Differential sirtuin expression patterns in amyotrophic lateral sclerosis (ALS) postmortem tissue: neuroprotective or neurotoxic properties of sirtuins in ALS?

BACKGROUND/AIMS: Sirtuins (SIRT1-7; class III histone deactylases) modulate fundamental mechanisms in age-related neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). We assessed the expression levels of sirtuins in human postmortem ALS and control brain and spinal cord. METHODS AND RESULTS: By quantitative real-time PCR, a significant reduction of SIRT1 and SIRT2 was detected in homogenates of the primary motor cortex (white and gray matter), while there were no differences in spinal cord homogenates. When specifically analyzing mRNA and protein expression in the gray matter (cortical layers I-VI of the precentral gyrus, ventral/dorsal horn of the spinal cord) by in situ hybridization histochemistry and immunohistochemistry, we found increased levels of SIRT1, SIRT2 and SIRT5 in ALS which were significant for SIRT1 and SIRT5 mRNA in the spinal cord. CONCLUSION: Our results indicate a general reduction of SIRT1 and SIRT2 in ALS primary motor cortex, while in situ hybridization histochemistry and immunohistochemistry showed neuron-specific upregulation of SIRT1, SIRT2 and SIRT5, particularly in the spinal cord. Opposed effects have been described for SIRT1 and SIRT2: while SIRT1 activation is mainly associated with neuroprotection, SIRT2 upregulation is toxic to neuronal cells. Novel therapeutic approaches in ALS could therefore target SIRT1 activation or SIRT2 inhibition.

NAD metabolism and sirtuins: metabolic regulation of protein deacetylation in stress and toxicity.

Sirtuins are recently discovered NAD(+)-dependent deacetylases that remove acetyl groups from acetyllysine-modified proteins, thereby regulating the biological function of their targets. Sirtuins have been shown to increase organism and tissue survival in diverse organisms, ranging from yeast to mammals. Evidence indicates that NAD(+) metabolism and sirtuins contribute to mechanisms that influence cell survival under conditions of stress and toxicity. For example, recent work has shown that sirtuins and increased NAD(+) biosynthesis provide protection against neuron axonal degeneration initiated by genotoxicity or trauma. In light of their protective effects, sirtuins and NAD(+) metabolism could represent therapeutic targets for treatment of acute and chronic neurodegenerative conditions. Our work has focused on elucidating the enzymatic functions of sirtuins and quantifying perturbations of cellular NAD(+) metabolism. We have developed mass spectrometry methods to quantitate cellular NAD(+) and nicotinamide. These methods allow the quantitation of changes in the amounts of these metabolites in cells caused by chemical and genetic interventions. Characterization of the biochemical properties of sirtuins and investigations of NAD(+) metabolism are likely to provide new insights into mechanisms by which NAD(+) metabolism regulates sirtuin activities in cells. To develop new strategies to improve cell stress resistance, we have initiated proof of concept studies on pharmacological approaches that target sirtuins and NAD(+) metabolism, with the goal of enhancing cell protection against genotoxicity.