Considerable evidence supports the proposal that the localisation of hemopoiesis to the bone marrow (BM) involves developmentally regulated adhesive interactions between primitive hemopoietic stem cells (HSC) and the hemopoietic microenvironment of the marrow. Previous studies in our laboratory demonstrate that HSC reside within an endosteal stem cell niche, and identified several key molecules that play critical roles in their attraction to, and retention and regulation within this region. We have previously described Hyaluronic acid (HA) as one of these key molecules and shown that human and murine HSC synthesise and express HA. HSC express the 3 HAS genes and synthesis of this glycosaminoglycan by HSC as well as by cells within the hemopoietic microenvironment is critical for HSC engraftment and for regulating the HSC pool in vivo. In addition, analysis of HAS 1 and 3 knockout mice identified HA synthesised by HAS 3 to be responsible for these effects. In the absence of HAS-3 synthase the endosteal region is devoid of HA, and the hemopoietic microenvironment is significantly impaired in its ability to attract and support HSC post-transplant as well as regulate the HSC pool in vivo. Furthermore, transplanted HSC isolated from HAS-3−/− mice have a reduced ability to lodge within the endosteal region and reconstitute hemopoiesis. In vitro, binding of HA by a surrogate ligand, HABP, also has a negative regulatory effect of HSC proliferation and differentiation. Together these data suggest that HA on the surface of HSC is critical in both their lodgement and subsequent quiescence within the hemopoietic stem cell niche. Analysis of a cohort of childhood ALL samples revealed a significant correlation between the length of first complete remission and HA synthesis, cell surface HA expression, HAS-3 expression, the distribution of leukemic cells following transplantation and the development of disease in a murine model. In addition, anti-sense inhibition of HAS-3 expression in the Pre-B ALL cell line NALM6, resulted in a significantly reduced time to onset of leukaemia within a NOD/SCID murine model. Furthermore, retroviral mediated overexpression of HAS-3 in this cell line resulted in 2.5-fold increase in mRNA, a 20-fold increase in cell surface HA and a significant decrease in the proliferation potential of these cells in culture. Furthermore, following transplantation of HAS-3 overexpressing NALM6 cells into NOD/SCID recipients there was a significant delay in the onset of leukemia compared to that seen following the transplant of unmanipulated NALM6 cells. This demonstrates a primary role of HAS-3 in the onset and progression of common ALL and that HA expression levels may provide a novel prognostic indictor for this leukaemia. Furthermore, initial studies of HA expression in other leukemic bone marrow samples revealed that cells from AML, CML and CLL also exhibit increased levels of HA, with a correlation between disease progression and HA synthesis and expression; HA levels were elevated at diagnosis, decreasing in remission and increasing again at relapse. Overall, our data is strongly suggestive of a key role for this polysaccharide in both normal and aberrant hemopoietic stem cell biology.

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