Biology Reference
In-Depth Information
molecule called stumpy inducing factor (SIF) promotes cell cycle arrest in G1/G0
phase and induces differentiation of trypomastigotes from the long slender form
into the short stumpy form. Treatment of the long slender form with SIF was
accompanied by an increase in cAMP, suggesting that this may be the trigger for
differentiation (Vassella et al.
1997
). The link between increased cAMP and its
effects on transformation and proliferation appeared to be supported by studies
using membrane-permeable analogues of cAMP, assumed at the time to be resistant
to hydrolysis. For example, pCPT-cAMP mimicked the effect of SIF to induce
differentiation of
T. brucei
trypomastigotes from the long slender to the short
stumpy form (Vassella et al.
1997
; Breidbach et al.
2002
). In
Leishmania
, exposure
to dibutyryl cAMP decreased
L. tropica
promastigote proliferation and induced
transformation of
L. donovani
amastigotes to promastigotes (Walter et al.
1978
).
However, a recent study has demonstrated that the membrane-permeable cAMP
analogue, 8-pCPT-cAMP is, in fact, efficiently hydrolyzed to 8-pCPT-5
0
AMP by
the kinetoplastid PDEs (Laxman et al.
2006
). Comparison of the effect of the cAMP
analogue and its corresponding AMP and adenosine analog breakdown products
suggested that slender to stumpy transformation was mediated by the cAMP
hydrolysis products 5
0
AMP and adenosine and not by cAMP per se (Laxman
et al.
2006
). These studies were conducted on a monomorphic strain of
T. brucei
that has lost the ability to transform from the slender to the stumpy form in vivo.
Monomorphic strains differ from pleomorphic strains in their cAMP responses;
unlike pleomorphic strains they do not show an increase in cAMP following
exposure to SIF (Vassella et al.
1997
). Thus, while many questions remain about
the role of cAMP in the transformation and differentiation of
T. brucei
, a consensus
is emerging that cyclic nucleotide signaling does play a role in kinetoplastid life
cycle progression. For more comprehensive reviews on cAMP signaling in kineto-
plastids, please refer to Seebeck et al. (
2004
), Laxman and Beavo (
2007
) and Kunz
et al. (
2009b
).
While the absence of effective pharmacological tools has been a drawback, this
limitation has partly been overcome with the use of gene knockdown via RNA
interference (RNAi) techniques and recent studies have begun to provide valuable
insights on the role of cAMP in the cell biology of these parasites (see Sect.
2
).
2 Kinetoplastid Phosphodiesterases
Although the presence of cAMP-PDEs in kinetoplastid parasites has been known
for at least three decades (Walter
1974
), the molecular analysis and exploration of
these enzymes as potential drug targets has only recently begun.
2.1 Primary Structure
The genomes of all currently sequenced kinetoplastids (
T. brucei
,
T. cruzi
,
T. congolense
,
T. vivax, Leishmania major
,
L.
infantum
,
L. braziliensis
, and
