When Pax5 expression commits these progenitors to monopotent pre-B lymphocytes the two microRNAs (miRNAs) are downregulated. Upon transplantation, stem cells and progenitors can reside in the BM, while pre-B cells, after their commitment, no longer do so. Retrovirally transduced, doxycycline-induced overexpression of either miR-221 or miR-222 in pre-B-I cells does not revert their monopotency to multipotency. However, upon transplantation miR-221, but not miR-222, transduced pre-B-I cells regain the capacity to reside in the BM. Upon subsequent termination of miR-221-expression by removal of doxycycline,

the transplanted cells leave the BM again. Microarray analyses identified 25 downregulated miR-221-target genes, which GW 572016 could function to localize phases of B-lymphocyte development in BM before and after commitment.

MicroRNAs (miRNAs) are noncoding RNAs that regulate gene expression programs of multiple biological processes on posttranscriptional levels. miRNAs exert their functions Selleck Stem Cell Compound Library by binding to cognate mRNA sequences, often in the 3′ untranslated region (UTR), thereby promoting mRNA instability or repression of productive translation [1]. Deletion of the miRNA processing machinery results in early embryonic lethality and dicer-deficient embryonic stem cells are defective in differentiation [2, 3], highlighting the importance of miRNAs IKBKE in development. Differentiation stage-specific expression of miRNAs in the mammalian hematopoietic system has been described [4-9]. Only

in a few cases has it been possible to identify direct targets for the regulatory action of an miRNA [5]. Mature B lymphocytes develop from pluripotent hematopoietic stem cells (pHSCs), over multipotent myeloid/lymphoid progenitors (MPPs), to common lymphoid progenitors (CLPs), Pax5 then commits the development to pre-B-I cells, pre-B-cell receptor positive (preBCR+) pre-B-II cells, and sIgM+ immature B cells [10, 11]. Consequently, Pax5-deficiency blocks B-cell development at an multipotent CLP-like, CD19− cell stage [12, 13]. CD19+Pax5+/+ pre-B-I cells [14] from fetal liver, but not from BM [15] and from CD19−Pax5−/− multipotent/CLP-like pro/pre-B cells [16, 17] can be established on stromal cells and with IL-7 as long-term-proliferating cell lines. Pax5+/+ pre-B-I cells can differentiate into B cells both in vitro as well as after transplantation in vivo. However, Pax5−/− multipotent CLP-like pro/pre-B cells, blocked in B-cell development, can be induced in the proper cytokine/stromal cell environment in vitro, as well as after transplantation in vivo to T cells, NK cells, and, although at lower efficiencies, to myeloid and erythroid cells.