A divergent protein kinase A regulatory subunit essential for morphogenesis of the human pathogen Leishmania.

Parasitic protozoa of the genus Leishmania cycle between the phagolysosome of mammalian macrophages, where they reside as rounded intracellular amastigotes, and the midgut of female sand flies, which they colonize as elongated extracellular promastigotes. Previous studies indicated that protein kinase A (PKA) plays an important role in the initial steps of promastigote differentiation into amastigotes. Here, we describe a novel regulatory subunit of PKA (which we have named PKAR3) that is unique to Leishmania and most (but not all) other Kinetoplastidae. PKAR3 is localized to subpellicular microtubules (SPMT) in the cell cortex, where it recruits a specific catalytic subunit (PKAC3). Promastigotes of pkar3 or pkac3 null mutants lose their elongated shape and become rounded but remain flagellated. Truncation of an N-terminal formin homology (FH)-like domain of PKAR3 results in its detachment from the SPMT, also leading to rounded promastigotes. Thus, the tethering of PKAC3 via PKAR3 at the cell cortex is essential for maintenance of the elongated shape of promastigotes. This role of PKAR3 is reminiscent of PKARIß and PKARIIß binding to microtubules of mammalian neurons, which is essential for the elongation of dendrites and axons, respectively. Interestingly, PKAR3 binds nucleoside analogs, but not cAMP, with a high affinity similar to the PKAR1 isoform of Trypanosoma. We propose that these early-diverged protists have re-purposed PKA for a novel signaling pathway that spatiotemporally controls microtubule remodeling and cell shape.

Novel kinetoplastid-specific cAMP binding proteins identified by RNAi screening for cAMP resistance in Trypanosoma brucei.

Cyclic AMP signalling in trypanosomes differs from most eukaryotes due to absence of known cAMP effectors and cAMP independence of PKA. We have previously identified four genes from a genome-wide RNAi screen for resistance to the cAMP phosphodiesterase (PDE) inhibitor NPD-001. The genes were named cAMP Response Protein (CARP) 1 through 4. Here, we report an additional six CARP candidate genes from the original sample, after deep sequencing of the RNA interference target pool retrieved after NPD-001 selection (RIT-seq). The resistance phenotypes were confirmed by individual RNAi knockdown. Highest level of resistance to NPD-001, approximately 17-fold, was seen for knockdown of CARP7 (Tb927.7.4510). CARP1 and CARP11 contain predicted cyclic AMP binding domains and bind cAMP as evidenced by capture and competition on immobilised cAMP. CARP orthologues are strongly enriched in kinetoplastid species, and CARP3 and CARP11 are unique to Trypanosoma. Localization data and/or domain architecture of all CARPs predict association with the T. brucei flagellum. This suggests a crucial role of cAMP in flagellar function, in line with the cell division phenotype caused by high cAMP and the known role of the flagellum for cytokinesis. The CARP collection is a resource for discovery of unusual cAMP pathways and flagellar biology.

A multi-adenylate cyclase regulator at the flagellar tip controls African trypanosome transmission.

Signaling from ciliary microdomains controls developmental processes in metazoans. Trypanosome transmission requires development and migration in the tsetse vector alimentary tract. Flagellar cAMP signaling has been linked to parasite social motility (SoMo) in vitro, yet uncovering control of directed migration in fly organs is challenging. Here we show that the composition of an adenylate cyclase (AC) complex in the flagellar tip microdomain is essential for tsetse salivary gland (SG) colonization and SoMo. Cyclic AMP response protein 3 (CARP3) binds and regulates multiple AC isoforms. CARP3 tip localization depends on the cytoskeletal protein FLAM8. Re-localization of CARP3 away from the tip microdomain is sufficient to abolish SoMo and fly SG colonization. Since intrinsic development is normal in carp3 and flam8 knock-out parasites, AC complex-mediated tip signaling specifically controls parasite migration and thereby transmission. Participation of several developmentally regulated receptor-type AC isoforms may indicate the complexity of the in vivo signals perceived.

Efficient flavinylation of glycosomal fumarate reductase by its own ApbE domain in Trypanosoma brucei.

A subset of flavoproteins has a covalently attached flavin prosthetic group enzymatically attached via phosphoester bonding. In prokaryotes, this is catalysed by alternative pyrimidine biosynthesis E (ApbE) flavin transferases. ApbE-like domains are present in few eukaryotic taxa, for example the N-terminal domain of fumarate reductase (FRD) of Trypanosoma, a parasitic protist known as a tropical pathogen causing African sleeping sickness. We use the versatile reverse genetic tools available for Trypanosoma to investigate the flavinylation of glycosomal FRD (FRDg) in vivo in the physiological and organellar context. Using direct in-gel fluorescence detection of covalently attached flavin as proxy for activity, we show that the ApbE-like domain of FRDg has flavin transferase activity in vivo. The ApbE domain is preceded by a consensus flavinylation target motif at the extreme N terminus of FRDg, and serine 9 in this motif is essential as flavin acceptor. The preferred mode of flavinylation in the glycosome was addressed by stoichiometric expression and comparison of native and catalytically inactive ApbE domains. In addition to the trans-flavinylation activity, the ApbE domain catalyses the intramolecular cis-flavinylation with at least fivefold higher efficiency. We discuss how the higher efficiency due to unusual fusion of the ApbE domain to its substrate protein FRD may provide a selective advantage by faster FRD biogenesis during rapid metabolic adaptation of trypanosomes. The first 37 amino acids of FRDg, including the consensus motif, are sufficient as flavinylation target upon fusion to other proteins. We propose FRDg(1-37) as 4-kDa heat-stable, detergent-resistant fluorescent protein tag and suggest its use as a new tool to study glycosomal protein import.

Metabolic selection of a homologous recombination-mediated gene loss protects Trypanosoma brucei from ROS production by glycosomal fumarate reductase.

The genome of trypanosomatids rearranges by using repeated sequences as platforms for amplification or deletion of genomic segments. These stochastic recombination events have a direct impact on gene dosage and foster the selection of adaptive traits in response to environmental pressure. We provide here such an example by showing that the phosphoenolpyruvate carboxykinase (PEPCK) gene knockout (Δpepck) leads to the selection of a deletion event between two tandemly arranged fumarate reductase (FRDg and FRDm2) genes to produce a chimeric FRDg-m2 gene in the Δpepck∗ cell line. FRDg is expressed in peroxisome-related organelles, named glycosomes, expression of FRDm2 has not been detected to date, and FRDg-m2 is nonfunctional and cytosolic. Re-expression of FRDg significantly impaired growth of the Δpepck∗ cells, but FRD enzyme activity was not required for this negative effect. Instead, glycosomal localization as well as the covalent flavinylation motif of FRD is required to confer growth retardation and intracellular accumulation of reactive oxygen species (ROS). The data suggest that FRDg, similar to Escherichia coli FRD, can generate ROS in a flavin-dependent process by transfer of electrons from NADH to molecular oxygen instead of fumarate when the latter is unavailable, as in the Δpepck background. Hence, growth retardation is interpreted as a consequence of increased production of ROS, and rearrangement of the FRD locus liberates Δpepck∗ cells from this obstacle. Interestingly, intracellular production of ROS has been shown to be required to complete the parasitic cycle in the insect vector, suggesting that FRDg may play a role in this process.

Culturing and Transfection of Pleomorphic Trypanosoma brucei.

Cultivation of pleomorphic Trypanosoma brucei strains was introduced in 1996 when matrix dependence of growth of natural isolates was recognized. Semisolid agarose or liquid methylcellulose are currently used and here we provide optimized protocols for these culture methods and for transfection of pleomorphic strains. Although more laborious than standard liquid culture, culture of native pleomorphic strains is important for a number of research questions including differentiation, virulence, tissue tropism, and regulated metabolism. Some subclones of pleomorphic strains have acquired matrix independence upon passage in culture but maintained a pleomorphic phenotype. It appears that matrix dependence and pleomorphism are not tightly linked traits, yet phenotypes have to be verified before choosing one of these subclones for given experiments. Based on direct comparisons, we give recommendations for pleomorphic strain selection and culture conditions that guarantee truly pleomorphic and differentiation competent Trypanosoma brucei.

A unified multi-kingdom Golden Gate cloning platform.

Assembling composite DNA modules from custom DNA parts has become routine due to recent technological breakthroughs such as Golden Gate modular cloning. Using Golden Gate, one can efficiently assemble custom transcription units and piece units together to generate higher-order assemblies. Although Golden Gate cloning systems have been developed to assemble DNA plasmids required for experimental work in model species, they are not typically applicable to organisms from other kingdoms. Consequently, a typical molecular biology laboratory working across kingdoms must use multiple cloning strategies to assemble DNA constructs for experimental assays. To simplify the DNA assembly process, we developed a multi-kingdom (MK) Golden Gate assembly platform for experimental work in species from the kingdoms Fungi, Eubacteria, Protista, Plantae, and Animalia. Plasmid backbone and part overhangs are consistent across the platform, saving both time and resources in the laboratory. We demonstrate the functionality of the system by performing a variety of experiments across kingdoms including genome editing, fluorescence microscopy, and protein interaction assays. The versatile MK system therefore streamlines the assembly of modular DNA constructs for biological assays across a range of model organisms.

Nucleoside analogue activators of cyclic AMP-independent protein kinase A of Trypanosoma.

Protein kinase A (PKA), the main effector of cAMP in eukaryotes, is a paradigm for the mechanisms of ligand-dependent and allosteric regulation in signalling. Here we report the orthologous but cAMP-independent PKA of the protozoan Trypanosoma and identify 7-deaza-nucleosides as potent activators (EC50 ≥ 6.5 nM) and high affinity ligands (KD ≥ 8 nM). A co-crystal structure of trypanosome PKA with 7-cyano-7-deazainosine and molecular docking show how substitution of key amino acids in both CNB domains of the regulatory subunit and its unique C-terminal αD helix account for this ligand swap between trypanosome PKA and canonical cAMP-dependent PKAs. We propose nucleoside-related endogenous activators of Trypanosoma brucei PKA (TbPKA). The existence of eukaryotic CNB domains not associated with binding of cyclic nucleotides suggests that orphan CNB domains in other eukaryotes may bind undiscovered signalling molecules. Phosphoproteome analysis validates 7-cyano-7-deazainosine as powerful cell-permeable inducer to explore cAMP-independent PKA signalling in medically important neglected pathogens.

Protein kinase A signaling during bidirectional axenic differentiation in Leishmania.

Parasitic protozoa of the genus Leishmania are obligatory intracellular parasites that cycle between the phagolysosome of mammalian macrophages, where they proliferate as intracellular amastigotes, and the midgut of female sand flies, where they proliferate as extracellular promastigotes. Shifting between the two environments induces signaling pathway-mediated developmental processes that enable adaptation to both host and vector. Developmentally regulated expression and phosphorylation of protein kinase A subunits in Leishmania and in Trypanosoma brucei point to an involvement of protein kinase A in parasite development. To assess this hypothesis in Leishmania donovani, we determined proteome-wide changes in phosphorylation of the conserved protein kinase A phosphorylation motifs RXXS and RXXT, using a phospho-specific antibody. Rapid dephosphorylation of these motifs was observed upon initiation of promastigote to amastigote differentiation in culture. No phosphorylated sites were detected in axenic amastigotes. To analyse the kinetics of (re)phosphorylation during axenic reverse differentiation from L. donovani amastigotes to promastigotes, we first established a map of this process with morphological and molecular markers. Upon initiation, the parasites rested for 6-12 h before proliferation of an asynchronous population resumed. After early changes in cell shape, the major changes in molecular marker expression and flagella biogenesis occurred between 24 and 33 h after initiation. RXXS/T re-phosphorylation and expression of the regulatory subunit PKAR1 correlated with promastigote maturation, indicating a promastigote-specific function of protein kinase A signaling. This is supported by the localization of PKAR1 to the flagellum, an organelle reduced to a remnant in amastigote forms. We conclude that a significant increase in protein kinase A-mediated phosphorylation is part of the ordered changes that characterise the amastigote to promastigote differentiation.

Cyclic AMP effectors in African trypanosomes revealed by genome-scale RNA interference library screening for resistance to the phosphodiesterase inhibitor CpdA.

One of the most promising new targets for trypanocidal drugs to emerge in recent years is the cyclic AMP (cAMP) phosphodiesterase (PDE) activity encoded by TbrPDEB1 and TbrPDEB2. These genes were genetically confirmed as essential, and a high-affinity inhibitor, CpdA, displays potent antitrypanosomal activity. To identify effectors of the elevated cAMP levels resulting from CpdA action and, consequently, potential sites for adaptations giving resistance to PDE inhibitors, resistance to the drug was induced. Selection of mutagenized trypanosomes resulted in resistance to CpdA as well as cross-resistance to membrane-permeable cAMP analogues but not to currently used trypanocidal drugs. Resistance was not due to changes in cAMP levels or in PDEB genes. A second approach, a genome-wide RNA interference (RNAi) library screen, returned four genes giving resistance to CpdA upon knockdown. Validation by independent RNAi strategies confirmed resistance to CpdA and suggested a role for the identified cAMP Response Proteins (CARPs) in cAMP action. CARP1 is unique to kinetoplastid parasites and has predicted cyclic nucleotide binding-like domains, and RNAi repression resulted in >100-fold resistance. CARP2 and CARP4 are hypothetical conserved proteins associated with the eukaryotic flagellar proteome or with flagellar function, with an orthologue of CARP4 implicated in human disease. CARP3 is a hypothetical protein, unique to Trypanosoma. CARP1 to CARP4 likely represent components of a novel cAMP signaling pathway in the parasite. As cAMP metabolism is validated as a drug target in Trypanosoma brucei, cAMP effectors highly divergent from the mammalian host, such as CARP1, lend themselves to further pharmacological development.

Adenylate cyclases of Trypanosoma brucei inhibit the innate immune response of the host.

The parasite Trypanosoma brucei possesses a large family of transmembrane receptor-like adenylate cyclases. Activation of these enzymes requires the dimerization of the catalytic domain and typically occurs under stress. Using a dominant-negative strategy, we found that reducing adenylate cyclase activity by about 50% allowed trypanosome growth but reduced the parasite's ability to control the early innate immune defense of the host. Specifically, activation of trypanosome adenylate cyclase resulting from parasite phagocytosis by liver myeloid cells inhibited the synthesis of the trypanosome-controlling cytokine tumor necrosis factor-α through activation of protein kinase A in these cells. Thus, adenylate cyclase activity of lyzed trypanosomes favors early host colonization by live parasites. The role of adenylate cyclases at the host-parasite interface could explain the expansion and polymorphism of this gene family.

Cytokinesis of Trypanosoma brucei bloodstream forms depends on expression of adenylyl cyclases of the ESAG4 or ESAG4-like subfamily.

Antigenic variation of the parasite Trypanosoma brucei operates by monoallelic expression of a variant surface glycoprotein (VSG) from a collection of multiple telomeric expression sites (ESs). Each of these ESs harbours a long polycistronic transcription unit containing several expression site-associated genes (ESAGs). ESAG4 copies encode bloodstream stage-specific adenylyl cyclases (AC) and belong to a larger gene family of around 80 members, the majority of which, termed genes related to ESAG4 (GRESAG4s), are not encoded in ESs and are expressed constitutively in the life cycle. Here we report that ablation of ESAG4 from the active ES did not affect parasite growth, neither in culture nor upon rodent infection, and did not significantly change total AC activity. In contrast, inducible RNAi-mediated knock-down of an AC subfamily that includes ESAG4 and two ESAG4-like GRESAG4 (ESAG4L) genes, decreased total AC activity and induced a lethal phenotype linked to impaired cytokinesis. In the Δesag4 line compensatory upregulation of apparently functionally redundant ESAG4L genes was observed, suggesting that the ESAG4/ESAG4L-subfamily ACs are involved in the control of cell division. How deregulated adenylyl cyclases or cAMP might impair cytokinesis is discussed.