br Experimental Procedures Full experimental procedures are

Experimental Procedures Full experimental procedures are provided in the Supplemental Information.
Author Contributions
Introduction Vertebrate embryonic development involves stepwise cell-fate conversion events. Neural induction is one of the most critical developmental events occurring during gastrulation, whereby a subset of epiblast cells acquires a neuroectodermal fate (Tam and Zhou, 1996). Neuroectodermal cells develop into all of the regional neural progenitors and ultimately the ibuprofen msds and spinal cord; thus, a failure of neural induction results in severe neural tube defects (NTDs) (Copp et al., 2003). Ninety percent of affected pregnancies will be terminated (Herrera-Araujo, 2016), with the survivors facing a risk of prenatal to postnatal lethality or lifelong disability, including neurological and cognitive complications. However, knowledge pertaining to abnormal neural induction in NTDs remains limited. Previous studies have revealed that the suppression of bone morphogenetic protein (BMP) pathways and coordinated regulation of the fibroblast growth factor (FGF), transforming growth factor β, and calcineurin signaling pathways are central to the extrinsic control of neural induction (Cho et al., 2014; Liu et al., 2016a; Stavridis et al., 2007). These extracellular signals subsequently converge on specific lineage-determining transcription factors to regulate neural induction. For example, BMP/SMAD signaling suppresses commitment to the neural lineage via the induction of ID proteins (Ying et al., 2003a). The FGF-ERK1/2-PARP1 cascade directly activates Pax6 expression to advance human neuroectodermal specification (Yoo et al., 2011; Zhang et al., 2010). Various transcription factors have also been found to lead undifferentiated cells to adopt a neural fate intrinsically. The zinc-finger nuclear protein Zfp521 is required and sufficient to drive the intrinsic transition of embryonic stem cell (ESC) differentiation into neuroectodermal cells (Kamiya et al., 2011). The POU domain transcription factor Oct6 initiates internal neural induction programs by directly activating neural-lineage genes, such as Zfp521 and Pax6 (Zhu et al., 2014), when expressed in the epiblast and the early neuroectoderm during development (Zwart et al., 1996). Disruption of the hierarchies of these signaling or gene-regulatory networks during early embryonic development may underlie abnormal neural induction in NTDs. Although NTDs have long been proposed to be primarily triggered by external environmental stresses, the specific environmental sensors that function to translate these environmental stresses into intracellular gene-regulatory network activity during the neural induction stage remain largely unexplored. Sirt1, a conserved NAD+-dependent lysine deacetylase, has been reported to be an environmental sensor that mediates intracellular responses to environmental states, such as redox state, nutrient availability, and DNA damage, in various organisms. In response to DNA damage or oxidative stress, Sirt1 is activated to repress p53-dependent apoptosis in mammalian cells (Luo et al., 2001), nuclear p53-mediated transactivation of proapoptotic genes in mouse cells (Han et al., 2008), and FOXO3-induced cell death in human cells (Brunet et al., 2004). Starvation-induced SIRT1 activation is required for the initiation of autophagy to cope with the lack of external nutrients in eukaryotes (Chang et al., 2015; Lee et al., 2008). Recently, Sirt1 was also found to be required to suppress neurogenesis or to induce oligodendrogenesis in neural stem/progenitor cells (Prozorovski et al., 2008; Stein and Imai, 2014), suggesting a profound role for Sirt1 during neural development.
Results
Discussion Annually, more than 300,000 neonates are born with NTDs worldwide. The risk of NTDs has long been proposed to be increased by environmental stresses, including maternal starvation, fever, and exposure to radiation or other teratogens, while the potential messenger between environmental stresses and NTDs remains unknown. SIRT1 has been reported to regulate the fate decision of neural stem/progenitor cells in response to environmental stimuli (Stein and Imai, 2014). However, NTDs arise when the process of neural tube formation is disrupted, which is an early developmental event. Therefore, whether SIRT1 is involved in environmental stress-induced NTDs remained to be explored. Our study strongly suggests that SIRT1 deacetylase activity is greatly modified by environmental stresses and is responsible for environmental stress-induced abnormal neural induction within the in vitro neural differentiation system of both hESCs and mESCs. Furthermore, in an in vivo radiation-induced NTD model, our results indicated that Sirt1 very likely translates environmental signals in developing embryos during early neural development and that SIRT1 activation critically contributes to environmental stress-induced NTDs.