Galectin-1 Induces Reversible Phosphatidylserine Exposure at the Plasma Membrane

Abstract

Cells normally undergo physiological turnover through the induction of apoptosis and phagocytic removal, partly through exposure of cell surface phosphatidylserine (PS). In contrast, neutrophils appear to possess apoptosis-independent mechanisms of removal. Here we show that Galectin-1 (Gal-1) induces PS exposure independent of alterations in mitochondrial potential, caspase activation, or cell death. Furthermore, Gal-1-induced PS exposure reverts after Gal-1 removal without altering cell viability. Gal-1-induced PS exposure is uniquely microdomain restricted, yet cells exposing PS do not display evident alterations in membrane morphology nor do they exhibit bleb formation, typically seen in apoptotic cells. Long-term exposure to Gal-1 prolongs PS exposure with no alteration in cell cycle progression or cell growth. These results demonstrate that Gal-1-induced PS exposure and subsequent phagocytic removal of living cells represents a new paradigm in cellular turnover. INTRODUCTIONCellular turnover represents one of the most fundamental homeostatic processes of multicellular organisms. Although many tissues experience cellular division and removal, cells of the immune system possess a unique capacity to rapidly proliferate in response to pathogenic challenge (Kaech and Ahmed, 2001). Significant expansion of leukocytes involved in both innate and adaptive immunity ultimately results in neutralization and removal of invading pathogens (Nathan, 2006). However, for effective immunological homeostasis to be maintained, efficient contraction of activated leukocyte populations must occur (Antia et al., 2005). Failure to appropriately eliminate activated leukocytes not only enhances the probability of cellular transformation, but also results in leukocyte-mediated damage of viable tissue and can eventually result in autoimmunity (Strasser et al., 2000;Danial and Korsmeyer, 2004).Many factors regulate leukocyte turnover, including members of the tumor necrosis factor (TNF) and galectin families (Liu and Rabinovich, 2005;Toscano et al., 2007).TNF family members, including Fas, TRAIL, and TNF-␣ effect leukocyte contraction through the induction of apoptotic cell death. Similarly, several galectin family members, including galectin-3 and -9 (Fukumori et al., 2003;Zhu et al., 2005; Gal-3 and -9), also induce leukocyte removal through apoptosis (Strasser, 2005;Zhu et al., 2005;Stowell et al., 2008c). Cells undergoing apoptotic cell death typically express phosphatidylserine (PS), a phospholipid normally confined to the inner leaflet of the plasma membrane, which serves as a ligand for receptor-mediated phagocytosis . Apoptotic cell death also results in DNA degradation and eventual cellular fragmentation (Jacobson et al., 1997;Stroh and Schulze-Osthoff, 1998). Importantly, apoptosis occurs in a coordinated manner, ultimately resulting in homeostatic cellular removal without inciting the deleterious consequences of an inflammatory response (Jacobson et al., 1997).In contrast to Gal-3, Gal-9, and members of the TNF family, sever...

Redundant and Antagonistic Functions of Galectin-1, -3, and -8 in the Elicitation of T Cell Responses

“…In contrast to our observations and those from other investigators, Matarrese et al (33) observed that Gal-1 sensitized human resting T lymphocytes to Fas-mediated cell death and that, at high doses, it was capable of inducing apoptosis in these primary cells. Discrepancies might reflect the different species used (human versus mouse), different protein preparations, or the method used to assess cell death, because phosphati-dylserine exposure is not necessarily associated with Gal-induced apoptosis (34).…”

Anionic Phospholipid Interactions of the Prion Protein N Terminus Are Minimally Perturbing and Not Driven Solely by the Octapeptide Repeat Domain

“…As the protective response evolves, perhaps as a requisite for eliciting specific signaling cascades (59), PrP C moves from lipid raft domains into the early endosome compartment. Simultaneously, the ATP-dependent enzyme aminophospholipid transferase, responsible for maintaining the normal membrane asymmetry of the lipid phosphatidylserine (60), is inhibited by lipid peroxidation (61), leading to increased expression of phosphatidylserine in the external leaflet of the plasmalemma, which is generally envisaged as an early marker of apoptosis (62), although other functional implications have been suggested (63,64). Surface expression of phosphatidylserine is not in itself sufficient to inevitably cause a cell to enter apoptosis (64), and surface expression of this moiety is reversible (65) and nonhomogeneous, with regions showing disproportionately higher concentrations (47), perhaps reaching the ratios used in our study.…”

“…Simultaneously, the ATP-dependent enzyme aminophospholipid transferase, responsible for maintaining the normal membrane asymmetry of the lipid phosphatidylserine (60), is inhibited by lipid peroxidation (61), leading to increased expression of phosphatidylserine in the external leaflet of the plasmalemma, which is generally envisaged as an early marker of apoptosis (62), although other functional implications have been suggested (63,64). Surface expression of phosphatidylserine is not in itself sufficient to inevitably cause a cell to enter apoptosis (64), and surface expression of this moiety is reversible (65) and nonhomogeneous, with regions showing disproportionately higher concentrations (47), perhaps reaching the ratios used in our study. Once trafficked, the acidic pH of the early endosome would favor the loss of any histidine-bound copper associated with PrP C and concomitantly provide a milieu more favorable to the N-terminal binding to recently externalized phosphatidylserine, with this organelle shown to be enriched in this lipid and thereby offering a potentially important cationic protein sorting capacity (47).…”

Antigen Modulation Confers Protection to Red Blood Cells from Antibody through Fcγ Receptor Ligation

“…When tolerance mechanisms fail, the cellular targets of immunity themselves can provide an additional layer of protection through resistance to effector mechanisms. Such cellular protection often reflects cell surface expression or release of soluble target cellderived factors that specifically modulate autoreactive cells (5)(6)(7), reducing the level of direct cell-mediated injury. Alternatively, some cell types have intrinsic resistance to the toxic effects of selfeffector molecules that otherwise potently destroy many pathogens (8).…”