Plasmodium Invasive Stages

ookinete (motile)• mosquito gut

epithelial cells

sporozoite (motile)• mosquito salivary

glands• hepatocytes

merozoite (non-motile)• erythrocytes

• Merozoite invasion involves specific interactions with the host erythrocyte.

• The actively growing parasite places metabolic and other demands on the host cell.

• Ultrastructural modifica-tions are evident in the infected erythrocyte.

Steps in Merozoite Invasion

Merozoite reorientation is accompanied by erythrocyte deformation.

Secretory (Apical) OrganellesOrganelle Shape Size (nm)Rhoptry Teardrop 300 x 600Microneme Ellipsoidal 40 x 100

Dense Granule Spherical 120 - 140

•Merozoite proteins:• EBA-175 (sialic binding protein of P. falciparum)• Duffy-binding protein (P. vivax and P. knowlesi)

•TRAP family*:• SSP2 (sporozoite surface protein-2) TRAP

(thrombospondin-related adhesive protein)• Toxoplasma, Eimeria and Cryptosporidium

proteins with homology to SSP2/TRAP• CTRP, circumsporozoite- and TRAP-related

protein (Plasmodium ookinete stage)

Proteins Localized to Micronemes

*Thrombospondin family functions in cell-cell and cell-matrix interactions.

Invasion Receptors/Ligands

SpeciesHost

ReceptorMerozoite

Ligand

P. falciparumglycophorins(sialic acid)

EBA-175

P. vivax,P. knowlesi

Duffy Ag DBP

Electron micrograph from Aikawa et al (1978) J. Cell Biol. 77:72

• microneme secretion• receptor-ligand

interactions• junction formation

Events correlated with entry

• clearance of erythrocyte membrane proteins

• host membrane invagination

• vacuolar membrane formation

• junction becomes an annulus

Are rhoptries involved in PVM formation?

• Initial Binding• merozoite surface proteins (eg, MSP-1)?

• Reorientation? • Microneme Discharge and Junction Formation

• receptor-ligand interactions• Ca2+ signal?

• Rhoptry Discharge and Vacuole Formation• clearing of host membrane proteins

• Parasite Entry• mediated by actin-myosin• MSP-1 proteolysis and shedding of surface coat?

• Closure of PVM and Erythrocyte Membrane

Merozoite invasion: a complex and ordered process

• Merozoite invasion involves specific interactions with the host erythrocyte.

• The actively growing parasite places metabolic and other demands on the host cell.

• Ultrastructural modifica-tions are evident in the infected erythrocyte.

UPTAKE AND PERMEABILITYThe malaria parasite has a high metabolic rate and has a large demand for small molecular metabolites that will serve as precursors for the synthesis of nucleic acids, proteins and lipids.

The erythrocyte has a rather sluggish metabolism and limited transport capabilities, but infected erythrocytes exhibit a substantial increase in permeability to low molecular weight solutes.

Metabolites need to cross the PVM and the parasite plasma membrane. A channel on the PVM has been implicated in the acquistion of nutrients. Others have proposed a direct connection to the host plasma via a 'parasitophorous duct‘. Presumably the parasite plasma membrane has transporters which are typical of other eukaryotes.

REDOX METABOLISM

A bi-product of metabolism and respiration are reactive oxygen intermediates (ROI) such as superoxide, hydroxyl radical and hydrogen peroxide. In particular, the digestion of oxy-hemoglobin results in the production of ROI. These ROI can damage lipids, proteins and nucleic acids and therefore need to be oxidized to oxygen and water.

Parasite enyzmes involved in redox metaboism have been identified. Superoxide dismutase (SOD), catalase, and glutathione peroxidase are involved in the detoxification of ROI. Oxidized glutathione is recycled by glutathione reductase and the reducing equivalents of NADPH are probably generated through the pentose phosphate cycle.

• Merozoite invasion involves specific interactions with the host erythrocyte.

• The actively growing parasite places metabolic and other demands on the host cell.

• Ultrastructural modifica-tions are evident in the infected erythrocyte.

Several Parasite Proteins Are Associated with Knobs

• KAHRP and PfEMP2 are believed to interact with the submembrane cytoskeleton of the host erythrocyte

• reorganization of the membrane skeleton may result in knob formation

• PfEMP1 crosses the erythrocyte membrane and is exposed on the surface• the acidic domain (C-terminus) interacts with the

basic KAHRP and cytoskeletal proteins

Invasion Receptors/Ligands

SpeciesHost

ReceptorMerozoite

Ligand

P. falciparumglycophorins(sialic acid)

EBA-175

P. vivax,P. knowlesi

Duffy Ag DBP

Human malaria: adaptations to the parasite

1) Sickle Cell- single point mutation- abnormal shape of a percentage of RBC

will not allow parasite development

RBC have bumps on surface- stick to capillary walls, loss of potassium, parasites inside die, damaged cells removed

Only benefit is to heterozygous individuals:

double dominant are susceptible

double recessive often die from anemia

Example of strong evolutionary pressure to respond to a parasite

2) G-6-Phosphate dehydrogenase deficiency: results in reduced parasitemias

3) Duffy Blood group: double recessive- completely resistant to P. vivax.

parasite cannot find receptors to enter RBC

Found in 80% of W. African black population

Future of malaria management

New drugs

New insecticides

Greater involvement by governments in vector control and monitoring

Habitat manipulation to reduce mosquito populations

Involve people in their own primary health care

Transgenic mosquitoes- resistant to Plasmodium sp.

Malaria Cases

Number of deaths / year Deaths/day # Jumbo Jets day

3,000,000 8,200 20

2,000,000 5,500 14

HOW MANY HAVE DIED IN THIS 1 HOUR LECTURE?