The Critical Role of the Cytoskeleton in the Pathogenesis of Giardia

Curr Clin Micro Rpt (2015) 2:155–162 DOI 10.1007/s40588-015-0026-y PARASITOLOGY (A VAIDYA, SECTION EDITOR) The Critical Role of the Cytoskeleton in the Pathogenesis of Giardia Christopher Nosala 1 & Scott C. Dawson 1 Published online: 8 October 2015 # Springer International Publishing AG 2015 Abstract 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. Keywords Giardia . Gut pathogen . Microtubule . Motility . Attachment . Drug targets This article is part of the Topical Collection on Parasitology * Scott C. Dawson Christopher Nosala 1 Department of Microbiology and Molecular Genetics, University of California Davis, One Shields Avenue, Davis, CA 95616, USA Introduction Giardia lamblia is one of the most common protozoan causes of diarrheal disease worldwide [1, 2•, 3]. The parasite belongs to the diplomonad group of microbial eukaryotes, defined by having two nuclei and eight flagella [4, 5]. The impact of Giardia on human health is global—over one billion people currently have acute or chronic giardiasis, with rates approaching 90 % in endemic areas [6, 7]. Giardia can also have an economic impact through the infection of farm animals and pets [8, 9]. Due to the high number of giardiasis cases worldwide and the comparative lack of research efforts toward prevention and treatment, Giardia was placed on the WHO’s Neglected Tropical Diseases Initiative in 2004 [1, 10, 11]. Despite its global importance, fundamental biological questions about Giardia remain and we know little about in vivo infection dynamics and pathogenesis. Giardiasis is a zoonotic disease with a wide range of mammalian hosts that serve as reservoirs for human infection [12, 13]. Giardia parasites have a dimorphic life cycle in all hosts (Fig. 1). The dormant cyst is resistant to UV-induced light damage and changes in water tonicity [14]. Cysts are commonly acquired from contaminated water sources in regions with poor water sanitation or from lakes and streams frequented by hikers [1, 14]. Ingestion of as little as ten cysts is reported to be sufficient for robust infection [15]. During passage through the gastrointestinal tract, cysts are exposed to changes in pH and the presence of bile and then “excyst” to become motile trophozoites. Using flagellar motility, trophozoites navigate the lumen of the small intestine until they encounter a suitable place for attachment to the intestinal microvilli [11]. Attachment, mediated by a suction cup-like cytoskeletal structure called the ventral disc, allows the parasite to resist peristaltic flow [16]. Giardia is an extracellular parasite; cells remain in the lumen and prefer the small intestine for 156 Curr Clin Micro Rpt (2015) 2:155–162 Giardiasis is especially problematic in children because it promotes malnutrition and, in some cases, affects both mental and physical development [2•]. At the cellular level, Giardia colonization of the host is known to result in villus shortening, enterocyte apoptosis, and intestinal barrier dysfunction [18]. Giardia has no known toxin and infection does not induce a robust inflammatory response, although T cells may be important for clearance [19]. The Giardia genome does encode over 300 variant surface protein (VSP) genes, and antigen switching of (VSPs) likely contributes to the evasion of immune screening [20, 21]. Giardiasis is most commonly treated with metronidazole (Flagyl); yet, metronidazole may induce side effects including nausea and vomiting. The failure rates for anti-giardial treatment are estimated to approach 20 % and symptoms sometimes return following anti-giardial treatment [7]. Furthermore, strains have been described that resist antibiotic treatment [6, 22, 23], emphasizing the need for new therapies for the future. The Giardia Cytoskeleton Is Critical for Parasite Motility, Host Attachment, Proliferation, Encystation and Excystation, and Dissemination Fig. 1 The Giardia life cycle in the human host. Giardia has two life cycle stages: the flagellated trophozoite that attaches to the intestinal microvilli and an infectious cyst that persists in the environment. Cysts are ingested by the host and excyst after transiting through the stomach. Excysting trophozoites complete cell division, and then trophozoites search for suitable sites for intestinal attachment and colonization using flagellar motility. To proliferate in the small intestine and resist peristaltic flow, trophozoites attach to the villi using the ventral disc and only briefly detach to complete cytokinesis. Trophozoites thus attach, divide, and proliferate and colonize the small intestine. Based on environmental cues, partially divided trophozoites encyst and mature cysts are shed in feces. This facilitates the dissemination of parasites and the infection of new hosts. Giardia is zoonotic, and this life cycle occurs in all mammalian hosts colonization. By factors not entirely defined, Giardia trophozoites are eventually induced to encyst and new infectious cysts exit the host via the feces. Trophozoites are also often detected in stool by antigen testing, PCR, or via microscopic examination due to their characteristic “tear drop” shape and two nuclei [17]. Giardia Colonizes and Proliferates in the Mammalian Small Intestine The mechanism by which Giardia colonization of the gastrointestinal tract induces diarrheal disease is unclear. Giardiasis may be either acute and/or chronic and Giardia infection is generally accompanied by abdominal cramps, gas, nausea, and weight loss. Giardiasis may also result in a severe form of malabsorptive diarrhea presenting as a fatty, watery stool. Giardia’s complex microtubule (MT) cytoskeleton, including its ventral disc and flagella, is of critical importance throughout both stages of its life cycle [14, 24]. Flagellar motility may play a mechanical role in the initial opening of the cyst, in addition to contractile or other MT-mediated forces [25, 26], and is also required for positioning of the trophozoite prior to attachment. The ventral disc, a spiral microtubule array, mediates trophozoite attachment via an a (...truncated)