**The Cytoskeleton of** *Giardia intestinalis*

**The Cytoskeleton of** *Giardia intestinalis*

Ana Paula Rocha Gadelha, Marlene Benchimol and Wanderley de Souza and Wanderley de Souza Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Ana Paula Rocha Gadelha, Marlene Benchimol

http://dx.doi.org/10.5772/intechopen.70243

#### **Abstract**

*Giardia intestinalis* is a pathogenic protozoan, which is the causative agent of giardiasis. The *Giardia* trophozoite presents a cytoskeleton formed by specialized microtubular structures such as the ventral disk, four pairs of flagella, the median body, and the *funis* that are involved in cell division and differentiation. Because trophozoite motility and adhesion to the host intestinal cells are important processes mediated by the parasite cytoskeleton, the fine regulation of these elements may be directly related to the mechanisms that underlie infection. The organization of *Giardia* cytoskeleton at the ultrastructural level has been analyzed by different classical microscopy methods, including negative stain and chemical fixation for scanning and transmission electron microscopy. In this chapter, we provide an overview of the *G. intestinalis* cytoskeleton, emphasizing its structural organization and proteins involved in the maintenance of the structures as well as their functional role. These structures have been recently analyzed in some detail using techniques such as electron microscopy tomography, cryoelectron microscopy, ultra-high resolution scanning electron microscopy (UHRSEM), and helium ion microscopy (HIM). In addition, genome survey and phylogenetic analysis as well as proteomic analysis have revealed the presence of several new and not yet well-characterized proteins.

DOI: 10.5772/intechopen.70243

**Keywords:** *Giardia intestinalis*, cytoskeleton, ventral disk, flagella, median body, *funis*, microfilaments

## **1. Introduction**

*Giardia intestinalis* (also known as *Giardia lamblia* and *Giardia duodenalis*) is a flagellated protist described for the first time in 1681 by Antony Van Leeuwenhoek, after a discovery he made while examining his own feces using a primitive light microscope. This species is the causative agent of a parasitic disease known as giardiasis, an intestinal illness characterized by chronic diarrhea and undernutrition [1]. Giardiasis is a waterborne disease with a worldwide distribution [2].

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

Approximately, 200 million people are currently infected with *G. intestinalis.* The prevalence of giardiasis is higher in areas where sanitation conditions are inadequate. The illness mainly affects children and immunocompromised individuals. The life cycle of *G. intestinalis* comprises of two developmental stages: the trophozoite, in which it inhabits the host's small intestine, and the cyst, in which it is immobile and resistant to stress conditions of the environmental milieu [3]. A more detailed description of basic biological aspects of this protozoan is presented in another chapter of this book. Host infection begins after ingestion of cysts present in contaminated water or food. When a cyst is subjected to the acidic pH and gastric enzymes of the stomach, a reorganization of the cyst wall takes place initiating the encystment process. Each cyst will differentiate into two trophozoites [4]. This process ends in the duodenum through the proteolytic action of pancreatic enzymes (specifically, chymotrypsin and trypsin) and alkalinization on the cyst wall [5]. When released into the small intestine, the trophozoites penetrate the intestinal mucus layer and attach to the epithelium of the duodenum and the upper jejunum. After division by binary fission, they form a monolayer that covers the entire intestinal surface. Some researchers have suggested that the physical attachment of *Giardia* trophozoites to the host intestinal epithelium may contribute to structural and functional changes in the host intestinal cells [6, 7]. Analysis of the *Giardia*-host cell interactions *in vitro* shows that this parasite is responsible for an increase in intestinal permeability due to the rearrangement of proteins of the tight adherens and desmosomal junctions [8–10]. Here, we will focus on the cytoskeleton of the trophozoite, which presents a half-pear shape with a bilateral symmetry and exhibits several unusual cytoskeleton structures such as the ventral disk, the median body, the *funis*, and the lateral crest, in addition to four pairs of flagella (**Figure 1a–c**).

**Figure 1.** A general view of *G. intestinalis* trophozoites by light and electron microscopy. (a) Dorsal side of the trophozoite as observed by differential interference contrast (DIC). The two nuclei (N) are observed in the anterior region of the cell. (b) Scanning electron microscopy of the ventral side of the trophozoites. Note that the parasite displays the pairs of flagella (anterior flagella—A, posterior flagella—P, ventral flagella—V, caudal flagella—C), the ventral disk (D), and the ventro-lateral flange. (c) Routine preparation for transmission electron microscopy (TEM) of the trophozoite showing the ventral disk (D), the two nuclei (N), peripheral vesicles (V), flagellar axonemes (A) and funis (arrows) [78]. Bars = 1 μm.

The maintenance and establishment of cell shape are fundamental roles of the cytoskeleton. Since the classic work by Elmendorf et al. [11], the cytoskeleton of *Giardia* has been considered to play an essential role in the development and maintenance of the infection, mainly because its main component, the ventral disk, is indispensable for the attachment of the protozoan to the intestinal epithelial cells.
