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Objectives: Although haematopoietic stem cells (HSCs) migrate to injured gut, therapeutic success clinically remains poor. This has been partially attributed to limited local HSC recruitment following systemic injection. Identifying site specific adhesive mechanisms underpinning HSC-endothelial interactions may provide important information on how to enhance their recruitment and thus potentially improve therapeutic efficacy. This study determined (i) the integrins and inflammatory cyto/chemokines governing HSC adhesion to injured gut and muscle (ii) whether pre-treating HSCs with these cyto/chemokines enhanced their adhesion and (iii) whether the degree of HSC adhesion influenced their ability to modulate leukocyte recruitment. Methods: Adhesion of HPC-7, a murine HSC line, to ischaemia-reperfused (IR) injured mouse gut or cremaster muscle was monitored intravitally. Critical adhesion molecules were identified by pre-treating HPC-7 with blocking antibodies to CD18 and CD49d. To identify cyto/chemokines capable of recruiting HPC-7, adhesion was monitored following tissue exposure to TNF-α, IL-1β or CXCL12. The effects of pre-treating HPC-7 with these cyto/chemokines on surface integrin expression/clustering, adhesion to ICAM-1/VCAM-1 and recruitment in vivo was also investigated. Endogenous leukocyte adhesion following HPC-7 injection was again determined intravitally. Results: IR injury increased HPC-7 adhesion in vivo, with intestinal adhesion dependent upon CD18 and muscle adhesion predominantly relying on CD49d. Only CXCL12 pre-treatment enhanced HPC-7 adhesion within injured gut, likely by increasing CD18 binding to ICAM-1 and/or CD18 surface clustering on HPC-7. Leukocyte adhesion was reduced at 4 hours post-reperfusion, but only when local HPC-7 adhesion was enhanced using CXCL12. Conclusion: This data provides evidence that site-specific molecular mechanisms govern HPC-7 adhesion to injured tissue. Importantly, we show that HPC-7 adhesion is a modulatable event in IR injury and further demonstrate that adhesion instigated by injury alone is not sufficient for mediating anti-inflammatory effects. Enhancing local HSC presence may therefore be essential to realising their clinical potential. [ABSTRACT FROM AUTHOR]

Product of the Itga2b gene, CD41 contributes to hematopoietic stem cell (HSC) and megakaryocyte/platelet functions. CD41 expression marks the onset of definitive hematopoiesis in the embryo where it participates in regulating the numbers of multipotential progenitors. Key to platelet aggregation, CD41 expression also characterises their precursor, the megakaryocyte, and is specifically up regulated during megakaryopoiesis. Though phenotypically unique, megakaryocytes and HSC share numerous features, including key transcription factors, which could indicate common sub-regulatory networks. In these respects, Itga2b can serve as a paradigm to study features of both developmental-stage and HSC- versus megakaryocyte-specific regulations. By comparing different cellular contexts, we highlight a mechanism by which internal promoters participate in Itga2b regulation. A developmental process connects epigenetic regulation and promoter switching leading to CD41 expression in HSC. Interestingly, a similar process can be observed at the Mpl locus, which codes for another receptor that defines both HSC and megakaryocyte identities. Our study shows that Itga2b expression is controlled by lineage-specific networks and associates with H4K8ac in megakaryocyte or H3K27me3 in the multipotential hematopoietic cell line HPC7. Correlating with the decrease in H3K27me3 at the Itga2b locus, we find that following commitment to megakaryocyte differentiation, the H3K27 demethylase Jmjd3 up-regulation influences both Itga2b and Mpl expression. [ABSTRACT FROM AUTHOR]

Although the expression of PECAM-1 (CD31) on vascular and haematopoietic cells within the bone marrow microenvironment has been recognized for some time, its physiological role within this niche remains unexplored. In this study we show that PECAM-1 influences steady state hematopoietic stem cell (HSC) progenitor numbers in the peripheral blood but not the bone marrow compartment. PECAM-1-/- mice have higher levels of HSC progenitors in the blood compared to their littermate controls. We show that PECAM-1 is required on both progenitors and bone marrow vascular cells in order for efficient transition between the blood and bone marrow to occur. We have identified key roles for PECAM-1 in both the regulation of HSC migration to the chemokine CXCL12, as well as maintaining levels of the matrix degrading enzyme MMP-9 in the bone marrow vascular niche. Using intravital microscopy and adoptive transfer of either wild type (WT) or PECAM-1-/- bone marrow precursors, we demonstrate that the increase in HSC progenitors in the blood is due in part to a reduced ability to migrate from blood to the bone marrow vascular niche. These findings suggest a novel role for PECAM-1 as a regulator of resting homeostatic progenitor cell numbers in the blood [ABSTRACT FROM AUTHOR]