Traumatic brain injury (TBI) can quadruple the risk of developing dementia, and increase the likelihood of developing neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). Scientists led by a team at the Keck School of Medicine, of the University of Southern California (USC) developed a lab-grown human brain organoid model that allowed them to uncover new insights into the cause of neuronal damage after TBI, and potentially how to mitigate the risk.

They found that mechanically injured organoids showed some of the same features observed in TBI patients, including nerve cell death and pathological changes in tau proteins, as well the dysfunction of a protein called TDP-43. This protein, their studies suggested, appears to drive nerve damage right after injury.

Further experiments showed that blocking the cell surface channel protein KCNJ2 can correct faulty TDP-43 and curb nerve death in mouse and human cells. The findings, the team noted, suggest that KCNJ2 inhibition could potentially represent an approach to both treating, and preventing TBI.

“Targeting KCNJ2 may reduce the death of nerve cells after TBI,” said Justin Ichida, PhD, who is the John Douglas French Alzheimer’s Foundation Associate Professor of Stem Cell Biology and Regenerative Medicine at USC, and a principal investigator at the Eli and Edythe Broad Center for Regenerative Medicine and Stem Cell Research at USC. “This could have potential as either a post-injury treatment or as a prophylactic for athletes and others at high risk for TBI.”

Ichida and colleagues reported on their findings in Cell Stem Cell, in a paper titled “KCNJ2 inhibition mitigates mechanical injury in a human brain organoid model of traumatic brain injury.”

Traumatic brain injury (TBI)—a condition long linked to contact sports and military services —is the leading environmental risk factor for neurodegenerative disease, the authors wrote. “Recent studies have associated TBI with the pathological accumulation of the neurotoxic proteins tau, TDP-43, and amyloid-beta, leading to progressive neurodegenerative diseases, including chronic traumatic encephalopathy (CTE), amyotrophic lateral sclerosis (ALS), Alzheimer’s disease, and other dementias.”

However, clinicians often find limited success in treating patients with traumatic brain injury. “There’s really nothing out there that can prevent the injury or trauma to the brain that cause nerve cell damage,” Ichida. “In more acute stages, patients can have difficulty concentrating and have extreme sensitivity to light and noise. Long term, there is a strong correlation between traumatic brain injury and neurodegenerative diseases, which can ultimately be fatal.”

To try to untangle what happens at impact, the researchers, including Jesse Lai, PhD, and Joshua Berlind in the Ichida lab, used patient-derived stem cells to grow rudimentary structures called brain organoids, pinhead-sized, tiny clusters of human neural cells that behave like a brain. They then hit the organoids with ultrasonic pulses, mimicking severe traumatic brain injuries.

Previous research has suggested that the protein tau may underlie the nerve damage, but the team’s studies using the brain organoid models uncovered a different culprit. “Although TBI is associated with tau dysfunction, we unexpectedly observed that TDP-43 proteinopathy and loss of function drive neuronal death in the early stages post-injury,” they wrote.

Researchers applied ultrasonic pulses to tiny clusters of neuron cells called brain organoid to mimic traumatic brain injury in humans [Jesse Lai, Ichida Lab]
Researchers applied ultrasonic pulses to tiny clusters of neuron cells called brain organoid to mimic traumatic brain injury in humans [Jesse Lai, Ichida Lab]
Their experiments showed that TDP-43, which edits the genetic script that carries protein-making DNA instructions, wanders astray in injured organoids and causes nerve death. “One of TDP-43’s physiological functions is to mediate the appropriate splicing of pre-mRNA for multiple neuronal genes,” the investigators explained. In healthy cells, TDP-43 usually resides within the nucleus, but Ichida et al., found that after injury the protein leaks out to the surrounding cytosol and cannot carry out its job. “Loss of nuclear TDP-43 therefore results in the incorporation of cryptic exons, which often lead to frameshift and nonsense-mediated decay of many TDP-43 target genes.”

The results also showed that neurons deep in the cortex of the brain are particularly vulnerable to trauma—“Interestingly, deep-layer excitatory neurons are more vulnerable to injury-induced TDP-43 dysfunction than upper-layer cortical neurons,” the investigators stated, while genetics can affect how the disorder progresses. Their collective data, they noted, “… suggest that TDP-43 dysfunction is a major driver of injury-induced neuronal death in organoids and preferentially affects deep-layer neurons … These findings may point to differences between deep- and upper-layer neurons that one could exploit to protect TBI patients against diseases in which deep-layer neurons are lost, including ALS.”

The scientists discovered that the pathological changes in TDP-43 were more prevalent in organoids derived from patients with ALS or frontotemporal dementia, making their nerve cells more suspectable to dysfunction and death following injury. This suggests that TBI might increase the risk of developing such diseases even more for patients with a genetic predisposition. The findings might also help explain why some individuals are at higher risk for developing these diseases after trauma.

To identify potential genetic modifiers of neuronal survival following mechanical injury, the investigators then conducted a genome-wide CRISPR interference screen using their model. “We then tested every gene in the human genome to see if we could rescue that injury by suppressing any individual gene,” Ichida continued. The screening came back with a hit—KCNJ2, a gene that encodes a mechanosensory potassium ion channel protein on cell surfaces. “If we suppressed the gene, it reversed all the problems associated with the injury and kept the nerve cells alive.”

Experiments showed that blocking both KCNJ2 gene activity and its protein, respectively, raised neuronal survival rate in organoids. The team saw similar effects in mouse models of traumatic brain injury when they targeted KCNJ2, reducing misplaced TDP-43. “Treating injured organoids from patients with neurodegenerative disease risk using KCNJ2 protein blockers before injury not only reduced nerve death but also lowered the TDP-43 buildup in cells. The results suggest dampening KCNJ2 activity may protect the brain from trauma. “Collectively, these data suggest that inhibition of KCNJ2 can effectively reduce the acute TDP-43 injury and degenerative responses following TBI in vitro and in vivo,” the authors stated on reporting their in vivo studies.

About five million Americans live with traumatic brain injury-related disabilities. With more research, Ichida can foresee new opportunities for improving prevention, diagnosis, and treatment. The findings may help inform people of their genetic risks and guide safety measures. TDP-43 may also serve as a biological marker to detect traumatic brain injury and monitor the damage one day.

“Collectively, we describe here a human organoid platform for the discovery and validation of modifiers of mechanical injury,” the team stated. “This injury model aims to bridge the gap between traditional in vitro systems and complex higher organisms, providing a scalable and genetically flexible system to identify potential disease mechanisms and therapies for the acute and chronic effects of TBI.”

Ichida concluded, “Our study suggests that one of the more effective ways of preventing the devastating effects of traumatic brain injury might be to fix TDP-43 and prevent its mislocalization early after injury.”

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