Targeting an aspect of Down syndrome

University of Michigan researchers have determined how a gene that is known to be defective in Down syndrome is regulated and how its dysregulation may lead to neurological defects, providing insights into potential therapeutic approaches to an aspect of the syndrome.

Normally, nerve cells called neurons undergo an intense period of extending and branching of neuronal protrusions around the time of birth. During this period, the neurons produce the proteins of the gene called Down syndrome cell-adhesion molecule, or Dscam, at high levels. After this phase, the growth and the levels of protein taper off.

However, in the brains of patients with Down syndrome, epilepsy and several other neurological disorders, the amount of Dscam remains high. The impact of the elevated Dscam amount on how neurons develop is unknown.

Bing Ye, a faculty member at U-M’s Life Sciences Institute, found that in the fruit fly Drosophila, the amount of Dscam proteins in a neuron determines the size to which a neuron extends its protrusions before it forms connections with other nerve cells. An overproduction of Dscam proteins leads to abnormally large neuronal protrusions.

Ye also identified two molecular pathways that converge to regulate the abundance of Dscam. One, dual leucine zipper kinase (DLK), which is involved in nerve regeneration, promotes the synthesis of Dscam proteins. Another, fragile X mental retardation protein (FMRP), which causes fragile X syndrome when defective, represses Dscam protein synthesis. Because humans share these genes with Drosophila, the DLK-FMRP-Dscam relationship presents a possible target for therapeutic intervention, Ye said.

Many genes are involved in neurological disorders like Down syndrome, and how molecular defects cause the disease is complex.

"But because of the important roles of Dscam in the development of neurons, its related defect is very likely to be an aspect of Down syndrome and it may be an aspect of the syndrome that can be treated," said Ye, an assistant professor in the Department of Cell and Developmental Biology at the U-M Medical School.

Ye’s next step is to test the effects of overexpression of Dscam in mice to see how it changes the development of the nervous system and the behavior of the animal.

Down syndrome occurs in about one in 830 newborns; an estimated 250,000 people in the U.S. have the condition, according to the National Library of Medicine’s Genetics Home Reference.

Neurochemical Traffic Signals May Open New Avenues for the Treatment of Schizophrenia

Researchers at Boston University School of Medicine (BUSM) have uncovered important clues about a biochemical pathway in the brain that may one day expand treatment options for schizophrenia. The study, published online in the journal Molecular Pharmacology, was led by faculty within the department of pharmacology and experimental therapeutics at BUSM.

Patients with schizophrenia suffer from a life-long condition that can produce delusions, disordered thinking, and breaks with reality. A number of treatments are available for schizophrenia, but many patients do not respond to these therapies or experience side effects that limit their use.

This research focused on key components of the brain known as NMDA receptors. These receptors are located on nerve cells in the brain and serve as biochemical gates that allow calcium ions (electrical charges) to enter the cell when a neurotransmitter, such as glutamate, binds to the receptor. Proper activation of these receptors is critical for sensory perception, memory and learning, including the transfer of short-term memory into long-term storage. Patients with schizophrenia have poorly functioning or “hypoactive” NMDA receptors, suggesting the possibility of treatment with drugs that positively affect these receptors. Currently the only way to enhance NMDA receptor function is through the use of agents called agonists that directly bind to the receptor on the outer surface of the cell, opening the gates to calcium ions outside the cell.

In this study, the researchers discovered a novel “non-canonical” pathway in which NMDA receptors residing inside the cell are stimulated by a neuroactive steroid to migrate to the cell surface (a process known as trafficking), thus increasing the number of receptors available for glutamate activation. The researchers treated neural cells from the cerebral cortex with the novel steroid pregnenolone sulfate (PregS) and found that the number of working NMDA receptors on the cell surface increased by 60 to 100 percent within 10 minutes. The exact mechanism by which this occurs is not completely clear, but it appears that PregS increases calcium ions within the cell, which in turn produces a green light signal for more frequent trafficking of NMDA receptors to the cell surface.

Although still in the early stages, further research in this area may be instrumental in the development of treatments not only for schizophrenia, but also for other conditions associated with malfunctioning NMDA receptors, such as age-related decreases in memory and learning ability.

Neuronal regeneration and the two-part design of nerves

Researchers at the University of Michigan have evidence that a single gene controls both halves of nerve cells, and their research demonstrates the need to consider that design in the development of new treatments for regeneration of nerve cells.

A paper published online in PLOS Biology by U-M Life Sciences Institute faculty member Bing Ye and colleagues shows that manipulating genes of the fruit fly Drosophila to promote the growth of one part of the neuron simultaneously stunts the growth of the other part.

Understanding this bimodal nature of neurons is important for researchers developing therapies for spinal cord injury, neurodegeneration and other nervous system diseases, Ye said.

Nerve cells look strikingly like trees, with a crown of “branches” converging at a “trunk.” The branches, called dendrites, input information from other neurons into the nerve cell. The trunk, or axon, transmits the signal to the next cell.

"If you want to regenerate an axon to repair an injury, you have to take care of the other end, too," said Ye, assistant professor in the Department of Cell and Developmental Biology at the U-M Medical School.

The separation of the nerve cell into these two parts is so fundamental to neuroscience that it’s known as the “neuron doctrine,” but how exactly neurons create, maintain and regulate these two separate parts and functions is still largely unknown.

While the body is growing, the neuronal network grows rapidly. But nerve cells don’t divide and replicate like other cells in the body (instead, a specific type of stem cell creates them). Adult nerve cells appear to no longer have the drive to grow, so the loss of neurons due to injury or neurodegeneration can be permanent.

Ye’s paper highlights the bimodal nature of neurons by explaining how a kinase that promotes axon growth surprisingly has the opposite effect of impeding dendrite growth of the same cell.

In the quest to understand the fundamentals of nerve cell growth in order to stimulate regrowth after injury, scientists have identified the genes responsible for axon growth and were able to induce dramatic growth of the long “trunk” of the cell, but less attention has been given to dendrites.

There are technical reasons that studying axons is easier than studying dendrites: The bundle of axons in a nerve is easier to track under the microscope, but to get an image of dendrites would require labeling single neurons.

Ye’s lab circumvented that obstacle by using Drosophila as a model. Using this simple model of the nervous system, the scientists were able to reliably label both axons and dendrites of single neurons and see what happened to nerve cells with various mutations of genes that are shared between the flies and humans.

One of the genes shared by Drosophila and people is the one that makes a protein called Dual Lucine Zipper Kinase, or DLK. As described previously by other groups, DLK is a product of the gene responsible for axon growth. Cells with more of the protein had very long axons, and those without the gene or protein had no regeneration after nerve injury. The DLK kinase seemed a promising target for therapies to regenerate nerve cells.

However, Ye’s lab found that the kinase had the opposite effect on the dendrites: Lots of DLK leads to diminished dendrites.

"This in vivo evidence of bimodal control of neuronal growth calls attention to the need to look at the other side of a neuron in terms of developing new therapies," Ye said. "If we use this kinase, DLK, as a drug target for axon growth, we’ll have to figure out a way to block its effect on dendrites."

Brain capable of making its own version of Valium

The oral drug Valium – also known by its generic name, diazepam – was once popular with doctors in the 1970s as a treatment for seizures brought on by epilepsy. However, the drug, also used to treat anxiety, has fallen out of favor in recent years as it is prone to abuse and often dangerous if taken in high doses.

Now, in light of a recent study, the need for Valium to treat epilepsy may be even further diminished. Researchers from Stanford University School of Medicine have discovered a naturally occurring protein in the brains of mammals that acts like Valium, stopping certain types of seizures from occurring. Researchers hope that if they are able to discover a way to boost this protein naturally, doctors would no longer have a need to prescribe Valium.

The protein, identified as diazepam binding inhibitor (DBI), essentially acts like the brain’s very own brake system, sensing when a seizure is about to occur and arresting the process before it can spiral out of control.

“Our thinking on brain circuits and epilepsy has been that our brains have their own ways to control seizures, and this is why most of us aren’t having seizures every day,” study author John Huguenard, professor of neurology and neurological sciences at Stanford, told FoxNews.com. “But what happens as a seizure starts, a few cells in the brain may get too active, and you get an avalanche of activity that eventually can take up most of the brain circuitry. The brain’s own ‘Valium’ is acting as an anti-avalanche method, checking things when they’re first starting.”

According to Huguenard, the brain has two main groups of nerve cells. The first type of cells – excitatory cells – are responsible for stimulating other cells and sending messages from one area of the brain to another. This messaging process, known as excitation, is responsible for communicating what we see, what we smell, what we do, etc.

The other key type of cells are known as inhibitory cells, which are responsible for keeping the brain circuitry under control. If one area of the brain gets too excited and starts to receive too many signals at once, the inhibitory cells kick into gear and slow the process in order to restore balance.

“In terms of this form of epilepsy we’ve been studying, if a certain group of brain cells can’t communicate well through this inhibitory process, then (the animals) have seizures,” Huguenard said.

The protein DBI is a crucial component of the inhibitory process, as it boosts the actions of an important neurotransmitter called gamma-aminobutyric acid (GABA). Roughly one-fifth of the inhibitory nerve cells in the brain operate by secreting GABA, which binds to receptors located on excitatory cells, rendering them temporarily unable to fire any more electrical signals.  

Without DBI, GABA cannot be enhanced, and the excitatory cells ultimately don’t get the message telling them to calm down. However, up until now, this function of DBI was not well understood by researchers.

To determine exactly how DBI operates in the brains of mammals, Huguenard and his team analyzed a group of bioengineered mice with the DBI gene mutation, meaning their brains were incapable of producing DBI.

“When we tested seizures in these animals and tested communication, we found that (the inhibitory process) was ineffective and that the animals had more seizures,” Huguenard said. “It told us that this gene is producing a product in the brain that is controlling the seizures.”

When they re-introduced the DBI-gene back into the brains of these mice, GABA-induced inhibition was restored and the mice suffered from fewer seizures.

Benzodiazepine drugs, like Valium, work in a very similar way to DBI by also enhancing GABA-induced inhibition. But they often come at a high cost. Many who take these medications long-term develop a physical dependence on the drug, experiencing serious withdrawal symptoms if they cease taking it. Some studies have also found Valium to have an adverse effect on both short-term and long-term cognition.

While the researchers only examined the brains of mice, they are optimistic DBI exists similarly in the brains of humans as well. If the results end up translating to the human mind, Huguenard hopes to find a way to naturally boost DBI in the brain, negating the need for Valium to help control seizures.

“The ultimate goal would be to develop new lines of therapy that would take this general approach – taking the brain’s mechanism for dealing with seizures and making them even more effective,” Huguenard said.

The research was published May 30 in the journal Neuron.

From trauma to tau - Researchers tie brain injury to toxic form of protein

University of Texas Medical Branch at Galveston researchers have uncovered what may be a key molecular mechanism behind the lasting damage done by traumatic brain injury.

The discovery centers on a particular form of a protein that neuroscientists call tau, which has also been associated with Alzheimer’s disease and other neurodegenerative conditions. Under ordinary conditions, tau is essential to neuron health, but in Alzheimer’s the protein aggregates into two abnormal forms: so-called “neurofibrillary tangles,” and collections of two, three, or four or more tau units known as “oligomers.”

Neurofibrillary tangles are not believed to be harmful, but tau oligomers are toxic to nerve cells. They are also are thought to have an additional damaging property — when they come into contact with healthy tau proteins, they cause them to also clump together into oligomers, and so spread toxic tau oligomers to other parts of the brain.

Now, in experiments with laboratory rats, using novel antibodies developed at UTMB, scientists have found that traumatic brain injuries also generate tau oligomers. The destructive protein assemblages formed within four hours after injury and persisted for at least two weeks — long enough to suggest that they might contribute to lasting brain damage.

Significantly, the rats used in the experiments were normal, unlike the genetically modified animals used in most tau research. The findings are thus likely to be more relevant to human traumatic brain injuries.

“Although people have given some attention to the formation of neurofibrillary tangles after traumatic brain injury, we were the first to look at tau oligomers, because we have an antibody that allows us to separate them out and see how much of the total tau is the toxic species,” said Bridget Hawkins, lead author of a paper on the research now online in the Journal of Biological Chemistry. “We saw that it’s a substantial amount — enough to play an important role in the effects of traumatic brain injury.”

Those effects can include memory deficits, which have been recently shown by UTMB researchers to be induced by tau oligomers. Other long-term ramifications of TBI include seizures, and disruptions in the sleep-wake cycle. The UTMB scientists hypothesize that these problems could be avoided if physicians had a way to stop the process of tau oligomerization.

One possibility is a treatment based on the antibodies used to label tau oligomers in this project, which were developed as part of an effort to develop a vaccine against different neurodegenerative disorders.

“We have antibodies that can specifically target these tau oligomers without interfering with the function of healthy tau,” said UTMB associate professor Rakez Kayed, the senior author on the paper. “This is a new approach — we’re starting by targeting them in animals — but we hope to eventually humanize these antibodies for clinical trials.”

Preventing ‘traffic jams’ in brain cells

Imagine if you could open up your brain and look inside.

What you would see is a network of nerve cells called neurons, each with its own internal highway system for transporting essential materials between different parts of the cell.

When this biological machinery is operating smoothly, tiny motor proteins ferry precious cargo up and down each neuron along thread-like roadways called microtubule tracks. Brain cells are able to receive information, make internal repairs and send instructions to the body, telling the fingers to flex or the toes to curl.

But when the neuron gets blocked, this delicate harmony deteriorates. One result: diseases like Alzheimer’s.

Understanding such blockages and how traffic should flow normally in healthy brain cells could offer hope to people with neurodegenerative diseases.

Toward that end, a research team led by University at Buffalo biologist Shermali Gunawardena, PhD, has shown that the protein presenilin plays an important role in controlling neuronal traffic on microtubule highways, a novel function that previously was unknown.

The research results were published online on May 24 in the journal Human Molecular Genetics. Gunawardena’s co-authors are Ge Yang of Carnegie Mellon University and Lawrence S. B. Goldstein of the Howard Hughes Medical Institute and the University of California, San Diego.

Inside the nerves of fruit fly larvae, presenilin helped to control the speed at which molecular motors called kinesins and dyneins moved along neurons. When the scientists halved the amount of presenilin present in the highway system, the motors moved faster; they paused fewer times and their pauses were shorter.

Given this data, Gunawardena thinks that tweaking presenilin levels may be one way to free up traffic and prevent dangerous neuronal blockages in patients with Alzheimer’s disease.

“Our major discovery is that presenilin has a novel role, which is to control the movement of motor proteins along neuronal highways,” said Gunawardena, an assistant professor of biological sciences. “If this regulation/control is lost, then things can go wrong. This is the first time a protein that functions as a controller of motors has been reported.

“In Alzheimer’s disease, transport defects occur well before symptoms, such as cell death and amyloid plaques, are seen in post-mortem brains,” she added. “As a result, developing therapeutics targeted to defects in neuronal transport would be a useful way to attack the problem early.”

The findings are particularly intriguing because scientists have known for several years that presenilin is involved in Alzheimer’s disease.

Presenilin rides along neuronal highways in tiny organic bubbles called vesicles that sit atop the kinesin and dynein motors, and also contain a second protein called the amyloid precursor protein (APP). Presenilin participates in cutting APP into pieces called amyloid beta, which build up to form amyloid plaques in patients with Alzheimer’s disease.

Such buildups can lead to cell death by preventing the transport of essential materials—like proteins needed for cell repair—along neurons.

The findings of the new study mean that presenilin may contribute to Alzheimer’s disease in at least two ways: not just by cleaving APP, but also by regulating the speed of the molecular motors that carry APP along neuronal highways.

“More than 150 mutations in presenilin have been identified in Alzheimer’s disease,” Gunawardena said. “Thus, understanding its function is important to understanding what goes wrong in Alzheimer’s disease.”

To track the movement of the kinesins and dyneins, the team tagged their cargo with a yellow fluorescent protein. This enabled the scientists to view the molecular motors chugging along inside the neuron under a microscope in a living animal. A special computer program then analyzed the motors’ paths, revealing more details about the nature of their movement and how often they paused.

Newly understood circuits add finesse to nerve signals

An unusual kind of circuit fine-tunes the brain’s control over movement and incoming sensory information, and without relying on conventional nerve pathways, according to a study published this week in the journal Neuron.

Researchers at the University of Alabama at Birmingham (UAB) discovered new details of a mechanism operating in the cerebellum, the brain region that processes nerve signals coming in from the spinal cord and cortex.

“Our results explain a second layer of nerve signal transmission that depends, not on whether a nerve cell is wired into a defined signaling pathway circuit, but instead on how close it is to the pathway,” said Jacques Wadiche, Ph.D., assistant professor in the Department of Neurobiology within the UAB School of Medicine, investigator in the Evelyn McKnight Brain Institute at UAB and senior study author. “It has become clear that this kind of nerve circuit is intimately linked with autism and certain movement disorders, and we hope the mechanisms detailed here contribute to the design of new treatments.”

Beyond nerve pathways

Nerve cells are known to occur in defined pathways that transmit messages in one direction. This pathway-specific view of nerve signaling has been reinforced by high-tech imaging studies yielding detailed connectivity maps. Along these lines, the Obama Administration will soon ask Congress for $100 million in research funding to further improve such maps.

Within nerve pathways, each nerve cell sends an electric pulse down an extension of itself called an axon until it reaches a synapse, a gap between itself and the next cell in line. When it reaches an axon’s end, the pulse triggers the release of chemicals called neurotransmitters that float across the gap, where they either cause the downstream nerve cell to “fire” and pass on the message, or stop the message. In this way, each synapse between nerve cells in a pathway “decides” whether or not a message continues on.

In recent years, studies have found that neurotransmitters also spill into tissue surrounding axons in a type signaling not restricted to synaptic connections. With the term itself implying a mess, “spillover” was thought to degrade the capacity of nerve cells to precisely pass on signals.

The current study adds to recent evidence arguing that spillover may instead enhance message transmission, with the results revolving around three nerve cell types in the cerebellum: climbing fibers, Purkinje cells and interneurons.

Climbing fibers, which carry information from the brainstem into the cerebellum, play key roles in motor timing and sensory processing. Within these fibers, nerve cells release the excitatory neurotransmitter glutamate into synapses that then strive to pass messages deeper into the cerebellum. Purkinje cells are paired with climbing fibers and intent on inhibiting their signals.

When excited by glutamate from climbing fibers at one end, Purkinje cells release another neurotransmitter called GABA at their downstream synapse to stop the message. An excitatory signal triggers an inhibitory one as a counter-balance, a form of feedback critical to the function of the central nervous system. Lack of inhibition, for instance, causes circuits to seize, seizures and the death of Purkinje cells, the latter of which has been linked by post mortem studies to a higher incidence of autism spectrum disorders.

Previously, researchers thought that incoming signals from climbing fibers caused a single, strong response in the cerebellum: the activation of Purkinje cells that released GABA. The current study argues that such signals also trigger the firing of interneurons, nearby inhibitory middlemen that connect sets of nerve cells.

Interneurons within, and outside of, the glutamate spill zone around climbing fibers may have different effects on the other interneurons and Purkinje cells they connect to, according to the current finding. The interactions either inhibit or excite many Purkinje cells surrounding an active climbing fiber and refine its messages in a feedback system more sophisticated than once thought.

Glutamate has its effect by fitting into AMPA and NMDA receptor proteins, like a key into a lock, on the surfaces of nerve cells it signals to. The consensus has been that glutamate receptors occur only within synapses. Finding them on nerve cells outside of synapse-defined pathways represents “a fundamental shift in understanding,” said Wadiche, and may result in longer-lasting inhibition within key signaling pathways.

“A 2007 study published in Nature Neuroscience found that many climbing fibers signal to interneurons in the outer layer of the cerebellum outside nerve pathways and exclusively through glutamate spillover,” said Luke Coddington, a graduate student in Wadiche’s lab and study author. “Our team built on that observation to show how spillover affects the function of interneurons, Purkinje cells, and ultimately, the entire cerebellum. Spillover-mediated signaling recruits local microcircuits to extend the reach and finesse of climbing fiber signaling.”

The Secret Lives (and Deaths) of Neurons

As the human body fine-tunes its neurological wiring, nerve cells often must fix a faulty connection by amputating an axon — the “business end” of the neuron that sends electrical impulses to tissues or other neurons. It is a dance with death, however, because the molecular poison the neuron deploys to sever an axon could, if uncontained, kill the entire cell.

Researchers from the University of North Carolina School of Medicine have uncovered some surprising insights about the process of axon amputation, or “pruning,” in a study published May 21 in the journal Nature Communications. Axon pruning has mystified scientists curious to know how a neuron can unleash a self-destruct mechanism within its axon, but keep it from spreading to the rest of the cell. The researchers’ findings could offer clues about the processes underlying some neurological disorders.

“Aberrant axon pruning is thought to underlie some of the causes for neurodevelopmental disorders, such as schizophrenia and autism,” said Mohanish Deshmukh, PhD, professor of cell biology and physiology at UNC and the study’s senior author. “This study sheds light on some of the mechanisms by which neurons are able to regulate axon pruning.”

Axon pruning is part of normal development and plays a key role in learning and memory. Another important process, apoptosis — the purposeful death of an entire cell — is also crucial because it allows the body to cull broken or incorrectly placed neurons. But both processes have been linked with disease when improperly regulated.

The research team placed mouse neurons in special devices called microfluidic chambers that allowed the researchers to independently manipulate the environments surrounding the axon and cell body to induce axon pruning or apoptosis.

They found that although the nerve cell uses the same poison — a group of molecules known as Caspases — whether it intends to kill the whole cell or just the axon, it deploys the Caspases in a different way depending on the context.

“People had assumed that the mechanism was the same regardless of whether the context was axon pruning or apoptosis, but we found that it’s actually quite distinct,” said Deshmukh. “The neuron essentially uses the same components for both cases, but tweaks them in a very elegant way so the neuron knows whether it needs to undergo apoptosis or axon pruning.”

In apoptosis, the neuron deploys the deadly Caspases using an activator known as Apaf-1. In the case of axon pruning, Apaf-1 was simply not involved, despite the presence of Caspases. “This is really going to take the field by surprise,” said Deshmukh. “There’s very little precedent of Caspases being activated without Apaf-1. We just didn’t know they could be activated through a different mechanism.”

In addition, the team discovered that neurons employ other molecules as safety brakes to keep the “kill” signal contained to the axon alone. “Having this brake keeps that signal from spreading to the rest of the body,” said Deshmukh. “Remarkably, just removing one brake makes the neurons more vulnerable.”

Deshmukh said the findings offer a glimpse into how nerve cells reconfigure themselves during development and beyond. Enhancing our understanding of these basic processes could help illuminate what has gone wrong in the case of some neurological disorders.