FGF21 and obesity due to melanocortin 4 receptor deficiency: Supporting a threshold effect of the myomitokines, a mild inhibition of OPA1 in muscle does not alter mitochondrial complex and supercomplex formation, thus inducing lower muscle-derived FGF21 serum levels than in the model where OPA1 is completely deleted in adulthood [45], resulting in FGF21-mediated beneficial metabolic changes in terms of resistance to diet-induced obesity [61].