Posts tagged astrocytes
Articles and news from the latest research reports.
Stopping tumours in their path
Glioblastoma (GBM) is the most common and deadly form of primary malignant brain cancer accounting for approximately 15% of all brain tumours and occurring mostly in adults between the ages of 45 and 70. The aggressive recurrent nature of this cancer is only temporarily contained by combined surgery, chemotherapy and radiation treatment. The recurrence of GBM is usually fatal, resulting in an average patient survival time of less than two years. A new study from the Montreal Neurological Institute and Hospital – The Neuro - at McGill University, published in Nature Communications, identifies two specific key players in the growth of GBM.
A GBM tumour contains a complex combination of different cell types, including ‘stem-like’ cells that are able to initiate brain tumour growth, even when present in very small numbers. These cells, known as brain-tumour initiating cells (BTICs), are believed to be among the cells that can re-initiate GBM if they are not completely eradicated through surgery, radiation and chemotherapy. Thus, BTICs represent an important therapeutic target for GBM treatment strategies.
“We wanted to find out how GBM-derived BTICs are able to initiate a tumour with the ultimate goal of preventing the re-growth of this deadly form of brain cancer,” says Dr. Stefano Stifani, neuroscientist at The Neuro and senior investigator on the paper. “What we found is that by impairing the activity of two transcription factors (proteins that control gene expression), termed FOXG1 and TLE, we can significantly reduce the ability of BTICs to give rise to brain tumours.” The researchers studied brain tumour growth in an in vivo mouse model using human GBM-derived BTICs. This approach provides what is called an in vivo environment that closely resembles the original human brain tumours. The demonstration that the FOXG1 and TLE proteins are important for the tumour-forming ability of human GBM-derived BTICs has long-term implications because FOXG1 and TLE control the expression of numerous genes. Identifying the genes whose expression is controlled by FOXG1 and TLE is expected to provide further information on the mechanisms involved in GBM tumourigenesis. In the long term, researchers hope to identify multiple important regulators, in order to find new potential therapeutic targets to impair the tumourigenic ability of BTICs.
“The implication of transcription factors FOXG1 and TLE in the tumour-forming ability of BTICs opens the door to possible strategies to block tumour growth – a major advance in the fight against GBM.”
(Image: ALAMY)
Concussion secrets unveiled in mice and people
There is more than meets the eye following even a mild traumatic brain injury. While the brain may appear to be intact, new findings reported in Nature suggest that the brain’s protective coverings may feel the brunt of the impact.
Using a newly developed mouse trauma model, senior author Dorian McGavern, Ph.D., scientist at the National Institute of Neurological Disorders and Stroke (NINDS), part of the National Institutes of Health, watched specific cells mount an immune response to the injury and try to prevent more widespread damage. Notably, additional findings suggest a similar immune response may occur in patients with mild head injury.
In this study, researchers also discovered that certain molecules, when applied directly to the mouse skull, can bypass the brain’s protective barriers and enter the brain. The findings suggested that, in the mouse trauma model, one of those molecules may reduce effects of brain injury.
Although concussions are common, not much is known about the effects of this type of damage. As part of this study, Lawrence Latour, Ph.D., a scientist from NINDS and the Center for Neuroscience and Regenerative Medicine, examined individuals who had recently suffered a concussion but whose initial scans did not reveal any physical damage to brain tissue. After administering a commonly used dye during MRI scans, Latour and his colleagues saw it leaking into the meninges, the outer covers of the brain, in 49 percent of 142 patients with concussion.
To determine what happens following this mild type of injury, researchers in Dr. McGavern’s lab developed a new model of brain trauma in mice.
"In our mice, there was leakage from blood vessels right underneath the skull bone at the site of injury, similar to the type of effect we saw in almost half of our patients who had mild traumatic brain injury. We are using this mouse model to look at meningeal trauma and how that spreads more deeply into the brain over time," said Dr. McGavern.
Dr. McGavern and his colleagues also discovered that the intact skull bone was porous enough to allow small molecules to get through to the brain. They showed that smaller molecules reached the brain faster and to a greater extent than larger ones. “It was surprising to discover that all these protective barriers the brain has may not be concrete. You can get something to pass through them,” said Dr. McGavern.
The researchers found that applying glutathione (an antioxidant that is normally found in our cells) directly on the skull surface after brain injury reduced the amount of cell death by 67 percent. When the researchers applied glutathione three hours after injury, cell death was reduced by 51 percent. “This idea that we have a time window within which to work, potentially up to three hours, is exciting and may be clinically important,” said Dr. McGavern.
Glutathione works by decreasing levels of reactive oxygen species (ROS) molecules that damage cells. In this study, high levels of ROS were observed at the trauma site right after the physical brain injury occurred. The massive flood of ROS set up a sequence of events that led to cell death in the brain, but glutathione was able to prevent many of those effects.
In addition, using a powerful microscopic technique, the researchers filmed what was happening just beneath the skull surface within five minutes of injury. They captured never-before-seen details of how the brain responds to traumatic injury and how it mobilizes to defend itself.
Initially, they saw cell death in the meninges and at the glial limitans (a very thin barrier at the surface of the brain that is the last line of defense against dangerous molecules). Cell death in the underlying brain tissue did not occur until 9-12 hours after injury. “You have death in the lining first and then this penetrates into the brain tissue later. The goal of therapies for brain injury is to protect the brain tissue,” said Dr. McGavern.
Almost immediately after head injury, the glial limitans can break down and develop holes, providing a way for potentially harmful molecules to get into the brain. The researchers observed microglia (immune cells that act as first responders in the brain against dangerous substances) quickly moving up to the brain surface, plugging up the holes.
Findings from Dr. McGavern’s lab indicate that microglia do this in two ways. According to Dr. McGavern, “If the astrocytes, the cells that make up the glial limitans, are still there, microglia will come up to ‘caulk’ the barrier and plug up gaps between individual astrocytes. If an astrocyte dies, that results in a larger space in the glial limitans, so the microglia will change shape, expand into a fat jellyfish-like structure and try to plug up that hole. These reactions, which have never been seen before in living brains, help secure the barrier and prevent toxic substances from getting into the brain.”
Studies have suggested that immune responses in the brain can often lead to severe damage. Remarkably, the findings in this study show that the inflammatory response in a mild traumatic brain injury model is actually beneficial during the first 9-12 hours after injury.
Mild traumatic brain injuries are a growing public health concern. According to a report from the Centers of Disease Control and Prevention, in 2009 at least 2.4 million people suffered a traumatic brain injury and 75 percent of those injuries were mild. This study provides insight into the damage that occurs following head trauma and identifies potential therapeutic targets, such as antioxidants, for reducing the damaging effects.
Common brain cell plays key role in shaping neural circuit
Stanford University School of Medicine neuroscientists have discovered a new role played by a common but mysterious class of brain cells.
Their findings, published online Nov. 24 in Nature, show that these cells, called astrocytes because of their star-like shape, actively refine nerve-cell circuits by selectively eliminating synapses — contact points through which nerve cells, or neurons, convey impulses to one another — much as a sculptor chisels away excess bits of rock to create an artwork.
“This was an entirely unknown function of astrocytes,” said Ben Barres, MD, PhD, professor and chair of neurobiology and the study’s senior author. The lead author was Won-Suk Chung, PhD, a postdoctoral scholar in Barres’ lab. More than one-third of all the cells in the human brain are astrocytes. But until quite recently, their role in the brain has remained obscure.
The study was performed on brain tissue from mice, but it is likely to apply to people as well, Barres said.
The discovery adds to a growing body of evidence that substantial remodeling of brain circuits takes place in the adult brain and that astrocytes are master sculptors of its constantly evolving synaptic architecture. The findings also raise the question of whether deficits and excesses in this astrocytic function could underlie, respectively, the loss of this remodeling capacity in old age or the wholesale destruction of synapses that erupts in neurodegenerative disorders, such as Alzheimer’s and Parkinson’s disease.
“Astrocytes are in the driver’s seat when it comes to synapse formation, function and elimination,” Barres said. In previous studies, he and his colleagues have shown that astrocytes play a critical role in determining exactly where and when new synapses are generated.
The new study showed that astrocytes’ synapse-gobbling behavior persists into adulthood and is triggered by activity in the neurons, suggesting astrocytes may be central to the constant fine-tuning and reconfiguring of brain circuits occurring throughout our lives in response to experiences such as learning, recollection, emotion and motion. While a healthy brain’s neurons remain intact for much a person’s lifetime, the connections between them — the synapses — are constantly forming, strengthening, weakening or dying.
The Barres team also has previously implicated another brain cell type, collectively known as microglia, in synaptic pruning in early development, when the young brain undergoes ongoing episodes of circuit remodeling. The role of astrocytes in synaptic refining, the new study shows, differs from that of microglia both in timing and mechanism.
Barres’ team began to suspect astrocytes’ participation in the pruning process when, having developed methods for isolating exceptionally pure populations of different types of brain cells, they saw that the genes for two separate biochemical pathways were active in astrocytes. Both of these pathways are involved in phagocytosis, the trash-collection process by which specialized cells in the body engulf, ingest and digest dead cells; foreign materials, including bacteria; debris from wounds; and so forth. At the leading end of the two pathways were two phagocytic receptors, MERKTK and MEGF10, which in other cell types have been shown to bind to particular proteins on targeted cells or materials, triggering the ensuing engulfment, ingestion and digestion of the targets.
It’s known that much of an astrocyte’s surface membrane is typically in close contact with neurons. In fact, a single astrocyte may ensheathe thousands of synapses. It was only natural, Barres said, to wonder whether astrocytes play some role in eliminating synapses.
The researchers first demonstrated that both MERKTK and MEGF10, along with their entire tool kits of cooperating proteins, are present in living astrocytes in the mouse brain. (In unpublished work, they have since confirmed this using human astrocytes.) Next, they showed that mouse astrocytes in a lab dish eagerly gobbled up synapses and dispatched them to their lysosomes, highly acidic internal garbage disposals found in most cells in the body. But this engulfment was dependent on astrocytes having functional MEGF10 and MERTK. Disabling one or the other receptor’s function cut in half astrocytes’ ability to engorge themselves on synapses; knocking out both receptors lowered the synapse-eating activity by about 90 percent.
To see if this happens in real life, Chung, Barres and their associates turned to a familiar experimental model: a brain area called the lateral geniculate nucleus, which is a critical component of the brain’s vision-processing system. The LGN receives inputs from neurons just a couple of steps downstream from the photoreceptors in the retina. In early development, neurons in the LGN are innervated by inputs from both eyes. But at a critical point in development, a highly selective synaptic-pruning process kicks in, resulting in each neuron from one side of the LGN being contacted pretty much only by neurons from a single eye. This pruning process in the LGN is dependent on the transmission of waves of spontaneous neuronal impulses originating in the retina.
Experimenting with mice that had entered the critical period for synaptic pruning in the LGN, the investigators labeled the incoming neurons in this system with different-colored stains so their synaptic regions could be identified within astrocytes if the astrocytes ate them up. And sure enough, a lot of this label turned up inside astrocytes’ lysosomes, indicating that astrocytes were actively ingesting synapses. Knocking out one or another or, especially, both of the two phagocytic receptors greatly reduced the amount of labeled synaptic material found in astrocytes. Impairing astrocytic MERKTK and MEGF10 function also caused a failure of LGN neurons to restrict their inputs to only neurons from just one eye, clearly implicating astrocytes in that process. Electrophysiology experiments proved that the LGN neurons in the MERKTK- and MEGF10-knockout mice retained an excessive number of synapses, demonstrating that astrocytes play an active role in pruning synapses during development.
Importantly, injection of a drug blocking the transmission of spontaneous waves of electrical impulses originating in the retina severely impaired astrocytes’ ability to eat synapses, showing that the synapse-pruning propensity is linked to neuronal activity. Other tests showed that astrocytic phagocytosis of synapses continues into adulthood.
Barres said this raises the question of whether astrocytes function throughout life to continually restructure our neuronal circuitry in response to experientially induced brain activity. If astrocytes’ synaptic snacking slows with aging, as that of other phagocytic cell types is known to do, it could reduce the aging brain’s capacity to adapt to new experiences, he said. “Maybe you need the astrocytes to gobble up old synapses to make room for new ones.”
If so, it may be possible someday to design drugs to keep astrocytes’ phagocytic process from slowing, Barres added. Such drugs might prevent the accumulation in aging brains of past-their-prime synapses, which are vulnerable to degeneration in Alzheimer’s, Parkinson’s and other neurodegenerative disease characterized by massive synapse loss.
Rare disease yields clues about broader brain pathology
Alexander disease is a devastating brain disease that almost nobody has heard of — unless someone in the family is afflicted with it. Alexander disease strikes young or old, and in children destroys white matter in the front of the brain. Many patients, especially those with early onset, have significant intellectual disabilities.
(Image: A mutant gene that causes the deadly Alexander disease creates an overgrowth of the protein GFAP in mouse brain cells called astrocytes (right) compared to normal brain cells (left))
Regardless of the age when it begins, Alexander disease is always fatal. It typically results from mutations in a gene known as GFAP (glial fibrillary acidic protein), leading to the formation of fibrous clumps of protein inside brain cells called astrocytes.
Classically, astrocytes and other glial cells were considered “helpers” that nourish and protect the neurons that do the actual communication. But in recent years, it’s become clear that glial cells are much more than passive bystanders, and may be active culprits in many neurological diseases.
Now, in a report in the Journal of Neuroscience, researchers at UW-Madison show that Alexander disease also affects neurons, and in a way that impacts several measures of learning and memory.
Mice were engineered to contain the same mutation in GFAP that is found in human patients. Their astrocytes spontaneously increased production of GFAP, the same response found after many types of injury or disease in the brain. In Alexander disease, the result is an increase in mutant GFAP that is “toxic to the cell, and unfortunately astrocytes respond by making more GFAP,” says first author Tracy Hagemann, an associate scientist with the university’s Waisman Center.
While GFAP is usually found in astrocytes, it also occurs in neural stem cells, a population of cells that persist in some areas of the brain to continually spawn new neurons throughout adulthood. In the mouse versions of Alexander disease, neural stem cells are present, but they fail to develop into neurons, Hagemann says. “Think of a garden where your green beans never sprouted. Was it too cold for them to sprout, or was there another problem? Something similar is happening with these neural stem cells. They are present, but inert, and we’re not sure why.”
The shortage of new neurons could explain why the mice with excess GFAP failed a test that required them to remember the location of a submerged platform in a tub of water.
The report is “the first to suggest that the problems in Alexander disease extend beyond just the white matter and astrocytes, and may provide a clue to the problems with learning and memory that are such prominent features in the human disease,” says lab leader Albee Messing, a professor of comparative biosciences in the UW School of Veterinary Medicine.
One immediate question that the team will try to answer is whether the same defect in stem cells can be found in autopsy samples stored over many years to allow just this kind of investigation.
Still to be clarified is whether the mutation affects the neural stem cells directly, or whether it acts through other astrocytes that are nearby. “We do know that the astrocytes become activated with this GFAP mutation,” Hagemann says. “That activation — a kind of inflammation — could be making the environment hostile to young neurons. Or the mutation could be changing the neural stem cells themselves in some other way.
"Medicine advances by teasing things apart," says Hagemann. "A single mutation can work in different ways — through different chains of cause and effect leading to different symptoms of a disease. In this case it’s like the old question of nature versus nurture. Was the stem cell born bad — was it genetically doomed? Or were the reactive astrocytes in the neighborhood a toxic influence? Or both? This is an important question for Alexander disease and other brain deteriorating disorders, especially with the current focus on stem cells as a source for new neurons and therapy."
Already, the Waisman group is screening drugs that might slow GFAP production. Eventually, Hagemann says, the work may illuminate the role of astrocyte dysfunction in other neural diseases featuring aggregates of misformed proteins, including ALS, Parkinson’s, and Alzheimer’s disease.
(Source: news.wisc.edu)
Drug Shows Early Promise in Treating Liver Failure-Related Seizures
A study out today in the journal Nature Medicine suggests a potential new treatment for the seizures that often plague children with genetic metabolic disorders and individuals undergoing liver failure. The discovery hinges on a new understanding of the complex molecular chain reaction that occurs when the brain is exposed to too much ammonia.
The study shows that elevated levels of ammonia in the blood overwhelm the brain’s defenses, ultimately causing nerve cells to become overexcited. The researchers have also discovered that bumetanide – a diuretic drug used to treat high blood pressure – can restore normal electrical activity in the brains of mice with the condition and prevent seizures.
“Ammonia is a ubiquitous waste product of regular protein metabolism, but it can accumulate in toxic levels in individuals with metabolic disorders,” said Maiken Nedergaard, M.D., D.M.Sc., co-director of the University of Rochester Medical Center (URMC) Center for Translational Neuromedicine and lead author of the article. “It appears that the key to preventing the debilitating neurological effects of ammonia toxicity is to correct a molecular malfunction which causes nerve cells in the brain to become chemically unbalanced.”
In healthy people, ammonia is processed in the liver, converted to urea, and expelled from the body in urine. Because it is a gas, ammonia can slip through the blood-brain-barrier and make its way into brain tissue. Under normal circumstances, the brain’s housekeeping cells – called astrocytes – sweep up this unwanted ammonia and convert it into a compound called glutamine which can be more easily expelled from the brain.
However, individuals with certain genetic metabolic disorders and people with impaired liver function because of chronic hepatitis, alcoholism, acetaminophen overdose, and other toxic liver conditions cannot remove ammonia from their bodies quickly enough. The result is a larger than normal concentration of ammonia in the blood, a condition called hyperammonemia.
When too much ammonia makes its way into the central nervous system, it can lead to tremors, seizures and, in extreme cases, can cause comas and even lead to death. In children with metabolic disorders the frequent seizures can lead to long-term neurological impairment.
While ammonia has long been assumed to be the culprit behind the neurological problems associated with inherited metabolic disorders and liver failure, the precise mechanisms by which it triggers seizures and comas have not been fully understood. The new study reveals that ammonia causes a chain of events that alters the chemistry and electrical activity of the brain’s nerve cells, causing them to fire in uncontrolled bursts.
One of the keys to unraveling the effects of ammonia on the brain has been new imagining technologies such as two-photon microscopy which allow researchers to watch this phenomenon in real time in the living brains of mice. As suspected, they observed that when high levels of ammonia enter the brain, astrocytes become quickly overwhelmed and cannot remove it fast enough.
The abundant ammonia in the brain mimics the function of potassium, an important player in neurotransmission, and tricks neurons into becoming depolarized. This makes it more likely that electrical activity in the brain will exceed the threshold necessary to trigger seizures.
Furthermore, the researchers observed that one of the neuron’s key molecular gatekeepers – a transporter known as NKCC1 – was also fooled into thinking that the ammonia was potassium. As a result, it went into overdrive, loading neurons with too much chloride. This in turn prevents the cells from stabilizing itself after spikes in activity, keeping the cells in a heightened level of electrical “excitability.”
The team found that the drug bumetanide, a known NKCC1 inhibitor, blocked this process and prevented the cells from overloading with chloride. By knocking down this “secondary” cellular effect of ammonia, the researchers were able to control the seizures in the mice and prolong their survival.
“The neurologic impact of hyperammonemia is a tremendous clinical problem without an effective medical solution,” said Nedergaard. “The fact that bumetanide is already approved for use gives us a tremendous head start in terms of developing a potential treatment for this condition. This study provides a framework to further explore the therapeutic potential of this and other NKCC1 inhibitors.”
(Source: urmc.rochester.edu)
Researchers Ferret Out Function Of Autism Gene
Findings in bacteria, yeast, mice show how flawed transport gene contributes to the condition
Researchers say it’s clear that some cases of autism are hereditary, but have struggled to draw direct links between the condition and particular genes. Now a team at the Johns Hopkins University School of Medicine, Tel Aviv University and Technion-Israel Institute of Technology has devised a process for connecting a suspect gene to its function in autism.
In a report in the Sept. 25 issue of Nature Communications, the scientists say mutations in one such autism-linked gene, dubbed NHE9, which is involved in transporting substances in and out of structures within the cell, causes communication problems among brain cells that likely contribute to autism.
“Autism is considered one of the most inheritable neurological disorders, but it is also the most complex,” says Rajini Rao, Ph.D., a professor of physiology in the Institute for Basic Biomedical Sciences at the Johns Hopkins University School of Medicine. “There are hundreds of candidate genes to sort through, and a single genetic variant may have different effects even within the same family. This makes it difficult to separate the chaff from the grain, to distinguish harmless variations from disease-causing mutations. We were able to use a new process to screen variants in one candidate gene that has been linked to autism, and figure out how they might contribute to the disorder.”
An estimated one in 88 children in the United States is affected by autism spectrum disorders, a group of neurological development conditions marked by varying degrees of social, communication and behavioral problems. Scientists for years have looked for the biological roots of the problem using tools such as genome-wide association studies and gene-linkage analysis, which crunch genetic and health data from thousands of people in an effort to pinpoint disease-causing genetic variants. But while such techniques have turned up a number of gene mutations that may be linked to autism, none of them appear in more than 1 percent of people with the condition. With numbers that low, researchers need a way to screen variants in order to make a definitive link, Rao says.
For the new study, Rao and her collaborators focused on NHE9, which other researchers had flagged as a suspect in attention-deficit hyperactivity disorder, addiction and epilepsy as well as autism spectrum disorders. The gene was already known to be involved in transporting hydrogen, sodium and potassium ions in and out of cellular compartments called endosomes, and the team wondered how this function might be related to neurological conditions.
Rao’s collaborators at Tel Aviv University and Technion-Israel Institute of Technology constructed a computer model of the NHE9 protein based on previous research on a distant relative in bacteria. They then used the model to predict how autism-linked variants in the NHE9 gene would affect the protein’s shape and function. Some of them were predicted to cause dramatic changes, while other changes appeared to be more subtle.
Rao’s team next tested how these variant forms of NHE9 would affect a relatively simple organism often used in genetic studies: yeast. “Using yeast to screen the function of variants was a quick, easy and inexpensive way of figuring out which were worth further study, and which we could ignore because they didn’t have any effect,” Rao says. To do that, the team engineered the yeast form of NHE9 to have the variants seen in autistic people.
For those mutations that did have a detectable effect on the yeast, the team moved on to a third and more challenging step, in mouse brains. They homed in on astrocytes, a type of brain cell that clears the signaling molecule glutamate out of the way after it has performed its job of delivering a message across a synapse between two nerve cells. Using lab-grown mouse astrocytes with variant forms of NHE9, the researchers found a change in the pH (acidity) inside cellular compartments called endosomes, which in turn altered the ability of cells to take up glutamate. Because endosomes are the vehicles that deliver cargo essential for communication between brain cells, changing their pH alters traffic to and from the cell surface, which could affect learning and memory, Rao says. “Elevated glutamate levels are known to trigger seizures, perhaps explaining why autistic patients with mutations in NHE9 and related genes also have seizures,” she notes.
Rao and her team hope that pinpointing the importance of this trafficking mechanism in autism spectrum disorders may lead to the development of new drugs for autism that alter endosomal pH. As the use of genomic data becomes increasingly commonplace in the future, the step-wise strategy devised by her team can be used to screen gene variants and identify at-risk patients, she says.
(Source: hopkinsmedicine.org)
Key target responsible for triggering detrimental effects in brain trauma identified
Researchers studying a type of cell found in the trillions in our brain have made an important discovery as to how it responds to brain injury and disease such as stroke. A University of Bristol team has identified proteins which trigger the processes that underlie how astrocyte cells respond to neurological trauma.
The star-shaped astrocytes, which outnumber neurons in humans, are a type of glial cell that comprise one of two main categories of cell found in the brain along with neurons. The cells, which have branched extensions that reach synapses (the connections between neurons) blood vessels, and neighbouring astrocytes, play a pivotal role in almost all aspects of brain function by supplying physical and nutritional support for neurons. They also contribute to the communication between neurons and the response to injury.
However, the cells are also known to trigger both beneficial and detrimental effects in response to neurological trauma. When the brain is subjected to injury or disease, the cells react in a number of ways, including a change in shape. In severe cases, the altered cells form a scar, which is thought to have beneficial, as well as detrimental effects by allowing prompt repair of the blood-brain barrier, and limiting cell death, but also impairing the regeneration of nerve fibres and the effective incorporation of neuronal grafts - where additional neuronal cells are added to the injured site.
The cells change shape via the regulation of a structural component of the cell called the actin cytoskeleton, which is made up of filaments that shrink and grow to physically manoeuvre parts of the cell. In the lab, the team cultured astrocytes in a dish and were able to make them change shape by chemically or genetically manipulating proteins that control actin, and also by mimicking the environment that the cells would be exposed to during a stroke.
By doing so the team found that very dramatic changes in cell shape were caused by controlling the actin cytoskeleton in the in vitro stroke model. The team also identified additional protein molecules that control this process, suggesting that a complex mechanism is involved.
Dr Jonathan Hanley from the University’s School of Biochemistry said: “Our findings are crucial to our understanding of how the brain responds to many disorders that affect millions of people every year. Until now, the details of the actin-based mechanisms that control astrocyte morphology were unknown, so we anticipate that our work will lead to future discoveries about this important process.”
(Source: eurekalert.org)
UC Davis stem cell study uncovers the brain-protective powers of astrocytes
One of regenerative medicine’s greatest goals is to develop new treatments for stroke. So far, stem cell research for the disease has focused on developing therapeutic neurons — the primary movers of electrical impulses in the brain — to repair tissue damaged when oxygen to the brain is limited by a blood clot or break in a vessel. New UC Davis research, however, shows that other cells may be better suited for the task.
Published today in the journal Nature Communications, the large, collaborative study found that astrocytes — neural cells that transport key nutrients and form the blood-brain barrier — can protect brain tissue and reduce disability due to stroke and other ischemic brain disorders.
“Astrocytes are often considered just ‘housekeeping’ cells because of their supportive roles to neurons, but they’re actually much more sophisticated,” said Wenbin Deng, associate professor of biochemistry and molecular medicine at UC Davis and senior author of the study. “They are critical to several brain functions and are believed to protect neurons from injury and death. They are not excitable cells like neurons and are easier to harness. We wanted to explore their potential in treating neurological disorders, beginning with stroke.”
Deng added that the therapeutic potential of astrocytes has not been investigated in this context, since making them at the purity levels necessary for stem cell therapies is challenging. In addition, the specific types of astrocytes linked with protecting and repairing brain injuries were not well understood.
The team began by using a transcription factor (a protein that turns on genes) known as Olig2 to differentiate human embryonic stem cells into astrocytes. This approach generated a previously undiscovered type of astrocyte called Olig2PC-Astros. More importantly, it produced those astrocytes at almost 100 percent purity.
The researchers then compared the effects of Olig2PC-Astros, another type of astrocyte called NPC-Astros and no treatment whatsoever on three groups of rats with ischemic brain injuries. The rats transplanted with Olig2PC-Astros experienced superior neuroprotection together with higher levels of brain-derived neurotrophic factor (BDNF), a protein associated with nerve growth and survival. The rats transplanted with NPC-Astros or that received no treatment showed much higher levels of neuronal loss.
To determine whether the astrocytes impacted behavior, the researchers used a water maze to measure the rats’ learning and memory. In the maze, the rats were required to use memory rather than vision to reach a destination. When tested 14 days after transplantation, the rats receiving Olig2PC-Astros navigated the maze in significantly less time than the rats that received NPC-Astros or no treatment.
The investigators used cell culture experiments to determine whether the astrocytes could protect neurons from oxidative stress, which plays a significant role in brain injury following stroke. They exposed neurons co-cultured with both types of astrocytes to hydrogen peroxide to replicate oxidative stress. They found that, while both types of astrocytes provided protection, the Olig2PC-Astros had greater antioxidant effects. Further investigation showed that the Olig2PC-Astros had higher levels of the protein Nrf2, which increased antioxidant activity in the mouse neurons.
“We were surprised and delighted to find that the Olig2PC-Astros protected neurons from oxidative stress in addition to rebuilding the neural circuits that improved learning and memory,” said Deng.
The investigators also investigated the genetic qualities of the newly identified astrocytes. Global microarray studies showed they were genetically similar to the standard NPC-Astros. The Olig2PC-Astros, however, expressed more genes (such as BDNF and vasoactive endothelial growth factor, or VEGF) associated with neuroprotection. Many of these genes help regulate the formation and function of synapses, which carry signals between neurons.
Additional experiments showed that both the Olig2PC-Astros and NPC-Astros accelerated synapse development in mouse neurons. The Olig2PC-Astros, however, had significantly greater protective effects over the NPC-Astros.
In addition to being therapeutically helpful, the Olig2PC-Astros showed no tumor formation, remained in brain areas where they were transplanted and did not differentiate into other cell types, such as neurons.
“Dr. Deng’s team has shown that this new method for deriving astrocytes from embryonic stem cells creates a cell population that is more pure and functionally superior to the standard method for astrocyte derivation,” said Jan Nolta, director of the UC Davis Institute for Regenerative Cures. “The functional improvement seen in the brain injury models is impressive, as are the higher levels of BDNF. I will be excited to see this work extended to other brain disease models such as Huntington’s disease and others, where it is known that BDNF has a positive effect.”
Deng added that the results could lead to stem cell treatments for many neurodegenerative diseases.
“By creating a highly purified population of astrocytes and showing both their therapeutic benefits and safety, we open up the possibility of using these cells to restore brain function for conditions such as Alzheimer’s disease, epilepsy, traumatic brain disorder, cerebral palsy and spinal cord injury,” said Deng.
Difficult-to-study diseases such as Alzheimer’s, schizophrenia, and autism now can be probed more safely and effectively thanks to an innovative new method for obtaining mature brain cells called neurons from reprogrammed skin cells. According to Gong Chen, the Verne M. Willaman Chair in Life Sciences and professor of biology at Penn State University and the leader of the research team, “the most exciting part of this research is that it offers the promise of direct disease modeling, allowing for the creation, in a Petri dish, of mature human neurons that behave a lot like neurons that grow naturally in the human brain.” Chen added that the method could lead to customized treatments for individual patients based on their own genetic and cellular information. The research will be published in the journal Stem Cell Research.
"Obviously, we don’t want to remove someone’s brain cells to experiment on, so recreating the patient’s brain cells in a Petri dish is the next best thing for research purposes and drug screening," Chen said. Chen explained that, in earlier work, scientists had found a way to reprogram skin cells from patients to become unspecialized or undifferentiated pluripotent stem cells (iPSCs). "A pluripotent stem cell is a kind of blank slate," Chen explained. "During development, such stem cells differentiate into many diverse, specialized cell types, such as a muscle cell, a brain cell, or a blood cell. So, after generating iPSCs from skin cells, researchers then can culture them to become brain cells, or neurons, which can be studied safely in a Petri dish."
Now, in their new research, Chen and his team have found a way to differentiate iPSCs into mature human neurons much more effectively, generating cells that behave similarly to neurons in the brain. Chen explained that, in their natural environment, neurons are always found in close proximity to star-shaped cells called astrocytes, which are abundant in the brain and help neurons to function properly. “Because neurons are adjacent to astrocytes in the brain, we predicted that this direct physical contact might be an integral part of neuronal growth and health,” Chen explained.
To test this hypothesis, Chen and his colleagues began by culturing iPSC-derived neural stem cells, which are stem cells that have the potential to become neurons. These cells were cultured on top of a one-cell-thick layer of astrocytes so that the two cell types were physically touching each other.
"We found that these neural stem cells cultured on astrocytes differentiated into mature neurons much more effectively," Chen said, contrasting them with other neural stem cells that were cultured alone in a Petri dish. "It was almost as if the astrocytes were cheering the stem cells on, telling them what to do, and helping them fulfill their destiny to become neurons."
To demonstrate the superiority of the neurons grown next to astrocytes, Chen and his co-authors used an electrophysiology recording technique to show that the cells grown on astrocytes had many more synaptic events — signals sent out from one nerve cell to the others. In another experiment, after growing the neural stem cells next to astrocytes for just one week, the researchers showed that the newly differentiated neurons start to fire action potentials — the rapid electrical excitation signal that occurs in all neurons in the brain. In a final test, the team members added human neural stem cells to a mixture with mouse neurons. “We found that, after just one week, there was a lot of ‘cross-talk’ between the mouse neurons and the human neurons,” Chen said. He explained that “cross-talk” occurs when one neuron contacts its neighbors and releases a chemical called a neurotransmitter to modulate its neighbor’s activity.
"Previous researchers could only obtain brain cells from deceased patients who had suffered from diseases such as Alzheimer’s, schizophrenia, and autism," Chen said. "Now, researchers can take skin cells from living patients — a safe and minimally invasive procedure — and convert them into brain cells that mimic the activity of the patient’s own brain cells." Chen added that, by using this method, researchers also can figure out how a particular drug will affect a particular patient’s own brain cells, without needing the patient to try the drug — eliminating the risk of serious side effects. "The patient can be his or her own guinea pig for the design of his or her own treatment, without having to be experimented on directly," Chen said.
Environment moulds behaviour - and not just that of people in society, but also at the microscopic level. This is because, for their function, neurons are dependent on the cell environment, the so-termed extracellular matrix. Researchers at the Ruhr-Universität have found evidence that this complex network of molecules controls the formation and activity of the neuronal connections. The team led by Dr. Maren Geißler und Prof. Andreas Faissner from the Department of Cell Morphology and Molecular Neurobiology reports in the “Journal of Neuroscience” in collaboration with the team of Dr. Ainhara Aguado, Prof. Christian Wetzel and Prof. Hanns Hatt from the Department of Cell Physiology.
Neurons and astrocytes in culture
In cooperation with Prof. Uwe Rauch from Lund University in Sweden, Bochum’s biologists examined cells from the brains of two mouse species: a species with a normal extracellular matrix and a species which lacked four components of the extracellular matrix due to genetic manipulation, namely the molecules tenascin-C, tenascin-R, neurocan and brevican. They took the cells from the hippocampus, a brain structure that is crucial for the long-term memory. The team not only examined neurons but also astrocytes, which are in close contact with the neurons, support their function and secrete molecules for the extracellular matrix.
Formation, stability and activity of the neuronal connections depend on the matrix
The researchers cultivated the neurons and astrocytes together for four weeks with a specially developed culture strategy. Among other things, they observed how many connections, known as synapses, the neurons formed with each other and how stable these were over time. If either the astrocytes or the neurons in the culture dish derived from animals with a reduced extracellular matrix, these synapses proved to be less stable in the medium term, and their number was significantly reduced. Together with the Department of Cell Physiology at the RUB and the University of Regensburg, the team also showed that the neurons with a mutated matrix showed lower spontaneous activity than normal cells. The extracellular matrix thus regulates the formation, stability and activity of the neuronal connections. The researchers also examined a special structure of the extracellular matrix, the so-called perineuronal nets, which the Nobel laureate Camillo Golgi first described more than a century ago. They were significantly reduced in the environment of genetically modified cells.