Scramblases Regulate Critical Cellular Events
The kinetics by which flippases/floppases and scramblases function is vastly different. Due to the active movement of phospholipids against an equilibrium gradient the necessary energy requirements and conformational changes results in a slower movement of phospholipids by flippases/floppases relative to scramblases (reviewed in ). Scramblases allow for the rapid change in phospholipids across its channels that can quickly abolish asymmetry. These types of drastic changes are necessary during critical cellular events. For example, Suzuki et al. identified that the Xkr8 scramblase plays a critical role in apoptosis, a form of highly regulated programmed cell death . When Xkr8 is activated through caspase cleavage it results in the rapid translocation of PS to the outer leaflet that functions as an “eat me” signal by macrophages. This event is a hallmark of apoptosis and is important for programmed death as engulfment by macrophages minimizes the release of toxic cytoplasmic substances that would be harmful to neighboring cells. Xkr8 was recently shown to be activated in a caspase-independent mechanism that utilizes phosphorylation and targets critical sites near the caspase cleavage site on the protein . Another well-known role for scramblases is their effect on blood coagulation . During secondary hemostasis, a scramblase known as transmembrane protein 16F (TMEM16F) promotes loss of lipid asymmetry through PS exposure on the outer leaflet. Abundant PS exposure on the outer leaflet on platelets recruits clotting factors that lead to the activation of thrombin and the formation of an insoluble fibrin clot . More recently, TMEM16F was shown to promote PS exposure during trophoblast fusion in a calcium-dependent fashion, and if the PS signal was suppressed, cell fusion was blocked . Knockout of TMEM16F using CRISPR-Cas9 methods completely blocked trophoblast fusion, which was rescued with the reintroduction of TMEM16F. Other TEMEM scramblases have been identified in several genetic diseases, neurological disorders, and even cancer (reviewed in ).
Summary and Future Directions
Asymmetric composition of phospholipids in the cell membrane is important for normal physiologic function, and acute changes by scramblases are observed in critical cellular events. In another example, a recent study showed how the SARS-CoV-2 promotes an increase in Ca2+ levels, activation of TMEM16F (ANO6), exposure of PS, and enhanced fusion of the viral and cell membranes . The group identified a compound A6-001 with high potency and selectivity to TMEM16F, and treatment with the compound blunted SARS-CoV-2 replication. Furthermore, novel scramblases continue to be identified, such as, the protein CLPTM1L that functions to translocate glucosaminylphatidylinositols from the cytosol to the lumen of ER and is critical for biosynthesis of GPIs which function as membrane anchors . Clearly, there is much to learn about this critical family of lipid regulating proteins, their impact on membrane composition, and how they affect critical cellular processes. Cytoskeleton has an array of live cell membrane probes to aide in cell membrane investigation; such as, the novel Flipper-TR membrane tension probes, the MemGlow polarity probes, and the fluorogenic MemGlow membrane labeling probes.
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