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Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2021)

The melanocortin 4 Receptor (MC4R) is a central component of the leptin-melanocortin pathway in the hypothalamus. MC4R plays a critical role in the long-term regulation of energy homeostasis and mutations in MC4R are the most common cause of monogenic obesity. However, the precise molecular and cellular mechanisms underlying the maintenance of energy balance within MC4R expressing neurons are unknown. The expression of MC4R in neurons from the Paraventricular Nucleus of the hypothalamus (PVN) has been shown to be both necessary and sufficient to regulate food intake and body weight, and we discovered that MC4R localizes at the primary cilium of those neurons. Primary cilia are unique microtubule-based organelles that protrude from the cell membrane to sense and relay extracellular signals. In Chapter one, we first mapped the expression of MC4R in the brain, to then determine the extent of MC4R localization at the primary cilia in some of the identified regions. We discovered that, in the PVN, MC4R localization at the primary cilium is dynamic and depends on age and physiological status relayed by MC4R ligands levels. Together, these results shed light on the complex regulation of MC4R bioavailability at the primary cilium. In Chapter two, we found that cilia were required specifically on MC4R-expressing neurons for the control of energy homeostasis. Moreover, these cilia were critical for pharmacological activators of MC4R to exert an anorexigenic effect. Using a combination of mouse genetic approaches, we demonstrated that MC4R signals via cilia of PVN neurons to control food intake and body weight. In Chapter three, we demonstrate that the Melanocortin Receptor Associated Protein 2 (MRAP2) is critical for the ciliary localization and weight-regulating function of MC4R. These findings demonstrate that targeting of MC4R to neuronal primary cilia is essential for the control of long-term energy homeostasis and suggests that genetic disruption of MC4R ciliary localization may frequently underlie inherited forms of obesity.

    Cover page: The melanocortin 4 receptor functions at the neuronal primary cilium to control long-term energy homeostasis
    • Article
    • Peer Reviewed
    UC San Francisco Previously Published Works (2018)
    Most monogenic cases of obesity in humans have been linked to mutations in genes encoding members of the leptin-melanocortin pathway. Specifically, mutations in MC4R, the melanocortin-4 receptor gene, account for 3-5% of all severe obesity cases in humans1-3. Recently, ADCY3 (adenylyl cyclase 3) gene mutations have been implicated in obesity4,5. ADCY3 localizes to the primary cilia of neurons 6 , organelles that function as hubs for select signaling pathways. Mutations that disrupt the functions of primary cilia cause ciliopathies, rare recessive pleiotropic diseases in which obesity is a cardinal manifestation 7 . We demonstrate that MC4R colocalizes with ADCY3 at the primary cilia of a subset of hypothalamic neurons, that obesity-associated MC4R mutations impair ciliary localization and that inhibition of adenylyl cyclase signaling at the primary cilia of these neurons increases body weight. These data suggest that impaired signaling from the primary cilia of MC4R neurons is a common pathway underlying genetic causes of obesity in humans.
      Cover page: Subcellular localization of MC4R with ADCY3 at neuronal primary cilia underlies a common pathway for genetic predisposition to obesity
      • Article
      • Peer Reviewed
      UC San Francisco Previously Published Works (2020)

      ABSTRACT

      The G protein-coupled receptor MC4R (Melanocortin-4 Receptor) and its associated protein MRAP2 (Melanocortin Receptor-Associated Protein 2) are both essential for the regulation of food intake and body weight in humans and mice. MC4R localizes and functions at the neuronal primary cilium, a microtubule-based organelle that senses and relays extracellular signals. Here, we demonstrate that MRAP2 is critical for the ciliary localization and weight-regulating function of MC4R. Our data reveal that GPCR localization to primary cilia can require specific accessory proteins that may not be present in heterologous cell systems. Our findings also demonstrate the essential role of neuronal primary cilia localization of MC4R for adequate control of energy homeostasis and the obesity-promoting effect of genetic disruption of this pathway.
        • Article
        • Peer Reviewed
        UC San Francisco Previously Published Works (2019)
        A wide range of human diseases result from haploinsufficiency, where the function of one of the two gene copies is lost. Here, we targeted the remaining functional copy of a haploinsufficient gene using CRISPR-mediated activation (CRISPRa) in Sim1 and Mc4r heterozygous mouse models to rescue their obesity phenotype. Transgenic-based CRISPRa targeting of the Sim1 promoter or its distant hypothalamic enhancer up-regulated its expression from the endogenous functional allele in a tissue-specific manner, rescuing the obesity phenotype in Sim1 heterozygous mice. To evaluate the therapeutic potential of CRISPRa, we injected CRISPRa-recombinant adeno-associated virus into the hypothalamus, which led to reversal of the obesity phenotype in Sim1 and Mc4r haploinsufficient mice. Our results suggest that endogenous gene up-regulation could be a potential strategy to treat altered gene dosage diseases.
          Cover page: CRISPR-mediated activation of a promoter or enhancer rescues obesity caused by haploinsufficiency
          • Article
          • Peer Reviewed
          UC San Francisco Previously Published Works (2023)
          The G protein-coupled receptor melanocortin-4 receptor (MC4R) and its associated protein melanocortin receptor-associated protein 2 (MRAP2) are essential for the regulation of food intake and body weight in humans. MC4R localizes and functions at the neuronal primary cilium, a microtubule-based organelle that senses and relays extracellular signals. Here, we demonstrate that MRAP2 is critical for the weight-regulating function of MC4R neurons and the ciliary localization of MC4R. More generally, our study also reveals that GPCR localization to primary cilia can require specific accessory proteins that may not be present in heterologous cell culture systems. Our findings further demonstrate that targeting of MC4R to neuronal primary cilia is essential for the control of long-term energy homeostasis and suggest that genetic disruption of MC4R ciliary localization may frequently underlie inherited forms of obesity.
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