Abstract
Invasion of hepatocytes by
Plasmodium
sporozoites initiates the pre-erythrocytic step of a malaria infection. Subsequent development of the parasite within hepatocytes and exit from them is essential for starting the disease-causing erythrocytic cycle. Identification of signaling pathways that operate in pre-erythrocytic stages provides insight into a critical step of infection and potential targets for chemoprotection from malaria. We demonstrate that
P
.
berghei
homologs of Calcium Dependent Protein Kinase 1 (CDPK1), CDPK4 and CDPK5 play overlapping but distinct roles in sporozoite invasion and parasite egress from hepatocytes. All three kinases are expressed in sporozoites. All three are required for optimal motility of sporozoites and consequently their invasion of hepatocytes. Increased cGMP can compensate for the functional loss of CDPK1 and CDPK5 during sporozoite invasion but cannot overcome loss of CDPK4. CDPK1 and CDPK5 expression is downregulated after sporozoite invasion. CDPK5 reappears in a subset of late stage liver stages and is present in all merosomes. Chemical inhibition of CDPK4 and depletion of CDPK5 in liver stages implicate these kinases in the formation and/or release of merosomes from mature liver stages. Furthermore, depletion of CDPK5 in merosomes significantly delays initiation of the erythrocytic cycle without affecting infectivity of hepatic merozoites. These data suggest that CDPK5 may be required for the rupture of merosomes. Our work provides evidence that sporozoite invasion requires CDPK1 and CDPK5, and suggests that CDPK5 participates in the release of hepatic merozoites.
The malaria-parasite
Plasmodium
begins its mammalian cycle by infecting hepatocytes in the liver. A single parasite differentiates into tens of thousands of hepatic merozoites which exit the host cell in vesicles called merosomes. Hepatic merozoites initiate the first round of erythrocytic infection that eventually causes disease. We show that optimal invasion of liver cells by
Plasmodium
requires the action of three closely-related parasite kinases, CDPK1, 4 and 5. Loss of any of the three enzymes in the parasite significantly reduces infection of liver cells. Furthermore, CDPK5 is likely required for release of hepatic merozoites from merosomes and therefore for initiation of the erythrocytic cycle. A better understanding of how these kinases function could lead to drugs that prevent malaria.