MECHANISMS OF GIARDIA-LAMBLIA DIFFERENTIATION INTO CYSTS

Microbiologists have long been intrigued by the ability of parasitic o rganisms to adapt to changes in the environment. Since most parasites occupy several niches during their journey between vectors and hosts, they have developed adaptive responses which allow them to survive und er adverse conditions. Therefore, the life cycles of protozoan and hel minthic parasites are excellent models with which to study numerous me chanisms involved in cell differentiation, such as the regulation of g ene expression, signal transduction pathways, and organelle biogenesis . Unfortunately, many of these studies ar-e very difficult because the conditions needed to elicit developmental changes in parasites remain undetermined in most cases. Recently, several interesting findings we re reported on the process of differentiation of Giardia lamblia troph ozoites into cysts. G. lamblia is a flagellated protozoan that inhabit s the upper small intestine of its vertebrate host and is a major caus e of enteric disease worldwide. It belongs to the earliest identified lineage among eukaryotes and therefore offers a unique insight into th e progression from primitive to more complex eukaryotic cells. The dis covery of a specific stimulus that induces trophozoites to differentia te into cysts, the identification and characterization of encystation- specific molecules, the elucidation of novel biochemical pathways, and the development of useful reagents and techniques have made this para site an excellent model with which to study differentiation in eukaryo tic cells. In this review, we summarize the most recent findings on se veral aspects of Giardia differentiation and discuss tile significance of these findings within the context of current knowledge in the fiel d.