Neuron loss
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Stem cell-derived neurons from combat veterans with post-traumatic stress disorder (PTSD) react differently to a stress hormone than those from veterans without PTSD, according to a new study.

The findings are detailed in the journal Nature Neuroscience  and led by scientists at the Icahn School of Medicine at Mount Sinai, the James J. Peters Veterans Affairs Medical Center, the Yale School of Medicine, and the New York Stem Cell Foundation Research Institute (NYSCF).

The study is the first to use human induced pluripotent stem cell (hiPSC) models to study PTSD, and demonstrated glucocorticoid hypersensitivity in PTSD neurons, and identified genes that contribute to this PTSD-dependent glucocorticoid response.

The researchers suggest that their findings could provide insights into how genetics can make someone more susceptible to developing PTSD following trauma exposure.

“We’re working on finding already-approved drugs that could reverse the hypersensitivity we’re seeing in neurons, said study co-lead Kristen Brennand, PhD, the Elizabeth Mears and House Jameson professor of psychiatry at Yale School of Medicine and an NYSCF–Robertson Stem Cell Investigator Alumna. “That way, any drugs we discover will have the fastest possible path to helping patients.”

Brennand and colleagues said their results suggest possible therapeutic strategies that could help to minimize the likelihood of PTSD following trauma exposure, which could be particularly valuable for first-responder, as well as military personnel. In their paper titled, “Modeling gene x environment interactions in PTSD using human neurons reveals diagnosis-specific glucocorticoid-induced gene expression,” the investigators concluded, “These findings suggest that induced neurons represent a platform for examining the molecular mechanisms underlying PTSD, identifying biomarkers of stress response, and conducting drug screening to identify new therapeutics.”

PTSD can develop following severe trauma and is an enormous public health problem for both veterans and civilians. However, while there is evidence to suggest that there is a heritable component to PTSD, the extent to which genetic and environmental factors contribute to individual clinical outcomes remains unknown, the authors noted. “Although unequivocally precipitated by environmental events, PTSD develops in only a minority of trauma survivors, prompting a search for risk factors that increase the probability of developing this condition following trauma exposure,” they wrote.

To bridge the existing knowledge gap, the research team studied a cohort of 39 combat veterans with and without PTSD who were recruited from the James J. Peters Veterans Affairs Medical Center in the Bronx. Most similar studies of PTSD to date have used blood samples from patients, but since PTSD is rooted in the brain, scientists need a way to capture how the neurons of individuals prone to the disorder are affected by stress. Brennand and colleagues opted to use stem cells, as they are uniquely equipped to provide a patient-specific, noninvasive window into the brain. “You can’t easily reach into a living person’s brain and pull out cells, so stem cells are our best way to examine how neurons are behaving in a patient,” said Brennand.

The induced pluripotent stem cells were generated by reprogramming skin cells biopsied from the recruited veterans. The NYSCF scientists used their scalable, automated, robotic system—the NYSCF Global Stem Cell Array®—to create the stem cells, and then differentiated them into glutamatergic neurons. Glutamatergic neurons help the brain send excitatory signals and have previously been implicated in PTSD. “Reprogramming cells into induced pluripotent stem cells is like virtually taking cells back in time to when they were embryonic and had the ability to generate all the cells of the body,” said research co-lead Rachel Yehuda, PhD, professor of psychiatry and neuroscience at Icahn Mount Sinai and director of mental health for the James J. Peters Veterans Affairs Medical Center. “These cells can then be differentiated into neurons with the same properties as that person’s brain cells had before trauma occurred to change the way they function. The gene expression networks from these neurons reflect early gene activity resulting from genetic and very early developmental contributions, so they are a reflection of the ‘pre-combat’ or ‘pre-trauma’ gene expression state.”

Study co-lead Daniel Paull, PhD, NYSCF senior vice president, discovery & platform development, added, “As this was the first study using stem cell models of PTSD, it was important to study a large number of individuals. At the scale of this study, automation is essential. With the Array, we can make standardized cells that allow for meaningful comparisons between numerous individuals, pointing to key differences that could be critical for discovering new treatments.”

To mimic the stress response that triggers PTSD, the scientists exposed the induced pluripotent stem cell-derived neurons to the stress hormone hydrocortisone, a synthetic version of the body’s own cortisol that is used as part of the “fight-or-flight” response.

“The addition of stress hormones to these cells simulates biological effects of combat, which allows us to determine how different gene networks mobilize in response to stress exposure in brain cells,” explained Yehuda. Using gene expression profiling and imaging, the scientists found that neurons from individuals with PTSD were hypersensitive to this pharmacological trigger.

The scientists also were able to identify the specific gene networks that responded differently following exposure to the stress hormones. They compared their results with those of a previous veterans PTSD study that had evaluated peripheral blood mononuclear cells (PBMC). “Transcriptional profiles of hiPSC neurons were compared with a well-matched and largely overlapping PBMC study of combat veterans with (n = 20 donors) and without (n = 20) donors) PTSD, the investigators noted.

“Two people can experience the same trauma, but they won’t necessarily both develop PTSD,” Brennand added. “This type of modeling in brain cells from people with and without PTSD helps explain how genetics can make someone more susceptible to PTSD.”

The team’s gene expression analysis revealed a set of genes that were particularly active in PTSD-prone neurons following their exposure to stress hormones. “In neurons only, we observed diagnosis-specific glucocorticoid-induced changes in gene expression corresponding with PTSD-specific transcriptomic patterns found in human postmortem brains,” the team noted. “We observed glucocorticoid hypersensitivity in PTSD neurons, and identified genes that contribute to this PTSD-dependent glucocorticoid response … This PTSD-specific neuronal glucocorticoid-response signature was enriched for transcriptomic patterns observed in postmortem (PM) brain tissue from PTSD cases.”

Graphic representation: skin cells being reprogrammed into induced pluripotent stem cells, then into neurons that were exposed to stress hormones, and subsequently observed by the study team. [Yale School of Medicine]

Paull added, “Importantly, the gene signature we found in the neurons was also apparent in brain samples from deceased individuals with PTSD, which tells us that stem cell models are providing a pretty accurate reflection of what happens in the brains of living patients.” As the team noted, “These findings are consistent with the glucocorticoid hypersensitivity hypothesis; for example, patients with PTSD have altered blood sensitivity to glucocorticoids and perturbations in glucocorticoid receptor signaling have been shown for PTSD in PM brain tissue.”

The differences between how PTSD and non-PTSD cells responded to stress could be informative in predicting which individuals are at higher risk for PTSD, the team suggests. “What’s really exciting about our findings is the opportunities they offer for accelerating the diagnosis and treatment of PTSD,” Paull continued. “Importantly, having a robust stem cell model provides an ideal avenue to drug screening ‘in the dish,’ even across diverse patient populations.”

The researchers plan to continue leveraging their induced pluripotent stem cell models to further investigate the genetic risk factors pinpointed by this study and to study how PTSD affects other types of brain cells, helping to broaden opportunities for therapeutic discovery. In their paper, they noted: “Critical aspects of glucocorticoid response are encoded in patient genetics, consistent with a clear genetic predisposition to PTSD … To translate our findings into clinically meaningful risk-stratification, it is imperative that future studies test whether our signatures relate to clinical severity and/or treatment responsiveness, and thereby uncover new therapeutic target(s) for PTSD.”

Their reported work indicates that if the biological mediators of environmental risk can be predicted, then hiPSC-based models could be used to test genotype-dependent and cell-type-specific environmental responses, the team suggested. “Our hope is that dissecting how disease risk variants interact with the environment across the diverse cell types that comprise the human brain will ultimately improve diagnostics, predict clinical trajectories and identify pathways that could serve as presymptomatic targets of therapeutic intervention.”

Brennand commented, “What’s special about this study is that it could have only been done by this group. It involved some of the best clinicians in this space, incredible stem cell biologists, and amazing psychiatric geneticists. Each group has unique expertise, and none of this could have been accomplished by any one team alone.”

“This study is a true testament to the power of team science,” stated Paull. “When researchers combine forces, we are able to ask bigger questions, make bigger discoveries, and hopefully, make a bigger difference for patients.”

Added NYSCF interim CEO Derrick Rossi, PhD, who is not one of the paper’s named authors, “This collaborative work underscores the unique value of stem cell modeling for studying and demystifying challenging diseases, and for discovering innovative strategies that could lead to urgently needed treatments.”

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