A conserved malaria parasite antigen Pb22 plays a critical role in male gametogenesis inPlasmodium berghei

Abstract

Gametogenesis, the formation of gametes from gametocytes, an essential step for malaria parasite transmission, is targeted by transmission‐blocking drugs and vaccines. We identified a conserved protein (PBANKA_0305900) in Plasmodium berghei, which encodes a protein of 22 kDa (thus named Pb22) and is expressed in both asexual stages and gametocytes. Its homologues are present in all Plasmodium species and its closely related, Hepatocystis, but not in other apicomplexans. Pb22 protein was localised in the cytosols of schizonts, as well as male and female gametocytes. During gamete‐to‐ookinete development, Pb22 became localised on the plasma membranes of gametes and ookinetes. Compared to the wild‐type (WT) parasites, P. berghei with pb22 knockout (KO) showed a significant reduction in exflagellation (~89%) of male gametocytes and ookinete number (~97%) during in vitro ookinete culture. Mosquito feeding assays showed that ookinete and oocyst formation of the pb22‐KO line in mosquito midguts was almost completely abolished. These defects were rescued in parasites where pb22 was restored. Cross‐fertilisation experiments with parasite lines defective in either male or female gametes confirmed that the defects in the pb22‐KO line were restricted to the male gametes, whereas female gametes in the pb22‐KO line were fertile at the WT level. Detailed analysis of male gametogenesis showed that 30% of the male gametocytes in the pb22‐KO line failed to assemble the axonemes, whereas ~48.9% of the male gametocytes formed flagella but failed to egress from the host erythrocyte. To explore its transmission‐blocking potential, recombinant Pb22 (rPb22) was expressed and used to immunise mice. in vitro assays showed that the rPb22‐antisera significantly inhibited exflagellation by ~64.8% and ookinete formation by ~93.4%. Mosquitoes after feeding on rPb22‐immunised mice also showed significant decreases in infection prevalence (83.3–93.3%) and oocyst density (93.5–99.6%). Further studies of the Pb22 orthologues in human malaria parasites are warranted.

Transmission-Blocking Strategies Against Malaria Parasites During Their Mosquito Stages

“…Further analysis showed that the defects of the Pb22 knockout (KO)line were limited to the male gametes, and the female gametes in Pb22-KO line were fertile at the wild-type level, indicating that Pb22 was indeed a key protein in the development of male gametes. Of the Pb22-KO line male gametocytes, 30% failed to assemble axonemes, whereas ~ 48.9% formed flagella but failed to egress from the host erythrocytes ( Liu et al, 2021 ). In addition, antibodies against the highly conserved Plasmodium berghei pleckstrin homology gene PbPH that is localized on the surface of gametes, zygotes, and ookinetes significantly inhibited the exflagellation of male gametocytes and the formation of ookinetes in a concentration-dependent manner ( Kou et al, 2016 ).…”

Gametogenesis in Plasmodium: Delving Deeper to Connect the Dots

“…Also, exhaustion of the Metallo-dependent protein phosphatase PPM1 in P. berghei causes impaired exflagellation ( Guttery et al., 2014 ), while reducing the Ca 2+ -dependent phosphatase calcineurin affects male gametogenesis and subsequent fertilization in the rodent malaria parasite ( Philip and Waters, 2015 ). Recently, another protein in P. berghei , Pb 22 is characterized to have an essential function in male gametogenesis by producing defective male gametes, which further affects egress, fertilization and ookinete formation ( Liu et al., 2021 ). Though Pb 22 is expressed in the membrane of both male and female gametes, it has no significant role in female gametogenesis, as proved by the cross-fertilization of male and female gametes of the Pb 22KO line with wild strains ( Liu et al., 2021 ).…”

Identification of Novel Malaria Transmission-Blocking Vaccine Candidates

“…Similarly, a gamete/ookinete surface protein of P. berghei, Pb22, was identified and recombinant Pb22 was expressed in E. coli. The Pb22-immunised mice had a significant TRA (93.5-99.6%) (Liu et al, 2021). 1) are categorized as pre-fertilization antigens (mainly expressed in the sexual stages of parasites before fertilization), and post-fertilization antigens (mainly expressed in the sexual stages of parasites after fertilization).…”

“…Similarly, a gamete/ookinete surface protein of P. berghei , Pb22, was identified and recombinant Pb22 was expressed in E. coli . The Pb22-immunised mice had a significant TRA (93.5-99.6%) ( Liu et al, 2021 ).…”