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Pro-opiomelanocortin (POMC) neurons function as key regulators of metabolism and physiology by releasing prohormone-derived neuropeptides with distinct biological activities. However, our understanding of early events in prohormone maturation in the ER remains incomplete. Highlighting the significance of this gap in knowledge, a single POMC cysteine-to-phenylalanine mutation at position 28 (POMC-C28F) is defective for ER processing and causes early onset obesity in a dominant-negative manner in humans through an unclear mechanism. Here, we report a pathologically important role of Sel1L-Hrd1, the protein complex of ER-associated degradation (ERAD), within POMC neurons. Mice with POMC neuron-specific Sel1L deficiency developed age-associated obesity due, at least in part, to the ER retention of POMC that led to hyperphagia. The Sel1L-Hrd1 complex targets a fraction of nascent POMC molecules for ubiquitination and proteasomal degradation, preventing accumulation of misfolded and aggregated POMC, thereby ensuring that another fraction of POMC can undergo normal posttranslational processing and trafficking for secretion. Moreover, we found that the disease-associated POMC-C28F mutant evades ERAD and becomes aggregated due to the presence of a highly reactive unpaired cysteine thiol at position 50. Thus, this study not only identifies ERAD as an important mechanism regulating POMC maturation within the ER, but also provides insights into the pathogenesis of monogenic obesity associated with defective prohormone folding.

Obesity is a chronic metabolic disorder affecting half a billion people worldwide. Major difficulties in managing obesity are the cessation of continued weight loss in patients after an initial period of responsiveness and rebound to pretreatment weight. It is conceivable that chronic weight gain unrelated to physiological needs induces an allostatic regulatory state that defends a supranormal adipose mass despite its maladaptive consequences. To challenge this hypothesis, we generated a reversible genetic mouse model of early-onset hyperphagia and severe obesity by selectively blocking the expression of the proopiomelanocortin gene (Pomc) in hypothalamic neurons. Eutopic reactivation of central POMC transmission at different stages of overweight progression normalized or greatly reduced food intake in these obesity-programmed mice. Hypothalamic Pomc rescue also attenuated comorbidities such as hyperglycemia, hyperinsulinemia, and hepatic steatosis and normalized locomotor activity. However, effectiveness of treatment to normalize body weight and adiposity declined progressively as the level of obesity at the time of Pomc induction increased. Thus, our study using a novel reversible monogenic obesity model reveals the critical importance of early intervention for the prevention of subsequent allostatic overload that auto-perpetuates obesity.

Null mutations of the proopiomelanocortin gene (Pomc) cause obesity in humans and rodents, but the contributions of central versus pituitary POMC deficiency are not fully established. To elucidate these roles, we introduced a POMC transgene (Tg) that selectively restored peripheral melanocortin and corticosterone secretion in Pomc mice. Rather than improving energy balance, the genetic replacement of pituitary POMC in PomcTg mice aggravated their metabolic syndrome with increased caloric intake and feed efficiency, reduced oxygen consumption, increased subcutaneous, visceral, and hepatic fat, and severe insulin resistance. Pair-feeding of PomcTg mice to the daily intake of lean controls normalized their rate of weight gain but did not abolish obesity, indicating that hyperphagia is a major but not sole determinant of the phenotype. Replacement of corticosterone in the drinking water of Pomc mice recapitulated the hyperphagia, excess weight gain and fat accumulation, and hyperleptinemia characteristic of genetically rescued PomcTg mice. These data demonstrate that CNS POMC peptides play a critical role in energy homeostasis that is not substituted by peripheral POMC. Restoration of pituitary POMC expression to create a de facto neuronal POMC deficiency exacerbated the development of obesity, largely via glucocorticoid modulation of appetite, metabolism, and energy partitioning.

Hypothalamic dopamine inhibits pituitary prolactin secretion and proliferation of prolactin-producing lactotroph cells by activating lactotroph dopamine D2 receptors (D2Rs). Conversely, prolactin (PRL) stimulates hypothalamic dopamine neurons via PRL receptors (PRLRs) in a short-loop feedback circuit. We used Drd2(-/-) and Prlr(-/-) mutant mice to bypass this feedback and investigate possible dopamine-independent effects of PRL on lactotroph function. The absence of either receptor induced hyperprolactinemia and large prolactinomas in females. Small macroadenomas developed in aged Prlr(-/-) males, but only microscopic adenomas were found in Drd2(-/-) male mice. Pharmacologic studies in Prlr(-/-) mice with D2R agonists and antagonists demonstrated a significant loss of endogenous dopamine tone, i.e., constitutive inhibitory signaling by the D2R, in the pituitary. However, Prlr(-/-) mice exhibited more profound hyperprolactinemia and larger tumors than did age-matched Drd2(-/-) mice, and there were additive effects in compound homozygous mutant male mice. In vitro, PRL treatment markedly inhibited the proliferation of wild-type female and male Drd2(-/-) lactotrophs, but had no effect on female Drd2(-/-) lactotrophs, suggesting a downregulation or desensitization of PRLR in response to chronic hyperprolactinemia. We conclude that PRL inhibits lactotrophs by two distinct mechanisms: (a) indirectly by activation of hypothalamic dopamine neurons and (b) directly within the pituitary in a dopamine-independent fashion.

Pulsatile growth hormone (GH) secretion differs between males and females and regulates the sex-specific expression of cytochrome P450s in liver. Sex steroids influence the secretory dynamics of GH, but the neuroendocrine mechanisms have not been conclusively established. Because periventricular hypothalamic somatostatin (SST) expression is greater in males than in females, we generated knockout (Smst(-/-)) mice to investigate whether SST peptides are necessary for sexually differentiated GH secretion and action. Despite marked increases in nadir and median plasma GH levels in both sexes of Smst(-/-) compared with Smst(+/+) mice, the mutant mice had growth curves identical to their sibling controls and retained a normal sexual dimorphism in weight and length. In contrast, the liver of male Smst(-/-) mice was feminized, resulting in an identical profile of GH-regulated hepatic mRNAs between male and female mutants. Male Smst(-/-) mice show higher expression of two SST receptors in the hypothalamus and pituitary than do females. These data indicate that SST is required to masculinize the ultradian GH rhythm by suppressing interpulse GH levels. In the absence of SST, male and female mice exhibit similarly altered plasma GH profiles that eliminate sexually dimorphic liver function but do not affect dimorphic growth.