Can malaria transmission be prevented through catastrophic failure of gametocyte quiescence?

Despite recent gains in 2000-20016, progress has stalled and malaria is still a devastating disease, killing ~405,000 people each year and infecting 228 million. Plasmodium falciparum, the parasite causing the most deadly form of malaria spreads when a female mosquito ingests specialised parasite cells called male and female gametocytes whilst biting an infected person. These gametocytes have no control over when a mosquito might bite, therefore, to maximise their chances of transmission they become "quiescent" (i.e. dormant) for up to 22 days in human blood. Most antimalarial drugs are not effective against quiescent gametocytes thus allowing the disease (and drug resistance genes) to escape and spread throughout the population.
Cellular quiescence is a process that is fundamental to all of life. In response to an unfavourable environment or a specific signal, cells can stop growing and become quiescent for a period of time. When conditions become more favourable, quiescent cells then exit this dormancy and resume their normal programmed growth. Cells carry out quiescence by a number of different methods, however common processes occur within the cell to keep them alive: 1. Reduced or efficient energy generation; 2. A shift in resource production from those needed for growth, to those necessary for survival; 3. Efficient damage and repair mechanisms.
I hypothesise that interfering with gametocyte quiescence mechanisms will have catastrophic effects on their infectiousness, leaving them unable to transmit to mosquitoes. Studying these processes will help the design and discovery of new transmission-blocking antimalarial therapies targeting gametocyte quiescence.
My fellowship focuses on how quiescent gametocytes regulate their energy production. Parasite stages in the mosquito generate energy by consuming glucose in a process called mitochondrial respiration, which is carried out in a specialised part of the cell called the mitochondrion. Mitochondrial respiration is essential for the parasite to survive in the mosquito but less important whilst it is in the human. Gametocytes must be ready to "switch on" mitochondrial respiration at a moment's notice. However, too much unwanted mitochondrial respiration is damaging for cells as it produces toxic "free radicals" that can kill the cell and thus would limit the lifespan of the gametocyte and lower its chances of transmission. Therefore, gametocytes appear to have several mechanisms to control their energy generation. It is hypothesised that one mechanism is to divert glucose away from the mitochondrion and out of the cell before it has been consumed. Alternatively, energy production could be reduced by replacing key enzymes (proteins that manufacture materials needed by the cell) involved in the process with less efficient alternatives. I have identified four enzymes made by gametocytes that may be responsible for this control. To study the role these play, I will genetically modify the parasite to lack these proteins and observe how this affects gametocytes and mosquito transmission. This will involve feeding parasites to live mosquitoes. I will also trace how glucose use by the parasite is affected in the mutant parasites using a technique called metabolomics which separates and identifies individual chemicals made by the cell. To identify additional proteins important for maintaining gametocytes in their quiescent state, I will label newly made proteins within the gametocyte with a chemical "tag" which will allow me to "capture" them and identify them using a technique called mass spectroscopy. By using this approach on male and female gametocytes individually, I will determine whether there is a sex difference in how gametocytes maintain quiescence. Finally, I will study how disrupting gametocyte energy metabolism impacts their ability to repair themselves.
Ultimately, my research will identify which steps in the quiescence pathway could by targeted by new therapeutics.

Malaria propagates when Plasmodium male and female gametocytes transmit from humans to mosquitoes. Interrupting this reduces new cases of malaria and the spread of drug resistance, therefore is a recognised therapeutic priority. Gametocytes have no control over when a mosquito bites, and so become quiescent for up to 22 days to maximise their chance of transmission. Mitochondrial (MT) respiration is essential for parasites in the mosquito, however blood stage parasites favour anaerobic respiration. Gametocytes develop a large cristate mitochondrion in readiness for transmission however they divert glucose metabolism away from the mitochondrion and excrete it as acetate. I hypothesise that strong MT activity in quiescent gametocytes will interfere with their dormant lifestyle in humans, and so gametocytes have been forced to develop novel (and potentially drug-targetable) mechanisms to tightly regulate glucose metabolism and thus prevent unwanted MT oxidative damage that would limit their lifespan. Through proteomic analysis I have identified 4 genes specifically expressed by gametocytes that replace or supplement key steps in glycolysis and parasite energy metabolism, including the enzyme putatively responsible for diverting glucose metabolism to acetate and an alternative isoform of phosphofructokinase lacking catalytic residues. I will study their function and essentiality through conditional gene deletion, phenotypic analysis during mosquito transmission, and activity of recombinant protein. I will also study their effects on glucose utilisation through 13C-glucose incorporation and metabolomic analysis. Finally, to discover further regulators of the gametocyte quiescence phenotype, I will track protein synthesis in male and female gametocytes using metabolic labelling to compare and contrast their quiescent proteomes by mass spectroscopy. In this way, I will identify new transmission-blocking drug targets for exploitation to ultimately aid malaria eradication.