Characterization of the scrambling domain of the TMEM16 family

In or out of the groove? Mechanisms of lipid scrambling by TMEM16 proteins

Phospholipid scramblases mediate the rapid movement of lipids between membrane leaflets, a key step in establishing and maintaining membrane homeostasis of the membranes of all eukaryotic cells and their organelles. Thus, impairment of lipid scrambling can lead to a variety of pathologies. How scramblases catalyzed the transbilayer movement of lipids remains poorly understood. Despite the availability of direct structural information on three unrelated families of scramblases, the TMEM16s, the Xkrs, and ATG-9, a unifying mechanism has failed to emerge thus far. Among these, the most extensively studied and best understood are the Ca2+ activated TMEM16s, which comprise ion channels and/or scramblases. Early work supported the view that these proteins provided a hydrophilic, membrane-exposed groove through which the lipid headgroups could permeate. However, structural, and functional experiments have since challenged this mechanism, leading to the proposal that the TMEM16s distort and thin the membrane near the groove to facilitate lipid scrambling. Here, we review our understanding of the structural and mechanistic underpinnings of lipid scrambling by the TMEM16s and discuss how the different proposals account for the various experimental observations.

The anoctamins: Structure and function

When activated by increase in intracellular Ca2+, anoctamins (TMEM16 proteins) operate as phospholipid scramblases and as ion channels. Anoctamin 1 (ANO1) is the Ca2+-activated epithelial anion-selective channel that is coexpressed together with the abundant scramblase ANO6 and additional intracellular anoctamins. In salivary and pancreatic glands, ANO1 is tightly packed in the apical membrane and secretes Cl-. Epithelia of airways and gut use cystic fibrosis transmembrane conductance regulator (CFTR) as an apical Cl- exit pathway while ANO1 supports Cl- secretion mainly by facilitating activation of luminal CFTR and basolateral K+ channels. Under healthy conditions ANO1 modulates intracellular Ca2+ signals by tethering the endoplasmic reticulum, and except of glands its direct secretory contribution as Cl- channel might be small, compared to CFTR. In the kidneys ANO1 supports proximal tubular acid secretion and protein reabsorption and probably helps to excrete HCO3-in the collecting duct epithelium. However, under pathological conditions as in polycystic kidney disease, ANO1 is strongly upregulated and may cause enhanced proliferation and cyst growth. Under pathological condition, ANO1 and ANO6 are upregulated and operate as secretory channel/phospholipid scramblases, partly by supporting Ca2+-dependent processes. Much less is known about the role of other epithelial anoctamins whose potential functions are discussed in this review.

VSI: The anoctamins: Structure and function: "Intracellular" anoctamins

Plasma membrane localized anoctamin 1, 2 and 6 (TMEM16A, B, F) have been examined in great detail with respect to structure and function, but much less is known about the other seven intracellular members of this exciting family of proteins. This is probably due to their limited accessibility in intracellular membranous compartments, such as the endoplasmic reticulum (ER) or endosomes. However, these so-called intracellular anoctamins are also found in the plasma membrane (PM) which adds to the confusion regarding their cellular role. Probably all intracellular anoctamins except of ANO8 operate as intracellular phospholipid (PL) scramblases, allowing for Ca2+-activated, passive transport of phospholipids like phosphatidylserine between both membrane leaflets. Probably all of them also conduct ions, which is probably part of their physiological function. In this brief overview, we summarize key findings on the biological functions of ANO3, 4, 5, 7, 8, 9 and 10 (TMEM16C, D, E, G, H, J, K) that are gradually coming to light. Compartmentalized regulation of intracellular Ca2+ signals, tethering of the ER to specific PM contact sites, and control of intracellular vesicular trafficking appear to be some of the functions of intracellular anoctamins, while loss of function and abnormal expression are the cause for various diseases.

Identification of determinants of lipid and ion transport in TMEM16/anoctamin proteins through a Bayesian statistical analysis

The TMEM16/Anoctamin protein family (TMEM16x) is composed of members with different functions; some members form Ca2+-activated chloride channels, while others are lipid scramblases or combine the two functions. TMEM16x proteins are typically activated in response to agonist-induced rises of intracellular Ca2+; thus, they couple Ca2+-signalling with cell electrical activity or plasmalemmal lipid homeostasis. The structural domains underlying these functions are not fully defined. We used a Naïve Bayes classifier to gain insights into these domains. The method enabled identification of regions involved in either ion or lipid transport, and suggested domains for possible pharmacological exploitation. The method allowed the prediction of the transport property of any given TMEM16x. We envisage this strategy could be exploited to illuminate the structure-function relationship of any protein family composed of members playing different molecular roles.

Subcellular distribution of membrane lipids revealed by freeze-fracture electron microscopy

Cell membranes are composed of a large variety of lipids and proteins. While the localization and function of membrane proteins have been extensively investigated, the distribution of membrane lipids, especially in the non-cytoplasmic leaflet of organelle membranes, remains largely unknown. Fluorescent biosensors have been widely used to study membrane lipid distribution; however, they have some limitations. By utilizing the quick-freezing and freeze-fracture replica labeling electron microscopy technique, we can uncover the precise distribution of membrane lipids within cells and assess the function of lipid-transporting proteins. In this review, I summarize recent progress in analyzing intracellular lipid distribution by utilizing this method.

Cell Fusion and Syncytia Formation in Cancer

The natural phenomenon of cell-cell fusion does not only take place in physiological processes, such as placentation, myogenesis, or osteoclastogenesis, but also in pathophysiological processes, such as cancer. More than a century ago postulated, today the hypothesis that the fusion of cancer cells with normal cells leads to the formation of cancer hybrid cells with altered properties is in scientific consensus. Some studies that have investigated the mechanisms and conditions for the fusion of cancer cells with other cells, as well as studies that have characterized the resulting cancer hybrid cells, are presented in this review. Hypoxia and the cytokine TNFα, for example, have been found to promote cell fusion. In addition, it has been found that both the protein Syncytin-1, which normally plays a role in placentation, and phosphatidylserine signaling on the cell membrane are involved in the fusion of cancer cells with other cells. In human cancer, cancer hybrid cells were detected not only in the primary tumor, but also in the circulation of patients as so-called circulating hybrid cells, where they often correlated with a worse outcome. Although some data are available, the questions of how and especially why cancer cells fuse with other cells are still not fully answered.

Transcriptomic profile of the mechanosensitive ion channelome in human cardiac fibroblasts

Human cardiac fibroblasts (HCFs) have mRNA transcripts that encode different mechanosensitive ion channels and channel regulatory proteins whose functions are not known yet. The primary goal of this work was to define the mechanosensitive ion channelome of HCFs. The most common type of cationic channel is the transient receptor potential (TRP) family, which is followed by the TWIK-related K+ channel (TREK), transmembrane protein 63 (TMEM63), and PIEZO channel (PIEZO) families. In the sodium-dependent NON-voltage-gated channel (SCNN) subfamily, only SCNN1D was shown to be highly expressed. Particular members of the acid-sensing ion channel (ASIC) (ASIC1 and ASIC3) subfamilies were also significantly expressed. The transcripts per kilobase million (TPMs) for Piezo 2 were almost 100 times less abundant than those for Piezo 1. The tandem of P domains in a weak inward rectifying K+ channel (TWIK)-2 channel, TWIK-related acid-sensitive K+ channel (TASK)-5, TASK-1, and the TWIK-related K1 (TREK-1) channel were the four most prevalent types in the K2P subfamily. The highest expression in the TRPP subfamily was found for PKD2 and PKD1, while in the TRPM subfamily, it was found for TRPM4, TRPM7, and TRPM3. TRPV2, TRPV4, TRPV3, and TRPV6 (all members of the TRPV subfamily) were also substantially expressed. A strong expression of the TRPC1, TRPC4, TRPC6, and TRPC2 channels and all members of the TRPML subfamily (MCOLN1, MCOLN2, and MCOLN3) was also shown. In terms of the transmembrane protein 16 (TMEM16) family, the HCFs demonstrated significant expression of the TMEM16H, TMEM16F, TMEM16J, TMEM16A, and TMEM16G channels. TMC3 is the most expressed channel in HCFs of all known members of the transmembrane channel-like protein (TMC) family. This analysis of the mechanosensitive ionic channel transcriptome in HCFs: (1) agrees with previously documented findings that all currently identified mechanosensitive channels play a significant and well recognized physiological function in elucidating the mechanosensitive characteristics of HCFs; (2) supports earlier preliminary reports that point to the most common expression of the TRP mechanosensitive family in HCFs; and (3) points to other new mechanosensitive channels (TRPC1, TRPC2, TWIK-2, TMEM16A, ASIC1, and ASIC3).

The role of lipid scramblases in regulating lipid distributions at cellular membranes

Glycerophospholipids, sphingolipids and cholesterol assemble into lipid bilayers that form the scaffold of cellular membranes, in which proteins are embedded. Membrane composition and membrane protein profiles differ between plasma and intracellular membranes and between the two leaflets of a membrane. Lipid distributions between two leaflets are mediated by lipid translocases, including flippases and scramblases. Flippases use ATP to catalyze the inward movement of specific lipids between leaflets. In contrast, bidirectional flip-flop movements of lipids across the membrane are mediated by scramblases in an ATP-independent manner. Scramblases have been implicated in disrupting the lipid asymmetry of the plasma membrane, protein glycosylation, autophagosome biogenesis, lipoprotein secretion, lipid droplet formation and communications between organelles. Although scramblases in plasma membranes were identified over 10 years ago, most progress about scramblases localized in intracellular membranes has been made in the last few years. Herein, we review the role of scramblases in regulating lipid distributions in cellular membranes, focusing primarily on intracellular membrane-localized scramblases.

Regulation of phospholipid distribution in the lipid bilayer by flippases and scramblases

Cellular membranes function as permeability barriers that separate cells from the external environment or partition cells into distinct compartments. These membranes are lipid bilayers composed of glycerophospholipids, sphingolipids and cholesterol, in which proteins are embedded. Glycerophospholipids and sphingolipids freely move laterally, whereas transverse movement between lipid bilayers is limited. Phospholipids are asymmetrically distributed between membrane leaflets but change their location in biological processes, serving as signalling molecules or enzyme activators. Designated proteins - flippases and scramblases - mediate this lipid movement between the bilayers. Flippases mediate the confined localization of specific phospholipids (phosphatidylserine (PtdSer) and phosphatidylethanolamine) to the cytoplasmic leaflet. Scramblases randomly scramble phospholipids between leaflets and facilitate the exposure of PtdSer on the cell surface, which serves as an important signalling molecule and as an 'eat me' signal for phagocytes. Defects in flippases and scramblases cause various human diseases. We herein review the recent research on the structure of flippases and scramblases and their physiological roles. Although still poorly understood, we address the mechanisms by which they translocate phospholipids between lipid bilayers and how defects cause human diseases.

A TMEM16J variant leads to dysregulated cytosolic calcium which may lead to renal disease

SIGIRR (single immunoglobulin IL-1 related receptor), PKP3 (plakophilin 3), and TMEM16J (anoctamin 9), a putative calcium-activated ion channel and phospholipid scramblase, control the immune response and the extent of inflammation. Variants of SIGIRR/PKP3/TMEM16J lead to severe inflammatory diseases such as pneumonia, enterocolitis, and kidney graft rejection. Meta-analysis of genome-wide association studies identified TMEM16J-T604A as a promotor for chronic kidney disease (CKD), but the disease mechanism and function of TMEM16J remain unknown. Here, we demonstrate TMEM16J as a calcium-activated calcium-permeable channel, which is expressed in the endoplasmic reticulum (ER). TMEM16J controls the intracellular distribution of calcium, and inhibits intracellular receptor-mediated Ca²⁺ signals and Ca²⁺ -dependent activation of ion channels, but augments transcription and release of pro-inflammatory cytokines. Renal epithelial cells expressing the variant TMEM16J-T604A show enhanced calcium signals when compared to cells expressing wt-TMEM16J, and demonstrate spontaneous transcription and release of cytokines. This study identifies TMEM16J as an important regulator of intracellular Ca²⁺ signals, ion channel activity, and cytokine release. TMEM16J may therefore affect immune response in renal tissue and immune cells.

The allosteric mechanism leading to an open-groove lipid conductive state of the TMEM16F scramblase

TMEM16F is a Ca²⁺-activated phospholipid scramblase in the TMEM16 family of membrane proteins. Unlike other TMEM16s exhibiting a membrane-exposed hydrophilic groove that serves as a translocation pathway for lipids, the experimentally determined structures of TMEM16F shows the groove in a closed conformation even under conditions of maximal scramblase activity. It is currently unknown if/how TMEM16F groove can open for lipid scrambling. Here we describe the analysis of ~400 µs all-atom molecular dynamics (MD) simulations of the TMEM16F revealing an allosteric mechanism leading to an open-groove, lipid scrambling competent state of the protein. The groove opens into a continuous hydrophilic conduit that is highly similar in structure to that seen in other activated scramblases. The allosteric pathway connects this opening to an observed destabilization of the Ca²⁺ ion bound at the distal site near the dimer interface, to the dynamics of specific protein regions that produces the open-groove state to scramble phospholipids.

Molecular dynamics simulations reveal the allosteric mechanism leading to an open, lipid scrambling competent state of a mammalian TMEM16F.

Role of Ion Channel Remodeling in Endothelial Dysfunction Induced by Pulmonary Arterial Hypertension

Endothelial dysfunction is a key player in advancing vascular pathology in pulmonary arterial hypertension (PAH), a disease essentially characterized by intense remodeling of the pulmonary vasculature, vasoconstriction, endothelial dysfunction, inflammation, oxidative stress, and thrombosis in situ. These vascular features culminate in an increase in pulmonary vascular resistance, subsequent right heart failure, and premature death. Over the past years, there has been a great development in our understanding of pulmonary endothelial biology related to the genetic and molecular mechanisms that modulate the endothelial response to direct or indirect injury and how their dysregulation can promote PAH pathogenesis. Ion channels are key regulators of vasoconstriction and proliferative/apoptotic phenotypes; however, they are poorly studied at the endothelial level. The current review will describe and categorize different expression, functions, regulation, and remodeling of endothelial ion channels (K+, Ca2+, Na+, and Cl− channels) in PAH. We will focus on the potential pathogenic role of ion channel deregulation in the onset and progression of endothelial dysfunction during the development of PAH and its potential therapeutic role.

Polymodal Control of TMEM16x Channels and Scramblases

The TMEM16A/anoctamin-1 calcium-activated chloride channel (CaCC) contributes to a range of vital functions, such as the control of vascular tone and epithelial ion transport. The channel is a founding member of a family of 10 proteins (TMEM16x) with varied functions; some members (i.e., TMEM16A and TMEM16B) serve as CaCCs, while others are lipid scramblases, combine channel and scramblase function, or perform additional cellular roles. TMEM16x proteins are typically activated by agonist-induced Ca2+ release evoked by Gq-protein-coupled receptor (GqPCR) activation; thus, TMEM16x proteins link Ca2+-signalling with cell electrical activity and/or lipid transport. Recent studies demonstrate that a range of other cellular factors—including plasmalemmal lipids, pH, hypoxia, ATP and auxiliary proteins—also control the activity of the TMEM16A channel and its paralogues, suggesting that the TMEM16x proteins are effectively polymodal sensors of cellular homeostasis. Here, we review the molecular pathophysiology, structural biology, and mechanisms of regulation of TMEM16x proteins by multiple cellular factors.

Influence of Anoctamin-4 and -9 on ADAM10 and ADAM17 Sheddase Function

Ca2+-activated Cl− channels (TMEM16, also known as anoctamins) perform important functions in cell physiology, including modulation of cell proliferation and cancer growth. Many members, including TMEM16F/ANO6, additionally act as Ca2+-activated phospholipid scramblases. We recently presented evidence that ANO6-dependent surface exposure of phosphatidylserine (PS) is pivotal for the disintegrin-like metalloproteases ADAM10 and ADAM17 to exert their sheddase function. Here, we compared the influence of seven ANO family members (ANO1, 4, 5, 6, 7, 9, and 10) on ADAM sheddase activity. Similar to ANO6, overexpression of ANO4 and ANO9 led to increased release of ADAM10 and ADAM17 substrates, such as betacellulin, TGFα, and amphiregulin (AREG), upon ionophore stimulation in HEK cells. Inhibitor experiments indicated that ANO4/ANO9-mediated enhancement of TGFα-cleavage broadened the spectrum of participating metalloproteinases. Annexin V-staining demonstrated increased externalisation of PS in ANO4/ANO9-overexpressing cells. Competition experiments with the soluble PS-headgroup phosphorylserine indicated that the ANO4/ANO9 effects were due to increased PS exposure. Overexpression of ANO4 or ANO9 in human cervical cancer cells (HeLa), enhanced constitutive shedding of the growth factor AREG and increased cell proliferation. We conclude that ANO4 and ANO9, by virtue of their scramblase activity, may play a role as important regulators of ADAM-dependent cellular functions.

Phospholipid scrambling by a TMEM16 homolog of Arabidopsis thaliana

Membrane asymmetry is important for cellular physiology and established by energy‐dependent unidirectional lipid translocases, which have diverse physiological functions in plants. By contrast, the role of phospholipid scrambling (PLS), the passive bidirectional lipid transfer leading to the break‐down of membrane asymmetry, is currently still unexplored. The Arabidopsis thaliana genome contains a single gene (At1g73020) with homology to the eukaryotic TMEM16 family of Ca²⁺‐activated phospholipid scramblases. Here, we investigated the protein function of this Arabidopsis homolog. Fluorescent AtTMEM16 fusions localized to the ER both in transiently expressing Arabidopsis protoplasts and HEK293 cells. A putative scrambling domain (SCRD) was identified on the basis of sequence conservation and conferred PLS to transfected HEK293 cells, when grafted into the backbone of the non‐scrambling plasma membrane‐localized TMEM16A chloride channel. Finally, AtTMEM16 ‘gain‐of‐function’ variants gave rise to cellular phenotypes typical of aberrant scramblase activity, which were reversed by the additional introduction of a ‘loss‐of‐function’ mutation into the SCRD. In conclusion, our data suggest AtTMEM16 works as an ER‐resident lipid scramblase in Arabidopsis.

Anoctamin 5 Knockout Mouse Model Recapitulates LGMD2L Muscle Pathology and Offers Insight Into in vivo Functional Deficits

Mutations in the Anoctamin 5 (Ano5) gene that result in the lack of expression or function of ANO5 protein, cause Limb Girdle Muscular Dystrophy (LGMD) 2L/R12, and Miyoshi Muscular Dystrophy (MMD3). However, the dystrophic phenotype observed in patient muscles is not uniformly recapitulated by ANO5 knockout in animal models of LGMD2L. Here we describe the generation of a mouse model of LGMD2L generated by targeted out-of-frame deletion of the Ano5 gene. This model shows progressive muscle loss, increased muscle weakness, and persistent bouts of myofiber regeneration without chronic muscle inflammation, which recapitulates the mild to moderate skeletal muscle dystrophy reported in the LGMD2L patients. We show that these features of ANO5 deficient muscle are not associated with a change in the calcium-activated sarcolemmal chloride channel activity or compromised in vivo regenerative myogenesis. Use of this mouse model allows conducting in vivo investigations into the functional role of ANO5 in muscle health and for preclinical therapeutic development for LGMD2L.

ANO7: Insights into topology, function, and potential applications as a biomarker and immunotherapy target

Anoctamin 7 (ANO7) is a member of the transmembrane protein TMEM16 family. It has a conservative topology similar to other members in this family, such as the typical eight-transmembrane domain, but it also has unique features. Although the ion channel role of ANO7 has been well accepted, evolutionary analyses and relevant studies suggest that ANO7 may be a multi-facet protein in function. Studies have shown that ANO7 may also function as a scramblase. ANO7 is highly expressed in prostate cancer as well as normal prostate tissues. A considerable amount of evidence has confirmed that ANO7 is associated with human physiology and pathology, particularly with the development of prostate cancer, which makes ANO7 a good candidate as a diagnostic and prognostic biomarker. In addition, ANO7 may be a potential target for prostate cancer immunotherapy. Antibody-based or T cell-mediated immunotherapies against prostate cancer by targeting ANO7 have been highly anticipated. ANO7 may also correlate with several other types of cancers or diseases, where further studies are warranted.

The Groovy TMEM16 Family: Molecular Mechanisms of Lipid Scrambling and Ion Conduction

The TMEM16 family of membrane proteins displays a remarkable functional dichotomy - while some family members function as Ca²⁺-activated anion channels, the majority of characterized TMEM16 homologs are Ca²⁺-activated lipid scramblases, which catalyze the exchange of phospholipids between the two membrane leaflets. Furthermore, some TMEM16 scramblases can also function as channels. Due to their involvement in important physiological processes, the family has been actively studied ever since their molecular identity was unraveled. In this review, we will summarize the recent advances in the field and how they influenced our view of TMEM16 family function and evolution. Structural, functional and computational studies reveal how relatively small rearrangements in the permeation pathway are responsible for the observed functional duality: while TMEM16 scramblases can adopt both ion- and lipid conductive conformations, TMEM16 channels can only populate the former. Recent data further provides the molecular details of a stepwise activation mechanism, which is initiated by Ca²⁺ binding and modulated by various cellular factors, including lipids. TMEM16 function and the surrounding membrane properties are inextricably intertwined, with the protein inducing bilayer deformations associated with scrambling, while the surrounding lipids modulate TMEM16 conformation and activity.

Caspase cleavage releases a nuclear protein fragment that stimulates phospholipid scrambling at the plasma membrane

Phospholipid scrambling in dying cells promotes phosphatidylserine exposure, a critical process for efferocytosis. We previously identified the Xkr family protein Xkr4 as a phospholipid-scrambling protein, but its activation mechanisms remain unknown. Here we show that Xkr4 is activated in two steps: dimer formation by caspase-mediated cleavage and structural change caused by activating factors. To identify the factors, we developed a new screening system, "revival screening," using a CRISPR sgRNA library. Applying this system, we identified the nuclear protein XRCC4 as the single candidate for the Xkr4 activator. Upon apoptotic stimuli, XRCC4, contained in the DNA repair complex, is cleaved by caspases, and its C-terminal fragment with an intrinsically disordered region is released into the cytoplasm. Protein interaction screening showed that the fragment interacts directly with the Xkr4 dimer to activate it. This study demonstrates that caspase-mediated cleavage releases a nuclear protein fragment for direct regulation of lipid dynamics on the plasma membrane.

The tertiary structure of the human Xkr8-Basigin complex that scrambles phospholipids at plasma membranes

Xkr8-Basigin is a plasma membrane phospholipid scramblase activated by kinases or caspases. We combined cryo-EM and X-ray crystallography to investigate its structure at an overall resolution of 3.8 Å. Its membrane-spanning region carrying 22 charged amino acids adopts a cuboid-like structure stabilized by salt bridges between hydrophilic residues in transmembrane helices. Phosphatidylcholine binding was observed in a hydrophobic cleft on the surface exposed to the outer leaflet of the plasma membrane. Six charged residues placed from top to bottom inside the molecule were essential for scrambling phospholipids in inward and outward directions, apparently providing a pathway for their translocation. A tryptophan residue was present between the head group of phosphatidylcholine and the extracellular end of the path. Its mutation to alanine made the Xkr8-Basigin complex constitutively active, indicating that it plays a vital role in regulating its scramblase activity. The structure of Xkr8-Basigin provides insights into the molecular mechanisms underlying phospholipid scrambling.

Structure-Function of TMEM16 Ion Channels and Lipid Scramblases

The TMEM16 protein family comprises two novel classes of structurally conserved but functionally distinct membrane transporters that function as Ca2+-dependent Cl- channels (CaCCs) or dual functional Ca2+-dependent ion channels and phospholipid scramblases. Extensive functional and structural studies have advanced our understanding of TMEM16 molecular mechanisms and physiological functions. TMEM16A and TMEM16B CaCCs control transepithelial fluid transport, smooth muscle contraction, and neuronal excitability, whereas TMEM16 phospholipid scramblases mediate the flip-flop of phospholipids across the membrane to allow phosphatidylserine externalization, which is essential in a plethora of important processes such as blood coagulation, bone development, and viral and cell fusion. In this chapter, we summarize the major methods in studying TMEM16 ion channels and scramblases and then focus on the current mechanistic understanding of TMEM16 Ca2+- and voltage-dependent channel gating as well as their ion and phospholipid permeation.

PHOSPHOINOSITIDES AND CALCIUM SIGNALING. A MARRIAGE ARRANGED IN ER-PM CONTACT SITES

Calcium (Ca²⁺) ions are critically important in orchestrating countless regulatory processes in eukaryotic cells. Consequently, cells tightly control cytoplasmic Ca²⁺ concentrations using a complex array of Ca²⁺-selective ion channels, transporters, and signaling effectors. Ca²⁺ transport through various cellular membranes is highly dependent on the intrinsic properties of specific membrane compartments and conversely, local Ca²⁺ changes have profound effects on the membrane lipid composition of such membrane sub-domains. In particular, inositol phospholipids are a minor class of phospholipids that play pivotal roles in the control of Ca²⁺-dependent signaling pathways. In this review, we will highlight some of the recent advances in this field as well as their impact in defining future research directions.

TMEM16K is an interorganelle regulator of endosomal sorting

Communication between organelles is essential for their cellular homeostasis. Neurodegeneration reflects the declining ability of neurons to maintain cellular homeostasis over a lifetime, where the endolysosomal pathway plays a prominent role by regulating protein and lipid sorting and degradation. Here we report that TMEM16K, an endoplasmic reticulum lipid scramblase causative for spinocerebellar ataxia (SCAR10), is an interorganelle regulator of the endolysosomal pathway. We identify endosomal transport as a major functional cluster of TMEM16K in proximity biotinylation proteomics analyses. TMEM16K forms contact sites with endosomes, reconstituting split-GFP with the small GTPase RAB7. Our study further implicates TMEM16K lipid scrambling activity in endosomal sorting at these sites. Loss of TMEM16K function led to impaired endosomal retrograde transport and neuromuscular function, one of the symptoms of SCAR10. Thus, TMEM16K-containing ER-endosome contact sites represent clinically relevant platforms for regulating endosomal sorting.

Targeting of Intracellular TMEM16 Proteins to the Plasma Membrane and Activation by Purinergic Signaling

Anoctamins such as TMEM16A and TMEM16B are Ca²⁺-dependent Cl⁻ channels activated through purinergic receptor signaling. TMEM16A (ANO1), TMEM16B (ANO2) and TMEM16F (ANO6) are predominantly expressed at the plasma membrane and are therefore well accessible for functional studies. While TMEM16A and TMEM16B form halide-selective ion channels, TMEM16F and probably TMEM16E operate as phospholipid scramblases and nonselective ion channels. Other TMEM16 paralogs are expressed mainly in intracellular compartments and are therefore difficult to study at the functional level. Here, we report that TMEM16E (ANO5), -H (ANO8), -J (ANO9) and K (ANO10) are targeted to the plasma membrane when fused to a C-terminal CAAX (cysteine, two aliphatic amino acids plus methionin, serine, alanin, cystein or glutamin) motif. These paralogs produce Ca²⁺-dependent ion channels. Surprisingly, expression of the TMEM16 paralogs in the plasma membrane did not produce additional scramblase activity. In contrast, endogenous scrambling induced by stimulation of purinergic P2X7 receptors was attenuated, in parallel with reduced plasma membrane blebbing. This could suggest that intracellular TMEM16 paralogs operate differently when compared to plasma membrane-localized TMEM16F, and may even stabilize intracellular membranes. Alternatively, CAAX tagging, which leads to expression in non-raft compartments of the plasma membrane, may antagonize phosphatidylserine exposure by endogenous raft-located TMEM16F. CAAX-containing constructs may be useful to further investigate the molecular properties of intracellular TMEM16 proteins.

TMEM16E/ANO5 mutations related to bone dysplasia or muscular dystrophy cause opposite effects on lipid scrambling

Mutations in the human TMEM16E/ANO5 gene are causative for gnathodiaphyseal dysplasia (GDD), a rare bone malformation and fragility disorder, and for two types of muscular dystrophy (MD). Previous studies have demonstrated that TMEM16E/ANO5 is a Ca²⁺ -activated phospholipid scramblase and that the mutation c.1538C>T (p.Thr513Ile) causing GDD leads to a gain-of-function phenotype. Here, using established HEK293-based functional assays, we investigated the effects of MD-related and further GDD-related amino acid exchanges on TMEM16E/ANO5 function in the same expression system. These experiments also revealed that the gradual changes in HEK293 cell morphology observed upon expression of TMEM16E/ANO5^GDD mutants are a consequence of aberrant protein activity. Our results collectively demonstrate that, on the level of protein function, MD mutations are associated to loss-of-function and GDD mutations to gain-of-function phenotypes, confirming conjectures made on the basis of inheritance modes.

Membrane lipids are both the substrates and a mechanistically responsive environment of TMEM16 scramblase proteins

Recent discoveries about functional mechanisms of proteins in the TMEM16 family of phospholipid scramblases have illuminated the dual role of the membrane as both the substrate and a mechanistically responsive environment in the wide range of physiological processes and genetic disorders in which they are implicated. This is highlighted in the review of recent findings from our collaborative investigations of molecular mechanisms of TMEM16 scramblases that emerged from iterative functional, structural, and computational experimentation. In the context of this review, we present new MD simulations and trajectory analyses motivated by the fact that new structural information about the TMEM16 scramblases is emerging from cryo-EM determinations in lipid nanodiscs. Because the functional environment of these proteins in in vivo and in in vitro is closer to flat membranes, we studied comparatively the responses of the membrane to the TMEM16 proteins in flat membranes and nanodiscs. We find that bilayer shapes in the nanodiscs are very different from those observed in the flat membrane systems, but the function-related slanting of the membrane observed at the nhTMEM16 boundary with the protein is similar in the nanodiscs and in the flat bilayers. This changes, however, in the bilayer composed of longer-tail lipids, which is thicker near the phospholipid translocation pathway, which may reflect an enhanced tendency of the long tails to penetrate the pathway and create, as shown previously, a nonconductive environment. These findings support the correspondence between the mechanistic involvement of the lipid environment in the flat membranes, and the nanodiscs. © 2019 Wiley Periodicals, Inc.

Structural Feature of Lipid Scrambling Model Transmembrane Peptides: Same-Side Positioning of Hydrophilic Residues and Their Deeper Position

Phospholipid scramblases that catalyze lipid transbilayer movement are associated with intercellular signaling and lipid homeostasis. Although several studies have shown that the hydrophilic residue-rich groove of the proteins mediates lipid scrambling, the interactions between the groove and the lipid bilayers remain poorly understood. Here we have revealed the structural features of model transmembrane peptides that conduct lipid scrambling as well as the interactions between the peptides and the surrounding lipids by means of experimental and simulation techniques. Peptides with two strongly hydrophilic residues located on the same side of the helices and at a deeper position in the membrane exhibited high scramblase activities. All-atom molecular dynamics simulations showed that the interactions between the hydrophilic residues and lipid head groups regulate the membrane thinning and disorder near the peptides in an order that correlates with the scramblase activity of the peptides. These results provide a basis for understanding the lipid scrambling mechanisms by transmembrane regions.

Exofacial phospholipids at the plasma membrane: ill-defined targets for early infection processes

The eukaryotic plasma membrane (PM) consists largely of phospholipids and proteins, and separates the intracellular compartments from the extracellular space. It also serves as a signaling platform for cell-to-cell communication and an interaction platform for the molecular crosstalk between pathogens and their target cells. Much research has been done to elucidate the interactions between pathogens and host membrane proteins. However, little is known about the interactions between pathogens and membrane phospholipids, although reports have described a contribution of phospholipids to cell recognition and/or invasion during early infection by diverse pathogens. Thus, during adhesion to the host cell, the obligate intracellular bacterial pathogens Chlamydia spp., the facultative intracellular pathogen Helicobacter pylori and the facultative aerobic pathogen Vibrio parahaemolyticus, interact with exofacial phospholipids. This review focuses on several prominent instances of pathogen interaction with host-cell phospholipids.

Dynamic modulation of the lipid translocation groove generates a conductive ion channel in Ca2+-bound nhTMEM16

Both lipid and ion translocation by Ca²⁺-regulated TMEM16 transmembrane proteins utilizes a membrane-exposed hydrophilic groove. Several conformations of the groove are observed in TMEM16 protein structures, but how these conformations form, and what functions they support, remains unknown. From analyses of atomistic molecular dynamics simulations of Ca²⁺-bound nhTMEM16 we find that the mechanism of a conformational transition of the groove from membrane-exposed to occluded from the membrane involves the repositioning of transmembrane helix 4 (TM4) following its disengagement from a TM3/TM4 interaction interface. Residue L302 is a key element in the hydrophobic TM3/TM4 interaction patch that braces the open-groove conformation, which should be changed by an L302A mutation. The structure of the L302A mutant determined by cryogenic electron microscopy (cryo-EM) reveals a partially closed groove that could translocate ions, but not lipids. This is corroborated with functional assays showing severely impaired lipid scrambling, but robust channel activity by L302A.

A membrane-exposed groove in Ca²⁺-gated TMEM16 scramblases forms the translocation pathway for ions and lipids. Here authors combine molecular dynamics with cryo-EM and functional assays to uncover the conformational transitions of the groove leading to a non-selective ion channel pore.

Platelets, Thrombo-Inflammation, and Cancer: Collaborating With the Enemy

Platelets are small anucleate cells present in the blood stream, their typical role in primary hemostasis has been well-described. However, new evidence suggests that they have critically important roles in cancer progression and inflammation. Cancer cells can activate platelets, thus using them as physical shields from blood shear forces and natural killer (NK) cells. The activated platelets may also regulate hematopoietic and immune cell migration toward the tumor site; therefore, contributing to the cancer-associated inflammation. The activation of platelets by cancer cells may also contribute to metastasis and cancer progression by stimulating deep venous thrombosis and neutrophil extracellular trap formations (NETs) that “hide” cancer cells. We strived to review the current literature to dissect the role of platelets in cancer-associated thrombosis and tumor microenvironment inflammation.

Dynamic change of electrostatic field in TMEM16F permeation pathway shifts its ion selectivity

TMEM16F is activated by elevated intracellular Ca2+, and functions as a small-conductance ion channel and as a phospholipid scramblase. In contrast to its paralogs, the TMEM16A/B calcium-activated chloride channels, mouse TMEM16F has been reported as a cation-, anion-, or non-selective ion channel, without a definite conclusion. Starting with the Q559K mutant that shows no current rundown and less outward rectification in excised patch, we found that the channel shifted its ion selectivity in response to the change of intracellular Ca2+ concentration, with an increased permeability ratio of Cl- to Na+ (PCl-/PNa+) at a higher Ca2+ level. The gradual shift of relative ion permeability did not correlate with the channel activation state. Instead, it was indicative of an alteration of electrostatic field in the permeation pathway. The dynamic change of ion selectivity suggests a charge-screening mechanism for TMEM16F ion conduction, and it provides hints to further studies of TMEM16F physiological functions.

Dysregulated calcium homeostasis prevents plasma membrane repair in Anoctamin 5/TMEM16E-deficient patient muscle cells

Autosomal recessive mutations in Anoctamin 5 (ANO5/TMEM16E), a member of the transmembrane 16 (TMEM16) family of Ca²⁺-activated ion channels and phospholipid scramblases, cause adult-onset muscular dystrophies (limb girdle muscular dystrophy 2L (LGMD2L) and Miyoshi Muscular Dystrophy (MMD3). However, the molecular role of ANO5 is unclear and ANO5 knockout mouse models show conflicting requirements of ANO5 in muscle. To study the role of ANO5 in human muscle cells we generated a myoblast line from a MMD3-patient carrying the c.2272C>T mutation, which we find causes the mutant protein to be degraded. The patient myoblasts exhibit normal myogenesis, but are compromised in their plasma membrane repair (PMR) ability. The repair deficit is linked to the poor ability of the endoplasmic reticulum (ER) to clear cytosolic Ca²⁺ increase caused by focal plasma membrane injury. Expression of wild-type ANO5 or pharmacological prevention of injury-triggered cytosolic Ca²⁺ overload enable injured patient muscle cells to repair. A homology model of ANO5 shows that several of the known LGMD2L/MMD3 patient mutations line the transmembrane region of the protein implicated in its channel activity. These results point to a role of cytosolic Ca²⁺ homeostasis in PMR, indicate a role for ANO5 in ER-mediated cytosolic Ca²⁺ uptake and identify normalization of cytosolic Ca²⁺ homeostasis as a potential therapeutic approach to treat muscular dystrophies caused by ANO5 deficit.

Predominant localization of phosphatidylserine at the cytoplasmic leaflet of the ER, and its TMEM16K-dependent redistribution

TMEM16K, a membrane protein carrying 10 transmembrane regions, has phospholipid scramblase activity. TMEM16K is localized to intracellular membranes, but whether it actually scrambles phospholipids inside cells has not been demonstrated, due to technical difficulties in studying intracellular lipid distributions. Here, we developed a freeze-fracture electron microscopy method that enabled us to determine the phosphatidylserine (PtdSer) distribution in the individual leaflets of cellular membranes. Using this method, we found that the endoplasmic reticulum (ER) of mammalian cells harbored abundant PtdSer in its cytoplasmic leaflet and much less in the luminal leaflet, whereas the outer and inner nuclear membranes (NMs) had equivalent amounts of PtdSer in both leaflets. The ER and NMs of budding yeast also harbored PtdSer in their cytoplasmic leaflet, but asymmetrical distribution in the ER was not observed. Treating mouse embryonic fibroblasts with the Ca2+ ionophore A23187 compromised the cytoplasmic leaflet-dominant PtdSer asymmetry in the ER and increased PtdSer in the NMs, especially in the nucleoplasmic leaflet of the inner NM. This Ca2+-induced PtdSer redistribution was not observed in TMEM16K-null fibroblasts, but was recovered in these cells by reexpressing TMEM16K. These results indicate that, similar to the plasma membrane, PtdSer in the ER of mammalian cells is predominantly localized to the cytoplasmic leaflet, and that TMEM16K directly or indirectly mediates Ca2+-dependent phospholipid scrambling in the ER.

TMEM16F/Anoctamin 6 in Ferroptotic Cell Death

Ca2+ activated Cl- channels (TMEM16A; ANO1) support cell proliferation and cancer growth. Expression of TMEM16A is strongly enhanced in different types of malignomas. In contrast, TMEM16F (ANO6) operates as a Ca2+ activated chloride/nonselective ion channel and scrambles membrane phospholipids to expose phosphatidylserine at the cell surface. Both phospholipid scrambling and cell swelling induced through activation of nonselective ion currents appear to destabilize the plasma membrane thereby causing cell death. There is growing evidence that activation of TMEM16F contributes to various forms of regulated cell death. In the present study, we demonstrate that ferroptotic cell death, occurring during peroxidation of plasma membrane phospholipids activates TMEM16F. Ferroptosis was induced by erastin, an inhibitor of the cystine-glutamate antiporter and RSL3, an inhibitor of glutathione peroxidase 4 (GPX4). Cell death was largely reduced in the intestinal epithelium, and in peritoneal macrophages isolated from mice with tissue-specific knockout of TMEM16F. We show that TMEM16F is activated during erastin and RSL3-induced ferroptosis. In contrast, inhibition of ferroptosis by ferrostatin-1 and by inhibitors of TMEM16F block TMEM16F currents and inhibit cell death. We conclude that activation of TMEM16F is a crucial component during ferroptotic cell death, a finding that may be useful to induce cell death in cancer cells.

Temperature-dependent increase in the calcium sensitivity and acceleration of activation of ANO6 chloride channel variants

Anoctamin-6 (ANO6) belongs to a family of calcium (Ca2+)-activated chloride channels (CaCCs), with three splicing variants (V1, V2, and V5) showing plasma membrane expression. Unlike other CaCCs, ANO6 requires a non-physiological intracellular free calcium concentration ([Ca2+]i > 1 μM) and several minutes for full activation under a whole-cell patch clamp. Therefore, its physiological role as an ion channel is uncertain and it is more commonly considered a Ca2+-dependent phospholipid scramblase. Here, we demonstrate that physiological temperature (37 °C) increases ANO6 Ca2+ sensitivity under a whole-cell patch clamp; V1 was activated by 1 μM [Ca2+]i, whereas V2 and V5 were activated by 300 nM [Ca2+]i. Increasing the temperature to 42 °C led to activation of all ANO6 variants by 100 nM [Ca2+]i. The delay time for activation of the three variants was significantly shortened at 37 °C. Notably, the temperature-dependent Ca2+-sensitisation of ANO6 became insignificant under inside-out patch clamp, suggesting critical roles of unknown cytosolic factors. Unlike channel activity, 27 °C but not 37 °C (physiological temperature) induced the scramblase activity of ANO6 at submicromolar [Ca2+]i (300 nM), irrespective of variant type. Our results reveal a physiological ion conducting property of ANO6 at 37 °C and suggest that ANO6 channel function acts separately from its scramblase activity.

Contribution of Anoctamins to Cell Survival and Cell Death

Before anoctamins (TMEM16 proteins) were identified as a family of Ca2+-activated chloride channels and phospholipid scramblases, the founding member anoctamin 1 (ANO1, TMEM16A) was known as DOG1, a marker protein for gastrointestinal stromal tumors (GIST). Meanwhile, ANO1 has been examined in more detail, and the role of ANO1 in cell proliferation and the development of different types of malignomas is now well established. While ANO5, ANO7, and ANO9 may also be relevant for growth of cancers, evidence has been provided for a role of ANO6 (TMEM16F) in regulated cell death. The cellular mechanisms by which anoctamins control cell proliferation and cell death, respectively, are just emerging; however, the pronounced effects of anoctamins on intracellular Ca2+ levels are likely to play a significant role. Recent results suggest that some anoctamins control membrane exocytosis by setting Ca2+i levels near the plasma membrane, and/or by controlling the intracellular Cl- concentration. Exocytosis and increased membrane trafficking induced by ANO1 and ANO6 may enhance membrane expression of other chloride channels, such as CFTR and volume activated chloride channels (VRAC). Notably, ANO6-induced phospholipid scrambling with exposure of phosphatidylserine is pivotal for the sheddase function of disintegrin and metalloproteinase (ADAM). This may support cell death and tumorigenic activity of IL-6 by inducing IL-6 trans-signaling. The reported anticancer effects of the anthelminthic drug niclosamide are probably related to the potent inhibitory effect on ANO1, apart from inducing cell cycle arrest through the Let-7d/CDC34 axis. On the contrary, pronounced activation of ANO6 due to a large increase in intracellular calcium, activation of phospholipase A2 or lipid peroxidation, can lead to ferroptotic death of cancer cells. It therefore appears reasonable to search for both inhibitors and potent activators of TMEM16 in order to interfere with cancer growth and metastasis.

Cryo-EM structures and functional characterization of the murine lipid scramblase TMEM16F

The lipid scramblase TMEM16F initiates blood coagulation by catalyzing the exposure of phosphatidylserine in platelets. The protein is part of a family of membrane proteins, which encompasses calcium-activated channels for ions and lipids. Here, we reveal features of murine TMEM16F (mTMEM16F) that underlie its function as a lipid scramblase and an ion channel. The cryo-EM data of mTMEM16F in absence and presence of Ca2+ define the ligand-free closed conformation of the protein and the structure of a Ca2+-bound intermediate. Both conformations resemble their counterparts of the scrambling-incompetent anion channel mTMEM16A, yet with distinct differences in the region of ion and lipid permeation. In conjunction with functional data, we demonstrate the relationship between ion conduction and lipid scrambling. Although activated by a common mechanism, both functions appear to be mediated by alternate protein conformations that are at equilibrium in the ligand-bound state.

Phosphorylation-mediated activation of mouse Xkr8 scramblase for phosphatidylserine exposure

The exposure of phosphatidylserine (PtdSer) to the cell surface is regulated by the down-regulation of flippases and the activation of scramblases. Xkr8 has been identified as a scramblase that is activated during apoptosis, but its exogenous expression in the mouse Ba/F3 pro B cell line induces constitutive PtdSer exposure. Here we found that this Xkr8-mediated PtdSer exposure occurred at 4 °C, but not at 20 °C, although its scramblase activity was observed at 20 °C. The Xkr8-mediated PtdSer exposure was inhibited by a kinase inhibitor and enhanced by phosphatase inhibitors. Phosphorylated Xkr8 was detected by Phos-tag PAGE, and a mass spectrometric and mutational analysis identified three phosphorylation sites. Their phosphomimic mutation rendered Xkr8 resistant to the kinase inhibitor for PtdSer exposure at 4 °C, but unlike phosphatase inhibitors, it did not induce constitutive PtdSer exposure at 20 °C. On the other hand, when the flippase genes were deleted, the Xkr8 induced constitutive PtdSer exposure at high temperature, indicating that the flippase activity normally counteracted Xkr8's ability to expose PtdSer. These results indicate that PtdSer exposure can be increased by the phosphorylation-mediated activation of Xkr8 scramblase and flippase down-regulation.

Drosophila Subdued is a moonlighting transmembrane protein 16 (TMEM16) that transports ions and phospholipids

Transmembrane protein 16 (TMEM16) family members play numerous important physiological roles, ranging from controlling membrane excitability and secretion to mediating blood coagulation and viral infection. These diverse functions are largely due to their distinct biophysical properties. Mammalian TMEM16A and TMEM16B are Ca²⁺-activated Cl^- channels (CaCCs), whereas mammalian TMEM16F, fungal afTMEM16, and nhTMEM16 are moonlighting (multifunctional) proteins with both Ca²⁺-activated phospholipid scramblase (CaPLSase) and Ca²⁺-activated, nonselective ion channel (CAN) activities. To further understand the biological functions of the enigmatic TMEM16 proteins in different organisms, here, by combining an improved annexin V-based CaPLSase-imaging assay with inside-out patch clamp technique, we thoroughly characterized Subdued, a Drosophila TMEM16 ortholog. We show that Subdued is also a moonlighting transport protein with both CAN and CaPLSase activities. Using a TMEM16F-deficient HEK293T cell line to avoid strong interference from endogenous CaPLSases, our functional characterization and mutagenesis studies revealed that Subdued is a bona fide CaPLSase. Our finding that Subdued is a moonlighting TMEM16 expands our understanding of the molecular mechanisms of TMEM16 proteins and their evolution and physiology in both Drosophila and humans.

TMEM16F activation by Ca2+ triggers plasma membrane expansion and directs PD-1 trafficking

TMEM16F is a Ca2+ -gated ion channel that is required for Ca2+ -activated phosphatidylserine exposure on the surface of many eukaryotic cells. TMEM16F is widely expressed and has roles in platelet activation during blood clotting, bone formation and T cell activation. By combining microscopy and patch clamp recording we demonstrate that activation of TMEM16F by Ca2+ ionophores in Jurkat T cells triggers large-scale surface membrane expansion in parallel with phospholipid scrambling. With continued ionophore application,TMEM16F-expressing cells then undergo extensive shedding of ectosomes. The T cell co-receptor PD-1 is selectively incorporated into ectosomes. This selectivity depends on its transmembrane sequence. Surprisingly, cells lacking TMEM16F not only fail to expand surface membrane in response to elevated cytoplasmic Ca2+, but instead undergo rapid massive endocytosis with PD-1 internalisation. These results establish a new role for TMEM16F as a regulator of Ca2+ activated membrane trafficking.

Chemically induced vesiculation as a platform for studying TMEM16F activity

Calcium-activated phospholipid scramblase mediates the energy-independent bidirectional translocation of lipids across the bilayer, leading to transient or, in the case of apoptotic scrambling, sustained collapse of membrane asymmetry. Cells lacking TMEM16F-dependent lipid scrambling activity are deficient in generation of extracellular vesicles (EVs) that shed from the plasma membrane in a Ca2+-dependent manner, namely microvesicles. We have adapted chemical induction of giant plasma membrane vesicles (GPMVs), which require both TMEM16F-dependent phospholipid scrambling and calcium influx, as a kinetic assay to investigate the mechanism of TMEM16F activity. Using the GPMV assay, we identify and characterize both inactivating and activating mutants that elucidate the mechanism for TMEM16F activation and facilitate further investigation of TMEM16F-mediated lipid translocation and its role in extracellular vesiculation.

Anoctamin-6 regulates ADAM sheddase function

ADAM17, a prominent member of the "Disintegrin and Metalloproteinase" (ADAM) family, controls vital cellular functions through cleavage of transmembrane substrates including TGF-alpha, Amphiregulin (AREG) and TNF-Receptor 1 (TNFR1). We recently presented evidence that surface exposure of phosphatidylserine (PS) is pivotal for ADAM17 to exert sheddase activity. Anoctamin-6 (ANO6) has Ca2+-dependent phospholipid scramblase activity and it followed that the functions of ANO6 and ADAM17 might be linked. We report that overexpression of ANO6 in HEK293T cells led to increased Ca2+-mediated PS-exposure that was indeed accompanied by enhanced release of AREG and TGF-alpha. The effect was not observed when cells were treated with the PKC-dependent ADAM17 activator PMA. Transformation of cells with a constitutively active ANO6 mutant led to spontaneous PS-exposure and to the release of ADAM17-substrates in the absence of any stimuli. Inhibitor experiments indicated that ANO6-mediated enhancement of substrate cleavage simultaneously broadened the spectrum of participating metalloproteinases. In complementary experiments, siRNA-mediated downregulation of ANO6 was shown to decrease ionophore-mediated release of TNFR1 in human umbilical vein endothelial cells (HUVECs). We conclude that ANO6, by virtue of its scramblase activity, may play a role as an important regulator of the ADAM-network in the plasma membrane.

Gating mechanism of the extracellular entry to the lipid pathway in a TMEM16 scramblase

Members of the TMEM16/ANO family of membrane proteins are Ca²⁺-activated phospholipid scramblases and/or Cl⁻ channels. A membrane-exposed hydrophilic groove in these proteins serves as a shared translocation pathway for ions and lipids. However, the mechanism by which lipids gain access to and permeate through the groove remains poorly understood. Here, we combine quantitative scrambling assays and molecular dynamic simulations to identify the key steps regulating lipid movement through the groove. Lipid scrambling is limited by two constrictions defined by evolutionarily conserved charged and polar residues, one extracellular and the other near the membrane mid-point. The region between these constrictions is inaccessible to lipids and water molecules, suggesting that the groove is in a non-conductive conformation. A sequence of lipid-triggered reorganizations of interactions between these residues and the permeating lipids propagates from the extracellular entryway to the central constriction, allowing the groove to open and coordinate the headgroups of transiting lipids.

Some TMEM16 family members are Ca²⁺-dependent phospholipid scramblases, which also mediate non-selective ion transport; however, the mechanism how lipids permeate through the TMEM16 remains poorly understood. Here, the authors combine biochemical assays and simulations to identify the key steps regulating lipid movement through the membrane-exposed groove.

Out-of-the-groove transport of lipids by TMEM16 and GPCR scramblases

Phospholipid scramblases externalize phosphatidylserine to facilitate numerous physiological processes. Several members of the structurally unrelated TMEM16 and G protein-coupled receptor (GPCR) protein families mediate phospholipid scrambling. The structure of a TMEM16 scramblase shows a membrane-exposed hydrophilic cavity, suggesting that scrambling occurs via the ‟credit-card" mechanism where lipid headgroups permeate through the cavity while their tails remain associated with the membrane core. Here we show that afTMEM16 and opsin, representatives of the TMEM16 and GCPR scramblase families, transport phospholipids with polyethylene glycol headgroups whose globular dimensions are much larger than the width of the cavity. This suggests that transport of these large headgroups occurs outside rather than within the cavity. These large lipids are scrambled at rates comparable to those of normal phospholipids and their presence in the reconstituted vesicles promotes scrambling of normal phospholipids. This suggests that both large and small phospholipids can move outside the cavity. We propose that the conformational rearrangements underlying TMEM16- and GPCR-mediated credit-card scrambling locally deform the membrane to allow transbilayer lipid translocation outside the cavity and that both mechanisms underlie transport of normal phospholipids.