The results are online and scheduled for publication in the April 2010 issue of the journal Cell Host & Microbe.
Postdoctoral fellow Jason Upton, PhD and graduate student William Kaiser are co-first authors of the paper. Both are in the laboratory of Edward Mocarski, PhD, Robert W. Woodruff Professor of Microbiology and Immunology at Emory University School of Medicine.
“This is the first example of a virus interfering with this death pathway, showing it is an important part of the host response to infection, and the lengths to which viruses will go to control those responses,” Upton says. “Programmed necrosis is still poorly understood, and this virus will be a great tool to dissect how it is regulated.” Understanding how programmed necrosis works may lead to new strategies to contain brain tissue damage after a stroke or heart attack.
Until recently, scientists divided cell death into two broad categories: necrosis, an unregulated process caused by injury or starvation, and apoptosis, a program of self-destruction deliberately set in motion by the cell itself. Recently, programmed necrosis has emerged as an alternative form of cell death that shares the morphological features with necrosis but like apoptosis is orchestrated by the cell itself rather than occurring by accident or neglect.
The discovery that a virus blocks this necrotic death pathway began with Upton, Kaiser and Mocarski’s investigation of a mutation in the common virus CMV (cytomegalovirus). For some types of cells, infection by the mutant virus results in premature cell death.
“When cells undergo apoptosis, their membranes bleb [create a blister] but stay intact and the nuclei fragment,” Kaiser says. “However, we observed that cells killed by mutant CMV infection had ruptured membranes and shrunken nuclei, which are characteristic features of necrosis.
“Normally, CMV infection doesn’t cause apoptosis or necrosis of infected cells, because the virus contains genes specifically for inhibiting those anti-viral pathways. Studying viruses that lack those inhibitors reveals what the cell would do in their absence,” Upton says.
Upton and Kaiser went on to show that the gene disabled in the mutant virus disrupts an enzyme called RIP3, recently identified by other scientists as being responsible for orchestrating programmed necrosis. Interfering with RIP3 is required for CMV to successfully infect cells. The levels of RIP3 dictate how sensitive cells are to virally induced necrosis.
“This study establishes RIP3-driven necrosis as a critical host defense against viral infection,” Kaiser says.
Apoptosis controls cell numbers, eliminating abnormal, harmful or injured cells. Scientists estimate that more than 10 million cells a day undergo apoptosis in the human body for homeostasis. Apoptosis occurs during development-for example, weeding out cells between the fingers in a human embryo’s developing hands. Apoptosis also limits how many white blood cells are produced in an immune response.
Stroke results in a mixture of apoptosis and necrosis in the brain: necrosis in the most oxygen-deprived areas, and apoptosis in others. That’s why understanding cell death pathways such as RIP3-mediated programmed necrosis could lead to new strategies to control tissue damage resulting from stroke.
It is estimated that a majority of the U.S. population is infected with CMV or has been exposed to it. However, most healthy people infected by CMV after birth have no symptoms. CMV is more a problem for infants or people with weakened immune systems, and CMV infection during pregnancy is a major cause of birth defects. Understanding how viruses activate and disrupt host cell death pathways may provide insight into how to limit or restrict other more damaging viral infections.
The research was supported by the National Institutes of Health.
Mocarski is currently on an uncompensated leave of absence from Emory University as Distinguished Fellow at MedImmune, LLC, the biologicals and vaccine division of AstraZeneca. There he provides administrative management to expand a vaccine pipeline research program with particular attention to herpes viruses, including cytomegalovirus. Mocarski will return to his research and teaching roles at Emory in November 2010 as Robert W. Woodruff Professor of Microbiology and Immunology in Emory University School of Medicine and the Emory Vaccine Center. His ongoing research group at Emory, currently under the temporary supervision of others, includes postdoctoral fellow Jason Upton and graduate student William Kaiser.
Writer: Quinn Eastman
The Robert W. Woodruff Health Sciences Center of Emory University is an academic health science and service center focused on missions of teaching, research, health care and public service. Its components include the Emory University School of Medicine, Nell Hodgson Woodruff School of Nursing, and Rollins School of Public Health; Yerkes National Primate Research Center; Emory Winship Cancer Institute; and Emory Healthcare, the largest, most comprehensive health system in Georgia. Emory Healthcare includes: The Emory Clinic, Emory-Children’s Center, Emory University Hospital, Emory University Hospital Midtown, Wesley Woods Center, and Emory University Orthopaedics & Spine Hospital. The Woodruff Health Sciences Center has a $2.5 billion budget, 17,600 employees, 2,500 full-time and 1,500 affiliated faculty, 4,700 students and trainees, and a $5.7 billion economic impact on metro Atlanta.