Scientists have identified a protein that could represent a target for preventing the progression of diseases caused by apicomplexan parasites such as Toxoplasma gondii, which causes toxoplasmosis, and Plasmodum falciparum, which causes malaria. Reporting in Science, a Boston College team of researchers studied a T. gondii mutant (FP-2) that couldn’t effectively invade host cells.
They identified a mutation in the gene for a protein similar to the double C2 proteins, which they designated DOC2.1. The mutation in this gene was associated with defective secretion by the organism’s micronemes, or organelles, which are unique to Apicomplexans and secrete proteins required for cellular processes including motility, active cell invasion, and migration through cells, biological barriers, and tissues.
Sequencing and structural comparisons indicated that DOC2.1 includes domains similar to those of proteins in other species of Apicomplexa and some ciliates, which are known to bind Ca2+ and function in Ca2+-mediated exocytosis. So to investigate the effects of DOC2.1 deficits in other Apicoplexans, the researchers generated a P. falciparum line that allowed inducible regulation of the orthologous DOC2.1 gene. These DOC2.1-deficient parasites demonstrated an 87% decrease in parasitemia over three life cycles, significantly reduced invasion capacity, and marked reductions in microneme protein secretion, “suggesting a global defect in invasion secondary to incomplete microneme discharge,” the researchers write.
Overall, knockdown of DOC2.1 in P. falciparum had a clear effect on invasion, but at the level of protein deficiency achieved, it didn’t stop the parasite from leaving red blood cells, “suggesting that Plasmodium merozoites may rely on microneme secretion for invasion but on qualitatively different exoneme secretion for egress from the erythrocyte,” the investigators write. In contrast, both invasion and egress were defective in the Toxoplasma F-P2 mutant, indicating that microneme secretion plays a central role in both of these processes and “highlighting divergent roles for secretory organelles in egress between these organisms.”
Calcium-dependent protein kinases (CDPK) have recently been implicated as key factors in microneme secretion and hence the motility of Toxoplasmosa species and P. falciparum. The Boston team claims their latest findings suggest that DOC2.1 constitutes a second level of Ca2+-dependent control of Toxoplasma microneme secretion. “Because DOC2.1 probably facilitates membrane fusion, it probably acts downstreamof the CDPKs.”
The results could also lead to development of new therapeutic approaches to diseases caused by Apicomplexan pathogens. “The mechanism of microneme secretion, which is required for host cell invasion, is a valid drug target," comments lead author Marc-Jan Gubbels, Ph.D., “Since neither microneme secretion nor invasion itself are currently targeted by any antimalaria drugs, a potentially new class of antimalarial reagents can be developed.”
The published work is titled “A DOC2 Protein Identified by Mutational Profiling Is Essential for Apicomplexan Parasite Exocytosis.”