SAT-564 The Role Of Se Recycling In Regulating Iodothyronine-deiodinases In The Skeletal Muscle

T3 action is locally modulated by the intracellular type 2 (D2) and type 3 (D3) iodothyronine-deiodinases which convert the prohormone T4 to active T3, and inactivate T4 and T3, respectively. The synchronized expression of D3 followed by D2 is necessary for proper muscle development and regeneration. D2 and D3 are selenoproteins containing selenium (Se) at their active site in the form of the rare amino acid selenocysteine (Sec) which is incorporated during translation by the recoding of an UGA stop codon. Consequently, dietary Se is essential for TH metabolism. During times of Se deficiency, selenocysteine lyase (Scly) decomposes Sec to Se allowing it to be recycled into new selenoproteins. We hypothesized that Scly is involved in preserving the production of the deiodinases and other selenoproteins when dietary Se levels are low. During the neonatal period when muscle is maturing, and deiodinase expression is high, we found that between P1 and P15 Scly expression increases 2-fold in soleus and 15-fold in the extensor digitalis longus (EDL) muscle. Further, we found that Scly mRNA expression was 2.5-fold higher in the adult slow oxidative soleus muscle versus the fast glycolytic EDL. Next, we fed male WT and Scly knockout (Scly-/-) mice a Se adequate or deficient diet and evaluated gene expression in the soleus and EDL via qPCR. In WT soleus, low Se diet decreased expression of many selenoproteins, likely in part due to nonsense-mediated decay of the non-translated UGA containing transcripts. In contrast, the expression of these selenoproteins was unchanged in the EDL. Scly-/- mice fed a Se adequate diet demonstrated a trend towards increased Dio2, SelenoN, SelenoM, SelenoP, Txnrd1, and Sephs2 expression in soleus when compared to WT mice on the same diet, but this was abolished under low Se conditions. In the soleus, diet had no effect on SelenoH in WT mice, while notably, SelenoH was significantly decreased in Scly-/- mice fed a low Se diet. In the EDL, expression of SelenoW and SelenoH was unchanged under low Se conditions in WT mice, but was decreased in Scly-/-. This suggests that the expression of a subset of genes relies upon the Se provided by Scly recycling when Se is limiting. Finally, to investigate the possible role of Scly in muscle differentiation, we induced the murine myoblast C2C12 cell line to differentiate via serum withdrawal in the presence of 100 nM total Se to control for the effects of decreased serum on Se levels. Scly mRNA expression reached peak induction by 48h after differentiation. Dio2, Gpx3, SelenoP, SelenoM, SelenoN, and the selenoprotein synthesis factor SBP2 exhibited similar expression profiles. Our results suggest a role for Scly in skeletal muscle development, differentiation, and adult muscle homeostasis. Further, expression of some muscle selenoproteins, including Dio2, appears to be sensitive to low dietary Se when Scly mediated Se recycling is impaired. Funding: NIH-NIDDK