Glial cells assist in the repair of injured nerves

Unlike the brain and spinal cord, the peripheral nervous system has an astonishing capacity for regeneration following injury. Researchers at the Max Planck Institute of Experimental Medicine in Göttingen have discovered that, following nerve damage, peripheral glial cells produce the growth factor neuregulin1, which makes an important contribution to the regeneration of damaged nerves.

From their cell bodies to their terminals in muscle or skin, neuronal extensions or axons in the peripheral nervous system are surrounded along their entire length by glial cells. These cells, which are known as Schwann cells, envelop the axons with an insulating sheath called myelin, which enables the rapid transmission of electrical impulses. Following injury to a peripheral nerve, the damaged axons degenerate. After a few weeks, however, they regenerate and are then recovered with myelin by the Schwann cells. For thus far unexplained reasons, however, the Schwann cells do not manage to regenerate the myelin sheaths completely. Thus the function of damaged nerves often remains permanently impaired and certain muscles remain paralysed in affected patients.

In a current research study, the scientists have succeeded in showing that the growth factor neuregulin1 supports nerve repair and the redevelopment of the myelin layer. This protein is usually produced by neurons and is localised on axons where it acts as an important signal for the maturation of Schwann cells and myelin formation. Because the axons rapidly degenerate after injury, the remaining Schwann cells lose their contact with the axons. They thus lack the neuregulin1 signal of the nervous fibres. “In the phase following nerve damage, in which the axons are missing, the Schwann cells must carry out many tasks without the help of axonal signals. If the Schwann cells cannot overcome this first major obstacle in the aftermath of nerve injury, the nerve cannot be adequately repaired,” explains Ruth Stassart, one of the study authors.

To prevent this, the Schwann cells themselves take over the production of the actual neuronal signal molecule. After nerve damage, they synthesise the neuregulin1 protein until the axons have grown again. With the help of genetically modified mice, the researchers working on this study were able to show that the neuregulin1 produced in Schwann cells is necessary for the new maturation of the Schwann cells and the regeneration of the myelin sheath after injury. “In mice that lack the neuregulin1 gene in their Schwann cells, the already incomplete nerve regeneration process is extensively impaired,” explains co-author Robert Fledrich.

The researchers would now like to examine in greater detail how the Schwann cells contribute to the complete repair of myelinated axons after nerve damage, so that this information can also be used for therapeutic purposes.

Socially Isolated Rats are More Vulnerable to Addiction

Rats that are socially isolated during a critical period of adolescence are more vulnerable to addiction to amphetamine and alcohol, found researchers at The University of Texas at Austin. Amphetamine addiction is also harder to extinguish in the socially isolated rats.

These effects, which are described this week in the journal Neuron, persist even after the rats are reintroduced into the community of other rats.

“Basically the animals become more manipulatable,” said Hitoshi Morikawa, associate professor of neurobiology in the College of Natural Sciences. “They’re more sensitive to reward, and once conditioned the conditioning takes longer to extinguish. We’ve been able to observe this at both the behavioral and neuronal level.”

Morikawa said the negative effects of social isolation during adolescence have been well documented when it comes to traits such as anxiety, aggression, cognitive rigidity and spatial learning. What wasn’t clear until now is how social isolation affects the specific kind of behavior and brain activity that has to do with addiction.

“Isolated animals have a more aggressive profile,” said Leslie Whitaker, a former doctoral student in Morikawa’s lab and now a researcher at the National Institute on Drug Abuse. “They are more anxious. Put them in an open field and they freeze more. We also know that those areas of the brain that are more involved in conscious memory are impaired. But the kind of memory involved in addiction isn’t conscious memory. It’s an unconscious preference for the place in which you got the reward. You keep coming back to it without even knowing why. That kind of memory is enhanced by the isolation.”

Researchers turn one form of neuron into another in the brain

A new finding by Harvard stem cell biologists turns one of the basics of neurobiology on its head – demonstrating that it is possible to turn one type of already differentiated neuron into another within the brain.

The discovery by Paola Arlotta and Caroline Rouaux “tells you that maybe the brain is not as immutable as we always thought, because at least during an early window of time one can reprogram the identity of one neuronal class into another,” said Arlotta, an Associate Professor in Harvard’s Department of Stem Cell and Regenerative Biology (SCRB).

The principle of direct lineage reprogramming of differentiated cells within the body was first proven by SCRB co-chair and Harvard Stem Cell Institute (HSCI) co-director Doug Melton and colleagues five years ago, when they reprogrammed exocrine pancreatic cells directly into insulin producing beta cells.

Arlotta and Rouaux now have proven that neurons too can change their mind. The work is being published on-line by the journal Nature Cell Biology.

In their experiments, Arlotta targeted callosal projection neurons, which connect the two hemispheres of the brain, and turned them into neurons similar to corticospinal motor neurons, one of two populations of neurons destroyed in Amyotrophic Lateral Sclerosis (ALS), also known as Lou Gehrig’s disease. To achieve such reprogramming of neuronal identity, the researchers used a transcription factor called Fezf2, which long as been known for playing a central role in the development of corticospinal neurons in the embryo.

What makes the finding even more significant is that the work was done in the brains of living mice, rather than in collections of cells in laboratory dishes. The mice were young, so researchers still do not know if neuronal reprogramming will be possible in older laboratory animals – and humans. If it is possible, this has enormous implications for the treatment of neurodegenerative diseases.

"Neurodegenerative diseases typically effect a specific population of neurons, leaving many others untouched. For example, in ALS it is corticospinal motor neurons in the brain and motor neurons in the spinal cord, among the many neurons of the nervous system, that selectively die," Arlotta said. "What if one could take neurons that are spared in a given disease and turn them directly into the neurons that die off? In ALS, if you could generate even a small percentage of corticospinal motor neurons, it would likely be sufficient to recover basic functioning," she said.

The experiments that led to the new finding began five years ago, when “we wondered: in nature you never seen a neuron change identity; are we just not seeing it, or is this the reality? Can we take one type of neuron and turn it into another?” Arlotta and Rouaux asked themselves.

Over the course of the five years, the researchers analyzed “thousands and thousands of neurons, looking for many molecular markers as well as new connectivity that would indicate that reprogramming was occurring,” Arlotta said. “We could have had this two years ago, but while this was a conceptually very simple set of experiments, it was technically difficult. The work was meant to test important dogmas on the irreversible nature of neurons in vivo. We had to prove, without a shadow of a doubt, that this was happening.”

The work in Arlotta’s lab is focused on the cerebral cortex, but “it opens the door to reprogramming in other areas of the central nervous system,” she said.

Arlotta, an HSCI principal faculty member, is now working with colleague Takao Hensch, of Harvard’s Department of Molecular and Cellular Biology, to explicate the physiology of the reprogrammed neurons, and learn how they communicate within pre-existing neuronal networks.

"My hope is that this will facilitate work in a new field of neurobiology that explores the boundaries and power of neuronal reprogramming to re-engineer circuits relevant to disease," said Paola Arlotta.

(Image courtesy Tulane University)

Who Decides in the Brain?

Whether in society or nature, decisions are often the result of complex interactions between many factors. Because of this it is usually difficult to determine how much weight the different factors have in making a final decision. Neuroscientists face a similar problem since decisions made by the brain always involve many neurons. Working in collaboration, the University of Tübingen and the Max Planck Institute for Biological Cybernetics, supported within the framework of the Bernstein Network, researchers lead by CIN professor Matthias Bethge have now shown how the weight of individual neurons in the decision-making process can be reconstructed despite interdependencies between the neurons.

When we see a person on the other side of the street who looks like an old friend, the informational input enters the brain via many sensory neurons. But which of these neurons are crucial in passing on the relevant information to higher brain areas, which will decide who the person is and whether to wave and say ‘hello’? A research group lead by Matthias Bethge has now developed an equation that allows them to calculate to what degree a given individual sensory neuron is involved in the decision process.

To approach this question, researchers have so far considered the information about the final decision that an individual sensory neuron carries. Just as an individual is considered suspicious if he or she is found to have insider information about a crime, those sensory neurons whose activity contains information about the eventual decision are presumed to have played a role in reaching the final decision. The problem with this approach is that neurons – much like people – are constantly communicating with each other. A neuron which itself is not involved in the decision may simply have received this information from a neighboring neuron and “joined in” the conversation. Actually, the neighboring cell sends out the crucial signal transmitted to the higher decision areas in the brain.

The new formula that has been developed by scientists addresses this by accounting not just for the information in the activity of any one neuron but also for the communication that takes place between them. This formula will now be used to determine whether only a few neurons that carry a lot of information are involved in the brain’s decision process, or whether the information contained in very many neurons gets combined. In particular, it will be possible to address the more fundamental question: In which decisions does the brain use information in an optimal way, and for which decisions is its processing suboptimal?

Reshaping the brain: scientists reprogram neurons after birth

The cerebral cortex—the gray matter that forms the outer layers of the mammalian cerebrum and cerebellum—is divided into six different layers based on the presence of specialized neurons, and we’ve known that since the early 1900s. Denis Jabaudon is interested in using the tools of modern biology to understand the genetic mechanisms that establish and maintain those layers. Over the past few years, his lab has published papers implicating various genes in the generation of specific neuronal subtypes.

Now they have gone a step further. They have developed a new electrochemical method to transfer genes into specific types of neurons—they call it iontoporation. Using it, they have transformed one type of neuron in a mature brain into a different type entirely.

Although Jabaudon and others have made some headway in working out how the different neurons arise, they still don’t know how plastic they are—if they can change fates after they started differentiating down one particular path. In the context of brain injury, it would be useful to know if certain neural circuits could be reprogrammed and repaired by having the neurons that are already present change fates to adapt to the damage. But this has been challenging to determine, because changing the fate of specific neurons in the latter stages of differentiation has been technically difficult.

Layer 4 mouse spiny neurons have round bodies, with many short dendrites (connections with other cells) that stay within their layer of the brain. They receive sensory signals from the thalamus. Layer 5B output neurons are pyramidal in shape, with a prominent dendrite that extends all the way to layer 1.

Fezf2 is a transcription factor that regulates the activity of other genes. It is expressed throughout the L5B neuron’s entire life, and it is necessary and sufficient for turning early cortical cells into L5B neurons.

When Jabaudon’s colleagues iontoporated Fezf2 into L4 neurons the day after mice were born, a week after the neurons had established their identity, it completely transformed them. You guessed it: the L4 neurons walked, talked, looked, and quacked just like L5B neurons. They looked like L5B’s and began transmitting signals to other nerves just as these cells did. Most significantly, however, they rearranged their intracortical inputs, meaning that they now received signals from layer 2/3 neurons instead of from the thalamus.

The researchers tried their iontoporation as late as ten days after the mice were born, and found that the neurons become less amenable to reprogramming with time, but some features were still malleable for a week after the mice were born—two weeks after the neurons originated.

The authors hope to further explore the molecular mechanisms responsible for the emergence and patterning of different cortical areas during brain development and their plasticity after injury. They hope that one day, reprogramming existing neurons could be a means of nervous system repair.

Lack of Protein Sp2 Disrupts Neuron Creation in Brain

A protein known as Sp2 is key to the proper creation of neurons from stem cells, according to researchers at North Carolina State University. Understanding how this protein works could enable scientists to “program” stem cells for regeneration, which has implications for neural therapies.

Troy Ghashghaei and Jon Horowitz, both faculty in NC State’s Department of Molecular Biomedical Sciences and researchers in the Center for Comparative Medicine and Translational Research, wanted to know more about the function of Sp2, a cell cycle regulator that helps control how cells divide. Previous research from Horowitz had shown that too much Sp2 in skin-producing stem cells resulted in tumors in experimental mice. Excessive amounts of Sp2 prevented the stem cells from creating normal cell “offspring,” or skin cells. Instead, the stem cells just kept producing more stem cells, which led to tumor formation.

“We believe that Sp2 must play a fundamental role in the lives of normal stem cells,” Horowitz says. “Trouble ensues when the mechanisms that regulate its activity are overwhelmed due to its excess abundance.”

Ghashghaei and his team – led by doctoral candidate Huixuan Liang – took the opposite approach. Using genetic tools, they got rid of Sp2 in certain neural stem cells in mice, specifically those that produce the major neurons of the brain’s cerebral cortex. They found that a lack of Sp2 disrupted normal cell formation in these stem cells, and one important result was similar to Horowitz’s: the abnormal stem cells were unable to produce normal cell “offspring,” or neurons. Instead, the abnormal stem cells just created copies of themselves, which were also abnormal.

“It’s interesting that both an overabundance of this protein and a total lack of it result in similar disruptions in how stem cells divide,” Ghashghaei says. “So while this work confirms that Sp2 is absolutely necessary for stem cell function, a lot of questions still remain about what exactly it is regulating, and whether it is present in all stem cells or just a few. We also need to find out if Sp2 deletion or overabundance can produce brain tumors in our mice as in the skin.

“Finally, we are very interested in understanding how Sp2 regulates a very important decision a stem cell has to make: whether to produce more of itself or to produce offspring that can become neurons or skin cells,” Ghashghaei adds. “We hope to address those questions in our future research, because these cellular mechanisms have implications for cancer research, neurodevelopmental diseases and regenerative medicine.”

The results appear online in Development. NC State graduate students Guanxi Xiao, and Haifeng Yin, as well as Dr. Simon Hippenmeyer, a collaborator with the Ghashghaei lab from Austria’s Institute of Science and Technology, contributed to the work. The work was funded by the National Institutes of Health and the American Federation for Aging Research.

My mistake or yours? How the brain decides

Humans and other animals learn by making mistakes. They can also learn from observing the mistakes of others. The brain processes self-generated errors in a region called the medial frontal cortex (MFC) but little is known about how it processes the observed errors of others. A Japanese research team led by Masaki Isoda and Atsushi Iriki of the RIKEN Brain Science Institute has now demonstrated that the MFC is also involved in processing observed errors.

The team studied the brains of monkeys while the animals performed the same task. Two monkeys sat opposite each other and took turns to choose between a yellow and green button, one of which resulted in a liquid reward for both. Each monkey’s turn consisted of two choices.

After blocks of between 5 and 17 choices, the button that resulted in reward was switched unpredictably, usually causing an error on the next choice. The choices made by each monkey immediately after such errors, or errors that were random, showed that they used both their own errors and their partner’s to guide their subsequent choices. While the monkeys performed this task, the researchers recorded activity of single neurons in their brains.

In this way they were able to determine which behavioural aspect was most closely associated with each neuron’s activity, explains Isoda. “We found that many neurons in the medial frontal cortex were not activated when the monkey made an error itself, but they became active when their partner made an error.” This brain activity shows that it is the MFC which processes observations of another’s error, and the corresponding behavior shows that observing and processing such errors guides subsequent actions.

“Such error identification and subsequent error correction are of crucial importance for developing and maintaining successful social communities,” says Isoda. “Humans are tuned into other people’s mistakes not only for competitive success, but also for cooperative group living. If non-invasive techniques become available in humans, then we should be able to identify medial frontal neurons that behave similarly.”

Having identified the MFC as being involved, Isoda now wants to delve deeper into the process. “The next steps will be to clarify whether the inactivation of medial frontal cortex neurons reduces the ability to identify others’ errors, and to determine whether other brain regions are also involved in the processing of others’ errors.”