How Does Picornavirus Infect Cells? Mechanisms & Stages Explained
The picornavirus is categorized among the largest and most important family of viruses. This class includes viruses such as enteroviruses, poliovirus and, parechoviruses. As the name suggests picornavirus is a small size RNA containing virus, pico refers to the small size of the virus which is about 27nm, and RNA in the name suggests that the nucleic material of this virus is RNA. This article deals with the understanding of the features of picornavirus, its structure, picornavirus replication, and its pathogenicity in the human body. This article also covers picornavirus classification and discussion of medically important genera of the virus.
Classification of Picornavirus
Picornavirus is the class of the virus that constitutes many genera of the virus, it is among one of the largest groups of the viral classification. Picornavirus is also known as Picornaviridae according to the taxonomic classification. The Picornaviridae is classified as a family of the virus. This family of the virus contains nine genera of the viruses out of which six are medically important. These six genera of the Picornaviridae family are as follows,
Enterovirus
Parechovirus
Hepatovirus
Aphthovirus
Rhinovirus
Cardiovirus
All these six genus of viruses are medically important as they play an important part in the pathogenicity in humans. They are responsible for causing hepatitis, polio, mouth, and foot disease in the human host.
Enterovirus- The enterovirus genus includes the poliovirus, coxsackievirus and, human and animal enteroviruses.
Parechovirus- The parechovirus genus of the virus includes the species of echovirus 22
Hepatovirus- The hepatovirus genus of the virus includes the hepatitis A virus, which causes hepatitis in humans.
Rhinovirus- The rhinovirus genus includes the viral species that contains RNA as genetic material or in other words, the species of virus that belongs to the retrovirus category are classified under the rhinovirus genus.
Aphthovirus - The genus aphthovirus contains viruses that are responsible for causing foot and mouth disease. A common example of the virus which belongs to the genus includes Equine rhinovirus type I.
Cardiovirus- The cardiovirus genus of the virus includes the following, Encephalomyocarditis, and Theilers murine encephalomyelitis virus. This genera of the virus generally infect rodents.
Characteristics of Picornavirus
There are certain characteristics according to which the picornavirus classification is performed, such as morphology, physicochemical and biologic properties, antigenic structures, genomic sequence, and mode of replication. This table lists some of those characteristics.
Some of the common sites for the isolation of such viruses include the nose, throat, and lower intestine. Some of the isolated viruses specifically infect rodents generally newborn mice. A common example of such viruses is Cardiovirus.
Structure of Picornavirus
The picornavirus is icosahedral in shape, the diameter of the virus of this family is about 28-30 nm. They have a cubic symmetry. They are generally nonenveloped. The virus capsid (the outer layer of the virus) is made up of 60 subunits of the protein. The capsid also contains the site of antibody binding, these are more commonly called as antibody binding sites. The capsid of the virus contains VP 1 - VP 4. The VP 1 and VP 4 are the main antibody binding proteins whereas the VP 3 acts as an integral protein. The capsid of the virus encloses the nucleic material in this case RNA of the virus. There are approximately 7,500 nucleotides in the RNA strand. A non-capsid viral protein commonly known as VPg is bound covalently to the RNA at the 5’ end and the 3’ polyadenylated tail. It is important to note that the ether resistant property of the virus can be attributed to the fact that they do not have any essential lipid.
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Replication in Picornavirus
The replication in picornavirus takes place in the cytoplasm of the host. The virus contains the single stranded RNA of (molecular weight, approximately 2 × 106 to 3 × 106.
The replication of the virus can only occur when the virus has entered the cell of the ghost as the virus uses the replication machinery of the host to complete its replication. The viral genome serves as an mRNA, thus prompting the translation of the protein using the components of the host.
The replication starts with the attachment of the virion to the specific cell receptor. The site of the attachment is in the plasma membrane. It is suggested that the hydrophobic N terminal of the integral protein of virion is inserted into the plasma membrane of the host.
After the insertion, the endosome engulfs the protein and endosomal translocation occurs. This results in reaching the viral nucleic acid at the cytoplasm.
It is followed by the uncoating of the viral genetic material, the VPg protein that is associated with the viral RNA is removed by the cellular enzymes of the host.
The uncoated RNA of the virus acts as viral mRNA, thus initiating the translation. The ribosome binds to the internal ribosomal entry site (IRES) within the 5′ UTR. it folds into a clover leaf structure allowing the binding of the host protein.
It is important to note that the viral genome does not contain the internal translational stop codons. This results in the synthesis of the single long polyprotein chain. This protein then undergoes translational modifications to ensure the production of the viral protein. The viral protein called proteases performs the cleavage of the encoded protein to obtain the desired end proteins.
The minus RNA strand is also known as antisense RNA strand. Its synthesis is initiated at the 3′ end of the virion RNA, it uses the VPg as a primer. As the synthesis of the antisense RNA is complete, the antisense RNA strand acts as a template for the synthesis of the sense RNA strand, which is also known as the plus strand. This sense RNA and the viral capsid proteins assemble to form the encasing into the viral head. Thus, it can be concluded that the virion replication is performed via two different replicative intermediates (RI).
Pathogenesis and Host Defense Mechanism
Picornavirus enters the body generally through the nose and mouth. The picornavirus generally replicates into the nasopharyngeal epithelium and another site of the virus replication in regional lymphoid tissues. The replication leads to asymptomatic infections or respiratory illnesses. It is seen that some of the viruses of this family can infect the lower intestine it can be attributed to the fact that some of the strains can grow in the intestine as they are resistant to intestinal and bile acidic content. The viral strain is then carried into the bloodstream to target organs such as the spinal cord, brain meninges, heart, liver, and skin. The virus also reaches the skeletal system through the CNS.
Diseases associated with Picornavirus
The picornavirus is associated with the following disease.
Undifferentiated febrile illness - This type of illness is accompanied by fever.
Respiratory tract infections
Gastrointestinal disturbances
Conjunctivitis
Mucous membrane disintegration
Lesions in mucous membranes and skin
Intrauterine infections
Pancreatitis
Generalize neonatal infections
Orchitis - It is the inflammation of one or both testicles
Host Defense Mechanism
The host response to the virus can be seen as an increase in the concentration of IgA antibodies in the mucous. This is the case in respiratory tract infections. Increased IgA concentration can also be seen in the gut of the human body. The first antibody produced by the body in response to the virus is the IgM antibody. After some time there is class switching, which results in the increased production of IgG antibodies in the human body. The concentration of IgG antibodies peek for about 2-3 weeks, but then the concentration starts to decrease when the stationary phase approaches. It is also important to note that clearance of antibody and antigen complex takes place by phagocytosis, digestion, and excretion.
Conclusion
Picornavirus is one of the largest and most important family of viruses that includes small sized RNA viruses. The class of virus constitutes nine genera in total, out of which six are medically important. We also studied the characteristics, structure, replication and pathogenesis of picornavirus.
FAQs on Picornavirus: Overview, Structure & Classification
1. What is a Picornavirus and what are its key structural characteristics?
A Picornavirus belongs to the family Picornaviridae, a group of small, non-enveloped viruses. Their name is derived from 'pico', meaning small, and 'RNA', referring to their genetic material. Key characteristics include a single-stranded positive-sense RNA genome enclosed within an icosahedral protein shell called a capsid. Being non-enveloped makes them highly resistant to environmental factors and disinfectants like ether.
2. What are the main types (genera) of Picornaviruses that infect humans?
The Picornaviridae family is divided into several genera, many of which contain significant human pathogens. The most prominent genera include:
- Enterovirus: This genus includes Poliovirus and Coxsackieviruses.
- Rhinovirus: This is the primary cause of the common cold.
- Hepatovirus: This genus includes the Hepatitis A virus.
- Parechovirus: Causes mild gastrointestinal or respiratory illnesses.
- Apthovirus: Primarily known for causing foot-and-mouth disease in animals but is a key example within the family.
3. What are the most common diseases caused by Picornaviruses?
Picornaviruses are responsible for a wide spectrum of human diseases, ranging from mild to severe. Common examples include the common cold (caused by Rhinoviruses), poliomyelitis (caused by Poliovirus), aseptic meningitis, myocarditis (inflammation of the heart muscle), herpangina, and Hepatitis A. The specific disease depends on the type of virus and the tissues it infects.
4. How are Picornaviruses typically transmitted between people?
The primary mode of transmission for many Picornaviruses, especially Enteroviruses like Poliovirus and Hepatitis A, is the fecal-oral route. This occurs through the ingestion of contaminated food or water. Other Picornaviruses, like Rhinoviruses, spread mainly through respiratory droplets from coughs and sneezes or contact with contaminated surfaces (fomites).
5. Is the common cold exclusively caused by a Picornavirus?
While Rhinoviruses, a genus of Picornaviruses, are the most frequent cause of the common cold, they are not the only ones. Other viruses, such as coronaviruses (different from SARS-CoV-2), adenoviruses, and even some enteroviruses can also produce cold-like symptoms. Therefore, a Picornavirus is the most probable, but not the exclusive, cause.
6. Why is Poliovirus classified as a Picornavirus?
Poliovirus is classified as a Picornavirus because it shares all the defining structural and genetic features of the Picornaviridae family. It is a small ('pico') virus containing an RNA genome. Specifically, it has a single-stranded, positive-sense RNA core, is non-enveloped, and possesses the characteristic icosahedral capsid structure. Within this family, it belongs to the genus Enterovirus due to its initial site of infection in the gastrointestinal tract.
7. How does the structure of a Picornavirus contribute to its ability to cause infection?
The structure of a Picornavirus is key to its success as a pathogen. Its non-enveloped nature makes it very stable and resistant to the harsh acidic environment of the stomach, allowing viruses transmitted via the fecal-oral route to survive and reach the intestines. The protein capsid not only protects the RNA genome but also has specific surface proteins that bind to receptors on host cells, enabling the virus to enter and inject its genetic material to begin replication.
8. Are there any treatments or preventive measures for Picornavirus infections?
There is no specific universal antiviral treatment for most Picornavirus infections; care is typically supportive, focusing on managing symptoms. However, prevention is highly effective for some of the most severe diseases. For example, vaccination has nearly eradicated Polio worldwide. For Hepatitis A, both a vaccine and good hygiene practices (hand washing, ensuring clean food and water) are crucial preventive measures. For Rhinoviruses, prevention relies on hygiene, such as frequent hand washing, to limit spread.
