WEHI researchers, in collaboration with researchers at Merck Sharp & Dohme (MSD) in the US, have captured the first three-dimensional images revealing at a molecular level how new agents work to stop malaria parasites multiplying in the bloodstream.

Plasmodium parasites responsible for malaria make enzymes called plasmepsin IX (PMIX) and plasmepsin X (PMX) that process key proteins, enabling parasites to move in and out of red blood cells.

Two novel compounds, the results of a six-year collaboration between WEHI and MSD, had previously been shown to inhibit the actions of these plasmepsins, blocking the parasite lifecycle and stopping replication.

Now WEHI’s Dr Anthony Hodder, Dr Janni Christensen (now at Denmark’s ExpreS2ion Biotechnologies) and Professor Alan Cowman – working with Dr David Olsen and the team at MSD – have been able to visualise the compounds binding at the active sites of the enzymes.

“PMIX and PMX are like molecular ‘scissors’ and their actions can be likened to the video game character Pac-Man,” lead researcher Dr Anthony Hodder said.

“Our world-first 3D images show exactly how these compounds bind in the Pac-Man’s ‘mouth’, stopping these molecular scissors from cutting other proteins.”

“The result is that the parasite cannot move out of one infected red blood cell and invade others,” explained Dr Hodder.

Dr Janni Christensen said the findings would not have been possible without the Australian Synchrotron and WEHI’s X-ray crystallography equipment. “This technology allowed us to capture images of these enzymes magnified up to 100 million times in size,” Dr Christensen said.

“We can see in exquisite molecular detail how these compounds block the activity of PMIX and PMX to stop Plasmodium parasites from spreading.”

Professor Alan Cowman, deputy director at WEHI, said the 3D images have laid the foundation for the development of new drugs to block the plasmepsins more effectively and prevent the replication of malaria parasites. “Knowing exactly how something works helps us design new and more potent antimalarial compounds,” said Professor Cowman.

Another WEHI research group has shown that chronic malaria infections without symptoms are not innocuous and can actually suppress the immune system, preventing it from eradicating parasites from the bloodstream.

Findings of a study led by WEHI PhD student Stephanie Studniberg and Associate Professor Diana Hansen challenge the long-held belief that asymptomatic malaria infections offer protection against more severe, symptomatic disease.

“Asymptomatic infections have often been left untreated in countries where malaria is endemic, based on this assumption,” said Associate Professor Hansen.

“Our findings suggest treating asymptomatic infections could enhance vaccine effectiveness and reduce malaria transmission.”

In collaboration with Indonesia’s Eijkman Institute for Molecular Biology, the Papuan Health and Community Foundation and the Menzies School of Health Research at Charles Darwin University, the research team analysed white blood cells of patients with asymptomatic and symptomatic infection in an area of Indonesia where malaria is endemic.

Their analysis showed genes that suppress the immune system are upregulated in patients with chronic asymptomatic infection. Production of proteins that help parasites survive is increased, and the body is unable to clear them from the bloodstream.

The study could have critical implications for malaria screening, vaccination and elimination strategies around the world.

“In a patient with asymptomatic malaria infection and immunosuppression, the effectiveness of a malaria vaccine is likely to be reduced as the immune system does not have the capacity to be ‘trained’ appropriately,” said Associate Professor Hansen.

“If we were to treat more individuals with asymptomatic malaria infections, we would also reduce the invisible parasite reservoir that perpetuates transmission and undermines malaria elimination campaigns.”

In an important advance for malaria screening, WEHI researchers have successfully modified a panel of markers for the disease to ensure greater accuracy in blood testing.

Malaria can be caused by several Plasmodium species, with the greatest burden of disease in humans caused by P. falciparum and P. vivax. Across much of Southeast Asia, P. vivax is co-endemic with a third species, P. knowlesi, making it important to have blood tests that minimise cross-reactivity and false positives.

A team led by WEHI’s Professor Ivo Mueller and Dr Rhea Longley, along with researchers from the Menzies School of Health Research at Charles Darwin University and the Queen Elizabeth II and Gleneagles Hospitals in Sabah, Malaysia, designed a panel of eight P. vivax proteins that have low levels of cross-reactivity with P. knowlesi, enhancing the specificity of testing for P. vivax.

Use of the enhanced panel of markers should improve the accuracy of screening for P. vivax infection and assist public health interventions to accelerate elimination of malaria in asymptomatic carriers.