Q344ter Mutation Causes Mislocalization of Rhodopsin Molecules That Are Catalytically Active: A Mouse Model of Q344ter-Induced Retinal Degeneration

Q344ter is a naturally occurring rhodopsin mutation in humans that causes autosomal dominant retinal degeneration through mechanisms that are not fully understood, but are thought to involve an early termination that removed the trafficking signal, QVAPA, leading to its mislocalization in the rod photoreceptor cell. To better understand the disease mechanism(s), transgenic mice that express Q344ter were generated and crossed with rhodopsin knockout mice. Dark-reared Q344terrho+/− mice exhibited retinal degeneration, demonstrating that rhodopsin mislocalization caused photoreceptor cell death. This degeneration is exacerbated by light-exposure and is correlated with the activation of transducin as well as other G-protein signaling pathways. We observed numerous sub-micrometer sized vesicles in the inter-photoreceptor space of Q344terrho+/− and Q344terrho−/− retinas, similar to that seen in another rhodopsin mutant, P347S. Whereas light microscopy failed to reveal outer segment structures in Q344terrho−/− rods, shortened and disorganized rod outer segment structures were visible using electron microscopy. Thus, some Q344ter molecules trafficked to the outer segment and formed disc structures, albeit inefficiently, in the absence of full length wildtype rhodopsin. These findings helped to establish the in vivo role of the QVAPA domain as well as the pathways leading to Q344ter-induced retinal degeneration

The structure of trp RNA-binding attenuation protein

The crystal structure of the trp RNA-binding attenuation protein of Bacillus subtilis solved at 1.8 Å resolution reveals a novel structural arrangement in which the eleven subunits are stabilized through eleven intersubunit β-sheets to form a β-wheel with a large central hole. The nature of the binding of L-tryptophan in clefts between adjacent β-sheets in the β-wheel suggests that this binding induces conformational changes in the flexible residues 25-33 and 49-52. It is argued that upon binding, the messenger RNA target forms a matching circle in which eleven U/GAG repeats are bound to the surface of the protein ondecamer modified by the binding of L-tryptophan

siRNA Screen Identifies Trafficking Host Factors that Modulate Alphavirus Infection

<div><p>Little is known about the repertoire of cellular factors involved in the replication of pathogenic alphaviruses. To uncover molecular regulators of alphavirus infection, and to identify candidate drug targets, we performed a high-content imaging-based siRNA screen. We revealed an actin-remodeling pathway involving Rac1, PIP5K1- α, and Arp3, as essential for infection by pathogenic alphaviruses. Infection causes cellular actin rearrangements into large bundles of actin filaments termed actin foci. Actin foci are generated late in infection concomitantly with alphavirus envelope (E2) expression and are dependent on the activities of Rac1 and Arp3. E2 associates with actin in alphavirus-infected cells and co-localizes with Rac1–PIP5K1-α along actin filaments in the context of actin foci. Finally, Rac1, Arp3, and actin polymerization inhibitors interfere with E2 trafficking from the trans-Golgi network to the cell surface, suggesting a plausible model in which transport of E2 to the cell surface is mediated via Rac1- and Arp3-dependent actin remodeling.</p></div

Alphavirus infection causes actin rearrangements into actin foci that are Rac1- and Arp3-dependent and that co-localize with Rac1, PIP5K1-α, and E2.

<p>(<b>A</b>) Representative confocal images of mock-, VEEV-, CHIKV-, or RVFV-infected HeLa cells (MOIs = 0.5, 5, or 3, respectively). Cells were fixed and stained with virus-specific antibodies (VEEV and CHIKV E2, RVFV nucleoprotein; shown in green) and phalloidin (red) 18 h (VEEV) or 24 h (CHIKV, RVFV) after infection. Nucleus staining is shown in blue. Representative actin foci are indicated by asterisks. (<b>B</b>) High-content quantitative image-based analysis was used to measure infection rates of VEEV, CHIKV, and RVFV (left panel), and the number of actin foci per cell (number of actin foci/total cell number, right panel). Analysis is based on single Z sections. ***, <i>p</i> < 0.0001, Student's <i>t</i> test (between the sample and mock). (<b>C</b>) VEEV-infected HeLa cells (MOI = 0.5) were fixed in formalin at the indicated time points, stained, and analyzed as in (<b>B</b>). (<b>B</b>–<b>C</b>) Values represent the mean ± SD, n ≥12. (<b>D</b>) Representative confocal images of VEEV-infected HeLa cells (MOI = 0.5) pretreated with the Rac1 inhibitor EHT1864 or Arp3 inhibitor CK548. Cells were fixed 18 h after virus addition and stained with VEEV E2-specific antibody (green), phalloidin (red), and a nuclear stain (blue). (<b>E</b>) High-content quantitative image-based analysis was used to measure infection rates of VEEV and the number of actin foci per cell. (<b>F</b>) Confocal images of VEEV-infected HeLa cells (MOI = 5). Co-localization of hemagglutinin (HA)-tagged PIP5K1-α (top panel) or Rac1 (bottom panel) (blue), actin (red), and VEEV E2 (green), at a single z section is shown (left panel). Insets: zoom on actin filaments indicated by white arrows. Single channel intensities were measured along lines crossing different actin clusters (right panel). VEEV was added to HeLa cells that were reverse-transfected with a plasmid encoding HA-tagged PIP5K1-α or tetracycline-induced T-Rex HeLa cells that expressed Rac1 fused to eGFP. Cells were fixed 20 h later, permeabilized, and stained with VEEV E2-specific antibody, phalloidin, and an antibody against HA.</p

Actin polymerization plays a role at a late stage of alphavirus infection.

<p>(<b>A</b> and <b>B</b>) High-content quantitative image-based analysis of relative VEEV and VEEV TC-83 infection rates in time-of-addition experiments. (<b>A</b>) VEEV-infected HeLa cells (MOI = 0.5) were treated with increasing concentrations of latrunculin A at the indicated time points prior to (-1 h) or after (+1–7 h) virus addition. Cells were fixed 20 h after addition of virus and stained for high-content quantitative image-based analysis with virus-specific antibodies. (<b>B</b>) VEEV TC-83 (MOI = 1)-infected HeLa cells were treated with cytochalasin D as in (<b>A</b>). Cells were fixed 12 h after addition of virus, stained, and analyzed as in (<b>A</b>). (<b>C</b>) HeLa cells were infected with VEEV (MOI = 0.5) for 3 h and then treated with increasing concentrations of cytochalasin B, cytochalasin D, latrunculin A, or nocodazole. Cells were fixed in formalin 17 h later, stained, and analyzed as in (<b>A</b>). (<b>A-C</b>) Results are normalized to DMSO-treated samples. (<b>D</b>) HeLa cells were infected as in (<b>C</b>) for 3 h and then treated with increasing concentrations of cytochalasin D or latrunculin A. After 17 h, virus titer in the supernatants was determined by plaque assay. Values represent the mean ± SD, n = 2. (<b>E</b>) Primary human astrocytes were infected with VEEV TC-83 (MOI = 0.005) for 5 h and then treated with increasing concentrations of inhibitors. After 6 h, virus titer in the supernatants was determined by plaque assay. (<b>F</b>) Aliquots of the cells treated in (<b>A</b>) were lysed and analyzed for E2 expression by immunoblotting (GAPDH was used as a loading control). Densitometric analysis of western blots was performed with ImageJ. (<b>G</b>) VEEV copy number (intracellular vRNA) in HeLa cells following treatment with inhibitors was determined by qRT-PCR. HeLa cells were inoculated with VEEV TC-83 (MOI = 2) and 5 h later treated with the indicated inhibitors. Cells were lysed and analyzed for virus copy number 11 h after virus addition. (<b>A-C, E, G</b>) Values represent the mean ± SD, n = 3. *, <i>p</i> < 0.05; **, <i>p</i> < 0.01; ***, <i>p</i> < 0.001; n.s., not significant, Student's <i>t</i> test (between the sample and DMSO-treated cells).</p