Secretion of rubella virions and virus-like particles in cultured epithelial cells

Molecular aspects of the teratogenesis of rubella virus

Rubella or German measles is an infection caused by rubella virus (RV). Infection of children and adults is usually characterized by a mild exanthematous febrile illness. However, RV is a major cause of birth defects and fetal death following infection in pregnant women. RV is a teratogen and is a major cause of public health concern as there are more than 100,000 cases of congenital rubella syndrome (CRS) estimated to occur every year. Several lines of evidence in the field of molecular biology of RV have provided deeper insights into the teratogenesis process. The damage to the growing fetus in infected mothers is multifactorial, arising from a combination of cellular damage, as well as its effect on the dividing cells. This review focuses on the findings in the molecular biology of RV, with special emphasis on the mitochondrial, cytoskeleton and the gene expression changes. Further, the review addresses in detail, the role of apoptosis in the teratogenesis process.

The key role of rubella virus glycoproteins in the formation of immune response, and perspectives on their use in the development of new recombinant vaccines

Rubella is a highly contagious viral disease which is mostly threatens to women of reproductive age. Existent live attenuated vaccines are effective enough, but have some drawbacks and are unusable for a certain group of people, including pregnant women and people with AIDS and other immunodeficiency. Thereby the development of alternative non-replicating, recombinant vaccines undoubtedly is needed. This review discusses the protein E1 and E2 role in formation of immune response and perspectives in development of new generation recombinant vaccines using them.

Rubella virus-like replicon particles: analysis of encapsidation determinants and non-structural roles of capsid protein in early post-entry replication

Rubella virus (RUBV) contains a plus-strand RNA genome with two ORFs, one encoding the non-structural replicase proteins (NS-ORF) and the second encoding the virion structural proteins (SP-ORF). This study describes development and use of a trans-encapsidation system for the assembly of infectious RUBV-like replicon particles (VRPs) containing RUBV replicons (self replicating genomes with the SP-ORF replaced with a reporter gene). First, this system was used to map signals within the RUBV genome that mediate packaging of viral RNA. Mutations within a proposed packaging signal did not significantly affect relative packaging efficiency. The insertion of various fragments derived from the RUBV genome into Sindbis virus replicons revealed that there are several regions within the RUBV genome capable of enhancing encapsidation of heterologous replicon RNAs. Secondly, the trans-encapsidation system was used to analyse the effect of alterations within the capsid protein (CP) on release of VRPs and subsequent initiation of replication in newly infected cells. Deletion of the N-terminal eight amino acids of the CP reduced VRP titre significantly, which could be partially complemented by native CP provided in trans, indicating that this mutation affected an entry or post-entry event in the replication cycle. To test this hypothesis, the trans-encapsidation system was used to demonstrate the rescue of a lethal deletion within P150, one of the virus replicase proteins, by CP contained within the virus particle. This novel finding substantiated the functional role of CP in early post-entry replication.

The Rubella virus capsid is an anti-apoptotic protein that attenuates the pore-forming ability of Bax

Apoptosis is an important mechanism by which virus-infected cells are eliminated from the host. Accordingly, many viruses have evolved strategies to prevent or delay apoptosis in order to provide a window of opportunity in which virus replication, assembly and egress can take place. Interfering with apoptosis may also be important for establishment and/or maintenance of persistent infections. Whereas large DNA viruses have the luxury of encoding accessory proteins whose primary function is to undermine programmed cell death pathways, it is generally thought that most RNA viruses do not encode these types of proteins. Here we report that the multifunctional capsid protein of Rubella virus is a potent inhibitor of apoptosis. The main mechanism of action was specific for Bax as capsid bound Bax and prevented Bax-induced apoptosis but did not bind Bak nor inhibit Bak-induced apoptosis. Intriguingly, interaction with capsid protein resulted in activation of Bax in the absence of apoptotic stimuli, however, release of cytochrome c from mitochondria and concomitant activation of caspase 3 did not occur. Accordingly, we propose that binding of capsid to Bax induces the formation of hetero-oligomers that are incompetent for pore formation. Importantly, data from reverse genetic studies are consistent with a scenario in which the anti-apoptotic activity of capsid protein is important for virus replication. If so, this would be among the first demonstrations showing that blocking apoptosis is important for replication of an RNA virus. Finally, it is tempting to speculate that other slowly replicating RNA viruses employ similar mechanisms to avoid killing infected cells.

Among the variety of defense systems employed by mammalian cells to combat virus infection, apoptosis or programmed cell death is the most drastic response. Some large DNA viruses encode proteins whose sole function is to block apoptosis. Conversely, very little is known about whether RNA viruses encode analogous proteins. In many cases, RNA viruses are able to replicate before cell death occurs, which may be one reason why so little thought has been given to this topic. However, a number of RNA viruses, some of which are important human pathogens, have slow replication cycles and it stands to reason that they must block apoptosis during this time period. Here we show that the multifunctional capsid protein of Rubella virus is a potent inhibitor of apoptosis. Data from reverse genetic experiments suggest that the anti-apoptotic function of a virus-encoded protein is important for replication of an RNA virus. We anticipate that other slowly replicating RNA viruses may employ similar mechanisms and, as such, these studies have implications for development of novel anti-virals and vaccines.

Interactions between rubella virus capsid and host protein p32 are important for virus replication

The distribution and morphology of mitochondria are dramatically affected during infection with rubella virus (RV). Expression of the capsid, in the absence of other viral proteins, was found to induce both perinuclear clustering of mitochondria and the formation of electron-dense intermitochondrial plaques, both hallmarks of RV-infected cells. We previously identified p32, a host cell mitochondrial matrix protein, as a capsid-binding protein. Here, we show that two clusters of arginine residues within capsid are required for stable binding to p32. Mutagenic ablation of the p32-binding site in capsid resulted in decreased mitochondrial clustering, indicating that interactions with this cellular protein are required for capsid-dependent reorganization of mitochondria. Recombinant viruses encoding arginine-to-alanine mutations in the p32-binding region of capsid exhibited altered plaque morphology and replicated to lower titers. Further analysis indicated that disruption of stable interactions between capsid and p32 was associated with decreased production of subgenomic RNA and, consequently, infected cells produced significantly lower amounts of viral structural proteins under these conditions. Together, these results suggest that capsid-p32 interactions are important for nonstructural functions of capsid that include regulation of virus RNA replication and reorganization of mitochondria during infection.

Virus-like particles: models for assembly studies and foreign epitope carriers

Virus‐like particles (VLPs), formed by the structural elements of viruses, have received considerable attention over the past two decades. The number of reports on newly obtained VLPs has grown proportionally with the systems developed for the expression of these particles. The chapter outlines the recent achievements in two important fields of research brought about by the availability of VLPs produced in a foreign host. These are: (1) The requirements for VLP assembly and (2) the use of VLPs as carriers for foreign epitopes. VLP technology is a rapidly advancing domain of molecular and structural biology. Extensive progress in VLP studies was achieved as the insect cell based protein production system was developed. This baculovirus expression system has many advantages for the synthesis of viral structural proteins resulting in the formation of VLPs. It allows production of large amounts of correctly folded proteins while also providing cell membranes that can serve as structural elements for enveloped viruses. These features give us the opportunity to gain insights into the interactions and requirements accompanying VLP formation that are similar to the assembly events occurring in mammalian cells. Other encouraging elements are the ability to easily scale up the system and the simplicity of purification of the assembled VLPs. The growing number of VLPs carrying foreign protein fragments on their surface and studies on the successful assembly of these chimeric molecules is a promising avenue towards the development of a new technology, in which the newly designed VLPs will be directed to particular mammalian cell types by exposing specific binding domains. The progress made in modeling the surface of VLPs makes them to date the best candidates for the design of delivery systems that can efficiently reach their targets.

The role of lipid microdomains in virus biology

Many of the highly pathogenic viruses including influenza virus, HIV and others of world wide epidemiological importance are enveloped and possess a membrane around the nucleocapsid containing the viral genome. Viral membrane is required to protect the viral genome and provide important functions for attachment, morphogenesis and transmission. Viral membrane is essentially composed of lipids and proteins. While the proteins on the viral envelope are almost exclusively virally encoded, lipids, on the other hand, are all of host origin and recruited from host membrane. However, lipids on the viral membrane are not incorporated randomly and do not represent average lipid composition of the host membrane. Recent studies support that specific lipid microdomains such as lipid rafts play critical roles in many aspects of the virus infectious cycle including attachment, entry, uncoating, protein transport and sorting as well as viral morphogenesis and budding. Lipid microdomains aid in bringing and concentrating viral components to the budding site. Similarly, specific viral protein plays an important role in organizing lipid microdomains in and around the assembly and budding site of the virus. This review deals with the specific role of lipid microdomains in different aspects of the virus life cycle and the role of specific viral proteins in organizing the lipid microdomains.

The missing link in coronavirus assembly. Retention of the avian coronavirus infectious bronchitis virus envelope protein in the pre-Golgi compartments and physical interaction between the envelope and membrane proteins

One missing link in the coronavirus assembly is the physical interaction between two crucial structural proteins, the membrane (M) and envelope (E) proteins. In this study, we demonstrate that the coronavirus infectious bronchitis virus E can physically interact, via a putative peripheral domain, with M. Deletion of this domain resulted in a drastic reduction in the incorporation of M into virus-like particles. Immunofluorescent staining of cells coexpressing M and E supports that E interacts with M and relocates M to the same subcellular compartments that E resides in. E was retained in the pre-Golgi membranes, prior to being translocated to the Golgi apparatus and the secretory vesicles; M was observed to exhibit similar localization and translocation profiles as E when coexpressed with E. Deletion studies identified the C-terminal 6-residue RDKLYS as the endoplasmic reticulum retention signal of E, and site-directed mutagenesis of the -4 lysine residue to glutamine resulted in the accumulation of E in the Golgi apparatus. The third domain of E that plays a crucial role in virus budding is a putative transmembrane domain present at the N-terminal region, because deletion of the domain resulted in a free distribution of the mutant protein and in dysfunctional viral assembly.

Rubella virus replication and links to teratogenicity

Rubella virus (RV) is the causative agent of the disease known more popularly as German measles. Rubella is predominantly a childhood disease and is endemic throughout the world. Natural infections of rubella occur only in humans and are generally mild. Complications of rubella infection, most commonly polyarthralgia in adult women, do exist; occasionally more serious sequelae occur. However, the primary public health concern of RV infection is its teratogenicity. RV infection of women during the first trimester of pregnancy can induce a spectrum of congenital defects in the newborn, known as congenital rubella syndrome (CRS). The development of vaccines and implementation of vaccination strategies have substantially reduced the incidence of disease and in turn of CRS in developed countries. The pathway whereby RV infection leads to teratogenesis has not been elucidated, but the cytopathology in infected fetal tissues suggests necrosis and/or apoptosis as well as inhibition of cell division of critical precursor cells involved in organogenesis. In cell culture, a number of unusual features of RV replication have been observed, including mitochondrial abnormalities, and disruption of the cytoskeleton; these manifestations are most probably linked and play some role in RV teratogenesis. Further understanding of the mechanism of RV teratogenesis will be brought about by the investigation of RV replication and virus-host interactions.

Rubella virus replication and links to teratogenicity

Rubella virus (RV) is the causative agent of the disease known more popularly as German measles. Rubella is predominantly a childhood disease and is endemic throughout the world. Natural infections of rubella occur only in humans and are generally mild. Complications of rubella infection, most commonly polyarthralgia in adult women, do exist; occasionally more serious sequelae occur. However, the primary public health concern of RV infection is its teratogenicity. RV infection of women during the first trimester of pregnancy can induce a spectrum of congenital defects in the newborn, known as congenital rubella syndrome (CRS). The development of vaccines and implementation of vaccination strategies have substantially reduced the incidence of disease and in turn of CRS in developed countries. The pathway whereby RV infection leads to teratogenesis has not been elucidated, but the cytopathology in infected fetal tissues suggests necrosis and/or apoptosis as well as inhibition of cell division of critical precursor cells involved in organogenesis. In cell culture, a number of unusual features of RV replication have been observed, including mitochondrial abnormalities, and disruption of the cytoskeleton; these manifestations are most probably linked and play some role in RV teratogenesis. Further understanding of the mechanism of RV teratogenesis will be brought about by the investigation of RV replication and virus-host interactions.