Researchers at Albert Einstein College of Medicine of Yeshiva University have helped identify a cellular protein that is critical for infection by the deadly Ebola virus. The findings, published in the August 24 online edition of Nature, suggest a possible strategy for blocking infection due to Ebola virus, one of the world’s most lethal viruses and a potential bioterrorism agent.
The study was a collaborative effort involving scientists from Einstein, the Whitehead Institute for Biomedical Research, Harvard Medical School, and the U.S. Army Medical Research Institute of Infectious Diseases
Ebola virus is notorious for killing up to 90 percent of the people it infects. Ebola hemorrhagic fever — the severe, usually fatal disease that Ebola virus causes in humans and in nonhuman primates — first emerged in 1976 in villages along the Ebola River in the Sudan and the Democratic Republic of the Congo, Africa. Since then, about two dozen outbreaks have occurred.
This drawing illustrates the sequence of events from the time the Ebola virus first enters the host cell (top) until the virus gains its release into the cytoplasm, where it can multiply (bottom). Researchers have shown that Ebola exists in the lysosome and enters the cytoplasm by interacting with NPC1 protein molecules (orange) embedded in the lysosomal membrane. (Credit: Image courtesy of Albert Einstein College of Medicine) (right)
Though Ebola and Marburg hemorrhagic fevers are fortunately rare diseases, “even small outbreaks of Ebola or Marburg virus can cause fear and panic,” said co-senior author Kartik Chandran, Ph.D., assistant professor of microbiology & immunology at Einstein “And then there’s the worry that these viruses could be used for bioterrorism.”
Ebola virus’s ability to enter cells is reminiscent of the Trojan Horse used by the ancient Greeks to defeat their archenemies. Ebola virus binds to the host cell’s outer membrane, and a portion of host cell membrane then surrounds the virus and pinches off, creating an endosome — a membrane-bound bubble inside the cell (see image). Endosomes carry their viral stowaways deep within the cell and eventually mature into lysosomes — tiny enzyme-filled structures that digest and recycle cellular debris.
The viruses captive in the lysosome manage to escape destruction by exploiting components of the cell to gain entry to the cytoplasm, the substance between the cell membrane and the nucleus where the virus can replicate. But the identities of many of these components have remained unknown.
In seeking the answer, Einstein researchers and colleagues searched for proteins that Ebola virus might exploit to enter the cell’s cytoplasm. One such cellular protein, known as Niemann-Pick C1 (NPC1), stood out.
“We found that if your cells don’t make this protein, they cannot be infected by Ebola virus,” said Dr. Chandran. “Obviously it’s very early days, but we think our discovery has created a real therapeutic opportunity.” At present, there are no drugs available to treat people who have been infected with Ebola virus or approved vaccines to prevent illness.”
The NPC1 protein is embedded within cell membranes, where it helps transport cholesterol within the cell. However, the absence of NPC1 due to gene mutations causes a rare degenerative disorder called Niemann-Pick disease, in which cells become clogged up with cholesterol and eventually die.
To confirm their finding that NPC1 is crucial for Ebola virus infection, the researchers challenged mice carrying a mutation in NPC1 with Ebola virus. Remarkably, most of these mutant mice survived the challenge with this normally deadly virus. Similarly, fibroblast cells (found in connective tissue) from people with Niemann-Pick disease were resistant to Ebola virus infection, as were human cells from other organs that were manipulated to reduce the amount of NPC1 they contained.
The researchers also tested whether other major viruses need NPC1 to infect human cells. Only Ebola virus and its close relative, Marburg virus, were found to require the presence of NPC1 protein for infection. Like Ebola virus, Marburg virus also needs NPC1 to kill mice.
“Our work suggests that these viruses need NPC1, which is embedded in the lysosomal membrane, to escape from the lysosome into the cytoplasm,” said Dr. Chandran. “We are now testing that hypothesis in the laboratory.”
The discovery of NPC1’s crucial role in Ebola infection raises the possibility that Ebola and Marburg virus outbreaks could be thwarted by a drug that blocks the action of NPC1. “Even though such a treatment would also block the cholesterol transport pathway, we think it would be tolerable,” said Dr. Chandran. “Most outbreaks are short-lived, so treatment would be needed for only a short time.” Einstein, in conjunction with the Whitehead Institute of Biomedical Research and Harvard Medical School, has filed a patent application related to this research that is available for licensing to partners interested in further developing and commercializing this technology.
Remarkably, an anti-Ebola virus inhibitor Dr. Chandran found as a postdoctoral fellow at the Brigham and Women’s Hospital in Boston, MA turns out to be just such an NPC1 blocker, as described in a separate manuscript by Côté and co-workers to be published in the same issue of Nature.
Note: Materials may be edited for content and length. For further information, please contact the source cited above.
Jan E. Carette, Matthijs Raaben, Anthony C. Wong, Andrew S. Herbert, Gregor Obernosterer, Nirupama Mulherkar, Ana I. Kuehne, Philip J. Kranzusch, April M. Griffin, Gordon Ruthel, Paola Dal Cin, John M. Dye, Sean P. Whelan, Kartik Chandran, Thijn R. Brummelkamp. Ebola virus entry requires the cholesterol transporter Niemann–Pick C1. Nature, 2011; DOI: 10.1038/nature10348
How Deadly Marburg Virus Silences Immune System: Breakthrough Findings Point to Targets for Drugs and Vaccines
Scientists at The Scripps Research Institute have determined the structure of a critical protein from the Marburg virus, a close cousin of Ebola virus. These viruses cause similar diseases and are some of the deadliest pathogens on the planet, each killing up to 90 percent of those infected.
The Marburg virus VP35 protein (beige) surrounds the virus’s double-stranded RNA (blue), masking it from immune system detection. (Credit: Image by Christina Corbaci, The Scripps Research Institute) (up)
Described in the Sept. 13, 2012 publication of the journal PLoS Pathogens, the new research reveals how a key protein component of the Marburg virus, called VP35, blocks the human immune system, allowing the virus to grow unchecked. The structure provides a major step forward in understanding how the deadly virus works, and may be useful in the development of potential treatments for those infected.
“The immune system is designed to recognize certain hallmarks of virus infection,” said Erica Ollmann Saphire, the Scripps Research scientist who led the effort. “When these are sensed, an immediate antiviral defense is launched. However, the Marburg and Ebola viruses mask the evidence of their own infection. By doing so, the viruses are able to replicate rapidly and overwhelm the patient’s ability to launch an effective defense.”
Ebola virus outbreaks have occurred in the last month in both Uganda and the Democratic Republic of the Congo, while Marburg virus broke out in Angola in 2005 to 2006 and again in Uganda in 2007. The Angolan Marburg virus outbreak began in a pediatric ward and killed 88 percent of those it infected. The virus has since been imported into the United States (Colorado) and the Netherlands by tourists who had visited Africa.
There is currently no cure for Marburg hemorrhagic fever. The virus is spread when people come into contact with the bodily fluids of a person or animal who is already infected. The best treatment consists of administering fluids and taking protective measures to ensure containment, like isolating the patient and washing sheets with bleach.
Most people, however, die within two weeks of exposure from a combination of dehydration, massive bleeding, and shock. A smaller number of people have stronger and immediate immune responses against the virus and survive.
A New Roadmap for Defense
The breakthrough described in the PLoS Pathogens article explains a key reason why the viruses are so deadly and provides the necessary templates to develop drugs to treat the infection.
The study’s lead author, Research Associate Shridhar Bale, explains that a key signature of Marburg virus infection is the double-stranded RNA that results from its replication inside cells. When human immune system proteins detect this virus-specific RNA, they sound an alarm to signal the rest of the immune system to respond. The new research describes how the VP35 protein of the Marburg virus binds to the viral double-stranded RNA and hides it to prevent the alarm from being sounded.
The new research also revealed a surprise. Images from the Marburg virus reveal the VP35 protein spirals around the double-stranded RNA, enveloping it completely. This is in contrast to previous images of the similar VP35 protein from Ebola virus that showed it only capping the ends of the RNA, leaving the center of the RNA helix exposed for possible recognition.
In addition to Ollmann Saphire and Bale, the article, “Marburg virus VP35 can both fully coat the backbone and cap the ends of dsRNA for interferon antagonism,” was authored by Jean-Philippe Julien, Zachary A. Bornholdt, Michelle A. Zandonatti, Gerard J.A. Kroon, Christopher R. Kimberlin, Ian J. MacRae, and Ian A. Wilson of The Scripps Research Institute, and Peter Halfmann, John Kunert, and Yoshihiro Kawaoka of the University of Wisconsin.
Support for the research was provided by grants from the Burroughs Wellcome Fund and The Skaggs Institute for Chemical Biology at Scripps Research.
Note: Materials may be edited for content and length. For further information, please contact the source cited above.
Bale S, Julien J-P, Bornholdt ZA, Kimberlin CR, Halfmann P, et al. Marburg Virus VP35 Can Both Fully Coat the Backbone and Cap the Ends of dsRNA for Interferon Antagonism. PLoS Pathog. PLoS Pathogens, 2012; 8(9): e1002916 DOI: 10.1371/journal.ppat.1002916
Between 50 million and 100 million dengue infections occur each year, according to the World Health Organization.
VIRUS CARRIER: This picture shows the presence of the dengue virus in the mosquitoes’ chemosensory (antennae and palp) and feeding organs (proboscis). (Photo: Johns Hopkins Bloomberg School of Public Health)
Mosquitoes are already blood-sucking machines, but new research indicates that the dengue virus, which the mosquitoes transmit to humans, makes them even thirstier for blood.
The virus specifically turns on mosquito genes that make them hungrier for a blood meal; the activated genes also enhance mosquitoes’ sense of smell, something that likely improves their feeding skills. The result is a mosquito better able to serve the virus by carrying it more efficiently to human hosts.
“The virus may, therefore, facilitate the mosquito’s host-seeking ability, and could — at least theoretically — increase transmission efficiency, although we don’t fully understand the relationships between feeding efficiency and virus transmission,” study researcher George Dimopoulus, of the Johns Hopkins Bloomberg School of Public Health, said in a statement. “In other words, a hungrier mosquito with a better ability to sense food is more likely to spread dengue virus.”
The virus doesn’t hurt the mosquitoes that carry it, a specific species called Aedes aegypti, but it lives in them. When the mosquito bites a human, it spreads the deadly disease through its saliva. More than 2.5 billion people live in areas where dengue fever-infected mosquitoes live. The World Health Organization estimates that between 50 million and 100 million dengue infections occur each year.
The researchers analyzed the mosquito genes before and after being infected with the virus, finding changes in 147 genes. These post-infection genes make proteins that are involved in processes that include virus transmission, immunity, blood feeding and host seeking, they found.
“Our study shows that the dengue virus infects mosquito organs, the salivary glands and antennae that are essential for finding and feeding on a human host,” Dimopoulus said. “This infection induces odorant-binding protein genes, which enable the mosquito to sense odors.”
“We have, for the first time, shown that a human pathogen can modulate feeding-related genes and behavior of its vector mosquito, and the impact of this on transmission of disease could be significant,” Dimopoulos said.
This is just one of many recent examples of a parasite taking control of an animal for its own benefit. Other examples include a fungus that turns ants into zombiesand a virus that causes caterpillars to dissolve and then rain virus particles down on other potential hosts.
The study was published on March 29 in the journal PLoS Pathogens.
http://www.mnn.com/earth-matters/animals/stories/dengue-virus-increases-mosquitos-lust-for-blood by Jennifer Welsh, LiveScience
Sim S, Ramirez JL, Dimopoulos G (2012) Dengue Virus Infection of the Aedes aegypti Salivary Gland and Chemosensory Apparatus Induces Genes that Modulate Infection and Blood-Feeding Behavior. PLoS Pathog 8(3): e1002631. doi:10.1371/journal.ppat.1002631