As small RNAs exert profound effects on development and regulation of gene expression, their own expression must be similarly constrained to the right level in the right cell at the right time. A collaboration between the labs of Michael Rosenfeld and Witold Filipowicz reports that the RNA-binding protein KSRP (KH-type splicing regulatory protein) regulates the precursor processing of a subset of microRNAs. If this regulatory mechanism is disrupted, effects can be seen on proliferation, differentiation and apoptosis. Thus KSRP is involved in both mRNA turnover and in promoting mRNA expression. This work has uncovered a further level of complexity in the regulation of gene expression by mi-RNA. MicroRNAs (miRNAs) have a role in down-regulating gene expression, and the levels of specific miRNAs are important for correct embryonic development; however, the mechanisms by which this is regulated are unknown. The splicing regulatory protein, KSRP, is now found to regulate the precursor processing of a subset of miRNAs, and its disruption leads to effects on proliferation, differentiation and apoptosis. Consistent with the role of microRNAs (miRNAs) in down-regulating gene expression by reducing the translation and/or stability of target messenger RNAs1, the levels of specific miRNAs are important for correct embryonic development and have been linked to several forms of cancer2,3,4. However, the regulatory mechanisms by which primary miRNAs (pri-miRNAs) are processed first to precursor miRNAs (pre-miRNAs) and then to mature miRNAs by the multiprotein Drosha and Dicer complexes5,6,7,8, respectively, remain largely unknown. The KH-type splicing regulatory protein (KSRP, also known as KHSRP) interacts with single-strand AU-rich-element-containing mRNAs and is a key mediator of mRNA decay9,10. Here we show in mammalian cells that KSRP also serves as a component of both Drosha and Dicer complexes and regulates the biogenesis of a subset of miRNAs. KSRP binds with high affinity to the terminal loop of the target miRNA precursors and promotes their maturation. This mechanism is required for specific changes in target mRNA expression that affect specific biological programs, including proliferation, apoptosis and differentiation. These findings reveal an unexpected mechanism that links KSRP to the machinery regulating maturation of a cohort of miRNAs that, in addition to its role in promoting mRNA decay, independently serves to integrate specific regulatory programs of protein expression.

There is evidence that eukaryotic miRNAs (hereafter called host miRNAs) play a role in the replication and propagation of viruses. Expression or targeting of host miRNAs can be involved in cellular antiviral responses. Most times host miRNAs play a role in viral life-cycles and promote infection through complex regulatory pathways. MiRNAs can also be encoded by a viral genome and be expressed in the host cell. Viral miRNAs can share common sequences with host miRNAs or have totally different sequences. They can regulate a variety of biological processes involved in viral infection, including apoptosis, evasion of the immune response, or modulation of viral life-cycle phases. Overall, virus/miRNA pathway interaction is defined by a plethora of complex mechanisms, though not yet fully understood. This article review summarizes recent advances and novel biological concepts related to the understanding of miRNA expression, control and function during viral infections. The article also discusses potential therapeutic applications of this particular host-pathogen interaction.

The importance of post‐transcriptional mechanisms for the regulation of the homoeostasis of the immune system and the response to challenge by microorganisms is becoming increasingly appreciated. We investigated the contribution of microRNAs (miRNAs) to macrophage activation induced by lipopolysaccharide (LPS). We first observed that Dicer knockout in bone marrow‐derived macrophages (BMDMs) increases the LPS‐induced expression of some inflammation mediators. miRNA microarray analysis in BMDMs revealed that LPS significantly induces the expression of a single miRNA, miR‐155, and this induction depends on enhanced miR‐155 maturation from its precursors. The single‐strand RNA‐binding protein KHtype splicing regulatory protein (KSRP) binds to the terminal loop of miR‐155 precursors and promotes their maturation. Both inhibition of miR‐155 and KSRP knockdown enhance the LPS‐induced expression of select inflammation mediators, and the effect of KSRP knockdown is reverted by mature miR‐155. Our studies unveil the existence of an LPS‐dependent post‐transcriptional regulation of miR‐155 biogenesis. Once induced, miR‐155 finely tunes the expression of select inflammation mediators in response to LPS.—Ruggiero, T., Trabucchi, M., De Santa, F., Zupo, S., Harfe, B. D., McManus, M. T., Rosenfeld, M. G., Briata, P., Gherzi, R. LPS induces KH‐type splicing regulatory protein‐dependent processing of microRNA‐155 precursors in macrophages. FASEB J. 23, 2898–2908 (2009). www.fasebj.org

There is a growing body of evidence about the presence and the activity of the miRISC in the nucleus of mammalian cells. Here, we show by quantitative proteomic analysis that Ago2 interacts with the nucleoplasmic protein Sfpq in an RNA-dependent fashion. By a combination of HITS-CLIP and transcriptomic analyses, we demonstrate that Sfpq directly controls the miRNA targeting of a subset of binding sites by local binding. Sfpq modulates miRNA targeting in both nucleoplasm and cytoplasm, indicating a nucleoplasmic commitment of Sfpq-target mRNAs that globally influences miRNA modes of action. Mechanistically, Sfpq binds to a sizeable set of long 3′UTRs forming aggregates to optimize miRNA positioning/recruitment at selected binding sites, including let-7a binding to Lin28A 3′UTR. Our results extend the miRNA-mediated post-transcriptional gene silencing into the nucleoplasm and indicate that an Sfpq-dependent strategy for controlling miRNA activity takes place in cells, contributing to the complexity of miRNA-dependent gene expression control.

The recent discovery of new classes of small RNAs has opened unknown territories to explore new regulations of physiopathological events. We have recently demonstrated that RNY (or Y RNA)-derived small RNAs (referred to as s-RNYs) are an independent class of clinical biomarkers to detect coronary artery lesions and are associated with atherosclerosis burden. Here, we have studied the role of s-RNYs in human and mouse monocytes/macrophages and have shown that in lipid-laden monocytes/macrophages s-RNY expression is timely correlated to the activation of both NF-κB and caspase 3-dependent cell death pathways. Loss- or gain-of-function experiments demonstrated that s-RNYs activate caspase 3 and NF-κB signaling pathways ultimately promoting cell death and inflammatory responses. As, in atherosclerosis, Ro60-associated s-RNYs generated by apoptotic macrophages are released in the blood of patients, we have investigated the extracellular function of the s-RNY/Ro60 complex. Our data demonstrated that s-RNY/Ro60 complex induces caspase 3-dependent cell death and NF-κB-dependent inflammation, when added to the medium of cultured monocytes/macrophages. Finally, we have shown that s-RNY function is mediated by Toll-like receptor 7 (TLR7). Indeed using chloroquine, which disrupts signaling of endosome-localized TLRs 3, 7, 8 and 9 or the more specific TLR7/9 antagonist, the phosphorothioated oligonucleotide IRS954, we blocked the effect of either intracellular or extracellular s-RNYs. These results position s-RNYs as relevant novel functional molecules that impacts on macrophage physiopathology, indicating their potential role as mediators of inflammatory diseases, such as atherosclerosis.