A molecule that treats multiple age-related diseases would have a major impact on global health and economics. The SIRT1 deacetylase has drawn attention in this regard as a target for drug design. Yet controversy exists around the mechanism of sirtuin-activating compounds (STACs). We found that specific hydrophobic motifs found in SIRT1 substrates such as PGC-1α and FOXO3a facilitate SIRT1 activation by STACs. A single amino acid in SIRT1, Glu230, located in a structured N-terminal domain, was critical for activation by all previously reported STAC scaffolds and a new class of chemically distinct activators. In primary cells reconstituted with activation-defective SIRT1, the metabolic effects of STACs were blocked. Thus, SIRT1 can be directly activated through an allosteric mechanism common to chemically diverse STACs.

Ca(2+) binding to synaptotagmin 1 triggers fast exocytosis of synaptic vesicles that have been primed for release by SNARE-complex assembly. Besides synaptotagmin 1, fast Ca(2+)-triggered exocytosis requires complexins. Synaptotagmin 1 and complexins both bind to assembled SNARE complexes, but it is unclear how their functions are coupled. Here we propose that complexin binding activates SNARE complexes into a metastable state and that Ca(2+) binding to synaptotagmin 1 triggers fast exocytosis by displacing complexin from metastable SNARE complexes. Specifically, we demonstrate that, biochemically, synaptotagmin 1 competes with complexin for SNARE-complex binding, thereby dislodging complexin from SNARE complexes in a Ca(2+)-dependent manner. Physiologically, increasing the local concentration of complexin selectively impairs fast Ca(2+)-triggered exocytosis but retains other forms of SNARE-dependent fusion. The hypothesis that Ca(2+)-induced displacement of complexins from SNARE complexes triggers fast exocytosis accounts for the loss-of-function and gain-of-function phenotypes of complexins and provides a molecular explanation for the high speed and synchronicity of fast Ca(2+)-triggered neurotransmitter release.

SIRT3 is a major mitochondrial NAD؉ -dependent protein deacetylase playing important roles in regulating mitochondrial metabolism and energy production and has been linked to the beneficial effects of exercise and caloric restriction. SIRT3 is emerging as a potential therapeutic target to treat metabolic and neurological diseases. We report the first sets of crystal structures of human SIRT3, an apo-structure with no substrate, a structure with a peptide containing acetyl lysine of its natural substrate acetyl-CoA synthetase 2, a reaction intermediate structure trapped by a thioacetyl peptide, and a structure with the dethioacetylated peptide bound. These structures provide insights into the conformational changes induced by the two substrates required for the reaction, the acetylated substrate peptide and NAD ؉ . In addition, the binding study by isothermal titration calorimetry suggests that the acetylated peptide is the first substrate to bind to SIRT3, before NAD ؉ . These structures and biophysical studies provide key insight into the structural and functional relationship of the SIRT3 deacetylation activity.Sirtuins are class III histone deacetylases that couple lysine deacetylation with NAD ϩ hydrolysis and are highly conserved in prokaryotes and eukaryotes (1). Mammals possess seven sirtuins, SIRT1-7, that occupy different subcellular compartments such as the nucleus (SIRT1, -6, -7), cytoplasm (SIRT2), and the mitochondria (SIRT3, -4, and -5) (2). They deacetylate lysines not only on histone substrates (3, 4) but also on nonhistone substrates such as p53 tumor suppressor protein (5), Foxo transcription factors (6, 7), PGC-1␣ (8), ␣-tubulin (9), acetyl-CoA synthetases (10 -12), and glutamate dehydrogenase (13). SIRT4 and SIRT6 have been shown to have ADP-ribosyltransferase activity (14 -16). Sirtuins have been reported to play important roles in gene silencing (17), cell cycle regulation (18,19), metabolism (8, 10 -12, 14, 20 -22), apoptosis (5, 23, 24), the lifespan-extension effects of calorie restriction (25,26), and circadian rhythms (27)(28)(29)(30) (50), and SIRT5 (51). Sirtuins contain a conserved enzymatic core with two domains; that is, a large Rossmann fold domain that binds NAD ϩ and a small domain formed by two insertions of the large domain that binds to a zinc atom. The acetylated peptide substrate binds to the cleft between the two domains. Some of the known structures are apo structures with sirtuin protein alone, whereas others are bound to acetylated peptide substrate and/or NAD ϩ and its analogs. These structures revealed the mechanism of action for the deacetylation activity and substrate specificity.SIRT3 localizes in mitochondria (13, 52-54) and is a major mitochondrial deacetylase. Hyperacetylation of mitochondrial proteins have been observed in SIRT3 knock-out mice (13, 55). Several key enzymes involved in energy production in the mitochondria have been identified as SIRT3 substrates. Acetyl-CoA synthetase 2 (AceCS2) 2 converts acetate into acetyl-CoA in the mitochondria. Deacetyla...

The NAD-dependent protein lysine deacylases of the Sirtuin family regulate various physiological functions, from energy metabolism to stress responses. The human Sirtuin isoforms, SIRT1-7, are considered attractive therapeutic targets for aging-related diseases, such as type 2 diabetes, inflammatory diseases and neurodegenerative disorders. We review the status of Sirtuin-targeted drug discovery and development. Potent and selective pharmacological Sirt1 activators and inhibitors are available, and initial clinical trials have been carried out. Several promising inhibitors and activators have also been described for other isoforms. Progress in understanding the mechanisms of Sirtuin modulation by such compounds provides a rational basis for further drug development.

SummaryThe function of synaptotagmin as a Ca 2+ sensor in neurotransmitter release involves Ca 2+ -dependent phospholipid binding to its two C 2 domains, but this activity alone does not explain why Ca 2+ binding to the C 2 B domain is more critical for release than Ca 2+ binding to the C 2 A domain. Synaptotagmin also binds to SNARE complexes, which are central components of the membrane fusion machinery, and displaces complexins from the SNAREs. However, it is unclear how phospholipid binding to synaptotagmin is coupled to SNARE binding and complexin displacement. Using supported lipid bilayers deposited within microfluidic channels, we now show that Ca 2+ induces simultaneous binding of synaptotagmin to phospholipid membranes and SNARE complexes, resulting in an intimate quaternary complex that we name SSCAP complex. Mutagenesis experiments show that Ca 2+ binding to the C 2 B domain is critical for SSCAP complex formation and displacement of complexin, providing a clear rationale for the preponderant role of the C 2 B domain in release. This and other correlations between the effects of mutations on SSCAP complex formation and their functional effects in vivo suggest a key role for this complex in release. We propose a model whereby the highly positive electrostatic potential at the tip of the SSCAP complex helps to induce membrane fusion during release.

The SM (Sec1/Munc18-like) protein Munc18-1 and the soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) proteins syntaxin-1, SNAP-25, and synaptobrevin/VAMP (vesicle-associated membrane protein) constitute the core fusion machinery for synaptic vesicle exocytosis. Strikingly, Munc18-1 interacts with neuronal SNARE proteins in two distinct modes (i.e., with isolated syntaxin-1 alone in a "closed" conformation and with assembled SNARE complexes containing syntaxin-1 in an "open" conformation). However, it is unclear whether the two modes of Munc18/SNARE interactions are linked. We now show that both Munc18/ SNARE interaction modes involve the same low-affinity binding of the extreme syntaxin-1 N terminus to Munc18-1, suggesting that this binding connects the two Munc18/SNARE interaction modes to each other. Using transfected cells as an in vitro assay system, we demonstrate that truncated syntaxins lacking a transmembrane region universally block exocytosis, but only if they contain a free intact N terminus. This block is enhanced by coexpression of either Munc18-1 or SNAP-25, suggesting that truncated syntaxins block exocytosis by forming an untethered inhibitory SNARE complex/Munc18-1 assembly in which the N-terminal syntaxin/Munc18 interaction is essential. Introduction of an N-terminal syntaxin peptide that disrupts this assembly blocks neurotransmitter release in the calyx of Held synapse, whereas a mutant peptide that does not disrupt the SNARE complex/Munc18 assembly has no effect. Viewed together, our data indicate that binding of Munc18 to the syntaxin N terminus unites different modes of Munc18/SNARE interactions and is essential for exocytic membrane fusion.

OBJECTIVETo test the graft-promoting effects of mesenchymal stem cells (MSCs) in a cynomolgus monkey model of islet/bone marrow transplantation.RESEARCH DESIGN AND METHODSCynomolgus MSCs were obtained from iliac crest aspirate and characterized through passage 11 for phenotype, gene expression, differentiation potential, and karyotype. Allogeneic donor MSCs were cotransplanted intraportally with islets on postoperative day (POD) 0 and intravenously with donor marrow on PODs 5 and 11. Recipients were followed for stabilization of blood glucose levels, reduction of exogenous insulin requirement (EIR), C-peptide levels, changes in peripheral blood T regulatory cells, and chimerism. Destabilization of glycemia and increases in EIR were used as signs of rejection; additional intravenous MSCs were administered to test the effect on reversal of rejection.RESULTSMSC phenotype and a normal karyotype were observed through passage 11. IL-6, IL-10, vascular endothelial growth factor, TGF-β, hepatocyte growth factor, and galectin-1 gene expression levels varied among donors. MSC treatment significantly enhanced islet engraftment and function at 1 month posttransplant (n = 8), as compared with animals that received islets without MSCs (n = 3). Additional infusions of donor or third-party MSCs resulted in reversal of rejection episodes and prolongation of islet function in two animals. Stable islet allograft function was associated with increased numbers of regulatory T-cells in peripheral blood.CONCLUSIONSMSCs may provide an important approach for enhancement of islet engraftment, thereby decreasing the numbers of islets needed to achieve insulin independence. Furthermore, MSCs may serve as a new, safe, and effective antirejection therapy.

The sirtuins SIRT1, SIRT2, and SIRT3 are NAD(+) dependent deacetylases that are considered potential targets for metabolic, inflammatory, oncologic, and neurodegenerative disorders. Encoded library technology (ELT) was used to affinity screen a 1.2 million heterocycle enriched library of DNA encoded small molecules, which identified pan-inhibitors of SIRT1/2/3 with nanomolar potency (e.g., 11c: IC50 = 3.6, 2.7, and 4.0 nM for SIRT1, SIRT2, and SIRT3, respectively). Subsequent SAR studies to improve physiochemical properties identified the potent drug like analogues 28 and 31. Crystallographic studies of 11c, 28, and 31 bound in the SIRT3 active site revealed that the common carboxamide binds in the nicotinamide C-pocket and the aliphatic portions of the inhibitors extend through the substrate channel, explaining the observable SAR. These pan SIRT1/2/3 inhibitors, representing a novel chemotype, are significantly more potent than currently available inhibitors, which makes them valuable tools for sirtuin research.