Radiation-Induced Cellular and Molecular Alterations in Asexual Intraerythrocytic Plasmodium falciparum

Exposure to ionizing radiation has a profound effect on the viability and further replication of living cells. Radiation is used for the treatment of cancer [1], generation of attenuated vaccines [2], and in food safety [3]. Radiation attenuation has also been one of the earliest methods used to generate whole parasite–based malaria vaccines [4–6]. Given the importance of ionizing radiation in the treatment of different forms of cancer, a large body of scientific data has accumulated on the effects of ionizing radiation on the cell cycle [7]. Such studies have shed light on the molecular and biochemical aspects of radiation-mediated cell death and survival mechanisms [1, 8]. These studies have also provided insight into the cell cycle checkpoints and the complex signaling pathways and associated molecules that ensure that each phase of the cell cycle is completed without errors [7, 9, 10]. Ionizing radiation is detrimental to the growth and survival of Plasmodium parasites. In murine models, asexual blood-stage parasites exposed to high doses of radiation failed to cause blood-stage parasite infection [11], and immunization with radiation-attenuated asexual-stage parasites protected against parasitemia [11–13] and experimental cerebral malaria [11]. In preerythrocytic-stage malaria, exposure of Plasmodium sporozoites to a threshold level of γ-radiation allows their invasion into liver cells and early development. However, these parasites undergo arrested development and fail to produce infectious liver-stage merozoites and, consequently, blood-stage parasite infection [14]. Irradiation-attenuated malaria sporozoites protect mice [4, 15] and humans [5, 16] against malaria in experimental challenge studies. Accordingly, despite the limited success of subunit-based malaria vaccines in clinical testing [17], there has been considerable interest in using radiation exposure to create an attenuated sporozoite-stage vaccine [14, 16]. In this study, we measured the effects of different doses of γ-irradiation on P. falciparum in short-term and long-term cultures. These studies were performed to evaluate both the immediate effect on survival and the ability of rare parasite populations to recover and repopulate. We also studied the effect of γ-irradiation on parasite structure, by fluorescence microscopy and transmission electron microscopy. In addition, we performed extensive genome-wide transcriptional profiling in P. falciparum exposed to increasing doses of γ-radiation, to identify parasite proteins that could be correlated with survival and cell death, and, on the basis of the data set, created a functionally diverse molecular signature of growth attenuation. Last, bioinformatic analyses were performed to decipher the biological systems altered by γ-irradiation that are associated with growth regulation and survival.