miRNAs Involved in Hematopoietic Lineage Differentiation
miRNA profiling studies, ectopic expression, conditional genetic ablation and transgene overexpression have been critical in the investigation of various mouse hematopoietic lineages. Studies using these techniques have provided evidence for the crucial roles that specific miRNAs have in the establishment, maintenance and function of all the hematopoietic lineages. Here, we describe a few key hematopoietic miRNAs.
The miR-17-92 Cluster
Genomic amplification and overexpression of the miR-17-92 cluster at 13q31.3 is found in B-cell lymphomas, in breast cancer and in lung cancer.[7,8] Two paralog clusters containing very similar or identical miRNAs, miR-106a-363 and miR-106b-25, are also differentially expressed in several malignancies.[7,8] Mice deficient for miR-17-92 are born with lung hypoplasia, ventricular septal defects and developmental abnormalities at the pro-B to pre-B-cell transition, and die shortly after birth. Ectopic expression of the first three miRNAs of the miR-17-92 cluster caused acceleration of myc-induced lymphomagenesis in a mouse model of human B-cell lymphoma. Mice with enhanced lymphocytic expression of the miR-17-92 cluster develop lymphoproliferative disorders leading to autoimmunity, and die during their first year of life. Lymphocytes from miR-17-92 transgenic mice have a lower threshold of activation after antigen recognition compared with those from control mice, and exhibit enhanced proliferation and reduced cell death, resulting in the accumulation of activated CD4+ T and B cells. By promoting the accumulation of activated T cells, it is possible that miRNAs from the miR-17-92 cluster create the setting for future malignancy. Bim (BCL-2-like 11 [an apoptosis facilitator], also known as Bcl2l11) and Pten (phosphatase and tensin homolog [mutated in multiple advanced cancers 1]), previously known to be essential for the depletion of self-reactive thymocytes, mature T cells and immature B cells, are proposed targets of the miR-17-92 cluster ( Table 1 ).[24,26]
Mainly expressed in spleen and lymph nodes, miR-150 is strongly upregulated during the differentiation of mature T and B cells, which indicates that it might participate in B-cell or T-cell lymphopoiesis, or both (Figure 2).[16,27] When ectopically expressed in hematopoietic stem cell (HSC) progenitors, miR-150 blocks B-cell development at the pro-B-cell to pre-B-cell stage of transition, and moderately enhances T-cell lymphopoiesis and myelopoiesis from hematopoietic stem and progenitor cells. miR-150-knockout mice have an increased number of B-1 cells and enhanced antibody production, but no defects in other types of lymphocytes have been detected. The proto-oncogene C-Myb (v-myb myeloblastosis viral oncogene homolog [avian], also known as Myb) is a critical target of miR-150. As with miR-155 (described below), the expression of miR-150 is modulated by stimulation of B cells with IgM-specific antibodies, CpG-containing DNA and LPS.[16,27]
Lineage-specific microRNAs. Schematic presentation of hematopoietic lineages and miRNAs that modulate the development of each of these lineages, as deduced from studies of miRNA mouse models. Abbreviations: BCP, B-cell progenitor; CLP, common lymphoid progenitors; CMP, common myeloid progenitors; GMP, granulocyte-macrophage progenitor; HSC, hematopoietic stem cell; MEP, megakaryocyte-erythrocyte progenitor; miRNA, microRNA; NK, natural killer; NKP, natural-killer progenitor; TCP, T-cell progenitor.
miR-155 expression is dramatically elevated in several human leukemias and lymphomas.[7,8,28] The BIC gene, the transcript from which miR-155 is processed, was first identified as a frequent site of integration for the avian leukosis virus, and when coexpressed with c-Myc (v-myc myelocytomatosis viral oncogene homolog [avian], also known as Myc), both have synergizing effects in lymphomagenesis.[7,8] Transfection of CD34+ HSC progenitors with miR-155 precursors results in a decrease in the number of myeloid and erythroid progenitors, suggesting that this miRNA has a role in normal myelopoesis and erythropoiesis (Figure 2).
Transgenic mice with B cells that overexpress miR-155 develop polyclonal B-cell-type pre-leukemia, while miR-155-knockout mice are unable to mount a proper T-cell or B-cell immune response and also display abnormal dendritic-cell function.[31,32] miR-155-knockout mice also have a reduced number of germinal centers, at least partly because of modifications of cytokine production. miR-155 overexpression results in the opposite phenotype. B-cell expression of miR-155 is mandatory for their antigen-driven maturation and for production of switched high-affinity antibodies. Absence of miR-155 has also been associated with impaired memory response, indicating that miR-155 might regulate the generation of memory B cells. In the myeloid compartment, miR-155 is upregulated when macrophages/monocytes are exposed to LPS or interferon, suggesting that this miRNA might have a crucial role in the innate immune response to both viral and bacterial infections[14,15] in mice and humans.
One probable role of miR-155 in response to LPS stimulation is the release of signals that inhibit TNF translation. miR-155 transgenic mice overproduce TNF when challenged with LPS. The list of putative and confirmed targets of miR-155 continues to grow ( Table 1 ). Of particular interest are the transcription factor targets previously implicated in hematopoiesis and hematological malignancies such as Bach1 (transcription regulator protein BACH1) and transcription factor PU.1, kinases such as IκB kinase ε and receptor-interacting serine/threonine protein kinase, as well as pro-survival proteins such as Fas-associated death domain and tumor protein 53.
Knockout and transgenic mouse models developed for miR-155 have already provided us with converging evidence of a key role for miR-155 in the development of innate and acquired immune responses. By elucidating the regulation of miR-155 expression and identifying additional targets we should shed a new light on the normal development of immune cells and their differentiation into tumor cells.
Highly expressed in spleen and thymus, miR-181 modulates T-cell and B-cell development when ectopically expressed in HSCs (Figure 2).[9,10] miR-181 levels are dynamic during thymocyte differentiation, when thymic-positive and thymic-negative selection processes take place. Thymic selections depend on the strength of signals generated by the T-cell receptor (TCR)-ligand complex and ensure the survival of only those thymocytes expressing a repertoire of antigen receptors responsive to foreign antigens, but not to self-antigens. Increasing levels of miR-181 result in stronger TCR signaling and, therefore, in higher T-cell sensitivity to antigens, a result reversed when miR-181 levels are diminished. The effects of miR-181 on TCR signaling result from its simultaneous targeting of the expression of multiple phosphatases known as 'signal terminators' ( Table 1 ). These phosphatases negatively regulate the signaling cascades triggered by the binding of different growth factors to tyrosine kinase receptors. Although there is no mouse model for miR-181, the crucial role of the phosphatases this miRNA affects in negatively regulating hematopoiesis is exemplified by the phenotype of motheaten mice, which results from the natural inactivation of the PTP1 (protein tyrosine phosphatase) locus. Motheaten mice die within the first several weeks of life (none survive longer than 8 weeks) due to excessive proliferation of lymphoid and myeloid cells. miR-181 modulates the sensitivity of developing thymocytes to antigens. This miRNA, therefore, has a pivotal role in thymic selection, as well as the development, tolerance and immune response of T cells in the periphery. Proper control of miR-181 levels might be critical for the establishment of selection processes in the thymus.
miR-223 is considered myeloid-specific because it is exclusively expressed in bone marrow. This miRNA targets NFI-A (nuclear factor I/A), which encodes an inhibitor of CCAAT/enhancer binding protein α, and Mef2c; both targets have key roles in myelopoiesis ( Table 1 ). Studies of miR-223-knockout mice demonstrated that miR-223 modulates granulocyte differentiation. In the absence of miR-223, increased production, differentiation and activation of granulocyte progenitors result in tissue inflammation and damage. These phenotypes are partly explained by the hypersensitivity that miR-223-knockout granulocytes have to different activation stimuli.
Nat Clin Pract Rheumatol. 2008;4(10):534-541. © 2008 Nature Publishing Group
Cite this: MicroRNAs, The Immune System and Rheumatic Disease - Medscape - Oct 01, 2008.