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

Genetic engineering study using TALEN-mediated mutagenesis in zebrafish to analyze the role of alpha-MSH in appetite and energy homeostasis.

“This article reports on zebrafish that have been genetically engineered to produce α-MSH mutants, α-MSH−7aa and α-MSH−8aa, selectively lacking 7 and 8 amino acids within the α-MSH region, but retaining most of the other normal melanocortin-signaling (Pomc-derived) peptides.”

Zebrafish (Danio rerio) with specific mutations in the pomca gene (α-MSH−7aa, α-MSH−8aa, and POMCa141a) compared to wild-type controls.

“Three pomca mutants were generated here. Two independent α-MSH-depleted mutant lines, α-MSH−7aa and α-MSH−8aa were generated with 21-bp and 24-bp bp in-frame deletions, respectively. One independent pomca mutant line, POMCa141a, was generated with a 13-bp frameshift deletion (Figure 1A,B).”

Twenty fish per diet were selected randomly for growth rate measurements; groups of five larvae were used for food intake quantification; at least 30 larvae were pooled for RNA sampling after injection.

“Twenty fish per diet (mixed sex) were selected randomly.”

Growth rate was recorded monthly from two months postfertilization to 12 months postfertilization; metabolic assays were conducted over 24 hours.

“The growth rate was recorded monthly starting at two months postfertilization (mpf) and ending at 12 mpf.”

α-MSH mutants exhibited hyperphagia (increased food intake) and significantly lower metabolic rates compared to wild-type controls.

“The α-MSH−7aa and α-MSH−8aa larvae exhibited obvious hyperphagia at 7 dpf (Figure 2B), when food intake significantly increased by 2.28-and 2.03-fold, respectively, compared with control WT zebrafish (Figure 2C).”

Adult α-MSH mutants developed obesity characterized by increased body weight, body mass index, and adipose tissue volume.

“Both the α-MSH−7aa and α-MSH−8aa adult fish had larger amounts of adipose tissue, with increases of 8- and 12-fold, respectively, compared with control WT zebrafish (Figure 4F,G).”

Injection of an α-MSH analog or Melanotan II into the hindbrain ventricle rescued the hyperphagic phenotype in mutant larvae.

“Both α-MSH−7aa and α-MSH−8aa fish could be rescued by the α-MSH analog and MTII—similar to normal phenotypes—although their anorexigenic effects were stronger than in WT controls (Figure 6B,C).”

The study notes that the mechanisms of the melanocortin system remain elusive and that hyperphagia alone does not fully explain the causal relationship with growth rate or weight gain due to the complexity of energy homeostasis.

“Because the energy homeostasis controlling system is highly complicated, simply increasing food intake might not explain the causal relationship with an acceleration of the growth rate or with weight gain [59,60,61].”