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The melanocortin-4 receptor (MC4R) is involved in energy homeostasis and is an important drug target for syndromic obesity. We report the structure of the antagonist SHU9119-bound human MC4R at 2.8-angstrom resolution. Ca 2+ is identified as a cofactor that is complexed with residues from both the receptor and peptide ligand. Extracellular Ca 2+ increases the affinity and potency of the endogenous agonist α-melanocyte-stimulating hormone at the MC4R by 37- and 600-fold, respectively. The ability of the MC4R crystallized construct to couple to ion channel Kir7.1, while lacking cyclic adenosine monophosphate stimulation, highlights a heterotrimeric GTP-binding protein (G protein)-independent mechanism for this signaling modality. MC4R is revealed as a structurally divergent G protein-coupled receptor (GPCR), with more similarity to lipidic GPCRs than to the homologous peptidic GPCRs., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

The melanocortin-4 receptor (MC4R) is essential for control of energy homeostasis in vertebrates. MC4R interacts with melanocortin receptor accessory protein 2 (MRAP2) in vitro, but its functions in vivo are unknown. We found that MRAP2a, a larval form, stimulates growth of zebrafish by specifically blocking the action of MC4R. In cell culture, this protein binds MC4R and reduces the ability of the receptor to bind its ligand, α-melanocyte-stimulating hormone (α-MSH). A paralog, MRAP2b, expressed later in development, also binds MC4R but increases ligand sensitivity. Thus, MRAP2 proteins allow for developmental control of MC4R activity, with MRAP2a blocking its function and stimulating growth during larval development, whereas MRAP2b enhances responsiveness to α-MSH once the zebrafish begins feeding, thus increasing the capacity for regulated feeding and growth.

D-fenfluramine (d-FEN) was once widely prescribed and was among the most effective weight loss drugs, but was withdrawn from clinical use because of reports of cardiac complications in a subset of patients. Discerning the neurobiology underlying the anorexic action of d-FEN may facilitate the development of new drugs to prevent and treat obesity. Through a combination of functional neuroanatomy, feeding, and electrophysiology studies in rodents, we show that d-FEN-induced anorexia requires activation of central nervous system melanocortin pathways. These results provide a mechanistic explanation of d-FEN's anorexic actions and indicate that drugs targeting these downstream melanocortin pathways may prove to be effective and more selective anti-obesity treatments.

The lethal yellow (AY/a) mouse has a defect in proopiomelanocortin (POMC) signaling in the brain that leads to obesity, and is resistant to the anorexigenic effects of the hormone leptin. It has been proposed that the weight-reducing effects of leptin are thus transmitted primarily by way of POMC neurons. However, the central effects of defective POMC signaling, and the absence of leptin, on weight gain in double-mutant lethal yellow (AY/a) leptin-deficient (lepob/lepob) mice were shown to be independent and additive. Furthermore, deletion of the leptin gene restored leptin sensitivity to AY/a mice. This result implies that in the AY/a mouse, obesity is independent of leptin action, and resistance to leptin results from desensitization of leptin signaling.

Melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH) regulate pigmentation and adrenal cortical function, respectively. These peptides also have a variety of biological activities in other areas, including the brain, the pituitary, and the immune system. A complete understanding of the biological activities of these hormones requires the isolation and characterization of their corresponding receptors. The murine and human MSH receptors (MSH-Rs) and a human ACTH receptor (ACTH-R) were cloned. These receptors define a subfamily of receptors coupled to guanine nucleotide-binding proteins that may include the cannabinoid receptor.

Recombinant retroviruses containing the complete genomic human beta globin gene (under the control of its own promoter) and the bacterial neomycin phosphotransferase gene (under the control of the normal or enhancerless viral promoter) were used to derive transgenic mouse strains by infection of preimplantation embryos. Expression of the beta globin gene in hematopoietic tissues was observed in all transgenic strains. In addition, one strain showed ectopic expression of beta globin in the same tissues that also expressed high levels of RNA from the viral promoter. It is likely that expression from the long terminal repeat (LTR), in contrast to expression from the internal promoter, is dependent on the site of integration. Thus, retroviral vectors can be used for tissue-specific expression of foreign genes in transgenic mice, as well as for the identification of loci that allow developmental activation of a provirus.

To explore the potential use of retrovirus vectors for the transfer of genomic DNA sequences into mammalian cells, recombinant retroviral genomes were constructed that encode a functionally rearranged murine lambda 1 immunoglobulin gene. Several of these genomes could be transmitted intact to recipient cells by viral infection, although successful transmission depended both on the orientation of the lambda 1 sequences and on their specific placement within vector sequences. The lambda 1 gene transduced by viral infection was expressed in a cell lineage-specific manner, albeit at lower levels than endogenous lambda 1 gene expression in cells from the B-lymphocyte lineage. Vectors yielding integrated proviruses that lacked viral transcriptional enhancer sequences were used to show that neither viral transcription nor the viral transcriptional sequences themselves had any effect on the tissue specificity of lambda 1 gene expression or the absolute amount of lambda 1 transcription. Vector transcription did, however, dramatically decrease the amount of lambda 1 protein that could be detected in tranduced cells. These results suggest that retrovirus vectors may be useful reagents not only for the expression of complementary DNA sequences but also for studies of tissue-specific transcription in mammalian cells.