It takes two to tango: NAD+ and sirtuins in aging/longevity control

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一个巴掌拍不响: NAD + 和 sirtuins 在衰老/寿命控制中

Abstract

摘要

The coupling of nicotinamide adenine dinucleotide (NAD+) breakdown and protein deacylation is a unique feature of the family of proteins called ‘sirtuins.’ This intimate connection between NAD+ and sirtuins has an ancient origin and provides a mechanistic foundation that translates the regulation of energy metabolism into aging and longevity control in diverse organisms. Although the field of sirtuin research went through intensive controversies, an increasing number of recent studies have put those controversies to rest and fully established the significance of sirtuins as an evolutionarily conserved aging/longevity regulator. The tight connection between NAD+ and sirtuins is regulated at several different levels, adding further complexity to their coordination in metabolic and aging/longevity control. Interestingly, it has been demonstrated that NAD+ availability decreases over age, reducing sirtuin activities and affecting the communication between the nucleus and mitochondria at a cellular level and also between the hypothalamus and adipose tissue at a systemic level. These dynamic cellular and systemic processes likely contribute to the development of age-associated functional decline and the pathogenesis of diseases of aging. To mitigate these age-associated problems, supplementation of key NAD+ intermediates is currently drawing significant attention. In this review article, we will summarize these important aspects of the intimate connection between NAD+ and sirtuins in aging/longevity control.

烟酰胺腺嘌呤二核苷酸分解和蛋白质去酰化的耦合是一种称为 sirtuins 的蛋白质家族的独特特征NAD + 和去乙酰化酶之间的这种密切联系有着古老的起源,并且提供了一个机械学基础,在不同的生物体中将能量代谢的调节转化为衰老和长寿的控制。虽然去乙酰化酶的研究领域经历了激烈的争论,但是越来越多的最近的研究已经使这些争论停止,并且充分确立了去乙酰化酶作为一个进化上保守的老化/长寿调节因子的意义。NAD + 和去乙酰化酶之间的紧密联系在几个不同的水平上被调节,这进一步增加了它们在新陈代谢和老化/寿命控制中协调的复杂性。有趣的是,已经证明 NAD + 的可用性随着年龄的增长而减少,减少去乙酰化酶的活性,并影响细胞水平上核与线粒体之间以及系统水平上下丘脑与脂肪组织之间的沟通。这些动态的细胞和系统过程可能有助于发展的年龄相关的功能下降和发病机制的老龄化疾病。为了减轻这些年龄相关的问题,补充关键 NAD + 中间体目前正在引起重要的注意。在这篇综述文章中,我们将总结 NAD + 和去乙酰化酶在衰老/长寿控制中密切联系的这些重要方面。

It’s so long ago

那是很久以前的事了

Since the first discovery of nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase activity of the silent information regulator 2 (Sir2) family (‘sirtuins’),1 the field of sirtuin biology has been evolving rapidly over the past 16 years. Many researchers from different fields have encountered sirtuins in their own research, enriching our knowledge of this fascinating family of enzymes. It is now clear that sirtuins are involved in the regulation of many fundamental biological processes throughout the body.2,3Furthermore, it has been revealed that sirtuins possess much broader enzymatic activities, namely, deacylases, including deacetylase, desuccinylase, demaloynylase, deglutarylase, long-chain deacylase, lipoamidase, and ADP-ribosyltransferase.4,5 All these enzymatic activities specifically require NAD+, and the catalytic mechanism of this NAD+dependency has been studied extensively.6 Clearly, sirtuins have evolved to respond to the availability of NAD+, an essential currency of cellular metabolism and DNA damage repair, and convert this information to many different biological outputs. In this particular review article, we will focus on this intimate connection between sirtuin function, aging/longevity control in particular, and their indispensable co-substrate, NAD+.

自从首次发现沉默信息调节因子2(sirtuins)家族(sirtuins)依赖于烟酰胺腺嘌呤二核苷酸(NAD +)的去乙酰化酶活性以来,去乙酰化酶生物学领域在过去16年中发展迅速。许多来自不同领域的研究人员在他们自己的研究中遇到了去乙酰化酶,丰富了我们对这个迷人的酶家族的知识。现在很清楚的是去乙酰化酶参与了许多基本的生物过程在整个身体的调节。2.3此外,还发现去乙酰化酶具有更广泛的酶活性,包括脱乙酰化酶、脱琥珀酸酶、脱酰化酶、脱戊二酰化酶、长链脱酰化酶、脂酰化酶和 adp- 核糖基转移酶。4.5所有这些酶活性都特别需要 NAD + ,这种 NAD + 依赖的催化机制已被广泛研究。6. 很明显,去乙酰化酶已经进化到对 NAD + 的可用性做出反应,NAD + 是细胞新陈代谢和 DNA 损伤修复的基本货币,并将这些信息转化为许多不同的生物输出。在这篇特别的综述文章中,我们将重点介绍 sirtuin 功能,特别是衰老/寿命控制,以及它们不可缺少的辅助基质 NAD + 之间的密切联系。

The origin of the connection between NAD+ and sirtuins is ancient. For instance, vibriophage KVP40 possesses a minimal set of genes for NAD+biosynthesis and consumption, namely, the genes encoding two key NAD+biosynthetic enzymes, nicotinamide phosphoribosyltransferase (NAMPT) and nicotinamide/nicotinic acid mononucleotide adenylyltransferase (NMNAT),2,3 and a sirtuin family protein (Figure 1).7 Although why such a minimalistic organism keeps these three genes in its genome remains unclear, one potential explanation is that controlling the host cell’s metabolism and proliferation in a NAD+-dependent manner could provide benefits for this particular vibriophage to efficiently produce progeny in the host cell. Such a NAD+/sirtuin-mediated virus–host relationship might be a prototype for the much more complex inter-tissue communication mediated by NAMPT and the mammalian sirtuin SIRT1.8 As discussed later in this review, NAMPT and SIRT1 comprise multiple layers of feedback regulatory loops inside cells and between tissues and organs, and contribute to the systemic regulation of mammalian aging and longevity.912 Another interesting example is the connection between the nicotinamidase Pnc1 and Sir2 proteins in yeast, worms, and flies. Whereas vertebrates and a limited number of bacterial species mainly use NAMPT to synthesize NAD+ from nicotinamide, invertebrates and most bacterial species use nicotinamidase to convert nicotinamide to nicotinic acid and synthesize NAD+ from nicotinic acid (Figure 1).13 Pnc1 regulates NAD+biosynthesis and affects lifespan in those organisms.1416

NAD + 和去乙酰化酶之间的联系起源很早。例如,噬病毒杆菌 KVP40具有 NAD + 生物合成和消费的最小基因组,即编码两个关键 NAD + 生物合成酶的基因,即烟酰胺磷酸核糖转移酶(NAMPT)和烟酰胺/烟酸腺苷酸转移酶(NMNAT) ,2,3和一个 sirtuin 家族蛋白(图1)。这种 NAD +/sirtuin 介导的病毒-宿主关系可能是由 NAMPT 和哺乳动物 sirtuin SIRT1介导的更为复杂的组织间通信的原型。8正如本文后面讨论的,NAMPT 和 SIRT1包括细胞内以及组织和器官之间的多层反馈调节环,有助于哺乳动物衰老和长寿的系统调节。9-12另一个有趣的例子是酵母、蠕虫和果蝇中的烟酰胺酶 Pnc1和 Sir2蛋白之间的联系。而脊椎动物和少数细菌种类主要使用 NAMPT 从烟酰胺合成 NAD + ,无脊椎动物和大多数细菌种类使用烟酰胺酶将烟酰胺转化为烟酸,并从烟酸合成 NAD + (图1)。13 Pnc1调节 NAD + 的生物合成并影响这些生物的寿命

figure1
Figure 1 图1

Although all genetic, pathophysiological, and pharmacological studies point out that sirtuin activity can be affected by changes in NAD+ levels, whether sirtuin activity is indeed regulated by the physiological fluctuation of NAD+has been of great debate. However, a recent detailed kinetic study has demonstrated that at least for deacetylase activities of SIRT1-3, each Km for NAD+ is consistent with the notion that changes in NAD+ levels in each subcellular compartment can directly regulate sirtuin activity.17 These findings further affirm the functional connection between NAD+ and sirtuins. We will now look into several important cases of this connection in diverse organisms.

虽然所有的遗传学、病理生理学和药理学研究都指出 sirtuin 活性会受到 NAD + 水平变化的影响,但是 sirtuin 活性是否真的受到 NAD + 生理波动的调节一直存在很大的争议。然而,最近一项详细的动力学研究表明,至少对于 SIRT1-3的去乙酰化酶活性来说,NAD + 的每个 Km 与每个亚细胞小室中 NAD + 水平的变化可以直接调节 sirtuin 活性的概念是一致的。我们现在将研究这种联系在不同生物体中的几个重要案例。

Learned all the games

学会了所有的游戏

The history of sirtuin research in the past 16 years has been a steep winding road. In this history, the biggest controversy was whether sirtuins are truly an evolutionarily conserved regulator for aging and longevity in diverse organisms. Although early studies demonstrated that Sir2 and its orthologs have a critical role in aging/longevity control in yeast, worms, and flies,1820subsequent studies reported contradictory results,21,22 bringing considerable debates on the role of sirtuins in aging/longevity control. However, this controversy has finally been put to the end because an increasing number of recent studies have successfully reconfirmed the original claims.2330 Even in mammals, the brain-specific SIRT1-overexpressing (BRASTO) transgenic mice have been demonstrated to exhibit significant delay in aging and lifespan extension in both male and female mice.27 Given that the whole-body SIRT1-overexpressing transgenic mice fail to show lifespan extension,22 this study strongly suggests that SIRT1 in the brain, more specifically in the hypothalamus, is a key to control aging and longevity in mammals.31 In addition, whole-body SIRT6 transgenic mice have been reported to show lifespan extension, although only males exhibit the phenotype.24 These recent studies have established a firm foundation for the significance of sirtuins as an evolutionarily conserved aging/longevity regulator.3234

抗衰老药物研究的历史在过去的16年中一直是一条崎岖陡峭的道路。在这段历史中,最大的争议是去乙酰化酶是否真的是一个进化上保守的调节因子,用于不同生物体的衰老和长寿。虽然早期的研究表明 Sir2及其直系亲属在酵母、蠕虫和果蝇的衰老/长寿控制中起着关键作用,但随后的18-20项研究报告了相互矛盾的结果,21,22就 sirtuins 在衰老/长寿控制中的作用引起了相当大的争论。然而,这场争论最终结束了,因为越来越多的近期研究已经成功地重新证实了最初的主张。23-30甚至在哺乳动物中,大脑特异性的 sirt1-过表达(BRASTO)转基因小鼠在雄性和雌性小鼠中都表现出明显的衰老和延长寿命的延迟。鉴于全身过度表达 SIRT1的转基因小鼠未能延长寿命,本研究有力地表明,大脑中的 SIRT1,特别是下丘脑中的 SIRT1,是控制哺乳动物衰老和长寿的关键。31此外,已报道全身 SIRT6转基因小鼠寿命延长,尽管只有雄性表现出表型。24. 这些最近的研究为去乙酰化酶作为一种进化上保守的老化/长寿调节因子的重要性奠定了坚实的基础。32-34

Interestingly, the connection between NAD+ and sirtuins has also been demonstrated to be crucial in aging/longevity control (Figure 2). In yeast, caloric restriction (CR), achieved by lowering glucose in the media from regular 2% down to 0.5% or 0.05%, substantially extends lifespan, and the lifespan extension by moderate CR (0.5% glucose) is dependent on Sir2 or nicotinic acid phosphoribosyltransferase Npt1.35 A more severe regimen of CR (0.05% glucose) extends lifespan by a different mechanism that is independent of Sir2 but dependent of the nutrient-responsive kinases, PKA, TOR, and the Akt ortholog Sch9.36 In addition, induction of nicotinamidase Pnc1, which is upstream of Npt1, is necessary and sufficient for lifespan extension by CR (Figure 2a).14 Nicotinamide riboside (NR), a key NAD+intermediate, is produced and secreted by yeast cells,37,38 and exogenous NR increases NAD+ biosynthesis and promotes lifespan through two independent pathways, the one mediated by NR kinase Nrk1 and the other mediated by uridine hydrolase Urh1 and purine nucleoside phosphorylase Pnp1.39 In worms, lifespan extension mediated by CR requires Sir-2.1 and Pnc1.40 In addition, NR is also able to promote lifespan in a Sir-2.1-dependent manner.25 Overexpression of the Drosophila nicotinamidase, D-NAAM, also extends lifespan in flies, and this lifespan extension is Sir2-dependent.15 These findings strongly suggest the importance of the connection between NAD+ and sirtuins in aging/longevity control in these organisms, which utilize nicotinic acid and nicotinamide ribose as main sources for NAD+ biosynthesis.

有趣的是,NAD + 和去乙酰化酶之间的联系也被证明在老化/寿命控制中是至关重要的(图2)。在酵母菌中,通过将培养基中葡萄糖含量从正常的2% 降低到0.5% 或0.05% 来实现热量限制(CR) ,大大延长了寿命,适度的 CR (0.5% 葡萄糖)延长寿命依赖于 Sir2或烟酸磷酸转移酶 Npt1.35 a 更严格的 CR (0.05% 葡萄糖)方案通过独立于 Sir2但依赖于营养激酶的不同机制延长寿命,此外,诱导 Npt1上游的烟酰胺酶 Pnc1是 CR 延长寿命的必要和充分条件(图2a)。14烟酰胺核糖苷(Nicotinamide riboside,NR)是由酵母细胞产生和分泌的 NAD + 关键中间体,37,38和外源性 NR 通过两个独立的途径增加 NAD + 的生物合成和促进寿命,其中一个途径由 NR 激酶 Nrk1介导,另一个途径由尿苷酸氢化酶 Urh1和嘌呤核苷磷酸化酶 Pnp1.39介导。25. 果蝇烟酰胺酶 D-NAAM 的过表达也延长了果蝇的寿命,这种延长寿命依赖于 sir2。15这些研究结果强烈表明 NAD + 和去乙酰化酶之间的联系在这些生物的衰老/寿命控制中的重要性,这些生物利用烟酸和烟酰胺核糖作为 NAD + 生物合成的主要来源。

figure2
Figure 2 图2

Different from yeast, worms, and flies, mammals do not have Pnc1 homologs but utilize NAMPT instead, converting nicotinamide, an amide form of vitamin B3, and 5′-phosphoribosyl-pyrophosphate to nicotinamide mononucleotide (NMN), another key NAD+ intermediate (Figure 1).41,42NMN is adenylated to NAD+ by three NMN adenylyltransferases, NMNAT1-3. It has been well established that the NAMPT-mediated NAD+biosynthetic pathway regulates sirtuin activity in the nucleus and mitochondria.4347 Just like Pnc1 in lower eukaryotes, NAMPT is also induced in rodents and humans by low glucose, fasting, CR, exercise, and a variety of stress and damage (Figure 2b).4751 An unique feature of this particular NAD+ biosynthetic pathway is that transcription of the Namptgene is mediated by the key circadian transcription factors CLOCK/BMAL, rendering NAD+ biosynthesis and the activity of sirtuins circadian. In addition, SIRT1 and SIRT6 feedback on the circadian oscillator in peripheral tissues and likely in the suprachiasmatic nucleus of the hypothalamus.43,45,52 The circadian oscillation of NAD+ drives at least SIRT1, SIRT3, and SIRT6 activities, impacting a variety of metabolic functions, including glucose, cholesterol, and fatty acid metabolism (Figure 3).44,53

与酵母、蠕虫和苍蝇不同,哺乳动物没有 Pnc1同源基因,而是利用 NAMPT,将烟酰胺,一种维生素 B3的酰胺形式,和5′-磷酸核糖焦磷酸转化为烟酰胺单核苷酸,另一种关键的 NAD + 中间体(图1)。43-47正如低等真核生物中的 Pnc1一样,NAMPT 也可以通过低糖、禁食、 CR、运动以及各种应激和损伤等途径在啮齿动物和人类中诱导(图2b)。47-51这种 NAD + 生物合成途径的一个独特特征是 NAMPT 基因的转录调控由关键的昼夜节律因子 CLOCK/BMAL 介导,从而呈现 NAD + 生物合成和 sirtuins 昼夜节律活性。此外,SIRT1和 SIRT6反馈对周围组织的昼夜节律振荡器,可能在下丘脑的视交叉上核

figure3
Figure 3 图3

Most recently, another interesting connection between NAMPT/NAD+ and SIRT1 has been found.8 Mammals have two different forms of NAMPT, intra- and extracellular NAMPT (iNAMPT and eNAMPT, respectively). iNAMPT is acetylated in white and brown adipose tissue. When iNAMPT is specifically deacetylated at lysine 53 by SIRT1, the protein is predisposed to secretion, and its enzymatic activity is enhanced. Surprisingly, eNAMPT secreted by adipose tissue remotely controls NAD+ biosynthesis, SIRT1 activity, and neural activity in the hypothalamus, which affects physical activity of mice during the dark phase and in response to fasting. These findings imply that adipose tissue communicates with other tissues through the secretion of eNAMPT, and regulates the spatial and temporal coordination of NAD+ biosynthesis at a systemic level. Given that the hypothalamus functions as a high-order ‘control center of aging’ in mammals,27,54 it is conceivable that adipose tissue functions as a ‘modulator’ of this control center (see Figure 5). Indeed, knock-in mice in which iNAMPT is overexpressed specifically in adipose tissue (ANKI mice) show significant increases in circulating eNAMPT levels, hypothalamic NAD+ and SIRT1 target gene expression, and physical activity, particularly in response to fasting.8 Therefore, it will be of great interest to examine whether these ANKI mice show a significant slowing in aging and lifespan extension.

最近,又发现了 NAMPT/NAD + 和 SIRT1之间的另一个有趣的联系。8哺乳动物有两种不同形式的 NAMPT,分别是细胞内和细胞外的 NAMPT (分别为 iNAMPT 和 eNAMPT)。在白色和褐色脂肪组织中乙酰化。当 iNAMPT 在赖氨酸53位被 SIRT1特异脱乙酰基时,蛋白质易于分泌,其酶活性增强。令人惊讶的是,脂肪组织分泌的烯胺 pt 远程控制 NAD + 的生物合成、 SIRT1活性和下丘脑的神经活性,这影响了小鼠在黑暗期和禁食时的身体活动。这些发现意味着脂肪组织通过分泌釉蛋白与其他组织进行交流,并在系统水平上调节 NAD + 生物合成的时空协调。考虑到哺乳动物的下丘脑作为老化的高级“控制中心”发挥作用,可以想象脂肪组织作为这个控制中心的“调节器”发挥作用(见图5)。事实上,iNAMPT 在脂肪组织特异性表达的基因敲入小鼠(ANKI 小鼠) ,在循环中的釉蛋白水平、下丘脑 NAD + 和 SIRT1靶基因表达,以及体力活动,特别是在禁食时都显著增加。8因此,研究这些 ANKI 小鼠在衰老和寿命延长方面是否表现出显著的减缓将是非常有意义的。

figure5
Figure 5 图5

Given that NAMPT and NAD+ levels display robust circadian oscillation in peripheral tissues,45 it is conceivable that eNAMPT secretion would also show circadian oscillation. Indeed, it has been reported that serum eNAMPT levels follow a diurnal rhythm, making a peak during early afternoon, in humans,55 which appears to be opposite to plasma eNAMPT oscillation in mice (a preliminary observation in the Imai lab). This eNAMPT oscillation would likely produce another oscillation of its enzymatic reaction product, NMN, in blood circulation. NMN is rapidly incorporated to major metabolic tissues and converted to NAD+.56 Therefore, in mammals, adipose tissue has an important role in generating the circadian oscillation of eNAMPT and possibly NMN in blood circulation, potentially synchronizing metabolic and neurobehavioral functions through other peripheral tissues and the hypothalamus in a circadian rhythm-dependent manner. In each tissue, NAD+ and sirtuins are critical mediators to orchestrate such inter-tissue communications. The loss of this orchestration is likely an important driver of aging in a wide variety of organisms.

考虑到 NAMPT 和 NAD + 水平在外周组织中表现出强烈的昼夜节律振荡,可以想象烯胺化酶的分泌也会表现出昼夜节律振荡。事实上,已经有报道说,血清烯胺铂水平遵循一个昼夜节律,在人类中在下午早些时候达到峰值,55似乎与小鼠血浆烯胺铂振荡相反(在今井实验室的初步观察)。这种烯胺铂振荡可能会在血液循环中产生另一种酶促反应产物 NMN 的振荡。NMN 在哺乳动物的主要代谢组织中迅速合并并转化为 NAD + . 56,因此,脂肪组织在血液循环中产生釉蛋白昼夜节律振荡,可能通过其他外周组织和下丘脑以昼夜节律依赖的方式同步代谢和神经行为功能。在每个组织中,NAD + 和去乙酰化酶是协调这种组织间通讯的关键介质。这种配器的丢失可能是各种生物衰老的重要驱动因素。

All the things that were said

所有说过的话

The functional connection between NAD+ and sirtuins is regulated at at least three levels: (1) regulation of NAD+ biosynthesis, (2) modulation of sirtuin activity by NAD+ substrates and derivatives, and (3) competitive utilization of NAD+ between sirtuins and other NAD+ consumers (Figure 4).

NAD + 和 sirtuins 之间的功能联系至少在三个水平上被调节: (1) NAD + 生物合成的调节,(2) NAD + 底物和衍生物对 sirtuin 活性的调节,(3) sirtuins 和其他 NAD + 消费者之间 NAD + 的竞争利用(图4)。

figure4
Figure 4 图4

First, the pathophysiological changes in NAD+ biosynthesis affect sirtuin activity. For instance, in mammals, NAMPT and NAD+ levels decline with age in multiple organs, such as pancreas, adipose tissue, skeletal muscle, liver, and brain.25,5658 Inflammatory, ischemic, and degenerative disease conditions also decrease NAMPT-mediated NAD+ biosynthesis.56,5962Although precise molecular mechanisms for the decline in NAMPT-mediated NAD+ biosynthesis remain unclear, it has been suggested that oxidative stress and/or inflammatory cytokines decrease NAMPT expression.56 Such pathophysiological decline in NAD+ biosynthesis decreases sirtuin activity, likely contributing to the development of age-associated pathophysiologies. For this reason, boosting NAD+ biosynthesis by using key NAD+ intermediates, such as NMN and NR, is now drawing significant attention as an efficient therapeutic intervention against diseases of aging, such as type 2 diabetes, Alzheimer’s disease, heart failure, and hearing loss.2,3 Thus, the regulation of NAD+ biosynthesis is one of the most important factors that affect the functional connection between NAD+and sirtuins.

首先,NAD + 生物合成的病理生理变化影响 sirtuin 的活性。例如,在哺乳动物中,NAMPT 和 NAD + 水平随着年龄的增长而下降,如胰腺、脂肪组织、骨骼肌、肝脏和大脑。25,56-58炎症、缺血和神经退行性疾病状态也会降低 NAMPT 介导的 NAD + 生物合成。56,59-62虽然 NAMPT 介导的 NAD + 生物合成下降的确切分子机制仍不清楚,但有人认为氧化应激和/或炎症细胞因子会降低 NAMPT 的表达。因此,通过使用关键的 NAD + 中间物,如 NMN 和 NR,促进 NAD + 的生物合成,作为一种有效的治疗衰老性疾病的干预手段,如2型糖尿病、阿尔茨海默病、心力衰竭和听力丧失,正在引起人们的极大关注。

Second, sirtuin activity can also be modulated by NAD+ substrates and derivatives, such as nicotinamide and NADH, and other molecules. Nicotinamide, which is released from NAD+ during the deacylation reaction of sirtuins, functions as a non-competitive inhibitor.63 The ability of nicotinamide to inhibit deacylation by sirtuins is dependent on the type of acyl substrates.17 For instance, for SIRT1, the IC50 for nicotinamide is 175 μM for the acetylated substrate but significantly less for longer chain acyl substrates. On the other hand, SIRT3 shows an opposite trend for nicotinamide inhibition. Whereas the calculated concentration of endogenous nicotinamide ranges from 10 to 150 μM in yeast,63 plasma nicotinamide concentrations range from 0.3 (human) to 5 μM (mouse) in mammals.6466 Therefore, nicotinamide might be more critical for the regulation of sirtuin activity in lower eukaryotes that tend to have higher endogenous nicotinamide concentrations. The NAD+/NADH ratio has also been proposed to be a critical parameter for the regulation of sirtuin activity due to the capability of NADH as a competitive inhibitor.67However, the subsequent studies have shown that the IC50 for NADH surpasses reported physiological ranges of NADH so that it is unlikely that NADH could function as a physiologically relevant competitive inhibitor for sirtuins, at least in mammals.44,68,69 Most recently, it has been demonstrated that the enzymatic activities of some sirtuins, particularly SIRT6, are regulated by long-chain fatty acids.70 Several free fatty acids, including myristic, oleic, and linoleic acids, at physiological concentrations are able to enhance SIRT6 deacetylase activity at K9 and K56 of histone H3 up to 35-fold. Taken together, these studies suggest that the connection between NAD+ and sirtuins can be modified by physiologically relevant endogenous NAD+ derivatives and other compounds.

其次,去乙酰化酶的活性也可以被 NAD + 底物和衍生物调节,如烟酰胺和 NADH,以及其他分子。63. 烟酰胺抑制 sirtuins 脱酰化的能力取决于酰基底物的类型。17例如,对于 SIRT1,烟酰胺的 IC50为乙酰化底物的175m,而对于长链酰基底物的 IC50明显较低。另一方面,SIRT3对烟酰胺的抑制呈现相反的趋势。内源性烟酰胺在酵母中的计算浓度在10ー150m 之间,而在哺乳动物体内的血浆烟酰胺浓度在0.3ー5m 之间。因此,在内源性烟酰胺浓度较高的低等真核生物中,烟酰胺对去乙酰化酶活性的调节可能更为关键。由于 NADH 具有竞争性抑制作用,NAD +/NADH 比值也被认为是调节去乙酰化酶活性的关键参数。67然而,随后的研究表明,NADH 的 IC50超过了报道的 NADH 的生理范围,所以 NADH 不太可能作为一种生理上相关的抗去乙酰化酶的竞争性抑制剂,至少在哺乳动物中是这样。44,68,69最近的研究表明,一些去乙酰化酶,特别是 SIRT6的酶活性受长链脂肪酸的调节。70几种游离脂肪酸,包括肉豆蔻酸、油酸和亚油酸,在生理浓度下能够提高组蛋白 H3的 K9和 K56的 SIRT6去乙酰化酶活性达35倍。综上所述,这些研究表明 NAD + 和去乙酰化酶之间的联系可以被生理上相关的内源 NAD + 衍生物和其他化合物修饰。

Lastly, the availability of NAD+ is under a fierce competition among NAD+-consuming enzymes, including sirtuins, poly-ADP-ribose polymerases (PARPs), and CD38/157 ectoenzymes.2,3 Particularly, SIRT1 and PARP1 compete with each other, and genetic ablation and pharmacological inhibition of PARP1 increase NAD+ content and SIRT1 activity and enhance oxidative metabolism.71 Similar findings were also reported in CD38 knockout mice.72,73 Interestingly, it has been shown that PARP is chronically activated in aging worms and mice, resulting in an increase in poly-ADP-ribosylation of cellular proteins.25 This chronic PARP activation, which might potentially be caused by an increase in chronic nuclear DNA damage, could lead to NAD+ depletion and decrease sirtuin activity, likely contributing to age-associated pathophysiologies.

最后,NAD + 的有效性受到去乙酰化酶、多 adp- 核糖聚合酶和 CD38/157胞外酶的激烈竞争。2,3特别是,SIRT1和 PARP1相互竞争,基因消融和药物抑制 PARP1增加 NAD + 含量和 SIRT1活性,增强唿吸作用。71在 CD38基因敲除小鼠中也有类似的发现。72,73有趣的是,已经证明 PARP 在老化的蠕虫和小鼠中被慢性激活,导致细胞蛋白质的多聚 adp 核糖基化增加。25这种慢性 PARP 激活,可能是由于慢性核 DNA 损伤的增加而引起的,可能导致 NAD + 耗竭和 sirtuin 活性降低,可能与年龄相关的病理生理学有关。

It takes two to tango

一个巴掌拍不响

How does this intimate interplay between NAD+ and sirtuins contribute to aging/longevity control? At a cellular level, the communication between the nucleus and mitochondria seems to be compromised when NAD+availability declines and thereby sirtuin activity decreases over age (Figure 5a). Interestingly, NAD+ deficiency causes a pseudohypoxic state through decreased SIRT1 activity.57 The defect in SIRT1 activity stabilizes HIF-1α, leading to an abnormal high level of HIF-1α. Because HIF-1α sequesters c-Myc, the gene encoding the mitochondrial transcription factor TFAM can no longer be activated by c-Myc (Figure 5a). In aged skeletal muscle, this defective TFAM expression causes significant reduction in mitochondrial gene expression, leading to mitochondrial metabolic dysfunction. Decreased SIRT1 activity also reduces the functions of PGC-1α and FOXO1, resulting in the reduction in mitochondrial biogenesis, oxidative metabolism, and anti-oxidant defense pathways.74,75 Furthermore, it has recently been demonstrated that SIRT1 and SIRT3 activities are involved in the mitochondrial unfolded protein response (UPRmt) pathway and mitophagy (Figure 5a).25,76 For instance, mammalian SIRT1, C. elegans sir-2.1, and NAD+ enhancement by NR all activate UPRmt genes, such as hsp-6(C. elegans) and HSP60 (mammals), in worms, mammalian cells, and skeletal muscle stem cells.25,77 Interestingly, the activation of UPRmt is tightly associated with mitonuclear protein imbalance, as defined by the decreased ratio between the nuclear DNA-encoded ATP5A and the mitochondrial DNA-encoded MTCO1 (cytochrome c oxidase subunit 1), and both UPRmtand mitonuclear protein imbalance are proposed to have a critical role in aging and longevity control.78 SIRT3 appears to be a part of UPRmt, and mitochondrial proteotoxic stress increases SIRT3 protein levels, inducing mitophagy and anti-oxidant response.76 Therefore, the breakdown of the intimate interplay between NAD+ and sirtuins causes serious mitochondrial dysfunction, a hallmark of aging, through these nuclear-mitochondrial communication mechanisms.

NAD + 和去乙酰化酶之间的这种亲密的相互作用是如何促进衰老/长寿控制的?在细胞水平上,当 NAD + 的可利用性下降时,细胞核和线粒体之间的通讯似乎受到损害,因此去乙酰化酶活性随着年龄的增长而减少(图5a)。有趣的是,NAD + 缺乏通过降低 SIRT1活性而导致假缺氧状态。 SIRT1活性缺陷稳定 hif-1,导致 hif-1异常高水平。因为 hif-1隔离 c-Myc,编码线粒体转录因子 TFAM 的基因不再能被 c-Myc 激活(图5 a)。在老年骨骼肌中,这种缺陷的 TFAM 表达导致线粒体基因表达显著降低,导致线粒体代谢功能障碍。SIRT1活性的降低也降低了 pgc-1和 FOXO1的功能,导致线粒体生物发生、唿吸作用和抗氧化防御通路的减少。74,75此外,最近的研究表明,SIRT1和 SIRT3的活性参与了线粒体未折叠蛋白反应(UPRmt)途径和吞噬功能(图5 a)。25,76例如,哺乳动物 SIRT1、秀丽隐杆线虫 sir-2.1和 NAD + 通过 NR 增强均能激活蠕虫、哺乳动物细胞和骨骼肌干细胞中的 UPRmt 基因,如热休克蛋白 -6(秀丽隐杆线虫)和热休克蛋白60(哺乳动物)。25,77有趣的是,UPRmt 的激活与线粒体蛋白失衡紧密相关,核 dna 编码的 ATP5A 和线粒体编码的 MTCO1(细胞色素c氧化酶单位1)之间的比例下降,UPRmt 和线粒体蛋白失衡被认为在衰老和寿命控制中起关键作用。78 SIRT3可能是 UPRmt 的一部分,线粒体蛋白毒性胁迫增加了 SIRT3蛋白水平,诱导了吞噬和抗氧化反应。76因此,NAD + 和去乙酰化酶之间亲密相互作用的破坏通过这些核-线粒体通讯机制导致严重的线粒体功能障碍,这是衰老的标志。

At a systemic level, the communication between the control center of aging (the hypothalamus) and its modulator (adipose tissue) might be compromised when systemic NAD+ biosynthesis decreases (Figure 5b). As discussed in the previous section, adipose tissue remotely controls the hypothalamus through the secretion of eNAMPT. On the other hand, the hypothalamus also appears to control adiposity through a SIRT1-dependent signaling pathway. In the dorsomedial hypothalamus (DMH), SIRT1 and its binding partner Nkx2-1 have an important role in regulating aging and longevity.27 PR domain containing 13 (Prdm13) has recently been identified as one of the DMH-specific downstream target genes in the SIRT1/Nkx2-1 signaling pathway.79 Interestingly, DHM-specific Prdm13-knockdown mice exhibit decreased sleep quality and increased adiposity with no change in food intake, an interesting mimicry of age-associated pathophysiology. When NAD+ availability declines with age, it is likely that the secretion of enzymatically active eNAMPT would be affected in adipose tissue so that the hypothalamus would not be able to synthesize adequate levels of NAD+from circulating NMN for its function (Figure 5b). This would lead to a decrease in Prdm13 expression in the DMH, as observed indeed in aged hypothalamus,79 and an increase in adiposity. An interesting question is whether circulating eNAMPT levels would also increase as a compensatory response to sustain hypothalamic NAD+ levels. Although the study that tries to answer this question is currently underway, the systemic feedback loop between the hypothalamus and adipose tissue, mediated by the connection between NAD+ and SIRT1 in each tissue, is critical, contributing to the maintenance of physiological robustness. Therefore, the breakdown of this intimate interplay between NAD+ and sirtuins, particularly SIRT1, in each tissue likely leads to the breakdown of the inter-tissue communication between the hypothalamus and adipose tissue, resulting in systemic functional decline over age and eventually limiting lifespan in mammals.

在全身水平上,当全身 NAD + 生物合成减少时,衰老控制中心(下丘脑)和它的调节器(脂肪组织)之间的通讯可能受到损害(图5b)。脂肪组织通过分泌釉蛋白远程控制下丘脑。另一方面,下丘脑似乎也通过 sirt1依赖的信号通路控制肥胖。在背内侧下丘脑(DMH)中,SIRT1及其结合伙伴 Nkx2-1在调节衰老和长寿方面具有重要作用。最近发现含有13(Prdm13)的27个 PR 结构域是 SIRT1/Nkx2-1信号通路中 DMH 特异性的下游靶基因之一。79有趣的是,dhm-Prdm13基因敲除小鼠表现出睡眠质量下降和肥胖,而食物摄入量没有改变,这是一个有趣的与年龄相关的病理生理学研究。当 NAD + 的利用率随着年龄的增长而下降时,脂肪组织中酶活性烯胺铂的分泌可能会受到影响,从而使下丘脑不能合成足够水平的 NMN 中的 NAD + 来维持其功能(图5b)。这将导致在 DMH 中 Prdm13表达的减少,正如在老化的下丘脑中观察到的,79和增加肥胖。一个有趣的问题是,作为维持下丘脑 NAD + 水平的代偿性反应,循环中的釉蛋白水平是否也会增加。虽然试图回答这个问题的研究目前正在进行,但是下丘脑和脂肪组织之间的系统反馈回路,通过在每个组织中 NAD + 和 SIRT1之间的联系来调节,是至关重要的,有助于维持生理健壮性。因此,NAD + 和 sirtuins (特别是 SIRT1)之间的这种亲密相互作用在每个组织中的破坏可能导致下丘脑和脂肪组织之间的组织间通讯的破坏,导致随着年龄的增长全身功能衰退,最终限制哺乳动物的寿命。

Concluding remarks

结语

The tight functional connection between NAD+ and sirtuins has a critical role in regulating physiological robustness, and contributing to aging/longevity control in diverse organisms. Recent studies have demonstrated that NAD+ availability declines over age due to the defect in NAMPT-mediated NAD+ biosynthesis and the PARP-mediated NAD+depletion, reducing sirtuin activities and affecting the communication between the nucleus and mitochondria at a cellular level and also the inter-tissue communication, particularly between the hypothalamus and adipose tissue, at a systemic level. These events likely cause age-associated pathophysiologies and contribute to the pathogenesis of diseases of aging. For this reason, supplementing key NAD+ intermediates, such as NMN and NR, is expected to mitigate age-associated functional decline and ameliorate a variety of age-associated pathophysiologies. The connection between NAD+ and sirtuins will provide deep mechanistic insight into how energy metabolism shapes the process of aging and determines lifespan in evolutionarily diverse organisms. Footnote1

NAD + 和去乙酰化酶之间紧密的功能联系在调节生理健壮性方面起着关键作用,并有助于多种生物体的衰老/寿命控制。最近的研究表明,NAD + 的可利用性随着年龄的增长而下降,这是由于 nampt 介导的 NAD + 生物合成缺陷和 parp 介导的 NAD + 缺失,减少了去乙酰化酶活性,影响了细胞水平上核与线粒体之间的通讯,以及在全身水平上下丘脑与脂肪组织之间的通讯,特别是下丘脑与脂肪组织之间的通讯。这些事件可能导致年龄相关的病理生理,并有助于老年疾病的发病机制。由于这个原因,补充主要 NAD + 中间物,如 NMN 和 NR,可望减轻年龄相关的功能衰退和改善各种年龄相关的病理生理学。NAD + 和去乙酰化酶之间的联系将为能量代谢如何影响衰老过程和决定进化多样的生物体的寿命提供深刻的机制洞察。脚注1

Notes

注释

  1. 1.Subtitles are cited from Todd Rundgren’s lyrics of ‘It Takes Two To Tango (This Is For The Girls)’ in his album ‘Something/Anything?’ released in 1972.字幕引用了托德 · 伦德格伦(Todd Rundgren)的专辑《 Something/Anything? 》中的歌词“ It Takes Two To Tango (This Is For The Girls)”1972年被释放。

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