trans

256 INTRACELLULAR SIGNALING
from intracellular stores. Mutation analysis
of T. gondii has identified cells that are resistant
to the effects of ionophore A23187. Further
characterization of the mutant cells should
reveal gene products that help sensitize the
egress process to Ca 2 . Invasion of host red
blood cells (RBCs) by Plasmodium also has a
Ca 2 sensitive component. Moreover, the
infected RBC also appears to be modified
in terms of its Ca 2 transport properties. Patch
clamp studies reveal an inwardly directed
Ca 2 current that is 18-fold greater than in the
uninfected RBC. A consequence of this change
is an elevation in [Ca 2 ] i in the infected RBC.
Kinetoplastids also are invasive to host cells.
T. cruzi makes contact by means of cell-surface
glycoconjugates. Contact is associated with
Ca 2 flux in the parasite cytoplasm. The elevated
[Ca 2 ] i appears to be important for invasion
since stabilization of [Ca 2 ] i with BAPTA
or Quin-2 inhibits this process. Calmodulin
antagonists are also inhibitory, but do not have
enough specificity to directly implicate calmodulin
in the invasion process. During invasion,
contact with parasites creates a bidirectional
signal. Association of host cells with specific
metacyclic glycoproteins, such as gp82, is
sufficient to initiate a Ca 2 flux. Additionally,
a parasite-derived serine endoprotease (oligopeptidase
B) generates a product that initiates
Ca 2 flux in the host cells. The product
appears to be recognized by a host-cell receptor
that is coupled to a pertussis toxin-sensitive
trimeric G protein. The subsequent production
of InsP 3 releases Ca 2 from the host ER
through the InsP 3 receptor. A consequence of
this activity is disruption of the cortical actin
network, and fusion of lysosomes with the
plasma membrane at the site of parasite entry.
Buffering of the host cell [Ca 2 ] i with BAPTA,
or depletion of intracellular pools with thapsigargin,
blocks parasite entry. The fusion
process involves the vesicle-associated Ca 2
sensor protein synaptotagmin VII. Cell entry
can be blocked with antibodies against its
C(2)A domain. Kinase cascades initiated by
glycosylphosphatidylinositols or by the transforming
growth factor (TGF) receptor have
also been implicated in T. cruzi invasion. How
the various Ca 2 dependent and independent
pathways work together to allow invasion to
occur is not known.
CYCLIC NUCLEOTIDES
Cyclic nucleotides (cAMP and/or cGMP) are
important second messengers in signal transduction
pathways, and can be involved in a
variety of functions ranging from simple environmental
responses in prokaryotes and lower
eukaryotes to more complex processes, such
as memory storage in Drosophila and hormonal
control of metabolism in mammals. In
parasitic protozoa, particularly trypanosomes
and Plasmodium, cyclic nucleotides have been
implicated in the control of differentiation.
Furthermore, many of the enzymatic components
of cyclic nucleotide pathways are
structurally distinct from their mammalian
counterparts. Multiple isoforms of purine
nucleotide cyclases, cyclic nucleotide-dependent
protein kinases and phosphodiesterases
contribute to further structural diversity.
This arrangement facilitates the fine tuning
of cellular responses, and also provides opportunities
for the development of highly specific
inhibitors. For example Viagra, the drug that
is used to treat male impotency, is an inhibitor
of a type 5 phosphodiesterase with cGMPspecific
activity. The major differences that have
now been identified between host and parasite
enzymes involved in cyclic nucleotide signaling
could therefore be exploitable in terms of
drug development.
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA