Exploring the biological basis for resilience

In a world in which it can sometimes feel that bad news lurks around every corner, it can be tough just to get out of bed. But some people seem uniquely able to weather even particularly traumatic or challenging experiences—abuse, natural disasters, war or even a yearslong global pandemic—that leave others with life-altering scars. Psychiatrist Victor Carrión, MD, wants to know why.

Carrión, the John A. Turner, MD, Endowed Professor for Child and Adolescent Psychiatry, studies psychological resilience, which helps people withstand stress and trauma without lasting damage to their mental health. Resilience is a dynamic trait molded by the intersection of one’s personality, social and family connections, physical health, and—intriguingly—Carrión suggests, genetics.

“Over the past decades, I and others have done research to elucidate the impact of stress on brain structure and function,” Carrión said. “One potential mechanism is the neurotoxic effects of the stress hormone cortisol. But we still don’t know much about the biological basis of resilience.”

Carrión has teamed up with Alexander Urban, Ph.D., an associate professor of psychiatry and behavioral sciences and of genetics, to use what are called neural organoids—small balls of cells grown in the laboratory that mimic the three-dimensional structure of the human brain—to ferret out the molecular foundations that help some people bounce back when others, despite their best efforts, fall flat.

The researchers are combining their laboratory research with a population study of schoolchildren in Puerto Rico, which has experienced multiple natural disasters including hurricanes, earthquakes and floods during the past decade. The stress has left many students struggling with anxiety, depression, poor sleep and, in some cases, post-traumatic stress disorder. Is it possible to help these children become more resilient?

Obviously, there’s only so much (or so little) experimentation that can be done on living human brains. Few people would volunteer a chunk of their gray matter so researchers can probe their specific genetic makeup. But because neural organoids are made from easily obtained skin or blood cells, they share the DNA sequences of the person from whom they are derived.

Presumably, any quirks and foibles in that DNA sequence that may cause a glitch in a person’s mental makeup will be reflected in the way cells in the organoid communicate (or don’t), organize themselves into functional (or dysfunctional) neighborhoods, or decorate their DNA with chemical tags that activate or deactivate gene expression in response to environmental cues—a level of regulation called epigenetics.

During the past two decades, genetic sequencing technology has advanced to the point that researchers can easily ascertain not just DNA sequences but also gene activity and the presence or absence of epigenetic tags. Urban and his laboratory have been exploring the frontiers of this technology, called high-throughput sequencing, to study the human brain. Now they’ve coupled it with the study of neural organoids to understand behaviors and emotions at the molecular level.

The promise of an up-close-and-personal peek under our psychological hoods has rocked the world of mental health research since neural organoids were developed for widespread use nearly a decade ago in the laboratory of Sergiu Pasca, MD, the Kenneth T. Norris, Jr. Professor II of Psychiatry and Behavioral Sciences.

“I wanted to push the boundaries and make the human brain accessible so that we can transform psychiatry through molecular biology,” Pasca said. “My dream is to ultimately find cures for some of the most devastating neuropsychiatric disorders and to understand what makes the human brain unique and, perhaps, uniquely susceptible to disease.”

Since his 2015 publication detailing the efficient generation of neural organoids and his later creation of more complex structures called assembloids to study human neural circuits outside the body, Pasca and his lab have shared their technique with hundreds of laboratories and even provided make-your-own-organoid kits to those unfamiliar with working with stem cells. “Ten to 15 years ago, we would not have been able to predict the types of experiments we are doing today,” Pasca said.

As their suffix suggests, organoids resemble, but don’t completely recapitulate, aspects of a naturally formed organ like a liver or, in this case, a brain. But they are more than just poorly made replicas. Think of them more as swanky, hard-to-identify forgeries of pricey designer bags rather than bargain-bin knockoffs of some once-trendy sunglasses.

That’s because, despite their similarity to a small boba tea bubble, neural organoids pack a big punch in a tiny package. They include many of the cell types found in a mature human brain, including neurons and support cells called glia, and they self-organize into a structure that roughly mirrors our cerebral cortex, where memories and emotions reside and thinking and learning occur.

Urban and Carrión are using the easily manipulated organoids to identify key genes involved in resilience and mental health as well as the environmentally influenced epigenetic tags that tune those genes’ activity.

“Cortisol exposure allows us to model the effects of stress on these cells,” Carrión said. “What innate characteristics in a person’s brain confer resilience, and are there ways to intervene to help people struggling with post-traumatic stress or anxiety? The cortisol model allows us to study genes that are activated or deactivated when the cells are ‘stressed.'”

Epigenetic tags act as an additional blanket of control between the DNA sequences we inherit from our parents and the actions of the genes that are encoded in those DNA sequences, and as mediators between the DNA sequence and the information gleaned from a cell’s environment.

“Next, we will expose organoids made from the cells of resilient and less-resilient people to increasing levels of cortisol and then analyze them with the high-throughput sequencing machines to determine which genes are activated or inactivated differently in response,” Urban said.

“We don’t have to artificially stress out a human in order to pin down pressure points in the genome and identify genetic variants or specific biochemical markers associated with resilience.”

Preliminary work in Urban’s lab has identified dozens of genes that change in their activity levels when organoids are exposed to increasing cortisol levels. More than one-third of these genes have been associated with stress response in humans, which strongly suggests the model accurately reflects at least part of what goes on in human brains.

Urban has also partnered with Laramie Duncan, Ph.D., an assistant professor of psychiatry and behavioral sciences. Duncan uses large genome-wide association studies to improve our understanding of the cellular roles played by genes associated with conditions such as schizophrenia and post-traumatic stress disorder. Overlaps between her datasets and Urban’s organoid studies further indicate the researchers are on the right track.

“It’s clear that these organoids respond to cortisol in ways that are quite similar to human brains, even on the molecular level,” Urban said. “That’s very exciting.”

Their preliminary findings also suggest a role for genes involved in collagen production, which influences atherosclerosis formation. “Some children under stress experience accelerated aging, including heart disease, so this is particularly interesting,” Carrión said.

He wants to find out if the right kinds of external intervention, including a preventive mindfulness and yoga curriculum he has studied extensively and a treatment approach he’s developed called cue-centered therapy, might help the Puerto Rican schoolchildren. If mind-body training affects how and when epigenetic tags associated with stress are applied, it may help a less resilient child become more resilient through mind-body training.

“Resilience and non-resilience aren’t binary states,” Carrión said. “They exist along a continuum of possible responses to environmental stress and trauma. Also, you can be resilient at certain times or in certain situations or parts of your life and less resilient in other situations.”

“Resilience is not just a hard-wired phenotype,” Urban said.

Examining differences between organoids from children more or less affected by stress, and their responses to intervention, should help Urban and Carrión further home in on genes involved in our collective stress responses.

“When we make stem cells and organoids from people who we know have a certain psychiatric diagnosis, we can pinpoint differences that become our key candidate genes for therapy or diagnosis,” Urban said. “This goes beyond resilience and can include other conditions including schizophrenia or autism. That’s why this approach is so powerful.”

Carrión agrees, “Most of us know about genes and our environment. Finally, we are beginning to understand how stress directly impacts our genetic code and whether we can intervene to help people be more resilient.”