microglia cells

The Brain’s Gardeners: Immune Cells ‘Prune’ Connections Between Neurons

A new study, published in the journal Nature Communications, shows that cells normally associated with protecting the brain from infection and injury also play an important role in rewiring the connections between nerve cells. While this discovery sheds new light on the mechanics of neuroplasticity, it could also help explain diseases like autism spectrum disorders, schizophrenia, and dementia, which may arise when this process breaks down and connections between brain cells are not formed or removed correctly.

(Image caption: Microglia (green) with purple representing the P2Y12 receptor which the study shows is a critical regulator in the process of pruning connections between nerve cells)

“We have long considered the reorganization of the brain’s network of connections as solely the domain of neurons,” said Ania Majewska, Ph.D., an associate professor in the Department of Neuroscience at the University of Rochester Medical Center (URMC) and senior author of the study. “These findings show that a precisely choreographed interaction between multiple cells types is necessary to carry out the formation and destruction of connections that allow proper signaling in the brain.”

The study is another example of a dramatic shift in scientists’ understanding of the role that the immune system, specifically cells called microglia, plays in maintaining brain function. Microglia have been long understood to be the sentinels of the central nervous system, patrolling the brain and spinal cord and springing into action to stamp out infections or gobble up dead cell tissue. However, scientists are now beginning to appreciate that, in addition to serving as the brain’s first line of defense, these cells also have a nurturing side, particularly as it relates to the connections between neurons.

The formation and removal of the physical connections between neurons is a critical part of maintaining a healthy brain and the process of creating new pathways and networks among brain cells enables us to absorb, learn, and memorize new information.  

“The brain’s network of connections is like a garden,” said Rebecca Lowery, a graduate student in Majewska’s lab and co-author of the study. “Not only does it require nourishment and a healthy environment, but every once in a while you need to prune dead branches and pull up weeds in order to allow new flowers to grow.”

While this constant reorganization of neural networks – called neuroplasticity – has been well understood for some time, the basic mechanisms by which connections between brain cells are made and broken has eluded scientists.

Performing experiments in mice, the researchers employed a well-established model of measuring neuroplasticity by observing how cells reorganize their connections when visual information received by the brain is reduced from two eyes to one.

The researchers found that in the mice’s brains microglia responded rapidly to changes in neuronal activity as the brain adapted to processing information from only one eye. They observed that the microglia targeted the synaptic cleft – the business end of the connection that transmits signals between neurons. The microglia “pulled up” the appropriate connections, physically disconnecting one neuron from another, while leaving other important connections intact.

This is similar to what occurs during an infection or injury, in which microglia are activated, quickly navigate towards the injured site, and remove dead or diseased tissue while leaving healthy tissue untouched.

The researchers also pinpointed one of the key molecular mechanisms in this process and observed that when a single receptor – called P2Y12 – was turned off the microglia ceased removing the connections between neurons.

These findings may provide new insight into disorders that are the characterized by sensory or cognitive dysfunction, such as autism spectrum disorders, schizophrenia, and dementia. It is possible that when the microglia’s synapse pruning function is interrupted or when the cells mistakenly remove the wrong connections – perhaps due to genetic factors or because the cells are too occupied elsewhere fighting an infection or injury – the result is impaired signaling between brain cells.

“These findings demonstrate that microglia are a dynamic and integral component of the complex machinery that allows neurons to reorganize their connections in the healthy mature brain,” said Grayson Sipe, a graduate student in Majewska’s lab and co-author of the study. “While more work needs to be done to fully understand this process, this study may help us understand how genetics or disruption of the immune system contributes to neurological disorders.”

Migrating immune cells promote nerve cell demise in the brain

The slow death of dopamine-producing nerve cells in a certain region of the brain is the principal cause underlying Parkinson’s disease. In mice, it is possible to simulate the symptoms of this disease using a substance that selectively kills dopamine-producing neurons. Scientists from the German Cancer Research Center (DKFZ) have now shown for the first time in mouse experiments that after this treatment, cells of the peripheral immune system migrate from the bloodstream into the brain, where they play a major role in the death of neurons. The investigators were able to reduce the level of neurodegeneration using a substance that blocks a specific surface molecule on these inflammatory cells.

A small area in the midbrain known as the substantia nigra is the control center for all bodily movement. Increasing loss of dopamine-generating neurons in this part of the brain therefore leads to the main symptoms of Parkinson’s disease – slowness of movement, rigidity and shaking.

In recent years, there has been increasing scientific evidence suggesting that inflammatory changes in the brain play a major role in Parkinson’s. So far, it has been largely unclear whether this inflammation arises inside the brain itself or whether cells of the innate immune system that enter the brain from the bloodstream are also involved.

At the DKFZ, a team led by Prof. Dr. Ana Martin-Villalba is investigating causes of cell death in the central nervous system. Neuroscientist Martin-Villalba has suspected that a specific pair of molecules, the CD95 system, is involved in neuronal death in Parkinson’s. This pair consists of the CD95 ligand and its corresponding receptor, CD95, also known as the “death receptor”.

Martin-Villalba recently showed that after spinal cord injury, inflammatory cells use these molecules to migrate to the injury site, where they cause damage to the tissue. Martin-Villalba then wanted to investigate whether peripheral inflammatory cells also play a role in chronic neurodegenerative processes such as Parkinson’s disease.

To investigate the process of neurodegeneration in mice, the scientists utilized a model system using the substance MPTP, which causes the selective death of dopamine-generating neurons in the human brain. In mice, MPTP typically causes Parkinson-like symptoms.

However, in mice whose inflammatory cells (monocytes, microglia) were unable to produce CD95L, MPTP treatment resulted in almost no neurodegeneration. This suggested that CD95L-bearing inflammatory cells are involved in the destruction of neurons. However, it remained unclear whether the true culprits are specific macrophages in the brain called microglia, or rather monocytes in the bloodstream that infiltrate the brain.

In order to make this distinction, the investigators used a chemical that blocks CD95L without being able to pass the blood-brain barrier. This substance therefore reaches only the inflammatory cells that circulate in the bloodstream and not the microglia that reside in the brain. Mice that had received this substance were also protected from MPTP-induced neurodegeneration.

“Thus, we have shown for the first time that peripheral inflammatory cells of the innate immune system also play a role in neurodegeneration,” say Liang Gao and David Brenner, first authors of the publication. “A key role in this process is played by CD95L, which enhances the mobility of these cells.”

Project leader Martin-Villalba speculates that a self-reinforcing vicious cycle arises in the brain: The breakdown of a few neurons that die from various causes attracts inflammatory cells that, in turn, further fuel the death of more neurons through inflammation-promoting signaling molecules.

At present, the researchers can only indirectly conclude that the results obtained in the artificial animal model are also relevant in human Parkinson’s disease. In collaboration with colleagues from Ulm, Martin-Villalba’s team recently found elevated quantities of inflammatory monocytes that were hyperactive in blood samples from Parkinson’s patients. Monocyte number correlated with the severity of disease symptoms. However, the researchers do not yet know whether these inflammatory cells also migrate into the brains of patients and contribute to the demise of neurons there, like they do in the mice with Parkinson’s.

“If this is the case, drugs that inhibit CD95L might mitigate Parkinson’s symptoms if administered early on – similar to what we observed in our experimental mice,” says Martin-Villalba. The substance required for this has already been investigated in clinical Phase II trials. Martin-Villalba also suspects that activated cells of the peripheral immune system might drive neurodegeneration not only in Parkinson’s disease but also in other neurodegenerative disorders such as Alzheimer’s.

New research findings on the brain’s guardian cells

Researcher Johan Jakobsson and his colleagues have now published their results in Nature Communications.

At present, researchers know very little about exactly how microglia work. At the same time, there is a lot of curiosity and high hopes among brain researchers that greater understanding of microglia could lead to entirely new drug development strategies for various brain diseases”, says Johan Jakobsson, research group leader at the Division of Molecular Neurogenetics at Lund University.

What the researchers have now succeeded in identifying is a deviation in the structure of the microglia cells, which makes it possible to visualise them and study their behaviour. By inserting a luminescent protein controlled by a microscopic molecule, microRNA-9, the researchers can now distinguish the microglia and monitor their function over time in the brains of rats and mice.

It has long been known that microglia form the first line of defence of the immune system in diseases of the brain. They move quickly to the affected area and release an arsenal of molecules that protect the nerve cells and clear away damaged tissue.

New research also suggests that microglia not only guard the nerve cells but also play an important role in their basic function.

“This represents a real step forward in technological development. Now we can view microglia in a way that has not been possible before. We and our colleagues now hope to be able to use this technique to study the role of the cells in different disease models, for example Parkinson’s disease and stroke, in which microglia are believed to play an important role”, explains Johan Jakobsson.

Microglia Can Be Derived From Patient-Specific Human Induced Pluripotent Stem Cells and May Help Modulate the Course of Central Nervous System Diseases

Today, during the 81st American Association of Neurological Surgeons (AANS) Annual Scientific Meeting, researchers announced new findings regarding the development of methods to turn human induced pluripotent stem cells (iPSC) into microglia, which could be used for not only research but potentially in treatments for various diseases of the central nervous system (CNS).

Microglia are the resident inflammatory cells of the CNS and can modulate the outcomes of a wide range of disorders including trauma, infections, stroke, brain tumors, and various degenerative, inflammatory and psychiatric diseases. However, the effective therapeutic use of microglia demonstrated in various animal CNS disease models currently cannot be translated to patients due to the lack of methods for procuring high-purity patient-specific microglia. Developing a method for obtaining these cells would be highly valuable.

In the study Differentiation of Induced Pluripotent Stem Cells to Microglia for Treatment of CNS Diseases, mouse and human iPSCs were generated and sequentially co-cultured on various cell monolayers and in the presence of added growth factors. The microglial identity of the resulting cells was confirmed using fluorescence activated cell sorting analyses, functional assays, gene expression analyses and brain engraftment ability. The study results will be shared by presenting author John K. Park, MD, PhD, FAANS, from 3:34-3:42 p.m. on Monday, April 29. Co-authors are Michael Shen, BS; Yong Choi, PhD; and Hetal Pandya, PhD.

In the results, researchers found mouse and human iPSCs co-cultured with OP9 cells differentiate into hematopoietic progenitor cells (HPCs). HPCs in turn co-cultured with astrocytes, generate cells that express CD11b, Iba-1 and CX3CR1; secrete the cytokines IL-6, IL-1ß and TNF-a; generate reactive oxygen species; and phagocytose fluorescent particles, all consistent with a microglial phenotype. Gene expression clustering using self-organizing maps indicates that iPSC-derived microglia more closely resemble normal microglia than other inflammatory cell types. The iPSC-derived microglia engraft and migrate to areas of injury within the brain. These finding have led researchers to conclude that iPSC-derived microglia may one day be useful as gene and protein delivery vehicles to the CNS.

“The actual results of our research were not surprising to us, but the overall importance of microglia in a wide variety of brain and spinal cord diseases was surprising. Microglia likely have a role in improving or worsening diseases such as multiple sclerosis, Alzheimer’s disease, Parkinson’s disease, obsessive compulsive disorder and Rett’s syndrome, just to name a few,” said John K. Park, MD, PhD, FAANS. “Microglia are the principal immune system cells of the brain and spinal cord, and help fight infections as well as help the healing process after injuries such as trauma and strokes. They also play a poorly understood role in many neurodegenerative and psychiatric diseases. We have developed methods to turn iPSCs into microglia. Because human iPSC can easily be obtained in large numbers, we can now generate large numbers of human microglia not only for use in experiments, but also potentially for use in treatments. The ability to study normal and diseased human microglia will lead to a greater understanding of their roles in healthy brains and various diseases. Diseases that are caused or exacerbated by defective microglia or a paucity of normal microglia may potentially be treated by microglia generated from a patient’s iPSC.”