Supranutritional Selenomethionine During Different Trimesters of Gestation in Beef Cows Alters Gene Expression...

Selenium (Se) is an essential micronutrient. We have previously shown that supranutritional Se-yeast supplementation improves growth and immune function in Se-replete cattle and sheep despite limited impact on selenoprotein and oxidoreductase gene expression, suggesting effects beyond the selenocysteine proteome. This study evaluated the impact of supranutritional Se-yeast during each trimester of pregnancy on transcriptome-wide gene expression in peripheral blood mononuclear cells (PBMCs) at parturition, and the underlying functional categories affected. We administered Se-yeast containing boluses weekly (105 mg Se/wk for 13 wks; 5× the level recommended by NASEM) to crossbred Angus cows during the first (TR1), second (TR2), or third (TR3) trimester of gestation. Control cows (CTR) had continuous access to 120 mg Se/kg mineral supplement as Na selenite (per US FDA regulations) throughout gestation. Blood was collected at parturition, and PBMCs were isolated for RNA sequencing. We identified 3,904 unique differentially expressed genes (DEGs), with 186 upregulated vs. 268 downregulated DEGs across all three Se-yeast TR vs. CTR comparisons, while selenoprotein and oxidoreductase gene expression remained unchanged. Downregulation increased across GO categorization levels, from balanced changes in Cellular Component terms (51 upregulated vs. 52 downregulated) through Molecular Function (13 vs. 139) and Biological Process terms (23 vs. 595), then decreased for KEGG pathways (28 vs. 76) and protein class terms (6 vs. 16). We hypothesize that supranutritional Se-yeast acts primarily through nonspecific SeMet incorporation into the broader proteome rather than through specific selenoprotein synthesis. These PBMC DEG changes may also affect disease risk in periparturient beef cattle, though further investigation is warranted to test these hypotheses.