The Endomembrane System of Giardia intestinalis

Abstract

Giardia intestinalis is a protozoan that colonizes the small intestine of virtually all mammals, adhering to the mucosal epithelial cells. It is a cosmopolitan parasite and agent of giardiasis, which can lead to human diarrheal diseases. The Giardia life cycle presents two forms-the trophozoite and the cyst-which are responsible for infection and transmission, respectively. This cell has been considered an excellent model for evolutionary studies, even though there are controversial hypotheses as to whether this parasite is an early eukaryote or not. G. intestinalis has a unique and very basic endomembrane system. The trophozoite gathers a very small pack of membrane-bounded structures: nuclei, endoplasmic reticulum (ER), peripheral vesicles (PV) and mitosomes. These organelles are involved in many functions from regulatory aspects in gene expression as well as membrane traffic events. Two functional nuclei are observed in the parasite; they are always located symmetrically in the anterior region of the trophozoite. The ER and PV commonly share and accumulate functions in the secretory pathway, they are responsible for endocytosis and digestion processes. The mitosome is a mitochondriarelated organelle that does not produce ATP and lacks several mitochondrial characteristics. During the parasite differentiation into cyst, different types of vesicles appear into the cell body: the encystation specific vesicles (ESVs) and the encystation carbohydratepositive vesicles (ECVs). These vesicles work together to form the parasite's cyst wall in order to ensure that the cell reaches the cyst stage. Interestingly, Giardia does not present a morphologically recognized Golgi apparatus. It has been claimed that during the encystation process, the ESVs could represent a Golgi-like structure, because this organelle presents some characteristics of that high eukaryotic Golgi apparatus. In this book chapter, we highlight the G. intestinalis endomembrane system, emphasizing their morphology, proteins involved in its organization as well as their functional role.