Smell and taste what is good
Depending on the immediate circumstances, the body needs nutrients in different quantities and compositions. Polyamines are those molecules that are increasingly needed where tissues develop, grow or regenerate. Low polyamine levels are associated with neurodegenerative diseases, aging, and fertility decline. Too many polyamines, on the other hand, may play a role in the development of cancer. In this research project, we are investigating how polyamines are detected by the sense of smell and taste.
Together with their Swedish colleagues, we have shown which receptors insects use to recognize polyamines in food. The study suggests that the ability to detect polyamines with the senses of taste and smell has influenced the survival and reproduction of animals.
Survival through the recognition of polyamines
Polyamines are small organic compounds involved in basic cellular functions such as cell division and growth. A polyamine deficiency can therefore have negative consequences for health, cognitive abilities, fertility, reproduction and life expectancy. But too high polyamine concentrations can be harmful. The body's polyamine supply should therefore match its immediate needs: During growth, injuries or increased physical stress such as pregnancy, the need is particularly high. The body can produce some of the polyamines it needs itself or with the help of intestinal bacteria. However, a significant proportion is absorbed through food. High polyamine concentrations are found, for example, in oranges, ripe cheese, but also in teas and some legumes. As the body's production of polyamines decreases with age, dietary polyamine intake becomes more important over time.
Some polyamines have characteristic names such as cadaverine, spermine or putrescine (Latin putridus means putrid). What smells unpleasant to us humans and many animals in higher concentrations and signals danger is essential for survival in smaller quantities. What role the smell and possibly also the taste of polyamines play in food selection, and how the molecules are recognized in the first place, was unclear until now. "The perception of smell and taste is quite similar in flies and humans," explains Ilona Grunwald Kadow. "We therefore used the fruit fly model to investigate whether and how the animals perceive polyamines and what this means for them." Now the neurobiologists have been able to show that polyamine-rich food, such as an overripe fruit, significantly increases the number and survival of the fly's offspring.
These three receptors recognize polyamines
We observed that flies were strongly attracted to the polyamine odor. Female flies preferred to lay their eggs on polyamine-rich, older fruit rather than fresh fruit. "So the flies had to perceive the polyamines - but how?" says Ashiq Hussain, one of the study's two lead authors, explaining the central question. The researchers found that the animals not only perceived the smell, but also used their sense of taste to find and examine polyamine-rich food sources. Using a microscope, the scientists observed which taste and olfactory cells became active when the flies perceived polyamines. Together with behavioral and genetic studies, they then identified three receptors that chemosensory neurons use to detect the smell and taste of polyamines.
The results show that flies first find a polyamine-rich food source by smell through two receptors, IR76b and IR41a. There, taste neurons recognize the quality of the polyamines found via the IR76b receptor together with a bitter receptor. Similar to us humans, a too high polyamine concentration seems to rather deter the flies. They ate or laid their eggs in polyamine-rich food only when additional food components present, such as sugar, masked the bitter taste of the polyamines. "This is a good mechanism to be able to detect the optimal concentration of these substances," explains Ashiq Hussain.
The three receptors that enable polyamine recognition belong to an evolutionarily very old class of proteins. They are related to receptors that control the synaptic activity of nerve cells. "It may therefore be that recognizing polyamines via these receptors early in evolutionary history improved animal survival," said Mo Zhang, the second lead author, summarizing the significance of the findings. "This allowed these important dietary components to be found and ingested in the right amounts." Next, the researchers plan to investigate whether mammals also recognize polyamines in food and ingest them as needed.