UIS2: A Unique Phosphatase Required for the Development of Plasmodium Liver Stages

Abstract

Plasmodium salivary sporozoites are the infectious form of the malaria parasite and are dormant inside salivary glands of Anopheles mosquitoes. During dormancy, protein translation is inhibited by the kinase UIS1 that phosphorylates serine 59 in the eukaryotic initiation factor 2α (eIF2α). De-phosphorylation of eIF2α-P is required for the transformation of sporozoites into the liver stage. In mammalian cells, the de-phosphorylation of eIF2α-P is mediated by the protein phosphatase 1 (PP1). Using a series of genetically knockout parasites we showed that in malaria sporozoites, contrary to mammalian cells, the eIF2α-P phosphatase is a member of the PP2C/PPM phosphatase family termed UIS2. We found that eIF2α was highly phosphorylated in uis2 conditional knockout sporozoites. These mutant sporozoites maintained the crescent shape after delivery into mammalian host and lost their infectivity. Both uis1 and uis2 were highly transcribed in the salivary gland sporozoites but uis2 expression was inhibited by the Pumilio protein Puf2. The repression of uis2 expression was alleviated when sporozoites developed into liver stage. While most eukaryotic phosphatases interact transiently with their substrates, UIS2 stably bound to phosphorylated eIF2α, raising the possibility that high-throughput searches may identify chemicals that disrupt this interaction and prevent malaria infection.

Inhibiting the Plasmodium eIF2α Kinase PK4 Prevents Artemisinin-Induced Latency

“…This portion of BIP is highly conserved among the Plasmodium species (Roobsoong et al, 2014). The anti-total eIF2α and anti-eIF2α-P sera were generated in the laboratory of William J. Sullivan Jr., and used in Plasmodium as previously described (Narasimhan et al, 2008a; Zhang et al, 2010; Zhang et al, 2016). …”

Translational Control in the Latency of Apicomplexan Parasites

“…and Toxoplasma and it has been shown to be activated in response to a variety of stresses including those that induce differentiation [31,35–39]. Furthermore, eIF2 phosphorylation is maintained in latent stages and those parasites that cannot maintain this mark display reduced viability to stressful insults and have an impaired ability to maintain quiescence [9,37,38,40,41], underscoring its importance as a driving force in managing apicomplexan latency. In metazoans, phosphorylation of eIF2 is mediated by a family of four different serine/threonine kinases, each activated by different cellular stresses (Box 2).…”

“…, eIF2 is phosphorylated in response to stressful insults and has been shown to accompany all latent stages thus far examined, which include extracellular tachyzoites, bradyzoites, rings, schizonts, gametocytes, and salivary gland sporozoites [9,31,36–41,47,48,50]. Furthermore, while replacement of endogenous eIF2 with a non-phosphorylatable version is lethal in Plasmodium berghei , this substitution results in reduced viability for extracellular Toxoplasma tachyzoites, implicating the ISR plays in the maintenance of latency [9,38].…”

“…For example, treatment of Toxoplasma with the small molecular inhibitors of eIF2 dephosphorylation, salubrinal or guanabenz, leads to an accumulation of phosphorylated eIF2 in the parasite, an induction of bradyzoite gene expression, and the conversion of tachyzoites into bradyzoites [37,51,52]. In keeping with this theme, salivary gland sporozoites convert prematurely to liver stage parasites when the eIF2 kinase eIK2 (UIS1) is knocked out and conversely, a knockout of the putative eIF2 phosphatase UIS2 impairs the ability of latent sporozoites to convert into liver stage parasites [9,41]. Taken together, these results illustrate the central role the ISR plays in parasite life cycle progression.…”

“…Typically, these transcripts encode proteins that are needed for a subsequent developmental stage such as metabolic enzymes, gene regulatory machinery, and cell signaling effectors. A few of the many examples include lactate dehydrogenase 1, which is repressed in bradyzoites, the ookinete-specific AP2-O transcription factor in specific Plasmodium gametocytes, and the UIS2 phosphatase that is transcriptionally upregulated in salivary gland sporozoite but not expressed at the protein level until the liver stages [41,63,79]. Most interestingly, at least some of the repressed transcripts can be sequestered in mRNA-containing granules in latent plasmodial stages [9,82,88].…”