Integration of Notch and Wnt signaling in hematopoietic stem cell maintenance.

A fundamental question in hematopoietic stem cell (HSC) biology is how self-renewal is controlled. Here we show that the molecular regulation of two critical elements of self-renewal, inhibition of differentiation and induction of proliferation, can be uncoupled, and we identify Notch signaling as a key factor in inhibiting differentiation. Using transgenic Notch reporter mice, we found that Notch signaling was active in HSCs in vivo and downregulated as HSCs differentiated. Inhibition of Notch signaling led to accelerated differentiation of HSCs in vitro and depletion of HSCs in vivo. Finally, intact Notch signaling was required for Wnt-mediated maintenance of undifferentiated HSCs but not for survival or entry into the cell cycle in vitro. These data suggest that Notch signaling has a dominant function in inhibiting differentiation and provide a model for how HSCs may integrate multiple signals to maintain the stem cell state.

A set of genes selectively expressed in murine dendritic cells: utility of related cis-acting sequences for lentiviral gene transfer.

Professional antigen presenting cells such as dendritic cells (DC) and macrophages (Mphi) share similar characteristics; however, they differ in their ability to initiate an immune response. DCs are much more potent in priming and stimulating nai;ve T-cells. Thus, DCs are good targets for the expression of foreign genes to elicit and specifically modify immune responses. To identify DC markers cDNA subtraction was performed using murine MHC class II(high), B7(high) bone marrow derived DCs as tester and interferon-gamma/E. coli lipopolysaccaride (LPS) treated bone marrow derived macrophages as driver. Analysis of 114 resulting clones revealed a diverse pattern of DC selective (DC(DeltaMphi)) gene expression including known genes whose expression in DCs had not been previously demonstrated as well as multiple novel genes. For several identified DC(DeltaMphi) genes, proximal promoter elements were isolated and incorporated into self-inactivating lentiviral GFP reporter vectors. Promoter activity was measured in bone marrow derived macrophages or dendritic cells. Of the promoters analyzed those for B7-DC and CCL17 drove strong GFP expression in DCs but not in resting or activated macrophages. The CCL17 promoter offered the highest level of expression in DCs and was further activated by culture with LPS or interleukin-4 (IL-4). In contrast, the B7-DC promoter was induced by IL-4 but not by LPS. Endogenous CCL17 and B7-DC mRNAs were increased similarly in IL-4 cultured DCs but only CCL17 was induced by LPS. Additionally, IL-4 increased cell surface expression of B7-DC in both immature and mature DCs.