Nicotinamide Pathway-Dependent Sirt1 Activation Restores Calcium Homeostasis to Achieve Neuroprotection in Spinocerebellar Ataxia Type 7.

Sirtuin 1 (Sirt1) is a NAD+-dependent deacetylase capable of countering age-related neurodegeneration, but the basis of Sirt1 neuroprotection remains elusive. Spinocerebellar ataxia type 7 (SCA7) is an inherited CAG-polyglutamine repeat disorder. Transcriptome analysis of SCA7 mice revealed downregulation of calcium flux genes accompanied by abnormal calcium-dependent cerebellar membrane excitability. Transcription-factor binding-site analysis of downregulated genes yielded Sirt1 target sites, and we observed reduced Sirt1 activity in the SCA7 mouse cerebellum with NAD+ depletion. SCA7 patients displayed increased poly(ADP-ribose) in cerebellar neurons, supporting poly(ADP-ribose) polymerase-1 upregulation. We crossed Sirt1-overexpressing mice with SCA7 mice and noted rescue of neurodegeneration and calcium flux defects. NAD+ repletion via nicotinamide riboside ameliorated disease phenotypes in SCA7 mice and patient stem cell-derived neurons. Sirt1 thus achieves neuroprotection by promoting calcium regulation, and NAD+ dysregulation underlies Sirt1 dysfunction in SCA7, indicating that cerebellar ataxias exhibit altered calcium homeostasis because of metabolic dysregulation, suggesting shared therapy targets.

Nar1 deficiency results in shortened lifespan and sensitivity to paraquat that is rescued by increased expression of mitochondrial superoxide dismutase.

Saccharomyces cerevisiae Nar1p is an essential Fe/S protein that exhibits striking similarity to bacterial iron-only hydrogenases. Nar1p is required for the maturation of cytosolic and nuclear, but not of mitochondrial Fe/S proteins, and plays a role in modulating sensitivity to oxygen in both yeast and Caenorhabditis elegans through unknown mechanisms. Here we report that Nar1 deficiency results in shortened lifespan and sensitivity to paraquat that is rescued by increased expression of mitochondrial superoxide dismutase. These data suggest that Nar1p promotes protection against oxidative stress and define a new role for Nar1p in promoting replicative lifespan.

Psidin is required in Drosophila blood cells for both phagocytic degradation and immune activation of the fat body.

Phagocytic blood cells are critical to innate immune defense: They internalize and destroy microbial invaders and produce signals that trigger other immune responses. Despite this central role, the in vivo contributions of phagocytosis to systemic immune activation are not well understood. Drosophila has proven a fruitful model for the investigation of evolutionarily conserved innate immune mechanisms, including NF-kappaB-dependent transcriptional induction, RNAi in antiviral responses, and phagocytosis. The phagocytes of Drosophila encounter bacterial invaders early in infection and contribute to survival of infection. Phagocytosis in flies and mammals is highly homologous: Both rely on scavenger receptors, opsonins, and actin rearrangements for engulfment; have phagosomal cysteine proteases active at low pH; and can be subverted by similar intracellular pathogens. Although the role of Drosophila phagocytes in the activation of other immune tissues has not been clear, we show that induction of the antibacterial-peptide gene Defensin in the fat body during infection requires blood-cell contributions. We identify a gene, psidin, that encodes a lysosomal protein required in the blood cells for both degradation of engulfed bacteria and activation of fat-body Defensin. These data establish a role for the phagocytic blood cells of Drosophila in detection of infection and activation of the humoral immune response.

Cooperative control of Drosophila immune responses by the JNK and NF-kappaB signaling pathways.

Jun N-terminal kinase (JNK) signaling is a highly conserved pathway that controls both cytoskeletal remodeling and transcriptional regulation in response to a wide variety of signals. Despite the importance of JNK in the mammalian immune response, and various suggestions of its importance in Drosophila immunity, the actual contribution of JNK signaling in the Drosophila immune response has been unclear. Drosophila TAK1 has been implicated in the NF-kappaB/Relish-mediated activation of antimicrobial peptide genes. However, we demonstrate that Relish activation is intact in dTAK1 mutant animals, and that the immune response in these mutant animals was rescued by overexpression of a downstream JNKK. The expression of a JNK inhibitor and induction of JNK loss-of-function clones in immune responsive tissue revealed a general requirement for JNK signaling in the expression of antimicrobial peptides. Our data indicate that dTAK1 is not required for Relish activation, but instead is required in JNK signaling for antimicrobial peptide gene expression.

TGF-beta activated kinase-1: new insights into the diverse roles of TAK1 in development and immunity.

A number of recent publications have examined the role of TAK1 in model systems ranging from fly to mouse. Rather than fit into a clearly defined linear molecular pathway, TAK1 seems to act in a signaling nexus that responds to a variety of upstream signals, including inflammatory molecules and developmental cues. TAK1 then influences a number of downstream processes ranging from innate immune responses to patterning and differentiation via JNK, NFkappaB and TCFbeta-catenin signaling. These differences in function are not simply a matter of cell type. For example, NFkappaB signaling in a particular cell may or may not require TAK1 depending on the nature of the activating signal. Interestingly, the multi-task functionality of TAK1 is conserved between vertebrate and invertebrate species. Studies of TAK1 in multiple experimental systems is likely to reveal more roles for this kinase and also elucidate mechanisms by which other signaling molecules fulfill diverse signaling roles. Here we provide an overview of the data concerning TAK1 from its discovery to more recent findings and provide a synthesis of the conclusions that have arisen from the multiple model systems and experimental approaches.

A peptide that antagonizes TCR-mediated reactions with both syngeneic and allogeneic agonists: functional and structural aspects.

We identify and consider some characteristics of a peptide antagonist for the Ag-specific receptor on 2C cells (the 2C TCR). The peptide, GNYSFYAL (called GNY), binds to H-2K(b), and a very high-resolution crystal structure of the GNY-K(b) complex at 1.35 A is described. Although the GNY peptide does not bind to L(d), the potency of GNY-K(b) as an antagonist is evident from its ability to specifically inhibit 2C TCR-mediated reactions to an allogenic agonist complex (QLSPFPFDL-L(d)), as well as to a syngeneic agonist complex (SIYRYYGL-K(b)). The crystal structure and the activities of alanine-substituted peptide variants point to the properties of the peptide P4 side chain and the conformation of the Tyr-P6 side chain as the structural determinants of GNYSFYAL antagonist activity.