The researchers have used a technique called
X-ray crystallography to learn the precise structure of the original
strain of EV-D68 on its own and when bound to an anti-viral compound
called "pleconaril." The ongoing research could lead to the development
of drugs that inhibit infections caused by the most recent strains of
the virus, said Michael G. Rossmann, Hanley Distinguished Professor of
Biological Sciences at Purdue University.
A molecule called a "pocket factor" is located
within a pocket of the virus's protective shell, called the capsid. When
the virus binds to a human cell, the pocket factor is squeezed out of
its pocket, resulting in the destabilization of the virus particle,
which then disintegrates and releases its genetic material to infect the
cell and to replicate itself.
The antiviral compound pleconaril also binds into the pocket, inhibiting infection.
"The compound and the normal pocket factor
compete with each other for binding into the pocket," Rossmann said.
"They are both hydrophobic, and they both like to get away from water by
going into the pocket. But which of these is going to win depends on
the pocket itself, the pocket factor and properties of the antiviral
compound."
The findings are detailed in a paper appearing in
the journal Science on Friday (Jan. 2). The paper was authored by Yue
Liu, a graduate student; Ju Sheng, a technical assistant; Andrei Fokine,
Geng Meng, Woong-Hee Shin, and Feng Long, post doctoral research
associates; Richard Kuhn, professor and head of Purdue's Department of
Biological Sciences; Daisuke Kihara, a professor of biological sciences
and computer science; and Rossmann.
A video interview with Rossmann and Liu is available at http://youtu.be/EXA01c0WL5o.
"In this work we only focused on the very
original EV-D68 isolate, which was discovered in 1962," Liu said.
"Strains in the current outbreaks have minor differences."
Although pleconaril is not active against current
strains of EV-D68 tested thus far, it is active against the original
isolate. Small changes in the structure of pleconaril are likely to lead
to anti EV-D68 inhibitors against a broader spectrum of isolates.
An upsurge of EV-D68 cases in the past few years
has been seen in clusters of infections worldwide. In August 2014 an
outbreak of mild-to-severe respiratory illnesses occurred among
thousands of children in the United States of which 1,149 cases have
been confirmed to be caused by EV-D68. The virus also has been
associated with occasional neurological infections and "acute flaccid
myelitis," characterized by symptoms including muscle weakness and
paralysis. Although EV-D68 has emerged as a considerable global public
health threat, there is no available vaccine or effective antiviral
treatment.
Research led by Rossmann, working with
pharmaceutical companies, has resulted in antiviral drugs for other
enteroviruses such as rhinoviruses that cause common cold symptoms.
These drugs include pleconaril, which was developed in the 1990s but not
approved by the U.S. Food and Drug Administration primarily because of a
side effect that puts women using birth control drugs at risk of
conception.
Purdue researchers became interested in studying
pleconaril's potential effectiveness against EV-D68 after an outbreak of
about 20 cases of acute flaccid paralysis was reported in California
between 2012 and 2014. Out of those cases, two tested positive for
EV-D68.
"This suggests the potential association of EV-D68 with polio-like illness," Liu said.
The researchers are working with the Centers for
Disease Control and Prevention and are studying the newer strains to
determine their structures.
This image shows the three-dimensional organization of enterovirus
D68 while it is bound to pleconaril, an antiviral compound against the
original strain of the virus. The bound pleconaril molecules are shown
in orange and bordered in black. Researchers are working to develop
anti-viral compounds that inhibit infection in current strains of the
virus. (Purdue University image/Yue Liu and Michael G. Rossmann) |
"The need for an effective antiviral agent for
treatment of EV-D68 infections was made apparent by the widespread and
large numbers of EV-D68 infections (in 2014), many of which were
associated with significant morbidity," said Mark A. McKinlay, director
of the Center for Vaccine Equity at the Task Force for Global Health.
"The determination of the structure of the EV-D68 reported here by
Michael Rossmann and his team represents an important step in this
direction. The strain of EV-D68 used in the study is from 1962, and
Michael's team, along with Steve Oberste's group at CDC have shown that
this strain is inhibited by pleconaril at clinically achievable
concentrations. Testing of pleconaril against the current circulating
strains at CDC thus far showed these strains are not susceptible to the
antiviral compound."
McKinlay, who collaborates with the CDC on polio
eradication efforts, has been a key figure in pharmaceutical-company
collaborations with Rossmann's group to discover and develop pleconaril.
Once the newer strains are better understood, the
ongoing research could yield compounds that are effective against these
strains.
"Designing the best possible compound for these
newer strains will take more time, but I hope that in a year or so we
might have something," Rossmann said.
Rossmann and Kuhn as well as David Stuart's team
in Oxford, England, working with Zihe Rao's group in Beijing were among
the first scientists to reveal important details of the structure of
enterovirus 71, or EV71, which causes hand, foot and mouth disease, and
is common throughout the world. Although that disease usually is not
fatal, the virus has been reported to cause fatal encephalitis in
infants and young children, primarily in the Asia-Pacific region.
Rossmann's and Kuhn's collaborative research has
looked at virus structures in complex with receptors that permit entry
of the virus into cells, and inhibitors of virus replication for a
variety of viruses.
Like EV-D68 and EV71, poliovirus is an
enterovirus and is within the large family called picornaviruses.
Non-polio enteroviruses are common viruses and cause about 10 to 15
million infections in the United States each year, but most infected
individuals have only mild illness, similar to a common cold, according
to the CDC.
The research was supported by the National Institutes of Health.