Endosome-to-cytosol transport of viral nucleocapsids.

During viral infection, fusion of the viral envelope with endosomal membranes and nucleocapsid release were thought to be concomitant events. We show here that for the vesicular stomatitis virus they occur sequentially, at two successive steps of the endocytic pathway. Fusion already occurs in transport intermediates between early and late endosomes, presumably releasing the nucleocapsid within the lumen of intra-endosomal vesicles, where it remains hidden. Transport to late endosomes is then required for the nucleocapsid to be delivered to the cytoplasm. This last step, which initiates infection, depends on the late endosomal lipid lysobisphosphatidic acid (LBPA) and its putative effector Alix/AIP1, and is regulated by phosphatidylinositol-3-phosphate (PtdIns3P) signalling via the PtdIns3P-binding protein Snx16. We conclude that the nucleocapsid is exported into the cytoplasm after the back-fusion of internal vesicles with the limiting membrane of late endosomes, and that this process is controlled by the phospholipids LBPA and PtdIns3P and their effectors.

Auxin inhibits endocytosis and promotes its own efflux from cells.

One of the mechanisms by which signalling molecules regulate cellular behaviour is modulating subcellular protein translocation. This mode of regulation is often based on specialized vesicle trafficking, termed constitutive cycling, which consists of repeated internalization and recycling of proteins to and from the plasma membrane. No such mechanism of hormone action has been shown in plants although several proteins, including the PIN auxin efflux facilitators, exhibit constitutive cycling. Here we show that a major regulator of plant development, auxin, inhibits endocytosis. This effect is specific to biologically active auxins and requires activity of the Calossin-like protein BIG. By inhibiting the internalization step of PIN constitutive cycling, auxin increases levels of PINs at the plasma membrane. Concomitantly, auxin promotes its own efflux from cells by a vesicle-trafficking-dependent mechanism. Furthermore, asymmetric auxin translocation during gravitropism is correlated with decreased PIN internalization. Our data imply a previously undescribed mode of plant hormone action: by modulating PIN protein trafficking, auxin regulates PIN abundance and activity at the cell surface, providing a mechanism for the feedback regulation of auxin transport.

CRISPR/Cas9-based gene targeting using synthetic guide RNAs enables robust cell biological analyses.

A key goal for cell biological analyses is to assess the phenotypes that result from eliminating a target gene. Since the early 1990s, the predominant strategy utilized in human tissue culture cells has been RNA interference (RNAi)-mediated protein depletion. However, RNAi suffers well-documented off-target effects as well as incomplete and reversible protein depletion. The implementation of CRISPR/Cas9-based DNA cleavage has revolutionized the capacity to conduct functional studies in human cells. However, this approach is still underutilized for conducting visual phenotypic analyses, particularly for essential genes that require conditional strategies to eliminate their gene products. Optimizing this strategy requires effective and streamlined approaches to introduce the Cas9 guide RNA into target cells. Here we assess the efficacy of synthetic guide RNA transfection to eliminate gene products for cell biological studies. On the basis of three representative gene targets (KIF11, CENPN, and RELA), we demonstrate that transfection of synthetic single guide RNA (sgRNA) and CRISPR RNA (crRNA) guides works comparably for protein depletion as cell lines stably expressing lentiviral-delivered RNA guides. We additionally demonstrate that synthetic sgRNAs can be introduced by reverse transfection on an array. Together, these strategies provide a robust, flexible, and scalable approach for conducting functional studies in human cells.

High-content classification of nucleocytoplasmic import or export inhibitors.

Transcription factors of the nuclear factor kappa B family are the paradigm for signaling dependent nuclear translocation and are ideally suited to analysis through image-based chemical genetic screening. The authors describe combining high-content image analysis with a compound screen to identify compounds affecting either nuclear import or export. Validation in silico and in vitro determined an EC(50) for the nuclear export blocker leptomycin B of 2.4 ng/mL (4.4 nM). The method demonstrated high selectivity (Z' >0.95), speed, and robustness in a screen of a compound collection. It identified the IkappaB protein kinase inhibitor BAY 11 7082 as an import inhibitor, the p38 mitogen-activated protein (MAP) kinase inhibitor PD98509 as an import enhancer, and phorbol ester as an export inhibitor. The results establish a robust method for identifying compounds regulating nucleocytoplasmic import or export and also implicate MAP kinases in nuclear import of nuclear factor kappa B.

Neointimal smooth muscle cells display a proinflammatory phenotype resulting in increased leukocyte recruitment mediated by P-selectin and chemokines.

Leukocyte recruitment is crucial for the response to vascular injury in spontaneous and accelerated atherosclerosis. Whereas the mechanisms of leukocyte adhesion to endothelium or matrix-bound platelets have been characterized, less is known about the proadhesive role of smooth muscle cells (SMCs) exposed after endothelial denudation. In laminar flow assays, neointimal rat SMCs (niSMCs) supported a 2.5-fold higher arrest of monocytes and "memory" T lymphocytes than medial SMCs, which was dependent on both P-selectin and VLA-4, as demonstrated by blocking antibodies. The increase in monocyte arrest on niSMCs was triggered by the CXC chemokine GRO-alpha and fractalkine, whereas "memory" T cell arrest was mediated by stromal cell-derived factor (SDF)-1alpha. This functional phenotype was paralleled by a constitutively increased mRNA and surface expression of P-selectin and of relevant chemokines in niSMCs, as assessed by real-time PCR and flow cytometry. The increased expression of P-selectin in niSMCs versus medial SMCs was associated with enhanced NF-kappaB activity, as revealed by immunofluorescence staining for nuclear p65 in vitro. Inhibition of NF-kappaB by adenoviral IkappaBalpha in niSMCs resulted in a marked reduction of increased leukocyte arrest in flow. Furthermore, P-selectin expression by niSMCs in vivo was confirmed in a hypercholesterolemic mouse model of vascular injury by double immunofluorescence and by RT-PCR after laser microdissection. In conclusion, we have identified a NF-kappaB-mediated proinflammatory phenotype of niSMCs that is characterized by increased P-selectin and chemokine expression and thereby effectively supports leukocyte recruitment.

Actin-based motility of endosomes is linked to the polar tip growth of root hairs.

Plant tip growth has been recognized as an actin-based cellular process requiring targeted exocytosis and compensatory endocytosis to occur at the growth cone. However, the identity of subcellular compartments involved in polarized membrane trafficking pathways remains enigmatic in plants. Here we characterize endosomal compartments in tip-growing root hair cells. We demonstrate their presence at the growing tip and differential distribution upon cessation of tip growth. We also show that both the presence of endosomes as well as their rapid movements within the tip region depends on an intact actin cytoskeleton and involves actin polymerization. In conclusion, actin-propelled endosomal motility is tightly linked to the polar tip growth of root hairs.

A high-content subtractive screen for selecting small molecules affecting internalization of GPCRs.

G-protein-coupled receptors (GPCRs) are pivotal in cellular responses to the environment and are common drug targets. Identification of selective small molecules acting on single GPCRs is complicated by the shared machinery coupling signal transduction to physiology. Here, we demonstrate a high-content screen using a panel of GPCR assays to identify receptor selective molecules acting within the kinase/phosphatase inhibitor family. A collection of 88 kinase and phosphatase inhibitors was screened against seven agonist-induced GPCR internalization cell models as well as transferrin uptake in human embryonic kidney cells. Molecules acting on a single receptor were identified through excluding pan-specific compounds affecting housekeeping endocytosis or disrupting internalization of multiple receptors. We identified compounds acting on a sole GPCR from activities in a broad range of chemical structures that could not be easily sorted by conventional means. Selective analysis can therefore rapidly select compounds selectively affecting GPCR activity with specificity to one receptor class through high-content screening.

Visual genome-wide RNAi screening to identify human host factors required for Trypanosoma cruzi infection.

The protozoan parasite Trypanosoma cruzi is the etiologic agent of Chagas disease, a neglected tropical infection that affects millions of people in the Americas. Current chemotherapy relies on only two drugs that have limited efficacy and considerable side effects. Therefore, the development of new and more effective drugs is of paramount importance. Although some host cellular factors that play a role in T. cruzi infection have been uncovered, the molecular requirements for intracellular parasite growth and persistence are still not well understood. To further study these host-parasite interactions and identify human host factors required for T. cruzi infection, we performed a genome-wide RNAi screen using cellular microarrays of a printed siRNA library that spanned the whole human genome. The screening was reproduced 6 times and a customized algorithm was used to select as hits those genes whose silencing visually impaired parasite infection. The 162 strongest hits were subjected to a secondary screening and subsequently validated in two different cell lines. Among the fourteen hits confirmed, we recognized some cellular membrane proteins that might function as cell receptors for parasite entry and others that may be related to calcium release triggered by parasites during cell invasion. In addition, two of the hits are related to the TGF-beta signaling pathway, whose inhibition is already known to diminish levels of T. cruzi infection. This study represents a significant step toward unveiling the key molecular requirements for host cell invasion and revealing new potential targets for antiparasitic therapy.

Automated genome-wide visual profiling of cellular proteins involved in HIV infection.

Recent genome-wide RNAi screens have identified >842 human genes that affect the human immunodeficiency virus (HIV) cycle. The list of genes implicated in infection differs between screens, and there is minimal overlap. A reason for this variance is the interdependence of HIV infection and host cell function, producing a multitude of indirect or pleiotropic cellular effects affecting the viral infection during RNAi screening. To overcome this, the authors devised a 15-dimensional phenotypic profile to define the viral infection block induced by CD4 silencing in HeLa cells. They demonstrate that this phenotypic profile excludes nonspecific, RNAi-based side effects and viral replication defects mediated by silencing of housekeeping genes. To achieve statistical robustness, the authors used automatically annotated RNAi arrays for seven independent genome-wide RNAi screens. This identified 56 host genes, which reliably reproduced CD4-like phenotypes upon HIV infection. The factors include 11 known HIV interactors and 45 factors previously not associated with HIV infection. As proof of concept, the authors confirmed that silencing of PAK1, Ku70, and RNAseH2A impaired HIV replication in Jurkat cells. In summary, multidimensional, visual profiling can identify genes required for HIV infection.

'Molecular farming' of antibodies in plants.

'Molecular farming' is the production of valuable recombinant proteins in transgenic organisms on an agricultural scale. While plants have long been used as a source of medicinal compounds, molecular farming represents a novel source of molecular medicines, such as plasma proteins, enzymes, growth factors, vaccines and recombinant antibodies, whose medical benefits are understood at a molecular level. Until recently, the broad use of molecular medicines was limited because of the difficulty in producing these proteins outside animals or animal cell culture. The application of molecular biology and plant biotechnology in the 1990s showed that many molecular medicines or vaccines could be synthesised in plants and this technology is termed 'molecular farming'. It results in pharmaceuticals that are safer, easier to produce and less expensive than those produced in animals or microbial culture. An advantage of molecular farming lies in the ability to perform protein production on a massive scale using hectares of cultivated plants. These plants can then be harvested and transported using the agricultural infrastructure. Thus, molecular farming allows rapid progress from genetic engineering to crop production, and new cash crops producing recombinant proteins are already being commercially exploited. We speculate that as functional genomics teaches us more about the nature of disease, molecular farming will produce many of the protein therapeutics that can remedy it.

Uptake of a fluorescent marker in plant cells is sensitive to brefeldin A and wortmannin.

We assessed FM1-43 [N-(3-triethylammoniumpropyl)-4-(4-[dibutylamino]styryl)pyridinium dibromide] as a fluorescent endocytosis marker in intact, walled plant cells. At 4 degrees C, FM1-43 stained the plasma membrane, and after 30 to 120 min of incubation at 26 degrees C, FM1-43 labeled cytoplasmic vesicles and then the vacuole. Fluorimetric quantitation demonstrated dye uptake temperature sensitivity (approximately 65% reduction at 16 degrees C, >90% at 4 degrees C). FM1-43 uptake in suspension cells was stimulated more than twofold by brefeldin A and inhibited approximately 0.4-fold by wortmannin. FM1-43 delivery to the vacuole was largely inhibited by brefeldin A, although overall uptake was stimulated, and brefeldin A treatment caused the accumulation of large prevacuolar endosomal vesicles heavily labeled with FM1-43. Three-dimensional time lapse imaging revealed that FM1-43-labeled vacuoles and vesicles are highly dynamic. Thus, FM1-43 serves as a fluorescent marker for imaging and quantifying membrane endocytosis in intact plant cells.

An unusual internal ribosomal entry site of inverted symmetry directs expression of a potato leafroll polerovirus replication-associated protein.

Potato leafroll polerovirus (PLRV) genomic RNA acts as a polycistronic mRNA for the production of proteins P0, P1, and P2 translated from the 5'-proximal half of the genome. Within the P1 coding region we identified a 5-kDa replication-associated protein 1 (Rap1) essential for viral multiplication. An internal ribosome entry site (IRES) with unusual structure and location was identified that regulates Rap1 translation. Core structural elements for internal ribosome entry include a conserved AUG codon and a downstream GGAGAGAGAGG motif with inverted symmetry. Reporter gene expression in potato protoplasts confirmed the internal ribosome entry function. Unlike known IRES motifs, the PLRV IRES is located completely within the coding region of Rap1 at the center of the PLRV genome.

Targeting tryptophan decarboxylase to selected subcellular compartments of tobacco plants affects enzyme stability and in vivo function and leads to a lesion-mimic phenotype.

Tryptophan decarboxylase (TDC) is a cytosolic enzyme that catalyzes an early step of the terpenoid indole alkaloid biosynthetic pathway by decarboxylation of L-tryptophan to produce the protoalkaloid tryptamine. In the present study, recombinant TDC was targeted to the chloroplast, cytosol, and endoplasmic reticulum (ER) of tobacco (Nicotiana tabacum) plants to evaluate the effects of subcellular compartmentation on the accumulation of functional enzyme and its corresponding enzymatic product. TDC accumulation and in vivo function was significantly affected by the subcellular localization. Immunoblot analysis demonstrated that chloroplast-targeted TDC had improved accumulation and/or stability when compared with the cytosolic enzyme. Because ER-targeted TDC was not detectable by immunoblot analysis and tryptamine levels found in transient expression studies and in transgenic plants were low, it was concluded that the recombinant TDC was most likely unstable if ER retained. Targeting TDC to the chloroplast stroma resulted in the highest accumulation level of tryptamine so far reported in the literature for studies on heterologous TDC expression in tobacco. However, plants accumulating high levels of functional TDC in the chloroplast developed a lesion-mimic phenotype that was probably triggered by the relatively high accumulation of tryptamine in this compartment. We demonstrate that subcellular targeting may provide a useful strategy for enhancing accumulation and/or stability of enzymes involved in secondary metabolism and to divert metabolic flux toward desired end products. However, metabolic engineering of plants is a very demanding task because unexpected, and possibly unwanted, effects may be observed on plant metabolism and/or phenotype.