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Biophysics in the Understanding, Diagnosis, and Treatment of Infectious Diseases Speaker Abstracts
29
Actions of Plasmodium Falciparum on Its Human Erythrocyte Host Studied by Electron
Tomography
Victoria Hale
1
, Jean Watermeyer
1
, Roland Fleck
2
, Fiona Hackett
3
, Michael Blackman
3
,
Helen
Saibil
1
.
1
ISMB, Birkbeck College, London, United Kingdom,
2
CUI, Kings College, London, United
Kingdom,
3
Crick Institute, London, United Kingdom.
In the blood stage of malaria infection, parasites invade erythrocytes and multiply inside the
parasitophorous vacuole. When mature, they escape from the host cell for further rounds of
invasion. This escape requires the sequential rupture of vacuolar and erythrocyte plasma
membranes, in a process called egress, which is triggered by a cascade of protease activation.
Selective inhibition of different steps in this cascade blocks parasite egress. The inhibitor
Compound 1 blocks the first stage of egress, leading to accumulation of mature parasites inside
the vacuole, whereas the cysteine protease inhibitor E64 blocks the second step of exit, resulting
in clusters of parasites contained only by the erythrocyte membrane. We used electron and X-ray
tomography, electron energy loss spectroscopy and fluorescence microscopy of late stage
infected cells to study membrane disruption in egress. We find evidence for leakage of cell
contents across the vacuole membrane when egress is blocked by compound 1, indicating
localised disruption to the vacuole membrane prior to its rupture. With E64 treatment the vacuole
membrane is ruptured to form extensive, multi-layered vesicles. The results reveal substages in
egress, starting with permeabilisation of the vacuole membrane immediately preceding its
breakdown into swirl-like fragments, followed by breakdown of the erythrocyte membrane.
We have also used electron tomography to examine the cell surface structures known as knobs, a
major virulence factor mediating cytoadherence in P. falciparum infection. In infected
erythrocyte ghosts, these knob structures are supported by a spiral framework with multiple
connections to the red cell cytoskeleton, suggesting how shear forces could be transmitted from
adhesion sites to the cytoskeleton.