A novel fungal metabolite inhibits Plasmodium falciparum transmission and infection

Leucinostatins target Plasmodium mitochondria to block malaria transmission

BACKGROUND

Malaria remains a critical disease. Leucinostatins from the fungus Purpureocillium lilacinum inhibited the transmission of Plasmodium falciparum to mosquitoes via contact.

METHODS

Here, we modified the leucinostatin B (LB) C-terminus to make derivatives and examined their inhibition against malaria transmission to mosquitoes. Fluorescence-labeled leucinostatins were incubated with intact gametocytes and were examined under microscopy to detect the targets of leucinostatins. We also analyzed leucinostatins' general cytotoxicity and hemolysis.

RESULTS

The results showed that the derivatives with -H, -CH3, -Atto495, and -Biotin at C-terminus had EC50 of 1.5 nM, 0.2 nM, 4.2 nM, and 42 nM, respectively. Atto495 and biotin are similar in size and much bigger than -CH3 and -H. Based on reverse-phase HPLC elution time, we found that LB-Biotin had much higher hydrophobicity than the others, consistent with its lowest malaria transmission-blocking activity. Fluorescence microscopy showed that LB-Atto495 colocalized with mitochondria inside intact P. falciparum gametocytes. We found that leucinostatin A significantly inhibited the proliferation of human nucleated cells with IC50 around 47 nM and it did not lyse erythrocytes at 100 μM.

CONCLUSIONS

We conclude that the leucinostatins pass through the cytoplasmic membrane without lysing cells and interact with molecules specifically in mitochondria. Therefore, leucinostatins should be ideal inhibitors against mobile parasites, such as ookinetes and sporozoites, during malaria transmission.

Leucinostatins from fungal extracts block malaria transmission to mosquitoes

BACKGROUND

Malaria is a mosquito-transmitted disease that kills more than half a million people annually. The lack of effective malaria vaccines and recently increasing malaria cases urge innovative approaches to prevent malaria. Previously, we reported that the extract from the soil-dwelling fungus Purpureocillium lilacinum, a common fungus from the soil, reduced Plasmodium falciparum oocysts in Anopheles gambiae midguts after mosquitoes contacted the treated surface before feeding.

METHODS

We used liquid chromatography to fraction fungal crude extract and tract the active fraction using a contact-wise approach and standard membrane feeding assays. The purified small molecules were analyzed using precise mass spectrometry and tandem mass spectrometry.

RESULTS

We isolated four active small molecules from P. lilacinum and determined them as leucinostatin A, B, A2, and B2. Pre-exposure of mosquitoes via contact with very low-concentration leucinostatin A significantly reduced the number of oocysts. The half-maximal response or inhibition concentration (EC50) via pre-exposure was 0.7 mg/m2, similar to atovaquone but lower than other known antimalarials. The inhibitory effect of leucinostatin A against P. falciparum during intraerythrocytic development, gametogenesis, sporogonic development, and ookinete formation, with the exception of oocyst development, suggests that leucinostatins play a part during parasite invasion of new cells.

CONCLUSIONS

Leucinostatins, secondary metabolites from P. lilacinum disrupt malaria development, particular transmission to mosquitoes by contact. The contact-wise malaria control as a nonconventional approach is highly needed in malaria-endemic areas.

Plasmodium parasitophorous vacuole membrane protein Pfs16 promotes malaria transmission by silencing mosquito immunity

With rising cases for the first time in years, malaria remains a significant public health burden. The sexual stage of the malaria parasite infects mosquitoes to transmit malaria from host to host. Hence, an infected mosquito plays an essential role in malaria transmission. Plasmodium falciparum is the most dominant and dangerous malaria pathogen. Previous studies identified a sexual stage-specific protein 16 (Pfs16) localized to the parasitophorous vacuole membrane (PVM). Here we elucidate the function of Pfs16 during malaria transmission. Our structural analysis identified Pfs16 as an alpha-helical integral membrane protein with one transmembrane domain connecting to two regions across PVM. ELISA assays showed that insect cell-expressed recombinant Pfs16 (rPfs16) interacted with An. gambiae midguts, and microscopy found that rPfs16 bound to midgut epithelial cells. Transmission-blocking assays demonstrated that polyclonal antibodies against Pfs16 significantly reduced the number of oocysts in mosquito midguts. However, on the contrary, feeding rPfs16 increased the number of oocysts. Further analysis revealed that Pfs16 reduced the activity of mosquito midgut caspase 3/7, a key enzyme in the mosquito Jun-N-terminal kinase (JNK) immune pathway. We conclude that Pfs16 facilitates parasites to invade mosquito midguts by actively silencing the mosquito's innate immunity through its interaction with the midgut epithelial cells. Therefore, Pfs16 is a potential target to control malaria transmission.

Arsinothricin Inhibits Plasmodium falciparum Proliferation in Blood and Blocks Parasite Transmission to Mosquitoes

Malaria, caused by Plasmodium protozoal parasites, remains a leading cause of morbidity and mortality. The Plasmodium parasite has a complex life cycle, with asexual and sexual forms in humans and Anopheles mosquitoes. Most antimalarials target only the symptomatic asexual blood stage. However, to ensure malaria eradication, new drugs with efficacy at multiple stages of the life cycle are necessary. We previously demonstrated that arsinothricin (AST), a newly discovered organoarsenical natural product, is a potent broad-spectrum antibiotic that inhibits the growth of various prokaryotic pathogens. Here, we report that AST is an effective multi-stage antimalarial. AST is a nonproteinogenic amino acid analog of glutamate that inhibits prokaryotic glutamine synthetase (GS). Phylogenetic analysis shows that Plasmodium GS, which is expressed throughout all stages of the parasite life cycle, is more closely related to prokaryotic GS than eukaryotic GS. AST potently inhibits Plasmodium GS, while it is less effective on human GS. Notably, AST effectively inhibits both Plasmodium erythrocytic proliferation and parasite transmission to mosquitoes. In contrast, AST is relatively nontoxic to a number of human cell lines, suggesting that AST is selective against malaria pathogens, with little negative effect on the human host. We propose that AST is a promising lead compound for developing a new class of multi-stage antimalarials.

Contemporary exploitation of natural products for arthropod-borne pathogen transmission-blocking interventions

An integrated approach to innovatively counter the transmission of various arthropod-borne diseases to humans would benefit from strategies that sustainably limit onward passage of infective life cycle stages of pathogens and parasites to the insect vectors and vice versa. Aiming to accelerate the impetus towards a disease-free world amid the challenges posed by climate change, discovery, mindful exploitation and integration of active natural products in design of pathogen transmission-blocking interventions is of high priority. Herein, we provide a review of natural compounds endowed with blockade potential against transmissible forms of human pathogens reported in the last 2 decades from 2000 to 2021. Finally, we propose various translational strategies that can exploit these pathogen transmission-blocking natural products into design of novel and sustainable disease control interventions. In summary, tapping these compounds will potentially aid in integrated combat mission to reduce disease transmission trends.

Crystal Structure Determination and Hirshfeld Analysis of a New Alternariol Packing Polymorph

A new polymorph of the mycotoxin alternariol is reported and characterized by single crystal X-ray diffraction. Structural data, Hirshfeld surface analysis, and 2D fingerprint plots are used to compare differences in the intermolecular interactions of the orthorhombic Pca21 Form I (previously reported) and the monoclinic P21/c Form II (herein reported). The polymorphs have small differences in planarity-7.55° and 2.19° between the terminal rings for Form I and Form II, respectively-that brings about significant differences in the crystal packing and O-H … H interactions.

Sterigmatocystin Limits Plasmodium falciparum Proliferation and Transmission

As part of our drug discovery program against malaria, the Penicillium janthinellum extract was discovered to inhibit P. falciparum proliferation in blood and transmission to mosquitoes. Bioactivity-guided fractionation of P. janthinellum extraction was carried out using chromatographic techniques. We determined the activities of fractions against Plasmodium falciparum asexual stage parasite proliferation in culture and sexual stage parasite transmission to mosquitoes using standard membrane feeding assays (SMFA). One active compound was isolated. Based on mass spectrometry and nuclear magnetic resonance profiles, the compound was structurally determined to be sterigmatocystin. Sterigmatocystin inhibited P. falciparum proliferation in the blood with an IC50 of 34 µM and limited the sexual parasites to infect mosquitoes with an IC50 of 48 µM. Meanwhile, sterigmatocystin did not show any acute toxicity to human kidney cells at a concentration of 64 µM or lower. Sterigmatocystin can be used as a drug lead for malaria control and as a probe to understand molecular mechanisms of malaria transmission.

Studies of the Parasite-Midgut Interaction Reveal Plasmodium Proteins Important for Malaria Transmission to Mosquitoes

Malaria transmission relies on parasite-mosquito midgut interaction. The interactive proteins are hypothesized to be ideal targets to block malaria transmission to mosquitoes. We chose 76 genes that contain signal peptide-coding regions and are upregulated and highly abundant at sexual stages. Forty-six of these candidate genes (60%) were cloned and expressed using the baculovirus expression system in insect cells. Six of them, e.g., PF3D7_0303900, PF3D7_0406200 (Pfs16), PF3D7_1204400 (Pfs37), PF3D7_1214800, PF3D7_1239400, and PF3D7_1472800 were discovered to interact with blood-fed mosquito midgut lysate. Previous works showed that among these interactive proteins, knockout the orthologs of Pfs37 or Pfs16 in P. berghei reduced oocysts in mosquitoes. Here we further found that anti-Pfs16 polyclonal antibody significantly inhibited P. falciparum transmission to Anopheles gambiae. Investigating these candidate proteins will improve our understanding of malaria transmission and discover new targets to break malaria transmission.