On the other hand, transfer of control bone marrow to iKO recipients resulted in competent development of thymocytes and bone marrow progenitor cells (Fig
On the other hand, transfer of control bone marrow to iKO recipients resulted in competent development of thymocytes and bone marrow progenitor cells (Fig. adult mammals, HSCs reside in the bone marrow, where they undergo proliferative renewal and differentiation to produce lineage-specific progenitor cells (2), including common lymphoid progenitors (CLPs), common myeloid progenitors (CMPs), granulocyte-macrophage progenitors (GMPs), and megakaryocyte-erythroid progenitors (MEPs) (3). The lineage commitment of the HSCs and the subsequent lineage-specific growth are subject EPZ011989 to tight regulations, since deregulation EPZ011989 of hematopoiesis can cause severe diseases, such as anemia and blood malignancies. A large number of transcription factors have been implicated in the regulation of different stages of hematopoiesis, although the signaling mechanism that controls the hematopoietic transcription network is usually poorly understood. Nevertheless, genetic evidence suggests an important role for the Wnt signaling pathway in the regulation of early stages of hematopoiesis (3,4). A central step of the canonical Wnt signaling pathway is usually stabilization of -catenin (5). In the absence of Wnt signaling, -catenin is constantly phosphorylated by a kinase, GSK-3, and targeted for degradation by the proteasome. Binding of Wnt proteins to their receptors causes inactivation of GSK-3 and stabilization of -catenin. The -catenin then forms a complex with T cell factor (TCF) family of transcription factors, TCF-1 and Lef-1, thereby inducing the transcription of a set of Wnt-target genes (5). Whereas, the Wnt signaling is usually important for specific actions of hematopoiesis and thymocyte development, this pathway is usually subject to tight control, since its deregulation impairs hematopoiesis at EPZ011989 the stage of HSCs (6,7). Although the mechanism of Wnt regulation remains largely unclear, a recent study suggests the involvement of lysine (K) 63 type of protein ubiquitination in the unfavorable control of Wnt signaling (8). Protein ubiquitination is an emerging mechanism that regulates signal transduction in different biological processes (9,10). Although traditionally viewed as a mechanism that targets proteins for degradation in the proteasome, ubiquitination is now known to also mediate various nondegradative functions, including signal transduction (911). Polyubiquitin chains are formed via linkage of the C-terminal glycine residue of one ubiquitin with an internal K residue of another ubiquitin, and the ubiquitin chains formed with different internal K residues may mediate distinct functions. In particular, K63-linked ubiquitin chains play an important role in mediating signal transduction (9). A ubiquitin-conjugating enzyme (E2), Ubc13, EPZ011989 specifically catalyzes K63-linked ubiquitin chains when forming a dimeric complex with a noncanonical E2, Uve1A (MMS2 in yeast) Rabbit polyclonal to IGF1R (12,13). RNA interference (RNAi) assays suggests a critical role of Ubc13 in the regulation of signal transduction mediating activation of NF-B (1416), a transcription factor involved in the activation of lymphocytes and innate immune cells (17,18). NF-B is also involved in the development of specific lineages of blood cells, although it appears to be dispensable for the early stage of hematopoiesis involving HSCs (1922). Recent studies demonstrate that conditional knockout of Ubc13 in lymphocytes attenuates NF-B activation by antigen receptors (23,24). Ubc13 appears to also regulate activation of NF-B and MAP kinase signaling pathway in innate immune cells by the Toll-like receptors (23,25). On the other hand, Ubc13 is largely dispensable for the development of T cells and the follicular B cells, although it regulates the formation of marginal zone B.