Reoviruses and Rotaviruses: Classification, structure and replication
The History of Discovery
First agents (later designated as reoviruses) were isolated in 1951 by US virologist W. Stanley from the feces of Australian child with fever, pneumonia and gastroenteritis. Further in 1959 A. Sabin termed them reoviruses (reo means respiratory enteric orphan viruses) as they were isolated from human airways and gastrointesinal tract without evident relations with certain human pathology.
These viruses were placed into separate family Reoviridae.
New members of this family – rotaviruses – were isolated by R. Bishop in 1973. They are common etiological agents of diarrheal diseases in infants.
Classification of Viruses
Reoviridae family currently comprises two subfamilies and 15 genera.
The genus Rotavirus pertains to subfamily Sedoreovirinae. This genus embraces 8 viral species (Rotavirus A-H).
Human infections are caused mainly by 3 species Rotavirus A, В and С, but more than 90% of infection cases are produced by Rotavirus A species members.
Depending on variations of external capsid proteins VP7 (G-protein) и VP4 (Р-protein) and their genes, all rotaviruses are divided into multiple serotypes and genotypes.
Other representatives of reoviruses, e.g. from genera Coltivirus and Orbivirus, are the transmissive zoonotic agents; in certain conditions they may also afflict humans.
Structure of Reoviruses and Rotaviruses
Virion size is 70-100 nm in diameter. Viruses possess two capsid shells, made of proteins; each capsid is of icosahedral symmetry.
Inner composition of viral particle has a triple-layered structure. Virion is devoid of lipid envelope (naked viruses).
Reoviral genome consists of double-stranded RNA, composed of 10-12 discrete segments. Rotaviruses contain 11 genome segments.
Virion of rotaviruses looks like wheel (Lat. rota – wheel).
Mature capsid of rotaviruses has 6 structural (VP1-VP7 except VP5) and 6 non-structural proteins (NSP1-NSP6).
External capsid is composed of proteins VP4 (or G-protein) and VP7 (or Р-protein). They play a role of viral receptors, binding to polysaccharides of enterocytes.
Protein VP1 is RNA-dependent RNA polymerase.
Certain non-structural proteins act as rotavirus virulence factors. The most active is viral enterotoxin NSP4 with multiple deleterious activities.
NSP4 activates chloride secretion by enterocytes and increases intracellular Ca concentration; the latter results in cytoskeleton impairment, damage of tight junctions between enterocytes and stimulation of proinflammatory cytokine secretion. Taken together, this leads to secretory and inflammatory diarrhea.
Protein VP3 inhibits interferon synthesis.
Segmented genomic RNA of rotaviruses encoding superficial VP4 and VP7 proteins is capable of gene segment recombination, known as genetic reassortment or genetic shift.
After possible coinfection of target cells by rotaviruses of different genotypes the reassortment can take place – it results in formation of recombinant virions with a new genome and new combination of VP4 and VP7 proteins within external coat. Recombination between human and animal rotaviruses is also possible.
Genetic drift means the minor genetic variations of viral VP4- and VP7-encoding sequences, ensuing from the point mutations in their genes. This exerts viral evasion from the host immune response.
Despite high genetic variability, human infections are caused by only limited number of genotypes and serotypes of rotaviruses.
Reoviruses and rotaviruses are stable in the environment – outside the host they maintain viability for several months. Virions withstand pH changes in the range of 3.0-9.0; therefore, they are more resistant in gastric juice. Nevertheless, rotaviruses are inactivated by heating at 50oC for 30 minutes. They are resistant to lipid solvents, but susceptible to 95% ethanol, phenol, chlorine, and glutaraldehyde. Limited treatment with proteolytic enzymes increases infectivity.
Rotavirus Replication Cycle
Viral particles attach via capsid receptors VP4 and VP7 to sialic acid residues and oligosaccharides of enterocyte membrane receptors.
Viruses enter the cell by endocytosis that is followed by removal of external capsid. Reproduction of rotaviruses occurs in cytoplasm of infected cells.
Core-associated viral RNA polymerase is activated. It transcribes mRNA molecules from the minus strand of each genome segment. Released mRNAs are translated into viral proteins and serve as templates for synthesis of negative-sense strands with subsequent double-stranded genome formation.
At the same time viral polypeptides self-assemble to form the inner and outer capsid shells.
Intensive reproduction and egress of progeny virions results in lysis of infected cells.
Rotaviruses are the fastidious agents to culture. They don’t propagate in embryonated eggs or experimental animals. Nevertheless, group A human rotaviruses are adapted to certain cell lines. They grow if pre-treated with trypsin that facilitates uncoating. Maturation of virions is incomplete and slow. Reoviruses produce the inclusion bodies in the cytoplasm of infected cells.