Mechanosensation and lipid scrambling news

In this issue of Blood, Liang et al, in a series of meticulous experiments, reveal a previously unrecognized connection between PIEZO1, a mechanically activated cation channel, and TMEM16F (also known as ANO6), a calcium-dependent phospholipid scramblase, in red blood cells. Their work not only demonstrates the role of TMEM16F in exposing phosphatidylserine (PS) on the extracellular membrane leaflet of red blood cells but also provides a crucial link between this exposure and the activation of PIEZO1.¹ 

The asymmetrical distribution of phospholipids is essential for maintaining the integrity of plasma membranes. Whereas P4-ATPases help to establish the phospholipid asymmetry, scramblases disrupt this asymmetry by exposing PS to the outer leaflet.² The exposure of PS is a signal for various biological processes. It is a well-known indicator of red blood cell senescence, marking these cells for phagocytosis by macrophages and subsequent clearance, a process contributing to the turnover of aging red blood cells.^3,4 Various conditions can lead to PS exposure independent of senescence, such as oxidative stress, blood storage, or inflammation, for example. Increased levels of red blood cells with PS exposure adversely affects blood rheology, promoting coagulation and enhancing adhesion to endothelial cells. Although previous characterization of TMEM16F activity and mutations in genetic diseases suggested that TMEM16F may play the role of a scramblase,⁵ its specific function and regulation in red blood cells were not well understood. The researchers diligently investigated the functional coupling between PIEZO1, a primary pathway for calcium uptake in red blood cells, and TMEM16F. Their comprehensive approach, incorporating studies on both human and mouse erythrocytes as well as expression experiments in HEK293T cells, convincingly establishes TMEM16F as the major Ca²⁺-dependent phospholipid scramblase in red blood cells. Moreover, they demonstrate that PIEZO1-mediated Ca²⁺ uptake activates TMEM16F, leading to PS exposure.

The study also explores the realm of clinical implications, particularly focusing on hereditary xerocytosis (HX), a rare autosomal dominant hemolytic anemia associated with mutations in PIEZO1.⁶ Patients with HX experience chronic hemolysis, with their red blood cells exhibiting increased cation permeability, dehydration, and higher susceptibility to lysis. Notably, splenectomized patients with hereditary xerocytosis show an increased risk of thrombosis, suggesting elevated red blood cell adherence properties.⁷ The authors postulate that gain-of-function mutations in PIEZO1 linked to HX may be responsible for hyperstimulating TMEM16F, thereby increasing PS exposure. This hypothesis was supported by their discovery of a novel PIEZO1 inhibitor, benzbromarone, a uricosuric drug used in gout treatment. Importantly, decoupling PIEZO1-TMEM16F prevented PS exposure in red blood cells from patients with HX, suggesting a direct link between PIEZO1 overactivity and loss of lipid asymmetry.

The identification of the TMEM16F-PIEZO1 interaction is interesting for red blood cell physiology and pathology. It provides crucial insight into the maintenance of membrane integrity, which is of major importance for blood storage. Furthermore, it enhances our comprehension of the molecular basis of red blood cell senescence, and it has direct implications for understanding and potentially treating hemolytic disorders. In the context of HX, it sheds light on potential factors contributing to thrombosis risk in these patients, providing avenues for therapeutic interventions. Notably, benzbromarone, a commercially available drug, appears to be a promising treatment to prevent hemolysis due to hyperactivity of PIEZO1 in case of HX. This drug, approved in countries other than the United States, had been used for decades to treat gout. In 2003, it was widely withdrawn from the market by its manufacturer after report of hepatotoxicity. However, there is a controversy about its risk-benefit balance, and it is still marketed in several countries (Brazil, Taiwan, Japan, some European countries). Given its action on PIEZO1, it would be worth exploring drug repurposing.

The TMEM16F-PIEZO1 coupling raises questions about the consequences of lipid scrambling on PIEZO1 activity. The interaction between PIEZO1 and lipids represents a finely tuned and active regulatory mechanism that governs the ion channel's functionality.^8,9 This interaction is not only structural but also functional, actively modulating the behavior of the ion channel. The alterations in membrane lipid distribution induced by the activity of TMEM16F present a potential way for modifying the functionality of PIEZO1. The dynamic nature of membrane lipids, influenced by TMEM16F, may act as a signaling cascade that communicates changes to PIEZO1, modulating its activity in response to Ca²⁺ changes. There are different pathways capable of increasing Ca²⁺ concentration in red blood cells,¹⁰ the activation of which would be expected to stimulate TMEM16F. Whether this could impair PIEZO1 functioning is an open question.

In summary, the intricate interplay between PIEZO1, TMEM16F, and membrane lipids introduces new opportunities to better characterize red blood cell physiology as well as treat clinical conditions like xerocytosis.

Conflict-of-interest disclosure: The author declares no competing financial interests.