Over-expression of recombinant human phospholipid scramblase 1 in E. coli and its purification from inclusion bodies

Identification and characterisation of a phospholipid scramblase in the malaria parasite Plasmodium falciparum

Recent studies highlight the emerging role of lipids as important messengers in malaria parasite biology. In an attempt to identify interacting proteins and regulators of these dynamic and versatile molecules, we hypothesised the involvement of phospholipid translocases and their substrates in the infection of the host erythrocyte by the malaria parasite Plasmodium spp. Here, using a data base searching approach of the Plasmodium Genomics Resources (www.plasmodb.org), we have identified a putative phospholipid (PL) scramblase in P. falciparum (PfPLSCR) that is conserved across the genus and in closely related unicellular algae. By reconstituting recombinant PfPLSCR into liposomes, we demonstrate metal ion dependent PL translocase activity and substrate preference, confirming PfPLSCR as a bona fide scramblase. We show that PfPLSCR is expressed during asexual and sexual parasite development, localising to different membranous compartments of the parasite throughout the intra-erythrocytic life cycle. Two different gene knockout approaches, however, suggest that PfPLSCR is not essential for erythrocyte invasion and asexual parasite development, pointing towards a possible role in other stages of the parasite life cycle.

Heavy-Metals-Mediated Phospholipids Scrambling by Human Phospholipid Scramblase 3: A Probable Role in Mitochondrial Apoptosis

Human phospholipid scramblases are a family of four homologous transmembrane proteins (hPLSCR1-4) mediating phospholipids (PLs) translocation in plasma membrane upon Ca²⁺ activation. hPLSCR3, the only homologue localized to mitochondria, plays a vital role in mitochondrial structure, function, maintenance, and apoptosis. Upon Ca²⁺ activation, hPLSCR3 mediates PL translocation at the mitochondrial membrane enhancing t-bid-induced cytochrome c release and apoptosis. Mitochondria are important target organelles for heavy-metals-induced apoptotic signaling cascade and are the central executioner of apoptosis to trigger. Pb²⁺ and Hg²⁺ toxicity mediates apoptosis by increased reactive oxygen species (ROS) and cytochrome c release from mitochondria. To discover the role of hPLSCR3 in heavy metal toxicity, hPLSCR3 was overexpressed as a recombinant protein in Escherichia coli Rosetta (DE3) and purified by affinity chromatography. The biochemical assay using synthetic proteoliposomes demonstrated that hPLSCR3 translocated aminophospholipids in the presence of micromolar concentrations of Pb²⁺ and Hg²⁺. A point mutation in the Ca²⁺-binding motif (F258V) led to a ∼60% loss in the functional activity and decreased binding affinities for Pb²⁺ and Hg²⁺ implying that the divalent heavy metal ions bind to the Ca²⁺-binding motif. This was further affirmed by the characteristic spectra observed with stains-all dye. The conformational changes upon heavy metal binding were monitored by circular dichroism, intrinsic tryptophan fluorescence, and light-scattering studies. Our results revealed that Pb²⁺ and Hg²⁺ bind to hPLSCR3 with higher affinity than Ca²⁺ thus mediating scramblase activity. To summarize, this is the first biochemical evidence for heavy metals binding to the mitochondrial membrane protein leading to bidirectional translocation of PLs specifically toward phosphatidylethanolamine.

Cathepsin B mediated scramblase activation triggers cytotoxicity and cell cycle arrest by andrographolide to overcome cellular resistance in cisplatin resistant human hepatocellular carcinoma HepG2 cells

Andrographolide regimen in single or in combination with anticancer drugs is a promising new strategy to reverse chemoresistance in heaptocellular carcinoma. Apoptosis inducing factor (AIF) may regulate a complementary, cooperative or redundant pathway, along with caspase cascades. Despite these findings, mechanisms underlying caspase-dependent and-independent signaling pathways in andrographolide -induced apoptosis in cisplatin-resistant human hepatocellular carcinoma cell line (HepG2CR) remain unclear. Andrographolide treatment effectively reduced NF-κβ nuclear localization by modulating protein kinase A- protein phosphatase 2 A- Iκβ kinase (PKA/PP2 A/IKK) axis that in turn maintains initiator caspase8 activity. Lysosomal distribution of tBid stimulates cytosolic cathepsin B resulting accumulation of truncated-AIF with induction in scramblase mediated phosphatidylserine exposure in HepG2CR cells. Andrographolide treatment thereby switch on subG1 phase arrest by modulating cellular check points (cyclin A, B, cyclin dependent kinase-1) cueing to the apoptosis event. Collectively, this study suggested antineoplastic potential of andrographolide through PKA/PP2 A/IKK pathway in HepG2CR cells.

The single C-terminal helix of human phospholipid scramblase 1 is required for membrane insertion and scrambling activity

Human phospholipid scramblase 1 (hPLSCR1) belongs to the ATP-independent class of phospholipid translocators which possess a single EF-hand-like Ca(2+)-binding motif and also a C-terminal helix (CTH). The CTH domain of hPLSCR1 was believed to be a putative single transmembrane helix at the C-terminus. Recent homology modeling studies by Bateman et al. predicted that the hydrophobic nature of this helix is due to its packing in the core of the protein domain and proposed that this is not a true transmembrane helix [Bateman A, Finn RD, Sims PJ, Wiedmer T, Biegert A & Johannes S. Bioinformatics 2008, 25, 159]. To determine the exact function of the CTH of hPLSCR1, we deleted the CTH domain and determined: (a) whether CTH plays any role beyond membrane anchorage, (b) the functional consequences of CTH deletion, and (c) any conformational changes associated with CTH in a lipid environment. In vitro reconstitution studies confirm that the predicted CTH is required for membrane insertion and scrambling activity. CTH deletion caused a 50% decrease in binding affinity of Ca(2+) for ∆CTH-hPLSCR1 (K(a) = 115 μM) compared with hPLSCR1 (K(a) = 249 μM). Far UV-CD studies revealed that the CTH peptide adopts α-helicity only in the presence of SDS micelles and negatively charged vesicles, indicating that electrostatic interactions are required for insertion of the peptide. CTH peptide-quenching studies confirm that the predicted CTH inserts into the membrane and its ability to interact with the membrane depends on the presence of charge interactions. TOXCAT assay revealed that CTH of hPLSCR1 does not oligomerize in the membrane. We conclude that CTH is required for membrane insertion and Ca(2+) coordination and also plays an important role in the functional conformation of hPLSCR1.

Biochemical evidence for lead and mercury induced transbilayer movement of phospholipids mediated by human phospholipid scramblase 1

Human phospholipid scramblase 1(hPLSCR1) is a transmembrane protein involved in bidirectional scrambling of plasma membrane phospholipids during cell activation, blood coagulation, and apoptosis in response to elevated intracellular Ca(2+) levels. Pb(2+) and Hg(2+) are known to cause procoagulant activation via phosphatidylserine exposure to the external surface in erythrocytes, resulting in blood coagulation. To explore its role in lead and mercury poisoning, hPLSCR1 was overexpressed in Escherichia coli BL21 (DE3) and purified using affinity chromatography. The biochemical assay showed rapid scrambling of phospholipids in the presence of Hg(2+) and Pb(2+). The binding constant (Ka) was calculated and found to be 250 nM(-1) and 170 nM(-1) for Hg(2+) and Pb(2+), respectively. The intrinsic tryptophan fluorescence and far ultraviolet circular dichroism studies revealed that Hg(2+) and Pb(2+) bind to hPLSCR1 and induce conformational changes. hPLSCR1 treated with protein modifying reagent N-ethylmaleimide before functional reconstitution showed 40% and 24% inhibition in the presence of Hg(2+) and Pb(2+), respectively. This is the first biochemical evidence to prove the above hypothesis that hPLSCR1 is activated in heavy metal poisoning, which leads to bidirectional transbilayer movement of phospholipids.

Recovery of functionally active recombinant human phospholipid scramblase 1 from inclusion bodies using N-lauroyl sarcosine

Human phospholipid scramblase (hPLSCR1) is a transmembrane protein involved in rapid bidirectional scrambling of phospholipids across the plasma membrane in response to elevated intracellular calcium (Ca(2+)) levels. Overexpression of recombinant hPLSCR1 in Escherichia coli BL21 (DE3) leads to its deposition in inclusion bodies (IBs). N-lauroyl sarcosine was used to solubilize IBs and to recover functionally active hPLSCR1 from them. Protein was purified to homogeneity by nickel-nitrilotriacetic acid (Ni(2+)-NTA) affinity chromatography and was >98% pure. Functional activity of the purified protein was validated by in vitro reconstitution studies, ~18% of 7-nitrobenz-2-oxa-1, 3-diazol-4-yl-phosphatidylcholine (NBD-PC) phospholipids was translocated across the lipid bilayer in the presence of Ca(2+) ions. Far ultraviolet circular dichroism (UV-CD) studies reveal that the secondary structure of protein is predominantly an α-helix, and under nondenaturing conditions, the protein exists as a monomer. Here we describe a method to purify recombinant membrane protein with higher yield than previously described methods involving renaturation techniques.

Regulatory role of nitric oxide in the reduced survival of erythrocytes in visceral leishmaniasis

BACKGROUND

Nitric oxide (NO) plays a vital role in maintaining the survivability of circulating erythrocytes. Here we have investigated whether NO depletion associated with visceral leishmaniasis (VL) is responsible for the reduced survival of erythrocytes observed during the disease.

METHODS

Infected hamsters were treated with standard anti-leishmanial sodium stibogluconate (SAG) and NO donor isosorbide dinitrate (ISD). Erythrophagocytosis by macrophages was determined by labelling the cells with FITC followed by flow cytometry. Aggregation of band3 was estimated from band3 associated EMA fluorescence. Caspase 3 activity was measured using immunosorbent assay kit. Phosphatidylserine (PS) externalization and cell shrinkage were determined using annexin V. Aminophspholipid translocase and scramblase activities were measured following NBD-PS and NBD-PC internalization, respectively.

RESULTS

Impairment of both synthesis and uptake of NO resulted in decreased bioavailability of this signaling molecule in erythrocytes in VL. NO level was replenished after simultaneous treatment with ISD and SAG. Combination treatment decreased red cell apoptosis in infected animals by deactivating caspase 3 through s-nitrosylation. Drug treatment prevented infection-mediated ATP depletion and altered calcium homeostasis in erythrocytes. Improved metabolic environment effectively amended dysregulation of aminophospholipid translocase and scramblase, which in turn reduced cell shrinkage, and exposure of phosphatidylserine on the cell surface under the diseased condition.

CONCLUSION AND GENERAL SIGNIFICANCE

In this study, we have identified NO depletion to be an important factor in promoting premature hemolysis with the progress of leishmanial infection. The study implicates NO to be a possible target for future drug development towards the promotion of erythrocyte survival in VL.