Thursday, September 06, 2007
Focus on Coronaviruses
Coronaviruses: Molecular and Cellular Biology
Coronaviruses are positive-strand, enveloped RNA viruses that are important pathogens of mammals and birds. This group of viruses cause enteric or respiratory tract infections in a variety of animals including humans, livestock and pets. The important discovery in 2003 that the causative agent of severe acute respiratory syndrome (SARS) was a new, potentially lethal coronavirus named SARS-CoV provided major impetus to coronavirus research. SARS-CoV spread within months to more than 30 countries causing the first epidemic of the new millennium and becoming a public health nightmare in the countries affected.
Binding and Entry
Coronaviruses bind to host cells primarily through interactions between viral spike glycoproteins and specific host cell surface glycoproteins. Some coronaviruses also bind to sialic acids on glycoproteins and glycolipids via their spike and/or hemagglutinin esterase glycoproteins. The interactions between coronaviruses and host cell receptors are critical determinants of species-specificity, tissue tropism, and virulence.
Replication
Coronaviruses have single-stranded, positive-sense RNA genomes of about 30 kilobases, by far the largest non-segmented RNA virus genomes currently known. The key functions required for coronavirus RNA synthesis are encoded by the viral replicase gene. The gene comprises more than 20,000 nucleotides and encodes two replicase polyproteins, pp1a and pp1ab, that are proteolytically processed by viral proteases. Over the past years, it has become clear that the unique size of the coronavirus genome and the special mechanism that coronaviruses (and several other nidoviruses) have evolved to produce an extensive set of subgenome-length RNAs is linked to the production of a number of nonstructural proteins (nsps) that is unprecedented among RNA viruses. Many of these replicase cleavage products in fact are multidomain proteins themselves, thus further increasing the complexity of protein functions and interactions. Structural studies suggest that several nsps, following their release from larger precursor molecules, form dimers or even multimers. The various pp1a/pp1ab precursors and processing products are thought to assemble into large, membrane-associated complexes that, in a temporally coordinated manner, catalyze the reactions involved in RNA replication and transcription and, most probably, serve yet other functions in the viral life cycle.
Genomic Cis-Acting Elements
In common with the genomes of all other RNA viruses, coronavirus genomes contain cis-acting RNA elements that ensure the specific replication of viral RNA by a virally encoded RNA-dependent RNA polymerase. The embedded cis-acting elements devoted to coronavirus replication constitute a surprisingly small fraction of the total genome, but this is probably a reflection of the fact that coronaviruses have the largest genomes of all RNA viruses. The boundaries of cis-acting elements essential to replication are fairly well-defined, and an increasingly well resolved picture of the RNA secondary structures of these regions is emerging. However, we are only in the early stages of understanding how these cis-acting structures and sequences interact with the viral replicase and host cell components, and much remains to be done before we understand the precise mechanistic roles of such elements in RNA synthesis
Genome Packaging
The assembly of infectious coronavirus particles requires the selection of viral genomic RNA from a cellular pool that contains an abundant excess of non-viral and viral RNAs. Among the seven to ten specific viral mRNAs synthesized in virus-infected cells, only the full length genomic RNA is packaged efficiently into coronavirus particles. Studies have revealed cis-acting elements and trans-acting viral factors involved in coronavirus genome encapsidation and packaging. Understanding the molecular mechanisms of genome selection and packaging is critical for developing antiviral strategies and viral expression vectors based on the coronavirus genome.
SARS
Human infection by SARS coronavirus appears to be limited to the respiratory tract where infection of susceptible cells leads to damage to the pneumocytes resulting in a histological picture of diffuse alveolar damage and a clinical picture of adult respiratory distress syndrome. Diarrhoea is also present but there is limited evidence of damage to the intestinal epithelium. The damage to the respiratory tree appears limited to the lower respiratory tract and there is evidence that the immune response plays a part in the outcome of patients with SARS.
Antiviral Research
Before the emergence of SARS-CoV, no efforts were put into the search for antivirals against coronaviruses. The rapid transmission and high mortality rate made SARS a global threat for which no efficacious therapy was available and empirical strategies had to be used to treat the patients. New insights into the field of the SARS-CoV genome structure and pathogenesis revealed novel potential anti-coronavirus targets. Several proteins encoded by the SARS-CoV could be considered as targets for therapeutic intervention: the spike protein, the main protease, the NTPase/helicase, the RNA dependent RNA polymerase and different other viral protein-mediated processes. Potential anti-SARS-CoV drugs are currently being developed in vivo. The development of effective drugs against SARS-CoV may also provide new strategies for the prevention or treatment of other coronavirus diseases in animals or humans.
Vaccine Development
The emergence and identification of several common and rare human coronaviruses that cause severe lower respiratory tract infection argues for the judicious development of robust coronavirus vaccines and vector platforms. Currently, limited information is available on the correlates of protection against SARS-CoV and other severe lower respiratory tract human coronavirus infections, a clear priority for future research. Passive immunization has been successful in establishing protection from SARS-CoV suggesting an important role for neutralizing antibodies. One important property of future vaccine candidates is the ability to confer protection against multiple variant strains of SARS-CoV, especially in senescent populations that are most at risk for severe disease. Many vaccine candidates are capable of inducing humoral and cellular responses. The development of infectious clones for coronaviruses has facilitated the identification of attenuating mutations, deletions and recombinations which could ultimately result in live attenuated vaccine candidates. Stable vaccine platforms should be developed that allow for rapid intervention strategies against any future emergence coronaviruses. Vaccine correlates that enhance disease after challenge should be thoroughly investigated and mechanisms devised to circumvent vaccine-associated complications.
Further Information
Coronaviruses: Molecular and Cellular Biology (Volker Thiel)
Animal Viruses: Molecular Biology (Thomas C. Mettenleiter and Francisco Sobrino)
Coronaviruses are positive-strand, enveloped RNA viruses that are important pathogens of mammals and birds. This group of viruses cause enteric or respiratory tract infections in a variety of animals including humans, livestock and pets. The important discovery in 2003 that the causative agent of severe acute respiratory syndrome (SARS) was a new, potentially lethal coronavirus named SARS-CoV provided major impetus to coronavirus research. SARS-CoV spread within months to more than 30 countries causing the first epidemic of the new millennium and becoming a public health nightmare in the countries affected.
Binding and Entry
Coronaviruses bind to host cells primarily through interactions between viral spike glycoproteins and specific host cell surface glycoproteins. Some coronaviruses also bind to sialic acids on glycoproteins and glycolipids via their spike and/or hemagglutinin esterase glycoproteins. The interactions between coronaviruses and host cell receptors are critical determinants of species-specificity, tissue tropism, and virulence.
Replication
Coronaviruses have single-stranded, positive-sense RNA genomes of about 30 kilobases, by far the largest non-segmented RNA virus genomes currently known. The key functions required for coronavirus RNA synthesis are encoded by the viral replicase gene. The gene comprises more than 20,000 nucleotides and encodes two replicase polyproteins, pp1a and pp1ab, that are proteolytically processed by viral proteases. Over the past years, it has become clear that the unique size of the coronavirus genome and the special mechanism that coronaviruses (and several other nidoviruses) have evolved to produce an extensive set of subgenome-length RNAs is linked to the production of a number of nonstructural proteins (nsps) that is unprecedented among RNA viruses. Many of these replicase cleavage products in fact are multidomain proteins themselves, thus further increasing the complexity of protein functions and interactions. Structural studies suggest that several nsps, following their release from larger precursor molecules, form dimers or even multimers. The various pp1a/pp1ab precursors and processing products are thought to assemble into large, membrane-associated complexes that, in a temporally coordinated manner, catalyze the reactions involved in RNA replication and transcription and, most probably, serve yet other functions in the viral life cycle.
Genomic Cis-Acting Elements
In common with the genomes of all other RNA viruses, coronavirus genomes contain cis-acting RNA elements that ensure the specific replication of viral RNA by a virally encoded RNA-dependent RNA polymerase. The embedded cis-acting elements devoted to coronavirus replication constitute a surprisingly small fraction of the total genome, but this is probably a reflection of the fact that coronaviruses have the largest genomes of all RNA viruses. The boundaries of cis-acting elements essential to replication are fairly well-defined, and an increasingly well resolved picture of the RNA secondary structures of these regions is emerging. However, we are only in the early stages of understanding how these cis-acting structures and sequences interact with the viral replicase and host cell components, and much remains to be done before we understand the precise mechanistic roles of such elements in RNA synthesis
Genome Packaging
The assembly of infectious coronavirus particles requires the selection of viral genomic RNA from a cellular pool that contains an abundant excess of non-viral and viral RNAs. Among the seven to ten specific viral mRNAs synthesized in virus-infected cells, only the full length genomic RNA is packaged efficiently into coronavirus particles. Studies have revealed cis-acting elements and trans-acting viral factors involved in coronavirus genome encapsidation and packaging. Understanding the molecular mechanisms of genome selection and packaging is critical for developing antiviral strategies and viral expression vectors based on the coronavirus genome.
SARS
Human infection by SARS coronavirus appears to be limited to the respiratory tract where infection of susceptible cells leads to damage to the pneumocytes resulting in a histological picture of diffuse alveolar damage and a clinical picture of adult respiratory distress syndrome. Diarrhoea is also present but there is limited evidence of damage to the intestinal epithelium. The damage to the respiratory tree appears limited to the lower respiratory tract and there is evidence that the immune response plays a part in the outcome of patients with SARS.
Antiviral Research
Before the emergence of SARS-CoV, no efforts were put into the search for antivirals against coronaviruses. The rapid transmission and high mortality rate made SARS a global threat for which no efficacious therapy was available and empirical strategies had to be used to treat the patients. New insights into the field of the SARS-CoV genome structure and pathogenesis revealed novel potential anti-coronavirus targets. Several proteins encoded by the SARS-CoV could be considered as targets for therapeutic intervention: the spike protein, the main protease, the NTPase/helicase, the RNA dependent RNA polymerase and different other viral protein-mediated processes. Potential anti-SARS-CoV drugs are currently being developed in vivo. The development of effective drugs against SARS-CoV may also provide new strategies for the prevention or treatment of other coronavirus diseases in animals or humans.
Vaccine Development
The emergence and identification of several common and rare human coronaviruses that cause severe lower respiratory tract infection argues for the judicious development of robust coronavirus vaccines and vector platforms. Currently, limited information is available on the correlates of protection against SARS-CoV and other severe lower respiratory tract human coronavirus infections, a clear priority for future research. Passive immunization has been successful in establishing protection from SARS-CoV suggesting an important role for neutralizing antibodies. One important property of future vaccine candidates is the ability to confer protection against multiple variant strains of SARS-CoV, especially in senescent populations that are most at risk for severe disease. Many vaccine candidates are capable of inducing humoral and cellular responses. The development of infectious clones for coronaviruses has facilitated the identification of attenuating mutations, deletions and recombinations which could ultimately result in live attenuated vaccine candidates. Stable vaccine platforms should be developed that allow for rapid intervention strategies against any future emergence coronaviruses. Vaccine correlates that enhance disease after challenge should be thoroughly investigated and mechanisms devised to circumvent vaccine-associated complications.
Further Information
