Two types of type IV P-type ATPases independently re-establish the asymmetrical distribution of phosphatidylserine in plasma membranes

Lipid flippases ATP9A and ATP9B form a complex and contribute to the exocytic pathway from the Golgi

Type IV P-type ATPases (P4-ATPases) serve as lipid flippases, translocating membrane lipids from the exoplasmic (or luminal) leaflet to the cytoplasmic leaflet of lipid bilayers. In mammals, these P4-ATPases are localized to distinct subcellular compartments. ATP8A1 and ATP9A, members of the P4-ATPase family, are involved in endosome-mediated membrane trafficking, although the roles of P4-ATPases in the exocytic pathway remain to be clarified. ATP9A and ATP9B are located in the TGN, with ATP9A also present in endosomal compartments. This study revealed the overlapping roles of ATP9A and ATP9B in transporting VSVG from the Golgi to the plasma membrane within the exocytic pathway. Furthermore, we demonstrated that the flippase activities of ATP9A and ATP9B were crucial for the transport process. Notably, we discovered the formation of homomeric and/or heteromeric complexes between ATP9A and ATP9B. Therefore, ATP9A and ATP9B play a role in the exocytic pathway from the Golgi to the plasma membrane, forming either homomeric or heteromeric complexes.

ATP8A1-translocated endosomal phosphatidylserine fine-tunes the multivesicular body formation and the endo-lysosomal traffic

P4-ATPases are phospholipid flippases responsible for the transbilayer lipid asymmetry. ATP8A1, a P4-ATPase family member, has been reported to be involved in phosphatidylserine (PS) translocation at the trans-Golgi network, early endosomes and recycling endosomes. However, the possible roles of the PS on late endosomes/lysosomes pathway and how they are regulated remain to be elucidated. This study showed enrichment of ATP8A1 in Rab7-positive late endosomal compartments, and that ATP8A1 primarily flips the endosomal PS from the luminal leaflet to the cytosolic leaflet but not the PS in the inner leaflet of the plasma membrane. ATP8A1 depletion accelerates the lysosome-destined cargo proteins transfer into the intraluminal vesicles (ILVs) of multivesicular bodies (MVBs) and alters the signaling of epidermal growth factor receptor. Mechanistically, ATP8A1 depletion leads to PS loading in the luminal leaflet of MVB's limiting membrane, which fine-tunes ILVs initiation and endosomal sorting complex required for transport (ESCRT) component recruitment.

Membrane structure-responsive lipid scramblase activity of the TMEM63/OSCA family

Phospholipids are asymmetrically distributed in the plasma membrane (PM), and scramblases disrupt this asymmetry by shuffling phospholipids. We recently identified mouse Tmem63b as a membrane structure-responsive scramblase. Tmem63b belongs to the TMEM63/OSCA family of ion channels; however, the conservation of the scramblase activity within this family remains unclear. We expressed human TMEM63 paralogs, TMEM63B orthologs, and plant OSCA1.1 in Tmem63b-deficient mouse pro-B cells and found that vertebrate TMEM63B orthologs exhibit scramblase activity at the PM. Previously, ten pathogenic human TMEM63B variants were identified, some of which exhibited constitutive scramblase activity. Upon expressing all variants, we found that nine variants displayed constitutive scramblase activity. These results suggest that membrane structure-responsive scramblase activity at the PM is conserved among vertebrate TMEM63B orthologs.

Integration of Mendelian Randomization to explore the genetic influences of pediatric sepsis: a focus on RGL4, ATP9A, MAP3K7CL, and DDX11L2

OBJECTIVE

This study aims to explore the genetic characteristics of pediatric sepsis through a combined analysis of multiple methods, including Mendelian Randomization (MR), differential gene expression analysis, and immune cell infiltration assessment. It explores their potential as biomarkers for sepsis risk and their involvement in immune-related pathways.

METHODS

Differential expression analysis was performed using public datasets to identify genes with significant expression changes between pediatric sepsis patients and healthy controls. MR analysis utilized genome-wide significant SNPs as instrumental variables to assess causal relationships between gene expression and sepsis risk. Bi-directional MR was conducted to assess both forward and reverse causality. FDR correction was applied to adjust for multiple comparisons in MR results. Immune cell infiltration analysis was performed to investigate the genes' roles in immune responses, and findings were validated with independent datasets. ROC curves were constructed to assess predictive performance.

RESULTS

Differential expression analysis identified significant changes in RGL4,ATP9A,MAP3K7CL, and DDX11L2. MR analysis revealed causal associations between these genes and sepsis risk, with RGL4 and ATP9A upregulated (inflammatory roles), and MAP3K7CL and DDX11L2 downregulated (protective roles). Bi-directional MR found no significant reverse causality. Immune cell analysis showed associations with key immune cell types, and ROC analysis demonstrated strong predictive potential.

CONCLUSION

RGL4,ATP9A,MAP3K7CL, and DDX11L2 play important roles in pediatric sepsis risk and immune response regulation, offering insights into genetic and immune mechanisms that may inform future sepsis research and treatment.

Membrane structure-responsive lipid scrambling by TMEM63B to control plasma membrane lipid distribution

Phospholipids are asymmetrically distributed in the plasma membrane (PM), with phosphatidylcholine and sphingomyelin abundant in the outer leaflet. However, the mechanisms by which their distribution is regulated remain unclear. Here, we show that transmembrane protein 63B (TMEM63B) functions as a membrane structure-responsive lipid scramblase localized at the PM and lysosomes, activating bidirectional lipid translocation upon changes in membrane curvature and thickness. TMEM63B contains two intracellular loops with palmitoylated cysteine residue clusters essential for its scrambling function. TMEM63B deficiency alters phosphatidylcholine and sphingomyelin distributions in the PM. Persons with heterozygous mutations in TMEM63B are known to develop neurodevelopmental disorders. We show that V44M, the most frequent substitution, confers constitutive scramblase activity on TMEM63B, disrupting PM phospholipid asymmetry. We determined the cryo-electron microscopy structures of TMEM63B in its open and closed conformations, uncovering a lipid translocation pathway formed in response to changes in the membrane environment. Together, our results identify TMEM63B as a membrane structure-responsive scramblase that controls PM lipid distribution and we reveal the molecular basis for lipid scrambling and its biological importance.

Impacts of P4-ATPase Deletion on Membrane Asymmetry and Disease Development

Phospholipids exhibit an asymmetrical distribution on the cell membrane. P4-ATPases, type IV lipid flippases, are responsible for establishing and maintaining this phospholipid compositional asymmetry. The essential β subunit CDC50 (also known as TMEM30) assists in the transport and proper functioning of P4-ATPases. Deletion of P4-ATPases and its β subunit disrupts the membrane asymmetry, impacting the growth and development and leading to various diseases affecting the nervous, skeletal muscle, digestive, and hematopoietic systems. This review discusses the crucial roles of P4-ATPases and their β subunit in Saccharomyces cerevisiae, Arabidopsis thaliana, Caenorhabditis elegans, and mammals, offering valuable insights for future research.

ATP11A Promotes Epithelial-mesenchymal Transition in Gastric Cancer Cells via the Hippo Pathway

Background: ATP11A, a P-type ATPase, functions as flippases at the plasma membrane to maintain cellular function and vitality in several cancers. However, the role of ATP11A in gastric cancer remains unknown. This study aimed to identify ATP11A related to the biological behavior of gastric cancer, and elucidate the underlying mechanism. Methods: The Cancer Genome Atlas (TCGA) and Genotype-Tissue Expression (GTEx) databases were used to analyze the expression and prognosis of ATP11A. The biofunctions of ATP11A were explored through Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Set Enrichment Analysis (GSEA). The expression of ATP11A were validated by immunohistochemistry (IHC), qRT-PCR and Western blotting. Transwell, wound healing, CCK8 and colony-formation were to detected the migration, invasion and proliferation of gastric cancer cells. The epithelial-mesenchymal transition (EMT) and Hippo pathway markers were examined by Western blotting. Results: The expression of ATP11A was higher in gastric cancer tissues than in normal tissues, and high ATP11A levels were related to worse prognosis of gastric cancer patients. Additionally, we proved that ATP11A promoted the migration, invasion and proliferation in gastric cancer cells. Furthermore, ATP11A was found to promote EMT by devitalizing the Hippo pathway. Conclusion: ATP11A promoted migration, invasion, proliferation and EMT via Hippo signaling devitalization in gastric cancer cells.

Flipping the script: Advances in understanding how and why P4-ATPases flip lipid across membranes

Type IV P-type ATPases (P4-ATPases) are a family of transmembrane enzymes that translocate lipid substrates from the outer to the inner leaflet of biological membranes and thus create an asymmetrical distribution of lipids within membranes. On the cellular level, this asymmetry is essential for maintaining the integrity and functionality of biological membranes, creating platforms for signaling events and facilitating vesicular trafficking. On the organismal level, this asymmetry has been shown to be important in maintaining blood homeostasis, liver metabolism, neural development, and the immune response. Indeed, dysregulation of P4-ATPases has been linked to several diseases; including anemia, cholestasis, neurological disease, and several cancers. This review will discuss the evolutionary transition of P4-ATPases from cation pumps to lipid flippases, the new lipid substrates that have been discovered, the significant advances that have been achieved in recent years regarding the structural mechanisms underlying the recognition and flipping of specific lipids across biological membranes, and the consequences of P4-ATPase dysfunction on cellular and physiological functions. Additionally, we emphasize the requirement for additional research to comprehensively understand the involvement of flippases in cellular physiology and disease and to explore their potential as targets for therapeutics in treating a variety of illnesses. The discussion in this review will primarily focus on the budding yeast, C. elegans, and mammalian P4-ATPases.

Deciphering Lipid Arrangement in Phosphatidylserine/Phosphatidylcholine Mixed Membranes: Simulations and Experiments

Phosphatidylserine (PS) exposure on the plasma membrane is crucial for many cellular processes including apoptotic cell recognition, blood clotting regulation, cellular signaling, and intercellular interactions. In this study, we investigated the arrangement of PS headgroups in mixed PS/phosphatidylcholine (PC) bilayers, serving as a simplified model of the outer leaflets of mammalian cell plasma membranes. Combining atomistic-scale molecular dynamics (MD) simulations with Langmuir monolayer experiments, we unraveled the mutual miscibility of POPC and POPS lipids and the intricate intermolecular interactions inherent to these membranes as well as the disparities in position and orientation of PC and PS headgroups. Our experiments revealed micrometer-scale miscibility at all mole fractions of POPC and POPS, marked by modest deviations from ideal mixing with no apparent microscale phase separation. The MD simulations, meanwhile, demonstrated that these deviations were due to strong electrostatic interactions between like-lipid pairs (POPC-POPC and POPS-POPS), culminating in lateral segregation and nanoscale clustering. Notably, PS headgroups profoundly affect the ordering of the lipid acyl chains, leading to lipid elongation and subtle PS protrusion above the zwitterionic membrane. In addition, PC headgroups are more tilted with respect to the membrane normal, while PS headgroups align at a smaller angle, making them more exposed to the surface of the mixed PC/PS membranes. These findings provide a detailed molecular-level account of the organization of mixed PC/PS membranes, corroborated by experimental data. The insights gained here extend our comprehension of the physiological role of PSs.

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.