Covering up a naked virus

Sabin type 2 poliovirusViruses can be broadly classified according to whether or not the particle is enveloped – surrounded by a membrane taken from the host cell – or naked. Some naked viruses apparently are more modest than we believed.

Members of the family Picornaviridae, which include Hepatitis A virus, poliovirus, and Coxsackieviruses, have non-enveloped particles that consist of a protein shell surrounding the viral RNA genome (poliovirus is illustrated). Examples of viruses that are enveloped include dengue virus, influenza virus, and measles virus.

Recently it was discovered that hepatitis A virus (HAV) particles are released from cells in membrane vesicles containing 1-4 virus particles. These membranous structures resemble exosomes, which are also released from uninfected cells and play roles in various biological processes. Enveloped hepatitis A virus particles are present in the blood of infected humans. However virus in the feces, which is transmitted to other hosts, is not enveloped.

Viral envelopes typically contain viral glycoproteins, such as the HA protein of influenza viruses, which serve important functions during replication, such as attachment to cell receptors. Envelope glycoproteins are also the target of antibodies that block viral infection. The presence of an envelope makes HAV resistant to neutralization with antibodies, because the membrane contains no viral proteins that can be blocked by antibodies.

Two other non-enveloped picornaviruses, Coxsackievirus B and poliovirus, are also released from cells within membrane vesicles. These virus particles are in vesicles derived from the autophagy pathway, which captures and recycles cytoplasmic contents by ejecting them from the cell.

What is the function of the membrane acquired by these naked viruses? Perhaps immune evasion: the presence of the cell membrane makes HAV and Coxsackievirus B virus particles resistant to neutralization with antibody. The ability to deliver multiple virus particles to a single cell might help to overcome genetic defects in the viral genome that are a consequence of the high mutation rates of these viruses.

An interesting problem is how these cloaked viruses enter cells, because there is no evidence that the membranes contain any viral proteins that could interact with a cell receptor. Nevertheless, entry of enveloped HAV and poliovirus into cells requires the known viral receptor. Perhaps the vesicles are taken into the cell by endocytosis, where viral particles are released from the vesicles, and then bind receptors to initiate escape of the genome.

Should HAV, poliovirus, and Coxsackievirus B be reclassified as enveloped viruses? Probably not, in part because the membranes surrounding these virus particles are not needed for infectivity. In contrast, removal of the membrane from influenza virus, dengue virus, or measles virus destroys their infectivity. Enveloped viruses acquire a membrane after the internal components have been assembled, whether they are helical or icosahedral nucleocapsids. In contrast, HAV, poliovirus, and Coxsackievirus B become fully infectious particles before they acquire an envelope.

Another argument against calling picornaviruses enveloped is that viral membranes contain viral glycoproteins that allow attachment to cell receptors and release of the viral genome into the cell. There is no evidence that the membranes of picornaviruses contain viral proteins.

The acquisition of a membrane may have taken place later in the evolution of picornaviruses, to allow more efficient infection or evasion of host responses. Alternatively, the membrane may simply be a by-product acquired when these viruses exit the cell by a non-lytic mechanism.

While the finding of membranes around picornavirus particles is intriguing, I am not yet convinced that these viruses should be considered to be enveloped. I would like to know if other non-enveloped viruses are similarly released from cells in membranous cloaks, and the function of this addition for viral replication in the host.

The neuraminidase of influenza virus

influenza virusThe influenza virus particle is made up of the viral RNA genome wrapped in a lipid membrane (illustrated). The membrane, or envelope, contains three different kinds of viral proteins. The hemagglutinin molecule (HA, blue) attaches to cell receptors and initiates the process of virus entry into cells. I have written about the HA and its function during infection (article one and two) but not about the neuraminidase (NA, red) or M2 (purple) proteins. Let’s first tackle NA.

An important function of the NA protein is to remove sialic acid from glycoproteins. Sialic acid is present on many cell surface proteins as well as on the viral glycoproteins; it is the cell receptor to which influenza virus attaches via the HA protein. The sialic acids on the HA and NA are removed as the proteins move to the cell surface through the secretory pathway. Newly released virus particles can still potentially aggregate by binding of an HA to sialic acid present on the cell surface. Years ago Peter Palese showed that influenza virus forms aggregates at the cell surface when the viral neuraminidase is inactivated. The NA is therefore an enzyme that is essential for release of progeny virus particles from the surface of an infected cell.

The NA protein also functions during entry of virus into the respiratory tract. The epithelial cells of the respiratory tract are bathed in mucus, a complex protective coating that contains many sialic acid-containing glycoproteins. When influenza virions enter the respiratory tract, they are trapped in mucus where they bind sialic acids. This interaction would prevent the viruses from binding to a susceptible cell were it not for the action of the NA protein which cleaves sialic acids from glycoproteins. When the virus particle encounters a cell, it binds the sialic acid-containing receptor and is rapidly taken into the cell before the NA protein can cleave the carbohydrate from the cell surface.

The essential nature of the NA for virus production has been exploited to develop new drugs designed to inhibit viral release. Both Tamiflu (Oseltamivir) and Relenza (Zanamivir) are structural mimics of sialic acid that bind tightly in the active site of the NA enzyme. When bound to drug, the NA cannot remove sialic acids from the cell surface, and consequently newly synthesized virus remains immobilized. The result is an inhibition of virus infection because virions cannot spread from one cell to another.

This article is part of Influenza 101, a series of posts about influenza virus biology and pathogenesis.

Virology lecture #5: Attachment and entry


Download: .wmv (386 MB) | .mp4 (131 MB)

There are some errors in this lecture – I’ll correct them during the next session.

Visit the virology W3310 home page for a complete list of course resources.

TWiV 33: Live in Philly

twiv-200Episode 33 of the podcast “This Week in Virology” was recorded before an audience at the ASM General Meeting in Philadelphia. Vincent, Alan, Dick, and Raul Andino discussed increased arterial blood pressure caused by cytomegalovirus infection, restriction of influenza replication at low temperature by the avian viral glycoproteins, first isolation of West Nile virus in Pennsylvania, and current status of influenza.

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