Abstract
The nutrient-sensor protein Sirtuin 1 (Sirt1; silent mating type information regulation 2 homolog 1) has been shown to have significant and opposing effects on insulin resistance, leptin resistance, and body weight in the periphery and the brain. In the hypothalamic arcuate nucleus (ARC) of the brain, Sirt1 increases in the obese state and acts to promote weight gain as well as insulin and leptin resistance by increasing the orexigenic neuropeptides Agouti-related protein (AgRP) and neuropeptide Y (NPY), and in a distinct set of ARC neurons, by decreasing POMC and thus its anorexigenic derivative alpha-melanocyte stimulating hormone (alpha-MSH) (1). Sirt1’s actions on these neuropeptides are mediated at least in part by the deacetylation of the transcription factor forkhead box O1 (FOXO1). Another mechanism by which Sirt1 regulates body weight appears to be through mediating changes in the synapses of these neuropeptide-producing ARC neurons. For example, a previous study demonstrated that Sirt1 inhibition with the specific Sirt1 inhibitor, Ex-527, decreased AgRP-NPY inhibitory synaptic input on POMC neurons, which suggests that the obesity-induced increase in ARC Sirt1 would increase AgRP-NPY inhibition of POMC neurons thus promoting weight gain (2). The present study investigated how Sirt1 regulates synapses specifically in POMC-producing N43-5 neurons. Results reveal that inhibition of Sirt1 with Ex-527 significantly increased the presynaptic marker Synapsin 1 (Syn1) in N43-5 neurons. Furthermore, we investigated whether the Sirt1 target, FOXO1, mediates these synaptic changes. FOXO1 overexpression significantly decreased Syn1 and transfection of mutant FOXO1 significantly increased Syn1. Overall, our results suggest that Sirt1 regulates synapses of POMC neurons and does so in a manner that differs from Sirt1’s regulation of AgRP-NPY neuronal synapses. Future work will elucidate the mechanisms and consequences of Sirt1 and FOXO1 regulation of POMC neuron synapses under different nutritional conditions in vitro and in vivo.
(1) Cyr, N. E., Steger, J. S., Toorie, A. M., Yang, J. Z., Stuart, R., Nillni, E. A. (2014). Central Sirt1 Regulates Body Weight and Energy Expenditure Along With the POMC-Derived Peptide α-MSH and the Processing Enzyme CPE Production in Diet-Induced Obese Male Rats, Endocrinology, 155(7), 2423–2435.
(2) Dietrich, M. O., Antunes, C., Geliang, G., Liu, Z., Borok, E., Nie, Y., . . . Horvath, T. L. (2010). Agrp neurons mediate Sirt1’s action on the melanocortin system and energy balance: Roles for Sirt1 in neuronal firing and synaptic plasticity. The Journal of Neuroscience, 30(35), 11815–11825.
Agrp Neurons Mediate Sirt1's Action on the Melanocortin System and Energy Balance: Roles for Sirt1 in Neuronal Firing and Synaptic Plasticity
