Developing genetically-engineered mosquitoes to fight malaria
Posted Jan. 19, 2011
Science is a team sport, especially when it comes to tackling malaria. In her 15 years as a malaria researcher, UC Davis molecular biologist Shirley Luckhart has engaged in several collaborations with universities and research labs across the country in order to understand and tame this global epidemic.
Malaria, a parasitic disease carried between humans by the Anopheles mosquito, can cause flu-like symptoms and anemia. If untreated, it can be fatal. Each year, it is contracted by an estimated 250 to 300 million people and kills 1 million to 3 million, mostly children in sub-Saharan Africa.
Controlling malaria is no small endeavor due to the parasite’s abilities to evolve and resist treatments. Outbreaks in the U.S. in recent years have been minimal, which is a testament to comprehensive public-health and patient-care efforts. The regions most affected by malaria today – Africa, India and Southeast Asia – typically lack these resources.
Renewed global interest in malaria over the past decade has fueled critically needed research on new drugs and potential vaccines. Transmission control efforts have also expanded, including bed-net programs, indoor insecticides and parasite-resistant, transgenic mosquitoes.
The last strategy – developing a so-called “super mosquito” – is where Luckhart and her UC Davis team together with colleagues at University of Arizona made a major breakthrough in 2010.
Creation of a super-mosquito
The project had two parts. For phase one, Michael Riehle, associate professor of entomology at the University of Arizona and the bioengineering lead, inserted a modified Akt gene into eggs of the species Anopheles stephensi, the major vector of malaria in India, parts of Asia and the Middle East. The Akt gene plays a key role in cellular processes that could impede parasite development, such as innate immunity (helping regulate the host’s anti-parasite defenses), antioxidant synthesis (playing a role in how the environment encourages parasite development), cell-damage repair (enhancing the ability to overcome cell damage caused by the parasite), cell-cycle arrest and growth control.
Phase two began with the delivery of the transgenic mosquito eggs to Luckhart’s lab, where the eggs were raised into mature mosquitoes. At the same time, the team cultivated the most dangerous human malaria parasite, a single-cell organism called Plasmodium falciparum, in human blood. They fed the blood to the female mosquitoes, and waited for the results.
Typically, development and transmission of Plasmodium happens only in female mosquitoes that feed on human and animal blood in order to produce eggs. When the mosquito feeds on malaria-infected blood, it ingests the parasite. The parasites then try to squeeze through the midgut lining, but most die in the process; those that survive become attached to the outer midgut wall, where they develop into oocysts. After about 10-12 days, thousands of new parasites are released from the oocysts. These new parasites migrate to the mosquito’s salivary glands, where they remain until the mosquito feeds on a human, transmitting the parasite to a new host and perpetuating the cycle.
After dissecting the adult females, the UC Davis team discovered that the modified Akt gene rendered the mosquito immune to Plasmodium falciparum. Following ingestion, the parasite was unable to survive in the mosquito’s midgut. The modified Akt also shortened the mosquito’s lifespan. Because it takes nearly two weeks for malaria parasites to invade the salivary glands, only older mosquitoes are full-fledged carriers, cutting off the parasite’s development can eliminate transmission.
The experiments delivered a 100 percent success rate with each mosquito – surprising Luckhart and her colleagues at the University of Arizona, who were hoping for, at best, a substantive effect on the mosquito’s growth and life span.
Ideally, the researchers say, the next generation of mosquitoes will carry this altered genetic code into future generations, significantly reducing the number of malaria cases. The ultimate goal is to release the transgenic insect into malaria-endemic regions after further research to assure that potential legal, social and economic issues are addressed.
More explorations into the human-mosquito cell connection
The UC Davis-University of Arizona partnership and the promising results of the study, published in the July issue of PLOS Pathogens and recognized as one of the “50 Best Inventions of 2010 by Time magazine, is one outcome of three complementary, malaria-focused research projects funded by the National Institutes of Health.
Luckhart specializes in cell signaling and the innate immunology related to host-parasite interactions, which is unique. While most malaria researchers focus exclusively either on the human host or the mosquito host, Luckhart and her lab do both. This possibility of “immunological cross-talk” – a term coined by Luckhart and a colleague to describe the functioning of human blood-derived immune factors inside a mosquito – was once a highly questionable idea. But, since her post-doctoral days, Luckhart has been excited about the possibility that human blood might carry important signals into the mosquito that dictate how the mosquito responds to malaria parasites.
Today, her lab is observing the factors in mammalian blood that are elaborated in response to a malaria parasite.
“When a mosquito feeds,” she explained, “it takes in all the protein factors in our blood. But how do mosquito cells recognize these human proteins? How do they process the information? The cell signaling pathways in mosquitoes activated by human blood factors actually regulate how the malaria parasite can develop and be transmitted from the mosquito. We are trying to uncover what those protein factors are and how they function inside the mosquito.”
One of those proteins is insulin. Blood insulin levels in human patients with malaria often are abnormally and significantly elevated. Mosquitoes have insulin signaling pathways similar to humans, so they respond to that ingested insulin in similar ways as humans do. Human insulin, for example, influences the action of mosquito hormones in a manner similar to the regulation of human hormones by insulin.
“You can extend science beyond its traditional limits when you develop strong collaborations. ... The best ideas develop from diverse perspectives on significant problems.”
— Shirley Luckhart
So far, 10 protein factors that function as specific physiological signals to the mosquito have been identified.
“There are probably many more yet to be discovered,” Luckhart said. “How they function is the million dollar question – literally, since our current grants are focused on exactly these questions.”
“In a nutshell we can say that many of these factors are ancient in evolutionary terms,” she added, “and the cellular architecture to recognize them is present in organisms that are 600 million years diverged from us. On a basic level, that’s really amazing. On an applied level, it tells us that the ‘primitive’ insect and ‘advanced’ mammalian hosts for malaria aren’t so dissimilar on some levels, and we had better understand that world if we want to develop successful strategies to block malaria parasite transmission.”
“Immunological cross-talk” leads to discoveries
Luckhart’s group is currently engaged in five grants totaling more than $10 million that focus on multiple avenues of malaria research.
“All of my grants are team-funded work. I think that the best science is done by people with complementary expertise,” Luckhart said. “You really can extend science beyond its traditional limits when you develop strong collaborations. Sometimes it’s challenging – people don’t always agree – but the best ideas emerge from diverse perspectives on significant problems.”
In 2009, Luckhart and Yoram Vodovotz, professor of surgery, immunology, and communications science and disorders at the University of Pittsburgh, and the colleague with whom Luckhart invented the term “immunological cross-talk” – became co-principal investigators on a project focusing on developing a systems biology approach to understanding the major signaling pathway that regulates malaria parasite transmission.
“Yoram and I are still finding new proteins, new chemical signals and new ways of thinking about how malaria is transmitted,” Luckhart said.
Luckhart and Renee Tsolis, associate professor of medical microbiology and immunology at UC Davis, are co-investigators on a two-year project that examines co-occurring infections, such as salmonella, in malaria-endemic countries.
Luckhart also is associate director, together with Gregory Lanzaro, professor of pathology, microbiology and immunology at the UC Davis School of Veterinary Medicine, for a five-year NIH grant on the biology of disease vectors that began in September 2009. This training grant funds four pre-doctoral and two post-doctoral scientists to work on various aspects of insect vector biology, from population genetics and epidemiology to the molecular cell biology of insect-pathogen interactions.
In January 2010, Luckhart and her team joined Lanzaro and associate professor Anthony Cornel of the UC Davis Department of Entomology on another NIH grant.
“We spend a lot of time working under carefully controlled, laboratory conditions,” she explains. “The work in this grant is the first significant attempt to understand if what we’ve observed in the laboratory that controls malaria transmission in mosquitoes is also what happens under naturally occurring conditions in Africa.”
Again, the team was coordinated based on complementary expertise. Lanzaro and Cornel have tremendous expertise in population genetics of mosquitoes in Africa, while Luckhart brings molecular cell biology knowledge of how malaria parasites are transmitted.
“Together we create a really strong team. We can’t address these questions by ourselves, but together there is huge potential.”
Developing the road map to conquering malaria
Many of Luckhart’s research ideas sprang to life early in her career.
With advanced degrees in entomology, Luckhart’s research has focused on parasites, including the African river-blindness nematode. For her post-doctoral training, she was hired to develop a new program in molecular entomology at the Walter Reed Army Institute of Research in Washington, D.C.
“Most people go to a post-doc where there’s an existing lab with existing project ideas,” Luckhart recalled. “I had to start from zero. It was really exciting, but scary, too.”
Her supervisor and mentor, retired U.S. Army Colonel Dr. Ron Rosenberg (now associate director for research at the Centers for Disease Control and Prevention in Fort Collins, Colo.), advised her to focus on malaria. He considered it one of the most significant human health problems and one of particular interest to the military in terms of protecting service people in the tropical belt.
In the early 2000s, Luckhart came to UC Davis as associate professor because, as she said, “I needed to be at an institution with greater research capacity in biomedical science. UC Davis has a tremendous reputation in biological sciences and the faculty here create a wonderful environment.”
Especially her own team.
“In my lab, the people are the most important part of what I do. They’re strong and enthusiastic researchers, and the ones responsible for my productivity,” she said.
Even with the persistence of malaria worldwide, Luckhart is optimistic. Public engagement has pressured governments to maintain critical research funding and aid to affected countries. Researchers and funding agencies also recognize that a crucial key to success is the training of scientists from endemic countries.
“Despite significant developments, there are still many obstacles and challenges,” said Luckhart. “Fortunately, I can participate as a mentor to help prepare the next generation of researchers for this work. There certainly will be work for them to do, so I take the training of my lab team very seriously.”