In Eleanor Raffan’s laboratory at the University of Cambridge, a portly Labrador retriever’s tail wags furiously as he desperately attempts to break into a clear plastic box containing an enticing sausage. This is not a misbehaving pup trying to steal someone’s lunch; instead, this chunky boy and his human are participating in important scientific research.
The aptly named sausage-in-a-box test is part of the battery of behavioral and physiological measures that Raffan, a canine geneticist, uses to explore the role of the pro-opiomelanocortin (POMC) gene in body weight regulation. In a recent study published in Science Advances, Raffan and her colleagues demonstrated that dogs carrying a specific POMC mutation displayed increased hunger and a 25 percent reduction in resting metabolic rate, and—likely subsequent these effects—increased adiposity.1
Investigations into canine genetics provide researchers like Raffan with unique opportunities to link genotypes to phenotypes and are especially crucial for studying the POMC pathway. POMC is a polypeptide produced by neurons in the hypothalamus. The large proteins are then chopped up into several smaller signalling peptides; in humans, these peptides include α- and β-melanocyte-stimulating hormones (MSH) and β-endorphin, which have complex and incompletely understood roles in energy balance.
β-MSH is particularly tricky to study, as human cases of β-MSH deletions are exceedingly rare. “Most of what we know about how POMC works in the brain is coming from studies in mice, and the β-MSH component is not there,” said Carmelo Quarta, who studies the neurobiology of obesity in rodent models at the University of Bordeaux and was not involved in the study. “It's interesting to see this dog model, where we have a genetic mutation that presumably is impacting [this neuropeptide].”
Dogs not only produce β-MSH; some also have naturally occurring mutations that affect its function. For example, more than ten percent of Labradors and flat-coated retrievers carry a POMC mutation that abolishes the production of β-MSH and β-endorphin but does not affect α-MSH. Therefore, these chunky puppies provide a natural model to explore the biology of these different POMC-derived neuropeptides, something that would be practically impossible to do in humans or rodents. “[This research] can tell us about this really quite niche bit of POMC biology that appeals to geeks in the field,” said Raffan.
During her training as a veterinary surgeon, Raffan saw plenty of overweight canines. When she started the Genetics of Obesity in Dogs (GOdogs) project more than a decade ago, she knew exactly where to begin her investigations. “Labradors are notoriously obesity prone, because they have such big appetites,” she said. “If you're going to study obesity in dogs, it’s a bit of a no brainer start there.”
In 2016, she discovered the POMC mutation associated with body weight in Labradors, but her curiosity wasn’t satisfied.2 There are many different processes related to energy intake and energy expenditure that contribute to an animal’s weight, and Raffan wanted to know which this mutation affected.
To determine if differences in satiety signals were contributing to increased body weight, the team fed the canines a can of dog food every 20 minutes until they couldn’t eat any more or, as was the case for one particularly gluttonous pup, until they reached the maximum amount allowed by the university’s Ethics and Welfare Committee. “The remarkable thing was that although all dogs ate an enormous amount of food—on the order of two kilograms of food per dog—there wasn't actually a difference between the dogs depending on their mutation,” said Raffan.
However, they received very different results on the sausage-in-a-box test, which was designed to assess the dogs’ food drive. “The dogs who carried the POMC mutation put much more effort into getting to the sausage and were much more interested in the box,” said Raffan. “The dogs who didn't have the mutation were more likely to spend their time exploring their new environment and were less persistent in their pursuit of the sausage.” (Raffan noted that experimenters allowed the dogs to eat the sausage at the end of the test.)
To examine the other half of the body weight equation, energy expenditure, Raffan used flow-through respirometry to calculate the dogs’ resting metabolic rates. Resting metabolism, said Raffan, accounts for about two-thirds of daily energy expenditure, although the exact percentage varies by activity levels. POMC mutation carriers had substantially lower resting metabolic rates, meaning that those dogs were not only more food motivated than their non-carrier counterparts, but they generally burned fewer calories as well.
α- and β-MSH are similar peptides and both bind to the melanocortin 3 and 4 receptors (MC3R and MC4R); yet, β-MSH deficiency significantly affects physiology. Raffan hypothesized that this might be because α- and β-MSH somehow produced different downstream effects when they bound to the melanocortin receptors. To explore this possibility, she examined the effects of canine and human α- and β-MSH on both types of melanocortin receptors in vitro. The results were not quite what she had expected. “We tested the major pathways downstream of the receptor, and actually, there's very little difference,” she said.
The reasons β-MSH deficiency has such a large influence on appetite and metabolism are likely much more complicated, said Raffan. After POMC synthesis, there may be variations in the enzymes that cleave it into different MSH forms, or the way the peptides are trafficked within cells, or the way they are released. “Probably, it has something to do with different cell populations processing POMC differently, releasing α- and β-MSH differently, in different parts of the brain,” said Raffan. “But we don't know the answer.”
Quarta also highlighted the growing interest in this complex system for developing new pharmacological treatments for obesity, including the 2020 approval of Setmelanotide, an MC4R agonist, for certain forms of obesity caused by single-gene mutations.3 “Given the clear effect on energy expenditure that they saw in this [canine] model when the melanocortin system is defective, it suggests that part of the way this drug works could be through activation of energy expenditure. And I think it's important to figure that out a little bit better,” he said.
“This could really [help us understand] how the circuit can be important for energy expenditure modulation,” said Quarta. “We need that because right now for obesity, we have many drugs that help us to lower food intake, but very few approved molecules that help people to actually improve the elimination of the energy.”
In the future, Raffan hopes that her work will be used to improve canine health, but also inform mechanistic follow-up of human genome-wide association studies. These studies have turned up hundreds of polymorphisms associated with obesity, but, said Raffan, “A lot of the genetic associations that we identify in human or dog association studies aren't in known obesity genes. They clearly have some important biology behind them, but which one do you spend thousands and thousands of pounds on to try to understand the mechanism?”4
Genes that seem to play a role in obesity in both dogs and humans could be a good place to start. Her team is currently working on identifying these genes to guide future explorations into the monumentally tangled web of molecular mechanisms that link genes to physiology.
“It's a complexity that I love and embrace,” said Raffan. “But it is fiendishly complicated and difficult to understand.”
- Dittmann MT, et al. Low resting metabolic rate and increased hunger due to β-MSH and β-endorphin deletion in a canine model. Sci Adv. 2024;10(10):eadj3823.
- Raffan E, et al. A deletion in the canine POMC gene is associated with weight and appetite in obesity-prone labrador retriever dogs. Cell Metab. 2016;23(5):893-900.
- Markham A. Setmelanotide: First approval. Drugs. 2021;81(3):397-403.
- Goodarzi MO. Genetics of obesity: What genetic association studies have taught us about the biology of obesity and its complications. Lancet Diabetes Endocrinol. 2018;6(3):223-236.
