A dispensable Plasmodium locus for stable transgene expression.

The ribosomal small subunit locus has been used for transgene expression in the rodent malaria parasites, Plasmodium berghei and Plasmodium yoelii, but this strategy utilizes single crossover integration and is thus prone to reversion by plasmid excision. Targeting of the ribosomal subunit locus may also have a negative effect on oocyst development in the mosquito. In P. berghei, the p230 paralog locus has been used for transgene expression. Here, we show that the P. yoelii S1 locus (sporozoite expressed gene 1) (PY05712) is dispensable and can be used for stable transgene expression throughout the parasite life cycle. P. yoelii s1(-) parasites show no defect in blood stage replication, oocyst formation, sporozoite production, or liver stage development when compared to P. yoelii wildtype parasites. Further, we show that a fluorescent transgene can be stably expressed from this site. This demonstrates that the S1 locus can be utilized for stable expression of heterologous genes in rodent malaria parasites.

Distinct malaria parasite sporozoites reveal transcriptional changes that cause differential tissue infection competence in the mosquito vector and mammalian host.

The malaria parasite sporozoite transmission stage develops and differentiates within parasite oocysts on the Anopheles mosquito midgut. Successful inoculation of the parasite into a mammalian host is critically dependent on the sporozoite's ability to first infect the mosquito salivary glands. Remarkable changes in tissue infection competence are observed as the sporozoites transit from the midgut oocysts to the salivary glands. Our microarray analysis shows that compared to oocyst sporozoites, salivary gland sporozoites upregulate expression of at least 124 unique genes. Conversely, oocyst sporozoites show upregulation of at least 47 genes (upregulated in oocyst sporozoites [UOS genes]) before they infect the salivary glands. Targeted gene deletion of UOS3, encoding a putative transmembrane protein with a thrombospondin repeat that localizes to the sporozoite secretory organelles, rendered oocyst sporozoites unable to infect the mosquito salivary glands but maintained the parasites' liver infection competence. This phenotype demonstrates the significance of differential UOS expression. Thus, the UIS-UOS gene classification provides a framework to elucidate the infectivity and transmission success of Plasmodium sporozoites on a whole-genome scale. Genes identified herein might represent targets for vector-based transmission blocking strategies (UOS genes), as well as strategies that prevent mammalian host infection (UIS genes).

Targeted deletion of SAP1 abolishes the expression of infectivity factors necessary for successful malaria parasite liver infection.

Malaria parasite sporozoites prepare for transmission to a mammalian host by upregulation of UIS (Upregulated in Infectious Sporozoites) genes. A number of UIS gene products are essential for the establishment of the intrahepatocytic niche. However, the factors that regulate the expression of genes involved in gain of infectivity for the liver are unknown. Herein, we show that a conserved Plasmodium sporozoite low-complexity asparagine-rich protein, SAP1 (Sporozoite Asparagine-rich Protein 1), has an essential role in malaria parasite liver infection. Targeted deletion of SAP1 in the rodent malaria parasite Plasmodium yoelii generated mutant parasites that traverse and invade hepatocytes normally but cannot initiate liver-stage development in vitro and in vivo. Moreover, immunizations with Pysap1(-) sporozoites confer long-lasting sterile protection against wild-type sporozoite infection. Strikingly, lack of SAP1 abolished expression of essential UIS genes including UIS3, UIS4 and P52 but not the constitutively expressed genes encoding, among others, sporozoite proteins CSP and TRAP. SAP1 localization to the cell interior but not the nucleus of sporozoites suggests its involvement in a post-transcriptional mechanism of gene expression control. These findings demonstrate that SAP1 is essential for liver infection possibly by functioning as a selective regulator controlling the expression of infectivity-associated parasite effector genes.

L-FABP is a critical host factor for successful malaria liver stage development.

The malaria parasite liver stage produces tens of thousands of red cell-infectious forms within its host hepatocyte. It is thought that the vacuole-enclosed parasite completely depends on the host cell for successful development but the molecular parasite-host cell interactions underlying this remarkable growth have remained elusive. Using a yeast two-hybrid screen and a yeast overexpression system we show that UIS3, a parasite protein essential for liver stage development, interacts directly with liver-fatty acid binding protein, L-FABP. Down-regulation of L-FABP expression in hepatocytes severely impairs parasite growth and overexpression of L-FABP promotes growth. This is the first identified direct liver stage-host cell protein interaction, providing a possible explanation for the importance of UIS3 in liver infection.