Regulation Of Hematopoietic Stem Cell Development Research Articles

Lrp5 and Lrp6 are required for maintaining self-renewal and differentiation of hematopoietic stem cells.

Hematopoietic stem cells (HSCs) have the capacity for self-renewal to maintain the HSCs' pool and the ability for multilineage differentiation, which are responsible for sustained production of multiple blood lineages. The regulation of HSC development is controlled precisely by complex signal networks and hematopoietic microenvironment, which has been termed the HSCs' niche. The Wnt signaling pathway is one of a variety of signaling pathways that have been involved in HSC self-renewal and maintenance. Previous studies are indeterminant on the regulation of adult HSCs upon canonical Wnt signaling pathways because of the different experimental systems and models used. In this study, we generated the conditional knockout Wnt coreceptor low-density lipoprotein receptor-related protein 5 (Lrp5) and low-density lipoprotein receptor-related protein 6 (Lrp6) mice in adult hematopoiesis via Vav-Cre Loxp system. Inactivation of Lrp5 and -6 in a hematopoietic system diminished the pool of HSCs, but there were no obvious defects in mature immune cells. Lrp5 and -6 double deficiency HSCs showed intrinsic defects in self-renewal and differentiation due to reduced proliferation and increased quiescence of the cell cycle. Analysis of HSC gene expression suggested that the quiescence regulators were significantly up-regulated, such as Egr1, Cdkn1a, Nr4a1, Gata2, Junb and Btg2, and the positive cell cycle regulators were correspondingly down-regulated, such as Ccna2 and Ranbp1. Taken together, we investigated the roles of Lrp5 and -6 in HSCs by functional and bioinformatic assays, and we demonstrated that Lrp5 and -6 are required for the self-renewal and differentiation of adult HSCs. The canonical Wnt pathway may contribute to maintaining the HSC pool and regulate the differentiation of adult HSCs by controlling cell cycle gene regulatory module.-Liu, J., Cui, Z., Wang, F., Yao, Y., Yu, G., Liu, J., Cao, D., Niu, S., You, M., Sun, Z., Lian, D., Zhao, T., Kang, Y., Zhao, Y., Xue, H.-H., Yu, S. Lrp5 and Lrp6 are required for maintaining self-renewal and differentiation of hematopoietic stem cells.

Role of Notch Signaling in Hematopoietic Stem Cell Function

Abstract SCI-15The Notch pathway controls the generation of different cell types in most tissues including blood, and dysregulation of this pathway is strongly associated with oncogenic processes. In many systems, Notch is also required for the maintenance of the stem cell populations. However, in the adult hematopoietic system this link between Notch and stemness has not been established. Instead, work of several groups, including ours, has clearly demonstrated that Notch has a prominent role in the generation of hematopoietic stem cells (HSC) during embryonic development. Although the first wave of blood cells appears in the mouse embryo around day 7.5 of development and is independent of Notch function, embryonic HSC are formed around day 10 of development from endothelial-like progenitors that reside in the embryonic aorta surrounded by the gonad and mesonephros, also called AGM region. By analyzing different Notch pathway mutant mouse embryos, we have demonstrated the involvement of the Jagged1-Notch1-GATA2 axis in this event. However, the formal demonstration that Notch regulates the GATA2 gene during HSC generation is still lacking. We have now found that GATA2 is a direct Notch target in vivo during embryonic HSC generation. However, whereas Notch positively activates GATA2 transcription in the HSC precursors, it simultaneously activates hes1 transcription, which acts a repressor of the same GATA2 gene. This finding directly implicates hes1 in the regulation of HSC development although further studies using loss-of-function mutant embryos are still needed. Altogether, our results indicate that both Notch and hes1 are required to finely regulate the levels, distribution, and likely the timing of GATA2 expression through an incoherent feed-forward loop. In parallel, we have identified other downstream targets of Notch in the AGM region by ChIP-on-chip and expression microarray analysis that we are currently characterizing. Disclosures:No relevant conflicts of interest to declare.

Identification of Novel Regulators of Hematopoietic Stem Cell Mobilization.

In an effort to identify the molecular changes during hematopoietic stem cell (HSC) mobilization we have analyzed genome wide changes in gene expression in HSCs in response to the chemotherapeutic agent cyclophosphamide (Cy) and the growth factor granulocyte colony stimulating factor (G). Cy/G treatment leads to an initial loss of proliferating precursors, followed by a rapid expansion of stem and progenitor cells and their migration to the peripheral blood. While this approach has been capitalized on for harvesting hematopoietic stem cells in clinical therapy, the molecular regulation of this process remains less well understood. To analyze changes in gene expression that occur in vivo as HSCs proliferate and prepare to migrate out of the bone marrow, we enriched for HSC fractions by isolating cells that express c-kit (K), Sca-1(S) and low levels of Thy1.1(T) but do not express lineage (L) markers (KTLS) from untreated mice and mice treated with Cy, G or Cy/G. The gene expression profile of these cells was examined by microarray analysis, relative to HSCs isolated from untreated mice. Interestingly, among the genes upregulated by Cy/G, a large majority (>60%) were a consequence of synergistic activity between Cy and G, while the rest of the upregulated genes could be attributed to activation by Cy or G alone. To test whether this screen allowed identification of novel candidates that regulate HSC function we analyzed the role of transforming growth factor beta induced gene (Big-h3), a gene highly upregulated after Cy/G treatment. Big-h3 is an extracellular matrix protein that mediates the adhesion and migration of various cell types, however its role in HSCs is unknown. Our experiments indicate that overexpression of Big-h3 in HSCs leads to enhanced adhesion to fibronectin as well as upregulation of specific integrins. These experiments suggest that changes in adhesion are an integral component of HSC mobilization, and that Big-h3 is one regulator of such processes. Further functional analysis of other candidates will likely lead to identification of other novel regulators of HSC development and mobilization.