Before they infect red blood cells and cause malaria, Plasmodium parasites undergo an obligate and clinically silent expansion phase in the liver that is supposedly undetected by the host. Here, we demonstrate the engagement of a type I interferon (IFN) response during Plasmodium replication in the liver. We identified Plasmodium RNA as a novel pathogen-associated molecular pattern (PAMP) capable of activating a type I IFN response via the cytosolic pattern recognition receptor Mda5. This response, initiated by liver-resident cells through the adaptor molecule for cytosolic RNA sensors, Mavs, and the transcription factors Irf3 and Irf7, is propagated by hepatocytes in an interferon-α/β receptor–dependent manner. This signaling pathway is critical for immune cell–mediated host resistance to liver-stage Plasmodium infection, which can be primed with other PAMPs, including hepatitis C virus RNA. Together, our results show that the liver has sensor mechanisms for Plasmodium that mediate a functional antiparasite response driven by type I IFN.

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The emergence of drug resistance is a major limitation of current antimalarials. The discovery of new druggable targets and pathways including those that are critical for multiple life cycle stages of the malaria parasite is a major goal for the development of the next-generation of antimalarial drugs. Using an integrated chemogenomics approach that combined drug-resistance selection, whole genome sequencing and an orthogonal yeast model, we demonstrate that the cytoplasmic prolyl-tRNA synthetase (PfcPRS) of the malaria parasite Plasmodium falciparum is a biochemical and functional target of febrifugine and its synthetic derivatives such as halofuginone. Febrifugine is the active principle of a traditional Chinese herbal remedy for malaria. We show that treatment with febrifugine derivatives activated the amino acid starvation response in both P. falciparum and a transgenic yeast strain expressing PfcPRS. We further demonstrate in the P. berghei mouse model of malaria that halofuginol, a new halofuginone analog that we developed, is highly active against both liver and asexual blood stages of the malaria parasite. Halofuginol, unlike halofuginone and febrifugine, is well tolerated at efficacious doses, and represents a promising lead for the development of dual-stage next generation antimalarials.

In an attempt to identify new genes implicated in the control of programmed cell death during limb development, we have generated a cDNA library from the regressing interdigital tissue of chicken embryos. We have analyzed 804 sequences from this library and identified 23 genes involved in apoptosis in different models. One of the genes that came up in the screening was the Bone Morphogenetic Protein family member, Bmp5, that has not been previously involved in the control of apoptosis during limb development. In agreement with a possible role in the control of cell death, Bmp5 exhibited a regulated pattern of expression in the interdigital tissue. Transcripts of Bmp5 and BMP5 protein were abundant within the cytoplasm of the fragmenting apoptotic interdigital cells in a way suggesting that delivery of BMPs into the tissue is potentiated during apoptosis. Gain-of-function experiments demonstrated that BMP5 has the same effect as other interdigital BMPs inducing apoptosis in the undifferentiated mesoderm and growth in the prechondrogenic mesenchyme. We have characterized both Smad proteins and MAPK p38 as intracellular effectors for the action of BMPs in the developing limb autopod. Activation of Smad signaling involves the receptor-regulated genes Smad1 and -8, and the inhibitory Smad6, and results in both the upregulation of gene transcription and protein phosphorylation with subsequent nuclear translocation. MAPK p38 is also quickly phosphorylated after BMP stimulation in the limb mesoderm. Treatment with the inhibitor of p38, SB203580, revealed that there are interdigital genes induced by BMPs in a p38-dependent manner (DKK, Snail and FGFr3), and genes induced in a p38-independent manner (BAMBI, Msx2 and Smads). Together, our results suggest that Smad and MAPK pathways act synergistically in the BMP pathway controlling limb development.

The causative agent of malaria, Plasmodium, replicates inside a membrane-bound parasitophorous vacuole (PV) that shields this intracellular parasite from the cytosol of the host cell1. One common threat for intracellular pathogens is the homeostatic process of autophagy through which cells capture unwanted intracellular material for lysosomal degradation2. During the liver stage of a malaria infection, Plasmodium parasites are targeted by the autophagy machinery of the host cell and the PV membrane (PVM) becomes decorated with several autophagy markers, including LC3 (microtubule-associated protein 1 light chain 3)3,4. Here we show that Plasmodium berghei parasites infecting hepatic cells rely on the PVM transmembrane protein UIS3 to avoid elimination by host cell-mediated autophagy. We found that UIS3 binds host LC3 through a non-canonical interaction with a specialised surface on LC3 where host proteins with essential functions during autophagy also bind. UIS3 acts as a bona fide autophagy inhibitor by competing with host LC3-interacting proteins for LC3 binding. Our work identifies UIS3, one of the most promising candidates for a genetically-attenuated vaccine against malaria5, as a unique and potent mediator of autophagy evasion in Plasmodium. We propose that the protein-protein interaction between UIS3 and host LC3 represents a target for antimalarial drug development.