Search

Osteopontin (OPN) is an important component in both bone and blood regulation, functioning as a bridge between the two. Previously, thrombin-cleaved osteopontin (trOPN), the dominant form of OPN in adult bone marrow (BM), was demonstrated to be a critical negative regulator of adult hematopoietic stem cells (HSC) via interactions with α 4 β 1 and α 9 β 1 integrins. We now demonstrate OPN is also required for fetal hematopoiesis in maintaining the HSC and progenitor pool in fetal BM. Specifically, we showed that trOPN is highly expressed in fetal BM and its receptors, α 4 β 1 and α 9 β 1 integrins, are both highly expressed and endogenously activated on fetal BM HSC and progenitors. Notably, the endogenous activation of integrins expressed by HSC was attributed to high concentrations of three divalent metal cations, Ca 2+ , Mg 2+ and Mn 2+ , which were highly prevalent in developing fetal BM. In contrast, minimal levels of OPN were detected in fetal liver, and α 4 β 1 and α 9 β 1 integrins expressed by fetal liver HSC were not in the activated state, thereby permitting the massive expansion of HSC and progenitors required during early fetal hematopoiesis. Consistent with these results, no differences in the number or composition of hematopoietic cells in the liver of fetal OPN -/- mice were detected, but significant increases in the hematopoietic progenitor pool in fetal BM as well as an increase in the BM HSC pool following birth and into adulthood were observed. Together, the data demonstrates OPN is a necessary negative regulator of fetal and neonatal BM progenitors and HSC, and it exhibits preserved regulatory roles during early development, adulthood and ageing.

With age, hematopoietic stem cells (HSC) undergo changes in function, including reduced regenerative potential and loss of quiescence, which is accompanied by a significant expansion of the stem cell pool that can lead to haematological disorders. Elevated metabolic activity has been implicated in driving the HSC ageing phenotype. Here we show that nicotinamide riboside (NR), a form of vitamin B3, restores youthful metabolic capacity by modifying mitochondrial function in multiple ways including reduced expression of nuclear encoded metabolic pathway genes, damping of mitochondrial stress and a decrease in mitochondrial mass and network-size. Metabolic restoration is dependent on continuous NR supplementation and accompanied by a shift of the aged transcriptome towards the young HSC state, more youthful bone marrow cellular composition and an improved regenerative capacity in a transplant setting. Consequently, NR administration could support healthy ageing by re-establishing a more youthful hematopoietic system. Aged hematopoietic stem cells (HSC) are characterised by reduced regenerative potential and a loss of quiescence. Here, the authors show nicotinamide riboside treatment shrinks the age-enlarged stem cell pool and shifts aged HSC functionally, metabolically and molecularly towards the young state. [ABSTRACT FROM AUTHOR]

Efficient homing of hematopoietic stem cells (HSC) from blood to the bone marrow (BM) following transplant is critical for achieving optimal engraftment. Indeed, HSC mobilized using the growth factor G-CSF are known to have reduced engraftment potential compared to native BM HSC, potentially due to impaired homing ability. Thus, the characterization of novel homing mechanisms will allow identification of improved mobilization strategies that are not solely reliant on HSC number but also HSC quality. In this study, we demonstrate junctional adhesion molecule A (JAM-A) has multi-faceted roles in HSC regulation during steady state and following mobilization. Specifically, JAM-A is critical for HSC homing and engraftment post-transplant and contributes to the maintenance of HSC quiescence in the endosteal niche. Furthermore, G-CSF mobilization significantly reduces JAM-A expression on HSC as a result of JAM-A cleavage by the enzyme TACE, which is upregulated following G-CSF. In addition, the significantly impaired homing potential of HSC mobilized with G-CSF is attributed to lower levels of JAM-A, CXCR4 as well as α4β1 and α9β1 integrins. In contrast, we show that HSC mobilized using the combination of the α4β1/α9β1 integrin antagonist BOP and the CXCR4 antagonist AMD3100 had significantly greater homing and long-term multi-lineage engraftment potential; in part due to higher levels of JAM-A, CXCR4 and α4β1/α9β1 integrins on mobilized cells. In addition, compared to G-CSF, which requires multiple doses over several days, mobilization with BOP plus AMD3100 occurred 1h following a single injection. Thus, our results identify JAM-A as a critical regulator of stem and progenitor trafficking to and maintenance in the BM and suggests the use of G-CSF independent mobilization regimes that do not abrogate JAM-A expression/function should result in improved stem cell transplants. [ABSTRACT FROM AUTHOR]