Effects of aging on plasma islet amyloid polypeptide basal level and response to oral glucose load

Association between amylin and amyloid-β peptides in plasma in the context of apolipoprotein E4 allele

Amylin, a pancreatic peptide that readily crosses the blood brain barrier (BBB), and amyloid-beta peptide (Aβ), the main component of amyloid plaques and a major component of Alzheimer's disease (AD) pathology in the brain, share several features. These include having similar β-sheet secondary structures, binding to the same receptor, and being degraded by the same protease. Thus, amylin may be associated with Aβ, but the nature of their relationship remains unclear. In this study, we used human samples to study the relationship between plasma amylin and Aβ in the context of the apolipoprotein E alleles (ApoE). We found that concentrations of Aβ1-42 (P<0.0001) and Aβ1-40 (P<0.0001) increased with each quartile increase of amylin. Using multivariate regression analysis, the study sample showed that plasma amylin was associated with Aβ1-42 (β = +0.149, SE = 0.025, P<0.0001) and Aβ1-40 (β = +0.034, SE = 0.016, P = 0.04) as an outcome after adjusting for age, gender, ethnicity, ApoE4, BMI, diabetes, stroke, kidney function and lipid profile. This positive association between amylin and Aβ1-42 in plasma was found regardless of the ApoE genotype. In contrast, the relationship between amylin and Aβ1-40 in plasma seen in ApoE4 non-carriers disappeared in the presence of ApoE4. Using AD mouse models, our recent study demonstrates that intraperitoneal (i.p.) injection of synthetic amylin enhances the removal of Aβ from the brain into blood, thus resulting in increased blood levels of both amylin and Aβ. The positive association between amylin and Aβ, especially Aβ1-42, in human blood samples is probably relevant to the findings in the AD mouse models. The presence of ApoE4 may attenuate amylin's capacity to remove Aβ, especially Aβ1-40, from the AD brain.

Plasma Amylin and Cognition in Diabetes in the Absence and the Presence of Insulin Treatment

Background

Plasma amylin is positively associated with cognitive function in humans. Amylin treatment improves memory in Alzheimer’s mouse models. However, the relationship between plasma amylin, diabetes and cognition is not clear.

Objectives

In this study we examined the concentration of plasma amylin, its relationship with diabetes and cognition.

Material and Method

A cross-sectional, homebound elderly population with data of plasma amylin under fasting condition and cognitive measurements was used.

Results

We found that subjects with a long and chronic duration of diabetes were more likely to take insulin treatment and have reduced secretion of amylin. Compared to non-diabetics, diabetic subjects without insulin treatment had a higher concentration, but those with insulin treatment had a lower concentration, of plasma amylin [median (Q1, Q3): 20 (11.0, 36.2) vs. 25.2 (13.2, 50.6) vs. 15.0 (4.9, 33.8), p<0.0001]. In the whole sample vs. in the absence of diabetes, plasma amylin was positively associated with logical memory delayed recall (β= +0.61, SE=0.25, p=0.02 vs. β=+0.80, SE=0.33, p=0.02) and block design (β=+0.62, SE=0.24, p=0.009 vs. β=+0.93, SE=0.31, p=0.003), and negatively associated with Trailmaking A scores (β= −6.21, SE=1.55, p<0.0001 vs. β=−7.51, SE=1.95, p=0.0001) and Trailmaking B (β= −4.32, SE=2.13, p=0.04 vs. β= −5.86, SE=2.73, p=0.04). All these relationships disappeared in the presence of diabetes regardless the treatment.

Conclusion

This study suggests that secretion of amylin by pancreas compensates and then deteriorates depending on the duration of diabetes. Amylin’s activities for cognition are impaired in the presence of diabetes.

Positive association between plasma amylin and cognition in a homebound elderly population

Our recent study reported that amylin, a pancreatic peptide that readily crosses the blood-brain barrier, improves learning and memory in Alzheimer's disease mouse models. However, the relationship between peripheral amylin and cognition in humans is unknown. In this follow-up study, using a cross-sectional, homebound elderly population, improvement in cognitive function with increasing quartiles of plasma amylin was suggested by positive association with verbal memory (p = 0.0002) and visuoconstruction tasks (p = 0.004), and inverse association with timed measures of attention (p < 0.0001) and executive function (p = 0.04). After adjusting for demographic information, apolipoprotein E4 allele, diabetes, stroke, kidney function, and lipid profile, log10 of plasma amylin remained associated with these cognitive domains. In contrast, plasma amyloid-β peptide was not associated with these specific cognitive domains. Our study suggests that peripheral amylin may be protective for cognitive decline, especially in the domains affected by Alzheimer's disease.

Anorexia of aging and gut hormones

We are expected to live longer than if we had been born 100 years ago however, the additional years are not necessarily spent in good health or free from disability. Body composition changes dramatically over the course of life. There is a gradual increase in body weight throughout adult life until the age of about 60-65 years. In contrast, body weight appears to decrease with age after the age of 65-75 years, even in those demonstrating a previous healthy body weight. This age related decrease in body weight, often called unintentional weight loss or involuntary weight loss can be a significant problem for the elderly. This has been shown to be related to decline in appetite and food intake is common amongst the elderly and is often referred to the anorexia of aging. Underlying mechanisms regulate energy homeostasis and appetite may change as people age. In this review, peripheral factors regulating appetite have been summarized in regards to their age-dependent changes and role in the etiology of anorexia of aging. Understanding the alterations in the mechanisms regulating appetite and food intake in conjunction with aging may help inform strategies that promote healthy aging and promote health and wellbeing in the elderly years, with the end goal to add life to the years and not just years to our lives.

Gastric emptying, diabetes, and aging

Gastric emptying is mildly slowed in healthy aging, although generally remains within the normal range for young people. The significance of this is unclear, but may potentially influence the absorption of certain drugs, especially when a rapid effect is desired. Type 2 diabetes is common in the elderly, but there is little data regarding its natural history, prognosis, and management. This article focuses on the interactions between gastric emptying and diabetes, how each is influenced by the process of aging, and the implications for patient management.

Tissue Expression and Secretion of Amylin

Amylin and insulin are co-localized within the same secretory granules of pancreatic beta-cells. Acutely, the secreted ratio of amylin:insulin is comparatively invariant, but long-standing hyperglycemia may favor induction of amylin synthesis and secretion over that of insulin. Amylin is also found in much lesser quantities in the gut and other tissues. In humans, both type 1 diabetes mellitus and the later stages of type 2 diabetes mellitus are characterized by deficiency of both insulin and amylin secretion. The severity of amylin deficiency appears to correlate with the severity of insulin deficiency. This concordance of deficiencies in amylin and insulin secretion observed with the progression of diabetes mellitus is consistent with their co-localization in pancreatic beta-cells. Amylin is cleared mainly by proteolytic degradation at the kidney. The terminal t1/2 for rat amylin in rats is approximately 13 min, and that for pramlintide in humans is approximately 20-45 min.

Food intake and ageing—the role of the gut

Healthy ageing is associated with decreased appetite and energy intake and this is generally associated with weight loss after about 70 years of age. The mechanisms responsible for this 'physiological' anorexia are not well understood, but it may predispose to the development of protein-energy malnutrition in older people, which is associated with increased morbidity and mortality. Many sensory and social factors, including olfactory changes and economic status, contribute to under-nutrition in older people; however, normal ageing is associated with a number of significant changes in gastrointestinal function. The control of appetite is complex but it is clear that gastrointestinal signals are important in the regulation of appetite and food intake. This review examines the role of small intestinal hormones and gastrointestinal motor function in the observed changes to appetite and food intake in older people.

Chronic low dose islet amyloid polypeptide infusion reduces food intake, but does not influence glucose metabolism, in unrestrained conscious rats: studies using a novel aortic catheterization technique

Islet amyloid polypeptide (IAPP) is a 37-amino acid polypeptide coproduced with insulin in the beta-cells of the pancreatic islets. The physiological effects of IAPP have not been established. Although effects on glucose metabolism are seen only at pharmacological doses both in vitro and in vivo, effects on food intake have been shown at near-physiological concentrations. The aim of the present study was to investigate the effects of similar elevations of circulating plasma IAPP levels on glucose metabolism in rats and to evaluate the function of a novel aortic catheterization technique. In a cross-over design, two sets of experiments in which conscious unrestrained rats received chronic IAPP infusions at 0 and 2 or 0 and 7 pmol/kg min were performed. Peripheral glucose disposal was determined by means of the hyperinsulinemic euglycemic clamp technique. Chronic elevations of circulating IAPP at concentrations that reduced food intake [43.5 +/- 6.2 g (control) vs. 35.7 +/- 8.2 g (IAPP; P < 0.01) and 34.0 +/- 2.2 g (control) vs. 28.8 +/- 1.4 g (IAPP; P = 0.07) for the 7 and 2 pmol/kg x min experiments, respectively] had no effect on the glucose metabolic rate [GMR; 18.5 +/- 0.6 mmol/kg x h (control) vs. 18.7 +/- 0.9 mmol/kg x h (IAPP) and 14.4 +/- 0.7 mmol/kg x h (control) vs. 15.6 +/- 0.7 mmol/kg x h (IAPP) for the 7 and 2 pmol/kg x min experiments, respectively]. Thus, effects on glucose metabolism are unlikely to explain the anorectic effect of IAPP.