The coming-out of malaria gametocytes

Abstract

The tropical disease malaria, which results in more than one million deaths annually, is caused by protozoan parasites of the genus Plasmodium and transmitted by blood-feeding Anopheline mosquitoes. Parasite transition from the human host to the mosquito vector is mediated by gametocytes, sexual stages that are formed in human erythrocytes, which therefore play a crucial part in the spread of the tropical disease. The uptake by the blood-feeding mosquito triggers important molecular and cellular changes in the gametocytes, thus mediating the rapid adjustment of the parasite from the warm-blooded host to the insect host and subsequently initiating reproduction. The contact with midgut factors triggers gametocyte activation and results in their egress from the enveloping erythrocyte, which then leads to gamete formation and fertilization. This review summarizes recent findings on the role of gametocytes during transmission to the mosquito and particularly focuses on the molecular mechanisms underlying gametocyte activation and emergence from the host erythrocyte during gametogenesis.

Figure 1

Ultrastructural changes in P. falciparum gametocytes during activation. (a) Transmission electron micrograph of a mature, non-activated gametocyte. The erythrocyte is reduced to an electron-light hem. The PVM is located adjacent to the PPM and the PV is therefore not discernable. No osmiophilic bodies are detectable, indicating that a male microgametocyte is pictured. Inset shows the pellicular membrane complex, depicting the SPM (1), the PPM (2) and the PVM (3). (b) A female macrogametocyte two-minute postactivation. The gametocyte is in the process of rounding up, thereby loosing its crescent shape. The osmiophilic bodies become closely associated to the parasite surface. At the poles, the PVM separates from the PPM and the erythrocyte is in the process of degrading (arrowheads). (c) Ultrastructure of an exflagellating microgametocyte. The axoneme of a forming microgamete is visible as longitudinal section, and several other cross sections of axonemes are detectable inside the microgametocyte. The PVM has disappeared and the EM ruptured. (d) A female macrogamete after emergence. This stage is marked by pronounced ER and the highly branched single mitochondrion. The SPM is in the process of disintegrating (arrowheads). A, axoneme; EM, erythrocyte membrane; ER, endoplasmic reticulum; FV, food vacuole; M, mitochondrion; N, nucleus; OB, osmiophilic body; PPM, parasite plasma membrane; PV, parasitophorous vacuole; PVM, PV membrane; SPM, subpellicular membrane. Bar, 1 μm.

Figure 2

Schematic overview of signaling pathways identified to date that are involved in gametocyte activation (modified from [83]). Gametocyte activation is induced by a decrease in temperature and the presence of the mosquito-derived molecule XA. So far, a receptor involved in activation has not been identified. Activation effects PLC and GC, resulting in an increase of IP₃ and cGMP. The latter activates a PKG. IP₃ mediates release of intracellular calcium from the ER, which activates CDPK4 and consequently Map-2 in the male microgametocyte, eventually leading to exflagellation. A possible link between PKG and calcium release has not yet been confirmed. Signaling pathways in the activated macrogametocyte that are downstream of calcium release were not yet identified. Black lines indicate direct interactions and dashed lines indicate indirect interactions. Ca²⁺, calcium ion; CDPK, calcium-dependent protein kinase; cGMP, cyclic guanosine monophosphate; DAG, diacylglycerol; DNA, deoxyribonucleic acid; E, erythrocyte; ER, endoplasmic reticulum; GC, guanylyl cyclase; GTP, guanosine triphosphate; IP₃, inositol triphosphate; Map-2, Mitogen-activated protein kinase 2; PDE, phosphodiesterase; PIP₂, phosphatidylinositol-4,5-bisphosphate; PKG, cGMP-dependent protein kinase; PLC, phospholipase C; PV, parasitophorous vacuole; R, receptor; T, temperature; XA, xanthurenic acid.

Figure 3

Morphological changes of malaria parasites during transmission from the human host to the mosquito vector. The intraerythrocytic gametocyte stages mature in the human host and are taken up by the blood-feeding female mosquito. By entering the mosquito midgut, the gametocytes become activated and round up, before emerging from the enveloping host erythrocyte. Proteases (P) are involved in these processes. During gametogenesis, the female macrogametocyte transforms into a macrogamete, while the activated male microgametocyte forms eight microgametes. Within approximately twenty-minute post-activation, the motile microgamete fertilizes the macrogamete and the resulting zygote transforms within a day into the infective ookinete. Two classes of sexual stage proteins are expressed in association with the parasite surface. A first class of proteins (shown in green) is expressed in the parasitophorous vacuole of the developing gametocyte, where some of them assemble to form adhesive multiprotein complexes. The proteins are later exposed on the surface of the newly emerged gametes, but expression ceases during fertilization. Expression of a second class of surface-associated proteins (shown in pink) is repressed in the gametocyte stage, but repression is released during fertilization (R) and protein expression persists to the ookinete stage.