The homeodomain transcription factor Six3 regulates hypothalamic Pomc expression and its absence from POMC neurons induces hyperphagia and mild obesity in male mice

Abstract

Objective: Proopiomelanocortin (POMC) neurons release potent anorexigenic neuropeptides, which suppress food intake and enhance energyexpenditure via melanocortin receptors. Although the importance of central melanocortin in physiological regulation is well established, theunderlying genetic mechanisms that define the functional identity of melanocortin neurons and maintain hypothalamic Pomc expression remain tobe fully determined. In this study, we investigate the functional significance of Six3, a transcriptional regulator notably expressed in embryonicand adult mouse POMC neurons, in the regulation of hypothalamic Pomc expression and downstream physiological consequences.Methods: We first evaluated the expression of Six3 in the developing and adult hypothalamus by double fluorescence in situ hybridization. Next,we assessed POMC immunoreactivity in mutant mice selectively lacking Six3 from Pomc-expressing neurons and quantified Pomc mRNA levels ina tamoxifen-inducible Six3 knockout mouse model activated at embryonic E9.5 days. We also determined glucose and insulin sensitivity, dailyfood intake, body composition and body weight in adult male and female mice lacking Six3 specifically from POMC neurons. Lastly, we assessedthe physiological consequences of ablating Six3 from POMC neurons in adult mice.Results: Six3 and Pomc were co-expressed in mouse hypothalamic neurons during development and adulthood. Mouse embryos deficient inSix3 showed reduced Pomc expression in the developing hypothalamus. Targeted deletion of Six3 specifically from POMC neurons resulted indecreased hypothalamic Pomc expression, increased daily food intake, enhanced glucose sensitivity and mild obesity in male but not in femalemice. Finally, conditional removal of Six3 from POMC neurons in adult mice led to a reduction in hypothalamic POMC immunoreactivity with nosignificant effects in body weight or food intake.Conclusions: Altogether, our results demonstrate that Six3 plays an essential role in the early establishment of POMC neuron identity and themaintenance of physiological levels of hypothalamic Pomc expression. In addition, our study demonstrates that the functional significance of Six3expression in POMC neurons is sexually dimorphic and age-dependent.