Ebola virus  (Source:
The team discovered a protein called Niemann-Pick C1 (NPC1), which they found was responsible for allowing Ebola virus to enter and replicate within cells

A team of scientists from different U.S. colleges and research institutes have discovered how and where Ebola virus enters a host cell, which has long been a mystery that deterred the creation of an effective vaccine.

Researchers from Whitehead Institute, Harvard Medical School, Albert Einstein College of Medicine and U.S. Army Medical Research Institute of Infectious Diseases have joined forces to better understand Ebola virus in order to eventually lead to the development of a vaccine that prevents Ebola hemorrhagic fever (EHF).

Ebola virus is classified as a category A bioterrorism agent by the U.S. Centers for Disease Control and Prevention (CDC). While Ebola virus outbreaks are rare, they can cause EHF and are extremely dangerous to humans. Those with EHF have symptoms such as fever, diarrhea, vomiting, intense weakness, joint and muscle pain, and sometimes external or internal bleeding due to the disintegration of blood vessels.

While the symptoms of EHF can be treated, there is currently no vaccine to prevent it. According to the World Health Organization (WHO), there have been 1,850 EHF cases with 1,200 deaths since the virus was found in 1976.

"Right now, people make therapeutics to inactivate the pathogen itself," said Thijn Brummelkamp, former Whitehead Fellow and current leader at the Netherlands Cancer Institute (NKI). "But the problem is that pathogens can quickly change and escape detection and elimination by the immune system. Here we get a good idea of the host genes that are needed for the pathogen to enter the cell for replication. Perhaps by generating therapeutics against those host factors, we would have a more stable target for antiviral drugs."

The team of researchers discovered how Ebola virus gained entry into a host cell by using an unusual human cell line. All humans receive one copy of each chromosome from each parent, but cell lines have a single set instead with only one copy of each individual gene. Researchers wanted to use gene disruption, which "knocks out" a gene function in host cells one-by-one, to  silence one copy of a gene.

Researchers used a technique that team member Jan Carette, of the Brummelkamp lab at Whitehead Institute, utilized while studying the cytolethal distending toxin (CDT) family. The CDT family secretes several pathogenic bacteria such as Escherichia coli, and each bacterial species has created different mutations of the CDT structure. Using a line of haploid cells, or KBM7 cells, which have only one copy of each chromosome and were isolated from a chronic myeloid leukemia patient, researchers could disrupt the expression of each gene while screening for mutants that had certain characteristics (specifically, those who could survive a lethal dose of the toxin).

While disrupting the normal structure of the gene, mutant KBM7 cells were exposed to the toxins and those that survived were studied. In the end, after studying the surviving cells' genomes, researchers were able to find ten human proteins used by CDT's during intoxication. Also, the host cells were adjusted for each CDT's target cell.

The research team utilized this same technique for Ebola virus using an unusual human cell line. Through the use of a harmless virus covered in the Ebola virus glycoprotein and the alteration of the haploid cells, the team discovered a protein called Niemann-Pick C1 (NPC1), which they found was responsible for allowing Ebola virus to enter and replicate within cells. The mutations of this gene cause a type of Niemann-Pick disease.

The effects of the active Ebola virus were tested on mice who had a knocked-out copy of the NPC1 gene and mice with two functioning NPC1 genes. Those with two functioning NPC1 genes were affected by the virus very quickly while those with a knocked-out NPC1 gene were significantly protected.

"This is pretty unexpected," said Carette. "This might imply that genetic mutations in the NPC1 gene in humans could make some people resistant to this very deadly virus. And now that we know that NPC1 is an Ebola virus host factor, it provides a strong platform from which to start developing new antivirals."

This study was published in Nature.

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