Malaria, a disease that continues to plague millions, with an estimated 263 million cases and 597,000 malaria deaths worldwide in 2023, remains a formidable global health challenge. According to India’s National Centre for Vector Borne Diseases Control, approximately 95% of the country's population resides in malaria-endemic areas, and 80% of malaria cases reported in the country are concentrated in areas of tribal, hilly, rugged, and inaccessible regions.
While recent breakthroughs, such as new vaccines, offer a glimmer of hope, the parasite responsible, Plasmodium falciparum, is a master of survival, constantly adapting and evolving. Understanding its life cycle, including how it multiplies rapidly in human blood to cause illness and then spreads through the bite of mosquitoes, is crucial for developing new strategies to combat it.
New research from the National Institute of Immunology, New Delhi, Yenepoya (Deemed to be University), Mangalore, and the Indian Institute of Science Education and Research (IISER) Pune has studied a critical part of the parasite’s lifecycle. The new study sheds light on a crucial protein, PfPPM2, that acts like a central control panel. It orchestrates both the parasite's relentless division and its essential switch to sexual forms, which are key for disease transmission.
Inside our red blood cells, the Plasmodium falciparum parasite undergoes asexual division, multiplying furiously, which is what makes us sick. However, to spread to new hosts, it must transform into sexual forms called gametocytes, which a mosquito can then pick up. This transformation is a bottleneck for the parasite, and disrupting it could be a potential game-changer in our malaria eradication efforts.
Scientists use a process called protein phosphorylation, a biochemical process where a phosphate group is added to a molecule, often a protein, DNA, or lipid. The process is akin to flipping an on-off switch, and it is vital for nearly all life processes, including the development of the malaria parasite. Enzymes called kinases add these tags, and phosphatases remove them. The new study focuses on a specific phosphatase, PfPPM2.
The researchers knew that PfPPM2 was important, as previous studies had hinted that it was essential for the parasite's survival during its human blood stage. But exactly how it worked was a mystery. Since turning off the PfPPM2 completely would kill the parasite, making it hard to study, the scientists used a technique called a conditional knockdown system. It involves giving the parasite a switch that they could activate with a specific sugar molecule, glucosamine (GlcN). When they added GlcN, the PfPPM2 protein levels plummeted, effectively silencing it.
Without enough PfPPM2, the parasites struggled to grow and multiply. They produced far fewer merozoites, the tiny asexual, invasive forms that burst out of red blood cells to infect new ones. Looking closer, the scientists saw that the parasite's internal machinery for cell division, particularly the spindles (which are like tiny cellular highways that pull chromosomes apart during division), were all messed up. It was clear that PfPPM2 was vital for the parasite's ability to divide and spread within the human body. More importantly, the researchers also observed a significant drop in the parasite's ability to convert into their sexual forms, the gametocytes.
To understand how PfPPM2 was accomplishing this, the team employed phosphoproteomics, which enabled them to identify which proteins had their phosphate tags altered when PfPPM2 was depleted. They found that PfPPM2 regulates the phosphorylation of several key proteins, especially one called Heterochromatin Protein 1 (HP1). In malaria parasites, HP1 is known to be critical for controlling which genes are on or off, influencing both asexual division and, importantly, the switch to sexual forms.
The study revealed that PfPPM2 typically removes a phosphate tag from HP1 at a specific site (designated as S33). When PfPPM2 was depleted, HP1 at S33 became hyperphosphorylated, with an excessive number of phosphate tags added, much like a switch stuck in the on position.
To confirm this, they created mutant versions of HP1: one that couldn't be phosphorylated at S33 and one that mimicked constant phosphorylation. The S33A mutant parasites, which mimicked the normal function of PfPPM2, showed a strong tendency to convert into sexual forms. Conversely, the S33D mutant, which mimics the absence of PfPPM2, caused severe arrest in asexual development, highlighting the delicate balance that PfPPM2 maintains.
Beyond HP1, PfPPM2 also influenced protein synthesis, the process by which cells build new proteins. It achieved this by controlling another protein, eIF2α-kinase PfPK4. PfPPM2 normally represses the phosphorylation of PfPK4, which in turn promotes protein synthesis. When PfPPM2 was absent, PfPK4 became hyperphosphorylated, resulting in impaired protein synthesis. Since making new proteins is essential for a cell to divide, this explains another way PfPPM2 controls parasite growth.
This research significantly advances our understanding of the biology of the malaria parasite. While previous studies had hinted at PPM2's role in the related rodent malaria parasite, P. berghei, the precise mechanisms, especially in P. falciparum, were unclear. This study identifies specific protein targets, such as HP1 and PfPK4, and describes how PfPPM2 regulates their phosphorylation states to control both asexual division and sexual conversion. However, the study faced some limitations, such as the difficulty in generating specific HP1 mutants due to their severe impact on parasite survival and technical challenges in directly studying gametocytes. Further direct evidence on how protein synthesis specifically impacts parasite division would also be beneficial.
By identifying PfPPM2 as a central regulator, scientists now have a clearer picture of potential new drug targets. Drugs and treatments can now be developed to specifically inhibit PfPPM2. If successful, we might be able to achieve a dual blow against malaria: first, by blocking the parasite's ability to multiply in human blood, thereby treating the disease, and second, by preventing its conversion to sexual forms, thus halting its transmission to mosquitoes. This could be a powerful new strategy in our ongoing battle against malaria.
This article is based on a Press Release from the National Institute of Immunology, New Delhi.
This article was written with the help of generative AI and edited by an editor at Research Matters.