Selective flexible packaging pathways of the segmented genome of influenza A virus.

The genome of influenza A viruses (IAV) is encoded in eight distinct viral ribonucleoproteins (vRNPs) that consist of negative sense viral RNA (vRNA) covered by the IAV nucleoprotein. Previous studies strongly support a selective packaging model by which vRNP segments are bundling to an octameric complex, which is integrated into budding virions. However, the pathway(s) generating a complete genome bundle is not known. We here use a multiplexed FISH assay to monitor all eight vRNAs in parallel in human lung epithelial cells. Analysis of 3.9 × 105 spots of colocalizing vRNAs provides quantitative insights into segment composition of vRNP complexes and, thus, implications for bundling routes. The complexes rarely contain multiple copies of a specific segment. The data suggest a selective packaging mechanism with limited flexibility by which vRNPs assemble into a complete IAV genome. We surmise that this flexibility forms an essential basis for the development of reassortant viruses with pandemic potential.

Comparing Agent-Based Delivery of DNA and PNA Forced Intercalation (FIT) Probes for Multicolor mRNA Imaging.

Fluorogenic oligonucleotide probes allow mRNA imaging in living cells. A key challenge is the cellular delivery of probes. Most delivery agents, such as cell-penetrating peptides (CPPs) and pore-forming proteins, require interactions with the membrane. Charges play an important role. To explore the influence of charge on fluorogenic properties and delivery efficiency, we compared peptide nucleic acid (PNA)- with DNA-based forced intercalation (FIT) probes. Perhaps counterintuitively, fluorescence signaling by charged DNA FIT probes proved tolerant to CPP conjugation, whereas CPP-FIT PNA conjugates were affected. Live-cell imaging was performed with a genetically engineered HEK293 cell line to allow the inducible expression of a specific mRNA target. Blob-like features and high background were recurring nuisances of the tested CPP and lipid conjugates. By contrast, delivery by streptolysin-O provided high enhancements of the fluorescence of the FIT probe upon target induction. Notably, DNA-based FIT probes were brighter and more responsive than PNA-based FIT probes. Optimized conditions enabled live-cell multicolor imaging of three different mRNA target sequences.

A bright FIT-PNA hybridization probe for the hybridization state specific analysis of a C → U RNA edit via FRET in a binary system.

Oligonucleotide probes that show enhanced fluorescence upon nucleic acid hybridization enable the detection and visualization of specific mRNA molecules, in vitro and in cellulo. A challenging problem is the analysis of single nucleotide alterations that occur, for example, when cellular mRNA is subject to C → U editing. Given the length required for uniqueness of the targeted segment, the commonly used probes do not provide the level of sequence specificity needed to discriminate single base mismatched hybridization. Herein we introduce a binary probe system based on fluorescence resonance energy transfer (FRET) that distinguishes three possible states i.e. (i) absence of target, (ii) presence of edited (matched) and (iii) unedited (single base mismatched) target. To address the shortcomings of read-out via FRET, we designed donor probes that avoid bleed through into the acceptor channel and nevertheless provide a high intensity of FRET signaling. We show the combined use of thiazole orange (TO) and an oxazolopyridine analogue (JO), linked as base surrogates in modified PNA FIT-probes that serve as FRET donor for a second, near-infrared (NIR)-labeled strand. In absence of target, donor emission is low and FRET cannot occur in lieu of the lacking co-alignment of probes. Hybridization of the TO/JO-PNA FIT-probe with the (unedited RNA) target leads to high brightness of emission at 540 nm. Co-alignment of the NIR-acceptor strand ensues from recognition of edited RNA inducing emission at 690 nm. We show imaging of mRNA in fixed and live cells and discuss the homogeneous detection and intracellular imaging of a single nucleotide mRNA edit used by nature to post-transcriptionally modify the function of the Glycine Receptor (GlyR).

Quantitative mRNA Imaging with Dual Channel qFIT Probes to Monitor Distribution and Degree of Hybridization.

Fluorogenic oligonucleotide probes facilitate the detection and localization of RNA targets within cells. However, quantitative measurements of mRNA abundance are difficult when fluorescence signaling is based on intensity changes because a high concentration of unbound probes cannot be distinguished from a low concentration of target-bound probes. Here, we introduce qFIT (quantitative forced intercalation) probes that allow the detection both of probe-target complexes and of unbound probes on separate, independent channels. A surrogate nucleobase based on thiazole orange (TO) probes the hybridization status. The second channel involves a nonresponsive near-IR dye, which serves as a reporter of concentration. We show that the undesirable perturbation of the hybridization reporter TO is avoided when the near-IR dye Cy7 is connected by means of short triazole linkages in an ≥18 nucleotides distance. We used the qFIT probes to localize and quantify oskar mRNA in fixed egg chambers of wild-type and mutant Drosophila melanogaster by wash-free fluorescence in situ hybridization. The measurements revealed a relative 400-fold enrichment of oskar within a 3000 µm3 large volume at the posterior pole of stage 8-9 oocytes, which peaked at a remarkably high 1.8 µM local concentration inside 0.075 µm3 volume units. We discuss detection limits and show that the number of oskar mRNA molecules per oocyte is independent of the oocyte size, which suggests that the final levels are attained already during the onset of oskar localization at stage 8.

In Vivo Visualization and Function Probing of Transport mRNPs Using Injected FIT Probes.

Fluorogenic hybridization methods, such as the use of FIT probes, enable the in vivo detection of specific mRNAs transcribed from their endogenous, genetically nonmodified loci. Here, we describe the design, synthesis and injection of nuclease resistant FIT probes into developing Drosophila oocytes to detect endogenous localizing mRNAs as wells as to probe function of structural RNA elements.

A Fluorescent RNA Forced-Intercalation Probe as a Pan-Selective Marker for Influenza†A Virus Infection.

The influenza A virus (IAV) genome is segmented into eight viral ribonucleoproteins, each expressing a negatively oriented viral RNA (vRNA). Along the infection cycle, highly abundant single-stranded small viral RNAs (svRNA) are transcribed in a segment-specific manner. The sequences of svRNAs and of the vRNA 5'-ends are identical and highly conserved among all IAV strains. Here, we demonstrate that these sequences can be used as a target for a pan-selective sensor of IAV infection. To this end, we used a complementary fluorescent forced-intercalation RNA (IAV QB-FIT) probe with a single locked nucleic acid substitution to increase brightness. We demonstrated by fluorescence in situ hybridization (FISH) that this probe is suitable and easy to use to detect infection of different cell types by a broad variety of avian, porcine, and human IAV strains, but not by other influenza virus types. IAV QB-FIT also provides a useful tool to characterize different infection states of the host cell.