Abstract
Malaria starts with the infection of the liver of the host by
Plasmodium
sporozoites, the parasite form transmitted by infected mosquitoes. Sporozoites migrate through several hepatocytes by breaching their plasma membranes before finally infecting one with the formation of an internalization vacuole. Migration through host cells induces apical regulated exocytosis in sporozoites. Here we show that apical regulated exocytosis is induced by increases in cAMP in sporozoites of rodent (
P. yoelii
and
P. berghei
) and human (
P. falciparum
)
Plasmodium
species. We have generated
P. berghei
parasites deficient in adenylyl cyclase α (
AC
α), a gene containing regions with high homology to adenylyl cyclases.
PbAC
α-deficient sporozoites do not exocytose in response to migration through host cells and present more than 50% impaired hepatocyte infectivity
in vivo
. These effects are specific to
AC
α, as re-introduction of
AC
α in deficient parasites resulted in complete recovery of exocytosis and infection. Our findings indicate that
AC
α and increases in cAMP levels are required for sporozoite apical regulated exocytosis, which is involved in sporozoite infection of hepatocytes.
Malaria is transmitted through the bite of an infected mosquito that deposits
Plasmodium
sporozoites under the skin. These sporozoites migrate from the skin into the circulation and then enter the liver to start a new infection inside hepatocytes. Sporozoites have the capacity to traverse mammalian cells. They breach their membranes and migrate through their cytosol. This process is required for infection of the liver and triggers the exposure of adhesive proteins in the apical end of sporozoites, a process that facilitates invasion of hepatocytes. We found that elevations of cAMP inside sporozoites mediate the exposure of adhesive proteins and therefore the infection process. Mutant sporozoites that do not express adenylyl cyclase, the enzyme that synthesizes cAMP, are not able to expose the adhesive proteins and their infectivity is reduced by half. Reinsertion of adenylyl cyclase gene in the mutant sporozoites recovers their capacity to expose adhesive proteins and to infect hepatocytes, confirming the specific role of this protein in infection. These results demonstrate the importance of cAMP and the exposure of adhesive proteins in sporozoites, but also show that
Plasmodium
sporozoites have other mechanisms to invade host hepatocytes that are not inhibited in the mutant parasites.