Around one third of children and two thirds of adults in the US are thought to be overweight or obese. By increasing the risk of disorders such as heart disease, stroke, diabetes and some types of cancer, obesity has become one of the leading causes of preventable death worldwide and accounts for an increasing proportion of all spending on healthcare.

Given the high costs of obesity for individuals and society, there is widespread interest in the development of drugs to aid weight loss. Three compounds are currently approved for this purpose, and they work either by reducing the body's ability to absorb fat or by acting on the brain to suppress appetite. However, all three have significant side effects.

Now, on the basis of experiments in fruit flies, Olds and Xu suggest an alternative strategy, namely targeting the ‘stretch-sensitive’ ion channels in the neurons in the digestive system that signal to the brain that the body has ingested enough food. By artificially activating these ion channels, it might be possible to induce feelings of fullness after smaller quantities of food have been consumed.

These ion channels—known as PPK1 ion channels—are present on posterior enteric neurons, which wrap around the muscles of the gut. Silencing these neurons caused fruit flies to eat too much, whereas activating them caused the flies to eat less. Deleting the gene that encodes the PPK1 ion channel had the same effect as silencing neurons, suggesting that drugs that act directly on PPK1 could help to regulate food intake. Consistent with this, insects ate more when their food was supplemented with a chemical that blocked the PPK1 ion channels.

By showing that PPK1 ion channels can be targeted pharmacologically, Olds and Xu have opened up a new avenue of anti-obesity research. A drug that can activate the equivalent ion channel in mammals would have the potential to aid weight loss, while avoiding the side effects associated with compounds that act directly on the brain.

Historically, the sensation of fullness has been documented as far back as Homer's Odyssey. Pioneering work by Cannon and Washburn revealed a correlation between stomach expansion and satiety in humans (Cannon and Washburn, 1911), which was later confirmed in rodents (Hargrave and Kinzig, 2012). Recently, several groups have shown that feeding-related neurons are sensitive to satiety state but not nutrients in Drosophila (Marella et al., 2012; Dus et al., 2013; Pool et al., 2014). These studies argue that non-metabolic inputs such as mechanic tension could regulate feeding.

Recent studies in Drosophila have identified neuronal regulation of food intake and nutrient sensing in the central nervous system (Marella et al., 2012; Dus et al., 2013). However, the potential contribution of the enteric neurons in the gastrointestinal tract has not been explored. To investigate this, we utilized four previously characterized Gal4 lines that are expressed in enteric neurons in the different parts of the Drosophila digestive system (Figure 1A,B). While the GMR48A05-Gal4 neurons project to the proventriculus and the anterior midgut, the GMR51F12-Gal4 neurons project to the anterior midgut and the crop (Figure 1A,B) (Jenett et al., 2012). In contrast, both HGN1-Gal4 and Ilp7-Gal4 neurons project to two posterior regions in the gut: (1) the hindgut pylorus and the connecting posterior midgut and anterior hindgut, and (2) the rectum pylorus and the rectum (referred to as the posterior enteric neurons or the PENs, Figure 1A,B) (Cognigni et al., 2011).

Figure 1

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We first used these Gal4 lines to activate specific enteric neurons by expressing the temperature-sensitive ion channel, TRPA1, and measured the effects on hemolymph glucose levels (UAS-TRPA1 [Hamada et al., 2008]). Activation of the Ilp7-Gal4 neurons significantly decreases glucose levels in comparison to the Ilp7-Gal4 and UAS-TRPA1 controls (Figure 1C). Since Ilp7-Gal4 is also expressed in the CNS and in neurons projecting to the reproductive organs (Yang et al., 2008), we did similar experiment using HGN1-Gal4, which expresses in the PENSs, but not in the other Ilp7-Gal4 neurons (Cognigni et al., 2011), and obtained similar results on glucose levels (Figure 1C). We next examined enteric neurons projecting to other regions of the digestive system using GMR48A05-Gal4 and GMR51F12-Gal4 and did not observe any obvious effect (Figure 1C). We then assayed the effects of silencing these neurons by expressing the temperature-sensitive Dynamin, Shi^TS1 (UAS-Shi^TS1 [Kitamoto, 2013]). Silencing of the PENs using both Ilp7-Gal4 and HGN1-Gal4 increased glucose levels in comparison to the controls, while no effect was observed by silencing the GMR48A05-Gal4 and GMR51F12-Gal4 neurons (Figure 1D). Together, the data indicate that activities of the PENs are integral in the regulation of hemolymph glucose levels.

The change of glucose levels could result from differences in food intake. Previous work by Miguel-Aliaga and colleagues revealed that silencing Ilp7-Gal4 neurons increases defecation (Cognigni et al., 2011), which could be the result of increased feeding. We therefore examined the hypothesis that the activities of the PENs regulate food intake. Consistent with the hypothesis, we silenced the PENs in the absence of food and found that this abolished the gains in glucose levels (Figure 1E). We next investigated this directly using the capillary feeding assay (Ja et al., 2007). Silencing the PENs dramatically increased food intake (Figure 2A), which is consistent with the gains in glucose levels seen earlier. Conversely, activating these neurons caused dramatic decreases in feeding (Figure 2B). Together, manipulation of the activities of the PENs results in an overall six-fold change in feeding in comparison to the controls. This change is significantly larger than alterations by modulation of neuropeptide F signaling (Hong et al., 2012). These data indicate that the activities of the PENs play a prominent role in feeding.

Figure 2

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As described above, experiments in mammals indicated that mechanic tension in the gastrointestinal tract could be a satiety signal (Cannon and Washburn, 1911; Hargrave and Kinzig, 2012). To determine whether the role of the PENs in feeding is related to mechanosensing activity, we first examined the projections of these neurons and found that they tightly wrap around the muscle layer rather than projecting into the lumen of the gut (Figure 2C). This anatomy favors an involvement of mechanosensory activity rather than in detecting nutritional signals in gastrointestinal tract.

Previous studies in Caenorhabditis elegans, Drosophila, and mice have shown that members of the Degenerin/Epithelial Sodium Channels (DEG/ENaCs) function as a conserved family of mechanosensory ion channels (O'Hagan et al., 2005; Hwang et al., 2007; Zhong et al., 2010). Mutants of the C. elegans DEG/ENaC Mec-4 and its Drosophila homolog, PPK1, are touch-insensitive and the affected neurons fail to generate action potentials in response to mechanic tension (O'Hagan et al., 2005; Hwang et al., 2007; Zhong et al., 2010). This raised the possibility that PPK1 could be involved in regulating feeding in the PENs. We thus examined PPK1 expression using PPK1-Gal4 driving mCD8::GFP and confirmed its presence in the PENs (Figure 2D). To test whether the function of PPK1 in the PENs is involved in the regulation of feeding, we first assayed the effect of RNAi knockdown of PPK1 expression (Ilp7-Gal4 or HGN1-Gal4//UAS-PPK1-RNAi). Knockdown of PPK1 in the PENs, but not knockdown of PPK29, a related family member (Thistle et al., 2012), dramatically increased feeding (Figure 2E), phenocopying the effects of silencing these neurons. Next, we examined the effect of PPK1 deficiency on feeding and found that PPK1 deficient flies have increased food intake (Figure 2F). This feeding defect is rescued by expressing a PPK1 transgene in the PENs (Ilp7-Gal4, UAS- PPK1 (Ainsley et al., 2014), Figure 2F). Finally, pharmacological inhibitor of DEG/ENaCs, benzamil, has been used to antagonize PPK1 in Drosophila and homologs in mice (Liu et al., 2003; Page et al., 2007). We therefore investigated the effect of benzamil on feeding and found it increased food consumption when supplemented in fly food, but not in flies where PPK1 is knocked down (Figure 2G). Together, these data indicate that the mechanosensory ion channel, PPK1, plays a critical role in the PENs for regulating feeding.

The identification of the involvement of the DEG/ENaC mechanosensory ion channels in enteric neurons for regulating feeding lays the groundwork for investigating mechanisms underlying the phenomenon of fullness sensation. Pharmacological interventions on appetite stimulation and suppression are important for many diseases including obesity, cancer, and AIDS. Currently, all three FDA-approved weight-loss drugs have significant side-effects that target either the hypothalamus or fat absorption (Manning et al., 2014). Enteric DEG/ENaCs provide an attractive alternative for drug development due to their druggability, pharmacological accessibility, and fewer side-effect complications than the central nervous system. Overall, our findings indicate an important role of the DEG/ENaC mechanosensory ion channels in the enteric nervous system in food intake and suggest an exciting therapeutic alternative for fighting obesity.

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Author details

1. William H Olds

   Department of Genetics, Howard Hughes Medical Institute, Boyer Center for Molecular Medicine, Yale University School of Medicine, New Haven, United States

   Contribution

   WHO, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   Competing interests

   The authors declare that no competing interests exist.

2. Tian Xu

   1. Department of Genetics, Howard Hughes Medical Institute, Boyer Center for Molecular Medicine, Yale University School of Medicine, New Haven, United States
   2. National Center for International Research, Fudan-Yale Center for Biomedical Research, Fudan University, Shanghai, China
   3. Institute of Developmental Biology and Molecular Medicine, Fudan-Yale Center for Biomedical Research, Fudan University, Shanghai, China

   Contribution

   TX, Conception and design, Analysis and interpretation of data, Drafting or revising the article

   For correspondence

   tian.xu@yale.edu

   Competing interests

   The authors declare that no competing interests exist.

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