Deletion of Six3 in post-proliferative neurons produces weakened SCN circadian output, improved metabolic function, and dwarfism in male mice

Abstract

Objective: The increasing prevalence of obesity makes it important to increase the understanding of the maturation and function of the neuronal integrators and regulators of metabolic function.

Methods: Behavioral, molecular, and physiological analyses of transgenic mice with Sine oculis 3 (Six3) deleted in mature neurons using the Synapsin^creallele.

Results: Conditional deletion of the homeodomain transcription factor Six3 in mature neurons causes dwarfism and weakens circadian wheel-running activity rhythms but increases general activity at night, and improves metabolic function, without impacting pubertal onset or fertility in males. The reduced growth in 6-week-old Six3^fl/fl:Synapsin^cre (Six3^syn) males correlates with increased somatostatin (SS) expression in the hypothalamus and reduced growth hormone (GH) in the pituitary. In contrast, 12-week-old Six3^syn males have increased GH release, despite an increased number of the inhibitory SS neurons in the periventricular nucleus. GH is important in glucose metabolism, muscle function, and bone health. Interestingly, Six3^syn males have improved glucose tolerance at 7, 12, and 18 weeks of age, which, in adulthood, is associated with increased % lean mass and increased metabolic rates. Further, 12-week-old Six3^syn males have reduced bone mineralization and a lower bone mineral density, indicating that reduced GH levels during early life cause a long-term reduction in bone mineralization.

Conclusion: Our study points to the novel role of Six3 in post-proliferative neurons to regulate metabolic function through SS neuron control of GH release.

Keywords: Circadian rhythm; Growth hormone; Metabolism; Six homeobox 3; Somatostatin; Suprachiasmatic nucleus.