Investigation of sirtuins as therapeutic targets in neurodegenerative disorders : studies on mechanisms and possible combined therapy

Up to now, current therapies for Alzheimer’s disease (AD) can treat the symptoms with modest effect and have little impact on the overall progression. AD is a complex, multifactorial disorder, featured by aggregation of toxic proteins, inflammation, oxidative stress, synaptic deficits, and cognitive decline. Thus, the multi-target therapeutic strategy is of particular interest as it uses a combination of drugs to affect different molecular targets and converge on neuroprotection or even disease modification. In drug discovery for neurodegenerative processes, an interesting role emerged for the histone-deacetylase enzymes Sirtuins (SIRTs), and SIRT1 and SIRT2 have been associated with neuroprotection and neurodegeneration, respectively. The availability of SIRTs small molecule modulators allowed the achievement of good results of SIRT1 activation or SIRT2 inhibition in models of neurodegeneration and AD, as both SIRTs are actively involved, to different extent, in the regulation of amyloidogenic processing, inflammatory and oxidative cascades. A direct intervention on such molecular players could be beneficial in obtaining cognitive or pathologic phenotype improvement deriving from multiple pathways targeting. In this work, we aimed at combining SIRT1 activation and SIRT2 inhibition to study a possible multi-target approach on AD models, both in vitro and in vivo. An initial drug dose-response and biochemical assessment was performed on the AD in vitro model H4-sw, focusing on the increase of the neuroprotective sAPPα fragment. Next, 3xTg-AD mice received the single or the combined treatment with SIRT1 activator SRT2104 and SIRT2 inhibitor AK7 for two weeks, and the effects on cognitive performance and key biochemical parameters were assessed. Results showed that both SIRT1 and SIRT2 single modulation independently improved cognitive performance in treated mice. Combined treatment showed complete memory recovery too. Some differences between single and double treatment emerged in biochemical assessments. Double-treated mice had increased NRF2, sAPPα and reduced Aβ oligomers in hippocampus, already seen upon SIRT2 inhibition, suggesting that cognitive improvement given by SIRT2 modulation could be attributed to those underlying effects. Together with memory recovery, SIRT1 activator treated mice showed decreased hippocampal CD11b and GFAP, increased synaptophysin, SOD1, sAPPα and NRF2, all seen upon double treatment too. Thus, SIRT1 and SIRT2 independent modulations could improve memory deficit acting both on peculiar or common protein targets. In conclusion, the combined treatment resumed all the behavioural and biochemical effects of single modulations. Our data support the concept that, with a multi-target strategy, it could be possible to take full advantage of the complementarity of SIRT1/SIRT2 treatments and obtain cognitive improvement based on changes on several underlying biochemical mechanisms that with single modulation could only be partially achieved. All these observations could open up new insights for research on SIRTs involvement in AD and neurodegeneration.