6.1 Ageing and clinical medicine 511

6.1 Ageing and clinical medicine 511

6.1 Ageing and clinical medicine Claire Steves and Neil Pendleton ESSENTIALS In 2017 there were, for the first time, more people older than 65 years than children under the age of 5 years. Despite the recent exponential increase in human lifespan, health-​span has not kept pace, and variability between countries in healthy lifespan exceeds that of life expectancy. The increase in morbidity as people age is largely explained by loss of physiological reserve capacity in multiple systems simul- taneously, which is termed frailty. Recent evidence suggests that different heritable (intrinsic factors) factors drive the ageing of dif- ferent organ systems, but diverse systems share environmental (or extrinsic) drivers. Ageing is associated with macromolecular changes (molecular damage); changes in nutrient sensing, metabolism, and metabolic signalling; senescence in stem cells; altered intercellular commu- nication, in particular changes associated with inflammaging; and changes in circadian rhythms and the hypothalamo-​pituitary-​ adrenal axis. We need to better understand these processes to meet the challenge set by extension in lifespan and achieve healthy ageing and reduction in age-​associated disease. Introduction Population ageing has seen unprecedented changes in human demographics across the world. Estimates are that in 2017 people older than 65  years will outnumber under 5-​year-​olds and, by 2050, more than a fifth of the world population will be over 60  years (Fig. 6.1.1). However, despite this recent exponential increase in human lifespan, health-​span has not kept pace, and variability between countries in healthy lifespan exceeds that of life expectancy (Fig. 6.1.2). The increase in morbidity is largely explained by loss of physiological reserve capacity in multiple sys- tems simultaneously, resulting in reduced resistance to stressors. This has been characterized as frailty, a multidimensional state which predicts adverse health events and mortality. Ageing and frailty are separate concepts. Any attempt to look at the mechanisms underpinning ageing must consider the relation- ship between time-​dependent functional decline despite optimal conditions—​‘intrinsic ageing’—​and the real-​life development of frailty syndromes, which may more reflect inactivity, stress, and exposure—​‘stressed ageing’. The extension in lifespan we have seen sets a new challenge to investigate the potential for healthy ageing and reduction in age-​associated disease. Heritability studies in twins show that population variance in frailty and many diseases of old age arise largely due to a combination of heritable effects (shared more by monozygotic twins) and environmental effects unique to individuals, which include stochastic variations. Few genetic links to longevity or frailty have been established in humans, which is likely to be due to a large number of small effects under the threshold of detection. Notable exceptions are the APOE genotype, and the INK4a-​INK4b locus. Studies in mammals have identified various interesting targets that appear to prolong life and re- duce disability—​in particular the target of rapamycin (TOR) pathway, and insulin or insulin-​like growth factor 1 signalling system, discussed next. Recent evidence suggests that different heritable (intrinsic factors) factors drive the ageing of different organ systems, but diverse systems share environmental (or ex- trinsic) drivers. All branches of clinical medicine are seeing increases both in older adult patients and in the combinations of co​morbidities in the context of whole system ageing, due in part to common risk factors (Fig. 6.1.3). An appreciation of gerontology is therefore relevant for all physicians. A more collaborative approach between those working on chronic diseases such as cardiovascular disease, cancer, and dementia with underlying processes of ageing is needed. This chapter aims to provide clinicians with concise description of the interplay between human ageing and age-​related diseases. In add- ition, new potential therapeutic avenues for age-​related disease are arising from research into ageing in model animals and humans, and these are also discussed in this chapter. Macromolecular changes Molecular damage underpins most models of ageing, and some have conceptualized ageing as a consequence of an imbalance be- tween molecular damage and repair. Experimental biogerontology

512 Section 6  Old age medicine Age <5 Age 65+ 20% 15% 10% 5% 0% 1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050 Fig. 6.1.1  Young children and older people as a percentage of Global Population 1950–​2050. From United Nations World Population Prospects, the 2010 Revision. Available at http://​esa.un.ord/​ unpd.wpp. (a) 20 24 22 20 18 16 14 12 10 8 HLYs at 50 years of age (years) 24 22 20 18 16 14 12 10 8 HLYs at 50 years of age (years) 22 24 26 28 30 32 Men (c) Men Women (b) Women NL GR MT DK LE HLYs LE HLYs BE LU PT SI CZ SK HU EE LT LV PL FR CY AT DE FI ES IE SE IT UK EE LT SK LV HU PT DE FI CY AT SI CZ BE IE NL SE GR UK PL MT DK LU ES FR IT 28 30 32 LE at 50 years of age (years) 34 36 14.53 18.42 15.92 14.77 23.64 9.05 12.86 18.01 13.56 19.78 10.78 18.91 20.63 11.02 11.49 17.99 21.68 20.21 16.48 14.90 12.28 15.34 19.16 20.22 19.74 33.70 33.39 32.86 30.72 31.94 30.52 34.15 35.37 33.41 33.02 29.40 33.24 35.31 29.32 29.90 33.60 32.74 33.28 31.23 32.92 29.96 32.44 35.02 34.05 32.69 15.66 18.66 13.71 16.26 24.12 10.42 13.87 19.74 13.55 20.81 11.39 20.17 20.86 12.74 11.86 18.16 22.58 20.40 20.16 12.67 13.07 17.25 18.62 20.31 20.78 Austria (AT) Belgium (BE) Cyprus (CY) Czech Republic (CZ) Denmark (DK) Estonia (EE) Finland (FI) France (FR) Germany (DE) Greece (GR) Hungary (HU) Ireland (IE) Italy (IT) Latvia (LV) Lithuania (LT) Luxembourg (LU) Malta (MT) Netherlands (NL) Poland (PL) Portugal (PT) Slovakia (SK) Slovenia (SI) Spain (ES) Sweden (SE) UK (UK) 29.08 28.67 29.52 25.61 28.30 22.42 28.48 29.57 28.96 29.43 22.72 29.50 30.37 21.31 21.74 28.78 29.07 29.14 24.62 28.12 23.68 26.81 29.48 30.28 29.46 Fig. 6.1.2  Inequalities in healthy life years in the 25 countries of the European Union in 2005, showing life expectancy (LE) and healthy life years (HLYs) at 50 years of age for all EU countries. HLYs, healthy life years; LE, life expectancy. (a) and (b) show scatter graphs for men and women, respectively. (C) Data for scatter graphs. From Jagger C, et al. (2008). Inequalities in healthy life years in the 25 countries of the European Union in 2005: a cross-​national meta-​regression analysis. Lancet, 372, 2124–​31, with permission from Elsevier.

6.1  Ageing and clinical medicine 513 has shown that damage to DNA, protein, lipids, and other molecules is associated with ageing in simple organisms, but it is not yet clear how the accumulation of molecular damage relates to age-​related disease and frailty in humans. Understanding the types and consequences of macromol- ecular damage associated with ageing will open up possibilities of intervening to redress the balance in favour of repair. However, protein, DNA, lipids, or other molecules do not function in isola- tion. Taking a network or systems approach allows integration of data across multiple levels of molecular change. This more holistic approach in biological gerontology has clear parallels with practice in clinical gerontology. One of the oldest theories in line with the damage/​repair balance has been the Free Radical/​Oxidative Stress Theory of Ageing, which postulated that oxidation damage decreased cellular func- tion resulting in reduction in system adaptability and an increase in development of pathology. However, it is becoming apparent that this theory is at least incomplete—​conflicting evidence comes from experimental models manipulating antioxidant enzyme systems, comparative studies in species which are naturally long-​lived, and age-​related disease and mortality in humans. Contradictions may be explained in part by the principle of hormesis—​that submaximal stress may actually be healthy. Genomic instability In DNA, damage has many forms, from point mutations to chromosomal gains and losses, and these changes accumulate with age and can manifest in age-​related cancer and cellular senescence, and link to pathological changes seen with age. Likewise loss of telomere length has long been associated with age. These protective caps of chromosomes shorten at each cell division, but can be rebuilt by repair enzymes in many cell types. Termination leads to replicative senescence and has been linked to diseases such as cancer. Whether telomere length acts as a marker or is a driver itself is not clear. Some evidence for the importance of DNA damage in ageing comes from progeroid genetic syndromes, which are characterized by some features of premature ageing. For example, loss of function of an acute thrombocytopenic purpura (ATP) dependent helicase in Werner’s syndrome leads to defects in DNA double strand breaks and enhanced telomere attrition, with the cellular consequences of increased cell death and senescence. Epigenetics Epigenetics can be considered operationally as a mitotically heritable feature or phenotype resulting from changes to the chromosome without alterations in DNA sequence. This involves changes in chromatin (DNA methylation and histone structure), transcription mechanisms, and non​coding ribonucleic acid (RNA). This is how molecular scientists can explain substantial differences in lifespan of animals such as worker ants and bees, versus the queen, who has the same genetic library, but differing reference systems. Recent research in epigenetics has identified some mechanisms underpinning intrinsic ageing. The Sirtuins family hold an im- portant role in age-​related diseases and organism lifespan. These are histone deacetylases and ADP-​ribosyltransferases which ap- pear to have a role in regulating genomic stability. Their effect on chromatin regulation demonstrates linkage between ageing and reproducible modifiers such as caloric restriction and exercise. The Sirtuins and specifically SIRT-​1 have been implicated in models of Alzheimer’s disease and diabetes mellitus. Using agents with direct effects on Sirtuins may mimic the positive ageing effect of calorie restriction. Organs/Systems Glucose intolerance Immune system dysfunction Sarcopenia Adiposity Endocrine system dysfunction Nervous system dysfunction Cardiovascular system dysfunction Renal dysfunction Respiratory system dysfunction Cells Genomic instability Mitochondria dysfunction Telomere attrition Cellular senescence Epigenetic alterations Loss of proteostasis Deregulated nutrient sensing Stem cell exhaustion Altered intercellular communication Organism Homeostasis breakdown with increased risk of failure Fig. 6.1.3  A hierarchical model of ageing from cells to organism. From Wu IC, et al. (2015). Biomedicine (Taipei), 5(1), 1.

514 Section 6  Old age medicine While at present there is no direct evidence that alterations in DNA methylation can extend animal lifespan, Horvath’s clock—​a combination of 353 methylation markers—​can pre- dict human age within four years, with a Pearson’s correlation coefficient of 0.96. As such it is currently the best biomarker for age. However, it is not yet clear whether this clock is helpful as a marker of age-​related morbidity, or whether the clock measures a biological process relevant to the variation in human lifespan. Epigenetics is thus seen as a exciting area of basic science re- search, translating the unrealized potential of the genome to understand mechanisms playing a role in human age-​related dis- ease. Such new insights may suggest targets for intervention. Modification in protein structure Maintenance of normal protein structure is a prerequisite of normal cellular function. Collectively the interactive network of functional proteins is called the proteome. Stressors constantly perturb the status quo by damaging proteins, resulting in aggregation. The cellular mechanism which monitors and manages the potentially damaging molecular aggregates of toxic material or misfolded proteins is called the proteostasis network. However, even within normal function, protein structural modifications need to occur to allow transport and assembly for correct function, with the resultant opportunity for damage in the form of altered conformational states. It is likely that this is why the complex system of proteostasis, with its chaperones and proteases, evolved. This network is both local and systemic. The ubiquitin-​based proteosome system is a key player in the management of cellular misfolded proteins, in which linkage of ubiquitin to the protein acts as signal for destruction. Although molecular chaperones such as ubiquitin can act by altering misfolding errors, their effects on damaged proteins is more di- verse. Chaperone molecules can also act as buffers to mutation-​ encoded abnormal protein structure. Ageing leads to greater levels of protein damage requiring management, but it also af- fects the efficiency of proteostasis. In this ‘Proteotoxicity’ model, damage can result in both enzymatic dysfunction and aggrega- tion, as is observed in several age-​related conditions such as car- diovascular, neurodegenerative disease, and cancer (Fig. 6.1.4). Proteosome management includes organelles, with mito- chondria being a key example. Mitochondria have a central role in cellular energy systems and generate reactive oxygen species. Delivering this function exposes these organelles to constant damage. In turn damaged mitochondria must be disaggregated and removed. Failure to adequately manage outputs from damaged mitochondria is seen with increasing age, and also in specific age-​ associated diseases such as Parkinson’s disease. While we understand the importance of individual proteins and the proteosome to healthy cell function, we know very little about systemic effects within organs or systems. We have some evidence of systemic effect of lifestyle via high fat intake, which can tip the balance in cellular proteostatic function leading to excess damaged aggregates. This can then lead to organ dysfunc- tion, disease phenotypes, and ageing. Autophagosome Heat shock Lysosome Chaperone-mediated autophagy Proteasomal degradation Unfolded protein Chaperone-mediated folding Refolded protein HSP HSF-1 Ageing Aggregation Oxidative stress Folded protein ER stress Hsc70 UbUbUbUb Macroautophagy Fig. 6.1.4  Age-​associated loss of proteostasis after stressors leading to accumulation/​aggregation driven toxicity.
ER stress, endoplasmic reticulum stress. From López-​Otín C, et al. (2013). The hallmarks of aging. Cell, 153(6), 1194–​217, with permission from Elsevier.

6.1  Ageing and clinical medicine 515 Nutrient sensing, metabolism, and metabolic signalling The close relationship between metabolic processes and ageing has been understood for decades. Specifically the effect of caloric re- striction on extension of lifespan has been demonstrated across species. These findings probably extend to primates: a study fol- lowing rhesus monkeys for 20 years in Wisconsin showed a sig- nificant improvement in lifespan on limiting monkeys to consume 70% of their own ad libitum diet (Fig. 6.1.5). Also, and crucially, they showed improvements in multiple ageing-​related diseases including diabetes, cancer, cardiovascular disease, and brain atrophy. The problem in translating these findings to humans is that it is unlikely, in a food-​rich era, that the general population will will- ingly restrict their diet to 30% less than ‘they would like’. Research into the mechanisms of energy restriction is important to deter- mine the most efficacious and acceptable means, and to identify ways to therapeutically ‘highjack’ the pathways involved. In add- ition, dissemination of knowledge of how caloric restriction works may help people to ‘own the idea’ and be more willing to take it on board in their lifestyle. The mechanistic effects of caloric restriction Metabolism is an essential process to the production of fuel energy, structural synthesis, growth, and reproduction. Core pathways such as glycolysis and fatty acid oxidation are altered in ageing and relatively spared in animal models of caloric re- striction. Diverse animal models have shown that a critical enzyme complex in metabolic signalling is the mammalian target of rapamycin complex 1 (mTORC1), which is activated in states of nutrient excess or by growth factors. The mTORC1 complex drives anabolic processes, building energy stores and macromolecular synthesis. Its actions have been studied using the inhibitor rapamycin, which was initially developed as an immunosuppressive agent. Several hypothetical mechan- isms for its life-​extending properties have been identified. One is the reduction of protein synthesis, resulting in lower burden on proteostatic systems. The mTORC1 complex inhibits cellular autophagy, including macroautophagy of older proteins and or- ganelles. Rapamycin increases this activity, which has important cell survival consequences. The adenosine monophosphate (AMP) dependant protein kinase (AMPK) is the reciprocal complex to mTOCR1 activation, re- sulting in catabolic adenosine triphosphate (ATP) generation and energy repletion in times of nutrient deprivation. A well-​known antidiabetic drug, metformin, is an indirect AMPK activator and has been used to explore the mechanism increasing the cellular AMP/​ADP ratio. Its use results in reduced mortality in many spe- cies, including man. Functionally it inhibits mTORC1 activity and promotes macroautophagy. These two key complexes in metabolic regulation altered by either rapamycin or metformin result in con- sequential effects on lifespan, but it is not yet known whether this comes with reduction in age-​associated pathology, with the excep- tion of metformin in diabetes mellitus, although this may not be due to this effect. Another relevant system is the nicotinamide adenine dinucleo- tide (NAD) coenzyme which, via redox reaction, forms redu- cing agent NADPH. NAD can also donate ADP ribose to poly ADP ribose polymerases (PARP) that are involved in DNA re- pair. Pharmacological agents focused on PARP molecules are currently being trialled in certain cancers. NAD also acts as a substrate for the Sirtuin family of protein deacetylases. Ageing reduces the activity of some Sirtuins, but caloric restriction can prevent this ageing decline in SIRT-​1 and SIRT-​3. The SIRT-​1 ac- tivator reservatrol has been a controversial agent in the study of ageing. It may act by increasing mitochondrial genesis, activating AMPK and increasing NAD levels, thereby demonstrating a link between these systems. The reservatrol lifespan extension in animal models seems to be mediated by reducing the negative effects of a high fat diet. The benefits of caloric restriction have led to studies examining limitation in specific nutritional components (Fig. 6.1.6). Not all behave similarly, and diets that predominantly restrict pro- tein may be much more efficacious than those which are high in protein, yet low in calories. The mechanisms that underlie these food-​type specific effects on longevity are currently under inves- tigation, and both the mTOR pathway and reduction in reactive oxygen species formation may be relevant. Futile cycles A counterintuitive observation in cellular metabolism is so-​called futile cycling. This is a process where two pathways flow in opposite directions, resulting in apparently useless energy transfer and en- ergy wastage through heat production. Some of these futile cyc- ling processes are increased with ageing, including the glycerol/​ free fatty acid cycle. The hydrolysation of triglycerides to free fatty acids can be used to generate ATP, while these free fatty acids can also be used to regenerate triglycerides. A connection with mol- ecules associated with ageing is present by virtue of this process delivering NAD levels required for PARP and sirtuin function. A further futile cycle-​related mechanism is uncoupling in mito- chondria, whereby there is loss of energy through incomplete coup- ling of ATP generation by oxidative phosphorylation. The resultant reactive oxygen species can damage cellular components and lead to an ageing phenotypic change, depending on their location. Age-related mortality (a) (b) Age (years) Percent survival 10 20 0 40 80 100 60 15 20 25 30 35 All-cause mortality Age (years) Percent survival 10 20 0 40 80 100 60 15 20 25 30 35 Control CR Control CR Fig. 6.1.5  Mortality curves in rhesus monkeys with sustained caloric restriction. CR, caloric restriction. Reproduced from Colman RJ, et al. (2009). Caloric restriction delays disease onset and mortality in rhesus monkeys. Science, 325(5937), 201–20​4.

516 Section 6  Old age medicine This observation connects the futile cycle process with the Free Radical/​Oxidative Stress Theory of Ageing. Senescence in stem cells There is currently substantial interest in research focused on the regenerative potential of stem cells, largely to repair degeneration of tissue associated with ageing. A challenge to this is the decline associated with ageing on stem cell functionality. There is considerable diversity of stem cell activity in normal tissues in an adult. Stem cells in some organs (e.g. the skin and blood) retain life long regeneration potential, whereas in others (such as neurological tissue) regeneration is limited. Data on Sirtuins and cancer suppressor genes in stem cells currently suggests that stem cells are generally more resistant to genetic damage. Nevertheless, quiescent stem cells accumulate other mo- lecular damage which may contribute to tissue ageing through impairing the stem cells’ productivity, or passing defects on to their cell progeny in older organs (Fig. 6.1.7). This could lead to tissue degeneration and may limit the potential to re-​activate stem cells in older organ systems. Reactivation of stem cells depends on their interaction with the local and systemic environment, which may be altered in aged or- gans, for example, by chronic inflammation. In organs that have limited intrinsic stem cell activity (e.g. brain and heart), trans- plantation holds more promise than activation. Parabiotic experi- ments, linking young and old animals (see next), show that old stem cells can be rejuvenated when exposed to circulating factors present in young animals. Considering all these data it is clear that stem cell therapy, whether using endogenous or implanted sources, presents potentially major opportunities to deal with the effects of ageing on human organs that lead to conditions such as arthritis and sarcopenia. They offer particular potential in diseases of organs with cellular loss and limited regenerative potential, such as the neurodegenerative con- ditions Parkinson’s and Alzheimer’s diseases. Insights from parabiosis There has been recent rejuvenation of interest into experiments joining young and old animals in the search for circulating fac- tors in younger animals with the ability to combat certain aspects of ageing. In such experiments, two genetically identical or inbred animals (most notably mice) of different ages have their circulations surgically linked (heterochronic parabiosis). Follow-​on screening for proteins in the blood of these mice have identified circulating factors which may be pro-​ageing or antiageing, in particular, for muscle tissues (the Wnt and TGF​β signalling pathways and oxy- tocin), and in stem cells (chemokines CCL11/​Eotaxin impeding neurogenesis in the hippocampus and subventricular tissue). Replication by different groups and in different species, as in all studies, is important to be certain if these effects are likely to be robust in humans. There are also concerns that factors stimulating stem cells (in particular) have the potential side effect of tumori- genesis. Nevertheless, in the study of potential circulating factors, parabiotic or serum transplant methodology could be extended, for example, by genetically altering one animal of a same aged pair in relation to the pathway of interest. Altered intercellular communication Inflammaging and immunosenesence The observation that ageing is accompanied with a chronic low-​ grade inflammatory state has been referred to as inflammaging (Fig. 6.1.8). This occurs in the absence of overt infection and is as- sociated with increased risk of mortality. This active inflammatory Control Caloric restriction Shift in energy metabolism Normal energy metabolism Decreased rate of ageing Longevity Morbidity & mortality Normal ageing Cellular damage dysfunction and loss Master regulators Fig. 6.1.6  Caloric restriction-​induced reprogramming of energy metabolism leading to reduced rate of ageing. From Anderson RM, et al. (2009). Caloric restriction and aging: studies in mice and monkeys. Toxicologic Pathology, 37(1), 47–​51. Loss of lineage specificity Progeny Stem cells ? YOUNG OLD Depletion due to loss of self-renewal Depletion due to senescence Malignant transformation Fig. 6.1.7  Decline in stem cell function with age. From Liu L and Rando TA (2011). Manifestations and mechanisms of stem cell aging. J Cell Biol., 193(2), 257–​66. © 2011 Liu and Rando, available under the Creative Commons BY-​NC-​SA 3.0 license.

6.1  Ageing and clinical medicine 517 state is evidenced in epidemiological studies by elevation of C-​ reactive protein (CRP) and interleukin 6 (IL6) levels. Although theoretically this observation may suggest a causal association with reduction in functional level, body composition changes, im- mune and organ function, this has yet to be proven. Nevertheless, inflammaging has the potential to offer interventions that may alter the morbidity associated with increased age, with transla- tional opportunities using existing agents. When either tissue damage or invasion by a foreign pathogen takes place, acute inflammation is an essential response to repair and management of the potential harmful consequences. Chronic inflammation, characterized by low level persistent activity in the immune system, has negative consequences leading to tissue damage and degeneration. There are several potential mechanisms by which this occurs. One is the production of reactive molecules by leukocytes which attack tissue cellular components. Both im- mune and other cells types that are damaged in this process can produce cytokines which amplify the inflammation. Inflammation leads to modification of anabolic signalling via insulin-​like growth factor and other similar molecules, aimed at protein synthesis. These mechanisms interact and lead to state of chronically in- flamed tissues in older adults. Further stimuli of immune function include the increased cellular molecular debris, such as damaged part-​organelles, peroxidized lipids, and glycated proteins, that result from increased production and inadequate elimination seen with ageing. An example of this is the damaged components of mitochondria, which being believed to be of bacterial origin are potentially potent stimuli to inflammation. Cellular senescence occurs in response to stress and damage, with senescent cells accumulating in older tissues. Persistent senescence leads to production of a collection of pro-​ inflammatory cytokines which comprise the senescence-​associated secretary phenotype (SASP). The immune system itself also under- goes immunosenesence, which leads to a mild increase in innate im- munity and a reduction in adaptive immunity, possibly enhanced by inactive infections such as cytomegalovirus (CMV) where T-​cell resources are expended. There is also an increased activity in the co- agulation system in aged animals and humans, leading to vascular pathology which also activates the immune system. Although we understand that increased active circulating in- flammatory molecules are associated with poor health and survival, we should consider whether this is through contributing to increased age-​related diseases. Core to the ageing immune phenotype are IL6 and tumour necrosis factor. These molecules, by interacting with cell surface receptors, activate transcription factors for multiple regulatory genes like nuclear factor kappa-β (NF-​kβ), which regulates most genes implicated in the senescence-​ associated secretary phenotype. NF-​kβ also directly drives several phenotypes associated with ageing, such as neurodegeneration and atherosclerosis. These observations provide potential common pathways from biological ageing to disease morbidity associated with older age. There is evidence for this in diseases such as cancer and vas- cular disease, in which inflammatory cytokines and senescence-​ associated secretary phenotypes are implicated. Complementary evidence for this comes from murine models where restoration of ‘young’ circulating inflammatory systemic factors can limit age-​ associated tissue changes. Examples include using whole ‘young serum’ to enhance muscle stem cells and maintaining comparable levels of circulating gonadotrophin-​releasing hormone to reverse ageing in muscle, brain, and skin of older animals. Immune ageing may not always be detrimental, but could be an adaptation that can optimize maintenance and repair. A relevant observation that supports a positive view of inflammaging is that long-​lived humans, centenarians, have high levels of circulating Il6 and hypercoagulability. This is in the face of a delay in the onset of many age-​associated diseases such as cardiovascular disease or cancer. This contradiction needs explanation, but possibly relates to differing genotypic sensitivities to pro-​inflammatory cytokines, or that inflammaging can be in a state of balanced activity within long-​lived adults. The substantial evidence that inflammation is important both in ageing and disease development prompts the obvious question as to whether modifying the process improves health. The non-​ steroidal anti-​inflammatory drug aspirin is the most widely used cardiovascular prevention intervention, acting at cardiac, cerebral, and systemic sites. There is observational evidence that it is asso- ciated with lower risk of several cancers seen in later life. Further evidence of prevention of mortality associated with inflammaging is the effect of exercise, which modifies chronic inflammation. The microbiome With the advent of genetic sequencing technology, a new area of re- search has sprung up investigating the health effects of the compos- ition of the human microbiome on health. Evidence is mounting that the gut microbiome, in particular, influences host metabolism Antigenic load and environmental free radicals Immune activation and tissue damage Inflammation & repair Oxidative metabolism Reactive oxygen species Further release of pro- inflammatory cytokines Remodelling and Inflammaging Fig. 6.1.8  The cycle of inflammaging. From Baylis D, et al. (2013). Understanding how we age: insights into inflammaging. Longev Healthspan, 2(1), 8.

518 Section 6  Old age medicine and the innate and adaptive immune system. This may have par- ticular relevance in inflammaging and frailty. Our gut, with an astounding surface area of about 32 square metres, is the largest interface with the external world, and contains 70% of the body’s lymphocytes. The gut microbiome has more than 150 times the genetic po- tential of the human genome by virtue of the trillions of micro- organisms, which are highly variable between people, yet stable in individuals. Age and frailty are both associated with alternations in the microbiota, which is likely to be a dynamic combination of host habitability, diet, and microbial effects. The identification of several classes of anti-​inflammatory commensal bacteria has ig- nited an interest in whether the microbiota of older subjects can be manipulated—​especially by diet—​to reduce systemic inflamma- tory levels and ensuing consequences. In conclusion, inflammaging is an important focus to the inter- play between ageing and chronic disease. Further understanding of the balance between positive features of immune response to harmful stimuli and the damaging effect of chronic low level in- flammation is needed. Translation of knowledge of the basic mech- anism of inflammaging to prevent age-​associated multimorbidity has been enhanced by the development of agents for organ-​specific inflammatory diseases. The well characterized health benefits of a balanced diet and adequate exercise may act through modulating the inflammatory system. Further insight into the mechanisms at play will enable more specific interventions, which may be more palatable and feasible on a population scale. Circadian rhythms, the hypothalmo-​pituitary-​adrenal axis, and ageing Mammals have clock genes that regulate an intrinsic cell cycle, most of these being transcription factors affecting gene expression. These clocks run on a 24-​hour cycle affecting the time-​dependant and location-​dependant cellular processes seen in cells from dif- ferent tissues. Epidemiological evidence indicates that perturbation of sleep-​ activity cycles in humans (e.g. through shift work) is associated with detrimental effects in many ageing related diseases and life- span. Epidemiological evidence also supports circadian clock influence on disease processes, such as cardiovascular event rela- tionships to time of day (Fig. 6.1.9). Experimental models suggest alteration in circadian rhythms can reduce lifespan, for example, in mice. The Sirtuins have con- nection with the clock genes of circadian cycles. SIRT-​1 expression Nutrition Circadian Clock Shift-work/nutritional disturbance/ clock gene mutation etc Circadian harmony/ homeostasis Circadian desynchrony Predictive/reactive synchrony Anabolism/ Catabolism Repair Metabolic dysregulation ROS Cell death Impaired repair Damage accumulation Combat Avoid Gating Metabolic homeostasis Promote healthy ageing Accelerates ageing and Age-related pathologies Disturbed homeostasis Predictive/reactive desynchrony Fig. 6.1.9  A schematic representation of the role of clocks in cellular tasks impacting cellular homeostasis and consequently affecting healthy ageing. From Bednářová A, et al. (2013). Nature’s timepiece—​Molecular coordination of metabolism and its impact on aging. Int J Mol Sci, 14(2), 3026–​49.

6.1  Ageing and clinical medicine 519 declines with increasing age and is associated with both length- ening of the circadian rhythm and inability to adapt to this cycle. The hypothalamus is responsible for regulating physiologic func- tions such as temperature regulation, thirst, hunger, sleep, mood, sex drive, and the release of other hormones within the body. There is a central clock located within the suprachiasmic nucleus of the hypo- thalamus which functions to maintain synergy of cellular clocks across the mammalian body. Light/​dark cycles influence the central clock signal, resulting in dissemination of day cycle effects on periph- eral cells. A major function of the clock gene transcription is to regu- late metabolic processes involved in glucose and lipid homeostasis. An additional hypothalamus-​mediated effect on lifespan is through nuclear factor-​kappaB (NF-​kB), inhibition of which in the mouse hypothalamus results in lifespan extension via modifying its inflammatory mediating effect. There is also direct impairment of neurogenesis and poor metabolic regulation resulting from NF-​ kB suppression of gonadotrophin-​releasing hormone. Following on the inflammaging theme, alterations in the gut microbial me- tabolism, in part mediated by dietary changes associated with shift work, provide another mechanism for circadian rhythms to impact inflammation and thereby alter the cause of ageing and lifespan-​ limiting morbidities such as metabolic or cardiovascular disease. The hypothalamus and circadian rhythm effects on lifespan and age-​associated pathological developments provide linkage to metabolic signalling. These may be an important target for the generation of interventions that result in systemic effect on age-​ associated disease and healthy longevity. Stress and ageing—​a synthesis between extrinsic and intrinsic factors Stress has an interesting and complex relationship with ageing. This is because stress can be considered in a variety of ways, from cellular injury arising from intrinsic factors to whole organism psychological and social stressors. Additionally stress can be biologically harmful (toxic stress) or physiologically beneficial (hormetic stress), leading to beneficial changes in cellular systems. First, we need to consider stress with a multilevel approach. In simple multicellular organisms stress focuses on clear physical agents such as heat, radiation, and reactive oxygen species. In hu- mans, ‘extrinsic’ stress, such as psychological and social challenges, may also contribute. Such stressors have been associated with sys- temic neuro-​hormonal and immune activation. The integration of diverse forms of stress in humans results in a more variable effect of stress across similar stressors, making estimation of the effects more complex. When considering stress we need to synthesize the laboratory level evidence of physical agents with systemic more complex social adversities to try to understand how these play out in human ageing. This is important as significant epidemiological data show evidence for an association between chronic stress and diseases. At a molecular cellular level much of our understanding of the effects of stress comes from simple organisms. An example of this is the work showing that ‘intrinsic’ genetic variation in the nematode Caenorhabditis elegans leads to significant variability both in life- span and resistance to harmful agents such as heat, heavy metals, and oxidative stress. Work on mechanisms behind this multiple stress resistance model show that molecular chaperones such as Heat Shock Factor-​1 are associated with protein homeostasis, redu- cing accumulation of harmful insoluble molecules. Added to this are observations that short-​term controlled exposure to stressful events, such as heat, can increase lifespan in nematodes and flies (hormesis). The effect of short-​term stressors include activation of genes involved in metabolic control. Thus, stress can be bene- ficial in these simple models, controlling for extent and period of exposure, and where other challenges to survival are eliminated. In models of higher level organisms, including humans, the story is more complex. Examples of a stress resistant or resilient phenotype come from research on psychological stressors in which not all individuals in situations of adversity suffer the same nega- tive consequences. Psychological research points to cognitive pro- cesses and social resources mediating the difference in outcome to stressors. What is not understood is whether these abilities are translated into resistance to cellular stresses. The harmful effect of ‘extrinsic’ stress on human health is far better appreciated. There is epidemiological evidence that adverse life events, including those in childhood, lead to higher risk of disease and mortality. The effects of prenatal stress has also been linked to later human health, as set out in the Barker hypothesis of the Developmental Origins of Disease. This process may func- tion though epigenetic alterations which can be transgenerational. Unlike simple organism research, in human studies, attempts to identify mechanisms mediating association between stressors and age-​associated disease are more challenging, but markers such as systemic inflammation, gene expression profile, and telomere changes have been identified. If we accept that stress has both positive and negative effects on ageing morbidity, then we can consider how we might intervene. Some consider that exercise is an example of a physiological and metabolic stress, acting possibly by free radical production and re- duction in inflammation, which has beneficial aspects for ageing. Alternatively, physical inactivity prevalent in modern society may constitute the ‘stressed’ state, especially considering the far greater population levels of physical activity in our evolutional history. We need to better understand both whether hormetic stress benefits ageing, and the mechanisms that underpin resilient pheno- types. This may include the integration of evidence-​based interven- tions such as exercise into models aimed at reducing stress-​related age-​associated poor health. Conclusion A connected systems-​level knowledge of ageing underpins many clinical and population health challenges of ageing, such as cancer, cardiovascular, and neurodegeneration (Fig. 6.1.10). Ageing, and age-​related diseases within different organ systems and multimorbidity should not be considered in isolation, but rather as components of whole systems’ ageing, with many sys- tems sharing extrinsic factors in particular. This is why clinicians specializing in ageing use a comprehensive approach to the frail older patient, who generally presents with non​specific problems such as falls, delirium, and immobility. Another reason for such an approach is the appreciation of how evidence-​based health promo- tion/​disuse prevention interventions, such as exercise, nutrition,

520 Section 6  Old age medicine and positive lifestyle choices, improve health and well-​being. These can be used to develop or refine how we practice personalized medicine in an ageing world. FURTHER READING Fontana L, Partridge L (2015). Promoting health and longevity through diet:  from model organisms to humans. Cell, 161,
106–​18. Franceschi C, Campisi J (2014). Chronic inflammation (inflammaging) and its potential contribution to age-​associated diseases. J Gerontol A Biol Sci Med Sci, 69 Suppl 1, S4–​9. Global Health and Ageing (2011). WHO and National Institute Ageing. NIH Publication no. 11–​7737 October 2011. He S, Sharpless NE (2017). Senescence in health and disease. Cell, 169(6), 1000–11. Kundu P, Blacher E, Elinav E, Pettersson S (2017). Our gut microbiome: the evolving inner self. Cell, 171(7), 1481–93. López-​Otín C, et  al. (2013). The hallmarks of aging. Cell, 153, 1194–​217. Moskalev AA, et al. (2014). Genetics and epigenetics of aging and longevity. Cell Cycle, 13, 1063–​77. Niccoli T, Partridge L (2012). Ageing as a risk factor for disease. Curr Biol, 22, R741–​52. Special Issue. Why we age (2015). Science, 350, Dec 4. HERITABLE GENETIC AND EPIGENETIC FACTORS EXCESSIVE LIFE STRESS PHYSICAL INACTIVITY Changes in HPA axis Senescence Inflammaging Changes in nutrient sensing Molecular damage INTRINSIC AGEING CALORIC EXCESS EXTRINSIC FACTORS Fig. 6.1.10  Schematic of ageing, indicating possible interactions between extrinsic factors, mechanisms of ageing, and intrinsic factors driving molecular ageing. HPA Axis, hypothalamic-​pituitary-​adrenal axis.