This microscope image of the brain region called the hippocampus shows the protein targeted by cannabis-derived CBD, GPR55 (red), and brain cells (blue) that send their extensions out to form the layers seen in the image. The interconnected nature of the hippocampus makes it a significant site for the initiation and spread of seizures.
Tsien et al, Courtesy of Cell Press

A study reveals a previously unknown way in which cannabidiol (CBD), a substance found in cannabis, reduces seizures in many treatment-resistant forms of pediatric epilepsy.  

Led by researchers at NYU Grossman School of Medicine, the new study found that CBD blocked signals carried by a molecule called lysophosphatidylinositol (LPI). Found in brain cells called neurons, LPI is thought to amplify nerve signals as part of normal function but can be hijacked by disease to promote seizures.

Published online February 13 in Neuron, the work confirmed a previous finding that CBD blocks the ability of LPI to amplify nerve signals in a brain region called the hippocampus. The current findings argue for the first time that LPI also weakens signals that counter seizures, further explaining the value of CBD treatment.   

“Our results deepen the field’s understanding of a central seizure-inducing mechanism, with many implications for the pursuit of new treatment approaches,” said corresponding author Richard W. Tsien, chair of the Department of Physiology and Neuroscience at NYU Langone Health.

“The study also clarified, not just how CBD counters seizures, but more broadly, how circuits are balanced in the brain,” added Tsien. “Related imbalances are present in autism and schizophrenia so that the paper may have a broader impact.”

Disease-Causing Loop

The study results build on how each neuron “fires” to send an electrical pulse down an extension of itself until it reaches a synapse, the gap that connects it to the next cell in a neuronal pathway. When it reaches the cell’s end before the synapse, the pulse triggers the release of compounds called neurotransmitters that float across the gap to affect the next cell in line. Upon crossing, such signals either encourage the cell to fire (excitation), or apply the brakes on firing (inhibition). Balance between the two are essential to brain function; too much excitation promotes seizures.   

The new study looked at several rodent models to explore mechanisms behind seizures, often by measuring information-carrying electrical current flows with fine-tipped electrodes. Other experiments looked at the effect of LPI by genetically removing its main signaling partner, or by measuring the release of LPI following seizures.

The tests confirmed past findings that LPI influences nerve signals by binding to a protein called G-coupled receptor 55 (GPR55), on neuron cell surfaces. This LPI-GPR55 presynaptic interaction was found to cause the release of calcium ions within the cell, which encouraged cells to release glutamate, the main excitatory neurotransmitter. Further, when LPI activated GPR55 on the other side of the synapse, it weakened inhibition, by decreasing the supply and proper arrangement of proteins necessary for inhibition. Collectively, this creates a “dangerous” two-pronged mechanism to increase excitability, say the authors.  

The research team found that either genetically engineering mice to lack GPR55, or treating mice with plant-derived CBD prior to seizure-inducing stimuli, blocked LPI-mediated effects on both excitatory and inhibitory synaptic transmission. While prior studies had implicated GPR55 as a seizure-reducing target of CBD, the current work provided a more detailed, proposed mechanism of action.

Scientists at Gladstone Institutes reports new findings that could guide the development of better therapeutic strategies for Dravet syndrome and related conditions. Shown here are the study’s first authors, Eric Shao (left) and Che-Wei Chang (right). CREDIT Photo: Michael Short/Gladstone Institutes

Children with Dravet syndrome, a severe form of epilepsy that begins in infancy, experience seizures, usually for their entire life. They are at high risk of sudden unexpected death in epilepsy (SUDEP) and can also develop intellectual disability and autism. Available treatments typically fail to improve these symptoms.

Now, a group of scientists at Gladstone Institutes led by Lennart Mucke, MD, reports new findings in the journal Science Translational Medicine that could guide the development of better therapeutic strategies for Dravet syndrome and related conditions.

The researchers previously discovered, in a mouse model of Dravet syndrome, that genetically removing the protein tau from the entire body during embryonic development reduces epilepsy, SUDEP, and autism-like behaviors. In the new study, they pinpoint the key cell type in the brain in which tau levels must be reduced to avoid these problems. They also show that lowering tau is still effective in mice when the intervention is delayed until after their birth.

“Our findings provide new insights into the cellular mechanisms by which tau reduction prevents abnormal overexcitation in the brain,” says Mucke, director of the Gladstone Institute of Neurological Disease. “They are also encouraging from a therapeutic perspective, since in humans, initiating treatment after birth is still more feasible than treating embryos in the womb.”

Tau is a promising therapeutic target not only for Dravet syndrome, but also for a variety of other conditions, including different types of epilepsy and some forms of autism, as well as Alzheimer’s disease and related neurodegenerative disorders.

Pinpointing the Crucial Brain Cells

A well-functioning brain depends on the correct balance between the activity of excitatory and inhibitory neurons—the former stimulate the activity of other neurons, while the latter suppress it. Dravet syndrome causes an imbalance between these types of cells, resulting in abnormally high and synchronized activity in brain networks that can manifest as seizures and other symptoms.

Mucke and his colleagues recently showed that removing tau from the entire brain changes the activities of both excitatory and inhibitory neurons, although in different ways. The current study aimed to determine whether it is more important to reduce tau in excitatory or inhibitory neurons.

For this purpose, the scientists used genetic tools to eliminate tau selectively from one or the other cell type in the Dravet mouse model. They found that removing tau from excitatory neurons reduced disease manifestations, whereas removing tau from inhibitory neurons did not.

“This means that tau production in excitatory neurons sets the stage for all these abnormalities to occur, including autistic behaviors, epilepsy, and sudden unexpected death,” says Mucke, who is also the Joseph B. Martin Distinguished Professor of Neuroscience and a professor of neurology at UC San Francisco.

Initiating Treatment after Birth

While the genetic approaches the scientists used to remove tau from specific cell types are effective and precise, they are not yet easy to use as a therapeutic intervention in humans. So, the team turned to a more practical option: global tau reduction in the brain with DNA fragments known as antisense oligonucleotides, or ASOs. The scientists delivered an anti-tau ASO into the brain of mice 10 days after birth and found that most symptoms of Dravet syndrome were gone 4 months later.

“We observed a robust reduction of SUDEP, seizure activity, and repetitive behaviors,” says Eric Shao, PhD, a scientist in Mucke’s lab and first author of the study.

In addition, the ASO treatment had no obvious side effects.

“We are excited about these findings, especially since another anti-tau ASO has already undergone a Phase I clinical trial in people with Alzheimer’s disease,” says Mucke. “It could be useful to consider this strategy also for Dravet syndrome and related conditions. However, defining the optimal timing for treatment initiation will be key, as the window of opportunity might be quite narrow.”

Although Alzheimer’s disease, epilepsy, and autism have diverse causes, they all seem to be associated with abnormally high ratios between excitatory and inhibitory neuronal activities—and this abnormality could potentially be fixed by tau-lowering therapeutics.

Still, a treatment based on anti-tau ASOs would involve repeated spinal taps, a procedure most people would rather avoid. Therefore, Mucke is partnering with Takeda Pharmaceuticals to develop small molecules that could reduce brain tau levels when administered as a pill.