Biology Reference
In-Depth Information
domain that may act as a receptor, and on the other by an intracellular catalytic
domain that shares homology with the catalytic domain of the class I ACs. The
kinetoplastid ACs are not regulated by G proteins, the absence of G-protein-
coupled receptors and heterotrimeric G proteins was confirmed when the complete
sequence of the first kinetoplastd genome (
T. brucei
) became available in 2005
(Berriman et al.
2005
). Kinetoplastid AC activity is unaffected by forskolin
or cholera toxin and neither agonists nor antagonists have yet been identified.
With regard to the downstream effectors of cAMP, while no counterparts for the
mammalian cyclic nucleotide-gated channels or exchange proteins activated by
cAMP (EPAC) have been identified in kinetoplastids, protein kinase A (PKA) is
present in a fairly conserved form in the trypanosomatid genomes (genes for one
regulatory and three catalytic subunits have been identified) (Siman-Tov et al.
1996
; Huang et al.
2002
,
2006
). Although early studies suggested kinetoplastid
PKA activity to be unresponsive to cAMP (Shalaby et al.
2001
), more recent studies
have revealed the presence of a cAMP-responsive PKA activity in
T. cruzi
(Huang
et al.
2006
). Moreover,
T. cruzi
cAMP-PDE, TcPDEC2, has been identified as an
interaction partner of the catalytic subunit of PKA from
T. cruzi
(TcPKAc) and the
recombinant TcrPDEC2 is phosphorylated by PKA (Huang et al.
2006
). Another
important component that is substantially conserved between mammals and kine-
toplastids is the cyclic nucleotide phosphodiesterase (PDE). There are four geneti-
cally distinct kinetoplastid PDE families and recent studies suggest that the enzyme
plays a vital role in the regulation and compartmentalization of cAMP signaling
(Zoraghi and Seebeck
2002
; Kunz et al.
2006
; Oberholzer et al.
2007
). These PDEs
are pharmacologically distinct from mammalian PDEs as none of the mammalian
PDE inhibitors tested to date have shown significant activity (Zoraghi et al.
2001
;
Laxman et al.
2006
; Johner et al.
2006
). Interestingly, an analysis of the flagellar
proteome of
T. brucei
has revealed the presence of several enzymes involved in
cAMP signaling, including two cAMP-PDEs, adenylate kinases, and putative
cAMP-binding proteins (Broadhead et al.
2006
). Experimental evidence points to
the PDEs being part of a larger signaling complex associated with the paraflagellar
rod, a highly organized filamentous structure running parallel to the axoneme
(Oberholzer et al.
2007
).
Although the results of early studies employing the use of mammalian PDE
inhibitors or class I AC agonists, now known to be ineffective against their
kinetoplastid counterparts, must be reassessed, changes in cAMP levels have
been observed to precede kinetoplastid transformation and differentiation.
T. brucei
bloodstream trypomastigotes exist in two forms, known as the long slender pro-
liferating form and the short stumpy nonproliferating form; the latter is preadapted
for survival in the tsetse fly (see Fig.
1
). Transformation from the long slender
to short stumpy form is induced at a critical density, corresponding to the peak of
parasitemia in the mammalian host. Cyclic AMP levels in the long slender form
at the peak of parasitemia were found to be five times higher than that in the short
stumpy form (Reed et al.
1985
). In
T. cruzi
, transformation of epimastigotes into
metacyclic trypomastigotes is also preceded by a threefold increase in cAMP
(Rangel-Aldao et al.
1988
). In a pleomorphic
T. brucei
strain, a quorum-sensing
