Aerobic exercise promotes hippocampal neurogenesis through skeletal myofiber‐derived vascular endothelial growth factor

The beneficial effects of exercise, including improved cognitive performance, memory, and mood, are well documented and thought to be a result of increased hippocampal neurogenesis through the proliferation and differentiation of neural precursor cells (NPCs) within the dentate gyrus, although the mechanisms by which this occurs are unknown. Following aerobic exercise, several factors are released into circulation. Of these, vascular endothelial growth factor (VEGF), along with insulin growth factor-1 (IGF-1), plays a key role in promoting angiogenesis and hippocampal neurogenesis (Fabel et al. 2003; Cotman et al. 2007); however, the source of this VEGF and its mechanism of action are not well defined. A recent study by Rich et al. (2017) posited that VEGF present in skeletal myofibers may be released during exercise and influence downstream signalling pathways, playing an important role in the neurogenic process. The potential role of skeletal myofiber VEGF in neurogenesis supports the utilization of exercise regimes in the treatment of certain central nervous system (CNS) disorders. In a series of elegantly designed experiments, Rich et al. (2017) demonstrated that skeletal myofiber VEGF is required to enhance hippocampal neurogenesis following exercise. To achieve this, conditional ablation of the skeletal myofiberspecific VEGF gene was achieved in male NES-GFP/VEGF LoxP mice, which were divided into two groups: (1) HSA-CREER positive, VEGF gene-ablated group (VEGFHSA−/−) and (2) HSA-CRE-ER negative, non-ablated group (VEGF). Each group was further subdivided into two subgroups: those that received exposure to a running wheel for 2 weeks or sedentary controls (Fig. 1). Cerebral blood flow (CBF), NPC and endothelial cell proliferation, and VEGF levels in skeletal muscle were compared between groups. Using an enzyme-linked immunosorbent assay (ELISA) on tissue from the plantaris, a skeletal muscle used during running, Rich et al. (2017) show that the VEGF ablation paradigm resulted in a 60% reduction of VEGF expressed in the skeletal muscle of VEGFHSA−/− mice compared to VEGF controls, confirming that the ablation paradigm effectively decreased the amount of skeletal myofiber VEGF. Interestingly, VEGF levels were not significantly increased in the plantaris of exercised mice compared to sedentary mice when measured following the exercise training period in either genotype. This could indicate that the skeletal myofiber VEGF was released into the circulation instead of being sequestered in the muscle, that transient expression of VEGF returned to baseline levels prior to measurement, or that exercise did not lead to increased VEGF expression. The presence of skeletal myofiber VEGF was necessary for improved performance in maximal speed and endurance following training. Not only were VEGF mice that received exercise training the only group to show improvements in performance, but they also outperformed all the other groups. The authors suggest that differences in performance may be due to blood flow variations between the VEFG and VEFGHSA−/− mice in response to exercise. Thus, the mechanism for improved physical performance in the exercise trained VEGF group may depend on increased peripheral VEGF-dependent angiogenesis (Delavar et al. 2014) leading to improved tissue oxygenation. To identify proliferating NPCs, 5-bromo2′deoxyuridine (BrdU) was administered during the exercise training phase. Mice were killed 1 day following the completion of the training phase and tissue was collected for analysis. Hippocampal NPC and endothelial cell proliferation was assessed with immunostaining for Nestin+ and CD31+ expression, respectively. Exercised VEGF mice had a higher number of proliferating NPCs in the hippocampus as measured by Nestin+ and BrdU+ immunostaining compared to both sedentary groups and the exercised VEGFHSA−/− group, confirming that skeletal myofiber VEGF is necessary for promoting exercise-mediated neurogenesis in the hippocampus. Notably, endothelial cell proliferation, as measured by CD31+/BrdU+ staining, was similar in all groups, indicating that proliferation was specific to NPCs and that angiogenesis was not the reason for increased neurogenesis. These findings suggest that a combination of skeletal myofiber VEGF and exercise is necessary for hippocampal-specific neurogenesis. One possible explanation for this observation is that VEGFHSA−/− mice had decreased hippocampal blood flow compared to VEGF mice, providing a potentially less favourable environment for neurogenesis, whereby fewer neurotrophic factors or signalling molecules, such as VEGF, IGF-1, and fibroblast growth factor 2, reach the dentate gyrus (Olson et al. 2006). This finding also demonstrates the importance of VEGF in the maintenance of normal CBF rates. One interesting consideration is that trained VEGF mice had both increased NPC proliferation and improved exercise performance in endurance and maximal speed. These findings allude to a link between neurogenesis in the dentate gyrus and physical performance, but the authors do not explicitly suggest the nature of that link. More work is required to determine whether these processes are directly related, indirectly related, or modulated by a common mechanism. This research provides compelling evidence that skeletal myofiber VEGF is crucial for stimulating NPC proliferation with aerobic exercise, as only mice with normal levels of skeletal myofiber VEGF exhibited increased neurogenesis following the training phase. The mechanism by which skeletal myofiber VEGF induces neurogenesis has yet to be elucidated; nonetheless, sources from outside the CNS can play a key role. Indeed, peripheral inhibition of VEGF prevents exercise-induced neurogenesis (Fabel et al. 2003), suggesting that circulating VEGF derived from skeletal muscle has the potential to induce direct