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)

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

“…Thus, we confirmed that Sepp1 ⌬240 -361 does not bind apoER2, in agreement with previous studies using immunostaining and 75 Se-Sepp1 uptake experiments (30). To characterize the Sepp1 and apoER2 binding, we conducted an ELISAbased solid phase binding assay using immunoaffinity-purified Sepp1.…”

“…The Sepp1 heparin-binding site is located in the N-terminal domain (residues 79 -86) (Fig. 6A) (10), and heparin binding facilitates Sepp1 uptake in the rat L8 muscle cell line (30). It is speculated that heparin binding may facilitate access of proteases to the two H-rich stretches.…”

“…Sepp1 or RAP proteins were immobilized on AminoLink coupling resin (Thermo Fisher Scientific Inc., Rockford, IL) at pH 7.4. A previously described membrane preparation was modified for mouse testis or placenta (30). Briefly, tissues were suspended in ice-cold TNI (150 mM NaCl, 25 mM Tris-HCl, pH 7.5, 2 mM benzamidine, 1 g/ml leupeptin, and 1 g/ml pepstatin) and homogenized with a Polytron homogenizer.…”

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

“…Brain and testis retained selenium better than kidney and muscle, presumably because they have higher expressions of apoER2, which is responsible for endocytosis of plasma Sepp1 (5).…”

“…A short Sepp1 isoform, consisting only of the N-terminal domain, has been identified (but not quantified) in rat plasma along with longer isoforms that contain up to the full 10 selenocysteine residues (4). Tissues take up "long isoform" Sepp1 from plasma via apoER2-mediated endocytosis and utilize its selenium for synthesis of selenoproteins (5,6).…”