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Diet, Dementia, Cognitive Impairment, Diabetes Intersect
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Reader Reference:
Advanced Glycation End products (AGEs)
Lipoxidation End Products (ALEs)
*are well-known glucose-derived factors contributing to diabetes-related complications
"Evidence suggests that the best way to reduce your risk of developing dementia is regular exercise, not smoking, and following a healthy diet."...... "Foods high in protein and fat, such as meat, cheese, and egg yolk, are rich in AGEs, and cooking at high temperatures, for example frying and barbecues, increases AGEs.
"For some scientists, the most exciting part of the study is the potential explanation of how diabetes and dementia might be linked. Diabetes is one of the few concrete risk factors for dementia, and doubles the risk of a person developing the disease, but how the two are connected has remained a mystery. Vlassara found that rodents and humans on a high glycotoxin diet had low levels of SIRT1 in the body, a protein that is thought to protect the brain from neurodegeneration."
"We report that age-related dementia (AD) and MS may be causally linked to high levels of food AGEs, specifically MG. The data extend our previous findings on AGEs promoting the MS in older animals and humans (22-24). The mouse study further reproduces the cognitive and metabolic conditions recently found to be linked in humans (1, 4, 5). The clinical study validates the mouse model and demonstrates that high sMG, a marker of dietary AGE intake and IR (23), may also be a determinant of dementia in older adults (17). It further validates the relevance of dietary AGEs to MS and AD in humans. Because AGEs can be modified in humans, recognition that this underappreciated risk factor plays a role in AD and MS may open unique therapeutic avenues.......Changes in the modern diet include excessive nutrient-bound AGEs, such as neurotoxic methyl-glyoxal derivatives (MG)......the highest AGE levels were observed in animal products high in protein and fat, such as meats and cheeses. Furthermore, high AGE levels were observed in (industrially) preprocessed foods from animal products like frankfurters, bacon, and powdered egg whites, compared with the unprocessed forms. Across all categories, exposure to higher temperature raised the AGE and ALE content (for equal food weights). The temperature level appeared to be more critical than the duration. Also, microwaving increased AGE content more rapidly compared with conventional cooking methods"
Fried and grilled meat may raise risk of diabetes and dementia
Study suggests changes in cooking habits might reduce levels of glycotoxins and help prevent diabetes and dementia
Ian Sample, science correspondent
theguardian.com, Tuesday 25 February 2014
Glycotoxins are found in fried or grilled meat, fried eggs and toasted bread. Photograph: Suzanne Plunkett/Reuters
Toxic chemicals found at high concentrations in fried and grilled meats may raise the risk of diabetes and dementia, researchers say.
US scientists found that rodents raised on a Western-style diet rich in compounds called glycotoxins showed early signs of diabetes, along with brain changes and symptoms that are seen in Alzheimer's disease.
The findings matched what the researchers saw in a small number of older people, where those with higher levels of glycotoxins in their circulation had memory and other cognitive problems, and signs of insulin resistance, which precedes diabetes.
The results are tentative, but if confirmed by other studies, the work could transform hopes for tackling two major diseases that have reached epidemic proportions in the developed world.
The study suggests that changes in cooking practices might lower the risk of both diabetes and dementia, while a greater understanding of the biological mechanisms could lead to drugs that delay their onset.
"The findings are very promising, but the question that needs to be answered is whether cutting down on glycotoxins can prevent or reverse dementia," said Helen Vlassara, who led the study at the Icahn School of Medicine at Mount Sinai in New York.
Vlassara raised groups of mice on diets that differed in their levels of a type of glycotoxin called advanced glycation end products, or AGEs. Animals that ate a Westernised diet rich in AGEs experienced a build-up of protein called amyloid in their brains, and developed cognitive and movement problems typical of dementia. The same changes were not seen in mice raised on a low-AGEs diet.
The researchers then turned to 93 healthy humans aged over 60. Over nine months of the study, they found that those with higher levels of AGEs in their bloodstream experienced greater levels of cogntive decline and insulin resistance. Details are published in Proceedings of the National Academy of Sciences.
Glycotoxins are widespread in animal products, including meat and dairy produce, and levels increase when food is fried, grilled, pasteurised or smoked, making them abundant in Western diets.
The sheer ubiquity of glycotoxins means dietary changes might not be easy or effective as public health interventions, but Vlassara said that cooking foods differently might help. Levels of glycotoxins rise when food is cooked dry at high temperature, but moisture prevents this.
"People will grill bacon and fry eggs for breakfast, or have a toasted bagel or muffin. But they could boil or poach the eggs, and have fresh bread. With meat, we recommend stewing and boiling, making sauces instead of exposing meat to very high dry heat," she said.
For some scientists, the most exciting part of the study is the potential explanation of how diabetes and dementia might be linked. Diabetes is one of the few concrete risk factors for dementia, and doubles the risk of a person developing the disease, but how the two are connected has remained a mystery. Vlassara found that rodents and humans on a high glycotoxin diet had low levels of SIRT1 in the body, a protein that is thought to protect the brain from neurodegeneration.
Simon Lovestone, a neuroscientist and leader of the dementia translational research collaboration at Oxford University and King's College London, said the finding was "fantastically interesting" but added that further studies were needed in people to see if reducing dietary glycotoxins staved off dementia.
Doug Brown, director of research and development at the Alzheimer's Society, said: "We are often told that burgers or fried chicken are bad for us and this study is not the first to link the chemicals in some cooked foods to Alzheimer's. However, this research adds to our understanding of how they might work and makes a strong case for further research.
"Diets with low levels of the compounds show promising effects in mice and should be further explored as a way to prevent dementia through changes in diet," he added. "Of course, we must not forget that the majority of research was conducted in mice and the human element of this study is too small to draw any conclusions.
"Evidence suggests that the best way to reduce your risk of developing dementia is regular exercise, not smoking, and following a healthy diet."
Derek Hill at University College London said the results were compelling and should encourage more work. "It is notoriously difficult to do experiments on mice that properly mimic Alzheimer's disease in humans," he said. "But it is grounds for optimism. This paper adds to the body of evidence suggesting that using preventative strategies might reduce the prevalence of Alzheimer's disease and other dementias in society. And that could have a very positive impact on us all."
Tom Dening, professor of dementia research at Nottingham University, said: "Foods high in protein and fat, such as meat, cheese, and egg yolk, are rich in AGEs, and cooking at high temperatures, for example frying and barbecues, increases AGEs. Dietary restriction of AGEs can bring about significant reductions. What isn't yet clear is how much these effects contribute to Alzheimer's disease compared to other factors, and we don't know whether dietary restriction of AGEs would be helpful in prevention."
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Glycotoxins: A Missing Link in the "Relationship of Dietary Fat and Meat Intake in Relation to Risk of Type 2 Diabetes in Men"
We have read with great interest the article entitled "Dietary Fat and Meat Intake in Relation to Risk of Type 2 Diabetes in Men," by Van Dam et al. (1), which suggests a relationship between increased consumption of animal fat and red and processed meats and higher risk of type 2 diabetes in men.
We propose that the recently recognized toxic derivatives of advanced glycation and lipoxidation abundant in diets may explain the associations observed. Advanced glycation end products (AGEs) and lipoxidation end products (ALEs) are well-known glucose-derived factors contributing to diabetes-related complications (2). In addition to endogenous glucose, diet constitutes an important exogenous source of reactive precursor and terminal AGE and ALE, including a-ß-dicarbonyl-containing derivatives. Common methods of food processing include heating, sterilizing, or ionizing, all of which tend to accelerate the nonenzymatic addition of nonreducing sugars to free NH2-groups of proteins and lipids, a chemical process known as the Maillard reaction (3). This process, also known as "browning" of foods, is largely responsible for the color and flavor of cooked foods that most people are drawn to.
Recent estimates of AGE levels in ~200 commonly consumed foods, based on immunoreactivity assays for specific AGEs (4G9; Alteon, Northvale, NJ) (4), found AGE and ALE content of these foods to be relative not only to food composition, but also to mode of cooking, temperature, and duration of exposure to heat. In particular, the presence of fats, which are major generators of free radicals that can enhance oxidative processes, including butter and margarine, attributes to high AGE and ALE levels. Thus, the highest AGE levels were observed in animal products high in protein and fat, such as meats and cheeses. Furthermore, high AGE levels were observed in (industrially) preprocessed foods from animal products like frankfurters, bacon, and powdered egg whites, compared with the unprocessed forms. Across all categories, exposure to higher temperature raised the AGE and ALE content (for equal food weights). The temperature level appeared to be more critical than the duration. Also, microwaving increased AGE content more rapidly compared with conventional cooking methods (5).
Studies in humans and animals have confirmed the significant intestinal absorption of consumed meal AGEs and their subsequent tissue retention (6,7). Restriction of food AGE intake in animals offered a marked protection against significant pathology observed in animal models of diabetic atherosclerosis, nephropathy, wound healing, and postinjury restenosis (femoral artery) (8-11). Recently, a marked improvement of various features of insulin resistance was demonstrated in db/db mice fed a diet low in AGEs (lower glucose and insulin responses to glucose challenge and improved lipid profiles) (12). Preliminary data from a 6-week study in patients with type 1 or type 2 diabetes, randomized to a high- or low-AGE diet, showed a significant reduction in the low-AGE diet group of circulating markers of inflammation, typical of diabetes vascular disease (13).
Based on the above data, we propose that dietary glycoxidation products may constitute an important link between the increased consumption of animal fat and meats and the subsequent development of type 2 diabetes.
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Oral glycotoxins are a modifiable cause of dementia and the metabolic syndrome in mice and humans
PNAS March 31 2014
"Modulation of the environment, i.e., caloric excess or caloric restriction (CR), can influence cognitive function.......Together with the animal data, these clinical findings reinforce the fact that chronic exposure to exogenous AGEs can weaken host defenses well in advance of cognitive or metabolic disturbances. A critical finding afforded by the animal studies is that AGE restriction prevented the loss of both conditions, highlighting glycotoxins as a modifiable risk for AD and MS in humans. Because aspects of the MS in humans may improve after AGE restriction (23, 24), it is possible that cognition can also improve in humans. Given the major public health potential of these findings, larger clinical trials are warranted......Age-related MS and diabetes are also causally associated with suppressed SIRT1 partly due to oxidant glycotoxins [advanced glycation end products (AGEs)]. Changes in the modern diet include excessive nutrient-bound AGEs, such as neurotoxic methyl-glyoxal derivatives (MG)."
Significance
Suppression of NAD+-dependent sirtuin 1 (SIRT1) is linked to dementia or Alzheimer's disease (AD) and the metabolic syndrome (MS). Because advanced glycation end products (AGEs) promote MS and neurotoxicity, we conducted studies of C57BL6 mice fed isocaloric diets containing defined AGEs [methyl-glyoxal derivatives (MG)] to determine whether food AGEs promote AD and MS. MG+-fed, but not MG--fed, mice developed brain SIRT1 deficiency, amyloid-ß deposits, cognitive and motor deficits, and MS. These findings were validated in older healthy humans with high baseline circulating MG levels by a time-dependent decline in cognition and insulin sensitivity. The data suggest that food-derived AGEs, an environmental factor, contribute to both AD and MS by causing chronic SIRT1 suppression. Importantly, reduction of food-derived AGEs is feasible and may provide an effective treatment strategy for both these epidemics.
Abstract
Age-associated dementia and Alzheimer's disease (AD) are currently epidemic. Neither their cause nor connection to the metabolic syndrome (MS) is clear. Suppression of deacetylase survival factor sirtuin 1 (SIRT1), a key host defense, is a central feature of AD. Age-related MS and diabetes are also causally associated with suppressed SIRT1 partly due to oxidant glycotoxins [advanced glycation end products (AGEs)]. Changes in the modern diet include excessive nutrient-bound AGEs, such as neurotoxic methyl-glyoxal derivatives (MG). To determine whether dietary AGEs promote AD, we evaluated WT mice pair-fed three diets throughout life: low-AGE (MG-), MG-supplemented low-AGE (MG+), and regular (Reg) chow. Older MG+-fed mice, similar to old Reg controls, developed MS, increased brain amyloid-ß42, deposits of AGEs, gliosis, and cognitive deficits, accompanied by suppressed SIRT1, nicotinamide phosphoribosyltransferase, AGE receptor 1, and PPARγ. These changes were not due to aging or caloric intake, as neither these changes nor the MS were present in age-matched, pair-fed MG- mice. The mouse data were enhanced by significant temporal correlations between high circulating AGEs and impaired cognition, as well as insulin sensitivity in older humans, in whom dietary and serum MG levels strongly and inversely associated with SIRT1 gene expression. The data identify a specific AGE (MG) as a modifiable risk factor for AD and MS, possibly acting via suppressed SIRT1 and other host defenses, to promote chronic oxidant stress and inflammation. Because SIRT1 deficiency in humans is both preventable and reversible by AGE reduction, a therapeutic strategy that includes AGE reduction may offer a new strategy to combat the epidemics of AD and MS.
Cognitive dysfunction is currently one of the most prevalent and important polygenic age-related diseases (1⇓-3). A link has been identified between dementia, the most frequent form of which is Alzheimer's disease (AD), and the metabolic syndrome (MS) or diabetes type 2 (T2D) (1, 3), conditions largely related to environmental factors (4, 5). An emerging view suggests that there is a compromise in innate defense mechanisms preceding these conditions that is due to sustained elevation of oxidant stress (OS) (6).
Modulation of the environment, i.e., caloric excess or caloric restriction (CR), can influence cognitive function (7, 8); however, the calorie-sensitive pathway(s) involved are unknown. NAD+-dependent sirtuin 1 (SIRT1), an NAD+-dependent deacetylase that positively regulates neuronal, immune, and endocrine responses, is down-regulated in aging-related diseases and is thought to contribute to cognitive decline (1, 9⇓-11). Restoration of brain SIRT1 is widely implicated in the benefits of CR on the aging brain (7⇓-9, 11).
Glycotoxins or advanced glycation end products (AGEs) are a class of OS-promoting agents implicated in diabetes and aging, including brain injury due to AD and stroke (6, 12⇓-14). Certain AGEs, such as the derivatives of methyl-glyoxal-imidazolone-H1 (MG-H1) amplify the proinflammatory properties of amyloid ß1-42 (Aß) or tau protein (15⇓-17). High MG levels in brain or the circulation are linked to cognitive decline in elderly subjects (15, 17, 18).
Food-derived AGEs have emerged as contributors to chronic diseases due to their abundance in thermally altered nutrients (19, 20). AGE restriction in nutritionally and nutritionally balanced diets delayed metabolic and vascular diseases and extended lifespan in mice (21, 22). The role of diet-derived AGEs in systemic AGE toxicity was confirmed in studies using a defined MG-supplemented low-AGE diet (MG+). Old MG+ mice, but not MG- mice, developed age-related MS and kidney and cardiac fibrosis, associated with inflammation and SIRT1 depletion in insulin-sensitive tissues (22). AGE restriction also improved insulin resistance and inflammation in humans (23, 24).
We therefore postulated that oral AGEs, in addition to causing MS, might also predispose to dementia, and these both might be prevented by AGE restriction (22). The MG+/MG- mouse model provides an opportunity to explore the link of oral AGEs to these chronic conditions, free of either genetic or caloric manipulations.
Herein, we show that cognitive dysfunction develops in parallel with metabolic changes in old mice fed defined AGEs (MG+) but not in AGE-restricted (MG-) mice. These findings were supported by clinical findings, introducing previously unidentified evidence of AGEs as a modifiable risk factor for both AD and MS.
Results
Chronic Oral MG+ Promotes Systemic and Brain Changes in Old WT Mice.
MG+ (18 mo) mice fed an MG-supplemented diet had higher body weight than pair-fed age-matched MG- mice fed a low-AGE diet (Table 1), a finding attributed to the higher amount of AGE-modified visceral fat found only in MG+ (and Reg 24-26 mo) mice (22). Higher serum AGEs [serum εN-carboxymethyl-lysine (sCML) and sMG], plasma 8-isoprostanes, and lower adiponectin levels were noted in MG+ and Reg mice (Table 1 and Fig. 1A), suggesting elevated OS in these two groups, but not in MG- mice (22). MG+ and Reg mice were also insulin resistant, based on higher fasting insulin and leptin levels (Table 1) and on an abnormal i.p. glucose tolerance test (IGTT) (SI Materials and Methods) (22).
Both brain protein- and lipid-associated AGE levels in Reg and MG+ mice were higher than in MG- mice (Table 1 and Fig. 1 B and C). Brain tissue from MG+ mice had reduced protein levels of SIRT1 and of the [NAD+/NADH]-regulating nicotinamide phosphoribosyltransferase (NAMPT), relative to MG- mice (Fig. 2 A and B), suggesting that exogenous AGEs induce parallel changes in brain and in the periphery (22). Brain AGE receptor 1 (AGER1) and PPARγ levels were reduced, and receptor for AGEs (RAGE) levels were enhanced in MG+, compared with MG-, brain tissue (Fig. 2 A and B) (22).
Oral MG+ Reduces ADAM10 Transcriptional Activity and Promotes Aß Accumulation.
A disintegrin and metalloproteinase binding protein 10 (ADAM10) modulates amyloid precursor protein (APP) and soluble APP-beta (APP, etc.) (sAPP-ß) levels, limiting the accumulation of Aß1-42, and is regulated by SIRT1 (25). In this context, ADAM10 mRNA and protein levels in MG+ and Reg brain were significantly lower than in MG- brain (Fig. 3 A and B, i and ii). Levels of total APP and sAPP-ß, the product cleaved by ß-secretase, were similar in MG+ and Reg brains. In contrast, sAPP-ß levels and the sAPP-ß:APP ratio were lower in MG- brains than in MG+ or Reg brains (Fig. 3C). Furthermore, the levels of Aß in the MG+ and Reg brains were significantly higher than in the MG- brain (Fig. 3D).
Morphometric analysis of hippocampal (HC) areas for anti-GFAP-positive glia indicated significantly more cells and levels of activation in MG+ than in MG- HC (Fig. 4 A, B, and Inset). MG+ HC had prominent AGE-positive aggregates colocalizing with GFAP-positive cells in areas of dense glial populations (Fig. 4 C and D, i and ii). In contrast, MG- HC sections displayed fewer cells and no AGE-positive clusters (Fig. 4C). No cortical differences were noted by specific nuclear protein staining.
Neocortical SIRT1 Expression Is Suppressed by Chronic MG+ Excess.
Chronically elevated MG levels could directly or indirectly predispose fetal neurons to injury. SIRT1 and NAMPT were suppressed in MG+ neuronal cells compared with cells from MG- cells (Fig. S1 A-C). Reduced AGER1 levels (Fig. S1C) were consistent with higher intraneuronal RAGE, AGEs, and reactive oxygen species (ROS) levels in Reg and MG+ neurons than in MG- neurons (Fig. S1 D and G). Moreover, ADAM10 was markedly suppressed in MG+ neurons but not in MG- neurons (Fig. S1E).
Prolonged ex vivo stimulation of Reg neurons with MG-BSA (>72 h) resulted in a dose-dependent suppression of SIRT1, AGER1, and ADAM10 (Fig. S2 A and B), changes that were associated with increased ROS (Fig. S2C).
Chronic MG+ Impairs Learning and Memory.
Basic motor coordination and balance learning skills were first evaluated with the rotarod test. MG- mice performed for a longer distance and at a higher speed before falling from the rod compared with MG+ mice (Fig. 5 A and B). MG+ mice showed a lower latency than MG- mice (Fig. 5C). On testing object recognition, MG+ fed mice showed poor exploratory behavior with a lower discriminatory capacity between a familiar and a novel object than MG- mice (Fig. 5D), which spent ~70% of the time exploring the new object. On testing object replacement, MG+-fed mice performed better, but this was not significant (Fig. 5E).
High MG Correlates with Dietary AGE Intake and SIRT1 Suppression in Older Humans.
At baseline, the cohort's body mass index (BMI) and metabolic and biochemical parameters (n = 93, ≥60 y old, educated, 68% female) were within the range expected for their age, as were calorie and dietary AGE intake (dAGE) (24, 26). Baseline cognitive function [by Mini Mental State Examination (MMSE)] was also normal (Table S1).
Baseline sMG levels correlated positively with dAGE intake (Fig. 6A) and inversely with mononuclear cell (MNC) SIRT1 mRNA levels (Fig. 6B and Table S2). In addition, baseline dAGE and sMG levels both correlated with sCML, plasma 8-isoprostanes, leptin, MNC TNFα protein, and RAGE mRNA, but inversely with SIRT1 mRNA and adiponectin levels (Table S2 and Fig. S3).
High MG Levels in Older Humans Correlate with Temporal Changes in Cognition and Insulin Sensitivity.
High baseline sMG levels predicted a cognitive decline over time (9 mo, P = 0.041; Fig. 7A), which remained significant after adjusting for age, sex, education, and baseline MMSE. Temporal changes in homeostasis model assessment (HOMA)-IR, a marker of insulin resistance, also correlated with changes in sMG (Fig. 7B and Fig. S4A), as well as with sCML (Fig. S4B). No other metabolic changes were noted.
Discussion
We report that age-related dementia (AD) and MS may be causally linked to high levels of food AGEs, specifically MG. The data extend our previous findings on AGEs promoting the MS in older animals and humans (22-24). The mouse study further reproduces the cognitive and metabolic conditions recently found to be linked in humans (1, 4, 5). The clinical study validates the mouse model and demonstrates that high sMG, a marker of dietary AGE intake and IR (23), may also be a determinant of dementia in older adults (17). It further validates the relevance of dietary AGEs to MS and AD in humans. Because AGEs can be modified in humans, recognition that this underappreciated risk factor plays a role in AD and MS may open unique therapeutic avenues.
Brain deposits of AGEs and Aß are thought to be age related (13⇓⇓-16, 27⇓⇓⇓-31). The current study shows that both of these elements were increased in brains of old MG+ mice to levels similar to those in old Reg controls. An important insight provided herein is that these changes cannot be attributed to aging or caloric intake alone, because the levels of AGEs and Aß were significantly lower in strain- and age-matched pair-fed MG- mice.
Brain dysfunction has also been associated with the MS and T2D, conditions linked to nutrient intake (8, 9, 11). We previously showed that MG+ mice had features of the MS, including AGE-modified white adipose tissue (WAT) accumulation and SIRT1 suppression (22). The effects of calories and CR on cognition were previously thought to be directly related to brain SIRT1 expression (10, 11). We found that modern diets are also replete with prooxidant AGEs, including MG (19⇓-21). These data, coupled with the fact that the MG+ diet induces systemic inflammation and SIRT1 suppression in this study independently of caloric intake (21, 22), provides a clear link between SIRT1 deficiency of the aging brain and glycotoxins. Mouse models of CR, AD, or SIRT1 expression in mice reveal that SIRT1 plays a central role in brain function (2, 8, 25). However, the fact that SIRT1 and NAMPT, AGER1, and PPARγ were suppressed in MG+ but not in MG- brains indicates that loss of multiple defense mechanisms in aging may reflect the impact of sustained high OS and that AGEs could play a seminal role (22). The MG- mouse data further demonstrate that SIRT1 deficiency may be preventable in mice regardless of caloric intake.
Changes in the SIRT1 pathway have been linked to AGE receptor levels (22⇓-24). AGE receptors are expressed in brain neurons, microglia, and endothelium. AGER1, an anti-AGE receptor, was up-regulated in the brain of MG- mice, whereas RAGE, a signaling receptor linked to oxidative stress and neurotoxicity, was decreased (30, 31). Because systemic AGER1 also inhibits SIRT1 suppression (22), it could have a similar effect on SIRT1 in the brain. In the current study, we found that neurons from MG- mice had higher AGER1 levels, which were associated with higher levels of SIRT1 and lower levels of intracellular AGE and ROS. In contrast, reduced AGER1 in Reg and MG+ neurons, by delaying the clearance of AGE-modified proteins such as AGE-Aß (13), could account for the increased amounts of AGE deposits, suppression of SIRT1, and glial activation seen in the brains of MG+ mice but not in MG- mice.
The data from neocortical neurons of MG+ mice further suggest a placental mode of transfer of excessive AGEs to the fetal brain, which might render the brain more susceptible to OS injury. The findings might be of relevance to the increasing incidence of dementia in younger adults with the MS or diabetes. Importantly, this injurious process appears to be preventable in brains of MG- mice, a finding of significant therapeutic import. Whether this involves epigenetic changes or an altered gut microbiome is a critical subject for further inquiry (32, 33).
SIRT1 deficiency leads to impaired insulin receptor signaling in adipose tissue from MG+ mice (22). It is not known if this pathway is altered in the brains of MG+ mice, although this might provide a mechanistic link to the insulin-resistant state shown to be associated with cognitive decline (1, 3, 34). Nonetheless, the prominent gliosis noted in the hippocampus of the MG+ mice, coupled with suppressed SIRT1 and AGER1, is consistent with an MG-mediated inflammatory response. The AGE aggregates observed in these mice could have elicited inflammatory responses (27⇓⇓-30), partly via RAGE activation (14, 35). Whether the effects in MG+ mice are a reflection of altered blood-brain barrier, high intracerebral OS, or both, remains to be established. However, the fact that lower MG levels in MG- brains were associated with lower OS and RAGE suggests that lowering external AGEs could exert significant benefits.
In this context, SIRT1 also regulates liver X receptor, forkhead box subgroup O, and PPARγ, important factors in brain plasticity (2, 11, 36). Additionally, PPARγ, which promotes amyloid clearance and suppresses glial activation (37), was decreased in MG+ and Reg compared with MG- brains. Thus, low PPARγ may delay Aß clearance, a hypothesis supported by higher levels of Aß levels and gliosis in brains of MG+ and Reg mice compared with MG- mice.
SIRT1 also limits Aß accumulation by directing APP processing via ADAM10 and α-secretase transcription (25). Because SIRT1 deficiency in MG+ mice was associated with reduced ADAM10 levels, this may partly account for the increased APPß/total APP ratio and the higher Aß generation in MG+ and Reg mice. The absence of these changes in the MG- mice further supports the view that altered brain homeostasis may stem from sustained exposure to neurotoxic AGEs.
Importantly, impaired spatial learning and recognition memory in MG+ mice mirrored cognitive changes in older humans (11, 38, 39). Significantly, these were absent in MG- mice, offering previously unidentified direct in vivo evidence that oral AGEs can impair cognition.
Because the MG+ mice also manifested in parallel metabolic (22) and cognitive changes, the data may identify MG as a causal link between AD and MS (3, 24). Herein, we found a significant temporal decline in cognition in subjects with high baseline sMG level, together with a strong inverse correlation between baseline levels of dietary or serum AGEs and MNC SIRT1 gene levels (24). Furthermore, changes in insulin resistance temporally correlated with changes in serum AGE levels. Together with the animal data, these clinical findings reinforce the fact that chronic exposure to exogenous AGEs can weaken host defenses well in advance of cognitive or metabolic disturbances. A critical finding afforded by the animal studies is that AGE restriction prevented the loss of both conditions, highlighting glycotoxins as a modifiable risk for AD and MS in humans. Because aspects of the MS in humans may improve after AGE restriction (23, 24), it is possible that cognition can also improve in humans. Given the major public health potential of these findings, larger clinical trials are warranted.
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Advanced glycation end products, dementia, and diabetes
PNAS March 31 2014
It is becoming abundantly clear that the insight into the pathological process of Alzheimer's disease (AD) provided through autosomal dominant variants of the condition is only a partial one. The formation and aggregation of Aß and the phosphorylation and aggregation of tau are clearly part of the core pathogenesis. However, although these may be necessary processes, and indeed in familial forms of the condition possibly also sufficient processes, they are not the whole story in the common late-onset forms of the disease. Here, mixed pathologies are the norm and at post mortem the plaques and tangles formed by Aß and tau are accompanied also by inflammation and by vascular disease. This finding is congruent with the epidemiology that has long pointed to only three substantial factors that alter risk of dementia other than age: head injury, anti-inflammatory drugs, and diabetes. It is reasonably clear why head injury and anti-inflammatory drugs might affect risk but the relationship between diabetes and dementia has been far less clear. It could be that diabetes simply increases risk of vascular and related damage to the brain as it does to limbs, kidneys, and other organs. More interesting to molecular scientists are the observations that insulin signaling modifies amyloid precursor protein (APP) metabolism and tau phosphorylation in cells and in animal models, suggesting a possible influence of metabolic pathways on the canonical pathway of AD. Most intriguing of all is the observation that such pathways and processes are related not only to metabolic disease and to AD, but also to aging or longevity itself. In PNAS, Cai et al. (1) shed light on these complex interactions and point to possible clinical implications, including both biomarkers and potential therapeutics. In line with the considerable evidence for an oxidant-mediated pathogenic effect, Cai et al. show that a pro-oxidant diet in mice induces ß-cleaved APP and Aß generation. At the same time these mice, fed methyl-glyoxyl (MG) derivatives, had increased weight and systemic insulin resistance. In both the mice and also in a human cohort, dietary MG levels and accompanying advanced glycation end products (AGEs) correlated positively with cognitive deficits or decline and inversely with survival factor sirtuin-1 (SIRT1) levels and other markers of insulin sensitivity.
The most striking, and sometimes neglected, observation in relation to risk of AD is that age increases it. This might be a simple inconsequential coincidence; it takes a lifetime of insult to result in neuronal dysfunction. However, the results of Cai et al. (1) hint at a more substantial possibility. The sirtuins, of which there are seven in mammals, have long been implicated as factors involved in longevity (2, 3). The most studied-SIRT1, -3 and -6-are all increased as a consequence of caloric restriction (CR), and CR increases both longevity and insulin sensitivity and also reduces certain features of AD pathology (4). It may indeed be the effect of CR on SIRT expression or function that is the critical factor, as mice lacking SIRT1 had a shortened lifespan even in the context of CR (5). These data suggest that the positive effects of CR are, at least in part, mediated by the induction of sirtuins.
Cai et al. (1) add another element to this growing story in showing that an oxidative diet of MG derivatives (MG+) decreases SIRT1 protein in mouse brain and, in man, there is evidence of increased MG in blood correlated with decreased SIRT1 mRNA levels in circulating monocytes. There is an apparent anomaly here though: in Cai et al. increased MG is associated with cognitive decline and decreased SIRT1, whereas in mouse models of neurodegeneration SIRT1 levels are generally increased (6). A plausible explanation for this otherwise puzzling observation would be that mice are able to respond protectively to the disease process. This theory has been suggested before and one of the earlier experiments to find such a protective factor pointed to a substantial increase in the insulin-signaling pathway that occurs in the brain of APP transgenic mice (7), a not unexpected effect if SIRT1-induced insulin sensitivity is part of a protective pathway.
If a decrease in the sirtuins, whether induced by diet or by some other factor, increases risk of both AD pathogenesis and insulin resistance, might this be the underlying mechanism to the undoubted link between AD and diabetes (8)? In line with such a hypothesis, in mice, SIRT1 increased insulin sensitivity through acetylation and phosphorylation of insulin receptor substrate-2 (IRS2) (5), whilst loss of IRS2, as well as resulting in insulin resistance, increased Aß-induced tau phosphorylation (9). Some recent evidence, however, suggests the effect of SIRT1 on insulin signalling might be different in neurons (10). The generation of AGEs is a common process in both diabetes and AD and might also stimulate the inflammatory response of both conditions (11).
The findings of Cai et al. (1), together with a substantial preceding literature, suggest a process whereby diet-and an excess of glycotoxins in particular-suppresses SIRT1 with adverse consequences for both systemic insulin sensitivity and AD pathogenic processes, including APP metabolism and the response of tau-kinase-regulating pathways leading to the phosphorylation of tau. Increased tau phosphorylation disrupts binding to microtubules resulting in a translocation from the axon, a loss of microtubule stability and function, and a tendency to increased aggregation (12), all of which are part of the tau-related toxicity of the disease (13). Taking these data together, our current understanding of the effect of glycotoxins is that they are able to cross-talk with pathways involved in glucose regulation and response to insulin, as well as with pathways of cytokine response and innate immunity (14), and so bring together in a network three of the four known environmental influences on AD: diabetes, inflammation, and age itself (Fig. 1). In fact, there is increasing evidence for glycotoxins being the lynchpin of the wheel of disease that includes diabetes, AD, and immunity; diets (such as those in the Mediterranean) that decrease glyocotoxins reduce risk not only of metabolic disease (15), but also of AD (16), as well as the inflammatory correlates of disease (17).
There are some fairly obvious practical considerations that follow from this integrative hypothesis; most obvious of these are public health measures to reduce calorie intake, obesity, and its associated complications, including type 2 diabetes. Another important step to consider now is to build credibility for the concept that it is not necessarily only increased caloric intake but, more specifically, increased intake of glycotoxins in the diet that plays an important role for these common human disorders. This process requires the initiation of sufficiently large controlled clinical trials specifically addressing the role of the glycotoxins, and the time seems ripe to consider this now. Might there also be other ways to build on the growing findings around glycotoxin-induced loss of sirtuin expression and function? One way might be to increase sirtuin expression directly, but which SIRT gene and in which target tissue? Another way might be to increase sirtuins indirectly, such as through resveratrol, a phytocompound found in, among other things, red wine. There is some evidence linking resveratrol directly and indirectly to the prevalence of dementia (18) and trials in man are underway. Another approach would be to target the signaling pathways downstream of SIRT1. These pathways include peroxisome proliferator-activated receptor-γ (PPARγ), and Cai et al. (1) observe, as would be expected, PPARγ suppression alongside a decrease in SIRT1 in glycotoxin-fed animals. Although trials of the PPARγ agonist Rosiglitazone did not prove efficacious in AD (19), this may be because they were conducted too late in the disease course to test the concept properly.
All of this points to an important piece of the jigsaw that many are searching for: a good biomarker of risk. If a marker, ideally a peripheral fluid marker, could be identified that would act as a herald of preclinical or incipient pathology, then secondary prevention trials in preclinical phases would become possible. There is now some evidence that the glycotoxin-induced pathway might be such a biomarker. Sirtuin 1 may be increased in serum in AD (20) and PPARγ gene variation might modify age of onset (21). AGEs and their receptors may be both risk factors and biomarkers.
It is probably too much to ask for a simple explanation of a complex disease, such as AD. Nonetheless, there is a growing body of work, substantially enhanced by Cai et al.'s report (1), that not only suggests an incidental link between metabolic disease and AD but begins to tease out the underlying molecular pathways. Specifically, the finding that a diet rich in glycotoxins increases the ß-cleavage of APP, resulting in Aß generation, and is accompanied by a decrease in SIRT1 does offer sight of a unifying hypothesis that is in line with previous evidence. More importantly, this finding offers some obvious experiments to test the hypothesis. Would a MG-rich diet have the same effects on learning and memory and on Aß generation in mice overexpressing sirtuins? Indeed, is an effect mediated simply by SIRT1 or are some of the other six mammalian sirtuins also involved? In man do these findings replicate; and might PPARγ agonism be more efficacious in a population not only early in disease but stratified by biomarkers relevant to this pathway including AGE, RAGE, and sirtuins; and might dietary change be additive to PPARγ agonists? These and other questions follow from these data, but for now another piece of evidence has been added to the link between diabetes, AD, inflammation, aging, and indeed diet.
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