The folate receptor, a protein expressed on the surface of many human cells,
could be capable of transporting much more into the cell than just folic acid
- the B vitamin involved in normal bodily functions and cell development. In
fact, the Ebola and Marburg viruses apparently can hitch a ride on the receptor
and find their way into a cell as the first step in the viral infection cycle,
researchers at the Gladstone Institute of Virology and Immunology and the
University of California, San Francisco are showing for the first time.
Once these deadly viruses enter the cell, they begin to replicate, spread
throughout the body, and cause the characteristic hemorrhagic fevers that lead
to death in up to 90 percent of people infected. Current treatments are largely
ineffective. Although for the most part Ebola and Marburg have exacted their
human toll in periodic outbreaks in Africa, incidental transmission to Europe
and the United States has also occurred.
In the study, the researchers used cells known to be naturally resistant to
infection. Genes from cells that are susceptible to infection were randomly
inserted into the resistant cells. When these cells were exposed to either the
Ebola or Marburg viruses, only those with the gene encoding the folate receptor
Published in the July 13 issue of the journal Cell, the study advances the
understanding of the Ebola and Marburg life cycles of which little is known.
Entering the cell is a first and essential step in the viruses’ move to
replicate itself. Finding a molecule to inhibit the viruses from binding to the
folate receptor could form the basis for a treatment, said senior author Mark
A. Goldsmith, MD, PhD, associate investigator at Gladstone and UCSF associate
professor of medicine.
“Targeting the virus in the first step in the replication cycle is an
intellectually compelling idea,” Goldsmith said. “It could stop the virus dead
in its tracks before it has an opportunity to multiply and seek new cells to
infect. A similar understanding of cell entry pathways used by another deadly
virus, the human immunodeficiency virus (HIV), has been an important foundation
for emerging new therapies for AIDS.”
In one experiment, the researchers were able to inhibit both the Ebola and
Marburg viruses from infecting a cell when they exposed the cells to high
concentrations of folic acid. Exposing cells to an antibody specifically
designed to bind to the folate receptor or to a “decoy” form of the folate
receptor also inhibited the viruses.
Goldsmith emphasized that much more study needs to be done before folate or
other binding proteins can be used as treatments. Other molecules could be
found that would do a better job of blocking entry, he said. What also remains
to be answered is whether the viruses can use other receptors to gain entry
into the cell. Goldsmith and colleagues strongly suspect that there are such
Studying the Ebola and Marburg viruses was possible through the use of
pseudotype viruses-viruses that can infect but can’t replicate, said lead
author Stephen Chan, PhD, former graduate student at Gladstone and UCSF medical
student. Traditionally, study of the two viruses has been hampered by the need
for a maximum safety facility, of which there are only a few in the world.
Pseudotype viruses are much safer to handle, which makes such research studies
possible without the use of high-level containment facilities.
Chan and Goldsmith are one of only a few research teams to publish studies
using pseudotype viruses for Ebola and Marburg. The ones developed by Chan use
selected components of HIV to generate disabled viruses with some special
features that make them useful for genetic experiments. Colleagues at the
United States Army Medical Research Institute for Infectious Diseases confirmed
the study’s key findings by experiments using live, natural viruses in a
maximum safety facility.
Co-investigators of the study include Cyril J. Empig, PhD, postdoctoral fellow,
Frank J. Welte, graduate student, Roberto F. Speck, MD, former postdoctoral
fellow and now assistant professor of medicine at the University Hospital in
Zurich, and Jason F. Kreisberg, graduate student, all of Gladstone; and Alan
Schmaljohn, PhD, United States Army Medical Research Institute for Infectious
Diseases chief of viral pathogenesis and immunology in the virology division.
The study was funded by the J. David Gladstone Institutes. Chan was supported
by the National Institutes of Health Medical Scientist Training Program and the
UCSF Biomedical Sciences Program.
The Gladstone Institute of Virology and Immunology is one of three research
institutes that comprise The J. David Gladstone Institutes, a private
biomedical research institution affiliated with UCSF. The institution is named
for a prominent real estate developer who died in 1971. His will created a
testamentary trust that reflects his long-standing personal interest in medical
education and research.