In the melanocortin pathway, melanocortin-4 receptor (MC4R) functions to control energy homeostasis. MC4R is expressed in a sub-population of Sim1 neurons (Sim1/MC4R neurons) and functions in hypothalamic paraventricular nuclei (PVN) to control food intake. Mapping sites of hypothalamic injury in obesity is essential to counteract the disease. In the PVN of male and female mice with diet-induced obesity (DIO) there is neuronal loss. However, the existing subpopulation of PVN Sim1/MC4R neurons is unchanged, but has a loss of mitochondria and MC4R protein. In mice of both sexes with DIO, dietary intervention to re-establish normal weight restores abundance of MC4R protein in Sim1/MC4R neurons and neurogenesis in the PVN. However, the number of non-Sim1/MC4R neurons in the PVN continues to remain decreased. Selective survival and recovery of Sim1/ MC4R neurons after DIO suggests these neurons as preferential target to restore energy homeostasis and of therapy against obesity.

Genetic obesity increases in liver phosphatidylcholine (PC)/phosphatidylethanolamine (PE) ratio, inducing endoplasmic reticulum (ER) stress without concomitant increase of ER chaperones. Here, it is found that exposing mice to a palm oil-based high fat (HF) diet induced obesity, loss of liver PE, and loss of the ER chaperone Grp78/BiP in pericentral hepatocytes. In Hepa1-6 cells treated with elevated concentration of palmitate to model lipid stress, Grp78/BiP mRNA was increased, indicating onset of stress-induced Unfolded Protein Response (UPR), but Grp78/BiP protein abundance was nevertheless decreased. Exposure to elevated palmitate also induced in hepatoma cells decreased membrane glycosylation, nuclear translocation of pro-apoptotic C/EBP-homologous-protein-10 (CHOP), expansion of ER-derived quality control compartment (ERQC), loss of mitochondrial membrane potential (MMP), and decreased oxidative phosphorylation. When PE was delivered to Hepa1-6 cells exposed to elevated palmitate, effects by elevated palmitate to decrease Grp78/BiP protein abundance and suppress membrane glycosylation were blunted. Delivery of PE to Hepa1-6 cells treated with elevated palmitate also blunted expansion of ERQC, decreased nuclear translocation of CHOP and lowered abundance of reactive oxygen species (ROS). Instead, delivery of the chemical chaperone 4-phenyl-butyrate (PBA) to Hepa1-6 cells treated with elevated palmitate, while increasing abundance of Grp78/BiP protein and restoring membrane glycosylation, also increased ERQC, expression and nuclear translocation of CHOP, non-mitochondrial oxygen consumption, and generation of ROS. Data indicate that delivery of PE to hepatoma cells under lipid stress recovers cell function by targeting the secretory pathway and by blunting pro-apoptotic branches of the UPR.

No abstract

The hypothalamus plays a central role in maintaining normal energy homeostasis. The arcuate nucleus (ARC) of the hypothalamus receives anorexigenic signals from the periphery mediated by increased circulating leptin and insulin, which bind to receptors expressed by proopiomelanocortin (POMC) neurons in the ARC. When activated, POMC neurons projecting to the paraventricular nucleus (PVN) of the hypothalamus release α‐Melanocyte‐stimulating hormone (α‐MSH) at this location. In the PVN, α‐MSH binding to the melanocortin 4 receptor (MC4R) signals to decrease food intake and increase energy expenditure. Exposure to HF diet induces, in male, but not female mice, injury to POMC neurons with neuronal loss and loss of α‐MSH abundance. It remains unclear whether other types of hypothalamic neurons are also being affected by exposure to HF diet. In this respect, we have found that exposure to HF diet induces loss of Single‐Minded Family BHLH Transcription Factor 1 (Sim1) neurons in the PVN of male and female mice. Sim1 neurons in the PVN include the population of MC4R neurons. Here we asked whether in the PVN of mice exposed to HF diet there is a loss of MC4R neurons and/or decreased abundance of MC4R protein. When exposed to HF diet, male and female Sapphire mice expressing GFP under the MC4R have increased body weight as compared to the control mice fed low fat (LF) diet (by 39.03 ± 5.37% and 24.19 ±4.18%, respectively). Both male and female Sapphire mice exposed to HF diet have the same number of MC4R neurons as compare to controls exposed to LF diet. These data indicate that MC4R neurons are resistant to HF diet. To test whether exposure to HF diet reduces the abundance of MC4R protein, we generated knock‐in mice with human influenza hemagglutinin (HA) epitope tag at the C‐terminus of endogenous MC4R (MC4R‐HA mice). As compared to control, male and female MC4R‐HA mice exposed to HF diet had increased body weight (by 53.84 ± 7.00% and 45.57 ± 6.23% respectively), increased caloric intake by (39.28 ± 16% and 37.52 ± 10.85%, respectively) and reduced MC4R protein abundance in the PVN (by 63.27 ± 18.54 % and 67.77 ± 18.00 %). The data indicate that loss of MC4R protein, rather than of MC4R neurons, is a feature of male and female mice exposed to HF diet, thereby suggesting expression of MC4R as a target for anti‐obesity therapy.Support or Funding InformationThis work was supported by National Institutes of Health Grants R01‐DK102206 (to G.B.), by UL1TR000039, and by Intramural Funding Support from the University of Arkansas for Medical Sciences College of Medicine Research Council.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

One feature of high-fat diet-induced neurodegeneration in the hypothalamus is an increased level of palmitate, which is associated with endoplasmic reticulum (ER) stress, loss of CoxIV, mitochondrial fragmentation, and decreased abundance of MC4R. To determine whether antidiabetic drugs protect against ER and/or mitochondrial dysfunction by lipid stress, hypothalamic neurons derived from pre-adult mice and neuronal Neuro2A cells were exposed to elevated palmitate. In the hypothalamic neurons, palmitate exposure increased expression of ER resident proteins, including that of SERCA2, indicating ER stress. Liraglutide reverted such altered ER proteostasis, while metformin only normalized SERCA2 expression. In Neuro2A cells liraglutide, but not metformin, also blunted dilation of the ER induced by palmitate treatment, and enhanced abundance and expression of MC4R at the cell surface. Thus, liraglutide counteracts, more effectively than metformin, altered ER proteostasis, morphology, and folding capacity in neurons exposed to fat. In palmitate-treated hypothalamic neurons, mitochondrial fragmentation took place together with loss of CoxIV and decreased mitochondrial membrane potential (MMP). Metformin, but not liraglutide, reverted mitochondrial fragmentation, and both liraglutide and metformin did not protect against either loss of CoxIV abundance or MMP. Thus, ER recovery from lipid stress can take place in hypothalamic neurons in the absence of recovered mitochondrial homeostasis.