G protein signaling in the parasite Entamoeba histolytica

Abstract

The parasite Entamoeba histolytica causes amebic colitis and systemic amebiasis. Among the known amebic factors contributing to pathogenesis are signaling pathways involving heterotrimeric and Ras superfamily G proteins. Here, we review the current knowledge of the roles of heterotrimeric G protein subunits, Ras, Rho and Rab GTPase families in E. histolytica pathogenesis, as well as of their downstream signaling effectors and nucleotide cycle regulators. Heterotrimeric G protein signaling likely modulates amebic motility and attachment to and killing of host cells, in part through activation of an RGS-RhoGEF (regulator of G protein signaling-Rho guanine nucleotide exchange factor) effector. Rho family GTPases, as well as RhoGEFs and Rho effectors (formins and p21-activated kinases) regulate the dynamic actin cytoskeleton of E. histolytica and associated pathogenesis-related cellular processes, such as migration, invasion, phagocytosis and evasion of the host immune response by surface receptor capping. A remarkably large family of 91 Rab GTPases has multiple roles in a complex amebic vesicular trafficking system required for phagocytosis and pinocytosis and secretion of known virulence factors, such as amebapores and cysteine proteases. Although much remains to be discovered, recent studies of G protein signaling in E. histolytica have enhanced our understanding of parasitic pathogenesis and have also highlighted possible targets for pharmacological manipulation.

Figure 1

Nucleotide cycle regulation of heterotrimeric and Ras superfamily G proteins. (a) Gα subunits cycle between guanosine diphosphate (GDP)- and guanosine triphosphate (GTP)-bound states. G protein-coupled receptors (GPCRs) serve as guanine nucleotide exchange factors (GEFs) for G protein heterotrimers, stimulating their release of GDP. Conversely, GoLoco motifs are guanine nucleotide dissociation inhibitors (GDIs) that slow nucleotide exchange by Gα subunits. RGS proteins are GTPase-accelerating proteins (GAPs) for Gα subunits, promoting signal termination by both activated Gα subunits and free Gβγ. (b) The small G protein nucleotide cycle parallels that of heterotrimeric G proteins, with GEF-stimulated and GDI-inhibited nucleotide exchange as well as GAP-mediated activation of GTP hydrolysis. RGS, regulators of G protein signaling.

Figure 2

Model of heterotrimeric G protein signaling in E. histolytica. Activated EhGα1, together with guanosine triphosphate (GTP)-bound EhRacC, engages the autoinhibited EhRGS-RhoGEF (E. histolytica regulator of G protein signaling–Rho guanine nucleotide exchange factor) to promote Rac GTPase in Drosophila S2 cells, although no specific E. histolytica Rho family substrate for EhRGS-RhoGEF has yet been identified. Both EhGα1 and EhRGS-RhoGEF alter trophozoite migration, host cell attachment and cell killing by altered cysteine protease secretion.^, An associated G protein-coupled receptor (GPCR) is postulated but not yet established within this signaling pathway; Despite its name, the protein EhGPCR-1 is more likely a Wnt-binding factor than a ligand-activated heterotrimeric G protein GEF. DH, Dbl homology; GDP, guanosine diphosphate; PH, pleckstrin homology.

Figure 3

EhRho1 and EhRacA signaling modulate pathogenic behaviors in E. histolytica. (a) Nucleotide exchange on EhRho1 is known to be catalyzed by E. histolytica guanine nucleotide exchange factor 1 (EhGEF1) in vitro. EhRho1 engages the GTPase-binding domain–formin homology 3 (GBD-FH3) domain tandem of the diaphanous-related and autoinhibited EhFormin1 to modulate actin polymerization. EhFormin1 has also been implicated in trophozoite proliferation and cytokinesis. (b) EhRacA nucleotide exchange is known to be accelerated by EhGEF3 in vitro. Constitutively active EhRacA perturbs phagocytosis and chemotaxis, as well as surface receptor capping in trophozoites. EhRacA·guanosine triphosphate (GTP) was also shown to bind E. histolytica p21-activated kinase 2 (EhPAK2), a likely effector whose kinase domain is implicated in collagen matrix invasion, cytokinesis and surface receptor capping. Stimulation of EhPAK2 kinase activity by EhRacA is postulated but has not yet been established. DH, Dbl homology; GDP, guanosine diphosphate; PH, pleckstrin homology.