Placenta-specific Slc38a2/SNAT2 knockdown causes fetal growth restriction in mice.

Fetal growth restriction (FGR) is a complication of pregnancy that reduces birth weight, markedly increases infant mortality and morbidity and is associated with later-life cardiometabolic disease. No specific treatment is available for FGR. Placentas of human FGR infants have low abundance of sodium-coupled neutral amino acid transporter 2 (Slc38a2/SNAT2), which supplies the fetus with amino acids required for growth. We determined the mechanistic role of placental Slc38a2/SNAT2 deficiency in the development of restricted fetal growth, hypothesizing that placenta-specific Slc38a2 knockdown causes FGR in mice. Using lentiviral transduction of blastocysts with a small hairpin RNA (shRNA), we achieved 59% knockdown of placental Slc38a2, without altering fetal Slc38a2 expression. Placenta-specific Slc38a2 knockdown reduced near-term fetal and placental weight, fetal viability, trophoblast plasma membrane (TPM) SNAT2 protein abundance, and both absolute and weight-specific placental uptake of the amino acid transport System A tracer, 14C-methylaminoisobutyric acid (MeAIB). We also measured human placental SLC38A2 gene expression in a well-defined term clinical cohort and found that SLC38A2 expression was decreased in late-onset, but not early-onset FGR, compared with appropriate for gestational age (AGA) control placentas. The results demonstrate that low placental Slc38a2/SNAT2 causes FGR and could be a target for clinical therapies for late-onset FGR.

Functional RNA interference (RNAi) screen identifies system A neutral amino acid transporter 2 (SNAT2) as a mediator of arsenic-induced endoplasmic reticulum stress.

Exposure to the toxic metalloid arsenic is associated with diabetes and cancer and causes proteotoxicity and endoplasmic reticulum (ER) stress at the cellular level. Adaptive responses to ER stress are implicated in cancer and diabetes; thus, understanding mechanisms of arsenic-induced ER stress may offer insights into pathogenesis. Here, we identify genes required for arsenite-induced ER stress response in a genome-wide RNAi screen. Using an shRNA library targeting ∼20,000 human genes, together with an ER stress cell model, we performed flow cytometry-based cell sorting to isolate cells with defective response to arsenite. Our screen discovered several genes modulating arsenite-induced ER stress, including sodium-dependent neutral amino acid transporter, SNAT2. SNAT2 expression and activity are up-regulated by arsenite, in a manner dependent on activating transcription factor 4 (ATF4), an important mediator of the integrated stress response. Inhibition of SNAT2 expression or activity or deprivation of its primary substrate, glutamine, specifically suppressed ER stress induced by arsenite but not tunicamycin. Induction of SNAT2 is coincident with the activation of the nutrient-sensing mammalian target of rapamycin (mTOR) pathway, which is at least partially required for arsenite-induced ER stress. Importantly, inhibition of the SNAT2 or the System L transporter, LAT1, suppressed mTOR activation by arsenite, supporting a role for these transporters in modulating amino acid signaling. These findings reveal SNAT2 as an important and specific mediator of arsenic-induced ER stress, and suggest a role for aberrant mTOR activation in arsenic-related human diseases. Furthermore, this study demonstrates the utility of RNAi screens in elucidating cellular mechanisms of environmental toxins.