Increased expression of phospholipid scramblase 1 in monocytes from patients with systemic lupus erythematosus

Identifying key genes in CD4+ T cells of systemic lupus erythematosus by integrated bioinformatics analysis

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by excessive activation of T and B lymphocytes and breakdown of immune tolerance to autoantigens. Despite several mechanisms including the genetic alterations and inflammatory responses have been reported, the overall signature genes in CD4⁺ T cells and how they affect the pathological process of SLE remain to be elucidated. This study aimed to identify the crucial genes, potential biological processes and pathways underlying SLE pathogenesis by integrated bioinformatics. The gene expression profiles of isolated peripheral CD4⁺ T cells from SLE patients with different disease activity and healthy controls (GSE97263) were analyzed, and 14 co-expression modules were identified using weighted gene co-expression network analysis (WGCNA). Some of these modules showed significantly positive or negative correlations with SLE disease activity, and primarily enriched in the regulation of type I interferon and immune responses. Next, combining time course sequencing (TCseq) with differentially expressed gene (DEG) analysis, crucial genes in lupus CD4⁺ T cells were revealed, including some interferon signature genes (ISGs). Among these genes, we identified 4 upregulated genes (PLSCR1, IFI35, BATF2 and CLDN5) and 2 downregulated genes (GDF7 and DERL3) as newfound key genes. The elevated genes showed close relationship with the SLE disease activity. In general, our study identified 6 novel biomarkers in CD4⁺ T cells that might contribute to the diagnosis and treatment of SLE.

Phospholipid scramblase 1: a protein with multiple functions via multiple molecular interactors

Phospholipid scramblase 1 (PLSCR1) is the most studied protein of the scramblase family. Originally, it was identified as a membrane protein involved in maintaining plasma membrane asymmetry. However, studies conducted over the past few years have shown the involvement of PLSCR1 in several other cellular pathways. Indeed, PLSCR1 is not only embedded in the plasma membrane but is also expressed in several intracellular compartments where it interacts with a diverse repertoire of effectors, mediators, and regulators contributing to distinct cellular processes. Although most PLSCR1 interactors are thought to be cell-type specific, PLSCR1 often exerts its regulatory functions through shared mechanisms, including the trafficking of different molecules within intracellular vesicles such as endosomes, liposomes, and phagosomes. Intriguingly, besides endogenous proteins, PLSCR1 was also reported to interact with exogenous viral proteins, thereby regulating viral uptake and spread. This review aims to summarize the current knowledge about the multiple roles of PLSCR1 in distinct cellular pathways.

RNA sequencing of whole blood in dogs with primary immune-mediated hemolytic anemia (IMHA) reveals novel insights into disease pathogenesis

Immune-mediated hemolytic anemia (IMHA) is a life-threatening autoimmune disorder characterized by a self-mediated attack on circulating red blood cells. The disease occurs naturally in both dogs and humans, but is significantly more prevalent in dogs. Because of its shared features across species, dogs offer a naturally occurring model for studying IMHA in people. In this study, we used RNA sequencing of whole blood from treatment-naïve dogs to study transcriptome-wide changes in gene expression in newly diagnosed animals compared to healthy controls. We found many overexpressed genes in pathways related to neutrophil function, coagulation, and hematopoiesis. In particular, the most highly overexpressed gene in cases was a phospholipase scramblase, which mediates the externalization of phosphatidylserine from the inner to the outer leaflet of cell membranes. This family of genes has been shown to be critically important for programmed cell death of erythrocytes as well as the initiation of the clotting cascade. Unexpectedly, we found marked underexpression of many genes related to lymphocyte function. We also identified groups of genes that are highly associated with the inflammatory response and red blood cell regeneration in affected dogs. We did not find any genes that distinguished dogs that lived vs. those that died at 30 days following diagnosis, nor did we find any relevant genomic signatures of microbial organisms in the blood of affected animals. Future studies are warranted to validate these findings and assess their implication in developing novel therapeutic approaches for dogs and humans with IMHA.

Transient Receptor Potential Canonical 5-Scramblase Signaling Complex Mediates Neuronal Phosphatidylserine Externalization and Apoptosis

Phospholipid scramblase 1 (PLSCR1), a lipid-binding and Ca²⁺-sensitive protein located on plasma membranes, is critically involved in phosphatidylserine (PS) externalization, an important process in cell apoptosis. Transient receptor potential canonical 5 (TRPC5), is a nonselective Ca²⁺ channel in neurons that interacts with many downstream molecules, participating in diverse physiological functions including temperature or mechanical sensation. The interaction between TRPC5 and PLSCR1 has never been reported. Here, we showed that PLSCR1 interacts with TRPC5 through their C-termini in HEK293 cells and mouse cortical neurons. Formation of TRPC5-PLSCR1 complex stimulates PS externalization and promotes cell apoptosis in HEK293 cells and mouse cerebral neurons. Furthermore, in vivo studies showed that PS externalization in cortical neurons induced by artificial cerebral ischemia-reperfusion was reduced in TRPC5 knockout mice compared to wild-type mice, and that the percentage of apoptotic neurons was also lower in TRPC5 knockout mice than in wild-type mice. Collectively, the present study suggested that TRPC5-PLSCR1 is a signaling complex mediating PS externalization and apoptosis in neurons and that TRPC5 plays a pathological role in cerebral-ischemia reperfusion injury.

Attenuation of endothelial phosphatidylserine exposure decreases ischemia-reperfusion induced changes in microvascular permeability

BACKGROUND

Translocation of phosphatidylserine from the inner leaflet to the outer leaflet of the endothelial membrane via phospholipid scramblase-1 (PLSCR1) is an apoptotic signal responsible for the loss of endothelial barrier integrity after ischemia-reperfusion injury (IRI). We hypothesized that inhibiting phosphatidylserine expression on endothelial cells would attenuate IRI induced increases in hydraulic permeability (Lp).

METHODS

Mesenteric Lp was measured in rat post-capillary mesenteric venules subjected to IRI via superior mesenteric artery (SMA) occlusion (45 minutes) and release (300 minutes) in conjunction with several inhibitors of phosphatidylserine exposure as follows: (1) inhibition of PLSCR1 translocation (dithioerythritol, n = 3), (2) inhibition of PLSCR1 membrane trafficking (2-bromopalmitate [2-BP], n = 3), and (3) inhibition of ion exchange necessary for PLSCR1 function (4,4'-Diisothiocyano-2,2'-stilbenedisulfonic acid [DIDS], n = 3). Under the same IRI conditions, rats were also administered targeted inhibitors of phosphatidylserine exposure including knockdown of PLSCR1 (n = 3) using RNA interference (RNAi), and as a potential therapeutic tool Diannexin, a selective phosphatidylserine blocker (n = 3).

RESULTS

During IRI net Lp increased by 80% (p < 0.01). Net reductions of Lp were accomplished by 2-BP (46% reduction, p = 0.005), combined DET + 2-BP + DIDS (32% reduction, p = 0.04), RNAi (55% reduction, p = 0.002), Diannexin administered pre-SMA artery occlusion (73% reduction, p = 0.001), and post-SMA occlusion (70% reduction, p = 0.002).

CONCLUSION

Phosphatidylserine exposure is a key event in the pathogenesis of microvascular dysfunction during IRI. Clinically, inhibition of phosphatidylserine exposure is a promising strategy that may 1 day be used to mitigate the effects of IRI.

Epigenome-wide association study of peripheral blood mononuclear cells in systemic lupus erythematosus: Identifying DNA methylation signatures associated with interferon-related genes based on ethnicity and SLEDAI

Systemic lupus erythematosus (SLE or lupus) is a heterogeneous autoimmune disease characterized by the involvement of multiple organs and the production of antinuclear antibodies. DNA methylation plays an important role in the pathogenesis of lupus. We have performed an epigenome-wide DNA methylation study in lupus and healthy control (non-lupus) subjects to identify epigenetic patterns in lupus characterized ethnicity and SLE disease activity index (SLEDAI). A total of fifty-seven lupus patients (39 African American (AA) and 18 European American (EA)) and 33 healthy controls (17 AA and 16 EA) were studied. Differential DNA methylation between lupus patients and controls was assessed for approximately 485,000 CpG sites across the genome. We identified 41 differentially methylated sites (associated with 30 genes) between lupus and control s subjects, 85% of which were hypomethylated. Significant hypomethylation of differentially methylated sites was associated with several interferon-related genes, including MX1, IFI44L, PARP9, DT3XL, IFIT1, IFI44, RSAD2, PLSCR1, and IRF7. Several of these associated genes were also hypomethylated in comparisons between AA lupus and AA non-lupus subjects and between lupus patients with SLEDAI>6 and non-lupus subjects. Our analysis of gene expression data through RT-PCR confirmed these findings. Thus, the results indicate epigenetics susceptibility in lupus, which may be associated with SLEDAI score and ethnicity. In addition, our findings support the importance of the Type 1 interferon pathway in lupus pathogenesis.

Phosphatidylserine Exposure Controls Viral Innate Immune Responses by Microglia

Microglia are the intrinsic immune sentinels of the central nervous system. Their activation restricts tissue injury and pathogen spread, but in some settings, including viral infection, this response can contribute to cell death and disease. Identifying mechanisms that control microglial responses is therefore an important objective. Using replication-incompetent adenovirus 5 (Ad5)-based vectors as a model, we investigated the mechanisms through which microglia recognize and respond to viral uptake. Transgenic, immunohistochemical, molecular-genetic, and fluorescence imaging approaches revealed that phosphatidylserine (PtdSer) exposure on the outer leaflet of transduced cells triggers their engulfment by microglia through TAM receptor-dependent mechanisms. We show that inhibition of phospholipid scramblase 1 (PLSCR1) activity reduces intracellular calcium dysregulation, prevents PtdSer externalization, and enables months-long protection of vector-transduced, transgene-expressing cells from microglial phagocytosis. Our study identifies PLSCR1 as a potent target through which the innate immune response to viral vectors, and potentially other stimuli, may be controlled.

Phospholipid Scramblase 1 Modulates FcR-Mediated Phagocytosis in Differentiated Macrophages

Phospholipid Scramblase 1 (PLSCR1) was initially characterized as a type II transmembrane protein involved in bilayer movements of phospholipids across the plasma membrane leading to the cell surface exposure of phosphatidylserine, but other cellular functions have been ascribed to this protein in signaling processes and in the nucleus. In the present study, expression and functions of PLSCR1 were explored in specialized phagocytic cells of the monocyte/macrophage lineage. The expression of PLSCR1 was found to be markedly increased in monocyte-derived macrophages compared to undifferentiated primary monocytes. Surprisingly, this 3-fold increase in PLSCR1 expression correlated with an apparent modification in the membrane topology of the protein at the cell surface of differentiated macrophages. While depletion of PLSCR1 in the monocytic THP-1 cell-line with specific shRNA did not inhibit the constitutive cell surface exposure of phosphatidylserine observed in differentiated macrophages, a net increase in the FcR-mediated phagocytic activity was measured in PLSCR1-depleted THP-1 cells and in bone marrow-derived macrophages from PLSCR1 knock-out mice. Reciprocally, phagocytosis was down-regulated in cells overexpressing PLSCR1. Since endogenous PLSCR1 was recruited both in phagocytic cups and in phagosomes, our results reveal a specific role for induced PLSCR1 expression in the modulation of the phagocytic process in differentiated macrophages.

Two c-Myc binding sites are crucial in upregulating the expression of human phospholipid scramblase 1 gene

Human phospholipid scramblase 1 (hPLSCR1) is a type II endofacial membrane protein which mediates bi-directional transport of phospholipids across the plasma membrane. hPLSCR1, a multifunctional protein with variety of roles in apoptosis, tumor progression, cell signaling and anti-viral defense. The expression of such a multifunctional protein should be under tight regulation. Apart from a single report showing snail mediated down regulation of hPLSCR1, the molecular mechanisms regulating the expression of scramblases are not well elucidated. In this study we identified c-Myc as a transcriptional regulator of hPLSCR1. Transcription factor prediction tool ConSite predicted three binding sites for c-Myc. Reporter gene assays and western blot analysis revealed c-Myc mediated up regulation of hPLSCR1 expression. Deletion construct -790 lacking one c-Myc binding site showed a 27% decrease in promoter activity while deletion construct -469 lacking two c-Myc binding sites showed a 62% decrease in promoter activity. Site directed mutagenesis revealed the importance of c-Myc binding sites from -751 to -756 and -548 to -553 on the promoter of hPLSCR1in transcriptionally regulating the expression of hPLSCR1. The results were further confirmed by shRNA mediated knock down of endogenous c-Myc and in vivo interactions by ChIP assay.

Roles and regulation of phospholipid scramblases

Phospholipid scramblase activity is involved in the collapse of phospholipid (PL) asymmetry at the plasma membrane leading to externalization of phosphatidylserine. This activity is crucial for initiation of the blood coagulation cascade and for recognition/elimination of apoptotic cells by macrophages. Efforts to identify gene products associated with this activity led to the characterization of PL scramblase (PLSCR) and XKR family members which contribute to phosphatidylserine exposure in response to apoptotic stimuli. Meanwhile, TMEM16 family members were identified to externalize phosphatidylserine in response to elevated calcium in Scott syndrome platelets, which is critical for activation of the coagulation cascade. Herein, we report their mechanisms of gene regulation, molecular functions independent of their scrambling activity, and their potential roles in pathogenic conditions.

Phospholipid scramblase 1 expression is enhanced in patients with antiphospholipid syndrome

OBJECTIVE

Thrombus formation is the key event of vascular manifestations in antiphospholipid syndrome (APS). Phosphatidylserine (PS) is normally sequestered in the inner leaflet of cell membranes. Externalization of PS occurs during cell activation and is essential for promoting blood coagulation and for the binding of antiphospholipid antibodies (aPL) to cells. One of the molecules involved in PS externalization is phospholipid scramblase 1 (PLSCR1). We evaluated PLSCR1 expression on monocytes from APS patients and analyzed the in vitro effect of monoclonal aPL on PLSCR1 expression.

PATIENTS AND METHODS

Forty patients with APS were investigated. In vitro experiments were performed in monocyte cell lines incubated with monoclonal aPL. PLSCR1 expression was determined by quantitative real-time polymerase chain reactions. PS exposure on CD14(+) cell surface was analyzed by flow cytometry.

RESULTS

Levels of full-length PLSCR1 messenger RNA (mRNA) were significantly increased in APS patients compared with healthy controls (2.4 ± 1.2 vs. 1.3 ± 0.4, respectively, p < 0.001). In cultured monocytes, interferon alpha enhanced tissue-factor expression mediated by β2-glycoprotein-I-dependent monoclonal anticardiolipin antibody.

CONCLUSIONS

Monocytes in APS patients had increased PLSCR1 mRNA expression.

Phospholipid scramblase 1 regulates Toll-like receptor 9-mediated type I interferon production in plasmacytoid dendritic cells

Toll-like receptor 9 (TLR9) senses microbial DNA in the endosomes of plasmacytoid dendritic cells (pDCs) and triggers MyD88-dependent type I interferon (IFN) responses. To better understand TLR9 biology in pDCs, we established a yeast two-hybrid library for the identification of TLR9-interacting proteins. Here, we report that an IFN-inducible protein, phospholipid scramblase 1 (PLSCR1), interacts with TLR9 in pDCs. Knockdown of PLSCR1 expression by siRNA in human pDC cell line led to a 60-70% reduction of IFN-α responses following CpG-ODN (oligodeoxynucleotide) stimulation. Primary pDCs from PLSCR1-deficient mice produced lower amount of type 1 IFN than pDCs from the wild-type mice in response to CpG-ODN, herpes simplex virus and influenza A virus. Following CpG-A stimulation, there were much lower amounts of TLR9 in the early endosomes together with CpG-A in pDCs from PLSCR1-deficient mice. Our study demonstrates that PLSCR1 is a TLR9-interacting protein that plays an important role in pDC's type 1 IFN responses by regulating TLR9 trafficking to the endosomal compartment.

Inhibition of Hepatitis B virus replication by phospholipid scramblase 1 in vitro and in vivo

Human Phospholipid scramblase 1 (PLSCR1) is an α/β interferon-inducible protein that mediates antiviral activity against RNA viruses including vesicular stomatitis virus (VSV) and encephalomyocarditis virus (EMCV). In the present study, we investigated the antiviral activity of PLSCR1 protein against HBV (Hepatitis B virus). Firstly, PLSCR1 mRNA and protein expression was found to be downregulated in HepG2 cells after HBV infection. Then by performing co-transient-transfection experiments in cells and hydrodynamics-based transfection experiments in mice using a HBV expression plasmid and a PLSCR1 expression plasmid, we found that PLSCR1 inhibited HBV replication in vitro and in vivo through a significant reduction in the synthesis of viral proteins, DNA replicative intermediates and HBV RNAs. We also demonstrated that the antiviral action of PLSCR1 against HBV occurs, partly at least, by activating the Jak/Stat pathway. In conclusion, our results suggest that the expression of PLSCR1 is involved in HBV replication and that PLSCR1 has antiviral activity against HBV.