults exhibit a mild compensated anemia, including enlarged spleens, increased red pulp, and a higher percentage of erythroid progenitors, together with elevated reticulocytes and abnormal erythrocytes in the peripheral blood. Impaired erythroid maturation is also observed in the fetal liver. We have found that KLF3 levels rise as erythroid cells mature to become TER119(+). Consistent with this, microarray analysis of both TER119(-) and TER119(+) erythroid populations revealed that KLF3 is most critical at the later stages of erythroid maturation and is indeed primarily a transcriptional repressor. Notably, many of the genes repressed by KLF3 are also known to be activated by EKLF. However, the majority of these are not currently recognized as erythroid-cell-specific genes. These results reveal the molecular and physiological function of KLF3, defining it as a feedback repressor that counters the activity of EKLF at selected target genes to achieve normal erythropoiesis.

Kruppel-like factor 1 (KLF1), KLF2, and Myc control a regulatory network essential for embryonic erythropoiesis

The Krüppel-like factor 1 (KLF1) and KLF2 positively regulate embryonic β-globin expression and have additional overlapping roles in embryonic (primitive) erythropoiesis. KLF1(-/-) KLF2(-/-) double knockout mice are anemic at embryonic day 10.5 (E10.5) and die by E11.5, in contrast to single knockouts. To investigate the combined roles of KLF1 and KLF2 in primitive erythropoiesis, expression profiling of E9.5 erythroid cells was performed. A limited number of genes had a significantly decreasing trend of expression in wild-type, KLF1(-/-), and KLF1(-/-) KLF2(-/-) mice. Among these, the gene for Myc (c-Myc) emerged as a central node in the most significant gene network. The expression of the Myc gene is synergistically regulated by KLF1 and KLF2, and both factors bind the Myc promoters. To characterize the role of Myc in primitive erythropoiesis, ablation was performed specifically in mouse embryonic proerythroblast cells. After E9.5, these embryos exhibit an arrest in the normal expansion of circulating red cells and develop anemia, analogous to KLF1(-/-) KLF2(-/-) embryos. In the absence of Myc, circulating erythroid cells do not show the normal increase in α- and β-like globin gene expression but, interestingly, have accelerated erythroid cell maturation between E9.5 and E11.5. This study reveals a novel regulatory network by which KLF1 and KLF2 regulate Myc to control the primitive erythropoietic program.

Integrating novel signaling pathways involved in erythropoiesis

Many extrinsic and intrinsic factors control the development of red blood cells from committed progenitors, with the Erythropoietin-receptor (Epo-R) signaling network being the primary controlling molecular hub. Although much is understood about erythroid signaling pathways, new and intriguing factors that influence different aspects of erythroid cell development are still being uncovered. New extrinsic effectors include hypoxia and polymeric IgA1 (pIgA1), and new Epo-R signaling pathway components include Lyn/Cbp and Lyn/Liar. Hypoxia directly activates committed erythroid progenitors to expand, whereas pIgA1 activates the Akt and MAP-Kinase (MAPK) pathways through transferrin receptors on more mature erythroid cells. The Lyn/Cbp pathway controls the activity and protein levels of Lyn through recruitment of Csk and SOCS1, as well as feeding into the control of other pathways mediated by recruitment of ras-GAP, PI3-kinase, PLCγ, Fes, and EBP50. Nuclear/cytoplasmic shuttling of Lyn and other signaling molecules is influenced by Liar and results in regulation of their intersecting signaling pathways. The challenge of future research is to flesh out the details of these new signaling regulators/networks and integrate their influences during the different stages of erythropoiesis.

The multifunctional role of EKLF/KLF1 during erythropoiesis

The cellular events that lead to terminal erythroid differentiation rely on the controlled interplay of extra- and intracellular regulatory factors. Their downstream effects are highly coordinated and result in the structural/morphologic and metabolic changes that uniquely characterize a maturing red blood cell. Erythroid Krüppel-like factor (EKLF/KLF1) is one of a very small number of intrinsic transcription factors that play a major role in regulating these events. This review covers 3 major aspects of erythropoiesis in which EKLF plays crucial functions: (1) at the megakaryocyte-erythroid progenitor stage, where it is involved in erythroid lineage commitment; (2) during the global expansion of erythroid gene expression in primitive and definitive lineages, where it plays a direct role in globin switching; and (3) during the terminal maturation of red cells, where it helps control exit from the cell cycle. We conclude by describing recent studies of mammalian EKLF/KLF1 mutations that lead to altered red cell phenotypes and disease.

KLF1 directly coordinates almost all aspects of terminal erythroid differentiation

The molecular events and transcriptional mechanisms that underlie erythropoiesis are of great interest to biologists and hematologists since disorders of erythrocytes are common and remain relatively poorly understood. Kruppel-like factor 1 (KLF1) is a critical transcription factor for erythropoiesis in mice and man. Recently the use of chromatin immunoprecipitation (ChIP) coupled to next-generation DNA sequencing (ChIP-seq) has led to an updated understanding of how KLF1 functions in vivo. The full extent of KLF1 target genes have provided new insights into erythropoiesis, and have established that KLF1 controls almost all aspects of erythroid cell development and maturation.

Mammalian Krüppel-like factors in health and diseases

The Krüppel-like factor (KLF) family of transcription factors regulates diverse biological processes that include proliferation, differentiation, growth, development, survival, and responses to external stress. Seventeen mammalian KLFs have been identified, and numerous studies have been published that describe their basic biology and contribution to human diseases. KLF proteins have received much attention because of their involvement in the development and homeostasis of numerous organ systems. KLFs are critical regulators of physiological systems that include the cardiovascular, digestive, respiratory, hematological, and immune systems and are involved in disorders such as obesity, cardiovascular disease, cancer, and inflammatory conditions. Furthermore, KLFs play an important role in reprogramming somatic cells into induced pluripotent stem (iPS) cells and maintaining the pluripotent state of embryonic stem cells. As research on KLF proteins progresses, additional KLF functions and associations with disease are likely to be discovered. Here, we review the current knowledge of KLF proteins and describe common attributes of their biochemical and physiological functions and their pathophysiological roles.

A global role for KLF1 in erythropoiesis revealed by ChIP-seq in primary erythroid cells

KLF1 regulates a diverse suite of genes to direct erythroid cell differentiation from bipotent progenitors. To determine the local cis-regulatory contexts and transcription factor networks in which KLF1 operates, we performed KLF1 ChIP-seq in the mouse. We found at least 945 sites in the genome of E14.5 fetal liver erythroid cells which are occupied by endogenous KLF1. Many of these recovered sites reside in erythroid gene promoters such as Hbb-b1, but the majority are distant to any known gene. Our data suggests KLF1 directly regulates most aspects of terminal erythroid differentiation including production of alpha- and beta-globin protein chains, heme biosynthesis, coordination of proliferation and anti-apoptotic pathways, and construction of the red cell membrane and cytoskeleton by functioning primarily as a transcriptional activator. Additionally, we suggest new mechanisms for KLF1 cooperation with other transcription factors, in particular the erythroid transcription factor GATA1, to maintain homeostasis in the erythroid compartment.

EKLF directly activates the p21WAF1/CIP1 gene by proximal promoter and novel intronic regulatory regions during erythroid differentiation

The switch from proliferation to differentiation during the terminal stages of erythropoiesis is a tightly controlled process that relies in part on transcription factor-mediated activation of cell cycle components. EKLF is a key transcription factor that is necessary for the initial establishment of the red cell phenotype. Here, we find that EKLF also plays a role during the subsequent differentiation process, as it induces p21(WAF1/CIP1) expression independent of p53 to regulate the changes in the cell cycle underlying erythroid maturation. EKLF activates p21 not only by directly binding to an EKLF site within a previously characterized GC-rich region in the p21 proximal promoter but also by occupancy at a novel, phylogenetically conserved region that contains consensus CACCC core motifs located downstream from the p21 TATA box. Our findings demonstrate that EKLF, likely in coordination with other transcription factors, directly contributes to the complex set of events that occur at the final erythroid cell divisions and accentuates terminal differentiation directly by activation of CDK inhibitors such as p21.

Inducible activation of IFI 16 results in suppression of telomerase activity, growth suppression and induction of cellular senescence

Expression of the human HIN-200 family member IFI 16 has been reported to suppress cell growth and contribute to the onset of cellular senescence. However the molecular events involved in this process have not been fully characterised. We fused IFI 16 to the estrogen receptor ligand-binding domain to establish an inducible model for studying the molecular events that cause these phenomena. In cells induced to express the ER-IFI 16 within the nucleus there was a decrease in cellular proliferation and concomitant growth arrest in the G1 phase of the cell cycle. Unlike previous reports, this did not appear to involve the p53-p21(WAF1/CIP1)-cdk2-pRb pathway. Following nuclear expression of ER-IFI 16 we noted senescence-like morphological changes and expression of senescence-associated beta-galactosidase in growth arrested cells. Importantly, we also found a marked reduction in telomerase activity in arrested cells compared to controls. Moreover, IFI 16 and hTERT co-localised within the nucleus and these two proteins physically interacted in vivo and in vitro. Together, these data suggest that IFI 16 may act as an endogenous regulator of telomerase activity and, through its interaction with hTERT, contributes to the inhibition of proliferation and induces a senescence-like state.

Role for parasite genetic diversity in differential host responses to Trypanosoma brucei infection

The postgenomic era has revolutionized approaches to defining host-pathogen interactions and the investigation of the influence of genetic variation in either protagonist upon infection outcome. We analyzed pathology induced by infection with two genetically distinct Trypanosoma brucei strains and found that pathogenesis is partly strain specific, involving distinct host mechanisms. Infections of BALB/c mice with one strain (927) resulted in more severe anemia and greater erythropoietin production compared to infections with the second strain (247), which, contrastingly, produced greater splenomegaly and reticulocytosis. Plasma interleukin-10 (IL-10) and gamma interferon levels were significantly higher in strain 927-infected mice, whereas IL-12 was higher in strain 247-infected mice. To define mechanisms underlying these differences, expression microarray analysis of host genes in the spleen at day 10 postinfection was undertaken. Rank product analysis (RPA) showed that 40% of the significantly differentially expressed genes were specific to infection with one or the other trypanosome strain. RPA and pathway analysis identified LXR/RXR signaling, IL-10 signaling, and alternative macrophage activation as the most significantly differentially activated host processes. These data suggest that innate immune response modulation is a key determinant in trypanosome infections, the pattern of which can vary, dependent upon the trypanosome strain. This strongly suggests that a parasite genetic component is responsible for causing disease in the host. Our understanding of trypanosome infections is largely based on studies involving single parasite strains, and our results suggest that an integrated host-parasite approach is required for future studies on trypanosome pathogenesis. Furthermore, it is necessary to incorporate parasite variation into both experimental systems and models of pathogenesis.

Megakaryocyte-erythroid lineage promiscuity in EKLF null mouse blood

Commitment towards megakaryocyte versus erythroid blood cell lineages occurs in the megakaryocyte-erythroid progenitor, where mutually exclusive expression of either EKLF (Klf1) or Fli1 defines alternative outcomes. Here we show there is a marked increase in the number of circulating platelets in mice lacking the erythroid transcription factor EKLF. In addition, committed erythroid cells retain key signatures of megakaryocytes both on the cell surface and at the mRNA level. We also show that the effect of EKLF on megakaryocyte-erythroid progenitor lineage decision and commitment is cell autonomous in bone marrow reconstitution assays where stem cells lacking EKLF favor the megakaryocyte differentiation pathway. We conclude the megakaryocyte program is aberrantly activated in EKLF null erythroid cells.

EKLF/KLF1 controls cell cycle entry via direct regulation of E2f2

Differentiation of erythroid cells requires precise control over the cell cycle to regulate the balance between cell proliferation and differentiation. The zinc finger transcription factor, erythroid Krüppel-like factor (EKLF/KLF1), is essential for proper erythroid cell differentiation and regulates many erythroid genes. Here we show that loss of EKLF leads to aberrant entry into S-phase of the cell cycle during both primitive and definitive erythropoiesis. This cell cycle defect was associated with a significant reduction in the expression levels of E2f2 and E2f4, key factors necessary for the induction of S-phase gene expression and erythropoiesis. We found and validated novel intronic enhancers in both the E2f2 and E2f4 genes, which contain conserved CACC, GATA, and E-BOX elements. The E2f2 enhancer was occupied by EKLF in vivo. Furthermore, we were able to partially restore cell cycle dynamics in EKLF(-/-) fetal liver upon additional genetic depletion of Rb, establishing a genetic causal link between reduced E2f2 and the EKLF cell cycle defect. Finally, we propose direct regulation of the E2f2 enhancer is a generic mechanism by which many KLFs regulate proliferation and differentiation.

TLX1 (HOX11) immortalization of embryonic stem cell-derived and primary murine hematopoietic progenitors

The ability to generate genetically engineered cell lines is of great experimental value. They provide a renewable source of material that may be suitable for biochemical analyses, chromatin immunoprecipitation assays, structure-function studies, gene function assignment, and transcription factor target gene identification. This unit describes protocols for TLX1 (HOX11)-mediated immortalization of murine hematopoietic progenitors derived from in vitro differentiated murine embryonic stem cells, or from primary mouse fetal liver or bone marrow. A wide variety of hematopoietic cell types have been immortalized using these procedures including erythroid, megakaryocytic, monocytic, myelocytic, and multipotential cell types. These lines are typically cytokine dependent for their survival and growth.

Acetylation of EKLF is essential for epigenetic modification and transcriptional activation of the beta-globin locus

Posttranslational modifications of transcription factors provide alternate protein interaction platforms that lead to varied downstream effects. We have investigated how the acetylation of EKLF plays a role in its ability to alter the beta-like globin locus chromatin structure and activate transcription of the adult beta-globin gene. By establishing an EKLF-null erythroid line whose closed beta-locus chromatin structure and silent beta-globin gene status can be rescued by retroviral infection of EKLF, we demonstrate the importance of EKLF acetylation at lysine 288 in the recruitment of CBP to the locus, modification of histone H3, occupancy by EKLF, opening of the chromatin structure, and transcription of adult beta-globin. We also find that EKLF helps to coordinate this process by the specific association of its zinc finger domain with the histone H3 amino terminus. Although EKLF interacts equally well with H3.1 and H3.3, we find that only H3.3 is enriched at the adult beta-globin promoter. These data emphasize the critical nature of lysine acetylation in transcription factor activity and enable us to propose a model of how modified EKLF integrates coactivators, chromatin remodelers, and nucleosomal components to alter epigenetic chromatin structure and stimulate transcription.

A network of Krüppel-like Factors (Klfs). Klf8 is repressed by Klf3 and activated by Klf1 in vivo

Transcription factors of the Sp/Klf (Krüppel-like factor) family regulate biological processes such as hematopoiesis, adipogenesis, and stem cell maintenance. Here we show that Bklf or Klf3 (Basic Krüppel-like factor) represses the Klf8 (Krüppel-like Factor 8) gene in vivo. Conversely, Eklf or Klf1 (Erythroid Krüppel-like factor) activates the Klf8 gene. Klf8 is driven by two promoters, both of which contain multiple CACCC sites. Klf3 can repress Klf1-mediated activation of both promoters. Chromatin immunoprecipitation experiments confirm that Klf3 occupies both Klf8 promoters in vivo. Interestingly, in Klf3 knock-out tissue Klf1 gains access, binds, and activates both Klf8 promoters. These results demonstrate direct competition between activating and repressing Klfs in vivo. Together with previous evidence that Klf1 directly activates the Klf3 gene, the results reveal an elaborate network of cross-talk within the Klf family. The recognition of cross-regulation and potential redundancy between Klf family members is critical to the interpretation of various Klf knock-out mice and the understanding of individual Klfs in particular contexts.

EKLF restricts megakaryocytic differentiation at the benefit of erythrocytic differentiation

Previous observations suggested that functional antagonism between FLI-1 and EKLF might be involved in the commitment toward erythrocytic or megakaryocytic differentiation. We show here, using inducible shRNA expression, that EKLF knockdown in mouse erythroleukemia (MEL) cells decreases erythrocytic and increases megakaryocytic as well as Fli-1 gene expression. Chromatin immunoprecipitation analyses revealed that the increase in megakaryocytic gene expression is associated with a marked increase in RNA pol II and FLI-1 occupancy at their promoters, albeit FLI-1 protein levels are only minimally affected. Similarly, we show that human CD34+ progenitors infected with shRNA lentivirus allowing EKLF knockdown generate an increased number of differentiated megakaryocytic cells associated with increased levels of megakaryocytic and Fli-1 gene transcripts. Single-cell progeny analysis of a cell population enriched in bipotent progenitors revealed that EKLF knockdown increases the number of megakaryocytic at the expense of erythrocytic colonies. Taken together, these data indicate that EKLF restricts megakaryocytic differentiation to the benefit of erythrocytic differentiation and suggest that this might be at least partially mediated by the inhibition of FLI-1 recruitment to megakaryocytic and Fli-1 gene promoters.

Kruppel-like factor 4 is essential for inflammatory monocyte differentiation in vivo

Several members of the Kruppel-like factor (KLF) family of transcription factors play important roles in differentiation, survival, and trafficking of blood and immune cell types. We demonstrate in this study that hematopoietic cells from KLF4(-/-) fetal livers (FL) contained normal numbers of functional hematopoietic progenitor cells, were radioprotective, and performed as well as KLF4(+/+) cells in competitive repopulation assays. However, hematopoietic "KLF4(-/-) chimeras" generated by transplantation of KLF4(-/-) fetal livers cells into lethally irradiated wild-type mice completely lacked circulating inflammatory (CD115(+)Gr1(+)) monocytes, and had reduced numbers of resident (CD115(+)Gr1(-)) monocytes. Although the numbers and function of peritoneal macrophages were normal in KLF4(-/-) chimeras, bone marrow monocytic cells from KLF4(-/-) chimeras expressed lower levels of key trafficking molecules and were more apoptotic. Thus, our in vivo loss-of-function studies demonstrate that KLF4, previously shown to mediate proinflammatory signaling in human macrophages in vitro, is essential for differentiation of mouse inflammatory monocytes, and is involved in the differentiation of resident monocytes. In addition, inducible expression of KLF4 in the HL60 human acute myeloid leukemia cell line stimulated monocytic differentiation and enhanced 12-O-tetradecanoylphorbol 13-acetate induced macrophage differentiation, but blocked all-trans-retinoic acid induced granulocytic differentiation of HL60 cells. The inflammation-selective effects of loss-of-KLF4 and the gain-of-KLF4-induced monocytic differentiation in HL60 cells identify KLF4 as a key regulator of monocytic differentiation and a potential target for translational immune modulation.

Targeted disruption of the basic Krüppel-like factor gene (Klf3) reveals a role in adipogenesis

Krüppel-like factors (KLFs) recognize CACCC and GC-rich sequences in gene regulatory elements. Here, we describe the disruption of the murine basic Krüppel-like factor gene (Bklf or Klf3). Klf3 knockout mice have less white adipose tissue, and their fat pads contain smaller and fewer cells. Adipocyte differentiation is altered in murine embryonic fibroblasts from Klf3 knockouts. Klf3 expression was studied in the 3T3-L1 cellular system. Adipocyte differentiation is accompanied by a decline in Klf3 expression, and forced overexpression of Klf3 blocks 3T3-L1 differentiation. Klf3 represses transcription by recruiting C-terminal binding protein (CtBP) corepressors. CtBPs bind NADH and may function as metabolic sensors. A Klf3 mutant that does not bind CtBP cannot block adipogenesis. Other KLFs, Klf2, Klf5, and Klf15, also regulate adipogenesis, and functional CACCC elements occur in key adipogenic genes, including in the C/ebpalpha promoter. We find that C/ebpalpha is derepressed in Klf3 and Ctbp knockout fibroblasts and adipocytes from Klf3 knockout mice. Chromatin immunoprecipitations confirm that Klf3 binds the C/ebpalpha promoter in vivo. These results implicate Klf3 and CtBP in controlling adipogenesis.

Erythroid defects in TRα−/− mice

Thyroid hormone and its cognate receptor (TR) have been implicated in the production of red blood cells. Here, we show mice deficient for TRα have compromised fetal and adult erythropoiesis. Erythroid progenitor numbers were significantly reduced in TRα−/− fetal livers, and transit through the final stages of maturation was impeded. In addition, immortalized TRα−/− erythroblasts displayed increased apoptosis and reduced capacity for proliferation and differentiation. Adult TRα−/− mice had lower hematocrit levels, elevated glucocorticoid levels, and an altered stress erythropoiesis response to hemolytic anemia. Most TRα−/− animals contained markedly altered progenitor numbers in their spleens. Strikingly, 20% of TRα−/− mice failed to elicit a stress erythropoiesis response and recovered very poorly from hemolytic anemia. We conclude that an underlying erythroid defect exists in TRα−/− mice, demon-strating the importance of TRα to the erythroid compartment.

Transcriptional control of erythropoiesis: emerging mechanisms and principles

Transcriptional networks orchestrate fundamental biological processes, including hematopoiesis, in which hematopoietic stem cells progressively differentiate into specific progenitors cells, which in turn give rise to the diverse blood cell types. Whereas transcription factors recruit coregulators to chromatin, leading to targeted chromatin modification and recruitment of the transcriptional machinery, many questions remain unanswered regarding the underlying molecular mechanisms. Furthermore, how diverse cell type-specific transcription factors function cooperatively or antagonistically in distinct cellular contexts is poorly understood, especially since genes in higher eukaryotes commonly encompass broad chromosomal regions (100 kb and more) and are littered with dispersed regulatory sequences. In this article, we describe an important set of transcription factors and coregulators that control erythropoiesis and highlight emerging transcriptional mechanisms and principles. It is not our intent to comprehensively survey all factors implicated in the transcriptional control of erythropoiesis, but rather to underscore specific mechanisms, which have potential to be broadly relevant to transcriptional control in diverse systems.

A global role for zebrafish klf4 in embryonic erythropoiesis

There are two waves of erythropoiesis, known as primitive and definitive waves in mammals and lower vertebrates including zebrafish. The founding member of the Kruppel-like factor (KLF) family of CACCC-box binding proteins, EKLF/Klf1, is essential for definitive erythropoiesis in mammals but only plays a minor role in primitive erythropoiesis. Morpholino knockdown experiments have shown a role for zebrafish klf4 in primitive erythropoiesis and hatching gland formation. In order to generate a global understanding of how klf4 might influence gene expression and differentiation, we have performed expression profiling of klf4 morphants, and then performed validation of many putative target genes by qRT-PCR and whole mount in situ hybridization. We found a critical role for klf4 in embryonic globin, heme synthesis and hatching gland gene expression. In contrast, there was an increase in expression of definitive hematopoietic specific genes such as larval globin genes, runx1 and c-myb from 24 hpf, suggesting a selective role for klf4 in primitive rather than definitive erythropoiesis. In addition, we show klf4 preferentially binds CACCC box elements in the primitive zebrafish beta-like globin gene promoters. These results have global implications for primitive erythroid gene regulation by KLF-CACCC box interactions.

Novel role for EKLF in megakaryocyte lineage commitment

Megakaryocytes and erythroid cells are thought to derive from a common progenitor during hematopoietic differentiation. Although a number of transcriptional regulators are important for this process, they do not explain the bipotential result. We now show by gain- and loss-of-function studies that erythroid Krüppel-like factor (EKLF), a transcription factor whose role in erythroid gene regulation is well established, plays an unexpected directive role in the megakaryocyte lineage. EKLF inhibits the formation of megakaryocytes while at the same time stimulating erythroid differentiation. Quantitative examination of expression during hematopoiesis shows that, unlike genes whose presence is required for establishment of both lineages, EKLF is uniquely down-regulated in megakaryocytes after formation of the megakaryocyte-erythroid progenitor. Expression profiling and molecular analyses support these observations and suggest that megakaryocytic inhibition is achieved, at least in part, by EKLF repression of Fli-1 message levels.

EKLF and KLF2 have compensatory roles in embryonic beta-globin gene expression and primitive erythropoiesis

The Krüppel-like C2/H2 zinc finger transcription factors (KLFs) control development and differentiation. Erythroid Krüppel-like factor (EKLF or KLF1) regulates adult beta-globin gene expression and is necessary for normal definitive erythropoiesis. KLF2 is required for normal embryonic Ey- and betah1-, but not adult betaglobin, gene expression in mice. Both EKLF and KLF2 play roles in primitive erythroid cell development. To investigate potential interactions between these genes, EKLF/KLF2 double-mutant embryos were analyzed. EKLF(-/-)KLF2(-/-) mice appear anemic at embryonic day 10.5 (E10.5) and die before E11.5, whereas single-knockout EKLF(-/-) or KLF2(-/-) embryos are grossly normal at E10.5 and die later than EKLF(-/-)KLF2(-/-) embryos. At E10.5, Ey- and betah1-globin mRNA is greatly reduced in EKLF(-/-)KLF2(-/-), compared with EKLF(-/-) or KLF2(-/-) embryos, consistent with the observed anemia. Light and electron microscopic analyses of E9.5 EKLF(-/-)KLF2(-/-) yolk sacs, and cytospins, indicate that erythroid and endothelial cells are morphologically more abnormal than in either single knockout. EKLF(-/-)KLF2(-/-) erythroid cells are markedly irregularly shaped, suggesting membrane abnormalities. EKLF and KLF2 may have coordinate roles in a common progenitor to erythroid and endothelial cells. The data indicate that EKLF and KLF2 have redundant functions in embryonic beta-like globin gene expression, primitive erythropoiesis, and endothelial development.

Evidence for linkage of red blood cell size and count: genome-wide scans in the Framingham Heart Study

Red blood cell (RBC) count and size are major criteria for evaluating anemia and related hematology disease diagnoses. While environmental factors influence RBC count (RBCC) and size, mean corpuscular volume (MCV), and mean corpuscular hemoglobin (MCH), studies have indicated that each of these measures has a substantial genetic component. So far, no linkage analysis or genome scan has been reported. We carried out 10 cM genome-wide scans on RBCC, MCV, and MCH in a community-based Caucasian cohort, the Framingham Heart Study, using 325 pedigrees with 1,144 individuals genotyped and phenotyped. Using variance-component linkage methods, heritabilities were estimated as 56, 52, and 52% after covariate adjusted for RBCC, MCV, and MCH, respectively. For RBCC, we found a maximum LOD score of 3.2 on chromosome 19, 24 cM (7.0 Mbp). Near this region, there lie a few important candidate genes, including erythropoietin receptor and erythroid Krüppel-like factor. For linkage analyses for MCV and MCH, there were coinciding maximized LOD scores on chromosome 11, 9 cM (5.2 Mbp) with values of 3.8 and 3.6, respectively. Under the peak resides the hemoglobin beta cluster - several beta-like genes, which are important candidates for RBC size. In subsequent analyses, we excluded individuals with low MCV to assess the possible influence of beta-thalassemia carriers, and there continued to be evidence for linkage in the same region on chromosome 11p15 (LOD scores of 2.6 and 2.7 for MCV and MCH, respectively). For MCV, we also identified a new region on chromosome 6q24 with a LOD score of 2.9. These findings suggest that further studies are warranted to identify potential causal genetic variants for RBC size and count and related erythrocyte indices.

Erythroid Kruppel-like Factor Regulates the G1 Cyclin Dependent Kinase Inhibitor p18INK4c

Erythroid Kruppel-like factor (EKLF, KLF1) is an essential erythroid cell specific C(2)H(2) zinc finger transcription factor that binds CACC box elements in promoters and distant regulatory elements to activate transcription. Forced expression of EKLF arrests cell division. The cyclin dependent kinase (Cdk) inhibitor p18(INK4c) was identified as a potential novel EKLF target gene from an expression profiling study. The p18(INK4c) protein functions as an inhibitor of Cdk4 and Cdk6 activity during early G1 phase of the cell cycle, thus acting as a physiological brake on cell division. We confirmed p18(INK4c) is downregulated in EKLF null mice by real-time PCR and Western blotting, and identified three closely associated and highly conserved EKLF binding sites (CCNCNCCCN) approximately 1 kb upstream of the p18(INK4c) transcriptional start site. We showed that EKLF binds to one of these elements by gel shift assay and demonstrated this site is capable of driving EKLF dependent transcription. We also determined by chromatin immunoprecipitation (ChIP) that this region of the p18(INK4c) promoter is bound by EKLF in erythroid cells. Thus, EKLF is a direct regulator of p18(INK4c) gene expression, and much of EKLF's role in driving erythroid cell differentiation may occur via p18(INK4c).

Erythroid Krüppel-like factor directly activates the basic Krüppel-like factor gene in erythroid cells

The Sp/Krüppel-like factor (Sp/Klf) family is comprised of around 25 zinc finger transcription factors that recognize CACCC boxes and GC-rich elements. We have investigated basic Krüppel-like factor (Bklf/Klf3) and show that in erythroid tissues its expression is highly dependent on another family member, erythroid Krüppel-like factor (Eklf/Klf1). We observe that Bklf mRNA is significantly reduced in erythroid tissues from Eklf-null murine embryos. We find that Bklf is driven primarily by two promoters, a ubiquitously active GC-rich upstream promoter, 1a, and an erythroid downstream promoter, 1b. Transcripts from the two promoters encode identical proteins. Interestingly, both the ubiquitous and the erythroid promoter are dependent on Eklf in erythroid cells. Eklf also activates both promoters in transient assays. Experiments utilizing an inducible form of Eklf demonstrate activation of the endogenous Bklf gene in the presence of an inhibitor of protein synthesis. The kinetics of activation are also consistent with Bklf being a direct Eklf target. Chromatin immunoprecipitation assays confirm that Eklf associates with both Bklf promoters. Eklf is typically an activator of transcription, whereas Bklf is noted as a repressor. Our results support the hypothesis that feedback cross-regulation occurs within the Sp/Klf family in vivo.

Genomic organisation and regulation of murine alpha haemoglobin stabilising protein by erythroid Kruppel-like factor

Alpha haemoglobin stabilising protein (AHSP) binds free alpha-globin chains and plays an important role in the protection of red cells, particularly during beta-thalassaemia. Murine ASHP was discovered as a GATA-1 target gene and human AHSP is directly regulated by GATA-1. More recently, AHSP was rediscovered as a highly erythroid Kruppel-like factor (EKLF) -dependent transcript. We have determined the organisation of the murine AHSP gene and compared it to orthologs. There are two CACC box elements in the proximal promoter. The proximal element is absolutely conserved, but does not bind EKLF as it is not a canonical binding site. In rodents, the distal element contains a 3 bp insertion that disrupts the typical EKLF binding consensus region. Nevertheless, EKLF binds this atypical site by gel mobility shift assay, specifically occupies the AHSP promoter in vivo in a chromatin immunoprecipitation assay, and transactivates AHSP through this CACC site in promoter-reporter assays. Our results suggest EKLF can occupy CACC elements in vivo that are not predictable from the consensus binding site inferred from structural studies. We also propose that absence of AHSP in EKLF-null red cells exacerbates the toxicity of free alpha-globin chains, which exist because of the defect in beta-globin gene activation.

Major erythrocyte membrane protein genes in EKLF-deficient mice

OBJECTIVES

Mice deficient in the transcription factor erythroid Krüppel-like factor, KLF1 (EKLF) die approximately 14.5 days postcoitum of anemia, attributed to decreased expression of the beta-globin gene. The objectives of this study were to rescue EKLF-deficient embryos with mice expressing gamma-globin from beta-spectrin or ankyrin promoters and to characterize expression of the major erythrocyte membrane genes in EKLF-deficient cells.

METHODS

Transgenic beta-spectrin/gamma-globin or ankyrin/gamma-globin mice were bred onto EKLF-deficient and wild-type backgrounds. Animals were genotyped, gamma-globin mRNA levels measured, and hemoglobin electrophoresis performed. Steady-state mRNA levels and transcriptional rates of the major erythrocyte membrane protein genes were assayed.

RESULTS

beta-spectrin/gamma-globin or ankyrin/gamma-globin mice on EKLF-deficient and wild-type backgrounds had identical levels of gamma-globin mRNA, indicating EKLF-independence of these promoters. gamma-Globin expression improved globin chain imbalance, but hemolysis was not improved and no live-born EKLF-deficient/(A)gamma-globin mice were obtained. Circulating erythroid cells from EKLF-deficient/(A)gamma-globin embryos exhibited hemolysis reminiscent of that seen in patients with severe erythrocyte membrane defects. Levels of beta-spectrin, ankyrin, and band 3 mRNA, but not alpha-spectrin, were decreased in EKLF-deficient fetal liver RNA. In a run-on assay, levels of transcription of the ankyrin and band 3 genes were decreased in EKLF-deficient fetal liver nuclei.

CONCLUSIONS

These results indicate that the EKLF-responsive regions of the ankyrin and beta-spectrin genes are outside their promoters and that EKLF is necessary for full transcriptional activity of the ankyrin and band 3 genes; the results also provide additional evidence that defects in addition to beta-globin deficiency, including an abnormal erythrocyte membrane, contribute to the anemia and embryonic lethality in EKLF-deficient mice.

Comparison of gene expression profiles in mouse primary T cells under normal and prolonged activation

In order to investigate the global transcriptional change of mouse primary T cells after prolonged activation, we took advantage of a Mouse Genome 430 2.0 Array to assess and compare the overall gene expression profiles of mouse T cells after activated with anti-CD3/CD28 for 18 or 48 h. The results demonstrated that most activation-related genes were preferentially up-regulated in mouse primary T cells after stimulated for 18 h; some apoptotic genes, however, were also found to be moderately up-regulated simultaneously. After the activation of T cells for 48 h, lots of apoptosis-related genes were dramatically up-regulated, followed by the augmentation of activation-induced cell death. In general, the number of differentially expressed genes in T cells after activation over 48 h declined almost in half as compared to that of 18 h. Both microarray and cytokine content analyses revealed that Th1 cytokines, rather than Th2 cytokines, were specifically up-regulated in activated mouse primary T cells. The present study also identified a number of genes that were dramatically up or down-regulated in T cells activated for 48 h for the first time, although the exact functions of these proteins are not known. Our studies provide detailed information on genes expression profiles of mouse primary T cells after normal (18 h) and prolonged activation (48 h); these data may accelerate the understanding of the T cell activation process and offer clues to the therapy of immune diseases.

Melatonin enhances alkaline phosphatase activity in differentiating human adult mesenchymal stem cells grown in osteogenic medium via MT2 melatonin receptors and the MEK/ERK (1/2) signaling cascade

The goals of this study were to determine (a) if melatonin enhances human adult mesenchymal stem cell (hAMSC) differentiation into osteoblasts as assessed by measuring alkaline phosphatase (ALP) enzyme activity, and (b) identify potential signal transduction pathways that mediate this process. ALP activity significantly increased in hAMSCs following a 10-day incubation in osteogenic medium, relative to hAMSCs incubated in basal growth medium alone. Melatonin (50 nm), added in combination with the osteogenic medium, significantly increased ALP activity relative to osteogenic medium alone. Co-exposure of hAMSCs to osteogenic medium supplemented with melatonin and either pertussis toxin or the melatonin receptor antagonists, luzindole or 4P-PDOT (MT2 receptor selective), inhibited the melatonin-induced increase in ALP activity, indicating the involvement of melatonin receptors, in particular, MT2 receptors. Assessment of melatonin receptor function following exposure to osteogenic medium containing either vehicle or melatonin produced dichotomous results. That is, if the differentiation of hAMSCs into an osteoblast was induced by osteogenic medium alone, then 2-[125I]-iodomelatonin binding and melatonin receptor function increased. However, examination of melatonin receptor function following chronic melatonin exposure, an exposure that resulted in a 50% enhancement in ALP activity, revealed that these receptors were desensitized. This was reflected by a complete loss in specific 2-[125I]-iodomelatonin binding as well as melatonin efficacy to inhibit forskolin-induced cAMP accumulation. Further characterization of the mechanisms underlying melatonin's effects on these differentiation processes revealed that MEK (1/2) and ERK (1/2), epidermal growth factor receptors, metalloproteinase and clathrin-mediated endocytosis were essential while PKA was not. Our results are consistent with a role for melatonin in osteoblast differentiation. If so, then, the decrease in plasma melatonin levels observed in humans during late adulthood may further enhance susceptibility to osteoporosis.

Genetic regulatory networks programming hematopoietic stem cells and erythroid lineage specification

Erythroid cell production results from passage through cellular hierarchies dependent on differential gene expression under the control of transcription factors responsive to changing niches. We have constructed Genetic Regulatory Networks (GRNs) describing this process, based predominantly on mouse data. Regulatory network motifs identified in E. coli and yeast GRNs are found in combination in these GRNs. Feed-forward motifs with autoregulation generate forward momentum and also control its rate, which is at its lowest in hematopoietic stem cells (HSCs). The simultaneous requirement for multiple regulators in multi-input motifs (MIMs) provides tight control over expression of target genes. Combinations of MIMs, exemplified by the SCL/LMO2 complexes, which have variable content and binding sites, explain how individual regulators can have different targets in HSCs and erythroid cells and possibly also how HSCs maintain stem cell functions while expressing lineage-affiliated genes at low level, so-called multi-lineage priming. MIMs combined with cross-antagonism describe the relationship between PU.1 and GATA-1 and between two of their target genes, Fli-1 and EKLF, with victory for GATA-1 and EKLF leading to erythroid lineage specification. These GRNs are useful repositories for current regulatory information, are accessible in interactive form via the internet, enable the consequences of perturbation to be predicted, and can act as seed networks to organize the rapidly accumulating microarray data.