Abstract
The role of Sirtuins in brain function is emerging, yet little is known about SIRT5 in this domain. Our previous work demonstrates that protein kinase C epsilon (PKCε)-induced protection from focal ischemia is lost in SIRT5−/− mice. Thus, metabolic regulation by SIRT5 contributes significantly to ischemic tolerance. The aim of this study was to identify the SIRT5-regulated metabolic pathways in the brain and determine which of those pathways are linked to PKCε. Our results show SIRT5 is primarily expressed in neurons and endothelial cells in the brain, with mitochondrial and extra-mitochondrial localization. Pathway and enrichment analysis of non-targeted primary metabolite profiles from Sirt5−/− cortex revealed alterations in several pathways including purine metabolism (urea, adenosine, adenine, xanthine), nitrogen metabolism (glutamic acid, glycine), and malate-aspartate shuttle (malic acid, glutamic acid). Additionally, perturbations in β-oxidation and carnitine transferase (pentadecanoic acid, heptadecanoic acid) and glutamate transport and glutamine synthetase (urea, xylitol, adenine, adenosine, glycine, glutamic acid) were predicted. Metabolite changes in SIRT5−/− coincided with alterations in expression of amino acid (SLC7A5, SLC7A7) and glutamate (EAAT2) transport proteins as well as key enzymes in purine (PRPS1, PPAT), fatty acid (ACADS, HADHB), glutamine-glutamate (GAD1, GLUD1), and malate-aspartate shuttle (MDH1) metabolic pathways. Moreover, PKCε activation induced alternations in purine metabolites (urea, glutamine) that overlapped with putative SIRT5 pathways in WT but not in SIRT5−/− mice. Finally, we found that purine metabolism is a common metabolic pathway regulated by SIRT5, PKCε and ischemic preconditioning. These results implicate Sirt5 in the regulation of pathways central to brain metabolism, with links to ischemic tolerance.
Highlights
The silent information regulator two homolog proteins (Sirtuins) are an evolutionarily conserved family of NAD+-dependent deacylases that regulate metabolic homeostasis in response to low nutrient availability and energetic stress (Houtkooper et al, 2012)
Recent studies demonstrate the emerging role of succinyl, malonyl, and glutaryl-lysine modifications, and their regulation by silent information regulator two-homolog 5 (SIRT5), within metabolically active tissues such as liver and heart (Hirschey and Zhao, 2015). The significance of these novel modifications and their regulation by SIRT5 is unclear in the brain, an organ with highly specialized and compartmentalized metabolic pathways
Given that SIRT5 is necessary for protein kinase c epsilon (PKCε)-induced ischemic tolerance against stroke (Morris-Blanco et al, 2016), these metabolic pathways are likely mediators of this protection
Summary
The silent information regulator two homolog proteins (Sirtuins) are an evolutionarily conserved family of NAD+-dependent deacylases that regulate metabolic homeostasis in response to low nutrient availability and energetic stress (Houtkooper et al, 2012). There are seven mammalian Sirtuins (SIRT1-7), 3 of which (SIRT3-5) are primarily localized in, but not limited to, the SIRT5 and PKCε Pathways in Brain mitochondrion (He et al, 2012). SIRT5 function is distinct from SIRT3, the major mitochondrial deacetylase, and SIRT4, which mediates ADP-ribosylation and has lipoamidase activity, in that it acts on succinyl- (Rardin et al, 2013), malonyl(Nishida et al, 2015), and glutaryl- (Tan et al, 2014) lysine posttranslational modifications. Metabolic pathways enriched with these modifications and with protein targets of SIRT5 include those central to liver function such as the urea cycle, betaoxidation and ketone body synthesis, among others (Hirschey and Zhao, 2015). Identifying the tissue-specific functions of SIRT5 and other sirtuins is essential to understand their global functionality under physiological and pathological conditions
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