Salt an Essential Nutrient: Advances in Understanding Salt Taste Detection Using Drosophila as a Model System

Common salt (NaCl) causes developmental, behavioral, and physiological defects in Drosophila melanogaster

PURPOSE

The incidence of obesity has surged to pandemic levels in recent decades. Approximately 1.89 million obesity are linked to excessive salt consumption. This study aims to check the toxicity of salt at different concentrations using an invertebrate model organism Drosophila melanogaster.

METHODS

Drosophila food was mixed with different salt concentrations (50, 200, 400, 800 µM). The toxicity of salt in third instar larvae was checked via different experiments such as trypan blue assay, crawling assay, and other histological staining was done to check the deposition of lipid droplets and amount of reactive oxygen species. Food intake analysis was performed to check the feeding rate, and body weight was also calculated to check the obesity index. Several behavioral assays are also performed in adult flies.

RESULTS

Most significant abnormalities were seen at 50 and 200 µM concentrations. Feeding rate increased up to 60%, body weight was increased up to 12% in larvae, and 27% in adult at 200 µM concentration. Approximately 60% larvae and 58% adult flies had defective response to extreme heat. 28% larvae and 38% adult flies were not responding to cold temperature. 55% flies had a defective phototaxis behavior and 40% of them showed positive geotaxis at those range. Salt stress leads to the buildup of free radicals, resulting in DNA damage in both the gut and hemolymph.

FINDINGS

Most toxic consequences are observed at the lower concentration range as the feeding rate was higher. Flies show aversive response to feed on the higher concentration of salt.

Diet influence on male sexual maturation through interplay between insulin signaling and juvenile hormone in insects

In animals, sexual maturation coincides with the development of sexual behaviors and reproductive system. These developmental events are influenced by diet and governed by endocrine signals. Here, for the first time in insects, we explored functional links between nutrition and juvenile hormone (JH) in the male reproductive physiology through the insulin signaling pathway (ISP) acting as a transducer of nutritional signals. We turned to the male moth Agrotis ipsilon for which sexual maturation, including accessory sex glands (ASGs) development concomitantly with antennal lobes (ALs) maturation for female sex pheromone processing and display of sexual behavior, is known to be JH- and diet-dependent. Indeed, a diet rich in sugars with sodium was previously shown to accelerate sexual maturation, which was achieved from the third day of adult life. In this study, we demonstrated that such a diet raised i) the expression of JH signaling actors (Methoprene-tolerant, Taiman, and Krüppel homolog 1) in ALs and ASGs, ii) the biosynthesis and circulating levels of JH, and iii) the expression of both insulin receptor (InR) and insulin-like peptides (ILPs) in corpora allata (CAs) and brain respectively. Insulin injection raised JH biosynthesis following increased HMG-CoA reductase expression in CAs; opposite effects were induced in InR-deficient males. Thus, we highlighted that promoting effects of a diet composed of sugars with sodium on male sexual maturation results from an early induction of ISP causing an increase in JH biosynthesis followed by a potentiation of JH actions on the development of ASGs and ALs in A. ipsilon.

Impact of high-salt diet in health and diseases and its role in pursuit of cancer immunotherapy by modulating gut microbiome

Cancer chemotherapy remains an area of concern, as many of the therapies are uncomfortable involving side effects and unpleasant experiences. These factors could further reduce patient's quality of life, and even endanger their life. Many therapeutic strategies have been tried to reduce the unpleasant side effects and increase the treatment effectiveness; however, none have shown to have promising effects. One of the main hindrances to cancer therapy is the escape strategies by tumor cells to the immune attack. Promoting inflammation in the tumor microenvironment is the cornerstone and key therapeutic target in cancer chemotherapy. High-salt diet (HSD) intake, though it has deleterious effects on human health by promoting chronic inflammation, is found to be advantageous in the tumor microenvironment. Studies identified HSD favors an increased abundance of Bifidobacterium species in the tumor environment due to gut barrier alteration, which, in turn, promotes inflammation and favors improved response to cancer chemotherapy. A review of the literature was carried out to find out the effects of an HSD on health and diseases, with special mention of its effect on cancer chemotherapy. Studies emphasized HSD would block the myeloid-derived suppressor cells which will enhance the tumor immunity. Exploration of the precise mechanism of simple HSD regime/ingestion of specific bacterial species as probiotics will be effective and essential to formulate the game-changing cancer chemotherapy. With the modern era of healthcare moving toward precision medicine where the physician can choose the treatment option suitable for the individual, HSD regime/ingestion of specific bacterial species can be considered.

High Salt Diet Impairs Male Fertility in Mice via Modulating the Skeletal Homeostasis

Male reproductive functions and bone health are both adversely affected by the high salt diet (HSD). Nevertheless, the underlying mechanism via which it alters the sperm function remains largely unknown. This study examines the mechanism by which HSD affects male fertility by impairing bone health. For investigating the same, male BALB/c mice were categorized into three groups-HSD group (fed with 4% NaCl), a low salt diet (LSD) group (fed with 0.4% NaCl), and a control group (fed with a normal diet) for 6 weeks and thereafter assessed for various sperm parameters, bone turnover markers, and testosterone levels. Furthermore, the quantitative assessment of testosterone biosynthesis enzymes was performed. Interestingly, we observed that mice fed with HSD showed significant alterations in sperm parameters-motility, count, and vitality, including morphological changes compared to both the LSD and the control groups. In addition, serum analysis showed an increase in bone resorption markers and a decrease in bone formation markers in the HSD group (p < 0.05). Further, HSD caused a decrease in the testosterone level and mRNA expression of testosterone biosynthesis enzymes. Importantly, a significant decrease in bone formation marker osteocalcin (OC) was observed to coincide with the dip in testosterone level in the HSD group. Given that OC plays a key role in maintaining male fertility, the above findings suggest that a decrease in OC levels may affect the testosterone biosynthesis pathway, reducing testosterone hormone secretion and thereby resulting in decreased spermatogenesis. The study for the first time delineates and bridges the mechanism of HSD-mediated bone loss (results in a deficiency of OC) with decreased testosterone biosynthesis and thus impaired male fertility.

Bacillus thuringiensis bioinsecticide influences Drosophila oviposition decision

Behavioural avoidance has obvious benefits for animals facing environmental stressors such as pathogen-contaminated foods. Most current bioinsecticides are based on the environmental and opportunistic bacterium Bacillus thuringiensis (Bt) that kills targeted insect pests upon ingestion. While food and oviposition avoidance of Bt bioinsecticide by targeted insect species was reported, this remained to be addressed in non-target organisms, especially those affected by chronic exposure to Bt bioinsecticide such as Drosophila species. Here, using a two-choice oviposition test, we showed that female flies of three Drosophila species (four strains of D. melanogaster, D. busckii and D. suzukii) avoided laying eggs in the presence of Bt var. kurstaki bioinsecticide, with potential benefits for the offspring and female's fitness. Avoidance occurred rapidly, regardless of the fraction of the bioinsecticide suspension (spores and toxin crystals versus soluble toxins/compounds) and independently of the female motivation for egg laying. Our results suggest that, in addition to recent findings of developmental and physiological alterations upon chronic exposure to non-target Drosophila, this bioinsecticide may modify the competitive interactions between Drosophila species in treated areas and the interactions with their associated natural enemies.

Gustatory function of sensilla chaetica on the labial palps and antennae of three tortricid moths (Lepidoptera: Tortricidae)

In adult Lepidoptera the labial palps are best known for their role in CO₂ detection, but they can also bear sensilla chaetica which function is unknown. The number and distribution of sensilla chaetica in labial palps was studied using a bright field microscope. To determine if these sensilla have a gustatory function, we performed single sensillum electrophysiology recordings from palp and antennal sensilla of adult moths of Cydia pomonella (L.), Grapholita molesta (Busck) and Lobesia botrana (Denis and Shieffermüller). Each sensillum was stimulated with 3 doses of one of four test stimulus (sucrose, fructose, KCl and NaCl). Overall, responses (spikes/s^-1) increased with dose, and were higher in the palps than in the antennae, and higher to sugars than to salts. With sugars the response increased with concentration in the palp but not in the antenna. With salts there was a drop in response at the intermediate concentration. The number and position of sensilla chaetica on labial palps was variable among individuals. Sensilla were located in the most exposed areas of the palp. Differences in sensilla distribution were detected between species. Such differences among species and between palps and antenna suggest that taste sensilla on the palps have an unforeseen role in adaptation.

Behavioural Nudges, Physico-Chemical Solutions, and Sensory Strategies to Reduce People’s Salt Consumption

This narrative historical review examines the wide range of approaches that has been trialled/suggested in order to reduce the consumption of salt. While sodium is an essential micronutrient, there is widespread evidence that high levels of consumption are leading to various negative health outcomes. This review summarises the evidence relating to the various approaches that have been put forward to date to help reduce salt consumption over the years, while also highlighting a number of important questions that remains for future research. Solutions to reducing salt consumption include everything from the gradual reduction in salt in foods through to the reduction in the number/size of holes in saltshakers (what one might consider a behavioural nudge). Physico-chemical solutions have included salt replacers, such as monosodium glutamate (MSG) through to the asymmetric distribution of salt in processed (e.g., layered) foods. A wide range of sensory approaches to modulating expected and perceived saltiness have also been suggested, including the use of salty aromas, as well as suggesting the use of colour cues, sonic seasoning, and even textural primes. It is currently unclear whether different salty aromas can be combined to increase odour-induced taste enhancement (OITE) effectiveness. In the years ahead, it will be interesting to assess how long such solutions remain effective, as well as whether different solutions can be combined to help reduce salt consumption without having to compromise on taste/flavour

Current Knowledge and Behavior towards Salt Reduction among Hong Kong Citizens: A Cross-Sectional Survey

Excessive dietary salt intake is prevalent in the Hong Kong community. Over the last decade, the Hong Kong Special Administrative Region Government has been actively promoting community participation to reduce salt intake. The aim of this study was to investigate the current knowledge levels and behaviors relating to dietary salt intake among Hong Kong adults. This cross-sectional survey involved 426 adults in Hong Kong. The findings of this study identified areas of knowledge deficit in the recommended upper limit of daily salt intake for an adult set by World Health Organization (n = 295, 69.2%) indicated a lack of awareness that the overconsumption of salt could cause coronary heart disease (n = 233, 54.7%). Disengagement with salt reduction behavior, such as rarely/never checking the sodium or salt content listed on the food label (n = 252, 59.2%) and rarely/never purchasing food labelled with low salt or no salt content (n = 292, 68.9%), was reported. Excessive salt intake in dietary habits remains an under-recognized non-communicable disease threat by Hong Kong citizens, indicating ineffective responsive risk communication. There is a need to refine existing salt reduction initiatives to aid in making appropriate decisions regarding dietary salt intake among Hong Kong citizens.

Salt Sensation and Regulation

Taste sensation and regulation are highly conserved in insects and mammals. Research conducted over recent decades has yielded major advances in our understanding of the molecular mechanisms underlying the taste sensors for a variety of taste sensations and the processes underlying regulation of ingestion depending on our internal state. Salt (NaCl) is an essential ingested nutrient. The regulation of internal sodium concentrations for physiological processes, including neuronal activity, fluid volume, acid–base balance, and muscle contraction, are extremely important issues in animal health. Both mammals and flies detect low and high NaCl concentrations as attractive and aversive tastants, respectively. These attractive or aversive behaviors can be modulated by the internal nutrient state. However, the differential encoding of the tastes underlying low and high salt concentrations in the brain remain unclear. In this review, we discuss the current view of taste sensation and modulation in the brain with an emphasis on recent advances in this field. This work presents new questions that include but are not limited to, “How do the fly’s neuronal circuits process this complex salt code?” and “Why do high concentrations of salt induce a negative valence only when the need for salt is low?” A better understanding of regulation of salt homeostasis could improve our understanding of why our brains enjoy salty food so much.

Preference and detrimental effects of high fat, sugar, and salt diet in wild-caught Drosophila simulans are reversed by flight exercise

High saturated fat, sugar, and salt contents are a staple of a Western diet (WD), contributing to obesity, metabolic syndrome, and a plethora of other health risks. However, the combinatorial effects of these ingredients have not been fully evaluated. Here, using the wild-caught Drosophila simulans, we show that a diet enriched with saturated fat, sugar, and salt is more detrimental than each ingredient separately, resulting in a significantly decreased lifespan, locomotor activity, sleep, reproductive function, and mitochondrial function. These detrimental effects were more pronounced in female than in male flies. Adding regular flight exercise to flies on the WD markedly negated the adverse effects of a WD. At the molecular level, the WD significantly increased levels of triglycerides and caused mitochondrial dysfunction, while exercise counterbalanced these effects. Interestingly, fruit flies developed a preference for the WD after pre-exposure, which was averted by flight exercise. The results demonstrate that regular aerobic exercise can mitigate adverse dietary effects on fly mitochondrial function, physiology, and feeding behavior. Our data establish Drosophila simulans as a novel model of diet-exercise interaction that bears a strong similarity to the pathophysiology of obesity and eating disorders in humans.