EncystR: A novel entry point for uncovering the regulation of encystation in Giardia

Project Summary Cyst formation is ubiquitous across the diversity of protists, yet its regulation is poorly understood in most species. Major morphological changes occur as the parasite Giardia transitions from proliferative trophozoites to infectious cysts. Importantly, these changes cause the parasites to release from the host intestine, thus this

process could be targeted to clear infections. Despite its importance, very little is understood about the signaling events that trigger and sustain Giardia encystation. In an exciting breakthrough, we identified EncystR, a negative regulator of encystation, which represents a novel control point in this developmental

program. EncystR is potentially the first encystation receptor identified for any parasite; how it connects with downstream regulation of encystation is unresolved. It has been shown that related proteins can function as sensors, protein trafficking receptors and solute transporters. EncystR localizes to the plasma membrane in

vegetative trophozoites, and, upon encystation stimuli, EncystR is internalized. By following EncystR trafficking we identified a novel acidic compartment. This stage induced compartment is marked by ESCRT components typically involved in multivesicular body (MVB) formation. Giardia is thought to lack MVBs and conventional

lysosomes, so this discovery was a surprise and represents another exciting therapeutic opportunity. EncystR is the furthest upstream regulator of encystation identified to date; therefore, uncovering its biology will lead to a deeper understanding of the regulation of encystation. Whether EncystR is a GPCR-like receptor that

changes conformation upon ligand binding to recruit effector proteins, has a role in solute transport, a role in trafficking proteins to the novel compartment, or some combination remains unknown. Here, we focus on uncovering the mechanistic basis of EncystR’s role in regulating differentiation as well as the purpose of its

trafficking to the novel acidic compartment. As the most upstream regulator of encystation identified to date, EncystR is a new entry point for probing this critical, disease-relevant process. The proposed studies will: (1) reveal the EncystR interactome; (2) determine whether or not EncystR transports small molecule metabolites;

(3) define the spatial relationship between EncystR and the novel acidic compartment; and (4) uncover the fate of EncystR after reaching the acidic compartment. The outcomes of this study will facilitate the targeting of future mechanistic studies of EncystR and the delineation of the full regulatory cascade needed for

encystation.