Deciphering the pathogen-host cell interactions during the liver stage of Plasmodium parasites
Many microorganisms have managed to escape from host immune responses by hiding inside cells. However, cells have evolved various intracellular, immune response-like mechanisms to eliminate pathogens. We spoke to Dr. Volker Heussler, who researches the development of the malaria parasite inside the hepatocytes, and the factors that influence its survival or elimination.
Malaria is a dangerous, potentially fatal disease, caused by the parasites of the genus Plasmodium. Malaria was responsible for 600,000 deaths in 2022. Every year, there are over 200 million cases of malaria worldwide. This devastating disease is transmitted exclusively via a bite by a female Anopheles mosquito.
The Plasmodium species undergoes many life cycle stages in the mammalian host and in the mosquito. Sporozoites are the highly-motile crescent-shaped form of the parasite that the mosquito injects into the skin of the mammalian host during a blood meal. The sporozoites glide in the skin until they find a blood vessel, which they enter and then get passively transported with the bloodstream to the liver. Inside liver cells also called hepatocytes, the sporozoites transform and start dividing their nuclei finally resulting in thousands of merozoites. The merozoites are released from the liver cell and then repeatedly infect and destroy red blood cells. The destruction of red blood cells is one of the causes of the symptoms of malaria. Some merozoites develop into gametocytes, which can be ingested by a mosquito. Gametocytes develop into sporozoites within the mosquito, and then the cycle starts again.
The liver stage of development is a crucial part of the Plasmodium life cycle and revealing the complexities behind its outcome is the core of Dr. Heussler’s project. Dr. Heussler’s past research has discovered that a large percentage of the sporozoites that invade hepatocytes, the main parenchymal cells of the liver, fail to form infectious merozoites. “There appears to be a delicate balance between parasite survival and
elimination and we now start to understand why this is so. It all depends on the fitness of the parasite and of course, on the state of the host cell,” explains Dr. Heussler. “When the parasites are in the liver, they transmigrate through a number of hepatocytes before they finally settle in one. This happens in a very special waythe parasite invaginates the host cell plasma membrane and forms a parasitophorous vacuole which is surrounded by a host cell membrane. So, the parasite lives in a special compartment in the hepatocyte which separates it from the host cell, but still is completely dependent on the host cell for nutrients. Therefore, the parasite must ensure that the host cell survives and does not undergo self-destruction (apoptosis).” continues Dr. Heussler.
Newly discovered intracellular immune response: Autophagy
His team has found that there is a type of intracellular immune response that is responsible for the selective elimination of
on, but just the selective autophagy. The damaged mitochondria are then surrounded by a membrane and this finally leads to elimination.” explains Dr. Heussler. Even more importantly, selective autophagy is used to eliminate intracellular pathogens. Selective autophagy is provoked while the parasite resides inside the parasitophorous vacuole that is surrounded by a membrane, the PVM for Parasitophorous Vacuole Membrane. “Autophagy has a lot to do with membranes. The PVM is completely covered with autophagy molecules and this results in the recruitment of lysosomes, the digestive organelles of a cell. We have shown that this is deleterious for about 50% of invading parasites. However, the interesting part is that the surviving parasites are also covered with these autophagy molecules but they still survive. The reason for that is that they can control the number of these autophagy molecules on the PVM by inducing a very high turnover of the PVM. They shed the PVM all the time and with it, they shed these autophagy molecules. The interesting part comes when we knocked out the host cell molecules which are responsible for the autophagy reaction, and we found that the number of surviving parasites is reduced. We were completely puzzled by this since we thought that stopping the autophagy reaction would provide an advantage for the parasite. And that is obviously not the case. This means that the parasite partially depends on this reaction” Dr. Heussler provides us with a detailed explanation.
researchers found in the first part of the project, with the help of super-resolution microscopy and many other cell biological techniques.
High throughput knockout screen of Plasmodium berghei genes
The second part of the project is focused entirely on the parasite side. The researchers have completed a high throughput knockout screen of parasite genes in collaboration with colleagues at the Sanger Institute in Kingston in the UK. The research group in Bern has established the entire life cycle of Plasmodium berghei, a model Plasmodium species that is infective for rodents, but not for humans.
“We introduced many bar-coded knockout constructs at the same time into blood stage parasites and then extracted the DNA at all the different lifecycle stages and sent them for sequencing to the Sanger institute. Sequencing of the barcodes told us exactly where these knockout parasites were eliminated at any given life cycle stage. A very simple but powerful system to follow these parasites. In this way we knocked out more than 1300 parasite genes and identified about 180 genes that are essential during the liver stage, which is the most interesting stage for us” says Dr. Heussler. The potential implications of this knockout screen could be the generation of genetically attenuated parasites, which could then be used as live vaccine strains. It also revealed essential metabolic pathways that can be the target for new anti-malarial drugs.
PAtHogeN - Host cell iNter ActioNs
Pathogen-host cell interactions during the liver stage of Plasmodium parasites
Project objectives
The project aims to decipher the mechanisms underlying the liver stage in the life cycle of the Plasmodium parasite, the causal agent of malaria. Dr. Heussler and his team are interested in the factors that influence the potential survival or elimination of the parasite during its development in the hepatocytes, focusing on autophagy, a newly discovered intracellular immune response.
Project Funding
This project is funded by the Swiss National Science Foundation (SNSF), Grant number 182465. https://data.snf.ch/grants/grant/182465
Project Partners
• Prof Oliver Billker, Umea University, Sweden, formerly at the Wellcome Trust Sanger Institute, UK
• Prof. Dominique Soldati, University of Geneva
• Prof. Chris Janse, Leiden University Medical Center, The Netherlands
• Prof. Vassily Hatzimanitakis, EPFL, Lausanne contact Details
Project Coordinator, Prof. Volker Heussler Director
Institute of Cell Biology
University of Bern Baltzerstr. 4
3012 Bern
t: +41 31 68 44650 (office) e: volker.heussler@unibe.ch W: https://www.izb.unibe.ch/research/ prof_dr_volker_heussler/index_eng. html#pane427241
intracellular pathogens, such as Plasmodium This mechanism is called autophagy. Autophagy is a cellular process for the natural elimination and degradation of unnecessary or dysfunctional cell components. It was originally considered to be a primordial degradation pathway that protects cells against starvation. Starved cells can digest part of their cytoplasm to provide enough nutrients for the most essential metabolic pathways of the cell so it can survive. However, it has been shown that autophagy has many other roles, and one of them includes the elimination of intracellular parasites, viruses, and bacteria.
“We call this elimination of intracellular parasites selective autophagy because it’s not approaching the host cell generally but very selectively the pathogen invader.
Selective autophagy was originally described for the elimination of damaged organelles.
For example, when a cell contains damaged mitochondria then it is not the whole canonical autophagy that is switched
There appears to be a delicate balance between parasite survival and elimination and we now start to understand why this is so. It all depends on the fitness of the parasite and of course, on the state of the host cell .
The researchers went further and managed to find the reason behind this paradoxical response. “By moderate recruitment of autophagy molecules to the PVM, the parasite attracts and activates host cell signaling molecules. This in turn activates a transcription factor that regulates the expression of survival factors. In the end, it makes sense that the parasite needs to somehow influence the survival pathways of the host cell because if the host cell cannot eliminate the invader, it could simply commit suicide, which we call apoptosis. To avoid host cell apoptosis, the parasite induces the survival pathways of the host by controlling its autophagy machinery” this is what the
From a more basic research point of view, the researchers hope that with this genome-wide knockout approach, they can identify the genes that are essential for parasite survival during the liver stage of development. “Our main interest is to understand what the function of these genes is. Once we have knocked out a gene, we then really look very carefully for the phenotype of the corresponding parasites.
Ideally, we would then finally combine the two big projects - the autophagy project and the parasite project, with the hope that the screen will reveal the parasite molecules that are responsible for attracting autophagy molecules to the PVM, and basically, understand the respective mechanism and the bigger picture behind it” concludes Dr. Heussler.
Dr. Volker Heussler is a Professor in Molecular Parasitology and Cell Biology at the Institute of Cell Biology, University of Bern. He is an acting director of the Institute of Cell Biology. His research is focused on the liver stage of the Plasmodium parasite, malaria’s causal agent.
