Brain-derived neurotrophic factor (BDNF) is the most abundant neurotrophin in the brain and its decreased levels are associated with the development of obesity and neurodegeneration. Our aim was to test the effect of dietary fat, its timing and the circadian clock on the expression of BDNF and associated signaling pathways in mouse brain and liver. Bdnf mRNA oscillated robustly in brain and liver, but with a 12-h shift between the tissues. Brain and liver Bdnf mRNA showed a 12-h phase shift when fed ketogenic diet (KD) compared with high-fat diet (HFD) or low-fat diet (LFD). Brain or liver Bdnf mRNA did not show the typical phase advance usually seen under time-restricted feeding (RF). Clock knockdown in HT-4 hippocampal neurons led to 86% up-regulation of Bdnf mRNA, whereas it led to 60% down-regulation in AML-12 hepatocytes. Dietary fat in mice or cultured hepatocytes and hippocampal neurons led to increased Bdnf mRNA expression. At the protein level, HFD increased the ratio of the mature BDNF protein (mBDNF) to its precursor (proBDNF). In the liver, RF under LFD or HFD reduced the mBDNF/proBDNF ratio. In the brain, the two signaling pathways related to BDNF, mTOR and AMPK, showed reduced and increased levels, respectively, under timed HFD. In the liver, the reverse was achieved. In summary, Bdnf expression is mediated by the circadian clock and dietary fat. Although RF does not affect its expression phase, in the brain, when combined with high-fat diet, it leads to a unique metabolic state in which AMPK is activated, mTOR is down-regulated and the levels of mBDNF are high.

In the liver, fructose bypasses the main rate-limiting step of glycolysis at the level of phosphofructokinase, allowing it to act as an unregulated substrate for de novo lipogenesis. It has been reported that consumption of large amounts of fructose increases de novo lipogenesis in the liver. However, the effect of fructose on ectopic deposition of muscle fat has been under dispute. Our aim was to study the effect of fructose on levels of genes and proteins involved in fatty acid oxidation and synthesis in hepatocytes vs. muscle cells. In addition, as fat accumulation leads to disruption of daily rhythms, we tested the effect of fructose treatment on clock gene expression. AML-12 hepatocytes and C2C12 myotubes were treated with fructose or glucose for 2 consecutive 24-h cycles and harvested every 6h. In contrast to glucose, fructose disrupted clock gene rhythms in hepatocytes, but in myotubes, it led to more robust rhythms. Fructose led to low levels of phosphorylated AMP-activated protein kinase (pAMPK) and high levels of LIPIN1 in hepatocytes compared with glucose. In contrast, fructose led to high pAMPK and low LIPIN1 and microsomal triacylglycerol transfer protein (MTTP) levels in myotubes compared with glucose. Analysis of fat content revealed that fructose led to less fat accumulation in myotubes compared to hepatocytes. In summary, fructose shifts metabolism towards fatty acid synthesis and clock disruption in hepatocytes, but not in myotubes.