Single-Cell Resolution Mapping of Malaria Parasites Sheds Light on Disease Transmission

Scientists at Stockholm University and collaborators say they have used high-resolution genomic tools to map the global repertoire of genes of gametocyte development toward the male or the female sexual fates in the malaria parasite. In order to transmit from the human host to the mosquito vector, Plasmodium falciparum has to differentiate to the gametocyte stage.

Unlike primary sex determination in mammals, which occurs at the chromosome level, it is not known what causes this unicellular parasite to form males and females.

The study, “Single-cell transcriptomics reveal transcriptional programs underlying male and female cell fate during Plasmodium falciparum gametocytogenesis,” published in Nature Communications, uncovers the genes that are expressed in P. falciparum, the deadliest among the malaria parasites, from the very onset of sexual stage development until they reach maturity. At this point, the male and female gametocytes are ready to be taken up by the female Anopheles mosquito to initiate the transmission cycle.

“We have combined state-of-the-art single-cell genomics with a novel computational approach to define the expression of several important genetic regulators along the developmental trajectory of male and female gametocytes,” explained Johan Ankarklev, PhD, associate professor in the department of molecular biosciences, the Wenner Gren Institute, Stockholm University, and senior author of the study.

The research is important for improving understanding of the genetics underlying malaria transmission. A widely conserved family of transcription factors called the ApiAP2, has emerged as key regulators of gene expression during Plasmodium lifecycle-stage differentiation and development.

“Our high-resolution data enabled us to computationally link the expression of several of these ApiAP2 genes with either the male or the female lineage, implicating their involvement in sexual cell fate determination,” continued Ankarklev. “Importantly, we also established a large set of novel candidate ‘driver’ genes of the male and the female cell fates, which we are currently further exploring in the lab using CRISPR technology.”

Treatment strategies have historically targeted the highly symptomatic, asexual blood stage of infection, with variable degrees of success. Current treatment strategies do not inhibit malaria transmission. This study provides new and important genetic markers for the future development of transmission-blocking therapies, which is the only way to inhibit the spread of malaria, according to the researchers.

From an evolutionary perspective, considering that Plasmodium is an ancient microbial eukaryote that produces males and females, the new data and analyses contribute novel information and clues regarding the evolution of sex in eukaryotes.

Enabling progress in malaria research

Single-cell transcriptome profiling allows a snapshot of a large array of genes expressed in one cell, in this case, one malaria parasite, at a given point of development. When adding thousands of single-cell transcriptomes to the analysis it becomes a powerful tool for determining genetic pathways and developmental bifurcations, which is essential for lineage tracing.

“By combining Pseudotime and RNA Velocity, two recently developed computational tools, we aligned the several thousands of cells along a branched pseudo-time axis, second, we used RNA velocity estimates to define the splicing kinetics among transcripts across the developmental axes,” said Mubasher Mohammed, PhD, a former doctoral student at the Ankarklev lab and lead author of the study. “This then allowed us to predict a large panel of putative “driver genes” for the male and the female sexual fates, and interestingly, a large number of these genes have previously not been annotated.

The malaria transmission stage marks a dramatic decrease in the parasite population numbers making it an attractive target for antimalarial control efforts.

“When such a bottleneck occurs in the population, it becomes more vulnerable to drugs and environmental factors,” pointed out Alexis Dziedziech, PhD, a previous postdoc at the Ankarklev lab and co-author on the study. “By delineating the molecular mechanisms of gametocyte development, we can target these pathways to develop effective transmission-blocking strategies, vital for malaria eradication efforts.”