Phospholipid Scramblase 1 Modulates FcR-Mediated Phagocytosis in Differentiated Macrophages

Abstract

Phospholipid Scramblase 1 (PLSCR1) was initially characterized as a type II transmembrane protein involved in bilayer movements of phospholipids across the plasma membrane leading to the cell surface exposure of phosphatidylserine, but other cellular functions have been ascribed to this protein in signaling processes and in the nucleus. In the present study, expression and functions of PLSCR1 were explored in specialized phagocytic cells of the monocyte/macrophage lineage. The expression of PLSCR1 was found to be markedly increased in monocyte-derived macrophages compared to undifferentiated primary monocytes. Surprisingly, this 3-fold increase in PLSCR1 expression correlated with an apparent modification in the membrane topology of the protein at the cell surface of differentiated macrophages. While depletion of PLSCR1 in the monocytic THP-1 cell-line with specific shRNA did not inhibit the constitutive cell surface exposure of phosphatidylserine observed in differentiated macrophages, a net increase in the FcR-mediated phagocytic activity was measured in PLSCR1-depleted THP-1 cells and in bone marrow-derived macrophages from PLSCR1 knock-out mice. Reciprocally, phagocytosis was down-regulated in cells overexpressing PLSCR1. Since endogenous PLSCR1 was recruited both in phagocytic cups and in phagosomes, our results reveal a specific role for induced PLSCR1 expression in the modulation of the phagocytic process in differentiated macrophages.

Dynamic Lipid-dependent Modulation of Protein Topology by Post-translational Phosphorylation

“…Phosphorylation events introduce a new topogenic signal within EMDs that could result in different structural organization of a protein within the changing lipid environment along the organelle trafficking pathway. These findings have crucial implications in membrane protein trafficking and proper targeting, as postulated for human CD38 (1) and phospholipid scramblase 1 (30), and could improve membrane protein topology prediction tools.…”

“…A phosphorylation-dephosphorylation cycle was suggested to control the orientation of the single TMD-spanning eukaryotic protein CD38 (1,29), suggesting a new topology-dependent mechanism regulating the signaling activity of CD38. Phospholipid scramblase 1 (PLSC1), which is also phosphorylated, undergoes topological changes during cell differentiation (30). However, no direct evidence linking post-translational phosphorylation to TMD orientation has been reported.…”

Flagging fusion: Phosphatidylserine signaling in cell–cell fusion

“…Early reconstitution of the scrambling activity of PLSCR in liposomes ( 81 ) suggested that this protein may directly mediate lipid redistribution between membrane leaflets; however, the timescales of scrambling observed in these experiments were nearly 20 times slower than the scrambling observed in native membranes. Moreover, normal levels of Ca 2+ -induced phospholipid scrambling are observed in mice deficient in PLSCR ( 81 ), in cells with suppressed PLSCR ( 83 ) and in flies lacking both of the PLSCR homologs in Drosophila ( 84 ). While these and other findings argue against the essential role of PLSCR as a PLSase (reviewed in ( 69 , 81 ), it remains possible that PLSCR deficiencies can be functionally compensated for by other types of scramblases or that PLSCR functions as a component of a protein complex that retains some scrambling activity in the absence of PLSCR.…”

Fungal Ice2p is in the same superfamily as SERINCs, restriction factors for HIV and other viruses

“…Therefore, the loop can only access lipid droplets in vivo if two or more TMHs change topology reversibly. Such changes have been reported for bacterial permeases and transporters, and they have been predicted in eukaryotes (Dowhan et al, 2019), with a few examples reported, though not yet studied in detail (Herate et al, 2016;Zhang et al, 2014).…”