Isoform-specific Binding of Selenoprotein P to the β-Propeller Domain of Apolipoprotein E Receptor 2 Mediates Selenium Supply

Sepp1 supplies selenium to tissues via receptor-mediated endocytosis. Mice, rats, and humans have 10 selenocysteines in Sepp1, which are incorporated via recoding of the stop codon, UGA. Four isoforms of rat Sepp1 have been identified, including full-length Sepp1 and three others, which terminate at the second, third, and seventh UGA codons. Previous studies have shown that the longer Sepp1 isoforms bind to the low density lipoprotein receptor apoER2, but the mechanism remains unclear. To identify the essential residues for apoER2 binding, an in vitro Sepp1 binding assay was developed using different Sec to Cys substituted variants of Sepp1 produced in HEK293T cells. ApoER2 was found to bind the two longest isoforms. These results suggest that Sepp1 isoforms with six or more selenocysteines are taken up by apoER2. Furthermore, the C-terminal domain of Sepp1 alone can bind to apoER2. These results indicate that apoER2 binds to the Sepp1 C-terminal domain and does not require the heparin-binding site, which is located in the N-terminal domain. Site-directed mutagenesis identified three residues of Sepp1 that are necessary for apoER2 binding. Sequential deletion of extracellular domains of apoER2 surprisingly identified the YWTD β-propeller domain as the Sepp1 binding site. Finally, we show that apoER2 missing the ligand-binding repeat region, which can result from cleavage at a furin cleavage site present in some apoER2 isoforms, can act as a receptor for Sepp1. Thus, longer isoforms of Sepp1 with high selenium content interact with a binding site distinct from the ligand-binding domain of apoER2 for selenium delivery.     

Sepp1UF forms are N-terminal selenoprotein P truncations that have peroxidase activity when coupled with thioredoxin reductase-1

Mouse selenoprotein P (Sepp1) consists of an N-terminal domain (residues 1–239) that contains one selenocysteine (U) as residue 40 in a proposed redox-active motif (-UYLC-) and a C-terminal domain (residues 240–361) that contains nine selenocysteines. Sepp1 transports selenium from the liver to other tissues by receptor-mediated endocytosis. It also reduces oxidative stress in vivo by an unknown mechanism. A previously uncharacterized plasma form of Sepp1 is filtered in the glomerulus and taken up by renal proximal convoluted tubule (PCT) cells via megalin-mediated endocytosis. We purified Sepp1 forms from the urine of megalin^−/− mice using a monoclonal antibody to the N-terminal domain. Mass spectrometry revealed that the purified urinary Sepp1 consisted of N-terminal fragments terminating at 11 sites between residues 183 and 208. They were therefore designated Sepp1^UF. Because the N-terminal domain of Sepp1 has a thioredoxin fold, Sepp1^UF were compared with full-length Sepp1, Sepp1^Δ240–361, and Sepp1^U40S as a substrate of thioredoxin reductase-1 (TrxR1). All forms of Sepp1 except Sepp1^U40S, which contains serine in place of the selenocysteine, were TrxR1 substrates, catalyzing NADPH oxidation when coupled with H₂O₂ or tert-butylhydroperoxide as the terminal electron acceptor. These results are compatible with proteolytic cleavage freeing Sepp1^UF from full-length Sepp1, the form that has the role of selenium transport, allowing Sepp1^UF to function by itself as a peroxidase. Ultimately, plasma Sepp1^UF and small selenium-containing proteins are filtered by the glomerulus and taken up by PCT cells via megalin-mediated endocytosis, preventing loss of selenium in the urine and providing selenium for the synthesis of glutathione peroxidase-3.     

Production of Selenoprotein P (Sepp1) by Hepatocytes Is Central to Selenium Homeostasis

Sepp1 is a widely expressed extracellular protein that in humans and mice contains 10 selenocysteine residues in its primary structure. Extra-hepatic tissues take up plasma Sepp1 for its selenium via apolipoprotein E receptor-2 (apoER2)-mediated endocytosis. The role of Sepp1 in the transport of selenium from liver, a rich source of the element, to peripheral tissues was studied using mice with selective deletion of Sepp1 in hepatocytes (Sepp1^c/c/alb-cre^+/− mice). Deletion of Sepp1 in hepatocytes lowered plasma Sepp1 concentration to 10% of that in Sepp1^c/c mice (controls) and increased urinary selenium excretion, decreasing whole-body and tissue selenium concentrations. Under selenium-deficient conditions, Sepp1^c/c/alb-cre^+/− mice accumulated selenium in the liver at the expense of extra-hepatic tissues, severely worsening clinical manifestations of dietary selenium deficiency. These findings are consistent with there being competition for metabolically available hepatocyte selenium between the synthesis of selenoproteins and the synthesis of selenium excretory metabolites. In addition, selenium deficiency down-regulated the mRNA of the most abundant hepatic selenoprotein, glutathione peroxidase-1 (Gpx1), to 15% of the selenium-replete value, while reducing Sepp1 mRNA, the most abundant hepatic selenoprotein mRNA, only to 61%. This strongly suggests that Sepp1 synthesis is favored in the liver over Gpx1 synthesis when selenium supply is limited, directing hepatocyte selenium to peripheral tissues in selenium deficiency. We conclude that production of Sepp1 by hepatocytes is central to selenium homeostasis in the organism because it promotes retention of selenium in the body and effects selenium distribution from the liver to extra-hepatic tissues, especially under selenium-deficient conditions.     

Megalin mediates selenoprotein P uptake by kidney proximal tubule epithelial cells

Selenoprotein P (Sepp1) contains most of the selenium in blood plasma, and it is utilized by the kidney, brain, and testis as a selenium source for selenoprotein synthesis. We recently demonstrated that apolipoprotein E receptor-2 (ApoER2) is required for Sepp1 uptake by the testis and that deletion of ApoER2 reduces testis and brain, but not kidney, selenium levels. This study examined the kidney Sepp1 uptake pathway. Immunolocalization experiments demonstrated that Sepp1 passed into the glomerular filtrate and was specifically taken up by proximal tubule epithelial cells. Neither the C terminus selenocysteine-rich domain of Sepp1 nor ApoER2 was required for Sepp1 uptake by proximal tubules. Tissue ligand binding assays using cryosections of Sepp1-/- kidneys revealed that the proximal tubule epithelium contained Sepp1-binding sites that were blocked by the receptor-associated protein, RAP, an inhibitor of lipoprotein receptor-ligand interactions. Ligand blotting assays of kidney membrane preparations fractionated by SDS-PAGE revealed that Sepp1 binds megalin, a lipoprotein receptor localized to the proximal tubule epithelium. Immunolocalization analyses confirmed the in vivo co-localization of Sepp1 and megalin in wild type kidneys and demonstrated the absence of proximal tubule Sepp1 uptake in megalin null mice. These results demonstrate that kidney selenium homeostasis is mediated by a megalin-dependent Sepp1 uptake pathway in the proximal tubule.     

The selenium-rich C-terminal domain of mouse selenoprotein P is necessary for the supply of selenium to brain and testis but not for the maintenance of whole body selenium

Selenoprotein P (Sepp1) has two domains with respect to selenium content: the N-terminal, selenium-poor domain and the C-terminal, selenium-rich domain. To assess domain function, mice with deletion of the C-terminal domain have been produced and compared with Sepp1-/- and Sepp1+/+ mice. All mice studied were males fed a semipurified diet with defined selenium content. The Sepp1 protein in the plasma of mice with the C-terminal domain deleted was determined by mass spectrometry to terminate after serine 239 and thus was designated Sepp1Delta240-361. Plasma Sepp1 and selenium concentrations as well as glutathione peroxidase activity were determined in the three types of mice. Glutathione peroxidase and Sepp1Delta240-361 accounted for over 90% of the selenium in the plasma of Sepp1Delta240-361 mice. Calculations using results from Sepp1+/+ mice revealed that Sepp1, with a potential for containing 10 selenocysteine residues, contained an average of 5 selenium atoms per molecule, indicating that shortened and/or selenium-depleted forms of the protein were present in these wild-type mice. Sepp1Delta240-361 mice had low brain and testis selenium concentrations that were similar to those in Sepp1-/- mice but they better maintained their whole body selenium. Sepp1Delta240-361 mice had depressed fertility, even when they were fed a high selenium diet, and their spermatozoa were defective and morphologically indistinguishable from those of selenium-deficient mice. Neurological dysfunction and death occurred when Sepp1Delta240-361 mice were fed selenium-deficient diet. These phenotypes were similar to those of Sepp1-/- mice but had later onset or were less severe. The results of this study demonstrate that the C terminus of Sepp1 is critical for the maintenance of selenium in brain and testis but not for the maintenance of whole body selenium.     

Mice Lacking Selenoprotein P and Selenocysteine Lyase Exhibit Severe Neurological Dysfunction,Neurodegeneration, and Audiogenic Seizures

Selenoproteins are a unique family of proteins, characterized by the co-translational incorporation of selenium as selenocysteine, which play key roles in antioxidant defense. Among selenoproteins, selenoprotein P (Sepp1) is particularly distinctive due to the fact that it contains multiple selenocysteine residues and has been postulated to act in selenium transport. Within the brain, Sepp1 delivers selenium to neurons by binding to the ApoER2 receptor. Upon feeding a selenium-deficient diet, mice lacking ApoER2 or Sepp1 develop severe neurological dysfunction and exhibit widespread brainstem neurodegeneration, indicating an important role for ApoER2-mediated Sepp1 uptake in normal brain function. Selenocysteine lyase (Scly) is an enzyme that plays an important role in selenium homeostasis, in that it catalyzes the decomposition of selenocysteine and allows selenium to be recycled for additional selenoprotein synthesis. We previously reported that constitutive deletion of Scly results in neurological deficits only when mice are challenged with a low selenium diet. To gain insight into the relationship between Sepp1 and Scly in selenium metabolism, we created novel transgenic mice constitutively lacking both genes (Scly^−/−Sepp1^−/−) and characterized the neurobehavioral phenotype. We report that deletion of Scly in conjunction with Sepp1 further aggravates the phenotype of Sepp1^−/− mice, as these mice needed supraphysiological selenium supplementation to survive, and surviving mice exhibited impaired motor coordination, audiogenic seizures, and brainstem neurodegeneration. These findings provide the first in vivo evidence that Scly and Sepp1 work cooperatively to maintain selenoprotein function in the mammalian brain.     

Long Isoform Mouse Selenoprotein P (Sepp1) Supplies Rat Myoblast L8 Cells with Selenium via Endocytosis Mediated by Heparin Binding Properties and Apolipoprotein E Receptor-2 (ApoER2)

In vivo studies have shown that selenium is supplied to testis and brain by apoER2-mediated endocytosis of Sepp1. Although cultured cell lines have been shown to utilize selenium from Sepp1 added to the medium, the mechanism of uptake and utilization has not been characterized. Rat L8 myoblast cells were studied. They took up mouse Sepp1 from the medium and used its selenium to increase their glutathione peroxidase (Gpx) activity. L8 cells did not utilize selenium from Gpx3, the other plasma selenoprotein. Neither did they utilize it from Sepp1(Δ240-361), the isoform of Sepp1 that lacks the selenium-rich C-terminal domain. To identify Sepp1 receptors, a solubilized membrane fraction was passed over a Sepp1 column. The receptors apoER2 and Lrp1 were identified in the eluate by mass spectrometry. siRNA experiments showed that knockdown of apoER2, but not of Lrp1, inhibited (75)Se uptake from (75)Se-labeled Sepp1. The addition of protamine to the medium or treatment of the cells with chlorate also inhibited (75)Se uptake. Blockage of lysosome acidification did not inhibit uptake of Sepp1 but did prevent its digestion and thereby utilization of its selenium. These results indicate that L8 cells take up Sepp1 by an apoER2-mediated mechanism requiring binding to heparin sulfate proteoglycans. The presence of at least part of the selenium-rich C-terminal domain of Sepp1 is required for uptake. RT-PCR showed that mouse tissues express apoER2 in varying amounts. It is postulated that apoER2-mediated uptake of long isoform Sepp1 is responsible for selenium distribution to tissues throughout the body.     

Deletion of selenoprotein P alters distribution of selenium in the mouse

Selenoprotein P (Se-P) contains most of the selenium in plasma. Its function is not known. Mice with the Se-P gene deleted (Sepp(-/-)) were generated. Two phenotypes were observed: 1) Sepp(-/-) mice lost weight and developed poor motor coordination when fed diets with selenium below 0.1 mg/kg, and 2) male Sepp(-/-) mice had sharply reduced fertility. Weanling male Sepp(+/+), Sepp(+/-), and Sepp(-/-) mice were fed diets for 8 weeks containing <0.02-2 mg selenium/kg. Sepp(+/+) and Sepp(+/-) mice had similar selenium concentrations in all tissues except plasma where a gene-dose effect on Se-P was observed. Liver selenium was unaffected by Se-P deletion except that it increased when dietary selenium was below 0.1 mg/kg. Selenium in other tissues exhibited a continuum of responses to Se-P deletion. Testis selenium was depressed to 19% in mice fed an 0.1 mg selenium/kg diet and did not rise to Sepp(+/+) levels even with a dietary selenium of 2 mg/kg. Brain selenium was depressed to 43%, but feeding 2 mg selenium/kg diet raised it to Sepp(+/+) levels. Kidney was depressed to 76% and reached Sepp(+/+) levels on an 0.25 mg selenium/kg diet. Heart selenium was not affected. These results suggest that the Sepp(-/-) phenotypes were caused by low selenium in testis and brain. They strongly suggest that Se-P from liver provides selenium to several tissues, especially testis and brain. Further, they indicate that transport forms of selenium other than Se-P exist because selenium levels of all tissues except testis responded to increases of dietary selenium in Sepp(-/-) mice.     

Selenoprotein P Is the Major Selenium Transport Protein in Mouse Milk

Selenium is transferred from the mouse dam to its neonate via milk. Milk contains selenium in selenoprotein form as selenoprotein P (Sepp1) and glutathione peroxidase-3 (Gpx3) as well as in non-specific protein form as selenomethionine. Selenium is also present in milk in uncharacterized small-molecule form. We eliminated selenomethionine from the mice in these experiments by feeding a diet that contained sodium selenite as the source of selenium. Selenium-replete dams with deletion of Sepp1 or Gpx3 were studied to assess the effects of these genes on selenium transfer to the neonate. Sepp1 knockout caused a drop in milk selenium to 27% of the value in wild-type milk and a drop in selenium acquisition by the neonates to 35%. In addition to decreasing milk selenium by eliminating Sepp1, deletion of Sepp1 causes a decline in whole-body selenium, which likely also contributes to the decreased transfer of selenium to the neonate. Deletion of Gpx3 did not decrease milk selenium content or neonate selenium acquisition by measurable amounts. Thus, when the dam is fed selenium-adequate diet (0.25 mg selenium/kg diet), milk Sepp1 transfers a large amount of selenium to neonates but the transfer of selenium by Gpx3 is below detection by our methods.     

Apolipoprotein E receptor-2 (ApoER2) mediates selenium uptake from selenoprotein P by the mouse testis

Selenium is a micronutrient that is essential for the production of normal spermatozoa. The selenium-rich plasma protein selenoprotein P (Sepp1) is required for maintenance of testis selenium and for fertility of the male mouse. Sepp1 trafficking in the seminiferous epithelium was studied using conventional methods and mice with gene deletions. Immunocytochemistry demonstrated that Sepp1 is present in vesicle-like structures in the basal region of Sertoli cells, suggesting that the protein is taken up intact. Sepp1 affinity chromatography of a testicular extract followed by mass spectrometry-based identification of bound proteins identified apolipoprotein E receptor 2 (ApoER2) as a candidate testis Sepp1 receptor. In situ hybridization analysis identified Sertoli cells as the only cell type in the seminiferous epithelium with detectable ApoER2 expression. Testis selenium levels in apoER2(-/-) males were sharply reduced from those in apoER2(+/+) males and were comparable with the depressed levels found in Sepp1(-/-) males. However, liver selenium levels were unchanged by deletion of apoER2. Immunocytochemistry did not detect Sepp1 in the Sertoli cells of apoER2(-/-) males, consistent with a defect in the receptor-mediated Sepp1 uptake pathway. Phase contrast microscopy revealed identical sperm defects in apoER2(-/-) and Sepp1(-/-) mice. Co-immunoprecipitation analysis demonstrated an interaction of testis ApoER2 with Sepp1. These data demonstrate that Sertoli cell ApoER2 is a Sepp1 receptor and a component of the selenium delivery pathway to spermatogenic cells.     

Hepatic selenoprotein P (SePP) expression restores selenium transport and prevents infertility and motor-incoordination in Sepp-knockout mice

SePP (selenoprotein P) is central for selenium transport and distribution. Targeted inactivation of the Sepp gene in mice leads to reduced selenium content in plasma, kidney, testis and brain. Accordingly, activities of selenoenzymes are reduced in Sepp(-/-) organs. Male Sepp(-/-) mice are infertile. Unlike selenium deficiency, Sepp deficiency leads to neurological impairment with ataxia and seizures. Hepatocyte-specific inactivation of selenoprotein biosynthesis reduces plasma and kidney selenium levels similarly to Sepp(-/-) mice, but does not result in neurological impairment, suggesting a physiological role of locally expressed SePP in the brain. In an attempt to define the role of liver-derived circulating SePP in contrast with locally expressed SePP, we generated Sepp(-/-) mice with transgenic expression of human SePP under control of a hepatocyte-specific transthyretin promoter. Secreted human SePP was immunologically detectable in serum from SEPP1-transgenic mice. Selenium content and selenoenzyme activities in serum, kidney, testis and brain of Sepp(-/-;SEPP1) (SEPP1-transgenic Sepp(-/-)) mice were increased compared with Sepp(-/-) controls. When a selenium-adequate diet (0.16-0.2 mg/kg of body weight) was fed to the mice, liver-specific expression of SEPP1 rescued the neurological defects of Sepp(-/-) mice and rendered Sepp(-/-) males fertile. When fed on a low-selenium diet (0.06 mg/kg of body weight), Sepp(-/-;SEPP1) mice survived 4 weeks longer than Sepp(-/-) mice, but ultimately developed the neurodegenerative phenotype. These results indicate that plasma SePP derived from hepatocytes is the main transport form of selenium supporting the kidney, testis and brain. Nevertheless, local Sepp expression is required to maintain selenium content in selenium-privileged tissues such as brain and testis during dietary selenium restriction.     

Modulation of β-amyloid precursor protein trafficking and processing by the low density lipoprotein receptor family

Selenium is an essential micronutrient that function through selenoproteins. Selenium deficiency results in lower concentrations of selenium and selenoproteins. The brain maintains it's selenium better than other tissues under low-selenium conditions. Recently, the selenium-containing protein selenoprotein P (Sepp) has been identified as a possible transporter of selenium. The targeted disruption of the selenoprotein P gene (Sepp1) results in decreased brain selenium concentration and neurological dysfunction, unless selenium intake is excessive However, the effect of selenoprotein P deficiency on the processes of memory formation and synaptic plasticity is unknown. In the present studies Sepp1(-/-) mice and wild type littermate controls (Sepp1(+/+)) fed a high-selenium diet (1 mg Se/kg) were used to characterize activity, motor coordination, and anxiety as well as hippocampus-dependent learning and memory. Normal associative learning, but disrupted spatial learning was observed in Sepp1(-/-) mice. In addition, severe alterations were observed in synaptic transmission, short-term plasticity and long-term potentiation in hippocampus area CA1 synapses of Sepp1(-/-) mice on a 1 mg Se/kg diet and Sepp1(+/+) mice fed a selenium-deficient (0 mg Se/kg) diet. Taken together, these data suggest that selenoprotein P is required for normal synaptic function, either through presence of the protein or delivery of required selenium to the CNS.     

Alternatively activated myeloid cells limit pathogenicity associated with African trypanosomiasis through the IL-10 inducible gene selenoprotein P

Uncontrolled inflammation is a major cause of tissue injury/pathogenicity often resulting in death of a host infected with African trypanosomes. Thus, comparing the immune response in hosts that develop different degrees of disease severity represents a promising approach to discover processes contributing to trypanosomiasis control. It is known that limitation of pathogenicity requires a transition in the course of infection, from an IFN-gamma-dependent response resulting in the development of classically activated myeloid cells (M1), to a counterbalancing IL-10-dependent response associated with alternatively activated myeloid cells (M2). Herein, mechanisms and downstream effectors by which M2 contribute to lower the pathogenicity and the associated susceptibility to African trypanosomiasis have been explored. Gene expression analysis in IL-10 knockout and wild-type mice, that are susceptible and relatively resistant to Trypanosoma congolense infection, respectively, revealed a number of IL-10-inducible genes expressed by M2, including Sepp1 coding for selenoprotein P. Functional analyses confirm that selenoprotein P contributes to limit disease severity through anti-oxidant activity. Indeed, Sepp1 knockout mice, but not Sepp1(Delta)(240-361) mice retaining the anti-oxidant motif but lacking the selenium transporter domain of selenoprotein P, exhibited increased tissue injury that associated with increased production of reactive oxygen species and increased apoptosis in the liver immune cells, reduced parasite clearance capacity of myeloid cells, and decreased survival. These data validate M2-associated molecules as functioning in reducing the impact of parasite infection on the host.     

Selenoprotein P is required for mouse sperm development

Selenoprotein P (SEPP1), an extracellular glycoprotein of unknown function, is a unique member of the selenoprotein family that, depending on species, contains 10-17 selenocysteines in its primary structure; in contrast, all other family members contain a single selenocysteine residue. The SEPP1-null (Sepp1(-/-)) male but not the female mice are infertile, but the cellular basis of this male phenotype has not been defined. In this study, we demonstrate that mature spermatozoa of Sepp1(-/-) males display a specific set of flagellar structural defects that develop temporally during spermiogenesis and after testicular maturation in the epididymis. The flagellar defects include a development of a truncated mitochondrial sheath, an extrusion of a specific set of axonemal microtubules and outer dense fibers from the principal piece, and ultimately a hairpin-like bend formation at the midpiece-principal piece junction. The sperm defects found in Sepp1(-/-) males appear to be the same as those observed in wild-type (Sepp1(+/+)) males fed a low selenium diet. Supplementation of dietary selenium levels for Sepp1(-/-) males neither reverses the development of sperm defects nor restores fertility. These data demonstrate that SEPP1 is required for development of functional spermatozoa and indicate that it is an essential component of the selenium delivery pathway for developing germ cells.     

Deletion of selenoprotein P upregulates urinary selenium excretion and depresses whole-body selenium content

Deletion of the mouse selenoprotein P gene (Sepp1) lowers selenium concentrations in many tissues. We examined selenium homeostasis in Sepp1(-/-) and Sepp1(+/+) mice to assess the mechanism of this. The liver produces and exports selenoprotein P, which transports selenium to peripheral tissues, and urinary selenium metabolites, which regulate whole-body selenium. At intakes of selenium near the nutritional requirement, Sepp1(-/-) mice had whole-body selenium concentrations 72 to 75% of Sepp1(+/+) mice. Genotype did not affect dietary intake of selenium. Sepp1(-/-) mice excreted in their urine approximately 1.5 times more selenium in relation to their whole-body selenium than did Sepp1(+/+) mice. In addition, Sepp1(-/-) mice gavaged with (75)SeO(2-)(3) excreted 1.7 to 2.4 times as much of the (75)Se in the urine as did Sepp1(+/+) mice. These findings demonstrate that deletion of selenoprotein P raises urinary excretion of selenium. When urinary small-molecule (75)Se was injected intravenously into mice, over 90% of the (75)Se appeared in the urine within 24 h, regardless of selenium status. This shows that urinary selenium is dedicated to excretion and not to utilization by tissues. Our results indicate that deletion of selenoprotein P leads to increased urinary selenium excretion. We propose that the absence of selenoprotein P synthesis in the liver makes more selenium available for urinary metabolite synthesis, increasing loss of selenium from the organism and causing the decrease in whole-body selenium and some of the decreases observed in tissues of Sepp1(-/-) mice.     

A potential screening factor for accumulation of cholesteyl ester transfer protein deficiency in East Asia: Schistosoma japonicum

Cholesteryl ester transfer protein (CETP)-deficiency manifests a unique plasma lipoprotein profile without other apparent symptoms. It is highly common in East Asia while rather rare anywhere else. A potential environmental screening factor(s) may therefore contribute to this eccentric distribution, such as its selective advantage against a regional illness, most likely an infectious disease, in relation to plasma lipoproteins. Blood flukes use the host plasma lipoproteins as nutrient sources through the lipoprotein receptor-like systems. Its Asian-specific species, Schistosoma (S) japonicum, which has been endemic in East Asia, takes up cholesteryl ester (CE) from high-density lipoprotein (HDL) for the embryonation of their eggs to miracidia, a critical step of the hepatic pathogenesis of this parasite, but poorly from HDL of CETP-deficiency. CD36-related protein (CD36RP) was cloned from the adults and the eggs of S. japonicum, with 1880-bp encoding 506 amino-acid residues exhibiting the CD36 domains and two transmembrane regions. Its extracellular domain selectively bound human HDL but neither LDL nor CETP-deficiency HDL, and the antibody against the extracellular domain suppressed the selective HDL-CE uptake and embryonation of the eggs. When infected with S. japonicum, wild-type mice developed less hepatic granulomatosis than CETP-transgenic mice by the ectopic egg embryonation. CD36RP is thus a candidate receptor of S. japonicum to facilitate uptake of HDL-CE necessary for egg embryonation. Abnormal HDL caused by CETP-deficiency retards this process and thereby protects the patients from development of hepatic lesions. S. japonicum infection is a potential screening factor for high prevalence of CETP deficiency in East Asia.     

Absence of selenoprotein P but not selenocysteine lyase results in severe neurological dysfunction

Dietary selenium restriction in mammals causes bodily selenium to be preferentially retained in the brain relative to other organs. Almost all the known selenoproteins are found in brain, where expression is facilitated by selenocysteine (Sec)-laden selenoprotein P. The brain also expresses selenocysteine lyase (Scly), an enzyme that putatively salvages Sec and recycles the selenium for selenoprotein translation. We compared mice with a genetic deletion of Scly to selenoprotein P (Sepp1) knockout mice for similarity of neurological impairments and whether dietary selenium modulates these parameters. We report that Scly knockout mice do not display neurological dysfunction comparable to Sepp1 knockout mice. Feeding a low-selenium diet to Scly knockout mice revealed a mild spatial learning deficit without disrupting motor coordination. Additionally, we report that the neurological phenotype caused by the absence of Sepp1 is exacerbated in male vs. female mice. These findings indicate that Sec recycling via Scly becomes limiting under selenium deficiency and suggest the presence of a complementary mechanism for processing Sec. Our studies illuminate the interaction between Sepp1 and Scly in the distribution and turnover of body and brain selenium and emphasize the consideration of sex differences when studying selenium and selenoproteins in vertebrate biology.     

Identification of small PDZK1-associated protein,DD96/MAP17, as a regulator of PDZK1 and plasma high density lipoprotein levels

Scavenger receptor class B, type I (SR-BI) is the high density lipoprotein (HDL) receptor essential for hepatic uptake of HDL cholesterol. SR-BI was shown to impact plasma HDL levels and be anti-atherogenic. Thus, the ability to regulate hepatic SR-BI may allow for the modulation of plasma HDL cholesterol and progression of atherosclerosis. However, regulation of SR-BI in liver is not well understood. Recently, the PDZ domain containing protein PDZK1 was shown to interact with SR-BI and may serve an essential role in SR-BI cell surface expression. Here we identify an in vivo PDZK1-interacting protein that we named small PDZK1-associated protein (SPAP; also known as DD96/MAP17). Unexpectedly, we found that hepatic overexpression of SPAP in mice resulted in liver deficiency of PDZK1. The absence of PDZK1 in SPAP transgenic mice resulted in a deficiency of SR-BI in liver and markedly increased plasma HDL. Metabolic labeling experiments showed that the proteasome plays a role in the turnover of newly synthesized PDZK1, but that SPAP overexpression in liver increased PDZK1 turnover in an alternate, proteasome-independent pathway. Thus, SPAP may be an endogenous regulator of cellular PDZK1 levels by regulating PDZK1 turnover.