The CREB Sirt1 axis mediates Neural Stem Cells modulation by nutrient availability

Background and objective. Calorie restriction improves memory and learning in rodents through the interplay between the c-AMP responsive Element Binding protein (CREB) and Sirtuin 1 (Sirt-1). Adult neurogenesis is also activated by calorie restriction and may contribute to the beneficial cognitive effects of this dietary regimen, but the underlying molecular interactions are largely unknown. We here investigated whether the CREB-Sirt1 axis is involved in Neural stem cell (NSC) response to nutrient availability in vitro, as a potential mechanism for the metabolic regulation of adult neurogenesis. Methods We used wild-type and CREBloxP/loxP mice as source of NCS. Inhibition of CREB or Sirt-1 expression were achieved by adenoviral transduction of, respectively, the Cre recombinase or a Sirt-1 specific shRNA construct. The Neuropshere assay (NSA) and 5-bromo-2'-deoxyuridine incorporation were used to assess NSC proliferative and self-renewal capacity under conditions of high (4,5 g/L glucose) and restricted (0.9g/L glucose) nutrients, the latter modeling calorie restriction. Gene expression, protein-DNA and protein-protein interactions were evaluated by standard procedures. Results. Proliferation of NSC in low glucose was significantly enhanced (doubled) compared to glucose-rich cultures, indicating an improved self-renewal capacity of stem cells in nutrient-restricted conditions. This behavior was mirrored by a reduced content of intracellular reactive oxygen species and an increased expression of the repressor and Notch target Hairy and Enhancer of Split 1 (Hes1), two biochemical determinants of stemness. Importantly, cell proliferation, neurosphere formation and Hes-1 expression were strongly reduced in CREB-depleted NSC, and were not or marginally affected by glucose availability. In wild type cells, low glucose induced the phosphorylation of CREB and increased its binding with Sirt-1; moreover, CREB was found to directly interact with the Hes-1 promoter in a fashion inducible by low glucose. Interestingly, Sirt-1 depletion, unlike CREB deletion, enhanced NSC proliferation in the NSA assay, but again disrupted responsiveness to low glucose. Conclusions These findings strongly suggest that CREB is activated in NSC in response to limited glucose availability and promotes self-renewal most likely through the induction of Hes-1. Sirt1 participates in this circuitry, presumably by increasing CREB activity and also by limiting NSC proliferation/exhaustion under nutrient restriction. The above results may have important implications for our understanding of how systemic energy metabolism affects adult neurogenesis