Children with autism have extra synapses in brain

In a study of brains from children with autism, researchers found that autistic brains did not undergo normal pruning during childhood and adolescence. 

The images show representative neurons from autistic (left) and control (right) brains; the spines on the neurons indicate the location of synapses.

Credit: Guomei Tang, PhD and Mark S. Sonders, PhD/Columbia University Medical Center

Children and adolescents with autism have a surplus of synapses in the brain, and this excess is due to a slowdown in a normal brain "pruning" process during development, according to a study by neuroscientists at Columbia University Medical Center (CUMC).

Because synapses are the points where neurons connect and communicate with each other, the excessive synapses may have profound effects on how the brain functions.

The study was published in the August 21 online issue of the journal Neuron.

A drug that restores normal synaptic pruning can improve autistic-like behaviors in mice, the researchers found, even when the drug is given after the behaviours have appeared.

"This is an important finding that could lead to a novel and much-needed therapeutic strategy for autism," said Jeffrey Lieberman, MD, Lawrence C. Kolb Professor and Chair of Psychiatry at CUMC and director of New York State Psychiatric Institute, who was not involved in the study.

Although the drug, rapamycin, has side effects that may preclude its use in people with autism, "the fact that we can see changes in behaviour suggests that autism may still be treatable after a child is diagnosed, if we can find a better drug," said the study's senior investigator, David Sulzer, PhD, professor of neurobiology in the Departments of Psychiatry, Neurology, and Pharmacology at CUMC.

David Sulzer

During normal brain development, a burst of synapse formation occurs in infancy, particularly in the cortex, a region involved in autistic behaviours; pruning eliminates about half of these cortical synapses by late adolescence.

Synapses are known to be affected by many genes linked to autism, and some researchers have hypothesized that people with autism may have more synapses.

To test this hypothesis, co-author Guomei Tang, PhD, assistant professor of neurology at CUMC, examined brains from children with autism who had died from other causes.

Thirteen brains came from children ages two to 9, and thirteen brains came from children ages 13 to 20. Twenty-two brains from children without autism were also examined for comparison.

Dr. Tang measured synapse density in a small section of tissue in each brain by counting the number of tiny spines that branch from these cortical neurons; each spine connects with another neuron via a synapse.

By late childhood, she found, spine density had dropped by about half in the control brains, but by only 16 percent in the brains from autism patients.

"It's the first time that anyone has looked for, and seen, a lack of pruning during development of children with autism," Dr. Sulzer said, "although lower numbers of synapses in some brain areas have been detected in brains from older patients and in mice with autistic-like behaviours."


Clues to what caused the pruning defect were also found in the patients' brains; the autistic children's brain cells were filled with old and damaged parts and were very deficient in a degradation pathway known as "autophagy."

Cells use autophagy (a term from the Greek for self-eating) to degrade their own components. Using mouse models of autism, the researchers traced the pruning defect to a protein called mTOR.

When mTOR is overactive, they found, brain cells lose much of their "self-eating" ability and without this ability, the brains of the mice were pruned poorly and contained excess synapses.

"While people usually think of learning as requiring formation of new synapses, "Dr. Sulzer says, "the removal of inappropriate synapses may be just as important."

The researchers could restore normal autophagy and synaptic pruning, and reverse autistic-like behaviors in the mice, by administering rapamycin, a drug that inhibits mTOR.

The drug was effective even when administered to the mice after they developed the behaviors, suggesting that such an approach may be used to treat patients even after the disorder has been diagnosed.

Because large amounts of overactive mTOR were also found in almost all of the brains of the autism patients, the same processes may occur in children with autism.

"What's remarkable about the findings," said Dr. Sulzer, "is that hundreds of genes have been linked to autism, but almost all of our human subjects had overactive mTOR and decreased autophagy, and all appear to have a lack of normal synaptic pruning.

This says that many, perhaps the majority, of genes may converge onto this mTOR/autophagy pathway, the same way that many tributaries all lead into the Mississippi River.

Overactive mTOR and reduced autophagy, by blocking normal synaptic pruning that may underlie learning appropriate behaviour, may be a unifying feature of autism."

Alan Packer, PhD, senior scientist at the Simons Foundation, which funded the research, said the study is an important step forward in understanding what's happening in the brains of people with autism.

"The current view is that autism is heterogeneous, with potentially hundreds of genes that can contribute."

"That's a very wide spectrum, so the goal now is to understand how those hundreds of genes cluster together into a smaller number of pathways; that will give us better clues to potential treatments," he said.

"The mTOR pathway certainly looks like one of these pathways. It is possible that screening for mTOR and autophagic activity will provide a means to diagnose some features of autism, and normalizing these pathways might help to treat synaptic dysfunction and treat the disease."

Journal Reference: 
Guomei Tang, Kathryn Gudsnuk, Sheng-Han Kuo, Marisa L. Cotrina, Gorazd Rosoklija, Alexander Sosunov, Mark S. Sonders, Ellen Kanter, Candace Castagna, Ai Yamamoto, Zhenyu Yue, Ottavio Arancio, Bradley S. Peterson, Frances Champagne, Andrew J. Dwork, James Goldman, David Sulzer. "Loss of mTOR-Dependent Macroautophagy Causes Autistic-like Synaptic Pruning Deficits." Neuron, 2014; DOI: 10.1016/j.neuron.2014.07.040

Autism-related disorder: Researchers have found new role for mTOR in Tuberous Sclerosis

Researchers have found a novel role for a protein that has been implicated in an autism-related disorder known as tuberous sclerosis complex (TSC). 

The disease, which affects 1 in about 8,000 children, manifests itself in the form of mental retardation in addition to severe epileptic episodes.

The disease is caused by mutations in two tumor-suppressing proteins, TSC1 and TSC2.

"Kids with this condition have benign tumors that grow all over the body," said Bernardo Sabatini  the Takeda Professor of Neurobiology at Harvard Medical School and senior author of the study, "but we wanted to know what happened in the brain."

The researchers found that when mutations in TSC1 and TSC2 adversely affected a third protein, mTOR, this mutation increased brain activity, which can result in epileptic seizures.

Bernardo Sabatini

The findings were published in the May 8 issue of Neuron. A protein kinase, mTOR is responsible for controlling cell growth in many parts of the body and has been widely implicated in epilepsy and autism.

TSC1 and TSC2 normally repress the activity of mTOR to keep cell growth in check.

In the case of TSC, there are mutations in TSC1 or TSC2, and mTOR's ability to promote cell growth goes unchecked, resulting in tumors in regularly dividing cells.

"But neurons don't divide," said Sabatini. "So it was important to note the changes in these non-dividing cells."

The researchers hypothesized that mTOR's function in the brain related to homeostasis, the brain's ability to maintain a controlled level of electrical activity.

When there's a lot of electrical activity, a negative feedback system switches on to suppress activity.

Conversely, when levels are too low, other positive feedback pathways are engaged that bring the activity level back up.

"We went into this study with the specific hypothesis that mTOR would be part of the homeostatic loop in the brain," explained Sabatini.

In the case of TSC patients, they thought that mTOR was incapable of maintaining homeostasis and kept adding to the level of electrical activity, leading to seizures. "But we were wrong," he added.

Helen Bateup

"What we actually found was that mTOR is part of a positive feedback pathway," said Helen Bateup, HMS research fellow in neurobiology and first author on the study.

"When a cell is active, mTOR gets turned on more frequently and makes the cell even more active by reducing the amount of inhibition that the neuron receives."

In cells where TSC proteins are mutated, this positive feedback gets out of control, and the neuronal circuit remains overactive despite all the pathways that normally shut down activity being turned on.

"It's like the circuit is trying to keep itself quiet, but it can't," said Sabatini. "The out-of-control mTOR causes some cells to loss all inhibition, something that can't be compensated for by turning down excitation."

The researchers think this key difference in how mTOR operates, in working to promote electrical activity, is important for the disease because patients end up with high levels of dysfunctional mTOR that makes for highly active circuits prone to epileptic fits.

Furthermore, "we know that once a person has one seizure, they're much more likely to have more, a concept known as kindling," said Sabatini.

These findings are among the first to show that contrary to scientific consensus, mTOR does not play a part in everything.

"We have shown that one of the few things that mTOR does not seem to partake in is this negative feedback pathway," said Sabatini.

Working in both in vitro and in vivo mouse models, the researchers think the next step would be tease out the molecular pathway of mTOR's involvement in this positive feedback loop.

"It's also important to compare how this pathway works in normal brains versus a diseased model," added Bateup.

"A huge challenge when studying the brain is that there are so many feedback pathways that a mutation in one gene can result in a hundred other secondary changes," said Sabatini.

Rapamycin (Sirolimus), a drug currently used to prevent organ rejection following transplants, targets mTOR and brings activity levels back to normal.

"We could use the drug to restore this excitatory-inhibitory balance in the brain," said Bateup. "

A lot of drugs that treat epilepsy try to make inhibition more powerful but given that the primary problem here is that a group of cells has lost inhibition, that approach won't work," she added.

"What we might need is to target the excitation side. Or find ways of changing the biochemistry of the cells to make inhibitory synapses again."

"For this disease, this is the right time to start looking at human cells," said Sabatini.

"We have really good data from the mouse model and it would be a really nice test to see if the mouse model is really predictive of human disorder and if it's worth being continued."

More information: www.sciencedirect.… 62731300264X