A versatile method for cell-specific profiling of translated mRNAs in Drosophila

In Drosophila melanogaster few methods exist to perform rapid cell-type or tissue-specific expression profiling. A translating ribosome affinity purification (TRAP) method to profile actively translated mRNAs has been developed for use in a number of multicellular organisms although it has only been implemented to examine limited sets of cell- or tissue-types in these organisms. We have adapted the TRAP method for use in the versatile GAL4/UAS system of Drosophila allowing profiling of almost any tissue/cell-type with a single genetic cross. We created transgenic strains expressing a GFP-tagged ribosomal protein, RpL10A, under the control of the UAS promoter to perform cell-type specific translatome profiling. The GFP::RpL10A fusion protein incorporates efficiently into ribosomes and polysomes. Polysome affinity purification strongly enriches mRNAs from expected genes in the targeted tissues with sufficient sensitivity to analyze expression in small cell populations. This method can be used to determine the unique translatome profiles in different cell-types under varied physiological, pharmacological and pathological conditions.     

Role of β-catenin in post-meiotic male germ cell differentiation

Though roles of β-catenin signaling during testis development have been well established, relatively little is known about its role in postnatal testicular physiology. Even less is known about its role in post-meiotic germ cell development and differentiation. Here, we report that β-catenin is highly expressed in post-meiotic germ cells and plays an important role during spermiogenesis in mice. Spermatid-specific deletion of β-catenin resulted in significantly reduced sperm count, increased germ cell apoptosis and impaired fertility. In addition, ultrastructural studies show that the loss of β-catenin in post-meiotic germ cells led to acrosomal defects, anomalous release of immature spermatids and disruption of adherens junctions between Sertoli cells and elongating spermatids (apical ectoplasmic specialization; ES). These defects are likely due to altered expression of several genes reportedly involved in Sertoli cell-germ cell adhesion and germ cell differentiation, as revealed by gene expression analysis. Taken together, our results suggest that β-catenin is an important molecular link that integrates Sertoli cell-germ cell adhesion with the signaling events essential for post-meiotic germ cell development and maturation. Since β-catenin is also highly expressed in the Sertoli cells, we propose that binding of germ cell β-catenin complex to β-catenin complex on Sertoli cell at the apical ES surface triggers a signaling cascade that regulates post-meiotic germ cell differentiation.     

Single injection recombinant vesicular stomatitis virus vaccines protect ferrets against lethal Nipah virus disease

Background

Nipah virus (NiV) is a highly pathogenic zoonotic agent in the family Paramyxoviridae that is maintained in nature by bats. Outbreaks have occurred in Malaysia, Singapore, India, and Bangladesh and have been associated with 40 to 75% case fatality rates. There are currently no vaccines or postexposure treatments licensed for combating human NiV infection.

Methods and results

Four groups of ferrets received a single vaccination with different recombinant vesicular stomatitis virus vectors expressing: Group 1, control with no glycoprotein; Group 2, the NiV fusion protein (F); Group 3, the NiV attachment protein (G); and Group 4, a combination of the NiV F and G proteins. Animals were challenged intranasally with NiV 28 days after vaccination. Control ferrets in Group 1 showed characteristic clinical signs of NiV disease including respiratory distress, neurological disorders, viral load in blood and tissues, and gross lesions and antigen in target tissues; all animals in this group succumbed to infection by day 8. Importantly, all specifically vaccinated ferrets in Groups 2-4 showed no evidence of clinical illness and survived challenged. All animals in these groups developed anti-NiV F and/or G IgG and neutralizing antibody titers. While NiV RNA was detected in blood at day 6 post challenge in animals from Groups 2-4, the levels were orders of magnitude lower than animals from control Group 1.

Conclusions

These data show protective efficacy against NiV in a relevant model of human infection. Further development of this technology has the potential to yield effective single injection vaccines for NiV infection.

     

Large favorable enthalpy changes drive specific RNA recognition by RNA recognition motif proteins

The RNA recognition motif (RRM) is a prevalent class of RNA binding domains. Although a number of RRM/RNA structures have been determined, thermodynamic analyses are relatively uncommon. Here, we use isothermal titration calorimetry to characterize single-stranded (ss)RNA binding by four representative RRM-containing proteins: (i) U2AF(65), (ii) SXL, (iii) TIA-1, and (iv) PAB. In all cases, ssRNA binding is accompanied by remarkably large favorable enthalpy changes (-30 to -60 kcal mol(-1)) and unfavorable entropy changes. Alterations of key RRM residues and binding sites indicate that under the nearly physiological conditions of these studies, large thermodynamic changes represent a signature of specific ssRNA recognition by RRMs.     

Durability of a Vesicular Stomatitis Virus-Based Marburg Virus Vaccine in Nonhuman Primates

The filoviruses, Marburg virus (MARV) and Ebola virus, causes severe hemorrhagic fever with high mortality in humans and nonhuman primates. A promising filovirus vaccine under development is based on a recombinant vesicular stomatitis virus (rVSV) that expresses individual filovirus glycoproteins (GPs) in place of the VSV glycoprotein (G). These vaccines have shown 100% efficacy against filovirus infection in nonhuman primates when challenge occurs 28–35 days after a single injection immunization. Here, we examined the ability of a rVSV MARV-GP vaccine to provide protection when challenge occurs more than a year after vaccination. Cynomolgus macaques were immunized with rVSV-MARV-GP and challenged with MARV approximately 14 months after vaccination. Immunization resulted in the vaccine cohort of six animals having anti-MARV GP IgG throughout the pre-challenge period. Following MARV challenge none of the vaccinated animals showed any signs of clinical disease or viremia and all were completely protected from MARV infection. Two unvaccinated control animals exhibited signs consistent with MARV infection and both succumbed. Importantly, these data are the first to show 100% protective efficacy against any high dose filovirus challenge beyond 8 weeks after final vaccination. These findings demonstrate the durability of VSV-based filovirus vaccines.     

Stress-Responsive Systems Set Specific Limits to the Overproduction of Membrane Proteins in Bacillus subtilis

Essential membrane proteins are generally recognized as relevant potential drug targets due to their exposed localization in the cell envelope. Unfortunately, high-level production of membrane proteins for functional and structural analyses is often problematic. This is mainly due to their high overall hydrophobicity. To develop new concepts for membrane protein overproduction, we investigated whether the biogenesis of overproduced membrane proteins is affected by stress response-related proteolytic systems in the membrane. For this purpose, the well-established expression host Bacillus subtilis was used to overproduce eight essential membrane proteins from B. subtilis and Staphylococcus aureus. The results show that the σ^W regulon (responding to cell envelope perturbations) and the CssRS two-component regulatory system (responding to unfolded exported proteins) set critical limits to membrane protein production in large quantities. The identified sigW or cssRS mutant B. subtilis strains with significantly improved capacity for membrane protein production are interesting candidate expression hosts for fundamental research and biotechnological applications. Importantly, our results pinpoint the interdependent expression and function of membrane-associated proteases as key parameters in bacterial membrane protein production.Membrane-embedded proteins are crucial for cellular homeostasis and life. Membrane proteins generally account for about 30% of the open reading frames in both prokaryotic and eukaryotic genomes (49), and they are involved in a wide range of different tasks. These include vital processes, such as energy transduction, phospholipid biosynthesis, protein translocation, cell wall biogenesis, cell division, and control of cell shape (52). Importantly, membrane proteins are partially exposed to the extracytoplasmic environment, which makes them readily accessible to drugs. For this reason, membrane proteins have become a major class of proteins in terms of current drug targets. Essential membrane proteins, which are indispensable for cell proliferation under specific conditions, are especially interesting from the pharmaceutical and biomedical perspectives because they represent prime targets for chemotherapy.Unfortunately, progress in the area of membrane protein research has so far been slow. This has been attributed primarily to the high hydrophobicity of membrane proteins, which complicates high-level production, purification, and crystallization (25). Consequently, yields are often frustratingly low, as underscored by a series of elegant screens for membrane protein overproduction in Escherichia coli (10, 11, 15, 47). Moreover, the accumulation of overproduced proteins in biological membranes may affect bilayer integrity, which would be toxic for the producing cell (33). Additional limitations are potentially caused by saturation of the cellular machinery for insertion of proteins into the membrane or by saturation of the membrane itself, resulting in the cytoplasmic accumulation of overproduced membrane proteins as well as native membrane proteins (46). Such overproduced proteins are usually misfolded and/or inactive, and they have a high tendency to form insoluble (micro)aggregates. These practical problems focus attention on the fundamental question of which cellular mechanisms set the key limits to membrane protein production.In the present studies, we show that important problems in membrane protein overproduction can be overcome by using different strains of the gram-positive bacterium Bacillus subtilis as the expression host, and we identify two key mechanisms that set limits to membrane protein production in this organism. B. subtilis is highly appreciated for biotechnological applications because it has a large capacity to secrete high-quality proteins into the culture medium and because it has the status of generally recognized as safe (18, 38, 50). Furthermore, B. subtilis is amenable to genetic engineering, and many expression systems are available (2, 16, 31, 40, 43, 44). This prompted us to investigate whether the secretion machinery of B. subtilis, which is also involved in membrane protein biogenesis (52), can be exploited for membrane protein overproduction. As model proteins for our studies, we selected essential membrane proteins that have a good potential to serve as targets for novel antimicrobial drugs. Accordingly, we not only overproduced B. subtilis membrane proteins but also their orthologues from the important human pathogen Staphylococcus aureus. Studies on these essential proteins are considered to be of major relevance, since S. aureus is rapidly gaining resistance against all available antibiotics and novel antibiotics against this pathogen are urgently needed (7, 17). The results of the present studies with homologous membrane proteins from B. subtilis and S. aureus show that, like in other expression hosts, bottlenecks in membrane protein production also do exist in B. subtilis. Importantly, however, at least some of the encountered bottlenecks can be overcome, because they relate to two dispensable membrane-associated stress-responsive systems: the σ^W regulon and the CssRS two-component regulatory system. Thus, the removal of at least one of these stress-responsive systems can result in drastically improved yields of particular membrane proteins.     

Two proteolytic modules are involved in regulated intramembrane proteolysis of Bacillus subtilis RsiW

Stress‐induced degradation of the Bacillus subtilis anti‐sigma factor RsiW results in the induction of genes controlled by the extracytoplasmic function sigma factor σ^W. RsiW is cleaved by the mechanism of regulated intramembrane proteolysis at site‐1 and ‐2 by PrsW and RasP respectively, and is then further degraded by cytoplasmic Clp peptidases. In a reconstituted Escherichia coli system, PrsW removes 40 amino acids from RsiW by cleaving between Ala168 and Ser169 of the extracytoplasmic domain, thereby generating RsiW‐S1. Further trimming of RsiW‐S1's C‐terminus by the periplasmic tail‐specific protease Tsp is crucial for subsequent RasP‐catalysed clipping. In B. subtilis, mutation of RsiW at Ala168 severely impairs site‐1 processing. RsiW‐S1 is undetectable in wild‐type B. subtilis and knockout strains lacking various extracytoplasmic proteases. While it can be stabilized by C‐terminal tagging, even this fusion protein is still attacked. Thus, several peptidases seem to be involved in trimming of RsiW downstream of PrsW and upstream of RasP in B. subtilis. Overall, the RsiW degradation pathway can be subdivided into two modules each consisting of a site‐specific peptidase that prepares RsiW for further degradation by downstream proteases.     

Deciliation Is Associated with Dramatic Remodeling of Epithelial Cell Junctions and Surface Domains

Stress-induced shedding of motile cilia (autotomy) has been documented in diverse organisms and likely represents a conserved cellular reaction. However, little is known about whether primary cilia are shed from mammalian epithelial cells and what impact deciliation has on polarized cellular organization. We show that several chemically distinct agents trigger autotomy in epithelial cells. Surprisingly, deciliation is associated with a significant, but reversible increase in transepithelial resistance. This reflects substantial reductions in tight junction proteins associated with “leaky” nephron segments (e.g., claudin-2). At the same time, apical trafficking of gp80/clusterin and gp114/CEACAM becomes randomized, basal-lateral delivery of Na,K-ATPase is reduced, and expression of the nonciliary apical protein gp135/podocalyxin is greatly decreased. However, ciliogenesis-impaired MDCK cells do not undergo continual junction remodeling, and mature cilia are not required for autotomy-associated remodeling events. Deciliation and epithelial remodeling may be mechanistically linked processes, because RNAi-mediated reduction of Exocyst subunit Sec6 inhibits ciliary shedding and specifically blocks deciliation-associated down-regulation of claudin-2 and gp135. We propose that ciliary autotomy represents a signaling pathway that impacts the organization and function of polarized epithelial cells.