Transmitted through infected mosquitos, the malaria parasite, once in the human bloodstream, synchronously ruptures red blood cells and rapidly infects new red blood cells, beginning the cycle anew. Controlled by an intrinsic clock, this cycle occurs in multiples of 24 hours, depending on the species. Humans infected with malaria parasites exhibit rhythmic fevers and chills every 24, 48 or 72 hours depending on the species of Plasmodium.
Is it possible that the parasite's biological clock is "in sync" with its human host's circadian clock?
A new study by Florida Atlantic University, Duke University and a team of researchers from the Armed Forces Research Institute of Medical Sciences has uncovered evidence of a "coupling" mechanism between the parasite and its host, which could one day lead to new treatments for a disease that claims the life of a child under age 5 every minute.
Researchers pondered the idea that it might be advantageous for the parasite to time its development and cell division to certain times when its host's immune system is the most suppressed. To test this theory of coupling, they relied on phase alignment – the timing and the pace of the two clocks – to determine if the parasite's 48-hour cycle lines up with its host's circadian processes.
Results, published in the Proceedings of the National Academy of Sciences, provide direct evidence that the parasite's intrinsic developmental cycle is aligned to the host's circadian rhythm during malaria infection. This alignment suggests an evolutionary adaptation that helps create synchrony between the parasites during the blood phase and a method for evading host defenses or optimizing utilization of host resources that are controlled by the host circadian clock.
In the future, the discovery of the molecular signals that coordinate this synchronization of the malaria parasite and the host will likely have important implications for new antimalarial therapies against a disease that continues to be increasingly resistant to existing drugs."
Francis Motta, Ph.D., first author and assistant professor in the Department of Mathematical Sciences within FAU's Charles E. Schmidt College of Science
For the study, whole blood from humans infected with malaria was cultured ex vivo (outside the host). Researchers simultaneously measured the gene expression of the malaria parasite and humans to observe which genes were turning on and off in rhythmic patterns at 48- and 24-hour periods. Looking at genetic oscillations over 48 hours enabled them to compare shifts in humans and parasites and their internal clocks to determine if they were advanced or delayed. For example, if a human's cycle was shifted ahead, researchers checked to see if the parasite's cycle also was ahead, and by the right amount. They did this across all pairs of human participants to measure how much correlation there was in the shift between the circadian cycle of the host and shifts in the parasite cycles.
Gene transcripts revealed that the phase of the host circadian cycle and the parasite cycle are correlated across multiple patients, supporting that the cycles are phase coupled. This suggests that the oscillators controlling the parasite intraerythrocytic cycle in human-infecting Plasmodium parasites are coupled to the host circadian rhythm.
"Knowing that two intrinsic clocks are aligning provides us with a mechanistic description of the fundamental underpinnings of this disease and how we might be able to use genetic targets for treatment," said Motta. "In mice, it's been shown that misaligning or disrupting the developmental cycle of a parasite with the host's circadian rhythm decreases the fitness or health of the parasite. If we can disrupt the parasite's development cycle so that it's less synchronized with its human host, thereby rendering it less fit, then it could be easier to treat malaria. That's the hope and the ultimate payoff if we can make this happen."
A key component of this research was the rigorous mathematical and statistical approach used to determine and compare phase shifts in highly dimensional and noisy data. Moreover, extracting circadian signals from whole blood was extremely complex because genes are not robustly expressed in whole blood.
"The math enabled us to reduce the number of assumptions we made about our data. The conclusions of this work may not have been possible otherwise, because we were dealing with data and a context, specifically whole blood from people affected with malaria, which is not well understood," said Motta. "We let the data tell the story without having to assume knowledge we don't yet have, and the math really empowered us to do that."
The researchers speculate one possibility of the mechanism of the intrinsic oscillator of the parasite and the circadian of the host couple is that there are chemical or physiological signals that can be read by either or both the host and pathogen that enable them to phase-align.
"There have to be some molecular signals that they're passing back and forth to each other," said Steve Haase, Ph.D., senior author and a professor of biology at Duke. "We don't know what they are, but if we can disrupt them, then we might have a shot at an intervention."
Study co-authors represent the University of North Carolina; University of California, San Francisco; Geisel School of Medicine at Dartmouth; Cymantix, Inc.; Memetics, LLC; Sana Biotechnology, Inc.; Mahidol University, Thailand; United States Armed Forces Research Institute of Medical Sciences, Thailand; Walter Reed National Military Medical Center; Walter Reed Army Institute of Research; and Geometric Data Analytics.
This research was supported by the Defense Advanced Research Projects Agency (D12AP00025 awarded to Haase and J. Harer, CEO, Geometric Data Analytics, Inc.), the National Institutes of Health (GM126555-0 awarded to Haase), and the National Science Foundation (DMS 1847144).
Motta, F. C., et al. (2023) The Parasite Intraerythrocytic Cycle and Human Circadian Cycle Are Coupled During Malaria Infection. PNAS. doi.org/10.1073/pnas.2216522120.