Malaria Flashcards
What are two key features if the biology of the blood stage that are responsible for severe disease?
- The ability of merozoites to invade RBCs causing high parasitaemia
- Extensive RBC remodelling allowing for the sequestration of infected RBCs to endothelial cells
What are the steps in the merozoite invasion process?
- Initial attachment: merozoite surface protein (MSP1) binds to RBC receptors
- Irreversible attachment and reorientation: proteins released from apical organelles (micronemes and rhoptries) lead to membrane deformation and begins pore formation. Two main invasion ligand families; EBA from micronemes and RH from rhoptries
- Tight junction formation between merozoite and RBC, apical membrane antigen (AMA1, microneme protein) is discharged and bins with RON to make AMA1-RON complex to form the tight junction. The tight junction is the anchoring structure through which the merozoite pushed itself into the emerging parasitphorous vacuole
- Active invasion; actin-myosin motility (rhoptries) and the tight junction moves along the surface of the merzoite until its completely in
What is the purpose of RBC remodelling?
In order to grow and replicate the intra-erythrocytic parasite needs to renovate the host cell in order to acquire nutrients and display cytoadherence factors on the surface of the RBC
What is the process of sequestration?
Parasites avoid spleen-dependent killing and immune response by sequestering away from circulation; it increases parasite survival and causes severe disease in host. The process involves the expression of cytoadherence factors on the RBC membrane that will adhere to endothelial cells in the tissues.
How are P.falciparum proteins exported from the parasite cytosol into the host RBC?
P.falciparum exported proteins carry a special motifs that are essential to protein export into the RBC. There are two major families of exported proteins; PEXEL and PNEPs. PEXEL and PNEPs reach the parasite membrane and parasitophorous vacuole by the conventional secretory pathway. Further export into the RBC is done via a transport channel in the parasitophorous vacuole membrane called the PTEX translocon. PTEX has three main components: EXP2 (the pore), PTEX150, HSP101 (ATPase). PTEX translocated PEXEL and PNEPs as well as soluble and transmembrane proteins into the RBC.
What are gametocytes and where are they found?
Gametocytes are sexual precursor cells formed through asexual parasites during intra-erythrocytic development. They are non-replicative and are essential for transmission to new hosts via the mosquito vector. During the maturation phase in humans, immature gametocytes sequester in extravascular spaces in bone marrow and spleen.
How are gametocytes generated?
Gametocyte generation starts with the trade-off between continuing asexual replication to sustain infection and differentiation of cells for gametocyte production necessary for transmission in the secondary host (i.e. humans). During each of the asexual erythrocytic cycles, a certain number of cells will be committed for gametogenesis. From the sexual ring stage there are 5 stages until maturation. Young asexual (ring stage) parasites are present in the blood but for maturation the immature gametocytes sequester in tissue to avoid immune detection. When they mature the return to the circulation with the aim of being picked up by a mosquito taking a blood meal to continue the transmission cycle, they are non-replicative so if they are not taken up that is a dead end.
What signals cause gamete formation?
Experiments with P.berghei elucidated the importance of AP2-G signal for gametocyte production. AP2-G binds to specific DNA motifs in plasmodium and promotes transcription of gametocyte specific genes. It is the master regulator of sexual-stage commitment development. So AP2-G controls the trade-off between asexual replication (infection) and producing gametocytes for tranmission. But how is the AP2-G expression regulated? AP2-G is under epigenetic control of heterochromatin protein (HP1) which acts as a silencing or activation factor.
Are parasites sensing their environment?
P.falciparum parasites can sense LysoPC in their host’s serum (microenvironment) and is a major building block of parasite phospholipid
metabolism which can influence cell commitment. High levels of LysoPC leads to low rates of gametocytes with the inverse being true too. P.falciparum induce switch to sexual differentiation in response to lysoPC depleteion (ap2-g activation). The parasite metabolism can also alter host LysoPC levels during infection (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5733390/pdf/main.pdf)
Are parasites monitoring the health state of their host?
In a way yes. Their sensitivity to lysoPC is an indirect sensing of host health. LysoPC levels drop when there is a lot of inflammation in the host. Low lysoPV-> gametocyte production. They are “jumping the ship” once inflammation is getting bad.
What is the difference between antigenic diversity and antigenic variation?
antigenic diversity or antigen polymorphisms refers to the different alleles of antigen-encoding genes which emerge through mutations and DNA recombination; different strains of the pathogen carry different alleles and express sequence variants of the surface antigen e.g. P.falciparum invasion factors AMA1, MSP1, MSP2. Antigenic diversity improves fitness of the species. Antigenic variation refers to the antigen being encoded by a gene family or gene paralogs; they encode sequence variants and expression switches in situ through epigenetic and DNA architecture e.g. P.falciparum PfEMP1 which improves the fitness of the pathogen strain and facilitates chronic infection by evading the immune system and has different cytoadherence phenotypes
What are the mechanisms behind antigenic variation?
Antigen variation occurs during blood infection, with var genes (the family which codes for PfEMP1) transcribed in the ring stage parasites. Only a single var gene is expressed at one time in individual parasites.
There are 60 var genes and 4 promoter types that code for var/PfEMP1 subgroups which are associated with different disease outcomes. ( e.g. upsA and upsE linked to severe disease). var gene switching occurs in situ and is independent of DNA recombination, instead it relies on nuclear architecture and epigenetics. The P.falciparum chromosome ends have a highly conserved structural set up where var genes are clustered. The active var gene “moves” to an active site at the nuclear periphery.
HP1, a silencing factor making heterochromatin. It is localised at these end-clusters. For a var gene to become active, it must be present in a euchromatin region. So there must occur a switch from heterochromatin-> euchromatin, i.e. HP1 cannot be active there.
HP1 probs localizes to clusters to silence the other, non active vars?
Juliettes interpretation: This (the moving to the active site in the nucleus) is controlled by epigenetic gene expression with reversible gene silencing and activation regulated by hetero chromatin proteins (HP1). HP1 localises to distinct spots at the nuclear periphery promoting gene switching allowing for antigenic variation.
