cAMP signalling in trypanosomatids: role in pathogenesis and as a drug target

Abstract

Despite recent research linking cAMP signalling to virulence in trypanosomatids and detailed studies of trypanosomatid adenylyl cyclases (ACs) and phosphodiesterases (PDEs) since their discoveries 40 years ago, downstream components of the pathway and their biological functions have remained remarkably elusive. However, in recent years, significant discoveries have been made: a role for parasite ACs has been proposed in cytokinesis, evasion of the host immune response, and social motility. cAMP phosphodiesterases PDEB1 and PDEB2 were found to be essential for survival and virulence of Trypanosoma brucei and, in Trypanosoma cruzi, PDEC2 was shown to be required for normal osmoregulation. As we discuss here, these breakthroughs have led to an ongoing surge in the development of PDE inhibitors as lead compounds for trypanocidal drugs.

Keywords: adenylyl cyclases; cAMP; drug target; phosphodiesterases; trypanosomatids.

Figure 1

cAMP signalling in the life cycle of Trypanosoma brucei. (1) Long slender bloodstream form in the mammalian bloodstream; the position of major organelles is indicated: flagellum (F), nucleus (N), and kinetoplast (K; contains mitochondrial DNA). The adenylyl cyclase (AC) expression site-associated gene 4 (ESAG4) has been implicated in modulation of the host innate immune response and members of the ESAG4 subfamily are important for control of cell division . (2) Short stumpy bloodstream form, which is pre-adapted to differentiate to procyclic forms following ingestion by a tsetse fly. (3) Early procyclic trypomastigote in the tsetse fly midgut. ACs have been implicated in the regulation of social motility . (4) Late procyclic trypomastigote. (5) Elongated trypomastigote, which migrates to the proventriculus. Here, a single asymmetric division produces one long epimastigote, which is thought to decay (not shown) and one short epimastigote (6), which goes on to colonise the tsetse fly salivary glands. (7) Epimastigote attached to the salivary gland. (8) Mammalian-infective metacyclic trypomastigote, which is released from the gland to infect a mammal during a blood meal. Curved arrows denote stages undergoing proliferative cell cycles.

Figure 2

An overview of potential drug targets in cAMP signalling pathways in trypanosomatids. Potential drug targets are highlighted as filled circles and colour coded as follows: pink, protein kinase A (PKA); red, phosphodiesterase (PDE); and dark blue, adenylyl cyclase (AC). (A) A cAMP signalling pathway in Leishmania mediates cellular adaptation and survival during hypoxia. In Leishmania major promastigotes, knockout of heme-containing AC (HemAC-Lm) under hypoxic conditions leads to increased cell death . It is unknown whether HemAC-Lm inhibition leads to cell death in mammalian-stage parasites, but it is possible that HemAC-Lm has a role in adaptation to changes in environmental oxygen in all life-cycle stages by catalysing the production of cAMP, which binds and activates PKA leading to multiple downstream effects. (B) cAMP signalling components regulate essential processes in Trypanosoma cruzi. Inhibition of PKA in epimastigotes induces cell death and is potentially implicated in immune evasion in the mammalian host by its interaction with trans-sialidases in trypomastigotes . PKA localises to both the cytosol and plasma membrane in trypomastigotes. The contractile vacuole complex (CVC) is part of the membranous spongiome network, which is essential for osmoregulation in T. cruzi. Inhibition of TcrPDEC2, which localises to the spongiome, in epimastigotes leads to dysregulation of the hyposmotic stress response , but it is unknown whether TcrPDEC2 also regulates this process in mammalian-stage parasites. (C) PDEB1 and 2 localise to the paraflagellar rod (PFR) and, when simultaneously knocked down, cell death occurs in vitro and the parasites are avirulent in mice . ESAG4 subfamily members are flagellar membrane-localised ACs that are highly expressed in bloodstream-form parasites. When the subfamily is simultaneously knocked down, cytokinesis defects occur, leading to cell death and the parasites have attenuated virulence in a mouse model .