Evolution of appetite regulating systems

All unapproved Citizendium articles may contain errors of fact, bias, grammar etc. A version of an article is unapproved unless it is marked as citable with a dedicated green template at the top of the page, as in this version of the 'Biology' article. Citable articles are intended to be of reasonably high quality. The participants in the Citizendium project make no representations about the reliability of Citizendium articles or, generally, their suitability for any purpose. This article is currently being developed as part of an Eduzendium student project in the framework of a course entitled Appetite and Obesity at University of Edinburgh. The course homepage can be found at CZ:(U00984) Appetite and Obesity, University of Edinburgh 2010. For the course duration, the article is closed to outside editing. Of course you can always leave comments on the discussion page. The anticipated date of course completion is 01 February 2011. One month after that date at the latest, this notice shall be removed. Besides, many other Citizendium articles welcome your collaboration!

Recently, there has been extensive research into the neuroendocrine mechanisms controlling appetite. The pro-opiomelanocortin (POMC) gene has an important role in these mechanisms, particularly through production of alpha melanocyte stimulating hormone (alpha-MSH). POMC and its end-products have not only been identified in humans, but also in many other vertebrates. This has lead to further research into the origins of the POMC gene and the evolution of appetite regulating systems. This article details the structure and function of the POMC gene. It highlights variations between species, allowing a potential evolutionary route, originating at a common ancestral gene, to be mapped out.

Human POMC

The human POMC gene encodes a hormone precursor protein, which itself is then cleaved, with the help of prohormone convertase enzymes, into a number of different peptides. These include the melanocyte-stimulating hormones (alpha-, beta-, gamma- MSH), adrenocorticotropic hormone (ACTH), the lipotropins, and beta-endorphin . ACTH and the MSHs are referred to as the melanocortins and all have the same core amino acid sequence, HFRW .

The POMC gene is found on chromosome 2p23, and is made up of three exons and two “large” introns. Only exons two and three are translated however. Exon two codes for the signal peptide and the initial N-terminal amino acids, while exon three codes for “most of the translated mRNA” .

POMC and its post-translational processing, adapted from Millington, and Raffin-Sanson et al.

After excision of the introns to form a “parent” POMC, this molecule is then cleaved into its various peptides, as mentioned above, by PCs, specifically PC1 and PC2. These enzymes act at cleavage sites consisting of paired basic residues, arginine and lysine, and end-products of their action depend on which sites are used. As PC1 and PC2 act on different sites, and their expression varies in different tissues, processing of POMC’s peptides is tissue-specific . For example, the anterior pituitary corticotroph cells only express PC1, which results in the cleavage of POMC into the NH2-terminal peptide (N-term), joining peptide (JP), ACTH, β-LPH, and some γ-LPH and β-endorphin (β-end) - See "POMC cleavage by PC1" diagram. The latter two peptides are produced because the “last cleavage site is only partially used”. However, in melanotroph cells, found in the intermediate lobe of the rodent pituitary, and the human hypothalamus, and placenta, both PC1 and PC2 are expressed. This means that all the cleavage sites are used and smaller peptides are produced ). N-term is therefore cleaved to the γ-MSHs, ACTH gives rise to α-MSH and CLIP (corticotropin-like intermediate lobe peptide) and γ-LPH to β-MSH.

The cleavage of POMC by PC1, adapted from Raffin-Sanson et al.

The POMC gene is expressed in number of tissues in man and animals. These include the anterior, and intermediate (only in rodents) pituitary of the central nervous system. It is also present in the nucleus tractus solitarius of the caudal medulla, and the hypothalamus, specifically the arcuate nucleus. Its expression has also been noted in the skin and the immune system,as well as other peripheral tissues.

The final products from the cleavage of POMC have a variety of functions. The melanocortins act on melanocortin receptors (MCRs) in different tissues. There are five types of MCR and melanocortin peptides bind to these with varying affinities, for example, mainly ACTH binds to MC2R (see table).

The Melanocortin Receptors, adapted from Yang et al.

β-endorphin, on the other hand, is involved in the pain pathway by acting as an inhibitory neurotransmitter in the central nervous system. Here it binds to opioid receptors, leading to an analgesic effect .

CLIP has been found in many areas of the brain, especially in nerve fibres. It has been shown that it plays an important role in REM sleep, which in turn helps with the consolidation of memories .

The functions of β- and γ-LPH however, are still uncertain, although lipotropins are known for their role in lipolysis, mobilizing lipids for energy production, and their importance has been implicated in haematopoiesis.

Physiology and Relation to the Hypothalamus and Appetite Regulation

In mammals, alpha MSH is generally assumed to be the main POMC product involved in appetite regulation. Alpha MSH acts as a potent inhibitor of appetite (anorexigenic), acting on MC3-R and MC4-R in the arcuate nucleus. In POMC null mice, alpha-MSH was found to be the most potent anorexigenic signaller.

Rodents do not express beta-MSH, but evidence has been found indicating an important role for beta-MSH in human appetite regulation. A study screening for mutations in the beta-MSH region of POMC found an increased incidence of the beta-MSH variant Try221Cys in obese subjects. The variant was shown to have altered binding and signalling through MC4-R (Lee et al).

Desacetyl-alpha-MSH is a precursor for alpha-MSH and is widely distributed in the brain (125). Although desacetyl-alpha-MSH displays a similar potency for receptor binding to alpha-MSH, it does not affect food intake except at extremely high doses (Abbott et al).

The arcuate nucleus (ARC) is found at the bottom of the hypothalamus and consists of several groupings of specific neurons (see diagram). There are two main populations of neurons regulating appetite; those co-expressing POMC and cocaine and amphetamine regulated transcript (CART) and those co-expressing Neuropeptide Y (NPY) and Agouti related peptide (AgRP). The POMC/CART neurons form part of the satiety pathway in the arcuate nucleus and neurons co-expressing NPY and AgRP are part of orexigenic signalling (48 Dhillo et al). AgRP acts as an inverse agonist at MC3-R and MC4-R, to help stimulate feeding (Dhillo et al).

Image Caption

POMC/CART and NPY/AgRP neuron populations both express leptin receptors (LepRb). Leptin has opposing effects on each neuron population, activating the anorexigenic POMC neurons and inhibiting the orexigenic NPY/AgRP neurons, in the arcuate nucleus. Using transgenic mice expressing green fluorescent protein (GFP), two CNS regions expressing POMC in response to leptin were found: the arcuate nucleus and the nucleus of the solitary tract (NTS) (Cowley MA et al). Another study used transgenic mice with POMC-GFP expression and NYP-GFP expression, measuring the number of inhibitory and excitatory postsynaptic currents. Leptin deficient (ob/ob) mice were shown to have an increased excitation of NPY neurons and increased inhibition of POMC neurons compared to their wild-type littermates. Treatment with leptin lead to normalisation of the synaptic signals to wild-type levels (Pinto S et al)

The full extent on the signalling between the neuron populations within the hypothalamus is not fully understood. It is thought that NPY neurons project to inhibit POMC expression, mediated through GABA (Cone). Using electron microscopy, co-expression of GABA and NPY was confirmed at these nerve terminals (Cowley MA et al). Leptin also acts at the GABA nerve terminals to reduce the inhibition of POMC neurons.

Ghrelin has been shown, using RT-PCR if hypothalamic RNA, expressed in the brain (Kojima et al). Ghrelin was identified as the endogenous ligand for the GHS-Receptor, expressed in the arcuate nucleus, and ghrelin-containing axons were found to in several hypothalamic nuclei. Ghrelin was shown using fluorescent protein tagged NPY neurons, to increase release of NPY and AgRP, as well as increasing the GABA-mediated inhibition of POMC neurons (Cowley MA et al 2).

There are five NPY receptors (Y1-Y5), which are also activated by the closely related peptide PYY. Using specific receptor agonists and measuring the effect on POMC mRNA levels, it was found that NPY down-regulates POMC expression through the Y2 receptor (yebenes et al). Interestingly, activation of a presynaptic Y2 autoreceptor suppresses NPY release. PYY3-36, a satiety signal released from the gut after a meal, acts as an agonist at the presynaptic Y2 receptors on the nerve terminals of NPY/AgRP/GABA neurons. This leads to suppression of NPY release and increased POMC release in the arcuate nucleus (Batterham RL et al).

POMC neurons involved in regulation of appetite are also found in the nucleus tractus solitarius (NTS), with projections to the paraventricular nucleus and others. Cholecystokinin (CCK) is another satiety-inducing peptide from the gut. Its effects are mediated through CCK1 receptors and vagal afferents to the NTS. Both feeding and intraperitoneal injection of CCK8 resulted in increased c-fos expression in NTS POMC neurons (fan et al). The PVN contains oxytocin neurons which activate a feedback mechanism to stomach, closing off the gastric sphincter as well as releasing oxytocin locally in the hypothalamus. The VMH has many oxytocin receptors, which leads to satiety (Gareth Lang lecture)

Species Variation in POMC Gene

evolutionary tree diagram Chordata (vertebrates)

   ~ Agnatha – Lamprey
   ~ Gnathostomes
         ~ Chondrichthyes (cartilaginous fish)
         ~ Osteichthyes (bony fish) 
              ~ Subclass Actinopterygii (ray-finned fish;) - paddlefish
              ~ Subclass Sarcopterygii (lobe-finned fish)
                    ~ Tetrapods (mammals, birds, reptiles?)

Invertebrates

Summary/Conclusion

Remember you are writing an encyclopedia article; it is meant to be readable by a wide audience, and so you will need to explain some things clearly, without using unneccessary jargon. But you don't need to explain everything - you can link specialist terms to other articles about them - for example adipocyte or leptin simply by enclosing the word in double square brackets.

About References

To insert references and/or footnotes in an article, put the material you want in the reference or footnote between <ref> and </ref>, like this:

<ref>Person A ''et al.''(2010) The perfect reference for subpart 1 ''J Neuroendocrinol'' 36:36-52</ref> <ref>Author A, Author B (2009) Another perfect reference ''J Neuroendocrinol'' 25:262-9</ref>.

Look at the reference list below to see how this will look.

If there are more than two authors just put the first author followed by et al. (Person A at al. (2010) etc.)

Select your references carefully - make sure they are cited accurately, and pay attention to the precise formatting style of the references. Your references should be available on PubMed and so will have a PubMed number. (for example PMID: 17011504) Writing this without the colon, (i.e. just writing PMID 17011504) will automatically insert a link to the abstract on PubMed (see the reference to Johnsone et al. in the list.)

Use references sparingly; there's no need to reference every single point, and often a good review will cover several points. However sometimes you will need to use the same reference more than once.

How to write the same reference twice:

Reference: Berridge KC (2007) The debate over dopamine’s role in reward: the case for incentive salience. Psychopharmacology 191:391–431 PMID 17072591

First time: <ref name=Berridge07>Berridge KC (2007) The debate over dopamine’s role in reward: the case for incentive salience. ''Psychopharmacology'' 191:391–431 PMID 17072591 </ref>

Second time:<ref name=Berridge07/>

This will appear like this the first time and like this the second time

Figures and Diagrams

You can also insert diagrams or photographs (to Upload files Cz:Upload)). These must be your own original work - and you will therefore be the copyright holder; of course they may be based on or adapted from diagrams produced by others - in which case this must be declared clearly, and the source of the orinal idea must be cited. When you insert a figure or diagram into your article you will be asked to fill out a form in which you declare that you are the copyright holder and that you are willing to allow your work to be freely used by others - choose the "Release to the Public Domain" option when you come to that page of the form.

When you upload your file, give it a short descriptive name, like "Adipocyte.png". Then, if you type {{Image|Adipocyte.png|right|300px|}} in your article, the image will appear on the right hand side.