Exploration of the role of CART/GPR160 in metabolism in the setting of Magel2 deficiency: Implications for Prader Willi Syndrome

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PROJECT SUMMARY Severe hyperphagia is a hallmark of Prader Willi Syndrome (PWS). Multiple neuronal circuits have been implicated in PWS-associated hyperphagia; however the complexity of the neural regulation of appetite has obfuscated the precise mechanisms underlying the dysregulation of feeding circuits in the setting of PWS. The neuropeptide cocaine and amphetamine regulated transcript (CART) is a key regulator of appetite and weight. Central infusion of CART decreased food intake and weight gain in obese rats, and global deletion of CART in mice led to predisposition to obesity and enhanced susceptibility to high fat diet-induced metabolic dysfunction. Likewise, mutations in the CART gene have been linked to obesity in humans. Interestingly, hypothalamic levels of CART peptide were reduced in an animal model of PWS. In spite of a wealth of evidence indicating an important role of CART in metabolism, the ability of CART to reduce food intake in models of PWS has not been reported. This lack of knowledge represents a major gap in the PWS field. Importantly, we recently identified the cognate receptor of CART as the G protein coupled receptor, GPR160, thus enabling chemical biology methods to design CART agonists for the treatment of PWS-associated hyperphagia. We hypothesize that central injection of CART will reduce food intake in the setting of paternally-inherited Magel2 deficiency, and that Magel2 deficiency will lead to reduced expression of CART in central feeding centers, particularly the hypothalamus. We will address our hypothesis in two Specific Aims. In Aim 1, we will inject CART icv into Magel2-deficient rats, and food and water intakes and meal patterning will be measured using our BioDAQ system. Rats will then be placed in an open field behavioral test to ensure that any observed effect of CART on food intake is due to a primary effect of CART on appetite, rather than a secondary effect on locomotor behavior. Although proCART mRNA levels likely are not modulated in PWS, the prohormone convertase responsible for the production of mature CART, PSCK1, has been shown to be reduced in individuals with PWS, which leads to reductions in the production of mature CART peptide. In Aim 2, we therefore will measure CART, GPR160, and PSCK1 levels in brains, CSF, and plasma of Magel2-deficient rats compared to WT rats. Because CART peptide levels fluctuate with metabolic state (i.e. reduced during fasting, normalized after re-feeding), we will determine if functional changes in CART peptide levels occur in the setting of Magel2 deficiency using global proteomic analyses. Lastly, we will determine if there are changes in the expression patterns of CART and GPR160 in the brains of Magel2-deficient rats compared to WT rats. If our hypotheses are correct, then replacement with CART agonists should at least partially control appetite in individuals with PWS. This strategy of targeting a system that is parallel to, and not dependent upon, other feeding circuits could revolutionize treatment of PWS-associated hyperphagia and offer an innovative solution to the severe obesity experienced by patients with PWS.