Depletion of Alpha-Melanocyte-Stimulating Hormone Induces Insatiable Appetite and Gains in Energy Reserves and Body Weight in Zebrafish

Abstract

The functions of anorexigenic neurons secreting proopiomelanocortin (POMC)/alpha-melanocyte-stimulating hormone (alpha-MSH) of the melanocortin system in the hypothalamus in vertebrates are energy homeostasis, food intake, and body weight regulation. However, the mechanisms remain elusive. This article reports on zebrafish that have been genetically engineered to produce alpha-MSH mutants, alpha-MSH-7aa and alpha-MSH-8aa, selectively lacking 7 and 8 amino acids within the alpha-MSH region, but retaining most of the other normal melanocortin-signaling (Pomc-derived) peptides. The alpha-MSH mutants exhibited hyperphagic phenotypes leading to body weight gain, as observed in human patients and mammalian models. The actions of several genes regulating appetite in zebrafish are similar to those in mammals when analyzed using gene expression analysis. These include four selected orexigenic genes: Promelanin-concentrating hormone (pmch), agouti-related protein 2 (agrp2), neuropeptide Y (npy), and hypothalamic hypocretin/orexin (hcrt). We also study five selected anorexigenic genes: Brain-derived neurotrophic factor (bdnf), single-minded homolog 1-a (sim1a), corticotropin-releasing hormone b (crhb), thyrotropin-releasing hormone (trh), and prohormone convertase 2 (pcsk2). The orexigenic actions of alpha-MSH mutants are rescued completely after hindbrain ventricle injection with a synthetic analog of alpha-MSH and a melanocortin receptor agonist, Melanotan II. We evaluate the adverse effects of MSH depletion on energy balance using the Alamar Blue metabolic rate assay. Our results show that alpha-MSH is a key regulator of POMC signaling in appetite regulation and energy expenditure, suggesting that it might be a potential therapeutic target for treating human obesity.