Age-dependent effects of short-term intermittent fasting and rapamycin treatment in Zebrafish (Danio Rerio) brain

World populations are rapidly aging, and there is an urgent need to develop interventions that prevent or reverse age-related deterioration of health. To date, several approaches have been developed to extend health span. Among these, non genetic interventions have a higher potential to be utilized in translational studies. Caloric restriction (CR) and its pharmacological mimetic rapamycin, are two applications that have been shown to reliably extend life and health span across species. Despite a growing body of knowledge on how CR and rapamycin show their beneficial effects, their molecular mechanisms in the brain are not completely understood. Furthermore, most studies applied life-long CR, which is not suitable for translational research. To fill this gap, we investigated whether short-term durations of a CR approach intermittent fasting (IF) or rapamycin altered cellular and molecular markers of critical processes in the brain as well as metabolic parameters in the body. To assess how the age of the subjects affect the outcome of the treatments, we included young (6-10 months old) and old (26-31 months) zebrafish, which has recently emerged as a suitable model for gerontological research. Our results demonstrated that IF decreased whole-body glucose and cortisol levels, and increased neural progenitor marker DCAMKL1 in young and old animals. While this proliferation-promoting effect was preceded by suppression of mTOR activity in young, the upregulation of foxm1 and reduced autophagic flux as measured by LC3 II/LC3-I ratio were observed in old animals. Rapamycin, on the other hand, did not alter the metabolic parameters and induced entirely different molecular profiles at young and old ages. The most notable changes in young animals were reduced mTOR activity, LC3-II/LC3-I ratio and expression levels of a global proliferation marker PCNA. In old animals, the marker of activated astrocytes (i.e. GFAP) was decreased, indicating lower neuroinflammation, whereas excitatory-inhibitory balance as measured by PSD-95/Gephyrin ratio was shifted towards a more excitatory state. These results suggested that IF and rapamycin induced distinct metabolic profiles in young and old animals. Furthermore, there was an age dependent reciprocal relationship between proliferation and autophagy, which might be partly due to differential regulation of mTOR activity. Interestingly, rapamycin treatment was more effective in suppressing mTOR activity in young animals, and compared to IF. Nevertheless, these results suggested that rapamycin crosses the blood-brain barrier in zebrafish, and that short-term durations of IF or rapamycin were sufficient to alter the expression levels of key proteins involved in critical mechanisms in the brain.