Disc-associated proteins mediate the unusual hyperstability of the ventral disc in Giardia lamblia.

Giardia lamblia, a widespread parasitic protozoan, attaches to the host gastrointestinal epithelium by using the ventral disc, a complex microtubule (MT) organelle. The 'cup-like' disc is formed by a spiral MT array that scaffolds numerous disc-associated proteins (DAPs) and higher-order protein complexes. In interphase, the disc is hyperstable and has limited MT dynamics; however, it remains unclear how DAPs confer these properties. To investigate mechanisms of hyperstability, we confirmed the disc-specific localization of over 50 new DAPs identified by using both a disc proteome and an ongoing GFP localization screen. DAPs localize to specific disc regions and many lack similarity to known proteins. By screening 14 CRISPRi-mediated DAP knockdown (KD) strains for defects in hyperstability and MT dynamics, we identified two strains - DAP5188KD and DAP6751KD -with discs that dissociate following high-salt fractionation. Discs in the DAP5188KD strain were also sensitive to treatment with the MT-polymerization inhibitor nocodazole. Thus, we confirm here that at least two of the 87 known DAPs confer hyperstable properties to the disc MTs, and we anticipate that other DAPs contribute to disc MT stability, nucleation and assembly.

Long-term in vivo monitoring of mouse and human hematopoietic stem cell engraftment with a human positron emission tomography reporter gene.

Positron emission tomography (PET) reporter genes allow noninvasive whole-body imaging of transplanted cells by detection with radiolabeled probes. We used a human deoxycytidine kinase containing three amino acid substitutions within the active site (hdCK3mut) as a reporter gene in combination with the PET probe [(18)F]-L-FMAU (1-(2-deoxy-2-(18)fluoro-ß-L-arabinofuranosyl)-5-methyluracil) to monitor models of mouse and human hematopoietic stem cell (HSC) transplantation. These mutations in hdCK3mut expanded the substrate capacity allowing for reporter-specific detection with a thymidine analog probe. Measurements of long-term engrafted cells (up to 32 wk) demonstrated that hdCK3mut expression is maintained in vivo with no counter selection against reporter-labeled cells. Reporter cells retained equivalent engraftment and differentiation capacity being detected in all major hematopoietic lineages and tissues. This reporter gene and probe should be applicable to noninvasively monitor therapeutic cell transplants in multiple tissues.

The domed architecture of Giardias ventral disc is necessary for attachment and host pathogenesis.

After ingestion of dormant cysts, the widespread protozoan parasite Giardia lamblia colonizes the host gastrointestinal tract via direct and reversible attachment using a novel microtubule organelle, the ventral disc. Extracellular attachment to the host allows the parasite to resist peristaltic flow, facilitates colonization and is proposed to cause damage to the microvilli of host enterocytes as well as disrupt host barrier integrity. The 9 um in diameter ventral disc is defined by a highly complex architecture of unique protein complexes scaffolded onto a spiral microtubule (MT) array of one hundred parallel, uniformly spaced MT polymers that bend approximately one and a quarter turns to form a domed structure. To investigate the role of disc-mediated attachment in causing epithelial cell damage, we used a new approach to rapidly create a stable quadruple knockout of Giardia of an essential ventral disc protein, MBP, using a new method of CRISPR-mediated gene disruption with multiple positive selectable markers. MBP quadruple KO mutant discs lack the characteristic domed architecture and possess a flattened crescent or horseshoe-shaped conformation that lacks the overlapping region, with severe defects in the microribbon-crossbridge (MR-CB) complex structure. MBP KO mutants are also unable to resist fluid flow required for attachment to inert surfaces. Importantly, MBP KO mutants have 100% penetrance off positive selection, which is essential for quantification of in vivo impacts of disc and attachment mutants with host cells. Using a new gastrointestinal organoid model of pathogenesis, we found that MBP KO infections had a significantly reduced ability to cause the barrier breakdown characteristic of wild-type infections. Overall, this work provides direct evidence of the role of MBP in creating the domed disc, as well as the first direct evidence that parasite attachment is necessary for host pathology, specifically epithelial barrier breakdown.

'Disc-o-Fever': Getting Down with Giardia's Groovy Microtubule Organelle.

Protists have evolved a myriad of highly specialized cytoskeletal organelles that expand known functional capacities of microtubule (MT) polymers. One such innovation - the ventral disc - is a cup-shaped MT organelle that the parasite Giardia uses to attach to the small intestine of its host. The molecular mechanisms underlying the generation of suction-based forces by overall conformational changes of the disc remain unclear. The elaborate disc architecture is defined by novel proteins and complexes that decorate almost all disc MT protofilaments, and vary in composition and conformation along the length of the MTs. Future genetic, biochemical, and functional analyses of disc-associated proteins will be central toward understanding not only disc architecture and assembly, but also the overall disc conformational dynamics that promote attachment.

Transcriptomic Profiling of High-Density Giardia Foci Encysting in the Murine Proximal Intestine.

Giardia is a highly prevalent, understudied protistan parasite causing significant diarrheal disease worldwide. Its life cycle consists of two stages: infectious cysts ingested from contaminated food or water sources, and motile trophozoites that colonize and attach to the gut epithelium, later encysting to form new cysts that are excreted into the environment. Current understanding of parasite physiology in the host is largely inferred from transcriptomic studies using Giardia grown axenically or in co-culture with mammalian cell lines. The dearth of information about the diversity of host-parasite interactions occurring within distinct regions of the gastrointestinal tract has been exacerbated by a lack of methods to directly and non-invasively interrogate disease progression and parasite physiology in live animal hosts. By visualizing Giardia infections in the mouse gastrointestinal tract using bioluminescent imaging (BLI) of tagged parasites, we recently showed that parasites colonize the gut in high-density foci. Encystation is initiated in these foci throughout the entire course of infection, yet how the physiology of parasites within high-density foci in the host gut differs from that of cells in laboratory culture is unclear. Here we use BLI to precisely select parasite samples from high-density foci in the proximal intestine to interrogate in vivo Giardia gene expression in the host. Relative to axenic culture, we noted significantly higher expression (>10-fold) of oxidative stress, membrane transporter, and metabolic and structural genes associated with encystation in the high-density foci. These differences in gene expression within parasite foci in the host may reflect physiological changes associated with high-density growth in localized regions of the gut. We also identified and verified six novel cyst-specific proteins, including new components of the cyst wall that were highly expressed in these foci. Our in vivo transcriptome data support an emerging view that parasites encyst early in localized regions in the gut, possibly as a consequence of nutrient limitation, and also impact local metabolism and physiology.

The Critical Role of the Cytoskeleton in the Pathogenesis of Giardia.

Giardia lamblia is a flagellated parasite of the gut and causes significant morbidity worldwide. Novel druggable targets are sorely needed due to Giardia's prevalence and the growing threat of antibiotic resistance. Giardia's conserved and unique cytoskeletal features, such as its eight flagella and ventral disc, are required for host colonization by facilitating motility, attachment, and cell division. Therapies that target these processes could interfere with trophozoite colonization, reduce the time or severity of the infection, and reduce the number of infectious cysts shed into the environment. This requires vetting and prioritizing critical cellular processes and identifying specific Giardia proteins in those processes as targets. It is time to leverage the wealth of data gathered through genome sequencing and proteomic studies, and new insights on the cytoskeleton of Giardia to design effective new drugs to treat giardiasis.