Roger Miesfeld and UA Assistant Research Scientist Jun Isoe evaluate a storage container with mosquitoes used to study ways to interfere with the insects’ ability to transmit diseases. Miesfeld’s son, Barrett (right), has joined the lab as a summer undergraduate researcher from North Carolina State University. Photo: Daniel Stolte/UANews |
Inhibiting
a molecular process cells use to direct proteins to their proper
destinations causes more than 90% of affected mosquitoes to die
within 48 hours of blood feeding, a University of Arizona (UA) team of biochemists found.
Mosquitoes
die soon after a blood meal if certain protein components are
experimentally disrupted, a team of biochemists at the University of
Arizona has discovered.
The
approach could be used as an additional strategy in the worldwide
effort to curb mosquito-borne diseases like dengue fever, yellow fever,
and malaria.
When
the researchers blocked a cellular process known as vesicle transport,
on which the mosquitoes rely to release digestive enzymes into the gut
among other functions, it caused the affected animals to die within two
days of blood feeding.
“The
idea behind our research is this: If we can kill the mosquito after she
bites the first person, she won’t be able to bite and infect a second,”
says Roger Miesfeld, a professor in the UA’s department of chemistry
and biochemistry, who led the research project.
“We
do this by blocking the mosquito’s ability to digest its blood meal,”
says Miesfeld, also a member of the UA’s BIO5 Institute.
The research team’s findings were recently published in Proceedings of the National Academy of Sciences, or PNAS.
“During
a blood meal, a mosquito ingests its body weight in blood. It’s the
equivalent of a 125-pound human consuming a 12-gallon smoothie made from
25 pounds of hamburger meat plus a half pound of butter and two
tablespoons of sugar,” Miesfeld says.
Miesfeld
and the research team had previously shown that the blood feeding
process poses a huge metabolic challenge to the female mosquito.
“By
disrupting any number of biochemical processes needed to fully utilize
the blood meal, the mosquito has a very difficult time completing the
egg production cycle,” he adds.
To
maintain their bodily needs, the insects rely on sugary nectar from
flowers, but when the time to make eggs comes, they need large amounts
of protein. Only female mosquitoes bite and feed on the blood of humans
or warm-blooded animals.
Recognized by white markings on the legs, a mosquito of the species Aedes aegypti feeds on human blood. Photo: CDC/James Gathany |
If
a mosquito finds enough victims to bite and avoids being squashed, it
can live as long as three weeks. During that time, it may lay up to five
clutches of more than 100 eggs each.
For
their studies, the team used mosquitoes of the species Aedes aegypti,
which originally hails from the sub-tropical and tropical regions of
Africa. These mosquitoes are now found in many parts of the world and
are particularly abundant in towns and cities where the climate is warm
and water is plentiful. A. aegypti mosquitoes buzz about at dawn and
dusk in search of their next blood meal, preferably from people’s
ankles.
A.
aegypti mosquitoes are the main vector for dengue fever, now the most
common viral disease transmitted by mosquitoes. Dengue fever has made a
comeback especially in subtropical and tropical regions due to the
geographical spread of the mosquitoes and the virus. Four strains, or
serotypes, of disease-causing dengue viruses are known.
When
infected for the first time, most people suffer through a bout of high
and painful fever, but usually the illness is not life threatening and
renders them immune to that particular strain of dengue virus. However, a
subsequent infection with any of the other dengue virus strains often
triggers an all-out immune response that leads to a much more severe
form, called dengue hemorrhagic fever, which can be fatal.
Miesfeld
says most mosquito-borne pathogens are not passed down from the female
mosquito to her offspring, but instead picked up by the mosquitoes when
they bite an infected human.
In
the case of A. aegypti mosquitoes, the pathogen is often dengue or
yellow fever viruses, whereas the Anopheles gambiae mosquito, which is
found in many parts of Africa, transmits the more deadly malaria
parasite. Miesfeld further explained that malaria and dengue pathogens
take about 10 to 12 days to complete their life cycle within the
mosquito before they can be transmitted to humans through blood feeding.
“The most dangerous mosquitoes are the older ones,” Miesfeld says.
Jun Isoe anesthetizes mosquitoes by placing them on ice before he prepares them for an experiment. For this study, Isoe has injected RNAi into about 5,000 mosquitoes. “I can process about 1,000 in an 8-hour day,” he says. Photo: Daniel Stolte/UANews |
His
team used a technique called RNAi to specifically target genes that are
required for the digestion process. The researchers homed in on a
protein complex called COPI, which stands for coatomer protein 1.
COPI
consists of several subunits that together make up the envelope of the
vesicles on which the cell relies for internal transport and for
secretion of enzymes into the gut.
When
a female mosquito takes a blood meal, the cells lining its gut secrete
enzymes to break down the blood proteins. The secretion process involves
packaging the enzymes in small droplets called vesicles that the cells
then release into the gut.
“We
thought, ‘Why don’t we knock out the whole process that allows the
proteases to be secreted?’ That’s where we got this amazing result,”
Miesfeld says. “Not only did we eliminate her ability to secrete
anything, we were surprised to find that about 90% of those
mosquitoes died within two days after feeding on blood.”
The COPI RNAi does not have an adverse effect on the female mosquitoes for 10 days – unless they decide to take a blood meal.
“When she does, all hell starts breaking loose, biochemically and anatomically speaking,” Miesfeld says.
“What
we think is happening is that if there is protein in her gut, it
induces the secretory machinery. It initiates this huge secretion
process but it’s defective and causes cells to disintegrate,” he adds.
“The whole lining of the gut starts to fall apart, allowing the blood to
seep into her body.”
In
looking at the potential causes, Miesfeld says his team found that
removing any one of the COPI subunits causes the whole complex to fall
apart.
“Based on what we know about the COPI system, it shouldn’t have that strong of an effect,” he says.
“As
scientists have been knocking out COPI to learn more about its function
over the past couple of years, they have achieved some interesting
results,” Miesfeld adds. “Together with our findings they suggest that
COPI does a lot more than what people thought.”
Miesfeld
envisions that the ultimate goal of this research is to develop a small
molecule that works in place of injected RNAi and acts as a specific
inhibitor of the secretion process.
For this to be an effective mosquito-selective insecticide, it must not have any effect on humans.
The
simplest use would be to soak it into mosquito nets like currently
available insecticides that target the mosquito’s nervous system. A
slightly more complex strategy would be to include it in a pill that
humans take, so the mosquitoes pick up the inhibitor drug when they
bite. As part of this strategy, the researchers are looking for genes
that are unique to the mosquito and could serve as targets without
affecting human health.
To explain, Miesfeld says to imagine a village in the tropics during a rainy season.
“As
the mosquitoes hatch in large numbers, the whole population of
villagers is ready,” he explains. “As soon as the insects start biting,
they take up the inhibitor and before they can bite again, they die in
large numbers. Over a few seasons, that can make a difference.”
Miesfeld
added it is unlikely there would ever be a silver bullet eliminating
mosquito-transmitted diseases like malaria and dengue fever altogether.
“One
potential issue with our strategy is genetic changes rendering the
mosquitoes immune over time,” Miesfeld says. “Many approaches from
different angles will be necessary, and ours could be another tool in
the toolbox.”
