P25 and P28 proteins of the malaria ookinete surface have multiple and partially redundant functions

Abstract

The ookinete surface proteins (P25 and P28) are proven antimalarial transmission-blocking vaccine targets, yet their biological functions are unknown. By using single (Sko) and double gene knock-out (Dko) Plasmodium berghei parasites, we show that P25 and P28 share multiple functions during ookinete/oocyst development. In the midgut of mosquitoes, the formation of ookinetes lacking both proteins (Dko parasites) is signi®cantly inhibited due to decreased protection against lethal factors, including protease attack. In addition, Dko ookinetes have a much reduced capacity to traverse the midgut epithelium and to transform into the oocyst stage. P25 and P28 are partially redundant in these functions, since the ef®ciency of ookinete/oocyst development is only mildly compromised in parasites lacking either P25 or P28 (Sko parasites) compared with that of Dko parasites. The fact that Sko parasites are ef®ciently transmitted by the mosquito is a compelling reason for including both target antigens in transmission-blocking vaccines.

High Mobility Group Protein HMGB2 Is a Critical Regulator of Plasmodium Oocyst Development

“…Ookinete formation was significantly decreased in ⌬pyh-mgb2 and comparable with that found for a double knock-out of the P28 and P25 P. berghei genes (27,32). In vivo, we observed a drastic impairment of oocyst formation (Table 2), although our microarray data indicate that PyHMGB2 regulates expression of genes transcribed in gametocytes.…”

“…The Pbs21 (or Pbs28) and Pbs25 genes, which encode surface proteins of ookinetes, have been shown to have redundant function as only a double KO exhibited a clear phenotype (27). The P. berghei strain with double KO of Pbs21 and Pbs25 has modest reduction of ookinete production and a drastic impairment of oocyst production in the mosquito (27). The ookinetes produced in the double KO are not able to cross the midgut epithelium and interact with the basal lamina and consequently do not produce oocysts (27).…”

“…The P. berghei strain with double KO of Pbs21 and Pbs25 has modest reduction of ookinete production and a drastic impairment of oocyst production in the mosquito (27). The ookinetes produced in the double KO are not able to cross the midgut epithelium and interact with the basal lamina and consequently do not produce oocysts (27). Similarly, PfCCp2 (P. falciparum) and its orthologue pblap4 (P. berghei) have been disrupted with a phenotype in the oocyst stage despite protein expression at the gametocyte stage (28,34).…”

“…Parasites Are Conserved and Expressed at the Gametocyte Stage-Among the genes down-regulated in the ⌬pyhmgb2 parasite, we found P. yoelii homologues of the P25 and P28 ookinete genes (PY00522 and PY00523, respectively) and of PfCCp2 (PY01580; also named lap4 in P. berghei) that have critical importance in oocyst development (27,28). Of the 32 genes, all but three were conserved among all Plasmodium species (Table 3 and supplemental Table I).…”

The development of malaria parasites in the mosquito midgut

“…P25 and P28 are targets of highly effective transmission‐blocking antibodies, which interfere with oocyst development. The proteins exhibit multiple and partially redundant functions, for example, they play a role in ookinete survival in the mosquito midgut, traversal of the epithelium and ookinete–oocyst transformation (Tomas et al ., 2001; reviewed in Pradel, 2007). …”