How Giardia Swim and Divide

Abstract

To determine how binuclear giardia swim, we used video microscopy to observe trophozoites of Giardia intestinalis, which were labeled with an amino-specific Alexa Fluor dye that highlighted the flagella and adherence disc. Giardia swam forward by means of the synchronous beating of anterior, posterolateral, and ventral flagella in the plane of the ventral disc, while caudal flagella swam in a plane perpendicular to the disc. Giardia turned in the plane of the disc by means of a rudder-like motion of its tail, which was constant rather than beating. To determine how giardia divide, we used three-dimensional confocal microscopy, the same surface label, nuclear stains, and antitubulin antibodies. Giardia divided with mirror-image symmetry in the plane of the adherence disc, so that the right nucleus of the mother became the left nucleus of the daughter. Pairs of nuclei were tethered together by microtubules which surrounded nuclei and prevented mother or daughter giardia from receiving two copies of the same nucleus. New adherence discs formed upon a spiral backbone of microtubules, which had a clockwise rotation when viewed from the ventral surface. These dynamic observations of the parasite begin to reveal how giardia swim and divide.Giardia intestinalis (also known as Giardia lamblia), which was likely first visualized by von Leeuwenhoek, is a protist that causes intestinal malabsorption and diarrhea (3, 21). Although giardia cause an unattractive disease, they are among the most beautiful organisms, as shown by scanning and transmission microscopy (4, 7, 10). Trophozoites of G. intestinalis have two similar-appearing nuclei, which are both transcriptionally active (13). The giardia nuclei are bilaterally symmetric, as are four other microtubule-associated structures: the ventral adherence disc, four pairs of flagella, the median body, and the funis (2, 7, 10). The ventral disc, by which giardia adhere to the surface of intestinal epithelial cells, is composed of ␣-and ␤-tubulin and at least three different unique cytoskeletal proteins called giardins (2,12,17,18,20). Giardia have four pairs of flagella (anterior, posterolateral, ventral, and caudal), which are composed of microtubules in a 9-plus-2 arrangement (7, 10). All four pairs of flagella originate from basal bodies, composed of microtubule triplets, which are located between the two nuclei and are dorsal to the adherence disc. The funis is a set of single microtubules, which run parallel to the caudal flagella from the disc to the tip of the tail (2). The median body, which is a bundle of microtubules bound by a unique protein called the median body protein, is perpendicular to the funis and caudal to the adherence disc (15).Holberton (10) used phase microscopy to show that the ventral flagella of adherent giardia were constantly beating in a synchronized manner in the plane of the adherence disc. Electron micrographs of adhering giardia to mouse intestines suggested that parasites were drawing the intestinal villi up to the adherence disc. Holberton pr...

Three-dimensional analysis of mitosis and cytokinesis in the binucleate parasiteGiardia intestinalis

“…This model of cell division can explain prior studies on nuclear partitioning during cytokinesis, albeit with an alternative mode of cytokinesis than that originally proposed (Ghosh et al, 2001).…”

“…To determine the pattern of nuclear partitioning and inheritance, we used fluorescence in situ hybridization (FISH) to visualize an episomal plasmid in the cenH3::GFP strain, which is maintained by one nucleus in each Giardia trophozoite (Ghosh et al, 2001;Singer et al, 1998;Yu et al, 2002). In each cell, we observed the episomal plasmid FISH signal in only one nucleus (either the left or the right) as previously reported (Ghosh et al, 2001).…”

“…including the role of non-spindle microtubular organelles in cell division (Benchimol, 2004b;Solari et al, 2003) or of the presence of multiple or incongruent planes of division (Benchimol, 2004a;Ghosh et al, 2001;Yu et al, 2002).…”

Participation of the Adhesive Disc during Karyokinesis in Giardia lamblia

“…Previous reports on Giardia mitosis stated conclusions based only in light microscopic observations (Filice, 1952;Cerva and Nohýnk-ová, 1992;Kabnick and Peattie, 1990;Nohýnková et al, 2000;Ghosh et al, 2001, Yu et al, 2002. The following suggestions have been made: (1) mitosis in Giardia corresponds to a semi-open orthomitosis (Nohýnková et al, 2000); (2) during mitosis the nuclear envelope is perforated in a sieve-like manner in both nuclei (Nohýnková et al, 2000); (3) spindle microtubules enter the nucleus through tiny holes (Nohýnková et al, 2000); (4) pairs of nuclei are tethered together by microtubules which surround the nuclei (Ghosh et al, 2001). However, all these proposals were not supported by any electron microscopic observation.…”

“…Most ultrastructural studies have focused on the cytoskeleton and adhesive disc of Giardia (reviewed in Gillin et al, 1996;Adam, 2001). Some studies have shown that the disc is fragmented at some stages of the Giardia cell cycle, such as during cell division (Ghosh et al, 2001) and encystation (Sheffield and Bjorvatn, 1977). However, careful study of each step of the Giardia cell cycle has not been carried out.…”