Roles of Selenoprotein M (SELENOM) in Hypothalamic Leptin Signaling and Calcium Regulation.

Abstract

Selenium is an essential trace element that is critical for human health. The biological effects of selenium are largely mediated by selenoproteins, a unique class of proteins that contain selenocysteine (Sec) as an integral part of their polypeptide chain. Selenoprotein M (SELENOM) is an ER-resident thiol-disulfide oxidoreductase that is most abundant in the brain. It contains a thioredoxin-like domain (cysteine-X-X-selenocysteine) that catalyzes thiol-disulfide exchange. It has been reported that SELENOM has neuroprotective functions and is implicated in regulation of Ca2+ homeostasis. Our group previously published that Selenom-/- mice display increased weight gain, elevated white adipose tissue deposition, and impaired hypothalamic leptin sensitivity compared to wild-type mice, suggesting a role for SELENOM in energy homeostasis. Therefore, we performed a series of studies using in vivo and in vitro models to investigate the specific influence of SELENOM on hypothalamic leptin signaling, ER stress, and Ca2+ signaling. The evidence gathered in this study revealed that SELENOM promotes hypothalamic leptin signaling, inhibits ER stress, and regulates Ca2+ influx. To further delineate the underlying mechanism, we then assessed the subcellular localization and binding partners of SELENOM. Using co-immunoprecipitation and liquid chromatography–mass spectrometry/mass spectrometry (LC-MS/MS) to screen for SELENOM-binding partners, we discovered and verified two proteins, microtubule associated protein 6 (MAP6) and choline-phosphate cytidylyltransferase 1 alpha (PCYT1A). In addition, we found that SELENOM localizes not only in ER but also in mitochondria-associated ER-membranes (MAMs), suggesting a potential role of SELENOM in regulation of Ca2+ signaling. Finally, we performed microarray analysis using both Selenom-/- hypothalamic tissue and mHypoE-44 cells to identify the genes and signaling pathways most affected by SELENOM. Our results revealed 11 genes that were significantly altered by SELENOM deficiency, including thioredoxin-interacting protein (TXNIP), a negative regulator of the thioredoxin (TXN) system. SELENOM deficiency also significantly reduced TXN activity in both hypothalamic tissue and mHypoE-44 cells. In summary, our studies reveal that SELENOM promotes hypothalamic leptin signaling, potentially due to its functions in ER stress, Ca2+ signaling, and the hypothalamic TXN system. In addition, we further determined that SELENOM localizes in MAMs and interacts with MAP6 and PCYT1A, suggesting an important role in MAMs-modulated Ca2+ homeostasis.