Dietary restriction and silent information regulator 2/Sirtuin 1: connecting the dots

To understand the molecular mechanisms by which DR prevents age-related diseases, it could be important to first understand what triggers age-related physiological decline. Aging leads to a constellation of molecular alterations, including accumulative oxidative damage, inflammation, mitochondrial dysfunction, lack of protein turnover or increased covalent modification of proteins⁽ Reference Canto and Auwerx ^{2 )}. Given the multifactorial nature of aging, it was surprising to find that single-gene mutations markedly contribute to extend lifespan in diverse models including yeast, worms and flies⁽ Reference Fontana, Partridge and Longo ^{3 )}. Many of these longevity-extending mutations decrease the activity of molecular pathways activated by nutrients, such as the insulin signalling pathway, suggesting that they promote a physiological state similar to that experienced during DR⁽ Reference Fontana, Partridge and Longo ^{3 )}.

It was particularly exciting to find that a protein called silent information regulator 2 (SIR2) could modulate yeast replicative lifespan⁽ Reference Kaeberlein, McVey and Guarente ^{21 )}. SIR2 was initially identified as a gene that silenced the extra copies of the mating-type information in yeast⁽ Reference Shore, Squire and Nasmyth ^{22 )}. Simultaneous work by independent groups established that the SIR2 gene product was a protein, Sir2, containing a NAD⁺-dependent enzymatic histone deacetylase activity⁽ Reference Imai, Armstrong and Kaeberlein ^{23 –} Reference Tanner, Landry and Sternglanz ^{26 )}. The deacetylation reaction catalysed by Sir2 is coordinated with the cleavage of NAD⁺ into nicotinamide and 1-O-acetyl-ADP-ribose⁽ Reference Tanner, Landry and Sternglanz ^{26 )}. Given that multiple metabolic paths use NAD⁺, or its reduced form NADH, as a cofactor, it was proposed that Sir2 could act as a metabolic sensor, capable of modulating gene expression according to the metabolic state of the cell⁽ Reference Imai, Johnson and Marciniak ^{27 )}.

Increasing the dosage of SIR2 increased replicative lifespan in yeast by 30 %, while its deletion shortened lifespan by 50 %. Several studies indicated that Sir2 could be a critical mediator of the effects of DR on yeast lifespan⁽ Reference Howitz, Bitterman and Cohen ^{28 ,} Reference Lin, Defossez and Guarente ^{29 )}. For example, SIR2 overexpression was enough to increase replicative lifespan in a similar, non-additive, manner to DR⁽ Reference Lin, Defossez and Guarente ^{29 )}. In an opposite fashion, DR was unable to increase lifespan in yeast where the gene coding for SIR2 was deleted⁽ Reference Lin, Defossez and Guarente ^{29 )}.

Following these initial discoveries, similar observations were made in nematodes and flies. The Caenorhabditis elegans genome contains four sirtuin genes, among which Sir-2·1 is the most homologous to yeast SIR2. Sir-2·1 overexpression was enough extended nematode lifespan⁽ Reference Tissenbaum and Guarente ^{30 –} Reference Viswanathan, Kim and Berdichevsky ^{32 )}. The fly Drosophila melanogaster has five sirtuin homologues, and dSir2 also extends lifespan when overexpressed⁽ Reference Rogina and Helfand ^{33 ,} Reference Wood, Rogina and Lavu ^{34 )}. However, these findings were recently challenged and attributed to a poor control of the genetic background⁽ Reference Burnett, Valentini and Cabreiro ^{35 )}. Indeed, when the transgenic worms and flies were backcrossed to identical backgrounds, the lifespan extension was lost⁽ Reference Burnett, Valentini and Cabreiro ^{35 )}, although a small but significant effect has still been reported under similar conditions for Sir-2·1 overexpression in C. elegans ⁽ Reference Viswanathan and Guarente ^{36 ,} Reference Mouchiroud, Houtkooper and Moullan ^{37 )}.

Irrespective of the ultimate effects on maximal lifespan, Sir-2·1 and dSir2 have been identified as critical regulators of the response to nutritional stress induced by DR. Indeed, when dSir2 was deleted, DR failed to extend lifespan in flies⁽ Reference Rogina and Helfand ^{33 ,} Reference Bauer, Morris and Chang ^{38 )}. Similarly, worms lacking Sir-2·1 did not live longer upon DR, and displayed shorter lifespan when exposed to hydrogen peroxide, heat stress, or UV radiation⁽ Reference Wang and Tissenbaum ^{39 )}. These results suggest that Sir2 homologues are key regulators of the metabolic and transcriptional adaptations to DR.

SIRT1 is the closest mammalian homologue of the yeast Sir2. While initially described as a transcriptional silencing enzyme via histone deacetylation, the actions of SIRT1 have unfolded far beyond histone modifications⁽ Reference Canto and Auwerx ^{40 )}. In the past decade, a large number of non-histone deacetylation targets have been identified, including key orchestrators of mitochondrial and lipid oxidation gene expression as the peroxisome-proliferator activated receptor γ coactivator 1α⁽ Reference Nemoto, Fergusson and Finkel ^{41 ,} Reference Rodgers, Lerin and Haas ^{42 )} and FOXO3a⁽ Reference Brunet, Sweeney and Sturgill ^{43 ,} Reference Motta, Divecha and Lemieux ^{44 )}. For an extended list of identified SIRT1 targets, the reader is referred to other recent reviews⁽ Reference Canto and Auwerx ^{40 ,} Reference Chang and Guarente ^{45 )}. The deacetylation of these targets by SIRT1 prompts their transcriptional activity on promoters encoding for genes aimed to extract energy through mitochondrial respiration⁽ Reference Canto and Auwerx ^{40 )}. Given the dual localisation of SIRT1, either in the cytoplasm and/or the nucleus, it is not surprising that SIRT1 targets have also been identified in both compartments⁽ Reference Canto and Auwerx ^{40 )}. This way, by sensing changes in NAD⁺ levels, SIRT1 is activated upon nutrient stress and triggers metabolic adaptations favouring energy production through non-carbohydrate energy sources and via oxidative processes. Recently, SIRT1 has been shown to trigger the mitochondrial unfolded protein response, which optimises mito-nuclear communication in order to ensure mitochondrial fitness through the efficient repair of damaged mitochondria⁽ Reference Mouchiroud, Houtkooper and Moullan ^{37 )}. Hence, SIRT1 activation could also contribute to alleviate mitochondrial function decay upon aging.

Dietary restriction and Sirtuin 1 in mammals (I): gain-of-function models

In addition to the prevention of age-related diseases, DR is known to promote a vast range of physiological and behavioural changes in mice. These effects include a reduction in body weight by a decrease in fat mass, enhanced insulin sensitivity and increased efficiency for energy production⁽ Reference Fontana, Partridge and Longo ^{3 )}. SIRT1 expression increases in many mammalian cells and tissues upon glucose deprivation or DR⁽ Reference Canto and Auwerx ^{40 ,} Reference Chen, Bruno and Easlon ^{46 ,} Reference Fulco, Cen and Zhao ^{47 )}. This, together with the data on lower eukaryotes, raised the hypothesis that forced SIRT1 activation could be used as a DR-mimetic strategy. In this section, we will discuss several approaches used to increase SIRT1 activity and whether they truly resemble the effects of DR.

The first SIRT1 gain-of-function model reported displayed several DR-like features: they were leaner and had improved glucose tolerance⁽ Reference Bordone, Cohen and Robinson ^{48 )}. One particularity of this model is that the overexpression of the SIRT1 protein occurred predominantly in the brain and in white- and brown-adipose tissues (BAT), but not in liver or muscle⁽ Reference Bordone, Cohen and Robinson ^{48 )}. Not much later, two mouse models for whole-body moderate SIRT1 overexpression were generated⁽ Reference Pfluger, Herranz and Velasco-Miguel ^{49 ,} Reference Banks, Kon and Knight ^{50 )}. In one of them, generated by the Serrano laboratory⁽ Reference Pfluger, Herranz and Velasco-Miguel ^{49 )}, the moderate overexpression of SIRT1 led to increased BAT function and thermogenic function, even on a low fat diet, which caused a modest increase in insulin sensitivity⁽ Reference Boutant, Joffraud and Kulkarni ^{51 )}. The involvement of SIRT1 on thermogenic functions was further strengthened by the second mouse model of moderate SIRT1 overexpression, generated by the Accili laboratory, where SIRT1 was shown to improve white adipose tissues ‘browning’ upon pharmacological or physiological adrenergic stimulation⁽ Reference Qiang, Wang and Kon ^{52 )}. In both models, however, no major effects on hepatic or muscle metabolism were observed under low-fat diets, despite a 2–4-fold increase in SIRT1 levels in both tissues⁽ Reference Pfluger, Herranz and Velasco-Miguel ^{49 –} Reference Boutant, Joffraud and Kulkarni ^{51 )}. Both mouse models, however, showed that a mild overexpression of SIRT1 prevented against high-fat diet induced hyperglycemia, insulin resistance and fatty liver, despite no significant differences in body weight⁽ Reference Pfluger, Herranz and Velasco-Miguel ^{49 ,} Reference Banks, Kon and Knight ^{50 )}. This was largely attributed to a protective effect of SIRT1 against high-fat diet-induced liver insulin resistance and inflammation⁽ Reference Pfluger, Herranz and Velasco-Miguel ^{49 ,} Reference Banks, Kon and Knight ^{50 )}. In line with this, liver-specific overexpression of SIRT1 is enough to effectively protect against insulin resistance in both dietary and genetic mouse models of obesity⁽ Reference Li, Xu and Giles ^{53 )}.

While initially surprising, the absence of effects of SIRT1 overexpression in muscle mitochondrial content and insulin sensitivity have been elegantly confirmed in mice with a selective overexpression of SIRT1 in muscle tissue⁽ Reference White, Philp and Fridolfsson ^{54 )}. However, it has recently been suggested that a 100-fold overexpression of SIRT1 in skeletal muscle can lead to decreased muscle mass, cross-sectional area and a shift towards a more oxidative, slow-twitch, muscle fibre types⁽ Reference Chalkiadaki, Igarashi and Nasamu ^{55 )}. Another recent report shows how a high, non-physiological, whole body SIRT1 overexpression prompts enhanced mitochondrial content in muscle, together with an improvement of whole-body glucose tolerance⁽ Reference Price, Gomes and Ling ^{56 )}. These observations suggest that endogenous SIRT1 is enough to account for the basal maintenance of mitochondrial content under physiological conditions in skeletal muscle, and that certain transcriptional programs might be forcedly enhanced under situations of massive SIRT1 overexpression.

Mice overexpressing SIRT1 in the β-cell display enhanced glucose-stimulated insulin secretion, even though whole-body moderate SIRT1 overexpression did not have any major effect on mice insulinemia on chow diet⁽ Reference Chang and Guarente ^{45 ,} Reference Pfluger, Herranz and Velasco-Miguel ^{49 –} Reference Boutant, Joffraud and Kulkarni ^{51 )}. These discrepancies could be attributed to differential SIRT1 overepression levels. However, whether SIRT1 overexpression at massive levels really mimics physiological SIRT1 activation upon DR is, unclear. Indeed, a caveat when comparing all these models is that higher SIRT1 levels do not necessarily have to correlate with SIRT1 activity, for example in situations where NAD⁺ might become rate-limiting, as can be the case during aging⁽ Reference Mouchiroud, Houtkooper and Moullan ^{37 ,} Reference Braidy, Guillemin and Mansour ^{57 )}. In this sense, it is interesting to note that the Serrano laboratory evaluated the impact of moderate SIRT1 overexpression on the aging process. While SIRT1 overexpression protected against diverse age-related pathologies including insulin resistance, osteoporosis, impaired wound healing and cancer, no effects on mouse lifespan were observed⁽ Reference Herranz, Munoz-Martin and Canamero ^{58 )}.

So, is SIRT1 overexpression comparable with DR? At a glance, one can clearly identify some overlaps (e.g. amelioration of glycemic profiles), but also some major discrepancies between both models. In most cases⁽ Reference Pfluger, Herranz and Velasco-Miguel ^{49 –} Reference Boutant, Joffraud and Kulkarni ^{51 )}, but not all⁽ Reference Bordone, Cohen and Robinson ^{48 )}, SIRT1 overexpression does not lead to reduced body weight. This is expected, as mice are not in a limited energy intake scenario. In some models, SIRT1 ovexpression enhanced insulin sensitivity under normal diet conditions⁽ Reference Bordone, Cohen and Robinson ^{48 ,} Reference Boutant, Joffraud and Kulkarni ^{51 )}, which would go in line with the expected DR-like effect. This increase in insulin sensitivity has been attributed to higher insulin-stimulated glucose uptake in BAT⁽ Reference Boutant, Joffraud and Kulkarni ^{51 )}. The positive influence of SIRT1 on BAT function and thermogenesis, however, is unlikely to be a feature of DR. Upon prolonged low food accessibility, organisms will tend to optimise ATP synthesis by decreasing mitochondrial proton leak (i.e. uncoupled respiration), which is the key feature of BAT through the expression of the Uncoupling Protein 1. This is a phenomenon observed even at the cellular level upon glucose deprivation⁽ Reference Gomes, Di Benedetto and Scorrano ^{59 )}. Some studies indeed indicate that DR in mice leads to reductions in BAT function⁽ Reference Li, Cope and Johnson ^{60 )}. Reduced BAT function generally manifested as lower energy expenditure, which is characteristic in rodents and human subjects upon DR regimes⁽ Reference Heilbronn, de Jonge and Frisard ^{19 ,} Reference Martin, Heilbronn and de Jonge ^{61 )}. This is opposite to the higher energy expenditure observed in some SIRT1 transgenic mouse lines⁽ Reference Bordone, Cohen and Robinson ^{48 ,} Reference Boutant, Joffraud and Kulkarni ^{51 )}. The impact of DR on behavioural traits is also not mimicked by SIRT1 overexpression, as DR promotes an increase in activity and foraging behaviour⁽ Reference Chen, Steele and Lindquist ^{62 )}, while daily activity was lower in some SIRT1 transgenic lines⁽ Reference Banks, Kon and Knight ^{50 ,} Reference Boutant, Joffraud and Kulkarni ^{51 )}.

SIRT1 biology has also been studied through the use of SIRT1 activating compounds (STAC), among which resveratrol and SRT1720 might be the most well-known. The ability of these compounds to specifically and directly activate SIRT1, however, is still a matter of debate⁽ Reference Pacholec, Bleasdale and Chrunyk ^{63 –} Reference Lombard, Pletcher and Canto ^{65 )}. Irrespectively of whether the action is direct or not, both compounds lead to SIRT1 activation and promote very overlapping effects in mice, including a large increase in mitochondrial biogenesis in skeletal muscle and BAT, enhanced energy expenditure and protection against high-fat diet-induced obesity⁽ Reference Lagouge, Argmann and Gerhart-Hines ^{66 ,} Reference Feige, Lagouge and Cantó ^{67 )}. In addition, STAC granted improved insulin sensitivity and a longer lifespan when mice were submitted to a high-fat diet⁽ Reference Baur, Pearson and Price ^{68 ,} Reference Minor, Baur and Gomes ^{69 )}. In the case of SRT1720, treated animals displayed enhanced lifespan even on chow diet conditions⁽ Reference Mitchell, Martin-Montalvo and Mercken ^{70 )}.

Other compounds used to activate SIRT1 are those aimed to elevate NAD⁺ bioavailability. This includes compounds that inhibit alternative NAD⁺ consuming activities, such as PARP-1⁽ Reference Pirinen, Canto and Jo ^{71 ,} Reference Cerutti, Pirinen and Lamperti ^{72 )} or CD38⁽ Reference Escande, Nin and Price ^{73 )}, or that enhance NAD⁺ synthesis, such as nicotinamide mononucleotide⁽ Reference Yoshino, Mills and Yoon ^{74 ,} Reference Gomes, Price and Ling ^{75 )} or nicotinamide riboside⁽ Reference Cerutti, Pirinen and Lamperti ^{72 ,} Reference Canto, Houtkooper and Pirinen ^{76 ,} Reference Khan, Auranen and Paetau ^{77 )}. In agreement with the ability of these strategies to increase NAD⁺ availability, all these compounds led to higher SIRT1 activity and ameliorations in glucose homeostasis. In the case of nicotinamide riboside or PARP inhibition, higher energy expenditure, decreased body weight gain upon high-fat feeding and enhanced mitochondrial biogenesis have also been reported⁽ Reference Pirinen, Canto and Jo ^{71 ,} Reference Cerutti, Pirinen and Lamperti ^{72 ,} Reference Canto, Houtkooper and Pirinen ^{76 ,} Reference Khan, Auranen and Paetau ^{77 )}.

It is interesting that NAD⁺ boosting strategies and STAC both converge at preventing high-fat diet-induced body weight gain, a phenomenon never observed in models of moderate SIRT1 overexpression⁽ Reference Pfluger, Herranz and Velasco-Miguel ^{49 –} Reference Boutant, Joffraud and Kulkarni ^{51 )}. Also, STAC promote dramatic increases in mitochondrial biogenesis, especially in skeletal muscle⁽ Reference Lagouge, Argmann and Gerhart-Hines ^{66 ,} Reference Feige, Lagouge and Cantó ^{67 )}, which, again, were not observed upon moderate SIRT1 overexpression⁽ Reference Boutant, Joffraud and Kulkarni ^{51 )}. Similarly, increases in skeletal muscle mitochondrial content are not generally seen upon DR, even though this is still a matter of debate⁽ Reference Hancock, Han and Higashida ^{78 )}. Therefore, all the earlier results indicate that, despite a significant overlap in the effects, there are remarkable differences between the different SIRT1 activating strategies. In some cases, such as the regulation of energy expenditure, the effect of SIRT1 activation might be even opposite to those of DR.

The earlier observations argue that forced SIRT1 activation and DR might lead to similar health benefits, yet not necessarily through similar means. This, however, does not rule out that SIRT1 participates in the adaptations to DR. This aspect can be tested through, at least, two strategies. For example, one could test the interaction between SIRT1 overexpression and the response to DR. In this sense, the overexpression of SIRT1 in skeletal muscle does not seem to alter the effects of DR on body weight, body composition or insulin sensitivity⁽ Reference White, McCurdy and Philp ^{79 )}. A possible explanation might be that endogenous SIRT1 levels could be enough to warrant full adaptability to DR in this tissue. A second, more conclusive, strategy is to evaluate the effects of DR in SIRT1 loss-of-function models, as discussed in the next section.

Dietary restriction and Sirtuin 1 in mammals (II): loss-of-function models

The whole-body deletion of SIRT1 leads to high prenatal lethality in inbred mice⁽ Reference McBurney, Yang and Jardine ^{80 ,} Reference Cheng, Mostoslavsky and Saito ^{81 )}. The very few pups that were born displayed marked cardiac and neurological problems, leading to death very early in the postnatal period⁽ Reference McBurney, Yang and Jardine ^{80 ,} Reference Cheng, Mostoslavsky and Saito ^{81 )}. In order to bypass this situation, SIRT1 deletion was performed in outbred mice⁽ Reference McBurney, Yang and Jardine ^{80 )}. These mice were viable and displayed a marked hypermetabolism due to decreased energy production efficiency, which, in turn, impeded the proper adaptation of metabolic rates to DR⁽ Reference Boily, Seifert and Bevilacqua ^{82 )}. DR is generally associated with behavioural changes at the level of food foraging activity. These changes, however, did not happen when DR was performed on SIRT1 deficient mice⁽ Reference Chen, Steele and Lindquist ^{62 )}. The role of SIRT1 in the influence of DR on mouse lifespan has been examined recently. For this purpose, SIRT1 knockout (KO), SIRT1 heterozygous and control wild-type mice, aged between 2 and 5 months old, were subjected to either 40 % DR or ad libitum diets. The results indicated that DR failed to increase lifespan in SIRT1 KO mice⁽ Reference Mercken, Hu and Krzysik-Walker ^{83 )}. Despite a trend for decreased maximum lifespan under DR conditions, DR extended the lifespan of both SIRT1 heterozygous or wild-type mice in a comparable manner⁽ Reference Mercken, Hu and Krzysik-Walker ^{83 )}. These results evoke two concepts: (1) SIRT1 is required in outbred mouse stocks for the lifespan extension caused by DR; and, (2) as suggested by the overexpression models, SIRT1 endogenous levels are largely enough to mediate DR-induced adaptations, since even a 50 % reduction in SIRT1 levels did not have a major impact on the lifespan extension promoted by DR.

One caveat of the earlier studies is that SIRT1 deficient mice display multiple defects on the basal state, including dwarfism, sterility, craniofacial abnormalities and several inflammatory conditions⁽ Reference McBurney, Yang and Jardine ^{80 ,} Reference Mercken, Hu and Krzysik-Walker ^{83 )}. Therefore, all these abnormalities could lead them to premature death irrespectively of feeding conditions. Given such detrimental effects on early development and global health, ensuring non-limiting SIRT1 levels in key central and peripheral tissues might have been naturally selected during evolutionary processes. Interestingly, an inducible model has been developed in order to genetically ablate SIRT1 exclusively in adulthood⁽ Reference Price, Gomes and Ling ^{56 )}. SIRT1 deletion in adult mice did not result in any overt phenotype or metabolic alteration⁽ Reference Price, Gomes and Ling ^{56 )}, making it a perfect model to precisely evaluate the impact of SIRT1 on DR-induced adaptations, even though efforts in this direction have not yet been reported.

While the requirement for SIRT1 in DR-induced adaptations in mice seems clear, there are still a number of open questions. For example: do all tissues require SIRT1 expression to allow DR-induced metabolic adaptations? Few efforts have been done in this direction, yet enough to conclude that this is the case in skeletal muscle. When submitted to a 60 % reduction in energy intake, mice display a marked increase in insulin sensitivity and insulin-induced glucose uptake in peripheral tissues⁽ Reference Schenk, McCurdy and Philp ^{84 )}. However, SIRT1 deletion specifically in skeletal muscle is enough to largely block the effects of DR on insulin-induced glucose disposal in skeletal muscle⁽ Reference Schenk, McCurdy and Philp ^{84 )}. In liver or adipocytes, where SIRT1 levels were similar between muscle-specific SIRT1 KO and control mice, DR-induced benefits on insulin signalling were comparable⁽ Reference Schenk, McCurdy and Philp ^{84 )}. This testifies that DR only failed to promote metabolic adaptations in skeletal muscle, where SIRT1 had been specifically deleted. Mechanistically, the authors proposed that DR improves the efficiency of insulin to trigger phosphoinositide 3-kinase signalling and that this is due to SIRT1-mediated deacetylation and inactivation of Stat3, a negative transcriptional regulator of the phosphoinositide 3-kinase regulatory subunits p55α and p50α⁽ Reference Schenk, McCurdy and Philp ^{84 )}. Hence, DR would enhance insulin signalling potency in skeletal muscle by increasing the expression of phosphoinositide 3-kinase regulatory subunits in a SIRT1-dependent manner.

The effects of DR have also been explored in liver-specific SIRT1 KO mice. In this case, liver-specific SIRT1 KO mice had similar body weight loss and fat reduction as control mice upon DR, and no major alterations were observed after the examination of different metabolic parameters, including fasting glycemia, insulinemia and glucose tolerance tests⁽ Reference Chen, Bruno and Easlon ^{46 )}. Hence, SIRT1 ablation in the liver does not seem to alter the physiological adaptation to DR. In part, this might be explained by the fact that, contrary to most tissues reported, SIRT1 expression is paradoxically decreased in livers upon DR⁽ Reference Chen, Bruno and Easlon ^{46 )}. As described earlier, SIRT1 has a key role in adipose tissue biology, where it enhances the action of ‘browning’ stimuli⁽ Reference Boutant, Joffraud and Kulkarni ^{51 ,} Reference Qiang, Wang and Kon ^{52 )} and prevents high-fat diet induced metabolic complications, at least at early stages⁽ Reference Chalkiadaki and Guarente ^{85 ,} Reference Mayoral, Osborn and McNelis ^{86 )}. Hence, it will be interesting to evaluate how SIRT1 deficiency in adipose tissues impacts on DR-induced metabolic adaptations.

The adaptation of mice to DR also relies on changes at the level of the central nervous system. In order to evaluate the role of SIRT1 expression in the brain in the adaptations to DR, mice lacking SIRT1 specifically in neurons, astrocytes and glial cells were generated⁽ Reference Cohen, Supinski and Bonkowski ^{87 )}. Wild-type and brain-specific SIRT1 KO mice lost weight similarly after a 7-month 40 % reduction in energetic intake. DR dramatically increased the insulin sensitivity of wild-type animals⁽ Reference Cohen, Supinski and Bonkowski ^{87 )}. However, DR induced only a modest improvement in insulin sensitivity in brain-specific SIRT1 KO mice⁽ Reference Cohen, Supinski and Bonkowski ^{87 )}, suggesting that SIRT1 expression in the brain is required to achieve the full benefits on insulin sensitivity conferred by DR. When placed on monitored running wheels, wild-type animals dramatically up-regulated their activity in response to DR. In contrast, while brain-specific SIRT1 KO mice were slightly more active than control mice when fed ad libitum, their activity markedly decreased upon DR⁽ Reference Cohen, Supinski and Bonkowski ^{87 )}. Therefore, SIRT1 activity in the brain is required to modulate several metabolic and behavioural adaptations to DR. A caveat of this model is that brain-specific SIRT1 KO mice, as whole-body SIRT1 deficient mice, display numerous abnormalities at the basal state, including dwarfism and a reduced somatotropic signalling (i.e. lower circulating growth hormone and insulin-like growth factor-1 levels)⁽ Reference Cohen, Supinski and Bonkowski ^{87 )}, which call for caution when interpreting the effects of DR in this model.

Conclusions and perspectives

Overall, most studies in genetically engineered mouse models indicate that SIRT1 is necessary to trigger many of the metabolic and behavioural adaptations to DR, even though the role of SIRT1 mediating these effects might differ from tissue to tissue. This fits nicely with the hypothesis that SIRT1 acts as an evolutionary conserved sensor of nutrient stress, promoting adaptations aimed to improve energy production efficiency. In line with this, SIRT1 activity directly impinges on many molecular pathways linked to longevity and that critically regulate metabolic adaptation to energy stress, including the FOXO family of transcription factors⁽ Reference Fontana, Partridge and Longo ^{3 ,} Reference Calnan and Brunet ^{88 )} and AMP-activated protein kinase signalling⁽ Reference Canto and Auwerx ^{89 )}.

Most results to date, however, also indicate that SIRT1 activation does not act per se a DR-mimetic. Several features of DR are not mimicked, or even opposed, by diverse SIRT1 gain-of-function models. Nonetheless, one should take into account the limitations of these models. For example, moderate SIRT1 overexpression might be aligned with the increases in SIRT1 observed in white adipose tissues or BAT upon DR, but, in fact, tissues such as brain or liver do not show increases in SIRT1 content on dietary restricted mice⁽ Reference Chen, Bruno and Easlon ^{46 ,} Reference Cohen, Supinski and Bonkowski ^{87 )}. STAC might push SIRT1 activity beyond physiological limits and might activate additional paths, such as the AMP-activated protein kinase, which could explain the strong effects on skeletal muscle mitochondrial biogenesis⁽ Reference Canto and Auwerx ^{89 )}. Therefore, two aspects will be worthy of our attention in the years to come: (1) the key elements influencing endogenous SIRT1 activity upon DR in diverse tissues and (2) a global landscape of the cooperation between SIRT1 and other pathways upon DR. While referring to SIRT1 activating strategies as ‘DR-mimetic’ might be misleading, the data in mouse models highlight that they bear an undeniable therapeutic potential for the amelioration of metabolic and age-related diseases. Novel approaches, such as enhancing NAD⁺ synthesis via nicotinamide mononucleotide or nicotinamide riboside supplementation are providing spectacular effects in the management of glucose homeostasis⁽ Reference Yoshino, Mills and Yoon ^{74 ,} Reference Canto, Houtkooper and Pirinen ^{76 )}, and their true potential in human subjects still needs to be unveiled.