Evidence of G-Protein-Coupled Receptors (GPCR) in the Parasitic Protozoa Plasmodium falciparum-Sensing the Host Environment and Coupling within Its Molecular Signaling Toolkit

Hijacking of GPCRs and RTKs by pathogens

Pathogens exploit multiple cellular and molecular pathways in the host organisms for their entry, survival and dissemination. The cell surface receptors such as G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) constitute the targets of many pathogens. This is due to the ubiquitous expression of these two receptor families in the organism and their pivotal role in various cellular and physiological processes. At the molecular level, receptor hijacking implies either direct or indirect interactions between pathogens' effectors or toxins with GPCRs and RTKs at the cell surface thereby interfering with their activation and their downstream signaling pathways inside the host cells. As a result, the pathogens manipulate and redirect GPCR/RTK-mediated signaling pathways and different aspects of cell function for their benefit. The review presents a compilation of the major examples of pathogen infections where GPCRs and RTKs and their related intracellular signaling pathways are targeted. This provides a molecular basis for pathogens hijacking cell signaling and their virulence. Our understanding of such complex host-pathogen interactions at the molecular level will open new opportunities to develop new prophylactic and therapeutic approaches against infections. In this context, the pharmacological targeting of GPCRs and RTKs may be a promising approach.

Molecular Characterization and Expression Analysis of Putative Class C (Glutamate Family) G Protein-Coupled Receptors in Ascidian Styela clava

In this study, we performed the genome-wide domain analysis and sequence alignment on the genome of Styela clava, and obtained a repertoire of 204 putative GPCRs, which exhibited a highly reduced gene number compared to vertebrates and cephalochordates. In this repertoire, six Class C GPCRs, including four metabotropic glutamate receptors (Sc-GRMs), one calcium-sensing receptor (Sc-CaSR), and one gamma-aminobutyric acid (GABA) type B receptor 2-like (Sc-GABABR2-like) were identified, with the absence of type 1 taste and vomeronasal receptors. All the Sc-GRMs and Sc-CaSR contained the typical "Venus flytrap" and cysteine-rich domains required for ligand binding and subsequent propagation of conformational changes. In swimming larvae, Sc-grm3 and Sc-casr were mainly expressed at the junction of the sensory vesicle and tail nerve cord while the transcripts of Sc-grm4, Sc-grm7a, and Sc-grm7b appeared at the anterior trunk, which suggested their important functions in neurotransmission. The high expression of these Class C receptors at tail-regression and metamorphic juvenile stages hinted at their potential involvement in regulating metamorphosis. In adults, the transcripts were highly expressed in several peripheral tissues, raising the possibility that S. clava Class C GPCRs might function as neurotransmission modulators peripherally after metamorphosis. Our study systematically characterized the ancestral chordate Class C GPCRs to provide insights into the origin and evolution of these receptors in chordates and their roles in regulating physiological and morphogenetic changes relevant to the development and environmental adaption.

G Protein-Coupled Receptors as Potential Intercellular Communication Mediators in Trypanosomatidae

Detection and transduction of environmental signals, constitute a prerequisite for successful parasite invasion; i.e., Leishmania transmission, survival, pathogenesis and disease manifestation and dissemination, with diverse molecules functioning as inter-cellular signaling ligands. Receptors [i.e., G protein-coupled receptors (GPCRs)] and their associated transduction mechanisms, well conserved through evolution, specialize in this function. However, canonical GPCR-related signal transduction systems have not been described in Leishmania, although orthologs, with reduced domains and function, have been identified in Trypanosomatidae. These inter-cellular communication means seem to be essential for multicellular and unicellular organism’s survival. GPCRs are flexible in their molecular architecture and may interact with the so-called receptor activity-modifying proteins (RAMPs), which modulate their function, changing GPCRs pharmacology, acting as chaperones and regulating signaling and/or trafficking in a receptor-dependent manner. In the skin, vasoactive- and neuro- peptides released in response to the noxious stimuli represented by the insect bite may trigger parasite physiological responses, for example, chemotaxis. For instance, in Leishmania (V.) braziliensis, sensory [Substance P, SP, chemoattractant] and autonomic [Vasoactive Intestinal Peptide, VIP, and Neuropeptide Y, NPY, chemorepellent] neuropeptides at physiological levels stimulate in vitro effects on parasite taxis. VIP and NPY chemotactic effects are impaired by their corresponding receptor antagonists, suggesting that the stimulated responses might be mediated by putative GPCRs (with essential conserved receptor domains); the effect of SP is blocked by [(D-Pro 2, D-Trp7,9]-Substance P (10^-6 M)] suggesting that it might be mediated by neurokinin-1 transmembrane receptors. Additionally, vasoactive molecules like Calcitonin Gene-Related Peptide [CGRP] and Adrenomedullin [AM], exert a chemorepellent effect and increase the expression of a 24 kDa band recognized in western blot analysis by (human-)-RAMP-2 antibodies. In-silico search oriented towards GPCRs-like receptors and signaling cascades detected a RAMP-2-aligned sequence corresponding to Leishmania folylpolyglutamate synthase and a RAMP-3 aligned protein, a hypothetical Leishmania protein with yet unknown function, suggesting that in Leishmania, CGRP and AM activities may be modulated by RAMP- (-2) and (-3) homologs. The possible presence of proteins and molecules potentially involved in GPCRs cascades, i.e., RAMPs, signpost conservation of ancient signaling systems associated with responses, fundamental for cell survival, (i.e., taxis and migration) and may constitute an open field for description of pharmacophores against Leishmania parasites.