Identification of a common mesenchymal stromal progenitor for the adult haematopoietic niche.

Microenvironment cues received by haematopoietic stem cells (HSC) are important in regulating the choice between self-renewal and differentiation. On the basis of the differential expression of cell-surface markers, here we identify a mesenchymal stromal progenitor hierarchy, where CD45-Ter119-CD31-CD166-CD146-Sca1+(Sca1+) progenitors give rise to CD45-Ter119-CD31-CD166-CD146+(CD146+) intermediate and CD45-Ter119-CD31-CD166+CD146-(CD166+) mature osteo-progenitors. All three progenitors preserve HSC long-term multi-lineage reconstitution capability in vitro; however, their in vivo fates are different. Post-transplantation, CD146+ and CD166+ progenitors form bone only. While Sca1+ progenitors produce CD146+, CD166+ progenitors, osteocytes and CXCL12-producing stromal cells. Only Sca1+ progenitors are capable of homing back to the marrow post-intravenous infusion. Ablation of Sca1+ progenitors results in a decrease of all three progenitor populations as well as haematopoietic stem/progenitor cells. Moreover, suppressing production of KIT-ligand in Sca1+ progenitors inhibits their ability to support HSCs. Our results indicate that Sca1+ progenitors, through the generation of both osteogenic and stromal cells, provide a supportive environment for hematopoiesis.

Evidence that ß7 Integrin Regulates Hematopoietic Stem Cell Homing and Engraftment Through Interaction with MAdCAM-1.

α4ß7 integrin is a cell adhesion receptor that is crucial for the migration of hematopoietic progenitors and mature effector cells in the periphery, but its role in adult hematopoiesis is controversial. We identified a subset of hematopoietic stem cells (HSCs) in the bone marrow (BM) that expressed ß7 integrin. These ß7(+) HSCs were capable of multilineage, long-term reconstitution and had an inherent competitive advantage over ß7(-) HSCs. On the other hand, HSCs that lacked ß7 integrin (ß7KO) had reduced engraftment potential. Interestingly, quantitative RT-PCR and flow cytometry revealed that ß7KO HSCs expressed lower levels of the chemokine receptor CXCR4. Accordingly, ß7KO HSCs exhibited impaired migration abilities in vitro and BM homing capabilities in vivo. Lethal irradiation induced expression of the α4ß7 integrin ligand-mucosal addressin cell adhesion molecule-1 (MAdCAM-1) on BM endothelial cells. Moreover, blocking MAdCAM-1 reduced the homing of HSCs and impaired the survival of recipient mice. Altogether, these data indicate that ß7 integrin, when expressed by HSCs, interacted with its endothelial ligand MAdCAM-1 in the BM microenvironment, thereby promoting HSC homing and engraftment.

Exosome-mediated microenvironment dysregulation in leukemia.

The hematopoietic stem cell (HSC) niche is composed of a complex set of stromal support cells that maintain HSCs and promote normal hematopoiesis. We now know that molecular changes within the hematopoietic niche contribute to leukemia development. Leukemia cells often reorganize the hematopoietic niche to promote and support their own survival and growth. Here we will summarize recent works that decipher the normal hematopoietic niche cellular components and describe how the leukemia-transformed niche contributes to hematological malignances. Finally, we will discuss recent publications that highlight a possible role for exosomes in the leukemia-induced niche reorganization. This article is part of a Special Issue entitled: Tumor Microenvironment Regulation of Cancer Cell Survival, Metastasis, Inflammation, and Immune Surveillance edited by Peter Ruvolo and Gregg L. Semenza.

Evidence for direct binding between HetR from Anabaena sp. PCC 7120 and PatS-5.

HetR, master regulator of heterocyst differentiation in the filamentous cyanobacterium Anabaena sp. strain PCC 7120, stimulates heterocyst differentiation via transcriptional autoregulation and is negatively regulated by PatS and HetN, both of which contain the active pentapeptide RGSGR. However, the direct targets of PatS and HetN remain uncertain. Here, we report experimental evidence for direct binding between HetR and the C-terminal RGSGR pentapeptide, PatS-5. Strains with a hetR allele coding for conservative substitutions at residues 250-256 had altered patterns of heterocysts and, in some cases, reduced sensitivity to PatS-5. Cysteine scanning mutagenesis coupled with electron paramagnetic resonance (EPR) spectroscopy showed quenching of spin label motion at HetR amino acid 252 upon titration with PatS-5, indicating direct binding of PatS-5 to HetR. Gel shift assays indicated that PatS-5 disrupted binding of HetR to a 29 base pair inverted-repeat-containing DNA sequence upstream from hetP. Double electron-electron resonance EPR experiments confirmed that HetR existed as a dimer in solution and indicated that PatS-5 bound to HetR without disrupting the dimer form of HetR. Isothermal titration calorimetry experiments corroborated direct binding of PatS-5 to HetR with a K(d) of 227 nM and a 1:1 stoichiometry. Taken together, these results indicated that PatS-5 disrupted HetR binding to DNA through a direct HetR/PatS interaction. PatS-5 appeared to either bind in the vicinity of HetR amino acid L252 or, alternately, to bind in a remote site that leads to constrained motion of this amino acid via an allosteric effect or change in tertiary structure.