Membrane transporters that clear the neurotransmitter glutamate from synapses are driven by symport of sodium ions and counter-transport of a potassium ion. Previous crystal structures of a homologous archaeal sodium and aspartate symporter showed that a dedicated transport domain carries the substrate and ions across the membrane. Here, we report new crystal structures of this homologue in ligand-free and ions-only bound outward- and inward-facing conformations. We show that after ligand release, the apo transport domain adopts a compact and occluded conformation that can traverse the membrane, completing the transport cycle. Sodium binding primes the transport domain to accept its substrate and triggers extracellular gate opening, which prevents inward domain translocation until substrate binding takes place. Furthermore, we describe a new cation-binding site ideally suited to bind a counter-transported ion. We suggest that potassium binding at this site stabilizes the translocation-competent conformation of the unloaded transport domain in mammalian homologues.DOI: http://dx.doi.org/10.7554/eLife.02283.001

Edited by Joel M. Gottesfeld Ethanol (EtOH) is a teratogen, but its teratogenic mechanisms are not fully understood. The alcohol form of vitamin A (retinol/ ROL) can be oxidized to all-trans-retinoic acid (RA), which plays a critical role in stem cell differentiation and development. Using an embryonic stem cell (ESC) model to analyze EtOH's effects on differentiation, we show here that EtOH and acetaldehyde, but not acetate, increase differentiation-associated mRNA levels, and that EtOH decreases pluripotency-related mRNAs. Using reporter assays, ChIP assays, and retinoic acid receptor-␥ (RAR␥) knockout ESC lines generated by CRISPR/ Cas9 and homologous recombination, we demonstrate that EtOH signals via RAR␥ binding to RA response elements (RAREs) in differentiation-associated gene promoters or enhancers. We also report that EtOH-mediated increases in homeobox A1 (Hoxa1) and cytochrome P450 family 26 subfamily A member 1 (Cyp26a1) transcripts, direct RA target genes, require the expression of the RA-synthesizing enzyme, aldehyde dehydrogenase 1 family member A2 (Aldh1a2), suggesting that EtOH-mediated induction of Hoxa1 and Cyp26a1 requires ROL from the serum. As shown with CRISPR/Cas9 knockout lines, the retinol dehydrogenase gene Rdh10 and a functional RARE in the ROL transporter stimulated by retinoic acid 6 (Stra6) gene are required for EtOH induction of Hoxa1 and Cyp26a1. We conclude that EtOH stimulates stem cell differentiation by increasing the influx and metabolism of ROL for downstream RAR␥-dependent transcription. In stem cells, EtOH may shift cell fate decisions to alter developmental outcomes by increasing endogenous ROL/RA signaling via increased Stra6 expression and ROL oxidation. Complex regulatory circuitry is required for maintaining the properties of stem cells so that symmetric self-renewal is not diverted prematurely to drive differentiation toward terminal cell fates (1). High levels of blood ethanol (EtOH) lead to aberrant regulation of normal differentiation in developing embryos and fetuses, making EtOH a teratogen (2-4). Unraveling the mechanisms by which EtOH interferes with endogenous cell signaling pathways that control differentiation is critical for understanding EtOH-mediated toxic effects which lead to disease states that arise during development, i.e. fetal alcohol spectrum disorders (FASD) 2 (5) and diseases such as cancers, which are caused in part by changes in cell plasticity (6). Of particular importance in stem cell differentiation is the interaction between EtOH and the vitamin A (retinol, ROL) metabolite all-trans-retinoic acid (RA), which lies at the nexus of physiological differentiation of stem cells (7, 8). Dysregulated RA signaling (from either supraphysiological or subphysiological levels) leads to several teratogenic phenotypes in common with EtOH (9-11). Several studies have shown interactions between EtOH and RA signaling (4, 12-16), possibly because of the similarities in metabolism between ROL and EtOH. Both EtOH and ROL metabolism rely on parallel two-step o...

Background: Ethanol (EtOH) is a teratogen that causes severe birth defects, but the mechanisms by which EtOH affects stem cell differentiation are unclear. Our goal here is to examine the effects of EtOH and its metabolites, acetaldehyde (AcH) and acetate, on embryonic stem cell (ESC) differentiation.Methods: We designed ESC lines in which aldehyde dehydrogenase (ALDH2, NCBI#11669) and acyl-CoA synthetase short-chain family member 2 (ACSS2, NCBI#60525) were knocked out by CRISPR-Cas9 technology. We selected these genes because of their key roles in EtOH oxidation in order to dissect the effects of EtOH metabolism on differentiation.Results: By using kinetic assays, we confirmed that AcH is primarily oxidized by ALDH2 rather than ALDH1A2. We found increases in mRNAs of differentiation-associated genes (Hoxa1, Cyp26a1, and RARb2) upon EtOH treatment of WT and Acss2 À/À ESCs, but not Aldh2 À/À ESCs. The absence of ALDH2 reduced mRNAs of some pluripotency factors (Nanog, Sox2, and Klf4). Treatment of WT ESCs with AcH or 4-hydroxynonenal (4-HNE), another substrate of ALDH2, increased differentiation-associated transcripts compared to levels in untreated cells. mRNAs of genes involved in retinoic acid (RA) synthesis (Stra6 and Rdh10) were also increased by EtOH, AcH, and 4-HNE treatment. Retinoic acid receptor-c (RARc) is required for both EtOH-and AcH-mediated increases in Hoxa1 and Stra6, demonstrating the critical role of RA:RARc signaling in AcH-induced ESC differentiation.Conclusions: ACSS2 knockouts showed no changes in differentiation phenotype, while pluripotency-related transcripts were decreased in ALDH2 knockout ESCs. We demonstrate that AcH increases differentiation-associated mRNAs in ESCs via RARc.

Ethanol (EtOH) is a recreationally ingested compound that is both teratogenic and carcinogenic in humans. Because of its abundant consumption worldwide and the vital role of stem cells in the formation of birth defects and cancers, delineating the effects of EtOH on stem cell function is currently an active and urgent pursuit of scientific investigation to explicate some of the mechanisms contributing to EtOH toxicity. Stem cells represent a primordial, undifferentiated phase of development; thus encroachment on normal physiologic processes of differentiation into terminal lineages by EtOH can greatly alter the function of progenitors and terminally differentiated cells, leading to pathological consequences that manifest as fetal alcohol spectrum disorders and cancers. In this review we explore the disruptive role of EtOH in differentiation of stem cells. Our primary objective is to elucidate the mechanisms by which EtOH alters differentiation-related gene expression and lineage specifications, thus modifying stem cells to promote pathological outcomes. We additionally review the effects of a reactive metabolite of EtOH, acetaldehyde (AcH), in causing both differentiation defects in stem cells as well as genomic damage that incites cellular aging and carcinogenesis.

Although there is insufficient evidence suggesting that infliximab is an appropriate alternative to conventional treatment options as first-line therapy for sarcoidosis, the preliminary outlook on its use in treatment-refractory cases is promising.