Bone marrow sinusoidal endothelium controls terminal erythroid differentiation and reticulocyte maturation

Building bones for blood and beyond: the growing field of bone marrow niche model development

The bone marrow (BM) niche is a complex microenvironment that provides the signals required for regulation of hematopoietic stem cells (HSCs) and the process of hematopoiesis they are responsible for. Bioengineered models of the BM niche incorporate various elements of the in vivo BM microenvironment, including cellular components, soluble factors, a three-dimensional environment, mechanical stimulation of included cells, and perfusion. Recent advances in the bioengineering field have resulted in a spate of new models that shed light on BM function and are approaching precise imitation of the BM niche. These models promise to improve our understanding of the in vivo microenvironment in health and disease. They also aim to serve as platforms for HSC manipulation or as preclinical models for screening novel therapies for BM-associated disorders and diseases.

Complementary and Inducible creERT2 Mouse Models for Functional Evaluation of Endothelial Cell Subtypes in the Bone Marrow

In the adult bone marrow (BM), endothelial cells (ECs) are an integral component of the hematopoietic stem cell (HSC)-supportive niche, which modulates HSC activity by producing secreted and membrane-bound paracrine signals. Within the BM, distinct vascular arteriole, transitional, and sinusoidal EC subtypes display unique paracrine expression profiles and create anatomically-discrete microenvironments. However, the relative contributions of vascular endothelial subtypes in supporting hematopoiesis is unclear. Moreover, constitutive expression and off-target activity of currently available endothelial-specific and endothelial-subtype-specific murine cre lines potentially confound data analysis and interpretation. To address this, we describe two tamoxifen-inducible cre-expressing lines, Vegfr3-creERT2 and Cx40-creERT2, that efficiently label sinusoidal/transitional and arteriole endothelium respectively in adult marrow, without off-target activity in hematopoietic or perivascular cells. Utilizing an established mouse model in which cre-dependent recombination constitutively-activates MAPK signaling within adult endothelium, we identify arteriole ECs as the driver of MAPK-mediated hematopoietic dysfunction. These results define complementary tamoxifen-inducible creERT2-expressing mouse lines that label functionally-discrete and non-overlapping sinusoidal/transitional and arteriole EC populations in the adult BM, providing a robust toolset to investigate the differential contributions of vascular subtypes in maintaining hematopoietic homeostasis.

Wnt/β-Catenin-Signaling Modulates Megakaryopoiesis at the Megakaryocyte-Erythrocyte Progenitor Stage in the Hematopoietic System

The bone marrow (BM) hematopoietic system (HS) gives rise to blood cells originating from hematopoietic stem cells (HSCs), including megakaryocytes (MKs) and red blood cells (erythrocytes; RBCs). Many steps of the cell-fate decision remain to be elucidated, being important for cancer treatment. To explore the role of Wnt/β-catenin for MK and RBC differentiation, we activated β-catenin signaling in platelet-derived growth factor b (Pdgfb)-expressing cells of the HS using a Cre-lox approach (Ctnnb1BM-GOF). FACS analysis revealed that Pdgfb is mainly expressed by megakaryocytic progenitors (MKPs), MKs and platelets. Recombination resulted in a lethal phenotype in mutants (Ctnnb1BM-GOFwt/fl, Ctnnb1BM-GOFfl/fl) 3 weeks after tamoxifen injection, showing an increase in MKs in the BM and spleen, but no pronounced anemia despite reduced erythrocyte counts. BM transplantation (BMT) of Ctnnb1BM-GOF BM into lethally irradiated wildtype recipients (BMT-Ctnnb1BM-GOF) confirmed the megakaryocytic, but not the lethal phenotype. CFU-MK assays in vitro with BM cells of Ctnnb1BM-GOF mice supported MK skewing at the expense of erythroid colonies. Molecularly, the runt-related transcription factor 1 (RUNX1) mRNA, known to suppress erythropoiesis, was upregulated in Ctnnb1BM-GOF BM cells. In conclusion, β-catenin activation plays a key role in cell-fate decision favoring MK development at the expense of erythroid production.

Regulation of erythropoiesis: emerging concepts and therapeutic implications

The process of erythropoiesis is complex and involves the transfer of cells from the yolk sac to the fetal hepar and, ultimately, to the bone marrow during embryonic development. Within the bone marrow, erythroid progenitor cells undergo several stages to generate reticulocytes that enter the bloodstream. Erythropoiesis is regulated by various factors, with erythropoietin (EPO) synthesized by the kidney being the promoting factor and hepcidin synthesized by the hepar inhibiting iron mobilization. Transcription factors, such as GATA and KLF, also play a crucial role in erythropoiesis. Disruption of any of these factors can lead to abnormal erythropoiesis, resulting in red cell excess, red cell deficiency, or abnormal morphological function. This review provides a general description of erythropoiesis, as well as its regulation, highlighting the significance of understanding the process for the diagnosis and treatment of various hematological disorders.

High-sensitivity hemoglobin detection based on polarization-differential spectrophotometry

Hemoglobin content is recognized as a momentous and fundamental physiological indicator, especially the precise detection of trace hemoglobin is of great significance for early diagnosis and prevention of tumors, cancer, organic injury, etc. Therefore, high-sensitivity hemoglobin detection is imperative. However, effective detection methods and reliable detection systems are still lacking and remain enormous challenges. Herein, we present a synthetical strategy to break through the existing bottleneck based on polarization-differential spectrophotometry and high-performance single-frequency green fiber laser. Importantly, this framework not only has precisely extracted the two-dimensional information of intensity and polarization during the interaction between laser and hemoglobin, but also has taken advantage of the high monochromaticity and fine directivity in the optimized laser source to reduce the undesirable scattered disturbance. Thus, the hemoglobin detection sensitivity of 7.2 × 10-5 g/L has advanced a hundredfold compared with conventional spectrophotometry, and the responsive dynamic range is close to six orders of magnitude. Results indicate that our technology can realize high-sensitivity detection of trace hemoglobin content, holding promising applications for precision medicine and early diagnosis as an optical direct and fast detection method.

Bone marrow sinusoidal endothelial cells are a site of Fgf23 upregulation in a mouse model of iron deficiency anemia

Iron deficiency is a potent stimulator of fibroblast growth factor 23 (FGF23), a hormonal regulator of phosphate and vitamin D metabolism, that is classically thought to be produced by bone-embedded osteocytes. Here, we show that iron-deficient transmembrane serine protease 6 knockout (Tmprss6-/-) mice exhibit elevated circulating FGF23 and Fgf23 messenger RNA (mRNA) upregulation in the bone marrow (BM) but not the cortical bone. To clarify sites of Fgf23 promoter activity in Tmprss6-/- mice, we introduced a heterozygous enhanced green fluorescent protein (eGFP) reporter allele at the endogenous Fgf23 locus. Heterozygous Fgf23 disruption did not alter the severity of systemic iron deficiency or anemia in the Tmprss6-/- mice. Tmprss6-/-Fgf23+/eGFP mice showed green fluorescence in the vascular regions of BM sections and showed a subset of BM endothelial cells that were GFPbright by flow cytometry. Mining of transcriptomic data sets from mice with normal iron balance revealed higher Fgf23 mRNA in BM sinusoidal endothelial cells (BM-SECs) than that in other BM endothelial cell populations. Anti-GFP immunohistochemistry of fixed BM sections from Tmprss6-/-Fgf23+/eGFP mice revealed GFP expression in BM-SECs, which was more intense than in nonanemic controls. In addition, in mice with intact Tmprss6 alleles, Fgf23-eGFP reporter expression increased in BM-SECs following large-volume phlebotomy and also following erythropoietin treatment both ex vivo and in vivo. Collectively, our results identified BM-SECs as a novel site for Fgf23 upregulation in both acute and chronic anemia. Given the elevated serum erythropoietin in both anemic models, our findings raise the possibility that erythropoietin may act directly on BM-SECs to promote FGF23 production during anemia.

Inhibition of FGF23 is a therapeutic strategy to target hematopoietic stem cell niche defects in β-thalassemia

Clinical evidence highlights a relationship between the blood and the bone, but the underlying mechanism linking these two tissues is not fully elucidated. Here, we used β-thalassemia as a model of congenital anemia with bone and bone marrow (BM) niche defects. We demonstrate that fibroblast growth factor 23 (FGF23) is increased in patients and mice with β-thalassemia because erythropoietin induces FGF23 overproduction in bone and BM erythroid cells via ERK1/2 and STAT5 pathways. We show that in vivo inhibition of FGF23 signaling by carboxyl-terminal FGF23 peptide is a safe and efficacious therapeutic strategy to rescue bone mineralization and deposition in mice with β-thalassemia, normalizing the expression of niche factors and restoring hematopoietic stem cell (HSC) function. FGF23 may thus represent a molecular link connecting anemia, bone, and the HSC niche. This study provides a translational approach to targeting bone defects and rescuing HSC niche interactions, with potential clinical relevance for improving HSC transplantation and gene therapy for hematopoietic disorders.

The tissue-specific transcriptional landscape underlines the involvement of endothelial cells in health and disease

Endothelial cells (ECs) that line vascular and lymphatic vessels are being increasingly recognized as important to organ function in health and disease. ECs participate not only in the trafficking of gases, metabolites, and cells between the bloodstream and tissues but also in the angiocrine-based induction of heterogeneous parenchymal cells, which are unique to their specific tissue functions. The molecular mechanisms regulating EC heterogeneity between and within different tissues are modeled during embryogenesis and become fully established in adults. Any changes in adult tissue homeostasis induced by aging, stress conditions, and various noxae may reshape EC heterogeneity and induce specific transcriptional features that condition a functional phenotype. Heterogeneity is sustained via specific genetic programs organized through the combinatory effects of a discrete number of transcription factors (TFs) that, at the single tissue-level, constitute dynamic networks that are post-transcriptionally and epigenetically regulated. This review is focused on outlining the TF-based networks involved in EC specialization and physiological and pathological stressors thought to modify their architecture.

ALK1 controls hepatic vessel formation, angiodiversity, and angiocrine functions in hereditary hemorrhagic telangiectasia of the liver

BACKGROUND AND AIMS

In hereditary hemorrhagic telangiectasia (HHT), severe liver vascular malformations are associated with mutations in the Activin A Receptor-Like Type 1 ( ACVRL1 ) gene encoding ALK1, the receptor for bone morphogenetic protein (BMP) 9/BMP10, which regulates blood vessel development. Here, we established an HHT mouse model with exclusive liver involvement and adequate life expectancy to investigate ALK1 signaling in liver vessel formation and metabolic function.

APPROACH AND RESULTS

Liver sinusoidal endothelial cell (LSEC)-selective Cre deleter line, Stab2-iCreF3 , was crossed with Acvrl1 -floxed mice to generate LSEC-specific Acvrl1 -deficient mice ( Alk1HEC-KO ). Alk1HEC-KO mice revealed hepatic vascular malformations and increased posthepatic flow, causing right ventricular volume overload. Transcriptomic analyses demonstrated induction of proangiogenic/tip cell gene sets and arterialization of hepatic vessels at the expense of LSEC and central venous identities. Loss of LSEC angiokines Wnt2 , Wnt9b , and R-spondin-3 ( Rspo3 ) led to disruption of metabolic liver zonation in Alk1HEC-KO mice and in liver specimens of patients with HHT. Furthermore, prion-like protein doppel ( Prnd ) and placental growth factor ( Pgf ) were upregulated in Alk1HEC-KO hepatic endothelial cells, representing candidates driving the organ-specific pathogenesis of HHT. In LSEC in vitro , stimulation or inhibition of ALK1 signaling counter-regulated Inhibitors of DNA binding (ID)1-3, known Alk1 transcriptional targets. Stimulation of ALK1 signaling and inhibition of ID1-3 function confirmed regulation of Wnt2 and Rspo3 by the BMP9/ALK1/ID axis.

CONCLUSIONS

Hepatic endothelial ALK1 signaling protects from development of vascular malformations preserving organ-specific endothelial differentiation and angiocrine signaling. The long-term surviving Alk1HEC-KO HHT model offers opportunities to develop targeted therapies for this severe disease.

Mechanical loading modulates phosphate related genes in rat bone

Mechanical loading determines bone mass and bone structure, which involves many biochemical signal molecules. Of these molecules, Mepe and Fgf23 are involved in bone mineralization and phosphate homeostasis. Thus, we aimed to explore whether mechanical loading of bone affects factors of phosphate homeostasis. We studied the effect of mechanical loading of bone on the expression of Fgf23, Mepe, Dmp1, Phex, Cyp27b1, and Vdr. Twelve-week old female rats received a 4-point bending load on the right tibia, whereas control rats were not loaded. RT-qPCR was performed on tibia mRNA at 4, 5, 6, 7 or 8 hours after mechanical loading for detection of Mepe, Dmp1, Fgf23, Phex, Cyp27b1, and Vdr. Immunohistochemistry was performed to visualise FGF23 protein in tibiae. Serum FGF23, phosphate and calcium levels were measured in all rats. Four-point bending resulted in a reduction of tibia Fgf23 gene expression by 64% (p = 0.002) and a reduction of serum FGF23 by 30% (p<0.001), six hours after loading. Eight hours after loading, Dmp1 and Mepe gene expression increased by 151% (p = 0.007) and 100% (p = 0.007). Mechanical loading did not change Phex, Cyp27b1, and Vdr gene expression at any time-point. We conclude that mechanical loading appears to provoke both a paracrine as well as an endocrine response in bone by modulating factors that regulate bone mineralization and phosphate homeostasis.

CSF1R as a Therapeutic Target in Bone Diseases: Obvious but Not so Simple

PURPOSE OF REVIEW

The purpose of the review is to summarize the expression and function of CSF1R and its ligands in bone homeostasis and constraints on therapeutic targeting of this axis.

RECENT FINDINGS

Bone development and homeostasis depends upon interactions between mesenchymal cells and cells of the mononuclear phagocyte lineage (MPS), macrophages, and osteoclasts (OCL). The homeostatic interaction is mediated in part by the systemic and local production of growth factors, macrophage colony-stimulating factor (CSF1), and interleukin 34 (IL34) that interact with a receptor (CSF1R) expressed exclusively by MPS cells and their progenitors. Loss-of-function mutations in CSF1 or CSF1R lead to loss of OCL and macrophages and dysregulation of postnatal bone development. MPS cells continuously degrade CSF1R ligands via receptor-mediated endocytosis. As a consequence, any local or systemic increase or decrease in macrophage or OCL abundance is rapidly reversible. In principle, both CSF1R agonists and antagonists have potential in bone regenerative medicine but their evaluation in disease models and therapeutic application needs to carefully consider the intrinsic feedback control of MPS biology.

Butanol-isopropanol fermentation with oxygen-tolerant Clostridium beijerinckii XH29

Acetone–butanol–ethanol (ABE) fermentation is a traditional way for solvents production through bioconversion by Clostridium species. It is still a challenge to obtain metabolic engineering strains with high ABE yield. Screening strains with remarkable characteristics from nature and improving ABE yield by mutation are viable approaches. Clostridium beijerinckii XH 0906, a newly isolated strain, produces butanol and isopropanol (BI) as the main end-products (9.1 g/L BI) during fermentation with glucose as the sole carbon source. The screening process for this strain was performed under aerobic conditions rather than anaerobic environment. Thus, it is a robust stain capable of oxygen-tolerant BI fermentation. Furthermore, C. beijerinckii XH 0906 fermented xylose and glucose simultaneously to produce BI. A mutant strain obtained by ultraviolet (UV) mutagenesis, C. beijerinckii XH 29, had improved BI production capacity and could produce 17.0 g/L BI and 18.4 g/L BI using glucose or corn stover hydrolysate, respectively as the carbon source. Interestingly, C. beijerinckii XH 29 also produced up to 19.3 g/L isopropanol through fermentation of a glucose–acetone mix. These results indicate that C. beijerinckii XH 29 is an excellent BI producer with great potential for industrial applications.

A newly isolated strain produces butanol and isopropanol (BI) rather than acetone butanol and ethanol (ABE).

The strain is oxygen-tolerant and robust in the fermentation.

A mutant obtained by ultraviolet mutagenesis produces higher levels of BI than the wild type strain using corn stover as a carbon source.