A curious thing happens as the brain develops. Well before the brain looks anything like what it will become – just a few weeks after the embryo has formed – a group of cells generated by your immune system move into the brain. Once there, they multiply, disperse, settle down, and send out delicate fine processes (imagine spidery legs extending from a small body). Then……they just sit there.
Or so we thought. For a long time this cell type – called microglia – was thought to react only when the brain was injured. Once that happened things changed fast. The cells retracted their processes, took on an amoeboid shape and actually began moving through the brain. When they got to the site of tissue damage they helped repair things, hoovering up cell debris. But scientists began to suspect they were missing something. Were the microglia really so dormant the rest of the time?
Regular readers of EiE will be familiar with the fact that the brain is not just billions of neurons but is also packed full of another cell type called glia. It is the activity of neurons and the fast electrical signals they tranmsit to one another (action potentials or “spikes”) which is the basis for brain function. But the neurons are nothing without their support cells. Astrocytes are the most abundant glia type. They are important for providing neurons with the nutrients they need and the busy “pumps” on their surfaces collect up excess neurotransmitter and recycle it. Oligodendrocytes wrap around the long processes of neurons (axons), increasing the speed of nerve conduction (the disease multiple sclerosis occurs when this insulating layer is damaged).
Back to microglia which, by the way, are not new to us – the famous histologist Cajal made drawings of microglia in the brain over a century ago (although his staining technique did not show up their delicate processes). Our understanding of microglia began to improve when techniques were developed that allowed us to label and visualise them in the brain (for example by making them fluorescent) and manipulate their numbers and activity (using drugs). The past couple of decades have taught us that microglia do many jobs early on in the brain, including helping with correct positioning neurons in various circuits and supporting their survival.
Microglia and epilepsy
Studies of microglia in epilepsy date back at least 40 years. Because they could be seen surrounding sites where seizures were occuring, they were assumed to be bad guys. This wasn’t necessarily wrong because studies showed that inhibiting microglia could reduce seizures in animal models. But it was certainly an over-simplification. In some studies, blocking microglia signals actually made seizures worse. And what are microglia doing before there is injury? In the “healthy” brain? Critically, we now know that once microglia settle in the brain, changing into their “ramified” shape, they are anything but “dormant”. It is increasingly clear that microglia are continuously sensing the local environment, using the delicate processes (filopodia) to “probe” the molecular enironment around synapses – the contact points for neurotransmission.
Getting close to understanding what microglia normally do
A major new study has shed light on what microglia normally do in the adult brain. It appears that a fundamental role of “resting” microglia is to dampen neuronal excitability. This appears to be independent but complementary to the traditional job that inhibitory neurons perform, which lie among excitatory neurons releasing substances such as the neurotransmitter GABA to block neurons from firing too much.
The study, published in Nature, was led by Anne Schaefer’s team at the Icahn School of Medicine in New York. They showed that microglia normally continually extend and retract their processes around neurons. Upon activation of neurons this motion changes. The microglia became more targeted in where they send their processes, increasingly directing them straight to synapses (where the neurotransmitter from one neuron crosses to the other). To understand how they do this, the team triggered short, controlled increases in neuron firing in the brain and then isolated the surrounding microglia. When they checked what genes were switched on or off they found it was genes that coded for proteins with “scaffolding” functions and controlled direction of movement (“chemotaxis”). Microglia physically react to increases in excitability and this is controlled by a series of genes that allow the cells to direct processes to where they need to be.
Missing microglia creates a storm
They wondered if this microglial mobility was important for brain excitability. To test this idea they treated mice with a drug that depleted the brain of microglia. By the way, this is an interesting phenomenon in itself. It turns out that microglia require a constant “signal” in order to stay alive. If that signal gets blocked – which can be done using a drug – the cells die off. The animals without microglia appeared largely normal but when they were given a low dose of a drug that can activate neurons, the animals lacking microglia developed seizures. Without microglia, an otherwise normal rise in neuronal activity quickly resulted in excessive firing of neurons. They explored this further, discovering that there are different sub-populations of microglia in different parts of the brain. Some lie around areas dense with the cell bodies of neurons while others lie among the tracts of axons, the wires that connect one region of the brain to another. Losing one or the other microglia population produced different effects on brain activity and some regions were more tolerant of a loss of microglia before problems emerged. They next recorded the firing of individual neurons while depleting microglia. Neuronal networks that lacked microglia began to synchronize, with neurons all firing at once – prolonged and high frequency neuronal events that spread across the brain (this is how a seizure starts).
Communicating through molecular messages
They turned last to the question of how neurons and microglia communicate with one another. What signal(s) cause the changes in microglia? They focused on a molecule that had previously been linked to how microglia sense their environmnet – ATP. ATP is the chemical that powers most reactions in cells. However, it is also packaged up with neurotransmitters and released from neurons. Higher firing or synchronisation causes more ATP release. There are specific receptors for ATP on various cells including microglia. But microglia also express an enzyme that breaks down ATP, resulting in formation of adenosine. Now, adenosine is a powerful suppressor of neuronal activity and seizures. We have known this for a long time (as an undergraduate student more than 25 years ago I remember applying it to brain slices and seeing electrical signals disappear before my eyes). A build up of adenosine is what helps seizures to stop – and if you block adenosine then seizures become more severe or prolonged. The team found that mice they had previously treated with the drug to deplete microglia had lower amounts of adenosine in the brain. Also, mice with normal numbers of microglia but lacking the enzyme that converts ATP to adenosine were more susceptible to seizures. Finally, they proved that neurons have a receptor for adenosine which, upon binding, suppresses firing. (N.B. For more on ATP and epilepsy, see our post ‘The curious multiple lives of ATP in epilepsy‘)
The study is a major advance in our understanding of what controls brain excitability. It tells us that microglia are intimately involved with communication in the brain and act as a brake on excitability. This has major implications for epilepsy where microglia are often highly reactive and in a state quite different to normal. If we could drive microglia back to their resting state, would hyperexcitability subside?
Some questions remain. What do the microglial processes do when they reach the synapse? Most of the studies were performed in otherwise healthy mice. Would the microglia-deficent mice develop a worse form of epilepsy? It is perhaps odd that the behaviour of the mice lacking microglia was reported as normal if they have such low levels of adenosine. Why didn’t they see hyperactivity or even seizures? Some further work is needed but this study moves us closer to new targets for treating or reversing epilepsy and some of the tools used here may not only help epilepsy researchers understand microglia better but could eventually be useful for treatment.
Finally, back to the origins story. Last year a study published in the journal Cell revealed that the maturation of microglia actually depended on signals coming from another cell type in the immune system. Without that communication, the cells remained in a state of immaturity and couldn’t do their job. And other studies show that immune cells communicate with microglia in the brain to prime them for disease. Maybe our microglia are responding a little bit as we do to receiving a long distance phone call from a relative on the other side of the world. There is bad news coming, so best prepare yourself for the worst.
‘Negative feedback control of neuronal activity by microglia‘ was published in Nature in September 2020
About the author
David is an Anglo-Irish neuroscientist who lives and works in Dublin. He studied Pharmacology for his undergraduate and PhD studies then moved to the USA where he spent six years between Pittsburgh and Portland. He came to RCSI in 2004 where he is currently Professor of Molecular Physiology and Neuroscience and the Director of the FutureNeuro Research Centre. His team are interested in the causes, diagnosis and treatment of epilepsy and neurodevelopmental disorders. In his free time, David enjoys running, reading, vegan cooking and as much time as possible with his family.