Abstract
Eighteen years ago, unexpected epileptic seizures in Selenop-knockout mice pointed to a potentially novel, possibly underestimated, and previously difficult to study role of selenium (Se) in the mammalian brain. This mouse model was the key to open the field of molecular mechanisms, i.e., to delineate the roles of selenium and individual selenoproteins in the brain, and answer specific questions like: how does Se enter the brain; which processes and which cell types are dependent on selenoproteins; and, what are the individual roles of selenoproteins in the brain? Many of these questions have been answered and much progress is being made to fill remaining gaps. Mouse and human genetics have together boosted the field tremendously, in addition to traditional biochemistry and cell biology. As always, new questions have become apparent or more pressing with solving older questions. We will briefly summarize what we know about selenoproteins in the human brain, glance over to the mouse as a useful model, and then discuss new questions and directions the field might take in the next 18 years.
A Brief History of the Field
When selenoprotein P (Selenop)-knockout mice were made independently in two laboratories, it was not expected that their most dramatic phenotype was to reveal the essential requirement of selenium (Se) in the brain (; ). However, the model was tricky: the neurological phenotype depended exquisitely on the level of dietary Se supply and, more precisely, on the timing of Se deficiency during the ontogeny of the animal. The neurological phenotype varied between none at all, epileptic seizures, movement phenotype with ataxia and/or dystonia, overt neurodegeneration with premature death, or death before weaning (; , ; ; ). Since Se levels in commercial diets may not always precisely reflect the printed specifications in every lot of mouse chow, and because the Selenop-knockout mice reflected so sensitively the dietary Se supply, working with the model was difficult, to say the least. At the time, antibodies against selenoproteins were not widely available, and enzymatic assays for glutathione peroxidase (GPX) and thioredoxin reductase (TXNRD), Se measurements, and metabolic labeling with 75Se were the methods of choice. At least, these methods clearly demonstrated, that inactivation of Selenop, a gene mostly expressed in liver and secreted into the plasma, reduced Se levels and selenoenzyme activities in the brain to a degree impossible to achieve with dietary Se restriction alone (; ). New selenoproteins were still being discovered, until a landmark genomic study fixed the number at the final 25 in humans and 24 in mice (). In an early review (), the key questions about the neurobiology of Se were: Which selenoproteins are expressed in the brain, in which regions and in which cell types? What are their functions? How does Se enter the brain and is it distributed in a hierarchical manner? Is there a causal connection to neurological disorders?
To cut the answers short: most selenoproteins are expressed in the brain, mainly in neurons, in all major brain regions (). Gene targeting in mice for all or single selenoproteins has revealed that GABAergic interneurons are particularly vulnerable, but also basal ganglia, cerebellum, cortex, and brain stem (; , ; ; ). At first it remained an open question whether myelination defects were caused by neuronal or oligodendroglial selenoprotein dysfunction (). Several selenoproteins are essential for the brain development and function, in particular glutathione peroxidase 4 (GPX4), but also thioredoxin reductase 1 (TXNRD1), and SELENOT (). Currently the function of GPX4 as a major regulator of ferroptosis in development and disease is receiving a great deal of attention (; ). It will be seen whether GPX4 is an indispensable protein for neurons in its own right or whether there is a causal connection to neurodegeneration in neurological disorders. What is now clear, is that Se enters the brain either as non-physiological selenite salt or, more physiologically, in the form of SELENOP, loaded in the liver with several atoms of Se (; ; ), and taken up at the blood-brain-barrier (BBB) and by individual cells via endocytosis using receptors of the LRP family (). The most important receptor is APOER2 (), but also LRP2/MEGALIN (; ) and, most likely, LRP1 contribute to SELENOP internalization. Among these receptors, APOER2 clearly is the most important, providing preferential Se supply at the BBB and in the testis. MEGALIN appears more involved in Se supply through the blood-cerebrospinal fluid barrier and in the kidney (). SELENOP is also expressed in the brain and may contribute to local Se storage and recycling (; ). SELENOP and its receptors contribute to a “hierarchy” of selenoprotein expression in organs in the sense that some organs, like the brain, can be preferentially supplied with Se at the expense of others, like the liver. There is, however, a second “hierarchy” of selenoprotein expression that depends on the relative sensitivity of individual selenoproteins to Se availability. Thus, GPX1 is more sensitive to cellular Se levels than, e.g., GPX4. The reasons behind this observation are multifarious and beyond the scope of this review. Both hierarchies may work hand-in-hand as GPX1 mRNA is highly expressed in liver and contains a highly efficient SECIS element. Depending on dietary Se supply, the amount of GPX1 protein covers orders of magnitude, and likely provides a safe Se storage device, until the Se is needed for distribution via SELENOP to preferred target tissues. The amount of Se in a tissue does not necessarily inform about its physiological importance, e.g., brain Se levels are much lower than liver Se levels, yet selenoproteins are essential in brain, but not liver (; ).
Genetic Deficiency of Single Selenoproteins
At that time, only one human disorder was genetically linked to deficiency of a selenoprotein-encoding gene, now known as selenoprotein N (SELENON)-related myopathies (; ), but the situation was soon to change with exome sequencing entering clinical practice. SELENON is an ER-resident membrane protein of unknown function. Mutations in SELENON that disrupt the gene or prevent selenocysteine (Sec) insertion into the protein lead to myopathy (). Mouse and zebrafish models of SELENON-deficiency reflect aspects of the muscular phenotype, in particular the preferential affection of axial muscles (). This example suggests that the mouse may represent an acceptable model for humans regarding selenoprotein deficiency. It should be noted that dietary deficiency for Se in livestock was recognized early as a cause for white muscle disease (). So far there is no evidence to link SELENON mutations to impaired neuronal or neurological function.
The syndromes associated with selenoprotein deficiency can be broadly divided into two categories: the first category represents mutations in single genes encoding selenoproteins. The second category is represented by mutations in genes involved in selenoprotein biosynthesis. A comprehensive compilation of individual mutations and patient phenotypes can be found in recent summaries of the state of the field (; ; ). Here, we will only briefly summarize these results and rather focus on new developments since then.
Conditional gene inactivation of Gpx4 in mice demonstrated that GPX4 is an essential selenoprotein for several types of neurons as discussed above (). Conditional inactivation of Gpx4 in forebrain neurons after development lead to cognitive decline and hippocampal neurodegeneration (). Furthermore, constitutive gene inactivation of Gpx4 lead to embryonic lethality around embryonic day 7 (; ). Thus, it came as a complete surprise to find newborn children affected with Sedaghatian-type spondylometaphyseal dysplasia with inactivating non-sense mutations in the GPX4 gene (; ). These patients show massive brain atrophy and usually die shortly after birth. In stark contrast to mice, where Gpx4-deficiency leads to early embryonic lethality, human fetuses obviously progress much further in their development.
A lesser sensitivity of humans compared to mice regarding the lack of selenoproteins was also observed with respect to TXNRD2. Txnrd2–/– mice died around embryonic day 13 with thinned ventricular walls in the heart and impaired hematopoiesis (). Conditional ablation of Txnrd2 in the heart lead to a fatal cardiomyopathy (). Similarly, heterozygous missense mutations were associated with dilated cardiomyopathy in humans () and were reminiscent of Keshan disease, a fatal cardiomyopathy observed in Se-deficient regions in China (; ). It thus came as a complete surprise that a homozygous truncating mutation in human TXNRD2 merely resulted in familial glucocorticoid deficiency without a cardiac phenotype ().
Similar to inactivation of Txnrd2, constitutive inactivation of Txnrd1 is embryonic lethal in mice (). Conditional ablation of Txnrd1 in neuronal precursors shows only a mild cerebellar defect (), while neuron-specific Txnrd1 ablation leads to neurodegeneration with aging (). Since our last review of this subject, we have found that homozygous mutations in TXNRD1, which reduce enzymatic activity, are associated with genetic generalized epilepsy in human (). In summary, upon Txnrd mutation the mouse phenotypes appear throughout more severe than the corresponding human phenotypes. This raises the question whether variants of TXNRDs may be able to compensate for the loss of the other TXNRD in humans, but not in mice, or whether, e.g., the glutaredoxin system may be able to partially compensate in some human cell types.
Ethanolamine-phosphotransferase 1 (EPT1/SELENOI) is one of two enzymes catalyzing the same step in phospholipid biosynthesis (). This enzyme is obviously important for myelin biosynthesis in the human brain (; ). Inactivation of the gene in mice is embryonic lethal (). Se deficiency of the brains of Selenop-deficient mice impaired the myelin sheath in the brain stem, at least under conditions of low Se diet (). Thus, it is possible that the myelination defect is a primary phenotype of myelin formation in oligodendrocytes and not a result of retrograde signaling from selenoprotein-deficient neurons, as we initially suspected.
The only other selenoprotein that has been shown to play an essential role in the brain is SELENOT (; ). How this relates to its role in protein glycosylation is not clear, yet (). It is intriguing, that other selenoproteins have also been implicated in protein glycosylation or protein folding, e.g., SELENOF and SELENOM. The respective knockout mouse models, however, did not show any apparent neurological defects (). SELENOS, another selenoprotein implicated in endoplasmic reticulum associated degradation of proteins (ERAD) (), has not been studied by gene targeting in mice. While selenoproteins have often simply been classified as “anti-oxidant,” it is remarkable that inactivation of a selenoenzyme with a defined reductase activity, methionine-R-sulfoxide-reductase B1 (MSRB1), has not produced a neurological phenotype (; ).
There is a rich literature on mouse models of neurodegenerative diseases whose phenotypes can be exacerbated by additional deficiency of “antioxidant” selenoproteins (; ). In such models there is always the conceptual question whether the mutation in the selenoprotein specifically abrogates (and thus reveals) a specific protective mechanism or whether the selenoprotein mutation simply tips over a dysbalanced system that is already vulnerable to any other possible stressor. Given the availability of many powerful genome-wide association studies on important neurodegenerative disorders, and their failure to identify mutations in selenoprotein genes, it seems unlikely for us that mutations in selenoproteins are important causes or modifiers of common neurological disorders. Yet, mutations in selenoproteins or their biosynthesis pathways may reveal specific cell biological or developmental functions of selenoproteins.
Selenoprotein Deficiency Resulting From Mutations Impairing Selenoprotein Biosynthesis
A landmark paper on the identification of mutations in the selenoprotein biosynthesis factor SECISBP2 in humans called into question the possibly simple-minded concept of selenoproteins as “anti-oxidants.” The key phenotype that brought the patients to medical attention, was a growth retardation in puberty (). Abnormal thyroid function tests (TFT), i.e., the constellation of thyroid-stimulating hormone (TSH) and thyroid hormone levels, guided the discovery of a congenital deficiency of selenoprotein biosynthesis. The pubertal growth spurt depends not only on growth hormone, but requires permissive action of thyroid hormone. The TFT suggested deficiency of deiodinase 2 (DIO2) activity in these patients which was confirmed in patient fibroblasts. Deiodinases are selenoenzymes capable of removing iodide from iodothyronines (; ). The prohormone thyroxine (T4) requires 5′-deiodination to yield triiodothyronine (T3), which binds the nuclear T3-receptors (Figure 1). 5-deiodination of T4 and T3 yields reverse T3 (rT3) and 3,3′-T2, respectively. Moreover, the two plasma selenoproteins SELENOP and GPX3 were reduced in these patients (). Thus, the congenital deficiency of selenoprotein biosynthesis revealed itself not in neurodegeneration, epilepsy, heart disease or a muscular disorder, but in altered thyroid hormone levels in the sense of a blunted response to T4! Later, more patients with apparently stronger mutations in SECISBP2 were identified (; ; ). Some of these patients exhibited a SELENON-related myopathy, infertility, and an immune phenotype (). The importance of local conversion of T3 is illustrated by the Thr92Ala polymorphism in DIO2. People with the homozygous Ala92 version of this polymorphism have a reduced ability to convert T4 to T3 (), hence when being treated for hypothyroidism, have improved psychological well-being on combination T4/T3 therapy than on T4 treatment alone (). Remarkably, the first patient with a mutation in the tRNASec gene (TCA-TRU in human, Trsp in mouse) showed the same phenotype of a blunted response to T4 (). Findings from Dio1- and Dio2-knock-out mouse models are entirely compatible with the above conclusions drawn from SECISBP2-deficiency (, ). Selenoprotein deficiency does not fundamentally impair thyroid gland function ().
FIGURE 1
This leads us to the obvious question whether other factors involved in selenoprotein biosynthesis have been found mutated in humans. And what kind of phenotypes are presented by affected individuals? The first patients with mutations in the selenocysteine synthase gene (SEPSECS) have been identified in . The patients presented with “progressive cerebello cerebral atrophy,” now systematically designated pontocerebellar hypoplasia 2D (PCH2D). The names of the syndromes capture quite well the observed phenotypes (). The predominantly neurological condition with neurodegeneration and epilepsy is likely based on dysfunction of GPX4 and other essential selenoproteins, possibly TXNRD1 or 2 (). Some patients display milder phenotypes and may grow into adulthood with intellectual disability, but no overt neurodegeneration (). Interestingly, we are not aware of reports of abnormal TFT in these patients. Likewise, we do not know of a SELENON-related myopathy in one of these patients.
Individuals with mutations in EEFSEC, SEPHS2, and PSTK have not yet been found (Figure 2). Selenoprotein expression in knockout mouse models for these genes have not been described. Conditional inactivation of the suggested biosynthesis factor SECP43, encoded by the Trnau1ap gene, in liver and in neurons did not support a role for this gene in selenoprotein expression (). Mouse models for hypomorphic mutations in the tRNASec have been generated. One model has a mutation in the promoter and, as a simple transgene, is inserted somewhere in the genome (). This mouse displays a neurological phenotype that resembles in several aspects of Selenop- and neuron-specific Trsp-knockout mutants (; , ). Point mutations have been made in Trsp affecting the anticodon loop of tRNASec. Overexpression of the A37G-Trsp mutant (made as a simple transgene) resulted in neurological defects, in particular when fed a high Se diet (). TFTs have not been determined for the Trsp mutant mouse models.
FIGURE 2
Looking at these results, there seems to exist a strange dichotomy of phenotypes being related either to neurobiology or endocrinology, when selenoprotein biosynthesis is impaired. This observation holds for both mice and humans. A naïve epistatic model of selenoprotein biosynthesis should predict more or less the same phenotypes, if selenoprotein translation is globally impaired (Figure 2). Clearly, we have not yet analyzed all possible mutants and not all available mutants have been systematically analyzed side by side. Yet, in humans, mutations most often come as missense, splicing or other mutations that may not completely abrogate 100% of gene/protein expression/activity. We have recently shown that the effect of a missense mutation in vitro and in vivo may differ, in particular stability of a mutated protein may depend on the cell type (). For the SECISBP2R543Q mutant, we demonstrated that the protein is a complete NULL in mouse liver, but partially functioning and supporting selenoprotein expression in neurons ().
Future Directions
In order to understand the neurobiology of Se, we need both, the precise biochemical or cell biological function of each selenoprotein and the full understanding of the phenotypes under conditions of its absence in an entire mammalian organism. This goal has only been achieved for a small subset of selenoproteins. For some of the others, we may have a biochemical reaction and a phenotype of cells grown in a dish, but we are convinced that nobody would have been able to predict the complex phenotype of patients with mutations in SECISBP2 based on the finding of reduced selenoprotein expression in SECISBP2-deficient cells in culture. If we just focus on the brain with its many neuronal and glial cell types, we are confronted with perplexing complexity (). All of these cell types are involved in mechanisms of development, exert a function in the mature organism, and may play are role in neurodegeneration. Thus, it is obvious how wide this field still is and how much expertise is required to address this question.
Another question related to the discussion above, is to what extent mice are valid models for humans with regard to understanding the functions of selenoproteins. The answer will again rely on the comparison of genetic models in mice and patients with congenital defects in selenoprotein genes or genes encoding selenoprotein biosynthesis factors. We can expect that exome-sequencing approaches that are now broadly available will help us identify patients with such mutations. A recent thought-provoking paper has looked at the same question from just the opposite perspective: Instead of searching for mutations in the genes of patients with clinical phenotypes, searched human genome data for inactivating mutations in selenoprotein genes. They found that humans can carry homozygous inactivating mutations in SELENOO without apparently presenting with a phenotype. SELENOO is a novel mitochondrial protein Ser/Thr-AMP transferase that has not yet been inactivated in mice ().
The question whether the selenoproteome is completely known seemed to have been solved through the landmark paper by who identified genes encoding selenoproteins based in part on the presence of the SECIS element. A recent proteome paper now suggested there are additional Sec-containing proteins with UGA/Sec codons, but lacking recognizable SECIS elements (). This provocative finding is, interestingly, in line with the demonstration of selenoprotein translation in the absence of functional SECISBP2 (; ). If mutations in SEPSECS, unlike mutations in SECISBP2, would also affect selenoproteins that do not depend on a SECIS for biosynthesis, the dichotomy of phenotypes could be explained and some of the non-canonical selenoproteins would likely be important for the brain.
The arguably most dynamic field of selenoprotein research, again related to the neurobiology of Se, is the wider context of the function of GPX4. The whole field of ferroptosis is blossoming. This type of cell death emerges as an important cell biological process on which much hope is placed in the context of cancer treatment and prevention of neurodegenerative disease. Can ferroptosis be modulated pharmacologically to the benefit of patients? Do other pathways related to selenoproteins play a role in these processes? What is the role of lipid peroxidation in this context? In mitochondria? This reminds one of us (US) of a lab rotation in organic chemistry long ago: during his undergraduate study he separated lipid-hydroperoxides and their respective alcohols on a chiral gas-chromatographic column and observed that the peroxides and alcohols were chiral. An enzymatic process seemed the most likely explanation, while biologists argued that spontaneous lipid peroxidation was most likely overinterpreted… Keeping this in mind, who knows what exciting findings there are just around the corner revealing a part of themselves seemingly as oddities or artifacts?
Statements
Author contributions
US wrote the initial draft of the manuscript. All authors refined and rewrote parts of the manuscript and contributed to research associated with the thoughts presented here.
Funding
Recent research on selenoproteins in the authors’ laboratory was supported by the Deutsche Forschungsgemeinschaft DFG (Schw914/2, Schw914/5, and Schw914/6), German Israeli Fund, and Medizinische Fakultät, Rheinische Friedrich-Wilhelms-Universität Bonn.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
References
1
AgamyO.Ben ZeevB.LevD.MarcusB.FineD.SuD. (2010). Mutations disrupting selenocysteine formation cause progressive cerebello-cerebral atrophy.Am. J. Hum. Genet.87538–544. 10.1016/j.ajhg.2010.09.007
2
AhmedM. Y.Al-KhayatA.Al-MurshediF.Al-FutaisiA.ChiozaB. A.Pedro Fernandez-MurrayJ. (2017). A mutation of EPT1 (SELENOI) underlies a new disorder of kennedy pathway phospholipid biosynthesis.Brain140547–554.
3
AnttonenA. K.HilanderT.LinnankiviT.IsohanniP.FrenchR. L.LiuY. (2015). Selenoprotein biosynthesis defect causes progressive encephalopathy with elevated lactate.Neurology85306–315. 10.1212/wnl.0000000000001787
4
AveryJ. C.YamazakiY.HoffmannF. W.FolgelgrenB.HoffmannP. R. (2020). Selenoprotein i is essential for murine embryogenesis.Arch. Biochem. Biophys.689:108444. 10.1016/j.abb.2020.108444
5
AygunC.CelikF. C.NuralM. S.AzakE.KucukodukS.OgurG.et al (2012). Simplified gyral pattern with cerebellar hypoplasia in Sedaghatian type spondylometaphyseal dysplasia: a clinical report and review of the literature.Am. J. Med. Genet. A158A1400–1405. 10.1002/ajmg.a.35306
6
AzevedoM. F.BarraG. B.NavesL. A.Ribeiro VelascoL. F.Godoy GarciaC. P.de CastroL. C. (2010). Rocha, selenoprotein-related disease in a young girl caused by nonsense mutations in the SBP2 gene.J. Clin. Endocrinol. Metab.954066–4071. 10.1210/jc.2009-2611
7
BiancoA. C.KimB. S. (2018). Pathophysiological relevance of deiodinase polymorphism.Curr. Opin. Endocrinol. Diabetes Obes.25341–346. 10.1097/med.0000000000000428
8
BoukhzarL.HamiehA.CartierD.TanguyY.AlsharifI.CastexM. (2016). Selenoprotein t exerts an essential oxidoreductase activity that protects dopaminergic neurons in mouse models of parkinson’s disease.Antioxid. Redox Signal.24557–574. 10.1089/ars.2015.6478
9
BurkR. F.HillK. E. (2015). Regulation of selenium metabolism and transport.Annu. Rev. Nutr.35109–134. 10.1146/annurev-nutr-071714-034250
10
CarlsonB. A.SchweizerU.PerellaC.ShrimaliR. K.FeigenbaumL.ShenL. (2009). The selenocysteine tRNA STAF-binding region is essential for adequate selenocysteine tRNA status, selenoprotein expression and early age survival of mice.Biochem. J.41861–71. 10.1042/bj20081304
11
CastetsP.LescureA.GuicheneyP.AllamandV. (2012). Selenoprotein N in skeletal muscle: from diseases to function.J. Mol. Med.901095–1107. 10.1007/s00109-012-0896-x
12
CastexM. T.AraboA.BenardM.RoyV.Le JoncourV.PrevostG. (2015). Selenoprotein T deficiency leads to neurodevelopmental abnormalities and hyperactive behavior in mice.Mol. Neurobiol.535818–5832. 10.1007/s12035-015-9505-7
13
Chiu-UgaldeJ.TheiligF.BehrendsT.DrebesJ.SielandC.SubbarayalP. (2010). Mutation of megalin leads to urinary loss of selenoprotein P and selenium deficiency in serum, liver, kidneys and brain.Biochem. J.431103–111. 10.1042/bj20100779
14
Chiu-UgaldeJ.WirthE. K.KleinM. O.SapinR.Fradejas-VillarN.RenkoK. (2012). Thyroid function is maintained despite increased oxidative stress in mice lacking selenoprotein biosynthesis in thyroid epithelial cells.Antioxid. Redox Signal.17902–913. 10.1089/ars.2011.4055
15
ConradM.JakupogluC.MorenoS. G.LipplS.BanjacA.SchneiderM. (2004). Essential role for mitochondrial thioredoxin reductase in hematopoiesis, heart development, and heart function.Mol. Cell. Biol.249414–9423. 10.1128/mcb.24.21.9414-9423.2004
16
CurranJ. E.JowettJ. B.ElliottK. S.GaoY.GluschenkoK.WangJ. (2005). Genetic variation in selenoprotein S influences inflammatory response.Nat. Genet.371234–1241. 10.1038/ng1655
17
Di CosmoC.McLellanN.LiaoX. H.KhannaK. K.WeissR. E.PappL.et al (2009). Clinical and molecular characterization of a novel selenocysteine insertion sequence-binding protein 2 (SBP2) gene mutation (R128X).J. Clin. Endocrinol. Metab.944003–4009. 10.1210/jc.2009-0686
18
DumitrescuA. M.LiaoX. H.AbdullahM. S.Lado-AbealJ.MajedF. A.MoellerL. C. (2005). Mutations in SECISBP2 result in abnormal thyroid hormone metabolism.Nat. Genet.371247–1252. 10.1038/ng1654
19
FomenkoD. E.NovoselovS. V.NatarajanS. K.LeeB. C.KocA.CarlsonB. A. (2008). Methionine-R-sulfoxide reductase 1 (MsrB1) knockout mice: roles of MsrB1 in redox regulation and identification of a novel selenoprotein form.J. Biol. Chem.2845986–5993. 10.1074/jbc.m805770200
20
Fradejas-VillarN. (2018). Consequences of mutations and inborn errors of selenoprotein biosynthesis and functions.Free Radic. Biol. Med.127206–214. 10.1016/j.freeradbiomed.2018.04.572
21
Fradejas-VillarN.SeeherS.AndersonC. B.DoengiM.CarlsonB. A.HatfieldD. L. (2017). The RNA-binding protein Secisbp2 differentially modulates UGA codon reassignment and RNA decay.Nucleic Acids Res.454094–4107. 10.1093/nar/gkw1255
22
Friedmann AngeliJ. P.KryskoD. V.ConradM. (2019). Ferroptosis at the crossroads of cancer-acquired drug resistance and immune evasion.Nat. Rev. Cancer19405–414. 10.1038/s41568-019-0149-1
23
GeK.XueA.BaiJ.WangS. (1983). Keshan disease-an endemic cardiomyopathy in China.Virchows Arch A Pathol. Anat. Histopathol.4011–15. 10.1007/bf00644785
24
GladyshevV. N.ArnerE. S.BerryM. J.Brigelius-FloheR.BrufordE. A.BurkR. F. (2016). Selenoprotein gene nomenclature.J. Biol. Chem.29124036–24040.
25
GuoL.YangW.HuangQ.QiangJ.HartJ. R.WangW. (2018). Selenocysteine-specific mass spectrometry reveals tissue-distinct selenoproteomes and candidate selenoproteins.Cell Chem. Biol.251380–1388.e4.
26
HambrightW. S.FonsecaR. S.ChenL.NaR.RanQ. (2017). Ablation of ferroptosis regulator glutathione peroxidase 4 in forebrain neurons promotes cognitive impairment and neurodegeneration.Redox Biol.128–17. 10.1016/j.redox.2017.01.021
27
HamiehA.CartierD.AbidH.CalasA.BurelC.BucharlesC. (2017). Selenoprotein T is a novel OST subunit that regulates UPR signaling and hormone secretion.EMBO Rep.181935–1946. 10.15252/embr.201643504
28
HillK. E.WuS.MotleyA. K.StevensonT. D.WinfreyV. P.CapecchiM. R. (2012). Production of selenoprotein P (Sepp1) by hepatocytes is central to selenium homeostasis.J. Biol. Chem.28740414–40424. 10.1074/jbc.m112.421404
29
HillK. E.ZhouJ.McMahanW. J.MotleyA. K.BurkR. F. (2004). Neurological dysfunction occurs in mice with targeted deletion of the selenoprotein p gene.J. Nutr.134157–161. 10.1093/jn/134.1.157
30
HillK. E.ZhouJ.McMahanW. J.MotleyA. K.AtkinsJ. F.GestelandR. F. (2003). Deletion of selenoprotein P alters distribution of selenium in the mouse.J. Biol. Chem.27813640–13646. 10.1074/jbc.m300755200
31
HoribataY.ElpelegO.EranA.HirabayashiY.SavitzkiD.TalG. (2018). EPT1 (selenoprotein I) is critical for the neural development and maintenance of plasmalogen in humans.J. Lipid Res.591015–1026. 10.1194/jlr.p081620
32
IwamaK.SasakiM.HirabayashiS.OhbaC.IwabuchiE.MiyatakeS. (2016). Milder progressive cerebellar atrophy caused by biallelic SEPSECS mutations.J. Hum. Genet.61527–531. 10.1038/jhg.2016.9
33
JakupogluC.PrzemeckG. K.SchneiderM.MorenoS. G.MayrN.HatzopoulosA. (2005). Cytoplasmic thioredoxin reductase is essential for embryogenesis but dispensable for cardiac development.Mol. Cell. Biol.251980–1988. 10.1128/mcb.25.5.1980-1988.2005
34
KasaikinaM. V.FomenkoD. E.LabunskyyV. M.LachkeS. A.QiuW.MoncasterJ. A. (2011). Roles of the 15-kDa selenoprotein (Sep15) in redox homeostasis and cataract development revealed by the analysis of Sep 15 knockout mice.J. Biol. Chem.28633203–33212. 10.1074/jbc.m111.259218
35
KasaikinaM. V.TuranovA. A.AvanesovA.SchweizerU.SeeherS.BronsonR. T. (2013). Contrasting roles of dietary selenium and selenoproteins in chemically induced hepatocarcinogenesis.Carcinogenesis341089–1095. 10.1093/carcin/bgt011
36
KöhrleJ.JakobF.ContempreB.DumontJ. E. (2005). Selenium, the thyroid, and the endocrine system.Endocr. Rev.26944–984. 10.1210/er.2001-0034
37
KryukovG. V.CastellanoS.NovoselovS. V.LobanovA. V.ZehtabO.GuigoR.et al (2003). Characterization of mammalian selenoproteomes.Science3001439–1443. 10.1126/science.1083516
38
KudinA. P.BaronG.ZsurkaG.HampelK. G.ElgerC. E.GroteA. (2017). Homozygous mutation in TXNRD1 is associated with genetic generalized epilepsy.Free Radic. Biol. Med.106270–277. 10.1016/j.freeradbiomed.2017.02.040
39
LeeB. C.PeterfiZ.HoffmannF. W.MooreR. E.KayaA.AvanesovA. (2013). MsrB1 and MICALs regulate actin assembly and macrophage function via reversible stereoselective methionine oxidation.Mol. Cell.51397–404. 10.1016/j.molcel.2013.06.019
40
LoscalzoJ. (2014). Keshan disease, selenium deficiency, and the selenoproteome.N. Engl. J. Med.3701756–1760. 10.1056/nejmcibr1402199
41
MahdiY.XuX. M.CarlsonB. A.FradejasN.GunterP.BraunD. (2015). Expression of selenoproteins is maintained in mice carrying mutations in SECp43, the tRNA selenocysteine 1 associated protein (Trnau1ap).PLoS One10:e0127349. 10.1371/journal.pone.0127349
42
McAninchE. A.JoS.PreiteN. Z.FarkasE.MohacsikP.FeketeC. (2015). Prevalent polymorphism in thyroid hormone-activating enzyme leaves a genetic fingerprint that underlies associated clinical syndromes.J. Clin. Endocrinol. Metab.100920–933. 10.1210/jc.2014-4092
43
MoghadaszadehB.PetitN.JaillardC.BrockingtonM.RoyS. Q.MerliniL. (2001). Mutations in SEPN1 cause congenital muscular dystrophy with spinal rigidity and restrictive respiratory syndrome.Nat. Genet.2917–18. 10.1038/ng713
44
MondalS.RajaK.SchweizerU.MugeshG. (2016). Chemistry and biology in the biosynthesis and action of thyroid hormones.Angew. Chem. Int. Ed. Engl.557606–7630. 10.1002/anie.201601116
45
MuthO. H.OldfieldJ. E.RemmertL. F.SchubertJ. R. (1958). Effects of selenium and vitamin E on white muscle disease.Science128:1090. 10.1126/science.128.3331.1090
46
OlsonG. E.WinfreyV. P.HillK. E.BurkR. F. (2008). Megalin mediates selenoprotein P uptake by kidney proximal tubule epithelial cells.J. Biol. Chem.2836854–6860. 10.1074/jbc.m709945200
47
OlsonG. E.WinfreyV. P.NagdasS. K.HillK. E.BurkR. F. (2007). Apolipoprotein E receptor-2 (ApoER2) mediates selenium uptake from selenoprotein P by the mouse testis.J. Biol. Chem.28212290–12297. 10.1074/jbc.m611403200
48
PittsM. W.RamanA. V.HashimotoA. C.TodorovicC.NicholsR. A.BerryM. J. (2012). Deletion of selenoprotein P results in impaired function of parvalbumin interneurons and alterations in fear learning and sensorimotor gating.Neuroscience20858–68. 10.1016/j.neuroscience.2012.02.017
49
PrasadR.ChanL. F.HughesC. R.KaskiJ. P.KowalczykJ. C.SavageM. O. (2014). Thioredoxin Reductase 2 (TXNRD2) mutation associated with familial glucocorticoid deficiency (FGD).J. Clin. Endocrinol. Metab.99E1556–E1563.
50
RederstorffM.CastetsP.ArbogastS.LaineJ.VassilopoulosS.BeuvinM. (2011). Increased muscle stress-sensitivity induced by selenoprotein N inactivation in mouse: a mammalian model for SEPN1-related myopathy.PLoS One6:e23094. 10.1371/journal.pone.0023094
51
RenkoK.WernerM.Renner-MullerI.CooperT. G.YeungC. H.HollenbachB. (2008). Hepatic selenoprotein P (SePP) expression restores selenium transport and prevents infertility and motor-incoordination in sepp-knockout mice.Biochem. J.409741–749. 10.1042/bj20071172
52
SantesmassesD.MariottiM.GladyshevV. N. (2020). Tolerance to selenoprotein loss differs between human and mouse.Mol. Biol. Evol.37341–354. 10.1093/molbev/msz218
53
ScharpfM.SchweizerU.ArzbergerT.RoggendorfW.SchomburgL.KöhrleJ. (2007). Neuronal and ependymal expression of selenoprotein P in the human brain.J. Neural. Transm.114877–884. 10.1007/s00702-006-0617-0
54
SchneiderM. J.FieringS. N.PalludS. E.ParlowA. F.St GermainD. L.GaltonV. A. (2001). Targeted disruption of the type 2 selenodeiodinase gene (DIO2) results in a phenotype of pituitary resistance to T4.Mol. Endocrinol.152137–2148. 10.1210/mend.15.12.0740
55
SchneiderM. J.FieringS. N.ThaiB.WuS. Y.St GermainE.ParlowA. F. (2006). Targeted disruption of the type 1 selenodeiodinase gene (Dio1) results in marked changes in thyroid hormone economy in mice.Endocrinology147580–589. 10.1210/en.2005-0739
56
SchoenmakersE.AgostiniM.MitchellC.SchoenmakersN.PappL.RajanayagamO. (2010). Mutations in the selenocysteine insertion sequence-binding protein 2 gene lead to a multisystem selenoprotein deficiency disorder in humans.J. Clin. Invest.1204220–4235. 10.1172/jci43653
57
SchoenmakersE.ChatterjeeK. (2018). Identification of genetic disorders causing disruption of selenoprotein biosynthesis.Methods Mol. Biol.1661325–335. 10.1007/978-1-4939-7258-6_23
58
SchoenmakersE.ChatterjeeK. (2020). Human disorders affecting the selenocysteine incorporation pathway cause systemic selenoprotein deficiency.Antioxid. Redox Signal.33481–497. 10.1089/ars.2020.8097
59
SchoenmakersE.CarlsonB.AgostiniM.MoranC.RajanayagamO.BochukovaE. (2016). Mutation in human selenocysteine transfer RNA selectively disrupts selenoprotein synthesis.J. Clin. Invest.126992–996. 10.1172/jci84747
60
SchomburgL.SchweizerU.HoltmannB.FlohéL.SendtnerM.KöhrleJ. (2003). Gene disruption discloses role of selenoprotein P in selenium delivery to target tissues.Biochem. J.370397–402. 10.1042/bj20021853
61
SchweizerU. (2016). “Selenoproteins in nervous system development, function, and degeneration,” in Selenium. its Molecular Biology and Role in Human Health, edsHatfieldD. L. S.TsujiP. A.GladyshevV. N. (Berlin: Springer).
62
SchweizerU.Fradejas-VillarN. (2016). Why 21? The significance of selenoproteins for human health revealed by inborn errors of metabolism.FASEB J.303669–3681. 10.1096/fj.201600424
63
SchweizerU.SchomburgL. (2006). “Selenium, selenoproteins, and brain function,” in Selenium: its Molecular Biology and Role in Human Health, edsHatfieldD. L.BerryM. J.GladyshevV. N. (New York: Springer), 233–248. 10.1007/0-387-33827-6_21
64
SchweizerU.BräuerA. U.KöhrleJ.NitschR.SavaskanN. E. (2004a). Selenium and brain function: a poorly recognized liaison.Brain Res. Brain Res. Rev.45164–178. 10.1016/j.brainresrev.2004.03.004
65
SchweizerU.MichaelisM.KöhrleJ.SchomburgL. (2004b). Efficient selenium transfer from mother to offspring in selenoprotein-P-deficient mice enables dose-dependent rescue of phenotypes associated with selenium deficiency.Biochem. J.37821–26. 10.1042/bj20031795
66
SchweizerU.SchomburgL.SavaskanN. E. (2004c). The neurobiology of selenium: lessons from transgenic mice.J. Nutr.134707–710. 10.1093/jn/134.4.707
67
SchweizerU.StreckfussF.PeltP.CarlsonB. A.HatfieldD. L.KöhrleJ.et al (2005). Hepatically derived selenoprotein P is a key factor for kidney but not for brain selenium supply.Biochem. J.386221–226. 10.1042/bj20041973
68
SeeherS.CarlsonB. A.MiniardA. C.WirthE. K.MahdiY.HatfieldD. L.et al (2014). Impaired selenoprotein expression in brain triggers striatal neuronal loss leading to co-ordination defects in mice.Biochem. J.46267–75. 10.1042/bj20140423
69
SeilerA.SchneiderM.ForsterH.RothS.WirthE. K.CulmseeC. (2008). Glutathione peroxidase 4 senses and translates oxidative stress into 12/15-lipoxygenase dependent- and AIF-mediated cell death.Cell Metab8237–248. 10.1016/j.cmet.2008.07.005
70
SibbingD.PfeuferA.PerisicT.MannesA. M.Fritz-WolfK.UnwinS. (2011). Mutations in the mitochondrial thioredoxin reductase gene TXNRD2 cause dilated cardiomyopathy.Eur. Heart J.321121–1133. 10.1093/eurheartj/ehq507
71
SmithA. C.MearsA. J.BunkerR.AhmedA.MacKenzieM.SchwartzentruberJ. A. (2014). Mutations in the enzyme glutathione peroxidase 4 cause sedaghatian-type spondylometaphyseal dysplasia.J. Med. Genet.51470–474. 10.1136/jmedgenet-2013-102218
72
SoerensenJ.JakupogluC.BeckH.ForsterH.SchmidtJ.SchmahlW. (2008). The role of thioredoxin reductases in brain development.PLoS One3:e1813. 10.1371/journal.pone.0001813
73
SreelathaA.YeeS. S.LopezV. A.ParkB. C.KinchL. N.PilchS. (2018). Protein AMPylation by an evolutionarily conserved pseudokinase.Cell175809–821.e19.
74
StockwellB. R.Friedmann AngeliJ. P.BayirH.BushA. I.ConradM.DixonS. J.et al (2017). Ferroptosis: a regulated cell death nexus linking metabolism, redox biology, and disease.Cell171273–285. 10.1016/j.cell.2017.09.021
75
ValentineW. M.AbelT. W.HillK. E.AustinL. M.BurkR. F. (2008). Neurodegeneration in mice resulting from loss of functional selenoprotein P or its receptor apolipoprotein E receptor 2.J. Neuropathol. Exp. Neurol.6768–77. 10.1097/nen.0b013e318160f347
76
ValentineW. M.HillK. E.AustinL. M.ValentineH. L.GoldowitzD.BurkR. F. (2005). Brainstem axonal degeneration in mice with deletion of selenoprotein p.Toxicol. Pathol.33570–576. 10.1080/01926230500243045
77
Villar-QuilesR. N.von, der HagenM.MetayC.GonzalezV.DonkervoortS.et al (2020). The clinical, histologic, and genotypic spectrum of SEPN1-related myopathy: a case series.Neurology95e1512–e1527.
78
WirthE. K.BharathiB. S.HatfieldD.ConradM.BrielmeierM.SchweizerU. (2014). Cerebellar hypoplasia in mice lacking selenoprotein biosynthesis in neurons.Biol. Trace Elem. Res.158203–210. 10.1007/s12011-014-9920-z
79
WirthE. K.ConradM.WintererJ.WoznyC.CarlsonB. A.RothS. (2010). Neuronal selenoprotein expression is required for interneuron development and prevents seizures and neurodegeneration.FASEB J.24844–852. 10.1096/fj.09-143974
80
YantL. J.RanQ.RaoL.Van RemmenH.ShibataniT.BelterJ. G. (2003). The selenoprotein GPX4 is essential for mouse development and protects from radiation and oxidative damage insults.Free Radic. Biol. Med.34496–502. 10.1016/s0891-5849(02)01360-6
81
ZhangY.ZhouY.SchweizerU.SavaskanN. E.HuaD.KipnisJ.et al (2008). Comparative analysis of selenocysteine machinery and selenoproteome gene expression in mouse brain identifies neurons as key functional sites of selenium in mammals.J. Biol. Chem.2832427–2438. 10.1074/jbc.m707951200
82
ZhangZ. H.ChenC.JiaS. Z.CaoX. C.LiuM.TianJ. (2020). Selenium restores synaptic deficits by modulating NMDA receptors and selenoprotein K in an Alzheimer’s disease model.Antioxid. Redox Signal.10.1089/ars.2019.7990Online ahead of print.
83
ZhaoW.BohleberS.SchmidtH.SeeherS.HowardM. T.BraunD. (2019). Ribosome profiling of selenoproteins in vivo reveals consequences of pathogenic Secisbp2 missense mutations.J. Biol. Chem.29414185–14200. 10.1074/jbc.ra119.009369
Summary
Keywords
genetics, neurodegeneration, GPX4, ferroptosis, epilepsy
Citation
Schweizer U, Bohleber S, Zhao W and Fradejas-Villar N (2021) The Neurobiology of Selenium: Looking Back and to the Future. Front. Neurosci. 15:652099. doi: 10.3389/fnins.2021.652099
Received
11 January 2021
Accepted
08 February 2021
Published
25 February 2021
Volume
15 - 2021
Edited by
Peter R. Hoffmann, University of Hawai’i, United States
Reviewed by
Arturo Hernandez, Maine Medical Center, United States; Margaret Rayman, University of Surrey, United Kingdom
Updates
Copyright
© 2021 Schweizer, Bohleber, Zhao and Fradejas-Villar.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Ulrich Schweizer, Ulrich.schweizer@uni-bonn.de; uschweiz@uni-bonn.de
This article was submitted to Neuroenergetics, Nutrition and Brain Health, a section of the journal Frontiers in Neuroscience
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