AES 2022 | Microglial surveillance required to regulate neuronal function: implications for epilepsy

Work in my lab focuses on neurovascular mechanisms of inflammation and tissue repair. And during the course of this work, we made discoveries at the neurovascular interface, discovering new mechanisms of how blood proteins interact with nervous system cells. And what we found is that microglia cells are key players that are activated by changes at the neurovascular interface in disease. And we found that these cells are mediating both neurodegeneration as well as also changes in brain hyperexcitability.

The work that I will be presenting at the conference focuses specifically on the role of microglia cells as this novel homeostatic role that we discovered in the regulation of brain network synchronization. So what we showed, we developed to the first chemogenetic model to be able to block microglia brain surveillance and dynamic motility. It has been known for several years that microglia cells can constantly survey the brain in the normal, physiological state. And why the brain expends so much energy to keep these cells in this constant motility remain unknown.

So our work filled that gap in knowledge by developing a chemogenetic model to stop this microglia processes from surveying the brain. And we did this by specifically expressing in a cell specific manner, the S1 subunit of pertussis toxin, and we blocked Gi signaling in the microglia cells. So the brain now of these mice could have microglia, but the cells could not perform anymore surveillance. And also they could not respond to stimuli like the trauma stimuli like laser ablation. Or when we encased glutamate in the brain, they also could not respond to this excitotoxic stimuli.

So we wanted to ask the question, what is the phenotype of this mouse? And to our surprise, some of these mice spontaneously developed seizure activity giving us the first indication that perhaps this constant microglia motility was responsible for suppressing this hyperexcitability in the brain. And indeed, when we induce seizures in mice by administration of a drug called pilocarpine, these mice had a really higher susceptibility to seizures.

We also combined this research by developing of high resolution in vivo two-photon imaging to be able to image simultaneously in an awake animal, both neuronal activity and also microglia surveillance. And there we made this unexpected observation that when we evoke physiologically neuronal activity in the mouse, microglia increased their surveillance to areas where there is increased activity. And at these areas that surveillance increases, the neuronal activity is suppressed, showing that microglia not only respond to increased neuronal activity, but this response is required to keep neuronal activity within a physiological range.

We also performed rescue experiments to be able to rescue these effects. And we found that when we keep Rho GTPases that are important for cytoskeletal rearrangements in an active state, we can rescue these effects, giving us perhaps some potential therapeutic strategies to reign in hyperexcitability by specifically interfering with this pathways in microglia.

This discovery opens new avenues of research, especially since impaired microglia dynamics and impaired microglia motility is a hallmark of many neurological diseases including neurological diseases like Alzheimer’s disease, multiple sclerosis, but also psychiatric disorders. So this, one of the immediate questions is to find whether these mechanisms that we found in physiology in the normal brain, whether these mechanisms are also active during disease pathogenesis. And if this is the case, then to explore whether the tools that we are developing, we developed from our research in brain physiology, whether these tools could be used to reign in hyperactivity in models of disease.

Because a way that we can think of therapeutically intervening with this pathway is whether we can make microglia cells resilient to any impaired microglia dynamics that could be induced by extrinsic mechanisms in the lesion environment in different diseases. An example of this is from our research on the blood-brain barrier. We found that blood-brain barrier leaks in the brain that occurs in autoimmune, neurodegenerative and traumatic diseases, this alter this surveillance of microglia and microglia cells perivascular cluster, specifically in the areas with impaired blood-brain barrier in the brain and increased extravasation of blood proteins.

So this is something that would be a really, I think, clinically relevant, whether we can make these cells resilient to these signals from the environment that impair their capacity to survey the brain and thus impair their capacity to regulate neuronal synchronization and suppress hyperexcitability.