Heteromeric interactions required for abundance and subcellular localization of human CDC50 proteins and class 1 P4-ATPases

Structural and functional properties of the N and C terminal segments of the P4-ATPase phospholipid flippase ATP8A2

ATP8A2 is a P4-ATPase that actively flips phosphatidylserine and to a lesser extent phosphatidylethanolamine across cell membranes to generate and maintain transmembrane phospholipid asymmetry. The importance of this flippase is evident in the finding that loss-of- function mutations in ATP8A2 are known to cause the neurodevelopmental disease known as cerebellar ataxia, intellectual disability, and dysequilibrium syndrome 4 (CAMRQ4) in humans and related neurodegenerative disorders in mice. Although significant progress has been made in understanding mechanisms underlying phospholipid binding and transport across the membrane domain, little is known about the structural and functional properties of the cytosolic N- and C-terminal segments of this flippase. In addition, there has been uncertainty regarding the methionine start site of ATP8A2 and accordingly the size of the N-terminal segment. Here, we have used mass spectrometry to show that bovine ATP8A2 like its human counterpart has an extended N-terminal segment not apparent in the mouse ortholog. This segment greatly enhances the expression of ATP8A2 without affecting its cellular localization or phosphatidylserine-activated ATPase activity. Using a cleavable C-terminal protein and site-directed mutagenesis, we further show that the conserved GYAFS motif in the C-terminal segment plays a role in autoinhibition as well as efficient folding of ATP8A2 into a functional protein.

Improved Broad Spectrum Antifungal Drug Synergies with Cryptomycin, a Cdc50-Inspired Antifungal Peptide

Fungal infections in humans are difficult to treat, with very limited drug options. Due to a confluence of factors, there is an urgent need for innovation in the antifungal drug space, particularly to combat increasing antifungal drug resistance. Our previous studies showed that Cdc50, a subunit of fungal lipid translocase (flippase), is essential for Cryptococcus neoformans virulence and required for antifungal drug resistance, suggesting that fungal lipid flippase could be a novel drug target. Here, we characterized an antifungal peptide, Cryptomycinamide (KKOO-NH2), derived from a 9-amino acid segment of the C. neoformans Cdc50 protein. A fungal killing assay indicated that KKOO-NH2 is fungicidal against C. neoformans. The peptide has antifungal activity against multiple major fungal pathogens with a minimum inhibitory concentration (MIC) of 8 μg/mL against C. neoformans and Candida glabrata, 16 μg/mL against Candida albicans and C. auris, and 32 μg/mL against Aspergillus fumigatus. The peptide has low cytotoxicity against host cells based on our hemolysis assays and vesicle leakage assays. Strikingly, the peptide exhibits strong drug synergy with multiple antifungal drugs, including amphotericin B, itraconazole, and caspofungin, depending on the specific species on which the combinations were assayed. The fluorescently labeled peptide was detected to localize to the plasma membrane, likely inhibiting key interactions of Cdc50 with membrane proteins such as P4 ATPases. Cryptococcus cells exposed to sub-MIC of peptide showed increased reactive oxygen species production and intracellular calcium levels, indicating a peptide-induced stress response. Decreased intracellular proliferation within macrophages was observed after 30 min of peptide exposure and 24 h coincubation with macrophages, providing a potential translational mechanism to explore further in vivo. In aggregate, the synergistic activity of our KKOO-NH2 peptide may offer a potential novel candidate for combination therapy with existing antifungal drugs.

CREB3L1 deficiency impairs odontoblastic differentiation and molar dentin deposition partially through the TMEM30B

Odontoblasts are primarily responsible for synthesizing and secreting extracellular matrix proteins, which are crucial for dentinogenesis. Our previous single-cell profile and RNAscope for odontoblast lineage revealed that cyclic adenosine monophosphate responsive element-binding protein 3 like 1 (Creb3l1) was specifically enriched in the terminal differentiated odontoblasts. In this study, deletion of Creb3l1 in the Wnt1+ lineage led to insufficient root elongation and dentin deposition. Assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) and RNA sequencing were performed to revealed that in CREB3L1-deficient mouse dental papilla cells (mDPCs), the genes near the closed chromatin regions were mainly associated with mesenchymal development and the downregulated genes were primarily related to biological processes including cell differentiation, protein biosynthesis and transport, all of which were evidenced by a diminished ability of odontoblastic differentiation, a significant reduction in intracellular proteins, and an even greater decline in extracellular supernatant proteins. Dentin matrix protein 1 (Dmp1), dentin sialophosphoprotein (Dspp), and transmembrane protein 30B (Tmem30b) were identified as direct transcriptional regulatory targets. TMEM30B was intensively expressed in the differentiated odontoblasts, and exhibited a significant decline in both CREB3L1-deficient odontoblasts in vivo and in vitro. Deletion of Tmem30b impaired the ability of odontoblastic differentiation, protein synthesis, and protein secretion in mDPCs. Moreover, overexpressing TMEM30B in CREB3L1-deficient mDPCs partially rescued the extracellular proteins secretion. Collectively, our findings suggest that CREB3L1 participates in dentinogenesis and facilitates odontoblastic differentiation by directly enhancing the transcription of Dmp1, Dspp, and other differentiation-related genes and indirectly promoting protein secretion partially via TMEM30B.

Crossing the membrane-What does it take to flip a phospholipid? Structural and biochemical advances on P4-ATPase flippases

Membrane asymmetry is critical for maintenance of several different processes such as cell signaling, apoptosis, and vesicular transport in various eukaryotic systems. Flippases of the P4-ATPase family are associated with flipping phospholipids from the luminal or exoplasmic leaflet to the cytosolic leaflet. P4-ATPases belong to the P-type ATPase family, which are activated by phosphorylation and couple ATPase activity to substrate translocation. These proteins possess a transmembrane domain responsible for substrate transport, while the cytosolic machinery performs the necessary ATP hydrolysis for this process. Several high-resolution structures of human or yeast P4-ATPases have recently been resolved, but a comprehensive overview of the changes for reaction cycle in different members was crucial for future research. In this review, we have compiled available data reflecting the reaction cycle-associated changes in conformation of P4-ATPases. Together, this will provide an improved understanding of the similarities and differences between these members, which will drive further structural, functional, and computational studies to understand the mechanisms of these flippases.

Novel phosphatidylinositol flippases contribute to phosphoinositide homeostasis in the plasma membrane

Phosphatidylinositol is a precursor of various phosphoinositides, which play crucial roles in intracellular signaling and membrane dynamics and have impact on diverse aspects of cell physiology. Phosphoinositide synthesis and turnover occur in the cytoplasmic leaflet of the organellar and plasma membranes. P4-ATPases (lipid flippases) are responsible for translocating membrane lipids from the exoplasmic (luminal) to the cytoplasmic leaflet, thereby regulating membrane asymmetry. However, the mechanism underlying phosphatidylinositol translocation across cellular membranes remains elusive. Here, we discovered that the phosphatidylcholine flippases ATP8B1, ATP8B2, and ATP10A can also translocate phosphatidylinositol at the plasma membrane. To explore the function of these phosphatidylinositol flippases, we used cells depleted of CDC50A, a protein necessary for P4-ATPase function and ATP8B1 and ATP8B2, which express in HeLa cells. Upon activation of the Gq-coupled receptor, depletion of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] was accelerated in CDC50A knockout (KO) and ATP8B1/8B2 double KO cells compared with control cells, suggesting a decrease in PtdIns(4,5)P2 levels within the plasma membrane of the KO cells upon stimulation. These findings highlight the important role of P4-ATPases in maintaining phosphoinositide homeostasis and suggest a mechanism for asymmetry of phosphatidylinositol in the cytoplasmic leaflet of the plasma membrane.

De Novo Missense Variations of ATP8B2 Impair Its Phosphatidylcholine Flippase Activity

P4-ATPases comprise a family of lipid flippases that translocate lipids from the exoplasmic (or luminal) to the cytoplasmic leaflet of biological membranes. Of the 14 known human P4-ATPases, ATP8B2 is a phosphatidylcholine flippase at the plasma membrane, but its physiological function is not well understood. Although ATP8B2 could interact with both CDC50A and CDC50B, it required only the CDC50A interaction for its exit from the endoplasmic reticulum and subsequent transport to the plasma membrane. Three de novo monoallelic missense variations of ATP8B2 were found in patients with intellectual disability. None of these variations affected the interaction of ATP8B2 with CDC50A or its localization to the plasma membrane. However, variations of either of two amino acid residues, which are conserved in all P4-ATPases, significantly reduced the phosphatidylcholine flippase activity of ATP8B2. Furthermore, mutations in the corresponding residues of ATP8B1 and ATP11C were found to decrease their flippase activities toward phosphatidylcholine and phosphatidylserine, respectively. These results indicate that the conserved amino acid residues are crucial for the enzymatic activities of the P4-ATPases.

Role of Enzymes Capable of Transporting Phosphatidylserine in Brain Development and Brain Diseases

Phosphatidylserine (PS) is a common type of phospholipid, typically located in the inner leaflet of the cell membrane, especially abundant in the nervous system. It is an important component of the neuronal membrane and is considered to play a regulatory role in various brain functions, including memory and emotional stability, because its exposure to the outer leaflet of the neuronal membrane can result in abnormalities in various neurobiological processes such as synaptic transmission and neuronal apoptosis. Recently, research on two types of membrane proteins that synergistically mediate the transmembrane transport of phospholipid molecules in eukaryotic cells has become more in-depth and detailed. This review mainly explores the regulation of the expression of phosphatidylserine transporters and their impact on brain development and diseases.

The lipid flippase ATP8A1 regulates the recruitment of ARF effectors to the trans-Golgi Network

Formation of transport vesicles requires the coordinate activity of the coating machinery that selects cargo into the nascent vesicle and the membrane bending machinery that imparts curvature to the forming bud. Vesicle coating at the trans-Golgi Network (TGN) involves AP1, GGA2 and clathrin, which are recruited to membranes by activated ARF GTPases. The ARF activation at the TGN is mediated by the BIG1 and BIG2 guanine nucleotide exchange factors (GEFs). Membrane deformation at the TGN has been shown to be mediated by lipid flippases, including ATP8A1, that moves phospholipids from the inner to the outer leaflet of the TGN membrane. We probed a possible coupling between the coating and deformation machineries by testing for an interaction between BIG1, BIG2 and ATP8A1, and by assessing whether such an interaction may influence coating efficiency. Herein, we document that BIG1 and BIG2 co-localize with ATP8A1 in both, static and highly mobile TGN elements, and that BIG1 and BIG2 bind ATP8A1. We show that the interaction involves the catalytic Sec7 domain of the GEFs and the cytosolic C-terminal tail of ATP8A1. Moreover, we report that the expression of ATP8A1, but not ATP8A1 lacking the GEF-binding cytosolic tail, increases the generation of activated ARFs at the TGN and increases the selective recruitment of AP1, GGA2 and clathrin to TGN membranes. This occurs without increasing BIG1 or BIG2 levels at the TGN, suggesting that the binding of the ATP8A1 flippase tail to the Sec7 domain of BIG1/BIG2 increases their catalytic activity. Our results support a model in which a flippase component of the deformation machinery impacts the activity of the GEF component of the coating machinery.

Functional and in silico analysis of ATP8A2 and other P4-ATPase variants associated with human genetic diseases

P4-ATPases flip lipids from the exoplasmic to cytoplasmic leaflet of cell membranes, a property crucial for many biological processes. Mutations in P4-ATPases are associated with severe inherited and complex human disorders. We determined the expression, localization and ATPase activity of four variants of ATP8A2, the P4-ATPase associated with the neurodevelopmental disorder known as cerebellar ataxia, impaired intellectual development and disequilibrium syndrome 4 (CAMRQ4). Two variants, G447R and A772P, harboring mutations in catalytic domains, expressed at low levels and mislocalized in cells. In contrast, the E459Q variant in a flexible loop displayed wild-type expression levels, Golgi-endosome localization and ATPase activity. The R1147W variant expressed at 50% of wild-type levels but showed normal localization and activity. These results indicate that the G447R and A772P mutations cause CAMRQ4 through protein misfolding. The E459Q mutation is unlikely to be causative, whereas the R1147W may display a milder disease phenotype. Using various programs that predict protein stability, we show that there is a good correlation between the experimental expression of the variants and in silico stability assessments, suggesting that such analysis is useful in identifying protein misfolding disease-associated variants.

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.

Direct evidence of lipid transport by the Drs2-Cdc50 flippase upon truncation of its terminal regions

P4-ATPases in complex with Cdc50 subunits are lipid flippases that couple ATP hydrolysis with lipid transport to the cytoplasmic leaflet of membranes to create lipid asymmetry. Such vectorial transport has been shown to contribute to vesicle formation in the late secretory pathway. Some flippases are regulated by autoinhibitory regions that can be destabilized by protein kinase-mediated phosphorylation and possibly by binding of cytosolic proteins. In addition, the binding of lipids to flippases may also induce conformational changes required for the activity of these transporters. Here, we address the role of phosphatidylinositol-4-phosphate (PI4P) and the terminal autoinhibitory tails on the lipid flipping activity of the yeast lipid flippase Drs2-Cdc50. By functionally reconstituting the full-length and truncated forms of Drs2 in a 1:1 complex with the Cdc50 subunit, we provide compelling evidence that lipid flippase activity is exclusively detected for the truncated Drs2 variant and is dependent on the presence of the phosphoinositide PI4P. These findings highlight the critical role of phosphoinositides as lipid co-factors in the regulation of lipid transport by the Drs2-Cdc50 flippase.

Adaptor protein AP-3 produces synaptic vesicles that release at high frequency by recruiting phospholipid flippase ATP8A1

Neural systems encode information in the frequency of action potentials, which is then decoded by synaptic transmission. However, the rapid, synchronous release of neurotransmitters depletes synaptic vesicles (SVs), limiting release at high firing rates. How then do synapses convey information about frequency? Here, we show in mouse hippocampal neurons and slices that the adaptor protein AP-3 makes a subset of SVs that respond specifically to high-frequency stimulation. Neurotransmitter transporters slot onto these SVs in different proportions, contributing to the distinct properties of release observed at different excitatory synapses. Proteomics reveals that AP-3 targets the phospholipid flippase ATP8A1 to SVs; loss of ATP8A1 recapitulates the defect in SV mobilization at high frequency observed with loss of AP-3. The mechanism involves recruitment of synapsin by the cytoplasmically oriented phosphatidylserine translocated by ATP8A1. Thus, ATP8A1 enables the subset of SVs made by AP-3 to release at high frequency.

Reconstitution of ATP-dependent lipid transporters: gaining insight into molecular characteristics, regulation, and mechanisms

Lipid transporters play a crucial role in supporting essential cellular processes such as organelle assembly, vesicular trafficking, and lipid homeostasis by driving lipid transport across membranes. Cryo-electron microscopy has recently resolved the structures of several ATP-dependent lipid transporters, but functional characterization remains a major challenge. Although studies of detergent-purified proteins have advanced our understanding of these transporters, in vitro evidence for lipid transport is still limited to a few ATP-dependent lipid transporters. Reconstitution into model membranes, such as liposomes, is a suitable approach to study lipid transporters in vitro and to investigate their key molecular features. In this review, we discuss the current approaches for reconstituting ATP-driven lipid transporters into large liposomes and common techniques used to study lipid transport in proteoliposomes. We also highlight the existing knowledge on the regulatory mechanisms that modulate the activity of lipid transporters, and finally, we address the limitations of the current approaches and future perspectives in this field.

Type IV P-Type ATPases: Recent Updates in Cancer Development, Progression, and Treatment

Adaptations of cancer cells for survival are remarkable. One of the most significant properties of cancer cells to prevent the immune system response and resist chemotherapy is the altered lipid metabolism and resulting irregular cell membrane composition. The phospholipid distribution in the plasma membrane of normal animal cells is distinctly asymmetric. Lipid flippases are a family of enzymes regulating membrane asymmetry, and the main class of flippases are type IV P-type ATPases (P4-ATPases). Alteration in the function of flippases results in changes to membrane organization. For some lipids, such as phosphatidylserine, the changes are so drastic that they are considered cancer biomarkers. This review will analyze and discuss recent publications highlighting the role that P4-ATPases play in the development and progression of various cancer types, as well as prospects of targeting P4-ATPases for anti-cancer treatment.

Knockdown of TMEM30A in renal tubular epithelial cells leads to reduced glucose absorption

The kidney reabsorbs large amounts of glucose through Na+-glucose cotransporter 2 (SGLT2). P4-ATPase acts together with the β-subunit TMEM30A to mediate the asymmetric distribution of phosphatidylserine (PS), phosphatidylethanolamine (PE), and other amino phospholipids, promoting plasma membrane and internal vesicle fusion, and facilitating vesicle protein transport. We observed reduced TMEM30A expression in renal tubules of DKD and IgA patients, suggesting a potential role of TMEM30A in renal tubular cells. To investigate the role of TMEM30A in renal tubules, we constructed a TMEM30A knockdown cell model by transfecting mouse kidney tubular epithelium cells (TCMK-1) with TMEM30A shRNA. Knockdown of TMEM30A in TCMK-1 cells attenuated vesicle transporter protein synthesis, resulting in reduced transport and expression of SGLT2, which in turn reduced glucose absorption. These data suggested that TMEM30A plays a crucial role in renal tubules.

Systematic crosstalk in plasmalogen and diacyl lipid biosynthesis for their differential yet concerted molecular functions in the cell

Plasmalogen is a major phospholipid of mammalian cell membranes. Recently it is becoming evident that the sn-1 vinyl-ether linkage in plasmalogen, contrasting to the ester linkage in the counterpart diacyl glycerophospholipid, yields differential molecular characteristics for these lipids especially related to hydrocarbon-chain order, so as to concertedly regulate biological membrane processes. A role played by NMR in gaining information in this respect, ranging from molecular to tissue levels, draws particular attention. We note here that a broad range of enzymes in de novo synthesis pathway of plasmalogen commonly constitute that of diacyl glycerophospholipid. This fact forms the basis for systematic crosstalk that not only controls a quantitative balance between these lipids, but also senses a defect causing loss of lipid in either pathway for compensation by increase of the counterpart lipid. However, this inherent counterbalancing mechanism paradoxically amplifies imbalance in differential effects of these lipids in a diseased state on membrane processes. While sharing of enzymes has been recognized, it is now possible to overview the crosstalk with growing information for specific enzymes involved. The overview provides a fundamental clue to consider cell and tissue type-dependent schemes in regulating membrane processes by plasmalogen and diacyl glycerophospholipid in health and disease.

Structural, topological, and functional characterization of transmembrane proteins TMEM213, 207, 116, 72 and 30B provides a potential link to ccRCC etiology

Due to their involvement in the development of various cancers Transmembrane Proteins (TMEMs) are the focus of many recent studies. Previously we reported TMEM de-regulation in clear cell Renal Cell Carcinoma (ccRCC) with TMEM213, 207, 116, 72 and 30B being among the most downregulated on mRNA level. TMEM down-regulation was also more pronounced in advanced ccRCC tumors and was potentially linked to clinical parameters such as: metastasis (TMEM72 and 116), Fuhrman grade (TMEM30B) and overall survival (TMEM30B). To further investigate these findings, first, we set off to prove experimentally that selected TMEMs are indeed membrane-bound as predicted in silico, we verified the presence of signaling peptides on their N-termini, orientation of TMEMs within the membrane and validated their predicted cellular localization. To investigate the potential role of selected TMEMs in cellular processes overexpression studies in HEK293 and HK-2 cell lines were carried out. Additionally, we tested TMEM isoform expression in ccRCC tumors, identified mutations in TMEM genes and examined chromosomal aberrations in their loci. We confirmed the membrane-bound status of all selected TMEMs, assigned TMEM213, and 207 to early endosomes, TMEM72 to early endosomes and plasma membrane, TMEM116 and 30B to the endoplasmic reticulum. The N-terminus of TMEM213 was found to be exposed to the cytoplasm, the C-terminus of TMEM207, 116 and 72 were directed toward the cytoplasm, and both termini of TMEM30B faced the cytoplasm. Interestingly, TMEM mutations and chromosomal aberrations were infrequent in ccRCC tumors, yet we identified potentially damaging mutations in TMEM213 and TMEM30B and found deletions in the TMEM30B locus in nearly 30% of the tumors. Overexpression studies suggested selected TMEMs may take part in carcinogenesis processes such as cell adhesion, regulation of epithelial cell proliferation, and regulation of adaptive immune response, which could indicate a link to the development and progression of ccRCC.

Regulation of plasmalogen biosynthesis in mammalian cells and tissues

Plasmalogens are a unique family of cellular glycerophospholipids that contain a vinyl-ether bond. Synthesis of plasmalogens is initiated in peroxisomes and completed in the endoplasmic reticulum. The absence of plasmalogens in several organs of patients with deficiency in peroxisome biogenesis suggests that de novo synthesis of plasmalogens contributes significantly to plasmalogen homeostasis in humans. Plasmalogen biosynthesis is spatiotemporally regulated by a feedback mechanism that senses the amount of plasmalogens in the inner leaflet of the plasma membrane and regulates the stability of fatty acyl-CoA reductase 1 (FAR1), the rate-limiting enzyme for plasmalogen biosynthesis. Dysregulation of plasmalogen synthesis impairs cholesterol synthesis in cells and brain, resulting in the reduced expression of genes such as mRNA encoding myelin basic protein, a phenotype found in the cerebellum of plasmalogen-deficient mice. In this review, we summarize the current knowledge of molecular mechanisms underlying the regulation of plasmalogen biosynthesis and the link between plasmalogen homeostasis and cholesterol biosynthesis, and address the pathogenesis of impaired plasmalogen homeostasis in rodent and humans.

Structural view of G protein-coupled receptor signaling in the retinal rod outer segment

Visual phototransduction is the most extensively studied G protein-coupled receptor (GPCR) signaling pathway because of its quantifiable stimulus, non-redundancy of genes, and immense importance in vision. We summarize recent discoveries that have advanced our understanding of rod outer segment (ROS) morphology and the pathological basis of retinal diseases. We have combined recently published cryo-electron tomography (cryo-ET) data on the ROS with structural knowledge on individual proteins to define the precise spatial limitations under which phototransduction occurs. Although hypothetical, the reconstruction of the rod phototransduction system highlights the potential roles of phosphodiesterase 6 (PDE6) and guanylate cyclases (GCs) in maintaining the spacing between ROS discs, suggesting a plausible mechanism by which intrinsic optical signals are generated in the retina.

Physiological and Pathological Functions of TMEM30A: An Essential Subunit of P4-ATPase Phospholipid Flippases

Phospholipids are asymmetrically distributed across mammalian plasma membrane. The function of P4-ATPases is to maintain the abundance of phosphatidylserine (PS) and phosphatidylethanolamine (PE) in the inner leaflet as lipid flippases. Transmembrane protein 30A (TMEM30A, also named CDC50A), as an essential β subunit of most P4-ATPases, facilitates their transport and functions. With TMEM30A knockout mice or cell lines, it is found that the loss of TMEM30A has huge influences on the survival of mice and cells because of PS exposure-triggered apoptosis signaling. TMEM30A is a promising target for drug discovery due to its significant roles in various systems and diseases. In this review, we summarize the functions of TMEM30A in different systems, present current understanding of the protein structures and mechanisms of TMEM30A-P4-ATPase complexes, and discuss how these fundamental aspects of TMEM30A may be applied to disease treatment.

Association of ATP8B3 gene polymorphisms with aspirin-exacerbated respiratory disease in asthmatics

BACKGROUND

Aspirin-exacerbated respiratory disease (AERD), an asthma phenotype, often presents with severe manifestations and it remains widely underdiagnosed because of insufficient awareness of the relationship between the ingestion of nonsteroidal anti-inflammatory drugs, including acetylsalicylic acid (ASA), and asthma exacerbation. Our previous genome-wide association study demonstrated an association between a single nucleotide polymorphism (SNP) of the ATP8B3 gene and the risk of AERD. This study examined AERD-related SNPs of the ATP8B3 gene in a large population.

METHODS

Twenty-five SNPs of ATP8B3 were genotyped with the GoldenGate assay using VeraCode microbeads in 141 asthmatics with AERD and 995 Aspirin-tolerant asthma (ATA). The genotype distribution was analyzed using logistic regression models. The declines in forced expiratory volume in 1 second (FEV1)following an ASA challenge were compared among the genotypes and haplotypes using a type III generalized linear model.

RESULTS

The minor allele frequencies (MAFs) of rs10421558 A>G in the 5'UTR and rs10403288 G>A in the intron were significantly lower in the AERD than the ATA [34.0% vs. 43.8%, OR = 0.66 (0.62-0.92), Pcorr = 0.03 and 28.4% vs. 35.4%, OR = 0.62 (0.59-0.89), Pcorr = 0.016, respectively]. BL1ht5 was significantly higher in the AERD [7.6% vs. 1.6%, OR = 12.23 (0.2-0.51), P = 4.7 × 10 -4 , Pcorr = 0.001]. Among them, rs10421558 A>G and BL1ht5 were associated with the percent decline in FEV1 on the oral ASA challenge test.

CONCLUSION

The minor allele of rs10421558 A>G in the 5'UTR may protect against the development of AERD via the increased production of ATP8B3.

ATP8B2-Mediated Asymmetric Distribution of Plasmalogens Regulates Plasmalogen Homeostasis and Plays a Role in Intracellular Signaling

Plasmalogens are a subclass of glycerophospholipid containing vinyl-ether bond at the sn-1 position of glycerol backbone. Ethanolamine-containing plasmalogens (plasmalogens) are major constituents of cellular membranes in mammalian cells and de novo synthesis of plasmalogens largely contributes to the homeostasis of plasmalogens. Plasmalogen biosynthesis is regulated by a feedback mechanism that senses the plasmalogen level in the inner leaflet of the plasma membrane and regulates the stability of fatty acyl-CoA reductase 1 (Far1), a rate-limiting enzyme for plasmalogen biosynthesis. However, the molecular mechanism underlying the localization of plasmalogens in cytoplasmic leaflet of plasma membrane remains unknown. To address this issue, we attempted to identify a potential transporter of plasmalogens from the outer to the inner leaflet of plasma membrane by focusing on phospholipid flippases, type-IV P-type adenosine triphosphatases (P4-ATPase), localized in the plasma membranes. We herein show that knockdown of ATP8B2 belonging to the class-1 P4-ATPase enhances localization of plasmalogens but not phosphatidylethanolamine in the extracellular leaflet and impairs plasmalogen-dependent degradation of Far1. Furthermore, phosphorylation of protein kinase B (AKT) is downregulated by lowering the expression of ATP8B2, which leads to suppression of cell growth. Taken together, these results suggest that enrichment of plasmalogens in the cytoplasmic leaflet of plasma membranes is mediated by ATP8B2 and this asymmetric distribution of plasmalogens is required for sensing plasmalogens as well as phosphorylation of AKT.

Lipid Dyshomeostasis and Inherited Cerebellar Ataxia

Cerebellar ataxia is a form of ataxia that originates from dysfunction of the cerebellum, but may involve additional neurological tissues. Its clinical symptoms are mainly characterized by the absence of voluntary muscle coordination and loss of control of movement with varying manifestations due to differences in severity, in the site of cerebellar damage and in the involvement of extracerebellar tissues. Cerebellar ataxia may be sporadic, acquired, and hereditary. Hereditary ataxia accounts for the majority of cases. Hereditary ataxia has been tentatively divided into several subtypes by scientists in the field, and nearly all of them remain incurable. This is mainly because the detailed mechanisms of these cerebellar disorders are incompletely understood. To precisely diagnose and treat these diseases, studies on their molecular mechanisms have been conducted extensively in the past. Accumulating evidence has demonstrated that some common pathogenic mechanisms exist within each subtype of inherited ataxia. However, no reports have indicated whether there is a common mechanism among the different subtypes of inherited cerebellar ataxia. In this review, we summarize the available references and databases on neurological disorders characterized by cerebellar ataxia and show that a subset of genes involved in lipid homeostasis form a new group that may cause ataxic disorders through a common mechanism. This common signaling pathway can provide a valuable reference for future diagnosis and treatment of ataxic disorders.

Autoinhibition and regulation by phosphoinositides of ATP8B1, a human lipid flippase associated with intrahepatic cholestatic disorders

P4-ATPases flip lipids from the exoplasmic to the cytosolic leaflet, thus maintaining lipid asymmetry in eukaryotic cell membranes. Mutations in several human P4-ATPase genes are associated with severe diseases, for example in ATP8B1 causing progressive familial intrahepatic cholestasis, a rare inherited disorder progressing toward liver failure. ATP8B1 forms a binary complex with CDC50A and displays a broad specificity to glycerophospholipids, but regulatory mechanisms are unknown. Here, we report functional studies and the cryo-EM structure of the human lipid flippase ATP8B1-CDC50A at 3.1 Å resolution. We find that ATP8B1 is autoinhibited by its N- and C-terminal tails, which form extensive interactions with the catalytic sites and flexible domain interfaces. Consistently, ATP hydrolysis is unleashed by truncation of the C-terminus, but also requires phosphoinositides, most markedly phosphatidylinositol-3,4,5-phosphate (PI(3,4,5)P₃), and removal of both N- and C-termini results in full activation. Restored inhibition of ATP8B1 truncation constructs with a synthetic peptide mimicking the C-terminal segment further suggests molecular communication between N- and C-termini in the autoinhibition and demonstrates that the regulatory mechanism can be interfered with by exogenous compounds. A recurring (G/A)(Y/F)AFS motif of the C-terminal segment suggests that this mechanism is employed widely across P4-ATPase lipid flippases in plasma membrane and endomembranes.

Structural insights into the activation of autoinhibited human lipid flippase ATP8B1 upon substrate binding

ATP8B1 is a P4 ATPase that maintains membrane asymmetry by transporting phospholipids across the cell membrane. Disturbance of lipid asymmetry will lead to the imbalance of the cell membrane and eventually, cell death. Thus, defects in ATP8B1 are usually associated with severe human diseases, such as intrahepatic cholestasis. The present structures of ATP8B1 complexed with its auxiliary noncatalytic partners CDC50A and CDC50B reveal an autoinhibited state of ATP8B1 that could be released upon substrate binding. Moreover, release of this autoinhibition could be facilitated by the bile acids, which are key factors that alter the membrane asymmetry of hepatocytes. This enabled us to figure out a feedback loop of bile acids and lipids across the cell membrane.

The human P4 ATPase ATP8B1 in complex with the auxiliary noncatalytic protein CDC50A or CDC50B mediates the transport of cell-membrane lipids from the outer to the inner membrane leaflet, which is crucial to maintain the asymmetry of membrane lipids. Its dysfunction usually leads to an imbalance of bile-acid circulation and eventually causes intrahepatic cholestasis diseases. Here, we found that both ATP8B1–CDC50A and ATP8B1–CDC50B possess a higher ATPase activity in the presence of the most favored substrate phosphatidylserine (PS), and, moreover, that the PS-stimulated activity could be augmented upon the addition of bile acids. The 3.4-Å cryo-electron microscopy structures of ATP8B1–CDC50A and ATP8B1–CDC50B enabled us to capture a phosphorylated and autoinhibited state, with the N- and C-terminal tails separately inserted into the cytoplasmic interdomain clefts of ATP8B1. The PS-bound ATP8B1–CDC50A structure at 4.0-Å resolution indicated that the autoinhibited state could be released upon PS binding. Structural analysis combined with mutagenesis revealed the residues that determine the substrate specificity and a unique positively charged loop in the phosphorylated domain of ATP8B1 for the recruitment of bile acids. Together, we supplemented the Post–Albers transport cycle of P4 ATPases with an extra autoinhibited state of ATP8B1, which could be activated upon substrate binding. These findings not only provide structural insights into the ATP8B1-mediated restoration of human membrane lipid asymmetry during bile-acid circulation, but also advance our understanding of the molecular mechanism of P4 ATPases.

Lipid flippase dysfunction as a therapeutic target for endosomal anomalies in Alzheimer's disease

Endosomal anomalies because of vesicular traffic impairment have been indicated as an early pathology of Alzheimer'| disease (AD). However, the mechanisms and therapeutic targets remain unclear. We previously reported that βCTF, one of the pathogenic metabolites of APP, interacts with TMEM30A. TMEM30A constitutes a lipid flippase with P4-ATPase and regulates vesicular trafficking through the asymmetric distribution of phospholipids. Therefore, the alteration of lipid flippase activity in AD pathology has got attention. Herein, we showed that the interaction between βCTF and TMEM30A suppresses the physiological formation and activity of lipid flippase in AD model cells, A7, and AppNL-G-F/NL-G-F model mice. Furthermore, the T-RAP peptide derived from the βCTF binding site of TMEM30A improved endosomal anomalies, which could be a result of the restored lipid flippase activity. Our results provide insights into the mechanisms of vesicular traffic impairment and suggest a therapeutic target for AD.

CDC50A is required for aminophospholipid transport and cell fusion in mouse C2C12 myoblasts

Myoblast fusion is essential for the formation of multinucleated muscle fibers and is promoted by transient changes in the plasma membrane lipid distribution. However, little is known about the lipid transporters regulating these dynamic changes. Here, we show that proliferating myoblasts exhibit an aminophospholipid flippase activity that is downregulated during differentiation. Deletion of the P4-ATPase flippase subunit CDC50A (also known as TMEM30A) results in loss of the aminophospholipid flippase activity and compromises actin remodeling, RAC1 GTPase membrane targeting and cell fusion. In contrast, deletion of the P4-ATPase ATP11A affects aminophospholipid uptake without having a strong impact on cell fusion. Our results demonstrate that myoblast fusion depends on CDC50A and may involve multiple CDC50A-dependent P4-ATPases that help to regulate actin remodeling.

Long-read assembly of the Chinese indigenous Ningxiang pig genome and identification of genetic variations in fat metabolism among different breeds

Advances in long-read sequencing technology and genome assembly provide an opportunity to improve the pig genome and reveal the full range of structural variations (SVs) between local Chinese and European pigs. To date, little is known about the genomes of some unique Chinese indigenous breeds, such as the Ningxiang pig. Here, we report the sequencing and assembly of a highly contiguous Ningxiang pig genome (NX) via an integration of PacBio single-molecule real-time sequencing, Illumina next-generation sequencing, BioNano optical mapping and Hi-C (chromosome conformation capture) approaches. The assembled genome comprises 2.44 Gb with a contig N50 of 26.1 Mb and 418 contigs in total. These contigs are organized into 121 scaffolds with a scaffold N50 of 139.0 Mb. More than 99.1% of the assembled sequence could be localized to 19 pseudochromosomes and is annotated with 20,914 protein-coding genes and 34.04% repetitive sequences. Comparisons between the NX and European Duroc assemblies revealed many SVs in genes involved in the immune system, nervous system, lipid metabolism and environmental adaptation. The genetic variants include 47 Chinese domestic pig-specific SVs and the associated 74 genes may contribute to the differences in domestic traits compared to European pigs. Moreover, single nucleotide polymorphisms (SNPs) identified from whole genome resequencing data of 73 Chinese pigs, representing 17 geographically isolated breeds, showed their specific genetic variations, population structure and evolutionary patterns. Finally, we explore transcriptional regulation in the first intron of the MYL4 gene, as the genomic SV (281-bp deletion) in Ningxiang pig promotes its subcutaneous fat compared to European pig breeds. This work identifies a set of Asian-specific SVs and SNPs, which will be important resources for modern pig breeding and genetic conservation.

Structural basis of the P4B ATPase lipid flippase activity

P4 ATPases are lipid flippases that are phylogenetically grouped into P4A, P4B and P4C clades. The P4A ATPases are heterodimers composed of a catalytic α-subunit and accessory β-subunit, and the structures of several heterodimeric flippases have been reported. The S. cerevisiae Neo1 and its orthologs represent the P4B ATPases, which function as monomeric flippases without a β-subunit. It has been unclear whether monomeric flippases retain the architecture and transport mechanism of the dimeric flippases. Here we report the structure of a P4B ATPase, Neo1, in its E1-ATP, E2P-transition, and E2P states. The structure reveals a conserved architecture as well as highly similar functional intermediate states relative to dimeric flippases. Consistently, structure-guided mutagenesis of residues in the proposed substrate translocation path disrupted Neo1’s ability to establish membrane asymmetry. These observations indicate that evolutionarily distant P4 ATPases use a structurally conserved mechanism for substrate transport.

The P4 ATPase lipid flippases play a crucial role in membrane biogenesis. Here the authors report the structure of the monomeric P4B ATPase Neo1 in several states, clarifying the mechanism of substrate transport.

P-Type ATPase Apt1 of the Fungal Pathogen Cryptococcus neoformans Is a Lipid Flippase of Broad Substrate Specificity

Lipid flippases of the P4-ATPase family are ATP-driven transporters that translocate lipids from the exoplasmic to the cytosolic leaflet of biological membranes. In the encapsulated fungal pathogen Cryptococcus neoformans, the P4-ATPase Apt1p is an important regulator of polysaccharide secretion and pathogenesis, but its biochemical characterization is lacking. Phylogenetic analysis revealed that Apt1p belongs to the subclade of P4A-ATPases characterized by the common requirement for a β-subunit. Using heterologous expression in S. cerevisiae, we demonstrate that Apt1p forms a heterodimeric complex with the C. neoformans Cdc50 protein. This association is required for both localization and activity of the transporter complex. Lipid flippase activity of the heterodimeric complex was assessed by complementation tests and uptake assays employing fluorescent lipids and revealed a broad substrate specificity, including several phospholipids, the alkylphospholipid miltefosine, and the glycolipids glucosyl- and galactosylceramide. Our results suggest that transbilayer lipid transport in C. neoformans is finely regulated to promote fungal virulence, which reinforces the potential of Apt1p as a target for antifungal drug development.

Deletion of phosphatidylserine flippase β-subunit Tmem30a in satellite cells leads to delayed skeletal muscle regeneration

Phosphatidylserine (PS) is distributed asymmetrically in the plasma membrane of eukaryotic cells. Phosphatidylserine flippase (P4-ATPase) transports PS from the outer leaflet of the lipid bilayer to the inner leaflet of the membrane to maintain PS asymmetry. The β subunit TMEM30A is indispensable for transport and proper function of P4-ATPase. Previous studies have shown that the ATP11A and TMEM30A complex is the molecular switch for myotube formation. However, the role of Tmem30a in skeletal muscle regeneration remains elusive. In the current study, Tmem30a was highly expressed in the tibialis anterior (TA) muscles of dystrophin-null (mdx) mice and BaCl₂-induced muscle injury model mice. We generated a satellite cell (SC)-specific Tmem30a conditional knockout (cKO) mouse model to investigate the role of Tmem30a in skeletal muscle regeneration. The regenerative ability of cKO mice was evaluated by analyzing the number and diameter of regenerated SCs after the TA muscles were injured by BaCl₂-injection. Compared to the control mice, the cKO mice showed decreased Pax7⁺ and MYH3⁺ SCs, indicating diminished SC proliferation, and decreased expression of muscular regulatory factors (MYOD and MYOG), suggesting impaired myoblast proliferation in skeletal muscle regeneration. Taken together, these results demonstrate the essential role of Tmem30a in skeletal muscle regeneration.

Lipid flippases as key players in plant adaptation to their environment

Lipid flippases (P4 ATPases) are active transporters that catalyse the translocation of lipids between the two sides of the biological membranes in the secretory pathway. This activity modulates biological membrane properties, contributes to vesicle formation, and is the trigger for lipid signalling events, which makes P4 ATPases essential for eukaryotic cell survival. Plant P4 ATPases (also known as aminophospholipid ATPases (ALAs)) are crucial for plant fertility and proper development, and are involved in key adaptive responses to biotic and abiotic stress, including chilling tolerance, heat adaptation, nutrient deficiency responses and pathogen defence. While ALAs present many analogies to mammalian and yeast P4 ATPases, they also show characteristic features as the result of their independent evolution. In this Review, the main properties, roles, regulation and mechanisms of action of ALA proteins are discussed.

Loss of phosphatidylserine flippase β-subunit Tmem30a in podocytes leads to albuminuria and glomerulosclerosis

The asymmetric distribution of phosphatidylserine (PS) in the cytoplasmic leaflet of eukaryotic cell plasma membranes is regulated by a group of P4-ATPases (named PS flippases) and the β-subunit TMEM30A. Podocytes in the glomerulus form a filtration barrier to prevent the traversing of large cellular elements and macromolecules from the blood into the urinary space. Damage to podocytes can disrupt the filtration barrier and lead to proteinuria and podocytopathy. We observed reduced TMEM30A expression in patients with minimal change disease and membranous nephropathy, indicating potential roles of TMEM30A in podocytopathy. To investigate the role of Tmem30a in the kidney, we generated a podocyte-specific Tmem30a knockout (KO) mouse model using the NPHS2-Cre line. Tmem30a KO mice displayed albuminuria, podocyte degeneration, mesangial cell proliferation with prominent extracellular matrix accumulation and eventual progression to focal segmental glomerulosclerosis. Our data demonstrate a critical role of Tmem30a in maintaining podocyte survival and glomerular filtration barrier integrity. Understanding the dynamic regulation of the PS distribution in the glomerulus provides a unique perspective to pinpointing the mechanism of podocyte damage and potential therapeutic targets.

AP-3-dependent targeting of flippase ATP8A1 to lamellar bodies suppresses activation of YAP in alveolar epithelial type 2 cells

Lamellar bodies (LBs) are lysosome-related organelles (LROs) of surfactant-producing alveolar type 2 (AT2) cells of the distal lung epithelium. Trafficking pathways to LBs have been understudied but are likely critical to AT2 cell homeostasis given associations between genetic defects of endosome to LRO trafficking and pulmonary fibrosis in Hermansky Pudlak syndrome (HPS). Our prior studies uncovered a role for AP-3, defective in HPS type 2, in trafficking Peroxiredoxin-6 to LBs. We now show that the P4-type ATPase ATP8A1 is sorted by AP-3 from early endosomes to LBs through recognition of a C-terminal dileucine-based signal. Disruption of the AP-3/ATP8A1 interaction causes ATP8A1 accumulation in early sorting and/or recycling endosomes, enhancing phosphatidylserine exposure on the cytosolic leaflet. This in turn promotes activation of Yes-activating protein, a transcriptional coactivator, augmenting cell migration and AT2 cell numbers. Together, these studies illuminate a mechanism whereby loss of AP-3-mediated trafficking contributes to a toxic gain-of-function that results in enhanced and sustained activation of a repair pathway associated with pulmonary fibrosis.

The phosphatidylserine flippase β-subunit Tmem30a is essential for normal insulin maturation and secretion

The processing, maturation, and secretion of insulin are under precise regulation, and dysregulation causes profound defects in glucose handling, leading to diabetes. Tmem30a is the β subunit of the phosphatidylserine (PS) flippase, which maintains the membrane asymmetric distribution of PS. Tmem30a regulates cell survival and the localization of subcellular structures and is thus critical to the normal function of multiple physiological systems. Here, we show that conditional knockout of Tmem30a specifically in pancreatic islet β cells leads to obesity, hyperglycemia, glucose intolerance, hyperinsulinemia, and insulin resistance in mice, due to insufficient insulin release. Moreover, we reveal that Tmem30a plays an essential role in clathrin-mediated vesicle transport between the trans Golgi network (TGN) and the plasma membrane (PM), which comprises immature secretory granule (ISG) budding at the TGN. We also find that Tmem30a deficiency impairs clathrin-mediated vesicle budding and thus blocks both insulin maturation in ISGs and the transport of glucose-sensing Glut2 to the PM. Collectively, these disruptions compromise both insulin secretion and glucose sensitivity, thus contributing to impairments in glucose-stimulated insulin secretion. Taken together, our data demonstrate an important role of Tmem30a in insulin maturation and glucose metabolic homeostasis and suggest the importance of membrane phospholipid distribution in metabolic disorders.

Novel variants in critical domains of ATP8A2 and expansion of clinical spectrum

ATP8A2 is a P4-ATPase that flips phosphatidylserine across membranes to generate and maintain transmembrane phospholipid asymmetry. Loss-of-function variants cause severe neurodegenerative and developmental disorders. We have identified three ATP8A2 variants in unrelated Iranian families that cause intellectual disability, dystonia, below-average head circumference, mild optic atrophy, and developmental delay. Additionally, all the affected individuals displayed tooth abnormalities associated with defects in teeth development. Two variants (p.Asp825His and p.Met438Val) reside in critical functional domains of ATP8A2. These variants express at very low levels and lack ATPase activity. Inhibitor studies indicate that these variants are misfolded and degraded by the cellular proteasome. We conclude that Asp825, which coordinates with the Mg²⁺ ion within the ATP binding site, and Met438 are essential for the proper folding of ATP8A2 into a functional flippase. We also provide evidence on the association of tooth abnormalities with defects in ATP8A2, thereby expanding the clinical spectrum of the associated disease.

The transport mechanism of P4 ATPase lipid flippases

P4 ATPase lipid flippases are ATP-driven transporters that translocate specific lipids from the exoplasmic to the cytosolic leaflet of biological membranes, thus establishing a lipid gradient between the two leaflets that is essential for many cellular processes. While substrate specificity, subcellular and tissue-specific expression, and physiological functions have been assigned to a number of these transporters in several organisms, the mechanism of lipid transport has been a topic of intense debate in the field. The recent publication of a series of structural models based on X-ray crystallography and cryo-EM studies has provided the first glimpse into how P4 ATPases have adapted the transport mechanism used by the cation-pumping family members to accommodate a substrate that is at least an order of magnitude larger than cations.

The N- or C-terminal cytoplasmic regions of P4-ATPases determine their cellular localization

Mammalian P4-ATPases specifically localize to the plasma membrane and the membranes of intracellular compartments. P4-ATPases contain 10 transmembrane domains, and their N- and C-terminal (NT and CT) regions face the cytoplasm. Among the ATP10 and ATP11 proteins of P4-ATPases, ATP10A, ATP10D, ATP11A, and ATP11C localize to the plasma membrane, while ATP10B and ATP11B localize to late endosomes and early/recycling endosomes, respectively. We previously showed that the NT region of ATP9B is critical for its localization to the Golgi apparatus, while the CT regions of ATP11C isoforms are critical for Ca²⁺-dependent endocytosis or polarized localization at the plasma membrane. Here, we conducted a comprehensive analysis of chimeric proteins and found that the NT region of ATP10 proteins and the CT region of ATP11 proteins are responsible for their specific subcellular localization. Importantly, the ATP10B NT and the ATP11B CT regions were found to harbor a trafficking and/or targeting signal that allows these P4-ATPases to localize to late endosomes and early/recycling endosomes, respectively. Moreover, dileucine residues in the NT region of ATP10B were required for its trafficking to endosomal compartments. These results suggest that the NT and CT sequences of P4-ATPases play a key role in their intracellular trafficking.

Mouse Models of Inherited Retinal Degeneration with Photoreceptor Cell Loss

Inherited retinal degeneration (RD) leads to the impairment or loss of vision in millions of individuals worldwide, most frequently due to the loss of photoreceptor (PR) cells. Animal models, particularly the laboratory mouse, have been used to understand the pathogenic mechanisms that underlie PR cell loss and to explore therapies that may prevent, delay, or reverse RD. Here, we reviewed entries in the Mouse Genome Informatics and PubMed databases to compile a comprehensive list of monogenic mouse models in which PR cell loss is demonstrated. The progression of PR cell loss with postnatal age was documented in mutant alleles of genes grouped by biological function. As anticipated, a wide range in the onset and rate of cell loss was observed among the reported models. The analysis underscored relationships between RD genes and ciliary function, transcription-coupled DNA damage repair, and cellular chloride homeostasis. Comparing the mouse gene list to human RD genes identified in the RetNet database revealed that mouse models are available for 40% of the known human diseases, suggesting opportunities for future research. This work may provide insight into the molecular players and pathways through which PR degenerative disease occurs and may be useful for planning translational studies.

Calpain cleaves phospholipid flippase ATP8A1 during apoptosis in platelets

The asymmetric distribution of phospholipids in the plasma/organellar membranes is generated and maintained through phospholipid flippases in resting cells, but becomes disrupted in apoptotic cells and activated platelets, resulting in phosphatidylserine (PS) exposure on the cell surface. Stable PS exposure during apoptosis requires inactivation of flippases to prevent PS from being reinternalized. Here we show that flippase ATP8A1 is highly expressed in both murine and human platelets, but is not present in the plasma membrane. ATP8A1 is cleaved by the cysteine protease calpain during apoptosis, and the cleavage is prevented indirectly by caspase inhibition, involving blockage of calcium influx into platelets and subsequent calpain activation. In contrast, in platelets activated with thrombin and collagen and exposing PS, ATP8A1 remains intact. These data reveal a novel mechanism of flippase cleavage and suggest that flippase activity in intracellular membranes differs between platelets undergoing apoptosis and activation.

ATPase reaction cycle of P4-ATPases affects their transport from the endoplasmic reticulum

P4-ATPases belonging to the P-type ATPase superfamily mediate active transport of phospholipids across cellular membranes. Most P4-ATPases, except ATP9A and ATP9B proteins, form heteromeric complexes with CDC50 proteins, which are required for transport of P4-ATPases from the endoplasmic reticulum (ER) to their final destinations. P-type ATPases form autophosphorylated intermediates during the ATPase reaction cycle. However, the association of the catalytic cycle of P4-ATPases with their transport from the ER and their cellular localization has not been studied. Here, we show that transport of ATP9 and ATP11 proteins as well as that of ATP10A from the ER depends on the ATPase catalytic cycle, suggesting that conformational changes in P4-ATPases during the catalytic cycle are crucial for their transport from the ER.

Expression and functional characterization of missense mutations in ATP8A2 linked to severe neurological disorders

ATP8A2 is a P4-ATPase (adenosine triphosphate) that actively flips phosphatidylserine and phosphatidylethanolamine from the exoplasmic to the cytoplasmic leaflet of cell membranes to generate and maintain phospholipid asymmetry. Mutations in the ATP8A2 gene have been reported to cause severe autosomal recessive neurological diseases in humans characterized by intellectual disability, hypotonia, chorea, and hyperkinetic movement disorders with or without optic and cerebellar atrophy. To determine the effect of disease-associated missense mutations on ATP8A2, we expressed six variants with the accessory subunit CDC50A in HEK293T cells. The level of expression, cellular localization, and functional activity were analyzed by western blot analysis, immunofluorescence microscopy, and ATPase activity assays. Two variants (p.Ile376Met and p.Lys429Met) expressed at normal ATP8A2 levels and preferentially localized to the Golgi-recycling endosomes, but were devoid of ATPase activity. Four variants (p.Lys429Asn, pAla544Pro, p.Arg625Trp, and p.Trp702Arg) expressed poorly, localized to the endoplasmic reticulum, and lacked ATPase activity. The expression of these variants was increased twofold by the addition of the proteasome inhibitor MG132. We conclude that the p.Ile376Met and p.Lys429Met variants fold in a native-like conformation, but lack key amino acid residues required for ATP-dependent lipid transport. In contrast, the p.Lys429Asn, pAla544Pro, p.Arg625Trp, and p.Trp702Arg variants are highly misfolded and undergo rapid proteosomal degradation.

TMEM30A deficiency in endothelial cells impairs cell proliferation and angiogenesis

Phosphatidylserine (PS) asymmetry in the eukaryotic cell membrane is maintained by a group of proteins belonging to the P4-ATPase family, namely, PS flippases. The folding and transporting of P4-ATPases to their cellular destination requires a β-subunit member of the TMEM30 protein family. Loss of Tmem30a has been shown to cause multiple disease conditions. However, its roles in vascular development have not been elucidated. Here, we show that TMEM30A plays critical roles in retinal vascular angiogenesis, which is a fundamental process in vascular development. Our data indicate that knockdown of TMEM30A in primary human retinal endothelial cells led to reduced tube formation. In mice, endothelial cell (EC)-specific deletion of Tmem30a led to retarded retinal vascular development with a hyperpruned vascular network as well as blunted-end, aneurysm-like tip ECs with fewer filopodia at the vascular front and a reduced number of tip cells. Deletion of Tmem30a also impaired vessel barrier integrity. Mechanistically, deletion of TMEM30A caused reduced EC proliferation by inhibiting VEGF-induced signaling. Our findings reveal essential roles of TMEM30A in angiogenesis, providing a potential therapeutic target.

Identification and functional analyses of disease-associated P4-ATPase phospholipid flippase variants in red blood cells

ATP-dependent phospholipid flippase activity crucial for generating lipid asymmetry was first detected in red blood cell (RBC) membranes, but the P4-ATPases responsible have not been directly determined. Using affinity-based MS, we show that ATP11C is the only abundant P4-ATPase phospholipid flippase in human RBCs, whereas ATP11C and ATP8A1 are the major P4-ATPases in mouse RBCs. We also found that ATP11A and ATP11B are present at low levels. Mutations in the gene encoding ATP11C are responsible for blood and liver disorders, but the disease mechanisms are not known. Using heterologous expression, we show that the T415N substitution in the phosphorylation motif of ATP11C, responsible for congenital hemolytic anemia, reduces ATP11C expression, increases retention in the endoplasmic reticulum, and decreases ATPase activity by 61% relative to WT ATP11C. The I355K substitution in the transmembrane domain associated with cholestasis and anemia in mice was expressed at WT levels and trafficked to the plasma membrane but was devoid of activity. We conclude that the T415N variant causes significant protein misfolding, resulting in low protein expression, cellular mislocalization, and reduced functional activity. In contrast, the I355K variant folds and traffics normally but lacks key contacts required for activity. We propose that the loss in ATP11C phospholipid flippase activity coupled with phospholipid scramblase activity results in the exposure of phosphatidylserine on the surface of RBCs, decreasing RBC survival and resulting in anemia.

Tmem30a deficiency leads to retinal rod bipolar cell degeneration

Phospholipids are asymmetrically distributed across the mammalian plasma membrane, with phosphatidylserine (PS) and phosphatidylethanolamine concentrated in the cytoplasmic leaflet of the membrane bilayer and phosphatidylcholine in the exoplasmic leaflet. This asymmetric distribution is dependent on a group of P4 ATPases called PS flippases. The proper transport and function of PS flippases require a β-subunit transmembrane protein 30A (TMEM30A). Disruption of PS flippases leads to several human diseases. Tmem30a is essential for photoreceptor survival. However, the roles of Tmem30a in the retinal rod bipolar cells (RBC) remain elusive. To investigate the role of Tmem30a in the RBCs, we generated a RBC-specific Tmem30a knockout (cKO) mouse model using PCP2-Cre line. The Tmem30a cKO mice exhibited defect in RBC function and progressive RBC death. PKCα staining of retinal cryosections from cKO mice revealed a remarkable dendritic sprouting of rod bipolar cells during the early degenerative process. Immunostaining analysis of PSD95 and mGluT6 expression demonstrated that rod bipolar cells in Tmem30a cKO retinas exhibited aberrant dendritic sprouting as a result of impaired synaptic efficacy, which implied a crucial role for Tmem30a in synaptic transmission in the retina. In addition, loss of Tmem30a led to reactive gliosis with increased expression of glial fibrillary acidic protein and CD68. TUNEL staining suggested that apoptotic cell death occurred in the retinal inner nuclear layer (INL). Our data show that loss of Tmem30a in RBCs results in dendritic sprouting of rod bipolar cells, increased astrogliosis and RBC death. Taken together, our studies demonstrate an essential role for Tmem30a in the retinal bipolar cells. Cover Image for this issue: doi: 10.1111/jnc.14492.

Yeast and human P4-ATPases transport glycosphingolipids using conserved structural motifs

Lipid transport is an essential process with manifest importance to human health and disease. Phospholipid flippases (P4-ATPases) transport lipids across the membrane bilayer and are involved in signal transduction, cell division, and vesicular transport. Mutations in flippase genes cause or contribute to a host of diseases, such as cholestasis, neurological deficits, immunological dysfunction, and metabolic disorders. Genome-wide association studies have shown that ATP10A and ATP10D variants are associated with an increased risk of diabetes, obesity, myocardial infarction, and atherosclerosis. Moreover, ATP10D SNPs are associated with elevated levels of glucosylceramide (GlcCer) in plasma from diverse European populations. Although sphingolipids strongly contribute to metabolic disease, little is known about how GlcCer is transported across cell membranes. Here, we identify a conserved clade of P4-ATPases from Saccharomyces cerevisiae (Dnf1, Dnf2), Schizosaccharomyces pombe (Dnf2), and Homo sapiens (ATP10A, ATP10D) that transport GlcCer bearing an sn2 acyl-linked fluorescent tag. Further, we establish structural determinants necessary for recognition of this sphingolipid substrate. Using enzyme chimeras and site-directed mutagenesis, we observed that residues in transmembrane (TM) segments 1, 4, and 6 contribute to GlcCer selection, with a conserved glutamine in the center of TM4 playing an essential role. Our molecular observations help refine models for substrate translocation by P4-ATPases, clarify the relationship between these flippases and human disease, and have fundamental implications for membrane organization and sphingolipid homeostasis.