2010 – MICROENVIRONMENTAL SFRP1 REGULATES REPOPULATING ACTIVITY OF HEMATOPOIETIC STEM CELLS VIA PP2A-MEDIATED REGULATION OF CTNNB1/EP300

We have previously found that a Sfrp1 knock-out environment fails to support the regeneration of hematopoetic stem cells (HSC) with serial to repopulate secondary recipients Renstrom et al., [1] . In order to dissect cell specific requirements of the Sfrp1 gene we established the Sfrp1flox/flox mouse strain and deleted Sfrp1 gene in Osterix+ (Sp7) osteolineage cells. In these Osx-Cre, Sfrp1 (OS1) mice, the number of MSCs is reduced, but these show an increased proportion of colony forming units (CFU-F). Furthermore, stromal cells grown ex-vivo show increased senescence-associated beta-galactosidase staining. In addition, the CFU-F-derived stromal cells differentiated spontaneously into adipocytes. These findings indicate altered functionality of stromal cells from OS1 mice. In the hematopoietic compartment of these mice, we found a decrease in myeloid progenitors in the bone marrow (BM) with concomitant increase in CD11b+ GR1hi granulocytes in peripheral blood. Although the number of primitive CD34- CD48- CD150+ HSCs (LT-HSCs) in the BM was unchanged, LT-HSCs from OS1 mice failed to repopulate in wild type recipients. In single cell cultures, we found that LT-HSCs from OS1 mice show decreased proliferation with concomitant increased differentiation into mature myeloid cells, which was associated with increased DNA damage as shown with comet tail assays and staining for gammaH2.AX. On a molecular level, we found that LT-HSCs from OS1 mice show increased CTNNB1 protein levels. CTNNB1 regulates cell differentiation and proliferation of stem cells by binding to Ep300 or CBP respectively Miyabayashi et al., [2] . Interestingly, CBP protein level was decreased in LT-HSCs from OS1 mice while Ep300 was increased, suggesting overactivation of the CTNNB1/Ep300 axis. Indeed, blocking CTNNB1/Ep300 binding with specific PP2A inhibitor IQ-1, we not only rescued the aberrant behavior of OS1 LT-HSC in vitro, but we also restored the repopulating activity of these LT-HSCs in vivo. Our results suggest that deletion of stromal SFRP1 diminishes the repopulating activity of LT-HSCs by increasing differentiation through PP2A-mediated dephosphorylation of the phospho-Ep300-binding site with CTNNB1.