Protein arginine methyltransferase 6 controls erythroid gene expression and differentiation of human CD34+ progenitor cells

Malat1 fine-tunes bone homeostasis by orchestrating cellular crosstalk and the β-catenin-OPG/Jagged1 pathway

The IncRNA Malat1 was initially believed to be dispensable for physiology due to the lack of observable phenotypes in Malat1 knockout (KO) mice. However, our study challenges this conclusion. We found that both Malat1 KO and conditional KO mice in the osteoblast lineage exhibit significant osteoporosis. Mechanistically, Malat1 acts as an intrinsic regulator in osteoblasts to promote osteogenesis. Interestingly, Malat1 does not directly affect osteoclastogenesis but inhibits osteoclastogenesis in a non-autonomous manner in vivo via integrating crosstalk between multiple cell types, including osteoblasts, osteoclasts and chondrocytes. Our findings substantiate the existence of a novel remodeling network in which Malat1 serves as a central regulator by binding to β-catenin and functioning through the β-catenin-OPG/Jagged1 pathway in osteoblasts and chondrocytes. In pathological conditions, Malat1 significantly promotes bone regeneration in fracture healing. Bone homeostasis and regeneration are crucial to well-being. Our discoveries establish a previous unrecognized paradigm model of Malat1 function in the skeletal system, providing novel mechanistic insights into how a lncRNA integrates cellular crosstalk and molecular networks to fine tune tissue homeostasis, remodeling and repair.

Development of Complementary Photo-arginine/lysine to Promote Discovery of Arg/Lys hPTMs Interactomes

Arginine and lysine, frequently appearing as a pair on histones, have been proven to carry diverse modifications and execute various epigenetic regulatory functions. However, the most context-specific and transient effectors of these marks, while significant, have evaded study as detection methods have thus far not reached a standard to capture these ephemeral events. Herein, a pair of complementary photo-arginine/δ-photo-lysine (R-dz/K-dz) probes is developed and involve these into histone peptide, nucleosome, and chromatin substrates to capture and explore the interactomes of Arg and Lys hPTMs. By means of these developed tools, this study identifies that H3R2me2a can recruit MutS protein homolog 6 (MSH6), otherwise repelDouble PHD fingers 2 (DPF2), Retinoblastoma binding protein 4/7 (RBBP4/7). And it is disclosed that H3R2me2a inhibits the chromatin remodeling activity of the cBAF complex by blocking the interaction between DPF2 (one component of cBAF) and the nucleosome. In addition, the novel pairs of H4K5 PTMs and respective readers are highlighted, namely H4K5me-Lethal(3)malignant brain tumor-like protein 2 (L3MBTL2), H4K5me2-L3MBTL2, and H4K5acK8ac-YEATS domain-containing protein 4 (YEATS4). These powerful tools pave the way for future investigation of related epigenetic mechanisms including but not limited to hPTMs.

Altered erythropoiesis via JAK2 and ASXL1 mutations in myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are driven by hyperactivation of JAK-STAT signaling but can demonstrate skewed hematopoiesis upon acquisition of additional somatic mutations. Here, using primary MPN samples and engineered embryonic stem cells, we demonstrate that mutations in JAK2 induced a significant increase in erythroid colony formation, whereas mutations in additional sex combs-like 1 (ASXL1) led to an erythroid colony defect. RNA-sequencing revealed upregulation of protein arginine methyltransferase 6 (PRMT6) induced by mutant ASXL1. Furthermore, genetic perturbation of PRMT6 exacerbated the MPN disease burden, including leukemic engraftment and splenomegaly, in patient-derived xenograft models, highlighting a novel tumor-suppressive function of PRMT6. However, augmented erythroid potential and bone marrow human CD71+ cells following PRMT6 knockdown were reserved only for primary MPN samples harboring ASXL1 mutations. Last, treatment of CD34+ hematopoietic/stem progenitor cells with the PRMT6 inhibitor EPZ020411 induced expression of genes involved in heme metabolism, hemoglobin, and erythropoiesis. These findings highlight interactions between JAK2 and ASXL1 mutations and a unique erythroid regulatory network in the context of mutant ASXL1.

Prmt6 represses the pro-adipogenic Ppar-gamma-C/ebp-alpha transcription factor loop

The feed-forward loop between the transcription factors Ppar-gamma and C/ebp-alpha is critical for lineage commitment during adipocytic differentiation. Ppar-gamma interacts with epigenetic cofactors to activate C/ebp-alpha and the downstream adipocytic gene expression program. Therefore, knowledge of the epigenetic cofactors associated with Ppar-gamma, is central to understanding adipocyte differentiation in normal differentiation and disease. We found that Prmt6 is present with Ppar-gamma on the Ppar-gamma and C/ebp-alpha promoter. It contributes to the repression of C/ebp-alpha expression, in part through its ability to induce H3R2me2a. During adipocyte differentiation, Prmt6 expression is reduced and the methyltransferase leaves the promoters. As a result, the expression of Ppar-gamma and C/ebp-alpha is upregulated and the adipocytic gene expression program is established. Inhibition of Prmt6 by a small molecule enhances adipogenesis, opening up the possibility of epigenetic manipulation of differentiation. Our data provide detailed information on the molecular mechanism controlling the Ppar-gamma-C/ebp-alpha feed-forward loop. Thus, they advance our understanding of adipogenesis in normal and aberrant adipogenesis.

Protein Arginine Methyltransferases as Therapeutic Targets in Hematological Malignancies

Arginine methylation is a common post-translational modification affecting protein activity and the transcription of target genes when methylation occurs on histone tails. There are nine protein arginine methyltransferases (PRMTs) in mammals, divided into subgroups depending on the methylation they form on a molecule of arginine. During the formation and maturation of the different types of blood cells, PRMTs play a central role by controlling cell differentiation at the transcriptional level. PRMT enzymatic activity is necessary for many cellular processes in hematological malignancies, such as the activation of cell cycle and proliferation, inhibition of apoptosis, DNA repair processes, RNA splicing, and transcription by methylating histone tails' arginine. Chemical tools have been developed to inhibit the activity of PRMTs and have been tested in several models of hematological malignancies, including primary samples from patients, xenografts into immunodeficient mice, mouse models, and human cell lines. They show a significant effect by reducing cell viability and increasing the overall survival of mice. PRMT5 inhibitors have a strong therapeutic potential, as phase I clinical trials in hematological malignancies that use these molecules show promising results, thus, underlining PRMT inhibitors as useful therapeutic tools for cancer treatment in the future.

Structure, Activity and Function of the Protein Arginine Methyltransferase 6

Members of the protein arginine methyltransferase (PRMT) family methylate the arginine residue(s) of several proteins and regulate a broad spectrum of cellular functions. Protein arginine methyltransferase 6 (PRMT6) is a type I PRMT that asymmetrically dimethylates the arginine residues of numerous substrate proteins. PRMT6 introduces asymmetric dimethylation modification in the histone 3 at arginine 2 (H3R2me2a) and facilitates epigenetic regulation of global gene expression. In addition to histones, PRMT6 methylates a wide range of cellular proteins and regulates their functions. Here, we discuss (i) the biochemical aspects of enzyme kinetics, (ii) the structural features of PRMT6 and (iii) the diverse functional outcomes of PRMT6 mediated arginine methylation. Finally, we highlight how dysregulation of PRMT6 is implicated in various types of cancers and response to viral infections.

PRMT6 activates cyclin D1 expression in conjunction with the transcription factor LEF1

The establishment of cell type specific gene expression by transcription factors and their epigenetic cofactors is central for cell fate decisions. Protein arginine methyltransferase 6 (PRMT6) is an epigenetic regulator of gene expression mainly through methylating arginines at histone H3. This way it influences cellular differentiation and proliferation. PRMT6 lacks DNA-binding capability but is recruited by transcription factors to regulate gene expression. However, currently only a limited number of transcription factors have been identified, which facilitate recruitment of PRMT6 to key cell cycle related target genes. Here, we show that LEF1 contributes to the recruitment of PRMT6 to the central cell cycle regulator CCND1 (Cyclin D1). We identified LEF1 as an interaction partner of PRMT6. Knockdown of LEF1 or PRMT6 reduces CCND1 expression. This is in line with our observation that knockdown of PRMT6 increases the number of cells in G1 phase of the cell cycle and decreases proliferation. These results improve the understanding of PRMT6 activity in cell cycle regulation. We expect that these insights will foster the rational development and usage of specific PRMT6 inhibitors for cancer therapy.

'Enhancing' red cell fate through epigenetic mechanisms

PURPOSE OF REVIEW

Transcription of erythroid-specific genes is regulated by the three-dimensional (3D) structure and composition of chromatin, which dynamically changes during erythroid differentiation. Chromatin organization and dynamics are regulated by several epigenetic mechanisms involving DNA (de-)methylation, posttranslational modifications (PTMs) of histones, chromatin-associated structural proteins, and higher-order structural changes and interactions. This review addresses examples of recent developments in several areas delineating the interface of chromatin regulation and erythroid-specific lineage transcription.

RECENT FINDINGS

We survey and discuss recent studies that focus on the erythroid chromatin landscape, erythroid enhancer-promotor interactions, super-enhancer functionality, the role of chromatin modifiers and epigenetic crosstalk, as well as the progress in mapping red blood cell (RBC) trait-associated genetic variants within cis-regulatory elements (CREs) identified in genome-wide association study (GWAS) efforts as a step toward determining their impact on erythroid-specific gene expression.

SUMMARY

As one of the best characterized and accessible cell differentiation systems, erythropoiesis has been at the forefront of studies aiming to conceptualize how chromatin dynamics regulate transcription. New emerging technologies that bring a significantly enhanced spatial and temporal resolution of chromatin structure, and allow investigation of small cell numbers, have advanced our understanding of chromatin dynamics during erythroid differentiation in vivo.

Epigenetic modifiers in normal and aberrent erythropoeisis

Erythroid differentiation program is comprised of lineage commitment, erythroid progenitor proliferation, and termination differentiation. Each stage of the differentiation program is heavily influenced by epigenetic modifiers that alter the epigenome in a dynamic fashion influenced by cytokines/humeral factors and are amicable to target by drugs. The epigenetic modifiers can be classified as DNA modifiers (DNMT, TET), mRNA modifiers (RNA methylases and demethylases) and histone protein modifiers (methyltransferases, acetyltransferases, demethylases, and deacetylases). Here we describe mechanisms by which these epigenetic modifiers influence and guide erythroid-lineage differentiation during normal and malignant erythropoiesis and also benign diseases that arise from their altered structure or function.

Stress erythropoiesis in atherogenic mice

Bone marrow erythropoiesis is mainly homeostatic and a demand of oxygen in tissues activates stress erythropoiesis in the spleen. Here, we show an increase in the number of circulating erythrocytes in apolipoprotein E^-/- mice fed a Western high-fat diet, with similar number of circulating leukocytes and CD41⁺ events (platelets). Atherogenic conditions increase spleen erythropoiesis with no variations of this cell lineage in the bone marrow. Spleens from atherogenic mice show augmented number of late-stage erythroblasts and biased differentiation of progenitor cells towards the erythroid cell lineage, with an increase of CD71⁺CD41CD34^-CD117⁺Sca1^-Lin^- cells (erythroid-primed megakaryocyte-erythroid progenitors), which is consistent with the way in which atherogenesis modifies the expression of pro-erythroid and pro-megakaryocytic genes in megakaryocyte-erythroid progenitors. These data explain the transiently improved response to an acute severe hemolytic anemia insult found in atherogenic mice in comparison to control mice, as well as the higher burst-forming unit-erythroid and colony forming unit-erythroid capacity of splenocytes from atherogenic mice. In conclusion, our work demonstrates that, along with the well stablished enhancement of monocytosis during atherogenesis, stress erythropoiesis in apolipoprotein E^-/- mice fed a Western high fat diet results in increased numbers of circulating red blood cells.

K562 erythroleukemia line as a possible reticulocyte source to culture Plasmodium vivax and its surrogates

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miR-26a and miR-30a knockdowns promote differentiation in Fy-transduced K562 cell lines.

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miR-26a and miR-30a knockdowns promote enucleation in Fy-transduced K562 cell lines.

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Data denote an interplay in the mode of action of miR-26a and miR-30a in erythropoiesis.

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Plasmodium cynomolgi and P. knowlesi invade, albeit inefficiently, Fy-transduced K562 cells.

Establishing an in vitro “red blood cell matrix” that would allow uninterrupted access to a stable, homogeneous reticulocyte population would facilitate the establishment of continuous, long-term in vitro Plasmodium vivax blood stage cultures. In this study, we have explored the suitability of the erythroleukemia K562 cell line as a continuous source of such reticulocytes and have investigated regulatory factors behind the terminal differentiation (and enucleation, in particular) of this cell line that can be used to drive the reticulocyte production process. The Duffy blood group antigen receptor (Fy), essential for P. vivax invasion, was stably introduced into K562 cells by lentiviral gene transfer. miRNA-26a-5p and miRNA-30a-5p were downregulated to promote erythroid differentiation and enucleation, resulting in a tenfold increase in the production of reticulocytes after stimulation with an induction cocktail compared with controls. Our results suggest an interplay in the mechanisms of action of miRNA-26a-5p and miRNA-30a-5p, which makes it necessary to downregulate both miRNAs to achieve a stable enucleation rate and Fy receptor expression. In the context of establishing P. vivax-permissive, stable, and reproducible reticulocytes, a higher enucleation rate may be desirable, which may be achieved by the targeting of further regulatory mechanisms in Fy-K562 cells; promoting the shift in hemoglobin production from fetal to adult may also be necessary. Despite the fact that K562 erythroleukemia cell lines are of neoplastic origin, this cell line offers a versatile model system to research the regulatory mechanisms underlying erythropoiesis.

Targeting protein methylation: from chemical tools to precision medicines

The methylation of proteins is integral to the execution of many important biological functions, including cell signalling and transcriptional regulation. Protein methyltransferases (PMTs) are a large class of enzymes that carry out the addition of methyl marks to a broad range of substrates. PMTs are critical for normal cellular physiology and their dysregulation is frequently observed in human disease. As such, PMTs have emerged as promising therapeutic targets with several inhibitors now in clinical trials for oncology indications. The discovery of chemical inhibitors and antagonists of protein methylation signalling has also profoundly impacted our general understanding of PMT biology and pharmacology. In this review, we present general principles for drugging protein methyltransferases or their downstream effectors containing methyl-binding modules, as well as best-in-class examples of the compounds discovered and their impact both at the bench and in the clinic.

FUSE binding protein 1 (FUBP1) expression is upregulated by T-cell acute lymphocytic leukemia protein 1 (TAL1) and required for efficient erythroid differentiation

During erythropoiesis, haematopoietic stem cells (HSCs) differentiate in successive steps of commitment and specification to mature erythrocytes. This differentiation process is controlled by transcription factors that establish stage- and cell type-specific gene expression. In this study, we demonstrate that FUSE binding protein 1 (FUBP1), a transcriptional regulator important for HSC self-renewal and survival, is regulated by T-cell acute lymphocytic leukaemia 1 (TAL1) in erythroid progenitor cells. TAL1 directly activates the FUBP1 promoter, leading to increased FUBP1 expression during erythroid differentiation. The binding of TAL1 to the FUBP1 promoter is highly dependent on an intact GATA sequence in a combined E-box/GATA motif. We found that FUBP1 expression is required for efficient erythropoiesis, as FUBP1-deficient progenitor cells were limited in their potential of erythroid differentiation. Thus, the finding of an interconnection between GATA1/TAL1 and FUBP1 reveals a molecular mechanism that is part of the switch from progenitor- to erythrocyte-specific gene expression. In summary, we identified a TAL1/FUBP1 transcriptional relationship, whose physiological function in haematopoiesis is connected to proper erythropoiesis.