Ultrastructural expansion microscopy (U-ExM) visualization of malaria parasite dense granules using RESA as a representative marker protein.

Dense granules (DG) are understudied apical organelles in merozoites, the malaria parasite stage that invades erythrocytes. Only six proteins have been identified which localize to DGs, despite that DG proteins play crucial roles in multiple steps of intraerythrocytic parasite development. To develop a tool for investigating DG structure and function, this study applied ultrastructural expansion microscopy (U-ExM) to visualize the ring-infected erythrocyte surface antigen (RESA) in Plasmodium falciparum merozoites. Merozoites were expanded to approximately four times their original size, allowing the identification of DGs without the need for electron microscopy. RESA localization in merozoite DGs was confirmed by staining with a combination of anti-RESA mAb and protein staining by NHS-ester. The translocation of RESA to the infected erythrocyte membrane was also observed in early ring-stage parasites. These results are in good agreement with the RESA localization reported using immunoelectron microscopy (IEM). By using U-ExM, the identification of novel DG proteins will be facilitated without time-consuming IEM, thereby enhancing our understanding of erythrocyte parasitism by P. falciparum.