Why do some combat soldiers exhibit posttraumatic stress disorder, whereas their comrades faced with the same situations do not? Why do some victims of violence remain scarred for life, when others emerge from similar experiences relatively unscathed?
Such frequently observed differences have fascinated a broad range of scientific researchers who have tried to discover what determines “resilience to stress,” a term describing the ability to adapt to difficult situations and to overcome adversity. Is it acquired through experience, or is a tendency to easily recover from stress possibly ingrained in us from a very early age, or even from birth?
A team of researchers at the Weizmann Institute of Science has revealed an important piece of this puzzle in a study of zebrafish, small fish whose natural habitat spans rivers, ponds and rice paddies in Pakistan, Myanmar, Nepal and India. Employing advanced behavioral and molecular analysis and gene editing in successive generations of fish, the researchers found that much of stress resilience is a heritable trait that is determined early on and remains constant throughout the fish’s life.
“To find out whether stress resilience is a trait that an animal has from a very early age, you need to study very young subjects with as little life experience as possible,” says Dr. Amrutha Swaminathan, who led experiments for the new study in Prof. Gil Levkowitz’s lab in Weizmann’s Molecular Cell Biology and Molecular Neuroscience Departments.
“Zebrafish are exceptionally suitable subjects for exploring such research questions, since they already show clear responses to stress as larvae, mere hours after hatching from eggs that are fertilized externally,” says Swaminathan. Zebrafish’s early response to stressful situations is essential for their survival: The parents don’t provide care for their young, which are independent and agile. Therefore, the behavior that scientists observe in their test subjects cannot be the result of nurture – that is, it’s not something they picked up from observing their parents.
“”By and large, offspring of the selected resilient parents tended to cope with stressful situations better than the average population, and the contrary was observed in offspring of susceptible parents. So we’re talking about a heritable trait”
Since zebrafish live in groups that inhabit freshwater environments, they are especially sensitive to increased salinity, isolation and physical contact (e.g., with a potential predator), among other stressors. When the scientists temporarily exposed newborn (6-day-old) larvae to one of such stress factors, the fish displayed clear stress responses as defense mechanisms, including diminished movement and freezing (playing dead). Their behavior normalized eventually, but a significant minority of 10 to 20 percent rebounded much faster than others, soon behaving as though they had a superior stress-coping ability, Levkowitz says. The scientists defined the fast rebounders as stress resilient, the others as susceptible.
Stressing once again
The identified resilient and susceptible groups of young fish were reared separately. The researchers exposed them to stress again days and even months later and found that both resilient and susceptible individuals remained that way throughout their lives, from the larval stage into adulthood.
Next, the researchers looked at the offspring of the fish from both groups in a series of experiments. The results were unambiguous. “By and large, offspring of the selected resilient parents tended to cope with stressful situations better than the average population, and the contrary was observed in offspring of susceptible parents. So we’re talking about a heritable trait,” says Levkowitz.
Immunity from stress
The study then went deeper, in an attempt to characterize the mechanisms that determine resilience in zebrafish. The researchers compared the genetic program activated throughout the bodies of fish in both groups, resilient and susceptible, in response to stress. This program was more extensive in the stress-resilient larvae: Their expression levels of about 250 genes decreased and those of about 100 increased, suggesting that resilience to stress is an active process.
When the scientists looked at the specific genes involved in the stress responses, they were surprised. They had expected to see the main changes in gene expression occurring in the brain, but they found that resilient and susceptible larvae also displayed differences in their immune systems. In resilient larvae, parts of the immune system were suppressed – particularly, proteins that are produced in the liver and that belong to the part of the innate immunity known as the complement system, which tags harmful microorganisms in our bodies and induces inflammatory responses that help fight infections.
“We had been mainly brain oriented, but it turns out that the liver is very much involved in regulating the response to stress,” Levkowitz said.
While the normal functioning of the complement immune system is crucial in fighting infections, the findings suggest that mitigation or inhibition of an exaggerated response of this system is beneficial when coping with adversities. To confirm this hypothesis, the researchers proceeded to tweak the zebrafish’s DNA through gene editing. They produced zebrafish mutants lacking crucial elements of the complement system. As they predicted, the mutants were significantly more likely to be resilient to stress than fish with functional complement systems. “We still don’t understand how complement factors affect resilience. However, we do know that stress influences the immune system and immune response affects brain function and behavior,” says Swaminathan.
Like fish, like humans?
Finally, to explore the similarities between the stress responses of fish and humans, the scientists checked for molecules previously linked to stress resilience in people.
They examined the effects on fish of neuropeptide Y, a chemical messenger in the brain that has been linked to resilience in humans. They found that fish lacking neuropeptide Y were less likely to be resilient than those with normal levels. “This gives us greater confidence that the way we test resilience in fish is relevant to humans,” Levkowitz said.