Nature Reviews Endocrinology | 2019
Neuronal clock coordinates appetite
Abstract
Nature reviews | Endocrinology The molecular mechanisms by which hypothalamic neurons coordinate the alignment of the sleep–wake cycle with appetite and metabolism are unclear. Research published in Cell Metabolism now demonstrates a transcriptional basis for the rhythmic control of hunger by the Agouti-related protein (AgRP) neuron molecular clock. “We used transgenic mice to target deletion of the molecular clock to the whole hypothalamus and, separately, to energy-sensing AgRP neurons,” explain study authors Jonathan Cedernaes and Joseph Bass. “Given that the molecular clock functions as a transcriptional–translational feedback loop, we were interested in understanding how transcriptional networks are altered across the sleep– wake cycle and by altered feeding state in [AgRP] neurons.” Brain-clock knockout (BKO) mice with genetic ablation of Bmal1 (which encodes a core molecular clock component) in the hypothalamus showed loss of diurnal feeding. “One of our most striking findings was that mice without a functional clock in the hypothalamus exhibit a remarkable disruption in their food intake rhythms. That is, BKO mice increased their food intake during the day, when they should normally be resting,” say Cedernaes and Bass. “This indicates a requirement for the hypothalamic clock in the proper circadian regulation of appetite.” In wild-type mice, gluconeogenesis occurs during sleep and BKO mice showed increased gluconeogenesis. Notably, night-time restricted feeding of BKO mice normalized gluconeogenesis, suggesting that the mistimed feeding in BKO mice was causing a metabolic defect. Mice with AgRP-specific ablation of Bmal1 (ABKO mice) showed disrupted feeding patterns, increased hepatic gluconeogenesis, altered metabolism and a trend towards increased body weight, highlighting the importance of the molecular clock in AgRP neurons. Next, Cedernaes and colleagues carried out whole cell transcriptional profiling of genetically tagged AgRP neurons across the sleep–wake cycle. These analyses revealed distinct morning and evening transcriptional networks in AgRP neurons in fasted wild-type mice. Moreover, leptin responsive pathways (such as focal adhesion pathways and JAK–STAT signalling) were differentially activated in the morning and evening. Genome-wide transcriptional analyses performed in ABKO mice at the time of day with maximal BMAL1 activity showed that loss of a functional clock from AgRP neurons resulted in upregulation of 41 genes and downregulation of 22 genes compared with wild-type mice. Interestingly, several of the differentially expressed genes (Akap12, Pfkl, Pik3c2a and Pcsk1n) have been previously linked with human obesity and type 2 diabetes mellitus. RiboTag analysis of ribosome-bound mRNAs in AgRP neurons showed that the ribosome-associated transcriptome exhibits an exaggerated response to fasting in the morning in reporter mice, providing further evidence of time-dependent post-transcription cycles in energy-sensing AgRP neurons. The researchers next assessed transcriptional mechanisms by which AgRP neurons respond to fasting compared with ad libitum feeding over the sleep–wake cycle in wild-type mice. 6,234 genes were differentially expressed in the morning and 2,818 genes in the evening between fed and fasted states. At the ribosomal mRNA level, several pathways were uniquely enriched (such as oxidatitve phosphorylation and RNA processing), suggesting AgRP-specific post-transcriptional control of bioenergetics in response to fasting. Interestingly, refeeding (in the morning or the evening) was associated with differential gene expression, suggesting time-of-day-dependent transcriptional responses to feeding in AgRP neurons in wild-type mice. Finally, the researchers assessed leptin-mediated transcriptional responses in BKO and ABKO mice. Surprisingly, the expression of a number of genes increased significantly in ABKO mice in response to leptin, suggesting that the molecular clock is required for appropriate regulation of leptin-mediated transcription. “We have established a roadmap for how the clock machinery regulates rhythms in energy-sensing neurons,” conclude Cedernaes and Bass. “We are now interested in identifying how rhythms in bioenergetic and peptidergic pathways in AgRP neurons can establishing rhythmic control of appetite and glucose metabolism, and the interplay with peripheral tissue metabolism.” Shimona Starling C I R C A D I A N R H Y T H M S