Sim1, or single-minded 1, is a basic helix-loop-helix Per-Arnt-Sim transcription factor crucial for neuronal development in the hypothalamus [3]. It is involved in the melanocortin pathway, where it contributes to the control of energy homeostasis [1]. Mutations or haploinsufficiency of Sim1 are associated with early-onset obesity, highlighting its significance in body weight regulation [2,3,4,5,6]. Genetic mouse models have been instrumental in studying its functions.

In Sim1-deficient mouse models, postnatal CNS deficiency of Sim1 leads to hyperphagic obesity, with conditional Sim1 heterozygotes mimicking germ-line Sim1 heterozygotes [5]. This phenotype is not due to global hypocellularity of the hypothalamic paraventricular nucleus (PVN), but rather a decrease in hypothalamic oxytocin and PVN melanocortin 4 receptor (Mc4r) mRNA [5]. Sim1 haploinsufficient mice show resistance to hypothalamic melanocortin signaling, with blunted suppression of feeding in response to the melanocortin agonist melanotan-2 despite normal energy expenditure [6]. Also, adult-onset Sim1 deletion increases bone formation through stimulating the sympathetic nervous system [3].

In conclusion, Sim1 plays essential roles in regulating energy homeostasis, feeding behavior, and bone remodeling. Studies using Sim1 knockout or conditional knockout mouse models have significantly enhanced our understanding of its functions and its implications in obesity and bone-related diseases.