Evidence that polyphenols do not inhibit the phospholipid scramblase TMEM16F

TMEM16 Ca²⁺-activated phospholipid scramblases (CaPLSases) mediate rapid transmembrane phospholipid flip-flop and as such play essential roles in various physiological and pathological processes such as blood coagulation, skeletal development, viral infection, cell-cell fusion, and ataxia. Pharmacological tools specifically targeting TMEM16 CaPLSases are urgently needed to understand these novel membrane transporters and their contributions to health and disease. Tannic acid (TA) and epigallocatechin gallate (EGCG) were recently reported as promising TMEM16F CaPLSase inhibitors. However, our present study shows that TA and EGCG do not inhibit the phospholipid-scrambling or ion conduction activities of the dual-functional TMEM16F. Instead, we found that TA and EGCG mainly acted as fluorescence quenchers that rapidly suppress the fluorophores conjugated to annexin V, a phosphatidylserine-binding probe commonly used to report on TMEM16 CaPLSase activity. These data demonstrate the false positive effects of TA and EGCG on inhibiting TMEM16F phospholipid scrambling and discourage the use of these polyphenols as CaPLSase inhibitors. Appropriate controls as well as a combination of both fluorescence imaging and electrophysiological validation are necessary in future endeavors to develop TMEM16 CaPLSase inhibitors.