Ka Sin Mak

PU.1 and Haematopoietic Cell Fate: Dosage Matters

The ETS family transcription factor PU.1 is a key regulator of haematopoietic differentiation. Its expression is dynamically controlled throughout haematopoiesis in order to direct appropriate lineage specification. Elucidating the biological role of PU.1 has proved challenging. This paper will discuss how a range of experiments in cell lines and mutant and transgenic mouse models have enhanced our knowledge of the mechanisms by which PU.1 drives lineage-specific differentiation during haematopoiesis.

Impaired B cell development in the absence of Krüppel-like factor 3

Krüppel-like factor 3 (Klf3) is a member of the Klf family of transcription factors. Klfs are widely expressed and have diverse roles in development and differentiation. In this study, we examine the function of Klf3 in B cell development by studying B lymphopoiesis in a Klf3 knockout mouse model. We show that B cell differentiation is significantly impaired in the bone marrow, spleen, and peritoneal cavity of Klf3 null mice and confirm that the defects are cell autonomous. In the bone marrow, there is a reduction in immature B cells, whereas recirculating mature cells are noticeably increased. Immunohistology of the spleen reveals a poorly structured marginal zone (MZ) that may in part be caused by deregulation of adhesion molecules on MZ B cells. In the peritoneal cavity, there are significant defects in B1 B cell development. We also report that the loss of Klf3 in MZ B cells is associated with reduced BCR signaling strength and an impaired ability to respond to LPS stimulation. Finally, we show increased expression of a number of Klf genes in Klf3 null B cells, suggesting that a Klf regulatory network may exist in B cells.

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

ABSTRACT 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.

Regions outside the DNA-binding domain are critical for proper in vivo specificity of an archetypal zinc finger transcription factor.

Transcription factors (TFs) are often regarded as being composed of a DNA-binding domain (DBD) and a functional domain. The two domains are considered separable and autonomous, with the DBD directing the factor to its target genes and the functional domain imparting transcriptional regulation. We examined an archetypal zinc finger (ZF) TF, Krüppel-like factor 3 with an N-terminal domain that binds the corepressor CtBP and a DBD composed of three ZFs at its C-terminus. We established a system to compare the genomic occupancy profile of wild-type Krüppel-like factor 3 with two mutants affecting the N-terminal functional domain: a mutant unable to contact the cofactor CtBP and a mutant lacking the entire N-terminal domain, but retaining the ZFs intact. Chromatin immunoprecipitation followed by sequencing was used to assess binding across the genome in murine embryonic fibroblasts. Unexpectedly, we observe that mutations in the N-terminal domain generally reduced binding, but there were also instances where binding was retained or even increased. These results provide a clear demonstration that the correct localization of TFs to their target genes is not solely dependent on their DNA-contact domains. This informs our understanding of how TFs operate and is of relevance to the design of artificial ZF proteins.

Generation of Mice Deficient in both KLF3/BKLF and KLF8 Reveals a Genetic Interaction and a Role for These Factors in Embryonic Globin Gene Silencing

ABSTRACT Krüppel-like factors 3 and 8 (KLF3 and KLF8) are highly related transcriptional regulators that bind to similar sequences of DNA. We have previously shown that in erythroid cells there is a regulatory hierarchy within the KLF family, whereby KLF1 drives the expression of both the Klf3 and Klf8 genes and KLF3 in turn represses Klf8 expression. While the erythroid roles of KLF1 and KLF3 have been explored, the contribution of KLF8 to this regulatory network has been unknown. To investigate this, we have generated a mouse model with disrupted KLF8 expression. While these mice are viable, albeit with a reduced life span, mice lacking both KLF3 and KLF8 die at around embryonic day 14.5 (E14.5), indicative of a genetic interaction between these two factors. In the fetal liver, Klf3 Klf8 double mutant embryos exhibit greater dysregulation of gene expression than either of the two single mutants. In particular, we observe derepression of embryonic, but not adult, globin expression. Taken together, these results suggest that KLF3 and KLF8 have overlapping roles in vivo and participate in the silencing of embryonic globin expression during development.

Loss of Krüppel-Like Factor 3 (KLF3/BKLF) Leads to Upregulation of the Insulin-Sensitizing Factor Adipolin (FAM132A/CTRP12/C1qdc2)

Krüppel-like factor 3 (KLF3) is a transcriptional regulator that we have shown to be involved in the regulation of adipogenesis in vitro. Here, we report that KLF3-null mice are lean and protected from diet-induced obesity and glucose intolerance. On a chow diet, plasma levels of leptin are decreased, and adiponectin is increased. Despite significant reductions in body weight and adiposity, wild-type and knockout animals show equivalent energy intake, expenditure, and excretion. To investigate the molecular events underlying these observations, we used microarray analysis to compare gene expression in Klf3+/+ and Klf3−/− tissues. We found that mRNA expression of Fam132a, which encodes a newly identified insulin-sensitizing adipokine, adipolin, is significantly upregulated in the absence of KLF3. We confirmed that KLF3 binds the Fam132a promoter in vitro and in vivo and that this leads to repression of promoter activity. Further, plasma adipolin levels were significantly increased in Klf3−/− mice compared with wild-type littermates. Boosting levels of adipolin via targeting of KLF3 offers a novel potential therapeutic strategy for the treatment of insulin resistance.

Changes in glycogen structure over feeding cycle sheds new light on blood-glucose control

Liver glycogen, a highly branched polymer of glucose, is important for maintaining blood-glucose homeostasis. It was recently shown that db/db mice, a model for Type 2 diabetes, are unable to form the large composite glycogen α particles present in normal, healthy mice. In this study, the structure of healthy mouse-liver glycogen over the diurnal cycle was characterized using size exclusion chromatography and transmission electron microscopy. Glycogen was found to be formed as smaller β particles, and then only assembled into large α particles, with a broad size distribution, significantly after the time when glycogen content had reached a maximum. This pathway, missing in diabetic animals, is likely to give optimal blood-glucose control during the daily feeding cycle. Lack of this control may contribute to, or result from, diabetes. This discovery suggests novel approaches to diabetes management.

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.

Differential regulation of the α-globin locus by Krüppel-like Factor 3 in erythroid and non-erythroid cells.

BackgroundKrüppel-like Factor 3 (KLF3) is a broadly expressed zinc-finger transcriptional repressor with diverse biological roles. During erythropoiesis, KLF3 acts as a feedback repressor of a set of genes that are activated by Krüppel-like Factor 1 (KLF1). Noting that KLF1 binds α-globin gene regulatory sequences during erythroid maturation, we sought to determine whether KLF3 also interacts with the α-globin locus to regulate transcription.ResultsWe found that expression of a human transgenic α-globin reporter gene is markedly up-regulated in fetal and adult erythroid cells of Klf3−/− mice. Inspection of the mouse and human α-globin promoters revealed a number of canonical KLF-binding sites, and indeed, KLF3 was shown to bind to these regions both in vitro and in vivo. Despite these observations, we did not detect an increase in endogenous murine α-globin expression in Klf3−/− erythroid tissue. However, examination of murine embryonic fibroblasts lacking KLF3 revealed significant de-repression of α-globin gene expression. This suggests that KLF3 may contribute to the silencing of the α-globin locus in non-erythroid tissue. Moreover, ChIP-Seq analysis of murine fibroblasts demonstrated that across the locus, KLF3 does not occupy the promoter regions of the α-globin genes in these cells, but rather, binds to upstream, DNase hypersensitive regulatory regions.ConclusionsThese findings reveal that the occupancy profile of KLF3 at the α-globin locus differs in erythroid and non-erythroid cells. In erythroid cells, KLF3 primarily binds to the promoters of the adult α-globin genes, but appears dispensable for normal transcriptional regulation. In non-erythroid cells, KLF3 distinctly binds to the HS-12 and HS-26 elements and plays a non-redundant, albeit modest, role in the silencing of α-globin expression.

Phosphorylation of Krüppel-like Factor 3 (KLF3/BKLF) and C-terminal Binding Protein 2 (CtBP2) by Homeodomain-interacting Protein Kinase 2 (HIPK2) Modulates KLF3 DNA Binding and Activity

Background: Krüppel-like factor 3 (KLF3) is a transcriptional repressor with multiple biological roles. Results: Phosphorylation of KLF3 by homeodomain-interacting protein kinase 2 (HIPK2) promotes DNA binding and transcriptional repression. Conclusion: Signal transduction pathways mediated by HIPK2 control and direct KLF3 activity. Significance: Determining the pathways that control KLF3 function offers potential for regulating its activity for therapeutic benefit. Krüppel-like factor 3 (KLF3/BKLF), a member of the Krüppel-like factor (KLF) family of transcription factors, is a widely expressed transcriptional repressor with diverse biological roles. Although there is considerable understanding of the molecular mechanisms that allow KLF3 to silence the activity of its target genes, less is known about the signal transduction pathways and post-translational modifications that modulate KLF3 activity in response to physiological stimuli. We observed that KLF3 is modified in a range of different tissues and found that the serine/threonine kinase homeodomain-interacting protein kinase 2 (HIPK2) can both bind and phosphorylate KLF3. Mass spectrometry identified serine 249 as the primary phosphorylation site. Mutation of this site reduces the ability of KLF3 to bind DNA and repress transcription. Furthermore, we also determined that HIPK2 can phosphorylate the KLF3 co-repressor C-terminal binding protein 2 (CtBP2) at serine 428. Finally, we found that phosphorylation of KLF3 and CtBP2 by HIPK2 strengthens the interaction between these two factors and increases transcriptional repression by KLF3. Taken together, our results indicate that HIPK2 potentiates the activity of KLF3.