A human cell-surface receptor for xenotropic and polytropic murine leukemia viruses: possible role in G protein-coupled signal transduction

A co-opted endogenous retroviral envelope promotes cell survival by controlling CTR1-mediated copper transport and homeostasis

Copper is a critical element for eukaryotic life involved in numerous cellular functions, including redox balance, but is toxic in excess. Therefore, tight regulation of copper acquisition and homeostasis is essential for cell physiology and survival. Here, we identify a different regulatory mechanism for cellular copper homeostasis that requires the presence of an endogenous retroviral envelope glycoprotein called Refrex1. We show that cells respond to elevated extracellular copper by increasing the expression of Refrex1, which regulates copper acquisition through interaction with the main copper transporter CTR1. Downmodulation of Refrex1 results in intracellular copper accumulation leading to reactive oxygen species (ROS) production and subsequent apoptosis, which is prevented by copper chelator treatment. Our results show that Refrex1 has been co-opted for its ability to regulate copper entry through CTR1 in order to limit copper excess, redox imbalance, and ensuing cell death, strongly suggesting that other endogenous retroviruses may have similar metabolic functions among vertebrates.

Inositol pyrophosphates activate the vacuolar transport chaperone complex in yeast by disrupting a homotypic SPX domain interaction

Many proteins involved in eukaryotic phosphate homeostasis are regulated by SPX domains. In yeast, the vacuolar transporter chaperone (VTC) complex contains two such domains, but mechanistic details of its regulation are not well understood. Here, we show at the atomic level how inositol pyrophosphates interact with SPX domains of subunits Vtc2 and Vtc3 to control the activity of the VTC complex. Vtc2 inhibits the catalytically active VTC subunit Vtc4 by homotypic SPX-SPX interactions via the conserved helix α1 and the previously undescribed helix α7. Binding of inositol pyrophosphates to Vtc2 abrogates this interaction, thus activating the VTC complex. Accordingly, VTC activation is also achieved by site-specific point mutations that disrupt the SPX-SPX interface. Structural data suggest that ligand binding induces reorientation of helix α1 and exposes the modifiable helix α7, which might facilitate its post-translational modification in vivo. The variable composition of these regions within the SPX domain family might contribute to the diversified SPX functions in eukaryotic phosphate homeostasis.

Role of transporters in regulating mammalian intracellular inorganic phosphate

This review summarizes the current understanding of the role of plasma membrane transporters in regulating intracellular inorganic phosphate ([Pi]In) in mammals. Pi influx is mediated by SLC34 and SLC20 Na+-Pi cotransporters. In non-epithelial cells other than erythrocytes, Pi influx via SLC20 transporters PiT1 and/or PiT2 is balanced by efflux through XPR1 (xenotropic and polytropic retrovirus receptor 1). Two new pathways for mammalian Pi transport regulation have been described recently: 1) in the presence of adequate Pi, cells continuously internalize and degrade PiT1. Pi starvation causes recycling of PiT1 from early endosomes to the plasma membrane and thereby increases the capacity for Pi influx; and 2) binding of inositol pyrophosphate InsP8 to the SPX domain of XPR1 increases Pi efflux. InsP8 is degraded by a phosphatase that is strongly inhibited by Pi. Therefore, an increase in [Pi]In decreases InsP8 degradation, increases InsP8 binding to SPX, and increases Pi efflux, completing a feedback loop for [Pi]In homeostasis. Published data on [Pi]In by magnetic resonance spectroscopy indicate that the steady state [Pi]In of skeletal muscle, heart, and brain is normally in the range of 1–5 mM, but it is not yet known whether PiT1 recycling or XPR1 activation by InsP8 contributes to Pi homeostasis in these organs. Data on [Pi]In in cultured cells are variable and suggest that some cells can regulate [Pi] better than others, following a change in [Pi]Ex. More measurements of [Pi]In, influx, and efflux are needed to determine how closely, and how rapidly, mammalian [Pi]In is regulated during either hyper- or hypophosphatemia.

Bioinformatics analyses proposed xenotropic and polytropic retrovirus receptor 1 as a potential diagnostic and prognostic biomarker and immunotherapeutic target in head and neck squamous cell carcinoma

OBJECTIVE

The role of Xenotropic and polytropic retrovirus receptor 1 (XPR1), a cell surface receptor for certain types of murine leukemia viruses, in human cancers has been rarely studied. We aimed to evaluate the values of XPR1 as a biomarker and therapeutic target in head and neck squamous cell carcinoma (HNSCC).

METHODS

Bioinformatics tools and online databases, including R packages, ONCOMINE, The Cancer Genome Atlas (TCGA), Human Protein Atlas (HPA), UALCAN, MethSurv, cBioPortal, and TIMER2.0 were applied in this study.

RESULTS

The mRNA and protein expression of XPR1 is significantly up-regulated in HNSCC tissues compared with normal tissues. The receiver operating characteristic (ROC) curve shows XPR1 has high specificity and accuracy in the diagnosis of HNSCC (AUC = 0.883). Patients with high-level expression of XPR1 have poorer overall survival (OS, P = 0.002), disease-specific survival (DSS, P = 0.014), and progress-free interval (PFI, P = 0.017). UALCAN analysis indicates that the methylation of XPR1 promoter in HNSCC is significantly down-regulated. MethSurve was used to investigate the impact of individual CpG islands on the prognosis of HNSCC patients. Low DNA methylation levels of cg11538848 and cg20948051 and high DNA methylation levels of cg23675362, cg18440470, and cg22026687 are significantly related to poor prognosis. The Gene Set Enrichment Analysis (GSEA), Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis indicate that XPR1 is involved in various important biological functions and signaling pathways closely related to cancer. The co-expression analysis of XPR1 and N6-methyladenosine (m6A) RNA methylation regulators shows that XPR1 is significantly related to the expression of main m6A regulators. Immune infiltration analysis shows that the expression of XPR1 is related to certain types of immune infiltrating cells and has a positive correlation with the expression of four immune checkpoint genes, PDCD1LG2, CD274, HAVCR2, and SIGLEC15.

CONCLUSION

In summary, these results indicate that XPR1 is a potential diagnostic and prognostic biomarker and immunotherapy target for HNSCC. This study sheds new light on understanding the formation and development of HNSCC and sets the basis for further studying the role of XPR1 in HNSCC and other types of cancers.

The Pathology of Primary Familial Brain Calcification: Implications for Treatment

Primary familial brain calcification (PFBC) is an inherited neurodegenerative disorder mainly characterized by progressive calcium deposition bilaterally in the brain, accompanied by various symptoms, such as dystonia, ataxia, parkinsonism, dementia, depression, headaches, and epilepsy. Currently, the etiology of PFBC is largely unknown, and no specific prevention or treatment is available. During the past 10 years, six causative genes (SLC20A2, PDGFRB, PDGFB, XPR1, MYORG, and JAM2) have been identified in PFBC. In this review, considering mechanistic studies of these genes at the cellular level and in animals, we summarize the pathogenesis and potential preventive and therapeutic strategies for PFBC patients. Our systematic analysis suggests a classification for PFBC genetic etiology based on several characteristics, provides a summary of the known composition of brain calcification, and identifies some potential therapeutic targets for PFBC.

Systematic profiling of conditional degron tag technologies for target validation studies

Conditional degron tags (CDTs) are a powerful tool for target validation that combines the kinetics and reversible action of pharmacological agents with the generalizability of genetic manipulation. However, successful design of a CDT fusion protein often requires a prolonged, ad hoc cycle of construct design, failure, and re-design. To address this limitation, we report here a system to rapidly compare the activity of five unique CDTs: AID/AID2, IKZF3d, dTAG, HaloTag, and SMASh. We demonstrate the utility of this system against 16 unique protein targets. We find that expression and degradation are highly dependent on the specific CDT, the construct design, and the target. None of the CDTs leads to efficient expression and/or degradation across all targets; however, our systematic approach enables the identification of at least one optimal CDT fusion for each target. To enable the adoption of CDT strategies more broadly, we have made these reagents, and a detailed protocol, available as a community resource.

Conditional Degron Tags are a valuable tool to validate and study novel therapeutic targets. Here, the authors compared 5 orthogonal tags across 16 unique proteins and provide a panel of vectors for users to systematically screen the tags with their own protein of interest.

Identification of Copper Transporter 1 as a Receptor for Feline Endogenous Retrovirus ERV-DC14

Vertebrates harbor hundreds of endogenous retroviral (ERV) sequences in their genomes, which are considered signs of past infections that occurred during evolution. On rare occasions, ERV genes like env are maintained and coopted by hosts for physiological functions, but they also participate in recombination events with exogenous retroviruses to generate rearranged viruses with novel tropisms. In domestic cats, feline leukemia virus type D (FeLV-D) has been described as a recombinant virus between the infectious FeLV-A and likely the ERV-DC14 env gene that resulted in an extended tropism due to the usage of a new uncharacterized retroviral receptor. Here, we report the identification of SLC31A1 encoding the copper transporter 1 (CTR1) as a susceptibility gene for ERV-DC14 infection. Expression of human CTR1 into nonpermissive cells was sufficient to confer sensitivity to ERV-DC14 pseudotype infection and to increase the binding of an ERV-DC14 Env ligand. Moreover, inactivation of CTR1 by genome editing or cell surface downmodulation of CTR1 by a high dose of copper dramatically decreased ERV-DC14 infection and binding, while magnesium treatment had no effect. We also investigated the role of CTR1 in the nonpermissivity of feline and hamster cells. While feline CTR1 was fully functional for ERV-DC14, we found that binding was strongly reduced upon treatment with conditioned medium of feline cells, suggesting that the observed resistance to infection was a consequence of CTR1 saturation. In contrast, hamster CTR1 was inactive due to the presence of a N-linked glycosylation site at position 27, which is absent in the human ortholog. These results provide evidence that CTR1 is a receptor for ERV-DC14. Along with chimpanzee endogenous retrovirus type 2, ERV-DC14 is the second family of endogenous retrovirus known to have used CTR1 during past infections of vertebrates. IMPORTANCE Receptor usage is an important determinant of diseases induced by pathogenic retroviruses. In the case of feline leukemia viruses, three subgroups (A, B, and C) based on their ability to recognize different cell host receptors, respectively, the thiamine transporter THTR1, the phosphate transporter PiT1, and the heme exporter FLVCR1, are associated with distinct feline diseases. FeLV-A is horizontally transmitted and found in all naturally infected cats, while FeLV-B and FeLV-C have emerged from FeLV-A, respectively, by recombination with endogenous retroviral env sequences or by mutations in the FeLV-A env gene, both leading to a switch in receptor usage and in subsequent in vivo tropism. Here, we set up a genetic screen to identify the retroviral receptor of ERV-DC14, a feline endogenous provirus whose env gene has been captured by infectious FeLV-A to give rise to FeLV-D in a process similar to FeLV-B. Our results reveal that the copper transporter CTR1 was such a receptor and provide new insights into the acquisition of an expanded tropism by FeLV-D.

Phosphate dysregulation via the XPR1-KIDINS220 protein complex is a therapeutic vulnerability in ovarian cancer

Despite advances in precision medicine, the clinical prospects for patients with ovarian and uterine cancers have not substantially improved. Here, we analyzed genome-scale CRISPR/Cas9 loss-of-function screens across 851 human cancer cell lines and found that frequent overexpression of SLC34A2 – encoding a phosphate importer – is correlated to sensitivity to loss of the phosphate exporter XPR1 in vitro and in vivo. In patient-derived tumor samples, we observed frequent PAX8-dependent overexpression of SLC34A2, XPR1 copy number amplifications, and XPR1 mRNA overexpression. Mechanistically, in SLC34A2-high cancer cell lines, genetic or pharmacologic inhibition of XPR1-dependent phosphate efflux leads to the toxic accumulation of intracellular phosphate. Finally, we show that XPR1 requires the novel partner protein KIDINS220 for proper cellular localization and activity, and that disruption of this protein complex results in acidic vacuolar structures preceding cell death. These data point to the XPR1:KIDINS220 complex and phosphate dysregulation as a therapeutic vulnerability in ovarian cancer.

Golub and colleagues identify the phosphate exporter XPR1 as a therapeutic vulnerability in ovarian and uterine cancers, and show that phosphate efflux inhibition reduces tumor cell viability through accumulation of intracellular phosphate.

The phosphate exporter XPR1/SLC53A1 is required for the tumorigenicity of epithelial ovarian cancer

Ovarian cancer is the fifth most common cause of cancer‐related death in women. Ovarian clear cell carcinoma (OCCC) is a chemotherapy‐resistant epithelial ovarian cancer with poor prognosis. As a basis for the development of therapeutic agents that could improve the prognosis of OCCC, we performed a screen for proteins critical for the tumorigenicity of OCCC using the CRISPR/Cas9 system. Here we show that knockdown of the phosphate exporter XPR1/SLC53A1 induces the growth arrest and apoptosis of OCCC cells in vitro. Moreover, we show that knockdown of XPR1/SLC53A1 inhibits the proliferation of OCCC cells xenografted into immunocompromised mice. These results suggest that XPR1/SLC53A1 plays a critical role in the tumorigenesis of OCCC cells. We speculate that XPR1/SLC53A1 might be a promising molecular target for the therapeutic treatment of OCCC.

Nano-Bio Interface-Guided Nanoparticle Protein Corona Antigen for Immunoassays and Immunoimaging in a Complex Matrix

Engineered nanoparticles are widely used in biological imaging and drug delivery because of their excellent physical and chemical properties, but almost all the original functions of engineered nanoparticles suffer from a complex matrix. Herein, we proposed a strategy of preparing nanoparticle protein corona antigens (NPCAgs) through exposing a magnetic core silicon shell (Fe₃O₄@SiO₂) fluorescent probe to an antigen protein solution, which could reduce the adsorption of nanoparticles (NPs) with other proteins in serum. In the presence of target anti-BSA IgG, a competitive-type displacement reaction was implemented between NPs@BSA and other proteins by target anti-BSA IgG through the specific antigen-antibody reaction. In addition, secondary structure analysis showed that almost all of the NPCAgs retained their natural conformation, which ensured the function of the NPCAgs, specifically capturing an antibody. Therefore, the NPCAgs showed good performance in immunoassays and immunoimaging, which should shed light on the application in imaging and identification of other nanomaterials.

The miR-4732-5p/XPR1 axis suppresses invasion, metastasis and epithelial-mesenchymal transition of lung adenocarcinoma by PI3K/Akt/GSK3β/Snail pathway

With the development in research, the importance of microRNAs (miRNAs) in the occurrence, metastasis, and prognosis of lung adenocarcinoma (LUAD) had received extensive attention. The aim of this study was...

Xenotropic and polytropic retrovirus receptor 1 regulates procoagulant platelet polyphosphate

Polyphosphate is a procoagulant inorganic polymer of linear-linked orthophosphate residues. Multiple investigations have established the importance of platelet polyphosphate in blood coagulation; however, the mechanistic details of polyphosphate homeostasis in mammalian species remain largely undefined. In this study, xenotropic and polytropic retrovirus receptor 1 (XPR1) regulated polyphosphate in platelets and was implicated in thrombosis in vivo. We used bioinformatic analyses of omics data to identify XPR1 as a major phosphate transporter in platelets. XPR1 messenger RNA and protein expression inversely correlated with intracellular polyphosphate content and release. Pharmacological interference with XPR1 activity increased polyphosphate stores, led to enhanced platelet-driven coagulation, and amplified thrombus formation under flow via the polyphosphate/factor XII pathway. Conditional gene deletion of Xpr1 in platelets resulted in polyphosphate accumulation, accelerated arterial thrombosis, and augmented activated platelet-driven pulmonary embolism without increasing bleeding in mice. These data identify platelet XPR1 as an integral regulator of platelet polyphosphate metabolism and reveal a fundamental role for phosphate homeostasis in thrombosis.

The fission yeast gmn2+ gene encodes an ERD1 homologue of Saccharomyces cerevisiae required for protein glycosylation and retention of luminal endoplasmic reticulum proteins

The gmn2 mutant of Schizosaccharomyces pombe has previously been shown to exhibit defects in protein glycosylation of N-linked oligosaccharides (Ballou, L. and Ballou, CE., Proc. Natl. Acad. Sci. USA, 92, 2790-2794 (1995)). Like most glycosylation-defective mutants, the S. pombe gmn2 mutant was found to be sensitive to hygromycin B, an aminoglycoside antibiotic. As a result of complementation analysis, the gmn2⁺ gene was found to be a single open reading frame that encodes a polypeptide of 373 amino acids consisting of multiple membrane-spanning regions. The Gmn2 protein shares sequence similarity with Kluyveromyces lactis and Saccharomyces cerevisiae Erd1 proteins, which are required for retention of luminal endoplasmic reticulum (ER) proteins. Although disruption of the gmn2⁺ gene is not lethal, the secreted glycoprotein showed a significant glycosylation defect with destabilization of the glycosyltransferase responsible for N-glycan elongation. It was also shown that a significant amount of BiP was missorted to the cell surface according to ADEL receptor destabilization. Fluorescent microscopy revealed that the functional Gmn2-EGFP fusion protein is mainly localized in the Golgi membrane. These results indicate that the Gmn2 protein is required for protein glycosylation and for retention of ER-resident proteins in S. pombe cells.

Biallelic XPR1 mutation associated with primary familial brain calcification presenting as paroxysmal kinesigenic dyskinesia with infantile convulsions

BACKGROUND

Mutations in the XPR1 gene are associated with primary familial brain calcifications (PFBC). All reported mutations are missense and inherited as an autosomal dominant trait. PFBC patients exhibited movement disorders, neuropsychiatric symptoms, and other associated symptoms with diverse severity, even within the same family.

MATERIALS AND METHODS

We identified and enrolled a patient with PFBC. Clinical data were comprehensively collected, including the age of onset, seizure types and frequency, trigger factors of paroxysmal dyskinesia, response to drugs, and general and neurological examination results. Whole-exome sequencing (WES) was performed to detect pathogenic variants. We further systematically reviewed the phenotypic and genetic features of patients with XPR1 mutations.

RESULTS

The patient showed bilateral calcification involving basal ganglia and cerebellar dentate. Clinically, he presented as paroxysmal kinesigenic dyskinesia with infantile convulsions (PKD/IC) with favorable outcome. We identified a compound heterozygous XPR1 mutation (c.786_789delTAGA/p.D262Efs*6, c.1342C>T/p.R448W), which were inherited from unaffected parents respectively. Further literature review shows a wide range of clinical manifestations of patients with XPR1 mutations, with movement disorders being the most common.

CONCLUSIONS

This is the first report of biallelic mutations in XPR1. The findings suggest for the first time a possible link between PKD/IC and XPR1 mutations.

Family-Specific Gains and Losses of Protein Domains in the Legume and Grass Plant Families

Protein domains can be regarded as sections of protein sequences capable of folding independently and performing specific functions. In addition to amino-acid level changes, protein sequences can also evolve through domain shuffling events such as domain insertion, deletion, or duplication. The evolution of protein domains can be studied by tracking domain changes in a selected set of species with known phylogenetic relationships. Here, we conduct such an analysis by defining domains as “features” or “descriptors,” and considering the species (target + outgroup) as instances or data-points in a data matrix. We then look for features (domains) that are significantly different between the target species and the outgroup species. We study the domain changes in 2 large, distinct groups of plant species: legumes (Fabaceae) and grasses (Poaceae), with respect to selected outgroup species. We evaluate 4 types of domain feature matrices: domain content, domain duplication, domain abundance, and domain versatility. The 4 types of domain feature matrices attempt to capture different aspects of domain changes through which the protein sequences may evolve—that is, via gain or loss of domains, increase or decrease in the copy number of domains along the sequences, expansion or contraction of domains, or through changes in the number of adjacent domain partners. All the feature matrices were analyzed using feature selection techniques and statistical tests to select protein domains that have significant different feature values in legumes and grasses. We report the biological functions of the top selected domains from the analysis of all the feature matrices. In addition, we also perform domain-centric gene ontology (dcGO) enrichment analysis on all selected domains from all 4 feature matrices to study the gene ontology terms associated with the significantly evolving domains in legumes and grasses. Domain content analysis revealed a striking loss of protein domains from the Fanconi anemia (FA) pathway, the pathway responsible for the repair of interstrand DNA crosslinks. The abundance analysis of domains found in legumes revealed an increase in glutathione synthase enzyme, an antioxidant required from nitrogen fixation, and a decrease in xanthine oxidizing enzymes, a phenomenon confirmed by previous studies. In grasses, the abundance analysis showed increases in domains related to gene silencing which could be due to polyploidy or due to enhanced response to viral infection. We provide a docker container that can be used to perform this analysis workflow on any user-defined sets of species, available at https://cloud.docker.com/u/akshayayadav/repository/docker/akshayayadav/protein-domain-evolution-project.

Interplay between primary familial brain calcification-associated SLC20A2 and XPR1 phosphate transporters requires inositol polyphosphates for control of cellular phosphate homeostasis

Solute carrier family 20 member 2 (SLC20A2) and xenotropic and polytropic retrovirus receptor 1 (XPR1) are transporters with phosphate uptake and efflux functions, respectively. Both are associated with primary familial brain calcification (PFBC), a genetic disease characterized by cerebral calcium-phosphate deposition and associated with neuropsychiatric symptoms. The association of the two transporters with the same disease suggests that they jointly regulate phosphate fluxes and cellular homeostasis, but direct evidence is missing. Here, we found that cross-talk between SLC20A2 and XPR1 regulates phosphate homeostasis, and we identified XPR1 as a key inositol polyphosphate (IP)-dependent regulator of this process. We found that overexpression of WT SLC20A2 increased phosphate uptake, as expected, but also unexpectedly increased phosphate efflux, whereas PFBC-associated SLC20A2 variants did not. Conversely, SLC20A2 depletion decreased phosphate uptake only slightly, most likely compensated for by the related SLC20A1 transporter, but strongly decreased XPR1-mediated phosphate efflux. The SLC20A2-XPR1 axis maintained constant intracellular phosphate and ATP levels, which both increased in XPR1 KO cells. Elevated ATP levels are a hallmark of altered inositol pyrophosphate (PP-IP) synthesis, and basal ATP levels were restored after phosphate efflux rescue with WT XPR1 but not with XPR1 harboring a mutated PP-IP-binding pocket. Accordingly, inositol hexakisphosphate kinase 1-2 (IP6K1-2) gene inactivation or IP6K inhibitor treatment abolished XPR1-mediated phosphate efflux regulation and homeostasis. Our findings unveil an SLC20A2-XPR1 interplay that depends on IPs such as PP-IPs and controls cellular phosphate homeostasis via the efflux route, and alteration of this interplay likely contributes to PFBC.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Regulation of porcine endogenous retrovirus by dual LTR1+2 (Long Terminal Region) miRNA in primary porcine kidney cells

Porcine endogenous retroviruses (PERVs) integrate into germline DNA as proviral genome that enables vertical transmission from parents to their offspring. The provirus usually survives as part of the host genome rather than as an infectious agent, but may become pathogenic if it crosses species barriers. Therefore, replication-competent PERV should be controlled through selective breeding or knockout technologies. Two microRNAs (miRNAs), dual LTR1 and LTR2, were selected to inhibit the expression of PERV in primary porcine kidney cells. The inhibition efficiency of the miRNAs was compared based on their inhibition of different PERV regions, specifically long terminal repeats (LTRs), gag, pol, and env. Gene expression was quantified using real-time polymerase chain reaction and the C-type reverse transcriptase (RT) activity was determined. The messenger RNA (mRNA) expression of the PERV LTR and env regions was determined in HeLa cells co-cultured with primary porcine kidney cells. The mRNA expression of the LTR, gag, pol, and env regions of PERV was dramatically inhibited by dual miRNA from 24 to 144 h after transfection, with the highest inhibition observed for the LTR and pol regions at 120 h. Additionally, the RT activity of PERV in the co-culture experiment of porcine and human cells was reduced by 84.4% at the sixth passage. The dual LTR 1+2 miRNA efficiently silences PERV in primary porcine kidney cells.

Murine Placental-Fetal Phosphate Dyshomeostasis Caused by an Xpr1 Deficiency Accelerates Placental Calcification and Restricts Fetal Growth in Late Gestation

Phosphorus is a necessary component of all living organisms. This nutrient is mainly transported from the maternal blood to the fetus via the placenta, and insufficient phosphorus availability via the placenta disturbs the normal development of the fetus, especially fetal bone formation in late gestation. Key proteins (phosphate transporters and exporters) that are responsible for the maintenance of placental-fetal phosphorus homeostasis have been identified. A deficiency in the phosphate transporter Pit2 has been shown to result in placental calcification and the retardation of fetal development in mice. What roles does XPR1 (the only known phosphate exporter) play in maintaining placental-fetal phosphorus homeostasis? In this study, we found that Xpr1 expression is strong in the murine placenta and increases with age during gestation. We generated a global Xpr1 knockout mouse and found that heterozygous (Xpr1^+/- ) and homozygous (Xpr1^-/- ) fetuses have lower inorganic phosphate (Pi) levels in amniotic fluid and serum and a decreased skeletal mineral content. Xpr1-deficient placentas show abnormal Pi exchange during gestation. Therefore, Xpr1 deficiency in the placenta disrupts placental-fetal Pi homeostasis. We also discovered that the placentas of the Xpr1^+/- and Xpr1^-/- embryos are severely calcified. Mendelian inheritance statistics for offspring outcomes indicated that Xpr1-deficient embryos are significantly reduced in late gestation. In addition, Xpr1^-/- mice die perinatally and a small proportion of Xpr1^+/- mice die neonatally. RNA sequence (RNA-Seq) analysis of placental mRNA revealed that many of the transcripts are significantly differentially expressed due to Xpr1 deficiency and are linked to dysfunction of the placenta. This study is the first to reveal that XPR1 plays an important role in maintaining placental-fetal Pi homeostasis, disruption of which causes severe placental calcification, delays normal placental function, and restricts fetal growth. © 2019 American Society for Bone and Mineral Research.

Mutational analysis and glycosylation sensitivity of restrictive XPR1 gammaretrovirus receptors in six mammalian species

Most viruses infect only a few hosts, but the xenotropic and polytropic mouse leukemia viruses (X/P-MLVs) are broadly infectious in mammalian species. X/P-MLVs use the XPR1 receptor for cell entry, and tropism differences are due to polymorphisms in XPR1 and the viral envelope. To characterize these receptor variants and identify blocks to cross-species transmission, we examined the XPR1 receptors in six mammalian species that restrict different subsets of X/P-MLVs. These restrictive receptors have replacement mutations in regions implicated in receptor function, and some entry restrictions can be relieved by glycosylation inhibitors. Mutation of the cow and hamster XPR1 genes identified a shared, previously unrecognized receptor-critical site. This G/Q503N replacement dramatically improves receptor function. While this substitution introduces an N-linked glycosylation site, XPR1 receptors are not glycosylated indicating that this replacement alters the virus-receptor interface independently of glycosylation. Our data also suggest that an unidentified glycosylated cofactor may influence X/P-MLV entry.

Gab2 promotes cancer stem cell like properties and metastatic growth of ovarian cancer via downregulation of miR-200c

Scaffolding adaptor Gab2 is overexpressed in a subset of high-grade ovarian cancer. Our published work shows that Gab2 via PI3K enhances migratory behaviors and epithelial to mesenchymal transition (EMT) features of ovarian cancer cells in vitro. However, it is still unclear how Gab2/PI3K pathway reuglates EMT characteristics and whether Gab2 promotes the growth of ovarian cancer stem cell (CSC)-like population and metastatic growth. In this study, we examined the effects of Gab2 expression on CSC-like cell growth using Aldefluor and tumorshpere assays commonly used for assessing ovarian cancer cells with CSC properties. Gab2 overexpression increased the number of ALDH⁺ cells and tumorsphere formation in two different ovarian cancer cell lines OVCAR5 and OVCAR8, whereas knockdown of Gab2 decreased the number of ALDH⁺ cells and tumorsphere formation in Caov-3 cells. Furthermore, Gab2 promoted metastatic tumor growth of OVCAR5 in nude mice. Mechanistically, we uncovered that Gab2 via PI3K specifically inhibited miR-200c expression. miR-200c downregulation contributed to the Gab2-enhanced cell migratory behaviors, EMT properties, and the expansion of ALDH⁺ cells and tumorspheres. Furthermore, Gab2 promoted CD44 expression and cell migration/invasion through miR-200c downregulation. Our findings support a model that Gab2-PI3K pathway via miR-200c downregulation promotes CD44 expression, EMT characteristics, and CSC-like cell growth. Therapies involving miR-200c or targeting CD44 should help treat ovarian cancer with high Gab2 expression.

Characterization of XPR1/SLC53A1 variants located outside of the SPX domain in patients with primary familial brain calcification

Primary familial brain calcification (PFBC) is a rare neurological disease characterized by deposits of calcium phosphate in the basal ganglia and other regions of the brain. Pathogenic variants in the XPR1/SLC53A1 gene, which encodes the only known inorganic phosphate exporter, cause an autosomal dominant form of PFBC. These variants are typically located in the SPX N-terminal domain of the protein. Here, we characterize three XPR1 variants outside of SPX in three PFBC patients with an apparently sporadic presentation: c.1375C > T p.(R459C), c.1855A > G p.(N619D) and c.1886T > G p.(I629S), with the latter identified as the first XPR1/SLC53A1 de novo mutation to occur in a PFBC proband. When tested in an in vitro physiological complementation assay, the three XPR1 variants were impaired in phosphate export function, although they were normally expressed at the cell surface and could serve as functional receptors for retrovirus entry. Moreover, peripheral blood cells from the p.N619D patient could be assayed ex vivo and displayed significantly impaired phosphate export. Our results establish for the first time the clinical and molecular characteristics of XPR1 variants located outside the SPX domain and assert a direct link between these variants, deficient phosphate export, and PFBC. Moreover, we unveiled new structural features in XPR1 C-terminal domain that play a role in phosphate export and disease.

Xenotropic and polytropic retrovirus receptor 1 (XPR1) promotes progression of tongue squamous cell carcinoma (TSCC) via activation of NF-κB signaling

Background

Xenotropic and polytropic retrovirus receptor 1 (XPR1), a previously identified cellular receptor for several murine leukemia viruses, plays a role in many pathophysiological processes. However, the role of XPR1 in human cancers has not yet been characterized.

Methods

Real-time PCR and western blotting assay were used to measure the expression of XPR1 in tongue squamous cell carcinoma (TSCC) tissues. Expression of XPR1 and p65 in clinical specimens was analyzed using immunohistochemical assay. The function of XPR1 on progression of TSCC was explored using in vitro and in vivo experiments. The molecular mechanism by which XPR1 helps to cancer progression was investigated by luciferase reporter activity, ELISA, PKA activity assay, immunofluorescence, western blotting and qPCR assay.

Results

Herein, we find that XPR1 is markedly upregulated in TSCC tissues compared to normal tongue tissues. High expression of XPR1 significantly correlates with the malignant features and poor patient survival in TSCC. Ectopic expression of XPR1 increases, while silencing of XPR1 reduces the proliferation, invasion and anti-apoptosis capacities of TSCC cells. Importantly, silencing of XPR1 effectively inhibits the tumorigenecity of TSCC cells. Moreover, we identified that XPR1 increased the concentration of intracellular cAMP and activated PKA. Thus, XPR1 promoted phosphorylation and activation of NF-κB signaling, which is required for XPR1-mediated oncogenic roles and significantly correlates with XPR1 expression in clinical specimens.

Conclusions

These findings uncover a critical role of XPR1 in TSCC progression via activation of NF-κB, and suggest that XPR1 might be a potential prognostic marker or therapeutic target.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1155-6) contains supplementary material, which is available to authorized users.

Phosphate as a Signaling Molecule and Its Sensing Mechanism

In mammals, phosphate balance is maintained by influx and efflux via the intestines, kidneys, bone, and soft tissue, which involves multiple sodium/phosphate (Na+/Pi) cotransporters, as well as regulation by several hormones. Alterations in the levels of extracellular phosphate exert effects on both skeletal and extra-skeletal tissues, and accumulating evidence has suggested that phosphate itself evokes signal transduction to regulate gene expression and cell behavior. Several in vitro studies have demonstrated that an elevation in extracellular Piactivates fibroblast growth factor receptor, Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular signal-regulated kinase) pathway and Akt pathway, which might involve the type III Na+/Picotransporter PiT-1. Excessive phosphate loading can lead to various harmful effects by accelerating ectopic calcification, enhancing oxidative stress, and dysregulating signal transduction. The responsiveness of mammalian cells to altered extracellular phosphate levels suggests that they may sense and adapt to phosphate availability, although the precise mechanism for phosphate sensing in mammals remains unclear. Unicellular organisms, such as bacteria and yeast, use some types of Pitransporters and other molecules, such as kinases, to sense the environmental Piavailability. Multicellular animals may need to integrate signals from various organs to sense the phosphate levels as a whole organism, similarly to higher plants. Clarification of the phosphate-sensing mechanism in humans may lead to the development of new therapeutic strategies to prevent and treat diseases caused by phosphate imbalance.

Xenotropic Mouse Gammaretroviruses Isolated from Pre-Leukemic Tissues Include a Recombinant

Naturally-occurring lymphomagenesis is induced by mouse leukemia viruses (MLVs) carried as endogenous retroviruses (ERVs). Replicating the ecotropic MLVs recombines with polytropic (P-ERVs) and xenotropic ERVs (X-ERVs) to generate pathogenic viruses with an altered host range. While most recovered nonecotropic recombinants have a polytropic host range, the X-MLVs are also present in the pre-leukemic tissues. We analyzed two such isolates from the AKR mice to identify their ERV progenitors and to look for evidence of recombination. AKR40 resembles the active X-ERV Bxv1, while AKR6 has a Bxv1-like backbone with substitutions that alter the long terminal repeat (LTR) enhancer and the envelope (env). AKR6 has a modified xenotropic host range, and its Env residue changes all lie outside of the domain that governs the receptor choice. The AKR6 segment spanning the two substitutions, but not the entire AKR6 env-LTR, exists as an ERV, termed Xmv67, in AKR, but not in the C57BL/6 mice. This suggests that AKR6 is the product of one, not two, recombination events. Xmv67 originated in the Asian mice. These data indicate that the recombinant X-MLVs that can be generated during lymphomagenesis, describe a novel X-ERV subtype found in the AKR genome, but not in the C57BL/6 reference genome, and identify residues in the envelope C-terminus that may influence the host range.

Tracking the Continuous Evolutionary Processes of an Endogenous Retrovirus of the Domestic Cat: ERV-DC

An endogenous retrovirus (ERV) is a remnant of an ancient retroviral infection in the host genome. Although most ERVs have lost their viral productivity, a few ERVs retain their replication capacity. In addition, partially inactivated ERVs can present a potential risk to the host via their encoded virulence factors or the generation of novel viruses by viral recombination. ERVs can also eventually acquire a biological function, and this ability has been a driving force of host evolution. Therefore, the presence of an ERV can be harmful or beneficial to the host. Various reports about paleovirology have revealed each event in ERV evolution, but the continuous processes of ERV evolution over millions of years are mainly unknown. A unique ERV family, ERV-DC, is present in the domestic cat (Felis silvestriscatus) genome. ERV-DC proviruses are phylogenetically classified into three genotypes, and the specific characteristics of each genotype have been clarified: their capacity to produce infectious viruses; their recombination with other retroviruses, such as feline leukemia virus or RD-114; and their biological functions as host antiviral factors. In this review, we describe ERV-DC-related phenomena and discuss the continuous changes in the evolution of this ERV in the domestic cat.

Transmission, Evolution, and Endogenization: Lessons Learned from Recent Retroviral Invasions

Viruses of the subfamily Orthoretrovirinae are defined by the ability to reverse transcribe an RNA genome into DNA that integrates into the host cell genome during the intracellular virus life cycle. Exogenous retroviruses (XRVs) are horizontally transmitted between host individuals, with disease outcome depending on interactions between the retrovirus and the host organism. When retroviruses infect germ line cells of the host, they may become endogenous retroviruses (ERVs), which are permanent elements in the host germ line that are subject to vertical transmission. These ERVs sometimes remain infectious and can themselves give rise to XRVs. This review integrates recent developments in the phylogenetic classification of retroviruses and the identification of retroviral receptors to elucidate the origins and evolution of XRVs and ERVs. We consider whether ERVs may recurrently pressure XRVs to shift receptor usage to sidestep ERV interference. We discuss how related retroviruses undergo alternative fates in different host lineages after endogenization, with koala retrovirus (KoRV) receiving notable interest as a recent invader of its host germ line. KoRV is heritable but also infectious, which provides insights into the early stages of germ line invasions as well as XRV generation from ERVs. The relationship of KoRV to primate and other retroviruses is placed in the context of host biogeography and the potential role of bats and rodents as vectors for interspecies viral transmission. Combining studies of extant XRVs and "fossil" endogenous retroviruses in koalas and other Australasian species has broadened our understanding of the evolution of retroviruses and host-retrovirus interactions.

Methods to Investigate the Role of Rho GTPases in Osteoclast Function

The actin cytoskeleton is essential for the biology of osteoclasts, in particular during bone resorption. As key regulators of actin dynamics, the small GTPases of the Rho family are very important in the control of osteoclast activity. The study of Rho GTPase signaling pathways is essential to uncover the mechanisms of bone resorption and can have interesting applications for the treatment of osteolytic diseases. In this chapter, we describe various techniques to obtain primary osteoclasts from murine bone marrow cells, to measure Rho GTPase activation levels, to monitor bone resorption activity of osteoclasts and to introduce the expression of proteins of interest using a retroviral approach. We illustrate the different methods with experimental examples of the effect of Rac1 activation by the exchange factor Dock5 on bone resorption by osteoclasts.

Recombinant Origins of Pathogenic and Nonpathogenic Mouse Gammaretroviruses with Polytropic Host Range

Ecotropic, xenotropic, and polytropic mouse leukemia viruses (E-, X-, and P-MLVs) exist in mice as infectious viruses and endogenous retroviruses (ERVs) inserted into mouse chromosomes. All three MLV subgroups are linked to leukemogenesis, which involves generation of recombinants with polytropic host range. Although P-MLVs are deemed to be the proximal agents of disease induction, few biologically characterized infectious P-MLVs have been sequenced for comparative analysis. We analyzed the complete genomes of 16 naturally occurring infectious P-MLVs, 12 of which were typed for pathogenic potential. We sought to identify ERV progenitors, recombinational hot spots, and segments that are always replaced, never replaced, or linked to pathogenesis or host range. Each P-MLV has an E-MLV backbone with P- or X-ERV replacements that together cover 100% of the recombinant genomes, with different substitution patterns for X- and P-ERVs. Two segments are always replaced, both coding for envelope (Env) protein segments: the N terminus of the surface subunit and the cytoplasmic tail R peptide. Viral gag gene replacements are influenced by host restriction genes Fv1 and Apobec3 Pathogenic potential maps to the env transmembrane subunit segment encoding the N-heptad repeat (HR1). Molecular dynamics simulations identified three novel interdomain salt bridges in the lymphomagenic virus HR1 that could affect structural stability, entry or sensitivity to host immune responses. The long terminal repeats of lymphomagenic P-MLVs are differentially altered by recombinations, duplications, or mutations. This analysis of the naturally occurring, sometimes pathogenic P-MLV recombinants defines the limits and extent of intersubgroup recombination and identifies specific sequence changes linked to pathogenesis and host interactions.IMPORTANCE During virus-induced leukemogenesis, ecotropic mouse leukemia viruses (MLVs) recombine with nonecotropic endogenous retroviruses (ERVs) to produce polytropic MLVs (P-MLVs). Analysis of 16 P-MLV genomes identified two segments consistently replaced: one at the envelope N terminus that alters receptor choice and one in the R peptide at the envelope C terminus, which is removed during virus assembly. Genome-wide analysis shows that nonecotropic replacements in the progenitor ecotropic MLV genome are more extensive than previously appreciated, covering 100% of the genome; contributions from xenotropic and polytropic ERVs differentially alter the regions responsible for receptor determination or subject to APOBEC3 and Fv1 restriction. All pathogenic viruses had modifications in the regulatory elements in their long terminal repeats and differed in a helical segment of envelope involved in entry and targeted by the host immune system. Virus-induced leukemogenesis thus involves generation of complex recombinants, and specific replacements are linked to pathogenesis and host restrictions.

Cyclophilins and nucleoporins are required for infection mediated by capsids from circulating HIV-2 primary isolates

HIV-2 groups have emerged from sooty mangabey SIV and entered the human population in Africa on several separate occasions. Compared to world pandemic HIV-1 that arose from the chimpanzee SIVcpz virus, the SIVsm-derived HIV-2, largely confined to West Africa, is less replicative, less transmissible and less pathogenic. Here, we evaluated the interactions between host cellular factors, which control HIV-1 infection and target the capsid, and HIV-2 capsids obtained from primary isolates from patients with different disease progression status. We showed that, like HIV-1, all HIV-2 CA we tested exhibited a dependence on cyclophilin A. However, we observed no correlation between HIV-2 viremia and susceptibility to hu-TRIM5alpha or dependence to CypA. Finally, we found that all CA from HIV-2 primary isolates exploit Nup358 and Nup153 for nucleus transposition. Altogether, these findings indicate that the ability to use the two latter nucleoporins is essential to infection of human cells for both HIV-1 and HIV-2. This dependence provides another molecular target that could be used for antiviral strategies against both HIV-1 and 2, based on both nucleoporins.

Comparative proteomics of a model MCF10A-KRasG12V cell line reveals a distinct molecular signature of the KRasG12V cell surface

Oncogenic Ras mutants play a major role in the etiology of most aggressive and deadly carcinomas in humans. In spite of continuous efforts, effective pharmacological treatments targeting oncogenic Ras isoforms have not been developed. Cell-surface proteins represent top therapeutic targets primarily due to their accessibility and susceptibility to different modes of cancer therapy. To expand the treatment options of cancers driven by oncogenic Ras, new targets need to be identified and characterized at the surface of cancer cells expressing oncogenic Ras mutants. Here, we describe a mass spectrometry-based method for molecular profiling of the cell surface using KRasG12V transfected MCF10A (MCF10A-KRasG12V) as a model cell line of constitutively activated KRas and native MCF10A cells transduced with an empty vector (EV) as control. An extensive molecular map of the KRas surface was achieved by applying, in parallel, targeted hydrazide-based cell-surface capturing technology and global shotgun membrane proteomics to identify the proteins on the KRasG12V surface. This method allowed for integrated proteomic analysis that identified more than 500 cell-surface proteins found unique or upregulated on the surface of MCF10A-KRasG12V cells. Multistep bioinformatic processing was employed to elucidate and prioritize targets for cross-validation. Scanning electron microscopy and phenotypic cancer cell assays revealed changes at the cell surface consistent with malignant epithelial-to-mesenchymal transformation secondary to KRasG12V activation. Taken together, this dataset significantly expands the map of the KRasG12V surface and uncovers potential targets involved primarily in cell motility, cellular protrusion formation, and metastasis.

Functional Interplay Between Murine Leukemia Virus Glycogag, Serinc5, and Surface Glycoprotein Governs Virus Entry, with Opposite Effects on Gammaretroviral and Ebolavirus Glycoproteins

Gammaretroviruses, such as murine leukemia viruses (MLVs), encode, in addition to the canonical Gag, Pol, and Env proteins that will form progeny virus particles, a protein called “glycogag” (glycosylated Gag). MLV glycogag contains the entire Gag sequence plus an 88-residue N-terminal extension. It has recently been reported that glycogag, like the Nef protein of HIV-1, counteracts the antiviral effects of the cellular protein Serinc5. We have found, in agreement with prior work, that glycogag strongly enhances the infectivity of MLVs with some Env proteins but not those with others. In contrast, however, glycogag was detrimental to MLVs carrying Ebolavirus glycoprotein. Glycogag could be replaced, with respect to viral infectivity, by the unrelated S2 protein of equine infectious anemia virus. We devised an assay for viral entry in which virus particles deliver the Cre recombinase into cells, leading to the expression of a reporter. Data from this assay showed that both the positive and the negative effects of glycogag and S2 upon MLV infectivity are exerted at the level of virus entry. Moreover, transfection of the virus-producing cells with a Serinc5 expression plasmid reduced the infectivity and entry capability of MLV carrying xenotropic MLV Env, particularly in the absence of glycogag. Conversely, Serinc5 expression abrogated the negative effects of glycogag upon the infectivity and entry capability of MLV carrying Ebolavirus glycoprotein. As Serinc5 may influence cellular phospholipid metabolism, it seems possible that all of these effects on virus entry derive from changes in the lipid composition of viral membranes.

Many murine leukemia viruses (MLVs) encode a protein called “glycogag.” The function of glycogag is not fully understood, but it can assist HIV-1 replication in the absence of the HIV-1 protein Nef under some circumstances. In turn, Nef counteracts the cellular protein Serinc5. Glycogag enhances the infectivity of MLVs with some but not all MLV Env proteins (which mediate viral entry into the host cell upon binding to cell surface receptors). We now report that glycogag acts by enhancing viral entry and that, like Nef, glycogag antagonizes Serinc5. Surprisingly, the effects of glycogag and Serinc5 upon the entry and infectivity of MLV particles carrying an Ebolavirus glycoprotein are the opposite of those observed with the MLV Env proteins. The unrelated S2 protein of equine infectious anemia virus (EIAV) is functionally analogous to glycogag in our experiments. Thus, three retroviruses (HIV-1, MLV, and EIAV) have independently evolved accessory proteins that counteract Serinc5.

Phosphate starvation promoted the accumulation of phenolic acids by inducing the key enzyme genes in Salvia miltiorrhiza hairy roots

Phosphate starvation increased the production of phenolic acids by inducing the key enzyme genes in a positive feedback pathway in Saliva miltiorrhiza hairy roots. SPX may be involved in this process. Salvia miltiorrhiza is a wildly popular traditional Chinese medicine used for the treatment of coronary heart diseases and inflammation. Phosphate is an essential plant macronutrient that is often deficient, thereby limiting crop yield. In this study, we investigated the effects of phosphate concentration on the biomass and accumulation of phenolic acid in S. miltiorrhiza. Results show that 0.124 mM phosphate was favorable for plant growth. Moreover, 0.0124 mM phosphate was beneficial for the accumulation of phenolic acids, wherein the contents of danshensu, caffeic acid, rosmarinic acid, and salvianolic acid B were, respectively, 2.33-, 1.02-, 1.68-, and 2.17-fold higher than that of the control. By contrast, 12.4 mM phosphate inhibited the accumulation of phenolic acids. The key enzyme genes in the phenolic acid biosynthesis pathway were investigated to elucidate the mechanism of phosphate starvation-induced increase of phenolic acids. The results suggest that phosphate starvation induced the gene expression from the downstream pathway to the upstream pathway, i.e., a feedback phenomenon. In addition, phosphate starvation response gene SPX (SYG1, Pho81, and XPR1) was promoted by phosphate deficiency (0.0124 mM). We inferred that SPX responded to phosphate starvation, which then affected the expression of later responsive key enzyme genes in phenolic acid biosynthesis, resulting in the accumulation of phenolic acids. Our findings provide a resource-saving and environmental protection strategy to increase the yield of active substance in herbal preparations. The relationship between SPX and key enzyme genes and the role they play in phenolic acid biosynthesis during phosphate deficiency need further studies.

Permissive XPR1 gammaretrovirus receptors in four mammalian species are functionally distinct in interference tests

Xenotropic/polytropic mouse leukemia viruses (X/P-MLVs) use the XPR1 gammaretrovirus receptor for entry. X/P-MLV host range is defined by usage of naturally occurring restrictive XPR1 receptors, and is governed by polymorphisms in the virus envelope glycoprotein and in XPR1. Here, we examined receptors of four mammalian species permissive to all X/P-MLVs (Mus dunni, human, rabbit, mink). Interference assays showed the four to be functionally distinct. Preinfection with X-MLVs consistently blocked all nine XPR1-dependent viruses, while preinfection with P-MLVs and wild mouse X/P-MLVs produced distinctive interference patterns in the four cells. These patterns indicate shared usage of independent, but not always fully functional, receptor sites. XPR1 sequence comparisons identified candidate sites in receptor-determining regions that correlate with some interference patterns. The evolutionary record suggests that the X/P-MLV tropism variants evolved to adapt to host receptor polymorphisms, to circumvent blocks by competing viruses or to avoid host-encoded envelope glycoproteins acquired for defense.

Soybean SPX1 is an important component of the response to phosphate deficiency for phosphorus homeostasis

Phosphate (Pi) homeostasis is required for plant growth and development, but the Pi-signaling pathways in plants still remain largely unknown. Proteins only containing the SPX domain are very important in phosphate (Pi) homeostasis and signaling transduction. In the T-DNA insertion Arabidopsis mutant spx3, AtPHT1-4, AtPHT1-5, AtACP5, AtRNS, and AtAT4 expression levels were increased under Pi-sufficient condition and low Pi condition compared with WT. Meanwhile, the expression levels of these phosphate starvation genes was inhibited in OXSPX1 and spx3/OXSPX1 compared with WT, only under Pi-sufficient condition. These imply that GmSPX1 may negatively control the transcription of Pi starvation responsive genes indirectly. However, there were no differences between expression levels of these PSI genes in spx3 and those in WT under -Pi conditions. These facts imply that the negative regulation of GmSPX1 and AtSPX3 on PSI genes is depending on Pi concentration. Consistent with this, GmSPX1 overexpression in the WT and spx3 decreased the total Pi concentration in plants and changed root hair morphology, suppressing the elongation and number of root hairs compared with the WT and spx3. The yeast two-hybrid assays and BiFC assays demonstrated that GmSPX1 could interact with GmMYB48.The qRT-PCR analysis showed that GmMYB48 is a new phosphate starvation induced transcription factor in soybean. Also, GmSPX1 overexpression led to decreased transcripts of AtMYB4, an ortholog of GmMYB48, in OXSPX1. Together, these results suggest that GmSPX1 is a negative regulator in the Pi signaling network of soybean, and the interaction of GmSPX1/GmMYB48 can be considered a potential candidate suppressor.

XPR1 mutations are a rare cause of primary familial brain calcification

Mutations in XPR1, a gene encoding an inorganic phosphate exporter, have recently been identified in patients with primary familial brain calcification (PFBC). Using Sanger sequencing, we screened XPR1 in 18 unrelated patients with PFBC and no SLC20A2, PDGFB, or PDGFRB mutation. XPR1 variants were tested in an in vitro physiological complementation assay and patient blood cells were assessed ex vivo for phosphate export. We identified a novel c.260T > C, p.(Leu87Pro) XPR1 variant in a 41-year-old man complaining of micrographia and dysarthria and demonstrating mild parkinsonism, cerebellar ataxia and executive dysfunction. Brain (123)I-Ioflupane scintigraphy showed marked dopaminergic neuron loss. Peripheral blood cells from the patient exhibited decreased phosphate export. XPR1 in which we introduced the mutation was not detectable at the cell surface and did not lead to phosphate export. These results confirm that loss of XPR1-mediated phosphate export function causes PFBC, occurring in less than 8 % of cases negative for the other genes, and may be responsible for parkinsonism.

Genome-wide association mapping and pathway analysis of leukosis incidence in a US Holstein cattle population

Bovine leukosis virus is an oncogenic virus that infects B cells, causing bovine leukosis disease. This disease is known to have a negative impact on dairy cattle production and, because no treatment or vaccine is available, finding a possible genetic solution is important. Our objective was to perform a comprehensive genetic analysis of leukosis incidence in dairy cattle. Data on leukosis occurrence, pedigree and molecular information were combined into multitrait GBLUP models with milk yield (MY) and somatic cell score (SCS) to estimate genetic parameters and to perform whole-genome scans and pathway analysis. Leukosis data were available for 11 554 Holsteins daughters of 3002 sires from 112 herds in 16 US states. Genotypes from a 60K SNP panel were available for 961 of those bulls as well as for 2039 additional bulls. Heritability for leukosis incidence was estimated at about 8%, and the genetic correlations of leukosis disease incidence with MY and SCS were moderate at 0.18 and 0.20 respectively. The genome-wide scan indicated that leukosis is a complex trait, possibly modulated by many genes. The gene set analysis identified many functional terms that showed significant enrichment of genes associated with leukosis. Many of these terms, such as G-Protein Coupled Receptor Signaling Pathway, Regulation of Nucleotide Metabolic Process and different calcium-related processes, are known to be related to retrovirus infection. Overall, our findings contribute to a better understanding of the genetic architecture of this complex disease. The functional categories associated with leukosis may be useful in future studies on fine mapping of genes and development of dairy cattle breeding strategies.

Xenotropic retrovirus Bxv1 in human pancreatic β cell lines

It has been reported that endogenous retroviruses can contaminate human cell lines that have been passaged as xenotransplants in immunocompromised mice. We previously developed and described 2 human pancreatic β cell lines (EndoC-βH1 and EndoC-βH2) that were generated in this way. Here, we have shown that B10 xenotropic virus 1 (Bxv1), a xenotropic endogenous murine leukemia virus (MuLV), is present in these 2 recently described cell lines. We determined that Bxv1 was also present in SCID mice that were used for in vivo propagation of EndoC-βH1/2 cells, suggesting that contamination occurred during xenotransplantation. EndoC-βH1/2 cells released Bxv1 particles that propagated to human 293T and Mus dunni cells. Mobilization assays demonstrated that Bxv1 transcomplements defective MuLV-based retrovectors. In contrast, common rodent β cell lines, rat INS-1E and RIN-5F cells and mouse MIN6 and βTC3 cells, displayed either no or extremely weak xenotropic helper activity toward MuLV-based retrovectors, although xenotropic retrovirus sequences and transcripts were detected in both mouse cell lines. Bxv1 propagation from EndoC-βH1/2 to 293T cells occurred only under optimized conditions and was overall poorly efficient. Thus, although our data imply that MuLV-based retrovectors should be cautiously used in EndoC-βH1/2 cells, our results indicate that an involuntary propagation of Bxv1 from these cells can be easily avoided with good laboratory practices.

Regulation of renal phosphate handling: inter-organ communication in health and disease

In this review, we focus on the interconnection of inorganic phosphate (Pi) homeostasis in the network of the bone-kidney, parathyroid-kidney, intestine-kidney, and liver-kidney axes. Such a network of organ communication is important for body Pi homeostasis. Normalization of serum Pi levels is a clinical target in patients with chronic kidney disease (CKD). Particularly, disorders of the fibroblast growth factor 23/klotho system are observed in early CKD. Identification of phosphaturic factors from the intestine and liver may enhance our understanding of body Pi homeostasis and Pi metabolism disturbances in CKD patients.

The EXS Domain of PHO1 Participates in the Response of Shoots to Phosphate Deficiency via a Root-to-Shoot Signal

The response of shoots to phosphate (Pi) deficiency implicates long-distance communication between roots and shoots, but the participating components are poorly understood. We have studied the topology of the Arabidopsis (Arabidopsis thaliana) PHOSPHATE1 (PHO1) Pi exporter and defined the functions of its different domains in Pi homeostasis and signaling. The results indicate that the amino and carboxyl termini of PHO1 are both oriented toward the cytosol and that the protein spans the membrane twice in the EXS domain, resulting in a total of six transmembrane α-helices. Using transient expression in Nicotiana benthamiana leaf, we demonstrated that the EXS domain of PHO1 is essential for Pi export activity and proper localization to the Golgi and trans-Golgi network, although the EXS domain by itself cannot mediate Pi export. In contrast, removal of the amino-terminal hydrophilic SPX domain does not affect the Pi export capacity of the truncated PHO1 in N. benthamiana. While the Arabidopsis pho1 mutant has low shoot Pi and shows all the hallmarks associated with Pi deficiency, including poor shoot growth and overexpression of numerous Pi deficiency-responsive genes, expression of only the EXS domain of PHO1 in the roots of the pho1 mutant results in a remarkable improvement of shoot growth despite low shoot Pi. Transcriptomic analysis of pho1 expressing the EXS domain indicates an attenuation of the Pi signaling cascade and the up-regulation of genes involved in cell wall synthesis and the synthesis or response to several phytohormones in leaves as well as an altered expression of genes responsive to abscisic acid in roots.

Genome-wide analysis of RING finger proteins in the smallest free-living photosynthetic eukaryote Ostreococus tauri

RING finger proteins and ubiquitination marks are widely involved in diverse aspects of growth and development, biological processes, and stress or environmental responses. As the smallest free-living photosynthetic eukaryote known so far, the green alga Ostreococus tauri has become an excellent model for investigating the origin of different gene families in the green lineage. Here, 65 RING domains in 65 predicted proteins were identified from O. tauri and on the basis of one or more substitutions at the metal ligand positions and spacing between them they were divided into eight canonical or modified types (RING-CH, -H2, -v, -C2, -C3HCHC2, -C2HC5, -C3GC3S, and -C2SHC4), in which the latter four were newly identified and might represent the intermediate states between RING domain and other similar domains, respectively. RING finger proteins were classified into eight classes based on the presence of additional domains, including RING-Only, -Plus, -C3H1, -PHD, -WD40, -PEX, -TM, and -DEXDc classes. These RING family genes usually lack introns and are distributed over 17 chromosomes. In addition, 29 RING-finger proteins in O. tauri share different degrees of homology with those in the model flowering plant Arabidopsis, indicating they might be necessary for the basic survival of free-living eukaryotes. Therefore, our results provide new insight into the general classification and evolutionary conservation of RING domain-containing proteins in O. tauri.

Selective sweeps versus introgression - population genetic dynamics of the murine leukemia virus receptor Xpr1 in wild populations of the house mouse (Mus musculus)

Background

The interaction between viruses and their receptors in the host can be expected to lead to an evolutionary arms race resulting in cycles of rapid adaptations. We focus here on the receptor gene Xpr1 (xenotropic and polytropic retrovirus receptor 1) for murine leukemia viruses (MLVs). In a previous screen for selective sweeps in mouse populations we discovered that a population from Germany was almost monomorphic for Xpr1 haplotypes, while a population from France was polymorphic.

Results

Here we analyze Xpr1 sequences and haplotypes from a broad sample of wild mouse populations of two subspecies, M. m. domesticus and M. m. musculus, to trace the origins of this distinctive polymorphism pattern. We show that the high polymorphism in the population in France is caused by a relatively recent invasion of a haplotype from a population in Iran, rather than a selective sweep in Germany. The invading haplotype codes for a novel receptor variant, which has itself undergone a recent selective sweep in the Iranian population.

Conclusions

Our data support a scenario in which Xpr1 is frequently subject to positive selection, possibly as a response to resistance development against recurrently emerging infectious viruses. During such an infection cycle, receptor variants that may convey viral resistance can be captured from another population and quickly introgress into populations actively dealing with the infectious virus.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0528-5) contains supplementary material, which is available to authorized users.

XPR1: a Gene Linked to Primary Familial Brain Calcification Might Help Explain a Spectrum of Neuropsychiatric Disorders

Primary familial brain calcifications (PFBC) compose a rare neurologic condition characterized by a bilateral pattern of hydroxyapatite deposits in basal ganglia, dentate nuclei, and thalamus. PFBC is identified through neuroimaging screenings such as computerized tomography. Patients with PFBC might present a wide variety of neurological symptoms such as mental and motor impairments, often misdiagnosed as Parkinson's disease, schizophrenia, Alzheimer's disease, and migraine. Four genes were confirmed as causative of PFBC: SLC20A2, PDGFB, PDGFRB, and XPR1. Curiously, other studies made occasional links between XPR1 variations or expression changes, in a few neuropsychiatric models. This letter is an assembly on XPR1 variants and expression change pattern data that were published in recent scientific reports, even before the current connection between that gene and brain calcification.

Mutations in XPR1 cause primary familial brain calcification associated with altered phosphate export

Primary familial brain calcification (PFBC) is a neurological disease characterized by calcium phosphate deposits in the basal ganglia and other brain regions and has thus far been associated with SLC20A2, PDGFB or PDGFRB mutations. We identified in multiple families with PFBC mutations in XPR1, a gene encoding a retroviral receptor with phosphate export function. These mutations alter phosphate export, implicating XPR1 and phosphate homeostasis in PFBC.

Sodium-dependent phosphate transporters in osteoclast differentiation and function

Osteoclasts are multinucleated bone degrading cells. Phosphate is an important constituent of mineralized bone and released in significant quantities during bone resorption. Molecular contributors to phosphate transport during the resorptive activity of osteoclasts have been controversially discussed. This study aimed at deciphering the role of sodium-dependent phosphate transporters during osteoclast differentiation and bone resorption. Our studies reveal RANKL-induced differential expression of sodium-dependent phosphate transport protein IIa (NaPi-IIa) transcript and protein during osteoclast development, but no expression of the closely related NaPi-IIb and NaPi-IIc SLC34 family isoforms. In vitro studies employing NaPi-IIa-deficient osteoclast precursors and mature osteoclasts reveal that NaPi-IIa is dispensable for bone resorption and osteoclast differentiation. These results are supported by the analysis of structural bone parameters by high-resolution microcomputed tomography that yielded no differences between adult NaPi-IIa WT and KO mice. By contrast, both type III sodium-dependent phosphate transporters Pit-1 and Pit-2 were abundantly expressed throughout osteoclast differentiation, indicating that they are the relevant sodium-dependent phosphate transporters in osteoclasts and osteoclast precursors. We conclude that phosphate transporters of the SLC34 family have no role in osteoclast differentiation and function and propose that Pit-dependent phosphate transport could be pivotal for bone resorption and should be addressed in further studies.

Origins of the endogenous and infectious laboratory mouse gammaretroviruses

The mouse gammaretroviruses associated with leukemogenesis are found in the classical inbred mouse strains and in house mouse subspecies as infectious exogenous viruses (XRVs) and as endogenous retroviruses (ERVs) inserted into their host genomes. There are three major mouse leukemia virus (MuLV) subgroups in laboratory mice: ecotropic, xenotropic, and polytropic. These MuLV subgroups differ in host range, pathogenicity, receptor usage and subspecies of origin. The MuLV ERVs are recent acquisitions in the mouse genome as demonstrated by the presence of many full-length nondefective MuLV ERVs that produce XRVs, the segregation of these MuLV subgroups into different house mouse subspecies, and by the positional polymorphism of these loci among inbred strains and individual wild mice. While some ecotropic and xenotropic ERVs can produce XRVs directly, others, especially the pathogenic polytropic ERVs, do so only after recombinations that can involve all three ERV subgroups. Here, I describe individual MuLV ERVs found in the laboratory mice, their origins and geographic distribution in wild mouse subspecies, their varying ability to produce infectious virus and the biological consequences of this expression.

MicroRNA-mediated surveillance of phosphate transporters on the move

Phosphate (Pi), which is indispensable for the structural and metabolic needs of plants, is acquired and translocated by Pi transporters. Deciphering the regulatory network of Pi signaling and homeostasis that involves the control of Pi transporters trafficking to, and their activity at, the plasma membrane provides insight into how plants adapt to environmental changes in Pi availability. Here, we review recent studies that revealed the involvement of microRNA399-PHOSPHATE 2 (PHO2) and microR827-NITROGEN LIMITATION ADAPTATION (NLA) modules in mediating the ubiquitination and degradation of PHOSPHATE TRANSPORTER 1 (PHT1) and/or PHOSPHATE 1 (PHO1). These discoveries show that miRNAs are an effective way for plants to monitor the turnover of Pi transporters in the membrane system by modulating the functioning of the membrane-associated ubiquitin machinery.