The role of microglia in epilepsy: Hero, villain, or both? Dr. Amy Brewster

Microglia can have protective and also potentially harmful effects in the brain. They seem to be involved in dampening the nervous system during acute seizures – but also appear to play a role in neuronal loss and cortical thinning. Dr. Cecilie Nome spoke with Dr. Amy Brewster about the many faces of microglia and the current understanding of their role in epilepsy and seizures.

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Transcript

[00:00:00] Dr. Cecilie Nome: Hello everyone, and welcome back to Sharp Waves. I'm your host, Cecilia, and today we have a very exciting episode lined up for you. We will dive into inflammation and microglia and how this relates to epilepsy.

[00:00:18] Dr. Amy Brewster: So I'm Amy Brewster. I am an associate professor in the Department of Biology at Southern Methodist University in Dallas, Texas, and my research focuses on neuroimmunology, microglia, and epilepsy.

[00:00:33] Dr. Cecilie Nome: Thank you so much. So I guess we will just start right on then. My first question will be, microglia: What are microglia? And what roles do they play in the brain?

[00:00:47] Dr. Amy Brewster: So microglia are very interesting cells. Microglia are the brain immune cells. They act like tiny guardians that protect the brain from harm. These cells constantly monitor the brain environment, extending and retracting their branches to check for problems. When they detect something wrong, they change shape and become more active in order to address the issue.

Some of their roles include cleaning up waste and dead cells, fighting infections by attacking harmful pathogens, helping with brain development by pruning unnecessary connections between neurons, responding to injuries by rushing to damaged areas, and supporting brain health throughout life. So they are very, very busy cells.

[00:01:39] Dr. Cecilie Nome: Thank you. Now you said that microglia can actually change shape. Could you talk more about that? How does that work? 

[00:01:49] Dr. Amy Brewster: Yeah, so once microglia typically react to signals in their environment that are “eat me” and “find me” signals, and other type of signals. And once they encounter those signals, they can change their morphology so they can react accordingly. They can change their phagocytic state, they can change their inflammatory state, and they can change their surveillance state, depending on the type of signal that they encounter. And according to that, then the morphology will change. They can become more branched, more hypertrophic, like they kind of swell, they can retract their branches and become small and amoeboid and all of these different morphological changes are thought to involve a change in function, even though I don't think there is a unique function that is linked to a particular morphology. We know that they become reactive and some people call them active.

[00:02:59] Dr. Cecilie Nome: So in regards to epilepsy researchers, they have observed morphological changes in microglia within the epileptic focus in the brain tissue and even brain tissue that has been obtained from patients with drug resistant epilepsy who has been going through surgery. What is happening and what, what is kind of known about microglia and inflammation in epilepsy?   

[00:03:23] Dr. Amy Brewster: I'll start with what we have observed from utilizing brain biopsies that have been surgically resected from patients with drug-resistant epilepsy. So histological analysis of these type of tissues have shown the presence of microglia of different shapes, as we just discussed, from highly branched to amoeboid. For example, hippocampal and cortical epileptic tissues have shown higher densities of amoeboid-shaped microglial cells within the seizure focus areas and less dense microglia populations surrounding those areas.

However, it is important to note, as I said before, that while the change in shape suggests alteration in the functional state, we don't really know the specific function. We do know that in those areas where microglia have accumulated in the seizure focus of human epilepsy cases, and we've seen it in animal models too, that omic base analysis have shown enrichment in pathways linked to microglia inflammatory profiles, fibrocytic profiles, and also they express more genes that are linked to those pathways, suggesting that, yes, microglia have an inflammatory phenotype within those seizure areas in the human brain.

Animal studies have also confirmed this phenomenon in rat and mouse models of both acquired and genetic epilepsies. 

[00:05:00] Dr. Cecilie Nome: Do you think microglia can influence neuronal excitability?

[00:05:05] Dr. Amy Brewster: Yes, absolutely. And I think it can do it in different ways, and it is context dependent. There is evidence that if we induce prolonged seizures in an otherwise healthy system that during the seizure, microglia extend their branches and make contact with the dendrites and spines. And if you disrupt the receptors that are modulating these interactions then you can decrease excitability of the cells, suggesting that in that particular context of acute seizures, microglia function to control or dampen neuronal excitability. 

However, in the context of an already established epilepsy network, microglia have a more inflammatory phenotype that is thought to contribute to exacerbated neuronal loss, and they can contribute to the hyperactivity that results in seizures. So they could do both, depending on their specific activation state, which will depend on the disease state and progression.

[00:06:32] Dr. Cecilie Nome: Okay, so does that mean that microglia can actually have a pro-epileptic effect on the surrounding brain tissue? And if I understand you correctly, this is something that changes from if you're having a seizure compared to if you're having an epileptic focus in your brain. Iis that right?

[00:06:48] Dr. Amy Brewster: That's what I think, you know, and that's based on the evidence that I have seen so far.

[00:06:56] Dr. Cecilie Nome: That's very interesting. You also mentioned that you've seen this both in, in acquired epilepsy, but also in genetic epilepsy. Do we know if there's any differences between those two, or is it kind of the same mechanism. 

[00:07:10] Dr. Amy Brewster: Although most studies evaluating microglial roles in seizures and epilepsy have been done in rodent models of status epilepticus and acquired epilepsies, with fewer investigations in genetic models and even human tissues, I think that the role of microglia under seizure conditions appears to be relatively consistent.

This conclusion is supported by numerous omics studies, as I mentioned earlier, that have consistently identified increased expression of microglia-related genes and enrichment of microglia-related pathways across different types of epilepsies in both humans and rodents. And within rodents, in mice and rats. So it transcends both seizure type and species type, suggesting that they are a mechanism that is linked to seizures and epilepsy.

[00:08:00] Dr. Cecilie Nome: Thank you. So that means that microglia will probably be important or play a significant role in many types of epilepsy, or maybe all types of epilepsy. I also read some basic neuroscience studies a while ago that have indicated that microglia may also be involved in synaptic removal and even in neuronal network remodeling. Could you expand on this?

[00:08:28] Dr. Amy Brewster: So, yes, microglia have been shown to contribute to synaptic refinement by pruning unwanted or unneeded synaptic elements during critical periods of development, particularly in the visual system. The processes are guided and modulated by proteins such as complement proteins like C1q and C3. While it is well established that these complement proteins increase in response to seizures, and in chronic epilepsy across both human and animal models, the exact role of microglia in potential synaptic stripping, in my view, remains unclear.

We have observed microglia dendritic and synaptic contacts in epilepsy, but I think there isn't enough conclusive evidence to definitely say that microglia engage in pathological synaptic pruning during seizures or in epileptic conditions. I think it's an intriguing question, and it's a question that my lab is actively investigating. Right now, we're still piecing together the puzzle, and our findings are preliminary. We're excited about the potential insights but we're not ready to make definitive claims about the mechanisms of microglia synaptic interactions in epilepsy. We need more studies. We need more evidence.

[00:09:52] Dr. Cecilie Nome: So there was also this other study that came out in 2021 by Altmann, et al. And here the researcher found that high density of microglial cells were found in areas characterized by reduced cortical thickness. And this was accomplished through system-level analysis of image-based cortical structural profile compared with gene expression data. Have you read this study? Would you be able to comment upon this? 

[00:10:20] Dr. Amy Brewster: Yes, I do find this is a fascinating study that suggests a potential link between high-density microglial populations and areas of neural loss and cortical thinning in epilepsy. This study, in my view, provided compelling evidence of increased gene expression of microglial genes, as I mentioned earlier, this is another one of these studies, and enrichment of microglial pathways, in both human and animal models. So it's, this is, it's good because it's consistently replicated across different studies, human and animal models, and this is one of them. 

I think that their experiments with microglia, when they depleted the microglia, the results are very intriguing in my view. By eliminating microglia, they found an attenuation of cortical neuronal loss, suggesting that microglia may indeed contribute to the cortical thinning in the context of epilepsy, what we saw in the humans, what they described in the humans.

However, it is important to note that several studies that have also depleted or reduced microglia populations in a similar fashion in acquired models of epilepsy, they have found conflicting results. Some studies demonstrated beneficial outcomes such as attenuating the neuropathology, including neuronal loss, reducing seizures, and improving memory. But other studies found no effect or even worsening of the both the neuropathology and the pathophysiology.

I think it is possible that these inconsistent observations may result from differences in the timing of microglial depletion across different studies and maybe different ages of the animals and perhaps different sexes of the animals. But together, I think these studies suggest that microglia roles in epilepsy are highly dynamic, that they evolve in a temporal and a spatial manner, and that we do need more research to identify both the beneficial and the detrimental signals that are particularly linked to microglia in the context of epilepsy.

[00:12:34] Dr. Cecilie Nome: Thank you. That was very well explained, I think. So I keep coming back to this question where, because we've been talking now and you told me that microglia are activated by seizures but also that microglia itself could contribute to seizures and to epilepsy. So do we know which one is sort of coming first or affecting the other one, or are they maybe both linked together? What do you think about this?

[00:13:03] Dr. Amy Brewster: That's a really good question. I think we're all trying to figure out which are the good versus the bad signals because microglia in my view, they're not bad guys. Microglia are designed to follow the instructions in their microenvironment and to act accordingly. So if those instructions are changed, microglia are going to respond accordingly, 

However, it is also possible that microglia themselves become dysfunctional in epilepsy. So, I think we really need to do more research to try to identify…it's too many signals, you know, which are the signals that are good versus which ones are the ones that are detrimental.

I mean, the goal is to find better treatments for epilepsy. What do we target and why and when? We could target the detrimental signals when they are detrimental. We could also find a way to enhance the beneficial signals to overcome the detrimental signals. So I think microglia are very complex and that we, again, we really need to do more studies to know what their, what their role is, what their role of the specific signals is in epilepsy.  

[00:14:27] Dr. Cecilie Nome: Thank you. You mentioned treatment. Are there any drug targets or specific drugs that could target microglia that are currently under investigation?

[00:14:38] Dr. Amy Brewster: Yes. So a drug that specifically targets microglia include the drugs that target the colony stimulating factor 1 receptor, the CSF1R receptor, which is the receptor that is targeted by the drugs that deplete microglia that have been used widely in animal models. Because this particular signal signaling pathway regulates microglia proliferation and survival. So it's ideal to try to eliminate microglia, but as we previously discussed, defeating microglia might not be the answer. We need to find the specific signals of microglia that may be harmful in epilepsy. But other drugs that can directly and indirectly modulate microglia functions include, for example, minocycline, which can reduce inflammation, and rapamycin and its analogues. 

So, rapamycin targets the mTOR signaling cascade, which we now know is also crucial for microglial survival and proliferation. And we know that rapamycin analogs are currently being used to treat some forms of epilepsy but have not really been evaluated. There are a lot of studies that have used rapamycin, for example, but have yet to look at how it influences microglia, which, you know, it can open more possibilities for drug discovery.

[00:16:10] Dr. Cecilie Nome: That's very interesting. And rapamycin, that's already used, right, for tuberous sclerosis, if I remember correctly. 

[00:16:19] Dr. Amy Brewster: Yes, that's correct.

[00:16:22] Dr. Cecilie Nome: Okay. Thank you so much. Do you think you could summarize the most important aspects of microglia in the context of epilepsy?

[00:16:30] Dr. Amy Brewster: Yes. Microglia in epilepsy are dynamic immune cells that undergo complex morphological and functional changes, contributing to neuroinflammation, synaptic remodeling, and neuronal damage. Their roles are not static, but they evolve throughout different stages of disease, exhibiting both protective and potentially harmful effects on neural networks that may also contribute to epilepsy. I think we need to do more research to fully understand which specific microglial signals are the critical targets to improve epilepsy outcomes.

[00:17:09] Dr. Cecilie Nome: So I was thinking about the other glial cells, like for example, astrocytes, which we did an episode on a time ago, I think like last year. How are they connected or do they talk to each other during seizures and epilepsy? 

[00:17:24] Dr. Amy Brewster: Yeah, I think there is evidence that the cells talk to each other. That's something that I started looking in my lab. I mean, there's plenty of people that are looking at the crosstalk between the different cell types, astrocytes with microglia and neurons. We have an ongoing project because we study complement signaling and complement signaling has been shown to help microglia and astrocytes talk to each other. We don't know what it means yet. But we are going to figure it out. 

[00:17:50] Dr. Cecilie Nome: Thank you. Is there anything else you would like to, to mention or to discuss?   

[00:17:56] Dr. Amy Brewster: Not for now. I think that we just need to get back to the lab and solve this mystery. 

[00:18:09] Dr. Cecilie Nome: That sounds like a very good idea, I think.