Malaria, caused by Plasmodium parasite species affected more than 250 million people and caused over half a million deaths in 2023 alone. Although young children in the African region are most susceptible to the disease, malaria during pregnancy can result in low birth weight, miscarriage, and stillbirth.1
These complications occur due to the ability of Plasmodium-infected red blood cells (RBCs) to adhere to the placental tissue and accumulate in the placenta.2 Previous research has shown that infected RBCs express a surface antigen, VAR2CSA, which helps the cells adhere to chondroitin sulfate A (CSA) molecules in the placental tissue.
“VAR2CSA is the main candidate to develop a placental malaria vaccine,” said Benoit Gamain, a parasitologist at the National Institute of Health and Medical Research (Inserm). As such, he and other researchers in the field have been trying to better understand how this surface protein binds to CSA. A few years ago, he and his team found that the phosphorylation of a part of VAR2CSA enhanced the adhesion of RBCs to placental molecules.3 “So, the next step was to identify which kinases could phosphorylate VAR2CSA.”
Recently, Gamain and his team showed that an enzyme from both RBCs and parasites help phosphorylate this surface protein.4 The findings, published in PLoS Pathogens, provide new molecular insights into placental malaria and identify potential targets for interventions.
“This is a really lovely piece of science,” said Alister Craig, a now-retired molecular parasitologist from the Liverpool School of Tropical Medicine, who was not involved with the study. “It is very robust, and they’ve done the experiments well.”
Researchers previously showed that casein kinase 2 (CK2) phosphorylates a surface protein on Plasmodium-infected RBCs, which helps the cells stick to other cells by a process called cytoadhesion.5 To determine whether this enzyme played a role in phosphorylating VAR2CSA, Gamain and his team treated Plasmodium-infected RBCs with CK2 inhibitors. Blocking the α subunit, casein kinase 2α (CK2α), reduced the binding of the cells to CSA, indicating that CK2α influences cytoadhesion.
The researchers created a lysate from infected RBCs and, separately, cloned and purified a domain of recombinant VAR2CSA that is known to get phosphorylated. Exposing the recombinant protein to the lysate resulted in phosphorylation, which was diminished by the addition of CK2α inhibitors, indicating that both human and P. falciparum enzymes phosphorylated VAR2CSA.
To determine if the two enzymes phosphorylated VAR2CSA similarly, they explored the phosphorylation of multiple protein domains after treatment with the enzymes separately. While human CK2α phosphorylated only one domain, the parasitic enzyme acted on multiple domains.
An enzyme modifies a protein (VAR2CSA, green) on the surface (red) of parasite-infected red blood cells, causing cell adhesion and accumulation in the placenta. Parasite nuclei are shown in blue.
Jean-Philippe Semblat, Inserm
Next, the team explored the residues phosphorylated by CK2α by mass spectrometry. They observed that, while the parasitic enzyme phosphorylated many sites, the human protein only modified one residue, a serine at position 1,068. Since both the enzymes acted on this residue, the researchers investigated the role of this phosphorylation site in infected RBC cytoadhesion.
They introduced a point mutation in the parasitic VAR2CSA gene that changed the serine at position 1,068 to a different residue and infected RBCs with these genome-edited parasites. Although the mutation did not affect the transcription of var2csa, flow cytometry analyses revealed reduced VAR2CSA levels at the RBC surface.
Gamain explained that this indicated that the phosphorylation of VAR2CSA by CK2α is also involved in the translocation of this protein on the infected RBC surface. He added that this could be an important consideration for drug designs. “The idea would be to [prevent] the translocation of the protein on the cell surface and also the binding of the parasite into the placenta,” Gamain said.
“One of the big ways [this study] advances the field is it's actually identified the residue that seems to be the key one [in cytoadhesion],” said Craig, who specialized in cytoadhesion in his research. Although this advances knowledge in the field, he noted that more work is required to use this knowledge for therapeutic purposes, especially to selectively target the parasitic CK2α without harming the human enzyme. Phosphorylation-specific monoclonal antibodies could be used for therapy, but there are technical challenges to studying this, he added.
Gamain noted that future studies could look at similar enzymes involved in the phosphorylation of other surface proteins implicated in malaria. “If this is true for VAR2CSA, it’s also possible that it is true for other [surface proteins] that are associated with forms of severe malaria, such as cerebral malaria.”
- Umbers AJ, et al. Malaria in pregnancy: Small babies, big problem. Trends Parasitol. 2011;27(4):168-175.
- Fried M, Duffy PE. Adherence of Plasmodium falciparum to chondroitin sulfate A in the human placenta. Science. 1996;272(5267):1502-1504.
- Dorin-Semblat D, et al. Phosphorylation of the VAR2CSA extracellular region is associated with enhanced adhesive properties to the placental receptor CSA. PLoS Biol. 2019;17(6):e3000308.
- Dorin-Semblat D, et al. Casein Kinases 2-dependent phosphorylation of the placental ligand VAR2CSA regulates Plasmodium falciparum-infected erythrocytes cytoadhesion. PLoS Pathog. 2025;21(1):e1012861.
- Hora R, et al. Erythrocytic casein kinase II regulates cytoadherence of Plasmodium falciparum-infected red blood cells. J Biol Chem. 2009;284(10):6260-6269.
