Entamoeba histolytica is a protozoan responsible for several pathologies in humans. Trophozoites breach the intestinal site to enter the bloodstream and thus traverse to a secondary site. Macropinocytosis and phagocytosis, collectively accounting for heterophagy, are the two major processes responsible for sustenance of Entamoeba histolytica within the host. Both of these processes require significant rearrangements in the structure to entrap the target. Rho GTPases play an indispensable role in mustering proteins that regulate cytoskeletal remodelling. Unlike phagocytosis which has been studied in extensive detail, information on machinery of macropinocytosis in E. histolytica is still limited. In the current study, using site directed mutagenesis and RNAi based silencing, coupled with functional studies, we have demonstrated the involvement of EhRho5 in constitutive and LPA stimulated macropinocytosis. We also report that LPA, a bioactive phospholipid present in the bloodstream of the host, activates EhRho5 and translocates it from cytosol to plasma membrane and endomembrane compartments. Using biochemical and FRAP studies, we established that a PI Kinase acts upstream of EhRho5 in LPA mediated signalling. We further identified EhGEF2 as a guanine nucleotide exchange factor of EhRho5. In the amoebic trophozoites, EhGEF2 depletion leads to reduced macropinocytic efficiency of trophozoites, thus phenocopying its substrate. Upon LPA stimulation, EhGEF2 is found to sequester near the plasma membrane in a wortmannin sensitive fashion, explaining a possible mode for activation of EhRho5 in the amoebic trophozoites. Collectively, we propose that LPA stimulated macropinocytosis in E. histolytica is driven by the PI Kinase-EhGEF2-EhRho5 axis.

Entamoeba histolytica causes amoebiasis which is a major health concern in developing countries. E. histolytica pathogenicity has been implicated to a large repertoire of small GTPases which switch between the inactive GDP bound state and the active GTP bound state with the help of guanine nucleotide exchange factors (GEFs) and GTPase activating protein (GAPs). Rho family of small GTPases are well known to modulate the actin cytoskeletal dynamics which plays a major role in E. histolytica pathogenicity. Here, we report an atypical amoebic RhoGEF, and its preferred substrate EhRho6, which, upon overexpression abrogated the pathogenic behavior of the amoeba such as adhesion to host cell, monolayer destruction, erythrophagocytosis, and formation of actin dots. A causative immunoblot analysis revealed actin degradation in the EhRho6 overexpressing trophozoites that could be inhibited by blocking the amoebic proteasomal pathway. A careful analysis of the results from a previously published transcriptomics study, in conjunction with our observations, led to the identification of a clade of Rho GTPases in this pathogenic amoeba which we hypothesize to have implications during the amoebic encystation.

Entamoeba histolytica is a unicellular enteric protozoan responsible for variety of gastrointestinal pathologies. The pathogenicity of the parasite is attributed to its dynamic cytoskeleton. Rho family GTPases, a major class of molecular switches, play an indispensable role in controlling cytoskeleton remodelling. Rho GTPases are known to be activated by specific Guanine nucleotide Exchange Factors (GEF) upon stimulation by various extracellular stimuli to regulate signalling. We have established that EhRho5 is important for LPA induced micropinocytosis in the amoebic trophozoites. EhRho5 translocated from cytosol to plasma membrane and endomembranous compartments when stimulated with LPA, a bioactive phospholipid known to activate Rho subfamily members (1,2). Our findings were strengthened when FRAP (Fluorescence recovery after photobleaching) study showed that LPA treated cells show 30% faster recovery of Rho5 when compared to serum starved cells. The translocation is accompanied by activation of the GTPase as revealed by effector pull down assay(2). LPA stimulation enhanced the macropinocytic efficiency of trophozoites in a Rho5 dependent manner. We then identified the GEF for Rho5 which too, shows translocation upon stimulation with LPA. Also, depletion of the GEF leads to decreased pinocytic efficiency of the trophozoites. Using inhibitor based studies, we further demonstrated that PI3K acts as an intermediate cue for EhRho5 activation during LPA induced macropinocytosis. Moreoever, Rho5 expression showed contrary relationship with random migration of the trophozoites suggesting a pivotal role of the GTPase in mutally antagonistic cellular processes(3) in the amoeba. Reference: Fleming IN, Elliott CM, Exton JH. Differential Translocation of Rho Family GTPases by Lysophosphatidic Acid, Endothelin‐1, and Platelet‐derived Growth Factor, J. Biol. Chem. 1996, 271: 33067–33073 Van Leeuwen FN, Olivo C, Grivell S, Giepmans BNG, Collard JG, Moolenaar WH. Rac Activation by Lysophosphatidic Acid LPA1 Receptors through the Guanine Nucleotide Exchange Factor Tiam1, J. Biol. Chem. 2003, 278:400–406 Veltman DM, Lemieux MG, Knecht DA, Insall RH. PIP3‐dependent macropinocytosis is incompatible with chemotaxis. Journal of Cell Biology. 2014 Feb 17;204(4):497‐505.