Cell Biology of Plasmodium

Cell Biology of Plasmodium
Mark F. Wiser
http://www.tulane.edu/~wiser/malaria/
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.
Plasmodium Invasive Stages
ookinete (motile)
mosquito gut epithelial cells

sporozoite (motile)
mosquito salivary glands
hepatocytes

merozoite (non-motile)
erythrocytes
Bannister et al (2003) J Cell Sci 116, 3825
Steps in Merozoite Invasion
accompanied by erythrocyte deformation
AMA-1 implicated*
apical membrane antigen-1
binds erythrocytes
antibodies inhibit invasion and reorientation
antibodies do not inhibit initial attachment
Reorientation
*Mitchell et al (2004) Inf. Imm. 72, 154.
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 characterized by von Willebrand factor type A domain. Functions in cell-cell and cell-matrix interactions.
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
parasitophorous vacuolar membrane (PVM) formation
junction becomes an annulus (ring)
Rhoptries are likely involved in PVM formation
Micrograph from Bannister et al (1986), Parasitology 92:291
entry requires force
cytochalasin inhibits invasion, but not attachment/junction
cytochalasin resistance maps to actin gene in Toxoplasma
unique Apicomplexan membrane associated myosin
Parasite Entry
force can be generated by actin/myosin
TRAP necessary for invasion and gliding motility
Soldati et al (2004) Current Opinion in Cell Biology 16,32-40
Initial Binding
merozoite surface proteins (eg. MSP-1)
Reorientation (AMA-1)
Microneme Discharge and Junction Formation
receptor-ligand interactions (adhesive proteins)
Ca2+ signal?
Rhoptry Discharge and Vacuole Formation
clearing of host membrane proteins
Parasite Entry
mediated by actin-myosin
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.
Permeability to metabolites is increased in the infected erythrocyte.
P. falciparum expresses ‘knobs’ on the surface of infected erythrocytes. Knobs mediate cytoadherence to endothelial cells.
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
KAHRP = knob associated histidine rich protein
EMP = erythrocyte membrane protein
family of ~60 var genes
conserved intracellular C-terminus
acidic terminal segment (ATS)
binds cytoskeleton + KAHRP
transmembrane domain
variable extracellular domain composed of modules
2-7 copies of Duffy-binding like domains
5 sequence types (a, b, g, d, e)
1-2 cys-rich interdomain regions
all have DBL1a + CIDR
participates in cytoadherence
PfEMP-1 Structure
CD36
Ig super-family
ICAM-1
VCAM-1
PECAM-1
E-selectin
thrombospondin
chondroitin sulfate A
hyaluronic acid

Rosetting Receptors
CR-1
glycosaminoglycan
blood group A
Possible Host Receptors
Roberts et al (1992) Nature 357:689
agglutinating anti-sera used to define antigenic types
antigenic variants obtained from a cloned parasite line
A high rate of antigenic variation is observed on the erythrocyte surface
clones also exhibited different ICAM1/CD36 binding phenotypes
A switch rate of 2% per generation in absence of immune pressure.
Antigenic switching is accompanied by changes in binding phenotype
Binding Phenotypes Can Be Selected In Vivo
Chondroitin sulfate A (CSA) is a complex carbohydrate found on the surface of endothelial cells in the placenta.
Differential expression of var genes in organs
Montgomery et al (2007) Mol. Microbiol. 65, 959-967
Var Gene Expression
one var gene is expressed at a time (allelic exclusion)
specific expression site in nucleus
repressor proteins bind promoters of non-expressed variants
switching mechanism?
Borst and Genest (2006) Nature 439, 926
SUMMARY
parasite modifies host via exported proteins
permeability changes
knobs + PfEMP-1
PfEMP-1 participates in cytoadherence
immune evasion accomplished through antigenic switching (var gene family)
other variant surface antigens?
rif and stevor in P. falciparum
in other species  cytoadherence