Malaria is a disease that affects the tropical areas of the world, killing 1-2 million people every year, particularly in Africa. In order to complete its life cycle, Plasmodium has to reproduce asexually in the vertebrate erythrocytes and sexually in an Anopheles vector. The invasion of host cells is an essential process in the life cycle and it requires parasite ligands and host cell receptors. Two invasive stages essential for the completion of the life cycle are the merozoites that invade the red blood cells and the sporozoites that invade the mosquito salivary glands and the vertebrate liver cells.
MAEBL is a homologue of the transmembrane Erythrocyte binding proteins (EBP) ligands, first described in erythrocytic stages. It is abundantly expressed in midgut sporozoites and was found to be essential for the invasion of Anopheles salivary glands in Plasmodium berghei. Alternative splicing generates different MAEBL isoforms and so it is unclear what form is functionally essential.
We examined the function of Plasmodium falciparum MAEBL in the erythrocytic stages by creating knockouts in W2mef and NF54 parasites. We found that MAEBL is not essential in erythrocytic stages but it may have a conditional role in P. falciparum blood-stage growth. Therefore, we investigated the function of P. falciparum MAEBL in the invasion of salivary glands. Using P. falciparum NF54 MAEBL knock outs, we confirmed that MAEBL is essential for the invasion of salivary glands. To identify the MAEBL isoform required for P. falciparum (NF54) sporozoite invasion of salivary glands, we created knockout and allelic replacements each carrying CDS of one of the major MAEBL isoforms. Only the transmembrane form of MAEBL is essential and is the first P. falciparum ligand validated as essential for invasion of Anopheles salivary glands. Understanding what P. falciparum sporozoite ligands are critical for mosquito transmission will help validate targets for vector-based transmission-blocking strategies.