Parasitology Unit, Max Planck Institute for Infection Biology, Berlin, Germany.
Metabolism of Microbial Pathogens, Robert Koch Institute, Berlin, Germany.
PLoS Pathog. 2020 Sep 21;16(9):e1008891. doi: 10.1371/journal.ppat.1008891. eCollection 2020 Sep.
The transitions between developmental stages are critical points in the Plasmodium life cycle. The development of Plasmodium in the livers of their mammalian hosts bridges malaria transmission and the onset of clinical symptoms elicited by red blood cell infection. The egress of Plasmodium parasites from the liver must be a carefully orchestrated process to ensure a successful switch to the blood stage of infection. Cysteine protease activity is known to be required for liver-stage Plasmodium egress, but the crucial cysteine protease(s) remained unidentified. Here, we characterize a member of the papain-like cysteine protease family, Plasmodium berghei serine repeat antigen 4 (PbSERA4), that is required for efficient initiation of blood-stage infection. Through the generation PbSERA4-specific antisera and the creation of transgenic parasites expressing fluorescently tagged protein, we show that PbSERA4 is expressed and proteolytically processed in the liver and blood stages of infection. Targeted disruption of PbSERA4 results in viable and virulent blood-stage parasites. However, upon transmission from mosquitoes to mice, Pbsera4(-) parasites displayed a reduced capacity to initiate a new round of asexual blood-stage replication. Our results from cultured cells indicate that this defect results from an inability of the PbSERA4-deficient parasites to egress efficiently from infected cells at the culmination of liver-stage development. Protection against infection with wildtype P. berghei could be generated in animals in which Pbsera4(-) parasites failed to establish infection. Our findings confirm that liver-stage merozoite release is an active process and demonstrate that this parasite-encoded cysteine protease contributes to parasite escape from the liver.
发育阶段的转变是疟原虫生命周期中的关键点。疟原虫在其哺乳动物宿主肝脏中的发育将疟疾传播和红细胞感染引起的临床症状的发作联系起来。疟原虫寄生虫从肝脏逸出必须是一个精心协调的过程，以确保成功切换到感染的血液阶段。已知半胱氨酸蛋白酶活性是肝脏阶段疟原虫逸出所必需的，但关键的半胱氨酸蛋白酶仍然未被确定。在这里，我们描述了木瓜蛋白酶样半胱氨酸蛋白酶家族的一个成员，即疟原虫伯氏疟原虫丝氨酸重复抗原 4（PbSERA4），它是有效起始血液阶段感染所必需的。通过产生针对 PbSERA4 的特异性抗血清和创建表达荧光标记蛋白的转基因寄生虫，我们表明 PbSERA4 在感染的肝脏和血液阶段表达并进行蛋白水解加工。靶向破坏 PbSERA4 会导致具有活力和毒力的血液阶段寄生虫。然而，当从蚊子传播到老鼠时，Pbsera4(-)寄生虫显示出降低的能力来启动新的一轮无性血液阶段复制。我们从培养细胞获得的结果表明，这种缺陷是由于 PbSERA4 缺陷寄生虫在肝脏阶段发育结束时不能有效地从受感染的细胞逸出所致。在 Pbsera4(-)寄生虫未能建立感染的动物中，可以针对野生型 P. berghei 感染产生保护。我们的发现证实了肝脏阶段裂殖体释放是一个主动的过程，并表明这种寄生虫编码的半胱氨酸蛋白酶有助于寄生虫从肝脏逃逸。
