Glucagon-Like Peptide 1 Stimulates Hypothalamic Proopiomelanocortin Neurons

Ma, X. et al.
University Laboratory of Physiology, Oxford, United Kingdom
The Journal of Neuroscience
July 4, 2007 [PubMed]

Central Conclusion of Paper (Reason for Being)
GLP1 exerts its appetite suppressive effect by activating proopiomelanocortin (POMC) neurons in the hypothalamus. The activation takes place by way of GLP1 binding to its receptor, activation of PKA, and increasing of L-type Ca2 current.

Discussion of Paper
This Brief Communication provides the experimental evidence for a hypothesis that already had substantial circumstantial evidence. It is known that GLP1 regulates appetite and weight. It is also known that the POMC neurons are the mediators of the appetite control by the hypothalamus. Finally, it is known that the receptor for GLP1 is expressed in the region of the hypothalamus that contains the POMC neurons. Now, this paper provides the data to show the direct interaction of GLP1 and the POMC neurons.

All of the experiments were done using brain slices containing eGFP-expressing POMC neurons. This allowed for the measurement of individual neurons. First the authors showed that treatment with GLP1 increased action potential firing (in 7 of 8 neurons surveyed). Culturing the neurons with both GLP1 with GLP1R antagonist exendin 9-39 abolished the GLP1 effect on the neurons. They repeated the experiment in the presence of blockers of AMPA receptors, NMDA receptors, and GABA receptors. GLP1 still increased firing indicating that GLP1 specifically acts through its receptor. Although, it had already been established, they showed that GLP1 stimulates electrical activities by increasing cytosolic calcium. Again, replicating others' results, the authors showed that GLP1 increased cAMP levels in the neurons. Forskolin, which also elevates cAMP, was also able to increase neuron firing indicating this is the likely signaling pathway that GLP1 uses. Blocking PKA activity with Rp-8-Bromo-cAMPS blocks the effects of GLP1.

Other Notes from the Paper
GLP1 is known to be produced by neurons in the nucleus of the solitary tract (NTS) in addition to the L-cells of the small intestine.

Intracerebroventricular and repeated central injections of GLP1 have been used to show that it reduces feeding and body weight in rats.

GLP1 levels are reduced in obese men and teenage girls (links to eating disorders) and rise with weight loss.

Subcutaneous injection of GLP1 over 5 days reduces weight in obese subjects.

GLP1R is densely expressed in the arcuate nucleus (ARC), paraventricular nucleus (PVN), and supraoptic nuclei of the hypothalamus

GLP-1 also stimulates the electrical activity of orexin/hypocretin neurons but has an opposite effect on Ca2 currents - activation of orexin/hypocretin neurons leads to hyperphagia.

Neurotrophic property of geniposide for inducing the neuronal differentiation of PC12 cells

Liu, J. et al.
Chongqing Technology and Business University, Chongqing , China
Int. J. Devl Neuroscience
Nov. 2006 [Pubmed]

This paper discusses the use of a cell-based assay to identify geniposide as a potential agonist of Glp1r and then establishes that geniposide can induce neurite outgrowth in a culture. The supposed Glp1r-screening assay seems to be very nonspecific. It is a luciferase reporter gene hooked up to a series of CREB response elements (CRE) so anything that induces CREB (which is many, many things) will generate a positive in the screen. The only indication that geniposide is acting through Glp1r is data from cell lines with and without the overexpression of Glp1r. Overexpression of Glp1r greatly increases the response of the reporter construct. This does not indicate that geniposide is specific for Glp1r. The authors tested cell lines overexpressing a serotonin and dopamine receptor and showed no increase in response but there are still many other more closely related receptors that geniposide could be acting through. The data from the neurite outgrowth is somewhat obvious. It is clear by the cell-based assay that geniposide activates the CREB pathway and it is already known that CREB pathway activities increases neurite outgrowth. The real question that remains and the critical one going forward is whether geniposide is a specific agonist of Glp1r. If you were to administer geniposide to an animal, will it only have the effect of other Glp1r agonists or will it have a number of unexpected consequences and side effects due to off target activity? This paper does not provide any guidance in this area. In fact, there are no papers other than this one even linking geniposide with the Glp1 pathway. Additionally, in the 100 some papers discussing geniposide, none of them link it to many of the hallmarks of Glp1 action link insulin response. This paper stands alone in providing this link.

Some other notes;

GLP1 is produced predominantly in the intestines.

GLP1R has been shown to be expressed in the brains of both humans and rodents including hypothalamus, thalamus, brainstem, lateral septum, subfornical organ.

GLP1R activation has been shown to induce neurite outgrowth by cyclic AMP signaling.

GLP1 and extendin-4 have been shown to have neuroprotective properties.

GLP1 and extendin-4 reduces A Beta when infused into the lateral ventricles.

In a well established rodent model of cholinergic neurodegeneration, infusion of GLP-1 had shown complete amelioration of an ibotenic acid-induced cholinergic marker deficit.

Geniposide has been shown to protect against neuronal cell death.

Small-molecule agonists for the glucagon-like peptide 1 receptor

Knudsen, L.B. et al.
Novo Nordisk Als, Maaloev, Denmark
Proceedings of the National Academy of Sciences
Jan. 16, 2007 [PubMed]

The GLP1 peptide lowers glucose and weight in type 2 diabetes patients.

The peptide has to be injected daily.

Have identified compounds that act as both independent agonists and increases affinity of GLP1 for its receptor.

Calls type 2 diabetes and associated obesity a worldwide pandemic.

GLP1, 30 AA protein, synthesized in the small intestine.

Nauck, MA et al. and Zander, M et al. have shown clinical efficacy of GLP1 for treating diabetes.

GLP1 stimulates glucose-independent release of insulin from the pancreas and insulin, glucokinase, and glucose transporter biosynthesis.

Also act to; (i) growth, proliferation, and antiapoptosis of pancreatic beta cells and neogenesis from ductal precursor cells; (ii) glucosedependent lowering of glucagon secretion, leading to lower hepatic glucose output; (iii) inhibition of gastric acid secretion and gastric emptying, the latter causing a reduction in postprandial plasma glucose excursions; and (iv) inhibition of appetite and lowering of food intake leading to decreased body weight.

Also has neuro and cardio protective activities.

High doses of GLP1 induce nausea and it has a short half-life

Exenatide, a GLP-1 analog originally isolated from the saliva of the Gila monster, recently was approved by the Food and Drug Administration as a twice-daily treatment regimen.

Liraglutide, another peptide analog is in phase 3 clinical trials for once daily treatments.

Identified a single small molecule that was specific for GLP1R and did not displace GLP1 binding.

Compound was not antagonized by Extendin.

Compound 2 (modified version of original compound) had no effect on insulin release in islet cells from Glp1r KO mice indicated specificity.

It is possible the compound is stimulating homodimerization of the receptor to produce the increased binding affinity of GLP1.

Authors state that compound is not potent enough to be used as drug.

Paper lacks any in vivo data. Can not determine if compound would have any activity in an animal or what dose would be needed for activity in an animal. Compound can bind and activate GLP1R in a cell based assay and ex vivo experiments show the expected effect on pancreatic tissue and cells (1mM in cells and 10microM in perfused tissues).