Study reveals brain circuit that helps us ‘push past’ nutritional stress

While it is well known that we have a canny ability to ignore hunger and push through the physical stress caused by a lack of food to complete something that is seen to be very important to us, the science behind exactly how the brain co-ordinates this sort of response to nutritional stresses is not very well understood.

Now researchers from the National Centre for Biological Sciences (NCBS) in Bangalore have identified brain circuitry that allows fruit flies to take a major developmental step in their lives despite nutritional stress.

Writing in the journal eLife​, the team noted that integrating nutritional sensing with the ability to carry out major developmental steps and important tasks is a common occurrence that, in general, ensures organisms have enough food, energy, and vital nutrients to complete a task before completing it.

But fruit flies also have an important bit of brain wiring that allows them to ‘push past’ a lack of food when needed.

"Larvae basically decide to pupate once they have eaten enough. But at a stage where they have already got enough protein in them, if you give them protein-deficient food, they hit a dilemma. Normal larvae have a neuronal circuit in their brains that let them get over this,”​ explained senior researcher Gaiti Hasan.

The circuit NCBS group identified is made up of three types of nerve cells - one set senses and reports the lack of protein in food, a second set integrates this input with metabolic information and the third set controls the hormonal signals necessary to begin pupation.

"What is really amazing about this work is that it demonstrates that a pure signalling pathway from a neuronal system can control a developmental switch - the one that tells a larva to become a pupa,"​ added lead author Siddharth Jayakumar.

Hasan and Jayakumar believe this ‘integrator circuit’ linking nutrition and development could be similar in insects and mammals. They noted that although the types of nerve cells involved in the systems maybe very different, the circuit patterns are ‘highly likely’ to be similar.

Fruit fly study​

From the time they hatch out of eggs, insects such as fruit flies undergo three major stages in their lives, explained the team. The first is the maggot-like larval stage during which the insect eats and builds up resources; the second is the transformatory pupa stage, from which the legless, wingless larvae emerge into the third and final adult stage of their lives as 6-legged, winged flies.

According to Agrawal and Hasan, integrating nutrition sensing with these developmental steps is important; the larva must know when it has enough resources to pupate and make the transformation into an adult.

Indeed, when deprived of food, pupation is often postponed until enough energy reserves have been built up.

But, the team noted that there are exceptions to this. For example, when faced with protein deprivation towards the end of their larval lives, fruit fly larvae are able to push themselves to pupate.

Now, Hasan and her team identified the integrative circuit of nerve cells in fruit fly brains that allows them to ignore the lack of proteins in their food to enter the pupal stage.

“We have a set of mutants that cannot pupate under this condition,"​ said Hasan.

"In actual fact, these mutants have everything in them to be able to pupate, but because this circuit is not functioning properly, they are unable to ignore the protein deficit,"​ she commented.

Source: eLife​

Published online, doi: 10.7554/eLife.07046​

“Maturation of a central brain flight circuit in Drosophila requires Fz2/Ca2+ signalling”​
Authors: Tarjani Agrawal, Gaiti Hasan