Biology subjects

Morriswood, B.

Publications and source records attributed to Morriswood, B..

4 recordsLinked to original sources

bioRxiv2024

The actomyosin system is essential for the integrity of the endosomal system in bloodstream form Trypanosoma brucei

The actin cytoskeleton is a ubiquitous feature of eukaryotic cells, yet its complexity varies across different taxa. In the parasitic protist Trypanosoma brucei, a rudimentary actomyosin system consisting of one actin gene and two myosin genes has been retained despite significant investment in the microtubule cytoskeleton. The functions of this highly simplified actomyosin system remain unclear, but appear to centre on the endomembrane system. Here, advanced light and electron microscopy imaging techniques together with biochemical and biophysical assays were used to explore the relationship between the actomyosin and endomembrane systems. The class I myosin (TbMyo1) had a large cytosolic pool and its ability to translocate actin filaments in vitro was shown here for the first time. TbMyo1 exhibited strong association with the endosomal system and was additionally found on glycosomes. At the endosomal membranes, TbMyo1 colocalised with markers for early and late endosomes (TbRab5A and TbRab7, respectively), but not with the marker associated with recycling endosomes (TbRab11). Actin and myosin were simultaneously visualised for the first time in trypanosomes using an anti-actin chromobody. Disruption of the actomyosin system using the actin-depolymerising drug latrunculin A resulted in a delocalisation of both the actin chromobody signal and an endosomal marker, and was accompanied by a specific loss of endosomal structure. This suggests that the actomyosin system is required for maintaining endosomal integrity in T. brucei.

cell biology↗

bioRxiv2022

Single-molecule fluorescence microscopy demonstrates fast dynamics of the variant surface glycoprotein coat on living trypanosomes

The fluidity of Trypanosoma bruceis dense coat of GPI-anchored variant surface glycoproteins (VSGs) is fundamental for the survival of the parasite. In order to maintain the integrity of the coat, it is recycled on the time scale of a few minutes. This is surprisingly fast as endo- and exocytosis take place in the same small membrane invagination called the flagellar pocket. Here, we present measurements of VSG dynamics on the single-molecule level in living trypanosomes. A large number of short protein trajectories sampling the parasites surface were analysed in two distinct scenarios: diffusion and directed motion. To this end, we employed a previously published algorithm and implemented two extensions to consider rim effects as well as localisations errors inherent to single-mole tracking. Neglect of the latter can have a significant distortive effect on the measured diffusion coefficient; in our case resulting in an underestimation by 20 %. We found large heterogeneity in the local diffusion coefficients and velocities with a surprisingly high average value of [Formula] and [Formula], respectively. To decide on the locally dominant motion mode, we present a guideline based on random walk simulations. We find that VSG dynamics is indeed dominated by diffusion. Complementary simulations on long time scales not accessible in the experiment showed that passive VSG randomisation is fast enough to prevent re-endocytosis newly exocytosed VSGs and to accomplish turnover of the full VSG coat within a few minutes. Author summarySingle-molecule tracking in biological systems often suffers from trajectories being too short to obtain statistically robust decisions on the present motion mode. We faced this issue when investigating the dynamics of the protein surface coat of African trypanosomes. To address the question whether diffusion or directed motion governs coat dynamics, we have adopted an algorithm based on temporal decomposition and spatial binning of an ensemble of single-molecule trajectories. We introduced several extensions to the original approach, including the consideration of localisation errors inherent to single-molecule tracking. This improvement alone already prevented the diffusion coefficient from being underestimated by 20 %. We analysed the coat dynamics in two scenarios, diffusion and directed motion, and offer a decision guideline to identify the locally dominating motion mode. Our extended algorithm is available to the scientific community via GitHub. For the trypanosome surface coat we found that the motion is indeed mainly characterised by diffusion with a surprisingly high diffusion coefficient. This finding solves a long-standing question how the parasite maintains its protein coat.

biophysics↗

bioRxiv2022

Endocytosis is required for access of surface-bound cargo to the flagellar pocket of trypanosomes

All endo- and exocytosis in the African trypanosome Trypanosoma brucei occurs at a single subdomain of the plasma membrane. This subdomain, the flagellar pocket, is a small vase-shaped invagination containing the root of the cells single flagellum. Several cytoskeleton-associated multiprotein complexes are coiled around the neck of the flagellar pocket on its cytoplasmic face. One of these, the hook complex, was proposed to affect macromolecule entry into the flagellar pocket lumen. In previous work, knockdown of the hook complex component TbMORN1 resulted in larger cargo being unable to enter the flagellar pocket. In this study, the hook complex component TbSmee1 was characterised in bloodstream form Trypanosoma brucei and was found to be essential for cell viability. TbSmee1 knockdown resulted in flagellar pocket enlargement and impaired access to the flagellar pocket membrane by surface-bound cargo, similar to depletion of TbMORN1. Unexpectedly, inhibition of endocytosis by knockdown of clathrin phenocopied TbSmee1 knockdown, suggesting that endocytic activity itself is a prerequisite for the entry of surface-bound cargo into the flagellar pocket. SummaryCharacterisation of the essential trypanosome protein TbSmee1 suggests that endocytosis is required for flagellar pocket access of surface-bound cargo.

cell biology↗

bioRxiv2019

A modification to the life cycle of the parasite Trypanosoma brucei

African trypanosomes cause sleeping sickness in humans and nagana in cattle. These unicellular parasites are transmitted by the bloodsucking tsetse fly. In the mammalian hosts circulation, tissues, and interstitium, at least two main life cycle stages exist: slender and stumpy bloodstream stages. Proliferating slender stage cells differentiate into cell cycle-arrested stumpy stage cells at high population densities. This developmental stage transition occurs in response to the quorum sensing factor SIF (stumpy induction factor), and is thought to fulfil two main functions. First, it auto-regulates the parasite load in the host. Second, the stumpy stage is regarded as pre-adapted for tsetse fly infection and the only stage capable of successful vector transmission. Here, we show that proliferating slender stage trypanosomes are able to complete the complex life cycle in the fly as successfully as the stumpy stage, and that a single parasite is sufficient for productive infection. Our findings not only propose a revision to the traditional rigid view of the trypanosome life cycle, but also suggest a solution to a long-acknowledged paradox in the transmission event: parasitaemia in chronic infections is characteristically low, and so the probability of a tsetse ingesting a stumpy cell during a bloodmeal is also low. The finding that proliferating slender parasites are infective to tsetse flies helps shed light on this enigma.

microbiology↗