Available online 30 March 2022

Blocking endocytosis triggers cell differentiation in Giardia trophozoites.

Inhibition of cholesterol entry is not sufficient to complete encystation.

Decreasing expression of the medium subunit Glµ2 produced immature cysts when induced to encyst.

GlAP2 is necessary to complete the final steps of encystation.

The protozoan parasite Giardia lamblia acquires cholesterol from the environment since it is unable to synthesize cholesterol de novo and this is vital for trophozoite growth. Conversely, the lack of cholesterol was described as an essential event to trigger encystation, the differentiation of trophozoites to mature cysts. During the G. lamblia cell cycle, cholesterol is acquired as a free molecule as well as through receptor-mediated endocytosis (RME) of lipoproteins. In this work, we describe the involvement of RME in the cell differentiation process of G. lamblia. We found that a reduction in the expression of the medium subunit (Glµ2) of the giardial adaptin protein GlAP2 impaired RME, triggering the process of encystation in growing cells. Contrary to expectations, decreasing Glµ2 expression produced a cohort of trophozoites that yielded significantly less mature cysts when cells were induced to encyst. Analysis of the subcellular localization of Glµ2 and the cyst wall protein 1 (CWP1) during encystation was later performed, to dissect the process. Our results showed, on one hand, that blocking RME by inhibiting Glµ2 expression, and probably cholesterol entry, is sufficient to induce cell differentiation but not to complete the process of encystation. On the other hand, we observed that GlAP2 is necessary to accomplish the final steps of encystation by sorting CWP1 to the plasma membrane for cyst wall formation. The understanding of the mechanisms involved in cyst formation should provide novel insights into the control of giardiasis, an endemic worldwide neglected disease.

Protozoan

Clathrin-adaptin protein 2

Cholesterol

Encystation

Endocytosis

LDL

© 2022 Australian Society for Parasitology. Published by Elsevier Ltd. All rights reserved.