Prader-Willi syndrome (PWS) is a multigene disorder associated with neonatal failure to thrive, developmental delay, and endocrine abnormalities suggestive of hypothalamic dysfunction. Children with PWS typically develop overt hyperphagia and obesity around 8 years of age, later than children with other genetic forms of obesity. This suggests a postnatal developmental or degenerative component to PWS-associated obesity. De novo inactivating mutations in one PWS candidate gene, MAGEL2, have been identified in children with features of PWS. Adult mice lacking Magel2 are insensitive to the anorexic effect of leptin treatment, and their hypothalamic pro-opiomelanocortin (POMC) neurons fail to depolarize in response to leptin. However, it is unclear whether this leptin insensitivity is congenital, or whether normal leptin sensitivity in neonatal Magel2-null mice is lost postnatally. We used in vitro cytosolic calcium imaging to follow the postnatal development of leptin responses in POMC neurons in these mice. Leptin caused an activation of POMC neurons in wild-type acute hypothalamic slice preparations at all ages, reflecting their normal leptin-invoked depolarization. Normal leptin responses were found in Magel2-null mice up to 4 weeks of age, but the proportion of leptin-responsive POMC neurons was reduced in 6 week-old Magel2-null mice. The number of α-melanocyte-stimulating hormone immunoreactive fibers in the paraventricular hypothalamic nucleus was also reduced in mutant mice at 6 weeks of age. A similar progressive loss of leptin sensitivity caused by loss of MAGEL2 in children with PWS could explain the delayed onset of increased appetite and weight gain in this complex disorder.
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