Identification of a Chemoattractant G-Protein-Coupled Receptor for Folic Acid that Controls Both Chemotaxis and Phagocytosis

Abstract

SUMMARY Eukaryotic phagocytes search and destroy invading microorganisms via chemotaxis and phagocytosis. The social amoeba Dictyostelium discoideum is a professional phagocyte that chases bacteria through chemotaxis and engulfs them as food via phagocytosis. G-protein-coupled receptors (GPCRs) are known for detecting chemoattractants and directing cell migration, but their roles in phagocytosis are not clear. Here, we developed a quantitative phosphoproteomic technique to discover signaling components. Using this approach, we discovered the long-sought-after folic acid receptor, fAR1, in D. discoideum. We showed that the seven transmembrane receptor fAR1 is required for folic acid-mediated signaling events. Significantly, we discovered that fAR1 is essential for both chemotaxis and phagocytosis of bacteria, thereby representing a chemoattractant GPCR that mediates not only chasing but also ingesting bacteria. We revealed that a phagocyte is able to internalize particles via chemoattractant-mediated engulfment process. We propose that mammalian phagocytes may also use this mechanism to engulf and ingest bacterial pathogens.

A Diaphanous -related formin links Ras signaling directly to actin assembly in macropinocytosis and phagocytosis

“…The initial steps of phagocytosis have been extensively studied and seem to be triggered by the recently identified folate receptor fAR1 (12) and its associated heterotrimeric G protein (13) to activate a subset of Ras proteins, which, in turn, leads to activation of their class-1 PI3K downstream effectors and production of PIP 3 (34,62). Due to a mutation of the RasGAP neurofibromin NF1, in axenic Dictyostelium strains, excessive Ras signaling appears to bypass the requirement for fAR1-receptor activation and makes macropinocytosis constitutive, allowing the cells to grow efficiently in liquid medium (10).…”

“…Due to its genetic tractability and ease of use, D. discoideum has become an attractive model organism that allows dissecting conserved mechanisms and signaling pathways of largescale endocytosis (11). Comparable to mammalian cells, the initial steps of the underlying signaling cascades are quite well understood and are initiated by G protein-coupled folate receptor fAR1 activation upstream of heterotrimeric G proteins (12,13). This step is followed by activation of small GTPases of the Ras family, which act as master regulators of diverse downstream signaling pathways, including target of rapamycin complex 2 (14) and class-I PI3-kinases (PI3Ks) (15), albeit Dictyostelium cells use ether-linked plasmanylinositides instead of phosphatidylinositides (16).…”

Acanthamoeba and Dictyostelium Use Different Foraging Strategies

“…Four cAMP receptors have been identified and two of these play a role in the cAMP chemotaxis involved with aggregated formation (Ginsburg et al 1995). Several other receptors have been genetically analyzed, including some close paralogs of the cAMP receptors, and recently a receptor responsible for folate chemotaxis has been identified (Anjard et al 2009; Pan et al 2016; Prabhu et al 2007; Raisley et al 2004). In regards to G proteins, folate responses require the Gα4 G protein subunit and cAMP responses require the Gα2 subunit (Hadwiger et al 1994; Hadwiger and Srinivasan 1999; Kumagai et al 1989).…”

Decoding the chemotactic signal

“…D. discoideum cARs signal through Gα2‐associated heterotrimeric G proteins, and may interact with arrestin‐domain containing proteins to modulate cAMP responses . To date, only 2 folate receptors have been identified in D. discoideum (fAR1 and fAR2), and only fAR1 has been shown to be required for folate‐induced signaling . Further, recent evidence suggests that the folate receptor fAR1 couples to Gα4‐associated heterotrimeric G proteins, rather than Gα2 …”