Reduced Gut Acidity Induces an Obese-Like Phenotype in Drosophila melanogaster and in Mice

Nutrient Sensing via Gut in Drosophila melanogaster

Nutrient-sensing mechanisms in animals' sense available nutrients to generate a physiological regulatory response involving absorption, digestion, and regulation of food intake and to maintain glucose and energy homeostasis. During nutrient sensing via the gastrointestinal tract, nutrients interact with receptors on the enteroendocrine cells in the gut, which in return respond by secreting various hormones. Sensing of nutrients by the gut plays a critical role in transmitting food-related signals to the brain and other tissues informing the composition of ingested food to digestive processes. These signals modulate feeding behaviors, food intake, metabolism, insulin secretion, and energy balance. The increasing significance of fly genetics with the availability of a vast toolbox for studying physiological function, expression of chemosensory receptors, and monitoring the gene expression in specific cells of the intestine makes the fly gut the most useful tissue for studying the nutrient-sensing mechanisms. In this review, we emphasize on the role of Drosophila gut in nutrient-sensing to maintain metabolic homeostasis and gut-brain cross talk using endocrine and neuronal signaling pathways stimulated by internal state or the consumption of various dietary nutrients. Overall, this review will be useful in understanding the post-ingestive nutrient-sensing mechanisms having a physiological and pathological impact on health and diseases.

Anatomy and Physiology of the Digestive Tract of Drosophila melanogaster

The gastrointestinal tract has recently come to the forefront of multiple research fields. It is now recognized as a major source of signals modulating food intake, insulin secretion and energy balance. It is also a key player in immunity and, through its interaction with microbiota, can shape our physiology and behavior in complex and sometimes unexpected ways. The insect intestine had remained, by comparison, relatively unexplored until the identification of adult somatic stem cells in the Drosophila intestine over a decade ago. Since then, a growing scientific community has exploited the genetic amenability of this insect organ in powerful and creative ways. By doing so, we have shed light on a broad range of biological questions revolving around stem cells and their niches, interorgan signaling and immunity. Despite their relatively recent discovery, some of the mechanisms active in the intestine of flies have already been shown to be more widely applicable to other gastrointestinal systems, and may therefore become relevant in the context of human pathologies such as gastrointestinal cancers, aging, or obesity. This review summarizes our current knowledge of both the formation and function of the Drosophila melanogaster digestive tract, with a major focus on its main digestive/absorptive portion: the strikingly adaptable adult midgut.

The Drosophila model for microbiome research

The gut microbiome is increasingly recognized to play an important role in shaping the health and fitness of animals, including humans. Drosophila is emerging as a valuable model for microbiome research, combining genetic and genomic resources with simple protocols to manipulate the microbiome, such that microbiologically sterile flies and flies bearing a standardized microbiota can readily be produced in large numbers. Studying Drosophila has the potential to increase our understanding of how the microbiome influences host traits, and allows opportunities for hypothesis testing of microbial impacts on human health. Drosophila is being used to investigate aspects of host-microbe interactions, including the metabolism, the immune system and behavior. Drosophila offers a valuable alternative to rodent and other mammalian models of microbiome research for fundamental discovery of microbiome function, enabling improved research cost effectiveness and benefits for animal welfare.

Proton pump inhibitors as risk factor for metabolic syndrome and hepatic steatosis in coeliac disease patients on gluten-free diet

BACKGROUND

Recent research has shown that patients with coeliac disease (CD) are at risk of developing metabolic syndrome (MS) and hepatic steatosis (HS) after commencing a gluten-free diet (GFD). This study aimed to evaluate the predictive factors for MS and HS in CD after 1 year of GFD.

METHODS

All consecutive newly diagnosed CD patients were enrolled. We prospectively collected data about BMI; waist circumference; blood pressure; cholesterol; triglycerides, glucose and insulin blood levels; insulin resistance (through the homeostatic model assessment HOMA-IR) and treatment with proton pump inhibitors (PPI). Diagnosis of MS was made in accordance with current guidelines and HS was diagnosed by ultrasonography. The prevalence of MS and HS was re-assessed after 1 year of GFD. A logistic regression analysis was performed to identify risk factors for MS and HS occurrence after 1 year of GFD.

RESULTS

Of 301 patients with newly diagnosed CD, 4.3% met criteria for diagnosis of MS and 25.9% presented with HS at the time of CD diagnosis; 99 subjects (32.8%) had long-term exposure to PPI during the study period. After 1 year, 72 (23.9%) patients had developed MS (4.3 vs 23.9%; p < 0.001, OR 6.9) and 112 (37.2%) had developed HS (25.9 vs 37.2%; p < 0.01, OR 1.69). At multivariate analysis, high BMI at diagnosis (OR 10.8; p < 0.001) and PPI exposure (OR 22.9; p < 0.001) were the only factors associated with the occurrence of MS; HOMA-IR (OR 9.7; p < 0.001) and PPI exposure (OR 9.2; p < 0.001) were the only factors associated with the occurrence of HS.

CONCLUSIONS

PPI exposure adds further risk of occurrence of MS and HS for patients with CD on GFD. The use of PPI in patients with CD on GFD should be limited to strict indications.

Drosophila as a model to study obesity and metabolic disease

Excess adipose fat accumulation, or obesity, is a growing problem worldwide in terms of both the rate of incidence and the severity of obesity-associated metabolic disease. Adipose tissue evolved in animals as a specialized dynamic lipid storage depot: adipose cells synthesize fat (a process called lipogenesis) when energy is plentiful and mobilize stored fat (a process called lipolysis) when energy is needed. When a disruption of lipid homeostasis favors increased fat synthesis and storage with little turnover owing to genetic predisposition, overnutrition or sedentary living, complications such as diabetes and cardiovascular disease are more likely to arise. The vinegar fly Drosophila melanogaster (Diptera: Drosophilidae) is used as a model to better understand the mechanisms governing fat metabolism and distribution. Flies offer a wealth of paradigms with which to study the regulation and physiological effects of fat accumulation. Obese flies accumulate triacylglycerols in the fat body, an organ similar to mammalian adipose tissue, which specializes in lipid storage and catabolism. Discoveries in Drosophila have ranged from endocrine hormones that control obesity to subcellular mechanisms that regulate lipogenesis and lipolysis, many of which are evolutionarily conserved. Furthermore, obese flies exhibit pathophysiological complications, including hyperglycemia, reduced longevity and cardiovascular function - similar to those observed in obese humans. Here, we review some of the salient features of the fly that enable researchers to study the contributions of feeding, absorption, distribution and the metabolism of lipids to systemic physiology.

Organ-to-Organ Communication: A Drosophila Gastrointestinal Tract Perspective

The long-term maintenance of an organism's homeostasis and health relies on the accurate regulation of organ-organ communication. Recently, there has been growing interest in using the Drosophila gastrointestinal tract to elucidate the regulatory programs that underlie the complex interactions between organs. Data obtained in this field have dramatically improved our understanding of how organ-organ communication contributes to the regulation of various aspects of the intestine, including its metabolic and physiological status. However, although research uncovering regulatory programs associated with interorgan communication has provided key insights, the underlying mechanisms have not been extensively explored. In this review, we highlight recent findings describing gut-neighbor and neighbor-neighbor communication models in adults and larvae, respectively, with a special focus on how a range of critical strategies concerning continuous interorgan communication and adjustment can be used to manipulate different aspects of biological processes. Given the high degree of similarity between the Drosophila and mammalian intestinal epithelia, it can be anticipated that further analyses of the Drosophila gastrointestinal tract will facilitate the discovery of similar mechanisms underlying organ-organ communication in other mammalian organs, such as the human intestine.

Preventing Age-Related Decline of Gut Compartmentalization Limits Microbiota Dysbiosis and Extends Lifespan

Compartmentalization of the gastrointestinal (GI) tract of metazoans is critical for health. GI compartments contain specific microbiota, and microbiota dysbiosis is associated with intestinal dysfunction. Dysbiosis develops in aging intestines, yet how this relates to changes in GI compartmentalization remains unclear. The Drosophila GI tract is an accessible model to address this question. Here we show that the stomach-like copper cell region (CCR) in the middle midgut controls distribution and composition of the microbiota. We find that chronic activation of JAK/Stat signaling in the aging gut induces a metaplasia of the gastric epithelium, CCR decline, and subsequent commensal dysbiosis and epithelial dysplasia along the GI tract. Accordingly, inhibition of JAK/Stat signaling in the CCR specifically prevents age-related metaplasia, commensal dysbiosis and functional decline in old guts, and extends lifespan. Our results establish a mechanism by which age-related chronic inflammation causes the decline of intestinal compartmentalization and microbiota dysbiosis, limiting lifespan.

Microbes without Borders: Decompartmentalization of the Aging Gut

The microbiota supports intestinal homeostasis in developing animals. With increased age, gut maintenance declines and microbes can stray from traditional zones, negatively impacting host health. In this issue of Cell Host & Microbe, Li et al. (2016) detail the mechanisms leading to the decline in intestinal health in aged flies.

Molecular mechanism and functional significance of acid generation in the Drosophila midgut

The gut of Drosophila melanogaster includes a proximal acidic region (~pH 2), however the genome lacks the H⁺/K⁺ ATPase characteristic of the mammalian gastric parietal cell, and the molecular mechanisms of acid generation are poorly understood. Here, we show that maintenance of the low pH of the acidic region is dependent on H⁺ V-ATPase, together with carbonic anhydrase and five further transporters or channels that mediate K⁺, Cl⁻ and HCO₃⁻ transport. Abrogation of the low pH did not influence larval survival under standard laboratory conditions, but was deleterious for insects subjected to high Na⁺ or K⁺ load. Insects with elevated pH in the acidic region displayed increased susceptibility to Pseudomonas pathogens and increased abundance of key members of the gut microbiota (Acetobacter and Lactobacillus), suggesting that the acidic region has bacteriostatic or bacteriocidal activity. Conversely, the pH of the acidic region was significantly reduced in germ-free Drosophila, indicative of a role of the gut bacteria in shaping the pH conditions of the gut. These results demonstrate that the acidic gut region protects the insect and gut microbiome from pathological disruption, and shed light on the mechanisms by which low pH can be maintained in the absence of H⁺, K⁺ ATPase.