Widespread context dependency of microRNA-mediated regulation.

Gene expression is regulated in a context-dependent, cell-type-specific manner. Condition-specific transcription is dependent on the presence of transcription factors (TFs) that can activate or inhibit its target genes (global context). Additional factors, such as chromatin structure, histone, or DNA modifications, also influence the activity of individual target genes (individual context). The role of the global and individual context for post-transcriptional regulation has not systematically been investigated on a large scale and is poorly understood. Here we show that global and individual context dependency is a pervasive feature of microRNA-mediated regulation. Our comprehensive and highly consistent data set from several high-throughput technologies (PAR-CLIP, RIP-chip, 4sU-tagging, and SILAC) provides strong evidence that context-dependent microRNA target sites (CDTS) are as frequent and functionally relevant as constitutive target sites (CTS). Furthermore, we found the global context to be insufficient to explain the CDTS, and that flanking sequence motifs provide individual context that is an equally important factor. Our results demonstrate that, similar to TF-mediated regulation, global and individual context dependency are prevalent in microRNA-mediated gene regulation, implying a much more complex post-transcriptional regulatory network than is currently known. The necessary tools to unravel post-transcriptional regulations and mechanisms need to be much more involved, and much more data will be needed for particular cell types and cellular conditions in order to understand microRNA-mediated regulation and the context-dependent post-transcriptional regulatory network.

Ultrashort and progressive 4sU-tagging reveals key characteristics of RNA processing at nucleotide resolution.

RNA synthesis and decay rates determine the steady-state levels of cellular RNAs. Metabolic tagging of newly transcribed RNA by 4-thiouridine (4sU) can reveal the relative contributions of RNA synthesis and decay rates. The kinetics of RNA processing, however, had so far remained unresolved. Here, we show that ultrashort 4sU-tagging not only provides snapshot pictures of eukaryotic gene expression but, when combined with progressive 4sU-tagging and RNA-seq, reveals global RNA processing kinetics at nucleotide resolution. Using this method, we identified classes of rapidly and slowly spliced/degraded introns. Interestingly, each class of splicing kinetics was characterized by a distinct association with intron length, gene length, and splice site strength. For a large group of introns, we also observed long lasting retention in the primary transcript, but efficient secondary splicing or degradation at later time points. Finally, we show that processing of most, but not all small nucleolar (sno)RNA-containing introns is remarkably inefficient with the majority of introns being spliced and degraded rather than processed into mature snoRNAs. In summary, our study yields unparalleled insights into the kinetics of RNA processing and provides the tools to study molecular mechanisms of RNA processing and their contribution to the regulation of gene expression.

Abortive lytic reactivation of KSHV in CBF1/CSL deficient human B cell lines.

Since Kaposi's sarcoma associated herpesvirus (KSHV) establishes a persistent infection in human B cells, B cells are a critical compartment for viral pathogenesis. RTA, the replication and transcription activator of KSHV, can either directly bind to DNA or use cellular DNA binding factors including CBF1/CSL as DNA adaptors. In addition, the viral factors LANA1 and vIRF4 are known to bind to CBF1/CSL and modulate RTA activity. To analyze the contribution of CBF1/CSL to reactivation in human B cells, we have successfully infected DG75 and DG75 CBF1/CSL knock-out cell lines with recombinant KSHV.219 and selected for viral maintenance by selective medium. Both lines maintained the virus irrespective of their CBF1/CSL status. Viral reactivation could be initiated in both B cell lines but viral genome replication was attenuated in CBF1/CSL deficient lines, which also failed to produce detectable levels of infectious virus. Induction of immediate early, early and late viral genes was impaired in CBF1/CSL deficient cells at multiple stages of the reactivation process but could be restored to wild-type levels by reintroduction of CBF1/CSL. To identify additional viral RTA target genes, which are directly controlled by CBF1/CSL, we analyzed promoters of a selected subset of viral genes. We show that the induction of the late viral genes ORF29a and ORF65 by RTA is strongly enhanced by CBF1/CSL. Orthologs of ORF29a in other herpesviruses are part of the terminase complex required for viral packaging. ORF65 encodes the small capsid protein essential for capsid shell assembly. Our study demonstrates for the first time that in human B cells viral replication can be initiated in the absence of CBF1/CSL but the reactivation process is severely attenuated at all stages and does not lead to virion production. Thus, CBF1/CSL acts as a global hub which is used by the virus to coordinate the lytic cascade.

Kaposi's sarcoma herpesvirus microRNAs target caspase 3 and regulate apoptosis.

Kaposi's sarcoma herpesvirus (KSHV) encodes a cluster of twelve micro (mi)RNAs, which are abundantly expressed during both latent and lytic infection. Previous studies reported that KSHV is able to inhibit apoptosis during latent infection; we thus tested the involvement of viral miRNAs in this process. We found that both HEK293 epithelial cells and DG75 cells stably expressing KSHV miRNAs were protected from apoptosis. Potential cellular targets that were significantly down-regulated upon KSHV miRNAs expression were identified by microarray profiling. Among them, we validated by luciferase reporter assays, quantitative PCR and western blotting caspase 3 (Casp3), a critical factor for the control of apoptosis. Using site-directed mutagenesis, we found that three KSHV miRNAs, miR-K12-1, 3 and 4-3p, were responsible for the targeting of Casp3. Specific inhibition of these miRNAs in KSHV-infected cells resulted in increased expression levels of endogenous Casp3 and enhanced apoptosis. Altogether, our results suggest that KSHV miRNAs directly participate in the previously reported inhibition of apoptosis by the virus, and are thus likely to play a role in KSHV-induced oncogenesis.

The miRNA-targetome of KSHV and EBV in human B-cells.

Micro-RNAs (miRNAs) are small non-coding RNA molecules which provide a subtle layer of regulation to thousands of cellular genes. The identification of virally encoded miRNAs added another layer of complexity to the dense interaction between viruses and their natural hosts. While it has been shown that viral miRNAs can regulate both cellular and viral gene expression, target identification has been a difficult and cumbersome task. The immunoprecipitation of Argonaute (Ago)-protein containing RNA-induced silencing complexes (RISC) followed by microarray analysis (RIP-Chip) allows the identification of miRNA-targetomes at whole transcriptome level. We applied Ago2-based RIP-Chip to identify cellular transcripts targeted by Kaposi's sarcoma-associated herpesvirus (KSHV, n=114), Epstein-Barr virus (EBV, n=44) and cellular miRNAs (n=2,337) in six latently infected or stably transduced human B-cell lines. While RIP-Chip yields a plethora of high-confidence miRNA targets and provides a quantitative estimate of miRNA function, additional biochemical methods like HITS-CLIP or PAR-CLIP and bioinformatic analysis are required to identify individual miRNA binding sites. Together, these methods will be useful to unravel the network of regulation exerted by both viral and cellular miRNAs, thereby providing the basis for functional studies on miRNA-mediated regulation of gene expression in herpesvirus infections.

Salmonella transforms follicle-associated epithelial cells into M cells to promote intestinal invasion.

Salmonella Typhimurium specifically targets antigen-sampling microfold (M) cells to translocate across the gut epithelium. Although M cells represent a small proportion of the specialized follicular-associated epithelium (FAE) overlying mucosa-associated lymphoid tissues, their density increases during Salmonella infection, but the underlying molecular mechanism remains unclear. Using in vitro and in vivo infection models, we demonstrate that the S. Typhimurium type III effector protein SopB induces an epithelial-mesenchymal transition (EMT) of FAE enterocytes into M cells. This cellular transdifferentiation is a result of SopB-dependent activation of Wnt/ß-catenin signaling leading to induction of both receptor activator of NF-κB ligand (RANKL) and its receptor RANK. The autocrine activation of RelB-expressing FAE enterocytes by RANKL/RANK induces the EMT-regulating transcription factor Slug that marks epithelial transdifferentiation into M cells. Thus, via the activity of a single secreted effector, S. Typhimurium transforms primed epithelial cells into M cells to promote host colonization and invasion.

Flavone synthase II (CYP93B16) from soybean (Glycine max L.).

Flavonoids are a very diverse group of plant secondary metabolites with a wide array of activities in plants, as well as in nutrition and health. All flavonoids are derived from a limited number of flavanone intermediates, which serve as substrates for a variety of enzyme activities, enabling the generation of diversity in flavonoid structures. Flavonoids can be characteristic metabolites, like isoflavonoids for legumes. Others, like flavones, occur in nearly all plants. Interestingly, there exist two fundamentally different enzymatic systems able to directly generate flavones from flavanones, flavone synthase (FNS) I and II. We describe an inducible flavone synthase activity from soybean (Glycine max) cell cultures, generating 7,4'-dihydroxyflavone (DHF), which we classified as FNS II. The corresponding full-length cDNA (CYP93B16) was isolated using known FNS II sequences from other plants. Functional expression in yeast allowed the detailed biochemical characterization of the catalytic activity of FNS II. A direct conversion of flavanones such as liquiritigenin, naringenin, and eriodictyol into the corresponding flavones DHF, apigenin and luteolin, respectively, was demonstrated. The enzymatic reaction of FNSII was stereoselective, favouring the (S)- over the (R)-enantiomer. Phylogenetic analyses of the subfamily of plant CYP93B enzymes indicate the evolution of a gene encoding a flavone synthase which originally catalyzed the direct conversion of flavanones into flavones, via early gene duplication into a less efficient enzyme with an altered catalytic mechanism. Ultimately, this allowed the evolution of the legume-specific isoflavonoid synthase activity.

Systematic analysis of viral and cellular microRNA targets in cells latently infected with human gamma-herpesviruses by RISC immunoprecipitation assay.

The mRNA targets of microRNAs (miRNAs) can be identified by immunoprecipitation of Argonaute (Ago) protein-containing RNA-induced silencing complexes (RISCs) followed by microarray analysis (RIP-Chip). Here we used Ago2-based RIP-Chip to identify transcripts targeted by Kaposi's sarcoma-associated herpesvirus (KSHV) miRNAs (n = 114), Epstein-Barr virus (EBV) miRNAs (n = 44), and cellular miRNAs (n = 2337) in six latently infected or stably transduced human B cell lines. Of the six KSHV miRNA targets chosen for validation, four showed regulation via their 3'UTR, while two showed regulation via binding sites within coding sequences. Two genes governing cellular transport processes (TOMM22 and IPO7) were confirmed to be targeted by EBV miRNAs. A significant number of viral miRNA targets were upregulated in infected cells, suggesting that viral miRNAs preferentially target cellular genes induced upon infection. Transcript half-life both of cellular and viral miRNA targets negatively correlated with recruitment to RISC complexes, indicating that RIP-Chip offers a quantitative estimate of miRNA function.

Quantitative polymerase chain reaction as a reliable method to determine functional lentiviral titer after ex vivo gene transfer in human mesenchymal stem cells.

BACKGROUND: Human mesenchymal stem cells (hMSCs) are a promising target for ex vivo gene therapy and lentiviruses are excellent gene transfer vehicles in hMSCs since they achieve high transduction rates with long-term gene expression. Nevertheless, senescence of hMSCs may limit therapeutic applications due to time-consuming cell selection and viral titration. Here, we describe a fast and reliable method to determine functional lentiviral titer by quantitative polymerase chain reaction (qPCR) after highly efficient ex vivo gene transfer in hMSCs. METHODS: Lentivirus production was tested with different types of packaging systems. Using p24 ELISA remaining viral particles were detected in the cell culture supernatant. The lentiviral gene transfer efficiency was quantified by FACS analysis. Lentiviral titers were determined by qPCR of expressed transgenes. RESULTS: Third-generation self-inactivating vectors showed highly efficient gene transfer in hMSCs. No viral antigen was detected in the cell culture supernatant after four media changes, suggesting the absence of infectious particles after 4 days. We observed a linear correlation between virus dilution and level of transgene expression by qPCR analysis, therefore allowing viral titering by quantification of transgene expression. Finally, we demonstrated that transduced hMSCs retained their stem cell character by differentiation towards adipogenic, osteogenic and chondrogenic lineages. CONCLUSIONS: Quantification of transgene copy numbers by qPCR is a fast and reliable method to determine functional lentiviral titer after ex vivo gene transfer in hMSCs.