Intermittent fasting (IF) has been suggested to have neuroprotective effects through the activation of multiple signaling pathways. Rodents fasted intermittently exhibit enhanced hippocampal neurogenesis and long‐term potentiation (LTP) at hippocampal synapses compared with sedentary animals fed an ad libitum (AL) diet. However, the underlying mechanisms have not been studied. In this study, we evaluated the mechanistic gap in understanding IF‐induced neurogenesis.
We evaluated the impact of 3 months of IF (12, 16, and 24 hr of food deprivation on a daily basis) on hippocampal neurogenesis in C57BL/6NTac mice using immunoblot analysis.
Three‐month IF significantly increased activation of the Notch signaling pathway (Notch 1, NICD1, and HES5), neurotrophic factor BDNF, and downstream cellular transcription factor, cAMP response element‐binding protein (p‐CREB). The expression of postsynaptic marker, PSD95, and neuronal stem cell marker, Nestin, was also increased in the hippocampus in response to 3‐month IF.
These findings suggest that IF may increase hippocampal neurogenesis involving the Notch 1 pathway.
Dietary restriction (DR) is defined as a decrease in energy consumption without reducing nutritional value. This simple dietary intervention has been shown in a wide range of experimental animals to extend lifespan and decrease the incidence of several age‐related diseases. The definition of DR has been expanded from an alternative description of caloric restriction (CR) to also encompass a broader scope of interventions, including short‐term starvation, periodic fasting, fasting‐mimetic diets, and intermittent fasting (IF; Mattson & Arumugam, 2018). IF has been proven to be advantageous to various organ systems in the body and acts as a mild metabolic stressor. It has been postulated that IF is able to cause powerful changes in the metabolic pathways in the brain via an increase in stress resistance, and breakdown of ketogenic amino acids and fatty acids (Bruce‐Keller, Umberger, McFall, & Mattson, 1999; Kim et al., 2018). Experimental studies have also shown that IF is neuroprotective against acute brain injuries such as stroke, and neurodegenerative diseases (Arumugam et al., 2010; Halagappa et al., 2007; Manzanero et al., 2014). In addition, recent studies have also shown that IF can lead to an increase in neurogenesis levels in the hippocampus (Manzanero et al., 2014).
In the adult brain, the niches of neuronal stem cells (NSCs) are located specifically at the subventricular zone (SVZ) of the lateral ventricles, and in the subgranular zone (SGZ) of the hippocampus. The ability of NSCs to maintain cerebral neurogenesis is controlled by the tight regulation of balanced events commencing from stem cell maintenance, to stem cell division and proliferation, to its differentiation into mature neurons, and finally their survival and functional integration into the brain parenchyma (Lathia, Mattson, & Cheng, 2008; Lledo, Alonso, & Grubb, 2006). The process of adult neurogenesis is highly regulated and is adaptable to environmental, morphological, and physiological cues, whereby cerebral performance is suited to function at optimal levels for a given environment. Studies have demonstrated that the proliferation of neural stem cells can be modified through metabolic perturbations experienced during high temperatures (Matsuzaki et al., 2009), physical activity (Niwa et al., 2016), and a high‐fat diet (Kokoeva, Yin, & Flier, 2005). Experimental studies from our group have also shown that IF increases neurogenesis in the hippocampus as a form of neuroprotection following acute brain injury such as ischemic stroke. Moreover, we established that the number of BrdU‐labeled cells in the dentate gyrus of IF mice was elevated (Manzanero et al., 2014). To measure cell proliferation without the confound availability of an exogenous marker BrdU, we established increases in the number of Ki67‐labeled cells in the dentate gyrus of mice on the IF diet, indicating enhancement of cell proliferation in these mice (Manzanero et al., 2014). In addition to our findings, previous work similarly demonstrated that using the every other day (EOD) IF regimen also increased BrdU‐labeled cell number in the hippocampus (Lee, Duan, & Mattson, 2002).
However, the molecular process involved in IF‐induced neurogenesis is not well understood. The Notch signaling pathway that is intricately involved in the determination of cell fate during brain development and adult neurogenesis may be a possible molecular process involved in IF‐induced neurogenesis (Lathia et al., 2008). In this study, we investigated the expression levels of molecular and cellular components of the hippocampal region, focusing specifically on Notch activation and associated proteins that are known to promote hippocampal neurogenesis such as brain‐derived neurotrophic factor (BDNF) and cAMP response element‐binding protein (CREB).[…]