A new cell receptor for rhinovirus

rhinovirus receptorsRhinovirus is the most frequent cause of the common cold, and the virus itself is quite common: there are over 160 types, classified into 3 species. The cell receptor has just been identified for the rhinovirus C species, which can cause more severe illness than members of the A or B species: it is cadherin-related family member 3.

Because viruses are obligate intracellular parasites, the genome must enter a cell before new particles can be made. The first step in this process is binding of the virus particle to a receptor on the plasma membrane. Two different membrane proteins serve as receptors for members of rhinovirus A and B species: intracellular adhesion molecule 1, and low-density lipoprotein receptor (illustrated).

It has not been possible to propagate species C rhinoviruses in conventional cell cultures, which has hampered research on how the virus replicates. The lack of a cell culture system required a different approach to identifying a cell receptor for this virus. It was known that the virus replicates in primary organ or cell cultures derived from sinus tissue, but not in a variety of epithelial and transformed cell lines (e.g. HeLa cells). In silico comparison of gene expression profiles revealed 400 genes that are preferentially expressed in virus-susceptible cells. This list was narrowed down to 12 genes that encode plasma membrane proteins. A subset of these genes were introduced into cells and tested for the ability to serve as a rhinovirus C receptor. Introduction of the gene encoding cadherin-related family member 3 (CDHR3) into HeLa cells allowed rhinovirus C binding and infection.

The cadherin family comprises cell surface proteins that are involved in cell-cell communication. The exact cell function of CDHR3 is not known, but the protein is found in human lung, bronchial epithelium, and cultured airway epithelial cells. A mutation in the gene encoding this protein is associated with wheezing illness and asthma in children. This mutation, which causes a change from cysteine to tyrosine at amino acid 529, was found to increase virus binding and virus replication in HeLa cells that synthesize CDHR3. It will be important to determine if this amino acid change increases rhinovirus C replication in humans, thereby leading to more serious respiratory illness.

The CDHR3 gene was used to establish a stable HeLa cell line that produces the receptor and which can be infected with species C rhinoviruses. This cell line will be useful for illuminating the details of viral replication in cells, which has so far been elusive due to lack of a susceptible and permissive cell line. It may also be possible to produce transgenic mice with the human CDHR3 gene, which could serve as a model for studying rhinovirus C pathogenesis. Transgenic mice that produce the receptor for the related polioviruses, CD155, are a model for poliomyelitis.

Blocking virus infection with soluble cell receptors

poliovirus + receptorWe recently discussed the development of a soluble receptor for HIV-1 that provides broad and effective protection against infection of cells and of nonhuman primates. Twenty-five years ago my laboratory published a paper which concluded that using soluble receptors to block virus infection might not be a good idea. In the first paragraph of that paper we wrote:

…it has been proposed that soluble cell receptors might be effective antiviral therapeutics. It has been suggested that mutants resistant to the antiviral effects of soluble receptors would not arise, because mutations that abrogate binding to receptors would be lethal.

We had previously shown that the cell receptor for poliovirus, CD155, produced in a soluble form, would bind to poliovirus (pictured – the very image from the banner of this blog), blocking viral infection. We then found that it was relatively easy to select for soluble receptor resistant (srr) virus mutants. These viruses still enter cells by binding to CD155, but the affinity of virus for the receptor is reduced. Poliovirus srr mutants replicate normally in cell cultures, and cause paralysis in a mouse model for poliomyelitis. We speculated that receptor binding might not be a rate-limiting step in viral infection, and short of  abolishing binding, the virus can tolerate a wide range of binding capabilities.

The amino acid changes that cause the srr phenotype map to both the exterior and the interior of the viral capsid. The changes on the virion surface are likely to directly interact with the cell receptor. Changes in the interior of the virus particle may be involved in receptor-mediated conformational transitions that are believed to be essential steps in viral entry.

When this work was done, clinical trials of soluble CD4 for HIV-1 infection were under way. We believed that our findings did not support the use of soluble receptors as antivirals, which we clearly stated in the last sentence of the paper:

These findings temper the use of soluble receptors as antiviral compounds.

HIV-1 mutants resistant to neutralization with soluble CD4 were subsequently isolated, and the compound was never approved to treat HIV-1 infection in humans for this and other reasons, including low affinity for the viral glycoprotein, enhancement of infection, and problems associated with using a protein as a therapeutic.

Recently a new soluble CD4 was produced which also includes the viral binding site for a second cell receptor, CCR5. This molecule overcomes many of the issues inherent in the original soluble CD4. It provides broad protection against a wide range of HIV-1 strains, and when delivered via an adenovirus-associated virus vector, protects nonhuman primates from infection. This delivery method circumvents the issues inherent in using a protein as an antiviral drug. Because this protein blocks both receptor binding sites on the viral envelope glycoprotein, it might be more difficult for viruses to emerge that are resistant to neutralization. The authors speculate that such mutants might not be efficiently transmitted among hosts due to defects in cell entry. Given the promising results with this antiviral compound, experiments to test this speculation are certainly welcome.

Measles in the brain: Fusion gone awry

paramyxovirus fusionThe entry of enveloped viruses into cells begins when the membrane that surrounds these virus particles fuse with a cell membrane. The process of virus-cell fusion must be tightly regulated, to make sure it happens in the right cells. The fusion activity of measles viruses isolated from the brains of AIDS patients is not properly regulated, which might explain why these viruses cause disease in the central nervous system.

Measles virus particles bind to cell surface receptors via the viral glycoprotein HN (illustrated). Once the viral and cell membranes have been brought together by this receptor-ligand interaction, fusion is induced by a second viral glycoprotein called F, and the viral RNA is released into the cell cytoplasm. The N-terminal 20 amino acids of F protein are highly hydrophobic and form a region called the fusion peptide that inserts into target membranes to initiate fusion. Because F-protein-mediated fusion can occur at neutral pH, it must be controlled, to ensure that virus particles fuse with only the appropriate cell, and to prevent aggregation of newly made virions. The fusion peptide of F is normally hidden, and conformational changes in the protein thrust the it toward the cell membrane (illustrated). These conformational changes in the F protein, which expose the fusion peptide, are thought to occur upon binding of HN protein to its cellular receptor.

During a recent outbreak of measles in South Africa, several AIDS patients died when measles virus entered and replicated in their central nervous systems. Measles virus normally enters via the respiratory route, establishes a viremia (and the characteristic rash) and is cleared within two weeks. The virus is known to enter the brain in up to half of infected patients, but without serious sequelae. The measles inclusion body encephalitis observed in these AIDS patients typically occurs in immunosuppressed individuals several months after infection with measles virus.

Measles virus isolated postmortem from these two individuals had a single amino acid change in the F glycoprotein, from leucine to tryptophan at position 454. This single amino acid change allowed viruses to fuse with cell membranes without having to first bind a cellular receptor via the HN glycoprotein. In other words, the normal mechanism for regulating measles virus fusion – binding a cell receptor – was bypassed in these viruses. This unusual property might have allowed measles virus to spread throughout the central nervous system, causing lethal disease.

How did these mutant viruses arise in the AIDS patients? Because these individuals had impaired immunity as a result of HIV-1 infection, they were not able to clear the virus in the usual two weeks. As a consequence, the virus replicated for several months. During this time, the mutation might have arisen that allowed unregulated fusion of virus and cell, leading to unchecked replication in the brain. Alternatively, the mutation might have been present in virus that infected these individuals, and was selected in the central nervous system.

An interesting question is whether these neurotropic measles viruses can be transmitted by aerosol between hosts – a rather unsettling scenario. Fortunately, we do have a measles virus vaccine that effectively prevents infection, even with these mutant viruses.

Receptor for new coronavirus-EMC identified

Coronavirus virionViruses are obligate intracellular parasites, which means that they must enter a cell to reproduce. As virions are too large to diffuse passively across the plasma membrane, cellular pathways for uptake of extracellular materials provide entry routes. The first step in entry is adherence of virus particles to the membrane, an interaction mediated by binding to one or more receptor molecules on the cell surface. Identification of cell receptors for viruses is an important objective because their study may lead to information about how the virus enters the cell, how it is targeted to specific tissues, and how it causes disease. The cell receptor for the recently identified coronavirus-EMC, which has so far infected 15 humans with 9 deaths, has been identified as dipeptidyl peptidase 4 (DPP4).

Enveloped viruses such as CoVs typically attach to cell receptors via spike glycoproteins embedded in the viral envelope. To identify the CoV-EMC receptor, part of the viral spike (S) glycoprotein was expressed as a fusion protein with the Fc domain of IgG antibody. The protein was mixed with lysates of cells known to be infected with CoV-EMC, and bound proteins were isolated by using agarose beads bound to protein A (which binds the Fc domain). A single polypeptide bound to the CoV-EMC spike protein was identified as DPP4. Four different lines of evidence indicate that DPP4 is a bona fide receptor for CoV-EMC:

  • Soluble DPP4 blocks infection of susceptible cells with CoV-EMC
  • Expression of DPP4 in non-susceptible cells renders them susceptible to infection
  • Antibody to DPP4 blocks infection of cells with CoV-EMC
  • Purified DPP4 protein binds CoV-EMC and inhibits infection

Considering that CoV-EMC was isolated in November 2012 from a sick patient, the identification of the cell receptor is indeed rapid progress.

DPP4 protein is expressed in primary human bronchiolar lung tissue and on primary bronchiolar epithelial cell cultures, consistent with the ability of the virus to infect the respiratory tract. The protein is also present on the epithelium of kidney, small intestine, liver and prostate. CoV-EMC has been detected in the respiratory tract and in urine. Whether the virus replicates in the respiratory tract, and then disseminates and replicates elsewhere (e.g. kidney) remains to be determined.

CoV-EMC is believed to have originated in bats. Consistent with this hypothesis, expression of bat DPP4 confers susceptibility to the virus, although not to the same extent as human DPP4. Once the bat precursor of CoV-EMC is identified, it will be interesting to determine how the viral spike glycoprotein has evolved to enable more efficient usage of human DPP4.

DPP4 is a transmembrane protein that regulates the activity of hormones and chemokines through proteolytic cleavage. The cell receptors for two other CoVs are also membrane-bound peptidases, but proteolytic activity is not needed for infection. A soluble form of DPP4 is also present in blood. The authors speculate that reduction of DPP4 protein levels by CoV-EMC infection could result in higher virus-induced disease. If DPP4 is important in regulating the activity of cytokines – major components of immune responses – their removal from the circulation could result in greater virus replication and more tissue damage. It will be important to study the levels of DPP4 in humans infected with CoV-EMC, and to determine whether levels of the receptor affect viral disease.