Role of C. elegans TAT-1 protein in maintaining plasma membrane phosphatidylserine asymmetry

Phospholipid flippase ATP8A2 is required for normal visual and auditory function and photoreceptor and spiral ganglion cell survival

ATP8A2 is a P4-ATPase that is highly expressed in the retina, brain, spinal cord and testes. In the retina, ATP8A2 is localized in photoreceptors where it uses ATP to transport phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the exoplasmic to the cytoplasmic leaflet of membranes. Although mutations in ATP8A2 have been reported to cause mental retardation in humans and degeneration of spinal motor neurons in mice, the role of ATP8A2 in sensory systems has not been investigated. We have analyzed the retina and cochlea of ATP8A2-deficient mice to determine the role of ATP8A2 in visual and auditory systems. ATP8A2-deficient mice have shortened photoreceptor outer segments, a reduction in photoresponses and decreased photoreceptor viability. The ultrastructure and phagocytosis of the photoreceptor outer segment appeared normal, but the PS and PE compositions were altered and the rhodopsin content was decreased. The auditory brainstem response threshold was significantly higher and degeneration of spiral ganglion cells was apparent. Our studies indicate that ATP8A2 plays a crucial role in photoreceptor and spiral ganglion cell function and survival by maintaining phospholipid composition and contributing to vesicle trafficking.

Phosphatidylserine-mediated cellular signaling

Phosphatidylserine (PS), a phospholipid with a negatively charged head group, is an important constituent of eukaryotic membranes. Rather than being a passive component of cellular membranes, PS plays an important role in a number of signaling pathways. Signaling is mediated by proteins that are recruited and/or activated by PS in one of two ways: via domains that stereospecifically recognize the head group, or by electrostatic interactions with membranes that are rich in PS and therefore display negative surface charge. Such interactions are key to both intracellular and extracellular signaling cascades. PS, exposed extracellularly, is instrumental in triggering blood clotting and also serves as an "eat me" signal for the clearance of apoptotic cells. Inside the cell, a number of pathways depend of PS; these include kinases, small GTPases and fusogenic proteins. This review will discuss the generation and distribution of PS, current methods of phospholipid visualization within live cells, as well as the current understanding of the role of PS in both extracellular and intracellular signaling events.

Phagocytosis executes delayed neuronal death after focal brain ischemia

Delayed neuronal loss and brain atrophy after cerebral ischemia contribute to stroke and dementia pathology, but the mechanisms are poorly understood. Phagocytic removal of neurons is generally assumed to be beneficial and to occur only after neuronal death. However, we report herein that inhibition of phagocytosis can prevent delayed loss and death of functional neurons after transient brain ischemia. Two phagocytic proteins, Mer receptor tyrosine kinase (MerTK) and Milk fat globule EGF-like factor 8 (MFG-E8), were transiently up-regulated by macrophages/microglia after focal brain ischemia in vivo. Strikingly, deficiency in either protein completely prevented long-term functional motor deficits after cerebral ischemia and strongly reduced brain atrophy as a result of inhibiting phagocytosis of neurons. Correspondingly, in vitro glutamate-stressed neurons reversibly exposed the "eat-me" signal phosphatidylserine, leading to their phagocytosis by microglia; this neuronal loss was prevented in the absence of microglia and reduced if microglia were genetically deficient in MerTK or MFG-E8, both of which mediate phosphatidylserine-recognition. Thus, phagocytosis of viable neurons contributes to brain pathology and, surprisingly, blocking this process is strongly beneficial. Therefore, inhibition of specific phagocytic pathways may present therapeutic targets for preventing delayed neuronal loss after transient cerebral ischemia.

Both the caspase CSP-1 and a caspase-independent pathway promote programmed cell death in parallel to the canonical pathway for apoptosis in Caenorhabditis elegans

Caspases are cysteine proteases that can drive apoptosis in metazoans and have critical functions in the elimination of cells during development, the maintenance of tissue homeostasis, and responses to cellular damage. Although a growing body of research suggests that programmed cell death can occur in the absence of caspases, mammalian studies of caspase-independent apoptosis are confounded by the existence of at least seven caspase homologs that can function redundantly to promote cell death. Caspase-independent programmed cell death is also thought to occur in the invertebrate nematode Caenorhabditis elegans. The C. elegans genome contains four caspase genes (ced-3, csp-1, csp-2, and csp-3), of which only ced-3 has been demonstrated to promote apoptosis. Here, we show that CSP-1 is a pro-apoptotic caspase that promotes programmed cell death in a subset of cells fated to die during C. elegans embryogenesis. csp-1 is expressed robustly in late pachytene nuclei of the germline and is required maternally for its role in embryonic programmed cell deaths. Unlike CED-3, CSP-1 is not regulated by the APAF-1 homolog CED-4 or the BCL-2 homolog CED-9, revealing that csp-1 functions independently of the canonical genetic pathway for apoptosis. Previously we demonstrated that embryos lacking all four caspases can eliminate cells through an extrusion mechanism and that these cells are apoptotic. Extruded cells differ from cells that normally undergo programmed cell death not only by being extruded but also by not being engulfed by neighboring cells. In this study, we identify in csp-3; csp-1; csp-2 ced-3 quadruple mutants apoptotic cell corpses that fully resemble wild-type cell corpses: these caspase-deficient cell corpses are morphologically apoptotic, are not extruded, and are internalized by engulfing cells. We conclude that both caspase-dependent and caspase-independent pathways promote apoptotic programmed cell death and the phagocytosis of cell corpses in parallel to the canonical apoptosis pathway involving CED-3 activation.

Caspases are cysteine proteases that in many cases drive apoptosis, an evolutionarily conserved and highly stereotyped form of cellular suicide with functions in animal development and tissue maintenance. The dysregulation of apoptosis can contribute to diseases as diverse as cancer, autoimmunity, and neurodegeneration. Caspases are often thought to be required for, or even to define, apoptosis. Although there is evidence that apoptosis can occur in the absence of caspase activity, caspase-independence can be difficult to prove, as most animals have multiple caspases. The nematode Caenorhabditis elegans has four caspases, CED-3, CSP-1, CSP-2, and CSP-3. CED-3 has a well-established role in apoptosis, but less is known about the functions of the CSP caspases. In this study, we show that CSP-1 promotes apoptosis in the developing C. elegans embryo and that CSP-1 is regulated differently than its homolog CED-3. Furthermore, we show that apoptosis and the engulfment of dying cells can occur in mutants lacking all four caspases, proving that neither apoptosis nor cell-corpse engulfment require caspase function and that caspase-independent activities can contribute to apoptosis of some cells during animal development.

Inactivation of Caenorhabditis elegans aminopeptidase DNPP-1 restores endocytic sorting and recycling in tat-1 mutants

This study identifies the Caenorhabditis elegans aspartyl aminopeptidase DNPP-1 as a regulator of endocytic sorting and recycling. The data reveal the involvement of an aminopeptidase in regulating endocytic sorting and recycling and suggest its possible roles in peptide signaling and/or protein metabolism in these processes.

In Caenorhabditis elegans, the P4-ATPase TAT-1 and its chaperone, the Cdc50 family protein CHAT-1, maintain membrane phosphatidylserine (PS) asymmetry, which is required for membrane tubulation during endocytic sorting and recycling. Loss of tat-1 and chat-1 disrupts endocytic sorting, leading to defects in both cargo recycling and degradation. In this study, we identified the C. elegans aspartyl aminopeptidase DNPP-1, loss of which suppresses the sorting and recycling defects in tat-1 mutants without reversing the PS asymmetry defect. We found that tubular membrane structures containing recycling cargoes were restored in dnpp-1 tat-1 double mutants and that these tubules overlap with RME-1–positive recycling endosomes. The restoration of the tubular structures in dnpp-1 tat-1 mutants requires normal functions of RAB-5, RAB-10, and RME-1. In tat-1 mutants, we observed alterations in membrane surface charge and targeting of positively charged proteins that were reversed by loss of dnpp-1. DNPP-1 displays a specific aspartyl aminopeptidase activity in vitro, and its enzymatic activity is required for its function in vivo. Our data reveal the involvement of an aminopeptidase in regulating endocytic sorting and recycling and suggest possible roles of peptide signaling and/or protein metabolism in these processes.

MARCKS-ED peptide as a curvature and lipid sensor

Membrane curvature and lipid composition regulates important biological processes within a cell. Currently, several proteins have been reported to sense and/or induce membrane curvatures, e.g., Synaptotagmin-1 and Amphiphysin. However, the large protein scaffold of these curvature sensors limits their applications in complex biological systems. Our interest focuses on identifying and designing peptides that can sense membrane curvature based on established elements observed in natural curvature-sensing proteins. Membrane curvature remodeling also depends on their lipid composition, suggesting strategies to specifically target membrane shape and lipid components simultaneously. We have successfully identified a 25-mer peptide, MARCKS-ED, based on the effector domain sequence of the intracellular membrane protein myristoylated alanine-rich C-kinase substrate that can recognize PS with preferences for highly curved vesicles in a sequence-specific manner. These studies further contribute to the understanding of how proteins and peptides sense membrane curvature, as well as provide potential probes for membrane shape and lipid composition.

Role for phospholipid flippase complex of ATP8A1 and CDC50A proteins in cell migration

Type IV P-type ATPases (P4-ATPases) and CDC50 family proteins form a putative phospholipid flippase complex that mediates the translocation of aminophospholipids such as phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the outer to inner leaflets of the plasma membrane. In Chinese hamster ovary (CHO) cells, at least eight members of P4-ATPases were identified, but only a single CDC50 family protein, CDC50A, was expressed. We demonstrated that CDC50A associated with and recruited P4-ATPase ATP8A1 to the plasma membrane. Overexpression of CDC50A induced extensive cell spreading and greatly enhanced cell migration. Depletion of either CDC50A or ATP8A1 caused a severe defect in the formation of membrane ruffles, thereby inhibiting cell migration. Analyses of phospholipid translocation at the plasma membrane revealed that the depletion of CDC50A inhibited the inward translocation of both PS and PE, whereas the depletion of ATP8A1 inhibited the translocation of PE but not that of PS, suggesting that the inward translocation of cell-surface PE is involved in cell migration. This hypothesis was further examined by using a PE-binding peptide and a mutant cell line with defective PE synthesis; either cell-surface immobilization of PE by the PE-binding peptide or reduction in the cell-surface content of PE inhibited the formation of membrane ruffles, causing a severe defect in cell migration. These results indicate that the phospholipid flippase complex of ATP8A1 and CDC50A plays a major role in cell migration and suggest that the flippase-mediated translocation of PE at the plasma membrane is involved in the formation of membrane ruffles to promote cell migration.

Mammalian P4-ATPases and ABC transporters and their role in phospholipid transport

Transport of phospholipids across cell membranes plays a key role in a wide variety of biological processes. These include membrane biosynthesis, generation and maintenance of membrane asymmetry, cell and organelle shape determination, phagocytosis, vesicle trafficking, blood coagulation, lipid homeostasis, regulation of membrane protein function, apoptosis, etc. P(4)-ATPases and ATP binding cassette (ABC) transporters are the two principal classes of membrane proteins that actively transport phospholipids across cellular membranes. P(4)-ATPases utilize the energy from ATP hydrolysis to flip aminophospholipids from the exocytoplasmic (extracellular/lumen) to the cytoplasmic leaflet of cell membranes generating membrane lipid asymmetry and lipid imbalance which can induce membrane curvature. Many ABC transporters play crucial roles in lipid homeostasis by actively transporting phospholipids from the cytoplasmic to the exocytoplasmic leaflet of cell membranes or exporting phospholipids to protein acceptors or micelles. Recent studies indicate that some ABC proteins can also transport phospholipids in the opposite direction. The importance of P(4)-ATPases and ABC transporters is evident from the findings that mutations in many of these transporters are responsible for severe human genetic diseases linked to defective phospholipid transport. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Mutations in a P-type ATPase gene cause axonal degeneration

Neuronal loss and axonal degeneration are important pathological features of many neurodegenerative diseases. The molecular mechanisms underlying the majority of axonal degeneration conditions remain unknown. To better understand axonal degeneration, we studied a mouse mutant wabbler-lethal (wl). Wabbler-lethal (wl) mutant mice develop progressive ataxia with pronounced neurodegeneration in the central and peripheral nervous system. Previous studies have led to a debate as to whether myelinopathy or axonopathy is the primary cause of neurodegeneration observed in wl mice. Here we provide clear evidence that wabbler-lethal mutants develop an axonopathy, and that this axonopathy is modulated by Wld^s and Bax mutations. In addition, we have identified the gene harboring the disease-causing mutations as Atp8a2. We studied three wl alleles and found that all result from mutations in the Atp8a2 gene. Our analysis shows that ATP8A2 possesses phosphatidylserine translocase activity and is involved in localization of phosphatidylserine to the inner leaflet of the plasma membrane. Atp8a2 is widely expressed in the brain, spinal cord, and retina. We assessed two of the mutant alleles of Atp8a2 and found they are both nonfunctional for the phosphatidylserine translocase activity. Thus, our data demonstrate for the first time that mutation of a mammalian phosphatidylserine translocase causes axon degeneration and neurodegenerative disease.

Axonal degeneration is an important pathological feature of many neurodegenerative diseases, such as Alzheimer disease, Parkinson's disease, and amyotrophic lateral sclerosis. In most of these disease conditions, molecular mechanisms of axonal degeneration remain largely unknown. Spontaneous mouse mutants are important in human disease studies. Identification of a disease-causing gene in mice can lead to the identification of the human ortholog as the disease gene in humans. This approach has the power to identify unexpected genes and pathways involved in disease. Our study centered on wabbler lethal (wl) mutant mice, which display axonal degeneration in both the central and peripheral nervous systems. We identified the disease-causing gene in mice with different wl mutations. The mutations are in Atp8a2, a gene encoding a phosphatidylserine translocase. This protein functions to keep phosphatidylserine enriched to the inner leaflet of the plasma membrane. Our study demonstrates a new role for phospholipid asymmetry in maintaining axon health, and it also reveals a novel function for phosphatidyleserine translocase in neurodegenerative diseases.

Phosphatidylserine exposure during apoptosis reflects bidirectional trafficking between plasma membrane and cytoplasm

Phosphatidylserine (PS) exposure on the external leaflet of the plasma membrane is widely observed during apoptosis and forms the basis for the annexin V binding assay to detect apoptotic cell death. Current efforts to explain PS exposure focus on two potential mechanisms, activation of a phospholipid scramblase or calcium-mediated trafficking of lysosomes to the cell surface. Here, we provide evidence that apoptotic PS exposure instead reflects bidirectional trafficking of membrane between the cell surface and cytoplasm. Using a series of cell lines, some of which expose large amounts of PS during apoptosis and some of which do not, we demonstrate that accumulation of plasma membrane-derived cytoplasmic vesicles in a dynamin-, clathrin- and Cdc42-independent manner is a previously undescribed but widely occurring feature of apoptosis. The apoptotic exposure of PS occurs when these vesicles traffic back to cell surface in a calcium-dependent process that is deficient in a substantial fraction of human cancer cell lines. These observations provide a new model for PS externalization during apoptosis and simultaneously identify an altered step that accounts for the paucity of apoptotic PS exposure in many cell lines.

CED-1, CED-7, and TTR-52 regulate surface phosphatidylserine expression on apoptotic and phagocytic cells

BACKGROUND

Phosphatidylserine (PS) normally confined to the cytoplasmic leaflet of plasma membrane (PM) is externalized to the exoplasmic leaflet (exPS) during apoptosis, where it serves as an "eat-me" signal to phagocytes. In addition, some living cells such as macrophages also express exPS.

RESULTS

A secreted Annexin V (sAnxV::GFP) PS sensor reveals that exPS appears early on apoptotic cells in C. elegans embryos and decreases in older or unengulfed apoptotic cells. This decrease in exPS expression is blocked by loss of CED-7, an ATP binding cassette (ABC) transporter, or TTR-52, a secreted PS binding protein. Phagocytic cells also express exPS, which is dependent on the activity of CED-7, TTR-52, and TTR-52-interacting phagocyte receptor CED-1. Interestingly, a secreted lactadherin PS sensor (sGFP::Lact(C1C2)) labels apoptotic cells but not phagocytes, prevents sAnxV::GFP from labeling phagocytes, and compromises phagocytosis. Immuno-electron micrographs of embryos expressing sAnxV::GFP or sGFP::Lact(C1C2) reveal the presence of extracellular PS-containing vesicles between the apoptotic cell and neighboring cells, which are absent or greatly reduced in the ced-7 and ttr-52 mutants, respectively, indicating that CED-7 and TTR-52 promote the generation of extracellular PS vesicles. Loss of the tat-1 gene, which maintains PS asymmetry in the PM, restores phagocyte exPS expression in ced-1, ced-7, and ttr-52 mutants and partially rescues their engulfment defects.

CONCLUSIONS

CED-7 and TTR-52 may promote the efflux of PS from apoptotic cells through the generation of extracellular PS vesicles, which lead to exPS expression on phagocytes via TTR-52 and CED-1 to facilitate cell corpse clearance.

C. elegans secreted lipid-binding protein NRF-5 mediates PS appearance on phagocytes for cell corpse engulfment

BACKGROUND

During programmed cell death, apoptotic cells are rapidly removed by phagocytes. How dying cells are recognized remains poorly understood.

RESULTS

Here we identify a secreted lipid transfer/LPS-binding family protein, NRF-5, which is required for efficient clearance of cell corpses. We observed that phosphatidylserine (PS), which is externalized to the outer leaflet of plasma membranes in apoptotic cells, is also detected on the surface of engulfing cells. Loss of NRF-5 function completely blocks PS appearance on engulfing cells but causes accumulation of PS on apoptotic cells, a phenotype observed in both ced-7(lf) and ttr-52(lf) mutants. The NRF-5 protein is expressed in and secreted from body wall muscle cells and clusters around apoptotic cells in a CED-7-dependent manner. NRF-5 associates with TTR-52, binds PS, and displays lipid transfer activity in vitro.

CONCLUSION

Our data suggest that NRF-5 may act with CED-7 and TTR-52 to mediate PS transfer from apoptotic cells to engulfing cells and thus promotes engulfment by phagocytes.

Eaten alive! Cell death by primary phagocytosis: 'phagoptosis'

Phagoptosis, also called primary phagocytosis, is a recently recognised form of cell death caused by phagocytosis of viable cells, resulting in their destruction. It is provoked by exposure of 'eat-me' signals and/or loss of 'don't-eat-me' signals by viable cells, causing their phagocytosis by phagocytes. Phagoptosis mediates turnover of erythrocytes, neutrophils and other cells, and thus is quantitatively one of the main forms of cell death in the body. It defends against pathogens and regulates inflammation and immunity. However, recent results indicate that inflamed microglia eat viable brain neurons in models of neurodegeneration, and cancer cells can evade phagocytosis by expressing a 'don't-eat-me' signal, suggesting that too much or too little phagoptosis can contribute to pathology. This review provides an overview of the molecular signals that regulate phagoptosis and the physiological and pathological circumstances in which it has been observed.

Integrin α PAT-2/CDC-42 signaling is required for muscle-mediated clearance of apoptotic cells in Caenorhabditis elegans

Clearance of apoptotic cells by engulfment plays an important role in the homeostasis and development of multicellular organisms. Despite the fact that the recognition of apoptotic cells by engulfment receptors is critical in inducing the engulfment process, the molecular mechanisms are still poorly understood. Here, we characterize a novel cell corpse engulfment pathway mediated by the integrin α subunit PAT-2 in Caenorhabditis elegans and show that it specifically functions in muscle-mediated engulfment during embryogenesis. Inactivation of pat-2 results in a defect in apoptotic cell internalization. The PAT-2 extracellular region binds to the surface of apoptotic cells in vivo, and the intracellular region may mediate signaling for engulfment. We identify essential roles of small GTPase CDC-42 and its activator UIG-1, a guanine-nucleotide exchange factor, in PAT-2–mediated cell corpse removal. PAT-2 and CDC-42 both function in muscle cells for apoptotic cell removal and are co-localized in growing muscle pseudopods around apoptotic cells. Our data suggest that PAT-2 functions through UIG-1 for CDC-42 activation, which in turn leads to cytoskeletal rearrangement and apoptotic cell internalization by muscle cells. Moreover, in contrast to PAT-2, the other integrin α subunit INA-1 and the engulfment receptor CED-1, which signal through the conserved signaling molecules CED-5 (DOCK180)/CED-12 (ELMO) or CED-6 (GULP) respectively, preferentially act in epithelial cells to mediate cell corpse removal during mid-embryogenesis. Our results show that different engulfing cells utilize distinct repertoires of receptors for engulfment at the whole organism level.

When cells undergo apoptosis, their corpses are quickly recognized and phagocytosed by engulfing cells. Although many cell types, such as muscle cells and epithelial cells, possess the ability to remove apoptotic cells, little is known about the receptors and signaling pathways used for apoptotic cell uptake by these “amateur” phagocytes. We show that, in Caenorhabditis elegans, integrin PAT-2/PAT-3 functions as an engulfment receptor in muscle cells. The integrin α subunit PAT-2 mediates both the recognition and subsequent phagocytosis of apoptotic cells. PAT-2 signals through UIG-1 for CDC-42 activation, leading to the cytoskeletal reorganization as the engulfing muscle cell extends pseudopods around the apoptotic cell. Furthermore, in contrast to PAT-2, the other integrin α subunit INA-1 and the engulfment receptor CED-1, both of which appear to act upstream of small GTPase CED-10 (RAC1), predominantly function in epithelial cells to mediate cell corpse removal. Therefore, epithelial cells and muscle cells employ different engulfment receptors for apoptotic cell recognition, downstream signaling, and specific GTPase activation during apoptotic cell removal.

Extracellular vesicles: budding regulated by a phosphatidylethanolamine translocase

Recent work on a Caenorhabditis elegans transmembrane ATPase reveals a central role for the aminophospholipid phosphatidylethanolamine in the production of a class of extracellular vesicles.

Reduced expression of BTBD10, an Akt activator, leads to motor neuron death

BTBD10, an Akt interactor, activates Akt by decreasing the protein phosphatase 2A-mediated dephosphorylation and inactivation of Akt. Overexpression of BTBD10 suppresses motor neuron death that is induced by a familial amyotrophic lateral sclerosis (ALS)-linked superoxide dismutase 1 (SOD1) mutant, G93A-SOD1 in vitro. In this study, we further investigated the BTBD10-mediated suppression of motor neuron death. We found that the small interfering RNA-mediated inhibition of BTBD10 expression led to the death of cultured motor neurons. In Caenorhabditis elegans (C. elegans), disruption of the btbd-10 gene caused not only loss of neurons, including both motor and touch-receptor neurons, but also a locomotion defect. In addition, we found that the expression of BTBD10 was generally decreased in the motor neurons from patients of sporadic ALS and transgenic mice overexpressing G93A-SOD1 (G93A-SOD1-transgenic mice). Collectively, these results suggest that the reduced expression of BTBD10 leads to motor neuron death both in vitro and in vivo.

Primary phagocytosis of neurons by inflamed microglia: potential roles in neurodegeneration

Microglial phagocytosis of dead or dying neurons can be beneficial by preventing the release of damaging and/or pro-inflammatory intracellular components. However, there is now evidence that under certain conditions, such as inflammation, microglia can also phagocytose viable neurons, thus executing their death. Such phagocytic cell death may result from exposure of phosphatidylserine (PS) or other eat-me signals on otherwise viable neurons as a result of physiological activation or sub-toxic insult, and neuronal phagocytosis by activated microglia. In this review, we discuss the mechanisms of phagocytic cell death and its potential roles in Alzheimer’s Disease, Parkinson’s Disease, and Frontotemporal Dementia.

Phospholipid flippases: building asymmetric membranes and transport vesicles

Phospholipid flippases in the type IV P-type ATPase family (P4-ATPases) are essential components of the Golgi, plasma membrane and endosomal system that play critical roles in membrane biogenesis. These pumps flip phospholipid across the bilayer to create an asymmetric membrane structure with substrate phospholipids, such as phosphatidylserine and phosphatidylethanolamine, enriched within the cytosolic leaflet. The P4-ATPases also help form transport vesicles that bud from Golgi and endosomal membranes, thereby impacting the sorting and localization of many different proteins in the secretory and endocytic pathways. At the organismal level, P4-ATPase deficiencies are linked to liver disease, obesity, diabetes, hearing loss, neurological deficits, immune deficiency and reduced fertility. Here, we review the biochemical, cellular and physiological functions of P4-ATPases, with an emphasis on their roles in vesicle-mediated protein transport. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

Atp8a1 deficiency is associated with phosphatidylserine externalization in hippocampus and delayed hippocampus-dependent learning

The molecule responsible for the enzyme activity plasma membrane (PM) aminophospholipid translocase (APLT), which catalyzes phosphatidylserine (PS) translocation from the outer to the inner leaflet of the plasma membrane, is unknown in mammals. A Caenorhabditis elegans study has shown that ablation of transbilayer amphipath transporter-1 (TAT-1), which is an ortholog of a mammalian P-type ATPase, Atp8a1, causes PS externalization in the germ cells. We demonstrate here that the hippocampal cells of the dentate gyrus, and Cornu Ammonis (CA1, CA3) in mice lacking Atp8a1 exhibit a dramatic increase in PS externalization. Although their hippocampi showed no abnormal morphology or heightened apoptosis, these mice displayed increased activity and a marked deficiency in hippocampus-dependent learning, but no hyper-anxiety. Such observations indicate that Atp8a1 plays a crucial role in PM-APLT activity in the neuronal cells. In corroboration, ectopic expression of Atp8a1 but not its close homolog, Atp8a2, caused an increase in the population (V(max) ) of PM-APLT without any change in its signature parameter K(m) in the neuronal N18 cells. Conversely, expression of a P-type phosphorylation-site mutant of Atp8a1 (Atp8a1*) caused a decrease in V(max) of PM-APLT without significantly altering its K(m) . The Atp8a1*-expressing N18 cells also exhibited PS externalization without apoptosis. Together, our data strongly indicate that Atp8a1 plays a central role in the PM-APLT activity of some mammalian cells, such as the neuronal N18 and hippocampal cells.

The P4-ATPase TAT-5 inhibits the budding of extracellular vesicles in C. elegans embryos

BACKGROUND

Cells release extracellular vesicles (ECVs) that can influence differentiation, modulate the immune response, promote coagulation, and induce metastasis. Many ECVs form by budding outwards from the plasma membrane, but the molecules that regulate budding are unknown. In ECVs, the outer leaflet of the membrane bilayer contains aminophospholipids that are normally sequestered to the inner leaflet of the plasma membrane, suggesting a role for lipid asymmetry in ECV budding.

RESULTS

We show that loss of the conserved P4-ATPase TAT-5 causes the large-scale shedding of ECVs and disrupts cell adhesion and morphogenesis in Caenorhabditis elegans embryos. TAT-5 localizes to the plasma membrane and its loss results in phosphatidylethanolamine exposure on cell surfaces. We show that RAB-11 and endosomal sorting complex required for transport (ESCRT) proteins, which regulate the topologically analogous process of viral budding, are enriched at the plasma membrane in tat-5 embryos, and are required for ECV production.

CONCLUSIONS

TAT-5 is the first protein identified to regulate ECV budding. TAT-5 provides a potential molecular link between loss of phosphatidylethanolamine asymmetry and the dynamic budding of vesicles from the plasma membrane, supporting the hypothesis that lipid asymmetry regulates budding. Our results also suggest that viral budding and ECV budding may share common molecular mechanisms.

Caenorhabditis elegans numb inhibits endocytic recycling by binding TAT-1 aminophospholipid translocase

Numb regulates endocytosis in many metazoans, but the mechanism by which it functions is not completely understood. Here we report that the Caenorhabditis elegans Numb ortholog, NUM-1A, a regulator of endocytic recycling, binds the C isoform of transbilayer amphipath transporter-1 (TAT-1), a P4 family adenosine triphosphatase and putative aminophospholipid translocase that is required for proper endocytic trafficking. We demonstrate that TAT-1 is differentially spliced during development and that TAT-1C-specific splicing occurs in the intestine where NUM-1A is known to function. NUM-1A and TAT-1C colocalize in vivo. We have mapped the binding site to an NXXF motif in TAT-1C. This motif is not required for TAT-1C function but is required for NUM-1A's ability to inhibit recycling. We demonstrate that num-1A and tat-1 defects are both suppressed by the loss of the activity of PSSY-1, a phosphatidylserine (PS) synthase. PS is mislocalized in intestinal cells with defects in tat-1 or num-1A function. We propose that NUM-1A inhibits recycling by inhibiting TAT-1C's ability to translocate PS across the membranes of recycling endosomes.

Phagocytic removal of cells that have become unwanted: implications for animal development and tissue homeostasis

Cells that have become unwanted need to be promptly, selectively, and safely removed. This is made possible by apoptosis-dependent phagocytosis, in which cells unnecessary, obstructive, or dangerous to organisms are induced to undergo apoptosis so that they are earmarked for phagocytosis. The phagocytic elimination occurs so quickly that cells with hallmarks of apoptosis are barely detectable in vivo. The removal of particular types of cells at appropriate stages of development not only contributes to the disposal of spent cells, the creation of space for morphogenesis, and the exclusion of pathogenic or noxious cells, but seems to actively control tissue renewal, tissue remodeling, tissue function, and pathogenic state. This event thus plays an indispensable role in the maintenance of animal development and tissue homeostasis.

Pumping lipids with P4-ATPases

While accumulating evidence indicates that P4-ATPases catalyze phospholipid transport across cellular bilayers, their kinship to cation-pumping ATPases has raised fundamental questions concerning the underlying flippase mechanism. Loss of P4-ATPase function perturbs vesicle formation in late secretory and endocytic compartments. An intriguing concept is that P4-ATPases help drive vesicle budding by generating imbalances in transbilayer lipid numbers. Moreover, activation of P4-ATPases by phosphoinositides and other effectors of coat recruitment provide a potential mechanism to confine flippase activity to sites of vesicle biogenesis. These developments have raised considerable interest in understanding the mechanism, regulation and biological implications of P4-ATPase-catalyzed phospholipid transport.

Critical role of the beta-subunit CDC50A in the stable expression, assembly, subcellular localization, and lipid transport activity of the P4-ATPase ATP8A2

P(4)-ATPases have been implicated in the transport of lipids across cellular membranes. Some P(4)-ATPases are known to associate with members of the CDC50 protein family. Previously, we have shown that the P(4)-ATPase ATP8A2 purified from photoreceptor membranes and reconstituted into liposomes catalyzes the active transport of phosphatidylserine across membranes. However, it was unclear whether ATP8A2 functioned alone or as a complex with a CDC50 protein. Here, we show by mass spectrometry and Western blotting using newly generated anti-CDC50A antibodies that CDC50A is associated with ATP8A2 purified from photoreceptor membranes. ATP8A2 expressed in HEK293T cells assembles with endogenous or expressed CDC50A, but not CDC50B, to generate a heteromeric complex that actively transports phosphatidylserine and to a lesser extent phosphatidylethanolamine across membranes. Chimera CDC50 proteins in which various domains of CDC50B were replaced with the corresponding domains of CDC50A were used to identify domains important in the formation of a functional ATP8A2-CDC50 complex. These studies indicate that both the transmembrane and exocytoplasmic domains of CDC50A are required to generate a functionally active complex. The N-terminal cytoplasmic domain of CDC50A appears to play a direct role in the reaction cycle. Mutagenesis studies further indicate that the N-linked oligosaccharide chains of CDC50A are required for stable expression of an active ATP8A2-CDC50A lipid transport complex. Together, our studies indicate that CDC50A is the β-subunit of ATP8A2 and is crucial for the correct folding, stable expression, export from endoplasmic reticulum, and phosphatidylserine flippase activity of ATP8A2.

Endocytic sorting and recycling require membrane phosphatidylserine asymmetry maintained by TAT-1/CHAT-1

Endocytic sorting is achieved through the formation of morphologically and functionally distinct sub-domains within early endosomes. Cargoes destined for recycling are sorted to and transported through newly-formed tubular membranes, but the processes that regulate membrane tubulation are poorly understood. Here, we identified a novel Caenorhabditis elegans Cdc50 family protein, CHAT-1, which acts as the chaperone of the TAT-1 P4-ATPase to regulate membrane phosphatidylserine (PS) asymmetry and endocytic transport. In chat-1 and tat-1 mutants, the endocytic sorting process is disrupted, leading to defects in both cargo recycling and degradation. TAT-1 and CHAT-1 colocalize to the tubular domain of the early endosome, the tubular endocytic recycling compartment (ERC), and the recycling endosome where PS is enriched on the cytosolic surface. Loss of tat-1 and chat-1 function disrupts membrane PS asymmetry and abrogates the tubular membrane structure. Our data suggest that CHAT-1 and TAT-1 maintain membrane phosphatidylserine asymmetry, thus promoting membrane tubulation and regulating endocytic sorting and recycling.

The process by which cells take up nutrients and other large molecules from the extracellular environment is known as endocytosis. At the cell surface, external molecules become enclosed in membrane spheres called endosomes. Early endosomes serve as a sorting station, directing the contents (cargo molecules) to the correct compartment within the cell. This is thought to be achieved by the formation of membrane structures with distinct shape and function. For example, cargoes destined for recycling and degradation are processed through tubular membrane structures and big vesicular compartments, respectively. However, it is poorly understood how early endosome membranes are shaped into different structures. Here we show that two proteins, CHAT-1 and TAT-1, regulate membrane structure and are important for normal endocytic transport in the nematode worm C. elegans. TAT-1 and CHAT-1 are found in tubular membrane structures along the sorting and recycling pathway, where they enrich the outer membrane layer with a lipid called phosphatidylserine (PS) and probably change the membrane curvature. Loss of tat-1 and chat-1 function disrupts the asymmetric distribution of PS, abolishes tubular membrane structures, and abrogates endocytic sorting/recycling. Our data support a role of TAT-1/CHAT-1–regulated membrane PS asymmetry in promoting membrane tubulation for endocytic cargo sorting and recycling.

Functions of phospholipid flippases

Asymmetrical distribution of phospholipids is generally observed in the eukaryotic plasma membrane. Maintenance and changes of this phospholipid asymmetry are regulated by ATP-driven phospholipid translocases. Accumulating evidence indicates that type 4 P-type ATPases (P4-ATPases, also called flippases) translocate phospholipids from the exoplasmic leaflet to the cytoplasmic leaflet of the plasma membrane and internal membranes. Among P-type ATPases, P4-ATPases are unique in that they are associated with a conserved membrane protein of the Cdc50 family as a non-catalytic subunit. Recent studies indicate that flippases are involved in various cellular functions, including transport vesicle formation and cell polarity. In this review, we will focus on the functional aspect of phospholipid flippases.

Biochemical and cellular functions of P4 ATPases

P4 ATPases (subfamily IV P-type ATPases) form a specialized subfamily of P-type ATPases and have been implicated in phospholipid translocation from the exoplasmic to the cytoplasmic leaflet of biological membranes. Pivotal roles of P4 ATPases have been demonstrated in eukaryotes, ranging from yeast, fungi and plants to mice and humans. P4 ATPases might exert their cellular functions by combining enzymatic phospholipid translocation activity with an enzyme-independent action. The latter could be involved in the timely recruitment of proteins involved in cellular signalling, vesicle coat assembly and cytoskeleton regulation. In the present review, we outline the current knowledge of the biochemical and cellular functions of P4 ATPases in the eukaryotic membrane.

CDC50 proteins are critical components of the human class-1 P4-ATPase transport machinery

Members of the P(4) subfamily of P-type ATPases catalyze phospholipid transport and create membrane lipid asymmetry in late secretory and endocytic compartments. P-type ATPases usually pump small cations and the transport mechanism involved appears conserved throughout the family. How this mechanism is adapted to flip phospholipids remains to be established. P(4)-ATPases form heteromeric complexes with CDC50 proteins. Dissociation of the yeast P(4)-ATPase Drs2p from its binding partner Cdc50p disrupts catalytic activity (Lenoir, G., Williamson, P., Puts, C. F., and Holthuis, J. C. (2009) J. Biol. Chem. 284, 17956-17967), suggesting that CDC50 subunits play an intimate role in the mechanism of transport by P(4)-ATPases. The human genome encodes 14 P(4)-ATPases while only three human CDC50 homologues have been identified. This implies that each human CDC50 protein interacts with multiple P(4)-ATPases or, alternatively, that some human P(4)-ATPases function without a CDC50 binding partner. Here we show that human CDC50 proteins each bind multiple class-1 P(4)-ATPases, and that in all cases examined, association with a CDC50 subunit is required for P(4)-ATPase export from the ER. Moreover, we find that phosphorylation of the catalytically important Asp residue in human P(4)-ATPases ATP8B1 and ATP8B2 is critically dependent on their CDC50 subunit. These results indicate that CDC50 proteins are integral part of the P(4)-ATPase flippase machinery.

The role of nucleotides in apoptotic cell clearance: implications for disease pathogenesis

Apoptosis occurs in many tissues, during both normal and pathogenic processes. Normally, apoptotic cells are rapidly cleared, either by neighboring or recruited phagocytes. The prompt clearance of apoptotic cells requires that the apoptotic cells announce their presence through the release of chemotactic factors, known as "find-me" signals, to recruit phagocytes to the site of death, and through the exposure of so-called "eat-me" signals, which are ligands for phagocytic uptake. The importance of prompt apoptotic cell clearance is revealed by findings that decreasing the efficiency of engulfment results in the persistence of apoptotic cells, which is often associated with chronic inflammation and autoimmunity. Additionally, the proper clearance of apoptotic cells is actively anti-inflammatory, which is thought to play a crucial role in immunologic tolerance. Therefore, defects associated with clearance of apoptotic cells may contribute to the pathogenesis of several inflammatory diseases, including autoimmunity and atherosclerosis. Here, we review the role of nucleotides in the apoptotic cell clearance process and discuss their implications for disease pathogenesis.

Find-me and eat-me signals in apoptotic cell clearance: progress and conundrums

Everyday we turnover billions of cells. The quick, efficient, and immunologically silent disposal of the dying cells requires a coordinated orchestration of multiple steps, through which phagocytes selectively recognize and engulf apoptotic cells. Recent studies have suggested an important role for soluble mediators released by apoptotic cells that attract phagocytes ("find-me" signals). New information has also emerged on multiple receptors that can recognize phosphatidylserine, the key "eat-me" signal exposed on the surface of apoptotic cells. This perspective discusses recent exciting progress, gaps in our understanding, and the conflicting issues that arise from the newly acquired knowledge.

Caenorhabditis elegans transthyretin-like protein TTR-52 mediates recognition of apoptotic cells by the CED-1 phagocyte receptor

During apoptosis, dying cells are swiftly removed by phagocytes. It is not fully understood how apoptotic cells are recognized by phagocytes. Here we report the identification and characterization of the Caenorhabditis elegans ttr-52 gene, which encodes a transthyretin-like protein and is required for efficient cell corpse engulfment. The TTR-52 protein is expressed in, and secreted from, C. elegans endoderm and clusters around apoptotic cells. Genetic analysis indicates that TTR-52 acts in the cell corpse engulfment pathway mediated by CED-1, CED-6 and CED-7 and affects clustering of the phagocyte receptor CED-1 around apoptotic cells. TTR-52 recognizes surface-exposed phosphatidylserine (PtdSer) in vivo and binds to both PtdSer and the extracellular domain of CED-1 in vitro. TTR-52 is therefore the first bridging molecule identified in C. elegans that mediates recognition of apoptotic cells by crosslinking the PtdSer 'eat me' signal with the phagocyte receptor CED-1.

A flippase-independent function of ATP8B1, the protein affected in familial intrahepatic cholestasis type 1, is required for apical protein expression and microvillus formation in polarized epithelial cells

Mutations in ATP8B1 cause familial intrahepatic cholestasis type 1, a spectrum of disorders characterized by intrahepatic cholestasis, reduced growth, deafness, and diarrhea. ATP8B1 belongs to the P(4) P-type adenosine triphosphatase (ATPase) family of putative aminophospholipid translocases, and loss of aminophospholipid asymmetry in the canalicular membranes of ATP8B1-deficient liver cells has been proposed as the primary cause of impaired bile salt excretion. To explore the origin of the hepatic and extrahepatic symptoms associated with ATP8B1 deficiency, we investigated the impact of ATP8B1 depletion on the domain-specific aminophospholipid translocase activities and polarized organization of polarized epithelial Caco-2 cells. Caco-2 cells were stably transfected with short hairpin RNA constructs to block ATP8B1 expression. Aminophospholipid translocase activity was assessed using spin-labeled phospholipids. The polarized organization of these cells was determined by pulse-chase analysis, cell-fractionation, immunocytochemistry, and transmission electron microscopy. ATP8B1 was abundantly expressed in the apical membrane of Caco-2 cells, and its expression was markedly induced during differentiation and polarization. Blocking ATP8B1 expression by RNA interference (RNAi) affected neither aminophospholipid transport nor the asymmetrical distribution of aminophospholipids across the apical bilayer. Nonetheless, ATP8B1-depleted Caco-2 cells displayed profound perturbations in apical membrane organization, including a disorganized apical actin cytoskeleton, a loss in microvilli, and a posttranscriptional defect in apical protein expression.

CONCLUSION

Our findings point to a critical role of ATP8B1 in apical membrane organization that is unrelated to its presumed aminophospholipid translocase activity, yet potentially relevant for the development of cholestasis and the manifestation of extrahepatic features associated with ATP8B1 deficiency.

Programmed cell clearance: molecular regulation of the elimination of apoptotic cell corpses and its role in the resolution of inflammation

Programmed cell clearance is a physiological process of elimination of apoptotic cell corpses. Recent studies have disclosed several ligand-receptor interactions that dictate the recognition or non-recognition of cells by macrophages and other phagocytes. The externalization of the anionic phospholipid, phosphatidylserine is effectively recognized by specific receptors on professional phagocytes and facilitates the clearance of apoptotic cells. Macrophage disposal of cells at sites of inflammation is believed to play an important role in the resolution of the inflammatory process, and recent studies have suggested a role for the NADPH oxidase in the process of macrophage elimination of activated neutrophils. The present review will focus on the molecular regulation of programmed cell clearance, and discuss the role of cell elimination in the resolution of inflammation.

Autoimmunity and the clearance of dead cells

To maintain organismal homeostasis, phagocytes engulf dead cells, which are recognized as dead by virtue of a characteristic "eat me" signal exposed on their surface. The dead cells are then transferred to lysosomes, where their cellular components are degraded for reuse. Inefficient engulfment of dead cells activates the immune system, causing disease such as systemic lupus erythematosus, and if the DNA of the dead cells is not properly degraded, the innate immune response becomes activated, leading to severe anemia and chronic arthritis. Here, we discuss how the endogenous components of dead cells activate the immune system through both extracellular and intracellular pathways.

Engulfment of apoptotic cells in C. elegans is mediated by integrin alpha/SRC signaling

BACKGROUND

Engulfment of apoptotic cells is important for cellular homeostasis and the development of multicellular organisms. Previous studies have shown that more than one engulfment receptors act upstream of the conserved signaling module CED-2/CrkII-CED-5/Dock180-CED-12/ELMO for cell corpse removal in C. elegans, but little is known about their identities, except for PSR-1.

RESULTS

We show that in C. elegans, integrin functions as an engulfment receptor in the recognition and subsequent phagocytosis of apoptotic cells. Mutations in the integrin alpha gene ina-1 result in inefficient engulfment of apoptotic cells. The INA-1 extracellular domain binds to the surface of apoptotic cells in vivo. This binding requires the phospholipid scramblase SCRM-1, which promotes the exposure of phosphatidylserine, a key "eat me" signal in apoptotic cells. Furthermore, we identify an essential role of the nonreceptor tyrosine kinase SRC-1 in INA-1-mediated cell corpse removal. INA-1 and SRC-1 both act in the engulfing cells during the engulfment process and are colocalized in the phagocytic cups extending around apoptotic cells. Finally, our genetic and biochemical data suggest that SRC-1 relays the scrm-1-dependent engulfment signal from INA-1 to the conserved motility-promoting signaling complex CED-2/CrkII-CED-5/Dock180-CED-12/ELMO for CED-10/Rac activation, probably by interactions with CED-2 and the INA-1 cytoplasmic domain, leading to the internalization of apoptotic cells.

CONCLUSIONS

Our findings provide evidence that integrin functions as an engulfment receptor at the whole-organism level and reveal a nonconventional signaling pathway in which SRC provides a FAK-independent linkage between integrin alpha and the common motility-promoting signaling module CED-2/CrkII-CED-5/Dock180-CED-12/ELMO to promote the internalization of apoptotic cells.

The Wnt pathway controls cell death engulfment, spindle orientation, and migration through CED-10/Rac

Wnt signalling pathways have extremely diverse functions in animals, including induction of cell fates or tumours, guidance of cell movements during gastrulation, and the induction of cell polarity. Wnt can induce polar changes in cellular morphology by a remodelling of the cytoskeleton. However, how activation of the Frizzled receptor induces cytoskeleton rearrangement is not well understood. We show, by an in depth 4-D microscopy analysis, that the Caenorhabditis elegans Wnt pathway signals to CED-10/Rac via two separate branches to regulate modulation of the cytoskeleton in different cellular situations. Apoptotic cell clearance and migration of the distal tip cell require the MOM-5/Fz receptor, GSK-3 kinase, and APC/APR-1, which activate the CED-2/5/12 branch of the engulfment machinery. MOM-5 (Frizzled) thus can function as an engulfment receptor in C. elegans. Our epistatic analyses also suggest that the two partially redundant signalling pathways defined earlier for engulfment may act in a single pathway in early embryos. By contrast, rearrangement of mitotic spindles requires the MOM-5/Fz receptor, GSK-3 kinase, and beta-catenins, but not the downstream factors LIT-1/NLK or POP-1/Tcf. Taken together, our results indicate that in multiple developmental processes, CED-10/Rac can link polar signals mediated by the Wnt pathway to rearrangements of the cytoskeleton.

Intracellular targeting signals and lipid specificity determinants of the ALA/ALIS P4-ATPase complex reside in the catalytic ALA alpha-subunit

Phospholipid flipping across cellular membranes contributes to vesicle biogenesis in eukaryotes and involves flippases (P₄-ATPases). However, the minimal composition of the flippase machinery remains to be determined. We demonstrate that cellular targeting and lipid specificity of P₄-ATPases require the α-subunit but are independent of the β-subunit.

Members of the P₄ subfamily of P-type ATPases are believed to catalyze flipping of phospholipids across cellular membranes, in this way contributing to vesicle biogenesis in the secretory and endocytic pathways. P₄-ATPases form heteromeric complexes with Cdc50-like proteins, and it has been suggested that these act as β-subunits in the P₄-ATPase transport machinery. In this work, we investigated the role of Cdc50-like β-subunits of P₄-ATPases for targeting and function of P₄-ATPase catalytic α-subunits. We show that the Arabidopsis P₄-ATPases ALA2 and ALA3 gain functionality when coexpressed with any of three different ALIS Cdc50-like β-subunits. However, the final cellular destination of P₄-ATPases as well as their lipid substrate specificity are independent of the nature of the ALIS β-subunit they were allowed to interact with.

The ins and outs of phospholipid asymmetry in the plasma membrane: roles in health and disease

A common feature of all eukaryotic membranes is the non-random distribution of different lipid species in the lipid bilayer (lipid asymmetry). Lipid asymmetry provides the two sides of the plasma membrane with different biophysical properties and influences numerous cellular functions. Alteration of lipid asymmetry plays a prominent role during cell fusion, activation of the coagulation cascade, and recognition and removal of apoptotic cell corpses by macrophages (programmed cell clearance). Here we discuss the origin and maintenance of phospholipid asymmetry, based on recent studies in mammalian systems as well as in Caenhorhabditis elegans and other model organisms, along with emerging evidence for a conserved role of mitochondria in the loss of lipid asymmetry during apoptosis. The functional significance of lipid asymmetry and its disruption during health and disease is also discussed.

Caenorhabditis elegans myotubularin MTM-1 negatively regulates the engulfment of apoptotic cells

During programmed cell death, apoptotic cells are recognized and rapidly engulfed by phagocytes. Although a number of genes have been identified that promote cell corpse engulfment, it is not well understood how phagocytosis of apoptotic cells is negatively regulated. Here we have identified Caenorhabditis elegans myotubularin MTM-1 as a negative regulator of cell corpse engulfment. Myotubularins (MTMs) constitute a large, highly conserved family of lipid phosphatases. MTM gene mutations are associated with various human diseases, but the cellular functions of MTM proteins are not clearly defined. We found that inactivation of MTM-1 caused significant reduction in cell corpses in strong loss-of-function mutants of ced-1, ced-6, ced-7, and ced-2, but not in animals deficient in the ced-5, ced-12, or ced-10 genes. In contrast, overexpression of MTM-1 resulted in accumulation of cell corpses. This effect is dependent on the lipid phosphatase activity of MTM-1. We show that loss of mtm-1 function accelerates the clearance of cell corpses by promoting their internalization. Importantly, the reduction of cell corpses caused by mtm-1 RNAi not only requires the activities of CED-5, CED-12, and CED-10, but also needs the functions of the phosphatidylinositol 3-kinases (PI3Ks) VPS-34 and PIKI-1. We found that MTM-1 localizes to the plasma membrane in several known engulfing cell types and may modulate the level of phosphatidylinositol 3-phosphate (PtdIns(3)P) in vivo. We propose that MTM-1 negatively regulates cell corpse engulfment through the CED-5/CED-12/CED-10 module by dephosphorylating PtdIns(3)P on the plasma membrane.

Clearance of dead cells is crucial for normal animal development. Cell corpses are recognized, engulfed, and removed by phagocytic cells. However, the mechanisms that regulate phagocytosis are still not well understood, especially the ways in which the process is inhibited (negatively regulated). We screened the nematode worm Caenorhabditis elegans for negative regulators of cell corpse engulfment and identified myotubularin MTM-1. Myotubularins (MTMs) are a family of highly conserved enzymes that remove phosphate groups from membrane lipids. Mutations in human MTM genes are associated with various severe diseases including X-linked myotubular myopathy and Charcot-Marie-Tooth disease, but the normal functions of MTMs are unknown. In this study, we found that MTM-1 inhibits cell corpse engulfment through a series of evolutionarily conserved signaling molecules (the bipartite GEF (CED-5/DOCK180-CED-12/ELMO) and the GTPase CED-10/Rac). The negative regulatory effect of MTM-1 requires both its lipid phosphatase activity and the function of another group of enzymes called PI3-kinases. We propose that MTM-1 acts through Rac GTPase CED-10 by dephosphorylating the lipid PtdIns(3)P on the plasma membrane. We have identified a negative regulatory mechanism of cell corpse engulfment and a previously unknown cellular function of MTM-1, which may provide further insights into the basis of human MTM-related diseases.

Localization, purification, and functional reconstitution of the P4-ATPase Atp8a2, a phosphatidylserine flippase in photoreceptor disc membranes

P(4)-ATPases comprise a relatively new subfamily of P-type ATPases implicated in the energy-dependent translocation of aminophospholipids across cell membranes. In this study, we report on the localization and functional properties of Atp8a2, a member of the P(4)-ATPase subfamily that has not been studied previously. Reverse transcription-PCR revealed high expression of atp8a2 mRNA in the retina and testis. Within the retina, immunofluorescence microscopy and subcellular fractionation studies localized Atp8a2 to outer segment disc membranes of rod and cone photoreceptor cells. Atp8a2 purified from photoreceptor outer segments by immunoaffinity chromatography exhibited ATPase activity that was stimulated by phosphatidylserine and to a lesser degree phosphatidylethanolamine but not by phosphatidylcholine or other membrane lipids. Purified Atp8a2 was reconstituted into liposomes containing fluorescent-labeled phosphatidylserine to measure the ability of Atp8a2 to flip phosphatidylserine across the lipid bilayer. Fluorescence measurements showed that Atp8a2 flipped fluorescent-labeled phosphatidylserine from the inner leaflet of liposomes (equivalent to the exocytoplasmic leaflet of cell membranes) to the outer leaflet (equivalent to cytoplasmic leaflet) in an ATP-dependent manner. Our studies provide the first direct biochemical evidence that purified P(4)-ATPases can translocate aminophospholipids across membranes and further implicates Atp8a2 in the generation and maintenance of phosphatidylserine asymmetry in photoreceptor disc membranes.

Expression, purification and use of recombinant annexin V for the detection of apoptotic cells

Apoptosis is a mode of programmed cell death that is widely used to eliminate cells during development, tissue homeostasis, infection or in response to injury. Alterations to the plasma membranes of apoptotic cells trigger recognition and engulfment of such cells by phagocytes. Measurement of plasma membrane phosphatidylserine externalization, using fluorescently labeled annexin V, is widely used for the detection of apoptotic cells. Here we describe protocols for bacterial expression, purification and FITC labeling of recombinant annexin V. By following the method outlined in this protocol, it is possible to produce milligram amounts of recombinant annexin V within 3 d. We also describe a method for the assessment of annexin V binding to cell populations by flow cytometry or fluorescence microscopy.

P-type ATPase TAT-2 negatively regulates monomethyl branched-chain fatty acid mediated function in post-embryonic growth and development in C. elegans

Monomethyl branched-chain fatty acids (mmBCFAs) are essential for Caenorhabditis elegans growth and development. To identify factors acting downstream of mmBCFAs for their function in growth regulation, we conducted a genetic screen for suppressors of the L1 arrest that occurs in animals depleted of the 17-carbon mmBCFA C17ISO. Three of the suppressor mutations defined an unexpected player, the P-type ATPase TAT-2, which belongs to the flippase family of proteins that are implicated in mediating phospholipid bilayer asymmetry. We provide evidence that TAT-2, but not other TAT genes, has a specific role in antagonizing the regulatory activity of mmBCFAs in intestinal cells. Interestingly, we found that mutations in tat-2 also suppress the lethality caused by inhibition of the first step in sphingolipid biosynthesis. We further showed that the fatty acid side-chains of glycosylceramides contain 20%–30% mmBCFAs and that this fraction is greatly diminished in the absence of mmBCFA biosynthesis. These results suggest a model in which a C17ISO-containing sphingolipid may mediate the regulatory functions of mmBCFAs and is negatively regulated by TAT-2 in intestinal cells. This work indicates a novel connection between a P-type ATPase and the critical regulatory function of a specific fatty acid.

Fatty acids serve diverse functions in organisms, including roles at the cell membrane to coordinate cell signaling processes. Monomethyl branched-chain fatty acids (mmBCFAs) are a special type of fatty acid that is commonly present in animals. Because mmBCFAs are a small component of the total fatty acid pool, their functions have not been a major research focus and are largely unclear. We tackled the problem using the nematode C. elegans. Our laboratory previously found that without mmBCFAs, worms cannot develop normally and die. To understand how these obscure fatty acids perform such important roles, we searched for other factors involved in the process by conducting a mutagenesis screen to uncover mutant worms that can recover the ability to grow without the presence of mmBCFAs. We found several such mutations in a single gene that codes for a protein called TAT-2. TAT-2 is one of several poorly understood P-type ATPases that likely help maintain the proper lipid structure in cell membranes. Our work indicates that TAT-2 antagonizes the growth regulatory function of mmBCFAs in intestinal cells. Studies on how mmBCFAs and this protein functionally interact explore a novel, interesting, and important problem that is only beginning to be understood.

Caenorhabditis elegans caspase homolog CSP-2 inhibits CED-3 autoactivation and apoptosis in germ cells

In Caenorhabditis elegans, apoptosis in germ cells is mediated by the same core apoptotic machinery that controls apoptosis in somatic cells. These include the CED-3 caspase, the CED-3 activator CED-4, and the cell death inhibitor CED-9. However, germline apoptosis also differs from somatic apoptosis in its regulation. We found that CSP-3, a caspase homolog that blocks CED-3 autoactivation and apoptosis in somatic cells, does not affect apoptosis in germ cells. Interestingly, the second C. elegans caspase homolog, CSP-2, shares sequence similarity to both catalytic subunits of the CED-3 caspase, and surprisingly, contains a stretch of sequence that is almost identical to that of CSP-3. Unlike CSP-3 that acts specifically in somatic cells, loss of CSP-2 causes increased apoptosis only in germ cells, suggesting that CSP-2 is a germ cell-specific apoptosis inhibitor. Moreover, like CSP-3, CSP-2 associates with the CED-3 zymogen and inhibits its autoactivation, but does not inhibit CED-4-induced CED-3 activation or the activity of the activated CED-3 protease. Thus, two different C. elegans caspase homologs use the same mechanism to prevent caspase autoactivation and apoptosis in different tissues, suggesting that this could be a generally applicable strategy for regulating caspase activation and apoptosis.

Cdc50p plays a vital role in the ATPase reaction cycle of the putative aminophospholipid transporter Drs2p

Members of the P(4) subfamily of P-type ATPases are believed to catalyze transport of phospholipids across cellular bilayers. However, most P-type ATPases pump small cations or metal ions, and atomic structures revealed a transport mechanism that is conserved throughout the family. Hence, a challenging problem is to understand how this mechanism is adapted in P(4)-ATPases to flip phospholipids. P(4)-ATPases form heteromeric complexes with Cdc50 proteins. The primary role of these additional polypeptides is unknown. Here, we show that the affinity of yeast P(4)-ATPase Drs2p for its Cdc50-binding partner fluctuates during the transport cycle, with the strongest interaction occurring at a point where the enzyme is loaded with phospholipid ligand. We also find that specific interactions with Cdc50p are required to render the ATPase competent for phosphorylation at the catalytically important aspartate residue. Our data indicate that Cdc50 proteins are integral components of the P(4)-ATPase transport machinery. Thus, acquisition of these subunits may have been a crucial step in the evolution of flippases from a family of cation pumps.

Viruses and endosome membrane dynamics

Cell surface molecules, ligands, and solutes can be endocytosed into animal cells via several pathways in addition to clathrin-mediated endocytosis, which all seem to lead to canonical endosomes. It seems that viruses can enter and infect cells through most of, if not all, endocytic routes, having evolved different, sometimes elaborate, strategies to (mis)use cellular machineries to their own benefit during infection. In this short review, I will discuss recent progress in understanding the pathways followed by animal viruses into cells, and how these studies are also providing novel insights into our understanding of some molecular mechanisms that control endocytic membrane transport.

A genetic strategy involving a glycosyltransferase promoter and a lipid translocating enzyme to eliminate cancer cells

The most common therapeutic strategy for the treatment of cancer uses antimetabolites, which block uncontrolled division of cancer cells and kill them. However, such antimetabolites also kill normal cells, thus yielding detrimental side effects. This emphasizes the need for an alternative therapy, which would have little or no side effects. Our approach involves designing genetic means to alter surface lipid determinants that induce phagocytosis of cancer cells. The specific target of this strategy has been the enzyme activity termed aminophospholipid translocase (APLT) or flippase that causes translocation of phosphatidylserine (PS) from the outer to the inner leaflet of the plasma membrane in viable cells. Efforts to identify the enigmatic, plasma membrane APLT of mammalian cells have led investigators to some P-type ATPases, which have often proven to be the APLT of internal membranes rather than the plasma membrane. By measuring kinetic parameters for the plasma membrane APLT activity, we have shown that the P-type ATPase Atp8a1 is the plasma membrane APLT of the tumorigenic N18 cells, but not the non-tumorigenic HN2 (hippocampal neuron x N18) cells. Targeted knockdown of this enzyme causes PS externalization in the N18 cells, which would trigger phagocytic removal of these cells. But how would we specifically express the mutants or antisense Atp8a1 in the cancer cells? This has brought us to a glycosyltransferase, GnT-V, which is highly expressed in the transformed cells. By using the GnT-V promoter to drive a luciferase reporter gene we have demonstrated a dramatic increase in luciferase expression selectively in tumor cells. The described strategy could be tested for the removal of cancer cells without the use of antimetabolites that often kill normal cells.