MicroRNAs: Small Molecules with a Pleiotropic Regulatory Function

MicroRNAs (miRNAs) are a new class of non-coding RNAs (~22nt) that mediate post-transcriptional repression of gene expression. It is now well established that miRNAs play essential roles in many basic physiologic processes, including organ development and tissue differentiation. More than 500 miRNAs have been identified in the human genome; however, there are only a few miRNAs that are currently understood functionally. Mouse models can be used to delineate the function of miRNAs.

In their paper, Thai and colleagues describe a mouse gain/loss-of-function model to investigate the function of miR-155 in the immune system. miR-155 has been demonstrated to be expressed in a variety of human hematopoietic cells, including activated lymphocytes. Its high expression was also found in B-cell lymphomas and in a miR-155+ mouse that develops B-cell malignancy. The miR-155 sequence is located in the non-protein coding region of bic gene. Using a transgenic approach, the authors generated miR-155 knock-out (bic/miR-155^-/-) and knock-in (B cell^miR-155) mutant mice. After an infectious challenge, an increased portion of germinal center B cells was found in the gut-associated lymphoid tissue of B-cell^miR-155 mice compared with bic/miR-155^-/-mice. Overexpression of miR-155 was found in T cells that were activated by TCR cross-linking, but not in naïve CD4+ T cells. In order to identify the gene targets of miR-155, the authors examined possible dysregulation of cytokine production and found low TNF production in bic/miR-155^-/-B cells at both mRNA and protein levels. The analyses of activated T cells revealed that the T_H cells from knock-out mice did not have impaired differentiation, but under T_H2 differentiation conditions bic/miR-155^-/-cells produced more IL-4 and less IFN-y, possibly reflecting their increased propensity to differentiation. Taken together, these findings suggest that miR-155 may be involved in the control of germinal center development and that this regulation can occur, at least in part, at the level of cytokine production.

In the second paper, Rodriguez and colleagues focused on determination of the bic/miR-155 role in vivo using bic-deficient mice. These mice developed lung airway defects. This phenotype was accompanied by a significantly increased number of leukocytes, suggesting a possible role of miR-155 in the immune system. To test the mechanism of this defect, the authors examined in vivo B-cell and T-cell responses in the knock-out mice after immunization with T-dependent antigens. Impaired B-cell response was observed and T cells produced less IL-2 and IFN-y. In addition, knock-out dendritic cells (DC) were not able to activate T cells, suggesting possible influence of miR-155 on DC function. Testing T_H differentiation led to the same findings as described by Thai et al. that bic/miR-155 is not required for T_H1 differentiation, but there was increased commitment to T_H2 lineage producing higher levels of IL-4. This group then searched for putative target genes of bic/miR-155 using microarray analysis. They characterized differentially expressed genes in bic-deficient T_H1 and T_H2 cells. Computational matching 3’UTRs of these genes (in silico analysis) with miR-155 sequence revealed a high probability of targeting of transcription factor c-MAF that may transactivate IL-4 promoter. This was verified by the increased expression in T_H2 cells with corresponding protein level. Further studies then revealed miR-155-dependent c-MAF expression.