Munich University. Germany: The deadly Ebola virus makes use of host mechanisms – including a specific type of membrane-bound calcium channel – to gain entry into the cell cytoplasm. LMU researchers now show that blocking this channel markedly inhibits infection.
In recent weeks, reports from West Africa, which has experienced by
far the worst Ebola epidemic yet seen, have signaled a turn for the
better in the fight against the disease. In the three countries worst
affected, the number of new infections has fallen significantly. Indeed,
the World Health Organization (WHO) has officially declared that the
epidemic in Mali has ended, although that announcement could yet prove
premature. After all, no previous outbreak has been so prolonged or so
extensive. Furthermore, in areas where the epidemic is still raging,
there has been little change in the incidence of lethality, and the
majority of infected patients succumb to the disease. There are still no
effective and readily available treatments, although several promising
lead candidates are now undergoing early clinical testing. The virus
causes a severe hemorrhagic fever, which leads to widespread internal
bleeding and ultimately results in death owing to multiple organ
failure.
How the virus infects host cells and exploits their metabolism for
the production of new virus particles is not fully understood. A
collaborative effort involving teams based in Germany and in the US has
now supplied one of the missing pieces of the puzzle – and uncovered a
new target for therapeutic drugs to combat the infection. The new work,
carried out by research groups led by pharmacologists Martin Biel and Christian Wahl
at LMU, and virologist Dr. Robert Davey at the Texas Biomedical
Research Institute in San Antonio, is reported in the leading American
journal “Science”.
The Ebola virus (EBOV) infects macrophages (whose normal function is
to dispose of pathogens that have been marked for destruction by other
cells of the immune system) by latching onto specific receptor molecules
found on their surfaces. Receptor binding causes the cell membrane to
fold inwards like a pouch which is then pinched off, engulfing the
receptors and the attached viruses in so-called endocytic vesicles.
These “endosomes” then fuse with another type of vesicles called
lysosomes. Specific ion-channel proteins in the lysosomal membrane,
known as two-pore channels (TPCs), are known to play an important role
in the fusion process. Biel and his colleagues have now shown that TPCs
are essential for the establishment of an Ebola infection: Upon binding
of an endosome, the TPCs release a stream of calcium ions into the
cytoplasm that serves as a signal for membrane fusion, which is required
to ensure that Ebola infection cycle can proceed. If the TPCs are
genetically defective or functionally inhibited, the viruses remain
trapped in the endosomes, effectively aborting the infection.
The researchers also found that tetrandrine, an alkaloid derived from
plants which has long been used in traditional Chinese medicine,
effectively inhibits infection of isolated macrophages by EBOV.
Experiments carried out by the American group in their state-of-the art
containment facility in San Antonio confirmed that the agent is
therapeutically active in mice inoculated with the virus, while
displaying relatively low toxicity. Meanwhile, the ion-channel experts
in Munich took a closer look at the interaction between tetrandrine and
TPCs and analyzed its effect on their function. The fact that the LMU
team had previously created mouse strains that lack individual TPCs was
the crucial element in this part of the project. Some of this work were
carried out under the auspices of the Center for integrated Protein
Science Munich (CiPSM) – a Cluster of Excellence – and the
Transregio-Collaborative Research Center 152 “TRiPs to Homeostasis:
Maintenance of Body Homeostasis by Transient Receptor Potential Channel
Modules”.
Martin Biel believes that targeting the TPCs represents a promising
strategy for fighting the virus. “Instead of trying to kill the virus,
we simply ensure that it is no longer infectious,” he points out. “We
don’t attack it directly; we take a roundabout route, interrupting the
progress of the infection.” This reduces the risk that the virus’s
propensity to mutate will allow it rapidly to become resistant to this
kind of inhibitor. The Munich team now plans to improve the
pharmacological and biochemical properties of tetrandrine and enhance
its specificity for the ion channel. “I am quite optimistic,” says Biel.
“I think there’s a pretty good chance that a useful drug candidate will
emerge from this work.” math
(Science 2015)