A new study empowered by advances in technology has provided what is likely the most detailed account to date of biological changes that take place in the brain when someone has post-traumatic stress disorder (PTSD). The new findings shed light on PTSD pathology, identify specific and potentially targetable genetic, cell-type, and functional alterations, and also shed light on factors distinguishing brain changes in PTSD and major depressive disorder (MDD).

Led by Matthew J. Girgenti, Ph.D., of Yale University, a two-time BBRF Young Investigator whose 2023 grant award helped support this research, a team that included 8 other recipients of BBRF grants examined a variety of changes at the single-cell level in 111 postmortem brains donated by people in three subgroups: those who had lived with PTSD, those diagnosed with MDD, and those who did not have a psychiatric diagnosis. Of the past BBRF grantees on the team, two are members of BBRF’s Scientific Council: its vice-president, John H. Krystal, M.D., and Kristen J. Brennand, Ph.D., both of Yale.

The data that contributed to the team’s analysis was derived from over 2 million individual cells from the brain’s dorsolateral prefrontal cortex (DLPFC), and specifically from the nuclei of those cells, which harbor the human genome and the regulatory elements that determine how and when genes are expressed. Prior studies have shed considerable light on genetic variations seen in PTSD patients, as well as molecular-level changes in PTSD brains, and have revealed alterations in a number of gene pathways affecting inhibitory neurotransmitter signaling, immunity, and neuroinflammation, as well as signaling in the glucocorticoid system, which processes the stress response that is perturbed in PTSD.

But until recently, it was not possible to study genetic variation in individuals affected by a given disorder at the level of individual cells in brain regions of interest like the DLPFC, which is part of the cerebral cortex and plays a major role in the regulation of emotions. “Advances in genome technologies now enable the study of chromatin assemblies in individual cells,” the team noted, referring to the bundling of DNA in the cell nucleus that helps determine which of our ~21,000 genes can be activated and which cannot be at a given moment.

When combined with an analysis of which genes in a cell are being expressed (i.e., activated so as to generate the manufacture of specific proteins), chromatin data can provide the resolution needed to identify how DNA variations associated with PTSD affect the process called transcription—the copying of genetic information to RNA—in individual cells. It is at this fine level of detail that progress can be made in learning how an illness like PTSD or depression alters neurons and the circuits they form, as well as other brain-cell types.

This new capability is especially significant in trying to understand the biological causes and effects of some psychiatric illnesses including PTSD and depression, which, unlike brain diseases like Alzheimer’s, are not associated with large-scale pathologies like plaques that can be readily imaged.

“We annotated and censused all major [brain] cell types, including excitatory and inhibitory neurons and non-neuronal cell types,” the team reported in the journal Nature. “We identified cell type-specific genes that were expressed differentially in PTSD and converging and diverging expression changes between PTSD and MDD.” They also “constructed the regulatory landscape” impacting gene expression in PTSD. These and other analyses led to a number of major findings.

Among the postmortem brains with PTSD, the team’s investigation revealed notable gene alterations in inhibitory neurons, which “fine-tune” excitatory brain circuits and in this way regulate them, among other things preventing them under normal conditions from firing too much. In brains affected by PTSD and MDD, the team observed a decrease in communication from inhibitory neurons—which may account for hyperexcitation in the prefrontal cortex. Following a traumatic experience, hyperexcitability might give rise to some of the symptoms seen in PTSD, such as hypervigilance or even nightmares.

The immune cells unique to the brain, microglia, were found to be overactive in the MDD brains and underactive in the PTSD brains. The apparent suppression of neuroimmune processes and microglial activity in the PTSD brains “is a finding that seems to differentiate MDD and PTSD,” Dr. Girgenti noted, despite a number of previously noted genetic overlaps.

The PTSD brains were found to have genomic changes associated with dysregulation in endothelial cells, which line the blood vessels. This was an unexpected finding. It is known, however, that cortisol, the primary stress hormone, is paradoxically present at unusually low levels in PTSD brains. The team speculated that this previously unknown neurovascular dimension of PTSD, mirrored by high levels of activity in endothelial cells of a previously identified PTSD risk gene called FKBP5, may prove to be a mechanism to compensate for the unusually low cortisol levels.

Taken together, the study “enabled us to identify genes and pathways associated with PTSD pathology.” These included stress hormones, immune, and neuroinflammatory mechanisms, in addition to the inhibitory neurotransmitter GABA.

About half of people with PTSD also suffer from MDD. The study helps identify “convergent and divergent molecular effects of both,” the team said. But future extensions of this work should include analysis of a greater number of brains from people diagnosed with PTSD but not MDD to further highlight changes that are specific to PTSD. They also noted that like all other studies reliant on postmortem brains, it was impossible to say in this study how other factors among individual donors might have perturbed their findings. One example is the use of medications as well as other substances, which can alter the brain and its functions significantly. A greater number and more diversity among brain donors will aid future studies of this kind, they said.

In addition to Drs. Girgenti, Krystal, and Brennand, the team included: Alicia Che, Ph.D., 2019 BBRF Young Investigator; Nenand Sestan, M.D., Ph.D., 2012 BBRF Distinguished Investigator, 2006 Young Investigator; Paul E. Holtzenheimer, M.D., 2016 BBRF Independent Investigator 2007 Young Investigator; David A. Lewis, M.D., 2008 BBRF Distinguished Investigator; Jill R. Glausier, Ph.D., 2015 BBRF Young Investigator; and Daniel Levey, Ph.D., 2019 BBRF Young Investigator.