Effect of dietary restriction beyond middle age: accumulation of altered proteins and protein degradation

Signaling pathways of dietary energy restriction and metabolism on brain physiology and in age-related neurodegenerative diseases

Several laboratory animal models have shown that dietary energy restriction (ER) can promote longevity and improve various health aspects in old age. However, whether the entire spectrum of ER-induced short- and long-term physiological and metabolic adaptions is translatable to humans remains to be determined. In this review article, we present recent evidence towards the elucidation of the impact of ER on brain physiology and in age-related neurodegenerative diseases. We also discuss modulatory influences of ER on metabolism and overall on human health, limitations of current experimental designs as well as future perspectives for ER trials in humans. Finally, we summarize signaling pathways and processes known to be affected by both aging and ER with a special emphasis on the link between ER and cellular proteostasis.

[Multidisciplinary Approach to Aging]

Age-related decreases of various physiological functions have significant influence on activities of daily living (ADL) and QOL in elderly populations. Mechanisms of aging are currently the focus of many researchers in a wide range of studies. Researchers are trying to find novel ways to attenuate or delay aging in humans as well as to develop interventions for age-associated diseases. In this review, we briefly discuss the need for a multidisciplinary approach in aging research.

Age-dependent modulation of fasting and long-term dietary restriction on acetylcholinesterase in non-neuronal tissues of mice

Dietary restriction (DR) without malnutrition is a robust intervention that extends lifespan and slows the onset of nervous system deficit and age-related diseases in diverse organisms. Acetylcholinesterase (AChE), a thoroughly studied enzyme better known for hydrolyzing acetylcholine (ACh) in neuronal tissues, has recently been linked with multiple unrelated biological functions in different non-neuronal tissues. In the present study, the activity and protein expression level of AChE in liver, heart, and kidney of young (1 month), adult (6 month), and aged (18 month) mice were investigated. We also studied age- and tissue-specific changes in AChE activity and protein expression level after the mice were subjected to 24-h fasting and long-term DR. Our results showed that AChE activity and protein expression in kidney and heart of aged mice decreased significantly in comparison with young mice. On the contrary, long-term DR decreases the AChE activity and the protein expression level in all tissues irrespective of ages studied. We summarized that changes in AChE with age in different tissues studied reflects its different roles at different phases of an organism's life. Conversely, the cumulative modulation manifested in the form of lowering AChE by long-term DR may prevent the futile synthesis and accumulation of unwanted AChE besides the added compensatory benefit of enhanced ACh availability needed during the period of starvation. This, in turn, may help in preventing the declining homeostatic roles of this important neurotransmitter in different tissues.

Age-Dependent Decrease in Renal Glucocorticoid Receptor Function Is Reversed by Dietary Restriction in Mice

The effects of age and dietary restriction (alternate days of feeding for 3 months) on the concentration, activation, and DNase I digestion of nuclear-bound glucocorticoid receptors (GRs) in the kidney of male mice at two different ages (5 months as adult and 20 months as old) were investigated. A significant decrease (30%) in the concentration of renal GRs was observed in older ad libitum (AL)-fed mice as compared to the adult mice. Dietary restriction (DR) of older mice significantly increased (28%) the level of GRs as compared to the AL-fed control animals. The affinity of the receptor for the hormone remained the same for both AL- and DR-fed animals at both ages. Scatchard and slot blot analyses of the data confirmed the decreased level of renal GRs in older mice compared to the adult mice as well as an increased level of receptor in older DR mice. Activation studies of GRs by both salt and heat indicated a decreased (15-20%) activation of renal GRs in older animals compared to the adult mice in the AL-fed group. It was further observed that DR significantly enhanced (30%) the degree of both salt- and heat-dependent activation of GRs in older animals compared to the AL-fed animals of the age-matched group. DNase I digestion and extraction of nuclear-bound GR complexes showed a lower degree (26%) of extraction in older AL-fed animals compared to the adult animals. However, DR did not alter the pattern of digestibility of bound GR complexes. These above findings indicate that DR could reverse the decrease of GR function in older animals and may provide better adaptability of kidney in water and electrolyte balance.

Carbonyl modification in rat liver histones: decrease with age and increase by dietary restriction

We studied carbonylation, a form of oxidative modification of proteins, of histones in rat livers. Histones H1, H2B/H2A, and H3 were significantly carbonylated but the modification was almost undetectable in H4. Contrary to the generally accepted view of increased protein carbonylation with age, the modification of histones was significantly lower in old (30-month-old) than in young (5-month-old) animals. Dietary restriction of older animals for 2 months resulted in increase in carbonylation comparable to that at the young level. These findings may have physiological implications in chromatin structure/function in aging and beneficial effects of DR by influencing transcription, replication, and/or repair activities.

Proteasome regulation of oxidative stress in aging and age-related diseases of the CNS

Proteasome-mediated protein degradation is responsible for a large percentage of bulk protein turnover, particularly the degradation of short-lived and oxidized proteins. Increasing evidence suggests that proteasome inhibition occurs during the aging of the central nervous system (CNS), and in a variety of age-related disorders of the CNS. The focus of this review is to discuss the role of the proteasome as a regulator of oxidative stress, with preservation of proteasome function playing an important role in preventing oxidative stress, and proteasome inhibition playing an important role as a mediator of oxidative stress. In particular, this review will describe experimental evidence that proteasome inhibition is sufficient to induce mitochondrial dysfunction, increase reactive oxygen species generation, elevate RNA and DNA oxidation, and promote protein oxidation. Taken together, these data indicate that the proteasome is an important regulator of oxidative damage in the CNS, and suggest that proteasome inhibition may serve as an important switch for the induction of oxidative stress in the CNS. Additionally we discuss the likelihood that the 20S proteasome and 26S proteasome may play different roles in regulating oxidative stress and neurotoxicity in the aging CNS, and in age-related disorders of the CNS.

Dietary restriction at old age lowers mitochondrial oxygen radical production and leak at complex I and oxidative DNA damage in rat brain

Previous studies in mammalian models indicate that the rate of mitochondrial reactive oxygen species ROS production and the ensuing modification of mitochondrial DNA (mtDNA) link oxidative stress to aging rate. However, there is scarce information concerning this in relation to caloric restriction (CR) in the brain, an organ of maximum relevance for ageing. Furthermore, it has never been studied if CR started late in life can improve those oxidative stress-related parameters. In this investigation, rats were subjected during 1 year to 40% CR starting at 24 months of age. This protocol of CR significantly decreased the rate of mitochondrial H(2)O(2) production (by 24%) and oxidative damage to mtDNA (by 23%) in the brain below the level of both old and young ad libitum-fed animals. In agreement with the progressive character of aging, the rate of H(2)O(2) production of brain mitochondria stayed constant with age. Oxidative damage to nuclear DNA increased with age and this increase was fully reversed by CR to the level of the young controls. The decrease in ROS production induced by CR was localized at Complex I and occurred without changes in oxygen consumption. Instead, the efficiency of brain mitochondria to avoid electron leak to oxygen at Complex I was increased by CR. The mechanism involved in that increase in efficiency was related to the degree of electronic reduction of the Complex I generator. The results agree with the idea that CR decreases aging rate in part by lowering the rate of free radical generation of mitochondria in the brain.

Minireview: the role of oxidative stress in relation to caloric restriction and longevity

Reduction of caloric intake without malnutrition is one of the most consistent experimental interventions that increases mean and maximum life spans in different species. For over 70 yr, caloric restriction has been studied, and during the last years the number of investigations on such nutritional intervention and aging has dramatically increased. Because caloric restriction decreases the aging rate, it constitutes an excellent approach to better understand the mechanisms underlying the aging process. Various investigations have reported reductions in steady-state oxidative damage to proteins, lipids, and DNA in animals subjected to restricted caloric intake. Most interestingly, several investigations have reported that these decreases in oxidative damage are related to a lowering of mitochondrial free radical generation rate in various tissues of the restricted animals. Thus, similar to what has been described for long-lived animals in comparative studies, a decrease in mitochondrial free radical generation has been suggested to be one of the main determinants of the extended life span observed in restricted animals. In this study we review recent reports of caloric restriction and longevity, focusing on mitochondrial oxidative stress and the proposed mechanisms leading to an extended longevity in calorie-restricted animals.

Murine Models of Life Span Extension

Mice are excellent experimental models for genetic research and are being used to investigate the genetic component of organismal aging. Several mutant mice are known to possess defects in the growth hormone/insulin-like growth factor 1 (GH/IGF-1) neurohormonal pathway and exhibit dwarfism together with extended life span. Their phenotypes resemble those of mice subjected to caloric restriction. Targeted mutations that affect components of this pathway, including the GH receptor, p66Shc, and the IGF-1 receptor (IGF-1R), also extend life span; mutations that affect IGF-1R or downstream components of the pathway decouple longevity effects from dwarfism. These effects on life span may result from an increased capacity to resist oxidative damage.

Oxidative stress and mitochondrial function with aging--the effects of calorie restriction

Accumulated oxidative stress resulting from a gradual shift in the redox status of tissues is now considered to be a key mechanism underlying the aging process. Calorie-restricted (CR) feeding, an experimental protocol to extend survival and delay aging in rodents, is recognized to slow the rate of accrual of age-related oxidative stress. This conclusion is based on the increase in tissues with age of the oxidation products of proteins, lipids and DNA. The functional consequence, however, of the accumulation of these non-specific oxidative markers is more difficult to determine. A shift in the redox status of tissues with age and calorie restriction feeding may have a greater impact on cell function through activation of redox sensitive transcription factors than through the accumulation of these non-specific oxidative markers. Activation of such transcription factors will stimulate signalling pathways that will lead to a change in the gene expression profile and cell functioning. Little research has been conducted in this area. It has been proposed that CR feeding slows the rate of accrual of oxidative damage because mitochondria in these animals have a lower rate of superoxide generation when compared with mitochondria from control animals. This proposal is based on in vitro observations using isolated mitochondria and clearly requires further confirmation in isolated cells or using an in vivo approach. The application of metabolic control analysis to identify in isolated mitochondria the mechanism underlying this response has suggested one possible explanation for the lower superoxide production rates observed.