A molecular switch controlling antigenic variation in malaria parasites

Malaria is a major cause of morbidity and mortality across the developing world. The major causative agent of severe malaria is the protozoan parasite Plasmodium falciparum. Key to its virulence is its effective use of antigenic variation in subverting the host immune system. P. falciparum expresses a transmembrane protein that is trafficked out of the parasite and into the membrane of the erythrocyte in which it resides. This protein, PfEMP1, is essential to the parasite as it mediates binding of the infected erythrocyte to the host vasculature, thereby preventing clearance by the spleen. At the same time, PfEMP1 is immunogenic and the host will mount an adaptive immune response specifically targeting this protein. The parasite evades this response through the expression of immunologically distinct isoforms of PfEMP1, each encoded by different members of the var multicopy gene family. There are approximately 40-90 var genes encoded in the genome of any given parasite isolate and they are expressed in a mutually exclusive fashion. While the epigenetic mechanisms controlling the activation and silencing of var genes have been identified, little is known regarding the regulation of transcriptional switching between genes. Due to the limited number of var genes available within the parasite genome, purely stochastic switching between genes would likely result in premature expenditure of the antigenic repertoire. Mathematical algorithms have provided models proposing an optimized switching network that enables the parasite to efficiently utilize its full complement of var genes over the course of an infection. These models have proposed a role for a “sink-node” within the network that coordinates transcriptional switching. There is an increasing body of evidence that a highly conserved, unique var gene, var2csa, is the hypothesized “sink-node”. var2csa encodes the PfEMP1 responsible for pregnancy associated malaria. Surprisingly, high levels of var2csa transcripts are found in non-pregnant individuals, suggesting an additional role for this gene. When the var2csa locus is deleted in cultured P. falciparum, the resulting parasites are unable to undergo a var gene transcriptional switch, indicating the potential for var2csa to serve as the “sink-node.”  var2csa is unique in that it is the only var gene with a highly conserved upstream open reading frame (uORF). The presence of a uORF is known to influence transcript stability, reduce downstream translation and can induce gene silencing. Specific Aim 1 is designed to investigate the role of the uORF in modulating var2csa mRNA stability and transcriptional activation through the nonsense mediated decay (NMD) pathway. Specific Aim 2 will examine the role of initiation factor conformation in regulating translation of the var2csa transcript. The phosphorylation state of eIF2α can respond to altered nutrient levels, resulting in changes in translational initiation. Together, this proposal will determine the role of the uORF in regulating var2csa’s proposed dual function.

Project details

Basic Science

Mar 2022 — Mar 2025

$94,446

United States