trans

CALCIUM 255
towards establishing the function of the novel
parasite proteins.
Ca 2 and cell function
Ca 2 signals are designed to trigger rapid
changes in cell activity. In parasitic protozoa,
a role for Ca 2 is most apparent in organisms
that attach to the host or become internalized
during the invasion process. For E. histolytica,
the invasion process involves a combination
of adhesion, secretion, phagocytosis and
growth. Aspects of each of these steps are
sensitive to Ca 2 . When E. histolytica makes
contact with the extracellular matrix (ECM),
surface proteins related to -integrins are
employed. The integrins associate with
fibronectin in the ECM and when this occurs,
fura-2-loaded E. histolytica exhibit a rise in
Ca 2 . If ionophores are added, the increase
in [Ca 2 ] i is by itself sufficient to reorganize
actin filaments and increase adhesion.
Ameba can also attach to other cells by interactions
involving surface lectins. When
E. histolytica makes contact with CHO cells,
Ca 2 oscillations occur. Ca 2 antagonists block
cytoadherence. Carbohydrates inhibit the
lectin interactions and prevent the elevation
in [Ca 2 ] i . In addition to the change in parasite
[Ca 2 ] i , the host cell also elevates [Ca 2 ] i as
a prelude to cell death. The rise in parasite
[Ca 2 ] i probably mediates dense granule
secretion where a variety of EF-hand Ca 2 -
binding proteins have been localized, including
calmodulin and granins. Calmodulin
antagonists prevent secretion. Other mediators
of the Ca 2 signal may also be involved.
For example, killing of CHO cells is enhanced
by TPA (12-o-tetradecanoylphorbol 13-acetate),
and blocked by sphingosine, an inhibitor of
PKC. Overall, the results suggest that both
calmodulin and PKC may be involved in parasite
invasiveness. Further work is needed to
clarify the role of specific signal pathways during
invasion.
In contrast with Entamoeba, the apicomplexans
are internalized by the host cell and
reside within the parasitophorous vacuole. The
process, typified by T. gondii, involves combinations
of apical attachment, actin/myosindependent
gliding motility, conoid extrusion,
release of micronemal proteins, formation
of the moving junction and internalization.
Replication of the parasite occurs within the
parasitophorous vacuole and egress must
occur before another round of cell infection
is initiated. Changes in [Ca 2 ] i are needed for
secretion of microneme contents, including
MIC2 and the T. gondii AMA-1. The MIC2 is
required for parasite adhesion in Toxoplasma
and Plasmodium. The secretion is triggered by
ionophore and prevented by intracellular Ca 2
buffers such as BAPTA. The release of stored
Ca 2 is sufficient for conoid extrusion and for
microneme secretion, since both processes can
occur in the presence of thapsigargin or upon
neutralization of acidic compartments with
NH 4 Cl. Additionally, a staurosporine-sensitive
protein kinase appears to stimulate microneme
secretion. The Ca 2 -sensitive kinase CDPK
may help transduce the Ca 2 signal. When
CDPK is inhibited with KT5926, invasion is also
blocked.
The converse of the invasion process is parasite
egress. Ionophores can stimulate egress,
implicating Ca 2 . Thiol reduction also appears
to be important, perhaps by activating an
oxidized, inactive nucleotide triphosphate
hydrolase of parasite origin. This enzyme can
be activated by dithiothreitol and results in a
concomitant Ca 2 flux from tubulovesicular
Ca 2 stores. Prior to egress, the host plasma
membrane becomes permeabilized. As a
consequence, extraparasitic K concentrations
decrease. The decrease in [K ] is sufficient
to stimulate PLC activity and release Ca 2
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA