The Role of Adenosine in Epilepsy

Adenosine is a well-characterized endogenous, anticonvulsant, and seizure terminator in the brain. Overexpression of adenosine kinase, the major adenosine metabolizing enzyme, directly contributes to seizure generation due to depletion of extracellular adenosine. Conversely, therapeutic levels of adenosine can suppress seizures in rodent models of temporal lobe epilepsy. In addition to the anti-seizure effect of adenosine mediated via the adenosine receptor, adenosine kinase may also have disease-modifying effects mediated through regulation of DNA methylation, making it an attractive therapeutic target.

Dr. Boison talks about the anti-seizure and disease-modifying effects of adenosine in epilepsy, as well as recent advances in developing adenosine kinase inhibitors as therapeutics for epilepsy.

This seminar is part of CURE Epilepsy’s Frontiers in Research Seminar Series. This program is generously supported by the Nussenbaum-Vogelstein Family and aims to help educate and expose researchers, clinicians, and students to exciting epilepsy research and also provide opportunities for young investigators to interact with leaders in the field.

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About the Speaker
The seminar is presented by Catalyst Award grantee Detlev Boison, PhD, professor at the Department of Neurosurgery at Rutgers University.

Q&A with Dr. Detlev Boison

Have you observed or looked into DNA methylation differences in response to adenosine?

Dr. Boison: That’s a good question. We have primarily seen changes in neurons. There are new data which I haven’t shown today, but we find the trends in ectopic expression of ADK-L in neurons during epileptogenesis. This is really a transient period and might be the reason why trends in treatment with an ADK inhibitor perfectly tailored to this trends in over-expression of ADK-L in neurons, which really coincides with increased 5-mC during the same time span might be an important role for the epileptogenic process.

Over time, do the medications change the seizure onset or property of the seizures?

Dr. Boison: The goal here is basically to have a prophylactic treatment to initiate, and yet to have trends in treatment during the latent period of epileptogenesis with the ultimate goal to prevent epilepsy all together. Now in most studies, which I’d shown previously, we compared the seizure phenotype at six weeks and nine weeks after initiating epilepsy and we did not find any differences. Of course, it would be worthwhile in future studies to extend that time period to longer time spent looking at brains at three months or six months and we will certainly do this in the future.

Are there any commercial assays available for measuring adenosine levels in human plasma?

Dr. Boison: No, there are not. Adenosine can be quantified by HPLC and by LCMS/MS methods. There are no commercial assays. The blood-brain barrier is not really penetrable for adenosine and adenosine in the circulation has an extremely sharp half-life in the range of seconds. So plasma adenosine is unlikely to be a representative for adenosine levels in the brain.

How is astrogliosis linked with epilepsy? Do we know if it causes epilepsy or is it just correlated with epileptic seizures?

Dr. Boison: There’s a lot of data, not just from my group, but also from other researchers that suggests that astrogliosis can truly be a cost for the generation of epileptic seizures. And astrogliosis is triggered by all the inflammatory processes that trigger the initial phases of epileptogenesis. So you get microglial activation and astroglial activation as a response basically to brain inflammation.

How specific is 5-ITU for ADK?

Dr. Boison: It’s one of the old inhibitors. It’s not selected for ADK-L versus ADK-S. According to our findings, it blocks both. That’s the reason why it prevents epileptogenesis, but it also has sedative side effects because it also increases extracellular adenosine.

Do you know of a mechanism by which adenosine may build up in the nucleus and make methylation?

Dr. Boison: The deficiency of adenosine in the nucleus would drive DNA methylation. If there’s a mechanism that increases adenosine in the nucleus, then it would block DNA methylation and this could be directly linked to factors that inhibit adenosine kinase in the nucleus. But this is currently unknown.

How did you go about designing a biopolymer that could release adenosine?

Dr. Boison: Whenever you have a release system, you get a logarithmic release profile. If you, let’s say, just encapsulate adenosine in the membrane, you will have a burst release: Most of it will come out immediately and then drop off. So in order to get a stable release, you basically need to combine several logarithmic release profiles in one implant.

So the way those silk polymers were engineered was: First we encapsulated adenosine as micro-vesicles in a silk membrane, and then those micro-vesicles were embedded in a 3D silk metrics. Then the whole construct was coated with alternative layers of adenosine and silk, always repeating, and then you can cap the whole thing with several additional layers of silk to slow down the release. So if you do it right, then you get the release profile.

The beauty of silk is it’s a biopolymer. It doesn’t have any toxins. It’s fully resolvable. Silk has been used as a suture material for decades and it’s a relatively boring amino acid repeat of glycine and alanine. So it’s very biocompatible.

What are good targets for DNA methylation to measure?

Dr. Boison: We are primarily interested just in the global DNA methylome, because based on the evolutionary principles, I think this is a primordial mechanism to regulate the entire DNA methylome. Because if you think in terms of evolution, if you want to regulate genes, you cannot start with transcription factors because transcription factors need their own genes, they need to be controlled by protein kinase pathway, which all needs their own genes which need to be regulated. This is way too complicated.

Now, if you really want to develop a regulatory system for the genome, the most simple way you can think of is just adding and removing methyl groups. This affects the entire genome and I believe this was probably one of the first mechanisms that came up in evolution to regulate the entire DNA methylome on a global level, and everything else that creates target specificity was invented later during the evolution.

There is a question about your MRS drug and whether it is BBB permeant: Does it permeate the blood-brain barrier?

Dr. Boison: We have evidence that it goes through the blood-brain barrier. We’ve also engineered ADK-L mice, which over-express ADK-L in the brain and we can find effects of MRS4203 in those ADK-L mice which is a direct proof that it has an effect in the brain.

Is there any application of your research yet for treating patients who have intractable epilepsy?

Dr. Boison: The primary goal right now with those new compounds is epilepsy prevention, to prevent them from becoming intractable. To treat intractable patients with adenosine is also doable. We have shown that adenosine can prevent pharmacoresistant seizures in our epilepsy models. The challenges are for intractable patients we need long-term therapies over months or years, which means the drugs we have to use need to be very, very safe and we need to come up with solutions to avoid side-effects based on extracellular adenosine. One way to achieve those goals would be focal therapies. One approach could be gene therapy to knock down adenosine kinase in an epileptogenic brain area, but that’s not all that quick solution most likely.

What is the role of AMP in epileptogenesis? Are some of the effects of adenosine kinase due to increase of AMP?

Dr. Boison: Most likely not because AMP levels in cells are 100,000 foot higher than adenosine. So if you change adenosine kinase, you have dramatic effects on the levels of adenosine without changing the levels of AMP significantly.

While A1R seem to have the dominant and inhibitory effect, what roles do other adenosine receptor subtypes have on seizures, particularly in the cortex, as opposed to the hippocampus?

Dr. Boison: We need to realize that the adenosine system is highly compartmentalized. There is one kind of global tissue tone of adenosine, or which also can be considered as an extracellular compartment, which is directly under the control of adenosine kinase expressed in astrocytes and which provides tonic inhibition through activation of the A1 receptors. This is just a global tissue tone of adenosine that primarily activates the A1 receptor to provide the tonic inhibition in the brain.

But on top of that, there is also a synaptic pool of adenosine. So neurons under high-frequency stimulation can directly release adenosine, and neurons can release ATP, which is rapidly broken down to adenosine. So there is a different source of adenosine at the synaptic level. And the A to A receptor has primarily a role to find you the activity of adenosine on the synaptic level. The rationale of this mechanism is basically the forearm. So if you have a globally inhibited networks through the A1 receptor, and then you have once synapse where you have adenosine providing activation of A to A receptors, you can improve the signal to noise ratio. You can even more specific signal on the synaptic level in a globally inhibited network.

The information contained herein is provided for general information only and does not offer medical advice or recommendations. Individuals should not rely on this information as a substitute for consultations with qualified health care professionals who are familiar with individual medical conditions and needs. CURE strongly recommends that care and treatment decisions related to epilepsy and any other medical condition be made in consultation with a patient’s physician or other qualified health care professionals who are familiar with the individual’s specific health situation.