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Accumulating evidence has suggested the involvement of oxidative stress in the pathogenesis of Alzheimer's disease (AD). The main endogenous antioxidant, glutathione (GSH), has been shown to decline with ageing and in several age-related degenerative diseases, including AD. Potential options for replenishing GSH levels as a therapeutic target to treat these conditions include the administration of GSH itself, and low toxicity forms of the limiting amino acid for GSH synthesis; cysteine. However, passive GSH uptake is limited due to an unfavourable concentration gradient between the plasma and cytosol. Similarly, cysteine prodrugs have demonstrated limited efficacy to elevate depleted GSH levels in several in vivo and in vitro models of disease. It has been suggested that the decline in GSH levels in AD, may be associated with down regulation of GSH homeostasis rather than substrate limitation. Cellular GSH homeostasis is regulated by non-allosteric feedback inhibition exerted by GSH on glutamate cysteine ligase (GCL), which is responsible for the synthesis of the GSH precursor γ-glutamylcysteine (GGC). In conditions involving down regulated GSH homeostasis, GGC serves as a crucialrate-limiting substrate for GSH synthetase, the main enzyme responsible for condensing glycine with GGC to form the final thiol tripeptide, GSH. In this review, we focus on the therapeutic potential of GGC to elevate cellular GSH levels. We also discuss the efficacy of GGC prodrugs which would be taken up and converted by the unregulated GS to GSH, and the administration of modified GSH compounds, such as GSH esters that could potentially overcome the concentration gradient that prohibits passive GSH uptake, in AD.

Ageing is associated with ultrastructural changes in the liver, including a reduction in hepatocyte mitochondrial number density and porosity of the liver sinusoidal cells. Histologically-verifiable changes include lipid and collagen accumulation, and increased perisinusoidal von Willebrand factor (VWF) expression. Sirtuin 1 (SIRT1), an NAD+-dependent deacetylase, is renowned for its range of anti-ageing actions. This study evaluated whether the anti-ageing effects of SIRT1 would extend to these liver ageing characteristics. Here the effects of whole-body SIRT1 overexpression on the livers of young (3-5 month) and old (13-20 month) SIRT1 transgenic mice were evaluated and compared to their wildtype controls. In addition, because NAD+ levels diminish with age, some old animals were supplemented with the NAD+ precursor, nicotinamide mononucleotide (NMN), to remove any limitation of naturally available NAD+ on SIRT1 overexpression. NAD+ is a SIRT1 cofactor. Old age is orthodoxly associated with reduced mitochondrial function and number in many tissues. However, transmission electron microscopy determination of liver mitochondrial numerical density (mitochondrial particles per cytosolic µm2) showed that there was no change with age. Additionally, there was no intervention effect of SIRT1 overexpression or NMN supplementation. Old age is also conventionally associated with reduced number and size of pores in the liver sinusoidal endothelial cell called fenestrae. However, analysis of the liver blood vessels with scanning electron microscopy was unable to confirm an effect of age on the number and size of fenestrae. Analysis did, however, reveal that SIRT1 overexpression was associated with increased sinusoidal porosity (%) in older but not younger mice (F(1,8) = 14.9, p < 0.01). Furthermore, although SIRT1-mediated increases in fenestrae diameter were not statistically verifiable in a 2-way ANOVA analysis, when plotting maximal fenestrae diameter size against groups, there was a trend suggesting a shift in the size distribution towards larger fenestrae for SIRT1-overexpressing animals. Histological examination of the mice livers showed that age was associated with an increase in lipid and collagen levels. However, there was no effect of age on the presence of activated stellate cells or perisinusoidal expression of VWF. Mice that were treated with NMN were 0.11 times less likely to have elevated α-SMA expression than controls, yet a combination of SIRT1 overexpression and NMN supplementation led to a 20.43 times greater likelihood of elevated α-SMA expression. The lack of canonical ageing-associated changes in a number of regards raises the question of whether the older mice were sufficiently old for such effects to emerge. Future studies should examine mice of at least 24 months of age. In summary, although SIRT1-overexpression shows promise in promoting liver sinusoidal endothelial porosity, the attendant risk related to stellate cell activation may be prohibitive in the use of SIRT1-elevating strategies with respect to the liver. The possibility for tissue- specific SIRT1-elevating strategies remains for other tissue types which do not incur this extra risk. Finally, NMN might reduce stellate cell activation by a SIRT1-independent pathway. Collectively, the results raise the possibility of a novel pathway leading to stellate cell activation and a new therapeutic approach to reducing stellate cell activation – both possibilities have important implications for the treatment of hepatic fibrosis. Keywords: sirtuin 1 (SIRT1), nicotinamide mononucleotide (NMN), nicotinamide adenine dinucleotide (NAD), ageing, anti-ageing, liver, gastrocnemius muscle, liver sinusoidal endothelial cells (LSECs), fenestrae, defenestration, pseudocapillarization, mitochondria, mitochondrial numerical density, fibrosis, steatosis, hepatosteatosis, lipid, collagen, α-smooth muscle actin (α-SMA), von Willebrand factor (VWF), scanning electron microscopy, transmission electron microscopy, histology, haematoxylin and eosin, Sirius Red, immunohistochemistry.