Flagellar membrane trafficking in kinetoplastids

Plumbagin, a plant-derived naphthoquinone metabolite induces mitochondria mediated apoptosis-like cell death in Leishmania donovani: an ultrastructural and physiological study

Naphthoquinones are known to exhibit a broad range of biological activities against microbes, cancer and parasitic diseases and have been widely used in Indian traditional medicine. Plumbagin is a plant-derived naphthoquinone metabolite (5-hydroxy-2-methyl-1,4-naphthoquinone) reported to inhibit trypanothione reductase, the principal enzyme and a validated drug target involved in detoxification of oxidative stress in Leishmania. Here, we report the mechanistic aspects of cell death induced by plumbagin including physiological effects in the promastigote form and ultrastructural alterations in both promastigote and amastigote forms of Leishmania donovani which till now remained largely unknown. Our observations show that oxidative stress induced by plumbagin resulted in depolarization of the mitochondrial membrane, depletion in ATP levels, elevation of cytosolic calcium, increase in caspase 3/7-like protease activity and lipid peroxidation in promastigotes. Apoptosis-like cell death induction post plumbagin treatment was confirmed by biochemical assays like Annexin V/FITC staining, TUNEL as well as morphological and ultrastructural studies. These findings collectively highlight the mode of action and importance of oxidative stress inducing agents in effectively killing both forms of the Leishmania parasite and opens up the possibility of exploring plumbagin and its derivatives as promising candidates in the chemotherapy of Leishmaniasis.

LdFlabarin, a new BAR domain membrane protein of Leishmania flagellum

During the Leishmania life cycle, the flagellum undergoes successive assembly and disassembly of hundreds of proteins. Understanding these processes necessitates the study of individual components. Here, we investigated LdFlabarin, an uncharacterized L. donovani flagellar protein. The gene is conserved within the Leishmania genus and orthologous genes only exist in the Trypanosoma genus. LdFlabarin associates with the flagellar plasma membrane, extending from the base to the tip of the flagellum as a helicoidal structure. Site-directed mutagenesis, deletions and chimera constructs showed that LdFlabarin flagellar addressing necessitates three determinants: an N-terminal potential acylation site and a central BAR domain for membrane targeting and the C-terminal domain for flagellar specificity. In vitro, the protein spontaneously associates with liposomes, triggering tubule formation, which suggests a structural/morphogenetic function. LdFlabarin is the first characterized Leishmania BAR domain protein, and the first flagellum-specific BAR domain protein.

KHARON1 mediates flagellar targeting of a glucose transporter in Leishmania mexicana and is critical for viability of infectious intracellular amastigotes

The LmxGT1 glucose transporter is selectively targeted to the flagellum of the kinetoplastid parasite Leishmania mexicana, but the mechanism for targeting this and other flagella-specific membrane proteins among the Kinetoplastida is unknown. To address the mechanism of flagellar targeting, we employed in vivo cross-linking, tandem affinity purification, and mass spectrometry to identify a novel protein, KHARON1 (KH1), which is important for the flagellar trafficking of LmxGT1. Kh1 null mutant parasites are strongly impaired in flagellar targeting of LmxGT1, and trafficking of the permease was arrested in the flagellar pocket. Immunolocalization revealed that KH1 is located at the base of the flagellum, within the flagellar pocket, where it associates with the proximal segment of the flagellar axoneme. We propose that KH1 mediates transit of LmxGT1 from the flagellar pocket into the flagellar membrane via interaction with the proximal portion of the flagellar axoneme. KH1 represents the first component involved in flagellar trafficking of integral membrane proteins among parasitic protozoa. Of considerable interest, Kh1 null mutants are strongly compromised for growth as amastigotes within host macrophages. Thus, KH1 is also important for the disease causing stage of the parasite life cycle.

The phosphoarginine energy-buffering system of trypanosoma brucei involves multiple arginine kinase isoforms with different subcellular locations

Phosphagen energy-buffering systems play an essential role in regulating the cellular energy homeostasis in periods of high-energy demand or energy supply fluctuations. Here we describe the phosphoarginine/arginine kinase system of the kinetoplastid parasite Trypanosoma brucei, consisting of three highly similar arginine kinase isoforms (TbAK1-3). Immunofluorescence microscopy using myc-tagged protein versions revealed that each isoform is located in a specific subcellular compartment: TbAK1 is exclusively found in the flagellum, TbAK2 in the glycosome, and TbAK3 in the cytosol of T. brucei. The flagellar location of TbAK1 is dependent on a 22 amino acid long N-terminal sequence, which is sufficient for targeting a GFP-fusion protein to the trypanosome flagellum. The glycosomal location of TbAK2 is in agreement with the presence of a conserved peroxisomal targeting signal, the C-terminal tripeptide ‘SNL’. TbAK3 lacks any apparent targeting sequences and is accordingly located in the cytosol of the parasite. Northern blot analysis indicated that each TbAK isoform is differentially expressed in bloodstream and procyclic forms of T. brucei, while the total cellular arginine kinase activity was 3-fold higher in bloodstream form trypanosomes. These results suggest a substantial change in the temporal and spatial energy requirements during parasite differentiation. Increased arginine kinase activity improved growth of procyclic form T. brucei during oxidative challenges with hydrogen peroxide. Elimination of the total cellular arginine kinase activity by RNA interference significantly decreased growth (>90%) of procyclic form T. brucei under standard culture conditions and was lethal for this life cycle stage in the presence of hydrogen peroxide. The putative physiological roles of the different TbAK isoforms in T. brucei are further discussed.

Flagellar membrane localization via association with lipid rafts

The eukaryotic flagellar membrane has a distinct composition from other domains of the plasmalemma. Our work shows that the specialized composition of the trypanosome flagellar membrane reflects increased concentrations of sterols and saturated fatty acids, correlating with direct observation of high liquid order by laurdan fluorescence microscopy. These findings indicate that the trypanosome flagellar membrane possesses high concentrations of lipid rafts: discrete regions of lateral heterogeneity in plasma membranes that serve to sequester and organize specialized protein complexes. Consistent with this, a dually acylated Ca(2+) sensor that is concentrated in the flagellum is found in detergent-resistant membranes and mislocalizes if the lipid rafts are disrupted. Detergent-extracted cells have discrete membrane patches localized on the surface of the flagellar axoneme, suggestive of intraflagellar transport particles. Together, these results provide biophysical and biochemical evidence to indicate that lipid rafts are enriched in the trypanosome flagellar membrane, providing a unique mechanism for flagellar protein localization and illustrating a novel means by which specialized cellular functions may be partitioned to discrete membrane domains.

The Trypanosoma brucei flagellum: moving parasites in new directions

African trypanosomes are devastating human and animal pathogens. Trypanosoma brucei rhodesiense and T. b. gambiense subspecies cause the fatal human disease known as African sleeping sickness. It is estimated that several hundred thousand new infections occur annually and the disease is fatal if untreated. T. brucei is transmitted by the tsetse fly and alternates between bloodstream-form and insect-form life cycle stages that are adapted to survive in the mammalian host and the insect vector, respectively. The importance of the flagellum for parasite motility and attachment to the tsetse fly salivary gland epithelium has been appreciated for many years. Recent studies have revealed both conserved and novel features of T. brucei flagellum structure and composition, as well as surprising new functions that are outlined here. These discoveries are important from the standpoint of understanding trypanosome biology and identifying novel drug targets, as well as for advancing our understanding of fundamental aspects of eukaryotic flagellum structure and function.

Actin-depolymerizing factor, ADF/cofilin, is essentially required in assembly of Leishmania flagellum

ADF/cofilins are ubiquitous actin dynamics-regulating proteins that have been mainly implicated in actin-based cell motility. Trypanosomatids, e.g. Leishmania and Trypanosoma, which mediate their motility through flagellum, also contain a putative ADF/cofilin homologue, but its role in flagellar motility remains largely unexplored. We have investigated the role of this protein in assembly and motility of the Leishmania flagellum after knocking out the ADF/cofilin gene by targeted gene replacement. The resultant mutants were completely immotile, short and stumpy, and had reduced flagellar length and severely impaired beat. In addition, the assembly of the paraflagellar rod was lost, vesicle-like structures were seen throughout the length of the flagellum and the state and distribution of actin were altered. However, episomal complementation of the gene restored normal morphology and flagellar function. These results for the first time indicate that the actin dynamics-regulating protein ADF/cofilin plays a critical role in assembly and motility of the eukaryotic flagellum.

The flagellum of Trypanosoma brucei: new tricks from an old dog

African trypanosomes, i.e. Trypanosoma brucei and related sub-species, are devastating human and animal pathogens that cause significant human mortality and limit sustained economic development in sub-Saharan Africa. T. brucei is a highly motile protozoan parasite and coordinated motility is central to both disease pathogenesis in the mammalian host and parasite development in the tsetse fly vector. Therefore, understanding unique aspects of the T. brucei flagellum may uncover novel targets for therapeutic intervention in African sleeping sickness. Moreover, studies of conserved features of the T. brucei flagellum are directly relevant to understanding fundamental aspects of flagellum and cilium function in other eukaryotes, making T. brucei an important model system. The T. brucei flagellum contains a canonical 9+2 axoneme, together with additional features that are unique to kinetoplastids and a few closely-related organisms. Until recently, much of our knowledge of the structure and function of the trypanosome flagellum was based on analogy and inference from other organisms. There has been an explosion in functional studies in T. brucei in recent years, revealing conserved as well as novel and unexpected structural and functional features of the flagellum. Most notably, the flagellum has been found to be an essential organelle, with critical roles in parasite motility, morphogenesis, cell division and immune evasion. This review highlights recent discoveries on the T. brucei flagellum.