Diabetes and aging: epidemiologic overview

Alterations in skeletal muscle fatty acid handling predisposes middle-aged mice to diet-induced insulin resistance

OBJECTIVE

Although advanced age is a risk factor for type 2 diabetes, a clear understanding of the changes that occur during middle age that contribute to the development of skeletal muscle insulin resistance is currently lacking. Therefore, we sought to investigate how middle age impacts skeletal muscle fatty acid handling and to determine how this contributes to the development of diet-induced insulin resistance.

RESEARCH DESIGN AND METHODS

Whole-body and skeletal muscle insulin resistance were studied in young and middle-aged wild-type and CD36 knockout (KO) mice fed either a standard or a high-fat diet for 12 weeks. Molecular signaling pathways, intramuscular triglycerides accumulation, and targeted metabolomics of in vivo mitochondrial substrate flux were also analyzed in the skeletal muscle of mice of all ages.

RESULTS

Middle-aged mice fed a standard diet demonstrated an increase in intramuscular triglycerides without a concomitant increase in insulin resistance. However, middle-aged mice fed a high-fat diet were more susceptible to the development of insulin resistance-a condition that could be prevented by limiting skeletal muscle fatty acid transport and excessive lipid accumulation in middle-aged CD36 KO mice.

CONCLUSION

Our data provide insight into the mechanisms by which aging becomes a risk factor for the development of insulin resistance. Our data also demonstrate that limiting skeletal muscle fatty acid transport is an effective approach for delaying the development of age-associated insulin resistance and metabolic disease during exposure to a high-fat diet.

Modifiers of cognitive function and brain structure in middle-aged and elderly individuals with type 2 diabetes mellitus

Cognitive deficits and hippocampal atrophy, features that are shared with aging and dementia, have been described in type 2 diabetes mellitus (T2DM). T2DM is associated with obesity, hypertension, dyslipidemia, hypothalamic pituitary adrenocortical (HPA) axis abnormalities and inflammation, all of which have been shown to negatively impact the brain. However, since most reports in T2DM focused on glycemic control, the relative contribution of these modifying factors to the impairments observed in T2DM remains unclear. We contrasted 41 middle-aged dementia-free volunteers with T2DM (on average 7 years since diagnosis) with 47 age-, education-, and gender-matched non-insulin resistant controls on cognition and brain volumes. HPA axis activity and other modifiers that accompany T2DM were assessed to determine their impact on brain and cognition. Individuals with T2DM had specific verbal declarative memory deficits, reduced hippocampal and prefrontal volumes, and impaired HPA axis feedback control. Diminished cortisol suppression after dexamethasone and dyslipidemia were associated with decreased cognitive performance, whereas obesity was negatively related to hippocampal volume. Moreover, prefrontal volume was influenced by worse glycemic control. Thus, obesity and altered cortisol levels may contribute to the impact of T2DM on the hippocampal formation, resulting in decreased verbal declarative memory performance.

Age-related decline in brain resources modulates genetic effects on cognitive functioning

Individual differences in cognitive performance increase from early to late adulthood, likely reflecting influences of a multitude of factors. We hypothesize that losses in neurochemical and anatomical brain resources in normal aging modulate the effects of common genetic variations on cognitive functioning. Our hypothesis is based on the assumption that the function relating brain resources to cognition is nonlinear, so that genetic differences exert increasingly large effects on cognition as resources recede from high to medium levels in the course of aging. Direct empirical support for this hypothesis comes from a study by Nagel et al. (2008), who reported that the effects of the Catechol-O-Methyltransferase (COMT) gene on cognitive performance are magnified in old age and interacted with the Brain-Derived Neurotrophic Factor (BDNF) gene. We conclude that common genetic polymorphisms contribute to the increasing heterogeneity of cognitive functioning in old age. Extensions of the hypothesis to other polymorphisms are discussed. (150 of 150 words)

Mitochondrial apoptotic signaling is elevated in cardiac but not skeletal muscle in the obese Zucker rat and is reduced with aerobic exercise

Mitochondrial apoptosis and apoptotic signaling modulations by aerobic training were studied in cardiac and skeletal muscles of obese Zucker rats (OZR), a rodent model of metabolic syndrome. Comparisons were made between left ventricle, soleus, and gastrocnemius muscles from OZR (n = 16) and aged-matched lean Zucker rats (LZR; n = 16) that were untrained (n = 8) or aerobically trained on a treadmill for 9 wk (n = 8). Cardiac Bcl-2 protein expression levels were approximately 50% lower in the OZR compared with the LZR, with no difference in either of the skeletal muscles. Bax protein expression levels were similar in skeletal muscles of the OZR compared with the LZR. Furthermore, mitochondrial apoptotic signaling was not different in skeletal muscles of OZR and LZR groups. However, there was an approximate sevenfold increase in the Bax protein accumulation in the myocardial mitochondrial-rich protein fraction of the OZR compared with the LZR. Additionally, there was an increase in cytosolic cytochrome c released from the mitochondria, caspase-9 and caspase-3 activity, with a corresponding elevation in DNA fragmentation in the cardiac muscles of the OZR compared with the LZR. Exercise training reduced cardiac Bax protein levels, the mitochondrial localization of Bax, cytosolic cytochrome c, caspase activity, and DNA fragmentation in cardiac muscles of the OZR after exercise, with no change in the skeletal muscles. These data show that mitochondrial apoptosis is elevated in the cardiac but not skeletal muscles of the OZR, but aerobic exercise training was effective in reducing cardiac mitochondrial apoptotic signaling.