Uticaj dugotrajne dijetalne restrikcije na insulinski signalni put u mozgu pacova tokom starenja
The effect of long-term dietary restriction on insulin signaling pathway in rat brain during aging
Doctoral thesis (Published version)
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Aging is a normal, physiological process that involves all organs and organic systems, and during which the organism faces a number of structural and functional changes. Brain aging is an extremely complex process, represented by a series of successive events that gradually result in loss of cognitive and motor functions. There are a number of pharmacological and environmental factors that are able to postpone and / or slow down many of the age-dependent processes, and dietary restriction (DR) is one of the most investigated and most widely used experimental intervention in aging research. There are numerous experimental and epidemiological data that support the beneficial effects of a restrictive diet, regardless of the type: a daily reduction or intermittent diet, reduction in food or in calories, all of those approaches seem beneficial. Nevertheless, there is a body of data that challenges current premise about comprehensive DR usefulness. In order for DR to achieve its optimal effect, it is necessary to take certain specificities into account while establishing a restrictive dietary regime. First of all, it has been shown that the percentage of restriction plays an important role in the effects it induces, and that a very restrictive diet may have negative consequences for the organism. In addition, not all the body systems are equally susceptible to aging processes, nor to the beneficial effects of DR. The effect of the restrictive diet can vary greatly, depending on the degree and type of restriction, gender and organs examined, as well as the age of the individual to which the restrictive regimen is applied. In order to investigate the effect of 60% DR on the energy homeostasis and insulin signaling in the brain, we first examined the effects of three different types of dietary restriction that varied in length and onset, on rat behavior during aging. The experiments were performed on 12-, 18- and 24-month-old male Wistar rats exposed to AL or DR type of feeding (60% of AL daily intake). A variety of parameters were monitored in the activities of animals and significant differences in their physical and cognitive performance were detected, both during aging and under the influence of different dietary regimes. Long-term dietary restriction that started in young adulthood led to the improvement of motor and cognitive performances and to the reduced frailty. By contrast, restrictive regimes of the same type, but with late-onset and/or the shorter duration, have a less pronounced positive impact on motor and cognitive functions during aging. A diet that started at old age and had the shortest duration has even led to negative consequences, causing additional difficulties in the habituation of animals, and increased their frailty score. Based on results mentioned above and the most favorable outcome of DR, we continued to investigate the longest dietary regime and its effect on brain metabolism during aging. Disruption of homeostasis in the brain energy metabolism is considered to be one of the leading causes that lie behind many age-related neurodegenerative diseases. The key proteins that regulate processes of glucose metabolism and energy homeostasis are: AMPK, as the main energy cell sensor, glucose transporters that facilitate glucose transport across the blood-brain-barrier and plasma membrane, and proteins involved in insulin signaling pathway, responsible for the regulation of nutrient intake. These proteins play an important role in processes involved in the effects of DR on age-dependent changes. For this reason, we examined changes in the expression of glucose transporters, AMPK protein, Neuropeptide Y, insulin, Insulin Receptor, Insulin Receptor Substrate, and Protein Kinase B. The analysis was performed in cerebral cortex, hippocampus and hypothalamus, structures most severely affected by process of aging and also with important role in energy homeostasis. Relevant biochemical parameters in the serum were also investigated, such as glucose, insulin, cholesterol, and triglycerides. The most pronounced changes elicited by long-term restrictive regime were related to increased levels of insulin and NPY proteins, as well as attenuation of the insulin signal pathway in the hypothalamus and the hippocampus, and improved energy supply of cortex and hippocampus. All the above changes resulted in a significant improvement of cognitive and motor performances of 24 month-old animals. The results of this doctoral dissertation point to the usefulness of a restrictive diet, but at the same time impose certain limits, regarding the time point when the diet should be applied and also the duration of diet, taking into account the negative effects that DR may have in certain cases.