For several years, my group has been studying the molecular mechanisms of Giardia lamblia adaptation and differentiation. Giardia is a flagellated protozoan that inhabits the upper small intestine of its vertebrate host and is the most common cause of defined waterborne diarrhea worldwide. Clinical manifestations of giardiasis vary from asymptomatic infection to acute or chronic disease associated with diarrhea and malabsorption. Besides of its medical importance, Giardia also is of great biological interest because it derives from one of the earliest branches of the eukaryotic lineage. Giardia trophozoites undergo fundamental biological changes to survive in aggressive environments. To survive outside the host's intestine, Giardia differentiates into a cyst, which is released with the feces and is responsible for transmitting the disease. Encystation entails the synthesis of cyst-wall components and the biogenesis of secretory organelles not present in nonencysting cells, such as Golgi apparatus and encystation-specific secretory vesicles (ESVs); ESVs transport the constituents necessary for assembling the extracellular cyst wall.
My research has focused on different aspects of Giardia differentiation. Recently, we discovered the environmental stimulus that triggers encystation, identified and characterized molecules that are differentially expressed during this process, studied the biogenesis of secretory organelles and protein trafficking during encystation, and gained insights into the basic biochemistry and cell biology of Giardia. Our current project addresses several important questions related to cyst-wall formation in Giardia, such as: (1) how these parasites sense the stimulus for differentiation and how specific gene expression takes place; (2) the composition of the extracellular cell wall; (3) how these materials are synthesized, transported, and released to the cell exterior in an organism that lacks fundamental protein transport organelles; and (4) how the wall components assemble into the extracellular superstructure. These questions share a common goal, which is to understand the molecular mechanisms of encystation in this primitive eukaryote and to use this knowledge to develop tools to block cyst wall formation and, consequently, transmission of infection among susceptible hosts.
We recently began to study another adaptive response developed by this parasite to survive within the host intestine: antigenic variation. Such clonal phenotypic variation of surface-exposed antigenic determinants is a major evasion mechanism that several pathogenic microorganisms have developed to maintain chronic infections under the continuous immune pressure generated by their hosts. Giardia undergoes antigenic variation by mechanisms that are unknown. In Giardia, antigenic variation accounts for the variable and often persistent course of some infections as well as the propensity for multiple reinfections and involves variant-specific surface proteins (VSPs); VSPs cover the surface of the trophozoites and are the major antigens recognized by the host immune system. The proteins vary in size from 20 to 200 kDa and possess a variable amino-terminal cysteine-rich region and a conserved carboxy-terminal region that includes a hydrophobic transmembrane region and a short cytosolic tail. There is a repertoire of about 150 VSP genes (vsps) in the parasite's genome, but only one VSP is expressed at any given time in each trophozoite. Once every 6–13 generations, even in the absence of an immune response, expression switches to a different VSP. Our work in this regard attempts to obtain insights into the molecular mechanisms involved in controlling the expression of these surface antigens in Giardia. We recognize that, if this defense mechanism could be blocked, the host would be able to develop an immune response against the full repertoire of VSPs and rapidly clear the infection caused by this parasite.
In summary, understanding protein trafficking and antigenic variation in Giardia not only is interesting from the point of view of cell biology, but knowledge of the genetic and biochemical basis of these mechanisms might also lead to the design of new chemotherapeutic agents, diagnostic tests, and vaccines. Results of our investigations might be applicable to other parasitic organisms and could provide information about mechanisms of intracellular transport and gene regulation in general.
Last updated September 2008