This article is written and edited by Tiago Medeiros Furquim for MindMint from his original SciFact blog.

Ok, it is finally time to talk about my favorite cells. They have been the focus of my research for as long as I can remember: the cute little cells in our brains called microglia. 

Microglia are unique cells that live in our brains and are the only ones that do not originate from there. That's right, they originate from the yolk sac (or vitelline sac/umbilical vesicle) during embryonic development when microglial precursors "invade" the brain even before the other types of glial cells (i.e., astrocytes and oligodendrocytes) are generated. Their origin is similar to other macrophages that live in tissues, and they also perform similar functions; therefore, they are, in effect, the macrophages of the brain. In a healthy brain, they are virtually the only immune cells, as our brain is tightly protected from things outside the central nervous system by the famous blood-brain barrier. It was recently reported that the COVID-19 virus SARS-CoV-2 induces massive activation of microglia. Would the microglia from such patients be changed forever, and would that mean anything to their health in the far future?  We will dive into this and other questions later on, but for now it is important to know that, in a healthy brain, microglia numbers are maintained by self-renewal: in humans, approximately 28% of all microglia cells are estimated to be renewed every year, with an average lifetime of 4.2 years, but some microglia are estimated to live as long as 20 years! (They really are nostalgic life companions to share memories with, huh).

As macrophages of the brain, microglia are sentinel and plastic cells that surveil their environment. Upon stimulation, they get activated and assume a broad range of activation states playing roles as warriors, cleaners, and nurturers (including synaptic pruners).

As sentinels and warriors, they are constantly surveilling the brain and are one of the first lines of defence to protect one of the most (if not the most) important systems in our bodies.  Not only that, but since they migrate to the brain at the early stages of embryonic development, microglia are super important to neurodevelopment. They help neurons mature by trimming synapses, i.e., giving neurons haircuts so they can properly meet each other. Microglia are also nurturers and nurses, producing growth factors that aid in tissue repair after a lesion, injury, or infection. Being macrophages, they also play the role of cleaners, like tiny Pac-Mans that eat and clear the brain of debris, dead cells, and unwanted misfolded proteins. Microglia are plastic and do not commit to any specific type of activation, assuming several states of activation.

You get the message, microglia are the handymen of the brain; they are versatile and quick, doing all kinds of jobs to keep your brain protected, clean, and functioning… or do they? If microglia are sentinels (always sensing their environment, ready to act) and play important roles in neurodevelopment and brain homeostasis (i.e., keeping the brain environment balanced and healthy), you can also imagine that changes in their activity can lead to disease, right? Microglia (as well as other macrophages) also have a dark side; they can have an inflammatory activation that, if sustained, can lead to damage of neurons and activation of other cells such as reactive astrocytes. Ok, it is not like they are either Jedi, servers of the light or dark lords such as Darth Vader. Activated microglia can assume a wide variety of states, increasing the expression of genes that will aid them in any of the functions mentioned above (warriors, cleaners, nurturers, sentinels). However, the specific role of each of these states in disease is still unclear.

A lot of research shows that microglia play roles in several neurodegenerative diseases such as Alzheimer's and Parkinson's, neurodevelopmental disorders (e.g. autism spectrum disorder), autoimmune diseases such as multiple sclerosis, and even brain tumors. In many neurodegenerative diseases, microglia activate in a certain way that we call "disease-associated microglia" or "neurodegeneration-associated microglia". Interestingly, the signature of these damage-associated microglia is also present during early neurodevelopment, probably because during this period, these cells are very active in clearing cell debris and pruning synapses.

Microglia activation is also transient, i.e. microglia should return to their homeostatic state after solving the problem and keep surveilling the brain for the next broken pipe that needs repairing. Indeed, being part of the innate immune system – not adaptive immune cells such as lymphocytes – these cells should not feature immune memory. However, recent research has shown that innate immune cells can develop immune memory, but differently from the adaptive immune system. 

In the adaptive immune system, the long-lasting immune memory comes from clonal expansion and the generation of long-lived memory cells that circulate in the body. The innate immune memory is more subtle and consists of reprogramming immune cells that makes them either more responsive or less responsive to a stimulus. In the more responsive state, we call this immune training, as the cells usually get more efficient (trained) into clearing infections and protecting the body. At the same time, one could also think trained microglia could be harmful to the brain, as they are overreacting in response to a stimulus, right? In the less responsive state, it is usually referred to as immune tolerance, and the cells do not respond with the same efficiency to a stimulus, which could be a good thing if that stimulus is not something bad, right? As said before, this is not black and white, and there is a lot we do not understand, especially the role of each of these states on the development of brain diseases. 

This is why my PhD project focuses on the innate immune memory of microglia and how this affects neuroinflammation and remyelination, which are hallmarks of multiple sclerosis.

After an initial stimulus, innate immune cells undergo epigenetic and metabolic reprogramming that makes them more (training) or less (tolerance) responsive to subsequent stimulus.

The reprogramming innate immune cells undergo during immune training and tolerance occurs at the level of epigenetic and metabolic modifications. Simply put, epigenetic modifications tell the cell how to read the DNA (more can be read in the article, It's in your DNA). Such epigenetic modifications make it easier or harder for the cell to read specific genes in their own DNA, hence making the cells more or less responsive to future challenges. On the metabolic side, the cells undergo shifts in their metabolism, which coordinates with their needs to react to a secondary stimulus, making them more or less responsive.

If microglia are so important but also hold grudges against what was done to them in the past, what does that mean for your health due to any insult you had or will have during your entire lifetime? Microglia not only to respond to brain infections but also respond to systemic (not brain) inflammation. Would any infection or brain injury leave a footprint in the microglia, forever changing them? It is known that infections or immune stimulation during pregnancy (called maternal immune activation) can lead to neurodevelopmental disorders and that these disorders can be ameliorated by targeting microglia activation in mice (reference 1 and reference 2). Additionally, as mentioned before, SARS-CoV-2 induces massive activation of microglia. Would this activation change the microglia forever? What would this mean to one’s health in the future? Well, we do not know exactly how long the reprogramming of microglia lasts, and it is difficult to investigate it in humans. Still, animal research suggests that microglia reprogramming lasts for at least 6 months in mice; it is a lot for a mouse's lifespan that usually lives up to only three years.

Still, there is a light at the end of the tunnel: ongoing clinical trials (NCT02829723 and NCT04066244) are actually making use of a drug that kills the microglia and allows them to repopulate the brain. The hope is the microglia replenishing in the brain would reset these microglia and allow for a brand-new start. However, we do not know whether when microglial cells replicate, their newly generated cells inherit this reprogramming or not, which impacts the efficacy of potential treatments that use this repopulation strategy.

This field of research is very recent, and there is a lot to be explored. I hope I have opened your eyes to how complex it can be to study one cell type and how important it is to answer fundamental questions in biology. Recent advances such as 3D-organoids  and induced pluripotent stem cells (iPSC) have really opened the possibilities to study such a system in a context closer-to-real-life. Such technological advancements deserve yet another post focusing on brain research. For now, that is all.

To hear more about microglia, check out this YouTube video from Neuro Transmissions