HIF prolyl hydroxylase inhibitor FG-4497 enhances mouse hematopoietic stem cell mobilization via VEGFR2/KDR

In normoxia, hypoxia-inducible transcription factors (HIFs) are rapidly degraded within the cytoplasm as a consequence of their prolyl hydroxylation by oxygen-dependent prolyl hydroxylase domain (PHD) enzymes. We have previously shown that hematopoietic stem and progenitor cells (HSPCs) require HIF-1 for effective mobilization in response to granulocyte colony-stimulating factor (G-CSF) and CXCR4 antagonist AMD3100/plerixafor. Conversely, HIF PHD inhibitors that stabilize HIF-1 protein in vivo enhance HSPC mobilization in response to G-CSF or AMD3100 in a cell-intrinsic manner. We now show that extrinsic mechanisms involving vascular endothelial growth factor receptor-2 (VEGFR2), via bone marrow (BM) endothelial cells, are also at play. PTK787/vatalanib, a tyrosine kinase inhibitor selective for VEGFR1 and VEGFR2, and neutralizing anti-VEGFR2 monoclonal antibody DC101 blocked enhancement of HSPC mobilization by FG-4497. VEGFR2 was absent on mesenchymal and hematopoietic cells and was detected only in Sca1+ endothelial cells in the BM. We propose that HIF PHD inhibitor FG-4497 enhances HSPC mobilization by stabilizing HIF-1α in HSPCs as previously demonstrated, as well as by activating VEGFR2 signaling in BM endothelial cells, which facilitates HSPC egress from the BM into the circulation.

Genomic insights into Mycobacterium simiae human colonization

Mycobacterium simiae (Karassova V, Weissfeiler J, Kraszanay E, Acta Microbiol Acad Sci Hung 12:275-82, 1965) is a slow-growing nontuberculous Mycobacterium species found in environmental niches, and recently evidenced as an opportunistic Human pathogen. We report here the genome of a clinical isolate of M. simiae (MsiGto) obtained from a patient in Guanajuato, Mexico. With a size of 6,684,413 bp, the genomic sequence of strain MsiGto is the largest of the three M. simiae genomes reported to date. Gene prediction revealed 6409 CDSs in total, including 6354 protein-coding genes and 52 RNA genes. Comparative genomic analysis identified shared features between strain MsiGto and the other two reported M. simiae genomes, as well as unique genes. Our data reveals that M. simiae MsiGto harbors virulence-related genes, such as arcD, ESAT-6, and those belonging to the antigen 85 complex and mce clusters, which may explain its successful transition to the human host. We expect the genome information of strain MsiGto will provide a better understanding of infective mechanisms and virulence of this emergent pathogen.

HIF-1α-stabilizing agent FG-4497 rescues human CD34+ cell mobilization in response to G-CSF in immunodeficient mice

Granulocyte colony-stimulating factor (G-CSF) is used routinely in the clinical setting to mobilize hematopoietic stem progenitor cells (HSPCs) into the patient's blood for collection and subsequent transplantation. However, a significant proportion of patients who have previously received chemotherapy or radiotherapy and require autologous HSPC transplantation cannot mobilize the minimal threshold of mobilized HSPCs to achieve rapid and successful hematopoietic reconstitution. Although several alternatives to the G-CSF regime have been tested, few are used in the clinical setting. We have shown previously in mice that administration of prolyl 4-hydroxylase domain enzyme (PHD) inhibitors, which stabilize hypoxia-inducible factor (HIF)-1α, synergize with G-CSF in vivo to enhance mouse HSPC mobilization into blood, leading to enhanced engraftment via an HSPC-intrinsic mechanism. To evaluate whether PHD inhibitors could be used to enhance mobilization of human HSPCs, we humanized nonobese, diabetic severe combined immune-deficient Il2rg^-/- mice by transplanting them with human umbilical cord blood CD34⁺ HSPCs and then treating them with G-CSF with and without co-administration of the PHD inhibitor FG-4497. We observed that combination treatment with G-CSF and FG-4497 resulted in significant mobilization of human lineage-negative (Lin^-) CD34⁺ HSPCs and more primitive human Lin^-CD34⁺CD38^- HSPCs into blood and spleen, whereas mice treated with G-CSF alone did not mobilize human HSPCs significantly. These results suggest that the PHD inhibitor FG-4497 also increases human HSPC mobilization in a xenograft mouse model, suggesting the possibility of testing PHD inhibitors to boost HSPC mobilization in response to G-CSF in humans.

Fms-like tyrosine kinase 3 (Flt3) ligand depletes erythroid island macrophages and blocks medullar erythropoiesis in the mouse

The cytokines granulocyte colony-stimulating factor (G-CSF) and Flt3 ligand (Flt3-L) mobilize hematopoietic stem and progenitor cells into the peripheral blood of primates, humans, and mice. We recently reported that G-CSF administration causes a transient blockade of medullar erythropoiesis by suppressing erythroblastic island (EI) macrophages in the bone marrow. In the study described here, we investigated the effect of mobilizing doses of Flt3-L on erythropoiesis in mice in vivo. Similar to G-CSF, Flt3-L caused whitening of the bone marrow with significant reduction in the numbers of EI macrophages and erythroblasts. This was compensated by an increase in the numbers of EI macrophages and erythroblasts in the spleen. However, unlike G-CSF, Flt3-L had an indirect effect on EI macrophages, as it was not detected at the surface of EI macrophages or erythroid progenitors.

Genome-wide discovery of human splicing branchpoints

During the splicing reaction, the 5′ intron end is joined to the branchpoint nucleotide, selecting the next exon to incorporate into the mature RNA and forming an intron lariat, which is excised. Despite a critical role in gene splicing, the locations and features of human splicing branchpoints are largely unknown. We use exoribonuclease digestion and targeted RNA-sequencing to enrich for sequences that traverse the lariat junction and, by split and inverted alignment, reveal the branchpoint. We identify 59,359 high-confidence human branchpoints in >10,000 genes, providing a first map of splicing branchpoints in the human genome. Branchpoints are predominantly adenosine, highly conserved, and closely distributed to the 3′ splice site. Analysis of human branchpoints reveals numerous novel features, including distinct features of branchpoints for alternatively spliced exons and a family of conserved sequence motifs overlapping branchpoints we term B-boxes, which exhibit maximal nucleotide diversity while maintaining interactions with the keto-rich U2 snRNA. Different B-box motifs exhibit divergent usage in vertebrate lineages and associate with other splicing elements and distinct intron–exon architectures, suggesting integration within a broader regulatory splicing code. Lastly, although branchpoints are refractory to common mutational processes and genetic variation, mutations occurring at branchpoint nucleotides are enriched for disease associations.