Impaired selenoprotein expression in brain triggers striatal neuronal loss leading to co-ordination defects in mice.

Secisbp2 [SECIS (selenocysteine insertion sequence)-binding protein 2] binds to SECIS elements located in the 3'-UTR region of eukaryotic selenoprotein mRNAs. It facilitates the incorporation of the rare amino acid selenocysteine in response to UGA codons. Inactivation of Secisbp2 in hepatocytes greatly reduced selenoprotein levels. Neuron-specific inactivation of Secisbp2 (CamK-Cre; Secisbp2fl/fl) reduced cerebral expression of selenoproteins to a lesser extent than inactivation of tRNA[Ser]Sec. This allowed us to study the development of cortical PV (parvalbumin)+ interneurons, which are completely lost in tRNA[Ser]Sec mutants. PV+ interneuron density was reduced in the somatosensory cortex, hippocampus and striatum. In situ hybridization for Gad67 (glutamic acid decarboxylase 67) confirmed the reduction of GABAergic (where GABA is γ-aminobutyric acid) interneurons. Because of the obvious movement phenotype involving a broad dystonic gait, we suspected basal ganglia dysfunction. Tyrosine hydroxylase expression was normal in substantia nigra neurons and their striatal terminals. However the densities of striatal PV+ and Gad67+ neurons were decreased by 65% and 49% respectively. Likewise, the density of striatal cholinergic neurons was reduced by 68%. Our observations demonstrate that several classes of striatal interneurons depend on selenoprotein expression. These findings may offer an explanation for the movement phenotype of selenoprotein P-deficient mice and the movement disorder and mental retardation described in a patient carrying SECISBP2 mutations.

Expression of Selenoproteins Is Maintained in Mice Carrying Mutations in SECp43, the tRNA Selenocysteine 1 Associated Protein (Trnau1ap).

Selenocysteine tRNA 1 associated protein (Trnau1ap) has been characterized as a tRNA[Ser]Sec-binding protein of 43 kDa, hence initially named SECp43. Previous studies reported its presence in complexes containing tRNA[Ser]Sec implying a role of SECp43 as a co-factor in selenoprotein expression. We generated two conditionally mutant mouse models targeting exons 3+4 and exons 7+8 eliminating parts of the first RNA recognition motif or of the tyrosine-rich domain, respectively. Constitutive inactivation of exons 3+4 of SECp43 apparently did not affect the mice or selenoprotein expression in several organs. Constitutive deletion of exons 7+8 was embryonic lethal. We therefore generated hepatocyte-specific Secp43 knockout mice and characterized selenoprotein expression in livers of mutant mice. We found no significant changes in the levels of 75Se-labelled hepatic proteins, selenoprotein levels as determined by Western blot analysis, enzymatic activity or selenoprotein mRNA abundance. The methylation pattern of tRNA[Ser]Sec remained unchanged. Truncated Secp43 Δ7,8mRNA increased in Secp43-mutant livers suggesting auto-regulation of Secp43 mRNA abundance. We found no signs of liver damage in Secp433-mutant mice, but neuron-specific deletion of exons 7+8 impaired motor performance, while not affecting cerebral selenoprotein expression or cerebellar development. These findings suggest that the targeted domains in the SECp43 protein are not essential for selenoprotein biosynthesis in hepatocytes and neurons. Whether the remaining second RNA recognition motif plays a role in selenoprotein biosynthesis and which other cellular process depends on SECp43 remains to be determined.

Secisbp2 is essential for embryonic development and enhances selenoprotein expression.

AIMS: The selenocysteine insertion sequence (SECIS)-binding protein 2 (Secisbp2) binds to SECIS elements located in the 3'-untranslated region of eukaryotic selenoprotein mRNAs. Selenoproteins contain the rare amino acid selenocysteine (Sec). Mutations in SECISBP2 in humans lead to reduced selenoprotein expression thereby affecting thyroid hormone-dependent growth and differentiation processes. The most severe cases also display myopathy, hearing impairment, male infertility, increased photosensitivity, mental retardation, and ataxia. Mouse models are needed to understand selenoprotein-dependent processes underlying the patients' pleiotropic phenotypes. RESULTS: Unlike tRNA[Ser]Sec-deficient embryos, homozygous Secisbp2-deleted embryos implant, but fail before gastrulation. Heterozygous inactivation of Secisbp2 reduced the amount of selenoprotein expressed, but did not affect the thyroid hormone axis or growth. Conditional deletion of Secisbp2 in hepatocytes significantly decreased selenoprotein expression. Unexpectedly, the loss of Secisbp2 reduced the abundance of many, but not all, selenoprotein mRNAs. Transcript-specific and gender-selective effects on selenoprotein mRNA abundance were greater in Secisbp2-deficient hepatocytes than in tRNA[Ser]Sec-deficient cells. Despite the massive reduction of Dio1 and Sepp1 mRNAs, significantly more corresponding protein was detected in primary hepatocytes lacking Secisbp2 than in cells lacking tRNA[Ser]Sec. Regarding selenoprotein expression, compensatory nuclear factor, erythroid-derived, like 2 (Nrf2)-dependent gene expression, or embryonic development, phenotypes were always milder in Secisbp2-deficient than in tRNA[Ser]Sec-deficient mice. INNOVATION: We report the first Secisbp2 mutant mouse models. The conditional mutants provide a model for analyzing Secisbp2 function in organs not accessible in patients. CONCLUSION: In hepatocyte-specific conditional mouse models, Secisbp2 gene inactivation is less detrimental than tRNA[Ser]Sec inactivation. A role of Secisbp2 in stabilizing selenoprotein mRNAs in vivo was uncovered.

Post-transcriptional control of selenoprotein biosynthesis.

Selenoproteins are defined as proteins containing the 21st proteinogenic amino acid, selenocysteine (Sec). Sec is encoded by UGA (STOP) codons which are re-coded to Sec by the presence of a selenocysteine insertion sequence (SECIS) element in the 3'-untranslated region of selenoprotein mRNAs. The SECIS element is bound by several proteins, including SECIS-binding protein 2 (SBP2). Translation of selenoproteins critically depends on the integrity of the SECIS element - SBP2 interaction. Mutations in a SECIS element can abrogate expression of the respective selenoprotein. Mutations in SBP2 impinge on biosynthesis of a subset of selenoproteins and lead to a syndrome including hormonal, neurological, immunological symptoms as well as myopathy. Several other RNA-binding proteins are involved in selenoprotein translation and mediate the hierarchical response of selenoproteins to selenium deficiency. Global inhibition of selenoprotein translation is lethal in the mouse and hypomorphic mutations in selenocysteine synthase in humans leads to Progressive Cerebello Cerebral Atrophy, a neurodevelopmental and neurodegenerative disease in pediatric patients.