Bone Marrow Hematopoietic Compartment Research Articles

Abstract MP153: TET2 Regulates Type I Interferon Signaling in the Hematopoietic Response to Myocardial Infarction

Myocardial infarction (MI) elicits a rapid and vigorous reaction from the bone marrow hematopoietic compartment, inducing a massive efflux of myeloid first responders into the bloodstream. These cells traffic to the infarct, where they mediate cardiac remodeling and repair through inflammatory signaling and recruitment of additional immune cells to the injured myocardium. A hyperinflammatory myeloid compartment, as is produced by mutations in epigenetic regulator TET2 associated with clonal hematopoiesis, can thus drive adverse cardiac remodeling after MI and accelerate progression to heart failure. Whether loss of TET2 alters the transcriptional landscape of MI-induced myelopoiesis remains to be investigated in an unbiased fashion. Here, we performed single-cell RNA sequencing of >16,000 bone marrow myeloid cells isolated from wild-type and Tet2 -/- mice after MI to characterize the emergency hematopoietic response in the presence and absence of TET2. Our data capture distinct transitional states of myeloid lineage commitment and maturation, originating from myeloid progenitors and progressing along divergent granulocytic and monocytic differentiation trajectories. Additionally, we delineate a subpopulation of interferon (IFN)-activated myeloid progenitors, monocytes, and neutrophils characterized by the concerted upregulation of various Type I IFN-stimulated genes, and find the fraction of IFN-activated cells, as well as the degree of activation, to be markedly higher in Tet2 -/- mice. We have previously described activation of this pathway after MI in mice, and demonstrated cardioprotective effects of its genetic or pharmacological inhibition. Our findings reveal heightened activation of the antiviral Type I interferon response among bone marrow myeloid cells of Tet2 -/- mice during MI-induced emergency hematopoiesis. This highlights IFN signaling as a potential candidate driver of cardiovascular pathologies (including atherosclerosis, myocardial infarction, and heart failure) associated with TET2-mediated clonal hematopoiesis. Further studies are necessary to investigate whether Tet2 -/- mice exhibit enhanced response to blockade of Type I IFN signaling after MI, and to determine whether myeloid cells of TET2 -mutant humans are similarly activated.

Pivotal role of PDK1 in megakaryocyte cytoskeletal dynamics and polarization during platelet biogenesis

AbstractDuring thrombopoiesis, megakaryocytes (MKs) form proplatelets within the bone marrow (BM) and release platelets into BM sinusoids. Phosphoinositide-dependent protein kinase-1 (PDK1) is required for Ca2+-dependent platelet activation, but its role in MK development and regulation of platelet production remained elusive. The present study explored the role of PDK1 in the regulation of MK maturation and polarization during thrombopoiesis using a MK/platelet-specific knockout approach. Pdk1-deficient mice (Pdk1−/−) developed a significant macrothrombocytopenia as compared with wild-type mice (Pdk1fl/fl). Pdk1 deficiency further dramatically increased the number of MKs without sinusoidal contact within the BM hematopoietic compartment, resulting in a pronounced MK hyperplasia and a significantly increased extramedullary thrombopoiesis. Cultured Pdk1−/− BM-MKs showed impaired spreading on collagen, associated with an altered actin cytoskeleton structure with less filamentous actin (F-actin) and diminished podosome formation, whereas the tubulin cytoskeleton remained unaffected. This phenotype was associated with abrogated phosphorylation of p21-activated kinase (PAK) as well as its substrates LIM domain kinase and cofilin, supporting the hypothesis that the defective F-actin assembly results from increased cofilin activity in Pdk1-deficient MKs. Pdk1−/− BM-MKs developed increased ploidy and exhibited an abnormal ultrastructure with disrupted demarcation membrane system (DMS). Strikingly, Pdk1−/− BM-MKs displayed a pronounced defect in DMS polarization and produced significantly less proplatelets, indicating that PDK1 is critically required for proplatelet formation. In human MKs, genetic PDK1 knockdown resulted in increased maturity but reduced platelet-like particles formation. The present observations reveal a pivotal role of PDK1 in the regulation of MK cytoskeletal dynamics and polarization, proplatelet formation, and thrombopoiesis.

Hottip Lncrna Reinforces CTCF Defined Chromatin Boundaries and Drives Wnt Target Gene Expression in AML Leukemogenesis

We reported recently that HOXA locus associated lncRNA, HOTTIP, is highly expressed in AML patients carrying MLL rearrangement and NPM1c+ mutations. The expression of HOTTIP positively correlates with posterior HOXA gene expression and poor patient survival. We further demonstrated that HOTTIP acts as an epigenetic regulator to define oncogenic HOXA topologically associated domain (TAD) and drive HOXA associated leukemic transcription program. However, it remains unclear whether and how HOTTIP lncRNA is involved in remodeling leukemic genome to facilitate AML leukemogenesis. Here, we showed that HOTTIP regulates a fraction of CTCF binding sites (CBSs) in the AML genome by directly interacting with CTCF and its binding motifs. We carried out CTCF ChIP-seq and HOTTIP ChIRP (chromatin isolation by RNA purification)-seq comparing WT and HOTTIP knockout (KO) MOLM13 cells. KO of HOTTIP in MLL-rearranged MOLM13 AML cells specifically impaired CTCF binding sites that were co-occupied by HOTTIP lncRNA, whereas loss of HOTTIP did not affect global CTCF binding. These target genes include posterior HOXA genes and Wnt target genes such as C-MYC, EVI1, AXIN, and TWIST1. Furthermore, we found that HOTTIP interacts with its putative target sites by formation of DNA: RNA hybridization structure triple helix and R-loop in vivo and in vitro. We then carried out DRIP (DNA-RNA immunoprecipitation)-seq and DRIPc(DNA-RNA immunoprecipitation followed by cDNA conversion)-Seq, which utilize a sequence independent but structure-specific S9.6 antibody for DRIP to capture global R-loops, by comparing WT and HOTTIP KO MOLM13 cells. The obtained DRIP-seq and DRIPc-seq data were then incorporated and integrated with the HOTTIP ChIRP-seq and CTCF ChIP-seq data to explore global collaboration between R-loop and HOTTIP associated CTCF binding sites. We found that HOTTIP interacts with CTCF binding motif that defines the TADs and the promoters of the HOTTIP target genes by formation of R-loop or triple helix structure. Loss of HOTTIP disrupted the R-loop formation at promoters and enhancers of the HOTTIP target genes to inhibit their expression. In MLL-rearranged AML genome, in addition to the HOXA locus, CTCF forms leukemic specific TADs that protect aberrant Wnt target genes. Depletion of HOTTIP lncRNA impaired CTCF defined TADs in the Wnt target gene loci and reduced Wnt target gene expression. In contrast, overexpression of Hottip lncRNA (Hottip-Tg) in the mice bone marrow hematopoietic compartment perturbs hematopoietic stem cell (HSC) self-renewal and differentiation leading to AML like disease by reinforcing CTCF defined TADs, enhancing chromatin accessibility within TADs, and upregulating gene transcription in the Wnt target loci. Finally, when we treated HOTTIP expressed primary patient AML cells carrying MLL-rearrangement and their derived PDX mouse model with a canonical Wnt inhibitor, ICG-001, ICG-001 inhibited AML LSC self-renewal in in vitro by LTC-IC assays and in vivo leukemogenesis in the PDX mouse models with an aberrant HOTTIP lncRNA expression, but not in HOTTIP negative/low non-MLL AML samples. Thus, HOTTIP lncRNA and CTCF cooperate to specifically reinforce CTCF defined WNT target locus TADs and drive Wnt target gene expression in the HOTTIP expressed AML. Disclosures No relevant conflicts of interest to declare.

Activation of Hottip lncrna Perturbs HSC Function Leading to AML like Disease in Mice

Several HOX loci associated long noncoding RNAs (lncRNAs) have been shown to regulate transcription of HOX genes through influencing epigenetic landscape. Especially, the posterior HOXA domain associated lncRNA HOTTIP acts as an epigenetic regulator that recruits WDR5/MLL complex to coordinate active chromatin modifications and HOXA genes expression in the development of animal digits. Despite HOX genes, especially HOXA genes, are highly expressed in many acute myeloid leukemia (AML) patients, it remains largely unknown whether and how HOTTIP lncRNA regulates hematopoietic stem cell (HSC) function and contributes to leukemogenesis.We showed previously that disruption of the CTCF boundary located between HOXA7 and HOXA9 genes (CBS7/9) resulted in reduced lncRNA HOTTIP and HOXA genes expression in MLL rearranged AML suggesting that HOTTIP may play a role in ectopic expression of the posterior HOXA gene. We employed a pooled CRISPR-Cas9 KO library to specifically screen lncRNAs in four HOX gene loci and identify HOTTIP as acritical regulator in controlling oncogenic HOX chromatin signature and associated gene expression patterns in AML by collaborating with posterior HOXA chromatin boundary. HOTTIP is upregulated in AML patients with MLL-rearrangement or NPM1 mutation. AML patients with high HOTTIP expression exhibits a significantly shortened survival compared to low HOTTIP expressing patients. To test whether HOTTIP acts to coordinate posterior chromatin domain and HOXA genes activation in AML, we manipulated HOTTIP lncRNA expression levels in the MLL-AF9 rearranged MOLM13 by loss-of-function KO and gain-of function rescue, as well as carried out genome wide chromatin and transcriptomic analysis to intterrogate the role of HOTTIP in control of AML specific posterior HOXA chromatin domain. We found that knock-out of HOTTIP lncRNA led to a loss of active chromatin structure and invasion of repressive H3K27me3 mark over the posterior HOXA domain. HOTTIP KO attenuated progression of AML in the transplanted AML mouse model resembling the effect of CBS7/9 boundary disruption, while transcriptional activation of HOTTIP lncRNA in the CBS7/9 boundary-disrupted AML cells restored HOXA locus chromatin signature and gene expression as well as reversed the CBS7/9-mediated anti-leukemic effects.To further determine the role of HOTTIP lncRNA in regulating HSC function and leukemogenesis, we generated transgenic mice that expresses Hottip lncRNA under the control of the hematopoietic specific Vav1 enhancer and promoter. The Hottip transgenic (Tg) mice exhibited increased WBC and neutrophil counts and developed splenomegaly indicating that enforced expression of Hottip lncRNA resulted in perturbation of hematopoiesis. Furthermore, overexpression of Hottip lncRNA in mice bone marrow hematopoietic compartment strongly perturbed hematopoietic stem and progenitor cell (HSC/HPC) function by altering self-renewal and differentiation property of HSC/HPCs through affecting homeotic gene associated oncogenic transcription program. Approximately 20% of Hottip lncRNA transgenic mice developed abnormal hematopoietic phenotypes resembling AML-like disease. RNA-seq and ATAC-seq analysis indicated that overexpression of Hottip enhanced promoter chromatin accessibility and stimulates transcription of genes and pathways involved in HSC function and leukemogenesis, including WNT signaling, hematopoietic cell lineage, cell cycle, Hoxa9, Hoxa13, and Meis1, Runx1, and Twist1 genes. Thus, Hottip lncRNA overexpression acts as an oncogenic event to promote HSC self-renewal and HPC proliferation by reprograming leukemic associated chromatin signature and transcription programs. DisclosuresNo relevant conflicts of interest to declare.

Megakaryocytic Maturation in Response to Shear Flow Is Mediated by the Activator Protein 1 (AP-1) Transcription Factor via Mitogen-activated Protein Kinase (MAPK) Mechanotransduction

Megakaryocytes (MKs) are exposed to shear flow as they migrate from the bone marrow hematopoietic compartment into circulation to release pro/preplatelets into circulating blood. Shear forces promote DNA synthesis, polyploidization, and maturation in MKs, and platelet biogenesis. To investigate mechanisms underlying these MK responses to shear, we carried out transcriptional analysis on immature and mature stem cell-derived MKs exposed to physiological shear. In immature (day (d)9) MKs, shear exposure up-regulated genes related to growth and MK maturation, whereas in mature (d12) MKs, it up-regulated genes involved in apoptosis and intracellular transport. Following shear-flow exposure, six activator protein 1 (AP-1) transcripts (ATF4,JUNB,JUN,FOSB,FOS, andJUND) were up-regulated at d9 and two AP-1 proteins (JunD and c-Fos) were up-regulated both at d9 and d12. We show that mitogen-activated protein kinase (MAPK) signaling is linked to both the shear stress response and AP-1 up-regulation. c-Jun N-terminal kinase (JNK) phosphorylation increased significantly following shear stimulation, whereas JNK inhibition reduced shear-induced JunD expression. Although p38 phosphorylation did not increase following shear flow, its inhibition reduced shear-induced JunD and c-Fos expression. JNK inhibition reduced fibrinogen binding and P-selectin expression of d12 platelet-like particles (PLPs), whereas p38 inhibition reduced fibrinogen binding of d12 PLPs. AP-1 expression correlated with increased MK DNA synthesis and polyploidization, which might explain the observed impact of shear on MKs. To summarize, we show that MK exposure to shear forces results in JNK activation, AP-1 up-regulation, and downstream transcriptional changes that promote maturation of immature MKs and platelet biogenesis in mature MKs.

Single‐cell technologies are revolutionizing the approach to rare cells

In the last lustrum single-cell techniques such as single-cell quantitative PCR, RNA and DNA sequencing, and the state-of-the-art cytometry by time of flight (CyTOF) mass cytometer have allowed a detailed analysis of the sub-composition of different organs from the bone marrow hematopoietic compartment to the brain. These fine-grained analyses have highlighted the great heterogeneity within each cell compartment revealing previously unknown subpopulations of cells. In this review, we analyze how this fast technological evolution has improved our understanding of the biological processes with a particular focus on rare cells of the immune system.

Abstract 3058: Bone marrow hematopoietic stem cell niche activation and mobilization fosters the metastatic niche

Abstract Metastasis remains the main cause of mortality for patients with cancer. The process whereby a localized tumor becomes metastatic remains poorly understood. Metastasis was previously thought to be a late-stage, unidirectional migration of cells from the primary tumor to a distant site, due to acquired genetic mutations that conferred necessary metastatic attributes to the cell. However, current research demonstrates that dissemination is not a late phenomenon and although loss of metastasis suppressor genes or gain of acquired mutations can contribute to the metastatic phenotype, the cross-talk between the tumor and its microenvironment has proven to be a major driver of this process. We have found that recruited bone marrow-derived cells and changes in resident stromal cells form a microenvironment that is conducive to metastatic progression, termed the pre-metastatic niche. The recruited and resident cells that form this microenvironment are active areas of investigation. Because the bone marrow is the source for most of the recruited cell types in these distant tissue sites, we postulated that activation of the bone marrow's hematopoietic compartment plays a key role in metastatic progression. We sought to profile the changes that occur within the hematopoietic stem cell niche of the bone marrow and subsequent changes in hematopoiesis in the setting of tumor development. Within the bone marrow of tumor bearing mice we found increased proliferation of hematopoietic stem and progenitor cells. These cells are also increased in the circulation of both tumor bearing mice and patients with malignancy. Furthermore, we have determined that elevated levels of circulating hematopoietic progenitor cells can be predictive of metastatic risk, as those patients with the highest levels at diagnosis developed metastases. This biomarker may be useful in stratifying therapy for patients with localized disease who have the highest metastatic risk. Better understanding of the steps and individual factors required to establish a metastatic microenvironment can lead to novel adjuvant therapies to target this process. Citation Format: Amber J. Giles, Meera Murgai, Yorleny Vicioso, Steven Highfil, Crystal Mackall, Leonard Wexler, David Lyden, Rosandra Natasha Kaplan. Bone marrow hematopoietic stem cell niche activation and mobilization fosters the metastatic niche. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3058. doi:10.1158/1538-7445.AM2014-3058

The Role of Circulating Endothelial Progenitor Cells in Tumor Angiogenesis

There is great controversy over the origin and definition of murine endothelial progenitor cells (EPCs). EPCs are reportedly important for the repair and remodeling of the vasculature and are implicated in tumor angiogenesis. Many conflicting reports exist as to whether these EPCs arise from the bone marrow hematopoietic compartment or whether they are non-hematopoietic in origin, and these differences could be attributed to the wide variance in assays used to identify the cells and time points at which the data are collected. Recently, circulating murine EPCs have been characterized as CD45(-)CD13(+)CD117(+)FLK-1(+) expressing cells via flow cytometry and this phenotype varies from prior descriptions. This review will focus on the changing phenotypic definition of murine circulating EPCs and the evidence that has been published in support of the lineage of origin of circulating EPCs and the role EPCs play in tumor angiogenesis in the adult mouse.

Hypoxia Preconditioned Mesenchymal Stem Cells Improve Vascular and Skeletal Muscle Fiber Regeneration After Ischemia Through a Wnt4-dependent Pathway

Mesenchymal stem cells (MSC) are multipotent postnatal stem cells, involved in the treatment of ischemic vascular diseases. We investigate the ability of MSC, exposed to short-term hypoxic conditions, to participate in vascular and tissue regeneration in an in vivo model of hindlimb ischemia. Transplantation of hypoxic preconditioned murine MSC (HypMSC) enhanced skeletal muscle regeneration at day 7, improved blood flow and vascular formation compared to injected nonpreconditioned MSC (NormMSC). These observed effects were correlated with an increase in HypMSC engraftment and a putative role in necrotic skeletal muscle fiber clearance. Moreover, HypMSC transplantation resulted in a large increase in Wnt4 (wingless-related MMTV integration site 4) expression and we demonstrate its functional significance on MSC proliferation and migration, endothelial cell (EC) migration, as well as myoblast differentiation. Furthermore, suppression of Wnt4 expression in HypMSC, abrogated the hypoxia-induced vascular regenerative properties of these cells in the mouse hindlimb ischemia model. Our data suggest that hypoxic preconditioning plays a critical role in the functional capabilities of MSC, shifting MSC location in situ to enhance ischemic tissue recovery, facilitating vascular cell mobilization, and skeletal muscle fiber regeneration via a paracrine Wnt-dependent mechanism.

In Vivo Platelet Production from Mature Megakaryocytes: Does Platelet Release Occur via Proplatelets?

Although platelets are universally accepted to be born from megakaryocytes (MKs), the mechanism by which platelets are formed and released from MKs in vivo remains controversial. One theory, known as the proplatelet theory, postulates that platelets are released from proplatelet processes protruding from MKs into sinusoids located in the bone marrow hematopoietic compartment. Proplatelet formation (PPF) has been observed in in vitro experiments involving detailed analyses of related molecular events. PPF has also been used as a marker of MK maturation. However, PPF is suggested to be a nonphysiological phenomenon. On the other hand, transmission electron microscopy (TEM) analyses have revealed platelet formation via explosive fragmentation of MK cytoplasm in bone marrow and lung capillaries prepared by immersion fixation. Moreover, TEM and scanning electron microscopy studies of liquid-cultured MKs kept in suspension show that platelet formation occurs without PPF. Rather, an explosive and global fragmentation of the MK cytoplasm composed of platelet territories has been reported as the mechanism of platelet formation. In addition, in vivo and ex vivo observations of platelet release from MKs with phase-contrast microscopy strongly support the explosive-fragmentation theory. With all observations taken into account, PPF may not be a prerequisite for platelet release from MKs under real-life conditions. In this review, a new "protoplatelet" concept is proposed to support the explosive-fragmentation theory. Additionally, the role of the lungs in platelet production is reviewed and discussed.

Modeling pO 2 Distributions in the Bone Marrow Hematopoietic Compartment. II. Modified Kroghian Models

Hematopoietic cells of various lineages are organized in distinct cellular architectures in the bone marrow hematopoietic compartment (BMHC). The homogeneous Kroghian model, which deals only with a single cell type, may not be sufficient to accurately describe oxygen transfer in the BMHC. Thus, for cellular architectures of physiological significance, more complex biophysical-transport models were considered and compared against simulations using the homogeneous Kroghian model. The effects of the heterogeneity of model parameters on the oxygen tension (pO 2) distribution were examined using the multilayer Kroghian model. We have also developed two-dimensional Kroghian models to simulate several cellular architectures in which a cell cluster (erythroid cluster) or an individual cell (megakaryocyte or adipocyte) is located in the BMHC predominantly occupied by mature granulocytes. pO 2 distributions in colony-type cellular arrangements (erythroblastic islets, granulopoietic loci, and lymphocytic nodules) in the BMHC were also evaluated by modifying the multilayer Kroghian model. The simulated results indicate that most hematopoietic progenitors experience low pO 2 values, which agrees with the finding that low pO 2 promotes the expansion of various hematopoietic progenitors. These results suggest that the most primitive stem cells, which are located even further away from BM sinuses, are likely located in a very low pO 2 environment.

Modeling pO 2 Distributions in the Bone Marrow Hematopoietic Compartment. I. Krogh's Model

Human bone marrow (BM) is a tissue of complex architectural organization, which includes granulopoietic loci, erythroblastic islets, and lymphocytic nodules. Oxygen tension (pO 2) is an important determinant of hematopoietic stem and progenitor cell proliferation and differentiation. Thus, understanding the impact of the BM architectural organization on pO 2 levels in extravascular hematopoietic tissue is an important biophysical problem. However, currently it is impossible to measure pO 2 levels and their spatial variations in the BM. Homogeneous Kroghian models were used to estimate pO 2 distribution in the BM hematopoietic compartment (BMHC) and to conservatively simulate pO 2-limited cellular architectures. Based on biophysical data of hematopoietic cells and characteristics of BM physiology, we constructed a tissue cylinder solely occupied by granulocytic progenitors (the most metabolically active stage of the most abundant cell type) to provide a physiologically relevant limiting case. Although the number of possible cellular architectures is large, all simulated pO 2 profiles fall between two extreme cases: those of homogeneous tissues with adipocytes and granulocytic progenitors, respectively. This was illustrated by results obtained from a parametric criterion derived for pO 2 depletion in the extravascular tissue. Modeling results suggest that stem and progenitor cells experience a low pO 2 environment in the BMHC.